Multivariate metal-organic frameworks for atmospheric water harvesting

Although the positions of water guests in porous crystals can be identified, determination of their filling sequence remains challenging. We deciphered the water-filling mechanism for the state-of-the-art water-harvesting metal-organic framework MOF-303 {[Al(OH)(PZDC)], where PZDC^2- = 1H-pyrazole-3,5-dicarboxylate; Fig. 1A} by performing a series of 22 single-crystal X-ray diffraction measurements at different water loadings. Our experimental findings were further supported by complementary density functional theory calculations. The first water molecules strongly bound to the pyrazole moieties at the polar organic linkers (Fig. 1B); they were followed by additional water molecules forming isolated clusters, then chains of clusters, and finally a water network. Equipped with the understanding of this evolution of water structures, we then modified the pores by the multivariate approach using different linker ratios within the framework lattice (Fig. 1C). Thereby, we were able to precisely modulate the binding strength of the first water molecules and deliberately shape the water uptake behavior. This resulted in higher water productivity, as well as tunability of regeneration temperature and enthalpy, without compromises to capacity and stability. Finally, we surveyed how the the multivariate approach affected the water mobility within the framework.

Fig. 1 | MOF-303 structure and tuning of its water sorption properties with the multivariate approach: A) View along the pore channel. B) Hydrophilic pocket at a loading of two water molecules per asymmetric unit [Al(OH)(PZDC)]₂. The first (I) and second (II) water adsorption sites are labeled with Roman numbers. The heteroatom–heteroatom distances between the hydrogen bond partners are given (in Å) next to the respective hydrogen bonds (red dashed lines). Al, blue polyhedron; O in the framework structure, pink; O in H₂O, red; C and H, gray; N, green. H atoms on the water molecules are omitted. C) Water sorption isotherm profiles of the multivariate MOF series. The nomenclature ‘n/m’ indicates the molar ratio between the two linkers. RH, relative humidity.