Rationally Designing Bifunctional Catalysts as an Efficient Strategy To Boost CO2 Hydrogenation Producing Value-Added Aromatics

Abstract

The efficient conversion of CO 2 to useful chemicals is a promising way to reduce atmospheric CO 2 concentration and also reduce reliance on fossil-based resources. Although much progress has been made toward the production of basic chemicals, like methanol, through CO 2 hydrogenation, the direct conversion of CO 2 to value-added aromatics, especially p- xylene (PX), is still a great challenge due to the inert nature of CO 2 and high barrier for C–C coupling. Herein, a bifunctional catalyst composed of Cr 2 O 3 and H-ZSM-5 zeolite (Cr 2 O 3 /H-ZSM-5) was designed for the direct conversion of CO 2 to aromatics. Due to the concertedly synergistic effect between the two components in this bifunctional catalyst, aromatics selectivity of ∼76% at CO 2 conversion of 34.5% was achieved, and there was no catalyst deactivation after 100 h of long-term stability testing. Moreover, a modified bifunctional catalyst Cr 2 O 3 /H-ZSM-5@S-1 (silicalite-1) consisting of a core–shell structured H-ZSM-5@S-1 zeolite capsule component was proposed to realize the target synthesis of BTX (benzene, toluene, and xylene), especially PX. The precise suppression of undesired side reactions was accomplished on Cr 2 O 3 /H-ZSM-5@S-1 because neutralizing acidic sites at outer surface of H-ZSM-5 by S-1 stopped isomerization of PX to o- or m- xylene as well as other side reactions. Consequently, the fractions of BTX and PX in aromatics products were increased from 13.2% and 7.6% to 43.6% and 25.3%, respectively, with almost unchanged catalyst activity and total aromatics selectivity.