Effect of isopropylidene replacement on gas transport properties of polycarbonates

The gas sorption and transport properties of a series of polycarbonates in which the isopropylidene unit of bisphenol A polycarbonate has been replaced with another molecular group are presented. Two new materials, bisphenol of norbornane polycarbonate (NBPC) and bisphenol Z polycarbonate (PCZ), are compared with several polymers which have been studied previously in this laboratory, including bisphenol A polycarbonate (PC), hexafluorobisphenol A polycarbonate (HFPC), and bisphenol of chloral polycarbonate (BCPC). The effect of molecular structure on chain mobility and chain packing is related to the gas transport properties. Dynamic mechanical thermal analysis and differential scanning calorimetry are used to judge chain mobility, while x-ray diffraction and free volume calculations give information about chain packing. Permeability measurements were made for He, H₂, O₂, N₂, CH₄, and CO₂ at 35°C over a range of pressures up to 20 atm. Sorption experiments were also done for N₂, CH₄, and CO₂ under the same conditions. The permeability coefficients of these polymers rank in the order HFPC ? NBPC>PC>BCPC ? PCZ for all of the gases. With the exception of BCPC, this order correlates well with fractional free volume. The low gas permeability of BCPC is attributed to a polarity effect. In general, bulky and relatively immobile substituents, as in HFPC and NBPC, can yield improved separation characteristics. The polar group of BCPC and the flexible cyclohexyl substituent of PCZ result in relatively low gas permeability.     

Analysis of trace levels of impurities and hydrogen isotopes in helium purge gas using gas chromatography for tritium extraction system of an Indian lead lithium ceramic breeder test blanket module

In the fusion fuel cycle, the accurate analysis and understanding of the chemical composition of any gas mixture is of great importance for the efficient design of a tritium extraction and purification system or any tritium handling system. Methods like laser Raman spectroscopy and gas chromatography with thermal conductivity detector have been considered for hydrogen isotopes analyses in fuel cycles. Gas chromatography with a cryogenic separation column has been used for the analysis of hydrogen isotopes gas mixtures in general due to its high reliability and ease of operation. Hydrogen isotopes gas mixture analysis with cryogenic columns has been reported earlier using different column materials for percentage level composition. In the present work, trace levels of hydrogen isotopes (∼100 ppm of H₂ and D₂) have been analyzed with a Zeolite 5A and a modified γ‐Al₂O₃ column. Impurities in He gas (∼10 ppm of H₂, O₂, and N₂) have been analyzed using a Zeolite 13‐X column. Gas chromatography with discharge ionization detection has been utilized for this purpose. The results of these experiments suggest that the columns developed were able to separate ppm levels of the desired components with a small response time (<6 min) and good resolution in both cases.     

Gas-separating properties of membranes based on star-shaped fullerene (C<Subscript>60</Subscript>)-containing polystyrenes

Based on regular star-shaped PSs differing in the structure of the branching center (one or two covalently bound fullerene C₆₀ molecules) and in the number of branchings (6 and 12), homogeneous gas-separation membranes have been produced. The transport behavior of the membranes with respect to several gases, such as H₂, He₂, N₂, CO, CO₂, and CH₄, has been studied by mass spectrometry. It has been found that the membranes prepared from six-arm PSs are characterized by a smaller density of macromolecular packing than the membranes obtained from 12-arm PCs and, consequently, they possess higher gas permeability. The starshaped PSs demonstrate a higher selectivity factor for separation of the O₂/N₂ gas pair compared to the corresponding characteristics of the linear PSs. The analysis of gas-separation characteristics by means of the Reitlinger-Robeson diagrams demonstrates that the transport behavior of star-shaped PSs qualitatively surpasses similar parameters of the known polymers in the separation of the CO/N₂ gas pair.     

Selective CO2 Sorption Using Compartmentalized Coordination Polymers with Discrete Voids**

Carbon capture and storage with porous materials is one of the most promising technologies to minimize CO₂ release into the atmosphere. Here, we report a family of compartmentalized coordination polymers (CCPs) capable of capturing gas molecules in a selective manner based on two novel tetrazole-based ligands. Crystal structures have been modelled theoretically under the Density Functional Theory (DFT) revealing the presence of discrete voids of 380 ų. Single gas adsorption isotherms of N₂, CH₄ and CO₂ have been measured, obtaining a loading capacity of 0.6, 1.7 and 2.2 molecules/void at 10 bar and at 298 K for the best performing material. Moreover, they present excellent selectivity and regenerability for CO₂ in mixtures with CH₄ and N₂ in comparison with other reported materials, as evidenced by dynamic breakthrough gas experiments. These frameworks are therefore great candidates for separation of gas mixtures in the chemical engineering industry.     

6FDA-containing polyimide-ionene + ionic liquid gas separation membranes

Polyimides (PI) synthesized from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with various diamines have been frequently studied as gas separation membranes. The use of 6FDA in polyimides creates a bent structure than can increase fractional free volume (FFV) and gas permeability. Here, we demonstrate that 6FDA is also a useful building block for PI-ionene materials, which contain cations directly within the polymer backbone. These new 6FDA-containing PI-ionenes were combined with several different imidazolium ionic liquids (ILs) to form thin membranes. The thermal properties of all the derivatives were investigated to determine the relationship between regiochemistry and degradation as well as the intermolecular forces that are present within these structures. The gas separation properties of these 6FDA-containing PI-ionene + IL materials were investigated, showing modest CO₂ permeabilities similar to other polyimide-ionenes and CO₂/CH₄ and CO₂/N₂ permselectivities that were relatively higher than other polyimide-ionenes.     

Transport properties of poly(phenylquinoxalines) containing heterocyclic moieties

The transport behavior of a new class of membrane materials—a series of poly(phenylquinoxalines) containing heterocyclic fragments in the backbone—has been studied. These polymers contain moieties of a common chemical structure. Therefore, it is possible to follow how the transport parameters change upon introduction of various moieties into the backbone chain. The coefficients of permeability, diffusion, and solubility for H₂, He, O₂, N₂, CO, CO₂, and CH₄ along with the separation factors for the corresponding pairs of gases have been determined. The results are compared with the data for previously studied polymers of the poly(phenylquinoxaline) series.     

Transport properties of polyimides containing phenylquinoxaline moieties

The gas permeabilities of a number of new structurally related polyimides containing phenylquinoxaline moieties were studied for the first time. The test polymers had different dianhydride units, whereas their diamine components differed in the presence of flexible ether bonds-O-in the main chain, a structure that is reflected in the effective packing of chains and, as a result, in transport parameters. The permeability, diffusion, and solubility coefficients for the gases H₂, He, O₂, N₂, CO, CO₂, and CH₄, as well as the ideal separation factors for gas pairs, were determined. The transport characteristics of polymers were compared within the given polymer series and with published data for other polymer series.     

The adsorption of acidic gaseous pollutants on metal and nonmetallic surface studied by first-principles calculation: A review

The acidic gases such as SO₂, NO_x, H₂S and CO₂ are typical harmful pollutants and greenhouse gases in the atmosphere, which are also the main sources of PM_2.5. The most widely used method of treating these gas molecules is to capture them with different adsorption materials, i.e., metal and nonmetallic materials such as MnO₂, MoS₂ and carbon-based materials. And doping transition metal atoms in adsorption materials are beneficial to the gas adsorption process. The first-principles calculation is a powerful tool for studying the adsorption properties of contaminant molecules on different materials at the molecular and atomic levels to understand surface adsorption reactions, adsorption reactivity, and structure-activity relationships which can provide theoretical guidance for laboratory researches and industrial applications. This review introduces the adsorption models and surface properties of these gas molecules on metal and nonmetallic surfaces by first-principles calculation in recent years. The purpose of this review is to provide the theoretical guidance for experimental research and industrial application, and to inspire scientists to benefit from first-principles calculation for applying similar methods in future work.     

Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks

Reducing anthropogenic CO₂ emission and lowering the concentration of greenhouse gases in the atmosphere has quickly become one of the most urgent environmental issues of our age. Carbon capture and storage (CCS) is one option for reducing these harmful CO₂ emissions. While a variety of technologies and methods have been developed, the separation of CO₂ from gas streams is still a critical issue. Apart from establishing new techniques, the exploration of capture materials with high separation performance and low capital cost are of paramount importance. Metal-organic frameworks (MOFs), a new class of crystalline porous materials constructed by metal-containing nodes bonded to organic bridging ligands hold great potential as adsorbents or membrane materials in gas separation. In this paper, we review the research progress (from experimental results to molecular simulations) in MOFs for CO₂ adsorption, storage, and separations (adsorptive separation and membrane-based separation) that are directly related to CO₂ capture.     

Correlation between Performances of Hollow Fibers and Flat Membranes for Gas Separation

This work presents an attempt at correlating the available permeability/selectivity literature data for hollow fibers and flat membranes. Therefore, this paper gathers the information pertaining to membrane materials for which membrane properties of flat membranes and hollow fibers have both been reported. An overview of the relations between selectivity and permeance of hollow fiber membranes for various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂, H₂/N₂, H₂/CO₂, H₂/CH₄ and He/N₂) is presented first. The upper bound lines are the ones proposed by Robeson, which were calculated by assuming a one-micron-thick skin layer as proposed by Robeson in 2008. From the results obtained, a relation between the selectivity ratio in both kinds of membranes (α_H/α_f) and skin layer thickness (l) calculated from flat membranes and hollow fibers gas permeation data for these pairs of gases is also presented. The skin layer thicknesses measured using seven different experimental techniques for six commercial membranes are compared. The influences of spinning parameters on the morphology and performance of hollow fiber membrane gas separation are discussed. Finally, an analysis is made of the reasons why the dense skin layer thicknesses of a hollow fiber calculated using permeance and permeability data vary for different gases and also differ from direct experimental measurements.     

INORGANIC POROUS MEMBRANES FOR LIQUID PHASE SEPARATION

Porous ceramic membranes are reviewed with reference to liquid phase separation. Methods for preparing porous ceramic membranes are summarized after a brief introduction to membranes and membrane processes. In the section on liquid phase separation, membrane materials are summarized from the viewpoint of pore size limits, since the pore sizes of porous membranes play an important role in determining separation performance. Various types of metal oxides and composite oxides have been developed by the sol-gel process; typical materials are Al₂O₃, TiO₂, SiO₂, ZrO₂, and composite oxides. The sol-gel process has a great advantage in terms of pore-size controllability over a wide range, from 0.5 ～ several ten nm, and therefore, is suitable for preparing membranes for use in liquid phase separation. Applications of inorganic membranes are reviewed in terms of water treatment, separation of nonaqueous systems, and photocatalysis.     

Preparation and characterisation of polyaniline based membranes for gas separation

Transport rates (permeability) and ideal separation factors for several gas pairs through dense polyaniline membranes are reported. The ideal separation factors for all gas pairs tested were found to be independent of the polyaniline membrane thickness whereas the permeability of the single gases showed significant variations. Both dedoped and redoped films (film thickness between 9 and 67 μm) were studied. The highest selectivities α_(A/B) found were 7.6 for the gas pair H₂/CO₂ in the case of the dedoped membrane and 10 for the gas pair H₂/CO₂, 6 for O₂/N₂ and 200 for H₂/N₂ in the case of the redoped membrane. Statistical analysis of a large number of membranes allowed the critical comparison with results obtained by other groups.Comparison with other membrane materials shows that an approximately sixfold enhancement of the respective separation factors is possible for gas pairs containing hydrogen. Similar separation factors are observed for the gas pairs CO₂/O₂, CO₂/N₂ and N₂/O₂.Membranes for which Knudsen diffusion was observed exhibited regularly distributed micropores (400 nm diameter).     

Crossover Sorption of C2H2/CO2 and C2H6/C2H4 in Soft Porous Coordination Networks

Porous sorbents are materials that are used for various applications, including storage and separation. Typically, the uptake of a single gas by a sorbent decreases with temperature, but the relative affinity for two similar gases does not change. However, in this study, we report a rare example of “crossover sorption,” in which the uptake capacity and apparent affinity for two similar gases reverse at different temperatures. We synthesized two soft porous coordination polymers (PCPs), [Zn₂(L1)(L2)₂]_n (PCP-1) and [Zn₂(L1)(L3)₂]_n (PCP-2) (L1= 1,4-bis(4-pyridyl)benzene, L2=5-methyl-1,3-di(4-carboxyphenyl)benzene, and L3=5-methoxy-1,3-di(4-carboxyphenyl)benzene). These PCPs exhibits structural changes upon gas sorption and show the crossover sorption for both C₂H₂/CO₂ and C₂H₆/C₂H₄, in which the apparent affinity reverse with temperature. We used in situ gas-loading single-crystal X-ray diffraction (SCXRD) analysis to reveal the guest inclusion structures of PCP-1 for C₂H₂, CO₂, C₂H₆, and C₂H₄ gases at various temperatures. Interestingly, we observed three-step single-crystal to single-crystal (sc-sc) transformations with the different loading phases under these gases, providing insight into guest binding positions, nature of host–guest or guest-guest interactions, and their phase transformations upon exposure to these gases. Combining with theoretical investigation, we have fully elucidated the crossover sorption in the flexible coordination networks, which involves a reversal of apparent affinity and uptake of similar gases at different temperatures. We discovered that this behaviour can be explained by the delicate balance between guest binding and host–guest and guest-guest interactions.     

Extraordinary Separation of Acetylene‐Containing Mixtures with Microporous Metal–Organic Frameworks with Open O Donor Sites and Tunable Robustness through Control of the Helical Chain Secondary Building Units

Acetylene separation is a very important but challenging industrial separation task. Here, through the solvothermal reaction of CuI and 5‐triazole isophthalic acid in different solvents, two metal–organic frameworks (MOFs, FJU‐21 and FJU‐22 ) with open O donor sites and controllable robustness have been obtained for acetylene separation. They contain the same paddle‐wheel {Cu₂(COO₂)₄} nodes and metal–ligand connection modes, but with different helical chains as secondary building units (SBUs), leading to different structural robustness for the MOFs. FJU‐21 and FJU‐22 are the first examples in which the MOFs’ robustness is controlled by adjusting the helical chain SBUs. Good robustness gives the activated FJU‐22 a , which has higher surface area and gas uptakes than the flexible FJU‐21 a . Importantly, FJU‐22 a shows extraordinary separation of acetylene mixtures under ambient conditions. The separation capacity of FJU‐22 a for 50:50 C₂H₂/CO₂ mixtures is about twice that of the high‐capacity HOF‐3, and its actual separation selectivity for C₂H₂/C₂H₄ mixtures containing 1 % acetylene is the highest among reported porous materials. Based on first‐principles calculations, the extraordinary separation performance of C₂H₂ for FJU‐22 a was attributed to hydrogen‐bonding interactions between the C₂H₂ molecules with the open O donors on the wall, which provide better recognition ability for C₂H₂ than other functional sites, including open metal sites and amino groups.     

Thermally stable polyimide isomers for membrane-based gas separations at elevated temperatures

The temperature dependence of gas sorption and transport properties is examined for two polyimide isomers. The permeabilities and solubilities of five gases in these materials are reported over an extensive temperature range from 35 to 325°C. Also, the activation energies for permeation, the heats of sorption, and the activation energies for diffusion obtained for both polyimides are compared and correlated with physical properties of the polymers and penetrants. The influence of temperature on the selective properties of these membrane materials is discussed for three gas separations; He/N₂, CO₂/CH_4, and O₂/N_2. Thorough analysis of these data provides insight into the influence of the subtle difference in chain structure of the two isomers. The performance of the 6FDA-6F_p DA as a separation membrane at high temperatures suggests that it is an outstanding candidate for use in novel elevated temperature applications. ©1995 John Wiley & Sons, Inc.     

Recent progress in applications of graphene oxide for gas sensing: A review

This paper is a review of the recent progress on gas sensors using graphene oxide (GO). GO is not a new material but its unique features have recently been of interest for gas sensing applications, and not just as an intermediate for reduced graphene oxide (RGO). Graphene and RGO have been well known gas-sensing materials, but GO is also an attractive sensing material that has been well studied these last few years. The functional groups on GO nanosheets play important roles in adsorbing gas molecules, and the electric or optical properties of GO materials change with exposure to certain gases. Addition of metal nanoparticles and metal oxide nanocomposites is an effective way to make GO materials selective and sensitive to analyte gases. In this paper, several applications of GO based sensors are summarized for detection of water vapor, NO₂, H₂, NH₃, H₂S, and organic vapors. Also binding energies of gas molecules onto graphene and the oxygenous functional groups are summarized, and problems and possible solutions are discussed for the GO-based gas sensors.     

Poly(dimethylsiloxane) networks modified with poly(phenylsilsesquioxane)s: Synthesis, structural characterisation and evaluation of the thermal stability and gas permeability

Self-supported translucent films constituted of semi-inorganic polymeric materials were prepared by sol-gel process from poly(phenylsilsesquioxane) (PPSQ) and poly(dimethylsiloxane) (PDMS), modified by diphenylsilanediol (DPS), phenyltriethoxysilane (PTES) and/or tetraethoxysilane (TEOS). These materials were characterized by infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermo-gravimetric analysis (TGA). Permeability to N₂, O₂, CH₄ and CO₂ and selectivity for a specific gas pair were investigated using the time-lag method. In the gas separation process high permeability and selectivity coefficients were observed, particularly for the membrane containing DPS and PTES as additives, which presented potential applications in the separation of CO₂/CH₄ and CO₂/N₂. The materials also showed good thermal stability, which could be correlated to the relative amounts between di-functional (D), tri-functional (T) and tetra-functional (Q) silicon units.     

The gas transport properties of amine-containing polyurethane and poly(urethane-urea) membranes

A series of amine-containing polyurethanes and poly(urethane-urea)s based on 4,4′-diphenylmethane diisocyanate and either poly(ethylene glycol) of molecular weights 400 or 600 were prepared as gas separation membranes. The amine functional groups of N-methyldiethanolamine (MDEA) and/or tetraethylenepentamine (TEPA) were introduced into the hard segment as a chain extender. The gas transport data of He, H₂, O₂, N₂, CH₄ and CO₂ in these polymer membranes were determined by using the Barrer's high-vacuum technique and the time-lag method. The restriction of chain mobility has been shown by the formation of hydrogen bonding in the soft segment and hard-segment domains, resulting in the increase in the density, glass transition temperature of soft segments (T_gs). The separation mechanism of various gas pairs used in industrial processes is also discussed. Effect of pressure on permeability of the gases above and below T_gs was studied. It was found that the gas permeability increased or decreased with upstream pressure above T_gs, and should be described by a modified free-volume model. On the other hand, the condensable CO₂ exhibits a minimum permeability at a certain upstream pressure below T_gs. The permeability of He and H₂ were pressure independent above and below the T_gs.     

MXene: A two-dimensional material in selective water separation via pervaporation

MXene, well-identified as Ti₃C₂T_X, belongs to the family of two-dimensional (2D) materials, which have been currently explored in various applications. Very recently, such materials have been pointed out as potential nanomaterials for advanced solute separations when introduced in membranes, such as ion separation, gas separation, nanofiltration, chiral molecular separation, and solvent separation. This latter separation, generally named Pervaporation (PV), is identified as a highly selective technology for water separations. To date, few pieces of research have been released but providing interesting insights into several solvent (including water) separations. Hence, this brief review aims to analyze and discuss the latest advances for utilizing MXenes for PV membranes. Particular emphasis has been devoted to the relevant outcomes in the field, along with the strategies followed by researchers to tailor membranes. Based on the current findings, the perspectives in the field are also stated.     

Effect of Molecular Structure on the GasPermeability of Cellulose Aliphatate Esters简

In this work, four kinds of cellulose aliphatate esters, cellulose acetate （CA）, cellulose propionate （CP）, cellulose butyrate （CB） and cellulose acetate butyrate （CAB） are synthesized by the homogeneous acylation reactions in cellulose/AmimC1 solutions. These cellulose aliphatate esters are used to prepare gas separation membranes and the effects of molecular structure, such as substituent type, degree of substitution （DS） and distribution of substituents, on the gas permeability are studied. For CAs, as the DS increases, their gas permeabilities for all five gases （02, N2, CH4, CO and CO2） increase, and the ideal permselectivity significantly increases first and then slightly decreases. At similar DS value, the homogenously synthesized CA （distribution order of acetate substituent： C6 〉 C3 〉 C2） is superior to the heterogeneously synthesized CA （distribution order of acetate substituent： C3 〉 C2 〉 C6） in gas separation. With the increase of chain length of aliphatate substituents from acetate to propionate, and to butyrate, the gas permeability of cellulose aliphatate esters gradually increases. The cellulose mixed ester CAB with short acetate groups and relatively long butyrate groups exhibits higher gas permeability or better permselectivity than individual CA or CB via the alteration of the DS of two substituents.