Tuning the Reactivity of Ethylene Oligomerization by HZSM-5 Framework Alf Proximity
Ethylene oligomerization with controlled chain lengthening over the shape-selective HZSM-5 catalyst provides alternative technologies for the C3+ light olefins production under current circumstances of shifting hydrocarbon feedstocks from naphtha to ethane. Here, by tuning the MFI framework Alf siting, either isolated or in close proximity, we investigated the influence of the Alf proximity on the kinetics and mechanisms of ethylene oligomerization. We demonstrate that the HZSM-5 catalyst with a high concentration of Alf that in close proximity (AlHProx) shows higher activity. The turnover frequency of the AlHProx is up to 14 fold of that for AlIso (HZSM-5 with isolated Alf). Steady-state kinetic study showed that the activation energy for the AlIso (104.8 kJ/mol) is lower than that for the AlHProx (144.5 kJ/mol). In addition, the kinetic compensation effect with the isokinetic point at Tiso 177°C and TOFiso 1.64×10-5 s-1 was identified. Further reaction transient analysis suggested that the dual-cycle hydrocarbon-pool (HP) mechanism to be 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. While the early-stage (HP build-up) behavior seems independent of the Alf proximity, the back-transient (HP clean-up) suggested decreasing of the aromatic cycle over the AlHProx sample.