Oxidative photo-decarboxylation in the presence of mesoporous silicas

FSM-16, a mesoporous silica, was found to catalyze oxidative photo-decarboxylation of alpha-hydroxy carboxylic acid, phenyl acetic acid derivatives and N-acyl-protected alpha-amino acids to afford the corresponding carbonyl compounds. Furthermore, FSM-16 proved to be re-usable by re-calcination at 450 degrees C after the reaction.     

Adsorption hysteresis in ordered mesoporous silicas

We review some recent progress in experimental studies of the adsorption hysteresis of simple molecules in ordered mesoporous silicas. We show that the nature of the adsorption hysteresis due to capillary condensation can be examined with less ambiguity by measuring the hysteresis loop for the ordered mesoporous silicas with three types of pore geometries (cylindrical, interconnected cylindrical, and interconnected spherical) over a wide temperature range. The adsorption hysteresis arises from the metastability of a confined phase and the temperature at which the hysteresis disappears is lower than the critical temperature of vapor-liquid equilibrium in pores. The hysteresis occurs mainly on the desorption rather than adsorption branch, irrespective of the pore geometries.     

Modifying the walls of mesoporous silicas prepared by supramolecular-templating

The present review is not exhaustive and rather endeavors to illustrate ways to improve the wall stability of ordered mesostructured silicas in presence of water. In situ X-ray diffraction of the phase transformations of hybrid (surfactant containing) silicas show how synthesis temperature, pH and duration control wall chemistry (thickness, condensation and hydrolysis). Ex situ studies of calcined (surfactant free, empty mesopores) silicas show how wall stability (in boiling water, under steam) is affected by synthesis conditions and/or post-synthesis treatments.     

Formation mechanism of achiral amphiphile-templated helical mesoporous silicas

A series of achiral cationic and anionic amphiphiles employed as templates for mesoporous silica, all showed an excellent ability to form highly ordered helical mesoporous silica (HMS) crystals. It was supposed that the helical mesostructure may originate from the helical propeller-like micelle formed by the achiral amphiphilic molecules due to their instantaneous asymmetric shape survived in the micelle. The existence of the mirror-imaged conformations of equal proportion may result in the racemic nature of the achiral amphiphile-templated HMSs. The helicity of the HMS greatly depends on the templating molecules which may be modulated by the twisting power of the helical propeller-like packing of the amphiphiles in the micelle. The dependence of the pitch length on different factors was explained by mechanical analysis indicating that the pitch length should be in direct proportion to the rod diameter but inversely proportional to the moment of micelle (M0). Additionally, chiral amphiphiles with opposite charge were used as chiral dopants to control the enantiopurity of the achiral amphiphile-templated HMSs. The absolute enantiomeric excess (ee) of the HMSs increased with increasing amounts of dopant.     

Molecular simulation of the adsorption and structure of benzene confined in mesoporous silicas

Grand Canonical Monte Carlo simulations are used to study the adsorption of benzene at 298 K in an atomistic cylindrical silica nanopore of a diameter 3.6 nm. The adsorption involves a transition from a partially filled pore (a two layers thick film at the pore surface) to a completely filled pore configuration. Strong layering of the benzene molecules at the pore surface is observed. It is found that the layering decays as the distance to the pore surface increases. The position of the peaks for the density of the C, H atoms and the center of mass of the molecules shows that benzene molecules prefer an orientation in which their ring is perpendicular to the pore surface. This result is corroborated by calculating orientational order parameters and examining the distribution of the distances between the H and C atoms of the benzene molecules and the H and O atoms of the silica substrate.     

Adsorption of a cationic porphyrin onto mesoporous silicas

The adsorption of a cationic porphyrin, tetrakis-(N-methyl-4-pyridiniumyl)porphine, into mesoporous silicas from solution of tetrakis-(N-methyl-4-pyridiniumyl)porphine p-toluene sulfonate was investigated. Irrespective of the pore size (2.4, 3.5 and 4.2 nm), the cationic porphyrin was adsorbed effectively onto mesoporous silicas to give brown-colored powders. Depending on the amounts adsorbed, which correlate with the average intermolecular distance, the porphines tend to aggregate (dimer).     

The application of modified mesoporous silicas in liquid phase catalysis

This review focuses on recent developments in the preparation, properties and catalytic applications of chemically modified mesoporous silicas. Over the last few years, this group of materials has been the subject of intense activity in the materials community, and many applications have been found for these fascinating materials. This non-exhaustive review aims to highlight the key features of the materials, which are relevant and important to catalysis, and illustrates their utility with a series of recent examples.     

Lower closure point of adsorption hysteresis in ordered mesoporous silicas

To examine the nature of the lower closure point of adsorption hysteresis in ordered mesoporous silicas, we measured the temperature dependence of the adsorption-desorption isotherm of nitrogen for three kinds of ordered silicas with cagelike pores and three kinds of ordered silicas with cylindrical pores. The lower closure point pressure of nitrogen in the cagelike pores with sufficiently small necks, that is, the cavitation pressure of a confined liquid, did not depend appreciably on the cage size in the temperature region far away from a hysteresis critical temperature (Tch) but its cage-size dependence was noticeable in the vicinity of Tch. The lower closure point in the cylindrical pores depended on the pore size, and its thermal behavior was totally different from that in the cagelike pores. Nevertheless, the hysteresis critical points of nitrogen in the ordered mesoporous silicas, which are defined as a threshold of temperatures (Tch) and pressure above which reversible capillary condensation takes place in a given size and shape of pores, fell on a common line in a temperature-pressure diagram regardless of the pore geometries. We consider this finding as evidence that capillary evaporation in the cylindrical pores follows a cavitation process in the vicinity of Tch in the same way as that in the cagelike pores and also that the low limit of the hysteresis loop that has been long recognized since 1965 is due to the occurrence of a vapor bubble in a stretched metastable liquid confined to the pores with decreasing pressure (cavitation).     

Sorption and diffusion of phenols onto well-defined ordered nanoporous monolithic silicas

The sorption of phenol, and o-, m-, and p-aminophenol (o-, m-, and p-AP) onto highly ordered mesoporous silicas (HOM) with cubic Im3m (HOM-1), hexagonal H(I) (HOM-2), 3-D hexagonal p6(3)/mmc (HOM-3), cubic Ia3d (HOM-5), lamellar L( infinity ) (HOM-6), and solid phase S (HOM-8) materials has been investigated kinetically. Nanostructured silica molecular sieves have been prepared at 25 and 60 degrees C with lyotropic liquid-crystalline phases of the nonionic surfactant (Brij 76) that was used as a structure-directing agent. Such nanostructured silicas have been studied by 29Si nuclear magnetic resonance (29Si NMR), powder X-ray diffraction (XRD), the Brunauer-Emmett-Teller (BET) method for nitrogen adsorption and surface area measurements, and transmission electron microscopy (TEM) techniques after synthesis and sorption. It was found that all materials exhibit well-defined long-range porous architectures without significant loss of the ordered texture during phenol sorption. The kinetics of phenol sorption has been studied spectrophotometrically at different temperatures (25-40 degrees C; +/-0.1 range). The sorption rate is zero order in all phenols sorbed, and increases directly in the pattern P >m-AP > o-AP > p-AP, which reflects the mobility of the phenol compounds on the particle pores. The isothermal sorption and the kinetic parameters were discussed and it was established that a diffusion-controlled process characterizes phenol sorption. Furthermore, the mechanism of phenol sorption was deduced to be predominantly particle diffusion. The diffusion coefficients were determined using Fick's equation. The trend of diffusion of all phenols onto nanoporous silica was HOM-8 > HOM-2 > HOM-6 > HOM-5 > HOM-1 > HOM-3, reflecting the effect of the uniform pore size distribution and the internal surface area of the nanostructured silicas on the diffusion process.     

The CO2 adsorption performance of aminosilane-modified mesoporous silicas

Journal of Thermal Analysis and Calorimetry - Aminosilane-modified MCM-41 and SBA-15 mesoporous silicas were synthesized using sodium silicate extracted from gold mine tailings slurry in this...     

Post-treatment and characterization of novel luminescent hybrid bimodal mesoporous silicas

A novel luminescent hybrid bimodal mesoporous silicas (LHBMS) were synthesized via grafting 1,8-Naphthalic anhydride into the pore channels of bimodal mesoporous silicas (BMMs) for the first time. The resulting samples were characterized by powder X-ray diffraction (XRD), N₂ adsorption/desorption measurement, Fourier transform infrared spectroscopy (FT-IR), Transmission electron microscopy (TEM), UV-vis absorption spectroscopy, and Photoluminescence spectroscopy (PL). The results show that 1,8-Naphthalic anhydride organic groups have been successfully introduced into the mesopores of the BMMs and the hybrid silicas are of bimodal mesoporous structure with the ordered small mesopores of around 3 nm and the large mesopores of uniform intra-nanoparticle. The excellent photoluminescent performance of LHBMS has a blue shift compared to that of 2-[3-(triethoxysilyl) propyl-1 H-Benz [de]isoquinoline-1, 3(2 H)-dione, suggesting the existence of the quantum confinement effectiveness.     

Photoprocesses in mesoporous silicas prepared by a supramolecular templating approach

Mesoporous solids derived from inorganic–surfactant mesostructured materials have been investigated extensively from both fundamental and practical viewpoints owing to their unusually high surface area and porosity, ordered pore arrangements, pore size uniformity, as well as possible surface engineering. These characteristic features of the mesoporous solids have motivated research to apply them practically. This class of materials was originally expected to find applications as adsorbents and catalysts, while other advanced materials applications such as in optics/electronics became a field of interest after their successful fabrication in controlled morphology. This review article summarizes the studies on the photochemistry and photophysics of guest species occluded in mesoporous silicas in order to highlight the characteristic features of the mesostructured and mesoporous solids and their potential for optical applications.     

Template synthesis of mesoporous silicas with the use of nanocrystalline cellulose

The porous structure of silica samples prepared via sol–gel synthesis with the use of nanocrystalline cellulose particles as a template has been studied by low-temperature nitrogen adsorption. The influence of the concentration of a nanocrystalline cellulose dispersion, template content in the nanocrystalline cellulose/SiO₂ composite, and the conditions of the sol–gel synthesis on the porous structure of silica has been investigated.     

Pore size engineering in mesoporous silicas using supercritical CO2

In this paper we investigate the use of supercritical carbon dioxide (sc-CO(2)) for synthesizing calcined mesoporous silicas with tunable pore sizes, wall thickness, and d spacings. Small angle neutron scattering was used to probe the controlled swelling of the triblock copolymer surfactant templating agents, P123 (PEO(20)PPO(69)PEO(20)), P85 (PEO(26)PPO(39)PEO(26)), and F127 (PEO(106)PPO(70)PEO(106)), as a function of CO(2) pressure. The transition from the liquid crystal phase to the calcined mesoporous silicas, formed upon condensation and drying, was also studied in detail. Powder X-ray diffraction, transmission electron microscopy, and nitrogen adsorption techniques were used to establish pore diameters, silica wall widths, and the hexagonal packing of the pores within the calcined silicas. Using a direct templating method, the diameters of mesopores and the spacing between the pores could be tuned with a high level of precision. The swelling process was observed to have no detrimental effects on the quality of silica formed, a distinct advantage over conventional swelling techniques, and all of the silicas synthesized in this study were highly ordered over distances of at least 2000 A.     

Pore size effects in the pyrolysis of 1,3-diphenylpropane confined in mesoporous silicas

A new method for derivatizing mesoporous silicas, SBA-15 and MCM-41, with a substituted phenol is described, and pore confinement and surface curvature are shown to impact the reaction rate and product selectivity for the pyrolysis of surface-immobilized 1,3-diphenylpropane.     

The template synthesis of mesoporous silicas with the thiourea functional group

Mono- and bifunctional mesoporous silicas containing the thiourea group ≡Si(CH₂)₃NHC(S)NHC₂H₅ or thiourea and various nitrogen-containing groups [≡Si(CH₂)₃NH₂, ≡Si(CH₂)₃NH(CH₂)₂NH₂, or {≡Si(CH₂)₃}₂NH], respectively, in the surface layer were prepared by template syntheses with cetylpyridinium chloride as a template. The synthesized samples had well-developed porous structures (S _sp = 750–1150 m²/g, V _c = 0.51–0.72 cm³/g, and d = 2.4–3.5 nm) and high functional group contents (1.0–2.0 mmol/g). Hydrothermal treatment of the mesophases in a mother liquor at 80°C (24 h) improved the structure-adsorption characteristics of the functionalized mesoporous silicas.     

Assessment of ordered and complementary pore volumes in polymer-templated mesoporous silicas and organosilicas

A method to determine the volumes of ordered mesopores and complementary small pores in polymer-templated ordered mesoporous silicas and organosilicas is proposed on the basis of the existing relation between the pore width and unit cell values obtained by the XRD structure modeling and the adsorption pore volume.     

Improved catalytic performance of lipase accommodated in the mesoporous silicas with polymer-modified microenvironment

The highly ordered mesoporous silicas with elaborately controlled microenvironment were synthesized via covalent incorporation of long-chain polymers (M(w) = 2000 g mol(-1)) bearing specific hydrophilic/hydrophobic balance. The microenvironment (hydrophilicity/hydrophobicity) of the mesoporous silicas was quantitatively determined by gas adsorption experiments and investigated by lysozyme (LYZ) adsorption. The relative activity of lipase from Pseudomonas cepacia (PCL) encapsulated in the mesoporous silica with moderate hydrophobic microenvironment (hereafter denoted as MHM) reaches up to 281% compared with the free PCL, notably higher than that of PCL accommodated in the mesoporous silicas with hydrophilic or strong hydrophobic microenvironment (20.7-26.2% relative to the free PCL). Moreover, PCL entrapped in the nanochannels with MHM affords the highest initial rate in the kinetic resolution of (R,S)-1-phenylethanol relative to other immobilized PCL. The above results suggest that the MHM could render the active center of PCL entirely exposed to the substrates without interrupting its native conformation in the "interfacial activation". In addition, the nanochannels with MHM could markedly improve the thermal stability of PCL (preserving nearly 60% of the initial activity after the incubation at 70 °C for 2 h) and facilitate the recycling of the immobilized PCL in both aqueous and organic media. Our work demonstrates that the subtle modulation of the microenvironment of mesoporous silicas for enzyme immobilization designates a very promising strategy to fabricate the highly active and stable heterogeneous biocatalysts for industrial application.     

Synthesis of mesoporous silicas inside large pores of inorganic matrix

Template syntheses of mesoporous silicas have been carried out inside large pores of inorganic matrix. Portions of tetraethoxysilane and cetyltrimethylammonium bromide micellar solution were incorporated step-by-step inside pore volume of silica gel with large pore size. Synthesized materials were characterized using thermal analysis, adsorption-desorption of nitrogen and X-ray diffraction scattering.