A kinetic study of the thermal degradation of cetyltrimethylammonium bromide inside the mesoporous SBA-3 molecular sieve

Abstract

An efficient removal of the organic component from the as-synthesized solid in order to access the internal free space is an important step in the final preparation of the porous materials. Recently, kinetic studies of the cetyltrimethylammonium bromide (CTMAB) removal from the MCM-41 and MCM-48 have shown that the CTMAB removal proceeds by very high activation energy indicating that the interactions of the CTMAB with silica framework are very strong. Preparation of mesoporous SBA-3 proceeds also in the presence of CTMAB; however, in contrast to the synthesis of MCM-41 and MCM-48 which is carried out under alkaline conditions, crystallization of the SBA-3 occurs in strongly acidic solution. This causes the interaction of CTMAB with the SBA-3 network to be different from its interaction with the MCM network since the SBA-3 network is electrically neutral while the MCM network is negatively charged. Thermal degradation of CTMAB inside the mesoporous SBA-3 has been studied under non-isothermal conditions. There are two distinct kinetic processes, which can be described by the Sestak-Berggren model. The main decomposition step proceeds with Ea = 116±2 kJ/mol which is significantly lower than for the MCM-41 and MCM-48, suggesting that the interaction between CTMAB and the silica network is the dominant factor in the removal of the surfactant. The second decomposition step has a higher Ea value (153±5 kJ/mol) than the first step, indicating that it is primarily controlled by the size of the pore openings.