Bottom up approach to study the gas separation properties of PIM-PIs and its derived CMSMs by isomer monomers

High-Performance Carbon Capture with Fluorine-Tailored Carbon Molecular Sieve Membranes

Increasing energy consumption and climate change present an urgent global challenge to achieve carbon neutrality, with CO2 capture as a top priority. Among various carbon capture technologies, CO2 membrane separation stands out for its simplicity and energy efficiency in applications including gas purification and industrial gas recovery. Herein, a series of fluorine-tailored porous carbon molecular sieve (CMS) membranes derived from precisely designed precursors, achieving a well-balanced high permeability and selectivity for CO2 separation are developed. Incorporating bent terphenyl monomers and both aliphatic/aromatic trifluoromethyl groups disrupted dense chain packing and promoted pore formation with enhanced permeability and selectivity for CO2 separation. The TFM-550 membrane, derived from a fluorinated stretched polymer backbone precursor, exhibits exceptional performance with a CO2 permeability of 47 190 ± 3204 Barrer and a CO2/N2 selectivity of 28.3 ± 5.7, while TFM-800 presented a higher selectivity of 71.8 ± 11.5, surpassing the 2019 upper bound. Furthermore, under flue gas conditions (CO2/O2/N2 = 1/1/4 in molar ratio), the CMS membrane demonstrate high CO2 permeability of 36,204 ± 2,235 Barrer and outstanding CO2/N2 selectivity of 35.3 ± 1.8. The results here highlight the effectiveness of fluorine tailoring and the potential of fluorinated CMS membranes for sustainable industrial carbon capture applications.

Carbon Molecular Sieve Membranes from Acenaphthenequinone-Biphenyl Polymer; Synthesis, Characterization, and Effect on Gas Separation and Transport Properties

A rigid, high temperature-resistant aromatic polymer, poly(1,1'-biphenyl)-6,8a-dihydroacenaphthylene-1(2H)-one (BDA) comprising acenaphthenequinone and biphenyl was successfully synthesized by superacid catalyzed polymerization. BDA has a high decomposition temperature (Td = 520 °C) that renders it a viable candidate for carbon molecular sieve membranes (CMSM) formation. BDA precursor pyrolysis at 600 °C (BDA-P600) leads to a carbon turbostratic structure formation with graphene-like amorphous strands in a matrix with micropores and ultramicropores, resulting in a carbon structure with higher diffusion and higher selectivity than dense BDA. When the BDA pyrolysis temperature is raised to 700 °C (BDA-P700), the average stacking number of carbon layers N increases, along with an increase in the crystallite thickness stacking Lc, and layer plane size La, leading to a more compact structure. Pure gas permeability coefficients P are between 3 and 5 times larger for BDA-P600 compared to the BDA precursors. On the other hand, there is a P decrease between 10 and 50% for O2 and CO2 between CMSM BDA-P600 and BDA-P700, while the large kinetic diameter gases N2 and CH4 show a large decrease in permeability of 44 and 67%, respectively. It was found that the BDA-P700 WAXD results show the emergence of a new peak at 2θ = 43.6° (2.1 Å), which effectively hinders the diffusion of gases such O2, N2, and CH4. This behavior has been attributed to the formation of new micropores that become increasingly compact at higher pyrolysis temperatures. As a result, the CMSM derived from BDA precursors pyrolyzed at 700 °C (BDA-P700) show exceptional O2/N2 gas separation performance, significantly surpassing baseline trade-off limits.

Tuning Fluorination of Carbon Molecular Sieve Membranes with Enhanced Reverse-Selective Hydrogen Separation From Helium

Membrane technology has been explored for separating helium from hydrogen in natural gas reservoirs, a process that remains extremely challenging due to the sub-Ångstrom size difference between H2 and He molecules. Reverse-selective H2/He separation membranes offer multiple advantages over conventional helium-selective membranes, which, however, suffer from low H2/He selectivity. To address this hurdle, a novel approach is proposed to tune the ultra-micropores of carbon molecular sieves (CMS) membranes through fluorination of the polymer precursor. By incorporating -CF3 units into the backbone of Tröger's base polymers, the microporosity of CMS is tailored and reverse-selective H2/He CMS membranes are deployed with remarkable separation performance, surpassing most reported membranes. These CMS membranes exhibit a H2 permeability of 1505.2 Barrer with a notable H2/He selectivity of 3.8. Barometric sorption tests reveal preferential sorption of H2 over He in the fluorinated CMS membranes, which also demonstrate a significantly higher H2/He diffusion selectivity compared to unfluorinated samples. Material studio calculations indicate that the "slim" hydrogen molecule penetrates ultra-micropores more readily than the spherical He molecule, thus achieving reverse H2/He selectivity. This design approach offers a promising pathway for developing molecularly sieving membranes to tackle the challenging helium separation from natural gas.

Regioisomeric Spirobifluorene CANAL Ladder Polymers and Their Gas Separation Performance

We synthesized and characterized two isomeric microporous hydrocarbon ladder polymers from catalytic arene norbornene annulation (CANAL) of regioisomeric bis-norbornene fused spirobifluorenes, where the ladder chains are connected either through the same fluorene unit or across two different fluorene units in spirobifluorene. This pair of isomeric polymers was used to investigate the effect of ladder macromolecular structures on the microporosity and transport properties. Both polymers form mechanically intact films with thermal stability up to 480 °C and relatively high BET surface areas. The polymer formed from 2,7-dibromospirobifluorene showed higher BET surface area and higher gas permeability than the polymer from 2,2'-dibromospirobifluorene despite similar intersegmental spacing as indicated by X-ray scattering. The aging behavior for both polymers followed the same trend as the previously reported CANAL-fluorene polymers, with dramatically increased permselectivities over time, resulting in gas separation performance above the 2008 upper bounds for H2/CH4 and O2/N2.

Nanoarchitectonics of carbon molecular sieve membranes with graphene oxide and polyimide for hydrogen purification

Hydrogen is an important energy carrier for the transition to a carbon-neutral society, the efficient separation and purification of hydrogen from gaseous mixtures is a critical step for the implementation of a hydrogen economy. In this work, graphene oxide (GO) tuned polyimide carbon molecular sieve (CMS) membranes were prepared by carbonization, which show an attractive combination of high permeability, selectivity and stability. The gas sorption isotherms indicate that the gas sorption capability increases with the carbonization temperature and follows the order of PI–GO-1.0%-600 °C > PI–GO-1.0%-550 °C > PI–GO-1.0%-500 °C, more micropores would be created under higher temperatures under GO guidance. The synergistic GO guidance and subsequent carbonization of PI–GO-1.0% at 550 °C increased H₂ permeability from 958 to 7462 Barrer and H₂/N₂ selectivity from 14 to 117, superior to state-of-the-art polymeric materials and surpassing Robeson's upper bound line. As the carbonization temperature increased, the CMS membranes gradually changed from the turbostratic polymeric structure to a denser and more ordered graphite structure. Therefore, ultrahigh selectivities for H₂/CO₂ (17), H₂/N₂ (157), and H₂/CH₄ (243) gas pairs were achieved while maintaining moderate H₂ gas permeabilities. This research opens up new avenues for GO tuned CMS membranes with desirable molecular sieving ability for hydrogen purification.

Hydrogen is an important energy carrier for the transition to a carbon-neutral society, the CMS membrane exhibited ultrahigh H₂/N₂ selectivity (117) and H₂ permeability, which have bright prospects for hydrogen purification.