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Abstract The capability of materials to interconvert between different phases provides more possibilities for controlling materials’ properties without additional chemical modification. The study of state-changing microporous materials just emerged and mainly involves the liquefication or amorphization of solid adsorbents into liquid or glass phases by adding non-porous components or sacrificing their porosity. The material featuring reversible phases with maintained porosity is, however, still challenging. Here, we synthesize metal-organic polyhedra (MOPs) that interconvert between the liquid-glass-crystal phases. The modular synthetic approach is applied to integrate the core MOP cavity that provides permanent microporosity with tethered polymers that dictate the phase transition. We showcase the processability of this material by fabricating a gas separation membrane featuring tunable permeability and selectivity by switching the state. Compared to most conventional porous membranes, the liquid MOP membrane particularly shows the selectivity for CO2 over H2 with enhanced permeability.

Abstract Structural flexibility is a critical issue that limits the application of metal-organic framework (MOF) membranes for gas separation. Herein we propose a mixed-linker approach to suppress the structural flexibility of the CAU-10-based (CAU = Christian-Albrechts-University) membranes. Specifically, pure CAU-10-PDC membranes display high separation performance but at the same time are highly unstable for the separation of CO2/CH4. A partial substitution (30 mol.%) of the linker PDC with BDC significantly improves its stability. Such an approach also allows for decreasing the aperture size of MOFs. The optimized CAU-10-PDC-H (70/30) membrane possesses a high separation performance for CO2/CH4 (separation factor of 74.2 and CO2 permeability of 1,111.1 Barrer under 2 bar of feed pressure at 35°C). A combination of in situ characterization with X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, as well as periodic density functional theory (DFT) calculations, unveils the origin of the mixed-linker approach to enhancing the structural stability of the mixed-linker CAU-10-based membranes during the gas permeation tests.

Metal-organic frameworks (MOFs) hold great promise as porous materials for pervaporation applications. However, the exploration of MOF membranes in this field is still in its early stages. One of the main challenges is the relatively low mass flux and stability of pure MOF membranes compared to other materials used in pervaporation. In this study, we propose a novel approach to enhance the separation performance of MOF membranes for water and ethanol separation. Our strategy involves incorporating the 2,5-thiophenedicarboxylic acid (TDC) linker into the MOF-303 structure, partially replacing the 3,5-pyrazoledicarboxylic acid (PDC) linker. The goal is to increase the aperture size of the microporous channels in the pristine MOF-303 membrane, thereby improving the mass flux. X-ray diffraction characterization, combined with Rietveld refinement, confirmed that the partial substitution of PDC with TDC resulted in an increased pore-limiting diameter (PLD) of MOF-303. For instance, the pristine MOF-303 exhibited a PLD of 5.78 Å, while MOF-303(70/30) with 70% TDC replacement displayed a PLD of 6.02 Å. To fabricate the mixed-linker MOF-303 membranes, we utilized a seeded growth method, which yielded membranes with dense layers, as confirmed by scanning electron microscopy and air permeation characterization. The prepared membranes were subjected to pervaporation tests to evaluate their performance in separating 90 wt.% ethanol at 60 °C. The pristine MOF-303 membrane exhibited notable separation capabilities, with an average flux of 0.071 kg·m−2·hr−1 and a water/ethanol separation factor of 5371. Surpassing the unmodified MOF-303, the mixed-linker MOF-303(50/50) membrane demonstrated improved mass flux and water/ethanol separation factor. Specifically, the MOF-303(50/50) membrane displayed an average flux of 0.092 kg·m−2·hr−1 and a water/ethanol separation factor of 8500. Importantly, the unmodified MOF-303 membrane exhibited instability during prolonged pervaporation operation, whereas the mixed-linker MOF-303(50/50) membrane effectively addressed this issue. Further analysis using in situ Fourier transform infrared spectroscopy and water adsorption characterization revealed that MOF-303(50/50) possessed a strong affinity for water, comparable to the pristine MOF-303. Overall, our study highlights the potential of the mixed-linker approach to optimize the separation performance and stability of MOF-based membranes for pervaporation application.

The separation of H2/CH4 or CO2/CH4 is critical to the purification of natural gas. Herein, we report on novel membranes with a metal-organic framework of CAU-10-PDC for the separation of these two mixtures. The dense CAU-10-PDC membranes are fabricated on a porous alumina support using the seeded growth method. An unexpected increase in selectivity was observed while testing mixed gas permeation with either H2/CH4 or CO2/CH4 at a molar ratio of 50:50. Steady-state selectivity reached 101 for H2/CH4 and 62 for CO2/CH4. Ideal selectivity measured from single gas permeation reached 475 for H2/CH4 and 288 for CO2/CH4. Molecular dynamics simulations and time-resolved X-ray diffraction with a synchrotron radiation source were used to probe conformational changes in CAU-10-PDC induced by exposure to CH4. When exposed to an atmosphere containing CH4, CAU-10-PDC presented a change in the space group (from I41/amd to I41), which drastically reduced the pore limiting diameter from 4.15 to 2.95 Å, rendering the channel nearly impermeable to CH4.

The expansion or compression of gas confined in a pistonand- cylinder device is a classic working example used for illustrating the First and Second Laws of Thermodynamics. The balance of energy and entropy enables the estimation of a number of thermodynamic properties. The entropy generation (also called entropy production) resulting from this process can also be used to determine the feasibility and reversibility of a process. In this work, we present an extended discussion involving quantitative analysis of the effects of friction between a piston and the interior wall of a cylinder. Our findings indicate that the friction force caused by the movement of the piston is a main source of entropy generation in this process. This explanation does not appear in most textbooks dealing with similar problems. We also discuss, from a quantitative perspective, the effects of friction on the dynamic physical and thermodynamic properties, including entropy generation. Our findings suggest that engaging students in a discussion related to piston-and-cylinder problems involving the effects of friction could provide valuable insight into entropy generation in practical applications. [ABSTRACT FROM AUTHOR]

Recently, we have reported the experimental results of DNA stretching by flow field in three microchannels (C. H. Lee and C. C. Hsieh, Biomicrofluidics 7(1), 014109 (2013)) designed specifically for the purpose of preconditioning DNA conformation for easier stretching. The experimental results do not only demonstrate the superiority of the new devices but also provides detailed observation of DNA behavior in complex flow field that was not available before. In this study, we use Brownian dynamics-finite element method (BD-FEM) to simulate DNA behavior in these microchannels, and compare the results against the experiments. Although the hydrodynamic interaction (HI) between DNA segments and between DNA and the device boundaries was not included in the simulations, the simulation results are in fairly good agreement with the experimental data from either the aspect of the single molecule behavior or from the aspect of ensemble averaged properties. The discrepancy between the simulation and the experimental results can be explained by the neglect of HI effect in the simulations. Considering the huge savings on the computational cost from neglecting HI, we conclude that BD-FEM can be used as an efficient and economic designing tool for developing new microfluidic device for DNA manipulation. [ABSTRACT FROM AUTHOR]

The CO2 adsorption characteristics of prototypical poly(ethyleneimine)/silica composite adsorbents can be drastically enhanced by altering the acid/base properties of the oxide support via incorporation of Zr into the silica support. Introduction of an optimal amount of Zr resulted in a significant improvement in the CO2 capacity and amine efficiency under dilute (simulated flue gas) and ultradilute (simulated ambient air) conditions. Adsorption experiments combined with detailed characterization by thermogravimetric analysis, temperature-programmed desorption, and in situ FT-IR spectroscopy clearly demonstrate a stabilizing effect of amphoteric Zr sites that enhances the adsorbent capacity, regenerability, and stability over continued recycling. It is suggested that the important role of the surface properties of the oxide support in these polymer/oxide composite adsorbents has been largely overlooked and that the properties may be even further enhanced in the future by tuning the acid/ base properties of the support. [ABSTRACT FROM AUTHOR]

Abstract Integration of metal‐organic frameworks (MOFs) as components of advanced electronic devices is at a very early phase of development and the fundamental issues related to their crystal growth on conductive substrate need to be addressed. Herein, we report on the structural characterization of a newly synthesized Sr‐based MOF {[Sr(2,5‐Pzdc)(H2O)2] ⋅ 3 H2O}n (1) and the uniform crystal growth of compound 1 on a conducting glass (fluorine doped tin oxide (FTO)) substrate using electrochemical deposition techniques. The Sr‐based MOF 1 was synthesized by the reaction of Sr(NO3)2 with 2,5‐pyrazinedicarboxylic acid dihydrate (2,5‐Pzdc) under solvothermal conditions. A single‐crystal X‐ray diffraction analysis revealed that 1 has a 3D structure and crystallizes in the triclinic P1‾ space group. In addition, the uniform crystal growth of this MOF on a conducting glass (FTO) substrate was successfully achieved using electrochemical deposition techniques. Only a handful of MOFs have been reposed to grown on conductive surfaces, which makes this study an important focal point for future research on the applications of MOF‐based devices in microelectronics.

Manipulating building-block nanomaterials to form an ordered superstructure in a dilute and spacer-free solution phase challenges the existing 5-nm node lithography and nanorobotics. The cooperative nature of nanocrystals, polymers, and cells can lead to superarrays or colloidal crystals. For known highly ordered systems, the characteristic length of materials, defined as the shortest dimension of objects, is generally larger than their separations. A spacer (small-molecule surfactant or polymer) is typically required to diminish short range van der Waals attraction, which results in a glassy or liquid state. Herein we propose a new concept of achieving highly ordered nano-objects in a dilute and spacer-free system via the synergistic effects of excellent solvation and appropriate constraints on rotational motion. As a proof of concept, this study demonstrates that aluminosilicate nanotubes (AlSiNTs) suspended in water under dilute conditions (e.g., 1.0 wt%) can spontaneously form hexagonal arrays with an intertubular distance as large as tens of nanometers. The separation distance of the ordered superstructure is also tunable via controlling the concentration and length of nanotubes. These superaligned structures are probed using small-angle x-ray scattering and cryo-TEM characterizations, with underlying mechanisms investigated at an atomic level using molecular dynamics simulations. The concept and discovery of this work can open up opportunities to a variety of applications including visible-UV photonics and nanolithography, and may be generalizable to other nano-object systems that fulfill similar requirements. [ABSTRACT FROM AUTHOR]