No abstract

This system illustrates "partial molecular sieve action". Methane has a higher affinity for the sorbent and hence is preferentially sorbed a t equilibrium, while nitrogen diffuses through the crystal more rapidly and thus is preferentially talcen up during the early stages of sorption.Measurements were made with the pure gases and with mixtures a t 0" and -79.4" C. The sorption isotherms fit Langmuir equations and the isosteric heats of sorption are essentially independent of concentration. The sorption rates for the pure gases may be characterized by diffusion coefficients, D , calculated in the usual manner assuming the flus of diffusion to be proportional to the concentration gradient. The resultant values for D increase with increasing sorbate concentration. Diffusion of a mixture may be formally characterized by D's for each component. While that for methane is approximately the same as for methane alone, D for nitrogen in mixtures is much larger than for the pure gas and also varies with con~position. This, as well as the existence of a temporary maximum in the amount of nitrogen sorbed, may be explained by considering the driving force for diffusion to be the gradient in chemical potential rather than in concentration.

The general case of a first-order catalytic reaction occurring during elution of a reactant through a chromatographic column is discussed. Under conditions of low reactant partial pressure and rapid adsorption relative to the rate of the surface reaction, the fractional conversion of a pulse of reactant passed through a chromatographic column is given by an equation analogous to that for the conversion under similar conditions in a steady-state flow reactor. A major advantage of the chromatographic technique is that it permits a determination of the extent of adsorption under reaction conditions, and thus of the rate constant for the reaction on the catalyst surface. The method is illustrated by a gas chromatographic study of the catalytic isomerization of cyclopropane on Linde Molecular Sieve 13X. The rate constant of the surface reaction was found to be k = 1.3 X 1010 exp( -30,000/TZF) sec.-1. The heat of adsorption of cyclopropane under reaction conditions was 11.0 kcal. mole-1. 1039 (1958).

No abstract

No abstract