Identification of mass transfer resistances in microporous materials using frequency response methods

The frequency response (FR) method, a pseudo-steady state relaxation technique employing perturbation frequency, plays an essential role in discriminating between multi-kinetic mechanisms in microporous materials for separation and catalytic processes. Experimental and theoretical principles are reviewed for three frequency response methods, including one commonly used batch system with volume perturbation and two recently developed flow-through systems with pressure or concentration oscillation. Even though these methods have different overall transfer functions, they can be linked closely through the adsorbed-phase functions, which account for individual or coupling of mass transfer resistances and heat effects. Mass transfer resistances include micropore diffusion, macropore diffusion, surface barriers, and external film resistance. By judicious application of FR methods, it is not only possible to identify dominating mass transfer resistances but also to extract reliable mass transfer coefficients based on corresponding mathematical models. Representative examples to display the ability of the FR methods in studies of zeolites, carbon molecular sieves, and other microporous materials are discussed. Mixture studies and future developments, including nonlinear frequency response and chemical reactions, have also been briefly described.