Title: Direct synthesis and pH adjustment approaches to incorporate zirconium species in SBA-15 mesoporous silica

Abstract

A comparative study of synthetic procedures by hydrothermal treatments at 60°C for the preparation of a series of Zr-SBA-15 materials (Si/Zr=∞, 10, 5, and 2) using direct synthesis (method A) and pH adjustment (method B) approaches is reported. The method B uses concentrated ammonia as a pH regulator and a further hydrothermal treatment. It shows a decrease in the Si/ Zr ratio measured by EDS and XPS as the Zr content increased in the preparation by both methods. XPS revealed lower values of Si/ Zr ratio with respect to EDS, which suggests more content of surface zirconium species than bulk. All N2-physisorption isotherms show type IV behaviors which are characteristic of mesoporous materials. Regardless of the preparation method, the BET specific surface area (SBET) and pore volume (Vp) values show a decreasing trend as Si/Zr ratio decreases. The pore size (dp) indicates diameters below 4 nm, except in the SB sample. Low-angle diffraction patterns show a peak associated with the (100) plane, which tends to widen and decrease in intensity for materials prepared with higher zirconium charges, revealing the presence of mesoporous structure. HRTEM corroborate the mesoporous arrangements of short- and long-range order, depending on the conditions. SEM-EDS mapping suggests a homogenous distribution between zirconium and silicon species over the SZA-10 and SZB-10 materials. It is concluded that the decrease of Si/Zr ratio modifies the structural, textural, and surface properties of the materials, and consequently, compromising the ordered arrangement of the mesopores. Finally, the pH adjustment method leads to better preservation of mesoporous order.