Modification of acidic groups over zeolites via calcination for the selective catalytic transformation of 2,3-butanediol in ethylene glycol

Abstract

Biomass and bio-derived chemicals can be transformed into ethylene glycol (EG) via catalytic hydrogenation, but a small amount of 2,3-butanediol (23BDO) is generated. 23BDO in EG was removed via the dehydration reaction over H-beta zeolite calcined at 800 °C, with a 23BDO conversion of 80.4% and corresponding SF value of 17.1, which was almost thrice that over the original zeolite due to the suppression of ketalization and oligomerization reactions of EG, especially the latter. The conversions of C_3–6 diols depended on the numbers of carbon atoms due to a positive effect of the alkyl group on their reactivity, with five-fold SF values compared to the pristine zeolite. The pore and crystal structures of the zeolite were disturbed to a small extent after calcination, resulting in its weakened catalytic activity. The NH₃-TPD result proved that the number of acid sites was significantly reduced after calcination, and the ratios of medium and strong acid sites were slightly decreased. The thermal decomposition of Si(OH)Al (Brønsted acid) and EFAL groups (Lewis acid) was confirmed by FTIR. The order of the EG oligomerization reaction with respect to catalyst dosage (third order) was higher than that of the ketalization reaction (first order), so the decrease of the number of acid sites was most unfavorable to EG oligomerization. Compared to the ketalization reaction, the dehydration reaction of 23BDO was less affected after the elimination of EFAL groups, which could not catalyze the dehydration reaction. The used catalyst was regenerated by calcination at 600 °C. The conversions of 23BDO and EG declined to a small and similar degree in the reuse experiment, indicative of the high stability of this modified catalyst.