Nickel Molybdate Catalysts and Their Use in the Selective Oxidation of Hydrocarbons

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Nickel Molybdate Catalysts and Their Use in theSelective Oxidation of HydrocarbonsL. M. Madeira a , M. F. Portela b & C. Mazzocchia ca LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia, Universidadedo Porto, Porto, Portugalb GRECAT (UQUIMAF, ICEMS, Lisboa), Departamento de Engenharia Química, InstitutoSuperior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049‐001, Lisboa,Portugalc Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, Milano,Italy

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To cite this article: L. M. Madeira, M. F. Portela & C. Mazzocchia (2004): Nickel Molybdate Catalysts and Their Use in theSelective Oxidation of Hydrocarbons, Catalysis Reviews: Science and Engineering, 46:1, 53-110

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Nickel Molybdate Catalysts and Their Use in theSelective Oxidation of Hydrocarbons

L. M. Madeira,1 M. F. Portela,2,* and C. Mazzocchia3

1LEPAE, Departamento de Engenharia Quımica, Faculdade de Engenharia,

Universidade do Porto, Porto, Portugal2GRECAT (UQUIMAF, ICEMS, Lisboa), Departamento de Engenharia Quımica,

Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Lisboa, Portugal3Dipartimento di Chimica, Materiali e Ingegneria Chimica,

Politecnico di Milano, Milano, Italy

CONTENTS

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

2. Preparation of Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.1. Coprecipitation Techniques . . . . . . . . . . . . . . . . . . . . . . 55

2.2. Other Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

2.2.1. Molybdenum-Enriched Catalysts . . . . . . . . . . . . . . 60

2.2.2. Nickel-Enriched Catalysts . . . . . . . . . . . . . . . . . . 61

2.3. Supported and Doped Catalysts . . . . . . . . . . . . . . . . . . . . 61

3. Thermal Activation—Transition of Phases . . . . . . . . . . . . . . . . . 63

4. Characterization of Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . 66

53

DOI: 10.1081/CR-120030053 0161-4940 (Print); 1520-5703 (Online)

Copyright # 2004 by Marcel Dekker, Inc. www.dekker.com

*Correspondence: M. F. Portela, GRECAT (UQUIMAF, ICEMS, Lisboa), Departamento de

Engenharia Quımica, Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Av. Rovisco Pais,

1049-001, Lisboa, Portugal; Fax: þ351-21-8477695; E-mail: [email protected].

CATALYSIS REVIEWS

Vol. 46, No. 1, pp. 53–110, 2004

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4.1. Composition of Phases for Catalysts with Different Ni :

Mo Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.2. Other Physicochemical Characterizations . . . . . . . . . . . . . . 67

4.2.1. Stoichiometric Nickel Molybdate . . . . . . . . . . . . . . 67

4.2.2. Catalysts with Excess Molybdenum or Nickel . . . . . . . 71

4.2.3. Catalysts Prepared Using Organic Precursors and

Sol–Gel Methods . . . . . . . . . . . . . . . . . . . . . . . 75

4.2.4. Doped and Supported Nickel Molybdates . . . . . . . . . 76

4.3. Characterization of the High Temperature b-Phase . . . . . . . . . 79

5. Applications of Ni–Mo–O Catalysts . . . . . . . . . . . . . . . . . . . . 80

5.1. Oxidation of Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . 81

5.2. Oxidative Dehydrogenation of Light Alkanes . . . . . . . . . . . . 85

5.2.1. Undoped Ni–Mo Catalysts . . . . . . . . . . . . . . . . . . 85

5.2.2. Doped and Supported Catalysts . . . . . . . . . . . . . . . 89

5.2.3. Kinetics and Mechanism . . . . . . . . . . . . . . . . . . . 93

5.3. Nature of Active Sites . . . . . . . . . . . . . . . . . . . . . . . . . 97

6. Conclusions and Future Trends . . . . . . . . . . . . . . . . . . . . . . . 98

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

ABSTRACT

This paper reviews the preparation techniques, characterization, and use of nickel

molybdate catalysts in the selective oxidation of hydrocarbons, particularly of light

alkanes. Catalysts with different Ni :Mo ratios, unsupported and supported, undoped

and doped, were considered. Particular attention is given to the thermal activation

process for the transition of the low temperature a-phase into the metastable b-phase,

which was shown to be more selective in some cases. Special reference is also made to

the results of kinetic studies performed, to the mechanisms proposed for some

important reactions, and to the nature of the active sites. Finally, after some general

conclusions, future trends are analyzed.

Key Words: Nickel molybdate; Preparation; Characterization; Selective oxidation;

Hydrocarbons; Oxidative dehydrogenation; Light alkanes.

1. INTRODUCTION

Olefins, aromatics, and many oxygenates are widely used as important raw materials

in industrial processes,[1,2] and thus the strong pressure of international markets has led

to constant optimization of production processes. Cost reduction can be achieved by

using cheaper raw materials (for instance alkanes), combined in some cases with the use

of more sophisticated catalysts. Indeed, in the last years a clear trend has been obser-

ved for the use of light alkanes for the direct production of oxygenates—through

Madeira, Portela, and Mazzocchia54

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partial oxidation[3–5]—or to manufacture olefins through dehydrogenation or oxidative

dehydrogenation (ODH) processes,[5–7] due to the ready availability and low price of

natural gas. However, this is a challenging problem for the chemical industry because

alkanes are less reactive than the products obtained, such as alkenes, dienes, or aldehydes

and acids, which are easily totally oxidized at the high temperatures required to activate

alkanes properly.

Therefore, around the world much effort has been put into developing new catalytic

systems providing selective oxidation of hydrocarbons, particularly light alkanes with

useful yields. However, the search for better and more effective catalyst compositions,

preparations, and processes continues and, up to now, few promising catalysts were found

for these applications. For instance, metal molybdates were successfully employed in

selective oxidation reactions and are quite versatile catalysts for important industrial

processes.[5] Among them, nickel molybdates show very interesting potential for oxidation

reactions, and particularly for ODH of light alkanes. A large number of papers and

patents is found in the literature regarding these applications (mentioned throughout this

text). But nickel (Ni)–molybdenum (Mo) catalysts are also very important for other

processes, such as the hydrodesulfurization and hydrodenitrogenation of petroleum

distillates;[8–17] the water–gas shift reaction;[18] the steam reforming, hydrogenolysis, and

cracking of n-butane;[19] the oxidative coupling of methane;[20] and other industrially

important hydrogenation and hydrotreating reactions.[8,21–24] Despite this large number of

important industrial applications, a review that systematically analyzes the preparation

techniques used, the more important characterization results, and the main catalytic

studies performed for oxidation reactions with Ni–Mo–O catalysts, is not found in the

open scientific literature. With this review we intend to fill this gap. We should remark

that we will only consider reaction investigations involving selective oxidation of

hydrocarbons.

2. PREPARATION OF CATALYSTS

2.1. Coprecipitation Techniques

During the preparation, through the coprecipitation method, of nickel–molybdenum

catalysts with different Ni :Mo ratios, Andrushkevich et al.[25] found in 1973 that both the

chemical composition and composition of phases of the obtained precipitate depend

strongly on the precipitation conditions (concentration of reactant ions in solution,

temperature, and duration of the aging process). The unsatisfactory aspects of the

coprecipitation method, involving direct mixture of the solutions, and particularly the lack

of reproducibility in the results, were eliminated by using an experimental setup that

allowed continuous preparation of the catalysts by precipitation.[25] The nickel nitrate and

ammonium paramolybdate solutions were mixed at constant flow rate at 868C. The NH4þ

ion concentration in the molybdenum-solution was equal to the NO32 ion concentration in

the nickel solution. The Ni :Mo ratio in the solution was varied by changing the respective

ratio in the original solutions. When steady-state conditions for precipitation were

established, the pH in the reaction volume was 5.4. The obtained precipitates were air

dried at room temperature and calcined at 5008C.[26]

Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 55

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Andrushkevich et al. also knew that, for preparation of nickel molybdates, the pH of

the medium during precipitation has a significant influence on the composition of the

precipitates. Thus, they decided to investigate the problem thoroughly.[27] They found that

by increasing the ammonia concentration in the paramolybdate, while the other

precipitation conditions were kept constant, an increase in the nickel concentra-

tion in the final precipitate was recorded due to solubilization of molybdenum with

ammonia.

After the 1980s, several works were published in which the NiO–MoO3 system was

studied because of its use as a hydrodesulfurization catalyst. However, the preparation

methods adopted varied slightly from one group to another:

Vagin et al.[28] prepared NiO–MoO3 samples with various compositions by

coprecipitation of analytical salts [Ni(NO3)2 and (NH4)6Mo7O24], from the

corresponding solutions at 908C and pH ¼ 6.0–6.5. The solutions containing the

precipitates were then evaporated in a water bath, dried at 1108C, and calcined at

6008C for 6 hr.

Brito et al.[29] also prepared a series of Ni–Mo mixed oxides by coprecipitation

(either in continuous or discontinuous mode), always controlling the precipitation

conditions in order to change the Ni :Mo ratio of the final product, namely by the

pH of the medium.

The hydrated precursor of the hydrodesulfurization catalysts[11] was synthesized by

coprecipitation of nickel nitrate (pH ¼ 4.7) and ammonium heptamolybdate

(pH ¼ 5.6) aqueous solutions. The methodology used to obtain the phase that is

stable at high temperatures (b-NiMoO4) will be described later (see Section 3).

The investigations carried out at the Polytechnic of Milan, Italy, have helped, among other

aspects, to clarify the experimental conditions that determine the formation of oxides with

different compositions in the NiO–MoO3 system. In two preliminary studies,[30,31] special

attention is given to the methodology that enables the precursor of the catalytically active

phase to be obtained. The solvated precursor was prepared by mixing, with stirring,

equimolar solutions of ammonium molybdate and nickel nitrate [Ni(NO3)2 . 6H2O] at pH

5.6 and at a temperature of 858C. The molybdenum solution was prepared by dissolution

of molybdic acid (H2MoO4. H2O) in an ammoniacal solution at 858C and pH of 6.2. The

precursor obtained, partially crystalline and with a pale yellow color, was dried at 1208Cand thermally activated at 5508C for 2 hr. It is noteworthy that the NiMoO4 prepared by

coprecipitation was also patented;[32] however, nickel molybdate was also synthesized by

the dry mode, from NiO and MoO3.[30,31]

As shown by Mazzocchia et al.,[30–34] the precipitation process can lead to different

precursors with the general formula:

xNiO yMoO3 nH2O mNH3 (1)

Small changes in experimental conditions such as pH, precipitation temperature,

H2MoO4/NH4OH ratio, filtration temperature, duration of aging of the precipitate in the

mother liquor, and duration and temperature of drying may lead to precursors with

different x, y, n, and m values. The effect of some of these parameters is shown in Fig. 1.


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Attention should be paid to the fact that each arrow refers to the effect of a given parameter

on the type of precursor obtained.

For formation of the precursors, the following equilibria are established between the

different species:[33]

7MoO2�4(aq:) þ 8Hþ(aq:) !Mo7O

6�24(aq:) þ 4H2O (2)

Ni2þ(aq:) þMoO2�4(aq:) !NiMoO4(s) (3)

Ni2þ(aq:) þMo7O6�24(aq:) !NiMo6O

10�24(aq:) þ (Mo6þ) (4)

Ni2þ(aq:) þ 2OH�(aq:) !Ni(OH)2(s) (5)

The conditions required for preparation of pure a-NiMoO4 are highly critical.[31] In order

to avoid polymerization of the molybdate ions it is not sufficient to control the

environmental conditions (Fig. 1). The rate at which the nickel solution is added is also a

determining factor, probably because the rate of Eq. (4) is a critical condition.

For formation of the precursor with Formula (1), the main reactions involved are the

following:

1

6,

x

y, 1)Eqs. (3)þ (4); x ¼ y)Eq. (3);

1 ,x

y, 1)Eqs. (3)þ (5)

Figure 1. Effect of the precipitation parameters on the type of precursor obtained. Each arrow

refers to the effect of a given parameter. (Information adapted from Refs.[30,33].)


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Thus, at pH 6 the G precursor is obtained (see Fig. 1), which becomes more green as pH is

increased due to coprecipitation of nickel hydroxide [cf. Eq. (5)], yielding an x/y ratio

higher than 1. In fact, in the patented preparation method, in which the temperature was

maintained at 858C, but with a pH of 6, the final catalyst had the following composition:

Ni1.5MoO4.5.[32] On the other hand, if room temperature is used, without changing the

other experimental conditions, a pale blue (B) precursor is obtained for which y/x ¼ 6

(cf. Fig. 1). After thermal activation, the sample exhibits the infrared spectrum and

x-ray diffraction (XRD) pattern characteristic of excess MoO3. The yellow precursor

(S—of stoichiometric), where x ¼ y ¼ 1, is obtained in the conditions already mentioned.

If, during filtration, the solution is allowed to cool (between 658C and 858C), a pale yellowprecursor (E) is obtained with a ratio y/x . 1. For this precursor in particular, both the

time of aging in the mother liquor and the temperature of the solution determine the y/xratio. Using, for instance, a pH of 5.6 but mixing the solutions at 708C, a precursor is

obtained, which after calcination (during 2 hr at 5508C) yields a catalyst with the formula

NiMo1.5O5.5.[32]

The stoichiometric catalyst has been used in several studies, but the preparation

procedure employed was not always exactly the same. For instance, 0.5M solutions of

Ni(NO3)2 . 6H2O and H2MoO4 were employed, with a final pH of 5.1 and at 858C.[35] Inother cases 0.25M solutions were used, with precipitation at pH 5.2 and a temperature of

908C,[36] with filtration of the precipitate at 858C and drying for 4 hr at 1108C, followed bycalcination for 2 hr at 5508C.[37]

The parameters m and n in the precursor with Formula (1) are affected by slight

variations in the temperature or in the amount of ammonia. For instance, in a study where

the yellow precursor (S) was prepared by using 0.25M solutions of molybdic acid and

nickel nitrate, at 858C and pH 5.4, thermogravimetric analyses revealed the following

composition for the precursor: NiMoO4. 3/4 H2O . 3/4 NH3.

[33] A green precursor (G)

was also prepared with the following composition: Ni1þdxMoO4. 1/3 H2O . 5/3 NH3,

from a 0.25M solution of ammonium heptamolybdate [(NH4)6Mo7O24. 4H2O] at 858C,

the pH of the solution being adjusted with ammonia in order to yield an NH3/Mo ratio of

1.5. The 0.25M solution of nickel nitrate, at the same temperature, was added at a

controlled rate (7mL/min). The green precipitate was immediately formed and the pH

dropped from its initial value of 8.48 to 7.09 in 1 hr.

2.2. Other Techniques

New techniques have been developed for preparation of catalysts that enable

clarification of specific aspects in multicomponent catalytic systems. Better control of

the contact between phases is achieved when compared with catalysts prepared by

precipitation or impregnation. In these situations, it is also difficult to control the thickness

and structure of the superficial layer. With these goals in mind, Zou and Schrader[38] used

reactive sputtering, an advanced technique for materials processing, in order to produce

catalysts of the NiMoO4–MoO3 system with controlled compositions and structures,

especially thin films with well-defined architectures. Another advantage of the samples

prepared in this way, compared to materials obtained by precipitation, is that they are more

easily characterized by several techniques. They prepared very thin films of MoO3, of

NiMoO4, and of combinations of both oxides over different supports (including SiO2),


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which were later characterized by various techniques. In the samples containing both

components, the films were prepared through a sequential process, the NiMoO4 being

deposited over the predeposited MoO3. More recently, they have examined in detail the

deposition parameters for the reactive sputtering technique and found that the multilayer

films of NiMoO4 on a-MoO3 include an interfacial material identified as b-NiMoO4,

which was detected at relatively low temperatures in the bilayer structures.[39]

The use of organic salts for preparation of active catalytic systems, such as

Ni–Mo–O, has the advantage of providing a lower crystallization temperature. Using an

oxalic precursor (a product that decomposes at a lower temperature than ammonia) and

different thermal treatments, Mazzocchia et al. have obtained several catalysts with

different compositions.[34] The NiC2O4. 2H2O and MoOC2O4

. 4H2O mixture was

prepared by adding ammonium heptamolybdate to 250mL of a solution containing a large

excess of oxalic acid. After dissolution, nickel nitrate is added at room temperature such

that the Ni :Mo ratio is 1. The solution (0.14M in Ni and in Mo) is then slowly warmed

under vacuum to 408C. Precipitation starts immediately and increases as the water

evaporates. The precursor is finally dried at 1208C for 15 hr.

Another method of catalyst preparation that has been recently used resorts to natural

substances or polymers. A polymeric network is created, containing the ionic compounds

of the active catalyst inside the organic matrix. In this context, Anouchinsky et al. tested

a new methodology for NiMoO4 preparation in which the precursor is an organic gel (agar-

agar) containing the Ni and Mo ions in a 1 : 1 atomic ratio.[40] This approach offers several

advantages: it is cheap and simple and multicomponent catalytic systems can be prepared

by simple dissolution of the desired elements, at the appropriate concentrations, in the

aqueous solution. In this way one may change, for instance, the Ni :Mo ratio with the

simultaneous presence of promoters. The gel was prepared from 0.5M solutions of nickel

nitrate and ammonium heptamolybdate and mixed at room temperature with continuous

stirring. A load of 1% (by weight) of powdered agar-agar is added and the solution warmed

at 808C to solubilize the agar-agar. Rapid cooling of the solution yields the gel, which is

subsequently dried by slow heating (108C/hr) from room temperature up to 1208C. Thistemperature is then maintained for 4 hr and finally the gel is calcined.

The agents that control pH in the synthesis of precursors of mixed oxides must be

easily removed from the precipitate. From this point of view, oxalic precursors have been

shown to be advantageous since they crystallize at low temperatures.[34] It would then be

expected that, for the Ni–Mo–O system, the use of the sol–gel technique could be

beneficial.

The sol–gel technique offers a low-temperature method for synthesizing materials

that are either totally inorganic in nature or both inorganic and organic. The process

offers many advantages, including the use of simple and inexpensive equipment,

excellent control of the stoichiometry of precursor solutions, and ease of compositional

modifications.

Good control of the stoichiometry may be very useful for fine control in the

preparation of Ni–Mo–O catalysts, as their composition is crucial for catalytic app-

lications. With this goal in mind, as well as the fact that precursor decomposition can be

achieved at low temperatures, several authors decided to apply the sol–gel route for

preparation of nickel molybdate catalysts. Although papers on this subject are somewhat

scarce, we should mention the work of Anouchinsky et al., who have prepared several

catalysts by the sol–gel method.[40] As expected, homogeneous dispersion of the Ni and


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Mo ions in the precursor was achieved, which led to formation of the NiMoO4 phases,

whose crystallization occurs at temperatures lower than those prepared by coprecipitation.

This process also leads to the stabilization of the b-phase at room temperature. Lezla

et al.[41] have also adopted the sol–gel methodology to prepare the stoichiometric catalyst

using citric acid (1mol/Ni), which was added to a solution of nickel nitrate (0.4M). Then

a solution of (NH4)6Mo7O24. 4H2O was added very slowly so as to avoid precipitation.

The solution was evaporated until a gel and then a solid were obtained. Finally, the solid

was ground and heated in air at 5008C for 24 hr.[41]

The sol–gel technique also was used recently for the preparation of supported catalysts,

with particular advantages in the case of the Ni–Mo–O system (see Section 2.3).

Nanocrystalline NiMoO4, among other molybdates, was also recently prepared from

the complete evaporation of a polymer-based metal-complex precursor solution.[42,43]

These fine-grained materials (with particle diameters less than 100 nm) are expected to

have potential applications in many technological areas.

2.2.1. Molybdenum-Enriched Catalysts

Catalysts containing excess MoO3 are often prepared by drying the final solution,

after mixing the reactants in appropriate ratios. However, this method gives rise to

numerous problems regarding the nature of the dried precursor, because the concentration

of the solution changes during the drying process, leading to precipitation of hetero-

polymolybdates or to a mixture of molybdate and molybdic acid.[44] Thus, Mazzocchia et

al. decided to prepare several catalysts with excess Mo, all with the same composition but

obtained from different precursors. They used decomposition of the heteropolymolybdate

or dry-mixing of nickel molybdate and excess of molybdenum trioxide. A catalyst studied,

derived from the heteropolymolybdate, was NiMoO4. 5MoO3, obtained through thermal

decomposition of (NH4)4H6NiMo6O24. 5H2O.

[44]

In other works a certain excess of MoO3 was introduced by cooling the solution of

the a-NiMoO4 precursor, with consequent coprecipitation of (NH4)4H6NiMo6O24. nH2O

(cf. Fig. 1).[31] Both the temperature and the cooling period depend on the excess of

molybdenum desired.

In a very interesting work published by Ozkan and Schrader, the synthesis of nickel

molybdates containing an excess of molybdenum through several methods is described in

detail.[45] The excess of Mo (relative to the stoichiometric) is present as a new phase,

MoO3, substantially increasing the complexity of the system. The authors report the fol-

lowing methods for incorporation of MoO3 into the catalyst: precipitation, solid state

reaction, and impregnation. Nickel molybdates prepared through precipitation were

obtained from aqueous solutions of ammonium heptamolybdate [(NH4)6Mo7O24. 7H2O]

and nickel nitrate [Ni(NO3)2 . 6H2O], the pH being changed with ammonium hydroxide or

nitric acid solutions. In order to obtain pure nickel molybdate, pH during addition and

reaction was kept at 6 (with a temperature of 638C). The catalysts with excess MoO3 are

obtained by acidification of the medium during addition of the solutions, the pH depending

on the excess of MoO3 desired. It should be stressed that with this procedure the catalyst

composition is insensitive to both concentration and composition of the reactants, the pH

of the medium during the precipitation being the key factor. The solid-state synthesis

basically consists of heating NiO together with MoO3, or MoO3 mixed with NiMoO4.

Nickel molybdate, obtained by precipitation, was also impregnated with ammonium


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heptamolybdate to provide catalysts with an excess of MoO3 between 2% and 55%.

Molybdenum trioxide was also impregnated with NiMoO4. The experimental procedure

used for these syntheses has been described in detail.[45]

More recently, Lezla et al. have also prepared Ni–Mo–O catalysts with Mo :Ni ratios

between 0.90 and 2.15 using several methods, which include precipitation, evaporation to

dryness, sol–gel, impregnation, and mechanical mixing. They analyzed the influence of

the preparation method on the catalytic performances for propane ODH.[41]

2.2.2. Nickel-Enriched Catalysts

Catalysts with an excess of Ni have usually been prepared in two ways: (i)

precipitation at 858C and pH 6.2, using 0.25M solutions of H2MoO4 and Ni(NO3)2, which

yields an Ni :Mo ratio of 1.40 and provides formation of NiO together with the a- and

b-phases of NiMoO4; and (ii) mechanical mixing of NiMoO4. H2O and Ni(OH)2, which

provides Ni :Mo ratios in the range 1.1–1.6, followed by activation at various

temperatures.[46] Chemical impregnation of a-NiMoO4, using an aqueous solution of

nickel acetate, was also adopted for preparation of Ni-enriched catalysts.[47] Using moly-

bdenum oxalate instead provided catalysts with excess molybdenum.[47]

2.3. Supported and Doped Catalysts

The use of supported nickel molybdate catalysts in ODH reactions is not very

common. Some exceptions are recent works in which TiO2 (anatase)[48] and SiO2

[49–52]

were used to support the active phase. In the first case, the catalysts were prepared using

two procedures: (i) wet impregnation of the support with an aqueous suspension of

NiMoO4, and (ii) direct precipitation of NiMoO4 on the support surface at 858C, usingsolutions of nickel nitrate and ammonium heptamolybdate.[48] The SiO2-supported

catalysts were also prepared by wet impregnation[52] or by direct precipitation of NiMoO4

on the support,[49,52] or even by sol–gel routes.[50,51]

Nickel–molybdenum catalysts are frequently used as supported catalysts in important

industrial processes like hydrodesulfurization or hydrogenation, so many works exist in

the literature regarding these issues. Conventional methods of preparation of hydro-

treatment catalysts usually consist of depositing transition-metal salts onto the support,

usually g-Al2O3, followed by calcination to produce stable oxidic materials that must be

sulfided either prior to or during the start-up of the hydrotreatment process. In a pioneering

work, Laine et al.[12] found that it is advantageous to impregnate alumina with nickel

before molybdenum. Later, Brito and Laine[53] prepared nickel–molybdenum catalysts

supported over g-Al2O3 through impregnation of commercial g-Al2O3. First Mo was

added—using an ammonium heptamolybdate solution—followed by drying (overnight at

1208C) and calcination (2 hr at 4008C). Portions of this solid were then submitted to dry

impregnation with nickel nitrate solutions with the purpose of obtaining solids with

several NiO loads. After drying, the samples were calcined at different temperatures

between 4008C and 8008C.It is well known that for the preparation of silica-supported catalysts, the sol–gel route

allows very good control of the composition, homogeneity, and textural properties of

the final products. In fact, the nanoscale chemistry involved in sol–gel methods appears to


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be the most straightforward way to prepare tailored nanocomposites, including organic–

inorganic hybrid materials. Moreover, sol–gel methods have been found to be effective for

dispersing small metal oxide particles in nonmetallic matrices. With these features in mind,

Cauzzi et al.[51] prepared NiMoO4/SiO2 composites by the sol–gel process via silicon

alkoxides, involving Si(OMe)4 (tetramethoxysilane), Ni(NO3)2, and (NH4)6Mo7O24 as

starting materials. The dried gels were treated at increasing temperatures until crystalline

grains of nickel molybdate highly dispersed in the amorphous silica matrix were formed

(6758C). Xerogels with different Ni :Mo ratios were synthesized and the preparation

procedures are described in detail.[51] It is noteworthy that besides leading to the support of

catalytic materials, the xerogel plays the important role of stabilizing b-nickel molybdate,

which otherwise would turn into the a-phase at room temperature.

Many other interesting investigations dealing with the preparation of Ni–Mo

supported catalyst could be mentioned. However, they are directed for use in processes

other than oxidation of hydrocarbons, which is outside the scope of the present review. It is

nonetheless important to stress that different materials have been used as supports for Ni–

Mo catalysts, namely alumina,[17,54,55] magnesia-alumina mixed oxides,[13] zeolites,[21,24]

titania-alumina mixed oxides,[14,56] activated-carbon,[15,16,57] or zirconia.[58]

In order to improve the catalytic performance of the Ni–Mo–O system in the

selective ODH of alkanes, particularly n-butane, nickel molybdate was doped with several

alkali (lithium, sodium, potassium, or cesium)[59] or alkaline-earth (calcium, strontium,

and barium)[60] promoters. The samples were prepared through wet impregnation of the

pure a-NiMoO4 material, using different loads of the respective nitrate solutions, followed

by filtration, drying, crushing, and calcination in dry air for 2 hr at 5508C. For propaneODH, promoters such as K, Ca, and P were frequently used, the catalysts being prepared

through an incipient wetness impregnation technique starting from an a-NiMoO4 calcined

pure catalyst.[61–63] Still for application in oxydehydrogenation processes, we should note

the preparation of catalyst compositions that contain other elements like phosphorus,

antimony, bismuth, or arsenic, and that are effective in converting paraffins or monoolefins

to a higher degree of unsaturation.[64] Methods described therein include coprecipitation,

impregnation, dry mixing, and similar methods the final catalyst compositions being

unsupported or supported.

While with the conventional impregnation method the doping element only stays on

the catalyst surface, the sol–gel route simultaneously produces surface and structural

modifications. In addition, better dispersion of the active species on the support can

frequently be achieved, as well as appropriate compositional homogeneity. This led Soares

et al.[65] to prepare, by the citric acid method, mixed Ni–Mg molybdates, which were

calcined under air flow at 5508C for 8 hr. These catalysts were tested for n-butane ODH

and exhibited a noteworthy catalytic performance.[65]

Dopants such as tellurium (Te) and phosphorus (P) were also added to Ni–Mo

catalysts, particularly for application in the direct oxidation of propane to acrolein and

acrylic acids. Reported techniques for preparation of the Te-doped catalysts include the

mechanical mixing of Te2MoO7 with NiMoO4–MoO3, the mixing of telluric acid with

NiMoO4–MoO3,[66,67] and the impregnation of nickel molybdate with ammonium

telluromolybdate.[68] For the P-doped system, the incipient wetness technique was used,

with (NH4)2HPO4.[66,67]

The preparation of doped nickel–molybdenum catalysts can also be found in US

Patent No. 3,968,054, by Cherry et al.,[69] who described an improved coprecipitation


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method for the preparation of antimony-doped Ni–Mo catalysts, useful for oxidation of

n-butane to maleic anhydride. Finally, Ferlazzo et al.[70] claimed a process for preparation

of a complex molybdenum-based catalytic system, which is comprised by one or two

crystalline phases (including beta nickel molybdate) and at least one modifying agent

(promoter element), useful, for instance, for the selective conversion of unsaturated

hydrocarbons into unsaturated aldehydes or diolefins.

3. THERMAL ACTIVATION—TRANSITION OF PHASES

The structure of some molybdates changes with temperature, while for others it

remains unchanged. For instance an irreversible structural conversion in Bi2MoO6 was

observed at temperatures higher than 5508C. Transformation is complete at 6408C.[71]

A reversible conversion in CoMoO4 occurs at 5008C while for NiMoO4 a temperature of

about 6908C is needed.[72,73]

In fact, as long ago as in 1973 Plyasova et al.[26] identified two polymorphic forms in

the nickel molybdate. One of them has a monoclinic crystalline network with the

molybdenum with number of coordination six and is stable at room temperature (then

named the P-phase). When heated to ca. 6508C, this form was converted into another

(then named the N-phase)—isomorphic with a-MgMoO4 and a-MnMoO4—in which the

molybdenum has number of coordination four, but which is not stable at room

temperature. After cooling a transition into P-phase was observed. The thermograms of

samples with Ni :Mo ratios close to 1 that were previously calcined at 5508C, showed an

endothermic effect at ca. 6508C when the solid was heated and an exothermic one at 508Cwhen the solid was cooled.[26] The former was attributed to conversion of the low

temperature into the high temperature phase (P! N), and the latter to the reverse

transformation, i.e., N! P.

The transformations of phases that occur when the precipitates are heated were

studied for the first time by Andrushkevich et al.[27] Differential thermal analysis showed

an endothermal effect due to the removal of crystallization water at about 1808C and

another at 4208C due to the decomposition of the hydrated molybdate. These results are in

very good agreement with the data shown in Fig. 2, obtained recently by Zavoianu et al.[48]

We can see that the thermal analysis performed over the precursor of NiMoO4 shows a loss

of weight below 473K, which corresponds to the desorption of water and ammonia. The

strong exothermic processes occurring at 723–773K are attributed to the decomposition

of NH4(NiMoO4)2OH .H2O and ammonium nitrate present in the structure.[48]

According to Andrushkevich et al.,[27] in samples with excess Mo the peak that

appears at 7808C coincides with the melting point of the molybdenum oxide present in the

catalyst. In their studies they still concluded that the crystallization temperature of the

fresh precipitate (4308C for Ni :Mo ratio ¼ 0.7) increases with the nickel content,

probably because the nonstoichiometric molybdate or the formed solid solution crystallize

at higher temperatures than the stoichiometric molybdate. Indeed, the infrared spectrum of

a sample with an Ni :Mo ratio ¼ 2.3 heated at 6508C shows the characteristic bands of the

N-phase (now named b-phase), showing that crystallization occurs at 6258C and is

accompanied by the exothermal effect observed in the thermal analyses. It should also be

noted that these authors found that in samples with a great excess of Ni (Ni :Mo ¼ 2.3) the

N! P transformation was not recorded when the sample was cooled.


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The Ni–Mo–O system indeed presents certain particularities. It is now well known

that NiMoO4 can have three different structures, two of them stable at atmospheric

pressure, while the other is observed at higher pressures. The two atmospheric pressure

isomorphs are now commonly named the a-phase—stable at room temperature and with

octahedral coordination of the Mo6þ ions—and the b-phase—high temperature phase,

metastable, and with tetrahedral coordination of the molybdenum.[74] The b-phase is

formed after heating the a-phase to ca. 7208C and undergoes reverse transition at low

temperature on cooling to ca. 2008C.[63] Figure 3 shows the differential thermal analysis

(DTA) cycle of phase transitions in the stoichiometric NiMoO4 system.

Figure 2. Thermal analysis of the precursor of NiMoO4. (Adapted from Ref.[48], with the kind

permission of Elsevier Science.)

Figure 3. The DTA cycle of stoichiometric NiMoO4 phase transitions. (From Ref.[63], with the

kind permission of Kluwer Academic Publishers.)


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The coordination of the molybdenum atoms in both phases was confirmed more

recently by Rodriguez et al. using x-ray absorption near-edge spectroscopy (XANES),

which has also shown that the Ni atoms are in octahedral sites.[75] However, in a

subsequent paper these authors reported that in the a-phase the molybdenum exhibits a

pseudo-octahedral coordination with two very long Mo–O distances (2.3–2.4 A).[76]

Regarding the stability of the phases, calculations of first-principles density functional

theory (DFT) have evidenced that the a-phase is �9 kcal/mol more stable than the

b-phase, with an energy barrier for the a to b transition of �50 kcal/mol, while time-

resolved XRD experiments point to an apparent activation energy of �80 kcal/mol.[76]

The phase stable at high temperature, b-NiMoO4, is frequently formed by heating the

precalcined a-NiMoO4 sample in situ, for instance at 7608C for 5min. The sample is then

quickly cooled to the desired temperature (which must always be higher than 2508C) forother treatments (e.g., sulfiding), characterization, or catalytic runs. It should be noted that

when more severe treatments were applied for b-phase formation (temperatures higher than

7608C or for more than 5min), after cooling to room temperature the sample exhibited not

only the a-phase, but also a more complex diffractogram, with peaks characteristic of both

phases. In this case the b-phase is stabilized at room temperature, which would be due, as

detailed below, to an excess of NiO as a result of the decomposition of the mixed compound

and sublimation of MoO3. When the normal treatment is applied for transition of phases,

b! a conversion is recorded when the sample is cooled to room temperature.

To use the high temperature b-phase of NiMoO4 in catalytic runs, Mazzocchia et al.

performed the a- to b-phase conversion in the reactor, by thermal activation of nickel

molybdate. The reactor was usually heated in 25min to 7008C under oxygen, and then this

temperature wasmaintained for 5–15min before cooling to the reaction temperature,[32,33,37]

but always avoiding excessive cooling to prevent the b to a-phase transition. The

temperature selected for transition of phases is in agreement with the high temperature XRD

data that show that at 5958C the b-phase is already present, but a temperature of about 7008Cis required to obtain full conversion into a pure b-phase.[37]

The data found in the literature reveal some discrepancies regarding the temperature

for phase transition in the NiMoO4 system. According to Di Renzo and Mazzocchia,[36]

this is due to the strong influence of the preparation conditions of the sample. Therefore, it

was decided to investigate, by differential thermal analysis, how thermal treatment of the

precursor affects the transition of phases in NiMoO4. It was found that the transition

temperature of the a- to the b-phase increases, and that for the b! a transition it

decreases, due to a temperature-induced relaxation. Thus, when the activation temperature

of the sample is increased, the activation energy for the a- to b-phase transition is

increased. It should, however, be noted that the endothermic peak that corresponds to this

transition (a! b) was not detected when the previous calcination thermal treatment was

performed at a temperature below 5508C. The temperature at which the exothermic peak

(relative to the b! a conversion) began ranged between 2578C and 2008C, depending onwhether the previous heating temperature was 7008C or 9008C, respectively.[36] Later, theuse of a high temperature diffraction camera showed that when activating NiMoO4 at

temperatures between 7008C and 9008C the temperature of the b to a transition at no point

differed significantly from 1808C, but the transition rate in the sample heated to 9008C was

slower than in the samples heated to lower temperatures. This effect was attributed to the

loss of MoO3 in the NiMoO4 with formation of a nickel-rich solid solution and with the

structure of the b-phase of NiMoO4.[46]


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4. CHARACTERIZATION OF CATALYSTS

4.1. Composition of Phases for Catalysts with Different Ni :Mo Ratios

Plyasova et al. studied, by XRD and infrared spectroscopy, the composition of phases of

the Ni–Mo–O system with Ni :Mo ratios from 0.2 to 2.0.[26] As described in the previous

section, two polymorphic forms have been identified in nickel molybdate catalysts: one that

is stable at room temperature (then named the P-phase), which when heated to about 6508Cis transformed into another (then named the N-phase), Which is unstable at room

temperature. After cooling, the transition to the P-phase is observed. It has been reported that

the high temperature modification reacts with excess nickel (relative to the stoichiometric),

forming a solid solution that is stable at room temperature. The existence of the N-phase at

room temperature and the absence of peaks characteristic of the P-phase or of NiO in the

x-ray diffractogram for samples with well-defined compositions indicates that, in certain

conditions, nickel is dissolved in the structure of the N-phase, stabilizing it at room

temperature, thereby forming a solid solution of nonstoichiometric composition.[26] It was

subsequently found that in samples containing excess nickel, relative to the stoichiometric

NiMoO4, the solid solution formed has a solubility limit in Ni in the range Ni :Mo ¼ 1.10–

1.20 (atomic).[77] A solid solution of the vacancy type is formed, i.e., the excess of dissolved

Ni ions occupies the normal octahedral positions in the structure of the N-phase, while some

tetrahedral positions of the Mo remain empty.

Other investigations have also been carried out in which differential thermal analysis

and XRD techniques were used to investigate the composition of the Ni–Mo–O system in

a wide range of Ni :Mo ratios (from pure NiO up to pure MoO3). The four phases detected

and identified were the following: nickel oxide, molybdenum trioxide, normal nickel

molybdate, and nonstoichiometric nickel molybdate.[78] In all the samples, with the

exception of the pure oxides, thermal analyses showed an irreversible exothermal effect at

about 430–4408C (which corresponds to crystallization of the nickel molybdate with

composition NiMoO4) and another at 620–6708C (which is presumed to be due to the

transition of phases of nickel molybdate). It was found that samples with m , 1

(m ¼ Ni :Mo ratio) present two phases: MoO3 and NiMoO4; the species with m close to 1

are mainly composed of normal nickel molybdate; the species with m . 1 are mixtures of

three phases: nickel oxide and normal and nonstoichiometric nickel molybdates.[78]

However, form . 1.6, only nickel oxide and the high temperature phase were detected.[26]

This identification of the phases that are present in Mo- or Ni-rich catalysts was

subsequently confirmed.[35] While in the stoichiometric catalyst, at room temperature, only

the low temperature phase was detected (with Mo in octahedral coordination), in Ni-rich

catalysts (Ni :Mo ¼ 1.0–1.3) a solid solution of nickel and both phases (high and low

temperature) were found, and the presence of nickel oxide was not detected in the x-ray

diffractograms or in the IR spectra. Therefore, the b-phase is stabilized at room temperature

due to the excess of nickel in the crystalline lattice of the molybdate. In catalysts where

Mo :Ni . 1, both the low temperature phase and MoO3 are present. For example, the com-

pound [(NH4)4H6NiMo6O24], prepared in well-defined conditions (see Fig. 1), at the drying

temperature presents very well-defined XRD diffraction patterns but at the thermal

activation temperature (5508C), only the a-NiMoO4 and MoO3 peaks were identified.[31]

For catalysts with excess nickel, in 1958 Corbet et al.[79] observed that, after heating

to 5008C the precursor obtained by precipitation of the solution containing Ni2þ and Mo6þ


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ions at pH higher than 6, a new phase was formed. This phase, then called the N-phase

with Ni :Mo . 1, according to Di Renzo et al.[46] corresponds to the high temperature

phase (b-NiMoO4), whose stabilization at room temperature is achieved by insertion of

excess Ni in the NiMoO4 lattice. The formation of the nickel-rich solid solution (NiO in

b-NiMoO4) entails an increase in the lattice parameters and leads to an increase in the

reducibility of the system. In fact, a catalyst with composition Ni :Mo . 1.40 is more

easily reduced than NiMoO4, which agrees with some results that will be described later

concerning the reducibility of this system.

The formation of the solid solution of NiO in NiMoO4 is demonstrated by the

stabilization of the b-phase at room temperature when the nickel-rich samples are

activated at temperatures around 550–7508C. The XRD data indicate that the solid

solution is responsible for the enlargement of the lattice parameters of the structure of the

b-NiMoO4 phase compared with the parameters of the stoichiometric phase.[46]

The Ni-rich samples prepared by coprecipitation, when heated to 5508C, mainly

showed the high temperature phase (b-NiMoO4), even when cooled to room temperature.

The longer this treatment lasts, the higher the percentage of that phase, and the smaller the

amount of crystalline NiO. However, when the temperature was increased, the percentage

of b-phase stabilizing at room temperature decreased and the proportion of NiO increased.

At very high temperatures the solid solution separates. Indeed, x-ray data showed that NiO

precipitation reached a significant rate at 8008C, contraction of the b-NiMoO4 cell

occurred at 9008C, and that after heating to 10008C the sample was composed, at room

temperature, only of a-NiMoO4 and NiO. This was confirmed by the results of diffuse

reflectance spectroscopy: the sample activated at 5508C showed the characteristic band of

the tetrahedral coordination of molybdenum (b-NiMoO4 phase) at 35,500 cm21, while

when activated at 9008C it presented the spectrum characteristic of stoichiometric

NiMoO4 with a band at 30,000 cm21 (typical of the octahedral coordination of

molybdenum in the a-phase) with the additional band of NiO at 14,000 cm21.[46]

4.2. Other Physicochemical Characterizations

4.2.1. Stoichiometric Nickel Molybdate

Stoichiometric nickel molybdate has been characterized by several research groups.

Usually the bulk composition of the catalyst is determined by inductively coupled plasma

spectroscopy, for molybdenum, and atomic absorption, for nickel. A typical BET surface

area for the stoichiometric catalyst is 44.1m2/g.[59] Since the material is crystalline, XRD

analysis (including high temperature XRD) has often been used to study its structure. A

typical diffractogram for both phases is presented in Fig. 4, which shows the characteristic

peak of the a-phase located at 2u ¼ 28.78 (JCPDS powder diffraction file card no. 33-948)

and of the b-phase at 26.48.The structure of the solid has also frequently been analyzed by infrared spectroscopy

because it provides useful information regarding, for instance, the stoichiometry of the

material. The Fourier transform infrared (FTIR) spectra of a-NiMoO4 (Ni :Mo ¼ 1.00) is

shown in Fig. 5, which is in good agreement with others found in the literature.[26,37] It is

characterized by bands at 608, 934, and 958 cm21. When, through stabilization, the

b-phase is also present, the spectrum at room temperature reveals a band at 950 cm21, and


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two new characteristic bands are also visible at 800 and 880 cm21 as a consequence of the

change in the Mo coordination from 6 to 4. This is an important feature in order to ensure

that the obtained nickel molybdate has a well-defined octahedral structure. Moreover, the

absence in the FTIR spectra of the characteristic MoO3 bands (at 980 cm21—attributed to

the vibration of the Mo–O bond—and at 870 and 812 cm21—attributed to the Mo–O

bond),[37] and the absence of those characteristic of the b-phase, are a good indication that

the prepared nickel molybdate does not contain excess of either Mo or Ni.

Regarding the electrical conductivity (s) of the solid, it is known that a-NiMoO4 is an

n-type semiconductor when prepared in quasistoichiometric conditions.[37,81] Studies

performed with this catalyst have shown that when the oxygen partial pressure in the

gas phase is lowered, at sufficiently high temperatures, the electrical conductivity

increases according to s / PO2

21/5.8.[37] The value of the exponent is close to 21/6,thus demonstrating that the main surface defects are compatible with the model of

doubly ionized vacancies,[82] whose formation can be described by the following

equilibria:

(OO)s !1

2O2(g)þ VO (6)

VO !VoO þ e� (7)

VoO !Voo

O þ e� (8)

Figure 4. X-ray diffraction patterns of a-NiMoO4 at 218C (A) and b-NiMoO4 at 7108C (B).

(Adapted from Ref.[80], with the kind permission of Elsevier Science.)


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where

(OO)s ¼ surface anion;

VO ¼ anionic vacancy with the two electrons trapped (neutral entity);

VOo and VO

oo ¼ singly and doubly ionized anionic vacancies, respectively.

From the equilibria of Eqs. (6)–(8), and taking into account that the corresponding

equilibrium constants follow Van’t Hoff’s law [Ki ¼ Koiexp (2DH i/RT)], it can be easily

deduced (e.g., Ref.[83]) that:

s ¼ A� e�½ � ¼ A� 2Ko6Ko7Ko8e�(DH6þDH7þDH8)=3RTP

�1=6O2

(9)

Thus, the exponent21/6 affecting the oxygen partial pressure is indicative of the existenceof doubly ionized anionic vacancies for a-NiMoO4, whose overall enthalpy of formation is

DH ¼ DH6 þ DH7 þ DH8 ¼ 3Ec. Mazzocchia et al.[37] found a value of 134 kJ/mol for Ec

(DH ¼ 402 kJ/mol), while Madeira et al. recorded an activation energy of 124.5 kJ/mol.[83]

Steinbrunn et al. also reported a value for Ec of 125.4 kJ/mol (at temperatures in the range

450–6508C), but unlike the previous authors, they found that the a-phase has two

conduction regimes: (1) in the range 450–6508C it is a p-type semiconductor; (2) at higher

temperatures (650–7008C) it is a n-type semiconductor with a higher activation energy of

conduction (Ec ¼ 182.4 kJ/mol).[73] Other studies concerning the electrical conductivity of

the Ni–Mo–O system can be found in the literature (e.g., Ref.[81,84–86]).

Due to the use of nickel–molybdenum catalysts in the hydrodesulfurization of

petroleum, the reducibility of nickel molybdate has been the subject of several studies. In

Table 1 some of them are summarized, which clearly illustrates that the mechanism of

Figure 5. The FTIR spectra of a-NiMoO4. (Adapted from Ref.[60], with the kind permission of

Academic Press, Inc.)


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Table

1.

Mechanismsandinterm

ediate

productsofNiM

oO4reductionbyhydrogen.

1ststep

2ndstep

Reference

NiM

oO

4��!

(4758C

)NiþNi-MoalloysþMoO

2þinterm

etallics

(e.g.,Ni 4MoÞ

MoO

2��!

(7008C

)Mo

[29]

NiM

oO

4��!

(400–5008C

)Ni-Moalloyþam

orphousmolybdenum

lower

oxide

Amorphousmolybdenum

lower

oxidephase��!

N2(6008C

)MoO

2

[87]

NiM

oO

4��!

(300–4508C

)NiþMoO

2[28,88]

NiM

oO

4��!

(300–5008C

)NiM

oxalloyþMoO

2NiM

oxalloyþMoO

2��!

(7008C

)MoþNi 3Mo

[22]

NiM

oO

4��!

(500–6008C

)NiþMo2O

3NiþMo2O

3�!

Interm

etallideofNiandMo

[89]

Source:

Ref.[87] .


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NiMoO4 reduction is far from being unambiguously established. More recently, Madeira

et al.[90] used the temperature-programmed reduction (TPR) technique followed by XRD

analysis in order to clarify the mechanism of NiMoO4 reduction by hydrogen. The TPR

profile is shown in Fig. 6 and is similar to those found in the literature,[9,11,22,29,46] with two

maxima at 5458C and 7258C. It was proposed that nickel molybdate reduction starts at low

temperatures (3008C), leading to metals (Ni and probably Ni4Mo) and amorphous

MoO2. Ni2þ, after being reduced to metallic nickel, activates molecular hydrogen, thus

favoring Mo6þ reduction. The amount of MoO2 formed becomes significant only at ca.

6208C. In this way, the peak at lower temperatures can be attributed to reduction of all

Ni2þ to metallic nickel and of Mo6þ to Mo4þ or to Ni4Mo. The second TPR peak would be

due to Mo4þ reduction. Metallic nickel activates hydrogen and induces molybdenum

reduction with formation of metallic Mo and an Ni–Mo alloy. Finally, at temperatures

higher than 7258C, a mixture of Mo, Ni–Mo alloy, and intermetallic Ni3Mo was found.[90]

Other techniques have also been applied for the characterization of NiMoO4, as will be

seen in the following sections. The use of surface techniques such as electron spin resonance

(ESR) and x-ray photoelectron spectroscopy (XPS) also deserves special mention, since they

have been very useful for identification of the active sites of the Ni–Mo–O catalytic system

for selective oxidation reactions. This will be dealt with in Section 5.3.

4.2.2. Catalysts with Excess Molybdenum or Nickel

Nickel molybdates with excess molybdenum have been characterized in detail by

Ozkan and Schrader,[45] who have particularly established that Mo in excess appears as a

new phase: MoO3. The solids were carefully characterized by several techniques,

Figure 6. The TPR profile of a-NiMoO4 with 5% H2 in argon. (Adapted from Ref.[90], with the

kind permission of Elsevier Science.)


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including BET surface area, Raman spectroscopy, XRD, x-ray fluorescence, photoelec-

tronic spectroscopy, and scanning electron microscopy. Curiously, an association of

particles was found, with crystallites of MoO3 covered superficially by NiMoO4 particles.

Other important results of this study were as follows:[45]

The BET surface areas of the catalysts decrease when the percentage of MoO3 excess

increases, varying between 37m2/g for pure NiMoO4 and 3m2/g for MoO3.

The percentage of MoO3 in the final product increases with the acidity of the medium

during precipitation (confirmed by x-ray fluorescence).

Two forms of crystallites, an irregular, round and porous form, attributed to NiMoO4,

and a hexagonal form, due to MoO3 were identified.

Ni–Mocatalystswith an atomic ratio ofMo :Ni . 1 are suitably characterized byFTIRandFT

Raman because new bands appear, typical of MoO3. As stated above, the infrared spectrum

exhibits additional bands at about 810, 860, and 990 cm21,[35] while the Raman peaks, char-

acteristic of MoO3, also become evident (shown in Table 2). This new phase is also clearly

visible inx-raydiffractograms.For instance, Fig. 7 shows theXRDpatterns at roomtemperature

of nickelmolybdates with different Ni :Mo ratios. It was found that all the Ni–Mo–O catalysts

show the diffraction patterns of the a-phase, and the x-ray diffractograms are practically

identical forNi :Mo ¼ 0.92or 1.00.[68]On the other hand, the strongpeaks corresponding to the

MoO3 phase are found in the diffraction patterns of samples with Ni :Mo ¼ 0.38.

Surface-sensitive techniques have also been used in order to investigate possible

variations in the oxidation states of samples with nonstoichiometric compositions. Ozkan

and Schrader[45] found that the band positions and bandwidths are identical for NiMoO4

samples containing excessMoO3, independently of the preparation technique used. Table 3

lists the observed binding energies (+0.2 eV) for such samples, showing that the Mo 3d

binding energies for NiMoO4 samples are identical to those of MoO3, while the Ni 2p band

positions are also very similar for the Ni–Mo–O catalysts, but entirely different from

those of NiO.[45] These results are important for demonstrating that the oxidation states of

molybdenum and nickel do not change in samples with different Mo :Ni ratios, and that

there is no NiO present in the observed samples. Furthermore, they can also exclude the

possibility of an entirely new compound in samples with excess MoO3.

All the precursors and corresponding catalysts presented in Fig. 1 were characterized

in detail by Mazzocchia et al. The E precursor, which contains excess Mo, deserved

special attention given the excellent catalytic properties revealed in the oxidation of

1-butene to maleic anhydride.[30] In this case, the presence of excess molybdenum of the

MoO3 type was revealed by the bands at 995, 865, 820, and 370 cm21 in the infrared

spectrum, and by the corresponding reflections at d ¼ 6.96, 3.82, 3.47, and 3.26 A in the

Table 2. Raman bands (cm21) of NiMoO4 and MoO3.

Sample

NiMoO4 963vs 916s 709s 494m 420m 389m 373m 179m

MoO3 998s 822vs 670s 381m 339m 293m 285s 160m

Source: Ref.[45].


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XRD patterns. When activated at 7608C the excess MoO3 is eliminated, and the resulting

pattern is characteristic of stoichiometric molybdate.[30]

Still using catalysts with excess Mo, but obtained from different precursors,

Mazzocchia et al. observed that in spite of an identical global atomic composition, the

physical–chemical properties of the solids and their catalytic behavior in propene

oxidation were completely different.[44] The differences in the electrical conductivities

suggested the existence of different phases in the two typical samples used. The sample

obtained by decomposition of the heteropolymolybdate presented a smaller activation

energy of conduction, a well-defined exothermal peak between 310–4308C in the

differential thermal analysis, due to the crystallization of MoO3; and the MoO3 showed a

preferential orientation of the crystalline planes in the (010) direction, compared with the

sample obtained by the dry mixture of NiMoO4 and MoO3.

Subsequently, several solids presenting more significant amounts of molybdenum than

the original product (in which Mo :Ni ¼ 0.98) were investigated. The materials exhibited

different concentrations of defects whose nature could be explained by electrical conductivity

measures and thermoluminescence experiments.[47,84] The Mo concentration varied between

Figure 7. The XRD diffraction patterns of nickel molybdates with various Ni :Mo atomic ratios.

(From Ref.[68], with the kind permission of Elsevier Science.)


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1% and 10% of the total number of Mo atoms in the initial product (a-NiMoO4). The

electrical conductivity measurements showed that the original product, containing nickel

vacancies, presents p-type conductivity. A small addition of Mo fills those vacancies.

However, at higher loads the Mo atoms occupy an interstitial position and free electrons

appear in the solid, responsible for the n-type conductivity. The recorded thermolumines-

cence emission was attributed to some of the atoms being in interstitial position.[84]

It should also be stressed that in catalysts containing excess Mo (or even Ni), Vagin

et al. found that the samples with the largest surface areas were those in which the

dominant phase was NiMoO4, with values varying between 22.0 and 29.4m2/g.[28] When

the NiO, and particularly the MoO3 content is increased, the surface area decreases.

Later Brito et al. investigated the reducibility of nickel and molybdenum catalysts, but

supported over g-Al2O3, and analyzed the effects of Ni concentration and calcination

temperature in the TPR profiles.[53] The acid character of these catalysts is noteworthy

(demonstrated by ammonia adsorption), especially when enriched in molybdenum, due to the

higher acidity of Mo6þ oxides compared to the alumina support. Characterization of the

catalysts showed a superficial interaction between Ni and Mo in catalysts calcined at

temperatures lower than 8008C, probably with formation of Ni–Mo–O phases. However, the

metastable phase (b-NiMoO4) was detected in diffractograms at room temperature of the

catalysts calcined at 8008C (characteristic line at 2u ¼ 26.88), which suggests that the supportplays a part in the stabilization of this phase. Regarding the reducibility of the catalysts, the

existence of a synergetic effect between Ni andMo should be noted since the reduction of any

of the species is facilitated by the presence of the other.[53]

Quite recently, Kaddouri et al. have also studied the reduction behavior of Ni–Mo–O

catalysts, and analyzed the effect of MoO3 (as well as the effect of tellurium and

phosphorous compounds) on the reducibility of NiMoO4 catalysts.[67] It is noteworthy that

when excess MoO3 is present, specifically above 0.5 (Mo : catalyst ratio by weight),

oxygen depletion from the solid decreases. When the MoO3 load is limited, the overall

reduction rate increases, compared to the stoichiometric NiMoO4 catalyst. It was found

that the 0.5 MoO3–NiMoO4 catalyst has the highest reduction rate.[67]

For catalysts with excess nickel, and particularly for Ni :Mo ¼ 1.7, Mazzocchia et al.

found that the thermal analyses did not show the exothermal peak that results from

NiMoO4 crystallization and, in addition, the XRD patterns showed that the degree of

crystallinity of the catalyst is lower, due to the formation of a NiMoO4 solid solution with

excess NiO.[31] It is known that excess Ni allows the stabilization of the b-phase at room

Table 3. Photoelectron spectra binding energies for pure compounds and catalysts (eV).

Sample Mo 3d5/2 Mo 3d3/2 Ni 2p3/2 Ni 2p1/2

MoO3 232.7 235.8

NiMoO4 232.6 235.7 855.7 873.3

NiO 853.3 871.3

NiMoO4 with 15% excess MoO3

(precipitation)

232.7 235.8 855.8 873.4

NiMoO4 with 15% excess MoO3

(impregnation)

232.7 235.9 855.7 873.4

Source: Ref.[45].


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temperature, which is confirmed by XRD (Ib/Ia ¼ I3.33/I3.09) and IR (presence of the

b-phase characteristic bands at 880 and 800 cm21) data.[31] For Ni-enriched catalysts,

a p-type semiconductor behavior has also been reported.[47]

The possibility of distinguishing the beta stoichiometric phase from the NiMoO4dNiO

phase, which is a solid solution, may lead to confusion in distinguishing the catalytic

properties of the NiMoO4 phases. Mazzocchia et al. considered this possibility and came

to the conclusion that there are indeed reasons to confuse the solid solution and the b-

NiMoO4 phase. One of these reasons is that an excess of NiO, even if very small, involves

the formation of an Ni1þ1MoO4þ1 solid solution, which is not very active for propane

ODH.[32–34] Another reason depends on how the thermal cycles were performed in order

to obtain the b-phase. If both the time and the temperature to which the catalyst is brought

are not strictly controlled, a loss of xMoO3 may occur with the consequent formation of an

Ni1þxMo12xO423x solid solution.

4.2.3. Catalysts Prepared Using Organic Precursors and Sol–Gel Methods

The agents that control pH in the synthesis of precursors of mixed oxides must be

easily eliminated from the precipitate. From this point of view, ammonia is usually

adopted instead of, for instance, alkaline or alkaline-earth hydroxides. However, the ease

of ammonia oxidation necessitates careful control of the conditions used for precursor

activation in order to avoid hot spots that induce heterogeneity in the properties of the final

oxide. In this context, the role of ammonium ions in the thermal activation of several

precursors of nickel molybdates, with Ni :Mo ratios smaller or larger than 1, has been the

subject of study by thermal and gravimetric analyses. Particularly, the different processes

(endothermal and exothermal) that occur in the activation of each precursor were clearly

defined.[33,91] It should be noted that elimination of ammonium ions can follow two

different pathways: ammonia removal or oxidation. The competition between the two

mechanisms depends on the oxygen partial pressure, kinetic factors, and on the heating

rate and the material of the cell. This last factor is related to the catalytic role of metals in

ammonia oxidation. While the decomposition of NH4NO3 in air is endothermal in alumina

cells, it becomes exothermal with metallic cells and occurs at a high rate if platinum is

used, which is also able to oxidize NH3 at temperatures of the order of 1508C.[91]

Later, Mazzocchia et al. decided to prepare an oxalic precursor, because organic

precursors have the advantage of crystallizing at lower temperatures, particularly the

oxalic precursor compared with ammonia.[34] The analyses performed revealed the

following processes in the precursor decomposition (when heated in O2):

MoOC2O4 � 4H2O��!1008C

MoOC2O4 þ 4H2O (10)

NiC2O4 � 2H2O��!2108C

NiC2O4 þ 2H2O (11)

MoOC2O4 þ1

2O2��!

2808CMoO2 þ 2CO2 (12)

MoO2 þ1

2O2��!

.2808CMoO3 (13)

NiC2O4 þ1

2O2��!

3508CNiOþ 2CO2 (14)

NiOþMoO3��!.4508C

b-NiMoO4 (15)


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The endothermal processes recorded are due to the loss of water and to oxalate

decomposition. The strong exothermal effect found was attributed to molybdenum

oxidation and to crystallization of MoO3. The final exothermal peak observed corresponds

to NiMoO4 crystallization. It should be stressed that the final composition of the catalyst

depends on the thermal treatment applied, and particularly on the heating rate and time.

For instance, to obtain NiMoO4 it is necessary to decompose the oxalate as quickly as

possible, usually introducing the precursor directly in a preheated oven at 5508C. It isnoteworthy that the catalyst obtained presented considerable stabilization of the b-phase at

room temperature.[34]

More recently, Anouchinsky et al. prepared several catalysts by the sol–gel method,

applying different thermal activations to the dry powder.[40] X-ray analysis indicated that

the dry gel is already a partially crystallized compound, several peaks of the a- and

b-phases of NiMoO4 were identified. Neither NiO nor MoO3 were found in the samples,

which confirms that the homogeneous dispersion of Ni and Mo ions in the precursor has

specifically led to the formation of the NiMoO4 phases, whose crystallization occurs at

temperatures lower than with precursors prepared by coprecipitation. Indeed, the main

exothermal effect recorded in the thermal analyses occurs at slightly over 2008C (either in

air or in nitrogen), and corresponds to the elimination of volatile compounds and to a

degree of crystallization of NiMoO4, concluded at 4708C. Water elimination is the

predominant phenomenon during thermal activation and the composition of phases of the

formed oxide varies as follows: fast heating (by direct introduction into the previously

warmed oven) favors b-NiMoO4 stabilization at room temperature, while slow heating

leads to the preferential formation of the more stable a-phase. This behavior was

explained based on the availability of Ni ions to form the solid solution with the

b-NiMoO4 phase, because when the sample is heated slowly, and therefore crystallization

occurs more slowly, the availability of Ni ions is reduced.[40]

4.2.4. Doped and Supported Nickel Molybdates

Alkali and alkali-earth metals have been widely and successfully used as promoters of

mixed oxides. In several works published by Portela et al. the effects of either

alkali[59,80,83,90] or alkaline-earth[60] promoters in the physical–chemical properties of the

stoichiometric nickel molybdate were analyzed. As already mentioned, these catalysts

were prepared by wet impregnation. In spite of the high metal content of the treating

solutions, in the bulk of the solids only traces of promoters were found when using alkali

metal salts. Consequently, they remain only on the catalyst surface, curiously in the overall

expected concentration, mainly affecting the NiMoO4 surface properties. The surface

promoter content was quantified by XPS, and will be herein denoted as X% (X is the

nominal metal/Mo atomic ratio in solution, which is equal to the surface content for

alkali-doped nickel molybdate). For alkali-earth elements (Ca, Sr, or Ba), the nominal

promoter content was found in the catalyst bulk.[60]

The above-mentioned promoters, particularly alkali metals, affect the NiMoO4 BET

surface area. It was found that higher promoter loadings lead to a greater decrease in SBETand that for a given loading of promoter the surface area decreases with increasing

promoter ionic radius. Surface areas as low as 26.7 and 26.6m2/g were found for 6%

Cs-a-NiMoO4[59] and 12% Ba-a-NiMoO4,

[60] respectively. However, the binding ener-

gies recorded for nickel and molybdenum showed no major changes after promoter


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addition (as compared to a-NiMoO4), demonstrating that the oxidation state of the

catalysts may remain unchanged.[59]

The use of cesium (Cs) deserved special attention because of its high selectivity in the

ODH of n-butane.[59,80] It is particularly noteworthy that Cs-doping does not affect the

nickel molybdate structure at room temperature (neither does Li, Na, or K), as revealed by

FTIR or XRD, but strongly affects the a- to b-phase transition. Indeed, high temperature

x-ray diffraction (HTXRD) analyses have shown that with a surface cesium loading of 3%

or 6% (atomic ratio Cs :Mo), the transition of phases at 7108C is only ca. 50%.[80] In our

opinion this behavior is probably related to the size of the promoter, which has an ionic

radius (r) of 1.67 A, because with either K or Ba (r ¼ 1.33 and r ¼ 1.34 A, respectively),

the a! b transition is complete, even when using higher promoter loadings.[60]

The reason for the choice of these kinds of promoters concerns the importance of the

adsorption bond strength of hydrocarbons to the surface in ODH reactions. It has been

proposed that for the selective oxidation of alkanes into alkenes, which are considered to

be nucleophilic molecules, a basic surface is crucial for obtaining higher selectivities

because it significantly decreases the chances of further oxidation into carbon oxides. This

led to doping NiMoO4 with basic promoters and using carbon dioxide as the probe

molecule in order to characterize the basicity of the surface of some Cs-doped catalysts

through temperature-programed desorption (TPD).[80] Figure 8 shows the TPD profiles for

a-NiMoO4 doped with different Cs loadings. It should be noted that Cs-doping

significantly increases the surface basicity, shown particularly by the area of the first peak,

with a maximum for a surface Cs loading of 3% (atomic ratio Cs :Mo). However, an

overdoping effect was recorded for the 6% Cs-NiMoO4 sample.

Such an overdoping effect was also noted in the electrical conductivity data recorded

for the same catalysts by Madeira et al.[83] (Fig. 9). It was found that cesium-doped

catalysts are much more conductive than unpromoted a-NiMoO4 due to surface Csþ ions

Figure 8. The TPD profiles of CO2 adsorbed at 308C for a-Ni :MoO4 doped with different Cs

loadings. (Adapted from Ref.[80], with the kind permission of Elsevier Science.)


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and associated oxygen species (ionic conductivity) and also exhibit smaller activation

energies of conduction (Ec), with values in the range 83–94 kJ/mol.[83] As noted above,

for the nickel molybdate catalyst the value obtained was Ec ¼ 124.5 kJ/mol, very close to

that obtained by other authors (Ec ¼ 134 kJ/mol).[37] This was, at the time, the main

evidence that led Madeira et al. to assume that under similar conditions the catalyst

exhibits the same electrical behavior with the same type of defects as those suggested by

Mazzocchia et al.,[37] i.e., doubly ionized anionic vacancies [cf. Eqs. (6)–(9)]. Such n-type

conductivity was confirmed subsequently by in situ electrical conductivity runs under

different atmospheres.[92]

Another interesting effect resulting from Cs-doping was the increase of the resistance of

nickel molybdate to reduction,[90] similarly to the effects recorded when doping molybdenum

catalysts with Li, Na, or K[93] or vanadium oxide with Cs.[94] Although the TPR profile

(cf. Fig. 6) was almost unaffected by the addition of Cs to the catalyst, an increase in the

temperature of onset of reduction was recorded when the surface Cs loading was increased,

with a value of 3008C for undoped a-NiMoO4 and 3508C for 6% Cs-NiMoO4.[90]

Concerning the characterization of alkali earth-doped a-NiMoO4, particularly

noteworthy is the formation of new oxygen species, detected as oxide and peroxide

compounds through XRD, FTIR, and FT Raman analyses.[60] In addition, CO2-TPD

results also showed an increase in the basicity of barium (Ba)-doped catalysts with

promoter loading, with an overdoping effect for Ba concentrations higher than 9%.[60]

Similar results were also recently reported by Liu et al. when doping an Ni0.9MoO4

catalyst with various barium loadings (molar ratio of Ba/Mo between 1% and 15%).[95] In

this paper, the XRD patterns and IR spectra demonstrate the formation of BaO2, the

concentration of which increases with barium loading up to 9%, decreasing for Ba

contents in the range 9–15% due to formation of BaMoO4.

Figure 9. Change of electrical conductivity at 3908C (B) and 4508C (W) as a function of surface

cesium contents on a-NiMoO4. (From Ref.[83], with the kind permission of Elsevier Science.)


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The deposition of coke over nickel molybdate has also been the subject of study and

has revealed interesting features, particularly the stabilization of the high temperature

b-phase at room temperature, as shown by XRD and FTIR analyses.[96] The role of coke

in this stabilization was demonstrated by completely eliminating it from the catalyst. After

this gasification, the solid again shows the typical a-phase structure at room temperature,

thus demonstrating the instability of the b-phase in the absence of coke. As mentioned

below, catalyst deactivation was not found after such forced coke deposition. On the

contrary, the effect of b-phase stabilization markedly improved catalyst performance,

particularly its selectivity for oxydehydrogenation of n-butane.[96]

The stabilization of b-NiMoO4 at low temperatures is very important because there is

much evidence that, for some reactions, and particularly for ODH of light alkanes, this

phase shows more interesting catalytic properties than the a-phase, especially selectivity

to dehydrogenation products. Thus, special reference should also be made to recent studies

that showed stabilization of b-NiMoO4, even at room temperature, when using TiO2[48] or

SiO2[49] as supports. Moreover, this effect was found both when preparing the cata-

lysts through precipitation[48,49] and through sol–gel routes.[50,51] The formation

and stabilization of this phase was also found over PNiMo :Al2O3 catalysts,[97] but in

this case it turned out to be a disadvantage because these catalysts are used in hydro-

desulfurization reactions in which the b-phase is much less active. Published data

on oxidic Ni-Mo :Al2O3 also point to a role of the alumina support in stabilizing

b-NiMoO4.[98]

The acid–base properties of SiO2-supported nickel molybdate catalysts were also

evaluated and compared with those of unsupported stoichiometric NiMoO4.[52,99]

Temperature-programed desorption experiments of NH3 and CO2 have showed that

supported catalysts with ca. one monolayer of the active phase are less acidic than the

unsupported nickel molybdate, but acidity increases with the number of monolayers.

The use of Ni–Mo-supported catalysts is widely reported in the literature, mainly for

applications in petroleum hydrotreatment processes, and so there are many examples of

interesting works in which the characterization of such materials is mentioned. A simple

example is the combination of TPR and ESR techniques, which have helped to clarify that

in Al2O3-supported Ni–Mo catalysts the good catalytic properties in the hydrode-

nitrogenation reaction of pyridine can be attributed to the improvement in the reducibility

of Mo, the formation of an Ni–Mo–O phase, and the creation of more anionic vacancies

when using Ni or W as promoters.[100,101] For the interested reader, other references are

provided, merely illustrative, to show that different supports have been used and the

catalytic systems characterized, namely alumina,[97] titania-alumina mixed oxides,[14,56]

alumina-magnesia mixed oxides,[13] activated-carbon,[15,16] and zirconia.[58]

4.3. Characterization of the High Temperature b-Phase

It is well known that the XRD signal at interplanar spacing around 3.33 A is

considered to be characteristic of b-NiMoO4 in Ni–Mo catalysts.[98] Thus, the use of a

heating camera has enabled several researchers, including the present authors, to record

the XRD patterns of both a- and b-phases of NiMoO4 at different temperatures, as well

as to study the transition of phases and their stability.[37,80] For instance, from Fig. 4 it is

clear that the a! b transformation is practically complete after 10min at 7108C in air, as


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shown by the relative intensities of the characteristic peaks (2uI2100 ¼ 28.78 and

2uI2100 ¼ 26.48, for a and b-phase, respectively). The HTXRD data have also provided

evidence that the a-phase is stable up to at least 6258C and that the b-phase is stable down

to at least 4258C,[80] the range of temperatures typical for ODH reactions.

The characterization of the b-phase through other techniques is not easy due to its

instability at low temperatures. The BET surface area, for instance, is estimated to be of the

same order of magnitude as the value obtained with the a-phase after thermal treatment

(e.g., 15min at 7008C), followed by quenching to room temperature. Mazzocchia et al.

obtained specific surface areas for the a- and b-phases of 40 and 15m2/g, respectively,[32,37]

very close to those obtained by Martin-Aranda et al. (44.1 vs. 16.0m2/g).[59]

Regarding electrical conductivity (s), it has been observed that the b-phase is also a

semiconductor of the n-type, like the a-phase, and it therefore obeys the same law (for

PO2. 19.7 kPa):

s ¼ so exp �DHa

RT

� �P�1=nO2

(16)

with a conduction enthalpy (DHa) of 125.4 kJ/mol.[37] For the b-phase the exponent n is

close to 4, demonstrating that the main surface defects are singly ionized anionic

vacancies, while with a-NiMoO4 a value close to 6 was obtained, indicative of doubly

ionized anionic vacancies [cf. Eqs. (6)–(8)].[37] Steinbrunn et al. also found that the

electrical transport for both nickel molybdate phases takes place via a classical intrinsic

band conduction mechanism, but they concluded that the b-phase behaves as a p-type

semiconductor in the temperature range 450–6508C, with an activation energy of

125.4 kJ/mol.[73]

Another property of the high temperature phase that was studied by Mazzocchia et al.

was its reduction rate by H2. A significant increase was reported in the reduction rate of

b-NiMoO4 phase compared to the a-phase.[31,67]

Among several results obtained by Brito et al.,[11] it should be noted that the BET

surface area of a-NiMoO4 (38m2/g)—obtained by calcination of the hydrated precursor

at 5508C—also decreases considerably after cooling b-NiMoO4 to room temperature

(26m2/g). However, the TPR profiles show clear differences in the reducibility of both

phases, with higher reduction temperatures for the b-phase due to the greater difficulty in

reducing Mo6þ in tetrahedral than in octahedral coordination.

5. Applications of Ni–Mo–O Catalysts

The multifunctional character of the Ni–Mo–O system is demonstrated by the wide

variety of reactions in which it is applied and through the great diversity of products

obtained with a given reactant. An example is butane oxidation, in which the possible

reactions involved include dehydrogenation, isomerization, oxidation with oxygen

insertion, partial oxidation with rupture of carbon–carbon bonds, and total oxidation.[35]

Nickel–molybdenum-based catalysts have been used in several reactions. As already

mentioned, this review essentially concerns oxidation reactions, and particularly ODH of

light alkanes. However, their use in reactions such as hydrogenation and hydrogeno-

lysis of toluene,[22] hydrodesulfurization of thiophene,[8–15] hydrodenitrogenation of


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pyridine,[16,17] water–gas shift,[18] steam reforming of hydrocarbons,[19] oxidative

coupling of methane,[20] COx hydrogenation,[55] and in other important hydrogenation and

hydrotreating reactions[8,21,23,24] should also be mentioned.

5.1. Oxidation of Hydrocarbons

According to Ozkan and Schrader,[45] there is strong evidence that the presence of

excess MoO3 is a key factor that determines the catalytic behavior of simple molybdates in

selective oxidation reactions. In this context, they decided to prepare nickel molybdates

with excess MoO3 through several techniques (see Section 2), and used these catalysts in

the conversion of some C4 hydrocarbons into maleic anhydride. The hydrocarbons used

were 1-butene,[102] butadiene, and furan.[103] The results recorded show that specific

concentrations of MoO3 are necessary in order to obtain high selectivities to maleic

anhydride (with a maximum for 15% molar excess of MoO3), and that pure NiMoO4 and

MoO3 are not selective. The most selective prepared catalyst for maleic anhydride

production was an MoO3 phase superficially covered with NiMoO4, which exhibited

stability in reaction conditions for 200 hr. Characterization of the catalysts used after such

long runs did not reveal any chemical or structural alteration, with no change in the

oxidation states of Mo 3d and Ni 2p (as found by XPS), but only a change of color (from

yellow to dark gray) due to carbon deposition on the surface.[102] Regarding the role of

each phase, it was clearly established that selectivity for maleic anhydride is determined

by competition between the processes of carbon oxides and maleic anhydride formation,

both occurring at different MoO3 sites. NiMoO4 is the component responsible for 1-butene

ODH, and moreover this phase selectively blocks the MoO3 sites that lead to total

oxidation, thus favoring selectivity to maleic anhydride.[103]

Zou and Schrader then decided to develop a technique to prepare thin films

(150–300 A) of NiMoO4 on the surface of (0 1 0) MoO3 previously deposited over a

support.[38] The catalysts thus prepared revealed excellent catalytic behaviors in the

oxidation of 1-butene to furan and maleic anhydride. It is noteworthy that when only

NiMoO4 was deposited a high selectivity to butadiene was obtained, with yields of about

48%, which was attributed to the presence of defects, probably in Ni–O–Mo sites.[38]

A synergetic effect between the a-phases of NiMoO4 and MoO3 was also detected in

1997 byMagaud et al. during propane oxidation.[104] Indeed, activity and selectivity to acetic

acid and acrylic acid are maxima when the ratio a-MoO3/(a-NiMoO4 þ a-MoO3) is close to

0.25, due to a specific arrangement of the two species, especially a reciprocal covering.

Mazzocchia et al. have also frequently used the Ni–Mo–O system in hydrocarbons

oxidation reactions. In a pioneering work, 1-butene oxidation to maleic anhydride was

studied with several catalysts.[30] Once again, the catalyst with excess Mo, relative to the

stoichiometric one (precursor E in Fig. 1), was shown to be particularly active and

selective. Moreover, pulsed-feed experiments have shown that ODH to butadiene is

possible due to the intervention of reticular oxygen (through a redox mechanism), while

the formation of partial and total oxidation products involves different forms of adsorbed

oxygen. In the conditions studied, the greatest selectivity to maleic anhydride was 64%,

which was attributed to the presence of Mo(V) sites that are able to activate the oxygen

molecule and that exist in catalysts with MoO3 present in an NiMoO4 matrix.


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The oxidation of propene has also been the subject of study, in particular the influence

of the Mo :Ni ratio on the behavior of the catalysts.[31] While all the catalysts tested have

produced acrolein, only catalysts with excess MoO3 enabled acrylic acid to be obtained. In

any case, the maximum yield was always obtained with catalysts containing excess MoO3,

which increases the superficial acidity of the catalyst and thus favors formation of acrylic

acid. In addition, the b-phase of NiMoO4 enabled propene to be converted into acrylic acid

and also oxidized acrolein into acrylic acid, while the NiMoO4 a-phase is practically

inactive in both reactions.

From the above-mentioned results it is clear that excess MoO3 is crucial in oxidation

reactions, the same being true for propene oxidation. However, the method of preparation

of the precursor significantly affects the catalytic behavior. Preparation of precursors with

identical atomic composition, but through different methods, leads to catalysts with quite

different properties.[44] For instance, the catalyst prepared by coprecipitation presents a

smaller electrical conduction activation energy and is catalytically more active than the

one prepared by a simple mixture of NiMoO4 and MoO3. The former is also more selective

to acrolein and to acrylic acid and presents a preferential orientation of the MoO3

crystalline planes in the (0 1 0) direction.[44]

Oxidation of propene with Ni–Mo-based catalysts has also been the subject of some

patents. For instance, US Patent No. 4,388,223, by Ferlazzo et al.,[70] describes the

preparation of a complex molybdenum-based catalytic system, which is formed of one

or two crystalline phases and at least one promoter element. When one of the phases

involves beta-nickel molybdate, runs performed using propene as the unsaturated

hydrocarbon resulted in a conversion of 95.3%, with a selectivity of 95.6% for the

acrolein and acrylic acid produced (feed containing the following percent volume

composition: propene/oxygen/steam ¼ 6.64/12.5/34.0, with a contact time of 2.3 sec at

3708C).[70] Finally, Umemura et al. reported a yield to acrolein as high as 91.5% using

an Mo–Co–Ni–Bi–Fe–Al–Ti–O type catalyst, which also has an excellent crushing

strength.[105]

The great interest in oxidizing propene directly to acrylic acid led Mazzocchia et al. to

study this reaction further. After determination of the optimum proportion between MoO3

and NiMoO4 in the binary system, i.e., NiMoO4. 2MoO3, the catalyst was promoted with

Te (Te2MoO7), which considerably increased the performance of the solid. This catalyst

was then used in a preliminary kinetic study.[106] As regards the effects of each oxide,

molybdic anhydride by itself dramatically increased the selectivity (and also activity) of

NiMoO4, while the presence of tellurium, in spite of decreasing conversion, increased the

selectivity to acrylic acid. It is possible that the decrease in conversion results from the fact

that tellurium molybdate accelerates the a! b-NiMoO4 transformation,[107] that leads to

a less active catalyst. The synergistic effect between MoO3 and a-NiMoO4 was related to

the large amount of Ni found by XPS on the surface of the catalyst, which was in

agreement with the above-mentioned results obtained by Ozkan and Schrader[45] that

demonstrated the presence of NiMoO4 on the surface of MoO3 crystals. With regard to the

effect of tellurium, it was considered that this promoter keeps Ni and Mo in high oxidation

states (e.g., Mo5þ þ Te4þ! Mo6þ þ Te3þ), necessary for easy desorption of acrolein,

whose formation probably represents the rate-determining step.[106]

More recently, the partial oxidation of propene was carried out with Ni–Mo–Te–O

ternary catalysts by Kaddouri et al.[67] It was concluded that the catalytic behavior is

governed by both the synergetic effect generated by combining NiMoO4 with Mo- and


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Te-oxides and by the oxygen partial pressure rather than lattice oxygen. Indeed, Te-doped

Ni–Mo–O catalysts have a high potential for partial propene oxidation, which is linked

also to the increase in catalyst reducibility induced by the presence of tellurium, although

the reaction seems to be governed by the molecular oxygen partial pressure. This was

concluded after a preliminary kinetic study, in which a power-law rate expression of the

form r ¼ kPmO2PC3H

n6showed a reaction order with respect to oxygen, m, of 0.39, 0.23, and

0.48 for CO, CO2, and acrolein formation, respectively. The order with respect to propene,

n, was 0.43, 0.49, and 0.32.[67] Thus, a conventional redox mechanism does not seem to

operate, and the yield of acrolein can be improved by lowering propene and increasing

oxygen partial pressures.

Even more interesting than the direct oxidation of propene to acrylic acid is the direct

use of propane, for which the Ni–Mo–Te–O system doped with P has been successfully

tested.[66] It was found that the catalytic system provides a wide product distribution,

leading to propene, acrolein, and acrylic acid formation according to a reaction pathway as

shown in Sch. 1. However, the reported yields were low, as compared to the more

promising study published by Fujikawa et al.[68]

Sautel et al. also tested nickel–molybdenum catalysts for such a reaction and

compared the results obtained by the almost stoichiometric catalyst (Mo :Ni ¼ 0.98) with

another one 5% enriched in Mo.[84] It was found that at 500 or 5308C, and with any phase,propene and acrolein formation rates were higher with the Mo-enriched sample, while the

CO2 formation rate was much lower. Therefore, the Mo-enriched compound was a better

catalyst for both conversion and selectivity, and this behavior was attributed to Mo atoms

in interstitial positions (demonstrated by electrical conductivity and thermoluminescence

measurements). While the higher conversion was attributed to increased propane

adsorption, the lower rate of CO2 formation was explained on the basis of smaller

availability of lattice oxygen due to the formation of interstitial Mo atoms.[84]

Investigating the oxidation of butane to butadiene and maleic anhydride, Mazzocchia

et al. found that significantly different results were obtained with respect to product

distribution by changing contact time, temperature, or the butane : oxygen ratio.[35] It was

also interesting that an excess of MoO3 is responsible for higher activity, despite reducing

selectivity to dehydrogenation products and increasing formation of carbon oxides. The

fact that practically the same yield in butenes was observed when the number of pure

butane pulses was increased suggested that butane dehydrogenation occurs without

the intervention of lattice oxygen. On the other hand, maleic anhydride formation is

related to the activation of gaseous oxygen in sites that disappear with strong reduction and

that cannot be regenerated by reoxidation. Such sites probably correspond to Mo(V)

Scheme 1. Reaction pathway proposed for propane oxidation with Ni–Mo–O based systems.

(From Ref.[66], with the kind permission of Elsevier Science.)


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sites.[35] The nickel molybdate catalysts used were prepared by coprecipitation and exhi-

bited low hydrocarbon conversions: at 4758C, 19% conversion of n-butane was recorded

with low selectivity to maleic anhydride.[35]

The great interest in producing maleic anhydride is due to its use as raw material for

products ranging from agricultural chemicals, paints, paper sizing, and food additives to

synthetic resins. To meet the high demand for this valuable chemical, a variety of

commercial processes and efficient catalysts have been developed, particularly from

n-butane oxidation. With this goal in mind, Cherry et al.[69] announced the preparation of a

catalyst composition useful for the vapor phase oxidation of butane to maleic anhydride.

The catalytic system used—an oxide composition containing Sn–Ni–Mo—was found to

be highly selective, stable, and long-lasting, providing selectivities for the desired maleic

anhydride in the range of about 25–35%. Surprisingly it was found that these catalysts are

more effective when unsupported. Table 4 shows some typical results obtained with

Ni–Mo catalysts, undoped or doped with antimony. For the Sb : Ni :Mo catalytic system,

the constant selectivity recorded with increasing conversion up to 70% is particularly

noteworthy, while with nickel molybdate the selectivity for maleic anhydride decreases

markedly above about 50–60% conversion.

In the United States patent by Kourtakis and Sullivan, several molybdenum-

containing oxides (including nickel–molybdenum-based materials) are described, which

can be used in a wider context for catalyzing other C4 oxidation processes.[108] They state

that such catalysts can be used advantageously with regard to conversion and selectivity in

a wide variety of conventional techniques and reactor configurations (i.e., fixed or

fluidized bed reactors or recirculating solids reactors) to perform the oxidation of C4

hydrocarbons to maleic anhydride.[108]

Other important oxidation processes in which Ni–Mo containing catalysts have been

successfully used include: (i) the oxidation of toluene, where a 70% yield of benzaldehyde

Table 4. Catalytic data of oxidation of butane to maleic anhydride.

Catalytic

system

T

(8C)Contact

time (sec)

Conversion

(%)

Selectivity

(%)

Yield

(%)

Sb/Ni/Moa 400 0.32 29 33 10

403 0.46 39 31 12

400 0.83 58 27 16

400 0.98 68 25 17

450 0.09 37 27 10

450 0.13 48 26 13

450 0.24 65 29 19

453 0.23 70 26 18

Ni/Mob 500 0.46 36 27 10

506 0.86 52 29 15

500 1.51 70 21 15

499 2.10 81 17 14

aAtomic ratio ¼ 1 : 0.24 : 0.14. Experimental conditions: butane/air ¼ 0.8/99.2 (mol%); W ¼ 29 g.bExperimental conditions: butane/air ¼ 0.9/99.1 (mol%); W ¼ 18.4 g.

Source: Ref.[69].


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per pass was achieved in a fixed bed reactor with NiMoO4 activated at 4508C;[109] (ii) theconversion of alcohols to aldehydes with an Ni–Mo catalyst containing a complex iron

molybdate;[110] (iii) methacrolein production by catalytic oxidation of isobutene;[105] and

(iv) the ammoxidation of olefins to unsaturated nitriles, notably acrylonitrile and metha-

crylonitrile production, using propene and isobutene as olefin reactants, respectively.[111]

5.2. Oxidative Dehydrogenation of Light Alkanes

5.2.1. Undoped Ni–Mo Catalysts

The search for catalysts with good performance in butadiene production has been

studied for a long time because of its use as a monomer in the production of synthetic

rubber. Although references exist since the 1960s on the use of the nickel–molybdenum

catalytic system in n-butane to butadiene ODH,[112] it was only after 1974 that a

considerable number of studies began to appear on this subjects. One of the pioneering

patents is that of Bertus et al., in which a stationary bed of nickel molybdate provided a

yield of butadiene of 4.2–13.5% by weight, with a selectivity of about 33% (at a

temperature within the range 550–5908C, a space velocity of the butane supply of

50–500 hr21 and at a molar ratio n-butane : oxygen : steam of 1 : 1 : 20).[113]

Around the same time, Pilipenko et al.[114] investigated the effect of the composition

of the nickel–molybdenum system in that reaction. n-Butane conversion at 6008C was

about 30–40% in samples with Ni :Mo atomic ratios m ¼ 17.3–0.48, drastically

decreasing when the MoO3 concentration was increased, reaching values lower than 0.6%

with the pure oxide. The pure nickel oxide is not also interesting for this ODH reaction

because its selectivity to butadiene was practically nil, although conversion is high. The

data obtained show that catalysts with compositions in the range m ¼ 1.92–1.28 present

the highest selectivities and yields butadiene, with values up to 54% and 17.1%,

respectively, decreasing strongly when the MoO3 loading is increased. Therefore,

catalysts with three phases, normal nickel molybdate, nickel oxide, and nonstoichiometric

nickel molybdate (phase N, now called b-phase), are the most efficient in butane to

butadiene ODH. The authors have proposed that an oxygenated nickel compound, which

may exist in several forms, is responsible for the catalytic activity. Finally, by performing

some runs from 1-butene and butadiene, they have suggested that nickel oxide is the

species responsible for the butane to butenes conversion step.[114]

The assignment of the active component of this system caused some controversy. Two

years later Itenberg et al. again studied this reaction with nickel–molybdenum

catalysts.[115] Although catalytic tests with the individual oxides have shown that

molybdenum trioxide and nickel oxide are active in n-butane ODH, the low recorded

conversions and selectivities to butadiene led them to conclude that they were not the

agents responsible for the catalytic activity of the NiO–MoO3 system. Then they

performed some runs with catalysts of different compositions and found that the maximum

selectivity (to butenes or butadiene), at equal butane conversion levels, was obtained with

samples with Ni :Mo atomic ratios between 1.0 and 1.2. A study of the composition of

phases led them to conclude that the active component was nickel molybdate or a solid

solution of nickel oxide in the molybdate lattice. A shift of the ratio for m . 1.2 or

m , 1.0 led to a decrease in selectivity, apparently due to the presence of the individual


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oxides. It was also reported that both nickel molybdate modifications (P and N, where Mo

presents octahedral and tetrahedral coordination, respectively) presented practically the

same activity and selectivity. That is, the crystalline structure of the nickel molybdate

lattice would not affect the behavior in butane ODH. Similar conclusions were also

presented by Cavani and Trifiro,[116] who reported that both crystalline forms of the

molybdate (now called a and b) present similar catalytic behaviors in butane ODH (for

T ¼ 5008C, butane conversion ¼ 12%, yield to butadiene ¼ 4.2%, yield to

butenes ¼ 4.8%). In addition, they reported that in the presence of excess MoO3 or NiO

relative to the stoichiometric molybdate, activity increases but selectivity decreases

considerably.[116]

The claim that a- and b-phases of NiMoO4 exhibit similar performances in ODH is

another subject of controversy. Indeed, neither the results obtained by Mazzocchia et al.

(mentioned in Table 5) nor those obtained by Madeira and Portela seem to support this.

For instance, in the first study reported by these authors it was reported that the a-phase is

more active (see Fig. 10) while the b-phase is much more selective for dehydrogenation

products at comparable conversions and similar temperatures.[59]

Most papers published on ODH reactions with undoped Ni–Mo–O catalysts concern

propane conversion. For catalysts with Mo :Ni ratios .1/1, Lezla et al. considered the

a-phase as the active one.[41] The addition of molybdenum oxide to nickel molybdate

significantly improved the behavior of the catalyst and the most effective composition found

had a Mo :Ni ratio ¼ 1.27/1, with which a selectivity to propene of 63% was obtained, at a

propane conversion level of 22% (at 5008C, t ¼ 3.8 sec, C3/O2/H2O/N2 ¼ 20/10/30/40).[41] They found that interfacial synergetic effects exist between the planes (0 1 0) of a-

NiMoO4 and (1 0 2) of MoO3, as already noticed in some oxidation reactions.

Thomas et al.[47] also tested nickel molybdates with different Ni :Mo ratios in propane

oxydehydrogenation. It was noticed that selectivity to propene is enhanced for Mo-rich

catalysts, which also have better catalytic activity as measured by reaction rates; although,

the surface areas were lower than those of Ni-rich products. Mo-rich catalysts were five

times more efficient than Ni-rich products and the high kinetic constants for propene

formation shown in Fig. 11 counterbalance the low values of their surface areas. This

effect was attributed to the existence of Mo atoms in interstitial positions, revealed by

electrical conductivity and thermoluminescence measurements.[47,84]

Table 5. Catalytic data of oxidative dehydrogenation of propane.a

Catalyst

T

(8C)Conversion of

propane (%)

Selectivity to

propene (%)

Yield to

propene (%)

a-NiMoO4 560 23.3 50.6 11.8

a-NiMoO4 600 37.1 33.8 12.5

b-NiMoO4 560 16.8 80.3 13.5

b-NiMoO4 600 29.0 62.5 18.1

NiMo1.5O5.5 600 37.2 28.6 10.6

Ni1.5MoO4.5 600 34.0 18.5 6.3

aExperimental conditions: Qt ¼ 15 LPTN/hr (25% propane, propane/O2 molar ratio ¼ 0.9);

W ¼ 0.5 g.

Source: Ref.[32].


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In another very interesting work the possibility of using nickel molybdate in

circulating bed reactors for propane ODH was tested.[117] In these reactors the catalyst is

regenerated by oxygen (in a regeneration zone) after leaving the reaction zone where it

oxidizes the hydrocarbon. The maximum amount of oxygen that can be reversibly

removed from a catalyst is a parameter that determines its applicability in these reactors. In

the case of NiMoO4 it corresponds to 2% of the total oxygen content, excessive reduction

leads to the irreversible transformation into MoO2 and Ni (NiO).[117]

More recently, Stern and Grasselli[72] have also studied propane to propene

conversion with several molybdates supported over SiO2. The results showed that the

Figure 10. Conversion and selectivity for n-butane ODH with the a (A) and b (B) phases of

NiMoO4. Experimental conditions: W ¼ 0.5 g; C4H10/O2/N2 molar ratio ¼ 4/9/87; W/F ¼

20 gcat hr/molbutane. (Adapted from Ref.[59], with the kind permission of Elsevier Science.)

Figure 11. Change of the kinetic constant for the formation of propene during propane ODHwith the

atomic composition in Ni–Mo–O catalysts. Experimental conditions:W ¼ 0.25 g; T ¼ 5008C; C3H8/O2/He% ¼ 15/18/67; QT ¼ 15L/hr. (From Ref.[47], with the kind permission of Elsevier Science.)


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reaction is catalytic and is not initiated with formation of radicals in the gas phase, the rate

controlling step being the breaking of the C–H bond with abstraction of a hydrogen from a

methylene group of the propane molecule. Among the simple molybdates tested of the

type AMoO4 (with A ¼ Ni, Co, Mg, Mn, or Zn), the nickel one presented the greatest

activity, even when normalized per unit of surface area (which was the highest: 39m2/g).The molybdenum–oxygen bond, which is influenced by the nature of the adjacent metal,

A, is probably responsible for propane activation and, consequently, bonds of the type

Ni–O–Mo–O are the most active. It is noteworthy that at equal conversion levels, the

highest selectivity was also found with the NiMoO4 catalyst (60% for propene at a

conversion of 27%).

Mazzocchia et al. have also studied the ODH of propane to propene with catalysts

containing Ni :Mo ratios below and above 1, as well as the behavior of the two NiMoO4

phases.[32] In Table 5 some of the data are presented. It is clear that the two phases of the

Ni–Mo–O system exhibit quite different catalytic behaviors. Further investigation

confirmed that although the a-phase is slightly more active, the b-phase is almost twice as

selective to propene at comparable conversions and identical temperatures.[37] This

selectivity difference was accounted for on the basis of the different types of oxygen bonds

at the active sites. Since the presence of M55O bonds is usually associated with the

formation of oxygen-containing products, and because in the b-phase the character of this

bond is weaker than in the a-phase (band at lower frequencies in the infrared spectrum),

the high temperature phase should be more selective to dehydrogenation products. This

different behavior of the two phases was very recently attributed by Kaddouri et al. to the

different reducibility of the two phases.[67] In fact, a relationship was found between the

reducibility of the catalysts and the catalytic behavior in propane ODH, demonstrating that

lattice oxygen plays an important role in the reaction, i.e., the process is governed by a

redox or Mars–van Krevelen mechanism.[118]

The formation of propene by reaction of propane with superficial O22 anions was

proposed, leading to the formation of anionic vacancies (VOoo) shown by electrical

conductivity measurements [Eq. (17)].[37] Regeneration of the oxygen species active in

propane ODH occurs through spontaneous reoxidation of the surface by gaseous O2

[Eq. (18)] according to a Mars–van Krevelen mechanism:

C3H8 þ O2�s �! C3H6 þ H2Oþ Voo

O þ 2e� (17)

1

2O2(g)þ Voo

O þ 2e� �! O2�s (18)

The fact that the b-phase is more selective than the a-phase led Mazzocchia et al.[33] to try

to prepare the former phase at lower temperatures than those generally used (about 7008C)in order to avoid sintering, which drastically reduces the surface area. It would also be

convenient to stabilize this phase but without excess NiO, because the precursor with

excess nickel (green), which stabilizes the b-phase at room temperature, does not exhibit

good performance in propane ODH. It was found that when calcining the yellow precursor

of stoichiometric nickel molybdate in situ, after thermal decomposition at 5008C, the b-

phase of NiMoO4 crystallizes.[33] The catalytic runs were quite interesting because the

precursor calcined in the reactor at 5508C showed a selectivity to propene at 5308C, whichwas almost as high as the b-phase (65.7% against 78.5%), which had been obtained

starting from a-NiMoO4 and heating from 258C to 7008C, then cooling later to the


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reaction temperature. Furthermore, the first catalyst presented a higher level of

propane conversion (18.3% against 10.0%), leading to a larger yield to propene (12.0%

against 7.8%).

The previous situation still presented a technical problem because the reactor had to

be continuously heated to keep the catalyst as the more selective b-phase for a long period,

and it could not be cooled to room temperature in order to prevent transition to the

a-phase. To overcome this drawback, an oxalic precursor was prepared which, after

thermal activation, led to stabilization of the NiMoO4 b-phase at room temperature.[34] In

this way the need for continuous heating of the reactor is avoided in possible industrial

applications. Moreover, the catalyst presented a specific surface area of 34m2/g, similar to

that of a-NiMoO4 (32m2/g).[34]

Later Anouchinsky et al. immobilized the Ni and Mo ions inside an organic gel (agar-

agar), which led to NiMoO4 crystallization at lower temperatures and to possible

stabilization of the b-phase at room temperature, depending on the thermal treatment

applied in the precursor activation.[40] Such stabilization may be due to the presence of

organic residues because, as mentioned above, stabilization of the b-phase at room

temperature was also found after forced coke deposition over NiMoO4.[96] The catalyst

obtained starting from the gel presented selectivity to propene similar to that obtained with

the pure b-phase, but it was less active.[40] Propene productivities (in mmol/hr) at 5008Cwere the following: 4.7 (a-NiMoO4), 2.5 (b-NiMoO4), and 1.9 (a þ b NiMoO4, obtained

from the gel). The catalytic results in propane ODH do not appear very promising,

although the preparation method has not yet been completely optimized.

5.2.2. Doped and Supported Catalysts

Due to the great industrial interest in butadiene, butenes have also been converted into

this product by ODH using Ni–Mo doped catalysts. Some interesting results are shown in

Table 6, obtained with catalysts claimed by Bertus in one of the pioneering patents dated

1978.[64] The effective conversion and yield of butadiene with both catalytic systems is

noteworthy, and it is also apparent that contact times must be short.

The use of promoted Ni–Mo catalysts in n-butane oxydehydrogenation is not recent.

For instance, US Patent No. 4,094,819 of 1978[64] presents good results using several

Ni–Mo doped-catalysts, those obtained with arsenic being particularly interesting

(Table 7). Even more efficient was a catalyst of oxides of molybdenum, cobalt, and nickel,

which provided a conversion of butane of about 20%, with a selectivity for n-butenes of

about 26% and of about 35% for butadiene, thus providing a total yield for C4s of

12.2%.[119]

Some studies on the use of promoted Ni–Mo catalysts for n-butane ODH have been

published by Madeira and Portela. In a preliminary study it was shown that all the alkali

metals studied (Li, Na, K, or Cs) significantly improve the selectivity of the NiMoO4

catalyst,[59] when promoting either the a- or b-phase. As shown in Fig. 12, this effect

increases in the sequence unpromoted , Li-doped , Na-doped , K-doped , Cs-doped,

the b-phase of this catalyst providing a yield to C4 products of 14.5% (for a conversion

level of 28.2%).[59] The fact that higher selectivities were obtained with Cs-doped nickel

molybdate led the authors to study the effect of this promoter in more detail.[80,83]

The effect of doping with alkali metals, i.e., the increase of selectivity to ODH

products, can be easily understood if one takes into account their basic nature. Thus, the


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more basic surface facilitates desorption of the nucleophilic olefins/di-olefins from the

catalyst surface (butenes and butadiene that have high electron densities at the p bonds),

thus avoiding their overoxidation into carbon oxides. Indeed, a good correlation was

recorded between surface basicity, measured by CO2-TPD experiments, and selectivity for

C4 products as shown in Fig. 13,[80] in which an overdoping effect is also visible.

Alkali-earth metals, namely Ca, Sr, and Ba, were also used by Madeira et al. as

promoters of the NiMoO4 catalyst in n-butane ODH.[60] In this case, and due to the

moderate basicity of such promoters (as compared with alkali metals and revealed by

CO2-TPD), a less pronounced increase in selectivity to C4s was recorded. However, and

because there is much interest in obtaining butadiene directly from butane, this “tuned”

basicity seems to be very interesting because a very high selectivity to butadiene

was achieved. In fact, alkali-earth doped catalysts were almost twice as selective to

butadiene as was undoped NiMoO4. Nonetheless, an overdoping effect was also detected

Table 6. Catalytic data of oxidative dehydrogenation of butenes to butadiene.

Catalytic

system

T

(8C)Contact

time (min)

Conversion

(%)

Selectivity

(%)

Yield

(%)

Ni/Mo/Pa 538 15 38.8 72.2 28.0

538 60 36.0 74.8 27.0

538 180 35.7 74.0 26.4

482 15 26.3 82.8 21.8

482 60 25.5 77.4 19.7

482 180 24.2 74.0 17.9

Ni/Mo/Sbb 538 15 28.3 83.0 23.5

538 60 19.1 92.2 17.6

538 180 16.2 94.3 15.3

aCatalyst composition ¼ 38.3/26.1/5.3 (wt%). Experimental conditions: butene/oxygen/steamfeed rate ¼ 300/264/5780GHSV.bCatalyst composition ¼ 35.7/24.3/15.2 (wt%). Experimental conditions: butene/oxygen/steamfeed rate ¼ 300/264/5400GHSV.Source: Ref.[64].

Table 7. Catalytic data of oxidative dehydrogenation of n-butane to butenes and butadiene.a

Catalytic

system

Conversion

(%)

Total

selectivity

(%)

Yield C4H8

(%)

Yield C4H6

(%)

Total yield

(%)

Ni/Mo/Bib 10.0 33.6 1.7 1.6 3.3

Ni/Mo/Sbc 15.0 23.0 3.6 0.3 3.9

Ni/Mo/Asd 15.9 59.0 6.0 3.4 9.4

aExperimental conditions: butane/oxygen/steam feed rate ¼ 50/50/500 GHSV; temperature ¼ 5668CbComposition ¼ 36.0/24.4/15.8 (wt%).cComposition ¼ 35.7/24.3/15.2 (wt%).dComposition ¼ 35.7/24.3/13.8 (wt%).

Source: Ref.[64].


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for barium-doped catalysts that exhibit a maximum of selectivity (as well as a maximum of

basicity) at a 9% promoter loading (atomic ratio Ba :Mo).[60] Similar conclusions were

also recently reported by Liu et al. for propane oxydehydrogenation.[95] They found that

selectivity to propene increased with the barium load from 1% to 9%, but decreased with

the increase of its content from 12% to 15%, i.e., a maximum was found once again for a

molar ratio Ba :Mo of 9%. The 9% Ba–Ni0.9MoO4 catalyst is thus the most interesting

one, with a very good yield to propene of 30.5%.[95]

Although alkali (and also alkaline-earth) metals improve selectivity in n-butane ODH,

the conversion level usually decreases, and this becomes more pronounced as the promoter

loading (or size) increases. This effect results mainly from the decrease in the BET surface

area after doping.[59,60]

Stern and Grasselli[72] have tested some nickel-containing binary molybdates with the

formulas Ni0.5A0.5MoO4 (with A ¼ Co, Mg, Mn, or Zn) in propane ODH. Although these

catalysts have not shown better performance than NiMoO4, the most active and selective

one (Ni0.5Co0.5MoO4) was used as a reference to investigate in more detail the Ni–Co–

Mo–O system due to its great ease of preparation. With this catalyst a kinetic study was

then performed and a mechanism proposed for propane ODH,[120] as mentioned below.

These authors decided to study in detail the following systems in propane ODH:

Ni12xCoxMoO4 (with x between 0 and 1), AMo1+xOy (where A ¼ Ni or Co and x is

between 0 and 0.1), and promoted Ni0.5Co0.5MoOx.[72] It is worth noting that the results of

the variation of molybdenum content (x) between 0.9 and 1.1 in catalysts of the type

NiMoxOy have shown that catalytic activity to partial oxidation products is strongly

sensitive to Mo content. The catalyst of stoichiometric composition, NiMoO4, presented

Figure 12. Effect of different alkali promoters (1% loading, surface atomic ratio metal/molybdenum) on the catalytic behavior of a- and b-phases of NiMoO4 for n-butane ODH.

Experimental conditions as in Fig. 10. (Adapted from Ref.[59], with the kind permission of Elsevier

Science.)


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by far the greatest activity and the highest yield, both activity and yield significantly

decreasing with increasing or decreasing Mo content.

Mazzocchia et al. have also used doped-nickel molybdate catalysts in ODH reactions,

namely K-, Ca-, and P-doped for propane[61–63] or isobutane conversion.[62] It is

particularly noteworthy that with calcium and potassium addition, propane conversion

decreases but selectivity to propene increases, showing once again that basic sites

(assessed by catalytic decomposition of isopropanol in the absence of oxygen) are crucial

to the oxydehydrogenation process. On the other hand, promotion with phosphorous

(a typical acidic element) has led to enhancement of propane conversion, which was

ascribed to an increase in alkane adsorption on the more acid catalyst surface.[61,63]

Similarly, to previous results obtained by Madeira et al.,[90] it was found that these

promoters significantly increase the reduction resistance of nickel molybdate, indicating

that the mobility of the lattice oxygen has become less important.[61,63] It seems that the

catalytic activity of Ni–Mo–O-based catalysts for ODH of light alkanes is related to

catalyst reducibility, while selectivity to dehydrogenation products depends mainly on the

acid–base character of the catalyst surface. However, Kaddouri et al. concluded that

selectivity also depends on the degree of catalyst reduction, since propene formation is

improved with the pulse period in a periodic-flow reactor operation.[61]

With regard to isobutane conversion, it was found that the catalytic performances of

stoichiometric nickel molybdate, in terms of selectivity for isobutene, can be improved by

the addition of potassium oxide, which avoids subsequent overoxidation of the reactive

isobutene formed. However, methacrolein formation is negatively affected (see Table 8).

The great interest in producing methacrolein, which is widely used to produce methacrylic

Figure 13. Selectivity to dehydrogenation products (at an n-butane conversion level of 5%) as

a function of catalyst surface basicity (amount of adsorbed CO2 at 308C) for unpromoted and

Cs-promoteda-NiMoO4 catalysts. (Adapted fromRef.[83], with the kind permission of Elsevier Science.)


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acid for the polymer industry, led Kaddouri et al. to try to avoid this negative effect.[62]

This was achieved by using low oxygen partial pressure in the feed, which also increases

isobutene selectivity, due to a decrease in formation of carbon oxides.

The use of supported catalysts in the oxydehydrogenation of light alkanes is

uncommon. Recent studies have been performed on isobutane ODH, using TiO2[48] or

SiO2[49,52] as supports. Such catalysts have been shown to be more selective to isobutene

than unsupported NiMoO4, which was attributed to the acid–base properties of the

surface[52] or to b-phase stabilization at low temperature.[48,49] This is very important

because the b-phase is metastable at room temperature and, in industrial practice, it is not

practical to keep reactor operation above 2508C to avoid the transition to the a-phase.

Furthermore, incorporation of excess nickel to stabilize the b-phase at low temperatures

favors competitive side reactions. Silica-supported nickel molybdate catalysts, prepared

through sol–gel procedures, were also tested in isobutane ODH,[50,51] but no significant

yields were obtained.

A special mention should be made of the unexpected effects found after coke deposition

on nickel molybdate catalyst.[96] First, and surprisingly, it was observed that, with this depo-

sition, deactivation was not found during n-butane ODH. On the contrary, conversion

increased (more than 40%), as well as selectivity to dehydrogenation products, and

particularly to butadiene. These effects were attributed to the stabilization of the more

selective b-phase at low temperatures and to the presence of catalytically active coke.[96]

5.2.3. Kinetics and Mechanism

One of the few kinetic and mechanistic studies found in the literature performed with

Ni–Mo–O catalysts for propane oxydehydrogenation was published by Stern and

Grasselli.[120] It was carried out with the above-mentioned Ni0.5Co0.5MoO4/SiO2 catalyst

and the results showed that the reaction proceeds through ODH, propene being formed as

Table 8. Typical results of oxidative dehydrogenation of isobutane to isobutene and methacrolein,

at 8% conversion level.a

Catalysts

Selectivity Surface

area

(m2/g)i-C4H8 CH255CCH3CHO CO CO2 (CH3)2CO

a-NiMoO4 25.5 12.5 27.7 34.3 0.0 32.2

a-NiMoO4/0.20%K 44.4 11.5 14.7 27.8 1.6 30.7

a-NiMoO4/0.25%K 52.2 9.5 11.8 24.6 1.9 29.4

b-NiMoO4 41.3 15.5 17.9 22.7 2.6 13.1

b-NiMoO4/0.20%K 66.0 6.4 5.7 20.9 1.0 10.6

b-NiMoO4/0.25%K 69.2 6.3 6.2 12.6 5.7 9.8

a0-NiMoO4/0.20%Kb 60.1 8.4 8.3 22.6 0.6 10.6

a0-NiMoO4/0.25%Kb 69.1 11.0 7.2 12.1 0.5 9.8

aExperimental conditions: W ¼ 0.5 g; %i-C4H10 ¼ 15; 4 , %O2 , 7; 4208C , T , 4808C;10 , F , 20L/hr.ba0-NiMoO4 is obtained after cooling the b phase to room temperature.

Source: Ref.[62].


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the primary and exclusive product. Formed propene is first oxidized to acrolein, which is

then oxidized to carbon oxides and acrylic acid. However, small amounts of COx are also

formed directly from propene. The selective oxidations of propane to propene and of

propene to acrolein are both zero-order in oxygen (a common behavior found in the

oxidation of hydrocarbons over metallic oxides and consistent with the redox mechanism

of Mars and van Krevelen)[118] and first-order with respect to the hydrocarbon (consistent

with a rate-limiting reaction between the hydrocarbon and an active site on the catalyst

surface). The deep oxidation of propane into CO and CO2 has an order of 1/2 in oxygen

and is of first-order with respect to propane, while the deep propene oxidation shows

dependence on the hydrocarbon concentration of the type r / kx/(1 þ Kx), that is, the

rate-limiting step involves a surface species that is in adsorption equilibrium with the gas

phase and is also of order 1/2 in oxygen. To conclude, the work by Stern and Grasselli[120]

showed that the partial and deep oxidation of the hydrocarbons on the Ni–Co–Mo–O

system occurs through two different mechanisms. The partial oxidation of propane to

propene and of this product to acrolein can be described by the Mars–van Krevelen

mechanism,[118] in which the adsorbed hydrocarbon reacts with the lattice oxygen

(nucleophilic). On the other hand, deep oxidation of propene to COx can be described by a

Langmuir–Hinshelwood mechanism in which the adsorbed hydrocarbon reacts with

adsorbed and dissociated oxygen (electrophilic). Finally, it should be noted that isotopic

studies in propane and propene activation have revealed that hydrogen abstraction from a

methylene group and in allylic position (a-hydrogen) are the respective rate-controlling

steps.[120]

The greater selectivity presented by the b-phase in some oxydehydrogenation

reactions was the reason that impelled Sautel et al. to perform a detailed kinetic study on

propane to propene ODH, trying likewise to clarify some important aspects of the reaction

mechanism.[121] For the conditions used, it was found that the propene formation rate

presents partial orders to propane and to oxygen close to one and zero, more precisely

0.95 + 0.01 and 0.03 + 0.02, respectively (see Fig. 14), and therefore gaseous oxygen is

not directly responsible for the ODH reaction. This is indicative of a Mars–van Krevelen

type mechanism. Over a-NiMoO4, Del Rosso et al. also found that a change in the oxygen

partial pressure does not affect propene formation, which is instead dependent on the

propane partial pressure. Indeed, for the propene formation rate partial orders with respect

to propane and oxygen of 1.2 + 0.01 and 0.04 + 0.01, respectively, were found.[122] The

involvement of lattice oxygen was then confirmed using continuous, transient, and

periodic operating systems.[122]

For the more selective b-NiMoO4, it was proposed that the global reaction:

C3H8 þ1

2O2 ! C3H6 þ H2O (19)

can be written in six steps:[121] (1) propane adsorption on the catalyst surface; (2) oxidation

of the adsorbed propane by the oxygen of the NiMoO4 lattice; (3) and (4) desorption of the

products (propene and water); (5) and (6) oxygen adsorption on the catalyst surface and

filling of the oxygen vacancies in the solid. Assuming a rate-determining step and that

the others are at near-equilibrium conditions, the theoretical rate equations were deduced

and were compared with the experimental data. It was possible to conclude that the

rate-limiting step that controls the overall reaction rate can be either propane adsorption


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(step 1) or the reaction between adsorbed propane and the oxygen of the NiMoO4 lattice

(step 2).[121]

A kinetic study with b-phase nickel molybdate was also reported for ethane ODH, in

which interesting and even uncommon results were obtained.[123] First, it was found that in

ethane oxydehydrogenation the a-phase is not only more active but also more selective

than the b-phase, therefore behaving differently from the case of propane ODH. Secondly,

it was found that for the b-phase the overall rate of ethane conversion can be described by:

rC2H6¼

d(C2H6)

dt¼ k(C2H6)

1:15(O2)0:21 (20)

with a reaction order of 0.16 for C2H4 formation with respect to oxygen. In addition, tests

performed under continuous flow in the absence of gas phase oxygen have led to the

conclusion that no ethylene is formed. Thus, and unlike the case of propane ODH, lattice

oxygen does not seem to guide the reaction towards dehydrogenation. For this case, a

mechanism that considers the intervention of surface O2 species was proposed, in which the

activation of ethane involves hydrogen abstraction by these species to give ethyl radicals.[123]

Regarding butane conversion, a kinetic study of the selective oxidation and

degradation of n-butane over undoped and cesium-doped nickel molybdates was recently

reported, covering a wide range of experimental conditions.[90] The rate data were fitted to

power-law rate equations, i.e.:

ri ¼ kPn1butaneP

n2O2

(21)

The computed reaction orders n1 and n2 showed that Cs doping only affects the partial

order with respect to butane, which increases for dehydrogenation products and decreases

for CO and CO2. The partial order with respect to oxygen is almost unaffected, in both

cases, i.e. with undoped or Cs-doped NiMoO4, showing zero-order dependence for

formation of C4s, which suggested the existence of a Mars–van Krevelen process.[90]

In a Mars–van Krevelen (or redox) model[118] it is assumed that hydrocarbon reacts

with the lattice oxygen of an oxidation catalyst, which becomes reduced. The reduced

catalyst then reacts with molecular oxygen from the gas phase to complete the catalytic

cycle.

Figure 14. Evolution of the rate of formation of propene with the partial pressure of propane (for

PO2¼ 18 � 103 Pa) and oxygen (for Ppropane ¼ 15 � 103 Pa) introduced with b-NiMoO4. (From

Ref.[121], with the kind permission of Elsevier Science.)


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A generalized Mars–van Krevelen model was applied by Madeira and Portela[124,125]

to the results of the above-mentioned kinetic study of the selective oxidation and

degradation of n-butane over undoped and cesium-doped nickel molybdates. By nonlinear

regression analysis, the following rate equations were obtained:

ri ¼kokrPbutane

ko þ akrPbutane

(22)

for the products of selective butane conversion (butenes and butadiene)[124] and

ri ¼kokrP

1=2O2

P2butane

koP1=2O2þ akrP

2butane

(23)

for the degradation products of butane (CO and CO2), over the pure NiMoO4 catalyst,[125]

where

ri ¼ butane conversion rate to specified products;

ko, kr ¼ kinetic constants for the reoxidation and reduction steps, respectively;

PO2, Pbutane ¼ oxygen and butane partial pressure, respectively;

a ¼ stoichiometric number of oxygen moles required in the reaction.

Other studies performed with nickel molybdate catalysts also supported a Mars–van

Krevelen mechanism for n-butane ODH. Evidence for such a mechanism includes: (i) the

zero-order dependence on the oxygen partial pressure for C4 formation,[90] (ii) the

relationship between catalytic activity and reducibility (inferred by the temperature of

onset of reduction) of several Cs-doped catalysts,[90] (iii) the similar apparent activation

energies for butane conversion with and without gas phase oxygen,[126] (iv) the fact that no

oxygen adsorption was observed by O2-TPD,[124] and (v) catalytic tests without O2 in the

feed, which showed that butane can be converted to C4 products with high selectivity even

without gas phase oxygen.[126] The tests performed in the absence of oxygen showed that

lattice oxygen plays a crucial role in selectivity during butane conversion, as is also the

case for propane ODH.[122] In addition, the very low butane conversion levels achieved

(typically below 1% as compared to runs with oxygen in the gas phase—between 2.9%

and 7.9%),[126] indicate that alkane conversion is limited by the reducibility of the catalyst

or by lattice oxygen availability and mobility. However, such low conversion levels

(,1%) are typically consistent with an oxygen consumption that corresponds to a small

percentage of the mobile oxygen content of the catalyst monolayer, as shown by Del Rosso

et al.[122] Therefore, the reaction seems to be controlled by the reducibility of the catalyst

(or by the oxygen diffusion within the solid).

More recently, the existence of a redox mechanism in n-butane ODH over nickel

molybdate catalysts was further supported by an in situ study of the catalyst’s electrical

conductivity.[92] When the catalysts were subjected to a sequence of gaseous atmospheres

of the type: oxygen–butane–oxygen-reaction mixture, a reversible redox process was

observed, for both undoped and Cs-doped NiMoO4 (Fig. 15). The sharp increase in the

electrical conductivity (s) recorded when pure butane is introduced in the cell is attributed

to the release of electrons into the conduction band during surface reduction:

C4H10 þ (OO)S �! C4H8 þ H2Oþ VooO þ 2e� (24)


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When oxygen is reintroduced, s reversibly decreases and practically returns to its initial

value, this process corresponding to the filling of the vacancies by gas phase oxygen in

accordance with the following equation:

VooO þ 2e� þ

1

2O2(g) �! (OO)S (25)

In the steady state, the electrical conductivity of the NiMoO4 catalyst under the reac-

tion mixture is much closer to that of the oxidized state than to that of the reduced one (see

Fig. 15), which agrees with the faster reoxidation as compared to reduction of the

catalyst.[92] The n-type semiconductor behavior of nickel molybdate was also

demonstrated, because (@s/@t)O 2, 0 and (@s/@t)C4H10

. 0.

The kinetics and mechanism of isobutane ODH over nickel molybdate were also

recently investigated.[127] It was concluded that isobutene is formed via a redox

mechanism with the participation of lattice oxygen, while the formation of carbon oxides

occurs with the participation of chemisorbed oxygen.

5.3. Nature of Active Sites

The nature of the active sites in the selective oxidation of hydrocarbons has been

widely investigated for molybdenum-containing catalysts,[7] and the use of surface-

sensitive techniques has made a crucial contribution. We should mention, for instance, the

very recent study by Watson and Ozkan,[128] in which ESR was used to investigate

changes in Mo(V) species upon contact with propane.

Figure 15. Kinetics of the changes in electrical conductivity of unpromoted and Cs-promoted

a-NiMoO4 catalysts under different atmospheres (at 3758C). (From Ref.[92], with the kind

permission of Elsevier Science.)


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The role of Mo5þ as an active species is well recognized by several authors. For

instance, Abello et al.[129,130] suggest the involvement of Mo5þ species (for Mg–Mo–O

catalysts) in propane ODH. Characterization by several techniques, particularly XPS and

EPR, gave clues that the active site would be a coordinatively unsaturated form of Mo5þ,

which could be generated on the surface by propane reduction. Khan and Somorjai[131]

proposed that the active sites in molybdenum-based catalysts are coupled pairs of Mo(V)

and Mo(VI) species, which are responsible for the redox mechanism. More recently, the

importance of the Mo(V)/Mo(VI) redox pair in propane ODH over molybdenum

phosphates was also pointed out.[132]

Although some consensus seems to exist regarding the active species for molybdenum-

containing catalysts, other active species are also suggested. For instance, Harlin et al.

claim that the oxidation state of molybdenum active in the dehydrogenation of n-butane

is either Mo5þ or MO4þ, which upon reduction to lower oxidation states leads to a

catalyst responsible for increased selectivity to cracking and coke formation.[133] Other

authors found instead that catalyst performance (Mg–Mo–O and Co–Mo–O systems)

during propane ODH increases in parallel with the increase of surface weak acid sites,

suggesting that these sites are involved in propane activation.[134] An excess of octa-

hedral molybdenum, partially covering the surface, seems to be responsible for the weak

acid sites.

For Ni–Mo–O catalysts, it is also generally accepted that the active site involves

Mo5þ species. Mazzocchia et al., while investigating butane to butadiene and maleic

anhydride oxidation, found practically the same yield of butenes when the number of

butane pulses was increased, suggesting that butane dehydrogenation occurs without the

intervention of lattice oxygen.[35] Moreover, maleic anhydride formation is related to

gaseous oxygen activation over sites that disappear by deep reduction and that cannot be

regenerated by reoxidation. Such sites probably correspond to Mo(V) sites. Although no

unequivocal and definitive proof exists that establishes the nature of the active and/orselective sites involved in selective oxidation reactions, other authors also seem to agree

that the redox couple Mo6þ/Mo5þ is necessary for the ODH step, particularly for propane

conversion.[41] However, further research using surface sensitive techniques for

characterization of NiMoO4 catalysts, of which there have so far been rather few, is urgent.

6. CONCLUSIONS AND FUTURE TRENDS

In this paper, the main scientific publications regarding the Ni–Mo–O catalytic

system and its applications for the selective oxidation of hydrocarbons were reviewed.

Particular attention was also dedicated to the preparation techniques that have been used

and to the main physical–chemical characterization data.

With regard to the preparation techniques, most studies found in the literature have

focused on the use of the coprecipitation method. These techniques have been

progressively refined considering that their preparation involves a sequence of unit

operations, each requiring the optimization of its specific parameters. In this context it is

important to note the work of Mazzocchia et al., which led to significant scientific and

technical results detailed in Refs.[32,37,106,121,135,136]. At the same time the application of

increasingly sophisticated characterization techniques was essential for fundamental

results contributing to the optimization and applicability of the prepared catalysts.[33,91,137]


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The use of supported Ni–Mo–O catalysts is now the subject of more inves-

tigation,[48,49] particularly catalysts prepared by sol–gel techniques.[50,51] This is a topic

that, in our opinion, should be further explored because interesting results have already

been obtained, particularly the stabilization of the high temperature b-phase at room

temperature. Since products obtained by sol–gel techniques depend strongly on all

experimental conditions, which play an important role in determining their physi-

cochemical characteristics and consequently their catalytic behavior, further fundamental

research on this subject is required.

Mixed Ni–Mg molybdates were also recently synthesized by Soares et al.[65] via the

sol–gel technique and revealed interesting catalytic behavior in n-butane ODH. C4 yields

were significantly improved using mixed molybdates, particularly for an Mg/Ni atomic

ratio of 0.31. But this behavior has not been fully clarified.

Nanosized materials are another promising field for research. It is known that the

catalytic properties of mixed metal oxides are largely dependent on their microstructure.

In the nanoparticle phase, the surface to volume ratio increases drastically and the surface

atoms include an increasing fraction of the total particulate volume with high defect

structures. Thus, they may exhibit interesting new or improved catalytic properties.

Nanocrystalline NiMoO4, with particle size of about 20 nm, has been prepared,[42,43] but

no catalytic data have been reported.

Regarding the application of Ni–Mo–O catalysts in oxidation reactions, the direct

oxidation of propane deserves special mention. It is known that the most important

industrial process for synthesis of acrylic acid involves two steps, the first one being

oxidation of propene to acrolein, which is further oxidized in the second stage to acrylic

acid. The most promising route would be the direct oxidation of propane to acrylic acid in

a single step. This was successfully tested by Kaddouri et al.[66] but their yields were not

sufficient for industrial application. More recently, Fujikawa et al.[68] found that nickel

molybdates modified with telluromolybdate are good catalysts for this reaction, with a

maximum yield of 20% (acrylic acid and acrolein). However, because of the instability

and toxicity of tellurium compounds, other catalysts should be tested in the future.

Several nickel–molybdenum-based catalytic formulations have already proved

promising for hydrocarbon oxidation reactions. The continuous screening of new classes

of catalytic materials and their optimization has been a great challenge, but it is very time

consuming. However, high-throughput experimentation methodology has been success-

fully used for other applications,[138] and is an important approach to be considered for

accelerated catalyst design, evaluation, and development. Besides, this technique is

particularly promising for the development of novel multicomponent mixed metal oxide

catalysts through rapid microscale synthesis and catalytic screening, using combinatorial

methods.

Although the data found in the literature for ODH reactions with Ni–Mo–O catalysts

do not point to very high levels of paraffin conversion, it seems that these oxides can operate

at low conversions, at which selectivity to dehydrogenation is quite favorable. Indeed,

according to Cavani and Trifiro,[116] the various molybdates (among them nickel) present a

common feature in ODH reactions, which is a decrease of selectivity to olefins when the

hydrocarbon conversion level increases (e.g., n-butane). For this reason these catalysts need

to operate at low conversions, with recycling of the nonconverted paraffin. But among the

required properties for heterogeneous oxidation catalysts (activity, selectivity, and stability),

selectivity to desired product(s) is crucial since the production of undesirable products


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increases costs at the industrial level.[5] Therefore, many authors have investigated the

applicability of nickel molybdates in such reactions and, among other aspects, have tried to

increase the selectivity of the catalyst to the desired oxidation products, particularly by the

use of promoters. Promising results have been achieved, but for very low hydrocarbon

conversion levels, and the yields obtained do not yet recommend the use of such processes

on an industrial scale, as an alternative to conventional dehydrogenation processes.

Despite the large number of studies found in the literature concerning the use of

Ni–Mo–O catalysts for the selective oxidation of hydrocarbons, and particularly for

the ODH of light alkanes, some important aspects, such as the nature of the active sites, the

hydrocarbon activation process, the kinetics and mechanism of the reaction, and the

factors that determine selectivity, are not yet sufficiently clear.

It has been shown that nickel–molybdenum catalysts are efficient for several oxidation

processes. Their use for isobutane conversion into isobutene is an application that should be

explored further in the future. Isobutene is a key reactant for the production of methyl t-butyl

ether (MTBE) and ethyl t-butyl ether (ETBE), which are used as lead-free octane boosters

for gasoline.[139,140] The use of MTBE has given rise to significant pollution problems not

found with ETBE, which is nowmuch in demand as a lead-free gasoline additive, improving

the combustion process, and decreasing carbon monoxide emissions. So, to produce the

amounts of ETBE required, additional sources of isobutene will be needed. Recently

some studies have been performed using TiO2-[48] and SiO2-supported

[49,52] NiMoO4 for the

oxydehydrogenation of isobutane. Although these catalysts have proved to be more selective

to isobutene than unsupported NiMoO4, which was attributed to b-phase stabilization at low

temperature[48,49] and to the acid–base properties of the surface,[52] the yields obtained were

not sufficiently high and the formation of many undesirable products was reported, namely

carbon oxides, products from cracking reactions (light alkanes), and heavy organic

compounds that condense at the reactor outlet or remain on the catalyst surface (coke). Other

studies have also been recently reported, but the isobutene yields attained were even

lower.[50,51,62] This subject is worth further investigation to achieve higher performances in

isobutane to isobutene conversion.

In the ODH of light alkanes, some studies suggest that, in practice, it would be

advantageous to operate with a high alkane/oxygen ratio in the gas-phase feed in order to

favor selectivity for dehydrogenation products. This agrees with the fact that a higher

partial order in oxygen was observed for formation of carbon oxides (CO and CO2)

compared with that found for the desired selective oxidation process (see Refs.[62,90]).

However, various groups have found difficulties in this operation. For instance, Madeira

et al.[90] found catalyst reduction and strong coke deposition when using high butane

concentrations in the feed of a tubular reactor containing undoped a-NiMoO4. Similar

problems were also found by Del Rosso et al. when operating a periodic flow reactor under

severe conditions (i.e., high temperature, long pulse periods, high propane partial pressure,

etc.) during propane ODH.[122,141] In this case, the deep reduction depleted more than the

surface oxygen monolayer, leading to irreversible deactivation of the catalyst with

formation of metallic nickel and coke filaments. A possible way to avoid this drawback

would be the use of promoters to inhibit such reduction, or the use of accelerators in the

gas phase. For instance, it was recently reported that Cs partially inhibits NiMoO4

reduction under pure butane, but completely inhibits it under a reaction mixture containing

butane and oxygen.[92] Thus, the promoter keeps the catalyst in a higher oxidation state,

allowing the use of higher alkane concentrations. Cs-doped NiMoO4 catalysts, therefore,


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seem to be promising for reactor designs in which operation is achieved in the absence of

gas-phase oxygen, with the consumed lattice oxygen periodically restored by exposure to

an oxygen-containing gas, i.e., in a cyclical process similar to that proposed by Boutry

et al.[142] This can be done in practice using multiple reactors or catalytic reactors with

mobile or fluidized beds. Concerning the use of accelerators in the gas phase, which are

applied in some industrial processes, Dury et al.[143] found very recently that the addition

of CO2 to the gas stream helps to maintain the catalytic surface of NiMoO4 in a high

oxidation state, under conditions for which the catalyst undergoes reduction with

consequent deactivation. In specific conditions it can also increase propene formation

from propane, thus suggesting the possible utilization of CO2 in industrial applications.

Some of the most important works concerning the ODH of alkanes have indicated the

possibility of using riser reactors in order to obtain results of industrial interest to alkenes.

The above-reviewed data have clearly demonstrated the redox properties of NiMoO4. Also,

tests performed by the periodic operation technique have determined the degree of optimal

reduction of NiMoO4 for its regeneration and also for the deposit conditions of coke. These

results make it possible to propose technologies based on riser reactors. The mechanical

properties of NiMoO4 catalytic systems are a fundamental factor for the success of these

technologies. In fact for such technologies the mechanical properties of the catalyst are

essential, since it has to resist both abrasion and circulation, because it has to be moved from

the reaction reactor to the regeneration reactor and back to the reaction reactor.

For the improvement of the mechanical properties of the catalysts there are some

noteworthy works in which microwave technology has been used for unit operations of

drying and calcination. The solids obtained after these operations, at the end of the

microwave thermal treatments, have better mechanical properties than those that have

undergone conventional thermal treatments.[144,145]

For technologies based on riser reactors there are two steps requiring optimization of

the reaction parameters:

1. During the first step a reaction between the alkane and lattice oxygen takes place

and, therefore, the contact time, temperature, and alkane/catalyst ratio have to beoptimized in the reactor to ensure maximum reduction of the catalyst allowing

both its reoxidation and optimal control of coke formation.

2. During the second step catalyst reoxidation occurs and it is essential to establish

the temperature, contact time, and oxygen partial pressure precisely, to prevent

harmful structural and morphological changes during reoxidation.

The use of catalytic membrane reactors (CMRs) is also a promising alternative since they

enable control of oxygen distribution, maintaining its partial pressure sufficiently low

within the catalytic bed.[146] In addition, generated heat is also more regularly distributed

along the bed, considerably decreasing hot-spot formation and leading to safer and more

stable operation. The catalyst charge can also be increased in this way.

The application of CMRs for oxidation reactions, particularly for partial oxidation of

hydrocarbons, was very recently patented by Schwartz et al.[147] Their invention relates to

the use of a gas-impermeable membrane for transport of oxygen anions, the membrane

separating oxidation and reduction zones. An oxygen-containing gas is reduced at the

membrane in the reduction zone (thus generating oxygen anions) while a species in a

reactant gas is oxidized in the oxidation zone of the reactor. This technology is claimed to


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be useful for several reactions, including partial oxidation of hydrocarbons to oxygenated

species and ODH of alkanes, using several catalysts and different configurations.[147] In

this approach, Ni–Mo–O based catalysts could also be considered. But for successful

industrial application, both catalyst development and process engineering, particularly

reactor design, must be considered simultaneously.

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Nickel Molybdate Catalysts and Their Use in the Selective Oxidation of Hydrocarbons

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