Preparation, Characterization And Catalytic Performance Of Acid-Modified SBA-15 Mesoporous Materials

As a newly reported silica mesoporous material, SBA-15 has regular pore array with uniform pore diameter, high surface area and pore volume. It is chemically inert but allow irreversible surface modification by reaction of the surface silanol groups. Herein, we report the preparation of acid-modified SBA-15 mesoporous molecular sieves by post systhesis methods. Esterification of acetic acid with n-butanol was carried out over the acid-modified SBA-15 catalysts in liquid phase conditions.The effects of the factors involved in catalysts compose, catalysts preparation methods and preparation conditions had been investigated. Characterization was done by powder X-ray diffraction (XRD),transmission electron micrographs (TEM), N₂ adsorption desorption, IR, thermogravimetry analysis.The total amount of acidity on loaded catalysts was estimated by NH3-TPD. The catalytic activity of this family of mesoporous solids had been studied by catalytic tests of esterification.The results show that :1. The preparation of acid-modified SBA-15sSBA-15 mesoporous molecular sieves were prepared by hydrothermal method using triblock copolymer Pluronic P123 (EO₂₀PO₇₀EO₂₀) as templating agent and tetraethoxysilane (TEOS) as silicon source under a strong acidic condition. Mesoporous silica materials were used as a solid support. The solid acid was prepared by impregnation or solid state grinding in absence of solvent respectively. The optimum conditions of the preparation by single factors are studied.La-SO₄^2-/SBA-15 and Ce-SO₄^2-/SBA-15 composite materials were prepared by incipient -wetness impregnation using the calcined host material SBA-15 molecular sieve and oxidation lanthanum and sulfuric cerium in sulfuric acid media respectively. The best conditions for preparation of La-SO₄^2-/SBA-15 were that the concentration of impregnation La₂(SO₄)₃ 0.03mol/L, calcination temperature 300℃, calcination time 4h. The best conditions for preparation of Ce-SO₄^2-/SBA-15 were that the concentration of impregnation Ce(SO₄)₂ 0.1mol/L, calcination temperature 300℃, calcination time 3h. TsOH/SBA-15 composite materials were prepared by impregnation using the calcined host material SBA-15 molecular sieve and TsOH in alcohol media. The best conditions were that the concentration of impregnation TsOH alcohol solution 0.5mol/L, calcination temperature 300℃, calcination time 4h.La-S₂O₈^2-/SBA-15, La-SO₄^2-/ SBA-15, Ce-S₂O₈^2-/SBA-15 and Ce-SO₄^2-/SBA-15 composite materials were prepared by solid state grinding in absence of solvent. The best conditions for preparation of La-S₂O₈^2-/SBA-15 and La-SO₄^2-/SBA-15 were that the mole ratio of La₂O₃ to (NH₄)₂S₂O₈ or (NH₄)₂SO₄ is 1 : 7 or 1 : 6 by manually grinding the corresponding the host SBA-15,calcination temperature 300℃, calcination time 3h.The best conditions for preparation of Ce-S₂O₈^2-/SBA-15 and Ce-SO₄^2-/SBA-15 were that the mole ratio of Ce(SO₄)₂ to (NH₄)₂S₂O₈ or (NH₄)₂SO₄ is 1 : 7 or 1 : 6 by manually grinding the corresponding the host SBA-15,calcination temperature 300℃, calcination time 3h.It can be concluded that calcination temperature play an important role in the form of acid sites on the surface of catalyst during the preparation of catalysts.2. The characterization of acid-modified SBA-15sThe properties of modified SBA-15 mesoporous molecular sieves were characterized by powder X-ray diffraction (XRD), transmission electron micrographs (TEM), N₂ adsorption desorption, IR, thermogravimetry analysis. The total amount of acidity of loaded catalysts was estimated by NH3-TPD. Results show that the active components have been incorporated into SBA-15 molecular sieve. The prepared catalysts keep highly ordered mesoporous two-dimensional hexagonal structure and do not change mesoporous channel structure of SBA-15 molecular sieve. The Acid strength on catalyst surface was determined by mean of Hammett indication method (-4.440≤3.30). The NH3-TPD profiles show that weak and medium strong acid sites on the surface of the modified mesporous materials.3. The catalysis performance for esterificationThe modified mesporous materials is an excellent catalyst to synthesize n-butyl acetate and methyl palmitate. Optimum conditions of esterification such as mole ratio of acid to alcohol, amount of catalyst and reaction time were studied by orthogonal experiments, it is observed that the straight correlation of the esterification behavior with the acidic properties of the catalysts. The catalytic activity increases with the rise of acid amount on the surface of catalysts.The results show that the catalysts can be recovered or regenerated for reuse.