(344r) DFT Analysis of Ethene Oligomerization on Metal Exchanged Zeolites

Ethylene oligomerization is gaining importance due to the abundance of ethene produced from steam cracking of ethane. The metal supported zeolitic materials, specifically the frameworks with larger pores, have emerged as promising catalysts for oligomerization reactions, due to their ease of recyclability as compared to homogeneous catalysts. Recent work from Joshi et al.¹ shows that on Ni-exchanged BEA catalyst, the Coordination Insertion (CI) mechanism is the major reaction pathway through which ethylene is converted to butenes. Further the authors show that isomerization of 1-butene to 2-butene can occur uniquely on the Ni sites in the absence of acid sites. Hence this study, focuses on using DFT in concurrence with microkinetic modelling on different metal exchanged zeolite catalysts to predict the isomer distribution of the oligomers, as a function of pressure & temperature.

Under ethylene oligomerization reaction conditions, it is proposed that the Ni-H coordinates with ethylene forming β-agostically bound ethyl complex, leaving a proton on the adjacent site. Hence the calculations were performed starting with Ni-ethyl complex as the active site. The isomerization pathway analysis shows that the there’s a branching pathway where 1-butyl complex can undergo either ethylene assisted desorption to form 1-butene or can undergo isomerization to form 2-butene. Furthermore, the trends in activity and selectivity towards butene formation across different metal cations was identified by comparing the calculations on Ni²⁺ with other cations like Co²⁺, Zn²⁺ and Ga³⁺.Finally, the effect of pressure was studied by increasing the ethylene coordination on Ni²⁺, which showed increased mobility of the Ni complex inside the framework. The mechanistic implications of increase in ethylene coordination, on isomerization and deactivation pathways will be discussed.

1. Joshi et al, ACS Catal., 2018, 8, 11407–11422.