Metal organic framework

Abstract

A porous metal-organic framework comprising metal ions coordinated to plural different linkers, wherein the first of the linkers comprises an aryl or alkyl ring and wherein a second of the linkers does not comprise an aryl or alkyl ring.

Description

[0001] METAL ORGANIC FRAMEWORK

[0002] This invention relates generally to metal-organic frameworks (MOFs). In particular, although not exclusively the invention relates to MOFs for water sorption-driven applications.

[0003] Providing fresh water to a rapidly growing world population is a global challenge, with more than half of the world expected to experience a shortage of water by the year 2050. Of all the water on earth only 2.5% is fresh water and only a small fraction of water is directly accessible in rivers and lakes (0.4%). The majority of fresh water is locked up in glaciers (68.7%) or stored in groundwater (30.1%). With two thirds of the world’s population experiencing water stress, it is unlikely that these sources of fresh water will suffice to address the global water problem in an energy efficient way.

[0004] Water in the atmosphere is a recyclable, natural resource and has the potential to provide water to arid regions of the world. However, the concentration of water in the air is low. Therefore, finding a way to trap and subsequently use this water is problematic.

[0005] Current technologies used to generate water from moist air and fog include: fog collection by means of large nets, cooling air below its dew point (the temperature at which the air is saturated with water) and sorbent-assisted water capture.

[0006] However, fog collectors require permanent high relative humidity levels and light winds to facilitate dewing on fine nets, severely restricting the geographical areas suitable for their application.

[0007] Whilst dewing has a much broader range of geographic applicability, its energy efficiency and water productivity are heavily dependent on the local climate. To initiate condensation, moist air needs to be cooled below the dew point through removal of the associated sensible heat (energy transfer required to cool down the air).

[0008] Alternatively, adsorption-based devices have a high potential for water harvesting, even from dry air, but are strongly dependent on the performance of the adsorbent. The water uptake of classical desiccants (e.g. CaCl2silica gel, or zeolites) employed in such devices is high but their strong affinity to water renders their regeneration energy intensive. It has been suggested previously that metal organic frameworks (MOFs) may be used to harvest water (MOFs are also known to be able to provide storage of gases and liquids other than water).

[0009] Accordingly, it would be beneficial to develop a MOF, and / or a method of forming MOFs, the properties of which are tuneable to allow for efficient use under different environmental conditions (e.g. temperatures, relative humidity etc.). In particular, it would be helpful to provide MOFs capable of performing many absorption / desorption cycles without deterioration, especially ones which have properties appropriate to a particular environment.

[0010] It is an object of the present invention to provide a MOF able to at least partially address one or more of the above issues.

[0011] Accordingly a first aspect of the invention provides a porous metal-organic framework comprising metal ions coordinated to plural different linkers, wherein a first of the linkers comprises an aryl or alkyl ring and wherein a second of the linkers does not comprise an aryl or alkyl ring.

[0012] Advantageously, a MOF comprising plural different linkers as specified above allows tunability of the properties of the as-formed MOF. For example, a series of MOFs having different hydrophilicities can be provided by having different proportions of linkers. Therefore, many multivariant MOFs can be formulated by varying the molar ratios of the first and second linker species. This may allow for MOFs to be provided with water adsorption and desorption properties which are tailored for specific applications and / or environments.

[0013] The first linker may comprise the following general formula: wherein A comprises a ring structure comprising at least one 5 or 6 membered ring; n1 and n2 are integers independently selected from the range of 0 to 20; and Ri and R2 are independently functional groups capable of coordinating to metal ions such as a carboxylate moiety, a carbonyl moiety, a hydroxy moiety, an amine moiety or an amide moiety.

[0014] The ring structure may comprise a single five-member ring, a single six-member ring, a five- member and six-member ring, two five-membered rings or two six-membered rings. Where there are two rings, the rings may be fused.

[0015] A may comprise a ring structure comprising at least one heteroatom, for example selected from O, S or N. The heteroatom is preferably nitrogen.

[0016] When the first linker comprises a 5-membered ring, the two side chains comprising the R1 and R2groups may be in the 3,5 position, and when the first linker comprises a 6-membered ring, the two side chains comprising the R1 and R2 groups may be in the 2,5 position.

[0017] The first linker may be selected from: wherein X1, X2, and X3are independently selected from N, NH, S or O;

[0018] Y1, and Y2, are selected from C( Rx)( RY) or C(Rx);

[0019] Rxand RYare selected from H or alkyl, e.g. lower alkyl; and n1 and n2 are independently integers selected from the range 0 to 20.

[0020] Alternatively, the first linker may be selected from: wherein at least one of Z1, Z2,, Z3, Z4, Z5and Z6is a heteroatom;

[0021] Z1, Z2, Z3, Z4, Z5and Ze are selected from N, NH, O, S, C(Rx) or C(Rx)(RY); wherein

[0022] Rxand RYare selected from H or alkyl, e.g. lower alkyl; and n1 and n2 are independently integers selected from the range 0 to 20.

[0023] Alternatively, the first linker may be selected from a linker which may have one of the following general structures:

[0024] wherein X1 ,, X2, X3 and X4 are individually selected from N , N H , S or O;Y1 , Y2, Y3 and Y4 are selected from C(Rx)(RY) or C(Rx);whereinRx and RY are selected from H or alkyl, e.g. lower alkyl; andn 1 and n2 are independently integers selected from the range 0 to 20.

[0025] For example, one or more of the linkers may have the following structure:

[0026] 5 In embodiments of the invention, the first linker may comprise two fused 6-membered rings.

[0027] The first linker may have one of the following general structures:

[0028] wherein X1, X2, X3, X4, X5and X6are selected from N, NH, O and S; Y1, Y2, Y3, Y4, Y5and Y6are selected from C(Rx)(RY) or C(Rx); wherein Rxand RYare selected from H or alkyl, e.g. lower alkyl; and n1 and n2 are independently integers selected from the range 0 to 20.

[0029] In embodiments of the invention, the first linker may comprise two fused 5-membered rings. The first linker may have one of the following general structures: wherein X1, X2, X3and X4are selected from N, NH, O and S; Y1, Y2, Y3and Y4 are selected C(Rx)(RY) or C(Rx); wherein Rxand RYare selected from H or alkyl; and n1 and n2 are independently integers selected from the range 0 to 20. The first linker may have one of the following general structures: wherein X1and X4are independently selected from NH, O and S; X2and X3are independently selected from NH, N, O and S; and n1 and n2 are independently integers selected from the range 0 to 20.

[0030] The first linker may have one of the following general structures: wherein X1, X2and X3are independently selected from N, NH, O and S; X4is selected from NH, O and S; and n1 and n2 are independently integers selected from the range 0 to 20.

[0031] For example, the first linker may have the following structure: The first linker may comprise a fused 5- and 6-membered ring.

[0032] The first linker may have one of the following general structures:

[0033] wherein X1is selected from NH, O or S;

[0034] X2, X3, X4and X5are independently selected from NH, N, O and S; Y1is C(RX)(RY); Y2, Y3, Y4, Y5and Y6are independently selected from C(Rx)(RY) or C(Rx); wherein Rxand RYare each selected from H or alkyl, e.g. lower alkyl; and n1 and n2 are independently integers selected from the range 0 to 20.

[0035] In each of the above structures of the first linker, groups R1 and R2 are shown as carboxylates. It will be appreciated that instead, in each case, R1, R2 might be independently selected from a carboxylate moiety, a carbonyl moiety, a hydroxy moiety, an amine moiety or an amide moiety.

[0036] Whilst it is possible for R1 and R2 to be different, typically they will be the same, e.g. both will be a carboxylate. Preferably, the first linker is a dicarboxylate.

[0037] The second linker may comprise the following general formula: wherein B comprises a C2-C5 hydrocarbon chain;

[0038] N1 and N2 are integers independently selected from the range of 0 to 20; and

[0039] R3and R4are independently functional groups capable of coordinating to metal ions such as a carboxylate group, a hydroxy group, a carbonyl group, an amine group or an amide group.

[0040] Whilst it is possible for R3and R4to be different, typically they will be the same, e.g. both will be a carboxylate. Preferably, the second linker is a dicarboxylate.

[0041] In the second linker B may be saturated or unsaturated. For example, B may comprise one or more C=C moieties.

[0042] Advantageously, the use of a first linker comprising an aryl group and a second linker comprising an alkyl group ensures that the first linker and second linkers have different properties, for example different hydrophilicities.

[0043] The sum of carbon atoms n1 and n2 in the first linker may be within + / - 2 carbon atoms compared to the sum of carbon atoms N1 and N2 in the second linker. The number of carbon atoms in the chain may be varied. For example n1 and n2, and / or N1 and N2 are independently integers selected from the range 0 to 15, or 0 to 10, or 0 to 5.

[0044] Accordingly, the chain lengths of the first linker and the second linker may be substantially the same, Advantageously this can reduce strain in the MOF and / or provide a consistent pore size. Alternatively, if a strained MOF is required, the lengths may be different ( / .e. the sum may be greater than + / - 2 carbon atoms).

[0045] The hydrocarbon chain length in the second linker may be X, the minimum number of atoms bridging the aryl group in the first linker may be X ± Y, where Y is selected from 1 , 2. For example, in the case of a five membered aryl group (substituted in the 3-5 positions) the number of bridging atoms is 3 whereas in the case of a six-membered aryl group (substituted in the 2-5 positions) is 4. Accordingly, in the case of a single 5 or 6 member ring in the first linker, the length of hydrocarbon chain in the second linker may be 2. By keeping Y to a low number the effective lengths of the A and B will be kept similar which may reduce strain in the MOF and / or help to provide a consistent pore size.

[0046] The metal ion of the MOF may be selected from aluminium, zirconium, nickel, iron, copper, manganese, magnesium, calcium, strontium, barium, titanium, zinc, indium, cadmium, hafnium, lead, cobalt and / or chromium.

[0047] The first linker may have the following structure:

[0048] The second linker may have the following structure:

[0049]

[0050] The porous metal-organic framework may have a Brunauer-Emmett-Teller (BET) surface area 1000 - 1600 m2g’1.

[0051] The porous metal-organic framework may have an estimated pore size of between 5 and 15 A, for example from 7 to 12 A , say from 9.0 - 10 A e.g. 9.0, 9.1 , 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9 or 10 A.

[0052] Advantageously, the MOFs of the invention may be able to house adsorbates such as water, fuel or gases (e.g. SOX, NOX, hydrocarbons, flue gases, methane, hydrogen) are capable of being housed in the pores of the porous metal-organic framework.

[0053] The porous metal-organic framework may exhibit one or more of the following: a) a fully saturated state with a water uptake of equal or greater than 45 wt% within a period of equal or less than 27 minutes at a relative humidity of 70% and at a temperature of 25°C; b) when fully saturated with water reaches full desorption within a period of equal or less than 15 minutes under dry conditions and at a temperature of 90°C; c) is fully regenerated from a state of full saturation with water molecules at temperatures in the range of 25 to 90°C, e.g. at 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 90°C; d) is capable of completing at least 16 full water sorption / desorption cycles with no decrease in the water absorption or desorption performance.

[0054] There is further provided a method of synthesising the porous metal-organic framework, the method comprising: a) providing, e.g. dissolving, two or more different linkers in a Bronsted-Lowry base, e.g. LiOH or NaOH, wherein a first linker comprises an aryl or alkyl ring and wherein a second linker does not comprise an aryl or alkyl ring; b) adding a source of metal ions to the mixture; and c) heating the mixture to produce a crystalline material.

[0055] The first linker and the second linker may have any of the structures disclosed above in respect of the first aspect of the invention.

[0056] The molar ratio of first linker and the second linker added to the Bronsted-Lowry base is in the range of 1 :20 - 20:1 , e.g. 1 :10 - 10:1.

[0057] Advantageously, by altering the molar ratio of the first linker and the second linker it is possible to alter the amounts of the linkers in the porous metal-organic framework.

[0058] The source of metal ions may be a metal salt, for example a metal halide, metal sulphate, metal nitrate, metal hydroxide, metal carbonate and so on. In an embodiment the metal may be aluminium. The source of metal ions may comprise any one of AlCl3AI2O3, AI2(SO4)3, AI(NO3)3, aluminium acetate or aluminium acetylacetonate.

[0059] Step (c) may comprise heating the mixture for 12 hours at a temperature of 100°C.

[0060] There may also be provided a further step of washing the recovered material with deionised water and methanol before heating the mixture for 12 hours at a temperature of 100°C.

[0061] In this specification the term ‘lower alkyl’ is intended to mean a branched or unbranched saturated monovalent hydrocarbon radical containing 1 to 6 carbon atoms.

[0062] In order that the invention may be more fully understood it will now be described, by way of example only, and with reference to the accompanying drawings, in which:

[0063] Figure 1 shows a fumaric acid linker used in synthesising a MOF according to the invention;

[0064] Figure 2 shows a pyrazole dicarboxylic acid (PZDC) linker used in synthesising a MOF according to the invention; Figure 3 is a graph displaying the gravimetric water adsorption and desorption isotherm of a MOF according to the invention at 25°C, 40°C and 55°C;

[0065] Figure 4 is a graph displaying the water adsorption kinetics of a MOF according to the invention;

[0066] Figure 5 is a graph displaying the water desorption kinetics of a MOF according to the invention;

[0067] Figure 6 is a graph showing the cyclic stability of a MOF according to the invention;

[0068] Figure 7 is a graph showing the volumetric nitrogen adsorption of a MOF according to the invention in powder and coated form;

[0069] Figure 8 shows scanning electron microscope (SEM) images of a MOF in powder and coated form according to the invention;

[0070] Figure 9 is a powder x-ray diffraction of a MOF according to the invention;

[0071] Figure 10 is a graph showing the thermal stability of a MOF according to the invention; and

[0072] Figure 11 is a graph showing the gravimetric water adsorption and desorption isotherm of a MOF according to the invention at 15°C, 25°C and 35°C.

[0073] Referring to Figure 1 , there is shown a fumaric acid linker used in forming a MOF according to the invention.

[0074] Referring to Figure 2, there is shown a pyrazole dicarboxylic acid (PZDC) linker used in forming a MOF according to the invention.

[0075] The fumaric acid linkers and PZDC linkers each coordinate to metal ions (M) to form a metal oxide cluster, resulting in the creation of a MOF having the general formula [M(OH)x(Fumaric)y(PZDC)z], where y and z may be determined by the molar amounts used in the synthesis.

[0076] Synthesis of MOFs

[0077] 4 mmol of fumaric acid and 4 mmol of PZDC were completely dissolved in 0.5 M aqueous LiOH solution (750 mL, 1.5 equivalents) in a 1000 mL screw-capped container. 4 mmol of AlCl3was introduced into the clear mixture, which was then heated for 12 hours in a preheated oven at 100 °C. The resultant white solid was then subjected to triple washes with deionised water and methanol. The MOF was dried by heating at 120 °C over a period of 12 hours, resulting in a pure, activated product with a 90% yield. In this case y and z were equal.

[0078] The synthesis method was repeated, varying the molar ratios of the linkers as set out in the table below to synthesise 2 distinct MOFs:

[0079] It will be appreciated by those of skill in the art that the invention is not limited to the ratios of linkers shown in the table above. The use of the linkers shown in Figure 1 and 2 may be used to synthesise multivariant MOFs covering the entire mixing range.

[0080] The MOFs were synthesised without centrifugation, densification, applied pressure, or binders.

[0081] Experimental Methods

[0082] Powder X-ray diffraction (PXRD) data was collected at room temperature on a D8 Bruker X-ray powder diffractometer, (CuKal radiation, A = 1.54056 A) equipped with a CuKal X- ray source operating at 40 kV and 40 mA. The PXRD pattern was collected over a 20 range of 20-50 degrees and over a period of 30 minutes.

[0083] The water sorption and desorption characteristics of the MOFs were measured using a dynamic vapor sorption (DVS) gravimetric analyser (Advantage DVS, Surface Measurement Systems, UK). The DVS device consists of a sealed chamber that can be controlled to maintain a specific relative humidity and temperature. The device was used to measure the weight changes of the MOF samples as a function of time, relative humidity and temperature. This allowed the mass of water vapour absorbed and desorbed onto the samples as a function of relative pressure and temperature to be measured and the kinetics of water adsorption and desorption and the cyclic performance of the MOFs to be determined. To analyse the MOF samples, 15-20 g of the samples were placed in the sample pan of the DVS analyser. The measurements were carried out at a relative humidity range of 0 to 0.9 and at a number of different temperatures ranging from 15°C to 55°C. Examples

[0084] In order for the invention to be more fully understood, reference is made to the following non-limiting examples. Examples 1 - 7 relate to a MOF synthesised using a fumaric acid to PZDC ratio of 7:3 (which we identify as NOV-4) . Example 8 relates to a MOF synthesised with a fumaric acid to PZDC ratio of 3:7.

[0085] Example 1 :

[0086] Figure 3 shows a graph of the gravimetric water adsorption and desorption of a MOF Nov- 4 at a relative humidity (RH) of up to 90%, and at temperatures of 25°C (blue) 40°C (green) and 55°C (red). At 25°C, the inflection point of steep water uptake of the MOF is observed at 24% relative humidity. This inflection point is seen to increase to 30% relative humidity at 40°C and to 35% relative humidity at 55°C.

[0087] Example 2:

[0088] Figure 4 shows a graph of the adsorption kinetics of a MOF Nov-4 at a relative humidity of 70% and a temperature of 25 °C. The MOF displays rapid adsorption kinetics, reaching a fully saturated state with a water uptake of 45 wt% within 30 minutes.

[0089] Example 3:

[0090] Figure 5 shows a graph of the desorption kinetics of a MOF Nov-4 at a temperature of 90°C under dry conditions (RH = 0%). The MOF displays rapid desorption kinetics, reaching full desorption within 17 minutes.

[0091] Example 4:

[0092] Figure 6 shows a graph of the cyclic stability of a MOF Nov-4 at a relative humidity of 70% and an adsorption temperature of 25°C. The MOF was contacted with air (for ~30 minutes) until it became fully saturated with water vapour (45 wt%). The MOF was then heated at a temperature of 90°C (for ~17 minutes) until full desorption of the captured water was observed. As a result of the short cycle time, the MOF is able to complete 16 adsorption / desorption cycles within 860 minutes. The MOF was fully regenerated with no decrease in performance over 16 cycles. Example 5:

[0093] Figure 7 shows a graph of the volumetric uptake of nitrogen gas of a MOF Nov-4 in powder form (red) and coated form (black). The volumetric nitrogen uptake of the MOF in the coated form is only marginally lower than that of the MOF in the powder form. This demonstrates that the porosity and surface area of the MOF is retained when the powder is made into a coating. Figure 8 shows SEM images of the MOF in powder form (8a) and coated form (8b).

[0094] The apparent surface area of the MOF Nov-4 was determined from nitrogen adsorption isotherms collected at 77 K on a Micromeritics Tristar II 3020 with the temperature held constant using a liquid nitrogen bath.

[0095] Brunauer-Emmet-Teller (BET) surface area measurements revealed a substantial surface area of 1310 m2g’1. The estimated pore size of the two distinct MOF compounds were similar and ranged from 9.1-9.8 A.

[0096] Example 6

[0097] Figure 9 shows a powder x-ray diffraction (PXRD) pattern of the neat nanoparticles of a MOF Nov-4. The sharp peak observed at 10 theta shows the presence of a pure phase (red) which is also observed after soaking the MOF for a period of 4 days (black).

[0098] Example 7

[0099] As shown in Figure 10, thermal stability testing of a MOF Nov-4 was carried out using thermogravimetric analysis (TGA). The MOF displayed no discernible weight loss due to structural decomposition up to approximately 400°C, with the weight loss observed below this temperature being attributed to contaminate desorption.

[0100] Example 8

[0101] Figure 11 shows a graph of the gravimetric water adsorption and desorption of a MOF synthesised with a fumaric acid to PZDC ratio of 3:7 at a relative humidity (RH) of up to 90%, and at temperatures of 15°C (blue) 25°C (yellow) and 35°C (black). At 15°C, the inflection point of steep water uptake of the MOF is observed at 12% relative humidity. This inflection point is seen to increase to 14% relative humidity at 25°C and to 17% relative humidity at 35°C. The inflection point of steep water uptake of the MOF at 25°C occurs at a relative humidity value that is 10% lower than the MOF Nov-4 (as shown in Figure 3). This demonstrates the tunability of the hydrophilic properties of the MOFs by varying the ratio of relatively hydrophobic to relatively hydrophilic linkers.

[0102] In alternative embodiments, the MOF synthesis may comprise AI2O3, AI2(SO4)3e.g. AI2(SO4)3■ 8H2O, AI(NO3)3, aluminium acetate or aluminium acetylacetonate as the metal source. Also, salts of metals other than aluminium may be used

[0103] Adsorbates such as water, fuel or gases including SOX, NOX, hydrocarbons, methane, hydrogen and flue gases, may be housed in the pores of the MOFs.

Claims

CLAIMS1 . A porous metal-organic framework comprising metal ions coordinated to plural different linkers, wherein the first of the linkers comprises an aryl or alkyl ring and wherein a second of the linkers does not comprise an aryl or alkyl ring.

2. A porous metal-organic framework according to Claim 1 , wherein the first linker has the following structure:

3. A porous metal-organic framework according to Claim 1 or 2, wherein the second linker has the following structure:

4. A porous metal-organic framework according to any of Claims 1 , 2 or 3, wherein the porous metal-organic framework has a Brunauer-Emmett-Teller (BET) surface area 1000 - 1600 m2g’1.

5. A porous metal-organic framework according to any preceding Claim, wherein the porous metal-organic framework has an estimated pore size of between 9.0 - 10 A e.g. 9.0, 9.1 , 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9 or 10 A.

6. A porous metal-organic framework according to Claim 1 , wherein the first linker comprises the following general formula:2 R2wherein A comprises a ring structure comprising at least one 5 or 6 membered ring; n1 and n2 are integers independently selected from the range of 0 to 20; and Ri and R2 are independently functional groups capable of coordinating to metal ions such as a carboxylate moiety, a hydroxy moiety, an amine moiety or an amide moiety.

7. A porous metal-organic framework according to Claim 6, wherein A comprises either a single five-membered ring, a single six-membered ring, a five-membered and a six-membered ring, two five-membered rings or two six-membered rings.

8. A porous metal-organic framework according to Claim 6 or 7, wherein the ring structure of A comprises at least one heteroatom, for example selected from O, S or N, preferably N.

9. A porous metal-organic framework according to any one of Claims 6 to 8, wherein when the first linker comprises a 5-membered ring, the side chains comprising the R1 and R2 groups are in the 3,5 position, and when the first linker comprises a 6- membered ring, the side chains comprising the R1 and R2 groups are in the 2,5 position.

10. A porous metal-organic framework according to any of one Claims 6 to 9, wherein the first linker is selected from one of the following general formulae:wherein X1, X2, and X3are selected from N, NH, S or O; Y1, and Y2, are selected from C(Rx)(RY) or C(Rx);Rxand RYare selected from H or alkyl, e.g. lower alkyl; and n1 and n2 are independently integers selected from the range 0 to 20.

11. A porous metal-organic framework according to any one Claims 6 to 9, wherein the first linker is selected from one of the following general formulae:wherein X1, X2, X3and X4are selected from N, NH, S or O; Y1, Y2, Y3and Y4 are selected from C(Rx)(RY) or C(Rx);Rxand RYare selected from H or alkyl, e.g. lower alkyl; and n1 and n2 are independently integers selected from the range 0 to 20.

12. A porous metal-organic framework according to any one Claims 6 to 9, wherein the first linker is selected from one of the following general formulae:wherein X1is selected from NH, O or S; X2, X3, X4and X5 are selected from NH, N, O and S; Y1is C(RX)(RY); Y2, Y3, Y4, Y5and Y6are selected from C(Rx)(RY) or C(Rx); wherein Rxand RYare selected from H or alkyl, e.g. lower alkyl; and n1 and n2 are independently integers selected from the range 0 to 20.

13. A porous metal-organic framework according to any one of Claims 6 to 9, wherein the first linker is selected from one of the following general formulae:wherein X1, X2, X3and X4are selected from N, NH, O and S; Y1, Y2, Y3and Y4 are selected C(Rx)(RY) or C(Rx); wherein Rxand RYare selected from H or alky; and n1 and n2 are independently integers selected from the range 0 to 20.

14. A porous metal-organic framework according to any one Claims 6 to 9, wherein the first linker is selected from one of the following general formulae:wherein X1, X2, X3, X4, X5and X6are selected from N, NH, O and S; Y1, Y2, Y3, Y4, Y5and Y6are selected from C(Rx)(RY) or C(Rx); wherein Rxand RYare selected from H or alkyl, e.g. lower alkyl; and n1 and n2 are independently integers selected from the range 0 to 20.

15. A porous metal-organic framework according to any one of Claims 1 or 6 to 14, wherein the second linker comprises the following general formula:wherein B comprises a C2-C5 alkyl chain; n1 and n2 are integers independently selected from the range of 0 to 20; andR3and R4are independently functional groups capable of coordinating to metal ions such as a carboxylate group, a hydroxy group, an amine group or an amide group.

16. A porous metal-organic framework according to Claim 15, wherein the sum of the carbon atoms n1 and n2 in the first linker is within + / - 2 carbon atoms compared to the sum of the carbon atoms N1 and N2 in the second linker.

17. A porous metal-organic framework according to any one of Claims 1 or 6 to 16, wherein the second linker comprises one or more C=C moieties.

18. A porous metal-organic framework according to any preceding Claim, wherein the metal ions may be selected from aluminium, zirconium, nickel, iron, copper,manganese, magnesium, calcium, strontium, barium, titanium, zinc, indium, cadmium, hafnium, lead, cobalt and / or chromium.

19. A porous metal-organic framework according to any one of Claims 6 to 17, wherein the porous metal-organic framework has a Brunauer-Emmett-Teller (BET) surface area 1000 - 1600 m2g-1.

20. A porous metal-organic framework according to any one of Claims 6 to 17, wherein the porous metal-organic framework has an estimated pore size of between 9.0 - 10 A e.g. 9.0, 9.1 , 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9 or 10 A.

21. A porous metal-organic framework according to any preceding Claim, wherein adsorbates such as water, fuel or gases (e.g. SOx, NOx, hydrocarbons, flue gases, methane, hydrogen) are capable of being housed in the pores of the porous metalorganic framework.

22. A porous metal-organic framework according to any preceding Claim, which exhibits one or more of the following: a) a fully saturated state with a water uptake of equal or greater than 45 wt% within a period of equal or less than 27 minutes at a relative humidity of 70% and at a temperature of 25°C; b) when fully saturated with water reaches full desorption within a period of equal or less than 15 minutes under dry conditions and at a temperature of 90°C; c) is fully regenerated from a state of full saturation with water molecules at temperatures in the range of 25 to 90°C, e.g. at 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 90°C; d) is capable of completing at least 16 full water sorption / desorption cycles with no decrease in the water absorption or desorption performance.

23. A method of synthesising the porous metal-organic framework, the method comprising: d) providing e.g. dissolving two or more different linkers in a Bronsted-Lowry base, e.g. LiOH or NaOH, wherein a first linker comprises an aryl or alkyl ring and wherein a second linker does not comprise an aryl or alkyl ring; e) adding a source of metal ions to the mixture; andf) heating the mixture to produce a crystalline material; wherein the first linker and the second linker may have any of the structures disclosed in any preceding Claim.

24. A method according to Claim 23, wherein the molar ratio of the first linker and the second linker is in the range of 1 :20 - 20: 1.

25. A method according to any one of Claims 23 or Claim 24, wherein the source of metal ions is a metal salt, for example a metal halide, metal sulphate, metal nitrate, or metal hydroxide, for example AlCl3.

26. A method according to any one of Claims 23 to 25, wherein step (c) comprises heating the mixture for 12 hours at a temperature of 100°C.

27. A method according to any of one Claims 23 to 26, further comprising washing the crystalline material with deionised water and methanol before heating the mixture for 12 hours at a temperature of 100°C.

Images