Highly Selective Transformation of CO₂ + H₂ into Para-Xylene via a Bifunctional Catalyst Composed of Cr₂O₃ and Twin-Structured ZSM-5 Zeolite

Abstract

The abundant C1 source CO2 can be utilized to produce value-added chemicals through hydrogenation technology. A bifunctional catalyst consisting of reducible metal oxide Cr2O3 and acidic zeolite ZSM-5 was designed for the direct conversion of CO2 + H2 into valuable aromatics, especially para-xylene (PX), via the methanol-mediated pathway. The twin structure of ZSM-5 (ZSM-5T), with sinusoidal channels that are predominantly exposed to the external surface, enhances the possibility of the transformation of methanol into PX due to the favorable diffusion dynamic of PX in the sinusoidal channels. Via the bifunctional catalyst Cr2O3&ZSM-5T, a PX selectivity of 28.7% and PX space-time yield (STY) of 2.5 gCH2 h−1 kgcat−1 are achieved at a CO2 conversion rate of 16.5%. Furthermore, we rationally modify the ZSM-5T zeolite via Cu species doping and amorphous SiO2 shell coating (Cu-ZSM-5T@SiO2). After combining with the Cr2O3 catalytic component, the CO2 conversion (18.4%) and PX selectivity (33.8%) are increased to some extent, which systematically increases the STY of PX to 3.0 gCH2 h−1 kgcat−1. The physicochemical property of the acidic zeolite and the corresponding structure-function relationship in enhancing the PX productivity are discovered. Our work provides a novel catalyst design idea to boost PX synthesis performance from CO2 hydrogenation.

Keywords

• CO₂ hydrogenation
• bifunctional catalysts
• para-xylene
• H-ZSM-5 zeolites