Influence of support acidity on the performance of size-confined Pt nanoparticles in the chemoselective hydrogenation of acetophenone

Abstract

Size-confined Pt nanoparticles of about 1.5 nm have been introduced into [Al]MCM-41 supports with similar acid strength but various population densities of acid sites by means of wet impregnation. The Pt nanoparticles covered preferentially the surface Brønsted acid sites (BAS) of the supports or were located near acid sites rather than on the bigger free space between acid sites even at a very low acid density (6 BAS per 1000 nm²). The free BAS around the Pt particles did not interact with Pt atoms and the electronic properties of the Pt nanoparticles as probed by DRIFTS combined with CO adsorption were similar for Pt/[Al]MCM-41 with and without nearby free BAS. Ionic effects were generated by the Pt-covered acid sites, whereas the population of BAS did not contribute significantly to the ionic effects induced on the Pt nanoparticles. The coverage of BAS of similar strength by platinum nanoparticles led to similar chemoselectivity and product distribution in acetophenone (Aph) hydrogenation, though the density of BAS on the supports increased by more than 11 times. However, increasing the number of BAS on the supports significantly changed the hydrogenation rate. TOFs continuously increased from 125 h⁻¹ up to 534 h⁻¹, when the population of free BAS increased from 18.2 BAS per 1000 nm² to 39.9 BAS per 1000 nm². When the free BAS density was further increased to 70.4 BAS per 1000 nm², the TOF then dropped to 176 h⁻¹. The hydrogenation pathway is similar for both monofunctional (Pt covering all BAS) and bifunctional catalysts (Pt with free BAS), and the reaction was initiated on the Pt surface. This finding indicates that proper tuning of the population density of acid sites on the support can significantly improve the catalytic performance of the supported metal catalysts while keeping similar product selectivities.