Temperature-Programmed Desorption of Pyridine on Zeolites in the Presence of Liquid Solvents

Selecting the proper solvent is a major challenge involved in biomass conversion processes as there is little predictive understanding of how solvent choice will affect reaction rates. To date, most attempts to understand solvent behavior are on a case by case basis, through catalytic activity testing and screening multiple solvent choices. There is a need for characterization techniques aimed toward fundamental understanding of solvent behavior in a simple and systematic fashion. In this work, the effect of solvent on the Brønsted acid site–pyridine interaction was isolated to improve understanding of a solvent’s ability to stabilize charge in nonideal environments like zeolite micropores. Pyridine temperature-programmed desorption (TPD) was applied to zeolites ZSM-5, Beta, and Y in back-pressurized, flowing solvent. The desorption temperatures of pyridine from Brønsted acid sites in vacuum were compared in liquid water, acetonitrile, and n-heptane. For H/ZSM-5 (Si/Al = 12), the pyridine desorption temperature profiles increased in the order water < acetonitrile < n-heptane ≈ vacuum. For these solvents, the TPD profiles shifted to lower temperature with increasing solvent dielectric constant, suggesting that stabilization of the proton is driving deprotonation and that liquid-phase pyridine TPD is a qualitative probe for proton stability in nonideal, porous environments. Pyridine TPD profiles were also compared across zeolites ZSM-5, Beta, and Y in liquid acetonitrile. Compared to pyridine TPD profiles on H/ZSM-5 (Si/Al = 12) across different solvents, the TPD profiles in one solvent (acetonitrile) across different zeolite frameworks (ZSM-5, Beta, and Y) displayed only minor differences. This suggests that acetonitrile solvated the proton in a similar way in most frameworks tested.