Tunable pores in mesoporous silica films studied using a pulsed slow positron beam

Positron annihilation lifetime spectroscopy (PALS) based on a pulsed slow positron beam was applied to study mesoporous silica films, synthesized using amphiphilic PEO–PPO–PEO triblock copolymers as structure-directing agents. The pore size depends on the loading of different templates. Larger pores were formed in silica films templated by copolymers with higher molecular-weights. Using 2-dimensional PALS, open porosity of silica films was also found to be influenced by the molecular-weight as well as the ratio of hydrophobic PPO moiety of the templates.     

Encapsulation method for the dispersion of NiO onto ordered mesoporous silica, SBA-15, using polyethylene oxide (PEO)

An effective method for dispersing NiO onto ordered mesoporous silica, SBA-15, is described. The procedure involves the use of polyethylene oxide (PEO) as an encapsulating agent. It can be expected that encapsulation between PEO and Ni2+ ions mainly involves complexes between PEO and Ni2+ ions. Both N2 adsorption/desorption isotherm and TEM analyses indicate that a 2-dimensionally hexagonal pore structure with a distinct pore symmetry (space group P6mm) is maintained throughout the procedure, even though both Ni2+ ions and PEO are present in the middle of the self-assembly of mesostructured silica. The particle size of the NiO increases slightly as a function of PEO concentration. When the ratio of PEO to the templating agent (triblock copolymer, polyethylene oxide-polypropylene oxide-polyethylene oxide) reaches a value of 4.5, ordered mesoporous silica with NiO is hardly formed.     

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.     

Mesoporous silica hybrid membranes for precise size-exclusive separation of silver nanoparticles

One-dimensional (1D) nanomaterials have unique applications due to their inherent physical properties. In this study, hexagonally ordered mesoporous silica hybrid anodic alumina membranes (AAM) were synthesized using template-guided synthesis with a number of nonionic n-alkyl-oligo(ethylene oxide), Brij-type (C(x)EO(y)), which are surfactants that have different molecular sizes and characteristics. The hexagonal mesoporous silicas are vertically aligned in the AAM channels with a predominantly columnar orientation. The hollow mesostructured silicas had tunable pore diameters varying from 3.7 to 5.1 nm. In this synthesis protocol, the surfactant molecular natures (corona/core features) are important for the controlled generation of ordered structures throughout AAM channels. The development of ultrafiltration membranes composed of silica mesostructures could be used effectively in separating silver nanoparticles (Ag NPs) in both aqueous and organic solution phases. This would be relevant to the production of well-defined Ag NPs with unique properties. To create a size-exclusive separation system of Ag NPs, we grafted hydrophobic trimethylsilyl (TMS) groups onto the inner pores of the mesoporous silica hybrid AAM. The immobilization of the TMS groups allowed the columnar mesoporous silica inside AAM to retain this inner pore order without distortion during the separation of solution-phase Ag NPs in organic solvents that may cause tortuous-pore membranes. Mesoporous TMS-silicas inside 1D AAM channels were applicable as a size-exclusive separation system to isolate organic solution-phase Ag NPs of uniform morphology and size.     

Effect of drying on the mesoporous structure of sol-gel derived silica with PPO-PEO-PPO template block copolymer

The effects of drying method on the pore structure of mesoporous silica were studied from the viewpoint of enhancing closed porosity in mesoporous silica. The mesoporous silica was prepared via a sol-gel process using polyethyleneoxide-polypropyleneoxide-polyethyleneoxide (PEO-PPO-PEO) triblock copolymer (Pluronic P123) as the structure-directing template. The closed porosity was evaluated from the apparent mass density of the sample measured by a helium pycnometer. These mesoporous silicas were also characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and nitrogen adsorption. The drying method was shown to be responsible for the finally templated mesoporous structure of the silica. More rapid drying is more preferable for enhancing the closed porosity of the mesoporous silica. The closed pores were formed by immediate immobilization of copolymer molecular assemblies in the silica matrix due to the instant removal of the solvent and solidification at higher temperatures. The drying method, mainly affecting the drying rate, is highly influential on the finally replicated mesoporous structure in silica.     

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.     

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.     

高温水热合成具有超低介电常数的规则介孔氧化硅材料

分别以高分子三嵌段共聚物P123(PEO₂₀-PPO₇₀-PEO₂₀)和F127(PEO₁₀₆-PPO₇₀-PEO₁₀₆)为模板剂, 通过高温水热法制备了具有超低介电常数的规则介孔氧化硅材料(OMSs). 当合成温度达到200℃时, 得到的产物仍可保持规则的介孔结构. X射线衍射和氮气吸附结果表明, OMSs系列材料具有规则的二维六方或体心立方介孔结构、 大的比表面积和孔容及均一的孔径分布. ²⁹Si MAS NMR分析表明, OMSs与低温(100℃)合成产物相比具有更高的骨架缩合度, 从而具有优异的水热稳定性. 由于具有大的孔容和高的骨架缩合度, OMSs表现出了超低的介电常数. 以P123为模板剂, 200℃下合成的OMS的介电常数可达1.31. OMSs作为一类稳定的超低介电常数材料, 对于绝缘材料的发展具有潜在的应用价值.     

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.     

Adsorption hysteresis of nitrogen and argon in pore networks and characterization of novel micro- and mesoporous silicas

We report results of nitrogen and argon adsorption experiments performed at 77.4 and 87.3 K on novel micro/mesoporous silica materials with morphologically different networks of mesopores embedded into microporous matrixes: SE3030 silica with worm-like cylindrical channels of mode diameter of approximately 95 angstroms, KLE silica with cage-like spheroidal pores of ca. 140 angstroms, KLE/IL silica with spheroidal pores of approximately 140 angstroms connected by cylindrical channels of approximately 26 angstroms, and, also for a comparison, on Vycor glass with a disordered network of pores of mode diameter of approximately 70 angstroms. We show that the type of hysteresis loop formed by adsorption/desorption isotherms is determined by different mechanisms of condensation and evaporation and depends upon the shape and size of pores. We demonstrate that adsorption experiments performed with different adsorptives allow for detecting and separating the effects of pore blocking/percolation and cavitation in the course of evaporation. The results confirm that cavitation-controlled evaporation occurs in ink-bottle pores with the neck size smaller than a certain critical value. In this case, the pressure of evaporation does not depend upon the neck size. In pores with larger necks, percolation-controlled evaporation occurs, as observed for nitrogen (at 77.4 K) and argon (at 87.3 K) on porous Vycor glass. We elaborate a novel hybrid nonlocal density functional theory (NLDFT) method for calculations of pore size distributions from adsorption isotherms in the entire range of micro- and mesopores. The NLDFT method, applied to the adsorption branch of the isotherm, takes into account the effect of delayed capillary condensation in pores of different geometries. The pore size data obtained by the NLDFT method for SE3030, KLE, and KLE/IL silicas agree with the data of SANS/SAXS techniques.     

Synthesis of mesoporous silicas of controlled pore wall thickness and their replication to ordered nanoporous carbons with various pore diameters

A synthesis strategy for the systematic control of the pore wall thickness has been developed for the mesoporous silicas with 2-D hexagonal order using ionic and nonionic surfactant mixtures. The mesoporous silicas have been used as templates for the synthesis of 2-D hexagonally ordered mesoporous carbons with controlled pore diameters. The synthesis strategy and results are useful not only for tailoring the properties of the mesoporous materials but also for extending our insights into the synthesis mechanism.     

Structural Control of Mesoporous 1,4-Phenylene-silica Using the Mixture of CTAB/SDS

The morphology, pore architecture and crystallinity of the mesoporous 1,4‐phenylene‐silicas were controlled using the mixtures of cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS). When the SDS/CTAB molar ratio increased from 0 to 1.0, the morphology of the mesoporous 1,4‐phenylene‐silicas changed in a sequence of sphere, hexagonal short rod, worm‐like, bent flake and flower‐like structure; the pore architecture of them changed from a hexagonal arranged tubular structure to a lamellar one; and the organization of the smallest repeat units within the wall changed from a random structure to a crystalline structure. At the SDS/CTAB molar ratios of 0.3 and 0.5, 1,4‐phenylene‐silica nanofibers with lamellar mesopores outside and tubular pore channels inside were obtained. The lamellar mesopores should be formed by merging the rod‐like micelles during the reaction process.     

Detailed Structural Characterizations of SBA‐15 and MCM‐41 Mesoporous Silicas on a High‐Resolution Transmission Electron Microscope

Using high‐resolution transmission electronic micrograph (HR‐TEM) observation, one can clearly see the pore geometry of the MCM‐41 and SBA‐15 mesoporous silicas to determine that their pore shapes are hexagonal and round, respectively. With the perpendicular orientations of the nanochannels to the electron beam, parallel line images of the (100) and (110) repeating spacings were observed. In the SBA‐15 mesoporous silicas, there are byproducts of the granular silica and disordered mesostructures, attributed to the weak hydrogen interactions between Pluronic 123 blockcopolymer and the silica species. There are also many different and significant +π disclination defects in SBA‐15 and MCM‐41 surfactant‐silica composites. The SBA‐15 with a thicker silica wall is more stable under irradiation by high‐energy electron beams compared to MCM‐41, which has thinner wall thickness. Some carbon nanostructure impurities were found in some carbon films on the metal grids.     

Ultrahigh surface area nanoporous silica particles via an aero-sol-gel process

We describe a new salt-assisted aero-sol-gel approach to produce spherical nanosized mesoporous silica particles. As an alternative to expensive templating mediums in prior works, salt (NaCl) was employed as a templating medium because it is thermally stable, recyclable, and easily leached. Furthermore, we demonstrate the ability to carry out traditional sol-gel chemistry within an aerosol droplet. The role of salt in sol-gel chemistry and aerosol processing was investigated as a function of hydrolysis time. It was verified that salt accelerates the kinetics of silica gelation, and simultaneously becomes an excellent templating medium to support nano-sized pores inside silica structures in the aerosol processing route. The presence of salt results in a roughly ten-fold increasing in the pore specific surface area and pore volume, subsequent to leaching of the salt matrix. The surface area and pore volume of the as-produced nanoporous silica particles was found to increase with increasing sol-gel hydrolysis time.     

Synthesis of mesoporous silicas inside large pores of inorganic matrix

Template syntheses of mesoporous silicas have been carried out inside large pores of inorganic matrix. Portions of tetraethoxysilane and cetyltrimethylammonium bromide micellar solution were incorporated step-by-step inside pore volume of silica gel with large pore size. Synthesized materials were characterized using thermal analysis, adsorption-desorption of nitrogen and X-ray diffraction scattering.     

Fabrication of MCM-41 mesoporous silica through the self-assembly supermolecule of β-CD and CTAB

Using the self-assembly β-cyclodextrin (β-CD) and cetyltrimethylammonium bromide (CTAB) as structure-directing agents, high-quality ordered MCM-41 silicas have been prepared. Small-angle X-ray diffraction (SXRD), N₂ adsorption-desorption and scanning electron microscope (SEM) techniques were used to characterize the calcined samples. Results showed that the pore structure of the resulting mesoporous silica belonged to the two-dimensional hexagonal structure (space group p6mm). The high-quality hexagonal structure was formed as n?1 (n denotes molar ratio of β-CD to CTAB). N₂ adsorption-desorption curves revealed type IV isotherms, H4 hysteresis loops, for all samples, and H1 hysteresis loops for samples at n=0, 0.1, 1 and 2, respectively. The pore size and the pore wall thickness of the samples increased with the increase in n values, respectively.     

Combined emulsion and solvent evaporation (ESE) synthesis route to well-ordered mesoporous materials

Control over morphology and internal mesostructure of surfactant templated silicas remains a challenge, especially when considering scaling laboratory syntheses up to industrial volumes. Here we report a method combining emulsification with the evaporation-induced self-assembly (EISA) method for preparing spherical, mesoporous silica particles. This emulsion and solvent evaporation (ESE) method has several potential advantages over classic precipitation routes: it is easily scaled while providing superior control over stoichiometric homogeneity of templating surfactants and inorganic precursors, and particle sizes and distributions are determined by principles developed for manipulating droplet sizes within water-in-oil emulsions. To demonstrate the method, triblock copolymer P104 is used as a templating amphiphile, generating unusually well-ordered 2D hexagonal (P6mm) mesoporous silica, while particle sizes and morphologies were controlled by varying the type of emulsifier and the method for emulsification.     

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.     

Porous Texture and Surface Character of Dehydroxylated and Rehydroxylated MCM-41 Mesoporous Silicas-Analysis of Adsorption Isotherms of Nitrogen Gas and Water Vapor

Four samples of MCM-41 mesoporous silicas whose average pore diameters are 2.4, 2.8, 3.2, and 3.6 nm were prepared using sodium orthosilicate and cationic surfactants of [CH(3)(CH(2))(n)N(CH(3))(3)]X (n=11, 13, 15, 17). These four samples were calcined at 1123 K in vacuo to obtain the dehydroxylated samples, which were further rehydroxylated at 298 K to obtain the rehydroxylated samples. The adsorption isotherms of nitrogen gas (77 K) for the 12 MCM-41 mesoporous silicas are of Type IVc, giving no adsorption hysteresis. On the other hand, the first adsorption isotherms of water vapor (298 K) for the dehydroxylated MCM-41 samples are quite different from those of nitrogen gas, giving the remarkable adsorption hysteresis. The second water isotherms for the rehydroxylated MCM-41 samples are of Type IV, showing slight hysteresis. Using the nitrogen isotherms, the relation between the pore size and carbon chain length of the surfactant has been determined, and the effect of dehydroxylation and rehydroxylation on the porous texture has been examined. Using the first and second water isotherms, the adsorption model of physisorbed waters adsorbed on the surface silanol groups has been proposed. From the pore size distribution curves of nitrogen and water, the presence of constrictions in the cylindrical pores has been predicted. Copyright 2000 Academic Press.