Low temperature conversion of linear C4 olefins with acid ZSM-5 zeolites of homogeneous composition

Conversion of linear butenes over ZSM-5 zeolites leads to a large variety of products, from olefins to aromatics. At low temperature, these products are mainly olefins from C3= to C13=, with a predominance of dimeric products. The analysis of the product stream obtained by the reaction of n-butenes over several materials with MFI topology, but with different aluminum content provides evidence for an oligomerization–cracking–realkylation mechanism. The reactions occur predominantly in the catalyst bulk or at the external surface, depending on the amount of coke developed inside the catalyst. This behavior is determined by the concentration of framework Al and associated silanol groups, inducing coking. For Al-rich MFI samples, the amount of hard coke is sufficient to block the access of reactants to internal acid sites. As a result, extra-crystalline catalysis dominates, while the appearance of the individual products is consistent with that predicted by classical carbocation theory. For siliceous samples, hard coke formation is less pronounced, and evidence for shape selective product formation is present. Expected relations among structural, morphological and acidic properties of all MFI materials have been confirmed by XRD, SEM and BET and by 27Al MAS NMR, NH3-TPD and FTIR.

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► n-Butenes dimerization at 220 °C with H-ZSM-5: dimerization–cracking–alkylation.
► Linear correlation between acid sites amounts and framework aluminum concentration.
► Presence of aromatic coke on Al-rich H-ZSM-5 fills the intracrystalline voids.
► On Al-rich catalysts, complete pore blockage with coke: extra-crystalline chemistry.
► On Al-poor ZSM-5, shape selective product distributions: intracrystalline catalysis.