关键词: 介孔材料, 固体酸, 硫酸化氧化锆, Zr-SBA-15, 酯交换反应

Ordered mesoporous Zr-SBA-15 material with a highly homogenous distribution of Zr within mesoporous walls was synthesized by a facile one-step high temperature hydrothermal treatment approach. In the proposed approach, the cooperative self-assembly between amphiphilic triblock copolymer P123 and silicon hydroxyl (Si-OH) species from the hydrolysis of tetraethyl orthosilicate (TEOS) was carried out to form an organic-inorganic composite with an ordered mesostructure (as-synthesized SBA-15, silica matrices with an embeddedness of P123 micelles) under a weak acidic medium originated from the hydrolysis of zirconium tetranitrate; during the followed high temperature hydrothermal treatment process, the further hydrolysis of zirconium precursor promotes more Zr⁴⁺ transferring to zirconium hydroxyl (Zr-OH) species, and the condensation between the Zr-OH species and Si-OH species, which exist in the synthesis solution and within the mesoporous walls of as-synthesized SBA-15, respectively, leads to a large amount of Zr atoms being highly homogenously grafted into the mesoporous walls of as-synthesized SBA-15 and the formation of framework Zr-O-Si bonds. After calcination to remove the organic P123 micelles, the ordered mesoporous Zr-SBA-15 material was obtained and used as a support for the immobilization of sulfuric acid by a traditional impregnation method. The characterization results confirmed that compared to supports SBA-15 and SBA-15-supported zirconia (Zr/SBA-15, prepared by the impregnation of SBA-15 with zirconium tetranitrate aqueous soliton followed drying and calcination treatments), the use of support Zr-SBA-15 can realize a stable immobilization of much more sulfuric acid groups, due to the highly homogenous dispersions of ZrO_x species on the mesoporous surface of Zr-SBA-15. In addition, the double attracting electron role from supported sulfuric acid group and Si element with higher electronegativity than that of Zr element can further induce an decrease in the electron cloud density of Zr⁴⁺ existing in the surface Zr-O-Si bonds, which plays an important role in promoting the formation of super strong Brönsted and Lewis acidic sites. More importantly, the formation of surface Zr-O-Si bonds can exert a significant influence on restraining the leaching of active sulfated ZrO_x species during the reaction. The resultant solid acid SO₄^2-/Zr-SBA-15 exhibits an ordered two-dimensional hexagonal mesostructure, a uniform mesoporous size, a high specific surface area, a large pore volume, and a prominently improved acidity. For soybean oil transesterification with methanol, the catalyst SO₄^2-/Zr-SBA-15 demonstrated a much higher soybean oil conversion (up to 100 %) along with a fatty acid methyl ester yield of 99.8 % and a desirable reusability, compared to the traditional supported sulfated zirconia catalysts. The superior catalytic performance of SO₄^2-/Zr-SBA-15 is associated with the outstanding structural and textural properties of support Zr-SBA-15 and the formation of surface Zr-O-Si bonds, thus benefiting the fast mass and heat transfer and the generation of a large number of stable active sites with a highly homogenous dispersion.

Key words: mesoporous material, solid acids, sulfated zirconia, Zr-SBA-15, transesterification