A unified interpretation of high-temperature pore size expansion processes in MCM-41 mesoporous silicas

A unified interpretation is, presented for high-temperature (ca. 423 K) unit-cell enlargement of MCM-41 synthesized in the presence of cetyltrimethylammonium (CTMA(+)) surfactants without auxiliary organics. Such processes during postsynthesis hydrothermal treatments and direct syntheses were reported by several research groups to afford large-unit-cell (d spacing up to 7 nm) and large-pore (up to 7 nn) MCM-41 materials, but their structures were claimed to be different and the proposed mechanisms of the processes were not consistent with each other. The current study demonstrates significant structural similarities of large-pore MCM-41 materials-prepared using direct syntheses and postsynthesis treatments, reveals the common mechanism of their formation, and discusses its implications for preparation of large-pore materials, including new synthesis procedures. The proposed comprehensive description of the mechanism of the unit-cell-enlargement processes:involves the in situ generation of N,N-dimethylhexadecylamine (DMHA) and its swelling action as-a driving force of the pore size enlargement. The swelling properties of DMHA were confirmed by a successful synthesis of large-pore MCM-41 in mild conditions in the presence of DMHA and preparation of silicas: with pore sizes up to 11 nm and extremely large-pore volumes up to 2.4 cm(3)/g via restructuring of 3.5 nm MCM-41 in aqueous emulsions of long-chain amines (dimethyldecylamine or DMHA). The neutral amine is formed during the high-temperature processes-as a result of decomposition of CTMA(+) cations, which is likely to be accompanied by migration of tetramethylammonium cations (TMA(+)) to the silica-surfactant interface or is possibly related with replacement of CTMA(+) by TMA(+) at the interface. The reasons for the pore volume increase and surface area decrease during die pore size enlargement are discussed. Moreover, it is suggested that the unit-cell expansion-in the direction perpendicular to the pore channels is accompanied by. shrinkage of the structure in the direction parallel to the channels.