EaStCHEM School of Chemistry and the Centre for Science at Extreme Conditions, The University of Edinburgh , King's Buildings, West Mains Road, Edinburgh EH9 3JJ, U.K.
J Am Chem Soc. 2014 Jun 18;136(24):8606-13. doi: 10.1021/ja411934f. Epub 2014 Jun 4.
Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally nonporous materials. Here we report a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC = 1,4-benzenedicarboxylate) and its nitro-functionalized derivative (Sc2(NO2-BDC)3) and compare it to direct compression behavior in a nonpenetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitro-form are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations, and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against nonpenetrative hydrostatic fluids at high pressure, this study demonstrates a novel high-pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favorable adsorption sites.
先前的高压实验表明，由小分子组成的传压流体可以被强制进入金属有机骨架材料的孔隙中，在那里它们可以引起相变和非晶化，甚至可以在传统的非多孔材料中诱导孔隙形成。在这里，我们报告了一个联合的高压衍射和计算研究，研究了在高压下甲醇吸收对钪对苯二甲酸 MOF（Sc2BDC3，BDC=1,4-苯二甲酸）及其硝基功能化衍生物（Sc2（NO2-BDC）3）结构响应的影响，并将其与非穿透静压流体 Fluorinert-77 中的直接压缩行为进行了比较。在 Fluorinert-77 中，Sc2BDC3 在 0.1 GPa 以上显示出非晶化，释放压力后可恢复，而 Sc2（NO2-BDC）3 经历了一个相转变（C2/c 到 Fdd2）到一个更密集但拓扑相同的同素异形体。在甲醇存在的情况下，Sc2BDC3 的可逆非晶化和硝基形式的位移相转变完全被抑制（至少在 3 GPa 以下）。在 Sc2BDC3 上吸收甲醇后，衍射表明甲醇分子占据两个位置，一个位置相对于另一个位置优先填充：广义正则蒙特卡罗模拟支持这些实验观察，分子动力学模拟揭示了甲醇分子的可能取向，这些取向至少部分由客体之间的氢键相互作用控制。这项研究不仅揭示了这些 MOFs 在高压下对非穿透静压流体的稳定的原子起源，还展示了一种新的高压方法，用于研究在多孔骨架中随客体含量增加的吸附作用，从而确定最具能量优势的吸附位置。
