Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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Leaching 2+ [Cu ] solv Al O 2 3 Al O 2 3 CHAPTER 2 Al O 2 3 Al O 2 3 Isolated Cu 2+ Figure 2‐2: Isolated Cu sites and CuO present simultaneously in a catalyst where CuO domains are often prone to leaching; schematic model of Cu‐doped montmorillonite consisting of regular layers and Al2O3 pillars [33]. Sol‐gel, hydrothermal and coprecipitation synthesis methods allow simultaneous synthesis of support and incorporation of the catalytically active metal and also afford Cu species with high dispersion when Cu is applied in low concentrations. All three methods are based on precipitating the catalyst from a homogeneous solution. Sol‐gel methods are based on the hydrolysis and subsequent condensation of a precursor (e.g. tetraethyl orthosilicate) often under controlled pH. The solvent can be removed by simple air‐drying but also more complex strategies like supercritical drying are used. High surface areas can be achieved by this approach. Addition of other metal ions allows doping; pore structures can be induced by bulky organic additives. Sol‐gel synthesis is often used for synthesizing Cu‐doped silica materials. Fast hydrolysis of the copper precursor compared to the SiO2 precursor may be problematic. Cu additives have a negative influence on MCM‐41 materials decreasing the surface area [37, 38]. This can partly be compensated by addition of Al ions [31, 38] forming acidic sites with ion exchange properties binding to Cu(II) [39] which also stabilizes Cu species against leaching (Figure 2‐3). Sol‐gel synthesis is of course not limited to doped silicas [40]. Hydrothermal synthesis, i.e. reaction of precursors to the desired product in an autoclave at elevated temperatures and pressures, is also useful for preparing homodisperse Cu species in a heterogeneous matrix as was done for the synthesis of copper‐substituted aluminophosphate molecular sieves [38, 41]. (Cu‐based) metal‐organic frameworks are also frequently synthesized via a solvothermal approach [42]. Cu incorporated in alumina can be synthesized by coprecipitating Cu and Al [43]. Coprecipitation requires no special (e.g. high‐pressure) equipment and starts from a solution of two (or more) metal oxides. Precipitation is (most often) induced by a change in pH. 12 Al2O3 Al O Al O Al O 2 3 Al O 2 3 2 3 Al O 2 3 CuO domains Al O 2 3 2 3
2.2 Catalyst synthesis for oxidation reactions Prominent examples for this synthesis route are the Cu‐ZnO‐Al2O3 catalysts for industrial methanol synthesis [11]. The final structure is tunable both by the pH during precipitation and the aging time. BET surface area (m 2 /g) 1050 1000 950 900 52 % Figure 2‐3: BET surface area of MCM‐41 as a function of Cu doping; (■) Cu doping only (●) Cu and Al doping (same wt.‐% as Cu); columns: 2,3,6‐trimethylphenol conversion with 2% Cu‐MCM‐41 (left) and 2% Cu‐2%Al‐ MCM‐41 (right) [38]. Coprecipitation is a flexible method. Where supported CuxO nanoparticles are desired these can be obtained by increasing the Cu concentration when aiming for CuO/Al2O3 [43, 44] and other catalysts [40] which is also true for the other synthesis methods described in the previous section, i.e. sol‐gel and hydrothermal synthesis. Another method frequently used is impregnation of a solid support [43, 45‐50]. Interestingly, CuO supported on Al2O3 is the catalyst synthesized most often by impregnation, though in many reports impregnation was shown to afford inferior catalysts compared to other synthesis techniques. A method which is often used for the synthesis of nanogold catalysts is deposition‐precipitation. This method is also useful for synthesizing metallic Cu nanoparticles on a support [51‐53]. Briefly, deposition‐precipitation is done by mixing a dispersion of the support with a solution of the Cu ion followed by subsequent increase of the pH to a moderately alkaline level causing precipitation. After filtration and drying, metallic finely dispersed Cu nanoparticles are obtained by hydrogen treatment at elevated temperatures. Colloidal methods are reported by the group of Schüth [54]. Nanoparticles in the range of 5 nm were obtained by reduction of copper acetylacetonate with trialkylaluminium which also serves as a stabilizer. Note that metallic copper particles are oxidized when exposed to oxygen. Their use is therefore limited to oxidation reactions under oxygen‐lean conditions. Of course, CuxO particles or metallic‐core/oxidic shell particles can be obtained from oxidation of metallic nanoparticles [55]. 0 1 2 3 4 5 13 64 % Cu (wt.-%)
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: 2.2.1 Synthesis of gold catalysts 2
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 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 75 and 76: 2.4 Strengths and opportunities of
Page 77 and 78:
2.4 Strengths and opportunities of
Page 79 and 80:
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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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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