Synthesis of supported bismuth molybdate catalyst and the application in selective oxidation of propylene to acrolein

Synthesis of supported bismuth molybdate catalyst and the application in selective oxidation of propylene to acrolein Synthesis of supported bismuth molybdate catalyst and the application in selective oxidation of propylene to acrolein luận văn tốt nghiệp thạc sĩ

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF TECHNOLOGY

Truong Duc Duc

SYNTHESIS OF SUPPORTED BISMUTH MOLYBDATE CATALYST AND THE APPLICATION IN SELECTIVE OXIDATION OF PROPYLENE TO

ACROLEIN

Tổ ng hợ p vậ t liệ u Bismuth Molybdate trên chấ t mang và ứ ng dụ ng làm xúc

tác cho phả n ứ ng oxi hóa chọ n lọ c propylene thành acrolein

Major: Chemical engineering

MSC THESIS

Supervisor:

Dr Le Minh Thang Invited supervisor :

Assoc Prof Anders Riisager

Hanoi, 10/2009

The subject of this thesis is the synthesis of high surface area bismuth molybdatebased catalysts Two synthesis directions this leads to the synthesis have beencarried out One is based on the idea of reducing the particle size of catalyst,nanometer sized -Bi2MoO6 crystals with dimension of 50nm were synthesized byhydrothemal treatment Another is based on the idea of impregnating bismuthmolybdate on high surface area supports, silica supported -Bi2Mo2O9 and zirconiasupported -Bi2Mo2O9 catalysts with various percentage of -Bi2Mo2O9 loadingcontents were therefore synthesized Both works aimed to investigate the influence

of surface area of catalyst on their catalytic activities in the selective oxidation ofpropylene to acrolein

In this thesis, bismuth molybdate catalysts and their applications in the selectiveoxidation of propylene to acrolein were presented very detail The Mars and vanKrevelen mechanism as well as the important role of lattice oxygen were alsodiscussed in detail

Synthesis of mesoporous ZrO2 support by hydrothermal method was also a studyingtask since the synthesis of mesoporous ZrO2 ordinary work and requires a specialattention in order to obtain a high surface area and a stable pore structure

The application of supported bismuth molybdate catalysts in selective propyleneoxidation results is provided a better understanding about the role of lattice oxygenand surface area of catalyst Both surface area and lattice oxygen of catalyst play askey roles in the reaction Supported bismuth molybdate samples possess both highsurface area and high oxygen mobility resulted in an extraordinary increase ofcatalytic activities Among supports, ZrO2 is more efficient due to the presence ofsynergy effect

Tóm tắ t

Nộ i dung củ a luậ n văn này là tổ ng hợ p vậ t liệ u xúc tác trên cơ sở bismuth

molybdate có diệ n tích bề mặ t riêng cao Hai hư ớ ng tổ ng hợ p vậ t liệ u đã đư ợ c thự c

hiệ n Hư ớ ng thứ nhấ t dự a trên ý tư ở ng làm giả m kích thư ớ c hạ t xúc tác, tinh thể

-Bi2MoO6 có kích thư ớ c hạ t khoả ng 50nm đã đư ợ c tổ ng hợ p thành công bằ ngphư ơ ng pháp kế t tinh thủ y nhiệ t Hư ớ ng thứ hai dự a trên ý tư ở ng mang bismuth

molybdate lên các chấ t mang có bề mặ t riêng cao, các xúc tác bismuth molybdate

ngâm tẩ m trên silica và zirconia đã đư ợ c điề u chế vớ i các hàm lư ợ ng chấ t mang

khác nhau Cả hai hư ớ ng đề u nhằ m để khả o sát ả nh hư ở ng củ a diệ n tích bề mặ t

riêng củ a xúc tác đế n hoạ t tính xúc tác củ a nó trong phả n ứ ng oxi hóa chọ n lọ c

propylene thành acrolein

Trong luậ n văn này, hệ xúc tác bismuth molybdate và ứ ng dụ ng củ a nó trong phả n

ứ ng oxi hóa chọ n lọ c propylen thành acrolein đã đư ợ c đư a ra rấ t chi tiế t Cơ chế

Mars van Krevelen cũng như vai trò đặ c biệ t quan trọ ng củ a oxy mạ ng lư ớ i củ a xúc

tác cũng đư ợ c thả o luậ n mộ t cách cụ thể

Tổ ng hợ p chấ t mang mao quả n trung bình zirconia bằ ng phư ơ ng pháp kế t tinh thủ y

nhiệ t cũng nằ m trong nộ i dung nghiên cứ u vì việ c tổ ng hợ p vậ t liệ u mao quả n trung

bình ZrO2 đòi hỏ i sự quan tâm đặ c biệ t để duy trì diệ n tích bề mặ t cao và cấ u trúc

mao quả n ổ n đị nh

Kế t quả nghiên cứ u ứ ng dụ ng vậ t liệ u xúc tác trên cơ sở bismuth molybdate có diệ n

tích bề mặ t riêng cao trong phả n ứ ng oxi hóa chọ n lọ c propylene đã cung cấ p thêm

nhữ ng hiể u biế t rõ hơ n về vai trò củ a oxy mạ ng lư ớ i và bề mặ t riêng củ a xúc tác Cả

diệ n tích bề mặ t riêng và oxy mạ ng lư ớ i củ a xúc tác đề u đóng vai trò quyế t đị nh

trong phả n ứ ng Các mẫ u xúc tác ngâm tẩ m bismuth molybdate có cả diệ n tích bề

mặ t riêng lớ n và oxy mạ ng lư ớ i linh độ ng dẫ n đế n hoạ t tính xúc tác tăng đáng kể

Trong hai loạ i chấ t mang này thì ZrO2 có hiệ u quả hơ n bở i vì sự xuấ t hiệ n củ a hiệ u

ứ ng hiệ p trợ xúc tác giữ a chấ t mang vớ i pha phả n ứ ng

This thesis is submitted in candidacy for Master degree from project

“Prj.104.DAN.8.L.1604 - Attractive routes for selective catalytic oxidation of

hydrocarbons” cooperated between Hanoi University of Technology (HUT) and

Technical University of Denmark (DTU) The work presented herein has beencarried out at Lab of Petrochemicals and Catalysis Materials, Adsorption, Faculty ofChemical Engineering, Hanoi University of Technology under the supervision of

Dr Le Minh Thang The work is also performed in the cooperation with Center forSustainable and Catalysis Chemistry, Technical University of Denmark (DTU),Denmark

First and foremost, I would like to thank my supervisor Dr Le Minh Thangfor her exceptional patience with me At the beginning, I was “clumsy” and made a

lot of my own mistakes, but in spite of this, she always gives me a patience andprovides valuable guidance and support But the most valuable thing I have learnt

from her is “how to do science” I also would like to thank Dr Nguyen Hong Lien,

director of Lab of petrochemicals and catalysis, adsorption materials for her supportand patience as well as the great opportunity which she offered me to work in herlab during the last two years

During my master study, I also have a chance to be trained for six months atCenter for Sustainable and Catalysis Chemistry, Denmark On this period, I learnedmany things about Supported Ionic Liquid Phase Catalysts (SILP) for thehydroformylation of ethylene, a new research area, which provided very usefulcomplimented knowledge for my research direction

I am deeply grateful to Prof Rasmus Fehrmann in the DTU Kemi Centre forCatalysis and Sustainable Chemistry for giving me the opportunity to work at hislab and his kindly guidance, support and patience to me during my stay in Denmark

I am also thankful to Associate Prof Anders Riisager who gave me invaluablethoughtful insights, advice, support, discussions and encouragements from the firstdays I came to DTU

I am very gratefully for the kindness and help from all my colleagues andpartners I must thank Msc Nguyen Ha Hanh for her friendly guidance and support

in the time I joined the project I would especially thank Post Doc Olivier Nguyen

Van Buu, Post Doc Eduardo García Suárez, Post Doc Jianmin Xiong from CSClab for their friendship and generous help during the time I stayed in Denmark.

Special gratitude must be given to all the graduate students with whom Ihave collaborated in this project at Hanoi University of Technology for their greatassistance and cooperation Vo Hoang Tung, Ho Si Dang, Le The Duy, NguyenHoang Hai I enjoyed working together with all of you!

I am also grateful to the Danida Fellowship Centre (DSC) for financialsupport I especially want to express my sincere gratitude to Prof Vu Dao Thang,Prof Le Van Hieu for their support, encouragement and useful discussions

To my mother, father, mother-in-law, father-in-law, brothers and my bestfriends my warmest thanks for their love, encouragement and support during all theyears of my education

I sincerely thank my best friends Christian Juel Adamsen, his wife MarieLahn ج lgaard and their little angel daughter Alberte for their true love, warm

friendship and encouragements

The most important of all, I would like to thank my wife, Thanh Thuy, withall my love Her endless love, support, understanding and patience to me wereimmeasurable More than anything else, her love has carried me through the manychallenges I faced during my graduate years

Truong Duc Duc Hanoi, 10.2009

Index of Tables

Table 1.1 Thermodynamic parameters of the formation of other propylene oxidationproducts

Table 1.2 Some examples of multi-component BiMo based catalysts

Table 1.3 Apparent Activation Energies of Partial Oxidation of Propylene toAcrolein over Bismuth Molybdate Catalysts [98]

Table 2.1 Raw chemicals for synthesis of unsupported and supported bismuthmolybdates

Table 2.2 Equivalent amounts of chemicals corresponding to different samples.Table 2.3 Strong lines corresponding to different phase of bismuth molybdates andcubic phase zirconia

Table 2.4 : IR bands (cm-1) of bismuth molybdates

Table 2.5: Raman frequencies in cm-1 of molybdate species

Table 2.6: Raman frequencies of bismuth molybdates

Table 2.7 Retention time of some products and related compounds

Table 3.1 Summary of synthesized zirconia samples

Table 3.2 Summary of synthesized bismuth molybdate samples

Table 3.3 Specific surface area (SBET) of support and supported samples

Index of Figures

Figure 1.1 Bismuth vacant sites in α -Bi2Mo3O12 along b axis projection The solid

lines show theα -Bi2Mo3O12 unit cell while the dashed line is a unit cell of scheelite.Large solid circles represent occupied Bi sites while the small circles are the empty

Bi sites (Adapted from van den Elzen and Rieck [11])

Figure 1.2 The unit cell structure of α -Bi2Mo3O12 (left figure) and atom map of theunit cell (right figure) (Adapted from van den Elzen and Rieck)

Figure 1.3 A unit cell of β -Bi2Mo2O9 (right figure) and atom map of the unit cell(left figure) (Adapted from H.-Y Chen and A.W Sleight [16])

Figure 1.4 Representation of the β -Bi2Mo2O9 structure projected along (010) [17].The figure shows clusters of MoO4 tetrahedral to form Mo4O16 The small circlerepresents a bismuth atom

Figure 1.5 From left to right, a unit cell ofγ -Bi2MoO6 and its atom map The figure

is adapted from Teller et al [21]

Figure 1.6 Phase equilibrium diagram of the bismuth molybdate system

Figure 1.7 Molecule structure of acrolein

Figure 1.8 The reaction paths of the partial oxidation of deuterium-labelledpropylene, Z=kD/kH, while 1 and 3 are the numbers of carbon atoms wherehydrogen is abstracted [46]

Figure 1.9 The reaction paths of the formation of side products [46]

Figure 1.10 A schematic of the Mars and van Krevelen mechanism on bismuthmolybdate catalysts [88]

Figure 1.11 A schematic of bridges and doubly bonded oxygen ions on bismuthmolybdate catalysts [72]

Figure 1.12 A schematic of steps in propylene oxidation into acrolein over bismuthmolybdate catalyst [82]

Figure 1.13 A reaction mechanism of propylene oxidation into acrolein, showingacid-base and redox steps [72]

Figure 1.14 Schematic P-T phase diagram of ZrO2 [105]

Figure 1.15 Three phases of ZrO2

Figure 1.16 Schematic diagram of the mechanism of preparation of mesoporousZrO2 (suggested by Guorong Duan et al [108])

Figure 1.17 Mechanism for mesoporous zirconia synthesis by hydrothermal methodsuggested by Bao-Lian Su et al [109]

Figure 2.1 Diagram of preparation of bismuth molybdate by hydrothermal methodsFigure 2.2 Diagram of soxhlet extraction

Figure 2.3 A X-ray generated tube (a) theory diagram (b) a real tube

Figure 2.4: illustrates how diffraction of X-rays by crystal planes allows one toderive lattice by using Bragg relation (a) and real XRD partten (b)

Figure 2.5: Principle of infrared absorption

Figure 2.6: Principle of Raman scattering

Figure 2.7 Some typical types of isotherm

Figure 2.8 The BET model of multilayers adsorption

Figure 2.9: The BET plot

Figure 2.10 The interaction between the primary electron beam and the sample in anelectron microscope leads to a number of detectable signals

Figure 2.11 Schematic diagram of a tranmission electron microscope

Figure 2.12 Diagram of reactor setup

Figure 3.1 Influence of H2O/Zr in gel on surface area of products

Figure 3.2 XRD partten of zirconia prepared by hydrothermal (Z6)

Figure 3.3 .Absorption – desorption curve and pore distribution curve of

synthesized ZrO2 sample (Z6)

Figure 3.4 XRD pattern of ZrO2 sample at different calcination temperatures

Figure 3.5 Influence of calcinations temperature on surface area of samples

Figure 3.6 SEM image of zirconia calcinated at 580oC (Z7)

Figure 3.7 XRD patterns of samples (M2 – M6) at various crystallizing

temperatures

Figure 3.8 SEM images of samples at various crystallizing temperatures

Figure 3.9 XRD patterns of samples (M4, M7, M8, M9) at various calcinationtemperatures

Figure 3.10 SEM images of samples at various calcination temperatures

Figure 3.11 FT-Raman of samples (M4, M7, M8, M9) at various calcinationtemperatures

Figure 3.12 TG/DSC diagram of -Bi2MoO6sample (M4)

Figure 3.13 XRD parttens of samples at various pH value in gel

Figure 3.14 X-ray diffraction patterns of samples synthesized by sol-gel andhydrothermal methods

Fig 3.15 FT-Raman spectra of samples synthesized by sol-gel and hydrothermalmethods

Figure 3.16 FE-SEM images of samples synthesized by sol-gel and hydrothermalmethods

Figure 3.17 Specific surface area (SBET) of samples prepared by solo-gel andhydrothermal methods

Fig 3.18 Conversion of propylene (a) and rate of acrolein formation (b) oversamples synthesized by sol-gel and hydrothermal methods

Figure 3.19 XRD patterns of zirconia supported -Bi2Mo2O9samples (a) and silicasupported -Bi2Mo2O9samples (b)

Figure 3.20 FT-Raman diagrams of zirconia supported -Bi2Mo2O9samples (a) andsilica supported -Bi2Mo2O9samples (b)

Figure 3.21 FT-IR spectra of silica supported -Bi2Mo2O9samples

Figure 3.22 SEM images of SiO2 support (a) 10%beta/SiO2 sample (b) and40%beta/SiO2 sample (c)

Figure 3.23 SEM image of 40%beta/SiO2 sample

Figure 3.24 TEM image of 10%beta/SiO2

Figure 3.25 Mechanism of -Bi2Mo2O9 formation on the surface of SiO2 support

Figure 3.26 SEM images of 10%beta/ZrO2 (a) and 40%bete/ZrO2 (b)

Figure 3.27 A -Bi2Mo2O9 formation on ZrO2 support mechanism

Figure 3.28 The absorption – desorption curve and pore distribution curve of

Figure 3.34 Rate of acrolein formation in selective oxidation of propylen overzirconia supported beta bismuth molybdates and pure beta bismuth molybdateFigure 3.35 Comparison of conversion of propylene (a) and rate of acroleinformation (b) in selective oxidation of propylene between 40%beta/SiO2 and40%beta/ZrO2samples at different temperature

Figure 3.36 Reaction rate for propylene consumption at 500oC of samples withdifferent percentage of beta mixing with ZrO2 and SiO2 supports

Figure 3.37 Comparison of activation energy of pure -Bi2Mo2O9, -Bi2Mo2O9impregnated on SiO2 and ZrO2 samples

Abtract………2

Tóm tắ t………3

Preface ……….4

INTRODUCTION………15

CHAPTER I: LITERATURE REVIEWS ……….17

I.1 Bismuth Molybdates……… 17

I.1.1 Crystal Structure of Bismuth Molybdates ………17

I.1.2 Synthesis of bismuth molybdate……….22

I.1.3 Relationship between structure of Bismuth Molybdate and catalytic activity………26

I.1.4 The catalytic performance of bismuth molybdate……….29

I 2 Selective oxidation of propylene to acrolein ……… 31

I.2.1 Acrolein and its production……… 31

I.2.2 Thermodynamic of selective oxidation of propylene to acrolein…………33

I.2.3 Catalysts for selective oxidation of propylene……… 34

I.2.4 The Reaction Mechanisms of Selective Oxidation of Propylene to Acrolein on Bismuth Molybdate Catalysts……… 36

I.2.5 Kinetics of Propylene Oxidation to Acrolein……… 43

I.3 Several catalytic supports……… 46

I.3.1 Mesoporous zirconium dioxide………46

I.3.1.1 General information………46

I.3.1.2 Synthesis and application of zirconia in catalysis field……… 48

I.3.2 Amorphous silica………51

I.4 Aim and main concentrations of the thesis……… …52

CHAPTER II: EXPERIMENTS……… 54

II.1 Catalyst Preparation ………54

II.1.1 Synthesis of Bismuth Molybdate with nano-size crystal……… 54

II.1.2 Impregnation of Bismuth Molybdate on high surface area supports… 56

II.1.3 Preparation of mesoporous zirconia support……….57

II.2 X-ray diffraction (XRD)……….59

II.2.1 Principles……… 59

II.2.2 Application in this thesis……… 61

II.3 Infrared spectroscopy………61

II.3.1 Principles……… 61

II.3.2 Application in this thesis……… 63

II.4 Raman spectroscopy……… 64

II.4.1 Principles……… 64

II.4.2 Application in this thesis……… 66

II.5 Physical absorption for the determination of surface area and pore distribution…….……… ……….67

II.5.1 Principles……… 67

II.5.2 Application in this thesis……… 69

II.6 Scanning electron microscopy (SEM)……….70

II.6.1 Principle……….70

II.6.2 Application in this thesis……… 72

II.7 Transmission Electron Microscopy……….73

II.7.1 Principle……….73

II.7.2 Application in this thesis……… 74

II.8 Thermal analysis……… 74

II.8.1 Principle……….74

II.8.2 Application in this thesis……… 75

II 9 temperature-programmed reoxidation (TPRO)………75

II.9.1 Principle………75

II.9.2 Application in this thesis……… 75

II 10 Evaluation of catalytic activity ……….75

II.10 1 Microactivity test……… 75

II.10.2 The Gas Chromatograph Analysis Method……… 76

CHAPTER III: RESULTS……….80

III.1 Synthesis and characterization of the support - mesoporous zirconia……… 80

III.1.1 Influence of H 2 O/Zr volume ratio……… 81

III.1.2 Influence of crystallized temperature ……… 83

III.1.3 Influence of calcination temperature on surface area and phase composition of samples……… 84

III.2 Synthesis and characterization of nano gama bismuth molybdate…… 87

III.2.1 The influence of crystallizing temperature……… 89

III.2.2 The influence of calcination temperature……….91

III.2.3 The influence of pH on phase composition of samples………94

III.2.4 Comparison of bismuth molybdate synthesized by sol-gel and hydrothermal methods ……… 95

III.3.Synthesis, characterization and catalytic activities of supported Bismuth molybdate system in selective oxidation of propylene…….100

III.3.1 Compositional characterization of bismuth molybdate on supports…100 III.3.2 Distribution of bismuth molybdate on supports……….103

III.3.3 Catalytic activities of beta bismuth molybdate impregnated on SiO 2

support samples ……… 111

III.3.4 Catalytic activities of beta bismuth molybdate impregnated on ZrO 2 support……… 114

III.3.5 Comparison of SiO 2 and ZrO 2 supports……… 116

CONCLUDING REMARKS……… 120

References……… 122

Appendix………133

More than one third of worldwide chemical products, are produced bycatalysed reactions with oxides types of catalysts [1] Among the catalysts, selectiveoxidation can be considered the most typical example of metal oxide-type materials

as heterogeneous catalysts, which produce around one quarter of total organicchemicals worldwide Acrolein has broad industrial and agricultural applicationsand had been produced by silica supported sodium catalysing vapour-phasecondensation of acetaldehyde and formaldehyde at temperatures between 300oC and

320oC [2] Large-scale production of acrolein process was later commercialized by

SOHIO (Standard Oil of OHIO) in 1950s using the Hearne and Adams’ catalytic

process

The use of metal oxide catalysts began with the realisation of the need toconvert low molecular-weight hydrocarbons, such as natural gas and refinery off-gas, to larger molecules of higher value Especially, the conversion of lighthydrocarbons into products containing oxygen or other hetero atoms producesimportant intermediates for the petrochemical industry A number of suchintermediates are derived from propylene, e.g., acrylonitrile, acrolein, acrylic acid,and propylene oxide It was Hearne and Adams [3] in 1948 who first reported thatcuprous oxide could selectively oxidize propylene into acrolein with a yield ofabout 50% at propylene/ oxygen ratios of about one However, the yield of acrolein

on cuprous oxide was low at that time Therefore, it was necessary to find the othercatalytic systems with higher effective catalytic performance In 1959, Idol, and in

1962, Callahan et al found that bismuth/molybdenum catalyst has higher catalyticperformance in selective oxidation of propylene to acrolein than that obtained in thecuprous oxide system Later, Veatch and co-workers [4] discovered that bismuthmolybdate-based catalysts (bismuth phosphomolybdates) are more superior tocuprous oxide The bismuth molybdate-based catalysts have since become themajor catalysts in commercial processes worldwide to produce acrolein [5,6,7,8]

Since the discovery of the process, a huge amount of reports of studies on theprocess have been published, mainly focusing on reaction mechanisms and catalystcharacterization to reveal the mystery behind the selective oxidation The mostimportant findings are the fact that the acrolein was formed via the formation of

allyl intermediate on the catalyst surface and the reaction uses lattice oxygen Interms of bismuth molybdate, it was also found that only three phases of purebismuth molybdate, namely , , and phases ( -Bi2Mo3O12, -Bi2Mo2O9, and -

Bi2MoO6, respectively) are active and selective for the reaction

Main disadvantage of bismuth molybdate is its extremely low specificsurface area (typically ~0.1 to 1.0 m2.g-1) due to its bulk structure It could be thereason for low conversion of light olefin over bismuth molybdates However, theselectivity of desired products is quite high resulted in bismuth molybdates are tillthe most effective catalysts for selective oxidation of olefin Recently, some authorsreported that they succeed in synthesis of bismuth molybdate with surface area of 2

– 4 m2.g-1 [9], but it was still low Few attempts have been done to increase thesurface area of bismuth molybdate by impregnation of bismuth molybdate (from0.5%wt to 5%wt) on silica gel [10] but results showed that catalytic performancedecreased significantly due to low selectivity of desired products Supportedbismuth molybdates were believed that its catalytic activities were not effectivebecause it was very difficult to obtain desired phases of bismuth molybdates on thesurface of supports by impregnation and because of low ion conductivity of mostsupports Therefore, the understanding of influence of specific surface area on thecatalytic activities as well as reaction mechanism has been still not clear up to now

The research in this thesis was designed to enlarge the surface area ofbismuth molybdates which achieved an improved understanding in the effect ofspecific surface area on catalytic performance of bismuth molybdates and kinetics.Two directions were offered One is to reduce bismuth molybdate crystal particle tonanometer sized one resulting in increasing the surface area On the other hand, theresearch also aimed to impregnate bismuth molybdate on some kind of high surfacearea supports to distribute better catalytic sites for the selective oxidation ofpropylene to acrolein High conductive support was also chosen to study since itwas believed that lattice oxygen plays an important role in the catalytic reaction

CHAPTER I: LITERATURE REVIEWS

In this chapter, we consider in detail the structure of bismuth molybdates andthe theories behind catalytic selective oxidation of propylene to acrolein overbismuth molybdate catalysts This will contains historical background of thereaction discussion in depth of the bismuth molybdates, how they play as catalyticrole in selective oxidation of propylene to acrolein and the background of thekinetic as well as the mechanisms of the reaction

I.1 Bismuth Molybdates

I.1.1 Crystal Structure of Bismuth Molybdates

The fact that only certain phases of known bismuth molybdate are active asselective oxidation catalysts means that the bulk structure determines the activity ofbismuth molybdate Before discussing the relation between structure and catalyticactivity, the crystal structures of the three phases of active bismuth molybdate,

namely α -Bi2Mo3O12, β -Bi2Mo2O9, and γ -Bi2MoO6 are briefly reviewed

i) α -Bi2 Mo 3 O 12

There are numerous reports on the crystal structure of α -Bi2Mo3O12 One of thewidely accepted reports is of van den Elzen and Rieck [11] who used a singlecrystalline X-ray diffraction technique and revealed that the structure was derivedfrom naturally occurring mineral CaWoO4 (scheelite) A neutron diffraction study

on a powder sample of α -Bi2Mo3O12 by Theobald et al [12] confirmed the structurefound by van den Elzen and Rieck

Scheelite crystalline structure is built from stacking up of Ca2+ and WO42- ions

In the case of α -Bi2Mo3O12, Ca2+ is replaced by Bi3+ ion while WO42- is replaced byMoO42- ion Both WO42- and MoO42- have the same structure To maintain thecharge balance within the crystal due to the replacement of a+2 ion (Ca2+) with a+3ion (Bi3+), the structure contains an ordered arrangement of vacancies in the Bipositions corresponding to the formation of Bi2Mo3O12 Figure 1.1 shows theprojection of the crystal along b axis and the bismuth vacation sites on the structure

Figure 1.1 Bismuth vacant sites in α -Bi2Mo3O12 along b axis projection The solid

lines show theα -Bi2Mo3O12 unit cell while the dashed line is a unit cell of scheelite.Large solid circles represent occupied Bi sites while the small circles are the empty

Bi sites (Adapted from van den Elzen and Rieck [11])

All MoO4 tetrahedron in α -Bi2Mo3O12 are in Mo2O8 form [11] There are twoforms of Mo2O8, one of which possesses a central symmetry while the other doesnot In addition, all Bi have 8 oxygen neighbours However, the Bi-O distances arenot equal and range from 2.12 to 2.93 Å The unit cell structure of α -Bi2Mo3O12according to van den Elzen and Rieck is shown in Figure 1.2

ii) β -Bi2 Mo 2 O 9

Growing a single crystal of β -Bi2Mo2O9 to the size suitable for singlecrystalline X-ray diffraction was not an easy task This is the reason why the crystal

structure of β -Bi2Mo2O9 was not well understood until 1975 In 1974, Chen [13]

grew a single crystalline β -Bi2Mo2O9, but the crystal was twinned and not suitablefor accurate X-ray structure analysis Due to the difficulties, van den Elzen andRieck did a comprehensive powder X-ray diffraction analysis on a high purity β -

Bi2Mo2O9 and obtained its structure

Figure 1.2 The unit cell structure of α -Bi2Mo3O12 (left figure) and atom map of theunit cell (right figure) (Adapted from van den Elzen and Rieck)

The crystal was prepared according to the method developed by Batist [14].Van den Elzen and Rieck noted that Bi3+ had octahedral coordination to oxygenanions at various distances as in the alpha phase, while Mo6+ had tetrahedralcoordination with oxygen van den Elzen and Rieck further proposed that Mo6+tetrahedral might be strongly distorted and linked together in pairs as in the alphaphase

The structure of β -Bi2Mo2O9 is built with square clusters of four MoO4tetrahedrons to become Mo4O16 They bound together by Bi ions located halfway onthe axis passing through the centre of the squares [15] Some Bi cations aresurrounded by eight oxygen ions from the MoO4 tetrahedrons There are also someoxygen ions associated only with Bi cation in the coordination sphere of others.Thus, the structure may be visualised as composed of rows of oxygen ions,connected only to Bi cations, running parallel to the (Mo4O16)-Bi-(Mo4O16) units.These resemble the ribbons of Bi2O from the Bi2O2 layers in koechlinite However,not all Bi sites are filled One in every four Bi sites is empty The structure may bethus represented by Bi(Bi3O2)(Mo4O16), where the first Bi is associated only withthe (Mo4O16) units and the Bi cations in parentheses are bonded to the oxygenatoms associated only with Bi as well as to those shared with the (Mo4O16) units

The structure of a unit cell of β -Bi2Mo2O9 and its crystal representation along the(010) face are shown in Figure 1.3 and 1.4, respectively

Figure 1.3 A unit cell of β -Bi2Mo2O9 (right figure) and atom map of the unit cell(left figure) (Adapted from H.-Y Chen and A.W Sleight [16]).

Figure 1.4 Representation of the β -Bi2Mo2O9 structure projected along (010) [17].The figure shows clusters of MoO4 tetrahedral to form Mo4O16 The small circlerepresents a bismuth atom

iii, γ −Bi 2 MoO 6

The gamma phase of bismuth molybdate is found naturally as mineralkoechlinite The structure was firstly determined by Zemann in 1956 [18] In 1973,van den Elzen and G.D Rieck [19] re-determined the structure using single crystalX-ray diffraction because they found that the crystal model from Zemann wasinaccurate due to a large linear absorption coefficient as a result of irregular and

poorly defined specimen Using the single crystal X-ray diffraction technique, theyfound that the actual symmetry was Pca21 rather than Cmca as suggested byZemann However, the model developed by van den Elzen and Rieck has twooxygen atoms that are separated by only 2.2 Å in O-O distance The distance is aphysically unrealistic feature, because the closest distance allowable for two-unbounded oxygen is 2.4Å, based on the effective ionic radii of oxygen [20] Thestructure of γ -Bi2MoO6 in Zeeman and van den Elzen and Rieck’s work were

determined from crystals of natural koechlinite mineral

The crystalline structure model developed by Teller et al [21] and Theobald et

al [22] was based on Zeeman and van den Elzen and Rieck’s work It contains

series of layers made up of (Bi2O2)n2+ and (MoO2)n2+ octahedron units connected by

O2- ions in the arrangement of (Bi2O2)2+n O2-n (MoO2)2+n O2-n An overview of thestructure is given in Figure 1.5

Figure 1.5 From left to right, a unit cell ofγ -Bi2MoO6 and its atom map The figure

is adapted from Teller et al [21]

Unlike the alpha and beta phases, the gamma phase of bismuth molybdate has

three polymorphs, namely, γ , γ ’, and γ ” The structure discussed above is the lowest

temperature polymorph At 570oC, the γ polymorph is reversibly turned into the γ ’polymorph and further to the irreversible γ ” polymorph if it is heated to 670oC [23,

24, 25] Thus, caution should be taken when preparing γ -Bi2MoO6 since only the γ

polymorph is active for partial oxidation of propylene to acrolein

I.1.2 Synthesis of bismuth molybdate

Many methods have been applied to prepare bismuth molybdates like solidstate, co-precipitation, sol-gel, hydrothermal, spray drying In this part, they arediscussed briefly some advantages and disadvantages of these methods

i, Solid state technique:

It is the oldest method for preparation of bismuth molybdate The procedure isquite simple, in which Bi2O3 and MoO3 are mixed in stoichiometric proportion.After that, mixture is calcinated at the temprature range of 500-650°C to gaindesired phase bismuth molybdate

A.Wantanabe et al [26] prepared Polycrystalline y(H)-Bi2MoO6 from startingmaterials Bi2O3 and MoO3, both 99.9% pure, which were thoroughly mixed instoichiometric proportion under ethanol After drying, the mixture was heated in acovered platinum crucible at 655°C for 40 hr Using HRTEM, they confirmed thatthe high firing temperature was need to make order structure of bismuth molybdate.R.N Vannier et al [27] prepared sample by solid state reaction fromstoichiometric amounts of Bi2O3 and MoO3 oxides in a covered gold crucible,calcination procedure was carried out after that They found new structural Bi-MoMixed Oxides with a Structure Based on [Bi12O14] Columns but it is verycomplicated and not to be easy to control

In general, solid state technique has some disadvantages like high calcinationtemperature, difficult to make homogeneous composition which is very important togain desired phases, product possesses low surface area (less than 2 m2.g-1) incommon

ii, Co-precipitation technique

Co-precipitation method is a popular method to prepare bismuth molybdatebecause its procedure is quite simple and it allows to easily control composition ofproducts The starting reactants are typically a solution of Bi(NO3)3 5 H2O in waterand a suitable quantity of HNO3 and a solution of (NH4)6Mo7O24 4H2O in water.The solution of bismuth nitrate is added to ammonium molybdate or vice versa pH

is adjusted in the range of about 1-9 by adding NH3 or HNO3, depending on eachauthor The precipitate is filtered and dried or the liquid is directly evaporated.Precipitates are calcined at some given temperature, normally from 450-580°C.

Ji Chul Jung et al [9] successful in preparation of bismuth molybdate bycoprecipitation followed the above procedure: A known amount of bismuth nitrate(Bi(NO3)3) was dissolved in distilled water that had been acidified withconcentrated nitric acid The solution was then added dropwise into an aqueoussolution containing a known amount of ammonium molybdate ((NH4)6Mo7O24)under vigorous stirring During the co-precipitation step, the pH of the mixedsolution was precisely controlled using known amounts of ammonia solution The

pH values were maintained at 1.5 and 3.0 in the preparation of -Bi2Mo3O12 and

-Bi2MoO6, respectively After the resulting solution was vigorously stirred at roomtemperature for 1 h, the precipitate was filtered to obtain a solid product The solidproduct was dried overnight at 110◦ C, and it was then calcined at 475 ◦ C for 5 h in

a stream of air to yield the final form The products both showed very highcrystallinity They also successed in prepared bismuth molybdate with surface area

of 1.9 and 3.5 m2.g-1 which has quite high surface area compare to another publicsbefore

Zhou Bing et al [28] also prepared bismuth molybdate by precipitation but athigh pH value (pH = 7) and various calcination temperature They found that themore calcination temperature increased, the lower surface area of products gained.The surface area of product decreased from 1.2 m2.g-1 to 0.8 m2.g-1 when increasingtemperature from 420oC to 570oC

Nianxue Song et al [29] tried to prepared three phases of bismuth molybdate

by co-precipitation method but they just achieved small surface area products(about 2 m2.g-1) and high impurities in products

One of disadvantages for this method is the filtration of the precipitant fromthe solution, this step is very easy to make loss of Bi or/and Mo ion which leads togain desired phase bismuth molybdate difficultly Almost results showed that verylow surface area products were achieved by co-precipitation

iii, Sol-gel method

Sol-gel is still new method for the preparation of bismuth molybdates It hasattracted a lot of attention of scientist recently, because sol-gel method has a lot ofadvantages like: easily controlling the composition of gel to gain the desired phase

of bismuth molybdates; synthesis conditions are simple, easily to put other

components into gel to make multiphase oxides…so on In common, gel was

prepared by following procedure: The precursor solutions were prepared fromaqueous solutions of (NH4)6Mo7O24 (solution A) and Bi(NO3)3/HNO3 (solution B)

A volume of solution A was slowly added into an equivalent amount of solution Bcorresponding to the desired Bi/Mo molar atomic ratios, and concentrated HNO3was continuously added in order to preserve the high acidity of the medium and toprevent the precipitation of bismuth molybdates Citric acid was added into solution

in order to result in complexes The obtained solutions were gellyfied at 60–80oCuntil the gels were completely formed The transparent yellow gels were then dried

at 110oC for 2 h, and the spongy solid precursors obtained were crushed Powdersobtained after the gelation were directly calcined in an air flow at 580oC for 2 hwith the heating rate of 10oC/min [30, 31, 32, 33]

M.T.Le et al [32,33] succeed in preparation of , , pure bismuth molybdates

by sol-gel method using citric acid as a complexes reagent The surface area of theproducts were depended on the amount of added citric acid The highest surfacearea reached to 12 m2.g-1 However, the disadvantage of this method is the burning

of citric acid which caused a local overheat It results in sintering and aggregation

E Godard et al, H.-G Lintz et al [34, 35] also tried to prepare bismuthmolybdates by sol-gel followed the above procedure Instead of gelation by heat inair, the water was evaporated under vacuum in a revolving flask at 30oC until aviscous solution was obtained After that, this solution was dried at 80oC undervacuum for 16 h in order to obtain a spongy precursor which was crushed,decomposed at 300oC for 16 h and calcined at 470oC for 18 h The products hadhigh crystallinity However, they found that it was very difficult to achieve highsurface area products The surface area of all of products were lower 2 m2.g-1

In summary, sol-gel is a good method to prepare desired bismuth molybdatephases easily However, it seemed to be very difficult to achieve high surface area

bismuth molybdates because desired bismuth molybdate phases were only formedduring calcinations, but calcination could lead to aggregation.

iiii, Hydrothermal method

Hydrothermal method is very famous in synthesis of zeolite and porousmaterials but, recently, the hydrothermal method has been also applied to preparebismuth molybdates An outstanding advantage point of hydrothermal was desiredbismuth molybdate phase formed directly in crystallization stage Therefore,aggregation could be prevented during calcinations

HongHua Li et al [36] suggested preparation of bismuth molybdates followingthe procedure: Bi(NO3)3.5H2O was dissolved into concentrated HNO3 solution,while stoichiometric amounts of (NH4)6Mo7O24.4H2O (Bi/Mo = 2/3, 2/2 and 2/1 for-Bi2Mo3O12, β -Bi2Mo2O9 and -Bi2MoO6 respectively) was dissolved in deionizedwater The two solutions were mixed under vigorous stirring The pH of the mixturewas adjusted to a specific value with ammonium hydroxide (1:1 volume fraction).After stirring for 0.5 h, this mother liquor was poured into a Teflon-lined stainlesssteel autoclave until 80% of the volume of the autoclave was occupied After that,the autoclave was sealed into a stainless steel tank and kept at 180◦ C for 24 h Then

the reactor was cooled to room temperature naturally Obtained samples werecollected and washed with deionized water and dried at 80 ◦ C in air In the case ofthe β -phase, it was necessary to calcined at 560 ◦ C since the pure material was

formed only after further heat treatment of the hydrothermally treated sample

-Bi2Mo3O12 was prepared successful at pH lower than 3 while -Bi2MoO6 easilyformed at high value of pH (7-9) β -Bi2Mo2O9 just only formed at pH of about 3with high purities Obtained products possessed quite small particle size, theirsurface areas were 10.9 m2.g-1 and 16.4 m2.g-1 after calcination

Andrew M Beale et al [37] prepared three phases of bimuth molybdates byhydrothermal method Using An EDXRD and combined XRD/XAS study theyconfimed that the and bismuth molybdate phases form directly after

hydrothermal treatment whereas the β phase need to be applied to an additional

heating to finally form the monophasic material This method also yielded materialswith a higher surface area (about 10 m2.g-1)

Therefore, hydrothermal method could be one of prospective methods toimprove specific surface area of bismuth molybdates

iiiii,Spray drying method:

M.T.Le et al [32, 38] suggested a novel spray drying method in which theprecursor solution for spray drying was prepared from a solution of (NH4)6Mo7O24

in water (solution A) and a solution of Bi(NO3)3 in water containing a mount ofconcentrated HNO3 in order to keep the Bi-salt dissolved (solution B) In order topreserve the high acidity of the medium together with the solubility of the products,solution A was slowly added into an equivalent amount of solution B During thisaddition, concentrated HNO3 was continuously added in order to prevent theprecipitation of bismuth molybdates The final solutions were homogeneous,transparent and stable for up to 24 h Thereafter, this precursor solution was spraydried using a Büchi 190 laboratory spray dryer with a 0.5mm nozzle and a feedingrate of 5 ml/min at a temperature of about 225◦ C The fine spray dried powder was

found to be amorphous Powders obtained after spray drying were directly calcined

in air at different temperatures from 200 to 650◦ C for 5 h phase were preparedsuccessfully even at temperatures as low as 200◦ C while pure phase was only

obtained when calcined at 550◦ C and pure phase at 600oC Spray drying seemed

to be very convenient method, which confirms itself as a more rapid way to preparebismuth molybdates, and results in higher purity of final products because the use of

a homogeneous precursor solution, where Bi3+ and Mo4+ ions interacted with eachother according to an accuracy set by the stoichiometric ratio of the solution which

is carried over into the fine spray dried powder In addition, the reproducibility ofspray drying is very high and it is not effected by external factors (thecoprecipitation of unwanted species, the lost of ions in the filtrate so on ).However, final products still have low surface area ( SBET of , , bismuthmolybdates were about 1.5 m2.g-1) because of aggregation occurred duringcalcinations

I.1.3 Relationship between structure of Bismuth Molybdate and catalytic activity

According to the literature on phase equilibrium studies [39, 40, 41, 42] asshown in Fig 1.6, there are eight phases known on binary bismuth molybdenumoxides, prepared by high temperature synthesis Among the eight phases, only three

of them are active and selective toward partial oxidation of olefin Hriljac et al

reported a possible fourth active binary bismuth-molybdenum oxide prepared undermild hydrothermal conditions, but no catalytic activity was reported.

Among the family of active bismuth molybdenum catalysts, several argumentshave been raised in deciding which one has the best activity and selectivity At leastthere are three opinions about the order of activity of the α , β , and γ phases The

first comes from Beres et al [43], Millet et al [44] and also Monnier and Keulks[45] who found that the β -phase was the most active and selective, followed by αand γ (β >α >γ ) The second comes from Batist et al [14] who observed that γ phasewas equally good as the beta phase (β = γ >α ) Yet another opinion from more recent

studies by Cullis et al [7] who observed that the order of propene activity decreased

in the order of α >β >γ whereas the order of selectivity was β >α >γ

In all active bismuth molybdate catalysts, lattice oxygen plays the key role inthe catalytic activity for partial oxidation of propylene The role of lattice oxygenhas been proven by tracer studies using 18O [46; 47] as well as photoelectroncharacterization of the catalyst surface [48; 49] The importance of lattice oxygen inthe catalytic selective oxidation led to the conclusion that an active catalyst has to

be able to provide lattice oxygen for oxidation

Batist et al [14] attempted to correlate the Bismuth-Molybdate activity andselectivity with the bulk crystal structure In their opinion, the catalytic activity for

9 0

8 0

7 0

6 0

5 0

4 0

3 0

2 0

90 0

80 0

70 0

60 0

50 0

1/1 + 2/3

L + 2/3

3/1(L) + 2/1(H)

3/1(H) + 2/1(H)

3/1(L) + 2/1(K)

selective oxidation of olefin was connected with the corner-sharing Mo-Ooctahedron, while the catalytic activity was largely absent in compounds containingedge-shared octahedron They believed that in the 1:1 bismuth to molybdenum ratiocompound, Mo ion has an octahedral coordination, which allows a high catalyticactivity In addition, they suggested that the interlinking of Mo-O octahedron byedge sharing is responsible for the low activity of the 2:3 molybdate However,detailed studies [11, 12] have shown that in the 2:3 compound, the molybdenum ion

is surrounded by a distorted tetrahedron Infrared and ultraviolet investigations bykumar and Ruckenstein [50] revealed the presence of both tetrahedral andoctahedral oxomolybdenum species on the surface of the 2:1 and 1:1 activeBismuth- Molybdates

The importance of edge sharing Mo-O octahedron was also reported in

Haber’s review [51] When the reaction occurred on the bismuth molybdate surface,

the uptake of oxygen from the surface caused oxygen vacancies The oxygenvacancies generated electrophilic oxygen, which then led to total oxidationproducts The oxygen vacancies, surprisingly, were not found on the reactingsurface The absence of elecrophilic surface oxygen is now believed to be caused bythe so-called “shear plane”, where the corner-sharing Mo-O turned into the edge-

sharing Mo-O The ability of bismuth molybdate to serve the change of the sharing

is believed to be the source of their selectivity for partial oxidation reaction

Buttrey et al [52] mentioned that the event that is taking place on the surfacecould lead to the structural reorganization They investigated the structure of

Bi2O3.nMoO3 crystal, covering n=3 (α phase), n=2 (β phase) and n=1 (γ phase) to

find the relation between the structure and catalytic activity and concluded that allactive phases were actually derived from the fluorite structure They furtherconcluded that fluorite structure was possibly responsible for the accommodation ofeither cation or anion vacancies in the crystal framework for the n = 2 and 3 anddetermined the catalyst activity toward the catalytic selective oxidation of olefin.For the phase with n = 1, fluorite-related structure did not exist and its activity wasdue to the facile path of lattice oxygen (O2- ) diffusion through MoO2 and Bi2O2layers [53] Buttrey et al [52], however, confessed that the structure identified intheir study might be different from the real structure under actual reaction

conditions This raises a question about the role of the structure of the catalyst underthe reaction condition This will be further investigated in more detail in this thesis.

I.1.4 Catalytic performance of bismuth molybdate.

Although bismuth molybdate have been the best catalyst for selectiveoxidation of olefin so far, but the catalytic activities has been still very low Fansuri,

H et al studied the selective oxidation of propylene to acrolein on - Bi2Mo2O9 andthey found the conversion of propylene just reached about 4% at 450oC with thebest sample M.T.Le et al [31]studied the oxidation of propylene to acrolein for verylong time, they also showed that conversion of propylene was only 3.8% at 425oCover 0.1 gram of pure phase bismuth molybdates Hence, improving the catalyticactivities of bismuth molybdate containing materials has attracted much attentionfrom scientists, recently This section briefly summarized researches which aimed

to improve the catalytic activities of bismuth molybdates recently

M.T.Le et al; Ji Chul Jung et al [30, 54] found the synergy effect between

-Bi2Mo3O12 and -Bi2MoO6 for the selective oxidation of propylene to acrolein Thesynergistic effect of these two catalysts was explained by a remote controlmechanism, in which oxygen species formed on the -Bi2MoO6 migrate onto thesurface of the -Bi2Mo3O12 to create active sites Therefore, it is believed that thesynergistic effect of the -Bi2Mo3O12 and -Bi2MoO6 catalysts in the selectiveoxidation of propylene was due to a combination of the facile oxygen mobility of -

Bi2MoO6 catalyst and the abundant adsorption sites of - Bi2Mo3O12 catalyst forpropylene This synergistic effect could help increase the selectivity significantly incomparison with that of pure and phases at the same reaction conditions

On the other hand, many attempts have been done to find out the multiphase

oxides systems (Bi, Mo, Co, Ti, Sn, Sb, Pb, Se, Si, …O) which was believed be

able to increase the activities of bismuth molybdate containing catalysts forselective oxidation of propylene [55] Insertion of various elements in bismuthmolybdate system which could make new materials and perhaps, good catalysts aremade Because modifiers affect the formation of new phases, increase theproportion of interfacial regions with a certain structure, and promote the growth ofmolybdate microcrystals in such a way that faces active in selective oxidation are inlarge excess Furthermore, modifiers can change the acid–base properties of the

catalyst and the rate of electron and oxygen transfer O V Udalova et al [55]studied Co6–8Mo12Fe2–3Bi0.5–0.75Sb0.1K0.1Ox catalysts for selective oxidation ofpropylene to acrolein The results obtained provide insights into the role of thecomponents of the catalyst CoMoO4 forms the structural framework of the catalyst.Iron molybdate can be stabilized on CoMoO4 as β -phase As its content is

increased, the catalyst gains activity but its selectivity declines Bismuth molybdate

is responsible for the selectivity of the process When present in small amounts,MoO3 raises the selectivity, binds free oxides, and converts reduced molybdatesinto their oxidized forms Excess molybdenum trioxide causes a dramatic fall in thecatalytic activity Potassium and antimony decrease the catalytic activity, but evensmall amounts of these elements raise the selectivity of the catalyst Chromium cansubstitute for iron atoms in the multicomponents catalyst Ni, Mn, and Mgsubstitute for Fe in iron molybdate to decrease the catalytic activity M.T.Le et al[31] suggested adding SnO2 and ZrO2 on beta phase bismuth molybdate to increaseconductivity of sample With 10%SnO2 adding, the conductivity increasedsignificantly which led to very strong synergy effects resulted in increasing catalyticperformance

Recently, some scientists have focused on increasing surface area of bismuthmolybdate catalysts by many ways [10, 56, 57] In order to overcome the surfacearea limitation of bismuth molybdates, two main directions, supported bismuthmolybdates on supports and nanometer sized bismuth molybdates, have beeninvestigated Yo-Han Han et al [10] studied lattice oxide ion-transfer effect insilica-supported phase bismuth molybdate catalysts for selective oxidation ofpropylene to acrolein The highly dispersed bismuth molybdate catalysts on silicawere found to be intrinsically active but poorly selective to acrolein When theyincreased the loading amount the oxidation activity drastically increased The pooracrolein selectivity of this catalyst was improved by continuous use in the catalyticoxidation for making the particle size of the dispersed bismuth molybdate larger.The catalytic activity and selectivity were little influenced by the loading amount.The results demonstrate that, for the activity and selectivity, bismuth molybdatecatalysts need to be of a certain particle size which can provide sufficient latticeoxide ions during the catalytic redox cycle Ashish Bhakoo et al [56] studiedsupported binary oxide monolayers 10%Bi2Mo3O12/TiO2 The selectivity at 500oC

is as good as that of unsupported -Bi2Mo3O12, implying that a large reservoir oflattice oxide ions is not a prerequisite for a selective oxidation catalyst, although theyield is only moderate Chao Xu et al [57] succeed in preparation of -Bi2MoO6nanoplates by hydrothermal with support of PVP (poly vinyl pyrrolidone) assurfactant The results indicated that as-prepared Bi2MoO6 product had a typicalplate-like structure with a thickness range of 100–150 nm This results promised to

open the ability to increase surface area of bismuth molybdates

In summary, it could be said that researches about improving catalyticactivities of bismuth molybdates have been achieved some of great successes.However, most of them just have been focused on changing composition of bismuthmolybdate contained catalyst systems which aimed to create synergy effects as well

as to change the rate of electron and oxygen transfer…so on A new idea is to

increase the specific surface area of pure bismuth molybdates or supported bismuthmolybdate catalysts has still attracted less attentions Few authors have reportedsome initial results about improving surface area of bismuth molybdates [10, 56,57] Although increasing the surface area of bismuth molybdates has seemed notbring the excellent results, the initial data gave us a chance to carry on studying

I 2 Selective oxidation of propylene to acrolein

I.2.1 Acrolein and its production

In order to understand the importance of acrolein products, it is necessary totake a look about it Acrolein was first prepared in 1843 by Redtenbacher by the drydistillation of fat [58] Commercial production of acrolein began in Germany in

1942, by a process based on the vapour-phase condensation of acetaldehyde andformaldehyde This method was used until 1959, when a process was introduced forproducing acrolein by vapourphase oxidation of propylene [59] Several catalystshave been used in the vapour-phase oxidation of propylene, including cuprousoxide, bismuth molybdate and antimony oxide [60] All commercial production ofacrolein is currently based on propylene oxidation [59] ln 1975, global production

of acrolein was about 59 000 tonnes [60] The worldwide capacity for production ofrefined acrolein is about 1 13 000 tonnes per year [61] Acrolein is produced bythree companies each in Japan and the United States of America and by onecompany each in France and Germany [62]

The principal use of acrolein is as an intermediate in the synthesis of acrylicacid, which is used to make acrylates, and of DL-methionine, an essential aminoacid used as an animal feed supplement Other important derivatives of acrolein areglutaraldehyde, pyridines, tetrahydrobenzaldehyde, allyl alcohol and glycerol, 1,4-butanedial and 1,4-butenediol, 1,3-propanediol, DL-glyceraldehyde, flavours andfragrances, polyurethane and polyester resins [59, 63] According to the report fromOccupational Safety and Health Administration of US Department of Labour, 50%

of acrolein production is for glycerine, 25% for methionine, and the rest for otherapplications The most important direct use of acrolein is as a biocide: It is used as aherbicide and to control algae, aquatic weeds and molluscs in recirculating processwater systems It is further used to control the growth of microorganisms in liquidfuel, the growth of algae in oil fields and the formation of slime in papermanufacture Acrolein has been used in leather tanning and as a tissue fixative inhistological work [59, 60, 61] Acrolein is widely known as a biocide Because ofits biocidal activity, acrolein is commonly applied as an aquatic herbicide andslimicide The molecular structure of acrolein is shown in Figure 1.7 Acrolein hastwo conjugative unsaturated carbon bonding, one from the vinyl group and the otherfrom aldehyde group Due to the existence of these groups, acrolein possessesreactions characteristic of both an unsaturated and an aldehyde compounds Someexamples of reaction characteristics of acrolein are Diels-Alder reaction betweentwo acrolein molecules, addition to carbon-carbon double bond, polymerisation, andreaction with amine compounds [2]

Figure 1.7 Molecule structure of acrolein

Basic chemical and physical properties (a) Description: Colourless to yellowish liquid with extremely acrid, irritating odour (b) Boilng-point: 52.5-53.5 °C

(c) Melting-point: -86.9 °C (d) Density: 0.8410 at 20 °C/4 °C

The activity of the aldehyde group to attack an amine-containing moleculemakes it reactive to a protein molecule The reaction underlies the activity ofacrolein as an anti microbial and biocide, where the reaction of acrolein with protein

on a cell wall can cause damage to the cell and kill it Acrolein is very toxic andflammable It also undergoes polymerization easily and exothermally Thepolymerization can be initiated by light, heat, air or peroxides It is alsopolymerized in the existence of alkaline solution such as amines, ammonia, andcaustic soda or by mineral acids such as sulphuric acid Acrolein polymerization,initiated by alkaline or acids, is very exothermic and no inhibitors can stop thepolymerization once it is initiated Acrolein reacts with water and therefore addition

of water to stored acrolein must be avoided, as the acrolein-containing water layer isparticularly prone to polymerization Acrolein vapour polymerises uponcondensation [2]

I.2.2 Thermodynamic of selective oxidation of propylene to acrolein

For industrial applications, acrolein is commercially produced byheterogeneously catalysed gas-phase oxidation of propylene [2] The catalyticpartial oxidation of propylene to form acrolein follows the reaction equation below.The reaction is exothermic and produces 340.8 kJ of heat per mol of reacted

propylene The Gibbs free energy (Δ G) shows that the reaction will spontaneously

occur, once the reaction is initiated

CH3CH=CH2 + O2 → CH2=CHCHO + H2O

Δ Ho = 340.8 kJ mol-1

Δ Go = -180.19 kJ mol-1Although the reaction is energetically spontaneous, acrolein is not the onlyproduct when propylene is reacted with oxygen Several other products such as

CO2, CO, acetaldehyde, formaldehyde, carboxylic acids etc can also form Somepossible products and their thermodynamic parameters are listed in Table 1.1

The thermodynamic parameters shown in Table 1.1 reveal that the formation of sideproducts (such as CO2 and CO) is more thermodynamically favourable than theformation of acrolein

Table 1.1 Thermodynamic parameters of the formation of other propylene oxidationproducts.

kJ.mol-1

Δ Ho298kJ.mol-1

C3H6(g) + 4.5O2(g)→ 3CO2(g) + 3H2O(l) -1942.089 -2058.43

C3H6(g) + 3O2(g)→ 3CO(g)+ 3H2O(l) -1276.765 -1209.48

C3H6(g) + O2(g)→ C3H4O2(l) + H2O(l) -550.2293 -404.21

C3H6(g) + O2(g)→ C3H4O(l) + H2O(l) -338.7959 -426.24

(* data taken from Grasselli [64])

I.2.3 Catalysts for selective oxidation of propylene

Commercial production of acrolein by heterogeneous catalytic selectiveoxidation of propylene started in 1948, when cuprous oxide catalysts were chosenfor Shell Chemical Company [65] The acrolein yield from this process, however,was less than 50% from the total input of propylene, which left enormousopportunity to develop better catalysts than cuprous oxide In the 1950s, SOHIOinitiated an extensive research into catalytic vapour phase oxidation of unsaturatedhydrocarbons to produce useful products such as acrolein, acrylonitrile,methacrylonitrile, and acrylic acid [6] The research was developed based on theLewis concept [66], in that lattice oxygen of reducible metal oxide would serve as abetter oxidant for hydrocarbon selective oxidation than the gas phase oxygen.Several catalysts, either single metal oxide or a mixture of two or more metal oxideswere tested for their activity towards acrolein formation from propylene In theearly stages of the development, SOHIO found that some mixtures of metal oxidesgave good selectivity as well as yield of acrolein However, the catalysts did notwork properly in the existent of molecular oxygen in the feed stream The extensiveresearch led the SOHIO team to discover bismuth-molybdate, which had betterselectivity for acrolein than cuprous oxide [2], even when the feed stream containedmolecular oxygen Since then, the bismuth-molybdate system has been significantlyimproved by adding some metal oxides to form multicomponent catalysts Thesecatalyst systems have even better activity and selectivity for the oxidation of

propylene to acrolein Some examples of bismuth-molybdate based component catalysts are presented in Table 1.2.

multi-Table 1.2 Some examples of multi-component BiMo based catalysts

Catalyst composition TReaction

(oC)

Conversion(%)

OnePassYield(mol %)

Cu2O is the most active catalyst for the selective oxidation of propylene to acroleinwhile oxygen rich Cu2O favours CO2 and H2O, which is in agreement with thereport by Krenzke [46] Extensive studies on copper oxide catalysts led to theconclusion that metal oxides catalysts for selective oxidation reactions shouldcontain two metal ion species in the form of MIMIIOx The MI ion is normally a

heavy metal and MII from transition metal groups, which have variable oxidationnumbers From this requirement, tin antimonite, uranium antimonite and bismuthmolybdate are among the most active and extensively studied binary metal oxidecatalysts Individual metal oxides do not have sufficient activity to convertpropylene to acrolein, but mixtures can have very good activity and selectivity.Uranium antimonite, as reported by Krenzke [46], has excellent selectivitytowards acrolein over a wide range of reaction temperatures There are two phasesfound in this catalyst system, namely USb3O10 and USbO5[73] where only SbV and

UV species are contained in the catalyst The first phase is selective and active foracrolein formation and hence, its concentration determines the overall performance

of catalyst, while the second phase has lower selectivity towards acrolein Matsuura[74] reported that the catalyst has special oxygen mobility, which favours surfacereaction of propylene partial oxidation to acrolein However, due to its highlyradioactive nature, this catalyst has little scope for commercial application

Tin antimonite is a mixture of SnO2 and a solid solution of α -Sb2O and β

-Sb2O4 where none of them, as individual, is active as a catalyst for propyleneoxidation to acrolein [75] It has been reported that the addition of only 6.8% Sb totin(IV) oxide increased the conductivity by three orders of magnitude [46] This isattributed to the substitution of Sb5+ for Sn4+ ions in the tin(IV) oxide lattice It isbelieved that the catalytic activity of these mixed oxides is related to the presence ofthe solid solution and the Sb2O4 species lying on the surface of SnO2

Bismuth molybdates are the second most studied catalyst after copper oxideand serve as the main ingredient in almost all commercial catalysts for propyleneoxidation to acrolein There are several known phases of binary bismuth molybdatesbut only three phases are active and selective for the partial oxidation of propylene

to acrolein [5, 76] The ratios of Bi to Mo in these catalysts are within the range of

2:3 and 2:1, namely α -Bi2Mo3O12, β -Bi2Mo2O9, and γ -Bi2MoO6

I.2.4 The Reaction Mechanisms of Selective Oxidation of Propylene to Acrolein

on Bismuth Molybdate Catalysts

The mechanism of propylene oxidation has been investigated through threeimportant components related to the reaction: propylene, oxygen and the catalystsystem An oxidation reaction may be considered as two groups of reactions:electrophilic oxidation, which proceeds through the activation of oxygen, and

nucleophilic oxidation, which involves hydrocarbon activation in the first step andnucleophilic oxygen insertion in one of the further steps The details were givenbelow.

i, Propylene Activation

It is probably Adams and Jennings [3] who first started the studies of thereaction mechanisms of propylene partial oxidation to acrolein over bismuthmoybdate catalysts In they work, Adam and Jenning used propylene labelled withdeuterium and used a kinetic isotope effect analysis to find out the probability ofdeuterium atoms being abstracted relative to hydrogen atoms The conclusions oftheir work were:

1 The first step in the oxidation of propylene is the abstraction of a hydrogenatom from the methyl group and the process is the rate-determining step

2 The hydrocarbon intermediate formed after the abstraction is a

symmetrical structure, probably similar to the π -allylic species This species is not

cyclic because no deuterium was found in the middle of carbon atoms of theresulting acrolein

3 Propylene underwent two successive hydrogen abstractions before theaddition of oxygen

The existence of the symmetrical intermediate was also found by Voge et al.[77] by using propylene labelled with13C on a cuprous oxide catalyst at 300oC Thesame conclusion was also made by Krenzke [46] Report on studies where allylradicals were generated in-situ, either by allyl iodide [78] or from gas phase radicals[79], confirmed the existence of the symmetrical allyl intermediate Figure 1.8 givesthe reaction path of the propylene activation via the formation of allylicintermediate

Figure 1.8 The reaction paths of the partial oxidation of deuterium-labelledpropylene, Z=kD/kH, while 1 and 3 are the numbers of carbon atoms wherehydrogen is abstracted The value of Z was calculated from the distribution ofdeuterium in acrolein Deuterated carbon was only found on terminal carbon atoms

A more recent study by Ono, Ogata, and Kuczkowski [83] using labelled oxygenand microwave spectroscopy also supported the existence of allylic intermediate

Several studies [84, 85] on the formation of side products also confirmed theallylic intermediate mechanism The side products normally accompany theselective oxidation of propylene to acrolein Keulks and Daniel [86] investigated theoxidation of 14C labelled and unlabelled acrolein They found that the carbondioxide is formed almost exclusively from the further oxidation of acrolein The

same result was also found earlier by Russian researchers [84, 85] Figure 1.9 showsthe reaction path of the formation of some side products.

Figure 1.9 The reaction paths of the formation of side products [46]

All of the above studies lead to the same conclusion that the selectiveoxidation of propylene occurs through the formation of the allylic radical Theformation of allylic intermediate in the reaction of partial oxidation of propyleneover bismuth molybdate catalysts has been widely accepted although the isolation

of the allylic and allyl-peroxiradicals was only proven in 1981 by Martir andLundsford [79] in their matrix isolation-EPR studies on α -Bi2Mo3O12 and γ -

Bi2MoO3

ii, Oxygen Insertion

The step after propylene activation is the abstraction of second hydrogen andthe oxygen insertion into allyl group The abstraction of the second hydrogen isunlikely to occur at the same time as the allylic formation is taking place becausethe reaction is energetically rate-determining step Studies using self-generatingallylic species [87, 78] have showed that the second hydrogen was abstracted fromthe allylic intermediate

The insertion of oxygen occurs before the second abstraction of hydrogenfrom the allylic intermediate In order to elucidate the key aspects of selectivepartial oxidation of propylene to acrolein over bismuth molybdate and relatedcatalysts, Burrington et al [80] used allyl alcohol-1,1-d2 and -3,3-d2 in theirinvestigation They concluded that the insertion of oxygen occurs before theabstraction of the second hydrogen and facilitated by the presence of a C─ O bond.

So far, no mention has been made of the source of oxygen for oxygeninsertion into the allylic intermediate For the formation of acrolein, there are twopossible sources of oxygen One involves the use of lattice oxygen and the other anadsorbed form of molecular or gas phase oxygen The first mechanism is referred to

as the redox mechanism where the catalyst itself acts as the oxidising agent In thismechanism, molecular oxygen serves only to re-oxidise the reduced catalyst Thesecond type of oxygen reacts with the allylic intermediate to form hydroperoxide.The hydroperoxide then decomposes to acrolein and water

The redox mechanism was first proposed by Mars and van Krevelen [88] andhas since been known as the Mars and van Krevelen mechanism In early 1954,Mars and van Krevelen [89] concluded that the catalytic oxidation of hydrocarbontook place in two steps: (a) the reaction between hydrocarbon and the oxide inwhich the latter is reduced and the former is oxidised, followed by (b) reoxidation

of the reduced catalyst by gaseous oxygen to the original state of activity andselectivity The Mars and van Krevelen mechanism is schematically given in Figure1.10 The concept where the lattice oxygen of a reducible metal oxide could serve

as a useful oxidising agent for hydrocarbons was actually the basis of the early work

at SOHIO which led to the development of bismuth molybdate catalyst [6]

The usefulness of lattice oxygen as the source of oxygen for oxidationreactions is due to the fact that the oxygen is relatively easy to be removed from thelattice Burlamacchi et al [90] gave evidence that the lattice oxygen ions on γ -

Bi2MoO6 surface could be easily removed by applying high vacuum at temperaturesabove 300oC Fattore et al [91] observed that acrolein could be formed without anygaseous oxygen over bismuth molybdate and several other metal oxides catalysts