New Strategies For Synthesizing Porous Materials

The inorganic porous materials have been paid much attention in catalysis,adsorption and separation, ion exchange and environmental processing field due totheir unique properties for high specific surface area, high porosity and open structure.In the last decade, mesoporous materials have become a fresh field, because of itslarge pore size (pore size between2nm-50nm), easy modification and othercharacteristics of the material, making mesoporous materials have greatapplication prospects.However, with the deeper research, many scientists found that lots of aspectsshould been improved before their practical applications: common poresize of mesoporous materials, in fact, are mostly distributed between2nm to10nm,mesoporous materials with large pore size are required for macromolecules catalyticreaction or adsorption and separation of biological macromolecules; mesoporousmaterials with a single pore size can no longer meet the needs; stability is thekey problem to limit the practical application of mesoporous materials;solvothermal synthesis inevitable use of solvents, not only increased the synthesisof cost, after getting the product, the formation of the liquid waste is still to beresolved.The present project try to obtain porous materials with different properties anddifferent pore sizes by increasing the temperature, adding inorganic salts underhigh-temperature system,"solid-phase" synthesis, changing the calcinationtemperature. The mechanism and the catalytic and adsorptive properties of theproducts are also studied systematically in present project. First, in order to obtain the meoporous materials with large poresize, scientists have done a lot of attempts. For example, by changing the carbonnumber of the surfactants, BJH pore diameter of MCM-41can be tuned from1.6to4.2nm. By addition of hydrophobic organic swelling agents such as trimethylbenzene(TMB) and dodecane in the system, OMSs with large pore sizes have beensuccessfully prepared. By increasing the preparation temperature, the PEO blocks ofpolymer surfactants become hydrophobic and retract from the silicate walls, whichresults in OMSs with larger pore sizes and thin walls. Among these routes, increasingthe preparation temperature for synthesis of OMSs with large pore sizes is simple.However, this route is always performed below140°C, and higher temperature (>140°C) is traditionally regarded as unfavorable condition for the existence of surfactants.Recently, we have reported direct hydrothermal synthesis of OMSs (SBA-15andMCM-41) at high temperatures (160-180°C) using solo hydrocarbon surfactant suchas P123or CTAB, and the mechanism for the formation of ordered mesostruture isreconsidered. It is suggested that the surfactants play an important role during thecooperative self-assembly of the silicates with surfactants in the initial stage for theformation of basic mesostructure, and surfactants are not indispensable any more inthe following hydrothermal treatment at high tempeatures. This mechanism offers anopportunity to adjust pore sizes of OMSs in a wide range by changing the synthetictemperatures regardless of the decomposition of surfactants in hydrothermaltreatments at high temperatures. We demonstrate here an example for synthesis ofordered cubic mesoporous silicas with very large pore sizes (LP-SBA-16) at very hightemperatures (180-220°C).In addition, three-dimensional ordered mesoporous silicas SBA-15withcontrollable large mesoporosity have been successfully synthesized athigh-temperature (200°C) in the presence of inorganic salts (KCl, MgCl₂, AlCl₃).Second, most of mesoporous aluminophosphates and heteroatom-substitutedaluminophophates synthesized in the early stage exhibit relatively low thermalstability, which strongly hinders their wide applications in catalysis and adsorption.To overcome this problem, many approaches such as adjusting surfactants, preparing inorganic precursors, using "acid-base pair" mechanism etc, have been employed tosuccessfully synthesize thermally stable mesoporous aluminophosphates underhydrothermal and solvothermal conditions. Normally, ordered mesoporousaluminophosphates have single mesopores, and recent results confirm thathierarchically porous materials are very helpful for practical applications in catalysisand adsorption. To create hierarchically porous aluminophosphate zeolites, both smallamines and bulky organosilane surfactants have been successfully used. It isparticularly emphasized that ordered mesostructured materials including orderedmesoporous aluminophosphates are usually assembled in the presence of solventssuch as water and alcohols, and the use of the solvent always produces liquid wastesfor the environment. Recently, we have reported a solvent-free route for preparingnanosized sulfated zirconia catalysts, and we demonstrate here a "solvent-free"synthesis of thermally stable and hierarchically porous aluminophosphates andheteroatom-substituted aluminophosphates from a mixture of raw materials for thefirst time. This unique approach not only saves solvent and energy in the synthesis,but also creates hierarchical pores including micropores and mesoporous from a solosurfactant template.Furthermore, in the same solvent-free conditions, we synthesize hierarchicallyporous aluminosilicate materials using CTAB surfanctant as a solo template.Third, the pore sizes of mesoporous materials are less than30nm, ultra-mesoporous materials is an important bridge for connection mesoporous materials tomacroporous materials. The ultra-mesoporous materials have more significance inthe selective catalytic, selective adsorption and separation. We successfully synthesizean ultra-mesoporous/macroporous-crystal wall (cristobalite) materials throughadjusting the calcination temperatures. Furthermore, the large pore size of the porousmaterials could be adjusted.