Proton proximity - New key parameter controlling adsorption, desorption and activity in propene oligomerization over H-ZSM-5 zeolites

- The distances of protons in H-ZSM-5 represent a new parameter in acid catalyzed reactions.
- Al atoms (H+) predominated at the channel intersections of H-ZSM-5.
- Propene adsorption and intrinsic oligomerization activity is faster over isolated protons.
- Proximate protons (5 Å) release faster olefins from alkoxides and π-bonded olefins.
- Turn-over frequency of oligomerization is up to 10 times higher over close protons.

The proximity of protons (∼5 Å) derived from the distribution of Al atoms in the H-ZSM-5 framework has been shown to substantially affect the rate of propene oligomerization. Two sets of H-ZSM-5 zeolites (Si/Al 13-30) greatly differing in the occurrence of Al pairs (AlSiSiAl sequences in 6MRs) or distant Al atoms in the framework (both located prevailingly at the channel intersections) were synthesized to analyze the effect of the proton proximity. Propene adsorption and formation of oligomeric alkoxides is faster over distant isolated protons, while the close (5 Å) positive charges of proximate protons result in faster olefin(s) release from H-bonded propene and oligomeric alkoxides. As the steady-state TOF of propene oligomerization to C4-C9 olefins at 473-533 K over close protons exceeds 2-10 fold (with increasing Si/Al) those over isolated protons, the release of olefins from alkoxides is suggested to be an important reaction step. The distribution of framework Al atoms determining the mutual distances of protons represents a new parameter for evaluating rates of acid-catalyzed reactions and individual steps over H-zeolites.

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