A mechanistic study on the synthesis of MCM-22 from SBA-15 by dry gel conversion to form a micro- and mesoporous composite

A composite material with ordered pores in both the micro- and mesopore ranges was prepared by a dry gel conversion technique known as vapor-phase transport (VPT). Mesoporous silica SBA-15 was used as the SiO2 source, and sodium aluminate was used as the Al2O3 source on the silica. VPT was then carried out under an atmosphere of hexamethyleneimine and water vapors. The unique microporous structure of MCM-22, a micropocket on the external surface of the zeolite, was generated at a late stage of crystallization. A zeolitic acid site, as strong as that of MCM-22 with mesopores originated from SBA-15, was achieved to create before the long-range ordering of the zeolite. A building unit of the zeolite was also formed at an early stage of crystallization. A drastic change in the morphology of SBA-15, promoted with the help of water vapor, took place during the course of crystallization. Slow crystallization and a structural support (e.g., carbon) inside the mesopores are necessary to prevent collapse of the mesostructure derived from the original SBA-15, and to obtain an ordered micro- and mesoporous composite material.