Above-Tg Annealing Benefits in Nanoparticle-Stabilized Carbon Molecular Sieve Membrane Pyrolysis for Improved Gas Separation

Nanoparticles can suppress asymmetric precursor support collapse during pyrolysis to create carbon molecular sieve (CMS) membranes. This advance allows elimination of standard sol-gel support stabilization steps. Here we report a simple but surprisingly important thermal soaking step at 400 °C in the pyrolysis process to obtain high performance CMS membranes. The composite CMS membranes show CO2 /CH4 (50 : 50) mixed gas feed with an attractive CO2 /CH4 selectivity of 134.2 and CO2 permeance of 71 GPU at 35 °C. Furthermore, a H2 /CH4 selectivity of 663 with H2 permeance of 240 GPU was achieved for promising green energy resource-H2 separation processes.

Selective permeation up a chemical potential gradient to enable an unusual solvent purification modality

The need for energy-efficient recovery of organic solutes from aqueous streams is becoming more urgent as chemical manufacturing transitions toward nonconventional and bio-based feedstocks and processes. In addition to this, many aqueous waste streams contain recalcitrant organic contaminants, such as pharmaceuticals, industrial solvents, and personal care products, that must be removed prior to reuse. We observe that rigid carbon membrane materials can remove and concentrate organic contaminants via an unusual liquid-phase membrane permeation modality. Surprisingly, detailed thermodynamic calculations on the chemical potential of the organic contaminant reveal that the organic species has a higher chemical potential on the permeate side of the membrane than on the feed side of the membrane. This unusual observation challenges conventional membrane transport theory that posits that all permeating species move from high chemical potential states to lower chemical potential states. Based on experimental measurements, we hypothesize that the organic is concentrated in the membrane relative to water via favorable binding interactions between the organic and the carbon membrane. The concentrated organic is then swept through the membrane via the bulk flow of water in a modality known as "sorp-vection." We highlight via simplified nonequilibrium thermodynamic models that this "uphill" chemical potential permeation of the organic does not result in second-law violations and can be deduced via measurements of the organic and water sorption and diffusion rates into the carbon membrane. Moreover, this work identifies the need to consider such nonidealities when incorporating unique, rigid materials for the separations of aqueous waste streams.

Appealing sheath-core spun high-performance composite carbon molecular sieve membranes

Carbon molecular sieve (CMS) membranes are attractive candidates meet energy demands for challenging gas separations. The added ability to maintain such intrinsic properties in an asymmetric morphology with a structure that we term a "Pseudo Wheel+ Hub & Spoke" asymmetric form offers new opportunities.  For CMS membrane, specifically, the structure provides both selective layer support and low flow resistance even for high feed pressures and fluxes in CO2 removal from natural gas. This capability is unavailable to even rigid glassy polymers due to the much higher modulus of CMS materials. Combining precursor asymmetric hollow fiber formation and optimized pyrolysis creates a defect free CMS proof-of-concept membrane for this application. Facile formation of the sheath-core spun precursor with a 6FDA-DAM sheath and Matrimid® core also avoids the need to seal defects before or after the carbonization of the precursors. The composite CMS membranes shows CO2/CH4 (50:50) mixed gas feed with an attractive CO2/CH4 selectivity of 64.3 and CO2 permeance of 232 GPU at 35 °C. A key additional benefit of the approach is reduction in use of the more costly high performance 6FDA-DAM in a composite sheath-core CMS membrane with the "Pseudo Wheel+ Hub & Spoke" structure.

Advanced carbon molecular sieve membranes derived from molecularly engineered cross-linkable copolyimide for gas separations

Carbon molecular sieve (CMS) membranes with precise molecular discrimination ability and facile scalability are attractive next-generation membranes for large-scale, energy-efficient gas separations. Here, structurally engineered CMS membranes derived from a tailor-made cross-linkable copolyimide with kinked structure are reported. We demonstrate that combining two features, kinked backbones and cross-linkable backbones, to engineer polyimide precursors while controlling pyrolysis conditions allows the creation of CMS membranes with improved gas separation performance. Our results indicate that the CMS membranes provide a versatile platform for a broad spectrum of challenging gas separations. The gas transport properties of the resulting CMS membranes are interpreted in terms of a model reflecting both molecular sieving Langmuir domains and a disordered continuous phase, thereby providing insight into structure evolution from the cross-linkable polyimide precursor to a final CMS membrane. With this understanding of CMS membrane structure and separation performance, these systems are promising for environmentally friendly gas separations.

Scalable Formation of Diamine-Appended Metal-Organic Framework Hollow Fiber Sorbents for Postcombustion CO2 Capture

We describe a straightforward and scalable fabrication of diamine-appended metal-organic framework (MOF)/polymer composite hollow fiber sorbent modules for CO₂ capture from dilute streams, such as flue gas from natural gas combined cycle (NGCC) power plants. A specific Mg-MOF, Mg₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), incorporated into poly(ether sulfone) (PES) is directly spun through a conventional "dry-jet, wet-quench" method. After phase separation, a cyclic diamine 2-(aminomethyl)piperidine (2-ampd) is infused into the MOF within the polymer matrix during postspinning solvent exchange. The MOF hollow fibers from direct spinning contain as high as 70% MOF in the total fibers with 98% of the pure MOF uptake. The resulting fibers exhibit a step isotherm and a "shock-wave-shock" breakthrough profile consistent with pure 2-ampd-Mg₂(dobpdc). This work demonstrates a practical method for fabricating 2-ampd-Mg₂(dobpdc) fiber sorbents that display the MOF's high CO₂ adsorption capacity while lowering the pressure drop during operation.

Key Features of Polyimide-Derived Carbon Molecular Sieves

Carbon molecular sieve (CMS) membranes have impressive separation properties; however, both chemical and morphology structures need to be understood better. Here we characterize CMS with the simplest polyimide (PI) PMDA/pPDA (PMDA=pyromellitic dianhydride, pPDA=p-phenylenediamine), using FTIR, solid-state ¹⁵ N-NMR and ¹³ C-NMR, XPS, XRD, and Raman spectra to study chemical structure. We also compare gas separation properties for this CMS to a CMS derived from a more conventional PI precursor. The detailed characterization shows the presence of aromatic pyridinic, pyrrolic rings as well as graphitic, pyridonic components and a few other groups in both CMS types derived from the very different precursors. The CMS morphologies, while related to precursor and pyrolysis temperature details, show similarities consistent with a physical picture comprising distributed molecular sieving plate-like structures. These results assist in understanding diverse CMS membrane separation performance.

Isomer-Tailored Carbon Molecular Sieve Membranes with High Gas Separation Performance

Partial substitution of the asymmetric 3,3',4,4'-biphenyl dianhydride monomer (aBPDA) into the backbone of a 6FDA-BPDA-DAM (6FDA=4,4'-hexafluoroisopropylidene, DAM=diaminomesitylene) diphthalic anhydride-based copolyimide based on symmetrical BPDA (sBPDA) was used to study membrane structure-processing-property relationships for gas separation. Properties of the polymer membrane as well as derived carbon molecular sieves (CMS) membranes were compared with copolyimides without the asymmetric monomer structure. CMS membranes derived from the copolyimides are very attractive for CO₂ /CH₄ separation. aBPDA provides the copolyimide with additional packing-inhibited structures compared with the symmetric ones and yields a corresponding CMS membrane with very high CO₂ permeability and good CO₂ /CH₄ selectivity. This work, therefore, outlines a new strategy for tuning CMS membrane structure to meet separation performance needs.

Ultraselective glassy polymer membranes with unprecedented performance for energy-efficient sour gas separation

Membrane-based separation of combined acid gases carbon dioxide and hydrogen sulfide from natural gas streams has attracted increasing academic and commercial interest. These feeds are referred to as "sour," and herein, we report an ultra H₂S-selective and exceptionally permeable glassy amidoxime-functionalized polymer of intrinsic microporosity for membrane-based separation. A ternary feed mixture (with 20% H₂S:20% CO₂:60% CH₄) was used to demonstrate that a glassy amidoxime-functionalized membrane provides unprecedented separation performance under challenging feed pressures up to 77 bar. These membranes show extraordinary H₂S/CH₄ selectivity up to 75 with ultrahigh H₂S permeability >4000 Barrers, two to three orders of magnitude higher than commercially available glassy polymeric membranes. We demonstrate that the postsynthesis functionalization of hyper-rigid polymers with appropriate functional polar groups provides a unique design strategy for achieving ultraselective and highly permeable membrane materials for practical natural gas sweetening and additional challenging gas pair separations.

Publisher Correction: Mixed matrix formulations with MOF molecular sieving for key energy-intensive separations

In the version of this Article originally published, the units of the y axis of Fig. 3b were incorrectly given as '10⁶ cm² s^-1'; they should have been '10^-8 cm² s^-1'. This has been corrected in the online versions of the Article.

Mixed matrix formulations with MOF molecular sieving for key energy-intensive separations

Membrane-based separations can improve energy efficiency and reduce the environmental impacts associated with traditional approaches. Nevertheless, many challenges must be overcome to design membranes that can replace conventional gas separation processes. Here, we report on the incorporation of engineered submicrometre-sized metal-organic framework (MOF) crystals into polymers to form hybrid materials that successfully translate the excellent molecular sieving properties of face-centred cubic (fcu)-MOFs into the resultant membranes. We demonstrate, simultaneously, exceptionally enhanced separation performance in hybrid membranes for two challenging and economically important applications: the removal of CO₂ and H₂S from natural gas and the separation of butane isomers. Notably, the membrane molecular sieving properties demonstrate that the deliberately regulated and contracted MOF pore-aperture size can discriminate between molecular pairs. The improved performance results from precise control of the linkers delimiting the triangular window, which is the sole entrance to the fcu-MOF pore. This rational-design hybrid approach provides a general toolbox for enhancing the transport properties of advanced membranes bearing molecular sieve fillers with sub-nanometre-sized pore-apertures.

Ultraselective Carbon Molecular Sieve Membranes with Tailored Synergistic Sorption Selective Properties

Membrane-based separations can reduce the energy consumption and the CO₂ footprint of large-scale fluid separations, which are traditionally practiced by energy-intensive thermally driven processes. Here, a new type of membrane structure based on nanoporous carbon is reported, which, according to this study, is best referred to as carbon/carbon mixed-matrix (CCMM) membranes. The CCMM membranes are formed by high-temperature (up to 900 °C) pyrolysis of polyimide precursor hollow-fiber membranes. Unprecedentedly high permselectivities are seen in CCMM membranes for CO₂ /CH₄ , N₂ /CH₄ , He/CH₄ , and H₂ /CH₄ separations. Analysis of permeation data suggests that the ultrahigh selectivities result from substantially increased sorption selectivities, which is hypothetically owing to the formation of ultraselective micropores that selectively exclude the bulkier CH₄ molecules. With tunable sorption selectivities, the CCMM membranes outperform flexible polymer membranes and traditional rigid molecular-sieve membranes. The capability to increase sorption selectivities is a powerful tool to leverage diffusion selectivities, and has opened the door to many challenging and economically important fluid separations that require ultrafine differentiation of closely sized molecules.