RESUMO
Urea oxidation reaction (UOR) is one of the promising alternative anodic reactions to water oxidation that has attracted extensive attention in green hydrogen production. The application of specifically designed electrocatalysts capable of declining energy consumption and environmental consequences is one of the major challenges in this field. Therefore, the goal is to achieve a resistant, low-cost, and environmentally friendly electrocatalyst. Herein, a water-stable fluorinated Cu(II) metalorganic framework (MOF) {[Cu2 (L)(H2 O)2 ]·(5DMF)(4H2 O)}n (Cu-FMOF-NH2 ; H4 L = 3,5-bis(2,4-dicarboxylic acid)-4-(trifluoromethyl)aniline) is developed utilizing an angular tetracarboxylic acid ligand that incorporates both trifluoromethyl (-CF3 ) and amine (-NH2 ) groups. The tailored structure of Cu-FMOF-NH2 where linkers are connected by fluoride bridges and surrounded by dicopper nodes reveals a 4,24T1 topology. When employed as electrocatalyst, Cu-FMOF-NH2 requires only 1.31 V versus reversible hydrogen electrode (RHE) to deliver 10 mA cm-2 current density in 1.0 m KOH with 0.33 m urea electrolyte and delivered an even higher current density (50 mA cm-2 ) at 1.47 V versus RHE. This performance is superior to several reported catalysts including commercial RuO2 catalyst with overpotential of 1.52 V versus RHE. This investigation opens new opportunities to develop and utilize pristine MOFs as a potential electrocatalyst for various catalytic reactions.
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Metal-Organic Frameworks (MOFs) with open metal sites (OMS) interact strongly with a range of polar gases/vapors. However, under ambient conditions, their selective adsorption is generally impaired due to a high OMS affinity to water. This led previously to the privilege selection of hydrophobic MOFs for the selective capture/detection of volatile organic compounds (VOCs). Herein, we show that this paradigm is challenged by metal(III) polycarboxylates MOFs, bearing a high concentration of OMS, as MIL-100(Fe), enabling the selective capture of polar VOCs even in the presence of water. With experimental and computational tools, including single-component gravimetric and dynamic mixture adsorption measurements, in situ infrared (IR) spectroscopy and Density Functional Theory calculations we reveal that this adsorption mechanism involves a direct coordination of the VOC on the OMS, associated with an interaction energy that exceeds that of water. Hence, MOFs with OMS are demonstrated to be of interest for air purification purposes.
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Porous materials with d3 electronic configuration open metal sites have been proved to be effective adsorbents for N2 capture and N2 /O2 separation. However, the reported materials remain challenging to address the trade-off between adsorption capacity and selectivity. Herein, we report a robust MOF, MIL-102Cr, that features two binding sites, can synergistically afford strong interactions for N2 capture. The synergistic adsorption site exhibits a benchmark Qst of 45.0â kJ mol-1 for N2 among the Cr-based MOFs, a record-high volumetric N2 uptake (31.38â cm3 cm-3 ), and highest N2 /O2 selectivity (13.11) at 298â K and 1.0â bar. Breakthrough experiments reveal that MIL-102Cr can efficiently capture N2 from a 79/21â N2 /O2 mixture, providing a record 99.99 % pure O2 productivity of 0.75â mmol g-1 . In situ infrared spectroscopy and computational modelling studies revealed that a synergistic adsorption effect by open Cr(III) and fluorine sites was accountable for the strong interactions between the MOF and N2 .
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The exploration of efficient electrocatalysts is the central issue for boosting the overall efficiency of water splitting. Herein, pertinently creating active sites and improving conductivity for metal-organic frameworks (MOFs) is proposed to tailor electrocatalytic properties for overall water splitting. An Ni(II)-MOF nanosheet array is presented as an ideal material model and a facile alkali-etched strategy is developed to break its NiO bonds accompanied with the introduction of extra-framework K cations, which contribute to creating highly active open metal sites and largely improving the electrical conductivity. As a result, the assembled defect-Ni-MOF||defect-Ni-MOF electrolyte cell delivers a lower and stable voltage of 1.50 V at 10 mA cm-2 in alkaline medium for overall water splitting, comparable to the combination of iridium and platinum as benchmark catalysts.
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Four isostructural CPO-54-M metal-organic frameworks based on the larger organic linker 1,5-dihydroxynaphthalene-2,6-dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO-27 (MOF-74) materials. Desolvated CPO-54-Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N2 , H2 , CO2 and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO-27. The plateau of high heat of adsorption decreases earlier in CPO-54-Mn as a function of loading per mole than in CPO-27-Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.
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Separation of ethane from ethylene is a very important but challenging process in the petrochemical industry. Finding an alternative method would reduce the energy needed to make 170 million tons of ethylene manufactured worldwide each year. Adsorptive separation using C2H6-selective porous materials to directly produce high-purity C2H4 is more energy-efficient. We herein report the "reversed C2H6/C2H4 adsorption" in a metal-organic framework Cr-BTC via the introduction of oxygen on its open metal sites. The oxidized Cr-BTC(O2) can bind C2H6 over C2H4 through the active Cr-superoxo sites, which was elucidated by the gas sorption isotherms and density functional theory calculations. This material thus exhibits a good performance for the separation of 50/50 C2H6/C2H4 mixtures to produce 99.99% pure C2H4 in a single separation operation.
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Although numerous porous adsorbents have been investigated for NH3 capture applications, these materials often exhibit insufficient NH3 uptake, low NH3 affinity at the ppm level, and poor chemical stability against wet NH3 conditions. The NH3 capture properties of M2 (dobpdc) complexes (M=Mg2+ , Mn2+ , Co2+ , Ni2+ , and Zn2+ ; dobpdc4- =4,4-dioxidobiphenyl-3,3-dicarboxylate) that contain open metal sites is presented. The NH3 uptake of Mg2 (dobpdc) at 298â K was 23.9â mmol g-1 at 1â bar and 8.25â mmol g-1 at 570â ppm, which are record high capacities at both pressures among existing porous adsorbents. The structural stability of Mg2 (dobpdc) upon exposure to wet NH3 was superior to that of the other M2 (dobpdc) and the frameworks tested. Overall, these results demonstrate that Mg2 (dobpdc) is a recyclable compound that exhibits significant NH3 affinity and capacity, making it a promising candidate for real-world NH3 -capture applications.
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Introduction of pore partition agents into hexagonal channels of MIL-88 type (acs topology) endows materials with high tunability in gas sorption. Here, we report a strategy to partition acs framework into pacs (partitioned acs) crystalline porous materials (CPM). This strategy is based on insertion of inâ situ synthesized 4,4'-dipyridylsulfide (dps) ligands. One third of open metal sites in the acs net are retained in pacs MOFs; two thirds are used for pore-space partition. The Co2 V-pacs MOFs exhibit near or at record high uptake capacities for C2 H2 , C2 H4 , C2 H6 , and CO2 among MOFs. The storage capacity of C2 H2 is 234â cm3 g-1 (298â K) and 330â cm3 g-1 (273â K) at 1â atm for CPM-733-dps (the Co2 V-BDC form, BDC=1,4-benzenedicarboxylate). These high uptake capacities are accomplished with low heat of adsorption, a feature desirable for low-energy-cost adsorbent regeneration. CPM-733-dps is stable and shows no loss of C2 H2 adsorption capacity following multiple adsorption-desorption cycles.
RESUMO
Acetylene (C2 H2 ) removal from ethylene (C2 H4 ) is a crucial step in the production of polymer-grade C2 H4 but remains a daunting challenge because of the similar physicochemical properties of C2 H2 and C2 H4 . Currently energy-intensive cryogenic distillation processes are used to separate the two gases industrially. A robust ultramicroporous metal-organic framework (MOF), Ni3 (pzdc)2 (7 Hade)2 , is reported for efficient C2 H2 /C2 H4 separation. The MOF comprises hydrogen-bonded linked one-dimensional (1D) chains, and features high-density open metal sites (2.7â nm-3 ) and electronegative oxygen and nitrogen sites arranged on the pore surface as cooperative binding sites. Theoretical calculations, inâ situ powder X-ray diffraction and Fourier-transform infrared spectroscopy revealed a synergistic adsorption mechanism. The MOF possesses S-shaped 1D pore channels that efficiently trap trace C2 H2 at 0.01â bar with a high C2 H2 uptake of 60.6â cm3 cm-3 and C2 H2 /C2 H4 selectivity.
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Materials that combine flexibility and open metal sites are crucial for myriad applications. In this article, we report a 2D coordination polymer (CP) assembled from CuII ions and a flexible meta-carborane-based linker [Cu2(L1)2(Solv)2]â¢xSolv (1-DMA, 1-DMF, and 1-MeOH; L1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane). 1-DMF undergoes an unusual example of reversible phase transition on solvent treatment (i.e., MeOH and CH2Cl2). Solvent exchange, followed by thermal activation provided a new porous phase that exhibits an estimated Brunauer-Emmett-Teller (BET) surface area of 301 m2 g-1 and is capable of a CO2 uptake of 41 cm3 g-1. The transformation is reversible and 1-DMF is reformed on addition of DMF to the porous phase. We provide evidence for the reversible process being the result of the formation/cleavage of weak but attractive B-HâââCu interactions by a combination of single-crystal (SCXRD), powder (PXRD) X-ray diffraction, Raman spectroscopy, and DFT calculations.
Assuntos
Boranos/química , Polímeros/química , Estruturas Metalorgânicas , PorosidadeRESUMO
Actinide based metal-organic frameworks (MOFs) are unique not only because compared to the transition-metal and lanthanide systems they are substantially less explored, but also owing to the uniqueness of actinide ions in bonding and coordination. Now a 3D thorium-organic framework (SCU-11) contains a series of cages with an effective size of ca. 21×24â Å. Th4+ in SCU-11 is 10-coordinate with a bicapped square prism coordination geometry, which has never been documented for any metal cation complexes. The bicapped position is occupied by two coordinated water molecules that can be removed to afford a very unique open Th4+ site, confirmed by X-ray diffraction, color change, thermogravimetry, and spectroscopy. The degassed phase (SCU-11-A) exhibits a Brunauer-Emmett-Teller surface area of 1272â m2 g-1 , one of the highest values among reported actinide materials, enabling it to sufficiently retain water vapor, Kr, and Xe with uptake capacities of 234â cm3 g-1 , 0.77â mmol g-1 , 3.17â mmol g-1 , respectively, and a Xe/Kr selectivity of 5.7.
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A 3D porous perchlorinated metal-organic framework (MOF), LIFM-26, featuring dual functionality, that is, functional polar groups and open metal sites, has been synthesized using perchlorinated linear dicarboxylate to link trigonal prismatic Fe3 (µ3 -O) units. LIFM-26 exhibits good thermal and chemical stability, and possesses high porosity with a BET surface area of 1513â m2 g-1 , compared with isoreticular MOF-235 and Fe3 O(F4 BDC)3 (H2 O)3 (F4 BDC=2,3,5,6-tetrafluorobenzene-1,4-dicarboxylate). Most strikingly, LIFM-26 features good gas sorption/separation performance at 298â K and 1â atm with IAST selectivity values reaching up to 36, 93, 23, 11, 46, and 202 for CO2 /CH4 , CO2 /N2 , C2 H4 /CH4 , C2 H6 /CH4 , C3 H8 /CH4 , and R22/N2 (R22=CHClF2 ), respectively, showing potential for use in biogas/natural gas purification and CO2 /R22 capture.
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For the design of adsorptive-separation units, knowledge is required of the multicomponent adsorption behavior. Ideal adsorbed solution theory (IAST) breaks down for olefin adsorption in open-metal site (OMS) materials due to non-ideal donor-acceptor interactions. Using a density-function-theory-based energy decomposition scheme, we develop a physically justifiable classical force field that incorporates the missing orbital interactions using an appropriate functional form. Our first-principles derived force field shows greatly improved quantitative agreement with the inflection points, initial uptake, saturation capacity, and enthalpies of adsorption obtained from our in-house adsorption experiments. While IAST fails to make accurate predictions, our improved force field model is able to correctly predict the multicomponent behavior. Our approach is also transferable to other OMS structures, allowing the accurate study of their separation performances for olefins/paraffins and further mixtures involving complex donor-acceptor interactions.
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Metal-organic frameworks (MOFs) are shown to be good examples of a new class of crystalline porous materials for guest encapsulation. Since the encapsulation/release of guest molecules in MOF hosts is a reversible process in nature, how to prevent the leaching of guests from the open pores with minimal and nondestructive modifications of the structure is a critical issue. To address this issue, we herein propose a novel strategy of encapsulating guests by introducing size-matching organic ligands as bolts to lock the pores of the MOFs through deliberately anchoring onto the open metal sites in the pores. Our proposed strategy provides a mechanical way to prevent the leaching of guests and thereby has less dependence on the specific chemical environment of the hosts, thus making it applicable for a wide variety of existing MOFs once the size-matching ligands are employed.
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Carbon capture and desulfurization of flue gases are crucial for the achievement of carbon neutrality and sustainable development. In this work, the "one-step" adsorption technology with high-performance metal-organic frameworks (MOFs) was proposed to simultaneously capture the SO2 and CO2. Four machine learning algorithms were used to predict the performance indicators (NCO2+SO2, SCO2+SO2/N2, and TSN) of MOFs, with Multi-Layer Perceptron Regression (MLPR) showing better performance (R2 = 0.93). To address sparse data of MOF chemical descriptors, we introduced the Deep Factorization Machines (DeepFM) model, outperforming MLPR with a higher R2 of 0.95. Then, sensitivity analysis was employed to find that the adsorption heat and porosity were the key factors for SO2 and CO2 capture performance of MOF, while the influence of open alkali metal sites also stood out. Furthermore, we established a kinetic model to batch simulate the breakthrough curves of TOP 1000 MOFs to investigate their dynamic adsorption separation performance for SO2/CO2/N2. The TOP 20 MOFs screened by the dynamic performance highly overlap with those screened by the static performance, with 76 % containing open alkali metal sites. This integrated approach of computational screening, machine learning, and dynamic analysis significantly advances the development of efficient MOF adsorbents for flue gas treatment.
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Separating CH4 from coal bed methane is of great importance but challenging. Adsorption-based separation often suffers from low selectivity, poor stability, and difficulty to scale up. Herein, a stable and scalable metal-organic framework [MOF, CoNi(pyz-NH2)] with multiple CH4 binding sites was reported to efficiently separate the CH4/N2 mixture. Due to its suitable pore size and multiple CH4 binding sites, it exhibits excellent CH4/N2 selectivity (16.5) and CH4 uptake (35.9 cm3/g) at 273 K and 1 bar, which is comparable to that of the state-of-the-art MOFs. Theoretical calculations reveal that the high density of open metal sites and polar functional groups in the pores provide strong affinity to CH4 than to N2. Moreover, CoNi(pyz-NH2) displays excellent structural stability and can be scale-up synthesized (22.7 g). This work not only provides an excellent adsorbent but also provides important inspiration for the future design and preparation of porous adsorbents for separations.
RESUMO
Pore topology and chemistry play crucial roles in the adsorption characteristics of metal-organic frameworks (MOFs). To deepen our understanding of the interactions between MOFs and CO2 during this process, we systematically investigate the adsorption properties of a group of pyrene-based MOFs. These MOFs feature Zn(II) as the metal ion and employ a pyrene-based ligand, specifically 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy). Including different additional ligands leads to frameworks with distinctive structural and chemical features. By comparing these structures, we could isolate the role that pore size, the presence of open-metal sites (OMS), metal-oxygen bridges, and framework charges play in the CO2 adsorption of these MOFs. Frameworks with constricted pore structures display a phenomenon known as the confinement effect, fostering stronger MOF-CO2 interactions and higher uptakes at low pressures. In contrast, entropic effects dominate at elevated pressures, and the MOF's pore volume becomes the driving factor. Through analysis of the CO2 uptakes of the benchmark materials âsome with narrower pores and others with larger pore volumesâit becomes evident that structures with narrower pores and high binding energies excel at low pressures. In contrast, those with larger volumes perform better at elevated pressures. Moreover, this research highlights that open-metal sites and inherent charges within the frameworks of ionic MOFs stand out as CO2-philic characteristics.
RESUMO
Efficient separation and purification of xenon (Xe) from krypton (Kr) represent an industrially crucial but challenging process. While the adsorption-based separation of these atomic gases represents an energy-efficient process, achieving highly selective adsorbents remains a difficult task. Here, we demonstrate a supramolecular assembly of coordination polymers, termed as M(II)-dhbq (M = Mg, Mn, Co, and Zn; dhbq = 2,5-dihydroxy-1,4-benzoquinone), with high-density open metal sites (5.3 nm-3) and optimal pore size (5.5 Å), which are able to selectively capture Xe among other chemically inert gases including Kr, Ar, N2, and O2. Among M(II)-dhbq materials, Mn-dhbq exhibits the highest Xe uptake capacity of 3.1 mmol/g and a Xe/Kr selectivity of 11.2 at 298 K and 1.0 bar, outperforming many state-of-the-art adsorbents reported so far. Remarkably, the adsorption selectivity of Mn-dhbq for Xe/O2, Xe/N2, and Xe/Ar at ambient conditions reaches as high as 70.0, 139.3, and 64.0, respectively. Direct breakthrough experiments further confirm that all M(II)-dhbq materials can efficiently discriminate Xe atoms from other inert gases. It is revealed from the density functional theory calculations that the strong affinity between Xe and the coordination polymer is mainly attributed to the polarization by open metal sites.
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The increasing recognition of the impact of CO2 emissions as a global concern, directly linked to the rise in global temperature, has raised significant attention. Carbon capture and storage, particularly in association with adsorbents, has occurred as a pivotal approach to address this pressing issue. Large surface area, high porosity, and abundant adsorption sites make metal-organic frameworks (MOFs) promising contenders for CO2 uptake. This review commences by discussing recent advancements in MOFs with diverse adsorption sites, encompassing open metal sites and Lewis basic centers. Next, diverse strategies aimed at enhancing CO2 adsorption capabilities are presented, including pore size manipulation, post-synthetic modifications, and composite formation. Finally, the extant challenges and anticipated prospects pertaining to the development of MOF-based nanomaterials for CO2 storage are described.
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In this study, we utilized an ultramicroporous metal-organic framework (MOF) named [Ni3(pzdc)2(ade)2(H2O)4]·2.18H2O (where H3pzdc represents pyrazole-3,5-dicarboxylic acid and ade represents adenine) for hydrogen (H2) adsorption. Upon activation, [Ni3(pzdc)2(ade)2] was obtained, and in situ carbon monoxide loading by transmission infrared spectroscopy revealed the generation of open Ni(II) sites. The MOF displayed a Brunauer-Emmett-Teller (BET) surface area of 160 m2/g and a pore size of 0.67 nm. Hydrogen adsorption measurements conducted on this MOF at 77 K showed a steep increase in uptake (up to 1.93 mmol/g at 0.04 bar) at low pressure, reaching a H2 uptake saturation at 2.11 mmol/g at â¼0.15 bar. The affinity of this MOF for H2 was determined to be 9.7 ± 1.0 kJ/mol. In situ H2 loading experiments supported by molecular simulations confirmed that H2 does not bind to the open Ni(II) sites of [Ni3(pzdc)2(ade)2], and the high affinity of the MOF for H2 is attributed to the interplay of pore size, shape, and functionality.