Synthesis And Characterization Of Hollow Spheres Of Composite Oxide With Mesopores In The Shell Wall

In recent years, there has been intense interest in the fabrication ofinorganic materials with hollow spherical structures for technologicalapplication and fundamental studies in chemistry, biotechnology, andmaterials science due to their advantages of good surface permeability, largespecific area, low density, and high mechanical stability. Inorganic materialswith hollow spherical structures have been widely used in many fields, suchas drugs delivery, protection of biologically active agents (e.g. proteins,enzymes, or DNAs), and controlled release capsules of various substances.Additionally, mesoporous materials have attracted much attention becauseof their possible uses in acting as molds for new materials syntheses, catalysts,low-dielectric coatings, and optical materials. They are proved to be animportant class of functional materials for their super performance comparedwith other porous materials. The question how to combine the advantages ofinorganic hollow spheres, mesoporous materials and binary composite oxidesis therefore very important in the research of material chemistry.There have been a number of methods for the synthesis of hollow spheres,such as nozzle-reaction system, sacrificial cores, emulsion/water extraction,and layer-by-layer self-assembly techniques. Though inorganic hollowspheres have shown many good potential properties, most of them prepared bythese methods lack thermal stability, and few of them are focuse on compositeoxides. Here we present a facile route for preparing hollow SiO2/Al2O3spheres by using TEOS and aluminum nitrate as silica and alumina sources, inwhich we combine a sol-gel process with an O/W/O microemulsion. PEGmolecules acts as an additive in this study. When the self-assembly process isgoing, PEG molecules uniformly disperse in the shell wall, thus a large amoutof uniform mesopores are formed in the shell wall after remove of PEG bycalcination at 550 °C. The mesopores in the shell wall make the hollowSiO2/Al2O3 spheres maintain their hollow structures after calcination,indicating the hollow SiO2/Al2O3 spheres have highly thermal stability basedon this approach. Characterizaion results show that the particle size of thehollow spheres and pore size of mesopores in the shell wall can be controlledby many factors, such as emulsion temperature, stirring speed, averagemoecular weight of PEG, the amounts of HNO3 and Al(NO3)3 in theprecursors, and the oil phase. The particle size of hollow spheres increasealong with the increase of stirring speed and the pore size of mesopores in theshell wall increase along with the increase of average molecular weight ofPEG. The sol viscosity is found to have a significant influence on theformation of hollow spheres. More recently, inorganic materials with hollow structures have attractedmore attention on controlled release of drugs. In application of controlledrelease of drugs the modification of inorganic materilals with organic reagentsis needed to realize the better controllable property. It is well known that thepresence of a large amount of silanol groups is an important factor that theinorganic materials can be further modified with organic reagents. However,most of inorganic spheres with hollow structures have less silanol groups afterremove of the templates or organic components by calcination becaue of thecondensation of silanols groups at high temperature. We prepare hollowSiO2/Al2O3 spheres with uniform mesopores in the shell wall and a largeamount of silanol groups base on the above-mentioned approach. It is the onlydifference that we wash the spheres with acetone, ethanol and boiling water toremove the organic componets in place of calcination. Characterization resultsindicate that although the hollow spheres show somewhat distorted shape theintact hollow structure and mesopores in the shell wall are still obvious. Moreimportantly, a large amount of silanol groups are retained for the absence ofcalcination. Additionally, we also prepare hollow SiO2/TiO2 spheres withuniform mesopores in the shell wall and a large amount of silanol groups bythe same method. Characterizaion results are very similar to those of hollowSiO2/Al2O3 spheres. Acetyl acetone is used as chelated reagent to control thehydrolysis of titanium source (TBOT). UV-vis spectra indicate thatfour-coordinated environment of titanium species similar to TS-1 can beobtained by controlling the hydrolysis of TBOT. Thus, the hollow SiO2/TiO2spheres we prepare by washing with boiling water have these advantages ofuniform mesopores in the shell wall, a large amount of silanol groups andfour-coordinated environment of titanium species. To realize the application of hollow spheres in catalysis, we synthesizestable hollow ZrO2/SiO2 spheres with mesopores in the shell wall by a sol-gelprocess in an oil/water/oil microemulsion. Here, zirconium nitrate and octaneare used as zirconium source and oil phase, respectively. The samples arecharacterized by transmission polarized light microscopy, SEM, TEM, and N2adsorption-desorption isotherms. The characterization results indicate that alarge number of mesopores are also present in the shell wall of the calcinedhollow ZrO2/SiO2 spheres, and that the diameter and shell thickness are ca.50and 15 μm, respectively. The hollow spheres exhibit high thermal stability andremain intact spherical structures even after calcination at 550 °C for 8 h,which is similar to hollow SiO2/Al2O3 spheres. In the cracking reactions ofcumene and 1,3,5-triisopropylbenzene the sulfated hollow ZrO2/SiO2 spheresshow very high catalytic activities. Especially, the higher catalytic activity of1,3,5-triisopropylbenzene cracking suggests the potential application incracking of bulky molecules for the presence of mesopores. A possibleformation mechanism of hollow spheres of binary composite oxide is alsoproposed. Additionally, it is proved that this approach for preparing hollow spheresof composite oxides by combining a sol-gel process with an O/W/Omicroemulsion is appropriate for the synthesis of Si-Al-Zr, Si-Al-Ti 和Si-Zr-Ti composite oxides. The characterization results are very similar tothose of hollow spheres of binary composite oxides. Although there are uniform mesopores in the shell wall of hollow spheresprepared by our approach, the mesopores are disordered. It is well known tous that mesoporous materials with ordered mesopores are proved to be animportant class of functional materials for their super performance comparedwith other porous materials. To obtain hollow spheres with ordered mesoporesin the shell wall, we present a novel route for combining sol-gel process andO/W microemulsion with hydrothermal technique. We successfully synthesizehollow SiO2 spheres with ordered mesopores in the shell wall by using CTABas template. Characterization results indicate the sample has cubic symmetry...