An Examination of Brønsted Acid Sites in H-[Fe]ZSM-5 for Olefin Oligomerization and Adsorption

The adsorption and reaction properties of an Al-free H-[Fe]ZSM-5 were examined and compared to an H-[Al]ZSM-5 sample with the same site density. H-[Fe]ZSM-5 was shown to have Brønsted acid sites in a concentration equal to the framework Fe concentration. Differential heats of adsorption for ammonia and pyridine were shown to be identical to that obtained in H-[Al]ZSM-5, with differential heats of ∼150 kJ/mol for ammonia and 200 kJ/mol for pyridine. For H-[Al]ZSM-5, adsorption of either propylene or 1-butene at room temperature resulted in rapid oligomerization. TPD–TGA measurements of the oligomers in H-[Al]ZSM-5 showed evidence for hydride-transfer reactions, in addition to simple oligomer cracking. By contrast, it was necessary to heat H-[Fe]ZSM-5 to 370 K for rapid oligomerization of propylene and oligomerization of 1-butene occurred only slowly at 295 K. TPD–TGA measurements of the oligomers in H-[Fe]ZSM-5 showed no evidence for hydride-transfer reactions and H-[Fe]ZSM-5 formed much less coke than H-[Al]ZSM-5 during steady-state reaction in 1-butene at 573 K. Adsorption measurements of 1-butene on D-[Fe]ZSM-5 suggested that the protonated complexes of 1-butene were formed but that these were relatively stable toward reaction, implying that the carbocation transition states were relatively unstable.