On the effects of K and La co-promotion on CuCl2/γ-Al2O3 catalysts for the oxychlorination of ethylene

Abstract

The performance of CuCl2/Al2O3 with and without La and K promoters, designated as Cu, K0.23Cu0.77, K0.29Cu0.71, La0.29Cu0.71 and K0.21La0.08Cu0.71 was studied in the ethylene oxychlorination at 503K and 1atm by in-situ UV–vis-spectroscopic kinetic study through a two-step approach. The effects of the dopants on the oxychlorination reaction were firstly deconvoluted to their influence on the Cu2+ reduction and Cu1+ oxidation in the redox catalyst cycle by transient kinetic study. It followed kinetic study using the rate diagram as well as the steady state kinetic study to investigate the effect of dopants on the steady state reaction rate and Cu2+ contents. All dopants increased the fraction of reducible Cu2+for ethylene oxychlorination. The results demonstrate that La significantly increased the reactivity of the active sites towards the reduction step, whilst K increased the rate of oxidation of Cu1+. The co-promoted catalyst improved the site activity by 40.6% during the reduction step compared to the single doped K catalyst. The combined effect of La and K in K0.21La0.08Cu0.71 substantially increased the oxidation of Cu1+. The kinetic study by the rate diagram from the transient kinetic data predicted a higher steady state rate and Cu2+ content on co-doped catalysts K0.21La0.08Cu0.71. The steady state experiments confirm the high Cu2+ content obtained, but a slightly higher reaction rate on K0.21La0.08Cu0.71 compared to K0.23Cu0.77, due to a suppressing effect by La promoted strong adsorption of HCl. The current study is expected to pave the way for exploring the potential benefits of co-promotion on CuCl2/Al2O3 based oxychlorination catalysts in order to enhance catalyst properties and performance. The rate diagram approach is a powerful tool for obtaining mechanistic insights and for guiding the design of co-doped oxychlorination catalysts.