Inovyn Norge (Norway)

The supported transition metal oxides, chlorides, carbides, and so forth (MXn), represent an important type of catalyst for redox processes. The MXn–support interaction, analogous to metal-support interaction, plays a critical role in redox reactions. Herein, we illustrated the nature of the monolayer CuCl2–γ-Al2O3 interaction, and its catalytic consequence induced by support facets in ethylene oxychlorination. Kinetic and theoretical studies, combined with operando spectroscopy demonstrated the effects of the interaction between CuCl2 with the various exposed facets of γ-Al2O3 (110) and (100). The catalyst with Al2O3(110) shows better performance with high activity, selectivity, and stability. The catalyst with the (111) facet deactivated rapidly, and the ethylene dichloride (EDC) selectivity decreased to 30%; while the activity on the catalyst with the (110) facet was much higher, with the EDC selectivity can be well kept at >99% in the long-term stability test. The multi-scale and in-depth characterizations revealed the concept of metal chloride–support interaction in determining the catalytic performance. We believe this method of tuning the reactivity of active species by manipulating their coordination with the support surfaces as a ligand shall spark inspiration for other catalytic applications.

Monitoring and simulating the events occurring on the catalysts under the real reaction conditions has significant meaning for elucidating the dynamic changes of the active sites during the reactions and for a better understanding of the reaction mechanisms. Herein, we use the operando ultraviolet–visible and near-infrared (UV–vis–NIR) spectroscopy to study the CuCl2/γ-Al2O3-based catalyst in ethylene oxychlorination, one of the most important processes for producing vinyl chloride in the industry, to elucidate the dynamic changes of the copper active sites. The full spectra of Cu species such as CuCl2, CuCl2 with vacancies, CuCl, and Cu2OCl2 were detected and identified for the first time, and their transient changes and contribution in the reduction, oxidation, and hydrochlorination steps as well as at the steady-state operation in the catalytic cycle can be accurately "imaged" by resolving the UV–vis–NIR spectra dataset using the multivariate curve resolution (MCR) analysis. The distribution and changes of the Cu species are correlated to the catalytic activity, selectivity, and stability. The time-resolved spectra and their correlation with the catalytic performance provide better insights into the dynamic nature of the active sites and their role in the Mars–van Krevelen-type reaction. This method is expected to be exploited to analyze the dynamics of the active sites and kinetic studies in other catalytic redox systems.

A kinetic model was developed by taking into account the dynamic nature of the active sites in Mars–van Krevelen type catalytic reactions to predict the evolution of the reactant and product composition in the gas phase and the CuCl2 concentration in the solid catalyst. The kinetic model at the steady-state of ethylene oxychlorination was obtained by combining transient experiments of the two half-reactions in the redox cycle, namely CuCl2 reduced to CuCl by ethylene and CuCl oxidation by oxygen on the K-promoted CuCl2/γ-Al2O3 catalyst. The dynamic transitions between CuCl2 and CuCl of the active sites during the reactions are also modeled, and the contributions of two active sites, namely Cu coordination numbers of 4 and 3 in CuCl2 were distinguished and included in the kinetic model. The kinetic models describe well the transient response of the reduction and oxidation steps as well as the reaction at the steady-state at different reaction conditions. Moreover, by combining the reactor modeling through a steady-state approach, the spatial-time resolved CuCl2 profile and the C2H4 reaction rate can be well predicted in comparison with the experimental results. The approach of both transient and steady-state kinetic modeling and simulation is supposed to have general relevance for a better understanding of Mars–van Krevelen type reactions.