Johann Gaube

Studies on product distributions of iron and cobalt catalyzed Fischer–Tropsch synthesis

Abstract The dependencies of hydrocarbon product distributions of iron and cobalt catalyzed Fischer–Tropsch synthesis on partial pressures of reactants have been studied. For cobalt catalysts particular attention has been focussed on the modification of distributions by secondary chain growth of readsorbed 1-alkenes while for iron catalysts secondary chain growth has been proved as negligible. The widely discussed concept of two superimposed Anderson–Schulz-Flory distributions has been applied for the representation of product distributions for both iron and cobalt catalysts. Based on 1-alkene and ethene cofeeding experiments with cobalt catalysts and the promoter effect of alkali on iron catalysts the conclusion has been drawn that superimposed distributions with different chain growth probabilities are the result of different chain growth mechanisms. These results and the dependence of product distribution on the partial pressures of hydrogen and carbon monoxide have lead to the conjecture that products associated with the lower growth probability are formed by the well accepted CH2 insertion mechanism.

Interpretation and kinetic modeling of product distributions of cobalt catalyzed Fischer–Tropsch synthesis

Carbon number distributions of Fischer–Tropsch products on iron and cobalt catalysts show deviations from the ideal Anderson–Schulz–Flory (ASF) distribution. For products obtained on cobalt catalysts these deviations are traced back by many authors to re-adsorption and incorporation of 1-alkenes followed by subsequent chain growth. In the present work, it could be shown by means of model calculations and based on experiments with co-feeding of ethene and 1-alkenes that such subsequent chain growth cannot be regarded as the main reason of observed deviations of the carbon number distribution from the ideal ASF distribution. The co-feeding experiments suggest that these deviations are the consequence of two different mechanisms of chain growth causing a superposition of two ASF distributions. Consequently, the carbon number distributions are represented by this superposition. In order to describe distributions as a function of reaction conditions the model parameter, the growth probabilities α1 and α2 as well as μ1, the fraction of distribution (α1) are presented as function of the partial pressures of hydrogen and carbon monoxide. Finally, the typical model parameters of products formed on cobalt and iron are compared.

Thermodynamics of aqueous poly(ethylene glycol)-dextran two-phase systems using the consistent osmotic virial equation

Abstract In this work we present a thermodynamic model for the prediction of the liquid-liquid phase behavior of aqueous poly(ethylene glycol) (PEG) - dextran two-phase systems. The model is based on the McMillan-Mayer solution theory (1945) and results in thermodynamically consistent expressions for the chemical potentials of the solutes derived from the osmotic virial equation (COVE). Applying the COVE, we have examined the predictability using a complete and reliable data- base of liquid-liquid equilibrium (LLE) and vapor-liquid equilibrium (VLE) data. As a result of this examination, we were able to demonstrate the essential influence of the molecular-weight distribution of polydisperse polymers on the LLE predictions. Accounting for the polydispersity in our calculations, the prediction of the compositions as well as the molecular-weight distributions in the coexisting phases is in good agreement with our experimental results, as illustrated for the system PEG 3000 + dextran 110000 + water at 293.15 K. It should be stressed, that these calculations are true predictions, since the LLE were calculated using model parameters determined from VLE measurements alone.

Consistent view of electrolytes in aqueous two-phase systems.

The effects of electrolytes in aqueous two-phase systems are investigated. It is shown that macroscopic and molecular models give a consistent view of electrolytes at interfaces. The electrostatic potential difference delta psi between coexisting phases is a common property at interfaces even though the phases are strictly electroneutral and delta psi can not be measured. It is shown how delta psi can be quantified under controlled conditions. Additionally, a molecular picture is presented based on computer simulations.

Virial coefficients of binary mixtures composed of polar substances

Abstract For the mixtures methanol—methyl acetate, methanol—diethyl ether, acetonitrile—methyl acetate and diethylamine — methyl acetate, the second cross virial coefficients and the second and third virial coefficients of the pure substances composing these mixtures have been determined. The new equipment for the measurement of virial coefficients of pure substances and binary mixture is described. The accuracy of the second virial coefficients is better than 20 cm 3 mol −1 .

GEQUAC, an excess Gibbs energy model describing associating and nonassociating liquid mixtures by a new model concept for functional groups

Abstract The physically founded excess Gibbs energy model, GEQUAC, allows the description of excess properties for associating as well as nonassociating liquid mixtures. This is achieved by explicitly accounting for different poles of molecular surface between which strong interactions take place in a mixture. In previous work, the model concept and the subsequent model equation were tested against results of Monte-Carlo computer simulations and proved the ability to describe gE, hE and TsE of ketone+n-alkane and n-alcohol+n-alkane mixtures. This work provides model parameters fitted to binary isothermal VLE and hE data for the mixtures butanone+n-heptane, 1-butanol+n-heptane and butanone+1-butanol as a first step in application. The model parameters show physical relevance and are used for prediction of isothermal VLE and hE data (own measurements) at 323 K of the ternary mixture butanone+n-heptane+1-butanol.

A study of the excess properties of mixtures 1-alkanol/n-alkane using a generalized chemical theory

Abstract A chemical theory based on the equation of Lichtenthaler, Donohue and Prausnitz for the entropy of mixing has been developed. For this theory a numerical procedure has been given which allows the consideration of association parameters Δ G 0 i and Δ H 0 i , depending on the degree of association i . The excess properties G E and H E of 1-alkanol/ n -alkane mixtures with varying carbon numbers of both the 1-alkanol and the n -alkane have been studied at moderate and high concentration of 1-alkanol. Whereas the chemical theory describes well H E , H E∞ A and C E p , a correction term to G E chem , due to an entropic effect, must be added for mixtures containing C 1 -, C 2 - and C 4 -alkanols. This effect may be explained by deviation from random mixing of the species caused by different molecular structure of associated alkanols and alkanes. The dependence of G E and H E on the carbon number of the 1-alkane can be well described by the chemical theory.

Kinetic studies on the hydrogenation of 1,3-butadiene, 1-butyne and their mixtures

The kinetics of the heterogeneously catalyzed hydrogenation of 1,3-butadiene and mixtures of 1-butyne/1,3-butadiene have been investigated for a Pd/Al 2 O 3 egg-shell catalyst. In contrast to the hydrogenation of butenes, the gas phase hydrogenation of 1,3-butadiene shows a restricted reproducibility. This behaviour is traced back to the very dense layer of strongly and rigidly adsorbed Butadiene molecules. The reaction order of the 1,3-butadiene hydrogenation is >1 with respect to hydrogen and

Selective Hydrogenation of Multiple Unsaturated Compounds

Selective hydrogenation reactions are wildly spread in chemical industry, such as the selective hydrogenation of alkadienes in petrochemical industry and the selective hydrogenation of α,β-unsaturated aldehydes in the production of Fine Chemicals. Considering the selective hydrogenation of alkadienes, tools for tailoring catalyst design are the modeling of the interrelation between chemical reaction and mass transfer based on fundamental studies of the kinetics and the reaction mechanism, where the focus is laid on the hydrogenation of 1,3-butadiene, 1,3-cyclooctadiene and benzene. Factors controlling the intramolecular selectivity of the hydrogenation of α,β-unsaturated aldehydes, such as acrolein, crotonaldehyde, citral and cinnamaldehyde, to unsaturated alcohols are the metal particle size and morphology of the supported metals, metal-support interactions and the influence of the nature of active sites in bimetallic catalysts. Surprisingly monometallic silver and gold catalysts are also convenient to produce unsaturated alcohols. Industrial applications of selective hydrogenations are given.

The role of adsorption in the oxidation of α,β-unsaturated aldehydes on Mo-V-oxide based catalysts

Publisher Summary The production of acrylic acid is a two stage process. The first stage is the oxidation of propene on catalysts of the type Mo–Bi–Fe-oxides to acrolein and second is the oxidation of acrolein on catalysts usually composed of Mo–V–Cu-oxides. The oxidation of acrolein is an appropriate model reaction to study partial oxidations, because of the moderate number of reaction products and high selectivity towards acrylic acid. Therefore, several investigations have been carried out for the elucidation of the function of catalyst components. Particularly, the dependence of activity and selectivity on the ratio of Mo 6+ /V 4+ has been discussed in the chapter. Kinetic and infrared studies have led to the development of a mechanism, including declarations, about specific interactions of adsorbed species with catalyst components, such as Mo and V. Infrared studies revealed that adsorbed acrylate anion is a relatively stable intermediate. The reaction of this anion to adsorbed acrylic acid by hydroxyl groups of the catalyst surface and desorption of acrylic acid are regarded as rate determining steps of the acrolein oxidation. This chapter discusses a more detailed kinetic study particularly of the role of the adsorption of the reactants. The reduced rate of acrolein oxidation at elevated acrolein concentration is interpreted by the adsorption of acrolein on reduced sites, and consequently described by a negative reaction order, with respect to acrolein in the reoxidation term r(O 2 ) 1 .