Single-Step Conversion of H2-Deficient Syngas into High Yield of Tetramethylbenzene

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.