Rational approach for an optimized formulation of silver-exchanged zeolites for iodine capture from first-principles calculations

Abstract

Ab initio calculations have been carried out to investigate in detail the effect of potential inhibiting species (CO, H₂O, CH₃Cl and Cl₂) on the adsorption of iodine species (I₂ and CH₃I) in silver-exchanged zeolites of different Si/Al ratios and structures (faujasite, mordenite, chabazite and clinoptilolite). We have found that the adsorption of iodine species becomes more favorable for a low Si/Al ratio. Indeed, a potentially strong inhibiting effect of CO for a high Si/Al ratio has been unravelled, while Cl₂ appears as an inhibitor for a low Si/Al ratio. In addition, a spontaneous dissociation is observed for I₂ and Cl₂ at a low Si/Al ratio which greatly increases the interaction energy. With the addition of sodium cations together with the silver ones to compensate for the charge in the zeolite structures, an improvement in the performance of mordenite for the selective trapping of iodine compounds is observed, in contrast to chabazite and clinoptilolite. Interestingly, we found that the presence of the Na cations in faujasite and mordenite prevents the spontaneous dissociation of the Cl₂ molecule, which leads to limitation of the strong inhibiting effect on the adsorption of iodine species, in particular CH₃I. Also, an efficient comparison between different structures using radar charts has been performed, where not only the Si/Al ratio is taken into consideration but also the silver loading in the zeolite structure for a given Si/Al ratio. Finally, we found that Ag-chabazite with a Si/Al of 5 and AgNa-mordenite with a Si/Al of 11 are the best candidates for selective trapping of iodine species, noticeably for I₂ trapping.