Synthesis and Property of Hierarchically Mesoporous Silica
|Course||Materials Physics and Chemistry|
|Keywords||hierarchically pore structure mesoporous silica dynamic template mesomorphous complexes single crystal alkylation of toluene and benzyl alcohol cationic surfactant anionic polyelectrolyte hydridosilica nanoparticles|
Hierarchically porous silica materials with well-defined morphologies havebeen of growing interest, because of their potential applications in catalysis,adsorption, separation, and biomedical systems. In the past decades, a variety oftemplating approaches have been proposed to synthesize hierarchically porous silicamaterials, and colloidal particles, polymers, emulsion droplets, and surfactants havebeen employed to create porous structures with different sizes. Bimodal or trimodalporous materials, including micro-meso, meso-meso, meso-macro, and evenmicro-meso-macro, have been reported. However, in most cases, the control ofwell-ordered mesostructure and well-defined morphology was difficult, because ofthe mixing of different types of templates in the syntheses. In this thesis, we aim todesign facile and effective preparation methods for synthesis of the hierarchicallyporous silica materials with well-defined morphologies, size and functionality andevaluate the application of the synthesized hierarchically porous silica materials inbioimmobilization and catalysis. The main contents are as follows:1. Synthesis of hierarchically mesoporous silica and periodic mesoporousorganosilica （PMO） and their bioimmobilized propertiesIn nature, some biominerals （for instance, sea urchin spines） possess complexand spongelike morphology but still remain single crystalline. The formationmechanism of the “porous” single crystals of biominerals can be ascribed to atime-dependent cooperative organization with complex organic matrix as dynamictemplates. Inspired by this, we try to explore facile and effective preparation methodsfor the synthesis of hierarchically porous silica materials with well-definedmorphologies via a “dynamic template mechanism”. In this section,single-crystal-like, hierarchically mesoporous silica SBA-1was synthesized withanionic polymer PAA and cationic surfactant hexadecylpyridinium chloride （CPC） mesomorphous complexes as dynamic template under alkaline condition. Scanningelectron microscopy （SEM）, power X-ray diffraction （XRD）, small angle X-rayscattering （SAXS）, transmission electron microscopy （TEM）, nitrogen adsorptionanalyzer were employed to characterize the morphology and mesostrucuture of thesynthesized hierarchically mesoporous silica. The results indicated that the obtainedmaterials possess global mesopore size of SBA-1and secondary interstitialnanoporous pores. The presence of a large amount of foamlike secondary nanoporesdid not disturb the single-crystal characteristic of the SBA-1particles. By adjustingthe amount of PAA in the synthesis, the silica particles involved fromsubmicrometer-sized spherical particles to micrometer-sized drum-like polyhedronparticles. The synthesized hierarchically mesoporous silica （SBA-1-4） was chosen asa typical support for the bioimmobilization of the papain and the digestibility of bullserum albumin （BSA） by the papain immobilized hierarchically mesoporous silicawas also evaluated. The results demonstrated that the papain immobilizedhierarchically mesoporous silica reveals a high enzyme immobilized amount andgood digestibility compare to that of the MCM-41. By means of pore-expanding with1,3,5-trimethylbenzene, the mesopore size of the SBA-1could be easily tuned from3.1nm to5.0nm, while the morphology of the particles was not changed. Moreover,hierarchically mesoporous PMO were also prepared by employing bridgedsilsesquioxane species （RO）3Si-CH2-CH2-Si（RO）3as a silica precursor. In order tostudy the bioimmobilization ability of the hierarchically nanoporous SBA-1andH-PMO, SBA-1-7.5and H-PMO-3.6were used as typical examples in theimmobilization of lysozyme and BSA. Due to the fact that PMO has not only ahierarchically nanoporous structure but also uniformly distributed bridged-ethylenegroups that provide a hydrophobic surface, the H-PMO showed a good adsorptioncapacity and rapid adsorption dynamics. To further explore the bioimmobilizationability of the hierarchically nanoporous SBA-1-7.5and H-PMO-3.6, theco-immobilization ability of SBA-1-7.5and H-PMO-3.6for two kinds of enzymewith different size were studied. Compare to that of the silica materials with singlepore structure, the hierarchical pore structure of the SBA-1-7.5makes it reveal preferable co-immobilization ability.2. Synthesis of hierarchically structured heteroatom molecular sieves andtheir propertiesHierarchically porous materials with well-defined morphologies have been ofgrowing interest in catalysis system, because of their hierarchical structure, highspecific surface area, high pore volume and superior thermal stability. Nevertheless,a main drawback of these materials with pure silica framework is that they oftendisplay relatively poor hydrothermal stability, which will restrain their extensiveindustrial application. In this section, mesoporous aluminosilicates Al-SBA-1withhierarchical pore structure have been successfully synthesized under alkalinecondition at120°C by employing organic mesomorphous complexes ofpolyelectrolyte （poly（acrylic acid）（PAA）） and cationic surfactant （hexadecylpyridinium chloride （CPC）） as template, aluminum isopropoxide as aluminumsource. The prepared Al-SBA-1exhibited submicrometer spherical morphology withthe presence of interstitial nanopores inside the submicrospheres as well as orderedalignment of mesopores through the whole particle, indicating that the incorporationof Al into the silica framework did not significantly disturb the interior texture andpore structure of the SBA-1. The molar ratio of the Si/Al could be effectivelycontrolled by adjusting the amount of the aluminum in the synthesis, and the Alcontaining could be as high as about20%（Si/Al=5）. In order to investigate theacidic properties of the obtained Al-SBA-1materials,27Al MAS NMR andNH3-TPD have been employed to characterize the synthesized Al-SBA-1materials.It is demonstrated that the incorporated Al atoms in the Al-SBA-1framework mainlyexist in the form of Al（IV） and the acid strength increase with the increasing of theAl content. Hydrothermal treatment showed that the Al-SBA-1materials exhibited ahigh hydrothermal stability and remained stable even after being treated in boilingwater for10days.The catalytic activity of the Al-SBA-1materials was investigated by employingAl-SBA-1as catalyst, commercial HZSM-5as reference sample, Friedel–Crafts alkylation of toluene with benzyl alcohol as a model reaction. Compared withcommercial HZSM-5which possess a strong acidity but a micropore structure, theAl-SBA-1exhibited higher catalytic activity due to the hierarchically mesoporousstructure. Moreover, the results indicated that the more acidic sites could efficientlypromote the catalytic reaction rate. Recycling experiment shows that the catalyticactivity of the Al-SBA-1has not significantly reduced after reused four times in thealkylation toluene with benzyl alcohol.3. Synthesis of hierarchically mesoporous silica nanoparticles andmesoporous hydridosilica nanoparticles and their propertiesMesoporous silica nanoparticles （MSNs） have received much attention inpractical fields, such as catalysis, adsorption, separation and biomedicineapplications due to their unique chemical, magnetic and optical properties, and highlevel of biocompatibility. It has been demonstrated that the control of structure,morphology, size, and surface properties of MSNs is important for their biomedicalapplications. However, the reported hierarchically mesoporous silica materials oftenhave relative large particle sizes （sub-micrometre or micrometer）. In this section,hierarchically structured single-crystal mesoporous silica SBA-1nanoparticles（HMSNs） were synthesized by using a cationic surfactant and anionic polymerpoly（acrylic acid）（PAA） mesomorphous complexes as a template, tetraethoxysilaneas silica source via the dilute solution method. The effect of different cationicsurfactant to the morphology and the interior structure of the nanoparticles wereinvestigated. The results indicated that hierarchically structured mesoporous silicaSBA-1nanoparticles could be successfully prepared by using CPC or CTAB assurfactant. The size of the nanoparticles was in the range of50-300nm, thesecondary interstitial nanopore and ordered mesopore could be observed. Moreover,mesoporous silica SBA-1nanoparticles synthesized with different silica source wasalso explored. It is found that when tetrapropyl orthosilicate was chosen as silicasource, only the single pore nanoparticles could be obtained and the particle size wasabout75nm. This was mainly due to that hydrolysis rate of tetrapropyl orthosilicate which was slower than that of the tetraethoxysilane. During the hydrolysis process,the polymer of PAA was completely extruded, and the PAA could not play a role asthe secondary template. The bioimmobilization ability of the synthesized HMSNswas investigated by chosen the lysozyme as a typical enzyme. The results revealedthat the HMSNs exhibit a rapid immobilization rates and high immobilized amountsthan that of the MSNs synthesized with tetrapropylorthosilicate as the silica source.The hydridosilica with Si-H groups could be transformed intophotoluminescent materials at high temperatures, and have potential applications insolar energy systems. It was reported that the mesoporous hydridosilica particleswith size of100-400nm could be prepared under acidic condition by using triblockcopolymer P123as surfactant, triethoxysilane as silica source. In this section, thefluorocarbon surfactant （FC-04） was used to control the growth of hydridosilicaparticles and the effect of FC-04to the morphology, particle size and meso-structureof the mesoporous hydrosilica was investigated. By adjusting the amount of FC-04,the morphology of the particles involved from solid to hollow, and the sizedecreased from few hundreds of nanometers to50nm. Moreover, by using thehollow mesoporous hydridosilica nanoparticle as support and reducing agent,mesoporous hydridosilica materials supported noble metal nanoparticles catalystswere prepared via one pot method. The performance of mesoporous hydridosilicamaterials supported noble metal nanoparticles catalysts were investigated usingcatalytic reduction of4-nitrophenol as a model reaction and they exhibited excellentcatalytic activity.