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Hydrocarbon separations using porous materials such as metal-organic frameworks (MOFs) have been proposed to reduce the energy demands associated with current distillation-based methods. Despite the potential of these materials to distinguish hydrocarbons through thermodynamic or kinetic mechanisms, experimental data quantifying hydrocarbon transport in MOFs is lacking. Such mass transfer measurements are vital to elucidate structure-property relationships and design future high-performing separation materials. In this work, we aim to isolate the influence of pore size on hydrocarbon diffusion by studying a pair of isoreticular MOFs, Co

Isothermal titration calorimetry (ITC) is a technique which directly measures the thermodynamic parameters of binding events. Although historically it has been used to investigate interactions in biological macromolecules and the kinetics of enzyme-catalyzed reactions, ITC has also been demonstrated to provide relevant thermodynamic information about interactions in synthetic systems, such as those in metal-organic frameworks (MOFs). MOFs are a family of crystalline porous materials that have been widely studied as supports for molecules ranging from gases to biomolecules through physisorption and chemisorption. Herein, we offer a perspective on the current applications of ITC in MOFs, including the mechanism of small molecule adsorption and the formation of MOF-based composite materials through noncovalent interactions. Experimental considerations specific to running ITC experiments in MOF systems are reviewed on the basis of existing reports. We conclude by discussing underexplored, but promising, MOF-related research directions which could be elucidated by ITC.

Metal-organic frameworks (MOFs) have been reported to effectively detoxify chemical warfare agents (CWAs) and their simulants. Early studies of Zr-MOF-catalyzed hydrolytic degradation of an organophospho-ester type CWA, Sarin, and its simulant, dimethyl-4-nitrophenylphosphate (DMNP), suggested that the activity originates from ZrIV-OH-ZrIV moieties that resemble the structure of the active-site phosphotriesterase enzyme. Measurements of pKa values for hexa-zirconium-node-sited bridging hydroxo, terminal hydroxo, and aqua ligands reveal essentially complete conversion of mu-hydroxo ligands to substitution-inert mu-oxo ligands at alkaline pHs, ruling out a primary role for ZrIV-OH-ZrIV moieties, despite the resemblance to the phosphotriesterase enzyme active-site. The measurements also rule out a secondary role as a hydrogen bond donor/stabilizer of bound DMNP. Additionally, the measurements show that reactant-displaceable node-aqua ligands become increasingly scarce in increasingly alkaline environments. Here, rates of catalyzed hydrolysis reaction were examined experimentally to ascertain reaction orders and, in turn, interrogate the mechanism. In striking contrast to the ubiquitous Zr-MOF, UiO-66, for which simulant displacement of water as a node ligand is rate-determining, and the rate of catalysis (in alkaline environments) increases with decreasing pH, catalysis by NU-1000 is rate-limited by solution hydroxide attack of the nitrophenoxide-phosphorous bond of the MOF-activated simulant, with the overall hy-drolysis rate being remarkably insensitive to pH (or pOH). Underlying the unusually weak sensitivity of rates of hydroly-sis to pH is almost exact counter-balancing of the prevalence of catalyst active-sites (Zr-OH2 sites) with activity of the re-action nucleophile, free hydroxide ion, as a function of pH. We ascribe the residual pH dependence of the overall rate to a charge-based modulation of the equilibrium constant for simulant binding to the node. In contrast to overall hydrolysis rates, the catalyst turnover frequencies per Zr-OH2 active-site are strongly pH dependent, exceeding 30,000 s-1 at pH 10.5.

We report the synthesis of novel charge transfer complexes consisting of TTF or TTN, and DPNI. A spectroscopic and computational approach is taken to elucidate charge transfer in these complexes.

We have screened an array of 23 metals deposited onto the metal–organic framework (MOF) NU-1000 for propyne dimerization to hexadienes under different reaction conditions for a total of ~1400 experiments. By a first-of-its-kind study utilizing data-driven algorithms and high-throughput experimentation (HTE) in MOF catalysis, yields on Cu-deposited NU-1000 were improved from 4.2% to 24.4%. Characterization of the most-performant catalysts reveal conversion to hexadiene to be due to the formation of large Cu nanoparticles, which is further supported by reaction mechanisms calculated with density functional theory (DFT). Our results demonstrate both the strengths and weaknesses of the HTE approach. As a strength, HTE excels at being able to find interesting and novel catalytic activity; any a priori theoretical approach would be hard-pressed to find success, as high-performing catalysts required highly specific operating conditions difficult to model theoretically, and initial naïve single-atom models of the active site did not prove representative of the nanoparticle catalysts responsible for conversion to hexadiene. As a weakness, our results show how the HTE approach must be designed and monitored carefully to find success; in our initial campaign (~six months and over half of the total experiments conducted) only minor catalytic performances (up to 4.2% yield) were achieved, which was only improved following a complete overhaul of our HTE approach and questioning our initial assumptions. Thus, the HTE approach is much less automated than it may seem, even if driven by machine learning algorithms – one must carefully design their HTE campaign to find success.

Reticular frameworks are crystalline porous materials that form via the self-assembly of molecular building blocks (i.e., nodes and linkers) in different topologies. Many of them have high internal surface areas and other desirable properties for gas storage, separation, and other applications. The notable variety of the possible building blocks and the diverse ways they can be assembled endow reticular frameworks with a near-infinite combinatorial design space, making reticular chemistry both promising and challenging for prospective materials design. Here, we propose an automated nanoporous materials discovery platform powered by a supramolecular variational autoencoder (SmVAE) for the generative design of reticular materials with desired functions. We demonstrate the automated design process with a class of metal-organic framework (MOF) structures and the goal of separating CO2 from natural gas or flue gas. Our model exhibits high fidelity in capturing structural features and reconstructing MOF structures. We show that the autoencoder has a promising optimization capability when jointly trained with multiple top adsorbent candidates identified for superior gas separation. MOFs discovered here are strongly competitive against some of the best-performing MOFs/zeolites ever reported. This platform lays the groundwork for the design of reticular frameworks for desired applications.

Understanding the mechanical properties of metal–organic frameworks (MOFs) is essential to the fundamental advancement and practical implementations of porous materials. Recent computational and experimental efforts have revealed correlations between mechanical properties and pore size, topology, and defect density. These results demonstrate the important role of the organic linker in the response of these materials to physical stresses. However, the impact of the coordination bond between the inorganic node and organic linker on the mechanical stability of MOFs has not been thoroughly studied. Here, we isolate the role of this node–linker coordination bond to systematically study the effect it plays in the compression of a series of isostructural MOFs, M-UiO-66 (M = Zr, Hf, or Ce). The bulk modulus (i.e. the resistance to compression under hydrostatic pressure) of each MOF is determined by in situ diamond anvil cell (DAC) powder X-ray diffraction measurements and density functional theory (DFT) simulations. These experiments reveal distinctive behavior of Ce-UiO-66 in response to pressures under one GPa. In situ DAC Raman spectroscopy and DFT calculations support the observed differences in compressibility between Zr-UiO-66 and the Ce- analogue. Monitoring changes in bond lengths as a function of pressure through DFT simulations provides a clear picture of those which shorten more drastically under pressure and those which resist compression. This study demonstrates that changes to the node–linker bond can have significant ramifications on the mechanical properties of MOFs.

Metal−organic frameworks (MOFs) with coordinatively unsaturated metal sites are appealing as adsorbent materials due to their tunable functionality and ability to selectively bind small molecules. Through the use of computational screening methods based on periodic density functional theory, we investigate O2 and N2 adsorption at the coordinatively unsaturated metal sites of several MOF families. A variety of design handles are identified that can be used to modify the redox activity of the metal centers, including changing the functionalization of the linkers (replacing oxido donors with sulfido donors), anion exchange of bridging ligands (considering μ-Br-, μ-Cl-, μ-F-, μ-SH-, or μ-OH- groups), and altering the formal oxidation state of the metal. As a result, we show that it is possible to tune the O2 affinity at the open metal sites of MOFs for applications involving the strong and/or selective binding of O2. In contrast with O2 adsorption, N2 adsorption at open metal sites is predicted to be relatively weak across the MOF dataset, with the exception of MOFs containing synthetically elusive V2+ open metal sites. As one example from the screening study, we predicted that exchanging the μ-Cl- ligands of M2Cl2(BBTA) (H2BBTA = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole) with μ-OH- groups would significantly enhance the strength of O2 adsorption at the open metal sites without a corresponding increase in the N2 affinity. Experimental investigation of Co2Cl2(BBTA) and Co2(OH)2(BBTA) confirms that the former exhibits weak physisorption of both N2 and O2, whereas the latter is capable of chemisorbing O2 at room temperature in a highly selective manner. The O2 chemisorption behavior is attributed to the greater electron-donating character of the μ-OH- ligands and the presence of H-bonding interactions between the μ-OH- bridging ligands and the reduced O2 adsorbate.

Construction of multifunctional metal-organic frameworks (MOFs) with asymmetric connectivity have the potential to expand the scope of their utilization. Herein, we report a robust 9-connected microporous Zr-based MOF, NU-300, assembled from asymmetric tri-carboxylate ligands and Zr6 nodes. As indicated by single-crystal X-ray diffraction analysis, there exist uncoordinated carboxylate groups in the structure of NU-300 that can participate in ammonia (NH3) sorption through acid-base interactions which yield high uptake of NH3 at low pressure regions (In situ infrared (IR) spectroscopy shows the interactions between Brønsted acidic sites and NH3, which suggests that NU-300 can be used as a sorbent for NH3 capture at low pressures.

Ball milling mechanochemistry enables enables targeted, rapid synthesis of mixed-metal metal-organic frameworks (MOFs) with controllable stoichiometric composition. Specifically, the use of ball milling enabled the use of pre-assembled coordination polymers of zinc, magnesium, nickel(II) and cobalt(II) as precursors in an innovative mechanochemical strategy for the deliberate assembly of mixed-metal MOF-74 materials comprised of pairs of transition of main group metals in a pre-determinmed 1:1 stoichiometric ratio, including ZnMg-, ZnCo, ZnCu, MgZn-, MgCo-, MgCa-, NiZn-,NiMg-, NiCo-, CoZn-, CoMg- and CoCu-MOF-74. While this is the first example of target-oriented MOF synthesis using mechanochemistry, it also provides an entry to controlling the stoichiometric composition of mixed-metal frameworks.

Solar selective films hold great promise for improving the efficiency of concentrated solar power (CSP) facilities. In this study, we used atomic layer deposition (ALD) to prepare solar selective films composed of metal-dielectric nanocomposites with tunable optical and electronic properties. We used niobium carbide (NbC) as the metallic component and Al2O3 as the dielectric component of the nanocomposite films, and these components were blended at the atomic scale by alternating between the NbC and Al2O3 ALD processes. In-situ quadrupole mass spectrometry and quartz crystal microbalance (QCM) measurements were performed to examine the growth of the NbC-AlO composite films as well as to establish the NbC ALD growth mechanism. These measurements revealed that the NbC inhibited the Al2O3 ALD, while the Al2O3 enhanced the NbC ALD. Next, NbC-AlO nanocomposite films wereprepared over the full range of 0-100% NbC in Al2O3 and the physical, optical and electrical properties were measured. We discovered that the band gap and electrical resistivity could be precisely tuned by controlling the composition, and that higher NbC contents yielded a lower band gap and a smaller resistivity. Based on the absorption spectra of the NbC-AlO composite films, we established that 10-20% NbC yield the highest selective absorption efficiencies due their high visible light absorption and low infrared absorption. However, the selective absorption properties of the NbC-AlO composite films were lost upon annealing to 400°C in air as a result of oxidation of the NbC. Our study demonstrates the efficacy for ALD preparing metal-dielectric nanocomposite films with tunable properties to achieve a high selective absorber efficiency. By applying this technique to more thermally robust metallic materials we hope to produce solar selective coatings suitable for deployment in CSP facilities.

Conformal coating of boron-containing films via atomic layer deposition (ALD) could be used as a shallow dopant source for advanced 3D-transistor structures in VLSI manufacturing. Aiming this application, fluorine (F) -free precursors of boron such as boric acid (B(OH)3) and trimethyl borate (B(OCH3)3) were explored as ALD precursors in a combination with Al2O3 ALD process. The ALD of Al2O3 was performed using trimethyl aluminum (Al(CH3)3) and Deionized H2O. The complex chemistry resulted in deposition of boron containing composite films of (BxAl2-xO3). In-situ quartz crystal microbalance study was performed to evaluate the optimum ALD precursors dose times as well as confirms the self-limiting behavior of the ALD processes. Growth rate of composite BxAl2-xO3 processes under various ALD conditions are in the range of 0.8-1.1Å. The resultant boron containing BxAl2-xO3 films was analyzed for composition by x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), ellipsometry, and electrical capacitance measurements. Atomic composition of B was tailored by adjusting relative ratio of the B to Al precursors cycle during ALD process. A rapid thermal annealing was performed as a function of time and temperature and confirms B-diffusion in the Si(wafer) as well as changes in optical properties of the BxAl2-xO3 layers. Here we will present detailed investigation of ALD grown BxAl2-xO3 layers.

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