While metal-organic frameworks (MOFs) already emerged about two decades ago, their nanoporous, hybrid structure composed of inorganic nodes connected through organic linkers still gives rise to unexpected and highly intriguing phenomena, which can, e.g., be harnessed in catalytic applications [1]. Amongst
these phenomena, phase transitions induced by temperature, pressure, light, or guest adsorption play an important role [2]. Moreover, their hybrid nature attracts researchers to discover mechanical trends by varying the nature of the linker
and the inorganic node, hence engineering the material's stability [3].
Within this rationale, we recently developed a thermodynamic protocol to predict the response of MOFs upon application of an external pressure by outlining its
pressure-versus-volume behavior P(V), furthermore revealing the metastable states of the material at a given temperature and pressure since the free energy profile F(V) can be constructed using thermodynamic integration [4]. This thermodynamic approach was applied to obtain unprecedented molecular-level insight in (i) the pressure-induced flexibility in MIL-53(Al) and MIL-47(V) [4,5] and (ii) the effect of systematically including linker vacancies on the loss-of-crystallinity in UiO-66-type materials [6].
For the flexible MIL-53(Al), our approach correctly indicates the presence of a pressure range at 300 K for which two metastable states with a vastly different unit cell exist. For MIL-47(V), it was shown that, while its phenyl-based ligand endows the material with rigidity, replacing the ligand by its biphenyl analogue results in a highly flexible COMOC-2 [5]. Finally, for the more rigid UiO-66-family, our model confirmed recent experiments indicating the amorphization of these materials at elevated pressures. We observed an outspoken loss of short-range symmetry in the material, as evidenced by a sudden decrease in the number of symmetry operators at elevated pressures. Moreover, by a judicious classification of the possible linker defects in the material, we succeeded in rationalizing the effect of the location and extent of linker defects on the equilibrium stability (bulk modulus) and on the loss-of-crystallinity pressure. These results may aid the engineering of materials toward specific applications relying on the formation of defects, such as heterogeneous catalysis [1], while ensuring the retention of the structural integrity of the catalyst.
[1] S.M.J. Rogge, V. Van Speybroeck, J. Gascon, et al.,
“Metal-organic and covalent organic frameworks as single-site
catalysts”, Chem. Soc. Rev., published online, 2017. DOI: 10.1039/C7CS00033B.
[2] F.-X. Coudert, Chem. Mater. 27:1905, 2015.
[3] M.R. Ryder, B. Civalleri, J.-C. Tan, Phys. Chem. Chem. Phys. 18:9079, 2016.
[4] S.M.J. Rogge, G. Maurin, V. Van Speybroeck, et al., J. Chem. Theory Comput. 11:5583, 2015.
[5] J. Wieme, L. Vanduyfhuys, S.M.J. Rogge, M. Waroquier, V. Van Speybroeck, J. Phys. Chem. C 120:14934, 2016.
[6] S.M.J. Rogge, M. Waroquier, V. Van Speybroeck, et al., Chem. Mater. 28:5721, 2016.
