Hybrid Periodic Mesoporous Organosilicas

In this study we report the synthesis of a new class of materials called hybrid periodic mesoporous organosilicas (HPMOs). By coupling a silsesquioxane precursor through at least two chemical linkages to the mesopore walls of a pre‐existing periodic mesoporous silica (PMS) or periodic mesoporous organosilica (PMO). Many of the problems of a conventional PMO material can be avoided while ensuring efficient use of the bridging organic functional groups of the silsesquioxane. We demonstrate this concept for PMS by anchoring various silsesquioxanes, such as ethene and ethane silsesquioxanes, to the mesopore walls of the PMS. The addition of anchored silsesquioxane monolayers and multilayers to the mesopore walls also allows for the strict control of the diameter of the mesopore as well as the mesopore wall thickness in the final HPMO material. Additionally it is shown that having the silsesquioxane located solely on the surface of the mesopores in HPMOs gives increased chemical accessibility of the organic bridge‐bonded moiety when compared with their PMO counterparts containing the bridge‐bonded organic both on the surface and within the pore walls.     

Polyamidoamine Dendrimers Prepared Inside the Channels of Pore‐Expanded Periodic Mesoporous Silica

Polyamidoamine dendrimers up to the fourth generation have been grown with unprecedentedly high loading within the channels of pore‐expanded (10.6 nm) MCM‐41 silica. In‐depth characterization using nitrogen adsorption, solid‐state NMR, FTIR, thermogravimetry, and elemental analysis showed that the dendrimers grow inside the channels with an average yield better than 99 %. The pore size and structure of the support have been found to be determining factors as to how much dendrimer growth can be achieved.     

A Novel Mesoporous CaO‐Loaded In2O3 Material for CO2 Sensing

A mesoporous CaO‐loaded In₂O₃ material (with Ca/In₂O₃ ratios ranging from 2.5 to 8.5 at %) has been synthesized and used as resistive gas sensor for the detection of CO₂. A nanostructured In₂O₃ matrix has been obtained by hard template route from the SBA‐15 silica template. Additive presence does not distort the lattice of In₂O₃, which crystallizes in the Ia3 cubic space group. It has been proved by XRD, HRTEM, Raman and XPS measurements that samples contain not only CaO but also CaCO₃ in calcite phase as a consequence of CaO carbonation. Pure In₂O₃ based sensors are low sensitive to CO₂, whereas those containing the additive show an important response in the 300–5000 ppm range of gas concentrations. As seen by DRIFTS, the electrical response arises from the interaction between CO₃^2– and CO₂, yielding bicarbonates products. The reaction is water‐assisted, so that hydration of the sensing material ensures sensor reliability whilst its dehydration would inhibit sensor response. The use of CaCO₃ instead of CaO does not cause significant differences in electrical and DRIFTS data, which corroborates the important role played by carbonate species in the sensing mechanism.     

Synthesis and Characterization of Chromium‐Doped Mesoporous Tungsten Oxide for Gas Sensing Applications

SBA‐15 (2D hexagonal structure) and KIT‐6 (3D cubic structure) silica materials are used as templates for the synthesis of two different crystalline mesoporous WO₃ replicas usable as NO₂ gas sensors. High‐resolution transmission electron microscopy (HRTEM) studies reveal that single‐crystal hexagonal rings set up the atomic morphology of the WO₃ KIT‐6 replica, whereas the SBA‐15 replica is composed of randomly oriented nanoparticles. A model capable of explaining the KIT‐6 replica mesostructure is described. A small amount of chromium is added to the WO₃ matrix in order to enhance sensor response. It is demonstrated that chromium does not form clusters, but well‐distributed centers. Pure WO₃ KIT‐6 replica displays a higher response rate as well as a lower response time to NO₂ gas than the SBA‐15 replica. This behavior is explained by taking into account that the KIT‐6 replica has a higher surface area as demonstrated by Brunauer–Emmett–Teller analyses and its mesostructure is fully maintained after the screen‐printing step involved in sensors preparation. The presence of chromium in the material results in a shorter response time and improved sensor response to the lowest NO₂ concentrations tested. Electrical differences related to mesostructure are reduced as a result of additive introduction.     

Spherical MSU‐1 Mesoporous Silica Particles Tuned for HPLC

Among the mesoporous silica micellar templated structures (MTSs), MSU‐X silica, obtained through an N⁰I⁰ assembly between non‐ionic polyethyleneoxide‐based surfactants (N⁰) and silica neutral inorganic precursors (I⁰), exhibits a regular ordered structure with a 3D wormhole porous framework and an easily controlled pore size. These materials have been tested for applications requiring both a narrow mesopore size distribution and isotropic properties. A specific double‐step synthesis that we developed recently for MSU‐X materials has allowed us to prepare mesoporous silica particles with the required shape, size, and properties. Both the particles’ synthesis and comparative HPLC separation tests with a commercial ungrafted silica HPLC powder of identical shape and size are reported.     

Colloidal Suspensions of Nanometer‐Sized Mesoporous Silica

Nanometer‐sized surfactant‐templated materials are prepared in the form of stable suspensions of colloidal mesoporous silica (CMS) consisting of discrete, nonaggregated particles with dimensions smaller than 200 nm. A high‐yield synthesis procedure is reported based on a cationic surfactant and low water content that additionally enables the adjustment of the size range of the individual particles between 50 and 100 nm. Particularly, the use of the base triethanolamine (TEA) and the specific reaction conditions result in long‐lived suspensions. Dynamic light scattering reveals narrow particle size distributions in these suspensions. Smooth spherical particles with pores growing from the center to the periphery are observed by using transmission electron microscopy, suggesting a seed‐growth mechanism. The template molecules could be extracted from the nanoscale mesoporous particles via sonication in acidic media. The resulting nanoparticles give rise to type IV adsorption isotherms revealing typical mesopores and additional textural porosity. High surface areas of over 1000 m² g^–1 and large pore volumes of up to 1 mL g^–1 are obtained for these extracted samples.     

Multiple Functionalization of Mesoporous Silica in One‐Pot: Direct Synthesis of Aluminum‐Containing Plugged SBA‐15 from Aqueous Nitrate Solutions

Aluminum‐containing plugged mesoporous silica has been successfully prepared in an aqueous solution that contains triblock copolymer templates, nitrates, and silica sources but without using mineral acid. The acidity of the solution can be finely tuned from pH 1.4 to 2.8 according to the amount of the introduced aluminum species which ranged from an Al/Si molar ratio of 0.25/1 to 4.0/1. The aluminum nitrate additive in the starting mixture, along with the weak acidity produced by the nitrates, contributes to the formation of plugged hexagonal structures and the introduction of different amounts of aluminum species into the mesostructure. Characterization by X‐ray diffraction, transmission electron microscopy, and N₂ sorption measurements show that the Al‐containing plugged silicas possess well‐ordered hexagonal mesostructures with high surface areas (700–860 m² g^–1), large pore volume (0.77–1.05 cm³ g^–1) and, more importantly, combined micropores and/or small mesopores in the cylindrical channels. Inductively coupled plasma–atomic emission spectrometry results show that 0.7–3.0 wt % aluminum can be introduced into the final samples. ²⁷Al MAS NMR results display that about 43–60% aluminum species are incorporated into the skeleton of the Al‐containing silicas and the amount of the framework aluminum increases as the initial added nitrates rises. Scanning electron microscopy images reveal that the directly synthesized Al‐containing plugged silica has a similar morphology to that of traditional SBA‐15. Furthermore, the Al‐containing plugged samples have excellent performances in the adsorption and the catalytic decomposition of isopropyl alcohol and nitrosamine. Finally, the direct synthesis method is used to produce plugged mesoporous silicas that contain other metals such as chromium and copper, and the resultant samples also show good catalytic activities.     

Thioether‐bridged Mesoporous Organosilicas: Mesophase Transformations Induced by the Bridged Organosilane Precursor

The achievement of structural control over thioether‐bridged mesoporous organosilicas is reported. The mesoporous materials have been synthesized by co‐condensation of bis[3‐(triethoxysilyl)propyl]tetrasulfide (TESPTS) and tetramethoxysilane (TMOS) in acetic acid/sodium acetate buffer solution (HAc–NaAc, pH 4.4), using the nonionic surfactant P123 as a template. The mesostructure of the material is mainly controlled by the molar ratio of TESPTS/TMOS in the initial gel mixture. A mesophase transformation, progressing from a highly ordered 2D hexagonal structure via a vesiclelike structure to a mesostructured cellular foam, can be clearly observed when the molar ratio of TESPTS/TMOS is increased in increments. Solid‐state NMR results show that TESPTS is not completely hydrolyzed and condensed at the applied buffer conditions. At low concentrations, the unhydrolyzed TESPTS can penetrate into the core of the surfactant micelles and change the packing parameter of the P123 surfactant. Above a certain concentration, the TESPTS can form a microemulsion with P123 surfactant molecules. Therefore, the vesiclelike structure or cellular foam structure can be synthesized by simply controlling the molar ratio of TESPTS/TMOS. This approach provides a novel method for the facile synthesis of organic–inorganic hybrid materials with a controllable mesostructure under mild synthetic conditions.     

Spin‐Coated Periodic Mesoporous Organosilica Thin Films—Towards a New Generation of Low‐Dielectric‐Constant Materials

Periodic mesoporous organosilica (PMO) thin films have been produced using an evaporation‐induced self‐assembly (EISA) spin‐coating procedure and a cationic surfactant template. The precursors are silsesquioxanes of the type (C₂H₅O)₃Si–R–Si(OC₂H₅)₃ or R′–[Si(OC₂H₅)₃]₃ with R = methene (–CH₂–), ethylene (–C₂H₂–), ethene (–C₂H₄–), 1,4‐phenylene (C₆H₄), and R′ = 1,3,5‐phenylene (C₆H₃). The surfactant is successfully removed by solvent extraction or calcination without any significant Si–C bond cleavage of the organic bridging groups R and R′ within the channel walls. The materials have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X‐ray diffraction (PXRD), and ²⁹Si and ¹³C magic‐angle spinning (MAS) NMR spectroscopy. The d‐spacing of the PMOs is found to be a function of R. Nanoindentation measurements reveal increased mechanical strength and stiffness for the PMOs with R = CH₂ and C₂H₄ compared to silica. Films with different organic‐group content have been prepared using mixtures of silsesquioxane and tetramethylorthosilicate (TMOS) precursors. The dielectric constant (k) is found to decrease with organic content, and values as low as 1.8 have been measured for films thermally treated to cause a “self‐hydrophobizing” bridging‐to‐terminal transformation of the methene to methyl groups with concomitant loss of silanols. Increasing the organic content and thermal treatment also increases the resistance to moisture adsorption in 60 and 80 %‐relative‐humidity (RH) environments. Methene PMO films treated at 500 °C are found to be practically unchanged after five days exposure to 80 % RH. These low dielectric constants, plus the good thermal and mechanical stability and the hydrophobicity suggest the potential utility of these films as low‐k layers in microelectronics.     

Surfactant‐Assisted Synthesis of Alumina with Hierarchical Nanopores

Using surfactant‐assisted synthesis, aluminas with hierarchical nanopores are produced. The hierarchical structures are composed of mesopores of 4 nm diameter, and macropores with diameters of about 300 nm. The structures were found to be stable to the thermal removal of the surfactant. Synthesis factors affecting the appearance of the hierarchically structured alumina material are presented. A potential mechanism for the formation of the uniquely structured aluminas is proposed.     

Hydrothermally Stable Thioether‐Bridged Mesoporous Materials with Void Defects in the Pore Walls

Hydrothermally stable thioether‐bridged mesoporous materials have been synthesized by one‐step co‐condensation of 1,4‐bis(triethoxysily)propane tetrasulfide (TESPTS) with tetramethoxysilane (TMOS) using cetyltrimethylammonimum bromide (CTAB) as the surfactant in basic conditions. The ordered mesoporous materials can be formed with a wide range of thioether concentrations in the mesoporous framework, as is seen by X‐ray diffraction (XRD) characterization. The results of N₂ sorption and transmission electron microscopy (TEM) reveal that the materials synthesized with TESPTS/TMOS molar ratios in the range 1:8–1:3 have extensive structural defect holes in the nanochannels. All materials exhibit enhanced hydrothermal stability, which is in proportion to the concentration of thioether bridging in the mesoporous framework. The thioether‐functionalized mesoporous materials are efficient adsorbents for removing Hg²⁺ and phenol from waste water. The Hg²⁺‐adsorption capacity of the material can be as high as 1500 mg g^–1.     

Synthesis of a Hydrothermally Stable,Periodic Mesoporous Material Containing Magnetite Nanoparticles,and the Preparation of Oriented Films

Magnetite nanoparticles modified covalently with triethoxysilane having a quaternary dicetyl ammonium ion are used together with tetraethylorthosilicate as building blocks to prepare a mesoporous material. Cetyltrimethylammonium bromide is used as a structure‐directing agent under conditions typically used for mesoporous MCM‐41 silicas. The resulting mesoporous material (MAG‐MCM‐41), containing up to 15 wt % of magnetite is characterized by transmission electron microscopy (TEM), isothermal gas adsorption, and X‐ray diffraction. In contrast to siliceous MCM‐41, mesoporous MAG‐MCM‐41 exhibits a remarkable hydrothermal stability. The magnetic properties of MAG‐MCM‐41 are characterized by DC and AC magnetic susceptibility, and by isothermal hysteresis cycles, confirming the long‐range magnetic ordering above 400 K. As evidenced by atomic force microscopy and TEM, the ability to respond to magnetic fields is used to orient films of MAG‐MCM‐41 with the channels perpendicular to a support.     

Facile Synthesis of Manganese‐Loaded Mesoporous Silica Materials by Direct Reaction Between KMnO4 and an In‐Situ Surfactant Template

A series of manganese oxide‐loaded SBA‐15 (MnSBA‐xh, x = 1, 2, 3, 4, 5, 6; h: hour(s)) mesoporous materials are synthesized via a facile, in‐situ reduction method with a surfactant template. The composite materials are characterized using Fourier‐transform infrared spectroscopy, X‐ray diffraction, N₂ sorption isotherms, X‐ray photoelectron spectroscopy (XPS), transmission electron microscopy, energy‐dispersive spectroscopy, and CO oxidation catalysis. The results show that a high content of manganese (an atomic ratio of Mn/Si from 0.12 up to approximately 1) could be loaded into the channels of SBA‐15 when treated with an aqueous solution of potassium permanganate, while retaining the ordered mesostructure and large surface area of SBA‐15. Increasing the manganese oxide content results in a gradual decrease in the specific surface area, pore size, and pore volume. XPS spectra are employed to confirm the redox reaction between KMnO₄ and the surfactant. CO‐conversion tests on the calcined MnSBA‐2h sample (MnSBA‐2h‐cal) shows that it has a repeatable, and relatively high, catalytic activity.     

Photonic Crystals from Ordered Mesoporous Thin‐Film Functional Building Blocks

Environment‐sensitive Bragg reflectors are built using functional mesoporous thin films as building blocks. Tuning of optical properties is achieved by changing the composition or porosity of the slabs or the introduction of planar defects. Sorption or capillary condensation of molecules into the pore system results in a 10–40 nm photonic bandgap (PBG) shift. Organic functions added to the pore surface change the response, permitting tailoring of the selectivity towards small‐size molecules.     

Uniform Rattle‐type Hollow Magnetic Mesoporous Spheres as Drug Delivery Carriers and their Sustained‐Release Property

A novel kind of rattle‐type hollow magnetic mesoporous sphere (HMMS) with Fe₃O₄ particles encapsulated in the cores of mesoporous silica microspheres has been successfully fabricated by sol–gel reactions on hematite particles followed by cavity generation with hydrothermal treatment and H₂ reduction. Such a structure has the merits of both enhanced drug‐loading capacity and a significant magnetization strength. The prepared HMMSs realize a relatively high storage capacity up to 302 mg g^?1 when ibuprofen is used as a model drug, and the IBU–HMMS system has a sustained‐release property, which follows a Fick's law.     

Synthesis of Phase‐Pure Ferromagnetic Fe3P Films from Single‐Source Molecular Precursors

A new method for the preparation of phase‐pure ferromagnetic Fe₃P films on quartz substrates is reported. This approach utilizes the thermal decomposition of the single‐source precursors H₂Fe₃(CO)₉PR (R = ^tBu or Ph) at 400 °C. The films are deposited using a simple, home‐built metal‐organic chemical vapor deposition (MOCVD) apparatus and are characterized using a variety of analytical methods. The films exhibit excellent phase purity, as evidenced by X‐ray diffraction, X‐ray photoelectron spectroscopy, and field‐dependent magnetization measurements, the results of which agree well with measurements obtained from bulk Fe₃P. Using scanning electron microscopy and atomic force microscopy techniques, the films are found to have thicknesses between 350 and 500 nm with a granular surface texture. As‐deposited Fe₃P films are amorphous, and little or no magnetic hysteresis is observed in plots of magnetization versus applied field. Annealing the Fe₃P films at 550 °C results in improved crystallinity as well as the observation of magnetic hysteresis.     

Sulfur‐Functionalized Mesoporous Carbon

A simple synthesis route to mesoporous carbons that contain heteroaromatic functionality is described. The sulfur‐functionalized mesoporous carbon (S‐FMC) materials that have been prepared show excellent thermal stability, as well as excellent hydrothermal stability, and stability over a wide range of pH values. These materials also show excellent mercury sorption performance over a broad range of pH, much broader than is possible with thiol‐based functionality or most silica‐based sorbents. The superior performance of these mesoporous heterocarbons as heavy‐metal sorbent material is demonstrated. These materials are shown to be stable at elevated temperatures and extreme pHs, making them ideally suited as a new class of absorbent material.     

Shape Selectivity in Adsorption of n‐ and Iso‐alkanes on a Zeotile‐2 Microporous/Mesoporous Hybrid and Mesoporous MCM‐48

The adsorption of linear and branched C5–C9 alkanes in the temperature range 50–250 °C on mesoporous MCM‐48 material and its microporous/mesoporous variant Zeotile‐2 at low surface coverage is investigated using the pulse chromatographic technique. On MCM‐48, the differences in adsorption between linear and branched alkanes are merely a result of differences in volatility, indicating that the MCM‐48 material does not present shape‐selective adsorption sites. On Zeotile‐2, there is a preferential adsorption of linear over branched alkanes. The difference arises from a difference in adsorption entropy rather than enthalpy. Upon their adsorption on Zeotile‐2 branched alkanes lose relatively more entropy than their linear isomers do. Zeolitic molecular pockets embedded in the walls of the mesoporous Zeotile‐2 impose steric constraints on the bulky isoalkanes. Zeotile‐2 combines adsorption properties from microporous and mesoporous materials. Compared to the nitrogen molecule, linear and branched C5–C9 alkanes are superior probes for investigating micropores and micropockets in hierarchical materials.     

Template‐Free Fabrication and Enhanced Photocatalytic Activity of Hierarchical Macro‐/Mesoporous Titania

Hierarchical macro‐/mesoporous titania is prepared without the addition of templates or auxiliary additives at room temperature by the simple dropwise addition of tetrabutyl titanate to pure water, and then calcined at various temperatures. The products are characterized by X‐ray diffraction, N₂‐adsorption–desorption analysis, scanning electron microscopy, and the corresponding photocatalytic activity is evaluated by measuring the photocatalytic oxidation of acetone in air. The results reveal that hierarchical macro‐/mesoporous structures of titania can spontaneously form by self‐assembly in alkoxide–water solutions in the absence of organic templates or auxiliary additives. The calcination temperature has a strong effect on the structures and photocatalytic activity of the prepared titania. At 300 °C, the calcined sample shows the highest photocatalytic activity. At 400 and 500 °C, the photocatalytic activity slightly decreases. When the calcination temperature is higher than 500 °C, the photocatalytic activity greatly decreases because of the destruction of the hierarchical macro‐/mesoporous structure of the titania and the drastic decrease of specific surface area. The hierarchically macro‐/mesostructured titania network with open and accessible pores is well‐preserved after calcination at 500 °C, indicating especially high thermal stability. The macroporous channel structures are even preserved after calcination at 800 °C. This hierarchical macro‐/mesostructured titania is significant because of its potential applications in photocatalysis, catalysis, solar‐cell, separation, and purification processes.     

Transparent Mesoporous Nanocomposite Films for Self‐Cleaning Applications

A versatile approach is studied for the elaboration of TiO₂ based photocatalytic coatings for self‐cleaning applications on transparent substrates. The basic principle of the synthesis relies on the use of preformed TiO₂ colloidal particles that are further dispersed within a transparent silica binder with a mesoporous structure. Film porosity in the nanometer range is controlled by achieving the sol–gel silica condensation around self‐organized micellar assemblies of a templating copolymer surfactant. The latter also acts as a stabilizer for the TiO₂ particles, thus preserving their high dispersion within the film so that excellent optical properties are maintained even for high TiO₂ loading (up to 50 %). Studies of photodegradation kinetics show that such mesoporous films are at least 15 times more active than films synthesized with a usual microporous silica binder. Moreover, the measured quantum‐yield efficiency (1.1 %) is found to be among the highest reported up to now. Improved photoactivity of the films is discussed as resulting from the closer proximity between the organic molecules and the surface of the TiO₂ crystallites as well as the improved diffusion rate of water and oxygen through the interconnected pore network.