Method of forming a multi-metallic metal-organic framework (MMMOF)

Abstract

There is provided a method of forming a multi-metallic metal-organic framework (MMMOF), the method comprising mechanochemical mixing a mixture comprising: (i) an oxide, hydroxide, acetate, or mixture thereof, comprising at least three metals; and (ii) an organic linker, for a pre-determined period of time. There is also provided an MMMOF formed from the method.

Description

[0001] Method of forming a multi-metallic metal-organic framework (MMMOF)

[0002] Technical Field

[0003] The present invention relates to a method of forming a multi-metallic metal-organic framework (MMMOF), and an MMMOF formed from the method.

[0004] Multi-metallic metal-organic frameworks (MMMOFs) are in high demand due to their use in gas storage, catalysis and separation. However, conventional methods of forming MMMOFs are limited to solvothermal methods that are not energy efficient and produces a large amount of liquid solvent waste. The existing methods are slow, typically requiring at least 2 days for completion of synthesis.

[0005] There is therefore a need for a method that can form MMMOFs efficiently with less amount of waste generated.

[0006] Summary of the invention

[0007] The present invention seeks to address these problems, and / or to provide an improved method for forming MMMOFs.

[0008] According to a first aspect, there is provided a method of forming a multi-metallic metalorganic framework (MMMOF), the method comprising mechanochemical mixing a mixture comprising: (i) an oxide, hydroxide, acetate, or mixture thereof, comprising at least three metals; and (ii) an organic linker, for a pre-determined period of time.

[0009] The mechanochemical mixing may comprise ball milling, crushing, shearing, grinding, or a combination thereof. According to a particular aspect, the mechanochemical mixing may comprise ball milling. The ball milling may be at 1500-4000 revolutions per minute (rpm). The ball milling may comprise liquid-assisted grinding. The liquid-assisted grinding may comprise adding a liquid to the mixture in a ratio (rj) from 0.05 to 5.0, wherein rj = mass of mixture (mg)

[0010] volume of liquid (jiL) The liquid may comprise a non-solvent.

[0011] According to a particular aspect, the pre-determined period of time may be 15-360 minutes.According to a particular aspect, the mechanochemical mixing may be at 20-45 °C.

[0012] The method may further comprise washing the mixture after the mechanochemical mixing. The method may further comprise drying the mixture after the washing.

[0013] According to a particular aspect, the method may further comprise doping the mixture with Group I metals.

[0014] The at least three metals may comprise transition metals, Group I metals, Group II metals, Group III metals, Group IV metals, or a combination thereof.

[0015] The organic linker may be selected from: tricarboxylic acids, imidazoles, terephthalic acid, melamine, aniline, and derivatives thereof.

[0016] According to a particular aspect, the mixture may not dissolve in a solvent.

[0017] According to a second aspect, there is provided a multi-metallic metal-organic framework (MMMOF) formed from the method according to the first aspect, wherein average particle size is 5-1500 nm.

[0018] The pores of the MMMOF may not comprise any solvent.

[0019] According to a particular aspect, the average pore size of the MMMOF may be 1-2 nm.

[0020] Brief Description of the Drawings

[0021] In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

[0022] Figure 1 shows a schematic representation of a mechanochemical synthesis of MMMOFs; and

[0023] Figure 2 shows PXRD analysis and comparison for the synthesised MMMOF.Detailed Description

[0024] As explained above, there is a need for an improved method of forming an MMMOF. MMMOFs are suitable for use as catalysts for the hydrogenation of CO2 to form alcohols, acetic acid, formic acid, or other oxygenates, and mixtures or combinations thereof. In general terms, the invention relates to an improved method of forming an MMMOF, and an MMMOF formed from the method. In particular, the MMMOF may be for, but not limited to, hydrogenation of CO2 to form alcohols, and may possess greater selectivity towards higher alcohols product formation. Further, the method of the present invention may be a simple and rapid method that generates less liquid waste, does not require toxic solvents, and does not require high energy input, and may therefore be easily scaled up at an industrial scale.

[0025] In the present disclosure, the use of the singular includes the plural unless specifically stated otherwise. It should be noted that, as used in the specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Further, the use of the term “including”, “comprising”, and “having” as well as other forms, such as “include”, “comprise”, “have” are not considered limiting.

[0026] According to a first aspect, there is provided a method of forming a multi-metallic metalorganic framework (MMMOF), the method comprising mechanochemical mixing a mixture comprising: (i) an oxide, hydroxide, acetate, or mixture thereof, comprising at least three metals; and (ii) an organic linker, for a pre-determined period of time.

[0027] In the present disclosure, references to a multi-metallic metal-organic framework (MMMOF) refers to a metal-organic framework that incorporates two or more different metals in its structure. In particular, the MMMOFs of the present disclosure may comprise three or more different metals in its structure. The MMMOFs may comprise any suitable structure. For example, the MMMOFs may comprise a structure similar to HKUST-1, MOF-5, MOF-74, and / or ZIF-8.

[0028] In the present disclosure, references to mechanochemical mixing refers to a process of using mechanical energy to induce reactions between reactants in the solid state. Themethod therefore do not require large amounts of organic solvents or high heat, instead using mechanical forces to alter chemical bonds.

[0029] The mixture may comprise any suitable oxide, hydroxide, acetate, or mixture thereof, having at least three metals. For example, the mixture may comprise an oxide of a first metal, a hydroxide of a second metal, and an acetate of a third metal. Alternatively, the mixture may comprise oxides of a first metal, a second metal, and a third metal, hydroxides of a first metal, a second metal, and a third metal, or acetates of a first metal, a second metal, and a third metal. Any suitable combinations of metal oxides, hydroxides, and acetates may be used.

[0030] The at least three metals may comprise transition metals, Group I metals, Group II metals, Group III metals, Group IV metals, or a combination thereof. For example, the at least three metals may comprise

[0031] In the present disclosure, references to an organic linker refers to a molecule used to connect metal ions in the MMMOF. The organic linker may be any suitable molecule that achieves linking of metal ions in the MMMOF.

[0032] The method advantageously achieves a high yield of MMMOFs in a short period of time, with simple steps that are environmentally friendly.

[0033] The mechanochemical mixing may comprise any suitable form of mechanochemical mixing. For example, the mechanochemical mixing may comprise ball milling, crushing, shearing, grinding, or a combination thereof. In particular, the mechanochemical mixing may comprise ball milling.

[0034] The ball milling may be at any suitable speed. For example, the ball milling may be at 1500-4000 revolutions per minute (rpm). In particular, the ball milling may be at 1600-3900 rpm, 1700-3800 rpm, 1800-3700 rpm, 1900-3800 rpm, 2000-3700 rpm, 2100-3600 rpm, 2200-3500 rpm, 2300-3400 rpm, 2400-3300 rpm, 2500-3200 rpm, 2600-3100 rpm, 2700-3000 rpm, 2800-2900 rpm. Even more in particular, the ball milling may be at 1740 rpm (29 Hz).

[0035] The ball milling may be performed without any solvents or reagents, also termed Neat Grinding (NG). Thus, the method may advantageously be simpler to perform, since solubility of the mixture becomes inconsequential without any requirement of additionalsolvents and reagents. Further, NG may also be conducted at room temperature with no external heating required, thereby saving energy.

[0036] According to a particular aspect, the ball milling may comprise liquid-assisted grinding (LAG). With LAG, a small amount of liquid is added to act as a lubricant, and / or to improve the mixing of the powders, reactivity and selectivity of the reaction. The liquid-assisted grinding may comprise adding a liquid to the mixture in a ratio (rj) from 0.05 to 5.0, wherein g =mass°fmixture(m) |nparticular, the g-value may range from 0.10 to volume of liquid (j-iL)

[0037] 4.50, 0.50 to 4.00, 1.00 to 3.50, 1.50 to 3.00, 2.00 to 2.50. Even more in particular, the g-value may range from 0.10 to 2.00.

[0038] The liquid may comprise a non-solvent, which is not a bulk solvent that dissolves all the reagents. For example, the liquid may comprise water, hexane, methanol, or any combination thereof.

[0039] According to a particular aspect, the pre-determined period of time may be 15-360 minutes. In particular, the pre-determined period of time may be 30-345 minutes, 45-330 minutes, 60-315 minutes, 75-300 minutes, 90-285 minutes, 105-270 minutes, 130-255 minutes, 145-240 minutes, 160-225 minutes, 175-210 minutes, 190-195 minutes. Even more in particular, the pre-determined period of time may be 60-90 minutes. The mechanochemical mixing may be intermittent, such that at every 10-15 minute interval, the mixing is paused, and the reagents are physically stirred before the mixing resumes. This overcomes any issue of the powders aggregating and causing poor mixing and incomplete reaction.

[0040] According to a particular aspect, the mechanochemical mixing may be at 20-45 °C. In particular, the mechanochemical mixing may be at a temperature of 25-40 °C, 30-35 °C. Thus, the method advantageously do not require high temperatures for reactions to take place.

[0041] The method may further comprise washing the mixture after the mechanochemical mixing. The washing may comprise adding any suitable liquid to the mixture, and mixing at a suitable speed for a suitable duration. For example, the liquid may be but not limited to, water. In particular, the washing may comprise adding water to the mixture, and ball milling at a lower speed of 600-1200 rpm, for 5-15 minutes.The method may further comprise drying the mixture after the washing. The drying may be at any suitable temperature. For example, the drying may be at 20-100 °C. In particular, the drying may be at 25-95 °C, 30-90 °C, 35-85 °C, 40-80 °C, 45-75 °C, SOTO °C, 55-65 °C. Even more in particular the drying may be at 20-25 °C, or 60-100 °C. According to a particular aspect, the method may further comprise doping the mixture with Group I metals. The doping may be via any suitable process. The doping may be performed simultaneously with the mechanochemical mixing step, or after the drying step. For example, the doping may be performed simultaneously by adding a Group (I) metal precursor to the mixture prior to the mixing. Alternatively, the doping may be performed after the drying step via ion exchange, solution impregnation, vapour phase infiltration, and / or chemical modification of the organic linker followed by heat treatment, such as, but not limited to, calcination. The Group I metals may be any suitable Group I metals, for example, but not limited to, sodium, potassium, or a combination thereof. According to a particular aspect, the at least three metals may comprise any suitable transition metals, Group I metals, Group II metals, Group III metals, Group IV metals, or a combination thereof. For example, the transition metal may be, but is not limited to, copper, iron, zinc, cobalt, nickel, vanadium, chromium, molybdenum, ruthenium, or a combination thereof. The Group I metals may be sodium, potassium, caesium, or a combination thereof. The Group II metals may be magnesium, calcium, strontium, barium, or a combination thereof. The Group III metals may be aluminum, scandium, yttrium, or a combination thereof. The Group IV metals may be titanium, zirconium, hafnium, or a combination thereof.

[0042] The organic linker comprised in the mixture may be any suitable organic linker. The organic linker may be selected from: tricarboxylic acids, imidazoles, terephthalic acid, melamine, aniline, and derivatives thereof. In particular, the organic linker may be terephthalic acid, 2,5-dihydroxyterephthalic acid, trimesic acid, 2-methylimidazole, or a combination thereof.

[0043] According to a particular aspect, the mixture may not dissolve in a solvent. Thus, any liquid added to the mixture during the process may not dissolve the mixture, and instead only improves the mixing of the powders, and reactivity and selectivity of the reaction.According to a second aspect, there is provided a multi-metallic metal-organic framework (MMMOF) formed from the method according to the first aspect, wherein average particle size is 5-1500 nm. In particular, the average particle size of the MMMOF may be ID-1400 nm, 50-1300 nm, 100-1200 nm, 200-1100 nm, 300-1000 nm, 400-900 nm, 500-800 nm, 600-700 nm. Even more in particular, the average particle size of the MMMOF may be 50-150 nm.

[0044] The pores of the MMMOF may not comprise any solvent. In particular, the MMMOF may have crystallites with empty pores, which can be characterized by methods such as, but not limited to, Thermogravimetric Analysis-Differential Scanning Calorimetry (TGA-DSC), Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM).

[0045] According to a particular aspect, the average pore size of the MMMOF may be 1-2 nm.

[0046] Example

[0047] Materials and methods

[0048] Synthesis of MMMOFs

[0049] A schematic of the reaction is shown in Figure 1. Different metal oxides, hydroxides, and acetates of Cu, Fe, Mg, Zn, Co, and / or Ni, along with corresponding organic linkers, were weighed and placed in 35 mL stainless steel jars, together with three stainless steel balls (10 mm diameter). The organic linkers used were terephthalic acid, 2,5-dihydroxyterephthalic acid, trimesic acid, and / or 2-methylimidazole. The total weight of the reagents were within the range of 3-7 g for a 35 mL jar. The reagents were gently mixed using a plastic spatula, then sealed in the jars and placed in a Retsch MM400 shaker mill, set at 29 Hz for a duration of 60-90 minutes.

[0050] In some examples, Liquid-Assisted Grinding (LAG) was used, whereby small measured amounts of non-solvent liquids were added to act as lubricants. LAG was used to improve the mixing of the powders, and reactivity and selectivity of the reaction. The amount of liquid added is dependent on the q-value, where q is defined as the mass of the reagents (in mg) / volume of the liquid added (in p L) .In some examples, intermittent ball milling was carried out, in which at every 10-15 minute intervals, the jars were opened and the reagents were physically mixed and the balls dislodged before milling was resumed.

[0051] Examples 1-6

[0052] Specifically, in Example 2 as detailed in Table 1 below, Cu(OH)2(1.46 g, 15 mmol), Co(OH)2(1.39 g, 15 mmol), trimesic acid (4.21 g, 20 mmol), Na2CO2 (265 mg, 2.5 mmol) and methanol as the non-solvent for LAG (7.06 mL, rj = 1) were placed in a 35 mL stainless steel jar with 3 stainless steel balls (10 mm diameter). The total weight of the reagents was 7.33 g. The reagents were gently mixed using a plastic spatula, then sealed in the jars and placed in a Retsch MM400 shaker mill, set at 29 Hz for a duration of 60-90 minutes.

[0053] In Examples 1-4 as detailed in Table 1 below, intermittent ball milling was carried out. After the milling was completed, the powders were either directly scraped from the milling jars, or about 5-10 mL of deionized water was added to the jar and subjected to further milling at 10-20 Hz for 5-15 minutes. This step was carried out to wash the products for easier transfer to another setup for drying.

[0054] The powders were rinsed with deionized water using a Buchner funnel setup in vacuo and dried for several days at room temperature or in a drying oven set at 60-100 °C for 6-16 hours. The dried MMMOF powders were placed in small dram vials and characterized using various solid-state techniques.

[0055] Example 7

[0056] MgO (138 mg, 3.46 mmol), ZnO (282 mg, 3.46 mmol), CU(OAC)2*H2O (691 mg, 3.46 mmol), CO(OAC)2(612 mg, 3.46 mmol), and 2-methylimidazole (2.27 g, 27.7 mmol) were placed in a 35 mL stainless steel jar with 3 stainless steel balls (10 mm diameter). The total weight of the reagents was 3.99 g. The reagents were gently mixed using a plastic spatula, then sealed in the jars and placed in a Retsch MM400 shaker mill, set at 29 Hz for a duration of 60-90 minutes.Synthesis of Comparative Example catalyst

[0057] For comparison of catalytic performance, a porous bimetallic Cu-Zn metal catalyst was synthesised as a Comparative Example. The catalyst was supported onto AI2O3 in Pluronic P-123 and was prepared by solution synthesis. Pluronic P-123 was used as a template to create a porous catalyst structure.

[0058] The metal nitrate precursors were dissolved in ethanol and mixed with Pluronic P-123 with nitric acid. The solution mixture was stirred for 4 hours at 60 °C, and subsequently air dried at room temperature over 8 days to remove the solvent. The dried mixture was then calcined in a muffle furnace and subsequently reduced in a tube furnace under flowing hydrogen gas.

[0059] Characterisation

[0060] Powder X-ray Diffraction (PXRD)

[0061] Powder X-ray diffraction (XRD) patterns of the synthesised MMMOF were collected on a powder diffractometer (Broker D8 Advance X-ray) with a Cu-Ka source (Ka = 0.154 nm) in the 20 range between 20-80°.

[0062] Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) Scanning electron microscopy (SEM) was conducted using a JEOL JSM-7900F Schottky Field Emission Scanning Electron Microscope fitted with Oxford Instruments X-Max 80 EDX spectrometer. SEM Images were taken at an acceleration voltage of 10 kV while EDX analysis at an acceleration voltage of 15 kV. Samples were sputter-coated with gold for better conductivity, using Cressington 208HR High Resolution Sputter Coater. Catalytic performance

[0063] Catalytic performance was measured for the synthesised catalysts of Examples 1-6 and the Comparative Example.

[0064] For the catalytic hydrogenation reaction of CO2, about 100-1000 mg of the catalysts were placed in a stainless steel 50 mL Parr 4792 reactor with a thermocouple attached. A magnetic stirrer bar was added to ensure adequate vortexing of the solids during the reaction. The reactions were either carried out solvent-less or with 10 mL of deionizedwater added as the solvent. The Parr reactor was sealed and purged three times with CO2 to remove O2 and N2, and then charged with 15 bar of CO2 and subsequently 45 bar of H2(total 60 bar).

[0065] The reaction was heated to 225 °C using a jacketed heater for 14-18 hours. After the reaction, the heating jacket was removed, and the Parr reactor was first cooled in air to room temperature and then further cooled in an ice bath to ensure adequate condensation of any liquids formed into the reactor. Droplets of the liquid products were siphoned, filtered to remove any solids, diluted with deionized water and analysed with gas chromatography.

[0066] Results and discussion

[0067] PXRD

[0068] Figure 2 shows a comparison of the PXRD patterns of the reagents and the synthesised MMMOF, with an absence of reagent peaks and presence of the small low angle peak at 5-8° (26) indicating formation of bimetallic MMMOF.

[0069] SEM-EDX

[0070] SEM-EDX images (not shown) revealed quad-metallic Cu / Fe / Zn / Mg-ZIF-8 with particle sizes of about 50-150 nm that are aggregated with relatively rough surfaces. The colours were well distributed in the EDX images, indicating that the various metallic elements were relatively homogenously distributed within the solids, forming a solid solution of the quad-metallic ZIF-8.

[0071] Catalytic performance

[0072] Table 1 shows the comparison of the catalytic performance of the Cu-Co trimesic MOFs (analogous to HKUST-1) catalysts against the Comparative Example Cu / Zn / AI P123.

[0073]

[0074] Table 1 : GC analysis of liquid products

[0075] As seen in Table 1 , the Comparative Example catalyst showed clean selectivity (99%) towards methanol, whilst the Cu-Co trimesic MOFs demonstrated selectivity towards higher carbon products such as ethanol (54-68%), acetone (51-60%), isopropanol (16-40%) and acetic acid (22-39%).Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention and that the embodiments are provided by way of illustration, and are not intended to be limiting.

Claims

Claims1 . A method of forming a multi-metallic metal-organic framework (MMMOF), the method comprising mechanochemical mixing a mixture comprising: (i) an oxide, hydroxide, acetate, or mixture thereof, comprising at least three metals; and (ii) an organic linker, for a pre-determined period of time.

2. The method according to claim 1 , wherein the mechanochemical mixing comprises ball milling, crushing, shearing, grinding, or a combination thereof.

3. The method according to claim 1 or 2, wherein the mechanochemical mixing comprises ball milling.

4. The method according to claim 3, wherein the ball milling is at 1500-4000 revolutions per minute (rpm).

5. The method according to claim 3 or 4, wherein the ball milling comprises liquid-assisted grinding.

6. The method according to claim 5, wherein the liquid-assisted grinding comprises adding a liquid to the mixture in a ratio (q) from 0.05 to 5.0, wherein q = mass of mixture (mgvolume of liquid7. The method according to claim 6, wherein the liquid comprises a non-solvent.

8. The method according to any preceding claim, wherein the pre-determined period of time is 15-360 minutes.

9. The method according to any preceding claim, wherein the mechanochemical mixing is at 20-45 °C.

10. The method according to any preceding claim, further comprising washing the mixture after the mechanochemical mixing.

11. The method according to claim 10, further comprising drying the mixture after the washing.

12. The method according to any preceding claim, further comprising doping the mixture with Group I metals.

13. The method according to any preceding claim, wherein the at least three metals comprise transition metals, Group I metals, Group II metals, Group III metals, Group IV metals, or a combination thereof.

14. The method according to any preceding claim, wherein the organic linker is selected from: tricarboxylic acids, imidazoles, terephthalic acid, melamine, aniline, and derivatives thereof.

15. The method according to any preceding claim, wherein the mixture does not dissolve in a solvent.

16. A multi-metallic metal-organic framework (MMMOF) formed from the method according to any preceding claim, wherein average particle size is 5-1500 nm.

17. The MMMOF according to claim 16, wherein pores of the MMMOF do not comprise any solvent.

18. The MMMOF according to claim 16 or 17, wherein average pore size is 1 -2 nm.

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