Synthesis of large-pore micelle-templated silico-aluminas at different alumina contents
Résumé
The EPR spectra of radical surfactant probes embedded in cetyltrimethylammonium bromide (CTAB) and trimethylbenzene (TMB) stable water emulsions (TMB/CTAB = 13) were analyzed to provide information on the kinetics of formation of micelle-templated silicoaluminas (MTSA) at 343 K, obtained by means of silica and alumina, solved in alkaline solutions, at different Si/Al ratios. Textural (surface area, pore volume, pore size, surfactant content) and structural characterization of both as-synthesized and calcined MTSA were performed by means of nitrogen sorption isotherms, TEM, and chemical analysis. This analysis showed that TMB worked as a swelling agent of the BTAB micelles, providing large-pore homogeneous and stable MTSA at TMB/CTAB = 13 for Si/Al from / 10. A demixing of the emulsion occurs at Si/Al < 10: at Si/Al = 7, a double wide-and-narrow pore structure was formed; then, at Si/Al = 5, an amorphous material was obtained. At Si/Al " 10, the computer-aided analysis of the EPR spectra as a function of the synthesis time indicated the distribution of the probes in two different environments: "micellar" probes inserted in the surfactant aggregates, whose mobility decreases over the synthesis time, thus reporting on the progressive modification of the surfactant aggregates structure and the solid condensation, and "interacting" probes due to probe-surfactant heads electrostatically interacting with the charged surface sites induced by alumina incorporation in the silica network. This last fraction increases its relative amount over the synthesis time, informing about the condensation and structuration of the MTSA. Increasing alumina contents slow down the synthesis process, but promote electrostatic interactions between the positively charged surfactant heads and the negatively charged silico-aluminate groups. Without alumina, the "interacting" component is absent in the EPR spectra because TMB preferentially interacts with the surfactant head groups by cation-Π interactions, thus preventing the interaction of these headgroups with silanols. When alumina is added, the negatively charged silicoaluminate at the surface promotes the interaction of the ammonium headgroups with the surface, and some Na+ cations also interact with TMB by cation-Π interaction and contribute to decreasing the interaction of headgroups with TMB. Therefore, increasing alumina contents promote electrostatic interactions between the positively charged surfactant heads and the negatively charged silicoaluminate groups. The strong interaction of the surfactant with the silicoaluminate surface allows the formation of a monolayerlike structure of surfactant, which is not observed in absence of alumina. The synthesis is slowed by increasing alumina contents due to a destructuration effect of alumina in the MTSA formation.