WO2017050936A1

WO2017050936A1 - Process

Info

Publication number: WO2017050936A1
Application number: PCT/EP2016/072608
Authority: WO
Inventors: Norbert Stock; Helge REINSCH; Karl Petter Lillerud; Steve WAISCHAT
Original assignee: Universitetet I Oslo; Christian-Albrechts-Universität Zu Kiel
Priority date: 2015-09-22
Filing date: 2016-09-22
Publication date: 2017-03-30
Also published as: GB201516781D0

Abstract

The invention relates to a continuous flow process for preparing a metal organic framework (MOF), comprising the steps (i) preparing a slurry reaction mixture comprising a metal salt and at least one organic linker compound in an aqueous solvent; (ii)supplying said reaction mixture to a flow reactor at a temperature of less than 150 °C and a pressure of less than 20 bar; and (iii) optionally repeating steps (i) and (ii).

Description

Process

Field of the Invention The present invention relates to a continuous flow process for preparing metal organic frameworks (MOFs), in particular to a continuous flow process for preparing Zr-MOFs. The invention further relates to an apparatus arranged to perform said process. 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 ZrLiO 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.

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 elevated temperatures and pressures not only increases the cost of the process but also means that scale-up to an industrial level poses many challenges. Apparatus suitable for withstanding the severe reaction conditions is often only compatible with small scale batch synthesis, rather than the continuous processes favoured for large scale production. Employing high pressures also carries with it safety concerns, particularly when combined with the use of corrosive liquids. Moreover, the use of organic solvents as the reaction medium is undesirable as such solvents are harmful to the environment and are expensive.

As MOFs become increasingly employed as alternatives to, for example, zeolites, there is a need for the development of novel processes for their production which are applicable to use on an industrial scale. Replacement of the organic solvent with an aqueous medium is reported in, for example, US 7411081 and US 8524932. These processes routinely involve the use of a base or require an alkaline reaction medium.

Moreover, reactions conditions which are suitable for the production of certain MOFs may not be transferable to others where different metals are used. For example, conditions found to be optimal for the production of MOFs containing main group metals from the second or third groups of the periodic table (e.g.

magnesium or aluminium) are often not appropriate for the preparation of analogous frameworks wherein a transition metal is used.

Recently, continuous flow chemistry has been investigated for the synthesis of MOFs, however these processes often still require the use of high temperatures and/or pressure. For example, a continuous flow process is disclosed in WO

2014/013274, however this reaction uses supercritical fluids as solvents and thus requires extreme conditions to maintain the solvents in this state throughout the reaction. The use of microfluidics in combination with flow reactors has also been reported which involves the production of MOFs within oil droplets. Product quality is reported to be low for microfluidic processing and the requirement for both oil and water phases does not eliminate the need to use harmful and costly organic solvents.

The present invention is particularly directed towards Zr-MOFs. An aqueous-based process for producing a Zr-MOF is reported by Yang et al in Angew. Chem. Int. Ed. 2013, 52, 10316-10320. This process involves a two-step synthesis. The product is obtained as a gel which, to be isolated, must be washed and recrystallised. This adds to the costs and timescale of the process, making it unsuitable for use on a larger industrial scale. There thus remains the need for the development of novel processes for the production of MOFs, and particularly Zr-MOFs, which are suitable for use on an industrial scale. The process should ideally be one which is "green" and thus considered environmentally friendly. 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. In particular, a process which involves fewer steps is desired. A process which avoids potential corrosion problems, such as the production of hydrochloric acid (HC1) as a by-product is particularly attractive. Ideally, a process which offers improvement in more than one of the above aspects would be developed.

The present inventors have surprisingly found that MOFs may be prepared in a straightforward continuous flow process utilising an aqueous solvent which avoids the need for high temperatures and pressures. In particular, the use of a flow reactor leads to a 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 continuous flow process for preparing a metal organic framework (MOF), comprising the steps:

(i) preparing a slurry reaction mixture comprising a metal salt and at least one organic linker compound in an aqueous solvent;

(ii) supplying said reaction mixture to a flow reactor at a temperature of less than 150 °C and a pressure of less than 20 bar; and (iii) optionally repeating steps (i) and (ii).

In a particularly preferable embodiment, the metal organic framework is a zirconium-based metal organic framework (Zr-MOF).

In a second aspect, the invention provides apparatus arranged to perform a process as hereinbefore defined comprising a flow reactor arranged to receive a slurry reaction mixture comprising a metal salt and at least one organic linker compound in an aqueous solvent, wherein the flow reactor is operated at a temperature of less than 150 °C and a pressure of less than 20 bar. Detailed Description

The present invention describes a continuous flow process for the

preparation of a metal organic framework (MOF). The process involves preparing a slurry reaction mixture comprising a metal salt and at least one organic linker compound in an aqueous solvent and supplying said reaction mixture to a flow reactor at a particular temperature and pressure. The process typically involves subsequently collecting and isolating the MOF.

MOF

As used herein, the term "MOF" is intended to cover any metal organic framework. MOFs typically comprise at least one metal ion or cluster of metal ions and at least one organic linker compound.

The metal ion or cluster of metal ions may be any suitable metal selected from Groups 1 to 16 of the Periodic Table, preferably a metal selected from the group consisting of transition metals, alkaline earth metals, lanthanides and mixtures thereof. The metal ion may have any valence appropriate for the specific metal. In a particularly preferable embodiment, the metal ion is selected from the group consisting of copper, zirconium, scandium, nickel, magnesium, bismuth, gallium, cobalt, zinc, aluminium, iron, cadmium, cerium and yttrium. Most preferably, the metal ion is zirconium. Whilst it is possible for a mixture of metal ions to be used, it is preferable if the MOF contains only a single type of metal ion.

In a particularly preferred embodiment, the process of the invention is used to prepare zirconium-based metal organic frameworks (Zr-MOFs). 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 0^2~ 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 at least bidentate 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.

The Zr-MOF may comprise additional metal ions other than zirconium, such as hafnium, titanium, or cerium, however preferably zirconium is 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%.

The Zr-MOFs of the invention particularly preferably have cornerstones having at least 12 coordination sites for the organic linkers, e.g. 12-24, 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 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 MOF.

In addition to the at least one metal ion or cluster of metal ions, the 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 metal ion. The organic linker compound may also be tridentate (i.e. containing three functional groups) or tetradentate (i.e. containing four functional groups).

The MOF may have a metal ion to organic linker molecule ratio of from 1 :0.45 to 1 :0.55, especially l :0.49 to 1 :0.51, particularly 1 :0.5. Other preferred metal ion to organic linker molecule ratios are 0.5 : 1 , 1 : 1, 3: 1 and 1 :3, especially 1 : 1.

The organic linker compounds of the 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 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 (i.e. 18 to 30 °C) and pressure (i.e. 0.5 to 3 bar) (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 (NH₂), 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 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_₂o alkyl group or a C₃_i₂ 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-l,4-benzene dicarboxylic acid (R3-BDC or H2N BDC), 1,2,4,5-benzene tetracarboxylic acid, 2-nitro- 1,4- benzene dicarboxylic acid Ι,Γ-azo-diphenyl 4,4'-dicarboxylic acid, eye lo butyl- 1,4- benzene dicarboxylic acid (R6-BDC), 1,2,4-benzene tricarboxylic acid, 2,6- naphthalene dicarboxylic acid (NDC), Ι,Γ-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, 1 ,4-bis(4- carboxyphenyl)butadiyne, nicotinic acid, terphenyl dicarboxylic acid (TPDC), 1,4- cyclohexane dicarboxylic acid (H2CDC) and N,N'- piperazinebis(methylenephosphonic acid). Other acids besides carboxylic acids, e.g. boronic acids may also be used.

Anhydrides may also be used, particularly those which are converted to dicarboxylic acids under the aqueous conditions used in the invention.

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 and 2-nitro- 1,4-benzene dicarboxylic acid, 1,4- cyclohexane dicarboxylic acid (H2CDC), N,N'-piperazinebis(methylenephosphonic acid) or mixtures thereof.

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

Where the MOF is a Zr-MOF, it is preferably of UiO-66 type. UiO-66 type Zr-MOFs cover structures in which the zirconium inorganic groups are

ΖΓ₆(0)₄(ΟΗ)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-l,4-benzene dicarboxylic acid, 2-nitro-l,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-l,4-benzene dicarboxylic acid, the resulting MOF may be referred to as UiO-66(Zr)-NH₂. 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.

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 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.

A particularly preferred functional group is halo, most preferably a fluoro group.

Preferably, the functionalised 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. Process

The process of the invention is a continuous flow process for preparing a metal organic framework (MOF), comprising the steps:

(ii) supplying said reaction mixture to a flow reactor at a temperature of less than 150 °C and a pressure of less than 20 bar; and

(iii) optionally repeating steps (i) and (ii).

By "continuous flow process" we mean one in which a chemical reaction is run in a continuously flowing stream rather than in a batch. Thus, it may be contrasted with a "batch process".

By "slurry" we mean a suspension of solid particles in a liquid, i.e. a heterogenous mixture of at least one solid and at least one liquid. Thus a "slurry" may be contrasted with a "solution", which is a homogenous mixture.

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 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 metal ion or metal ion cluster the organic linker will be deprotonated. Thus all preferable embodiments defined above relating to the organic linker in the context of the MOF apply equally to this compound as a starting material.

The metal ions may be provided in any conventional way, and thus any conventional source of metal ions may be used. However, the metal ions will typically be provided in the form of at least one metal salt, which may or may not be in its hydrated form. 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 metal salt is used, the salt is usually water soluble, i.e. preferably having a solubility of at least 1 g/L at room temperature (i.e. 18 to 30 °C) and pressure (i.e. 0.5 to 3 bar) (RTP), preferably at least 2 g/L, more preferably at least 5 g/L. Preferable metal ions are discussed above in the context of the MOF and all embodiments discussed therein apply equally here. Suitable counter-ions will be familiar to the skilled worker and may include halides (e.g. chlorides), acetates, nitrates, oxides, acrylates, carboxylates, sulfates, hydroxides, oxynitrates and oxychlorides. Examples of metal salts include zirconium sulfate, zirconium hydroxide, zirconium chloride (ZrC ) and zirconyl chloride (ΖΓ0Ο₂·ηΗ₂0, wherein n is an integer from 1 to 10, preferably 8).

In a preferable embodiment, the metal ions are zirconium metal ions. These zirconium metal ions may be provided in the form of a zirconium salt selected from the group consisting of hydroxide, sulfate or mixtures thereof. Most preferably, the salt is zirconium hydroxide (e.g. Zr(OH)₄) or zirconium sulfate (e.g. Zr(S0₄)₂ or Zr(S0₄)₂.4H₂0), especially zirconium sulfate. The zirconium ions will typically be in the +4 oxidation state, i.e. Zr⁴⁺ ions.

The use of zirconium salts such as sulfate and hydroxide has cost advantages because these starting materials are relatively cheap to obtain. Moreover, these preferable salts are safer to handle than certain other common salts and do not lead to the production of highly corrosive hydrochloric acid as a side-product.

A slurry reaction mixture of the metal salt and the at least one organic linker compound is prepared in aqueous solvent, i.e. a solvent comprising water. The pH of the aqueous solvent is preferably acidic, i.e. having a pH less than 7, more preferably pH 0-5, such as pH 0-3. In one embodiment, the aqueous solvent consists of water. Alternatively, the aqueous solvent may comprise a mixture of water and acetic acid. The water and acetic acid may be present in a volume ratio of between 1 :5 and 5: 1, more preferably 1 :2 to 2: 1 , most preferably 1 : 1.

In all embodiments of the invention it is preferred if the slurry reaction mixture does not comprise an organic solvent.

The reaction mixture prepared in step (i) of the processes of the invention is typically prepared by mixing the various components together in the aqueous solvent. Mixing may be carried out by any known method in the art, e.g.

mechanical stirring. Alternatively, the mixture may be prepared by mixing a first component comprising an aqueous solvent and the metal salt and a second component comprising an aqueous solvent and the at least one organic linker compound. The mixing may occur in a mixing vessel such as a beaker.

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 3 bar, especially 1 bar.

In step (ii) of the process, the reaction mixture prepared in step (i) is supplied to a flow reactor at a temperature of less than 150 °C and a pressure of less than 20 bar.

Flow reactors are well known in the art. They carry material as a flowing stream. Reactants are continuously fed into the reactor and emerge as continuous stream of product. The flow reactor of the present invention may be any suitable type of flow reactor known in the art, but is typically tube-like and is made from a material which will not react with the reagents of the continuous flow process. Examples of suitable materials include stainless steel, glass and polymers.

The flow reactor is at a temperature of less than 150 °C. Preferably, the temperature is less than 120 °C, more preferably less than 110 °C, such as less than 100 °C. The temperature may be at least 35 °C, preferably at least 40 °C, more preferably at least 50 °C. An example temperature is 85 °C.

The pressure of the flow reactor in step (ii) of the process is preferably less than 15 bar, more preferably less than 10 bar, even more preferably less than 5 bar, such as 0.5 to 3 bar, especially 1 to 2 bar.

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.

In step (ii), the reaction mixture is typically supplied to the flow reactor by way of at least one pump, which pumps the mixture through the flow reactor.

Preferably, a single pump is used. Where more than one pump is present, they are preferably connected in series.

It will be appreciated that a suitable pump speed may be chosen to achieve a desired flow rate through the reactor and thus the residence time (i.e. reaction time) in the flow reactor. Naturally, the residence time is also dependent upon the volume of the flow reactor. The skilled man would be able to select appropriate values based on the desired reaction time. The volume of the flow reactor may be in the range 100 ml to 5 litres, preferably 250 ml to 3 litres, more preferably 500 ml to 2 litres, such as 1.5 litres.

Typical pumping speeds would be in the range 4 ml/min to 300 ml/min, preferably 8 ml/min to 150 ml/min, more preferably 10 ml/min to 50 ml/min, such as 20 ml/min.

The residence time (i.e. reaction time/heating time) may be from 10 minutes to 10 hours, preferably 30 minutes to 5 hours, more preferably 45 minutes to 2 hours, such as 75 minutes.

As an example, for a flow reactor with a volume of 1.5 litres a pump speed of 20 ml/min may be used, giving a reaction time of 75 minutes.

The mild reactions conditions used in the process of the invention offer numerous advantages over those of previous methods wherein organic solvents were used as the reaction medium. The processes may be carried out in the absence of high pressures, temperatures or reaction times. This offers improvements in terms of costs, safety and suitability for industrial scale-up.

Moreover, the use of a slurry reaction mixture means that a wider range of starting materials may be employed compared with processes which require homogenous solutions. The process is thus potentially compatible with a large variety of MOFs. There is also a cost and time advantage, since slurries may be cheaper and easier to prepare than solutions.

In the context of Zr-MOFs it has surprisingly been found that the specific combination of zirconium ions and sulfate ions in the reaction mixture enable the direct formation of the Zr-MOF as an easy-to-handle powder. Thus, the preparation process may take place over significantly shorter timescales compared to the methods of the prior art.

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.

It will be appreciated that the MOF product forms during step (ii) of the process. The processes of the invention additional comprise step (iii) in which steps (i) and (ii) are optionally repeated. Steps (i) and (ii) may be repeated once or more than once, depending on factors such as the scale of the reaction or the nature of the apparatus employed. Step (iii) may be used when the reaction is performed on an industrial scale, for example, when large tanks or hoppers are refilled with the slurry reaction mixture to enable a continuous flow of said mixture to the flow reactor.

The processes of the invention usually comprise a further step (iv) of isolating the MOF.

Advantageously, the MOF is usually formed as a crystalline product which can be isolated quickly and simply by methods such as filtration, or centrifugation. This offers an improvement over some methods of the prior art which produce an amorphous or gel-like product which must be further recrystallized before it can be isolated. The processes of the present invention thus preferably eliminate the need for these additional steps.

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 MOF is preferably obtained as a fine crystalline powder having crystal size of 0.1 to 100 μιη, such as 10 to 50 μιη.

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

In all embodiments of the invention, it is preferred the process is carried out in the absence of a base.

In a further embodiment, the invention relates to a metal organic framework

(MOF) produced or formable by the processes as herein described.

Apparatus The present invention also relates to an apparatus arranged to perform the processes as hereinbefore defined. The apparatus comprises a flow reactor arranged to receive a slurry reaction mixture comprising a metal salt and at least one organic linker compound in an aqueous solvent, wherein the flow reactor is operated at a temperature of less than 150 °C and a pressure of less than 20 bar.

It will be appreciated that all embodiments relating to the nature of the flow reactors such as pump speed, volume, temperature and pressure etc. discussed above in the context of the process of the invention apply equally to the apparatus of the invention. Similarly, all embodiments discussed above relating to the metal salt, organic linker and solvent apply equally to the apparatus embodiment.

Typically, the flow reactor comprises tubing in fluid communication with at least one pump, which pumps the reaction mixture through the reactor. The pump may be placed at one end of the tubing or part way along the length of the tubing. Preferably a single pump is used. If more than one pump is present, they are preferably connected in series.

In one embodiment, the apparatus comprises at least one pump connected via a first tube to a reaction vessel containing a slurry reaction mixture comprising a metal salt and at least one organic linker compound in an aqueous solvent, and a second tube to a flow reactor.

Thus, in a particularly preferred embodiment, the invention relates to an apparatus arranged to perform the process as hereinbefore defined, wherein said apparatus comprises:

(i) a reaction vessel containing a slurry reaction mixture comprising a metal salt and at least one organic linker compound in an aqueous solvent; (ii) a first tube in fluid communication with the reaction vessel and at least one pump so as to allow the reaction mixture to flow from the reaction vessel to the at least one pump;

(iii) a second tube in fluid communication with the at least one pump and a flow reactor so as to allow the reaction mixture to flow from the at least one pump to the reactor;

wherein the flow reactor is operated at a temperature of less than 150 °C and a pressure of less than 20 bar.

Typically, in this embodiment, the flow reactor is heated using an oven or microwave reactor. The pump may be a magnetic membrane pump. An example of this embodiment of the apparatus of the invention is shown in Figure 1. In all embodiments, the apparatus of the invention may further comprise other components such as a sonicator, which can be used to enhance mixing.

Preferably, the apparatus of the invention does not comprise a back pressure regulator. This is because the apparatus is most preferably operated at atmospheric pressure, e.g. 1 to 3 bar.

Applications

The 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.

Figures

Figure 1: Example apparatus of the invention

Figure 2: Powder X-ray diffraction pattern measured with a wavelength of 1.5406 A of UiO-66-COOH prepared as described in the first example.

Figure 3: Powder X-ray diffraction pattern measured with a wavelength of 1.5406 A of UiO-66-COOH prepared as described in the second example.

Figure 4: Sorption isotherm of nitrogen measured at 77 K on UiO-66-COOH prepared as described in the second example.

Figure 5: Powder pattern from [Al(OH)(FUM)] synthesised in a flow reactor.

Figure 6: N2-Isotherme of [Al(OH)(FUM)] synthesised in a flow reactor.

Examples In order to pump a defined amount of slurry through the flow reactor, a pressure transmitting medium is necessary. Here, water was utilized and prior to the reaction the reactor was filled with water and preheated to the temperature given. After pumping the slurry into the reactor, additional water was again used to push the slurry to the end of the reactor. In a fully continuous mode the pressure transmitting medium would be the slurry itself.

Example 1

Trimellitic acid (16.8 g) and Zr(S04)2^»4H20 (7.1 g) were mixed with 100 mL water using a magnetic stirrer. The resulting slurry was transferred into the reactor using a magnetic membrane pump into the reactor which was preheated to 85 °C. Adjusting the pumping frequency (12 per minute) and the idle stroke (50 %) to suitable values, a residence time of 75 minutes in the reactor was achieved (Volume 1850 mL). The product was first collected in a beaker and separated by filtration while still hot. Afterwards the raw material was mixed with water (50 mL par 5 g) and heated for 15 minutes to 85 °C in order to remove residual linker molecules. The slurry was again filtrated while still hot and dried under ambient conditions. The PXRD pattern is shown in Figure 2. Example 2

Trimellitic Anhydride (15.4 g) was thoroughly ground. The fine powder and Zr(S04)2^»4H20 (7.1 g) were mixed with 100 mL water using a magnetic stirrer. The resulting slurry was transferred into the reactor using a magnetic membrane pump into the reactor which was preheated to 85 °C. Adjusting the pumping frequency (12 per minute) and the idle stroke (50 %) to suitable values, a residence time of 75 minutes in the reactor was achieved (Volume 1850 mL). The product was first collected in a beaker and separated by filtration while still hot. Afterwards the raw material was mixed with water (50 mL par 5 g) and heated for 15 minutes to 85 °C in order to remove residual linker molecules. The slurry was again filtrated while still hot and dried under ambient conditions. 6.1 g of a white powder were obtained. The PXRD pattern is shown in Figure 3. The sorption isotherm measured with nitrogen at 77 K after activation at 120 °C in vacuum (0.1 mbar) is shown in Figure 4. The apparent specific surface area evaluated with the BET method is 760 m²/g.

Example 3

Terephtalic acid (0.42 g), NaOH (0.2 g) and Α1(Ν0₃)₃·9 H₂0 (0.94 g) were mixed with 25 mL water using a magnetic stirrer. The resulting solution was transferred into the reactor using a magnetic membrane pump into the reactor which was preheated to 80 °C. Adjusting the pumping frequency (65 per minute) and the idle stroke (50 %) to suitable values, a residence time of 15 minutes in the reactor was achieved (Volume 1850 mL). The product was first collected in a beaker and separated by filtration while still hot.

Example 4

Fumaric acid (H₂FUM, 2.9 g), NaOH (3 g) and A1(S0₄)₂- 18 H₂0 (8.33 g) were mixed with 50 mL water using a magnetic stirrer. The resulting solution was transferred into the reactor using a magnetic membrane pump into the reactor which was preheated to 80 °C. Adjusting the pumping frequency (65 per minute) and the idle stroke (50 %) to suitable values, a residence time of 15 minutes in the reactor was achieved (Volume 1850 mL). The product was first collected in a beaker and separated by filtration while still hot. The PXRD pattern is shown in Figure 5. The sorption isotherm measured with nitrogen at 77 K after activation at 120 °C in vacuum (0.1 mbar) is shown in Figure 6.

Claims

1. A continuous flow process for preparing a metal organic framework (MOF), comprising the steps:

(iii) optionally repeating steps (i) and (ii).

2. A process as claimed in claim 1, wherein the metal salt comprises a metal ion selected from the group consisting of copper, zirconium, scandium, nickel, magnesium, bismuth, gallium, cobalt, zinc, aluminium, iron, cadmium, cerium and yttrium, preferably zirconium.

3. A process as claimed in claim 1 or 2, wherein the metal salt is selected from the group consisting of zirconium sulfate, zirconium hydroxide, zirconium chloride and zirconyl chloride.

4. A process as claimed in any of claims 1 to 3, wherein the temperature of the flow reactor is less than 120 °C, preferably less than 110 °C, more preferably less than 100 °C.

5. A process as claimed in any of claims 1 to 4, wherein the pressure of the flow reactor is less than 15 bar, preferably less than 10 bar, more preferably less than 5 bar.

6. A process as claimed in any of claims 1 to 5, wherein the at least one organic linker compound comprises at least two functional groups selected from the group consisting of carboxylate (COOH), amine (NH₂), anhydride and hydroxyl (OH) or a mixture thereof.

A process as claimed in any of claims 1 to 6, wherein the at least one organic linker compound comprises a linear or branched Ci_₂o alkyl group, a C_3-12 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 7, wherein the organic linker compound is selected from the group consisting of 1 ,4-benzene dicarboxylic acid (BDC), 2-amino-l ,4-benzene dicarboxylic acid, 1 ,2,4-benzene tricarboxylic acid, 2-nitro-l ,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 8, wherein the aqueous solvent consists of water or is a mixture of water and acetic acid.

A process as claimed in any of claims 1 to 9, wherein the reaction mixture does not comprise an organic solvent.

A process as claimed in any of claims 1 to 10, wherein 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.

12. A metal organic framework (MOF) produced or formable by the process as defined in any of claims 1 to 1 1.

13. Apparatus arranged to perform the process as defined in any of claims 1 to 1 1 , comprising a flow reactor arranged to receive a slurry reaction mixture comprising a metal salt and at least one organic linker compound in an aqueous solvent, wherein the flow reactor is operated at a temperature of less than 150 °C and a pressure of less than 20 bar.

14. Apparatus as claimed in claim 13, wherein said apparatus comprises

apparatus comprises: (i) a reaction vessel containing a slurry reaction mixture comprising a metal salt and at least one organic linker compound in an aqueous solvent;

(ii) a first tube in fluid communication with the reaction vessel and at least one pump so as to allow the reaction mixture to flow from the reaction vessel to the at least one pump;