Gases In Membranes Research Articles

PIM-1 based mixed matrix membranes with MOF-808(Ce) for CO2 separations

Metal-organic frameworks (MOFs) have shown potential as nanofillers for the selective capture of specific gases in mixed matrix membranes (MMMs). This study introduces an efficient method, utilizing a mild (100 °C) reaction condition and a short reaction time (20 min), for the synthesis of MOF-808(Ce) nanocrystals. The MOF-808(Ce) that is obtained exhibits porous characteristics, a notable affinity for CO2, and robust intermolecular interactions with PIM-1. Furthermore, this study pioneers the incorporation of MOF-808(Ce) as a filler into PIM-1, with varying loadings, to create PIM/MOF-808 MMMs for the purpose of CO2 separation. Consequently, the MMMs containing 2 wt% MOF-808(Ce) demonstrate high CO2 permeability of 6854 Barrer and CO2/N2 selectivity of 23.2, which is far surpasses 2008 upper bound from Robeson and approaches the refined 2019 upper bound from MeKenow. A comparative analysis demonstrates the efficacy of MOF-808(Ce) as a filler in PIM-1 for the purpose of CO2 separation.

Permeance of Condensable Gases in Rubbery Polymer Membranes at High Pressure.

The gas transport properties of thin film composite membranes (TFCMs) with selective layers of PolyActive™, polydimethylsiloxane (PDMS), and polyoctylmethylsiloxane (POMS) were investigated over a range of temperatures (10-34 °C; temperature increments of 2 °C) and pressures (1-65 bar abs; 38 pressure increments). The variation in the feed pressure of condensable gases CO2 and C2H6 enabled the observation of peaks of permeance in dependence on the feed pressure and temperature. For PDMS and POMS, the permeance peak was reproduced at the same feed gas activity as when the feed temperature was changed. PolyActive™ TFCM showed a more complex behaviour, most probably due to a higher CO2 affinity towards the poly(ethylene glycol) domains of this block copolymer. A significant decrease in the permeate temperature associated with the Joule-Thomson effect was observed for all TFCMs. The stepwise permeance drop was observed at a feed gas activity of p/po ≥ 1, clearly indicating that a penetrant transfer through the selective layer occurs only according to the conditions on the feed side of the membrane. The permeate side gas temperature has no influence on the state of the selective layer or penetrant diffusing through it. The most likely cause of the observed TFCM behaviour is capillary condensation of the penetrant in the swollen selective layer material, which can be provoked by the clustering of penetrant molecules.

Activated Gas Transport in Polymer-Grafted Nanoparticle Membranes

Carbon capture and related gas separation processes are critical tools in our efforts to combat climate change. While polymer membranes are seen as a central construct to achieve these goals, their performance needs further improvement to meet current sustainability goals. It is in this context that membranes composed of polymer-grafted nanoparticles (GNPs) become highly germane. Previous work has shown that gas transport in pure GNP membranes can be strongly enhanced relative to that in the corresponding neat polymer. Additionally, we found that larger gases display greater degrees of permeability enhancement than smaller ones. There is currently no clear understanding of these two underpinning facts, and to address this issue, we have characterized the activation energy driving the permeability of multiple gases in several GNP membranes. Our results show that gases smaller than a critical size display the same activation energy as in the native polymer─any enhancement therefore is associated with changes in local gas/chain dynamics, e.g., the speeding up of polymer dynamics due to the local stretching in a brush conformation. Our results are consistent with the notion that small gases are present in the whole polymer layer. Conversely, larger gases show substantially lower activation energies and, in contrast to the neat polymer, exhibit essentially no dependence on penetrant size. Sorption measurements illustrate that this is a result of the enhanced solubility of larger gases in GNPs, relative to that in the pure polymer, which is reflected in their significantly more favorable enthalpies of solvation. Larger gases thus preferentially sorb into the interstitial spaces in the GNP assembly, thereby resulting in even higher permeability enhancements in the GNP layers relative to smaller gases. GNPs, therefore, provide us with multiple handles to control gas transport, a fact that offers critical insights into their utility for gas separations.

Comprehensive characterization of gas diffusion through graphene oxide membranes

Graphene oxide (GO) based multi-layered membranes have shown outstanding molecular-sieving properties for gas separation, surpassing the upper bound for polymeric membranes especially for hydrogen decarbonization. At the same time, the mechanism of gas permeation through such 2D GO membranes is very different in comparison to the traditional polymeric membranes due to multilayer, laminated nature of the former. For strategical design of novel membranes based on two-dimensional materials, it is important to understand the mechanism and to be able to measure two key parameters for gas transport: diffusivity and solubility. Such measurements are well established for the characterization of transport properties of bulk polymeric membranes. However, it is still a challenge to measure gas diffusion coefficients directly and accurately in ultra-thin multi-layered membranes. The lack of characterization limits our understanding of the mechanisms of gas transport though such membranes. In this work, we applied a time-lag method to determine the diffusivities for He, H2, O2, N2, CH4, CO2 and H2/CO2 equimolar mixture by on-line mass spectrometry. In contrast to polymeric membranes, the diffusivity and diffusion activation energy for all gases in 2D membranes are exponentially dependent on pathway length. Thus, in 2D membrane we can use an easy strategy to precisely regulate permeability and selectivity by the adjustment of the number of nanolayers and the size of 2D flakes, which is not possible using traditional polymeric membranes. This study is important for both the characterization and the standardization of gas transport properties of multi-layered membranes, and the design of novel membranes based on 2D materials.

Mixed matrix membranes comprising 6FDA-based polyimide blends and UiO-66 with co-continuous structures for gas separations

Membrane gas separation is considered a highly promising alternative to conventional gas separation. Recently, mixed matrix membranes containing UiO-66 metal–organic frameworks (MOFs) have been demonstrated to achieve high gas separation performance, especially in CO2-related separation. To date, few studies have used morphological control strategies to enhance the gas transport properties of mixed matrix membranes. In this study, we fabricated mixed matrix membranes comprising porous UiO-66 nanoparticles and immiscible 6FDA-based polyimide blends with co-continuous structures. The selective location of UiO-66 nanoparticles within one polymer phase of the co-continuous polymer blend was observed. However, this co-continuous structure cannot be referred to as a double-percolative morphology since the percolative UiO-66 networks did not successfully form. The gas permeability and gas sorption properties of pure gases (H2, CO2, N2, O2, and CH4) in the dense membranes and mixed matrix membranes were measured using the constant-volume variable-pressure method and the dual-volume pressure decay method, respectively. On the basis of the solution–diffusion mechanism, the contributions of gas sorption and gas transport to the overall gas permeation properties were determined and examined. When the UiO-66 content reached 35 wt%, the CO2 permeability of the mixed matrix membranes was significantly increased from 16.5 Barrer to 104.7 Barrer (by 635 %) without scarifying the permeability selectivities of CO2/N2 (16.5 ± 1.5) and CO2/CH4 (31.7 ± 5.8). The results suggest that both the addition of porous UiO-66 nanoparticles and the co-continuous structures contributed to the strongly enhanced gas separation performance of the mixed matrix membranes.

Modelling Sorption and Transport of Gases in Polymeric Membranes across Different Scales: A Review.

Professor Giulio C. Sarti has provided outstanding contributions to the modelling of fluid sorption and transport in polymeric materials, with a special eye on industrial applications such as membrane separation, due to his Chemical Engineering background. He was the co-creator of innovative theories such as the Non-Equilibrium Theory for Glassy Polymers (NET-GP), a flexible tool to estimate the solubility of pure and mixed fluids in a wide range of polymers, and of the Standard Transport Model (STM) for estimating membrane permeability and selectivity. In this review, inspired by his rigorous and original approach to representing membrane fundamentals, we provide an overview of the most significant and up-to-date modeling tools available to estimate the main properties governing polymeric membranes in fluid separation, namely solubility and diffusivity. The paper is not meant to be comprehensive, but it focuses on those contributions that are most relevant or that show the potential to be relevant in the future. We do not restrict our view to the field of macroscopic modelling, which was the main playground of professor Sarti, but also devote our attention to Molecular and Multiscale Hierarchical Modeling. This work proposes a critical evaluation of the different approaches considered, along with their limitations and potentiality.

An approach for evaluating gas sorption in polymer matrix membranes based on balancing incoming and outgoing membrane mass fluxes

The transport of gases in polymeric membranes can be described by a solution–diffusion mechanism in which the individual transport parameters are determined from independent measurements of the gas permeability and diffusivity and further measurements of the gas solubility in the polymer of interest. Herein, we propose a new method for determining all three transport parameters from a single permeation experiment performed using a modified classical permeation cell with very small volumes above and below the membrane (1.192 and 0.658 mL, respectively). The new concept is based on the evaluation of gas sorption in the membrane determined from the balance of mass fluxes entering and leaving the membrane. The amount of gas sorbed in the membrane can then be calculated by integrating the area between the fluxes. This arrangement allows for the rapid and complex characterization of the gas transport and separation properties of polymeric materials using a single experiment. The novel approach was tested on a polydimethylsiloxane membrane for three gases having considerably different solubilities in the polymer, and the results were consistent with those obtained using classical procedures.

Gas Mass-Transport Coefficients in Ionomer Membranes Using a Microelectrode.

Gas permeability, the product of gas diffusivity and Henry's gas-absorption constant, of ionomer membranes is an important transport parameter in fuel cell and electrolyzer research as it governs gas crossover between electrodes and perhaps in the catalyst layers as well. During transient operation, it is important to divide the gas permeability into its constituent properties as they are individually important. Although transient microelectrode measurements have been used previously to separate the gas permeability into these two parameters, inconsistencies remain in the interpretation of the experimental techniques. In this work, a new interpretation methodology is introduced for determining independently diffusivity and Henry's constant of hydrogen and oxygen gases in ionomer membranes (Nafion 211 and Nafion XL) as a function of relative humidity using microelectrodes. Two time regimes are accounted for. At long times, gas permeability is determined from a two-dimensional numerical model that calculates the solubilized-gas concentration profiles at a steady state. At short times, permeability is deconvoluted into diffusivity and Henry's constant by analyzing transient data with an extended Cottrell equation that corrects for actual electrode surface area. Gas permeability and diffusivity increase as relative humidity increases for both gases in both membranes, whereas Henry's constants for both gases decrease with increasing relative humidity. In addition, results for Nafion 211 membranes are compared to a simple phase-separated parallel-diffusion transport theory with good agreement. The two-time-regime analysis and the experimental methodology can be applied to other electrochemical systems to enable greater precision in the calculation of transport parameters and to further understanding of gas transport in fuel cells and electrolyzers.

Tuning the Transport Properties of Gases in Porous Graphene Membranes with Controlled Pore Size and Thickness

AbstractPorous graphene membranes have emerged as promising alternatives for gas‐separation applications due to their atomic thickness enabling ultrahigh permeance, but they suffer from low gas selectivity. Whereas decreasing the pore size below 3 nm is expected to increase the gas selectivity due to molecular sieving, it is rather challenging to generate a large number of uniform small pores on the graphene surface. Here, a pore‐narrowing approach via gold deposition onto porous graphene surface is introduced to tune the pore size and thickness of the membrane to achieve a large number of small pores. Through the systematic approach, the ideal combination is determined as pore size below 3 nm, obtained at the thickness of 100 nm, to attain high selectivity and high permeance. The resulting membrane shows a H2/CO2 separation factor of 31.3 at H2 permeance of 2.23 × 105 GPU (1 GPU = 3.35 × 10−10 mol s−1 m−2 Pa−1), which is the highest value reported to date in the 105 GPU permeance range. This result is explained by comparing the predicted binding energies of gas molecules with the Au surface, −5.3 versus −21 kJ mol−1 for H2 and CO2, respectively, increased surface–gas interactions and molecular‐sieving effect with decreasing pore size.

Tuning the transport properties of gases in porous graphene membranes with controlled pore size and thickness

Tuning the transport properties of gases in porous graphene membranes with controlled pore size and thickness

Application of the fractal model to estimate the diffusion and solubility of gases in non-porous membranes based on aromatic polynorbornene dicarboximides

Application of the fractal model to estimate the diffusion and solubility of gases in non-porous membranes based on aromatic polynorbornene dicarboximides

Improving Gas Selectivity in Membranes Using Polymer-Grafted Silica Nanoparticles

Herein, we demonstrate that tuning of structural parameters and material chemistry enables control of the permeability and selectivity of gases (CO2, He, and CH4) in membranes based on poly(butyl methacrylate)-grafted nanoparticles (PGNs). Our data show that the presence of nanoparticles and the overall dense packing of grafted chains noticeable in PGNs with low grafting density have an adverse effect on the diffusivity of gases. This effect is compensated by an improvement in the solubility of CO2 gas promoted by the silica nanoparticle surface, yielding a substantial improvement in the permeability of CO2 versus CH4. In membranes with high grafting density, changes in the structural arrangements and alterations in the membrane porosity, evident from small-angle X-ray scattering and positron annihilation lifetime spectroscopy, positively influence the permeability of He and CO2 gases. In contrast, CH4 permeability in the same membranes is significantly suppressed, suggesting the formation of a unique, highly selective environment for gas separation. As a result, an improvement of up to 50% in selectivity for gas pairs containing large CH4 molecules is observed. Our studies provide fundamental insights into the role that structural parameters play in gas transport through polymer membranes, laying a foundation for the rational design of membranes with improved permeability and selectivity.

Experimental and modeling study of CO2 separation by modified Pebax 1657 TFC membranes

Ionic liquid modified Pebax 1657 thin-film composite (TFC) membranes were synthesized on different porous supports for improved separation of CO2 from CH4 and N2. XRD, SEM, FTIR, TGA, DFT, and BJH tests were utilized for the characterization of TFC membranes and supports. The incorporation of 1,3-Di-n-butyl-2-methylimidazolium chloride ionic liquid into the Pebax 1657 solution led to better compatibility between selective layer and support. The results of gas permeation tests showed an enhanced CO2/CH4 and CO2/N2 selectivity for most of the ionic liquid modified TFC membranes. To derive a predictive model for the permeability of gases in TFC membranes, correction factors (β) for fractional free volumes of TFC membranes were linearly correlated against correction factors (α) for porosity and tortuosity of the substrate in the dusty gas model using CO2 experimental permeance data at 20, 60, 100 and 140 °C. The resulting modified model showed remarkable effectiveness in the prediction of permeability of CH4 and N2 in the TFC membrane at investigated temperatures. The comparison of model predictive performance with previous models showed the supremacy of the new model in terms of average absolute relative errors (AARE) and the standard deviation of relative errors (σ).

Accurate prediction of solubility of gases within H2-selective nanocomposite membranes using committee machine intelligent system

In-depth knowledge about the gas sorption within hydrogen (H2) selective nanocomposite membranes at various conditions is crucial, particularly in petrochemical and separation processes. Hence, various artificial intelligence (AI) methods such as multilayer perceptron artificial neural network (MLP-ANN), adaptive neuro-fuzzy inference system (ANFIS), the adaptive neuro-fuzzy inference system optimized by genetic algorithm (GA-ANFIS), Genetic Programming (GP) and Committee Machine Intelligent System (CMIS) were applied to predict the sorption of gases in H2-selective nanocomposite membranes consist of porous nanoparticles as the dispersed phase and polymer matrix as continuous phase. The momentous purpose of this paper was to estimate the sorption of C3H8, H2, CH4 and CO2 within H2-selective nanocomposite membranes considering the effect of nanoparticles loading, critical temperature (gas type characteristics) and upstream pressure. Obtained data were categorized into two parts including training and testing data set. The CMIS method showed more precise results rather than other intelligent models. Having developed different intelligent approaches rely on algorithms, a powerful successor for labor-intensive experimental processes of solubility was revealed. The prediction results and experimental data were significantly consistent in approach with a correlation coefficient (R2) of 0.9999, 0.9987, 0.9998, 0.9995, and 0.9997 for CMIS, GP, GA-ANFIS, ANFIS and ANN models respectively.

Gas Transport in Glassy Polymers: Prediction of Diffusional Time Lag.

The transport of gases in glassy polymeric membranes has been analyzed by means of a fundamental approach based on the nonequilibrium thermodynamic model for glassy polymers (NET-GP) that considers the penetrant chemical potential gradient as the actual driving force of the diffusional process. The diffusivity of a penetrant is thus described as the product of a purely kinetic quantity, the penetrant mobility, and a thermodynamic factor, accounting for the chemical potential dependence on its concentration in the polymer. The NET-GP approach, and the nonequilibrium lattice fluid (NELF) model in particular, describes the thermodynamic behavior of penetrant/polymer mixtures in the glassy state, at each pressure or composition. Moreover, the mobility is considered to follow a simple exponential dependence on penetrant concentration, as typically observed experimentally, using only two adjustable parameters, the infinite dilution penetrant mobility L10 and the plasticization factor β, both determined from the analysis of the dependence of steady state permeability on upstream pressure. The available literature data of diffusional time lag as a function of penetrant upstream pressure has been reviewed and compared with model predictions, obtained after the values of the two model parameters (L10 and β), have been conveniently determined from steady state permeability data. The model is shown to be able to describe very accurately the experimental time lag behaviors for all penetrant/polymer pairs inspected, including those presenting an increasing permeability with increasing upstream pressure. The model is thus more appropriate than the one based on Dual Mode Sorption, which usually provides an unsatisfactory description of time lag and required an ad hoc modification.

Coupled GCMC and LBM simulation method for visualizations of CO2/CH4 gas separation through Cu-BTC membranes

A fully dynamic model for mixture gas separation is built in Cu-BTC membranes. A multi-scale method that couples the lattice Boltzmann method with grand canonical Monte Carlo (GCMC) is proposed to investigate the mass transfer process of CO2/CH4 mixture gases in Cu-BTC membranes. The convection and diffusion in the interparticle flow field and the intraparticle diffusion and adsorption in the particle interior are simultaneously considered. The membrane morphology is reconstructed by a sphere-based simulated annealing method. The effects of membrane porosity and particle size on the mass transfer and selectivity of CO2/CH4 mixture gases are predicted. Results show that the selectivity of CO2/CH4 is mainly determined by interparticle and intraparticle mass transfer resistances. Meanwhile, the time of saturation adsorption for CO2 and CH4 both decrease with an increase in porosity but decreases for CO2 and increases for CH4 with an increase in particle size. The selectivity of CO2/CH4 in Cu-BTC membranes decreases with an increase in porosity and particle size. Therefore, membranes with small porosity and particle size should be utilized. Compared with the traditional binary GCMC and ISAT methods based on saturation adsorption, the proposed coupled method is closer to the physical essence of the process because it considers dynamic competitive adsorption. The present method can be helpful in the design of efficient metal–organic framework (MOF) membranes.

Gas permeation modeling of mixed matrix membranes: Adsorption isotherms and permeability models

In order to calculate the permeability of gases in mixed matrix membranes (MMMs) filled with porous particles, the first step is to calculate the permeability of filler particles. Because particle pore sizes of the considered sodium zeolite Y (NaY) in this study is greater than the molecular sizes of the tested gases (CO2 and N2), it is assumed that diffusion of the gases in the zeolite particles is from Knudsen type. Moreover, the gas sorption data (isotherms) were measured experimentally and then fitted with Langmuir and Freundlich model equations to calculate the related constants. Finally, the experimental CO2 and N2 permeabilities of cellulose acetate (CA)/NaY MMMs were compared with the corresponding data predicted by the well‐known permeability models in this field. It was concluded that the Bruggeman model has better predictions among the other models, especially for the higher particle loadings. POLYM. COMPOS., 39:4560–4568, 2018. © 2017 Society of Plastics Engineers

Molecular transport through mixed matrix membranes: A time‐dependent density functional approach

The transport properties of gases in mixed matrix membranes (MMMs) are important in materials design. Here, a novel time‐dependent density functional theory (TDDFT) method to study the transport properties of gases in MMMs is developed. The MMM is modeled by inserting a spherical filler into the continuous polymer phase, which is similar to the Maxwell model; additionally, the inhomogeneity of the filler and the molecular correlations were taken into account in the TDDFT method. Transport properties such as permeation, density profile, flux, and chemical potential are examined and discussed. TDDFT prediction of the permeation is found to be higher than that of the Maxwell model, and the filler‐polymer interface is key to tuning this effect, which also seems to be the dominating factor in the transport process on both the microscopic and macroscopic scale. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4586–4594, 2017

Determination of Gas Transport Coefficients of Mixed Gases in 6FDA-TMPDA Polyimide by NMR Spectroscopy

The solubility and diffusion coefficients of pure carbon dioxide and its mixtures with methane, ethylene, and propylene in poly(4,4′-hexafluoroisopropylidene diphthalic anhydride-2,3,5,6-tetramethyl-1,4-phenylenediamine) (6FDA-TMPDA) membranes were determined with proton and 13C NMR spectroscopy and pulsed-field gradient NMR, respectively. In addition, the solubility and permeability of the pure gases in the polyimide membranes were also measured by sorption and permeation methods. The results showed that NMR measurements on pure gases are very reproducible and in relatively good agreement with those determined by the other methods. Furthermore, the NMR method allows the independent measurement of the solubility and diffusion coefficients for each component in mixtures of two or three gases and subsequently the gas permeability. The main conclusion of this work is that NMR spectroscopy is a suitable tool to determine all transport coefficients of multicomponent gas mixtures in polymer membranes, provided ...

Extending effective medium theory to finite size systems: Theory and simulation for permeation in mixed-matrix membranes

We present a novel theory for estimation of the effective permeability of pure gases in flat mixed-matrix membranes (MMMs), in which effective medium theory (EMT) is extended to systems with finite filler size and membrane thickness. We introduce an inhomogeneous filler volume fraction profile, which arises due to depletion of the filler in regions adjacent to the membrane ends, into the MMM permeation model. In this way, the effective medium approach (EMA) can still be applied to systems where the dispersant size is not small in comparison to the membrane thickness, and for which a permeability profiles arises in the MMM that is dependent on both filler size and membrane thickness, besides the filler-polymer equilibrium constant. It is found that increase in particle size reduces the effective membrane permeability at fixed membrane thickness, and that the effective membrane permeability increases with increase of the membrane thickness to asymptotically reach the value predicted by existing models. The present theory is validated against detailed simulations of the transport in MMMs, and theoretical predictions are found to be in agreement with those obtained from the exact calculations. Further, comparison of the exact effective permeability at different filler volume fractions is made for different packing configurations, showing variations in dispersant packing structure to have only a very weak effect on MMM performance.