Highly Active and Stable CeO2-Promoted CuCl2/Al2O3 Oxychlorination Catalysts Developed by Rational Design Using a Rate Diagram of the Catalytic Cycle

In this study, we have developed a method to predict the steady-state rate and Cu oxidation state during ethylene oxychlorination from a reaction rate diagram of the individual steps involved in the catalytic oxychlorination cycle. The steady state of the redox cycle is represented by a cross point of the reaction rates of the reduction and oxidation steps as a function of the Cu2+ in the rate diagram. Transient kinetics of elementary reactions and steady-state kinetics of the overall catalytic cycle were investigated in an operando study using combined mass and UV-vis-NIR spectrophotometry. The catalytic consequence of the promoters was then evaluated in terms of reduction and oxidation activity as well as number of active sites, site activity, and the catalyst oxidation state at steady state. Results revealed that the neat CuCl2 catalysts operated at low Cu2+ at the steady-state conditions with stoichiometric feed composition, as a result of relatively low oxidation rate of Cu1+ As a consequence of a high content of Cu1+, ethylene conversion and selectivity are low, and the catalyst deactivates rapidly. By the promotion of the CuCl2 catalyst by K, the reactor operates at a high Cu2+ concentration with much improved stability as a result of enhanced oxidation rate, but the catalyst has low activity due to significantly reduced reduction rate. Therefore, the rate diagram has been applied as a tool for a rational design of the CuCl2-ased oxychlorination catalysts, and Ce was proposed as the promoter due to its high promotion to the oxidation and low reactivity with Cu ions. It was found that the activity of the Ce-promoted catalysts increased 8 times compared to the neat CuCl2 catalyst and moreover significantly improved the stability for the oxychlorination catalyst at steady state, due to the enhancement of both the rates of the reduction and oxidation. It is anticipated that the methodology developed here paves the way for a general method for catalyst design of heterogeneous catalysts where the catalyst undergoes oxidation state changes, in particular in redox reactions.