MOF-5: Enthalpy of formation and energy landscape of porous materials

The first experimental thermodynamic analysis of a metal-organic framework (MOF) has been performed. Measurement of the enthalpy of formation of MOF-5 from the dense components zinc oxide (ZnO), 1,4-benzenedicarboxylic acid (H ₂BDC), and occluded N,N-diethylformamide (DEF) (if any) gave values of 78.64 ± 2.95 and 99.47 ± 3.62 kJ·[mol of Zn ₄O(BDC)₃·xDEF]^-1 for the as-made form and the desolvated structure, respectively. These as-made and desolvated enthalpies correspond to the values 19.66 ± 0.74 and 24.87 ± 0.94 kJ· (mol of Zn)^-1, respectively. The energetics of desolvated MOF-5 per mole of Zn falls in line with trends relating the enthalpy of inorganic porous materials (zeolites, zeotypes, and mesoporous materials) to molar volume. MOF-5 extends a plateauing trend first suggested by thermodynamic studies of mesoporous materials. This leveling off of the destabilization energetics as the void space swells suggests that additional void volume beyond a certain point may begin to act as a parameter "external" to the structure and not destabilize it further. This could help explain the rich landscape of large-volume MOFs and their ease of desolvation.