The present invention relates to metal-organic framework (MOF) materials and in particular to methods of preparing MOFs.

MOFs are materials having a metallic ion or metallic cluster and a bridging organic ligand. The ligands act as spacers, creating an open porous three-dimensional structure. MOFs have been described in many publications in the scientific literature and patents including, for example, US 5,648,508, in which the materials are prepared by mixing together a metal compound with at least one ligand containing multidentate functional groups and optionally a ligand containing a monodentate functional group with a solvent and a templating agent (which may comprise the solvent or ligand).

We have now discovered an alternative method of making MOF materials which offers a convenient method for scale-up.

According to the invention, a method of making a metal-organic framework material comprises the steps of mixing a solution of a metal compound and a solution of at least one multidentate organic ligand compound and maintaining the mixture at a suitable reaction temperature for sufficient time to form said metal-organic framework material, wherein the solvent used in the reaction comprises either water or a mixture of water and an organic solvent.

According to the invention we also provide a crystalline MOF material which is made by, or is obtainable by, the process of the invention. According to the invention we provide a metal-organic framework (MOF) material comprising a metallic ion or metallic cluster and at least one multidentate organic ligand, wherein said MOF material has a surface area of at least 800 m ² per gram, preferably at least 1000 m ² per gram, as measured by nitrogen adsorption at 77K, using a BET method after drying the sample in a vacuum oven at 90°C and outgassing (activation) at a temperature of at least 150°C for at least 20 hours.

Any metal which is capable of bonding (including through covalent and/or coordinate and/or hydrogen bonding) to a multidentate organic ligand may be used. Suitable metals, include Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Rh, Ir, Ni, Co, Cu, Zn, Pd, Ag, Au, Pt, Cd, Hg, Al, Ga, In, TI, Si, Ge, Sn, Pb, As, Sb, and Bi. Preferred metals which have been used to synthesise MOFs include Sc, Zr, Pd, Ag, In, Be, Mg, Ca, Sr, Ba, Ga, Cu, Fe, Zn, Co, Ni, Al, Cd, Cr, Ti, Pt, Ru, Rh, Ir and Os. More than one metal or metal compound may be included in a MOF. The metal may, in particular, comprise Cu, Ni, Zn or Fe.

The metal compound may be selected from any compound which is soluble in the solvent used. Suitable compounds include inorganic metal salts, such as metal nitrates, sulphates, halogenates, and organic salts such as acetates, formates or oxalates. The multidentate ligand may be selected from many different compounds. The published prior art contains many examples of multidentate ligands which are useful for making MOFs, any of which could be used in the method of the present invention. Suitable compounds which have been used to form MOFs include alkyl, cycloalkyl, aryl or heterocyclic compounds having functional groups capable of bonding to a metal atom. The functional group may, for example be selected from carboxy, hydroxy, sulphonate, silicate, thio, phosphoxy, nitro, amino and nitrile. Examples of particular compounds which may be used to form the ligand include those containing carboxylic acid-functional groups, including dicarboxylic acids such as terephthalic acids, substituted terephthalic acids such as amino, hydroxy or dihydroxy terephthalic acid, benzene tricarboxylic acids biphenyldicarboxylic acids, naphthalene di and tri carboxylic acids, adamantane tetracarboxylic acids; nitrogen- and/or oxygen-containing heterocyclic compounds such as substituted pyridines (for example pyridine carboxylic acids, eg pyridine 4-carboxylic acid or nicotinamides), bipyridines and substituted bipyridines, adenine, pyrimidine compounds and imidazole, tetrazole and pyrazole analogues. Examples of thio compounds include substituted thiophenes, e.g. thiophene 2,5 dicarboxylic acid. Preferred multidentate ligand- forming compounds include compounds having at least two carboxylic acid functional groups. The multidentate ligand may be derived from an aromatic carboxylic acid including at least two carboxylic acid groups, including, for example an aromatic dicarboxylic acid or an aromatic tricarboxylic acid. Aromatic carboxylic acids may also include one or more hydroxyl or amino groups. For example, the ligand may be derived from benzenetricarboxylic acid,

dihydroxyterephthalic acid or 2-amino terephthalic acid. Carboxylic acid functional groups may be used in the form of a derivative. Carboxylic acid derivatives include salts.

The MOF material may comprise or consist of a compound represented by the formula Mx(dhtp) or a hydrate thereof, where the metal is selected from Cu, Ni, Fe or Zn and dhtp represents dihydroxy terephthalate and x is 4/the valency of the metal. Alternatively the MOF material may comprise or consist of a compound represented by the formula Mx(BTC), where the metal is selected from Cu, Ni, Fe or Zn and BTC represents benzenetricarboxylate and x is 3 /the valency of the metal.

One or more monodentate ligands may also be present in addition to the multidentate ligand. The solvent used in the reaction comprises either water or a mixture of water and an organic solvent. The reaction solvent preferably comprises a mixture of water and organic solvent having a composition, by volume of from 100:0 - 40:60, expressed as the volume % of water: volume % organic solvent. Water : organic volume ratios of 100:0 - 50:50 are more preferred, in particular water: organic volume ratios in the range 100:0 - 70:30, especially solvents containing at least 80% water. The use of water or solvent mixtures having a high content of water provides a benefit in terms of cost and environmental and safety considerations when the reaction is conducted at industrial scale. The organic solvent may be any solvent in which the ligand is soluble. We have found that, for the manufacture of CPO-27-Ni, in which the ligand is an aromatic dicarboxylic acid, polar protic solvents are capable of yielding high surface area, thermally stable MOFs. Therefore polar protic solvents, such as alkyl alcohols and diols for example, are preferred. The organic solvent may be selected from the group consisting of alkyl alcohols and alkyl polyols. Ethylene glycol, glycerol, n-butanol and 2-butanone have been particularly effective in the presence of water. Preferably the organic solvent has a relative polarity (relative to water) of at least 0.7. The most preferred organic solvents include ethylene glycol and glycerol. Water is also a preferred solvent, in the absence of any organic solvent. The reaction may be carried out in the absence of an added base. The absence of an added base in the reaction mixture simplifies the washing procedure and avoids the contamination of the product by salts formed in the reaction.

The relative amount of metal compound and multidentate ligand should be at least sufficient to provide a stoichiometric ratio of metal to ligand to form the desired MOF. An excess of the metal compound may be used. Normally the metal compound is dissolved in water before the reaction. Normally the ligand is dissolved in the organic solvent, although when it is desired to carry out the reaction in water alone, the ligand may be dissolved in water. The reaction may be carried out in an open reaction system, for example under reflux, in which the pressure is ambient pressure or a pressure which is substantially ambient. Alternatively the reaction may be carried out in a sealed or partially sealed system in which the pressure is greater than ambient pressure. An example of such a reaction system is a solvothermal reaction. The reaction is preferably carried out under reflux, although other methods may be employed.

The reaction temperature may be greater than or equal to 50 °C. The reaction temperature may be less than or equal to 100 °C. The reaction temperature is preferably in the range from 50 - 100°C. We have found that at temperatures < 50°C, the reaction may be very slow. When the temperature used is greater than about 100°C, the properties of the MOF begin to decline. Without wishing to be bound by theory, we postulate that this may be due to the change in the dielectric properties of water above 100°C, leading to a change in the pH of the solution.

The characteristics of the organic solvent appear to determine the characteristics of the material produced, in particular the crystallite size, surface area and thermal stability. Using the preferred method it is possible to form crystals of metal-organic framework material having a crystallite size, as determined by X-Ray diffraction, of at least 100 nm. The crystallite size is estimated using Pawley analysis. A preferred MOF material according to the invention has an average crystallite size of at least 100 nm. The MOF material may have a generally rod-shaped crystalline structure. The MOF material may have a rod-shaped crystalline structure having a polygonal, especially a hexagonal, cross-section. By rod-shaped we mean an anisotropic 3- dimensional shape having a length (L) at least 2* diameter (or width), i.e. L/D> 2.

We have found that the method of the invention may produce a crystalline MOF product having a surface area of at least 1000 m ² per gram, as measured by nitrogen adsorption at 77K, using a BET method after drying the sample in a vacuum oven at 90°C and outgassing (activation) at a temperature of at least 150°C for at least 20 hours. The method of the invention may produce crystalline MOF product having a surface area of at least 1 100 m ² per gram, A preferred MOF material according to the invention has a surface area of at least 800 m ² per gram, preferably 1000 m ² per gram, especially at least 1 100 m ² per gram. The invention will be further described in the following examples.

Example 1 - 4: Preparation of CPO-27-Ni at varying scale

The MOF known as CPO-27-Ni has the composition Ni ₂ (dhtp)(H ₂ 0) ₂ , where dhtp is 2,5- dihydroxyterephthalate. CPO-27-Ni was prepared by the following method. Ni(ll) acetate-4H ₂ 0 (Alfa Aesar, 98%) was dissolved in a volume of distilled water, shown in Table 1 . 2,5- dihydroxyterephthalic acid (dhtp) (Aid rich, 98%) was dissolved in the same volume of tetrahydrofuran (THF) (Sigma-Aldrich, 99.9%). The two solutions were stirred until a clear solution was obtained and then placed in a round-bottom-flask, which was subsequently attached to a reflux condenser. Each of the two precursors was dissolved in three quarters of the amount of solvent shown in Table 1 , the remaining volume of solvent being used to rinse the beakers to ensure that all of the reactants were washed into the flask. The molar ratio Ni(ll)/2,5 dihydroxyterephthalic acid was 2:1 . The mixture was kept under reflux with stirring for 24 hours. The temperature of reaction was 67°C. Once the reaction was completed, the final product was washed three times with 500ml distilled water for 20 minutes each time. The product was filtered by vacuum filtering using ceramic funnels and then dried in air for a day, then under vacuum at 80°C for 24 h. The dried product consisted of yellow-ochre crystals.

The product was characterised by inductively-coupled plasma optical emission spectrometry (ICP-OES) to determine the metal content. The yield was calculated on the basis of the nickel, using a nickel assay determined by ICP. X-Ray diffraction (Cu K a radiation, Ni filter, scanned from 10 to 130° 2Θ at ambient temperature) was used for identification of crystalline phases and measurement of crystallite size, by Pawley analysis, calculated at the width at half maximum height of the relevant peak. The surface area was measured by nitrogen adsorption at 77K, using a BET method on a Quantachrome Autosorb 1 instrument, after drying the sample in a vacuum oven at 90°C and outgassing (activation) at 150°C for 22 hours in the adsorption apparatus. The nitrogen adsorption was measured at ten points between a relative pressure P/Po of 0.01 and 0.1 . The total pore volume was determined by measuring the gas adsorbed per gram of sample at a relative pressure ρ/ρθ: 0.5. Thermal behaviour and stability was analysed by thermogravimetric analysis in air between 35°C and 600°C at a heating rate of 10°C/minute using a Diamond™ TG/DTA instrument from Perkin Elmer. The temperature of onset of thermal degradation was calculated using instrument-specific software. SEM images were obtained using a Zeiss Ultra 55 instrument. The scale bar shown at the bottom left hand side of the images represents 100 nm. All images are shown at approximately the same magnification. Without wishing to be bound by theory, we postulate that when very low volumes of solvent are present, the amount of solvent trapped within the pores is less, and that this affects the surface area, in particular. Samples 2 - 5 began to decompose at temperatures between 240 and 250°C, whilst sample 1 appeared more thermally stable, having a decomposition temperature above 290°C.

Table 1

Example 6 - 12: Preparation of CPO-27-Ni in different solvents

The preparation described above was repeated using 20 ml water as solvent for the nickel acetate (1 .492g) and 20 ml of an organic solvent as shown in Table 2, for dissolving the dhtp (0.596g). Example 10 used only water. The reactions were carried out at boiling temperature unless the solvent mixture boiled above 100°C, in which case the temperature was less than boiling point. Once the reaction was completed, the final product was washed three times with 500ml distilled water for 20 minutes each time. The product was filtered by vacuum filtering, dried in air for a day, then dried under vacuum at 80°C for 24 h. The dried product consisted of yellow-ochre crystals.

Characterisation was done as previously described. Table 3 shows the surface area measured for each sample by the BET method after activation at the temperature shown in the table and also the measured pore volume, crystallite size and temperatures of the start of decomposition and total decomposition from the DSC measurements.

The samples were also studied by scanning electron microscopy (SEM). Images for samples prepared in Examples 6 - 12 are shown in Figure 1A - 1 G, respectively.

The XRD patterns of samples from Examples 6 - 12 are shown in Fig 2, together with a reference pattern for CPO-27-Ni (bottom trace, on horizontal axis). All of the traces are similar to each other and to the reference pattern. Table 2

Table 3

Examples13 & 14: Preparation of CPO-27-Ni at different temperatures

The preparation was repeated using, as a solvent, a 1 :1 by volume mixture of ethylene glycol and water following the general method described in Example 1 . The reaction temperatures used were 1 10°C (boiling point of the solvent mixture) and 67°C. Characterisation was done as previously described, including surface area measurement by BET following outgassing (activation) at 150°C. The results are shown in Table 4 and compared with Example 6 above.

Table 4

Example 15: Preparation of CPO-27-Ni at Ni:dhtp molar ratio 3:1

22.4 g of Ni(ll) acetate-4H ₂ 0 (Alfa Aesar, 98%) was dissolved in 100 ml distilled water. 5.9 g of 2,5-dihydroxyterephthalic acid (Aldrich, 98%) was dissolved in 100 ml of ethylene glycol (Alfa Aesar, 99%) in a separate beaker. The two solutions were stirred for 10 minutes and added to a round-bottom-flask, which was subsequently attached to a reflux condenser. The molar ratio Ni(ll)/2,5 dihydroxyterephthalic acid was 3:1 . The mixture was kept under reflux and stirring for 24 hours. The temperature of reaction was set at 94°C. Once the reaction was completed, the final product was washed three times with distilled water. 9.2 g of CPO-27-Ni was collected, weighed before activation. The surface area of the product was measured as 1335 m ² /g.

Example 16: Preparation of Fe(BTC) at Fe(ll) : BTC molar ratio of 3:1

2.61 g of Fe(ll)acetate (anhydrous) was partially dissolved in 50 ml deionised H ₂ 0. 2.13 g of benzenetricarboxylic acid (H ₃ BTC) was dissolved in 50 ml ethanol. The solutions were mixed in a round bottomed flask and the mixture appeared to form a turbid suspension. After reflux at 78°C, under stirring, for 24 h, 4.64g of a salmon pink-coloured material was collected by filtration. XRD analysis showed that the so obtained material was crystalline, matching the structure of MIL-100. The X-ray diffractogram is shown in Fig 3. This synthesis of MIL-100 can be contrasted with synthesis reported in the literature requiring HF solvent and harsh conditions. Materials obtained by this method had measured surface areas of 1300 - 1700 m2/g.

Example 17: Preparation of Fe(BTC) at Fe(lll) : BTC molar ratio of 1 .5:1

26.9 g of Fe(ll)acetate were dissolved in 500 ml deionised H ₂ 0. Upon heating at 80°C the metal salt was completely dissolved. 21 .3 g of benzenetricarboxylic acid were dissolved in 500 ml methylated spirits. The solutions were mixed in a round bottomed flask and the reflux was set at 78°C, under stirring, for 24 h. After 15 min of reaction a precipitate was observed which was collected by filtration the following day and washed with 250 ml of methylated spirits. The following day the material was washed and filtered twice with 250 ml of methylated spirits. A range of surface areas between 400 - 540 m ² /g were measured for materials made in this way, depending if the N ₂ or Ar were used in the adsorption measurement.

Example 18

0.6059g of 2,5-dihydroxyterephthalic acid was partially dissolved in 16ml water. 1 .5037g of Ni(ll) acetate-4H ₂ 0 was dissolved in 14 ml water. The two solutions were mixed with a pipette and then 2ml of ethylene glycol were added. The mixture was refluxed at 94°C for 24h. The obtained material was washed with water. The material contained 23.8% Ni and had a BET surface area measured in the range 1 100 - 1300 m ² /g. The XRD pattern was found to be entirely consistent with CPO-27-Ni made by a conventional solvothermal method. The crystallite size, estimated by Pawley analysis, was 108nm. The SEM image of the material is shown in Fig 4.

Example 19 0.6021 g of 2,5-dihydroxyterephthalic acid was partially dissolved in 20ml water and 1 .4996 g of Ni(ll) acetate-4H ₂ 0 was dissolved in 16 ml water. The two solutions were mixed with a pipette and then 4ml of ethylene glycol were added. The mixture was refluxed at 94°C for 24h. The obtained material was washed with water. The material contained 23.8% Ni and had a BET surface area measured in the range 1 100 - 1300 m ² /g. The XRD pattern was found to be entirely consistent with CPO-27-Ni made by a conventional solvothermal method. The crystallite size, estimated by Pawley analysis, was 1 18nm. The SEM image of the material is shown in Fig 5.