WO2013093519A2 - Aérogels et xérogels de nanotubes de carbone pour la capture de co2 - Google Patents

Aérogels et xérogels de nanotubes de carbone pour la capture de co2 Download PDF

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WO2013093519A2
WO2013093519A2 PCT/GB2012/053260 GB2012053260W WO2013093519A2 WO 2013093519 A2 WO2013093519 A2 WO 2013093519A2 GB 2012053260 W GB2012053260 W GB 2012053260W WO 2013093519 A2 WO2013093519 A2 WO 2013093519A2
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aerogel
xerogel
carbon nanotube
carbon nanotubes
ldh
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WO2013093519A3 (fr
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Milo Shaffer
Ainara GARCIA GALLASTEGUI
Abdulrahman O ALYOUBI
Abdullah Yousef OBAID
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Bio Nano Consulting
King Abdulaziz University
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Publication of WO2013093519A3 publication Critical patent/WO2013093519A3/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
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    • C01B32/158—Carbon nanotubes
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    • C—CHEMISTRY; METALLURGY
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    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
    • Y02C20/00—Capture or disposal of greenhouse gases
    • Y02C20/40—Capture or disposal of greenhouse gases of CO2

Definitions

  • the present invention relates to carbon nanotube networks which are selected from aerogels and xerogels comprising layered double hydroxides (LDHs).
  • LDHs layered double hydroxides
  • the invention is also concerned with the method of preparing such carbon nanotube networks which are selected from aerogels and xerogels and use of such carbon nanotube networks which are selected from aerogels and xerogels 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 ⁇ x M 3 OH) 2 ] x+ [A m" x/ m A7H 2 0] x" , 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
  • present invention seeks to provide such a system by supporting LDHs onto carbon nanotube aerogels and xerogels.
  • Carbon nanotubes are a new form of carbon with an intrinsically high aspect ratio and nanoscale diameter, as well as high strength and high modulus. These characteristics make it possible to create robust, low density networks with a high degree of connectivity and high surface area.
  • Carbon nanotube networks which are selected from aerogels and xerogels open a new interesting possibility: electrical swing adsorption.
  • the carbon nanotube networks which are selected from aerogels and xerogels are 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 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 carbon nanotube network which is selected from an aerogel and xerogel.
  • the mechanical properties and high thermal stability of carbon nanotubes (CNTs) are able to produce hybrids that can withstand harsher industrial conditions.
  • Layered double hydroxide (LDH) may be defined in various ways.
  • LDH based on a combination of divalent and trivalent metal cations, they may have the general formula [ ⁇ " 1 . ⁇ ⁇ ⁇ " ⁇ ( ⁇ ) 2 ][ ⁇ nH 2 0] or [ ⁇ "- ⁇ "'- X] or [ ⁇ "- ⁇ "'], where [M'Vx M'" x (OH) 2 ] ([M"-M'"]) represents the layer, and [ ⁇ nH 2 0] the interlayer composition.
  • A C0 2" 3 , benzoate, succinate, halide, non-metal oxoanions (B0 3 3" , C0 3 2" N0 3 " , Si 2 05 2” , HP0 4 2” , S0 4 2” , CIO “ 4 , As0 4 3” , Se0 4 2” , Br0 4 " ), oxometallate anions (V0 4 3” , Cr0 4 2” , Mn0 4 " , V10O 28 6" , Cr 2 0 7 2” , Mo 7 0 24 6” , PW 12 O 40 3” ), anionic complexes of transition metals (Fe(CN) 6 2” ), volatile organic anions (CH 3 COO " , C 6 H 5 COO " , Ci 2 H 25 COO " , C 2 0 4 2” , C 6 H 5 S0 3 " ), anionic polymers (PSS, PVS, etc.) or NO " 3 ; S0 2"
  • x is between 0.17 and 0.33;
  • n is between 0.1 and 4.0
  • M 3+ Al 3+ , Fe 3+ , Cr 3+ , Ga 3+ .
  • LDH is preferably Mg-AI-LDH which has the general formula [M 2 ⁇ x M 3 ⁇ (OH) 2 ] x+ [A m nH 2 Of, 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.
  • LDH is Mg 2 AI(OH) e (C03)o.5-0.15H 2 0.
  • the formula for any given LDH may be calculated according to Example 4.
  • the ratio of LDH:carbon nanotube (w/w) is between 100:0.01 and 0.01 :100, preferably between 20:0.1 and 0.1 :20, even more preferably between 20:0.1 and 0.5: 1. Therefore, the ratio of LDH:carbon nanotube (w/w) may be 4:1 , 3: 1 , 2:1 , 1 :1 or 1 :2.
  • LDH is present within the carbon nanotube network which is selected from an aerogel and xerogel in an amount of between 10 to 85 wt%, preferably between 30 and 70 wt% and even more preferably between 40 and 60 wt% by total weight of the LDH supported carbon nanotube network which is selected from an aerogel and xerogel.
  • a method of preparing a carbon nanotube network which is selected from an aerogel and xerogel comprising LDH, comprising the steps of:
  • the LDH may be deposited onto the carbon nanotube network which is selected from an aerogel and xerogel by any method known to those skilled in the art. In a preferred method, LDH is directly grown or precipitated on the carbon nanotube network which is selected from an aerogel and xerogel. In a more preferred embodiment, 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.
  • carbon nanotubes are generally impure and have a very low solubility/dispersion in common solvents, particularly in water. Therefore, they are often chemically modified to aid their purification and to increase their solubility.
  • a common strategy is the use of acids or other oxidising reagents to remove impurities such as metal catalytic particles, amorphous carbon, and graphitic nanoparticles.
  • these treatments produce molecular debris that can remain adsorbed onto the nanotube walls even after conventional water washing.
  • the carbon nanotube are oxidised prior to the deposition of LDH.
  • the oxidation step introduces new oxygen- containing acidic groups that improve water solubility.
  • the oxidised carbon nanotube ensures a favourable interaction and hence a good dispersion with the LDH.
  • the oxidised carbon nanotube may be treated with a base to convert the acidic groups into their conjugate salts, further increasing the solubility of both nanotubes and their oxidation debris, in water.
  • Such treatment steps provide sites to coordinate metal ions, and balance charge during LDH synthesis. Furthermore, it allows for homogeneous aqueous dispersions of carbon nanotubes for the deposition of LDH crystals by co-precipitation of Mg 2+ and Al 2+ ions, under alkaline conditions.
  • LDH can be synthesised onto the crosslinked carbon nanotube network gel.
  • the coprecipitation can be made at neutral pH to avoid the hydrolysis of the ester bonds between the nanotubes and hence, to avoid the loss of the network structure.
  • the synthesised LDH/carbon nanotube gel hybrid is purified by water solvent exchange.
  • the resulting LDH/carbon nanotube gel hybrid is freeze dried or dried at ambient pressure after solvent exchange processes (water/acetone/hexane) to avoid the collapse of the carbon nanotube gel structure obtaining a carbon nanotube xerogel and finally carbon nanotube aerogel/LDH hybrid.
  • the oxidised carbon nanotubes may be cross-linked by forming covalent bonds between the existing oxide groups on the nanotube surface using a direct condensation reaction. Direct condensation between the existing surface oxides occurs only at the contact points between the nanotubes, leaving the remaining oxidised surface unchanged, or available for subsequent LDH deposition. Carboxylates are particularly preferred for their compatibility with LDH.
  • the nanotube gel is formed by using linking molecules that directly bond to the pristine nanotube 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 network which is selected from an aerogel and xerogel comprising LDH.
  • a further aspect of the present invention involves the use of the carbon nanotube network which is selected from an aerogel and xerogel comprising LDH for C0 2 sorption, capture and storage.
  • Heating Carbon Nanotube Network which is selected from an Aerogel and Xerogel Comprising LDH
  • the carbon nanotube network which are selected from aerogels and xerogels comprising LDH may be heated using an electrical current.
  • Any current may be applied to effect the heating.
  • a current of up to 1 A preferably between 3 and 500 mA, more preferably between 5 and 100 mA, more preferably between 6 and 18 mA is applied.
  • any voltage may be applied to effect the heating.
  • voltage of up to 240 V preferably between 0.5 and 150 V, more preferably between 5 and 100 V, more preferably between 10 and 30 V is applied.
  • a current density of up to 500 A/cm 2 preferably between 0.1 mA/cm 2 and 100 A/cm 2 , more preferably between 1 mA/cm 2 and 100 mA/cm 2 , more preferably between 5 mA/cm 2 and 20 mA/cm 2 is achieved.
  • electric fields of up to 100 V/cm preferably between 0.001 and 20 V/cm, more preferably between 0.005 and 10 V/cm, more preferably between 0.1 and 1 V/cm is achieved.
  • the distance separating the electrodes is between 0.1 cm and 50 cm, preferably between 1 cm and 20 cm, more preferably between 2 cm and 10 cm.
  • the electrode surface area of the electrodes is between 0.5 cm 2 and 100 cm 2 , preferably between 1 cm 2 and 50 cm 2 , more preferably between 2 cm 2 and 10 cm 2 .
  • Electrodes may be sheets, films, rods, wires, coatings, or other morphology of metallic conductor suitable for the application and aerogel geometry required. During fabrication, the aerogel may be gelled around or against the electrodes (which may be porous to assist mechanical interlocking). Alternatively, the electrodes may be pressed against or inserted into the aerogel after fabrication. Inert electrode materials, such as noble metals or conductive carbons, are preferred. The electric field may then be calculated using the equation below:
  • the power required to produce a specific heating effect may be calibrated as follows.
  • the voltage may be measured using a conventional power supply and a conventional ammeter may be used to measure the current at each temperature.
  • the carbon nanotube networks which are selected from aerogels and xerogels comprising LDH may be heated in an inert atmosphere to a temperature of up to 3000 °C, preferably between 100 to 1000 °C, more preferably between 200 and 500 °C. If the aerogels or xerogels are heated in air, they may be heated to a temperature of up to 600 °C, preferably between 100 and 550 °C, more preferably between 150 and 500 °C, even more preferably between 200 and 450 °C.
  • Heating the carbon nanotube networks which are selected from aerogels and xerogels comprising LDH above approximately 773 K in inert atmosphere would carbonise any polymer structure, forming a more stable carbonaceous binding of the junctions.
  • Heating above 673 K 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 (Hibino, T.; Yamashita, Y.; Kosuge, K.; Tsunashima, A.
  • the carbon nanotubes may graphitise and fuse together to form an inherently continuous and robust structure with a high degree of graphiticity.
  • heating of the aerogels or xerogels 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 nanotube 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.
  • Carbon nanotube networks which are selected from aerogels and xerogels comprising LDH provide high electrical and thermal conductivity, to generate a robust, high surface area network that can be electrically-heated.
  • the temperature may be monitored by one or more external thermocouple(s) placed in the aerogel or xerogel comprising LDH or embedded during fabrication, or by optical pyrometry.
  • the measured resistance of the carbon nanotube network which is selected from an aerogel and xerogel comprising LDH itself may be used as an indication of temperature.
  • the shape of the carbon nanotube network which is selected from an aerogel and xerogel 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 ( Figure 1).
  • the aerogel or xerogel comprising LDH 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 carbon nanotube surface (or, optionally, additional material supported on the carbon nanotubes) 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 CNT 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 carbon nanotube surface can act as a sorbent/filter either to purify a gas/liquid stream, or to store (a fraction of) it.
  • heating of the carbon nanotube network which is selected from an aerogel and xerogel comprising LDH will regenerate the sorbent by decomposition or desorption of the trapped species, being the trapped species composed of liquid or gas phase molecules, preferably gas phase molecules, more preferably C0 2 , CO, NO, N 2 0, SO x , CH 4 , 0 3 , H 2 and water vapour or volatile organic compounds (VOCs)
  • the carbon nanotube networks which are selected from aerogels and xerogels comprising LDH provide advantages such as rapid and homogeneous heating, fast cycling, rapid emission of stored species, and minimal thermal degradation.
  • a preferred embodiment according to the present invention further comprises a system for the fractionation of adsorbed species by adsorbate/adsorbent interaction using an accurate temperature control by varying the electrical current passing through the carbon nanotube networks which are selected from aerogels and xerogels.
  • the carbon nanotube networks which are selected from aerogels and xerogels according to the present invention may be produced by any method known to those skilled in the art.
  • the aerogel or xerogel according to the present invention may be produced by a method comprising the steps of: (a) dispersing carbon nanotubes in a solvent compatible with said carbon nanotubes; (b) cross-linking said carbon nanotubes using functional groups already present on the carbon nanotubes or with a linking molecule comprising at least two functional sites capable of reacting with the surface of said carbon nanotubes, to form a covalently cross-linked gel network; and (c) optionally removing said solvent to give a carbon nanotube aerogel/xerogel with a solvent content of less than 10% by weight.
  • the cross-linked carbon nanotube network which is selected from an aerogel and xerogel 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 solvent content of the carbon nanotube network which is selected from an aerogel and xerogel is preferably less than 2% by weight, more preferably less than 0.5% by weight, more preferably less than 0.1 % by weight.
  • the carbon nanotubes according to the present invention are reacted using functional groups already present on the carbon nanotubes or with a linking molecule comprising at least two functional sites capable of reacting with the surface of said carbon nanotubes, to form a covalently cross-linked gel network.
  • the linking molecule may have functionalities that can couple directly to the nanotube sidewalk
  • functionalities for example, bis(diazonium) salts, or multifunctional molecules suitable for 1 ,3-dipolar cycloadditions, or Bingel condensations using known nanotube surface chemistry.
  • Such reductions may be carried out by Birch reduction (Chemical Attachment of Organic Functional Groups to Single-Walled Carbon Nanotube Material, Chen Y., Haddon R. C, Fang S., Rao A. M., Eklund P.
  • Carbon nanotube reduction may be also carried out using alkali metals and the radical anions generated from naphthalene (Spontaneous Dissolution of a Single-Wall Carbon Nanotube Salt, Penicaud A., Poulin P., Derre A., Anglaret E., Petit P., J. Am. Chem. Soc, 2005, 127, 8) or benzophenone (Covalent Sidewall Functionalization of Single-Walled Carbon Nanotubes via One-Electron Reduction of Benzophenone by Potassium, Wei L, Zhang Y., Chem. Phys.
  • such a linking molecule may react with oxide groups of oxidised carbon nanotubes 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 .
  • aromatic diamines Ci_ 2 o alkyl diols, C5-20 aromatic diols, C2-100 polyols, bis-sodium C1-20 alkoxides, C2-20 dicarboxylic acids, C2-20 di acid chlorides, more preferably C M0 alkyl diamines, C 6 -is aromatic diamines, C2-10 alkyl diols, C 6 -is aromatic diols, C2-20 polyols, bis-sodium C2-10 alkoxides, C2-10 dicarboxylic acids, C2-10 di acid chlorides, and the like.
  • the two reactive groups are located on different atoms of the linking molecule, more preferably at some distance, to maximise the chance of reacting with two different nanotubes.
  • the use of small rigid molecules may maximise the chance of establishing a cross-link, for example using 1 ,4-diamino benzene, by limiting the possibility of reacting twice with the same nanotube.
  • the oxidised carbon nanotubes 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 nanotube surface.
  • This approach has the advantage of bringing the nanotubes 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 embodiments, additional linking molecules will saturate the entire surface, although nanotube cross-links will only occur relatively rarely.
  • These molecules may be wasteful and may, undesirably, occlude the conductive surface that is desirable in certain applications such as electrochemical electrodes. Direct condensation between the existing surface oxides occurs only at the contact points between the nanotubes, leaving the remaining surface unchanged, or available for subsequent differential functionalisation. Carboxylates are particularly preferred for their compatibility with LDH.
  • oxidised carbon nanotubes may be obtained commercially or, more usually, be those that have further been oxidised according to any standard method.
  • oxidised carbon nanotubes refers to any carbon nanotube with one or more oxide groups present on the surface of the carbon nanotube.
  • 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 oxides may be produced during the original synthesis reaction, or be deliberately introduced in a subsequent step, involving liquid or gas phase oxidation.
  • Gas phase oxidation can be carried out at elevated temperature in any suitable oxidising gas, such as air, oxygen, water vapour, carbon monoxide, or carbon dioxide; the temperature depends on the gas and is typically in the range 250°C to 800°C.
  • the oxidised carbon nanotubes are prepared using a system of mixed acids or oxidising agents, preferably being selected from the group consisting of H 2 S0 4 , HN0 3 , H 2 0 2 , KMn0 4 , K 2 Cr 2 0 7 , Os0 4 , and Ru0 4 .
  • the mixed acid system is a combination of sulphuric and nitric acid.
  • the level of oxidation of the carbon nanotubes will vary according to the desired mechanical and electrical properties required. Typically, the level of oxidation on the oxidised carbon nanotubes is between 0.001 - 10 mmol/g, preferably 0.1 mmol/g or greater.
  • the oxidised carbon nanotubes are base-washed before the cross-linking step.
  • Such 'base-washing' may be an important step to remove oxidation 'debris' and to expose groups directly bound to the nanotubes, such that the nanotubes are covalently connected during the cross- linking step. Removal methods for such debris are disclosed in Purification of Single Walled Carbon Nanotubes: The Problem with Oxidation Debris, Fogden S., Verdejo R., Cottam B., Shaffer M., Chem. Phys.
  • base washing is carried out using weak aqueous base, more preferably a weak aqueous solution of sodium hydroxide or potassium hydroxide.
  • the oxidised carbon nanotubes 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 carbon nanotubes 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 carbon nanotubes 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), N.N'-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, O-(benzotriazoM-yl)- ⁇ , ⁇ , ⁇ ', ⁇ '-tetramethyluronium hexaflu
  • 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 1-ethyl-3-(3- dimethylaminopropyl)carbodiimide, ⁇ , ⁇ '-dicyclohexyl carbodiimide, N,N'-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 ⁇ , ⁇ '- dicyclohexylcarbodiimide, N.N'-diisopropylcarbodiimide and 1-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 reaction is carried out at a temperature of between 15 to 60 °C, preferably 20 to 30 °C.
  • 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 carbon nanotubes that are available directly from synthesis or conventional purification regimes. Such groups include ether linkages which may be formed by dehydration at a temperature greater than 120 °C, preferably greater than 130 °C and even more preferably greater than 150 °C, using an acid catalyst.
  • Cross-linking process will require a solvent with a high boiling point, greater than the reaction temperature. In a preferred embodiment, the boiling point of the solvent is greater than 120 C, preferably greater than 130 °C and even more preferably greater than 150 °C.
  • the carbon nanotubes 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 nanotubes 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(nTr/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 nanotube surface, only a small proportion (approximately 1-3%) of the surface oxides react to form the cross-links between the carbon nanotubes.
  • the cross-linked carbon nanotube network which is selected from an aerogel and xerogel thus obtained will have unreacted oxide groups on the surface of the carbon nanotubes. These groups impart hydrophilicity (i.e. tendency to interact with or be dissolved by water and other polar substances) to the resulting carbon nanotube network which is selected from an aerogel and xerogel.
  • the method can further comprise a step of capping residual surface oxides on the oxidised carbon nanotubes.
  • 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 CM 2 haloalkyl and CMS alkyl, most preferably a CM 2 - haloalkyl and more preferably CM 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 C1-12 hydroxyhaloalkyl compound, preferably trifluoroethanol.
  • hydrophobic it is meant that the group imparts increased hydrophobic character to the carbon nanotube, thereby reducing the solid surface tension.
  • 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. In this case 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 carbon nanotubes may be used (Towards Solutions of Single Walled Carbon Nanotubes in Common Solvents. Bergin S. D., Nicolosi V., Streich P. 3, Giordani S., Sun Z. 1 , Windle A.H. 4, Ryan P. 5, Nirmalraj P.P.N. 5, Wang Z.T. 4, Carpenter L, Blau W.J., Boland J.J. 4, Hamilton J. P. 3, Coleman J.N., Advanced Materials, 2008, 20, 10, 1876).
  • the term "compatible” refers to any solvent in which the carbon nanotubes form a substantially homogeneous solution or dispersion.
  • the solvent which is compatible with the carbon nanotubes 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 carbon nanotubes may be present in the solvent at any given concentration.
  • the nanotubes 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 nanotubes dispersion in the chosen solvent.
  • the carbon nanotubes 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.
  • the need for low surface tension decreases. Therefore in principle, some nanotube gels may be dried without solvent exchange and others will need very low surface tensions.
  • solvent exchange is carried out using acetone, followed by C 3 -Ci 0 hydrocarbon, preferably hexane.
  • the aerogel or xerogel is synthesised without covalent crosslinking of the carbon nanotubes; instead relying on a non-covalent binder to create the stabilse the required monolith.
  • the aerogel or xerogel may be synthesised in the presence of a one or more gelling agents to provide greater support to the gel-forming process.
  • a one or more gelling agents may include polymers such as those disclosed in Chitosan Abarrategi A., Gutierrez M. C, Moreno-Vicente C, Hortiguela M. J ., Ramos V., Lopez-Lacomba J. L, Ferrer M. L, del Monte F., Biomaterials, 2008, 29, 94.
  • a preferred polymer according to the present invention is carboxymethyl cellulose (CMC).
  • gelling agents include gelatine such as those disclosed in Nabeta M., Sano M., Langmuir, 2005, 21 , 1706- 1708; ionic liquids such as those disclosed in Fukushima T., Kosaka A., Ishimura Y., Yamamoto T., Takigawa T., Ishii N., Aida T. ,Science, 2003, 300, 2072; and surfactants such as those disclosed in Leroy CM., Cam F., Backov R., Trinquecoste M., Delhaes P., Carbon, 2007, 45, 2307-2320 and Bryning M. B., Milkie D. E., Islam M.F ., Hough L.
  • the aerogel or xerogel may be synthesised by providing pristine carbon nanotube and CMC or other polymer in water or other solvent, and freeze drying (Nakagawa K, Yasumura Y., Thongprachanb N., Sano N., Chemical Engineering and Processing, 2011 , 50, 22- 30; Zou J., Liu J., Karakoti A.
  • the aerogel or xerogel/gel may be synthesised by providing oxidised carbon nanotube and CMC or other polymer in water or the solvent, and freeze drying (Nabeta M., Sano M..
  • the aerogel or xerogel may be synthesised by providing oxidised cross- linked carbon nanotube and CMC in water, and freeze drying.
  • the aerogel or xerogel may be synthesised by providing oxidised cross-linked carbon nanotube in water, and freeze drying.
  • the aerogel or xerogel may be synthesised by providing oxidised cross-linked carbon nanotube in water, carrying out solvent exchange (preferably using water-acetone-hexane) and drying at ambient temperature and pressure.
  • the carbon nanotube network according to the present invention is 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 nanotube "gel” will be used, rather than the dried form.
  • an "aerogel” comprises a carbon nanotube 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 high aspect ratio nanotubes may have ultra-low densities, high surface areas, but large pore sizes; in principle, the pore size may approach the scale of the individual nanotube 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 nanotubes and optional hydrophobic functionalisation of the nanotube 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 nanotubes limits the process, yielding a more useful, more porous, less dense structure, than obtained from drying physical gels or other nanotube suspensions.
  • the aerogel network is based around carbon nanotubes with aspect ratio of between 100 and 10000, preferably with aspect ratio of between 200 and 1000.
  • aspect ratio is meant the ratio between the length and diameter of the carbon nanotubes.
  • carbon nanotubes have a high aspect ratio since the length of carbon nanotubes is typically in the order of 1-100 ⁇ .
  • each carbon nanotube 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 carbon nanotubes is therefore thought to display excellent electrical conductivity and current density, compared to existing carbon aerogels.
  • carbon nanotubes have desirable intrinsic mechanical characteristics, including high strength, stiffness, and flexibility, at low density. These properties make carbon nanotubes desirable for many industrial applications, and lend desirable properties to the resulting aerogel networks.
  • the shape of the aerogel or xerogel 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 nanotubes within the initial gel.
  • catalysts comprising a carbon nanotube aerogel and xerogel prepared any of the above methods.
  • a further embodiment according to the present invention involves the use of carbon nanotube networks which are selected from aerogels and xerogels comprising LDH for sorption and gas storage.
  • 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.
  • carbon nanotubes refers to nano-scale tubes made substantially of carbon atoms, having a structure based on graphite basal planes that are wrapped or curled to become a tube. The definition therefore encompasses carbon nanotubes of any type, including common carbon nanotubes, variants of common carbon nanotubes, and carbon nanotubes with various modifications.
  • the term encompasses those that are not exactly shaped like a tube, such as a carbon nanohorn (a horn-shaped carbon nanotube whose diameter continuously increases from one end toward the other end); a carbon nanocoil (a coil-shaped carbon nanotube forming a spiral when viewed in entirety); a carbon nanobead (a spherical bead made of amorphous carbon or the like with its centre pierced by a tube) and a cup-stacked nanotube. It may also encompass structures that are not pure carbon, such as those doped with nitrogen or boron, or functionalised with surface groups such as those containing oxygen, hydrogen, or other derivatives.
  • Carbon nanotubes may further be submicron fibres with primarily graphitic (sp 2 ) or amorphous carbon structures arranged in any crystallographic orientation such as platelet nanofibres and bamboo nanofibres.
  • the present invention uses common carbon nanotubes in which the carbon atoms are ideally sp 2 bonded into a graphene-like sheet with cylindrical geometry.
  • Suitable nanotubes may be those that are commercially available such as the products of Applied Sciences Inc., Bayer Chemicals, Cheaptubes Inc., Chengdu Organic Chemicals, Future Carbon, Nanocyl S. A., Nanoshel, Arry International Group Limited, Carbon Nano Materials R&D Center, Carbon Solutions Inc., NanocarbLab (NCL), Nanocs, Thomas Swan Ltd.
  • carbon nanotubes may be single-walled carbon nanotubes, double-walled carbon nanotubes or multi-walled carbon nanotubes having more than two layers.
  • the nanotubes used in the present invention preferably have 5 to 15 walls. Whether one or more types of carbon nanotube are used (and, if only one type is to be used, which type is chosen) is selected appropriately taking into consideration the particular end use. For example, carbon nanotubes with a smaller diameter favour larger surface area and those with a larger diameter display greater resistance to collapse during drying of the gel network.
  • the carbon nanotubes are multi-walled carbon nanotubes having more than two layers. Preferably, greater than 75 wt% of the nanotubes in the network have more than two layers.
  • the carbon nanotubes used in the present invention have a diameter range of from about 0.5 to 200 nm, preferably from about 0.5 to 100 nm, more preferably about 80 to 160 nm.
  • the carbon nanotubes used in the present invention have a length of from about 0.05 to 1000 ⁇ , preferably from about 1 to 300 ⁇ , most preferably in the range of about 5 to 100 ⁇ .
  • greater than 75 wt% of the nanotubes in the network have dimensions in the ranges set out immediately above.
  • the aerogels and xerogels according to the present invention may be produced by directly, covalently crosslinking carbon nanotubes to form a gel network, followed by solvent removal.
  • the gel structure is created by rigid nanotubes directly bound to each other or with another gel-forming component. Direct binding between the carbon nanotubes 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 nanotube network.
  • the aerogel is heated due to the Joule effect.
  • An aerogel is introduced or synthesised in a cuvette that has two of its sides composed by aluminium foil.
  • a DC power supply with controllable voltage was used together with a portable ammeter to monitor the current while increasing the voltage.
  • the temperature was monitored using an IR camera as the voltage was increased. After applying each voltage step, the temperature increases and rapidly stabilised, whereupon the temperature was recorded.
  • Figure 4 IR camera images of the cuvette with the aerogel inside at each voltage showing the temperature change.
  • An aerogel according to the invention may be produced under the following conditions:
  • multi-walled carbon nanotubes are used (commercial ARKEMA Graphistrength® Multi-Wall Carbon Nanotubes). Oxidation of the nanotubes
  • the sample was suspended in dimethylformamide (ACS, Sigma-Aldrich) and filtrated using 10 ⁇ PTFE membranes (LCW Mitex membrane filter, Millipore). The dimethylformamide washing step was repeated up to 3 times.
  • the contact angle between the solvent and the nanotubes was increased by introducing hydrophobic functional groups onto the nanotube 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 (ReagentPlus, ⁇ 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. Exchange the pore fluid with a selected solvent
  • 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. Since dimethylformamide and n-hexane are immiscible, 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 resulting nanotube-hexane gel was recovered with the aid of a spatula and immediately placed in a glass syringe (20 ml volume and 2 mm nozzle diameter). Extrusion of the gel from the syringe produced a long (up to 10 cm) cylindrical sample that supported its own weight in air and retained its shape in hexane (see the images below). After 15 minutes the resulting 2 mm diameter cylinders were separated with tweezers, taken out from hexane and dried at room temperature. No shrinkage was observed during the drying of these carbon nanotube based cylindrical Aerogels which had a density of 0.3 g/cm 3 .
  • 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 nanotubes within the gel. For example, between at least the 20 vol % value described in the specific example and the percolation threshold of these specific crosslinked nanotubes in dimethylformamide (estimated to be around 1 vol %).
  • MWNTs Long, CVD-grown, Multi-Walled Carbon Nanotubes (MWNTs) were purchased from ARKEMA (Graphistrength®), with an average diameter of 10-15 nm. Mg(N0 3 ) 2 .6H 2 0 (99%) and AI(N0 3 ) 3 .9H 2 0 (98%) were purchased from Sigma- Aldrich; NaOH, H 2 S0 4 (95%), HN0 3 (65%) were purchased from AnalaR and Na 2 C0 3 was purchased from Riedel-de Haen. Polycarbonate membranes were from Millipore (HTTP Isopore membrane).
  • carbon nanotube gel/xerogel/aerogel were synthesised using cross-linked carbon nanotubes.
  • Carbon nanotube gel/xerogel/aerogel can be also synthesised using polymers to reinforce the aerogel structure.
  • 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 carbon nanotube 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/Carbon nanotube gel/xerogel hybrid is dried at room temperature to obtain LDH/Carbon nanotube aerogel.
  • Mg/AI ratio 2 is estimated by EDX (Energy- dispersive X-ray spectroscopy)

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Abstract

La présente invention concerne des réseaux de nanotubes de carbone qui sont choisis parmi les aérogels et les xérogels comprenant des hydroxydes à double couche (LDH). L'invention concerne également le procédé de préparation de tels réseaux de nanotubes de carbone qui sont choisis parmi les aérogels et les xérogels et l'utilisation de tels réseaux de nanotubes de carbone qui sont choisis parmi les aérogels et les xérogels pour la sorption et le stockage de gaz.
PCT/GB2012/053260 2011-12-22 2012-12-21 Aérogels et xérogels de nanotubes de carbone pour la capture de co2 WO2013093519A2 (fr)

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GBGB1122199.1A GB201122199D0 (en) 2011-12-22 2011-12-22 Carbon nanotube aerogels/xerogels/gels for CO2 capture
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