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

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

Intermediate Transfer Rates and Solid-State Ion Exchange are Key Factors Determining the Bifunctionality of a Tandem CO2 Hydrogenation Catalyst

Probing the interaction between different active sites and transfer of reaction intermediates in bifunctional catalysts for tandem hydrogenation of CO2 is crucial for optimal catalyst design that maximize synergy to achieve high rates and product selectivity. Herein, thermocatalytic conversion of CO2 to hydrocarbon (HC) via a methanol (CH3OH) intermediate was investigated by modulating the placement of In2O3 and HZSM-5 in bifunctional admixtures at temperatures between 350 to 450 °C and 500 psig, to probe the key factors that drive synergy in these bifunctional systems. Analysis of the intermediate CH3OH transfer rates showed that although a millimeter scale placement of In2O3 and HZSM-5 yields a simple tandem reaction with a total HC and methanol CH3OH space-time yield of 8×10-6 molCgcat-1min-1, a microscale placement exhibits a ten-fold increase in catalytic activity with a total HC and CH3OH space-time yield of 8×10-5 molCgcat-1min-1 (at 400 °C) due to a faster advective and diffusive transfer rate of CH3OH. A combination of reactivity, spectroscopy with Raman, X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD) patterns, microscopy with scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and control experiments on methanol to hydrocarbons (MTH) revealed that further enhancing the reaction intermediate transfer at a nanoscale placement was counteracted by solid-state ion exchange (SSIE) between Brønsted acid sites (H+) of the HZSM-5 with the Inδ+ ions from In2O3, and that the formation of CH4 at the nanoscale placement was likely through CH3OH hydrogenolysis and not CO2 methanation at these intimate distances. Overall, our data showed the interconnected and subtle ways through which bifunctionality of catalysts could be regulated and paves the way for the development of design principles for designing more effective bifunctional catalysts for tandem CO2 hydrogenation