WO2018046925A1

WO2018046925A1 - Process for preparing a cerium-zirconium mof and materials so obtained

Info

Publication number: WO2018046925A1
Application number: PCT/GB2017/052614
Authority: WO
Inventors: Norbert Stock; Martin LAMMERT
Original assignee: Universitetet I Oslo; Christian-Albrechts-Universität Zu Kiel; Golding, Louise
Priority date: 2016-09-08
Filing date: 2017-09-07
Publication date: 2018-03-15
Also published as: GB201615268D0

Abstract

There is provided a process for preparing a cerium-zirconium metal organic framework (Ce/Zr-MOF), the process comprising the steps (i) preparing a reaction mixture comprising zirconium (IV) ions, cerium (IV) ions, a carboxylic acid, and at least one organic linker compound in a solvent; and (ii) heating the reaction mixture from step (i), wherein, when the organic linker comprises a carboxylate functional group, said organic linker is different to the carboxylic acid.

Description

PROCESS FOR PREPARING A CERIUM-ZIRCONIUM MOF

AND MATERIALS SO OBTAINED

Field of the Invention

The present invention relates to a process for preparing mixed-metal org frameworks (MOFs), in particular to a process for preparing cerium/zirconium MOFs. The invention also relates to the Ce/Zr-MOFs themselves.

Background

MOFs or "metal organic frameworks" are compounds having a lattice structure having vertices or "cornerstones" which are metal -based inorganic groups, for example metal oxides, linked together by organic linkers. The linkers are usually at least bidentate ligands which coordinate to the metal-based inorganic groups via functional groups such as carboxylate and/or amine. The porous nature of MOFs renders them promising materials for many applications such as gas storage and catalyst materials.

Perhaps the best known MOF is MOF-5 [Zn₄0(BDC)₃] in which each

[Zn₄0]⁶⁺ cornerstone is coordinated by six terephthalate (BDC^2") linkers. Other MOFs in which the inorganic cornerstone is for example chromium, copper, vanadium, cadmium or iron are also known.

It is also possible to prepare mixed-metal organic frameworks in which one metal cation is at least partially substituted by another. These types of structure offer the possibility to tune the properties of the framework for a desired application. For example, frameworks containing mixed lanthanide cations can be tailored with particular luminescent properties and doping an aluminium MOF with lithium has been found to increase its hydrogen adsorption capabilities.

Numerous processes are known in the prior art for the production of MOFs.

The most commonly used techniques involve the solvothermal reaction of a metal salt with the desired organic linker in a suitable solvent, usually organic, such as dimethylformamide (DMF). High pressures and temperatures are commonly required to facilitate the reaction. Typical methods are disclosed in, for example, WO 2009/133366, WO 2007/023134, WO2007/090809 and WO 2007/118841.

When mixed metal-frameworks are desired, several synthetic options are available. Each metal may be added as a separate salt in the solvothermal processes described above. In addition pre-synthesized mixed-metal clusters can be employed. Alternatively, post-synthesis metal exchange may be carried out in which some of the base metal is replaced by the second cation after the framework has been formed.

The processes of the present invention are directed to zirconium-based MOFs (Zr-MOFs) and specifically to mixed metal cerium/zirconium MOFs. Zr- MOFs are well studied materials, due in part to their attractive thermal, mechanical and hydrothermal properties. Zr-MOFs find application in many areas, such as gas adsorption and separation as well as catalysis. Mixed-metal frameworks based on zirconium have been reported and include titanium/zirconium structures as well as cerium/zirconium materials. Cerium in particular has been identified as an attractive candidate as it tends to adopt a similar coordination geometry to zirconium and, moreover, Ce0₂ is known to have good catalytic properties.

A process for preparing Ce/Zr-UiO-66 is described by Nouar et al in Chem Commun., 2015, 51, 14458-14461. This method involves the solvothermal reaction of a zirconium (IV) salt with a cerium (III) salt in DMF. The reaction mixture is heated to approximately 120 °C for 24 hours. However, the amount of cerium incorporated into the UiO-66 framework was only 5 at%. Analysis of the oxidation state of cerium showed a 20/80% content of Ce³⁺/Ce⁴⁺. A similar process is reported by Ebrahim et ai m ACS Applied Materials and Interfaces, 2013, 5, 10565-10573. Ce³⁺ contents of up to 13.3 at% are achieved. Both these synthetic methods result in MOFs in which zirconium remains the dominant metal ion and cerium is incorporated only as Ce³⁺ or mixed-valent Ce^3+/4+. Given the known catalytic activity of cerium in the oxidation state IV it may be valuable to investigate structures in which higher amounts of exclusively cerium(IV) can be incorporated.

There thus remains the need for the development of novel processes for the production of Ce/Zr-MOFs with a wide range of cerium contents. The process should ideally be one which can be carried out cheaply and efficiently, thus rendering it suitable for use on an industrial scale. It would also be advantageous to have a process which can be carried out more rapidly than those of the prior art. Ideally, a process which offers improvement in more than one of the above aspects would be developed.

The present inventors have surprisingly found that Ce/Zr-MOFs with varying

Ce:Zr ratios may be prepared in a rapid solvothermal process. In particular, the specific combination of zirconium(IV) ions, cerium(IV) ions and a carboxylic acid in the reaction mixture unexpectedly leads to a procedure which can be carried out using short reaction times and which allows for facile variation in the Zr:Ce ratio in the final product.

Summary of the Invention

Thus, in a first aspect, the invention provides a process for preparing a cerium-zirconium metal organic framework (Ce/Zr-MOF), said process comprising the steps:

(i) preparing a reaction mixture comprising zirconium (IV) ions, cerium (IV) ions, a carboxylic acid and at least one organic linker compound in a solvent; and

(ii) heating the reaction mixture from step (i),

wherein, when the organic linker comprises a carboxylate functional group, said organic linker is different to the carboxylic acid.

In a second aspect, the invention provides a cerium-zirconium metal organic framework (Ce/Zr-MOF) produced or formable by the processes as herein described.

In a further aspect, the invention provides a cerium-zirconium metal organic framework (Ce/Zr-MOF), wherein said metal organic framework comprises cerium(IV) in an amount of at least 5 at% relative to the total number of metal atoms in the framework.

In another aspect, the invention provides a cerium -zirconium metal organic framework (Ce/Zr-MOF), wherein said metal organic framework comprises cerium in an amount of at least 15 at% relative to the total number of metal atoms in the framework.

In yet another aspect, the invention provides a cerium-zirconium metal organic framework (Ce/Zr-MOF), wherein said metal organic framework comprises cerium solely in the form Ce(IV).

Detailed Description

The present invention describes a process for the preparation of a mixed metal cerium-zirconium metal organic framework (Ce/Zr-MOF). The process involves preparing a reaction mixture comprising zirconium (IV) ions, cerium(IV) ions, a carboxylic acid and at least one organic linker compound in a solvent and heating the reaction mixture. The process typically involves subsequently isolating the Ce/Zr-MOF.

Ce/Zr-MOF

As used herein, the term "Ce/Zr-MOF" is intended to cover any metal organic frameworks (MOFs) which comprise at least one zirconium metal ion and at least one cerium metal ion. The MOFs of the invention have "cornerstones" which are zirconium or cerium inorganic groups. Typical cerium/zirconium inorganic groups include cerium/zirconium ions connected by bridging oxygen or hydroxide groups. These inorganic groups are further coordinated to at least one organic linker compound. In some cases, the inorganic groups may be further connected to non- bridging modulator species, complexing reagents or ligands (e.g. sulfates or carboxylates such as formate, benzoate or acetate) and/or solvent molecules. The metal oxide unit is usually based on an idealized octahedron of metal ions which are μ3-bridged by O^2" and/or OH^" ions via the faces of the octahedron and further saturated by coordinating moieties containing O-atoms, like carboxylate groups.

In the mixed-metal frameworks of the invention it will be appreciated that the cerium and zirconium occupy equivalent sites in the crystal structure. Thus, when comparing a Ce/Zr-MOF with a Zr-MOF prepared under identical reaction conditions, the structure of the framework will be largely similar with cerium ions occupying some of the zirconium ion sites.

The cerium and zirconium metal ions present in the MOFs of the invention are typically Ce(IV) and Zr(IV). Preferably, both cerium and zirconium are present solely in the +4 oxidation state (i.e. as Ce(IV) and Zr(IV))

Zr-MOFs are well known in the art and cover structures in which the zirconium cornerstone is usually linked to an at least bidentate organic linker compound to form a coordinated network. The structures may be one-, two- or three-dimensional. The Ce/Zr-MOFs of the present invention will have a similar construction. The MOFs usually comprise pores which are present in the voids between the coordinated network of cerium/zirconium ions and organic linker compounds. The pores are typically micropores, having a diameter of 2 nm or less, or mesopores, having a diameter of 2 to 50 nm.

The idealised Zr oxide cluster is considered to be a Zr₆0₃₂-cluster which comprises between 6 and 12 (preferentially as close as possible to 12) carboxylate groups. However, in practice, there is a degree of flexibility in the structure of the cluster. The cluster may be represented by the formula [Zr₆O_x(OH)_8-x]^("x+16)+ wherein x is in the range 0 to 8. For example, the cluster may be represented by the formula [Zr₆(0)4(OH)₄]¹²⁺. The MOFs of the invention are typically based on this hexanuclear framework. The skilled man will appreciate that in Ce/Zr-MOFs some zirconium ions will be replaced by cerium ions in the cluster.

Whilst it is not outside the bounds of the present invention for the Zr-MOF to comprise additional metal ions other than zirconium and cerium, such as hafnium or titanium, it is preferable if zirconium and cerium are the only metal ions present. If additional metal ions are present these may be present in an amount of up 50 wt% relative to total amount of metal ions, preferably up to 25 wt%, more preferably up to 10 wt%, e.g. up to 5 wt%.

The Ce/Zr-MOFs of the invention particularly preferably have cornerstones having at least 12 coordination sites for the organic linkers, e.g. 12-36, especially preferably at least 14, 16 or 18, most especially 24. In this way at least 6, more preferably at least 8, especially at least 12 bidentate ligand groups of the organic linkers can bind to each cornerstone. In all embodiments, the surface area of the Ce/Zr-MOF is preferably at least 400 m²/g, more preferably at least 500 m²/g, especially at least 700 m²/g, such as at least 1020 m²/g, for example at least 1050 m²/g, e.g. at least 1200 m²/g. The surface area may be up to 10000 m²/g, especially up to 5000 m²/g. It will be understood that, where functionalised organic linker compounds are used, the presence of additional, and often bulky, groups may affect (i.e. reduce) the surface area of the Ce/Zr-MOF.

In addition to the inorganic zirconium and cerium "cornerstones", the Ce/Zr- MOFs of the invention comprise at least one organic linker compound. The organic linker compound is typically at least bidentate, i.e. has at least two functional groups capable of coordinating to the zirconium or cerium cornerstones. The organic linker compound may also be tridentate (i.e. containing three functional groups) or tetradentate (i.e. containing four functional groups).

The Ce/Zr-MOF may have a total metal ion to organic linker molecule ratio of from 1 :0.45 to 1 :0.55, especially 1 :0.49 to 1 :0.51, particularly 1 :0.5. Other preferred metal ion to organic linker molecule ratios may be in the range 1 : 1 to 1 :0.33. Specific ratios can include 0.5: 1, 1 : 1, 3 : 1 and 1 :3, especially 1 : 1.

The organic linker compounds of the Ce/Zr-MOFs of the invention may be any organic linker molecule or molecule combination capable of binding to at least two inorganic cornerstones and comprising an organic moiety. By "organic" moiety we mean a carbon based group which comprises at least one C-H bond and which may optionally comprise one or more heteroatoms such as N, O, S, B, P, Si.

Typically, the organic moiety will contain 1 to 50 carbon atoms.

The organic linker compound may be any of the linkers conventionally used in MOF production. These are generally compounds with at least two cornerstone binding groups, e.g. carboxylates, optionally with extra functional groups which do not bind the cornerstones but may bind metal ions on other materials it is desired to load into the MOF. The introduction of such extra functionalities is known in the art and is described for example by Campbell in JACS 82:3126-3128 (1960).

The organic linker compound may be in the form of the compound itself or a salt thereof, e.g. a disodium 1,4-benzenedicarboxylate salt or a monosodium 2- sulfoterephthalate salt. The organic linker compound may be soluble in aqueous solvents (i.e. ones comprising, preferably consisting of, water) and/or in organic solvents. By soluble we mean that it preferably has a solubility in a given solvent which is high enough to enable the formation of a homogenous solution. The solubility of the organic linker compound may be at least 1 g/L at room temperature and pressure (RTP), preferably at least 2 g/L, more preferably at least 5 g/L.

The organic linker compound typically comprises at least two functional groups capable of binding to the inorganic cornerstone. By "binding" we mean linking to the inorganic cornerstone by donation of electrons (e.g. an electron pair) from the linker to the cornerstone. Preferably, the linker comprises two, three or four functional groups capable of such binding.

Typically, the organic linker comprises at least two functional groups selected from the group of carboxylate (COOH), amine ( H₂), nitro (N0₂), anhydride and hydroxyl (OH) or a mixture thereof. In a preferable embodiment, the linker comprises two, three or four carboxylate or anhydride groups, most preferably carboxylate groups.

If the organic linker comprises a carboxylate functional group, it will be understood that said organic linker is different to the carboxylic acid employed in the processes of the invention. Thus, the same compound cannot serve the purpose of the organic linker and the carboxylic acid.

The organic linker compound comprising said at least two functional groups may be based on a saturated or unsaturated aliphatic compound or an aromatic compound. Alternatively, the organic linker compound may contain both aromatic and aliphatic moieties.

In one embodiment, the aliphatic organic linker compound may comprise a linear or branched Ci_-20 alkyl group or a C₃.₁₂ cycloalkyl group. The term "alkyl" is intended to cover linear or branched alkyl groups such as all isomers of propyl, butyl, pentyl and hexyl. In all embodiments, the alkyl group is preferably linear. Particularly preferred cycloalkyl groups include cyclopentyl and cyclohexyl.

In a particularly preferred embodiment, the organic linker compound comprises an aromatic moiety. The aromatic moiety can have one or more aromatic rings, for example two, three, four or five rings, with the rings being able to be present separately from one another and/or at least two rings being able to be present in condensed form. The aromatic moiety particularly preferably has one, two or three rings, with one or two rings being especially preferred, most preferably one ring. Each ring of said moiety can independently comprise at least one heteroatom such as N, O, S, B, P, Si, preferably N, O and/or S.

The aromatic moiety preferably comprises one or two aromatic C6 rings, with the two rings being present either separately or in condensed form. Particularly preferred aromatic moieties are benzene, naphthalene, biphenyl, bipyridyl and pyridyl, especially benzene.

Examples of suitable organic linker compounds include oxalic acid, ethyloxalic acid, fumaric acid (H₂Fum), benzene-l,3,5-tricarboxylic acid (H₃BTC), 1,3,5-benzene tribenzoic acid (H₃BTB), benzene tribiphenylcarboxylic acid

(H₃BBC), 5, 15-bis (4-carboxyphenyl) zinc (II) porphyrin (H₂BCPP), tetrakis- 5,10, 15,20-(4-carboxyphenyl)porphyrin (H₆TCPP), 1,4-benzene dicarboxylic acid (H₂BDC), 2-amino- 1,4-benzene dicarboxylic acid (H₂BDC- H₂), 1,2,4,5-benzene tetracarboxylic acid, 2-nitro- 1,4-benzene dicarboxylic acid l,l'-azo-diphenyl 4,4'- dicarboxylic acid, cyclobutyl-l,4-benzene dicarboxylic acid, 1,2,4-benzene tricarboxylic acid, 2,6-naphthalene dicarboxylic acid (H₂ DC), 1,1 '-biphenyl 4,4'- dicarboxylic acid (H₂BPDC), 2,2'-bipyridyl-5,5'-dicarboxylic acid (H₂BPyDC), adamantane tetracarboxylic acid (H₄ATC), adamantane dibenzoic acid (H₂ADB), dihydroxyterephthalic acid (H₂BDC-(OH)₂), biphenyltetracarboxylic acid

(H₄BPTC), tetrahydropyrene 2,7-dicarboxylic acid (H₂HPDC), pyrene 2,7- dicarboxylic acid (H₂PDC), pyrazine dicarboxylic acid, thiophene-2,5-dicarboxylic acid (H₂TDC), 4,4'-diphenyl-dicarboxylate (H₂BDPC), pyrazole-3,5-dicarboxylic acid (H₂PZDC), 4,4',4",4" '-methanetetrayltetrabenzoic acid (H₄MTB), 4,4'-[(2,5- dimethoxy-l,4-phenylene)bis(ethyne-2,l-diyl)]dibenzoic acid (H₂PEDB-(OMe)₂), naphthalene dicarboxylic acid (H₂ DC), acetylene dicarboxylic acid (H₂ADC), camphor dicarboxylic acid, l,4-bis(4-carboxyphenyl)butadiyne, nicotinic acid, and terphenyl dicarboxylic acid (H₂TPDC). Other acids besides carboxylic acids, e.g. boronic acids may also be used. Anhydrides may also be used.

In a particularly preferred embodiment, the organic linker compound is selected from the group consisting of benzene-1, 3, 5-tri carboxylic acid (H₃BTC), 1,4-benzene dicarboxylic acid (H₂BDC), 2-amino-l,4-benzene dicarboxylic acid, 1,2,4-benzene tricarboxylic acid, 1,2,4,5-benzene tetracarboxylic acid and 2-nitro- 1,4-benzene dicarboxylic acid or mixtures thereof.

A mixture of two or more of the above-mentioned linkers may be used. It is preferably, however, if only one type of linker is used.

The Ce/Zr-MOF is preferably of UiO-66 type. UiO-66 type Ce/Zr-MOFs cover structures in which the zirconium and cerium inorganic groups are

M₆(0)4(OH)₄ (wherein M can be zirconium, cerium or a mixture thereof) and the organic linker compound is 1,4-benzene dicarboxylic acid or a derivative thereof. Derivatives of 1,4-benzene dicarboxylic acid used in UiO-66 type Ce/Zr-MOFs include 2-amino- 1,4-benzene dicarboxylic acid, 2-nitro- 1,4-benzene dicarboxylic acid, 1,2,4-benzene tricarboxylic acid and 1,2,4,5-benzene tetracarboxylic acid.

When the linker is 1,4-benzene dicarboxylic acid, the resulting MOF may be referred to as UiO-66(Ce/Zr). When the linker is 2-amino- 1,4-benzene dicarboxylic acid, the resulting MOF may be referred to as UiO-66(Ce/Zr)- H₂. When the linker is 1,2,4-benzene tricarboxylic acid, the resulting MOF may be referred to as UiO- 66(Ce/Zr)-COOH. When the linker is 1,2,4,5-benzene tetracarboxylic acid, the resulting MOF may be referred to as UiO-66(Ce/Zr)-(COOH)₂.

A mixture of linkers may be used to introduce one or more functional groups within the pore space, e.g. by using aminobenzoic acid to provide free amine groups or by using a shorter linker such as oxalic acid. This introduction of functionalised linkers is facilitated by having a Ce/Zr-MOF with inorganic cornerstones with a high number of coordination sites. Where the number of these coordination sites exceed the number required to form the stable 3D MOF structure, functionalisation of the organic linkers may be effected, e.g. to carry catalytic sites, without seriously weakening the MOF structure.

By "functionalised MOF" we mean a MOF wherein one or more of the backbone atoms of the organic linkers carries a pendant functional group or itself forms a functional group. Functional groups are typically groups capable of reacting with compounds entering the MOF or acting as catalytic sites for reaction of compounds entering the MOF. Suitable functional groups will be apparent to a person skilled in the art and in preferred embodiments of the invention include amino, nitro, thiol, oxyacid, halo (e.g. chloro, bromo, fluoro) and cyano groups or heterocyclic groups (e.g. pyridine), each optionally linked by a linker group, such as carbonyl. The functional group may also be a phosphorus-or sulfur-containing acid.

Preferably, the functionalised Ce/Zr-MOF has a surface area of at least 400 m²/g, more preferably at least 500 m²/g, especially at least 700 m²/g, such as at least 1020 m²/g.

Whilst it is within the ambit of the invention for the Ce/Zr MOF of the invention to comprise cerium in a mixture of oxidation states, it is most preferably if the cerium is solely in the form of Ce (IV).

In one embodiment, the Ce/Zr-MOFs of the invention may comprise cerium in an amount of 0.1 to 99.9 at%, relative to the total number of metal atoms in the framework. Preferably, the amount of cerium is in the range 5 to 95 at%, more preferably 10 to 90 at%, such as 15 to 85 at% relative to the total number of metal atoms in the framework.

Alternatively, in other embodiments, the Ce/Zr-MOFs of the invention comprise cerium (IV) in an amount of at least 5 at%, relative to the total number of metal atoms in the framework. In this embodiment, the amount of cerium is preferably in the range 5 to 95 at%, more preferably 15 to 90 at % relative to the total number of metal atoms in the framework.

In a further alternative embodiment, the Ce/Zr-MOFs of the invention comprise cerium in an amount of at least 15 at%, relative to the total number of metal atoms in the framework. In this embodiment, the amount of cerium is preferably in the range 15 to 95 at%, more preferably 20 to 90 at % relative to the total number of metal atoms in the framework.

The Ce/Zr MOFs of the invention can comprise zirconium in an amount of

0.1 to 99.9 at%, relative to the total number of metal atoms in the framework.

Preferably, the amount of zirconium is in the range 5 to 95 at%, more preferably 10 to 90 at%, such as 15 to 85 at% relative to the total number of metal atoms in the framework. Process

The process of the invention comprises at least the steps of:

(i) preparing a reaction mixture comprising zirconium(IV) ions, cerium (IV) ions, a carboxylic acid and at least one organic linker compound in a solvent; and

(ii) heating the reaction mixture from step (i),

The organic linker compound may be any organic linker as hereinbefore defined. It will be understood that the organic linker described in the context of the Ce/Zr-MOF produced by the processes of the invention is the same organic linker which is used as a starting material in step (i) of the process of the invention, albeit that once bound to the inorganic cornerstone the organic linker will be deprotonated. Thus all preferable embodiments defined above relating to the organic linker in the context of the Ce/Zr-MOF apply equally to this compound as a starting material.

The zirconium ions are in the +4 oxidation state, i.e. Zr⁴⁺ ions.

The zirconium ions may be provided in any conventional way, and thus any conventional source of zirconium ions may be used. However the zirconium ions will typically be provided in the form of at least one zirconium salt. Example salts include zirconium nitrate, zirconium sulfate, zirconium chloride, zirconium oxychloride and zirconium oxynitrate, or mixtures thereof. Especially preferable zirconium salts include zirconium nitrate and zirconium oxynitrate, or mixtures thereof. Without wishing to be bound by theory, it is thought that the use of a zirconium nitrate helps to stabilise the cerium ion in the +4 oxidation state. Whilst the use of a mixture of two or more different salts is encompassed by the invention, it is preferable if one salt is used.

Where a zirconium salt is used, the salt is usually soluble in organic solvents, i.e. preferably having a solubility of at least 1 g/L at room temperature and pressure (RTP), preferably at least 2 g/L, more preferably at least 5 g/L.

The cerium ions are in the 4+ oxidation state, i.e. Ce⁴⁺ ions. The cerium ions may be provided in any conventional way, and thus any conventional source of cerium ions may be used. However the cerium ions will typically be provided in the form of at least one cerium salt. Example cerium salts include cerium sulfate, cerium fluoride and cerium ammonium nitrate. A preferable cerium salt is cerium ammonium nitrate. Whilst the use of a mixture of two or more different salts is encompassed by the invention, it is preferable if one salt is used.

The carboxylic acid may be any carboxylic acid known in the art, however it must be different to the organic linker. By "carboxylic acid" we mean any organic compound which contains at least one carboxyl functional group. The presence of the carboxylic acid in the reaction mixture is thought to provide better crystalline products. Moreover, the acid molecules have the ability to coordinate to the zirconium and/or cerium metal ions during the processes of the invention before they are replaced by the organic linker. This coordination is considered to help to modulate the synthesis of the MOFs.

Preferably, the carboxylic acid will be one represented by the general formula (I) below:

wherein R is selected from the group consisting of hydrogen, linear or branched Ci_-2o alkyl groups, C₃.₁₂ cycloalkyl groups, and optionally substituted C_6-20 aryl groups.

The term "alkyl" is intended to cover linear or branched alkyl groups such as methyl, ethyl and propyl. In all embodiments, the alkyl group is preferably linear.

Particularly preferred cycloalkyl groups include cyclopentyl and cyclohexyl.

Examples of the substituted aryl groups include aryl groups substituted with at least one substituent selected from halogens, alkyl groups having 1 to 8 carbon atoms, acyl groups, or a nitro group. Particularly preferred aryl groups include substituted and unsubstituted phenyl, benzyl, phenylalkyl or naphthyl.

In a preferable embodiment R is selected from the group consisting of hydrogen, linear C_1-4 alkyl groups and unsubstitited C₆.₁₂ aryl groups. Most preferably the carboxylic acid is selected from the group consisting of hydrogen, methyl and benzyl, especially hydrogen. Examples of carboxylic acids which are ideally used are therefore formic acid, acetic acid and benzoic acid.

The zirconium ions, cerium ions, carboxylic acid and at least one organic linker are mixed in a solvent. The solvent may be an aqueous solvent (i.e. one comprising, preferably consisting of, water), an organic solvent, or a mixture thereof. The organic solvent may be any suitable organic solvent known in the art to be compatible with MOF synthetic methods. Preferably, the organic solvent is an amide solvent, especially alkyl amide solvents, wherein "alkyl" is intended to cover linear or branched alkyl groups such as methyl, ethyl and propyl. Particular examples of organic solvents include dimethyl formamide, diethyl formamide, dimethyl acetamide, acetonitrile or mixtures thereof.

Ideally, the solvent is a mixture of an aqueous solvent and an organic solvent, such as a mixture of water and DMF.

The reaction mixture prepared in step (i) of the processes of the invention is typically prepared by mixing the various components together in the solvent.

Mixing may be carried out by any known method in the art, e.g. mechanical stirring. The mixing is preferably carried out at room temperature, i.e. 18 to 30 °C. Usually, step (i) is carried out at or around atmospheric pressure, i.e. 0.5 to 2 bar, especially 1 bar.

In step (ii) of the process, the reaction mixture prepared in step (i) is heated. Preferably, the temperature is increased to 70 - 120 °C, more preferably 75 - 115 °C, such as 80-100 °C. Usually, step (ii) is carried out at or around atmospheric pressure, i.e. 0.5 to 2 bar, especially 1 bar.

The reaction mixture is preferably heated for a period of time of at least 2 minutes, more preferably at least 10 minutes, even more preferably at least 15 minutes. The reaction mixture is preferably heated for not more than 90 minutes, preferably not more than 75 minutes, even more preferably not more than 60 minutes.

In one particularly preferable embodiment, step (ii) is carried out for 15 minutes at 100 °C. The method of heating in step (ii) may be by any known method in the art, such as heating in a conventional oven, a microwave oven or heating in an oil bath or using an aluminium heater block.

The short reaction times used in the process of the invention offer numerous advantages over those of previous methods wherein reaction times of the order of 24 hours were reported. This offers improvements in terms of costs, safety and suitability for industrial scale-up.

Moreover, it has surprisingly been found that the specific combination of zirconium (IV) ions, cerium (IV) ions and the carboxylic acid in the reaction mixture enable the direct formation of the Ce/Zr-MOF with cerium present primarily, preferably exclusively, as cerium (IV) ions. Moreover, by varying the amount of zirconium and cerium ions added to the reaction mixture, MOFs with wide ranging cerium contents can be prepared, which was not possible via previous methods.

The molar ratio of zirconium ions to cerium ions added to the reaction mixture may be in the range 1 : 100 to 100: 1, such as 1 :50 to 50: 1, for example 1 : 10 to 10: 1, especially 1 :5 to 5: 1. It will be appreciated that an appropriate molar ratio can be selected based on the relative amounts of each metal desired in the end product.

The molar ratio of total metal ions to total organic linker compound(s) present in the reaction mixture prepared in step (i) is typically 1 : 1, however in some embodiments an excess of the organic linker compound may be used. Thus, in some embodiments, the molar ratio of total metal ions to total organic linker compound(s) in the reaction mixture is in the range 1 : 1 to 1 : 5, such as 1 :4.

The carboxylic acid is typically added in an amount corresponding to a molar ratio of at least 10: 1 relative to the total number of metal ions. The carboxylic acid is therefore added in excess. Preferable ratios include at least 20: 1 and at least 40: 1.

It will be appreciated that the Ce/Zr-MOF product forms during step (ii) of the process.

The processes of the invention usually comprise a further step (iii) isolating the Ce/Zr-MOF. Advantageously, the Ce/Zr-MOF is usually formed as a crystalline product which can be isolated quickly and simply by methods such as filtration, or centrifugation. The isolation step (iii) is typically carried out by centrifugation, but isolation may also be performed by processes such as filtration, solid-liquid separations or extraction. After isolation, the Ce/Zr-MOF is preferably obtained as a fine crystalline powder having crystal size of 0.1 to 100 μπι, such as 10 to 50 μπι.

In addition to steps (i), (ii) and (iii), the processes of the invention may comprise additional steps, such as drying and/or cooling. Typically, there will be a cooling step between steps (ii) and (iii). Cooling usually involves bringing the temperature of the reaction mixture back to room temperature, i.e. 18-30 °C.

In a further embodiment, the invention relates to a cerium-ziconium metal organic framework (Ce/Zr-MOF) produced or formable by the processes as herein described. Applications

The Ce/Zr-MOF produced or formable by the processes of the present invention may be employed in any known application for such materials.

Applications therefore include, but are not restricted to, electrode materials, drug reservoirs, catalyst materials, adsorbents and cooling media.

Examples

Procedures EDX

Energy-dispersive X-ray spectroscopy (EDX) data were recorded on a Philips XL30 FEG microscope. Each sample was measured three times at different spots. From the data the average value in at% of Ce and Zr was calculated. Ce/Zr-MOF-808

Mixed Ce/Zr-MOF-808 solid solutions were synthesized using Pyrex glass reaction tubes (maximum volume 14 mL). 1,3,5-benzenetricarboxylic acid (H₃BTC, 67.2 mg), was introduced into the glass reactor and Ν,Ν-dimethylformamide (DMF, 1.6 mL) and concentrated formic acid (HCOOH, 100 %, 4.12 mL) were added. The starting conditions are identical for all syntheses.

To incorporate different amounts of Ce and Zr in the MOFs different ratios of aqueous solutions of cerium(IV) ammonium nitrate (0.533 M) and zirconium(IV) dinitrate oxide hydrate (0.533 M) were used. (Table 1)

Table 1 : Conditions for the synthesis of solid solutions of Ce/Zr-UiO-66.

Ratio

Ce Zr H₃BTC HCOOH DMF

Sample Ce:Zr:H₃BTC:HCOOH

[μί] \fiL] [mg] [mL] [mL]

Ce Zr H₃BTC HCOOH

Ml 1 5 3 1024 200.0 1000.0 67.2 4.12 1.6

M2 2 4 3 1024 400.0 800.0 67.2 4.12 1.6

M3 3 3 3 1024 600.0 600.0 67.2 4.12 1.6

M4 4 2 3 1024 800.0 400.0 67.2 4.12 1.6

M5 5 1 3 1024 1000.0 200.0 67.2 4.12 1.6

After all starting materials were added, the glass reactors were sealed and heated using an aluminum heater block under stirring for 20 min at 100 °C. The light yellow precipitate was centrifuged in the mother liquor, which was then decanted off, before being re-dispersed and centrifuged twice in DMF (2 mL). To remove DMF from the product, the solid was washed and centrifuged with acetone (2 mL) four times. The resulting white solid was dried in air at 70 °C.

The ratio of incorporated Ce/Zr was determined using energy-dispersive X- ray spectroscopy (EDX). Results are shown in Table 2. Table 2: Results of the EDX analysis of the samples M1-M5.

Sample Zr / at% Ce / at%

Ml 80 20

M2 74 26

M3 62 38

M4 49 51

M5 33 67

Ce/Zr-UiO-66

Mixed Ce/Zr-UiO-66 solid solutions were synthesized using Pyrex glass reaction tubes (maximum volume 14 mL). 1,4-benzendicarboxylic acid (H₂BDC, 127.6 mg, was introduced into the glass reactor and Ν,Ν-dimethylformamide (DMF, 3.6 mL) and concentrated formic acid (HCOOH, 100 %, 1.03 mL) were added. The starting conditions are identical for all syntheses. The variation of the Ce/Zr ratio is possible by varying the molar ratio of the starting materials, i. e. the ratio of aqueous solutions of cerium(IV) ammonium nitrate (0.533 M) and zirconium(IV) dinitrate oxide hydrate (0.533 M) (Table 3).

Table 3 : Conditions for the synthesis of solid solutions of Ce/Zr-UiO-66.

Ratio

Ce Zr H₂BDC HCOOH DMF

Sample Ce:Zr:H₂BDC:HCOOH

[μί] [mg] [mL] [mL]

Ce Zr H₂BDC HCOOH

PI 1.5 4.5 7.2 256 300.0 900.0 127.6 1.03 3.6

P2 2.0 4.0 7.2 256 400.0 800.0 127.6 1.03 3.6

P3 2.5 3.5 7.2 256 500.0 700.0 127.6 1.03 3.6

P4 3.0 3.0 7.2 256 600.0 600.0 127.6 1.03 3.6

P5 3.5 2.5 7.2 256 700.0 500.0 127.6 1.03 3.6

P6 4.0 2.0 7.2 256 800.0 400.0 127.6 1.03 3.6

P7 4.5 1.5 7.2 256 900.0 300.0 127.6 1.03 3.6 P8 5.0 1.0 7.2 256 1000.0 200.0 127.6 1.03 3.6

P9 5.5 0.5 7.2 256 1100.0 100.0 127.6 1.03 3.6

After all starting materials were added the glass reactors were sealed and heated using an aluminum heater block under stirring for 15 min at 100 °C. The light yellow precipitate was centrifuged in the mother liquor, which was then decanted off, before being re-dispersed and centrifuged twice in DMF (2 mL). To remove DMF from the product, the solid was washed and centrifuged with acetone (2 mL) four times. The resulting white solid was dried in air at 70 °C.

The molar ratios in the product were determined by EDX analysis (Table 4). Table 4: Results of the EDX analysis of the samples P1-P9.

Sample Zr / at% Ce / at%

PI 81 19

P2 78 22

P3 71 28

P4 65 35

P5 61 39

P6 47 53

P7 41 59

P8 28 72

P9 14 86

Claims

A process for preparing a cerium-zirconium metal organic framework (Ce/Zr-MOF), said process comprising the steps:

(i) preparing a reaction mixture comprising zirconium (IV) ions, cerium (IV) ions, a carboxylic acid, and at least one organic linker compound in a solvent; and

(ii) heating the reaction mixture from step (i),

2. A process as claimed in claim 1, further comprising step (iii) isolating the Ce/Zr-MOF, wherein step (iii) is preferably carried out by centrifugation.

A process as claimed in claim 1 or 2, wherein the zirconium ions are provided in the form of at least one zirconium salt.

A process as claimed in claim 3, wherein said zirconium salt(s) is selected from the group consisting of zirconium nitrate and zirconium oxynitrate, or mixtures thereof.

A process as claimed in any of claims 1 to 4, wherein the cerium (IV) ions are provided in the form of at least one cerium (IV) salt.

A process as claimed in any of claims 5, wherein the cerium salt is cerium ammonium nitrate.

A process as claimed in any of claims 1 to 6, wherein the carboxylic acid is selected from the group consisting of formic acid, benzoic acid and acetic acid, or mixture thereof, preferably formic acid.

8. A process as claimed in any of claims 1 to 7, wherein the at least one organic linker compound comprises at least two functional groups selected from the group consisting of carboxylate (COOH), amine ( H₂), anhydride and hydroxyl (OH) or a mixture thereof.

A process as claimed in any of claims 1 to 8, wherein the at least one organic linker compound comprises a linear or branched Ci_-20 alkyl group, a C₃.₁₂ cycloalkyl group and/or an aromatic moiety, preferably an aromatic moiety such as benzene, naphthalene, biphenyl, bipyridyl or pyridyl.

A process as claimed in any of claims 1 to 9, wherein the organic linker compound is selected from the group consisting of 1,4-benzene dicarboxylic acid (H₂BDC), 2-amino- 1,4-benzene dicarboxylic acid, 1,2,4-benzene tricarboxylic acid, 2-nitro- 1,4-benzene dicarboxylic acid and 1,2,4,5-benzene tetracarboxylic acid, or mixtures thereof.

A process as claimed in any of claims 1 to 10 wherein in step (ii) of the process, the reaction mixture from step (i) is carried out at room temperature.

A process as claimed in any of claims 1 to 11, wherein step (ii) is performed for a time period of up to 90 minutes.

A process as claimed in any of claims 1 to 12, wherein the solvent is an aqueous solvent, an organic solvent or a mixture thereof, preferably a mixture thereof.

A process as claimed in claim 13, wherein the organic solvent is an amide solvent.

A process as claimed in claim 14, wherein the amide solvent is selected from the group consisting of diemethyl formamide, diethyl formamide and dimethyl acetamide, or mixtures thereof.

16. A cerium-zirconium metal organic framework (Ce/Zr-MOF) produced by the process as defined in any of claims 1 to 15.

A cerium-zirconium metal organic framework (Ce/Zr-MOF), wherein said metal organic framework comprises cerium in an amount of at least 15 at% relative to the total number of metal atoms in the framework.

A cerium-zirconium metal organic framework (Ce/Zr-MOF), wherein said metal organic framework comprises cerium (IV) in an amount of at least 5 at% relative to the total number of metal atoms in the framework.

19. A cerium-zirconium metal organic framework (Ce/Zr-MOF), wherein said metal organic framework comprises cerium solely in the form Ce(IV).