Transesterification on “imprinted” silica

The transesterification reaction of ethyl phenylacetate to hexyl phenylacetate on an imprinted silica catalyst of the type first reported by Heilmann and Maier is investigated. In the presence of sulfuric acid co-catalysts, the data obtained here reveal that catalytic activity can be due to phosphorous species leached into solution; no evidence for molecular recognition and catalysis from an imprinted site in the silica is observed. In the absence of co-catalysts, no conclusion regarding the presence of an imprinted site in the silica can be ascertained because of the low level of activity relative to background autocatalysis.     

Effects of silica aerogel particle sizes on the thermal–mechanical properties of silica aerogel – unsaturated polyester composites

Silica aerogels with a surface area as high as 773?m²?g^?1 and a density of 0.077?g?cm^?3 were produced from rice husk via sol–gel process and ambient pressure drying. A particulate composite material was prepared by adding silica aerogel particles of three different particle sizes (powder, granules and bead) to unsaturated polyester resin with a fixed volume fraction of 30%. Thermogravimetric and thermal conductivity studies revealed that silica aerogel composites were having higher thermal stability and thermal insulation than the neat resin. It was suggested that the preservation of aerogel pores from resin intrusion is important for better thermal properties. Larger silica aerogel particles have more porous area (unwetted region) which results in a lower degradation rate and lower thermal conductivity of the base polymer. However, the addition of silica aerogel into resin has reduced the tensile modulus of the polymer matrix where smaller particle size displayed higher toughness than those with bigger particle size.     

Ultrahydrophobic silica films by sol–gel process

Wettability of solid surfaces is a crucial concern in our daily life as well as in engineering and science. The present research work describes the room temperature (27 °C) synthesis of adherent and water repellent silica films on glass substrates using vinyltrimethoxysilane (VTMS) as a hydrophobic reagent by a single step sol–gel process. The silica sol was prepared by keeping the molar ratio of tetraethoxysilane (TEOS), methanol (MeOH), water (H₂O) constant at 1:14.69:5, respectively, with 0.01 M NH₄F throughout the experiments and the VTMS/TEOS molar ratio (M) was varied from 0 to 0.97. The effects of M on the surface structure and hydrophobicity have been researched. The static water contact angle as high as 144° and water sliding angle as low as 14° was obtained for silica film prepared from M = 0.97. The hydrophobic silica films retained their hydrophobicity up to a temperature of 255 °C and above this temperature the films became superhydrophilic. The prepared silica films were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Fourier Transform Infrared (FT-IR) spectroscopy, humidity test and static and dynamic water contact angle measurements.     

Synthesis of superhydrophobic core–shell mesoporous silica nanoparticles

Mesoporous silica nanoparticles (MSNPs) have been used in variety of applications due to their morphology and porous structure. This work reports the one-pot synthesis of ultrahydrophobic MSNPs using N-cetyl-n,n,n trimethyl ammonium bromide as a cationic surfactant template and ethanol (EtOH) as a cosolvent to form mesopores in the MSNPs. The effects of EtOH on the size and the pore structure of the MSNPs were studied by scanning electron microscopy and transmission electron microscopy. The results show that an addition of EtOH led to an enlargement of the MSNPs and a change in pore structure from a lamellar structure to a radially oriented structure. Co-condensation with two different types of fluoroalkyl silanes; trimethyl(fluoromethyl)silane, and trichloro(1H,1H,2H,2H-perfluorooctyl)silane provided low surface energy MSNPs with a core–shell structure. An assembly on the surface of these F-MSNPs generated nanostructure surface roughness rendering an improvement in surface wettability with water contact angle of 158.6°, which is a characteristic of oleophobic and ultrahydrophobic material.     

Synthesis and characterization of microporous silica–alumina membranes

The development of microporous ceramic thin layers is of prime interest for sensors or gas separation membranes working at high temperature. Microporous silica membranes can be easily prepared by the sol–gel process. However the microporosity of pure silica is rapidly modified by steam at high temperature. One way to improve hydrothermal stability is to use mixed-oxide membranes. In this work, microporous silica–alumina membranes were prepared by a simple and robust sol–gel method. Tetraethoxysilane was mixed with an acidic alumina hydrosol. Urea was added for preparing the alumina hydrosol, for controlling the mixed-oxide network polycondensation and also as porogen agent. FTIR and ²⁷Al NMR spectroscopic analyses showed that for Si/Al molar ratios up to 6/1, homogeneous mixed oxides were obtained with a random distribution of Al and Si atoms in the oxide lattice based on tetrahedral units. The derived supported layers were crack-free as demonstrated by scanning electron microscopy (SEM) observations. Their microporosity was investigated using ellipsoporosimetry (EP) with films supported on flat dense substrates. He, N₂ and CO₂ permeance measurements were performed for membranes deposited on porous tubular substrates. The measured values of He/N₂ and He/CO₂ ideal selectivities are in agreement with the microporous nature of the prepared layers.     

Selective photoepoxidation of propylene over hydroxyapatite–silica composites

Two types of hydroxyapatite (HAP)–silica composites were prepared from aqueous solutions of Ca(NO₃)₂ and (NH₄)₂HPO₄ in the presence of SiO₂ in a buffer (pH 7.5) and an alkaline condition. Photooxidation of propylene with oxygen was carried out over the composites at 303 K. The major product was propylene oxide which exceeded 80% among C3 oxygenated products. The conversion increased with an increase in HAP content, but the selectivity of propylene oxide decreased. This was caused by that propylene oxide was strongly adsorbed on HAP followed by consecutive photooxidation. The thin-layered HAP in the HAP–SiO₂ composite is more effective for photoepoxidation of propylene than the bulk HAP.     

Cobalt–silica interaction and the dehydrogenation of 2-propanol

The interaction between silica and cobalt was studied on supported catalysts with low silica loading. Below a threshold cobalt level of 0.41 wt%, the catalysts were inactive for dehydrogenation of 2-propanol at 450 K. Inactivity was attributed to irreducibility of cobalt ions. Samples that were impregnated at a level below the threshold, dried, calcined, then reimpregnated below the threshold level, redried and recalcined such that the total cobalt content exceeded the threshold, were inactive. These results are not consistent with a model in which a portion of the cobalt interacts with specific silica sites, forming an irreducible species. Rather, they suggest that strongly interacting cobalt ions are incorporated into the silica surface.     

Lime–alumina–silica vitreous ceramic processing incorporating wollastonite

Abstract

Improvements in mechanical and physical properties of advanced ceramics have been undermined by high processing costs and the reputation of unpredictable reliability. One potential technique to address this situation is to use a naturally occurring mineral to develop new ceramic microstructures using a simple processing technique. The system chosen for potential development was an alumina ceramic modified using 20 wt-% wollastonite (CaSiO₃ ). Green body production was carried out by wet processing optimised by rheological measurements. The best conditions were found to be a pH of 3 using 1500 ppm BETZ (dispersant) processed at ～0·35 volume fraction of solids in a water based suspension. Thermal analysis revealed that the optimum sintering procedure involved vitreous composite sintering for 15 min at 1500°C followed by an air quench. This glass ceramic was then devitrified using a heat treatment at 1032°C (4 h) followed by 4 h at 1200°C. The final material of 68 wt-% alumina, with anorthite and gehlenite, had a high indentation toughness (6·2 MPa m^1/2 ) and an improved thermal expansion coefficient. Thus, the new material should be attractive as a low cost material for anti-abrasion applications at elevated temperatures.     

Hydrophobic titania–silica aerogels: epoxidation of cyclic compounds

Topics in Catalysis - Titania–silica mixed oxides with covalently bound methyl or phenyl groups were prepared from the corresponding alkyltrimethoxysilane and tetramethoxysilane using a...     

Preparation of transparent ultrahydrophobic silica film by sol�Cgel process

Transparent ultrahydrophobic films were synthesized by sol–gel process with organic silicones modified into silica sol and cured under UV irradiation. The effects of hydrolysis temperature, hydrolysis time, molar ratio of organic silicone to silica sol, and surface morphology on the hydrophobicity of the films were discussed in detail using FTIR spectroscopy, scanning electron microscopy (SEM), AFM, optical transmission, and contact angle measurement, respectively. The AFM and SEM images indicated that the surface roughness enhanced the hydrophobicity of the films. The results revealed that methyl-trimethoxysilane (MTMS)-modified silica film prepared at 50°C for 2 h with an MTMS/silica sol molar ratio of 1:10 had a very high contact angle (130°). However, the higher hydrolysis temperature and longer reaction time might have accelerated the self-condensation of silanol and decreased the contact angle of the films.     

Porous pyridine based polyimide–silica nanocomposites with low dielectric constant

Highly porous polymer–silica hybrid materials were prepared based on the organo-soluble polyimides of four various dianhydride and 2,5-diaminopyridine. 3-Aminopropyltriethoxysilane (APS) was used to increase the intrachain chemical bonding and interchain hydrogen bonding between the polyimide and silica moieties, respectively. The chemical interaction would significantly affect the morphologies and properties of the prepared films. The produced polyimide–silica composites were investigated by X-ray diffraction analysis, scanning electron microscope and thermal analysis tecniques. The effect of silica modified with functional group of 3-aminopropyltriethoxy silane on the porous structure and dielectric properties as well as the thermal stability of films were investigated. Capacitances were determined with a HP4294A at a frequency between 1 kHz and 1 MHz. The dielectric constant was significantly reduced with increasing silica modified with APS. The result indicates that the composite materials are potentially useful in low dielectric materials.     

Preparation and characterization of PLLA–ESO/surface-grafted silica nanocomposites

Various amounts of surface-grafted silica (g-SiO₂) and un-grafted (SiO₂) nanoparticles were solution blended with a copolymer of l-lactide and epoxidized soybean oil (PLLA–ESO) or PLLA. Chemical reaction between the low molecular weight (LMW) PLLA and surface of silica nanoparticles is confirmed by FTIR and TGA analyses. The amount of grafted LMW PLLA investigated by thermal gravimetric analysis (TGA) was about 14.9%–28.2% in weight. g-SiO₂ nanoparticles can be easily dispersed into PLLA–ESO matrix to form a uniform PLLA–ESO/g-SiO₂ composite. Thermal properties of PLLA–ESO/g-SiO₂ and PLLA/g-SiO₂ nanocomposites were subsequently investigated by the differential scanning calorimeter measurements (DSC). DSC analyses indicated that g-SiO₂ nanoparticles can serve as a nucleating agent for the crystallization of PLLA–ESO in the composites, while the melting temperature (T _m) and the glass transition temperature (T _g) of PLLA–ESO/g-SiO₂ nanocomposites seemed to be independent of loading of g-SiO₂ particles. The DSC curves of PLLA/g-SiO₂ nanocomposite obviously showed double melting peaks, while that of PLLA–ESO/g-SiO₂ nanocomposites only a single melting peak. PLLA–ESO/g-SiO₂ composites exhibited a higher tensile strength and elongation than that of PLLA–ESO/SiO₂ composites.     

Thermally activated sintering–coarsening–coalescence-polymerization of amorphous silica nanoparticles

An onset sintering–coarsening–coalescence-polymerization (SCCP) event of amorphous SiO₂ nanoparticles (ca. 40–100 nm in size) by isothermal firing in the 1150–1300 °C range in air was characterized by an N₂ adsorption–desorption hysteresis isotherm coupled with X-ray diffraction and vibrational spectroscopy. The apparent activation energy of such a rapid SCCP process was estimated as 177±32 kJ/mol, based on 30% reduction of a specific surface area with an accompanied change of medium range orders, i.e. forming Si₂O₅ while retaining the Si–2ndO yet losing the Si–2ndSi without appreciable crystallization. The minimum temperature of the SCCP process, as of concern to industrial silica applications and sedimentary/metamorphosed sandstone formation, is 1120 °C based on the extrapolation of steady specific surface area reduction rates to null.     

Sol–gel synthesis of nano-sized silica in confined amorphous space of polypropylene: Impact of nano-level structures of silica on physical properties of resultant nanocomposites

Polypropylene (PP)/silica nanocomposites were prepared by the sol–gel reaction of silicon alkoxide that was impregnated in the confined amorphous nanospace of PP with the aid of supercritical carbon dioxide. This novel technique enabled us to prepare nanocomposites having a variety of silica morphology without altering the higher-order structures of PP, being ideal to study relationships between the silica morphology and mechanical properties of the nanocomposites. The synthesized silica particles were highly dispersed in PP with dimensions comparable to the amorphous thickness (<10 nm), while their mass fractal dimension acquired by small-angle X-ray scattering was dependent on the sol–gel conditions. We found that the Young's modulus as well as the storage modulus in melt viscoelastic measurements was negatively correlated with the mass fractal dimension of silica nanoparticles: A lower mass fractal dimension resulted in not only higher reinforcement but also percolation network formation at a lower silica loading.     

Blending silica bodies in centrifugal runner mills, “model 115”

Conclusions The output of model 115 centrifugal runner mills for mixing dinas bodies is 1.7 times higher than that of runner mills with heavy rollers. Goods pressed from bodies prepared in centrifugal runner mills are no different in quality from those pressed from bodies made in runner mills with heavy rollers. The difference in output of acceptable product is slight.In the preparation of bodies in model 115 runner mills there are few fines in the batch. When formulating the batch it is necessary to introduce 20–30°% quartzite ground in a tube mill with a content of not less than 80% fraction finer than 0.088 mm. With this, the fraction finer than 0.5 and 3–2 mm should be considered; air blowing of the body should be started 2–2.5 min after the start of blending.To observe the norms for alkalinity and moisture content of the batch, with a moisture content of 2–2.4% the total content of CaO in the lime-iron slip is 23.5–24.5% and with a wet weight of 700–750 kg the amount of slip varies from 43 to 50 liters, the blending time lies in the 4–5 min range.Scrapers and the lining of the runner mills should be made from manganese steel and the bottom of the mills from carbon steel.A drawback of model 115 runner mills is the sticking of the lime-iron slip and the need to clean them more than once during 24 h.     

Negatively charged organic–inorganic hybrid silica nanofiltration membranes for lithium extraction

Effective extraction of lithium from high Mg~(2+)/Li+ratio brine lakes is of great challenge. In this work, organic–inorganic hybrid silica nanofiltration(NF) membranes were prepared by dip-coating a 1,2-bis(triethoxysilyl)ethane(BTESE)-derived separation layer on tubular TiO_2 support, for efficient separation of LiC l and MgCl_2 salt solutions. We found that the membrane calcinated at 400 °C(M1–400) could exhibit a narrow pore size distribution(0.63–1.66 nm) owing to the dehydroxylation and the thermal degradation of the organic bridge groups. All as-prepared membranes exhibited higher rejections to LiCl than to MgCl_2, which was attributed to the negative charge of the membrane surfaces. The rejection for LiCl and MgCl_2 followed the order: LiCl N MgCl_2, revealing that Donnan exclusion effect dominated the salt rejection mechanism. In addition, the triplecoated membrane calcined at 400 °C(M3–400) exhibited a permeability of about 9.5 L·m~(-2)·h~(-1)·bar~(-1) for LiCl or MgCl_2 solutions, with rejections of 74.7% and 20.3% to LiCl and MgCl_2,respectively, under the transmembrane pressure at 6 bar. Compared with the previously reported performance of NF membranes for Mg~(2+)/Li+separation, the overall performance of M3–400 is highly competitive. Therefore, this work may provide new insight into designing robust silica-based ceramic NF membranes with negative charge for efficient lithium extraction from salt lakes.     

A computational linear elastic fracture mechanics-based model for alkali–silica reaction

A fracture mechanics model for alkali–silica reaction (ASR) is presented that deals with the case of a concrete made up of dense spherical aggregates. Chemistry and diffusion (of ions and gel) are not modelled. The focus is put on the mechanical consequences of the progressive replacement of the aggregates by a less dense gel. A ring-shaped crack then appears in the cement paste depending on the pressure build-up, according to an incremental energy criterion. The stored elastic energy and deformation of each configuration are determined assuming that each aggregate is embedded in an infinite cement paste matrix, through Finite Element Analysis. We note a very different behaviour of aggregates of different sizes. Adding the contributions of different aggregates leads to an estimate of the free expansion of a concrete of given aggregate size distribution. Parameters of the model are identified, providing a good fit to experiments taken from Multon's work.     

Bienzyme system immobilized in biomimetic silica for application in antifouling coatings☆

Antifouling coatings are used extensively on vessels and underwater structures. Conventional antifouling coat-ings contain toxic biocides and heavy metals, which may induce unwanted adverse effects such as toxicity to non-target organisms, imposex in gastropods and increased multiresistance among bacteria. Therefore, enzyme-based coatings could be a new alternative solution. A H2O2-producing bienzyme system was developed in this study. H2O2 can be produced from starch by the cooperation ofα-amylase and glucose oxidase, which pro-motes the hydrolysis of polymeric chain and oxidizes the glucose to produce H2O2, respectively. The encapsulated bienzyme (A-G@BS) exhibits enhanced stabilities of thermal, pH, recycling and tolerance of xylene. The A-G@BS-containing coating releases H2O2 at rates exceeding a target of 36 nmol·cm?2·d?1 for 90 days in a laboratory assay. The results demonstrate that the method is a promising coating technology for entrapping active enzymes, presenting an interesting avenue for enzyme-based antifouling solutions.