Are MOF membranes better in gas separation than those made of zeolites?

Despite much progress in the development of zeolite molecular sieve membranes, there is so far no industrial gas separation by zeolite membranes, with the exception of the de-watering of bio-ethanol by steam permeation using LTA membranes. During the last 5 years, metal-organic framework (MOF) membranes have been developed and tested in gas separation. The complete tool box of techniques originally developed for the preparation of zeolite membranes could be applied for the preparation of MOF membranes, such as the use of macroporous ceramic or metal supports, seeding, intergrowth-supporting additives, and microwave heating. There are some structure-related properties of MOFs which recommend them as suitable material for molecular sieve membranes. On the other hand, the structural flexibility of MOFs apparently prevents a sharp molecular sieving with a pore size estimated from the 'rigid' crystallographic structure by size exclusion. An application of MOFs is predicted in so-called mixed matrix membranes which show improved performance in comparison with the pure polymer membranes. Different from zeolites as organic-inorganic material, the MOF nanoparticles can be easily embedded into organic polymers, and standard shaping technologies to hollow fibers or spiral wound geometries can be applied.