Hierarchically Ordered Nanoporous Pd/SBA-15 Catalyst
for the Aerobic Selective Oxidation of Sterically Challenging Allylic
Alcohols
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Abstract
The
utility of a hierarchically ordered nanoporous SBA-15 architecture,
comprising 270 nm macropores and 5 nm mesopores (MM-SBA-15), for the
catalytic aerobic selective oxidation of sterically challenging allylic
alcohols is shown. Detailed bulk and surface characterization reveals
that incorporation of complementary macropores into mesoporous SBA-15
enhances the dispersion of sub 2 nm Pd nanoparticles and thus their
degree of surface oxidation. Kinetic profiling reveals a relationship
between nanoparticle dispersion and oxidation rate, identifying surface
PdO as the catalytically active phase. Hierarchical nanoporous Pd/MM-SBA-15
outperforms mesoporous analogues in allylic alcohol selective oxidation
by (i) stabilizing PdO nanoparticles and (ii) dramatically improving
in-pore diffusion and access to active sites by sesquiterpenoid substrates
such as farnesol and phytol