Control over microporosity of ordered microporous-mesoporous silica SBA-15 framework under microwave-hydrothermal conditions: Effect of salt addition

The crystallization of ordered microporous-mesoporous silica, SBA-15, has been investigated in the presence of an ionic salt, such as sodium chloride, under microwave-hydrothermal conditions. The crystallization was performed by templating a silica precursor (tetraethyl orthosilicate or sodium silicate) with pluronic123 copolymer (EO₂₀PO₇₀EO₂₀; MW 5800) with varied amounts of salt content at 373 K. The crystallized samples were characterized by means of powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and nitrogen adsorption-desorption measurements at 77 K. The textural properties of the SBA-15 samples, such as pore volume, pore size, and specific surface area, were found to decrease with an increase in salt concentration. Furthermore, the microporosity of the crystallized samples was found to be a function of the added salt concentration, that is, micropore volume has been found to decrease with an increase in salt concentration. Interestingly, SBA-15 samples crystallized in the presence of higher salt content were found to exhibit a relation between the pore size, pore volume, and specific surface area which is similar to that observed for cylindrical or hexagonal disconnected pores. Such a behavior has demonstrated for the first time that the pore size and microporosity within the pore walls of ordered mesoporous silica SBA-15 can be tuned by means of salt addition under microwave-hydrothermal conditions.