Separation of water using a membrane

Abstract

This invention relates to uses of graphene oxide, and in particular graphene oxide on a porous support, and a membrane comprising these materials. This invention also relates to methods of dehydration, which include vapour phase separation and pervaporation. Pervaporation is a method of separating mixtures of liquids using a membrane. Pervaporation consists of two basic steps: permeation of the permeate through the membrane and evaporation of the permeate from the other side of the membrane. Pervaporation is a mild which can be used to separate components which would not survive the comparatively harsh conditions needed for distillation (high temp, and/or low pressure).

Claims

1. A method of separating water from a product, the method comprising: contacting a mixture of the water and the product with a first surface of a graphene oxide membrane, wherein the graphene oxide membrane comprises a layer of graphene oxide supported on a layer of a porous membrane; and wherein the layer of graphene oxide comprises a stack of a plurality of randomly oriented single molecular graphene oxide layers; removing the water from a second surface of the graphene oxide membrane.

2. The method of claim 1, wherein the layer of graphene oxide layer is between 100 nm and 10 μm thick.

3. The method of claim 1, wherein the porous membrane comprises a polymer.

4. The method of claim 1, wherein the porous membrane comprises an inorganic material.

5. The method of claim 4, wherein the inorganic material is a ceramic.

6. The method of claim 4, wherein the inorganic material comprises alumina.

7. The method of claim 1, wherein the mixture of the water and the product is a gaseous mixture of water vapour and the product, the product being a gas.

8. The method of claim 7, wherein the method is a method of detecting a gaseous product and, once the water has passed through the graphene oxide membrane, the method comprises the step of detecting the product.

9. The method of claim 1, wherein the method is a method of separating the water from the product by pervaporation.

10. The method of claim 9, wherein the mixture of the water and the product is a fermentation broth or has been extracted from a fermentation broth.

11. The method of claim 1, wherein the porous membrane is formed of a polymeric material selected from polytetrafluoroethylene(PTFE), poly(vinylidene difluoride) (PVDF) and cyclopore polycarbonate.

12. The method of claim 1, wherein the porous membrane has a surface roughness which is the same as or smoother than as polytetrafluoroethylene(PTFE).

13. The method of claim 1, wherein the ratio of the thickness of the graphene oxide layer to the thickness of the porous membrane is in the range from 10:1 to 1:10.

14. The method of claim 1, wherein the porous membrane is no more hydrophobic than polytetrafluoroethylene(PTFE).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 shows He-leak measurements for a freestanding submicrometer-thick GO membrane and a reference PET film (normalized per square centimeter).

(3) FIG. 2 shows weight loss for a container sealed with a GO film.

(4) FIG. 3 shows weight loss for a container sealed with a 500 nm thick GO on Anopore membrane (20 mm diameter).

(5) FIG. 4 shows the propanol leak rate for different support membranes with and without GO coating.

(6) FIG. 5 shows a schematic view of the graphene oxide and graphite oxide layered structures.

DETAILED DESCRIPTION

(7) This invention relates to the separation of water from other chemical entities. In the context of this application, ‘separation’ can be understood to mean that the proportion of the product mixture which is water is lower after the separation than it was before the separation. In some cases, water will be substantially or even entirely removed from the product. In other cases, water will be partially removed from the product. The water which has been separated will, in some cases, be substantially or entirely pure (i.e. free of the product). In other cases, the water will contain some product.

(8) The invention relates to the separation of water from one or more products. The term ‘product’ may mean any chemical species. In an embodiment, a product may be any gas, e.g. an elemental gas. A product may be an organic molecule. Thus, a product may be any species which is a gas at standard temperatures and pressures, e.g. H.sub.2, N.sub.2, O.sub.2, methane, ethane, ethene, ethyne, ethylene oxide, propane, butane, He, Ar, Ne, CO.sub.2, CO H.sub.2S, SO.sub.2, NO.sub.x, etc. A product may be a liquid at standard temperatures and pressure such as pentane, hexane, decane, ethanol, methanol, propanol, acetone, butanol etc. The water may be separated from any single product, or a mixture of any two, three or four products or a complex mixture of many products.

(9) In some embodiments, at least some of any one or more products are in the form of a gas or a vapour. In other words, a product may be partially in a gas or vapour form and partially in a liquid form. It may be that all of the products are partially in the gas or vapour form and partially in a liquid form. It may also be that one or more products are entirely in the gas or vapour form and one or more other products are partially in the gas or vapour form and partially in a liquid form. Thus, if at least some of the water is in the form of water vapour, then that means that the water is partially in the liquid phase and partially in the form of water vapour. It is within the scope of this invention that any product or water may be present in the gas or vapour phase, the liquid phase and the solid phase. Likewise, it is within the scope of this invention that any product may be partially present in the solid phase and partially present in the gas or vapour phase.

(10) It is within the scope of this invention that the water and one or more products are in the liquid phase and are in contact with the porous membrane and/or the graphene oxide.

(11) Support materials with pore size in the range 0.1 to 10 micrometers are commonly referred to as microfiltration membranes. Membrane materials with pore size in the range 0.001 to 0.1 micrometers are commonly referred to as ultrafiltration membranes. However, porous structures having a pore size in the range 100 nm (0.1 micrometers) to 500 nm are also effective in the membranes of the present invention and we have used membranes in the range of 150 nm to 300 nm e.g. 200 nm to demonstrate impermeability even at larger pore sizes (see examples).

(12) The porous membrane may comprise a synthetic polymer. Examples of synthetic polymers include: polysulfones (e.g., PALL HT TUFFRYN®); polyethersulfones (e.g., PALL SUPER®, MILLIPORE EXPRESS®, SARTORIUS PES); polyvinylidene difluoride (PVDF; e.g., PALL FP VERICEL™, MILLIPORE DURAPORE®); polypropylene (e.g., PALL GH POLYPRO); acrylic 15 polymers (e.g., PALL VERSAPOR®); polyamide (Nylon) (e.g, PALL NYLAFLO™, SARTORIUS NY); and polytetrafluoroethylene (PTFE; e.g. MILLIPORE OMNIPORE™).

(13) The porous membrane may comprise a natural polymer or modified natural polymer. Examples of natural polymer and modified natural polymer polymers include: cellulose esters (e.g., MILLIPORE MF-MILLIPORE™); cellulose nitrate (e.g., SARTORIUS CN); cellulose acetate (e.g., SARTORIUS CA); and regenerated cellulose (e.g., SARTORIUS RC).

(14) The porous membrane may comprise a carbon monolith. An example of a suitable monolith would be those prepared by carbonization of polymerized high internal phase emulsions (see D. Wang, N. L. Smith and P. M. Budd, Polymer Int., 2005, 54, 297-303).

(15) The porous membrane may comprise an inorganic material. Examples of appropriate inorganic materials include: Aluminum oxide (Al.sub.2O.sub.3, Alumina; e.g., ANODISK; ANOPORE™); Metal oxide/ceramic (e.g., VEOLIA WATER SOLUTIONS CERAIVIEM®); Silicon carbide (SiC; e.g., VEOLIA WATER SOLUTIONS CERAMEM®); Zirconium oxide; Silicon dioxide; Titanium dioxide.

(16) The graphene oxide for use in this application can be made by any means known in the art. In a preferred method, graphite oxide can be prepared from natural graphite flakes by treating them with potassium permanganate and sodium nitrate in concentrated sulphuric acid. This method is called Hummers method. Another method is the Brodie method, which involves adding potassium chlorate (KClO.sub.3) to a slurry of graphite in fuming nitric acid. For a review see, Dreyer et al. The chemistry of graphene oxide, Chem. Soc. Rev., 2010, 39, 228-240.

(17) Individual graphene oxide (GO) sheets can then be exfoliated by dissolving graphite oxide in water with the help of ultrasound, and bulk residues can then be removed by centrifugation.

(18) The preparation of graphene oxide supported on a porous membrane can be achieved using filtration, spray coating, casting or dip coating techniques. For large scale production of supported graphene based membranes or sheets it is preferred to use spray coating techniques. One benefit of spray coating is that spraying GO solution in water on to the porous support material at an elevated temperature produces a large uniform GO film.

(19) Previously, graphene had been believed to be impermeable to all gases and liquids but in the present application we demonstrate that, surprisingly, a composite structure made from graphene oxide provided on a porous support can selectively allow permeation of water whilst being impermeable to gases, such as helium, and other vapours and liquids. However, we have also found that the properties of the graphene oxide composite as a whole can be modulated so as to allow selective passage of a limited number of other materials by changing the porous support.

(20) We have found that the nature of the porous support on to which the graphene oxide is deposited is important to the overall performance of the graphene oxide membrane composite structure and its ability to allow permeation. In other words, the porous support can modulate the ability of the overall graphene oxide membrane to allow selective permeation of liquid or vapour such as water. However, other small polar molecules may permeate through the membrane or may be prevented entirely by the membrane. In this respect the term “small polar molecules” specifically excludes water.

(21) The graphene oxide membrane according to the invention will always allow the permeation of water and the membrane materials thus govern the extent to which other small polar molecules such as C1-4 alcohols and the like will permeate or be excluded.

(22) One factor governing the permeation is the smoothness of the surface of the porous support. A smooth porous support such as alumina is better at resisting transmission of a small polar molecule such as propanol than a rougher porous support such as PTFE or PVDF (polyvinyldifluoride) is when used with graphene oxide in a graphene oxide membrane composite. Thus the choice of the porous support will be determined in part by the relative smoothness of its surface and the identity of the material which is intended to be allowed to permeate or to be stopped by the membrane structure. Ideally, the support should have a surface roughness which is the same as or less than that of PTFE. Preferably, the surface roughness is less than that of PTFE.

(23) Another important factor is the hydrophilicity of the porous support. A more hydrophilic support performs better in allowing selective permeation of small polar molecules such as propanol when used in combination with graphene oxide than a less hydrophilic support does. In this respect, alumina is better as a support than a polymeric material such as PTFE is if preventing permeation of small polar molecules to a higher degree or totally is the intention. The choice of porous support will thus depend on the hydrophilicity of the material used as the support in a graphene oxide membrane composite structure and the identity of the material which is intended to be allowed to permeate. Ideally, the support should be no more hydrophobic than PTFE, and preferably it is more hydrophilic than PTFE.

(24) We are thus able to produce a membrane whose permeation characteristics can be tailored to allow the passage of water and, in some circumstances, other small polar solvents such as methanol, ethanol and propanol whilst remaining completely impermeable to other fluids such as gases and liquids.

(25) Methanol and ethanol are small polar molecules which are of particular interest in terms of the ability to allow under certain circumstances selective transmission through the membrane. In other cases, denial of passage through the membrane structure of small polar molecules such as these in their entirety is desirable.

(26) Membranes of this type will have a number of uses in applications for gas drying and liquid drying.

EXPERIMENTAL SETUP

(27) Metal containers for permeation experiments were fabricated from an aluminum alloy and sealed by using two O rings. For gravimetric measurements, the containers were specially designed to minimize their mass. The weight loss was monitored by using a computer-controlled precision balance (ADAM Equipment Ltd; accuracy 1 mg). All the gravimetric experiments were carried out in an argon atmosphere in a glove box with a negligible water pressure (<10.sup.−3 mbar). If the containers were sealed with submicron GO membranes, no weight loss could be detected for any liquid other than water. For the case of an open aperture, evaporation rates for other liquids were higher than for water (for example, ≈1.3, 6.0 and 8.3 mg/h/mm.sup.2 for ethanol, hexane and acetone at room temperature (T) respectively).

Example 1

(28) This example relates to the permeation properties of GO. The studied GO membranes were prepared as follows: We employed Hummer's method to obtain graphite oxide that was dispersed in water by sonication to make a stable suspension of GO crystallites. We then used this suspension to produce laminates by spray- or spin-coating. Scanning electron microscopy and x-ray analysis reveal that such GO films have a pronounced layered structure and consist of crystals with typical sizes L of a few micrometers, which are separated by a typical distance d of ˜10 {acute over (Å)}. For the Example 1 permeation experiments, Cu foils of several centimeters in diameter were uniformly covered with the GO laminates. Then, we chemically etched Cu to produce apertures of diameter D ≈1 cm fully covered by freestanding GO films. Finally, a metal container was sealed by using the Cu disks. We studied membranes with thicknesses h from 0.1 to 10 μm. Even submicrometer-thick membranes were strong enough to withstand a differential pressure ΔP up to 100 mbar.

(29) As an initial test, we filled the containers with various gases under a small overpressure (<100 mbar) and recorded its changes over a period of several days. We observed no noticeable reduction in ΔP for any tested gas including He, H.sub.2, N.sub.2, and Ar. This allowed an estimate for the upper limit on their permeation rates Pr as ≈10.sup.−11 g/cm.sup.2.Math.s.Math.bar, which is close to the value reported for micron-sized “balloons” made from continuous graphene monolayers.

(30) We used mass spectrometry and found no detectable permeation of He. The accuracy was limited only by digital noise of our He spectrometer and a slightly fluctuating background, which yielded Pr<10.sup.−12 g/cm.sup.2.Math.s.Math.bar. Using hydrogen mass spectrometry, no permeation was found either, albeit the accuracy was three orders of magnitude lower than for He, due to a larger background. A 12-μm thick film of polyethylene terephthalate (PET) was used as a reference barrier and exhibited a He leakage rate 1000 times higher than our detection limit (FIG. 1) yielding PET's bulk permeability Π.sub.He=Pr.Math.h≈10.sup.−11 mm.Math.g/com.sup.2.Math.s.Math.bar, in agreement with literature values.

(31) To evaluate the permeation barrier for liquid substances, we employed weight-loss measurements. FIG. 2 shows examples for evaporation rates from a metal container with an aperture covered by a 1-μm-thick GO membrane. No weight loss could be detected with accuracy of <1 mg for ethanol, hexane, acetone, decane, and propanol in the measurements lasting several days. This sets an upper limit for their Π as ≈10.sup.−11 mm.Math.g/cm.sup.2.Math.s.Math.bar. We observed a huge weight loss if the container was filled with water.

(32) Moreover, the evaporation rate was practically the same as as through an open aperture i.e. in the absence of the GO film; (h≈1 μm; aperture's area≈1 cm.sup.2).

Example 2

(33) This example relates to the permeation properties of GO on a permeable membrane. A 500 nm thick GO layer supported on an Anopore membrane (20 mm diameter) was prepared by vacuum filtration of graphene oxide solution in water through the alumina membrane. The pore size of the Anopore alumina membrane was 200 nm. The permeability of water, ethanol and methanol through the membrane was determined by measuring the weight loss from a metal container sealed with the GO/membrane composite. A comparative experiment was also performed to determine the permeability of water through an Anopore membrane. The measurements were carried out at room temperature and zero humidity.

(34) FIG. 3 shows the evaporation rates. As can be seen, the evaporation rate through the GO on Anopore composite and the reference Anopore membrane are practically the same. On the other hand the GO on Anopore composite is completely impermeable to the methanol and ethanol vapours.

Example 3

(35) This example shows 2-propanol permeation through supported GO membranes.

(36) To enhance the mechanical stability of GO membranes, we have deposited graphene oxide on different polymer/ceramic porous support and studied their influence on the graphene oxide's membrane property. We have used anodisc alumina, Polytetrafluoroethylene (PTFE), Polyvinylidene fluoride (PVDF) and Cyclopore Polycarbonate (PC) support. We measured the vapour permeation of 2-propanol (2-propanol leak test) at room temperature and room humidity to study the influence of support membrane on property of graphene oxide membrane. Table 1 below shows the 2-propanol leak test performed for GO on different support membranes. The porosity of all the support membranes was identical and for each was 200 nm so that a direct comparison could be made. The table demonstrates that the support material has an effect on graphene oxides membrane properties. The best substrate found so far is the Anodisc alumina membrane. Graphene oxide on Anodisc alumina shows the same property as that of a free standing graphene oxide (impermeable to propanol).

(37) TABLE-US-00001 TABLE 1 Propanol leak test for graphene oxide (4 μm thick) on different support membranes Flux with- Support porous out GO Flux with support coating GO coating Hydrophilicity/ (200 nm pore) mg/h/cm.sup.2 mg/h/cm.sup.2 hydrophobicity Smoothness PTFE 12.8 2.6 hydrophobic Rough PVDF 13.7 2.1 hydrophilic Smooth Cyclopore 28.8 1.0 hydrophilic Smooth Polycarbonate (smoother than PVDF) Anodisc 30.6 undetectable hydrophilic Very smooth alumina

(38) It can also be seen that the propanol vapour barrier for GO on cyclopore Polycarbonate (PC) membranes were found better than that of PTFE and PVDF. Four micron thick GO coating on PC membranes increase the propanol barrier nearly 30 times compared to five and seven times improvement for PTFE and PVDF membranes. Table 1 also shows smoothness of different GO coating on different support obtained by microscopic or visual inception. In general very smooth hydrophilic substrates are found to be a better candidate for GO support material.

(39) FIG. 4 shows the propanol leak rate for different support membranes with and without a GO coating.

(40) Supported GO membranes for Dehydration/concentration

(41) 8 micron thick GO on Anodisc alumina membrane

(42) $\mathrm{Separation}\mathrm{factor}=\frac{\left(Y_i/Y_j\right)}{\left(X_i/X_j\right)}$

(43) Where X is the weight fraction of components i and j in the feed and Y is the weight fraction of a component in the permeate

(44) TABLE-US-00002 Initial alco- Feed hol concen- Total flux Water flux Alcohol flux Sep. sample tration wt % gm.sup.−2h.sup.−1 gm.sup.−2h.sup.−1 gm.sup.−2h.sup.−1 Factor water 0 146.64 146.64 — — Ethanol 100 Undetect- — Undetect- — able able Ethanol 40 69.44 63.64 5.81 7.3 (aq) 2-propanol 40 75.71 74.96 0.75 66.9 (aq) 1-propanol 40 100.29 96.70 3.58 18.0

(45) GO on hydrophobic substrate

(46) TABLE-US-00003 GO 2-propanol thickness Total flux Water flux flux Sep. (microns) Feed sample gm.sup.−2h.sup.−1 gm.sup.−2h.sup.−1 gm.sup.−2h.sup.−1 Factor 0 2-propanol 136.95 41.85 95.10 0.3 (40% aq) 1 2-propanol 54.52 27.60 26.92 0.7 (40% aq) 3 2-propanol 36.39 26.66 9.72 1.8 (40% aq)

(47) GO on hydrophilic substrate

(48) TABLE-US-00004 GO 2-propanol thickness Total flux Water flux flux Sep. (microns) Feed sample gm.sup.−2h.sup.−1 gm.sup.−2h.sup.−1 gm.sup.−2h.sup.−1 Factor 0 2-propanol 41.56 21.58 19.98 0.7 (40% aq) 1 2-propanol 37.51 29.35 8.17 2.4 (40% aq) 5 2-propanol 26.93 17.89 9.04 2.5 (40% aq)

(49) A separation factor above one corresponds to the membrane enriching the retentate in alcohol and a separation factor lower than one corresponds to the membrane enriching the retentate in water.

(50) There are a number of uses for the composite membranes of the invention having these characteristics e.g. detector devices, the pervaporation of fermentation broths, concentration of liquids (e.g. fruit juices), liquid drying (e.g. of hydrocarbon based fuels), gas drying, gas humidification.

(51) Even though the atomic structure and chemical composition of graphene oxide and graphite oxide membranes are same, the membrane properties are very different. For example, bulk graphite oxide membranes (Boehm et al. Journal of Chimie Physique 58, 141 (1961)) allow water and other polar solvents (eg. ethanol) to permeate but it is completely impermeable to all other gases. Graphene oxide on the other hand is completely impermeable to all gases and liquid (including polar solvents) except water. This unique property of graphene oxide membranes is due to its perfect layered structure. Permeation of polar solvents through graphite oxide membranes can be originated from the difference in their layered structure. FIG. 5 below shows schematic view of the graphene oxide and graphite oxide layered structures.

(52) FIG. 5 shows a schematic view of the layered structure of graphene oxide (A) and graphite oxide (B) membranes

(53) Graphite oxide membranes are consists of micrometer size thick perfectly staked graphite oxide flakes (defined by the starting graphite flakes used for oxidation, after oxidation it gets expanded due to the attached functional groups) and can be considered as a polycrystalline material. Exfoliation of graphite oxide in water into individual graphene oxide was achieved by the sonication technique followed by centrifugation at 10000 rpm to remove few layers and thick flakes. Graphene oxide membranes were formed by restacking of these single layer graphene oxides by a number of different techniques such as spin coating, spray coating, road coating and vacuum filtration.

(54) Graphene oxide membranes according to the invention consist of overlapped layers of randomly oriented single layer graphene oxide sheets with smaller dimensions (due to sonication). These membranes can be considered as a centimeter size single crystal (grain) formed by parallel graphene oxide sheets. Due to this difference in layered structure, the atomic structure of the capillary entrance of graphene oxide and graphite oxide membranes are different. For membranes the edge functional groups are located over the functionalised regions of another graphene oxide sheet while in graphite oxide membranes mostly edges are aligned over another graphene oxide edge. These differences unexpectedly may influence the membrane properties of graphene oxide membranes as compared to those of graphite oxide.

(55) The word ‘harmful’ may mean capable of doing harm. A chemical entity which is harmful to the environment may be a greenhouse gas or it could be harmful or toxic to flora or fauna or other organisms. A harmful chemical entity might be one that is considered harmful according to an accepted international safety standard. For instance it could be a chemical entity which has been assigned a risk code described as ‘harmful’ as defined in Annex III of European Union Directive 67/548/EEC, i.e. has been afforded a risk code selected from R20, R21, R22 or R52.

(56) The word ‘toxic’ may mean capable of causing death, illness or injury. A toxic chemical entity might be one that is considered toxic or very toxic according to an accepted international safety standard. For instance it could be a chemical entity which has been assigned a risk code described as toxic or very toxic as defined in Annex III of European Union Directive 67/548/EEC, i.e. has been afforded a risk code selected from R23, R24, R25, R26, R27, R28, R50, R51, R54, R55, R56 or R57.

(57) A detector system is a system which is used for the detection of one or more chemical entities. It will comprise a detector. The composite materials of the present invention are particularly useful in combination with a detector which can be damaged by water or which requires the product which is being detected to be present at a certain concentration.

(58) The term chemical entity is not intended to exclude biological entities, nor is it intended to exclude radioactive material. A chemical entity may by organic or it may be inorganic.

(59) Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(60) Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

(61) The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.