Mechanism of benzene diffusion in MOF-5: A molecular dynamics investigation

In this contribution, the diffusion of benzene in the porous metal organic framework MOF-5 is investigated by molecular dynamics simulations. Previously, we have shown that by using a first principles derived fully flexible force field the experimentally determined self-diffusion coefficients DselfDself could be well reproduced [S. Amirjalayer, M. Tafipolsky, R. Schmid, Angew. Chem. Int. Ed. 46 (2007) 463]. Here, we use the same methodology to determine the loading dependence on the diffusion. It is found that diffusivity, which is in the range of liquid benzene, slightly increases up to a load of 32 molecules per unit cell and then falls off at higher load. Free energy maps reveal that additional sites appear at higher load due to attractive guest–guest interactions. The topology of these sites is very close to the experimentally determined locations of ferrocene molecules in MOF-5, which corroborates that attractive π–ππ–π interactions govern these systems. The site–site and site-phenylene distances are very similar to the first solvation radius of liquid benzene. For the very open MOF-5, the main barrier for diffusive transport is to overcome the attractive interaction in the binding pockets, which is in contrast to zeolitic microporous systems, where the barrier for diffusion is the hindrance of the pore window. Spatial free energy maps are used to investigate the diffusion pathway on a molecular level and the load dependence of the free energy barriers for these transport processes.