Insights of the Dynamic Copper Active Sites in Ethylene Oxychlorination Studied by the Multivariate UV–vis–NIR Resolution Kinetic Approach

Abstract

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.

Toward Fully Selective Ethylene Oxychlorination through Engineering the Cu Oxidation State Spatial Profile

“…37 Operando experiments have further confirmed that La does not act as an active site in participating in the redox cycle. 38 It exists in its chloride form, where Cl is directly bonded with La. La is considered as a chemical promoter to alter the electronic structure of the Cu active site by influencing the Cl bonded with La and Cu.…”

Studying, Promoting, Exploiting, and Predicting Catalyst Dynamics: the Next Frontier in Heterogeneous Catalysis

“…The data correspond to the reduction (left), oxidation (middle), and hydrochlorination (right) steps, respectively. Adapted with permission from reference . Copyright 2021 American Chemical Society.…”

“…In this context, operando UV–vis–NIR measurements can offer an accessible way of studying the dynamics of catalytic cycles involving species with strong visible light absorption. , This technique would be advantageous in systems with strong d–d transitions and charge transfer regions, such as Cu-halide-based systems. Researchers recently demonstrated the utility of this approach in studying the dynamics of ethylene oxychlorination on CuCl 2 –Al 2 O 3 -based catalysts . Specifically, they combined a multivariate UV–vis–NIR resolution kinetic approach (MURKA) with multivariate curve resolution (MCR) to analyze the spectroscopic evolution of the catalyst under operating conditions.…”

“…Researchers recently demonstrated the utility of this approach in studying the dynamics of ethylene oxychlorination on CuCl 2 −Al 2 O 3 -based catalysts. 109 Specifically, they combined a multivariate UV−vis−NIR resolution kinetic approach (MURKA) with multivariate curve resolution (MCR) to analyze the spectroscopic evolution of the catalyst under operating conditions. This approach allowed them to quantify the various Cu species involved in the reaction, including the intermediates in the oxidation, reduction, and hydrochlorination steps (Figure 9B).…”

Facet-Induced Strong Metal Chloride−Support Interaction over CuCl2/γ-Al2O3 Catalyst to Enhance Ethylene Oxychlorination Performance

“…Many efforts have been devoted to understanding the chemistry of the catalyst, the reaction mechanism, and the role of promoters in affecting the catalytic performance and the active sites, − and also the influence of different supports on this reaction. − γ-Al 2 O 3 is widely used and recognized as the superior support due to the strong interaction with CuCl 2 than the other supports like SiO 2 and α-Al 2 O 3 , enhancing the catalyst stability and minimizing the volatility of CuCl than other supports . Moreover, the interaction is quite important in the catalyst stability, especially for the Cu loss and migration.…”