Solid-Solution Mixed-Linker Synthesis of Isoreticular Al-Based MOFs for an Easy Hydrophilicity Tuning in Water-Sorption Heat Transformations

The solid-solution mixed-linker approach, where a linker is partially replaced by a similar one under retention of the isoreticular metal-organic framework (MOF) structure, offers an easy and inexpensive way to fine-tune MOF properties to design tailored compounds. A total of 10 aluminum mixed-linker MOFs, [Al(OH)(X)a(Y)1-a] (X = IPA, isophthalate; Y = FDC, 2,5-furandicarboxylate) spanning between the isostructural MOFs CAU-10-H (a = 1) and MIL-160 (a = 0), were synthesized by employing different ratios of the aforementioned linkers. CAU-10-H and MIL-160 have been reported as highly promising materials for cycling water sorption for heat transformation applications. A detailed characterization with a focus on the changes in the sorption properties for water vapor showed that the hydrophilicity is readily and easily tuned through the mixed-linker approach between the limits of MIL-160 and CAU-10-H. An increasing fraction of IPA shifts the steep increase in the S-shaped water adsorption isotherm in small steps from p/p0 = ∼0.05 for MIL-160 to p/p0 = ∼0.18 for CAU-10-H. Higher coefficient of performance (COPH) values for the mixed-linker materials over MIL-160 illustrate the well-balanced hydrophobicity/hydrophilicity of the former under the exemplary calculation conditions.