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.
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