Understanding the Stability in Water of Mesoporous SBA-15 and MCM-41

Abstract

Surprisingly, SBA-15 mesoporous silicas are not as stable as expected in water, even at room temperature, despite their thick walls. The microporosity of SBA-15, synthesized at a temperature below 110 °C, is lost during water treatment, leading to a strong decrease in specific surface area and an increase in mesopore size. Only SBA-15s without microporosity, such as the ones synthesized at 130 °C, are stable under water treatment. Investigations by nitrogen adsorption isotherms and hyperpolarized 129 Xe NMR spectroscopy have been performed in an effort to understand the silica dissolution/redeposition processes occurring during water treatment at room temperature and at the boiling point for three SBA-15s synthesized at different temperature levels: 60, 100, and 130 °C. The differences between the local curvatures of silica in the different structures explain the difference of behavior in water with respect to silica dissolution/redeposition. Similar experiments on MCM-41 lead to a totally different dissolution/redeposition process because of its thinner walls: decrease of pore size, surface area and pore volume.

High‐Temperature Synthesis and Formation Mechanism of Stable, Ordered MCM‐41 Silicas by Using Surfactant Cetyltrimethylammonium Tosylate as Template

“…[47] As is already known, the dissolution rate of silica nanoparticles drastically depends on the size of particles, alkalinity, and reaction temperature. [13] It is important to note that, before and after crystallization, the pH of the system remained constant, close to 11.5. At higher crystallization temperatures and higher alkalinity, the smaller silica nanoparticles can be easily decomposed to the silicate species, such as monomer or oligomer.…”

“…Generally, MCM-41 silica is synthesized at mild temperatures of less than 130°C, which usually results in imperfectly condensed mesoporous walls with a thickness of less than 1.0 nm and large amounts of terminal hydroxy groups, which makes the mesostructure unstable. [11] Direct synthesis of mesoporous silica (SBA-15) by using nonionic copolymer as template has been found the most obvious [a] Shanghai Key Laboratory of Green Chemistry and Chemical(XRD), N 2 adsorption, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), 13 C crosspolarization magic-angle spinning (CPMAS) solid-state NMR spectroscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM), a new mechanism was proposed to understand the formation mechanism of highly ordered MCM-41 silicas. The enlargement of pore-wall thickness is attributed to the migration and subsequent deposition of the silicate species in the inner pore channel.…”

“…[12] However, a recent study by Galarneau et al showed that SBA-15 mesoporous silicas are not as stable as expected in water, even at room temperature, probably due to the presence of micropores in the channels. [13] It should be mentioned that the studies on the synthesis of mesoporous silicas at high temperature mostly concentrate on SBA-15. [14][15][16][17][18] Very few studies focus on MCM-41 silica.…”

Pore size distribution of micelle-templated silicas studied by thermoporosimetry using water and n-heptane

“…The final suspension was stirred for 1 h. A hydrothermal treatment of the obtained gel was carried out in a stainless steel autoclave for 2 weeks at 115°C. The solid was filtered, washed with deionized water until neutral pH and dried overnight at 80°C [23].…”

“…When the polymer was totally dissolved 9.45 g of tetraethyl orthosilicate (TEOS, Aldrich) were added and the final solution was stirred during 5 h. The resulting gel was aged in a teflon-lined stainless steel autoclave at 60°C for 24 h. The obtained solid was filtered, washed with deionized water until neutral pH and dried overnight at 80°C [23].…”