Impact of mechanical deformation on guest diffusion in zeolitic imidazolate frameworks

Abstract

The effect of the mechanical deformation of metal-organic frameworks on guest diffusion was investigated by employing molecular dynamics simulations. Two basic deformation modes, uniaxial tensile and shear deformation, were considered. The computed shear modulus of the zeolitic imidazolate framework-8 (ZIF-8) model system was much lower than the Young's modulus, which is in agreement with the experimental results. The diffusion rate in ZIF-8 was calculated for two types of guest molecules: the nonpolar H2 and the quadrupolar CO2. Under tensile strain, the diffusion of both H2 and CO2 was found to be enhanced, whereas the diffusion rates did not change significantly under shear loading. The evolution of the internal structure of ZIF-8 was studied to determine its effect on guest diffusion. The organic-inorganic connection was identified as the source of the framework's flexibility, and therefore we focused on the N-Zn bond and the N-Zn-N angle. Under stretching deformation, the N-Zn bond is elongated and the N-Zn-N angle remains constant. Thus, the length of the C2-C2 long bond, determining the size of the 6-membered ring (6MR) gate, increases and the gate is opened, allowing for faster guest diffusion. Under shear deformation, the N-Zn bond length changes very little and the N-Zn-N angle is distorted. This results in the occurrence of three peaks in the C2-C2 bond length distribution. Although the 6MR gate is distorted, the variation of its average size is small, resulting in a very small effect on the guest diffusivity. In addition, we found that the fluctuation of the ZIF-8 cell can enhance the impact of the mechanical deformation of the host on guest diffusion.

Theoretical prediction of the mechanical properties of zeolitic imidazolate frameworks (ZIFs)

“…We then successfully synthesized ZIF-I (ESI †) to prove that it is not just a hypothetical structure. In previous studies, 40 we have argued that the axial stretching and shear deformation of ZIFs depend on the Zn-N bond length and the N-Zn-N angle, respectively. Here, we used the cluster model to quantify the enhancing effect of the terminal group on the bond length and angle in ZnN 4 ( Table 3).…”

Structural Insight of MOFs under Combined Mechanical and Adsorption Stimuli

“…One appealing possibility is the use of mechanical stress, through the application of an external force [16] . This option has shown promise to tune the effective pore size of not only breathing MOFs [17–20] but also flexible carbons [21, 22] …”

“…[17,18,21,22] Structural responses under combined σ mech /P gas were also predicted by molecular simulations using fully flexible force fields for the MOF frameworks. [19,23,24] These preliminary studies highlighted the importance of this synergistic stimuli effect to substantially boost the separation performance of flexible adsorbents. However, from a fundamental perspective there is no direct evidence of the structural changes upon combined σ mech /P gas at the origin of these improved adsorption/separation properties.…”

“…[14,15] One appealing possibility is the use of mechanical stress, through the application of an external force. [16] This option has shown promise to tune the effective pore size of not only breathing MOFs [17][18][19][20] but also flexible carbons. [21,22] Insofar, the interplay between mechanical stress (σ mech ) and gas pressure (P gas ) has been mostly studied through indirect means, by observing the effect of external force on adsorption behaviour of MOFs as changes in isotherm steps.…”

Heterogeneous growth of continuous ZIF-8 films on low-temperature amorphous silicon

“…Its elastic framework is characterized by large openings of 11.6 Å accessible through six-membered ring apertures of 3.4 Å, which makes ZIF-8 structure suited for gas separation and adsorption [12,13]. Furthermore, for its high specific surface area (~ 1700 m 2 g -1 ), remarkable thermal resistance (above 550°C) and good chemical stability in water and organic solvents, ZIF-8 has also great potentialities for gas separation, chemical sensing, and energy devices [11,[14][15][16][17]. Most of the synthetic methods used for ZIF-8 powders synthesis such as solvothermal, microwave-based, mechanochemical and sonochemical syntheses [18][19][20][21] are not suitable for film growth because they lead to cracks and defects [22,23].…”