STUDY ON STRUCTURAL TRANSFORMATION OF MOLYBDENUM OXIDE CATALYST FOR PROPANE
OXYDEHYDROGENATION (ODH) REACTION
DURGA DEVI A/P SUPPIAH
FACULTY OF SCIENCE UNIVERISTY MALAYA
KUALA LUMPUR
2012
STUDY ON STRUCTURAL TRANSFORMATION OF MOLYBDENUM OXIDE CATALYST FOR PROPANE
OXYDEHYDROGENATION (ODH) REACTION
DURGA DEVI A/P SUPPIAH
DISSERTATION SUBMITTED IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE
OF MASTERS OF SCIENCE
DEPARTMENT OF CHEMISTRY FACULTY OF SCIENCE UNIVERSITY OF MALAYA
KUALA LUMPUR
2012
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ABSTRACT
Catalytic oxidative dehydrogenation (ODH) of propane is recognized as an attractive alternative process as compared to propane dehydrogenation which the latter requires higher reaction temperature. For ODH of propane, key points in synthesizing an active catalyst are the preparation method, surface reducibility, and acid-base properties.
However, existing ODH catalysts gives low activity and poor selectivity. Combination of optimal activation thermal analysis and structural control can help to guide and gain insight into the structure-activity relationship of the nanostructured catalyst system. Molybdenum and vanadium oxide based catalyst has been identified as one of the most suitable catalyst for the reaction. However it is difficult to control the key properties such as crystal size, structure, particle shape and surface area that influence the catalysts performances. In these studies, molybdenum oxides based catalysts (MoOx and MoVOx) were synthesized using controlled precipitation method. Parameters varied were pH, concentration, temperature and also rate of addition. For MoOx based catalysts, the phase obtained were
‗supramolecular‘ phase (Mo36O112)and hexagonal phase (h-MoO3). Protonation encourages the growth of the catalytic structure where Mo7O24 acts as a nucleus creating the polyoxomolybdates. Both phases have bulk structural corner sharing pentagonal channels as structural motif which is catalytically active. For MoVOx based catalyst, amorphous phase was observed for all spray dried precursors. Highly crystalline hexagonal phase (MoV2O8) and tetragonal phase [(MoV)5O14] were obtained after activation under static air and inert respectively when vanadyl was used as the vanadium source. Dispersion of vanadium creates the ‗site isolation‘ effect which is important to avoid olefins transforming to neighboring oxidized sites. When vanadates were used as the vanadium source, different phases were observed at varying of vanadium loading. Orthorhombic phase was observed
at low vanadium loading whilst mixed phase of monoclinic and triclinic were obtained at higher vanadium loading. Temperature programmed activation using in-situ XRD was used to study the dynamics of structural transformation of selected synthesized molybdenum oxide-based precursor. The structural evolution for MoOx precursor takes place from
‗supramolecular‘ to metastable hexagonal phase at 300 ºC. The structural changed finally to the stable orthorhombic phase at 450 ºC. For MoVOx precursor, the structural transformation takes place from amorphous to nanocrystalline phase at 400 ºC. At 500 ºC, the catalyst morphology transforms finally to thermodynamically stable crystallized tetragonal phase. By correlating in-situ XRD reactivity studies with temperature programmed in-situ DSC, catalytic activity in MoOx catalyst was not observed whilst for reaction using MoVOx catalyst, catalytic activity was observed at the nanocrystalline phase.
v
ABSTRAK
Propylene adalah bahan mentah yang digunakan secara meluas dalam industri petrokimia. Permintaan yang tinggi dan kepelbagaian dalam pengunaan propylene merangsang pembangunan teknologi pengeluaran terkini. Oksidatif pengdehidrogenan propana yang menggunakan mangkin merupakan alternatif yang menarik berbanding pengdehidrogenan propana yang memerlukan suhu yang tinggi meyebabkan kos yang mahal. Dalam ODH propana, perkara utama yang perlu diberikan perhatian untuk mensintesis mangkin yang sempurna adalah kaedah penyediaan, aspek penurunan permukaan dan sifat asid-bes. Walau bagaimanapun, Mangkin ODH yang ada pada masa kini masih mempunyai aktiviti dan pemilihan yang rendah. Pemilihan produk sintesis adalah berdasarkan struktur mangkin. Gabungan analisis terma dan kajian struktur boleh membantu dalam membimbing dan mendapat pemahaman atas hubungan struktur-aktiviti sistem pemangkin nanostruktur. Mangkin Molibdenum dan Vanadium oksida telah dikenal pasti sebagai salah satu mangkin yang sesuai untuk tindak balas ini. Walau bagaimanpun, adalah sukar untuk mengawal sifat-sifat utama seperti saiz dan struktur kristal, bentuk dan luas permukaan zarah yang menentukan prestasi mangkin tersebut. Dalam kajian ini, mangkin berasaskan Molibdenum oksida (MoOx dan MoVOx) telah disintesis mengunakan kaedah pemendapan terkawal. Parameter seperti pH, kepekatan, suhu dan kadar penambahan telah diubah-ubah. Untuk mangkin MoOx, fasa yang telah diperolehi adalah fasa ‗supramolekular‘ (Mo36O112) dan fasa heksagon (h-MoO3). Protonasi menggalakkan pembesaran struktur mangkin dimana Mo7O24 selaku nukleus membentuk polioxymolibdates. Kedua-dua fasa mempunyai sudut berstruktur pukal dan berkongsi saluran pentagonal sebagai motif stuktur mangkin yang aktif. Untuk mangkin berasaskan MoVOx, fasa amorfus didapati untuk semua prekursor yang diperolehi melalui semburan
kering. Fasa hexagon (MoV2O8) dengan kehabluran yang tinggi dan fasa tetragon [(MoV)5O14] telah diperoleh selepas pengaktifan masing-masing di bawah udara statik dan gas lengai apabila vanadil telah digunakan sebagai sumber vanadium. Oksida Mo5O14
struktur dengan fasa tetragon juga mempunyai saluran pentagon dan herotan fasa dioksida meningkat dengan kandungan vanadium. Penyebaran vanadium mewujudkan kesan 'tapak terasing' yang penting untuk mengelakkan olefin berubah ke tapak teroksida bersebelahan.
Apabila vanadate digunakan sebagai sumber vanadium, fasa berbeza diperolehi pada muatan vanadium yang berbagai. Fasa ortorombik diperhatikan pada peratus muatan vanadium rendah manakala fasa campuran monoklinik dan triklinik didapati pada peratus muatan vanadium yang lebih tinggi. Pengaktifan dengan suhu terprogram menggunakan XRD in-situ telah digunakan untuk mengkaji transformasi dinamik struktur prekursor mangkin molibdenum oksida. Evolusi struktur untuk prekursor MoOx berlaku dari fasa
‗supramolekular‘ ke fasa heksagon metastabil pada suhu 300 ºC. Struktur tersebut akhirnya berubah ke fasa ortorombik yang stabil dari segi termodinamik pada suhu 450 ºC. Bagi prekursor MoVOx, transformasi adalah dari amorfus ke fasa nanokristal pada 400 ºC. Pada 500 ºC, morfologi mangkin berubah ke tetragon iaitu fasa yang stabil dari segi termodinamik. Dengan menghubungkaitkan kajian reaktiviti XRD in-situ bersama dengan DSC in-situ dengan program suhu, aktiviti pemangkinan pada mangkin MoOx tidak diperhatikan manakala untuk mangkin MoVOx, aktiviti pemangkinan telah diperhatikan pada fasa nanokristal.
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ACKNOWLEDGEMENT
First and foremost I would like to thank God for guiding me in completing this dissertation successfully. I would also like to give great thanks to my supervisor, Prof. Dr.
Sharifah Bee Abd Hamid for all the advice, knowledge and helpful support in completing this project. I am also very grateful to Dr. Muralithran Kutty, Dr. Looi Ming Hong and Fazliana Abd Hamid for all their encouragement, assistance and suggestions in helping me to complete my MSc studies. Not to forget, the COMBICAT technical service team for all their contributions in samples characterization. I would also want to expresses sincere gratitude to the National Science Foundation (NSF) for financially supporting me and my MSc program and PPP Grant (PS374/2010B) for providing financial support for this project. Finally I am very grateful to my parents and friends for all their understanding and support throughout my MSc program.
TABLE OF CONTENTS
Abstract iii
Abstrak v
Acknowledgement vii
Table of Contents viii
List of Figures xi
List of Tables xv
List of Symbols and Abbreviations xvii
List of Appendices xix
1.0 INTRODUCTION 1
1.1 Overview of Malaysia‘s Petrochemical Industry 2
1.2 Propene and Its Derivatives 3
1.3 Propene Production Technology 5
1.4 Propene Market and Demand 7
1.5 Oxidative dehydrogenation (ODH) of propane 8
1.6 Molybdenum Based-Catalyst 10
1.7 Project Design Motivation 11
1.8 Objectives 12
2.0 LITERATURE REVIEW 13
2.1 Selective Oxidation 14
2.1.1 Propane Selective Oxidation 14
2.2 Oxydehydrogenation of Propane 16
2.3 Catalyst 18
2.3.1 Heterogeneous catalyst 18
2.3.2 Catalyst Support 19
2.3.3 Nanostructured Catalyst 20
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2.3.4 Bulk Catalyst 20
2.4 Catalyst Synthesis 21
2.4.1 Hydrothermal 22
2.4.2 Sol-Gel 22
2.4.3 Impregnation 23
2.4.4 Precipitation 23
2.4.4.1 Co-precipitation 25
2.5 Transition Metal Oxides (TMO) 26
2.5.1 Mixed Metal Oxide (MMO) 27
2.5.2 Molybdenum Oxides 28
2.5.2.1 Polyoxomolybdates 31
2.5.3 Vanadium Oxides 32
2.5.4 Molybdenum Vanadium Oxides 35
2.6 Structure-Activity Relationship 40
2.7 In situ Structural Techniques Studies 43
3.0 METHODOLOGY 47
3.1 Chemicals and Gases 48
3.2 Synthesis of Mo based Catalyst Precursors 49
3.2.1 Synthesis of MoOx Catalyst 49
3.2.2 Synthesis of MoVOx Catalyst 50
3.2.2.1 MoVOx Activation 51
3.3 Structural and Elemental Characterization 52
3.3.1 X-Ray Diffractogram (XRD) 52
3.3.2 Scanning Electron Microscope (SEM) Imaging 52
3.3.3 Energy Dispersive X-ray (EDX) 53
3.4 Thermal Analysis 53
3.4.1 Thermogravimetric Analysis (TGA) 53
3.4.2 Differential Scanning Calorimetry (DSC) 53
3.5 Reactivity Studies 54
3.5.1 In-situ XRD 54
3.5.2 In-situ DSC 55
4.0 RESULTS AND DISCUSSION 56
PART A 57
4.1 Synthesis and Characterization of MoOx Catalyst 57 4.1.1 Titration Curves 57 4.1.2 Structural and Elemental Characterization 62
4.1.2.1 X-Ray Diffractogram (XRD) Analysis 62 4.1.2.2 Scanning Electron Microscope (SEM) Imaging 69 4.1.2.3 Energy Dispersive X-ray (EDX) 75
4.1.3 Catalytic Thermal Analysis 76
PART B
4.2 Synthesis and Characterization of MoVOx Catalyst 83 4.2.1 Structural and Elemental Characterization of MoVOx 83
4.2.1.1 X-Ray Diffractogram (XRD) Analysis 83
4.2.1.2 Scanning Electron Microscope (SEM) Imaging 96
4.2.1.3 Energy Dispersive X-ray (EDX) 104
4.2.2 Catalytic Thermal Analysis 107
PART C
4.3 Reactivity Studies 117
4.3.1 In-situ X-Ray Diffractogram (XRD) Analysis 117 4.3.2 In-situ Differential Scanning Calorimetry (DSC) Analysis 127
5.0 CONCLUSION 131
BIBLIOGRAPHY 135
APPENDICES 147
xi
LIST OF FIGURES
Page
Figure 1.1: Global Propene Demand by Derivative 4 Figure 1.2: Global Propene Consumption Trends 8
Figure 1.3: Propane ODH Reaction Network 8
Figure 2.1: Propane reaction pathways and reaction enthalpies 15 Figure 2.2: Polyhedra representations of {Mo36} and a {Mo8} unit 40
Figure 2.3: h-MoO3 (xy) projection 41
Figure 2.4: Crystal structure of Mo5O14 43 Figure 3.1: In-situ X-Ray Diffractometer (XRD) 54 Figure 3.2: In-situ Differential Scanning Calorimeter (DSC) 55
Figure 4.1: Titration of 0.10 M AHM with 1.0 M HNO3 at different 57 temperature
Figure 4.2: Titration of AHM at different concentration with 1.0 M HNO3 59 at 1 mL/min
Figure 4.3: Titration of 0.10 M AHM with 1.0 M HNO3 at different rate 60 of addition
Figure 4.4: Titration of 0.10 M AHM with HNO3 at different concentration 61 Figure 4.5: XRD Diffractograms of MoOx showing ‗Supramolecular‘ 62
structure peak characteristics.
Figure 4.6: XRD Diffractograms of MoOx showing hexagonal structure 65 peak characteristics.
Figure 4.7: SEM Imaging of M014 (Supramolecular structure) 71 Figure 4.8: SEM Imaging of M033 (Supramolecular structure) 71
Figure 4.9: SEM Imaging of M064 (Supramolecular structure) 72 Figure 4.10: SEM Imaging of M065 (Supramolecular structure) 72 Figure 4.11: SEM Imaging of M043 (Supramolecular structure) 73 Figure 4.12: SEM Imaging of M021 (Supramolecular structure) 73 Figure 4.13: SEM Imaging of M035 (Mixed Hexagonal and
Supramolecular structure) 74
Figure 4.14: SEM Imaging of M039 (Hexagonal structure) 74 Figure 4.15: TG/DTG Analysis of M033 from 30 °C to 500 °C 76 Figure 4.16: MS Evaluation of M033 from 30 °C to 500 °C 77 Figure 4.17: DSC Analysis of M033 from 30 °C to 500 °C 77 Figure 4.18: TG/DTG Analysis of M039 from 30 °C to 500 °C 79 Figure 4.19: MS Evaluation of M039 from 30 °C to 500 °C 79 Figure 4.20: DSC Analysis of M039 from 30 °C to 500 °C 80 Figure 4.21: TG/DTG Analysis of M035 from 30 °C to 500 °C 81 Figure 4.22: MS Analysis of M035 from 30 °C to 500 °C 81 Figure 4.23: DSC Analysis of M035 from 30 °C to 500 °C 82 Figure 4.24: XRD Diffractograms of MoVOx spray dried precursors synthesized 83
using (a) vanadyl oxalate (b) ammonium metavanadate
Figure 4.25: XRD Diffractograms of M038 before and after calcination 85 (i) spray Dried (ii) calcined in static air (iii) calcined in Helium
Figure 4.26: XRD Diffractograms of M042 before and after calcination 85 (i) spray Dried (ii) calcined in static air (iii) calcined in Helium Figure 4.27: XRD Diffractograms of MoVOx synthesized using ammonium 91
metavanadate calcined in Nitrogen
Figure 4.28: SEM imaging for M038 (a,b,c) and M042 (d,e,f) before calcination 99
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Figure 4.29: SEM imaging for M038 after calcination (a,b,c) in air & 100 calcination (d,e,f) in Helium
Figure 4.30: SEM imaging for M042 after calcination (a,b,c) in air & 101 calcination (d,e,f) in Helium
Figure 4.31: SEM imaging for M044 before (a,b,c) & after calcination 102 in Nitrogen (d,e,f)
Figure 4.32: SEM imaging for M056 before (a,b,c) & after calcination 103 in Nitrogen (d,e,f)
Figure 4.33: Elemental Mapping for MoVOx, M038 (calcination in Helium) 105 Figure 4.34: Elemental Mapping for MoVOx, M042 (calcination in Helium) 106 Figure 4.35: TG/DTG Analysis of M038 from 30 °C to 500 °C. 108 Figure 4.36: MS Evaluation of M038 from 30 °C to 500 °C. 108 Figure 4.37: DSC Analysis of M038 from 30 °C to 500 °C. 109 Figure 4.38: TG/DTG Analysis of M042 from 30 °C to 500 °C. 110 Figure 4.39: MS Evaluation of M042 from 30 °C to 500 °C. 110 Figure 4.40: DSC Analysis of M042 from 30 °C to 500 °C. 111 Figure 4.41: TG/DTG Analysis of M044 from 30 °C to 500 °C. 112 Figure 4.42: MS Evaluation of M044 from 30 °C to 500 °C. 112 Figure 4.43: DSC Analysis of M044 from 30 °C to 500 °C. 113 Figure 4.44: TG/DTG Analysis of M056 from 30 °C to 500 °C. 114 Figure 4.45: MS Evaluation of M056 from 30 °C to 500 °C. 114 Figure 4.46: DSC Analysis of M056 from 30 °C to 500 °C. 115 Figure 4.47: In-situ XRD of MoOx precursor (M033) activation from 118
50 °C-500 °C in Helium Gas
Figure 4.48: In-situ XRD of MoOx precursor (M039) activation from 121
50 °C-500 °C in Helium Gas
Figure 4.49: In-situ XRD of MoVOx precursor (M038) activation from 124 50 °C-500 °C in Helium Gas
Figure 4.50: In-situ DSC of M033 activation from 30 °C -500 °C 127 Figure 4.51: MS of In-situ DSC of M033 activation from 30°C -500 °C 127 Figure 4.52: In-situ DSC of M038 activation from 30 °C -500 °C 129 Figure 4.53: MS of In-situ DSC of M038 activation from 30 °C -500 °C 129
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LIST OF TABLES
Page
Table 1.1: Location of Oil Refineries in Malaysia 3
Table 1.2: Molybdenum compounds in catalysts 10
Table 3.1: List of Chemicals & Gases Used 48
Table 3.2: MoOx precursors synthesized using various conditions 50
Table 3.3: MoVOx precursors synthesized using various conditions 51
Table 4.1: X-Ray Data of M014 (MoOx) 63
Table 4.2: X-Ray Data of M033 (MoOx) 63
Table 4.3: X-Ray Data of M064 (MoOx) 63
Table 4.4: X-Ray Data of M065 (MoOx) 63
Table 4.5: X-Ray Data of M043 (MoOx) 63
Table 4.6: X-Ray Data of M021 (MoOx) ` 63
Table 4.7: X-Ray Data of M039 (MoOx) 67
Table 4.8: X-Ray Data of M035 (MoOx) 67
Table 4.9: Crystallite size of MoOx catalyst precursors 68
Table 4.10: EDX Analysis of MoOx catalyst precursors 75
Table 4.11: X-Ray Data of M038 (calcined in air) 87
Table 4.12: X-Ray Data of M038 (calcined in He) 87
Table 4.13: X-Ray Data of M042 (calcined in air) 89
Table 4.14: X-Ray Data of M042 (calcined in He) 89
Table 4.15: X-Ray Data of M044 (calcined in Nitrogen) 92
Table 4.16: X-Ray Data of M047 (calcined in Nitrogen) 92
Table 4.17: X-Ray Data of M045 (calcined in Nitrogen) 93
Table 4.18: X-Ray Data of M056 (calcined in Nitrogen) 93 Table 4.19: Crystallite size of MoVOx samples 95 Table 4.20: EDX Analysis of MoVOx catalyst precursors 104 Table 4.21: X-Ray Data of M033 Activation (300 °C) 119 Table 4.22: X-Ray Data of M033 Activation (500 °C) 119 Table 4.23: Crystallite size of M033 after in-situ XRD activation 120 Table 4.24: X-Ray Data of M039 Activation (240 °C) 122 Table 4.25: X-Ray Data of M039 Activation (500 °C) 122 Table 4.26: Crystallite size of M039 after in-situ XRD activation 123 Table 4.27: X-Ray Data of M038 Activation (500 °C) 126 Table 4.28: Crystallite size of M038 after in-situ XRD activation 126
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LIST OF SYMBOLS AND ABBREVIATIONS
2 Two theta
ABS Acrylonitrile-butadiene-styrene AHM Ammonium HeptaMolybdate C3 Propane
C4 Butane C5 Pentane
d value Interplanar spacing
DSC Differential Scanning Calorimetry DTG Differential Thermogravimetric EDX Energy Dispersive X-ray FCC Fluid Catalytic Cracking FWHM Full Width Half Maximum LPG Liquefied Petroleum Gas m/e Mass/charge ratio
MoOx Molybdenum based Oxides
MoVOx Molybdenum Vanadium based Oxides MS Mass Spectroscopy
ODH Oxidative Dehydrogenation PDF Powder Diffraction File PDH Propane Dehydrogenation PP Polypropylene
SAN Styrene acrylonitrile
SAXS Small-Angle X-ray Scattering
SEM Scanning Electron Microscope Tcf Trillion cubic feet
TG Thermogravimetric Ts Sample Temperature
WAXS Wide-Angle X-ray Scattering XRD X-Ray Diffraction
XRF X-Ray Fluorescence
xix
List of Appendices
Page
1. List of Proceedings 148
2. List of Publication 149