Cluster-Size-Dependent Interaction between Ethylene and CuCl₂ Clusters Supported via γ-Alumina

Abstract

Alumina-supported copper chloride serves as an industrial catalyst for ethylene oxychlorination, resulting from its high activity and selectivity. A better understanding of the detailed active site structure and reaction mechanism is highly desired. The present work aims to explore the dependence of the structure of active sites and the adsorption of ethylene on differently sized (CuCl 2 ) n (n = 1−4) clusters supported by γ-Al 2 O 3 . The effect of the support facets (i.e., ( 110) and ( 100) surfaces) on the interface structures between the active component CuCl 2 and the support was also investigated. The stronger CuCl 2 −support interaction was found on the (110) surface compared to the (100) surface, which is attributed to the stronger Lewis acidity of Al of the (110) surface. The adsorption strength of (CuCl 2 ) n) (n = 1−4) clusters becomes weak with the increment of cluster size on the (110) surface. The cluster size has a profound influence on the interaction between ethylene and the clusters. Ethylene binds to a copper atom on the small clusters (i.e., CuCl 2 and (CuCl 2 ) 2 ), while it extracts two chlorine atoms to form dichloroethane from the large clusters (i.e., (CuCl 2 ) 3 and (CuCl 2 ) 4 ), which explains the high activity of catalysts with high loadings upon exposure to ethylene. The effects of cluster size and alumina facets on the short d-band center and the Bader charge of the active sites result in the distinct formation energy of the chlorine vacancy and the interaction energy between C 2 H 4 and the clusters. Thus, an improved catalyst could be achieved by the modification of the surface electronic structure via finetuning the support or adding promoters.

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

“…CuCl 2 reacts with C 2 H 4 in the reduction step (eq ) to form EDC and CuCl in principle. It is consistent with the observation of density functional theory (DFT) , and kinetic studies, , where ethylene first attracted two Cl atoms from CuCl 2 with a very low energy barrier. However, CuCl 2 is rapidly converted to an intermediate instead of CuCl, as shown in Figure b.…”

“…The intermediate should be Cu species with an oxidation state between CuCl 2 and CuCl. The coordination numbers of Cu with the Cl were found to be 4 and 2 for CuCl 2 and CuCl, respectively. ,− The reaction of CuCl 2 with C 2 H 4 removed the lattice Cl to form Cl vacancy on CuCl 2 catalysts. DFT study suggested that CuCl 2 with vacancy could have coordination numbers of 3 and 2 for low and high concentrations of Cl vacancy, respectively. , Lattice Cl is rather mobile in CuCl 2 and can rapidly redistribute on the surface .…”

“…The coordination numbers of Cu with the Cl were found to be 4 and 2 for CuCl 2 and CuCl, respectively. ,− The reaction of CuCl 2 with C 2 H 4 removed the lattice Cl to form Cl vacancy on CuCl 2 catalysts. DFT study suggested that CuCl 2 with vacancy could have coordination numbers of 3 and 2 for low and high concentrations of Cl vacancy, respectively. , Lattice Cl is rather mobile in CuCl 2 and can rapidly redistribute on the surface . Therefore, it is reasonable to identify the intermediate as CuCl 2 -□ (□: vacancy), where Cu has most likely a coordination number of 3 with Cl.…”

“…However, CuCl 2 is rapidly converted to an intermediate instead of CuCl, as shown in Figure 3b. Moreover, it shows an unstable character of the intermediate where the fraction increases first following a decrease with the 27,39 Lattice Cl is rather mobile in CuCl 2 and can rapidly redistribute on the surface. 39 Therefore, it is reasonable to identify the intermediate as CuCl 2 -□ (□: vacancy), where Cu has most likely a coordination number of 3 with Cl.…”

“…Moreover, it shows an unstable character of the intermediate where the fraction increases first following a decrease with the 27,39 Lattice Cl is rather mobile in CuCl 2 and can rapidly redistribute on the surface. 39 Therefore, it is reasonable to identify the intermediate as CuCl 2 -□ (□: vacancy), where Cu has most likely a coordination number of 3 with Cl. With increasing extent of reduction, the concentration of CuCl 2 -□ decreases due to the conversion to CuCl.…”