Assessing the Influence of Void Environment in MFI Zeolites on Propene Oligomerization Kinetics Using a Combined Computational and Experimental Approach

The consequences of Brønsted acid site location in MFI zeolites were investigated for propene oligomerization. Adsorbates in MFI may reside in smaller channels (∼5.5 Å diam.) or larger channel intersections (∼7 Å diam.), with tighter confinement expected to enhance both stabilizing dispersive interactions and destabilizing steric constraints. MFI samples synthesized using tetrapropylammonium (TPA⁺) as the structure directing agent have higher fractions of acid sites near intersections, while samples synthesized with 1,4-diazabicyclo[2.2.2]octane (DABCO) or ethylenediamine (EDA) have higher fractions of acid sites within channels. Measured propene dimerization rates (per H⁺, 315 kPa C₃H₆, 503 K, 1.6 H⁺/u.c.) are ∼9 times higher on MFI-DABCO/MFI-EDA than MFI-TPA. Because propene oligomerization is transport-limited in MFI at these conditions, experimentally measured rates are proportional to both effective kinetic (k_eff) and diffusion (D_e) constants. DFT was therefore used to investigate the kinetic influences of void environment in isolation of transport effects. Adsorption free energies for C₃ and C₆ alkenes and dimerization free energy barriers were calculated at all 12 T-sites present in the MFI framework and all accessible O-sites around each T-site. C₃ preferentially adsorbs as H-bonded propene with similar energies in all void environments, while C₆ alkenes are destabilized by 10–56 kJ mol^–1 and dimerization transition states are destabilized by 29–102 kJ mol^–1, on average, in the channels relative to intersections. The stability of C₃ and C₆ alkenes and dimerization transition states is largely governed by steric penalties arising from distortion of the MFI framework that outweigh stronger dispersive interactions with decreasing void size, even for species as small as C₃. Given that DFT predicts k_eff values are lower at acid sites in smaller channel voids of MFI, higher measured rates on MFI samples synthesized using DABCO or EDA must reflect less severe diffusion restrictions and, in turn, higher D_e values.