(617i) Molecular Insights into the Adsorption and Diffusion Behavior of Hybrid GO/MOF Adsorbents for CO2 Capture: Systematic Study for GO/CuBTC and GO/UTSA-16 Structures

In the context of net zero emissions, the development of high-performance adsorbents appears as a promising approach to promote the application of adsorption processes for CO₂ capture, utilization and storage (CCUS). In this regard, the hybridization of graphene oxide (GO) with metal-organic frameworks (MOFs) has proven to be a successful strategy to improve the CO₂ adsorption performance of MOFs [1,2]. However, although several studies on the adsorption behavior of such hybrid materials have been published, no experimental or simulation studies have reported the diffusion properties of CO₂ in GO/MOFs, which is critical to understand the behavior of GO/MOFs for mixture separations (e.g., CO₂/N₂): both the adsorption and diffusion properties are important for the systematic evaluation of the potential application of GO/MOFs in CO₂ capture.

Therefore, in this contribution, Grand Canonical Monte Carlo (GCMC) and equilibrium Molecular Dynamics (MD) simulations in the canonical ensemble were performed by means of the RASPA and LAMMPS packages, respectively, to obtain the adsorption isotherms and self-diffusivity (D_self) coefficients of CO₂ and N₂ in selected GO/MOF structures under process conditions. The MOFs studied include CuBTC and UTSA-16, at hybridization concentrations in the range of 9-65%wt. Hence, sandwich-like hybrid models were developed by our group [2] and validated against experimental results from the literature [3,4]. Lennard-Jones parameters, charges for adsorbents and adsorbate molecules, and additional simulation details can be found in previously published work [2].

From the simulation results, it was found that hybridization with GO can actually enhance the CO₂ adsorption capacity of MOFs, especially under low-pressure conditions; however, such enhancement in adsorption capacity is counterbalanced by the diffusion: the impenetrable GO sheets and stronger adsorption regions created by GO hybridization make CO₂ and N₂ tend to hinder their diffusion along the direction parallel to the GO sheets, resulting in a preferential direction for diffusion and a lower overall diffusivity.

Moreover, the different MOF topologies also significantly affect the adsorption capacity and diffusivity of CO₂ in different hybrid GO/MOFs. For instance, the D_self of CO₂ in GO/UTSA-16 continues to decrease with increasing the CO₂ loading, and the D_self of CO₂ and N₂ in the parent UTSA-16 structure is higher than that of GO/UTSA-16 in most of the cases. This is because the inherent pore size in UTSA-16 and the additional micro-pores generated during the GO hybridization process are not large enough to accommodate more than two layers of molecules, which saturates the region at very low pressures and limits the movement of the molecules.

Overall, results presented in this work reinforce the use of molecular simulations as a valuable tool to explore emerging materials for CO₂ capture at process conditions and guiding in the selection of the best structure/composition to achieve an optimal performance. The approach used here can be used to understand the working mechanism of GO/MOFs in other application fields such as catalysis, water purification, other gas separations, etc., thus promoting further applications of GO/MOFs.

We acknowledge the financial support of this work by Khalifa University through the Research and Innovation Center on CO₂ and hydrogen (project RC2-2019-007).

Keywords: CO₂ capture; molecular dynamics; adsorption; diffusion; Graphene oxide; MOFs.

[1] R. Kumar, D. Raut, U. Ramamurty, and C.N.R. Rao, “Remarkable improvement in the mechanical properties and CO₂ uptake of MOFs brought about by covalent linking to graphene”. Angewandte Chemie International Edition, vol. 55, no. 27, pp.7857-7861, 2016.

[2] H. Zhao, D. Bahamon, M. Khaleel, L. F. Vega, “Insights into the performance of hybrid graphene oxide/MOFs for CO₂ capture at process conditions by molecular simulations”. Chemical Engineering Journal, vol. 449, pp.137884, 2022.

[3] S. Liu, L. Sun, F. Xu, J. Zhang, C. Jiao, F. Li, Z. Li, S. Wang, Z. Wang, X. Jiang and H. Zhou, “Nanosized Cu-MOFs induced by graphene oxide and enhanced gas storage capacity”. Energy & Environmental Science, vol. 6, pp.818-823, 2013.

[4] Y. Shen, Z. Li, L. Wang, Y. Ye, Q. Liu, X. Ma, Q. Chen, Z. Zhang, S. Xiang, Cobalt–citrate framework armored with graphene oxide exhibiting improved thermal stability and selectivity for biogas decarburization, Journal of Materials Chemistry A, vol.3, pp.593-599, 2015