(314e) Catalytic Diversity in MFI Zeolites for Brønsted Acid-Catalyzed Propene Oligomerization: Consequences of Crystallite Properties and Active Site Proximity

Brønsted acid zeolites catalyze alkene oligomerization, a reaction relevant for upgrading light alkenes to transportation fuels. Acid site proximity and density (H⁺/u.c.), and crystallite size are reported to affect propene oligomerization rate and selectivity in MFI zeolites; yet consensus is lacking among literature reports, in part reflecting concurrent variations in active site and crystallite properties in commercially available materials. Herein, we combine experiment and theory to identify the origins of catalytic diversity within MFI zeolites for propene oligomerization.

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 μm) 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 (10×) with increasing crystallite size for MFI samples of similar active site density (Si/Al 250-300) and different crystallite sizes (0.3-2.7 μm), 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 (15×). 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 mol^−1) 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.