Analysis of CO2 adsorption dynamics of MOF using NEB method

Introducition

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

The subjects of the calculations were MOF①, which does not adsorb CO ₂ or N ₂, and MOF②, which does not adsorb N ₂ but adsorbs CO _2. Both have pores that are isolated spaces, and at first glance, it seems impossible for molecules to enter the pores.

To identify the CO₂ adsorption pathway, the Cl-NEB method, the method for calculating the transition state of a chemical reaction, was used. The initial state (IS) consisted of one CO₂ molecule captured in one pore of the MOF, and the final state (FS) consisted of the same CO₂ molecule relocated into an adjacent closed pore. By the CI-NEB method, their transition states were optimized. and the state between the initial state and the final state was calculated.

Results and Discussion

The calculation results for CO₂ adsorption in MOF② are shown in the figure on the right. Only at the moment that CO₂ passes through does a minute change occur in the MOF's framework structure, and a passage through which CO₂ can move appears. When CO₂ passes through, the passage closed behind and CO₂ is trapped within the pores. This state was more energetically unstable than initial and final states, suggesting the existence of an activation barrier for CO₂ adsorption.

Similar calculations were performed on the adsorption of CO ₂ and N ₂ in MOF ① and on the adsorption of N ₂ in MOF ②. It was found that the activation barrier was larger than that for CO ₂ adsorption in MOF ②, confirming the consistency with the experimentally observed results. Furthermore, X-ray crystallography did not confirm any clear structural transition of the MOF before, during, or after adsorption, and the behavior of the atoms observed in the simulation also supports the experiment.

Because of their complicates structure, the calculations of MOFs were computationally expensive and inaccurate. Thus, calculations of MOF adsorption properties have been widely used models that extract only partial structures of MOFs or models in which the atomic positions of the MOF are fixed. On the other hand, it is well known that MOFs often undergo structural changes during gas adsorption and that the flexibility of the MOF contributes greatly to the gas adsorption properties of MOFs. The ability of Matlantis to perform high-speed calculations with structural flexibility of entire MOF structures is expected to provide important insights for the design of new MOFs in the future.