Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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4.1 Introduction CHAPTER 4 Selective catalytic oxidation by inexpensive reagents such as oxygen, hydrogen peroxide or t‐butyl hydroperoxide is a viable way of introducing functional groups and thereby upgrading saturated and unsaturated hydrocarbons [1‐3]. Finding catalytic systems for these oxidation reactions is of significant industrial importance [4], e.g. for p‐xylene and cyclohexane oxidation to terephthalic acid and adipinic acid, respectively, which are monomers for commodity polymers [5]. The high industrial impact is also reflected by the large amount of related patent literature [e.g. 6‐8]. Nevertheless, these catalytic oxidation reactions still require harsh conditions in industry [4]. As an example, a very effective but highly corrosive and environmentally harmful homogeneous Co/Mn/Br system is employed for p‐xylene oxidation [9] in liquid phase using acetic acid as solvent which is partly decomposed during reaction. The oxidation follows a radical chain mechanism with the transition metal promoting the chain initiation and the decomposition of peroxo‐species [4, 9]. Active catalysts for the selective side‐chain oxidation of alkyl aromatic compounds are reported mainly for liquid phase oxidations. Only a few examples are reported in the gas phase [10‐ 12] or in supercritical media [13, 14]. The selective liquid phase oxidation can be conducted both homogeneously and heterogeneously involving mostly metals with two or more stable oxidation states typically under acidic conditions. Most catalytic systems are based on Co often modified with Mn and a Br source [9, 15, 16] but other examples based on e.g. Pt [17] and Cu [18, 19] are also known. Typical oxidizing agents are organic peroxides, hydrogen peroxide, and oxygen offering a cost‐effective route to important industrial intermediates such as benzoic acid, acetophenone, benzophenone, toluic acid etc. [5]. Interestingly, ring and side chain oxidation can be controlled by the choice of oxidant [20, 21]. Due to the relative inertness of the corresponding C‐H bonds, the reactions are often performed at high pressure and in an additional solvent. Avoiding elevated pressures and working under neat conditions would be the next steps to render the oxidation process more cost‐efficient. Based on the homogeneous Co/Mn/Br system used in the AMOCO process many heterogeneous catalysts employ Co as a catalytically active metal. In the present study, the potential of silver as an active material is further investigated after showing considerable activity as an alcohol oxidation catalyst in previous studies [22] (cf. Chapter 3). The impregnation route for catalyst synthesis used previously left considerable potential for improvement as it afforded a broad silver particle size distribution with particles ranging from a few nm up to 50 nm. Flame spray pyrolysis (FSP) has extensively been studied for the preparation of different metal oxide catalysts [23, 24] as well as supported noble metal catalysts often affording high metal dispersions with particle sizes of less than 5 nm on the support [25]. In this way, a good mixing of the oxide particles can be achieved. Therefore, the Ag/SiO2‐CeO2 catalyst system was not only prepared by impregnation but also by 102
4.2 Experimental single‐step flame spray pyrolysis and investigated for its potential for the oxidation of aryl aromatic compounds. Comparing Ag/CeO2‐SiO2 catalysts made by these two routes, the FSP‐made catalysts exhibited an improved performance even at significantly lower silver loadings for the oxidation of toluene, p‐xylene, ethylbenzene and cumene. The structure was further elucidated by X‐ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM). 4.2 Experimental 4.2.1 Catalyst synthesis The impregnated catalyst was prepared by adding 25.0 mL of an aqueous AgNO3 (Fluka, >99.5 %) solution (31 mg/mL) to 5.00 g of SiO2 (Aerosil 200, Degussa, 200 m 2 /g) resulting in a Ag:SiO2 ratio of 1:10. The mixture was stirred with a glass rod and sonicated for 1 h. After drying at 80 °C for 24 h the catalyst was calcined in air at 900 °C for 1 h in a muffle oven controlled by a Eurotherm 904 controller. The catalyst is denoted as imp 10%Ag/SiO2‐900. For catalysts made by flame spray pyrolysis, silver nitrate (Fluka, >99.5 %) dissolved in ethanol (EtOH) and diethyleneglycol‐monobutylether (DEGME, Fluka, >98 %; 1:1, Ag 0.5 M), hexamethylsiloxane (HMDSO, Fluka >98 %) and cerium‐2‐ethylhexanoate (40 % Ce w/v, Strem) were used as silver, silica and ceria precursors, respectively. The appropriate amounts of the above mentioned precursor solutions were mixed with solvents, i.e. DEGME (Fluka, >98 %) and 2‐ ethylhexanoic acid (Aldrich, >98 %) while keeping the solvent composition constant at 1:1 by volume. The total support metal (Si + Ce) concentration was kept constant at 0.6 M in these solutions. The nominal CeO2 weight fraction in the product powder ranged from 0 to 50 wt.‐%. The silver precursor was added accordingly to reach a nominal 1 wt.‐%‐ Ag loading in the final powder product. The Ag/CeO2‐SiO2 powders were produced in a laboratory scale FSP reactor as described elsewhere [26]. The liquid and dispersion gas feed rate were kept constant at 5 mL/min and 5 L/min, respectively. The pressure drop above the nozzle was adjusted to 1.7 bar and a sheath gas flow of 5 L/min O2 was applied through a metal ring (11 mm i.d., 18 mm o.d.) with 32 holes (0.8 mm i.d.) to ensure complete combustion of the precursor. The powders were collected with the aid of a vacuum pump (Busch SV 1025 B) on a glass microfiber filter (Whatman GF/D, 257 mm in diameter). The catalysts are denoted as FSP 1%Ag/SiO2 FSP 1%Ag/10%CeO2‐SiO2, FSP 1%Ag/30%CeO2‐SiO2 and respectively. Details are given in Table 4‐1. Flame spray pyrolysis was performed by Björn Schimmöller (ETH Zürich). 103 FSP 1%Ag/50%CeO2‐SiO2,
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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
Page 65 and 66: Table 2-8: Oxidation of hydroquinon
Page 67 and 68: 2.3 Selective liquid‐phase oxidat
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
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: Chapter 4 Selective Side-Chain Oxid
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
Page 127 and 128: Absorption (a.u.) Absorption (a.u.)
Page 129 and 130: 4.3 Results atomic mixing of Ag and
Page 131 and 132: 4.3 Results Figure 4‐7: Influence
Page 133 and 134: 4.4 Discussion and mechanistic cons
Page 135 and 136: 4.5 Conclusions heterogeneous radic
Page 137 and 138: 4.6 References [29] S.I. Zabinsky,
Page 139 and 140: Chapter 5 Experimental Determinatio
Page 141 and 142: 5.2 Experimental The most important
Page 143 and 144: 5.2 Experimental Eurotherm 2216e te
Page 145 and 146: Figure 5-1: Schematic view of the e
Page 147 and 148: (Eq. 5‐5) AB i j ε AB i j β 5.2
Page 149 and 150: (a) (b) (c) (d) (e) (f) (g) (h) (i)
Page 151 and 152: 5.3 Results two association sites o
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
Page 165 and 166: 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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