Preparation And Characterization Of Porous SiO₂ Gel And The Derived Bulky Materials

Porous SiO₂ gels have large specific surface area and pore volume.They have a potential application in adsorption,catalysis,separation,etc.In this work,a porous composite SiO₂ gel was prepared by sol-gel method.Monolith porous gel materials were prepared by mold pressing the composite gel powder as raw material.The adsorption of methylene blue（MB）in aqueous solutions by both the composite and monolithic gels was studied.The gel synthesis process does not generate waste liquid.Because the drying and pressing are convenient and efficient,the results provide new idea and method for preparing porous SiO₂ gel monoliths.Tetraethylorthosilicate（TEOS）,n-butylamine（BA）,water and ethanol were used as silica source,catalyst and co-solvents,respectively.Porous SiO₂ composite materials were rapidly prepared under ambient pressure and low temperature（≤100 ^oC）conditions.The gel material has a maximum specific surface area,pore volume and pore size of 656 m²·g^-1,1.92cm³·g^-1 and 68 nm,respectively.The structure and properties of the obtained porous SiO₂ gel materials were characterized by scanning electron microscopy（SEM）,N₂-sorption,TG-DSC,infrared spectroscopy and X-ray photoelectron spectroscopy（XPS）.The results show that the porous SiO₂ gel is composed of spherical nanoparticles.The voids between the particles constitute porous channel of the gel.The SiO₂ gel contains organic substances such as amine and solvents,which benefit the gel in plastic forming and adsorption of cationic dyes in water.The effect of adsorption time（t）,amount of adsorbent（m）,adsorption temperature（T）,initial concentration of MB（C₀）on the adsorption of MB in aqueous solution was investigated.The results show that the amine released by the composite gel in water led to an alkaline adsorbate solution（pH=8.73-10.36）and promoted the adsorption of MB by the gel.The maximum adsorption capacity of MB was 56.15 mg·g^-1 when the adsorption time,adsorbent amount,adsorption temperature,and initial concentration of MB were 3 h,1 g,25^oC,and 1000 mg·L^-1,respectively.The porous SiO₂ gel was re-used up to five times.And the adsorption at the fifth times is better than the first adsorption.The adsorption of MB by the porous SiO₂ gel fits the Langmuir model and pseudo-second-order kinetic model.Thermodynamic analysis showed that the adsorption process was spontaneous and exothermic.Porous SiO₂ gel monolith materials were prepared by conventional dry mold and isostatic pressing,respectively.The influence of molding pressure and dwell time on the preparation of porous SiO₂ gel monoliths was investigated.The mechanical strength,thermal stability and hydrothermal stability of the porous SiO₂ gel monoliths prepared by dry-pressing were characterized.The results indicate that calcination,pressing increase the mechanical strength of the porous SiO₂ gel monoliths,although companied by the reduction in surface area.Under hydrothermal conditions,amine and other organic substances can be released into solution,but the internal pore structure of the porous SiO₂ gel monoliths do not change significantly.After a calcination at 950 ^oC for 4 h,dwell time and pressure of 15 s and 34 MPa,respectively,the specific surface area,pore volume and pore size of the monolith material prepared by the dry-pressing can still reach 117 m²·g^-1,0.57 cm³·g^-1,and 17 nm.These results show that the obtained porous SiO₂ gel monoliths have very good mechanical and thermal stability,which is due to the fact that the pores inside the material were composed by the interparticle pores rather than templated by porogens.The porous SiO₂ gel monoliths prepared by the two molding methods were compared in the experiment.The experimental results showed that the porous SiO₂ gel monoliths prepared by conventional pressing have relatively large mechanical strength,specific surface area and better adsorption effect on MB.However,the porous SiO₂ gel monoliths prepared by isostatic pressing have a relatively lower thermal conductivity.The adsorption of MB by porous SiO₂ monoliths was also studied experimentally.The experimental results showed that the maximum unit adsorption capacity was 10.25 mg·g^-1when the adsorption time,the amount of adsorbent,the initial concentration of MB were 4 h,1 g,200 mg·L^-1.The adsorption of MB by the porous SiO₂ gel monoliths better fits the Freundlich and the pseudo-first-order kinetic models.The adsorption performance of porous SiO₂ monoliths is not as good as porous SiO₂ gel,but it is easy to use and recycle after application.