Kinds Of Hydroxyl Groups Research Articles

Exploration of a two-stage polymerization mechanism in construction of dense polyester membranes for drinking water treatment

Due to the relatively low reactivity of hydroxyls with acyl chloride groups, the synthesized polyester (PE) membranes commonly have loose structures, only capable of removing large-molecule organics with low rejection of inorganic salts. In this study, the pore size reduction of the synthesized PE membrane was successfully achieved by optimizing the interfacial polymerization (IP) reaction between erythritol and trimesoyl chloride (TMC). The optimized membrane was able to maintain a high xylose rejection (∼60 %) and acceptable water permeance (7.85 L m−2 h−1 bar−1), which was suitable for drinking water treatment. Through comprehensive characterizations, a two-stage membrane formation process was demonstrated regarding the reaction between aqueous monomers with different reactive functional groups and TMC. The highly reactive primary hydroxyls more preferentially participated in the initial reaction stage, leading to a linear cross-linking pattern of polyester. When the molecular diffusion was inhibited, the participation of secondary hydroxyls made the reaction proceed toward partial or full cross-linking. The distinct roles of the two kinds of hydroxyl groups were further verified by the energy barrier differences obtained by density functional theory (DFT) calculation. The exploration of membrane-optimized preparation and formation mechanisms can promote the structural regulation and tailored synthesis of PE membranes for various applications.

Unraveling the Surface Hydroxyl Network on In2O3 Nanoparticles with High-Field Ultrafast Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy.

Hydroxyl groups are among the major active surface sites over metal oxides. However, their spectroscopic characterizations have been challenging due to limited resolutions, especially on hydroxyl-rich surfaces where strong hydroxyl networks are present. Here, using nanostructured In2O3 as an example, we show significantly enhanced discrimination of the surface hydroxyl groups, owing to the high-resolution 1H NMR spectra performed at a high magnetic field (18.8 T) and a fast magic angle spinning (MAS) of up to 60 kHz. A total of nine kinds of hydroxyl groups were distinguished and their assignments (μ1, μ2, and μ3) were further identified with the assistance of 17O NMR. The spatial distribution of these hydroxyl groups was further explored via two-dimensional (2D) 1H-1H homonuclear correlation experiments with which the complex surface hydroxyl network was unraveled at the atomic level. Moreover, the quantitative analysis of these hydroxyl groups with such high resolution enables further investigations into the physicochemical property and catalytic performance characterizations (in CO2 reduction) of these hydroxyl groups. This work provides insightful understanding on the surface structure/property of the In2O3 nanoparticles and, importantly, may prompt general applications of high-field ultrafast MAS NMR techniques in the study of hydroxyl-rich surfaces on other metal oxide materials.

Spectroscopic Expression of the External Surface Sites of H-ZSM-5

The nature and spectroscopic expression of external surface sites of zeolites, in particular ZSM-5, is a long-debated question. Herein, we use three cutting-edge techniques: Fourier-transform infrared spectroscopy (FTIR) with Fourier self-deconvolution (FSD), high magnetic field proton NMR spectroscopy under fast magic-angle spinning (MAS) and periodic boundary Density Functional Theory (DFT) calculations to study external surface models and analyze the effect of crystallite size. This provides an unequaled description of the various kinds of hydroxyl groups and of their proximities. The hydrogen-bond donor, acceptor or isolated nature of the hydroxyls results in distinct signals both in FTIR and NMR spectra, but the peak assignment is not the same from one technique to the other when the chemical nature of the hydroxyl changes. Bridging Si–(OH)–Al groups and Al–(H2O) lead to overlapping signals in one-dimensional 1H MAS NMR, whereas their contributions are strongly different in FTIR spectra. However, quantification and proximity assessment could only be obtained by 1H MAS NMR. With DFT, we confirm previous assignments for silanols and Si–(OH)–Al bridging OH groups. Other signals (between 3750 and 3600 cm–1, and between 1 and 4 ppm) are not only assigned to extra-framework species (which we confirm with dedicated models), but also enclose the signature of sites exposed at the external surface of ZSM-5. In particular, Al–(H2O) species (∼3665 cm–1; 3.8, 2.6 ppm) and silanol–Al (∼3740, 3720, 3665 cm–1; 2.6, 2.2 ppm) contribute to several features depending on their environment. μ1-Al–OH are also present at the external surface in low amount, with a 3780 cm–1 signal in IR, and weak signals in the 0–2 ppm interval in 1H MAS NMR.

Dehydroxylation and Structural Distortion of Kaolinite as a High-Temperature Sorbent in the Furnace

As a high-temperature sorbent, kaolinite undergoes the flash calcination process in the furnace resulting in the dehydroxylation and structural distortion, which are closely related to its heavy metal/alkali metal adsorption characteristics. We investigated the flash calcination of kaolinite by the experiments using a drop tube furnace and by the characterization of flash-calcined products using thermogravimetric-differential scanning calorimeter (TG-DSC), X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer (FTIR)and nuclear magnetic resonance (NMR). There were three kinds of hydroxyl groups in kaolinite during flash calcination at 800–1300 °C, E-type (~50%, easy), D-type (~40%, difficult) and U-type (~10%, unable) according to the removal difficulty. The hydroxyl groups activation was believed to be the first step of the removal of E-type and D-type hydroxyl groups. The kinetics model of dehydroxylation groups at 900–1200 °C was established following Arrhenius equation with the activation energy of 140 kJ/mol and the pre-exponential factor of 1.32 × 106 s−1. At 800 °C, the removal of E-type hydroxyl groups resulted in the conversion of a part of VI-coordinated Al in kaolinite to V-coordinated Al and the production of meta-kaolinite. When the temperature rose up to 1200 °C, mullite was produced and a part of V-coordinated Al converted to IV-coordinated Al and VI-coordinated Al. Finally, the adsorption characteristics of kaolinite was discussed according to the results of dehydroxylation and structural distortion.

Amphiprotic properties of a bis(μ-hydroxo)divanadium(IV)-substituted γ-Keggin-type silicodecatungstate containing two different kinds of hydroxyl moieties.

A bis(μ-hydroxo)divanadium(IV)-substituted γ-Keggin-type silicodecatungstate, (TBA)4[γ-SiV(IV)2W10O36(μ-OH)4] (1), possesses two different kinds of hydroxyl groups and can work as an amphiprotic species to accept and donate proton(s). Dehydrative condensation reactions of 1 with methanol and formic acid proceed on more basic hydroxyl groups between two vanadium atoms without the deprotonation of more acidic hydroxides between two tungsten atoms to form (TBA)4[γ-SiV(IV)2W10O36(μ-OH)3(μ-OR)] (2·R, R = Me, Et, Pr; 3, R = C(O)H), showing Brønsted base properties of the hydroxyl groups between two vanadium atoms. On the other hand, the hydroxyl groups between tungsten atoms exhibit Brønsted acid properties and react with pyridine (Py) and TBAOH to form (TBA)4X[γ-SiV(IV)2W10O37(μ-OH)3] (PyH·4, X = PyH; TBA·4, X = TBA). DFT calculations for [γ-SiV(IV)2W10O36(μ-OH)4](4-) in water also support both the acidic and basic nature of hydroxyl groups in 1.

The amphoteric behavior of water in silicate melts from the point of view of their ionic-polymeric constitution

Dissociation of water into protons and hydroxyl ions is a fundamental feature of aqueous solutions. Although it exerts a profound influence on properties of magmas, this autoprotolysis reaction has been hitherto neglected for water dissolved in silicate melts. As made here with an acid–base model, in fact one has to deal with molecular water (H2Omol) and two kinds of hydroxyl groups, bonded or not to network-forming cations (OH and OH−, respectively) in hydrous silicate melts. By mixing cations and anions on distinct sublattices and quantifying the disproportionation of water dissolved in silicate melts into its ionic products, H+ and OH−, we reconcile spectroscopic determinations of water speciation, and highlight the main compositional features involving chemical exchanges between H2Omol, TOH and MOH groups (T and M being a former and a modifier, respectively). In particular, water addition to depolymerized systems, such as basalts, determine a relative predominance of OH− with respect to silicic systems, such that the increase of water concentration tends to immediately limit depolymerization rather than enhance it. This opens new perspectives to the understanding of the chemical control of hydrous magmas and their physical properties, as well as the attainment of saturation in hydrous minerals such as amphiboles or micas, particularly in depolymerized melts.

Synthesis and Characterization of Crystalline Structures Based on Phenylboronate Ligands Bound to Alkaline Earth Cations

We describe the preparation of the first crystalline compounds based on arylboronate ligands PhB(OH)(3)(-) coordinated to metal cations: [Ca(PhB(OH)(3))(2)], [Sr(PhB(OH)(3))(2)]·H(2)O, and [Ba(PhB(OH)(3))(2)]. The calcium and strontium structures were solved using powder and single-crystal X-ray diffraction, respectively. In both cases, the structures are composed of chains of cations connected through phenylboronate ligands, which interact one with each other to form a 2D lamellar structure. The temperature and pH conditions necessary for the formation of phase-pure compounds were investigated: changes in temperature were found to mainly affect the morphology of the crystallites, whereas strong variations in pH were found to affect the formation of pure phases. All three compounds were characterized using a wide range of analytical techniques (TGA, IR, Raman, XRD, and high resolution (1)H, (11)B, and (13)C solid-state NMR), and the different coordination modes of phenylboronate ligands were analyzed. Two different kinds of hydroxyl groups were identified in the structures: those involved in hydrogen bonds, and those that are effectively "free" and not involved in hydrogen bonds of any significant strength. To position precisely the OH protons within the structures, an NMR-crystallography approach was used: the comparison of experimental and calculated NMR parameters (determined using the Gauge Including Projector Augmented Wave method, GIPAW) allowed the most accurate positions to be identified. In the case of the calcium compound, it was found that it is the (43)Ca NMR data that are critical to help identify the best model of the structure.

The reaction of o-hydroxybenzyl alcohol with phenyl isocyanate in polar solvents

The reaction of o-hydroxybenzyl alcohol with phenyl isocyanate has been investigated in different polar solvents with in situ FT-IR. The rate constants for the reactions of the phenolic hydroxyl group and the aliphatic hydroxyl group were calculated as k 1 and k 2, respectively. It is found that the phenolic hydroxyl group reacts more easily than the aliphatic hydroxyl group. It is also found that k 1 increases with increasing solvent polarity, while the trend of k 2 is the opposite. Moreover, the reaction kinetics is second-order with respect to toluene, butyl acetate, cyclohexanone and pyridine, but is first-order with respect to NMP and DMF without distinction for the two kinds of hydroxyl groups.

Skeletal Isomerization of Linear Butenes on Boron Promoted Ferrierite: Effect of the Catalyst Preparation Technique

Potassium and acid ferrierites were impregnated with boron species by wet and incipient wetness techniques. All samples display a medium-intensity band at 3,450–3,470 cm−1 associated to Si−OH···O groups corresponding to boron-containing units. The 1,398–1,404 cm−1 band assigned to the B–O stretching in BO3 units does not appear on boron–potassium–ferrierite prepared by wet impregnation. Catalytic performance during the linear butene skeletal isomerization was measured. At 300 °C, boron impregnated by incipient wetness technique on acid ferrierite reduces both linear butene conversion at a short time and isobutene yield in all time range. Boron–potassium–ferrierite prepared by wet impregnation has a suitable acidity to promote isobutene production. At 450 °C, this sample shows the best performance, being the isobutene yield 1.7 times higher than the acid-ferrierite one and reaching the highest isobutene selectivity (92%). This performance is maintained with time. Both isobutene yield and by-product distribution are strongly affected by temperature; dimer intermediates are formed. Finally, both kinds of hydroxyl groups corresponding to 3,466 and 3,635 cm−1 bands influence the isobutene production whereas BO3 sites are inactive for this reaction.

Salt-induced thickening and gelation of a poly(carboxylate) having three kinds of hydroxyl groups

A salt-induced physical gelation was found for the aqueous solution of poly(carboxylate)s having three kinds of hydroxyl groups, i.e., primary, tertiary and one on a hemiacetal ring. The gelation point (critical salt concentration) was positively correlated with the content of the hemiacetal component, while the previously confirmed hydrogen bond between the hemiacetal OH and –COO− group does not seem to essentially contribute to the physical gelation. Above a critical polymer concentration, the solution viscosity was first decreased and then increased with increasing NaCl concentration, leading to gelation. However, below the critical polymer concentration, the viscosity decreased. These different behaviors of the solution viscosity depending on the polymer concentration were ascribed to a preferential promotion of intermolecular or intramolecular hydrogen bonds among the hemiacetal OH groups above and below the critical polymer concentration, respectively.

Specific dissociation behavior of a polycarboxylate having three kinds of hydroxyl groups

AbstractDissociation behavior of a hydrophilic copolymer having three kinds of hydroxyl groups, ie primary OH on α‐hydroxyallyl alcohol component, hemiacetal OH on cyclic hemiacetal component and tertiary OH on α‐hydroxyacrylate component, was investigated as a function of the polymer constitution to study effects from possible intramolecular hydrogen bonds between the carboxyl group and those hydroxyl groups. The apparent dissociation constant, pKa, decreased with increasing fraction of the hemiacetal component, suggesting that the carboxyl anion is stabilized by intramolecular hydrogen bonding with the hemiacetal OH group. Furthermore, the decrease in pKa value was more significant at the lower degrees of dissociation, α. This dissociation behavior was found to be consistent with IR peak shifts to higher wavenumbers observed for the asymmetric vibration band of COO−, which were more significant with decreasing α and for copolymers with larger hemiacetal contents. Effects from the lactone ring, formed during titration, on the pKa values are also discussed. Copyright © 2005 Society of Chemical Industry

Preparation of a new hydrophilic copolymer from a polylactone and the structure analyses

A new hydrophilic copolymer having three kinds of hydroxyl groups, i.e. primary OH on α-hydroxy allyl alcohol component, secondary OH on cyclic hemiacetal component and tertiary OH on α-hydroxy acrylate component, was prepared by reducing a ‘polylactone’, poly(1-oxa-2-oxobutane-1,4:3,3-tetrayl), with NaBH 4 in mixed solvents containing an alcohol. The structural analyses were performed by the solution 1H NMR and the solid state 13C NMR spectroscopies of the reduced samples. The composition of the three components turned out to be variable to the alcohol species used for the mixed solvent.

Cluster quantum-chemical MINDO/3 study of the nature of hydroxyl groups on a calcium oxide surface

A modified MINDO/3 method was applied to study water molecule interaction with a pure calcium oxide using a supermolecular approach. The calcium oxide was modelled by a Ca 32O 32 four-layer molecular cluster containing all sets of the most chemically active low-coordinated calcium and oxygen ions (Ca 2+ LC and O 2− LC). It is shown that the dissociative adsorption of water molecule on a pair of acid-base centers of CaO is energetically more favorable than the molecular forms of adsorption on an acidic sites. OH stretching frequencies of six kinds of hydroxyl groups on a CaO calculated by a harmonic oscillator approach can be ascribed to the corresponding local structures.

Use of isotopic labelling in a SIMS study of the hydroxylation of TiO 2(100) surfaces

The interaction of TiO 2(100) surfaces with hydrogen and water vapour has been studied using mainly secondary ion mass spectrometry (SIMS). The influence of the stoichiometry of the surface on its reactivity has been checked. The existence of two kinds of hydroxyl groups is pointed out. The behaviour of these hydroxyl populations with the temperature was studied. Moreover the ability of SIMS to distinguish different isotopes of the same element was used: exposure of TiO 2(100) surfaces to isotope labelled molecules, D 2 and H 2 18O, was performed in order to specify the nature of these two hydroxyl groups.

Magnetic Resonance Studies of Hydrogen Adsorbed on Zinc Oxide. II. Proton NMR of Chemisorbed Hydrogen and Molecular Hydrogen Adsorbed Ionically

The broad line NMR measurement of protons of adsorbed hydrogen and hydroxyl groups on fine ZnO powder was performed in connection with effects of adsorbed hydrogen on ESR of conduction electrons (CESR) in the accumulation layer. The NMR spectra from the ZnO out-gassed at 400°C showed the existence of two kinds of hydroxyl groups on the surface: one is passively adsorbed when present in high density and the other is in a state with free motions. States of chemisorbed hydrogen are similar to those of hydroxyl groups at 22°C, but chemisorbed hydrogen formed at -120°C is in a state of a migrating motion. When H2 gas was introduced to the ZnO at -120°C, the NMR spectrum became asymmetric, which revealed the presence of a weakly adsorbed hydrogen with 15 ppm of the chemical shift to the low field direction. This species is concluded to be H3+ molecules, which causes an increase in the CESR intensity with discharge of their electrons to the accumulation layer.

Polycondensation equilibrium and the kinetics of the catalyzed transesterfication in the formation of polyethylene terephthalate

AbstractThe polycondensation equilibrium constant for bis(β‐hydroxyethyl) terephthalate was found to be 0.50. By means of this value for determining the relative reactivity of two kinds of hydroxyl groups, the kinetic data were found to fit a third‐order reaction for the transesterification: one order each with respect to hydroxyl group, methoxy group, and catalyst. An order with respect to hydroxyl group, which had previously been in doubt, was proved in a series of experiments in which the concentration of ethylene glycol was varied. Third‐order rate constants were determined for the reactions catalyzed by lead, zinc, and calcium salts, and the activation energies were found to be 12.5, 15.0, and 14.5 kcal./mole, respectively. The results are consistent with the author's theory of aromatic transition states and are difficult to explain on the basis of ionic theories of reaction mechanism.

Hydroxyl groups on silica, alumina, and silica-alumina catalysts

In order to characterize the surface hydroxyls on silica, alumina, and silica-alumina catalysts, several kinds of organometallic compounds were used. Experiments were carried out with a closed system under normal nitrogen atmosphere and the volume of ethane produced was measured. For the silica-alumina catalyst, the results obtained with all the reagents were the same and were comparable to the amount of hydroxyl group measured by another method. For alumina and silica, the results obtained with ethyllithium, ethylmagnesium bromide, and triethylaluminum were also the same. The amounts obtained with diethylaluminum chloride and ethylaluminum dichloride were smaller. It seems that the hydroxyl groups on silica-alumina are different from those on alumina or silica, which are composed of two or more kinds of hydroxyl groups. Alkali-poisoned, ion-exchanged, or catalysts supported on other materials were also examined.