Parallel-Consecutive CO Formation during CO₂ Hydrogenation to Olefins in a Tandem ZnZrOx/SSZ-13

Tandem systems with metal oxides and zeolites as the catalyst duo hold promise for the methanol-mediated synthesis of olefins from carbon dioxide. Tuning the selectivity of the tandem carbon dioxide to an olefins reaction requires knowledge of selectivity at the optimal temperature and metal/zeolite loading, secondary hydrogenation of olefins to paraffins in the presence of hydrogen, and catalyst stability. Herein, reference ZnZrOx and steamed SSZ-13 were subjected to different reaction conditions by varying reaction temperatures, space velocities, and ZnZrOx-to-zeolite ratios. Tandem coupling was proven to increase the yield of the CO-free products. The consumption of the intermediate (methanol) was quantified, and a parallel-consecutive reaction to CO (via methanol) is proposed to explain variations in ZnZrOx/zeolite loading. Both low and high gas hour space velocity regimes offer opportunities to avoid the CO (production) selectivity. This work suggests that CO is mainly formed via methanol decomposition rather than from reverse water gas shift, and this has implications for catalyst design. While ZnZrOx is responsible for methanol and CO production, the zeolite determines olefin and paraffin selectivity. Several acidity-lowering modification strategies have been proven to decrease the undesired secondary hydrogenation of olefins to paraffins (O/P) without hampering the olefin formation. Further, it was found that the feed composition also greatly influences the catalyst lifetime. Understanding of the reaction network, including the complex “dynamic hydrocarbon pool” mechanism, is important for the rational design of the catalysts functions, especially in integrated tandem systems for the one-step conversion of CO2 to olefins.