Adsorption of Carbon Dioxide on Unsaturated Metal Sites in M₂(dobpdc) Frameworks with Exceptional Structural Stability and Relation between Lewis Acidity and Adsorption Enthalpy

Abstract

A series of metal-organic frameworks (MOFs) M2 (dobpdc) (M=Mn, Co, Ni, Zn; H4 dobpdc=4,4'-dihydroxy-1,1'-biphenyl-3,3'-dicarboxylic acid), with a highly dense arrangement of open metal sites along hexagonal channels were prepared by microwave-assisted or simple solvothermal reactions. The activated materials were structurally expanded when guest molecules including CO2 were introduced into the pores. The Lewis acidity of the open metal sites varied in the order Mn<Co<Ni>Zn, as confirmed by C=O stretching bands in the IR spectra, which are related to the CO2 adsorption enthalpy. DFT calculations revealed that the high CO2 binding affinity of transition-metal-based M2 (dobpdc) is primarily attributable to the favorable charge transfer from CO2 (oxygen lone pair acting as a Lewis base) to the open metal sites (Lewis acid), while electrostatic effects, the underlying factor responsible for the particular order of binding strength observed across different transition metals, also play a role. The framework stability against water coincides with the order of Lewis acidity. In this series of MOFs, the structural stability of Ni2 (dobpdc) is exceptional; it endured in water vapor, liquid water, and in refluxing water for one month, and the solid remained intact on exposure to solutions of pH 2-13. The DFT calculations also support the experimental finding that Ni2 (dobpdc) has higher chemical stability than the other frameworks.

High Ammonia Uptake of a Metal–Organic Framework Adsorbent in a Wide Pressure Range

“…The activation energy of the desorption process was roughly estimated to be 159.0 kJ mol −1 for M=Mg, 146.7 kJ mol −1 for M=Mn, 153.8 kJ mol −1 for M=Co, and 168.6 kJ mol −1 for M=Ni. The NH 3 desorption temperature and activation energy trend was as follows: Ni > Mg > Co > Mn, which closely resembles the CO 2 adsorption enthalpy trend observed in the M 2 (dobpdc) series [29] . Furthermore, this order directly reflects the Lewis acidity strength of the metal ions of these systems.…”

High Ammonia Uptake of a Metal–Organic Framework Adsorbent in a Wide Pressure Range

“…TheNH 3 uptake of the Mg and Ni compounds was observed to decrease.T his could be associated with the significant interactions between NH 3 and the metal centers,as Mg and Ni have the strongest Lewis acidities among the metal Angewandte ions in the M 2 (dobpdc) series. [29] After the cyclic adsorption test, all samples,e xcept for Zn, retained their structural integrity,asconfirmed by PXRD,IR, and N 2 sorption results (Supporting Information, Figures S16-S22, Table S4). The peaks of the Zn analog in the PXRD pattern significantly weakened after the first cycle,suggesting that the capacity loss originates from structural collapse (Supporting Information, Figures S16 and S22).…”

Performance of Mixed Matrix Membranes Containing Porous Two-Dimensional (2D) and Three-Dimensional (3D) Fillers for CO2 Separation: A Review

“…In the case of CO 2 , for example, it has been reported that the presence of unsaturated open metal sites can greatly enhance the CO 2 sorption capacity due to considerable polarizability and quadrupole moment. Open metal cations play as Lewis acidic nodes that strongly favors CO 2 [ 40 , 41 ]. The occurrence of breathing is reported to significantly affect the pore volume, and, therefore, the gas sorption ability.…”