Effects of Brønsted-acid site proximity on the oligomerization of propene in H-MFI

The oligomerization of propene was investigated over H-MFI zeolites with varying Si/Al ratios. For a constant space time per active site, the conversion of propene as well as the selectivity to products of different carbon number was affected by the density of the sites within the zeolite. In particular, as the Si/Al ratio decreased, corresponding to an increase in site proximity, the rate of oligomerization per site decreased but the selectivity to dimers relative to cracking products increased. These effects were shown to arise from the effects of molecular crowding on the rate coefficient for propene trimer formation and were confirmed by quantum chemical analysis of the energetics of propene oligomerization. It was found that the activation for propene dimerization is unaffected by the presence of oligomers on nearby sites, but the activation energy for propene trimerization relative to desorption of hexene increases by 19 kcal mol−1 when two next nearest neighbor sites are occupied by oligomers. In situ IR spectroscopy observations showed the buildup of aromatic species with time-on-stream. The accumulation of these species increases with decreasing Si/Al ratio, suggesting that increasing proximity of Brønsted-acid sites enhances the formation of aromatic species.