Tuning the reactivity of ethylene oligomerization by HZSM-5 framework Alf proximity

Abstract

Ethylene oligomerization with controlled chain lengthening over the shape-selective HZSM-5 catalyst provides alternative technologies for the production of C₃₊ light olefins under the current circumstances of shifting hydrocarbon feedstocks from naphtha to ethane. Here, by tuning the MFI framework Al_f site to either isolated or in close proximity, we investigated the influence of the Al_f proximity on the kinetics and mechanisms of ethylene oligomerization. We demonstrate that the HZSM-5 catalyst with a high concentration of Al_f in close proximity (Al_HProx) shows higher activity. The reaction rate of Al_HProx is up to 14-fold that of Al_Iso (HZSM-5 with isolated Al_f). Steady-state kinetic studies showed that the activation energy for Al_Iso (104.8 kJ mol⁻¹) was lower than that for Al_HProx (144.5 kJ mol⁻¹). In addition, the kinetic compensation effect with the isokinetic point (T_iso) at 177 °C and Rate_iso at 0.015 μmol g⁻¹ s⁻¹ was identified. Further reaction transient analysis suggested that the dual-cycle hydrocarbon-pool (HP) mechanism was involved. Unlike the steady state, the transient state favors the formation of alkanes and aromatics, indicating the establishment of cyclic carbocations through oligomerization, cyclization, and hydride transfer. The early-stage (HP build-up) behavior seemed to be independent of the Al_f proximity, but the back-transient (HP clean-up) suggested decrease in the aromatic cycle over the Al_HProx sample.