The Dynamic Catalytic Activity of Phosphorus-containing Catalysts
The active site speciation of phosphorus-containing all-silica zeolites, P-zeosils, involves a dynamic distribution that is manipulated through controlled hydrolysis of the catalytic material. While water vapor is known to enhance the catalytic activity of phosphorus-based catalysts through phosphorous linkage hydrolysis, here we demonstrate that hydrolysis using a combination of water and sufficiently basic nitrogen-containing compounds like alkylamines increases the acid site density by more than an order of magnitude relative to water-only hydrolysis. The approach of base-assisted hydrolysis was found to be applicable to multiple catalytic chemistries, facilitating both the rates of alcohol dehydration (300× rate enhancement) and alkylamine Hofmann elimination (15× rate enhancement) over P-zeosils. Through a broad array of in situ and ex situ characterization, the enhancement of catalytic activity via base-facilitated hydrolysis was attributed to enhanced acid site density as a result of shifting the dynamic distribution of P-moieties to less condensed states. Specifically, more recalcitrant P-O-P linkages not cleaved through water only hydrolysis, were readily hydrolyzed in the presence of a base. Sufficient basicity in the nitrogen-containing group is critical to access the highest possible density of acid sites, which greatly exceeds that achievable through hydrolysis alone.