Alkaline Earth and Transition Metal Oxides Supported on Mesostructured Carbon for Selective Hydrogen Transfer Reactions

The performance of Co3O4, ZnO, and MgO nanoparticles supported on mesoporous carbon (MC) as catalysts for hydrogen transfer reactions has been studied in this thesis. A series of model catalysts with different loadings and metal oxide combinations has been synthesized and extensively characterized. The metal precursors were introduced into a mesostructured polymer framework via ion-exchange process; the composites were subsequently pyrolyzed and mildly oxidized. This enabled the obtention of narrow particle size distribution and homogeneous dispersion of Co3O4 and ZnO. MgO was also homogeneously dispersed, although with the presence of some larger needle shaped particles. The simultaneous incorporation of Co/Zn and Co/Mg led to formation of homogeneously dispersed CoO and MgO nanoparticles, and ZnO agglomerations. Overall, the cobalt loaded catalysts presented an increase in porosity, probably caused by a partial combustion of the carbon support catalyzed by cobalt during the pyrolysis step. The catalytic activity of the materials has been evaluated for citral hydrogenation via hydrogen transfer reaction, using 2-propanol in excess as the hydrogen donor. The reactions were done in batch reactor, using a stainless-steel Teflon-lined autoclave, performing a 6 h reaction at 120 °C. The results from the catalytic test show that the highest activity (i.e. highest average TOF value) is displayed by the Co3O4 catalyst with the lowest Co loading. Although ZnO and MgO presented lower conversions than the Co3O4, they were highly selective towards the unsaturated alcohol products of citral (nerol and geraniol) reaching values of up to 80%, surpassing so the performance of almost all Pd and Pt catalysts used in conventional H2 hydrogenation reactions. Moreover, these results were achieved without the use of hydrogen gas and its related high-pressure reactions. These values present MgO and ZnO as promising alternative catalysts for the selective hydrogenation of the C=O bond in unsaturated aldehyde, which is a critical reaction step in several industrial processes, i.e. in the flavor, fragrance, cosmetic, and pharmaceutical industry