Department of Inorganic Chemistry, Technische Universität Dresden, Bergstrasse 66, 01062 Dresden, Germany.
Department Sample Environments, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
Nature. 2016 Apr 21;532(7599):348-52. doi: 10.1038/nature17430. Epub 2016 Apr 6.
Adsorption-based phenomena are important in gas separations, such as the treatment of greenhouse-gas and toxic-gas pollutants, and in water-adsorption-based heat pumps for solar cooling systems. The ability to tune the pore size, shape and functionality of crystalline porous coordination polymers--or metal-organic frameworks (MOFs)--has made them attractive materials for such adsorption-based applications. The flexibility and guest-molecule-dependent response of MOFs give rise to unexpected and often desirable adsorption phenomena. Common to all isothermal gas adsorption phenomena, however, is increased gas uptake with increased pressure. Here we report adsorption transitions in the isotherms of a MOF (DUT-49) that exhibits a negative gas adsorption; that is, spontaneous desorption of gas (methane and n-butane) occurs during pressure increase in a defined temperature and pressure range. A combination of in situ powder X-ray diffraction, gas adsorption experiments and simulations shows that this adsorption behaviour is controlled by a sudden hysteretic structural deformation and pore contraction of the MOF, which releases guest molecules. These findings may enable technologies using frameworks capable of negative gas adsorption for pressure amplification in micro- and macroscopic system engineering. Negative gas adsorption extends the series of counterintuitive phenomena such as negative thermal expansion and negative refractive indices and may be interpreted as an adsorptive analogue of force-amplifying negative compressibility transitions proposed for metamaterials.
吸附现象在气体分离中非常重要，例如处理温室气体和有毒气体污染物，以及在基于水吸附的太阳能制冷系统中使用的热泵。调节晶体多孔配位聚合物（或金属有机骨架）的孔径、形状和功能的能力，使它们成为这种基于吸附的应用的有吸引力的材料。MOFs 的灵活性和客体分子依赖性响应导致了意想不到的、通常是理想的吸附现象。然而，所有等温热力学气体吸附现象的共同特点是，随着压力的增加，气体的吸收量增加。在这里，我们报告了在一个 MOF（DUT-49）的等温线上的吸附转变，该 MOF 表现出负吸附，即在定义的温度和压力范围内，随着压力的增加，气体（甲烷和正丁烷）自发解吸。原位粉末 X 射线衍射、气体吸附实验和模拟的结合表明，这种吸附行为是由 MOF 的突然滞后结构变形和孔收缩控制的，这会释放出客体分子。这些发现可能使能够利用能够进行负气体吸附的框架的技术在微观和宏观系统工程中实现压力放大。负气体吸附扩展了一系列反直觉的现象，如负热膨胀和负折射率，并且可以解释为对拟议的用于超材料的力增强负压缩性转变的吸附类似物。
