Water in metal-organic frameworks: structure and diffusion of H2O in MIL-53(Cr) from quantum simulations

The structural and dynamical properties of water in the nanopores of the MIL-53(Cr) metal-organic framework are examined using classical and quantum molecular dynamics simulations. The results indicate that, depending on the number of molecules adsorbed per unit cell as well as on the shape of the nanopores, the explicit inclusion of nuclear quantum effects can either enhance or inhibit the molecular mobility. At low loadings, when MIL-53(Cr) exists in a narrow pore configuration, the translational and rotational motion of the water molecules is largely suppressed. Importantly, it is found that nuclear quantum effects make the reorientation of the water molecules within the narrow nanopores slower than in the classical limit. This is attributed to the quantum delocalisation that effectively increases the molecular volume and, consequently, reduces the free space available in the nanopores. As the number of molecules per unit cell increases and the nanopores start opening, the dynamics of quantum water becomes faster again. Independent of the loading, the molecular mobility in MIL-53(Cr) is reduced with respect to bulk water.