Room-temperature synthesis of silicate mesoporous MCM-41-type materials: influence of the synthesis pH on the porosity of the materials obtained

Abstract

Pure silica MCM-41 materials were synthesized at room temperature at different pH values ranging from 8.5 to 12. The solids were characterized by XRD, ²⁹Si-MAS-NMR spectroscopy and N₂ and Ar adsorption measurements. When the synthesis pH is too high, the degree of polycondensation of the silicate species is too low and, after removal of the organic species by calcination, amorphization occurs. For an intermediate pH value (11), the N₂ and Ar adsorption isotherms at 77 K, determined on the corresponding solid, are ‘type I-b like’ and are characteristic of the presence of primary and secondary micropores (d<2 nm). At a low pH value (8.5), a typical MCM-41 material with a mesopore diameter of ~2.6nm is obtained. The adsorption isotherms are of type IV. The influence of the thermal treatment on the as-synthesized samples was studied, and an effect was observed in the material containing micropores. Whereas a short calcination time at 873 K leads to a decrease in the intensity of the XRD peaks, a prolonged calcination time at this temperature leads to an improvement in the degree of organization and to an increase in the total microporous volume of about 18%. This increase can be explained by the greater homogeneity of the micropore surface after such a thermal treatment.