(420c) Modeling of Synthesis, Stability and Base Characteristics of Nitrogen-Substituted FAU and MFI

Zeolites in protonic forms have traditionally been used as acid catalyst due to large surface area and shape selective properties. Substituting oxygen with isoelectronic ?-NH-? group inside zeolite, one can incorporate high basicity into the framework. Amine functionalized zeolites have been synthesized in other laboratories, and have shown strong catalytic activity for reactions that rely on basic sites. Despite these advances, the synthesis mechanism and atomic nature of basic and catalytic sites remain poorly understood. Also stability of these materials in presence of water is in debate. We have performed Density Functional Theory calculations (DFT) to understand the synthesis mechanism in HY and Silicalite zeolites. The mechanism found is a two step process with overall barriers of about 350 KJ/mol. The formation process occurs at temperatures >600 ^OC which is in qualitative agreement with the experiments. The yield of substituted sites depends on water content, especially for Silicalite. The base catalytic sites were found to be stable at high temperatures (<350 ^OC) even for saturation water loadings. Using BH₃ as a probe of base strength, we have found nitrogen-substituted zeolites are highly basic in comparison to the corresponding zeolite without substitution. Base strength of such zeolites is increased by ion exchange in the order Li<Na<K~Rb~Cs, suggesting that K-N-zeolites may optimize cost/activity of amine-zeolites.