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Study of modified mesoporous silica for the capture of iodine

Abstract : Since the recent nuclear accident in Fukushima (2011), many R&D works were focused on the design of improved dry filters for the capture of radioactive fission products such as volatile iodine species (mainly I2 and CH3I). In this study, a novel class of organic/inorganic hybrid adsorbents with high affinity for iodine was investigated. About 20 different organically-modified silica sorbents with periodic mesoporosity (SBA-15 and PMOs) were prepared using one-pot or two-pot synthesis strategies. These sorbents were characterized by elemental analysis, TGA, N2 porosimetry at 77K, SAXS, DR-UV-Vis, Raman, ATR/IR and DRIFT spectroscopies to establish the relationships between their chemical, structural and textural properties on one hand, and their iodine adsorption behavior on the other hand. Their adsorption capacities and thermal trapping stabilities, were obtained from liquid and gas-phase (breakthrough) experiments carried out at different temperatures (20-100°C). After-test characterization of the spent sorbents was used to shed light on the nature of surface interactions existing between amine species and iodine. The syntheses of different lots of parent SBA-15 material at a scale of ≈ 10 g was found to be rather reproducible with specific surface areas ≈ 700-800 m2/g, porous volume ≈ 0.8-0.9 cm3/g and uniform pore size distribution ca 7-8 nm (from BJH method). The post-incorporation of different types of amines, aminosilanes, or thiols with N (or S) content in the range 2-8 wt% by reflux in toluene at 90oC led to a rather significant decrease of textural characteristics (by 60-80%) and in general an acceptable preservation of the hexagonal SBA-15 structure (SAXS d100= 9.7 nm and a0= 11.2 nm). However, one-pot synthesis strategies (co-condensation and direct synthesis of Periodic Mesoporous Organosilicas (PMOs) led to sorbents with a lower degree of organization, with sometimes very low textural characteristics and probably less accessible –N or –S adsorption sites embedded within the pore walls. Iodine adsorption tests carried out in cyclohexane solvent at 20°C or using breakthrough experiments at 100°C led to rather similar conclusions. The modification with amine- and thiol-containing compounds is essential to enhance the adsorption capacities, especially in the case of I2, while CH3I was about 10 times less easily adsorbed. The most influential material parameter on I2 adsorption capacities is the N (or S) elemental content of the adsorbents, with sometimes quasi-linear relationships existing between both sets of data. On the other hand, other parameters such as the textural characteristics and the type (primary, secondary…) and density of amine groups were found to be less important, except when the surface specific area is very low. SBA-15 adsorbents impregnated with different loadings (19, 38, 51 wt%) of branched polyethyleneimine (PEI) were also investigated for iodine adsorption. Owing to a high density of amine groups and high N content (up to 16.6 wt%), these adsorbents displayed improved characteristics for I2 adsorption, in a way rather similar to what was shown in the literature for CO2 adsorption. For a 38 wt% PEI loading, an exceptionally high I2 adsorption capacity exceeding 2g/g was found for an extended range of adsorption conditions. Above this threshold, the presence of excess PEI led to a total collapse of textural characteristics and a decrease of adsorption capacity. The usable temperature range of these sorbents is however limited to 150-200°C due to limited thermal stability of organic groups. The after-test spectroscopic investigations of different amine-modified SBA-15 sorbents, shows that besides physisorption phenomena, iodine adsorption proceeds via charge-transfer complex formation. Once formed, these CT complexes partially transformed in presence of humidity or silanol groups to ionic species of higher thermal stability.
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Submitted on : Monday, June 8, 2020 - 3:06:37 PM
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  • HAL Id : tel-02860699, version 1


Amal Hijazi. Study of modified mesoporous silica for the capture of iodine. Chemical Sciences. Université de Lorraine, 2019. English. ⟨NNT : 2019LORR0307⟩. ⟨tel-02860699⟩



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