Structural Insight of MOFs under Combined Mechanical and Adsorption Stimuli

Soft crystalline porous materials, such as metal-organic frameworks (MOFs) are known to undergo substantial structural-level changes under a variety of stimuli. These unique materials can switch their pore sizes and shape in response to light, temperature changes, electric or magnetic fields and more. This has made them highly interesting as they promise unprecedented performance in applications like gas storage, gas separation, catalysis and sensing. Even more, they can serve as molecular-level factories or nanomachines, mimicking biological systems.
However, there is still more to learn about how to best tune and make use of this behaviour. One appealing possibility is the use of mechanical stress, through the application of an external force. Essentially, the crystals are squeezed in order to change their pores at a microscopic level, a phenomenon which has previously been found mainly in MOFs, as well as other materials like carbons.
So far, the interplay between mechanical stress (σ_mech) and gas pressure (P_gas) has been mostly studied through indirect experimental means by observing the effects on gas uptake or by molecular simulations. In order to directly observe structural changes upon the combination of the two stimuli, advanced instrumentation was required. To this end we constructed a novel powder X-ray diffraction (PXRD) apparatus, combining stress-pressure clamp (CSPC) cell as seen in Fig. 1a, capable of simultaneously applying uniaxial mechanical stress up to 1 GPa and gas pressure up to 20 bar, which was applied to follow structural changes on soft crystalline materials through in-situ measurements at the XPRESS beamline of Elettra.
As a showcase, we considered the prototypical breathing MOF, MIL-53(Al) in our cell. This system displays complex stimuli-responsiveness to various guests, mechanical stress and temperature. Its wine-rack 1D porous network allows the pores to switch between well-defined phases with differing characteristic openings of the microporous channels. At ambient pressure and temperature, the activated material exists in a open pore state (op). Both mechanical stress and CO₂adsorption can “close” the structure, resulting in a closed pore (cp) or a narrow pore form (np) respectively. However, by dosing more CO₂, the structure re-opens, to its original open pore form (op).
 

Figure 1.  (a) Combined mechanical Stress and gas Pressure Clamp (CSPC) cell design. (b) Explored phase space of mechanical stress and gas pressure. Colour denotes existing phases, as tentatively observed from characteristic Bragg Peaks.
 

We used our developed cell to observe the MIL-53(Al) system under combined mechanical stress and CO₂pressure. We observed that by increasing the force applied on the powder material, we could prevent the reopening of the structure at high CO₂pressure. This is the first direct evidence of the effect of crystallite strain on the flexibility of MIL-53(Al), which could be recorded in-situ with the help of the CSPC cell and the advanced instrumentation of the XPRESS beamline at Elettra. Moreover, we could map an entire σ_mech+ P_gas phase space, showing the existence domains of different phases as depicted in Fig. 1b.
We hope that through the use of this novel cell a fundamental understanding of the interplay between external stress and guest adsorption in a broader range of crystalline responsive materials can be achieved. This new working mode opens the door to other opportunities for innovative applications, such as external control over separation performance, stress-swing adsorbent regeneration, actuated gas storage and fluid-solid barocaloric or energy storage systems, just to name a few. Moreover, the CSPC cell bridges the gap towards other types of combined stimuli use for influencing the dynamics of soft matter.

This research was conducted by the following research team:

Paul Iacomi¹, Frederico Alabarse², Roger Appleyard³, Thomas Lemaire³, Christophe Thessieu³, Sujing Wang⁴, Christian Serre⁴, Guillaume Maurin¹, andPascal G. Yot¹

¹ ICGM, Univ. Montpellier, CNRS, ENSCM, Montpellier, France 
² Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
³ Almax-easyLab, Wagenmakerijstraat, Diksmuide. Belgium 

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