Richard Thede

Diffusion, adsorption and catalytic studies by gas chromatography

Three topics important in many areas of physical chemistry, namely measurements of diffusion coefficients, adsorption kinetics and adsorption isotherms, as well as rates and conversions in heterogeneous catalysis, are reviewed from the gas chromatography viewpoint. After a general introduction, various theoretical sections include synopses of important mathematical models and developments, followed by experimental sections describing basic instrumentation and set-ups, and sections exemplifying the main results with brief discussions. The article is concluded with a methodology embracing simultaneous determination of all three properties (diffusion, adsorption and catalytic rates) in a single experiment by GC.

Theoretical treatment of first-order reversible reactions occurring in a chromatographic reactor, on the basis of consecutive reactions

SummaryAnalytical peak-shape equations were derived for first-order reversible reactions occurring in a chromatographic reactor by treating the reversible reactions as consecutive reactions with alternating products. The results of the analytical peak-shape equations were compared with those from a numerical solution of the partial differential equation system modeling the chromatographic reactor. For small to medium conversions the correspondence was found to be sufficiently close to enable substitution of the numerical solution in fitting procedures for the determination of rate constants.

Parametric Studies on the Determination of Enantiomerization Rate Constants from Liquid Chromatographic Data by Empirical Peak Shape Equations for Multi-Step Consecutive Reactions

On-column enantiomerization is a well documented and practically important special case of the occurrence of a reversible reaction in a chromatographic reactor. The present paper explains the possibility to obtain enantiomerization rate constants from liquid chromatographic data using analytical equations derived for multi-step consecutive reactions. The results are compared with results from a finite difference method, and the application to experimental data is demonstrated.

Synthesis of Naphthylpyridines from Unsymmetrical Naphthylheptadiynes and the Configurational Stability of the Biaryl Axis

A series of different unsymmetrically substituted naphthyl-based diynes were synthesized. These substrates formed the foundation for the assembly of novel biaryls containing pyridine moieties with differently substituted five-membered rings in the backbone of the newly formed heterobiaryl system. The key step for their efficient construction was the photo- and cobalt-catalyzed [2 + 2 + 2] cycloaddition reaction between the corresponding naphthyldiyne and aceto- or benzonitrile. The heterobiaryl products have been isolated and investigated with respect to the configurational stability of their biaryl axis using dynamic chiral HPLC; subtle effects of the substitution pattern on the stability of the axis were observed. For several compounds the activation barriers (ΔG(‡)) of racemization were determined. Suitable substitution of the five-membered ring backbone exemplarily allowed the Co-catalyzed enantioselective cyclization to yield the enantiomerically enriched heterobiaryl.

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

SummaryThe 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.

Kinetic models for catalytic selection processes as applied to asymmetric hydrogenation

Abstract Kinetic terms for product formation in selection processes are evaluated for the example of asymmetric hydrogenation. The investigated models are characterized by preceding intermolecular equilibria to 2–4 stereoisomeric substrate complexes as well as direct intramolecular interconversion between corresponding diastereomeric intermediates. In the case of isobaric conditions the gross-hydrogen consumption can be described in each case by an equation analogous to simple Michaelis-Menten-type kinetics. The interpretation of the resulting constants is briefly demonstrated for a few practical examples. The temperature dependence of the enantiomeric ratio for the resulting products, as for example for a selection process, is also considered for the investigated models.

Investigations into the formation and stability of cationic rhodium diphosphane η6-arene complexes.

Rhodium-η(6)-arene complexes can be generated in the presence of arenes following the hydrogenation of the diolefin in rhodium catalyst precursors of the type [Rh(PP*)(diolefin)]X (PP* = chelating diphosphane, X = noncoordinating anion). In this paper we report the characterization of such arene complexes with the ligands DuPhos, dipamp, dppe, Tangphos, dppf, and diop by means of NMR spectroscopy ((31)P, (103)Rh) and X-ray analysis. A procedure that follows the approach to equilibrium as a function of time monitored by using an UV/Vis diode array was used to determine 20 stability constants. Analyses were accomplished directly from the spectra by either a numeric and/or a new analytic solution of the underlying system of differential equations. Additionally thermodynamic parameters were determined in the temperature range between 278 and 318 K.

Second-order kinetics in the liquid chromatographic reactor

Equations for the determination of second order rate constants with the pulse overlay method, based on the application of empirical peak shape equations are derived. This approach was then applied in an experimental study of the rection of pyridine with tetrachloroterephthaloyl chloride in a reversed-phase HPLC system. Rate constants evaluated from reaction chromatograms were found to be in good agreement with those obtained earlier for this reaction by Chu and Langer in chromatographic reactor experiments under pseudo-first order conditions.

Autocatalytic reactions in a liquid chromatographic reactor

The possibility of extending the treatment of second-order on-column reactions with two different reactants by empirical peak shape equations to autocatalytic on-column reactions was investigated. An approximate analytical solution is presented, which for small to medium conversions agrees with the solution of the ideal non-equilibrium model of chromatography by a finite difference method.

Determination of rate constants of consecutive first order reactions occurring on chromatographic columns

Abstract Approximate analytical equations were developed for the products of consecutive reactions occurring on a chromatographic column. The equations were tested both by numerically evaluated chromatograms and the acid catalyzed hydrolysis of phthalic dichloride in a liquid chromatographic reactor.