Design of ionic liquid involved heterogeneous catalytic system for effective oxidative transformation
Date of Issue2014
School of Chemical and Biomedical Engineering
Ionic liquids (ILs), defined as low-melting salts showed great potential in improving the efficiency of catalytic processes due to their unique physicochemical properties. The primary reasons of employing ILs in catalytic processes originated from the facts that ILs have extremely low vapor pressure as well as excellent dissolving capacity, which afforded them promising candidates of green reaction media. ILs have been successfully integrated with an variety of advanced catalytic technologies except for being reaction solvent. Concerning the atom economy and environmental demand, heterogeneous catalysis was preferred as a greener alternative to replace the conventional stoichiometric procedures. Therefore, in this Ph. D. work, taking both the advantages of IL and heterogeneous catalysis, we intended to develop effective and recyclable IL-involved heterogeneous catalytic processes for the oxidative transformation with particular highlights on the specific effects of the involved IL during the reaction process. Based on the study of the solvent-free aerobic alcohol oxidation over a carbon nanotube supported palladium catalyst, IL 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide ([emim][NTf2]) was introduced in the reaction mixture as solvent/additive and dramatically improved the catalytic performances. The enhanced catalytic activity was attributed to the stabilization effect of ILs on the active sites-Pd nanoparticles as well as IL’s intrinsic physical-chemical properties, e.g., electronegativity. In a parallel work, [emim][NTf2] was applied as a reaction solvent in the alkene epoxidation over vanadium-exchanged faujasite zeolite catalysts (V-X), using tert-butyl hydroperoxide (TBHP) as the oxidant. Compared with the conventional organic solvent such as DMF, [emim][NTf2] significantly improved the catalytic activity, which was elucidated by examining the interaction between IL and the components in the reaction, specifically the hydrogen bonding interaction between IL and t-butabol which is one of the by-products. As indicated above, the combination of heterogeneous catalyst and IL as a reaction solvent has made remarkable progress, while the consumption of a large amount of IL as a solvent is bound to high cost of the process. Therefore from the economic point of view, the strategy of reducing the amounts of IL meanwhile maintaining its unique properties to obtain effective heterogeneous catalysis was urged to be developed. Recent works have shown several strategies to synthesize the heterogeneous catalyst with IL directly integrated, among which the strategy of synthesizing IL-based polyoxometalate (IL-POM) composites by ion exchange method was studied in our work. A series of IL-POM catalytic materials were successfully synthesized and found to be able to effectively transform the cellobiose into levulinic acid (LA) and formic acid (FA) in one pot with high selectivities in the presence of oxygen. The IL cation introduced in the hybrid material on one hand acted as the strong Brønsted acidic sites, responsible for their catalytic activity in the hydrolytic degradation of cellobiose. On the other hand afforded the IL-POM catalyst high melting point as well as low solubility in water, which can recrystallize into solid for the ease of recycling catalyst for more runs.