Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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CHAPTER 6 conditions where, however, free organic radicals could not be detected. Potential free radicals could be EPR inactive due to coupling or their concentration is below the limit of detection. The inhibiting effect of radical scavengers might also be due to adsorption or coordination to free Co sites [48]. A signal at g = 2.0076 was found which can be ascribed to Co III ‐O2* ‐ (superoxo‐) species formed from reaction of Co II with oxygen [49]. These can already be found in the solid catalyst measured ex situ and thus are not formed specifically under reaction conditions. Hence, their formation is likely not connected to the induction phase. Note that Co III ‐peroxo‐species, which might also form from reaction of Co II with O2, are not EPR active [50]. 6.3.7 Co-epoxidation of styrene and (E)-stilbene Driven by the observation that styrene epoxidation does not only proceed faster but also features a shorter induction phase, the co‐epoxidation of styrene and (E)‐stilbene was investigated thus doubling the substrate/catalyst ratio (Figure 6‐10a). Although the catalyst amount was not increased, styrene induced a much faster conversion of (E)‐stilbene. At the same time, the conversion of styrene became only slightly but significantly slower at medium conversions. Co‐ epoxidation with styrene had the strongest effect on the conversion of (Z)‐stilbene which increased from 11 % to 58 % with styrene (Table 6‐3, entry 1,2). This effect was not observed for the homogeneous Co catalysts neither for (E)‐ nor (Z)‐stilbene epoxidation (Table 6‐3, entry 3, 4) and might be an effect related to the special features of the MOF. Styrene being smaller than (E)‐stilbene can diffuse more easily into the micropores of the MOF. However, a transfer‐epoxidation from styrene oxide to (E)‐stilbene does not account for the styrene promotion since styrene oxide as an additive did not have any promoting effect even in large quantities. Oxygen transfer from styrene oxide as the potential oxidizing agent to (E)‐stilbene was also not observed under reaction conditions with flowing N2 instead of O2. 168
6.3 Results Figure 6‐10: Co‐epoxidation of styrene and (E)‐stilbene and the effect of benzaldehyde. (a) Comparison of only styrene and (E)‐stilbene conversion (solid symbols) with co‐epoxidation of styrene and (E)‐stilbene (open symbols), (■,□) (E)‐stilbene; (•,○) styrene conversion; (b) effect of benzaldehyde addition (0.2 mmol) on (E)‐stilbene conversion. <strong>Reactions</strong> conditions: 2.0 mmol (E)‐stilbene and/or styrene, 100 mg biphenyl, 30 mL DMF, 50 mL min ‐1 O2, 2.0 mg STA‐12(Co), 100 °C. Table 6‐3: Epoxidation of (Z)‐stilbene with heterogeneous and homogeneous Co catalysts. Reaction conditions: 2.0 mmol (Z)‐stilbene, 6.8 µmol Co as catalyst, 100 mg biphenyl, 30 mL DMF, 10 h, 50 mL min ‐1 O2. Optional styrene addition: 2.0 mmol. Entry Catalyst Styrene Promotion Conversion (%) Selectivity (%) 1 STA‐12(Co) NO 11 92 2 STA‐12(Co) YES 58 86 3 Co(acac)3 NO 66 80 4 Co(acac)3 YES 65 92 5 Co3O4 NO 9.4 53 Two major differences between MOF‐catalyzed styrene and (E)‐stilbene conversion are the higher reaction rate and the higher benzaldehyde selectivity (styrene ca. 12 %; (E)‐stilbene ca. 5 %) for the former. Thus, styrene conversion affords a comparably high benzaldehyde concentration already in the beginning of the reaction. Indeed, benzaldehyde as the major by‐product can explain the observed promoting effect – as with styrene, the (E)‐stilbene reaction rate was enhanced by addition of small substoichiometric amounts of benzaldehyde (Figure 6‐10b). In analogy to the previous observations, benzaldehyde did not have a promoting effect on homogeneous Co catalysts. 6.3.8 Formation of oxidizing species (a) (b) Triphenylphosphine is frequently used to scavenge thermally unstable peroxides for GC analysis. Also in the present study, triphenylphosphine oxide was found when PPh3 was added to the samples taken for GC analysis indicating the presence of peroxides. Over time the amount of peroxides increased steadily. Similar to FMF‐formation, peroxides also formed in lower amounts in the absence 169
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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
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2.6 References [56] H. Tumma, N. Na
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2.6 References [113] C. Keresszegi,
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2.6 References [169] Q. Zhu, Y. Lia
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2.6 References [223] M. Fetizon, M.
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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
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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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Page 133 and 134: 4.4 Discussion and mechanistic cons
Page 135 and 136: 4.5 Conclusions heterogeneous radic
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Page 139 and 140: Chapter 5 Experimental Determinatio
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Page 143 and 144: 5.2 Experimental Eurotherm 2216e te
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Page 147 and 148: (Eq. 5‐5) AB i j ε AB i j β 5.2
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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
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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
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Page 187 and 188: 6.4 Discussion would need to be for
Page 189 and 190: 6.5 Conclusions Formation of the ep
Page 191 and 192: 6.6 References [28] J. Perles, N. S
Page 193 and 194: Chapter 7 Concluding Remarks 7.1 Co
Page 195 and 196: 7.2 Final remarks and outlook speci
Page 197 and 198: Acknowledgements Acknowledgements T
Page 199: Curriculum Vitae Matthias Josef Bei
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