Field of Invention

The invention relates to a method for producing a catalyst, in particular for use in hydrogenation of carbon dioxide to methanol.

Background

Traditionally, methanol is commercially produced by reacting carbon monoxide and hydrogen over a catalyst, typically a mixture of copper and zinc oxides supported by alumina.

An alternative method is to use carbon dioxide instead, using Cu/ZnO/silica as a catalyst, which has the added benefit of utilising carbon emissions and increasing carbon offset efforts.

However, the alternative method described above disadvantageous^ results in a low yield of less than 1%. While there have been attempts to improve the yield, such as with the use of co-precipitated Cu/Zn/Al ₂ O3 catalyst which may increase the yield to >20%, high pressure is required (e.g. 360 barg) which is impractical and costly to implement.

An aim of the invention therefore is to provide a method for producing a catalyst suitable for use in hydrogenation of carbon dioxide to methanol which overcomes at least some of the above issues. Summary of Invention

In an aspect of the invention, there is provided a method for producing a catalyst comprising the steps of: mixing a metal precursor with an organic ligand and a solvent to form a precursor solution; heating the precursor solution at a predetermined temperature and time to form a metal organic framework; and washing and drying the metal organic framework; characterised in that a solution containing zinc and copper ions is added to the metal organic framework by incipient wetness impregnation, whereby the resulting loaded support is dried and then calcined to form the catalyst.

Advantageously the incipient wetness impregnation ensures that the volume of precursor solution added to the dried metal organic framework is substantially equal to the pore volume thereof, whereby capillary action draws the solution into the pores, such that when dried and calcined to remove volatile components, the zinc/copper is deposited on the surface within the pores, resulting in a more structured and homogenous catalyst with a significantly higher surface area (and thus activity) compared to the prior art.

In one embodiment the metal organic framework comprises UiO-66, wherein the metal precursor comprises ZrOCh or ZrOfNCh , the organic ligand comprises 1,4-benzene dicarboxylic acid (H2BDC), and the solvent comprises dimethylformamide (DMF), and further includes a modulator such as formic acid (FA).

In one embodiment the predetermined temperature and time for heating the precursor solution for UiO-66 is about 100-150°C and around 12-48 hours respectively, typically about 120°C and around 24 hours respectively.

In a further embodiment the metal organic framework comprises NU-1000, wherein the metal precursor comprises ZrOCh or ZrO(NO3)2, the organic ligand comprises 4,4',4",4"'-(pyrene-l,3,6,8-tetrayl)tetrabenzoic acid (FUTBAPy), and the solvent comprises dimethylformamide (DMF), and further includes a modulator such as benzoic acid (BA).

In one embodiment the predetermined temperature and time for heating the precursor solution for NU-1000 is about 80-120°C and around 12-48 hours respectively, typically about 100°C and around 24 hours respectively.

In a further embodiment the metal organic framework comprises MIP-206, wherein the metal precursor comprises ZrC’h. the organic ligand comprises isophthalic acid (IP A), and the solvent comprises formic acid (FA) or acetic acid. Typically the solvent further comprises water.

Typically the predetermined temperature and time for heating the precursor solution for MIP-206 is about 100-200°C and around 12-48 hours respectively, typically about 180°C and around 24 hours respectively.

In one embodiment the metal organic framework is washed with dimethylformamide and acetone.

In one embodiment the washed metal organic framework is dried by centrifugation or fdtration, and then solvent removal under reduced pressure, typically in a vacuum desiccator for 2 hours.

In one embodiment the metal organic framework undergoes anion exchange to remove chloride ions in between the washing and drying steps. Advantageously this prevents catalyst poisoning due to the presence of chloride (which would significantly reduce the yield) as well as contamination/leaching which can cause corrosion at downstream processing. It will be appreciated that UiO-66 and NU-1000 may use chloride-free precursors, such that the anion exchange is not required.

In one embodiment the anion exchange is conducted by dispersing the metal organic framework in methanol, and solvent exchanging with ammonium formate in methanol. In one embodiment the loaded support is dried at about 80°C for around 24 hours, but it will be appreciated that other temperatures and times could be used.

In one embodiment the solution containing zinc and copper ions comprises copper nitrate and zinc nitrate. Typically the solution further comprises ammonium niobate oxalate, zirconium nitrate, and manganese nitrate.

In one embodiment the loaded support is calcined at about 200°C-400°C for around 2-6 hours. Typically where the temperature is >250°C, oxidative catalyst is formed. In addition copper sintering may occur if the temperature is >300°C.

In one embodiment where the temperature is about 250°C, the resulting pristine precatalyst can be reduced in situ using 5% H2/N2 or Fh/Ar at about 250°C for around 2-6 hours to form pristine catalyst.

In one embodiment pristine catalyst is dosed with F^CCh in the ratio range of 3: 1 to 10: 1, typically 3: 1, at about 250°C and around 40 bar for about 5 hours to form reductive catalyst.

In another aspect of the invention there is provided a catalyst made in accordance with the method herein described.

In a yet further aspect of the invention there is provided a process of making methanol by reacting carbon dioxide with hydrogen over a catalyst as herein described.

In one embodiment the temperature of the process is about 200-300°C, the pressure is around 20-100 bar, and the FtCCh ratio is 3-10: 1.

Typically the temperature of the process is about 225°C, the pressure is around 40 bar, and the FtCCh ratio is 3: 1, for about 5 hours. Typically the temperature of the process is about 225°C, the pressure is around 80 bar, and the F^CCh ratio is 3: 1 in a continuous flow system. This compares favourably with the alternative method using Cu/ZnO/silica as a catalyst, where the temperature of the process is about 200°C, the pressure is around 40 bar, and the TtCCh ratio is 3: 1, for about 5 hours. However, whereas the yield of the alternative method is <1%, the yield for the invention is >20% because of the improved catalyst.

Brief Description of Drawings

It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

Figure 1 is a schematic view of the methods of making a catalyst according to an embodiment of the invention.

Detailed Description

With regard to Figure 1, a schematic overview of the process for making a catalyst is illustrated.

The precursors comprising a metal precursor, an organic ligand and a solvent modulator are mixed 2 and then heated 4 in a Teflon-lined sealed reactor, in this example at 180°C for 24 hours. The resulting metal organic framework (MOF) is then washed 6 with DMF and acetone. If chloride ions are present, the product is anion exchanged 8 in methanol with ammonium formate. Then product is then dried.

The MOF was then loaded 10 with Cu/Zn solution using incipient wet impregnation (IWI), dried at 80°C overnight, and then calcined 12 at 250°C for 2 hours, resulting in a catalyst 14 such as UiO-66, NU-1000 or MIP-206.

Further details are provided below. Catalyst Preparation

Synthesis of Metal-organic Framework Supports

The reported synthesis and handling procedures for UiO-66, NU-1000 and MIP-206 have been optimised from several literature protocols to ensure the synthesis of high- quality MOF samples reproducibly on a large lab scale. ¹ " ³ Conditions for the synthesis of MOFs were optimised with the chloride free precursor ZrOfNCh and an anion exchange protocol was developed for MIP-206 in order to prevent chloride poisoning of the catalyst. The reported conditions exemplify small scale MOF synthesis (0.5 g to 2 g), larger scale MOF synthesis can be achieved through subtle modification of reaction conditions (increased reagent concentration) and by scaling reagent amounts, an example condition is stated below for UiO-66 at 100 g.

UiO-66-Cl free (ZrOfNOjf). (1 g scale synthesis):

ZrO(NO3)2 (1.803 g, 7.8 mmol) was added into a solvent mixture of DMF/formic acid (40 ml/ 15 ml) in a round-bottom flask and sonicated for 15 minutes. H2BDC 1.296 g (7.8 mmol, MW 166.13 g/mol) was added to the solution which was further sonicated for 15 min. The solution was separated evenly between six 20 ml Teflon capped vials. The vials were then transferred to a pre-heated oven and kept at 120°C for 16 h (overnight). The white crystalline material was collected via centrifugation, washed three times with DMF (50 ml x 3) and then the bulk sample was solvent exchanged with acetone (30 ml x 4) and left solvated until ready for use.

UiO-66-Cl free (ZrOfNOjf). (100 g scale synthesis):

ZrO(NO3)2 (90.15 g, 0.39 mol) was added into a solvent mixture of DMF/formic acid (200 ml/ 75 ml) in a round-bottom flask and stirred for 15 minutes. H2BDC 64.8 g (0.39 mol, MW 166.13 g/mol) was added to the solution which was stirred for a further 15 min. The solution was heated at 120°C for 16 h (overnight), whilst stirring constantly. The white crystalline material was collected via filtration, washed three times with DMF (100 ml x 3) and then the bulk sample was solvent exchanged with acetone (60 ml x 4) and left solvated until ready for use. The sample was dried from acetone (2 d at 100 deg), yielding 104.5 g of white crystalline UiO-66 powder (62.78 mmol, 96% yield). NU-1000-CI free (ZrOfNOjf). (500 mg scale synthesis):

ZrO(NO3)2 (1.343 g, 0.72 mmol) and benzoic acid (28.8 g) were added to DMF (85.3 ml) and sonicated until fully dissolved. The solution was heated at 80°C for 1 h, then cooled to room temperature for 30 min. To this solution FftTBAPy (426.4 mg) was added, and the solution was sonicated until dispersed, separate into ten 20 ml Teflon capped vials and then heated at 100°C for 16 h (overnight). The yellow crystalline material was cooled to room temperature, collected via centrifugation, then washed with DMF (10 ml x 4) and acetone (10 ml x 4) and left solvated in acetone until ready for use.

MIP-206 synthesis (2 g scale):

MIP-206 was synthesized solvothermally by the reaction of ZrC’h and isophthalic acid (IP A) in formic acid. IPA (1.1 g, 6.6 mmol) was weighed into a 23 mb Teflon reactor, formic acid (5 mb) was added followed by stirring at room temperature for 5 minutes until a homogeneous suspension was formed. ZrCU (2 g, 8.6 mmol) was added to the suspension followed by 10 minutes stirring at room temperature to disperse the reactants uniformly. Afterwards, the reaction was sealed in an autoclave and heated to 180 °C in 2 hours and was kept at 180 °C for 24 hours. After cooling down to room temperature, the expected product of MIP-206 (1.98 g) was collected by centrifugation, washed with DMF (3 x 10 ml), acetone (3 x 10 ml) and stored solvated in acetone until ready for use.

MOF drying:

Solvated MOF samples were centrifuged, the solvent was decanted and the residual solvent from the MOF pores was removed under reduced pressure in a vacuum desiccator for 2 hours.

MIP-206 Anion exchange:

Dried MIP-206 was dispersed in methanol (30 ml / 1 g of MOF), centrifuged and was then solvent exchanged with ammonium formate in methanol (0.05 M, 30 ml x 5), centrifuging in between washes to ensure solvent removal. The MOF was washed with methanol (30 ml x 3) and stored in methanol prior to use.

Catalyst Synthesis

Preparation ofCu/ZnO nanocluster MOF catalysts (('u ZnO'aMOl ¹ ) :

The following protocol was optimised to achieve highly crystalline and well dispersed Cu/ZnO at 10 wt% relative to the MOF support. ⁵ A stock solution of Cu(NO3)2 6H2O : Zn(NC>3)2 6H2O (70 : 30, 1.08 M) in methanol was prepared using 262 mg and 109 mg of Cu and Zn salts per 1 ml of methanol, respectively. MOF supports are dried in a desiccator to remove solvent (2 h under vacuum) and the dried MOF supports were weighed in 20 ml vials and 1.38 ml of the stock solution was added to the dried MOF per gram of MOF support. The resultant slurry was stirred until homogeneous, and air dried at 80°C for 24 h and then calcined at 250°C for 2 h and stored in a dry desiccator prior to analysis. Acetone was used as the solvent for MIP-206 samples.

Activation Protocol for Gas Sorption:

Samples were dried from acetone in a vacuum desiccator for 1 h then transferred into sorption analysis tubes, the samples were then dried under a high vacuum (1 pbar) at 120°C for 3 h to yield activated samples.

Oxidative MOF-derived catalyst (Cu/ZnO@MOF (Ox. MDC)) preparation:

Approximately 50 mg of dried Cu/ZnO@MOF catalyst were placed in a crucible and then calcined (250-400°C) for 2 h and stored in a dry desiccator prior to analysis.

Reductive MOF-derived catalyst (Cu/ZnO@MOF (Red. MDC)) preparation:

Approximately 10 mg of dried Cu/ZnO@MOF catalyst were placed in catalyst and loaded into the sample holder, which was then evacuated for 1 h, then dosed with 1 bar of 5% PF/ Ar and heated at 250°C for 2 h. After in situ reduction the sample was then cooled to room temperature and evacuated for 1 h at 30°C. The reaction cell was then dosed with reactant gasses CO2 : H2 (1 : 3) to the desired reaction pressure (40 bar), for a 40 bar reaction the cell was dosed with 7.5/30 bar of CO2/H2 at 30°C, respectively and the cell was heated to 250°C for 5 h. The reaction cell was then cooled to room temperature and evacuated overnight, and the sample was ready for use without additional reduction steps.

Catalyst Testing

General Details of the Catalysis Testing Rig

High temperature and pressure batch gas phase catalytic experiments were carried out using a custom-built high temperature and high-pressure pulse-gas sampling equipment coupled with a mass spectrometer for real-time analysis of the reaction mixture. ⁶

Reaction Monitoring

Reaction monitoring was conducted using a residual gas analyser and gas chromatography .

Procedures for CO2 Hydrogenation Reactions:

Approximately 10 mg (accurately weighed) of catalyst is loaded into the sample holder, which is then evacuated for 1 h, then dosed with 1 bar of 5% H2/Ar and heated at 250°C for 2 h. After in situ reduction* the sample is then cooled to room temperature and evacuated for 1 h at 30°C. The reaction cell is then dosed with reactant gasses CO2 : H2 (1 : 3) to the desired reaction pressure (22.5 - 40 bar), for example for reactions run at 22.5 bar, the cell is dosed with 5.625/16.875 bar of CO2/H2, respectively. The reaction is held at 30°C for 1 h (to establish a background for the RGA) and then the reaction cell is heated to the reaction temperature (200 - 250°C) and held at constant temperature for 120-300 min. The reaction mixture is analysed in real time via RGA (by pulsing a small amount of gas into the vacuum chamber, 2 x 10‘ ⁶ Torr) to follow methanol production. After the reaction has concluded the gaseous reaction mixture was analysed via gas chromatography (GC-FID/TCD).

*Note-. All samples are reduced in situ prior to catalytic experiments except for Cu/ZnO@MOF (Red. MDC) samples as these samples are reduced during their formation. Cu/ZnO@MOF (Red. MDC) samples are formed by reducing Cu/ZnO@MOF catalysts in situ (1 bar 5% PF/ Ar at 250°C for 2 h), and then exposed these samples to reductive reaction conditions (250°C, 40 bar, 3: 1 F^CCh) for 5 h.

Table 1 illustrates the catalyst performance data where under comparative conditions, the invention with an MOF support achieves a yield of >20%, whereas the alternative silica support (which has a ratio of Nb:Mn:Zr (1: 1: 1) with 0.09 wt% of overall Cu/Zn weight) has a yield of <1%. The only yield identified in the prior art literature requires a much higher pressure of 360 bar, which is expensive and impractical.

It will be appreciated by persons skilled in the art that the present invention may also include further additional modifications made to the system which does not affect the overall functioning of the system.

References:

1. Albalad, J.; Xu, H.; Gandara, F.; Haouas, M.; Martineau-Corcos, C.; Mas- Balleste, R.; Barnett, S. A.; Juanhuix, J.; hnaz, I.; Maspoch, D., Single-Crystal-to- Single-Crystal Postsynthetic Modification of a Metal-Organic Framework via Ozonolysis. Journal of the American Chemical Society 2018, 140 (6), 2028-2031.

2. Wang, T. C.; Vermeulen, N. A.; Kim, I. S.; Martinson, A. B. F.; Stoddart, J. F.; Hupp, J. T.; Farha, O. K., Scalable synthesis and post-modification of a mesoporous metal-organic framework called NU-1000. Nature Protocols 2016, 11 (1), 149-162.

3. Wang, S.; Chen, L.; Wahiduzzaman, M.; Tissot, A.; Zhou, L.; Ibarra, I. A.; Gutierrez-Alejandre, A.; Lee, J. S.; Chang, J.-S.; Liu, Z.; Marrot, J.; Shepard, W.; Maurin, G.; Xu, Q.; Serre, C., A Mesoporous Zirconium-Isophthalate Multifunctional Platform. Matter 2021, 4 (1), 182-194.

4. Mota, N.; Guil-Lopez, R.; Pawelec, B. G.; Fierro, J. L. G.; Navarro, R. M., Highly active Cu/ZnO-Al catalyst for methanol synthesis: effect of aging on its structure and activity. RSC Advances 2018, 8 (37), 20619-20629.

5. Lippi, R.; Howard, S. C.; Barron, H.; Easton, C. D.; Madsen, I. C.; Waddington, L. J.; Vogt, C.; Hill, M. R.; Sumby, C. J.; Doonan, C. J.; Kennedy, D. F., Highly active catalyst for CO2 methanation derived from a metal organic framework template. Journal of Materials Chemistry A 2017, 5 (25), 12990-12997.

6. Alvino, J. F.; Bennett, T.; Kier, R.; Hudson, R. J.; Aupoil, J.; Nairn, T.; Golovko, V. B.; Andersson, G. G.; Metha, G. F., Apparatus for the investigation of high- temperature, high-pressure gas-phase heterogeneous catalytic and photo-catalytic materials. Review of Scientific Instruments 2017, 88 (5), 054101.