Adsorption-based phenomena are important in gas separations1,2, such as the treatment of greenhouse-gas3 and toxic-gas4 pollutants, and in water-adsorption-based heat pumps5 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 applications3,6,7,8. The flexibility and guest-molecule-dependent response9,10 of MOFs give rise to unexpected and often desirable adsorption phenomena11,12,13,14. 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 expansion15,16 and negative refractive indices17 and may be interpreted as an adsorptive analogue of force-amplifying negative compressibility transitions proposed for metamaterials18.
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V.B. thanks the German Federal Ministry for education and research (project BMBF number 05K13OD3). S. Krause, V.B., I.S. and S. Kaskel thank the Helmholtz-Zentrum Berlin for financial support and allocation of synchrotron radiation beam time at the KMC-2 beamline. F.-X.C. thanks GENCI (grant number x2015087069) for access to High-Performance Computing resources. G.M. thanks Institut Universitaire de France for its support. We thank U. Koch for scanning electron microscope images, as well as L. Sarkisov and A. Fuchs for discussions.
This video clip shows macroscopic movement of a DUT-49 sample bed filmed in a glass capillary during adsorption of n-butane at 298 - 299 K.
This video clip illustrates in an animation the framework structural transition including visualization of ligand deformation and changes of pore size occurring upon contraction of DUT-49op to DUT-49cp.
About this article
Zeitschrift für Kristallographie - Crystalline Materials (2019)