Isomerisation of (+)citronellal over Zn(II) supported catalysts

A series of acid catalysts prepared by impregnation of commercial silicas with a solution of ZnBr2 have been characterised with several techniques (BET, adsorption-desorption porosimetry, scanning electron microscopy (SEM), energy dispersive analysis (EDAX), X-ray diffraction (XRD), atomic absorption spectroscopy (AAS)). The results have shown that ZnBr2 is evenly distributed over the surface of the supports regardless of their surface properties. The catalytic surface contains two different sites: physically adsorbed ZnBr2 and Zn(II) sites, obtained from the partial decomposition of the precursor. No correlation between the surface area of the support and the amount of Zn(II) sites has been observed. The ZnBr2/SiO2 based catalysts have been also tested in the isomerisation of (+)citronellal. The catalytic activity results have shown that the structural properties of the support have a scarce influence on the catalytic performance. On all the investigated samples, the selectivity to isopulegols is 100%, whereas the stereoselectivity to (-)isopulegol ranges between 70 and 86%. Zn(II) are the most active sites for the catalytic isomerisation while ZnBr2 has a negligible catalytic activity. However, the presence of ZnBr2 is of fundamental importance to achieve higher selectivity toward the formation of (-)isopulegol. (C) 2002 Elsevier Science B.V. All rights reserved.