Metal-organic frameworks are of interest in a number of host-guest applications [1]. However, their weak coordination bonding often leads to instability in aqueous environments, particularly at extreme pH, and hence, is a challenging topic in the field [2]. In this work, a two-step, post-synthetic polymerization method is used to create a series of highly hydrophobic, stable MOF composites [3]. The MOFs are first coated with thin layers of polydopamine from free-base dopamine under a mild oxygen atmosphere, which then undergoes a Michael addition to covalently graft hydrophobic molecules to the external MOF surface (Fig. 1). This novel technique, which entails the use of free-base dopamine to graft hydrophobic HSF molecules onto the external MOF surface, significantly enhances MOF stability in water and in acidic or basic conditions. Free-base dopamine was selected as a binder between the MOF and HSF because it is a strong adhesive and polymerizes under a mild oxygen atmosphere at room temperature making it suitable to modify even the most unstable MOFs.

This easy, mild post-synthetic could be applied to a number of composites using a structurally diverse set of MOFs with varying metals, ligands, and topologies. The as-prepared composites, including HKUST-1@PDA-SF, ZIF-67@PDA-SF, ZIF-8@PDA-SF, UiO-66@PDA-SF, Cu-TDPAT@PDA-SF, Mg-MOF-74@PDA-SF, and MIL-100-Fe@PDA-SF, show excellent stability even under harsh acid or basic conditions. Considering the novel results obtained from HKUST-1 and the many other structurally dissimilar frameworks, it is thought that this facile, mild method can be universally applied to improve the stability most MOFs and any other materials known to have water/pH instability. It is expected that this work can open doors to numerous new applications that were before inaccessible, and/or enhance MOF performance in existing areas coupled to catalysis, selective gas separation, delivery etc. by inhibiting decomposition, enhancing hydrophobicity and decreasing water permeability.

[1] a) H. Furukawa, K. E. Cordova, M. O'Keeffe, O. M. Yaghi, Science, 2013, 341, 1230444; b) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450-1459.
[2] A. J. Howarth, Y. Liu, P. Li, Z. Li, T. C. Wang, J. T. Hupp, O. K. Farha, Nat. Rev. Mater., 2016, 1, 15018.
[3] S. Yang, L. Peng, D. Sun, M. Asgari, E. Oveisi, O. Trukhina, S. Bulut, A. Jamali, W. L. Queen, Chem. Sci., 2019, 10, 4542-4549.