(51d) Metal Oxide Mobility in Zeolite: Impact on Hydrocarbon Pools and Its Inhibition Via Silicalite-1 Coating during CO? Hydrogenation

The tandem hydrogenation of CO₂ to hydrocarbons (HC) combines (i) methanol (CH₃OH) synthesis from CO₂ and H₂ over redox sites of metal oxides and (ii) methanol-to-hydrocarbons (MTH) conversion over Brønsted acid sites (BAS) of zeolites in a single-reactor. We have shown that the efficacy of oxide/zeolite systems could be improved by increasing the proximity between redox sites and BAS. However, metal oxide mobility can dramatically influence their reactivity. We aim to probe how metal oxide mobility can affect HC pool (HCP) mechanism within zeolite pores and how their mobility can be inhibited.

We show that HC space-time-yield (STY) can be enhanced ~8 by increasing proximity of redox sites and BAS from milliscale to microscale on In₂O₃ and HZSM-5 system. However, increasing the proximity further to nanoscale caused the migration of In₂O₃ into HZSM-5 micropores and subsequent ion exchange of BAS with In^δ+, inhibiting the acidity of HZSM-5 and its C-C coupling ability. This led to our hypothesis that metal oxide mobility dictates i) the likelihood of BAS exchange with cations, and ii) affects HCP. Hence, we probed C₃/C₂ HC ratio and paraffin-to-olefins (P/O) ratio during reaction. We revealed that while In^δ+ inhibited HCP yielding C₃/C₂~0 and P/O~0, Zn^δ+ enhanced hydrogen-transfer yielding ~5 higher P/O ratio and decreased olefin selectivity, compared to its microscale proximity admixture where ion exchange did not occur. We then aimed at coating metal oxides with S-1 shell. The performance of S-1 coated In₂O₃ (In₂O₃@S-1) at nanoscale proximity with HZSM-5 (In₂O₃@S-1/HZSM-5_n) exhibited ~5 higher yield of C₂₊ HC as compared to In₂O₃/HZSM-5_n, indicating ion exchange of BAS with In^δ+ was likely inhibited. Additionally, In₂O₃@S-1/HZSM-5_n exhibited ~2 higher C₅₊ HC than microscale proximity (In₂O₃/HZSM-5_m), indicating efficient transfer of CH₃OH favored methylation, enhancing C₅₊ selectivity at nanoscale proximity.