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Water and other small molecules frequently coordinate within metal‐organic frameworks (MOFs). These coordinated molecules may actively engage in mass transfer, moving together with the transport molecules, but this phenomenon has yet to be examined. In this study, we explore a unique water transfer mechanism in UTSA‐280, where an incoming water molecule can displace a coordinated molecule for mass transfer. We refer to this process as the "knock‐off" mechanism. Despite UTSA‐280 possessing one‐dimensional channels, the knock‐off transport enables water movement along the other two axes, effectively simulating a pseudo‐three‐dimensional mass transfer. Even with a relatively narrow pore width, the knock‐off mechanism enables a high water flux in the UTSA‐280 membrane. The knock‐off mechanism also renders UTSA‐280 superior water/ethanol diffusion selectivity for pervaporation. To validate this unique mechanism, we conducted 1H and 2H solid‐state NMR on UTSA‐280 after the adsorption of deuterated water. We also derived potential energy diagrams from the density functional theory to gain atomic‐level insight into the knock‐off and the direct‐hopping mechanisms. The simulation findings reveal that the energy barrier of the knock‐off mechanism is marginally lower than the direct‐hopping pathway, implying its potential role in enhancing water diffusion in UTSA‐280. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) layered porous materials are of a considerable interest to researchers due to differences in the sizes of in-plane and out-of-plane confinement. These layered materials have been blended with polymers for the fabrication of mixed-matrix membranes (MMMs) for applications involving separation. The polymers in MMMs serve as a matrix and the 2D layered materials serve as filler used to enhance the separation performance (diffusivity and selectivity) of the polymeric matrix. Unfortunately, the existing models for estimating the effective diffusivity of MMMs with layered fillers are unable to provide reliable predictions. This study proposed a numerical approach to the estimation of effective diffusivity and the selectivity of MMMs using layered fillers based on the mass transfer simulations. In the proposed models, we took into account two critical parameters of the layered fillers: anisotropic diffusivity and orientation distributions. The predictive ability of the proposed approach was evaluated via comparison with experimental data. Finally, we outline the design of highly selective MMMs with layered porous materials based on the proposed methodology. [ABSTRACT FROM AUTHOR]

We report a method for estimating the effective molecular diffusivity of mixed matrix membranes (MMMs) with tubular fillers. This method is based on constructing three-dimensional MMM models for solving Fick׳s diffusion equation to obtain the concentration profiles. The effective diffusivity of the MMMs was thus derived from the concentration distributions. Using this approach, the effects of various structural parameters, including the tubular filler volume fraction, spatial distribution, orientation, aspect ratios, and diffusivity ratios, were assessed. The results suggest that the tubular filler alignment and spatial distributions are critical to the mass transfer in MMMs. We compared the diffusivity estimated by the presented method to the experimental data. Our approach shows improved predictions compared with the Maxwell model and the Kang–Jones–Nair (KJN) model. Overall, this work presents a useful tool for understanding and designing MMMs with tubular fillers. [ABSTRACT FROM AUTHOR]

Abstract: Nanotubes have been shown to possess intriguing mass transport properties, and are being incorporated into polymeric membranes for molecular separations. Although models have been developed to predict the effective permeability and selectivity of composite membranes with non-spherical fillers, they only apply to fillers with isotropic transport properties. However, molecular transport in tubular fillers is essentially one-dimensional. Here, an analytical model is developed – including the orientational distribution, aspect ratio, volume fraction, permeability of the fillers, and membrane non-idealities – to describe molecular transport in composite membranes with tubular fillers. Using the model, the effects of various structural and operational parameters of the composite membrane are assessed. It is found that the filler volume fraction, as opposed to the filler orientation, intrinsic permeability, and aspect ratio, has the most significant impact on the membrane permeability. Several case studies of binary mixture separations by ideal and non-ideal composite membranes with tubular fillers are discussed. Finally, an advanced composite membrane configuration that allows exploitation of the unique transport properties of tubular fillers is discussed. [Copyright &y& Elsevier]

Mixed matrix membranes (MMMs) are composed of a polymeric matrix, which gives them good processability, and crystalline microporous fillers, which give them high separation performance. The need to predict the separation performance of MMMs has led to the development of a number of analytical and numerical methods for the estimation of diffusivity in MMMs, based on the intrinsic diffusivity of the matrix and filler phases. However, current reports on the estimations of diffusivity in MMMs are only for flat dense MMMs (FDMMMs), rather than hollow fiber MMMs (HFMMMs), a scalable membrane configuration used in industrial membrane separation applications. In this paper, we presented the first numerical method for the estimation of effective diffusivity in HFMMMs. We determined that the curved geometry of HFMMMs results in mass transport pathways different from those found in FDMMMs, which produces higher effective diffusivity. We also investigated how the diffusivity and size of the filler as well as the interior diameter and thickness of the selective skin layer of HFMMMs affect diffusivity. The proposed methodology provides an effective tool for the design of high-performance HFMMMs applicable to cases involving separation. [ABSTRACT FROM AUTHOR]