Isotropic and anisotropic properties of adsorption-induced deformation of porous carbon materials

In the present work, the issues of isotropic/anisotropic character of adsorption-induced deformation of carbon adsorbents were addressed. A simple model of microporous zones of carbon adsorbents was developed in terms of their structure determined by a raw material and activation conditions. The model made it possible to evaluate the number of micropores (similar to 10(20) g(-1)) and the number of microporous nanocrystals with micropores (denoted as an elementary microporous zone, EMZ) similar to 10(11) g(-1 )for four carbon adsorbents differing in raw material and activation conditions. The equiprobable orientation of the EMZ in activated carbons nullifies the influence of the anisotropic structure of nanocrystallites on the changes in macroscopic dimensions of the adsorbents stimulated by adsorption. The assumption was confirmed by the dilatometry measurements of the CO2 adsorption-induced deformation for the commercial activated carbon Sorbonorit-4 granules, which were deposited parallel and perpendicular to the vertical axis of the dilatometer. The adsorption-induced strain isotherms did not depend on the orientation of the Sorbonorit-4 granules and exhibited the non-monotonic character. The initial contraction of the adsorbent was followed by its expansion with increasing pore filling. The contraction-expansion transition, as well as the contraction magnitude, were found to be temperature-dependent within the temperature range from 216.6 to 393 K. The compressibility and triaxial compression modulus of Sorbonorit-4 were evaluated over the temperature range from 216.6 to 293 K. The temperature dependences of both parameters were approximated by exponential functions. The triaxial compression modulus of Sorbonorit-4 decreased from 42 to 10 GPa and the compressibility increased by five times with a rise in temperature from 216.6 to 293 K.