Kinetic studies of alkyl formate hydrolysis using formic acid as a catalyst

BACKGROUND: The hydrolysis of methyl formate is the major industrial process for the production of formic acid. The aim of the work is to determine the reaction kinetics quantitatively in the presence of formic acid catalyst, develop a mathematical model for the reaction system and estimate the kinetic parameters for the purpose of optimization. RESULTS: Liquid phase hydrolysis kinetics of alkyl formates (ethyl and methyl formate) was studied in an isothermal batch reactor at 80–110 °C and 20 bar nitrogen pressure. The catalyst of choice was formic acid. The reaction rate was enhanced but the formic acid product yield was slightly suppressed relative to the uncatalysed system. A kinetic model comprising mass balances and rate equations was developed and the kinetic and equilibrium parameters included in the rate equations were estimated from the experimental data with non‐linear regression analysis. CONCLUSION: The model was able to predict the experimental results successfully. The results obtained were compared quantitatively with an earlier model involving alkyl formate hydrolysis in a neutral aqueous solution. Copyright © 2011 Society of Chemical Industry     

Kinetic study of urease-catalysed urea hydrolysis

The kinetics of the urease-catalysed hydrolysis of urea in phosphate and citrate buffers have been studied. The effects of urease concentration, substrate concentration, pH and temperature of solutions, and particularly the concentrations of buffers and product on the reaction rate of urea hydrolysis have been examined. The activation energies in two buffer systems indicate that the reaction mechanism catalysed by urease may be the same in each case. A substrate inhibition model has been developed that describes the effect of substrate concentration ranging from 0.001 to 4 mol dm^?3. The experimental data indicated that the inhibition of urease by phosphate is of partially mixed type, whilst that by citrate is uncompetitive. In contrast ammonium ions exhibited no significant influence on the rate of urea hydrolysis by urease.     

Kinetic study of the enantioselective hydrolysis of a meso‐diester using pig liver esterase

A kinetic study of the hydrolysis of the diester dimethyl cis‐cyclohex‐4‐ene‐1,2‐dicarboxylate, to the (1S,2R)‐monoester, catalysed by the enzyme Pig Liver Esterase (PLE) was performed. The effects of the most relevant parameters that influence the enzymatic conversion were studied, such as pH, temperature and concentration of substrate and reaction products. It was concluded that the pH at which the enzyme exhibits a maximum activity is pH 7. At 25 °C PLE presents a better long‐term stability and enantioselectivity than at higher temperatures, although the reaction rate is slower. The kinetic results obtained are well described by the Michaelis–Menten equation, although a slight deviation to this model was observed for low substrate concentrations. Methanol, a co‐product of the enzymatic hydrolysis, was found to act as a non‐competitive inhibitor of the reaction. The Michaelis–Menten parameters were determined and a comprehensive kinetic model, which already accounts for methanol inhibition, is presented. © 2000 Society of Chemical Industry     

Kinetic study of cellulose hydrolysis with tungsten-based acid catalysts

Tungsten plays an important role in transforming cellulose to C2 C3 polyols. In previous reports, the research focus was mainly on the C C cleavage reactions of cellulose catalyzed by various tungsten-containing catalysts, but less on its catalytic role in cellulose hydrolysis although it is usually considered as the rate-determining step in cellulose conversion. In this article, the method of determining kinetics parameters for hydrolyzing cellulose into glucose was developed. The effects of reaction temperature, different tungsten-based acid catalysts, and H⁺ concentration on reaction rate of hydrolyzing cellulose into glucose were quantitatively addressed. The relevant reaction rate equations with using H₃O₄₀PW₁₂, H₄O₄₀SiW₁₂, and H₂WO₄ as tungsten acid catalysts were obtained in developed batch continuous stirred tank reactors and validated by experimental data. The simulating analysis indicates that the reaction mechanism of cellulose hydrolysis can change with the temperature. H₃O₄₀PW₁₂ is the best candidate catalyst for obtaining the maximum glucose concentration.     

pH-stat methodology in continuous monitoring of the kinetics of hydrolysis of phosphate esters catalysed by alkaline phosphatase from human placenta: II. Kinetic aspects

The adaptation of the pH-stat to continuous monitoring of the in-vitro hydrolase activity of alkaline phosphatase in solution, an activity which until now has only been analysed by means of spectrophotometric methods in a continuous or static state, is described. To control the reliability of the method in the monitoring of these enzymic reactions, a series of kinetics of hydrolysis of chromogenic (p-nitrophenyl and o-carboxyphenyl phosphates) and non-chromogenic (ATP) substrates was carried out, comparing the results obtained via continuous spectrophotometric analysis of the corresponding phenol released or via a discontinuous technique by the phosphate produced with those results obtained via pH-stat analysis of the H⁺ released or utilized by the enzymic reaction of hydrolysis. It can be concluded that kinetic studies carried out on the pH-stat of in-vitro alkaline phosphatase activity offer results analogous to those obtained for the same system through classic spectrophotometric methods, offering as well notable advantages of speed and simplicity in the kinetic assays since it is always possible to monitor the enzymic activity continuously, with chromogenic or non-chromogenic substrates. The pH-stat methodology affords additional information on the kinetics of action of alkaline phosphatase from placenta acting on non-chromogenic biological substrates. In this sense, kinetic studies with the pH-stat on the enzymic hydrolysis of ATP were begun, determining their rate versus pH profile (optimum pH at 9.1) and proposing possible ‘non-Michaelian’ type kinetic behaviour of the enzyme in solution as deduced from the graphical analysis of the data v([ATP]).     

Enzymatic hydrolysis and detection of 3-monochloropropane-1,2-diol esters

There are more and more studies on the detection method of 3-chloro-1,2-propanediol fatty acid esters (3-MCPD esters) at present, by comparing these methods for the determination of 3-MCPD esters. Indirect methods, which determine total amount of 3-MCPD after hydrolysis of the esters, have an advantage over direct methods. The existing indirect methods, however, may yield unreliable results or require long hours of alkaline methanolysis. In contrast, the indirect enzymatic hydrolysis method has mild conditions and more accurate results. In this study, we developed a reliable and rapid indirect method for determination of 3-MCPD esters. 3-MCPD esters were enzymolysis to 3-MCPD by indirect enzymatic hydrolysis method, and the conditions of enzymatic hydrolysis were optimized, the content of 3-MCPD after enzymatic hydrolysis was detected by gas chromatography–mass spectrometry (GC–MS) and the yield was calculated. Finally, the optimum conditions for enzymatic hydrolysis of 3-MCPD esters were determined. According to the optimal enzymatic hydrolysis condition, the contents of 3-MCPD esters in four food oils were determined. The method is simple and sensitive and can meet the requirement of 3-MCPD esters detection in general oils.     

Kinetic modeling of hemicellulose hydrolysis in the presence of homogeneous and heterogeneous catalysts

Kinetic models were developed for the hydrolysis of O‐acetyl‐galactoglucomannan (GGM), a hemicellulose appearing in coniferous trees. Homogeneous and heterogeneous acid catalysts hydrolyze GGM at about 90°C to the monomeric sugars galactose, glucose, and mannose. In the presence of homogeneous catalysts, such as HCl, H₂SO₄, oxalic acid, and trifluoroacetic acid, the hydrolysis process shows a regular kinetic behavior, while a prominent autocatalytic effect was observed in the presence of heterogeneous cation‐exchange catalysts, Amberlyst 15 and Smopex 101. The kinetic models proposed were based on the reactivities of the nonhydrolyzed sugar units and the increase of the rate constant (for heterogeneous catalysts) as the reaction progresses and the degree of polymerization decreases. General kinetic models were derived and special cases of them were considered in detail, by deriving analytical solutions for product distributions. The kinetic parameters, describing the autocatalytic effect were determined by nonlinear regression analysis. The kinetic model described very well the overall kinetics, as well as the product distribution in the hydrolysis of water soluble GGM by homogeneous and heterogeneous catalysts. The modelling principles developed in the work can be in principle applied to hydrolysis of similar hemicelluloses as well as starch and cellulose. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1066–1077, 2014     

有机相酶法不对称水解乳酸乙酯

探讨了在有机介质中酶催化外消旋乳酸乙酯不对称水解制备L-乳酸的可行性。 研究了反应介质、不同来源的酶、摇床转速、水含量、乳酸乙酯浓度及温度等因素对反应的影响。结果表明,N435的活性和选择性较高,叔丁醇和异辛烷的混合溶剂（体积比为1∶1）为最合适的反应介质,其余最适条件为：摇床转数200 r•min^-1、水含量（水与底物的质量比）1∶5、乳酸乙酯浓度0.27 g•ml^-1、酶浓0.8 g•mol^-1、反应温度60℃,在此条件下反应16 h得产物ee值为90.02% ,产率为28.69%。 最后研究了乳酸乙酯不对称水解的动力学,结果表明反应符合米氏方程,属于双底物抑制的双乒乓机制,并考察了D-乳酸乙酯和D-乳酸在建立反应动力学模型时的影响。     

Kinetic modelling of the hydrolysis of carbonyl sulfide catalyzed by either titania or alumina

The reaction rates for the hydrolysis of COS on both alumina (Kaiser 201) and titania (CRS 31) were measured from 270 to 330°C using a laboratory Berty reactor over ranges of water concentration, 2-12 mol %, and COS concentrations, 0.5-2 mol %. Four mechanism-related model functions were tested in the search for the best fitting function by using nonlinear regression techniques. The results indicate that an Eley-Rideal model provides the best fitting rate function for the hydrolysis of COS for both alumina and titania, even though their catalytic activities are different. The Bates and Watt's (1980) curvature measure along with simulation studies were used in the nonlinear regression search for the best values of the model parameters. The kinetic parameters obtained confirm that CRS 31 appears to be the better of the two catalysts for COS hydrolysis. The adsorption of water vapor significantly retards the hydrolysis rate, more so for Kaiser 201.     

Kinetic modeling of the hydrolysis of carbon disulfide catalyzed by either titania or alumina

The reaction rates for the hydrolysis of CS₂ on either alumina (Kaiser 201) or titania (CRS 31) were measured at 150 kPa and from 270 to 330°C using a laboratory Berty reactor over ranges of water concentration (2–12 mol%) and CS₂ concentration (0.5–2 mol%). Four mechanism-related model functions were tested in the search for the best-fitting rate function by using non-linear regression techniques. The results show that the Eley-Rideal rate function best-fitted the kinetics using either Kaiser 201 or CRS 31 with different parameters. The kinetic parameters obtained confirm that CRS 31 appears to have the higher activity of the two catalysts for CS₂ hydrolysis. The strong adsorption of water vapor on each of the two catalysts studied inhibits the hydrolysis rate and affords an effective reaction order close to zero for H₂O, possibly by saturating the catalysis surface and reducing the fraction of catalyst surface available for CS₂ and COS adsorption.     

Kinetic studies of xylan hydrolysis of corn stover in a dilute acid cycle spray flow-through reactor

Xylan of corn stover was pretreated with 1%, 2% and 3% (w/w) sulfuric acid at relatively low temperatures (90°C, 95°C and 100°C) in a dilute acid cycle spray flow-through reactor (DCF). The hydrolysis of xylan to its monomeric xylose was modeled by a series of first-order reactions. Both biphasic and Saeman hydrolysis models were applied to fit the experimental data. The results confirmed that the kinetic data of xylan hydrolysis fitted a first-order irreversible reaction model and the experimental data. The reaction rates of xylose monomer formation and degradation were sensitive to catalyst concentration and temperature. Higher catalyst concentration and lower reaction temperature result in high xylose yield. The activation energy for xylose formation and degradation were determined to be 112.9 and 101.0 kJ·mol^-1, respectively. Over 90% theoretical xylose obtained from corn stover can be used to produce ethanol, xylitol and fumaric acid by fermentation.     

pH-stat methodology in continuous monitoring of the kinetics of hydrolysis of phosphate esters catalysed by alkaline phosphatase from human placenta: Limitations,advantages and theoretical aspects

The appropriateness of pH-stat methodology in the dynamic monitoring of the kinetics of hydrolysis of phosphate esters catalysed by alkaline phosphatase (from human placenta) has been studied. This methodology involves autotitration of the H⁺ released or consumed during a given reaction time. The lack of specificity of the method, titrating the total H⁺, together with the known effect of the dissolution of environmental CO₂ on the reacting system, giving rise to carbonic acid, implies an overtitration of the H⁺ released in the hydrolysis reaction. Because of this, parallel to each series of reaction kinetics, the dissolution kinetics of environmental CO₂ under working conditions are quantified with suitable blanks, thus enabling one to subtract them from the global kinetics of the release of H⁺. Although no background buffers are added to the reacting system in order to maintain a constant pH, the buffering capacity of the system itself obliges one to quantify it by calibrations in order to calculate the true release rate of H⁺ in the reaction. These two principal limitations of the pH-stat methodology, the dissolution of environmental CO₂ and the inherent buffering capacity of the system, can be partially mitigated by working within an inert atmosphere of N₂ and having a good knowledge of the acid/base characteristics of the reagents and reaction products, which together will permit the choice of a working pH range in which the second limitation will be negligible. Having studied theoretically the acid/base balances of the system in reaction, it is concluded that optimum working conditions for the pH-stat methodology are between pH 8·5 and 11·0, since over this pH range the buffering capacity is null (thus eliminating the need for calibrations) and the stoichiometry of H⁺ (n_H⁺) is close to unity. These theoretical predictions have been confirmed experimentally with calibrations and measurements of n_H⁺ at different pH values.     

Triazine reaction of cyanate ester resin systems catalyzed by organic tin compound: kinetics and mechanism

The mechanism and kinetics of the thermal cure reaction of two cyanate esters (CEs), 1,1′bis(4‐cyanatophenyl)ethane (AroCy L‐10) and bisphenol A dicyanate ester (BADCy), in the presence of dibutyl tin dilaurate (DBTDL) has been investigated using Fourier‐transform infrared spectroscopy (FTIR) and High‐performance liquid chromatography (HPLC). It was found that the organic tin compound (H₉C₄)₂Sn(NCO—R—OCN)₂, an active catalyst, has high catalytic efficiency in the polymerization of cyanate esters. The consuming rate of cyanate concentration showed a first‐order dependence on both active catalyst and the cyanate ester monomer concentration. The apparent activation energies (E_a) and frequency factors of both AroCy L‐10 and BADCy were calculated. A mechanism of cyclotrimerization was proposed, based on the kinetic data and FTIR spectra, which involves the formation of an active catalyst and the catalysis of the active catalyst. Copyright © 2004 Society of Chemical Industry     

盐酸水解玉米秸秆木聚糖的动力学研究

对稀盐酸水解秸秆半纤维素生成木糖水解液过程进行动力学研究。分别测定了不同温度下,不同水解时间水解液中木糖质量浓度以及其降解产物糠醛的质量浓度。利用Saeman模型拟合木糖生成过程。实验表明,该模型能够较好地描述木糖生成过程以及其降解产物糠醛的产生过程。通过曲线拟和确定了不同水解温度下木糖的生成速率以及分解速率常数,同时利用Arrhenius方程确定木糖生成活化能Ea为116 kJ/mol。综合比较不同温度下水解液中的木糖以及糠醛质量浓度,确定使用质量分数为2%的稀盐酸于120℃下水解60 min为最佳水解条件。其水解液中木糖质量浓度可达20.99 g/L,糠醛质量浓度可维持在1.46 g/L以下。     

A study on molecular transport of organic esters and aromatics into viton fluoropolymers

Molecular transport of organic liquids into Viton fluoropolymers has been investigated by a sorption—desorption gravimetric method. Diffusion coefficients have been calculated from Fick's equation. The sorption—desorption results have been used to calculate the concentration profiles by solving Fick's diffusion equation under suitable boundary conditions. A numerical method based on the finite difference technique was also used to calculate the concentration profiles of liquids as a function of sorption time and thickness of the Viton fluoropolymers. The dependence of sorption, desorption, diffusion, and permeation properties of the liquids on temperature showed a significant effect. The Arrhenius activation parameters have been estimated for diffusion, permeation, and sorption processes. The experimental and calculated results are discussed to study the type and nature of interactions between Viton fluoropolymers and the solvent molecules. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 717–723, 1997     

Kinetics and mechanism of homogeneous catalytic hydroxylation of maleic acid by hydrogen peroxide: III. Kinetics of the hydrolysis of cis‐epoxysuccinic acid

The objective of this work was to study the hydrolysis kinetics and also the character of the involvement of the epoxidation catalyst (Na₂WO₄ – sodium tungstate) on the hydrolysis of cis‐epoxysuccinic acid (the initial product in the hydroxylation reaction of maleic acid by hydrogen peroxide). The results obtained at 65 °C clearly revealed that the hydrolysis reaction exhibits a considerably low rate in the absence of a catalyst whilst the rate is significantly enhanced by the introduction of catalytic quantities of Na₂WO₄. The phenomenon of end‐product inhibition was observed in this study and the results obtained permitted the development of a kinetic model consistent with experimental observations. Analysis of the kinetic model shows that the reaction is first order with respect to the concentrations of the catalyst and the epoxide. However, tartaric acid has a strong inhibitive influence on the overall reaction rate. © 1999 Society of Chemical Industry     

苯酐水解速率实验测定

对邻苯二甲酸酐水解反应动力学进行了研究.在线测定了溶液中pH值的变化,根据pH值与溶液浓度之间一一对应的关系,得到苯酐在水解过程中邻苯二甲酸浓度的变化.由水解结束时溶液的平衡浓度与此时溶液的pH值,计算了不同温度下邻苯二甲酸的一级电离常数Ka,从而确定了水解反应级数α和不同温度下反应速率常数k.得到以下结论:随着温度的升高,水解速率常数也随之增加.根据阿伦尼乌斯方程,求得了水解反应的活化能Ea为9.45 kJ/mol、指前因子A为2.30.     

Facile optical resolution of amino acid esters via hydrolysis by an industrial enzyme in organic solvents

Racemic amino acid esters were optically resolved via hydrolysis in organic solvents by the catalysis of an industrial alkaline protease, “Alcalase”. The products which were composed mainly of L-amino acids were insoluble and easily separated by filtration. The activity of the enzyme and enantiomeric excess of the products were significantly dependent on the nature of solvent and the water content in the reaction media. Generally, high values of enantiomeric excess were obtained at low water contents. Many natural and unnatural amino acids were resolved by this method.