Early stages of phase selection in MOF formation observed in molecular Monte Carlo simulations

Metal-organic frameworks (MOF) comprising metal nodes bridged by organic linkers show great promise because of their guest-specific gas sorption, separation, drug-delivery, and catalytic properties. The selection of metal node, organic linker, and synthesis conditions in principle offers engineered control over both structure and function. For MOFs to realise their potential and to become more than just promising materials, a degree of predictability in the synthesis and a better understanding of the self-assembly or initial growth processes is of paramount importance. Using cobalt succinate, a MOF that exhibits a variety of phases depending on synthesis temperature and ligand to metal ratio, as proof of concept, we present a molecular Monte Carlo approach that allows us to simulate the early stage of MOF assembly. We introduce a new Contact Cluster Monte Carlo (CCMC) algorithm which uses a system of overlapping "virtual sites" to represent the coordination environment of the cobalt and both metal-metal and metal-ligand associations. Our simulations capture the experimentally observed synthesis phase distinction in cobalt succinate at 348 K. To the best of our knowledge this is the first case in which the formation of different MOF phases as a function of composition is captured by unbiased molecular simulations. The CCMC algorithm is equally applicable to any system in which short-range attractive interactions are a dominant feature, including hydrogen-bonding networks, metal-ligand coordination networks, or the assembly of particles with "sticky" patches, such as colloidal systems or the formation of protein complexes.</p

Evaluation of Ideal Adsorbed Solution Theory as a Tool for the Design of Metal–Organic Framework Materials

As a class of porous materials, metal–organic frameworks (MOFs) show promise for the adsorption-based separation of mixtures of gases. The design of any process involving selective adsorption requires knowledge of mixture adsorption isotherms. Ideal adsorbed solution theory (IAST) predicts mixture adsorption equilibria using only single-component data, thereby minimizing the need for experimental adsorption data. In this work we perform a systematic study of the applicability of IAST to MOFs by using grand canonical Monte Carlo (GCMC) simulations to investigate the suitability of IAST for the prediction of the adsorption of mixtures of molecules of differing sizes, asphericities, and polarities in a range of structurally different MOFs. We show that IAST is generally accurate for MOFs. Where we find IAST is less accurate, deviations result from both mixture effects, in the form of nonidealities in the adsorbed phase, and characteristics of the adsorbent structures. In terms of the MOF structure, departures from IAST are a consequence of heterogeneities both on the scale of the unit cell and on shorter length scales, whereby competition for adsorption sites has a strong influence

Protecting group and switchable pore-discriminating adsorption properties of a hydrophilic-hydrophobic metal-organic framework

Formed by linking metals or metal clusters through organic linkers, metal-organic frameworks are a class of solids with structural and chemical properties that mark them out as candidates for many emerging gas storage, separation, catalysis and biomedical applications. Important features of these materials include their high porosity and their flexibility in response to chemical or physical stimuli. Here, a copper-based metal-organic framework has been prepared in which the starting linker (benzene-1,3,5-tricarboxylic acid) undergoes selective monoesterification during synthesis to produce a solid with two different channel systems, lined by hydrophilic and hydrophobic surfaces, respectively. The material reacts differently to gases or vapours of dissimilar chemistry, some stimulating subtle framework flexibility or showing kinetic adsorption effects. Adsorption can be switched between the two channels by judicious choice of the conditions. The monoesterified linker is recoverable in quantitative yield, demonstrating possible uses of metal-organic frameworks in molecular synthetic chemistry as 'protecting groups' to accomplish selective transformations that are difficult using standard chemistry techniques