Catalytic performance in selective hydrogenation of citral of bimetallic Pt-Sn catalysts supported on MgAl2O4 and γ-Al2O3

The liquid phase citral hydrogenation, using Pt and PtSn catalysts supported on MgAl2O4 and gamma-Al2O3, was studied in a stirred reactor at 70 degrees C and atmospheric pressure. It was found that the addition of Sn to the Pt catalysts increases the selectivity to double unsaturated alcohols for both catalyst series. Besides, monometallic catalysts hydrogenate the alpha,beta-unsaturation with a high selectivity in absence of cyclization secondary products. The performance of these catalysts in the citral hydrogenation was related with the characteristics of the metallic phase. Results from test reactions - cyclohexane dehydrogenation (CHD) and cyclopentane hydrogenolysis (CPH) - H-2 chemisorption, 2-propanol dehydration, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS), were used to explain the influence of the support and the Sn loading and postulate the models of the catalytic metallic surface. Results indicated that, a fraction of ionic Sn would be deposited near Pt, thus increasing the polarization of the carbonyl group, and a fraction of metallic Sn could form Pt-Sn alloy phases that would hinder the hydrogenation of the olefinic bonds and would be active to the hydrogenation of the carbonyl group. Both effects contribute to a higher selectivity to unsaturated alcohols in bimetallic PtSn/Al2O3 catalysts than PtSn/MgAl2O4 ones, which display a low alloy formation. (C) 2010 Elsevier B.V. All rights reserved.