Influence of acid strength on olefin selectivity of chabazite (CHA) framework zeolite/zeotype during tandem CO2 hydrogenation

The role of Brønsted acid sites (BAS) strength of chabazite (CHA) framework on olefin selectivity during methanol-to-olefin (MTO) and tandem CO2 hydrogenation was investigated over an aluminosilicate, SSZ-13 and a silicoaluminophospate, SAPO-34 and their bifunctional admixtures with In2O3. During MTO, SSZ-13 and SAPO-34 yielded primarily olefins (cumulative selectivity of ∼60 % and ∼90 %, respectively at cumulative turnover number, TON over 500 molC/molH+). Interestingly, an interpellet admixture of In2O3/SSZ-13 (distance between redox sites and BAS of 260–900 µm) predominantly yielded paraffins (cumulative selectivity of ∼93 % at cumulative TON over 40) via the secondary hydrogenation of olefins as seen from the cumulative paraffin-to-olefin (P/O) ratio of ∼21 during CO2 hydrogenation. In comparison, an interpellet In2O3/SAPO-34 admixture yielded majority olefins (cumulative selectivity of ∼67 % at cumulative TON over 60) due to a lesser degree of secondary hydrogenation (cumulative P/O ratio of ∼0.2) on the BAS in SAPO-34, which has a lower acid strength as compared to SSZ-13. Interestingly, both interpellet admixtures of In2O3/SSZ-13 and In2O3/SAPO-34 remained stable during tandem CO2 hydrogenation by favoring the olefin cycle and suppressing the formation of deactivation-inducing-aromatics as probed via occluded hydrocarbon analysis, unlike MTO, where both admixtures showed fast deactivation. Ion-exchange of BAS (H+) with Inδ+ (from In2O3) in intrapellet admixtures (distance between redox sites and BAS of 270–1500 nm) of In2O3/SSZ-13, and In2O3/SAPO-34, inhibited C–C coupling and predominantly formed CH4. Overall, our study related the product selectivity and deactivation in MTO and tandem CO2 hydrogenation over CHA framework zeolite/zeotype to the aromatic and olefin cycle propagation in the hydrocarbon pool mechanism. These underpinnings will help with rational catalyst design for tandem CO2 hydrogenation.