Exploiting Large‐Pore Metal–Organic Frameworks for Separations through Entropic Molecular Mechanisms

Abstract

We review the molecular mechanisms behind adsorption and the separations of mixtures in metal-organic frameworks and zeolites. Separation mechanisms can be based on differences in the affinity of the adsorbate with the framework and on entropic effects. To develop next-generation adsorbents, the separation efficiency of the materials needs to be improved. The performance under industrially relevant conditions largely depends on two factors: 1) the separation selectivity and 2) the pore volume capacity of the material. Enthalpic mechanisms can lead to increased selectivities, but these are mostly restricted to the low loading regime, and hence these mechanisms are unable to make use of all of the large-pore volume that a metal-organic framework can provide. Industrial processes routinely operate in the pore saturation regime. In this Review, we focus on entropic molecular separation mechanisms that are effective under these conditions and, in particular, on a recent methodology to obtain high selectivities at high pore loading.

Using Aliphatic Alcohols to Tune Benzene Adsorption in MAF‐6

“…At low loading, the competition of these two molecules can be explained by the combination of the higher heat of adsorption and lower Henry coefficient of 1-propanol compared to these of benzene (enthalpic effect), while at saturation pressure length and size entropies become important. [47] For a deepest understanding of the adsorption mechanism, we analyze the preferential position of the hydroxyl groups of the alcohols in the structure. As it happens with many metal-organic frameworks, the open metal sites in the structure could act as preferential adsorption sites for polar molecules, affecting therefore the adsorption of non-polar molecules as benzene.…”

MOF catalysts in biomass upgrading towards value-added fine chemicals

“…46 Torres-Knoop and Dubbeldam compared the pore sizes and nitrogen surface areas of ZIFs, COFs, zeolites and MOFs in the context of separation and adsorption mechanisms. 48 MOFs possess a large variety of different pore MOFs in comparison to the total number of publications. Source: Scifinder (search carried out in May 2016), search terms "catalysis by MOFs", "catalysis by zeolites" compared to "MOF" and "zeolite".…”

Behavior of the Enthalpy of Adsorption in Nanoporous Materials Close to Saturation Conditions

“…Many industrial separation processes based on adsorption work close to saturation conditions to operate cost efficiently. 10−13 Because separations are driven by entropic forces in this regime, 14−17 it is important to understand and study how entropy behaves in these systems to design a new generation of nanoporous materials for separations. 18,19 Separations based on adsorption rely on the equilibrium loading differences of the mixture components.…”