Effect of crystal size on butenes oligomerization over MFI catalysts

• Intracrystalline diffusion affects butenes oligomerization over MFI polycrystals.
• Small polycrystals show higher activity in oligomerization.
• The activity is governed by Brønsted sites on external surface and subsurface.
• Selectivity shifts toward lighter hydrocarbons with the decrease of polycrystal size.
• Coke formation and deactivation increase over small polycrystals.

Oligomerization of butene–butane feedstock into gasoline-range fuels has been studied under 1.5 MPa and 573 K over H-MFI zeolites with similar acid site density (Si/Al ∼ 30) and different size of polycrystals composed of nanocrystals of 40–70 nm. The variation of the polycrystalline size in the range of 0.2–3 μm was achieved by adjusting the composition of the synthesis gel and application of the seeding technique. The structural, textural and morphological properties of the catalysts were characterized by XRD, SEM and nitrogen adsorption–desorption. The internal and external acidity was studied using infrared spectroscopy of adsorbed pyridine and 2,6-di-tert-butylpyridine. It has been demonstrated that the decrease of polycrystalline size from 3 to 0.2 μm leads to the significant increase of activity, accompanied by the intensive coke formation and the shift of selectivity toward light hydrocarbons. The results point to significant intracrystalline diffusion limitations in large polycrystals and suggest that the activity of MFI catalysts in butene oligomerization is governed by the active sites located on the external surface and subsurface polycrystals.

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