One-step generation of highly selective hydrogenation catalysts involving sub-nanometric Cu2O supported on mesoporous alumina:: strategies to control their size and dispersion

A series of binary Cu-Al surfactant-assisted mesoporous precursors have been prepared through direct synthesis and catalytically evaluated in the selective hydrogenation of conjugated alpha,beta-unsaturated aldehyde (cinnamaldehyde) to the corresponding unsaturated alcohol in the liquid phase. The resulting material consists in a mesoporous alumina substrate involving a ripple sheet type-texture onto which Cu hydroxynitrate domains of uniform size were dispersed. After calcination, these particles yield sub-nanometer (<1 nm) and larger (20-40 nm) CuO particles. An original model implying a homogeneous temperature-induced particle accretion is proposed to explain this bimodal dispersion. A specific re-evaluation of some synthesis parameters allowed us to optimize the relative size, dispersion and final stabilization of these particles. Upon reduction, CuO yields sub-nanometric Cu2O and metallic Cu particles of relatively uniform size. The remarkable selectivity (70%) of the binary mesoporous samples towards the hydrogenation of the carbonyl group of cinnamaldehyde is due to the presence of Cu2O particles that are probably in relatively strong interaction with the alumina pore walls. By contrast, solids involving Cu nitrate impregnates on either mesoporous and commercial (bulky) gamma-alumina performed poorly towards the formation of cinnamyl alcohol, arguing for a fundamental difference in the electronic states of the various supported Cu particles with respect to those generated on samples by direct synthesis. (C) 2004 Elsevier B.V. All rights reserved.