The Preparation, Characterization And Performances Study Of SO₄～(2-)/Zr-SBA-15 Mesoporous Superacid Materials

SBA-15 mesoporous molecular sieves have regular mesoporous structure, high surface area, thick channel walls and higher thermostability, but its amorphous framework in long-range order leads to its weak acidity and low catalytic activity. Meanwhile, solid superacid SO₄^2-/ZrO₂ has become the focus of acid-catalysized catalysts in catalysis field because of its strong acidity and the advantages over the liquid acid, but its surface area is lower and easily deactivated in the reactions. So the combination of the porous structure of mesoporous molecular sieves and the acid properties of superacids is greatly likely to obtain more prospective molecular superacid materials.In the paper, SO₄^2-/Zr-SBA-15 mesoporous superacids were firstly prepared by impregnation-crystallination-immersion method (method I) and then two-step crystallination-immersion method (method II), and the textural properties and catalytic performances were further studied on the basis of previous experiments. In addition, the one-pot method (method III) was originally put forward, and the SO₄^2-/Zr-SBA-15 mesoporous superacids were successfully synthesized and characterized by various means, then the acid catalysis performances were studied in details.1). The samples prepared by method I showed regular porous structure and double mesoporous distribution by XRD and N₂ adsorption and desorption. They have B acid sites and L acid sites, and the amount of L was more than that of B, and the ratio of superacid sites to medium-acid sites was 0.66 at Zr/Si=2.15, which verified the SO₄^2-/Zr-SBA-15 mesoporous materials were superacidic. Besides, most of Zr were incorporated into the framework of SBA-15 with only small part bonded and highly dispersed on the surface by Uv-vis scattered reflection measurement. When Zr/Si was about 2.15, the 180min conversion of n-pentane could reach to 46.6%, which was higher far and away than 1.4% of 5h reported, and its BET surface area was 313.51m²/g, the mass content of ZrO₂ 45.83 Wt.%, the efficiency of Zr loaded 56.49% and the S mass content 0.9 Wt.%.2). The samples prepared by method II owned the basic features of SBA-15 presented by XRD, N₂ adsorption-desorption and HRTEM images. The species in the samples should be further associating with each other and crystallizing in the during of calcination. With the increase in Zr/Si, the regularity of the samples gradually descended, and BET surface area and pore volume decreased. The fact that the regularity of the samples prepared by method II was lower and the strength of tetragonal ZrO₂ characteristic peaks was higher than that of method I indicated the amount of Zr loaded was higher by method II than method I . The pore size distributions presented the materials had double mesoporous, and the proportion of about 3.5nm pores was associated with the amount of the Zr loaded. The type of acid sites was the same as that of method I , B and L, and the amount of L was more than B. The ratio of superacid sites to medium-acid sites was about 1.82 at Zr/Si=1.3. There was noticeable ZrO₂ absorption observed at Zr/Si=0.7 in the Uv-vis scattered reflection spectra, and it confirmed the amount of Zr loaded was higher than that of method I . The acid catalysis activity of samples with about Zr/Si=1.3 was optimal, affording 180min conversion of n-pentane 57.6% which was higher than method I by 11%, and its BET surface area was 229.77m²/g, the mass content of ZrO₂ was about 65%, and the loading efficiency of Zr was more than 90%. Besides, the stability of acid catalysis performances was fine, and the optimal time of impregnating in the H₂SO₄ solution was 0.5h.3). The regularity of samples synthesized by method III was best at V_H2SO4/V_HCl=2:2 showed by low-angle XRD patterns, and the SO₄^2-/Zr-SBA-15 mesoporous superacids could be successfully synthesized in the Zr/Si scale of 0-3.0 with 0 excluded. With the increase of V_H2SO4/V_HCl ,the strength of the characteristic peaks for tetragonal ZrO₂ at Zr/Si=0.3 were gradually fell down to that for amorphous structure at V_H2SO4/V_HCl=∞and the difference of intensity of ZrO₂ characteristic peaks gradually reduced with the increase in Zr/Si. When the V_H2SO4/V_HCl was constant, the regularity of samples descended and the intensity of tetragonal ZrO₂ characteristic peaks enhanced with increase in Zr/Si. Compared with method II, the regularity of samples was lower, and the intensity of tetragonal ZrO₂ characteristic peaks were higher and wider than the former, which possibly was owing to the SO₄^2- effects of stabilizing tetragonal ZrO₂ and dispersing the ZrO₂ particles.Further, the samples had regular porous structure presented by N₂ adsorption-desorption and HRTEM. With increase in V_H2SO4/V_HCl, BET surface area and pore volume raised at the same Zr/Si, and with Zr/Si grew, regularity, BET surface area and pore volume descended and reduced, respectively, at the same V_H2SO4/V_HCl, which were explained by the results of elements analysis. Pore size distribution was characteristic of double mesoporous constitution.The samples synthesized by method III had B and L acid sites, and the ratio of L/B gradually decreased with increase in V_H2SO4/V_HCl at the same Zr/Si, which was in agreement with that of the ratio H/L of superacid sites to medium-acid sites and indicated the superacid sites were mainly offered by L acid sites. There was not noticeable tetragonal ZrO₂ absorption peak observed in the Uv-visible scattered reflection spectra which proved that most of the Zr species were incorporated into the framework of mesoporous materials and highly diffused.The results of reactions showed us that the samples with Zr/Si=1.5, V_H2SO4/V_HCl=∞and 312.14 m²/g BET surface area had the best acid catalysis activity, and the 180min conversion of n-pentane was 44.8%.In a word, the acid catalysis activity of samples prepared by method III were lower than other two methods, but it has superiority of simpler procedures and shorter preparation period, therefore it will be perfected by further researches.,...