New mixed ligand coated platinum nanoparticles for heterogeneous catalytic applications

A series of platinum nanoparticles was prepared for the first time in presence of one hydrophobic ligand and one hydrophilic ligand containing terminal sulfonic acid headgroup. This mixed ligand combination causes stripe-like domains circumscribing the nanoparticles, thereby enhancing the catalytic efficiency compared to single ligand coated Pt nanocatalysts. This could be achieved by strategically replacing the hydrophilic ligand (3-mercaptopropane-1-sulfonic acid or MPSA) with hydrophobic ligands (linear alkanethiols like 1-octanethiol or OT) on the surface of the nanoparticles. Apart from the ligand composition, the metallic core of the nanocomposite also influences the catalytic efficiency of the ligand shell through a synergistic effect. Herein we report the effects of ligand shell composition and structure on the efficiency of the nanocomposites on catalytic acylation reactions. Quite high conversion of benzyl alcohol to benzyl acetate (with >99% selectivity) was observed when acetic acid was used as the acetylating reagent and MPSA–OT–Pt as nanocatalyst at 80 °C. The turnover frequency of reaction using MPSA–OT–Pt nanocatalyst was found to be an order of magnitude higher than that using well known solid acid catalyst zeolite USY under identical reaction conditions. Further, the selectivity toward benzyl acetate using USY was ≤80%, the remaining 20% being dibenzylether. It was also found that the MPSA–OT–Pt nanocatalyst system shows higher activity than gold nanoparticles coated with same set of ligands (such as MPSA–OT–Au nanocatalyst). Additionally, the effects of changing the substrate (such as 1-butanol) and the acylating agent (such as propionic acid) were also studied.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (80 K)Download as PowerPoint slideHighlights► Formation of striped mixed ligand shell circumscribing Pt nanoparticles. ► A new generation of acid catalyst comprising of platinum nanoparticles functionalized with sulfonic acid and a variety of hydrophobic alkanethiols. ► About 90–95% conversion of substrate and >99% product selectivity in <3 h at 80 °C. Active for a variety of acylation reactions. ► Both the stripe-like environment at the ligand shell and the influence of Pt metal core are dominant factors for the activation of the reactant molecules due to enhanced surface energy.