Proton conductivity versus acidic strength of one-pot synthesized acidic functionalized SBA-15 Mesoporous silica

International audienceThis paper reports the one-pot synthesis and characterization of functionalized mesoporous SBA-15 silica, containing two loadings of different acid groups (-CO2H, -PO(OH)2 and -SO3H). The thermodynamic features of the water confined in these porous silicas is investigated by Differential Scanning Calorimetry (DSC). The results show that the melting behaviour of the confined water is mainly governed by the pore diameter and, as a consequence, indicate that the chemical "decoration" of the porous surface does not play any key role on water thermodynamics in that case. On the contrary, the proton conductivity of the hydrated mesoporous materials, examined in a wide range of temperatures (-100 to 70°C), turns out to be strongly dependent on both the physical state of the confined water and the acidity of the functions located at the porous surface. The proton conductivity is shown to be directly related to the pKa and the density of the functional groups attached to the mesopore surface. The high conductivity values obtained at low temperature when the confined water is frozen, let us think that the -SO3H functionalized SBA-15 investigated here could be promising candidates for electrolyte solids applications in fuel cells

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

Effect of surface chemistry on the thermodynamics and conductivity of water in silica nanopores

International audienceThe thermodynamics and conductivity of water confined in nanoporous silicas SBA-15 are studied using differential scanning calorimetry and impedance spectroscopy. The effect of the water/surface interaction is addressed by investigating samples with different surface chemistries