Synthesis of metal-organic frameworks

Abstract

A method of forming a metal organic framework (MOF) having a UTSA-16 structure comprising: (i) providing a mixing reactor comprising a first inlet, a second inlet and an outlet; (ii) delivering a first fluid to the first inlet, the first fluid comprising a metal salt solution; (iii) delivering a second fluid to the second inlet, the second fluid comprising a ligand solution; and (iv) extracting a MOF solution from the outlet. Preferably the metal salt solution comprises a zinc salt, more preferably the metal salt solution comprises two salts, a zinc salt and a salt chosen from copper, cobalt, iron, magnesium or manganese. Also disclosed is a magnesium-zinc MOF with a USTA-16 framework wherein the atomic ratio of magnesium to zinc is 1: at least 9. Also disclosed is a magnesium-zinc MOF with a USTA-16 framework comprising from 0.1 to 1 atomic % of magnesium and from 5 to 10 atomic % of zinc.

Description

The invention relates to metal-organic frameworks (MOFs) having a UTSA-16 structure and to a method of making said MOFs. In particular, the invention relates to UTSA-16 (Mg, Zn) MOFs having good CO; capture performance. Background Metal-organic frameworks (MOFs) are porous crystalline materials with a modular synthetic chemistry. Due to high compositional tuneability, they are amenable to precise materials engineering allowing excellent performance in numerous commercially interesting applications. For example, certain MOFs have exhibited large CO? uptake capacities, high CO? selectivity and prolonged stability making them excellent CO2 capture sorbents. The development of scalable and sustainable synthetic protocols is an essential step towards actual commercial applications of MOFs. This is because most intended MOF applications entail significant volumes of material (e.g., -3000 tonnes sorbent for a capture unit integrated with a 500 MW coal-based power facility). As such, the environmental impact and cost of manufacture substantially affect the overall economic viability of the process. UTSA-16 is a highly promising MOF material for CO2 capture by adsorption due to the isotherm features, mechanism of adsorption, and stability. Almost all reported syntheses for this material are based on the protocol in Journal of the American Chemical Society, 2005, 127, 16352-16353, where only the vessel and solvent volume are adjusted to meet the required scale of synthesis. This reported solvothermal synthetic protocol requires specialized reactors, introducing additional capital costs for production, and precludes the use of common glass equipment, where the typical pressure rating is below 2 bar. Ongoing attempts to scale-up this material are hindered by failure to reduce the synthetic temperature below the boiling point of the solvent. In addition, the aforementioned protocol also necessitates extended crystallization time (up to 2 days). Transitioning from closed-solvothermal conditions to continuous synthesis is an important step towards scalable protocols. Gaikwad, R., et al.. Journal of Industrial and Engineering Chemistry, 2022, 111, 346-355, describes a series of bimetallic UTSA-16 (Zn, X; X = Mg, Mn, and Cu) MOFs synthesized using a microwave irradiation method. The process disclosed in Gaikwad requires 14 washing steps using a wide range of solvents, has a reaction temperature of 90°C as a result of microwaves and is a batch process, so it is not a scalable method from a production perspective. There is therefore a need for improved methods which can produce UTSA-16 in a faster time and under milder reactions, while retaining good CO2 capture performance. The capital equipment cost, safety compliance cost and energy cost may all be reduced when the known solvothermal processes are replaced with the process of the present invention. Summary of the invention In a first aspect the invention provides a method of forming a metal-organic framework (MOF) having a UTSA-16 structure, the method comprising: (i) providing a mixing reactor comprising a first inlet, a second inlet and an outlet; (ii) delivering a first fluid to the first inlet, the first fluid comprising a metal salt solution; (iii) delivering a second fluid to the second inlet, the second fluid comprising a ligand solution; and (iv) extracting a metal-organic framework (MOF) dispersion from the outlet. In a second aspect the invention provides a metal-organic framework (MOF) having a UTSA-16 structure obtainable by the method according to the first aspect of the invention. The metal organic-framework (MOF) having a UTSA-16 structure obtainable by method according to the first aspect of the invention may be a UTSA-16 magnesium-zinc (Mg, Zn) metal-organic framework (MOF) when the first fluid of the method comprises zinc salt and magnesium salt in a molar ratio of 1:1. In a third aspect, the invention provides a magnesium-zinc (Mg, Zn) metal-organic framework (MOF) having a UTSA-16 structure wherein the atomic % ratio of magnesium to zinc is Fat least 9. In a fourth aspect, the invention provides a magnesium-zinc (Mg, Zn) metal-organic framework (MOF) having a UTSA-16 structure comprising: from 0.1 to 1 atomic % of magnesium; and from 5 to 20 atomic % of zinc. In a fifth aspect, the invention provides a method of capturing CO: comprising the step of exposing a material comprising a metal organic-framework (MOF) to an environment containing CO:, wherein the metal organic-framework (MOF) is obtainable by a method according to the first aspect of the invention or the metal organic-framework (MOF) is according to the third or fourth aspect of the invention. Brief description of the drawings Figure 1: shows a schematic diagram of a method according to the present invention. Figure 2: shows an example of a reactor that may be used in the method according to the present invention. Figure 3: shows X-ray diffraction (XRD) patterns of bimetallic UTSA-16-type MOFs obtained according to the method of the present invention and their calculated BET surface area and the CO2 uptake measured by thermogravimetric analysis (TGA). Figure 4: shows N2 adsorption isotherm graphs of the UTSA-16 metal-organic frameworks (MOFs) of example 3. Figure 5: shows the CO2 uptake from a 15% CO2 gas stream, quantified using TGA, of the UTSA-16-type (Mg, Zn) MOF of example 3. Detailed description The invention is based on the finding that metal-organic frameworks (MOFs) having a UTSA-16 structure can be synthesised at room temperature in continuous flow. The method of making UTSA-16 MOFs of the present invention is significantly faster under milder reaction conditions than known methods of making MOFs, provides instantaneous and homogeneous mixing and has millisecond reaction times and excellent scale-up reproducibility. The method is consistent and thorough, leading to a satisfactory yield of UTSA-16 MOF. The method of the present invention can be used to produce UTSA-16 MOFs having good CO2 capture performance and surface area. In particular, UTSA-16 (Mg, Zn) MOFs unexpectedly show high surface area and CO2 uptakes. In a first aspect the invention provides a method of forming a metal-organic framework (MOF) having a UTSA-16 structure, the method comprising: (i) providing a mixing reactor comprising a first inlet, a second inlet and an outlet; (ii) delivering a first fluid to the first inlet, the first fluid comprising a metal salt solution; (iii) delivering a second fluid to the second inlet, the second fluid comprising a ligand solution; and (iv) extracting a metal-organic framework (MOF) dispersion from the outlet. In the method of the present invention the mixing reactor may be a reactor suitable for mixing two fluids. Typically, it will be suitable for forming metal-organic framework (MOF) particles having a UTSA-16 structure. The mixing reactor may be a counter current mixing reactor. In the method of the present invention the mixing reactor may be a counter current mixing reactor for continuously mixing two fluids. In the method of the present invention the second inlet of the mixing reactor may be diametrically opposed to the first inlet. In the method of the present invention the mixing reactor may comprise a body having a first inlet, a second inlet and an outlet. The first inlet may be at a first end of the body and the second inlet may be at a second end of the body. The mixing reactor may comprise a body having the first inlet, the second inlet, the outlet, an inner passage through the body from the first inlet to the second inlet and an outer passage closer to a surface of the body than the inner passage. The inner passage and outer passage as described here may meet at a junction where the first and second fluids are mixed. In the method of the present invention the mixing reactor may comprise a body having a first inlet, a second inlet and an outlet, in which there is an inner passage through the body from the first inlet at a first end of the body to the second inlet at a second end of the body, the inner passage may further have a side wall along a length of the body, and an outer passage closer to a surface of the body than the inner passage, the outer passage running from the outlet at the second end of the body along the length of the body and meeting the inner passage at a junction at the first end, the outer passage joining the inner passage through the side wall at the junction. The inner and outer passages may be symmetrical about the centreline of the inner passage. The body may be made of a metal material or a chemically resistant polymer. The metal material may be a metal, such as stainless steel, or alloys such as Hastelloy, Inconel, Monel or Nimonic. The first fluid may be pumped into the body through the first inlet at the first end of the body. The second fluid may be pumped into the inner passage of the body through the second inlet at the second end of the body. The first fluid may be flowed up into the body through the first inlet. The second fluid may be flowed down into the inner passage of the body through the second inlet. Because of the opposed introduction of both fluids, turbulence will be induced at the junction, leading to mixing of the first and second fluids and formation of MOFs. Thus the junction provides a mixing point. Consequent mixing continues as both fluids meet at the junction and the reacted MOF dispersion is pushed towards the outlet. The MOF dispersion can then be extracted from the outlet. The method of forming a metal-organic framework (MOF) having a UTSA-16 structure of the present invention may be a method of forming a (X, Zn; X=Mg, Co, Mn. Fe or Cu) metal-organic framework (MOF) having a UTSA-16 structure. In one embodiment the invention provides a method of forming (Mg, Zn) MOF, (Mn, Zn) MOF, (Fe, Zn) MOF, (Cu, Zn) MOF or (Co, Zn) MOF having a UTSA-16 structure. The first and second fluids may be liquids, including solutions, dispersions or suspensions. In the method of the present invention the metal salt solution may be a bimetallic salt solution. The first fluid comprises a metal salt solution. The metal salt solution may comprise, for example, a solution of metal nitrates, metal sulphates, metal acetates, metal acetylacetonates, metal halides, metal carbonates or combination thereof. The metal salt solution may comprise a copper salt, cobalt salt, iron salt, magnesium salt, manganese salt, zinc salt, or combinations thereof. In one embodiment, the metal salt solution may comprise zinc salt. The zinc salt may be zinc acetate dihydrate. The metal salt solution may comprise magnesium acetate tetrahydrate, cobalt acetate tetrahydrate, manganese acetate, iron acetate, copper acetate monohydrate, zinc acetate dihydrate or combinations thereof. In the method of the present invention the metal salt solution may be a bimetallic salt solution comprising a first metal salt and a second metal salt. The metal salt solution of the first fluid may comprise a first metal salt selected from magnesium salt, manganese salt, iron salt, copper salt or cobalt salt and a second metal salt being zinc salt. The metal salt solution may comprise a mixture of a first metal salt selected from magnesium acetate tetrahydrate, cobalt acetate tetrahydrate, manganese acetate, iron acetate or copper acetate monohydrate and a second metal salt being zinc acetate dihydrate. The first fluid may further comprise water, methanol, ethanol or combinations thereof. The first fluid may further comprise water and ethanol. The first fluid may comprise a magnesium salt, a zinc salt, water and ethanol. The first fluid may comprise magnesium acetate tetrahydrate, zinc acetate dihydrate, water and ethanol. The first fluid may comprise a cobalt salt, a zinc salt, water and ethanol. For example, the first fluid may comprise cobalt acetate tetrahydrate, zinc acetate dihydrate, water and ethanol. In the method of the present invention the metal salt solution may comprise a first metal salt selected from magnesium salt, manganese salt, iron salt, copper salt or cobalt salt, and a second metal salt being zinc salt, wherein the molar ratio of first metal salt to second metal salt is 1:1 or 2:7. For example, the metal salt solution may comprise magnesium salt and zinc salt in a molar ratio 1:1 or cobalt salt and zinc salt in a molar ratio 1:1. The metal salt solution may comprise magnesium acetate tetrahydrate and zinc acetate dihydrate in a molar ratio 1:1 or cobalt acetate tetrahydrate and zinc acetate dihydrate in a molar ratio 1:1. The metal salt solution of the present invention may comprise a first metal salt selected from manganese salt, iron salt, or copper salt, and a second metal salt being a zinc salt, wherein the molar ratio of first metal salt to second metal salt is 2:7. The metal salt solution of the present invention may comprise a first metal salt selected from manganese acetate, iron acetate or copper acetate monohydrate, and a second metal salt being zinc acetate dihydrate, wherein the molar ratio of first metal salt to second metal salt is 2:7. The second fluid comprises a ligand solution. The ligand solution may comprise a ligand and a basic solution. The ligand may be terephthalic acid, citric acid, fumaric acid, isophthalic acid, dihydroxyisophthalic acid, trimesic acid, 2-methylimidazole, 2-aminoterephthalic acid, 2,5- dihydroxyterephthalic acid or 2,5- pyrazole-2,5-dicarboxylic acid. The second fluid may further comprise potassium hydroxide, sodium hydroxide, triethylamine, ammonium hydroxide, pyridine, water or combinations thereof. The second fluid may comprise citric acid, potassium hydroxide and water. The metal-organic framework (MOF) dispersion may be particle-bearing suspension or dispersion. The metal-organic framework (MOF) dispersion may comprise MOF particles in solution, dispersion or suspension. The mixing reactor will mix the first and second fluids, so that both fluids mix together and MOF particles are formed. The initial mixing location may be where the two fluids firstly meet, for example at the junction. The mixing reactor may be operated with at least 50 mL / min flow rate measured at the outlet, such as at least 100 mL / min, or at least 140 mL / min, or at least 200 mL / min, or at least 1 L / min, or at least 5 L / min, or at least 10 L / min flow rate measured at the outlet. The mixing reactor may be operated with from 50 mL / min to 10 L / m flow rate measured at the outlet, such from 50 mL / min to 8 L / m, or from 50 mL / min to 7 L / m, or from 50 mL / min to 6 L / m, or from 50 mL / min to 5 L / m, or from 50 mL / min to 4 L / m, or from 50 mL / min to 2 L / m, or from 100 mL / min to 10 L / m, or from 100 mL / min to 8 L / m, or from 100 mL / min to 7 L / m, or from 100 mL / min to 6 L / m, or from 100 mL / min to 5 L / m, or from 100 mL / min to 4 L / m, or from 100 mL / min to 3.5 L / m, or from 100 mL / min to 1 L / m, or from 500 mL / min to 10 L / m, or from 500 mL / min to 8 L / m, or from 500 mL / min to 6 L / m, or from 500 mL / min to 4 L / m flow rate measured at the outlet. The method may comprise heating or cooling the mixed fluid as it passes through the outer passage of the body as describe herein. The mixing reactor may further comprise a heater coupled to the surface, such as a band heater. This will heat the outer passage rather than the inner passage or the first inlet. In this case, the body may be made of heat-conductive material, such as a metal material, such as stainless steel, typically stainless steel, or alloys such as Hastelloy, Inconel, Monel or Nimonic. The mixing reactor may be operated at ambient temperature or a temperature between 2 °C and 200 °C, such as between 2 °C and 150 °C, or between 2 °C and 100 °C, or between 2 °C and 50 °C, or between 5 °C and 150 °C, or between 5 °C and 100 °C, or between 5 °C and 70 °C, or between 5 °C and 50 °C, or between 5 °C and 40 °C, or between 10 °C and 200 °C, or between 10 °C and 100 °C, or between 10 °C and 70 °C, or between 10°C and 60 °C, or between 10 °C and 50 °C, or between 10 °C and 40 °C, or between 20 °C and 30 °C, or between 20 °C and 70 °C, or between 20°C and 60 °C, or between 20 °C and 50 °C, or between 20 °C and 40 °C, or between 20 °C and 30 °C, or between 20 °C and 28 °C, or between 20 °C and 25 °C, or between 22 °C and 28 °C, or between 24 °C and 26 °C. In one embodiment the mixing reactor is operated at 25 °C. The method described herein may also be carried out without heating the reactor or without preheating the first and / or second fluids. In one embodiment the method of the present invention does not require heating. The method described herein may be carried out at ambient or room temperature, for example between 20 °C and 28 °C. In one embodiment, the first and / or second fluids are not heated before entering the reactor. The mixing reactor may be operated from 0.5 bar to 240 bar, or from 1 bar to 240 bar, or from 0.5 bar to 200 bar, or from 0.5 bar to 150 bar, or from 0.5 bar to 100 bar, or from 0.5 bar to 150 bar, or from 1 bar to 200 bar, or from 1 bar to 150 bar, or from 1 bar to 10 bar. The mixing reactor may be operated at 1 bar. The method of the present invention may be used to form bimetallic UTSA-16 metalorganic frameworks (MOFs) comprising zinc and a metal (X) selected from magnesium (Mg), manganese (Mn), iron (Fe), copper (Cu) or cobalt (Co). The method of the present invention may be used to form UTSA-16 (Mg, Zn) MOF, UTSA-16 (Mn, Zn), UTSA-16 (Fe, Zn), UTSA-16 (Cu, Zn) and UTSA-16 (Co, Zn) MOF. The UTSA-16 MOFs obtained by the process of the present invention may be UTSA-16 (Mg, Zn) MOF, when the first fluid comprises zinc salt and magnesium salt in a molar ratio of 1:1, or the UTSA-16 MOFs obtained by the process of the present invention may be UTSA-16 (Co, Zn) MOF, when the first fluid comprises zinc salt and cobalt salt in a molar ratio of 1:1. The UTSA-16 MOFs obtained by the process of the present invention may be UTSA-16 (X, Zn; X=Mn, Fe, Cu) MOFs, when the first fluid comprises a metal salt solution comprising a first metal salt selected from iron salt, copper salt or manganese salt and a second metal salt being zinc salt, and the molar ratio of first metal salt to second metal salt is 2:7. The method described here may be a method of forming a magnesium-zinc (Mg, Zn) metal-organic framework (MOF) having a UTSA-16 structure, the method comprising: (i) providing a mixing reactor comprising a first inlet, a second inlet and an outlet; (ii) delivering a first fluid to the first inlet, the first fluid comprising magnesium salt and zinc salt; (iii) delivering a second fluid to the second inlet, the second fluid comprising a ligand (e.g. citric acid); and (iv) extracting a UTSA-16 (Mg, Zn) MOF dispersion from the outlet. The mixing reactor used in the method of obtaining the UTSA-16 (Mg, Zn) MOF is as described herein, so for example, the reactor may have the second inlet diametrically opposed to the first inlet. The mixing reactor may be operated at ambient temperature as described herein. The mixing reactor may be operated at ambient temperature, for example between 20 °C and 28 °C. The first fluid may comprise a magnesium salt to zinc salt molar ratio of 1:1. The first fluid may comprise magnesium acetate tetrahydrate, zinc acetate dihydrate, water and ethanol, as described herein. The second fluid may comprise citric acid, potassium hydroxide and water. In the methods of the present invention the first fluid may comprise from 0.0 IM to 0.4M magnesium acetate tetrahydrate (Mg(OAc)2.4H2O), such as from 0.03M to 0.4M, or 0.05M to 0.4M, or from 0.07M to 0.4M, or from 0.0IM to 0.3M, or from 0.0IM to 0.2M, or from 0.03M to 0.3M, or 0.05M to 0.3M, or from 0.07M to 0.3M, or from 0.03M to 0.2M, or 0.05M to 0.2M, or from 0.07M to 0.2M, or from 0.08M to 0.4M magnesium acetate tetrahydrate (Mg(OAc)2.4H2O). In the method of the present invention the first fluid may comprise from 0.0 IM to 0.4M zinc acetate dihydrate (Zn(OAc)2.2H2O), such as from 0.03M to 0.4M, or 0.05M to 0.4M, or from 0.07M to 0.4M, or from 0.0IM to 0.3M, or from 0.0IM to 0.2M, or from 0.03M to 0.3M, or 0.05M to 0.3M, or from 0.07M to 0.3M, or from 0.03M to 0.2M, or 0.05M to 0.2M, or from 0.07M to 0.2M, or from 0.08M to 0.4M zinc acetate dihydrate (Zn(OAc)2.2H2O). In the method of the present invention the first fluid may comprise from 0.0 IM to 0.4M magnesium acetate tetrahydrate (Mg(OAc)2.4H2O) and from 0.0IM to 0.4M zinc acetate dihydrate (Zn(OAc)2.2H2O), wherein the molar ratio of magnesium acetate tetrahydrate to zinc acetate dihydrate is 1:1. In the method of the present invention the second fluid may comprise between 0.05M and IM of citric acid, such as between 0.05M and 0.8M , or between 0.05M and 0.7M , or between 0.05 M and 0.6 M, or between 0.05M and 0.55M, or between 0.08M and 0.8M, or between 0.08M and 0.7M, or between 0.08M and 0.6M, or between 0. IM and 0.8M, or between 0. IM and 0.6M, or between 0. IM and 0.55M, or between 0.13M and 0.8M, or between 0.13M and 0.55M, or between 0.1333M and 0.5332M, or between 0.13M and 0.4M of citric acid. In the method of the present invention the second fluid may comprise between 0.1M and 2M of potassium hydroxide (KOH), such as between 0.1M and 1.8M, or between 0.1M and 0.16M, or between 0.1M and 1.5M, or between 0.2M and 2M, or between 0.2M and 1.8M, or between 0.2M and 1.6M, or between 0.2M and 1.5M, or between 0.3M and 2M, or between 0.3M and I.8M, or between 0.3M and 1.6M, or between 0.3M and 1.5M, or between 0.35M and 2M. or between 0.35M and 1.8M, or between 0.35M and 0.16M, or between 0.35M and 1.5M. or between 0.4M and 2M, or between 0.4M and 1.8M, or between 0.4M and 1.5M of potassium hydroxide (KOH). In the method of the present invention the first fluid may comprise a ratio of ethanol to water of 20:80, or 30:70, or 40:60, or 50:50. The method of the present invention may be carried out at ambient temperature. The method of the present invention may be carried out at 25 °C. The method of the present invention may further comprise separating the metal-organic framework (MOF) particles (e.g. the UTSA-16 (Mg, Zn) MOF particles) from the solution. The method may further comprise separating the MOF particles from the solution by centrifugation. The method of the present invention may further comprise separating the metal-organic framework (MOF) particles (e.g. the UTSA-16 (Mg, Zn) MOF particles) from the supernatant and washing and drying the UTSA-16 MOF particles. The method may further comprise pelletizing the UTSA-16 metal-organic framework (MOF) particles (e.g. the UTSA-16 (Mg, Zn) MOF particles) using a binder. In a second aspect, the invention provides a metal-organic-framework (MOF) having a UTSA-16 structure, obtainable by the method as described herein. The metal-organic framework (MOF) having a UTSA-16 structure, obtainable by the method described herein may be a bimetallic metal organic framework, wherein the metal salt solution of the first fluid may comprise a first metal salt and a second metal salt, and the first metal salt may be selected from copper salt, cobalt salt, iron salt, magnesium salt or manganese salt and the second meal is zinc salt. The mixing reactor, conditions, and first and second fluids may be as described herein. The metal organic framework (MOF) having a UTSA-16 structure, obtainable by process described herein may be a UTSA-16 (X, Zn; X = Mg, Co) metal-organic framework (MOF), wherein the metal salt solution of the first fluid comprises zinc salt and magnesium salt in a molar ratio of 1:1 or zinc salt and cobalt salt in a molar ratio of 1:1. The metal organic framework (MOF) having a UTSA-16 structure, obtainable by the process described herein may be a UTSA-16 (X, Zn; X = Fe, Mb, Cu) metal organic framework (MOF), wherein the metal salt solution of the first fluid comprises a zinc salt and a metal salt (X) selected from iron salt, manganese salt or copper salt and the molar ratio of metal salt (X) to zinc salt is 2:7. The invention further provides a magnesium-zinc (Mg, Zn) metal organic framework (MOF) having a UTSA-16 structure, obtainable by a process comprising: (i) providing a mixing reactor comprising a first inlet, a second inlet and an outlet, (ii) delivering a first fluid to the first inlet, wherein the first fluid comprises a metal salt solution comprising magnesium salt and zinc salt, and wherein the molar ratio of magnesium salt to zinc salt is 1:1; (iii) delivering a second fluid to the second inlet, the second fluid comprising a ligand (e.g. citric acid); and (iv) extracting a UTSA-16 magnesium-zinc (Mg, Zn) metal organic framework (MOF) dispersion from the outlet. The mixing reactor, conditions and first and second fluids may be as described herein. In a third aspect, the invention provides a magnesium-zinc (Mg, Zn) metal organic framework (MOF) having a UTSA-16 structure comprising an atomic % ratio of magnesium to zinc is l:least 9. The magnesium:zinc atomic % ratio is Fat least 9. For example, the atomic % ratio of magnesium to zinc may be from 1:9 to 1:30, or from 1:11 to 1:30, or from 1:13 to 1:30, or from 1:15 to 1:30, or from 1:9 to 1:25, or from 1:11 to 1:25, or from 1:13 to 1:25, or from 1:15 to 1:25, or from 1:9 to 1:20, or from 1:11 to 1:20, or from 1:13 to 1:20, or from 1:15 to 1:20, or from 1:9 to 1:18, or from 1:11 to 1:18, or from 1:13 to 1:18, or from 1:15 to 1:18. In a fourth aspect, the invention provides a magnesium-zmc (Mg, Zn) metal organic framework (MOF) having a UTSA-16 structure comprising: from 0.1 to 1 atomic % of magnesium; and from 5 to 20 atomic % of zinc. Magnesium (Mg) is present in the UTSA-16 (Mg, Zn) MOF in an atomic % from 0.1 to 1. For example, the atomic % of magnesium in the (Mg, Zn) MOF may be from 0.1 to 0.9, such as from 0.1 to 0.8, or from 0.1 to 0.7, or from 0.1 to 0.6, or from 0.1 to 0.5, or from 0.2 to 1, or from 0.2 to 0.9, or from 0.2 to 0.8, or from 0.2 to 0.7, or from 0.2 to 0.6, or from 0.3 to 1, or from 0.3 to 0.9, or from 0.3 to 0.8, or from 0.3 to 0.7, or from 0.3 to 0.6, or from 0.4 to 1, or from 0.4 to 0.9, or from 0.4 to 0.8, or from 0.4 to 0.7, or from 0.4 to 0.6. Zinc (Zn) is present in the UTSA-16 (Mg, Zn) MOF in an atomic % from 5 to 20. For example, the atomic % of zinc in the (Mg, Zn) MOF may be from 5 to 18, such as from 5 to 16, or from 5 to 14, or from 5 to 12, or from 5 to 10, or from 5 to 9, or from 6 to 18, or from 6 to 16, or from 6 to 14, or from 6 to 12, or from 6 to 10, or from 6 to 9, or from 7 to 18, or from 7 to 16, or from 7 to 14, or from 7 to 12, or from 7 to 10, or from 7 to 9, or from 8 to 18, or from 8 to 16, or from 8 to 14, or from 8 to 12, or from 8 to 10, or from 8 to 9. As mentioned, the MOFs described herein may be formed under milder conditions (such as 25 °C and 1 bar). The MOFs described herein or made by the method according to the invention, including the UTSA-16 (Mg, Zn) MOF, may have a Brunauer-Emmett-Teller (BET) surface area from 300 to 1500 m2 / g, such as from 300 to 1500 m2 / g, or from 300 to 1000 m2 / g, or from 300 to 800 m2 / g, or from 300 to 700 m2 / g, or from 400 to 1500 m2 / g, or from 400 to 1000 m2 / g, or from 400 to 800 m2 / g, or from 400 to 700 m2 / g, or from 450 to 1500 m2 / g, or from 450 to 1000 m2 / g. or from 450 to 800 nr / g, or from 450 to 700 m2 / g, or from 500 to 1500 m2 / g, or from 500 to 1000 m2 / g, or from 500 to 800 nr / g, or from 500 to 700 m2 / g, or from 550 to 1500 m2 / g, or from 550 to 1000 m2 / g, or from 550 to 800 m2 / g. To measure the surface area of the MOFs, N2 adsorption isotherms were recorded using a Tristar II instrument (Micromeritics, USA) at 77 K, and at partial pressures between 0.00 and 0.90. Prior to measurement, approximately 60-100 mg of MOF materials were degassed under vacuum overnight at 150 °C using a VacPrep 061 (Micromeritics, USA), before being allowed cool to room temperature for analysis. Following measurement of N2 isotherms, surface areas were calculated using the BET method, through the analysis procedure contained within the instrument software (Microactive). The MOFs described herein, including the UTSA-16 (Mg, Zn) MOFs, may be useful in CO2 capture. The ability of the sorbent to capture CO2 can be demonstrated by measuring adsorption by thermogravimetric analysis (TGA). As such, the MOFs of the current invention may have a CO2 uptake (from a 15% CO2 gas stream) greater than 0.5 mmol / g, such as greater than 0.8mmol / g, or greater than 1 mmol / g, or greater than 1.2 mmol / g, or greater than 1.5 mmol / g, or greater than 1.8 mmol / g, or greater than 2 mmol / g. The MOFs of the present invention, including the UTSA-16 (Mg, Zn) MOFs, may have a 15% CO2 uptake from 0.5 to 3 mmol / g, or from 0.5 to 2.7 mmol / g, or from 0.5 to 2.5 mmol / g, or from 0.8 to 3 mmol / g, or from I to 3 mmol / g, or from I to 2.5 mmol / g, or from 1.2 to 3 mmol / g, or from 1.2 to 2.5 mmol / g, or from 1.5 to 3 mmol / g, or from 1.5 to 2.5 mmol / g. The MOFs described herein may be shaped into pellets using a binder. The binder may be polyvinyl butyral (PVB). The MOF of the present invention may be useful in capturing CO2. Thus, in a fifth aspect of the invention, there is provided a method of capturing CO2 comprising the step of exposing a material comprising a MOF as described herein to an environment containing CO2. Examples The following examples illustrate various aspects of the invention. The examples should, of course, be understood to be merely illustrative of only certain embodiments of the invention and not to constitute limitations upon the scope of the invention. Results are also presented and described in the Figures and Figure legends. Example 1: UTSA-16 (Mg, Zn) metal organic frameworks (MOFs) were synthesised using a counter-current reactor (1) as shown in the schematic diagram of Figure 1. The first fluid (2) comprising the metal salt solution was prepared dissolving magnesium acetate tetrahydrate (5.471 g, 25 mmol) and zinc acetate dihydrate (5.599 g, 25 mmol) in 50 mL of water, followed by addition of 200 mL of ethanol. In a separate beaker, the second fluid (3) comprising a ligand solution was prepared mixing citric acid (6.466 g, 33.3 mmol) and potassium hydroxide (6.233 g, 100 mmol) dissolved in 250 mL water. The first fluid (2) was placed in the upflow feed (4) and the second fluid (3) in the downflow feed (5) (see Figure 1). The fluids were pumped into the reactor (1) and the reaction product, namely the metal organic framework (MOF) dispersion (6) collected at the outlet feed. The reaction was run with a flow rate of 140 mL / min at each flow using ambient conditions (25 °C and 1 bar). The reaction product was collected from the outflow feed using a suitable collection vessel. The product was centrifuged at 2500 rpm to remove the supernatant. The isolated solid reaction product was redispersed / washed with methanol and centrifuged a second time. The solution was removed and the centrifuged solids oven dried at 100°C for 1 hour to remove the excess solvent. Example 2: Figures 2 show an example of a counter current mixing reactor (1) that may be used in the method of the present invention. It comprises a body (7) having a first inlet (8), a second inlet (9) and an outlet (10). There is an inner passage (11) through the body (7) from the second inlet (9) at a second end (13) of the body (7) towards the first inlet (8) at a first end (12) of the body (7), the inner passage (11) having a side wall (14) along a length of the body (7), and an outer passage (15) closer to a surface (16) of the body (7) than the inner passage (11), the outer passage (15) running from the outlet (10) at the second end (13) of the body (7) along the length of the body (7) and meeting the inner passage (11) at a junction (17) at the first end (12), the outer passage (15) joining the inner passage (11) through the side wall (14) at the junction (17). The mixing reactor (1) has been used with the following reactants to produce UTSA-16 (Mg, Zn) MOF: 0.1M Mg(OAc)2.4H2O, 0.IM Zn(OAc)2.2H2O, 40 / 60 EtOH / H2O (from total volume) 0.1333M citric acid, 0.4M KOH 3:2:6 metal salt solution: citric acid: KOH ratio (wherein the metal salt solution is the result of dissolving the Mg(OAc)2.4H2O and Zn(OAc)2.2H2O in the water and ethanol) The reaction was run at ambient conditions (25 °C and 1 bar). Other MOFs were synthesised using the above method but replacing magnesium acetate by cobalt acetate tetrahydrate, manganese acetate, iron acetate or copper acetate monohydrate. UTSA-16 (Co, Zn) MOF was synthesised using 1:1 molar ratio of cobalt acetate tetrahydrate to Zn(OAc)2.2H2O. For UTSA-16 (X, Zn; Fe, Mn and Cu) MOFs the ratio of manganese acetate, iron acetate or copper acetate monohydrate to Zn(OAc): 2H2O was 2:7. Example 3 The XRD patterns of bimetallic UTSA-16-type (X, Zn; X=Mg, Mn, Fe, Cu, Co) MOFs obtained according to the method of the present invention, their BET surface area and CO2 uptake were calculated and are shown in Figure 3. Each MOF with reactant metal salt molar ratios of 1:1 and 2:7 (molar ratios of first metal salt to second metal salt in the first fluid) was analysed using a Rigaku MiniFlex600 X-ray Diffractometer (Rigaku, Europe SE) with Cu Ka radiation (X = 1.5418A). Approximately 50 mg of MOF material was ground into a fine powder and loaded into a sample holder for analysis. Diffraction patterns were recorded across a 20 range of 5-40°, with a step size of 0.02° and 0.1s per step. All analysis were completed using the Rigaku SmartLab Studio II software, which allows comparison to known reference crystalline patterns. To measure the surface area of the UTSA-16 MOFs listed in Figure 3, V adsorption isotherms were recorded using a Tristar II instrument (Micromeritics, USA) at 77 K, and at partial pressures between 0.00 and 0.90. Prior to measurement, approximately 60-100 mg of MOF materials were degassed under vacuum overnight at 150 °C using a VacPrep 061 (Micromeritics, USA), before being allowed cool to room temperature for analysis. Following measurement of N; isotherms, surface areas were calculated using the BET method, through the analysis procedure contained within the instrument software (Microactive). The calculated surface areas of each MOF are listed in Figure 3 and the BET isotherm adsorption graph of the MOFs of Figure 3 are shown in Figure 4. The CO2 uptake from a 15% CO2 gas stream of the UTSA-16 (Mg, Zn) MOF of example 3 is shown in Figure 5. CO2 uptake measurements were conducted using a Q500 TGA (TA Instruments, USA), equipped with both N2 and CO2 gas cylinders (CO2 / N2 gas cylinder if using 15%) (sourced from BOC). During experiments, the sample gas flow (either N2 or CO2) was fixed at 133 mL min4 whilst the associated balance flow was fixed at 5 ml min . Analysis was conducted in open Pt pans, which had been previously cleaned by thorough rinsing with DI water and exposure to a blow torch flame (to volatilise organic contaminants) for five seconds. The mass of individual clean pans was recorded before MOF loading through the standard TGA taring process. For a single uptake measurement, approximately 20 mg of candidate MOF material was loaded into a previously fared pan. Under a flow of pure N2, the sample temperature was rapidly increased to 150 °C, where it was held stable for 30 minutes to fully activate the material. After this 30 minutes, the sample was cooled and equilibrated at 25 °C, still under continuous N2 sample gas flow. Sample gas flow was then switched from N2 to 15% CO2 (balance of 85% N2), where it was held for 30 minutes, during which CO2 uptake occurred. Sample mass was recorded as a function of time throughout the above. CO2 uptake was derived from the increase in sample mass upon CO2 introduction, as marked on Figure 5. Example 4 Experimental details: SEM was conducted using a JSM IT-200 (Jeol, Japan) operated at 10 kV and a working distance of 10 mm. Micrographs were collected using a secondary electron detector at magnification levels specified in individual images. Samples were prepared for SEM by adhering a small amount of MOF material to metal stub using a carbon tab. EDS data were collected and analysed using Aztec Nanoanalysis (Oxford Instruments, UK). Table 1 shows weight percentages of element and atomic percentages obtained from SEM analysis of UTSA-16 (Mg, Zn) MOF with a reactant metal salt molar ratio of 1:1. Element Element wt. % Atomic % Carbon (C) 31.22 50.29 Oxygen (O) 32.57 35.29 Potassium (K) 11.14 5.66 Magnesium (Mg) 0.63 0.50 Zinc (Zn) 27.77 8.25 Total 100% 100% Mg:Zn ratio 1:44.08 1:16.50

Claims

1. A method of forming a metal organic framework (MOF) having a UTSA-16 structure, the method comprising:(i) providing a mixing reactor comprising a first inlet, a second inlet and an outlet;(ii) delivering a first fluid to the first inlet, the first fluid comprising a metal salt solution;(iii) delivering a second fluid to the second inlet, the second fluid comprising a ligand solution; and(iv) extracting a metal organic framework (MOF) dispersion from the outlet.

2. The method of claim 1, wherein the metal salt solution comprises a zinc salt.

3. The method of claim 1 or claim 2, wherein the metal salt solution comprises a first metal salt and a second metal salt, wherein the first metal salt is selected from copper salt, cobalt salt, iron salt, magnesium salt or manganese salt and the second metal is a zinc salt.

4. The method of claim 3 wherein the molar ratio of first metal salt to second metal salt is 1:1 or 2:7.

5. The method of claim 3 or claim 4, wherein the first metal salt is cobalt salt or magnesium salt and the second meal is a zinc salt and, wherein optionally the molar ratio of first metal salt to second metal salt is 1:1.

6. The method of any one of claims 3 to 5, wherein the first metal salt is selected from magnesium acetate, cobalt acetate, manganese acetate, iron acetate or copper acetate.

7. The method of any one of claims 3 to 6, wherein the zinc salt is zinc acetate dihydrate.

8. The method of any one of claims 1 to 7, wherein the metal salt solution comprises water, methanol, ethanol or combinations thereof, wherein optionally the metal salt solution comprises water and ethanol.

9. The method of any one of claims 1 to 8, wherein the ligand solution comprises a ligand and a basic solution.

10. The method of any one of claims 1 to 9, wherein the ligand solution comprises terephthalic acid, citric acid, fumaric acid, isophthalic acid, dihydroxyisophthalic acid, trimesic acid, 2-methylimidazole, 2-aminoterephthalic acid, 2,5- dihydroxyterephthalic acid, 2,5-pyrazole-2,5-dicarboxylic acid or combinations thereof.

11. The method of any one of claims 1 to 10, wherein the second fluid comprises potassium hydroxide and water.

12. The method of any one of claims 1 to 11, wherein the mixing reactor is a counter current mixing reactor.

13. The method of any one of claims 1 to 12, wherein the mixing reactor further comprise a body having a first end and a second end, wherein the first inlet is at the first end of the body and the second inlet is at the second end of the body.

14. The method of any one of claims 1 to 13, wherein the second inlet of the mixing reactor is diametrically opposed to the first inlet.

15. The method of any one of claims 1 to 14, wherein the metal organic framework (MOF) dispersion comprises a bimetallic metal organic framework (MOF) having a UTSA-16 structure.

16. The method of any one of claims 1 to 15, wherein the metal organic framework (MOF) dispersion comprises a (X, Zn) metal organic framework (MOF) having a UTSA-16 structure, wherein X is a metal selected from magnesium, manganese, iron, copper or cobalt.

17. A metal organic framework (MOF) having a UTSA-16 structure, obtainable by the method according to the any one of claims 1 to 16.

18. The metal organic framework (MOF) of claim 17, comprising a first metal selected from magnesium, manganese, iron, copper or cobalt and a second metal being zinc.

19. The metal organic framework (MOF) of claim 17 or claim 18, wherein the metal salt solution comprises zinc salt and magnesium salt in a molar ratio of 1:1 and the metal organic framework (MOF) is a (Mg, Zn) MOF having a UTSA-16 structure.

20. A magnesium-zinc (Mg, Zn) metal organic framework (MOF) having a UTSA-16 structure, wherein the atomic % ratio of magnesium to zinc is Fat least 9.

21. The magnesium-zinc (Mg, Zn) metal organic framework (MOF) of claim 20, wherein the atomic % ratio of magnesium to zinc is from 1:9 to 1:30.

22. A magnesium-zinc (Mg, Zn) metal organic framework (MOF) having a UTSA-16 structure comprising:from 0.1 to 1 atomic % of magnesium; andfrom 5 to 20 atomic % of zinc.

23. The magnesium-zinc (Mg, Zn) metal organic framework (MOF) of claim 22 comprising:from 0.3 to 0.8 atomic % of magnesium; andfrom 5 to 10 atomic % of zinc.

24. The magnesium-zinc (Mg, Zn) metal organic framework (MOF) of any one of claims 20 to 23, wherein the (Mg, Zn) MOF has (BET) surface area from 300 to 1500 nr / g.

25. The magnesium-zinc (Mg, Zn) metal organic framework (MOF) of any one of claims20 to 24, wherein the (Mg, Zn) MOF has a CO2 uptake from 0.5 to 3 mmol / g.

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