(376i) Evaluation of Desorption and Diffusion in Zeolite Membrane with Nano-Perm Porometry

Zeolite is an aluminosilicate crystal having a pore structure of molecular size. It is expected that zeolite membranes can be applied to separation of small molecule mixtures based on the differences of molecular size and/or adsorption. It is important to evaluate the adsorption, diffusion and desorption of molecules to understand the mechanism of permeation through zeolite membrane. Though there have been a lot of reports on adsorption in zeolite, methods for evaluating diffusion and desorption in zeolite membrane have not yet been established. In this study, we aimed to evaluate diffusion and desorption of hydrocarbon adsorbed on silicalite-1 membrane by using Nano-Perm Porometry.

Silicalite-1 membrane was synthesized by a hydrothermal synthesis method after application of seed crystals by dip coating on α-alumina tubular support (outer diameter 10 mm, length 30 mm, and average pore diameter 150 nm). Silicalite-1 membrane was set in a module and heated to a given temperature while flowing He in a Nano-Perm Porometer. Hydrocarbon, which was condensable in zeolite pore, was supplied at different relative pressures with He. Then, after stopping hydrocarbon supply, we measured the He permeance every 1 second. Hydrocarbons used were n-hexane and 3-methylpentane. The measurement temperature was 373 - 473 K.

He permeance through silicalite-1 membrane was about 3.0 × 10^-7 mol m^-2 s^-1 Pa^-1 after pretreatment. The He permeance with n-hexane or 3-methylpentane substantially decreased to less than 4.0 × 10^-8 mol m^-2 s^-1 Pa^-1. Clearly, hydrocarbon adsorbed in the pore of silicalite-1 membrane and inhibited the permeation of He. After the supply of hydrocarbon stopped, the He permeance recovered to about 3.0 × 10^-7 mol m^-2 s^-1 Pa^-1. This result showed that hydrocarbon diffused and desorbed from silicalite-1 membrane and thereby He was able to permeate through silicalite-1 membrane. We found that the recovery rate of the He permeance became faster as the membrane temperature increased. With increasing membrane temperature, the adsorption equilibrium of hydrocarbon was shifted to desorption and its diffusion in the micropore increased. These results indicated that this method can evaluate differences of hydrocarbon diffusion and desorption rate in zeolite membrane. Moreover, the recovery rate of He permeance was extremely slow when we used 3-methylpentane at 373 K, suggesting that diffusion and/or desorption of 3-methylpentane in silicalite-1 membrane was extremely slow. In summary, this method can evaluate the rates of diffusion and desorption, possibly leading to the understanding of diffusion of hydrocarbon and its desorption in zeolite membrane.