Types Of OH Groups Research Articles

Selective route for enhancing liquid crystal-based hydroxylpropyl cellulose by esterification

PurposeThis paper aims to study the effect of hydrolysis route of hydroxypropyl cellulose (HPC) on its esterification performance as liquid crystal material. The assessment was carried out from the data of spectra (Fourier-transform infrared analysis [FTIR] and 1H-nuclear magnetic resonance [1H-NMR]), thermal stability as well as optical properties via forming ordered mesophases at lower concentration than HPC.Design/methodology/approachThe HPC was hydrolyzed by hydrochloric acid-methanol at times 9 and 18 h, and the products were esterified by decanoyl chloride. The products of hydrolysis and the esterification were characterized by FTIR, NMR, nonisothermal analysis, thermo-gravimetric analysis (TGA) and polarizing microscope to evaluate the role of degree of substitution of HPC as a result of hydrolysis, on esterification degree, thermal stability and thermal and liquid crystal behavior of the final esterified HPC.FindingsThe pretreatment by acid hydrolysis of HPC was successful for synthesizing novel cholesteric hydroxypropyl cellulose ester. The data of FTIR and TGA thermal analysis proved that hydrolysis and esterification of HPC with the decanoyl chain significantly enhanced crystallinity of this cellulose derivative from 0.57 to (1.7–1.9). Moreover, they provided products with superior thermal stability than pure HPC, as noticed from increasing the activation energy of degradation (Ea) from 514.3 to 806.2 kJ/mol. The NMR measurement proved that hydrolysis of HPC for 9 and 18 h decreased the degree of substitution from 3 to 2.1 and 1.3, respectively. Moreover, the esterified HPC showed a promising birefringence texture (chiral nematic) besides decreasing the critical concentration from 30% for HPC to 10% for the esterified unhydrolyzed HPC, while superior decreasing to 1–5% was observed for the esterified hydrolyzed HPC.Research limitations/implicationsThere are two stages for preparation of decanoyl ester hydroxypropyl cellulose. At the first stage, HPC was treated by hydrochloric acid-methanol in ratio 1:10 at times 9 and18 h. At the second stage, HPC and hydrolyzed HPC were refluxed with decanoyl chloride (1:6) in presence of nitrogen atmosphere. The final product was precipitated by distilled water.Practical implicationsThere are two stages for preparation of decanoyl ester hydroxypropyl cellulose. At the first stage, HPC was treated by hydrochloric acid-methanol in ratio 1:10 at times 9 and18 h. At the second stage, HPC and hydrolyzed HPC were refluxed with decanoyl chloride (1:6) in presence of nitrogen atmosphere. The final product was precipitated by distilled water.Originality/valueThe novelty of this work was focused on enhancing the crystallinity, thermal stability and liquid crystal behavior of esterified HPC, via decreasing the degree of substitution and consequently the type of OH group subjected to esterification. The decanoyl ester formation from the hydrolyzed hydroxypropyl cellulose is able to form ordered mesophases at even low concentration (promising birefringence texture at concentrations 1–5%). It is worthy to notice that the investigated route is able to omit the role of graphene oxide in promoting the liquid crystal behavior of HPC, as it hasn't any effect on critical concentration. This work will promote the use of HPC in technological applications, e.g. high modulus fibers and electronic devices.

Adsorption of polycyclic aromatic hydrocarbons on FeOOH polymorphs: A theoretical study

In this work we studied interactions of polycyclic aromatic hydrocarbons (PAHs) with surfaces of two iron oxyhydroxide polymorphs, goethite and lepidocrocite, by means of density functional theory (DFT) method. The first focus here was to obtain adsorption energies of selected PAH molecules with the (110) goethite surface and the (010) lepidocrocite surface by performing DFT calculations with the inclusion of dispersion interactions. To this end we have performed assessment of six different methods to show their efficiency and predictability of nonbonding interactions. The second objective was to compare adsorption capability of two FeOOH polymorphs, goethite and lepidocrocite, as common minerals in several soil types and to relate obtained adsorption energies to different surface structure and topology of the most stable surfaces of both polymorphs. The goethite (110) surface has more complicated topology and three different types of OH groups compared to the lepidocrocite (010) surface formed by a plane of OH groups of the same type. For goethite, adsorption energy did not increase monotonically with the increasing molecular size of PAHs and the strongest adsorption was observed for the linearly-shaped anthracene molecule. On the other hand, adsorption of PAHs at the lepidocrocite surface increased energetically with increasing molecular weight, and the strongest adsorption was achieved for the pyrene molecule. The results showed that the goethite (110) surface is more attractive than the (010) lepidocrocite surface, which is explained by the different surface topology, and types and surface density of the OH groups.

Insights into crystal chemistry of the vesuvianite-group: manaevite-(Ce), a new mineral with complex mechanisms of its hydration

Manaevite-(Ce), a new vesuvianite-group mineral has been investigated by means of electron microprobe, TGA and DSC and CHN analysis of H2O, powder X-ray diffraction, single-crystal X-ray structure analysis, 139La NMR, 57Fe Mossbauer spectroscopy, IR spectroscopy and optical measurements. Tetragonal unit-cell parameters are a = 15.9247(13) A, c = 11.9661(10) A, space group P4/nnc. The structure model was solved and refined to R1 = 3.35% for 1757 independent observed reflections with I > 4σ(I). Manaevite-(Ce) is the first vesuvianite-group mineral with the species-defining role of REE3+ at the X3 site. The mineral contains two different kinds of hydroxyl anions. The first type of hydroxyl groups is associated with the O10 and O11 sites as observed for other vesuvianite-group members. Another type of OH groups is due to the hydrogarnet substitution (H4O4)4‒ ↔ (SiO4) 4‒ associated with the Z1 and Z2 sites. The incorporation of REE3+ into the crystal structure of manaevite-(Ce) proceeds via the substitution schemes 2Ca2+ ↔ Th4+ + □ and 3Ca2+ ↔ 2REE3+ + □, which results in the formation of vacancies at the X3 site and the presence of H2O molecules.

Surface Investigation of Tungstophosphoric Acid Supported on Ordered Mesoporous Aluminosilicates for Biodiesel Synthesis.

Aluminosilicates with ordered hexagonal mesopores denoted as MAS-7 and MAS-9 with uniform size were synthesized from zeolite β assembly and ZSM-5 precursors, respectively, with P123 as a surfactant, via the cooperative self-assembly pathway. A series of 12 tungstophosphoric acid (TPA) supported on MAS-7 and MAS-9 (H3PW12O40/MAS-7/MAS-9) catalysts for biodiesel production were synthesized via the wet impregnation technique. The characterization of the supports and catalysts was done using N2 adsorption–desorption analysis, X-ray diffraction (XRD), and pyridine adsorption Fourier transform infrared and Raman spectroscopies. Also, the 29Si magic-angle spinning and cross-polarization/MAS nuclear magnetic resonance (NMR) techniques were employed to study MAS-7/MAS-9 and the supported solid acid catalyst surfaces. The nitrogen sorption analysis and XRD patterns indicated the formation of well-defined mesoporous materials, whereas IR spectroscopy confirmed the presence of four distinct types of OH groups with varying degrees of acidity. 29Si MAS NMR signified a stronger interaction between the framework of mesoporous aluminosilicates and H3PW12O40. The X-ray absorption near-edge spectra of L1- and L3-edge tungsten showed that W in the H3PW12O40/MAS-9 sample exists as W6+, indicating the tungsten environment similar to that of H2WO4 with Oh symmetry. The catalytic activity of the synthesized catalysts was investigated for biodiesel synthesis using unrefined green seed canola oil as a feedstock, giving a methyl ester yield of 76.5–88.7 wt % under optimized reaction conditions. The catalyst activities were strongly correlated with the surface chemistry of the TPA-supported MAS-7 and MAS-9 catalysts.

Pure and Fe-doped CeO2 nanoparticles obtained by microwave assisted combustion synthesis: Physico-chemical properties ruling their catalytic activity towards CO oxidation and soot combustion

A sample of pure CeO2 and two samples of Fe-doped CeO2 containing either 3 or 6 at.% Fe were obtained by microwave assisted combustion synthesis. The powders were extensively characterized by several techniques and tested as catalysts for both CO oxidation and soot combustion.As-synthesized CeO2 nanoparticles have a mostly squared shape and size well below 100nm; they are characterized by the presence of surface Ce3+ species likely due to the occurrence of very defective ceria crystalline phases, as revealed by HR-TEM. Oxidation at 400°C leads to the formation of a hydroxyls-rich surface, with several types of OH groups related to both Ce4+ and Ce3+ species; reduction in H2 at mild temperature (200°C) leads both to reduction of surface Ce4+ to Ce3+ and formation of new OH groups.With respect to CeO2 nanoparticles, Fe-doped ones have, as a whole, a larger size and less abundant surface OH species. A core-shell structure is inferred where Fe is mostly present in the shell, both in a secondary phase (CeFeO3) and as a dopant, finally lowering the band gap of the material. The presence of Fe improved samples reducibility, as shown by the lowering of the onset of temperature programmed reduction.Catalytic tests of CO oxidation showed that surface Fe species significantly improve the catalytic performance of the samples, by lowering the onset of CO conversion to CO2 especially at low Fe content (i.e. 3 at.%), whereas at 6 at.% Fe loading, the preferential formation of the secondary phase CeFeO3 occurs, finally lowering the CO conversion with respect to the sample containing 3 at.% Fe.Conversely, the soot combustion activity was higher for pure CeO2 nanoparticles, likely due to their smaller size, which increases the amount of solid–solid contact points between soot and the catalyst. However, a positive effect of the presence of surface Fe species on the catalytic activity towards soot combustion was observed, as well.

An Atomic-Scale Study of TiO2(110) Surfaces Exposed to Humid Environments

Rutile titanium dioxide (TiO2) (110)-(1 × 1) surfaces prepared in an ultrahigh vacuum (UHV) were transferred to humid environments and thereafter returned to UHV to be examined by the scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) techniques. STM images showed that hydroxyl groups terminating 5-fold-coordinated Ti atoms (OHt groups) and hydroperoxyl (OOH) groups, with densities of 0.9 and 0.2 nm–2, respectively, formed after exposure to water vapor at a pressure of several tens of pascals. The density of the OHt groups was found to exceed that predicted when they were assumed to have originated from another type of OH groups involved in bridging O atoms (OHb groups) intrinsically present on the (1 × 1) surface. On the basis of this observation, we hypothesize that dissociation of H2O molecules by evacuation of the water layer produces OHt groups. The OHb groups, produced with the OHt groups by this dissociation, react with O2 molecules dissolved in the water layer, forming ...

Polyethylene glycol ratio effect on magnesium oxide prepared by chemical precipitation: Impact on structure, morphology, and electrical properties

In this work, we investigate the properties of magnesium oxide powders synthesized via simple chemical precipitation (SPC). The effect of polyethylene glycol PEG (300) ratio in electrical behavior of MgO is the main part of this study. Crystalline structure and particle size were investigated through X-ray diffraction (XRD). Crystallite size of obtained MgO powders was calculated using Scherer׳s formula. Fourier transforms infrared spectroscopy (FTIR), Differential Thermal Analysis (DTA) and Gravimetric Analysis (TGA) were used to investigate PEG (300) effect. In fact the mass loss decreases when the PEG (300) ratio increases. Two type of OH group in the MgO surface sites: Type A and Type B with different coordinations, one for the first type and more (2, 3, 4) for the second, were highlighted. The number of OH group decreases when the ratio of MgO/PEG (300) increases. Ratio effect, on dielectric phenomena and electrical conductivity properties of MgO powders, was studied to improve the conductivity. XRD revealed that obtained MgO powders are crystalline and of high purity. Complex impedance spectroscopy was used to examine electrical properties. The activation energies of the MgO powders in the temperature range of study were correlated with PEG (300) ratio.

FTIR spectroscopic study of natural andalusite showing electronic Fe–Ti charge-transfer processes: zoning and thermal evolution of OH-vibration bands

We investigated a natural Brazilian Fe–Ti-containing andalusite and its thermal behavior by polarized infrared and optical spectroscopy. Polarized infrared spectra of the Brazilian andalusite and their evolution at thermal annealing in air clearly evidence that there are several types of OH-groups in the structure. Optical spectra and their evolution with temperature indicate that the incorporated iron (about 0.43 wt% calculated as FeO) is in the ferrous and ferric state. Incorporation of ferrous iron in the Al-sites of andalusite is discussed as a possible incorporation mechanism for hydrogen. The weakening and disappearance of the Fe2+/Ti4+ IVCT band in the andalusite spectra under annealing in air is caused by oxidization of Fe2+ to Fe3+ in Fe2+/Ti4+ IVCT pairs. The process of oxidation is accompanied by a rearrangement of the hydroxyl groups and dehydration of the sample, especially vivid at the final stage of the thermal annealing at 1,200 °C. During thermal annealing, structural hydroxyls of different types apparently transform into each other: the most distinct are the hydroxyls causing the doublet at 3,516 and 3,527 cm−1 (i.e., H bonded to O1) which seem to transform into the hydroxyls causing the line at 3,461 cm−1 (i.e., H bonded to O2). The infrared spectra scanned across differently colored zones of the crystal clearly show that some amount of hydroxyls is related to Fe2+/Ti4+ IVCT pairs which are the cause of the red-to-black coloration of the sample in E||c-polarized illumination: it is evident that in a part of the hydroxyl groups, OH-vector changes orientation aligning directly along crystallographic a-axis due to some kind of interaction with Fe2+/Ti4+ IVCT pairs.

Sorption of 2,4-dinitroanisole (DNAN) on lignin

The present study describes the use of two commercially available lignins, namely, alkali and organosolv lignin, for the removal of 2,4-dinitroanisole (DNAN), a chemical widely used by the military and the dye industry, from water. Sorption of DNAN on both lignins reached equilibrium within 10 hr and followed pseudo second-order kinetics with sorption being faster with alkali than with organosolv lignin, i.e. k2 10.3 and 0.3 g/(mg·hr), respectively. In a separate study we investigated sorption of DNAN between 10 and 40C and found that the removal of DNAN by organosolv lignin increased from 0.8 to 7.5 mg/g but reduced slightly from 8.5 to 7.6 mg/g in the case of alkali lignin. Sorption isotherms for either alkali or organosolv lignin best fitted Freundlich equation with enthalpy of formation, ΔH0 equaled to 14 or 80 kJ/mol. To help understand DNAN sorption mechanisms we characterized the two lignins by elemental analysis, BET nitrogen adsorption-desorption and 31P NMR. Variations in elemental compositions between the two lignins indicated that alkali lignin should have more sites (O- and S-containing functionalities) for H-bonding. The BET surface area and calculated total pore volume of alkali lignin were almost 10 times greater than that of organosolv lignin suggesting that alkali lignin should provide more sites for sorption. 31P NMR showed that organosolv lignin contains more phenolic -OH groups than alkali lignin, i.e., 70% and 45%, respectively. The variations in the type of OH groups between the two lignins might have affected the strength of H-bonding between DNAN and the type of lignin used.

Biomolecule−Biomaterial Interaction: A DFT-D Study of Glycine Adsorption and Self-Assembly on Hydroxylated Cr2O3 Surfaces

The adsorption of glycine, the building block of amino acids, on hydroxylated (0001)-Cr2O3 model surfaces, representing the stainless steel passive film surface, was modeled by means of the GGA + U method. The roles of glycine coverage and surface termination (hydroxylated Cr- and O-terminated surfaces) on the adsorption mode and self-assembly properties were explored. The hydroxylated Cr-terminated Cr2O3 surface, which presents two types of (H)OH groups exhibiting different acidic character, is more reactive than the hydroxylated O-terminated surface, where one single type of OH group is present, for all adsorption modes and coverages considered. Outer sphere adsorption occurs in the zwitterion form, stabilized at low coverage through H-bond formation with coadsorbed water molecules, and at the monolayer coverage by glycine self-assembling. The OH substitution by glycinate is favored on the hydroxylated Cr-terminated surface and not on the O-terminated one. The inclusion of dispersion forces does not change the observed tendencies. An atomistic thermodynamics approach suggests that outer sphere adsorption is thermodynamically favored over inner sphere adsorption in the whole domain of glycine concentration. The obtained SAM's free energies of formation are rationalized in a model considering the balance between sublimation and solvation free energies, and extrapolated to other amino acids, to predict the SAMs formation above hydroxylated surfaces. It is found that hydrophobic AA tend to self-assemble at the surface, whereas hydrophilic ones do not.

New insights in reactivity of hydroxyl groups in water gas shift reaction on Pt/ZrO 2

The reactivity of mono- and multi-coordinated hydroxyl groups on Pt/ZrO 2 with CO was investigated with in-situ transmission FT-IR. It was found that mono-coordinated hydroxyl groups on the ZrO 2 support reacted with CO to formate. Multi-coordinated hydroxyl groups were needed for formate decomposition, producing H 2 and CO 2, taking place exclusively in the presence of Pt. The observed differences in reactivity of different types of OH groups are relevant for the WGS mechanism, assuming that formate is a reactive intermediate under the conditions used in this study (300–400 °C). The fact that Pt is needed for decomposition of formate indicates that the reactivity of formate depends on the position on the ZrO 2 surface, i.e. in close vicinity or remote from metal particles. Our results show that reduction of the support was limited to structural defective zirconia sites at edges and kinks, hosting the mono-coordinated hydroxyl groups. These sites can be re-oxidized with water, re-establishing the hydroxyl groups.

Model study on sorption of polycyclic aromatic hydrocarbons to goethite

A systematic DFT study of interactions between a set of mono- and polyaromatic hydrocarbons (PAHs) and the (110) goethite surface have been performed in this work. It was found that PAHs form relatively weak surface complexes having their molecular plane practically parallel to the surface plane. The origin of the interactions is in the polarization of the π-system by polar OH groups and in the formation of weak hydrogen bonds where the π-system acts as a proton acceptor. The computed perpendicular distances of the molecular plane of PAHs to the hydrogen atoms of the surface OH groups range from 2.3 to 2.7 Å. Computed interaction energies regularly increase for the linearly shaped molecules from benzene to anthracene. Two other PAHs with a nonlinear shape, phenanthrene and pyrene, are less strongly bound to the surface although they have a similar (phenanthrene) or even larger size (pyrene) than anthracene. These differences were explained by the specific configuration of the surface hydroxyl groups of goethite. The three types of OH groups, μ-OH, μ 3 -O IIH, and OH, form a valley, the width of it fits very well the molecular shape of the linear PAHs. It was found that with anthracene as example the linear PAHs can easily slide along the valley of OH groups with practically no barrier. In summary it is concluded from our results that the (110) goethite surface will withhold linear PAHs significantly better than nonlinear ones.

Single-crystal Raman investigation of vesuvianite in the OH region

Vesuvianite, a ortho–diortho silicate with the simplified formula Ca 19(Al, Mg, Fe, Ti, Mn, …) 13(B, Al, Fe, □) 5(SiO 4) 10(Si 2O 7) 4O 1–2(OH, F, O, Cl) 9 is a model object for IR investigations on the OH region of minerals. Vesuvianite contains two structural types of OH groups. There are four to six bands in its vibration spectra that are explained as due to different coordination effects. In this paper, the results of Raman investigations on the OH region in vesuvianite are given for the first time. Investigations on ferrous vesuvianite in rodingites from the Urals show that analogous modes are active on both FTIR and Raman spectra. Using a sample of super-clear vesuvianite with P4/ n symmetry and ordered structure [ a = 15.523(1) Å, c = 11.810(1) Å, P4/ n, R 1 = 0.0343] from Kazakhstan, it is demonstrated that the character and position of OH bands on vibration spectra are connected not only with coordination effects but also with statistically disordered proton sites and with the number and character of hydrogen bonds. The true symmetry of the vesuvianite from Kazakhstan is lower than P4/ n as determined on the basis of the X-ray single-crystal diffraction data.

Some physico-chemical alterations caused by mechanochemical treatments in kaolinites of different structural order

Two kaolinites of different structural order designated KGa-2 and KC were milled for variable times (15, 30, 45, 60, 75, 90, 105 and 120 min). The ensuing changes were monitored using particle size distribution (PSD) analyses, pH measurements and thermal methods [thermogravimetric analysis (TGA), and differential thermal analysis (DTA)]. The size mode for the initial particles ranged from 0.31 to 0.36 μm in KGa-2 and from 6.6 to 7.7 μm in KC. After 120 min of milling, however, both kaolinites exhibited similar PSD results and modes (26.2–30.5 μm). Milling caused the transformation of surface charges and produced water molecules. This resulted in a decreased pH in KC and an increased one in KGa-2; however, both tended to converge on pH 6 after several minutes of milling. The new larger particles were formed by bonding between the new water molecules formed. Based on the temperature at which they disappeared, the water molecules were coordinated as zeolitic or interlayer water. The effects of milling became apparent at an earlier stage in the less ordered kaolinite. In well-ordered kaolinites consisting of large particles, the inner-to-inner surface hydroxyl ratio results in a symmetric dehydroxylation effect; thus, milling increased structural disorder and decreased the peak temperature, symmetry and width. Also, T p − T on increased by effect of an increased presence of edges and inner surface hydroxyls, whereas T off − T p decreased with decreasing proportion of inner hydroxyls relative to other types of OH groups.

Miscibility of blends of poly(methyl methacrylate) and oligodiols based on a bisphenol A nucleus and ethylene oxide or propylene oxide branches

Cloud-point curves of blends of poly(methyl methacrylate) (PMMA) with a series of oligodiols based on a bisphenol A nucleus and short branches of poly(ethylene oxide) or poly(propylene oxide) (BPA-EO or BPA-PO), and with PEO and PPO oligomers, were obtained using a light transmission device. Experimental results were fitted with the Flory–Huggins model using an interaction parameter depending on both temperature and composition. For PMMA/PEO and PMMA/PPO blends, the miscibility increased when increasing the size of the diol, due to the significant decrease in the entropic and enthalpic terms contributing to the interaction parameter. This reflected the decrease in the self-association of solvent molecules and in the contribution of terminal OH groups to the mismatching of solubility parameters. For PMMA/BPA-EO blends, a decrease of the entropic contribution to the interaction parameter when increasing the size of the oligodiol was also found. However, the effect was counterbalanced by the opposite contribution of combinatorial terms leading to cloud-point curves located in approximately the same temperature range. For PMMA/BPA-PO blends, the interaction parameter exhibited a very low value. In this case, the effect of solvent size was much more important on combinatorial terms than on the interaction parameter, leading to an increase in miscibility when decreasing the oligodiol size. For short BPA-PO oligodiols no phase separation was observed. The entropic contribution of the interaction parameter exhibited an inverse relationship with the size of the oligodiols, independent of the nature of the chains bearing the hydroxyls and the type of OH groups (primary or secondary). This indicates that the degree of self-association of solvent molecules through their OH terminal groups, was mainly determined by their relative sizes.

Use of Hydroxyl Functionalized (Meth)acrylic Cross‐Linked Polymer Microparticles as Chain Transfer Agent in Cationic Photopolymerization of Cycloaliphatic Epoxy Monomer, 1

AbstractIn the case of cationic‐type photopolymerized epoxy networks, a new type of chain transfer agent based on hydroxyl functional acrylic cross‐linked polymer microparticles (CPM, also called microgels) has been tested. The CPM functionality was obtained through hydroxyethyl acrylate (HEA) monomer, used as comonomer with butyl acrylate (BA) and hexane diol diacrylate (HDDA). Stabilizing monomers were also required for the synthesis of CPM. In order to compare their particular effects, 4 sets of CPM were synthesized with 4 different stabilizing agents, either hydroxyl functional or not. Consequently, two types of OH groups were present in the particles: primary groups coming from HEA and preferentially located in the particles, and optional hydroxyl groups due to the hydroxyl functional stabilizing agents which were mainly placed onto the particles' surface. The viscoelastic properties of the photopolymerized films have been used to obtain information about the chain transfer reaction and the network microstructure. In all cases, the rubber modulus, $E'_{\rm r}$ was improved because of the decrease of the number of dangling epoxy chains in the epoxy network. When hydroxyl groups were only present in the particles, the mobility of the linkages was assumed to be low, and the mechanical relaxation temperature, Tα, strongly increased. However, CPM aggregation occurred at high CPM concentrations, lowering $E'_{\rm r}$ and Tα. On the contrary, when OH groups were located both on the surface as well as inside the particles, no large CPM aggregation took place, even with [CPM] = 40 wt.‐%. Nevertheless, the presumably higher mobility of the linkages on the particles' surface prevented any Tα increase. The water absorption of all systems based on CPM was very low, around 2% whatever the concentration of chain transfer agent.Schematic description of the transfer reaction between a propagating cationic‐type epoxy chain and a hydroxyl functional CPM.magnified imageSchematic description of the transfer reaction between a propagating cationic‐type epoxy chain and a hydroxyl functional CPM.

Hydrogen-Bond Interactions in Organically-Modified Polysiloxane Networks Studied by 1D and 2D CRAMPS and Double-Quantum 1H MAS NMR

Hydrogen-bonding interactions, distribution of various hydroxy groups, and surface morphology in organically modified polysiloxane networks were studied by solid-state NMR techniques based on 1H spin-exchange, double-quantum, and 1H−29Si heteronuclear MAS NMR spectroscopy. 1H CRAMPS experiments revealed four main types of OH groups differing in hydrogen-bond strength, order, and dynamics, which are mutually dipolar-coupled (their interatomic distances are not larger than 0.5 nm), however, not involved in fast chemical exchange even in the fully hydrated state at room temperature. The resulting hydrogen-bonding network is inhomogeneous in the entire set of hydroxyl groups. These findings were correlated with the quantum chemical geometry optimization of hydrogen-bonded local structures and subsequent calculations of 1H NMR chemical shifts. 2D 1H spin-diffusion experiments were used to determine the 1H−1H interatomic distances and to probe the average size of OH clusters, which is ca. 1−2 nm. Intimate mixing of strongly hydrogen-bonded OH and methyl groups was confirmed by 2D double-quantum 1H MAS NMR spectra. Hydrogen-bonding strength of various hydroxyl clusters was evaluated with respect to the type of siloxane structure units and geometry of the siloxane matrix by 2D 1H−29Si heteronuclear experiments.

OH frequency calculations for the hydroxylated MgO(001) surface

We have performed periodic Hartree-Fock calculations for OH groups adsorbed on the MgO(001) surface considering different surface coverages. Six types of OH groups are discussed: OH m, OH . , H+, H . and hydrogen-bonded OH and H. It is found that when both OH and H are present on the surface, the two groups are best described as OH m . We suggest that the highest-frequency fundamental band (∼3750 cm m 1 in the experimental OH spectrum) is assigned to OH m groups adsorbed on top of Mg 2+ , while H + adsorbed on top of O 2 m give rise to the broader band at ∼3550 cm m 1 .

Hydrated layered manganese dioxide: Part I. Synthesis and characterization of some hydrated layered manganese dioxides from α-NaMnO 2

Hydrated layered manganese dioxides with the birnessite structure (monoclinic Na-birnessite, hexagonal Na-birnessite and hexagonal K-birnessite) were synthesized from the ternary phase α-NaMnO 2. The synthesis of monoclinic Na-birnessite was carried out through hydrolysis in air. Hexagonal Na-birnessite was prepared by oxidation of monoclinic Na-birnessite with excess Br 2, 6 M NaOH, while hexagonal K-birnessite was prepared by oxidation of monoclinic Na-birnessite with K 2S 2O 8. The characterization of the synthesized samples was carried out using chemical analysis, X-ray diffraction (XRD) measurement, IR and ion exchange. Samples were found to possess the layered structure with good crystallinity and reversible changes of the interlayer distance. Moreover, type and amount of the guest ion affect the lattice symmetry. IR spectra of the hydrated phases enhance the presence of at least two types of OH groups.

Characterization of Acidic OH Groups in Y-Type Zeolites by Means of Different Methods of Temperature-Programmed Desorption (TPD) of Ammonia

The acidic properties of HNaY zeolites with different degrees of exchange have been studied by means of two complementary techniques of temperature-programmed desorption (TPD) of ammonia, namely, FTIR-TPD and conventional TPD. In HNaY zeolites with low exchange levels only a single type of OH group is observed. The analysis of the spectra in the region of NH bending modes during NH3-FTIR-TPD on zeolites with higher proton content (40% or higher) shows that ammonia is bound in two different adsorption states. This assessment is further confirmed by evaluation of the OH stretching region. Furthermore, desorption energy distribution functions for the different types of OH groups have been calculated using a numerical regularization method.