Elucidating the olefin formation mechanism in the methanol to olefin reaction over AlPO-18 and SAPO-18

Abstract

The mechanism of the methanol to olefin (MTO) reaction over AlPO-18 (without Brønsted acid sites) and two SAPO-18 (with different Brønsted acid site densities) catalysts has been investigated. The Brønsted acid site density of AlPO-18 and SAPO-18 catalysts was determined by ¹H MAS NMR spectroscopy. Methanol conversion over the catalysts showed that the catalytic activity of the catalysts was strongly influenced by their Brønsted acid site density. Using ¹³C magic angle spinning (MAS) NMR, we directly observed the pentamethylcyclopentenyl cation (pentaMCP⁺) over SAPO-18 under real MTO reaction conditions, but no carbenium ion was detected over AlPO-18. Furthermore, analysis of confined organics by ¹³C MAS NMR and GC-MS clearly demonstrated that higher Brønsted acid site density improved the formation and accumulation of some important and reactive hydrocarbon pool species, such as pentaMCP⁺ and polymethylbenzenes. With the aid of the ¹²C/¹³C-methanol switch technique, the detailed olefin formation mechanism was elucidated. During the MTO reaction, light olefin generation over SAPO-18 mainly followed the aromatic-based hydrocarbon pool mechanism; however, the olefin methylation and cracking mechanism accounted for the production of light olefins over AlPO-18.