Flexible regulation of C3^=/C2^= ratio in methanol-to-hydrocarbons by delicate control of acidity of ZSM-5 catalyst

Avery wide range of the C3^=/C2^= ratio from 0.72 to 7.56 with high C2^= ＋ C3^= selectivity of around 66%in the methanol-to-hydrocarbons process can be realized over ZSM-5 catalyst in a fixed-bed reactor.We firstly conduct a single factor experiment of acidity,demonstrating that the acidity control of MTH catalyst is crucial to adjusting light olefins selectivity.Weak Bronsted acid sites favor to high C3^= selectivity（59.0%）due to the suppression of the conversion reactions from the alkene-based to arene-based cycle,while Lewis acid sites conduce to high C2^= selectivity（39.6%） due to the promotion of the conversion reactions for the aromatics formation and steric constraints of Lewis acid sites making the aromatics crack more efficiently to C2^=.

Progressive steps and catalytic cycles in methanol-to-hydrocarbons reaction over acidic zeolites

Understanding the complete reaction network and mechanism of methanol to-hydrocarbons remains a key chal-lenge in the field of zeolite catalysis and C1 chemistry.Inspired by the identification of the reactive surface methoxy species on solid acids,several direct mechanisms associated with the formation of the first C-C bond in methanol conversion have been recently disclosed.Identifying the stepwise involvement of the initial intermedi-ates containing the first CC bond in the whole reaction process of methanol to-hydrocarbons conversion becomes possible and attractive for the further development of this important reaction.Herein,several initial unsaturated aldehydes/ketones containing the C-C bond are identified via complementary spectroscopic techniques.With the combination of kinetic and spectroscopic analyses,a complete roadmap of the zeolite catalyzed methanol-to-hydrocarbons conversion from the initial CC bonds to the hydrocarbon pool species via the oxygen-containing unsaturated intermediates is clearly ilustrated.With the participation of both Bronsted and Lewis acid sites in H-ZSM-5 zeolite,an initial aldol-cycle is proposed,which can be closely connected to the well-known dual-cycle mechanism in the methanol-to-hydrocarbons conversion.

Effect of external surface of HZSM-5 zeolite on product distribution in the conversion of methanol to hydrocarbons

The external surface of HZSM-5 zeolite was passivated by liquid siliceous deposition and by acidic sites poisoning with lepidine, respectively. Then methanol-to-hydrocarbons （MTH） reaction was investigated over the above as-prepared catalysts and the dissoluble coke on these used catalysts was analyzed by GC-MS, to study the role of the external surface of HZSM-5 in the catalytic reaction. Comparison with the experi- mental results based on parent ZSM-5 showed that the product distribution of MTH reaction was obviously influenced by the external surface. Evidences were listed as follows： （1） the final product on parent HZSM-5 showed higher aromatic selectivity, lower olefin selectivity, lower ra- tio of C2/C3＋ aliphatics and higher ratio of C3/C4＋ aliphatics than the reaction mixture produced by the sole catalysis of acidic sites in HZSM-5 channel; （2） a little of pentamethylbenzene and hexamethylbenzene in the product on parent HZSM-5, was produced via multi-methylation of methylbenzene on the external surface. The above conclusion may also be suitable for MTH reaction over other zeolites with 10-ring channel.