Brønsted acid zeolites catalyze alkene oligomerization, a reaction relevant for upgrading light alkenes to transportation fuels. Acid site proximity and density (H+/u.c.) are reported to affect propene oligomerization rate and selectivity in MFI zeolites; yet, literature reports are discordant, in part reflecting concurrent variations in active site and crystallite properties among commercially available zeolite materials. Herein, we synthesize Al-MFI materials with independent variations in active density and crystallite size, and combine theory and experiments to rationalize the catalytic diversity observed for propene oligomerization among MFI zeolites of fixed composition. MFI zeolites were synthesized with independently varied active site density and crystallite size using different structure-directing agents and silica sources. Initial and steady-state C3H6 dimerization rates (503 K, 7-610 kPa, &lt; 5% conversion) measured on MFI samples of low active site density (Si/Al 60-300) were apparent negative order in C3H6 pressure. Dimerization rates measured for MFI of fixed Al content (Si/Al 250-300) decreased (10×) with increasing crystallite size (0.2-2.7 μm), suggesting rates measured in this regime are influenced by intracrystalline mass transfer. In contrast, samples with high Al content (Si/Al 13-31) and similar crystallite sizes (0.1-0.3 μm) exhibited apparent first-order dependences on C3H6 pressure with rate constants (per H+) that varied (3×), but non-systematically with Al density, suggesting sites of different reactivity may be present within these materials. Intrinsic dimerization free energy barriers for different T-sites in MFI were estimated by DFT. Intrinsic dimerization free energy barriers varied for different T-sites (53-115 kJ mol-1), indicating that dimerization rates (per H+) are influenced T-site location. Overall, these findings highlight catalytic diversity in MFI for alkene oligomerization and demonstrate how insights from theory and experiment can aid in developing synthesis-structure-function relations for alkene oligomerization in Brønsted acid zeolites.

Catalytic diversity in MFI zeolites for Brønsted acid-catalyzed propene oligomerization: Consequences of active site density and crystallite size

ACS Spring 2022

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