Characterization of Mo 2C–WO 2 composite catalysts for bifunctional isomerization: A new pulse method to quantify acid sites

Abstract

New bimetallic catalysts containing molybdenum carbide (Mo 2C) and tungsten dioxide (WO 2) with three different Mo/W ratios were synthesized by temperature programmed reaction in flow of C 2H 6/H 2 mixture and used for the bifunctional isomerization of n-heptane. They are characterized by X-ray diffraction (XRD), electron diffraction (ED), transmission electron spectroscopy (TEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), N 2 sorption, CO chemisorption and adsorption–desorption of iso-propylamine. New pulse method based on the adsorption–desorption of iso-propylamine was used to estimate the total amount of acid sites of which are differentiated the number of Lewis and Brönsted acid sites. XPS shows that only molybdenum and a limited part of tungsten are reduced while the major part of tungsten is under oxide form as WO 2. The adsorption of CO allows estimating the number of metallic sites (Mo 2C) and coke is formed during the isomerization of n-heptane on Brönsted acid sites (WO 2). A kinetic study gives that the reaction order is first for n-heptane conversion. The stabilisation of activity and high selectivity to iso-C 7 from 86 to 94% for the isomerization of n-heptane are observed after induction period of 9 h. The turnover rate (TOR) is 0.1 s −1 at 300 °C.