Designing a bifunctional ZrCuOx/HZSM-5 catalyst for selective methylation of toluene with carbon dioxide to para-xylene

Abstract

Direct synthesis of valuable aromatics, such as xylene from CO2 hydrogenation is of highly interest for carbon emission reduction and reutilization, but it remains a challenge due to the poor product yield limited by low CO2 conversion. On the other hand, toluene methylation with methanol suffers from low methylation efficiency due to severe side reactions of methanol. Herein, a tandem catalysis that combines CO2 methanol synthesis and toluene methylation using CO2/H2 mixture as C1 source with a Zr-modified CuO/HZSM-5 bifunctional catalyst has been evaluated. Introducing Zr provided abundant sites for CO2 adsorption, more oxygen defects and surface area, thus enhancing the conversion of CO2 and toluene. The competitive adsorption of CO2 and toluene on ZrCuOx caused the phase transition of bulk CuO, and further catalytic deactivation. This issue was successfully solved by configuring a core (metal)-shell (zeolite) like composite catalyst to minimize the exposure of toluene on ZrCuOx surface. Toluene conversion rate can reach 45.94% when CO2 conversion is 31.99%, and the yield of xylene can reach 0.6 g gcat−1⋅h−1. The selective of high value product para-xylene in xylene isomers can get higher than 90 % by silicon coating modification.