Simple Chemical Reactions in the Solid State: Towards Elaborating a Conception

In contrast to the conventional homogeneous kinetics, there is no conception of a simple reaction in the solid-state reaction kinetics. The geometric-probabilistic phenomenology currently in use is not adequate for describing the interplay between the chemical mechanism and the observed kinetic behaviour. An attempt is made to formulate a conception of simple reaction in the solid state as a basis for constructing kinetic models of involved reactions.     

Kinetics of the Oxidation of Tartrazine with Peroxydisulfate in the Presence and Absence of Catalysts

Summary.  The food dye tartrazine is oxidized with peroxydisulfate in the absence and in the presence of Ag(I) and Fe(III) catalysts. In the absence of these metal ions, the reaction shows second-order kinetics, first-order in each of the reacting species. With the Ag(I) ion in the medium the reaction proceeds considerably faster, but still follows second-order kinetics. The reaction rate depends on the concentration of Ag(I) and S₂O₈ ²⁻, but is independent of the concentration of the substrate. When Fe(III) acts as the catalyst, a marked enhancement in the reaction rate is observed, and the reaction proceeds through two parallel pathways, the catalyzed and the noncatalyzed. The catalyzed path follows third order kinetics, being first-order in substrate, oxidant, and catalyst concentration. Mechanisms of the noncatalyzed as well as the Ag(I) and Fe(III) catalyzed reaction systems are proposed. Received June 28, 1999. Accepted (revised) September 27, 1999     

Selective CO2 Conversion into Fuels on Nanochannels

The purpose of this research idea is to develop a method to electrochemically convert carbon dioxide into higher alcohol chains such as ethanol to be used as fuel. Electrochemical CO₂ reduction has low yields and poor product selectivity, being able to improve this reaction would have an impact in the energy and food market. We propose the use of a modified nanofluidic transistor to block reaction steps that are thermodynamically favored by constraining the kinetics of the reaction when the reaction takes place in a geometrically restricted environment with different double layer properties to those found in conventional planar electrosynthesis.     

Kinetics of Inactivation of Aminoacylase I During Modification of Its Thiol Groups by DPDS and PCMB

The kinetics theory of the substrate reaction during modification of enzyme activity previously described by Tsou has been applied to a study on the kinetics of the course of inactivation of aminoacylase I by DPDS and PCMB.From the results obtained we have found that the inactivation reaction of aminoacylase I by DPDS is noncomplexing inhibition,and PCMB reaction is complexing inhibition.The microscopic constants for the reaction of the inactivator with free enzyme and the enzyme-substrate complex were determined.     

微波作用下的多肽固相缩合反应及动力学研究

分别在微波作用以及传统加热两种方式下, 研究了Fmoc-Val-OH与NH₂-Tyr(t-Bu)-Wang树脂的固相缩合反应及其动力学. 测定了温度变化对反应速率的影响, 并获得了两种方式下的缩合反应的宏观动力学参数: 300 W微波作用下表观缩合反应级数为2.3, 活化能为104.7 kJ/mol; 传统方法中表观反应级数为2.9, 活化能为142.4 kJ/mol. 微波作用将常规条件下的连接率由68%提高到95%, 而所需时间降为常规条件的1/14.     

膨胀型无卤阻燃HIPS热分解动力学及阻燃机理研究

利用动态热失重法(TGA)研究了一种新型的膨胀型无卤阻燃高抗冲聚苯乙烯(HIPS)热降解反应动力学及阻燃机理, 通过对Kissinger模型和Coat-Redern (C-R)模型求解的热降解反应的动力学参数对比, 最终确定反应的动力学参数. 其中, 反应级数n的确定是通过一般反应对Ea/RTmax取值范围的限定, 利用最大热降解速率所对应的失重率αmax与n的关系, 确定其取值. 并采用TGA-FTIR及Py-GC/MS对材料气相产物及热裂解产物进行了阻燃机理的研究. 研究表明, 两种反应的热降解反应动力学参数基本一致, 其中阻燃HIPS的平均表观活化能小于纯HIPS, 说明在HIPS分解之前, 无卤阻燃剂已经开始分解, 释放的难燃气体(氨气及其衍生物、水蒸气等)在气相中起到阻燃的作用. 同时阻燃剂的添加, 促使反应向链转移反应飘移, 使燃烧产物中非单体化合物增加, 而在凝聚相中形成的致密的炭层结构也起到阻燃的效果.     

Kinetics and mechanism of the heterogeneous catalyzed oxidative decolorization of Acid-Blue 92 using bimetallic metal-organic frameworks

The kinetics study of the oxidative decolorization of Acid-Blue 92 has been investigated by hydrogen peroxide catalyzed with bimetallic metal-organic frameworks. The used metal-organic frameworks (MOF) are [Ph3SnCu(CN)2·L] where L=pyrazine (pyz) 1, methylpyrazine (mepyz) 2, 4,4'-bipyridine (bpy) 3, trans-1,2-bis(4-pyridyl)ethene (tbpe) 4 or 1,2-bis(4-pyridyl)ethane (bpe) 5. The reaction was followed by conventional UV-Vis spectrophotometer at λmax=571 nm. The reaction exhibited first-order kinetics with respect to [dye] and [H2O2]. The reactivity of the catalysts depends on the type of the medium and thereafter decreases in strong alkaline media. Addition of NaCl enhances the reaction rate. Also, the irradiation of the reaction with UV-light enhanced the rate of AB-92 mineralization by about 86.9%. The reaction was entropy-controlled as confirmed by the isokinetic relationship. A reaction mechanism was proposed with the formation of free radicals as an oxidant.     

Comparative study of reaction kinetics in membrane and agarose bead affinity systems.

Compared to conventional agarose bead affinity supports, a microporous nylon membrane exhibits greatly improved reaction kinetics as quantified in the reaction between gamma-globulin and immobilised protein A. The improvement is only observed when the solution of gamma-globulin is forced through the membrane pores. In the absence of flow in the pores, it is possible to relate approximately the rate of uptake onto either type of affinity support to independently determined diffusion coefficients. In the presence of flow, the reaction rate is similar for membranes having 0.45 and 3.0 microns diameter pores, and considerably smaller than predicted by the Smoluchowski formula.     

The absorption and metabolism of modified amino acids in processed foods

The chemical reactions involved in the modifications of amino acids in processed food proteins are described. They concern the Maillard reaction, reaction with polyphenols and tannins, formation of lysinoalanine during alkaline and heat treatments, formation of isopeptides, oxidation reaction of the sulfur amino acids, and isomerization of the L-amino acids into their D-form. Information on the digestion, absorption, and urinary excretion of the reaction products obtained by using conventional nutritional tests is given. The studies that have been made on the metabolism of these molecules by using a radioisotopic approach to follow their kinetics in the organism after ingestion are also reviewed. This approach provides unique data on the quantitation of the metabolic pathways and on the kinetics of the metabolic processes involved.     

Proton diffusion reaction in a protein polyelectrolyte membrane and the kinetics of electromechanical forces

A theoretical model is developed for proton transport in a charged, protein polyelectrolyte membrane. Protons bind to fixed charge groups of the membrane. This binding process also modulates electromechanical tensile forces in the membrane, due to the change in the electrostatic interactions between the charged membrane molecules. We used this force as an experimental measure of the space—time evolution of proton transport inside the membrane. A step change in the bath pH produced a measurable change in the isometric tensile force of the membrane. H⁺ ion transport was modelled by a diffusion reaction theory. The kinetics of the measured force were compared to the H⁺ diffusion reaction dynamics predicted by the theory. Experimental and theoretical time constants were in good agreement for reasonable values of the H⁺ diffusivity, the equilibrium dissociation reaction constant and the number of available binding sites. This suggests that the diffusion reaction of H⁺ into the membrane is rate-limiting in force generation. Other nonequilibrium processes, including molecular reconformation and osmotic swelling, must be proceeding at least as rapidly as H⁺ transport. The combination of theory and experiment provides a useful, non-destructive technique for characterizing the kinetics of binding, and other transport processes, in charged polymers and in certain biological tissues.     

Efficient mixing in membrane bioreactors:minimizing dilution effects

Thermodynamically unfavorable enzyme-catalyzed synthesis reactions can attain higher levels of conversion with enhanced kinetics in membrane bioreactors as opposed to conventional biochemical reactors. Dilute reactants in separate solutions are mixed efficiently using permselective membranes, with the result that the reaction can be carried out to a high level of conversion. Pertinent operating parameters are identified and model calculations are presented showing thermodynamic and kinetic advantages.     

Kinetics and thermodynamics of synthesis of palm oil-based trimethylolpropane triester using microwave irradiation

Enhancement of reaction performance utilizing microwave irradiation has drawn so much interest due to its shorter reaction time and low catalyst loading. These advantages are particularly significant from kinetics and thermodynamics perspectives. This study aimed to investigate the kinetics and thermodynamics of microwave-assisted transesterification of palm oil-based methyl ester into biolubricant. The transesterification reaction of palm oil methyl ester (PME) and trimethylolpropane (TMP) was conducted at 110–130 °C for 90 min under vacuum condition. Sodium methoxide was employed as the catalyst at 0.6 wt% of reactants fixed at molar ratio of 4:1 (PME: TMP). The experimental data were fitted with the second-order reversible reaction kinetics mechanisms. The data were solved via Runge-Kutta 4,5 order using MATLAB software. Analysis on the data revealed that the reaction rate constants at temperatures of 110–140 ℃ were in the range of 0.01–0.63 [(w/w)(min)]⁻¹, with standard errors of 0.0026–0.0228 within 99.99% prediction interval. Microwave-assisted reaction obtained 17.0 kcal/mol of activation energy. This method reduced activation energy by 49% as compared to the conventional heating. Activation energy and time-periodic energy assessment showed that the reaction was endothermic. The reaction at 130 °C is the easiest to activate. The positive Gibbs free energy (ΔG > 0) found using Eyring-Polanyi equation indicated that the transesterification was non-spontaneous and endergonic.     

A new look at fluorescence depolarization and the dynamics of anisotropic rotational diffusion

Existing derivations of the time-dependent fluorescence anisotropy of an asymmetric molecule constitute a straightforward application of Favro's work on the rotational diffusion equation (RDE), and make no contribution to the elucidation of rotational dynamics as such. A new approach is developed, and the problem formulated in the parlance of conventional reaction kinetics by demonstrating, with the aid of the method of moments, that the RDE is completely equivalent to a set of ordinary linear differential equations, formally identical with those used to describe a first-order series-parallel reaction scheme.     

The kinetics of the polymerization of methyl methacrylate initiated by t-butylmagnesium bromide in THF,toluene or mixed solvent

The kinetics of the solution polymerization of methyl methacrylate in THF, toluene and their mixtures were studied between 200 and 300 K using dilatometry (in the systems where it was valid), gravimetric determination and monitoring monomer and polymer concentrations by NMR spectrometry. The reaction followed zero order kinetics at 200 K, first order kinetics at 275 K and mixed order in between. At both the limits and intermediate range, the reaction followed an integrated rate equation consistent with terminationless propagation proceeding through a complex between monomer and the propagating species. Above 275 K, termination and side reactions were evident and the yields of high mol. wt polymer were small. Density-temperature calibrations for monomer in THF, toluene and mixtures were constructed for the range 190–283 K. However for polymerizations in toluene-rich mixtures, where very high mol. wt polymer forms, the contraction did not correlate linearly with conversion.     

Can the absence of solvation of neutral reagents by ionic liquids be responsible for the high reactivity in base-assisted intramolecular nucleophilic substitutions in these solvents?

[reaction: see text] The kinetics of the rearrangement of the Z-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole (1a) into the relevant 4-benzoylamino-2,5-diphenyl-1,2,3-triazole (2a) induced by amines have been studied in two room-temperature ionic liquids (IL-1, [BMIM][BF4] and IL-2, [BMIM][PF6]). The data collected show that the reaction occurs faster in ionic liquids than in other conventional solvents previously studied (both polar or apolar, protic or aprotic). Presumably, this could depend on their peculiar ability to minimize the strong substrate-solvent, amine-solvent and amine-amine interactions occurring in conventional solvents.     

Kinetic studies of CO2dissociation on supported Ni catalysts

Summary In order to investigate the kinetics of CO₂dissociation on supported nickel catalysts, a novel technique, which can give the surface reaction rate constants with no information on the number of active sites, was developed. It was revealed that CO₂dissociation was more enhanced on TiO₂support than on other metal oxide ones. The activity pattern and activation energies were in good agreement with those obtained by a conventional pulse technique using the number of active sites, suggesting the validity of the present technique for investigating the kinetics of the surface reaction.</o:p>     

Comparison of various coupling reagents in solid-phase aza-peptide synthesis

Aza-peptides are promising drug leads, however extensive study of their properties is hampered by low yielding aza-peptide bond formation during conventional Fmoc SPPS. The kinetics of aza-peptide bond formation in the model peptide H-Ala-AzAla-Phe-NH₂ was compared with various conventional amino acid activators. The reaction rates and yields were dependent on the activator structure. The reaction time of aza-peptide formation using oxyma-based agents was approximately 30 times longer than in typical peptide synthesis. Therefore, new activators are required to increase the reactivity of the activated amino acid to achieve effective acylation of the semicarbazide moiety during aza-peptide bond formation.     

In‐Situ Kinetic Pursuit of Emulsion Crosslinking Copolymerizations of Monomethacrylate and Dimethacrylate by Means of ReactIR®

The usefulness of the ReactIR® reaction analysis system as a powerful tool to in‐situ monitor the kinetics of the emulsion polymerization of vinyl monomers was verified by comparison with the conventional gravimetric method and by checking the rate dependencies on initiator and surfactant concentrations in the emulsion polymerization of butyl methacrylate (BMA). The system was then applied to the kinetic monitoring of emulsion crosslinking copolymerizations of monomethacrylate and dimethacrylate, including methyl methacrylate/ethylene dimethacrylate and BMA/1,6‐hexanediol dimethacrylate systems of different hydrophilicities.     

Extrapolating H-atom transfer rate constants via thermochemical kinetics: The effect of tunneling

One of the important applications of chemical kinetics is the attempt to understand complex processes through kinetic modeling. This process frequently requires that rate constants be obtained by extrapolation of data either to higher or lower temperatures than the experimental, or by estimation or correlation with such data. Thermochemical kinetics combined with conventional transition state theory forms a framework from which this may be done. However, rate data for H transfer reactions may have a significant contribution from tunneling. In this work, a one dimensional approach to tunneling, consistent with conventional transition state theory, is taken to show how tunneling affects the extrapolation and correlation of rate constants in thermochemical kinetics. It is concluded that extrapolation and correlation are both quite good even when tunneling comprises 80% of the reaction. However, this is not without limitations, which are discussed. © 1993 John Wiley & Sons, Inc.