Adjacent Silanol Groups Research Articles

Characterization of H4SiW12O40 supported on mesoporous silica (SBA-15), non-structured amorphous silica and γ-alumina

H4SiW12O40 (HSiW) was supported on mesoporous silica (SBA-15), non-structured amorphous silica (AEROSIL) and γ-alumina (Al2O3) by impregnation. The supported HSiW samples were characterized by X-ray diffraction (XRD), N2 adsorption-desorption isotherms, infrared (IR) spectroscopy and ethanol dehydration reaction as a model reaction. HSiW was not highly dispersed even at 29 wt% loading, but the hexahydrate (HSiW·6H2O) was stable under evacuation below 150 °C on silica supports, while destruction of Keggin units occurred on Al2O3. IR spectra of HSiW·6H2O loaded SBA-15 and AEROSIL were analyzed in detail in combination with changes in surface area and XRD patterns. The loaded HSiW·6H2O crystals possessed weak secondary interaction with adjacent silanol groups. The crystal size was increased in three dimensions on AEROSIL by increasing loading amount, while the extension of the size to the pore direction was found on SBA-15. The number of protons in supported HSiW·6H2O was quantified by the integrated intensity of the key band of pyridinium cation (C5H5N+H) at 1538 cm−1 in IR spectra. At high loading (>29 wt%) four hydrogen atoms per Keggin unit in HSiW·6H2O were detected as proton on AEROSIL and SBA-15 with negligible amount of Lewis acid sites, while less than a half on Al2O3 with a majority of Lewis acid sites. Thus, IR method using pyridine as a probe can quantify Brønsted and Lewis acid sites in the absence of thermal decomposition of loaded HSiW·6H2O. It was also found that all the protons detected by pyridine adsorption could not catalyze the reaction over HSiW/Al2O3, suggesting the necessity of a caution to relate the acid amount measured by probe molecules to catalysis.

Ethanol Conversion to Butadiene over Isolated Zinc and Yttrium Sites Grafted onto Dealuminated Beta Zeolite

Zinc and Yttrium single sites were introduced into the silanol nests of dealuminated BEA zeolite to produce Zn-DeAlBEA and Y-DeAlBEA. These materials were then investigated for the conversion of ethanol to 1,3-butadiene. Zn-DeAlBEA was found to be highly active for ethanol dehydrogenation to acetaldehyde and exhibited low activity for 1,3-butadiene generation. By contrast, Y-DeAlBEA was highly active for 1,3-butadiene formation but exhibited no activity for ethanol dehydrogenation. The formation of 1,3-butadine over Y-DeAlBEA and Zn-DeAlBEA does not occur via aldol condensation of acetaldehyde but, rather, by concerted reaction of coadsorbed acetaldehyde and ethanol. The active centers for this process are ≡Si-O-Y(OH)-O-Si≡ or ≡Si-O-Zn-O-Si-O≡ groups closely associated with adjacent silanol groups. The active sites in Y-DeAlBEA are 70 times more active than the Y sites supported on silica, for which the Y site is similar to that in Y-SiO2 but which lacks adjacent hydroxyl groups, and are 7 times more active than the active sites in Zn-DeAlBEA. We propose that C-C bond coupling in Y-DeAlBEA proceeds via the reaction of coadsorbed acetaldehyde and ethanol to form crotyl alcohol and water. The dehydration of crotyl alcohol to 1,3-butadiene is facile and occurs over the mildly Brønsted acidic ≡Si-OH groups present in the silanol nest of DeAlBEA. The catalysts reported here are notably more active than those previously reported for both the direct conversion of ethanol to 1,3-butadiene or the formation of this product by the reaction of ethanol and acetaldehyde.

Adsorption of 2-Chloroethyl Ethyl Sulfide on Silica: Binding Mechanism and Energy of a Bifunctional Hydrogen-Bond Acceptor at the Gas–Surface Interface

This work investigates the fundamental nature of sulfur mustard surface adsorption by characterizing interfacial hydrogen bonding and other intermolecular forces for the surrogate molecule (simulant) 2-chloroethyl ethyl sulfide (2-CEES). Adsorption at the surface of amorphous silica is the focus of this work because of silica’s low chemical reactivity, well-known properties, and abundance in the environment. 2-CEES has two polar functional groups, the chloro and thioether moieties, available to accept hydrogen bonds from free surface silanol groups. Diethyl sulfide and chlorobutane are also investigated to independently assess the role of the chloro and thioester functionalities in the overall adsorption mechanism and to explore the interplay between the charge transfer and electrostatic contributions to total hydrogen-bond strength. Our approach utilizes infrared spectroscopy to study specific surface–molecule interactions and temperature-programmed desorption to measure the activation energy for desorption of adsorbed molecules. Our results indicate that 2-CEES adsorbs to silica by hydrogen bonding through either the chloro or thioether moieties but is unable to form a more stable configuration in which both polar groups interact simultaneously with adjacent silanol groups. The activation energy for desorption of 2-CEES is nearly 43 kJ/mol, driven by both strong hydrogen bonding and other non-bonding interactions. A systematic study of chloroalkanes reveals that each methylene group contributes approximately 5–8 kJ/mol to the overall desorption energy.

Reversible wettability control of silicon nanowire surfaces: From superhydrophilicity to superhydrophobicity

Bio-inspired superhydrophobic surfaces have attracted considerable attention due to their potential applications. Although various techniques to fabricate artificial superhydrophobic surfaces have been demonstrated, most of the methods lack water adherence or controllable wetting properties of the surfaces, which hinders their practical usage. In this paper, we present a simple approach to fabricate water-adhesive superhydrophobic silicon nanowire (Si NW) surfaces by applying a thermal annealing treatment in oxygen ambient. The Si NW arrays were fabricated using a metal assisted chemical etching method. After the cycled rapid thermal annealing (RTA) process at 1000°C under oxygen ambient, the water contact angle of the Si NW surface changed dramatically from 0 to 154.3° with high water-adhesive properties. This drastic change of the wettability could be attributed to the formed siloxane groups (−Si–O–Si–) on the thermally-treated Si NW surfaces; H2O is released from two adjacent silanol groups (–Si–O–H) to form siloxane groups during the RTA process. When the annealed Si NW was exposed in air, the wettability of the superhydrophobic Si NW was reconverted due to the re-formation of silanol groups (–Si–O–H). The wettability conversion of Si NW between superhydrophilic and superhydrophobic was repeated with good reversibility.

Gas phase nitridation of silicate fiber glass materials with ammonia:Three case studies

Three types of the leached fiber glass materials of silicate origin modified with Zr, Al and rare earth metals (REM) were nitridated with ammonia at temperatures ranging from 673 to 973K. DRIFTS (Diffuse Reflectance Infrared Fourier-Transform Spectroscopy) and H/D exchange using ND3 were applied for characterization of the formed NHx species. Along with nitridation of silanol groups their dehydroxilation as well as hydrolysis of NHx species take place. In situ DRIFTS study showed that the limiting steps are slow chemical reactions, but not a diffusion of NH3 or H2O molecules in the bulk of glasses. The concept on strained siloxane bridges formed due to dehydroxylation of two adjacent silanol groups was used for explanation of the reaction kinetics features.

First-principles molecular-dynamics investigation of the hydration mechanisms of the (0001) α-quartz surface

Using first-principles molecular dynamics, we investigate the interactions of water with the (0001) α-quartz surface. Both the cleaved (unreconstructed) surface, which presents non-bridging oxygen atoms, and the dense surface, which is characterized by 3-membered and 6 membered-rings with siloxane bonds at the top, are considered. The cleaved surface is found to be very hydrophilic. When a water molecule arrives at the surface, its oxygen atom bonds to an under-coordinated Si atom. One of its hydrogen atoms is then transferred to a neighboring non-bridging oxygen through the formation of a hydrogen bond. When the surface is already partially hydrated, the formation of hydrogen-bond chains is observed between the water molecule and adjacent silanol groups. By contrast, the dense surface is shown to be hydrophobic. Our calculations provide qualitative information on the possible reaction paths for the dehydration mechanisms, and more generally on the interactions of water with amorphous SiO2 surfaces.

Preparation of Monodisperse Crosslinked Organic‐Inorganic Hybrid Copolymer Particles by Dispersion Polymerization

AbstractThis study presents a new method for producing monodisperse crosslinked organic‐inorganic hybrid polymer particles in micron‐size by using simple dispersion polymerization. Firstly, highly monodisperse hybrid copolymer particles were prepared by conventional dispersion polymerization of styrene and 3‐(trimethoxysilyl)propyl methacrylate (TMSPM) in a methanol/water medium. It was very interesting to find that slightly crosslinked hybrid polymer particles were formed by this simple dispersion polymerization due to a partial condensation reaction between the adjacent silanol groups of the TMSPM during the polymerization. Secondly, for the hydrolytic condensation of the remaining trimethoxysilyl groups, post treatment by sol‐gel process was carried out to form an inorganic siloxane network, which provides the particle with high crosslinking density. In the proper conditions, monodisperse crosslinked particles could be produced in high TMSPM concentrations, up to 20 wt.‐%. In addition, the size monodispersity of the hybrid polymer particles was maintained after the sol‐gel process. Crosslinking with inorganic networks formed by the sol‐gel process was confirmed by a thermal analysis and a 29Si NMR spectroscopy.Synthesis of crosslinked organic‐inorganic hybrid polymer particles by dispersion copolymerization and post sol‐gel process.magnified imageSynthesis of crosslinked organic‐inorganic hybrid polymer particles by dispersion copolymerization and post sol‐gel process.

Plasma Polymer Films as Adhesion Promoting Primers for Aluminum Substrates. Part I: Characterization of Films and Film/Substrate Interfaces

Plasma-polymerized hexamethyldisiloxane (HMDSO) films (∼800 Å in thickness) were deposited onto aluminum substrates (6111-T4 alloy) in radio frequency (RF) and microwave (MW) powered reactors to be used as primers for structural adhesive bonding. Processing variables such as substrate pre-treatment, carrier gas, and film post-treatment were adjusted to produce films that had different structures and properties. The plasma polymerized films were characterized by reflection-absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and ellipsometry. Films deposited using argon as the carrier gas were siloxane-like. When siloxane-like films were post-treated with an oxygen plasma, a silica-like surface layer was produced. Silica-like films were deposited using oxygen as the carrier gas. Plasma polymerized silica-like films were formed of spheroidal particles deposited next to each other and impinging upon one another. These spheres were thought to be islands formed by the growth of stable nuclei. Etching aluminum substrates in argon and argon/hydrogen plasmas before deposition of the primer removed adsorbed water from the surface, possibly creating a more stable oxide surface to which the silica-like primer could bond. The effect of etching silica-like films in an Ar plasma was to promote the condensation of adjacent silanol groups to form Si—O—Si bonds but with little change in the film thickness or the concentration of isolated silanol groups.

Near-surface composition of acid-leached labradorite investigated by SIMS

The composition of the near-surface of acid-leached labradorite was investigated by depth-profiling with secondary ion mass spectrometry for H, D, O, Na, Al, Si, Cl, and Ca. Specimens were leached at room temperature at pH 1 for 500 h, and pH 2 and 3 for 1200 h, in solutions acidified with HCl. Complete leaching of Na, Ca, and Al occurred to a depth of 100 nm after 1200 h of reaction at pH 3 and produced sigmoidal depletion/concentration profiles. Depletion depths of these cations are shown to increase further with H +-activity and time. The presented element profiles, therefore, represent non-steady-state conditions. Calcium is removed from between 10 and 30 nm deeper within a single profile than the corresponding Al depletion depth. After removal of Na + and Ca +, a sequence of reaction steps results in preferential leaching of Al over Si and formation of silanol groups around the tetrahedral vacancy. This is immediately followed by spontaneous condensation of adjacent silanol groups, which eliminates a fraction of oxygen from the leached layer. The O/Si ratio of the residual structure is thus reduced, and a high degree of cross-linkage between Si-tetrahedra is maintained. Throughout the leached layer, the O/Si atomic ratio is reduced from 3.3 (fresh labradorite) to values between 2.5 and 2.0. The leached layer is enriched in H in all specimens, with average concentration plateus of about 7·10 21 atoms/cm 3, or 11 at%, if the leached layer is assumed to have a density equal to amorphous SiO 2. At the time of analysis the leached layer is anhydrous and composed solely of H, Si, and O. The H concentrations are in accord with an O/Si atomic ratio of 2.2 in terms of charge balance. The analytical composition corresponds to SiO 1.8(OH) 0.4. Good agreement between leaching depths determined by SIMS profiling and as calculated from element release rates during kinetic dissolution experiments suggests that the sites for ion-exchange and depolymerization reactions are uniformly distributed over the entire mineral surface, and that any changes in reactive surface area have been small during these experiments.

On the atomic mechanisms of water-enhanced silicon wafer direct bonding

The quantum-chemical methods of modified neglect of diatomic overlap (MNDO) and Austin model 1 (AM1) are discussed and applied to the investigation of the different steps of direct bonding of silicon or silica wafers enhanced by water molecules. Concerning the hydrophilization, the reaction enthalpy of joining a water molecule to the silica surface was found to be 4.3 kcal mol −1, which is insufficient for a molecule to disintegrate, therefore higher concentrations of OH − ions are necessary to form silanol bonds, realized, e.g., by NH 4OH treatment. The strength of the H 2O interaction with an oxidized surface is nearly five times higher than that with pure silicon. At low bond densities, i.e., if only one siloxane bond is formed from two adjacent silanol groups, the transformation of silanol bonds into siloxane ones is exothermal, leading to an easy procedure, implying, however, low adhesion (fixing). Stronger bonding is provided by higher densities of reacting silanol bonds, but then the process becomes endothermal because of spatial restrictions, requiring a thermal treatment. At the hydrophilized silica surface a specific arrangement of silanol bonds and water clusters results. Its thermodynamic stability is determined by the equilibrium of core-core repulsion and the exchange interaction of the electron shells. As the formation enthalpy depends non-linearly on the bond number the stable configuration is attained at a density of four silanol bonds per 100 Å 2, coupled with 2–4 water molecules per silanol bond.

The role of hydrogen in excimer-laser-induced damage of fused silica

Hydrogen nuclear magnetic resonance (NMR) was used to probe defect reactions in fused silica during 193 nm ultraviolet irradiation and thermal treatment. Hydrogen concentration and Carr–Purcell measurements indicate that the majority of hydrogen atoms are isolated and do not segregate or evolve from the fused silica during annealing or irradiation. Previously, NMR spin-lattice relaxation measurements have revealed that a small fraction of hydrogen in fused silica exists as centers that undergo molecular motion and are consumed during annealing. Here, a detailed analysis of the NMR spin-lattice relaxation behavior is used to support the identification of this center as a pair of adjacent silanol groups. These silanol pairs react during annealing to form a strained bond, thus increasing the susceptibility of fused silica to E′ formation. Excimer laser irradiation, like thermal treatment, is shown to directly reduce the number of relaxation centers in the material. The measured dependence of spin-lattice relaxation as a function of irradiation for identically prepared samples is consistent with silanol pair center consumption by a two-photon process.

Infrared study of the adsorption of fluoroalcohols on silica

Infrared spectra have been recorded of ethanol, 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoropropan-2-ol adsorbed on silica at the solid/vapour interface and on silica immersed in carbon tetrachloride. The predominant surface–adsorbate interactions at ca. 306 K involved the formation of hydrogen bonds between surface silanol groups and hydroxy groups in adsorbed molecules. The strongest adsorption of this type occurred for single alcohol molecules adsorbed on to pairs of adjacent silanol groups. Isolated silanol groups acted as donors of hydrogen bonds, each to a single alcohol acceptor molecule. For ethanol, at high surface coverages lateral interactions between adjacent adsorbed ethanol molecules occurred and led to the build-up of aggregates of ethanol on the silica surface. Some chemisorption occurred for all three alcohols primarily through addition reactions at exposed strained siloxane groups.

Infrared study of the adsorption of oleic and linolenic acids onto the surface of silica immersed in carbon tetrachloride

The adsorption of oleic acid and linolenic acid onto silica from carbon tetrachloride solution has been studied by infrared spectroscopy. At the lowest surface coverages studied adsorption of acid monomer onto pairs of adjacent silanol groups occurred. As the concentration of acid in solution was increased adsorption of monomer onto isolated surface silanol groups became predominant. At high solute concentrations some evidence was also observed for adsorbed acid dimers. The primary adsorptive interaction involved the formation of hydrogen-bonds between surface hydroxyl groups and the carboxylic acid groups of the adsorbate molecules. However simultaneous interaction of the oxide surface with the alkene residues of the alkyl chains occurred. Evacuation and heat treatment of silica covered with an adsorbed layer of linolenic acid led to the elimination of molecular water and the formation of chemisorbed linolenate groups on the oxide surface.