Synthesis And Characterization Of Novel Mesoporous Or Supermicroporous Silica Materials Using Semi-fluorinated Surfactants As Templates

Since the discovery of MCM-41 by Mobil scientists in 1992, mesoporousperiodic materials have attracted considerable attention for their potential use asversatile catalysts and supports. During the synthesis of mesoporous materials,a number of surfactants have been used as templates. Recently, it is reported thatfluorocarbon surfactants are also good templates for preparation of mesoporousmaterials.We show here disordered mesoporous silica materials with "worm-like" porestructure which were synthesized in both strongly acidic (JLU-11)and neutral(JLU-12) media using semi-fluorinated surfactant FSN-100 (CF₃(CF₂)₅(EO)₁₄).More importantly, when 1,3,5-trimethylbezene (TMB) is introduced into thesynthesis mixture, ordered mesoporous silica materials are successfully obtained.However, in our case, when a semi-fluorinated surfactant mixed with TMB isused as a template, obtained mesoporous silica materials (JLU-11(o) andJLU-12(o) samples) exhibit very similar pore size distribution with themesoporous silica materials without using TMB (JLU-11 and JLU-12 samples).Apparently, TMB is not a swelling agent, which is quite different from thesynthesis of MCM-41 and SBA-15. Compared with hydrocarbon surfactants inaqueous solution, TMB is difficult to dissolve in the alkyl core ofsemi-fluorinated surfactants (CF)x(EO)y, because of special hydrophobic andlipophobic properties of fluorocarbon chains in the semi-fluorinated surfactant.Therefore, TMB molecules have to interact with hydrocarbon part of thesemi-fluorinated surfactant, resulting in the change in packing parameter (g) ofthe surfactant micelle, which are related to the hydrophilic-hydrophobic ratiosbetween the EO headgroups and carbon tail groups in amphiphilic blockcopolymer surfactants.Mesoporous silica-based materials (JLU-11,JLU-12,JLU-14 and JLU-15)with tailorable size are successfully synthesized by changing crystallizationtemperatures in both strongly acidic media and neutral conditions usingsemi-fluorinated surfactants(FSN-100 (CF3(CF2)5(EO)14 and FSO-100(CF3(CF2)4(EO)10). Pore size of these materials can be changed from 1.6nm to4.0nm when the synthesis temperature is changed from room temperature to100°C. As we know, preparation of pore-controlled product is one of the purposesof mesoporous materials research. Scientists have found that when blockedoligomer is used as a surfactant, the pore size can vary regularly with thesynthesis temperature. This phenomenon is attributed to the hydrophobicity of thePEO group changes with temperature. But there is no report that control the poresize in the range of micoporous (<2nm) and mesoporous ( >2nm), and our workbring it into practice.Ordered hexagonal smaller supermicroporous silica (JLU-14L and JLU-11L)has been successfully synthesized at ultra-low temperature (0 -20 °C) bysemi-fluorinated surfactant. Supermicrcoporous materials are very important inindustrial application because they bridge the gap of microporous zeolites andmesoporous materials. These materials are expected to have the potential of sizeand shape selectivity for those organic molecules that are too large to assess tothe pores of zeolites and are too small to be separated by mesopores. It has beenreported that synthesis temperature strongly influences the pore sizes ofmesoporous materials, and lower temperatures in the synthesis generally result inthe formation of mesoporous silica materials with smaller pore sizes. Furthermore,it is worthy to note that the samples synthesized at the ultra-low temperaturescontain uniform micropores. Such hierarchically porous materials withmicro/supermicroporosity should be potentially important for fast diffusion ofreactants and products in catalytic reactions. In conclusion, theultralow-temperature synthesis is a crucial factor for the formation of orderedsmaller supermicropores and the control of microporosity in JLU-14L.Ordered mesoporous aluminosilicates designated as MAS-3 and MAS-8 withhexagonal mesopores have been hydrothermally synthesized from preformedzeolite L precursors under alkaline and strongly acidic conditions, respectively.MAS-3 and MAS-8 show good thermal and hydrothermal stability. Catalyticresults in cracking cumene show that these mesoporous aluminosilicates are moreactive than conventional mesoporous aluminosilicates (Al-MCM-41).Characterization of infrared (IR), N2 adsorption isotherms, andtemperature-programmed-desorption of ammonia (NH3-TPD) have also beenused to investigate these materials and the results indicate that the walls ofMAS-3 and MAS-8 contain the primary and secondary structural building units,which might be responsible for the high hydrothermal stability and goodactivities in catalysis. Therefore, we propose that during the preparation ofMAS-3 and MAS-8, the aluminum sites are fixed in the framework of the zeoliteL nanoclusters in the first step and are introduced into the mesoporous structurewhen the nanoclusters self-assemble with the template in the second step.