Determination and tailoring the pore entrance size in ordered silicas with cage-like mesoporous structures

A practical approach to the determination of the pore entrance size in ordered silicas with cage-like mesoporous structures (OSCMSs) is proposed. A fundamental insight into the OSCMS pore connectivity is gained, including the control of the pore entrance size by post-synthesis surface modification, and by selection of appropriate synthesis temperature. These findings show a new promise for the synthesis of mesoporous solids with molecular size- and shape-selective properties.     

MCM-48-like large mesoporous silicas with tailored pore structure: facile synthesis domain in a ternary triblock copolymer-butanol-water system

Assembly of mesostructured silica using Pluronic P123 triblock copolymer (EO(20)-PO(70)-EO(20)) and n-butanol mixture is a facile synthesis route to the MCM-48-like ordered large mesoporous silicas with the cubic Iad mesostructure. The cubic phase domain is remarkably extended by controlling the amounts of butanol and silica source correspondingly. The extended phase domain allows synthesis of the mesoporous silicas with various structural characteristics. Characterization by powder X-ray diffraction, nitrogen physisorption, scanning electron microscopy, and transmission electron microscopy reveals that the cubic Iad materials possess high specific surface areas, high pore volumes, and readily tunable pore diameters in narrow distribution of sizes ranging from 4 to 12 nm. Moreover, generation of complementary pores between the two chiral channels in the gyroid Iad structure can be controlled systematically depending on synthesis conditions. Carbon replicas, using sucrose as the carbon precursor, are obtained with either the same Iad structure or I4(1)/a (or lower symmetry), depending on the controlled synthesis conditions for silica. Thus, the present discovery of the extended phase domain leads to facile synthesis of the cubic Iad silica with precise structure control, offering vast prospects for future applications of large-pore silica materials with three-dimensional pore interconnectivity.     

Pyrolysis of mesoporous silica-immobilized 1,3-diphenylpropane. Impact of pore confinement and size

Mesoporous silicas such as SBA-15 and MCM-41 are being actively investigated for potential applications in catalysis, separations, and synthesis of nanostructured materials. A new method for functionalizing these mesoporous silicas with aromatic phenols is described. The resulting novel hybrid materials possess silyl aryl ether linkages to the silica surface that are thermally stable to ca. 550 degrees C, but can be easily cleaved at room temperature with aqueous base for quantitative recovery of the organic moieties. The materials have been characterized by nitrogen physisorption, FTIR, NMR, and quantitative analysis of surface coverages. The maximum densities of 1,3-diphenylpropane (DPP) molecules that could be grafted to the surface were less than those measured on a nonporous, fumed silica (Cabosil) and were also found to decrease as a function of decreasing pore size (5.6-1.7 nm). This is a consequence of steric congestion in the pores that is magnified at the smaller pore sizes, consistent with parallel studies conducted using a conventional silylating reagent, 1,1,3,3-tetramethyldisilazane. Pyrolysis of the silica-immobilized DPP revealed that pore confinement leads to enhanced rates and altered product selectivity for this free-radical reaction compared with the nonporous silica, and the rates and selectivities also depended on pore size. The influence of confinement is discussed in terms of enhanced encounter frequencies for bimolecular reaction steps and pore surface curvature that alters the accessibility and resultant selectivity for hydrogen transfer steps.     

Organic-inorganic hybrid mesoporous silicas: functionalization, pore size, and morphology control

Topological design of mesoporous silica materials, pore architecture, pore size, and morphology are currently major issues in areas such as catalytic conversion of bulky molecules, adsorption, host-guest chemistry, etc. In this sense, we discuss the pore size-controlled mesostructure, framework functionalization, and morphology control of organic-inorganic hybrid mesoporous silicas by which we can improve the applicability of mesoporous materials. First, we explain that the sizes of hexagonal- and cubic-type pores in organic-inorganic hybrid mesoporous silicas are well controlled from 24.3 to 98.0 A by the direct micelle-control method using an organosilica precursor and surfactants with different alkyl chain lengths or triblock copolymers as templates and swelling agents incorporated in the formed micelles. Second, we describe that organic-inorganic hybrid mesoporous materials with various functional groups form various external morphologies such as rod, cauliflower, film, rope, spheroid, monolith, and fiber shapes. Third, we discuss that transition metals (Ti and Ru) and rare-earth ions (Eu(3+) and Tb(3+)) are used to modify organic-inorganic hybrid mesoporous silica materials. Such hybrid mesoporous silica materials are expected to be applied as excellent catalysts for organic reactions, photocatalysis, optical devices, etc.     

Nanoporous silica films containing aluminum and titanium

By utilizing surfactant aggregates as supramolecular templates, mesoporous and mesostructured silicas with highly ordered structures became available. The resulting mesoporous silicas are promising candidates to host various photo- and electro-active species along with catalytically active species, due to their large and controllable pore sizes, highly ordered pore arrangements with low dimensional geometries, and reactive surfaces. We have developed the rapid solvent evaporation method, which is a modified sol-gel process, for synthesizing the mesostructured silica-surfactant films as well as the mesoporous silica films. Supported thin films, self-standing films and bubbles of mesoporous silicas have been synthesized by the rapid solvent evaporation method. The microstructures of the films have also been successfully controlled by changing the synthetic conditions. Taking advantage of the ease of synthetic operation and the transparency and homogeneity of the resulting materials, we have been interested in the introduction of functional units into the mesostructured materials. This paper reports the synthesis of transparent films of titanium- and aluminum-containing nanoporous silicas to modify the surface properties (such as adsorptive and catalytic) of nanoporous silicas. The incorporation of Al led to the formation of cation exchange or acidic sites on the mesopore surface, as revealed by the cationic dye adsorption experiments. The photocatalytic reactions of the Ti-containing nanoporous silica films were also examined.     

A general strategy for the rational design of size-selective mesoporous catalysts

A series of functionalized mesoporous silicas with cagelike pore topology has been synthesized and screened for size-selective catalytic transformations. The aluminum-catalyzed Meerwein-Ponndorf-Verley (MPV) reduction of differently sized aromatic aldehydes (benzaldehyde and 1-pyrenecarbox-aldehyde) has been investigated as a test reaction. The catalysts were synthesized in a two-step grafting sequence comprising pore-size engineering of mesoporous silicas (SBA-1, SBA-2, SBA-16) with long-chain alkyl dimethylaminosilanes and surface organoaluminum chemistry with triethylaluminum [{Al(CH(2)CH(3))3}2]. Size-selective reaction behavior was found for small pore SBA-1 materials, and the selectivity could be efficiently tuned by selecting a silylating reagent of appropriate size. The results are compared with the catalytic performance of a large-pore periodic mesoporous organosilica PMO[SBA-1] and the nonporous high-surface-area silicas Aerosil 300/380.     

Assessment of ordered and complementary pore volumes in polymer-templated mesoporous silicas and organosilicas

A method to determine the volumes of ordered mesopores and complementary small pores in polymer-templated ordered mesoporous silicas and organosilicas is proposed on the basis of the existing relation between the pore width and unit cell values obtained by the XRD structure modeling and the adsorption pore volume.     

Effect of pore packing defects in 2-d ordered mesoporous carbons on ionic transport

Ordered mesoporous materials show great importance in energy, environmental, and chemical engineering. The diffusion of guest species in mesoporous networks plays an important role in these applications, especially for energy storage, such as supercapacitors based on ordered mesoporous carbons (OMCs). The ion diffusion behavior in two different 2-D hexagonal OMCs was investigated by using cyclic voltametry and electrochemical impedance spectroscopy. In addition, transmission electron microscopy, small-angle X-ray diffraction, and nitrogen cryosorption methods were used to study the pore structure variations of these two OMCs. It was found that, for the OMC with defective pore channels (termed as pore packing defects), the gravimetric capacitance was greatly decayed when the voltage scan rate was increased. The experimental results suggest that, for the ion diffusion in 2-D hexagonal OMCs with similar mesopore size distribution, the pore packing defect is a dominant dynamic factor.     

Mesoporous Silica Layer: Preparation and Opportunity

Periodic mesoporous silicas, which were prepared from silica‐surfactant mesostructured materials, have been investigated for a wide range of application due to their very large surface area, high porosity, pore size uniformity and variation, periodic pore arrangement and possible pore surface modification, after the pioneering papers on the formation of mesoporous silicas (MCM‐41 and FSM‐16). Morphosyntheses from such macroscopic morphologies as bulk monolith and film to nanoscopic ones, nanoparticles and their stable suspension, make mesoporous materials more attractive for applications and detailed characterization. Mesoporous silicas have been studied initially for such applications as adsorbent and catalyst, and more recently, optical, electronic, and bio‐related applications have been investigated. This review summarizes the studies on mesoporous silica film to highlight the present status and future of the preparation, characterization and application of the mesoporous silica film.     

Effect of salts on synthesis of mesoporous materials with mixed cationic and anionic surfactants as templates

Mesoporous silica materials were synthesized using tetraеthoxysilane as precursor and liquid crystals formed in aqueous mixtures of cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) as templates, without and with the addition of NaBr or Na₂SO₄. For this purpose, the formation of liquid crystals as a function of the ratio of CTAB and SDS under different conditions was studied. It was found that liquid crystals formed in the mixed system of CTAB and SDS at certain mixing ratios are well-structured templates for the synthesis of mesoporous silicas. The synthesized silica materials were characterized by transmission electron microscope and nitrogen adsorption/desorption analysis. The pore size of mesoporous silicas could be controlled between 3 to 6 nm by simply changing the concentration of NaBr in solution. The mesoporous silicas exhibited lamellar structure and the order of structural arrangement was promoted with addition of NaBr. However, addition of Na₂SO₄ led to ink-bottle type pores of mesoporous silica with a narrow pore size distribution of around 2 nm and a higher specific surface area of 610 m² g^–1.     

A "teardown" method to create large mesotunnels on the pore walls of ordered mesoporous silica

A "teardown" method to create large mesotunnels (approximately 9 nm) on the pore walls of ordered mesoporous silicas is demonstrated by digesting the organic constituents from polymer-silicate nanocomposites. The ordered mesostructured polymer-silicate composites were first obtained via the evaporation-induced triconstituent co-assembly method by using a low-molecular-weight phenolic resin (resols) as an organic precursor; prehydrolyzed TEOS as an inorganic precursor, and triblock copolymer F127 as a template. All of organic components including F127 and phenolic resins are removed by the microwave digestion (MWD) method from mesostructured polymer-silica composites. While the removal of triblock copolymer F127 generates main pore channels, the phenolic resins can also be torn down from the pore walls, yielding mesotunnels between the channels. The resulting silica products exhibit ordered 2-D hexagonal mesostructure, large pore volume (up to 1.92 cm(3)/g), and very large pore size (up to 22.9 nm), which is even larger than their mesostructural cell parameter (14.2 nm). TEM images confirm the existence of mesotunnels on the silica pore walls. FT-IR and (29)Si solid-state NMR results reveal that these silica products have a large number of silanol groups.     

Synthesis of ordered and disordered silicas with uniform pores on the border between micropore and mesopore regions using short double-chain surfactants.

Silica molecular sieves with uniform pores on the borderline between micropore (diameter <2 nm) and mesopore (from 2 to 50 nm) ranges were synthesized by a novel method using judiciously chosen mixtures of short double-chain alkylammonium surfactants. These silicas were characterized using X-ray diffraction (XRD), thermogravimetry, and nitrogen and argon adsorption. The calcined materials exhibited either 2-dimensional (2-D) hexagonal or disordered structures with XRD interplanar spacing from 2.51 to 2.93 nm, including the value of as small as 2.69 nm for highly ordered 2-D hexagonal silica. The dependence of the pore size and surfactant content on the surfactant chain length provided strong evidence for supramolecular templating being operative in the formation of small-pore silicas, even for the surfactant chain length of six carbon atoms. Both hexagonally ordered and disordered calcined samples were shown to exhibit narrow pore size distributions with maxima in the range from 1.96 to 2.61 nm (reliably evaluated on the basis of the unit-cell dimension and pore volume for 2-D hexagonal materials, and calculated using a properly calibrated procedure), tailored by the surfactant chain length. The samples exhibited primary pore volumes from 0.28 to 0.54 cm(3) g(-1) and specific surface areas from 730 to 930 m(2) g(-1). Because of their small yet uniform pore size and large specific surface area, the silicas reported herein promise to be useful in applications in adsorption and catalysis. Adsorption studies of these materials provided a unique new insight into the pore-filling mechanism for small-pore materials. Moreover, the approach proposed herein is expected to facilitate the synthesis of not only small-pore silicas but also materials with other framework compositions, thus largely contributing to bridging the gap in attainable pore sizes between micropore and mesopore ranges.     

以EDTANa2为矿化剂近中性条件下室温合成介孔二氧化硅

利用EDTANa2作为矿化剂, 在近中性条件下于室温合成了厚壁(约3 nm)蠕虫介孔分子筛. 在反应物配比为n(CTAB):n(TEOS):n(EDTANa2):n(H2O)=(0.05~0.2):(0.1~2):(0.1~0.5):(100~500)范围内都能得到介孔二氧化硅. 以XRD、N2吸附、FTIR、SEM和TEM详细考察了体系配比和温度对介孔二氧化硅结构和形貌的影响, 发现CTAB用量越大, 介孔d值相应增大. 温度对介孔二氧化硅的结构和形貌有很大的影响, 温度在2~15 ℃范围内都能生成孔径分布较规则的介孔, 介孔材料的形貌随温度的升高由空心管变成小颗粒聚集体; 合成的介孔分子筛中的模板剂可以通过乙醇萃取的方式除去. 提出了介孔的生成遵从(S+-E-)0-I0中性模板机理.     

Effect of pore structure in mesoporous silicas on VOC dynamic adsorption/desorption performance

The dynamic adsorption/desorption behavior of volatile organic compounds (VOCs) such as toluene (C7H8) and benzene (C6H6) was evaluated for three kinds of mesoporous silicas of SBA-15, all having almost the same mesopore size of ca. 5.7 nm, and a MCM-41 silica with a smaller pore size of 2.1 nm using a continuous three-step test. The fiberlike SBA-15 silica exhibited exceptionally good breakthrough behavior, a higher VOC capacity, and easier desorption. The fiberlike silica was composed through the catenation of rodlike particles. The rodlike silicas, by comparison, were proven to be less useful in dynamic adsorption processes because of lower dynamic VOC capacities despite having comparative porous parameters with the fiberlike silica. The large dynamic VOC capacity of the fiberlike silica was attributed to the presence of a bimodal pore system consisting of longer, one-dimensional mesopore channels connected by complementary micropores.     

Post-treatment and characterization of novel luminescent hybrid bimodal mesoporous silicas

A novel luminescent hybrid bimodal mesoporous silicas (LHBMS) were synthesized via grafting 1,8-Naphthalic anhydride into the pore channels of bimodal mesoporous silicas (BMMs) for the first time. The resulting samples were characterized by powder X-ray diffraction (XRD), N₂ adsorption/desorption measurement, Fourier transform infrared spectroscopy (FT-IR), Transmission electron microscopy (TEM), UV-vis absorption spectroscopy, and Photoluminescence spectroscopy (PL). The results show that 1,8-Naphthalic anhydride organic groups have been successfully introduced into the mesopores of the BMMs and the hybrid silicas are of bimodal mesoporous structure with the ordered small mesopores of around 3 nm and the large mesopores of uniform intra-nanoparticle. The excellent photoluminescent performance of LHBMS has a blue shift compared to that of 2-[3-(triethoxysilyl) propyl-1 H-Benz [de]isoquinoline-1, 3(2 H)-dione, suggesting the existence of the quantum confinement effectiveness.     

有机-无机杂化氧化硅基介孔材料

有机基团可以通过嫁接或共聚的方法引入到氧化硅基介孔材料的孔表面或材料的骨架中,形成表面结合型和桥键型两大类有机-无机杂化氧化硅基介孔材料.本文综述了有机-无机杂化氧化硅基介孔材料的最新研究进展,介绍了其合成方法、应用及潜在的应用领域,详细总结了目前已报道的有机-无机杂化氧化硅基介孔材料的种类,展望了桥键型有机-无机杂化氧化硅基介孔材料的发展及应用前景.     

Study on the capacity of mesoporous alumina with different pore sizes absorb silane inhibitors

By modifying and optimizing the procedures, which were well described and understood for the synthesis of macroporous alumina, mesoporous alumina–based film has been successfully prepared, In this paper, the orderly mesoporous Al₂O₃ thin film was prepared by electrochemical workstation, and via supported N‐octyltriethoxysilane (NOS) coupling agent, corrosion inhibitors be loaded into the different pore sizes of mesoporous alumina films. The physicochemical properties of this thin inhibitors carrier film were characterized. Corrosion resistance of mesoporous alumina and honeycomb ceramic macroporous alumina were compared; the conclusion shows that mesoporous alumina film can be used as good corrosion inhibitors carrier and bring out a high‐efficiency inhibition result. Simultaneously, by compared with corrosion inhibition of different pore sizes (20‐50 nm) mesoporous alumina who absorbed NOS, and a general relationship between the different mesoporous alumina pore sizes and the adsorption capacity of NOS was obtained.     

基于长链离子液体模板合成Fe(Co、Ni)-MCM-48

以两亲长链离子液体(氯化-1-十六烷基-3-甲基咪唑)为模板剂,采用水热合成方法,分别合成了含铁、钴和镍的立方介孔分子筛MCM-48。通过XRD、TEM、ICP-AES、FTIR、UV-Vis和N₂吸附/脱附实验对其结构和形态进行了表征,并考察了钴掺杂量和模板剂用量对合成立方相结构的影响。结果表明,用该方法合成的试样具有较高的比表面积和规则的立方介孔孔道结构等特征;相对于铁和镍的掺杂试样,钴物种能以四面体的结构稳定地存在于立方介孔分子筛骨架中。     

Nanoparticulated silicas with bimodal porosity: chemical control of the pore sizes

Nanoparticulated bimodal porous silicas (NBSs) with pore systems structured at two length scales (meso- and large-meso-/macropores) have been prepared through a one-pot surfactant-assisted procedure by using a simple template agent and starting from silicon atrane complexes as hydrolytic inorganic precursors. The final bulk materials are constructed by an aggregation of pseudospherical mesoporous primary nanoparticles process, over the course of which the interparticle (textural) large pore system is generated. A fine-tuning of the procedural variables allows not only an adjustment of the processes of nucleation and growth of the primary nanoparticles but also a modulation of their subsequent aggregation. In this way, we achieve good control of the porosity of both the intra- and interparticle pore systems by managing independent variables. We analyze in particular the regulating role played by two physicochemical variables: the critical micelar concentration (cmc) of the surfactant and the dielectric constant of the reaction medium.