Metal organic framework coating methods

Abstract

A method of producing a coating for a structure comprising mixing a metal organic framework (MOF) with a solvent to form a coating mixture. The coating is used in methods for coating articles or structures used in direct air capture (DAC) apparatus to remove carbon dioxide (CO2) from the atmosphere. The coating is made and applied without the use of a binder, binding agent, or an adhesive.

Description

[0001] TITLE

[0002] METAL ORGANIC FRAMEWORK COATING METHODS

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to metal organic framework (MOF) coating methods and articles coated by such methods. In particular, the present invention relates to methods for coating articles or structures used in direct air capture (DAC) apparatus to remove carbon dioxide (CO₂) from the atmosphere.

[0005] BACKGROUND

[0006] Direct air capture (DAC) is a method of extracting molecules from the air using an adsorbent material which has an affinity for those molecules. In the case of carbon capture, the molecules are carbon dioxide (CO₂). The direct air capture process has two main states - the adsorption state and the desorption state. During the adsorption state air is directed across the adsorbent material and the molecules of interest bond to the adsorbent material releasing energy. This energy is often referred to the isosteric heat of adsorption / desorption (Qst) representing all the energy necessary to bind the molecules of interest to and release the molecules of interest from the adsorbent material.

[0007] During the desorption phase the environment surrounding the adsorbent material is changed so that the molecules of interest are released from the adsorbent material, and in the process adsorb the Qstenergy. The desorption phase is carried out at a gas pressure of near zero absolute, and by adding heat at the temperature which causes the molecules of interest to disassociate from the adsorbent material. This process is commonly referred to as temperature vacuum swing adsorption (TVSA). This process is similar to the process of boiling water in that at a particular temperature and pressure water molecules disassociate from the liquid and become gas, and in the process require latent heat energy to release the water-water bond.One class of adsorbent materials identified as being particularly suited for DAC processes are metal organic frameworks (MOFs) due to their high porosity, very high surface area and their ability to reversibly adsorb CO₂. MOFs are a class of porous polymers consisting of inorganic metal clusters, also referred to as secondary building units (SBUs), coordinated to organic ligands, also referred to as "struts'' or "linkers", to form one-, two- or three- dimensional structures MOFs have an extended structure wherein the sub¬ units occur in a constant ratio and are arranged in a repeating pattern.

[0008] MOFs are typically applied to a structure using a binder, binding agent, adhesive or the like, such as, but not limited to ethyl cellulose, to ensure that the MOF adheres to the structure. However, one problem with using binders, binding agents, adhesives or the like is that they block or partially block some of the pores in the MOF thus reducing the surface area available for CO₂ adsorption.

[0009] Another problem encountered with MOFs is the propensity of MOFs to clump together, as illustrated in the image in FIG. 1, which shows a MOF in the form of NbOFFIVE-1-Ni (KAUST-7) in its raw manufactured form and the crystals clumped together. The same problem could be observed with other MOFs in general and in particular MOF808-lysine (MOF-3). This problem can also severely impact the effectiveness of MOFs for DAC applications, for example, because the surface area available for CO₂ adsorption is significantly reduced.

[0010] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge in the art.

[0011] OBJECT OF THE INVENTION

[0012] It is a preferred object of the present invention to provide a method of coating metal organic frameworks (MOFs) that addresses or at least ameliorates the aforementioned problems and / or provides a useful commercial alternative.In particular, it is a preferred object of the present invention to provide a method of coating structures and articles with metal organic frameworks (MOFs), and structures coated with such methods, for direct air capture (DAC) systems and / or methods and / or apparatus to remove carbon dioxide from the atmosphere.

[0013] SUMMARY OF THE INVENTION

[0014] Generally, embodiments of the present invention are directed to methods of coating metal organic frameworks (MOFs) to structures and articles and structures and articles coated by such methods, in particular, for direct air capture systems, methods and apparatus for removing carbon dioxide from the atmosphere.

[0015] According to one aspect, but not necessarily the broadest or the only aspect, the present invention is directed to a method of producing a coating for a structure, the method comprising mixing a metal organic framework (MOF) with a solvent to form a coating mixture;

[0016] wherein the structure is a metal, or comprises metal; and

[0017] wherein the coating mixture is produced and applied to the structure without the use of a binder, binding agent, or adhesive.

[0018] According to another aspect, but not necessarily the broadest or the only aspect, the present invention is directed to a method of coating a structure with a metal organic framework (MOF), the method comprising:

[0019] mixing a metal organic framework (MOF) with a solvent to form a coating mixture; and

[0020] coating the structure with the coating mixture

[0021] wherein the structure is a metal, or comprises metal; and

[0022] wherein the coating mixture is produced and applied to the structure without the use of a binder, binding agent, or adhesive.

[0023] According to a further aspect, but not necessarily the broadest or the only aspect, the present invention is directed to a structure coated with a coating mixture formed from a metal organic framework (MOF) mixed with a solvent;

[0024] wherein the structure is a metal, or comprises metal; andwherein the coating mixture is produced and applied to the structure without the use of a binder, binding agent, or adhesive.

[0025] Preferably, the structure is a metal selected from the following: aluminium; magnesium copper; silver; gold; iron; steel.

[0026] Suitably, the structure is a honeycomb structure.

[0027] Suitably, the MOF is NbOFFIVE-1-Ni (KAUST-7) or MOF-808, or any functionalised variants of MOF-808 such as MOF-808-lysine / MOF-3.

[0028] Suitably, the MOF is selected from any class of MOF which exhibits static electric fields within the crystal structure and / or clumps together into groups of crystals.

[0029] Suitably, the solvent is selected from the following: water; acetone; isopropyl alcohol (IPA); ethanol; methanol; methyl cyanide (MeCN) (acetonitrile).

[0030] Preferably, the method includes subjecting the coating mixture to stirring and / or high frequency sound waves, in particular ultrasonic waves, prior to and / or during coating of the structure with the coating mixture.

[0031] Suitably, coating the structure with the coating mixture includes one or more of the following: dipping the structure in the coating mixture; spraying the structure with the coating mixture.

[0032] Suitably, the method includes repeating the coating step.

[0033] Suitably, mixing of the metal organic framework (MOF) with the solvent to form the coating mixture and / or coating the structure with the coating mixture are conducted at atmospheric pressure and ambient temperature.

[0034] Preferably, the method includes evaporating the solvent after the coating step, in particular to leave no residue of the solvent behind.

[0035] Preferably, the coating mixture is subjected to heat before and / or during the coating process, for example to raise a temperature of the coating mixture to around 45°C.Preferably, a rate of evaporating the solvent is increased by increasing a flow of ambient air over the coated structure.

[0036] Preferably, the structure is pretreated with either an acid or a base or both.

[0037] Suitably, the acid is hydrochloric acid and the base is sodium hydroxide.

[0038] According to another aspect, but not necessarily the broadest or the only aspect, the present invention is directed to a structure comprising: a repeating array of hollow cells; and an adsorbent material to adsorb carbon dioxide (CO₂) from the atmosphere;wherein the adsorbent material is in the form of a coating, disposed on the hollow cells, made from mixing a metal organic framework (MOF) with a solvent; wherein the structure is a metal, or comprises metal; and wherein the coating is produced and applied to the structure without the use of a binder, binding agent, or adhesive.

[0039] Preferably, the structure is a honeycomb structure.

[0040] Preferably, the structure is housed within at least one enclosure of a direct air capture (DAC) apparatus to remove carbon dioxide (CO₂) from the atmosphere.

[0041] Further features and / or aspects of the present invention will become apparent from the following detailed description.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described with reference to the accompanying drawings, which are provided by way of example only, wherein like reference numerals refer to like features. In the drawings:

[0043] FIG 1 is a scanning electron micrograph (SEM) image of a metal organic framework (MOF) in the form of NbOFFIVE-1-Ni (KAUST-7) in its raw manufactured form illustrating clumping of the MOF;

[0044] FIG 2 is a graph showing uptake of CO₂ at various pressures for five different samples of the synthesised NbOFFIVE-1-Ni (KAUST-7);FIG 3A to 3F illustrate the coating of aluminium plates with a coating mixture in the form of the synthesised NbOFFIVE-1-Ni (KAUST-7) mixed with six different solvents;

[0045] FIG 4A to 4F show SEM images at the same magnification of aluminium foil sheets sprayed with a coating mixture comprising NbOFFIVE-1- Ni (KAUST-7) and different solvents;

[0046] FIG 5 is a graph showing uptake of CO₂ at various pressures for threesamples having different coating concentrations of NbOFFIVE-1-Ni (KAUST-7) and a different number of coating dips compared with bulk synthesised NbOFFIVE-1-Ni (KAUST-7);

[0047] FIG 6 shows the graph of FIG 5 at a smaller range of pressures;

[0048] FIG 7 is a graph showing uptake of CO₂ at various pressures for threesamples having different coating concentrations of NbOFFIVE-1-Ni (KAUST-7) and a different number of coating dips, and a spray-coated sample, compared with bulk synthesised NbOFFIVE-1-Ni (KAUST-7);

[0049] FIG 8 shows the graph of FIG 7 at a smaller range of pressures;

[0050] FIG 9 shows an example of an aluminium honeycomb structure prior to coating with a coating mixture;

[0051] FIG 10 is a graph illustrating the CO₂ uptake of a structure coated according to an embodiment of the present invention over 32 cycles of temperature swing;

[0052] FIG 11 shows a sample structure in the form of an aluminium honeycomb after cleaning and prior to coating;

[0053] FIG 12A to 12C show a spray coating process of MOF808-lysine applied to the sample structure shown in FIG 11;

[0054] FIG 13 shows a dip coating process of MOF808-lysine;

[0055] FIG 14A and 14B show the result of dip coating of MOF808-lysine applied to the sample structure shown in FIG 11;FIG 15 shows the results of dip coating aluminium bars for various times for coating mixtures that were stirred and / or sonicated;

[0056] FIG 16 shows a PXRD graph of NbOFFIVE-1-Ni (KAUST-7) in various solvents;

[0057] FIG 17 shows a thermogravimetric analysis of aluminium foil coated with a coating mixture comprising NbOFFIVE-1-Ni (KAUST-7) and IPA under N₂ and air;

[0058] FIG 18 shows a thermogravimetric analysis of aluminium foil coated with a coating mixture comprising MOF808-lysine and IPA under N₂ and air;

[0059] FIG 19 is an image showing three honeycomb structures dip-coated with a coating mixture comprising NbOFFIVE-1-Ni (KAUST-7) and IPA;

[0060] FIG 20 is an image showing a honeycomb structure dip coated with a coating mixture comprising MOF808-lysine and IPA;

[0061] FIG 21 shows a thermogravimetric analysis of bulk MOF808-lysine mixed with a binder under N₂ and air; and

[0062] FIG 22 shows a thermogravimetric analysis of bulk MOF808-lysine mixed with a binder of FIG 21 under N₂ and air after a 6-month aging period.

[0063] FIG 23 is an image showing a heat exchanger structure before coating. FIG 24 is an image showing the heat exchanger structure shown in FIG 23 dip coated with a coating mixture comprising NbOFFIVE-1-Ni (KAUST-7) and IPA.

[0064] FIG 25 is an image showing details of the coating on the dip coated heat exchanger shown in FIG 24.

[0065] FIG 26 is an image showing a drying system for drying the heat exchanger structure shown in FIG 24 between the deposition of each layer of NbOFFIVE-1-Ni (KAUST-7).

[0066] FIG 27 is an image of a Fin Tube Radiator (FTR) coated with KAUST-7. FIG 28 is an image of the Fin Tube Radiator (FTR) coated with KAUST- 7 shown in FIG 27.Skilled addressees will appreciate that elements in the drawings are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the relative dimensions of some elements in the drawings may be distorted to help improve understanding of embodiments of the present invention. Embodiments of the present invention may be represented schematically and / or the drawings may omit one or more features for the sake of clarity.

[0067] DETAILED DESCRIPTION

[0068] Generally, embodiments of the present invention are directed to methods of coating metal organic frameworks (MOFs) to structures and articles and structures and articles coated by such methods, in particular, for direct air capture systems, methods and apparatus for removing carbon dioxide from the atmosphere.

[0069] According to one aspect, embodiments of the present invention are directed to a method of producing a coating for a structure, the method comprising mixing a metal organic framework (MOF) with a solvent to form a coating mixture.

[0070] According to another aspect, embodiments of the present invention are directed to a method of coating a structure with a metal organic framework (MOF), the method comprising mixing a metal organic framework (MOF) with a solvent to form a coating mixture and coating the structure with the coating mixture.

[0071] According to a further aspect, embodiments of the present invention are directed to a structure coated with a coating mixture formed from a metal organic framework (MOF) mixed with a solvent.

[0072] In some preferred embodiments, the structure is a metal, or comprises metal. However, it is envisaged that the coating methods of the present invention will work with non-metallic structures and surfaces, such as, but not limited to plastics, amorphous solids, such as glass, organic structures, such as wood, masonry such as brickwork and blockwork, ceramic monoliths.In some preferred embodiments, the structure is a metal selected from the following: aluminium; magnesium; copper; silver; gold; iron; steel. In particularly preferred embodiments, the structure is made from aluminium.

[0073] In some preferred embodiments, the structure is a honeycomb structure. In particularly preferred embodiments, the structure is an aluminium honeycomb structure comprising a plurality of tessellating, hollow, hexagonal shaped channels, which provides a large surface area per unit volume to maximize the surface are for absorption, in particular for the absorption of carbon dioxide (CO₂) from the atmosphere for direct air capture (DAC) systems, methods and apparatus.

[0074] In some preferred embodiments, the MOF is NbOFFIVE-1-Ni (KAUST- 7).

[0075] In some preferred embodiments, the MOF is MOF-808 or any functionalised variants of MOF-808.

[0076] In some embodiments, the MOF is selected from any class of MOF which exhibits static electric fields within the crystal structure and / or clumps together into groups of crystals

[0077] In some preferred embodiments, the solvent is selected from the following: water; acetone; isopropyl alcohol (IPA); ethanol; methanol; methyl cyanide (MeCN) (acetonitrile).

[0078] In some preferred embodiments, the method includes subjecting the coating mixture to stirring and / or high frequency sound waves, in particular ultrasonic waves, prior to and / or during coating of the structure with the coating mixture.

[0079] In some preferred embodiments, coating the structure with the coating mixture includes one or more of the following: dipping the structure in the coating mixture; spraying the structure with the coating mixture.

[0080] In some preferred embodiments, the method includes repeating the coating step. In particular, the method includes dipping the structure in the coating mixture or spraying the structure with the coating mixture the followingnumber of times: 2; 3; 4; 5; 6; 7; 8; 9; 10; more than 10 times. The coating is allowed to dry before repeating the dipping or spraying process.

[0081] In some embodiments, the method includes repeating the coating step by alternately dipping the structure in the coating mixture followed by spraying the structure with the coating mixture, or spraying the structure with the coating mixture, followed by dipping the structure in the coating mixture. The coating is allowed to dry before repeating the dipping or spraying process. In some embodiments, multiple coatings are produced by dipping followed by producing multiple coatings by spraying and / or vice versa. For example, the coating step can include producing two coats by dipping followed by producing two coats by spraying.

[0082] In some preferred embodiments, mixing of the metal organic framework (MOF) with the solvent to form the coating mixture and / or coating the structure with the coating mixture are conducted at atmospheric pressure and ambient temperature. However, the mixing and / or coating processes can be carried out at pressures other than atmospheric, and at temperatures other than ambient. For example, in some embodiments, the temperature of the coating mixture is elevated by any suitable means before and / or during the coating process. For example, the coating mixture can be elevated to around 45° and maintained around this temperature for the coating process.

[0083] In some preferred embodiments, the method includes evaporating the solvent after the coating step to avoid leaving a residue of the solvent which could affect the adsorption function of the MOF.

[0084] In some embodiments, evaporation of the solvent is permitted to occur naturally or unassisted at ambient temperature. In other embodiments, evaporation of the solvent is assisted with the application of heat. In some embodiments, evaporation of the solvent is assisted by increasing the flow rate of ambient air over the coated structure, for example, with an exhaust fan or a table top fan.

[0085] According to another aspect, embodiments of the present invention are directed to a honeycomb structure comprising an adsorbent material to adsorb carbon dioxide (CO₂) from the atmosphere, wherein the adsorbent material isin the form of a coating made from mixing a metal organic framework (MOF) with a solvent.

[0086] In some preferred embodiments, the honeycomb structure is housed within at least one enclosure of a direct air capture (DAC) apparatus to remove carbon dioxide (CO₂) from the atmosphere.

[0087] In some preferred embodiments, the coating mixture is produced and applied to the structure without the use of a binder, adhesive or the like thus avoiding the aforementioned problem of the pores of the MOF becoming blocked or partially blocked with the binder, adhesive or the like.

[0088] Further details of embodiments of the invention and examples will now be provided with reference to the accompanying drawings, images and graphs.

[0089] Samples of a metal organic framework (MOF) in the form of NbOFFIVE-1~Ni (KAUST-7) were synthesized by adding a 0.7 M solution of pyrazine (4004.5 mg in 71.428 mL H₂O) to a 0.7 M solution of NiNbOF5·6H₂O (9267.5 mg in 35.71 mL H₂O) and stirred for 72h at 700rpm. The resulting product, shown in FIG 1, was filtered, washed with methanol (MeOH), and left to air dry. Other methods can also be used for the synthesis of NbOFFIVE-1- Ni (KAUST-7) or other MOFs such as MOF808-lysine.

[0090] FIG 2 is a graph showing uptake of CO₂ at various pressures for five different samples of the synthesised NbOFFIVE-1-Ni (KAUST-7), samples CLM158E, CLM158H, CLM158I, CLM158K and CLM158L, measured using a 3Flex adsorption analyser at 298K. The uptake of CO2 at a pressure of 4x1 O’4atm for the samples CLM158E, CLM158H, CLM158I, CLM158K and CLM158L are shown in Table 1 below:

[0091] Sample Uptake, cm3 / g Uptake mmol / g CLM158E 18.6 0.83

[0092] CLM158H 22.9 1.02

[0093] CLM158I 18.8 0.84

[0094]

[0095] CLM158K 20.9 0.93

[0096] CLM158L 20.9 0.93

[0097]

[0098] Table 1

[0099] To prepare the coating solution, a set mass of MOF (NbOFFIVE-1-Ni (KAUST-7)) was suspended in a polar solvent beyond its solubility limit to form a supersaturated solution otherwise referred to as a coating mixture or a coating solution. Examples of the polar solvents trialled include water, acetone, isopropyl alcohol (IPA), ethanol (EtOH), methanol (MeOH) and acetonitrile (MeCN). Other MOFs, such as Zeolite 13X, were considered for coating structures for CO2 adsorption.

[0100] In accordance with some embodiments of the present invention, coating the structure with the coating mixture includes spraying the structure in the coating mixture, otherwise referred as spray coating. In one example, 5cm x 5cm aluminium plates were spray coated with the different coating mixtures of NbOFFIVE-1-Ni (KAUST-7) dissolved in different polar solvents mentioned above. The spray coating method involves spraying the corresponding coating mixture, at an angle of about 30 degrees until 5m L of the coating mixture was used up. About 5 cycles of spraying were performed. Each coat was allowed to dry before the next coat was applied. FIGS 3A to 3F illustrate the results of spray coating of six 5cm x 5cm aluminium plates with a coating mixture in the form of NbOFFIVE-1-Ni (KAUST-7) mixed with six different solvents: water, acetone, isopropyl alcohol (IPA), ethanol (EtOH), methanal (MeOH) and acetonitrile (MeCN). The spray coating solution were prepared by suspending NbOFFIVE-1~Ni (KAUST-7) in solvents with a concentration of 60g / L and sonicated for 1.5h to produce a coating mixture.

[0101] The amount of NbOFFIVE-1-Ni (KAUST-7) deposited by the spray coating method on the aluminium plates and the calculated layer thicknesses for each of the polar solvents used are shown in Table 2:

[0102] Solvent Amount deposited, mg Calculated layer thickness, pm

[0103]

[0104] Isopropanol 12.1 2.3 Methanol 10.8 2.1

[0105] Water 14.9 2.9

[0106] Ethanol 13.8 2.7

[0107] MeCN 14.2 2.8 Acetone 14.3 2.8

[0108]

[0109] Table 2

[0110] In accordance with some other embodiments of the present invention, coating the structure with the coating mixture includes dipping the structure in the coating mixture, otherwise referred as dip coating. In the following examples, pieces or sheets of aluminium foil 2.5cm x 4cm were partially dipped in different coating mixtures made from mixing NbOFFIVE-1-Ni (KAUST-7) with different polar solvents.

[0111] FIGS 4A to 4F show scanning electron microscope (SEM) images of the coatings of the sheets of aluminium foil achieved by dip coating using different polar solvent such as isopropanol, acetone, acetonitrile (MeCN), methanol, ethanol and water as shown in FIGS 3A to3F. The aluminium foil coated with a coating mixture comprising the MOF NbOFFIVE-1~Ni (KAUST- 7) in isopropyl alcohol (IPA) appears to provide the smoothest and most uniform coating compared with the other solvents used.

[0112] The effect of different concentrations of the MOF NbOFFIVE-1-Ni (KAUST-7) and the number of dips on the thickness of the coating was also investigated. Five sheets or pieces of aluminium foil 2.5cm x 4cm were used for each test. NbOFFIVE-1-Ni (KAUST-7) was mixed with isopropyl alcohol (IPA) to form a suspension at a “low” concentration of 20g / L and a “high” concentration of 60g / L. The sheets of aluminium were dipped 10 times and 20 times allowing drying between each dip. Table 3 below shows the parameters used for each sample:Experiment code NbOFFIVE-1-Ni Number of dips

[0113] cone, g / L

[0114] CLM208A 60 10

[0115] CLM208B 60 20

[0116] CLM207C 20 10

[0117] CLM207D 20 20

[0118]

[0119] Table 3

[0120] FIG 5 is a graph showing the CO2 uptake at various pressures for three of the samples shown in table 3 and the bulk synthesised NbOFFIVE-1-Ni (KAUST-7). The measurements have been made using a 3Flex adsorption analyser at 298K.

[0121] FIG 6 shows the graph of FIG 5 at a smaller range of pressures.

[0122] FIGS 5 and 6 illustrate that the CO2 uptake of the dip coated sample remains within the same range as the CO2 uptake of the bulk synthesised NbOFFIVE-1-Ni (KAUST-7). Therefore, the performance of the NbOFFIVE-1- Ni (KAUST-7) is not altered by the dip coating process, especially for a CO2 concentration of 400ppm. Furthermore, comparing the data from CLM208A (10 layers with a NbOFFIVE-1-Ni concentration of 60g / L) and CLM207C (10 layers with a NbOFFIVE-1-Ni concentration of 20g / L) on FIGS 5 and 6 reveals that the concentration may have only a marginal impact as the uptake of CO2 measured with both samples are almost identical. However, comparing the data from CLM207D (20 layers with a NbOFFIVE-1-Ni concentration of 60g / L) and CLM207C (10 layers with a NbOFFIVE-1-Ni concentration of 20g / L) on FIGS 5 and 6 shows that increasing the number of layers substantially increases the uptake of CO2.

[0123] In another experiment, a larger aluminium plate (15cm x 5cm) was spray coated to deposit more MOF on the structure to enable volumetric CO2 measurement. FIG 7 shows a graph of the uptake of CO2 at various pressures for four samples having different coating concentrations of NbOFFIVE-1~Ni (KAUST-7), three of which have a different number of coating dips and one orwhich is spray coated, compared with bulk synthesised NbOFFIVE-1-Ni (KAUST-7) measured using a 3Flex adsorption analyser at 298K. The parameters relating to the different samples are shown in the table 4 below:

[0124] Experiment code NbOFFIVE-1-Ni Number of dips

[0125] cone, g / L

[0126] CLM-01-208 A 60 10

[0127] CLM-01-207 C 20 10

[0128] CLM-01-207 D 20 20

[0129] AAB705B spray 60 Spray coated as

[0130] coated descripted above

[0131]

[0132] Table 4

[0133] FIG 8 shows the graph of FIG 7 at a smaller range of pressures.

[0134] FIGS 7 and 8 illustrate that the CO2 uptake performance of a spray coated sample is comparable to the dip coated samples and the bulk synthesised NbOFFIVE-1-Ni (KAUST-7) sample, as shown in experiment / sample CLM-158H above.

[0135] A number of different techniques were tested in some example coating methods where the MOF was in the form of NbOFFIVE-1-Ni (KAUST-7) or MOF808-lysine to investigate coating efficiencies. The structure coated was in the form of an aluminium honeycomb structure of the type shown in FIG 9. The aluminium honeycomb structure comprised 22 rows and 36 columns of tessellating, hollow, hexagonal shaped channels each having a perimeter of 3.6cm and a height of 10cm resulting in a total surface area of the structure of 2.8(5)m2.

[0136] To create a 2pm coating requires a mass of 12g of NbOFFIVE-1-Ni (KAUST-7):

[0137] 2.8m2*2*10'6m = 5.7*10’6m3

[0138] 5.7*1 O’6m3*2062kg / m3(NbOFFIVE-1-Ni density) = 11.7 *10’3kg = 12 gFor one dip coating method 58 mg of the MOF was deposited out of 500 mg of the MOF used for a suspension, which is approximately 12 wt.% of material deposited. For a spray coating method performed in a fume hood for safety reasons, 12 mg of the MOF was deposited out of 100 mg used for a suspension, which is 12 wt.% of material deposited. It is suspected that some of the spray was being blown away from the sample by the constant air flow in the fume hood during the deposition process, thereby reducing the mass of MOF being deposited. In another spray coating method performed in a shielding container in the form of a box without a constant air flow, 48 mg of the MOF was deposited out of 100 mg used for a suspension, which is 48 wt.% of material deposited. This method thus resulted in 4 times more material being deposited than the previous method.

[0139] Therefore, for the coating methods above, the following masses of MOF were required to coat the aforementioned structure in coating 2pm thick:

[0140] Dip coating: 12g / 12% wt.% = 100g of the MOF

[0141] Spray coating (in a fume hood): 12g / 12% wt.% = 100 g of the MOF Spray coating (in a box): 12g / 48% wt.% = 25g of the MOF.

[0142] Structures coated according to embodiments of the present invention were tested using isometric gravimetric analysis (IGA) over 100 cycles of temperature swing. FIG 10 is a graph illustrating the CO₂ uptake of a structure coated according to an embodiment of the present invention over 32 cycles of temperature swing. An aluminium honeycomb structure as described above was coated with a coating mixture comprising NbOFFIVE-1-Ni (KAUST-7) powder mixed with isopropyl alcohol (IPA) as a solvent. Remarkably, FIG 10 illustrates a constant CO2 uptake at a pressure of 1 bar of 9.4mg over 32 cycles of temperature swing (with adsorption at 27°C, CO2 1 bar and desorption at 60°C, N2 1 bar) with no sign of degradation of the MOF adsorption performances.

[0143] Regarding the structure in the form of an aluminium honeycomb structure of the type shown in FIG 9, each sample washed with isopropyl alcohol (IPA) and dried in an oven to clean the surface.Some of the samples of the aluminium honeycomb structure used to compare coating mixtures and processes had an end face area 5.5cm x 5.5-6.0cm. Such a sample after cleaning and prior to coating is shown in FIG 11.

[0144] For some samples, 1 g of the mixture of MOF808-lysine was sonicated for 2h in 50mL IPA. The spray coating process of MOF808-lysine and the result of spray coating of MOF808-lysine is shown in FIGS 12A-12C. In this example only 68mg of the of MOF808-lysine was coated and the process is time consuming at this scale.

[0145] For some other samples, 12g of the mixture of MOF808-lysine was sonicated for 1h in 600mL of isopropyl alcohol (IPA) and then stirred for 1h. A sample as shown in FIG 11 was dip coated in the MOF808-lysine mixture. Each dip in the mixture lasted for 30s. The coating was allowed to dry for 10 min after each dip and before a further dip. This process resulted in 575.4mg of the MOF coating the sample or an absolute amount of 4.5% (0.52g of 12g) with a coating thickness of approximatively 2pm. The dip coating process of MOF808-lysine is shown in FIG 13 and the result of 10 dips via the dip coating process is shown in FIGS 14A and 14B.

[0146] The spray coating methods produced a more homogeneous coating of the structure, with an absolute amount deposited of 6.8% (68mg of 1g) with a coating thickness of approximately 0.3pm. However, spray-coating was timeconsuming at this scale, and recovering the unused MOF powder was found to be harder. It was also found that the coating produced by dip coating was less homogeneous than the coating produced by spraying. However, while spray coating could be used for coating small intricate parts such as the honeycomb structure shown in FIG. 11, the spray coating method becomes impractical for larger honeycomb structures shown in FIG 9. The dip coating method is not subjected to the same limitation and can be applied with ease to deposit a coating regardless of the size of the honeycomb structure.

[0147] FIG 15 shows the results of dip coating structures in the form of aluminium bars for various times for coating mixtures that were stirred and / or sonicated. The coating mixture comprises a MOF in the form of NbOFFIVE-1-Ni (KAUST-7) and IPA at a concentration of 100g / L. The aluminium bars wereall coated with 7 layers via dip coating and dried with a tabletop fan. Larger particles on the coating surface were observed when the MOF bath was both stirred and sonicated, as shown in FIG 15a. When the bath was either stirred or sonicated, the surface of the coating was found to be smoother and more even, as shown in FIGS 15b and 15c. However, the sonicated bath started to remove the coating after multiple rounds of dip coating. The loss of MOF from the coating was more significant when the dipping time was increased, as shown in FIG 15d, which shows the results of dipping for 5 seconds.

[0148] Generally, fast dipping, meaning less than 1 second, and a stir only solution bath was found to give a smooth and substantially flawless coating, as shown in FIG 15b.

[0149] In another example, a structure in the form of a honeycomb cartridge made of aluminium was dip coated using a solution of MOF808-lysine and IPA at a concentration of 100g / L. The solution was sonicated until the MOF was completely dispersed in the solvent to form a saturated suspension at a temperature ranging from 40°C to 45°C. Prior to dipping the honeycomb cartridge, the sonicating bath was turned off to prevent any adverse effect the sonication may have on the deposited layer whilst maintaining the temperature of the MOF808-lysine solution to facilitate the subsequent drying of the deposited layer. It was also found that maintaining the MOF808-lysine solution at a temperature ranging from 40°C to 45°C can improve the evenness of the MOF dispensed. The honeycomb cartridge was quickly dipped and removed from the MOF808-lysine solution as described above and gently shaken to remove any excess of the MOF808-lysine solution from a honeycomb cartridge. The honeycomb cartridge was subsequently blow dried with an exhaust fan producing an air flow propagating within the channels of the honeycomb cartridge. The same process can be repeated multiple times to increase the number of deposited layers thereby increasing the potential CO2 uptake. The coating looked smooth and even. Whilst it has been found that up to 25 layers could be deposited using this dip coating method, potentially more layers could be deposited before the coating starts becoming rough or flaky.FIG 16 shows a powder X-ray diffraction (PXRD) graph of a MOF in the form of NbOFFIVE-1~Ni (KAUST-7) in various solvents. The PXRD results showed that NbOFFIVE-1-Ni (KAUST-7) is stable in the following solvents: acetone; water; and IPA. This is evident from the lack of peak broadening in the distinctive peak at around 13°.

[0150] In another example, a coating mixture or solution was produced by mixing 1g of NbOFFIVE-1-Ni (KAUST-7) in 10mL of IPA or an effective concentration of 100g / L. A structure in the form of 5 mmx5 mm aluminium foil of mass 1.1 mg was dipped in the NbOFFIVE-1-Ni (KAUST-7) solution. 25 layers were coated (sample not shown). FIG 17 shows a thermogravimetric analysis of the aluminium foil coated with the coating mixture comprising NbOFFIVE-1-Ni (KAUST-7) and IPA under N2and air. The 1stcycle was run under N2: Mass sample: 4.137 mg; Mass sample + N2: 4.194 mg; Mass MOF: 3.037 mg; Mass N2: 0.057 mg. The 2ndcycle comprised adsorption under air and desorption under N2 (110 °C):_Mass sample: 4.134 mg; Mass sample + Air: 4.243 mg; Mass Air: 0.109 mg; Mass CO₂:0.0634 mg; Wt%: 2.09 wt%; mmol / g: 0.475 mmol / g; Mass sample after desorption: 4.132 mg; % Sample regenerated: 100 %; Surprisingly, this actually corresponds to a decrease in uptake compared to the uncoated powder, which showed an uptake of 0.84 mmol / g.

[0151] In a further example, a coating mixture or solution was produced by mixing 1g of MOF808-lysine (MOF-3) in 10mL of IPA or an effective concentration of 100g / L. A structure in the form of 5 mmx5 mm aluminium foil of mass 1.5 mg was dipped in the MOF / IPA solution (sample not shown). 15 layers were coated. FIG 18 shows a thermogravimetric analysis of the aluminium foil coated with the coating mixture of MOF808-lysine / IPA under N2and air. The 1stcycle was run under N2: Mass sample: 5.714 mg; Mass sample + N2: 5.813 mg; Mass MOF: 4.214 mg; Mass N2: 0.099 mg; Wt%: 2.35 wt%; mmol / g: 0.839 mmol / g; Mass sample after desorption:5.705 mg; % Sample regenerated: 100 %. The 2ndcycle adsorption under air and desorption under N2 (110°C): Mass sample: 5.705mg; Mass sample + Air: 5.801 mg; Mass Air: 0.083 mg; Mass CO₂:0.0038 mg; Wt%: 0.09 wt%; mmol / g: 0.020 mmol / g; Mass sample after desorption: 5.700 mg; % Sampleregenerated: 100 %. This result is in agreement with the lower end of the CO2 uptake monitored for MOF808-lysine (MOF-3). NMR digestions of this material show that lysine is still present at the expected ratio. This material had been sitting in ambient air for a long time and possibly needed a more intensive activation procedure to fully regenerate the material.

[0152] In a further example, a honeycomb structure was dipped in a coating mixture in the form of a solution comprising NbOFFIVE-1-Ni (KAUST-7) and IPA with a concentration of 100g / L. A bath of the solution was maintained at 40°C and stirred every time before dipping. The IPA was topped up after 5thand 10thcycle to account for IPA evaporation overtime. An uncoated honeycomb structure had a mass of 1071,49g, and with a handle a mass of 1472.07g. The honeycomb structure was dipped 12 times resulting in a coated honeycomb structure (without the handle) of mass 1158.27g, i.e. a net mass of 86.78g of the MOF coating the structure. The results of each cycle are shown in TABLE 5 below:

[0153] Cycle Weight with handle (g) MOF coated (g) MOF mass increased (g) 1 1483.17 11.1 11.1

[0154] 2 1490.55 18.48 7.38

[0155] 3 1498.72 26.65 8.17

[0156] 4 1502.6 30.53 3.88

[0157] 5 1511.4 39.33 8.8

[0158] 6 1519.4 47.33 8

[0159] 7 1524.3 52.23 4.9

[0160] 8 1532.11 60.04 7.81

[0161] 9 1540.42 68.35 8.31

[0162] 10 1545.69 73.62 5.27

[0163] 11 1554.94 82.87 9.25

[0164] 12 1561.97 89.9 7.03

[0165]

[0166] TABLE 5Prior to the 7thcoating cycle an emulsifier was used to resuspend the MOF in the IPA. This did not seem as effective as stirring the solution with the large rod.

[0167] FIG 19 is an image showing three honeycomb structures coated with a coating mixture comprising NbOFFIVE-1-Ni (KAUST-7) and IPA at 100g / L FIG 20 is an image showing a honeycomb structure coated with a coating mixture comprising MOF808~lysine (MOF-3) and IPA at 100g / L.

[0168] Hence, the methods and structures according to embodiments of the present invention address, or at least ameliorate one or more of the aforementioned problems of the prior art and provide a useful commercial alternative.

[0169] Thermogravimetric analysis of bulk MOF808-lysine (MOF-3) mixed with a binder under N2and air is shown in FIG 21. The same thermogravimetric analysis repeated after a 6-month aging period of the same bulk MOF8O8- lysine (MOF-3) mixed with a binder under the same conditions is shown in FIG 22. Whilst the initial CO2 uptake of the bulk MOF808-lysine (MOF-3) mixed with a binder was 0.584 wt%, after a 6-month aging period, the CO2 uptake dropped to 0.14 wt%, thereby demonstrating the adverse effects of the binder on the performance of the MOF808-lysine (MOF-3) overtime.

[0170] Besides the honeycomb structure which could be dip coated with MOF as shown in FIGS 11 to 14B, other structures could be used. In another example, FIG 23 shows an image showing a heat exchanger structure before coating whilst FIG 24 shows the heat exchanger structure shown in FIG 23 dip coated with a coating mixture comprising NbOFFIVE-1-Ni (KAUST-7) and IPA described above. The heat exchanger typically comprises a plurality of hollow cells, similarly to the honeycomb structure previously described, to increase the heat transfer efficacy. FIG 25 shows details of the coating on the dip coated heat exchanger shown in FIG 24. As previously described, a bath of the coating solution (KAUST-7 dissolved in IPA with a concentration of 100g / L) was stirred and sonicated for 30 minutes at 45°C. The heat exchanger was repeatedly dipped into the bath, drained and dried to increase the number of deposited layers. The drying system for drying the heatexchanger between each coating is shown in FIG 26. In FIG 26, the air expelled from the drying system is at room temperature. However, using air at temperature above room temperature could be envisioned to further increase the solvent’s evaporation rate and therefore reduce the time required for drying the applied coating. Furthermore, the air flux could be directed downward as shown in FIG 26 or in any other directions.

[0171] Number Total mass of the Mass increase Comments

[0172] of dip heat exchanger (g) (g)

[0173] 0 1265.5

[0174] 1 1293.0 27.5 small pebbles picked up on 1st dip

[0175] 2 1299.8 6.8

[0176] 3 1305.7 5.9

[0177] 4 1311.6 5.9 clumping observed

[0178] 5 1320.8 9.2 short dry, residual IPA 6 1324.5 3.7 long dry, more evaporation 7 1331.2 6.7 accumulation observed in fins

[0179] 8 1335.4 4.2

[0180]

[0181] TABLE 6

[0182] Table 5 shows the measured mass of the heat exchanger prior coating and after each dip coat with a total of 8 layer of MOF deposited. The average mass increase, which correspond to the amount of deposited MOF, after each coating is 6g whilst the total amount of MOF is 70g. It is noted that the first dip coat resulted in a larger mass increase which is attributed to small pebbles of unproperly dissolved MOF. Furthermore, as shown in FIG 25, some MOF accumulation occurred between fins and at the apex. Nevertheless, FIGS 24 and 25 demonstrate that the MOF coating could be applied on structures with different geometries.

[0183] In another example, multiple layers of KAUST-7 were deposited on a larger Fin Tube Radiator, with a surface of about 15m2, as shown in FIGS 27 and 28 to further demonstrate the applicability of the dip coating method tolarger structures. A 60g / L solution of KAUST-7 dissolved in IPA was used as coating solution. The coating solution was agitated with an overhead stirrer before the coating. The Fin Tube Radiator was lowered into the coating solution and then raised above the solution, shaken on all sides, and then dried by an industrial fan. Once dry, the Fin Tube Radiator was then lowered on a bath of IPA for further rinsing. Each coat took about 10 minutes and a total of 18 coats were applied resulting in 250 g of KAUST-7 successfully deposited onto the Fin Tube Radiator. The same process was repeated multiple time on identical Fin Tube Radiator, yielding a coated mass of KAUST-7 ranging from 200g to 340g. It would be appreciated that both the honeycomb structure and heat exchanger discussed above as well as the Fin Tube Radiator of the present example are essentially structures with a repeating array of hollow cells. Any structures with a repeating array of hollow cells could be coated with a MOF using the method described above.

[0184] In yet another example, attempts were made to recover the MOF coating deposited on honeycomb structure to investigate the durability of the deposited MOF coating using the dip coating method. To that extent, a MOF808-lysine (MOF-3) coated honeycomb structure was immersed in an IPA bath at 45°C and sonicated for 30 minutes. It was observed that far less than expected MOF808-lysine (MOF-3) was removed from the honeycomb structure. Whilst this shows that recovery of the deposited MOF808-lysine (MOF-3) using the dip-coating method may not be a viable option, it also demonstrates the resilience of the MOF808-lysine (MOF-3) coating once deposited on the dip coating method discussed above.

[0185] In a further example, the metallic surface upon which the MOF coating is deposited was pre-treated either with an acid (HCI) or a base (NaOH) or both to improve the efficacy of the dip coating process. Such treatment includes for example exposing the metallic surface to an acid followed by a base, exposing the metallic surface to a base followed by an acid and any other sequential exposure of the metallic surface to an acid and a base.

[0186] In contrast, the coating mixture of the present invention is produced and applied to the structure without the use of a binder, binding agent,adhesive or the like thus avoiding the aforementioned problem of the pores of the MOF becoming blocked or partially blocked with the binder, binding agent, adhesive or the like.

[0187] Also, use of the solvent, which evaporates after coating the structure leaving no residue of the solvent behind, and in some embodiments, stirring and / or the use of high frequency sound waves, in particular ultrasound, to uniformly suspend MOF particles throughout the solvent prior to application of the coating mixture, counteracts the propensity of MOFs to clump together, thus “un-clumping” the MOF in an effort to maximize the surface area of the MOF available for CO₂ adsorption.

[0188] Hence, the coating methods and structures coated according to embodiments of the present invention provide a very promising solution to the clumping problem of MOFs.

[0189] Furthermore, binders or binding agents, adhesives or the like can have a long-term detrimental effect on the MOF capacity to efficiently capture CO2. Such detrimental effect could be related to aging of the binding agents or adhesives related to their chemical instabilities which could inadvertently alter the MOF properties. Therefore, coating methods and structures coated according to embodiments of the present invention which do not use binding agents or adhesives can mitigate the limitations of the current technology and ensure longer term performances of the MOF, for example, in adsorption applications, and in particular, even more effective in DAC applications and the adsorption of CO2.

[0190] Any of the features of the methods and structures described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

[0191] In this specification, adjectives such as first and second, and the like may be used solely to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order. Where the context permits, reference to an integer or a component or step (or the like) is not to be interpreted as being limited to only one of thatinteger, component, or step, but rather could be one or more of that integer, component, or step etc.

[0192] In this specification, the terms “comprises”, “comprising” or similar terms are intended to mean a non-exclusive inclusion, such that an apparatus that comprises a list of elements does not include those elements solely but may well include other elements not listed.

[0193] Throughout the specification the aim has been to describe the invention without limiting the invention to any one embodiment or specific collection of features. Persons skilled in the relevant art may realize variations from the specific embodiments that will nonetheless fall within the scope of the invention.

Claims

CLAIMS1. A method of producing a coating for a structure, the method comprising mixing a metal organic framework (MOF) with a solvent to form a coating mixture;wherein the structure is a metal, or comprises metal; andwherein the coating mixture is produced and applied to the structure without the use of a binder, binding agent, or adhesive.

2. A method of coating a structure with a metal organic framework (MOF), the method comprising:mixing a metal organic framework (MOF) with a solvent to form a coating mixture; andcoating the structure with the coating mixture;wherein the structure is a metal, or comprises metal; andwherein the coating mixture is produced and applied to the structure without the use of a binder, binding agent, or adhesive.

3. A structure coated with a coating mixture formed from a metal organic framework (MOF) mixed with a solvent.wherein the structure is a metal, or comprises metal; andwherein the coating mixture is produced and applied to the structure without the use of a binder, binding agent, or adhesive.

4. The method or structure of any one of the preceding claims, wherein the structure is a metal selected from the following: aluminium; magnesium; copper; silver; gold; iron; steel.

5. The method or structure of any one of the preceding claims, wherein the structure is a honeycomb structure.

6. The method or structure of any one of the preceding claims, wherein the MOF is NbOFFIVE-1-Ni (KAUST-7) or MOF-808 or any functionalised variants of MOF-808 such as MOF-808-lysine / MOF-3.

7. The method or structure of any one of the preceding claims, wherein the MOF is selected from any class of MOF which exhibits static electric fields within the crystal structure and / or clumps together into groups of crystals.

8. The method or structure of any one of the preceding claims, wherein the solvent is selected from the following: water; acetone; isopropyl alcohol (IPA); ethanol; methanol; methyl cyanide (MeCN) (acetonitrile).

9. The method or structure of any one of the preceding claims, wherein the method includes subjecting the coating mixture to stirring and / or high frequency sound waves, in particular ultrasonic waves, prior to and / or during coating of the structure with the coating mixture.

10. The method or structure of any one of the preceding claims, wherein coating the structure with the coating mixture includes one or more of the following: dipping the structure in the coating mixture; spraying the structure with the coating mixture.

11. The method or structure of any one of the preceding claims, wherein the method includes repeating the coating step.

12. The method or structure of any one of the preceding claims, wherein the mixing of the metal organic framework (MOF) with the solvent to form the coating mixture and / or coating the structure with the coating mixture are conducted at atmospheric pressure and / or ambient temperature.

13. The method or structure of any one of the preceding claims, further comprising evaporating the solvent after the coating step, in particular to leave no residue of the solvent behind.

14. The method or structure of any one of the preceding claims, wherein the coating mixture is subjected to heat before and / or during the coating process, for example to raise a temperature of the coating mixture to around 45°C15. The method or structure of claim 13, wherein a rate of evaporating the solvent is increased by increasing a flow of ambient air over the coated structure.

16. The method or structure of any one of the preceding claims wherein the structure is pretreated with either an acid or a base or both.

17. The method or structure of claim 16 wherein the acid is hydrochloric acid.

18. The method or structure of claim 16 wherein the base is sodium hydroxide.

19. A structure comprising:a repeating array of hollow cells; andan adsorbent material to adsorb carbon dioxide (CO2) from the atmosphere;wherein the adsorbent material is in the form of a coating, disposed on the hollow cells, made from mixing a metal organic framework (MOF) with a solvent;wherein the structure is a metal, or comprises metal; andwherein the coating is produced and applied to the structure without the use of a binder, binding agent, or adhesive.

20. The structure of claim 19 wherein the structure is a honeycomb structure.

21. The structure of claim 19 or 20, wherein the structure is housed within at least one enclosure of a direct air capture (DAC) apparatus to remove carbon dioxide (CO₂) from the atmosphere.

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