Selective methylation of toluene using CO<sub>2</sub> and H<sub>2</sub> to <i>para</i>-xylene.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 32937362.
- Also identified by DOI 10.1126/sciadv.aba5433 and PMC identifier 7442476.
- Licence recorded as CC BY-NC.
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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, CO<sub>2</sub> and H<sub>2</sub> replace methanol in toluene methylation over a class of ZnZrO <i><sub>x</sub></i> -ZSM-5 (ZZO-Z5) dual-functional catalysts. Results demonstrate that the reactive methylation species (H<sub>3</sub>CO*; * represents a surface species) are generated more easily by CO<sub>2</sub> 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 H<sub>2</sub>/D<sub>2</sub> and CO<sub>2</sub>/<sup>13</sup>CO<sub>2</sub> indicate that xylene product is substantially generated from toluene methylation rather than disproportionation. A mechanism involving generation of reactive methylation species on ZZO by CO<sub>2</sub> hydrogenation and migration of the methylation species to Z5 pore for the toluene methylation to form xylene is proposed.