Li Jiahong, Goncharov Vitaliy G, Strzelecki Andrew C, Xu Hongwu, Guo Xiaofeng, Zhang Qiang
Department of Chemistry, Washington State University, Pullman, Washington 99164, United States.
Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164, United States.
Inorg Chem. 2022 Sep 26;61(38):15152-15165. doi: 10.1021/acs.inorgchem.2c02345. Epub 2022 Sep 13.
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. 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 (Δ) of MIL-96, MIL-100, and MIL-110 from AlO HBTC, and HO (only AlO and HBTC 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 μ-oxo-bridged (Al(μ-O)) secondary building units (SBUs) promoting stabilization of Al polyhedra by the ligands within these frameworks, in comparison to the sterically strained Al octamer cluster cores formed in MIL-110. Furthermore, similar Δ 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 + OH)/ligand ratios. This trend has been successfully extended to the previously measured Δ of several ZnO-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, HO, 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.
通过一系列量热法和X射线技术，对铝（III）-1,3,5-苯三甲酸酯（Al-BTC）金属有机框架（MOF）（包括MIL-96、MIL-100和MIL-110）的热稳定性和热力学性质进行了研究。高温X射线衍射（HT-XRD）和热重分析结合差示扫描量热法（TGA-DSC）表明，这些MOF在配体燃烧之前会发生热非晶化。基于估计的非晶化温度，Al-BTC MOF的热稳定性顺序为MIL-110 < MIL-96 < MIL-100。它们的热力学稳定性通过高温滴燃烧量热法直接测量。由AlO HBTC和HO（MIL-100仅用AlO和HBTC）生成MIL-96、MIL-100和MIL-110的归一化（每摩尔Al）生成焓（Δ）分别确定为-56.9±13.7、-36.2±17.9和62.8±11.6 kJ/mol·Al。我们的结果表明，MIL-96和MIL-100在热力学上是有利的，而MIL-110是亚稳的，这与热稳定性和水热稳定性趋势一致。MIL-96和MIL-100的焓偏好可能归因于它们共享的三核μ-氧桥联（Al(μ-O)）二级结构单元（SBU），与MIL-110中形成的空间应变Al八聚体簇核相比，这些框架内的配体促进了Al多面体的稳定。此外，MIL-96和MIL-100相似的Δ解释了它们在合成过程中经常作为物理混合物同时形成的现象，这意味着动力学因素可能促进Al-BTC框架异构体形成的重要性。更重要的是，Al-BTC MOF异构体的归一化生成焓与带电配位取代基与连接体的比例（以MOF化学式单元内每摩尔Al归一化）呈负相关，焓偏好较小（O + OH）/配体比例的体系。这一趋势已成功扩展到先前测量的几种基于ZnO的框架（如MOF-5、MOF-5(DEF)、MOF-177、UMCM-1）的Δ，所有这些框架相对于它们的致密相（ZnO、HO和配体）都是亚稳的。结果表明，具有较高金属配位环境的羧酸盐MOF从配位配体中获得更多的焓稳定。因此，一些镧系/锕系、过渡金属和主族羧酸盐框架的形成在能量上可能更有利，然而，这需要进一步研究。
