Preparation And Token Of Mesoporous Molecular Sieves MCM-41 From Sepiolite

Mesoporous molecular sieves have attracted considerable attention in the region of chemical engineering,industrial catalysis,biomedicine,environmental protection,and fabrication of novel nano-object materials due to high surface area,well-defined and adjustable pore structure.The hydrothermal method is the most prevailing and effective route for synthesis of mesoporous sieves.The traditional silica-containing molecular sieves are prepared using water glass,tetraethoxysilane (TEOS) as silica sources,the resource consumption is expensive.Rencently, attention has been paid to a new process to prepare mesoporous silica from natural clays.The mesoporous molecular sieve MCM-41 was synthesized by hydrothermal synthesis method with post-Mg-extraction sepiolite as silica source and hexadecyl-trimethylammonium bromide (CTAB) as template. The mixture was stirred vigorously. The pH value of the slurry was adjusted by NaOH solution. The resultant slurry was transferred into a Teflon-lined autoclave and reacted under appropriate conditions. After crystallization, the solid product was filtrated, washed with large amounts of warm deionized water and dried, and finally calcined at 540℃for 4 h in air with a heating rate of 3℃/min to obtain MCM-41. The products were characterized by SAXRD, BET,DSC,TEM,SEM and IR. In this paper, the effects of crystallization conditions (crystallization time, pH value, crystallization temperature and the ratio of CTAB/SiO₂) on the formation of MCM-41, the reaction mechanism,the thermal stability and surface acidity were investigated. The results are as follows:(1) The effects of crystallization conditions on the formation of MCM-41 were:â‘ the hexagonal MCM-41 were formed in the conditions of pH=6.0～12.0 (crystallization temperature 100℃,crystallization time 24h, SiO₂:CTAB=1:0.2). The products prepared in the range of pH8.0～11.0 were ordered. The results of N₂ adsorption show that the products have higher BET surface area and pore volume.â‘¡MCM-41 was formed with CTAB/SiO₂ in the range of 0.05～0.6. It was favored for the synthesize of MCM-41 in the solution with higher template concentration.â‘¢MCM-41 could be formed at the range of 65℃～140℃. Obviously, the promotion of crystallization appeared when the crystallization temperature was ascended from 65℃to 100℃.â‘£It was favored for the increase of relative crystallinity with prolonging crystallization time.The longer the crystallization time, the higher the specific surface area of MCM-41. The specific surface area of MCM-41 treated for 96h hydrothermally was 1036 m²/g,the pore diameter was 3.0nm.(2) Combination with SAXRD,SEM and IR spectra, the results reveal that there is≡Si-OH on the surface of post-Mg-extraction sepiolite, the hydroxyl of≡Si-OH was exchangeable. The CTA⁺ exchanged with exchangeable H+ and formed a≡SiO—CTA⁺ complex during crystallization, and then condensed to form ordered mesoporous materials though hydrothermal treatment, and MCM-41 came into being after calcination. It was in accord with LCT mechanism.(3) DSC and XRD results show that Mg content was very important to the thermal stability of MCM-41.The synthesized MCM-41 has higher thermal stability with fewer Mg content (0%, 0.36%) , the collapsed temperature was about 1030℃; in the contrast, it has lower thermal stability with higher Mg content (2.27%), the structure was collapsed at about 800℃. The surface acidity of MCM-41 was measured by IR after the MCM-41 was treated with 1 mol/L NH4C1 solution and treated by pyridine subsequently.The results show that the samples have a strong absorption band at 1448cm^-1 assigned to Lewis acidic centres. The sample containing Mg ions has a small quantity of Bronsted acidic centre at about 1542cm^-1, and Lewis acidic centre still appeared strong absorption band.