Study of intracrystalline diffusion in zeolites 6. Comparison of the results obtained by the method of densitometry of light and the adsorption method

The use of the method of densitometry of light passing through a layer of zeolite crystals permits determining the limiting stage of diffusion during adsorption by zeolites. Diffusion in transport pores and external heat exchange play the basic role in adsorption of benzene by NaX zeolite, while diffusion in crystals is the determining mechanism of transport in adsorption of water by NaA and NaX zeolites. The diffusion coefficients of water in NaX zeolite have an order of magnitude of 10^–17 m²/sec and increase with an increase in the degree of filling. An explanation for the anomalous behavior of the kinetic adsorption curve for brief times of adsorption of water by zeolite is proposed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1224–1228, June, 1990.     

Collocation Methods for Weakly Singular Second-kind Volterra Integral Equations with Non-smooth Solution

Collocation type methods are studied for the numerical solutionof the weakly singular Volterra integral equation of the secondkind: where the solution (t) is assumedto have the form f(t) = x(t)+r^?(t), x and being sufficientlysmooth. The solution is approximated near zero by a linear combinationof powers of t?, and away from zero by the usual polynomialrepresentation. Convergence is proved and many numerical experimentsare carried out with examples from the literature. A comparisonis made with a method of Brunner & Norsett (1981), originallydeveloped for (1) with a smooth solution. Special attentionis paid to the numerical approximation of the so-called momentintegrals which emerge in the collocation scheme.     

Structured mesoporous Mn, Fe, and Co oxides: Synthesis, physicochemical, and catalytic properties

Structured mesoporous Mn, Fe, and Co oxides are synthesized using “soft” and “hard” templates; the resulting materials are characterized by XRD, SEM, TEM, BET, and TG. It is shown that in the first case, the oxides have high surface areas of up to 450 m²/g that are preserved after calcination of the material up to 300°C. Even though, the surface area of the oxides prepared by the “hard-template” method does not exceed 100 m²/g; it is, however, thermally stable up to 500°C. Catalytic activity of mesoporous oxides in methanol conversion was found to depend on both the nature of the transition metal and the type of template used in synthesis.     

Comprehensive core-level study of the effects of isomerism, halogenation, and methylation on the tautomeric equilibrium of cytosine

Core-level X-ray photoemission and near-edge X-ray absorption fine structure spectra of 5-methylcytosine, 5-fluorocytosine, and isocytosine are presented and discussed with the aid of high-level ab initio calculations. The effects of the methylation, halogenation, and isomerization on the relative stabilities of cytosine tautomers are clearly identified spectroscopically. The hydroxy-oxo tautomeric forms of these molecules have been identified, and their quantitative populations at the experimental temperature are calculated and compared with the experimental results and with previous calculations. The calculated values of Gibbs free energy and Boltzmann population ratios are in good agreement with the experimental results characterizing tautomer equilibrium.