(569ci) Surface Acidity Characterization of Phosphotungstic Acid

Solid heteropoly acids (HPAs) are interesting catalytic materials due to their high acid strength and proton conduction. The catalytic and conducting properties of HPAs are highly sensitive to the amount of hydration water present in the system, as water molecules interact with the HPA protons to form [H₃O]⁺ and [H₅O₂]⁺ ions. These protonic species are the active species that dictate the catalytic reaction pathways and the proton migration mechanism. A major challenge in characterization of heteropoly acid catalysts involves investigating surface acidity owing to the acid site dynamics attributed to proton mobility. How the surface acidity depends on the protonic species and their mobility has not been established. In this work, surface acidity of HPAs was characterized by temperature-programmed desorption (TPD) of 2,6-di-tert-butylpyridine (DTBP) rather than the typical ammonia TPD. It is a selective titrant for Brønsted acid sites, which has been widely used to characterize zeolite surfaces. Surface acid site density and acid strength of bulk (HPW) with various hydration levels (0 < n < 6) were investigated. It was found that acid strength depended on the type of protonic species (H+, [H₃O]⁺_, [H₅O₂]⁺) and their mobility. Understanding the dependence of surface acidity on protonic species and their mobility can contribute to developing catalytic systems where require solid acids with high acid strength such as biomass upgrading.