The origin of the surface barrier in nanoporous materials

Surface barriers are influencing the mass transfer in nanopores, but their origin is unclear and can be quite different in different materials. For MFI and CHA membranes studied here, we show that the surface barrier may be a surface diffusion process with higher activation energy than the surface diffusion process in the pores, but other possible mechanisms such as pore blocking and pore narrowing has not been ruled out. The higher activation energy is probably a result of less interaction between adsorbed molecules at the pore mouth than inside the pores, i.e. the barrier is simply a geometrical effect in these materials. For pure components at low concentration in MFI zeolite, we found that barrier is proportional to the product of the molecular weight and heat of desorption. For MFI and CHA zeolite, we observed that the barrier is a function of concentration and approach zero at high concentration and that the barriers of the components become more similar due to interaction between the components in mixtures, which explains the high and selective mass transfer displayed by these nanoporous materials at high concentration.