Showing papers on "Silica gel published in 2004"

Room-temperature ionic liquids as template to monolithic mesoporous silica with wormlike pores via a sol-gel nanocasting technique

Abstract: This paper reports a novel room-temperature ionic liquid (RTIL), [C4mim]+ BF4-, as template to monolithic mesoporous silica with wormhole framework via a convenient nanocasting technique. In contrast with the applied liquid crystal self-assembly of long-chain surfactants on the preparation of mesoporous nanostructures, a new so-called hydrogen bond-co-π−π stack mechanism was proposed to be responsible for the present self-assembly of the RTIL in the reaction system for the formation of the wormlike mesopore, in which both the hydrogen bonds formed between the [BF4]- and silano group of silica gel and the π−π stack interaction of the neighboring imidazolium rings play crucial roles in the formation of the wormhole framework of mesporous silica. The proposed hydrogen bond-co-π−π stack mechanism with the RTIL as template may open a new pathway to prepare mesoporous materials.

Synthesis of new nanoporous carbon materials using nanostructured silica materials as templates

Abstract: In this review, we summarize recent reports on the synthesis of various nanoporous carbon materials. Many nanoporous carbon materials having variable pore sizes and pore structures have been synthesized using appropriate nanostructured silica materials as templates. Nanoporous carbons with high pore volumes and uniform pore sizes have been produced using silica sol and silica gel as templates. Mesoporous carbons with several different pore structures have been synthesized using mesoporous silica materials such as MCM-48, HMS, SBA-15, MSU, and MCF as templates. Some of these nanoporous carbon materials were successfully used as adsorbents for bulky pollutants and electrodes for supercapacitors and fuel cells.

A luminescent tris(2-thenoyltrifluoroacetonato)europium(III) complex covalently linked to a 1,10-phenanthroline-functionalised sol–gel glass

Abstract: Lanthanide doped sol–gel glasses are an attractive type of luminescent material which can be processed at ambient temperatures. However, the solubility of the lanthanide complexes in the sol–gel matrix can be a problem and it is difficult to obtain a uniform distribution of the complexes (avoidance of cluster formation). These problems can be solved by covalently linking the lanthanide complex to the glass matrix. In this study, a strongly luminescent europium β-diketonate complex was immobilized on a 1,10-phenanthroline-functionalised silica sol–gel glass. The glass matrix was prepared by first reacting 5-amino-1,10-phenanthroline with 3-(triethoxysilyl)propyl isocyanate. The resulting compound, tetramethoxysilane (TMOS) and diethoxydimethylsilane (DEDMS) were hydrolysed and condensed at a neutral pH to a sol–gel glass. A tris(2-thenoyltrifluoroacetonato)europium(III) dihydrate complex was bound to the 1,10-phenanthroline groups on the silica gel and the coordinated water was expelled. High-resolution luminescence spectra were recorded and the radiative lifetimes were measured. It is shown that the spectroscopic behaviour of the luminescent materials is very comparable with that of the pure [Ln(tta)3(phen)] complex.

Feasibility study of hydrophilic and hydrophobic silica aerogels as drug delivery systems

Abstract: The feasibility of silica aerogels as oral drug delivery systems was investigated. Silica aerogels were loaded with several drugs by adsorption from their solutions in supercritical CO2. It was demonstrated that for all three drugs investigated here, high loading of the aerogel could be achieved. The loaded aerogels were characterized by IR- and UV spectroscopy and X-ray diffraction in order to show that no degradation of the drugs occurred during the loading procedure. The release profiles of two drugs (ketoprofen and griseofulvin) from loaded aerogels were measured. It was found that the drugs adsorbed on hydrophilic silica aerogels dissolve faster than the corresponding crystalline drugs. This fact can be explained by both an increase in the specific surface area of the drug adsorbed on the aerogel and its non-crystallinity in this state. The influence of density and hydrophobicity of aerogels on both the adsorption and release of drugs were studied.

Isocyanate-Crosslinked Silica Aerogel Monoliths: Preparation and Characterization

Abstract: Polymerization of di- and tri-isocyanates can be templated onto the mesoporous surface of a preformed network of sol–gel-derived silica nanoparticles, resulting in a conformal ‘crosslinked’ coating that renders the interparticle neck zone wider. Upon drying, these crosslinked networks yield aerogels which are up to ∼3× more dense than native aerogels based on the underlying silica framework, but also up to 10× less hygroscopic and they may take more than 300× the force to break. These results have been obtained with one-step based-catalyzed sol–gel silica networks, as well as with gels derived through a two-step process involving an acid-catalyzed sol and a based-catalyzed gel. Furthermore, it has been also found that crosslinking increases the dielectric constant only by ∼35% relative to values reported in the literature for native silica aerogels of about the same porosity. Chemical investigations into the polymerization reaction have shown that the process of crosslinking involves reaction of the isocyanate with: (a) OH groups at the surface of silica to form carbamate; and (b) adsorbed water, to form an amine and carbon dioxide. This amine then reacts with additional isocyanates resulting in polymer chain extension and bridging of particles with urethane-terminated polyurea.

Encapsulation of functional cells by sol–gel silica: actual progress and perspectives for cell therapy

Abstract: The conjugation of inorganic materials with biological systems has attracted research interest, as a consequence of the availability of the sol–gel process, operating under experimental conditions compatible with biomolecules and whole cells. In Section 2, the merits and limitations of alkoxide-based sol–gel silica as an immobilization material are presented and discussed with particular attention to the confinement of living cells and cell clusters. Biosil technology is based on the encapsulation of whole cells by a sol–gel silica layer deposited on the cell surface using silica precursors in the gas phase. It is presented in Section 3, which describes some important Biosil features such as mechanical stability, versatility for porosity control with the aim of immunological protection, and maintenance of cell viability with free expression of cell functions. Some recent unpublished results are presented to substantiate the properties of Biosil materials for encapsulation of functional animal cells. Extension of the Biosil process to alginate microencapsulation is then presented in Section 4, with emphasis on biocompatibility within the perspective of cell grafts and therapy. Section 5 covers some clinical transplantations concerning grafts with allogenic pancreatic islets and adenovirus infected cells. The conclusions (Section 6) present some potential guidelines for future trends, specifically for whole cell immobilization by silica and, more in general, for advanced biomaterials.

Photodegradation of dye pollutants on silica gel supported TiO2 particles under visible light irradiation

Abstract: In this paper, silica gel supported titanium dioxide particles (TiO2/SiO2) prepared by acid-catalyzed sol–gel method was as photocatalyst in the degradation of acid orange 7 (AO7) in water under visible light irradiation. The particles were characterized by X-ray diffraction, BET specific surface area determination, and point of zero charge measurement. The supported catalyst had large surface area, high thermal stability and good sedimentation ability. The photodegradation rate of AO7 under visible light irradiation depended strongly on adsorption capacity of the catalyst, and the photoactivity of the supported catalyst was much higher than that of the pure titanium dioxides. The photodegradation rate of AO7 using 31% TiO2/SiO2 particles was faster than that using P-25 and TiO2 (Shanghai) as photocatalyst by 2.3 and 12.3 times, respectively. The effect of the calcination temperature and the TiO2 loading on the photoactivity of TiO2/SiO2 particles was also discussed.

Selective water sorbent for solid sorption chiller: experimental results and modelling

Abstract: In this paper the experimental results of a lab-scale chilling module working with the composite sorbent SWS-1L (mesoporous silica gel impregnated with CaCl2) are presented. The interesting sorption properties of this material yield a high COP=0.6 that gives a promising alternative to the common zeolite or silica gel for application in solid sorption units driven by low temperature heat (T 4100 � C). The measured low specific power of the device is a result of not optimised geometry of the adsorber and of the pelletised shape of the adsorbent. Heat transfer optimisation is currently under progress to increase the specific power. The experimental results are compared with those of a mathematic model able to describe the dynamic behaviour of the system. The model is used to study the influence of the main operating

Fabrication of magnetically separable mesostructured silica with an open pore system

Abstract: Magnetically separable mesostructured silica with an unobstructed pore system was fabricated through the deposition of cobalt nanoparticles on the outer surface of the submicron-sized silica particles. These cobalt nanoparticles were further protected by a nanometer-thick carbon shell against acid erosion. Due to the fact that the magnetic particles are grafted on the outer surface of the porous silica, the pores are still accessible for further modification, which could widen the application range of porous silica.

Silica functionalised sulphonic acid groups: synthesis, characterization and catalytic activity in acetalization and acetylation reactions

Abstract: A simple synthesis procedure for the immobilization of propyl thiol groups on silica is investigated using various concentrations of 3-mercapto propyl trimethoxy silane (3-MPTS) in the range of 5–40%. The thiol group functionalised silicas (SiO2–SH) were then oxidized to Bronsted sulphonic acid silica materials (SiO2–SO3H) using aqueous H2O2 as oxidizing agent. The surface structures of the functionalized catalysts were analyzed by a series of characterization techniques like elemental analysis, FTIR, TG-DTA, Surface area measurements, XPS, 13 C CP MAS NMR and 29 Si MAS NMR. The 13 C CP MAS NMR analyses confirm that disulphide species are not formed under the present preparation condition of catalysts. The acidity of the synthesized catalysts were further confirmed by the temperature programmed desorption of ammonia. The catalytic activity of the sulphonic acid functionalized silicas was evaluated in the liquid phase acetalization of ethyl acetoacetate with ethylene glycol and in the acetylation of anisole with acetic anhydride. The catalysts were found to be active in the acetalization reaction, which needs mild acidic sites, while the acetylation reactions gave lower activity, probably it needs stronger acid sites. A 30 wt.% SO3H loaded silica (SiO2–SO3H30) was recycled two times in the acetalization of ethyl acetoacetate and no major change in the conversion of EAA and selectivity to fructone is seen, which further argue against the possibility of leaching of the anchored sulphur containing species during reactions.

A condensable amphiphile with a cleavable tail as a "Lizard" template for the sol-gel synthesis of functionalized mesoporous silica.

Abstract: Use of a dialkoxysilane-containing, condensable alanine amphiphile with a cleavable hexadecyl ester tail (1) as a “lizard” template for sol−gel synthesis results in the formation of a mesoporous silicate (2) whose channels are filled with an organic group of the template. Treatment of 2 with aqueous HCl allows selective cleavage of the ester moiety to leave nanochannels (3) whose surface is densely covered with alanine−CO2H. According to XRD and TEM, 2, on conversion into 3, can preserve its regular hexagonal structure. 3 displays a clear N2 adsorption/desorption isotherm with a BET surface area of 536 m2 g-1. 3 can also adsorb a basic guest such as NH3 up to 0.7 mmol g-1, which is 7 times greater than that observed for mesoporous silica obtained by calcination of 2.

Great Improvement of Chromatographic Performance Using MCM-41 Spheres as Stationary Phase in HPLC

Abstract: Grafted MCM-41 materials are ordered mesoporous adsorbents suitable for reversed-phase liquid chromatography applications (RP-HPLC): they possess high surface area, which is a great advantage to enhance the thermodynamic behavior of the classical stationary phase by increasing solute retention. Hence, MCM-41s allow the separation of poorly retained solutes which are hard to separate on conventional silica gels. Furthermore, the ordered porosity in MCM-41 enhances the kinetic properties of the classical stationary phase. MCM-41s afford higher and more homogeneous molecular diffusivity, which increases the column efficiency for high flow rates as compared to that of classical silica-based columns. Besides, RP-HPLC provides a meaningful description of the particle size distribution at a macroscopic scale and of the pore ordering by probing the diffusivity of solutes. MCM-41s obtained by pseudomorphic synthesis of silica gel have successfully passed all the chromatographic tests and the results show that thi...

Protic acid (HClO4 supported on silica gel)-mediated synthesis of 2,3-unsaturated-O-glucosides and a chiral furan diol from 2,3-glycals.

Abstract: Perchloric acid supported on silica gel acts as an excellent reagent system in converting glucals into 2,3-unsaturated-O-glucosides in good to excellent yields in short reaction time with good a selectivity. Primary, secondary, and allylic alcohols, phenols, and thiols react with 3,4,6-tri-O-acetyl glucal with equal ease. In addition to this, a chiral furan diol is obtained from unprotected D-glucal or D-galactal in good yields.

Pd(II)-Schiff Base Complexes Heterogenised on MCM-41 and Delaminated Zeolites as Efficient and Recyclable Catalysts for the Heck Reaction

Abstract: We present a widely applicable approach for the preparation of Pd-complexes supported on silica-based mesoporous and laminar inorganic solids (silica gel, purely siliceous and acidic MCM-41, and delaminated zeolites ITQ-2 and ITQ-6) which involves the use of the Schiff bases 2-tert-butyl-4-methyl-6{(E)-[(2S)-1-(1-arylmethyl)pyrrolidinyl]imino}methylphenol (aryl=phenyl, 1-naphthyl, 2-naphthyl) as anchored ligands. As an extension of our research in “recyclable catalytic systems” we report results demonstrating the reactivity and recyclability of these systems for the Heck reaction (in a biphasic mode using ethylene glycol and toluene) and Suzuki coupling under phosphine-free conditions. The complexes are insensitive to oxygen or moisture, and no change of their activity was observed when exposed to an open system during the usual operation. No palladium black was observed after an extended reaction time and no residual palladium was detected from the filtrate at the end of the reaction.

Adsorption Equilibria of Aromatic Compounds on Activated Carbon, Silica Gel, and 13X Zeolite

Abstract: The adsorption of volatile organic compounds (VOCs) on the activated carbon, silica gel, and type-13X synthetic zeolite were investigated by a gravimetric adsorption method. The sorbates (VOCs) include benzene, toluene, o-xylene, p-xylene, and m-xylene. The equilibrium isotherm data obtained from this study were fitted by the Freundlich equation. The standard deviations for the predicted adsorption isotherms in the activated carbon bed were from 0.44 to 0.92, and the standard deviations were from 0.25 to 4.79 in the silica gel bed and were from 0.12 to 0.87 in the 13X zeolite bed.

Crystal morphology control of AFI type molecular sieves with microwave irradiation

Abstract: AFI type molecular sieves with various morphologies such as aggregated sphere, plate, rod, prism and barrel were synthesized under varying reaction conditions under microwave irradiation. Hexagonal plate-like crystals with an aspect ratio of about 0.2 were obtained in the presence of silica under alkaline conditions. Rod-like crystals with an aspect ratio of about 40 were obtained with the addition of fluoride and an increase of template and water concentrations. The addition of silica to an alkaline precursor gel hindered the crystal growth in the c-direction and the presence of fluoride ions retarded the nucleation and crystal growth.

Extraction of uranium by amine, amide and benzamide grafted covalently on silica gel

Abstract: Organic complexing moieties such as amine, 2°-amide and benzamide functional groups were covalently linked to silica gel by performing organomodification on commercially available silica gel. The extraction of uranium as a function of pH, time, concentration of uranium and the effect of ions such as Fe2+, Co2+, Ni2+ and CO3 2- on the distribution coefficient of uranium by sorbent was studied. The data was fitted into Langmuir adsorption isotherm. The selectivity of the sorbents towards uranium were found to decrease in the order gel-amine >> gel-benzamide ≅ gel-2°-amide at all pHs. The presence of metal ions such as Fe2+, Co2+, Ni2+ and CO3 2- were found to decrease the distribution coefficients (K d, ml/g) of uranium and the decrease was very high when Fe2+ was present. The performance of the sorbent, gel-amine, under a given column condition was tested by following the breakthrough curve of uranium up to C/C 0 = 0.025. The sorbent was found to be good for the quantitative pre-concentration of uranium from a large amount of sodium.