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Designing Bifunctional Nano-Alloy Catalysts for Bio-Renewable Feedstock Valorisation

Final Report Summary - BIFUNC-NANOCAT (Designing Bifunctional Nano-Alloy Catalysts for Bio-Renewable Feedstock Valorisation)

Supported bimetallic nanoalloys based catalysts, especially gold-palladium nanoalloys, have been reported to be exceptionally active for many reactions including (a) CO oxidation, (b) primary alcohol oxidation, (c) polyol oxidation, (d) selective oxidation of C-H bonds and many more. The catalytic activities of these bimetallic catalysts have been reported to be several folds higher than that of their monometallic counterparts. These catalysts have been reported mostly for single step organic transformations like oxidation or hydrogenation only. The objectives of BIFUNC NANOCAT are (a) to expand the synthesis strategies of gold-palladium based bimetallic nanoalloys to ruthenium-palladium nanoalloys based catalysts, (b) characterize these supported bimetallic nanoalloys using advanced microscopic/spectroscopic methods, (c) utilize these supported bimetallic nanoalloys as catalysts for one-pot synthesis (cascade reactions) of imines and secondary amines involving biomass derived platform molecules and (d) understand the mechanism of these catalysed reactions using advanced in situ spectroscopic techniques.
Before the start of this project, at Cardiff Catalysis Institute, I had reported an “anion-excess” modification for the wet-impregnation method (MIm) for preparing supported gold-palladium nanoalloys based catalysts. Catalysts prepared by this new methodology have been found to be superior in activity and stability compared to the catalysts prepared using conventional wet impregnation (CIm) and polymeric stabilizer-ligand assisted sol-immobilization methodologies (SIm). During the course of the Marie Curie IEF project, I successfully expanded this MIm strategy to the synthesis of supported ruthenium-palladium nanoalloys. Supported AuPd and RuPd catalysts have been characterized using advanced spectroscopic / microscopic techniques like X-ray Absorption Spectroscopy (XAS) including both Extended X-Ray Absorption Fine Structure (EXAFS) & X-ray Absorption Near Edge Structure (XANES) and Scanning Transmission Electron Microscopy (STEM) combined with X-ray energy dispersive (XEDS) spectroscopy. These two techniques are complimentary in nature and detailed analyses of the results revealed that supported ruthenium-palladium nanoalloys, prepared by MIm, have smaller metal particles with better control over the size distribution compared to the gold-palladium nanoalloys. Supported gold-palladium and ruthenium-palladium nanoalloys based catalysts have been tested for a new one-pot synthesis (cascade reactions) of imines and secondary amines directly from primary alcohols and nitrobenzene without using any base or external hydrogen. This cascade reaction involves dehydrogenation of primary alcohol to carbonyl compound and simultaneously reduction of nitro compound to primary amines followed by the coupling of carbonyl compound and amines to form imines which further reduces to form secondary amines. Thus, this particular catalyst acts as a multi-functional catalyst catalysing (a) dehydrogenation, (b) coupling and (c) hydrogenation reactions in one pot. Based on the characterization data and the catalytic data I found that homogeneous random alloy structure of these bimetallic nanoalloys is crucial to observe the “synergistic” effect. Thus a structure-activity correlation has been arrived for this system. Finally, I used three different in situ spectroscopic techniques (in situ ATR-IR, in situ DRIFT and in situ Inelastic Neutron Scattering (INS)) to elucidate the mechanism of (a) oxidative dehydrogenation of benzyl alcohol and (b) disproportionation of benzyl alcohol. Though we used these advanced spectroscopic techniques we could not prove the mode of adsorption of benzyl alcohol over the surface of these supported nanoalloys based catalysts. However, this study has been predicted to inspire the development of more advanced spectroscopic techniques to elucidate the mechanism of these complex catalytic reactions.
The successful execution of this Marie Curie IEF project demonstrates the importance of combining catalyst designing with catalyst characterization using advanced spectroscopic/microscopic methods. I strongly believe that this research project will inspire many catalysis scientists to design synthesis strategies to achieve specific nanostructures that are catalytic very active, especially bimetallic nanostructures. Two well-known research groups in Europe (Prof. G. J. Hutchings, Cardiff University and Prof. B. M. Weckhuysen, Utrecht University) have collaborated for the first time because of this Marie Curie IEF project. I believe that this scientific cooperation will continue in future as well.