Operando spectroscopy unravels the nature of deactivating species and their precursors in alkene oligomerization catalysis†

Abstract

Alkene oligomerization represents an attractive methodology to produce environmentally friendly synthetic fuels, free of aromatics. However, the materials that catalyze this reaction undergo complex deactivation, the understanding of which remains elusive. To better understand deactivation, its mechanism and pathways, operando UV-vis spectroscopy has been implemented alongside solid-state NMR spectroscopy in the oligomerization of propene (at 523 K and 50–100 kPa of propene pressure) over highly acidic ZSM-5 and zeolite beta. Measured spectra reveal that deactivation is initiated by the formation of an allylic hydrocarbon pool comprising dienes and cyclopentenyl cations. This hydrocarbon pool acts as a scaffold for the formation of alkylated benzenes (e.g., 1,3-di-tert-butylbenzene) which, due to spatial limitations, end up retained as coke species. The hydrocarbon pool also mediates further growth of alkylated benzenes into polycyclic aromatic hydrocarbons, a process that forms larger coke species. However, as in the case of ZSM-5, this process can be retarded by the shape selectivity of the zeolite. The spectra also show that the formation of long oligomers (C₁₄–C₁₆), irrespective of their degree of branching, renders them entrapped within zeolite pores.