Mesostructured Silica Research Articles

Periodic mesoporous organosilicas with bis(8-quinolinolato) dioxomolybdenum(VI) inside the channel walls

Novel periodic mesostructured organometallic silicas of MCM-41 type bearing homogeneously distributed bis(8-quinolinolato)dioxomolybdenum(VI) inside the channel walls (denoted as MoO2Q2@PMO-x) are synthesized via a convenient one-pot method and examined as catalysts in the epoxidation of cyclooctene. The ordered mesoporous structures as well as the organometallic groups incorporated into the framework are fully determined by comprehensive characterization techniques such as XRD, TEM, N2 adsorption/desorption, SEM, FT-IR, UV–vis spectroscopy, solid-state NMR, ICP-AES, XPS and TG/DSC. MoO2Q2@PMO-6% catalyst exhibits higher activity for the epoxidation of cyclooctene with tert-butyl hydroperoxide than other MoO2Q2@PMO materials and its homogeneous or randomly grafted analogue.

Facile Fabrication of Hollow Mesoporous Silica Nanospheres for Superhydrophilic and Visible/Near‐IR Antireflection Coatings

A series of hierarchically mesostructured silica nanoparticles (MSNs) less than 100 nm in size were fabricated by means of a one-step synthesis using dodecanethiol (C(12)-SH) and cetyltrimethylammonium bromide (CTAB) as the dual template, and trimethylbenzene (TMB) as the swelling agent. Silica nanoparticles with varied morphologies and structures, including mesoporous silica nanoparticles with tunable pore size, mesoporous silica nanoparticles with a thin solid shell, hollow mesoporous silica nanoparticles with tunable cavity size, and hollow mesoporous silica nanoparticles with a thin solid shell, were obtained by regulating the TMB/CTAB molar ratio and the stirring rate with the assistance of C(12)-SH. Silica particulate coatings were successfully fabricated by using MSNs with varied morphologies and structures as building block through layer-by-layer dip-coating on glass substrates. The thickness and roughness of the silica particulate coatings could be tailored by regulating the deposition cycles of nanoparticles. The silica particulate coatings composed of hollow mesoporous silica nanoparticles with a thin shell (S2) increased the maximum transmittance of slide glass from 90 to 96%, whereas they reduced its minimum reflection from 8 to 2% at the optimized wavelength region that could be adjusted from visible to near-IR with a growing number of deposition cycles. The coatings also exhibited excellent superhydrophilic and antifogging properties. These mesostructured silica nanoparticles are also expected to serve as ideal scaffolds for biological, medical, and catalytic applications.

Modelling of optical absorption of silver NP’s produced by UV radiation embedded in mesostructured silica films

Hexagonal mesostructured films containing silver ions were obtained by sol–gel method. Brij 58 was used to produce channels into the film, which house these ions. The films were exposure to UV radiation to produced silver metallic nanoparticles. The presence of the metallic nanoparticles was determined by infrared spectroscopy and optical absorption. Besides, these nanoparticles and core–shell structures of silver–silver oxide nanoparticles were identified by high-resolution transmission electronic microscopy. From these measurements, the obtained size range for silver nanoparticles was 6.1 nm. The absorption spectrum located at 440 nm was modelled and well fitted with the Gans theory considering refractive index higher than the one coming from host matrix. This index is explained because the silver oxide shell modifies the local surrounding medium of the metallic nanoparticles.

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over palladium catalyst supported on SO3H-functionalized MCF silica: Effect of calcination temperature of mesostructured cellular foam silica

Palladium catalysts supported on SO3H-functionalized MCF silica (Pd/SO3H-MCF-T (T=450, 550, 650, 750, 850, and 950)) were prepared with a variation of calcination temperature (T, °C) of MCF silica. They were then applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Conversion of hydrogen, selectivity for hydrogen peroxide, and yield for hydrogen peroxide showed volcano-shaped curves with respect to calcination temperature of MCF silica. Yield for hydrogen peroxide increased with increasing acid density of Pd/SO3H-MCF-T catalysts. Thus, acid density of Pd/SO3H-MCF-T catalysts played an important role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. Pd/SO3H-MCF-T catalysts efficiently served as an acid source and as an active metal catalyst in the direct synthesis of hydrogen peroxide.

Experimental Evidence of Plasmophores: Plasmon-Directed Polarized Emission from Gold Nanorod–Fluorophore Hybrid Nanostructures

We show that the fluorescence emission from individual hybrid nanostructures composed of Au nanorod cores and oxazine 725-embedded mesostructured silica shells is strongly polarized, with the degree of polarization being equal to that of the light scattered by the nanorod and varying from 0 to 1 as the longitudinal plasmon resonance wavelength is increased. Our analyses indicate that the interactions of the plasmon resonance of the nanorod with the excitation and emission processes of the fluorophores are temporally separated under unsaturated excitation conditions. The emission polarization is found through electrodynamic calculations to arise from the plasmon-coupled emission instead of the plasmon-enhanced excitation polarization. The emission carries the direction and polarization properties that are essentially determined by the dipolar plasmon of the nanorod antenna. Our results therefore provide direct and concrete evidence for the plasmophore that has been proposed recently for plasmon-enhanced fluorescence.

Mesoporous silica iron-doped as stable and efficient heterogeneous catalyst for the degradation of C.I. Acid Orange 7 using sono–photo-Fenton process

Iron-containing mesostructured silica (Fe 2O 3/SBA-15) was prepared and used as heterogeneous catalyst in the integrated sono–photo-Fenton process. The effect of hydrogen peroxide concentration, initial pH, Fe 2O 3/SBA-15 loading, ultrasonic power, and initial solution concentration on the decolorization of C.I. Acid Orange 7 (AO7) by the integrated heterogeneous sono–photo-Fenton process was investigated in order to optimize process efficiency. The results showed that the decolorization efficiency increased with the increase of hydrogen peroxide concentration, ultrasonic power, Fe 2O 3/SBA-15 loading, but decreased with the increase of initial pH and initial dye concentration. Catalyst stability was evaluated by measuring iron leaching in solution. The particles size distribution of spent catalyst evidenced a remarkable size reduction during reaction, leading to more reactive surface and higher mass transfer rate. The main reaction intermediates were separated and identified using GC–MS and a possible degradation pathway was proposed.

Support-Enhanced Selective Aerobic Alcohol Oxidation over Pd/Mesoporous Silicas

The influence of silica mesostructure upon the Pd-catalyzed selective oxidation of allylic alcohols has been investigated for amorphous and surfactant-templated SBA-15, SBA-16, and KIT-6 silicas. Significant rate enhancements can be achieved via mesopore introduction, most notably through the use of interconnected porous silica frameworks, reflecting both improved mass transport and increased palladium dispersion; catalytic activity decreases in the order Pd/KIT-6 ≈ Pd/SBA-16 > Pd/SBA-15 > Pd/SiO2. Evidence is presented that highly dispersed palladium oxide nanoparticles, not zerovalent palladium, are the catalytically active species. © 2011 American Chemical Society.

Decoupling particle formation from intraparticle ordering in mesostructured silica colloids

A synthesis methodology that decouples the formation of silica particles from their internal mesoordering has been developed. This was possible by employing synthesis conditions that combine fast nucleation and growth of particles with slow silica condensation. The synthesis developed is based on an inorganic silica source and the nonionic block copolymer Pluronic P123 using a synthesis pH of 3 and conditions that result in a stable colloidal suspension. Using a two-step procedure the particles can be first formed and then triggered into developing an ordered internal mesostructure by subsequent addition of silica or Pluronic. This is possible because under the conditions employed the particles are stabilized by relatively weak interactions before the formation of a stable silica network.

Characteristics and Catalytic Properties of Mesocellular Foam Silica Supported Pd Nanoparticles in the Liquid-Phase Selective Hydrogenation of Phenylacetylene

An ultra-large pore mesocellular foam silica (MCF) was employed as a support for preparation of supported Pd catalysts for the liquid-phase selective hydrogenation of phenylacetylene. The catalysts were prepared by three different routes: (i) incipient wetness impregnation using Pd(II)acetate solution (Pd/MCF-imp), (ii) impregnation of colloidal Pd nanoparticles obtained by the solvent reduction method (Pd/MCF-col), and (iii) in situ synthesis of MCF in the presence of the Pd colloid (Pd/MCF-ss). The conventional impregnation method resulted in more agglomeration of Pd particles and partial collapse of MCF structure, hence the Pd/MCF-imp exhibited the lowest selectivity towards styrene at total conversion of phenylacetylene. Only the Pd/MCF-ss, in which most of the Pd nanoparticles were encapsulated by the silica matrix, was found to retain high styrene selectivity (>80%) after complete conversion of phenylacetylene. Comparing to the other highly efficient Pd catalysts reported in the literature under similar reaction conditions, it can be emphasized that coverage of Pd surface by the support produces great beneficial effect for enhancing styrene selectivity, regardless of the type of supports used (i.e., TiO2, carbon nanotubes, or mesostructured silica). The direct incorporation of palladium nanoparticles before hydrolysis of the silica precursor during the synthesis of mesocellular foam silica (Pd/MCF-ss) resulted in superior catalytic performance in the selective hydrogenation of phenylacetylene, comparing to those obtained via impregnation of Pd acetate solution (Pd/MCF-imp) and deposition of Pd colloid (Pd/MCF-col). High styrene selectivity (> 80%) was retained after complete conversion of phenylacetylene over the Pd/MCF-ss.

Orientation of spheroid-shaped silver nanoparticles in mesostructured silica films studied by polarized absorption and low-frequency Raman spectroscopy

Orientation of spheroid-shaped silver (Ag) nanoparticles (NPs) embedded in mesostructured silica sol gel films is studied by polarized absorption measurements and low-frequency Raman scattering, two complementary techniques well-adapted to characterize nanostructures in amorphous matrix. Comparison is made with analysis of mesostructured silica sol gel films containing nanospheres. Polarization-dependent absorption spectra clearly evidence spheroid-shaped NP orientation and their mean aspect ratio is estimated from corresponding polar plots. Mean diameter of spherical NPs is deduced from low-frequency Raman scattering spectra as well as size of elongated spheroid NPs.

Mesostructured Silica Aerosol Particles: Comparison of Gas-Phase and Powder Deposit X-ray Diffraction Data

We report on the characterization of mesostructured aerosol silica particles in the gas phase using in situ synchrotron small-angle X-ray scattering (SAXS) in order to unveil the influence of the basic production parameters. The investigated system was based on tetraethylorthosilicate (TEOS) as the inorganic precursor and on cetyltrimethyl-ammonium bromide (CTAB) as the surfactant. The heating temperature, surfactant to silicate ratio, and particle flow rate were thoroughly investigated, and for this purpose, an in-house-built aerosol reactor equipped with a special X-ray observation chamber was used. Complementary fine structural analysis was applied on dried deposits of the silica aerosols comprising direct Fourier transforms as well as simple two-phase model fits. This resulted in robust estimates for the silica wall thickness and surfactant core radius of the hexagonally ordered mesostructure. The particle shape and size distribution were examined by scanning electron microscopy (SEM). The quality of the inner nanostructure was revealed from an analysis of the peak width. The comparison of data from the gas phase and powder deposit shows that, in general, slower drying conditions (heating temperature about 80 °C) and a medium surfactant to Si ratio (about 0.14) lead to nanostructures of the best quality in terms of well-defined long-range organization.

Sulfonic Acid-Functionalized Catalysts for the Valorization of Glycerol via Transesterification with Methyl Acetate

Sulfonic acid-functionalized mesostructured SBA-15 silicas have been demonstrated to be active for the catalytic transesterification of glycerol with methyl acetate to produce di- and triacetylglyc...

Periodic Mesoporous Benzene−Silicas Prepared Using Boric Acid as Catalyst

Highly ordered periodic mesoporous benzene−silicas with 2D p6mm symmetry were synthesized through sol−gel self-assembly in the presence of a nonionic poly(ethylene oxide)−poly(dl-lactic acid-co-glycolic acid)−poly(ethylene oxide) (PEO−PLGA−PEO, EO16(L29G7)EO16) triblock copolymer template using boric acid (weak Lewis acid) as a catalyst instead of strong hydrochloric acid. 1,4-Bis(triethoxysilyl)benzene (BTEB) was used as an organosilica precursor, and the reaction and aging temperatures were set at 40 and 100 °C, respectively. The molar concentration of boric acid was varied from 0.5 to 3 M to find the minimum acidity required and to check the effect of this acidic catalyst on the formation of benzene−silica mesostructures. This study shows that the ordered mesostructure of benzene−silica was observed even at 1 M concentration of boric acid ([B(OH)3]/[BTEB] = 12), and highly ordered hexagonal mesostructures were obtained at higher concentrations of this acid. Thermal stability of the benzene−silica mater...

Comparison between Ti- and Si-based mesostructures for the removal of phosphorous from aqueous solution

The aim of this study was to explore the feasibility of utilizing inorganic mesostructures for the removal of phosphorous in laboratory experiments. The emphasis of the experiments was on the comparison of the efficiency of phosphorous adsorption between mesostructures synthesized using two different inorganic materials. Powder X-ray diffraction, Fourier transformed infrared spectroscopy, and Brunauer-Emmett-Teller(BET) methods were used to characterize the inorganic mesostructures. The efficiencies of silica and titanium mesostructures for the removal of phosphorous from aqueous solutions were investigated on a batch scale. Equilibrium data were analyzed using the Langmuir isotherm. The maximum adsorption capacities of the mesostructured adsorbents were found to be 49.3 and 19.5 mg g−1 for the titanium and silica mesostructures, respectively. The adsorption kinetics was best described by a pseudo-third-order kinetic model. The results from this study indicated that the titanium mesostructure, because of low cost and high capability, has the potential to be utilized for the cost-effective removal of phosphorous from sewage or wastewater. © 2011 American Institute of Chemical Engineers Environ Prog, 2012

Geometrical confinement effect in the liquid-phase semihydrogenation of phenylacetylene over mesostructured silica supported Pd catalysts

Abstract Three types of mesostructured silica with different pore sizes (MCM-41, SBA-15, and MCF) were employed as supports for deposition of colloidal Pd nanoparticles obtained by the solvent reduction method. As determined by transmission electron microscopy (TEM), average Pd particle sizes on the various supports were quite similar and were not significantly different from the colloidal particles (~ 2.3–2.5 nm). There was limited access of the reactants to Pd active sites and suppression of CO chemisorption for the Pd/SBA-15, probably because most of the Pd particles were located inside the pores. This geometrical confinement effect in the case of Pd/SBA-15, however, resulted in an improved selectivity towards styrene at complete conversion of phenylacetylene. Such effect was similar to those reported in the literature for the Pd nanoparticles encapsulated in support matrices (simultaneous synthesis) and the presence of strong metal-support interaction effect in Pd/TiO2.

Formation and characterization of mesostructured silica nanotubes

The multi-walled mesoporous silica nanotubes are prepared using cetyltrimethylammonium bromize (CTAB) as the surfactant micellar template and tetraethylorthosilicate (TEOS) as the silica precursor via a one-step wet chemical approach. The synthesized tubes are found to be double/triple walled and of amorphous nature. Their diameter and the length are about 100 nm to 1 μm and about 0.1–20 μm, respectively. The specific surface area approaches 1,488 m2/g. Based on the transmission electron microscopy analysis, it is inferred that the formation of the double/triple walled silica nanotubes is associated with the lamellar curling mechanism. A striking photoluminescence effect is detected in the mesostructured silica nanotubes. These nanotubes are expected to be a promising material for various applications such as gas storage, catalyst, or catalyst supports.

Biomimetic morphogenesis of micropottery: helical coiling of mesostructured silica nanofibers

Biomimetic morphogenesis of sophisticated mesostructured silica micropottery with a novel 3D hierarchically ordered structure and diverse morphologies was realized by hydrolyzing tetraethylsiloxane (TEOS) in concentrated HNO3 solutions containing cetyltrimethylammonium bromide (CTAB). It was proposed that the unique micropottery was self-organized by helical coiling of hexagonally ordered mesostructured silica nanofibers with the assistance of the NO3− counterion, which is amazingly reminiscent of the manufacture of pottery vessels by wheel shaping in ancient Oriental Asia. The observed self-organization of the mesostructured silica micropottery may open new avenues towards the micro-fabrication of advanced materials with hierarchical 3D architectures.

Preparation of mesostructured silica–micelle hybrids and their conversion to mesoporous silica modified controllably with immobilized hydrophobic blocks by using triethoxysilyl-terminated PEO–PPO–PEO triblock copolymer

Mesostructured silica having mesopore surface functionalized with poly(ethylene oxide)20-b-poly(propylene oxide)70-b-poly(ethylene oxide)20 (EO20PO70EO20, P123) micelles was synthesized by using triethoxysilyl-terminated P123 (TES-P123) with dual functions of templating and surface anchoring. The amount of anchored P123 was controlled by mixing TES-P123 and conventional P123 as a cotemplate and the subsequent removal of the cotemplate by extraction. All the samples show the presence of ordered mesopores after the extraction. Unexpectedly, the d-spacing and the pore size increased after the extraction when the ratio of TES-P123/(TES-P123 + P123) was over 75%, which is explained by an osmotic force caused by anchored P123 swollen with THF during the extraction. To evaluate the stability of anchored P123, ibuprofen was simultaneously incorporated into mesopores during the formation of mesostructured silica. When TES-P123 was exclusively used, the percentage of retained P123 after the extraction of IBU was higher than that found for the case of conventional P123. This approach will open a new way to stabilize assembled amphiphiles in regularly ordered siloxane frameworks.

Novel synthesis of graphene foils in mesostructured silica between hexagonal and lamellar phases

The mesostructured silica template between hexagonal and lamellar phases derived from self-assembled tetraethyl-ortho-silicate (TEOS) with uniform adsorption of iron-group metal ions provides an ideal environment for growing highly crystalline graphene foils at a low temperature.

Plasmon-induced modulation of the emission spectra of the fluorescent molecules near gold nanorods

Both the excitation and emission processes of a fluorescent molecule positioned near a noble metal nanocrystal can interact strongly with the localized surface plasmon resonance of the metal nanocrystal. While the effects of this plasmon-fluorophore interaction on the intensity, polarization, and direction of the fluorescence emission have been intensively investigated, the plasmonic effect on the emission spectrum has barely been explored. We show, on the single-particle level, that the localized surface plasmon resonance of Au nanorods can strongly alter the spectral profile of the emission from adjacent fluorescent molecules. The fluorescent molecules are embedded in a mesostructured silica shell that is uniformly coated on each Au nanorod. The longitudinal plasmon resonance wavelengths of the nanorods are deliberately shifted away from the intrinsic fluorescence emission peak wavelength by synthetically tuning the nanorod aspect ratio. The resultant emission spectra of the fluorescent molecules are found to be remarkably modulated. Besides the intrinsic fluorescence peak, a plasmon-induced new peak emerges at the plasmon resonance wavelength. The intensity of this plasmon-induced fluorescence peak increases as the size of the Au nanorod is increased. This spectral modulation can be understood by depicting the decay process of the fluorophore with multiple vibrational energy levels. The plasmon with a specific resonance energy will enhance the transition rate between the energy levels that have the transition energy approximately equal to the plasmon energy. This plasmon-enhanced transition rate results in a modulated spectral profile of the fluorescence emission.