Adsorption of volatile organic and iodine compounds over silver-exchanged mordenites: A comparative periodic DFT study for several silver loadings

摘要

The ability of Ag-exchanged mordenite (Ag-MOR) to capture iodine species such as I-2 and CH3I as released for instance during a nuclear accident could be severely limited by the presence of volatile organic compounds (VOCs) that can disrupt their adsorption in the zeolite. Here, using density functional theory (DFT), we investigate in detail the adsorption process of several hydrocarbons (methane, cyclohexane, benzene, and 1,3-dimethylbenzene) and oxygenated compounds (methanol, ethanol, propan-2-ol and propanone) and compare it with the adsorption of I-2 and CH3I for different values of the Si/Al ratio of the Ag-exchanged mordenite. It is found that the adsorption process of the iodine species becomes favorable as the Si/Al ratio is decreased. Indeed at high Si/Al ratio, some VOCs such as benzene, 1, 3-dimethylbenzene and propan-2-ol exhibit strong inhibiting effects, while at low Si/Al ratio, only the adsorption of CH3I is limited by the presence of aromatic hydrocarbons. Also, in addition to the elongation of the I-I bond upon the adsorption of the iodine molecule in Ag-MOR with a high silver content, we found that there is a significant electron transfer between the Ag and I atoms. Thus, our results indicate that the I-I bond becomes weaker during the adsorption and therefore the formation of AgI precipitates is favored.