Selective methylation of toluene using CO ₂ and H ₂ to <i>para</i> -xylene

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 <i>_x</i> -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% <i>para</i>-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.