Boosting Benzene Alkylation Conversion with CO2/H2 via a Triple Composite Catalyst

The alkylation of benzene with CO2/H2 to synthesize toluene and xylene is of great significance for alleviating carbon emissions and upgrading light aromatics toward value-added chemicals. However, the alkylation reagent from CO2 hydrogenation showed relatively weak alkylation activity and severe self-reaction, leading to a low alkylation efficiency. Here, a high-performance triple composite catalytic system was constructed by using ZnZrOx oxide, Al2O3 oxide, and H-ZSM-5 zeolite as catalyst components for the alkylation of benzene with CO2/H2 to toluene and xylene. According to the results, CO2 is hydrogenated to methanol on oxygen vacancies, methanol is dehydrated to dimethyl ether (DME) on Lewis acid sites, and benzene is then alkylated with the formed DME to toluene and xylene over the acidic sites of the zeolite. The combined selectivity of toluene and xylene among all hydrocarbons reached 97% at a benzene conversion of 9.7%, surpassing that of most reported traditional catalysts. The methanol dehydration to DME is responsible for the high alkylation activity, providing more alkylation reagents with high activity and suppressing the self-reaction of methanol to light hydrocarbons (C1–C5). More importantly, water is formed by methanol dehydration before the alkylation step and outside the zeolite, thus reducing the competitive adsorption of benzene and water on the acidic sites in the zeolite channel, consequently increasing the conversion of benzene and improving the catalytic stability. The establishment of the triple composite catalytic system in this work opens valuable horizons for enhancing the alkylation of benzene by coupling with CO2 hydrogenation.