Supported liquid-phase catalysts containing ruthenium complexes for selective hydrogenation of α, β-unsaturated aldehyde:: importance of interfaces between liquid film, solvent, and support for the control of product selectivity

Selective hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) has been investigated with supported liquid-phase catalysts (SLPC), which contain Ru-TPPTS complexes in water film supported on a porous silica gel, in organic solvent (toluene). It was shown that COL can be mainly produced with small quantities of hydrocinnamaldehyde (HCAL) and hydrocinnamyl alcohol (HCOL). The influence of various reaction and catalyst preparation parameters on the overall rate of reaction and the product selectivity have been examined. In addition, liquid-phase adsorption of CAL on water-loaded silica samples has also been measured. It is believed that the hydrogenation of CAL to COL occurs at the interface between the water film and the organic solvent while that to HCAL at the interface between silica, water, and toluene. The hydrophilic nature of the C=O bond of CAL is significant for the selective formation of COL at the water/toluene interface, to which polar C=O bonds of CAL molecules are pointing. The product, COL, also has a polar OH group, and so it is difficult for COL to be hydrogenated to HCOL at this interface. At the other silica/water/toluene interface, CAL molecules may be adsorbed with their C=C bonds as well, which enables the hydrogenation to HCAL. On recycling of SLPC, its activity and COL selectivity decrease. Both oxygen dissolved in toluene and change of active Ru complexes in the water film are responsible for the decrease of catalytic activity, and the change of the state of dispersion of the water film is significant for the decrease of COL selectivity. (C) 2004 Elsevier Inc. All rights reserved.