SiOH Bending Research Articles

Behavior of Hydrogarnet‐Type Defects in Hydrous Stishovite at Various Temperatures and Pressures

AbstractDense polymorphs of silica have been demonstrated experimentally to incorporate from 1.5 wt% to as much as 11.6 wt% H2O as OH groups, with implications for the hydrogen budgets of Earth and other planets. This OH is thought to enter the SiO2 structure via a charge‐balanced substitution in which silicon vacancies (VSi) are compensated by protonating four of the surrounding six oxygen atoms, often referred to as a hydrogarnet‐type defect. There are many possible configurations for this defect structure in dense silica, but the nature of these configurations and whether they can be distinguished experimentally is unknown. We present here density functional theory calculations that systematically assess the possible configurations of a hydrogarnet‐type defect in stishovite (rutile‐type SiO2), with direct comparisons to experimental vibrational spectroscopy data. We predict that stishovite synthesized at 450 K and 10 GPa quenched to room temperature is dominated by a single defect type with tetrahedral geometry. This leads to OH stretching modes (2,500–3,000 cm−1) and SiOH bending modes (∼1,400–1,450 cm−1) largely consistent with experimentally observed modes. One remaining issue is that our calculations produce results compatible with experimental data on H to D exchange, but do not explain why a considerable fraction of the 1,420 cm−1 mode shifts by only 40 cm−1 in deuterated samples. At elevated pressures and temperatures, we find that a second square planar defect configuration also becomes favorable, leading to modes that should allow differentiation from the tetrahedral configuration.

Synthesis and Structural Characterization of Fly Ash Based Mineral Polymer Precursors

Preparation of mineral polymer materials with fly ash is one of the effective ways to realize the green and comprehensive utilization of fly ash. Using the sol-gel method mineral aggregate material precursor, swe tudied the different sources of silicon and aluminum, synthesis temperature, different Si/Al ratio, different dosages of ethanol, and other influence factors under the conditions of the synthesis, and the structure of the sample using the means of XRD, SEM, TG, IR on the synthesis of silica-alumina precursor sample performance and structure are analyzed. The results showed that the optimum preparation conditions were obtained when the Si/Al ratio was 10:1, the synthesis temperature was 70°C, and the sample was calcined at 850°C for 3 h with the highest synthesis rate of 81.33%. The Si-Al precursors prepared are amorphous—phase amorphous grid—space structures. The infrared analysis indicated that the synthesized sample had a Si-O-Si bond, but the absorption peak and characteristic peak were obviously different under the conditions of different synthesis temperatures, different Si/Al ratios, and different ethanol dosage. For example, at 806 cm−1 and 1,395 cm−1, they correspond to the absorption peak of Si-OH bending vibration and shear bending vibration and plane bending vibration of C-H without complete reaction respectively. SEM observation shows that the size of the synthesized Silicon aluminum precursors sample is small and the distribution is uniform. The in-depth study on the preparation process and reaction mechanism of mineral polymer precursorswill bring broad prospects for the application and development of mineral polymer precursors themselves.

Novel epoxidized natural rubber composites with geopolymers from fly ash waste

Geopolymer (GP) based on fly ash waste from electric power generating plants has been applied in natural rubber composites as a sustainable solution to deal with this industrial solid waste. This alternative type of rubber filler can reduce hazards and environmental impacts of that waste. Novel elastomer composites based on geopolymer (GP) filled epoxidized natural rubber (ENR) were prepared with various GP loadings. Cure characteristics, mechanical, dynamic, thermal, and morphological properties of the ENR/GP composites were characterized. It was found that incorporating GP significantly affected cure characteristics of ENR compounds, decreasing scorch and cure times by accelerating the cure rate via the excessive metal oxides in GP. Furthermore, the ENR/GP composites with 15 phr GP showed the highest tensile strength due to maximal chemical linkages of hydroxyl groups and metal oxides on GP surfaces and epoxide groups in ENR. This was confirmed by a new FTIR peak for SiOH bending vibrations at the wavenumber 970 cm−1. Furthermore, the Payne effect in ENR/GP composites was assessed from the relationship of shear modulus and strain amplitude at a fixed oscillation frequency. It was found that the Payne effect, or the level of filler-filler interactions, increased with GP loading. In addition, it was also found that the glass transition temperature (Tg) of ENR/GP composites increased with GP loading. Furthermore, TGA and TSSR results suggest that the metal oxide in GP activated degradation of the rubber network at elevated temperatures.

Inelastic Neutron Scattering Study on the Location of Brønsted Acid Sites in High Silica LTA Zeolite

The presence of Bronsted acid sites (SiOHAl) is crucial in many of the applications of zeolites in petroleum refining and related industries. This acidity relies on the number and location of acid sites, but the latter is difficult to know accurately in spite of a large number of experimental techniques available. Zeolites usually contain many types of acid sites depending on the crystallographic different O-sites and their chemical environment. In the present work we show how inelastic neutron scattering (INS) can be employed for an accurate determination of the location of Bronsted acid sites in zeolites. This is achieved using a sample of LTA zeolite with few defects of connectivity, few (O1H, O2H, O3H) crystallographically different oxygens, and high Si/Al ratio. The narrow and intense peaks corresponding to SiOH bending modes ca. 1100 cm–1 have been shown, using periodic DFT calculations, to strongly depend on the type of oxygen to which the proton is bonded. A comparison between calculations and INS...

Observation of temperature dependence of the IR hydroxyl absorption bands in silica optical fiber

Abstract This study reports on the temperature dependent behavior of silica based optical fibers upon exposure to high temperatures in hydrogen and ambient air. The hydroxyl absorption bands in the wavelength range of 1000–2500 nm of commercially available multimode fibers with pure silica and germanium doped cores were examined in the temperature range of 20–800 °C. Two hydroxyl-related infrared absorption bands were observed: ∼2200 nm assigned to the combination of the vibration mode of Si-OH bending and the fundamental hydroxyl stretching mode, and ∼1390 nm assigned to the first overtone of the hydroxyl stretching. The absorption in the 2200 nm band decreased in intensity, while the 1390 nm absorption band shifted to longer wavelengths with an increase in temperature. The observed phenomena were reversible with temperature and suspected to be due, in part, to the conversion of the OH spectral components into each other and structural relaxation.

Hydrogen dissolution and structural analysis in high temperature TiOx–MnO–SiO2 oxide melts

The effects of the composition and temperature on the hydrogen solubility and melt structure in the TiOx–MnO–SiO2 welding flux system were investigated. The redox reaction between Ti3+ and Ti4+ was distinguished within TiOx based on the substitutional effects of TiOx in SiO2. A higher TiOx content compared with SiO2 decreased the hydrogen solubility because the dominant hydrogen dissolution mechanism changed from an incorporated hydroxyl to a mixed region that included both the incorporated and free hydroxyl mechanisms. The Ti3+/Ti4+ trend and the decrease in the absolute amount of SiO2 appear to cause this mechanistic change. Based on the effects of the basic and acidic components on the hydrogen solubility in the melt, the effect of the extended basicity ((Ti2O3+MnO)/(TiO2+SiO2)) was also analyzed and exhibited a parabolic behavior. Three regions were observed from low to high extended basicity, which correspond to the incorporated to free hydroxyl modification with a mixed region. FTIR analysis of the as-quenched melts provided insight into the changes in the Si–O tetrahedral stretching bands and Si–OH bending, which were more pronounced as the SiO2 content increased. The FTIR and XPS results obtained for the melts quenched from 1773 to 1823 K were compared to identify the effect of temperature on the unit structures, and these results indicated that Si–OH was more pronounced at low TiOx/SiO2 and less pronounced at high TiOx/SiO2 at higher temperatures. The changes in the structural units as a function of the composition and temperature correlated well with the hydrogen solubility. Basic oxides of MnO additions can depolymerize the melt by providing O2−, which can increase or decrease the hydrogen solubility depending on the dominant hydrogen dissolution mechanism.

Vibrational mode frequencies of H4SiO4, D4SiO4, H6Si2O7, and H6Si3O9 in aqueous environment, obtained from ab initio molecular dynamics

We report the vibrational properties of H(4)SiO(4), D(4)SiO(4), H(6)Si(2)O(7), and H(6)Si(3)O(9) in aqueous solution at 300 K and 1000 K, obtained from the combination of ab initio molecular dynamics (MD) and a mode-decomposition approach. This combination yields vibrational subspectra for selected vibrational modes at finite temperatures. We also performed normal-mode analysis (NMA) on numerous configurations from the same MD run to sample the effect of the variable molecular environment. We found good agreement between both approaches. The strongest effect of temperature is on the SiOH bending mode δSiOH, which is at about 1145 cm(-1) in solution at 300 K, opposed to about 930 cm(-1) in solution at 1000 K. The frequency of the δSiOH vibration also depends on environment, shifting from 1145 cm(-1) in solution to about 845 cm(-1) in the gas-phase. We found both in the mode-decomposition approach and in multiple-configuration NMA that the H(6)Si(2)O(7) dimer shows a vibrational mode at about 790 cm(-1), which we consider to be responsible for a hitherto unexplained shoulder of the monomer Raman band at 770 cm(-1) in dilute silica solutions. Our results demonstrate the importance of temperature and solvation environment in calculations that aim to support the interpretation of experimental Raman spectra of dissolved silica.

Hydrogen Dissolution in the TiO2–SiO2–FeO and TiO2–SiO2–MnO Based Welding-Type Fluxes

The hydrogen solubility of TiO2–SiO2–FeO and the TiO2–SiO2–MnO welding-type flux systems have been studied to identify and compare the hydrogen dissolution behavior in molten welding fluxes at high temperatures of 1823 K in a wide range of compositions. The dependence of the water vapor pressure and the hydrogen solubility showed a slope of 1/2 suggesting thermodynamic equilibrium was obtained. For an acidic slag composition, the hydrogen was found to be incorporated within the silicate network structure and decreased with higher TiO2 additions due to the dilution effect of SiO2 and subsequent decrease in incorporation sites. The additions of FeO and MnO, which acts as a basic component to the slag system, provided free oxygen ions, and decreased the incorporation sites for hydrogen dissolution. FTIR analysis of as quenched molten flux samples showed more pronounced bands for Si–OH bending vibrations when hydrogen solubility increased. For a basic slag composition, the hydrogen existed as a free hydroxyl and the addition of basic components such as FeO and MnO to the slag system increased the hydrogen solubility in the molten slag. Furthermore, it was observed that the hydrogen solubility was slightly lower for FeO than MnO containing fluxes.

Vibrational properties of silanol group: From alkylsilanol to small silica cluster: Effects of silicon substituents

Structural and vibrational features of silanol group are investigated in detail by quantum calculations and normal mode analysis. The structural parameters, charge distributions, force fields, vibrational wavenumbers, potential energy distributions of normal modes and derivatives of the electric dipole moment are analyzed in relation to the nature of the substituents adjacent to the silanol group. The calculations results are discussed in light of available experimental data. Although the O H stretching mode has already been well localized in various silanols, both the Si (OH) stretching and SiOH bending vibrations have not been yet finely analyzed leading to some discrepancies reported in literature. Clarified assignments of these vibrations are proposed on the basis of normal mode analysis and of SiOH → SiOD isotopic exchange. The following spectral ranges are determined: 790–1030 cm −1 for νSi (OH), 790–1010 cm −1 for νSi (OD), 790–900 cm −1 for δOH and 580–640 cm −1 for δOD. The νSi (OH)/ νSi (OD) wavenumbers are highly dependent on silicon substituents: electron-withdrawing groups induce shifts to higher wavenumbers while electron-releasing groups induce shifts to lower wavenumbers. In alkylsilanols, the SiOH bending is observed at higher wavenumber than the stretching vibration. Analysis of infrared intensities and dipole derivatives in internal coordinates gives explanations to spectral “anomalies” observed in experimental measurements such as well defined and intense νSi (OD) absorption in contrast with very low intensity for νSi (OH). Numerous empirical correlations are established allowing reconstruction of both SiOH force field and SiOH structural parameters with knowledge of few experimental data.

Vibrational spectra of trimethylsilanol: The problem of the assignment of the SiOH group frequencies

The assignment of the SiOH group vibrations of trimethylsilanol, which is still controversial, is proposed. This assignment is based on theoretical B3LYP force field scaled using the constants of the (CH 3) 3Si group optimized to fit experimental vibrational frequencies of (CH 3) 3SiF and (CD 3) 3SiF molecules as well as the OH stretching scale factor from methanol. The ab initio force field defined in this way gives a good agreement of the theoretical vibrational frequencies of trimethylsilanol with the positions of IR and Raman bands observed in the gas phase. This force field predicts the greatest contribution of the δ SiOH coordinates to the vibration with frequency of 804 cm −1. The elimination of the coupling of the SiOH deformation with methyl rocking modes by the normal coordinate treatment of (CD 3) 3SiOH gives 832 cm −1 for silanol deformation which is in a good agreement with the 834 cm −1 value proposed earlier for the bending mode of free silanol groups. The geometry and force field of the open chain H 3SiOH trimer is computed to model the change of the δ SiOH frequencies upon formation of the hydrogen-bonded polymers. This model predicts a significant shift of SiOH bending frequencies to the 1000–1200 cm −1 range while those of SiOD to the 800–850 cm −1 range. These predictions allow us to ascribe the 1087 cm −1 band observed in the IR spectrum of crystalline (CH 3) 3SiOH and the Raman 775 cm −1 band of the liquid (CH 3) 3SiOD to deformations of the hydrogen-bonded silanol groups.

The two-component bands at about 4500 and 800 cm −1 in infrared spectra of hydroxyl-containing silicas. Interpretation in terms of Fermi resonance

The band at about 4550 cm −1 in the near-infrared (IR) spectra of OH-containing high-surface area silicas is known to resolve into a doublet under certain experimental conditions. The interpretation of this by Tsyganenko [Zh. Fiz. Khim. (Moscow) 56 (1982) 2330], which has received no recognition so far, is advocated and is elaborated as follows: (1) there is Fermi resonance interaction between the SiOH bending fundamental and a doubly excited vibration giving rise to a mixed-state doublet around 800 cm −1, (2) the Fermi doublet propagates in the near IR in combinations with the OH stretching vibration (≈3750 cm −1). From a review of available IR transmission, inelastic neutron scattering (INS), and Raman scattering experimental data and model densities of vibrational states it is inferred that the doubly excited state causing Fermi resonance is likely to be either the first overtone of the bridging-oxygen rocking mode (400 cm −1 in the density of vibrational states) or the binary summation of the bridging-oxygen symmetric stretching in bent Si–O–SiOH units (the D 1 Raman band near 490 cm −1, in the interpretation of Mulder et al.) with the A ′ component of the Si–OH wagging mode (expected at about 300 cm −1). In the near-IR spectra of OH-containing vitreous silica and natural opals, the irregular shape of the band near 4500 cm −1, as argued, has a similar origin.

Near infrared and ab initio study of the vibrational modes of isolated silanol on silica

Near infrared spectroscopy was used to study the vibrational modes of isolated silanol groups in porous silica monoliths that had been heated in acuo at 750°C. A set of bands is assigned to overtone and combination transitions for normal modes involving mainly the SiOH group: the νOH stretching, the νSi–OH stretching, the δOH bending and the τOH torsion around the SiO bond. The spectra of a deuterated sample were also analysed. The wavenumbers for the fundamental transitions were deduced and some anharmonicity coefficients were calculated. The δOH bending is coupled with a symmetric stretching of SiOSi bridges in an (SiO)3SiOH unit. Pairs of bands involving some SiOH bending character (at 4510/4580 cm−1 for combinations with νOH and at 8100/8165 cm−1 for combinations with 2νOH) were extrapolated to fundamental transitions at 760/835 cm−1. This coupling was not observed for the deuterated sample because the δOD absorption at 605 cm−1 is well separated from SiOSi stretching centred at 815 cm−1. The experimental spectra are discussed with the help of ab initio calculations of the vibrational modes of a molecule modelling an isolated silanol group on silica. The contribution of hot transitions at room temperature, from low lying levels such as τOH and its overtones, is also analysed.

Attenuated Toatal Reflectance Fourier‐Transform Infrared Spectra of a Hydrated Sodium Soilicate Glass

Attenuated total reflectance Fouriertransform infrared (ATR‐FTIR) spectra were measured in the region from 4300 to 400 cm−1 for a hydrated Na2O–SiO2 glass containing 35 wt% water. The Si–OH bending vibration mode was observed. It was found that the incorporated water, molecular water as well as hydroxyls, affected the Si–O vibrations. The effect of incorporated water upon the glass structure is discussed.

An ab initio quantum chemical investigation on the effect of the magnitude of the T-O-T angle on the Broensted acid characteristics of zeolites

Ab initio molecular orbital calculations are used to monitor the effect of increasing SiOAl angle (SiOAl) on the acidic properties of bridged hydroxyl groups in zeolites. A 3-21G basis set is employed in the calculations, and the bridged hydroxyl group is modeled via an aluminosiloxane unit. Increasing angle SiOAl is shown to give rise to decrease in the stretching frequency of the OH group (anti v/sub OH/) with a corresponding decrease in the ionicity of the OH bond. The decrease in ionicity of the OH bond indicates that increasing angle SiOAl gives rise to a decrease in acidity. The decreased strength of the OH bond as evidenced by the decreased anti v/sub OH/ value suggests that the OH bond is unstable when situated at a high angle SiOAl link with the decreased polarity of the bond tending to favor homolytic dissociation over heterolytic dissociation. It is also shown that angle SiOH exhibits decreased flexibility with increasing angle SiOAl, suggesting that the SiOH bending frequency should be greater for zeolites with higher TOT angles.

Vibrational relaxation of adsorbed molecules: Comparison with relaxation rates of model compounds

Time-resolved measurements of vibrational relaxation rates are reviewed. Depopulation of the vibrationally excited OH oscillator (on high surface area SiO2, ZSM-5, and ZnO) was found to occur on a time scale of 200 ps or less, depending on the substrate. The temperature dependence of both the OH and OD vibrational relaxation rates on SiO2 was found to follow simple multiphonon relaxation models. However, the relative rates for the two isotopes did not scale as predicted by the same theoretical models. Measurements of isotopically substituted silanols and alcohols [i.e., R3SiOH(D) and R3COH(D) dilute in CCl4] also find OD decay times comparable to those of OH. Based on results from these model compounds, the observed relaxation rates for OH/SiO2 appear to be dominated by processes which proceed through a SiOH bending mode.

Infrared Absorption Spectra of Silica Gel-H216O, D216O, and H218O Systems

Abstract Infrared absorption spectra of the systems of silica gel - H216O,D216O,H218O, and some organic vapors were measured in order to assign the bands senstive to adsorption at 870 and around 950 cm−1. Deuteration of silica gel caused the absorption decrease at 870 cm−1 and the appearance of a band at 620 cm−1. The band around 950 cm−1 appearing by adsorption not only of water but also of organic vapors showed no shift on deuteration, but shifted toward low frequency side by 25 cm−1 on the substitution of 16O of surface silanol groups with 18O. From these facts the bands at 870, 620 and 950 cm−1 were assgned to Si–OH bending, Si–OD bending and Si–O stretching vibrations of surface silanol groups, respectively. The oxygen atom of silanol groups of silca gel was found to exchange with that of adsorbed water molecules even at room temperature for a sample treated at temperature below 300°C. On the other hand, higher reaction temperature of ca. 200°C was necessary for a sample calcined at 800°C. mechanism of the oxygen atom exchange recation was discussed.