Thermodynamics of metal-organic frameworks

• Thermochemical data on various MOF structures were experimentally determined.
• MOFs are moderately unstable relative to their dense phase assemblage.
• Overall energetic landscape of porous materials was revealed.
• Guest–host interactions in MOFs were evaluated directly using calorimetry.
• Confinement effect and defined chemical binding lead to strong interactions.

Although there have been extensive studies over the past decade in the synthesis and application of metal-organic frameworks (MOFs), investigation of their thermodynamic stability and of the energetics of guest–host interactions has been much more limited. This review summarizes recent progress in experimental (calorimetric) determination of the thermodynamics of MOF materials. The enthalpies of MOFs relative to dense phase assemblages suggest only modest metastability, with a general increase of enthalpy with increasing molar volume, which becomes less pronounced at higher porosity. The energy landscape of nanoporous materials (inorganic and hybrid) consists of a pair of parallel patterns within a fairly narrow range of metastability of 5–30 kJ per mole of tetrahedra in zeolites and mesoporous silicas or per mole of metal in MOFs. Thus strong thermodynamic instability does not seem to limit framework formation. There are strong interactions within the chemisorption range for small molecule–MOF interactions with defined chemical binding at the metal centers or other specific locations. Coexistence of surface binding and confinement can lead to much stronger guest–host interactions.

Energy landscape of inorganic and hybrid porous materials.Figure optionsDownload as PowerPoint slide