Screening of Metal-Organic Frameworks (MOFs) for Mixture Separations

There has been an exponential growth in the development of metal-organic frameworks (MOFs) for use as adsorbents in mixture separations. For large scale applications, such separations are commonly performed using the pressure swing adsorption (PSA) technology. PSA processes consist of a series of fixed bed adsorption devices, operated in a transient mode consisting of adsorption and desorption cycles. The primary objective of this article is to discuss the variety of metrics and tools that are available for evaluation and ranking of MOFs with regard to their separation efficacy in PSA operations. In the simplest scenario, the MOFs may be evaluated using a combination of metrics such as adsorption selectivity and uptake capacities, that are calculable from the Ideal Adsorbed Solution theory (IAST) for mixture adsorption equilibrium using unary isotherms as input data. The appropriate metric to be employed depends on whether the desired product, of required purity, is recoverable in the adsorption cycle or desorption cycle of PSA operations. For MOFs in which the guest molecules are tightly confined, intra-crystalline diffusion may also strongly influence the separation performance. Indeed, diffusional influences may be harnessed to effectively exclude one of the constituents from entering the pores. Such exclusion is a strategy that is of vital importance in recovery of high purity alkene from alkene/alkane mixtures. The important message to emerge from this article is that a basic understanding of PSA operations is of vital importance in the screening and ranking of MOFs.