Kinetic stability ofnitrogen-substituted sites in HY and silicalite from first principles

We have modeled the formation kinetics of nitrogen-substituted (nitrided) zeolites HY and silicalite; we have also modeled the stability of nitrided sites to heat and humidity. These kinetic calculations are based on mechanisms computed from DFT-computed pathways reported in our previous work. Reactant ammonia and product water concentrations were fixed at various levels to mimic continuous nitridation reactors. We have found that zeolite nitridation — replacing Si–O–Si and Si–OH–Al linkages with Si–NH– Si and Si–NH2–Al, respectively — proceeds only at high temperatures (>600 C for silicalite and >650 C for HY) due to the presence of large overall barriers. These threshold temperatures are in good agreement with experiments. Nitridation yields were found to be sensitive to water concentration, especially for silicalite where nitridation is more strongly endothermic. As a result, overall nitridation yields in silicalite are predicted to be much lower than those in HY. The stability of nitrided sites was investigated by modeling the kinetics of nitridation in reverse, going back to untreated zeolite plus ammonia. Using 10 h as a benchmark catalyst lifetime, nitrided silicalite and HY half-lives exceeded 10 h for temperatures below 275 and 500 C, respectively, even at saturation water loadings. As such, our calculations suggest that nitrided silicalite and HY zeolites require high temperatures to form, but once formed, they remain relatively stable, auguring well for their use as shape-selective base catalysts. 2010 Elsevier Inc. All rights reserved.