Carbon Molecular Sieve Membranes Research Articles

Hierarchically porous and single Zn atom-embedded carbon molecular sieves for H2 separations

Hierarchically porous materials containing sub-nm ultramicropores with molecular sieving abilities and microcavities with high gas diffusivity may realize energy-efficient membranes for gas separations. However, rationally designing and constructing such pores into large-area membranes enabling efficient H2 separations remains challenging. Here, we report the synthesis and utilization of hybrid carbon molecular sieve membranes with well-controlled nano- and micro-pores and single zinc atoms and clusters well-dispersed inside the nanopores via the carbonization of supramolecular mixed matrix materials containing amorphous and crystalline zeolitic imidazolate frameworks. Carbonization temperature is used to fine-tune pore sizes, achieving ultrahigh selectivity for H2/CO2 (130), H2/CH4 (2900), H2/N2 (880), and H2/C2H6 (7900) with stability against water vapor and physical aging during a continuous 120-h test.

Controlled organic solvent transport in ultramicroporous carbon membranes

The drive to decarbonize chemical processes necessitates exploring low-energy solutions for separating complex liquid mixtures ubiquitous in food, pharmaceuticals, petrochemical, and synthetic fuel production industries. This report comprehensively investigates conceptualizing and implementing two novel organic solvent separation processes: Organic Solvent Forward Osmosis (OSFO) and Organic Solvent Pressure Assisted Osmosis (OSPAO). The intrinsic molecular specificity of ultramicroporous carbon molecular sieve (CMS) membranes allowed the unprecedented separation of a complex mixture of short-chain alcohols (e.g., methyl alcohol, ethyl alcohol, and isopropyl alcohol) at room temperature. A new hybridized membrane-based separation methodology, OSPAO, that combines the elements of organic solvent nanofiltration (OSN) and OSFO is introduced. CMS-based OSPAO demonstrated a marked enhancement in the flux of organic species permeate, considerably improving separation efficiencies. The separation modalities presented in this report underscore the essential role of osmosis in the differentiation of organic solvents, thereby contributing valuable insights to osmosis-based separation technologies.

The influence of debromination and TR on the microstructure and properties of CMSMs

One unique challenging in achieving advanced carbon molecular sieve membranes (CMSMs) is tailoring their gas separation properties from the precursor. To clarify this, we designed two homo-polyimides with a bromine (PI-Br) and a hydroxyl (PI-OH) groups in the ortho position of imide group, and carbonized them into CMSMs at 550 °C (PI-Br-550 and PI-OH-550). There are debromination (510 °C) and carbonization occurred for PI-Br-550 whereas the PI-OH underwent thermally rearrangement (TR, 450 °C) and then pyrolysis to CMSM. After carbonization, both CMSMs showed excellent gas separation properties, with their CO2/CH4 separation performance surpassed the latest 2019 pure-gas and 2018 mixed-gas trade-off curves, additionally, the PI-Br-550 demonstrated a relatively smaller CO2 permeability (12462 vs 14,253 Barrer), lower PCO2 increment ratio (199 vs 1480), and less CO2/CH4 dropping ratio (69.7 to 45.5 vs 87.5 to 36.1) than the PI-OH-550 from their precursors. Their XPS, Raman, and WXRD characterization results indicated that debromination boosts the gas separation performance of CMSM by creating more ultra-micropore “plate” (pyrrolic-N of 36.0 % vs 19.0 %) and smaller d-spacing (3.89 vs 4.15 Å) than the TR precursor, which boosts the permeability of CMSM is due to the high stability of PBO intermediates that retarded the decomposition of the polymer main chian and caused some big pores. The above results provided a good route to get highly efficient CMSM by molecular designing of their precursors.

Controlling the curing process of novolac phenolic resin during preparation of carbon molecular sieve membrane for propylene/propane separation

A carbon molecular sieve (CMS) membrane was prepared for propylene/propane separation using a phenolic resin as a cost-effective polymer precursor. However, to achieve a desirable separation performance, the pre-treatment process of phenolic resin should be affordably carried out. In this study, novolac phenolic resin was cured with hexamine as a curing agent. The curing conditions were varied in terms of the curing time and concentration of the curing agent, both of which led to a higher degree of crosslinking of the phenolic resin, resulting in improved thermal stability. The propylene/propane separation factor of 6.33 for the CMS membrane derived from the phenolic resin cured for 1 h increased to 18.5 when the curing time was 4 h, whereas the propylene permeance decreased from 153 to 53.5 GPU. A similar trend was observed for higher concentrations of the curing agent. Moreover, the long-term stability of the CMS membrane was significantly improved after prolonged curing with a higher concentration of the curing agent. The CMS membranes derived from cured phenolic resins possess more selective carbon structures and better structural stabilities.

Carbon Molecular Sieve Membranes with Ultra‐Thin Selective Skin Layer by Pyrolysis of Porous Hollow Fibers (Small 20/2024)

Carbon Molecular Sieve Membranes with Ultra‐Thin Selective Skin Layer by Pyrolysis of Porous Hollow Fibers (Small 20/2024)

Mechanically stable polymer molecular sieve membranes with switchable functionality designed for high CO2 separation performance.

The development of high-performance membranes selective for carbon dioxide is critically important for advancing energy-efficient carbon dioxide capture technologies. Although molecular sieves have long been attractive membrane materials, turning them into practical membrane applications has been challenging. Here, we introduce an innovative approach for crafting a polymeric molecular sieve membrane to achieve outstanding carbon dioxide separation performance while upholding the mechanical stability. First, a polymer molecular sieve membrane having high gas permeability and mechanical stability was fabricated from a judiciously designed polymer that is solution-processable, hyper-cross-linkable, and functionalizable. Then, the carbon dioxide selectivity was fine-tuned by the subsequent introduction of various amine-based carriers. Among the diverse amines, polyethyleneimine stands out by functionalizing the larger pore region while preserving ultramicropores, leading to improved carbon dioxide/dinitrogen separation performance. The optimized membrane demonstrates exceptional carbon dioxide/dinitrogen separation performance, outperforming other reported polymer molecular sieve membranes and even competing favorably with most carbon molecular sieve membranes reported to date.

Hollow Fiber Carbon Molecular Sieve Membranes for Gas Separation: A Mini Review

Abstract: Owing to the advantages of rapid adsorption and desorption characteristics, excellent gas separation performance, as well as good thermal and chemical resistance, carbon molecular sieve (CMS) membranes have been developed as a promising gas separation tool. Over the past 30 years, hollow fiber carbon molecular sieve (HFCMS) membranes have become the preferred choice for industrial applications due to their high surface area-to-volume ratio and the ability to assemble lightweight membrane modules. The gas transport mechanism behind the HFCMS is dominated by molecular sieving function. They can be prepared by pyrolysis of the polymeric hollow fiber precursors. Post-treatments can tailor the ultramicropores structure to improve the separation performance. This paper aims to review the recent progress in the preparation of HFCMS membranes from aspects of precursor selection, pyrolysis conditions and post-treatment. Moreover, a brief perspective in terms of future investigation of HFCMS membrane is also proposed.

Carbon Molecular Sieve Membranes with Ultra-Thin Selective Skin Layer by Pyrolysis of Porous Hollow Fibers.

Translating carbon molecular sieve (CMS) membranes into highly scalable hollow fiber geometry with ultra-thin selective layer (<1µm) for gas separation remains as great challenge. The porous support layer of precursor hollow fiber membranes is prone to collapse during pyrolysis, which induces thick skin layer (15-50µm) of CMS hollow fiber membranes. Here, a novel strategy is present to obtain an ultra-thin selective skin layer by carbonization of hollow fiber membranes with porous skin. P84-based defect-free CMS hollow fiber membranes with ultra-thin selective skin layer (0.9µm) for gas separation are prepared without any coating or complex chemical pretreatment. Compared with the carbon membranes derived from defect-free fibers, the H2 permeance (93.9 GPU) of CMS membranes derived from the porous fibers increases ≈1353% with comparable selectivity of H2/CH4 (143) and higher H2/N2 (120). Furthermore, the porous fibers are pre-aged near the Tg in N2 conditions before carbonization, and the H2 permeance of the derived CMS hollow fiber membranes reached 147 GPU (increased 2180%). It is a new facile way to prepare CMS hollow fiber membranes with ultra-thin selective layer by porous fibers, demonstrating its versatile potential in gas separation or organic liquids separation.

In-situ crosslinking of a novel conjugated dialkynyl-based polyimide for high performance carbon molecular sieve membrane

High-performance and reliable membranes for efficient gas separation and their design strategies are key points for membrane-based carbon capture. Here, we reported a high-performance carbon membrane design protocol from a microporous, highly rigid polyimide precursor (6FBD1-CR), which was produced by in-situ crosslinking a polyimide (6FBD1) containing conjugated dialkynyl group at 300 °C. The possible crosslinking mechanisms are the [2+2+2] trimerization of three ethynyl groups to form a benzene ring and the Diels-Alder reaction between the ethynyl bond and phenylacetylene to generate the fused naphthalene ring. After crosslinking, the membrane shows a ∼ 8-fold improved microporosity (37.1–298 m2 g−1). When pyrolyzed at 550 °C, the resulting 6FBD1-CMS demonstrates very high pure- and mixed-gas separation performance. E.g., the CO2 permeability reaches 7661 Barrer and CO2/CH4 selectivity of 44, additionally, even at the CO2/CH4 mixed-gas pressure of 30 bar, the PCO2 can still reach 4920 Barrer combined with a αCO2/CH4 of 28, which is well above the latest CO2/CH4 mixed-gas Upper Bound line. We consider this is due to the acetylene crosslinking induced a very stable microporous structure composed of rigid fused rings that alleviate the micropore collapse during the carbonization process, and thus, resulting in a relatively higher BET surface area, larger d-spacing, relatively higher concentration of continuous disordered phase (higher sp3/sp2) and small activation energy (Ep) for gas transport than the CMSM derived from polyimide without pre-crosslinking. This finding provides new opportunity for the structural design of high-performance CMSMs.

Field-Scale Testing of a High-Efficiency Membrane Reactor (MR)-Adsorptive Reactor (AR) Process for H2 Generation and Pre-Combustion CO2 Capture.

The study objective was to field-validate the technical feasibility of a membrane- and adsorption-enhanced water gas shift reaction process employing a carbon molecular sieve membrane (CMSM)-based membrane reactor (MR) followed by an adsorptive reactor (AR) for pre-combustion CO2 capture. The project was carried out in two different phases. In Phase I, the field-scale experimental MR-AR system was designed and constructed, the membranes, and adsorbents were prepared, and the unit was tested with simulated syngas to validate functionality. In Phase II, the unit was installed at the test site, field-tested using real syngas, and a technoeconomic analysis (TEA) of the technology was completed. All project milestones were met. Specifically, (i) high-performance CMSMs were prepared meeting the target H2 permeance (>1 m3/(m2.hbar) and H2/CO selectivity of >80 at temperatures of up to 300 °C and pressures of up to 25 bar with a <10% performance decline over the testing period; (ii) pelletized adsorbents were prepared for use in relevant conditions (250 °C < T < 450 °C, pressures up to 25 bar) with a working capacity of >2.5 wt.% and an attrition rate of <0.2; (iii) TEA showed that the MR-AR technology met the CO2 capture goals of 95% CO2 purity at a cost of electricity (COE) 30% less than baseline approaches.

A new segment-level mixing strategy to improve the gas separation performances of carbon molecular sieve membrane derived from polymer blends

A novel mixing strategy for optimizing the structures and gas separation properties of carbon molecular sieve membranes (CMSMs) through the precise segment-level mixing of polyimide-co-siloxane (PSI) blends is proposed. PSI copolymers with different contents of siloxane were synthesized and blended, denoted as PSI-0, PSI-1, PSI-2, and PSI-3 based on the molar ratio of DABA:siloxane, 10:0, 9:1, 8:2, and 7:3, respectively. This study focuses on unraveling the intricate interplay between repeating segment distribution and resulting carbonization structures, with a specific emphasis on the gas separation performances of CMSMs. Of particular interest is the PSI-0:PSI-3 (1:2) blend, which, despite containing an identical siloxane amount to neat PSI-2, yields a CMSM with comparable hydrogen permeability and significantly enhanced selectivity (232 % H2/CH4 and 184 % H2/N2 selectivity). These findings underscore the critical role of segment distribution in achieving superior gas separation performances, presenting a promising avenue for the precise engineering of polymer precursor structures to optimize CMSM properties.

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.

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

AbstractNanoparticles 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.

Carbon Molecular Sieve Membranes from Multiring Highly Aromatic Functional Copolymers

Carbon Molecular Sieve Membranes from Multiring Highly Aromatic Functional Copolymers

Enhancing gas separation performance of polyimide with Tröger’s bases: Unveiling the impact on polymer and carbon molecular sieve membranes

The potential of carbon molecular sieve membranes (CMSM) for gas separation is widely recognized, and the key precursor for these membranes is polyimide (PI). This study focuses on enhancing the gas separation properties of CMSMs by introducing a unique structural element, Tröger’s bases, into polyimides, resulting in the creation of five distinct polyimides containing Tröger’s bases (PI-TBs). The incorporation of Tröger’s bases has a profound impact on the arrangement of polymer chains within the PIs, leading to a substantial improvement in gas permeability without compromising selectivity. Following carbonization at 600 °C, the resultant CMSMs exhibit significantly enhanced gas permeability while preserving, and in some cases, improving gas selectivity. Among the various types of diamines considered, the 6FDA variant of PI-TB stands out, boasting outstanding gas permeability with a H2 permeability of 4991.0 Barrer and a H2/CH4 selectivity of 30.7; this level of performance surpasses the 2015 Robeson’s upper bound. Furthermore, thermal analysis coupled with mass spectrometry (TG-MS) reveals that the pyrolysis of Tröger’s bases commences prior to that of polyimide. This sequential degradation process facilitates the formation of pyridine N and promotes the π-stacking of PI chains; both of which are instrumental in maintaining excellent gas selectivity. These findings provide valuable insights into the roles of Tröger’s bases in enhancing the gas separation performance of PIs and CMSMs, offering a promising avenue for further advancements.

Techno-economic analysis of carbon molecular sieve membranes to produce oxygen enrichment air

Oxygen Enriched Air (OEA) is essential for medical applications, metal production, chemical production, the petrochemical industry, and energy processes. In this work, carbon molecular sieve membranes (CMSM) and membrane systems are analyzed concerning their technical and economic feasibility to produce OEA for medical, post-combustion, and oxy-combustion applications. Process parameters, such as stage-cut, feed-to-permeate pressure ratio, membrane area, and the number of stages, were studied to assess the technical and economic feasibility of this technology. In this work, it was observed that for each membrane, there is a permeating oxygen concentration that minimizes the specific oxygen cost. The production of 56 tonO2·day−1 for a combustion enhancement process with a concentration of 72 % was obtained in a single-stage with a specific cost of 61.0 €·tonO2−1. A single-stage is also effective for producing OEA for medical applications (required concentration of 82 %) at a minimum separation cost of 45.8 €·tonO2−1. However, a two-stage system is required to produce OEA with a concentration higher than 95 % for oxy-combustion with an OEA production cost of 83.8 €·tonO2−1. These production costs are lower than those obtained with two different commercial membranes and are competitive with medium-scale cryogenic distillation and pressure swing adsorption processes.

Influence of the coagulation bath on the nanostructure of cellulose films regenerated from an ionic liquid solution.

Cellulose membranes were prepared from an EMIMAc ionic liquid solution by nonsolvent-induced phase separation (NIPS) in coagulation baths of water-acetone mixtures, ethanol-water mixtures and water at different temperatures. High water volume fractions in the coagulation bath result in a highly reproducible gel-like structure with inhomogeneities observed by small-angle neutron scattering (SANS). A structural transition of cellulose takes place in water-acetone baths at very low water volume fractions, while a higher water bath temperature increases the size of inhomogeneities in the gel-like structure. These findings demonstrate the value of SANS for characterising and understanding the structure of regenerated cellulose films in their wet state. Such insights can improve the engineering and structural tuning of cellulose membranes, either for direct use or as precursors for carbon molecular sieve membranes.

Breaking the Permeability-Selectivity Trade-Off: Advanced carbon molecular sieve membranes derived from thermally rearranged Mixed-Matrix membrane precursors

A crucial problem in membrane-based gas separation is to fabricate membranes with excellent permeability, selectivity, and stability. Here, a high-performance carbon molecular sieves membrane of TR-CMS/SSZ-13 was successfully produced by pyrolyzing a thermally rearranged (TR) mixed matrix membrane (MMM) that obtained from a copolyimide precursor (6FDA/ODA/6FAP(3:2)) and SSZ-13 zeolite as filler at 450 °C. The effect of SSZ-13 zeolite content and structures of precursors on the transport behavior of derived CMS membranes was thoroughly studied. After TR process, the as formed polybenzoxazole (PBO) chain resulted in an enhanced rigidity, higher fractional free volume (0.15 vs 0.17), and a significantly increased (1898 times) BET surface area. Further enhancing the treatment temperature to 650 °C, the formed TR-CMS membranes exhibited a huge improved BET surface area from 228.2 to 639.3 m2/g, delivering a remarkable gas separation performance with H2 permeability of 7089 Barrer and H2/CH4 selectivity of 136. When doping with SSZ-13 till 15 wt%, the TR-CMS/SSZ-13–15 % showed an even higher H2 (9726 Barrer) and CO2 permeability (6932 Barrer), as well as H2/CH4 (150 vs 136) and CO2/CH4 (107 vs 57) selectivity than the pure TR-CMS, which surpassed the newest 2019 Robeson Upper bound. This can associate with the strong interaction between the modified SSZ-13 and the polymer matrix that prevent the collapse of the aromatic strand during the pyrolysis process, thereby create more ultra-micropore volume and concentration. Conclusively, pyrolyzing TR-MMM precursor is an effective way to form high-performance membranes with good potential for CO2 separation and H2 purification.

Directly measured pyrolysis temperature dependence of sorption selectivity for carbon dioxide/methane in Matrimid® polyimide-derived carbon molecular sieve membranes

In this study, we augment our analysis of pyrolysis temperature dependence of carbon molecular sieve (CMS) membranes derived from a polyimide, Matrimid® useful for advanced CMS membranes. Our direct sorption results prove that the strong pyrolysis temperature dependence of permselectivity for the CO2/CH4 gas pair is promoted by an unusual trend in sorption selectivity reported here. This trend complements conventional diffusion selectivity trends for this pair with increasing pyrolysis temperatures. Our results show that the elimination of pyridinic and pyrrolic groups over the same pyrolysis temperature range occurs. Elimination of such CO2-philic pyridinic and pyrrolic groups typically would be expected to cause a trend opposite to that actually measured directly for CO2/CH4 selectivity. The resulting trend for pyrolysis temperatures between 550 °C and 900 °C, however, can be explained in terms of microporous and ultramicroporous properties of the CMS for increasing pyrolysis temperatures. Specifically, apparent thermodynamic exclusion selectivity of the 0.5 Å larger CH4 vs. the slim 3.3 Å CO2 provides a consistent explanation of the trends. This discovery provides additional options to tune permselectivity of CMS membranes to complement molecular sieving based on diffusion contributions.

High-performance hydrogen separation using cellulose-based carbon molecular sieve membranes

Cellulose-based carbon molecular sieve membranes were designed for hydrogen deblending from natural gas and hydrogen recovery from other relevant industrial sources. The crystallinity of the cellulose precursor was increased by adding propylene glycol, which originated a carbon membrane with a permeability to the hydrogen of 544 barrer and an H2/CH4 permselectivity of ca. 3500 (Robeson index = 100). A careful optimization of the carbonization conditions of the cellulose precursor boosted the carbon membrane permeability to hydrogen to 1144 barrer and the H2/CH4 permselectivity to 1080 (Robeson index = 62). The membrane with the highest permeability to hydrogen presented the highest micropore volume of 0.443 cm3·g−1 and a micropore surface area of 1326 m2·g−1.A detailed techno-economic analysis of the hydrogen recovery from the natural gas was performed. It was concluded that a single-stage carbon membrane module can recover hydrogen with a concentration of 99.8 % and a specific cost of 0.26 €·kgH2−1 for a hydrogen recovery of 68 %. A two-stage membrane process was simulated to deliver a purity >99.997 % and a recovery of 96 %, with a specific cost of 1.8 €·kgH2−1. The carbon membranes prepared in this work are highly competitive with other separation processes for the recovery and purification of hydrogen from different industrial processes essential for the decarbonization.