Isomerisation of n-hexane over bifunctional Pt-heteropoly acid catalyst: Enhancing effect of gold

Abstract

Isomerisation of n-hexane was studied in the presence of acid and bifunctional metal-acid catalysts based on Keggin-type heteropoly acids (HPA), in particular focusing on Cs2.5H0.5PW12O40 (CsPW) and Pt/CsPW as the catalysts, using a fixed-bed microreactor under differential conditions (n-hexane conversion ≤ 10%) at 180–220 °C, ambient pressure and a ratio of n-hexane and H2 partial pressures of 0.06–0.24. The turnover rate of HPA-catalysed isomerisation was found to correlate with the acid strength of HPA (initial enthalpy of ammonia adsorption). Bifunctional Pt-HPA catalysts were more efficient than monofunctional HPA catalysts. In the isomerisation over Pt/CsPW bifunctional catalyst, n-hexane dehydrogenation step was found to equilibrate at a molar ratio of Pt and H+ surface sites Pts/H+ ≥ 0.8, corresponding to a Pt loading ≥6%. Bimetallic PtAu/CsPW catalyst showed higher activity in n-hexane isomerisation than Pt/CsPW, although the Au alone without Pt was inert. In the presence of Au, the turnover rate at Pt sites increased more than twofold. The effect of Au is attributed to PtAu alloying. Scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM-EDX) and X-ray diffraction (XRD) analyses of PtAu/CsPW indicated the presence of bimetallic PtAu nanoparticles with a wide range of Pt/Au atomic ratios.