Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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CHAPTER 2 being oxidatively removed under aerobic conditions. Both effects limit the selectivity and the catalytic activity, respectively, and were not reported over Ag and Cu. 2.3.1.2 Aerobic alcohol oxidation While with Ag/HT [86] also aerobic oxidation was performed very effectively (TOF 6000 h ‐1 for benzyl alcohol), similar experiments for the corresponding Cu catalysts are not reported. A reason might be that the usually metallic copper nanoparticles are oxidized and hence deactivated under elevated temperature aerobic conditions while silver is stable against deep oxidation. Hence, aerobic alcohol oxidation is reported only with oxidic copper species. An early example is the oxidation of 2‐propanol to acetone with Cu‐exchanged octahedral molecular sieves (Table 2‐1, entry 11) [84]. In Cu1.5Mn1.5O4 spinel supported on Al2O3 a collaborative effect of Mn and Cu was found where Cu was assumed to activate oxygen [61]. TOFs obtained with this catalyst are quite low (entry 12). Cu was also used as a mixed oxide together with Mg as a support for Au [98] forming a highly active catalyst. The exact nature of the beneficial influence of Cu could not be identified and might be connected to its influence on surface basicity, redox activity or an influence on the net charge of Au particles (which were, however, assumed to be the primary active species). In another study, Y‐zeolite encorporated CuO particles were used at very high temperatures (200 °C) for the oxidation of e.g. 1‐phenylethanol [103] for which CuO and CuO/Y exhibited significantly different behavior: CuO afforded acetophenone (entry 13a), CuO/Y gave mainly 2,3‐diphenyl‐but‐2‐ene (entry 13b). The difference in selectivity was assigned to the interaction of lewis‐acid/ ‐base sites on the zeolite and CuO particles, respectively. Cu‐containing mixed oxides were further used with H2O2 (entry 14) [104] and TBHP (entry 15) [105] though this approach appears less attractive given the activities obtained already with molecular oxygen. Room temperature aerobic oxidation was described with Cu‐Al‐hydrotalcite which was co‐impregnated with rac‐BINOL as a ligand (entry 16) [106]. Due to the low temperatures long reaction times were required but aldehydes were obtained with excellent selectivity under neat conditions. Steric constraints afforded a high selectivity for primary alcohols. The catalyst was reusable so apparently leaching of copper and the required co‐impregnated ligand was low. Compared to gold catalysis, the literature on silver catalysts for the selective aerobic alcohol oxidation is scarce. This is astonishing since silver has long been used as an industrial methanol oxidation catalyst in gas phase reactions proving the principal feasibility of silver. In one of the first studies, polystyrene crosslinked Ag‐exchanged natural zeolite was successfully used under mild conditions for the oxidation of simple alcohols such as iso‐propanol and 2,3‐butandiol (Table 2‐1, entry 17). In the latter case, C‐C cleavage was observed as the main reaction pathway thus giving acetaldehyde [84]. The reactions were carried out under mild conditions (55 °C) in water. Various reports found unmodified metallic silver to be inactive for alcohol oxidation [69‐71]. Liotta et al. 24
2.3 Selective liquid‐phase oxidation reactions found that metallic silver can be activated by treatment in air with molecular oxygen at 500 °C (entry 18) [69] which was confirmed in a recent study (entry 19) [71]; however, air treatment did not bulk‐ oxidize silver. The complex interaction of silver with oxygen and thus the formation of stable silver‐ subsurface oxygen (so‐called γ‐oxygen) species might account for the prominent change in reactivity [74, 75, 115, 116]. These species are also connected to the catalytic activity in gas phase methanol oxidation [73]. Palladium is a highly active metal in alcohol oxidation. Combinations of Pd and Ag supported on pumice were tested for benzyl alcohol oxidation [69]. Addition of Ag to Pd/pumice dramatically decreased the alcohol oxidation activity. On the other hand, Ag/pumice and Pd/pumice applied as a physical mixture exhibited a remarkable synergistic effect. Leaching of the metal species as a source for this observation was not investigated. Similarly, the addition of CeO2 nanoparticles to Ag/SiO2 increased the benzyl alcohol conversion more than ten‐fold [71]. High reaction temperatures were required and the catalytic activity was only moderate likely connected to the poor Ag dispersion; here, EXAFS analysis significantly underestimated particle sizes (2‐3 nm) while TEM and XRD agreed well suggesting particle sizes around 30 nm. However, 10 % Ag/SiO2 was similarly active as gold catalysts (containing only 1 wt.‐% gold thus giving higher TOFs) and exhibited high selectivity. Details on this study can also be found in Chapter 3. Later, Ag catalysts synthesized via flame‐spray pyrolysis with smaller Ag particles (cf. Figure 2‐7) resulted in similar TOFs [90]. CeO2 turned out to be a general, easily applicable dopant, i.e. also Au/TiO2, Au/Al2O3, Au/ZnO and Pd/Al2O3 exhibited a significantly higher catalytic performance when used with nanosized ceria. The effect of the dopant was assumed to be via direct physical contact; leaching of either Ag or Ce species could be excluded. No alcohol dehydrogenation activity was observed under anaerobic conditions. Silver in combination with vanadium is a frequently used oxidation catalyst. Also silver‐exchanged molybdovanado phosphoric acid showed considerable catalytic activity for the selective oxidation of various allylic alcohols with moderate to high selectivity. Neither leaching nor deactivation was observed. Both silver and copper can interact with TEMPO and thus mediate alcohol oxidation. Due to steric constraints, primary alcohols are most susceptible to reaction. In the presence of peroxodisulfate, the oxidation of sugars proceeded at room temperature [107], CuCl2 dissolved in an ionic liquid used in a SILP catalyst required low reaction temperatures (65 °C) with O2 as the oxidant [108]. Finally, it should be noted that the aerobic N‐acylation of amines with methanol (methanol thus being oxidized) is also facilitated by coinage metal catalysts, i.e. with supported Au nanoparticles [117], copper hydroxyl salts as heterogeneous catalysts [56] and principally also by metallic silver surfaces [118]. The acylation occurs via transformation of methanol to H2 and CO2 reacting with the amine. [119] 25
Page 1 and 2: TECHNICAL UNIVERSITY OF DENMARK (DT
Page 3 and 4: Preface The present dissertation su
Page 5 and 6: the carboxylic acid, ceria inhibite
Page 7 and 8: Resume Denne afhandling giver indle
Page 9 and 10: hvorimod mængden af katalysator ku
Page 11 and 12: 2.3.8 Oxidation of sulfides .......
Page 13 and 14: 5.3.5 Macroscopic mass transport in
Page 15 and 16: Chapter 1 Introduction With its gre
Page 17 and 18: 1. Introduction Scheme 1‐1: High
Page 19 and 20: 1. Introduction Figure 1‐2: Phase
Page 21 and 22: 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 37: 2.3 Selective liquid‐phase oxidat
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 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
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3.2 Experimental 3.2.1 Catalyst pre
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3.3 Results Scattering amplitudes a
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Absorption (a.u.) 1.2 1.0 0.8 0.6 0
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Conversion (%) 100 90 80 70 60 50 4
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3.3 Results Figure 3‐5: In situ X
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3.3 Results Table 3‐2: Conversion
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3.3 Results In order to gain insigh
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3.3 Results the cost of selectivity
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3.4 Discussion synergetic interacti
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3.5 Conclusions to corroborate the
Page 113 and 114:
3.6 References [29] T. Mitsudome, Y
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Chapter 4 Selective Side-Chain Oxid
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4.2 Experimental single‐step flam
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4.2 Experimental alcohol (Aldrich,
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4.3 Results Scheme 4‐1: Oxidation
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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
Page 171 and 172:
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
Page 185 and 186:
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
Page 199:
Curriculum Vitae Matthias Josef Bei
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