Thermodynamics Drives the Stability of the MOF-74 Family in Water

The stability of functional materials in water-containing environments is critical for their industrial applications. A wide variety of metal-organic frameworks (MOFs) synthesized in the past decade have strikingly different apparent stabilities in contact with liquid or gaseous H₂O, ranging from rapid hydrolysis to persistence over days to months. Here, we show using newly determined thermochemical data obtained by high-temperature drop combustion calorimetry that these differences are thermodynamically driven rather than primarily kinetically controlled. The formation reaction of a MOF from metal oxide (MO) and a linker generally liberates water by the reaction MO + linker = MOF + H₂O. Newly measured enthalpies of formation of Mg-MOF-74_(s) + H₂O_(l) and Ni-MOF-74_(s) + H₂O_(l) from their crystalline dense components, namely, the divalent MO (MgO or NiO) and 2,5-dihydroxyterephthalic acid, are 303.9 ± 17.2 kJ/mol of Mg for Mg-MOF-74 and 264.4 ± 19.4 kJ/mol of Ni for Ni-MOF-74. These strongly endothermic enthalpies of formation indicate that the reverse reaction, namely, the hydrolysis of these MOFs, is highly exothermic, strongly suggesting that this large thermodynamic driving force for hydrolysis is the reason why the MOF-74 family cannot be synthesized via hydrothermal routes and why these MOFs decompose on contact with moist air or water even at room temperature. In contrast, other MOFs studied previously, namely, zeolitic imidazolate frameworks (ZIF-zni, ZIF-1, ZIF-4, Zn(CF₃Im)₂, and ZIF-8), show enthalpies of formation in the range 20-40 kJ per mole of metal atom. These modest endothermic enthalpies of formation can be partially compensated by positive entropy terms arising from water release, and these materials do not react appreciably with H₂O under ambient conditions. Thus, these differences in reactivity with water are thermodynamically controlled and energetics of formation, either measured or predicted, can be used to assess the extent of water sensitivity for different possible MOFs.