Metal organic frameworks for the capture of volatile organic compounds

Abstract

The use of porous crystalline solids constituted of a metal-organic framework (MOF) for the capture of polar volatile organic compounds (VOCs). In particular, the MOF of interest are material having an average pores sizes of 0.4 to 0.6 nm and an hydrophobic core formed by a metal oxide and/or hydroxide network connected by linkers, the linkers being selected from the group including (i) C.sub.6-C.sub.24 aromatic polycarboxylate linkers, such as benzyl or naphtyl di-, tri- or tetracarboxylate, and (ii) C.sub.6-C.sub.16 polycarboxylate aliphatic linkers; the linkers bearing or not apolar fluorinated groups, e.g. —(CF.sub.2)n—CF.sub.3 groups, n being a integer from 0 to 5, preferably 0 ou 3, and/or apolar C.sub.1-C.sub.20 preferably C.sub.1-C.sub.4 alkyl groups, e.g. —CH.sub.3 or —CH.sub.2—CH.sub.3, grafted directly to the linkers and pointing within the pores of the MOF. The MOF solids used in the present invention can be used for the purification of air, for example for the capture of polar VOCs like acetic acid and aldehydes from indoor air in cars, museums and archives, much more efficiently than common adsorbents, particularly in presence of above normal levels of humidity. They can in particular be used for the preservation of cultural heritage.

Claims

1. A process for adsorbing polar volatile organic compounds present at a concentration in the range of 10 ppb to 100 ppm (volume/volume) in a gaseous environment comprising the step of contacting with said environment a porous Metal-Organic Framework (MOF) material comprising an average pore size of 0.4 to 0.6 nm and an hydrophobic core formed by a metal oxide and/or hydroxide network connected by linkers, said linkers being selected from the group consisting of: C.sub.6-C.sub.24 aromatic polycarboxylate linkers; C.sub.6-C.sub.16 polycarboxylate aliphatic linkers; C.sub.4-C.sub.16 polycarboxylate aliphatic linkers; and imidazole-based linkers; each of the aforementioned linkers optionally bearing apolar fluorinated groups and/or apolar C.sub.1-C.sub.20 groups grafted directly to the linkers and pointing within the pores of the MOF.

2. The process according to claim 1, wherein the C.sub.6-C.sub.24 aromatic polycarboxylate linkers are selected from the group consisting of C.sub.6H.sub.4(CO.sub.2.sup.−).sub.2 (terephthalate), C.sub.10H.sub.6(CO.sub.2.sup.−).sub.2 (naphthalene-2,6-dicarboxylate), C.sub.12H.sub.8(CO.sub.2.sup.−).sub.2 (biphenyl-4,4′-dicarboxylate), C.sub.6H.sub.3(CO.sub.2.sup.−).sub.3 (benzene-1,2,4-tricarboxylate), C.sub.6H.sub.3(CO.sub.2.sup.−).sub.3 (benzene-1,3,5-tricarboxylate), C.sub.24H.sub.15(CO.sub.2.sup.−).sub.3 (benzene-1,3,5-tribenzoate), C.sub.6H.sub.2(CO.sub.2.sup.−).sub.4 (benzene-1,2,4,5-tetracarboxylate, C.sub.10H.sub.4(CO.sub.2.sup.−).sub.4 (naphtalene-2,3,6,7-tetracarboxylate), C.sub.10H.sub.4(CO.sub.2.sup.−).sub.4 (naphtalene-1,4,5,8-tetracarboxylate), C.sub.12H.sub.6(CO.sub.2.sup.−).sub.4 (biphenyl-3,5,3′,5′-tetracarboxylate), and modified analogues selected from 2-methyl terephthalate, 2,5-dimethyl terephthalate, tetramethyl terephthalate, perfluoromethyl terephthalate, diperfluoromethyl terephthalate, 2-chloroterephthalate, 2-bromoterephthalate, 2,5-tetrafluoroterephthalate, tetrafluoroterephthalate, dimethyl-4,4′-biphenyldicarboxylate, tetramethyl-4,4′-biphenyldicarboxylate, dicarboxy-4,4′-biphenyldicarboxylate, azobenzene dicarboxylate, and azobenzene tetracarboxylate.

3. The process according to claim 1, wherein the C.sub.4-C.sub.16 polycarboxylate alkyl linkers are selected from di-, tri- and tetracarboxylate or carboxylic acid linkers.

4. The process according to claim 2, wherein the linkers optionally bear apolar fluorinated —(CF.sub.2)—CF.sub.3 or —CF.sub.3 groups grafted directly to the linkers and pointing within the pores of the MOF.

5. The process according to claim 1, wherein the linkers optionally bear —CH.sub.3 or —CH.sub.2—CH.sub.3, groups grafted directly to the linkers and pointing within the pores of the MOF.

6. The process according to claim 1, wherein the metal atom of the metal oxide and/or hydroxide is selected from Li, Na, Rb, Mg, Ca, Sr, Ba, Sc, Ti, Zr, Ta, Cr, Mo, W, Mn, Fe, Ru, Os, Co, Ni, Pd, Pt, Cu, Au, Zn, Al, Ga, In, Si, Ge, Sn, Bi, Cd, Mn, Tb, Gd, Ce, La, and Cr.

7. The process according to claim 1, wherein the MOF is selected from the group consisting of MIL-140B, MIL-140C, UiO-66-2CF.sub.3, UiO-NDC, UiO-66-(CH.sub.3).sub.2, ZIF, ZIF-8, MIL-53, MIL-69 and MIL-88B-4CH.sub.3.

8. The process according to claim 1, wherein the polar volatile organics compounds are selected from the group consisting of acetic acide, acetaldehyde, formaldehyde and a mixture of two or three thereof.

9. The process according to claim 1, wherein the MOF is in the form of a powder or granules or embedded in the form of a composite material, or embedded in or applied onto the surface of a paper sheet or a polymer or a fiber.

10. The process according to claim 1, wherein the gaseous environment is air having >30% relative humidity.

11. The process according to claim 1, wherein the linkers are selected from the group consisting of: C.sub.6-C.sub.24 aromatic polycarboxylate linkers, C.sub.6-C.sub.16 polycarboxylate aliphatic linkers, and C.sub.4-C.sub.16 polycarboxylate aliphatic linkers.

12. The process according to claim 1, wherein the C.sub.6-C.sub.24 aromatic polycarboxylate linkers are benzyl or naphthyl di-, tri- or tetracarboxylates.

13. The process according to claim 1, wherein the apolar fluorinated groups are —(CF.sub.2).sub.n—CF.sub.3 groups, n being an integer from 0 to 5.

14. The process according to claim 1, wherein the apolar C.sub.1-C.sub.20 groups are C.sub.1-C.sub.4 alkyl groups.

15. The process according to claim 13, wherein the apolar C.sub.1-C.sub.20 groups are —CH.sub.3 or —CH.sub.2—CH.sub.3.

16. The process according to claim 3, wherein the C.sub.4-C.sub.16 polycarboxylate alkyl linkers are selected from C.sub.2H.sub.2(CO.sub.2.sup.−).sub.2 (fumarate), C.sub.2H.sub.4(CO.sub.2.sup.−).sub.2 (succinate), C.sub.3H.sub.6(CO.sub.2.sup.−).sub.2 (glutarate), (C.sub.4H.sub.4)(CO.sub.2.sup.−).sub.2 (muconate), and C.sub.4H.sub.8(CO.sub.2.sup.−).sub.2 (adipate).

17. The process according to claim 7, wherein the MOF is selected from the group consisting of MIL-140B, MIL-140C, UiO-66-2CF.sub.3, UiO-NDC, UiO-66-(CH.sub.3).sub.2, MIL-53, and MIL-88B-4CH.sub.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: shows the obtained nitrogen adsorption isotherms, at −196° C., on a) UiO-66, UiO-66-NH.sub.2, UiO-66-2CF.sub.3 and UiO-NDC and MIL-53(Fe)-2CF.sub.3; and b) MIL-101-Cr, MIL-101-Cr—NH.sub.2, MIL-140A, MIL-140B and ZIF-8.

(2) FIG. 2: shows, acetic acid adsorption isotherms, at 25° C., expressed as equivalent liquid adsorbed volume.

(3) FIG. 3: shows the nitrogen and water adsorption isotherms results, at −196° C. and 30° C. respectively, on: a) UiO-66, UiO-66-NH.sub.2, UiO-66-2CF.sub.3 and UiO-NDC; and b) MIL-101-Cr, MIL-101-Cr—NH.sub.2, MIL-140A and MIL-140B. Adsorbed volume assuming a liquid like state at the adsorption temperature.

(4) FIG. 4: shows water adsorption isotherms results, at 30° C., on: a) UiO-66, UiO-66-NH.sub.2, UiO-66-2CF.sub.3 and UiO-NDC; and b) MIL-101-Cr, MIL-101-Cr—NH.sub.2, MIL-140A and MIL-140B, amounts expressed by surface area of each material

(5) FIG. 5: shows: a) results of a blanc experiments with no injection of acetic acid (baseline) and the injection of acetic acid without the presence of adsorbents. b) results with acetic acid concentration profiles inside a closed chamber after the injection of 1 μL of acetic acid, at 23° C., 40% RH, with the presence of 50 mg of MIL-140A, MIL-140B, MIL-140C, UiO-66-2CF.sub.3, MIL-53-2CF.sub.3 and ZIF-8.

(6) FIG. 6: shows acetaldehyde concentration profiles results inside a closed chamber after the injection of 14 of acetaldehyde, at 23° C., 40% RH, with the presence of 50 mg of UiO-66-2CF.sub.3, MIL-140B, and a standard commercial activated carbon RB4. The best results are obtained for MIL-140B and C UiO-66-2CF.sub.3.

(7) FIG. 7: shows nitrogen adsorption isotherms, at −196° C., of UiO-66-2CH.sub.3 of Example 6.

(8) FIG. 8: shows acetic acid concentration profiles results inside a closed chamber after the injection of 1 μL of acetic acid, at 27° C., 40% RH, with the presence of 50 mg of UiO-66-2CH.sub.3.

EXAMPLES

(9) According to the present disclosure, the usable MOFs materials and their preparation can be understood further by the examples that illustrate some of the processes by which these materials are prepared or used. It will be appreciated, however, that these examples should not be construed to limit the disclosure. Variations of the disclosure, now known or further developed, are considered to fall within the scope of the present disclosure as described herein and as hereinafter claimed.

Example 1

Materials Synthesis

(10) 1.1. MIL-101(Cr): MIL-101(Cr) was obtained via a hydrothermal treatment of a mixture of terephthalic acid (166 mg, 1 mmol), Cr(NO.sub.3).sub.3.9H2O (400 mg, 1 mmol), HF (0.2 mL, 1 mmol) and deionized water (4.8 mL, 265 mmol) heated up to 220° C. for 8 h as disclosed in document G. Férey, et al. Science 2005, 309, 2040-2042 [27]. After cooling down the autoclave, a green powder can be removed and washed. Removing the large excess of unreacted terephthalic acid from the powder is performed by following this purification process. First, with a glass filter whose the pore size is between 40 and 100 μm, the solution is filtered off twice to remove insoluble terephthalic acid from the solution. Then, the product is put into an autoclave to be washed with ethanol at 80° C. for 24 h. After this step, the solid is mixed into a solution of 1M of NH.sub.4F at 70° C. for 24 h followed by a filtration and a wash with hot water. The resulting product is then dried overnight at 150° C.

(11) 1.2. MIL-101(Cr)-EN: In order to get ethylenediamine grafted MIL-101(Cr), after dehydratation of 0.5 g of MIL-101(Cr) heated at 150° C. for 12 h, the solid was suspended in 30 mL of anhydrous toluene, as disclosed in document Y. K. Hwang, et al. Angew. Chem. Int. Ed. Engl. 2008, 47, 4144-4148 [28]. Afterwards, ethylenediamine (0.05 mL, 0.75 mmol) was added to the suspension and stirred under reflux for 12 h. After the reaction, the material was filtered out, washed with deionized water and ethanol, and dried at room temperature (i.e. in the present examples at a temperature between 18° C. and 28° C.).

(12) 1.3. ZIF-8: For the synthesis of ZIF-8, Zn(NO.sub.3).sub.2.6H.sub.2O (2.933 g, 9.87 mmol) was firstly solubilized in 200 mL of methanol. The same operation was carried out for the ligand by putting 2-methylimidazole (6.489 g, 79.04 mmol) into 200 mL of methanol, as disclosed in document A. Demessence et al., Adsorption properties in high optical quality nanoZIF-8 thin films with tunable thickness, J. Mater. Chem., 2010, 20, 7676-7681 [29]. After the solubilization of the species, the solution with the metal was quickly poured into the ligand mixture under stirring at room temperature (i.e. in the present examples at a temperature between 18° C. and 28° C.). Slowly, the solution became less translucent. After 1 h, the reaction was stopped and the solid was separated from the liquid by centrifugation for 15 min at 20000 rpm. The particules were then washed with absolute ethanol and centrifugated three times to remove the excess of unreacted salt and ligand. The solid was then dried at room temperature (i.e. in the present examples at a temperature between 18° C. and 28° C.) overnight.

(13) 1.4. UiO-66: UiO-66(Zr) was synthesized by mixing ZrCl.sub.4 (5.825 g, 25 mmol), terephthalic acid (8.300 g, 50 mmol), HCl (1.54 mL, 50 mmol, 37%) in 150 mL of N,N-Dimethylformamide (DMF). The solution was then transferred into a 750 mL Teflon liner and heated overnight at 220° C. in oven. The solid obtained was filtered off, washed with DMF twice followed by two washed with acetone and dried at room temperature (i.e. in the present examples at a temperature between 18° C. and 28° C.).

(14) 1.5. UiO-66-NH.sub.2: In order to get UiO-66-NH.sub.2, a solution composed of ZrCl.sub.4 (233 mg, 1 mmol) and 2-aminoterephthalic acid (181 mg, 1 mmol) was prepared in 3 mL of DMF and put into a 23 mL Teflon liner, as disclosed in document C. Gomes Silva, et al. Chem. Eur. J. 2010, 16, 11133-11138 [30]. The mixture was heated in oven at 100° C. for 24 h. The solid was then recovered after filtration and treated with DMF. The material was then left in DMF at room temperature (i.e. in the present examples at a temperature between 18° C. and 28° C.) overnight under stirring. After a new filtration, the solid was washed twice with THF and dried at room temperature (i.e. in the present examples at a temperature between 18° C. and 28° C.).

(15) 1.6. UiO-66-2CF.sub.3: The synthesis conditions of UiO-66-2CF.sub.3 was like to those of UiO-66 previously mentioned. ZrCl.sub.4 (582 mg, 2.5 mmol), diperfluoromethyl terephthalic acid (595 mg, 2.5 mmol), HCl (0.077 mL, 2.5 mmol, 37%) were dissolved and mixed in a 125 mL Teflon liner and then heated in oven at 100° C. for 24 h. The obtained product was filtered off followed by two washes with DMF and two others with THF. The product was left at room temperature (i.e. in the present examples at a temperature between 18° C. and 28° C.) overnight for the evaporation of the solvent.

(16) 1.7. UiO-NDC: In a 50 mL round-bottom flask, UiO-NDC was obtained by firstly solubilizing 2,6-naphthalenedicarboxylic acid (1.296 g, 6 mmol) in 108 mL of DMF at 90° C. as disclosed in document S. Kaskel et al., Cryst Eng Comm 2013, 15 (45), 9572-9577 [31]. Afterwards, benzoic acid (7.32 g, 60 mmol) and HCl (0.98 mL, 32 mmol, 37%) were put into the solution followed by ZrCl.sub.4 (1.398 g, 6 mmol). The mixture was left at 90° C. for 6 h. The solid was separated by centrifugation and washed three times with DMF and twice with EtOH.

(17) 1.8. MIL-140A: ZrCl.sub.4 (13.980 g, 60 mmol), terephthalic acid (19.96 g, 120 mmol) and acetic acid (34 mL, 600 mmol) were put into a 500 mL round-bottom flask filled with 400 mL of DMF as disclosed in document V. Guillerm et al. Angew. Chemie Int. Ed. 2012, 51, 9267-9271. The solution was kept under reflux and stirring for 6 h. The solid obtained after filtration was washed first with 1 L of DMF at 120° C. for 2 h30, filtrated and washed a last time with 1.5 L of MeOH for 12 h.

(18) 1.9. MIL-140B: In a 500 mL round-bottom flask, ZrCl.sub.4 (2.77 g, 11.9 mmol), 2,6-naphthalenedicarboxylic acid (7.08 g, 32.8 mmol) and acetic acid (25.5 mL, 450 mmol) were mixed in 430 mL of DMF and kept under reflux for 7 h. The solution was then filtrated and the resulting solid was washed with 200 mL of DMF at 120° C. for 2 h30 followed with filtration and a last wash with 400 mL of MeOH for 12 h to get after filtration the material of interest, MIL-140B.

(19) 1.10 MIL-53-2CF.sub.3: In a 100 mL Teflon-lined reactor, 0.755 g (2.5 mmol) of 2,5-diperfluoroterephthalic acid, 0.675 g (2.5 mmol) of iron(III) chloride hexahydrate and 25 mL of deionized water were mixed. The resulting mixture is, stirred for ten minutes followed by placing the reactor in a microwave heated to 100° C. for a 20 min period (heating rate 60° C./min). The product is a yellow crystalline solid that can be recovered by centrifugation, and dried in air. The activation was done by heating at 250° C. under vacuum for two days.

Example 2

Materials Characterization

(20) Three series of analysis have been carried out to confirm the obtention of the various materials synthetized in above Example 1: nitrogen adsorption at −196° C.; PXRD patterns of the synthesized materials; and Thermogravimetric analysis of the synthesized materials.

(21) 2.1 Nitrogen adsorption at −196° C.: Nitrogen (Air Liquid, 99.999%) adsorption-desorption isotherms were measured at −196° C. using a liquid nitrogen cryogenic bath, in a volumetric automatic apparatus (Micromeritics, ASAP 2010). Prior to the measurement the samples were outgassed at 150° C. for 8 h at a pressure lower than 0.133 Pa.

(22) 2.2 PXRD patterns of the synthesized materials: The X-ray powder diffraction patterns were obtained with a high resolution D5000 Siemens X'Pert MDP diffractometer (λCu, Kα1, Kα2) from 5 to 20° (2θ) using a step of 0.02° and 10 s of accumulation per step in continuous mode.

(23) 2.3 Thermogravimetric analysis of the synthesized materials: In order to get the TGA profile of each material synthesized, the sample (about 10 mg) were analysed with a gravimetric analyser (Model Perkin Elmer STA 6000) in air at a constant rate of 2° C./min.

(24) All results of these analysis confirm the obtention of the various materials synthetized in example 1.

Example 3

Experiments on Adsorption Capacity of the Different Materials

(25) 3.1. Protocol Used for Water Adsorption Measurements

(26) The adsorption isotherms of water were determined at 30.0±0.1° C. in an automated apparatus, model Omnisorp 100cx (Coulter, USA), using a fixed dosing method. All samples were outgassed at 150° C. during 4 h at a vacuum lower than 10.sup.−2 Pa. The amounts adsorbed on an empty cell were used to correct the data of the adsorption isotherms.

(27) 3.2. Protocol Used for Acetic Acid Adsorption Measurements

(28) Adsorption isotherms of were measured by the volumetric method at low relative pressure, up to 0.06 p/p.sup.0, on about 50 mg different samples of the materials synthetized in example 1, each outgassed as described above. Adsorption temperature was maintained with a water bath (Grant GD120) at 25° C. The pressure was measured with a capacitance transducer from Pfeiffer Vacuum (CMR 262). Nonideality of the phase was accounted by the use of the compressibility factor z, expressed as function of the pressure p, given by the equation z=0.351+0.729 p.sup.−0.176, which was obtained by fitting data published in document F. H. MacDougall, J. Am. Chem. Soc. 1936, 58, 2585 [32].

(29) 3.3. Protocol Used for Acetic Acid Adsorption in a Controlled Relative Humidity Environment

(30) About 100 mg of materials were placed inside 10 cm.sup.3 glass vials and kept in an oven at 100° C. over night. The vials were removed and closed tightly with polyethylene caps until being used in the experiments. 50 mg of adsorbent material was weighted (Mettler AE240) in a watch glass and immediately placed inside a glass chamber (2.9 dm.sup.3) with controlled humidity. The humidity was controlled to about 40% relative humidity by means of a saturated solution of potassium carbonate (BDH Prolabo, 99.6%) in an open petri dish, as disclosed in document L. Greenspan, J. Res. Natl. Bur. Stand. Sect. A Phys. Chem. 1977, 81A, 89 [33]. The chamber was flushed with nitrogen flow during 15 minutes and the material was allowed to equilibrate with the humidified atmosphere for 1.5 hour. After this time, a syringe (Hamilton 7001 KH) was used to inject 1 μL of acetic acid (Riedel-de Haën, 99.8%) inside the chamber trough a rubber septum injection port, in the surface of a clean paper filter to improve the spreading and evaporation of the small droplet. Immediately before injection of acetic acid, the humidifier with potassium carbonate was removed to assure that acetic acid removal is only due to the tested MOF. The total volatile organic compounds (TVOC) concentration, temperature and relative humidity inside the chamber were measured (Graywolf TG-502 TVOC ppb) and were recorded at fixed time interval (15 s) during one hour using computer software (Wolfsense LAP). During experiments, the temperature was 22.8±0.8° C. and the relative humidity was 39.6±3.5%. A blank experiment with no injection of acetic acid was preformed and a control experiment was preformed with injection of acetic acid without any MOF to demonstrate the tightness of the chamber during the experiments time frame, and ascertain the TVOC signal response obtained by 1 μL injection.

Example 4

Results of the Analysis

(31) 4.1 Nitrogen Adsorption

(32) Annexed FIG. 1 shows the obtained nitrogen adsorption isotherms, at −196° C., on a) UiO-66, UiO-66-NH.sub.2, UiO-66-2CF.sub.3 and UiO-NDC; and b) MIL-101-Cr, MIL-101-Cr—NH.sub.2 MIL-140A, MIL-140B and ZIF-8.

(33) 4.2 Acetic Acid Adsorption

(34) Annexed FIG. 2 shows, acetic acid adsorption isotherms, at 25° C., expressed as equivalent liquid adsorbed volume.

(35) 4.3 Water Adsorption and Assessment of Hydrophobic/Hydrophilic Character of Surfaces

(36) The water adsorption isotherms represented on annexed FIG. 3 display the different hydrophobic/hydrophilic character of the different materials. This figure shows nitrogen and water adsorption isotherms, at −196° C. and 30° C. respectively, on: a) UiO-66, UiO-66-2CF.sub.3 and UiO-NDC; and b) MIL-101-Cr, MIL-101-Cr—NH.sub.2, MIL-140A and MIL-140B, adsorbed volume assuming a liquid like state at the adsorption temperature. The water amounts adsorbed in each material were not dependent on the pore volume (or surface area), in agreement with the results reported by other authors for some of the studied MOF and demonstrating the strong influence of the chemical nature of the surface on the results. Also, the different shapes of the isotherms indicate this strong influence. For example, for UiO-66 and UiO-66-NH.sub.2 that have similar microporous volume and pore sizes, the absorbed amounts at 0.7 p/p.sup.0 are very similar (about 22 mmol g.sup.−1), but the inflection points of both isotherms occur at significantly different relative pressures (FIG. 3). The UiO-66-NH.sub.2 exhibit one jump at 0.25 p/p.sup.0, while UiO-66 exhibit two consecutive inflection points at 0.35 and 0.45 p/p.sup.0. On the contrary, UiO-66-2CF.sub.3 displays a lower adsorbed amount than UiO-66 and an inflexion point at about 0.5 p/p.sup.0. Such comparison of inflection points in water isotherms proved helpful for evaluating porous materials' hydrophobicity with different types of surface chemistry and can be correlated with the interaction energy of water with the surface. Thus, results indicate that functionalization of UiO-66 with amines decreases the hydrophobicity of the material, while the contrary occurs with the presence of CF.sub.3 groups. The changing of benzene to naphthalene dicarboxylate linker increases considerably the hydrophobicity of this type of structure as can be seen from the comparison of the UiO-66 and UiO-NDC isotherms (FIG. 3).

(37) Further confirmation of this change in water affinity can be seen if the results are represented as amounts adsorbed by surface area of materials instead of mass, as shown on annexed FIG. 4. FIG. 4 shows water adsorption isotherms, at 30° C., on: a) UiO-66, UiO-66-NH.sub.2, UiO-66-2CF.sub.3 and UiO-NDC; and b) MIL-101-Cr, MIL-101-Cr—NH.sub.2, MIL-140A and MIL-140B, amounts expressed by surface area of each material. This representation is better for comparing the nature of the surface of materials with significantly different specific surface areas. The results depicted in annexed FIG. 4 show that at all the studied materials of the UiO-66 family tend to about the same covering of the surface (between 0.020 and 0.025 mmol m.sup.−2) at high relative pressures, but they are significantly different below the 0.6 p/p.sup.0 pressure region, being UiO-2CF.sub.3 the most hydrophobic one (FIG. 4).

(38) In MIL-101-Cr water adsorption isotherm (FIG. 3), an initial step below 0.1 p/p.sup.0 attributed to the adsorption of water in the open metal sites and Cr metal clusters is observed, followed by an intermediate plateau. The second inflection is observed at a high relative pressure (between 0.6 to 0.75 p/p.sup.0) and the magnitude of the step indicates that most of the surface of the material is hydrophobic. Comparing the amounts adsorbed by surface area on the MIL-101-Cr with those of the UiO-66 family (FIG. 4), it can be concluded that the former is adsorbing much less water per surface. In fact, the nitrogen and water isotherms can be compared considering the adsorbed phase in a liquid like state and use the respective liquid density to compare the adsorbed volumes (FIG. 3). The volumes of water adsorbed on the materials of the UiO-66 type approach those obtained with nitrogen (FIG. 3), although it occurs at different relative pressures for each material. Nevertheless, we can conclude that the micropore volume of these materials become filled with water at pressures above 0.6 p/p.sup.0. On the contrary, for MIL-101-Cr it is evident that, even at 0.8 p/p.sup.0 (after the second inflection point), the porous volume is still far from being completely filled with water (FIG. 3).

(39) The main differences observed in water adsorption between the MIL-101-Cr—NH.sub.2 and the parent MIL-101-Cr are the disappearance of the first inflection point at low pressures and a slow raise in the adsorbed amounts without a defined step at intermediate pressures, for the amine functionalized material. The absence of the first inflection point is most surely related with the presence of the ethylenediamine that occupies the open metal sites, were adsorption occurs at very low pressures. But, the amine groups decrease the hydrophobicity of the material since the adsorbed amounts per surface are higher than those observed for the parent MIL-101-Cr, at intermediate pressures (FIG. 4). Nevertheless, at high pressures the pore volume the MIL-101-Cr—NH.sub.2 (FIG. 3) is still not saturated, similarly to the behaviour of MIL-101-Cr. FIG. 4 puts in evidence the more hydrophobic nature of MIL-101-Cr and MIL-101-Cr—NH.sub.2 materials in relation to the UiO-66 type materials.

(40) The MIL-140A and MIL-140B adsorb significantly less water amounts than the other tested materials. Even when taking into account the low surface area of this material, the amounts adsorbed per surface are significantly below the other materials, except ZIF-8 (FIG. 4), confirming the hydrophobic nature of these materials, which was studied by infrared techniques. Although they are formed by the same type of metal clusters and linker as UiO-66 and UiO-NDC, the structural features of MIL-140A and MIL-140B renders a much more hydrophobic surface. ZIF-8 presents very low adsorbed amounts confirming the hydrophobic character of this MOF, being the most hydrophobic MOF from the tested samples. This can also be confirmed by the analysis of FIGS. 3 and 4.

(41) 4.4 Removal of Acetic Acid with MOFs in the Presence of Moisture

(42) Annexed FIG. 5 shows: a) Blanc experiments with no injection of acetic acid (baseline) and the injection of acetic acid without the presence of adsorbents. b) Acetic acid concentration profiles inside a closed chamber after the injection of 1 μL of acetic acid, at 23° C., 40% RH, with the presence of 50 mg of MIL-140A, MIL-140B MIL-140C, UiO-66-2CF.sub.3 and ZIF-8. Regarding the MIL-140 family of materials, FIG. 5 shows an increase in the efficiency of removal from the MIL-140A to the MIL-140B, because the concentration inside the chamber after one hour is significantly lower in the latter case. When going from the MIL-140B to the larger pores MIL-140C the efficiency of the removal decreases. These results show that MIL-140B have the most suitable properties among the MIL-140 family, by a combination of proper pore size and hydrophobicity, for the removal of acetic acid. FIG. 5b demonstrates the advantage of the presence of the perfluoro methyl groups in the structure of the MOFs. The UiO-66-2CF.sub.3 is the most efficient of the cases presented in FIG. 5b, even more than the very hydrophobic ZIF-8. The effect of these groups in MIL-53-2CF.sub.3 is only noticed after some time because this structure need to change from the closed pore (initial) to the open pore (final) form. This justifies the sharp rise followed by a sharp decrease in the concentration.

(43) Comparing the Adsorption Isotherm Data with the Data Obtained in the Closed Chamber

(44) The Henry's constant reflects the affinity of the MOFs for acetic acid and can be used to estimate adsorbed amounts at very low concentrations. These amounts can be compared with the values estimated in the closed chamber experiments. The values calculated from the concentrations after one hour (assuming equilibrium and no influence from the presence of water) for the best materials (Table 1 below) are ranging from 4.57 to 0.18 μmol g.sup.−1. Comparing with the amounts deduced from the closed chamber measurements (Table 1), one estimates that ZIF-8 is approaching equilibrium.

(45) Remarkably, the estimations based on the acetic acid isotherm are agreeing fairly with observations preformed under the presence of moisture, probably due to the very hydrophobic character of ZIF-8 which prevents the interference of moisture with the acetic acid adsorption. On the contrary, for UiO-66-2CF.sub.3 and MIL140B, differences between the amount estimated from the isotherms and the one measured in the chamber (Table 1) are significant and indicate a strong influence of water on the acetic acid adsorption and that the systems are not close to equilibrium after one hour. In fact, a considerable slope of the concentration profiles inside the chamber at 1 hour is seen, which leads to a drop to 4.2 μg dm.sup.−3 and 9.5 μg dm.sup.−3 after 1.5 hour, for UiO-66-2CF.sub.3 and MIL140B respectively.

(46) TABLE-US-00001 TABLE 1 Comparison of the concentration of acetic acid in the chamber after one hour, respective relative pressure and the adsorbed amounts in the materials. Concentration n.sup.ads n.sup.ads in the chamber Relative from Henry's from FIG. after 1 hour pressure constants .sup.a) 1 .sup.b) Material μg dm.sup.−3 p/p.sup.0 μmol g.sup.−1 μmol g.sup.−1 UiO66- 7.2 1.41 × 10.sup.−4 4.57 0.20 2CF.sub.3 UiO-NDC 20.5 4.00 × 10.sup.−4 0.83 0.18 MIL-140B 12.2 2.38 × 10.sup.−4 4.47 0.19 ZIF-8 20.2 3.94 × 10.sup.−4 0.18 0.18 .sup.a) Adsorbed amounts in the materials estimated from the Henry's constant, at the relative pressure after one hour, assuming equilibrium and only acetic acid adsorption; .sup.b) Adsorbed amounts in the materials estimated from the difference between the concentration in the chamber with materials and the blank experiment, after one hour.

(47) 4.5 Removal of Acetaldehyde with MOFs in the Presence of Moisture

(48) Annexed FIG. 6 shows acetaldehyde concentration profiles inside a closed chamber after the injection of 14 of acetaldehyde, at 23° C., 40% RH, with the presence of 50 mg of UiO-66-2CF.sub.3, MIL-140B, and activated carbon RB4. The best results are obtained for MIL-140B and UiO-66-2CF.sub.3.

Example 5

Results

(49) The removal of low concentrations of acetic acid from indoor air at museums poses serious conservation problems that current adsorbents cannot easily solve due to the competitive adsorption of water. In this work, several topical MOFs with different pore sizes, topologies and pending functional groups have been studied to demonstrate what features are more effective to the challenge of capturing this very polar volatile organic compound in the presence of water. Results show that although increasing the hydrophobicity can have a positive effect in the removal efficiency, it is not sufficient if not accompanied by an increased interaction with acetic acid. The two best materials, MIL-140B and UiO-66-2CF.sub.3, confirm that two strategies are possible to increase selectively the interaction with acetic acid. For MIL-140B, the hydrophobicity combined with the proper pore width promote the acetic acid adsorption by a confinement effect. In the UiO-66-2CF.sub.3, the acetic acid adsorption was enhanced by the introduction of the CF.sub.3 groups that increase the hydrophobicity and the interaction with acetic acid.

Example 6

UiO-66-2CH.SUB.3

(50) Synthesis Procedures

(51) The reaction mixture of 178.13 mg (1 mmol) of Zirconyl chloride octahydrate, (98%), 194.18 mg (1 mmol) of 2,5-Dimethylterephthalic acid (97%) and 3.77 mL (100 mmol) of Formic acid (99%) were dispersed in 8.05 mL (104 mmol) of dimethylformamide (98%). The mixture was placed in a Teflon-lined autoclave (23 mL) for 24 hours at 150° C. Then, the white solid was recovered by centrifugation and washed 3 times with 50 mL of ethanol.

(52) Experimental Section

(53) The X-ray powder diffraction patterns were obtained with a high resolution D5000 Siemens X'Pert MDP diffractometer (λ.sub.Cu, Kα.sub.1, Kα.sub.2). Thermogravimetric analysis was performed with a thermogravimetric analyzer (Model Perkin Elmer STA 6000) in air at a constant heating rate of 2° C./min. Transmission IR spectra were measured using Nicolet 6700 spectrometer. Nitrogen physisorption isotherms were measured at T=77K with a Micromeritics 3Flex surface characterisation analyser. Prior to the measurements, the powders (50-80 mg) were outgassed for 6 h at T=373K under a 10.sup.−1 mbar vacuum.

(54) Results

(55) The successful synthesis of UiO-66-2CH.sub.3 was confirmed by powder X-ray diffraction (PXRD) studies. Characteristic peaks at 2θ=7.34°, 8.48° confirm UiO-66 structure. The UiO-66-2CH.sub.3 has a face-centered cubic (fcc) unit cell, space group: Fm-3m.

(56) The IR spectrum of UiO-66-2CH.sub.3 showed: an absorption band at 1574 cm.sup.−1 indicating the existence of the reaction of COOH with Zr.sup.4+, the aromatic bound C═C from ligand is referred at 1493 cm.sup.−1 to C═C from aromatic; and bands at 2969 and 2933 cm.sup.−1 representing asymmetric stretchings of methyl groups.

(57) Using thermogravimetric analysis (TGA) the thermal stability of UiO-66-2CH.sub.3 was investigated. Three weight loss steps were observed. The first weight loss of 3.4 wt % occurred between 20 and 60° C. due to vaporization of water and ethanol. The second step of weight loss was 4.1 wt % at 60-300° C. due to dehydroxylation of OH.sup.−. The third step of weight loss was 49 wt % at 300-550° C. due to decomposition of material.

(58) FIG. 7 shows N.sub.2 adsorption isotherms collected on the material. BET surface area is 1563±8 m.sup.2/g, the maximum pore volume is 0.625566 cm.sup.3/g and the median pore width: 5,231 Å. It can be seen that UiO-66-2CH.sub.3 presented higher surface area (calculated using the BET theory), than theoretical value 1200 m.sup.2/g. The higher surface area can be explained by presence of the defects in the structure, like «missing linker defects» (which incidentally also explains its surface area higher than the bare UiO-66 MOF). The presence of this type of defects is attributed to a high degree of connectivity of the clusters.

(59) In addition, UiO-66-2CH.sub.3 (bearing methyl groups, as compared to UiO-66) enxhibits an enhanced/improved capture performance of acetic acid. FIG. 8 represents the decrease of the acetic acid concentration as a function of time, when 1 μL is injected in the chamber (as in the protocol of example 3, at 27° C.). The UiO-66-2CH.sub.3, which is the equivalent structure of UiO-66 but with two methyl groups per linker, therefore presents an advantage over the structure without methyl groups. In addition, the UiO-66-2CH.sub.3 material is advantageous due to the easy preparation and consequently lower production costs (the carboxylic acid used for the UiO-66-2CH.sub.3 synthesis (terephthalic acid with two methyl substitution on the aromatic ring; (HCO.sub.2).sub.2—C.sub.6H.sub.2—(CH.sub.3).sub.2) is available commercially) than the equivalent UiO-66-2CF.sub.3 with perfluoro groups instead of methyl groups. The results for UiO-66-2CH.sub.3 were obtained at a temperature (27° C.) slightly higher than those for UiO-66 (23° C.), implying that a slightly better comparative performance is expected for UiO-66-2CH.sub.3 if the acetic acid capture is performed at the same temperature.

(60) It is proposed that alternative synthesis routes to UiO-66-2CH.sub.3 (without inhibitors such as monocarboxylic acids) may lead to a lower defect content, a lower surface area but a more hydrophobic character, that may improve significantly the acetic acid capture of this MOF.