Silica-supported PtSn Catalysts Obtained Through Surface Organometallic Chemistry on Metals Techniques Using a Hydrosoluble Organotin Promoter. Application to the Selective Hydrogenation of α, β-Unsaturated Aldehydes and Ketones

Abstract

Abstract: In this paper we propose the preparation of a PtSn bimetallic catalyst through techniques derived from Surface Organometallic Chemistry on Metals (SOMC/M) using water as solvent. The system, PtSn0.2 ac , was employed in the chemoselective liquid-phase hydrogenation of acetophenone, cinnamaldehyde and benzaldehyde. The results were com-pared with those obtained with a PtSn catalyst also prepared via SOMC/M, but using a conventional paraffinic solvent. The aqueous medium-prepared catalyst resulted to be as active in and selective to the desired product (unsaturated alco-hols) as that obtained from n- heptane. This catalyst has the advantage of being prepared in a solvent compatible with the environment, without losing the superior characteristics of SOMC/M-based systems. Keywords: PtSn catalysts, SOMC/M, Hydrosoluble organotin promoter, Acetophenone, Cinnamaldehyde, Chemoselective hydrogenation. INTRODUCTION The reduction of α,β-unsaturated aldehydes and ketones leads to the production of alcohols used as intermediates in the production of compounds of interest in fine chemistry [1,2]. The catalytic hydrogenation of unsaturated carbonyl compounds is relatively simple with respect to the C=C bond, due to the well-known fact that its hydrogenation is thermodynamically favored. However, the desired product from these reactions is generally the unsaturated alcohol. Due to this fact, th e design of catalysts having the ability of inhibiting t he hydrogenation of the C=C bond and/or favor-ing the hydrogenation of the C=O group remains an issue of permanent interest. Most reports found in the tliterature for the hydrogenation of unsaturated carbonyl tcompounds are based on catalysts involving metals of 8, 9, 10 or 11 groups (mostly of the Pt group), modified with a more electroposi-tive metal, generating a bimetallic system [2, 3]. The second metal may exist as adatom, forming an alloy, partially oxi-dized or in ionic state. The difference in electronegativity between the two metals may favor the polarization of the carbonyl bond causing high selectivities to unsaturated alco-hols. In some systems, bimetallic geometric effects caused by the presence of the second metal have also been observed [2]. Thus, the addition of a second metal may lead to varia-tions in the activity and/or selectivity of the catalytic system, either through electronic interactions with the first metal or by modifying the architecture of the active site.