Carbon Adsorbents with Molecular Sieve Properties

Abstract

IT has been found possible to obtain an adsorbent with typical molecular sieve properties by thermal processing of granulated carbonaceous material (prepared on the basis of pyrolysis of wood material at about 500° C) at 970–980° C in the atmosphere of gases preventing the oxidation of the carbon material. The adsorption of oxygen at −196° C occurs in its micropores, whereas nitrogen and argon are adsorbed at this temperature on the surface of transitional pores and macropores¹ only. The volume of micropores of this adsorbent is 0.15 cm³/g and the value of the specific surface of transitional pores and macropores is nearly 17 m²/g. Adsorption of nitrogen and argon is substantially increased by raising the temperature to −78° C. This effect is completely analogous to that observed in the adsorption of nitrogen on ‘Zeolite NaA’ (type 4A)². At 20° C water vapour is adsorbed in the micropores of the adsorbent, whereas benzene vapour is adsorbed at this temperature only on the surface of transitional pores and macropores. The values of adsorption are presented in Table 1. From the adsorption data it is evident that the dimensions of openings of the micropores are close to 4 Å. The adsorbent may thus be considered the carbon analogy of the ‘Zeolite NaA’ (4A). The shape of the adsorption isotherm of water vapour is characteristic for the system water-active charcoal¹. The steep rise of the adsorption isotherm is observed in the region of relative pressures 0.4–0.5 and the isotherm reveals a typical hysteresis.