Negative gas adsorption transitions and pressure amplification phenomena in porous frameworks

Abstract

Nanoporous solids offer a wide range of functionalities for industrial, environmental, and energy applications. However, only a limited number of porous materials are responsive, i.e. the nanopore dynamically alters its size and shape in response to external stimuli such as temperature, pressure, light or the presence of specific molecular stimuli adsorbed inside the voids deforming the framework. Adsorption-induced structural deformation of porous solids can result in unique counterintuitive phenomena. Negative gas adsorption (NGA) is such a phenomenon which describes the spontaneous release of gas from an "overloaded" nanoporous solid via adsorption-induced structural contraction leading to total pressure amplification (PA) in a closed system. Such pressure amplifying materials may open new avenues for pneumatic system engineering, robotics, damping, or micromechanical actuators. In this review we illustrate the discovery of NGA in DUT-49, a mesoporous metal-organic framework (MOF), and the subsequent examination of conditions for its observation leading to a rationalization of the phenomenon. We outline the development of decisive experimental and theoretical methods required to establish the mechanism of NGA and derive key criteria for observing NGA in other porous solids. We demonstrate the application of these design principles in a series of DUT-49-related model compounds of which several also exhibit NGA. Furthermore, we provide an outlook towards applying NGA as a pressure amplification material and discuss possibilities to discover novel NGA materials and other counterintuitive adsorption phenomena in porous solids in the future.

Fig. 1. Negative gas adsorption transitions in DUT-49: (a) volumetric methane physisorption on DUT-49(Cu) at 111 K and GCMC simulated isotherms for DUT-49 op and cp phases; (b) NGA range of the adsorption isotherm indicating the expelled amount and pressure amplification in the cell; (c) gravimetrically measured adsorption of n-butane on DUT-49(Cu) at 298 K; (d) crystal structures of DUT-49op and DUT-49cp and the mechanism of the structural contraction and reopening; (e) volumetrically measured methane adsorption/desorption on DUT-49(Cu) at 111 K (large circles identify the points in which PXRD is measured) and (f) PXRD patterns, measured in selected points of the adsorption (bottom, blue contour) and desorption (top, red contour) isotherm; (g) evolution of the unit cell parameter a upon adsorption and desorption of methane at 111 K; (h) volumetrically measured adsorption and desorption of n-butane on DUT-49(Cu) at 298 K; (j) macroscopic changes of the adsorption bed upon n-butane physisorption at 298 K (the figure is adapted from ref. . Copyright Springer Nature 2016).

Scheme 1. Major factors influencing NGA (the figure is adapted from ref. 27–30).

Fig. 2. Ligand elongation strategy in the isoreticular series of DUT-49(Cu) for the fine control over mechanical stress: (a) ligand structures and resulting frameworks; (b) strain–stress analysis of the linker molecules; (c) free energy profiles calculated for the isoreticular series showing two distinct minima for each MOF; (d) pore size distributions for the isoreticular series of DUT-49-related MOFs; (e) nitrogen physisorption at 77 K measured on the DUT-49 and related frameworks (the figure is adapted from ref. . Open access Springer 2019).

Scheme 2. Rationalization of the NGA regime with the view of primary structural deformability.

Fig. 3. The role of temperature for breathing and NGA transitions in DUT-49(Cu): (a) Δn_NGA for inert gases and hydrocarbons as a function of temperature; (b) linear correlation of Δn_NGA and critical point of the fluids; (c)–(f) linear correlation of T_high, T_low, T_NGA and T average from the critical point (the figure is adapted from ref. Copyright RSC 2021).

Fig. 4. In situ ¹²⁹Xe NMR spectra of DUT-49 measured at 200 K. Note the sudden chemical shift change by about 100 ppm in the narrow relative pressure range between 0.13 and 0.18. Reproduced and adapted with permission from ref. . Copyright American Chemical Society.

Fig. 5. Physisorption of CO₂ on DUT-49(Cu) and pressure amplification experiments: (a) physisorption of CO₂ on DUT-49(Cu) at 230 K; (b) physisorption of CO₂ on DUT-49(Cu) at 240K; (c) principle of the pressure amplifiers using the NGA effect in DUT-49(Cu); (d) pressure amplification experiment upon physisorption of CO₂ on DUT-49(Cu) at 230 K (reproduced and adapted with permission from ref. . Copyright Wiley-VCH).

Fig. 6. Computed osmotic surface of methane adsorption on DUT-49 at 120 K, as a function of unit cell volume (a) and methane gas pressure (b). Examples of the 1D osmotic surface at specific gas pressures (c). Reproduced and adapted with permission from ref. . Copyright American Chemical Society 2021.

Simon Krause

Jack D. Evans

Volodymyr Bon

François-Xavier Coudert

Guillaume Maurin

Stefan Kaskel