(48d) Optimizing Base Strength of Nitrogen-Substituted FAU Zeolite

Nitrogen-substituted zeolites, those with Si-O-Si/Si-O-Al- linkages replaced with Si-NH-Si/Si-NH-Al- groups, can act as next generation strongly basic, shape-selective catalysts, with likely application in biomass processing for fuels and products. Nitrided zeolites have been successfully synthesized, and have shown strong catalytic activity for reactions that rely on basic sites. Despite empirical reports demonstrating use of nitrogen-substituted zeolites as base catalysts, there has been no systematic study revealing the variation of base strength versus aluminum content or alkali cation in these systems. We have studied the base strengths of nitrogen-substituted X and Y zeolites by calculating adsorption energies of probe molecules BF₃ and BH₃, using DFT with mixed basis sets via the ONIOM embedded-cluster approach. Although BF₃ is a stronger lewis acid, BH₃ is found to be a better probe of base strength as it eliminates metal-fluorine interaction. For a particular Si:Al ratio, we have found that base strength varies in the order Li<Na<K~Rb~Cs. Base strength was found to decrease with decreasing Al content (Si:Al ratio increasing from 1 to 3); this is consistent with conventional wisdom. However, base strength was found to increase with further decrease in Al content to a Si:Al ratio of 11; this non-monotonic trend is new to zeolite science. In practice, we predict that K-N-Y (Si:Al = 11.0) provides a good choice to optimize cost/activity/stability.