A study of the behaviour of mesoporous silicas in OH/CTABr/H2O systems: phase dependent stabilisation, dissolution or semi-pseudomorphic transformation

Abstract

An investigation of the behaviour of various calcined mesoporous silicas (MCM-41, MCM-48, SBA-15, HMS) in OH/surfactant/H₂O systems, at room temperature or under hydrothermal (150 °C) conditions, has shown that their stability (i.e., whether or not they are dissolved) depends on the match between the pore channel structure (i.e., pore geometry and pore size) and the surfactant micelles that can form under the prevailing conditions. In general a close match allows the surfactant to protect the silica framework from dissolution. The key feature in the stabilisation of the calcined mesoporous silicas is the diffusion of surfactant molecules into the pores. Optimum protection of the silica is achieved when the micelles that can form within the pores favourably match the pore geometry and size of the pore channel structure. We found that under conditions generally used for the synthesis of MCM-41, the surfactant (cetyltrimethyammonium bromide) molecules adequately stabilise calcined MCM-41, partially stabilise calcined MCM-48 but offer no stabilisation for calcined SBA-15 and HMS silicas. The SBA-15 and HMS silicas are dissolved and transformed into MCM-41. The SBA-15 is transformed in a semi-pseudomorphic manner thereby providing a route for morphological control of MCM-41. The MCM-41 yield derived from SBA-15 is made up of unusual rope-like particles that are not typical of MCM-41 materials prepared under basic conditions.