Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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CHAPTER 5 experimentally observed in the batch experiments. This flexible approach can predict the phase behavior for a broad range of conditions and thus offers an alternative to often laborious experimental phase behavior determination requiring special equipment. The model predictions for the phase transition furthermore fell in the pressure regime where a considerable change in catalytic activity was observed and gave a good indication of the optimal system pressure. Thus, the CPA EoS allows for a more effective reaction optimization. This can be especially useful where little is known about the phase behavior. In the present case, the phase behavior modeling suggested that the catalytic reaction should be studied at higher pressures than previously done in order to reach a single phase. Under very similar conditions as in ref. [7] also an increase in reaction rate between 140 bar and 150 bar (0.9 % benzyl alcohol, 0.45 % O2, rest CO2, cf. Figure 5‐7) was found here. Only when extending the studies to pressures >160 bar a decrease in reaction rate was concurring with the system being in a single phase which is similar to results reported for the oxidation of cinnamyl alcohol [8]. Both at 140 bar and at 150 bar the mixture was biphasic though the phase behavior in the catalyst pores may be different as indicated by infrared spectroscopic studies [7]. Pd catalysts used in alcohol oxidation are sensitive to the availability of oxygen [41, 44]. A low availability of oxygen causes blocking of surface sites by adsorbed hydrogen and strongly adsorbing CO which are both removed by oxidation. High amounts of oxygen favor overoxidation of Pd which is connected to a lower catalytic activity [41, 44, 45]. Hence, a general recommendation is to operate the reactor in the oxygen mass‐transport limited regime [3]. The lower catalytic activity observed under single phase conditions might be connected to the oxidative deactivation of the palladium particles on the catalyst surface which underlines that the oxygen mass transport to the catalyst surface is improved compared to the biphasic situation. This is further corroborated by the significantly higher selectivity to overoxidation products. Note that oxidized palladium can be re‐ reduced by the alcohol [46] so no permanent (short‐term) deactivation is caused by oxygen treatment. Under biphasic conditions, XAS measurements showed that Pd was mainly reduced [41, 42] but analogue measurements under single phase conditions are still missing. If the oxidation state of the active metal is the primary reason for the pronounced response to the phase behavior, then the results made at 100 °C where no sudden change in conversion was observed might indicate that palladium is always reduced at that temperature. The oxidation of palladium occurs rapidly [6, 41] and does not explain the long time required for reaching a steady‐state when operating in a single phase. A side product in the oxidation of primary alcohols is the corresponding carboxylic acid which is only poorly soluble in CO2. Hence, adsorption of benzoic acid on alumina and/or Pd during single phase conditions could cause deactivation and was previously suggested from IR investigations [7]. In the presence of two phases, the benzyl alcohol rich layer around the catalyst particles has a higher eluting power than the 146
5.4 Discussion corresponding CO2‐rich single phase cleaning the catalyst surface from overoxidation products. In addition, less benzoic acid is formed under biphasic conditions. The phase transition was accompanied by a change in the product distribution; the major side product at low pressures was toluene while benzoic acid and benzyl benzoate were the preferred side products at high pressures. Cleavage of the C‐O bond in benzyl alcohol is caused by surface adsorbed hydrogen on palladium [47] and thus the difference in selectivity is related to a low availability of oxygen which first has to diffuse through a layer of the second benzyl alcohol (or benzaldehyde) rich phase covering the catalyst particles. Under single phase conditions, the availability of oxygen is obviously improved. Though more data are necessary to corroborate this trend, the selectivity also appeared to depend on the flow. At high flows, the selectivity to benzaldehyde was ca. 70 %, while the composition studied at the same pressure at a lower flow oxidized benzyl alcohol with ca. 90 % selectivity. Additionally, the system exhibited only a small response to the pressure within the two phase region at high flow. With lower flow, however, a maximum in conversion was observed at the high pressure end of the two phase region. At this point, the amount of data allow no clear definition of this trend and so the explanation sketched in Figure 5‐12 remains highly speculative: at very low pressures the solubility of benzyl alcohol is poor and the catalyst particles are covered in a layer of substrate which is intensified by the observed accumulation of the substrate in the reactor and the additional low solubility of the benzaldehyde product (cf. Figure 5‐5). The layer protects the catalyst particles keeping Pd in its active state. The low availability of oxygen causes significant formation of toluene and may also hinder the conversion of substrate. When the catalyst surface is exposed directly to the CO2‐rich phase, the selectivity to toluene is low and the high availability of oxygen may in principal afford higher reaction rates over the catalyst in its active state, but the catalyst (gradually) deactivates. At high pressures in the biphasic region both scenarios may coexist, if the flow (and therefore the substrate accumulation) in the reactor is not too high. Additionally, the solubility for benzaldehyde is high which is rapidly extracted to the CO2‐rich phase. The advantages of contact with both phases may add up: droplets of benzyl alcohol maintain the catalyst in its active state while a high benzaldehyde selectivity and potentially also a higher conversion is obtained from catalyst sites directly in contact with the supercritical phase. Note that due to deactivation, the reaction rate over not deactivated palladium under single phase conditions was not accessible. The difference in conversion in short‐ time (i.e. low deactivation) batch experiments between biphasic and single phase conditions was however low. Previous IR studies further indicated that the phase transition might be (slightly) different within the pore system [7] hinting at the possible coexistence of single and biphasic conditions. Once again, more data and extensive ATR‐IR as well as EXAFS studies are necessary to corroborate these speculations. 147
Page 1 and 2:
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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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
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: 5.4 Discussion catalytic reactions
Page 163 and 164: 5.5 Conclusions 5.5 Conclusions The
Page 165 and 166: 5.6 References [29] I. Tsivintzelis
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
Page 177 and 178: 6.3 Results Figure 6‐6: Compariso
Page 179 and 180: 6.3 Results Figure 6‐8: Influence
Page 181 and 182: 6.3 Results investigated the epoxid
Page 183 and 184: 6.3 Results Figure 6‐10: Co‐epo
Page 185 and 186: 6.3 Results Figure 6‐11: EXAFS an
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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