H2 physisorption in fluorinated MOF-74: The role of fluorine from the perspective of electronic structure calculations

Fluorine has been shown to be a promising candidate of H2 physisorption center. Firstprinciples calculations reported here provide a fundamental insight into the role of fluorine in H2 physisorbing performance of a fluorinated metal-organic framework Mg-MOF-74. When a fluorine atom caps an open metal site of the framework, the fluorine p states weakly interact with the linker states so that they are highly localized, a inducing highly charged fluorine center. Meanwhile, the fluorine substituent on the aromatic ring of the organic linker has its p states strongly interacting with the linker states. Therefore, the fluorine p states become so dispersive that the fluorine site is weakly charged. The ab initio molecular dynamics (AIMD) simulation demonstrates that the highly charged capping fluorine acts as a dominating adsorption center that can attract four H2 molecules. The binding strengths of the H2 molecules with the capping fluorine are higher than with the organic linker of the original framework by up to 3.9 kJ/mol. Whereas, the weakly charged fluorine substituent does not act as a dominating adsorption center. In this case, the AIMD simulation produces similar adsorption positions as those in the original framework. Although the fluorine substituent shows some contribution to the H2 affinity, it indirectly reduces the host-guest binding strengths by reducing the electron density on the p bonds of the aromatic ring as well as delocalizing the electron density on oxygen atoms. Consequently, the H2 affinity of the linker fluorinated framework is weaker than that of the original framework by up to 3.0 kJ/mol. (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study employs first-principles calculations to investigate the role of fluorine in the physisorption performance of H2 in a fluorinated metal-organic framework (Mg-MOF-74). The results show that when a fluorine atom caps an open metal site of the framework, it acts as a highly charged adsorption center for H2 molecules. On the other hand, fluorine substituents on the aromatic ring of the organic linker do not act as dominating adsorption centers. The presence of these substituents weakens the binding strengths between the host framework and H2 molecules. Overall, the fluorinated framework exhibits weaker H2 affinity compared to the original framework.