Progress in adsorption-based CO2 capture by metal-organic frameworks

Metal-organic frameworks (MOFs) have recently attracted intense research interest because of their permanent porous structures, large surface areas, and potential applications as novel adsorbents. The recent progress in adsorption-based CO(2) capture by MOFs is reviewed and summarized in this critical review. CO(2) adsorption in MOFs has been divided into two sections, adsorption at high pressures and selective adsorption at approximate atmospheric pressures. Keys to CO(2) adsorption in MOFs at high pressures and low pressures are summarized to be pore volumes of MOFs, and heats of adsorption, respectively. Many MOFs have high CO(2) selectivities over N(2) and CH(4). Water effects on CO(2) adsorption in MOFs are presented and compared with benchmark zeolites. In addition, strategies appeared in the literature to enhance CO(2) adsorption capacities and/or selectivities in MOFs have been summarized into three main categories, catenation and interpenetration, chemical bonding enhancement, and electrostatic force involvement. Besides the advantages, two main challenges of using MOFs in CO(2) capture, the cost of synthesis and the stability toward water vapor, have been analyzed and possible solutions and path forward have been proposed to address the two challenges as well (150 references).     

Control of the coordination status of the open metal sites in metal-organic frameworks for high performance separation of polar compounds

Metal-organic frameworks (MOFs) with open metal sites have great potential for enhancing adsorption separation of the molecules with different polarities. However, the elution and separation of polar compounds on such MOFs packed columns using nonpolar solvents is difficult due to too strong interaction between polar compounds and the open metal sites. Here, we report the control of the coordination status of the open metal sites in MOFs by adjusting the content of methanol (MeOH) in the mobile phase for fast and high-resolution separation of polar compounds. To this end, high-performance liquid chromatographic separation of nitroaniline, aminophenol and naphthol isomers, sulfadimidine, and sulfanilamide on the column packed with MIL-101(Cr) possessing open metal sites was performed. The interaction between the open metal sites of MIL-101(Cr) and the polar analytes was adjusted by adding an appropriate amount of MeOH to the mobile phase to achieve the effective separation of the polar analytes due to the competition of MeOH with the analytes for the open metal sites. Fourier transform infrared spectra and X-ray photoelectron spectra confirmed the interaction between MeOH and the open metal sites of MIL-101(Cr). Thermodynamic parameters were measured to evaluate the effect of the content of MeOH in the mobile phase on the separation of polar analytes on MIL-101(Cr) packed column. This approach provides reproducible and high performance separation of polar compounds on the open metal sites-containing MOFs.     

Dispersion corrected-DFT investigation on the adsorption and selective detection of organic solvent vapors by Cu2(HCOO)4 paddle wheel of metal organic framework

Porous metal organic frameworks (MOFs) has shown large surface area and high micropore volume making it a promising electrode material for sensing devices. Adsorption and electronic sensitivity of copper-based open metal sites paddlewheel (Cu₂(HCOO)₄) towards polar, moderately polar, and non-polar organic solvent vapors (OSVs) were was investigated using density functional theory, employing B3LYP. The most stable adsorption structures were those with the OSVs interacting with the metal node of PW. Based on calculations, the adsorption energy of molecules is in the range of ?7.8 to ?24.8 kcal/mol, ?9.2 to ?25.7 kcal/mol, and ?6.6 to ?10.9 kcal/mol for polar, moderately polar, and non-polar OSVs, respectively. Also sensing activities of PW were studied from three points of view band gap changing, sensing factor, and work function changes. From the standpoint of conductivity changing, Cu-PW has (i) low sensitivity to acetonitrile, acetone, dimethyl formamide, dimethyl ether, benzene, and ethanol; (ii) moderate sensitivity to toluene, and (iii) strong sensitivity to THF detection so that its HOMO/LUMO gap of the PW is significantly decreased from 1.63 to 0.97 eV which may increase the electrical conductivity, sensing factor is 1.4 * 10¹¹, and work function changing is 0.45 eV after THF adsorption. Thus, we suggest that Cu-PW may be used as a highly sensitive/selective and multi-time reusable sensor material for THF detection.     

Effect of Central Metal Ions of Analogous Metal–Organic Frameworks on Adsorption of Organoarsenic Compounds from Water: Plausible Mechanism of Adsorption and Water Purification

The adsorptive removal of organoarsenic compounds such as p‐arsanilic acid (ASA) and roxarsone (ROX) from water using metal–organic frameworks (MOFs) has been investigated for the first time. A MOF, iron benzenetricarboxylate (also called MIL‐100‐Fe) exhibits a much higher adsorption capacity for ASA and ROX than activated carbon, zeolite (HY), goethite, and other MOFs. The adsorption of ASA and ROX over MIL‐100‐Fe is also much more rapid than that over activated carbon. Moreover, the used MIL‐100‐Fe can be recycled by simply washing with acidic ethanol. Therefore, it is determined that a MOF such as MIL‐100‐Fe can be used to remove organoarsenic compounds from contaminated water because of its high adsorption capacity, rapid adsorption, and ready regeneration. Moreover, only one of three analogous MIL‐100 species (MIL‐100‐Fe, rather than MIL‐100‐Al or MIL‐100‐Cr) can effectively remove the organoarsenic compounds. This selective and high adsorption over MIL‐100‐Fe, different from other analogous MIL‐100 species, can be explained (through calculations) by the facile desorption of water from MIL‐100‐Fe as well as the large (absolute value) replacement energy (difference between the adsorption energies of the organoarsenic compounds and water) exhibited by MIL‐100‐Fe. A plausible adsorption/desorption mechanism is proposed based on the surface charge of the MOFs, FTIR results, calculations, and the reactivation results with respect to the solvents used in the experiments.     

金属有机骨架材料在吸附分离大气污染物中的应用

大气污染问题是关系人民生命健康和经济社会和谐发展的重大问题.因此需要开发高效的吸附材料用于大气污染物的吸附和分离.金属有机骨架材料(MOFs)是一类新型的多孔材料,该材料具有结构多样、孔结构有序、大比表面积和高孔隙率等结构特点.MOFs通过调节有机配体的长度和官能团调节孔径和孔道尺寸,并进行功能化修饰在孔道中引入功能性...     

Performance evaluation of functionalized ordered mesoporous silica for VOCs adsorption by molecular dynamics simulation

Volatile organic compounds (VOCs) are growing pollutants now that cause the serious environmental pollution and threaten human health. The functionalized ordered mesoporous silica (FOMS) has attracted considerable attention in adsorbing VOCs. In this paper, the molecular dynamics simulation was used to simulate the adsorption performance of FOMS on VOCs (acetone, ethyl acetate and toluene). After simulating different pore sizes (2 nm, 3 nm and 4 nm) adsorption performances of ordered mesoporous silica (OMS) on VOCs, OMS with a pore size of 4 nm was selected to further study the influence of functional groups (vinyl, methyl, and phenyl). The following law was obtained: the saturated adsorption capacities of vinyl-functionalized OMS (V-FOMS) to acetone, ethyl acetate and toluene were 3.045 mmol.g^?1, 2.568 mmol.g^?1 and 1.976 mmol.g^?1 respectively; the saturated adsorption capacities of methyl-functionalized OMS (M-FOMS) to acetone, ethyl acetate and toluene were 2.798 mmol.g^?1, 2.312 mmol.g^?1 and 1.698 mmol.g^?1 respectively; the saturated adsorption capacities of phenyl-functionalized OMS (P-FOMS) to acetone, ethyl acetate and toluene were 2.124 mmol.g^?1, 1.941 mmol.g^?1 and 1.539 mmol.g^?1 respectively. These results show that the adsorption ability of FOMS for different adsorbates follows the sequence of acetone > ethyl acetate > toluene. Furthermore, the interaction between functional groups (vinyl, methyl and phenyl) in FOMS and VOCs was explored. It is found that the interaction between different functional groups and adsorbates is different (interaction energy effect). This interaction energy effect promotes FOMS to better adsorb VOCs. This work would provide fundamental understanding and guidance for the development of novel adsorption materials for the adsorption of VOCs.     

Comparative study of adsorbents performance in ethylene/ethane separation

Some potential adsorbents for ethylene/ethane separation are ethylene selective while the others are ethane selective. Among different adsorbents, i.e., zeolites and metal organic frameworks (MOFs), a comparative study is critical to find the more suitable adsorbent for the separation. In this paper, binary ethylene/ethane adsorption performances of zeolites and MOFs, i.e., equilibrium selectivities and adsorption capacities are investigated utilizing ideal adsorbed solution theory (IAST). IAST model is applied at different gas compositions (0.1–0.9 ethylene mole fractions) and pressures up to 100 kPa. The results revealed that the most selective adsorbent toward ethylene is 5A zeolite while MOFs have higher equilibrium adsorption capacities. Among zeolites and MOFs, 5A and Fe₂(dobdc) have the highest selectivity (27.4 and 13.6) and capacity (≈2.8 and 5.8 mmol ethylene/g) at 100 kPa and 298 K for a 50/50 mixture. Among ethane selective adsorbents, Silicalite-1 zeolite and UTSA-33a (MOF) have the highest selectivity and capacity (≈2.9 and ≈1.5 mmol ethane/g) at 100 kPa and 298 K for a 50/50 mixture, respectively. Investigation showed that adsorption capacity of ethylene selective adsorbents is higher than that of ethane selective ones.     

Clicked Isoreticular Metal–Organic Frameworks and Their High Performance in the Selective Capture and Separation of Large Organic Molecules

Three highly porous metal–organic frameworks (MOFs) with a uniform rht‐type topological network but hierarchical pores were successfully constructed by the assembly of triazole‐containing dendritic hexacarboxylate ligands with Zn^II ions. These transparent MOF crystals present gradually increasing pore sizes upon extension of the length of the organic backbone, as clearly identified by structural analysis and gas‐adsorption experiments. The inherent accessibility of the pores to large molecules endows these materials with unique properties for the uptake of large guest molecules. The visible selective adsorption of dye molecules makes these MOFs highly promising porous materials for pore‐size‐dependent large‐molecule capture and separation.     

Functionalization effect of Fe-type MOF for methylene blue adsorption

Water pollutant such as dyes had danger the water quality. Todays, porous materials are great potential for dye adsorption from water bodies. In this study, the iron-based metal–organic framework (MOF-Fe) of MIL-101 is synthesized through a facile solvothermal method. The amine-functionalization effect of the MOF-Fe (amine-MOF-Fe) is evaluated for the adsorptive removal of methylene blue (MB) from aqueous solution. The adsorption behaviour had shown a rapid MB adsorption within the first hour of the process due to the pore-filling mechanism of the porous MOF-Fe structure. The electrostatic interaction between the amino group of amine-MOF-Fe and MB had contributed to the high adsorption capacity. The amine-functionalization effect also found the amine-MOF-Fe is having two times higher adsorption capacity when used with the loading two times lower than non-functionalized MOF-Fe. The maximum equilibrium adsorption capacities were measured at 149.25 and 312.5 mg/g with optimum MOFs loading of 0.8 and 0.4 g/L for MOF-Fe and amine-MOF-Fe, respectively. The adsorption mechanism proposed includes the electrostatic interaction, pore filling, hydrogen bonding, and π–π stacking. The regeneration study showed the MOFs could be recycled without interfering with the removal efficiency. Hence, the results indicate that the MOFs had desirable reusability for the practical adsorption of cationic dyes with its features of fast adsorption and high capacity.     

Capture of nerve agents and mustard gas analogues by hydrophobic robust MOF-5 type metal-organic frameworks

In this communication, a series of observations and data analyses coherently confirms the suitability of the novel metal-organic framework (MOF) [Zn(4)(μ(4)-O)(μ(4)-4-carboxy-3,5-dimethyl-4-carboxy-pyrazolato)(3)] (1) in the capture of harmful volatile organic compounds (VOCs). It is worthy of attention that 1, whose crystal structure resembles that of MOF-5, exhibits remarkable thermal, mechanical, and chemical stability, as required if practical applications are sought. In addition, it selectively captures harmful VOCs (including models of Sarin and mustard gas, which are chemical warfare agents), even in competition with ambient moisture (i.e., under conditions mimicking operative ones). The results can be rationalized on the basis of Henry constant and adsorption heat values for the different essayed adsorbates as well as H(2)O/VOC partition coefficients as obtained from variable-temperature reverse gas chromatography experiments. To further strengthen the importance of 1, its performance in the capture of harmful VOCs has been compared with those of well-known materials, namely, a MOF with coordinatively unsaturated metal sites, [Cu(3)(btc)(2)] and the molecular sieve active carbon Carboxen. The results of this comparison show that coordinatively unsaturated metal sites (preferential guest-binding sites) are ineffective for the capture of VOCs in the presence of ambient moisture. Consequently, we propose that the driving force of the VOC-MOF recognition process is mainly dictated by pore size and surface hydrophobicity.     

Strong and reversible binding of carbon dioxide in a green metal-organic framework

The efficient capture and storage of gaseous CO(2) is a pressing environmental problem. Although porous metal-organic frameworks (MOFs) have been shown to be very effective at adsorbing CO(2) selectively by dint of dipole-quadruple interactions and/or ligation to open metal sites, the gas is not usually trapped covalently. Furthermore, the vast majority of these MOFs are fabricated from nonrenewable materials, often in the presence of harmful solvents, most of which are derived from petrochemical sources. Herein we report the highly selective adsorption of CO(2) by CD-MOF-2, a recently described green MOF consisting of the renewable cyclic oligosaccharide γ-cyclodextrin and RbOH, by what is believed to be reversible carbon fixation involving carbonate formation and decomposition at room temperature. The process was monitored by solid-state (13)C NMR spectroscopy as well as colorimetrically after a pH indicator was incorporated into CD-MOF-2 to signal the formation of carbonic acid functions within the nanoporous extended framework.     

CH(4) storage and CO(2) capture in highly porous zirconium oxide based metal-organic frameworks

A series of porous Zr oxoclusters-based MOFs was computationally explored for their gas storage/capture performances. The highly porous UiO-67(Zr) and UiO-68(Zr) solids show exceptionally high CH(4) and CO(2) adsorption capacities under operating conditions that make these thermal, water and mechanical resistant materials very promising for physisorption-based processes.     

金属有机框架材料在分离分析领域的研究进展

金属有机框架材料是由金属离子节点和有机配体通过配位键连接形成的具有序多孔骨架的材料, 因其具有比表面积大、 孔隙可调及表面性质可控等优点而备受关注. 通过对有机配体和金属离子进行选择及对金属有机框架材料进行后修饰处理, 可实现对金属有机框架材料表面性质的调控, 以提升其选择性吸附及特异性识别等性能, 进而拓展其在分离分析等领域的应用. 本文从金属有机框架材料的表面性质调控出发, 介绍了其表面性质与分离分析性能的关系, 总结了近年来该领域的代表性工作, 并展望了金属有机框架材料在分离分析领域的应用前景.     

Adsorption on Mesoporous Metal–Organic Frameworks in Solution: Aromatic and Heterocyclic Compounds

Adsorption and desorption play major roles in separations, purification of water, waste streams, liquid fuels, catalysis, biomedicine and chromatography. Mesoporous metal–organic frameworks (MOFs) with pore sizes 2–50 nm are particularly suitable for adsorption of organic compounds in solution. Tens of thousands of aromatic and heterocyclic compounds are major components of liquid fuels, feedstock for industrial synthesis, solvents, dyestuffs, agricultural chemicals, medicinal drugs, food additives, and so forth. This Review provides a systematization and analysis of studies on adsorption/desorption on mesoporous MOFs in solution and their underlying chemical mechanisms. The (in)stability of mesoporous MOFs in water is critically discussed. Adsorption capacity and selectivity are covered for organic dyes, medicinal drugs, major components of liquid fuels, and miscellaneous industrial chemicals. Ionic interactions, Brønsted acid–base interactions, hydrogen bonding, coordination bonding, π–π interactions, and non‐specific interactions are covered amongst adsorption mechanisms. The effects of post‐synthetic modifications of mesoporous MOFs on their stability, adsorption capacity, selectivity, and mechanisms of adsorption and desorption are analyzed. To encourage research in this quickly growing field, we identify “niches” for which no application‐oriented and/or mechanistic studies were reported. Perspectives and limitations of a wide use of mesoporous MOFs as industrial sorbents are discussed.     

Microwave‐Assisted Synthesis of HKUST‐1 and Functionalized HKUST‐1‐@H3PW12O40: Selective Adsorption of Heavy Metal Ions in Water Analyzed with Synchrotron Radiation

A simple, rapid and efficient synthesis of the metal‐organic framework (MOF) HKUST‐1 [Cu₃(1,3,5‐benzene‐tri‐carboxilic‐acid)₂] by microwave irradiation is described, which afforded a homogeneous and highly selective material. The unusually short time to complete the synthesis by microwave irradiation is mainly attributable to rapid nucleation rather than to crystal growth rate. Using this method, HKUST‐1‐MW (MW=microwave) could be prepared within 20 min, whereas by hydrothermal synthesis, involving conventional heating, the preparation time is 8 h. Work efficiency was improved by the good performance of the obtained HKUST‐1‐MW which exhibited good selective adsorption of heavy metal ions, as well as a remarkably high adsorption affinity and adsorption capacity, but no adsorption of Hg²⁺ under the same experimental conditions. Of particular importance is the preservation of the structure after metal‐ion adsorption, which remained virtually intact, with only a few changes in X‐ray diffraction intensity and a moderate decline in surface area. Synthesis of the polyoxometalate‐containing HKUST‐1‐MW@H₃PW₁₂O₄₀ afforded a MOF with enhanced stability in water, due to the introduced Keggin‐type phosphotungstate, which systematically occluded in the cavities constituting the walls between the mesopores. Different Cu/W ratios were investigated according to the extrusion rate of cooper ions concentration, without significant structural changes after adsorption. The MOFs obtained feature particle sizes between 10–20 μm and their structures were determined using synchrotron‐based X‐ray diffraction. The results of this study can be considered important for potentially wider future applications of MOFs, especially to attend environmental issues.     

A Water‐Stable Ionic MOF for the Selective Capture of Toxic Oxoanions of SeVI and AsV and Crystallographic Insight into the Ion‐Exchange Mechanism

Selectively capturing toxic oxoanions of selenium and arsenic is highly desired for the remediation of hazardous waste. Ionic metal–organic frameworks (iMOFs) especially cationic MOFs (iMOF‐C) as ion‐exchange materials, featuring aqueous phase stability, present a robust pathway for sequestration of the oxoanions owing to their ability to prevent leaching because of their ionic nature. On account of scarcity of water‐stable cationic MOFs, the capture of oxoanions of selenium and arsenic has been a major challenge and has not been investigated using iMOFs. Herein, we demonstrate large scale synthesis of cationic MOF, viz. iMOF‐1C that exhibits selective capture of oxoanions of Se^VI (SeO₄^2?) and As^V (HAsO₄^2?) in water with a maximum sorption capacity of 100 and 85 mg g^?1, respectively. This represents among the highest uptake capacities observed for selenate oxoanion in MOFs. Further, the ion‐exchange mechanism was directly unveiled by single crystal analysis, which revealed variable modes of host–guest binding.     

Separation of Acetylene from Carbon Dioxide and Ethylene by a Water‐Stable Microporous Metal–Organic Framework with Aligned Imidazolium Groups inside the Channels

Separation of acetylene from carbon dioxide and ethylene is challenging in view of their similar sizes and physical properties. Metal–organic frameworks (MOFs) in general are strong candidates for these separations owing to the presence of functional pore surfaces that can selectively capture a specific target molecule. Here, we report a novel 3D microporous cationic framework named JCM‐1 . This structure possesses imidazolium functional groups on the pore surfaces and pyrazolate as a metal binding group, which is well known to form strong metal‐to‐ligand bonds. The selective sorption of acetylene over carbon dioxide and ethylene in JCM‐1 was successfully demonstrated by equilibrium gas adsorption analysis as well as dynamic breakthrough measurement. Furthermore, its excellent hydrolytic stability makes the separation processes highly recyclable without a substantial loss in acetylene uptake capacity.     

Recent Progress of Atmospheric Water Harvesting Using Metal-Organic Frameworks

Atmospheric water harvesting based on vapor adsorption is a newly emerged and potential technology to supply portable water for arid areas.To efficiently harvest vapor from the air,sorbents are required to have conside-rable adsorption capacity,easy regeneration and high stability.With the advantages of porous structure,tunable pore size and tailorable hydrophilicity,metal-organic frameworks(MOFs)have demonstrated excellent performance in vapor adsorption and water generation.In this review,we first discuss the degradation mechanisms of MOFs exposed to water and summarize the structure-stability relationship;by centering on the adsorption isotherms,the connection between the structure of MOFs and the water adsorption property is illuminated;finally,some prospects are suggested in order to push forward the progress of this technology.     

金属有机框架材料对Cr(Ⅵ)离子的吸附去除研究进展

重金属离子污染问题一直备受关注。开发利用多孔材料吸附去除水中重金属离子一直是材料、环境等相关学科领域的研究热点之一。金属有机框架材料(metal?organic frameworks,MOFs)是一类新型的多孔材料,具有结构多样、比表面积大、孔径可调、孔表面特征易设计调控等特点,在气体分离、催化、传感等领域表现出极大的应用潜力。近年来,高稳定MOF材料的构筑取得了许多重大突破,大量研究工作探索了这类材料在水中的应用,包括水中重金属离子的吸附去除。Cr(Ⅵ)离子是一类毒性大、分布广的重金属离子,不同条件下存在形态多样,其吸附去除研究具有理论和实际意义。本文主要综述了近年来利用MOF材料吸附去除水中Cr(Ⅵ)离子的研究工作,并将这些材料归属为:(1)高稳定的锆基MOF、(2)阳离子框架型MOF、(3)后修饰的MOF及(4)MOF基复合材料4类;也对这些材料的Cr(Ⅵ)离子吸附机理、吸附量、材料再生性等进行了概括;最后分析了MOF材料在重金属离子吸附去除实际应用上存在的问题并展望了今后的重点研究方向。     

SERS-Active MIL-100(Fe) Sensory Array for Ultrasensitive and Multiplex Detection of VOCs

The application of metal–organic frameworks (MOFs) as SERS-active platforms in multiplex volatile organic compounds (VOCs) detection is still unexplored. Herein, we demonstrate that MIL-100 (Fe) serves as an ideal SERS substrate for the detection of VOCs. The limit of detection (LOD) of MIL-100(Fe) for toluene sensing can reach 2.5 ppm, and can be even further decreased to 0.48 ppb level when “hot spots” in between Au nanoparticles are employed onto MIL-100 (Fe) substrate, resulting in an enhancement factor of 10¹⁰. Additionally, we show that MIL-100(Fe) substrate has a unique “sensor array” property allowing multiplex VOCs detection, with great modifiability and expandability by doping with foreign metal elements. Finally, the MIL-100(Fe) platform is utilized to simultaneously detect the different gaseous indicators of lung cancer with a ppm detection limit, demonstrating its high potential for early diagnosis of lung cancer in vivo.