Analysis of CO₂ adsorption dynamics of MOF using NEB method

Case Study Provider:
Tokyo Institute of Technology

Overview

In order to achieve a carbon-neutral society, there is an urgent need for the development of materials to capture and separate the greenhouse gas CO₂. Metal–organic frameworks (MOFs), which are porous materials composed of organic multidentate ligands combined with metal ions, have attracted attention as promising candidates because of their readily modifiable structures. 

In the study of reference 1, two kinds of MOFs with isolated voids which cannot be directly accessed were obtained. Despite the apparent absence of CO₂ diffusion pathways in these structures, one of the MOFs was able to selectively capture and store CO₂. Simulations using Matlantis^TM were performed to elucidate the mechanism of this specific adsorption phenomenon.

Calculation models and methods

Here, MOF(1) and MOF(2) were used in the simulations. MOF(1) does not adsorb CO₂ or N₂, and MOF(2) does not adsorb N₂ but does adsorb CO₂. Both have isolated pores which seem to be inaccessible to even small gas molecules.

The Nudged Elastic Band (NEB) method was used to evaluate the diffusion path of CO₂ through the MOF. The initial state (IS) consisted of one CO₂ molecule enclosed inside a pore of a unit cell. Whereas in the final state (FS) , the same molecule was relocated into an adjacent closed pore. By the NEB method, their transition states were optimized.

Calculation Results and Discussion

The CO₂ diffusion simulation result of MOF(2) is as the right fiture. In the transition state, their frameworks were slightly adjusted, opening up a channel between the neighboring pores. After CO₂ passed through the channel, the passage closed behind and the MOF backbone returned to the initial state. This state was more energetically unstable than the initial and final states, suggesting that an energy barrier must be overcome as CO₂ crosses between the pores.

Similar simulations for CO₂ and N₂ adsorption on MOF(1) and for N₂ adsorption on MOF(2), the diffusion barriers are larger than for CO₂ adsorption on MOF(2), which is consistent with the experimentally observed results. Additionally, X-ray crystallography did not confirm any clear structural transitions in the MOF before, during, or after adsorption, and the atomic behavior observed in the simulations supports the experimental findings.

Because of their complicated structure, the calculations of MOFs were computationally expensive and inaccurate. Thus, calculations of MOF adsorption properties have been widely used models that extract only substructures of MOFs, or that fixed atomic positions of MOFs. However, it is well known that MOFs often undergo structural changes during gas adsorption and that the flexibility contributes to their adsorption properties of them. The ability of Matlantis to perform high-speed calculations with flexibility of entire MOF structures is expected to have important implications for the future design of new MOFs.

Calculation Conditions