Mechanisms of Light Alkene Oligomerization on Metal-Modified Zeolites

Zeolites modified with metal (Zn, Ga, Ni, etc.) species of various nature and composition perform oligomerization of light alkenes with high conversion and selectivity to dimeric products. Despite the significant difference in catalytic properties of metal-modified zeolites compared to unmodified H-form zeolite performance, the ability of metal-containing sites to catalyze alkene oligomerization has been debated, with the key role attributed to zeolite Brønsted acid sites (BAS). However, recent spectroscopic and theoretical investigations provide evidence for metal species involvement in alkene transformations. In this Review, we have outlined the features of BAS-assisted alkene oligomerization and further discussed the characteristics of oligomerization assisted by metal-containing sites. The possible mechanisms of metal-assisted hydrocarbon chain elongation have been categorized into several types depending on the nature of the key intermediate: alkyl-based, vinyl-based, allyl-based, and metallacycle. The emphasis has been placed on spectroscopic (13C MAS NMR and FTIR) characteristics of these intermediates, both experimentally available and also hypothetically expected, that serve as the indicators of the reaction occurrence with the assistance of BAS and metal sites. The remaining problems and challenges concerning the detection and identification of the reaction intermediates of light alkene oligomerization on zeolite catalysts and possible strategies to solve them in future studies are also outlined.