GB2573886A - Process of preparing metal-organic framework material - Google Patents

Process of preparing metal-organic framework material Download PDF

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Publication number
GB2573886A
GB2573886A GB1905526.8A GB201905526A GB2573886A GB 2573886 A GB2573886 A GB 2573886A GB 201905526 A GB201905526 A GB 201905526A GB 2573886 A GB2573886 A GB 2573886A
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metal
process according
abtc
ligand precursor
metal salt
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GB201905526D0 (en
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Matheson Ken
K Ozdemir Osman
Tischler Andrew
Robert Kaufman Joel
Thompson Raul
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Vapor Point LLC
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Vapor Point LLC
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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C245/00—Compounds containing chains of at least two nitrogen atoms with at least one nitrogen-to-nitrogen multiple bond
    • C07C245/02—Azo compounds, i.e. compounds having the free valencies of —N=N— groups attached to different atoms, e.g. diazohydroxides
    • C07C245/06—Azo compounds, i.e. compounds having the free valencies of —N=N— groups attached to different atoms, e.g. diazohydroxides with nitrogen atoms of azo groups bound to carbon atoms of six-membered aromatic rings
    • C07C245/08—Azo compounds, i.e. compounds having the free valencies of —N=N— groups attached to different atoms, e.g. diazohydroxides with nitrogen atoms of azo groups bound to carbon atoms of six-membered aromatic rings with the two nitrogen atoms of azo groups bound to carbon atoms of six-membered aromatic rings, e.g. azobenzene
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07F—ACYCLIC, CARBOCYCLIC OR HETEROCYCLIC COMPOUNDS CONTAINING ELEMENTS OTHER THAN CARBON, HYDROGEN, HALOGEN, OXYGEN, NITROGEN, SULFUR, SELENIUM OR TELLURIUM
    • C07F11/00—Compounds containing elements of Groups 6 or 16 of the Periodic System
    • C07F11/005—Compounds containing elements of Groups 6 or 16 of the Periodic System compounds without a metal-carbon linkage
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07F—ACYCLIC, CARBOCYCLIC OR HETEROCYCLIC COMPOUNDS CONTAINING ELEMENTS OTHER THAN CARBON, HYDROGEN, HALOGEN, OXYGEN, NITROGEN, SULFUR, SELENIUM OR TELLURIUM
    • C07F15/00—Compounds containing elements of Groups 8, 9, 10 or 18 of the Periodic System
    • C07F15/02—Iron compounds
    • C07F15/025—Iron compounds without a metal-carbon linkage
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07F—ACYCLIC, CARBOCYCLIC OR HETEROCYCLIC COMPOUNDS CONTAINING ELEMENTS OTHER THAN CARBON, HYDROGEN, HALOGEN, OXYGEN, NITROGEN, SULFUR, SELENIUM OR TELLURIUM
    • C07F5/00—Compounds containing elements of Groups 3 or 13 of the Periodic System
    • C07F5/06—Aluminium compounds
    • C07F5/069—Aluminium compounds without C-aluminium linkages
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07F—ACYCLIC, CARBOCYCLIC OR HETEROCYCLIC COMPOUNDS CONTAINING ELEMENTS OTHER THAN CARBON, HYDROGEN, HALOGEN, OXYGEN, NITROGEN, SULFUR, SELENIUM OR TELLURIUM
    • C07F7/00—Compounds containing elements of Groups 4 or 14 of the Periodic System
    • C07F7/003—Compounds containing elements of Groups 4 or 14 of the Periodic System without C-Metal linkages
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07F—ACYCLIC, CARBOCYCLIC OR HETEROCYCLIC COMPOUNDS CONTAINING ELEMENTS OTHER THAN CARBON, HYDROGEN, HALOGEN, OXYGEN, NITROGEN, SULFUR, SELENIUM OR TELLURIUM
    • C07F7/00—Compounds containing elements of Groups 4 or 14 of the Periodic System
    • C07F7/28—Titanium compounds

Abstract

A process of preparing a porous metal-organic framework (MOF) is provided, wherein the process comprises the steps of (i) providing a metal salt of a ligand precursor having at least two carboxylic acid groups; (ii) mixing the metal salt of a ligand precursor with acid to form a protonated ligand precursor having at least two carboxylic acid groups; and (iii) mixing the protonated ligand precursor with a metal salt compound to form a metal-organic framework comprising a metal ion cluster and at least one ligand having at least two carboxylic acid groups. The metal salt compound may comprise a metal having a charge of 3+ or 4+. The metal may be iron, aluminium, chromium, titanium or zirconium. The metal salt compound is preferably iron nitrate nonahydrate. The ligand precursor may be 3,3’,5,5’-azobenzene tetracarboxylic acid (ABTC, H4-TazBz). The metal salt ligand precursor may be obtained by mixing 5-nitroisophtalic acid with sodium hydroxide in the presence of water and glucose to provide Na4ABTC as an aqueous suspension. Said aqueous suspension may be employed directly in step (ii) of said process.

Description

- 1 Process of Preparing Metal-Organic Framework Material
The present invention relates to processes for preparing metal-organic framework materials and the use thereof. In particular, the present invention is concerned with 5 an economical process for preparing a metal-organic framework on an industrial scale.
Metal-Organic Frameworks (MOFs) have garnered significant interests in the last two decades due to their promising potential in many applications such as gas adsorption, 10 separation, catalysis and sensing.
Compared with other porous materials such as zeolite and mesoporous silica, MOFs are based on crystalline porous structures tunable on the atomic scale, which can be designed and functionalized by judicious choice of metal nodes and modification of 15 the organic linkers. However, one of the limitations of most MOFs is their low chemical stability, which undoubtedly hampers their application in industry. A rule of thumb for the construction of stable MOFs comes from the simple Hard and Soft Acid and Base Theory, which guides the selection of the metal-ligand combination for a MOF. For example, see Pearson, R.G. J. Am. Chem. Soc. 1963, 85, 3533. Because 20 the carboxylate group is a hard Lewis base, hard Lewis acids such as Fe3+, Cr3+, Zr»+ and Ti4+ are usually considered good candidates for the construction of robust MOFs.
This method has become the focus of some recent research efforts but very few stable MOFs have been obtained, especially in single crystal form. The main reason is that MOFs based on these metal ions of high valence are difficult to crystallize.
Occasionally, MOFs in the form of crystalline powder were obtained, but structure solution and refinement based on Powder X-Ray Diffraction (PXRD) data is not straightforward. Furthermore, the incorporation of rarely reported metal nodes into MOFs is less predictable and controllable.
Metal-organic frameworks are coordination polymers having an inorganic-organic hybrid framework that comprises metal ions and organic ligands coordinated to the metal ions. These materials maybe three-dimensional, i.e. have three-dimensional lattices in which the metallic species are joined together periodically by spacer ligands.
Metal-organic frameworks have many applications including, for example, in the field of adsorption, storage, separation or controlled release of chemical substances, such
-2as, for example gases, or in the field of catalysis. Metal-organic frameworks may also be useful in the field of pharmaceuticals (controlled release of medicaments) and in the field of cosmetics.
There are in fact a growing number of applications for metal-organic frameworks and as such there is an ever growing need for new such materials with a variety of properties and a need for new metal-organic frameworks having improved properties.
In addition, there is a need to develop new processes for preparing metal-organic frameworks that allow for the preparation of a wide variety of metal-organic frameworks and/or improve the quality of the metal-organic frameworks obtained.
For example, a number of iron metal organic frameworks have been synthesised but 15 there remains a need to develop a synthesis that is robust and can be used to prepare a wide range of metal organic frameworks.
There is also a need to provide commercially viable processes that allow the synthesis of metal-organic frameworks to be scaled up. Examples of challenges encountered in 20 scaling up the synthesis of metal-organic frameworks include slow reaction kinetics of the ligand when larger batches are used, and the metal salt forms of the precursor ligand compounds are usually micron-sized and difficult to filter using conventional techniques. It is therefore an objective of the present invention to overcome these challenges.
These objectives are overcome by the process of the invention.
According to one aspect, the invention provides a process of preparing a porous metal-organic framework, the process comprising the steps of:
(i) providing a metal salt of a ligand precursor having at least two carboxylic acid groups;
(ii) mixing the metal salt of a ligand precursor with acid to form a protonated ligand precursor having at least two carboxylic acid groups; and (iii) mixing the protonated ligand precursor with a metal salt compound to 35 form a metal-organic framework comprising a metal ion cluster and at least one ligand having at least two carboxylic acid groups.
-3In particular, the process is on an industrial scale.
In one embodiment the metal ion cluster comprises the unit M3O, wherein M is independently a metal ion selected from the group consisting of Group 2 through
Group 16 metals.
For example, M maybe selected from Fe(II), Fe(III), Al(III), Cr(III), Ti(IV), V(III), V(IV), V(V), Sc(III), In(III), Ga(III), and mixtures thereof.
In one embodiment, the M3O unit contains at least two metal ions which are different to each other.
In one embodiment, the M3O unit comprises a second metal ion selected from Al(III), Fe(II,III), Co(II), Ni(II), Mn(II), Zn(II), Mg(II), Cr(III), V(III), Sc(III), Ca(II), Ba(II) or In(III), preferably Fe(II,III), Co(II), Ni(II), Mn(II), Zn(II), or Mg(II).
In one embodiment, the M3O unit comprises three metal ions, wherein at least one metal ion is selected from iron, aluminium, chromium, titanium, vanadium, scandium, indium and gallium.
In one embodiment, the M3O unit comprises three metal ions, wherein at least two metal ions are selected from iron, aluminium, chromium, titanium, vanadium, scandium, indium and gallium.
In one embodiment, the M3O unit has the formula M2X0, wherein each M is independently a metal ion selected from iron, aluminium, chromium, titanium, vanadium, scandium, indium and gallium, and X is a metal ion selected from the group consisting of Group 2 through Group 16 metals.
In one embodiment, M is a metal ion selected from iron and aluminium, and X is a metal ion selected from Fe(II,III), Al(III), Co(II), Ni(II), Mn(II), Zn(II), Mg(II), Cr(III), V(III), Sc(III), and In(III).
In one embodiment, the M3O unit has formula Fe2X0, wherein X is Fe(II,III), Co(II), 35 Ni(II), Mn(II), Zn(II), or Mg(II).
In one embodiment, the M3O unit has formula Fe2CoO or Fe3O.
-4In one embodiment, the M3O unit has formula Al2X0, wherein X is a metal ion selected from Al(III), Fe(III), Cr(III), V(III), Sc(III) or In(III).
In one embodiment, the M3O unit has a formula A13O.
In one embodiment, the metal salt compound and the metal ion cluster comprise a metal having a charge of 2+ or 3+ or 4+.
In one embodiment, the metal salt compound employed in the process of the invention is an iron nitrate, such as iron nitrate nonahydrate.
In one embodiment, the one or more ligands are derived from a dicarboxylic acid, a tricarboxylic acid, a tetracarboxylic acid, a hexacarboxylic acid, or a octacarboxylic 15 acid.
In one embodiment, the one or more ligands are derived from a carboxylic acid selected from Li to L30 or a combination of ligands selected from L31 and L32:
L9
L13
L14
L15 L16 L17 L18
L30
COOH
CO OH
L32
In one embodiment, the one or more ligands are derived from a carboxylic acid:
In one embodiment, the ligand is derived from a carboxylic acid selected from:
-ΊCOOH
COOH
COOH
COOH
HOOC
COOH
-8In one embodiment, each metal ion cluster is coordinated with 4,5, or 6 ligands.
In one embodiment, the metal-organic framework is in crystalline form.
In one embodiment, the ligand precursor is ABTC:
In one embodiment, the metal salt of a ligand precursor is a sodium salt of a ligand precursor having at least two carboxylic acid groups.
In one embodiment, the metal organic framework comprises Fe3O(ABTC)3 repeating unit.
In one embodiment, the acid employed in step (ii) is hydrochloric acid. However, any suitable strong acid may be employed. Suitable strong acids include hydrochloric acid (HC1), hydroiodic acid (HI), hydrobromic acid (HBr), perchloric acid (HC1O4), nitric 20 acid (HNO3) and sulfuric acid (H2SO4)
In one embodiment, step (iii) is carried out in the presence of organic acid, such as acetic acid, and organic solvent, such as DMF. The organic acid employed may be any suitable organic acid, particularly any suitable strong organic acid. Examples include 25 CH3COOH (acetic acid), CC13COOH, CHCUCOOH, CF3COOH, CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H.
The process of the present invention provides a simplified ligand synthesis process by direct use of the sodium form of the ligand, e.g. sodium salt of ABTC, in the synthesis of 30 the metal-organic framework and/or combining the synthesis of the ligand and the synthesis of the metal-organic framework by adding the metal-organic framework
-9synthesis reactants, e.g. iron nitrate, to the ligand slurry directly before any filtration steps.
The solvent employed in the process may be any suitable organic solvent. Examples of 5 suitable solvents include but are not limited to alcohols (e.g. methanol, ethanol, propanols such as i-propyl alcohol), water, dimethylformamide (DMF), dimethylacetamide (DMA), dimethylsulfoxide (DMSO).
In one embodiment, the protonated ligand precursor is provided in solution comprising 10 water and organic solvent, such as DMF.
In one embodiment, the metal salt of the ligand precursor having at least two carboxylic acid groups is provided as an alkaline slurry comprising the metal salt of the ligand precursor and water.
In one embodiment, step (ii) is carried out in the presence of organic solvent, such as DMF.
In one embodiment, the metal salt of the ligand precursor having at least two carboxylic 20 acid groups is the sodium salt of 3, 3’, 5,5’ - azobenzene tetracarboxylic acid (Na4ABTC).
In one embodiment, the sodium salt of 3,3’, 5, 5’ - azobenzene tetracarboxylic acid is prepared by mixing 5-nitroisophthalic acid with sodium hydroxide in the presence of 25 water and a reducing agent to provide an aqueous suspension of the sodium salt of 3, 3’, 5,5’ - azobenzene tetracarboxylic acid in water.
The reducing agent reduces the nitro groups of 5-nitroisophthalic acid. In one embodiment, the reducing agent is D-glucose.
In one embodiment, the aqueous suspension obtained is employed directly in step (ii), or the aqueous suspension obtained is dried before being employed in step (ii).
In one embodiment, the aqueous suspension is not filtered before being dried.
- 10 In one aspect, the invention provides a process for preparing a metal-organic framework comprising a Fe3O metal ion cluster and a ligand which is ABTC, the process comprising the steps of:
(a) reacting 5-nitroisophthalic acid with sodium hydroxide in an aqueous solution;
(b) adding D-glucose;
(c) cooling and aerating the reaction mixture to provide an alkaline slurry of ABTC sodium in water;
(d) adding hydrochloric acid and an organic solvent to the alkaline slurry to form a solution of protonated ABTC;
(e) mixing the solution of protonated ABTC with iron nitrate in the presence of acetic acid to provide said metal organic framework;
(f) filtering the reaction mixture; and (g) drying the filtered material.
In one aspect, the invention provides a process for preparing a metal-organic framework comprising a Fe3O metal ion cluster and a ligand which is ABTC, the process comprising the steps of:
(a) reacting 5-nitroisophthalic acid with sodium hydroxide in an aqueous solution;
(b) adding D-glucose;
(c) cooling and aerating the reaction mixture to provide an alkaline slurry of ABTC sodium in water;
(d) drying the alkaline slurry without filtering it to provide dry ABTC sodium;
(e) adding the dried ABTC sodium to aqueous hydrochloric acid to form protonated ABTC;
(f) adding DMF to provide a solution of protonated ABTC;
(g) mixing the solution of protonated ABTC with iron nitrate in the presence of acetic acid to provide said metal organic framework;
(h) filtering the reaction mixture; and (i) drying the filtered material.
In one aspect, the invention provides a process for preparing the sodium salt of 3, 3’, 5, 5’ - azobenzene tetracarboxylic acid (Na4ABTC), the process comprising mixing 5nitroisophthalic acid with sodium hydroxide in the presence of water and a reducing agent to provide an aqueous suspension of the sodium salt of 3,3’, 5, 5’ - azobenzene 35 tetracarboxylic acid in water.
- 11 In one embodiment, the reducing agent is D-glucose.
The process employed to prepare the ligand material is sufficiently robust that the results are not adversely affected by using low quality reaction materials. For example, lower concentration of materials can be employed, and the presence of impurities, such as salts, does not adversely affect the result.
The process(es) of the invention described herein address a number of drawbacks associated with the scale up of laboratory7 syntheses. Some of the differences observed 10 are highlighted below.
A metal-organic framework that has been found to be useful in a number of applications is so-called PCN-250 (Fe3). PCN-250 (Fe3) is a metal-organic framework comprising an Fe3O metal ion cluster and is prepared from ligand precursor ABTC. The advantages of the present invention will be illustrated and discussed below in the context of PCN-250 (Fe3) however this is merely to exemplify the benefits of the present invention and should not in any way be construed as limiting such benefits only to the synthesis of PCN-250 (Fe3).
The preparation of the sodium salt of ABTC is carried out by reacting 5-nitroisophthalic acid (5-NIPA) with sodium hydroxide (50% aqueous solution). Water maybe employed as solvent. The reaction is heated to, for example, about 333 °K with stirring. In large scale processes, mixing of the 5-NIPA and the NaOH will raise the temperature of the reaction (exothermic reaction) and to an extent will reduce the need to heat the reaction to the required temperature.
It has been found that the processes described herein allow lower concentrations of 5NIPA to be employed. Lower concentrations of 5-NIPA are cheaper to provide. Hence, this improves the economic benefits of the process, particularly on an industrial scale where the benefits of cheaper starting materials are greater. In addition, the synthesis of 5-NIPA (5-nitroisophthalic acid) may be carried out employing the following process.
Isophthalic acid may be reacted with nitric or fuming nitric acid to provide 5-NIPA. More specifically, a solution of isophthalic acid in nitric or fuming nitric acid is stirred for 24 hours at 20°C and poured into ice water. Precipitate of 5-NIPA is filtered off, washed with water and dried. It has been observed that is not necessary to recrystallize
- 12 the 5-NIPA obtained from this process before employing it in the reaction process described above for preparing metal-organic framework. 5-NIPA that is not recrystallized produces metal-organic framework (e.g. PCN-250 (Fe3)) that exhibits greater gas (e.g. nitrogen) adsorption. For example, greater adsorption is exhibited by a PCN-250 (Fe3) synthesised using 5-NIPA prepared using the fuming nitric acid process described above (i.e. not recrystallized) compared to PCN-250 (Fe3) synthesised using recrystallized, pharma grade 5-NIPA.
To the reaction mixture, D-glucose (mixed with water) is then added slowly. The reaction is then cooled to 293°K and then stirred and aerated. Aeration may be carried out using a sintered polypropylene, 2” x 18”, 50-micron pore size to aerate the reaction mixture. The use of such a sintered tube is found to introduce air into the reaction mixture more evenly, and as a result it is possible to reduce the reaction time and increase yield. The reaction mixture is then cooled to provide a suspension of sodium
ABTC in water. In the reoxidation step, any suitable method, such as aeration gas could be employed, e.g. air, in order to agitate/reoxidise the N-N bonds to form N=N of 3,3’, 5, 5’ - azobenzene tetracarboxylic acid in the reaction mixture.
Whereas previous syntheses would require filtration of this suspension before reacting the sodium ABTC (i.e. the ligand pre-cursor) with a metal salt compound, the inventors have discovered that in the context of the process of the present invention filtration is not required. The processes described herein therefore avoid the unattractive step of filtering the ligand pre-cursor.
In one embodiment, the ligand slurry obtained above, i.e. the aqueous suspension of sodium ABTC, having a pH greater than 7 (i.e. alkaline) is treated with concentrated hydrochloric acid, deionised water and DMF to provide Solution la. Solution 2a is prepared by mixing iron nitrate nonahydrate with acetic acid and DMF. Solution la and Solution 2a were mixed, stirred for 20 mins with heating and then poured into a sealed solvothermal vessel. The solvothermal treatment was carried out at 150 °C for 12 hours. PCN-250 (Fe3) was obtained.
In an alternative embodiment, the ligand slurry obtained above, i.e. the aqueous suspension of sodium ABTC, is dried. Dry sodium ABTC is treated with hydrochloric acid, deionised water and DMF to form protonated ABTC in acidic solution (pH < 7) to provide Solution ib. For example, sodium ABTC can be added to an aqueous HC1 bath
-13and then DMF added. Solution 2b is prepared by mixing iron nitrate nonahydrate with acetic acid and DMF. Solution ib and Solution 2b were mixed, stirred for 20 mins with heating and then poured into a sealed solvothermal vessel. The solvothermal treatment was carried out at 150 °C for 12 hours. PCN-250 (Fe3) was obtained.
In both embodiments, the role of the DMF is to dissolve the ligand, notably in the protonated form of the ligand to facilitate the reaction of the protonated ligand precursor with iron nitrate. In this context, other conventional organic solvents can be used. For example, isopropyl alcohol (1-propanol), DMA (dimethylacetamide) or dimethylsulfoxide could also be used. In addition, preparing protonated ABTC in situ avoids a filtration step which can give rise to difficulties. The use of DMF as well as the use of DMA allows the solvents to be recycled. It has also been noted that during the synthesis of PCN-250 (Fe3) conversion of DMF to DMA may be observed. Even if this occurs, the solvents may still be recycled as the synthesis can be carried out using a mixture of DMF and DMA.
It has been discovered that the ratio of organic solvent to organic acid (e.g. DMF to acetic acid) present when the ligand precursor (ABTC) reacts with the metals salt compound (iron nitrate) can influence the size of the crystals obtained. Suitable ratios of organic solvent (DMF) to organic acid (acetic acid) include from about 15:1 to about 1:3, from about 10:1 to about 2:3, from about 9:1 to about 1:1, from about 8:1 to about 2:1, from about 7:1 to about 3:1, from about 6:1 to about 4:1, or about 5:1. The largest crystals were obtained suing 35:1 mixture of DMF:acetic acid.
It has also been discovered that carrying out the reaction steps at different pressures (or the absence of pressure) and different temperatures, the reaction dynamics can change and this can have an effect on the resulting structure size and yield.
PCN-250 (Fe3) can be extracted from the reaction mixture using a filter press sustaining cross flow filtration which has tangential feed solution flow, while washing with organic solvent, for example DMF, while PCN-250 (Fe3) is collected in the filter press. In contrast to filter press, where the feed is tangential to the filter, conventional filtration methods utilize dead-end filtration where a single feed stream runs perpendicular to the surface of the membrane causing blockage and significant downtime due to caking. This has been found to be highly efficient washing process and leads to increased surface area of the final metal-organic framework product.
-14The waste slurry obtained from the metal-organic framework synthesis may contain useful materials such as unreacted ligand and monomer material. The waste slurry obtained from the metal-organic framework synthesis may contain useful materials 5 such as unreacted inorganic and/or organic materials and metal salts. The waste slurry obtained from the metal-organic framework synthesis may contain useful coordinated materials combining inorganic and organic precursors to the metal-organic framework. The waste slurry may therefore be employed to recycle/recover these materials, or reused in processes that employ such materials as starting materials.
It has also been observed that a moderate amount of mixing in large scale synthesis of PCN-250 (Fe3) avoids/reduces temperature discrepancies throughout the reaction mixture, which brings with it advantages.
PCN-250 (Fe3) can then be dried. For example, it can be dried in a 50 gallon steam heated vessel. Also, PCN-250 (Fe3) can be dried in open trays using heating lamps.
A significant portion of the cost requirements for MOF synthesis lie in the production of the ligand. The ligand required for PCN-250 (Fe3) synthesis, ABTC, is synthesized 20 from commercial starting materials in two steps. The first step is a homocoupling of 5nitroisophthalic acid under basic conditions, using glucose as a reductant followed by subsequent oxidation to produce the final diazo bridge. The ABTC produced by this step is in the deprotonated form Na4ABTC because of the basic conditions of the synthesis. In addition, Na4ABTC is highly soluble in the reaction solution, water, and so 25 collection of the final product requires refrigeration of the solution to get it to crash out.
Typically we have utilized the acidified form of the ligand, H4ABTC, in PCN-250 (Fe3) synthesis. As such we must first acidify the material and then wash the resulting bright orange-colored material with water to remove the sodium chloride formed in the 30 reaction. The resultant orange-colored solid is then dried before being used in the synthesis of PCN-250 (Fe3).
Part of our work in improving the efficiency of this process has led us to attempt to combine a few of the ligand preparation steps with the final MOF production. We tried 35 two variations 1) use of Na4ABTC in the place of H4ABTC in the synthesis of PCN-250
-ι5(Fe3), and 2) in situ acidification of the sodium ligand slurry during PCN-250 (Fe3) synthesis.
Na4ABTC can be utilized in the reaction conditions with few alterations to the reaction conditions. In brief, the ligand is slurried in a portion of DMF, producing a solution that is roughly 0.034 M in concentration. To this is added a 0.22 M solution of Fe(NO3)»9H2O in acetic acid. This mixture is stirred for approximately 5 minutes at ~ioo°C which produces a homogeneous orange solution. The solution is then sealed in a reactor and heated to 15O°C for 12 hours, resulting in a reddish/orange powder.
Product yields of PCN-250 (Fe3) are -85-100% of the yields of H4ABTC based PCN-250 (Fe3).
Using the Na4ABTC ligand slurry requires the addition of concentrated HC1 to acidify the solution, as the solution is quite basic due to the excess NaOH used in the synthesis.
In brief, 6omL of a 0.0678 M Na4ABTC reaction slurry is acidified with 10 mL concentrated HC1, an additional lomL of H20 is then added to help dissolve the reaction byproducts. To this slurry is added lOOmL of DMF giving a uniform black solution. 5.4 g of Fe(NO3)«9H2O is dissolved in 40 mL of a one to one DMF:acetic acid solution and is then added to the ligand solution. The reaction mixture is stirred for 20 20 minutes to produce a homogenous black solution. The mixture is then placed in a 200 mL autoclave and heated to 15O°C for 12 hr. The material produced is a black solid in a yield ~io% that of a traditional PCN-250 (Fe3) synthesis. Based on the pXRD analysis, both methods produce PCN-250 (Fe3), although the peaks for the ligand slurry are somewhat broader and less intensive compared to the predicted spectrum.
N2 adsorption analysis of the two materials shows that the material produced from the ligand slurry has a much lower uptake than samples produced by other methods .
The BET surface areas of the samples are as follows: Na4ABTC sourced PCN-250: 1011 m2/g, Na4ABTC synthesis slurry PCN-250 (Fe3): 475 m2/g, standard PCN-250 (Fe3):
1433 m2/g· The reduction in total uptake of the Na4ABTC based prep is likely due to the inclusion of sodium chloride within the pores, reducing the total pore volume. The ligand slurry based PCN-250 (Fe3) shows some defect formation as can be seen by the high pressure region (-0.8-0.9 P/Po) of the N2 isotherm, in addition to its low total uptake. We have also compared the particle size of the Na4ABTC based PCN-250 (Fe3)
-16with the standard synthetic procedure. Typically we find that Na4ABTC- PCN-250 (Fe3) has a smaller average particle size compared to the traditional synthesis .

Claims (17)

  1. Claims
    1. A process of preparing a porous metal-organic framework, preferably on an industrial scale, the process comprising the steps of:
    5 (i) providing a metal salt of a ligand precursor having at least two carboxylic acid groups;
    (ii) mixing the metal salt of a ligand precursor with acid to form a protonated ligand precursor having at least two carboxylic acid groups; and (iii) mixing the protonated ligand precursor with a metal salt compound to io form a metal-organic framework comprising a metal ion cluster and at least one ligand having at least two carboxylic acid groups.
  2. 2. The process according to claim 1, wherein the metal salt compound and the metal ion cluster comprise a metal having a charge of 3+ or 4+.
  3. 3. The process according to claim 1 or claim 2, wherein the metal is iron, aluminium, chromium, titanium, or zirconium.
  4. 4. The process according to claim 3, wherein the metal salt compound is an iron
    20 salt compound such as iron nitrate, such as iron nitrate nonahydrate.
  5. 5. The process according to claim 3, wherein the metal salt compound is an aluminium salt compound such as aluminium chloride, aluminium sulphate or aluminium nitrate.
  6. 6. The process according to any of the proceeding claims, wherein the ligand precursor is ABTC:
    HOOC
    COOH
    HOOC nn
    COOH
  7. 7- The process according to any of the proceeding claims, wherein the metal salt of a ligand precursor is a sodium salt of a ligand precursor having at least two carboxylic acid groups.
    5
  8. 8. The process according to any of the proceeding claims, wherein the metal organic framework comprises Fe3O(ABTC)3 repeating unit, wherein ABTC is:
    io
  9. 9. The process according to any of claims 1-7, wherein the metal organic framework comprises Fe2CoO(ABTC)3 repeating unit, wherein ABTC is:
    15 10. The process according to any of claims 1-7, wherein the metal organic framework comprises A13O(ABTC)3 repeating unit, wherein ABTC is:
    20 11. The process according to any of the proceeding claims, wherein the acid is hydrochloric acid.
    12. The process according to any of the proceeding claims, wherein step (iii) is carried out in the presence of organic acid, such as acetic acid, and organic solvent, such as DMF.
    13. The process according to any of the proceeding claims, wherein the protonated ligand precursor is provided in solution comprising water and organic solvent, such as DMF.
  10. 10 14. The process according to any of the proceeding claims, wherein the metal salt of the ligand precursor having at least two carboxylic acid groups is provided as an alkaline slurry comprising the metal salt of the ligand precursor and water.
  11. 15. The process according to claim 14, wherein step (ii) is carried out in the
    15 presence of organic solvent, such as DMF.
  12. 16. The process according to claim 14 or claim 15, wherein the metal salt of the ligand precursor having at least two carboxylic acid groups is the sodium salt of 3,3’, 5, 5’ - azobenzene tetracarboxylic acid (Na4ABTC).
  13. 17. The process according to claim 16, wherein the sodium salt of 3,3’, 5,5’ azobenzene tetracarboxylic acid is prepared by mixing 5-nitroisophthalic acid with sodium hydroxide in the presence of water and a catalyst to provide an aqueous suspension of the sodium salt of 3, 3’, 5,5’ - azobenzene tetracarboxylic acid in water.
  14. 18. The process according to claim 17, wherein the reducing agent is D-glucose.
  15. 19. The process according to claim 17 or claim 18, wherein the aqueous suspension obtained is employed directly in step (ii).
  16. 20. The process according to claim 17 or claim 18, wherein the aqueous suspension obtained is dried before being employed in step (ii).
  17. 21. The process according to claim 20, wherein the aqueous suspension is not
    35 filtered before being dried.
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WO2015177511A1 (en) * 2014-05-23 2015-11-26 The Texas A&M University System Method of preparing a metal organic framework
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WO2015079230A1 (en) * 2013-11-26 2015-06-04 The Texas A&M University System Aluminium metal organic framework materials
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