(349v) P-Zeosils: Are Weaker Acid Sites Better?

Owing to their weaker acidity, the acid sites of phosphoric acid supported on all silica zeolite surfaces (P-zeosils) offer a high degree of selectivity towards multiple acid catalyzed chemistries, particularly for dehydrations relevant to biomass upgrading. The enhanced selectivity, however, comes at the price of reduced catalytic activity compared to other solid acid catalysts. To enhance the catalytic activity of P-zeosils, we sought to understand the kinetic nature of their acidic active sites. Through a combination of temperature programmed surface reactions, probe molecule spectroscopy, and computational chemistry, we demonstrate that P-zeosils are Brønsted acidic in nature. Using both alkylamine Hofmann elimination and alcohol dehydration as probe chemistries, we kinetically interrogate the phosphorous based active sites, relative to more standard bearer aluminosilicate zeolite catalysts. Despite the significantly distinct acidity of P-zeosils and aluminosilicates, adsorbed alkylammonium reaction intermediates completely abstract the Brønsted acidic proton of the active sites, resulting in identical alkylamine Hofmann elimination kinetics over both material families. Once the proton of the Brønsted acid site is completely abstracted, the catalytic cycle is insensitive to the identity of the conjugate base (SiO^-Al or PO^-). The weaker Brønsted acidity of the phosphorous acid sites results in weaker binding of reaction intermediates, which significantly alters the surface reaction energetics of alcohol dehydration. Despite the milder acidity of P-zeosils, and slower catalytic rate of alcohol dehydration, they exhibit smaller apparent activation energies relative to aluminosilicates. This apparent discrepancy is rationalized by recognizing the apparent nature of the macroscopically measured activation energies, which are lower on P-zeosils relative to aluminosilicates on account of the less favorable adsorption of alcohols and their reaction intermediates. We verify this hypothesis through kinetic isotope experiments, which reveal an identical mechanistic pathway for alcohol dehydration over aluminosilicates and P-zeosils, despite their significantly distinct catalytic turnover frequencies and reaction energetics.