Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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CHAPTER 4 peroxide dissociation thereby generating a higher radical concentration and promoting the chain initiation. Scheme 4‐2: Schematic reaction sequence in the radical autoxidation of p‐xylene and potential roles of Ag, CeO2 and p‐toluic acid promoters. Cumene oxidation with FSP synthesized CeO2‐SiO2 supported catalysts proceeded with a low reaction rate and gave mainly acetophenone as a product. This was not the case when imp 10%Ag/SiO2‐900 was used in the presence of ceria nanoparticles, but here leaching of silver is important. In both cases the product distribution differs from typical cumene autoxidations for which the main product is cumene hydroperoxide. Thermal decomposition of the hydroperoxide alone cannot be the reason since in the absence of ceria and benzoic acid, cumene hydroperoxide was by far the dominating product. Ceria was used before as a catalyst for alkyl aromatics oxidation with bromate affording the carbonyl compound as the main product [58]. Acetophenone can be formed from cumene hydroperoxide either by reaction with a reducing agent [62] or generation of a hydroxyl radical via cleavage of the oxygen‐oxygen bond [57] resulting an intermediate α,α‐ dimethylbenzyloxy radical. According to Bhattacharya et al. [57] this radical preferably reacts to form acetophenone which could be supported by ceria. Addionally, the initiation by homogeneous silver as observed with the impregnated catalyst is likely faster compared to the chain termination by a 120
4.5 Conclusions heterogeneous radical scavenger explaining the higher activity of the impregnated catalyst as compared to the FSP catalysts. 4.5 Conclusions Silver particles supported on silica effectively promoted the side chain oxidation of toluene, p‐xylene, ethylbenzene and cumene without any solvent. Except for toluene, no elevated pressures were needed to facilitate the reaction. Addition of a carboxylic acid considerably enhanced the reaction rate. CeO2 had an ambivalent influence on the reaction and could both promote and inhibit product formation depending on substrate and reaction conditions. Except for cumene oxidation silver catalysts prepared via impregnation were inferior to flame‐made catalysts which gave TONs up to 2000. In addition, the flame‐synthesized catalysts were significantly less prone to leaching compared to the impregnated catalyst. This may be exploited in the future to prepare more stable catalysts for liquid phase reactions where leaching should be circumvented. In order to be active, silver is required to be metallic as suggested by EXAFS analysis. The oxidation reaction is most likely based on a radical mechanism where CeO2 possibly assists silver as a chain initiator, while the carboxylic acid influences the dissociation of peroxides to radical species to initiate radical chains. Ceria as a radical scavenger additionally induces chain termination. 121
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TECHNICAL UNIVERSITY OF DENMARK (DT
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Preface The present dissertation su
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the carboxylic acid, ceria inhibite
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Resume Denne afhandling giver indle
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hvorimod mængden af katalysator ku
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2.3.8 Oxidation of sulfides .......
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5.3.5 Macroscopic mass transport in
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Chapter 1 Introduction With its gre
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1. Introduction Scheme 1‐1: High
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1. Introduction Figure 1‐2: Phase
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1. Introduction [28] S. Bawaked, N.
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2.1 Introduction 2.1 Introduction E
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2.2.1 Synthesis of gold catalysts 2
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2.2 Catalyst synthesis for oxidatio
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2.2 Catalyst synthesis for oxidatio
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2.3 Selective liquid‐phase oxidat
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Table 2-1: Overview over alcohol ox
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Ph O O Ph (4) (5) 2.3 Selective liq
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Table 2-4: Side-chain oxidation of
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2.3.4 Oxidation of cyclohexane 2.3
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Table 2-5: Oxidatin of cyclohexane.
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Table 2-6: Ring hydroxylation of ar
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Table 2-7: Aerobic oxidation of ami
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Table 2-8: Oxidation of hydroquinon
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Table 2-9: Oxidation of silanes wit
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Table 2-10: Oxidation reactions cat
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2.4 Strengths and opportunities of
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2.6 References 2.6 References [1] M
Page 83 and 84: 2.6 References [56] H. Tumma, N. Na
Page 85 and 86: 2.6 References [113] C. Keresszegi,
Page 87 and 88: 2.6 References [169] Q. Zhu, Y. Lia
Page 89 and 90: 2.6 References [223] M. Fetizon, M.
Page 91 and 92: Chapter 3 Selective Liquid-Phase Ox
Page 93 and 94: 3.2 Experimental 3.2.1 Catalyst pre
Page 95 and 96: 3.3 Results Scattering amplitudes a
Page 97 and 98: Absorption (a.u.) 1.2 1.0 0.8 0.6 0
Page 99 and 100: Conversion (%) 100 90 80 70 60 50 4
Page 101 and 102: 3.3 Results Figure 3‐5: In situ X
Page 103 and 104: 3.3 Results Table 3‐2: Conversion
Page 105 and 106: 3.3 Results In order to gain insigh
Page 107 and 108: 3.3 Results the cost of selectivity
Page 109 and 110: 3.4 Discussion synergetic interacti
Page 111 and 112: 3.5 Conclusions to corroborate the
Page 113 and 114: 3.6 References [29] T. Mitsudome, Y
Page 115 and 116: Chapter 4 Selective Side-Chain Oxid
Page 117 and 118: 4.2 Experimental single‐step flam
Page 119 and 120: 4.2 Experimental alcohol (Aldrich,
Page 121 and 122: 4.3 Results Scheme 4‐1: Oxidation
Page 123 and 124: 4.3 Results Figure 4‐2: Formation
Page 125 and 126: 4.3 Results oxidation by either 2,6
Page 127 and 128: Absorption (a.u.) Absorption (a.u.)
Page 129 and 130: 4.3 Results atomic mixing of Ag and
Page 131 and 132: 4.3 Results Figure 4‐7: Influence
Page 133: 4.4 Discussion and mechanistic cons
Page 137 and 138: 4.6 References [29] S.I. Zabinsky,
Page 139 and 140: Chapter 5 Experimental Determinatio
Page 141 and 142: 5.2 Experimental The most important
Page 143 and 144: 5.2 Experimental Eurotherm 2216e te
Page 145 and 146: Figure 5-1: Schematic view of the e
Page 147 and 148: (Eq. 5‐5) AB i j ε AB i j β 5.2
Page 149 and 150: (a) (b) (c) (d) (e) (f) (g) (h) (i)
Page 151 and 152: 5.3 Results two association sites o
Page 153 and 154: Pressure (bar) 150 145 140 135 5.3
Page 155 and 156: 5.3 Results Figure 5‐7: Oxidation
Page 157 and 158: Weight (g) 5.3 Results Figure 5‐8
Page 159 and 160: 5.4 Discussion catalytic reactions
Page 161 and 162: 5.4 Discussion corresponding CO2‐
Page 163 and 164: 5.5 Conclusions 5.5 Conclusions The
Page 165 and 166: 5.6 References [29] I. Tsivintzelis
Page 167 and 168: 6.1 Introduction 6.1 Introduction A
Page 169 and 170: 6.2 Experimental [38]. In a typical
Page 171 and 172: 6.3 Results out in 1 mm quartz tube
Page 173 and 174: 6.3 Results Figure 6‐2: Structure
Page 175 and 176: 6.3 Results Figure 6‐4: Epoxidati
Page 177 and 178: 6.3 Results Figure 6‐6: Compariso
Page 179 and 180: 6.3 Results Figure 6‐8: Influence
Page 181 and 182: 6.3 Results investigated the epoxid
Page 183 and 184: 6.3 Results Figure 6‐10: Co‐epo
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6.3 Results Figure 6‐11: EXAFS an
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6.4 Discussion would need to be for
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6.5 Conclusions Formation of the ep
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6.6 References [28] J. Perles, N. S
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Chapter 7 Concluding Remarks 7.1 Co
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7.2 Final remarks and outlook speci
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Acknowledgements Acknowledgements T
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Curriculum Vitae Matthias Josef Bei
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