ABSTRACT
Recycling Kr and Xe from used nuclear fuel (UNF) is conducive to regenerating economy and protecting the environment, and it is urgent to screen or design high-performance cutting-edge metal-organic framework (MOF) materials for Kr/Xe adsorption separation. After grand canonical Monte Carlo (GCMC) simulations of Kr/Xe adsorption separation on 11,000 frameworks in CoRE MOFs (2019), the important structure-adsorption property relationship (SAPR) was induced; that is, the porosity (φ) at 0.30-0.40, LCD/PLD at 1.00-1.49, density (ρ) range between 1.20 and 2.30 g/cm3, and PLD at 2.40-3.38 Å can be utilized to screen for high-performance G-MOFs and hMOFs. In addition, the key "genes" (metal nodes and linkers) of MOFs determining the Kr/Xe adsorption separation were data-mined by a machine learning technique, which were assembled into novel MOFs. After comprehensive consideration of thermal stability and the adsorbent performance score (APS), eight promising MOFs on Kr/Xe separation with the APS more than 1290.89 were screened out and assembled, which are better than most of the reported frameworks. Note that the adsorption isotherms of these MOFs on Kr and Xe belong to type I curve with the thermodynamic equilibrium mechanism on Kr/Xe based on the confinement effect. Furthermore, according to the electronic structure calculations of the independent gradient model based on Hirshfeld partition (IGMH) and energy decomposition analysis, it is found that the interactions between guests and frameworks are vdW forces with dominant induction energy (Eind). In addition, the electrostatic potential gradients of frameworks are generally linearly negative correlated with Kr uptakes. Therefore, both the geometrical and electronic structures dominate the adsorption separation performance on Kr/Xe. Interestingly, these eight MOFs are also suitable for the separation of CH4/H2 with considerable selectivities and CH4 uptakes of up to 2566.67 and 3.04 mmol/g, respectively. Herein, the accurately constructed SAPR and material genomics strategy should be helpful for the experimental discovery of novel MOFs on Kr/Xe separation experimentally.
ABSTRACT
The solvent extraction, complexing ability, and basicity of tetradentate N-donor 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4-BT- Phen) and its derivatives functionalized by Br, hydroxyphenyl, nitryl were discussed and compared. It was demonstrated that four BTPhen ligands are able to selectively extract Am(lll) over Eu(lll). It was notable that the distribution ratio of 5-nitryl-CyMe4-BTPhen for Eu(lll) was suppressed under 0.02, which was much lower compared to DEu(lll) = 1 by CyMe4-BTPhen. The analysis of the effect of the substituent on the affinity to lanthanides was conducted by UV/vis and fluorescence spectroscopic titration. The stability constants of various ligands with Eu(lll) were obtained by fitting titration curve. Additionally, the basicity of various ligands was determined to be 3.1 ± 0.1, 2.3 ± 0.2, 0.9 ± 0.2, 0.5 ± 0.1 by NMR in the media of CD3OD with the addition of DClO4. The basicity of ligands follows the order of L1 > L2 > L3 > L4, indicating the tendency of protonation decreases with the electron-withdrawing ability increase.
ABSTRACT
The separation of Xe/Kr mixtures in used nuclear fuel (UNF) has attracted lots of attention, but no report on the adsorption and separation of Kr from mixed Kr/Xe at room temperature can be found. From grand canonical Monte Carlo (GCMC) simulation, it is found that by replacing the metal center Ca of SBMOF-1 with Mg, due to the appropriate pore size, the adsorption selectivity (S Kr/Xe) was extremely high (250 000) and the adsorption capacity for Kr on Mg-SBMOF-1 modified with -NH2 was increased by 300% to 1.020 from 0.248 mmol g-1. Based on the calculations of density functional theory (DFT), we found that the stronger electron-donating ability of a functional group will increase the polarizability of the ligand, and thus increase the adsorption capacity to Kr. In addition, the analysis of electronic structures with independent gradient model (IGM) and energy decomposition analysis (EDA) indicates that van der Waals forces will be responsible for the interaction of Mg-SBMOF-1 and Kr gas. Among them, the interaction of Mg-SBMOF-1 and Kr gas is mainly an induction force, while that of modifications with -CH3 and -NH2 is mainly a dispersion force. The present theoretical study represents the first report of the separation of Kr from Xe with MOF adsorption at room temperature. We hope this work may promote the experimental synthesis of Mg-SBMOF-1 for efficient separation of Kr and Xe.
ABSTRACT
Enzymatic heme and non-heme Fe(iv)-O species usually play an important role in hydrogen abstraction of biocatalytic reactions, yet duplicating the reactivity in biomimicry remains a great challenge. Based on Xiao et al.'s experimental work [Nat. Chem., 2014, 6(7), 590], we theoretically found that in the presence of the oxidant N2O, the enzyme-like metal organic framework, i.e., magnesium-diluted Fe-MOF-74 [Fe/(Mg)-MOF-74] can activate the C-H bonds of 1,4-cyclohexadiene (CHD) into benzene with a two-step hydrogen abstraction mechanism based on the density functional theory (DFT) level. It is shown that the first transition state about the cleavage of the N-O bond of N2O to form the Fe(iv)-O species is the rate-determining step with activation enthalpy of 19.4 kcal mol-1 and the complete reaction is exothermic by 62.8 kcal mol-1 on quintet rather than on triplet PES. In addition, we proposed a rebound mechanism of cyclic cyclohexane (CHA) hydroxylation to cyclohexanol which has not been studied experimentally. Note that the activation enthalpies on the first hydrogen abstraction for both cyclic CHD and cyclohexane are just 8.1 and 3.5 kcal mol-1, respectively, which are less than that of 13.9 kcal mol-1 for chained ethane. Most importantly, for the hydrogen abstraction of methane catalyzed by M/(Mg)-MOF-74 (M = Cu, Ni, Fe, and Co), we found that the activation enthalpies versus the C-H bond length of methane of TSs, NPA charge of the reacting oxyl atom have linear relationships with different slopes, i.e., shorter C-H bond and less absolute value of NPA charge of oxyl atom are associated with lower activation enthalpy; while for the activation of methane, ethane, propane and CHD catalyzed by Fe/(Mg)-MOF-74, there also exists positive correlations between activation enthalpies, bond dissociation energies (BDEs) and C-H bond lengths in TSs, respectively. We hope the present theoretical study may provide the guideline to predict the performance of MOFs in C-H bond activation reactions.
