Modified polymer gas separation membrane

Abstract

A polymer composition for carbon dioxide gas separation, the polymer composition comprising a polymer and a plurality of supramolecular structures, wherein the supramolecular structures are configured to selectively and reversibly adsorb carbon dioxide.

Description

[0001] Modified polymer gas separation membrane

[0002] FIELD OF THE INVENTION

[0003] The invention relates to polymer compositions and membranes comprising supramolecular structures, preferably cucurbituril compounds, that demonstrate enhanced carbon dioxide gas separation. The invention also relates to methods of forming these improved polymer membranes, and to the use of these improved polymer membranes in carbon dioxide gas separation.

[0004] BACKGROUND OF THE INVENTION

[0005] Escalating greenhouse gas concentrations in the atmosphere due to fossil fuel consumption has resulted in a ~1.0 °C increase in global mean surface temperature beyond pre-industrial levels. This global warming has been linked to increased instances of extreme weather and climate change, motivating the establishment of the Paris Climate Accord in 2015. To meet the ambitious target of the Paris Accord of confining further global temperature increases to 1 .5 °C, the capture and sequestration of the most prominent greenhouse gas - carbon dioxide (CO2) - from anthropogenic emissions is imperative.

[0006] Amine scrubbing is one of the most common technologies used to capture CO2 from flue gas and has been found in commercial applications for over 90 years. However, amine scrubbing is energy intensive and accounts for over 20% of a typical power plant energy output, offsetting the original goal of CO2 capture and hampering further commercialization. An alternative strategy is using membrane technology to selectively separate CO2 from the many other species present in the flue gas. Membrane technology offers some intrinsic merits over amine scrubbing, chiefly by operating at ambient temperatures (saving ca. 90% of the cost associated with heating), as well as affording continuous operation and greater simplicity and modularity in process designs.

[0007] Polymer membrane based gas separation technology has commercial applications in several settings, including air separation to obtain technical grade nitrogen or oxygen enriched air, natural gas "sweetening" to remove carbon dioxide from methane, and direct air capture. However, CO2 separation and capture using membrane technology has not been widely achieved.

[0008] For CO2 separation from a mixture of other gases, polymer membranes with both high permeability and selectivity are desirable. However, gas separation properties of polymer membrane materials follow a distinct trade-off relationship: more permeable polymers are generally less selective and vice versa. To overcome the trade-off problem and enhance the gas separation performance of polymer membranes, two methods are conventionally employed. The first one is to prepare membranes using special-designed polymers such as polymers of intrinsic microporosity (PIMs). The drawback of this method is that PIMs undergo physical aging which leads to a decrease in permeability when used in membranes. The other method, which attracts more attention in the membrane community, is to incorporate porous inorganic particles within polymer matrix as filler to form so-called mixed matrix membrane (MMM). The drawbacks of MMM include lack of processability and deterioration of gas separation performance due to formation of non-ideal interfaces between the inorganic particles and polymers, which limits the commercial application of mixed matrix membranes for gas separation.

[0009] The present invention utilises supramolecular structures to enhance the CO2gas separation performance of polymeric membranes without altering their physical characteristics. The polymer membranes according to the present invention can achieve over fivefold increases in permeability and elevated selectivity to more than 220% compared to unmodified polymer membranes.

[0010] SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide polymer compositions and membranes comprising supramolecular structures, preferably cucurbituril compounds, that demonstrate enhanced carbon dioxide gas separation.

[0012] According to the present invention there is provided a polymer composition for carbon dioxide gas separation, the polymer composition comprising a polymer and a plurality of supramolecular structures, wherein the supramolecular structures are configured to selectively and reversibly adsorb carbon dioxide.

[0013] The supramolecular structures may be colloids and / or macrocycles.

[0014] A macrocycle according to the present invention may be a chemical structure containing a ring of twelve or more members (i.e. atoms). The present invention also provides a membrane comprising a polymer composition according to the invention. When the polymer composition of the present invention is used to form a carbon dioxide gas separation membrane according to the present invention, the supramolecular structures (e.g. cucurbituril compounds) attract carbon dioxide gas (CO2) towards the surface of the membrane. This acts to selectively lower the membrane surface energy barrier for CO2 molecules, making it easier for CO2 molecules to permeate through the membrane. The attraction of carbon dioxide gas towards the surface of the membrane also acts to increase the CO2 concentration gradient across the membrane, which selectively increases the number of CO2 molecules that permeate through the membrane. The supramolecular structures (e.g. cucurbituril compounds) are also capable of functioning as molecular level CO2 pumps, which act to selectively pump CO2 molecules through the membrane.

[0015] Preferably the supramolecular structures are cucurbituril compounds.

[0016] Cucurbituril compounds according to the present invention are a group of organic compounds having the Formula (I): wherein n is 5, 6, 7, 8 or 10; and for each glycoluril unit, R1is selected from hydrogen, halogen, optionally substituted -(Ci-Ce)alkyl, optionally substituted -(Ci-C6)haloalkyl or -OH.

[0017] Cucurbituril compounds are a group of organic compounds comprising repeating glycoluril monomer units linked by methylene bridges (where n denotes the number of repeating glycoluril units). Each glycoluril unit may be substituted at the ring fusion position, for example substituted with halogen, optionally substituted -(Ci-Ce)alkyl, optionally substituted -(Ci- Ce)haloalkyl or -OH, or un-substituted having the Formula (II):

[0018] (n = 5, 6, 7, 8, 10) Preferably, for each glycoluril unit, R1is hydrogen and the cucurbituril compound has the Formula (II) above.

[0019] As used herein, the term cucurbituril (CB[n]) refers to cucurbituril compounds with unsubstituted glycouril units (where n denotes the number of repeating glycoluril units), according to Formula (II) above. Cucurbituril compounds comprise a hydrophobic cavity accessed by two portals surrounded by polar carbonyl groups, at opposing ends of the cavity.

[0020] Specifically, cucurbituril compounds comprising 6, 7 or 8 repeating glycoluril monomers have been found to selectively adsorb CO2 gas within their cavities. Without wanting to be bound by theory, it is believed that the high selectively of cucurbituril compounds for CO2 is due to the high enthalpy of CO2 adsorption caused by the CO2 molecules and cucurbituril compounds interacting through hydrogen bonding and / or dipole-quadrupole interactions.

[0021] When the polymer composition of the present invention is used to form a carbon dioxide gas separation membrane according to the present invention, the cucurbituril compounds attract carbon dioxide gas (CO2) towards the surface of the membrane. This acts to selectively lower the membrane surface energy barrier for CO2 molecules, making it easier for CO2 molecules to permeate through the membrane. The attraction of carbon dioxide gas towards the surface of the membrane also acts to increase the CO2 concentration gradient across the membrane, which selectively increases the number of CO2 molecules that permeate through the membrane. The cucurbituril compounds are also capable of functioning as molecular level CO2 pumps, which act to selectively pump CO2 molecules through the membrane.

[0022] Preferably the cucurbituril compounds comprise 6, 7 or 8 glycoluril or substituted glycouril units. These cucurbituril compounds have been found to provide particularly good CO2 gas separation performance when used in a membrane according to the present invention.

[0023] In another aspect of the invention, the supramolecular structures are colloids, wherein the colloids have been functionalised with cucurbituril compounds. Preferably, the supramolecular structures are functionalised with cucurbituril compounds comprising 6, 7 or 8 glycoluril or substituted glycouril units.

[0024] Preferably, the supramolecular structures are MCM-41 colloids that have been functionalised with cucurbituril compounds. A membrane according to the present invention comprising MCM-41 colloids that have been functionalised with cucurbituril compounds (e.g. cucurbituril compounds comprising 6, 7 or 8 glycoluril or substituted glycouril units) has been shown to double the CO2 separation performance of the membrane while maintaining the same level of CO2 selectivity. Preferably, the channels of the MCM-41 colloids are functionalised with cucurbituril compounds.

[0025] Preferably, the supramolecular structures (e.g. cucurbituril compounds) comprise cavities. Preferably, the cavities are nanometer or sub-nanometer in diameter. Preferably, the cavities are from 1 to 10 angstroms in diameter. A cavity of this dimension is similar in size to carbon dioxide molecules, and so provides tight and specific binding that enable the selective capture of CO2 molecules within the cavity.

[0026] The supramolecular structures (e.g. cucurbituril compounds) provide an increase in the carbon dioxide permeability and / or selectively of the polymer composition relative to the polymer alone. The supramolecular structures (e.g. cucurbituril compounds) may provide an increase in the carbon dioxide permeability of the polymer composition relative to the polymer alone while maintaining the same level of CO2 selectivity. The supramolecular structures (e.g. cucurbituril compounds) may provide a 50% to 3400% increase in carbon dioxide permeability of the polymer composition relative to the polymer alone. Preferably, the supramolecular structures (e.g. cucurbituril compounds) provide a 100% to 1000% increase in carbon dioxide permeability of the polymer composition relative to the polymer alone, for example the supramolecular structures (e.g. cucurbituril compounds) provide a 100% to 600% increase in carbon dioxide permeability of the polymer composition relative to the polymer alone

[0027] The supramolecular structures may (e.g. cucurbituril compounds) provide a 0% to 200% increase in carbon dioxide selectively of the polymer composition relative to the polymer alone. Preferably, the supramolecular structures (e.g. cucurbituril compounds) provide a 10% to 150% increase in carbon dioxide selectively of the polymer composition relative to the polymer alone.

[0028] Preferably the supramolecular structures are organic supramolecular structures. A membrane according to the present invention comprising organic supramolecular structures (e.g. cucurbituril compounds) provides enhanced carbon dioxide gas separation without altering the physical characteristics of the membrane.

[0029] In a preferred embodiment of the invention, the supramolecular structures (e.g. cucurbituril compounds) are positioned on a surface of the polymer such that the surface is functionalized with supramolecular structures (e.g. cucurbituril compounds). Preferably, the supramolecular structures (e.g. cucurbituril compounds) are distributed uniformly on a surface of the polymer. The supramolecular structures (e.g. cucurbituril compounds) may be present in a concentration from 0.05 mg to 32 mg per square meter surface area of the polymer composition. Preferably, the supramolecular structures (e.g. cucurbituril compounds) are present in a concentration from 0.1 mg to 10 mg per square meter surface area of the polymer composition. Preferably, the supramolecular structures (e.g. cucurbituril compounds) are present in a concentration from 0.1 mg to 5 mg per square meter surface area of the polymer composition. A membrane according to the present invention that has a surface functionalised with supramolecular structures (e.g. cucurbituril compounds) can provide a 6 times increase in the CO2 permeability of the membrane while also doubling the selectivity towards CO2.

[0030] In an alternative embodiment of the invention, the supramolecular structures (e.g. cucurbituril compounds) are distributed within and throughout the polymeric matrix of the polymer. Preferably, the supramolecular structures (e.g. cucurbituril compounds) are distributed uniformly within and throughout the polymeric matrix of the polymer. The supramolecular structures (e.g. cucurbituril compounds) may be present in a concentration from 0.1 to 5 mol% of the polymer composition. Preferably, the supramolecular structures (e.g. cucurbituril compounds) are present in a concentration from 0.5 to 4 mol% of the polymer composition. The supramolecular structures (e.g. cucurbituril compounds) can be used as filler in commercial-available polymer membranes to prepare hybrid polymer membranes. This type of membrane is similar to the mixed matrix membrane (MMM) while avoiding the formation of non-ideal interfaces between the filler and the polymer matrix as the supramolecular structures (e.g. cucurbituril compounds) are organic and in sub-nanometer sizes. The CO2 permeability in such hybrid membranes is enhanced at least 1.4 times (depending on the nature and the amount of supramolecular structures (e.g. cucurbituril compounds) added into the membrane) while keeping the selectivity towards CO2.

[0031] A wide variety of polymer may be used for the present invention. The polymer may be a rubbery polymer or a glassy polymer. Examples of suitable rubbery polymers according to the present invention are polydimethylsiloxane (PDMS), polyvinylamine (PVAm) and polyether oxide (PEO) based polymers. Examples of glassy polymers according to the present invention are polyamides, polyimides, polymers of intrinsic porosity (PIMs) and poly(1 -trimethylsilyl-1 -propyne) (PTMSP). The polymer may be a Pebax Elastomer polymer. Pebax Elastomer polymers are made up of rigid polyamide blocks and soft polyether blocks. The polymer may be selected from the group comprising a polysiloxane (e.g., a polymethylsiloxane (PMS) or a polydimethylsiloxane (PDMS)), poly(1 -trimethylsilyl-1 - propyne) (PTMSP), cellulose acetate (CA), polyethylene (PE) and polyimide (PI).

[0032] Preferably the polymer is a polysiloxane e.g. a polymethylsiloxane or a polydimethylsiloxane. Preferably, the polylsiloxane polymer is formed from a 6:1 ratio of prepolymer to curing agent. The prepolymer used may be Dow Corning Sylgard 184 silicone elastomer base (Sylgard 184A) and the curing agent used may be Sylgard 184 silicone elastomer curing agent (Sylgard 184B).

[0033] Alternatively, preferably the polymer is poly(1 -trimethylsilyl-1 -propyne) (PTMSP).

[0034] The polymer composition may be in the form of a flat sheet membrane or a hollow fibre membrane.

[0035] According to the present invention there is also provided a membrane comprising a polymer composition according to the present invention. The membrane may have a thickness from 7 pm to 6000 pm. Preferably, the membrane has a thickness from 100 pm to 5000 pm. Preferably, the membrane is a polymeric membrane.

[0036] The loading of the supramolecular structures on the membrane may be from 0.002 mg / cm2to 5 mg / cm2. For example, from 0.002 mg / cm2to 4 mg / cm2, for example from 0.05 mg / cm2to 3 mg / cm2.

[0037] The inventors have found that a loading of from 0.7 mg / cm2to 0.8 mg / cm2led to high CO2 permeability and CO2 selectivity, as compared to the unmodified membrane. Further, the inventors have found that a loading of from 0.05 mg / cm2to 0.6 mg / cm2led to high CO2 permeability compared to the unmodified membranes, and a loading of from 0.9 mg / cm2to 3.5 mg / cm2led to high CO2 selectivity, compared to the unmodified membranes.

[0038] A further object of the present invention is the provision of a membrane comprising a polymer and a plurality of supramolecular structures (e.g. cucurbituril compounds), wherein the supramolecular structures (e.g. cucurbituril compounds) are configured to selectively and reversibly adsorb carbon dioxide.

[0039] The plurality of supramolecular structures may be disposed on the surface of the membrane or may be disposed within the membrane.

[0040] The supramolecular structures may be colloids and / or macrocycles. A macrocycle according to the present invention may be a chemical structure containing a ring of twelve or more members (i.e. atoms).

[0041] The supramolecular structures (e.g. cucurbituril compounds) attract carbon dioxide gas (CO2) towards the surface of the membrane. This acts to selectively lower the membrane surface energy barrier for CO2 molecules, making it easier for CO2 molecules to permeate through the membrane. The attraction of carbon dioxide gas towards the surface of the membrane also acts to increase the CO2 concentration gradient across the membrane, which selectively increases the number of CO2 molecules that permeate through the membrane. The supramolecular structures (e.g. cucurbituril compounds) are also capable of functioning as molecular level CO2 pumps, which act to selectively pump CO2 molecules through the membrane.

[0042] Preferably the supramolecular structures are cucurbituril compounds.

[0043] Cucurbituril compounds according to the present invention are a group of organic compounds having the Formula (I): wherein n is 5, 6, 7, 8 or 10; and for each glycoluril unit, R1is selected from hydrogen, halogen, optionally substituted -(Ci-Ce)alkyl, optionally substituted -(Ci-C6)haloalkyl or -OH.

[0044] Cucurbituril compounds are a group of organic compounds comprising repeating glycoluril monomer units linked by methylene bridges (where n denotes the number of repeating glycoluril units). Each glycoluril unit may be substituted at the ring fusion position, for example substituted with halogen, optionally substituted -(Ci-Ce)alkyl, optionally substituted -(Ci- Ce)haloalkyl or -OH, or un-substituted having the Formula (II):

[0045] (n = 5, 6, 7, 8, 10)

[0046] Preferably, for each glycoluril unit, R1is hydrogen and the cucurbituril compound has the Formula (II) above.

[0047] Specifically, cucurbituril compounds comprising 6, 7 or 8 repeating glycoluril monomers have been found to selectively adsorb CO2 gas within their cavities. Without wanting to be bound by theory, it is believed that the high selectively of cucurbituril compounds for CO2 is due to the high enthalpy of CO2 adsorption caused by the CO2 molecules and cucurbituril compounds interacting through hydrogen bonding and / or dipole-quadrupole interactions.

[0048] The cucurbituril compounds attract carbon dioxide gas (CO2) towards the surface of the membrane. This acts to selectively lower the membrane surface energy barrier for CO2 molecules, making it easier for CO2 molecules to permeate through the membrane. The attraction of carbon dioxide gas towards the surface of the membrane also acts to increase the CO2 concentration gradient across the membrane, which selectively increases the number of CO2 molecules that permeate through the membrane. The cucurbituril compounds are also capable of functioning as molecular level CO2 pumps, which act to selectively pump CO2 molecules through the membrane.

[0049] Preferably the cucurbituril compounds comprise 6, 7 or 8 glycoluril or substituted glycouril units. These cucurbituril compounds have been found to provide particularly good CO2 gas separation performance when used in a membrane according to the present invention.

[0050] Alternatively, the supramolecular structures may be colloids, wherein the colloids have been functionalised with cucurbituril compounds. Preferably, the supramolecular structures are functionalised with cucurbituril compounds comprising 6, 7 or 8 glycoluril or substituted glycouril units.

[0051] Preferably, the supramolecular structures are MCM-41 colloids that have been functionalised with cucurbituril compounds. A membrane according to the present invention comprising MCM-41 colloids that have been functionalised with cucurbituril compounds (e.g. cucurbituril compounds comprising 6, 7 or 8 glycoluril or substituted glycouril units) has been shown to double the CO2 separation performance of the membrane while maintaining the same level of CO2 selectivity. Preferably, the channels of the MCM-41 colloids are functionalised with cucurbituril compounds.

[0052] Preferably, the supramolecular structures (e.g. cucurbituril compounds) comprise cavities. Preferably, the cavities are nanometer or sub-nanometer in diameter. Preferably, the cavities are from 1 to 10 angstroms in diameter. A cavity of this dimension is similar in size to carbon dioxide molecules, and so provides tight and specific binding that enable the selective capture of CO2 molecules within the cavity.

[0053] The supramolecular structures (e.g. cucurbituril compounds) provide an increase in the carbon dioxide permeability and / or selectively of the membrane relative to the membrane alone. The supramolecular structures (e.g. cucurbituril compounds) may provide an increase in the carbon dioxide permeability of the membrane relative to the membrane while maintaining the same level of CO2 selectivity. The supramolecular structures (e.g. cucurbituril compounds) may provide a 50% to 3400% increase in carbon dioxide permeability of the membrane relative to the membrane alone. Preferably, the supramolecular structures (e.g. cucurbituril compounds) provide a 100% to 1000% increase in carbon dioxide permeability of the membrane relative to the membrane alone, for example the supramolecular structures (e.g. cucurbituril compounds) provide a 100% to 600% increase in carbon dioxide permeability of the membrane relative to the membrane alone.

[0054] The supramolecular structures may (e.g. cucurbituril compounds) provide a 0% to 200% increase in carbon dioxide selectively of the membrane relative to the membrane alone. Preferably, the supramolecular structures (e.g. cucurbituril compounds) provide a 10% to 150% increase in carbon dioxide selectively of the membrane relative to the polymer alone.

[0055] Preferably, the supramolecular structures are organic supramolecular structures. A membrane according to the present invention comprising organic supramolecular structures (e.g. cucurbituril compounds) provides enhanced carbon dioxide gas separation without altering the physical characteristics of the membrane.

[0056] In a preferred embodiment of the invention, the supramolecular structures (e.g. cucurbituril compounds) are positioned on a surface of the polymer such that the surface is functionalized with supramolecular structures (e.g. cucurbituril compounds). Preferably, the supramolecular structures (e.g. cucurbituril compounds) are distributed uniformly on a surface of the polymer.

[0057] A membrane according to the present invention that has a surface functionalised with supramolecular structures (e.g. cucurbituril compounds) can provide a 6 times increase in the CO2 permeability of the membrane while also doubling the selectivity towards CO2.

[0058] In an alternative embodiment of the invention, the supramolecular structures (e.g. cucurbituril compounds) are distributed within and throughout the polymeric matrix of the polymer. Preferably, the supramolecular structures (e.g. cucurbituril compounds) are distributed uniformly within and throughout the polymeric matrix of the polymer.

[0059] Preferably, the membrane is a polymeric membrane.

[0060] A wide variety of polymer may be used for the present invention. The polymer may be a rubbery polymer or a glassy polymer. Examples of suitable rubbery polymers according to the present invention are polydimethylsiloxane (PDMS), polyvinylamine (PVAm) and polyether oxide (PEO) based polymers. Examples of glassy polymers according to the present invention are polyamides, polyimides, polymers of intrinsic porosity (PIMs) and poly(1 -trimethylsilyl-1 -propyne) (PTMSP). The polymer may be a Pebax Elastomer polymer. Pebax Elastomer polymers are made up of rigid polyamide blocks and soft polyether blocks.

[0061] Preferably the polymer is a polysiloxane e.g. a polymethylsiloxane or a polydimethylsiloxane. Preferably, the polylsiloxane polymer is formed from a 6:1 ratio of prepolymer to curing agent. The prepolymer used may be Dow Corning Sylgard 184 silicone elastomer base (Sylgard 184A) and the curing agent used may be Sylgard 184 silicone elastomer curing agent (Sylgard 184B).

[0062] Alternatively, preferably the polymer is poly(1 -trimethylsilyl-1 -propyne) (PTMSP).

[0063] The membrane may have a thickness from 7 pm to 6000 pm. Preferably, the membrane has a thickness from 100 pm to 5000 pm.

[0064] The membrane may be a flat sheet membrane or a hollow fibre membrane. The membrane may be a surface modified membrane.

[0065] The loading of the supramolecular structures may be from 0.002 mg / cm2to 5 mg / cm2. For example, from 0.002 mg / cm2to 4 mg / cm2, for example from 0.05 mg / cm2to 3 mg / cm2. A further object of the present invention is the provision of a method for preparing a carbon dioxide gas separation membrane comprising; (i) providing a membrane comprising a polymer and (ii) applying a plurality of supramolecular structures (e.g. cucurbituril compounds) to a surface of the membrane, wherein the supramolecular structures (e.g. cucurbituril copounds) are configured to selectively and reversibly adsorb carbon dioxide.

[0066] Preferably, the method prevents deformation of the polymer membrane structure during preparation of the carbon dioxide gas separation membrane.

[0067] The plurality of supramolecular structures (e.g. cucurbituril compounds) may be applied to the surface of the membrane by immersing the membrane in a solution containing the supramolecular structures.

[0068] The plurality of supramolecular structures (e.g. cucurbituril compounds) may be applied to the surface of the membrane by directly applying a solution of the supramolecular structures to the membrane and spreading the solution across the surface of the membrane.

[0069] The plurality of supramolecular structures (e.g. cucurbituril compounds) may be applied to the surface of the membrane by dispersing the solid supramolecular structures on to the surface of the membrane. Such a method may, for example, be used for functionalising a membrane with cucurbituril compounds or a colloid as described herein.

[0070] The surface of the membrane may be treated with a plasma prior to application of the supramolecular structures.

[0071] The method may include a step following application of the supramolecular structures, in which an acidic solution or polar solvent is applied to the modified membrane to remove some of the supramolecular structures. The acidic solution may comprise hydrochloric acid. This step may assist with tuning the final loading of supramolecular structures.

[0072] The method may include a step of heating the membrane following application of the supramolecular structures. The supramolecular structures (e.g. cucurbituril compounds), may be applied as a solution having a supramolecular structure (e.g. cucurbituril compound) concentration of from 0.05 mg / mL to 35 mg / mL.

[0073] The inventors have found that at concentrations of at least 5 mg / mL, aggregation of the supramolecular structures occurs which led to high CO2 permeability, as compared to the unmodified membrane. The supramolecular structures (e.g. cucurbituril compounds), may be applied to a surface of the membrane so as to provide a loading of from 0.002 mg / cm2to 5 mg / cm2. For example, from 0.002 mg / cm2to 4 mg / cm2, for example from 0.05 mg / cm2to 3 mg / cm2.

[0074] The membrane may be a flat sheet membrane or a hollow fibre membrane. Preferably, the membrane is a polymeric membrane.

[0075] These methods have been shown to prevent changes of the membrane's matrix structure. In other words, the method of forming the membrane does not affect the mechanical properties of the membranes that are formed, which broadens the applicability of the method to prepare functional membranes with enhanced CO2 separation performance. At the same time, compared with the traditional method of improving film performance through complex material design, surface modification has better scalability and processibility. The surface modification method has been applied to fabricate functional membranes for CO2 separation using either the rubbery polymers or glassy polymers which included most polymeric membranes available on the market.

[0076] Preferably, the supramolecular structures are cucurbituril compounds. Preferably the cucurbituril compounds comprise 6, 7 or 8 glycoluril or substituted glycouril units. These cucurbituril compounds have been found to provide particularly good CO2 gas separation performance when used in a membrane according to the present invention.

[0077] In another aspect of the invention, the supramolecular structures are colloids and / or macrocycles, wherein the colloids and / or macrocycles have been functionalised with cucurbituril compounds. Preferably, the supramolecular structures are functionalised with cucurbituril compounds comprising 6, 7 or 8 glycoluril or substituted glycouril units. Preferably, the supramolecular structures are MCM-41 colloids that have been functionalised with cucurbituril compounds.

[0078] A further object of the present invention is the provision of a method for preparing a carbon dioxide gas separation membrane comprising a polymer, comprising the steps of; (i) mixing a prepolymer with a plurality of supramolecular structures (e.g. cucurbituril compounds), wherein the supramolecular structures (e.g. cucurbituril compounds) are configured to selectively and reversibly adsorb carbon dioxide and (ii) adding a curing agent to the prepolymer and supramolecular structures (e.g. cucurbituril compounds) mixture and polymerising the mixture to form a membrane. A further object of the present invention is the provision of a process for separating carbon dioxide gas from a gas mixture comprising; (i) contacting the gas mixture to a membrane comprising a polymer and a plurality supramolecular structures (e.g. cucurbituril compounds), wherein the supramolecular structures (e.g. cucurbituril compounds) are configured to selectively and reversibly adsorb carbon dioxide; and (ii) recovering a permeate gas having a higher concentration of carbon dioxide than in the gas mixture.

[0079] A further object of the present invention is the use of a membrane in a process for separating carbon dioxide gas from a gas mixture, wherein the membrane comprises a polymer and a plurality of supramolecular structures (e.g. cucurbituril compounds), wherein the supramolecular structures (e.g. cucurbituril compounds) are configured to selectively and reversibly adsorb carbon dioxide.

[0080] Preferably, the supramolecular structures are cucurbituril compounds. Preferably the cucurbituril compounds comprises 6, 7 or 8 glycoluril or substituted glycouril units. These cucurbituril compounds have been found to provide particularly good CO2 gas separation performance when used in a membrane according to the present invention.

[0081] In another aspect of the invention, the supramolecular structures are colloids and / or macrocycles, wherein the colloids and / or macrocycles have been functionalised with cucurbituril compounds. Preferably, the supramolecular structures are functionalised with cucurbituril compounds comprising 6, 7 or 8 glycoluril or substituted glycouril units. Preferably, the supramolecular structures are MCM-41 colloids that have been functionalised with cucurbituril compounds.

[0082] DETAILED DESCRIPTION OF THE INVENTION

[0083] Definition of terms

[0084] A Supramolecular structure according to the present invention is a host structure capable of forming a supramolecular complex with a guest molecule of carbon dioxide.

[0085] A MCM-41 (Mobil Composition of Matter No. 41) colloid is a mesoporous material with a hierarchical structure consisting of a regular arrangement of cylindrical mesopores that form a one-dimensional pore system.

[0086] “(Ci-Ce)” means a carbon radical having 1 , 2, 3, 4, 5 or 6 carbon atoms.

[0087] In this specification, unless stated otherwise, the term “halo” or “halogen” may be fluoro, chloro, bromo or iodo.

[0088] In this specification, unless stated otherwise, the term “alkyl” includes both straight and branched chain alkyl radicals and may be methyl, ethyl, n-propyl, / -propyl, n-butyl, / -butyl, s- butyl, f-butyl, n-pentyl, / -pentyl, f-pentyl, neo-pentyl, n-hexyl, / -hexyl or t-hexyl. The term “optionally substituted” means an alkyl radical as defined above which is substituted.

[0089] In this specification, unless stated otherwise, the term “haloalkyl” means an alkyl radical as defined above, substituted with one or more halo radicals. The term “(Ci-C6)haloalkyl” may include, but is not limited to, fluoromethyl, difluoromethyl, trifluoromethyl, fluoroethyl and difluoroethyl.

[0090] Particular non-limiting examples of the present invention will now be described with reference to the following Figures, in which:

[0091] Figure 1 a) is a schematic representation of the synthesis of glycouril and b) is a 1 H NMR spectrum of the synthesized glycoluril.

[0092] Figure 2 a) is a schematic representation of the synthesis of CB[n] and b) is a 1 H NMR spectrum of the crude CB[n] reaction mixture.

[0093] Figure 3 a) is the molecular structure of CB[6] and b) is a 1 H NMR spectrum of the CB[6] solid.

[0094] Figure 4 is a schematic representation of the condensation reaction between glycoluril and formaldehyde and the resulting structure of cucurbit[n]urils (CB[n]s).

[0095] Figure 5 is a schematic representation of CB[n]-modified MCM-41 colloids.

[0096] Figure 6 is a schematic representation of the surface-modification of polymer membranes using CB[n],

[0097] Figure 7 is a schematic representation the synthesis of CB[n]-based mixed matrix modified membrane.

[0098] Figure 8 is the piping and instrumentation diagram (P&ID) for the gas separation process. Figure 9 is a graph indicating the relationship between permeability and selectivity of membranes with different proportions of PDMS.

[0099] Figure 10 is a graph indicating CO2 separation performance of a surface-modified PDMS membrane with varying amount of CB[6] (Membrane thickness: 150um; Testing pressure AP=1 bar; Composite of the feed gas:CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min).

[0100] Figure 11 is a graph indicating CO2 separation performance of a surface-modified PDMS membrane with varying amount of CB[7] (Membrane thickness: 150um; Testing pressure AP=1 bar; Composite of the feed gas:CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min).

[0101] Figure 12 is a graph indicating CO2 separation performance of a surface-modified PDMS membrane with varying amount of CB[8] (Membrane thickness: 150um; Testing pressure AP=1 bar; Composite of the feed gas:CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min).

[0102] Figure 13 is a graph summarising the CO2 separation performance of surface modified PDMS membranes with CB[n] (n = 6,7,8, separately). (Membrane thickness: 150um; Testing pressure AP=1bar; Composite of the feed gas:CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min).

[0103] Figure 14 is a graph indicating the effect of membrane thickness of CB[6]-modified PDMS membranes on CO2 / CH4 separation. (Testing pressure AP=1 bar; Composite of the feed gas:CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min).

[0104] Figure 15 is a graph indicating the effect of membrane thickness of CB[7]-modified PDMS membranes on CO2 / CH4 separation. (Testing pressure AP=1 bar; Composite of the feed gas:CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min).

[0105] Figure 16 is a graph indicating the CO2 separation performance of mixed matrix membranes incorporating CB[6] . (Membrane thickness: 200um; Testing pressure AP=1bar; Composite of the feed gas: CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min).

[0106] Figure 17 is a graph indicating the CO2 separation performance of mixed matrix membranes incorporating CB[7] . (Membrane thickness: 200um; Testing pressure AP=1bar; Composite of the feed gas: CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min). Figure 18 is a graph indicating the CO2 separation performance of mixed matrix membranes incorporating CB[8] . (Membrane thickness: 200um; Testing pressure AP=1bar; Composite of the feed gas: CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min).

[0107] Figure 19 is a graph summarising the CO2 separation performance of PDMS mixed matrix membranes incorporating various CB[n] (n=6,7,8). (Membrane thickness: 200um; Testing pressure AP=1 bar; Composite of the feed gas: CC>2:CH4=3:7; Flow rate of the sweep gas:20ml / min).

[0108] Figure 20 is a graph summarising the CO2 separation performance of PDMS membranes that have been surface modified with CB[6] functionalised MCM-41 colloids.

[0109] Figure 21 is an FTIR spectra of the CB[n]-based mixed matrix membranes.

[0110] Figure 22 is a graph indicating the contacting angle testing result for the CB[n]-based mixed matrix membranes.

[0111] Figure 23 are Atomic Force Microscopy (AFM) images of (a) pure PDMS (roughness average:0.7451); and (b) 1 % CB[6] mixed PDMS membrane (roughness average:0.5842).

[0112] Figure 24 are Scanning Electron Microscope (SEM) images showing the cross-section of PDMS membranes that have been surface modified with CB[n] at a concentration of 0.2 mg / cm2.

[0113] Figure 25 are Scanning Electron Microscope (SEM) images showing the top surface of PDMS membranes that have been surface modified with CB[n] at a concentration of (a) 0.6 mg / cm2and (b) 0.2 mg / m2.

[0114] Figure 26 is a graph summarising the CO2 separation performance of PTMSP membranes that have had their surface modified with CB[7] and also of CB[7]-mixed matrix PTMSP membranes.

[0115] Figure 27 is a graph showing CO2 permeability (permeance) and CO2 / CH4 selectivity for a non-functionalised (pristine) cellulose acetate membrane, together with cellulose acetate membranes functionalised with varying CB[7] loadings. Figure 28 is a graph showing CO2 permeability (permeance) and CO2 / CH4 selectivity for a non-functionalised (pristine) cellulose acetate membrane, together with cellulose acetate membranes functionalised with varying CB[7] loadings.

[0116] Figure 29 is a graph showing CO2 flux as a function of partial pressure difference for a cellulose acetate membrane functionalised with CB[7] by applying a CB[7] solution having a concentration of 10mg / mL CB[7] and a final CB[7] loading of 0.798mg / cm2.

[0117] Figure 30 is a graph showing the CO2 permeability of a cellulose acetate membrane (CA_HS) functionalised with CB[7] over a one month period.

[0118] Figure 31 is a graph showing the CO2 permeability and CO2 / CH4 selectivity for an unmodified polyethylene membranes and polyethylene membranes functionalised with CB[7],

[0119] Figure 32 is a graph showing the CO2 permeability and CO2 / N2 selectivity for an unmodified cellulose acetate membrane (CA_HS) and a CA_HS membrane functionalised with CB[7] with a loading of 0.795 mg / cm2. Figure 32 also shows the CO2 permeability and the CO2 / N2 selectivity of an unmodified polyimide membrane and a polyimide membrane functionalised with CB[7] with loading of 0.795mg / cm2.

[0120] Figure 33 is a graph showing the CO2 permeability and CO2 / N2 selectivity of an unmodified polyimide hollow fibre membrane and a polyimide hollow fibre membrane functionalised with CB[7],

[0121] Figure 34 is a graph showing the CO2 permeability and CO2 / N2 selectivity of an unmodified cellulose acetate membrane, and cellulose acetate membrane functionalised with a hollow polystyrene nanoparticle colloid.

[0122] The term cucurbituril (CB[n]) refers to cucurbituril compounds with un-substituted glycouril units (where n denotes the number of repeating glycoluril units), according to Formula (II).

[0123] Method of synthesis

[0124] 1. Synthesis and purification of cucurbiturils (CBfnl)

[0125] 1.1 Synthesis of glycoluril

[0126] The glycoluril is first synthesized as it is the starting material for CB[n] synthesis. One typical example is listed below in Fig.1a. Initially, 300 g of urea was dissolved in 500 mL water followed by a 250 g of 40% aq. solution of glyoxal and 43 mL cone. HCI (12 M). This mixture was heated at ~87 °C until a heavy precipitate formed, which is separated by centrifugation after it had cooled down to room temperature, and was washed with deionized water and acetone. The resulting white solid was dried under a high vacuum. The 1 H NMR spectrum of the synthesized glycoluril is measured and shown in Fig. 1 b.

[0127] 1.2 Synthesis of CB[n]

[0128] The CB[n] were synthesized by condensation reaction between glycoluril and formaldehyde according to Fig 2a. Initially, 196 g of the prepared glycoluril and 86 g of paraformaldehyde were mixed thoroughly. A 260 mL of an ice-cold con. HCI solution was added gradually while stirring. After the addition of -100 mL HCL solution, the stirring was no longer possible as the reactants were transformed into a brick-like material. At this point, the reaction becomes highly exothermic. This heterogeneous mixture was then gradually heated to 80 °C for 2.5 h and remains for an additional 2.5 h to dissolve the solids homogeneously. The resultant red solution was further heated to 100 °C for 19 h to precipitate the CB[n] mixture. Fig 2b shows the 1 H NMR spectrum of the synthesized CB[n] mixture, confirming the formation of CB[5], CB[6], CB[7], and CB[8],

[0129] 1.3 Purification of CB[n]

[0130] The CB[n] are purified based on their different solubilities in different solvents. Specifically, the CB[6] was obtained by purifying the CB[n] based on the solubility difference in water and ethyl alcohol. A 3 L of deionized water was added into the as-synthesised CB[n] mixture and stirred for 48 h to dissolve CB[5] and CB[7] into the water, whereas CB[6] and CB[8] remains in the solid precipitate. To separate CB[6] from the CB[6] and CB[8] mixture, 4.5 L of 3 M HCI has added into the CB[6] and CB[8] mixture, and then the excess of ethyl alcohol was added. This process precipitates the CB[6] and the CB[8] dissolves in the solution. The obtained CB[6] was washed 3 times by ethyl alcohol followed by acetone and then recrystallized from concentrated HCI. The chemical structure and the proton NMR of the CB[6] are shown in Fig.3a and 3b, respectively.

[0131] Fig 4 summarises the condensation reaction between glycoluril and formaldehyde and the resulting structure of cucurbit[n]urils (CB[n]s).

[0132] 2. Synthesis of CBfnl modified MCM-41 colloids The CB[n] modified MCM-41 colloids are synthesised according to the following procedure. The CB[n] molecules are firstly evenly dispersed in a solvent. The MCM-41 colloids are then added to the solution, and they are mixed together. Due to their electrostatic properties, the CB[n] molecules are drawn to the channels of the MCM-41 colloids resulting in the channels of the MCM-41 colloids being functionalised with CB[n] molecules. The solution is then freeze-dried to yield a CB[n]-modified MCM-41 powder. These CB[n]-modified MCM-41 colloids are indicated in Figure 5.

[0133] 3. Surface-modification of PDMS membranes using CBfnl (CBfnl-surface-membranes)

[0134] The CB[n] molecules are employed to modify the surfaces of commercial-available polymer membranes. In these examples, polydimethylsiloxane (PDMS) membranes have been used. The typical protocol of the surface modification method is as follow:

[0135] 1 . Synthesize the PDMS membrane using a precursor to curing agent ratio of 6: 1 . Mix the precursor and curing agent thoroughly.

[0136] 2. Pour the mixture into a Petri dish with a diameter of 6 cm and bake it at 80°C for 5-10 minutes.

[0137] 3. Grind the CB[n] and place it in an oven at 100°C overnight to remove any residual moisture.

[0138] 4. Evenly spray the CB[n] powder onto the surface of the membrane that has been baked for 5 minutes.

[0139] 5. Place the modified membrane back in the oven and bake it for an additional 30- 60 minutes at 80°C.

[0140] 6. After baking, the modified membrane is ready for testing.

[0141] The schematic of Figure 6 demonstrates the surface-modification of polymer membranes using CB[n],

[0142] 4. Synthesis of CBfnl-based mixed matrix modified membrane

[0143] The CB[n] are used as filler in commercial-available polymer membranes to prepare hybrid polymer membranes. In these examples, polydimethylsiloxane (PDMS) membranes have been used. The protocol for preparation of CB[n]-based mixed matrix membranes:

[0144] 1 . Grind the CB[n] and place it in an oven at 100°C overnight to remove any residual moisture.

[0145] 2. Mix the precursor of PDMS with CB[n]s powder. 3. Synthesize the PDMS membrane using a CB[n]s-mixed precursor to curing agent ratio of 6: 1 . Mix the precursor and curing agent thoroughly.

[0146] 4. Pour the mixture into a Petri dish with a diameter of 6 cm and bake it at 80°C for 30-60 minutes.

[0147] 5. Place the membrane in fuming cupboard 10 mins for cooling down. Then the membrane will be ready for testing.

[0148] The schematic of Figure 7 demonstrates the synthesis of CB[n]-based mixed matrix modified membrane.

[0149] 5. Surface-modification of PDMS membranes using CBfnl modified MCM-41 colloids

[0150] The surface of a PDMS membrane can be modified with CB[n] modified MCM-41 colloids according to multiple methods according to the present invention.

[0151] Dipping Method: This involves coating the solution onto the polymer membrane surface and allowing the solvent to evaporate. This results in the CB[n] modified MCM-41 colloids being adhered to the polymer surface. Plasma Activation: The surface of the polymer membrane is activated via plasma processing to introduce hydroxy (-OH) groups onto the surface of the polymer membrane. The CB[n]-modified MCM-41 powder is then applied to the activated surface of the polymer membrane. Excess powder is shaken off, leaving CB[n]-modified MCM-41 particles that are adhered to the surface of the membrane through hydrogen bonds formed between the CB[n]-modified MCM-41 colloids and the activated membrane surface.

[0152] Direct Application: The CB[n]-modified MCM-41 powder can also be directly applied to the polymer membrane surface which results in the surface being functionalised with CB[n]- modified MCM-41.

[0153] Surface modification of polymeric membranes with CBfnl using the dipping method

[0154] A polymeric membrane in the form of a flat sheet or a hollow fibre is immersed in a CB[n[] solution having a concentration ranging from 0.05 mg / mL to 35 mg / mL for over 5 seconds. Following immersion, excess solution is removed and the membranes dried at a temperature of between 25°C and 80°C in an oven.

[0155] Surface modification of polymeric membranes with CBfnl using the loading and spreading method A CB[n] solution is applied directly to the surface of a membrane and evenly distributed across the surface of the membrane. The membranes are then transferred to an oven and baked at elevated temperatures, resulting in CB[n] loadings ranging from 0.002 mg / cm2to 3 mg / cm2.

[0156] Surface modification of polymeric membranes with CBfnl using the sprinkling method

[0157] CB[n] powder is directly sprinkled onto the membrane surface. The CB[n] molecules are able to attach to the membrane surface by physical adsorption caused by intermolecular hydrogen bonding and the electric potential difference between the molecule and the membrane surface.

[0158] Surface modification of polymeric membrane with CBfnl utilising plasma treatment

[0159] A polymeric membrane is first treated using a plasma treatment process. The treated membrane is then loaded with CB[n] powder and the CB[n] spread over its surface. Excess CB[n] powder is removed.

[0160] In situ surface modification of polymeric membrane with CBfnl while the membrane is in a membrane module.

[0161] A solution of CB[n] can be provided in a tank, and the solution continuously pumped into the membrane module at a controlled flow rate and circulated back into the tank. Alternatively, the circulation can be paused and the membrane left to soak in the CB[n] solution within the membrane module. Following circulation of the CB[n] solution or soaking of the CB[n] solution, air is introduced into the membrane module to dry the membrane and thereby provide a membrane functionalised with CB[n],

[0162] This method enables in situ functionalising of the polymer membrane while the polymer membrane is in use.

[0163] CBfnl removal from membranes

[0164] A HCI solution or a polar solvent can be used to wash the surface of the functionalised membrane and remove CB[n] molecules. This technigue can enable precise control of the CB[n] loading, and enables optimising of a given functionalised membrane. Additionally, this washing process can be used to completely remove CB[n] molecules and regenerate the membrane. C02Separation Performance Analysis

[0165] 1 . Method for testing gas separation performance of membranes

[0166] This method involves placing the prepared membrane between a circle shaped aluminium foil. A hole of 4 mm is punched through the centre of the foil to allow for the inlet gas to only be fed through the membrane. The thickness of the membrane is measured using a micrometre. Hot glue is used to create an air-tight frame to prevent any leakages of gas passing through the membrane. The foil with the membrane inside is then placed in the membrane holder which is part of a gas separation system. The membrane is secured in the membrane holder with a rubber seal and three screws to prevent any gas leakage. The gas separation system is turned on by switching on the valve for the inlet feed gas, which contains a composition of 70% methane and 30% carbon dioxide, as well as the valve for sweep gas (nitrogen), which is fed to the permeate side of the membrane. This gas possesses the function of lowering the partial pressure of the permeating species which is carbon dioxide which therefore increases the driving force for separation. The piping and instrumentation diagram (P&ID) of this procedure is shown as Fig 8.

[0167] 2. Testing ratio of PDMS membrane prepolymer to curing agent

[0168] An experiment was performed to determine the ratio of PDMS membrane prepolymer to curing agent. As observed in the Fig 9, a ratio of 6 to 1 is preferable for gas separation since it has a better carbon dioxide permeability and selectivity than a ratio of 10 to 1. While the selectivity is less than 1.5 at 14 to 1 , the penetration rate is slightly higher than 5000 Barrer, which is a poor choice. The optimal ratio to fill with CB molecules in order to create a combination of matrix membranes is therefore thought to be 6 to 1.

[0169] 3. CO2 separation performed of CBlnl-surface-membranes

[0170] Experiments were performed to test the performance of separating CO2 from a CO2 / CH4 mixture using CB[n]-surface-membranes (n=6,7,8) and pure PDMS membrane, separately.

[0171] 3. 1 CO2 separation performance of CB[6]-surface-membranes

[0172] Fig 10 demonstrates that with the increase in CB[6] amount, the CO2 permeability of the membrane enhanced to eight times.

[0173] 3.2 CO 2 separation performance of CB[7]-surface-membranes Fig 11 demonstrates that modifying the membrane surface with CB[7] significantly enhances the permeability of the membrane without significantly affecting its selectivity. The permeability increases to four times with the CB[7] amount on the membrane surface increasing and the selectivity of the membrane decrease to 85% comparing to the pure PDMS membrane function mentioned in Fig. 13. When considering CBs as molecular-level CO2 pumps, CB[7] seems to possess a more potent pumping power for CO2 compared to CB[6], This could be attributed to the presence of more functional groups in CB[7], which can absorb carbon dioxide more effectively than CB[6], as well as provide a more suitable channel for size-sieving diffusion. With a larger pore size, CB[7] offers a more substantial channel compared to CB[6], which allows for a faster flux in the chamber, contributing to the enhanced membrane performance.

[0174] 3.3 CO2 separation performance of CB[8]-surface-membranes

[0175] Fig 12 demonstrates that by increasing the CB[8] amount on the membrane surface, the permeability of the membrane increased 15 times; however, the selectivity also decreased to almost one after 0.6mg point. The observed results are akin to those of CB[7], demonstrating a substantially greater enhancement in permeability in comparison to CB[6] and CB[7], Nonetheless, the enlarged channel afforded by CB[8] precludes it from delivering size-sieving selectivity. While CB[8] exhibits the most potent capacity to augment membrane permeability, it concurrently possesses the lowest selectivity.

[0176] 3.4 Summary of separation performance

[0177] Fig 13 compares the CO2 separation performance for surface modified PDMS membranes having the same amount of specific CB[n] (n=6,7,8, separately) on the PDMS membrane surface. CB[7] offers the best performance without sacrificing the membrane's selectivity. CB[8] appears to provide the most significant improvement in permeability, while CB[6] is the most cost-effective option.

[0178] In summary, it is evident that CB[n]s (n=6,7,8) possess significant potential for surface modification of CO2 gas separation membranes. In the following studies, CB[6] and CB[7] will be adopted to further optimize the conditions for CO2 separation.

[0179] 3.5 Effect of membrane thickness on CB[6]-surface-membrane CO2 separation performance

[0180] Fig. 14 demonstrates a correlation between increased membrane thickness and the CO2 separation performance. In this experiment, to fully explore the potential of surface modification, we immobilized minimum amounts of CB[n]s on the membrane. The first point in Fig.10 is the second point in Fig.14. As evidenced in Fig.14, the membrane permeability exhibited a substantial increase of over 4.3 times, while selectivity improved by approximately 65%. The separation efficiency of the membrane is enhanced ten time after this modification.

[0181] 3.6 Effect of membrane thickness on CB[7]-surface-membrane CO2 separation performance

[0182] Fig 15 demonstrates a correlation between increased membrane thickness and the CO2 separation performance. In this experiment, to fully explore the potential of surface modification, we immobilized minimum amounts of CB[n]s on the membrane. The first point in Fig.11 is the first point in Fig.15. For CB[7], it exhibits even greater potential due to its sizesieving capability within its channel compared to CB[6], This unique property allows CB[7] to selectively facilitate the transport of CO2 molecules, thereby enhancing the membrane's overall efficiency and effectiveness. As evidenced in Fig 15, the membrane permeability exhibited a substantial increase of over 4.5 times, while selectivity improved by approximately 127%.

[0183] 4. CO2 separation performed of CBfnl-based mixed matrix membranes

[0184] Experiments were performed to test the performance of separating CO2 from a CO2 / CH4 mixture using CB[n]- based mixed matrix membranes (n=6,7,8) and pure PDMS membrane, separately.

[0185] 4. 1 CO2 separation performance of CB[6]-based mixed matrix membrane

[0186] Fig.16 demonstrates that integrating CB[6] into the membrane evidently enhances its flux. When the CB[6] amount in the membrane increases, the permeability of the membrane gradually enhanced to 3 times compared to the pure PDMS membrane (first data point in Fig.16). However, at the same time, the membrane's selectivity begins to decline when the CB6 concentration exceeds 1%. Consequently, without sacrificing selectivity, the membrane's flux can be increased by 1.8 times compared to the original PDMS membrane by incorporating CB[6],

[0187] 4.2 CO2 separation performance of CB[7]-based mixed matrix membrane

[0188] Fig 17 shows that that adding CB[7] to the membrane improves its permeability. With the CB[7] amount in the membrane increase, the permeability of the membrane gradually enhanced to 2.7 times compared to the pure PDMS membrane (first data point in Fig.17), the membrane's selectivity keep constant until the CB[7] concentration exceeds 0.5% and then start to decreases. Interestingly, without sacrificing selectivity, CB[7] enhances the membrane's permeability by 1.8 times compared to the unmodified membrane.

[0189] 4.3 CO2 separation performance of CB[8]-based mixed matrix membrane

[0190] Figure 18 demonstrates that adding CB[8] to the membrane results in an improvement in permeability. With the amount of CB[8] in membrane increased, the permeability of the membrane enhance to 1 .35 times compared to the original membrane, and the selectivity of the membrane is stable.

[0191] 4.4 Summary of separation performance

[0192] To systematically evaluate the efficacy of the mixed matrix modification method, we incorporated equal amounts of various CB[n] types into PDMS membranes and compared their performance. As shown in Fig.19, the integration of CB[6], CB[7] and CB[8] led to an improvement in membrane function. This result demonstrates that incorporating CB[6], CB[7] and CB[8] into PDMS mixed matrix membranes can effectively enhance their gas separation performance.

[0193] 5. CO2 separation performed of CBf61 functionalised MCM-41 -surface-membranes

[0194] Figure 20 summarises the CO2 separation performance of PDMS membranes that have been surface modified with CB[6] functionalised MCM-41 colloids. In this experiment, it was observed that when CB[6] functionalised MCM-41 colloids were used to modify both sides of a PDMS membrane, the membrane’s permeability doubled while maintaining the same level of selectivity. This modification effectively doubled the separation efficiency of the membrane compared to the original PDMS membrane.

[0195] 6. CO2 separation performance of CBR1 functionalised PTMSP membranes

[0196] Experiments were performed to analyse the CO2 separation performance of CB[7] functionalised PTMSP membranes. Figure 26 demonstrates the CO2 separation performance of PTMSP membranes that have had their surface modified with CB[7] and also of CB[7]-mixed matrix PTMSP membranes. After matrix modification, the membrane's permeability increased by 54.7% and selectivity went up by 94.2%. With the surface modification, the permeability increased to 338,720 Barrer and 713,185 Barrer (16.3 and 34.3 times that of the original PTMSP membrane). Additionally, when the membrane's thickness grew from 150um to 507um, the effects of the surface modification were more evident, with the thicker membrane showing a 2.1 times greater impact on CO2 separation performance.

[0197] CO2 / CH4 gas separation performance of CBR1 functionalised cellulose acetate membranes

[0198] Experiments were performed to analyse the CO2 separation performance of CB[7] functionalised cellulose acetate membranes.

[0199] Unmodified cellulose acetate membranes were functionalised with CB[7] at varying loadings using the loading and spreading method as described above. Two cellulose acetate membranes were tested. The first cellulose acetate membrane demonstrated a higher selectivity and lower permeability before functionalisation, and is referred to as CA_HS. The second cellulose acetate membrane demonstrated a lower selectivity before functionalisation and is referred to as CA_LS.

[0200] The results of the experiments carried out on the first cellulose acetate membrane (CA_HS) are illustrated in Figure 27.

[0201] The unmodified cellulose acetate membrane (pristine) demonstrated a permeability of 28 GPU and CO2 / CH4 selectivity of 25. At low CB[7] loadings, the CO2 permeability of the membrane increased, while at higher CB[7] loadings, CO2 selectivity also increased. A CB[7] loading of 0.795 mg / cm2led to high CO2 permeability and CO2 selectivity, as compared to the unmodified membrane.

[0202] The concentration of the CB[7] solution that is applied to the membrane also had an effect on the CO2 permeability and the CO2 / CH4 selectivity. It is thought that at higher CB[7] concentrations, the CB[7] molecules aggregate and thereby form channels that facilitate the passage of CO2 molecules through the membrane.

[0203] The results of the experiments carried out on the second cellulose acetate membrane (CA_LS) are illustrated in Figure 28.

[0204] The Ca_LS membrane was functionalised with CB[7] at varying loadings using the loading and spreading method as described above.

[0205] As illustrated in Figure 28, functionalisation with CB[7] led to increased CO2 permeability and selectivity of the cellulose acetate film compared to the unfunctionalized film (pristine CA_LS). Stability studies

[0206] The functionalised cellulose acetate membranes were also tested for stability and the results of these experiments are set out in figures 29 and 30.

[0207] Figure 29 illustrates a stability study carried out on a cellulose acetate (CA_LS) membrane to which CB[7] was applied in a concentration of 10mg / ml leading to a CB[7] loading of 0.798mg / cm2. The functionalised membrane was prepared using the loading and spreading method as described above. The functionalised membrane demonstrated excellent stability. The modified CA_LS membrane demonstrated a linear relationship between CO2 flux and applied pressure, and the CO2 / CH4 selectivity remained stable under elevated pressure.

[0208] The functionalised CA_LS membrane was able to maintain high CO2 flux at 2.22x1 O'5m3(STP) m-2s'1and CO2 / CH4 selectivity of 18, even with transmembrane pressure difference approaching 0 (FC02=4.06x1 O'5m3(STP) m-2s’1, SC02 / CH4= 22 when applied pressure is 1 bar). This means that a process of CO2 separation using this membrane could have a low energy consumption.

[0209] Long term durability was also tested and the results of this experiment are shown in Figure

[0210] 30. The CA_HS membrane was functionalised by applying CB[7] at a concentration of 10mg / ml, leading to a loading of 0.798mg / cm2. The functionalised membrane was prepared using the loading and spreading method as described above.

[0211] As demonstrated in Figure 30, there was no significant decrease in CO2 permeability or CO2 / CH4 selectivity observed over a one month period.

[0212] CO2 / CH4 gas separation performance of CBR1 functionalised polyethylene membranes

[0213] Experiments were performed to analyse the CO2 separation performance of CB[7] functionalised polyethylene membranes. The results of this experiment are shown in Figure

[0214] 31.

[0215] A polyethylene membrane was functionalised by applying CB[7] at a concentration of 10mg / ml, leading to a loading of 0.798mg / cm2. The functionalised membrane was prepared using the loading and spreading method as described above. As shown in Figure 31 , functionalising the polyethylene film with CB[7] led to a significant increase in CO2 permeability while also maintaining CO2 selectivity of the polyethylene membrane.

[0216] CO2 / N2 gas separation performance of CBR1 functionalised cellulose acetate flat sheet membranes, polyimide flat sheet membranes and polyimide hollow fibre membranes

[0217] The flat sheet CA_HS membrane, described above, was functionalised using CB[7] to achieve a CB[7] loading of 0.795mg / cm2. The functionalised membrane was prepared using the loading and spreading method as described above.

[0218] A flat sheet polyimide membrane and polyimide hollow fibres were each functionalised using CB[7], The flat sheet polyimide membrane was functionalised using CB[7] to achieve a CB[7] loading of 0.795mg / cm2. The functionalised membrane was prepared using the loading and spreading method as described above.

[0219] The polyimide hollow fibres were functionalised with CB[7] using the dipping method described above.

[0220] As shown in Figure 32, modification of the cellulose acetate sheet led to comparable CO2 permeability and increased CO2 selectivity. Additionally, as shown in Figure 32, modification of the flat polyimide sheet membrane led to a significant increase in CO2 selectivity.

[0221] As shown in Figure 33, modification of the polyimide hollow fibres led to increased CO2 selectivity for both gaseous mixtures of CO2 / N2 and CO2 / CH4.

[0222] CO2 / N2 gas separation performance of cellulose acetate sheet membranes functionalised with a hollow polystyrene nanoparticle colloid

[0223] The solid hollow polystyrene nanoparticles were deposited on the surface of the cellulose acetate membrane.

[0224] CO2 / N2 separation performance was tested and the results are shown in Figure 34. As shown in Figure 34, functionalisation with hollow polystyrene nanoparticles led to improved CO2 permeability while maintaining the selectivity of the unfunctionalized membrane.

[0225] Membrane Characteristics Analysis

[0226] 1. CBfnl-based mixed matrix modified membrane characteristics 1. 1 Fourier Transform infrared spectroscopy (FTIR)

[0227] The fabricated membrane was characterized to confirm the composition of the membranes. Upon testing the CB[n]-mixed PDMS membrane, no peak at 1732 cm-1 (carbonyl group of the CB[n]) can be observed, as shown in Fig 21. This finding suggests that there is a negligible presence of CB[n] on the membrane surface. This observation supports the notion that the function enhancement of the membrane is provided by the mixed matrix method, where CB[n] is incorporated within the membrane structure, rather than a surface modification method that would involve CB[n] presence on the surface.

[0228] 1.2 Contacting Angle

[0229] Fig.22 presents the contact angle testing results for mixed matrix membranes containing varying amounts of CB[n], The contact angles on the membrane surface remain almost unchanged compared to pure PDMS membrane, indicating no significant alterations in the hydrophilicity of the modified membranes. This observation suggests that the modification with CB[n] does not considerably impact the membrane surface properties. In conclusion, the incorporation of different CB[n] amounts into the mixed matrix membranes does not appear to substantially affect the surface hydrophilicity.

[0230] 1.3 Atomic Force Microscope (AFM) images

[0231] Fig.23 presents the Atomic Force Microscopy (AFM) images of both the modified membrane and the pure PDMS membrane. Upon examination, it is evident that the surface roughness of these two membranes is nearly identical, with the modified membrane appearing slightly smoother. Considering this similarity in surface morphology, it can be inferred that the modification introduced in the membrane's structure does not significantly impact the surface roughness. Consequently, it is reasonable to assume that this modification will not adversely affect the membrane's separation efficiency due to any alteration in surface morphology. This observation suggests that the modified membrane may maintain its performance in the separation process while potentially offering additional benefits, such as enhanced mechanical or chemical properties, arising from the modification.

[0232] 2. CBfnl-surface-membrane characteristics

[0233] Figures 24 and 25 are Scanning Electron Microscope (SEM) images showing the crosssection and top surface of PDMS membranes that have been surface modified with CB[n] molecules. Surface modification of a variety of membrane types, including cellulose acetate sheets, polyimide sheets and hollow fibres led to minimal impact on the original non-functionalised membrane structure.

[0234] Conclusions These experiments indicate the successful development of innovative CB[n]-enhanced membranes by incorporating cucurbituril compounds as fillers or surface modifiers with commercially available polymeric membranes for CO2 separation. Surface modification has shown up to a tenfold increase in separation efficiency with minimum amount of CB[n]s. These functional membranes can be used in advanced gas separation materials.

Claims

Claims1. A polymer composition for carbon dioxide gas separation, the polymer composition comprising a polymer and a plurality of supramolecular structures, wherein the supramolecular structures are configured to selectively and reversibly adsorb carbon dioxide.

2. A polymer composition according to claim 1, wherein the supramolecular structures are colloids and / or macrocycles.

3. A polymer composition according to claim 2, wherein the supramolecular structures are cucurbituril compounds.

4. A polymer composition according to claim 3, wherein the cucurbituril compounds comprise 6, 7 or 8 glycoluril or substituted glycouril units.

5. A polymer composition according to claims 1 or 2, wherein the supramolecular structures have been functionalised with cucurbituril compounds.

6. A polymer composition according to claim 5, wherein the supramolecular structures are MCM-41 colloids functionalised with cucurbituril compounds.

7. A polymer composition according to any preceding claim, wherein the supramolecular structures are organic supramolecular structures.

8. A polymer composition according to any preceding claim, wherein the supramolecular structures are positioned on a surface of the polymer such that the surface is functionalised with supramolecular structures.

9. A polymer composition according to claim 8, wherein the supramolecular structures are distributed uniformly on a surface of the polymer.

10. A polymer composition according to any of claims 1 to 7, wherein the supramolecular structures are distributed within and throughout the polymeric matrix of the polymer.11 . A polymer composition according to claim 10, wherein the supramolecular structures are distributed uniformly within and throughout the polymeric matrix of the polymer.

12. A polymer composition according to any preceding claim, wherein the supramolecular structures comprise cavities.

13. A polymer composition according to claim 12, wherein the cavities are nanometer or subnanometer in diameter.

14. A polymer composition according to claim 13, wherein the cavities are from 1 to 10 angstroms in diameter.

15. A polymer composition according to any preceding claim, wherein the supramolecular structures provide an increase in the carbon dioxide permeability and / or selectively of the polymer composition relative to the polymer alone.

16. A polymer composition according to claim 15, wherein the supramolecular structures provide a 100% to 1000% increase in carbon dioxide permeability of the polymer composition relative to the polymer alone.

17. A polymer composition according to claims 15 or 16, wherein the supramolecular structures provide a 10% to 150% increase in carbon dioxide selectively of the polymer composition relative to the polymer alone.

18. A polymer composition according to claims 8 or 9, wherein the supramolecular structures are present in a concentration from 0.1 mg to 10 mg per square meter surface area of the polymer composition.

19. A polymer composition according to claims 10 or 11 , wherein the supramolecular structures are present in a concentration from 0.5 to 4 mol% of the polymer composition.

20. A polymer composition according to any preceding claim, wherein the polymer is selected from the group comprising of a polysiloxane (e.g., a polymethylsiloxane (PMS) or a polydimethylsiloxane (PDMS)), poly(1 -trimethylsilyl-1 -propyne) (PTMSP), cellulose acetate (CA), polyethylene (PE) and polyimide (PI).21 . A polymer composition according to claim 20, wherein the polymer is a polysiloxane e.g. a polymethylsiloxane or a polydimethylsiloxane.

22. A polymer composition according to claim 21 , wherein the polysiloxane polymer is formed from a 6:1 ratio of prepolymer to curing agent.

23. A polymer composition according to any of claims 1 to 20, wherein the polymer is poly(1 - trimethylsilyl-1 -propyne) (PTMSP).

24. A polymer composition according to any preceding claim, wherein the polymer composition is in the form of a flat sheet membrane or a hollow fibre membrane.

25. A membrane comprising a polymer composition according to any of claims 1 to 23.

26. A membrane according to claim 25, wherein the membrane has a thickness from 1000 pm to 5000 pm.

27. A membrane according to claim 25 or 26, wherein the loading of the supramolecular structures is from 0.002 mg / cm2to 5 mg / cm2.

28. A method for preparing a carbon dioxide gas separation membrane comprising;(i) providing a membrane comprising a polymer;(ii) applying a plurality of supramolecular structures (e.g. cucurbituril compounds) to a surface of the membrane, wherein the supramolecular structures (e.g. cucurbituril compounds) are configured to selectively and reversibly adsorb carbon dioxide.

29. A method according to claim 28, wherein the method prevents deformation of the membrane structure during preparation of the carbon dioxide gas separation membrane.

30. A method according to claim 28 or 29, wherein the plurality of supramolecular structures are applied to a surface of the membrane to provide a loading of from 0.002 mg / cm2to 5 mg / cm2.31 . A method according to any one of claims 28 to 30, wherein the plurality of supramolecular structures are applied to a surface of the membrane as a solution at a concentration of from 0.05 mg / mL to 35 mg / mL.

32. A method according to any one of claims 28 to 31 wherein the membrane is a flat sheet membrane or a hollow fibre membrane.

33. A process for separating carbon dioxide gas from a gas mixture comprising;(i) contacting the gas mixture to a membrane comprising a polymer and a plurality of supramolecular structures (e.g. cucurbituril compounds), wherein the supramolecular structures (e.g. cucurbituril compounds) are configured to selectively and reversibly adsorb carbon dioxide;(ii) recovering a permeate gas having a higher concentration of carbon dioxide than in the gas mixture.

34. The use of a membrane in a process for separating carbon dioxide gas from a gas mixture, wherein the membrane comprises a polymer and a plurality of supramolecular structures (e.g. cucurbituril compounds), wherein the supramolecular structures (e.g. cucurbituril compounds) are configured to selectively and reversibly adsorb carbon dioxide.

35. A membrane comprising a polymer and a plurality of supramolecular structures (e.g. cucurbituril compounds), wherein the supramolecular structures (e.g. cucurbituril compounds) are configured to selectively and reversibly adsorb carbon dioxide.

36. The membrane according to claim 35, wherein the supramolecular structures are colloids and / or macrocycles.

37. The membrane according to claim 36, wherein the supramolecular structures are cucurbituril compounds.

38. The membrane according to claim 37, wherein the cucurbituril compounds comprise 6, 7 or 8 glycoluril or substituted glycouril units.

39. The membrane according to claims 35 or 36, wherein the supramolecular structures have been functionalised with cucurbituril compounds.

40. The membrane according to claim 39, wherein the supramolecular structures are MCM- 41 colloids functionalised with cucurbituril compounds.

41. The membrane according to any one of claims 35 to 40, wherein the supramolecular structures are organic supramolecular structures.

42. The membrane according to any one of claims 35 to 41 , wherein the supramolecular structures are positioned on a surface of the polymer such that the surface is functionalised with supramolecular structures.

43. The membrane according to claim 42, wherein the supramolecular structures are distributed uniformly on a surface of the polymer.

44. The membrane according to any one of claims 35 to 41 , wherein the supramolecular structures are distributed within and throughout the polymeric matrix of the polymer.

45. The membrane according to claim 44, wherein the supramolecular structures are distributed uniformly within and throughout the polymeric matrix of the polymer.

46. The membrane according to claim 45, wherein the supramolecular structures comprise cavities.

47. The membrane according to claim 46, wherein the cavities are nanometer or subnanometer in diameter.

48. The membrane according to claim 47, wherein the cavities are from 1 to 10 angstroms in diameter.

49. The membrane according to any one of claims 35 to 48, wherein the supramolecular structures provide an increase in the carbon dioxide permeability and / or selectively of the membrane relative to the membrane alone.

50. The membrane according to claim 49, wherein the supramolecular structures provide a 100% to 1000% increase in carbon dioxide permeability of the membrane relative to the membrane alone.

51. The membrane according to claims 49 or 50, wherein the supramolecular structures provide a 10% to 150% increase in carbon dioxide selectively of the membrane relative to the membrane alone.

52. The membrane according to any one of claims 35 to 51 , wherein the polymer is selected from the group comprising of a polysiloxane (e.g., a polymethylsiloxane (PMS) or a polydimethylsiloxane (PDMS)), poly(1 -trimethylsilyl-1 -propyne) (PTMSP), cellulose acetate (CA), polyethylene (PE) and polyimide (PI).

53. The membrane according to claim 52, wherein the polymer is a polysiloxane e.g. a polymethylsiloxane or a polydimethylsiloxane.

54. The membrane according to claim 53, wherein the polysiloxane polymer is formed from a 6:1 ratio of prepolymer to curing agent.

55. The membrane according to any one of claims 35 to 52, wherein the polymer is poly(1 - trimethylsilyl-1 -propyne) (PTMSP).

56. The membrane according to any one of claims 35 to 55, wherein the membrane is a flat sheet membrane or a hollow fibre membrane.

57. The membrane according to any one of claims 35 to 56, wherein the membrane is a surface modified membrane.

58. The membrane according to claim 57, wherein the surface modified membrane is modified with a plurality of supramolecular structures (e.g. cucurbituril compounds).

59. The membrane according to any one of claims 35 to 58, wherein the loading of the supramolecular structures is from 0.002 mg / cm2to 5 mg / cm2.

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