Carbon molecular sieve (CMS) membranes are candidates for the separation of organic molecules due to their stability, ability to be scaled at practical form factors, and the avoidance of expensive ...supports or complex multi‐step fabrication processes. A critical challenge is the creation of “mid‐range” (e.g., 5–9 Å) microstructures that allow for facile permeation of organic solvents and selection between similarly‐sized guest molecules. Here, we create these microstructures via the pyrolysis of a microporous polymer (PIM‐1) under low concentrations of hydrogen gas. The introduction of H2 inhibits aromatization of the decomposing polymer and ultimately results in the creation of a well‐defined bimodal pore network that exhibits an ultramicropore size of 5.1 Å. The H2 assisted CMS dense membranes show a dramatic increase in p‐xylene ideal permeability (≈15 times), with little loss in p‐xylene/o‐xylene selectivity (18.8 vs. 25.0) when compared to PIM‐1 membranes pyrolyzed under a pure argon atmosphere. This approach is successfully extended to hollow fiber membranes operating in organic solvent reverse osmosis mode, highlighting the potential of this approach to be translated from the laboratory to the field.
The pyrolysis of PIM‐1‐derived carbon molecular sieve membranes in an H2‐included environment results in well‐defined “mid‐sized” micropores and a drastic increase in p‐xylene permeability with a little loss in p‐xylene/o‐xylene selectivity.
Far‐from‐equilibrium thermodynamic systems that are established as a consequence of coupled equilibria are the origin of the complex behavior of biological systems. Therefore, research in ...supramolecular chemistry has recently been shifting emphasis from a thermodynamic standpoint to a kinetic one; however, control over the complex kinetic processes is still in its infancy. Herein, we report our attempt to control the time evolution of supramolecular assembly in a process in which the supramolecular assembly transforms from a J‐aggregate to an H‐aggregate over time. The transformation proceeds through a delicate interplay of these two aggregation pathways. We have succeeded in modulating the energy landscape of the respective aggregates by a rational molecular design. On the basis of this understanding of the energy landscape, programming of the time evolution was achieved through adjusting the balance between the coupled equilibria.
Finding the right balance: The energy landscape of a supramolecular polymerization in which the supramolecular assembly transforms from a J‐aggregate to an H‐aggregate over time has been modulated by a rational molecular design. Based on this, kinetic control over pathway complexity was achieved through adjusting the balance between the coupled equilibria.
Gas separation membranes have been studied for several decades and are starting to find commercial acceptance. This review will focus on polymer functionalization to improve gas separation ...performance, namely, permeability, permselectivity, or both. The review will cover both “diffusivity controlled” and “solubility controlled” functionalization strategies; each strategy refers to the different mode of gas transport through the membrane. Diffusivity controlled functionalization strategies mainly involve control over free volume elements in amorphous polymers via promotion or inhibition of chain packing through functional groups. As such, the effects of this functionalization are typically confined to the well-known selectivity/permeability tradeoff. Solubility controlled modification strategies utilize functional groups that have strong chemical interactions with some of the penetrant molecules and offers an enhanced solution-diffusion or a non-solution-diffusion permeation pathway. This functionalization can potentially exceed the Robeson upper bound, but is often challenged by impurities and deactivation of the chemical functionality.
The transport of water through nanoscale capillaries/pores plays a prominent role in biology, ionic/molecular separations, water treatment and protective applications. However, the mechanisms of ...water and vapor transport through nanoscale confinements remain to be fully understood. Angstrom-scale pores (~2.8-6.6 Å) introduced into the atomically thin graphene lattice represent ideal model systems to probe water transport at the molecular-length scale with short pores (aspect ratio ~1-1.9) i.e., pore diameters approach the pore length (~3.4 Å) at the theoretical limit of material thickness. Here, we report on orders of magnitude differences (~80×) between transport of water vapor (~44.2-52.4 g m
day
Pa
) and liquid water (0.6-2 g m
day
Pa
) through nanopores (~2.8-6.6 Å in diameter) in monolayer graphene and rationalize this difference via a flow resistance model in which liquid water permeation occurs near the continuum regime whereas water vapor transport occurs in the free molecular flow regime. We demonstrate centimeter-scale atomically thin graphene membranes with up to an order of magnitude higher water vapor transport rate (~5.4-6.1 × 10
g m
day
) than most commercially available ultra-breathable protective materials while effectively blocking even sub-nanometer (>0.66 nm) model ions/molecules.
Vertically-aligned carbon nanotube (VaCNT) membranes allow water to conduct rapidly at low pressures and open up the possibility for water purification and desalination, although the ultralow viscous ...stress in hydrophobic and low-tortuosity nanopores prevents surface interactions with contaminants. In this experimental investigation, steroid hormone micropollutant adsorption by VaCNT membranes is quantified and explained via the interplay of the hydrodynamic drag and friction forces acting on the hormone, and the adhesive and repulsive forces between the hormone and the inner carbon nanotube wall. It is concluded that a drag force above 2.2 × 10
pN overcomes the friction force resulting in insignificant adsorption, whereas lowering the drag force from 2.2 × 10
to 4.3 × 10
pN increases the adsorbed mass of hormones from zero to 0.4 ng cm
. At a low drag force of 1.6 × 10
pN, the adsorbed mass of four hormones is correlated with the hormone-wall adhesive (van der Waals) force. These findings explain micropollutant adsorption in nanopores via the forces acting on the micropollutant along and perpendicular to the flow, which can be exploited for selectivity.
The electrosynthesis of value‐added multicarbon products from CO2 is a promising strategy to shift chemical production away from fossil fuels. Particularly important is the rational design of gas ...diffusion electrode (GDE) assemblies to react selectively, at scale, and at high rates. However, the understanding of the gas diffusion layer (GDL) in these assemblies is limited for the CO2 reduction reaction (CO2RR): particularly important, but incompletely understood, is how the GDL modulates product distributions of catalysts operating in high current density regimes > 300 mA cm−2. Here, 3D‐printable fluoropolymer GDLs with tunable microporosity and structure are reported and probe the effects of permeance, microstructural porosity, macrostructure, and surface morphology. Under a given choice of applied electrochemical potential and electrolyte, a 100× increase in the C2H4:CO ratio due to GDL surface morphology design over a homogeneously porous equivalent and a 1.8× increase in the C2H4 partial current density due to a pyramidal macrostructure are observed. These findings offer routes to improve CO2RR GDEs as a platform for 3D catalyst design.
Multiscale structural control in 3D‐printable fluoropolymers enables highly tunable gas‐diffusion electrodes. By modulating the diffusion of reactants to and products from the catalyst layer during electroreduction, the product distribution is shifted in favor of higher carbon products. Insights into the rational design of electrodes offer a platform for developing and optimizing next‐generation flow reactors.
Rising atmospheric CO2 levels have triggered recent research into the science of amine materials supported on hard, porous materials such as mesoporous silica or alumina. While such materials can ...give high CO2 uptakes and good sorption kinetics, they are difficult to utilize in practical applications due to difficulty in contacting large volumes of CO2-laden gases with powder materials without significant pressure drops or sorbent attrition. Here, we describe a simple approach based on the impregnation of a permanently microporous polymer, PIM-1, with poly(ethylene imine) (PEI), removing the need for use of the hard oxide. PEI/PIM-1 composites demonstrate comparable performance to more traditionally studied oxide sorbents, with the benefit that PIM-1 is soluble in common solvents, making it eminently more viable for processing into morphologies that can facilitate heat and mass transfer and fabrication into low pressure drop contactors. In addition to adsorption studies performed on a variety of PEI/PIM-1 architectures, spin diffusion NMR studies were performed to suggest that PEI is well-dispersed within the PIM-1, allowing for rapid CO2 adsorption.
Emerging commercial applications of vertically aligned, single-walled carbon nanotube (SWCNT) “forests” require synthesis that minimizes nanotube diameter while maximizing number density across ...substrate areas exceeding centimeter scale. To address this need, we synthesized SWCNT forests on full silicon wafers with notable reproducibility and uniformity, and co-optimized growth for small diameters and high densities across large areas to access new territory in this 3D parameter space. We mapped the spatial uniformity of key structural features using Raman microscopy, synchrotron X-ray scattering, and Rutherford backscattering spectrometry. Low C2H2 flux over sub-nm Fe/Mo catalysts produced small-diameter SWCNTs (2.1 nm) at high number densities (2.26 × 1012 cm−2) on wafers up to 6 in. Although removing Mo resulted in larger SWCNT diameters and lower densities (<0.7 × 1012 cm−2), mass conversion rates from C2H2 to SWCNT product were high and remarkably invariant for catalyst compositions and densities (i.e., 47.7% or 1.30 × 106% g-catalyst−1 on 4-in. wafers). These carbon conversion efficiencies far exceed typical benchtop reactors and are on par with the best reported literature values. Our detailed elucidation of correlations among structural characteristics within this resource-efficient process is expected to guide future scale-up efforts of SWCNT forest growth beyond wafer scale.
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Polymers of intrinsic microporosity (PIMs) are solution-processable, rigid, and sterically hindered polymers. Since their inception almost two decades ago, this new class of materials has grown ...substantially and found prominence in membrane separations. Indeed, the large internal free volume of PIMs has led to the development of highly permeable and moderately selective membranes. However, this modest selectivity often requires further improvements to reach industrial targets for separation performance. Here, we highlight the latest advancements in PIM membrane functionalization to improve the membrane separation performance including the development of molecularly mixed composite matrix membranes, thin film composite carbon molecular sieve membranes, and vapor phase infiltrated membranes.
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