Selective methylation of toluene using CO2 and H2 to para-xylene

Jiachang Zuo; Weikun Chen; Jia Liu; Xinping Duan; Linmin Ye; Youzhu Yuan

doi:10.1126/sciadv.aba5433

Selective methylation of toluene using CO₂ and H₂ to para-xylene

Sci Adv. 2020 Aug 21;6(34):eaba5433. doi: 10.1126/sciadv.aba5433. Print 2020 Aug.

Authors

Jiachang Zuo¹, Weikun Chen¹, Jia Liu¹, Xinping Duan¹, Linmin Ye¹, Youzhu Yuan²

Affiliations

¹ State Key Lab of Physical Chemistry of Solid Surfaces, National Engineering Lab for Green Chemical Productions of Alcohols-Ethers-Esters, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China.
² State Key Lab of Physical Chemistry of Solid Surfaces, National Engineering Lab for Green Chemical Productions of Alcohols-Ethers-Esters, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China. yzyuan@xmu.edu.cn.

Abstract

Toluene methylation with methanol to produce xylene has been widely investigated. A simultaneous side reaction of methanol-to-olefin over zeolites is hard to avoid, resulting in an unsatisfactory methylation efficiency. Here, CO₂ and H₂ replace methanol in toluene methylation over a class of ZnZrO _x -ZSM-5 (ZZO-Z5) dual-functional catalysts. Results demonstrate that the reactive methylation species (H₃CO*; * represents a surface species) are generated more easily by CO₂ hydrogenation than by methanol dehydrogenation. Catalytic performance tests on a fixed-bed reactor show that 92.4% xylene selectivity in CO-free products and 70.8% para-xylene selectivity in xylene are obtained on each optimized catalyst. Isotope effects of H₂/D₂ and CO₂/¹³CO₂ indicate that xylene product is substantially generated from toluene methylation rather than disproportionation. A mechanism involving generation of reactive methylation species on ZZO by CO₂ hydrogenation and migration of the methylation species to Z5 pore for the toluene methylation to form xylene is proposed.

Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).