Preparation of Pt nanoparticles in the presence of a chiral modifier and catalytic applications in chemoselective and asymmetric hydrogenations

Abstract

Pt/bentonite (Pt/B) samples with 1% of Pt content were synthesized via successive cation-exchange, by using the layer-structured, synthetic cationic clay Bentolite H as a host material, the precursor [Pt(NH₃)₄](NO₃)₂ and the chiral modifier cinchonidine dihydrochloride (CD·2HCl). The samples Pt/B1 and Pt/B2, reduced before and after CD ion-exchange, respectively, were investigated, together with the unmodified sample Pt/B0. Structural characterization was performed by ICP-AES, XRD and TEM measurements. The intercalation of the chiral modifier, which took place for both Pt/B1 and Pt/B2, was less affected by the sequence of the reduction and the ion-exchange steps than the location and the size distribution of the Pt crystallites. For Pt/B1, the size distribution was considerably narrower and the amount of the interlamellar Pt particles was possibly higher than for Pt/B2. The samples were tested as catalysts for the chemoselective hydrogenation of 2-cyclohexene-1-one and for the asymmetric hydrogenation of ethyl pyruvate. It was established for the transformation of 2-cyclohexene-1-one that both the external and interlayer Pt particles participated in the reaction and the presence of the CD modifier improved both the catalytic activity and the chemoselectivity. In contrast, the chirally modified active sites participating in the asymmetric reaction were situated in the interlamellar region of the clay host, as revealed by a substantial difference between the catalytic performances of the modified samples. Pt/B1 proved to be a more active and enantioselective catalyst than Pt/B2. The enantioselectivity of Pt/B1 exceeded that of Pt/B0, which was studied in the presence of the free modifier.