Title: Low Temperature Reaction Network of n-Butenes with Acid Zeolites of MFI Topology
Other Titles: Lage Temperatuur Reactienetwerk van n-Butenen over Zure Zeolieten met MFI Topologie
Authors: Henry, Mathieu
Issue Date: 29-May-2012
Abstract: <w:latentstyles deflockedstate="false" defunhidewhenused="true" defsemihidden="true" defqformat="false" defpriority="99" The one-step catalytic transformation of n-butenes into p-xylene would constitute the basis for a sustainable process allowing conversion of cheap gases into a high value base chemical. According to classical carbenium ion chemistry, this conversion should be possible if catalyzed by solid acids. According to literature, the transformation in presence of sterical constraints, viz. with H-ZSM-5 catalysts, still leads depending on the reaction temperature to a large variety of products, ranging from olefins over alkylnaphthenes to aromatics. The large variety of competing reactions over acid sites explains this behavior.It was the aim of the present study to disentangle the complex reaction network, by working at unusually low reaction temperatures, viz. 220 &#176;C, and by using a typical zeolite catalyst with shape selective properties, reducing the number of molecules in the product slate. For the latter purpose, the most common modifications of the ZSM-5 zeolite framework were made. The samples obtained were then characterized by a pleiade of state-of-the-art techniques, with emphasis on the quantitative description of the acidity spectrum.First a series of ZSM-5 samples with varying number of Br&oslash;nsted acid sites were prepared by changing the Si/Al ratio of the framework from 12 till 400. All framework Al atoms were distributed homogeneously across the crystals. Only one of the Al-rich samples contained significant amounts of extra-framework Al. Selected samples were subjected to a desilication treatment with an aqueous NaOH solution, showing reduced crystallinity and enhanced mesoporosity. Alternatively, specific ZSM-5 samples were prepared either with an aluminum depleted external surface or with a pronounced Al gradient in the crystals. The n-butenes were converted at 220 &#176;C in a high throughput continuous flow reactor with 16 parallel reactors. The products in such conditions devoid of aromatics were analyzed on-line. To achieve maximum chromatographic separation power, the products were passed over a hydrogenation reactor, allowing determination of the structure of the hydrocarbon skeletons of all products. Important intermediates in the products occasionally were fed to the reactor as feedstock.The acidic properties of the H-ZSM-5 zeolites as determined by 27Al and 29Si MAS NMR, NH3-TPD, FT-IR of silanols and adsorbed pyridine, were found to be consistent among each other, namely, one framework aluminum atom yielding one (Br&oslash;nsted) acid site. Despite the wide variety of ZSM-5 modifications, the carbon number product distributions can be explained by a oligomerization – cracking – re-alkylation mechanism. To rationalize quantitatively the selectivity to all products and the distributions in all fractions, the amount of coke remaining after reaction has to be taken into account. From thermogravimetric analysis (TGA) and FT-IR of used H-ZSM-5 catalysts, it follows that coke residues are composed of soft coke (large paraffinic residues) and of hard coke (heavy aromatic residues), bulkier than soft coke. Both specimens are desorbed at significantly different temperatures from the catalysts, the aromatic species being retained at the higher temperatures.H-ZSM-5 catalysts with a high concentration of framework aluminum atoms in the bulk systematically retain an amount of hard coke substantially higher than 3 wt%, sufficient to eliminate the microporosity determined by low temperature nitrogen physisorption. This way, access to acid sites present in the micropores is restricted. Steady state catalysis occurs therefore at the external surface, the mesopores and macropores, yielding distributions in many product fractions that match thermodynamically expected values. Despite the loss of acid sites in the micropores, these catalysts still present high reaction rates, as the aluminum concentration at the external surface is high.On the other hand, H-ZSM-5 materials, with a low concentration of framework aluminum atoms in the bulk, show partially accessible micropores. A hard coke amount lower than 3 wt% is typical for these catalysts. The reaction rate is rather low, as the aluminum concentration is proportional to the concentration of Br&oslash;nsted acid sites. The selectivity to cracked products is high as a result of the ß-scission of feed oligomers located in the micropores. The product distributions are generally affected by the shape-selectivity exerted by the sites in the intracrystalline voids. Mono-methylbranched molecules with the branching at the chain end, viz. 2-methyl isomers, are abundantly present. This different behavior among the two catalyst classes is again encountered when different C8 feeds are used, the existence of pore mouth effects being more pronounced due to the large size of the feedstocks compared to that of n-butenes. Whereas Al-rich catalysts present blocked H-ZSM-5 micropores restricting the catalysis over the external surface or at the pore mouth, Si-rich H-ZSM-5 show pronounced effects of shape selectivity and consequently the presence of intracrystalline catalysis.
Table of Contents: Acknowledgments – Remerciements III
Abstract V
Samenvatting IX
List of abbreviations XIII
Table of contents XVII
Chapter 1: Literature Overview 1
1.1. Introduction 1
1.2. ZSM-5 materials 1
1.2.1. ZSM-5 structure 2
1.2.2. ZSM-5 acidity 3 Nature of acid sites 3 Correlation between acidic properties and the aluminum content 6 Methodology for measuring acidic properties 7
1.2.3. Dealumination of ZSM-5 9
1.2.4. Alkali-treatment of ZSM-5 materials 10
1.2.5. Generation of an aluminum depleted external surface 12
1.3. One-pot n-butenes aromatization 12
1.3.1. Influence of ZSM-5 acid properties 13
1.3.2. Influence of ZSM-5 morphological properties 14
1.4. Dimerization and olefin interconversion 15
1.4.1. n-Butenes dimerization 15
1.4.2. Skeletal isomerization of n-butenes 18
1.4.3. Skeletal (hydro)isomerization of large paraffins 22
1.4.4. Cracking reactions 25
1.4.5. Cyclization 28
1.4.6. Hydride transfer 30
1.4.7. Isomerization of xylenes 34
1.5. Coking reactions 39
1.6. Conclusions 42
1.7. Scope of the work 45
Chapter 2: n-Butenes Conversion at Low Temperature over Acid ZSM-5 Zeolites of Homogeneous Composition 47
2.1. Introduction 47
2.2. Experimental 49
2.2.1. Materials 49
2.2.2. Material characterization 49 X-Ray Diffraction (XRD) 49 Scanning Electron Microscopy (SEM) 50 Specific surface and porosity 50 27Al Magic Angle Spinning Nuclear Magnetic Resonance (27Al MAS NMR) 50 Temperature Programmed Desorption of ammonia (NH3-TPD) 51 Fourier Transform Infra-Red spectroscopy (FTIR) 52 Thermogravimetric analysis (TGA) 52
2.2.3. Catalytic tests 54
2.2.4. Thermodynamic equilibrium calculations 55
2.3. Results and discussion 57
2.3.1. Physico-chemical characterization 57 Crystallinity and morphological properties 57 Acid properties 60 Consistence of various ways of determining acid properties 63
2.3.2. Change of n-butenes dimerization activity with physico-chemical characteristics of MFI catalysts 66
2.3.3. Changes in reaction selectivity and product distribution 69 Reaction scheme 69 Carbon deposition on catalysts 72 Reaction selectivities 77 Product Distributions 79 Product Distribution in C8= fraction 79 Product Distribution in C5= fraction 82 Product distributions in the C7= fraction 85
2.4. Conclusions 87
Chapter 3: n-Butenes Conversion over Acid ZSM-5 Zeolites Containing a Large Amount of EFAl 91
3.1. Introduction 91
3.2. Experimental 91
3.3. Physico-chemical characterization of samples 93
3.3.1. Topology, morphology and porosity 94
3.3.2. Distribution of acid sites 96 27Al MAS NMR 96 NH3-TPD 97 IR spectroscopy of pyridine adsorbed H-ZSM-5 99 IR spectroscopy of pure H-ZSM-5 101
3.3.3. Quantification of acidity 102
3.4. Catalytic results and discussion 103
3.4.1. Characterization of used catalysts 103
3.4.2. Dimerization rates 104
3.4.3. Change of low temperature selectivity 104
3.4.4. Change of low temperature product distributions 105 Octenes 105 Pentenes 111 Heptenes 112
3.5. Conclusions 117
Chapter 4: n-Butenes Conversion at Low Temperature over Acid ZSM-5 Catalysts Containing an Aluminum Profile 119
4.1. Introduction 119
4.2. Experimental 121
4.2.1. Materials 121
4.2.2. Material characterization 123
4.2.3. Catalytic tests 124
4.3. Results and discussion 125
4.3.1. Physico-chemical characterization 125
4.3.2. Change of n-butene dimerization activity of MFI catalysts with physico-chemical characteristics 130
4.3.3. Changes in reaction selectivity and product distribution 132 Reaction scheme 132 Carbon deposition on catalysts 135 Reaction selectivities 136 Product Distributions 137 Product Distribution in C8= fraction 137 Product Distribution in C5= fraction 141 Product distributions in the C7= fraction 142
4.4. Conclusions 143
Chapter 5: n-Butenes Conversion at Low Temperature over Mesoporous Acid ZSM-5 Zeolites 147
5.1. Introduction 147
5.2. Experimental 149
5.2.1. Materials 149
5.2.2. Materials characterization 150
5.2.3. Catalytic Tests 150
5.3. Results and discussion 152
5.3.1. Physico-chemical characterization of mesoporous catalysts 152 Crystallinity and morphological properties 152 Acid properties 156
5.3.2. Changes of n-butenes dimerization activity with physico-chemical characteristics of mesoporous catalysts 161
5.3.3. Products selectivities 162 Reaction scheme 162 Carbon deposition on catalysts 163 Reaction selectivities 164 Products Distributions 166 Product Distributions in the C8= fraction 166 Product Distributions in the C5= fraction 167
5.4. Conclusion 169
Chapter 6: Octene Feed Conversion at Low Temperature over H-ZSM-5 Zeolites with Different Composition 171
6.1. Introduction 171
6.2. Characterization of used materials 172
6.3. Change of reaction selectivities 174
6.4. Conversion of 2,4-dimethyl-1-hexene 176
6.5. Conversion of 2,5-dimethyl-1,5-hexadiene 186
6.6. Conversion of 1,7-octadiene 191
6.7. Conversion of 1-octene 197
6.8. Conclusions 200
Chapter 7: General Conclusions and Perspectives 203
7.1. H-ZSM-5 zeolites with various amounts of framework Al 204
7.2. H-ZSM-5 zeolites with extra-framework Alx-Oy species 206
7.3. H-ZSM-5 zeolites with concentration gradient of framework Al across the crystals 207
7.4. H-ZSM-5 zeolites with mesopores 209
7.5. Use of potential intermediates as reaction feeds 210
7.6. Perspectives for future work 212
List of references 215
Appendix: Interpretation of chromatograms 227
List of publications 233
List of oral presentations 233
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis

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