Two-dimensional materials for gas separation membranes

The next generation of gas separation membranes requires from novel membrane materials with superior performance, sufficient mechanical stability, and long-term stability under harsh operation conditions. Two-dimensional (2D) materials offer several advantages over conventionally used polymeric materials. However, gas separation membranes containing 2D materials have not reached commercialization yet, despite having been discovered almost two decades ago. Difficulties in membrane scalability and high costs associated with the manufacturing processes are the main challenges. This review focuses on the current state and prospects of the technology and highlights novel 2D materials and strategies to fabricate ultrathin membranes that have been developed during the last three years. A multidisciplinary approach, covering the fields of physics, chemistry, and chemical engineering, needs to be taken to achieve the preparation of robust, large-scale, and economically affordable (2D material)-based membranes capable of breaking into the gas separation market

Separation of H2 and CO2 Containing Mixtures with Mixed Matrix Membranes Based on Layered Materials

Some membrane separation processes are gradually taking over conventional processes such as distillation, evaporation or crystallization as the technology progresses from bench-scale tests to large-scale prototypes. However, membranes for H2 and CO2 separation constitute a daring technology still under development. This overview focuses on mixed matrix membranes (MMMs), a special type of membranes in which a filler is dispersed in a polymer matrix, as a successful strategy to improve their permeability-selectivity performance while keeping the polymer processability. In particular, among all the possible fillers for MMMs, layered materials (porous zeolites and titanosilicates and graphite derivatives) are discussed in detail due to the several advantages they offer regarding selective microporosity, crystallinity and, what is most important, high specific surface area and aspect ratio. In fact, a selective and as thin as possible, i.e. with high aspect ratio, filler would help to develop high performance MMMs

Desarrollo de un sistema de separación de gases mediante membranas híbridas polímero - zeolita

En la Industria Química existe la necesidad de separar corrientes de gases, siendo los procesos habituales muy costosos energéticamente. Actualmente se están estudiando alternativas menos costosas como la utilización de membranas poliméricas. El grupo CREG de la Universidad de Zaragoza, está desarrollando membranas híbridas polímero-zeolita, que mejoran las propiedades separativas, haciéndolas mucho más competitivas frente a los métodos empleados habitualmente. En este proyecto se ha estudiado la separación de la mezcla H2/CH4, una mezcla muy interesante debido a la emergente economía del hidrógeno, mediante la utilización de membranas híbridas fabricadas con polímeros comerciales de polisulfona y poliimida y la zeolita Nu-6(2) de alta área superficial, sintetizada por el grupo CREG, cuyos tamaños de poro permiten el paso de moléculas de H2 pero discriminan las de CH4. Para ello se ha realizado el diseño y la puesta a punto del un sistema de separación de la mezcla binaria de gases H2/CH4, que incluye la utilización de un microcromatógrafo de gases en línea, en el cual se han analizado las propiedades separativas de las membranas mixtas fabricadas. Además, estas membranas se han caracterizado mediante análisis termogravimétrico, espectroscopía infrarroja por transformada de Fourier, microscopía electrónica de barrido y de transmisión, microscopía óptica y calorimetría diferencial de barrido. Se ha concluido que se pueden preparar con éxito membranas híbridas con los polímeros comerciales Udel® y Matrimid® y la zeolita Nu-6(2) de alta área superficial, las cuales presentan una buena adherencia entre el material inorgánico y los polímeros empleados debido a la naturaleza hidrofóbica de la zeolita. Se ha observado que para bajos porcentajes en peso de zeolita se obtiene una distribución homogénea de ésta en la matriz polimérica (sección transversal y horizontal), y que al aumentar los porcentajes se produce una ligera deposición de las partículas en la parte inferior de las membranas. El sistema de separación de gases diseñado permite determinar con éxito los valores de permeabilidad de H2 y CH4 y selectividad H2/CH4 de las membranas preparadas. Las membranas híbridas de Udel® preparadas con porcentajes de zeolita Nu-6(2) exfoliada de hasta un 8% en peso muestran un aumento de la selectividad con respecto al polímero puro para la mezcla H2/CH4 sin una gran reducción en la permeabilidad de H2. Se obtiene un valor máximo de selectividad de 89 para las membranas preparadas con un 15% en peso, en estas la permeabilidad de H2 aumenta, mientras que la permeabilidad de CH4 se mantiene prácticamente constante. En el caso de las membranas de Matrimid® de hasta un 8% en peso de carga adicionada se observa una disminución de las permeabilidades de H2 y CH4 con respecto a la poliimida pura, obteniéndose el máximo valor de selectividad (160) para este porcentaje. En las membranas preparadas con 15% en peso, se produce un aumento tanto de la permeabilidad de H2 como la de CH4, obteniéndose valores de selectividad similares a los del polímero puro. Adicionando porcentajes menores de carga inorgánica se obtienen valores de selectividad H2/CH4 mayores en el caso de las membranas híbridas de Udel® y similares en las membranas híbridas de Matrimid® comparados con los publicados por otros autores

Comparative life cycle assessment of seawater desalination technologies enhanced by graphene membranes

Graphene oxide (GO)-enhanced membranes are being developed to solve major limitations in both reverse osmosis (RO) and membrane distillation (MD) technologies, which include high electricity and thermal energy consumption. This study performed, for the first time, a life cycle assessment to determine the effects of using GO-enhanced membranes on the environmental impacts of seawater desalination via RO and MD. Four scenarios were evaluated and eighteen environmental impacts were quantified according to the ReCiPe impact assessment method. The average impacts for the RO-GO scenarios were lower than those of RO by 3–7 %. The reduction in the climate change impact was 3–8 %, which could avoid the release of 380–850 kt CO2 eq. per year globally if these membranes were used in current seawater RO systems. The MD-GO scenarios had, on average, 27–34 % lower impacts than the MD scenarios. Overall, the RO-GO systems were the most favourable, with lower impacts than MD-GO for most categories. However, using solar-thermal energy instead of natural gas in MD desalination would lead to 43–93 % lower impacts in nine categories than RO powered predominantly by fossil fuels. This includes climate change, which would be 64 % lower; however, freshwater ecotoxicity would be more than four-times higher. The results of this work indicate the potential environmental benefits of GO-enhanced membranes and discuss the future developments needed to improve the performance of RO and MD

PVDF membranes containing alkyl and perfluoroalkyl-functionalized graphene nanosheets for improved membrane distillation

Polyvinylidene fluoride (PVDF) membranes containing hydrophobic graphene nanofillers were prepared and tested for membrane distillation applications. The nanofillers were obtained by a two-step process: 1st) chemical grafting of hydrophobic molecules, either octylamine (OA) or perfluoroctylamine (PFOA), to graphene oxide (GO) nanosheets, and 2nd) chemical reduction of functionalized GO (rGO) to remove unreacted oxygen-containing functional groups. This resulted in OA-functionalized reduced GO (OA-rGO) and PFOA-functionalized rGO (PFOA-rGO). The addition of these nanomaterials to PVDF membranes prepared by the phase inversion process led to an increase in the membrane contact angle, and therefore higher hydrophobicity, as well as an increase in the membrane porosity. When comparing both nanofillers, OA-rGO and PFOA-rGO, the latter was more efficient in achieving higher contact angles due to the presence of fluorine atoms, whereas OA-rGO led to a greater enhancement in membrane porosity as compared to PFOA-rGO. MMMs containing 0.7 wt% nanofiller loadings of OA-rGO and PFOA-rGO achieved the highest water fluxes of 9.1 and 8.8 L m−2 h−1, respectively and salt rejection above 99.9%, which was monitored for at least 162 h of operation for the former. In comparison with pure PVDF (flux of 5 L m−2 h−1), the addition of OA-rGO and PFOA-rGO nanofillers results in a flux increment of 82% and 76%, respectively

The use of carbon nanomaterials in membrane distillation membranes: a review

From Springer Nature via Jisc Publications RouterHistory: registration 2020-01-23, received 2020-04-01, accepted 2020-07-20, pub-electronic 2021-01-25, online 2021-01-25, pub-print 2021-08Publication status: PublishedAbstract: Membrane distillation (MD) is a thermal-based separation technique with the potential to treat a wide range of water types for various applications and industries. Certain challenges remain however, which prevent it from becoming commercially widespread including moderate permeate flux, decline in separation performance over time due to pore wetting and high thermal energy requirements. Nevertheless, its attractive characteristics such as high rejection (ca. 100%) of nonvolatile species, its ability to treat highly saline solutions under low operating pressures (typically atmospheric) as well as its ability to operate at low temperatures, enabling waste-heat integration, continue to drive research interests globally. Of particular interest is the class of carbon-based nanomaterials which includes graphene and carbon nanotubes, whose wide range of properties have been exploited in an attempt to overcome the technical challenges that MD faces. These low dimensional materials exhibit properties such as high specific surface area, high strength, tuneable hydrophobicity, enhanced vapour transport, high thermal and electrical conductivity and others. Their use in MD has resulted in improved membrane performance characteristics like increased permeability and reduced fouling propensity. They have also enabled novel membrane capabilities such as in-situ fouling detection and localised heat generation. In this review we provide a brief introduction to MD and describe key membrane characteristics and fabrication methods. We then give an account of the various uses of carbon nanomaterials for MD applications, focussing on polymeric membrane systems. Future research directions based on the findings are also suggested

Porous silica nanosheets in PIM-1 membranes for CO2 separation

PIM-1-based freestanding mixed matrix membranes (MMMs) and thin film nanocomposites (TFNs) were prepared by incorporating porous silica nanosheets (SN) and exfoliated SN (E-SN) derived from natural vermiculite (Verm) in the PIM-1 polymer matrix. In addition, SN were functionalized by sulfonic acid and amine groups (S-SN and N-SN, respectively) and were also used as fillers for the preparation of MMMs. The gas separation performance was evaluated using CO2/CH4 and CO2/N2 (1:1, v:v) binary gas mixtures. Among freestanding membranes, fresh ones (i.e. tested right after preparation) containing 0.05 wt% functionalized SN and E-SN outperformed the neat PIM-1, surpassing the 2008 Robeson upper bound. At the same filler concentration, fresh MMMs with sulfonic acid-functionalized SN (S-SN) exhibited 40% higher CO2 permeability, 20% higher CO2/N2 selectivity and almost the same CO2/CH4 selectivity as neat PIM-1 membranes. Moreover, after 150 days of aging, these membranes were capable of maintaining up to 68% of their initial CO2 permeability (compared to 37% for neat PIM-1). When prepared as TFN membranes, the incorporation of 0.05 wt% of S-SN led to 35% higher initial CO2 permeance and five times higher CO2 permeance after 28 days

Mitigation of Physical Aging with Mixed Matrix Membranes Based on Cross-Linked PIM-1 Fillers and PIM-1

A low cross-link density (LCD) network-PIM-1, which offers high compatibility with the polymer of intrinsic microporosity PIM-1, is synthesized by a modified PIM-1 polycondensation that combines both a tetrafluoro- and an octafluoro-monomer. To maximize the advantages of utilizing such cross-linked PIM-1 fillers in PIM-1-based mixed matrix membranes (MMMs), a grafting route is used to decorate the LCD-network-PIM-1 (dispersed phase) with PIM-1 chains, to further enhance compatibility with the PIM-1 matrix. Mixed-gas CO2/CH4 (1:1, v/v) separation results over 160 days of membrane aging confirm the success of a relatively short (24 h) grafting reaction in improving the initial CO2 separation performance, as well as hindering the aging of PIM-1/grafted-LCD-network-PIM-1 MMMs. For MMMs based on a 24 h grafting route, all the gas separation data surpass the 2008 Robeson upper bound by a significant margin, and the 160-day aged membranes show only 29% reduction from the initial CO2 permeability, which is substantially less than the equivalent losses of nearly 70% and 48% for PIM-1 and traditionally fabricated MMMs counterparts, respectively. These results demonstrate the potential of network-PIM components for obtaining much more stable gas separation performance over extended periods of time

PIM-1/graphene pervaporation membranes for bioalcohol recovery

Biofuels are an alternative to more traditional fuels, such as those derived from crude oil. Bioalcohols, including bioethanol and biobutanol, are produced from biomass through sugar fermentation and purification processes and thus they are a more sustainable alternative to reducing the CO2 footprint of transportation and mitigating climate change. In the short term they will find it difficult to replace hydrocarbon fuels due to direct competition with the food supply chain, although as new alternative raw materials and production processes are developed this hurdle will be overcome. The recovery of bioalcohols from fermentation broths includes a series of very challenging steps that need more attention. In this regard, membrane-based technologies with lower energy consumption, such as pervaporation (PV), have emerged as potential candidates for the replacement of energy intensive distillation operations. In this work we present the development of novel organophilic membranes based on polymers of intrinsic microporosity (PIMs) and graphene for the separation of ethanol and butanol from aqueous solutions. PIM-1 is one of the few polymers that offer selectivity for organic compounds over water [1-3]. However, excessive swelling limits its performance and the addition of graphene nanoparticles can have a positive effect in preventing it [4,5]. For the preparation of mixed matrix membranes (MMMs) PIM-1 and graphene were first synthesized. Graphene oxide (GO) was obtained from natural flake graphite via a modified Hummer’s method, functionalized with octylamine (OA) and octadecylamine (ODA), 8 and 18 carbons, respectively and subsequently reduced with hydrazine monohydrate. PIM-1 was prepared by the polycondensation of monomers 3,3,3’,3’-tetramethyl-1,1’’-spirobisindane-5,5’,6’,6’-tetrol with 2,3,5,6-tetrafluorophthalonitrile with a molecular ratio of 1:1 [6]. The preparation of freestanding membranes was done via a casting-evaporation technique using chloroform as solvent (one of the very few that dissolve PIM-1). The functionalization of GO with OA or ODA allowed its dispersion in chloroform and therefore a homogeneous casting solution was obtained. Membranes of thicknesses up to 40 µm with loadings of graphene from 0.01 to 0.5 wt.% were prepared and characterized via contact angle measurements, FTIR, TGA, and SEM. PV tests of aqueous feed solutions containing 5wt% of alcohol were performed at 65 ˚C and a pressure of 10 mbar on the permeate side of the membrane. An increase in the separation factor of ethanol and butanol over water was achieved for both amine-functionalized GO with maximum values of 7 and 40, respectively. The maximum flux achieved of ~ 2 kg m-2 h1 was obtained for membranes with graphene loadings of 0.5 wt.%. [1] Mason, C.R., et al. Polymer, 2013. 54(9), 2222-2230. [2] Žák, M., et al. Separation and Purification Technology, 2015. 151, 108-114. [3] Adymkanov, S.V., et al. Polymer Science Series A, 2008. 50(4), 444-450. [4] A. Gonciaruk, et al., Microporous Mesoporous Mater., 2015. 209, 126-134. [5] M.M. Khan, et al. J. Membr. Sci. 2013. 436, 109-120. [6] Budd, P.M., et al. Advanced Materials, 2004. 16(5), 456-459