Effects of the cation and Si/Al ratio on CH3I adsorption by faujasite zeolites

Abstract

In this study, we aimed to investigate the effects of some specific zeolitic parameters on the retention properties of metal-exchanged faujasites (X and Y) for methyl iodide. In the first part, the nature of exchangeable cation (H + , Na + , Cu + /Cu 2+ , Ag + , Pb 2+) is discussed. In order to link the materials properties with the adsorption behaviour, the state and location of the exchanged transition metals after calcination at 500°C were first investigated using N 2 sorptiometry at 77K, DRS-UV-Vis spectroscopy, XRD and FTIR of adsorbed CO and NO. Adsorption capacities at breakthrough and saturation were obtained from CH 3 I breakthrough curves obtained at 100°C, this temperature being relevant to severe nuclear accident conditions. From the I/M ratio computed at saturation of the adsorbents by CH 3 I vapors, it was found that the efficiency of iodine trapping follows the order Cu + /Cu 2+ > Ag + >> Pb 2+ > Na + > H +. This efficiency roughly followed the trend of the different cations to form stable MI x precipitates from CH 3 I. Nevertheless, for nuclear application, it was found that Ag/FAU are preferred over Cu/FAU, due to the propension of the latter to oxidize CH 3 I to I 2 , which slips out of the sorbent bed at 100°C. The effect of the preparation method (ion-exchange vs impregnation) is then discussed for Ag/Y zeolites having different Si/Al ratio (2.5 and 40) but similar silver content (23 wt%). It is found that the silver oxidation state and silver dispersion have important effects on CH 3 I adsorption/desorption.

Structure–Activity Relationships between the State of Silver on Different Supports and Their I2 and CH3I Adsorption Properties

“…DR-UV-Vis spectroscopy was used in order to provide complementary information on the electronic state of silver species incorporated within the investigated supports prior to calcination. According to the literature studies by us and others [ 14 , 15 , 16 , 17 , 30 , 33 , 34 , 35 ], the absorptions relevant to Ag + cations in different environments are usually found in the range 200–240 nm, whereas the plasmon resonance of silver nanoparticles is responsible of broad absorption bands in the visible region. Other signals are more difficult to assign, but it is generally admitted that charged and metallic clusters (Ag n δ+ and Ag ° m ) yield absorptions at 240–280 nm and 280–350 nm, respectively [ 14 , 15 , 16 , 17 , 30 , 33 , 34 , 35 ].…”

“…Actually, the quasi-absence of exchange sites for the almost pure silica zeolite (with Si/Al = 40) made impossible to ensure a good dispersion of silver species, leading to poor CH 3 I adsorption capacity (23 mg/g). By contrast, silver nanoparticles were not formed at lower Si/Al ratio (2.5) due to the possibility to disperse silver as Ag + cations in exchange positions, leading to much greater CH 3 I trapping under similar conditions (up to 210 mg/g) [ 17 ].…”

“…[ 9 , 10 ]. Indeed, we and others have tested a large panel of silver zeolite configurations (nature of cation, Si/Al ratio, zeolitic structure) by experimental and computational means in order to gain insights about the most prominent parameters affecting the retention of volatile iodine [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. Some silver zeolites are now still currently investigated at industrial scale in France for their future incorporation in Filtered Containment Venting Systems (FCVS).…”

“…Only recently, the relationships existing between the adsorption performances and the main physicochemical characteristics of the materials were identified for silver zeolites [ 14 , 15 , 16 ]. In that respect, we found that the silver oxidation state (Ag° or Ag + ) as well as its content and dispersion in the pore system of the host support are the most important parameters governing the adsorption performances for I 2 or CH 3 I [ 17 ]. The present study aims to expand these findings by designing a comparative study to investigate different classes of silver-based materials on a common basis.…”

Surface Interactions and Mechanisms Study on the Removal of Iodide from Water by Use of Natural Zeolite-Based Silver Nanocomposites

“…The existence of nanoparticles in the Ag 2 O@NZU sample shows that the ion exchanged Ag + is either oxidized to Ag 2 O or it forms silver clusters. The photo-reduction of Ag + species and consequent formation of Ag m n+ clusters under light exposure during Ag + ion exchange has been reported for ZSM-5, NaX and NaY zeolites [ 43 ] and auto-reduction of Ag + to Ag m n+ and Ag m 0 has been reported for faujasite [ 31 ].…”