A model for multilayer adsorption of small molecules in microporous materials

In the paper applicability of BET approach to modeling of adsorption of small nearly spherical molecules in submicro- and microporous sorbents is discussed. The BET assumptions are analyzed from physical point of view, and properly generalized to handle geometrical as well as energetic conditions for multilayer adsorption in microporous structures. A formal description of multilayer adsorption is derived, by using the thermodynamic approach, with any energy profile across the layers being admitted. To get an analytical formula describing the process one proposes to assume an exponential distribution of pore capacity, and apply the BET equation with limited number of layers. Formal properties of the obtained formula, referred to as LBET model, are carefully examined and compared with those of Dubinin-Radushkievith and BET equations. The examination showed that the LBET model describes adequately adsorption process on materials of submicroporous and microporous structure with dominant fraction of small micropores. It provides reliable information on the porous surface capacity and on the first layer adsorption energy. It gives also a semiquantitative characterization of pore volume distribution. Application of the LBET model to interpretation of sorption isotherms of water and methanol on a hard coal and of carbon dioxide on activated carbon is presented.