Chapter -4 Nitrobenzene Hydrogenation -...

Chapter -4 Nitrobenzene

Hydrogenation

Chapter-4 Nitrobenzene hydrogenation

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4.1 Section-A: Advantage of Ni/SBA-15 catalyst over Ni/MgO catalyst in

terms of catalyst stability due to release of water during nitrobenzene

hydrogenation to aniline

4.1.1. Abstract

Two catalysts, Ni/SBA-15 and Ni/MgO were prepared by wet impregnation

technique with 5 weight% Ni loading in each catalyst. These were tested for nitrobenzene

hydrogenation to aniline reaction at atmospheric pressure under vapour phase conditions.

Ni/MgO suffered a huge loss in the activity (conversion) during time on stream study even

though the Ni dispersion is larger compared to that in SBA-15 supported catalyst. TPR

studies of used catalysts confirm coking of the catalysts. The superior catalytic activity in

terms of conversion and selectivity during time on stream operation exhibited by Ni/SBA-

15 catalyst is due to the hydrophobic nature of this catalyst (resistant to water generated

during the reaction) and the effect of coking is not significant.

4.1.2. Introduction

Aniline (AN) is an important intermediate for preparation of polyurethanes, dyes,

pharmaceuticals, explosives, methylene diphenyl diisocyanate (MDI) and agricultural

products. Hydrogenation of nitrobenzene (NB) is a convenient route to produce aniline

(AN) over a variety of supported metal catalysts both in vapour and in liquid phase

conditions [1–5]. The catalyst deactivation is mainly due to coking of the catalyst or

sintering of active metal. There is no systematic study on the influence of water released

during the reaction on the activity. The present investigation highlights this point for which

Ni/SBA-15 and Ni/MgO catalysts have been selected.

The present investigation highlights the advantage of SBA-15 support over MgO in

yielding high amount of aniline in the vapour phase hydrogenation of nitrobenzene at

atmospheric pressure for the first time.


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4.1.3. Experimental

4.1.3.1. Catalyst preparation

The mesoporous SBA-15 support was prepared by using triblock co-polymer

(P123) as a structure directing agent and Tetraethylorthosilicate (M/s. Sigma Aldrich

Chemicals, USA) as reported by Zhao et al, [6]. MgO was prepared by precipitation of

Mg(NO3)2 (M/s. Loba chemie, India) with 5% Na2CO3 solution (M/s. Loba chemie, India

followed by washing, drying (393 K for 12 h) and calcining (773 K for 5 h) the precipitate.

The Ni based catalysts were prepared by wet impregnation method using Ni(NO3)2 6H2O

(M/s. Loba chemie, India) and the excess water was removed by evaporation followed by

drying at 393 K for 12 h and calcination at 723 K for 5 h.

4.1.3.2. Catalyst characterisation

All the above catalysts are characterized by XRD, BET method, H2 pulse

chemisorption and TPR. Details of all above techniques and procedures have already been

described in chapter-3.

4.1.3.3. Activity studies

The hydrogenation of nitrobenzene was carried out in a fixed bed glass reactor (14

mm i.d and 300 mm long) at atmospheric pressure in the temperature range of 498-573 K

with a liquid feed rate of 1 cm3 h-1 and H2 flow rate of 18 cm3 min-1 over 1 g of catalyst. In a

separate experiment, co-feeding of H2O along with NB (H2O: NB = 2:3) is maintained.

The product mixture was collected in ice trap. The product components were confirmed by

GCMS-QP 5050A and analyzed by FID equipped GC-17A (M/s. Shimadzu Instruments,

Japan).

4.1.4. Results and Discussion

Table-4.1 indicates the BET surface area, pore volume, Average pore diameter, Ni

dispersion by H2 pulse chemisorption, Ni particle size and Water adsorption uptake of the

two catalysts. The decrease in BET surface area of Ni/SBA-15 is due to the pore blockage.

This is clear from the fact that the Ni particle size (10.6 nm) is too big to enter into the

pores of SBA-15 whose average pore diameter (3.9 nm). It was reported that the


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considerable decrease in the surface area of the Ni/SBA-15 prepared by impregnation

technique can be attributed to either the loss of crystallites or pore blockage [7]. MgO

possesses low surface area and slight decrease in the surface area after incorporation of Ni

may be due to the pore blockage. However, Ni dispersion and active metal area of this

catalyst is nearly 3 times that of Ni/SBA-15 catalyst. This clearly indicates a strong

interaction between Ni and MgO. The particle size of Ni in this catalyst is ~ 3nm.

Property Support Ni/SBA-15 Ni/MgO

SBA-15 MgO Fresh Used Fresh Used

BET Surface Area (m2g-1) 546 51 427 325 45 42

Pore Volume (cm3g-1) 0.5284 0.0098 0.2981 0.2514 0.0843 0.0280

Average Pore Diameter (nm) 3.9 0.77 2.8 3.1 8.2 2.7

Ni Dispersion (%) - - 9.5 - 31.5 -

Ni Particle size (nm) - - 10.6 - 3.2 -

Ni-Metal area (m2g-1) - - 3.2 - 10.5 -

H2O adsorption capacity

(µmol g-1)

1.7 6.7 - - - -

Table-4.1.Physico-chemical properties of Ni/SBA-15 and Ni/MgO catalysts.

4.1.4.1 X-ray diffraction studies

Figure-4.1.1 shows the SAXS patterns of Ni/SBA-15 the catalysts in reduced form

(reduced catalysts) and after the reaction (spent catalysts). In Ni/MgO catalysts, reflections

due to Ni metal are absent (Figure-4.1.2). The smaller Ni particle size as obtained from H2

chemisorption experiments prompts to think that Ni is in amorphous form (below the

detection ability of the XRD unit i.e., < 4 nm). The reflections at 2 values of 37.04, 62.49,

78.88 may be due to the presence of NiMgO2 (ICDD No.24-0712) or MgO. It is reported

that the XRD patterns of the (Mg, Ni)O system (MgNiO2) phase (cubic system, spatial

group Fm3m and parameter a = 4.1922 A0), its symmetry and unit cell parameter are

essentially the same as those for the MgO (cubic system, spatial group Fm3m, parameter a


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= 4.209 A ° ) phase and the incorporation of nickel to the structure of the MgO produces

the shift of the diffraction peaks to higher 2 values (lower crystalline plane distances)

when the nickel content is higher. These findings indicate the formation of a solid solution,

NiO–MgO [8-10]. In the case of Ni/SBA-15 catalyst, the reflections of Ni metal can be

clearly seen in reduced and spent catalysts. A decrease in the intensity of reflections at

lower 2 with lower long-range order than the others(Figure-4.1.1) for catalyst Ni/SBA-15

has been reported due to partial collapse of SBA-15 structure after the introduction of Ni

salt by impregnation technique [7,11,12]. However in the present case, the low angle XRD

patterns (Figure-4.1.1) of reduced and spent Ni/SBA-15 catalysts indicates the presence of

mesoporous structure of SBA-15. This indicates the robustness of SBA-15 particularly in

maintaining the mesoporous structure during the reaction.

1 2 3 4 5

b

aInte

nsity

(a.u

.)

2 Theta (deg)

Figure-4.1.1. SAXS Pattern of a) Ni/SBA-15 (Reduced) b) Ni/SBA-15 (Used).


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20 40 60 80

dc

b

a

Inte

nsity

(a.u

.)

2 theta(deg)

Figure-4.1.2. WAXS Pattern of a) Ni/MgO (Reduced) b) Ni/MgO (Used) c) Ni/SBA-15 (Reduced) d) Ni/SBA-15 (Used)

4.1.4.2 Temperature programmed reduction studies

TPR profiles of supported Nickel catalysts (calcined) are depicted in Figure-4.1.3A.

In the case of Ni/SBA-15 catalyst, a single stage reduction peak at Tmax of 473 °C is

observed which corresponds to hydrogen consumption due to the reduction of NiO to Ni0.

It is reported that the TPR pattern of Ni/SiO2 catalyst presents two reduction peaks with

Tmax at 450 and 610 °C. The low temperature signal is due to the reduction of NiO species

of low interaction with the support, whereas the high temperature signal is due to the

reduction of nickel oxide interacted chemically with the support which can be ascribed to

the formation of nickel silicate [13,14]. Probably, a low intense shoulder peak at higher

temperature region may be ascribed to the strongly interacted NiO with SBA-15. Where as

in the TPR pattern of Ni/MgO catalyst, a broad peak is observed that could be attributed to

strong interaction between the support and metal as well as formation of solid solution at


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the inter phase NiO-MgO i.e., NiMgO2 [15]. It is reported that in the TPR pattern of

Ni/MgO catalyst, three reduction peaks at around 325, 600 and 775 °C respectively can be

ascribed to the reduction of NiO located on the MgO surface, some form of Ni2+ ions

having square-pyramidal coordination in the outermost layer of the catalyst structure, and

the NiO–MgO (MgNiO2) solid solution lattice [16]. The defect sites of MgO are assumed

to be more reactive and the adsorption properties for metal species can be qualitatively

different from those of regular surface sites [17]. The defect sites of MgO may be

responsible for the presence of strongly adsorbed Ni species on MgO. TPR profiles of spent

catalysts (Fig.ure-4.1.3B) indicate the presence of carbon/coke on both the catalysts which

is formed during the course of reaction. The negative signals in the TPR patterns suggest

that the nature of carbon is a H2 rich carbon (CHx moiety) [18]. Coking in the catalytic

hydrogenation of nitro groups runs through condensation of intermediates, partially

hydrogenated nitro-groups with already formed amino-groups.

200 400 600 800 1000

Temparature (OC)

H2 C

onsu

mpt

ion

(A)

c

b

a

200 400 600 800 1000

(B)

b

a

Temparature (OC)

H2 C

onsu

mpt

ion

Figure-4.1.3. TPR patterns of Ni catalysts Figure-4.1.3A: Calcined catalysts a) 5Ni/MgO b) 10Ni/MgO c) Ni/SBA-15

Figure-4.1.3B: Used catalysts a) Ni/MgO b) Ni/SBA-15

4.1.4.3. TG/DTA analysis

The rate of weight loss as well as the heat affects associated with decomposition of

dried catalysts as a function of temperature is important because it is responsible to find out

the minimum temperature at which the weight loss is negligible. This gives an idea of the

catalyst where it is become thermally stable. The TG/DTA profiles of dried catalysts are

depicted in Figure-4.1.4. The profiles show an endothermic peak at 1000C attributed to the


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thermal decomposition of water as reported [19] .The peak in the temperature range of 250-

3500C attributed to the loss of crystalline water from nickel nitrate hydrate (Ni (NO3)2

6H2O) and the thermal decomposition of nickel hydroxide into NiO and H2O as reported

[20]. From the TG/DTA profiles, it is found that the catalyst can be calcined at 4500C.

Figure-4.1.4. TG/DTA Profiles of (A) 5NiMgO (B) 5NiSBA-15

4.1.4.4. SEM studies

The SEM images of both fresh and used catalysts were shown in Figure-4.1.5. The

SEM morphology of both fresh and used Ni/SBA-15 catalysts almost intact wherin the

SEM morphology of both fresh and used Ni/MgO is quiet different may be attributed to the

brucite-periclase transition. It is interesting to note that the brucite-periclase transition is

common in MgO supported catalysts where water is a by-product.


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Figure-4.1.5 .SEM images of a) Ni/SBA-15 calcined b) Ni/SBA-15 used c) Ni/MgO

calcined d) Ni/MgO used.

4.1.4.5. XPS studies

The XPS spectra of Ni species in reduced catalysts are shown in Figure-4.1.6. XPS

is a useful technique in order to know the chemical state of Ni. All the reduced samples

show two major peaks at binding energies (BE) of 852.5 eV and 869.7 eV corresponding to

the core level Ni 2P3/2 and Ni 2P1/2 transitions indicates the presence of Nickel in the

metallic state.


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Figure-4.1.6. XPS patterns of Ni catalysts (a) Ni/SBA-15 (b) Ni/MgO

NO2NH2

3H2 2H2O H= -443 KJ/Mole

Scheme-1. Nitrobenzene hydrogenation over Ni catalysts.

4.1.4.6. Catalytic activity studies

Because of the exothermic nature of nitrobenzene hydrogenation reaction (Scheme-

1), decline in the activity (initial nitrobenzene conversion) at higher temperatures (above

250 °C) is observed on both the catalysts (Figure-4.1.7). However, the selectivity to aniline


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remains at 100% on both the catalysts at all temperatures. The high dispersion of Ni on

MgO support results in high conversion (nearly 90% as against ~50% on Ni/SBA-15

catalyst) at low temperatures, i.e., at 225 °C.

225 250 275 300

50556065707580859095

100105

b

a

Con

vers

ion(

%)

Temperature(0C)

Figure-4.1.7. Effect of Temperature on Nitrobenzene hydrogenation a) Ni/MgO b) Ni/SBA-15

4.1.4.6.1. Influence of Ni Loading

The NB hydrogenation reaction pathway over Ni/SBA-15 catalysts can be seen in

Scheme-1. Generally for NB hydrogenation, H2 in higher concentration (more than the

required stoichiometric concentration) is required. However, in the present case, only 4

moles per mole of NB is pumped at 225 °C which is close to the boiling point of NB.


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225 250 275 300

50

60

70

80

90

100

Con

vers

ion(

%)

Temparature(0C)

5Ni/SBA-15 10Ni/SBA-15 20Ni/SBA-15 30Ni/SBA-15 40Ni/SBA-15 50Ni/SBA-15

Figure-4.1.8. Influence of Ni loading on the hydrogenation of nitrobenzene over Ni/SBA-

15 catalysts

Effect of temperature on the conversion of NB over Ni/SBA-15 catalysts is

displayed in Figure-4.1.8. The conversion of NB increases upto 250 °C and remains

constant beyond that temperature. As the Ni loading increases from 5 wt% to 20 wt%, the

conversion of NB increases from 53% to 100% at 225 °C. Beyond 20 wt% loading, the

conversion of NB remains same. The selectivity to aniline remains at 100% on all the

catalysts at all temperatures. The reason for the catalysts with low Ni loading (5 and 10

wt%) exhibiting lower activity is not surprising because these catalysts possess fewer

surface Ni sites compared to the other catalysts. These results clearly indicates that the 20


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wt% Ni loading enough for getting good conversions and selectivity. The increase in

conversion with increase in loading is due to increase in active species.

4.1.4.6.2. Time on stream study and effect of water vapor

The time on stream study on these catalysts at 250 °C (Figure-4.1.9) reveals a

drastic and continuous decrease in the conversion over Ni/MgO catalyst. On the other hand,

Ni/SBA-15 catalyst shows a very high and constant activity (>99% NB conversion) during

the 10h time on stream study. This clearly indicates the robustness of Ni/SBA-15 catalyst.

On both the catalysts, the selectivity to aniline remains at 100% during the 10h operation.

10987654321 0

20

40

60

80

100

dc

ba

Time(h)

Con

vers

ion(

%)

Figure-4.1.9. Time on stream study in the Nitrobenzene hydrogenation a) Ni/MgO (NB + H2O) co-feeding b) Ni/MgO

c) Ni/SBA-15 (NB + H2O) co-feeding d) Ni/SBA-15


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In our earlier publication, the decline in the NB conversion during time on stream

study over Pd/HT catalyst is attributed to either coke formation or water generated during

the course of reaction (1mole of NB yields 2 mole of H2O) [21]. It appears that the

deactivation of catalyst due to sintering of Ni particles during the course of reaction is ruled

out because of the similarity in the XRD patterns before and after the reaction (Figure-

4.1.2). From water adsorption uptakes (Table-4.1), it is clear that MgO support possesses

more water vapor adsorption uptake (6.65 µ moles per gram support) is nearly 4 times that

on SBA-15 support (1.7 µ moles per gram support). Though the amount of water adsorbed

on either MgO or SBA-15 support is too small when compared to the amount of water

released during 10h operation, one can guess that adsorption of water may block the

adsorption of NB on the surface of the catalyst. Moreover in the case of MgO support,

because of water adsorption, brucite-periclase transformation may not be ruled out. Such

transformation may result in the formation of MgO or Mg(OH)2 over-layer on Ni particles

which blocks the adsorption of NB molecules on Ni sites. This type of transformation is

common in MgO supported catalysts [22-24]. The water adsorption capacity of Ni/MgO

catalyst (Table-4.1) is 1.2μmol g−1 catalyst. No water is adsorbed over Ni/SBA-15 catalyst

(Table-4.1). In addition to the water adsorption experiments, co-feeding of water along

with nitrobenzene was maintained on both the catalysts. During the co-feeding of H2O

along with nitrobenzene (Figure-4.1.9) confirms a drastic decrease in the conversion of

nitrobenzene which is due to the poisoning effect of water over Ni/MgO catalyst. Over

Ni/SBA-15 catalyst, the conversion of nitrobenzene is high and steady, indicating its

resistance to water. The hydrophobic nature of Ni/SBA-15 cannot prevent coking during

the reaction. However, the coke deposition as evidenced from the TPR patterns of both the

catalysts has no or little effect on the activity because Ni/SBA-15 catalyst experiences a

steady conversion of ~90% during the entire 10 h operation of time on stream with water

co-feeding along with nitrobenzene and hydrogen. The reason for the poor performance of

the Ni/MgO catalyst may be due to the incomplete reduction of Ni–MgO interacted species.

However during the co-feeding of water along with nitrobenzene results zero activity at the

second hour over Ni/MgO which is not the case when nitrobenzene alone is injected.


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4.1.5. Conclusions

Though the Ni dispersion is lower on SBA-15 support than on MgO support, its

hydrophobic character is advantageous in yielding high and steady activity in the vapour

phase nitrobenzene hydrogenation. The SEM studies reveal the transformation of brucite-

periclase transition. Moreover, the coke deposition (as evidenced from the TPR of used

catalysts) is insignificant on the catalytic performance. Furthermore, water adsorption

capacity of Ni/MgO enables that this catalytic system is not suitable for the hydrogenation

reaction especially water as a byproduct.

4.1.6. References

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[3]. A. Nieto-Ma´rquez, S Gil, A. Romero, J. L.Valverde,S. G. mez-Quero, M. A. Keane,

Appl. Catal. A: Gen. 2009, 363, 188.

[4]. J. Wang, Z. Yuan, R. Nie, Z. Hou, X. Zheng, Ind. Eng. Chem. Res., 2010,49, 4664.

[5]. P. Sangeetha, K. Shanthi, K.S. Rama Rao, B. Viswanathan, P. Selvam, Appl. Catal. A:

Gen. 2009, 353, 160.

[6]. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D.

Stucky, Science. 1998, 279, 548.

[7]. Y. Park, T. Kang, J. Lee, P. Kim, H. Kim, J.Yi, Catal. Today., 2004, 97, 195.

[8]. J. Barbero, M.A. Pen˜a, J.M. Campos-Martin, J.L.G. Fierro, P.L. Arias, Catal. Lett.,

2003, 87, 211.

[9]. E. Ruckenstein, Y.H. Hu, Appl. Catal. A: Gen.,1999, 183, 85.

[10]. J. Requies, M. A. Cabrero, V. L. Barrio, M. B. Guemez, J. F. Cambra, P. L. Arias, F.

J. Perez-Alonso, M. Ojeda, M. A. Pena, J. L. G. Fierro, Appl. Catal. A: Gen., 2005,

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[11] Y.S. Cho, J.C. Park, B. Lee, Y. Kim, J. Yi, Catal. Lett., 2002, 81, 89.

[12]. Y. Park, T. Kang, Y. Cho, P. Kim, J. Park, J. Yi, Stud. Surf. Sci. Catal., 2003, 146,

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[14]. H. V. Fajardo, E. Longo, D. z. Mezalira, G. B. Nuernberg, G. L. Almerindo, A.

Collasiol, L. F. D. Probst, I. T. S. Garcia, N. L. V. Carreno, Environ. Chem. Lett.,

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[16]. T. Furusawa, A. Tsutsumi, Appl. Catal. A: Gen., 2005, 278, 207.

[17]. A. V. Matveev, K. M. Neyman, I. V. Yudano, N. Rosch, Surf. Sci. 1999, 426, 123.

[18]. S. Chandra Shekar, J. Krishna Murthy, P. Kanta Rao, K.S. Rama Rao, J. Mol.

Catal. A: Chemical, 2003, 191, 45.

[19]. G. Jianjun, H. Lou, H. Zhao, D. Chai, X. Zheng, Appl. Catal. A: Gen., 2004, 273, 75.

[20]. B. Parathasarathi, M. Rajamathi, M.S. Hegde, P. V. Kamath, Bull. Mater. Sci., 2000,

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[21]. P. Sangeetha, P. Seetharamulu, K. Shanthi, S. Narayanan, K.S. Rama Rao, J. Mol.

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4.2 Section-B: A comparative study of nitrobenzene hydrogenation over

impregnated and reductive deposition precipitation method catalysts

4.2.1. Abstract

Highly dispersed nickel nanoparticles on supports such as ZrO2 and TiO2 were

prepared by reductive deposition method by using hydrazine as reducing agent. The

highlight of this work is to compare the characteristics and activity of these catalysts with

the catalysts of same composition prepared by impregnation method. All catalysts were

characterised by various techniques such as BET, H2 Pulse chemisorption, SEM

(Morphology), XRD (Crystalline nature), reduction behaviour (TPR) and state of nickel

species (XPS). Ni/TiO2 catalyst prepared by reductive deposition method show excellent

conversion of nitrobenzene to aniline. This is due to the presence of higher number of

surface Ni species than other catalysts as evidenced by H2-chemisorption. TPR results

reveal the formation of metallic Ni species in the reductive deposition method. XRD results

suggest that the all catalytic systems showing peaks corresponding to the supports only and

not due to the metallic Ni because of its presence in highly dispersed form.

4.2.2. Introduction

Nanomaterials are of topical interest, because of their fascinating properties which

are different from the corresponding bulk materials. Due to their unique properties,

nanomaterials are employed in electronic, optical, catalytic, coating, medical and sensor

applications [1-3]. A plethora of research work has been cited on the synthesis Ni

nanoparticles by various methods such as chemical reduction [4], radiolytic reduction [5],

UV photolysis [6], solvent extraction reduction [7], microemulsion technique [8], polyol

reduction method [9] and sonochemical decomposition [10]. Among these methods,

chemical reduction appears to be the most promising method for the large-scale

nanoparticle production, although simple methods for preparing metal supported

nanocatalysts under mild and inexpensive conditions have not still been definitely

addressed. Among the chemical reduction reagents, utilisation of NaBH4 leaves Na+ ions

and B in the catalysts that effects the reaction while the utilisation of Na2HPO4 leaves both


98

Na+ ions and P in the catalysts which influences the catalytic activity. In our present study,

we used hydrazine as a reducing agent which on decomposition produces H2 as well as N2.

The liberated N2 is enough to act as inert atmosphere coverage to the reaction vessel.

Recently, supported nickel catalysts were reported for various organic

transformations such as hydrogenation [11, 12], steam reforming reactions [13] reductive

amination of alcohols [14] hydrodechlorination [15, 16] partial oxidation [17], and dry

reforming of methane [18]. Amongst, hydrogenation is of great interest for both industry

and academia.

The synthesis of aniline is generally done by the nitrobenzene hydrogenation.

Aniline is an important raw material for synthesis of more than 300 different end products

industrially, aniline is mainly used for the production of methylene diphenyl diisocyanate

(MDI), a key compound in the production of polyurethane rigid and/or semi-rigid foams

and elastomers. It is also used as a solvent, and has been used as an antiknock compound

for gasoline [19, 20].

Aniline finds application in various end-use markets including construction, rubber

products, transportation, consumer, adhesives/sealants, packaging, agriculture, textiles,

coatings, photography, dyes, and pharmaceuticals.

A predominant proportion of the worldwide aniline production is utilized in the

manufacture of polyurethanes, which are used in a wide array of applications including

footwear, insulation and furniture.

The global aniline market is projected to reach 6.2 million tons by the year 2015,

due to the increasing demand from various end-user markets. In particular, the rising

demand from methylene diphenyl diisocyanate (MDI), the chief ingredient in polyurethane

foam, is expected to fuel consumption of aniline [21].

The present section highlights the synthesis of Ni catalysts by reductive deposition

method without using any capping agent as an efficient catalyst for the hydrogenation of

nitrobenzene at atmospheric pressure in vapour phase.


99

4.2.3. Experimental

4.2.3.1. Catalyst preparation

Materials used in this research work are Ni(NO3)2 6H2O (M/s. LOBA Chemie,

India, 98%); hydrazine hydrate solution (M/s. LOBA Chemie, India, 80%); Sodium

Hydroxide(M/s. Sigma Aldrich, USA); de-ionized water in the preparation of all the

solutions. The nano-sized supported nickel particles were synthesized by dissolving

requisite quantities of Ni(NO3)2 6H2O in deionised water and placing the respective

supports in a vessel. The pH of the solution was then increased to 10 by adding a mixture

of sodium hydroxide and hydrazine hydrate solution. The solution is stirred for 6h. During

the stirring, we have observed a change in colour of the solution from green to violet to

colourless and then to grey/black nickel nanoparticles with the evolution of N2 gas. The

liberated N2 is enough to act as inert atmosphere blanket. In order to avoid the expansion of

volume of N2 during the stirring, precaution has been taken by selecting a flask of bigger

volume. The resultant catalysts were designated as Ni/TiO2 (R) and Ni/ZrO2 (R)

respectively. In comparison, we have also prepared the catalysts by impregnation method

with similar Ni loading. They were designated as Ni/TiO2 (I) and Ni/ZrO2 (I) respectively.

The Ni loading in all the catalysts was maintained at 5 wt%.

4.2.3.2. Catalyst characterisation

All the above catalysts were characterized by XRD, BET method, H2 pulse

decomposition, TPR and SEM. Details of all the above techniques and procedures have

already been described in chapter-3.

4.2.3.3. Activity studies

A layer of silicon beads has been placed to act as a preheating zone, above catalyst

bed taken in a fixed bed reactor. Isothermal conditions were ensured by diluting the catalyst

bed with ground silica beads. Reaction temperature was continuously monitored by a

thermocouple inserted in a thermowell which was placed within the catalyst bed. The liquid

reactant was delivered to the reactor from a glass / Teflon air-tight syringe and Teflon tube

using a microprocessor controlled infusion pump (M/s. B.Braun Co, Germany) at a

required flow rate. NB and ultra pure H2 were maintained at a WHSV ≈ 1 with a inlet NB


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flow =1mlh-1 and H2 flow of 1 time in excess of the stoichiometric requirement (H2/NB

molar ratio = 4).

In a blank test, passage of NB reactant along with stream of H2 through an empty

reactor or over the support alone did not result in any appreciable NB conversion. The

product mixture was cooled in a ice cooled trap and the analysis were performed on a gas

chromatograph (GC-17A, M/s. Shimadzu Instruments Co, Japan) equipped with a

programmed split injector and a flame ionization detector, with a OV-1 (30 m length 0.53

mm i.d., 0.33 mm film thickness) capillary column and the product components were

confirmed by Shimadzu-17A gas chromatograph equipped along with the mass spectrum

QP-8080.

4.2.4. Results and discussion

Hydrazine is a normally used as a reducing agent. It can be oxidised depending

upon the medium. In acid medium, it easily undergoes oxidation to NH3 while in case of

basic medium it is easily underwent oxidation to N2 and H2. Hence, it is necessary to use

the basic medium to produce N2 and H2 .For this purpose we have chosen NaOH as a base.

The H2 produced is further utilised for the reduction of nickel precursors which are present

in the solution.

4.2.4.1. X-ray diffraction studies

The XRD patterns of all the catalyst samples displayed in Figure-4.2.1. In case of

Ni/ZrO2 catalyst, it is reported that peaks at 2θ = 30.3° and 35.14° reveal the presence of

indices (111) and (200) planes, respectively corresponds to tetragonal zirconia phase [22].

Major Peak at 2θ = 28.3° reveals the part of the sample containing the monoclinic phase of

zirconia [23]. It is reported that the incorporation of di positive or tetra positive metal ions

into zirconia promotes the transition of tetragonal to monoclinic phase [24,25].

Furthermore, the PH of solution during the synthesis was maintained at 10. PH also

influences the transformation of zirconia phase [23]. In case of Ni/TiO2 catalyst, the peaks

at 2θ = 25.32°, 37.84°, 48.07°, 53.95°, 55.10°, 62.16°, 70.34° and 75.12° reveal the

presence of indices (101), (004), (200), (105), (211) (Tetragonal TiO2, Space group

141/amd 141)) corresponding to titania (anatase) (ICDD .No.84-1286). The peaks at


101

around 2θ = 27.54°, 37.06° and 43.18° are corresponding to rutile phase of TiO2. The peaks

at around 2θ = 22.87°, 34.09° and 49.51° are attributed to NiTiO3 species [26, 27].

Figure-4.2.1. XRD patterns of Ni catalysts a) Ni/ZrO2 (R) b) Ni/ZrO2 (I) c) Ni/TiO2 (R) d) Ni/TiO2 (I)

4.2.4.2. N2 physisorption studies

The surface area, pore volume and pore size of the catalysts were shown in Table-

4.2.1. The N2 adsorption-desorption isotherms of all the catalyst samples were shown in

Figure-4.2.2. The surface areas of the catalysts prepared by reductive deposition

precipitation are smaller than that of impregnation method. The surface area of the catalysts

prepared by reductive deposition precipitation is half of the surface area of the catalysts

prepared by impregnation method. Although the surface area of the catalysts prepared by

reductive deposition method is smaller compared to that of impregnation method, the

catalysts show good yields towards aniline. The domination of t-ZrO2 could partially

20 30 40 50

mm m

m

t t

t

d

c

b

a

R R @@@

RA

AA

A

Inte

nsity

(a.u

.)

2 Theta (deg)


102

contribute to the higher surface area of the Ni/ZrO2 catalyst prepared by impregnation

method [28, 29].

0.0 0.2 0.4 0.6 0.8 1.0

Vol

ume

@ST

P (c

c/g)

Relative Pressure (P/P0)

d

cb

a

Figure-4.2.2. N2 adsorption-desorption isotherms of Ni catalysts

a) Ni/ZrO2(I) b) Ni/ZrO2 (R) c) Ni/TiO2(I) d) Ni/TiO2(R)

Table.4.2.1. Physico-chemical properties of Ni catalysts prepared by impregnation and

reductive deposition precipitation method.

Catalyst Surface area

(m2/g)

Pore volume

(cm3/g)

Average Pore

diameter

(A0)

NiZrO2 ( R ) 21.08 0.082 155.68

NiZrO2 ( I ) 46.90 0.128 109.48

NiTiO2 ( R ) 7.97 0.047 234.90

NiTiO2 ( I ) 14.60 0.053 147.23


103

4.2.4.3. Temperature programmed reduction studies

200 300 400 500 600 700

c

b

aH2 C

onsu

mpt

ion

(a.u

.)

Temperature (0C)

d

Figure-4.2.3. TPR profiles of Ni catalysts a) Ni/TiO2 (I) b) Ni/ZrO2 (I) c) Ni/TiO2 (R) d) Ni/ZrO2 (R)

Typical profiles of TPR are shown in Figure-4.2.3. For Ni/TiO2, the reduction peaks

at temperatures in the range of 360 to 480° C are probably due to the reduction of bulk NiO

and NiO with significant interaction with the titania surface [30]. In the TPR patterns of

Ni/ZrO2, reduction peaks at a Tmax of about 443 and 503 °C are observed. The former one

can be assigned to the reduction of relatively bulk NiO species and the latter assumed to be

interacted species with ZrO2 [31]. Presence of strong chemical bonds between the Ni metal

and the ZrO2 has been revealed based on a DFT calculation by Beltran [32]. The reduction

peak in the TPR pattern at 306 °C is attributed to the reduction of bulk NiO [32] while the

reduction peaks at 420–450 °C were related to NiOx species that have strong interaction

with ZrO2 [33, 34]. The strong interaction between the nickel metal and the zirconia could

be probably due to an increased interfacial area [33, 34]. While in case of reductive


104

precipitation catalysts, no significant consumption of hydrogen was observed which clearly

indicates the presence of Ni is in metallic state.

4.2.4.4. H2 pulse chemisorption studies

Table- 4.2.2. No. of surface Ni species, dispersion, metal area and particle size of Ni

catalysts

H2 Chemisorption results are complied in Table-4.2.2. Based on the assumption that

each Ni atom chemisorbs one H-atom, Ni dispersion, and its particle size are determined.

The number of surface Ni species on the TiO2 supported catalysts is more compared to that

of ZrO2 support and the activity is in good agreement with the H2 chemisorption results.

The metal dispersion follows a trend: Ni/ZrO2 (I) <Ni/ZrO2 (R) <Ni/TiO2 (I) < Ni/TiO2 (R).

This clearly indicates that the metallic dispersion depends not only on the preparation

method but also on the nature of the support. The particle size of the Ni species follows a

opposite trend to that of the metal dispersion. The smaller particles and higher dispersion

are generally useful for the catalytic hydrogenation of nitrobenzene.

4.2.4.5. Microscopic studies

The SEM images of catalysts prepared by reductive deposition precipitation method

shows the presence of well dispersed Ni particles than in the impregnated catalysts. The

Catalyst H2 Uptake

(µmoles/g)

Dispersion

(%)

Average

particle

size(nm)

NiZrO2 ( I ) 68.66 16.12 6.276

NiZrO2 ( R ) 99.72 23.41 4.31

NiTiO2 ( I ) 119.98 28.17 3.591

NiTiO2 ( R ) 186.82 43.86 2.307


105

greater nucleation in the impregnation method leads to agglomeration of metal

nanoparticles.

Figure-4.2.4. SEM images of Ni catalysts

a) Ni/TiO2(R) b) Ni/TiO2(R) c) Ni/ZrO2(R) d) Ni/ZrO2 (I)

4.2.4.6. XPS studies

The XPS profiles of catalyst samples were displayed in Figure-4.2.5. XPS is a useful

technique in order to know the chemical state of Ni. All the reduced samples show two

major peaks observed corresponding to the core level Ni 2P3/2 and Ni 2P1/2 transitions at a

binding energies (BE) of 852.5 eV and 869.7 eV which is due to the presence of Nickel in

the metallic state.


106

Figure-4.2.5. XPS patterns of reduced Ni catalysts

a) Ni/TiO2 (I) b) Ni/ZrO2 (I) c) Ni/TiO2 (R) d) Ni/ZrO2 (R)

4.2.4.6. Catalytic activity studies

4.2.4.6.1. Effect of temperature

The catalysts prepared by reductive deposition method are found to be efficient for

the production of aniline than for the impregnated catalysts (Figure-4.2.6). Moreover,

among all the catalysts Ni/TiO2 (R) catalyst prepared by reductive deposition method is

found to be an efficient catalyst due to high Ni dispersion as well as smaller Ni particle size


107

(Table-5.2.2) than the others. Additionally, the SEM result also supports the presence of

smaller Ni particles in this catalyst. Among all the temperatures, 250 °C is an optimum

temperature for yielding the aniline at cent percent conversion.

Figure-4.2.6. Effect of support and preparation method on the NB conversion over Ni based

catalysts.

4.2.4.6.2. Effect of time on stream

The stability of Ni/TiO2 catalyst prepared by reductive deposition method can be

observed from the Figure-4.2.7 during the time on stream for 10 h at 250 °C. The

conversion of NB over this catalyst is decreasing slightly during the course of time, may be

due to the condensation of reaction intermediates (coke formation) or water released during

the reaction. In the previous section, we observed the catalytic performance of Ni/SBA-15

catalyst, during the time on stream for 10 h remains constant due to hydrophobic nature of

SBA-15 while in case of Ni/MgO catalyst, the drastic decline in the catalytic performance

of nitrobenzene hydrogenation was observed due to the water adsorption on the MgO

support [35]. The adsorbed water may promote brucite- periclase transformation in case of

MgO. The formation of coke is also another main cause for the deactivation of reaction

0102030405060708090

100

Ni/ZrO2(R)Ni/ZrO2 (I)

Ni/TiO2 (R)Ni/TiO2 (I)

225 0C

250 0C

275 0C

300 0C


108

during the course of the reaction. Lin et al studied the nitrobenzene hydrogenation over

Ni/TiO2 and observed a decline in the activity in subsequent cycles. Carbon coating of

catalyst was reported to improve the performance of the catalyst [36].

1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

Con

vers

ion

& S

elec

tivity

(%)

Time (h)

Conversion Selectivity

Figure-4.2.7. Time on stream study on Ni/TiO2 (R) for the Nitrobenzene hydrogenation.

4.2.5. Conclusions

In summary, the catalysts prepared by reductive precipitation method are found to be

efficient for the hydrogenation of nitrobenzene. Among all the catalytic systems, Ni/TiO2

(R) catalyst shows good conversion of NB as well as selectivity towards aniline which is

attributed to the high dispersion.


109

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