(482a) Kinetic Stability of Nitrogen-Substituted Siliceous FAU Zeolite From First Principles

Acidic zeolite catalysts have long been back-bone of petroleum industry because of their high surface area, large adsorption capacities, and shape selective properties. Substituting oxygen with isoelectronic ?-NH-? group inside zeolite can incorporate high basicity, which could find likely applications in bio-fuel industry. We have performed Density Functional Theory calculations (DFT) to understand the synthesis mechanism of nitrogen substitution in siliceous FAU zeolite. We have used a combination of nudged band elastic method and ONIOM method of treating embedded clusters to obtain the full nitridation reaction pathway, as implemented in our previous work [1]. The mechanism is found to be a five step process as compared to four step processes found in silicalite and HY zeolites [1]. The overall barrier for the process is about 200 kJ/mol, which is much less compared as to barriers found in silicalite and HY zeolites [350 kJ/mol]. We have found that zeolite nitridation in siliceous FAU takes place temperatures > 450 °C. We have also found that presence of moisture can greatly affect nitridation process in accordance with experimental results. We also investigated the stability of nitrided sites by modeling the kinetics of nitridation in reverse, going back to untreated zeolite plus ammonia. Using 10h as the benchmark catalyst lifetime, we have found that catalyst remains stable below 175 °C even at saturation water loadings. This temperature is lower than that found in Silicalite and HY zeolites [2], however, such catalyst can find application in reactions involving very low moisture content.

References

1. V. Agarwal, G. W. Huber, W. C. Conner, Jr. and S. M. Auerbach, "DFT Study of Nitrided Zeolites: Mechanism of Substitution in HY and Silicalite," J. Catal. 269, 2009, 53-63.

2. V. Agarwal, G. W. Huber, W. C. Conner, Jr. and S. M. Auerbach, "Kinetic Stability of Nitrogen-substituted sites in HY and Silicalite from First Principles," J. Catal. 270, 2010, 249-255.