Characterization of pore systems of mesoporous silicas templated by cetylpyridinium under mild-alkaline condition: the effect of low-temperature ammonia post-treatment

Mesoporous silicas with ordered hexagonal MCM-41 structure were synthesized under mildly alkaline condition at room temperature in the presence of the surfactant cetylpyridinium chloride. Low-temperature post-synthesis treatment of these silicas in ammonia solution was investigated. The pore size was expanded from 2 to 4 nm by post-treatment at temperatures lower than 373 K, accompanied by a subtle change of morphology, and could be fine-tuned by varying the post-treatment temperature. A bimodal mesopore system with an interconnected three-dimensional was obtained, when the post-treatment occurred at 373 K. The control of the pore system and the creation of larger secondary mesopores, leading the unidimensional channels interconnected, are important and useful for catalysis. The role of ammonia in the extension of the pore size during low-temperature treatment and the possible generation of the secondary pore system by the decomposition of the thermally unstable cetylpyridinium surfactant are discussed in detail. The present work can serve as an excellent example for pore system engineering.