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Estudio cinético de la oxidación parcial de tolueno a benzaldehído,
empleando como catalizador un óxido mixto de Ce – Zr – Mo
Geraldine Díaz Chaparro
Universidad Nacional de Colombia
Facultad de Ingeniería
Departamento de Ingeniería Química y Ambiental
Bogotá, Colombia
2016
Estudio cinético de la oxidación parcial de tolueno a benzaldehído,
empleando como catalizador un óxido mixto de Ce – Zr – Mo
Geraldine Díaz Chaparro
Trabajo final de grado, presentado como requisito parcial para optar al título de:
Magíster en Ingeniería – Ingeniería Química
Director:
Julio Cesar Vargas Sáenz.
Campo de estudio:
Catálisis heterogénea
Grupo de investigación:
Procesos Químicos y Bioquímicos
Universidad Nacional de Colombia
Facultad de Ingeniería
Departamento de Ingeniería Química y Ambiental
Bogotá, Colombia
2016
Kinetic study of toluene partial oxidation to benzaldehyde, using a Ce – Zr – Mo mixed oxide as catalyst
Geraldine Díaz Chaparro
A dissertation submitted in partial fulfilment of the requirements for the degree of
Magister en Ingeniería – Ingeniería Química
Advisor:
Julio Cesar Vargas Sáenz.
Research field:
Heterogeneous Catalysis
Research Group:
Chemical and Biochemical Processes
Universidad Nacional de Colombia
School of Engineering
Department of Chemical and Environmental Engineering
Bogotá, Colombia
2016
Dedicated to
My parents and my sister, who have been the best mentors and trip allies.
Hernán and Juliana, my little angels.
Acknowledgments
While I am responsible for this thesis, it is nonetheless at least as much of a product of
years of interaction with, and inspiration by friends and colleagues as it is my own work.
For this reason, I wish to express my warmest gratitude to all the people whose comments,
questions, criticism, support and encouragement, personal and academic, have left a mark
on this work. I also wish to thank the institutions that have supported me during the
development on this thesis. Regrettably, the following list of names will be incomplete, and
I hope that those who are missing will forgive me, and will accept my sincere appreciation
for their influence on my work.
First, I praise God for providing me with this opportunity and granting me the skills to
succeed.
I would like to thank my advisor, Professor Julio César Vargas, for the support and
persistent help he gave me. Many thanks also to Professor Anne - Cécile Roger, who
provided me with an opportunity to join her team as an intern, and who gave me access to
the ICPEES & CNRS laboratory and research facilities. Without their precious support, it
would not be possible to conduct this research.
I gratefully acknowledge the funding received during my master program from the
Universidad Nacional de Colombia by the scholarship “Asistentes Docentes”. I am also
grateful for the funding received through the program “Mobilité IdEx” from the Université de
Strasbourg.
I would like to say a heartfelt thank you to my Mum, Dad, Ángela, Hernán and Juliana for
always believing in me and encouraging me to follow my dreams. Special thanks goes to
Nicolas Daza and Diana Sánchez, for helping me to see the light at the end of scary tunnels.
Finally, Laura, Luisa, Adriana and Darwin, thank you for helping in whatever way you could
during this challenging period.
Abstract
The kinetic study of Ce-Zr-Mo catalysts on toluene partial oxidation to benzaldehyde in
vapor phase is presented in this work. Three different families of mixed oxides prepared by
pseudo sol gel method were characterized. The effect of the synthesis method on the
catalysts performance was evaluated by changing procedure steps and the interaction
between the active phase and the support of the materials. For the modified pseudo sol gel
method samples, the changes of structural properties evidence the predominant role of the
Mo species on the catalyst surface promoting total oxidation. The synthesis effect was also
evidenced on materials which exhibit ceramic structures, leading an irregular surface for
deposited catalysts and dispersed grains for integrate catalysts. The theoretical
composition proposed, as well as the synthesis methodology lead to solids, which exhibit
multiple phases and polycrystallinity. The kinetic study was made for the most selective
catalyst evaluated (Ce2.65Zr0.35Mo3O14). The mathematical regression fitting for Mars & Van
Krevelen kinetic model present less relative error with respect to the experimental data,
compared with three LHHW kinetic models and power law rate model evaluated.
Keywords: Mixed oxides, catalysts, toluene, benzaldehyde, kinetic study.
Resumen El estudio cinético de la oxidación parcial de tolueno a benzaldehído en fase de vapor
usando catalizadores de Ce-Zr-Mo es presentado en este trabajo. Tres diferentes familias
de óxidos mixtos preparados por el método pseudo sol gel fueron caracterizadas. El efecto
del método de síntesis en el desempeño de los catalizadores fue evaluado mediante el
cambio de pasos de la metodología y la interacción entre la fase activa y el soporte de los
materiales. Para los catalizadores obtenidos por el método pseudo sol gel modificado, los
cambios en las propiedades estructurales evidencian el papel predominante de las
especies de molibdeno en la superficie del material, promoviendo la oxidación total. El
efecto del método de síntesis fue también evidenciado en materiales que mostraron
estructuras cerámicas, obteniendo como resultado una superficie irregular para los
catalizadores depositados y granos dispersos para los catalizadores integrados. La
composición teórica propuesta, al igual que el método de síntesis usado permitieron la
formación de sólidos que exibieron múltiples fases y policristalidad. El estudio cinético fue
hecho para el catalizador más selectivo entre los evaluados (Ce2.65Zr0.35Mo3O14). La
regresión matemática ajustó para el modelo cinético Mars & Van Krevelen, presentando
menor error relativo con respecto al conjunto de datos experimentales, comparado con tres
diferentes modelos cinéticos LHHW y el modelo de ley de potencias que también se
evaluaron.
Palabras claves: Óxidos mixtos, catalizadores, tolueno, benzaldehído, estudio cinético.
Contents
Page
Acknowledgments ................................................................................................................. ix
Abstract ................................................................................................................................. xi
List of figures ...................................................................................................................... xvii
List of tables ........................................................................................................................ xix
Introduction ......................................................................................................................... xxi
1. Toluene oxidation: a value alternative for the crude oil market ..................................... 23
1.1. Heavy oil: an option to dwindling conventional oil supplies .................................... 23
1.2. Heavy oils market challenge ................................................................................... 24
1.3. Adding value to refinery by-products ....................................................................... 26
1.4. Benzaldehyde as a compound of major interest ..................................................... 27
1.5. Benzaldehyde production processes ...................................................................... 30
1.6. Benzaldehyde production by toluene partial oxidation ........................................... 31
1.7. Mixed oxides as selective oxidation catalysts ......................................................... 33
1.8. Cerium, zirconium and molybdenum mixed oxides ................................................ 34
1.9. Modelling the reaction ............................................................................................. 36
1.10. Scope ..................................................................................................................... 40
2. Vapor phase toluene partial oxidation: the reaction at laboratory scale ........................ 43
2.1. Thermodynamic study ............................................................................................. 43
2.2. Reaction system ...................................................................................................... 45
2.3. Ce–Zr–Mo oxydes as catalysts ............................................................................... 47
2.3.1. Modified pseudo sol gel method catalysts ....................................................... 49
2.3.2 Pseudo sol gel method catalysts ....................................................................... 56
2.3.3 Influence of cation integration vs deposition ..................................................... 60
3. Kinetic study .................................................................................................................... 67
3.1. Data collection ......................................................................................................... 67
3.1.1. Experimental procedure ................................................................................... 67
3.1.2 Experimental results .......................................................................................... 68
3.2 Determination of kinetic parameters ........................................................................ 69
3.2.1 Power law rate expression ................................................................................ 70
3.2.2 LHHW kinetic expression with two active sites ................................................. 72
3.2.3 LHHW kinetic expression with dissociative oxygen adsorption ........................ 73
Contents xvi
3.2.4 LLHW kinetic expression with non-dissociative oxygen adsorption ................. 74
3.2.5 Mars and Van Krevelen kinetic expression ....................................................... 75
4. Conclusions and findings ................................................................................................ 81
References .......................................................................................................................... 83
Annexes .............................................................................................................................. 91
A. Chromatographic methods ................................................................................... 91
i. Non-condensable products analysis .................................................................... 91
ii. Condensable products analysis ........................................................................... 92
B. Catalytic tests results ........................................................................................... 93
C. Kinetic experimental results ................................................................................. 94
xvii
List of figures
Figure 1. Proven vs recoverable and unconventional world oil reserves of crude oil. ...... 24
Figure 2. Global capacity requirements by process type 2011 – 2035 ............................. 25
Figure 3. Exports of aldehydes-ethers, aldehydes-phenols and aldehydes with oxygen
radicals ................................................................................................................................ 28
Figure 4. Imports of aldehydes-ethers, aldehydes-phenols and aldehydes with oxygen
radicals ................................................................................................................................ 28
Figure 5. Aldehydes-eters, aldehydes-phenols and aldehydes with oxygen radicals’
imports and exports in Colombia ........................................................................................ 29
Figure 6. Hydrogen transfer mechanism for the initial steps in catalytic oxidation of
toluene ................................................................................................................................ 37
Figure 7. Electron transfer mechanism for the initial steps in catalytic oxidation of toluene
............................................................................................................................................ 38
Figure 8. Toluene molar conversion as function of reaction conditions. ........................... 43
Figure 9. CO molar selectivity as function of reaction conditions ...................................... 44
Figure 10. CO2 molar selectivity as function of reaction conditions ................................... 44
Figure 11. Experimental arrangement for toluene oxidation .............................................. 45
Figure 12. Recovered volume fraction of the main outlet condensable compounds in
function of the trap temperature ......................................................................................... 46
Figure 13. Global catalyst preparation procedure following the pseudo sol-gel method .. 49
Figure 14. Catalyst preparation procedure following the modified pseudo sol-gel method
............................................................................................................................................ 49
Figure 15. Synthetized CZM2(I), CZM3
(I), CZM4(I) catalysts samples TPR analysis results 50
Figure 16. Synthetized catalysts samples TPO analysis results. ...................................... 52
Figure 17. Diffractograms of fresh mixed oxides: CZM2(I), CZM3
(I), CZM4(I) ....................... 53
Figure 18. Raman spectra of fresh mixed oxides: CZM2(I), CZM3
(I), CZM4(I) ...................... 53
Figure 19. SEM micrographs of fresh mixed oxides: a. CZM2(I), b. CZM3
(I), c. CZM4(I) ...... 54
Figure 20. Catalytic test temperature profile vs time.......................................................... 55
Figure 21. Diffractograms of fresh mixed oxides: CZM2(II), CZM3
(II), CZM4(II). .................... 57
Figure 22. Electron diffraction of fresh mixed oxides: a. CZM2(I), b. CZM2
(II) ..................... 58
Figure 23. TEM micrographs of fresh mixed oxides: a. CZM2(I), b. CZM2
(II) ....................... 58
Figure 24. SEM micrographs of fresh mixed oxides: a. CZM2(I), b. CZM2
(II) ...................... 59
Figure 25. Global catalyst preparation procedure for deposited catalysts, following the
pseudo sol gel method ........................................................................................................ 61
Figure 26. Diffractograms of fresh mixed oxides: CM/CZ(I), CM/CZ(II). .............................. 62
Figure 27. Electron diffraction of fresh mixed oxides: a.CM/CZ(I), b. CM/CZ(II). ................ 62
Figure 28. TEM micrographs of fresh mixed oxides: a. .CM/CZ(I), b. CM/CZ(II). ................ 63
Figure 29. SEM micrographs of fresh mixed oxides: a. .CM/CZ(I), b. CM/CZ(II). ................ 64
Figure 30. Test methodology schema. ............................................................................... 68
Figure 31 Kinetic rate’s toluene partial pressure dependency evaluated at 7mL/min
oxygen feed rate. ................................................................................................................ 68
Contents xviii
Figure 32 Kinetic rate’s oxygen partial pressure dependency at 30L/min toluene feed
rate. ..................................................................................................................................... 69
Figure 33. Parity diagram for power law kinetic model ...................................................... 70
Figure 34. Parity diagram for modified power law kinetic model ....................................... 71
Figure 35. Parity diagram for two active sites LHHW kinetic model .................................. 73
Figure 36. Parity diagram for oxygen dissociative adsorption LHHW kinetic model ......... 74
Figure 37. Parity diagram for non- dissociative oxygen adsorption LHHW kinetic model. 75
Figure 38. Parity diagram for M&VK kinetic model ............................................................ 77
Figure 39. Parity diagram for modified M&VK kinetic model ............................................. 78
Figure 40. Parity diagram for M&VK kinetic regression for experiments made at 475°C . 78
Figure 41. M&VK fitting model for experiments made at 7mL/min oxygen feed rate. ....... 79
Figure 42. M&VK fitting model for experiments made at 30L/min toluene feed rate....... 79
xix
List of tables
Table 1. Catalyst proposal development ............................................................................ 48
Table 2. Solids synthetized by the modified pseudo sol-gel method ................................. 50
Table 3. Catalytic test results of: CZM2(I), CZM3
(I), CZM4(I) samples. ................................. 55
Table 4. Samples synthetized by the pseudo sol-gel method ........................................... 56
Table 5. Catalytic test results of: ZM2(I), CZM3
(I), CZM4(I) samples ..................................... 59
Table 6. Third catalyst family synthetized .......................................................................... 61
Table 7. Catalytic test results of .CM/CZ(I) and CM/CZ(II)samples ..................................... 64
Table 8. Power law model kinetic parameters obtained .................................................... 70
Table 9. Modified Power law model kinetic parameters obtained ..................................... 71
Table 10. Two active sites LHHW model kinetic parameters obtained ............................. 72
Table 11 Dissociative oxygen adsorption LHHW kinetic parameters obtained ................. 74
Table 12. Non-dissociative oxygen adsorption LHHW kinetic parameters obtained ........ 75
Table 13. M&VK kinetic parameters obtained .................................................................... 76
Table 14. Modified M&VK kinetic parameters obtained ..................................................... 77
Table 15. Retention times and response factors utilised for the non-condensable
products. ............................................................................................................................. 91
Table 16. Retention times and response factors utilised for the condensable products. .. 92
Table 17. Catalytic tests results summary ......................................................................... 93
Table 18. Experimental data results. .................................................................................. 94
Introduction
Toluene represent a very important feedstock to petrochemical industry. This hydrocarbon
may be use as solvent or as a precursor to other chemicals, e.g. polyurethane foam, the
explosive trinitrotoluene (TNT) and numerous synthetic drugs. By oxidation of toluene, a
wide range of products may be generated, such as benzyl alcohol, benzaldehyde, benzoic
acid, benzyl benzoate, and phenyl benzene. All of the above listed products have more
commercial value than toluene. The most versatile of them is benzaldehyde, which is not
produced in Colombia and could satisfy a continuous growing market.
The oxidation of toluene to benzaldehyde could take place in vapor phase using mixed
oxides as catalyst. However, the synthesized solids had low surface area and show various
phases that may not be the most active or may become unstable. Moreover, in catalytic
partial oxidation, several variables affect toluene conversion and benzaldehyde selectivity,
such as the oxygen to toluene feed ratio, the reaction temperature and the space velocity
(W/F). Therefore it is necessary to study the influence of different conditions in the behavior
of the reactants and the products involved. This study is related to the reaction rate and
could be modelled by mathematical expressions in order to fit the experimental data.
This work aims to contribute with the characterization of a cerium- zirconium and
molybdenum mixed oxide as an active material for the partial oxidation of toluene to
benzaldehyde, and the adjustment of reaction phenomena by a kinetic expression. In order
to present the main results, this document has been divided into three main chapters:
The first one is a brief introduction to the state of the art. This section comprises an overview
about the current scenario of the global crude oil market, the partial oxidation as an
alternative for adding value to toluene, a refinery by-product; and the generalities about the
catalysts used in the toluene to benzaldehyde vapor phase reaction, being the Ce-Zr-Mo
mixed oxide a striking choice.
The second chapter exhibits the results of the experimental work performed during the initial
stage of this project. It includes the formulation, characterization and catalytic test of the
materials produced, as well as the assessments of catalyst obtained by different synthesis
methods and compositions.
Introduction xxii
The chapter three states the behavior of the most selective and stable catalyst evaluated.
Mathematical adjust of different parameters such as temperature and partial pressure of
the reactants related to reaction rates is given by kinetic expressions.
Finally, some conclusions and findings for further research were summarized.
1. Toluene oxidation: a value alternative for the crude oil market
1.1. Heavy oil: an option to dwindling conventional oil supplies
Crude oil is a mix of alkanes, cycloparaffins, aromatic compounds and non-hydrocarbon
heterocompounds. Appreciable property differences appear between crude oils as result of
the variable ratios of the crude oil components. In order to identify the behavior of the
different kind of crude oils, the American Petroleum Institute stablishes a way to express
the crude’s relative gravity (°API). A low API gravity indicates a higher content of heavy
fractions such as cycloparaffins and aromatic compounds, thus it means a heavier crude
oil, while a higher API gravity leads a lighter crude oil [1].
The crude oil found in conventional reserves is mostly light and intermediate crude oils,
with high market value and whose production and processing consist of technically well-
established methods [2]. However, the continuous increase in world energy demand, driven
by the economic development and the population growth recorded in recent decades, has
caused the scarcity of traditional reserves and nowadays new discoveries of conventional
oil are insufficient to supply the estimated future energy demand. Since most of the oil
reserves are situated on the Middle East, it generates a wide gap in the worldwide energy
supply with global economic impacts, such as the one caused by the Organization of the
Petroleum Exporting Countries (OPEC) later in 2015, when its crude oil production growth
and global prices fell sharply [3].
The crude oil global distribution is presented in Figure 1. The darker bars represent the
proven crude oil reserves, most of them traditional reserves with more than 90% of
probability of being productive. The lighter bars represent the recoverable crude oil
reserves, it means heavy crude oil very difficult to recover by traditional methods, so the
probability of being productive is lower. And finally, the bigger bar shows the global
unconventional crude oil reserves, which includes tar sands, shade oil and bitumen, among
others [4].
24 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
Figure 1. Proven vs recoverable and unconventional world oil reserves of crude oil [4].
As it is shown, recoverable crude oil reserves present more homogeneous distribution
comparing with proven reserves. On a commercial basis, the exploitation of these reserves
would be of enormous benefit to different countries, contributing with the energy
independence in the intermediate term and providing secure energy supplies to the
Western Hemisphere for the near future. Achieve a better scenario depends on the oil
industry’s capacity to transform potential resources into commercial exploitable reserves
developing more technologies as Enhanced Oil Recovery (EOR), mining or fracking [2].
1.2. Heavy oils market challenge
The global current tendency is to obtain more product on the heavy fractions in the refining
of recovered oils [3], it implies higher content of nitrogen, oxygen, sulphur and heavy metals
hetero-compounds and a wider quantity of naphthenic and aromatic compounds. Therefore
the process requires even more heating and dilution strategies during recovery, which
increases the production costs, compared to light and intermediate oils [5]. The high contain
1. Toluene oxidation: a value alternative for the crude oil market 25
of heavy fractions changes the conditions of refining processes, so, unconventional oils
refining requires great specificity and more severe conditions of pressure and temperature
to obtain high added value products, such as liquefied petroleum gas (LPG), gasoline,
kerosene, and diesel oil [2].
Agreeing with this situation, the OPEC estimated the refining sector would have to increase
its complexity in order to guarantee the products specifications, so this organization
presented the crude oil industry forecast to 2035, given by the Figure 2. Major refinery
process are collected by four groups, showing the current projects and the additional
requirements taking into account the composition changes of the raw materials. In this way,
desulphurization requirements comprises the largest volume of capacity additions to 2035,
nearly 1.5 times those for distillation [6].
Figure 2. Global capacity requirements by process type 2011 – 2035 [6]
In this context, the development of new technologies to handle the increment of sub
products becomes crucial for the economical processing of unconventional resources such
as heavy and extra-heavy oils.
26 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
1.3. Adding value to refinery by-products
Crude oil refining includes distillation, coking and thermal units, catalytic cracking and
hydrocracking, hydro treating, reforming, isomerization, alkylation and polymerization
processes [7]. The complexity of each arrangement depends on the market to supply and
the crude oil characteristics. When part of the products are fuels such as gasoline, it is
necessary to produce other fractions with higher octane number to improve their properties,
for example aromatic compounds [1].
Benzene’s derivate hydrocarbons or aromatic compounds are refinery products since early
40’s, before so, their production was by dry distillation of coal [8]. These derivatives
represent a very important feedstock to petrochemical industry, as the main obtained
compounds are benzene, toluene and xylenes (BTX), all of them platform hydrocarbons.
Nevertheless, most of the BTX products were used to improve the fuel’s octane rating, so
since the restrictions against health hazardous components contained in formulated
gasoline are rising, the aromatic compounds are becoming low-value by products in places
where the petrochemical industry is not well developed.
Taking into account the progressive crude oil composition changes, the production of BTX
will increase because of the growth on fuels demand, not because of the complexity of
producing it, as it is shown in the Figure 2: The requirement of octane units is the smallest,
and because of aromatic compounds contained in the current crude oil are increasing, so
its overproduction. This scenario is the one to focus on add value to BTX products as
toluene, a platform compound.
Toluene could be used to obtain valuable products through partial oxidation. In that case,
catalysts are needed to prioritize the wanted reaction with respect to total oxidation and
other possible interactions. Furthermore, heterogeneous gas phase – solid catalysis is
preferred since those kind of processes avoid the need for solvents and complex
procedures to purify products. Liquid – solid heterogeneous catalysis or liquid phase
homogeneous catalysis are the second choice processes, also depending on the oxidant
agent used [9].
1. Toluene oxidation: a value alternative for the crude oil market 27
Nowadays, the interest to improve the oxidation of different compounds is higher than ever.
The study of different reactions and alternative mechanisms for traditional processes have
increased recently. This particular change is taking place mainly because of the
popularization of the green chemistry concept and the emergent concern on processes
intensification and energy safety [10]. These particular interests affect different areas,
among them, chemistry, engineering and biology, because selective oxidation could be a
solution to different problems, such as the deficiency of particular enzymes, the inability of
natural synthesis of specific compounds or the growing demand of a limited product.
By selective oxidation of toluene a wide range of products such as benzyl alcohol,
benzaldehyde, benzoic acid, benzyl benzoate and phenyl benzene may be generate. All of
the listed products have more commercial value than toluene and most of them are used
in cosmetic and polymer industries. The selectivity depends on the catalyst, the solvent,
reaction temperature, and pressure [11].
The most versatile product of the toluene’s selective oxidation is benzaldehyde. This
compound, best known for having the characteristic odor of essential oil of almonds, is used
in several applications of food, cosmetic and pharmaceutical industries [12].Nevertheless,
it is not produced in Colombia and could represent an alternative of management to the
increasing refinery by-products and an option to a growing market.
1.4. Benzaldehyde as a compound of major interest
Globally, the aldehydes are compounds of greater economic attention and have a high
mobility between countries, reporting an increase of over 179% in exports for Germany,
176% in the case of India and 45% for China in the period 2005-2011 [13]. Figure 3 presents
the trades of the bigger aldehyde exporter countries during the named period.
28 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
Figure 3. Exports of aldehydes-ethers, aldehydes-phenols and aldehydes with oxygen radicals [13]
The main importers are Switzerland, UK and India (marketer country). They reported an
increment in their imports higher than 2%, 127% and 150% respectively for the period 2005-
2011 [14]. The main trades of these countries are shown in Figure 4.
Figure 4. Imports of aldehydes-ethers, aldehydes-phenols and aldehydes with oxygen radicals [14]
For most Latin American countries, including Colombia, the trend is to import the necessary
amount of functionalized aldehydes to meet domestic needs and thus be the final
consumers, supplied mostly by the United States. The Colombian case, shows an import
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1. Toluene oxidation: a value alternative for the crude oil market 29
growth of 30% in the period 2005-2010, furthermore, on average only 0.8% of the material
arrived to the country came out [13, 14]. Figure 5 present the major Colombian money
trades referring to aldehydes and oxygen radicals for the named period.
Figure 5. Aldehydes-ethers, aldehydes-phenols and aldehydes with oxygen radicals’ imports and exports in Colombia [13, 14]
Those tendencies show the importance of compounds such as benzaldehyde, whose
global production for 2009 was estimated at 90,000 tons per year [15]. Benzaldehyde has
been used in organic synthesis as feedstock for several products, including compounds
such as mandelic acid, benzylic acid, cinnamic acid and benzyl alcohol, various aldehydes
(methyl, butyl, amyl and hexyl cinnamic aldehyde) and some amino acids such as
phenylglycine [12]. Recent developments in the use of benzaldehyde have occurred in the
healthcare industry, for the production of blood pressure regulators and influenza drugs,
also in the agriculture, as part of the formulation of insecticides and in the malachite green
pigment synthesis [16].
Benzaldehyde, which is found naturally in several foods such as the tonsillar glucoside,
which is a compound characteristic of bitter almonds, can also be gotten by different ways,
for example, with the partial oxidation of toluene, as it has been discussed before. In
countries with undeveloped petrochemical industry such as Colombia, toluene is imported,
even when it has been reported residual toluene productions higher than 4% in the last
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30 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
years (quantified as a product of commercial interest) [17], and there is not a market
established to use when it is obtained as by-product.
When this scenario is contextualized in a country directly affected by the crude oil dynamic,
like Colombia, the study of valuable products production processes from oil refining and
chemical industry to strengthen the country’s economy becomes a priority. For the specific
case arises, the benzaldehyde production from oxidation of toluene.
1.5. Benzaldehyde production processes
Benzaldehyde can be extracted naturally from almonds, apples and cherries kernels, since
these fruits contain amygdalin-glucoside. Under enzymatic catalysis this glycoside
produces benzaldehyde, hydrogen cyanide and two glucose molecules [12]. However, in
areas with limited production of the fruits mentioned, benzaldehyde is synthesized by
several ways, including alkaline hydrolysis of cinnamaldehyde, the partial oxidation of
benzyl alcohol, the alkaline hydrolysis of benzyl - chloride and benzene carbonylation [18].
Among the major reactive mechanisms of scientific interest, there are highlighted the
reaction between benzene with dichloromethyl methyl ether in the presence of AlCl3, the
formylation of benzene with formyl fluoride, using as catalyst boron trifluoride, and the
oxidation of benzyl alcohol in the presence of Cu(NO3)2, having as intermediary the
chloromethylbenzene. Benzaldehyde production by these chemical routes is generally
carried at laboratory scale due to the low conversions, the high costs associated with
purification of the benzaldehyde obtained and the environmental implications involving the
effluent treatment [19]. Industrially, the production is carried by the Etard reaction [11],
which includes chlorine compounds as by-products, involving high costs associated with
the purification of the benzaldehyde. Also, it is environmentally unfavorable because metal
chlorinated complexes derivate of CrO2Cl2 are generated, and the end uses of produced
benzaldehyde are limited [20].
However, a chemical pathway that has attracted great interest in recent years is the partial
oxidation of toluene. This reaction involves only two reagents, neither of them halogenated,
and occurs in the presence of a selective catalyst to prevent complete oxidation of the
hydrocarbons [1, 20].
1. Toluene oxidation: a value alternative for the crude oil market 31
1.6. Benzaldehyde production by toluene partial oxidation
The oxidation of toluene can take place in liquid phase, involving the use of solvents, low
conversions and high reaction times, or in vapor phase, which promotes the contact
between the reactants. However, the last one demands higher temperatures than liquid
phase reaction and may generate mixtures of toluene and oxygen near to the explosion
point, entailing the develop of nontrivial control systems [21].
Considering the limitations outlined for the current process carried out at industrial scale
and the interest on produce benzaldehyde without impurities, which restrict their use and
increase the purification costs associated; recent studies have focused on the partial
oxidation of toluene in liquid phase or vapor phase, particularly focusing on the
development of selective catalysts and strategies to improve yield and selectivity. The main
problem with this kind of reactions is that each successive oxidation is faster than the
previous one, so the tendency is to total oxidation, which is related to formation of acidic
hydrogens after the insertion of the oxygen’s heteroatom or carbon oxides [1].
The selective oxidation of toluene to benzaldehyde in the liquid phase is generally carried
in the presence of organic solvents, liquid acid catalysts and in most of the cases using as
oxidizing agent hydrogen peroxide. In order to promote the conversion and selectivity of
the reaction, it has been suggested to change the oxidizing agent for compounds, such as
tert-butyl hydroperoxide [22].
Toluene liquid phase oxidation implies significant reaction time, leading to higher operating
costs. Concerned with this problem, research have directed their efforts to improve the
contact between the reactants using oxygen as oxidizing agent and a variety of catalysts,
such as ionic liquids [23], tetra hydrate cobalt acetate [24], and different metal oxides as
Fe-V-O [25], MnOx over SBA-15 [11], and vanadium oxide supported on SBA-15 [26].
Considering the results reported by several studies, the economic viability of production of
benzaldehyde by partial oxidation of toluene in liquid phase is still a matter of discussion
because it is essential to use solvents, which hinders the transport mass, promotes the
generation of by-products [27], and leads to low reaction conversion and selectivity.
32 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
Recently research have focused on the vapor phase reaction, although the development of
catalysts to prevent complete oxidation is a key point [1, 28]. The oxidation have been
developed using vanadium oxide as active phase without support [29], supported on
different materials as metal oxides [30] or silica [31, 32], enriched by cations as potassium
[33], iron and cerium [34] and mixed with other metals as silver [35] . Vanadium oxide is
used because of its redox properties, but it shows low conversions and selectivity to
benzaldehyde. It seems the selectivity of toluene transformations into benzaldehyde
decrease as the vanadium containing catalysts ability to generate the singlet form of
molecular oxygen grows [36].
Moreover, the feasibility of using oxides of lanthanum [37], cerium [38] and iron [39]
separately and supported on MoO4 [40] where studied. Molybdenum trioxide has been
suggested to be a selective, if not very active, catalyst because the only active chemisorbed
oxygen entity appears to be O2-: other diatomic species that are known to exist on the
catalyst are not involved in the catalytic oxidation [41]. In addition, it was proven structure
the sensitivity of molybdenum oxides for oxidation reactions. Studies clearly demonstrated
how the geometry of the reacting molecule and atomic arrangements at the oxide surface
are important factors in oxidation reactions. E. g. it was shown that propene gives almost
exclusively acrolein on the (1 0 0) lateral face and CO2 on the apical (1 0 1), (1̅ 0 1) and
basal (0 1 0) planes [42]. However, at high concentrations of molybdenum, increasingly
sized MoO3 crystals preferentially expose non-selective (0 1 0) crystal faces, responsible
for undesired C-C bond cleavage and total oxidation reactions [43]
While the presence of stronger Lewis acid sites in vanadium-based catalysts active the
carbon atom of the carbonyl groups, thus facilitate its nucleophilic attack to form benzoate
species which further degrade to carbon oxides [44]; molybdenum based catalysts present
high performance, even when the preparation conditions and the catalyst structure exert
great influences on the catalytic selectivity [40].
1. Toluene oxidation: a value alternative for the crude oil market 33
1.7. Mixed oxides as selective oxidation catalysts
Catalytic hydrocarbons oxidation determining rate step is the hydrocarbon adsorption on
the active phase, since it is necessary to include a new type of atom in the structure. The
compounds that facilitate this process are metal oxides, due to the bonding force between
carbon and metal oxide [45]. Additionally, oxygen mobility in such solids assists the
reaction, preventing further interaction between the hydrocarbon and molecular oxygen [46,
47].
Oxygen mobility is related with metallic ions estate of valence. If the metallic ion has several
state of valence, different oxides could be present and therefore, compensation effect
promotes the generation of vacancies [48]. Cerium oxides are frequently desired, because
its fluorite type structure enables cycle types like: [CeO2↔ CeO2- + (/2)O2 for =0 to 0.5].
Its oxygen storage capacity (OSC) improves by substitution of cations, doping or changing
the meso and nano solid architecture [49]. Among those reported are Mo - Ce supported
oxides [50, 51], Mo-Zr-Ce oxides [52], Co-Ce-Zr mixed oxides [53], Ce-Zr-Y-Mn oxides [54]
Pr-Zr-Ce oxides [55], and Mn-Ni-Co-Ce-Zr oxides [56].
One of the most popular modifications has been the inclusion of zirconium cation in the
cerium solid matrix (CeO2- ZrO2, Ce1-xZrxO2, or CZ) because this solid solution present
higher OSC and thermal stability, making it more versatile at the inclusion of cations to
provide other characteristics to the oxide [49].
The use of ternary oxides based on Ce-Zr-Mo for the synthesis of benzaldehyde in gas
phase has been proposed [57], in order to achieve better catalytic behavior by improve the
thermal stability of the structures changing their redox properties and oxygen mobility [58].
However, the synthesized catalysts had low surface area and shows various phases that
may not be the most active or may become unstable [55, 56]. Still, a well understand
structure giving by cerium, zirconium and molybdenum mixed oxides is not reported so far
and however represent an option as oxidation catalyst.
34 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
1.8. Cerium, zirconium and molybdenum mixed oxides
The main problem for plurimetallic mixed oxides, is to find the components ratio to obtain a
stable solid phase, because within one structure type the size and crystal structure can
modify some of their functional properties therefore the size effect and morphological
characteristics may serve as factors determining the practical use of one or another
structural form [59].
Cerium is the second element of the rare earth series with electronic configuration: (Xe) 6s2
5d1 4f1. In such configuration, the volume of 6s and 5d orbitals are greater than that of the
4f orbital and, therefore, the three 6s2 and 5d1 electrons are the only ones participating for
chemical bonds. The valence of the lanthanide elements in their configurations is habitually
3+. In the particular case of Ce, of which the electronic configuration is (Xe) 4f1, the f
electron is easily released, acquiring the more stable structure of the tetravalent ion Ce4+.
Effectively, cerium dioxide (CeO2) is the thermodynamic stable phase in nature. Its crystal
structure is cubic containing four formula units per unit cell. Each cation is coordinated to
eight oxygen atoms and each anion is coordinated to four cerium atoms [60].
There are three well-defined polymorphs of pure ZrO2, namely, the monoclinic, tetragonal,
and cubic phases. The monoclinic phase is stable up to about 1100°C temperature range
to the tetragonal phase. At approximately 2370°C, the compound adopts the cubic structure
[61].
The structural units of all МоО3 phases are [МоО6] octahedral linked by vertices into layered
(α-МоО3) or framework (β-МоО3) structures. Among them, the hexagonal metastable h-
МоО3 phase has a unique tunnel structure and consists of zigzag chains of the octahedral
sharing vertices and edges. The tunnels are 1D cavities with a diameter of ∼3 Å formed by
the adherence of 12 octahedral towards the с axis [59].
In the case of the CeO2, the reason to mix with other metallic cations is its poor thermal
stability, then support CeO2 based catalyst on ZrO2 is presented as a simple and effective
method to improve the properties of the oxide and its mixtures, especially for the exothermic
oxidation reactions. Thus the ZrO2 acts as a carrier who stabilize the surface active
structure of CeO2 - based oxides by formation of Zr0.88Ce0.12O2 solid solution in the interface,
1. Toluene oxidation: a value alternative for the crude oil market 35
also it can enhance mobility of active oxygen and improve catalytic performance of total
oxidation [62].
Although most conventional CeO2-ZrO2 systems which exhibit cubic fluorite or tetragonal
zirconia structures have a disordered arrangement of Ce and Zr within the crystal structure,
CeO2–ZrO2 pyrochlore-type compounds have been discovered which arrange these two
atoms in an ordered manner along the [1 1 0] direction in Ce2Zr2O7+ crystallites [63].
Pyrochlore structure is generally obtained through high-temperature reduction, resulting in
a decrease in specific surface area due to sintering and a less functional material for
catalyst. Even so, there are several preparation methods proposed in order to improve the
thermal stability of the compound [64].
CeO2-MoO3 system have also many applications such as methanation catalysts [65], most
of them to improve the thermal behavior of the catalyst and the poison resistance. The
highly dispersed molybdate, crystalline Ce8Mo12O49 and Ce2Mo4O15 are formed in the
MoO3-CeO2 complex oxide calcined, increasing MoO3 content [66]. Nevertheless, the
stability of the compounds is limited by the sublimation of the Mo compounds [67].
Binary compounds of those oxides have been studied in other reactions [68], some of them
similar reactions, as the case of Mo-Zr oxide to promote the selective oxidation of methane
to formaldehyde [69], the results showed the direct relation between the catalytic
performance of the solids and their physicochemical properties with the content of Mo in
the bulk of ZrO2. At the end, the Zr(MoO4)2 structure was the only one stable and the one
with better results.
The behavior of Ce-Zr binary oxides on ternary systems have also been studied. CeO2–
DyO1.5–ZrO2 system [70], present the different phases between the CeO2–ZrO2 binary
mixtures (Fluorite type cubic, fluorite type cubic and tetragonal, monoclinic and tetragonal),
similar with the results reported for the CeO2–ThO2–ZrO2 system [71] and the HfO2–ZrO2–
CeO2 system [72].
The Ba-Zr-Mo oxides ternary system shows that at reducing conditions, the Mo compounds
are reduced into Mo0 in Mo-rich compositions and as a function of composition and
temperature, Mo0, Mo4+ and Mo6+ compounds can thus coexist at these conditions. At
36 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
oxidizing conditions all the identified Mo compounds show that the stable Mo valence is
Mo6+ then, solid solutions between Zr4+ and Mo6+ compounds are practically non-existent
[73]. Related studies shows that isolate MoO42- species is the primary molybdenum species
at low loading amount of Mo in Ce-Zr oxides matrix, but polymeric molybdate species
predominates when the Mo6+ loading amount is increased. The molybdena is preferentially
dispersed on the surface of CeO2 at lower loading because the base strength of CeO2 is
stronger than that of ZrO2 [74].
Even without a single phase, the expectations for the well development of the ternary
catalyst in partial oxidations is increase taking into account the good results reported for
another reactions, as the selective reduction of NOX [75]. The addition of Mo controlled the
growth of CeO2 particle size in a mixed matrix of Ce – Zr oxides, improved the redox ability
and increased the amount of surface acidity, especially the Lewis acidity [76]. In fact,
studies conclude that solids with the presence of the three metallic cations exhibits excellent
catalytic activity for the condensation of various aromatic and hetero aromatic anilines
which are similar compounds to the aromatic hydrocarbons [77].
1.9. Modelling the reaction
In catalytic oxidation of toluene several variables affect the conversion of the hydrocarbon
and the benzaldehyde selectivity, as the oxygen to toluene feed ratio, the reaction
temperature and reciprocal of the space velocity (W/F) [27]. Therefore it is necessary to
study at different conditions the behavior of the reactants and the products involved. This
kind of analysis is related to the reaction rate and may be modelled by different
mathematical expressions in order to represent the reaction mechanism which is taken
place in the catalyst surface.
However, thorough discussions of complete reaction schemes seldom appear, precise rate
expressions are difficult to obtain because of the existence of reaction networks [29]. As
example, the simplified initial steps in toluene oxidation by hydrogen transfer mechanism
are show in Figure 6. In the first step, loosely bound, physically adsorbed or hydrogen-
bonded toluene is subjected to a rate-determining hydrogen abstraction forming an OH
group and m-complexed benzyl species of radical character. If the benzyl radicals react
1. Toluene oxidation: a value alternative for the crude oil market 37
with neutral toluene molecules the coupling products of Route 2 will be obtained. A high
concentration of the benzyl radicals would favoring the formation of dibenzyl by
recombination of two radicals. The same reactions will also occur to some extent in the
vapor phase due to desorption of benzyl radicals. As a parallel path it is found Route 3
since the benzylic species will form resonance structures with the charge or radical
character to some extent delocalized to the ring system in preferentially ortho and para
positions. A direct hydrogen abstraction in the nucleus requires a much higher energy than
in the methyl position and the total process is thought to be unfavorable. Nevertheless,
route 3 is almost negligible [78].
Figure 6. Hydrogen transfer mechanism for the initial steps in catalytic oxidation of toluene [78]
Some of the more reasonable paths are: (1) electron transfer, (2) electrophilic substitution,
(3) nucleophilic substitution within the coordination sphere of “soft” metal ions. These
mechanisms depend on the nature of the metal compound and the ionization potential of
the aromatic compound. The electron transfer mechanism is show in Figure 7. In process
38 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
(1), the first step is a reversible transfer of one electron to the metal oxidant with the
formation of a cation radical. This is considered to be a slow and rate-determining step
which is followed by a faster one in which the cation radical subsequently releases one
proton forming the benzyl radical. There is a certain probability for a direct substitution
reaction with nucleophilic oxygen, which will lead to route 3. A proton release (vide supra)
from the nucleus as a competing process to methyl proton release is also feasible. The
methyl-phenyl radical thus formed will also lead to route 3 [78].
Figure 7. Electron transfer mechanism for the initial steps in catalytic oxidation of toluene [78]
1. Toluene oxidation: a value alternative for the crude oil market 39
Results of steady-state and transient kinetics studies have helped to propose several
mechanisms, i.e. a parallel-consecutive reaction scheme for partial toluene oxidation over
vanadia/titania catalyst [79].
Kinetic expressions are usually of the power-rate law type and are applicable within limited
experimental ranges [80]. The oxidation of toluene on copper catalysts doped with
molybdenum and tungsten oxides on silt between 350 and 450 °C showed that reaction
was zero order with respect to the hydrocarbon (at concentrations from 5% to 30%) and
first order with respect to oxygen (at concentrations from 5% to 25%) [81]
Mars & Van Krevelen redox mechanism is very often used for selective and total oxidation
reaction. In their study utilizing vanadium oxide catalysts, Mars and Van Krevelen
investigated the oxidation of benzene to benzoquinone, maleic anhydride, CO2, CO and
H2O, toluene to benzaldehyde and benzoic acid, naphthalene to naphthoquinone and
phthalic anhydride, and anthracene to anthraquinone and phthalic anhydride. Also applied
their rate equation to data reported in the literature for SO2 oxidation to SO3 over various
vanadium oxide-based catalysts [82]. Their reaction mechanism can be written as:
I. Aromatic compound + oxidized catalyst 𝑘1→ oxidation products + reduced catalyst
II. Reduced catalyst + oxygen 𝑘2→ oxidized catalyst
The following assumptions were stablished [82]:
(1) The reaction in step I is first order with respect to the reactant and the fraction of sites
covered by oxygen ();
(2) Certain lattice O ions at the surface are involved in the oxidation reaction in step I;
(3) Only O ions (atoms) are assumed to exist on these sites (no reactant or products);
(4) The rate of surface re-oxidation (step II), i.e., O2 adsorption, is proportional to 𝑃𝑂2𝑛 and
to the concentration of active sites not covered by oxygen (1-).
The reaction network for toluene oxidation have been modelled assuming the redox
mechanism proposed by Mars and van Krevelen, the approach is oversimplified with
40 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
respect to the true reaction mechanism, but provides a better description of the
experimental data than the empirical approaches (power-law equations) [81]. Kinetic
measurements in different kind of oxides supports this affirmation, i. e. Co –Ni - Cu- Zn
oxides catalysts [83], WO3-MoO3 catalyst [84], and V2O5 catalyst, which showed that the
reaction was second order with respect to the hydrocarbon and zero order with respect to
oxygen. The mechanism fitted the experimental data, but the authors did not take into
account the existence of secondary reactions [81].
The oxidation of toluene by air over pure bismuth molybdate catalyst, considering a
complex reaction system and indicated the named mechanism fitted the experimental data.
The reaction was found to be first order in the hydrocarbon partial pressure and first order
in oxygen partial pressure [85].
In this way, kinetic studies of the oxidation of unsubstituted and substituted toluene over
molybdenum trioxide showed that the reaction was controlled by the rate of the reduction
of the catalyst, and the initial rate was first order in toluene and was independent of the
concentration of oxygen. The results were interpreted in terms of a kinetic expression of
the power-rate law type [41].
1.10. Scope
As it was mentioned, the current oil global market tends are pushing the refinery facilities
to change in order to improve their processes and to manage different raw materials. This
scenario leads new tasks to accomplish, for instance to handle toxic sub-products steams.
Such a case is the toluene, a platform molecule which could be oxidize to obtain
benzaldehyde, a simple hydrocarbon well known and used in fine chemistry.
Nevertheless, there are many factors affecting the toluene partial oxidation to
benzaldehyde when this is performed in vapor phase so it avoid mass and heat problems.
Most of the issues in this reaction are related to the catalyst, leading vanadia and molibdena
catalyst as better choices to achieve high selectivity and conversions.
Since vanadium compounds are related to hydrogen transfer mechanism which leads to
total oxidation, molybdenum compounds are preferred because of the great selectivity,
1. Toluene oxidation: a value alternative for the crude oil market 41
especially mixed with others oxides in order to stabilize the material and control its reduction
rate.
This work aims to contribute with the characterization of a cerium- zirconium - molybdenum
mixed oxide as an active material for the partial oxidation of toluene to benzaldehyde, and
the adjust of reaction phenomena by a kinetic expression.
2. Vapor phase toluene partial oxidation: the reaction at laboratory scale
The partial oxidation of toluene in vapor phase involves different considerations to take into
account not only the type of catalyst to use but the reaction conditions in order to prevent
possible oxygen – toluene explosive mixtures and to improve reaction selectivity and yield.
In this chapter, different considerations are selected to study the reaction and the
equipment to guarantee them are present.
2.1. Thermodynamic study
Duarte [57] presented the equilibrium species distribution for reactions given between
200°C and 800°C, changing the toluene to oxygen molar ratio between 10:1 to 1:10,
concluding that the most selective to benzaldehyde temperature up from 400°C until 600°C
using a toluene to oxygen feeding ratio of 1:1.
Figure 8. Toluene molar conversion as function of temperature and Oxygen/Toluene molar feed.
Using Aspen Plus ® this information was corroborate simulating the equilibrium state by
minimizing the formation free Gibbs energy of the different reactants between 330°C to
330
415
500
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.30 0.40 0.79 0.991.32
1.98
Temperature (°C)
Tolu
ene
con
vers
ion
Oxygen/Toluene molar feed ratio
44 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
500°C (this range was set taking into account the results reported by Duarte [57] and
avoiding possible Mo species sublimation [84]). Results confirm the increasing toluene
conversion at higher temperatures and oxygen feeding concentration, present by figure 8.
Figure 9. CO molar selectivity as function of reaction conditions
Also, the results confirm the high selectivity to CO at high temperatures and great selectivity
to CO2 at low temperatures, as it is present in figures 9 and 10.
Figure 10. CO2 molar selectivity as function of reaction conditions
0.3
0.4
0.8
1.01.32.0
0
0.2
0.4
0.6
0.8
330373
415458
500
Oxygen/Toluene molar feed ratio
CO
mo
lar
sele
ctiv
ity
Temperature
0.3
0.4
0.81.0
1.32.0
0.00
0.20
0.40
0.60
0.80
1.00
330373
415458
500
Oxygen/Toluene molar feed ratio
CO
2m
ola
r se
lect
ivit
y
Temperature (°C)
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 45
CO, CO2 and water are the main products at equilibrium conditions, the benzaldehyde
production increase at high temperatures and small oxygen to toluene feeding ratios, but it
is almost negligible. This information is very important, because it implies the great
importance of the selectivity to benzaldehyde that should improve the catalyst. Also,
according with this brief analysis, the possible conditions to study the reaction were fixed,
and will be present later on.
2.2. Reaction system
The reaction arrangement were carefully chosen, including the feeding system, the reactor
and the analytical equipment, as it is shown in the Figure 11.
At normal conditions (P=1 atm, T=25°C) toluene is liquid, so it is necessary to evaporate it
by changing pressure or temperature. Oxygen may be used pure or mixed (i.e. air) [84,85],
nevertheless, the employment of pure oxygen lead others compounds as possible intern
standards to quantify the outflows. Since not only toluene and oxygen should be injected in
the system, inert compounds whom dilute the reactants, prevent risky mixtures and stabilize
the inlet flow pressure should be part of the feed. Therefore, three individual lines for the
Toluene
Pump
Mixer
Ar
O2
N2
F
F
F
GC
Atmosphere
Reactor
Condensable
products
Non- condensable
products
Cold trap Isopropanol
trap
Catalytic bed
Quartz
wool
Quartz
chips
Oven
Gas lines
Liquid introduction
Products
Temperature
control
Figure 11. Experimental arrangement for toluene oxidation
46 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
gases were arranged: the first one for the introduction of carrier gas (argon), the second
one for the oxygen and a third line for the internal standard (nitrogen).
To guarantee stable conditions the flow rate was controlled using mass flow controllers
(Brooks Instrument). The different gases were mixed before the reaction zone in a ¼ ID
tube packed with crushed quartz (pink lines). The toluene (blue line) was pumped using a
305 Gilson micro pump and introduced into the reactor by a needle. Before the reactor, a
“T” connection was disposed to allow the introduction of liquid and gases.
The feeding system was coupled to the reactor, the one used was a quartz straight reactor
with a length of 30 cm and an internal diameter (ID) of 7 mm. A cylindrical oven of 20 cm
of length was used coupled to a k-type thermocouple.
After the reactor, the line was heated and controlled at 200 °C. The reaction products (red
line) were separated into non-condensable and condensable products. The condensable
products were recovered in liquid phase using a dry trap maintained at 0 °C. Recovery of
condensable products giving by temperature only, leads to toluene lost, as it was calculate
using the software Aspen Plus ®. Figure 12 presents the recovered volume fraction of the
main outlet condensable compounds in function of the trap temperature.
Figure 12. Recovered volume fraction of the main outlet condensable compounds in function of the trap temperature
0
0.2
0.4
0.6
0.8
1
1.2
-100 -80 -60 -40 -20 0 20 40 60 80
Rec
ove
r v
olu
me
frac
tio
n
Trap Temperature (°C)
Benzaldehyde Toluene
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 47
Therefore, it was placed a second trap which contained isopropanol so the affinity of the
hydrocarbons reduce the mass losses [86].
The gas permanent products were analyzed by on-line gas phase chromatography. The
condensable products were stocked (4°C) and analyzed later on.
The analysis of products was performed according to its recuperation after reaction. The
non-condensable products present in gaseous phase were characterized online by gas
phase chromatography (GPC). The analysis was performed every 25 min using a Network
GC system Agilent Technologies 6890N with TCD detector. The column used was a
Carbosieve II. The products analyzed were H2, O2, CO, CO2, and N2 (internal standard).
The chromatographic method is presented in Annex A.
The analysis of condensable products recuperated in liquid phase, was performed by gas
chromatography using a Network GC system Agilent Technologies 6890N with a FID
detector. The column was a SGE Sol gel Wax (dimensions: 60m length, 0.25mm internal
diameter and 0.25m film thickness). The products analyzed were toluene, benzene and
benzaldehyde. The external standard used was isopropanol. The chromatographic method
is presented in annex A.
2.3. Ce–Zr–Mo oxydes as catalysts
Not only the reaction setup was carefully chosen, the catalysts to evaluate were proposed
in order to obtain better structural stability, keeping the catalytic performance at reaction
conditions. This is not a simple target considering the behavior of each metal oxide to mix,
but there is information to account in order to formulate possible catalytic solids.
As it was present before, the different phases between the CeO2–ZrO2 binary mixes (fluorite
type cubic, fluorite type cubic and tetragonal, monoclinic and tetragonal) are known [70-
72]. At low loading amount of Mo in Ce-Zr oxides matrix, the isolate MoO42- species is the
primary molybdena species, but polymeric molybdate species predominates when the Mo6+
loading amount is increased. The molybdena is preferentially dispersed on the surface of
CeO2 at lower loading because the base strength of CeO2 is stronger than that of ZrO2 [74].
48 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
This information is very important, but it didn’t clarify much the scene. Other way to
formulate the catalysts is to start by known stable compounds and try to include the third
metal compound, assuming the oxidation state to make the oxygen approach. In this way,
three formulations were proposed to synthetize, starting by Ce0.5Zr0.5O2. The first one
substituting half of the oxygen by molybdenum oxide, the second one substituting three
quarters of the oxygen and finally by the substitution of all the oxygen, as follows:
Table 1. Catalyst proposal development
4(𝐶𝑒𝑂2)
4(𝐶𝑒0.5𝑍𝑟0.5𝑂2)
𝑪𝒆𝟐𝒁𝒓𝟐𝑶𝟖
𝐶𝑒2𝑍𝑟2𝑂4(𝑀𝑜𝑂4)4
𝐶𝑒2𝑍𝑟2𝑀𝑜4𝑂20
𝑪𝒆𝒁𝒓𝑴𝒐𝟐𝑶𝟏𝟎
𝐶𝑒2𝑍𝑟2𝑂2(𝑀𝑜𝑂4)6
𝐶𝑒2𝑍𝑟2𝑀𝑜6𝑂26
𝑪𝒆𝒁𝒓𝑴𝒐𝟑𝑶𝟏𝟑
𝐶𝑒2𝑍𝑟2(𝑀𝑜𝑂4)8
𝐶𝑒2𝑍𝑟2𝑀𝑜8𝑂32
𝑪𝒆𝒁𝒓𝑴𝒐𝟒𝑶𝟏𝟔
There are several ways to synthetize plurimetallic oxides, e. g. co-precipitation, electrostatic
adsorption and impregnation [87], like those, there is the pseudo sol-gel method, a
derivation of the standard sol-gel scheme, where the reactivity of the departure precursors
is affected before reaction in order to form a polymeric mixed precursor in solution. This
solution is later transformed into a solid resin by controlled evaporation, which after drying
and calcination leads to a mixed oxide with a well-defined structure and homogeneity
composition [88].
This method was the one selected for synthetize the catalysts, the precursor salts used
were cerium (III) acetate hydrate, zirconium (IV) acetylacetonate and bis-acetylacetonate
of molybdenum dioxide. The precursor salts were dissolved in propionic acid, with a cation
concentration of 0.12 mol.L-1 [89], but for the molybdenum salt the solution the cation
concentration was 0.006 mol.L-1. The different solutions formed were then mixed and kept
under total reflux for 1h. This procedure ensured the effective formation of the polymeric
precursor in solution, and, a homogeneous mixture, as it was probed [88]. The elimination
of the organic solvent was carried out by vacuum distillation. After distillation the gel
obtained was calcined at 550°C to prevent Mo-compounds sublimation [84] for 6 h, using
a heating rate of 2 °C.min-1. The global procedure is presented in the Figure 13.
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 49
time (h)
Task 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Dissolve precursor salts
Mix the propionates
Mix Ce propionate
Mix Zr propionate
Mix Mo propionate
Vacuum distillation
Calcination
Figure 13. Global catalyst preparation procedure following the pseudo sol-gel method [89]
Since, considerable amounts of propionic acid should be used during each synthesis
procedure, mainly related to the molybdenum salt dilution, a modification in the methods is
implemented. The catalyst proposed were done as it is show below, adding the
molybdenum salt by equal batches during the vacuum distillation (formation of the resin)
and using the evaporated and recuperate solvent of each batch to dilute the next one.
time (h)
Task 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Dissolve precursor salts
Mix the propionates
Mix Ce propionate
Mix Zr propionate
Mix Mo propionate
Vacuum distillation
Calcination
Figure 14. Catalyst preparation procedure following the modified pseudo sol-gel
method
2.3.1. Modified pseudo sol gel method catalysts
Once it was synthetized the catalysts and knowing their expected composition and their
real composition, calculated with the quantity and purity of the primary salts, which is
presented in Table 2.
.
50 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
Table 2. Solids synthetized by the modified pseudo sol-gel method
The catalytic behavior of mixed metal oxides depend on the species of the surface, which
are related to the nature of the solid, the content of each metal oxide and the calcination
temperature [66]. Then, it is very important to characterize the synthetized catalysts by
different methods in order to correlate the catalytic performance with the chemical
properties.
The redox properties of the catalyst are of great importance thus they have been proven to
affect the catalytic activity and stability of metal catalysts. These properties may be studied
by temperature programmed analysis, like Temperature Programmed Reduction (TPR) and
Temperature Programmed Oxidation (TPO).
Figure 15. Synthetized CZM2(I), CZM3
(I), CZM4(I) catalysts samples TPR analysis
results
0
100
200
300
400
500
600
700
800
900
1000
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120 140
Tem
per
atu
re (
°C)
TCD
sig
nal
(a.
u.)
time (mins)
Theoretical molecular formula
Name
Theoretical metal ratio
(M mol/Zr mol)
Expected metal ratio
(M mol/Zr mol)
Ce Zr Mo Ce Zr Mo
CeZrMo2O10 CZM2I 1 1 2 1.04 1.00 2.00
CeZrMo3O13 CZM3I 1 1 3 1.04 1.00 2.99
CeZrMo4O16 CZM4I 1 1 4 1.05 1.00 4.01
CZM2(I)
CZM4(I)
CZM3(I)
Temperature
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 51
There are four principal picks expected in a TPR analysis of a solid composed of the metal
oxides involved in the presented formulations, the 500°C peak and the 852°C peak related
with the Ce4+ to Ce3+ change (surface and bulk, respectively) [90]; as well as the 700-770°C
peak and the 900°C peak, related with the Mo6+ to Mo4+ change and the Mo4+ to Mo0
change, respectively [91].
Certainly, the zirconia has also changes, at 610°C and 690°C approx.(related with the Zr4+
to Zr3+ in the surface and in the bulk, respectively), but the signal usually is weak [92].
Samples of the three formulations were analyzed and the results are showed in the Figure
15, the TCD signal profiles present only two principal peaks, which may be related to the
mixed of the different peaks already listed.
Different authors noticed changes in the reduction behaviour of a single metallic oxide with
the presence of a metal or a guest metallic cation in the oxide structure, resulting in the
shift in the temperature of the main H2 consumptions [90]. The profiles of temperature
reduction change as the molybdenum content increase, making every catalyst more stable
each time. And that is the main effect present by the TPR profiles, the CZM2I sample, whom
contain of Mo is the lowest, may exhibit changes in the work operation range temperature
(represent in Figure 15 by the red shape), but the conditions of the reduction are different.
Its reduction begins at 450°C, meanwhile CZM3I sample starts to reduce at 500°C and
CZM4I sample reduces from 650°C.
The presence of oxygen vacancies in the structure of metal oxides of transition metals is
decisive in the redox reactions in which they participate, aside from being fundamental in
the reaction mechanism in catalytic oxidation processes [93]. It has been reported that the
activity of the catalysts is influenced by the presence of the type of oxygen species on the
solid surface, which are consumed in the course of the reaction and can be regenerated
through the presence of an oxidative atmosphere [93]. Then, it is very important to stablish
if the metal species change dramatically because of the excess of oxygen and that
information is giving by TPO analysis results.
Figure 16 present the TPO results for CZM2(I), CZM3
(I) and CZM4(I) samples. As it is present,
there is a small weight loss before 100°C for the CMZ2I sample that may be associate with
the presence of water on the surface. The interaction between and oxidative atmosphere
and the selected samples did not change the weight of any of them, so, there is not reasons
to assume changes in the initial structures giving by the metal- oxygen interaction or the
52 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
temperature changes (it has been report sublimation of molybdenum species up to 600°C
[28, 59]).
Figure 16. Synthetized catalysts samples TPO analysis results: (blue) CZM2(I), (red)
CZM3(I), (green) CZM4
(I).
Characterization using X-ray Diffraction (XRD), Raman spectroscopy and Scanning
Electron Microscopy (SEM) were performed to study the some physicochemical properties
of the mixed oxides such as crystallinity, surface morphology, among others. The XRD
analysis results for the three fresh samples, CMZ2(I), CMZ3
(I) and CMZ4(I) are present in
Figure 17. No single oxides corresponding to diffraction peaks were detected (CeO2, ZrO2,
MoO3) then, it may suggest that preparation method allows the crystallization of the mixed
oxides without any single phases rejection. Nevertheless, only corresponding peaks to one
cerium-molybdenum oxide triclinic structure (Ce2(MoO4)2(Mo2O7) SS-VVV-PPP: 01-076-
1040) fit the two main peaks, but the matching is not precise. Peaks related to Ce8Mo12O49
are also present and fit one of the main peaks but their match is also not precise and
predominate at low Mo load, contrary to Ce(MoO4)2 related peaks. Binary Zr – Mo
compounds are evidenced at low Mo load, exhibiting monoclinic structures.
According with these X-ray diffraction patterns, peaks related to Ce2(MoO4)2(Mo2O7)
structure are present but the peak shift at 24.2° and the 26.5° peak lack may occur as effect
of Zr insertion in the structure. Then, it is possible to suggest the integration of the three
metal oxides.
95
96
97
98
99
100
101
102
0 100 200 300 400 500 600 700 800
Wei
ght
per
cen
taje
Temperature (°C)
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 53
Figure 17. Diffractograms of fresh mixed oxides: CZM2(I), CZM3
(I), CZM4(I).
■Ce(MoO4)2 ♦Ce8Mo12O49 ●Ce2(MoO4)2(Mo2O7) ▲Zr(MoO4)2
Figure 18 present the Raman analysis result of the three fresh samples, CMZ2(I), CMZ3
(I)
and CMZ4(I). The predominant bands associated to MoO3 are 285, 335, 378, 667, 818, 995
cm-1, their intensity decrease when the sample Mo content increases.
Figure 18. Raman spectra of fresh mixed oxides: CZM2(I), CZM3
(I), CZM4(I),
15 25 35 45 55 65 75 85
Inte
nsi
ty (
a.u
.)
2 (°)
0 100 200 300 400 500 600 700 800 900 1000 1100 1200
Inte
nsi
ty (
a.u
.)
Raman shift (cm-1)
CZM2(I)
CZM4(I)
CZM3(I)
CZM2(I)
CZM4(I)
CZM3(I)
54 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
Meanwhile, the highly dispersed molybdate species became more important as the bands
at 332, 856, 946 cm-1 [66] turn out to be the main bands when the sample Mo content
increases.
The bands attributed to Ce8Mo12O49 are present but shifted, although normally appear at
280, 410, 680, 850 and 946 cm-1 [66]. That result may be attributed to the effect of the
zirconium inclusion on the Ce-Mo oxide structure or to the generation of oxygen deficiency,
which changes the molybdate species presented on the surface.
CeO2–ZrO2 mixed oxides bands are no present in the Raman spectra of the different
samples, these bands are usually centered at 130, 300, 460, 540 and 620 cm-1 [94].
The Raman spectra suggest that molybdenum oxide species variably exist as highly
dispersed molybdate species and MoO3 with increasing of Mo content.
Figure 19. SEM micrographs of fresh mixed oxides: a. CZM2(I), b. CZM3
(I), c. CZM4(I)
CZM2(I), CZM3
(I) and CZM4(I) samples morphology was evidenced by Scanning Electron
Microscopy, Figure 19 present the micrographs.
These images exhibit the preference for define structure as the Mo content grows, forming
grains of 0.25m average diameter. Meanwhile at low Mo – content, the structure is not
defined and present high porosity.
According to the analysis made from the results of X-ray Diffraction (XRD), Raman
spectroscopy and Scanning Electron Microscopy (SEM), the changes of structural
properties evidence the predominant role of the Mo species and the presence of the
modified Ce8Mo12O49 structure.
The catalytic behavior of the CZM2(I), CZM3
(I) and CZM4(I) samples was also evaluated at
three temperatures (450°C, 475°C and 500°C) using the setup presented in Figure 11. The
toluene’s feed rate was 30L/min in liquid phase and oxygen’s feed rate was 5mL/min in
gas phase, nitrogen as internal standard and argon as balance gas were also feed. Taking
a b c
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 55
into account the stabilization of the pump, it was started at the same time as the reactor
oven, but the reactants where mixed after the reactor reach desired temperature. Once the
reactants mixture stabilized, the tramps were connected and every 30 minutes the liquid
products where collected. The other two temperatures were evaluated by the same
procedure but the reactor temperature changes was performed without toluene feed. The
reactants were feeding and mixed until stable regime after reach the desirable conditions.
Test methodology schema is resumed in figure 20.
Figure 20. Catalytic test temperature profile vs time
For each test 0.1 g of powder sample of solid catalyst was load. W/F ratio was keep at
0.162 kgh-1m-3, and the constant GHSV relation was fixed at 20360 h-1. The catalytic
behavior results of the different mixed oxides are summarize in Table 3.
Table 3. Catalytic test results of: CZM2(I), CZM3
(I), CZM4(I) samples.
Catalyst Conversion (%) Selectivity (%)
450°C 475°C 500°C 450°C 475°C 500°C
CZM2(I) 31 35 39 05 05 05
CZM3(I) 32 32 36 13 12 11
CZM4(I) 36 37 37 12 12 10
0
100
200
300
400
500
600
0 100 200 300 400 500
Tem
per
atu
re (
°C)
Time (minutes)
O2, C7H8, N2 and Ar feeding
O2, C7H8, N2 and Ar feeding
O2, C7H8, N2 and Ar feeding
56 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
As it is present, the conversion variation between the different samples is less than 10%,
then the selectivity may be the key result to take into account to choose the catalyst whit
best performance.
Catalytic test results present small variations related to cerium, zirconium or molybdenum
load of the samples, but it is not conclude. It does not exhibit a direct relation between metal
composition and selectivity, neither to benzaldehyde nor CO and CO2, as it is present in
annex B.
By the other hand, selectivity results are not favorable comparing with reported results for
other catalysts (20-80% [57]). As it was state by the characterization analysis, the
molybdenum species are predominant on the catalyst surface, promoting total oxidation,
then the change during the materials synthesis may affect the catalysts performance.
2.3.2 Pseudo sol gel method catalysts
A second family of catalyst of three different Ce-Zr-Mo oxides with the same theoretical
molecular formula, but using the pseudo sol gel method, presented in Figure 13, was
synthetized. The information of the samples is summarized in Table 4.
Table 4. Samples synthetized by the pseudo sol-gel method
Theoretical molecular formula
Name
Expected metal ratio
(Mmol/Zrmol)
Real metal ratio (M mol/Zr mol)
Ce Zr Mo Ce Zr Mo
CeZrMo2O10 CZM2II 1 1 2 1.04 1.00 2.01
CeZrMo3O12 CZM3II 1 1 3 1.05 1.00 3.01
CeZrMo4O16 CZM4II 1 1 4 1.0 1.0 3.90
The three new catalysts were analyzed by X-ray Diffraction (XRD). The CMZ2(II), CMZ3
(II)
and CMZ4(II) XRD diffraction patterns are present in figure 21.
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 57
Figure 21. Diffractograms of fresh mixed oxides: CZM2(II), CZM3
(II), CZM4(II).
–MoO3 ■Ce(MoO4)2 ♦Ce8Mo12O49 ●Ce2(MoO4)2(Mo2O7) ▲Zr(MoO4)2
The preparation method allows the crystallization of the mixed oxide with phase rejection,
as MoO3 is detected in CMZ4(II) X-ray diffraction pattern. By the other hand, the
diffractograms change significantly comparing with the presented for the samples, CMZ2(I),
CMZ3(I) and CMZ4
(I). In this case, peaks change drastically as the composition variate. For
low Mo content (sample CMZ2(II)) X-ray diffraction patterns evidence the presence of binary
compounds of the different oxides. As the Mo content grows, Ce - Mo binary oxides are
preferred. For CMZ3(II) sample, Ce(MoO4)2 is predominant, but as Mo load increase for
CMZ4(II) its diffractogram became more likely to the CMZX
(I) samples X-ray patterns.
The main difference between the X - ray diffraction patterns of the family samples lead a
structural effect as result of the synthesis method. This assumption was confirmed by
electron diffraction pattern of two samples with the same theoretical formula, but different
synthesis procedure. The images, presented in Figure 22 where taken on CZM2(I) and
CZM2(II) samples. These micrographics confirm the poly crystallinity of both materials,
although CZM2(I) sample present less phases than CZM2
(II) sample.
15 25 35 45 55 65 75 85
Inte
nsi
tiy
(a.u
.)
2 (°)
CZM4(II)
CZM3(II)
CZM2(II)
58 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
Figure 22. Electron diffraction of fresh mixed oxides: a. CZM2(I), b. CZM2
(II)
The same result was obtain by Transmission electron microscopy (TEM). The micrographs
of CZM2(I) and CZM2
(II) samples are present in Figure 23. The images corroborate the poly
crystallinity of both materials, although CZM2(I) sample present less different kind of crystals
with 20 nm average diameter. .
Figure 23. TEM micrographs of fresh mixed oxides: a. CZM2(I), b. CZM2
(II)
The synthesis method effect was also evidenced by Scanning Electron Microscopy, Figure
24 present the micrographs of the CZM2(I).and CZM2
(II) samples with equivalent theoretical
a b
a b
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 59
formula, but different synthesis procedure, these images exhibit structures completely
different, with more dispersed grains (internal diameter of 0.3m) for the traditional
synthesis methodology.
Figure 24. SEM micrographs of fresh mixed oxides: a. CZM2(I), b. CZM2
(II)
Finally, at the same conditions, given by figure 20, the catalytic tests were made,
maintained the catalyst load at 0.1 g for each test (particle diameter between 315m and
500m), W/F ratio was keep at 0.162 kg.h-1.m-3, and GHSV relation constant at 20360 h-1.
The general results of the different catalytic test are summarize in Table 5.
Table 5. Catalytic test results of: ZM2(I), CZM3
(I), CZM4(I) samples
Catalyst Conversion (%) Selectivity (%)
450°C 475°C 500°C 450°C 475°C 500°C
CZM2(II) 33 40 41 23 17 13
CZM3(II) 36 38 39 24 24 18
CZM4(II) 35 38 41 26 16 06
The conversion present for the different samples had not change dramatically, leading once
again the selectivity as the key factor to evaluate the catalysts performance. The results
clearly change comparing with the obtained for CMZ2(I), CMZ3
(I) and CMZ4(I)samples. For
a b
60 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
these cases is evidenced an inverse relation between selectivity and temperature, but it is
not precise the role of each metal on the reaction selectivity.
Once again none of the evaluated metal oxides reach high selectivity comparing with similar
catalyst (20-80%) [57]. The main reason for this situation may be the metal ratio proposed
for the formulated catalyst, previous studies referred Mo/(Ce+Zr)=1 as a suitable relation
for toluene partial oxidation catalyst [57]. However, only CMZ2(I) and CMZ2
(II) samples have
this metal ratio, but both of them exhibit the lowest selectivity comparing with CMZ3(x) and
CMZ4(x) samples. Then, even when the catalyst metal ratio match this relationship, the
solids formulations proposed so far may not have the most selective metal oxides
proportions.
It is important to notice that not only the metal content of the solids affects the catalytic
performance, but also the species remaining on the surface of the materials as it was
showed. For CMZx(I) samples, the synthesis method lead Mo- compounds on the surface
which improve the CO and CO2 selectivity comparing with CMZx(II) samples.
2.3.3 Influence of cation integration vs deposition
Ce-Mo mixed oxides deposits on Ce – Zr mixed oxides supports have been proposed [57].
Among the different formulations valuated, the best results obtained was for
Ce2(Mo3O4)3/(Ce0.65Zr0.35O2). Giving the great influence of the synthesis method on the
catalyst performance, two samples of this formulation were prepared:
The first one was deposits, the Ce-Zr oxide support was previously synthetized using the
pseudo sol like method and subsequently the active phase was introduced on the support
by the inclusion of the Ce-Mo propionate resins, as it is present by figure 26. The second
sample, with the same Ce-Mo-Zr oxides content was prepared following the pseudo sol gel
method (Figure 13).
For both cases the precursor salts used were cerium (III) acetate hydrate, zirconium (IV)
acetylacetonate and bis-acetylacetonate of molybdenum dioxide, the elimination of the
organic solvent was carried-out by vacuum distillation and after so the gel obtained was
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 61
calcined at 550°C to prevent Mo-compounds sublimation [84] for 6 h, using a heating rate
of 2 °C.min-1.
time (h)
Task 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Precursor salts
disolution
Mix of the propionates
Ce propianate
mix
Zr propianate
mix
Mo propianate
mix
Vacuum distillation
Mix of support and
active phase
Calcination
Figure 25. Global catalyst preparation procedure for deposited catalysts, following the pseudo sol gel method
Table 6 presents the expected composition of the samples and their real composition
calculated with the quantity and purity of the primary salts.
Table 6. Third catalyst family synthetized
Theoretical molecular formula Name
Theoretical metal ratio
(Mmol/Zrmol)
Expected metal ratio
(M mol/Zr mol)
Ce Zr Mo Ce Zr Mo
Ce2(Mo3O4)3/(Ce0.65Zr0.35O2) CM/CZI 7.6 1 8.6 7.78 1.00 8.50
Ce2.65Zr0.35Mo3O14 CM/CZII 7.6 1 8.6 7.75 1.0 8.55
The CM/CZI and CM/CZII catalysts were analyzed by X-ray Diffraction (XRD) (see Figure
26). The preparation method allows the crystallization of the mixed oxides without any
62 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
phase rejection and no single oxides are detected (CeO2, ZrO2,MoO3). For both samples it
is present peaks related with binary compound of the three metals, for the CM/CZ(I) sample
the Mo based compounds diffraction pattern are preferred, meanwhile for the CM/CZ(II)
sample, present less well defined peaks, related with less crystallinity.
Figure 26. Diffractograms of fresh mixed oxides: CM/CZ(I), CM/CZ(II). ■Ce(MoO4)2 ♦Ce8Mo12O49 ●Ce2(MoO4)2(Mo2O7) ▲Zr(MoO4)2
The difference between the X - ray diffraction patterns of the samples lead a structural effect
as result of the synthesis method. This correlation is also confirm by electron diffraction
made to both samples, showed in Figure 27.
Figure 27. Electron diffraction of fresh mixed oxides: a.CM/CZ(I), b. CM/CZ(II).
15 25 35 45 55 65 75 85
Inte
nsi
tiy
(a.u
.)
2 (°)
a b
CM/CZ(II)
CM/CZ(I)
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 63
The electron diffraction of fresh CM/CZ(I) and CM/CZ(II) samples, confirm the poly
crystallinity of both materials. Although CM/CZ(II) sample present more intense peaks,
perhaps related to binary well crystalize compounds.
Transmission electron microscopy (TEM) analysis images of fresh CM/CZ(I) and CM/CZ(II)
samples are present in Figure 28.
Figure 28. TEM micrographs of fresh mixed oxides: a. .CM/CZ(I), b. CM/CZ(II).
The micrographs confirms the presence of different kind of crystals. For both materials, well
faceted crystals are evidenced. In the images trapezoidal well defined grains are present
(particle dimensions: 10nm to 20 nm average) related to Mo-compounds. For the CM/CZ(II)
sample another well-defined kind of grain is evidence (internal diameter: 20nm average)
related to fluorite type cubic Ce-Zr compounds.
Scanning Electron Microscopy (Figure 29) present the micrographs of fresh CM/CZ(I) and
CM/CZ(II) samples, which exhibit ceramic structures completely different, leading
heterogeneous surface for CM/CZ(I) sample and dispersed grains for CM/CZ(II) sample.
Therefore, it is possible to state that synthesis method affect the final oxide obtained.
a b
64 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
Figure 29. SEM micrographs of fresh mixed oxides: a. .CM/CZ(I), b. CM/CZ(II).
Finally, at the same conditions, given by figure 20, the catalytic tests were made,
maintained the same stipulations of catalyst load at 0.1 g for each test (particle diameter
between 315m and 500m), W/F ratio was keep at 0.162 kg.h-1.m-3, and GHSV relation
constant at 20360 h-1. The general results of the different catalytic test are summarize in
table 7 and annex B.
Table 7. Catalytic test results of .CM/CZ(I) and CM/CZ(II)samples
Catalyst Conversion (%) Selectivity (%)
450°C 475°C 500°C 450°C 475°C 500°C
CM/CZ(I) 41 45 47 12 11 07
CM/CZ(II) 37 37 40 28 24 20
When the formulated catalysts were evaluated at the same conditions as the others
catalysts, the results remained in similar ranges. This behavior implies that the GHSV
condition affects directly the conversion reach by the evaluated catalyst, as Duarte stated
[57].
The catalytic test results are very promising, because the highest selectivity was reach by
mixture of the three metal oxides, not by the deposited catalyst. It is also important to notice
the similar results obtained by CMZ3(I) sample and CM/CZ(I) sample, which may suggest a
different relationship between the metal content and the selectivity to benzaldehyde.
As it was state before, the synthesis method lead different surface species which affect the
catalytic performance of the materials. For instance, the effect of cation deposition leads
a b
2. Vapour phase toluene partial oxidation: the reaction at laboratory scale 65
Mo-Ce compounds on the solid surface as it was proved by XRD, SEM and TEM analysis.
These species improve the CO and CO2 selectivity, unlike the cation integrate samples.
As the best catalytic results were obtained with CM/CZ(II) sample, this one was select to do
the kinetic study.
3. Kinetic study
The design of a catalytic reactor involves the need for a correlation relating the reaction
rates and the partial pressures of the reactants and products, for a particular type of
catalyst. The most widely used rate expressions are those based on the extended Langmuir
Hinschelwood theory, which are often referred to as Hougen and Watson rate expressions
(LHHW) [95]. The selection of the most appropriate rate expression is a lengthy process,
involving postulation of reaction mechanisms, isothermal regression for discarding
obviously inappropriate mechanisms, nonisothermal regression, and the use of
physicochemical criteria for selecting the most appropriate one from among competing
mechanisms [95].
Global expressions takes into account mass and heat transfer phenomena, which is related
to extrinsic effects such as diffusional limitations. Also, it considers temperature and
interactions between reactants and catalytic surface, among others effects which influences
directly the reaction rate and are associate to the intrinsic phenomena [96]. When reactor
and physical effects are not include in the mathematical equation, the expression is related
to the intrinsic rate [24]. In this chapter, an intrinsic kinetic correlation is developed.
3.1. Data collection
3.1.1. Experimental procedure
To obtain the kinetic model, three experiments were carried-out using the setup presented
in Figure 11 using 0.1g of catalyst (particle diameter between 315m and 500m), at three
different temperatures (450°C, 475°C and 500°C). For each experiment liquid toluene feed
was fixed (30, 40 or 50 L), and oxygen feed was increasing from 5 to 9 mL, nitrogen as
internal standard and argon as balance gas were also feed to the reactor, keeping constant
the gas flow (25mL/min) . Each arrangement was carried at three different temperatures
(450°C, 475°C and 500°C)
Taking into account the stabilization of the pump, it was started at the same time as the
heating procedure of the reactor oven using a by-pass. The reactants where mixed after
the reactor reach the desired temperature. Once the reactants mixture stabilized, the
tramps were connected and every 50 minutes the liquid products where collected. At each
condition two samples of recollected liquid products were taken.
68 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
The other two oxygen partial pressure were evaluated by the same procedure: for 20
minutes the oxygen inlet changes without toluene feeding, after, the reactants were feeding
and mixed until stable regime and then it was the data taken. Test methodology schema is
resumed in figure 30.
Figure 30. Test methodology schema.
The information related for each experiment is summarized in annex C.
3.1.2 Experimental results
The different experiments lead the oxygen and toluene dependency for the kinetic rate, as
it is present in figure 31, which summarize the results obtained for experiments made at
fixed oxygen feed rate of 7mL/min.
Figure 31 Kinetic rate’s toluene partial pressure dependency evaluated at 7mL/min oxygen feed rate. ▲450° ●475°C ♦500°C
0
1
2
3
4
5
6
7
8
9
10
0 100 200 300 400 500
Oxy
gen
fee
din
g (m
L)
time (min)
0
5
10
15
20
25
0 0.05 0.1 0.15 0.2 0.25 0.3
r (
mo
l/s*
g) (
E-6)
Toluene partial pressure (atm)
O2, C7H8, N2 and Ar feeding
O2, C7H8, N2 and Ar feeding
O2, C7H8, N2 and Ar feeding
3..Kinetic study 69
Figure 32 presents the results for experiments made at fixed toluene feed rate of 30L/min.
Figure 32 Kinetic rate’s oxygen partial pressure dependency at 30L/min toluene feed rate. ♦450°C ■475°C●500°C
It is important to notice the different effect that implies the increment of each
reactant in the kinetic rate. The direct relationship implies an important role in the
kinetic mechanism for each case.
3.2 Determination of kinetic parameters
Different models may be evaluated using the method of Least Squares, which is a
procedure to determine the best fit line to data. The proof uses simple calculus and linear
algebra. The basic problem is to find the best fit straight line y = ax + b given that, for n ∈
{1, . . . , N}, the pairs (xn, yn) are observed.
The method easily generalizes to finding the best fit of the form:
𝑦 = 𝑎1𝑓1(𝑥) + ⋯+ 𝑐𝑘𝑓𝑘(𝑥);
It is not necessary for the functions fk to be linearly in x – all that is needed is that y is to be
a linear combination of these functions.
The parameters for each model were obtained minimizing the relative error between the
calculated rate and the experimental rate.
0
2
4
6
8
10
12
14
16
18
20
0.00 0.05 0.10 0.15 0.20 0.25 0.30
r (m
ol/
s*g)
(E-
6)
Oxygen partial pressure (atm)
70 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
3.2.1 Power law rate expression
The power law rate model is the simplest expression, which links the reaction rate and the
reactants pressure by reaction orders. The relation is giving by equation 1:
𝑟𝑇 = 𝐾𝑃𝑂2𝛼 𝑃𝑇
𝛽 Eq.1
In this case K is the specific reaction rate constant and is given by the Arrhenius Equation
[96]. The reaction orders are and . These number may be enters but it is not mandatory.
Table 8 summarize the parameters obtained by mathematical regression.
Table 8. Power law model kinetic parameters obtained
Temperature (°C)
Kinetic parameter
K
450 6.91E-4 0.1157 2.5329 475 4.33E-4 0.11088 2.0429 500 7.66E-4 0.1136 2.3202
The fitting of the model may be evaluated by parity diagram. The adjust of the calculated
rates and the experimental data is giving by the coefficient of determination, denoted R2
which is a number between 0 to 1 that indicates the proportion of the variance in the
regression. The results are present in figure 33.
Figure 33. Parity diagram for power law kinetic model
3..Kinetic study 71
As the coefficient of determination is not near to 1, which implies good accuracy of the
model, the orders were fixed to entire numbers as it was proposed for others studies which
showed that the reaction was second order with respect to the hydrocarbon and zero order
with respect to oxygen [81] and the obtained numbers were near to those ciphers. The new
calculated parameters are present in table 9.
Table 9. Modified Power law model kinetic parameters obtained
Temperature (°C)
Kinetic parameter
K
450 2.58E-4 0 2 475 4.01E-4 0 2 500 4.83E-4 0 2
The parity diagram present a higher determination coefficient, nevertheless, the correlation
is very small, which implies the assumptions to simplify the model are not accurate
Figure 34. Parity diagram for modified power law kinetic model
R² = 0.6603
5.0E-06
1.0E-05
1.5E-05
2.0E-05
2.5E-05
3.0E-05
3.5E-05
4.0E-05
5.0E-06 1.0E-05 1.5E-05 2.0E-05 2.5E-05
calc
ula
ted
rat
e (m
ol/
s*g)
observate rate (mol/s*g)
72 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
3.2.2 LHHW kinetic expression with two active sites
The Langmuir – Hinshelwood – Hougen –Watson (LHHW) model is based on a continuum
model, in which the surface of the catalyst is described as an array of equivalent sites which
do interact neither before nor after chemisorption. Furthermore, the derivation of rate
equations assumes that both reactants and products are equilibrated with surface species
and that a rate-determining step can be identified. Surface coverages are correlated with
partial pressures in the fluid phase by means of Langmuir adsorption isotherms.[96, 97].
As the model leads several hypotheses, the first approach will be taking into account the
following assumptions:
1. Two active site type
2. Non dissociative toluene adsorption.
3. Dissociative oxygen adsorption
4. Surface reaction as controlling step
The LLHW kinetic expression associate to these statements is represented by equation 2:
𝑟𝑇 =𝐾𝑘𝑇𝑃𝑇𝑘𝑂2𝑃𝑂2
(1+𝑘𝑇𝑃𝑇)∗(1+√𝑘𝑂2𝑃𝑂2) Eq.2
Table 10 summarize the parameters obtained by mathematical regression.
Table 10. Two active sites LHHW model kinetic parameters obtained
The parity diagram presented by figure 35 exhibit low determination coefficient, as the fitting
is not good enough, then the assumptions should be changed.
Temperature (°C)
Kinetic parameter K kT KO2
450 1.18E-2 1.21E-2 2.23E-2 475 7.87E-3 2.37E-1 2.37E-1 500 1.57E-2 1.85E-1 1.77E-1
3..Kinetic study 73
Figure 35. Parity diagram for two active sites LHHW kinetic model
3.2.3 LHHW kinetic expression with dissociative oxygen adsorption
Several theories may be consider in order to obtain a kinetic expression, nevertheless,
taking into account the probable reaction mechanism, the proposed assumptions for the
second LHHW model evaluated are:
1. Only one active site type
2. Non dissociative toluene adsorption.
3. Dissociative oxygen adsorption
4. Competitive toluene and oxygen adsorption.
5. Surface reaction as controlling step
The LLHW kinetic expression associate to these assumptions is giving by equation 3:
𝑟𝑇 =𝐾𝑘𝑇𝑃𝑇𝑘𝑂2𝑃𝑂2
(1+𝑘𝑇𝑃𝑇+√𝑘𝑂2𝑃𝑂2)2 Eq.3
The mathematical treatment of the data leads the regression of the different constants and
as result, the parity diagram presented by figure 36 exhibit higher determination coefficient,
but the fitting may improve, then the assumptions should be changed.
R² = 0.7447
5.0E-06
1.0E-05
1.5E-05
2.0E-05
2.5E-05
3.0E-05
5.0E-06 7.0E-06 9.0E-06 1.1E-05 1.3E-05 1.5E-05 1.7E-05 1.9E-05 2.1E-05 2.3E-05 2.5E-05
calc
ula
ted
rat
e (m
ol/
s*g)
observate rate (mol/s*g)
74 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
Figure 36. Parity diagram for oxygen dissociative adsorption LHHW kinetic model
The kinetic parameters for the proposed model are present in Table 11 11.
Table 11 Dissociative oxygen adsorption LHHW kinetic parameters obtained
Temperature (°C)
Kinetic parameter K kT KO2
450 7.50E-3 1.43E-2 32.95 475 1.02E-2 1.99E-2 11.18 500 1.06E-2 2.05E-2 14.50
3.2.4 LLHW kinetic expression with non-dissociative oxygen adsorption
The last LHHW model evaluated includes the following assumptions:
1. Only one active site type
2. Non dissociative toluene adsorption.
3. Non dissociative oxygen adsorption
4. Competitive toluene and oxygen adsorption.
5. Surface reaction as controlling step
The LLHW kinetic expression associate to these assumptions is:
𝑟𝑇 =𝐾𝑘𝑇𝑃𝑇𝑘𝑂2𝑃𝑂2
(1+𝑘𝑇𝑃𝑇+𝑘𝑂2𝑃𝑂2)2 Eq.4
R² = 0.9517
1.0E-05
1.2E-05
1.4E-05
1.6E-05
1.8E-05
2.0E-05
2.2E-05
2.4E-05
2.6E-05
1.0E-05 1.2E-05 1.4E-05 1.6E-05 1.8E-05 2.0E-05 2.2E-05 2.4E-05
r ca
lc (
mo
l/s*
g)
observate rate (mol/s*g)
3..Kinetic study 75
Table 12 summarize the parameters obtained by Least Squares mathematical regression
of non-dissociative oxygen adsorption LHHW kinetic model.
Table 12. Non-dissociative oxygen adsorption LHHW kinetic parameters obtained
Temperature (°C)
Kinetic parameter
K kT kO2
450 6.77E-2 3.69E-2 34.01 475 1.55E-2 1.86E-1 27.50 500 6.76E-3 6.84E-1 40.14
The parity diagram for the non-dissociative oxygen adsorption LHHW kinetic model,
presented by figure 37 have almost the same determination coefficient than dissociative
oxygen adsorption LHHW kinetic model parity diagram. Although, only the specific reaction
rate constant “K” present a direct relationship with the temperature change and its variations
exhibit a clear tendency. The other two parameters kT and kO2 do not show any tendency
and may be product of mathematical processes but not reveal physical meaning.
Figure 37. Parity diagram for non- dissociative oxygen adsorption LHHW kinetic model
3.2.5 Mars and Van Krevelen kinetic expression
For M&VK mechanism the assumptions [82] were already listed:
R² = 0.9579
5.0E-06
1.0E-05
1.5E-05
2.0E-05
2.5E-05
3.0E-05
5.0E-06 1.0E-05 1.5E-05 2.0E-05 2.5E-05
calc
ula
ted
rat
e (m
ol/
s*g)
observate rate (mol/s*g)
76 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
(1) The reaction in step I is first order with respect to the reactant and the fraction of sites
covered by oxygen ();
(2) Certain lattice O ions at the surface are involved in the oxidation reaction in step I;
(3) Only O ions (atoms) are assumed to exist on these sites (no reactant or products);
(4) The rate of surface re-oxidation (step II), i.e., O2 adsorption, is proportional to 𝑃𝑂2𝑛 and
to the concentration of active sites not covered by oxygen (1-).
The M&VK kinetic expression associate to these assumptions is represented by the
equation 5:
𝑟𝑇 =1
1
𝑘1𝑃𝑇+
𝛽
𝑘2𝑃𝑂2𝑛
=𝑘1𝑘2𝑃𝑇𝑃𝑂2
𝑛
𝑘1𝑃𝑇+𝑘2𝑃𝑂2𝑛 Eq.5
The mathematical regression for M&VK kinetic model leads the parameters present in
Table 13.
Table 13. M&VK kinetic parameters obtained
Temperature (°C)
Kinetic parameter n k1 k2
450 2.2184 6.10E-05 6.04E-3 475 2.4672 8.81E-05 5.85E-3 500 1.9727 9.53E-05 6.37E-3
The parity diagram for the M&VK kinetic model, presented by figure 38. The regression
leads second order respect the oxygen, then it is fixed to an entire number to recalculate
the parameters.
3..Kinetic study 77
Figure 38. Parity diagram for M&VK kinetic model
The new results are summarized in table 14:
Table 14. Modified M&VK kinetic parameters obtained
Temperature (°C)
Kinetic parameter n k1 k2
450 2 6.47E-05 2.33E-3 475 2 8.87E-05 2.62E-3 500 2 1.01E-04 4.10E-3
The parity diagram for the modified M&VK kinetic model, presented by figure 39 have
almost the same determination coefficient than non-dissociative oxygen adsorption LHHW
kinetic model and M&VK model without modifications, nevertheless, the new parameter
obtained present a clear tendency with the increment of the temperature, which is a regular
behavior for kinetic constants (Arrhenius theory).
78 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
Figure 39. Parity diagram for modified M&VK kinetic model
As long, as the kinetic parameters where regressed separately for each temperature
evaluated, Figure 40 presents the parity diagram for experiments made at 475°C.
Figure 40. Parity diagram for M&VK kinetic regression for experiments made at 475°C
The good accuracy of the model is also evidenced in figure 41 and 42.
R² = 0.967
9.0E-06
1.1E-05
1.3E-05
1.5E-05
1.7E-05
1.9E-05
2.1E-05
2.3E-05
2.5E-05
2.7E-05
1.0E-05 1.2E-05 1.4E-05 1.6E-05 1.8E-05 2.0E-05 2.2E-05 2.4E-05
calc
ula
ted
rat
e (m
ol/
s*g)
observate rate (mol/s*g)
R² = 0.9987
9.0E-06
1.1E-05
1.3E-05
1.5E-05
1.7E-05
1.9E-05
2.1E-05
2.3E-05
1.0E-05 1.2E-05 1.4E-05 1.6E-05 1.8E-05 2.0E-05 2.2E-05
cula
ted
rat
e (m
ol/
s*g
)
observate rate (mol/s*g)
3..Kinetic study 79
Figure 41 presents the experimental data related to experiments made at oxygen feed rate fixed and the fitting for M&VK model.
Figure 41. M&VK fitting model for experiments made at 7mL/min oxygen feed rate. ▲450° ●475°C ♦500°C
Figure 42 presents the experimental data related to experiments made at toluene feed rate fixed and the fitting for M&VK model.
Figure 42. M&VK fitting model for experiments made at 30L/min toluene feed rate. ♦450°C ■475°C●500°C
0.0E+00
5.0E-06
1.0E-05
1.5E-05
2.0E-05
2.5E-05
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
r (
mo
l/s*
g)
Toluene partial pressure (atm)
0.0E+00
5.0E-06
1.0E-05
1.5E-05
2.0E-05
2.5E-05
0.00 0.10 0.20 0.30
r (m
ol/
s*g)
Oxygen partial pressure (atm)
4. Conclusions and findings
It was synthetized three different families of Ce-Zr-Mo mixed oxides, two by pseudo sol gel
like method and a third one by a modified pseudo sol gel procedure. For all the cases the
calcination temperature was 550°C.
The theoretical composition proposed, as well as the synthesis methodology lead to stable
oxides at reaction conditions as it was proved by TPO and TPR analysis. Nevertheless, the
solids exhibit different phases and polycrystallinity.
After the characterization and catalytic evaluation of the Ce-Zr-Mo catalyst it is possible to
state that synthesis method affect the final oxide obtained and its catalytic performance.
The synthesis method lead different surface species which affect the catalytic performance
of the materials. The modified pseudo sol gel method and the cation deposited strategy
leads Mo-Ce compounds on the solid’s surface as it was proved by XRD, SEM and TEM
analysis. These species on the catalyst surface promoting total oxidation.
For further researches it is suggested to evaluate the influence of each metal load.as the
catalytic test results present small variations related to cerium, zirconium or molybdenum
load of the samples, but it is not conclude. It does not exhibit a direct relation between metal
composition and conversion or selectivity, neither to benzaldehyde nor CO and CO2.
The kinetic study was made for the most selective catalyst evaluated (Ce2.65Zr0.35Mo3O14).
The mathematical regression for Mars & Van Krevelen kinetic model present less relative
error respect with the experimental data, than three LHHW kinetic models proposed and
power law rate model. The M&VK parameters obtained present a clear tendency with the
increment of the temperature, which is a regular behavior for kinetic constants (Arrhenius
theory).
Determination coefficient for Mars & Van Krevelen kinetic model obtained was 0.96, which
implies low correlation with the model proposed. Nevertheless, the kinetic expression only
consider the reactants effect. It is suggest for further research to include the influence of
the main products (benzaldehyde, CO, CO2 and water) on the kinetic study.
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Annexes 91
Annexes
A. Chromatographic methods
The non-condensable and condensable products were analysed by gas phase
chromatography (GPC), using in both cases Network GC system Agilent Technologies
6890N. For the non-condensable products a TCD detector was used; while for the
condensable products a FID detector was employed. Next are presented the
chromatographic method, retention times and response factors of the different products
analysed for each case.
i. Non-condensable products analysis
The non-condensable products analysed were H2, O2, CO, CO2, and N2 (internal standard).
The column used was a Carbosieve II of dimensions: 2m, i.d. 1/8".
The chromatographic method was the following:
Injection: 150 °C
Carrier Gas: Ar, flow: 15.3 ml min-1
Oven program: 90 °C for 5 min
from 90°C to 210 °C at 10 °C min.-1
210 °C for 3 min
Detector: TCD 250 °C
In Table 15 are reported the retention times according the method above described. The
molar response factors with N2 as internal standard are also presented.
Table 15. Retention times and response factors utilised for the non-condensable products.
Product Retention time
(tR) Response factor
(molar)
H2 1.3 0.099 O2 4.7 0.88 N2 5.0 1 CO 5.9 1.104 CO2 11.0 0.078
92 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
ii. Condensable products analysis
The condensable products analysed were benzene, toluene and benzaldehyde isopropanol
(internal standard). The column was a SGE Sol gel Wax of dimensions: 60m x 0.25mm,
i.d.0.25m.
The chromatographic method was the following:
Injection: 250 °C; split ratio: 120:1; Flow: 109 ml min-1
Carrier Gas: He, flow: 0.9 ml min-1
Oven program: 100 °C for 2 min
from 100 °C to 130 °C at 20 °C min-1
from 130 °C to 280 °C at 60 °C min-1
280 °C for 4 min
Detector: FID 250 °C; 40 ml min-1 H2
In table 11 are reported the retention times of the condensable products, according the
method above described. The molar response factors with n-propanol as internal standard
are also included.
Table 16. Retention times and response factors utilised for the condensable products.
Product Retention time
(tR) Response factor
(molar)
Isopropanol 3.4 1 Benzene 3.46 0.313 Toluene 3.6 0.173
Benzaldehyde 5.5 0.219
Annexes 93
B. Catalytic tests results
Complement information about the catalytic tests made to the different evaluated materials is summarized in table 12.
Table 17. Catalytic tests results summary
Conversion Selectivity to Benzaldehyde Selectivity to CO Selectivity to CO2
Catalyst 450°C 475°C 500°C 450°C 475°C 500°C 450°C 475°C 500°C 450°C 475°C 500°C
CZM2(I) 31 35 39 05 05 05 29 28 29 17 15 16
CZM3(I) 32 32 36 13 12 11 24 24 24 13 13 14
CZM4(I) 36 37 37 12 12 10 16 21 21 17 20 20
CZM2(II) 33 40 41 23 17 13 20 21 23 9 10 14
CZM3(II) 36 38 39 24 24 18 26 16 20 5 11 9
CZM4(II) 35 38 41 26 16 06 18 25 33 8 4 4
CM/CZ(I) 41 45 47 12 11 07 26 27 27 18 18 17
CM/CZ(II) 37 37 40 28 24 20 19 17 19 5 8 12
94 Kinetic study of toluene partial oxidation to benzaldehyde,
using a Ce – Zr – Mo mixed oxide as a catalyst
C. Kinetic experimental results
The information taken for each experiment is summarize in Table 13:
Table 18. Experimental data results.
Experiment number
Temperature Oxygen feeding
Toluene feeding
W/FA0 X toluene rate
°C mL/min L/min kg*h/m3 mol/s.g (E-06)
1 450 5 30 0.31593 0.23 9.10
2 450 7 30 0.31593 0.27 10.64
3 450 9 30 0.31593 0.28 10.85
4 450 5 40 0.24294 0.22 11.08
5 450 7 40 0.24294 0.25 12.97
6 450 9 40 0.24294 0.23 11.56
7 450 5 50 0.1863 0.20 13.54
8 450 7 50 0.1863 0.21 13.89
9 450 9 50 0.1863 0.22 14.34
10 475 5 30 0.31593 0.33 13.00
11 475 7 30 0.31593 0.36 14.18
12 475 9 30 0.31593 0.38 14.85
13 475 5 40 0.24294 0.29 15.02
14 475 7 40 0.24294 0.30 15.40
15 475 9 40 0.24294 0.35 18.03
16 475 5 50 0.1863 0.24 16.30
17 475 7 50 0.1863 0.27 17.86
18 475 9 50 0.1863 0.31 20.67
19 500 5 30 0.31593 0.37 14.52
20 500 7 30 0.31593 0.46 18.21
21 500 9 30 0.31593 0.56 21.91
22 500 5 40 0.24294 0.35 17.96
23 500 7 40 0.24294 0.36 18.61
24 500 9 40 0.24294 0.38 19.42
25 500 5 50 0.1863 0.32 21.13
26 500 7 50 0.1863 0.33 21.72
27 500 9 50 0.1863 0.35 22.99