(395ac) Free Energy Analysis of Adsorption-Induced Structural Transition in ZIF-8: A Molecular Simulation Study

ZIF-8 is one member of a family of zeolitic imidazolate frameworks [1,2], and has attracted much attention in many applications because of its high thermal stability and intra-framework flexibility. Recent experimental and simulation studies have revealed that reorientation of the imidazolate linkers of ZIF-8 is induced by molecular adsorption, which lead to increase in the accessible pore volume of ZIF-8. However, the mechanism of the structural transition is still unclear because of the lack of knowledge about free energy change due to the reorientation of the imidazolate linkers. We have therefore focused attention on argon adsorption in ZIF-8, and performed the free energy analysis [3] with the aid of grand canonical Monte Carlo (GCMC) simulations to investigate the adsorption-induced structural transition of ZIF-8.

   Adsorption isotherms of argon in ZIF-8 were measured at 79 K, 83 K, 87 K, and 91 K. A hysteresis loop was observed in all the adsorption isotherms, and its width increased with increasing temperature, which should provide important insight into the mechanism of the adsorption-induced structural transition of the argon/ZIF-8 system.

   We constructed atomistic ZIF-8 models by rotating the imidazolate linkers about an axis through two nitrogen atoms from 0 ° to 30 °, and a series of adsorption isotherms of argon in the respective ZIF-8 models were obtained by the GCMC simulations. We used the following force fields with modifications: the universal force field for argon-ZIF-8 interactions and the Amber force field to calculate the potential energy of the ZIF-8 framework. The free energy of the system was calculated as a sum of the Helmholtz free energy of the ZIF-8 framework and the contribution of adsorbed argon obtained by integrating the simulated adsorption isotherm. In the adsorption process, with increasing pressure, the global minimum of the free energy shifts to a larger rotational angle of the imidazolate linker without any activation processes, and a second local minimum appears at around 25.5 º. Further increase in pressure provides a switch of the global minimum, and a spontaneous structural transition takes place when the energy barrier lying between the metastable state at the smaller rotational angle (10.5 º) and the global minimum at the larger rotational angle (25.5 º) becomes less than the energy fluctuation of the system of 0.51 kT per an imidazolate linker. The energy fluctuation of the system determined from the comparison with the experimental data is quite suggestive because its value is comparable in magnitude to the energy of one rotational degree of freedom of the imidazolate linker. Then, in the desorption process, the system stays at the local minimum at 25.5 º regardless of the decrease in pressure, and finally an equilibrium structural transition occurs from 25.5 º to 10.5 º. The fact that the equilibrium structural transition is accomplished in the desorption process differently from the adsorption process is an indication that the energy fluctuation of the system is larger than 0.51 kT/linker. This is reasonable because the energy fluctuation of the system should be proportional to the adsorption amount of argon. Actually, the absorbed amount before the equilibrium structural transition in the desorption process is about 1.2 times larger than that before the spontaneous structural transition in the adsorption process. Finally, we constructed a theoretical adsorption isotherm of argon in ZIF-8 by using the free energy analysis, and obtained a good agreement with the experimental isotherm over the entire range of pressures. This suggests that our scenario for the adsorption and desorption processes obtained from the free energy analysis is quite appropriate to describe the mechanism of the adsorption-induced structural transition of ZIF-8.

[1] X. C. Huang et al., Angew. Chem. Int. Ed. 45, 1557 (2006).

[2] K. S. Park et al., Proc. Natl. Acad. Sci. U.S.A. 103, 10186 (2006).

[3] S. Watanabe et al., J. Chem. Phys. 130 164707 (2009).