Acid Sites of Phosphorus-Modified Zeosils

The acid sites of phosphorus-containing
zeosils were probed through
a combination of solid acid characterization, density functional theory
calculations, and kinetic interrogations, establishing their weakly
Brønsted-acidic character. Because of the disparity in the acid-site
strength, P-zeosils catalyzed the probe chemistry of isopropanol dehydration
slower than aluminosilicate zeolites by an order of magnitude on an
active-site basis. Propene selectivity during isopropanol dehydration
remained 20–30% higher than that of aluminosilicates, illustrating
the distinct nature of the weakly acidic phosphorus active sites that
favored unimolecular dehydration routes. Regardless of the confining
siliceous environment, the nature of phosphorus active sites was unchanged,
as indicated by the identical apparent uni- and bimolecular dehydration
energy barriers. Kinetic isotope experiments with deuterated isopropanol
feeds implicated an E2-type elimination in propene formation on phosphorus-containing
materials. The comparison of kinetic isotope effects between phosphorus-containing
zeosils and aluminosilicates pointed to an unchanged isopropanol dehydration
mechanism, with changes in the apparent energetic barriers attributed
to weaker binding on phosphorus-active sites that lead to a relatively
destabilized alcohol dimer adsorbate. Both ex situ alkylamine Hofmann elimination and in situ pyridine
titration characterization methods exhibited a phosphorous acid site
count that was dependent on the probe molecules’ identity or
concentration, underpinning the limitations of extending common characterization
techniques for Brønsted acid catalysis to weakly acidic materials.