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

Catalysis Science & Technology Pub Date: 2014-06-10 DOI: 10.1039/C4CY00551A

Abstract

The mechanism of the methanol to olefin (MTO) reaction over AlPO-18 (without Br?nsted acid sites) and two SAPO-18 (with different Br?nsted acid site densities) catalysts has been investigated. The Br?nsted 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 Br?nsted 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 Br?nsted 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.

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