Gas chromatographic determination of rate constants in bimolecular gaseous reactions

Summary Rate constants for bimolecular reactions in the gas phase, under diffusion controlled conditions, can easily be determined by the reversed-flow gas chromatography (RF-GC) technique. The analysis of the diffusion band by means of a simple PC programme gives directly an apparent, second-order rate constant for gaseous reactions. By varying the amounts of the reactants, one can calculate the true order of the reaction and the true non-first-order rate constant of gaseous reactions. The calibration problem of the analytical techniques in non-first-order reaction kinetics is absent as are other disadvantages connected with carrier gas flow, peak shape and their instrumental spreading. The method can be used for atmospheric reactions and was applied in the gaseous reaction systems: SO₂+NO₂, SO₂+Br₂, C₆H₆+NO₂, C₆H₅CH₃+NO₂ and C₃H₆+NO₂ with various concentrations of reactants in nitrogen. The effect of the NO₂ concentration on the apparent second-order rate constant of C₂H₄+NO₂ at 333.2 K was also studied. Finally, the effect of sun light pre-irradiation of C₂H₂+NO₂ in nitrogen was investigated.     

Hhdrodenitrogenation catalysis studied by reversed-flow gas chromatography

Summary Many important parameters of surface catalysed reactions can be determined simultaneously, under nonsteady state conditions using Reversed-Flow Gas Chromatography. A simple, slightly modified gas chromatograph is required. The distorted diffusion bands, obtained experimentally for reactant and product(s), can be analysed mathematically, using simple PC programs, to give the pre-exponential factors and the exponential coefficients of a function consisting of the sum of two-four exponential functions of time. From these, and some geometrical and diffusional characteristics of the reaction cell, the values of adsorption, desorption and reaction rate constants, the overall mass transfer coefficients in the gas and in the solid catalyst, and the adsorption equilibrium constant, for both reactant and product(s) can be calculated.The above parameters were determined at various temperatures and over three catalysts for the hydrodenitrogenation of piperidine ton-pentane, an industrially important hydrotreating process. The results obtained can help to understand the mechanism of reactions on solid surfaces and to confirm experimentally theoretical calculations on adsorption and surface reactions.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

A differential method for the analysis of chemical kinetics results based on reversed-flow gas chromatography

Summary The reversed-flow gas chromatographic technique has been applied to the determination of the apparent rate constant and the reaction order of a reaction between two gases or vapors. For seven hydrocarbons (ethane, ethene, ethyne, propene, butene, benzene and toluene) reacting with nitrogen dioxide, the above mentioned kinetic parameters have been determined. For these determinations, the necessary mathematical formulation of the problem has been written and solved, leading to simple expressions which describe the height of the chromatographic sample peaks as a function of time.     

Ring opening of cyclohexane over ZSM-zeolites followed by reversed-flow gas chromatography

Summary Kinetic parameters for the ring opening of cyclohexane over modified ZSM-5 zeolites can be measured simultaneously under non-steady state conditions using reversed-flow gas chromatography. At relatively low temperatures, the main product detected and measured was propene.The mathematical relations used for the calculation of adsorption-desorption rate constants, surface reaction rate constants and adsorption equilibrium constants for the reactant cyclohexane are different from previously studied catalytic reactions, owing to the different experimental arrangement for feeding the catalytic bed. The diffusion bands obtained experimentally for reactant or product are described by the sum of two or three exponential functions of time, respectively. From the exponential coefficients of time and the pre-exponential factors, all determined by simple PC programs, the above kinetic parameters are calculated.     

Ring opening of cyclohexane over ZSM-zeolites followed by reversed-flow gas chromatography

Summary Kinetic parameters for the ring opening of cyclohexane over modified ZSM-5 zeolites can be measured simultaneously under non-steady state conditions using reversed-flow gas chromatography. At relatively low temperatures, the main product detected and measured was propene. The mathematical relations used for the calculation of adsorption-desorption rate constants, surface reaction rate constants and adsorption equilibrium constants for the reactant cyclohexane are different from previously studied catalytic reactions, owing to the different experimental arrangement for feeding the catalytic bed. The diffusion bands obtained experimentally for reactant or product are described by the sum of two or three exponential functions of time, respectively. From the exponential coefficients of time and the pre-exponential factors, all determined by simple PC programs, the above kinetic parameters are calculated.     

Solvent retention in thin polymer films studied by gas chromatography

Thin polystyrene films were produced by spin coating from solutions in toluene. The amount of solvent retained in the films after drying for different times was measured using gas chromatography. Whereas for thicker films (thickness > 200 nm), the relative amount of solvent in the films is less than a few percent, the proportion of toluene increases significantly in thinner films. The thickness dependence of the mass of retained solvent shows that the solvent is mainly retained at the polymer–substrate interface. The solvent desorption rate exhibits no variation on the film thickness.     

Analysis of trace components in gases by gas chromatography

Summary Relationships are derived describing how the detection limit of a chromatographic system depends on the minimum detectable limit of the detector and the chromatographic parameters such as column length, efficiency, carrier gas flow rate and the capacity factor. Performance data of detectors developed in the last 25 years at the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences are given and a few selected application examples are listed. These include trace analysis by preconcentration and by direct analysis and the utilization of multidimensional gas chromatography with two columns, two detectors, a 12-port valve, and a catalytic conversion reactor.     

New uses of thermionic ionization detectors in gas chromatography

Summary A newly designed thermionic detector was operated in two modes of thermionic ionization detection (TID), and a third mode of catalytic flame ionization detection (CFID). Operating modes were varied by changing the composition of the electrically-heated thermionic/catalytic source, and the composition of gases supplied to the detector. A TID-2 source activated by a low concentration of Cs/ceramic was operated in a dilute H₂/air environment and provided specific responses to nitrogen and phosphorus compounds. A TID-1 source activated by a high concentration of Cs/ceramic was operated in a N₂ environment and provided very high specificities and picogram detectivities to compounds containing electronegative functional groups such as the NO₂ group. A CFID source formed from Ni/ceramic was operated in a H₂/air flame environment and provided universal responses to all organics with enhanced responses to some heteroatom compounds.Presented at the 14th International Symposium on Chromatography London, September, 1982     

气相色谱法研究胶体金负载纳米金属氧化物催化氧化CO行为

将胶体金负载于不同的金属氧化物纳米材料上,分别制备了Au/TiO2、Au/γ-Al2O3、Au/Fe2O3和Au/ZnO纳米材料,采用透射电子显微镜(TEM)对负载前后的材料进行了表征。利用气相色谱法检测CO的浓度,在选定的色谱条件下,5 min内就实现了快速分离测定,该检测方法的线性范围为1.0×10-6%~10%(体积分数),回收率为96.0%~103.6%,精密度为1.2%~2.3%。应用自制的催化活性评价装置对氧化物纳米材料催化氧化CO的行为进行了研究,结果表明:负载金氧化物纳米材料催化氧化CO的性能在50~200℃均优于相应的未负载金的材料,其中Au/TiO2的催化氧化效果最佳,且在一定气体流速范围内具有稳定的催化活性。     

Time separation of adsorption sites on heterogeneous surfaces by inverse gas chromatography

Summary The measurement of local (homogeneous) adsorption energiesε _i , local monolayer capacities,c _max ^* , local adsorption isotherms,θ _i (p, T, ε), and probability density functions for adsorption, f(ε) and ϕ(ε,t), can be used to study the mechanism of adsorption of five gaseous hydrocarbons on the heterogeneous surface of magnesium oxide. The method does not use analytical or numerical solutions of a classical integral equation comprisingf(ε) as unknown, but it depends on a time function of gas chromatographic peaks obtained by short flow-reversals of the carrier gas. The results for adsorption of ethane, ethylene, acetylene, propene, and l-butene on MgO, in the absence and presence of O₃ are given and discussed on the basis of a mechanism proposed earlier for argon on titatium dioxide.     

Improvements in preconcentration methods for determining trace impurities in ultra-pure gases by gas chromatography

Summary The blanks in preconcentration methods for determining trace amounts of impurities in ultra-pure gases, which include the preconcentration volume of sample gases, carrier gas impurities and atmospheric contamination are discussed and three concentration methods for eliminating blank errors are proposed. These are, the differential volume method by concentrating at the same flow-rate but different times (DVMSF), the differential time method by concentrating the same volumes at different flow-rates (DTMSV) and the differential volume method by concentrating for the same times but different flow-rates (DVMST). DVMST is proposed as the best method for its ability to eliminate all blank errors described. The methods are used to determine trace amounts of Ar+O₂ and N₂ in ultra-pure hydrogen. Calculations demonstrate that the methods can effectively improve analytical accuracy.     

Physicochemical characterization of dilute n-alcohol/biodiesel mixtures by inverse gas chromatography

Inverse gas chromatography (IGC) has been used to determine the physicochemical parameters that characterize solution thermodynamic interactions in biodiesel-n-alcohol solute systems. Such data is of value to chemical engineers and separation scientists in optimizing separation processes to separate alcoholic solutes at low concentrations in soybean oil methyl ester mixtures (biodiesel). The derived activity and Henry's Law coefficient data can be used to rationalize the interaction of four members of an n-alcoholic homologous series and the soya-based methyl ester solvent in terms of such esters as "green" renewable solvents. Sorption isotherm data confirm linear behavior in most cases between the solute (alcohol) vapor state concentrations and their uptake into the biodiesel phase. Overall, the experimentally determined activity coefficients agree well with those predicted by solution thermodynamic theories as well as correlative chemical engineering equations.     

Chromatographic determination of the rate and extent of absorption of air pollutants by sea water

Summary A simple chromatographic method is developed to determine the rate constant for expulsion of an air pollutant from water or its diffusion parameter in the liquid, the rate constant for chemical reaction of the pollutant with water, its mass transfer coefficient in the liquid, and the partition coefficient between liquid water and air. From these physicochemical parameters, the absorption rate by sea water and, therefore, the depletion rate of a polluting substance from the air can be calculated, together with the equilibrium state of this absorption. The method has been applied to nitrogen dioxide being absorbed by triple-distilled water and by sea water, at various temperatures. From the temperature variation of the reaction rate constant and of the partition coefficient, the activation energy for the reaction and the differential heat of solution were determined.     

An alternative approach for predicting peak width in gas chromatography

Summary The linear relationship between natural logarithm of width factor (lnp′)and natural logarithm of retention factor (lnk) is demonstrated. This relationship is then used to establish the relationship between (lnp′), absolute temperature (T), and carbon number (z), as follows: Inp′=A+bz+c/T+dz/T where A, b, c and d are thermodynamically related constants. The above equation is used to predict the unadjusted widths (w _R ) ofn-alkanes, fatty alcohols and fatty acid methyl esters (FAMEs) at various temperatures, predicted values are in good agreement with experimental values. The above equation can be used to predict the width of FAMEs from rice bran oil. The largest difference between the experimental and predicted values is 0.66 s or 6.32%.     

Early evolution of gas adsorption chromatography. Part I: Displacement development and the beginnings of elution-type investigations

Summary This two-part article reviews the early evolution of gas-adsorption chromatography, before the advent of partition chromatography. After reporting on the early tests to determine the light hydrocarbon content of natural gas and the methodology described by P.Schuftan, the research activities of S. Claesson and C.S.G. Phillips on displacement-type gas-adsorption chromatography and early work on elution-type analysis by G. Damk?hler, E. Wicke, E. Glueckauf and G. Hesse are discussed. Part II of this review article will be published in the forthcoming issue ofChromatographia.     

Advances in the determination of volatile organic solvents and other organic pollutants by gas chromatography with thermal desorption sampling and injection

Summary The problem of the separation of 34 volatile organic chlorinated compounds is solved by using three different GC columns selected according to the needs of the particular separation required. The effect of water vapor contained as moisture in the trapped air on the retention of some characteristic compounds is studied. The influence of dead volumes on trap injection is also studied.Dedicated to Prof. Dr. A. Liberti on the occasion of his 70th birthday.     

邻氨基苯腈气相色谱分析法的研究

采用邻苯二甲酸二壬酯为固定液,Chromsorb W/AW为载体的色谱柱和热导检测器,用苯乙腈为内标物,对邻氨基苯腈试样进行了分析,其中邻氨基苯腈的回收率为100.5%,试样浓度小于1%时测定的相对标准偏差小于8.6%。     

Novel stationary phases based on asphaltenes for gas chromatography

We present the results of investigations on the possibility of the application of the asphaltene fraction isolated from the oxidized residue from vacuum distillation of crude oil as a stationary phase for gas chromatography. The results of the investigation revealed that the asphaltene stationary phases can find use for the separation of a wide range of volatile organic compounds. The experimental values of Rohrschneider/McReynolds constants characterize the asphaltenes as stationary phases of medium polarity and selectivity similar to commercially available phases based on alkyl phthalates. Isolation of asphaltenes from the material obtained under controlled process conditions allows the production of a stationary phase having reproducible sorption properties and chromatographic columns having the same selectivity. Unique selectivity and high thermal stability make asphaltenes attractive as a material for stationary phases for gas chromatography. A low production cost from a readily available raw material (oxidized petroleum bitumens) is an important economic factor in case of application of the asphaltene stationary phases for preparative and process separations.