Number Of Silanol Groups Research Articles

Ultrasonic Pretreatment as a Tool for the Preparation of Low-Defect Zeolite Mordenite.

The effects of the ultrasonic (US) pretreatment of synthesis gel for the preparation of mordenite zeolite were studied in comparison with the classical stirring method. Even though the US pretreatment was performed before the hydrothermal crystallization, it significantly affected the properties of the obtained mordenite crystals. The US-assisted procedure resulted in a material with improved textural characteristics, in particular, the micropore volume accessible for nitrogen molecules in the as-made form. On the other hand, mordenite prepared with the classical stirring method demonstrated comparable sorption properties only after a postsynthetic treatment. Moreover, in the case of US-pretreated mordenite, altered crystal shape and more homogeneous morphology were observed. 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) demonstrated that the US pretreatment introduced structural changes on the atomic level, resulting in fewer defects (reflected in the number of silanol groups) and less pore blockage (affected by Na+ cations) for the as-made sample.

Laser Modification of Silicon and Borosilicate Glass Wettability for Micro-Fluidic Systems

A laser modification method to control the surface wettability of a silicon and borosilicate glass substrate is proposed and demonstrated. The wettability of the silicon surface decreases after 2.5 W laser irradiation without a change in surface morphology. When the laser power is greater than 4 W, it results in the formation of a 10-nm-deep groove and the wettability increases. These phenomena are caused by changes in the number of surface groups and morphology, respectively. On the other hand, the glass surface is modified by infrared laser irradiation and the treated surface is highly hydrophilic. Surface analysis by FT-IR indicates that the modification is the result of an increase in the number of silanol groups. The proposed modification method is applied to micro-fluidic systems. A modified line can function as a surface micro-channel. Furthermore, with a gradient in wettability, the micro-channel has the ability to self-transport water droplets.

Gamma Globulins Adsorption on Carbofunctional Polysiloxane Microspheres

Polysiloxane microspheres containing a large number of silanol groups were functionalized by reactions of these groups with organic alkoxy or chlorosilanes having chloro, amine or imidazole functions in their organic parts. The obtained imidazole groups on the microspheres were ionized by the reaction with n-octyl iodide and with methyl iodide while the chloro functions were used for quaternization of a tertiary amine. Adsorption of gamma globulins on these functionalized microspheres was studied in aqueous suspension at pH 5.2, 7.4 and 9.2. Hydrophilic–hydrophobic properties of these microspheres were examined by measuring adsorption of a hydrophobic dye, Rose Bengal. The functionalized microspheres showed higher adsorption of globulins and higher hydrophobicity than the not functionalized ones rich in silanol groups. In the case of microspheres with ionic functional groups the electrostatic forces also contribute to attractive interactions between proteins and microspheres.

A novel one-step synthesis of mesostructured silica-pillared clay with highly ordered gallery organic–inorganic hybrid frame

A new class of organic–inorganic hybrid mesostructured silica-pillared clay (HSPC) has been prepared through the surfactant directed assembly of organosilica in the galleries of montmorillonite. The surfactant templates were removed from the pores by solvent-extraction. The products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), nitrogen adsorption–desorption measurements, solid-state 29Si and 13C NMR, and Fourier-transform infrared (FT-IR) spectra. XRD patterns indicated a regular interstratifications of the clay layers for HSPCs. Depending on loading of phenyl groups, HSPCs had BET surface areas of 395–602m2g−1, pore volumes of 0.34–0.79cm3g−1, and the framework pore sizes in the supermicropore to small mesopore range (1.3–2.6nm). HSPCs were hydrophobic and acidic. The number of silanol groups on the surface of HSPC materials has been titrated by a surface reaction with hexamethyldisilazane, followed by quantification of the liberated NH3. Based on this method extracted HPCHs have high silanol numbers, a very important feature with respect to the amount of catalytic sites that can eventually be grafted onto the surface.

Chromium (VI) removal from aqueous solutions using mercaptosilane functionalized sepiolites

Natural and acid-activated sepiolite samples were functionalized with (3-Mercaptopropyl)trimethoxysilane. By X-ray powder diffraction, Fourier transform infrared spectroscopy, and differential thermal analysis it was shown that functionalization of sepiolites by grafting silane reagents occurs mainly on the surface, whereby their crystalline structure remained unchanged. The efficiency of the adsorbents in Cr(VI) removal from aqueous solutions follows the order: functionalized acid-activated sepiolite > functionalized natural sepiolite > acid-activated sepiolite > natural sepiolite. As the number of silanol groups on the sepiolite surface increased during the acid activation, the silane functionalization increased and this led to an increase in Cr(VI) adsorption. The adsorption capacity strongly depended on the pH of the solution from which the adsorption occurred. Maximum Cr(VI) removal was ca. 8.0 mg Cr(VI) per g of functionalized acid-activated sepiolite at initial pH 3, when the pH of solution reached 4.7, and ca. 2.7 mg Cr(VI) per g of functionalized natural sepiolite at initial pH 2, when the pH of the solution reached 2.5. Cr(VI) removal by the functionalized sepiolites was mainly attributed to electrostatic attraction between the protonated mercapto groups and the negatively charged Cr(VI) species. The sorption isotherms showed that Cr(VI) removal by both functionalized sepiolites for all initial investigated solution pH values is well described using the Dubinin–Radushkevich model. The thermodynamic data suggest spontaneity of the major physical adsorption process at 298 K.

Hydrothermal stability of mesostructured silica prepared using a nonionic fluorinated surfactant

In this study, we have investigated the influence of the temperature during the synthesis on the hydrothermal stability of mesostructured silica prepared from a system based on nonionic fluorinated surfactant. Results obtained by SAXS, nitrogen adsorption–desorption analysis and infrared spectroscopy show that when the temperature is increased the hydrothermal stability of the recovered material is enhanced. Indeed, as soon as the treatment into boiling water begins, the structure of the sample synthesized at 80 °C is altered, while the material synthesized at 100 °C is very stable up to 24 h. This behavior has been attributed to a higher degree of silica condensation and to a decrease of both the number of silanol groups and the microporosity. A comparison of the characteristics of the materials obtained with both hydrogenated and fluorinated surfactants also evidences that the lather are more stable when treated with boiling water.

Influence of Structural Differences of Dextromethorphan and its Three Metabolites on their Simultaneous Separation using Various Silica Columns with a Simple Aqueous Mobile Phase

The retention behaviors and efficiencies of the chromatographic separation of dextromethorphan (DM) and its metabolites (dextrorphan (DX), 3‐methoxymorphinan (MM), and 3‐hydroxymorphinan (HM)), on three different silica columns (Inertsil, μ‐Porasil, and Lichrospher), were compared using a simple aqueous mobile phase consisting of an organic solvent (methanol or acetonitrile) and water at different volume ratios (1∶9 to 9∶1) containing triethylamine (TEA) and acetic acid (ACH). Results demonstrated that the retention capacities for basic compounds with the same ionization conditions of the silanol group were the largest for the Lichrospher column, followed by the μ‐Porasil one, with the Inertsil column exhibiting the lowest level. Based on their physical characteristics, the larger retention capacities for DM and its three metabolites in the LiChrospher column compared to those of the Inertsil and μ‐Porasil columns may have been because the LiChrospher column has the largest surface area for interaction. However, since the surface areas of the Inertsil and μ‐Porasil columns are similar, the greater retention capacities for DM and its three metabolites in the μ‐Porasil column can probably be attributed to the existence of a greater number of silanol groups than in the Inersil column. This also demonstrates that the interaction of tertiary amines with ionized silanol groups is greater than that of secondary amines resulting in the elution order being MM>DM and HM>DX. However, O‐demethylation to expose metabolites with a phenolic hydroxy group seemed to decrease the retention capacity, thus yielding the elution order of DX>DM (tertiary amine) and HM>MM (secondary amine). This might be attributable to the negative charge repulsion between phenoxyl groups (PhO−) and ionized silanol groups (SiO−) decreasing the affinity of basic compounds.

Glass bead grafting with poly(carboxylic acid) polymers and maleic anhydride copolymers

AbstractGlass beads were etched with acids and bases to increase the surface porosity and the number of silanol groups that could be used for grafting materials to the surfaces. The pretreated glass beads were functionalized using 3‐aminopropyltriethoxysilane (APS) coupling agent and then further chemically modified by reacting the carboxyl groups of carboxylic acid polymers with the amino groups of the pregrafted APS. Several carboxylic acid polymers and poly(maleic anhydride) copolymers, such as poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), poly(styrene‐alt‐maleic anhydride) (PSMA), and poly(ethylene‐alt‐maleic anhydride) (PEMA) were grafted onto the bead surface. The chemical modifications were investigated and characterized by FT‐IR spectroscopy, particle size analysis, and tensiometry for contact angle and porosity changes. The amount of APS and the different polymer grafted on the surface was determined from thermal gravimetric analysis and elemental analysis data. Spectroscopic studies and elemental analysis data showed that carboxylic acid polymers and maleic anhydride copolymers were chemically attached to the glass bead surface. The improved surface properties of surface modified glass beads were determined by measuring water and hexane penetration rates and contact angle. Contact angles increased and porosity decreased as the molecular weights of the polymer increased. The contact angles increased with the hydrophobicity of the attached polymer. The surface morphology was examined by scanning electron microscopy (SEM) and showed an increase in roughness for etched glass beads. Copyright © 2007 John Wiley & Sons, Ltd.

Silicon‐29 and carbon‐13 nuclear magnetic resonance identification of intermediates developed during the formation of a hybrid based on tetraethoxysilane (TEOS) and 4‐[(5‐dichloromethyl)(silyl)pentyloxy]cyanobenzene (DCN)

AbstractDuring the formation of organic–inorganic hybrid materials by the sol–gel process, the observation of multiphase systems at different stages of the reaction is not uncommon. An assessment of the chemical species formed before and after gelation is not an easy task because complex mixtures are generally found in these systems. In this study, the identification of intermediates developed during the formation of a hybrid obtained from tetraethoxysilane (TEOS) and 4‐[(5‐dichloromethylsilyl)pentyloxy]cyanobenzene monomer has been undertaken. Solution and solid‐state 29Si‐ and 13C‐NMR spectra, recorded at different degrees of conversion, allow the elucidation of the mechanism of hybrid formation. 4‐[(5‐dichloromethylsilyl)pentyloxy]cyanobenzene, (DCN), DCN–TEOS cyclosiloxanes and low‐molecular‐weight linear copolysiloxanes are formed in the early stages of the process and before gelation occur. Before the gel is formed, cyclic species undergo an acid‐catalyzed ring‐opening copolymerization with silica to produce a random copolymer. Other intermediates of the Qij type (where Q indicates a four‐functional silicon, the first subscript indicates the number of siloxane bridges connected to the silicon site, and the second subscript indicates the number of silanol groups) that develop before the gel is formed have also been identified. Spectral peak patterns of the final hybrid, observed in solid‐state 29Si and 13C‐NMR spectra, are also discussed. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 520–531, 2006

Direct gas-phase epoxidation of propene with nitrous oxide over modified silica supported [formula omitted] catalysts

Vapour phase epoxidation of propene with nitrous oxide over alkaline containing silica supported iron oxide catalysts was experimentally investigated. A reaction network is deduced from the trends of product selectivities as function of propene conversion using different catalysts. Important side reactions are parallel propanal formation and consecutive isomerisations of propylene oxide (PO) to propanal, acetone and allyl alcohol. Rates of these reactions were affected by catalyst composition, which influenced acid-base properties, number of silanol groups and surface area. A PO selectivity of ∼ 75 % and PO space–time yields of 0.02– 0.03 g PO g cat - 1 h - 1 were obtained with a catalyst containing ironoxo-species and caesium ions/-oxides. These results can only be obtained at a high surplus of N 2 O versus propene concentration in the reactor feed. Likely reasons for that is an inhibition of propylene oxide's formation rate by propene and propylene oxide itself.

Effects of pH and Dye Concentration on the Optical and Structural Properties of Coumarin-4 Dye-Doped SiO2-PDMS Xerogels

Optical and structural properties of coumarin-4 dye-doped SiO2-PDMS xerogels synthesized by acid catalyzed (one-step) and acid-base catalyzed (two-step) sol-gel routes with varying pH (0.6 to 7) and dye content ( 5 × 10−4 to 5 × 10−2 mole) are reported. Spectroscopic methods such as photo-luminescence, FT-IR and FT-Raman were used for characterizations. The acid catalyzed xerogels prepared with pH 2.5. The number of silanol groups was higher for the samples with pH > 2.5.

Template extraction from porous clay heterostructures: Influence on the porosity and the hydrothermal stability of the materials

The template extraction of two porous clay heterostructures (PCHs), derived from natural montmorillonite (PMH) and synthetic saponite (PSH), has been studied. Extraction in methanol solvent in the presence of cations (H+) removes up to 90% of the templates in a reproducible way without a subsequent calcination step for both PMH and PSH. The extracted materials have a high surface area (896 m2 g−1 for PMH and 1122 m2 g−1 for PSH) and pore volume (0.84 cm3 g−1 for PMH and 1.13 cm3 g−1 for PSH) and are structurally very stable when exposed to hydrothermal conditions. The number of silanol groups on the surface of PCH materials have been titrated by a surface reaction with hexamethyldisilazane, followed by quantification of the liberated NH3. Based on this method both extracted PMH and PSH have slightly higher silanol numbers than their calcined forms, a very important feature with respect to the amount of catalytic sites that can eventually be grafted onto the surface. The hydrothermal stability under a N2 flow containing 25% steam for 5 days at different temperatures between 400 °C and 700 °C for both calcined and extracted PMH and PSH was investigated using X-ray diffraction and N2-adsorption data. A remarkable hydrothermal stability was observed for all samples until 600 °C, a promising characteristic for applications of porous clay heterostructures.

Water-repellent coating films on glass prepared from hydrolysis and polycondensation reactions of fluoroalkyltrialkoxylsilane

Water-repellent glasses for automotive windows are new glass products in which several kinds of fluoroalkylsilanes are used to modify the surface properties of the glass substrates. We examined the hydrolysis and polycondensation reactions of a fluoroalkyltrimethoxysilane (FAS), CF 3(CF 2) 7CH 2CH 2Si(OCH 3) 3, by using gas chromatography, gel permeation chromatography and time-of-flight mass spectroscopy. Dimers, trimers and higher polymerized species of FAS were formed by dehydrating the water-repellent coating solutions after a hydrolysis process. The polymerized species of FAS are preferable for improvement of the durability of the water-repellent glasses. However, the excess polycondensation of FAS deteriorates the abrasion resistance, because of the decrease in the number of silanol groups, which are bound to the glass surface. Therefore, appropriate amounts of the polymerized species of FAS play an important role in improving both the abrasion resistance and the UV-light resistance of the water-repellent glasses. The water-repellent glasses obtained from the dehydrated water-repellent coating solution thus show higher durability than those obtained from the non-dehydrated one.

15N-pyridine—a mobile NMR sensor for surface acidity and surface defects of mesoporous silica

experiments performed under magic angle spinning (MAS) conditions in the absence and the presence of pyridine-d5 reveal that the large majority of silanol groups are located in the inner surfaces, isolated from each other but able to form hydrogen bonds with pyridine. On the other hand, low- and room-temperature 15 N CPMAS and MAS experiments (CP cross-polarization) performed on pyridine- 15 N show that at low concentrations all pyridine molecules are involved in hydrogen bonds with the surface silanol groups. In the presence of an excess of pyridine, a non-hydrogen-bonded pyridine phase is observed at 120 K in the slow hydrogen bond exchange regime and associates with an inner core phase. From these measurements, the number of pyridine molecules bound to the inner surfaces corresponding to the number of silanol groups could be determined to be nOH 3n m -2 for MCM-41 and 3.7 nm -2 for SBA-15. At room temperature and low concentrations, the pyridine molecules jump rapidly between the hydrogen-bonded sites. In the presence of an excess of pyridine, the hydrogen-bonded binding sites are depleted as compared to low temperatures, leading to smaller apparent numbers nOH. Using a correlation established previously between the 15 N and 1 H chemical shifts and the NHO hydrogen bond geometries, as well as with the acidity of the proton donors, the distances in the pyridine-hydroxyl pairs were found to be about rHN ) 1.68 A,rOH ) 1.01 A, andrON ) 2.69 A. This geometry corresponds in the organic solid state to acids exhibiting in water a pKa of about 4. Roomtemperature 15 N experiments on static samples of pyridine- 15 N in MCM-41 at low coverage show a residual 15 N chemical shift anisotropy, indicating that the jumps of pyridine between different different silanol hydrogen bond sites is accompanied by an anisotropic reorientational diffusion. A quantitative analysis reveals that in this regime the rotation of pyridine around the molecular C2 axis is suppressed even at room temperature, and that the angle between the Si-O axes and the OH axes of the isolated silanol groups is about 47°. These results are corroborated by 2 H NMR experiments performed on pyridine-4-d1. In contrast, in the case of SBA-15 with the larger pore diameters, the hydrogen bond jumps of pyridine are associated with an isotropic rotational diffusion, indicating a high degree of roughness of the inner surfaces. This finding is correlated with the finding by 29 Si CPMAS of a substantial amount of Si(OH)2 groups in SBA-15, in contrast to the MCM-41 materials. The Si(OH)2 groups are associated with surface defects, exhibiting not only silanol groups pointing into the pore center but also silanol groups pointing into other directions of space including the pore axes, leading to the isotropic surface diffusion. All results are used to develop molecular models for the inner surface structure of mesoporous silica which may be a basis for future simulations of the surfaces of mesoporous silica.

Influence of fluorination of xerogels on the intensity of photoluminescence of gamma-irradiated sol-gel quartz glasses

We investigated the influence of fluorination of xerogel on the intensity of luminescence with a maximum at 1.9 eV attributable to nonbridging oxygen atoms in gamma-quanta irradiated sol-gel quartzglasses. It is established that in samples with a high content of hydroxyl groups (more than 8 ppm) the intensity of this luminescence correlates with the content of hydroxyl in the initial samples. Fluorination of gels at the technological stage before sintering substantially reduces the number of silanol groups, improves the quality of glass, and increases their radiative-optical stability.

Processing of Inorganic-Organic Hybrids From Metal Alkoxides and Phenyltriethoxysilane

AbstractIn the synthesis of inorganic-organic hybrids from metal alkoxides and phenyltriethoxysilane (PTES), metal alkoxides were found to influence the phenyl siloxane network formation. The presence of metal alkoxides in the initial stage of hydrolysis and condensation of PTES promote the formation of phenyl siloxane networks to provide a dense hybrid. However, the presence of metal alkoxides in the stage of hydrolyzed PTES provide a loose structure of phenyl siloxane networks involving a number of silanol groups.

Analysis of Chemically Bonded Silica Gel by Computational Chemistry

Abstract Several forms of silica gels were constructed by Molecular Mechanics calculation. The structure of a designed silica gel is limited because of the requirement of physical strength and the maximum bonding. The number of silanol groups was a half that of the silicon atoms on the surface of the silica gels. All the silanol groups can be completely silanized with chloro- silane reagent to make a chemically stable bonded phase from the theory.

Mordenite acidity: dependence on the silicon/aluminum ratio and the framework aluminum topology. 1. Sample preparation and physicochemical characterization

A series of dealuminated mordenites (with Si/Alf ratios between 7 and 48) was prepared by acid leaching and characterized by using different physicochemical methods. The A1 content of the framework and the distribution of the tetrahedral and octahedral aluminum were determined by 27Al MAS NMR measurements and chemical analysis. Attempts to quantify the number of SiOH defect groups by '%i MAS NMR measurements (following a procedure given in the literature) failed. The number of silanol groups (and Bransted sites) were determined using 'H MAS NMR investigations. The micro- and mesopore volumes of the samples were derived from measurements of the benzene adsorption isotherms. It was found that low dealumination of the H-mordenites transforms small- into large-port mordenites. The micropore volume diminished again at a high degree of dealumination.

Spectroscopy of prismatic and cube- shaped ZSM-5 crystals

Cubic and prismatic single crystals of ZSM-5 were prepared according to the method of Jansen et al. (ACS Symp. Ser. 398, 1989, p. 257) and compared with microcrystalline ZSM-5. From the viewpoint of XRD and i.r. spectroscopy, all the materials are well crystallized. 29Si MAS n.m.r. shows, however, that the cubic crystals have superior crystallinity. The cubes are composed of oriented prismatic crystals. The Si/Al ratio is 500; most of the Al of the synthesis mixture is incorporated into analcime crystals. The same holds for the prismatic crystals, but not for the microcrystalline material. For the three materials, the number of silanol groups is 3–7%, which is small compared to the numbers given in the literature or theoretically predicted. Cu2+ is ion-exchanged via a solid-state reaction with Cu or CuCl2·2 H2O. The Cu2+ coordinates at two distinct sites in prismatic and cubic crystals and three sites in the microcrystalline material. The sites have e.s.r. parameters, which are similar to those for Cu2+ in mordenite.

Diffuse Reflectance Spectra of V(CP)2+on Silica

Abstract Diffuse reflectance spectra of vanadocene adsorbed on silica were recorded in the spectral region 4000-30 000 cm−1. A d-d band around 13 000 cm−1 indicated, that vanadocene formed V(cp)2 + on the support when the number of silanol groups was decreased. This cation is suggested to be stabilized by the negative charge of deprotonated silanol groups.