Question

For gas separation membranes, which transport mechanism relies on the difference in the rate at which gas molecules penetrate the membrane material based on their molecular size?

Accepted Answer

The transport mechanism that relies on the difference in the rate at which gas molecules penetrate a membrane based on their molecular size is known as molecular sieving. This process occurs primarily in rigid, porous membrane materials such as zeolites, carbon molecular sieves, or metal-organic frameworks. These materials contain pores with precise, uniform dimensions that are similar in scale to the kinetic diameter of gas molecules. The kinetic diameter is the effective size of a gas molecule as it moves through space. In molecular sieving, the membrane acts as a physical barrier where molecules smaller than the pore diameter pass through the material freely, while larger molecules are excluded or significantly slowed down because they are physically too large to enter the pores. This mechanism differs from the solution-diffusion mechanism, where separation is governed by how well a molecule dissolves into a polymer and how fast it diffuses through the material segments. In molecular sieving, the separation efficiency is determined by the strict geometric relationship between the size of the gas molecule and the dimensions of the internal membrane pores.

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For gas separation membranes, which transport mechanism relies on the difference in the rate at which gas molecules penetrate the membrane material based on their molecular size?