(344m) Effects of MFI Crystallite Properties on Propene Oligomerization Rate and Selectivity

Brønsted acid zeolites catalyze alkene oligomerization, a reaction relevant for upgrading light hydrocarbons to fuels.¹ First-order propene dimerization rate constants increase with decreasing zeolite void size, because transition states are larger than their alkoxide precursors and thus benefit preferentially from van der Waals stabilization.² First-order propene dimerization rate constants are independent of Al content on H-form zeolites (MFI, TON, Beta),² although some studies report rates that are sensitive to the bulk density and spatial distribution (i.e., proximity) of framework Al in MFI.^3,4 Here, we report propene dimerization rates (503 K) on commercial Al-MFI (Si/Al = 13-140) and B-Al-MFI (Si/Al = 60-150, Si/B = 26-32) synthesized in the presence of ethylenediamine (EDA) and tetrapropylammonium (TPA⁺) structure-directing agents.⁵ This suite of MFI samples contain different Al distributions (void environments and proximity). Propene dimerization rates (503 K, 169 kPa C₃H₆) on B-Al-MFI (>1.5 μm) are independent of Al content, but systematically decrease with increasing diffusion parameter, suggesting intracrystalline propene concentration gradients influence measured rates on these samples. Propene dimerization rates (503 K, 169 kPa C₃H₆) on commercial Al-MFI samples vary by 4× but are independent of diffusion parameter, suggesting these differences are kinetic in origin. These results demonstrate the need to account for intracrystalline diffusion effects on measured rates, and motivate the development of zeolite synthesis approaches to independently vary crystallite and active site propertoes.

References

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