Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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CHAPTER 4 Table 4‐6: <strong>Oxidation</strong> of cumene with silver catalysts in the presence of CeO2 and carboxylic acid. Reaction conditions: 122 mmol cumene, 3 mol‐% benzoic acid, 100 mg biphenyl, 100 mg catalyst, 140 °C, 3 h reaction time. Catalyst indicates a shift from heterogeneous to homogeneous catalysis and explains also the inferior performance and the different product distribution over the FSP‐catalysts that tend to leach less. Note that peroxo/peroxide species formed during the presence of the catalyst may also maintain the reaction to a certain extent. 4.4 Discussion and mechanistic considerations The oxidation of different alkyl aromatic compounds can be promoted by heterogeneous silver catalysts prepared via impregnation or flame spray pyrolysis. Yields are similar to other heterogeneous catalyst systems [19] but there is still a large gap in performance compared to the industrial homogeneous AMOCO process. In the case of p‐xylene and ethylbenzene the FSP catalysts were considerably more active with respect to the Ag loading. In addition, the FSP catalysts exhibited a significantly lower leaching tendency which might originate from a stronger metal‐support interaction established during synthesis by combustion of the liquid precursor. In contrast, the impregnated catalyst performed superior in the cumene oxidation and gave another product distribution than the FSP catalysts thus indicating a different mechanism changing from heterogeneous to homogeneous catalysis. Yields (%) The suppression of the oxidation reaction by radical scavengers and the formation of hydroperoxides strongly suggest that the investigated p‐xylene oxidation reaction is based on a radical reaction mechanism, which is supported by many examples from the literature for analogous cases [e.g. 9, 54, 55]. Radical reactions comprise mainly three steps, namely initiation, radical chain reaction and termination (Scheme 4‐2). The ratio of rate constants for these steps determines the overall catalytic performance and is dependent on the catalyst. Conversion of starting material was 118 Combined imp 10%Ag/SiO2‐900 4.2 9.1 9.5 22.8 330 1 FSP 1%Ag/10%Ce‐SiO2 1.6 0.1 0.6 2.3 310 2 FSP 1%Ag/30%Ce‐SiO2 1.7 0.1 0.5 2.3 310 3 FSP 1%Ag/50%Ce‐SiO2 2.4 0.1 0.4 2.9 390 4 SiO2 O OH OOH TON < 0.1 0.1 1.7 1.8 ‐ 5 Entry
4.4 Discussion and mechanistic considerations already observed when only the impregnated silver catalyst was used. The addition of peroxides to start the reaction as reported in other cases [56] was not necessary. Hence, silver facilitates the formation of initial radicals (Scheme 4‐2, step 1). The chain reaction is likely not to be supported by the silver catalyst, firstly, because the product distribution in the absence of carboxylic acid is typical for uncatalyzed autoxidations [57, 58]. In addition, chain propagation is fast compared to initiation. This would also require a catalyst in the same phase as e.g. in case of the homogeneous Co/Mn/Br catalyst system [9] but a correlation between leaching and catalyst activity could not be established at least for p‐xylene. Diffusion of the radicals to Ag particles and CeO2 would be likely to occur in the same time regime so that the rate constants for chain propagation and termination would be similar if chain propagation relies on silver. Silver may, however, play a role in the decomposition of hydroperoxides influencing the radical concentration. FSP synthesized catalysts featured both metallic silver and Ag‐O species which might be reduced under reaction conditions since the used FSP 1%Ag/10%CeO2‐SiO2 catalyst showed a stronger Ag‐Ag backscattering amplitude in the EXAFS spectrum. Silver in the active impregnated catalyst was found to be only metallic. Thus, metallic silver appears to be required for an active catalyst. The ambivalent influence of ceria suggests that ceria is interacting with two parts of the overall radical reaction one of which is obviously the chain termination (Scheme 4‐2, step 3), since the reaction is suppressed when only ceria and the silver catalyst is used. Initiation by silver might be influenced by ceria (step 1). Ceria is known to activate oxygen [39], which could then support breaking of C‐H bonds in p‐xylene which are stronger compared to ethylbenzene and cumene. Thus, CeO2 had no beneficial effect in the latter cases. Additionally, the latter more stable peroxo radicals [59] might have a longer half life which makes quenching by scavengers, which CeO2 is, more likely. Molecular oxygen does not play a dominant role in the radical chain reaction since the reaction of alkyl radicals with molecular oxygen is fast [57]. Addition of carboxylic acid did not only increase the overall yield but also influenced the product distribution and hence changed the overall mechanism either directly via the propagation mechanism [54] (Scheme 4‐2, step 2) or via destabilization of the termination product, i.e. the hydroperoxide, in the presence of radicals [60]. Indeed, in the case of p‐xylene, ethylbenzene and cumene oxidation the hydroperoxide exhibited transient behavior. A higher degree of hydroperoxide dissociation would also result in a higher radical concentration which explains why not only the product distribution was shifted but also a higher overall yield was obtained. The higher O‐H bond dissociation energies (BDE) of benzoic acid and toluic acid compared to the C‐H BDE of alkyl aromatic compounds and the O‐H BDE of the corresponding peroxides [59] makes the presence of carboxyloxo radical species generated from the added carboxylic acid unlikely. Thus, the carboxylic acids presumably promote the peroxide dissociation. Additionally, the capability of carboxylic acids to act as an H‐bond acceptor can stabilize ∙OH radicals [61]. This lowers the activation energy for e.g. 119
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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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Page 81 and 82: 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: 4.3 Results Figure 4‐7: Influence
Page 135 and 136: 4.5 Conclusions heterogeneous radic
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
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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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