CD-MOF: A Versatile Separation Medium Hartlieb, Karel J; Holcroft, James M; Moghadam, Peyman Z ...
Journal of the American Chemical Society,
02/2016, Volume:
138, Issue:
7
Journal Article
Peer reviewed
Open access
Porous metal–organic frameworks (MOFs) have been studied in the context of a wide variety of applications, particularly in relation to molecular storage and separation sciences. Recently, we reported ...a green, renewable framework material composed of γ-cyclodextrin (γ-CD) and alkali metal saltsnamely, CD-MOF. This porous material has been shown to facilitate the separation of mixtures of alkylaromatic compounds, including the BTEX mixture (benzene, toluene, ethylbenzene, and the regioisomers of xylene), into their pure components, in both the liquid and gas phases, in an energy-efficient manner which could have implications for the petrochemical industry. Here, we report the ability of CD-MOF to separate a wide variety of mixtures, including ethylbenzene from styrene, haloaromatics, terpinenes, pinenes and other chiral compounds. CD-MOF retains saturated compounds to a greater extent than their unsaturated analogues. Also, the location of a double bond within a molecule influences its retention within the extended framework, as revealed in the case of the structural isomers of pinene and terpinine, where the isomers with exocyclic double bonds are more highly retained than those with endocyclic double bonds. The ability of CD-MOF to separate various mono- and disubstituted haloaromatic compounds appears to be controlled by both the size of the halogen substituents and the strength of the noncovalent bonding interactions between the analyte and the framework, an observation which has been confirmed by molecular simulations. Since CD-MOF is a homochiral framework, it is also able to resolve the enantiomers of chiral analytes, including those of limonene and 1-phenylethanol. These findings could lead to cheaper and easier-to-prepare stationary phases for HPLC separations when compared with other chiral stationary phases, such as CD-bonded silica particles.
The zeolitic imidazolate framework ZIF-4 is a metal–organic framework made of tetrahedral Zn2+ nodes connected by imidazolate ligands to form zeolite-like pores. ZIF-4 exhibits many interesting ...phenomena such as framework flexibility and phase transitions. Recently, the “window effect” has also been demonstrated experimentally in ZIF-4. The window effect refers to an unusual, nonmonotonic trend in the diffusivity of chain molecules in nanoporous materials. In ZIF-4, the diffusivities decrease with chain length from methane to n-butane but with n-pentane having a higher diffusivity than n-butane and n-hexane. This is hypothesized to be due to the difficulty of the longer chains (n-pentane, n-hexane) to coil up inside the cages, causing them to extend out of the cage through the flexible cage windows. As a result, the diffusion mechanisms could change as the chain length increases from n-butane to n-pentane. We investigate this possibility by simulating the adsorption of C1–C6 n-alkanes in ZIF-4 using hybrid Monte Carlo simulations and analyzing the adsorbed configurations for molecular siting differences among the n-alkanes from methane to n-hexane.
Metal-organic frameworks--a class of porous hybrid materials built from metal ions and organic bridges--have recently shown great promise for a wide variety of applications. The large choice of ...building blocks means that the structures and pore characteristics of the metal-organic frameworks can be tuned relatively easily. However, despite much research, it remains challenging to prepare frameworks specifically tailored for particular applications. Here, we have used computational modelling to design and predictively characterize a metal-organic framework (NU-100) with a particularly high surface area. Subsequent experimental synthesis yielded a material, matching the calculated structure, with a high BET surface area (6,143 m(2) g(-1)). Furthermore, sorption measurements revealed that the material had high storage capacities for hydrogen (164 mg g(-1)) and carbon dioxide (2,315 mg g(-1))--gases of high importance in the contexts of clean energy and climate alteration, respectively--in excellent agreement with predictions from modelling.
Hydrogen is an appealing energy storage solution for electric vehicles due to its low environmental impact and faster recharge times compared to batteries. However, there are many engineering ...challenges involved in safely storing a sufficient amount of hydrogen onboard a vehicle with a reasonable volumetric density. Nanoporous materials such as metal-organic frameworks (MOFs) have the potential to store hydrogen at high density and only moderate pressure. Considerable research has been devoted to finding new MOFs for hydrogen storage in recent years; however, a MOF that provides sufficient hydrogen density and is suitable to commercial applications has not yet been found. Much of this research makes use of molecular modelling to screen thousands of materials in a high-throughput way. Computational screening can be an effective tool for gaining insight into structure-performance relationships as well as finding specific candidates for an application. Recently, some research groups have also used machine learning to analyze data more effectively and accelerate the screening process. In this review, we discuss some recent advances in using molecular modelling and machine learning to find materials for hydrogen storage. We also discuss and compare some popular models for the hydrogen molecule and the accuracy of different equations of state, which are important considerations for accurate molecular simulations.
Water stability in metal-organic frameworks (MOFs) is critical for several practical applications. While water instability is mainly thought to stem from linker hydrolysis, MOFs with strong, ...hydrolysis-resistant metal-linker bonds can be susceptible to damage by capillary forces, which cause cavities and channels to collapse during activation from water. This study utilizes metal node functionalization as a strategy to create vapor-stable and recyclable MOFs.
Recent studies have suggested that the gas-phase hydrolysis of nerve agents by Zr-based metal–organic frameworks (MOFs) may be limited by product inhibition resulting from strong bidentate binding of ...the hydrolysis products to the Zr6-nodes. A potential method to avoid this problem is to deposit single-atom catalysts on the nodes so that the products bind in a more favorable monodentate mode. Such catalytic active sites can be characterized with atomic precision, enabling detailed computational mechanistic studies. Thus, we used density functional theory to perform a comprehensive screening of single-atom transition-metal catalysts, in varying oxidation states, deposited on NU-1000 nodes for the gas-phase hydrolysis of the nerve agent sarin. By calculating the complete reaction pathways for M–NU-1000 systems, we discovered that the highest reaction barrier varies between catalysts, highlighting the need to consider more than a single reaction step when screening a large number of diverse materials. The single-metal catalysts are predicted to exhibit lower product desorption energies than unfunctionalized NU-1000. By comparing their relative turnover frequencies using the energetic span model, we identified several catalysts that are predicted to be more active than the parent MOF for this reaction. Finally, we explored periodic trends and molecular descriptors for their effect on catalytic activity.
To investigate postcombustion capture of CO2 in the presence of water, we developed a pressure/vacuum swing adsorption (P/VSA) cycle model consisting of a system of partial differential algebraic ...equations incorporating mass and energy balances, the Ergun equation for pressure changes, competitive Langmuir isotherms, and the linear driving force model. Four potential adsorbents, zeolites 13X and 5A and the MOFs HKUST-1 and Ni-MOF-74, are investigated, evaluated, and compared. Using this simulation, a two-stage Skarstrom cycle, coupled with an upstream dehydration unit and a downstream compression unit, is optimized using a nondominant sorting genetic algorithm, NSGA-II, to minimize the overall cost of capturing 90% of CO2 from flue gas at a purity of 90% and compressing it for pipeline transportation at 110 bar. The results show that under dry flue gas conditions, zeolite 13X is the best performing adsorbent with an overall cost of $32.1/ton of CO2. Under humid flue gas conditions, zeolites 13X and 5A performed equally well with overall costs of capturing CO2 of approximately $34.1/ton of CO2.
Multivariate metal–organic frameworks (MTV-MOFs) contain multiple linker types within a single structure. Arrangements of linkers containing different functional groups confer structural diversity ...and surface heterogeneity and result in a combinatorial explosion in the number of possible structures. In this work, we carried out high-throughput computational screening of a large number of computer-generated MTV-MOFs to assess their CO2 capture properties using grand canonical Monte Carlo simulations. The results demonstrate that functionalization enhances CO2 capture performance of MTV-MOFs when compared to their parent (unfunctionalized) counterparts, and the pore size plays a dominant role in determining the CO2 adsorption capabilities of MTV-MOFs irrespective of the combinations of the three functional groups (−F, −NH2, and −OCH3) that we investigated. We also found that the functionalization of parent MOFs with small pores led to larger enhancements in CO2 uptake and CO2/N2 selectivity than functionalization in larger-pore MOFs. Free energy contour maps are presented to visually compare the influence of linker functionalization between frameworks with large and small pores.
Storing an acceptable density of hydrogen in porous materials by physisorption at room temperature and reasonable pressures is a challenging problem. Metal-organic frameworks (MOFs) are a new class ...of nanoporous materials that have shown early promise for meeting this goal. They have extremely large specific surface areas, but the heats of adsorption to date are too low to provide significant storage at room temperature. In this work, molecular simulations are used to provide guidelines for the design of MOFs for hydrogen storage. To learn how much the heat of adsorption must be increased to meet current targets, we artificially increase the hydrogen/MOF Lennard-Jones attraction. The correlation of the amount of hydrogen adsorbed with the heat of adsorption, the surface area, and the free volume is revisited. We also review the distinction between excess and absolute adsorption and show that comparing the density of hydrogen within the free volume of materials provides useful insight. The simulation results yield a graph showing the required heats of adsorption as a function of the free volume to meet gravimetric and volumetric storage targets at room temperature and 120 bar.
Metal−organic frameworks (MOFs) synthesized in a building-block approach from organic linkers and metal corner units offer the opportunity to design materials with high surface areas for adsorption ...applications by assembling the appropriate building blocks. In this paper, we show that the surface area calculated in a geometric fashion from the crystal structure is a useful tool for characterizing MOFs. We argue that the accessible surface area rather than the widely used Connolly surface area is the appropriate surface area to characterize crystalline solids for adsorption applications. The accessible surface area calculated with a probe diameter corresponding to the adsorbate of interest provides a simple way to screen and compare adsorbents. We investigate the effects of the probe molecule diameter on the accessible surface area and discuss the implications for increasing the surface area of metal−organic frameworks by the use of catenated structures. We also demonstrate that the accessible surface area provides a useful tool for judging the quality of a synthesized sample. Experimental surface areas can be adversely affected by incomplete solvent removal during activation, crystal collapse, or interpenetration. The easily calculated accessible surface area provides a benchmark for the theoretical upper limit for a perfect crystal.
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