WO2018046930A1

WO2018046930A1 - Process for the preparation of zirconium based mofs.

Info

Publication number: WO2018046930A1
Application number: PCT/GB2017/052620
Authority: WO
Inventors: Karl Petter Lillerud; Unni Olsbye; Sachin CHAVAN
Original assignee: Universitetet I Oslo; Golding, Louise
Priority date: 2016-09-08
Filing date: 2017-09-07
Publication date: 2018-03-15
Also published as: GB201615270D0

Abstract

There is provided a process for preparing a zirconium-based metal organic framework (Zr-MOF), comprising the steps: (i) mixing a Zr-MOF having fcu topology with aqueous solvent, wherein said Zr-MOF comprises n linkers L1 and 12-n linkers L2; (ii) isolating a Zr-MOF having bcu topology from the mixture in step (i), wherein said Zr-MOF having bcu topology comprises n-4 linkers L1 and 12-n linkers L2; and (iii) preparing a reaction mixture comprising the Zr-MOF having bcu topology and either linker L2 or linker L3 in a solvent; wherein linkers L1, L2 and L3 are a first, second and third linker which are all different and n is 4, 8 or 12. There is further provided a process for the preparation of a Zr-MOF having fcu topology comprising the steps: (i) mixing a Zr-MOF having bcu topology with an alkali metal salt in a solvent, wherein said Zr-MOF comprises 8 linkers L1 and at least one charge balancing anion coordinated to the Zr cluster; (ii) isolating a Zr- MOF having bcu topology from the mixture in step (i), wherein said Zr-MOF having bcu topology does not comprise any charge balancing anions coordinated to the Zr cluster; and (iii) preparing a reaction mixture comprising the Zr-MOF having bcu topology from step (ii) and either linker L1 or linker L2 in a solvent; wherein linkers L1 and L2 are a first and second linker which are different.

Description

PROCESS FOR THE PREPARATION OF ZIRCONIUM BASED MOFS.

Field of the Invention The present invention relates to a process for preparing mixed-linker metal organic frameworks (MOFs), in particular to a process for preparing mixed-linker Zr-MOFs. The invention also relates to Zr-MOFs produced by such processes.

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 in which each Zn₄0 cornerstone is coordinated by six bis-carboxylate organic linkers. Other MOFs in which the inorganic cornerstone is for example chromium, copper, vanadium, cadmium or iron are also known. The processes of the present invention are directed primarily to zirconium-based MOFs (Zr-MOFs).

Numerous processes are known in the prior art for the production of MOFs. The most commonly used techniques involve the 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. The use of aqueous media in place of organic solvents is also reported in, for example, US 7411081 and US 8524932. These processes routinely involve the use of a base or require an alkaline reaction medium. One recent development in MOF synthesis has been the preparation of multifunctional structures. Multifunctional MOFs typically contain more than one type of linker, each bearing a different functional group. These mixed-linker frameworks offer the potential for enhanced tunability, but their use has been restricted due to poor structural reproducibility, limited stability and synthetic protocols being compatible with only a small number of ligand classes.

Initial methods to create multifunctional MOFs involved a one-pot synthesis where all ligands were added simultaneously. One of the main challenges of one-pot syntheses is how to control ligand distribution, making it difficult to achieve the necessary homogeneity between unit cells as well as reproducibility between batches. One of the reasons for this is the competition between kinetically and thermodynamically favoured products.

An alternative procedure involves post synthetic ligand installation, as described by Yuan et al in J. Am. Chem. Soc, 2015, 137, 3177-3180, for example. In this process the MOF is prepared with a particular ligand set before treatment with one or more ligands of a different functionality with the aim that the new ligand(s) will be added to the network structure. However, this approach is time consuming, often requires very controlled reaction conditions and only works with a limited number of linkers. It also seems that, for these methods to work, the original MOF structure must have a body centred cubic unit cell formation so that there are "pockets" into which the new ligand(s) can be placed. There is no change to the structure of the MOF during the reaction.

A further alternative to introduce functionality into an MOF structure is to use post-synthetic modification where heterogeneous chemical reactions are employed to functionalise preassembled MOFs. These are discussed by Deria et al in Chem. Soc. Rev., 2014, 43, 5896-5912. Other post-synthetic modifications include solvent assisted ligand exchange, also described by Deria et al, however again very specific reaction conditions are often required to achieve the removal and replacement of the desired ligands. Controlling the extent of ligand exchange can also be challenging (i.e. directing the reaction towards ligand exchange rather than ligand installation into defect sites) and solvent choice is often very limited. There thus remains the need for the development of novel processes for the production of multi-linker MOFs which are reproducible and which are compatible with a broader range of linker moieties. It is particularly desirable for a process to be capable of generating MOFs which possess a consistent structure across all unit cells. Ideally, the process will be suitable for use on an industrial scale. It would also be advantageous to have a process which can be carried out quickly and cheaply and which offers improvements in terms of complexity over those processes already known in the art. Ideally, a process which offers improvement in more than one of the above aspects would be developed.

The present inventors have surpri singly found that by making use of the ability of MOFs to reversibly transform between two structures, multi -linker frameworks may be prepared in a straightforward and reliable process. In particular, the reversible transformation of an MOF between a face centered cubic (feu) structure and a body centered cubic (BCU) structure enables a fixed proportion of the ligands to be exchanged for ones of an alternative functionality. This

unexpectedly leads to a reliable procedure which is applicable to use on an industrial scale and offers an environmentally friendly and cheap route to these valuable materials. Summary of the Invention

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

(i) mixing a Zr-MOF having feu topology with aqueous solvent, wherein said Zr-MOF comprises n linkers LI and 12-n linkers L2;

(ii) isolating a Zr-MOF having bcu topology from the mixture in step (i), wherein said Zr-MOF having bcu topology comprises n-4 linkers LI and 12-n linkers L2; and

(iii) preparing a reaction mixture comprising the Zr-MOF having bcu topology and either linker L2 or linker L3 in a solvent; wherein linkers LI, L2 and L3 are a first, second and third linker which are all different and n is 4, 8 or 12. In a second aspect, the invention provides a process for preparing a zirconium-based metal organic framework (Zr-MOF), comprising the steps:

(i) mixing a Zr-MOF having feu topology with aqueous solvent, wherein said Zr-MOF comprises 12 linkers LI;

(ii) isolating a Zr-MOF having bcu topology from the mixture in step (i), wherein said Zr-MOF having bcu topology comprises 8 linkers LI;

(iii) preparing a reaction mixture comprising the Zr-MOF having bcu topology and a linker L2 in a solvent;

(iv) isolating a Zr-MOF having feu topology from the mixture in step

(iii), wherein said Zr-MOF comprises 8 linkers LI and 4 linkers L2;

(v) mixing said Zr-MOF having feu topology from step (iv) with aqueous solvent;

(vi) isolating a Zr-MOF having bcu topology from the mixture in step (v), wherein said Zr-MOF having bcu topology comprises 4 linkers LI and 4 linkers L2;

(vii) preparing a reaction mixture comprising the Zr-MOF having bcu topology from step (vi) and linker L2 in a solvent;

(viii) isolating a Zr-MOF having feu topology from the mixture in step (vii), wherein said Zr-MOF comprises 4 linkers LI and 8 linkers L2;

(ix) mixing said Zr-MOF having feu topology from step (viii) with

aqueous solvent;

(x) isolating a Zr-MOF having bcu topology from the mixture in step (ix), wherein said Zr-MOF having bcu topology comprises 8 linkers L2;

(xi) preparing a reaction mixture comprising the Zr-MOF having bcu topology from step (x) and linker L2 in a solvent; and

(xii) isolating a Zr-MOF having feu topology from the mixture in step (xi), wherein said Zr-MOF comprises 12 linkers L2;

wherein linkers LI and L2 are a first and second linker which are different. In another aspect, the invention provides a process for the preparation of a Zr-MOF having feu topology comprising the steps:

(i) mixing a Zr-MOF having bcu topology with an alkali metal salt in a solvent, wherein said Zr-MOF comprises 8 linkers LI and at least one charge balancing anion coordinated to the Zr cluster;

(ii) isolating a Zr-MOF having bcu topology from the mixture in step (i), wherein said Zr-MOF having bcu topology does not comprise any charge balancing anions coordinated to the Zr cluster; and

(iii) preparing a reaction mixture comprising the Zr-MOF having bcu topology from step (ii) and either linker LI or linker L2 in a solvent;

wherein linkers LI and L2 are a first and second linker which are different.

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

Detailed Description

The present invention describes a process for the preparation of a zirconium- based metal organic framework (Zr-MOF). The process involves at least a three stage process wherein a Zr-MOF having feu topology is added to aqueous solvent, a Zr-MOF having bcu topology is isolated from this mixture and subsequently mixed with additional linker material to form a new Zr-MOF. The process typically involves subsequently isolating the new Zr-MOF. The process is applicable to the preparation of mixed-linker Zr-MOFs and Zr-MOFs in which only a single linker moiety is present. In this latter embodiment, the process of the invention can be used to perform complete exchange of one linker for another.

Zr-MOF As used herein, the term "Zr-MOF" is intended to cover any metal organic frameworks (MOFs) which comprise at least one zirconium metal ion. The Zr- MOFs of the invention have "cornerstones" which are zirconium inorganic groups. Typical zirconium inorganic groups include 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 zirconium oxide unit is usually based on an idealized octahedron of Zr-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. 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 wherein x is in the range 0 to 8. For example, the cluster may be represented by the formula Zr₆(0)₄(OH)₄,

Zr-MOFs are well known in the art and cover structures in which the zirconium cornerstone is linked to an organic linker compound to form a

coordinated network. The structures may be one- two- or three-dimensional. The Zr-MOF usually comprises pores which are present in the voids between the coordinated network of 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.

Whilst it not outside the bounds of the present invention for the Zr-MOF to comprise additional metal ions other than zirconium, such as hafnium, titanium, or cerium, zirconium may be the only metal ion 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%.

Zr-MOFs can have a range of topologies. The present invention is concerned with Zr-MOFs which have either a face centered cubic (feu) topology or a body centered cubic (bcu) topology. Within the context of the present invention, by "Zr-MOF having feu topology" we mean a zirconium cluster coordinated to between 9 and 12 linker molecules, preferably 12 linker molecules, which typically has the formula Zr₆0₄(OH)₄Li2. By "Zr-MOF having bcu topology" we mean a zirconium cluster coordinated to between 6 and 8 linker molecules, preferably 8 linker molecules, which typically has the formula Zr₆0₄(OH)₈L₈. The principle of the present invention is based around the development of a method in which a Zr-MOF can be reversibly transformed from an feu structure to a bcu structure, and vice versa. This is illustrated in Figure 1.

It will be appreciated that where we refer to the number of linkers moieties present in a Zr-MOF within the context of the present invention it is intended that we mean the number of linkers attached to each zirconium cluster in the Zr-MOF. Thus, a Zr-MOF comprising 12 linkers LI may be understood to possess 12 linkers LI coordinated to the Zr cluster. Similarly, a Zr-MOF comprising 8 linkers LI and 4 linkers L2 may be understood to possess 8 linkers LI coordinated to the Zr cluster and 4 linkers L2 coordinated to the Zr cluster.

In all embodiments, the surface area of the Zr-MOF is preferably at least 400 m²/g, more preferably at least 450 m²/g, especially at least 500 m²/g, such as at least 550 m²/g. The surface area may be up to 10000 m²/g, especially up to 5000 m²/g. It will be understood that the presence of bulky functional groups may affect (i.e. reduce) the surface area of the Zr-MOF.

In addition to the inorganic zirconium "cornerstones", the Zr-MOFs of the invention comprise at least one linker. This linker can be an organic linker which is monodentate or at least bidentate, i.e. has at least two functional groups capable of coordinating to the zirconium cornerstones. The organic linker may also be tridentate (i.e. containing three functional groups) or tetradentate (i.e. containing four functional groups).

The Zr-MOF may have a Zr 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 Zr metal ion to organic linker molecule ratios are 0.5: 1, 1 : 1, 3 : 1 and 1 :3, especially 1 : 1.

The organic linkers of the Zr-MOFs of the invention may be any organic linker molecule or molecule combination capable of binding to at least one inorganic cornerstone (e.g. 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 one cornerstone binding group, e.g. carboxylate, 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 is preferably water soluble. By water soluble we mean that it preferably has a solubility in water which is high enough to enable the formation of a homogenous solution in water. The solubility of the organic linker compound in water 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 comprises at least one functional group 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.

Where the linker comprises only one functional group it is preferable if this is a carboxylate group. Typically, however, 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.

The organic linker compound comprising said at least one functional group 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_-2o 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 particularly 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, 1,3,5-benzene tribenzoic acid (BTB), benzene tribiphenylcarboxylic acid (BBC), 5, 15-bis (4-carboxyphenyl) zinc (II) porphyrin (BCPP), 1,4-benzene dicarboxylic acid (BDC), 2-amino- 1,4-benzene dicarboxylic acid (R3-BDC or H2N BDC), 1,2,4,5-benzene tetracarboxylic acid, 2-nitro-l,4- benzene dicarboxylic acid Ι, Γ-azo-diphenyl 4,4'-dicarboxylic acid, cyclobutyl- 1,4- benzene dicarboxylic acid (R6-BDC), 1,2,4-benzene tricarboxylic acid, 2,6- naphthalene dicarboxylic acid ( DC), 1, 1 '-biphenyl 4,4'-dicarboxylic acid (BPDC), 2,2'-bipyridyl-5,5'-dicarboxylic acid, adamantane tetracaboxylic acid (ATC), adamantane dibenzoic acid (ADB), adamantane teracarboxylic acid (ATC), dihydroxyterephthalic acid (DHBDC), biphenyltetracarboxylic acid (BPTC), tetrahydropyrene 2,7-dicarboxylic acid (HPDC), pyrene 2,7-dicarboxylic acid (PDC), pyrazine dicarboxylic acid, acetylene dicarboxylic acid (ADC), camphor dicarboxylic acid, fumaric acid, benzene tetracarboxylic acid, l,4-bis(4- carboxyphenyl)butadiyne, aspartic acid, succinic acid, 2,5-furane dicarboxylic acid, 2-nitro- 1,4-benzene dicarboxylic acid, 2-bromo- 1,4-benzene dicarboxylic acid, monosodium 2-sulfoterephthalic acid, acetic acid, benzoic acid, salicylic acid, nicotinic acid, and terphenyl dicarboxylic acid (TPDC). 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 1,4-benzene dicarboxylic acid (BDC), 2- amino- 1,4-benzene dicarboxylic acid, 1,2,4-benzene tricarboxylic acid, 1,2,4,5- benzene tetracarboxylic acid, aspartic acid, succinic acid, 2,5-furane dicarboxylic acid, 2-nitro-l,4-benzene dicarboxylic acid, 2-bromo- 1,4-benzene dicarboxylic acid, monosodium 2-sulfoterephthalic acid, benzoic acid, salicylic acid and 2-nitro- 1,4- benzene dicarboxylic acid or mixtures thereof.

The Zr-MOF is preferably of UiO-66 type. UiO-66 type Zr-MOFs cover structures in which the zirconium inorganic groups are Zr₆(0)₄(OH)₄ 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 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(Zr). When the linker is 2-amino- 1,4-benzene dicarboxylic acid, the resulting MOF may be referred to as UiO-66(Zr)- H₂. When the linker is 1,2,4-benzene tricarboxylic acid, the resulting MOF may be referred to as UiO- 66(Zr)-COOH. When the linker is 1,2,4,5-benzene tetracarboxylic acid, the resulting MOF may be referred to as UiO-66(Zr)-2COOH.

The methods of the present invention involve at least two different linkers LI and L2. In some embodiments, a third linker L3 is employed. Each of LI, L2 and L3 may independently be selected from any of the linkers listed above. It will be appreciated that suitable linkers will be chosen such that the desired linker is preferentially displaced into solution on conversion to the bcu structure. This will be affected by factors such as the nature and position of the functional groups present on the linker as well as the overall structure and properties, such as solubility, of the material. Those skilled in the art would be capable of selecting appropriate linkers based on these features. In all embodiments it is preferred if LI is selected from the group consisting of 1,2,4,5-benzene tetracarboxylic acid, monosodium-2-sulfoterephthalic acid and 1,2,4-benzene tricarboxylic acid. Without wishing to be bound by theory it is considered that the number of carboxylate functional groups on these linkers result in a degree of preference for aqueous solution over binding to the zirconium cluster. This results in relatively facile removal of a proportion of these linkers, thereby facilitating the transformation from feu to bcu structure.

L2 and L3 are each preferentially selected from the group consisting of oxalic acid, 1,4-benzene dicarboxylic acid, 2-amino- 1,4-benzene dicarboxylic acid (R3-BDC or H2N BDC), 2,5-pyridine dicarboxylic acid, dihydroxyterephthalic acid, 1,2,4,5-benzene tetracarboxylic acid, 1,4-naphthalene dicarboxylic acid (1,4-NDC), pyrazine dicarboxylic acid, 2,6-naphthalene dicarboxylic acid (2,6-NDC), 1 4- cyclohexanedicarboxylic acid, sodium 1,5-naphthalenedisulfonate, sodium 1,4- benzenedi sulfonate, 1,4-benzene di boronic acid, 2, 5-thiophene dicarboxylic acid, biphenyl-4,4 '-dicarboxylic acid, monosodium 2-sulfoterephthalic acid and salts thereof.

In one particularly preferred embodiment wherein both L2 and L3 are employed in the processes of the invention, L2 is 1,4-benzene dicarboxylic acid and L3 is 2-amino-l,4-benzene dicarboxylic acid.

Process

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

(iii) preparing a reaction mixture comprising the Zr-MOF having bcu topology and either linker L2 or linker L3 in a solvent; wherein linkers LI, L2 and L3 are a first, second and third linker which are all different and n is 4, 8 or 12. The Zr-MOFs employed and prepared using the processes of the invention may be any Zr-MOF as defined above. Thus all preferable embodiments defined above relating to the Zr-MOF apply equally to this compound as a starting material, intermediate or final product in the processes of the invention.

Each linker may be any organic linker as hereinbefore defined. It will be understood that the organic linker described in the context of the Zr-MOF produced by the processes of the invention is the same organic linker which may be added in step (iii) 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 Zr- MOF apply equally to this compound as a starting material.

In one embodiment n is 12, i.e. the Zr-MOF having feu topology added in step (i) of the process comprising 12 linkers LI and zero linkers L2. In this scenario it will be appreciated that the process of the invention may be employed to prepare a mixed-linker Zr-MOF with two different linker moieties LI and L2 (when linker L2 is added in step (iii)) or LI and L3 (when linker L3 is added in step (iii)). Each of these products may be referred to as a "bifunctionalised Zr-MOF". The

bifunctionalised Zr-MOF product prepared in this way will have 8 linkers LI and 4 linkers L2 (or L3).

In an alternative embodiment n is 8, i.e. the Zr-MOF having feu topology added in step (i) of the process comprising 8 linkers LI and 4 linkers L2. In this scenario it will be appreciated that the process of the invention may be employed to prepare a mixed-linker Zr-MOF with two different linker moieties LI and L2 (when linker L2 is added in step (iii)) or three different linker moieties LI, L2 and L3 (when linker L3 is added in step (iii)). Thus, the product may be referred to as a

"bifunctionalised Zr-MOF" or "trifunctionalised Zr-MOF". The bifunctionalised Zr- MOF product prepared in this way will have 4 linkers LI and 8 linkers L2. The trifunctionalised Zr-MOF product prepared in this way will have 4 linkers LI, 4 linkers L2 and 4 linkers L3.

Finally, n may be 4, i.e. the Zr-MOF having feu topology added in step (i) of the process comprising 4 linkers LI and 8 linkers L2. In this scenario it will be appreciated that the process of the invention may be employed to prepare a mixed- linker Zr-MOF with two different linker moieties L2 and L3 (when linker L3 is added in step (iii)) or a single linker moiety L2 (when linker L2 is added in step (iii)). Thus, the product may be referred to as a "bifunctionalised Zr-MOF" or "monofunctionalised Zr-MOF". The bifunctionalised Zr-MOF product prepared in this way will have 8 linkers L2 and 4 linkers L3. The monofunctionalised Zr-MOF product prepared in this way will have 12 linkers L2.

Figure 2 illustrates the various embodiments which form part of the invention.

Step (i) of the processes of the invention involves mixing a Zr-MOF having feu topology with an aqueous solvent (i.e. comprising, preferably consisting of, water). Mixing may be carried out by any known method in the art, e.g. mechanical stirring. Usually, step (i) is carried out at or around atmospheric pressure, i.e. 0.5 to 2 bar, especially 1 bar.

Step (i) is preferably carried out for a period of time of at least 24 hours, more preferably at least 30 hours, even more preferably at least 36 hours, i.e. at least 48 hours. The reaction mixture is preferably heated for not more than 108 hours, more preferably not more than 96 hours.

Step (i) is generally carried out at room temperature (i.e. 18-30 °C) but may also be performed by heating. The heating step may involve mild heating (e.g. 30 to 70 °C, such as 40 to 60 °C) using, for example, ultrasound, or more aggressive heating under reflux. The skilled man will appreciate that heating under reflux is a routine procedure with which anyone working in the field of the invention would be familiar. The method of heating 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.

The isolation step (ii) is typically carried out by filtration, but isolation may also be performed by processes such as centrifugation, solid-liquid separations, spray drying or extraction. After isolation, the Zr-MOF having bcu topology is preferably obtained as a fine crystalline powder having crystal size of 0.1 to 100 μπι, such as 10 to 50 μπι.

The reaction mixture in step (iii) of the processes of the invention is prepared by mixing the Zr-MOF having bcu topology with a further linker in a solvent. Mixing may be carried out by any known method in the art, e.g. mechanical stirring. The mixing is preferably carried in the temperature range 30- 80 °C, more preferably 50-60 °C. Usually, step (iii) is carried out at or around atmospheric pressure, i.e. 0.5 to 2 bar, especially 1 bar.

Step (iii) is preferably carried out for a period of time of at least 4 hours, more preferably at least 8 hours, even more preferably at least 12 hours. The reaction is preferably not carried out for more than 24 hours.

The solvent employed in step (iii) of the process may be an aqueous solvent (i.e. one comprising, preferably consisting of, water) or an organic solvent.

Example organic solvents include dimethlyformamide (DMF), dimethyl sulfoxide (DMSO), dimethyle acetamide, ethanol, acetonitrile, methanol, propanol, isopropanol, tetrahydrofuran (THF), N-methyl-2-pyrolidone and propylene carbonate. Ideally, however, the solvent is an aqueous solvent.

The molar ratio of total zirconium ions to total organic linker compound(s) present in the reaction mixture prepared in step (iii) 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 zirconium ions to total organic linker compound(s) in the reaction mixture is in the range 1 : 1 to 1 :5, such as 1 :2 and 1 :4.

It will be appreciated that a new Zr-MOF product forms during step (iii) of the process.

The processes of the invention usually comprise a further step (iv) isolating the new Zr-MOF. This product is usually formed as a crystalline material which can be isolated quickly and simply by methods such as filtration, or centrifugation.

The isolation step (iv) is typically carried out by filtration, but isolation may also be performed by processes such as centrifugation, solid-liquid separations or extraction. After isolation, the 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 (i) and (iii) or only after step (iii). Cooling usually involves bringing the temperature of the reaction mixture back to room temperature, i.e. 18-30 °C. In one embodiment, the process of the invention may be employed to carry out the complete exchange of one linker LI for another, L2. Thus, the invention also provides a process for preparing a zirconium-based metal organic framework (Zr-MOF), comprising the steps:

(iv) isolating a Zr-MOF having feu topology from the mixture in step (iii), wherein said Zr-MOF comprises 8 linkers LI and 4 linkers L2;

(v) mixing said Zr-MOF having feu topology from step (iv) with aqueous solvent;

(ix) mixing said Zr-MOF having feu topology from step (viii) with

aqueous solvent;

wherein linkers LI and L2 are a first and second linker which are different. In this embodiment, steps (i), (v) and (ix) may be carried out as defined above for step (i). Steps (ii), (iv), (vi), (viii), (x) and (xii) may be carried out as defined above for step (ii) and optional step (iv). Steps (iii), (vii) and (xi) may be carried out defined above for step (iii).

The Zr-MOF having feu topology employed in step (i) of the processes of the present invention may be prepared by any known method in the art. For example by refluxing aqueous mixture of ZrS0₄.4H₂0 and 1,2,4-benzene tricarboxylic acid (BTC) for 4-5h as reported in WO 2016/046383. The two stage water based process reported by Yang et al in Angew. Chem. Int. Ed. 2013, 52, 10316-10320 can also be used to prepare FCU materials. Alternatively, the starting FCU material can also be prepared using organic solvent such as N,N-dimethyl formamide and Ν,Ν-dimethyl acetamide, NN-diethylformamide or acetonitrile. For example Biswas et al in Dalton Trans. 2013, 42, 4730-4737 reported the synthesis using ZrON0₃.xH₂0 and 1,2,4-benzene tricarboxylic acid (BTC) in N,N- dimethylformamide heating at 150 °C for 24 h.

Alternatively, the Zr-MOF having feu topology may be prepared by transforming a suitable Zr-MOF having bcu topology into the desired structure having feu topology. The present inventors have surprisingly found that by treating a Zr-MOF with bcu topology, which has both a linker LI and at least one charge balancing anion coordinated to the Zr cluster, with an alkali meta lsalt solution, the anion moieties are preferentially lost, leading to a bcu framework which does not contain any anions coordinated to the Zr cluster. Subsequent treatment with additional linker LI or a second linker L2 in a solvent results in a transformation from bcu to feu topology and leads to the production of a Zr-MOF having feu topology, wherein said Zr-MOF comprises 12 linkers LI or 8 linkers LI and 4 linkers L2 depending on whether the bcu framework was treated with additional linker LI or a second linker L2.

Thus, in a further aspect, the invention provides a process for the preparation of a Zr-MOF having feu topology comprising the steps:

(i) mixing a Zr-MOF having bcu topology with an alkali metal salt in a solvent, wherein said Zr-MOF comprises 8 linkers LI and at least one charge balancing anion coordinated to the Zr cluster; (ii) isolating a Zr-MOF having bcu topology from the mixture in step (i), wherein said Zr-MOF having bcu topology does not comprise any charge balancing anions coordinated to the Zr cluster; and

wherein linkers LI and L2 are a first and second linker which are different.

Typically, the linker LI or L2 employed in step (iii) is added as the linker itself or a salt thereof, such as a sodium salt.

In this embodiment, the Zr-MOF and linkers LI and L2 may be as defined above and thus all preferable aspects discussed previously are equally applicable to this embodiment.

In one embodiment, L2 is added in step (iii), LI is preferably 2-amino-l,4- benzene dicarboxylic acid, adipic acid, fumaric acid, 2-aminoterephthalic acid or 1,2,4,5-benzene tetracarboxylic acid (most preferably 2-amino-l,4-benzene dicarboxylic acid) and L2 is preferably selected from the group consisting of 1,4- benzene dicarboxylic acid, benzoic acid or salicylic acid (or salt thereof).

In an alternative embodiment, LI is added in step (iii) and LI is 1,2,4,5- benzene tetracarboxylic acid.

The solvent employed in steps (i) and (iii) of the process may be an aqueous solvent (i.e. one comprising, preferably consisting of, water) or an organic solvent.

Example organic solvents include ethanol, methanol, isopropanol, acetonitrile, N,N- dimethyl sulfoxide, Ν,Ν-dimethyl formamide and Ν,Ν-dimethyl acetamide, N,N- diethylformamide or acetonitrile. Ideally, however, the solvent is an aqueous solvent.

The alkali metal salt may be any comprising an alkali metal (i.e. a salt of lithium, sodium, potassium, rubidium, caesium or francium). Preferable salts include sodium salts, such as sodium bicarbonate, sodium acetate or sodium hydroxide and sodium salts of mono- or di-carboxylic acids.

It will be understood that the at least one charge balancing anion serves to balance the charge of the Zr-MOF such that it has no overall charge. The anion may be any suitable anion known in the art, such as sulfate, chloride, nitrate and carbonate, especially sulfate. The starting material Zr-MOF having bcu topology may be prepared by any method known in the art such as those described by Reinsch et al in

CrystEngComm, 2015, 17, 4070-4074 or CrystEngComm, 2015, 17, 331-337.

In a further embodiment, the invention relates to a zirconium -based metal organic framework (Zr-MOF) produced or formable by the processes as herein described.

Applications The Zr-MOFs 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. Figures

Figure 1 : Reversible transformation of a Zr-MOF between FCU and BCU structures Figure 2: Flowchart illustrating embodiments of the invention

Figure 3 : Powder X-ray diffraction pattern of the starting FCU (A), intermediate BCU (B) and the final FCU product obtained after washing BCU with aqueous solution of 1,2,4-Benzene tricarboxylic acid (C). Figure 4: Thermo-gravimetric analysis of intermediate BCU (grey curve) and the final FCU product (Black curve) obtained after washing with aqueous solution of 1,2,4-benzene tricarboxylic acid(BDC-COOH). The horizontal dashed lines show the theoretically expected level (wt%) of MOF decomposition step. Figure 5: Powder X-ray diffraction pattern of the final FCU product obtained after washing BCU (500g) with aqueous solution of 1,2,4-Benzene tricarboxylic acid inside 3L reactor. Figure 6: Powder X-ray diffraction pattern of the intermediate BCU(A) and final FCU product obtained in step (iii) after washing with aqueous solution of : 2-amino- 1,4-benzene dicarboxylic acid (B), 2,5-pyridine dicarboxylic acid (C), 2,5-dihydroxy 1,4-Benzene diicarboxylic acid (D).

Figure 7: Powder X-ray diffraction pattern of the intermediate BCU(A) and final FCU product obtained in step (iii) after washing with aqueous solution of : 1,4- naphthalene dicarboxylic acid (B), 1,2,4,5-benzene tetracarboxylic acid (C), 2,5- Pyrazinedicarboxylic acid (D).

Figure 8: Powder X-ray diffraction pattern of the intermediate BCU(A) and final FCU product obtained in step (iii) after washing with aqueous solution of : 1,4- cyclohexane dicarboxylic acid (B), 2,6-naphthalene dicarboxylic acid (C), 2,6- Naphthalenedisulfonic acid disodium salt (D).

Figure 9: Powder X-ray diffraction pattern of the intermediate BCU(A) and final FCU product obtained in step (iii) after washing with aqueous solution of : Oxalic acid (B), Benzene- 1,4-diboronic acid (BOC) (C), 2,5-thiophene dicarboxylic acid (D).

Figure 10: 1H- MR spectra of BDC-COOH linker (black curve) and bifunctional FCU product prepared with 2-amino- 1,4-benzene dicarboxylic acid (BDC-NH2) solution in step (iii) (grey curve).

Figure 11 : 1H- MR spectra of BDC-COOH linker (black curve) and bifunctional MOF prepared with 2,5-pyridine dicarboxylic acid (2,5-PDC) solution in step (iii) (grey curve). Figure 12: 1H- MR spectra of BDC-COOH linker (black curve) and bifunctional MOF prepared with 1,4-naphthalene dicarboxylic acid (1,4-NDC) solution in step (iii) (grey curve). Figure 13 : 1H- MR spectra of BDC-COOH linker (black curve) and bifunctional MOF prepared with 2,5-Pyrazinedicarboxylic acid (2,5-PzDC) solution in step (iii) (grey curve).

Figure 14: 1H- MR spectra of BDC-COOH linker (black curve) and bifunctional MOF prepared with 1,2,4,5-benzene tetracarboxylic acid (BDC-2COOH) solution in step (iii) (grey curve). Figure 15: 1H- MR spectra of BDC-COOH linker (black curve) and bifunctional MOF prepared with 2,6-naphthalene dicarboxylic acid (2,6-NDC) solution in step (iii) (grey curve).

Figure 16: Thermo-gravimetric analysis of intermediate BCU (grey curve) and the bifunctional FCU product (Black curve) obtained after step (iii) with aqueous solution 2-amino-l,4-benzene dicarboxylic acid (BDC- H2). The horizontal dashed lines show the theoretically expected level (wt% ) of MOF decomposition step.

Figure 17: Thermo-gravimetric analysis of intermediate BCU (grey curve) and the bifunctional FCU product (Black curve) obtained after step (iii) with aqueous solution 2,5-Pyridinedicarboxylic acid (2,5-PDC). The horizontal dashed lines show the theoretically expected level (wt% ) of MOF decomposition step.

Figure 18: Thermo-gravimetric analysis of intermediate BCU (grey curve) and the bifunctional FCU product (Black curve) obtained after step (iii) with aqueous solution 1,4-naphthalene dicarboxylic acid (1,4-NDC). The horizontal dashed lines show the theoretically expected level (wt% ) of MOF decomposition step.

Figure 19: Thermo-gravimetric analysis of intermediate BCU (grey curve) and the bifunctional FCU product (Black curve) obtained after step (iii) with aqueous solution of 2,5-Pyrazinedicarboxylic acid (2,5-PzDC). The horizontal dashed lines (upper and lower) show the theoretically expected level (wt%) of MOF decomposition step for defect free MOF. The horizontal dashed line (middle) shows the expected level (wt%) of MOF decomposition step for MOF with linker composition (20%) derived from H-NMR spectroscopy results. Figure 20: Nitrogen sorption isotherm recorded at 77K on BCU intermediate (Black curve) and bifunctional MOF prepared with 2-amino-l,4-benzene dicarboxylic acid (BDC-NH2) solution in step (iii) (grey curve). Prior to the N2 adsorption samples were activated at 150 °C for 2h. Figure 21 : Nitrogen sorption isotherm recorded at 77K on BCU intermediate (Black curve) and bifunctional MOF prepared with 2,5-Pyridinedicarboxylic acid (2,5- PDC) solution in step (iii) (grey curve). Prior to the N2 adsorption samples were activated at 150 °C for 2h Figure 22: Nitrogen adsorption isotherm recorded at 77K on BCU intermediate

(Black curve) and bifunctional MOF with 1,4-naphthalene dicarboxylic acid (1,4- NDC) solution in step (iii) (grey curve). Prior to the N2 adsorption samples were activated at 150 °C for 2h. Figure 23 : Nitrogen adsorption isotherm recorded at 77K on BCU intermediate

(Black curve) and bifunctional MOF with 2,5-Pyrazinedicarboxylic acid (2,5-PzDC) solution in step (iii) (grey curve). Prior to the N2 adsorption samples were activated at 150 °C for 2h. Figure 24: Powder X-ray diffraction pattern of the intermediate BCU(A) and final FCU product obtained in step (iii) after washing with solution of (clockwise) : 1,4- benzene dicarboxylic acid in water, 1,4-benzene dicarboxylic acid in DMF, disodium terephthalate in water. Figure 25: Thermo-gravimetric analysis of intermediate BCU (grey curve) and the bifunctional FCU product (Black curve) obtained after step (iii) with solution 1,4- benzene dicarboxylic acid in DMF. The horizontal dashed lines show the

theoretically expected level (wt% ) of MOF decomposition step

Figure 26: Powder X-ray diffraction pattern of the intermediate BCU (top left) and final FCU product obtained in step (iii) with solution of monosodium-2- sulfoterephthalic acid in water (top right) and in DMF (bottom right).

Figure 27: Powder X-ray diffraction pattern of the intermediate BCU (top left) and final FCU product obtained in step (iii) with solution of Biphenyl-4,4'-dicarboxylic acid in water (top right) and in DMF (Bottom right).

Figure 28: Powder X-ray diffraction pattern of the starting bifunctional FCU (A) containing both BDC-COOH and BDC linker, the intermediate BCU (B) containing 1,2,4-benzene tricarboxylic acid and 1,4-benzene dicarboxylic acid and final trifunctional FCU product (C) containing 1,2,4-benzene tricarboxylic acid, 1,4- benzene dicarboxylic acid and 2-amino- 1,4-benzene dicarboxylic acid (BDC-NH2).

Figure 29: ¹H- MR spectra of trifunctional FCU product containing 1,2,4-benzene tricarboxylic acid, 2-amino- 1,4-benzene dicarboxylic acid and 1,4-benzene dicarboxylic acid.

Figure 30: Powder X-ray diffraction pattern of the final FCU product containing 1,4- benzene dicarboxylic acid (89%) and 1,2,4-benzene tricarboxylic acid(l 1%) obtained in complete exchange experiment.

Figure 31 : ¹H- MR spectra of the final FCU product containing 1,4-benzene dicarboxylic acid (89%) and 1,2,4-benzene tricarboxylic acid(l 1%) obtained in complete exchange experiment. Figure 32: Powder X-ray diffraction patterns (in ascending order) of the final FCU product obtained in step (iii) with aqueous solution of Benzoic acid, Salicylic acid, L-Histidine and mixture of L-Histidine and L-cysteine. Figure 33 : Powder X-ray diffraction pattern of the starting BCU structure containing sulphate (lower curve), BCU intermediate obtained after treating starting BCU with 0.1M sodium acetate solution (middle curve) and final FCU product obtained after stirring BCU intermediate in aqueous solution of disodium terephthalate (upper curve).

Figure 34: Powder X-ray diffraction pattern of the starting BCU structure containing sulphate (lower curve), BCU intermediate obtained after treating starting BCU with 0.1M sodium hydroxide solution (middle curve) and final FCU product obtained after stirring BCU intermediate in aqueous solution of disodium terephthalate (upper curve).

Figure 35: Powder X-ray diffraction pattern of the starting BCU structure containing sulphate (lower curve), BCU intermediate obtained after treating starting BCU with 0.1M sodium acetate solution (middle curve) and final FCU product obtained after stirring BCU intermediate in aqueous solution of 2-amino-l,4-benzene dicarboxylic acid (upper curve). Examples

Techniques

Powder X-Ray Diffraction

The crystal structure was investigated by means of powder X-ray diffraction under ambient conditions. PXRDs patterns were recorded on a Bruker D8 Discovery diffractometer equipped with a focusing Ge-monochromator, using Cu_ai radiation and Bruker LYNXEYE detector. PXRD were collected in reflectance Bragg- Brentano geometry in the 2Θ range from 3 to 55°. ^JH NMR spectroscopy

For NMR measurement samples were prepared by weighing 20 mg of MOF into a centrifuge tube. A 1ml solution of 1M NaOH in D₂0 was then added to the tube. The digestion medium used in the majority of this work was 1M NaOH in D₂0. Upon addition of the digestion medium, the centrifuge tubes were capped and shacked on IKA KS260 instrument for 30 min before leaving the samples to digest over a period of 24 hours. This OH^" based procedure dissolves only the organic portion of the MOF (linker, modulator, solvent etc.), while the inorganic component precipitated as Zirconium hydroxide. The precipitate was separated by centrifugation at 3000 rpm for 30 min and the 700 μΐ of top solution was sampled in NMR. Liquid 1H NMR spectra were recorded with a Bruker Avance DPX-400 NMR Spectrometer (300 MHz). The relaxation delay (dl) was set to 20 seconds to ensure that reliable integrals were obtained, allowing for the relative concentrations of the molecular components to be accurately determined. The number of scans was 64. The molar ratios between the various linker species within the MOF was determined by integrating the proton NMR signal associated with linker species.

The results for the bifunctional Zr-MOFs were used to calculate the percentage incorporation (by mol) of linker L2 in the final material using the equation:

(lA of H_L2 )

incorporation (IA of H_L2 ) (IA of H_L1 )

4·

no,of H_L2 no.of H_L1 wherein

IA of H_L2 = integrals(numerical values) obtained for the proton signals of L2 no. of H_L2 = number of equivalent nuclei contributing to the proton signals of L2 IA of H_Li = integrals(numerical values) obtained for the proton signals of LI no. of H_Li = number of equivalent nuclei contributing to the proton signals of LI Surface Area measurement

The specific surface area was determined by means of N₂ physisorption measured on a Belsorp-mini apparatus at 77 K. Prior to the measurement the sample was activated at 150 °C under vacuum for 2 h to remove occluded water molecules. The surface area was calculated by the BET-method.

Thermogravimetric analysis

The thermal stability was investigated by means of thermogravimetry. The sample was heated up with a rate of 5 °C/min under a flow of synthetic air (N2/02= 20/5 ml min^"1), constantly monitoring weight loss.

Example 1 The principle of FCU to BCU reversible transformation was demonstrated by transforming UiO-66-COOH with FCU structure to BCU by suspending it in water for 3-4 days. The BCU structure was isolated by filtration and converted back to FCU structure by treating lg of it with BDC-COOH aqueous solution (0.025M). The final product was isolated by filtration and dried at 60 °C. PXRD data were obtained for the FCU starting material; BCU intermediate and FCU product are shown in Figures 3a-c. The starting material and final products gave the same result, thus showing that the original FCU structure can be regenerated. Thermogravimetric analysis of the BCU intermediate and FCU product is shown in Figure 4. The weight loss at MOF decomposition step matches with weight loss expected for ideal FCU structure (horizontal dashed line), also confirms the reversible transformation. The scale-up potential was also demonstrated by repeating the process using 500g of UiO-66 in 3 litres water. The PXRD pattern obtained for the final product of reaction is shown in Figure 5. This is consistent with Figure 3 demonstrating successful reversibility even when performed on very large scales. Example 2 - Bifunctional MOFs

The bifunctional MOFs were prepared as follows. In step (i) UiO-66-COOH containing BDC-COOH (LI) was suspended in water to convert to BCU structure which was isolated in step (ii) by filtration. In step (iii) this BCU structure (lg) was stirred in 50 ml aqueous solution (0.025 M) of linkers (L2) at 50-60 °C for 12h. Various linkers (L2) aqueous solutions were used in step (iii). In cases where 0.025 M solution does not lead to the structure transformation higher concentration solutions were tried. The final FCU structure was isolated by filtration and dried at 60 °C.

The PXRD patterns for the bcu topology Zr-MOF intermediate and each multi- linker product are shown in Figures 6 to 9. 1H MR spectroscopy, Thermogravimteric analysis (TGA) and N₂ sorption for the bcu topology Zr-MOF intermediate and some of the multi-linker products are shown in Figures 10 to 23. The amount of linker 2 (L2) incorporated in each of these examples and BET surface area are given in Table 1.

Table 1

Linker 2 (L2) % L2 in bifunctional Surface area product (by mol) (BET) m2/g

BCU intermediate 0 612

2-amino-l,4-benzene dicarboxylic acid 32.9 777

2,5-pyridine dicarboxylic acid 32.7 730

1,4-naphthalene dicarboxylic acid 35.27 580

2,5-Pyrazinedicarboxylic acid 20.86 892

1,2,4,5-benzene tetracarboxylic acid 32.07

2,6-naphthalene dicarboxylic acid 33.3 Example 3 - Solvent and Linker Source Dependence

The process of Example 2 was repeated using 1,4 -benzene dicarboxylic acid (BDC) and the sodium salt of 1,4 -benzene dicarboxylate (BDC-Na) as linker L2. The reaction with BDC was carried out in water and no incorporation of L2 or transformation to feu topology was observed. When the analogous reaction was carried out using the sodium salt as linker L2 a successful transformation was observed. Replacing the water with DMF also resulted in a successful transformation. The PXRD patterns for the bcu topology Zr-MOF intermediate and each multi-linker product are shown in Figure 24. The thermogravimetric analysis shown in Figure 25 also supports the observation of structure transformation in DMF using 1,4 -benzene dicarboxylic acid.

The process of Example 2 was also repeated using monosodium-2- sulfoterephthalic acid and biphenyl-4,4 '-dicarboxylic acid as linker L2. In both cases, when the reaction was carried out in water, no incorporation of L2 or transformation to feu topology was observed. When the analogous reaction was carried out using DMF as a solvent, a successful transformation was observed.

PXRD data for these reactions are shown in Figures 26 and 27.

Example 4 - Tri -functional MOFs

The process of Example 2 was repeated using a bi-functional Zr-MOF having feu topology as the starting material. This Zr-MOF comprised 1,2,4 - benzene tricarboxylic acid (BDC-COOH) as linker LI and 1,4 -benzene dicarboxylate (BDC) as linker L2. 2-amino-l,4-benzene dicarboxylic acid (BDC- H₂) was added as linker L3 in water to generate a tri -functional mixed-linker Zr- MOF product. The PXRD patterns for the bifunctional feu starting material, bifunctional bcu intermediate and trifunctional FCU product are shown in Figure 28A-C. 1H MR spectroscopy for the multi-linker product is shown in Figure 29. The amount of each linker in the final product is set out in Table 2. Table 2

Example 5 - Complete exchange of linker

Complete exchange of linker BDC-COOH from the UiO-66-COOH, FCU structure was performed by repeating in sequence 3 times the steps (i), (ii) and (iii) and using every time aqueous solution disodium terephthalate in step (iii). The final product was isolated by filtration and dried at 60 °C. The PXRD pattern and H- MR spectra of the final product are shown in Figures 30 and 31. The amount of each linker in the final product is set out in Table 3.

Table 3

Example 6 -Structure transformation with linker containing one carboxylic acid functionality

The process of Example 2 was repeated using aqueous solution of different linkers containing one -COOH (e.g. amino acids) capable of coordinating to Zr-cluster. Linkers benzoic acid, salicylic acid and L-Histidine were used in step (iii) of the process. In typical synthesis lg of BCU MOF was stirred in 50 ml solution of linker 2 (0.050M) at 50 °C for 12h. The final product was recovered by filtration and dried at 60 °C. The PXRD patterns of the final products are shown in Figure 32 which confirms the successful structure transformation. A mixture of monocarboxylic acids was also shown to successfully transform the structure for example: Aqueous solution of containing L-Histidine and L-cysteine used in step (iii) also transform the BCU structure to FCU (Figure 32, upper curve).

Example 7 - Synthesis of feu framework from bcu framework

The synthesis of FCU structure starting from BCU framework was demonstrated with two different Zr-MOF frameworks. Two BCU starting materials containing BDC- H2 and BDC-2COOH linker were prepared following the procedure reported by Reinsch et al CrystEngComm, 2015, 17, 4070-4074. In as prepared form both frameworks contained sulfate (S0₄ ^2") coordinated to Zr-cluster. To remove the sulfate, these BCU frameworks were stirred in solution of sodium salt at 50-60 °C for 12h. Sodium acetate (0.1M) and sodium hydroxide (0.1M) solutions were used. Both solutions lead to the removal of sulfate from the structure without change in structure. Theses sulfate free BCU framework materials were stirred in aqueous solution of linker 2 (0.025M) at 50-60 °C for 12h. The BCU-BDC- H2 was stirred with disodium terephthalate and BCU-BDC-2COOH was stirred with BDC- H2. The final product was isolated by filtration and dried at 60 °C. The PXRD patterns for the BCU starting material, BCU intermediate and bifunctional products are shown in Figures 33 to 35. The sample obtained with O. lM Na-acetate solution appeared more crystalline than the sample obtained with 0.1M NaOH.

Claims

1. A process for preparing a zirconium-based metal organic framework (Zr- MOF), comprising the steps:

(ii) isolating a Zr-MOF having bcu topology from the mixture in step (i), wherein said Zr-MOF having bcu topology comprises n-4 linkers LI and 12- n linkers L2; and

(iii) preparing a reaction mixture comprising the Zr-MOF having bcu topology and either linker L2 or linker L3 in a solvent;

wherein linkers LI, L2 and L3 are a first, second and third linker which are all different and n is 4, 8 or 12.

2. The process as claimed in claim 1, further comprising step (iv) isolating the Zr-MOF.

3. The process as claimed in claim 1 or 2, wherein linker LI is selected from the group consisting of 1,2,4,5-benzene tetracarboxylic acid, monosodium-2- sulfoterephthalic acid and 1,2,4-benzene tricarboxylic acid.

4. The process as claimed in any of claims 1 to 3, wherein L2 and L3 may each independently be selected from the group consisting of oxalic acid, 1,4- benzene dicarboxylic acid, 2-amino-l,4-benzene dicarboxylic acid (R3-BDC or H2N BDC), 2,5-pyridine dicarboxylic acid, dihydroxyterephthalic acid, 1,2,4,5-benzene tetracarboxylic acid, 1,4-naphthalene dicarboxylic acid (1,4- DC), pyrazine dicarboxylic acid, 2,6-naphthalene dicarboxylic acid (1,4- DC), 1 4-cyclohexanedicarboxylic acid, sodium 1,5- naphthalenedisulfonate, sodium 1,4-benzenedisulfonate, 1,4-benzene di boronic acid, 2, 5-thiophene dicarboxylic acid, biphenyl-4,4'-dicarboxylic acid, monosodium 2-sulfoterephthalic acid and salts thereof.

5. The process as claimed in any of claims 1 to 4, wherein n is 8, L2 is 1,4- benzene dicarboxylic acid and L3 is 2-amino-l,4-benzene dicarboxylic acid.

The process as claimed in any of claims 1 to 5, wherein the solvent in step (iii) is an aqueous solvent or an organic solvent selected from the group consisting of dimethlyformamide (DMF), dimethyl sulfoxide (DMSO), dimethyle acetamide, ethanol, acetonitnle, methanol, propanol, isopropanol, tetrahydrofuran (THF), N-methyl-2-pyrolidone and propylene carbonate.

The process as claimed in any of claims 1 to 6, wherein step (i) is carried out at room temperature.

8. The process of any of claims 1 to 7, wherein step (i) is carried out for a

period of time of at least 24 hours, preferably at least 30 hours, more preferably at least 36 hours, such as at least 48 hours.

9. The process of any of claims 1 to 8, wherein step (iii) is carried out at a

temperature in the range 30-80 °C, preferably 50-60 °C.

10. A process for preparing a zirconium-based metal organic framework (Zr-

MOF), comprising the steps:

(iv) isolating a Zr-MOF having feu topology from the mixture in step (iii), wherein said Zr-MOF comprises 8 linkers LI and 4 linkers L2; (v) mixing said Zr-MOF having feu topology from step (iv) with aqueous solvent; (vi) isolating a Zr-MOF having bcu topology from the mixture in step (v), wherein said Zr-MOF having bcu topology comprises 4 linkers LI and 4 linkers L2;

(ix) mixing said Zr-MOF having feu topology from step (viii) with

aqueous solvent;

wherein linkers LI and L2 are a first and second linker which are different.

11. The process of claim 10, wherein LI and L2 are as defined in claim 3 and/or 4.

12. A process for the preparation of a Zr-MOF having feu topology comprising the steps:

(iii) preparing a reaction mixture comprising the Zr-MOF having bcu topology from step (ii) and either linker LI or linker L2 in a solvent; wherein linkers LI and L2 are a first and second linker which are different.

13. The process as claimed in claim 12, wherein the alkali metal salt is a sodium salt, such as sodium acetate or sodium hydroxide.

14. A zirconium-based metal organic framework (Zr-MOF) produced by the process as defined in any of claims 1 to 13.