(509e) Impact of Intermediate Transfer Rates, Metal Cation Mobility, and Hydrocarbon Pool Mechanisms on the Rates and Selectivity for Tandem CO2 Hydrogenation to Olefins and Fuels

The influence of metal oxides on methanol (CH₃OH)-mediated conversion of CO₂ to olefin was investigated using SAPO-34 (a silicoaluminophospate) with three representative metal oxides (e.g., In₂O₃, ZnZrO_x, Cr₂O₃). The placement of redox sites of metal oxides and Brønsted acid sites (BAS) of SAPO-34 was modulated to probe their proximity-dependent reactivity. Analysis of the intermediate CH₃OH transfer rates revealed that CH₃OH transfer could be faster over intrapellet admixtures (distance between redox and BAS ~255-1500 nm) as compared to interpellet admixtures (~200-325 µm), resulting in a higher space-time yield (STY) of hydrocarbons over ZnZrO_x/SAPO-34 (17x10^-5 and 11x10^-5 mol_C g_cat^-1 min^-1 over intrapellet and interpellet admixtures, respectively) and Cr₂O₃/SAPO-34 (1.4x10^-5 and 1.2x10^-5 mol_C g_cat^-1 min^-1 over intrapellet and interpellet admixtures, respectively) at 400°C and 3 MPa. However, for intrapellet In₂O₃/SAPO-34, the catalytic activity was found to diminish yielding mostly CH₄(95%). While powder X-ray diffraction (PXRD), Ar physisorption, and transmission electron microscopy (TEM) revealed no change to the SAPO-34 structure in intrapellet admixtures, X-ray photoelectron spectroscopy (XPS) revealed the occurrence of solid-state ion exchange (SSIE) between BAS of SAPO-34 with In^δ+ and Zn^δ+ ions from In₂O₃ and ZnZrO_x, respectively. Although ion-exchanged In^δ+ species diminished the acidity of SAPO-34 and inhibited the propagation of olefin and aromatic cycles in the hydrocarbon pool mechanism, it promoted CH₄ formation via CH₃OH hydrodeoxygenation (HDO). Interestingly, ion-exchanged Zn^δ+ species in intrapellet admixture enhanced secondary hydrogenation of olefins and exhibited ~5 higher paraffin-to-olefin ratio in HCP, as compared to its interpellet admixture. Conversely, ion exchange was not observed in Cr₂O₃/SAPO-34 systems. The likelihood of the formation of cationic species from the metal oxides was further demonstrated in terms of metal vacancy formation energies, which was found to be consistent with our reactivity data (ZnO (1.2 eV) >In₂O₃ (4.6 eV) > Cr₂O₃ (5.8 eV) >ZrO₂ (20.43 eV)).