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.

The zeolite Si/Al ratio was observed to affect the activity and selectivity of propene oligomerization over H-MFI. Both a kinetic model as well as a quantum mechanic/molecular mechanic model illustrate that this effect is caused by the proximity of adsorbed oligomers restricting the formation of the trimer. This causes the rate coefficient for dimer desorption relative to the rate coefficient for trimer formation to increase with decreasing Si/Al ratio explaining the observed activity and selectivity behavior.Figure optionsDownload high-quality image (76 K)Download as PowerPoint slideHighlights
► Decreasing MFI Si/Al ratio increases dimer selectivity but decreases propene conversion.
► Kinetic analysis indicates that change in selectivity and activity is caused by change in relative rate coefficients for trimerization and dimer desorption.
► QM/MM model indicates that steric crowding of nearby adsorbed dimer species increases activation energy of trimerization faster than dimer desorption.
► Larger amounts of aromatics/Al were produced in the more sterically crowded, low Si/Al ratio zeolites.