(654h) Effect of Proximity on Hydrocarbon Selectivity in the Tandem Hydrogenation of CO2 Utilizing Zeolite-Tailored Bifunctional Catalyst

The direct conversion of CO₂ to C₂₊ hydrocarbons through tandem catalysis presents a promising solution for mitigating CO₂ emissions while producing value-added hydrocarbons and fuels. Recent studies corroborate that ZnZrO in tandem with zeolite converts CO₂ to C₂₊ hydrocarbons through CH₃OH intermediate. However, the low conversion and selectivity to C₂₊ hydrocarbons demand a further understanding of the catalytic process to allow improvements in catalyst design. Herein, the effect of the proximity of ZnZrO and zeolites (HZSM-5 and SAPO-34) to catalyst activity is investigated. We control the proximity of interaction between the ZnZrO and zeolite by different integration manners, e.g., dual-bed arrangement (millimeter scale spacing), granular mix (microscale), and mortar mix (nanoscale). The ZnZrO/HZSM-5 system shows the highest selectivity for C₅₊ hydrocarbons (~40%) for granular mix while mortar mix exhibits the highest selectivity of short-chain paraffins (C₂-C₄) (~50%) at 400 °C. In the case of ZnZrO/SAPO-34, granular mix shows the highest selectivity for olefins (C₂⁼-C₄⁼) (~30%) while mortar mix exhibits the highest selectivity of short-chain paraffins (~80%). With microscale proximity, the chain length of end products is mostly affected by the pore size of zeolites. Interestingly, nanoscale proximity exhibits over-hydrogenation resulting in short-chain saturated hydrocarbons. Hence, we hypothesize that Zn sites could get ion-exchanged with protons of zeolite under reaction conditions. We investigate the ion-exchange effect of Zn²⁺ by using granular mix of ZnZrO and Zn-ion-exchanged zeolite (Zn-ZSM-5 and Zn-SAPO-34) that results in high paraffin selectivity, similar to mortar mix. Characterization of the fresh and spent zeolites with ammonia and pyridine TPD reveal that Brønsted acid sites (BAS) decrease and Lewis acid sites (LAS) form in mortar mix. This LAS increase zeolite acidity causing over-hydrogenation of olefins. To conclude, proximity affects the hydrocarbon selectivity during CO₂ hydrogenation and migration of Zn²⁺ ions occurs at nanoscale proximity resulting in over-hydrogenation of products.