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

Abstract

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 SiO 2 source, and sodium aluminate was used as the Al 2O 3 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.