Controlling
the selectivity in single-step conversion of syngas
to single aromatic hydrocarbon to enhance CO utilization is a big
challenge. By adapting the reaction coupling methodology, which allows
the precise control of C–C coupling reaction, we obtained a
high selectivity of ∼70% of a single product, tetramethylbenzene
(TeMB), in hydrocarbons, at total CO conversion of 37%. This was enabled
by the reaction of H2-deficient syngas over a composite
catalyst of physically mixed nanosized ZnCr2O4 and H-ZSM-5. The H-ZSM-5 employed in this work appeared as a coffin
shape with short straight channels [010] along the b-axis that exhibit low molecular-diffusion resistance, resulting
in high selectivity of aromatics, particularly TeMB. Due to selective
methanol formation and enhanced molecular diffusion, we observed an
aromatic vacancy created inside H-ZSM-5 pores, which boosts the transformation
of olefins into aromatics, thus making the aromatic cycle dominant
in a dual-cycle mechanism and giving a high yield of aromatics and
TeMB. Furthermore, no catalyst deactivation was observed within 600
h of reaction time using H2-deficient syngas. Therefore,
by rejecting the need for extra H2 addition into the syngas-to-aromatics
(STA) reaction system, direct conversion of H2-deficient
syngas derived from coal/biomass into TeMB makes an attractive industrial
process.
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