Role of Catalyst Domain Size in the Hydrogenation of CO2 to Aromatics over ZnZrOx/ZSM-5 Catalysts

Abstract

The direct conversion of carbon dioxide into aromatic compounds may provide an environmentally friendly resource for a family of chemical building blocks with high global demand─aromatic chemicals. An array of bifunctional catalysts composed of ZnZrOx for the conversion of CO2 to methanol and ZSM-5 for methanol aromatization is evaluated considering multiple operational and catalytic parameters, with a focus on the particle size of each catalyst functionality. Aromatic yields generally increase as the ZSM-5 domain size decreases for different configurations of catalyst domains─mixed powders, mixed pellets, and separate beds. However, for a physical mixture of the catalyst components prior to pelletization (mixed powder case), varying the ZSM-5 crystal size inversely impacts the particle size of ZnZrOx, leading to enlarged metal oxide domains and enhanced intermediate diffusion barriers for very small crystals of ZSM-5. Larger crystals of ZSM-5 can be employed to obtain a higher concentration of benzene, toluene, and xylene (BTX) among aromatics. Upon employing a physical mixture of the ZnZrOx/ZSM-5 catalyst with a Si/Al ratio of 300 and a ZSM-5 crystal size of 300 nm, a total aromatics selectivity of 44% at CO2 conversion of 6% was obtained at 320 °C, WHSV of 7200 mL gcatalyst–1 h–1, and H2/CO2 ratio of 3.