Methodologies for evaluation of metal–organic frameworks in separation applications

Abstract

The separation performance of fixed-bed adsorbers is governed by a number of factors that include (a) adsorption selectivity, (b) uptake capacity, and (c) intra-crystalline diffusion limitations.

Abstract

Metal–organic frameworks (MOFs) offer considerable potential for separating a wide variety of mixtures. For any given separation, there are several MOFs that could be employed. Therefore, there is a need for reliable procedures for screening and ranking MOFs with regard to their anticipated performance in fixed-bed adsorbers, commonly used in industry. Such fixed-bed adsorbers are invariably operated in a transient mode. The separation performance of fixed-bed adsorbers is governed by a number of factors that include adsorption selectivity, uptake capacity, and intra-crystalline diffusion limitations. We undertake a detailed analysis of the separations of several mixtures that include: C ₂H ₂/CO ₂, CO ₂/N ₂, CO ₂/CH ₄, H ₂S/CO ₂/CH ₄, H ₂/CO ₂/CO/CH ₄/N ₂, Xe/Kr, C ₂H ₂/C ₂H ₄, C ₂H ₄/C ₂H ₆, C ₃H ₆/C ₃H ₈, O ₂/N ₂, N ₂/CH ₄, hexane isomers, xylene isomers, and styrene/ethylbenzene. For each separation, we compare the performance of a few carefully selected MOFs by using transient breakthrough simulations that are representative of practical operations. These case studies demonstrate that screening MOFs on the basis of adsorption selectivity alone, as is common practice, often leads to wrong conclusions as regards their separation capability in fixed-bed adsorbers. High uptake capacities often compensate for low selectivities. Conversely, low uptake capacities diminish the separation performance of MOFs with high selectivities. Intra-crystalline diffusion limitations lead to distended breakthroughs, and diminished productivities in a number of cases. We also highlight the possibility of harnessing intra-crystalline diffusion limitations to reverse the adsorption selectivity; this strategy is useful for selective capture of nitrogen from natural gas, and in air separations.