J.Am.Chem.Soc.http://doi.org/p7q(2013)

Credit: © 2013 ACS

Porous solids offer rich 'host–guest' chemistry that is being increasingly exploited, in particular for molecular recognition and gas-separation uses. These applications rely on understanding structure–property relationships to enable tuning of the materials' properties. Crystalline solids are well suited to these investigations, but amorphous ones consisting of organic cages packed in a disordered manner also show promising high porosity. This is because, in addition to the cages having hollow cavities, they have accessible space between them. The porous domains of such amorphous molecular solids, however, are much less straightforward to characterize and tailor than those of their crystalline counterparts.

Andrew Cooper at the University of Liverpool and co-workers have now used molecular dynamics simulations to study the porosity of amorphous molecular solids, and their ability to support gas diffusion. Amorphous materials are difficult to model, partly because they are metastable; however, the researchers devised a four-step procedure to generate amorphous structure models for a given organic cage. In this procedure an ensemble of 40 cages is considered in an initial configuration, and subsequently stabilized into a low-density structure. The volume of that structure is in turn compressed until no further reduction is observed and the resulting structure is geometry-optimized. Six initial ensembles give six models that are averaged to take into account the non-homogeneity of the structure.

Cooper and colleagues investigated two isostructural cages — with either ethane or cyclohexane vertices — experimentally and computationally. The materials were synthesized by 'freeze-drying', which involves rapid precipitation of the cages from a solution and subsequent removal of the frozen solvent. Gas-sorption measurements showed different selectivities for hydrogen and nitrogen. The models were used to determine the materials' pore size distribution and connectivity — the extent to which a guest can diffuse through the void — and their gas-diffusion behaviour. The computational results were in good agreement with the experiments and point to a 'gas hopping' mechanism through the pores.