Use of Raman spectroscopy as an in situ tool to obtain kinetic data for organic transformations

Raman spectroscopy has been used as an in situ tool to obtain kinetic data for an organic transformation. The model reaction studied was the synthesis of 3-acetylcoumarin from the condensation between salicylaldehyde and ethyl acetoacetate with piperidine as a catalyst. The study shows that precise kinetic data can be obtained quickly and reproducibly, allowing for the facile determination of both overall reaction order and reaction order with respect to each component of the reaction. Additionally, Arrhenius parameters such as activation energy for a reaction can be readily obtained. In conjunction with computational modeling, this data-rich analysis technique also allows for in-depth probing of mechanistic aspects of reactions. Microwave heating proves to be an ideal tool for aiding in kinetic studies. It offers reproducible noncontact heating as well as precise temperature monitoring and data recording.     

Variable Time Normalization Analysis: General Graphical Elucidation of Reaction Orders from Concentration Profiles

The recent technological evolution of reaction monitoring techniques has not been paralleled by the development of modern kinetic analyses. The analyses currently used disregard part of the data acquired, thus requiring an increased number of experiments to obtain sufficient kinetic information for a given chemical reaction. Herein, we present a simple graphical analysis method that takes advantage of the data‐rich results provided by modern reaction monitoring tools. This analysis uses a variable normalization of the time scale to enable the visual comparison of entire concentration reaction profiles. As a result, the order in each component of the reaction, as well as k_obs , is determined with just a few experiments using a simple and quick mathematical data treatment. This analysis facilitates the rapid extraction of relevant kinetic information and will be a valuable tool for the study of reaction mechanisms.     

Reaction Schemes That Are Easily Confused with a Reversible First–Order Reaction

A detailed kinetic analysis of two schemes, one involving coupled consecutive processes and another featuring the simultaneous association reaction and decay of a component, is presented here using Taylor series expansion. It is shown that both of these schemes are easily confused with the reversible second–order reaction in a routine kinetic study. The kinetic traces predicted by both schemes are sufficiently close to pseudo–first–order curves so that it is practically impossible to identify the deviations based on data with the usual experimental errors, which was also demonstrated by fitting simulated theoretical curves to exponential functions. The dependence of the pseudo–first–order rate constants on the concentration of the excess reagent features the same trend as in the case of a reversible reaction: A straight line with an intercept is observed. This analysis emphasizes that the reversible nature of reactions should be demonstrated by direct equilibrium studies when elements of reversibility are implied by kinetic results.     

On the application of open circuit relaxation spectroelectrochemistry to the diagnosis and evaluation of the kinetics of succeeding second order chemical reactions

The spectroelectrochemical technique of open circuit relaxation (OCR) has found limited application in the kinetic characterization of systems wherein the electrogenated absorbing species is consumed in a second or higher order reaction. This limited usage arises from the inapplicability of conventional homogeneous kinetic data analysis techniques to the absorbance-time transients observed in the OCR experiment. Treatment of such transients by conventional kinetic expressions (e.g., 1/A vs. t for second order reactions) results in non-linear plots which neither serve as diagnostic criteria for the assignment of kinetic order nor provide meaningful rate constants. This paper presents an empirical method for the treatment of spectroelectrochemical OCR data arising from post-electron transfer kinetic processes which exhibit second order dependence on the concentration of the electrogenerated absorbing species. This procedure, which takes into account the inhomogeneous distribution of the reactant species, affords reduced data plots which not only are linear, hence diagnostic of kinetic order, but also provide a valid measure of the bimolecular rate parameters characteristic of this type of dynamic system. The procedure has been applied to the treatment of OCR data for the reaction of 9,10-diphenylanthracene cation radical with 4-cyanopyridine in acetonitrile. Excellent agreement between the resulting kinetic parameters and those obtained using stopped-flow techniques has been demonstrated.     

裂化催化剂水热失活动力学及装置平衡活性模型

根据裂化催化剂水热失活过程伴随着超稳化过程的特点，确定了对应不同自抑制函数的催化剂水热失活动力学模型方程。利用裂化催化剂水热失活实验数据进行参数估值，建立了裂化催化剂水热失活动力学模型。统计检验结果表明，二级自抑制的一级水热失活模型能很好地模拟实验数据，是较理想的水热失活动力学模型。考虑工业装置中裂化催化剂呈全混流，建立了裂化催化剂平衡活性模型方程，并且装置平衡催化剂微反活性的模型计算值与实测值相当吻合。该模型的预测结果表明，随着再生器温度或催化剂藏量的提高，平衡剂的微反活性逐步降低；平衡剂微反活性随催化剂单耗的提高先快速提高，然后缓慢提高。     

非等温法研究TGDDM/DDS体系固化反应动力学

采用DSC对4,4′-四缩水甘油基二氨基二苯基甲烷(TGDDM)和3,3′-二氨基二苯基砜(DDS)体系的固化反应动力学进行了研究.分别通过n级反应法和Malek的最大概然机理函数法确定了固化反应机理函数,求解了固化反应动力学参数,得到了固化反应动力学模型.结果表明,通过Kissinger,Crane方法求解动力学参数所得到的n级反应模型与实验值差别较大;而采用Malek方法判别机理,表明该固化反应按照自催化反应机理进行,实验得到的DSC曲线与模型计算所得到的曲线吻合的较好,所确立的模型在5~20K/min的升温速率下能较好地描述TGDDM/DDS体系的固化反应过程,并为工艺参数的选择和工艺窗口的优化提供了理论依据.     

Extraction of the contribution of kinetic profile for reaction of a chemical species in unknown matrices by standard addition method

A new and simple strategy is applied to resolve kinetic profile for the reaction of an analyte in unknown matrices, using standard addition method (SAM). The proposed method uses kinetic spectrophotometric data obtained by standard addition of analyte into unknown mixtures followed by the reaction of analyte with a proper reagent. The proposed method extracts kinetic profile for the reaction of an analyte by averaging the kinetic profiles obtained by subtraction of kinetic profiles after and before standard addition. The rate constant can be obtained using computational curve fitting. The performance of method was evaluated by using synthetic data as well as several experimental data sets. The proposed method can be applied to obtain kinetic profiles of the reactions in the presence of additive interference as well as multiplicative interferences. Hydroxylation reaction of diphenylcarbazide (DPCI) in the presence of diphenylcarbazone (DPCO) as a real system at various pHs was also studied by the present method. The rate constant and the order of the hydroxylation reaction were determined from extracted kinetic profiles.     

速率常数-秩分析法在化学反应过程分析中的应用

针对化学反应动力学谱-吸收光谱组成的两维数据,提出了以优化速率常数而消去反应物波谱信息为减秩手段的速率常数-秩分析法(RCRA).结果表明,RCRA在一次优化过程中可同时获得两个最优解,分别对应于两步速率常数.在获得动力学参数的前提下,利用最小二乘回归可解出包括中间体在内的各组分的吸收光谱.该方法用于处理苯胺电解降解的两维数据,发现苯胺降解过程中有一种表观中间体存在,降解过程符合一级连串反应模型.     

利用减秩因子分析法同时优化确定化学反应级数和速率常数

通过对反应过程中在线测得的动力学谱-光谱二维数据矩阵进行主成分分析，可确定化学反应过程存在的组分数。提出用优化动力学参数-减秩因子分析法解析二维数据矩阵，对未知动力学模型的复杂反应可同时优化求解第一步反应的级数和速率常数。模拟二维数据验证了该方法的可行性。该方法用于高锰酸钾氧化溴化钠的反应过程中测得的二维数据的解析，结果表明：高锰酸钾的还原过程符合0级反应模型。     

Chemometrics study of the kinetics of the Griess reaction

The kinetics of the Griess reaction in which 3‐nitroaniline acts as a nitrosation agent and 1‐naphtylamine as a coupling reagent was studied by chemometrics methods. The kinetic reaction was investigated under pH 1.0 and 25°C by UV‐vis spectrophotometry. The concentrations of nitrite, 3‐nitroaniline and 1‐naphtylamine were such that a second‐order kinetic reaction took place. Data explorations based on principal component analysis and multivariate curve resolution–alternating least squares were performed to obtain information about the reaction. Calculation of band boundaries of the multivariate curve resolution–alternating least squares solutions showed that the rotational ambiguities associated with the calculation of spectra and concentration profiles have been completely removed. The decrease in the ambiguity of the recovered solutions was closely related to the application of the equality constraint. The results of the exploratory data analysis showed that the kinetic reaction proceeds through a two‐step mechanism. Moreover, the two‐steps are second order. Data analysis approaches based on hard modeling and global hard modeling were used to resolve profiles of the reactants, intermediates and products and to evaluate the rate constants. Copyright © 2013 John Wiley & Sons, Ltd.     

About compensation effect by thermal decomposition of some catalyst precursors

Summary In order to obtain catalysts, the thermal decomposition of the precursors is a compulsory step. However, kinetic analysis of the decomposition data obtained under non-isothermal conditions lead very seldom to the intimate reaction mechanism. There is also a lack of information because in non-isothermal kinetics, the compensation effect, is rather a rule and unfortunately a source of debate. In order to discriminate between these processes, and the influence of conversion, respectively temperature on the reaction rate, the NPK (non-parametric kinetic - Sempere and Nomen) method was used. This method is based on the singular value decomposition algorithm (SVD) applied on the matrix of reaction rate at corresponding conversion and temperature. This method allows a less speculative determination of the conversion functions and of the kinetic parameters.     

Thermal analysis of Beypazari lignite

Beypazari lignite was investigated by differential scanning calorimetry (DSC), thermogravimetry (TG), high pressure thermogravimetry (HPTG) and combustion cell experiments. All the experiments were conducted at non-isothermal heating conditions with a heating rate of 10°C min^?1, in the temperature range of 20–700°C. DSC-TG data were analysed using an Arrhenius-type reaction model assuming a first-order reaction. For the HPTG data the Coats and Redfern equation was used for kinetic analysis. In the combustion cell experiments the Fassihi and Brigham approach was used in order to calculate kinetic data. Finally a comparison is made between the kinetic results.     

Combined kinetic analysis of solid-state reactions: a powerful tool for the simultaneous determination of kinetic parameters and the kinetic model without previous assumptions on the reaction mechanism

The combined kinetic analysis implies a simultaneous analysis of experimental data representative of the forward solid-state reaction obtained under any experimental conditions. The analysis is based on the fact that when a solid-state reaction is described by a single activation energy, preexponetial factor and kinetic model, every experimental T-alpha-dalpha/dt triplet should fit the general differential equation independently of the experimental conditions used for recording such a triplet. Thus, only the correct kinetic model would fit all of the experimental data yielding a unique activation energy and preexponential factor. Nevertheless, a limitation of the method should be considered; thus, the proposed solid-state kinetic models have been derived by supposing ideal conditions, such as unique particle size and morphology. In real systems, deviations from such ideal conditions are expected, and therefore, experimental data might deviate from ideal equations. In this paper, we propose a modification in the combined kinetic analysis by using an empirical equation that fits every f(alpha) of the ideal kinetic models most extensively used in the literature and even their deviations produced by particle size distributions or heterogeneities in particle morphologies. The procedure here proposed allows the combined kinetic analysis of data obtained under any experimental conditions without any previous assumption about the kinetic model followed by the reaction. The procedure has been verified with simulated and experimental data.     

Kinetic Studies of Polyethylene Terephthalate Synthesis with Titanium-Based Catalyst

Summary: Thermogravimetric analysis and stirred tank reactor techiques were used to study the kinetic of polycondensation of bis-hydroxy ethylene terephthalate catalyzed by titanium tetrabutylate. Polycondensation reaction can be modelled best with a second order reaction with respect to hydroxyl end groups concentration. Titanium tetrabutylate is a precursor and needs preactivation for polycondensation catalysis. Kinetic data depends on the mode of operation in thermogravimetric analysis. In nonisothermal mode, the overall activation energy was determined by model-free method and the pre-exponential factor was found to be affected by catalyst concentration. The kinetic study of isothermal reaction in thermogravimetric analysis is complex due to lack of a precise way to reach desired isothermal temperature and high activity of the catalyst. Titanium catalyst shows higher activity and lower selectivity than antimony catalyst but its activity is affected by the nature of ligands.     

Determination of Polyurethane Foam Growth Kinetics by a Simple Column Height Test

In the present work, the height of foam column has been used to monitor the reaction of the isocyanate group. Kinetic modeling describes the conversion versus time curves. The global reaction can be divided into two different paths: the blowing reaction and the polymerization. The kinetics of the reaction of toluene diisocyanate with a polypropylene glycol has been studied with auxiliary blowing agents; the concentration of NCO and OH groups could be considered equivalent during all the experiments as a model for similar polyurethane systems. Despite the complexity of the system in which different kinetic orders are possible, as well a physically controlled diffusion process, data were fitted to a simple kinetic model of an apparent order. The results obtained have enabled us to propose a reaction path and catalyzed second‐order kinetic model.     

Algorithm for error-compensated kinetic determinations without prior knowledge of reaction order or rate constant

The development and evaluation of a new algorithm for error-compensating predictive kinetic determinations are described. With the new algorithm it is possible to fit kinetic data without prior knowledge of the rate constant, reaction order or initial and final values of the detector signal. A curve-fitting method is used to obtain values of these parameters that give the best fit of the model to the data. Although intended to be used primarily to compute signal changes between t=0 and ∞, the method can also be used to determine reaction orders. Although the algorithm fails for reaction orders of zero and unity, it works well for orders between these values and orders greater than unity. Simulated data are used to evaluate the effects of reaction order, signal noise and data range on computed values of signal change and, to a lesser extent, reaction order.     

Automation of the information content analysis of kinetic parameters

The aim of this work has been to automate the analysis of kinetic measurements in solving inverse problems in order to recognize the number and form of the independent combinations of reaction rate constants. Software for determining the basis of nonlinear parametric functions of the kinetic parameters of a mathematical model for the mechanism of a complex chemical reaction has been developed and described. The main stages of program construction with the use of a search algorithm for the independent combinations of the rate constants of elementary steps in the mechanism of a complex chemical reaction based on experimental data on the concentrations of reactants are considered.     

A computer program system for kinetic analysis of non-isothermal thermogravimetric data: II. Generalized kinetic analysis and application to coal pyrolysis

A FORTRAN program is presented which enables the kinetic analysis of extent and rate of reaction data resulting from transposed experimental TG/DTG data obtained under non-isothermal conditions. This software allows one to perform Arrhenius, Friedman and Kissinger analyses for up to nine different solid-state rate-controlling reactions, including nth order, Avrami-Erofeev, phase boundary movement and diffusional models. Data from an investigation of the pyrolysis of a bituminous coal serve as an example of the application of the program.     

An in situ Fourier transform infrared spectroscopy method for collecting real-time data for the liquid-phase reaction of CO2 and mono-ethanolamine in alcoholic solvent systems

A novel Fourier transform infrared (FTIR) spectroscopy method of analysis was developed to study the industrially important reaction of CO₂ with the primary amine, mono-ethanolamine (MEA), in n-propanol as representative alcoholic solvent. A semi-batch reactor with a piston like ‘floating’ head was specifically designed in order to study the reaction in the liquid phase, thereby eliminating the mass transfer limitations of a gas-liquid system. An attenuated total reflectance (ATR) probe was used to monitor the change in reagent and product concentrations in-situ. Successful spectral peak identification and calibrations were performed in order to collect real time, reaction kinetic data. Data analysis confirmed that the zwitterion reaction mechanism most accurately describes the reaction in non-aqueous systems.     

Correction of temperature variations in kinetic-based determinations by use of pruning computational neural networks in conjunction with genetic algorithms

The joint use of genetic algorithms and pruning computational neural networks is shown to be an effective means for selecting the number of inputs required to correct temperature variations in kinetic-based determinations. The genetic algorithm uses a pruning procedure based on Bayesian regularization and is highly efficient as a feature selector; it provides quite good results in the generalization process without the need to use a validation set. The fitness function is defined as the sum of two subfunctions: one controls the learning ability of the network and the other its complexity. The training, pruning, and generalization processes were initially tested with simulated data in order to acquire preliminary information for the ensuing work with real data. The performance of the proposed method was assessed by applying it to the determination of the amino acid L-glycine by its classical spectrophotometric reaction with ninhydrin. A straightforward network topology including temperature as input (40+T:2:1 with 19 connections after the pruning process) was used to estimate the L-glycine concentration from kinetic curves affected by temperature variations over the range 60-75 degrees C, using kinetic data acquired up to only 1.5 half-lives. The trained network estimates this concentration with a standard error of prediction for the testing set of ca. 8%, which is much smaller than those provided by a classical parametric method such as nonlinear regression (even if kinetic data acquired at longer half-lives are used). Finally, a kinetic interpretation of the pruning process is provided in order to better demonstrate its potential for kinetic analysis.