Optimization of silica/surfactant ratio in MCM-41 synthesis

Abstract

A series of MCM-41 silicas Was synthesized using a pH adjustment method in the presence of mixed cetyltrimethylammonium/cetyltriethylammonium surfactant template. The molar ratio of silica to surfactant in the reaction mixture was varied from 1 : :1. (surfactant excess) to 15 : I (silica excess). It was shown that the silica : surfactant ratio in the as-synthesized MCM-41 decreases only slightly when the initial silica : surfactant ratio increases from 1 : I to 8 : 1, but it significantly decreases for higher initial proportions of silica with respect to surfactant. The calcined MCM-41 silicas obtained for higher initial proportions of silica had significantly lower primary mesopore volume and specific surface area, which was attributed to the formation of amorphous non-templated silica in the absence of the sufficient amount of surfactant. The yield of silica was high (86-97%) for all initial silica : surfactant ratios. However, the templating efficiency, defined as the primary mesopore volume of MCM-41 per gram of the alkylammonium ions used, significantly varied and was the highest (1.20 cm(3) per gram of alkylammonium) for the ratios of 8 : I and 10 : 1, while only the 8 : I ratio produced MCM-41 of very good quality.