1 Department of Chemical and Biochemical Engineering, Technical University of Denmark2 CHEC Research Centre, Department of Chemical and Biochemical Engineering, Technical University of Denmark3 CERE – Center for Energy Ressources Engineering, Department of Chemical and Biochemical Engineering, Technical University of Denmark4 Department of Chemistry, Technical University of Denmark5 Center for Energy Resources Engineering, Center, Technical University of Denmark
Heterogeneously catalyzed selective oxidation reactions have attracted a lot of attention in recent time. The first part of the present thesis provides an overview over heterogeneous copper and silver catalysts for selective oxidations in the liquid phase and compared the performance and catalytic properties to the widely discussed gold catalysts. Literature results were summarized for alcohol oxidation, epoxidation, amine oxidation, phenol hydroxylation, silane and sulfide oxidation, (side-chain) oxidation of alkyl aromatic compounds, hydroquinone oxidation and cyclohexane oxidation. It was found that both copper and silver can function as complementary catalyst materials to gold showing different catalytic properties and being more suitable for hydrocarbon oxidation reactions. Potential opportunities for future research were outlined. In an experimental study, the potential of silver as a catalyst for the selective oxidation of alcohols was investigated. By using a straightforward screening approach, silver supported on silica prepared by impregnation was found to be catalytically active in a mixture with nano-sized ceria. The collaborative effect between ceria and silver was traced back to direct physical contact while leaching could be excluded. The silver catalyst was most active when calcined over a short time at 500 °C potentially due to the formation of silver-oxygen species. Removal of these species might be a deactivation mechanism as was suggested by X-ray absorption spectroscopy (XAS) analysis. XAS revealed that silver was in the metallic state. Silver particle sizes estimated from XAS were significantly smaller (2- 3 nm) than from transmission electron microscopy (TEM) and X-ray diffraction (XRD; ca. 30 nm). It was proposed that silver-oxygen species might cause local disorder which would lead to an underestimated particle size by XAS. Based on catalytic studies, a simplified preliminary mechanism was proposed following the dehydrogenation mechanism over gold and palladium catalysts. Comparison of the performance of the silver catalyst with commercial palladium and gold catalysts revealed that all catalysts were more active in combination with ceria nanoparticles and that under the tested reaction conditions silver was equally or even more efficient than the gold catalysts. Calcination at 900 °C of silver on silica prepared by impregnation afforded a catalyst which was used in the selective side chain oxidation. Toluene, p-xylene, ethylbenzene and cumene were oxidized to the corresponding hydroperoxides, alcohols, carbonyl compounds and carboxylic acids most likely following a radical autoxidation mechanism. p-Xylene conversion was promoted by ceria nanoparticles in combination with a carboxylic acid resulting in TONs of up to 2000. In the absence of the carboxylic acid, ceria inhibited the reaction exhibiting radical scavenger properties. Contrary to p-xylene, neither ethylbenzene nor cumene conversion was promoted by ceria even in the presence of a carboxylic acid. Substantial leaching of silver was observed with the impregnated silver catalyst. By using silver supported on CeO2-SiO2 prepared by flame spray pyrolysis, leaching could be limited significantly. XAS investigation revealed that the active catalyst is most likely metallic silver. Compared to silver on silica calcined at 500 °C (vide supra) the particle size obtained from XAS for the impregnated catalyst calcined at 900 °C (> 5 nm) was in qualitative agreement with TEM and XRD results. Oxidation reactions like the previously described alcohol oxidation can profit from the use of pressurized CO2 as a solvent. Phase behavior in CO2 containing systems was previously shown to be important for catalytic reactions. Using the Cubic plus Association (CPA) Equation of State the phase behavior of ternary systems consisting of benzyl alcohol – O2 – CO2 and benzaldehyde – H2O – CO2 were modeled. Dew points of the latter system occurred in general at lower pressures than the former. Good agreement between experimental data measured at catalysis-relevant conditions and the model was found. Experimental dew points occurred at (slightly) lower pressures as predicted by the model. The usefulness of the model was further evaluated in a continuous catalytic study with Pd/Al2O3 as catalyst where the conversion of benzyl alcohol was monitored as a function of pressure. Indeed the reaction rate differed significantly depending on whether the system was mono- or biphasic. The CPA model predictions therefore assist in locating the pressure regime with the highest catalytic activity in a rational manner making extensive trial-and-error experiments unnecessary. Biphasic conditions close to the dew point gave the highest catalytic activity for benzyl alcohol oxidation over Pd/Al2O3 while rates were lower under single phase conditions. A reason for this behavior might be overoxidation of palladium or blocking of surface sites by byproducts with low solubility under single phase conditions. Under biphasic conditions the substrate was found to accumulate in the reactor (and the overall experimental setup) leading to longer residence times. Compared to alcohol oxidation the epoxidation of olefins with molecular oxygen is more difficult. Using N,N-dimethylformamide (DMF) as a solvent the Co-based metal-organic framework (MOF) STA-12(Co) catalyzed the epoxidation of styrene, (E)- and (Z)-stilbene. While the stilbene isomers were converted with good selectivity, styrene probably underwent oligomerization as a major side reaction. DMF was oxidized correlating with the olefin conversion thus functioning as a sacrificial reductant. Due to the high Co loading of the MOF, high conversions were achieved with considerably lower absolute catalyst amounts compared to previous literature results. Higher reaction rates were observed with increasing temperature, substrate concentration and oxygen supply while the amount of catalyst only had a limited influence. Some cobalt leaching was observed though the MOF in general exhibited good stability as suggested by scanning electron microscopy (SEM), XRD and XAS. Amines (from DMF) deactivated the catalyst. The MOF catalyst featured an induction period which was not observed with homogeneously dissolved cobalt. During the induction, the catalyst was activated by benzaldehyde forming as a side-product which led to the interesting effect that co-epoxidation of (E)- stilbene proceeded faster in the presence of styrene since styrene is more easily oxidized to benzaldehyde. Further catalytic studies allowed the formulation of a tentative reaction mechanism.