Types Of Hydroxyl Groups Research Articles

Specific Acidities of the Surface Hydroxyl Groups on Maghemite

Abstract Two types of surface hydroxyl groups on maghemite have been identified and their acidities evaluated. One of these surface groups bonds to the surface iron ions of the octahedral sites and shows a stretching vibration mode of infrared absorption at 3690 cm−1. The other bonds to the tetrahedral sites and vibrates at 3630 cm−1. The acidity of the groups positioned at the octahedral sites was evaluated as 12.5 in the pKa value while that of those on the tetrahedral sites was found to be 7.9. A charge transfer mechanism between these neighboring two types of hydroxyl groups is proposed as an explanation for these acidities.

FT-IR analysis of the photooxidation of styrene-acrylonitrile copolymers

Infrared spectroscopy was used to study the photooxidation of the styrene-acrylonitrile (SAN) copolymer samples that were exposed to ultraviolet radiation having a minimum wavelength of 290 nm. It was determined that the photooxidation of SAN occurs exclusively at the styrene repeat units within the copolymer structure. The major infrared spectral changes resulting from the degradation process involve the appearance of new peaks within the hydroxyl and carbonyl regions. Using factor analysis results, it was concluded that there are a minimum of four different types of hydroxyl groups and three different types of carbonyl groups formed during the photooxidation of SAN. Furthermore, it was found that after 200 h exposure the rate of increase in the concentration of the carbonyl groups within the degraded material is greater than the rate of increase in the concentration of the hydroxyl groups.

Hydration and preferential molecular adsorption on titanium in vitro

Surface sensitive spectroscopies, Auger electron and X-ray photoelectron (XPS), were used to determine changes in titanium oxide composition, oxide stoichiometry and adsorbed surface species as a function of exposure to human serum in a balanced electrolyte (serum/SIE) and 8.0 mM ethylenediaminetetraacetic acid in a balanced electrolyte (EDTA/SIE) at 37°C. Before immersion, the oxide was near ideal TiO 2, covered by two types of hydroxyl groups: acidic OH(s) with oxygens doubly coordinated to titanium and basic Ti-OH groups singly coordinated. After extended exposure to both solutions, up to 5000 h (ca. 208 d), the surface concentration of OH groups increased and non-elemental P appeared. The P LVV Auger transition and P 2p spectra indicated the peak positions were similar to reference phosphate compounds. The adsorbed phosphate species were presumed to be either Ti-H 2PO 4 or Ti-HPO 4 −. The XPS data suggested that a lipoprotein and/or glycolipid film was adsorbed to the specimens exposed to serum/SIE. Analysis of the preferential lipoprotein/glycolipid adsorption using electrostatic bonding concepts contributed to the refinement of the hierarchical model for the Ti-tissue interface. The salient features are that Ti metal is not in direct contact with the biological milieu, rather there is a gradual transition from the bulk metal, near-stoichiometric oxide, Ca and P substituted hydrated oxide, adsorbed lipoproteins and glycolipids, proteoglycans, collagen filaments and bundles to cells.

Polyesters from lignin. 1. The reaction of kraft lignin with dicarboxylic acid chlorides

AbstractKraft lignin was treated with dicarboxylic aliphatic (sebacoyl) or aromatic (terephthaloyl) acid chlorides in N,N‐dimethylacetamide or N‐methyl‐2‐pyrrolidone as solvents in the presence of triethylamine or pyridine as catalysts. The polyester gels obtained were characterized by IR after specific extractions and the chemical incorporation of lignin was proved unambiguously. Their thermal behaviour was determined by DSC and TGA; they are amorphous and stable up to 200°C in a nitrogen atmosphere. The reactivities of the two types of hydroxyl group (aliphatic and phenolic) were assessed from model reactions carried out in the same conditions.

Solvent dependence of intramolecular hydrogen-bonds in ethylene glycol derivatives: an FT-IR study

Abstract A number of mono-methyl ethers of (poly) ethylene glycols have been investigated by FT-IR in the gas phase as well as in dilute solutions of CCl 4 , CS 2 , CHCl 3 and CH 3 I. It appears that both nucleophilic and electrophilic solvent interaction causes a decrease in the stretching frequency of an OH-donor. The behaviour of an OH-donor in a 5-RH-bond is similar to that of a monohydroxy-OH as both types of hydroxyl groups have an “open” character towards the solvent. As the OH-proton is strongly shielded in the 8-R and 11-R conformations, the frequencies in CCl 4 , CS 2 and CH 3 I are mainly determined by the positions of the acceptors and the bulk effect of the solvents. In solution, conformations are preferred in which the OH-proton exhibits a large accessibility to the solvent.

Estimation of the distribution of surface hydroxyl groups on silica gel, using chemical modification with trichlorosilane

An estimate of the relative distribution of the free and bridged surface hydroxyl groups is made, using a surface modification with trichlorosilane according to the following main reaction Si—OH + HSiCl3→Si—O—SiHCl2+ HCl. The amount of chemisorbed chloride groups on the substrates is compared with data on the total amount of surface hydroxyl groups and with hydroxyl group information derived from infrared spectra. The interpretation of these data gives an indication of the reactivity, distribution and concentration of the different types of hydroxyl groups on the surface of silica gel.Trichlorosilane reacts mainly with bridged hydroxyl groups on silica gel, pretreated at low temperatures and exclusively with free hydroxyls at high pretreatment temperatures.Extrapolation of the reactivity of trichlorosilane towards the surface hydroxyl groups demonstrates that bridged hydroxyl groups are present up to pretretment temperatures of 973 K.

Polymerization of propylene oxide by activated monomer mechanism - kinetics

The kinetics of propagation in the polymerization of propylene oxide proceeding by the activated monomer (AM) mechanism is studied. Propagation in this system involves addition of a protonated monomer molecule to the terminal hydroxyl group of the growing oligomer. A kinetic analysis of the reaction allowed the determination of the equilibrium constant K, governing the distribution of protons between monomer and HO- groups, which in turn enables to calculate the propagation rate constant kp. This composite value has been further separated into the rate constants of four reactions, involving two different modes of ring opening in reactions with two different types of hydroxyl groups, viz. primary and secondary ones. The corresponding values are (all rate constants in mol−1 · L · s−1 and for CH2Cl2 at 25°C): K = 5,3; k1,1 = 0,170, k1,2 = 0,190, k2,1 = 0,105, k2,2 = 0,135

Type, stability, and acidity of hydroxyl groups of HNaK-erionites

The concentration and acid strength of the hydroxyl groups of a series of HNaK erionites have been investigated by means of temperature programmed activation (t.p.a.), temperature programmed desorption (t.p.d.) of ammonia, and transmission absorption i.r. spectroscopy. With an increasing degree of NH 4 + exchange, primarily Na +, at first, and, subsequently, K + cations are replaced. During activation of these materials, NH 3 desorbs in a temperature interval from 540 to 900 K, leading to hydroxyl groups. However, it is impossible to decompose NH 4 + quantitatively without dealumination, if the degree of NH 4 + exchange is higher than 85%. Five types of hydroxyl groups are present, characterized by i.r. bands at approximately 3741, 3691, 3660, 3618-3608, and 3566 cm −1. The assignment of these bands is discussed. Only the OH groups represented by the bands at 3660, 3618–3608, and 3566 cm −1 adsorb ammonia under the conditions employed. The concentration and strength of the bridging hydroxyls (3618-3608 cm −1), associated with framework Al 3+, increase with increasing degree of ion exchange.

Characterization of the acidity of zeolite Beta by FTi.r. spectroscopy and t.p.d. of NH 3

The acidity of zeolite Beta has been characterized by FTi.r. spectroscopy and temperature-programmed desorption (t.p.d.) of NH 3. Three types of hydroxyl groups, absorbing at 3740, 3602, and 3540 cm −1, have been identified and assigned to terminal/internal SiOH, framework (OH), and hydrogen-bonded (OH) groups, respectively. The influence of adsorption of NH 3, D 2O, benzene, and pyridine on these hydroxyl groups has been evaluated. From a comparison of these results with those obtained with zeolites Y and ZSM-5, the following decreasing order for the acidic strength of the framework hydroxyls is derived: ZSM-5 > Beta > Y.

In situ infrared spectroscopic investigations of platinum-loaded mordenite catalysts.

A platinum-loaded hydrogen mordenite catalyst was investigated by means of IR spectroscopy, both in vacuo and flowing hydrogen, and the effect of hydration water, pretreatment temperature, and n-heptane isomerization reaction on the hydroxyl groups were reported. At least three different types of hydroxyl groups having absorption frequencies of 3734, 3660(s), and 3595cm-1 existed on the surface. The evolution of the 3595-cm-1 band as a function of temperature, during hydrogen activation, was interpreted from the view point of catalytic dehydroxylation mechanism. The hydroxyl groups associated with this band interacted preferentially with heptane and represented more likely the potentially active centers. The nature of surface acidity was also studied, using pyridine as a probe molecule. Both Bronsted and Lewis acid sites were detected, and their temperature dependence was discussed.

Nature of acid sites in SAPO5 molecular sieves. Part 1.—Effects of the concentration of incorporated silicon

Three SAPO5 samples of similar chemical composition, but with varying degrees of the concentration of silicon incorporated and hence varying concentrations of amorphous material, have been characterized by i.r. spectroscopy, temperature-programmed desorption and scanning electron microscopy. Several types of hydroxyl groups were observed and attributed to SiOH groups (3745 cm–1), POH groups (3678 cm–1) and SiOHAl groups (3626, 3605 and 3518 cm–1). The relative intensities of the latter bands increased with increasing crystallinity of the material. The band at 3626 cm–1 was attributed to SiOHAl groups pointing into the twelve-membered ring. Because the band at 3518 cm–1 disappeared after the material was put in contact with ammonia, but not benzene, and because of its low wave-number, we conclude that the corresponding hydroxyl group is located in a six-membered ring. The shoulder at 3605 cm–1 could not be assigned unequivocally. These three strong Brönsted-acid sites are caused by formal substitution of phosphorus by silicon in the ALPO5 lattice. Their acid strength is considerably lower than that of the Brönsted-acidic hydroxyl groups in high-silica zeolites and does not depend upon the concentration of silicon incorporated into the lattice or the amorphous phases present.

A method for the quantitative determination of the basic hydroxyl groups on an alumina surface

It is shown that with alumina both F − exchange and reaction with MoO 2(acac) 2 (acac = acetylacetonate) occur selectively with one type of hydroxyl group only: the one corresponding to the high-frequency (3775 cm −1) band in the IR spectrum of the alumina. In view of the chemistry involved—F −: anion exchange; MoO 2(acac) 2: very rapid reaction with OH −, no reaction with H +—this band must be due to the basic hydroxyl groups. It is established that the number of these groups can be determined via the reaction with MoO 2(acac) 2. VO(acac) 2 behaves in very much the same way toward alumina as MoO 2(acac) 2.

13C‐NMR spectral studies on the distribution of substituents in some cellulose derivatives

Abstract13C‐NMR spectra of trityl cellulose (Tr‐Cell), tosyl cellulose (Ts‐Cell), cellulose S‐methyl xanthate (Cell‐M‐Xan), and cellulose formate (CF) in dimethylsulfoxide‐d6 were analyzed at 50.4 MHz. It was found that the distribution of substituents in the anhydroglucose units of these cellulose derivatives can be estimated from their ring carbon spectra. The results showed that (i) in Tr‐Cell having degree of substitution (DS) lower than 1, the hydroxyl groups at C‐6 carbon position are selectively tritylated, (ii) in the case of Ts‐Cell, the difference in the relative DS value among three different types of hydroxyl groups is not large, although the relative reactivities of hydroxyl groups toward tosylation decrease in the order C‐6 > C‐2 > C‐3, (iii) in Cell‐M‐Xan, the hydroxyl groups at C‐3 carbon position are mainly substituted, and (iv) the ease of formylation is C‐6 > C‐2 > C‐3. The 100.8 MHz 13C‐NMR spectra of O‐methyl cellulose (MC) revealed that the reactivity order in commercial MC prepared from alkali cellulose is C‐6 ⋍ C‐2 > C‐3. Concerning MC, its water solubility was also discussed in terms of the distribution of substituents along the cellulose chain.

Infrared studies of adsorbed species on rutile at the solid—vapour and solid—liquid interfaces

Infrared spectra of rutile confirm the existence of several types of surface hydroxyl group with different reactivities towards adsorbate molecules. One type of hydroxyl group is unreactive towards adsorbates and exists at sub-surface lattice sites. Water, amines and carbonyl compounds are adsorbed in part on rutile through coordinative interactions with exposed Ti 4+ ions which act as Lewis acidic surface sites. Oxide ions on rutile exhibit basic or nucleophilic character and chemisorption reactions often involve the oxidation of adsorbate molecules. The formation of oxidation products on the surface was promoted by the pretreatment of rutile in hydrogen at high temperature. In contrast, reduction in hydrogen decreased the Lewis acidity of the rutile surface. Reaction of rutile with silicon tetrachloride and water led to a modified surface which exhibited Brønsted acidity in the presence of adsorbed pyridine. Infrared spectra of rutile immersed in solutions with liquid hydrocarbon as the predominant component suggest that the general characteristics of the surface are unchanged by the transfer from the solid/gas to the solid/liquid interface.

A fusion method for the preparation of steroid sulfates

A method for the preparation of steroid triethylammonium sulfates is outlined which involves the fusion of triethylamine-sulfur trioxide and steroids. Experimental details are presented which define the process as a thermal equilibrium resulting in the preferential sulfation of aliphatic hydroxyl groups. Sulfation of an aromatic hydroxyl group can be achieved in the absence of an aliphatic hydroxyl group. With excess reagent both types of hydroxyl groups in the same molecule can be sulfated.

13C nuclear magnetic resonance studies of cellulose acetate

Abstract13C‐NMR spectra of ring carbons and O‐acetyl carbonyl carbons of cellulose acetate (CA) in dimethyl sulfoxide‐d6 were analyzed. The CA samples with the degree of substitution (DS) ranging from 0.84 and 1.91 were prepared by homogeneous acetylation of cellulose with acetic anhydride in a 10% LiCl/dimethyl acetamide solvent. It was found that the use of these low DS samples permitted easier assignments not only of the ring carbon but also of the O‐acetyl carbonyl carbon signals. The assignments were confirmed by comparing with the 1H‐NMR spectra of the samples obtained by complete acetylation of the corresponding CA samples with acetyl‐d3 chloride. Two methods for determining the distribution of O‐acetyl groups of CA, i.e., the relative DS at the three different types of hydroxyl groups, were developed. One is based on the measurements of the relative intensities of the signals for the ring carbons and the other is based on the measurements of the relative intensities of the signals for the O‐acetyl carbonyl carbons.

Infrared study of the adsorption of triethylamine at the rutile/vapour and rutile/heptane interfaces

The non-dissociative adsorption of triethylamine on rutile involves the formation of hydrogen bonds between surface hydroxyl groups and triethylamine molecules and coordinative interactions between triethylamine and Lewis-acidic surface sites. There is no evidence for the Bronsted acidity of hydroxyl groups.Dissociative chemisorption of triethylamine involves surface hydroxyl groups and generates adsorbed diethylamine, ethoxide ions and acetate ions as products. One type of hydroxyl group is completely unreactive towards triethylamine.

Observation of surface characteristics by DSC and DTA

The three types of hydroxyl groups on surfaces of goethite crystals have previously been shown to be distinguishable by inflections on the leading edge of the dehydroxylation endotherm on DSC curves. The same inflections can be observed on DTA curves and it is now established that the main prerequisite for their observation is rapid removal of water vapour from particle surfaces, although small particle size (i.e. large surface area) and uncontaminated surfaces are also important.

ChemInform Abstract: INFRARED STUDY OF THE ADSORPTION OF ETHYL ACETATE ON SILICA IMMERSED IN CARBON TETRACHLORIDE

The adsorption of ethyl acetate onto silica immersed in carbon tetrachloride has been studied by infrared spectroscopy. For silica which had been preheated at 863 K adsorption involved the formation of hydrogen bonds between isolated surface silanol groups and the oxygen atoms of carbonyl groups in ethyl acetate molecules. For silica preheated at 453 K the existence of other types of hydroxyl group on the oxide surface did not affect the adsorption of ethyl acetate onto isolated silanol group sites. However, a second mode of adsorption involved the formation of hydrogen bonds between pairs of adjacent hydroxyl groups and single ethyl acetate molecules. The adsorption of ethyl acetate onto pair sites constituted a stronger mode of adsorption than that involving single silanol groups and was particularly significant at low surface coverages.

Infrared, microcalorimetric, and electron spin resonance investigations of the acidic properties of the H-ZSM-5 zeolite

The infrared spectra of the H-ZSM-5 zeolite calcined at temperatures up to 1173 K, and the corresponding electron spin resonance and microcalorimetric data are discussed. Two types of hydroxyl groups are characterized by absorption bands at 3720 and 3605 cm−1, a small shoulder being present at 3665 cm−1. Infrared spectra were also recorded after pyridine adsorption, showing the presence of Brønsted acid sites and Lewis acid sites. After calcination at increasing temperatures, dehydroxylation of the zeolite is observed: above 675 K, the number of Brønsted acid sites decreases, while that of strong Lewis acid sites increases; however, a small dealumination occurs as shown by chemical analysis measurements and XPS data. Moreover, dehydroxylation enhances the constraint character of this zeolite, as observed by pyridine inability to titrate the total Lewis sites. Electron spin resonance studies of trapped hydrogen atoms, of adsorbed NO, and of adsorbed benzene radical cations formed on H-ZSM-5 at various calcination temperatures are discussed in terms of the number and strength of the acid sites. It is suggested that the acid sites which are present are very similar to those of H-mordenite although slightly stronger. A microcalorimetric study of ammonia adsorption confirms the very strong acidic character of the acid sites and shows their dependence in strength and heterogeneity upon calcination temperature.