MOF-5: Enthalpy of Formation and Energy Landscape of Porous Materials

Abstract

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(2)BDC), and occluded N,N-diethylformamide (DEF) (if any) gave values of 78.64 ± 2.95 and 99.47 ± 3.62 kJ·[mol of Zn(4)O(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.

Kinetic stability of metal–organic frameworks for corrosive and coordinating gas capture

“…However, an increasing body of work suggests that crystalline porous materials with empty pores are metastable with respect to their dense phases (Figure 1). [16][17][18][19][20][21] The dense phase is a hypothetical assemblage of the same constituent atoms, ions, or ligands, but carries negligible porosity. For an all-silica zeolite, the dense phase is easily envisaged as nonporous amorphous silica, which can be accessed via simple heating.…”

Electroactive Nanoporous Metal Oxides and Chalcogenides by Chemical Design

“…For reference, the formation enthalpies for all materials examined here are comparable to the formation enthalpy of other metastable materials (e.g., MOF-5). 31 …”

Little Thermodynamic Penalty for the Synthesis of Ultraporous Metal Organic Frameworks

“…for zeolites but the conclusion of this investigation is that at least some structures of low energy, lying on the observed plateau, probably exist for very high porosity. A rich energy landscape of structures with similar energies (on the plateau) suggests that preference among configurations and/or structures which have comparable energies is controlled by kinetic factors rather than by thermodynamics . These small energetic differences permit a diversity of synthesized frameworks …”