Caractérisations psysicochimiques de silices mésoporeuses fonctionnalisées du type SBA-15

Ce travail est consacré à l'étude des propriétés physico-chimiques de silices mésoporeuses fonctionnalisées du type SBA-15. Il a consisté à caractériser i) la structure mésoporeuse par DRX et par isothermes d'adsorption d'azote, ii) les états de surface de la silice par RMN lorsqu'on passe d'une fonction propyle à une chaîne -CH2-CH2-CH2-PO(OX)2 où X=Et, H, Li, Na et K, iii) les propriétés de confinement de trois adsorbats (eau, 1-pentanol et n-heptane) par analyses thermiques et iv) les propriétés de conduction ionique par spectroscopie d'impédance complexe. L'examen de la microstructure par RMN du solide a permis de mettre en évidence que la substitution des silanols de surface des pores des silices étudiées par des groupements tétraméthylsilyles (-Si(CH3)3) n'était pas totale. Les analyses par DSC des silices exposées la vapeur saturante d'eau, de 1-pentanol ou de n-heptane ont révélé que les températures de transitions de phase du fluide confiné dans les pores sont modifiées par rapport à celles du fluide en volume étendu. Ceci est dû à l'effet combiné d'une dimension réduite et de l'interface fluide mur, siège d'interactions spécifiques liées à la nature (polaire ou apolaire) des groupements introduits à la surface des pores. En présence d'eau confinée, les analyses RMN ont montré que celle-ci conserve sa coordination tétraédrique et que la mobilité des chaînes à la surface des pores augmente. L'étude de la conductivité par SIC des silices soumises à la vapeur saturante d'eau, de 1-pentanol ou de n-heptane indique que tous les solides étudiés sont des isolants tant qu'il n'y a pas de phase adsorbée. Lorsqu'une conductivité apparait, elle est de type ioniqueThis work is devoted to the study of physico-chemical properties of mesoporous functionalized silica SBA-15. It has particularly been axed to the characterization of i) the mesoporous structure by XRD and N2 adsorption- desorption isotherms, ii) the analysis by solid NMR of the surface states, iii) the properties of confinement of three adsorbates (water, 1-pentanol, n-heptane) by thermal analysis and iv) the properties of ionic conduction by complex impedance spectroscopy. The microstructure examination by solid NMR has revealed that the substitution of the silanols of nanopores surface by tetramethylsilyle -Si(CH3)3 groups was not complete. This work explored the surface functionalization when moving gradually from a -CH2-CH2-CH3 groups to -CH2-CH2-CH2-PO(OX)2 or X = Et, H, Li, Na and K groups . The DSC analysis of silica exposed vapour pressure of water, 1-pentanol or n-heptane showed that temperatures of phase transitions of the fluids are modified by confinement compared to those of the bulk state. These changes are related to a combination of small size and the fluid-wall interactions which depend on the nature (polar or apolar) of functional groups introduced on the surface pores. In the presence of confined water the NMR analysis indicates that the mobility of the functional chains on the surface of pores increases. The confined water retains its tetrahedral coordination. The study of conductivity by CIS of the silica samples previously dried and then exposed to vapour pressure of water, 1-pentanol and n-heptane indicates that all samples studied are insulators if there is no adsorbed phase. When a conductivity appears suggests that it is of ionic typeMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Insights into cation exchange selectivity of a natural clinoptilolite by means of dielectric relaxation spectroscopy

International audiencePurified natural clinoptilolite from the Tasajeras deposit, Cuba, and some of its metal exchanged forms are studied, at the dehydrated state, by means of Dielectric Relaxation Spectroscopy (DRS) using two different modus operandi: Complex Impedance Spectroscopy and Dielectric Dynamic Thermal Analysis. Data analysis yields the determination of the extra-framework cation (EFC) population into the various possible crystallographic sites of the zeolitic framework as well as of the activation energy characterizing the localized hopping mechanism of EFC. First, it is shown that the DRS responses obtained here match well with the previous reported data which were previously localized EFCs in positions close to M1 and M2 sites when the clinoptilolite is modified to almost homoionic form. From this outcome, it can be concluded that all EFCs are in the same crystallographic situation regarding solvation or, in other terms, that no steric effect can be taken into account to explain cationic selectivity. Second, based on the assumption that the activation energy for EFC hopping is directly connected to the EFC/framework interaction and on simple thermodynamics consideration, we show this interaction does not govern the EFC exchange reaction. So, it is emphasized that EFC/H2O interaction is the key factor for cation exchange selectivity

Chemical bond and ionic conductivity: alkali metal cations in various porous or dense oxydes

A simple model for the alkali metal bonding energy in some solid oxides is proposed based on a simplified approach of the Electronegativity Equalization Method. The bonding energy can be written as a linear combination of three independent energy contributions namely, an electrostatic, a covalent and a polarization term which are expressed as a function of the alkali metal cation radius. the model is then used to fit the evolution of the activation energy for the dc conductivity with the various alkali metal cations in the following test cases: a clay mineral (montmorillonite), two zeolites (faujasites Fau-X and Fau-Y), three glassy oxides (a silicate with two alkali contents and a triborate). Two typical behaviours can be distinguished: i) the considered solid oxide is either porous or supposed to have enough free volume for the displacement of the cations and then the model can be applied, ii) the compactness of the structure is such that the analytical expression for the activation energy of dc conductivity contains an additional term and then the model cannot be directly used. However, if this additional energy is known, then it is possible to estimate the alkali metal bonding energy and to properly reproduce it from the model. Finally, it is shown that the model yields significant information about both the nature of the alkali metal bond and the microscopic mechanism involved in the dc conductivity

Multi-functional hybrid materials for proton conductivity

International audienc

Measurements of radiation induced defects in quartz material by Dielectric Relaxation Spectroscopy

International audienc

WATER AND ETHANOL DESORPTION IN THE FLEXIBLE METAL ORGANIC FRAMEWORKS, MIL-53 (Cr, Fe), INVESTIGATED BY COMPLEX IMPEDANCE SPECTROCOPY AND DENSITY FUNCTIONAL THEORY CALCULATIONS

International audienceThe breathing behaviour of MIL-53(Cr) and MIL-53(Fe) upon water and ethanol desorption has been investigated by combining complementary experimental techniques including ThermoGravimetry Analysis (TGA), Differential Scanning Calorimetry (DSC) and Complex Impedance Spectroscopy (CIS). It was shown that two stages of solvent departure are involved in the desorption process, as revealed by (i) a change of the weight loss gradient in the TGA curve, (ii) the existence of a second endothermic peak in the DSC signal and (iii) a sudden drop and/or profile change of the ac conductivity in CIS. All these features are observed around a typical temperature Tc, for which the framework contractions, caused by the solvent desorption, occur. Moreover, it is shown that these modifications are more pronounced when the magnitude of the breathing is higher, as illustrated by the comparison of the water/MIL-53(Cr), ethanol/MIL-53(Cr) and water/MIL-53(Fe) systems. CIS data were further analyzed in the light of DFT calculations which provided us the preferential arrangements of the molecules within the pores and the resulting host/guest interactions. It could then be proposed that (i) the polarization conductivity results from the local re-orientation of the 2-OH dipoles bonded to the metal atom from the hybrid solid, i.e. Fe or Cr, and (ii) that dc conductivity, which can be ascribed to a proton propagation via a Grotthus mechanism, is favoured when the solvent molecules form strong hydrogen bonds between each other