Number Of Surface Silanol Groups Research Articles

Effect of Surface Silanol Density on the Proton Conductivity of Polymer-Surface-Functionalized Silica Nanoparticles

We have developed a polymer electrolyte membrane (PEM) material using polymer-coated silica nanoparticles (NPs) by the reversible addition-fragmentation chain-transfer polymerization with particles (RAFT PwP) method. In this paper, we controlled the number density of surface silanol groups on the silica NPs that not only maintain the structure of the surface adsorbed polymers by RAFT PwP but also form fast proton-conducting interface to study the silanol density effect on proton conductivity. The number of surface silanol groups was successfully increased by NaOH surface treatment and decreased by heat treatment. Then, we clarified that silanol-rich silica NPs with a polyacrylic acid and polystyrene block copolymer (PAA-b-PS) applied by RAFT PwP exhibit larger proton conductivity. This result implies that hydrophilicity of the filler is one of the important factors in the design of filler-functionalized PEM with high proton conductivity.

Enhancement of ionic conductivity of aqueous solution by silanol groups over zeolite surface

The ionic conduction of zeolite powder in ion-exchanged water and various other solutions was investigated. H-BEA-92.5, which contained many silanol groups, showed the highest conductivity of 1.1 × 10−1 S cm−1 at 95 °C in a 10,000 ppm NaCl aqueous solution. After controlling the amount of surface silanol groups by calcination or dealumination, zeolites with a relatively large number of surface silanol groups showed the highest conductivity in water. The improvement of conductivity with zeolite in water was also observed for zeolite-related materials with surface silanol groups. Therefore, we conclude that surface silanol groups play an important role in increasing the conductivity of solution with zeolite in water. Furthermore, it was revealed that the mechanism of conduction was similar to the Grötthuss mechanism because of its apparent activation energy.

Model of a Reversed Phase Grafted on Silica Gel

A model is proposed for the surface structure of reversed phase sorbents obtained via the chemical modification of silica gel with alkyldimethylchlorosilanes. The structure of the surface is consistent with the known data on the properties of silica gel and n-alkanes. The model notes that the coupling of silicon-oxygen tetrahedra (silica gel structural units) is such that the tetrahedra alternate in the order top up/top down. At the same time, the number of surface silanol groups on the model’s surface, which can be obtained for the orthorhombic structure of cristobalite, is close to the experimental value (slightly less than five groups per nm2). By comparing the parameters of such a surface and those of alkane packing in the solid phase, it is concluded that all surface silanol groups can be subjected to derivatization according to a hydride scheme. However, due to steric reasons (the presence of two methyl groups in the anchor group), only half the surface groups can be replaced upon silylating the surface of silica gel with alkyldimethylchlorosilane. At the same time, alkyl groups of sorbates or planar molecules can easily be introduced into the graft phase with no need for the conformational rearrangement of grafted alkyl radicals.

Efficient Tertiary Amine/Weak Acid Bifunctional Mesoporous Silica Catalysts for Michael Addition Reactions

AbstractWe describe the development and application of efficient bifunctional acid–base mesoporous silica catalysts, denoted hereafter as Ext‐SBA‐15‐NMe2, comprising tertiary amine and silanol (weak acid) groups for the Michael addition reaction. The catalysts were synthesized by grafting tertiary amine containing organosilane into the mesoporous channel pores of SBA‐15 mesoporous silica in polar protic solvent (isopropyl alcohol) or nonpolar solvent (toluene). The resulting materials, Ext‐SBA‐15‐NMe2‐IPA or Ext‐SBA‐15‐NMe2‐Tol, respectively, were used as catalysts for the Michael addition reactions between trans‐β‐nitrostyrene(s) and active methylene compounds, such as malononitrile, acetylacetone, and diethylmalonate, at different temperatures. A variable‐temperature NMR‐array technique was used to monitor the reactant conversion and the reaction progress. Among the active methylene compounds tested with trans‐β‐nitrostyrene, malononitrile gave the highest conversion of 90 % in 0.3 h at 0 °C if catalyzed by the catalyst Ext‐SBA‐15‐NMe2‐IPA, the silanol groups of which were not passivated by trimethysilyl (SiMe3) groups. Compared with Ext‐SBA‐15‐NMe2‐Tol, the Ext‐SBA‐15‐NMe2‐IPA material (whose the amine groups were grafted in polar‐protic solvents), in particular, demonstrated an effective cooperative bifunctional catalysis because of its optimized proportions of tertiary amine groups and a significant number of surface silanol groups that were judiciously left behind on the material with this optimized synthetic strategy. The catalytic activity of this material was found to be significantly higher than that of the corresponding material that was grafted in toluene and that contained less optimized proportions of these two catalytic groups (i.e., amine and silanol groups). The bifunctional catalysts also showed good recyclability upon washing with acetone, which offered an effective way of regenerating the catalyst free from any undesired product/substrate bound on the surface of the catalysts.

Porosity and surface properites of SBA-15 with grafted PNIPAAM: a water sorption calorimetry study.

Mesoporous silica SBA-15 was modified in a three-step process to obtain a material with poly-N-isopropylacrylamide (PNIPAAM) grafted onto the inner pore surface. Water sorption calorimetry was implemented to characterize the materials obtained after each step regarding the porosity and surface properties. The modification process was carried out by (i) increasing the number of surface silanol groups, (ii) grafting 1-(trichlorosilyl)-2-(m-/p-(chloromethylphenyl) ethane, acting as an anchor for (iii) the polymerization of N-isopropylacrylamide. Water sorption isotherms and the enthalpy of hydration are presented. Pore size distributions were calculated on the basis of the water sorption isotherms by applying the BJH model. Complementary measurements with nitrogen sorption and small-angle X-ray diffraction are presented. The increase in the number of surface silanol groups occurs mainly in the intrawall pores, the anchor is mainly located in the intrawall pores, and the intrawall pore volume is absent after the surface grafting of PNIPAAM. Hence, PNIPAAM seals off the intrawall pores. Water sorption isotherms directly detect the presence of intrawall porosity. Pore size distributions can be calculated from the isotherms. Furthermore, the technique provides information regarding the hydration capability (i.e., wettability of different chemical surfaces) and thermodynamic information.

Effect of Nanoscale Pore Space Confinement on Cadmium Adsorption from Aqueous Solution onto Ordered Mesoporous Silica: A Combined Adsorption and Flow Calorimetry Study

The competitive adsorption of Cd(II) ions has been studied from aqueous solutions of cadmium nitrate in the presence of 0.1 M sodium nitrate at 298 K onto nonporous Spherosil XO75LS and ordered porous silica of the SBA-15 type with the purpose of tracking the effect of confinement in pores. Four SBA-15 mesoporous samples, prepared using pluronic P103 and P123 as templates at two temperatures of 303 and 373 K and differing in both the pore size (from 3.8 to 6.4 nm) and the areal density of acid sites reactive toward gaseous ammonia, were used. The equilibrium adsorption isotherms and the enthalpy curves for the related displacement process were determined by means of solution depletion method and liquid flow microcalorimetry, respectively. The experimental adsorption curves were transformed into normalized plots showing the ratio between the amount of cadmium retained at the solid–liquid interface and the number of surface silanol groups. Estimates of the standard molar Gibbs free energy and entropy of adsorption were made in the pseudoplateau adsorption region. It was postulated that the adsorbing Cd2+ cations ion exchanged with the initially adsorbed Na+ ions, the overall displacement process being endothermic and entropy-driven in the whole concentration range studied, mostly due to small modifications of the hydration shells of adsorbing and desorbing species. The areal density of surface silanols, influenced by the surface curvature within pore space, appeared to be the major factor that induced significant changes in the molar enthalpy of displacement.

NMR Provides Checklist of Generic Properties for Atomic-Scale Models of Periodic Mesoporous Silicas

MCM-41 and SBA-15 silicas were studied by (29)Si solid-state NMR and (15)N NMR in the presence of (15)N-pyridine with the aim to formulate generic structural parameters that may be used as a checklist for atomic-scale structural models of this class of ordered mesoporous materials. High-quality MCM-41 silica constitutes quasi-ideal arrays of uniform-size pores with thin pore walls, while SBA-15 silica has thicker pore walls with framework and surface defects. The numbers of silanol (Q(3)) and silicate (Q(4)) groups were found to be in the ratio of about 1:3 for MCM-41 and about 1:4 for our SBA-15 materials. Combined with the earlier finding that the density of surface silanol groups is about three per nm(2) in MCM-41 (Shenderovich, et al. J. Phys. Chem. B 2003, 107, 11924) this allows us to discriminate between different atomic-scale models of these materials. Neither tridymite nor edingtonite meet both of these requirements. On the basis of the hexagonal pore shape model, the experimental Q(3):Q(4) ratio yields a wall thickness of about 0.95 nm for MCM-41 silica, corresponding to the width of ca. four silica tetrahedra. The arrangement of Q(3) groups at the silica surfaces was analyzed using postsynthesis surface functionalization. It was found that the number of covalent bonds to the surface formed by the functional reagents is affected by the surface morphology. It is concluded that for high-quality MCM-41 silicas the distance between neighboring surface silanol groups is greater than 0.5 nm. As a result, di- and tripodical reagents like (CH(3))(2)Si(OH)(2) and CH(3)Si(OH)(3) can form only one covalent bond to the surface. The residual hydroxyl groups of surface-bonded functional reagents either remain free or interact with other reagent molecules. Accordingly, the number of surface silanol groups at a given MCM-41 or SBA-15 silica may not decrease but increase after treatment with CH(3)Si(OH)(3) reagent. On the other hand, nearly all surface silanol groups could be functionalized when HN(Si(CH(3))(3))(2) was used.

Dispersion of Si 3N 4 powders in aqueous media

The stability of Si 3N 4 suspensions was studied and characterized in terms of zeta potential, sedimentation, apparent viscosity and rheology measurements. Zeta potential measurement showed that the isoelectric point (IEP) of the as-treated Si 3N 4 particles was pH ∼ 4.6. The ratio between the number of surface amine groups per unit area N B, and the number of surface silanol groups per unit area, N A, was calculated as 0.007. As silanol groups are expected to show acidic behavior, a cationic polymer, PEI was selected as dispersant. After the adsorption of PEI, The IEP of Si 3N 4 particles shifted to pH 10.4. The surface dissociation behavior of Si 3N 4 particles in aqueous media was also studied and related to the slurry stability. Based on the sedimentation, apparent viscosity and rheological measurements, the optimal slurry for tape-casting process can be obtained at pH 9.3 with 1 wt.% PEI as dispersant. It was shown that PEI is an effective dispersant for Si 3N 4 particles in aqueous media.

Selective synthesis of Bisphenol-A over mesoporous MCM silica catalysts functionalized with sulfonic acid groups

Mesoporous MCM-41 and -48 silicas anchored with sulfonic acid (SO 3H) groups via postsynthesis modification are very effective for the synthesis of Bisphenol-A by liquid-phase condensation of phenol with acetone. Higher amounts of thiol groups can be incorporated in MCM-48 silicas presumably due to the presence of larger number of surface silanol groups. However sulfur K-edge XANES spectroscopy reveals that effective oxidation of the precursor thiol (SH) groups to the sulfonic acid (SO 3H) groups was necessary for obtaining samples of good catalytic activity. It was noted that when sulfur loadings exceed 1.5 meq/g solid a part of the sulfur atoms remains in the reduced form even after prolonged oxidation. MCM-41 silica anchored with sulfonic acid groups has comparable catalytic activity to that of commercial ion-exchange resin Amberlite-120 and the former also showed higher selectivity toward the desired p, p′ isomer. MCM-48 silica-anchored samples are equally effective for selective synthesis of Bisphenol-A.

Surface Oxidation States in Si/SiO2 Nanostructures Prepared from Si/SiO2 Mixtures

Silica nanospheres have been produced by a novel technique where surface Si oxidation states can be adjusted using the ratio of metalloid ions/metalloid atoms in the starting mixture. When the proportions of Si4+/Si0 are equal in the synthesis, the resulting solid is considerably more reactive than Cab-O-Sil toward the phenol hydroxylation reaction and the surface shows an average Si oxidation state of +3. On the other hand, those silica nanospheres, produced from a mixture of Si4+/Si0 = 0.25, showed a lower reactivity comparable to that of Cab-O-Sil which XPS demonstrates has a surprisingly low average Si oxidation state close to +1. We speculate that the silicon surface oxidation state and the number of surface silanol groups play important roles in determining the activity of the solid toward the phenol hydroxylation reaction. In expanding our earlier report4 on the copper-silica system, we establish that the surface chemistry of the silica nanospheres is apparently different from that of fumed, amorphous silica. These results suggest that we are developing a technique that can be generalized to create supported, mixed metal oxides having tunable average surface oxidation states.

Effect of Aqueous Phase Composition on the Properties of a Silica-Stabilized W/O Emulsion

A high volume fraction silica stabilized w/o emulsion with small droplet size (≈3 μm) has been prepared using a commercially available hydrophobic silica. Addition of hydroxypropyl cellulose to the dispersed aqueous phase was found to improve the monodispersity of the emulsion by suppressing the production of larger droplets. The droplet size distribution showed complex behavior as the silica concentration was varied, which was explained using a simple kinetic argument. The effect of varying the acidity and ionic strength of the internal water phase was investigated. It was found that the presence of strong acid or strong alkali in the internal aqueous phase increased the gelation of the emulsions by promoting flocculation. This could be explained by acid- and alkali-catalyzed cleavage of surface siloxane groups increasing the number of surface silanol groups. If emulsions of strong acid and strong alkali were mixed, substantial additional gelation (again caused by flocculation) occurred. A possible explanation, preferred by the author, was an electrostatic one involving the interaction of dipoles in close proximity in the flocculated emulsion.

Preparation and catalytic properties of new mesoporous materials

The development of ordered mesoporous solids is revised. Acid, base, and redox catalytic properties are presented together with their application for a series of organic reactions of fundamental and practical interest. Finally it is explained that it is possible to make use of the high surface and regular porosity of MCM–41 to prepare supported metals, and bifunctional catalysts. The above properties together with the presence of a large number of surface silanol groups allow the grafting of catalytically active transition metal complexes.

Highly dispersed precipitated silicas as agents improving products made on the base of polymers and as thickening agents in toothpastes

Results of studies were presented on the effects of surfactants and coagulants on the quality of silica, precipitated out of sodium metasilicate solution under the effect of gaseous carbon dioxide. Particular attention was paid to obtaining silicas of high paraffin oil adsorbing capacity and low capacity to adsorb water. Results were also presented of studies on the effect of new nitrogen and sulphur atoms containing oxyethylene compounds on the modification of the precipitated silicas. Modification of the silicas was evaluated by estimating their wettability, number of surface silanol groups and by observations in electron microscope. Moreover, studies were conducted on the application of the obtained silica, both unmodified and modified, in butadiene-styrene rubbers, polyurethanes and toothpastes.

Characterisation of the surface of a new amorphous microporous silica

Physical and chemical properties of a new microporous amorphous silica were characterised by adsorption studies with molecules of different size, titration with sodium hydroxide and reaction with Grignard reagent supported by loss-on-ignition studies. BET nitrogen data gave no reliable information about the surface area of this silica. From methylene blue adsorption studies the surface area accessible to this adsorbate can be accurately determined. By reaction with Grignard reagent the number of surface silanol groups was determined; the results agree with alkalimetric titrations, provided the latter were evaluated using Frumkin's isotherm and the appropriate interaction parameter. The silica was found to be an amorphous material with a specific surface area of ca. 400 m 2 g −1, which is completely covered with silanol groups. The major part of this surface area is within slot-shaped micropores < 0.5 nm in width. Transmission electron microscopy supported this opinion. This silica is a useful source of reactive SiO 2. Moreover the special pore size and structure suggest its application as a catalyst base or an adsorbent.

Properties of chemically bonded phases

Retention on non-polar chemically-bonded stationary phases depends on the silica carrier, the kind of bonded silane, the surface concentration of bonded alkyl groups and the number of surface silanol groups before and after chemical modification. The influence of carbon content of bonded phases, column temperature and how the eluent mixture has been prepared on solute retention is demonstrated. For comparison of different bonded phases standardization for the amount of bonded carbon per unit column volume is recommended. The problems in attributing specific selectivities to different functional groups in chemically bonded phases are discussed. The advantages and specific selectivities of some phases with functional groups are demonstrated.