Surface Of Porous Vycor Glass Research Articles

ESR study of alkyl radicals adsorbed on porous Vycor glass: I. Build-up of methyl and ethyl radicals

Methyl and ethyl radicals have been produced on the surface of porous Vycor glass (PVG) at 77 K by UV photolysis of adsorbed azomethane and azoethane, respectively. The build-up of the radicals as a function of irradiation time has been studied by using the technique of electron spin resonance (ESR). Ethyl radicals are found to be slightly more difficult to generate than methyl radicals and both radicals were observed not to approach saturation within the period of irradiation employed. The variation of the rate of formation of the radicals with surface coverage and their formation during re-irradiation after previous decay for several hours at 77 K are discussed.

Novel Organic-Vycor Glass Composite Membrane Having Si-C Bonds.

A novel organic-ceramic membrane having stable Si-C bonds was developed for separation of high temperature organic solutions. Porous vycor glass was employed for a substrate which was modified by a series of chemical vapor deposition (CVD) and hydrosilylation. 1,3,5,7-Tetramethylcyclotetrasiloxane (H4) was used as a CVD source in order to introduce Si-H bonds on the surface of porous vycor glass. The Si-H bonds were then catalytically hydrosilylated with olefins. As a result, organic functional groups were introduced via Si-C bonds. A compact membrane was obtained when CVD treatment was carried out at 343 K. The membrane that was modified by the hydrosilylation of styrene showed benzene permselectivity in benzene/cyclohexane vapor permeation test at 373 K. The separation factor of benzene over cyclohexane was 2.9 and benzene flux was 1.2 × 10-8 mol m-2s-1Pa-1, and the permeation properties are stable for more than 1000 min at 373 K.

Preparation of H2 Permselective Silica Membranes by Alternating Reactant Vapor Deposition

Thin SiO_2 layers were deposited on the surface of porous Vycor glass by alternating vapor phase reaction with SiCl_4 and H_2O. The membranes prepared by this technique had a H_2 permeance of 0.3-0.4 cm^3(STP)/(min•cm^2•atm) and a H_2:N_2 selectivity of 500-1000 at 600 °C. The SiCl_4 dosage at each silylation cycle, the concentration of initial surface OH groups, and the reaction temperature significantly influence the deposit layer thickness. After two weeks of heating at 550 °C under 3 atm of water vapor, the membrane H_2 permeance decreased by about 20%, and the selectivity increased to more than 2000. The membrane properties after this hydrothermal treatment are superior to those of membranes prepared earlier by one-sided (steady flow) deposition. A simple model incorporating diffusion and surface reaction was used to study the effect of various parameters on the formation of the deposit layer.

Resonance Raman investigation of iodine and bromine adsorbed on porous vycor glass

AbstractResonance Raman spectra have been recorded for bromine and iodine supported on the surface of porous Vycor glass. They show that the halogens are mainly physisorbed on the Vycor glass. Other features indicate the presence of polyhalide species of type X3−. When the amount of polyhalide is large, we observe peaks which can be attributed to a halogen molecule coordinated to a Lewis base, which is probably X3−.

ChemInform Abstract: RAMAN SCATTERING FROM AMMONIA ADSORBED ON THE AMORPHOUS SILICA SURFACE

AbstractDas Raman‐Spektrum von adsorbiertem NH3 wird für Bedeckungen von etwa 0.01 Monoschichten bis zu einigen Schichten auf porösem Vycor‐Glas untersucht.

Raman scattering from ammonia adsorbed on the amorphous silica surface

We have performed measurements of the perturbations of the Raman spectrum of ammonia molecules due to adsorption on the surface of porous Vycor glass. The spectrum of hydrogen bonded NH3 is studied as a function of coverage and related to changes in the substrate surface phonon spectrum. More strongly chemisorbed ammonia species are also observed. When NH3 is adsorbed on a partially deuterated surface we observe a slow chemical reaction in which the adsorbed NH3 exchanges hydrogen with a surface deuteroxyl group: we observe in addition to the changes due to NH3 interacting with OH groups the appearance of adsorbed NH2D and the near complete replacement of surface OD with OH.

ChemInform Abstract: NH2 RADICALS TRAPPED ON THE SURFACE OF POROUS VYCOR GLASS AT 77 K

AbstractDie Ergebnisse der ESR‐Untersuchungen von durch Photolyse von NH3 erzeugten und an porösen Vicorglasoberflächen bei 77 K adsorbierten NH2‐ Radikalen werden behandelt.

NH2 Radicals Trapped on the Surface of Porous Vycor Glass at 77 K

Abstract The results of ESR studies of the amino radicals produced by the photolysis of ammonia adsorbed on the surface of porous Vycor glass at 77 K are described. The observed ESR signal patterns of the amino radicals were remarkably affected by the amount of ammonia on the surface. The isotropic and anisotropic spectra were analyzed by the use of isotopically-substituted amino radicals.

Carbon-13 hyperfine splittings of normal and abnormal methyl radicals trapped on the porous Vycor glass surface

To clarify the electronic structure and motional state of the abnormal methyl radical, we studied the 13C-hf splittings of normal and abnormal methyl radicals photoproduced and stabilized on the surface of porous Vycor glass, as well as that of the methyl radical-iodide ion adduct prepared by γ-irradiation of 13CH3I in a CD3CN matrix at 77 K. The ESR spectra of all the methyl radicals quoted above exhibited axially symmetric features with respect to the 13C-hf (and 127I-hf) tensors at low temperatures. A significant decrease in the isotropic 13C-hf constant of the abnormal methyl radical compared with that of the normal one is interpreted in terms of the spin-delocalization onto the oxygen atom of the surface siloxane bridge, keeping its structure planar. The fact that the 13C-hf tensors of all the methyl radicals show a trend of A∥ (13C) > A⊥ (13C) implies that the normal and abnormal methyl radicals interact with the surface via the 2pπ unpaired-spin orbital and rotate around the C3 axis. The temperature dependence of the isotropic 13C-hf splitting of the normal methyl radical was also reexamined in the temperature range 77–300 K, and, accordingly, the result was found to agree well with the theoretical calculation by Schrader based on the extra coupling due to the out-of-plane vibration.

Electron spin resonance studies of methyl radicals stabilized on porous VYCOR glass: various surface interactions, second-order splitting, and a linewidth temperature study

Electron spin resonance studies have been performed on methyl radicals stabilized on the surface of porous VYCOR glass. As previously reported, two very different methyl radicals, denoted Me and Me′, were seen. Results indicate that the radical Me′ is most probably weakly bound to the boroxane group (=B—O—B=) which is prominent on the surface of high temperature (700–900 °C) pretreated porous VYCOR glass. Four satellite lines about each Me line, denoted previously as radical X, were observed when the surface was pretreated at lower temperatures (400–500 °C). In an earlier publication radical X was attributed to the interaction between methyl radicals and the isotopic surface species 11B. In this study two additional satellite lines about each Me line are reported as well as a reassignment of the origin of all satellite lines. Four of the six satellite lines have been assigned to forbidden "spin–flip" transitions while the other two have been assigned to direct interaction between methyl radicals and the surface hydroxyl protons. Support for the irreversibility of high temperature dehydroxylation of porous VYCOR glass is also presented.A partial resolution of the second-order splitting of each of the central pair of lines was achieved for the normal methyl radical. The value of the second-order splitting was between 220 and 230 mG. The linewidths of the two central lines were measured at 77 °K. The samples were then allowed to warm up to various temperatures and then recooled to 77 °K. The linewidths were smaller after completing this cycle. Storage of the sample at 77 °K allowed the linewidths to return to their original values. These linewidths effects are explained by postulating the existence of two different types of potential wells on the surface.

Temperature Dependences of the Hyperfine Interactions, Linewidths, and Line Asymmetries in Methyl Radicals Stabilized on Porous Glass Surfaces

Methyl radicals were stabilized on the surface of porous Vycor glass at low temperatures by the uv photolysis of methyl iodide, deuterium-enriched methyl iodide, and carbon-13-enriched methyl iodide. The proton, deuterium, and carbon-13 hyperfine splittings were measured as a function of temperature in order to make comparisons with the dependences calculated by Schrader. In addition, the linewidths, line asymmetries, and relative line intensities in the methyl radical ESR spectra were measured as a function of temperature. The absolute values of the temperature coefficients of the proton and deuterium splittings were found to be slightly larger than those predicted by theory. The carbon-13 hyperfine splittings were found to agree with theory above 200°K but a large deviation was observed below 200°K. Below 275°K the proton lines become increasingly asymmetric. The relative line intensities approach but do not attain the binomial values as the temperature is increased. The proton linewidths were found to have different temperature dependences below 200°K, while having the same dependences above 200°K. The observed results may be explained by a physical model based on increased surface interaction at low temperatures.

Electron Paramagnetic Resonance of Methyl Radicals on Porous Glass Surface

The photolysis of methyl iodide on a porous glass surface has been studied by electron paramagnetic resonance. A large number of methyl radicals can be stabilized over a wide range of temperature. The surface of porous Vycor glass is an excellent host for trapping free radicals.