An examination of Bronsted-Acid Sites in H[Fe]ZSM-5 for oligomerization and adsorption

KRESNAWAHJUESA,O.; KUHL, G.H.; GORTE, R.J.; QUERINI, C.A.

JOURNAL OF CATALYSIS

ACADEMIC PRESS INC ELSEVIER SCIENCE

Año: 2002 vol. 210 p. 103 - 115

0021-9517

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. 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. ∼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.