(314e) Catalytic Diversity in MFI Zeolites for Brønsted Acid-Catalyzed Propene Oligomerization: Consequences of Crystallite Properties and Active Site Proximity
AIChE Annual Meeting
2021 Annual Meeting
Catalysis and Reaction Engineering Division
Hydrocarbon Conversion: Catalytic Pathways
Tuesday, November 9, 2021 - 1:24pm to 1:42pm
MFI zeolites were synthesized with independently varied active site density and proximity, and crystallite size. Initial C3H6 dimerization rates (503 K, 7-610 kPa, < 5% conversion) measured on MFI samples of similar crystallite size (0.1-0.3 Î¼m) but varying Al content (Si/Al 13-300) showed different apparent C3H6 pressure dependences (Fig. 1a). Samples with dilute Al content (Si/Al 60-300) were negative order in C3H6 pressure and transitioned to first order at higher C3H6 pressures; these distinct kinetic regimes suggest the presence of multiple active site types. C3H6 dimerization rates in the negative-order regime decreased (10Ã) with increasing crystallite size for MFI samples of similar active site density (Si/Al 250-300) and different crystallite sizes (0.3-2.7 Î¼m), suggesting rates are influenced by intracrystalline mass transfer in this regime. Samples with high Al content (Si/Al 13-43) exhibited first-order dependences on C3H6 pressure with varying rate constants (15Ã). Dimerization barriers at isolated Al and at Al-Al site pairs predicted to favorably titrate Co2+ (54 pairs considered) were estimated by DFT and intrinsic C3H6 dimerization barriers increased (11-35 kJ molâ1) for T-11 pairs, indicating that rates (per H+) are influenced by proximal adsorbates (Fig. 1b). These findings highlight catalytic diversity in MFI for alkene oligomerization and demonstrate the utility of synthetic methods that enable independently varying crystallite and active site properties to build synthesis-structure-function relations in zeolites.