Understanding the Adsorption Mechanism of C₂H₂, CO₂, and CH₄ in Isostructural Metal–Organic Frameworks with Coordinatively Unsaturated Metal Sites

Abstract

An computational study using density functional theory and grand-canonical Monte Carlo simulation that explore the adsorption mechanism of C 2 H 2 , CO 2 , and CH 4 to metal−organic frameworks (MOFs) with coordinatively unsaturated metal sites (M-MOF-74, M = Mg and Zn) has been carried out. The theoretical studies reveal that open metal sites have important roles in adsorption. The high CO 2 adsorption ability of M-MOF-74 is due to the strong Lewis acid and base interactions between metal ions and oxygen atom of CO 2 , as well as carbon atom of CO 2 with oxygen atoms in organic linkers. Meanwhile, the high C 2 H 2 adsorption for M-MOF-74 is contributed by the strong complexation between the metal ions and the π orbital of C 2 H 2 . The different adsorption mechanisms of CO 2 , C 2 H 2 , and CH 4 in M-MOF-74 can qualitatively explain the high CO 2 selectivity in CO 2 /CH 4 mixture and high C 2 H 2 selectivity in C 2 H 2 /CH 4 mixture. Energy decomposition analysis reveals that electrostatic energy, exchange energy, and repulsive energy are key factors in the binding strength of gas molecules on M-MOF-74. The preferential adsorption sites are confirmed to be located near the five-coordinate metal ions decorating the edges of the hexagonal channels. The elucidation of the adsorption mechanism at the molecular level provides key information for designing novel MOFs with high capacity and selectivity for CO 2 from light hydrocarbon mixtures.

Metal Coordination ofCO2

“…A bridged coordination has also been suggested by Hou et al 90 in their DFT study of the adsorption mechanism of CO 2 in M-MOF-74 (M = Mg and Zn). The high CO 2 adsorption of this solid is due to strong Lewis acid and base interactions between the metal ions and the oxygen atom of CO 2 , as well as between the carbon atom of CO 2 with oxygen atoms of the organic linkers of the MOF.…”

The Carbon Dioxide Molecule and the Effects of Its Interaction with Electrophiles and Nucleophiles

“…9). The CO 2 ligand is bent with a COC angle equal to 133.4 , and the CO bond lengths are 1.255 and 1.257 Å. Side-on bonded CO 2 has been found also in metal organic frameworks systems such as M-MOF-74 (M¼Mg and Zn) [109]. On the basis of DFT calculation, metals should interact with the O atoms of CO 2 , while the carbon atom gives strong interactions with the O atoms of the organic linkers of the MOF.…”

Microporous metal-organic frameworks with suitable pore spaces for acetylene storage and purification

“…By immobilizing specific sites, such as open metal sites [23,24] and Lewis acidic and basic sites [25,26] into the frameworks, either high acetylene storage capacity or separation capacity has been reached in dozens of MOFs, but there are a few MOFs which exhibit both high acetylene storage capacity and separation capacity at the same time [27,28]. As for acetylene separation, optimization of both separation selectivity (mainly determined by the functional sites and size exclusive effect) and storage capacity (mainly determined by the gas storage capacities) is very important and still a challenge for researchers now.…”

“…As well-known, the MOFs with open metal sites usually exhibit high-performance for acetylene storage[23,24,26]. Both Cu 2 TPTC-Me and Cu 2 TPTC-OMe possess a large amount of open metal sites (~3.3 mmol g -1 ) to absorb acetylene molecules.…”