Thermal stability and surface acidity of mesoporous silica doubly doped by incorporation of sulfate and zirconium ions

A sulfated Si-Zr-MCM-41 solid with highly ordered mesostructure was synthesized through a templated synthesis route where the CTAB surfactant was used as template. During the synthesis procedure, various amounts of (NH₄)₂SO₄ were added into the mixed solution of Zr and Si precursors to in situ sulfate the MCM-41 solids, aiming to enhance the acidity and thermal stability. The resultant materials showed a long-range ordered hexagonal arrangement with high surface area larger than 797 m²/g and an average pore size distributed at approximate 2.5-2.8 nm. The high-resolution TEM observations confirmed that the order of the mesostructure gained when the molar ratio of SO₄²⁻/(ZrO₂ + SiO₂) increased from 0.1 to 0.3 but decreased as it reached 0.5, which is consistent with the results of ²⁹Si MAS-NMR and XRD analysis. Compared to Si-MCM-41, the (Q² + Q³)/Q⁴ ratio derived from the NMR spectra of the Zr-doped sample was higher, indicating that zirconium atoms were incorporated into the silica framework. Unexpectedly, in situ sulfation does not enhance the surface Brönsted acidity, most likely due to the sulfur retained within the bulk of the materials; however, it indeed improved the thermal stability of the solid and long-range order of the structure.