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

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

PIM-1/Holey Graphene Oxide Mixed Matrix Membranes for Gas Separation: Unveiling the Role of Holes

PIM-1/holey graphene oxide (GO) mixed matrix membranes (MMMs) have been prepared and their gas separation performance for CO2/CH4 mixtures assessed. Nanopores have been created in the basal plane of gas-impermeable GO by chemical etching reactions, and the resulting holey flakes have been further chemically functionalized, either with octadecylamine (ODA) or with PIM-1 moieties, to aid their dispersion in PIM-1. It is found that nanopores barely promote gas transport through the graphene-like nanofiller for fresh membranes (tested right after preparation); however, the prepared hybrid PIM-1/holey GO membranes exhibit higher CO2 permeability and CO2/CH4 selectivity than the pure polymer membrane 150 days after preparation and 13 and 15% higher CO2 permeability for filler contents of 0.1% of octadecylamine-functionalized holey GO and 1% of (PIM-1)-functionalized holey GO, respectively. The most significant improvement is observed for the mitigation of physical aging, as MMMs using 10% of (PIM-1)-functionalized holey GO nanofillers are capable of maintaining up to 70% of their initial CO2 permeability after 150 days, whereas only 53% is kept for pure PIM-1 after the same period. The gas permeability of the nanofiller has been rationalized with the aid of the Maxwell–Wagner–Sillars equation.The authors are grateful to EPSRC for funding under grant numbers EP/K016946/1 and EP/M01486X/1. J. M. Luque-Alled is grateful to the Department of Chemical Engineering and Analytical Science─The University of Manchester for funding his Ph.D. studies. P. Gorgojo acknowledges the Spanish Ministry of Economy and Competitiveness and the European Social Fund through the Ramon y Cajal programme (RYC2019-027060-I/AEI/10.13039/501100011033).Peer reviewe

Study on the formation of thin film nanocomposite (TFN) membranes of polymers of intrinsic microporosity and graphene-like fillers: effect of lateral flake size and chemical functionalization

Thin film nanocomposite (TFN) membranes of polymer of intrinsic microporosity PIM-1 incorporating graphene oxide (GO) nanosheets of different sizes and chemistries are presented. These membranes show an improved separation performance for the recovery of n-butanol from aqueous solutions through pervaporation; an improvement of ca. a third of the value achieved for pristine PIM-1 thin films is obtained for TFN membranes filled with nanometer-sized reduced octyl-functionalized GO. In addition, these nanometer-sized fillers lead to a maximum increase in total flux of approximately 40%. The thickness of the supported films is in the range 1–1.5 µm, and fillers used are micrometer- and nanometer-sized alkyl-functionalized GO nanosheets and their chemically reduced counterparts. As evidenced by a superior overall membrane performance, the interfacial interaction between the filler and the polymer matrix is enhanced for those whose lateral size is in the nanometer range. Moreover, an enhancement in the separation performance and productivity of such membranes is observed for higher operating temperatures and higher contents of n-butanol in the feed