Mesostructured Silica Research Articles

Mesoporous isocyanurate-containing organosilica–alumina composites and their thermal treatment in nitrogen for carbon dioxide sorption at elevated temperatures

Composite mesostructures consisting of organosilica with isocyanurate bridging groups and alumina have been synthesized using evaporation-induced self-assembly (EISA) in the presence of a triblock copolymer, Pluronic P123, in absolute ethanol solution. These mesostructures were prepared using aluminum isopropoxide and aluminum nitrate nonahydrate as alumina precursors and tris[3-(trimethoxysilyl)propyl]isocyanurate (ICS). The triblock copolymer was removed by extraction with 95% ethanol solution followed by additional thermal treatment of the extracted sample at 300 °C in flowing nitrogen; this process assured a complete removal of the polymeric template without degradation of the ICS bridging groups. The use of aluminum nitrate nonahydrate and a N-containing ICS precursor with a small amount of 3-aminopropyltriethoxysilane (AP) led to the hybrid materials with well-developed porosity and high specific surface area (200–450 m2 g−1). A controlled heating of these materials in nitrogen resulted in N-doped alumina–silica mesostructures showing high affinity towards CO2 at elevated temperatures. The use of inexpensive aluminum nitrate instead of aluminum alkoxides in the EISA synthesis had a significant impact on the pore structure, surface area and adsorption properties of the resulting composite materials.

Lattice Matching in the Epitaxial Formation of Mesostructured Silica Films

Crystallographic orientation of mesostructured silica films on a substrate drastically changes when the substrate is modified with an anisotropic surface. The [01] axis of a two-dimensional (2D) hexagonal structure of the film prepared on a polyimide surface using C(22)EO(20) as a structure-directing agent changes from perpendicular to parallel with respect to the substrate after a rubbing treatment of polyimide, which is accompanied by the simultaneous unidirectional alignment of the cylindrical pores in the plane of the film. The normal direction of the film is [21¯], which has never been observed in the mesostructured silica films reported so far including those with controlled in-plane alignment of the mesochannels. The change of the orientation with respect to the substrate can be explained by the increased lateral distance between the adjacent surface micelles, which is caused by the elongation of the alkyl chains of the surfactant molecules induced by the adsorption onto the polymer surface with a molecular-level anisotropy. These results show that the total structural orientation of the mesostructured silica film is determined by the matching of the intrinsic lattice constant of the mesostructured silica with that of the surface micelle structure on a substrate.

Mesoporous Silica Hollow Spheres with Ordered Radial Mesochannels by a Spontaneous Self-Transformation Approach

We demonstrate a self-transformation approach for the synthesis of ordered mesoporous silica hollow spheres with radially oriented mesochannels. The method is simple and facile, in which mesostructured silica spheres synthesized in a Stober solution can spontaneously transform to hollow structure when they are incubated with water. The formation of the hollow structure does not require any sacrificial templates, emulsion droplets, or surface protective agents. The obtained mesoporous silica hollow spheres possess controllable diameter, tunable shell thickness, high specific surface area, and uniform mesopore. Transmission electron microscopy (TEM) observations show that the formation of the hollow spheres undergoes a selective etching process in the inner section. 29Si NMR spectra and detailed reactions demonstrate that the solid-to-hollow transformation of the Stober silica spheres in water is attributed to the difference in the degree of condensation of silica between their outer layer and inner section...

Investigation of High-Pressure and Temperature Behavior of Surfactant-Containing Periodic Mesostructured Silicas

Surfactant-containing periodic mesostructured silica materials, namely SBA-16 and FDU-12, were studied under pressures between 1 and 4 GPa and temperatures between 100 and 400 °C. At 4 GPa crystallization of coesite can be achieved already at 200 °C. The mild transition of amorphous to crystalline silica is believed to be accomplished by the inbuilt hydroxyl groups present in the starting material. At 2 GPa the crystallization of quartz is accomplished at a temperature of 400 °C. Both quartz and coesite are obtained in nanocrystalline form.

Catalytic applications of recyclable silica immobilized NHC–ruthenium complexes

A monosilylated Hoveyda–Grubbs ruthenium alkylidene has been prepared and grafted through the NHC ligand to a mesostructured silica, in refluxing toluene or at room temperature, giving two new organic–inorganic hybrid silica materials M2 and M3, respectively. While M3 exhibited good performances in several metathesis reactions, M2 showed good selectivity in the hydrosilylation of terminal alkynes, where the β-(Z)-vinylsilane was obtained as major product. Recycling of the supported catalysts without significant decrease in activity and selectivity was proven for at least three cycles in both transformations.

Direct catalytic conversion of glycerol to liquid-fuel classes over Ir–Re supported on W-doped mesostructured silica

Glycerol was converted to liquid fuels through a one-pot process catalyzed by mesostructured Ir–Re–W/SiO2. The reaction was performed over 0.4g of catalyst suspended in an aqueous solution of glycerol (4.72mmol/ml) at 553K under a hydrogen pressure of 6.0MPa for 50h. A hydrocarbon-enriched organic phase was self-separated from the aqueous solution after the reaction. The total yield of oil products was greater than 40%, and the selectivity toward hydrocarbons was approximately 90%. The conversion proceeds through a complicated mechanism that involves a CC coupling reaction and hydrodeoxygenation. Doped tungsten oxide species in a silica matrix are active acidic sites for acid-catalyzed dehydration. Supported iridium species are catalytic active sites for hydrodeoxygenation and CC coupling reactions. The evolution of their chemical states during the catalytic process was detected by X-ray photoelectron spectroscopy. Rhenium species are important co-catalysts for iridium species. These active catalytic species work together to achieve the direct conversion of glycerol in aqueous solution to liquid fuels under a hydrogen atmosphere.

Synthesis of highly dispersed mesostructured cellular foam silica sphere and its application in high-performance liquid chromatography

Highly dispersed mesostructured cellular foam silica spheres of relatively uniform micrometer size (3.5–4.5μm) were successfully prepared using a triblock copolymer EO20PO70EO20 as the structure-directing agent accompanied by TMB and K2SO4. Both two additives have effective influence and K2SO4 have better performance than other inorganic salts such as KCl and NaCl. The pore size and surface area were tuned by TMB and NH4F. The resultant S-MCFs were modified with C18 group before being used as the HPLC stationary phase for effective separation of aromatic compounds and phthalic acid esters. The perfect spherical morphology and large pore volume gave rise to low and stable back pressure. High surface area, ultralarge pore size and unique pore structure would permit high flow rates and afford the possibility for fast separation.

Synthesis of Nickel Phosphide Nanorods as Catalyst for the Hydrotreating of Methyl Oleate

Arrays of nickel phosphide nanorods were successfully synthesized by nanocasting using mesostructured silica SBA-15 as a hard template (HT-Ni2P). After temperature-programmed reduction of the phosphate precursor infiltrated within the pore walls of SBA-15, the unsupported material was obtained by removing the silica matrix with diluted HF. The pore channel of the SBA-15 template stabilizes the Ni2P particles, preventing sintering after the high reduction temperature and shaping their elongated morphology. Moreover, HT-Ni2P catalyst shows an improvement in the textural properties with a significantly higher surface area than the reference sample synthesized in the absence of template. X-ray diffraction revealed that the only crystalline phase present in this material was Ni2P. On the other hand, transmission electron microscopy shows that the catalyst is mainly constituted by agglomerates of nanorods. Through EDX microanalysis the efficient removal of silicon was confirmed. Under hydrotreating conditions, nanorods of Ni2P show a fourfold enhancement in the conversion of methyl oleate with respect to conventional Ni2P synthesized in absence of hard template. Nevertheless, when these data are normalized to surface area, the specific activity of HT-Ni2P nanorods is significantly lower than that of the conventionally prepared sample. Differences in selectivity were also observed as Ni2P nanorods favored the decarboxylation reaction leading to a higher yield of n-heptadecane.

Sulfonated mesoporous silica–carbon composites and their use as solid acid catalysts

The synthesis of highly functionalized porous silica–carbon composites made up of sulfonic groups attached to a carbon layer coating the pores of three types of mesostructured silica (i.e. SBA-15, KIT-6 and mesocellular silica) is presented. The synthesis procedure involves the following steps: (a) removal of the surfactant, (b) impregnation of the silica pores with a carbon precursor, (c) carbonization and (d) sulfonation. The resulting silica–carbon composites contain ∼30wt % of carbonaceous matter with a high density of acidic groups attached to the deposited carbon (i.e.SO3H, COOH and OH). The structural characteristics of the parent silica are retained in the composite materials, which exhibit a high surface area, a large pore volume and a well-ordered porosity made up uniform mesopores. The high density of the sulfonic groups in combination with the mesoporous structure of the composites ensures that a large number of active sites are easily accessible to reactants. These sulfonated silica–carbon composites behave as eco-friendly, active, selective, water tolerant and recyclable solid acids. In this study we demonstrate the usefulness of these composites as solid acid catalysts for the esterification of maleic anhydride, succinic acid and oleic acid with ethanol. These composites exhibit a superior intrinsic catalytic activity to other commercial solid acids such as Amberlyst-15.

Citral hydrogenation on high surface area mesoporous TiO2–SiO2 supported Pt nanocomposites: Effect of titanium loading and reduction temperature on the catalytic performances

Platinum-based catalysts supported on TiO2-modified mesostructured silica were prepared using the direct co-condensation of silica and titania precursors to synthesize the mixed-oxide support. The physico-chemical properties of the Pt/xTi-SBA15 samples with various xTi contents (in mol%) were further evaluated using several techniques, including elemental analysis, X-ray diffraction, N2-physisorption, H2-chemisorption, transmission electronic microscopy and the probe reaction of cyclohexane dehydrogenation to evaluate the metal–support interaction (SMSI effect). All the Pt/xTi-SBA15 samples display high specific surface areas (650–820 m2g−1) and high mesopore volumes (0.44–0.68cm3g−1), with the formation of TiO2 anatase nanoparticles since the low titanium content, i.e. 2mol%. The hexagonal organized pore structure of SBA silica is also strongly altered by the titanium adding, but mixed oxides so obtained present very particular morphology with maintaining of high surface areas.The catalytic performances of the Pt/xTi-SBA15 catalysts were estimated for the citral hydrogenation performed at 70°C under hydrogen pressure (7MPa), and discussed in terms of activity and unsaturated alcohols (UA: nerol and geraniol) selectivity. A synergetic effect on the UA selectivity was obtained on the Pt/xTi-SBA15 catalysts, compared to both Pt/SBA15 and Pt/TiO2 P25 reference samples (reduction temperature=300°C), and was explained by the specific role of the reducible TiO2 species, which under the nano-anatase form generate a strong metal–support interaction (SMSI effect) with platinum after reduction at 300°C. The formation of partially reduced support species was finally modulated by varying the reduction temperature of the catalysts. It is then possible to achieve high selectivity after reduction at 350°C, while maintaining high conversions.

Amino modified mesostructured silica nanoparticles for efficient adsorption of methylene blue

In this work, mesostructured silica nanoparticles (MSNAP) with high adsorptivity were prepared by a modification with 3-aminopropyl triethoxysilane (APTES) as a pore expander. The performance of the MSNAP was tested by the adsorption of MB in a batch system under varying pH (2–11), adsorbent dosage (0.1–0.5gL−1), and initial MB concentration (5–60mgL−1). The best conditions were achieved at pH 7 when using 0.1gL−1 MSNAP and 60mgL−1MB to give a maximum monolayer adsorption capacity of 500.1mgg−1 at 303K. The equilibrium data were evaluated using the Langmuir, Freundlich, Temkin, and Harkins–Jura isotherms and fit well to the Freundlich isotherm model. The adsorption kinetics was best described by the pseudo-second order model. The results indicate the potential for a new use of mesostructured materials as an effective adsorbent for MB.

Fe3O4‐Nanoparticle‐Embedded Multifunctional Hollow Mesoporous Silica Capsules

AbstractMagnetic nanoparticle‐embedded hollow mesoporous silica capsules (Mag‐HMSCs) with a large surface hole are prepared by a cholesterol‐assisted emulsion method. The resulting Mag‐HMSC materials have a hollow capsular geometry with a mesostructured silica wall. They exhibit a very high intracellular delivery efficiency of the representative large protein, antibody immunoglobulin G (IgG). As a result of the embedded Fe3O4 nanoparticles, Mag‐HMSC materials also serve as magnetic resonance imaging (MRI) contrast agents. Good T2‐weighted MR image contrasts were observed.

Development of Sinter-Resistant Core–Shell LaMnxFe1–xO3@mSiO2 Oxygen Carriers for Chemical Looping Combustion

This work investigates the possibility of using LaMn0.7Fe0.3O3.15@mSiO2 as oxygen carriers for chemical looping combustion (CLC). CLC is a new combustion technique with inherent separation of CO2 from atmospheric N2. LaMn0.7Fe0.3O3.15@mSiO2 core–shell materials were prepared by coating a layer of mesostructured silica around the agglomerated perovskite particles. The oxygen carriers were characterized using different methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 sorption, hydrogen temperature-programmed reduction (H2-TPR), and temperature-programmed desorption of oxygen (TPD-O2). The reactivity and stability of the carrier materials were tested in a special reactor, allowing for short contact time between the fluidized carrier and the reactive gas [Chemical Reactor Engineering Centre (CREC) fluidized riser simulator]. Multiple reduction–oxidation cycles were performed. TEM images of the carriers showed that a perfect mesoporous silica layer was formed around samples with 4, 32, and 55 nm in thickness. The oxygen carriers having a core–shell structure showed higher reactivity and stability during 10 repeated redox cycles compared to the LaMn0.7Fe0.3O3.15 core. This could be due to a protective role of the silica shell against sintering of the particles during repeated cycles under CLC conditions. The agglomeration of the particles, which occurred at high temperatures during CLC cycles, is more controllable in the core–shell-structured carriers, as confirmed by SEM images. XRD patterns confirmed that the crystal structure of all perovskites remained unchanged after multiple redox cycles. Methane conversion and partial conversion to CO2 were observed to increase with the contact time between methane and the carrier. Indeed, more oxygen from the carrier surface, grain boundaries, and even from the bulk lattice was released to react with methane. Upon rising the contact time, less CO was formed, which is desirable for CLC application. Increasing the reaction temperature and methane partial pressure lead to enhanced conversions of CH4 under CLC conditions.

Amino functionalized mesostructured SBA-15 silica for CO2 capture: Exploring the relation between the adsorption capacity and the distribution of amino groups by TEM

The distribution of amino groups on amino-functionalized SBA-15 materials for CO2 adsorption was studied by transmission electron microscopy (TEM) in combination with a staining technique using RuO4 in order to analyze the influence of the aminated organic chains location on the CO2 adsorption properties. Mesostructured amino-functionalized SBA-15 materials were obtained by co-condensation, grafting and impregnation using aminopropyl, AP(N), ethylene-diaminepropyl, ED(NN), diethylene-triaminepropyl, DT(NNN) and polyethyleneimine, PEI, as functionalizing agents. CO2 adsorption isotherms of functionalized samples at 45°C showed that both the adsorption capacity (mgCO2/g ads) and the efficiency of amino groups (mol CO2/mol N) depend on the functionalization technique and the amount of organic compound used. While samples synthesized by co-condensation showed negligible CO2 uptake and efficiency, adsorbents prepared by grafting and impregnation presented significant CO2 adsorption capacities but a dissimilar efficiency. Key differences in the location of aminated chains explained the performance of CO2 capture for every adsorbent, being grafted samples the adsorbents where amino groups were better distributed, favouring CO2 diffusion trough the whole structure.

Plasmon-Controlled Förster Resonance Energy Transfer

The localized plasmons of metal nanocrystals have been widely utilized to control a variety of optical signals, such as Raman, fluorescence, and circular dichroism, from proximal dye molecules. We show, on the single-particle level, that the Forster resonance energy transfer between two different fluorophores can be modulated by adjacent plasmonic nanocrystals. The donor and acceptor fluorophore molecules are embedded in a mesostructured silica shell that is uniformly coated on Au–Ag core–shell nanocrystals. The longitudinal plasmon wavelengths of the core–shell metal nanocrystals are synthetically tailored by varying the aspect ratio. Comparison of the scattering and fluorescence spectra taken from the different hybrid nanostructures indicates that the energy transfer efficiency can be controlled by the plasmon wavelength. When the plasmon peak overlaps with the emission peak of the donor, the energy transfer channel is turned off. When the plasmon peak is red-shifted to be in between the emission peak o...

Ni2P/SBA-15 As a Hydrodeoxygenation Catalyst with Enhanced Selectivity for the Conversion of Methyl Oleate Into n-Octadecane

A novel hydrodeoxygenation catalytic system, Ni2P/SBA-15, has been synthesized by temperature-programmed reduction of a nickel phosphate precursor impregnated in the mesostructured silica support. The formation of this active phase was verified by X-ray diffraction, whereas the study by transmission electron microscopy revealed that the catalyst is mainly constituted of nickel phosphide particles of relatively uniform size dispersed within the SBA-15 channels. Both Ni2P/SBA-15 and a reference Ni/SBA-15 catalyst were tested for the hydrodeoxygenation of methyl oleate (C17H33–COO–CH3) in a fixed-bed continuous flow reactor. This compound was used as a convenient surrogate of triglyceride molecules present in vegetable oils that following catalytic hydrotreating yields n-alkanes as the main products. In the whole range of pressure studied (3–40 bar) and for temperatures higher than 290 °C, both systems achieve more than 80% ester conversion at 20 h–1 WHSV, although the Ni/SBA-15 catalyst presents a slightly ...

Poly-l-lysine Functionalized Large Pore Cubic Mesostructured Silica Nanoparticles as Biocompatible Carriers for Gene Delivery

Large pore mesoporous silica nanoparticles (LP-MSNs) functionalized with poly-L-lysine (PLL) were designed as a new carrier material for gene delivery applications. The synthesized LP-MSNs are 100-200 nm in diameter and are composed of cage-like pores organized in a cubic mesostructure. The size of the cavities is about 28 nm with an entrance size of 13.4 nm. Successful grafting of PLL onto the silica surface through covalent immobilization was confirmed by X-ray photoelectron spectroscopy, solid-state (13)C magic-angle spinning nuclear magnetic resonance, Fourier transformed infrared, and thermogravimetric analysis. As a result of the particle modification with PLL, a significant increase of the nanoparticle binding capacity for oligo-DNAs was observed compared to the native unmodified silica particles. Consequently, PLL-functionalized nanoparticles exhibited a strong ability to deliver oligo DNA-Cy3 (a model for siRNA) to Hela cells. Furthermore, PLL-functionalized nanoparticles were proven to be superior as gene carriers compared to amino-functionalized nanoparticles and the native nanoparticles. The system was tested to deliver functional siRNA against minibrain-related kinase and polo-like kinase 1 in osteosarcoma cancer cells. Here, the functionalized particles demonstrated great potential for efficient gene transfer into cancer cells as a decrease of the cellular viability of the osteosarcoma cancer cells was induced. Moreover, the PLL-modified silica nanoparticles also exhibit a high biocompatibility, with low cytotoxicity observed up to 100 μg/mL.

Highly Efficient Enzyme Immobilization and Stabilization within Meso-Structured Onion-Like Silica for Biodiesel Production

Meso-structured onion-like silica (Meso-Onion-S) was synthesized and used as a host of enzyme immobilization. Meso-Onion-S has a 200–300 nm sized primary meso-structured onion building unit, and each onion unit has highly curved mesopores of 10 nm diameter in a multishell structure. Nanoscale enzyme reactors (NERs) in Meso-Onion-S were prepared via a two-step process of enzyme adsorption and subsequent enzyme cross-linking, which effectively prevents the leaching of cross-linked enzyme aggregates from highly curved mesopores of Meso-Onion-S. As a result, NERs in Meso-Onion-S significantly improved the enzyme stability as well as the enzyme loading. For example, NER of lipase (NER-LP) was stable under rigorous shaking for 40 days, while the control sample of adsorbed LP (ADS-LP) with no enzyme cross-linking showed a rapid inactivation due to rigorous enzyme leaching under shaking. Stable NER-LP was successfully employed to produce biodiesels and fatty acid methyl esters, from the LP-catalyzed transesterification of soybean oil with methanol. Interestingly, the specific activity of NER-LP was 23 and 10 times higher than those of free LP and ADS-LP, respectively, revealing the importance of LP stabilization in the form of NER-LP in the presence of organic solvents.

Intracellular protein delivery by hollow mesoporous silica capsules with a large surface hole

We prepared cell membrane-permeable hollow mesoporous silica capsules (HMSCs) by a simple new method. CTAB micellar assembly in cholesterol emulsion gave rise to a novel capsular morphology of the HMSC particles. The HMSCs consisted of mesostructured silica walls with a large surface hole (25–50 nm) and the average particle dimension was 100–300 nm. They exhibited high surface areas of up to 719.3 m2 g−1 and a mesoporous range of pores of 2.4–2.7 nm. The surface-functionalized HMSCs could also be prepared by a similar co-condensation method using tetraethoxysilane with various organoalkoxysilane precursors in the presence of cholesterol. These organically modified HMSCs could be further modified on demand. For example, a carboxy-functionalized HMSC could be surface-functionalized by a green fluorescent 5-aminofluorescein (AFL) through an amidation reaction to afford a fluorescent AFL–HMSC. The hollow capsular morphology of the HMSCs with a large surface hole enabled us to develop very efficient intracellular delivery systems for membrane-impermeable ions, molecules, and various functional proteins. Non-covalent sequestration and delivery of proteins as well as covalent linkage of fluorescent molecules on the silica surface are effective for this system. The highly negatively charged green fluorescent probe mag-fluo-4 could be intracellularly delivered into HeLa cells by HMSC without any difficulty. The HMSCs could also effectively transport large functional proteins such as antibodies into HeLa cells. The efficiency of protein delivery by HMSC seems to be 3–22-fold higher than that of mesoporous silica nanospheres (MSNs) based on confocal laser scanning microscopy (CLSM) analysis.

Formation of a nanohybrid composite between mesostructured cellular silica foam and microporous copper trimesate

A metal-organic framework HKUST-1 or Cu3(BTC)2 was synthesized in mesopores of a COOH functionalized mesostructured cellular silica foam (MCF(M)-COOH), resulting in a hydrophobic nanocomposite. The Cu3(BTC)2 formed in the MCF(M)-COOH has been prepared by a microwave method with sequential incorporation of copper nitrate and 1,3,5-benzenetricarboxylic acid in a water/ethanol mixture containing the MCF(M)-COOH. The nanocomposite was characterized by XRD, BET, FT-IR, TEM and solid state 13C NMR. The nanocomposite had a higher hydrophobic property than pure Cu3(BTC)2 and MCF(M)-COOH as a result of their hybridization. Moreover, the formation of the nanocomposite increased the sorption rate of cyclohexane vapor as compared with those of pure Cu3(BTC)2 and MCF(M)-COOH due to the increased hydrophobicity.