Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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2.3.4.2 Silver catalysts CHAPTER 2 Silver catalysts were investigated at similarly harsh conditions using molecular oxygen as the oxidant. Ag supported on MCM‐41 (entry 10), TS‐1 and Al2O3 was an effective initiator for the aerobic radical autoxidation reaction [80]. At 140 °C, high amounts of cyclohexylhydroperoxide were formed while at 155 °C, K/A oil was formed in 80 % selectivity with a conversion around 10 %. Compared to examples with gold, an attractive advantage of silver is the relatively high K/A ratio > 1. With respect to the industrial process, these values are high though the promising results must be taken with care when product analysis is not carried out as accurately (i.e. gas and liquid phase) as reported in ref. [156] (cf. section 2.3.4.1 previously discussed) though an internal standard was used. Lower K/A selectivities were reported for Ag decamolybdovanadophosphate in pressurized CO2 in combination with MeOH as a solvent (entry 11) [167]. On the other hand, overall organic oxygenates were formed with high selectivity (96 % at 10 % conversion), i.e. over‐oxidation to CO and CO2 was low even at 180 °C. K/A ratios around 2 were obtained. Interestingly, the polyoxymetallate catalyst was more suitable for this reaction compared to the homogeneous Co/Mn/Br system applied in the AMOCO process [146] giving mainly carbon oxides as products. 2.3.4.3 Copper catalysts Contrary to silver and gold most studies focusing on copper applied milder reaction conditions at the cost of oxygen being necessary in an activated form as H2O2 or TBHP in an additional solvent. Cu is active in its fully oxidized form as Cu(II). The activation of peroxides by copper complexes is intensively studied [168‐171] and rather complex and may include intermediate Cu(I) but also Cu(III) [171] species. Hydroxy, alkyloxy and alkyperoxy radicals are generated from TBHP or H2O2, respectively, suggesting that the cyclohexane oxidation over Cu(II) follows a radical autoxidation mechanism. Many Cu catalysts are supported on or integrated in silica frameworks where significant leaching is often encountered as a problem and a deactivation pathway. A good example is Cu/MCM‐ 41 [37]. Cu being applied during the synthesis of MCM‐41 resulted in low surface areas and an almost inactive catalyst. CuxO particles (thus not integrated in the framework) were rapidly washed out under reaction conditions. With 3‐aminopropyl‐linkers, the stability of the catalyst towards leaching was improved (though not eliminated) and TOFs around 2 h ‐1 with H2O2 as oxidant at 100 °C were obtained (Table 2‐5, entry 12). Product formation was already found at room temperature. K/A ratios were around 2, K/A ratios > 1 being in general observed with peroxides over Cu catalysts. Leaching was also a problem for Cu(II) on another mesoporous silicate, TUD‐1 [172]. With TBHP as oxidizing agent, TOFs around 7 h ‐1 were obtained in the absence of an additional solvent at 70 °C (entry 13). Substantial Cu leaching was also encountered with sol‐gel derived Cu/SiO2 which could be slightly improved by calcination [173, 178]. Also here, no additional solvent was necessary with TBHP as the 42
2.3 Selective liquid‐phase oxidation reactions oxidant (entry 14). TOFs were comparable to TUD‐1. Despite the leaching, homogeneous Cu species were not catalytically active. A problem of Cu can be the low efficacy in the use of peroxides due to unselective peroxide decomposition [37, 174]. The other side of the medal is a desirable low selectivity to cyclohexyl peroxides [37, 172‐174]. Selectivities of up to 100 % to K/A‐oil were reported e.g. with Cu‐ZSM‐5 in an ionic liquid [174] with moderate TOFs (6 h ‐1 ) at 90 °C (entry 15). Of course, these high selectivities are only reliable when obtained by very thorough product quantification [156]. As an exception, Cu supported on magnesium manganese oxide octahedral molecular sieves featured a high selectivity to cyclohexane peroxide and t‐butyl cyclohexyl perether with TBHP [32] though at higher reaction temperatures cyclohexane peroxide selectivity decreased significantly (Table 2‐5, entry 16). Remarkably, no leaching of Cu was found with this material but TOFs were only around 1 h ‐1 . Another mixed oxide, Cu1‐xZnxCr2O4 spinel, was found to be catalytically active with TBHP where ACN was required as a solvent (entry 17) [175]. This system was fully recyclable in contrast to many other Cu catalysts where leaching is an issue. Also in ACN but with H2O2 as oxidizing agent copper pyrophosphate (Cu2P2O7) as a bulk material exhibited considerable catalytic activity [176] already at 65 °C (TOF ~2 h ‐1 , entry 18) and might therefore be interesting as a supported catalyst. The authors further stated that surface hydrophobicity is a crucial parameter for this material. There are only few studies investigating cyclohexane oxidation focusing on molecular oxygen with Cu catalysts. By using a sophisticated deposition method (including evaporation of Cu and trapping in acetone) metallic Cu nanoparticles in the size range of 2‐6 nm were supported on TiO2 (entry 19a), Al2O3 (entry 19b) and Fe2O3 (entry 19c) [177] and used as catalysts. Cu particles likely oxidized at the elevated pressures (20 bar) and temperatures (120 °C), the latter still being significantly lower than those used in the industrial case and in Au catalysis. Copper leaching with TiO2 and Al2O3 was only observed in the first run making these catalysts highly recyclable. Only cyclohexanol and cyclohexanone yields were reported and the reaction times were long (TOF 10 h ‐1 over 24 h) which may be related to the low temperatures. Interestingly, high K/A ratios of up to 11.5 could be obtained at relatively high yields around 10 %. Cu/ZSM‐5 catalyzed the oxidation with molecular oxygen already at 100 °C (entry 20) [27]. Calculated TOFs were in the order of 100‐200 h ‐1 and thus significantly higher than in the previous case. Lower K/A ratios around 1 were obtained which remained virtually unaffected when adding substoichiometric amounts of TBHP as a radical initiator. The unchanged product distribution at higher conversions suggests that Cu acts as a radical initiator. Calcination of Cu‐ZSM‐5 decreased its catalytic performance. Co‐ZSM‐5 studied in the same project was always more active; nevertheless, the activity of Cu‐ZSM‐5 was promising and it would be interesting to see its performance under industrial conditions. Unfortunately, leaching studies for Cu‐ ZSM‐5 were not performed. As another Cu‐modified zeolite NaY was selectively converting 43
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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 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: 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
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
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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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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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