Field of the Invention

[0001] The present invention is broadly directed to a method of producing a modified zirconium-based metal organic framework (MOF). The invention is also more generally directed to a modified zirconium-based MOF.

Summary of Invention

[0002] According to a first aspect of the present invention there is provided a method of producing a modified zirconium-based metal organic framework (MOF), said method comprising the steps of: synthesizing a zirconium-based MOF; exposing the zirconium-based MOF to an amine agent thereby loading said MOF with the amine agent to produce the modified zirconium-based MOF being effective in preferential chemisorption of carbon dioxide (CO ₂ ) from air.

[0003] Preferably the step of exposing the zirconium-based MOF to an amine agent involves impregnation of a relatively high boiling point polymeric amine into said MOF. Alternatively the step of exposing the MOF to an amine agent involves grafting or otherwise appending a relatively small molecule amine to said MOF.

[0004] Preferably the method also comprises the step of functionalising the zirconium- based MOF by exposing it to silanes to increase the internal hydrophobicity of the zirconium- based MOF and enhance the CO ₂ /water selectivity of the modified zirconium-based MOF. More preferably this step of functionalising the MOF occurs prior to the step of exposing the MOF to an amine agent. Still more preferably or alternatively, the method further comprises the step of incorporating a super-hydrophobic agent into the zirconium-based MOF either prior to or after exposure to the amine agent thereby improving the CO ₂ /H ₂ O selectivity of the improved zirconium-based MOF.

[0005] Preferably the method further comprises the step of exposing the zirconium- based MOF to an etching solution being effective in generating defects in the zirconium- based MOF thereby increasing the CO ₂ adsorption capacity of the modified zirconium-based MOF. More preferably this step of etching the zirconium-based MOF occurs prior to the step of exposing the MOF to an amine agent. [0006] Preferably the method comprises a preliminary step of activating the synthesized zirconium-based MOF prior to the loading or functionalisation steps.

[0007] According to a second aspect of the invention there is provided a modified zirconium-based metal organic framework (MOF) comprising a zirconium-based MOF loaded with an amine agent to produce the modified zirconium-based MOF being effective in preferential chemisorption of carbon dioxide (CO ₂ ) from air.

[0008] Preferably the zirconium-based MOF is a functionalised zirconium-based MOF including silanes being effective in increasing the hydrophobicity of said zirconium-based MOF.

[0009] Preferably the zirconium-based MOF is an etched zirconium-based MOF including defects being effective in increasing the CO ₂ adsorption capacity of the modified zirconium-based MOF.

Brief Description of Drawings

[0010] In order to achieve a better understanding of the nature of the present invention a preferred embodiment of a method of producing a modified zirconium-based organic framework (MOF) and the modified Zr-based MOF itself will now be described, by way of example only, with reference to the accompanying illustrations in which:

Figure 1 is a schematic of a method of producing a modified Zr-based MOF according to an example of a first aspect of the invention;

Figure 2 is a schematic of a method of producing a modified Zr-based MOF according to another example of the first aspect of the invention;

Figure 3 depicts reaction schemes for the chemical absorption or chemisorption of carbon dioxide (CO ₂ ) utilising the modified Zr-based MOF of first and second aspects of the invention;

Figure 4 is a Powder X-Ray Diffraction (PXRD) pattern of various modified Zr-based MOFs according to embodiments of both aspects of the invention together with and predicted from single crystal simulations of a pristine Zr-based MOF;

Figure 5 is a thermal gravimetric analysis (TGA) of the modified Zr-based MOF of one of the embodiments of figure 4; Figure 6 shows a CO ₂ adsorption isotherm depicting preferential chemisorption of CO ₂ for the modified Zr-based MOF of the embodiment of figures 4 and 5;

Figure 7 illustrates CO ₂ adsorption isotherms for two of the modified Zr-based MOFs of the preceding embodiments;

Figure 8 depicts a PXRD of various modified Zr-based MOFs according to another embodiment of both aspects of the invention together with and predicted from single crystal simulations of a pristine Zr-based MOF;

Figure 9 shows a PXRD of a modified Zr-based MOF of the preferred embodiment together with and predicted from single crystal simulations of a pristine Zr-based MOF;

Figure 10 depicts CO ₂ gas sorption isotherms for modified Zr-based MOFs of alternative embodiments of select of the preceding figures;

Figure 1 1 illustrates a PXRD of a Zr-based MOF synthesized as a potential large-scale candidate for loading of an amine agent according to one or more of the earlier embodiments of the invention;

Figure 12 depicts a PXRD of various functionalised Zr-based MOFs suitable for loading with an appropriate amine agent to produce a modified Zr-based MOF according to both aspects of the invention together with a photograph of its hydrophobicity under a water drop test;

Figure 13 is a TGA of the various functionalised Zr-based MOFs of figure 12;

Figure 14 is a TGA of various embodiments of a modified Zr-based MOF derived from the functionalised Zr-based MOFs of figures 12 and 13;

Figure 15 illustrates a PXRD of the modified Zr-based MOFs of the embodiments of figure 14 together with and predicted from single crystal simulations of a pristine Zr-based MOF;

Figure 16 shows various CO ₂ sorption isotherms for the various modified Zr-based MOFs of the embodiments of figures 14 and 15 compared to a pristine equivalent of the Zr-based MOF;

Figure 17 shows PXRDs and CO ₂ sorption isotherms for other embodiments of a modified Zr- based MOF loaded with amine agents according to both aspects of the invention under shortened reaction times; Figure 18 depicts CO ₂ sorption isotherms and PXRDs for a further embodiment of a modified Zr-based MOF according to both aspects of the invention together with and predicted from single crystal simulations of a pristine Zr-based MOF;

Figure 19 shows PXRDs and CO ₂ sorption isotherms for earlier embodiments of the modified Zr-based MOF derived from a functionalised Zr-based MOF together with and predicted from single crystal simulations of a pristine Zr-based MOF;

Figure 20 is a TGA of an etched Zr-based MOF including defects;

Figure 21 illustrates a CO ₂ isotherm for the etched and loaded Zr-based MOF of the embodiment of figure 20 compared with a pristine Zr-based MOF equivalent;

Figure 22 illustrates a CO ₂ sorption isotherm of another embodiment of a Zr-based MOF under the influence of another etching solution for the introduction of defects compared with a pristine Zr-based MOF;

Figure 23 shows PXRD patterns for the embodiment of figures 20 and 21 together with the other embodiment of figure 22 compared with the pristine or as-synthesized version of the Zr- based MOF.

Detailed Description

[0011] As seen in the schematics of figures 1 and 2, there are alternative examples of a method of producing a modified zirconium-based metal organic framework (MOF). In the illustrated examples, the modified zirconium-based MOF designated at 10 is derived from a Zr-based MOF in the form of UiO-66 designated at 12 although it will be understood that in the preferred embodiment MOF-808 replaces UiO-66. In the examples depicted, the method broadly comprises the steps of:

1 . synthesizing a Zr-based MOF, such as UiO-66 designated at 12;

2. exposing the synthesized UiO-66 MOF 12 or a derivative thereof to an amine agent such as polyethylenimine (PEI) and/or tetraethylenepentamine (TEPA) designated at 14.

[0012] Importantly, by exposing the Zr-based MOF 12 to the amine agent 14, said MOF 12 is loaded with the amine agent 14 to produce the modified Zr-based MOF 10. The modified Zr-based MOF 10 is effective in preferential chemisorption of carbon dioxide (CO ₂ ) from air. It is understood that this chemisorption mechanism involves reaction of the amines associated with the modified Zr-based MOF with CO ₂ in air to form either carbamates (2 amines: 1 CO ₂ ) or bicarbonates (1 amine: 1 CO _2: 1 H ₂ O). This chemisorption mechanism for direct air capture (DAC) of CO ₂ can be seen in the reaction schemes of figure 3.

[0013] Returning to figure 2, the method of the preferred embodiment may also comprise the step of functionalising the Zr-based MOF 12. The Zr-based MOF 12 is functionalised by exposing it to salines such as PhSiH ₃ designated at 16. In this example, the functionalised MOF is in the form of a super-hydrophobic MOF designated at 18 formulated for the chemisorption of CO ₂ in preference to water or other moisture. The super-hydrophobic MOF 18 is thereafter exposed to the amine agent such as PEI and/or TEPA 14 to produce the modified Zr-based MOF 10. Although not illustrated, the improved Zr-based MOF may be further functionalised by incorporating a super-hydrophobic agent within the MOF, such as by covalent grafting, to improve the CO ₂ /H ₂ O selectivity

[0014] Zr-based MOFs 12 are selected as hosts for chemisorption agents for CO ₂ in air on the understanding that they are chemically and thermally robust. The Zr-based MOF selected is also understood to be suitable for DAC of CO ₂ insofar as it is both air and water stable. In the preferred embodiments, the step of exposing the Zr-based MOF to an amine agent involves either:

1 . impregnation of a relatively high boiling point polymeric amine into the MOF; or

2. grafting or otherwise appending a relatively small molecule amine to the MOF.

[0015] In 1 , the impregnation of high boiling point polymeric amines is expected to depend on the pore aperture of the Zr-based MOF selected. In 2, the small molecule amines are preferably grafted onto organic ligands or open metal sites present in the Zr-based MOF. As an alternative approach to these examples of amine loading, small molecule amines are polymerized within pores of the Zr-based MOF to generate a high boiling point polymeric amine. This alternative strategy is understood to be more appropriate for MOFs with relatively small pore apertures.

[0016] To increase the internal hydrophobicity of the improved Zr-based MOF, the presence of both -OH functional groups and open metal sites alters the ratio of superhydrophobic and alkyl amine groups incorporated. The -OH groups can be functionalised using silanes (e.g. phenylsilane) to integrate internal hydrophobicity. PEI or other alkyl amine groups and their analogues can be impregnated, or alternatively grafted onto the -OH groups themselves, leading to a mixed hydrophobic-amine material. According to another strategy, covalent grafting of the super-hydrophobic groups to a Zr-based MOF may be accompanied by covalent or other grafting of an amine agent to produce the modified Zr-based MOF.

PEI Impregnation of ‘Pristine’ MOF-808

[0017] MOF-808 was synthesised by combining ZrCL (116.5 mg), trimesic acid (35.3 mg), acetic acid (2.8 mL) in DMF (5 mL). The resulting mixture was sonicated for 10 minutes prior to being heated at 135°C in a teflon lined steel bomb for 12 hours. A gelatinous white solid was obtained, filtered, and washed with with DMF (2 x 30 mL), acetone (2 x 30 mL) and ethanol (3 x 30 mL) before being dried in a vacuum dessicator for 12 hours.

[0018] Activated MOF-808 (50 mg) was suspended in a solution of TEPA or T ris(2- aminoethyl)amine (TREN) (at 25% or 50% in methanol (10 mL)). The sample was stirred for 5 days at room temperature. Following stirring, the methanol was removed under reduced pressure to yield the composite material, which was used without further purification.

[0019] PEI was infiltrated into MOF-808 by stirring the activated framework in a 50 w/w % PEI MeOH solution for 12 hours. The solid was then collected by vacuum filtration and dried in a vacuum desiccator overnight. Powder X-Ray Diffraction (PXRD) patterns for both stages of synthesis are shown in figure 4 and confirm that structure was maintained during the PEI infiltration step.

[0020] Three different concentrations of PEI were used. PEI(800) (5 mg or 10 mg) were dissolved in methanol (10 mL), then added to MOF-808 (20 mg). Alternatively, PEI(800) (7.5 mg) was dissolved in methanol (5 mL), then added to MOF-808 (10 mg). Samples were stirred at RT for 5 d. Following stirring, the solvent was removed to yield the composite material. Figure 4 is a PXRD of MOF-808 predicted from single crystal simulations (black), and PEI@MOF-808 (25 wt% (red), 50 wt%(blue) and 75 wt% (green).

[0021] In an alternative synthetic protocol MOF-808 (20 mg) was activated by heating at 150 °C under high vacuum (0.4 mbar) on a Schlenk line for 6 hours. The sample was allowed to cool to 50 °C under vacuum, then left under an inert N ₂ atmosphere. A solution of PEI (800) (10 mg) was dissolved in methanol (10 mL) and sonicated for 20 minutes. The solution was then added to activated MOF-808 under N ₂ . The suspension was stirred for 5 days at 55 °C.

[0022] PEI was infiltrated into MOF-808 using a 50 w/w % MeOH solution. However, thermal gravimetric analysis (TGA) revealed that PEI uptake was closer to 20 w/w %. Low pressure CO ₂ uptake for this framework appears promising. Figure 5 is a TGA of PEI@MOF- 808 and figure 6 shows CO ₂ uptake at 298 K of PEI@MOF-808.

[0023] Figure 7 illustrates CO ₂ adsorption isotherms measured at 298 K on PEI@MOF- 808 at loadings of 25 wt% (black) and 50 wt% (red). Closed and open shapes represent adsorption and desorption isotherms, respectively.

TEPA and TREN Impregnation of MOF-808

[0024] The impregnation of MOF-808 using TEPA and TREN was conducted. PXRD measurements indicate that TEPA and MOF-808 are compatible, while TREN appears to degrade the MOF. Figure 8 is a PXRD of MOF-808 predicted from single crystal simulations (black), TREN@MOF-808 (25 wt% (red) and 50 wt% (blue) and TEPA@MOF-808 (25 wt% (green) and 50 wt% (pink)). By way of comparison, figure 9 is a PXRD of simulated MOF-808 (black) and PEI@MOF-808 (50 wt%) synthesised using more rigorous activation conditions.

[0025] Both amine impregnated MOF-808 materials exhibit improved Type I behaviour. The CO ₂ adsorption capacities are 0.4 mmol g-1 for PEI800 and 0.6 mmol g-1 for TEPA. Figure 10 shows CO ₂ gas sorption isotherms (298 K) for MOF-808 impregnated with PEI-800 (50 wt% - black) and TEPA (50 wt% - red). Closed and open shapes represent adsorption and desorption isotherms, respectively.

Postsynthetic Modification of MOF-808 Frameworks

[0026] The preceding examples explored amine incorporation into a stable zirconium- based MOF-808 framework to increase CO ₂ uptake capacity (acetate form X = CH ₃ ). The pore dimensions and interior pore surface of the MOF-808 frameworks are readily tuned by varying the modulating acid used in synthesis. With the aim to increase amine or for example PEI loading, pore volume and CO ₂ selectivity, a series of MOF-808 frameworks were synthesised, from large pore volume and pore dimension to smaller pore volume but with superhydrophobic trifluoromethyl (CF ₃ ) groups. The following equations are exemplary of schemes for snythesizing the MOF-808 framework using different modulating acids.

[0027] Synthesis of MOF-8Q8(X) (X = H, CH ₃ , CF ₃ ): ZrCI ₄ (116.5 mg) and H ₃ btc (35.3 mg) were added to DMF (5 mL) and sonicated 5 minutes to dissolve. The solution was transferred to a Teflon-lined stainless steel autoclave, then an appropriate quantity of modulating organic acid was added (formic acid (X = H) 1 .85 mL); acetic acid (X = CH ₃ ) 2.8 mL; trifluoroacetic acid (X = CF ₃ ) 3.75 mL). The reactions were heated in a thermostated oven (135 °C) for 24 hours, then cooled slowly to room temperature.

[0028] Large-scale Synthesis of MOF-8Q8(CH ₃ /CI): As seen in the equation below, a large-scale synthesis of MOF-808 involves heating at atmospheric pressure in acetic acid water mixtures, yielding MOF-808 with chloride and acetate non-structural ligands (termed MOF-808(CH ₃ /CI)). Post-synthetic modification with hydrophobic organic acids is then used to modify hydrophobicity or CO2 selectivity. The refluxing synthesis yielded a large quantity of microcrystalline MOF-808(CH ₃ /CI).

[0029] To a solution of zirconium oxychloride (4.83 g) in a mixture of acetic acid (42.9 mL) and water (16.2 mL), H ₃ btc (1 .05 g) was added. The reaction was immediately heated to 95 °C, then stirred 18 hours under reflux. After cooling, the precipitated white solids were isolated by centrifugation, then washed with water (100 mL), again isolating the solids by centrifugation. The framework was activated by stirring 12 hours in refluxing acetone (100 mL), after which the solids were isolated by centrifugation and washed with acetone (3 x 60 mL). The white solid was dried briefly in vacuo then at 85 °C overnight, yielding 1 .747 g white powder. [0030] Figure 12 depicts PXRD of MOF-808(CH ₃ ) calculated from single crystal data (black) and experimental pattern of MOF-808(CH ₃ /CI) synthesised using large-scale aqueous synthesis. Inset shows magnification of higher angle data.

[0031] Hydrophobicity Modification of MOF-808(CH3/CI): Post-synthetic modification to replace the non-structural acetate and chloride ligands was performed by stirring as- synthesised MOF-808(CH ₃ /CI) with an excess of modifying organic acid in DMF. The acids were chosen to alter the hydrophobicity of the framework. The non-structural ligands were chosen to either yield a large quantity of MOF-808(CH ₃ ) for amine impregnation (using sodium acetate), or to produce hydrophobic or fluorinated analogues of MOF-808 (using benzoic, trifluoroacetic or perfluorooctanoic acids).

[0032] MOF-808(CH ₃ ): AS seen above in equation (a), to MOF-808(CH ₃ /CI) (500 mg) suspended in water (55 mL), sodium acetate (443 mg) was added. The suspension was stirred 24 hours at 25 °C, then the solids isolated by centrifugation. The white solid was washed with water (2 x 60 mL) then acetone (3 x 60 mL), then isolated by centrifugation and dried in vacuo.

[0033] MOF-8Q8(X), X = Ph, CF ₃ , C7F15: As seen above in equations (b, c, d), MOF-

808(CH ₃ /CI) (200 mg) was suspended in DMF (22 mL), then benzoic (X = Ph, 269 mg), trifluoroacetic (X = CF ₃ , 159 pL) or perfluorooctanoic (X = C7F15, 911 mg) acids were added. The reactions were stirred 24 hours at 60 °C. The solids were isolated by centrifugation, washed with DMF (30 mL), then resuspended in DMF (20 mL) and heated (60 °C) with stirring for 12 hours. Hydrophobicity Modification of MOF-808(CH ₃ /CI)

[0034] Post-synthetic modification to replace the non-structural acetate and chloride ligands was performed with characterisation of the products achieved by diffraction and TGA analysis. Crystallinity is maintained in all the compounds following non-structural linker modification as shown by PXRD of figures 12(a). The PXRD of MOF-808(CH ₃ ) is calculated from single crystal data (black) and experimental patterns of various modified MOF-808(X) synthesised via post-synthetic modification with organic acids. The left panel of figure 12(a) shows magnification of high angle data, noting the change in reflection intensity for MOF- 808(C ₇ FI ₅ ) due to addition of electron rich C7F15 chains in the framework voids. Water drop tests show use of perfluorooctanoic acid (X = C7F15) modifier produces hydrophobic MOF- 808(C ₇ FIS) as seen in figure 12(b). Other acid modifiers only give compounds completely wetted by water. Optimisation of perfluoroalkyl chain length could be considered to balance hydrophobicity and CO ₂ uptake.

[0035] TGA indicates the modified MOF-808(X) frameworks remain stable up to ca. 300 °C. The thermal stability of the fluoroalkyl (X = CF3, C ₇ FIS) is slightly less than the alkyl and aryl modified frameworks, likely due to decomposition of the perfluoroalkyl chain. The as- synthesised frameworks show significant solvent content, as shown by mass loss below ca. 150 °C. Assuming this solvent corresponds exclusively to water and complete non-structural modification, this corresponds to ca. 40, 50, 46 and 26 water molecules per unit formula for X = CH ₃ , Ph, CF ₃ , C ₇ FI ₅ . Reduced solvent content of MOF-808(C ₇ FI ₅ ) is consistent with the presence of hydrophobic perfluoroalkyl chains in the pores. Figures 16 shows TGA of MOF- 808(CH ₃ ) (red), MOF-808(Ph) (blue), MOF-808(CF ₃ ) (green), and MOF-808(C ₇ FI ₅ ) (purple) at a heating rate of 5 °C/minute in a flow of N ₂ .

Synthesis of Modified Zr ₆ O ₄ (OH)4(btc)2(XCOO)6 Frameworks (MOF-808(X), X = CH ₃ , Ph,

CF ₃ , C7F15)

[0036] MQF-808(CH ₃ ): AS seen above in equation (a), to MOF-808(CH ₃ /CI) (500 mg) suspended in water (55 mL), sodium acetate (443 mg) was added. The suspension was stirred 24 hours at 25 °C, then the solids isolated by centrifugation. The white solid was washed with water (2 x 60 mL) then acetone (3 x 60 mL), then isolated by centrifugation and dried in vacuo.

[0037] MOF-8Q8(X), X = Ph, CF ₃ , C7F15: As seen above in equations (b, c, d), MOF-

808(CH ₃ /CI) (200 mg) was suspended in DMF (22 mL), then benzoic (X = Ph, 269 mg), trifluoroacetic (X = CF ₃ , 159 pL) or perfluorooctanoic (X = C7F15, 911 mg) acids were added. The reactions were stirred 24 hours at 60 °C. The solids were isolated by centrifugation, washed with DMF (30 mL), then resuspended in DMF (20 mL) and heated (60 °C) with stirring for 12 hours. The solids were isolated by centrifugation then resuspended DMF (20 mL) and heated (60 °C) with stirring 12 hours. The solids were isolated by centrifugation, then washed with acetone (3 x 30 mL) then dried in vacuo.

Amine Loading in MOF-808(X) Frameworks

Zr _s O ₄ (OH ) (bfc) ₂ (XCOO) _s

MeOH, 25 C [0038] As seen in the above equation, MOF-808(X) (100 mg) was suspended in methanolic solution of PEI-800 (7.5 mg/mL, 10 ml) then stirred vigorously for 4 days. After 4 days, the reactions were concentrated to dryness by rotary evaporation, then dried in vacuo (25 °C, 1 .5 mbar) for 24 hours before TGA. Workup of amine-samples of MOF-808(X, X =CH ₃ , Ph, CF ₃ and C7F15,) according to equation (a, b, c, d) proceeded and these are termed PEI@MOF-808(X). The initially synthetic trials used 100 mg each modified MOF-808 material with loading of 75 wt% PEI-800. Workup of these reactions yielded tacky powders (paste in case of MOF-808(C7F15)). These samples were characterised by TGA to determine stability to desolvation/activation procedures prior to gas sorption. As evident from figure 14, the PEI@MOF-808(X) compounds have relatively low thermally stability, undergoing decomposition at approximately 100 °C lower temperature compared with the pristine MOF- 808(X) compounds. Figure 14 depicts TGA of PEI@MOF-808(CH ₃ ) (red), PEI@MOF- 808(Ph) (blue), PEI@MOF-808(CF ₃ ) (green), and PEI@MOF-808(C7Fi5) (purple) at a heating rate of 5 °C/minute in a flow of N ₂ .

[0039] PXRD studies are shown in figure 15 indicating that all pristine MOF-808(X) samples are crystalline. PEI impregnation only affects the synthesis of PEI@MOF-808(C ₇ FI ₅ ), where a reduction in crystallinity is observed. The remaining composite samples show no appreciable loss in crystallinity. Figure 15 shows PXRD of pristine MOF-808(X) and PEI@MOF-808(X) samples.

[0040] Figures 16(a) to 16(d) illustrate CO ₂ sorption results on all materials indicating that PEI impregnation improves the low pressure uptake of CO ₂ from 0 to 0.4 mmol g ^-1 . The best CO ₂ performances are observed in MOF-808(CF ₃ ) and MOF-808(Ph) seen in figures 16(b) and 16(c).

Shortening Reaction Times for Impregnation

[0041] Alternative reaction methods for amine impregnated were explored for shortening the reaction time with overnight amine impregnation at 60 °C. MOF-808(X) (X = CF ₃ , Ph) were the chosen MOFs for impregnation as they showed the most promising gas sorption data. After loading the sample at approximately 56 wt% (of the entire composite material), PXRD studies revealed that the composites retained their crystallinity, see figures 17(a) and 17(b). The composites were also crystalline following gas sorption studies. The use of ethanol at 60 °C appears to lead to a phase change upon activation for gas sorption, evidenced by sharper peaks at 20 = 4° and a merging of the two peaks around 20 = 8°. CO ₂ sorption results show a modest improvement in CO ₂ uptake at lower pressures when the composite is treated at 60 °C in ethanol, see figures 17c and 17d. For the MOF-808(CF ₃ ) composites, there is an increase at lower pressure by 0.05 mmol/g if the composite is synthesised in ethanol. For the MOF-808(Ph) composite, the improvement is 0.1 mmol/g. These experiments will be performed using TEPA as the amine agent on the MOF-808(X).

[0042] Figure 17 shows PXRD of a) PEI@MOF-808(CF ₃ ) and b) PEI@MOF-808(Ph) samples, loaded at 56 wt% of the final composite material mass. PXRD patterns were taken directly before and directly after CO ₂ sorption experiments and are compared to predicted MOF-808 (black). CO ₂ sorption isotherms at 298 K of c) PEI@MOF-808(CF ₃ ) and d) PEI@MOF-808(Ph), synthesised either stirring in methanol for four days at room temperature (black) or stirring in ethanol overnight at 60 °C (red). Closed and open symbols represent adsorption and desorption, respectively.

TEPA Loading in MOF-808 Frameworks

[0043] When loaded with TEPA (at 25 wt% of the MOF), a CO ₂ capacity of 0.4 mmol/g was observed. This could be improved to 0.6 mmol/g when the loading of TEPA was increased to 50 wt% of the MOF. Importantly, PXRD of the samples following gas sorption studies revealed that the composites retained their crystallinity. The basic nature of TEPA suggests there is a maximum loading that can be used for MOF-808 before the MOF loses its crystallinity (MOF-808 is stable to around pH 8). Figure 18 depicts a) CO ₂ soprtion isotherms at 298 K of TEPA@MOF-808 (25 wt% of MOF - black, 50 wt% of MOF - red). Closed and ope symbols represent adsorption and desorption, respectively, b) PXRD of as-synthesised MOF-808 (black) and TEPA@MOF-808 (25 wt% (red) and 50 wt% (blue) of the MOF) following gas sorption analysis.

TEPA@MOF-808(X) (X = CH ₃ , CF ₃ , Ph and C7F15) with lower capacities

[0044] It should be understood that TEPA@MOF-808(X) (70 wt% of the final composite) yielded CO ₂ capacities of up to 1 .3 mmol/g at low pressures. However, the crystallinity was compromised due to the basic nature of the amine. With this in mind, the synthesis of the TEPA@MOF-808 loading was revisited to reduce the amount of amine. A reduction in TEPA to 50 wt% of the final compound demonstrated improved crystallinity of the final composite, see figure 19a. There appears to be a dependence of the modulator on the stability of MOF- 808(X) to TEPA, where the fluorinated modulators maintain good crystallinity. Crystallinity is maintained in the composites following gas sorption. CO ₂ sorption studies at 298 K reveal capacities between 0.7-1.1 mmol/g CO ₂ at low pressures, see figure 19b, which do not significantly deviate from those capacities of the 70 wt% loaded composites at 0.8-1 .3 mmol/g. It should be noted that there is little deviation in capacity of the 50 wt% loaded samples containing fluorinated modulators from their 70 wt% analogues. Figure 19 shows a) PXRD of pristine MOF-808 and TEPA@MOF-808(X) samples, loaded at 50 wt% of the final composite material mass. XRD patterns were taken directly before and directly after CO ₂ sorption experiments b) CO ₂ sorption isotherms at 298 K of TEPA@MOF-808(CH ₃ ) (black), TEPA@MOF-808(CF ₃ ) (red), TEPA@MOF-808(Ph) (blue) and TEPA@MOF-808(C ₇ FI ₅ ) (green) loaded at 50 wt%. Closed and open symbols represent adsorption and desorption, respectively.

Defect Generation in MOF-808

MOF-808 EDTA infiltration

[0045] MOF-808 was infiltrated with ethylenediaminetetraacetric acid (EDTA) by stirring the MOF in an EDTA solution. TGA analysis was carried out to determine whether infiltration had been successful, see figure 20. There is a large mass loss at the start of the analysis. After this initial mass loss region there are three subsequent losses. The loss at 550°C corresponds to framework thermal decomposition, the loss at 350°C corresponds to modulator loss (in this case acetic acid), while the loss at 250°C likely corresponds to loss of the infiltrated amine. Despite the EDTA infiltration, the low-pressure CO ₂ sorption did not materially improve, see figure 21 .

MOF-808 Etching in NH ₄ HCO ₃

[0046] MOF-808 was etched by stirring in a 0.08 M NH ₄ HCO ₃ solution for 10 minutes in order to break the Zn-O bonds thus increasing the number of defects present in the MOF-808 structure. The CO ₂ sorption isotherm shows an increase in low pressure CO2 uptake, see figure 22.

[0047] Based on the improvement in low pressure CO ₂ sorption, the etching process can be tuned. NH ₄ HCO ₃ concentrations of between around 0.02 M to 0.32 M are expected to be suitable and, depending on the concentration of the etching agent used, etching times of between around 5 minutes to 80 minutes are appropriate. MOF-808 was stable to both the etching process and the EDTA impregnation as demonstrated by the PXRD of figure 23. Some loss in crystallinity was observed upon etching as evidenced by the increase in peak width. Figure 23 shows PXRD patterns of MOF-808_EDTA, MOF-808_etched, and as synthesized MOF-808. [0048] Now that various embodiments of both aspects of the invention have been described it will be apparent to those skilled in the art that they have at least the following advantages:

1 . the method produces a modified Zr-based MOF being effective in chemisorption of CO ₂ from air in preference to water;

2. the method in functionalising the Zr-based MOF allows for tuning of the modified Zr- based MOF influencing its chemisorption of CO ₂ and hydrophobicity;

3. the method provides effective amine loading of the Zr-based MOF without degradation;

4. the Zr-based MOF can be functionalised to increase its internal hydrophobicity or etched for the generation of defects to increase the CO ₂ adsorption capacity of the modified Zr-based MOF with immaterial losses in crystallinity.

[0049] Those skilled in the art will appreciate that the invention as described herein is susceptible to variations and modifications other than those specifically described. All such variations and modifications are to be considered within the scope of the present the nature of which is to be determined from the foregoing description.