Mesostructured Silica Materials Research Articles

Pyrrole-Containing ABA Triblock Brush Polymers as Dual Functional Molecules to Facilely Access Diverse Mesostructured Materials

We report a new type of ABA-type brush-like triblock copolymer (PEGMA-b-PPHMA-b-PEGMA), which is characterized by two hydrophilic end segments (A) and a hydrophobic midblock (B) bearing polymerizable pyrrole moieties. This kind of symmetric triblock copolymers can not only serve as a structure-directing agent for the formation of mesoporous silicates but also function as a polymerizable monomer or carbon source for further chemical conversion to deliver mesostructured polymer or carbon materials within the preformed silicate framework. By utilizing such dual functional macromolecules, rich morphologies of mesostructures are accessible through microphase separation, and the order–order transition among cubic, lamellar, 2D hexagonal, and bicontinuous mesostructures is sensitively achievable by varying the hydrophilic volume fraction of copolymers and TEOS dosage. These features enable facile access to diverse mesostructured silica, polymer, and carbon materials. Remarkably, due to the brush-like polymer architecture and the formation of loop configuration, a widely tunable pore size from mesoscale to macroscale is realizable through using a relatively lower molecular weight copolymer. Importantly, it is found that the brush-like hydrophilic segments of our triblock copolymers can firmly anchor into the silica walls without appreciable escape of copolymer template in commonly used solvents even under reflux extraction, providing a prerequisite for reliable confined chemical transformation into mesostructured polymer and carbon materials. Our work demonstrates that the design and synthesis of functional amphiphilic block copolymers will bring great opportunity for the development of novel mesostructured materials.

Evaluation of mesostructured silica materials with different structures and morphologies as carriers for quercetin and naringin encapsulation

Two mesostructured silicas with wormhole-like pore arrangement (HMS and MSU-2) were synthesized and evaluated for the first time as carriers for the encapsulation of two bioactive flavonoids (quercetin and naringin). For comparative purposes, a hexagonal mesostructured SBA-15 silica type frequently used as encapsulating support was also prepared and tested. All the materials were characterized before and after the loading with the analytes. Different silica/analyte ratios were evaluated to determine the loading and encapsulation kinetics of the different materials. Both flavonoids were successfully loaded inside the pores of the three silicas. The quercetin loading capacity of HMS was higher than SBA-15 and MSU-2 silicas, whereas for naringin SBA-15 and MSU-2 were slightly more effective. These differences could be attributed to the molecular size of the analytes and the textural properties of the different materials. Nevertheless, HMS was the silica that enabled to release the highest amount of both analytes. Thus, it could be considered a suitable carrier of these flavonoids and an alternative to other materials such as SBA-15. Moreover, the release process was performed under controlled conditions (pH 2.0 and 7.4) to simulate digestive conditions. Quercetin was delivered faster and more efficiently from the encapsulated at pH 2.0, whereas no differences were observed for naringin at both pHs. Finally, the antioxidant activity of the resulting encapsulates was determined. The results obtained suggested the potential use of wormhole-like mesostructured silicas as carriers to enhance the stability and bioavailability of flavonoids, so they can be used in future food and biomedical applications.

Efficient Fabrication of Diverse Mesostructured Materials from the Self-Assembly of Pyrrole-Containing Block Copolymers and Their Confined Chemical Transformation

Block copolymers (BCPs) have been intensively used as structure-directing agents for the synthesis of mesostructured materials because of their rich microphase separation and excellent thermal stability. Incorporating functional moieties into BCPs to extend the currently dominated monotonic role may open new opportunities in the creation of mesostructured materials using one single BCP system. In this work, we reported a new kind of pyrrole-containing BCP (PPHMA-b-PEO), finding that PPHMA-b-PEO could produce the effect of killing more birds with one stone and efficient fabrication of diverse mesostructured materials could be facilely realized. As a structure-directing agent, different ordered mesoporous silicas were produced as expected by using PPHMA-b-PEO. Importantly, the pyrrole-containing template molecules (PPHMA-b-PEO) confined in silica channels could also be used as a polymerizable monomer or precursor for further chemical transformation, leading to a mesostructured polymer and carbon materials. Moreover, because of the reductive activity of pyrrole, numerous oxidative agents could be explored to carry out the related polymerization process, making it possible to facilely access the diverse-doped mesostructured polymer or carbon materials. In addition to mesostructured silica, polymer, and carbon materials, it is also found that the uniform carbon coating derived from PPHMA-b-PEO could serve as a robust support for realizing the mesoporous metal oxides with high crystallinity, as demonstrated by the formation of high crystalline TiO2. All of our results indicate that the rational incorporation of functional moieties into BCPs will bring new opportunities and show great potential for the efficient fabrication of diverse mesostructured materials.

Synthesis, characterization and catalytic study of mesoporous carbon materials prepared via mesoporous silica using non-surfactant templating agents

Mesostructured silica materials with surface area in the range of ~700–900 m2/g have been prepared using hydroxy-carboxylic acid compounds such as tartaric acid, malic acid and citric acid (low cost non-surfactant template/pore forming agents) and tetraethylorthosilicate (TEOS) as silica source by sol-gel reaction. The templates were removed by either soxhlet extraction or calcination method. Mesoporous carbon molecular sieves were then prepared by carbonizing sucrose inside the pores of the above prepared mesoporous silica using sulfuric acid as a catalyst. The materials were characterized by FTIR spectroscopy, powder X-ray diffraction (XRD), N2-sorption studies, microanalysis, thermal analysis and transmission electron microscopy (TEM). The resulting carbon material shows relatively higher surface area (~1100 m2/g), narrow pore size distribution and pore diameter of 4–5 nm. The mesoporosity of carbon material arises from interconnecting channels arrangements of mesoporous silica template. The mesoporous carbon material was used as a support for the immobilization of rhodium complex [HRhCO(TPPTS)3] by ossification method. The prepared catalyst has been tested for the hydroformylation of higher olefins. The activity of the catalyst was improved by 20–30% as compared to the catalyst prepared from a conventional activated carbon support.

Immobilization of Cellulolytic Enzymes in Mesostructured Silica Materials

Mesostructured silica nanoparticles offer a unique opportunity in the field of biocatalysis thanks to their outstanding properties. The tunable pore size in the range of mesopores allows for immobilizing bulky enzyme molecules. The large surface area improves the catalytic efficiency by increasing enzyme loading and finely dispersing the biocatalyst molecules. The easily tunable pore morphology allows for creating a proper environment to host an enzyme. The confining effect of mesopores can improve the enzyme stability and its resistance to extreme pH and temperatures. Benefits also arise from other peculiarities of nanoparticles such as Brownian motion and easy dispersion. Fossil fuel depletion and environmental pollution have led to the need for alternative sustainable and renewable energy sources such as biofuels. In this context, lignocellulosic biomass has been considered as a strategic fuel source. Cellulases are a class of hydrolytic enzymes that convert cellulose into fermentable sugars. This review is intended to survey the immobilization of cellulolytic enzymes (cellulases and β-glucosidase) onto mesoporous silica nanoparticles and their catalytic performance, with the aim to give a contribution to the urgent action required against climate change and its impacts, by biorefineries’ development.

Insights of the kolliphor/water system for the design of mesostructured silica materials

Abstract Herein, mesostructured silica materials have been prepared through the Cooperative Templating Mechanism (CTM) using kolliphor EL (KEL) as biocompatible surfactant. First, the behavior of KEL in water has been studied and the phase diagram has been determined. KEL forms micelles (L1) in water and in higher concentrations a liquid crystal domain appears, which is constituted of a cubic phase (Pm3n) and a direct hexagonal phase. Small Angle X-ray Scattering (SAXS) experiments were performed to study the structure of the Organized Molecular Systems. We show that KEL leads to spherical micelles in water, which have a radius comprised between 6.2 and 6.5 nm, depending on the surfactant concentration. Then, mesoporous materials were synthesized and characterized by SAXS measurements and nitrogen adsorption-desorption analysis. The structure of the recovered material is affected by the surfactant/silica molar ratio. Indeed, the mesopore ordering is detected only for ratios in the range between 0.017 and 0.031. As long as mesostructures are obtained, the mesopores diameters are in accordance with the dimension of the hydrophobic size of the micelles.

Physicochemical Properties of Highly Dispersed Iron Oxides Formed Inside Mesoporous Silica

Nanocomposites based on nanoparticles of iron oxides formed inside porous matrices were studied. Mesostructured silica material SBA-15 impregnated with an aqueous solution of iron(III) oxalate was heattreated in oxidizing (air) and reducing (Ar + 5% H2) media. The resulting samples exhibited small oxide particle sizes (ca. 5 nm) and sufficiently uniform distribution of iron whose content was ~16 wt %.

Functionally Active Membrane Proteins Incorporated in Mesostructured Silica Films.

A versatile synthetic protocol is reported that allows high concentrations of functionally active membrane proteins to be incorporated in mesostructured silica materials. Judicious selections of solvent, surfactant, silica precursor species, and synthesis conditions enable membrane proteins to be stabilized in solution and during subsequent coassembly into silica-surfactant composites with nano- and mesoscale order. This was demonstrated by using a combination of nonionic ( n-dodecyl-β-d-maltoside or Pluronic P123), lipid-like (1,2-diheptanoyl- s n-glycero-3-phosphocholine), and perfluoro-octanoate surfactants under mild acidic conditions to coassemble the light-responsive transmembrane protein proteorhodopsin at concentrations up to 15 wt % into the hydrophobic regions of worm-like mesostructured silica materials in films. Small-angle X-ray scattering, electron paramagnetic resonance spectroscopy, and transient UV-visible spectroscopy analyses established that proteorhodopsin molecules in mesostructured silica films exhibited native-like function, as well as enhanced thermal stability compared to surfactant or lipid environments. The light absorbance properties and light-activated conformational changes of proteorhodopsin guests in mesostructured silica films are consistent with those associated with the native H+-pumping mechanism of these biomolecules. The synthetic protocol is expected to be general, as demonstrated also for the incorporation of functionally active cytochrome c, a peripheral membrane protein enzyme involved in electron transport, into mesostructured silica-cationic surfactant films.

Novel Amine-impregnated Mesostructured Silica Materials for CO2 Capture

In this work, mesostructured KIL-2 silica materials were synthesized and functionalized by impregnation with two different amino sources, polyethylenimine (PEI) and Tetraethylenepentamine (TEPA), for their application to low temperature CO2 capture. Infrared spectroscopy verified that amino groups were successfully incorporated on the surface of the silica support and within its interparticle porosity. N2 sorption analysis of the synthesized silica materials showed a decrease in surface area and total pore volume as a result of the impregnation with the amine groups. The CO2 capture capacity of prepared sorbents was evaluated by CO2 adsorption/desorption isotherms at 25°C and 90°C. The highest sorption capacity was achieved at 90°C for KIL-2-TEPA material (4.35 mmol/g ads), which is significantly higher than values previously reported in the literature for similar type of disordered mesopore structure materials under similar conditions.

9,10-Diphenylanthracene immobilized in mesostructured silica materials with a highly efficient deep-blue fluorescent property

The fluorescent solid material, which emits highly efficient deep-blue light, was successfully fabricated by stabilized 9,10-diphenylanthracene (DPA) molecules in mesostructured silica material (MSM) (referred to as DPA-MSM). The DPA molecules are favorably introduced into the hydrophobic domain of a self-assembled amphiphilic triblock copolymer (poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), Pluronic P123) template through a one-pot synthetic route based on the van der Waals interaction. Powder X-ray diffraction patterns show that the unit-cell parameter of DPA-MSM was 1.2nm larger than that of pure MSM, along with the well-ordered hexagonal structure. The photophysical properties of DPA-MSM have been studied and compared with those of DPA in solution (0.15mM) and powdered states, respectively. Both absorption and emission maxima of the immobilized DPA show a remarkable hypsochromic shift as compared with those of pure DPA powder. In addition, we have observed distinct spectral features in the DPA-MSM, which resemble those of DPA dissolved in cyclohexane, indicating that the immobilized DPA is molecularly separated by the interactions with the poly(propylene glycol) part in P123. The fluorescence decay behavior showed that DPA-MSM has a longer lifetime because the intramolecular motions of DPA are strongly restricted by the confined environment within the pore channel of rigid MSM. These results also show that the immobilized DPA has a higher fluorescence quantum yield than did DPA in dissolved or powdered states. The fabricated blue LED chip with DPA-MSM exhibited outstanding deep-blue color with CIE coordinates of (0.15, 0.03). It was found that the incorporation of DPA in MSM provides an efficient blue emitter in the solid-state.

Mesoporous silica materials from diluted and concentrated solutions of nonionic fluorinated and ionic hydrogenated surfactants mixtures

Herein, mesostructured silica materials have been prepared through both the cooperative self-assembly and liquid crystal templating mechanisms by using mixtures of polyoxyethylene fluoroalkylether and cationic quaternary ammonium surfactants as building blocks. The nonionic surfactant/ionic surfactant/water phase diagram shows that a micellar phase (L1) was formed. The SAXS spectra of the two surfactants micellar phases are different from that of micelles constituted of only one component. This provides informations about the nature of the micelles, which are really mixed. A detailed investigation of the liquid crystals domain was also conducted, and results indicate the presence of a hexagonal mixed phase.Then, mesoporous silica materials with high specific surface area have been synthesized, using surfactant mixtures, and characterized by small angle X-ray scattering, transmission electron microscopy and nitrogen adsorption-desorption analysis. The SAXS patterns show two hexagonal porous networks, showing the presence of the dual porosity.

Investigation of a novel fluorinated surfactant-based system for the design of spherical wormhole-like mesoporous silica

In contrast to hydrogenated based systems that led to many studies, fluorinated surfactants have been little reported. Thanks to their high chemical and thermal stability, these compounds are considered as suitable candidates for the synthesis of porous materials with an enhanced hydrothermal stability. This study reports the synthesis of a new fluorinated surfactant, 2-trifluoromethyl-7,7,8,8,9,9,10,10,11,11,12,12,12-tridecafluoro-4-thia-1-dodecanoic acid (FSC) obtained from the thiol-ene radical addition of 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanethiol onto 2-trifluoromethyl acrylic acid in 85% yield. In the aim of achieving micelles in water to design mesoporous materials according to the cooperative templating mechanism, FSC was modified with water-soluble telechelic diamine (Jeffamine) ED-600. The modified surfactant was deeply characterized by spectroscopic methods and the FSC-Jeffamine ED-600 micellar system was used as porogen to prepare mesoporous materials via the cooperative templating mechanism. Spherical wormhole-like mesostructured silica materials of high specific surface area (850m2/g) and homogeneous pore size distribution (ca. 3.4nm) were obtained by conveniently adjusting the porogen/silica molar ratio and the hydrothermal conditions.

Non-Platinum Group Metal Catalysts for Oxygen Reduction in Fuel Cell Applictaions

Over the last two decades the attention to alternative, low cost catalysts for fuel cells has been steadily rising. Non-Platinum Group Metal (Non-PGM) catalysts of several different types have been proposed, with those based on transition metal and nitrogen-carbon matrix (M-N-C) gaining the most attention. These catalysts are almost exclusively synthesized by pyrolysis in inert or reactive (ammonia-containing) atmosphere. Often a templating approach is used whether explicitly (hard template) or as a part of the processing conditions (soft template or pore-forming agents). Lately, such Non-PGM catalyst have started to be tested in Proton Exchange Membrane (PEM) and Alkaline Exchange Membrane (AEM) fuel cells, often aiming automotive applications. UNM has developed and demonstrated a family of Non-PGM catalysts from the M-N-C family of materials derived with the use of hard (oxide) templates. These catalysts were transferred as a technology to Pajarito Powder Co. and deployed as products for variety of applications. In this overview we will address the critical challenges that the UNM team has faced on the way to introduction of such catalysts for both acid and alkaline polymer electrolyte fuel cells. UNM catalysts synthesis is based on a Sacrificial Support Method (SSM). This is a templated synthesis approach for obtaining hierarchically structured materials based on the "lost wax technique" practiced through pyrolysis. In this method the catalysts precursors are being absorbed on mono-dispersed silica (or other oxide powder), or on meso-structured silica material, obtained by micro-emulsion templating, aerosol synthesis routes or evaporation-induced self-assembly. This oxide template is impregnated with the solution of the catalyst precursors: transition metal salt and a nitrogen-containing organic substance (usually an amine, nitrogen-containing polymer or macrocycle). Alternatively, a mechanochemical approach is used based on mechanically mixing (ball-milling) the support (usually mono-dispersed or meso-structured structured silica) and the transition metal, and nitrogen-carbon precursor. The mix is then thermally processed (pyrolyzed) and then the silica support is removed by etching (in KOH or HF). This results in a open frame structure of a “self-supported” material that consists of the catalyst only. The support has been removed by chemical dissolution. Synthesis of Non-PGM catalysts based on transition metals and N-containing carbonaceous materials obtained by both methods will be discussed. 1-5 Such hierarchical structures are advantageous in enhancement of the fuel cell performance since they correspond to the different levels of transport in the corrugated electrode matrixes. The fact that these materials consist of catalytic substance only (the support has been etched away) allows detail structural characterization. Using several methods: XPS, XAFS/XANES, Mossbauer Spectroscopy, BET and adsorption isotherm-derived pose size distribution, etc. we have built a hypothesis of the active site that includes graphene-like material with N-defects that include transition metal atomically dispersed and associated with the nitrogen defects in graphene (see Fig.). The mechanism of the oxygen reduction reaction on these M-N-C materials will be discussed as well. A wide variety of materials can be made by these methods in which not only the composition but also the microstructure can be varied. It is the combination of these attributes - control over microstructure at a number of different length scales and composition, simultaneously - that is extremely important to the performance of the electrocatalyst materials in a fuel cells. This paper will bring examples of successful practical applications of our materials in automotive technologies using both Polymer Electrolyte (PEMFC) and Alkaline Membrane (AMFC) Fuel Cells with gaseous and liquid fuels. 6-10 References 1 K. Artyushkova, et al., Top. Catalysis, 46 (2007) 263-275 2 S. Pylypenko, et al., Electrochim. Acta, 53 (2008) 7875-7883 3 J. Ziegelbauer, et al., J. Phys. Chem. C, 112 (2008) 8839–8849 4 T. Olson, et al., J. Power Sources, 183 (2008) 557–563 5 T. Olson, et al., J. Electrochem. Society, 157 (2010) B54-B63 6 A. Serov, et al., Electrochem. Comm., 22 (2012) 53-56 7 A. Serov, et al., Appl. Catalysis B: Env., 127 (2012) 300-306 8 M. Robson, et al., Electrochim. Acta, 90 (2013) 656–665 9 P. Atanassov and A. Serov, Japanese Society of Automotive Engineers, 67 (2013) 68-71 A. Serov, et al., Advanced Energy Materials, 4 (2014) DOI: 10.1002/aenm.201301735 Figure 1

Fabrication of Mesostructured Silica Materials through Co-Structure-Directing Route

Abstract This account describes our recent advances on the fabrication of mesoporous/mesostructured silica materials through the co-structure-directing route. This route establishes critical interactions between the head groups of templates and co-structure-directing agent (CSDA) to form highly ordered mesostructures and well-defined morphologies. Three particular approaches are described based on the different kinds of templates: anionic surfactants, cationic surfactants, and biomolecules (DNA and peptides). Notably, because of the pairing effect between templates and CSDA, the functional groups can be arranged regularly in the surface of the mesopores. Moreover, the applications and future perspective of mesoporous materials synthesized through co-structure-directing route are briefly described.

Synthesis and application of fatty acid derived templates for the preparation of mesostructured silica material

Bio-Organic-Salts (BOSs) synthesized from fatty acid methyl esters (FAMESs) have been developed for use as a bio-derived template for mesostructured material.

Hybrid polarizing solids for pure hyperpolarized liquids through dissolution dynamic nuclear polarization.

Hyperpolarization of substrates for magnetic resonance spectroscopy (MRS) and imaging (MRI) by dissolution dynamic nuclear polarization (D-DNP) usually involves saturating the ESR transitions of polarizing agents (PAs; e.g., persistent radicals embedded in frozen glassy matrices). This approach has shown enormous potential to achieve greatly enhanced nuclear spin polarization, but the presence of PAs and/or glassing agents in the sample after dissolution can raise concerns for in vivo MRI applications, such as perturbing molecular interactions, and may induce the erosion of hyperpolarization in spectroscopy and MRI. We show that D-DNP can be performed efficiently with hybrid polarizing solids (HYPSOs) with 2,2,6,6-tetramethyl-piperidine-1-oxyl radicals incorporated in a mesostructured silica material and homogeneously distributed along its pore channels. The powder is wetted with a solution containing molecules of interest (for example, metabolites for MRS or MRI) to fill the pore channels (incipient wetness impregnation), and DNP is performed at low temperatures in a very efficient manner. This approach allows high polarization without the need for glass-forming agents and is applicable to a broad range of substrates, including peptides and metabolites. During dissolution, HYPSO is physically retained by simple filtration in the cryostat of the DNP polarizer, and a pure hyperpolarized solution is collected within a few seconds. The resulting solution contains the pure substrate, is free from any paramagnetic or other pollutants, and is ready for in vivo infusion.

ChemInform Abstract: Advancements in Solid Acid Catalysts for Biodiesel Production

AbstractReview: 150 refs.

Efficient cyclohexyl acrylate production by direct addition of acrylic acid and cyclohexene over SBA-15-SO3H

A set of propysulfonic acid modified mesostructured silica materials (SBA-15-SO3H) were prepared by a convenient one-pot co-condensation method. Samples were characterized by XRD, FT-IR, Py-FTIR, N2 adsorption–desorption, elemental analysis and acid–base titration. The catalysts exhibited excellent activities and selectivities for addition esterification of acrylic acid and cyclohexene. The highest catalytic activity (82.8 %) and selectivity (92.6 %) were obtained over SBA-15-SO3H (15 %). The efficient catalytic performance of SBA-15-SO3H (15 %) was attributed to the high surface area, proper mesopore texture and presence of sulfonic acid groups in its structure. SBA-15-SO3H catalyst was easily separated and recycled with high stability.

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.

Synthesis, characterization and biocompatibility evaluation of iron oxide incorporated magnetic mesoporous silica

On the basis of a thermal process, a facile, low cost, one-step approach for preparing iron oxide (Fe(2)O(3)) incorporated ordered magnetic mesoporous silica nanocomposites by a co-operative self-assembly approach is presented. Various mesostructured silica materials incorporated with different amounts of iron oxide (nSi/nFe = 1/1, 1/0.5, 1/0.25 and 1/0.123) at various pH (<1, 3, 5 and 7) were synthesized and characterized by electron microscopy and X-ray diffractometry. Further, the surface area and magnetic properties were evaluated using N(2)-sorption analyses, and a superconducting quantum interference device interfaced with a vibrating sample magnetometer (SQUID-VSM) respectively. The transmission electron micrographs and nitrogen sorption analysis indicated that most of the Fe(2)O(3) domains of several nanometers were embedded in the silica walls, rather than dispersed in the mesopores. The incorporation of iron oxide into the mesopores without compromising the structural and textural properties was achieved at pH < 1. These structures have great potential in diagnostics and therapeutics. However, the acceptance of this material by the biological host is a critical issue for such biomedical applications. In this study, we have also evaluated the in vivo biocompatibility of these magnetic mesoporous materials in a rat model. The histopathological results show that this magnetic material can be classified as a level 2 biomaterial that can be safely used for short term applications such as MRI imaging, hyperthermia, targeted drug delivery, etc.