Intermediate Transfer Rates and Solid-State Ion Exchange are Key Factors Determining the Bifunctionality of In <sub>2</sub> O <sub>3</sub> /HZSM-5 Tandem CO <sub>2</sub> Hydrogenation Catalyst
Fatima Mahnaz, Jasan Robey Mangalindan, Balaji C. Dharmalingam, Jenna Vito, Yuting Lin, Mustafa Akbulut, Jithin John Varghese, Manish Shetty
Abstract
High Resolution Image Download MS PowerPoint Slide 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 CO 2 to hydrocarbons (HC) for sustainable fuel production. Herein, we combined CH 3 OH synthesis from CO 2 and H 2 on In 2 O 3 and methanol-to-hydrocarbons (MTH) conversion on HZSM-5 and discerned the descriptors by leveraging the distance-dependent reactivity of bifunctional In 2 O 3 and HZSM-5 admixtures. We modulated the distance between redox sites of In 2 O 3 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 CH 3 OH (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 CH 3 OH 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 CH 3 OH transfer at a nanoscale distance (∼300 nm), the sole product formed was CH 4 . 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 In 2 O moieties, inhibiting C–C coupling and promoting CH 4 formation through CH 3 OH hydrodeoxygenation (HDO). Density functional theory (DFT) calculations further revealed that CH 3 OH adsorption on the In 2 O moiety with preadsorbed and dissociated H 2 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 CH 3 OH transfer and prevention of ion exchange are the key descriptors in achieving catalytic synergy in bifunctional In 2 O 3 /HZSM-5 systems.