Porous crystalline molecular-sieve membranes for xenon separation from krypton and air

Abstract

The main objective of this work is the development of continuous crystalline microporous molecular sieve membranes to separate Kr/Xe and air/Xe gas mixtures. Specifically, for Kr/Xe separation we demonstrate that ZIF-8 and AlPO-18 membranes can effectively separate Kr/Xe gas mixtures. For air/Xe separation, we demonstrate the first examples of any porous crystalline membrane to separate air/Xe gas mixtures. In the case of Kr/Xe separation, the best ZIF-8 membranes separated Kr/Xe mixtures with Kr permeances as high as 50.8 GPU and separation selectivities as high as 16.1. AlPO-18 membranes showed the highest Kr/Xe separation selectivity of 7.9 and an unprecedented Kr permeance as high as 940 GPU. For air/Xe separation we demonstrate that ZIF-8 and SAPO-34 membranes can separate this gas mixture effectively. Specifically, our best ZIF-8 membranes showed air permeances as high as 118 GPU and separation selectivities as high as 12.4 for air/Xe gas mixtures. SAPO-34 membranes’ air/Xe separation performance exceeded that of ZIF-8, showing air permeances as high as 690 GPU and separation selectivities of 30.1. Molecular sieving, competitive adsorption, and differences in diffusivities were identified as the separation mechanisms for both gas mixtures. Among these mechanisms, molecular sieving and differences in diffusivities were the dominant mechanisms leading to Kr and air selective membranes. This work represents one of the first known examples of microporous crystalline membranes with molecular sieving properties to separate Kr/Xe and air/Xe gas mixtures. The proposed separation technology represents an attractive alternative route to cryogenic distillation, typically employed to separate these gases.