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.     

Carbon–Al2O3–Ag composite molecular sieve membranes for gas separation

Phenolic resins loaded with two different inorganic fillers (boehmite (γ-AlO(OH)) and silver (Ag)) were used to prepare composite carbon membranes. Polymer solutions containing γ-AlO(OH) and AgNO₃ were prepared and the effect of Ag on the transport properties of the composite membrane was evaluated. The polymer solutions were coated on α-Al₂O₃ tubes and carbonized in a single dipping-drying-pyrolysis step. After pyrolysis at 550 °C, γ-AlO(OH) yielded γ-Al₂O₃, and Ag agglomerated forming spherical nanoparticles of 30 nm in diameter. Ag loading enhanced the carbon membrane performance for several gas pairs of interest, especially for C₃H₆/C₃H₈ separation, where the C₃H₆/C₃H₈ permselectivity increased from a maximum of 15 to 38.     

Matrimid® 5218 in preparation of membranes for gas separation: Current state-of-the-art

Over the last decades, different polymers have been used as continuous phase for preparing selective membranes for gas separation. Today, some of these materials have been consolidated commercially; however, the necessity to improve the performance (in terms of permeability/selectivity) of polymeric membranes above Robeson’s upper bound has been conducted by blending polymers, use of additives, implementation new methods, development of new materials, coating films, development of mixed matrix membranes, and so on. One of the most recent approaches is the use of polymers such as polyimides, i.e., Matrimid® 5218, which has demonstrated, to provide remarkable gas separation performance using the attempts aforementioned. The aim of this work is to present the current state-of-the-art of the use of Matrimid® 5218 in preparation of membrane for gas separation. The progress in this field is summarized and discussed chronologically in two periods, decade (from 1998 to 2008) and current (from 2009 up to now) frameworks. This contribution leads to take a complete and compelling overview of the state-of-the-art based on Matrimid. Furthermore, the main approaches, aim of study, gas separation evaluated, main techniques used for membrane characterization, main supplier of the polymer, main secondary materials for blending, fillers incorporated into the matrix, and remark of the study are summarized in detail. Finally, it denotes the prospects and future trends on use of Matrimid® 5218 for membrane applications.     

Beyond graphene oxides: Emerging 2D molecular sieve membranes for efficient separation☆

Recent decades witnessed the significant progress made in the research field of 2D molecular sieve membranes.In comparison with their 3D counterparts, 2D molecular sieve membranes possessed several unique advantages like significantly reduced membrane thickness(one atom thick in theory) and diversified molecular sieving mechanisms(in-plane pores within nanosheets interlayer galleries between nanosheets). M. Tsapatsis first carried out pioneering work on fabrication of lamellar ZSM-5 membrane. Since then, diverse 2D materials typically including graphene oxides(GOs) have been fabricated into membranes showing promising prospects in energy-efficient gas separation, pervaporation, desalination and nanofiltration. In addition to GOs, other emerging 2D materials, including 2D zeolites, 2D metal–organic frameworks(MOFs), 2 D covalent-organic frameworks(COFs), layered double hydroxides(LDHs), transition metal dichalcogenides(TMDCs), MXenes(typically Ti_3C_2TX), graphitic carbon nitrides(typically g-C_3N_4), hexagonal boron nitride(h-BN) and montmorillonites(MT) are showing intriguing performance in membrane-based separation process. This article summarized the most recent developments in the field of 2D molecular sieve membranes aside from GOs with particular emphasis on their structure–performance relationship and application prospects in industrial separation.     

Structure–performance characterization for carbon molecular sieve membranes using molecular scale gas probes

Understanding the relationship between carbon molecular sieve (CMS) pore structure and corresponding gas separation performance enables optimization for a given gas separation application. The final pyrolysis temperature and starting polymer precursor are the two critical parameters in controlling CMS performance. This study considers structure and performance changes of CMS derived from a commercially available polymer precursor at different pyrolysis temperatures. As reviewed in this paper, most traditional characterization methods based on microscopy, X-ray diffraction, spectroscopy, sorption-based pore size distribution measurements etc. provide limited information for relating separation performance to the CMS morphology and structural changes. A useful alternative approach based on different sized gases as molecular scale probes of the CMS pore structure was successfully used here in conjunction with separation data to provide critical insights into the structure–performance relationships of the engineered CMS.     

Fabrication and characterization of silica modified polyimide–zeolite mixed matrix membranes for gas separation properties

In this study, new monomers having silica groups were synthesized as an intermediate for the preparation of poly(imide siloxane)-zeolite 4A and 13X mixed matrix membranes (MMMs). The effects of membrane preparation steps, zeolite loading, precursor’s composition, and pore size of zeolite on the gas separation performance of these mixed matrix membranes were studied. The new diamine monomer was prepared from 3,5-diaminobenzoic acid (3,5-DABA), 3-aminopropyltrimethoxysilane (3-APTMS), and zeolite 4A and zeolite 13X in N-methyl-2-pyrollidone (NMP) at 180 °C. Poly(imide siloxane)-zeolite 4A and 13X MMMs were synthesized from pyromellitic dianhydride (PMDA) and 4,4-oxydianiline (ODA) in NMP using a two-step thermal imidization. SEM images of the MMMs show the interface between polymer and zeolite phases getting closer when surface modified zeolite is used. The increase in glass transition temperature (T _g) confirms the polymer chain becoming more rigid induced by the presence of zeolite. The experimental results indicated that a higher zeolite loading resulted in a decrease in gas permeability and an increase in gas pair selectivity. In terms of O₂ and N₂ permeance and ideal selectivity, the separation performances of poly(imide siloxane)-zeolite MMMs were related to the zeolite type and zeolite pore dimension.     

Hierarchical amphiphilic high-efficiency oil–water separation membranes from fermentation derived cellulose and recycled polystyrene

A high-efficiency separation of oil and water can be achieved by using specially designed amphiphilic porous membrane. However, the preparation of such membranes often involves complex multistep chemical processes. Herein, we report an amphiphilic composite membrane (polystyrene [PS]/bacterial cellulose [BC] membrane) consisting of hydrophobic recycled PS and hydrophilic BC, fabricated by a facile in situ fermentation process. Not only these membranes exhibit a combination of contrasting wettability but also comprise of a hierarchical network of microfibers and nanofibers, which makes them ideal for oil–water separation. The structural and morphological properties of as-produced BC, recycled PS membrane, and PS/BC composite membrane were studied by Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. The ability of the membranes to separate oil and water was tested by using an emulsion of hexane-in-water as the feed and the collected filtrates were characterized by optical microscopy and UV–Vis spectroscopy. PS membranes were unable to separate oil and water, while the PS/BC membrane efficiently separated water from the emulsion. PS/BC composite membranes showed a high water recovery of more than 90%, against only 57% recovery shown by BC. Mechanisms of oil–water separation for each membrane are discussed. The reusability of the PS/BC composite membrane was also demonstrated.     

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.     

Mixed matrix membranes of Matrimid 5218 loaded with zeolite 4A for pervaporation separation of water–isopropanol mixtures

Mixed matrix membranes were prepared by incorporating zeolite 4A into polyimide of Matrimid 5218 using solution-casting technique. The fabricated membranes were characterized by scanning electron microscopy (SEM), differential scanning calorimeter (DSC) and thermo gravimetric analysis (TGA). It was found that the higher annealing temperature of 250 °C is more favorable to improve adhesion between zeolite and polymer phases. Effects of different parameters such as temperature (30–60 °C), water content in feed (10–40 wt.%), zeolite loading (0–15 wt.%) and polymer content (10 and 15 wt.%) on pervaporation dehydration of isopropanol were studied. Sorption studies were carried out to evaluate degree of swelling of the membranes in feed mixtures of water and isopropanol. The experimental results showed that both pervaporation flux and selectivity increase simultaneously with increasing the zeolite content in the membranes. The membrane containing Matrimid 5218 (10 wt.%)–zeolite 4A (15 wt.%) exhibits the highest separation factor (α) of 29,991 with a substantial permeation flux (J) of 0.021 kg/m² h at 30 °C for 10 wt.% of water in the feed. The PV performance was also studied in term of pervaporation separation index (PSI). Permeation flux was found to follow the Arrhenius trend over the investigated temperature range.     

Torlon® hollow fiber membranes for organic solvent reverse osmosis separation of complex aromatic hydrocarbon mixtures

Defect-free polyamide-imide (Torlon®) hollow fiber membranes were fabricated to investigate the potential for polymer-based organic solvent reverse osmosis (OSRO) separations. The quality of the membranes was assessed by gas permeation, and the membranes were found to be defect-free. Low molecular weight cut-offs of ~180 g/mol were obtained using a complex mixture of aromatic hydrocarbons relevant to refinery separations. We demonstrate bulk OSRO-type separations of 80/20 (mol%) mixtures of toluene and 1,3,5-triisopropylbenzene (TIPB, 204 g/mol). At an upstream pressure of 80–90 bar, we find that the permeate concentration was approximately 98.5–99.0 mol% toluene and that the TIPB rejection coefficient was approximately 90% in the permeate. We observed low solvent permeances of 0.01 L/m² hr bar, which can be attributed to the low OSRO driving forces and the low permeability of Torlon®. The membranes were found to provide stable performance up to pressures of 95 bar and temperatures of 60°C.     

Investigation of the mechanism for the separation of nitrogen—oxygen mixtures on carbon molecular sieves

Sorption separation of nitrogen-oxygen mixtures by carbon molecular sieves, e.g. to produce nitrogen using the pressure swing adsorption principle, proceeds under the condition of non-equilibrium. Experimental data based upon measurements of sorption equilibria and kinetics for nitrogen and oxygen on several samples of carbon molecular sieve particles in the temperature range 0–50°C suggest the existence of a surface barrier in the pore mouths of slit-like micropores for these particular samples. The conclusion on this phenomenon is supported by the results of canonical and grand canonical Monte Carlo simulations. Both types of analysis give a deeper insight into the complex non-equilibrium separation process.     

Preparation of ZIF-8—Pebax 1657 nanocomposite membranes for n-hexane/nitrogen separation as a representative of gasoline vapour recovery

Gasoline vapour emission is hazardous to both human health and the ecosystem and also results in capital loss, altogether revealing the necessity of its recovery. Some ZIF-8–Pebax flat nanocomposite membranes were fabricated by the method of solution casting and used for gasoline vapour recovery as represented by n-hexane vapour/nitrogen separation. Microporous ZIF-8 nanoparticles were synthesized and characterized by Fourier transform infrared (FTIR) and Brunauer–Emmett–Teller (BET) analysis. BET results revealed specific surface area, total volume, and average pore diameter of 940.8 m² · g⁻¹, 0.36 cm³ · g⁻¹, and 1.54 nm, respectively. Pure nitrogen and n-hexane vapour/nitrogen gas mixture permeabilities were measured through the membranes. There was a decline in both permeation rate and selectivity up to 5.0 wt.% of ZIF-8 loading and the next increment at their higher loadings to considerably more values that the pristine membrane. The maximum n-hexane vapour permeability and selectivity at 10.0 wt.% loading of ZIF-8 nanoparticles, the feed flow rate of 173 mL · min⁻¹, and permeate side pressure of −200 mbar were observed as 280.1 Barrer and 106.7, respectively, revealing 60.0% and 36.9% improvements compared with those of the pristine Pebax membrane. Observed 86%–92% n-hexane vapour recovery approves the successful application of the ZIF-8–Pebax nanocomposite membranes for n-hexane/nitrogen separation. The long-term separation performance of 5.0 wt.% ZIF-8 loaded nanocomposite membrane was improved by 76.5% compared with that of the pristine Pebax membrane.     

Molecular simulation,experiments and modelling of single adsorption capacity of 4A molecular sieve for CO2–CH4 separation

Monte Carlo simulation of CO₂ and CH₄ adsorption on zeolite 4A is carried out in grand canonical Monte Carlo (GCMC) simulation. LTA framework was used to reproduce the structure of zeolite 4A. A comparison between the structure and properties of this zeolite and 13X, ZSM-5, 4A and 3A is performed and the results are included in the article. Universal force field was used for calculation of intermolecular forces. Our own experiments were also carried out to reinvestigate the simulation results. Ewald summation method was used for calculating electrostatic forces and atom based method was applied for van der Waals forces. The simulation results show good agreement with experimental results. Highest CO₂ adsorption capacity of zeolite 4A was in good agreement with experiments at the same pressure ranges, and was found to be 3.17 mol/kg from GCMC. Isosteric heat of adsorption was calculated to find the heat released during adsorption of each gas. Finally simulation results were fitted to four isotherms to find the best fit.     

Preparation of blend polyethersulfone/cellulose acetate/polyethylene glycol asymmetric membranes for oil–water separation

Blend PES/CA hydrophilic membranes were prepared via a phase-inversion process for oil–water separation. PEG-400 was introduced into the polymer solution in order to enhance phase-inversion and produce high permeability membranes. A gas permeation test was conducted to estimate mean pore size and surface porosity of the membranes. The membranes were characterized in terms of morphology, overall porosity, water contact angle, water flux and hydraulic resistance. A cross-flow separation system was used to evaluate oil–water separation performance of the membranes. From FESEM examination, the prepared PES/CA membrane presented thinner outer skin layer, higher surface porosity with larger pore sizes. The outer surface water contact angle of the prepared membrane significantly decreased when CA was added into the polymer solution. The higher water flux of the PES/CA membrane was related to the higher hydrophilicity and larger pore sizes of the membrane. From oil–water separation test, the PES/CA membrane showed stable oil rejection of 88 % and water flux of 27 l/m² s after 150 min of the operation. In conclusion, by controlling fabrication parameters a developed membrane structure with high hydrophilicity, high surface porosity and low resistance can be achieved to improve oil rejection and water productivity.     

UTSA-280 metal–organic framework incorporated 6FDA-polyimide mixed-matrix membranes for ethylene/ethane separation

Mixed-matrix membranes (MMMs), judiciously combining processability of polymer and remarkable separation performance of nanofillers, have been extensive pursuits for molecular separation process. Permeability matching between filler and polymer is one of the necessary requisites to desirable mixed-matrix effect. Considering the superior molecular sieving effect of UTSA-280 metal-organic frameworks on C₂H₄ and C₂H₆, here, we report two types of UTSA-280/6FDA-polyimide MMMs toward C₂H₄/C₂H₆ separation. The molecular sieving effect of UTSA-280 endowed 6FDA-DAM:DABA(3:2) membrane with simultaneous improvements in C₂H₄ permeability and C₂H₄/C₂H₆ selectivity. Optimally, when the filler reached 21.80 wt%, C₂H₄ permeability and C₂H₄/C₂H₆ selectivity was increased to 6.49 Barrer (by 15%) and 4.94 (by 32%), respectively. On the contrary, UTSA-280/6FDA-DAM MMMs showed undesirable mixed-matrix effect that C₂H₄ permeability decreased meanwhile C₂H₄/C₂H₆ selectivity nearly kept at polymeric pristine membrane level. It was found that permeability matching between two phases was responsible to these opposite mixed-matrix effects. More specifically, UTSA-280 had a relatively low gas permeability so that it required a less permeable polymeric matrix like 6FDA-DAM:DABA(3:2) to exert its molecular sieving effect. Furthermore, the optimal-matching 6FDA-matrix in permeability with UTSA-280 fillers was predicted by theoretical model. This work not only reports improving C₂H₄/C₂H₆ separation performance via mixed-matrix formulation, but also emphasizes the importance of permeability matching between polymer and filler to realize the mixed-matrix effect.     

Pervaporation of water/acetic acid through polyimide membranes from 3,3′, 4,4′-benzophenone tetracarboxylic dianhydride and 4,4′-oxydianiline

Summary Pervaporation performance of polyimide (PI) membrane from 3,3, 4,4-benzophenone tetracarboxylic dianhydride (BTDA) and 4,4-oxydianiline (ODA) was investigated at 80wt% feed acetic acid concentration and at 65°C. Imide contents in PI film were estimated by thermogravimetric analysis (TGA) method. The separation factor of PI membrane increases with the degree of imidization, while the flux was almost constant. The separation factor toward water through PI-94 membrane was about 417 with the flux of 47 g/m². hr measured at 65°C and with 80wt% acetic acid as a feed. PI-94 showed the best pervaporation performance toward the separation of water from acetic acid solution among the PI membranes investigated. The swelling behaviors at various feed concenration were also examined.     

Mixed matrix metal–organic framework membranes for efficient CO2/N2 separation under humid conditions

Mixed matrix metal–organic framework (MOF) membranes show excellent application prospects in gas separation. However, their stability in various practical application scenarios is poor, especially under humid conditions. Herein, we encapsulated a hydrophobic ionic liquid (IL) into the cavity of MOFs, which effectively mitigated the competition between H₂O and CO₂ in humid gas mixtures, leading to stable and high-performance gas separation. For this reason, the resulting membranes using polymer of intrinsic miroporosity-1 (PIM-1) as a polymer matrix show good CO₂/N₂ separation performance and long-term test stability under humid environment. In particular, the 20 wt% IL-UiO/PIM-1 shows a high permeability of 13,778 Barrer and competitive CO₂/N₂ separation factor of ~35.2, transcending the latest upper bound. Besides, the according membrane module exhibits slightly decreased CO₂ permeability and selectivity, promoting the application of self-supporting membranes. This work provides a reliable strategy for the rational design of MOF-based hybrid membranes under extreme conditions.     

Using a simple method to prepare UiO-66-NH2/chitosan composite membranes for oil–water separation

Chitosan (CS) was used as a cross-linking agent to modify UiO-66-NH₂, and the modified UiO-66-NH₂ was fixed on the mixed cellulose membrane (MCE) through vacuum filtration technology to prepare a new type of membrane. The membrane exhibited excellent hydrophilicity in the air and excellent super-oleophobic performance underwater, and effectively separated various oil–water emulsions. When separating petroleum ether-water emulsion, the filtration flux of the modified membrane was 2000 L m⁻² h⁻¹ higher than that of MCE, and the separation efficiency can reach more than 95%. After 10 cycles, the flux of the modified membrane was about four times of that MCE, which was 500 L m⁻² h⁻¹. Most importantly, the membrane still maintained underwater superoleophobicity in the environment of strong acid, strong base, and salt solution.