(677f) On the Acid Sites of Phosphorous Modified All-Silica Zeolites
Phosphoric acid supported all-silica zeolites have recently been utilized as catalysts in biomass conversion including the catalytic conversion of dimethyl furan to p-xylene, and (methyl) tetrahydrofuran to C4-C5 dienes, respectively. As the understanding of these solid acids evolves, there is a need to benchmark their performance in simple probe chemistries, and in doing so, investigate whether traditional site-counting methods to calculate turnover frequencies can be extended to them. To that extent, the acid sites of P-zeosils were probed by assessing the kinetics of isopropanol dehydration. A conventional Brønsted acid site counting technique, namely alkylamine Hofmann elimination, resulted in probe-dependent acid site counts in a manner consistent with the stability of resulting carbenium ions formed during the reaction. This was a direct consequence of enhanced molecular desorption rates of the amines from the active phosphorus sites viz-a-viz bridged hydroxyls in aluminosilicate zeolites. The active P-sites catalyzed IPA dehydration at more than twenty-fold lower heteroatom-normalized rates than aluminosilicate zeolites, while exhibiting propene selectivities 20-30% higher than aluminosilicates under identical reaction conditions. Akin to alkylamine Hofmann elimination results, in-situ pyridine titration methods during IPA dehydration exhibited acid site counts dependent on pyridine partial pressures, underpinning the challenges in the measurement of accurate turnover frequencies (TOFs) using these traditional techniques. Kinetic isotope experiments with deuterated IPA feeds evinced an E2-type elimination as the rate-determining step (RDS) of unimolecular IPA dehydration on P-SPP. Propene and di-isopropyl ether (DIPE) formation kinetics enabled the calculation of unimolecular and bimolecular dehydration activation barrier on P-SPP (ca.~25 kcal/mol, and ~17 kcal/mol, respectively), which were independent of support and phosphorus loadings, and indicated an identical nature of catalytic P-site in different microporous (CHA, MFI, BEA), and mesoporous (SPP, MCM-41, Stöber) confining environments. Furthermore, the active P-content could be sited in microporous environments, differentiating these P-zeosils from solid-phosphoric acid (SPA) catalysts.