Effect of Amine Functionalization of MOF Adsorbents for Enhanced CO2 Capture and Separation: A Molecular Simulation Study

Different types of amine-functionalized MOF structures were analyzed in this work using molecular simulations in order to determine their potential for post-combustion carbon dioxide capture and separation. Six amine models -of different chain lengths and degree of substitution- grafted to the unsaturated metal sites of the M2(dobdc) MOF [and its expanded version, M2(dobpdc)] were evaluated, in terms of adsorption isotherms, selectivity, cyclic working capacity and regenerability. Good agreement between simulation results and available experimental data was obtained. Moreover, results show two potential structures with high cyclic working capacities if used for Temperature Swing Adsorption processes: mmen/Mg/DOBPDC and mda-Zn/DOBPDC. Among them, the -mmen functionalized structure has higher CO2 uptake and better cyclability (regenerability) for the flue gas mixtures and conditions studied. Furthermore, it is shown that more amine functional groups grafted on the MOFs and/or full functionalization of the metal centers do not lead to better CO2 separation capabilities due to steric hindrances. In addition, multiple alkyl groups bonded to the amino group yield a shift in the step-like adsorption isotherms in the larger pore structures, at a given temperature. Our calculations shed light on how functionalization can enhance gas adsorption via the cooperative chemi-physisorption mechanism of these materials, and how the materials can be tuned for desired adsorption characteristics.

High yield electrooxidation of 5-Hydroxymethyl furfural catalysed by unsaturated metal sites in CoFe Prussian Blue Analogue Films

Prussian Blue Analogues (PBAs) are promising electrocatalysts in oxidation reactions due to its binary metal composition and tuneable redox properties. Herein, we reported the generation of coordinative unsaturated metal sites...

Stability of Zeolitic-imidazolate Frameworks in Humid Environments: Disentangling the Roles of Confined vs. Surface Water

<div>Most of the chemistry in nanoporous materials with small pore sizes and windows are known to occur on the surface of the material which is in immediate contact with substrate/solvent, rather than inside the pores and channels. Experimentally, it is not straightforward to distinguish the chemistry of confinement from the surface. Comprehensive molecular dynamics simulations coupled with quantum mechanical calculations are employed to decipher stability of zeolitic-imidazolate frameworks in aqueous solutions. Water adsorption properties are compared and contrasted in crystalline bulk vs. nanopoarticles of ZIF-8 as a representative of the ZIF family in order to fully disentangle how water interacts with the surface of the material which contains coordinatively unsaturated metal sites compared to the pristine bulk. </div><div>Our following detailed mechanistic study reveals the significantly higher propensity of the surface with coordinatively unsaturated Zn$^{2+}$ sites toward water attack and hydrolysis. Our results presented in this work are general and are applicable to other nanoporous materials with small particle sizes, pores and windows and are useful in devising plans for synthesis of more robust water stable materials for applications that involve atmospheric and/or bulk water.</div>

Stability of Zeolitic-imidazolate Frameworks in Humid Environments: Disentangling the Roles of Confined vs. Surface Water

<div>Most of the chemistry in nanoporous materials with small pore sizes and windows are known to occur on the surface of the material which is in immediate contact with substrate/solvent, rather than inside the pores and channels. Experimentally, it is not straightforward to distinguish the chemistry of confinement from the surface. Comprehensive molecular dynamics simulations coupled with quantum mechanical calculations are employed to decipher stability of zeolitic-imidazolate frameworks in aqueous solutions. Water adsorption properties are compared and contrasted in crystalline bulk vs. nanopoarticles of ZIF-8 as a representative of the ZIF family in order to fully disentangle how water interacts with the surface of the material which contains coordinatively unsaturated metal sites compared to the pristine bulk. </div><div>Our following detailed mechanistic study reveals the significantly higher propensity of the surface with coordinatively unsaturated Zn$^{2+}$ sites toward water attack and hydrolysis. Our results presented in this work are general and are applicable to other nanoporous materials with small particle sizes, pores and windows and are useful in devising plans for synthesis of more robust water stable materials for applications that involve atmospheric and/or bulk water.</div>

Tuning Nitric Oxide Adsorption in Cobalt–Triazolate Frameworks

<p>Nitric oxide (NO) is an important signaling molecule in biological systems, and as such the ability of certain porous materials to reversibly adsorb NO is of interest for medical applications. Metal–organic frameworks have been explored for their ability to reversibly bind NO at coordinatively-unsaturated metal sites, however the influence of metal coordination environment on NO adsorption has yet to be studied in detail. Here, we examine NO adsorption in the frameworks Co₂Cl₂(bbta) and Co₂(OH)₂(bbta) (H₂bbta = 1<i>H</i>,5<i>H</i>-benzo(1,2-<i>d</i>:4,5-<i>d</i>′)bistriazole) via gas adsorption, infrared spectroscopy, powder X-ray diffaction, and magnetometry measurements. While NO adsorbs reversibly in Co₂Cl₂(bbta) without electron-transfer, adsorption of low pressures of NO in Co₂(OH)₂(bbta) is accompanied by charge transfer from the cobalt(II) centers to form a cobalt(III)–NO⁻ adduct, as supported by diffraction and infrared spectroscopy data. At higher pressures of NO, characterization data support additional uptake of the gas and disproportionation of the bound NO to form a cobalt(III)–nitro (NO₂⁻) species and N₂O gas, a transformation that appears to be facilitated in part by stabilizing hydrogen bonding interactions between the bound NO₂⁻ and framework hydroxo groups. This reactivity represents a rare example of reductive NO-binding in a metal–organic framework and demonstrates that NO binding can be tuned by changing the coordination environment of the framework metal centers.</p>

Tuning Nitric Oxide Adsorption in Cobalt–Triazolate Frameworks

<p>Nitric oxide (NO) is an important signaling molecule in biological systems, and as such the ability of certain porous materials to reversibly adsorb NO is of interest for medical applications. Metal–organic frameworks have been explored for their ability to reversibly bind NO at coordinatively-unsaturated metal sites, however the influence of metal coordination environment on NO adsorption has yet to be studied in detail. Here, we examine NO adsorption in the frameworks Co₂Cl₂(bbta) and Co₂(OH)₂(bbta) (H₂bbta = 1<i>H</i>,5<i>H</i>-benzo(1,2-<i>d</i>:4,5-<i>d</i>′)bistriazole) via gas adsorption, infrared spectroscopy, powder X-ray diffaction, and magnetometry measurements. While NO adsorbs reversibly in Co₂Cl₂(bbta) without electron-transfer, adsorption of low pressures of NO in Co₂(OH)₂(bbta) is accompanied by charge transfer from the cobalt(II) centers to form a cobalt(III)–NO⁻ adduct, as supported by diffraction and infrared spectroscopy data. At higher pressures of NO, characterization data support additional uptake of the gas and disproportionation of the bound NO to form a cobalt(III)–nitro (NO₂⁻) species and N₂O gas, a transformation that appears to be facilitated in part by stabilizing hydrogen bonding interactions between the bound NO₂⁻ and framework hydroxo groups. This reactivity represents a rare example of reductive NO-binding in a metal–organic framework and demonstrates that NO binding can be tuned by changing the coordination environment of the framework metal centers.</p>