Intermediate Transfer Rates and Solid-State Ion Exchange are Key Factors Determining the Bifunctionality of...

Identifying the descriptors for the synergistic catalytic activity of bifunctional oxide-zeolite catalysts constitutes a formidable challenge in realizing the potential of tandem hydrogenation of CO₂ to hydrocarbons (HC) for sustainable fuel production. Herein, we combined CH₃OH synthesis from CO₂ and H₂ on In₂O₃ and methanol-to-hydrocarbons (MTH) conversion on HZSM-5 and discerned the descriptors by leveraging the distance-dependent reactivity of bifunctional In₂O₃ and HZSM-5 admixtures. We modulated the distance between redox sites of In₂O₃ and acid sites of HZSM-5 from milliscale (∼10 mm) to microscale (∼300 μm) and observed a 3-fold increase in space-time yield of HC and CH₃OH (7.5 × 10^-5 mol_C g_cat^-1 min^-1 and 2.5 × 10^-5 mol_C g_cat^-1 min^-1, respectively), due to a 10-fold increased rate of CH₃OH advection (1.43 and 0.143 s^-1 at microscale and milliscale, respectively) from redox to acid sites. Intriguingly, despite the potential of a three-order-of-magnitude enhanced CH₃OH transfer at a nanoscale distance (∼300 nm), the sole product formed was CH₄. Our reactivity data combined with Raman, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) revealed the occurrence of solid-state-ion-exchange (SSIE) between acid sites and In^δ+ ions, likely forming In₂O moieties, inhibiting C-C coupling and promoting CH₄ formation through CH₃OH hydrodeoxygenation (HDO). Density functional theory (DFT) calculations further revealed that CH₃OH adsorption on the In₂O moiety with preadsorbed and dissociated H₂ forming an H-In-OH-In moiety is the likely reaction mechanism, with the kinetically relevant step appearing to be the hydrogenation of the methyl species. Overall, our study revealed that efficient CH₃OH transfer and prevention of ion exchange are the key descriptors in achieving catalytic synergy in bifunctional In₂O₃/HZSM-5 systems.