WO2013132259A1 - Aérogels/xérogels de graphène et d'oxyde de graphène pour la capture de co2 - Google Patents

Aérogels/xérogels de graphène et d'oxyde de graphène pour la capture de co2 Download PDF

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WO2013132259A1
WO2013132259A1 PCT/GB2013/050570 GB2013050570W WO2013132259A1 WO 2013132259 A1 WO2013132259 A1 WO 2013132259A1 GB 2013050570 W GB2013050570 W GB 2013050570W WO 2013132259 A1 WO2013132259 A1 WO 2013132259A1
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graphene
aerogel
xerogel
ldh
graphene oxide
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Ainara Garcia GALLASTEGUI
Milo Shaffer
Abdulrahman O ALYOUBI
Sulaiman BASAHEL
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Bio Nano Consulting
King Abdulaziz University
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Priority to GB1417256.3A priority Critical patent/GB2515425B/en
Publication of WO2013132259A1 publication Critical patent/WO2013132259A1/fr

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/194—After-treatment
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/198—Graphene oxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/20—Graphite
    • C01B32/21—After-treatment
    • C01B32/23—Oxidation

Definitions

  • the present invention relates to graphene materials, particularly to aerogels and xerogels which comprise graphene or graphene oxide, and also contain layered double hydroxides (LDHs).
  • the invention is also concerned with the method of preparing such graphene or graphene oxide aerogels and xerogels and use of such materials for sorption and gas storage.
  • C0 2 adsorbent must have high selectivity and adsorption capacity for C0 2 , fast adsorption-desorption kinetics, adequate multicycle stability and good performance in the presence of competing species, such as water.
  • C0 2 adsorbents have previously been proposed. Zeolites, activated carbons, organic-inorganic hybrids (e.g. amines covalently bound to silica) and metal-organic frameworks have been reported to be competitive at temperatures below -393 K.
  • chemisorbents such as layered double hydroxides (-473 K-723 K), lithium zirconates (-673 K-873 K) and calcium oxides (-723 K-973 K) are promising C0 2 adsorbents for high temperature CCS and other applications involving C0 2 equilibria (Yong, Z., Mata, V., Rodrigues, A. E., Sep. Purif. Technol., 2002, 26, 195; Choi, S., Drese, J. H., Jones, C. W., Chem. Sus. Chem., 2009, 2, 796; Wang, Q., Luo, J., Zhong, Z., Borgna, A., Energy Environ. Sci., 2011, 4, 42).
  • LDHs Layered double hydroxides
  • 2D two-dimensional nanostructured basic, anionic clays.
  • Their structure is composed of positively charged brucite-like Mg(OH) 2 layers in which a fraction of divalent cations, octahedrally coordinated by hydroxyls, are partially substituted by trivalent cations.
  • the excess of positive charge is balanced by intercalated anions. Loosely bound water molecules may occupy the remaining free space in the interlayer regions.
  • the charge-neutral LDH structure can be represented by the general formula [M 2+ i_ where M 2+ , M 3+ and A m" commonly represent Mg 2+ , Al 3+ and C0 3 2" respectively, and x is usually between 0.17 and 0.33. LDHs require less energy to be regenerated and show better multicycle stability than other potential C0 2 solid adsorbents (e.g. calcium oxides) (Choi, S., Drese, J. H., Jones, C. W., Chem. Sus. Chem., 2009, 2, 796).
  • C0 2 solid adsorbents e.g. calcium oxides
  • LDHs exhibit relatively low C0 2 adsorption capacities which limit their commercial use.
  • alkali dopants such as potassium and caesium
  • Graphene as an ideal atomic-thick 2D material provides an extremely large surface area (theoretical specific surface area is up to 2600 m 2 g 1 ) (Chen Y., Zhang X., Yu P., Y. W. Ma Chem. Commun., 2009, 4527).
  • Co-AI LDH-NS Graphene oxide and Co-AI layered double hydroxide nanosheet
  • Co-AI LDH-NS has been synthesised as electrode material for application as a pseudocapacitor where exfoliated host layers of LDHs (i.e., nanosheets) have been used as 2D building blocks (Wang L., Wang D., Dong X. Y., Zhang Z. J., Pei X. F., Chen X. J. Chem. Commun., 2011, 47, 3556-3558).
  • the exfoliated graphite oxide (GO) is reduced to graphene using glucose as the reductant, and then Ni/AI LDH platelets are formed in situ on the surfaces of the graphene nanosheets.
  • the as-obtained graphene nanosheets/LDH composite exhibited a high specific capacitance (781.5 F/g at 5 mV.s "1 ) (Gao Z., Wang J., Li Z., Yang W., Wang B., Hou M., He Y., Liu Q., Mann T., Yang P., Zhang M., Liu L. Chem. Mater.2011, 23, 3509- 3516).
  • NiCo 2 0 4 -reduced graphene oxide composite shows a very high specific capacitance of 1050 F.g "1 , thus showing great potential as an electrode material for high performance supercapacitors(Wang H. W., Hu Z. A., Chang Y. Q., Chen Y. L, Wu H. Y., Zhang Z. Y., Yang Y. Y. J. Mater. Chem., 2011, 21, 10504).
  • magnetite-graphene and Mg/AI LDHs were synthesised to remove arsenate from aqueous solutions (Wu X. L, Wang L, Chen C. L, Xu A. W., Wang X. K. J. Mater. Chem., 2011, 21, 17353-17359).
  • Inorganic nanostructures (ZnO) grown directly on graphene layers can also been found in the literature (Won II Park W., Lee C. H., Lee J. M., Kimb N. J., Yi G. C, Nanoscale, 2011, 3, 3522).
  • the exfoliated graphite oxide (GO) is simultaneously reduced to graphene in company with the homogeneous precipitation of Ni 2+ -Fe 3+ LDH (Li H., Zhu G., Liu Z. H., Yang Z., Wang Z. Carbon, 2010, 48, 4391-4396).
  • multilayer hybrid films has been fabricated comprising polyvinyl alcohol)/graphene and LDH hybrid (Chen D., Wang X., Liu T., Wang X., Li J. Applied Materials & Interfaces, 2010, 2, 2005-2010).
  • Graphene aerogel with high electrical conductivity (lxlO 2 S m "1 ) has been synthesised by sol-gel polymerization of resorcinol ( ) and formaldehyde (F) with sodium carbonate as a catalyst (C) in an aqueous suspension of graphene oxide (GO) (M. A. Worsley, P. J. Pauzauskie, T. Y. Olson, J. Biener, J. H. Satcher, T. F. Baumann, J. Am. Chem. Soc, 2010, 132, 14067-14069).
  • Graphene oxide sponges were synthesised by vacuum centrifugal evaporating system (F. Liu, T. S. Seo, Adv. Funct. Mater., 2010, 20, 1930-1936).
  • Graphene hydrogel has been prepared by an hydrothermal process under high pressure, and the obtained hydrogel is electrically conductive, mechanically strong, and exhibits a high specific capacitance (Y. X. Xu, K. X. Sheng, C. Li, G. Q. Shi, ACS Nano, 2010, 4, 4324- 4330).
  • 3D architectures of graphene have been fabricated via an in situ self-assembly of graphene obtained by mild chemical reduction of graphene oxide in water under atmospheric pressure (W. Chen, L.Yan, 2011, Nanoscale, 3, 3132-3137).
  • graphene oxide aerogel comprising a layered double hydroxide
  • graphene xerogel comprising a layered double hydroxide
  • graphene oxide xerogel comprising a layered double hydroxide
  • the C0 2 adsorption capacity of the LDH is increased by enhancement of particle dispersion and gas accessibility.
  • the regeneration and stability after continuous absorption- desorption cycles is increased by the supporting LDH onto a high surface area material that separates and stabilises the active particles.
  • the mechanical properties and high thermal stability of graphene-based porous materials are able to produce hybrids that can withstand harsher industrial conditions.
  • the term "layered double hydroxide" will be hereinafter referred to as LDH.
  • a method of producing a material selected from graphene aerogel comprising LDH, graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, and mixtures thereof there is provided a catalytic system comprising a catalyst and a material selected from graphene aerogel comprising LDH, graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, and mixtures thereof.
  • a gas adsorption medium comprising a material selected from graphene aerogel comprising LDH.
  • graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, graphene gel comprising LDH, graphene oxide gel comprising LDH, and mixtures thereof.
  • a gas adsorption medium comprising a material selected from graphene aerogel and xerogel where the aerogel or xerogel may comprise any other materials suitable for carrying out the stated objective.
  • the compositions may comprise resins, gelling agents, polymers, fillers and the like.
  • a gas adsorption medium comprising a material selected from graphene or graphene oxide comprising LDH.
  • the use of a material selected from graphene aerogel and xerogel where the aerogel or xerogel may comprise any other materials suitable for carrying out the stated objective.
  • the compositions may comprise resins, gelling agents, polymers, fillers and the like.
  • the material selected from graphene aerogel comprising LDH, graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, and mixtures thereof, are used for gas sorption, capture and/or storage, preferably for C0 2 sorption, capture and/or storage.
  • graphene aerogel or “graphene oxide aerogel” means an aerogel comprising graphene or graphene oxide respectively. Further materials may be present in the aerogel. Alternatively, no further materials are present other than residual reagent materials, gas and/or solvents.
  • graphene xerogel or “graphene oxide xerogel” means a xerogel comprising graphene or graphene oxide respectively. Further materials may be present in the xerogel. Alternatively, no further materials are present other than residual reagent materials, gas and/or solvents. Unless further specified herein, reference herein to an "aerogel” is deemed to refer to an aerogel comprising graphene or graphene oxide.
  • xerogel refers to a xerogel comprising graphene or graphene oxide.
  • Graphene as an ideal atomic-thick 2D material provides an extremely large surface area. Aerogels and xerogels containing graphene open a new interesting possibility: electrical swing adsorption.
  • the graphene or graphene oxide network which is selected from an aerogels/xerogels may be selected to be electrically conductive such that when a power supply is applied across the network, a current passes through the matrix, with a resulting desorption of the adsorbed component. The desorption may be due to a resistive heating of the matrix, or due to a direct electrical effect on the sorbate-sorbent interactions.
  • the present invention seeks to provide a method of obtaining cross-linked graphene or graphene oxide networks, which are in the form of aerogels or xerogels.
  • the present invention also seeks to provide cross-linked graphene or graphene oxide networks which are in the form of aerogels or xerogels, and which allow more control over the density, shape, conductivity and internal surface of the graphene or, so that they display desirable electrical and mechanical properties.
  • the present invention seeks to provide a method of obtaining graphene or graphene oxide aerogels or xerogels in the presence of a one or more gelling agents to provide greater support to the gel-forming process. These agents may include polymers.
  • a preferred polymer according to the present invention is carboxymethyl cellulose (CMC).
  • the present invention also seeks to provide graphene or graphene oxide aerogels or xerogels in the presence of a one or more gelling agents to provide greater support to the gel-forming process.
  • These agents may include polymers.
  • a preferred polymer according to the present invention is carboxymethyl cellulose (CMC).
  • the present invention seeks to provide LDH supported onto graphene or graphene oxide aerogels or xerogels as an improved system for gas sorption and storage, in particular carbon dioxide capture and storage.
  • the graphene or graphene oxide aerogels or xerogels of the invention can act as adsorbents for liquid ions and gas molecules in general.
  • the graphene or graphene oxide aerogels or xerogels of the invention can act as solid base catalysts.
  • the viability of such catalysts can be improved by supporting the LDH on the graphene or graphene oxide aerogels or xerogels to maximize the dispersion and stability of the catalyst (LDH) over which the active phase can be distributed.
  • LDH catalyst
  • the resultant aerogels, and xerogels can be used as a heterogeneous catalyst for preparing compounds selected from the group consisting of aldols, ⁇ , ⁇ -unsaturated nitriles, ⁇ , ⁇ -unsaturated esters, transesterified products, ⁇ -nitroalkanols, Michael adducts and epoxides by reacting with corresponding aldehydes with acetone (Aldol condensation), aldehydes with activated nitriles or esters (Knoevenagel condensation), alcohols with ⁇ - keto or simple esters (transesterification), aldehydes with nitro alkanes (Henry reaction), activated methylenes with ⁇ , ⁇ -unsaturated compounds (Michael addition), and epoxidation of olefins.
  • the obtained products are important intermediates for the preparations of materials selected from the group consisting of drugs, pharmaceuticals, perfumes, cosmetics, oils, paint and fine chemicals.
  • materials selected from the group consisting of drugs, pharmaceuticals, perfumes, cosmetics, oils, paint and fine chemicals for example the products of benzylidene derivatives prepared by Knoevenagel condensation are used to inhibit tyrosine proteinase kinase, fine chemicals such as styrene oxide, 1-decene oxide, 1-octene oxide, 1-hexene oxide, cyclohexene oxide, cyclopentene oxide, epoxy chalcones by epoxidation of olefins, can be obtained by this method.
  • graphene refers preferably to non-functionalised graphene, and other functionalised graphene, such as carboxylated graphene.
  • functionalised graphene may be used instead of graphene oxide to avoid the carboxylated polyaromatic hydrocarbons generated during the conventional oxidation process because it has been observed that these carboxylated polyaromatic hydrocarbons may inhibit some reactions as a results of contamination of LDH surface.
  • Polar functionalised graphene may be prepared by adapting a variety of well-known chemical methods. Polar groups, such as carboxylates that can coordinate to LDH, can be introduced without the other oxidation debris associated with graphene oxide production by acid oxidation techniques.
  • aerogels and xerogels synthesised from graphene oxide are preferable because, unlike graphene, graphene oxide is easy to disperse in water due to the hydrophilic groups on the surface.
  • Graphene oxide (GO) nanosheets are stable in an aqueous solution as a result of ionization of the carboxylic acid and phenolic hydroxyl groups that are known to exist on the GO sheets.
  • the C0 2 adsorption capacity of the LDH is increased by enhancement of particle dispersion and gas accessibility.
  • modification of the LDH particles due to changes in dimensions, increases in surface area, defect concentration/edge sites, or even intrinsic properties, for example, due to the intercalation into the graphene or graphene oxide layers.
  • the regeneration and stability after continuous absorption-desorption cycles is increased by the supporting LDH onto graphene or graphene oxide aerogel or xerogel.
  • the mechanical properties and high thermal stability of graphenes and graphene oxide are able to produce hybrids that can withstand harsher industrial conditions.
  • polar-functional graphene aerogel comprising LDH > graphene oxide aerogel comprising LDH > graphene aerogel comprising LDH > polar-functional graphene xerogel comprising LDH > graphene oxide xerogel comprising LDH > graphene xerogel comprising LDH.
  • the LDH comprises a mixed metal oxide.
  • the mixed metal oxide is formed by calcinations of the LDH at elevated temperature (typically 450 °C).
  • Mixtures of LDHs may be preferably used in the present invention.
  • LDH may be defined in various ways.
  • LDH based on a combination of divalent and trivalent metal cations may have the general formula [M"i -X M'"x(OH) 2 ] [X Q ⁇ X/Q nH 2 0] or [M"-M'"-X] or [M"-M im ], where [M"i -X M"' X (OH) 2 ] ([M"-M im ]) represents the layer, and [X Q" X / Q nH 2 0] the interlayer composition.
  • LDH preferably refers to those having the general formula [M z+ 1 .
  • LDH is or comprises Mg-AI-LDH which has the general formula wherein M 2+ , M 3+ and A m" are Mg 2+ , Al 3+ and C0 3 2" respectively, n is between 0.5 and 4.0 and x is between 0.15 and 0.35.
  • Mg-AI-LDH is prepared using the method according to Example 3.
  • the LDH is or comprises Mg 2 AI(OH) 6 (CO 3 ) 0 . 5 -0.15H 2 O.
  • LDHs need to be calcined or thermally decomposed in an inert atmosphere in order to obtain the corresponding metal oxides that have the surface area/basic sites required for the C0 2 adsorption.
  • the typical calcinations temperature is 673K.
  • the ratio of LDH:graphene or graphene oxide aerogel or xerogel (w/w) is preferably between 100:0.01 and 0.01:100, more preferably between 100:0.01and 0.01:100, even more preferably between 20:1 and 0.1:1. Therefore, the ratio of LDH:graphene (w/w) may be 0.1/1, 1/1, 10/1, 20/1.
  • LDH is present within the graphene or graphene oxide aerogel or xerogel in an amount of between 0.1 to 99.99 wt%, preferably between 1 and 99 wt% and even more preferably between 10 and 95 wt% by total weight of the LDH supported graphene or graphene oxide aerogel or xerogel.
  • the present invention further provides a composition comprising a material selected from graphene aerogel comprising LDH, graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, further comprising at least one adjunct material, preferably a support material, for example, alumina.
  • the aerogel and xerogel compositions of the present invention may comprise any other materials suitable for carrying out the stated objective.
  • the compositions may comprise resins, gelling agents, polymers, fillers and the like.
  • a method of preparing a graphene aerogel, graphene xerogel, graphene oxide aerogel or graphene oxide xerogel, comprising LDH comprising the steps of: a) providing a material selected from the group consisting of graphene aerogel, graphene xerogel, , graphene oxide aerogel or graphene oxide xerogel; and b) depositing LDH onto said material.
  • the LDH may be deposited onto the graphene or graphene oxide aerogel or xerogel by any method known to those skilled in the art.
  • the deposition may be by a method selected from the group consisting of coprecipitation, the urea method, induced hydrolysis reconstruction, sol-gel technique, hydrothermal, microwave, ultrasounds treatments and anion exchange reactions.
  • LDH is directly grown or precipitated on the graphene or graphene oxide aerogel or xerogel.
  • LDH is directly grown or precipitated on the graphene or graphene oxide.
  • the deposition of LDH is carried out by co-precipitation of Mg 2+ and Al 2+ ions under alkaline conditions.
  • the LDH is prepared using the method according to Example 3.
  • the graphene is oxidised prior to the deposition of LDH in order to produce graphene oxide.
  • the carboxylic acid concentration on the graphene oxide is in the range of 0.01 mmol.g “1 to 100 mmol.g “1 , more preferably 1 mmol.g “1 to 50 mmol.g “1 . Most preferably, the carboxylic acid concentration on the graphene oxide is about 1.23 mmol.g “1 .
  • the first is a one-step process, and is achieved through direct oxidisation of graphite with strong oxidants such as concentrated sulfuric acid, concentrated nitric acid, or potassium permanganate.
  • the second is a two-step process, in which graphite is oxidized through Hummers', Brodies', Staudenmaiers', or modified Hummer's methods, or (W. Hummers and . Offema, J. Am. Chem. Soc, 1958, 80, 1339; W. F. Chen, L. F. Yan and P. R. Bangal, Carbon, 2010, 48, 1146-1152) electrochemical oxidation, followed by exfoliating or thermally expanding the graphite oxide obtained.
  • the third is a physicochemical process: graphene oxide nanoribbons are created through lengthwise cutting and unravelling the sidewalls of multiwalled carbon nanotube (MWCNTs) by oxidative processes (L. Yan, Y. B. Zheng, F. Zhao, S. Li, X. Gao, B. Xu, P. S. Weiss, Y. Zhao, Chem. Soc. Rev., 2012, 41, 97-114).
  • MWCNTs multiwalled carbon nanotube
  • the acid oxidation of graphite generates oxygenated species like carboxyl, epoxy and hydroxyl on the material, generating graphene oxide (W. Gao, L. B. Alemany, L. J. Ci and P. M. Ajayan, Nat. Chem., 2009, 1, 403; W. Cai, R. D. Piner, F. J. Stadermann, S. Park, M. A. Shaibat, Y. Ishii, D. Yang, A. Velamakanni, S. J. An and M. Stoller,Science, 2008, 321, 1815).
  • LDH can be synthesised onto the crosslinked graphene network gel.
  • the coprecipitation can be made at neutral pH to avoid the hydrolysis of the ester bonds between the graphenes and hence, to avoid the loss of the network structure.
  • the synthesised LDH/graphene gel hybrid is purified by water solvent exchange.
  • the resulting LDH/graphene gel hybrid is freeze dried or dried at ambient pressure after solvent exchange processes (water/acetone/hexane) to avoid the collapse of the graphene gel structure obtaining a graphene xerogel and finally graphene aerogel/LDH hybrid.
  • graphene oxide may be cross-linked by forming covalent bonds between the existing oxide groups on the graphene surface using a direct condensation reaction. Direct condensation between the existing surface oxides occurs only at the contact points between graphene oxide, leaving the remaining oxidised surface unchanged, or available for subsequent LDH deposition. Carboxylates are particularly preferred for their compatibility with LDH.
  • the graphene gel is formed by using linking molecules that directly bond to the graphene surface, without using oxidised groups, using chemistries described below.
  • Non- crosslinked regions will be surface functionalised by saturation with unsatisfied linking molecules; the remaining unreacted terminus can be converted to carboxylate, or other polar group, using standard chemistry, before LDH or other secondary phase deposition.
  • a method may include the use of a blend of graphene or graphene oxide and a carbon nanotube network which is selected from an aerogel and xerogel, crosslinked by a mutually compatible chemistry as described for (oxidised) nanotubes. LDH may then subsequently be deposited on the graphene-carbon nanotube blend to provide a carbon nanotube-graphene network which is selected from an aerogel and xerogel comprising LDH.
  • a method may include the use of a blend of graphene and a carbon nanotube network which is selected from an aerogel and xerogel, crosslinked using linking molecules that directly bond to the graphene and nanotube surface using chemistries described below. LDH may then subsequently be deposited on the graphene-carbon nanotube blend to provide a carbon nanotube-graphene network which is selected from an aerogel and xerogel comprising LDH.
  • LDH can be synthesised onto graphene oxide. The coprecipitation can be made at neutral or basic pH. These chemical routes provide sites to coordinate metal ions, and balance charge during LDH synthesis.
  • the synthesised LDH/graphene oxide is preferably purified by vacuum filtration and water washing.
  • the resultant LDH/graphene oxide can be dried.
  • a further aspect of the present invention involves the use of the graphene or graphene oxide aerogel or xerogel comprising LDH for C0 2 sorption, capture and storage.
  • a further aspect of the present invention involves the use of the graphene or graphene oxide comprising LDH for C0 2 sorption, capture and storage.
  • the graphene or graphene oxide aerogel or xerogel comprising LDH may be heated in an inert atmosphere to a temperature of up to 3273 K, preferably between 373 K and 1273 K, more preferably between 373 K and 737 K. If the aerogel or xerogel are heated in air, they may be heated to a temperature of up to 873 K, preferably between 373 K and 823 K, more preferably between 150 and 737 K, even more preferably between 673 K and 723 K.
  • Heating the aerogel or xerogel comprising LDH above approximately 773 K in inert atmosphere would likely carbonise any polymer structure such as the CMC mentioned before, forming a more stable carbonaceous binding of the junctions.
  • thermal decomposition plays a crucial role in the C0 2 adsorption properties of LDHs.
  • LDHs are dehydrated below 473 K, partially decarbonated between 473 K and 673 K, dehydroxylated between 673 K and 873 K and further decarbonated, becoming amorphous metastable mixed oxide solid solutions above 873 K. It has been found that a calcination temperature of about 673 K produces LDH derivatives with an optimum balance between surface area and basic sites, which maximizes their C0 2 adsorption capacities and favored reversible adsorption.
  • the graphenes may graphitise and fuse together to form an inherently continuous and robust structure with a high degree of graphiticity.
  • heating of the graphene or graphene oxide aerogel or xerogel comprising LDH may be carried out by cycling between two or more temperatures. Accordingly, in one aspect of the invention, cycling is carried out between 100°C and 1000°C, more preferably between 200°C and 600°C, even more preferably between 300°C and 400°C repeating the adsorption/desorption process up to 10000 cycles, preferably 5000 cycles, more preferably 2000 cycles even more preferably 1000 cycles.
  • the number of cycles will be dependent upon a number of factors such as its application or scale. Although it would be favourable to have as many cycles as possible, this must be balanced against loss of performance. The rate of decline will depend on conditions and application.
  • the desire is for a stable structure, wherein the graphene aerogel network is itself robust, and helps to support and stabilise the LDH through as many cycles as possible whilst avoiding typical losses in performance due to sintering, ripening, leaching or recrystallisation as other phases, etc.
  • Graphene or graphene oxide aerogel or xerogel comprising LDH provide high electrical and thermal conductivity, to generate a robust, high surface area network that can be electrically-heated.
  • a continuously-connected homogeneous network is provided, through which current can flow. Due to Joule heating within the branches of the network, the local temperature is raised throughout, by internal heating. The need for thermal diffusion is minimised and any local variations in temperature is reduced by the high thermal conductivity of the graphene and graphene oxide. Furthermore, graphenes and graphene oxides are stable to high temperatures, (at least 773 K even under oxidising conditions, and much higher temperatures in inert atmospheres). The temperature within the network can be rapidly adjusted by varying the current or applied voltage to immediately vary the local Joule heating effect. In addition, aerogel or xerogel comprising LDH according to the invention have very low heat capacity, helping to reduce response time.
  • the temperature may be monitored by one or more external thermocouple(s) placed in the aerogel or xerogel comprising LDH of the invention or embedded during fabrication, or by optical pyrometry.
  • the measured resistance of the aerogel or xerogel comprising LDH of the invention itself may be used as an indication of temperature.
  • the shape of the aerogel or xerogel of the invention comprising LDH and type of electrical contacts can be varied widely to suit particular applications. Examples include a volume monolith within a reaction chamber, a disc shape (akin to a sintered glass frit), or a high aspect ratio filling for a pipe.
  • the aerogel or xerogel comprising LDH of the invention may be used in a flow-through geometry, in which gas or liquid passes through the pores in the structure, coming into contact with the graphene surface (or, optionally, additional material supported on the graphenes) and therefore also being heated to the desired temperature.
  • the resistance at each temperature may be calculated from the current and voltage values.
  • the temperature can be measured using an optical pyrometer, an embedded thermocouple, or other methods known in the art.
  • the result will be a calibration relation between aerogel or xerogel resistance and temperature; this dependence will vary with device geometry, aerogel or xerogel density, and graphene or graphene oxide type (both intrinsic structure and cross-linking chemistry).
  • the resistance can then be used to determine the temperature, and hence provide appropriate feedback control. In this way, the requisite heating can be predetermined for a given aerogel or xerogel comprising LDH by adjusting the applied voltage/current accordingly.
  • the graphene or graphene oxide surface can act as a sorbent/filter either to purify a gas/liquid stream, or to store (a fraction of) it. Subsequently, heating of the graphene or graphene oxide aerogel or xerogel comprising LDH will regenerate the sorbent by decomposition or desorption of the trapped species.
  • the aerogel or xerogel comprising LDH of the invention provides advantages such as rapid and homogeneous heating, fast cycling, rapid emission of stored species, and minimal thermal degradation. Furthermore, accurate temperature control allows fractionation of adsorbed species by type.
  • the graphene aerogel or graphene xerogel comprising LDH provide enhanced C0 2 adsorption capacity and cycling stability.
  • graphene or Graphene Oxide Aerogel/ Xerogel The graphene oxide aerogel, graphene oxide xerogel, graphene aerogel or graphene xerogel according to the present invention may be produced by any method known to those skilled in the art.
  • a method of producing a graphene or graphene oxide network which is selected from an aerogel and a xerogel comprising the steps of: (a) dispersing graphene or graphene oxide in a solvent compatible therewith; (b) cross-linking said graphene or graphene oxide using functional groups already present thereon or with a linking molecule comprising at least two functional sites capable of reacting with the pure surface of said graphene or graphene oxide, to form a covalently cross-linked gel network; and (c) removing said solvent to give a cross-linked graphene or graphene oxide network which is selected from an aerogel and a xerogel with a solvent content of less than 10% by weight.
  • the solvent content of the cross-linked graphene network which is selected from an aerogel and a xerogel is less than 2% by weight, more preferably less than 0.5% by weight, more preferably less than 0.1% by weight.
  • Graphene according to the present invention is reacted using functional groups already present thereon or with a linking molecule comprising at least two functional sites capable of reacting with the surface of said graphene or graphene oxide, to form a covalently cross- linked gel network.
  • acylation reactions are among the most common approaches used for linking molecular moieties onto oxygenated groups at the edges of graphene oxide.
  • the acylation reaction between the carboxyl acid groups of graphene oxide and octadecylamine (after SOCI 2 activation of the COOH groups) can be used to modify graphene oxide by long alkyl chains.
  • graphene nanosheets can be functionalized with polymers like polyvinyl alcohol) (PVA) through the carbodiimide-activated esterification reaction between the carboxylic acid moieties on the nanosheets and hydroxyl groups on PVA using N,N-dicyclohexylcarbodiimide (DCC), 4-(dimethylamino)-pyridine (DMAP), and N- hydroxybenzotriazole (HOBT) in DMSO.
  • PVA polyvinyl alcohol
  • DCC N,N-dicyclohexylcarbodiimide
  • DMAP 4-(dimethylamino)-pyridine
  • HOBT N- hydroxybenzotriazole
  • the linking molecule may have functionalities that can couple directly to the graphene or graphene oxide surface
  • functionalities that can couple directly to the graphene or graphene oxide surface
  • radical addition, electrophile addition or cycloaddition, or all the reactions involving the reduction of the graphenes followed by the reaction of the graphene with an electrophilic cross-linking molecule is of interest.
  • reductions may be carried out by the addition of electrons (reduction) to the graphene layers.
  • Direct coupling to the sidewalls avoids the necessity for damaging pre-oxidation steps.
  • a more thermally and chemically stable framework can be produced, of greater versatility in application, for example, by creating an aerogel linked by only carbon-carbon bonds.
  • Direct coupling is particularly amenable to creating conjugated linking systems that aid electrical conductivity.
  • some direct chemistries such as alkylation following the reductive charging in Birch reactions, are particularly suitable for improving the dispersion of the graphenes and graphene oxide to form a good gel and hence a homogeneous aerogel.
  • such a linking molecule may react with oxide groups of graphene oxide to produce, for example, ester, ether, or amide linkages.
  • Suitable cross-linking agents include alkyl diamines, aromatic diamines, alkyl diols, aromatic diols, polyols, bis-sodium alkoxides, dicarboxylic acids, di acid chlorides, di siloxane halides, di siloxane alkoxides, preferably Ci_ 2 o alkyl diamines, C 5 .
  • Ci_ 2 o alkyl diols Ci_ 2 o alkyl diols, C5.20 aromatic diols, C 2 _ioo polyols, bis-sodium Ci -2 o alkoxides, C 2 _ 20 dicarboxylic acids, C 2 _ 20 di acid chlorides, more preferably Ci_i 0 alkyl diamines, C 6 .i 8 aromatic diamines, C 2- io alkyl diols, C 6 _i8 aromatic diols, C 2 _ 20 polyols, bis-sodium C 2- io alkoxides, C 2 _i 0 dicarboxylic acids, C 2 _i 0 di acid chlorides, and the like.
  • the two reactive groups are located on different atoms of the linking molecule, more preferably remote atoms (for example, at least 3 or more atoms apart), to maximise the chance of reacting with two different graphene or graphene oxide molecules.
  • the use of small rigid molecules is thought to maximise the establishment of a cross-link.
  • a preferred linker is 1,4-diamino benzene. Such a compound limits the possibility of reacting twice with the same graphene or graphene oxide molecule.
  • the graphene oxide according to the present invention are cross-linked using any linking groups which are capable of forming covalent bonds by direct reaction between the oxides on the graphene surface.
  • This approach has the advantage of bringing the graphenes into close contact, maximising the electrical conductivity of the junction, and minimising both the additional reagents required and subsequent parasitic mass added to the network. It is worth noting that, in the previous embodiment, the additional linking molecules will saturate the entire surface, although graphene cross-links will only occur relatively rarely.
  • the aerogel/xerogel may be synthesised in the presence of a one or more gelling agents to provide greater support to the gel-forming process.
  • These agents may include polymers.
  • a preferred polymer according to the present invention is carboxymethyl cellulose (CMC).
  • the cross-linked aerogel or xerogel of the invention may be advantageous for some applications since they provide more physical stability and lower electrical resistance.
  • the chemical structure of the cross-link can be tuned to adjust its resistance, and hence the rate of energy dissipation within the structure, for example by modulating the degree of conjugation and/or the molecular weight.
  • the preferred embodiments according to the present invention may involve the use of graphene oxide which may be obtained commercially or, more usually, be those that have further been oxidised according to any standard method.
  • graphene oxide refers to any graphene with one or more oxide groups present on the surface of the graphene.
  • oxide groups are selected from the group consisting of quinones, ketones, lactones, pyrones, carboxylic acids, carboxylates, hydroxides and hydroxyl groups, and groups derivable from these via oxidation, and mixtures of two or more thereof.
  • the surface oxides are carboxylic and/or hydroxide groups.
  • the level of oxidation of the graphenes will vary according to the desired mechanical and electrical properties required. Typically, the level of oxidation on the oxidised graphene is between 0.001 - 100 mmol/g, preferably 0.1 mmol/g or greater.
  • the oxidised graphenes are cross-linked to form an ester or ether bond, most preferably an ester bond.
  • the reaction is preferably a condensation reaction, one that releases a small molecule byproduct such as water, rather than introducing additional atoms into the resulting linkage.
  • the surface oxides may be converted to other simple functional groups for direct condensation.
  • the surface alcohols on the graphenes may be converted to, for example, an amine functionality, which subsequently allows the cross-links to be formed via an amide bond.
  • Other direct molecular condensations such as those to form imines, thioethers, thioesters, and ureas, also fall within the scope of the present invention.
  • the cross-links between the oxidised graphenes may be formed using a coupling agent.
  • the term "coupling agent” as used herein does not have the conventional meaning often used in polymer resin chemistry but refers to any substance capable of facilitating the formation of a bonding link between two reagents, as in the field of organic chemistry.
  • Such compounds include ⁇ , ⁇ '-dicyclohexylcarbodiimide (DCC), ⁇ , ⁇ '- diisopropylcarbodiimide (DIC), ethyl-(N',N'-dimethylamino)propylcarbodiimide hydrochloride (EDC) [adding an equivalent of 1-hydroxybenzotriazole (HOBt) to minimize the racemisation], 4-(N,N-dimethylamino) pyridine (DMAP), (benzotriazol-1- yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), (benzotriazol-1- yloxy)tripyrrolidinophosphonium hexafluorophosphate, bromotripyrrolidinophosphonium hexafluorophosphate, 0-(benzotriazol-l-yl)-N,N,N',N'-tetramethyluronium hex
  • a carbodiimide is used to couple a suitable functional group and a carbonyl group such as an ester or an acid.
  • Preferred examples of carbodiimides include but are not limited to l-ethyl-3-(3-dimethylaminopropyl)carbodiimide, ⁇ , ⁇ '- dicyclohexyl carbodiimide, ⁇ , ⁇ '-diisopropyl carbodiimide, bis(trimethylsilyl)carbodiimide and N-cyclohexyl-N'-( -[N-methylmorpholino]ethyl)carbodiimide p-toluenesulfonate.
  • the coupling agent is selected from the group consisting of N,N'-dicyclohexylcarbodiimide, ⁇ , ⁇ '-diisopropylcarbodiimide and l-ethyl-3-(3- dimethylaminopropyl)carbodiimide.
  • the coupling agent may be supplemented by an additional agent such as those known to enhance extra selectivity or yield of such condensation reactions, such as N- hydroxybenzotriazole or N-hydroxysuccinimide.
  • the cross-linking process may be carried out at any reasonable temperature and left for any length of time necessary to complete the reaction, so long as the reaction is carried out at a temperature below the boiling point of the reaction solvent(s).
  • the cross-linking reaction is carried out at a temperature of between 288 K to 333 K, preferably 293 K to 303 K.
  • the reaction time is preferably between 0.1 to 50 hours and more preferably between 1 and 12 hours.
  • the cross-linking process may be carried out by dehydration.
  • dehydration refers to a chemical reaction which involves the loss of water from the reacting molecule(s).
  • dehydration is carried out by using groups on the graphene oxide.
  • groups include ether linkages which may be formed by dehydration at a temperature greater than 393 K, preferably greater than - 403 K and even more preferably greater than 423 K, preferably using an acid catalyst.
  • Cross- linking process will require a solvent with a high boiling point, greater than the reaction temperature.
  • the boiling point of the solvent is greater than 293 K, preferably greater than 403 K and even more preferably greater than 423 K.
  • the graphenes or graphene oxides are cross-linked to form a gel phase.
  • gel refers to what those skilled in the art understand by the term, and preferably refers to a composition which retains its shape during the drying process.
  • the term “gel” as used herein (in isolation) more preferably refers to a precursor of the aerogel/xerogel prior to the removal of the solvent or drying step.
  • gel in itself is not intended to cover an aerogel or a xerogel.
  • the gel phase is formed by a continuous network of covalently bound graphenes within the solvent. Under small shear deformations the response is predominantly elastic rather than viscous; in dynamic shear rheology experiments, at the gel point there is a characteristic crossover of G' and G"/tan(nn/2) given by the equation below:
  • G' is the storage modulus
  • G" is the loss modulus
  • ⁇ is the gamma function
  • n is the relaxation exponent
  • S g is the gel strength
  • ⁇ is the frequency.
  • cross-linking is carried out by direct reaction between the oxides on the graphene surface
  • only a small proportion (approximately 1-3%) of the surface oxides react to form the cross-links between the graphenes or graphene oxide.
  • the cross-linked aerogel or xerogel thus obtained will have unreacted oxide groups on the surface of the graphene oxide. These groups impart hydrophilicity (i.e. tendency to interact with or be dissolved by water and other polar substances) to the resulting aerogel/xerogel. Also the ionic interaction between the negatively-charged graphene oxide and positively-charged LDH nanosheets plays an important role in the LDH growth process.
  • the method can further comprise a step of capping residual surface oxides on the oxidised graphenes.
  • capping refers to any step which alters or transforms the surface oxides into other functionalities. In this respect, it can be any functional group which is able to react with the surface oxide group such as a metal, haloalkanes, acid halides and the like. In a preferred embodiment, the surface oxides are capped using a hydrophobic functional group.
  • the hydrophobic functional group is preferably selected from the group consisting of haloalkyl, alkyl and siloxane, more preferably Ci_i 2 haloalkyl and Ci_i 8 alkyl, most preferably a Ci_i 2 - haloalkyl and more preferably Ci_i 0 haloalkyl.
  • the hydrophobic functional group is a haloalkyl containing more than 1 fluorine atom, preferably 3 to 20 fluorine atoms, preferably 8 to 16 fluorine atoms, more preferably 10 to 14 fluorine atoms, for example 13 fluorine atoms.
  • the capping group reagent is a hydroxyhaloalkyl compound, preferably a Ci_i 2 hydroxyhaloalkyl compound, preferably trifluoroethanol.
  • hydrophobic it is meant that the group imparts increased hydrophobic character to the graphene, thereby reducing the solid surface tension.
  • a linking molecule is used to form the cross-links, although only a small proportion of the carbon surface is involved with cross-linking, unlike the direct condensation reactions, the remaining surface will already be saturated with excess linking molecules. Since, both sides are saturated, these molecules are unlikely to covalently cross-link during drying (depending on the reagent), but may well be relatively polar and form undesirable non-covalent interactions that encourage collapse.
  • a further reaction with a capping agent as described above, could be used to lower the surface tension, where the hydrophobic end group is reacted with the remaining unreacted end of the excess linking molecules.
  • a solvent which is compatible with the graphene may be used.
  • the term “compatible” refers to any solvent in which the graphenes or graphene oxides form a substantially homogeneous solution or dispersion.
  • the solvent which is compatible with the graphenes or graphene oxide is miscible therewith.
  • the coupling agent is also substantially soluble in the solvent.
  • the solvent is selected from dimethyl formamide, benzene, dichloromethane, chlorobenzene, chloroform, toluene, xylene, dioxane, dimethylsulfoxide, tetrahydrofuran, amide solvents and mixtures thereof, most preferably dimethyl formamide.
  • amide solvents refers to any solvent which contains an amide group.
  • Preferred amide solvents include N-methyl-2-pyrrolidone and cyclohexyl pyrrolidone.
  • the graphenes or graphene oxide may be present in the solvent at any given concentration.
  • the graphenes or graphene oxide must be sufficiently concentrated that they can form a continuous connected network across the whole composition.
  • this concentration is above the rheological percolation threshold for the chosen graphenes dispersion in the chosen solvent.
  • the graphenes or graphene oxide are present in the solvent at a concentration of between 0.01-30 vol.%, more preferably 0.1-20 vol.%, more preferably 1-5 vol.%.
  • the removal of solvent is carried out by solvent exchange with at least one solvent having lower surface tension than the initial solvent.
  • surface tension refers to the attractive force in any liquid exerted by the molecules below the surface upon those at the surface/air interface, which force tends to restrain a liquid from flowing.
  • low surface tension refers to liquids having a surface tension of less than or equal to about 30 mN/m as measured at 25 °C and atmospheric pressure. However, this value may be more or less, since the critical tolerable surface tension to avoid collapse during the drying step will depend on the network.
  • any one of the graphene thickness the cross-link density, or degree of hydrophobicity increases the need for low surface tension decreases. Therefore in principle, some graphene gels may be dried without solvent exchange and others will need very low surface tensions. Whether a particular network requires such solvent exchange will depend on the individual properties of the gels. The lower density, higher surface area networks have more desirable properties but tend to be less robust so need solvent exchange or other controlled drying technique.
  • solvent exchange is carried out using acetone, followed by C 3 -Ci 0 hydrocarbon, preferably hexane.
  • the aerogel/xerogel is synthesised without covalent crosslinking of the graphenes; instead relying on a non-covalent binder to create the stabilse the required monolith.
  • the aerogel/xerogel may be synthesised in the presence of a one or more gelling agents to provide greater support to the gel-forming process.
  • These agents may include polymers.
  • a preferred polymer according to the present invention is carboxymethyl cellulose (CMC).
  • the aerogel or xerogel may be synthesised by providing pristine graphene and CMC or other polymer in water or other solvent, and freeze drying.
  • the aerogel or xerogel of the invention may be synthesised by providing oxidised graphene and CMC or other polymer in water or the solvent, and freeze drying.
  • the aerogel or xerogelof the invention may be synthesised by providing oxidised cross-linked graphene and CMC in water, and freeze drying.
  • the aerogel or xerogel of the invention may be synthesised by providing oxidised cross- linked graphene or graphene oxide in water, and freeze drying.
  • the aerogel or xerogel, of the invention may be synthesised by providing oxidised cross-linked graphene or graphene oxide in water, carrying out solvent exchange (preferably using water-acetone-hexane) and drying at ambient temperature and pressure.
  • the graphene or graphene oxide network according to the present invention is preferably an aerogel or xerogel, most preferably an aerogel. Aerogels may be more advantageous for particular applications given their higher porosity and surface area.
  • the term "aerogel” refers to a highly porous material of low density, which is prepared by forming a chemically-crosslinked gel and then removing liquid from the gel while substantially retaining the gel structure.
  • the usual solvent removal step may optionally be omitted, if the system is to be used in a liquid-related application; if necessary, the gel fabrication solvent may be exchanged with the intended application solvent, by means of one or more solvent exchange steps; thus the cross-linked graphene or graphene oxide "gel” will be used, rather than the dried form.
  • an "aerogel” comprises a graphene or graphene oxide network wherein the volume change on drying of the gel is less than 30%, preferably less than 20%, preferably less than 10%, preferably less than 5%.
  • Aerogels have open-celled microporous or mesoporous structures. Typically, they have pore sizes of less than 1000 nm and surface areas of greater than 100 m 2 per gram. Preferably they have pore sizes of less than 200 nm and surface areas of greater than 400 m 2 per gram. They often have low densities, e.g., from 500 mg/cm 3 down to as little as 1 mg/cm 3 preferably in the range of 15 to 300 mg/cm 3 .
  • those produced from graphenes or graphene oxides may have ultra-low densities, high surface areas, but large pore sizes; in principle, the pore size may approach the scale of the individual graphene lengths which can reach millimetres or even centimetres.
  • aerogels are materials in which the liquid has been removed from the gel under supercritical conditions.
  • removal of solvent may be carried out by supercritical drying or lyophilisation (freezing-vacuum process) to form an aerogel.
  • supercritical drying involves the removal of the solvent with supercritical carbon dioxide, and this may be used in the present invention.
  • the drying process is carried out at room temperature and/or ambient pressure.
  • This method is a more versatile procedure to fabricate an aerogel since it does not require supercritical C0 2 , or a lyophilisation (freezing-vacuum process).
  • the aerogel can be obtained by simply drying the gel. The objective is to evaporate the solvent producing the minimum volume reduction when obtaining the aerogel from the gel.
  • a method involves cross-linking between the graphenes and optional hydrophobic functionalisation of the graphene surface, this may help the process.
  • the method may further comprise a solvent exchange process to a solvent with lower surface tension. The functionalisation during the preparation of the gel permits simplification of the later drying step.
  • xerogel refers to a type of aerogel in which the volume change on drying of the gel is greater than approximately 30%. In this case, although the gel partially collapses during drying, the strong covalent network of graphenes limits the process, yielding a more useful, more porous, less dense structure, than obtained from drying physical gels or other graphene suspensions.
  • each graphene or graphene oxide used in the present invention has high electric conductivity and allows a current flow at a current density of greater than 0.1 mA/cm 2 , preferably greater than 500 A/cm 2 or more.
  • a network of graphenes is therefore thought to display excellent electrical conductivity and current density, compared to existing carbon aerogels.
  • graphenes and graphene oxide have desirable intrinsic mechanical characteristics, including high strength, stiffness, and flexibility, at low density. These properties make them desirable for many industrial applications, and lend desirable properties to the resulting aerogel networks.
  • the shape of the aerogel or xerogel of the invention can be controlled by controlling the shape of the vessel used during the gelation step.
  • the density of the final aerogel can be controlled by varying the volume fraction of graphenes or graphene oxides within the initial gel.
  • the present invention also provides catalysts, catalyst supports, fluid heaters and electrically-regenerable filters/sorbents comprising an graphene aerogel and or xerogel according to the present invention.
  • a further embodiment according to the present invention involves the use of an aerogel, or xerogel of the invention comprising LDH for sorption and/or gas storage.
  • composition “comprising” encompasses “including” as well as “consisting” e.g. a composition “comprising” X may consist exclusively of X or may include something additional e.g. X + Y.
  • alkyl refers to a straight or branched saturated monovalent hydrocarbon radical, having the number of carbon atoms as indicated.
  • suitable alkyl groups include propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, and the like.
  • monolayer graphene stands for a single atomic sheet of graphene, or a sample composed substantially of such material.
  • graphene is usually taken to include “few layer graphene", including graphite substantially composed of platelets of 5 atomic graphene monolayers or fewer.
  • graphene oxide monolayer stands for a single sheet of graphene with one or more oxide groups present on the surface of the graphene.
  • graphene oxide refers to any graphene with one or more oxide groups present on the surface of the graphene.
  • a wide range of surface oxides are known in carbon chemistry, including quinones, ketones, lactones, pyrones, carboxylic acids, carboxylates, hydroxides and hydroxyl groups etc., and groups derivable from these via oxidation.
  • the surface oxides are carboxylic and/or hydroxide groups.
  • the aerogels and xerogels according to the present invention may be produced by directly, covalently crosslinking graphene to form a gel network, followed by solvent removal.
  • the gel structure is created by rigid graphenes directly bound to each other or with another gel- forming component. Direct binding between the graphene provides high strength, high electrical conductivity, high temperature conductivity and a large accessible surface area - all of which are beneficial across a number of applications.
  • the gel phase allows casting into any desired moulded shape and controllable density.
  • the gel can be synthesised directly in a cuvette where the electrical power will be applied to generate an electrical current through the graphene network.
  • the aerogel is heated due to the Joule effect.
  • An aerogel according to the invention may be produced under the following reaction scheme 1:
  • the contact angle between the solvent and the graphenes was increased by introducing hydrophobic functional groups onto the graphene surface.
  • This hydrophobisation was achieved through an additional esterification with a fluorinated alcohol; specifically, 0.8 ml (33 % carbon mol) of 2,2,2-trifluoroethanol ( eagentPlus, >99%, Sigma-Aldrich) was added to the gel. After 12 hours the supernatant was set aside. In order to wash the sample, 2 ml of dimethylformamide were added to the gel and after 5 minutes the supernatant was set aside. The washing step was repeated up to 3 times.
  • the objective in this case is to exchange the pore fluid with the more hydrophobic n-hexane to reduce the effective surface tension during the drying of the gel.
  • acetone is used as an intermediate exchange agent as it is completely soluble in both liquids.
  • Solvent exchange of pore-filled dimethylformamide with acetone and subsequently, of acetone with hexane was carried out. For this purpose 2 ml of the solvent were added to the gel and after 5 minutes the supernatant was set aside. The same process was repeated 3 times with each solvent. The sample was dried at room temperature to obtain the resulting graphene oxide aerogel.
  • the shape of the Aerogel can be modulated by controlling the shape of the vessel during the gelation step.
  • the density of the final Aerogel can be modulated by varying the volume fraction of graphenes within the gel. For example, between at least the 15 vol % value described in the specific example and the percolation threshold of these specific crosslinked graphenes in dimethylformamide (estimated to be around 1 vol %).
  • Graphene oxide was purchased from (Nanoinnova Technologies, L. T. D.).Mg(N0 3 )2.6H 2 0 (99%) and AI(N0 3 )3.9H 2 0 (98%) were purchased from Sigma-Aldrich; NaOH, was purchased from AnalaR and Na 2 C0 3 was purchased from Riedel-de Haen. Polycarbonate membranes were from Millipore (HTTP Isopore membrane).
  • Unsupported Layered Double Hydroxides were prepared via co-precipitation. An Mg/AI ratio of 2 was selected as it has been reported to be optimal for C0 2 sorption. An aqueous solution (50 mL) of 0.1 mol Mg(N0 3 ) 2 .6H 2 0 and 0.05 mol AI(N0 3 ) 3 .9H 2 0 was added to an aqueous solution (75 mL) containing 0.35 mol of NaOH and 0.09 mol of Na 2 C0 3 . The resulting white suspension was heated at 333 K for 12 hours under stirring (300 rpm).
  • LDHs Unsupported Layered Double Hydroxides
  • Graphene oxide was dispersed in an aqueous solution (2.06 mL) containing 9.9 mmol NaOH and 2.5 mmol Na 2 C0 3 . Subsequently, 1.39 mL of a salt solution of 2.8 mmol Mg(N0 3 ) 2 .6H 2 0 and 1.4 mmol AI(N0 3 ) 3 .9H 2 0 was added. The resulting black suspension was aged at 333 K for 12 hours under stirring (300 rpm). The sample was filtered and dried as explained above for the preparation of unsupported LDHs.
  • LDH/graphene oxide hybrids were prepared containing varying graphene oxide weight percentages [LDH/graphene oxide mass ratios] 9 wt% [0.1/1], 50 wt% [1/1], 90 wt% [10/1] and 95 wt% [20/1] obtained varying the volume of the base solution and nitrate based solution.
  • LDH/graphene oxide mass ratios 9 wt% [0.1/1], 50 wt% [1/1], 90 wt% [10/1] and 95 wt% [20/1] obtained varying the volume of the base solution and nitrate based solution.
  • the LDH is synthesised (in alkaline conditions or at variable pH conditions) and once the co-precipitation takes place and the LDH is formed, the LDH can be added directly onto a graphene gel.
  • the LDH/gel was washed by solvent exchange with water as explained in the EXAMPLE 2.
  • the pore-filled water was eliminated by freeze drying or by solvent exchange/room temperature drying.
  • solvent exchange of pore-filled water with acetone and subsequently, of acetone with hexane was carried out.
  • 2 ml of the solvent were added to the gel and after 5 minutes the supernatant was set aside. The same process was repeated 3 times with each solvent.
  • LDH/Graphene gel/xerogel hybrid is dried at room temperature to obtain LDH/Graphene aerogel.
  • the pH of the solution has to be kept neutral (without NaOH) to avoid the hydrolysis of the ester functional groups bonding the graphene.

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Abstract

La présente invention concerne des matériaux de graphène, en particulier des aérogels et des xérogels qui contiennent du graphène ou de l'oxyde de graphène, ainsi que des hydroxydes doubles lamellaires (LDH). L'invention concerne également un procédé de préparation de ces aérogels et xérogels de graphène et d'oxyde de graphène et leur utilisation comme matériaux de sorption et de stockage de gaz.
PCT/GB2013/050570 2012-03-09 2013-03-07 Aérogels/xérogels de graphène et d'oxyde de graphène pour la capture de co2 WO2013132259A1 (fr)

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