Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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CHAPTER 2 using microwave irradiation [79]. This underlines the great potential of silver in selected cases. Note that the support alone gave considerably lower conversions at high selectivity so there might be a specific synergistic interaction between Ag particles and Ti sites though this was not investigated. Note additionally that other catalysts required more expensive TBHP as epoxidizing agent. Silver catalysts are also useful in the aerobic oxidation of cyclohexane and alkyl aromatic compounds; especially in the latter case high TOFs were obtained for p‐xylene and ethylbenzene oxidation with Ag supported on CeO2‐SiO2 [91] though the catalyst activity was still no‐where near the industrial catalyst. Massive leaching was encountered with impregnated catalysts; flame‐spray pyrolysis as an alternative synthesis technique gave more stable catalysts [91]. The capability of Ag to form peroxides from alkyl aromatic compounds would also allow epoxidation reactions as proposed by ref. [124]. Within silver catalysis, special attention has to be drawn to Ag‐V materials which are very universal catalysts catalyzing alcohol oxidation [94], sulfide oxidation [226], epoxidation [92], benzene hydroxylation [188] and cyclohexane oxidation [167], the latter with high selectivity to oxygenated products even under harsh reaction conditions. For comparison, the prominently used Co/Mn/Br oxidation system afforded mainly carbon oxides. Both catalysts with metallic and oxidic silver can be synthesized (though the oxidation state under reaction conditions has not yet been investigated). Silver has recently found an application as hydroquinone oxidation catalyst with H2O2 [218] (while other catalysts can employ oxygen) demonstrating its principal suitability for this reaction. Amine oxidation certainly is also a field of interest for silver catalysts though silver appears to be especially promising in alcohol oxidation and epoxidation reactions. 2.4.4 Cascade reactions Combining two reaction steps (or more) to one step simplifies the production of target compounds, can potentially result in higher yields and is therefore a field of research which has started to attract attention resulting in an increasing amount of literature [246‐249]. Therefore, materials catalyzing multistep cascade reactions need to be multifunctional. Bifunctionality is e.g. given where a redox active noble metal is combined with a basic [249] or acidic [250] support. Additionally (and probably more problematic), the reaction conditions need to be adjusted to suit both reactions. This compromise may prevent the combination of the two most reactive catalysts for one reaction and is therefore a true chance for catalysts with apparently lower efficacy active under different reaction conditions. Examples for cascade reactions can also be found for the coinage metals. An example for gold was reported for epoxidation [124] where in a first step cumene was oxidized by Au/CeO2 to cumene hydroperoxide which epoxidized olefins over Ti‐MCM‐41 also present in the reaction system (thus combining two catalysts rather than having a bifunctional catalyst). For the first step, the 62
2.4 Strengths and opportunities of coinage metals as oxidation catalysts activation of cumene is crucial which can also be facilitated by copper [150] and silver [91] catalysts. Also other alkyl aromatic compounds might be applicable resulting in more useful byproducts compared to α,α‐dimethylbenzyl alcohol obtained from cumene. Since additionally silver supported on SiO2 is a suitable catalyst [91], an Ag/Ti‐SiO2 catalyst could provide true bifunctionality for both reaction steps. Cu catalysts for the anaerobic alcohol oxidation can conduct two reaction steps concomitantly as was shown for carveol [100] being principally an isomerization reaction. Based on this principle, the amination of alcohols over CuAl‐HT also gave good results, running through the steps alcohol dehydrogenation – imine formation – imine hydrogenation [102]. Due to the lack of hydrogenation activity [86] similar reactions on silver catalysts under anaerobic conditions seem unlikely. (Di)amination was also successfully achieved during hydroquinone oxidation [216] resulting in highly functionalized reaction products. Memoquin (a drug candidate) could be synthesized in 71 % yield compared to 17 % from the conventional synthesis. A prerequisite for this reaction is a good chemoselectivity of the catalyst for hydroquinone oxidation leaving the amine group untouched. 2.4.5 Catalyst deactivation During oxidation of organic substrates, suitable ligands like carboxylic acids are formed. Leaching of the active species is therefore a common deactivation pathway for copper catalysts, especially where CuxO particles are present (Scheme 2‐19) [37, 172, 173, 178]. Since these species are often not necessary from a catalytic point of view, an optimized catalyst synthesis offers a good opportunity for circumventing at least strong leaching. Catalysts with both isolated Cu species and CuxO particles may exhibit significant leaching problems only during the first catalytic run after which the catalyst is stable [139, 177]. Therefore, undesired CuxO could be removed by a suitable pretreatment procedure preceding the first use as catalyst. In some cases Al [38] and potentially also Fe [150] may stabilize the Cu species against leaching. Leaching studies may be further complicated since leached Cu can precipitate again as Cu2O resulting in an apparently underestimated leaching [43]. On the other hand, reduced copper catalysts used in anaerobic oxidation often show good reusability indicating low leaching. 63
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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
Page 25 and 26: 2.2.1 Synthesis of gold catalysts 2
Page 27 and 28: 2.2 Catalyst synthesis for oxidatio
Page 29 and 30: 2.2 Catalyst synthesis for oxidatio
Page 31 and 32: 2.3 Selective liquid‐phase oxidat
Page 33 and 34: Table 2-1: Overview over alcohol ox
Page 45 and 46: Ph O O Ph (4) (5) 2.3 Selective liq
Page 51 and 52: Table 2-4: Side-chain oxidation of
Page 53 and 54: 2.3.4 Oxidation of cyclohexane 2.3
Page 55 and 56: Table 2-5: Oxidatin of cyclohexane.
Page 59 and 60: Table 2-6: Ring hydroxylation of ar
Page 63 and 64: Table 2-7: Aerobic oxidation of ami
Page 65 and 66: Table 2-8: Oxidation of hydroquinon
Page 69 and 70: Table 2-9: Oxidation of silanes wit
Page 71 and 72: Table 2-10: Oxidation reactions cat
Page 73 and 74: 2.4 Strengths and opportunities of
Page 75: 2.4 Strengths and opportunities of
Page 79 and 80: 2.4 Strengths and opportunities of
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
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Absorption (a.u.) Absorption (a.u.)
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4.3 Results atomic mixing of Ag and
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4.3 Results Figure 4‐7: Influence
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4.4 Discussion and mechanistic cons
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4.5 Conclusions heterogeneous radic
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4.6 References [29] S.I. Zabinsky,
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Chapter 5 Experimental Determinatio
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5.2 Experimental The most important
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5.2 Experimental Eurotherm 2216e te
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Figure 5-1: Schematic view of the e
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(Eq. 5‐5) AB i j ε AB i j β 5.2
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(a) (b) (c) (d) (e) (f) (g) (h) (i)
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5.3 Results two association sites o
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Pressure (bar) 150 145 140 135 5.3
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5.3 Results Figure 5‐7: Oxidation
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Weight (g) 5.3 Results Figure 5‐8
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5.4 Discussion catalytic reactions
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5.4 Discussion corresponding CO2‐
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5.5 Conclusions 5.5 Conclusions The
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5.6 References [29] I. Tsivintzelis
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6.1 Introduction 6.1 Introduction A
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6.2 Experimental [38]. In a typical
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6.3 Results out in 1 mm quartz tube
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6.3 Results Figure 6‐2: Structure
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6.3 Results Figure 6‐4: Epoxidati
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6.3 Results Figure 6‐6: Compariso
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6.3 Results Figure 6‐8: Influence
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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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