(349h) Investigating the Consequences of Brønsted Acid Site Proximity on Propene Oligomerization in MFI Zeolites with DFT

Propene oligomerization is relevant for producing transportation fuel-range molecules from light alkenes. This process is catalyzed by Brønsted acid zeolites, such as H-ZSM-5 (MFI framework), which can be synthesized with varying fractions of proximal Al–Al site pairs capable of titrating Co²⁺ ions. Prior work has shown that methanol dehydration rates increase (in CHA framework, H-SSZ-13) because H-bonding between proximal Brønsted acid sites stabilizes conjugate base structures formed at critical transition states. Here, we use density functional theory (DFT) to understand the effect of proximity on adsorbates and reactions relevant to propene oligomerization, which lack these H-bonding capabilities. Fully periodic DFT calculations were performed at 54 Al–Al site pairs in MFI with DFT-predicted Co²⁺ exchange energies below 60 kJ mol^−1. Four proximal species were investigated at each site pair: 1-propyl–Z, 2-propyl–Z, pi-bonded propene, and a proton (H–Z). Binding energies for 1-propyl–Z and 2-propyl–Z indicate that no combination of adsorbate and proximal species is consistently preferred; instead, the most stable proximal species depends on the site pair. Intrinsic free energy barriers (503 K, kJ mol^−1) for propene dimerization were also calculated at site pairs with different proximal species. At the T11–T11 site pair, the presence of a proximal 1-propyl–Z or 2-propyl–Z increases the intrinsic dimerization barrier by 30 and 31 kJ mol^−1, respectively, relative to the isolated T11 site (Fig. 1). However, the presence of a proximal pi-bonded propene or H–Z decreases the intrinsic barrier by 7 and 9 kJ mol^−1, respectively (Fig. 1), suggesting the effect of proximal sites on propene dimerization depends on the nature of the proximal adsorbate species. These findings help elucidate the influence of site pair geometry and proximal species identity on the stability of adsorbates and reaction barriers crucial for understanding propene oligomerization reactions.