Microwave synthesis of delaminated acid saponites using quaternary ammonium salt or polymer as template. Study of pH influence

Abstract

Mesoporous saponites were prepared at pH8 and 13 without and with template (surfactant or polymer) at 453K and autogenic pressure using microwaves or conventional oven during the hydrothermal ageing treatment. Acidity was obtained by calcination of the NH<inf>4</inf>-form. The effect of dilution (H<inf>2</inf>O/Si molar ratios of 250, 125 and 50) was studied for the samples prepared at pH8 with surfactant. In order to compare the effect of introducing surfactant during or after saponite synthesis, several samples prepared at pH13 were modified after synthesis by refluxing or by stirring at room temperature in a surfactant solution. Preparation of samples at pH13 favoured the ordering in the stacking direction, improved Al^T/Al^O ratio and led to samples with lower amorphous siliceous material content. The use of microwaves and surfactant for the synthesis at pH13 afforded a saponite with the highest surface area (603m²/g) and the smallest lamellae crystallite size (about 4nm). The properties of the samples synthesized at pH8 with surfactant depended on the dilution. Thus, the degree of delamination, the BET area and the Al^T/Al^O ratio increased whereas the amorphous siliceous material content decreased at lower slurry dilution. The incorporation of the surfactant by post-synthesis resulted in some degree of delamination especially when using refluxing. However, the degree of delamination was higher for the saponites prepared with the addition of the surfactant during the hydrothermal synthesis.