Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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CHAPTER 2 effective catalyst for styrene epoxidation with TBHP (even better than Au catalysts) but leaching studies were unfortunately not reported. Since styrene oligomerization was not taken into account, however, a final conclusion cannot be drawn. Cu2(OH)PO4 as an example for Cu phosphates also oxidized styrene under aerobic conditions [60] though the selectivity was low. Cu phosphates are in general good oxidation catalysts (useful e.g. also in benzene hydroxylation [59]) which might be due to isolation of the Cu ions by interjacent phosphate groups. Cu catalysts are also suitable for hydroquinone oxidation with oxygen; a good example being Cu/Al(OH) giving full conversion under mild conditions already at room temperature [216]. Cu3(BTC)2 also exhibited high TOFs [214]. Though for hydroquinone oxidation a gold catalyst gave the highest TOFs [219], the necessity of chloroform makes this approach less attractive favoring Cu catalysts especially when taking – once more – the difference in price into account. As outlined before, alcohol oxidation is certainly a reaction where gold is highly suitable – under aerobic conditions. Cu affects alcohol oxidation under anaerobic conditions and among the oxidation reaction this also the only reaction where metallic copper is the active phase. Therefore, alcohol oxidation is in principal the reaction where the three metals are most comparable. This reaction requires high temperatures and the reaction rates are usually significantly lower than in aerobic alcohol oxidation. Cu (and also Ag, vide infra) should thus not be seen as competitors but rather as a complementary tool to aerobic alcohol oxidation catalyzed by Au (or Pt‐group metals): oxygen‐free conditions are safer to handle, aldehydes which are highly prone to overoxidation can be synthesized with high selectivities (e.g. octanal [51]) and most importantely different chemoselectivities can be obtained with Cu catalysts [52, 100]. Also Pd catalysts catalyze the anaerobic oxidation of alcohols but both rapid poisoning by CO [114] and the hydrogenation activity of Pd (giving e.g. toluene from benzyl alcohol) were observed as a problem. The hydrogen remaining in the system also offers opportunities – thus, Cu catalysts can also catalyze isomerization reactions under anaerobic, acceptor‐free conditions [101]. On a mechanistic level (Scheme 2‐18), alcohol oxidation on e.g. palladium catalysts [114] is assumed to occur via a dehydrogenation mechanism [7] where the O‐H bond is broken upon adsorption followed by β‐C‐H bond scission affording the carbonyl compound. Oxygen (or another hydrogen acceptor) regenerates the metallic surface either by removal of hydrogen or oxidative cleansing of the metal surface from adsorpt catalyst poisons like CO. Pre‐adsorpt oxygen may facilitate breaking of O‐H bonds [242, 243] which can also be supported by bases as often necessary for the Au catalyzed alcohol oxidation [7]. This may be the reason why anaerobic alcohol oxidation has not been described over gold catalysts so far. Both for Au [244] and for Ag [67] a positively polarized transition state was proposed. Since alcohol oxidation proceeds over Cu (and Ag) in the absence of an oxidant, this also makes the dehydrogenation mechanism likely. From high‐vacuum experiments it is known that alcohol 60
2.4 Strengths and opportunities of coinage metals as oxidation catalysts oxidation proceeds both on clean Cu and Ag surfaces without pre‐adsorbed oxygen though in the case of Ag with a higher activation energy [10, 245]. O ad (2 OH ad) Scheme 2‐18: Schematic alcohol dehydrogenation mechanism [7], reproduced with permission from the American Chemical Society © 2004. 2.4.3 Catalysis by silver RCH 2OH ad H ad RCH 2O ad Ag is also active under anaerobic conditions [86]. In contrast to Cu, no hydrogenation activity was observed thus double bonds in alcohols remained untouched: only the C‐O bond was oxidized in cinnamic alcohol. An important design parameter for active Ag catalysts appears to be the abundance of both acidic and basic sites on the support [67] but in general studies shedding light into reaction mechanisms are still scarce both for Cu and Ag catalysts. Contrary to Cu, supported Ag particles are also active in aerobic alcohol oxidation. Given that there are only few studies with Ag, the activity of Ag/SiO2 prepared by both impregnation [71] and flame‐spray pyrolysis [90] comparable to Au reference catalysts is promising. Also aliphatic alcohols were oxidized with high activity, and TOFs as a high as 6000 h ‐1 were reported with Ag/HT for benzyl alcohol oxidation (though this may be overestimated given the very low reported silver loading) [86]. γ‐O species may be important during alcohol oxidation [67‐69, 71] though the Ag/HT catalyst was prepared under conditions were γ‐O was not formed. When used for silane oxidation, silver exhibited a high chemoselectivity to aromatic silanes. The aryl moiety was necessary for adsorption of the substrate and therefore it would be interesting to see whether the high chemoselectivity prevails for molecules bearing both aromatic and aliphatic silane groups. Silver in combination with peroxides is useful for styrene epoxidation but as previously described, many publications did not consider styrene oligomerization. Noticeably, argon atmosphere is required in some cases as oxygen can inhibit product formation. Two epoxidation catalysts in connection with Ag are especially noteworthy; one example employing metallic silver particles can epoxidize cyclohexene with (considering aerobic conditions) remarkable selectivities of 50 % to the epoxide [92] but harsh reaction conditions and rapid catalyst deactivation also limit the scope of this reaction. By making the concession of using activated oxygen in the form of H2O2 very attractive results were obtained: silver supported on TiO2‐ SiO2 was reactive in water as a solvent with very high selectivities (90 % and more) at the price of 61 H ad H 2O ad (2 H 2O ad) RCHO ad
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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.
Page 23 and 24: 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: 2.4 Strengths and opportunities of
Page 77 and 78: 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
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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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