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Silica Surface (silica + surface)
Selected AbstractsReactions of Cl,/Cl2,, Radicals with the Nanoparticle Silica Surface and with Humic Acids: Model Reactions for the Aqueous Phase Chemistry of the AtmospherePHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2007Paula Caregnato ABSTRACT Reactions of chlorine radicals might play a role in aqueous aerosols where a core of inorganic components containing insulators such as SiO2 and dissolved HUmic-LIke Substances (HULIS) are present. Herein, we report conventional flash photolysis experiments performed to investigate the aqueous phase reactions of silica nanoparticles (NP) and humic acid (HA) with chlorine atoms, Cl,, and dichloride radical anions, Cl2,,. Silica NP and HA may be taken as rough models for the inorganic core and HULIS contained in atmospheric particles, respectively. Both Cl, and Cl2,, were observed to react with the deprotonated silanols on the NP surface with reaction rate constants, k ± ,, of (9 ± 6) × 107 M,1 s,1 and (7 ± 4) × 105 M,1 s,1, respectively. The reaction of Cl, with the surface deprotonated silanols leads to the formation of SiO, defects. HA are also observed to react with Cl, and Cl2,, radicals, with reaction rate constants at pH 4 of (3 ± 2) × 1010 M,1 s,1 and (1.2 ± 0.3) × 109 M,1 s,1, respectively. The high values observed for these constants were discussed in terms of the multifunctional heterogeneous mixture of organic molecules conforming HA. [source] Replication and Coating of Silica Templates by Hydrothermal Carbonization,ADVANCED FUNCTIONAL MATERIALS, Issue 6 2007M.-M. Titirici Abstract Hierarchical carbon materials with functional groups residing at the surface are prepared for the first time by using nanostructured silica materials as templates in combination with hydrothermal carbonization at mild temperatures (180,°C). Different carbon morphologies (e.g., macroporous casts, hollow spheres, carbon nanoparticles, and mesoporous microspheres) can be obtained by simply altering the polarity of the silica surface. The surface functionality and hydrophilicity of the resulting materials are assessed by Fourier transform IR spectroscopy, X-ray photoelectron analysis, and water porosimetry. Raman spectroscopy and X-ray diffraction measurements show that the materials are of the carbon-black type, similar to charcoal. [source] Silica-Supported Zirconium Complexes and their Polyoligosilsesquioxane Analogues in the Transesterification of Acrylates: Part 1.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2009Characterization, Synthesis Abstract Various silica-supported acetylacetonate and alkoxy zirconium(IV) complexes have been prepared and characterized by quantitative chemical measurements of the surface reaction products, quantitative surface microanalysis of the surface complexes, in situ infrared spectroscopy, CP-MAS 13C,NMR spectroscopy and EXAFS. The complex (SiO)Zr(acac)3 (acac=acetylacetonate ligand) (1) can be obtained by reaction of zirconium tetraacetylacetonate [Zr(acac)4] with a silica surface previously dehydroxylated at 500,°C. The complexes (SiO)3Zr(acac) (2) and (SiO)3Zr(O- n- Bu) (n- Bu=butyl ligand) (3) can be synthesized by reaction of (SiO)3ZrH with, respectively, acetylacetone and n -butanol at room temperature. The spectroscopic data, including EXAFS spectroscopy, confirm that in compound 1 the zirconium is linked to the surface by only one SiOZr bond whereas in the case of compounds 2 and 3 the zirconium is linked to 3 surface oxygen atoms which are sigma bonded. EXAFS data indicate also that the acetylacetonate ligands behave as chelating ligands leading to a hepta-coordination around the zirconium atom in 1 and a penta-coordination in 2. In order to provide a molecular analogue of 1, the synthesis of the following polyoligosilsesquioxane derivative (c -C5H9)7Si8O12(CH3)2Zr(acac)3 (1,) was achieved. The compound 1, is obtained by reacting (c -C5H9)7Si8O11(CH3)2(OH), 4, with an equimolecular amount of Zr(acac)4. In the same manner, syntheses of complexes (c -C5H9)7Si7O12Zr(acac) (2,) and of (c-C5H9)7Si7O12Zr(O- n- Bu) (3,) were achieved by reaction of the unmodified trisilanol, (c -C5H9)7Si7O9(OH)3, with respectively Zr(acac)4 and Zr(O- n- Bu)4 at 60,°C in tetrahydrofuran. Compounds 1,, 2, and 3, can be considered as good models of 1, 2 and 3 since their spectroscopic properties are comparable with those of the surface complexes. The synthetic results obtained will permit us to study the catalytic properties of these surface complexes and of their molecular analogues with the ultimate goal of delineating clear structure-activity relationships. [source] Highly Selective Oxidation of Alkylphenols to Benzoquinones with Hydrogen Peroxide over Silica-Supported Titanium Catalysts: Titanium Cluster Site versus Titanium Single SiteADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2009Oxana Abstract Titanium-silica catalysts have been prepared by supporting titanium(IV) precursors with different nuclearity {mononuclear titanocene dichloride Ti(Cp)2Cl2, dinuclear titanium diethyl tartrate and the tetranuclear titanium peroxo complex (NH4)8[Ti4(C6H4O7)4(O2)4],8,H2O} onto the surface of silica materials with different textural characteristics. The supported catalysts have been explored as highly active and reusable catalysts for the oxidation of 2,3,6-trimethylphenol (TMP) and 2,6-dimethylphenol (DMP) to 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, vitamin E key intermediate) and 2,6-dimethyl-1,4-benzoquinone (DMBQ), respectively, using aqueous hydrogen peroxide as green oxidant. Catalysts prepared by grafting mononuclear Ti(Cp)2Cl2 revealed a strong dependence of the product selectivity on the surface concentration of titanium active centers. Mesoporous materials with titanium surface concentration in the range of 0.6,1.0,Ti/nm2 were identified as optimal catalysts for the transformation of alkylphenols to benzoquinones. Catalysts having <0.6,Ti/nm2 produced a mixture of benzoquinones and dimeric by-products. Conversely, when di-/tetranuclear titanium precursors were employed for the catalyst preparation, a diminution of the titanium surface concentration had no impact on the benzoquinone selectivity, which was typically as high as 96,99%. DR-UV spectroscopic studies revealed that the catalysts capable of producing alkylbenzoquinones with nearly quantitative yields possess titanium dimers and/or subnanometer-size clusters homogeneously distributed on a silica surface. On the contrary, catalysts with isolated titanium sites give a considerable amount of dimeric by-products. This is the first example which clearly demonstrates the advantages of titanium cluster-site catalysts over titanium single-site catalysts in hydrogen peroxide-based selective oxidation reaction. [source] Mechanochemical modification of silica with poly(1-vinyl-2-pyrrolidone) by grinding in a stirred media millJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2007Voronov A. Abstract The polymerization of 1-vinyl-2-pyrrolidone (VP) mechanochemically initiated by grinding silica was investigated in a wet stirred media mill. The polymerization itself proceeds from the silica grinding without any additional initiator. We have found that the amount of grafted polymer increases with an increase in total ground silica surface. The suspension of polymer-modified silica nanoparticles showed high colloidal stability in water because of the appearance of grafted hydrophilic PVP on the surface during the reactive grinding. Because the nanoparticles SiO2 - graft -PVP are biocompatible, the developed polymer nanocomposite material can be of particular interest for the performance of membranes and for the fabrication of biocompatible hydrogels with enhanced mechanical properties and porosity. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3708,3714, 2007 [source] Solvatochromism of carbenium,arene EDA (electrondonor,acceptor) complexes and their behaviour on silicaJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2001Stefan Spange Abstract Spectral characteristics of two different types of electron donor,acceptor (EDA) complexes with charged components (arene,carbenium and anion,, acceptor) are presented. The UV,Vis absorption maxima of the charge-transfer (CT) band (,max,CT) of the EDA complexes were measured in various solvents and after adsorption on silica. Tropylium, triarylmethylium and diarylmethylium ions in combination with aromatic , donors, e.g. pyrene, acenaphthene and methoxy-substituted benzene derivatives and also the tetraethylammonium iodide,1,3,5-trinitrobenzene complex have been investigated by means of a special UV,Vis technique in transparent slurries of silica nanoparticles in 1,2-dichloroethane. Multiple linear regression analyses of the ,max,CT values of the EDA complexes in various solvents and on silica with the Kamlet,Taft solvent parameters and structure,reactivity terms of the complex components, e.g. the oxidation potential of the arene component or the Hammett substituent constant, ,p+, of the arylmethyl component, show their related properties to the solution complexes. The influence of the electron-pair donating capacity of a solvent on ,max,CT of carbenium/arene complexes was compared with the property of the silica surface in order to link EDA complexes. For the diarylmethyl carbenium ion,pyrene complexes on silica, the presence of a ,-like structure is discussed. Copyright © 2001 John Wiley & Sons, Ltd. [source] Immobilization of flame retardant onto silica nanoparticle surface and properties of epoxy resin filled with the flame retardant-immobilized silicaJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 22 2009Takeshi Yamauchi Abstract To prepare silica nanoparticle having flame retardant activity, the immobilization of flame retardant onto hyperbranched poly(amidoamine) (PAMAM)-grafted silica was investigated. Grafting of PAMAM onto a silica surface was achieved in a solvent-free dry-system using PAMAM dendrimer synthesis methodology. The immobilization of bromine flame retardant, poly(2,2,,6,6,-tetrabromobisphenol-A) diglycidyl ether (PTBBA), was successfully achieved by the reaction of terminal amino groups of PAMAM-grafted silica (Silica-PAMAM) with epoxy groups of PTBBA. The immobilization of PTBBA was confirmed by FTIR and thermal decomposition GC-MS. The amount of PTBBA immobilized onto Silica-PAMAM was determined to be 60 wt %. PTBBA-immobilized Silica-PAMAM (Silica-PAMAM-PTBBA) was dispersed uniformly in a epoxy resin, and the epoxy resin was cured in the presence of hexamethylenediamine. Flame retardant activity of the epoxy resin filled with Silica-PAMAM-PTBBA was estimated by limiting oxygen index (LOI). The LOI of epoxy resin filled with Silica-PAMAM-PTBBA was higher than that filled with untreated silica and free PTBBA. It was confirmed that the flame retardant activity of epoxy resin was improved by the addition of the Silica-PAMAM-PTBBA. The elimination of PTBBA from the epoxy resin filled with Silica-PAMAM-PTBBA into boiling water was hardly observed by immobilization of PTBBA onto silica surface. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6145,6152, 2009 [source] Grafting of polymers onto and/or from silica surface during the polymerization of vinyl monomers in the presence of ,-ray-irradiated silicaJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 9 2006Akiko Tsuchida Abstract The effects of radicals on silica surface, which were formed by ,-ray irradiation, on the polymerization of vinyl monomers were investigated. It was found that the polymerization of styrene was remarkably retarded in the presence of ,-ray-irradiated silica above 60 °C, at which thermal polymerization of styrene is readily initiated. During the polymerization, a part of polystyrene formed was grafted onto the silica surface but percentage of grafting was very small. On the other hand, no retardation of the polymerization of styrene was observed in the presence of ,-ray-irradiated silica below 50 °C; the polymerization tends to accelerate and polystyrene was grafted onto the silica surface. Poly(vinyl acetate) and poly(methyl methacrylate) (MMA) were also grafted onto the surface during the polymerization in the presence of ,-ray-irradiated silica. The grafting of polymers onto the silica surface was confirmed by thermal decomposition GC-MS. It was considered that at lower temperature, the grafting based on the propagation of polystyrene from surface radical ("grafting from" mechanism) preferentially proceeded. On the contrary, at higher temperature, the coupling reaction of propagating polymer radicals with surface radicals ("grafting onto" mechanism) proceeded to give relatively higher molecular weight polymer-grafted silica. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2972,2979, 2006 [source] Preparation of Scratch- and Abrasion-Resistant Polymeric Nanocomposites by Monomer Grafting onto Nanoparticles, 4,MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 3 2003Frank Bauer Abstract To obtain transparent, scratch- and abrasion-resistant coating materials a high content of nanosized silica and alumina filler was embedded in radiation-curable acrylate formulations by acid catalyzed silylation using trialkoxysilanes. 29Si magic angle spinning (MAS) NMR spectroscopy and matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) were used to elucidate the structure of the surface-grafted methacryloxypropyl-, vinyl-, and n -propyltrimethoxysilane. 29Si MAS NMR measurements revealed a predominance of T2 and T3 structures of silicon atoms, i.e., silane oligomers have been formed by an extensive loss of water. For methacryloxypropyltrimethoxysilane, the proportion of the highly condensed T3 silicon atoms was estimated to be 75%. In accordance with these NMR findings, MALDI-TOF MS showed highly condensed oligomeric siloxanes of more than 20 monomeric silane units. The degree of silane condensation is higher on alumina than on silica, irrespective to the addition of maleic acid as catalyst. Moreover, completely condensed silsesquioxanes, e.g., octamers, decamers, and dodecamers, were detected. Based on the observed MS pattern, a ladder-like arrangement of two linked siloxane chains forming connected eight-membered rings is proposed, which is similar for all of the condensed organosilanes under study. The grafted polysiloxanes lead to an organophilation of inorganic nanofillers and improve their dispersibility in acrylate suspensions. Proposed ladder-like arrangement of the T3 structure of silicon atoms in polysiloxanes grafted on the silica surface. [source] Introducing a Flat Model of the Silica-Supported Bis(imino)pyridyl Iron(II) Polyolefin CatalystMACROMOLECULAR RAPID COMMUNICATIONS, Issue 4 2006Wei Han Abstract Summary: A well-defined flat model of a supported homogeneous polyolefin catalyst is prepared on the basis of an immobilized bis(imino)pyridyl iron complex on a super flat silica surface. The amount of supported catalyst precursor is quantified using XPS. This model catalyst remains active over extended periods, i.e., an average activity of 0.25,×,103 kg PE,·,(molCat,·,h,·,bar),1 is obtained for 24 h of ethylene polymerization. The morphology of the nascent polyethylene film is investigated by SEM. A side-view SEM image of the PE produced from the supported bis(imino)pyridyl Fe catalyst. [source] In situ elevated-temperature drift spectroscopyof ,-aminopropyltriethoxysilane as a condensed phase and a coated film on silica surfacePOLYMER COMPOSITES, Issue 3 2001W. Noobut It is the purpose of this study to determine the structure of ,-amlnopropyltriethoxysilane (APS) on the surface of silica. Diffuse Reflectance spectroscopy (DRIFT) was employed to study the molecular structure of the partially condensed (APS) and Al's-coated silica at 90°C. The condensed APS and Al's-coated silica were prepared at the natural pH (10.3) of the APS and in the ambient environment. At room temperature, amino groups of the condensed APS and the APS-coated silica form the amine carbonate and amine bicarbonate salts. Upon heat treatment, the amino salt structures are eliminated, leaving the amino groups in non-bonded form. [source] Influence of filler type and content on properties of styrene-butadiene rubber (SBR) compound reinforced with carbon black or silicaPOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 3 2004Sung-Seen Choi Abstract Rubber compounds are filled with reinforcing fillers to improve their physical properties. Carbon black and silica have different surface chemistries to each other. Differences in properties of carbon black- and silica-reinforced styrene-butadiene rubber (SBR) compounds were studied. Variation of properties of carbon black- or silica-filled compounds with the filler content was also investigated. The silica-filled compounds without any coupling agent and dispering agent were prepared to investigate the influence of polar materials-adsorption on the silica surface. Viscosity and crosslink density increased with increase of the filler content. Hardness, modulus, tensile strength, and wear property were improved more and more by increasing the filler content. Viscosity of the silica-filled compound was higher than that of the carbon black-filled one. Cure rate of the silica-filled compound became slower as the filler content increased, while that of the carbon black-filled one became faster. Difference in properties between the carbon black- and silica-filled compounds were explained by the poor silica dispersion and the adsorption of cure accelerator on the silica surface. Copyright © 2004 John Wiley & Sons, Ltd. [source] Supporting mechanism of non-toxic chromium (III) acetate on silica for preparation of Phillips ethylene polymerization catalystsASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Pengyuan Qiu Abstract Phillips catalyst is an important kind of industrial polyethylene catalyst. As early as in the late 1970s, CrO3 was substituted by chromium (III) acetate for the preparation of Phillips catalyst on the industrial scale owing to health and environmental considerations. There is still considerable research focusing on the relations between the preparation process and catalyst properties in academics. In this work, the supporting mechanism of chromium (III) acetate on silica has been studied by Thermogravimetry,Differential Thermal Analysis (TG-DTA), and Electron Spin Resonance (ESR), in comparison with that of supporting CrO3 on SiO2. The basic chromium (III) acetate supported on high surface area silica gel decomposed differently from that for bulk basic chromium acetate when decomposition temperature was decreased by 15 °C. The decomposition temperature was 299 °C for Cr3(OH)2(Ac)7/SiO2 catalyst precursor, which would be firstly transferred into CrO3 followed by supporting on silica surface as chromate species. The further weight loss came from thermal inductive reduction of chromate species into Cr2O3, which was also supported by the results of colors of catalysts. Moreover, with the increase of chromium loading of Cr3(OH)2(Ac)7/SiO2, such thermal inductive reduction became more severe. ESR spectra of Cr3(OH)2(Ac)7/SiO2 and CrO3/SiO2 catalyst precursors showed that a small amount of supported Cr5+ can exist stably on silica gel surface at temperatures higher than 200 °C. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Layer-By-Layer Dendritic Growth of Hyperbranched Thin Films for Surface Sol,Gel Syntheses of Conformal, Functional, Nanocrystalline Oxide Coatings on Complex 3D (Bio)silica TemplatesADVANCED FUNCTIONAL MATERIALS, Issue 17 2009Guojie Wang Abstract Here, a straightforward and general method for the rapid dendritic amplification of accessible surface functional groups on hydroxylated surfaces is described, with focus on its application to 3D biomineral surfaces. Reaction of hydroxyl-bearing silica surfaces with an aminosilane, followed by alternating exposure to a dipentaerythritol-derived polyacrylate solution and a polyamine solution, allows the rapid, layer-by-layer (LBL) build-up of hyperbranched polyamine/polyacrylate thin films. Characterization of such LBL-grown thin films by AFM, ellipsometry, XPS, and contact angle analyses reveals a stepwise and spatially homogeneous increase in film thickness with the number of applied layers. UV,Vis absorption analyses after fluorescein isothiocyanate labeling indicate that significant amine amplification is achieved after the deposition of only 2 layers with saturation achieved after 3,5 layers. Use of this thin-film surface amplification technique for hydroxyl-enrichment of biosilica templates facilitates the conformal surface sol,gel deposition of iron oxide that, upon controlled thermal treatment, is converted into a nanocrystalline (,9.5,nm) magnetite (Fe3O4) coating. The specific adsorption of arsenic onto such magnetite-coated frustules from flowing, arsenic-bearing aqueous solutions is significantly higher than for commercial magnetite nanoparticles (,50,nm in diameter). [source] Synthesis and properties of organic/inorganic hybrid nanoparticles prepared using atom transfer radical polymerizationJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2008Tzong-Liu Wang Abstract The synthesis of organic/inorganic hybrid materials was conducted by atom transfer radical polymerization (ATRP) of styrene and methyl methacrylate (MMA) from the surface of silica colloids. Colloidal initiators were prepared by the functionalization of silica nanoparticles with (3-(2-bromoisobutyryl)propyl) dimethylethoxysilane (BIDS). Well-defined polymer chains were grown from the nanoparticle surfaces to yield individual particles composed of a silica core and a well-defined outer polystyrene (PS) or poly(methyl methacrylate) (PMMA) layer. Fourier transform infrared (FTIR) and solid state 13C and 29Si-NMR spectroscopy confirmed the successful modification of nanosilica surfaces. Subsequent grafting of polymers on silica surfaces by ATRP was also performed with success based on FTIR and NMR data. Scanning electron microscopy (SEM) and silicon mapping showed both hybrid materials were homogeneous dispersion systems. Energy dispersive X-ray spectrometer (EDS) analysis indicated that the BIDS initiator was covalently attached on surfaces of silica nanoparticles and ATRP of styrene and MMA were accomplished. Thermogravimetric analysis (TGA) results displayed higher thermal stabilities for both nanohybrids in comparison with the linear-type vinyl polymers. Contact angle measurements revealed the nanomaterials character for both silica-based hybrid materials. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Stereospecific polymerization of methacrylates by metallocene and related catalystsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 14 2004Eugene Y.-X. Abstract Stereospecific,isospecific, syndiospecific, and diastereospecific,polymerizations of methacrylates using group 4 metallocene and related catalysts produce polymethacrylates with controlled stereo-microstructures. The versatility and stereospecificity of these cat- alysts for methyl methacrylate polymerization were demonstrated not only in solution-phase polymerization, but also in polymerizations on silica surfaces and inside silicate nanogalleries. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3395,3403, 2004 [source] Hydroxylation and Dehydroxylation Behavior of Silica Glass Fracture SurfacesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002Andrew S. D'Souza The hydroxylation and dehydroxylation behavior of amorphous silica fracture surfaces was studied using temperature-programmed static SIMS. The results show that vacuum heat treatments result in more extensive condensation of silanol groups on the silica glass fracture surface as compared to fumed silica (Cabosil). This is attributed to differences in the distribution of silanol groups on the two silica surfaces. The rehydration kinetics of the dehydroxylated silica fracture surfaces showed two distinct reaction rates,an initial rapid increase in the silanol concentration, followed by a slower rehydration for longer dosing times. The slower rehydration reaction was shown to follow first-order reaction kinetics with the reaction rate constant, suggesting hydrolysis of strained siloxane bonds on three-membered silicate ring structures. The much faster initial rehydration is attributed to the hydrolysis of extremely strained siloxane bonds in two-membered, edge-shared tetrahedral rings. The effect of the dehydration time and temperature (i.e., thermal history of the surface) on the rehydration kinetics is also discussed. [source] | |||||||||||||