High-stability, zirconium-based metal−organic frameworks are attractive as heterogeneous catalysts and as model supports for uniform arrays of subsequently constructed heterogeneous catalystsfor example, MOF-node-grafted metal−oxy and metal− sulfur clusters. For hexa-Zr(IV)-MOFs characterized by nodes that are less than 12-connected, sites not used for linkers are ideally occupied by reactive and displaceable OH/H 2 O pairs. The desired pairs are ideal for grafting the aforementioned catalytic clusters, while aqua-ligand lability renders them effective for exposing highly Lewis-acidic Zr(IV) sites (catalytic sites) to candidate reactants. New single-crystal X-ray studies of an eight-connected Zr-MOF, NU-1000, reveal that conventional activation fully removes modulator ligands, but replaces them with three node-blocking formate ligands (from solvent decomposition) and only one OH/H 2 O pair, not foura largely overlooked complication that now appears to be general for Zr-MOFs. Here we describe an alternative activation protocol that effectively removes modulators, avoids formate, and installs the full complement of terminal OH/H 2 O pairs. It does so via an unusual isolatable intermediate featuring eight aqua ligands and four non-ligated chloridesagain as supported by single-crystal X-ray data. We find that complete replacement of node-blocking modulators/formate with the originally envisioned OH/OH 2 pairs has striking consequences; here we touch upon just three. First, elimination of unrecognized formate renders aqua ligands much more thermally labile, enabling open Zr(IV) sites to be obtained at lower temperature. Second, in the absence of formate, which otherwise links and locks pairs of node Zr(IV) ions, reversible removal of aqua ligands engenders reversible contraction of MOF meso-and micropores, as evidenced by X-ray diffraction. Third, formate replacement with OH/OH 2 pairs renders NU-1000 ca.10× more active for catalytic hydrolytic degradation of a representative simulant of G-type chemical warfare agents.
Membrane-based separation is an emerging technology to separate different components. [1] Owing to not only the functions of separation, concentration, and purification, but also the properties of energy efficiency, environmental friendliness, high efficiency, simplicity, manufacturing scalability, small footprint, and ease of operation and control, this technology has been widely used in chemical industry, environmental protection, medicine, food, and desalination. [2,3] Membrane is regarded as a significant method to solve the far-reaching issues such as water shortages, environmental pollution, and energy crisis.Along with the rapid industrialization and sharp increase in population, the problems of water shortages, environmental pollution, and energy crisis become more and more severe and complex, leading to the higher technology requirement for membrane separation. Although the frequently used traditional polymer membrane featured with low cost, good mechanical strength, excellent flexibility, and ease of processing, the two key parameters of permeability and selectivity that are generally used to evaluate membrane separation performance are interinhibitive because of the well-known trade-off effect. [4] One of the main reasons for the trade-off effect is the wide distribution of pore size in polymeric membranes. Therefore, materials with uniform and well-ordered pores are considered as perfect candidates for fabricating highperformance separation membranes. Keeping this in perspective, many meaningful studies have been conducted for constructing homoporous membranes [5] with traditional polymers as starting materials. The obtained membranes have a pore size ranging from 5 to 50 nm and possess great potential for ultrafiltration (UF). [6] However, in some practical application systems such as gas separation, dyes removal, protein and drug recovery as well as desalination, the kinetic dimensions of components to be separated usually are less than 5 nm. Therefore, it is very important that a homoporous structure with a pore size of less than 5 nm is constructed to be used in membrane separation.Covalent organic frameworks (COFs) [7][8][9][10][11] are an emerging class of organic porous crystalline materials with pores that range from 0.5 to 5 nm, [12,13] which are entirely composed by light elements, such as C, H, O, N, B, and Si and are connected by strong covalent bonds. Owing to the unique nature of their well-ordered and tailorable pore channels, high and permanent porosity, excellent thermostability and chemical stability, and ease of functionalization, COF materials have been demonstrated promising potential in many applications, including separation, [14] catalysis, [15] sensor, [16] optoelectronics, [17] energy conversion, [18] and semiconductors, [19] among others. Particularly, these characteristics of COF materials perfectly meet the requirements for making advanced separation membrane. For example, the uniform aperture, high porosity, and excellent stability are beneficial
Crystalline metal−organic frameworks (MOFs) are promising synthetic analogues of photosynthetic light-harvesting complexes (LHCs). The precise assembly of linkers (organic chromophores) around the topology-defined pores offers the evolution of unique photophysical behaviors that are reminiscence of LHCs. These include MOF excited states with photoabsorbed energy that is spatially dispersed over multiple linkers defining the molecular excitons. The multilinker molecular excitons display super-radiancea hallmark of coupled oscillators seen in LHCswith radiative rate constant (k rad ) exceeding that of a single linker. Our theoretical model and experimental results on three zirconium MOFs, namely, PCN-222(Zn), NU-1000, and SIU-100, with similar topology but varying linkers suggest that the size of such molecular excitons depends on the electronic symmetry of the linker. This multilinker exciton model effectively predicts the energy transfer rate constant; corresponding single-step exciton hopping time, ranging from a few picoseconds in SIU-100 and NU-1000 to a few hundreds of picoseconds in PCN-222(Zn), matches well with the experimental data. The model also predicts the anisotropy of exciton displacement with preferential migration along the crystallographic c-axis. Overall, these findings establish various missing links defining the exciton size and dynamics in MOF-assembled linkers. The understandings will provide design principles, especially, positioning the catalysts or electrode relative to the linker orientation for low-density solar energy conversion systems.
Effective permeation into, and diffusive mass transport within, solvent-filled metal–organic frameworks (MOFs) is critical in applications such as MOF-based chemical catalysis of condensed-phase reactions. In this work, we studied the entry from solution of a luminescent probe molecule, 1,3,5,7-tetramethyl-4,4-difluoroboradiazaindacene (BODIPY), into the 1D channel-type, zirconium-based MOF NU-1008 and subsequent transport of the probe through the MOF. Measurements were accomplished via in situ confocal fluorescence microscopy of individual crystallites, where the evolution of the fluorescence response from the crystallite was followed as functions of both time and location within the crystallite. From the confocal data, intracrystalline transport of BODIPY is well-described by one-dimensional diffusion along the channel direction. Varying the chemical identity of the solvent revealed an inverse dependence of probe-molecule diffusivity on bulk-solvent viscosity, qualitatively consistent with expectations from the Stokes–Einstein equation for molecular diffusion. At a more quantitative level, however, measured diffusion coefficients are about 100-fold smaller than expected from Stokes–Einstein, pointing to substantial channel-confinement effects. Evaluation of the confocal data also reveals a non-negligible mass transport resistance, i.e., surface barrier, associated with the probe molecule leaving the solution and permeating the exterior surface of the MOF. Permeation by the probe entails displacement of solvent from the MOF channels. The magnitude of the resistance increases with the size of the solvent molecule. This work draws attention to the importance of MOF structure, external-surface barriers, and solvent molecule identity to the overall transport process in MOFs, which should assist in understanding the performance of MOFs in applications such as condensed-phase heterogeneous catalysis.
Guest transport through metal−organic frameworks (MOFs) is a critical process in the application of MOFs for catalysis. Understanding the interplay between transport behavior and a MOF's structure is of fundamental importance to further tailor MOFs for optimal catalysis. Here, we investigated dye transport processes through two solvent-filled Zr-MOFs, NU-600 and NU-1008, which are compositionally the same but display different topologies, i.e., she and csq, respectively. Dye transport through individual MOF crystallites was monitored spatially and temporally by confocal fluorescence microscopy. In both MOF crystals, dye molecules permeated the external-surface barrier first, then diffused along channels. Transport in NU-600 is three dimensional due to orthogonal channels, while diffusion in NU-1008 is primarily one dimensional owing to parallelly aligned channels. Quantitatively, the diffusivity of dye molecules in NU-600 is smaller than in NU-1008, which is attributed to the narrower channels and tortuous pore network of NU-600. However, comparing crystals of the same volume, macroscopic uptake of dye in NU-600 is significantly more efficient than in NU-1008, highlighting that the she-net NU-600, which features intersecting channels, affords efficient pathways for substrate transport. Additionally, for NU-600 and NU-1008, the nanoscale topologies of the compounds qualitatively govern the resulting macroscopic crystallite morphologies, including aspect ratios. The morphology difference is crucial to conferring a transport efficiency advantage on NU-600. Atomistic simulations of solvated dye diffusion in the two MOFs indicate energetically favorable interaction between the linkers and dye. Molecular dynamics trajectories reveal that the dye molecule spends more time on the linkers in NU-600 relative to NU-1008, which supports the smaller diffusivity in NU-600 measured by experiments. In this work, we combined experiments and simulations to demonstrate the interplay between MOF structure and guest transport behavior both microscopically and macroscopically, which provides insights for selecting or designing MOF topologies to enhance guest transport through MOFs intended, for example, for chemical catalysis.
Ammonia capture by porous materials is relevant to protection of humans from chemical threats, while ammonia separation may be relevant to its isolation and use following generation by emerging electrochemical schemes. Our previous work described both reversible and irreversible interactions of ammonia with the metal–organic framework (MOF) material, NU-1000, following thermal treatment at either 120 or 300 °C. In the present work, we have examined NU-1000-Cl, a variant that features a modified node structure–at ambient temperature, Zr6(μ3-O)4(μ3-OH)4(H2O)8 12+ in place of Zr6(μ3-O)4(μ3-OH)4(OH)4(H2O)4 8+. Carboxylate termini from each of eight linkers balance the 8+ charge of the parent node, while four chloride ions, attached only by hydrogen bonding, complete the charge balance for the 12+ version. We find that both reversible and irreversible uptake of ammonia are enhanced for NU-1000-Cl, relative to the chloride-free version. Two irreversible interactions were observed via in situ diffuse-reflectance infrared Fourier-transform spectroscopy: coordination of NH3 at open Zr sites generated during thermal pretreatment and formation of NH4 + by proton transfer from node aqua ligands. The irreversibility of the latter appears to be facilitated by the presence chloride ions, as NH4 + formation occurs reversibly with chloride-free NU-1000. At room temperature, chemically reversible (and irreversible) interactions between ammonia and NU-1000-Cl result in separation of NH3 from N2 when gas mixtures are examined with breakthrough instrumentation, as evinced by a much longer breakthrough time (∼9 min) for NH3.
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