Adsorption and vibrational spectroscopy of ammonia at mordenite: Ab initio study

An ab initio density functional theory was used to investigate the adsorption of ammonia at various active centers at the outer and inner surfaces of mordenite, involving BrØnsted acid (BA) sites, terminal silanol groups, and Lewis sites. It was shown how ammonia formed an ammonium ion when adsorbed at strong BA sites. It was also found that the calculated adsorption energies for different BA sites varied in the interval from 111.5 to 174.7 kJ/mol depending on the local environment of the adduct. The study showed that the lowest adsorption energy, found for a monodentate complex was the main channel, which was also the highest for a tetradenate configuration in the side pocket.