Influence of Acid Strength on Olefin Selectivity of Chabazite (CHA) Framework Zeolite/Zeotypes during Tandem CO2 Hydrogenation

Abstract

The role of the 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 turn-over number, TON over 500). 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, 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 to the product selectivity and deactivation in MTO and tandem CO2 hydrogenation over CHA framework zeolite/zeotype to the aromatic and olefin pool in the hydrocarbon pool mechanism. These underpinnings will help with rational catalyst design for tandem CO2 hydrogenation