Chapter 4. Recent High Performance Polymer Membranes for CO2 Separation

To prevent greenhouse emissions into the atmosphere, separations like CO2/CH4 and CO2/N2 from natural gas, biogas, and flue gasses are crucial. Polymer membranes gained a key role in gas separations over the past decades, but these polymers are often not organized at a molecular level, which results in a trade-off between permeability and selectivity. In this work, the effect of molecular order and orientation in liquid crystals (LCs) polymer membranes for gas permeation is demonstrated. Using the self-assembly of polymerizable LCs to prepare membranes ensures control over the supramolecular organization and alignment of the building blocks at a molecular level. Robust freestanding LC membranes were fabricated that have various, distinct morphologies (isotropic, nematic cybotactic, and smectic C) and alignment (planar and homeotropic), while using the same chemical composition. Single gas permeation data show that the permeability decreases with increasing molecular order while the ideal gas selectivity of He and CO2 over N2 increases tremendously (36-fold for He/N2 and 21-fold for CO2/N2) when going from randomly ordered to the highly ordered smectic C morphology. The calculated diffusion coefficients showed a 10-fold decrease when going from randomly ordered membranes to ordered smectic C membranes. It is proposed that with increasing molecular order, the free volume elements in the membrane become smaller, which hinders gasses with larger kinetic diameters (Ar, N2) more than gasses with smaller kinetic diameters (He, CO2), inducing selectivity. Comparison of gas sorption and permeation performances of planar and homeotropic aligned smectic C membranes shows the effect of molecular orientation by a 3-fold decrease of the diffusion coefficient of homeotropic aligned smectic C membranes resulting in a diminished gas permeation and increased ideal gas selectivities. These results strongly highlight the importance of molecular order and orientation in LC polymer membranes for gas separation.

Thermally rearranged (TR) polymer membranes for CO 2 separation

Unique Ligand Exchange Dynamics of Metal–Organic Polyhedra for Vitrimer-like Gas Separation Membranes

Superglassy polymers have emerged as potential membrane materials for several gas separation applications, including acid gas removal from natural gas. Despite the superior performance shown at laboratory scale, their use at industrial scale is hampered by their large drop in gas permeability over time due to physical aging. Several strategies are proposed in the literature to prevent loss of performance, the incorporation of fillers being a successful approach. In this work, we provide a comprehensive economic study on the application of superglassy membranes in a hybrid membrane/amine process for natural gas sweetening. The hybrid process is compared with the more traditional stand-alone amine-absorption technique for a range of membrane gas separation properties (CO2 permeance and CO2/CH4 selectivity), and recommendations for long-term membrane performance are made. These recommendations can drive future research on producing mixed matrix membranes (MMMs) of superglassy polymers with anti-aging properties (i.e., target permeance and selectivity is maintained over time), as thin film nanocomposite membranes (TFNs). For the selected natural gas composition of 28% of acid gas content (8% CO2 and 20% H2S), we have found that a CO2 permeance of 200 GPU and a CO2/CH4 selectivity of 16 is an optimal target.

Recent progress on functional polymeric membranes for CO2 separation from flue gases: A review

Aging in super glassy polymers such as poly(trimethylsilylpropyne) (PTMSP), poly(4-methyl-2-pentyne) (PMP), and polymers with intrinsic microporosity (PIM-1) reduces gas permeabilities and limits their application as gas-separation membranes. While super glassy polymers are initially very porous, and ultra-permeable, they quickly pack into a denser phase becoming less porous and permeable. This age-old problem has been solved by adding an ultraporous additive that maintains the low density, porous, initial stage of super glassy polymers through absorbing a portion of the polymer chains within its pores thereby holding the chains in their open position. This result is the first time that aging in super glassy polymers is inhibited whilst maintaining enhanced CO2 permeability for one year and improving CO2/N2 selectivity. This approach could allow super glassy polymers to be revisited for commercial application in gas separations.

Sorption, diffusion, and permeability of humid gases and aging of thermally rearranged (TR) polymer membranes from a novel ortho-hydroxypolyimide

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/AmimCl 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 (O2, 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.

Membrane-based gas separation has attracted a great deal of attention recently due to the requirement for high purity gasses in industrial applications like fuel cells, and because of environment concerns, such as global warming. The current methods of cryogenic distillation and pressure swing adsorption are energy intensive and costly. Therefore, polymer membranes have emerged as a less energy intensive and cost effective candidate to separate gas mixtures. However, the use of polymeric membranes has a drawback known as the permeability-selectivity tradeoff. Many approaches have been used to overcome this limitation including the use of polymer blends. Polymer blending technology synergistically combines the favorable properties of different polymers like high gas permeability and high selectivity, which are difficult to attain with a single polymer. During polymer mixing, polymers tend to uncontrollably phase separate due to unfavorable thermodynamics, which limits the number of completely miscible polymer combinations for gas separations. Therefore, compatibilizers are used to control the phase separation and to obtain stable membrane morphologies, while improving the mechanical properties. In this review, we focus on immiscible polymer blends and the use of compatibilizers for gas separation applications.

Soluble aromatic polyimides containing ortho-positioned hydroxy groups were synthesized as precursors for thermal rearrangement (TR) to polybenzoxazoles (PBOs). Fully imidized polyimides with high-molecular-weights were afforded via a ‘one-pot’ solution imidization technique (i.e., ester-acid method). The poly(hydroxyimide)s were designed to vary in their glass transition temperatures (Tg) by carefully selecting dianhydride–bisaminophenol combinations to introduce various levels of chain rigidity. TR conversion (imide-to-benzoxazole conversion) occurred in solid-state films only under inert atmosphere and over a temperature range of 300–450 °C, depending on the chemical structure (chain rigidity) of precursors. The effect of the precursor Tg on TR conversion was studied using TGA, DSC, FTIR and gel fraction measurements. The TR conversion temperature of imide-to-benzoxazole rearrangement strongly depended on the precursor Tg. Thus, for example, the feasible TR temperature was successfully reduced by ∼100 °C by lowering the precursor Tg by using a bisphenol A type dianhydride in the polymer synthesis. Gas permeation properties of representative TR systems are also reported. The TR process significantly increased gas permeabilities while maintaining good selectivities. By correlating the TR conversion degree with gas transport properties, there appears to be an optimal TR conversion degree that can maximize both gas permeability and selectivity. Systematic studies on TR polymers derived from low Tg precursors were suggested to further explore this correlation.

Three acetate-functional polyimides based on 2,2-bis(3-amino-4-hydroxy-phenyl)hexafluoropropane (APAF) and dianhydrides with different linking groups (-O-, -CO-, and -C(CF3)2-) were prepared, from which films were prepared and thermally rearranged (TR) to form poly(benzoxazole) dense membranes. The polymers were characterized by 1H nuclear magnetic resonance, Fourier transform infrared, differential scanning calrimetry, thermogravimetric analysis-mass spectrometry, and wide-angle X-ray diffraction. Transport properties of the TR membranes were investigated, and effects of the linking group and TR conversion rate on the membrane performance were discussed. Among the TR polymers studied, those bearing larger linking groups have higher d-spacing, fractional free volume, and gas permeability. The membrane permeability to gases increases with TR conversion rate and follows the order H2 > CO2 > O2 > N2 > CH4, except that CO2 surpasses H2 in APAF-6FDA-TR450. The CO2/CH4 separation performance of all of the TR po...

Post-synthesis defect abatement of inorganic membranes for gas separation

Cellulose acetate in fabrication of polymeric membranes: A review

Predictive transport modelling in polymeric gas separation membranes: From additive contributions to machine learning

Energy-efficient separation of gases has attracted intensive attentions both in research and in industry.nThe separation of gases by membranes is more effective and energy saving with lower productionnand equipment cost than some traditional gas separation methods, such as adsorption or distillation.nHowever, most of the polymeric membranes suffer from the trade-off between mass transport ratesnand separation efficiency. Polymeric membranes show high gas permeation flux but low selectivity,nand vice versa. To overcome such weakness, mixed matrix membranes (MMMs) can providenpromising potentials in high performance gas separation, by combining high separation properties ofnthe inorganic filler with low cost and flexible of the polymers. For filler selection, metal-organicnframeworks (MOFs) are promising adsorbents for gas storage and separation due to their high surfacenarea and porosity, adjustable pore sizes and controllable surface functionality.This thesis is focused on developing novel MOFs-based MMMs for gas separation with highnpermeability and selectivity, as well as good thermal and chemical stability. The studies includenfabrication and optimization of MOFs-based MMMs, designing MMMs with good filler/polymerninteraction and good interfacial morphology, as well as evaluating the permeation and selectivitynperformance of all the prepared membranes. It aims to establish the guidance for the MOF/polymernpair selection and interface manipulation in the fabrication of MMMs to greatly increase thenmembrane permeability and selectivity.In the first part of the experimental chapters, novel MMM with dispersed MOF into polyimide matrixnwere fabricated and the derived membranes are employed for gas separation. MMMs werensynthesized from 2,2-bis(3,4-anhydrodicarboxyphenyl) hexafluoropropane (6FDA) and 4,4(-diaminodiphenyl ether (oxydianiline, ODA) into which were incorporated Cd-6F MOF filler. Cd-6Fnwas synthesized by using 4,4p-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) as the organicnlinker which is also one of the monomers in the 6FDA-ODA synthesis. A specific interfacialninteraction between MOF crystals and polymer chains was innovatively targeted with this specificnMOF/polymer pair by controlling the in-situ 6FDA-ODA polymerization procedure with thenexistence of MOF particles. The enhanced adhesion between filler particles and polymer phase wasnachieved and the improved interfacial interaction between MOF and polymeric matrix was confirmednby FTIR, NMR and XPS. Moreover, it was found that the MOF/polymer interfacial morphologynstrongly affects the gas permeability and selectivity of the membrane. The MMM prepared by in-situnpolymerization displays excellent interfaces between micron-ized Cd-6F crystals and polymernmatrix as well as increased permeability and selectivity compared to pure 6FDA-ODA polymericmembrane. The interaction between MOF crystals and polymeric matrix can be controlled for theninterfacial voids elimination and optimal membrane transport properties.The second part of the experimental chapters focuses on MMMs with MOF/CNT composite. NovelnMMM filler CNT-MIL was synthesized by in-situ growth of NH2-MIL-101(Al) on the externalnsurface of carbon nanotubes (CNT) and applied to fabricate MMMs for CO2/CH4 separation. Extranamino groups and active sites were introduced to the surface of CNT. Compared to pure polymericnmembrane, MMMs containing synthesized MOF/CNT composite displayed significantly increasednCO2 permeance (up to 150%) and selectivity (up to 37.5%). The separation performance of derivednMMMs clearly transcends the 1991 upper bound and being on the 2008 upper bound for polymericnmembrane performance. MMMs for efficient C3H6/C3H8 separation were fabricated by embeddingnZIF-8/CNT composite into 6FDA-durene polymer matrix. The volume of filler and voids in thenpolymeric matrix as well as the distribution of the fillers and their contact with the polymeric matrixnwere quantitatively evaluated by using the tomographic focused ion beam scanning electronnmicroscopy (FIB-SEM). The dispersion of ZIF-8 in 6FDA-durene polymer was enhanced by growthnof ZIF-8 on the CNT surfaces. Meanwhile, good adhesion between the synthesized MOF/CNT fillersnand polymer matrix was observed, only 0.086% voids volume fraction is determined, even at a highnfiller loading. The improved ZIF-8 dispersion and filler/polymer interface lead to the efficient C3H6separation in MMMs. MMMs containing synthesized MOF/CNT composite displayed significantlynincreased C3H6 permeability (up to 105%) and selectivity (up to 96%) compare to pure 6FDA-durenenmembrane.The third part of the experimental chapters focuses on elimination of interfacial voids bynincorporating ionic liquid (IL) as the MOFs/polymer interfacial binder into MMM. Thin layer of ILnhas been fabricated on CuBTC fillers for MMMs fabrication. With the aid of ionic liquid, theninterfacial voids of the CuBTC-IL MMM was significantly reduced due to the favourable MOF/ILnand IL/polymer adhesion, thus improving CO2 selectivity compare to pure polymer membrane andnMMMs with untreated CuBTC. The performance of 10% CuBTC-IL/6FDA-durene MMM clearlyntranscends the 2008 upper bound for polymer membrane. The strategy of IL decoration method cannprovide an effective way to eliminate interfacial voids and enhance CO2 selectivity and can be appliednin large sized fillers with better gas separation properties.n