Mesostructured Silica Research Articles

PW$_{12}$-APTES@MCF: effective nanosized mesoporous composites for the oxidation of benzyl alcohols

The surface of mesocellular silica foam (MCF) was modified by grafting 3-aminopropyl-triethoxysilane (APTES) to have a positive charge, and thus to provide sites for the immobilization of H_3PW_{12}O_{40} (PW_{12}). This modified nanosized mesoporous silica (PW_{12}-APTES@MCF) was characterized by FTIR, XRD, BET, and TEM. XRD and adsorption--desorption analysis shows that the mesostructure of silica remains intact after various modifications, while the spectral technique shows the successful grafting of the neat PW_{12} inside the porous silica support. The oxidation of alcohols occurs effectively and selectively with H_2O_2 as the oxidant. PW_{12}-APTES@MCF was used as the catalyst. The catalyst can be reused several times but it will be less active.

User-friendly synthesis of highly selective and recyclable mesoporous titanium-silicate catalysts for the clean production of substituted p-benzoquinones

Mesoporous titanium-silicates have been prepared following the evaporation-induced self-assembly (EISA) methodology and characterized by elemental analysis, XRD, N2 adsorption, SEM, DRS UV–Vis and Raman techniques. The use of acetylacetone during synthesis allowed the formation of highly dispersed dimeric and/or small oligomeric Ti species, within the mesostructured silica network, to be realized. The materials catalyse oxidation of alkylsubstituted phenols to corresponding p-benzoquinones with 100% selectivity using the green oxidant – 30% aqueous hydrogen peroxide. The titanium-silicates prepared by the convenient and versatile EISA-based procedure reveal the true heterogeneous nature of the catalysis and do not suffer from titanium leaching. They show advantages over other types of mesoporous Ti,Si-catalysts, such as TiO2–SiO2 mixed oxides and grafted Ti/SiO2, in terms of the catalyst stability and reusability.

Tuning Function of the Light-Driven Proteorhodopsin Proton Pump by Formation of Oligomeric and Surfactant-Based Synthetic Complexes

The Proteorhodopsin (PR) membrane protein equips marine bacteria worldwide with the capability of converting light energy into chemical energy. The proton pump absorbs light via an intrinsic retinal molecule, followed by subsequent conformational changes that store energy in the form of a proton gradient across the bacterial cell membrane. Because PR exists in the complex membrane environment, a multitude of factors could contribute to its light absorption and energy-transducing properties, including protein-protein, protein-lipid, and electrostatic interactions. Here we demonstrate by steady-state and time-resolved optical absorption spectroscopy that there are profound functional effects (e.g. altered activity and photocycle kinetics) modulated by the association of PR with other PR molecules to form oligomeric complexes, and by reconstitution in membrane-mimetic surfactant environments with varying chemical and physical characteristics.We study the molecular-level properties accompanying these functional consequences of environment with magnetic resonance techniques, specifically amino acid side-chain dynamics probed by Electron Paramagnetic Resonance (EPR) and local hydration dynamics by Overhauser Dynamic Nuclear Polarization (ODNP). Such information lends insight into the mechanisms tuning PR function and can be applied towards the design of function-enhancing synthetic environments. For example, surface water diffusion is faster in a model lipid bilayer than in surfactant micelles, concurrent with faster conformational motion, and possibly efficiency (Hussain, et.al, Angew. Chem. 52, 2013). We further have developed a synthetic platform for potential device applications by the incorporation of active PR into surfactant-directed mesostructured silica films, in both its hexameric and monomeric forms, and using different surfactants. This provides a unique opportunity to apply results from the biophysical characterization of PR to the design of an optimized solar energy-harvesting biomaterial, tuning molecular interactions to match the desired functional properties.

Nanoporous silica-supported organocatalyst: a heterogeneous and green hybrid catalyst for organic transformations

Organically modified hybrid silica mesostructures are becoming increasingly promising candidates as catalysts for various kinds of organic reactions and green chemistry.

Synthesis of cubic Ia-3d mesoporous silica in anionic surfactant templating system with the aid of acetate

Mesoporous silica with three-dimensional (3D) bicontinuous cubic Ia-3d structure and fascinating caterpillar-like morphology was synthesized by using anionic surfactant N-lauroylsarcosine sodium (Sar-Na) as the template and 3-amionpropyltrimethoxysilane (APS) as the co-structure-directing agent (CSDA) with the aid of acetate. A phase transformation from high interfacial curvature 2D hexagonal to low interfacial curvature 3D cubic Ia-3d occurred in the presence of a proper amount of acetate. Other species of salts (excluding acetate) had the ability to induce the caterpillar-like morphology, but failed to induce the cubic Ia-3d mesostructure. Furthermore, [3-(2-aminoethyl)-aminopropyl]trimethoxysilane (DAPS) was also used as the CSDA to synthesize Ia-3d mesostructured silica under the aid of sodium acetate. After extraction of the anionic surfactants, amino and di-amine functionalized 3D bicontinuous cubic Ia-3d mesoporous silicas were obtained and used as supports to immobilize Pd nanoparticles for supported catalysts. The catalytic activity of the catalysts was tested by catalytic hydrogenation of allyl alcohol.

Butadiene production from bioethanol and acetaldehyde over tantalum oxide-supported ordered mesoporous silica catalysts

A series of ordered mesoporous silica (OMS) supported tantalum oxide samples (Ta/OMS) were tested as catalysts for the production of 1,3-butadiene (BD) from ethanol and acetaldehyde. To study the influence of the type of mesostructure, the pore size, and the particle morphology on the performance of the catalyst, Ta/OMS catalysts were prepared using two different mesostructured silica supports (2-D hexagonal SBA-15 and 3-D cubic KIT-6) and five different 2-D hexagonal OMS supports (SBA-15 series and MMS) with varied mesopore diameters in the range of 2.5–10.9nm. The obtained Ta/OMS catalysts were characterized by a nitrogen physisorption analysis and X-ray diffraction, as well as by scanning electron micrography. The catalytic results showed that the pore size and crystal size of OMS samples are more important than mesopore structure such as pore dimension and pore shape to optimize the catalytic performance of Ta/OMS catalysts for BD production from ethanol and acetaldehyde.

Protonation of Al-grafted mesostructured silica nanoparticles (MSN): Acidity and catalytic activity for cumene conversion

The hexagonal structure of the mesostructured silica nanoparticles (MSN) based solid acid catalyst was synthesized using 1,2-propanediol as a co-solvent by sol–gel method, followed by aluminum grafting and protonation. The activity of the catalysts was tested for cumene conversion in a pulse microcatalytic reactor at 323–573K. XRD, TEM and N2 physisorption results confirmed the hexagonal ordered structure with a pore diameter of 3.4–4.0nm, a particle size of 70–120nm and a surface area of 588–995m2/g. Solid state NMR and IR results confirmed that the aluminum grafting and protonation form framework and extra-framework aluminums which led to generating strong Brønsted and Lewis acidic sites. High activity in the cumene conversion was only observed on HAlMSN producing propylene, benzene, toluene via a cracking on protonic acid sites and producing a main product of α-methylstyrene via a dehydrogenation on Lewis acidic sites at high reaction temperature. While only α-methylstyrene and higher hydrocarbon (⩾C10) were produced at low reaction temperature showing the permanent Brønsted acid sites did not involve in the cumene conversion. It is suggested that the presence of hydrogen and strong Lewis acid sites increased the stability and activity of the HAlMSN catalyst in the cumene conversion. Although the small deactivation of HAlMSN was observed during the reaction due to the formation of small coke deposits on the surface, the reactivation recovered the activity of catalyst and the high activity was still observed after 60h of reaction. The high stability and activity of HAlMSN in the cumene conversion can be considered as a method for the production of α-methylstyrene via a dehydrogenation process.

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.

Aqueous mercury adsorption in a fixed bed column of thiol functionalized mesoporous silica

The aim of this work was to investigate the aqueous mercury adsorption in a fixed bed of mesostructured silica SBA-15 functionalized with propylthiol by co-condensation (SBA-15-SH). Powdered synthesized adsorbents were used to prepare pellets with sizes ranging from 0.5 to 1 mm. The physicochemical properties determined from N2 adsorption and chemical analysis were compared for powder and pellets. Batch static experiments were carried out to obtain the equilibrium mercury adsorption isotherms, resulting that although the maximum adsorption capacity was reduced from powder to pellets, the materials maintained high efficiency for mercury removal even at very low aqueous metal concentration. Dynamic experiments were carried out in a fixed bed column by modifying the volumetric flow rate, bed length, inlet concentration, and amount of propylthiol groups incorporated to the adsorbent, and analyzing the temporal scale and the mercury adsorption capacities. The elution of the fixed bed was carried out chemically by circulating an aqueous 2 M hydrobromic acid stream for 2 h so achieving a complete recovery of the mercury previously adsorbed. Simplified dynamic equations of Bohart–Adams and Wolborska were used for modeling the breakthrough curves.

Highly active Ni-promoted mesostructured silica nanoparticles for CO2 methanation

Mesostructured silica nanoparticles (MSN) and Ni loaded onto MSN (Ni/MSN) for the methanation of CO2 were prepared by the sol–gel and impregnation methods. Catalytic testing was conducted in the temperature range of 423–723K under atmospheric pressure in the presence of H2. Ni supported on MSN was compared with others types of support such as MCM-41 (Mobile Crystalline Material), HY (protonated Y zeolite), SiO2 and γ-Al2O3. The activity of CO2 methanation followed the order: Ni/MSN>Ni/MCM-41>Ni/HY>Ni/SiO2>Ni/γ-Al2O3. The nitrogen physisorption and pyrrole adsorbed IR spectroscopy results indicated that the high activity of Ni/MSN is due to the presence of both intra- and inter-particle porosity which led to the high concentration of basic sites. In addition, the correlation between N–H band intensity and the turnover frequency revealed that the methanation activity increased with increasing of the concentration of basic sites. The presence of defect sites or oxygen vacancies in MSN was responsible for the formation of surface carbon species, while Ni sites dissociated hydrogen to form atomic hydrogen. The surface carbon species then interacted with atomic hydrogen to form methane. The Ni/MSN catalyst performed with good stability and no deactivation up to 200h.

Sequential desilication–isomorphous substitution route to prepare mesostructured silica nanoparticles loaded with ZnO and their photocatalytic activity

A simple electrochemical method followed by impregnation was employed to prepare a coral-like zinc oxide catalyst loaded on mesostructured silica nanoparticles (ZnO/MSN). The introduction of zinc species onto a silica framework was found to form an interaction between the host and support material. XRD analysis suggested the presence of zinc on the internal pore walls of MSN. FE-SEM and TEM analyses displayed nanorods, nanoparticles and coral-like shapes of ZnO, MSN and ZnO/MSN, respectively. 29Si NMR and FTIR results showed that desilication occurred in the silica framework of the MSN accompanied by isomorphous substitution of Zn2+ cations to form an active species Zn–O–Si bond. The photocatalytic activity of the ZnO/MSN was tested by decolorizing methyl orange (MO). It was found that increasing the ZnO loading led to a higher recombination rate of photoinduced electron–hole pairs, which resulted in decreased photocatalytic activity. The highest decolorization rate was obtained using 1gL−1 of 5wt% ZnO/MSN with an optimum dosage of 3.06×10−2mM MO after 8h contact time at pH 2 under UV irradiation. A kinetic study demonstrated that the photocatalytic reaction followed the pseudo first-order model. The photocatalyst was still stable after five cycling runs with a small amount of Zn was leached (<3.0%).

Self-Assembly Mechanism of Folate-Templated Mesoporous Silica

A method to form ordered mesoporous silica based on the use of folate supramolecular templates has been developed. Evidence based on in situ small-angle X-ray scattering (SAXS), electron microscopy, infrared spectroscopy, and in situ conductivity measurements are used to investigate the organic-inorganic interactions and synthesis mechanism. The behavior of folate molecules in solution differs distinctively from that of surfactants commonly used for the preparation of ordered mesoporous silica phases, notably with the absence of a critical micellar concentration. In situ SAXS studies reveal fluctuations in X-ray scattering intensities consistent with the condensation of the silica precursor surrounding the folate template and the growth of the silica mesostructure in the initial stages. High-angle X-ray diffraction shows that the folate template is well-ordered within the pores even after a few minutes of synthesis. Direct structural data for the self-assembly of folates into chiral tetramers within the pores of mesoporous silica provide evidence for the in register stacking of folate tetramers, resulting in a chiral surface of rotated tetramers, with a rotation angle of 30°. Additionally, the self-assembled folates within pores were capable of adsorbing a considerable amount of CO2 gas through the cavity space of the tetramers. The study demonstrates the validity of using a naturally occurring template to produce relevant and functional mesoporous materials.

Assembling photoluminescent tri(8-quinolinolato)aluminum into periodic mesoporous organosilicas

Mesostructured and mesoporous materials are emerging as a new class of optical materials. However, their synthesis is nontrivial. In this work, periodic mesostructured metal complex-containing silicas of MCM- and SBA-type bearing homogeneously distributed photoluminescent tri(8-quinolinolato)aluminum inside the channel walls (denoted as Alq3@PMO-MCM and Alq3@PMO-SBA, respectively) have been achieved via one-pot co-assembling of inorganic/surfactant/optically active species. A comprehensive multianalytical characterization of the structural and optical properties demonstrates that both Alq3@PMO-MCM and Alq3@PMO-SBA series gainfully combine the photoluminescent properties of Alq3 with the porous features of PMOs. Regularly arranged pores provide high surface area to disperse optically active components well and render Alq3-containing PMOs promising materials for optoelectronic applications.

Ordered mesoporous silica and aluminosilicate-type matrix for amikacin delivery systems

Amikacin, which belong to aminoglycosides class of antibiotics, was used to obtain drug delivery systems based on MCM-41-type supports. In order to study the factors that influence the adsorption and in vitro release of amikacin, a series of mesostructured silica and aluminosilicates were employed as carriers. The toxicity of the supports was evaluated by proliferation tests on murine fibroblastic cells (3T3) and no significant toxic effect was observed. Small-angle and wide-angle X-ray diffraction data, N2 adsorption/desorption isotherms, as well as DSC analyses proved the presence of drug molecules in amorphous state into the channels of mesostructured MCM-41-type supports. It was found that the amikacin uptake depends mainly on the total pore volume of the carrier. The aluminum content in the mesostructured matrix with average pore size smaller than 2.4nm is favorable to the drug uptake. In vitro release profiles in phosphate buffer solution, pH 7.4, exhibited a burst effect of 40% drug in the first hour of the assay, followed by a slow rate of amikacin delivery. Slower release kinetics and a lower amikacin cumulative release amount from mesostructured aluminosilicates than silica carriers were noticed.

Novel synthetic sol–gel CaO based pellets using porous mesostructured silica in cyclic CO2 capture process

In this study, four CO2 carriers were examined through 31 carbonation–calcination cycles. The carriers included one natural limestone (Cadomin) and three modified calcium oxide-based sorbents, Cadomin–calcium aluminate cement pellets (CD–CA-14), Cadomin–silica-sol pellets (CD–Si), and Cadomin-mesostructured silica core/shell pellets (CD–CS). The modified sorbents were not only prepared using two different binders (calcium aluminate cement, CA-14, and silica-sol, HS-40), but the CD–CS sample was provided with a protective porous shell (using a mesoporous silica layer). Natural Cadomin limestone was the base common to the modified sorbents. The surface morphology and porosity of these sorbents before and after different cycles were investigated by scanning electron microscopy (SEM) and nitrogen physisorption (BET/BJH). X-ray diffraction (XRD) was also used to identify the crystal phase composition of the sorbents before and after calcination. The results showed that the presence of the Mayenite phase (Ca12Al14O33) in the CD–CA-14 pellets led to slower decay in sorbent activity with increased number of cycles. Although the CD–CS pellets showed the best performance in the retention of CO2 uptake activity over 31 cycles, with an activity loss of only 46%, the pellet shells experienced severe disintegration during the first few cycles.

Role of 3-aminopropyltriethoxysilane in the preparation of mesoporous silica nanoparticles for ibuprofen delivery: Effect on physicochemical properties

The development of mesoporous silica nanoparticle-based platforms for a controlled drug delivery was studied. Mesoporous silica nanoparticles (MSN) was synthesized and modified with 3-aminopropyltriethoxysilane (APTES) using co-condensation (MSNco) and post-grafting (MSNpost) methods. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data confirmed the formation of ordered mesostructured silica nanoparticles. The performance of MSNs was then tested on an ibuprofen immobilization and release. The results revealed that unmodified MSN demonstrated the best immobilization rate and capacity of ibuprofen (98%), MSNpost exhibited higher ibuprofen adsorption (78%) as compared to MSNco (71%), suggesting the modification method is not the dominant factor for the adsorption studied. In fact, according to the FT-IR results, the silanol groups density was found to be the contributing factor that affected the adsorption. The in vitro drug release was also investigated with simulated biological fluids. In 20h, MSNco showed the fastest release of ibuprofen (100%), followed by MSN (50%) and MSNpost (38%). Both pore size and amine groups influenced the rate of the release process. From the results, MSN and MSNpost is suggested to have suitable features for slow drug release which provides constant release over a defined period to avoid repeated administration. While MSNco could be best employed as a fast release system that provides initial burst of drug release to achieve rapid and maximum relief.

Synthetic Chemistry with Periodic Mesostructures at High Pressure

Over the last two decades, researchers have studied extensively the synthesis of mesostructured materials, which could be useful for drug delivery, catalytic cracking of petroleum, or reinforced plastics, among other applications. However, until very recently researchers used only temperature as a thermodynamic variable for synthesis, completely neglecting pressure. In this Account, we show how pressure can affect the synthetic chemistry of periodic mesoporous structures with desirable effects. In its simplest application, pressure can crystallize the pore walls of periodic mesoporous silicas, which are difficult to crystallize otherwise. The motivation for the synthesis of periodic mesoporous silica materials (with pore sizes from 2 to 50 nm) 20 years ago was to replace the microporous zeolites (which have pore sizes of <2 nm) in petroleum cracking applications, because the larger pore size of mesoporous materials allows for faster transport of larger molecules. However, these mesoporous materials could not replace zeolite materials because they showed lower hydrothermal stability and lower catalytic activity. This reduced performance has been attributed to the amorphous nature of the mesoporous materials' channel walls. To address this problem, we developed the concept of "nanocasting at high pressure". Through this approach, we produced hitherto-unavailable, periodic mesostructured silicas with crystalline pore walls. In nanocasting, we compress a periodic mesostructured composite (e.g. a periodic mesoporous silica with carbon-filled pores) and subsequently heat it to induce the selective crystallization of one of the two phases. We attain the necessary high pressure for synthesis using piston-cylinder and multianvil apparatuses. Using periodic mesostructured silica/carbon nanocomposites as starting material, we have produced periodic mesoporous coesite and periodic mesoporous quartz. The quartz material is highly stable under harsh hydrothermal conditions (800°C in pure steam), verifying that crystallinity in the channel walls of periodic mesoporous silicas increases their hydrothermal stability. Even without including the carbon phase in the silica pores, we could obtain mesoporous coesite materials. We found similar behavior for periodic mesoporous carbons, which convert into transparent, mesoporous, nanopolycrystalline diamond at high-pressure. We also show that periodic mesoporous materials can serve as precursors for nanocrystals of high-pressure phases. We obtained nearly monodisperse, discrete stishovite nanocrystals from periodic mesoporous silicas and coesite nanocrystals from periodic mesoporous organosilicas. The stishovite nanocrystals disperse in water and form colloidal solutions of individual stishovite nanocrystals. The stishovite nanocrystals could be useful for machining, drilling, and polishing. Overall, the results show that periodic mesoporous materials are suitable starting materials for the synthesis of nanoporous high-pressure phases and nanocrystals of high pressure phases. The substantially enhanced hydrothermal stability seen in periodic mesoporous silicas synthesized at high pressure demonstrates that high pressure can be a useful tool to produce porous materials with improved properties. We expect that synthesis using mesostructures at high pressure can be extended to many other materials beyond silicas and carbons. Presumably, this chemistry can also be extended from mesoporous to microporous and macroporous materials.

Efficient conversion of levulinic acid into alkyl levulinates catalyzed by sulfonic mesostructured silicas

Sulfonic mesoporous silicas have demonstrated an outstanding catalytic performance in the esterification of levulinic acid with different alcohols to produce alkyl levulinates, a family of chemicals considered to be excellent oxygenated fuel extenders for gasoline, diesel and biodiesel. Catalyst screening indicated that propylsulfonic acid-modified SBA-15 material was the most active one, among tested materials, due to a combination of moderately strong sulfonic acid sites with relative high surface hydrophobicity. Under optimized reaction conditions (T=117°C, ethanol/levulinic acid molar ratio=4.86/1 and catalyst/levulinic acid=7wt%) almost 100% of levulinic acid conversion was achieved after 2h of reaction, being negligible the presence of levulinic acid by-products or ethers coming from intermolecular dehydration of alcohols. The catalyst has been reused, without any regeneration treatment, up to three times keeping almost the high initial activity. Interestingly, a close catalytic performance to that achieved using ethanol has been obtained with bulkier alcohols.

Preparation of silicas impregnated with HPBI, HPMSP and DEHPA and their application in the solid–liquid extraction of Cu(II) and Zn(II)

The synthesis of mesostructured silicas impregnated with acidic chelators, an acylisoxazolone 3-phenyl-4-benzoyl-5-isoxazolone (HPBI), an acylpyrazolone 1-phenyl-3-methyl-4-stearoyl-5-pyrazolone (HPMSP) and an organophosphoric acid di-(2-ethylhexyl)-phosphoric acid (DEHPA) was undertaken. The different solids were characterized by physico-chemical methods: XRD, FTIR, thermal analysis, TEM, and N2-sorption. The porous integrity of the silica was preserved after impregnation. The amount of ligand was evaluated by UV–Visible analysis of washings, weight loss after calcination and thermal analysis. More than 90% of the ligand used for the impregnation was trapped into the functionalized solids. These materials achieved encouraging extractive performances in removing copper with HPBI below pH 1 and in removing zinc with DEHPA below pH 3 in sulfate media.

Strict In-Plane Alignment Control of Block-Copolymer-Templated Mesostructured Silica Films Using an Alignment-Controlling Agent

An alkoxysilane with an alkyl chain is introduced as an alignment-controlling agent of a block-copolymer-templated mesostructured silica film. Use of the alkylalkoxysilane achieves the alignment of the mesochannels of a triblock-copolymer-templated film by an intermolecular interaction with a rubbing-treated polyimide film. Co-use of an alkoxysilane with a hydroxymethyl group as a hydrophobicity reducing agent improves the alignment close to that of the film prepared using an alkyl surfactant. This concept widens the range of the structural period of aligned mesoporous films and thus widens the useful range of the anisotropic optical properties.