Energetic Systematics of Metal-Organic Frameworks: A Case Study of Al(III)-Trimesate MOF Isomers

Jiahong Li; Vitaliy G Goncharov; Andrew C Strzelecki; Hongwu Xu; Xiaofeng Guo; Qiang Zhang

doi:10.1021/acs.inorgchem.2c02345

Energetic Systematics of Metal-Organic Frameworks: A Case Study of Al(III)-Trimesate MOF Isomers

Inorg Chem. 2022 Sep 26;61(38):15152-15165. doi: 10.1021/acs.inorgchem.2c02345. Epub 2022 Sep 13.

Authors

Jiahong Li¹, Vitaliy G Goncharov^{1

2

3}, Andrew C Strzelecki^{1

2

3

4}, Hongwu Xu^{4

5}, Xiaofeng Guo^{1

2

3}, Qiang Zhang^{1

2}

Affiliations

¹ Department of Chemistry, Washington State University, Pullman, Washington 99164, United States.
² Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164, United States.
³ Alexandra Navrotsky Institute for Experimental Thermodynamics, Washington State University, Pullman, Washington 99164, United States.
⁴ Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States.
⁵ School of Molecular Sciences and Center for Materials of the Universe, Arizona State University, Tempe, Arizona 85287, United States.

PMID: 36099470
DOI: 10.1021/acs.inorgchem.2c02345

Abstract

Thermal stability and thermodynamic properties of aluminum(III)-1,3,5-benzenetricarboxylate (Al-BTC) metal-organic frameworks (MOFs), including MIL-96, MIL-100, and MIL-110, have been investigated through a suite of calorimetric and X-ray techniques. In situ high-temperature X-ray diffraction (HT-XRD) and thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC) revealed that these MOFs undergo thermal amorphization prior to ligand combustion. Thermal stabilities of Al-BTC MOFs follow the increasing order MIL-110 < MIL-96 < MIL-100, based on estimated amorphization temperatures. Their thermodynamic stabilities were directly measured by high-temperature drop combustion calorimetry. Normalized (per mole of Al) enthalpies of formation (ΔH*_f) of MIL-96, MIL-100, and MIL-110 from Al₂O_3, H₃BTC, and H₂O (only Al₂O₃ and H₃BTC for MIL-100) were determined to be -56.9 ± 13.7, -36.2 ± 17.9, and 62.8 ± 11.6 kJ/mol·Al, respectively. Our results demonstrate that MIL-96 and MIL-100 are thermodynamically favorable, while MIL-110 is metastable, in agreement with thermal and hydrothermal stability trends. The enthalpic preferences of MIL-96 and MIL-100 may be attributed to their shared trinuclear μ³-oxo-bridged (Al₃(μ₃-O)) secondary building units (SBUs) promoting stabilization of Al polyhedra by the ligands within these frameworks, in comparison to the sterically strained Al₈ octamer cluster cores formed in MIL-110. Furthermore, similar ΔH*_f of MIL-96 and MIL-100 explain their concurrent formation as physical mixtures often encountered during synthesis, implying the importance of kinetic factors that may facilitate the formation of Al-BTC framework isomers. More importantly, the normalized formation enthalpies of Al-BTC MOF isomers follow a negative correlation with the ratio of charged coordinated substituents to linkers (normalized per mole of Al within the MOF formula unit), with enthalpic preference given to systems with smaller (O^2- + OH^-)/ligand ratios. This trend has been successfully extended to the previously measured ΔH*_f of several Zn₄O-based frameworks (e.g., MOF-5, MOF-5(DEF), MOF-177, UMCM-1), all of which have been found to be metastable with respect to their dense phases (ZnO, H₂O, and ligands). The result suggests that carboxylate MOFs with higher metal coordination environments attain more enthalpic stabilization from the coordinated ligands. Thus, the formation of some lanthanide/actinide, transition metal, and main group carboxylate frameworks may be energetically more favored, which, however, requires further studies.