Liquid structure of acetic acid-water and trifluoroacetic acid-water mixtures studied by large-angle X-ray scattering and NMR

The structures of acetic acid (AA), trifluoroacetic acid (TFA), and their aqueous mixtures over the entire range of acid mole fraction xA have been investigated by using large-angle X-ray scattering (LAXS) and NMR techniques. The results from the LAXS experiments have shown that acetic acid molecules mainly form a chain structure via hydrogen bonding in the pure liquid. In acetic acid-water mixtures hydrogen bonds of acetic acid-water and water-water gradually increase with decreasing xA, while the chain structure of acetic acid molecules is moderately ruptured. Hydrogen bonds among water molecules are remarkably formed in acetic acid-water mixtures at xA相似文献   

Solubility of benzoic acid in acetone, 2-propanol,acetic acid and cyclohexane: Experimental measurement and thermodynamic modeling

In this study the solubility of benzoic acid in acetone, 2-propanol, acetic acid and cyclohexane was experimentally determined over the temperature range of (277–346) K at around 5 K intervals by employing a gravimetric method. The experimental results showed that acetone had the best solubility for benzoic acid followed by 2-propanol, acetic acid and cyclohexane and the solubility in acetic acid had the strongest positive temperature dependency. The newly measured solubility data was also compared with the available literature data. Several commonly used thermodynamic models, including the empirical Van’t Hoff equation, the λh equation, the Wilson, NRTL and UNIQUAC equations, were applied to correlate the experimental solubility data. The adjustable parameters for each model were optimized by fitting the solubility data measured in this work. It was found that the three-parameter NRTL equation could give the best correlation results. Better predictions of the solubility of benzoic acid in acetic acid at higher temperature were observed using the Wilson and UNIQUAC equations than the other three equations which all gave underestimations.     

Ethanol, acetic acid, and water adsorption from binary and ternary liquid mixtures on high-silica zeolites

Adsorption isotherms were measured for ethanol, acetic acid, and water adsorbed on high-silica ZSM-5 zeolite powder from binary and ternary liquid mixtures at room temperature. Ethanol and water adsorption on two high-silica ZSM-5 zeolites with different aluminum contents and a high-silica beta zeolite were also compared. The amounts adsorbed were measured using a recently developed technique that accurately measures the changes in adsorbent/liquid mixture density and liquid concentration. This technique allows the adsorption of each compound in a liquid mixture to be measured. Adsorption data for binary mixtures were fit with the dual-site extended Langmuir model, and the parameters were used to predict ternary adsorption isotherms for each compound with reasonable accuracy. In ternary mixtures, acetic acid competed with ethanol and water for adsorption sites and reduced ethanol adsorption more than it reduced water adsorption.     

Dissociation Constant of Acetic Acid in (N,N-Dimethylformamide + Water) Mixtures at the Temperature 298.15 K

In the present work the thermodynamic dissociation constants of acetic acid were determined in (N,N-dimethylformamide (DMF) + water) mixtures over the DMF mole fraction range from 0 to 0.65 at the temperature 298.15 K by the potentiometric titration method. The dissociation constant in pure DMF was obtained by extrapolation and comparative calculation methods. The dependence of the acetic acid dissociation constant on the mixed solvent composition was fitted with linear multiple regression of the solvatochromic parameters of (DMF + water) mixtures at every studied composition.     

Structure of aqueous solutions of sucrose,derived from viscosimetry data and IR spectroscopy

The structural features of aqueous sucrose solutions are investigated via viscosimetry and IR spectroscopy in the 293.15–323.5 K range of temperatures at concentrations of 0–60%. Concentration dependences of the activation parameters of viscous flow, structural temperature, and energy and length of intermolecular hydrogen bonds are calculated based on the experimental data. It is shown that the solution became more structured with an increase in the sucrose concentration.     

3. Acetate manufacturing,process and technology 3.1 Chemistry of cellulose acetylation

Nowadays Celluloseacetate is mainly produced with the acetic acid process. After an activation with acetic acid and sulfuric acid the acetylation of the cellulose starts by adding acetic anhydride. The temperature and the catalyst concentration play an important role for the reaction. Beneath acetylation also degradation of the cellulose chains occurs. In the first step of the process cellulosetriacetate is formed. In a second step, the hydrolysis, several acetyl groups are removed to achieve an average degree of substitution of 2,5. The water content in this step influences the acetyl distribution.     

Comparative analysis of molecular interactions in quaternary fluid system performed by classical and ab initio molecular dynamics

Molecular interactions in the quaternary fluid system acetic acid–n-propanol–n-propyl acetate–water were analyzed by classical and ab initio molecular dynamics methods. It was shown that ab initio molecular dynamics simulation can reproduce the molecular mobility tendency and structural features of a multicomponent system without empirical parameters.     

Liquid–liquid equilibria of water/acetic acid/1-butanol system — effects of sodium (potassium) chloride and correlations

Sodium and potassium chloride were experimentally shown to be effective in modifying the liquid–liquid equilibrium (LLE) of water/acetic acid/1-butanol system in favour of the solvent extraction of acetic acid from an aqueous solution with 1-butanol, particularly at high salt concentrations. Both the salts enlarged the area of the two-phase region; decreased the mutual solubilities of water and marginally decreased the concentrations of 1-butanol and acetic acid in the aqueous phase while significantly increased the concentrations of the same components in the organic phase. These effects essentially increased the heterogeneity of the system, which is an important consideration in designing a solvent extraction process. The equilibrium data were well correlated by Eisen–Joffe equation with respect to the overall molar ratio of salt to water in the liquid phases. By expressing the salt–solvent interaction parameters as a third order polynomial of salt concentration in the liquid phase, Tan's modified NRTL model [T.C. Tan, Trans. Inst. Chem. Eng., Part A 68 (1990) 93–103.] for solvent mixtures containing salts or dissolved non-volatile solutes was able to provide good correlation of the present LLE data. Using the regressed salt concentration coefficients for the salt–solvent interaction parameters and the solvent–solvent interaction parameters obtained from the same system without salt, the calculated phase equilibria compared satisfactorily well with the experimental data.     

A vibrational sum frequency spectroscopy study of the liquid-gas interface of acetic acid-water mixtures: 1. Surface speciation

Aqueous acetic acid solutions have been studied by vibrational sum frequency spectroscopy (VSFS) in order to acquire molecular information about the liquid-gas interface. The concentration range 0-100% acetic acid has been studied in the CH/OH and the C-O/C=O regions, and in order to clarify peak assignments, experiments with deuterated acetic acid and water have also been performed. Throughout the whole concentration range, the acetic acid is proven to be protonated. It is explicitly shown that the structure of a water surface becomes disrupted even at small additions of acetic acid. Furthermore, the spectral evolution upon increasing the concentration of acetic acid is explained in terms of the different complexes of acetic acid molecules, such as the hydrated monomer, linear dimer, and cyclic dimer. In the C=O region, the hydrated monomer is concluded to give rise to the sum frequency (SF) signal, and in the CH region, the cyclic dimer contributes to the signal as well. The combination of results from the CH/OH and the C-O/C=O regions allows a thorough characterization of the behavior of the acetic acid molecules at the interface to be obtained.     

Redox polymerization-kinetics of the reaction inititated by the system CH2(CH)2?MN(III) in aqueous H2SO4 and the system CH2(CN)2?MN(OAC)3 in DMF and acetic acid

Polymerization of acrylonitrile and methyl methacrylate by the redox systems propanedinitrile-Mn(III) in aqueous sulfuric acid and propanedinitrile-Mn(OAc)₃ in DMF and glacial acetic acid was investigated in the temperature range 20–40°C. The kinetics are consistent with the formation of an intermediate complex whose irreversible decomposition yields the initiating radical. With both monomers mutual termination predominates. The main difference between aqueous H₂SO₄ and acetic acid compared with DMF lies in the ease of oxidation of the primary radicals by the oxidant. Rate and equilibrium parameters were estimated and their significance discussed.     

Liquid-vapor distribution of amines and acid ionization constants of their ammonium salts in aqueous systems at high temperature

Ammonia and volatile organic amines are used to treat the water in steam generating systems to afford protection of the steam- and feedwater circuits from corrosion by acidic condensate. The base strength and the volatility of the reagent used are two important parameters that have bearing on its ability to inhibit corrosion; they determine the pH of the solution produced from condensing steam. The distribution of thirteen amines between water and steam in the temperature range 150 to 325°C was studied. The amines were chosen on the basis of their known room-temperature base strength. The acid ionization constant of the ammonium salts of seven of these amines was determined by conductivity measurement in the temperature range 25 to 275°C. Amines with high room-temperature base strength and distribution coefficient in the optimum range were chosen for the high-temperature base strength determination. The results of these studies were used to estimate the effectiveness of these amines in raising the pH of the condensate from steam containing carbon dioxide and acetic acid, to provide the basis for further studies on the corrosion inhibition properties of the amines.     

Comparative Analysis of Acyclovir Esters Stability in Solutions: The Influence of the Substituent Structure,Kinetics, and Steric Effects

Reversed‐phase high‐performance liquid chromatography has been applied to the determination of acyclovir (ACV) esters such as acetate, isobutyrate, pivalate, ethoxycarbonate, and nicotinate. All analyses were carried out at laboratory temperature using a column LiChrospher RP‐18 (250 × 4 mm, 5 µm) and a proper mobile phase consisting of acetonitrile and phosphate buffer (pH 6 or 6.7) or acetonitrile and potassium dihydrogen phosphate, and acetic acid. The methods were validated by the determination of the following parameters: selectivity, precision, accuracy, and linearity. Kinetic studies on the hydrolysis were investigated in solutions at 310 K over the pH range 0.42–1.38. The pH‐profiles indicated specific acid‐catalyzed and spontaneous water‐catalyzed degradation. The stability of the studied ACV esters were determined not only by steric factors. In the case of ethoxycarbonyl ester of ACV, the hydrolysis was a two‐step reaction.     

Synthesis and kinetic study of hydrolysis of di‐2‐chloroaniline phosphate ester in acidic medium

The kinetics and mechanism of the acid‐catalyzed hydrolysis of di‐2‐chloroaniline phosphate ester were studied in 0.5–7.0 mol dm^?3 hydrochloric acid at 80°C in 20/80 (v/v) dioxane–water medium. The log rate profile shows rate maximum at 4.0 mol dm^?3 hydrochloric acid. The results show that di‐2‐chloroaniline phosphate is reactive mainly via conjugate acid species. Ionic strength data show a positive salt effect. The effect of different parameters such as temperature, solvent, and substrate concentration on the rate of hydrolysis was studied. Molecularity and order of the reaction have been supported by Arrhenius parameters, the Zucker–Hammett hypothesis. The rate values shown by experimental and theoretical computational hold good agreements in the entire acid range. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 126–131, 2010     

Kinetic Study of Empty Fruit Bunch Using Hot Liquid Water and Dilute Acid

Empty fruit bunch (EFB), a residual product of the palm plantation, is an attractive biomass for biorefinery. As xylan is susceptible to high temperature pretreatment, it is important to setup a proper pretreatment condition to maximize the sugar recovery from EFB. Kinetic parameters of mathematical models were obtained in order to predict the concentration of xylose, glucose, furfural, and acetic acid in the hydrolysate and to find production conditions of xylose. We investigated the kinetics of hot liquid water and dilute sulfuric acid hydrolysis over a 40-min period using a self-designed setup by measuring the concentrations of released sugars (xylose, glucose) and degradation products (acetic acid and furfural). The reaction was performed within the range 160～180 °C, under reaction conditions of various concentration of sulfuric acid (0.1～0.2%) and 1:7 solid-liquid ratio in a batch reactor. The kinetic constants can be expressed by the Arrhenius equation with the activation energy for the hydrolysis of sugar and decomposition of sugar. The activation energy of xylose was determined to be 136.2187 kJ mol(-1).     

基于苯并硫氮杂卓的亚胺、烯胺在质子溶剂中的稳定性及其互变异构

用核磁共振法研究了亚胺型杂卓1和烯胺型杂卓2在不同质子溶剂—甲醇、乙醇、叔丁醇、乙酸及CF3CO2D的DMSO-d6溶液中的稳定性及互变异构.结果表明,烯胺型杂卓2在上述溶剂中稳定,亚胺型杂卓1在酸性较弱的质子溶剂中(乙醇、叔丁醇)及较低的温度下(≤30℃)基本稳定,在酸性较强的质子溶剂中(甲醇、乙酸、CF3CO2D的DMSO-d6溶液)向烯胺转化,而且随着溶剂酸性的增强、温度的升高以及反应物与溶剂接触时间的延长,亚胺向烯胺的转化进一步加强.本文还以1a和1b为例,研究了在乙酸溶液中亚胺型杂卓向烯胺型杂卓转化的热力学和动力学特征.研究表明,在283~333 K范围内,该转化的焓变(△H)和熵变(△S)均大于零,吉布斯自由能变(△G)小于零,因此其是吸热、自发及熵增加的过程,该过程可用二级动力学方程来描述,质子对亚胺向烯胺的转化起催化作用.     

Solubility of terephthalic acid in aqueous acetic acid from 423.15 to 513.15 K

Using the steady-state method, the solubilities of terephthalic acid(1) in binary acetic acid(2) + water(3) solvent mixtures in a specially contrived vessel have been measured as a function of temperature in the temperature range 423.15–513.15 K and solvent composition range from x₂ = 1.000 to 0.3103 (molar fraction). The experimental solubilities are correlated with the Apelblat equation. The calculated results show good agreement with the experimental solubilities.     

Development of kinetic models for acid‐catalyzed methyl acetate formation reaction: Effect of catalyst concentration and water inhibition

The reaction kinetics of reversible liquid‐phase esterification of acetic acid with methanol is investigated in the temperature range 26–50°C using sulfuric acid catalyst. The main goal of this work is to study the effect of catalyst concentration and sensitivity to the presence of water on the rate expression of this industrially important reaction. Experiments are conducted in an isothermal batch reactor and a second‐order kinetic model is used to correlate the experimental data, which are found to fit well with the assumed kinetic model in terms of the concentrations of reactants and products. Furthermore, an activity‐based kinetic model is also developed employing the UNIQUAC (universal quasi‐chemical equation) model to compute the activities. It is observed that the rate constant is influenced by the concentration of catalyst, and the reaction rate increased with an increase in the catalyst concentration. It is also observed that the catalyst activity is slightly inhibited by the water present in the reaction mixture. The performance of the proposed models is compared with that of other models reported in the literature, and it is found that the proposed models outperformed all the other models reported in the literature. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 263–277, 2011     

Hydrogen Bonding of Carboxylic Acids in Aqueous Solutions—UV Spectroscopy, Viscosity, and Molecular Simulation of Acetic Acid

The UV spectra of aqueous acetic acid solutions up to 2M were investigated. At these wavelengths, the carboxylic acids exhibit an absorption peak, attributed to the C=O group, which shifts when hydrogen bonds are formed.. The measured spectra were best fitted to several bands, either of Gaussian or Lorentzian shape, which can be explained as several types of structural units formed by hydrogen bonds established between acetic acid and water molecules and between acetic acid molecules themselves. Molecular dynamics simulation of these mixtures was also performed, confirming the occurrence of several types of hydrogen bonds and showing the presence of dimers at higher concentrations. The viscosity and density of these solutions were also measured at different concentrations and temperatures. These results give a more complete picture of the hydrogen bond network of the system.     

Electric conductivity of concentrated aqueous solutions of propionic acid, sodium propionate and their mixtures

Specific electric conductivity (EC) of concentrated aqueous solutions of propionic acid (PA), sodium propionate (SP), and water/PA/SP mixtures is measured in the temperature range of 15–90°C. Specific EC passes a maximum at the increase in the electrolyte concentration in the mixtures of water/PA, water/SP, and water/PA/SP containing a similar PA concentration. The maximum EC value of the aqueous PA solution at the given temperature is used as the generalizing term. It is shown that the values of reduced EC (ratio of EC and its maximum value at the given temperature) fall on a single curve in the whole studied range of temperatures and concentrations of the water/PA mixture. The EC activation energy is calculated for all the studied solutions. It is found that the EC activation energy of these solutions decreases at the temperature increase and grows at the increase of the concentration of electrolyte.     

乙酸、卤代乙酸在水溶液中电离焓的测量和热力学过程的研究

本文用LKB-2277BioActivityMonitor以5℃的间隔测量了20-45℃温度区间乙酸、氯乙酸、溴乙酸和碘乙酸在水溶液中的电离焓。通过每一温度下测量五个不同浓度酸的热效应并外推至酸浓度无限稀,得到了电离焓ΔH~i^ⅲ。实验结果表明,在实验温度范围内,ΔH~i^ⅲ为:乙酸>氯乙酸>溴乙酸>碘乙酸;上述各个酸的ΔH~i^ⅲ跟温度呈线性变化的关系。本文用溶液中溶质与溶剂的相互作用及"内部-环境"模型较圆满地解释了取代基和温度对乙酸、氯乙酸、溴乙酸和碘乙酸的电离焓和电离熵的影响。