Vornholt Simon M, Chen Zhihengyu, Hofmann Jan, Chapman Karena W
Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States.
J Am Chem Soc. 2024 Jun 26;146(25):16977-16981. doi: 10.1021/jacs.4c05313. Epub 2024 Jun 14.
In metal-organic frameworks (MOFs) the interplay between the dynamics of individual components and how these are constrained by the extended lattice can yield unusual emergent phenomena. For the archetypal Zr-MOF, UiO-66, we explore the cooperative dynamics of a Zr-node transformation that gives rise to negative thermal expansion (NTE). Using synchrotron X-ray scattering, with powder diffraction and pair distribution function (PDF) analyses, we identify lattice hysteresis and a thermal ramp-rate-dependence of the thermal expansion. Specifically, kinetic trapping of distorted node states formed at high temperature, leads to broad variability in the apparent thermal expansion which ranges from large positive to large negative thermal expansion with coefficients of thermal expansion (CTE) from +45 to -80 × 10K. Time-resolved relaxation studies at selected temperatures suggest that when equilibrated UiO-66 is intrinsically NTE, with a CTE of -35 × 10K. Kinetic trapping of the node-distorted state following high temperature activation has broad implications for characterization and applications of these Zr-MOFs; the nonequilibrium node state depends on the thermal history of the sample with quench vs slow cooling likely to impact gas binding, pore volume, and accessible catalytic sites.
在金属有机框架（MOF）中，各个组分的动力学与这些组分如何受到扩展晶格的限制之间的相互作用，可能会产生不同寻常的涌现现象。对于典型的锆基金属有机框架UiO-66，我们探究了导致负热膨胀（NTE）的锆节点转变的协同动力学。通过同步加速器X射线散射、粉末衍射和对分布函数（PDF）分析，我们确定了晶格滞后现象以及热膨胀对热升温速率的依赖性。具体而言，高温下形成的扭曲节点态的动力学捕获，导致表观热膨胀出现广泛变化，其范围从大的正热膨胀到负热膨胀，热膨胀系数（CTE）从+45到-80×10⁻⁶K⁻¹。在选定温度下进行的时间分辨弛豫研究表明，平衡态的UiO-66本质上是负热膨胀的，热膨胀系数为-35×10⁻⁶K⁻¹。高温活化后节点扭曲态的动力学捕获对这些锆基金属有机框架的表征和应用具有广泛影响；非平衡节点态取决于样品的热历史，淬火与缓慢冷却可能会影响气体吸附、孔体积和可及的催化位点。
