Beta zeolite is one of the most promising green catalysts for the isobutane (C4) alkylation with the advantages of eco-friendliness and facile separation. Although Al-rich beta zeolites are found to exhibit a longer catalytic lifetime in C4 alkylation, they suffer from low selectivity owing to the severe cracking reactions induced by their numerous strong Brønsted acid sites (BAS). In this work, cobalt modification has been used to modulate the acidity of Al-rich beta zeolite to improve the selectivity of C4 alkylation. The highly dispersed, tetrahedrally coordinated isolated Co(II) species are confirmed to be located at the zeolite cationic sites. These cobalt species exist as Lewis acid sites, with the formation of Co–H species activating the C–H bonds. Among Co@HB-1 catalysts with various cobalt loadings, the 1.7Co@HB-1 sample exhibits the highest catalytic productivity. Moreover, in the optimization of process conditions, a lower olefin WHSV slows coke accumulation to extend lifetime but intensifies cracking, which in turn reduces RON; a higher I/O ratio accelerates hydride transfer kinetics and thus enhances RON. Under optimal conditions, 1.7Co@HB-1 achieves a RON of 96 at an I/O ratio of 100 and an olefin WHSV of 0.08 h–1. This study provides fundamental insights into tuning the acidity of zeolites for significantly enhanced C4 alkylation selectivity.
Wen et al. (Mon,) studied this question.