Contributions of ZSM-5 morphology over hybridized ZnO-ZrO₂/ZSM-5 for direct CO₂ hydrogenation activity to aromatics

Direct synthesis of aromatics through CO₂ hydrogenation to oxygenates and its successive conversion to aromatics was investigated on hybridized ZnO-ZrO₂ metal oxides with ZSM-5. Hydrothermally synthesized micro-mesoporous ZSM-5 having four different morphologies with different Al distribution within ZSM-5 frameworks from small agglomerates to large elongated spherical particles such as nano (N-ZSM-5), hierarchically-structured hexagonal (H-ZSM-5), sheet-like (S-ZSM-5) and conventional cube-like (C-ZSM-5) structures. A larger number of strong Brønsted acid sites and small crystallite size of N-ZSM-5 in the hybridized ZnO-ZrO₂/N-ZSM-5 were responsible for a largely enhanced CO₂ conversion of 22.9% and aromatics selectivity of 69.8% through methanol-to-aromatics reaction. The H-ZSM-5, having a larger number of external Brønsted acid sites, revealed a higher selectivity to alkylated tetramethyl benzene (TeMB) with 88.6% due to an enhanced surface C–C coupling reaction of aromatics formed from the tandem reaction. In addition, the selective distributions of Al species in the intersection micropores of sinusoidal and straight channels of the S-ZSM-5 showed an increased selectivity to xylene and trimethyl benzene (TriMB) and inhibited the formation of TeMB by further alkylation of surface intermediates through steric hinderance effects. The distributions of acidic sites and ZSM-5 morphology significantly altered local residence time resulted in different selectivity to aromatics and heavier hydrocarbons during gas-phase CO₂ conversion, which provided an efficient approach for direct CO₂ conversion to aromatics.