How to Get the Best Gas Separation Membranes from State-of-the-Art Glassy Polymers

Melamine-Copolymerization Strategy Engineered Fluorinated Polyimides for Membrane-Based Sour Natural Gas Separation

Membrane-based gas separation provides an energy-efficient approach for the simultaneous CO2 and H2S removal from sour natural gas. The fluorinated polyimide (PI) membranes exhibited a promising balance between permeability and permselectivity for sour natural gas separation. To further improve the separation efficiency of fluorinated PI membranes, a melamine-copolymerization synthetic approach is devised that aims to incorporate melamine motifs with high sour gas affinity into the structure of the PI membranes. The fluorinated copolyimide membranes that are structurally engineered exhibited excellent solution-processability and enhanced sweet-mixed gas selectivity compared to their original PI membranes. Additionally, under a five-component sour mixed-gas feed, these melamine-copolymerized fluorinated PI membranes provided superior combined H2S and CO2 removal efficiency in comparison to conventional glassy polymer membranes. The melamine-copolymerization strategy provides an easily operable and generally effective approach to developing performance-enhancing PI membranes for sour natural gas separation.

Insights into Cis-Amide-Modified Carbon Nanotubes for Selective Purification of CH4 and H2 from Gas Mixtures: A Comparative DFT Study

Among a plethora of mixtures, the methane (CH4) and hydrogen (H2) mixture has garnered considerable attention for multiple reasons, especially in the framework of energy production and industrial processes as well as ecological considerations. Despite the fact that the CH4/H2 mixture performs many critical tasks, the presence of other gases, such as carbon dioxide, sulfur compounds like H2S, and water vapor, leads to many undesirable consequences. Thus purification of this mixture from these gases assumes considerable relevance. In the current research, first-principle calculations in the frame of density functional theory are carried out to propose a new functional group for vertically aligned carbon nanotubes (VA-CNTs) interacting preferentially with polar molecules rather than CH4 and H2 in order to obtain a more efficient methane and hydrogen separations The binding energies associated with the interactions between several chemical groups and target gases were calculated first, and then a functional group formed by a modified ethylene glycol and acetyl amide was selected. This functional group was attached to the CNT edge with an appropriate diameter, and hence the binding energies with the target gases and steric hindrance were evaluated. The binding energy of the most polar molecule (H2O) was found to be more than six times higher than that of H2, indicating a significant enhancement of the nanotube tip's affinity toward polar gases. Thus, this functionalization is beneficial for enhancing the capability of highly packed functionalized VA-CNT membranes to purify CH4/H2 gas mixtures.

All-atom molecular dynamics simulation of solvent diffusion in an unentangled polystyrene film

The diffusion behavior of low molecular weight solvents within an unentangled polystyrene film below and above its glass transition temperature is investigated. The diffusion behavior in the glassy state exhibits a distinct behavior known as case II or class II diffusion, noticeably diverging from conventional Fickian diffusion observed above the glass transition temperature of the polymer film. In the context of case II diffusion, the primary experimental observation entails the emergence of a well-defined concentration front moving at a constant speed, delineating a swollen, rubbery region from a glassy region within the polymer system. Despite the prevalence of this phenomenon in experimental settings, simulating case II diffusion has posed a significant challenge, primarily due to the computationally intensive nature of the diffusion process. To address this, we have developed an all-atom molecular dynamics simulation approach for the observation of case II diffusion in glassy polymers. This method aims to unravel the intricacies of the diffusion process, providing valuable insights into the dynamic interactions between solvents and the polymer matrix.

Investigation of cellulose acetate and ZIF-8 mixed matrix membrane for CO2 separation from model biogas

The separation of CO2 from the biogas mixtures (CH4/CO2) is essential for biogas upgradation. However, polymer membranes used for CO2 separation exhibit low permeability. Mixed Matrix Membranes (MMMs) incorporating inorganic filler in the polymer enhance CO2 separation. In this work, bio-degradable cellulose acetate (CA) based MMMs with varying filler weight percentages (2-20 wt.%) of ZIF-8 were studied for the separation of CO2 from a model biogas (CH4/CO2) mixture. The MMMs were characterized by analysis of TGA and DSC for thermal stability and FTIR for alteration or formation of any new functional group. FESEM was done to evaluate the dispersion and interaction of ZIF-8 in the CA polymer matrix. Considering the economic aspect, the fabricated MMMs were tested for gas separation performance at reasonably lower feed pressure (1.5, 2 bar). MMM with 5 and 10 wt.% of ZIF-8/ CA MMMs showed the best performance with CO2 permeability of 9.65 Barrer and 9.5 Barrer, approximately two folds as compared to pure CA, and CO2/CH4 selectivity was 10.37 and 15.3. The experimental results were compared with the predicted gas permeation results determined using MMM transport predictive models, and found that the permeabilities were higher than the model predictions.

Mitigation of Physical Aging of Polymeric Membrane Materials for Gas Separation: A Review

The first commercial hollow fiber and flat sheet gas separation membranes were produced in the late 1970s from the glassy polymers polysulfone and poly(vinyltrimethyl silane), respectively, and the first industrial application was hydrogen recovery from ammonia purge gas in the ammonia synthesis loop. Membranes based on glassy polymers (polysulfone, cellulose acetate, polyimides, substituted polycarbonate, and poly(phenylene oxide)) are currently used in various industrial processes, such as hydrogen purification, nitrogen production, and natural gas treatment. However, the glassy polymers are in a non-equilibrium state; therefore, these polymers undergo a process of physical aging, which is accompanied by the spontaneous reduction of free volume and gas permeability over time. The high free volume glassy polymers, such as poly(1-trimethylgermyl-1-propyne), polymers of intrinsic microporosity PIMs, and fluoropolymers Teflon^® AF and Hyflon^® AD, undergo significant physical aging. Herein, we outline the latest progress in the field of increasing durability and mitigating the physical aging of glassy polymer membrane materials and thin-film composite membranes for gas separation. Special attention is paid to such approaches as the addition of porous nanoparticles (via mixed matrix membranes), polymer crosslinking, and a combination of crosslinking and addition of nanoparticles.