Crystal-Facet Modulated CrOx/γ-Al2O3: Quasi-Liquid Surface Modification by Bonded Polydimethylsiloxane for Catalytic Oxidation of Propene.

The crystal-facet effect of catalytic supports plays a crucial role in tailoring the physicochemical properties of active sites and the surface chemically bonded polymer can also regulate the local environment around active sites for optimizing catalytic performance. Herein, we report the effect of exposed facets of γ-Al2O3 supports and further modification by surface bonded long-chain polydimethylsiloxane (PDMS) on the properties of CrOx/γ-Al2O3 catalysts for selective oxidation of propene. The {111} facets of γ-Al2O3 stabilize "non-redox Cr3+" and promote the overall oxidation rates compared with catalysts on {110} facets of γ-Al2O3. The surface bonded PDMS, with grafting density being about 0.13 chains/nm2, endows a hydrophobic environment to facilitate the enrichment of the hydrophobic substrate and the desorption of hydrophilic products and occupies some acid sites on catalysts to limit acid-catalyzed side reactions. The inherent liquidlike nature of bonded PDMS also forms a setting that can regulate the redox ability of surface Cr species, that lead to modified activation of oxygen toward more surface adsorbed species. As a result, the modified catalysts enhance the whole oxidation process with favorable formation of epoxide product at low reaction temperatures (<225 °C). Our findings highlight the impact of surface chemically bound polydimethylsiloxane (PDMS) upon tailoring the surroundings of the catalyst surface, and that combined with facet-effect of supports can tune the reaction process toward selective ones.