(349h) Investigating the Consequences of Brønsted Acid Site Proximity on Propene Oligomerization in MFI Zeolites with DFT
AIChE Annual Meeting
2021
2021 Annual Meeting
Catalysis and Reaction Engineering Division
Microporous and Mesoporous Materials III: Reaction Pathways and Effects Beyond the Binding Site
Thursday, November 11, 2021 - 5:18pm to 5:36pm
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 AlâAl 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 AlâAl site pairs in MFI with DFT-predicted Co2+ exchange energies below 60 kJ molâ1. Four proximal species were investigated at each site pair: 1-propylâZ, 2-propylâZ, pi-bonded propene, and a proton (HâZ). Binding energies for 1-propylâZ and 2-propylâZ 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 molâ1) for propene dimerization were also calculated at site pairs with different proximal species. At the T11âT11 site pair, the presence of a proximal 1-propylâZ or 2-propylâZ increases the intrinsic dimerization barrier by 30 and 31 kJ molâ1, respectively, relative to the isolated T11 site (Fig. 1). However, the presence of a proximal pi-bonded propene or HâZ decreases the intrinsic barrier by 7 and 9 kJ molâ1, 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.