Advancing our understanding of the defect formation mechanism in metal–organic frameworks (MOFs) is critical for the rational design of the material’s structure. In particular, the defects in the UiO-66 framework have been shown to have a significant impact on the framework functionality and stability. However, the effects of synthesis conditions on defect formation are elusive and our understanding of missing-ligand and missing-cluster defects in UiO-66 is far from clear. In this work, we demonstrate that the formation of missing-cluster (MC) defects is due to the large number of partially deprotonated ligands in synthesis solution. The proposed mechanism is verified by a series of syntheses controlling the defect formation. The results show that the quantity of MC defects is sensitive to deprotonation reagents, synthesis temperature, and reactant concentration. The pore size distribution derived from the N2 adsorption isotherm at 77 K allows accurate and convenient characterization of the defects in UiO-66. The existence of defects in the UiO-66 framework can cause significant deviations in its pore size distribution from the results derived from the theoretically perfect crystal structure. The extra cavities generated by MC defects are demonstrated to allow deposition of a large functional molecule, ferrocene (3.5 Å × 4.5 Å × 4.5 Å). The successful incorporation is proven by the tuning of the original N2-selective framework to become an O2-selective framework.

The most common products obtained in the synthesis of zirconium-based metal−organic frameworks (ZrMOFs) are fine powders. The particle size of a typical ZrMOF UiO-66 was first reported to be around 200 nm, so the original crystal structure was only solved by powder XRD coupled with Rietveld refinement due to the incapability of single crystal XRD to solve such small crystals with poor crystallinity. One may ask the reason why the particle size of UiO-66 is so small compared to that of other common MOFs and what the key factor terminating the growth of UiO-66 is. In this work, we try to answer this question by proposing a hypothesis that the partially deprotonated ligand caused by the accumulated protons in the reaction solution is the key factor preventing the continuous growth of the UiO-66 crystal. The hypothesis is verified by growth reactivation with the addition of a deprotonating agent in an in situ biphase solvothermal reaction. As long as the protons were sufficiently coordinated by the deprotonating agent, the continuous growth of UiO-66 is guaranteed. Moreover, the modulation effect can impact the coordination equilibrium and nucleation so that an oriented attachment growth of UiO-66 film was achieved in membrane structures.

Electrospun poly­(vinyl cinnamate) (PVCi) nanofibers were cross-linked for varying times to study the impact it has on fiber stability at elevated temperatures and in dimethylformamide solutions. UiO-66 impregnated PVCi was cross-linked, and secondary growth of UiO-66 crystals was performed. The impact of temperature and number of growths was analyzed through powder X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. A membrane that underwent three growth cycles at 100 °C was further characterized for potential uses as a gas membrane through inert gas permeation studies and nitrogen porosimetry.

A novel method for increasing the effective nanoparticle loading in electrospun fibers is presented involving the electrospinning of polymer blends with suspended ZIF-8 crystals. Initially, varying ratios of Matrimid 5218 and poly-(ethylene oxide) (PEO) are electrospun, followed by methanol washes to remove PEO to form porous Matrimid nanofibers. After an optimum surface area is found from the ratio of 1:1 Matrimid:PEO by weight, the metal-organic-framework ZIF-8 is suspended in the polymer blend and electrospun to form ZIF-8 impregnated fibers. After PEO removal from ZIF-8 impregnated fibers, it is found that the ZIF-8 remains in the porous fibers, resulting in drastic increases in ZIF-8:nanofiber loadings, increased gas uptake, and increased accessible ZIF-8 within the fibers. This method is anticipated to work for many different nanoparticle− polymer systems, having implications in the fields of filtration, sensing, catalysis, and adsorption.

The quality of nanoparticle dispersion in a polymer matrix significantly influences the macroscopic properties of the composite material. Like general polymer-nanoparticle composites, electrospun nanofiber nanoparticle composites do not have an adopted quantitative model for dispersion throughout the polymer matrix, often relying on a qualitative assessment. Being such an influential property, quantifying dispersion is essential for the process of optimization and understanding the factors influencing dispersion. Here, a simulation model was developed to quantify the effects of nanoparticle volume loading (ϕ) and fiber-to-particle diameter ratios (D/d) on the dispersion in an electrospun nanofiber based on the interparticle distance. A dispersion factor is defined to quantify the dispersion along the polymer fiber. In the dilute regime (ϕ < 20%), three distinct regions of the dispersion factor were defined with the highest quality dispersion shown to occur when geometric constraints limit fiber volume accessibility. This model serves as a standard for comparison for future experimental studies and dispersion models through its comparability with microscopy techniques and as a way to quantify and predict dispersion in electrospinning polymer-nanoparticle systems with a single performance metric.

In this study, a new cobalt‐based metal‐organic framework (MOF), [ (μ3‐OH)2(ipa)5(C3O2)(DMF)2] (CoIPA) was synthesized. The crystal structure analysis shows that CoIPA is constructed by Co6(μ3‐OH)2 units linked by isophthalic acid forming a sxb topology and it possesses a small pore size of about 4 Å. The new MOF has been characterized using multiple experimental methods. Monte Carlo and Molecular Dynamic simulations were employed to investigate adsorption equilibrium and kinetics in terms of capacity and diffusivity of CO2, N2, and CH4 on CoIPA. The gas adsorption isotherms collected experimentally were used to verify the simulation results. The activated CoIPA sample exhibits great gas separation ability at ambient conditions for CO2/N2 and CO2/CH4 with selectivity of around 61.4 and 11.7, respectively. The calculated self‐diffusion coefficients show a strong direction dependent diffusion behavior of target molecules. This high adsorption selectivity for both CO2/N2 and CO2/CH4 makes CoIPA a potential candidate for adsorptive CO2 separation. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4532–4540, 2017

Direct air capture (DAC) of CO 2 is an emerging technology in the battle against climate change. Many sorbent materials and different technologies such as moisture swing sorption have been explored for this application. However, developing efficient scaffolds to adopt promising sorbents with fast kinetics is challenging, and very limited effort has been reported to address this critical issue. In this work, the availability and kinetic uptake of CO 2 in sorbents embedded in various matrices are studied. Three scaffolds including a commercially available industrial film containing ion-exchange resin (IER), IER particles embedded in dense electrospun fibers, and IER particles embedded in porous electrospun fibers are compared, in which a solvothermal polymer additive removal technique is used to create porosity in porous fibers. A frequency response technique is developed to measure the uptake capacity, sorbent availability, and kinetic uptake rate. The porous fiber has 90% IER availability, while the dense fibers have 50% particle accessibility. The sorption half time for both electrospun fiber samples is 10 AE 3 min. Our experimental results demonstrate that electrospinning polymer/sorbent composites is a promising technology to facilitate the handleability of sorbent particles and to improve the sorption kinetics, in which the IER embedded in porous electrospun fibers shows the highest cycle capacity with an uptake rate of 1.4 mol CO 2 per gramhour.

Ion hydration is a fundamental process in many natural phenomena. This paper presents a quantitative analysis, based on atomistic modeling, of the behavior of ions and the impact of hydration in a novel CO2 sorbent. We explore moisture-driven CO2 sorbents focusing on diffusion of ions and the structure of ion hydration complexes forming inside water-laden resin structures. We show that the stability of the carbonate ion is reduced as the water content of the resin is lowered. As the hydration cloud of the carbonate ion shrinks, it becomes energetically favorable to split a remaining water molecule and form a bicarbonate ion plus a hydroxide ion. These two ions bind less water than a single, doubly charged carbonate ion. As a result, under relatively dry conditions, more OH− ions are available to capture CO2 than in the presence of high humidity. Local concentrations of dissolved inorganic carbon and water determine chemical equilibria. Reaction kinetics is then driven to a large extent by diffusion rates that allow water and anions to move through the resin structure. Understanding the basic mechanics of chemical equilibria and transport may help us to rationally design next-generation efficient moisture-driven CO2 sorbents.