Mesostructural Ordering Research Articles

Influence of silica source in the catalytic activity and heterogenity of mesoporous vanadosilicates

Vanadium-substituted ordered mesoporous silicates (V-OMS) were synthesized hydrothermally using two common silica sources, viz. fumed silica and tetraethyl orthosilicate, and their surface properties and catalytic activities were evaluated in the liquid phase oxidation of 1-napthol using aqueous H 2O 2 as an oxidant. The catalysts were characterized by XRD, N 2 adsorption–desorption, DRUV–vis and TPR to evaluate the mesostructural ordering and the local environment of vanadium in the MCM-41 matrix. Characterization data of the fresh calcined catalysts reveal a more ordered hexagonal structure for the V-MCM-41 catalyst prepared by using tetraethyl orthosilicate as the silica source while spent catalyst showed that the material is less stable than the V-MCM-41 catalyst prepared by using fumed silica as the silica source. Further, the observed differences in the selectivity behavior of extracted and calcined forms of both vanadium catalysts shows that the treatment conditions had a decisive role in the formation of extra framework metal species and further in the mesoscopic structural ordering.

Ordering improvement of mesoporous silica materials templated from semi-fluorinated nonionic surfactants by introduction of organic additives

Worm-like mesoporous silica materials are synthesized in both acidic (JLU-11) and neutral (JLU-12) media using a semi-fluorinated surfactant of CF 3(CF 2) 5(EO) 14 as a template. After introduction of 1,3,5-trimethylbenzene (TMB) in the synthesis, ordered hexagonal mesoporous silica materials (JLU-11(o) and JLU-12(o)) are obtained. The additive of TMB in the synthesis of mesoporous materials (JLU-11(o) and JLU-12(o)) is actually used as an effective agent for improving the order of mesostructure for the samples, rather than a swelling agent for the expansion of mesopore size, which is quite different from the results of mesoporous materials templated from hydrocarbon surfactants (MCM-41 and SBA-15). Furthermore, the ordering improvement of the samples is well characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and N 2 isotherms.

Generalized and Facile Synthesis Approach to N-Doped Highly Graphitic Mesoporous Carbon Materials

The synthesis and characterization of porous graphitic carbon materials via chemical vapor deposition (CVD) using various mesoporous silicas including SBA-12, SBA-15, MCM-48, MCM-41, and HMS as solid templates is presented. The use of acetonitrile as carbon precursor generates N-doped carbon (CNx type) materials with nitrogen content of ca. 8 wt %. The N-doped carbon materials exhibit both well-ordered mesoporosity and high levels of graphitic character. The mesostructural ordering and extent of graphitization depend on the CVD (i.e., carbonization) temperature and the nature of the silica template. In general, higher CVD temperatures (>900 °C) generated high levels of graphitic character but compromised the mesostructural ordering of the carbon materials. The mesostructural ordering of the CNx materials (and replication of pore channel ordering from the silica template) depends on the nature of the mesoporous silica used as solid template. We achieved structural replication and high surface area (440−1000 m2/g) CNx materials from SBA-12, MCM-48, and SBA-15 silica templates. MCM-41 and HMS silicas did not function effectively as templates and generated low surface area (<320 m2/g) CNx materials. The particle morphology of the silica template was in all cases replicated in the carbon materials, and we observed hollow particles for samples prepared at CVD temperature of 1000 °C or higher.

Highly ordered mesostructured Ni particles prepared from lyotropic liquid crystals by electroless deposition: the effect of reducing agents on the ordering of mesostructure

Mesostructured Ni and Co particles are formed by metal deposition in the aqueous domains of lyotropic liquid crystals (LLC) using different reducing agents. We have investigated the role of reducing agents in metal deposition for the synthesis of highly-ordered mesoporous metals by electroless deposition. Slow metal deposition accompanied by the autocatalytic reaction of dimethylamineborane (DMAB), caused by the combination of Ni and DMAB, led to the formation of highly-ordered mesostructured Ni.

Direct Synthesis and Catalytic Reactivity of Highly Ordered Large-Pore Methylaminopropyl-Functionalized SBA-15 Materials

Highly ordered large-pore SBA-15 materials functionalized with a high loading of amino groups were synthesized for the first time by co-condensation of tetraethyl orthosilicate (TEOS) and [3-(methylamino)propyl]trimethoxysilane (MAPTMS) using an amphiphilic block copolymer. Addition of inorganic salt to the initial mixture greatly enhanced the mesostructure ordering and stability of the mesoporous materials. The materials thus obtained showed high catalytic activity and selectivity for the synthesis of flavanones by means of the Claisen–Schmidt condensation in the absence of solvent.

Mesostructured Hollow Spheres of Graphitic N-Doped Carbon Nanocast from Spherical Mesoporous Silica

Mesostructured hollow spheres of graphitic N-doped carbon (CNx) materials may be nanocast from solid core mesoporous silica SBA-15 spheres via a chemical vapor deposition (CVD) route. The hollow spheres are generated when the SBA-15 silica/carbon composite obtained after CVD is subjected to silica etching in hydrofluoric (HF) acid. Hollow spheres are only obtained for CVD temperatures above 900 °C; here we present data on materials prepared at 1000 °C. The use of acetonitrile as carbon precursor results in N-doped (CNx) materials with nitrogen content of ca. 6.5 wt %. The CNx hollow spheres prepared at 1000 °C exhibit a high level of graphitization as evidenced by powder X-ray diffraction analysis and Raman spectrocopic studies. The CNx hollow spheres also exhibit good mesostructural ordering and have high surface area (779 m2/g) and pore volume (0.66 cm3/g). We propose a mechanism for the formation of the hollow spheres and clarify the importance of the CVD temperature in their formation.

Enhanced Mesostructural Order and Changes to Optical and Electrochemical Properties Induced by the Addition of Cerium(III) to Mesoporous Titania Thin Films.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Enhanced Mesostructural Order and Changes to Optical and Electrochemical Properties Induced by the Addition of Cerium(III) to Mesoporous Titania Thin Films

The addition of up to 10 mol % cerium(III) during the synthesis of cubic mesoporous titania thin films significantly increases the degree of mesostructural order in the final material. Photocurrent measurements, cyclic voltammetry, and spectroelectrochemistry show that the addition of cerium introduces a large number of interfacial states into the mesoporous material, which act as electron traps. Electron trapping by individual cerium ions also causes changes in the electrochemical properties of the mesoporous films. The mesoporous cerium-doped titania films show an increased electrochromic response that is indicated by an increase in absorbance at 550 nm with respect to pure titania. This effect is counter to the behavior of sol−gel ceria−titania films, which generally show a loss of electrochromism compared to pure titania.

Mixed Cationic‐nonionic Surfactants and pH Adjustment Route to Synthesize High‐quality MCM‐48

AbstractUsing the mixture of cetyltrimethylammonium bromide (CTAB) and p‐Octyl polyethylene glycol phenyl ether (OP‐10) as templates, siliceous MCM‐48 materials can be synthesized with low molar ratio of CTAB to silica (0.139:1) and low concentration of mixed surfactants (ca.5%) and within a wide range of OP‐10/CTAB ratio (0.08˜0.25). The materials were characterized by X‐ray powder diffraction, N2 adsorption/desorption isotherm, TEM, TG‐DSC and 29Si MAS NMR. Measurements indicated that the use of mixed surfactants allowed better condensation and higher ordering of the cubic mesostructure; at the same time, some properties of these materials were sensitive to the OP‐10/CTAB ratio. It was also found that the reduced pH of the gel which had been crystallized for a certain time gave a highly reproducible synthesis with a high silica yield (about 95%). Furthermore, the reaction mechanism of the synthesis is discussed in detail.

Novel route to periodic mesoporous aminosilicas, PMAs: ammonolysis of periodic mesoporous organosilicas.

A new route to periodic mesoporous aminosilicas (PMAs) that contain amine functional groups in the framework of a mesoporous network is reported. The materials are prepared via thermal ammonolysis of periodic mesoporous organosilicas (PMOs) under a flow of ammonia gas. PMOs integrate similar or even higher quantities of nitrogen-containing groups upon ammonolysis than similarly treated ordered mesoporous silicas (MCM-41). The quantity of amine groups introduced into the materials was found to depend strongly on the ammonolysis temperature. The largest loading of amine groups was obtained when a well-ordered cubic methylene PMO material without prior vacuum-drying was thermolyzed in ammonia. The ordered mesoporosity of PMOs was preserved during the ammonolysis with only a slight decrease in the mesopore size and the degree of mesostructural ordering. The extent of substitution of framework oxygen by amine and nitride groups was established by solid-state (29)Si CP-MAS, (29)Si MAS, (15)N MAS, and (13)C CP-MAS NMR spectroscopies, elemental analysis, and X-ray photoelectron spectroscopy. In some cases, methylene and methyl functional groups were also present in the PMAs along with amine functional groups, as inferred from elemental analysis and gas adsorption, particularly in cases where PMOs were subjected to ammonolysis at 400 and 550 degrees C for several hours. This resulted in new multifunctional mesoporous organoaminosilica nanomaterials with properties that could be tuned by systematically varying the relative amounts of hydrophilic amine and hydrophobic hydrocarbon pendent and framework groups. The stability upon storage was found to be much higher for PMAs obtained from PMOs than for those obtained from MCM-41 silicas under the same conditions.

Crystal Morphology of Mesoporous Silica Thin Films Synthesized by the Spin‐Coating Method Using PEO–PPO–PEO Triblock Copolymer

Mesoporous thin films on Si substrates with thicknesses of about 460–610 nm have been synthesized by the spin‐coating method using a Pluronic EO77PO29EO77 (F68), EO104PO39EO104 (F88), and EO133PO50EO133 (F108) triblock copolymer system. The triblock copolymers were preserved within the synthesized mesoporous thin films. Transmission electron microscopy (TEM) characterization of these films clearly demonstrates that long‐range mesostructural ordering strongly depends on the molecular weight of the poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymer, with lower molecular weight producing higher degrees of order. Plane and cross‐sectional high‐resolution TEM studies coupled with X‐ray diffraction (XRD) analysis also show that highly ordered F68 mesoporous silica thin film forms a cubic structure with a lattice spacing a= 6.70 nm.

Molecular and Mesoscopic Structures of Transparent Block Copolymer−Silica Monoliths

Mesoscopically ordered, transparent silica-surfactant monoliths have been prepared using amphiphilic triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymer species to organize polymerizing silica networks. The block copolymer acts as a structure- directing agent, as the aqueous silica cations partition within the hydrophilic regions of the self-assembled system and associate preferentially with the PEO blocks. Subsequent polymerization of the silica precursor species under strongly acidic conditions (pH 1) produces a densely cross-linked silica network that may be mesoscopically organized by the block copolymer species into composites with characteristic ordering length scales of >10 nm. When this is accompanied by slow evaporation of the aqueous solvent, such composite mesostructures can be formed into transparent and crack-free monoliths (e.g., 2.5 cm diameter 3 mm thick). Distributions and dynamics of the PEO and PPO copolymer blocks within the silica matrix were investigated in situ using 29 Si{ 1 H} and 13 C{ 1 H} two-dimensional solid-state heteronuclear correlation NMR techniques and 1 H NMR relaxation measurements. Mesostructural ordering was determined by X-ray diffraction and transmission electron microscopy. The degree of microphase separation and the resulting mesostructure of bulk samples were found to depend strongly upon the concentration of block copolymer, with higher concentrations producing higher degrees of order.

Structural characterization of organically-modified porous silicates synthesized using CTA+ surfactant and acidic conditions

Organically-modified silicates have been synthesized from RSi(OEt) 3 (R=CH 3 , C 2 H 5 , C 8 H 17 , CH 2 =CH or C 6 H 5 ) and TEOS under acidic conditions, in the presence of cetyltrimethylammonium bromide (CTAB). The introduction of RSiO 1.5 units with R/Si=0.2:1, prevents the system from adopting a high degree of hexagonal mesostructural order, except when R=C 6 H 5 . For the sample containing the phenyl functionality, removal of the surfactant by calcination at 350°C appears to be more complete, and less disruptive to mesoscopic ordering than solvent extraction using EtOH. Nitrogen adsorption measurements on the hexagonally ordered calcined sample that still contains phenyl groups, yields a type I isotherm, typical of a microporous solid with a pore size distribution centered at 15 A.