Investigation of porosity of carbon materials and related effects on gas separation properties

Carbon molecular sieving membranes are chemically robust materials with tailorable gas transport properties for O₂/N₂, CO₂/CH₄ and C₃H₆/C₃H₈ separations. Such carbon materials were formed in this study by the pyrolysis of polyimide precursors. The final pyrolysis temperature was varied to alter the carbon structure, which changed the average pore size. Characterization of the porosity of these materials and how this feature changes when pyrolysis conditions are varied could guide the systematic control of these materials. However, the carbon is an amorphous, microporous material, which makes it difficult to characterize compared to crystalline materials. From separation studies of penetrants on these materials it appears that these materials have both ultramicropores (<7 Å) and larger micropores. The ultramicropores are believed to be mainly responsible for molecular sieving while the micropores provide negligible resistance to diffusion but provide high capacity sorption sites for penetrants. Techniques such as wide angle X-ray diffraction and the analysis of carbon dioxide adsorption isotherms using density functional theory were employed to characterize the microporosity of the material. The small dimensions of the key ultramicropores make accurate determination of their pore size distribution difficult. Therefore, to effectively discuss the differences in transport properties when different pyrolysis temperatures are used as well as penetrants with different dimensions, a hypothetical ultramicropore size distribution was used as a tool to discuss and interpret a combination of parameter effects and trends of separation properties.     

Synthesis and characterization of nanostructured carbons with controlled porosity prepared from sulfonated divinylbiphenyl copolymers

A series of porous carbons have been prepared by the carbonization of spherical porous sulfonated divinylbiphenyl (DVBPh) copolymers. Carbons in spherical bead form were obtained by the pyrolysis of H⁺, Na⁺, Cs⁺, Cu²⁺, Co²⁺ and Fe³⁺ forms of the sulfonated DVBPh beads. Thermogravimetric analysis (TGA) in an inert nitrogen atmosphere (25-900 °C) was carried out on the DVBPh copolymer precursor, the sulfonated copolymer sample and various ionic forms of the resin. The TGA data provides evidence that the sulfonation process thermally stabilized the polymer resulting in a higher final carbon yield. It was found that the pyrolysis yield was ca. 40% for the sulfonic acid derivative and between 40% and 65% for the various sulfonic acid salts. The highest yield was observed for the monovalent sodium and cesium ionic forms of the sulfonated DVBPh copolymers. Low temperature nitrogen adsorption/desorption isotherms provided information on the porous structure of the polymer precursors and the carbons prepared from them. The pore structure in the carbons was found to a large extent to be similar to the porous structure of the starting sulfonated resin material, however, the metal form was found to impact on the micropore structure of the resulting carbons. The carbon materials prepared were characterized by X-ray photoelectron spectroscopy (XPS) to provide information on the form of the residual sulfur in the carbons. XPS results suggest that the ionic form of the sulfonic resin influences the amount and the form of the sulfur and this may be correlated with the yield of the final carbon.     

Carbon molecular sieve dense film membranes derived from Matrimid® for ethylene/ethane separation

Development of dense film carbon molecular sieve (CMS) membranes for ethylene/ethane (C₂H₄/C₂H₆) separation is reported. A commercial polyimide, Matrimid®, was pyrolyzed under vacuum and inert argon atmosphere, and the resultant CMS films were characterized using pure C₂H₄ and C₂H₆ permeation at 35 °C, 50 psia feed pressure. The effects on C₂H₄/C₂H₆ separation caused by different final vacuum pyrolysis temperatures from 500 to 800 °C are reported. For all pyrolysis temperatures separation surpassed the estimated ‘upper bound’ solution processable polymer line for C₂H₄ permeability vs. C₂H₄/C₂H₆ selectivity. C₂H₄ permeability decreased and selectivity increased with increasing pyrolysis temperature until 650–675 °C where an optimum combination of C₂H₄ permeability ～14–15 Barrer with C₂H₄/C₂H₆ selectivity ～12 was observed. A modified heating rate protocol for 675 °C showed further increase in permeability with no selectivity loss. CMS films produced from argon pyrolysis showed results comparable to vacuum pyrolysis. Further, mixed gas (63.2 mol% C₂H₄ + 36.8 mol% C₂H₆) permeation showed a slightly lower C₂H₄ permeability with C₂H₄/C₂H₆ selectivity increase rather than a decrease that is often seen with polymers. The high selectivity of these membranes was shown to arise from a high ‘entropic selection’ indicating that the ‘slimmer’ ethylene molecule has significant advantage over ethane in passing through the rigid ‘slit-shaped’ CMS pore structure.     

Physical and chemical properties of carbons synthesized from xylan, cellulose, and Kraft lignin by H3PO4 activation

Physical and chemical properties of activated carbons produced from commercial xylan, cellulose, and Kraft lignin by H₃PO₄ activation at various process conditions were studied. The results show that the more reactive the precursor under acidic conditions, the easier the porosity development, particularly mesoporosity. In addition, Boehm titration and Fourier Transform Infrared Spectroscopy (FTIR) characterization results demonstrated that the functional groups on the surfaces of these carbons consist of both temperature-sensitive and temperature-insensitive components. The temperature-sensitive component is primarily caused by the hydrolysis of raw materials under acidic conditions at low temperature, and the reaction between activation mixture and oxygen in the process of activation, particularly at low impregnation ratio. These surface groups decompose at high temperature. The temperature-insensitive contribution is mainly composed of phosphorus-containing groups arising from the reaction of H₃PO₄ (or pyro- and polyphosphoric acids) with precursor, and carbonyl-containing groups. This part of surface functional group is stable, even at high activation temperatures. This study also confirmed that the nature of precursor, impregnation ratio between H₃PO₄ and precursor, and activation temperature are important factors affecting the properties of final activated carbon products.     

Carbon molecular sieve membranes derived from Matrimid® polyimide for nitrogen/methane separation

A commercial polyimide, Matrimid® 5218, was pyrolyzed under an inert argon atmosphere to produce carbon molecular sieve (CMS) dense film membranes for nitrogen/methane separation. The resulting CMS dense film separation performance was evaluated using both pure and mixed N₂/CH₄ permeation tests. The effects of final pyrolysis temperature on N₂/CH₄ separation are reported. The separation performance of all CMS dense films significantly exceeds the polymer precursor dense film. The CMS dense film pyrolyzed at 800 °C shows very attractive separation performance that surpasses the polymer membrane upper bound line, with N₂ permeability of 6.8 Barrers and N₂/CH₄ permselectivity of 7.7 from pure gas permeation, and N₂ permeability of 5.2 Barrers and N₂/CH₄ permselectivity of 6.0 from mixed gas permeation. The temperature dependences of permeabilities, sorption coefficients, and diffusion coefficients of the membrane were studied, and the activation energy for permeation and diffusion, as well as the apparent heats of sorption are reported. The high permselectivity of this dense film is shown to arise from a significant entropic contribution in the diffusion selectivity. The study shows that the rigid ‘slit-shaped’ CMS pore structure can enable a strong molecular sieving effect to effectively distinguish the size and shape difference between N₂ and CH₄.     

A review of the control of pore structure in MgO-templated nanoporous carbons

Nanoporous carbons with a high surface area were directly prepared from various carbon precursors without any stabilization and activation processes. Various carbon precursors, including poly(vinyl alcohol), poly(ethylene terephthalate), polyimide, coal tar pitch, were used and MgO itself, Mg acetate, Mg citrate, Mg gluconate and Mg hydroxy-carbonate were employed as MgO precursor. Carbon precursor was mixed with MgO precursor in different ratios either in powder (powder mixing) or in solution (solution mixing), and heat-treated at 900 °C in inert atmosphere. MgO formed in the carbonization products was dissolved out using a diluted acid. BET surface area of the carbons obtained could be reached to high value, as high as 2000 m²/g, even though any activation process was not applied. Most carbons prepared through this method were rich in mesopores. Size of mesopores in the resultant carbons was tunable by selecting MgO precursor and relative volume between mesopores and micropores was controlled by carbon precursor.     

Preparation and properties of catalyzed polyimide/dicyanate semi‐interpenetrating networks for polymer gas membrane with suppressed CO2‐plasticization

The work presents an approach to reduce the plasticization of polymeric membranes caused by condensable gases, and particularly the effect of plasticization caused on polyimides by CO₂ at high pressure. A technical polyimide, Matrimid^®, was chosen as a reference of polyimide membrane and the approach applied consisted of incorporating reactive oligomers to have cross‐linkable mixed systems, which do not plasticize at high CO₂ pressure. Films of semi‐interpenetrating networks (semi‐IPNs) based on Matrimid^® and phenolphthalein dicyanate as cross‐linking monomer in ratios 90/10, 80/20, and 70/30, were prepared using a catalyst to lower the curing temperature from 280 to 180°C. Semi‐IPNs properties such as thermal stability, mechanical properties, glass transition temperatures, or density were measured to characterize the films and were correlated with the dicyanate monomer content. The CO₂ gas permeation behavior of the three semi‐IPNs was studied using a CO₂ feed pressure ranging from 1 to 30 atm. The gas separation properties were mainly explained attending to the density of the films, which depended on the dicyanate content used. In the three catalyzed semi‐IPNs, the CO₂ permeability coefficients remained almost constant all along the investigated range of CO₂ pressure while Matrimid^® treated at 180°C did show a clear tendency to plasticization over a critical feed pressure of about 17 bar. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Polyimides cross‐linked with amino group‐containing compounds

The commercially available linear polyimide Matrimid^® 9725 was crosslinked with amino groups containing both high‐molecular‐weight and low‐molecular‐weight compounds. The multi‐functional amine‐terminated hyperbranched polyimide precursor (hyperbranched polyamic acid), based on 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride and 4,4′,4″‐triaminotriphenylmethane, and its fully imidized form (amine‐terminated hyperbranched polyimide), bifunctional amine, 4,4′‐diaminodiphenylamine and trifunctional amine, 4,4′,4″‐triaminotriphenylamine, were used as the crosslinkers. Theoretically, 10% or 20% of the Matrimid imide groups was reacted with the amino groups of the crosslinking agent during the formation of the amide groups. The insoluble content (gel) in the final materials was very low at the crosslinking temperature of 80°C and was in the 55–90% range at the crosslinking temperature of 200°. The permeability coefficients of hydrogen, carbon dioxide and methane in the self‐standing, mechanically tough film (membrane) based on the combination of Matrimid and hyperbranched polyimide were approximately 30–45% higher compared with those in the membrane made of pure Matrimid at a comparable separating ability (selectivity). POLYM. ENG. SCI., 57:1367–1373, 2017. © 2017 Society of Plastics Engineers     

Enhanced propylene/propane separation by carbonaceous membrane derived from poly (aryl ether ketone)/2,6-bis(4-azidobenzylidene)-4-methyl-cyclohexanone interpenetrating network

An innovative combination of a photosensitive crosslinker, 2,6-bis(4-azidobenzylidene)-4-methyl-cyclohexanone (Azide) with poly (aryl ether ketone) (PAEK) is utilized to form a semi-interpenetrating network (IPN) as the precursor for carbon membranes. Low temperature pyrolysis (450-650 °C) of this precursor produces carbon membranes with excellent olefin/paraffin separation performance that surpasses the conventional trade-off line. The carbon membranes have reasonably good flexibility since excessive closure of the micropores is avoided. This is evident from the mechanical properties of the carbon membranes obtained from nanoindention to the pore size distribution derived from CO₂ adsorption. By varying the composition of Azide/PAEK and optimizing the low-temperature pyrolysis protocol, it was found that PAEK/Azide (80:20) pyrolysed at 550 °C exhibits the best propane/propylene separation performance with C₃H₆ permeability of 48 barrer and ideal C₃H₆/C₃H₈ selectivity of 44. Due to strong competitive sorption of propane and propylene molecules, the C₃H₆ permeability is lowered to 3.6 barrer and the C₃H₆/C₃H₈ selectivity to 32 in mixed gas experiments. However, this separation performance is still above the trade-off line. Even though both Azide and PAEK cannot form useful carbon membranes, their IPN is a unique precursor that can produce carbon membranes with comparable performance.     

Synthesis of Fe/Fe3C nanoparticles encapsulated in nitrogen-doped carbon with single-source molecular precursor for the oxygen reduction reaction

Ammonium ferric citrate (AFC) was used as a single-source molecular precursor to prepare Fe/Fe₃C nanoparticles encapsulated in nitrogen-doped carbon by pyrolysis in Ar atmosphere followed by acid-leaching. Comparative studies, using citric acid and ferric citrate as the precursors, indicated that the ammonia and ferric ion in AFC and the pyrolysis temperature affected the composition of iron species and the properties of carbon in AFC-derived materials. Above the pyrolysis temperature of 600 °C, the iron species were Fe/Fe₃C, and the carbon had a hollow graphitic nanoshell structure in AFC-derived materials. The specific surface area and content of nitrogen element decreased with increasing pyrolysis temperature. The AFC-derived material pyrolyzed at 600 °C had the optimal graphitization degree, specific surface area (489 m² g⁻¹) and content of nitrogen (1.8 wt.%), thus resulted in the greatest activity for oxygen reduction reaction among the AFC-derived materials pyrolyzed at different temperatures. The AFC-derived material pyrolyzed at 600 °C exhibited improved methanol-resistance ability compared with Pt/C catalyst.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

Quantitative gas evolution kinetics of coal primary pyrolysis at high heating rates is critical for developing predictive coal pyrolysis models. This study aims to investigate the gaseous species evolution kinetics of a low rank coal and a subbituminous coal during pyrolysis at a heating rate of 1000 °C s^− 1 and pressures up to 50 bar using a wire mesh reactor. The main gaseous species, including H₂, CO, CO₂, and light hydrocarbons CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, were quantified using high sensitivity gas chromatography. It was found that the yields of gaseous species increased with increasing pyrolysis temperature up to 1100 °C. The low rank coal generated more CO and CO₂ than the subbituminous coal under similar pyrolysis conditions. Pyrolysis of the low rank coal at 50 bar produced more gas than at atmospheric pressure, especially CO₂, indicating that the tar precursor had undergone thermal cracking during pyrolysis at the elevated pressure.     

Hydrothermal and conventional H3PO4 activation of two natural bio-fibers

Two phosphoric acid activation procedures; Activation after Hydrothermal Impregnation (recently published) and Activation after Incipient Wetness Impregnation instead of conventional impregnation are analyzed in two natural bio-fiber precursors: banana pseudostem and coconut fiber matting. Both procedures are compared analyzing, in both precursors, the influence that variables such as H₃PO₄/precursor ratio, activation temperature and impregnation time have on the resulting activated carbons (ACs) properties. The work also pays special attention to the mesoporosity development and the application of these ACs to adsorb gasoline vapors.Both H₃PO₄ activation procedures develop activated carbons having suitable activation yields and porosity developments, giving the Activation after Incipient Wetness Impregnation method better results than the Activation after Hydrothermal Impregnation. Both natural bio-fibers are good precursors, rendering the coconut fiber matting better results than the banana pseudostem. The variables studied affect the porosity development, being precursor and H₃PO₄/precursor ratio the variables that most affect. By a suitable selection of these variables, activated carbons having high adsorption capacities (BET above 2500 m² g^?1 and micropore volume above 1.00 cm³ g^?1) and well developed mesoporosity (reaching 1.41 cm³ g^?1), can be prepared. Most of the samples prepared perform very well for adsorbing gasoline vapors, showing a linear relationship with their resulting volumes.     

Exploiting pyrolysis protocols on BTDA-TDI/MDI (P84) polyimide/nanocrystalline cellulose carbon membrane for gas separations

Tubular carbon membranes were fabricated by the blending of BTDA-TDI/MDI (P84) polyimide with nanocrystalline cellulose in a controlled pyrolysis process, specifically the pyrolysis environment (He, Ar, and N₂) and the thermal soak time (30–120 min). The carbon membrane layer on a tubular support is converted to carbon matrix at 800 °C with a heating rate of 3 °C min⁻¹. The effects of these controlled pyrolysis conditions on the gas permeation properties have been investigated. The results revealed that the pyrolysis under Ar gas environment at 120 min of thermal soak time have the best gas permeation performance with the highest CO₂/CH₄ selectivity of 68.2 ± 3.3 and CO₂ permeance of 213.6 ± 2.2 GPU. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46901.     

Modeling particle inflation from poly(amic acid) powdered precursors. II. Morphological development during bubble growth

The morphological development of cellular polyimide microstructures from poly(amic acid) powders has been shown to depend on the processing conditions throughout the inflation process and the morphological characteristics of the precursor particles. In an earlier publication the authors presented a numerical study of the preliminary stages prior to particle inflation when the processing temperature is below the glass transition temperature, T_g. In the present article, a second numerical scheme is presented for behavior above T_g in which bubble growth is modeled to account for the effect of multiple phenomena in the final stages of morphological development. The bubble growth kinematics and subsequent cessation of growth are predicted as a function of process parameters and material properties. Morphological characteristics of the precursor particles have also been shown to influence the kinematics of inflation. These results provide a clearer understanding of the solid‐state foaming processes for polyimide cellular materials. POLYM. ENG. SCI., 47:572–581, 2007. © 2007 Society of Plastics Engineers.     

Investigation of direct methanol fuel cell performance of sulfonated polyimide membrane

Sulfonated polyimide (SPI) membranes have been evaluated as electrolyte membranes in direct methanol fuel cells (DMFCs). The membrane-electrode assembly (MEA) was made by hot-pressing the membrane, an anode and a cathode, catalyzed with PtRu/CB (PtRu dispersed on carbon black) and Pt/CB bound with Nafion^® ionomer, respectively. The performance of the cell based on SPI was compared with that of Nafion^® 112 in various operation conditions such as cell temperature (T_cell), cathode relative humidity (RH), and methanol concentration (C_MeOH). The methanol crossover at the cell based on SPI was a half of Nafion^® 112, resulting in the improved cell efficiency. Advantage of the use of SPI became much distinctive from the conventional Nafion^® 112 when the DMFC was operated at a higher T_cell or a higher C_MeOH.     

Identification and spectral properties of PAHs in carbonaceous material produced by laser pyrolysis

Carbonaceous materials have been prepared by laser-induced pyrolysis of a mixture of hydrocarbons (C₂H₂, C₂H₄, and benzene) under different conditions. We have investigated the soluble and insoluble part of the condensed carbon powders with several techniques, such as UV/VIS and IR spectroscopy, mass spectroscopy, gas chromatography combined with mass and IR spectrometry and high-performance liquid chromatography (HPLC) in order to obtain information on the total content and composition of the extracted soluble part and on the influence of the soluble component on the spectroscopic properties of the condensed carbon nanopowder. It has been found that the extract contains more than 64 different polycyclic aromatic hydrocarbons (PAHs). The most abundant PAH molecules are those containing 3–5 rings. The total amount of aromatic soluble components depends on the temperature in the condensation zone whereas the ratio between high- and low-mass PAHs is influenced by both, the temperature and the precursor gases. IR spectroscopic investigations of the extract have shown partial hydrogenation of PAHs leading to the formation of CH₂ groups, at the edge of PAH molecules. The IR spectral properties of the carbonaceous materials and of the PAHs are influenced by the adsorption process.     

One-step synthesis of in situ nanocarbon-containing calcium aluminate cement in reducing atmosphere

In order to overcome the problems caused by poor water wettability/dispersion and oxidation resistance of carbons for improving properties of carbon-containing refractory castables, we prepare in situ nanocarbon-containing cement through the carbon-bed sintering method with calcium citrate tetrahydrate (CCT, C₁₂H₁₀Ca₃O₁₄ · 4H₂O) and Al₂O₃ as raw materials, and FeCl₃ as a catalyst. In this paper, the carbonization process of CCT was analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and thermogravimetric analysis coupled with a FTIR. In addition, the synthesized cement was characterized through XRD, Raman spectroscopy, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy. The results show that the in situ carbon was generated through carbonization of calcium aconitic acid which was generated by pyrolysis of CCT, accompanying with overflowing of H₂O, CO₂, CO, and CH₄. Interestingly, as the sintering temperature increases, the freshly generated carbons were transformed into nanofiber and nanoflake carbons under the action of Fe catalyst. After sintering at 1500°C for 4 hours, the phase compositions of the synthesized product approached to that of the commercial cement Secar71, and the in situ carbons with nanofiber and nanoflake morphologies dispersed in calcium aluminate grains.     

One-step purification of ethylene from acetylene and carbon dioxide by ultramicroporous carbons

The one-step adsorptive separation of high-purity ethylene (C₂H₄) from a ternary gas mixture (C₂H₂/C₂H₄/CO₂) is challenging and has not been reported on porous carbons. Herein, we report camphor seeds husk-derived ultramicroporous carbons (CSHs) show high affinities toward acetylene (C₂H₂) and carbon dioxide (CO₂) over C₂H₄. The optimized CSH-2-700 with high heteroatom contents and centered pore size distributions shows high C₂H₂ adsorption capacity (2.24 mmol g⁻¹) and record ideal adsorbed solution theory (IAST) C₂H₂/C₂H₄ selectivity (10.2) among one-step C₂H₄ purification adsorbents. Meanwhile, CSH carbons are the only carbon adsorbents that preferentially adsorb CO₂ over C₂H₄, with a CO₂/C₂H₄ selectivity of 1.9 under ambient conditions. Furthermore, dynamic breakthrough experiments verified its feasibility for one-step C₂H₄ purification from a three-component C₂H₂/C₂H₄/CO₂ gas-mixture.     

Chemical Production of Activated Carbon from Nutshells and Date Stones

The effect of chemical reagent (H₃PO₄, KOH, and NaOH), temperature (400 °C, 475 °C, 550 °C), and impregnation ratio (100 %, 150 %, 200 %) was investigated on the specific surface area and iodine uptake of the carbons produced from almond, walnut, and pistachio‐nut shells and date stones. The effect of mesh size and holding time was also studied in the case of almond shell. While the alkali activation of the precursors resulted in such fine powders that purifying them of contaminants was almost impossible, the acid activation of the raw materials produced carbons with high iodine numbers (about 1000 mg I₂/g carbon). To further characterize their porosity, the almond‐based carbons underwent BET measurements, with the results showing comparatively high surface areas (about 1400 m²/g). The carbons were rather mesoporous, and thus more suitable for liquid applications, which was confirmed by using the carbons in chromium (VI) uptake in another study [1].     

Preparation and characterization of SAPO-34 – Matrimid 5218 mixed matrix membranes for CO2/CH4 separation

Different SAPO-34 zeolite loaded Matrimid^® 5218 mixed matrix membranes (MMMs) were prepared by solution casting method and characterized using XRD and SEM analysis. Findings showed that semi crystalline neat polymer becomes more crystalline after thermal treatment at higher temperatures close to Matrimid^® 5218 glass transition temperature. Furthermore, incorporation of crystalline filler particles of SAPO-34 zeolite resulted in more and more crystallinity of the MMMs. SEM images also exhibited acceptable contacts between the filler particles and the polymer chains. Permeation measurement showed that CO₂ permeabilities and CO₂/CH₄ selectivities of the MMM with 20 wt% loading of SAPO-34 zeolite particles up to 6.9 (Barrer) and 67, respectively. This can be attributed to size discrimination of SAPO-34 pores that falls between CO₂ and CH₄ kinetic diameters.