CO2 adsorption-induced anisotropic mechanical response in an 8-fold interpenetrated diamondoid metal–organic framework

Abstract

Adsorption in nanoporous adsorbent materials is considered a viable technology for CO₂ capture. With intrinsic structural transition and tunable working capacity, flexible metal–organic frameworks (MOFs) hold great potential for energy-efficient CO₂ capture. Transitioning MOFs from laboratory-scale to practical capture requires in-depth understanding of their structural behavior and adsorption performance. Nevertheless, interpreting the fundamental mechanism underlying their flexibility from a molecular level poses a significant challenge. Herein, we employ hybrid Monte Carlo/molecular dynamics simulations to explore CO₂-induced structural transition of a flexible MOF (X-dia-2-Cd). Stepped isotherms are predicted at different response pressures during CO₂ adsorption, which agree well with experiments at both 195 K and 273 K. Structural transition from a narrow pore (Np) phase to a large pore (Lp) phase is observed and revealed to be driven primarily by the deformation of Cd metal nodes. CO₂ diffusion is substantially accelerated upon the structural transition from the Np to Lp phase. Moreover, the mechanical strength of X-dia-2-Cd is found to be preserved during the structural transition. From this study, we provide quantitative understanding in the flexibility of X-dia-2-Cd and unravel the microscopic mechanism underlying CO₂-induced structural transition by linking its local elastic behavior to multiphase stability. These microscopic insights offer valuable guidance for the rational design of new flexible MOFs for CO₂ capture and other industrially important gas adsorption processes.