Role of coupling and zeolite acidity in the methanol-mediated CO2 conversion to olefins over ZnZrOx-AEI zeolite tandem catalysis

Abstract

The tandem conversion of carbon dioxide to olefins (CTO) via methanol over a combination of metal oxide plus zeolite catalysts is a considerable alternative to fossil-based routes to light olefins. Here, AEI type small pore zeolites i.e., SSZ-39 and SAPO-18, in combination with ZnZrOx, are put forward as the excellent tandem catalysts for CTO. We deconvolute the influences of acidity and framework composition on the product selectivity and productivity of light olefins. Post-synthesis steaming was utilized as a strategy to tune the acidity and structure of the zeolites. SSZ-39 steamed at 750 °C showed the highest olefin/paraffin ratio (O/P = 2.1) as compared to the unsteamed SSZ-39 with an O/P ratio of 0.2. In contrast, SAPO-18 steamed at 650 °C showed a maximum O/P ratio of 2.1, whereas steaming at higher temperatures resulted in decreased activities. Propylene was observed as the major olefin for both SSZ-39 and SAPO-18 in line with the cage-defining ring size of AEI. It was found through FT-IR and NH₃-TPD that the increase in the steaming temperature decreases the Brønsted acidity of SSZ-39 which inhibits both the secondary hydrogenation of olefins to paraffins and the methanol-assisted hydrogen transfer reaction. Coupling of CO₂ to methanol (CTM) with methanol to olefins (MTO) reaction increased the CO₂ conversion and selectivity towards methanol equivalents by suppressing CO production in tandem systems as compared to the sole CTM reaction. The effect of catalyst bed configuration on product selectivity over the best performing catalyst was also studied, and the highest O/P ratio (3.7) was observed for a powder mixed system opposed to dual bed or mixed bed systems.