Carbon Molecular Sieve Membranes Research Articles

Novel Approach to Fabricate Carbon Molecular‐Sieve Membranes Based on Consideration of Interpenetrating Networks

AbstractSummary: Carbon molecular‐sieve membranes (CMSMs) have shown great potential for gas separation. They exhibit high selectivity by permitting effective size‐ and shape‐separation between gas molecules of similar molecular dimensions. Hence, the control of their pore size is very important. While previous studies have focused on the conditions of pyrolysis and its effect on CMSM properties, a novel approach is reported here whereby the precursor polymer is chemically modified prior to pyrolysis and the resultant CMSM was investigated for its gas separation properties. Pyrolysis of chemically crosslinked and uncrosslinked Matrimid® resulted in a change in d‐spacing from 5.6 to 3.6 and 3.7 Å, respectively. The crosslinked CMSM also exhibited greater ordering in its packing. The Matrimid‐derived CMSMs exhibited excellent separation properties for CO2/CH4.Mechanism of chemical crosslinking modification.imageMechanism of chemical crosslinking modification.

Preparation of a Platinum and Palladium/Polyimide Nanocomposite Film as a Precursor of Metal-Doped Carbon Molecular Sieve Membrane via Supercritical Impregnation

The synthesis of a Pt- and Pd-nanoparticle-dispersed polyimide film as a precursor for the preparation of a metal-doped carbon molecular sieve (CMS) membrane for hydrogen separation has been performed. Impregnation of Pt(II) acetylacetonate and Pd(II) acetylacetonate dissolved in supercritical CO2 yielded Pt and Pd nanoparticles (diameters 12 and 5 nm, respectively) that were highly dispersed inside polyimide films under optimized conditions. The impregnation temperature and varieties of polyimide films strongly influenced the dispersion of the metal particles. The Pd-doped CMS membrane was successfully prepared from the polyimide and Pd(II) acetylacetonate composite. It showed good hydrogen separation performance.

Molecular dynamics simulations of transport and separation of carbon dioxide-alkane mixtures in carbon nanopores.

The configurational-bias Monte Carlo method, which is used for efficient generation of molecular models of n-alkane chains, is combined for the first time with the dual control-volume grand-canonical molecular-dynamics simulation, which has been developed for studying transport of molecules in pores under an external potential gradient, to investigate transport and separation of binary mixtures of n-alkanes, as well as mixtures of CO2 and n-alkanes, in carbon nanopores. The effect of various factors, such as the temperature of the system, the composition of the mixture, and the pore size, on the separation of the mixtures is investigated. We also report the preliminary results of an experimental study of transport and separation of some of the same mixtures in a carbon molecular-sieve membrane with comparable pore sizes. The results indicate that, for the mixtures considered in this paper, even in very small carbon nanopores the energetic effects still play a dominant role in the transport and separation properties of the mixtures, whereas in a real membrane they are dominated by the membrane's morphological characteristics. As a result, for the mixtures considered, a single pore may be a grossly inadequate model of a real membrane, and hence one must resort to three-dimensional molecular pore network models of the membrane.

The gas separation properties of carbon molecular sieve membranes derived from polyimides having carboxylic acid groups

Carbon molecular sieve (CMS) membranes were prepared by pyrolysis of polyimides containing carboxylic acid groups, in order to investigate their permeation properties for He, CO 2, O 2 and N 2. The polyimides were synthesized with a varying number of carboxylic acid groups in the diamine. Wide-angle X-ray diffraction and nitrogen gas sorption isotherms were employed to characterize the carbon structures. From pure gas permeation experiments, it was found that the removal of carboxylic acid pendent side groups during the pyrolysis resulted in a large pore volume in the carbon matrix, which significantly affected the gas separation performance of the final CMS membranes. The gas permeabilities of the CMS membranes pyrolyzed at 700 °C improved with increasing carboxylic acid group content. The CMS membranes derived from polyimide containing 50 mol% diamines showed maximum gas permeability for O 2 of 707 Barrers (1×10 −10 cm 3 (STP) cm/cm 2 s cmHg) and an O 2/N 2 selectivity of 9 at 25 °C.

Separation of CO 2/CH 4 through carbon molecular sieve membranes derived from P84 polyimide

The separation of CO 2/CH 4 separation is industrially important especially for natural gas processing. In the past decades, polymeric membranes separation technology has been widely adopted for CO 2/CH 4 separation. However, polymeric membranes are suffering from plasticization by condensable CO 2 molecules. Thus, carbon molecular sieve membranes (CMSMs) with excellent separation performance and stability appear to be a promising candidate for CO 2/CH 4 separation. A commercially available polyimide, P84 has been chosen as a precursor in preparing carbon membranes for this study. P84 displays a very high selectivity among the polyimides. The carbonization process was carried out at 550–800 °C under vacuum environment. WAXD and density measurements were performed to characterize the morphology of carbon membranes. The permeation properties of single and equimolar binary gas mixture through carbon membranes were measured and analyzed. The highest selectivity was attained by carbon membranes pyrolyzed at 800 °C, where the pyrolysis temperatures significantly affected the permeation properties of carbon membranes. A comparison of permeation properties among carbon membranes derived from four commercially available polyimides showed that the P84 carbon membranes exhibited the highest separation efficiency for CO 2/CH 4 separation. The pure gas measurement underestimated the separation efficiency of carbon membranes, due to the restricted diffusion of non-adsorbable gas by adsorbable component in binary mixture.

Relationship between chemical structure of aromatic polyimides and gas permeation properties of their carbon molecular sieve membranes

A series of aromatic polyimides was prepared via two-step polymerization and subsequent thermal imidization to manufacture carbon molecular sieve (CMS) membranes. In order to know the effect of microstructural changes of the polyimides on the gas permeation properties of their CMS membranes, three copolyimides were designed and synthesized using a dianhydride and diamines with a different number of methyl substituent groups. The density, fractional free volume (FFV), and glassy transition temperature ( T g) of the polyimides were characterized to investigate the microstructural changes of the polyimides which led to the difference in gas permeation properties as well as that in their physical properties. The introduction of methyl substituent group in the rigid polyimide backbone increased FFV of polyimides, and the gas permeabilities increased typically with FFV. After an inert pyrolysis of these polyimides, similar gas permeation behaviors were also observed in their CMS membranes pyrolyzed at 600 and 800 °C, respectively. That is, the gas permeation characteristics of the polyimide due to the structural change were well kept even after pyrolysis within the temperature range used in this work. In all cases, the gas permeabilities increased in the order He>CO 2>O 2>N 2, indicating that the gas permeabilities are in agreement with the order of the kinetic gas diameters. The CMS membranes prepared in this study exhibited good separation properties on He/N 2, O 2/N 2, and CO 2/N 2 with satisfactory gas permeabilities. Thereby, the present work will provide an insight into the molecular design of the polymeric precursors for the effective preparation of the CMS membranes.

Carbon molecular sieve membranes derived from metal-substituted sulfonated polyimide and their gas separation properties

Carbon molecular sieve (CMS) membranes containing alkali metal ions (Li +, Na +, and K +) were prepared by the pyrolysis of metal-substituted sulfonated polyimide (M-SPI) precursors. We have determined the effect of substituted metal ions in the polymeric precursor on the gas separation performance of CMS membranes containing metal ions. Thermogravimetric analysis showed that thermal stability of sulfonated polyimide was improved by substituting metal ions in the polymeric precursor. In single gas permeation experiments using He, O 2, and N 2 gases having kinetic diameters from 2.6 to 3.64 Å, the gas permeabilities through the M-SPI membranes increased in the order: Li-SPI < Na-SPI < K-SPI. The presence of metal ions in their carbonized membranes also affected their gas permeation properties. The interplanar d spacing of the CMS membranes increased with increasing ionic radius of the substituted metal ion, and this was the cause of the effect on the gas permeation properties. At a constant concentration of metal ions, the gas permeabilities increased with ionic radius of substituted metal ions (Li + < Na + < K +), whereas the ideal gas separation factor decreased in the same order. A CMS membrane pyrolyzed at 590 °C derived from K-SPI precursor showed maximum gas permeabilities for He, O 2, and N 2 of 248, 6.8 and 1.7 Barrer, respectively, while one pyrolyzed at 590 °C derived from Li-SPI showed maximum He/N 2 and O 2/N 2 mixture selectivities of 174 and 5.7.

The structural characterization of carbon molecular sieve membrane (CMSM) via gas adsorption

The microstructure of a carbon molecular sieve membrane (CMSM) is characterized using adsorption equilibrium information. The pore size distributions of the CMSM derived from N 2 and CH 4 adsorption isotherm are found to be consistent with each other and in agreement with the results of gas permeation experiments as well as the general characteristics of such molecular sieve materials.

Functionalized Carbon Molecular Sieve membranes containing Ag-nanoclusters

In Carbon Molecular Sieve (CMS) membranes, the separation of O 2 and N 2 is primarily based on the difference in size between the gas molecules. To enhance the separation properties of these CMS membranes it is necessary to functionalize the carbon matrix with materials that show a high affinity to one of the permeating gas species. Adding Ag-nanoclusters increases the selectivity of O 2 over N 2 by a factor 1.6 compared to a non-functionalized CMS membrane prepared by the same pyrolysis procedure. We have analyzed the structure of Ag-nanocluster ( d cluster≈50 nm) containing membranes produced from different Ag sources, AgNO 3 and AgAc, and with different Ag content (0, 6, 25, and 40 wt.%). By measuring the pure gas permeabilities of He, CO 2, O 2, and N 2 we have determined the effect of Ag-nanoclusters in the carbon matrix, concluding that in the case of pure gases, the Ag-nanoclusters act primarily as a spacer at pyrolysis end temperatures up to 600 °C, increasing the O 2 permeability by a factor of 2.4. However, they enhance the separation of O 2 over N 2 at higher pyrolysis end temperatures (700 and 800 °C). It was shown that the build up of an Ag layer on the surface of the membrane reduces the permeability, but does not affect the selectivity.

Preparation and gas permeation properties of carbon molecular sieve membranes based on sulfonated phenolic resin

Effects of pyrolysis and preparation conditions on the gas permeation properties of pyrolytic membranes derived from sulfonated phenolic resin were investigated. Pyrolysis temperature, dip-coating conditions and coating/pyrolysis (C/P) cycle had significant influence on the gas permeation properties of pyrolytic membranes. Membrane obtained under optimum preparation conditions exhibited O 2 permeance of 30 GPU and ideal O 2/N 2 separation factor of 12 at 35 °C, which are comparable to the O 2/N 2 separation performances of carbon molecular sieve (CMS) membranes on the upper-bound line in the plots of permeance versus selectivity. N 2 adsorption and desorption at 77 K suggested that the pore structures of CMS membranes pyrolyzed around 500 °C seemed to be made up of interconnected pores with different size and shape.

Carbon molecular sieve membranes: a promising alternative for selected industrial applications.

Carbon molecular sieve (CMS) membranes (hollow fibers) have been studied for application as possible separation units for selected industrial gas streams. Gas streams at petrochemical plants (polypropene and polyethene) and upgrading of biogas to fuel specifications have been in focus. Gases present in biogas (N(2), CO(2), H(2)O(vap), and CH(4)) and gas streams at polyolefin plants (C(2)H(4), C(3)H(6), and C(3)H(8)) have been measured; both as pure gases and in mixtures. Aging of the CMS-membranes as a function of humidity and pore blocking is discussed; likewise, possible regeneration methods when flux decrease is experienced. Transport mechanisms depending on pore size and molecular properties are also discussed. Excellent separation properties were documented for these applications, but also the need for frequent regeneration of the membrane in order to maintain permeability flux. The mixed gas experiments documented clearly the need for careful pore tailoring in order to optimize selectivity when the membranes were used for alkane-alkene separation.

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 O 2/N 2, CO 2/CH 4 and C 3H 6/C 3H 8 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.

The characterization of CO2 permeation in a CMSM derived from polyimide

Abstract Carbon molecular sieve membrane (CMSM) is prepared from Kapton polyimide. The CMSM presents strong molecular sieving effect for gas molecules with different dimensions. CO 2 sorption and permeation experiments were performed on the CMSM over a wide pressure range (up to 40 atm). The permeation time lag and steady state permeation flux were employed to characterize the concentration dependence of the diffusion coefficient of the adsorbate. A new functional form is proposed for the concentration dependence of the surface diffusivity [ D μ ( C )/ D μ 0 =1/(1− θ ) n , where θ is the ratio of surface coverage and n ≥1] to cope with the experimental observations. With the sorption–diffusion mechanism, both transient time lag and steady state flux indicates that the surface diffusivity of CO 2 takes a concentration dependence much stronger than that described by the traditional HIO model, [i.e. D μ ( C )/ D μ 0 =1/(1− θ )]. The reason for this abnormality is discussed.

Knudsen diffusion in microporous carbon membranes with molecular sieving character

Measurements were done on a relatively wide pore carbon molecular sieve membrane (CMSM) by means of a range of probe molecules (He, H 2, N 2, CH 4, CO 2, C 5H 12) between 25 and 500 °C. At high temperatures, where adsorption effects are attenuated, the permeance of the carbon membrane to pure gases displayed the molecular weight and temperature dependence expected from Knudsen diffusion, even though the pore size distribution lies demonstrably below 0.55 nm. Utilizing a concept of an effective diameter, the authors analyzed the temperature dependence of the probe molecules’ permeabilities and separated out the relative contribution of the different gas transport mechanisms in the pores of the carbon microporous membranes. Calculations of the total gas permeance from two different idealized membrane pore structures were made and compared to measured permeances. The comparison showed that an idealized pore structure comprising larger and smaller diameter pore regions essentially connected in series can reasonably explain the measurements. The estimate for the larger pore diameter was found by using the temperature dependence of n-pentane permeance, which showed behavior approaching that to be expected for activated diffusion typical of sieving membranes. The larger diameter region of the pore system was estimated to be 0.58 nm and the smaller diameter region to be 0.43–0.48 nm. After correction for the contribution of Knudsen diffusion, the gases tested could be ranked according to the contribution of adsorption-mediated transport as follows: CO 2>CH 4>N 2>H 2>He based on the calculated net energies for pore transport. This is consistent with the ranking to be found for adsorption isotherms of these gases on activated glassy carbon.

Carbon molecular sieve membranes prepared from porous fiber precursor

Carbon molecular sieve (CMS) membranes are usually prepared from dense polymeric precursors that already show intrinsic gas separation properties. The rationale behind this approach is that the occurrence of any kind of initial porosity will deteriorate the final CMS performance. We will show that it is not necessary to produce a non-porous precursor in order to obtain a selective CMS membrane. We used tight ultra-filtration (UF) fiber membranes as a precursor. These fibers did not have any gas separation properties before the pyrolysis treatment, nor were coatings applied to these fibers before or subsequent to the pyrolysis. After a heat treatment in air followed by a pyrolysis in a nitrogen atmosphere CMS fiber membranes were obtained. The CMS fibers were analyzed using scanning electron microscopy, thermo gravimetrical analysis, and gas permeation. From the permeation rates and permselectivity values measured for He, H 2, CO 2, Ar, O 2, N 2, CH 4, C 2H 4, C 2H 6, C 3H 6, C 3H 8 and SF 6 the evolution of the mean pore diameter was investigated. It was found that the pore diameter increases with pyrolysis temperature up to 800 °C, but decreases as the temperature is raised to 900 °C. The overall porosity reaches its highest value at 900 °C.

Carbon Molecular Sieving Membranes Derived from Lignin-Based Materials

Carbon molecular sieving membranes were prepared by pyrolysis of lignocresol derived from lignin by the phase-separation method. Lignocresol membranes formed by a dip process on a porous α-alumina tubing were carbonized at 400–800°C under nitrogen atmosphere. The thickness of the membrane formed on the outer surface of the substrate was about 400 nm judging from SEM observation. Gas-evolving behavior of lignocresol was measured using thermogravimetry-mass spectrometry (TG-MS). The gaseous products evolved from lignocresol included a number of fragments with higher molecular weights; whereas those from phenolic resin are mainly due to phenol and methylphenol. These evolved pyrolysis fragments effectively contribute to micropore formation of carbonized lignocresol membranes. Gas permeation rates through the membrane decreased in the order of increasing kinetic molecular diameter of the penetrant gas, and the membrane behaved like a “molecular sieve.” The permeation properties were dependent on heating conditions, and a pyrolysis temperature of 600°C gave the best membrane performance. Gas selectivities of the membrane prepared at 600°C were 50, 8, 290, and 87 for CO2/N2, O2/N2, H2/CH4, and CO2/CH4 at 35°C, respectively.

Ag functionalized Carbon Molecular Sieves membranes for separating O2 and N2

ABSTRACTIn the last two decades substantial progress has been made in the preparation of Carbon Molecular Sieve (CMS) membranes for gas separation. Today, researchers actively study precursor materials and pyrolysis routes to fully explore the merits of CMS membranes. Successful separation of permanent gas mixtures in which the gaseous components posses only little to no affinity to adsorb onto the internal CMS surface relies highly on the exact tailoring of microsieving regions. Preparation of CMS structures, which are highly permselective towards one of such mixture components, is especially cumbersome if the molecular sizes differ only slightly (e.g. O2/N2).To facilitate the separation of O2 and N2 we have chosen to functionalize the carbon matrix. By introducing nano-sized (40 nm) metallic Ag-clusters, the affinity of the membrane matrix for O2 significantly increases. We have added a silver containing salt, AgNO3, to a solution of BMTA-TDI/MDI co-polyimide (P84, Lenzing) in NMP in the absence of light to obtain homogeneous flat film polymeric precursors. These precursors were pyrolysed, reducing Ag+ to Ag, at different temperatures (350, 500, 600, 700, and 800 °C) in a N2 atmosphere and characterized using Scanning Electron Microscopy, Atomic Force Microscopy, and gas permeation. Thermo Gravimetrical Analysis was used to follow the pyrolysis in detail. From this we observed an increase of the ideal separation factor of 1.6. Moreover, we observed an increase of the permeability to a maximum of 240 %.

Ion Beam Modification of Matrimid® Gas Separation Membrane—Evolution in Chemical Structure, Microstructure and Gas Permeation Properties

ABSTRACTMatrimid® is a widely used polyimide that has both attractive thermal and gas transport properties. In addition, it has been used as a precursor polymer for production of carbon molecular sieving membranes with commercially interesting gas separations. Ion beam irradiation over a wide range of doses was used to modify a series of Matrimid® membranes. A combined analysis of impact of ion irradiation on the chemical structure, microstructure and gas transport properties of Matrimid® will be presented. Specifically, the evolution in gas permeation properties following irradiation over a wide range of doses will be discussed. Ion irradiation resulted in combined increased permeability and permselectivity for several gas pairs of interest.

Carbon molecular sieve membranes derived from a phenolic resin supported on porous ceramic tubes

The preparation of a composite carbon membrane from a phenolic resin is described. The membrane is formed by a thin microporous carbon layer (thickness: 2 μm) obtained by pyrolysis (700°C, under vacuum) of a phenolic resin film supported on the inner face of a porous alumina tube. The separation characteristics of the resulting carbon membranes were analysed from permeation experiments with pure gases of different molecular size (He, CO2, O2, N2 and CH4) and separation of binary gas mixtures O2–N2 and CO2–CH4. An almost defect-free carbon membrane is obtained in only one casting step. The effective micropore size was estimated to be around 4.4 Å. The prepared carbon membranes have demonstrated to be effective for separating gas mixtures such as O2–N2 (O2 permeance: 100 Barrer; O2–N2 separation factor: 12) and CO2–CH4 (CO2 permeance: 400 Barrer; CO2–CH4 separation factor: 150). The oxidation of the phenolic resin film with air at temperatures ranging from 150 to 300°C improves the gas permeance and originates a decrease in the permselectivity.

Carbon Molecular Sieve Membranes Derived from Phenolic Resin with a Pendant Sulfonic Acid Group

A thermosetting phenolic resin with a pendant sulfonic acid group was prepared by reacting a resol-type phenolic resin (PF) with a Novalak-type sulfonated phenolic resin (SPF). Large amounts of gaseous molecules with similar and small size such as H2O and SO2 evolved in the range of 110 and 350 °C during the pyrolysis of this thermosetting phenolic resin (PF/SPF). Highly permeable carbon molecular sieve (CMS) membranes were obtained by pyrolysis of PF/SPF(45/55) precursor membranes which were dip-coated on porous alumina tubes. For example, the membrane pyrolyzed at 500 °C for 1.5 h displayed H2, CO2, and O2 permeances of 1950, 800, and 240 [GPU (gas permeation units) = 10-6 cm3(STP)·s-1·cm-2·cmHg-1], respectively, and ideal H2/CH4, CO2/CH4, and O2/N2 separation factors of 65, 27, and 5.2 at 35 °C and 1 atm, respectively. Sulfonic acid groups linked to thermostable polymer chains might act as “bonded templates” and showed attractive potential in the preparation of CMS membranes.