Tuning palladium selective oxidation catalysts via mesoporous supports

Surfactant templating provides a facile route to mesoporous materials with tuneable architectures, whilst simultaneously allowing control over complementary macro and micro porosity. The impact of these varying physical properties, on both catalyst synthesis and during catalytic reactions, was one principal area of investigation. Catalyst series were prepared on mesoporous SBA-15, SBA-16 and KIT-6, meso-macroporous SBA-15, true liquid crystal templated SBA-15 and a commercial low surface area silica support. Additionally two mesoporous alumina series, with mesoporosity akin to SBA-15, were produced. The catalytic activity of the materials was screened for the selective oxidation of allylic alcohols, which represents a class of industrially relevant chemical upgrading reactions. Nanoparticulate palladium is widely recognised as an able catalytic species, although the active site nature is still debated with both metallic and oxidic surfaces proposed. Further insight into the active species was another major area of investigation. Extensive characterisation confirmed successful support synthesis and mesopore stability after palladium impregnation. Irrespective of support, decreasing metal loading elevates dispersion (particles sizes are typically less than 2nm), which correlates with the increase of surface PdO content. In relation to the silica supports these trends escalate via support transition in the order of: Pd/low surface area commercial silica < Pd/true liquid crystal templated SBA-15 < Pd/SBA-15 < Pd/mesomacroporous SBA-15 < Pd/KIT-6 ≈ Pd/SBA-16. Initially increasing support surface area is critical and later rising mesopore accessibility dictates. Catalytic activity, for cinnamyl and crotyl alcohol selective oxidation, reveals significant rateenhancements with PdO content, with turnover frequencies providing compelling proof of a PdO active species. Alumina supports, even with lower surface areas than equivalent silicas, allow further gains in metal dispersion, surface oxidation state and resulting catalytic activity. In conclusion, tuning the physical and chemical properties of the support is paramount if highly active catalysts are to be produced.