WO2013182208A1

WO2013182208A1 - Process for preparation of metal borohydrides

Info

Publication number: WO2013182208A1
Application number: PCT/DK2013/050185
Authority: WO
Inventors: Bo RICHTER; Torben René JENSEN
Original assignee: Aarhus Universitet
Priority date: 2012-06-08
Filing date: 2013-06-10
Publication date: 2013-12-12

Abstract

The present invention relates to a novel low temperature process for the preparation of metal borohydrides or an isotopically enriched analogue thereof. The present invention also relates to the use of said metal borohydrides as hydrogen storage materials or reducing agents in organic chemical processes.

Description

Process for preparation of metal borohydrides Field of invention

The present invention relates to a novel low temperature process for the preparation of metal borohydrides. The present invention also relates to the use of said metal borohydrides as hydrogen storage materials or reducing agents in organic chemical processes.

Background of invention

Metal borohydrides are being widely used for example as hydrogen storage materials and as reducing agents in organic chemistry processes. Hence a number of processes for synthesizing metal borohydrides are known.

US application No. US19560619112 describes a process for the preparation of alkali metal borohydrides which comprises (1) reacting a dispersion on finely divided alkali metal in an essential inert diluent with hydrogen to produce a suspension of alkali metal hydride, (2) reacting said suspended alkali metal hydride with an alkyl borate to form alkali metal borohydrides and by-products, (3) dissolving said alkali metal borohydride in dimethyl ether of diethylene glycol by adding said ether to the reaction mixture containing said alkali metal borohydride and by-product suspended in said inert diluent whereby two separate liquid phases form, and (4) separating the dissolved alkali metal borohydride-ether phase and recovering the alkali metal borohydride from said phase. The reaction is generally conducted over a temperature ranging from 140-400°C. US application No. US19660530367 discloses a process, where an alkali metal borohydride is produced by heating, under anhydrous conditions, stoichiometric amounts of an alkali metal hydride or alkali metal and hydrogen, and an alkali metal tetraborate, as sole reactants. The reactants are preferably heated in an atmosphere of hydrogen at 2-10 cm Hg gauge with intimate mixing to 200-450°C.

All processes, which are known today, suffer from one or more drawbacks. For instance, in some of the known processes it is crucial to add an excess of one reagent to avoid formation of by-products, and thereby prevent a lower yield and decreased purity of the final product. Other known processes involve the use of diborane gas and hydrides at high temperatures and pressures. These processes further suffer from problems regarding formation of a so-called passivation layer, hampering further reaction progress. This problem, however, has been circumvented using a procedure in which simultaneous ball milling is performed, but still the reaction requires elevated temperature and/or use of diborane gas or coordinating solvents such as diethyl ether or amines.

These known problems are overcome by the process of the present invention, in which the reaction proceeds below or at room temperature with near full conversion. The major advantage of the process of the present invention when compared to the known processes is that the reaction proceeds at low temperature with near full conversion and with no or only a slight excess of borane reagent. For practical reasons the process of the present invention is superior to other existing processes, because no external heating source is required, handling and/or liberation of dangerous and toxic gaseous species during the process is avoided, and complicated arrangement of equipment, such as high pressure reaction chambers or sophisticated extraction apparatus and chemicals, are avoided.

Summary of invention

One object of the present invention is to provide a process for the preparation of metal borohydrides characterized by comprising the steps of:

a) providing a metal hydride as the starting material,

b) activating said metal hydride,

c) mixing said activated metal hydride with a borane reagent at a temperature ranging from 0 - 30°C,

d) stirring said mixture of activated metal hydride and borane reagent at room temperature under inert conditions, whereby the metal borohydride is produced, and e) collecting the metal borohydride produced in step d. Another object of the present invention is the use of the product obtained by said process as a material for hydrogen storage or as a reducing agent in organic chemical processes. Description of Drawings

Figure 1 shows the x-ray diffraction spectrum of the products y-Mg(BH₄)₂ and

Mg(BH₄)₂ ½Me₂S produced in example 1. Figure 2 shows the x-ray diffraction spectrum of the product a-Ca(BH₄)₂ produced in example 2.

Figure 3 shows the x-ray diffraction spectrum of the product LiBH₄ produced in example 3.

Figure 4 shows the SR-PXRD spectrum of the product LiBH₄ produced in example 4. Detailed description of the invention

The present invention relates to a novel process for the preparation of metal borohydrides. This novel process comprises the steps of:

a) providing a metal hydride as the starting material,

b) activating said metal hydride,

d) stirring said mixture of activated metal hydride and borane reagent at room temperature under inert conditions, whereby the metal borohydride is produced, and e) collecting the metal borohydride produced in step d.

The process of the present invention may be described by the following reaction scheme:

Mⁿ⁺H_n + nR BH₃ Mⁿ⁺(BH₄)_n + nR

where,

M represents a metal, such as Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Y, La, Eu, Yb, Er, Tb and Sc which forms part of the metal hydride, Mⁿ⁺H_n, where n designates the valence of the metal, and

R BH₃ designates the borane reagent.

Isotopically enriched analogues of the metal hydride and/or the borane reagent may also be employed. Such analogues include isotopes such as for example ¹¹ B, ¹⁰B, ²D and ⁷Li. In the process of the present invention a metal hydride is used as the starting material. By the term "metal hydride" as used herein is meant alkali metal hydrides, earth alkali metal hydrides as well as hydrides formed by metals from group 3 of the periodic table and part of the lanthanides. The term includes hydrides formed by one metal cation such as for example LiH, NaH, KH, RbH, CsH, BeH₂, MgH₂, CaH₂, SrH₂, BaH₂, YH₃, LaH₃, EuH₃, YbH₃, ErH₃, TbH₃ and ScH₃. The term also includes isotopically enriched metal hydrides. In one embodiment the starting material is selected from the group consisting of LiH, MgH₂ and CaH₂. The product produced are the metal borohydrides such as for example LiBH₄, NaBH₄, KBH₄, RbBH₄, CsBH_4, Be(BH₄)₂, Mg(BH₄)₂, Ca(BH₄)₂, Sr(BH₄)₂, Ba(BH₄)₂, Y(BH₄)₃, La(BH₄)₃, Eu(BH₄)₃, Yb(BH₄)_3, Er(BH₄)₃, Tb(BH₄)₃ and Sc(BH₄)₃. The product produced may also be an isotopically enriched metal borohydrides. By the term "isotopically enriched metal hydride" and "isotopically enriched metal borohydride" is meant metal hydrides and metal borohydrides where the metal ion has been replaced by its metal isotope, such as for examples where Li has been replaced by ⁷Li, and/or where hydrogen has been replaced by deuterium and/or where boron has been replaced by one of its isotopes, ¹¹ B or ¹⁰B.

It is essential for the process that the metal hydride starting material is activated before use in order for the process to occur at low temperature. This activation process may be performed by any known method, but preferably the activating of the starting material is performed by ball milling the metal hydride. By the term ball milling as used herein is meant a mechanical grinding process in which the particle sizes are significantly reduced, increasing the reactivity of the starting materials. A ball mill is a device consisting of a number of fixed vials containing a number of metal balls (e.g.

Wolfram-Carbide) rotating on a planetary disc, which is situated on another larger disc. The discs are rotating in opposite directions at a given speed and results in the balls grinding along the vial-walls and sling-impacting the opposite wall. This provides the energy for deforming and decreasing the particle sizes. The prerequisite that the starting materials are in the form of small particles is essential for the reaction to proceed to completion. The activation process may also be performed by

nanoconfinement of the metal hydride in an inert scaffold material such as carbon aerogels. Alternatively, hand-grinding or lower-energy ball milling followed by removal of aggregates using 100-300 μηι sieves has proven to be sufficient in some cases. The borane reagent used in the process of the present invention is designated by the formula R BH₃ and is preferably a reagent that is less hazardous than e.g. borane gas, being easier and relatively safe to handle. Examples of suitable borane reagents include Me₂S BH₃, isoamylsulfide-BH₃ and tetrahydrofuran- BH₃. Preferably the borane reagent is Me₂S BH₃. In some embodiments the borane reagent is Me₂S BH₃, which is found as a solution (1.0 - 10.0M) in toluene, dichloromethane, diethyl ether, tetrahydrofuran or dimethyl sulfide. In a preferred embodiment the borane reagent is a 2M-5M solution of Me₂S BH₃ in toluene. The concentration of the borane reagent can be altered as desired, although increased/decreased reaction times and exothermicity should be evaluated. The use of toluene as the specific solvent is preferred, but any hydrocarbon, non metal-coordinating, solvent would be suitable. It is important, however, that the solvent is anhydrous grade so that the risk of hydrolysing starting materials and/or products is minimized. Any anhydrous aprotic solvent can be used in the process since the role of the solvent is to provide suspension of the reactants and products rather than to solvate the reactants and products. Examples of compatible solvents include toluene, pentane, hexane, heptane, benzene, acetonitrile, methylene chloride, diethyl ether, tetrahydrofuran (THF) and dimethyl sulfide. The latter 3 solvents dissolve some borohydrides and as such the end-product will be a solvate, e.g.

Ca(BH₄)₂-2THF, from which the solvent can be subsequently removed. Toluene is preferred due to its economical and practical characteristics. It is relatively cheap and considered to be a relatively green solvent as compared to many other hydrocarbons.

One significant advantage of the process of the present invention is that it is possible to mix the entire amount of activated metal hydride and the entire amount of borane reagent in one step. In known processes the reagents must be mixed slowly and dropwise due to heat evolution, which makes the known processes rather tedious and potentially dangerous. However, for reactions involving higher concentrations, e.g. 5M Me₂S BH₃, the reaction produces heat and it is recommended to cool using e.g. an ice bath while adding the reactants only. The reactants should be added dropwise but can still be added in one portion.

A major advantage of the process of the present invention is that the reaction occurs at low temperature. By the term "low temperature" as used herein is meant a temperature ranging from 0 - 30°C. More precisely, the reaction occurs, i.e. the stirring of the reaction mixture in step d, occurs at room temperature. However, in some cases it may be necessary to cool the reactants before mixing in step c, because a slight excess of heat may be produced during mixing. This is particular the case when the borane reactant is for instance 5 or 10 M Me₂S BH₃, By the term "room temperature" as used herein is meant the ambient temperature, which is usually ranging from 20-25°C.

In the process of the present invention the reaction mixture is stirred at inert conditions. By the term "inert conditions" as used herein is meant an atmosphere excluding moisture, oxygen and C0₂. Examples of such atmospheres include dry N₂ and Ar. Moreover, inert conditions demand the use of substantially dry equipment. It lies within the skills of a practitioner to perform a process under inert conditions. In a preferred embodiment the inert conditions are obtained by stirring the reaction mixture in a closed reaction vessel inside a glovebox. It is also viable to use Schlenk techniques or any other standard technique for obtaining inert conditions, i.e. maintain an inert gas flow by a steady supply/flow of N₂/Ar to a stirred solution.

The chemical reaction will take place during stirring in step d. The reaction time, which is required in order to obtain full conversion of starting material into product will depend on the concentration ratio between the activated metal hydrides and the borane reagent as well as the degree of activation. The reaction time when using the 2M reagent may vary from 72 to 144 hours depending on the metal hydride used. When using the 5M reagent the reaction time is decreased and may be as low as 24 hours.

After complete reaction the produced solid metal borohydrides are collected from the reaction mixture. A skilled person would know how to separate a solid product from a reaction mixture. In a preferred embodiment, however, the metal borohydride is collected by filtration of the mixture. It is also possible to simply decant the solution inside a glovebox, and leave the remaining slurry to evaporate the remaining solvent and collect the materials subsequently. Preferably, the product is dried before storage. A skilled person would know how to dry the product without affecting the quality and purity of the product. In a preferred embodiment the obtained product is dried in vacuo at room temperature. Alternatively, drying the material can be achieved by slow evaporation either at room temperature, or with heating below decomposition temperatures for the metal borohydride. All manipulation and handling must be performed under inert conditions. It is a very important advantage of the process of the present invention that essentially no excess of the borane reagent is required, although a slight excess may be included in order to account for unexpected evaporation and to decrease the reaction time. Hence, in a preferred embodiment of the process of the present invention the activated metal hydrides and the borane reagent is mixed in equivalent amounts. In another preferred embodiment of the process of the present invention the activated metal hydrides and the borane reagent is mixed in a ratio where the borane reagent is slightly in excess of the activated metal hydride, such as in a ratio of borane reagent : activated metal hydride of 1.1 : 1. A skilled person knows that the relative ratio of the reagents can be tuned to afford faster reaction times.

The products obtained are all crystalline and exhibit distinct structural features, which are easily assessed by standard X-ray diffraction methods. Furthermore, complete phase-selectivity is achieved by the process of the present invention. For instance pure 0-LJBH₄, a-Ca(BH₄)₂ and Mg(BH₄)₂-½Me₂S is obtained, when the process is carried out at room temperature, the borane reagent is Me₂S BH₃ (2M) and the starting material is LiH, CaH₂ or MgH₂, respectively. It is well known that it is trivial to access all other phases of these materials from o-LiBH₄, a-Ca(BH₄)₂ and Mg(BH₄)₂ ½Me₂S,

respectively. This fact is a significant advantage as compared with products obtained from known processes, where mixtures of phases are obtained, from which

transformation to certain polymorphs is not possible. Hence, the known processes have essentially no control over the structure of the product and desolvation of the product is required in order to obtain a substantially pure product. In particular for Ca(BH₄)₂ the method is clearly superior in the sense of phase selectivity since all other methods result in mixtures of polymorphs.

In a preferred embodiment the product produced by the process of the present invention is obtained in the form of a pure crystalline single phase. In a preferred embodiment the product obtained by the process of the present invention is selected from the group consisting of o-LiBH₄, a-Ca(BH₄)₂ and Mg(BH₄)₂-½Me₂S.

When LiH is used as starting material, the obtained product is pure o-LiBH₄, which does not require further desolvation before use. When CaH₂ is used as the starting material the produced product is pure a-Ca(BH₄)₂, from which all other known phases can be obtained. When Ca(BH₄)₂ is produced by any of the known methods the products will also contain p-Ca(BH₄)₂ to some extent due to the well-known fact that a-Ca(BH₄)₂ converts to p-Ca(BH₄)₂ upon heating. This conversion is avoided in the process of the present invention, because no heating is necessary at any step in the process (the desolvation of Ca(BH₄)₂-2THF can be performed at temperatures above 160 °C).

When MgH₂ is used as the starting material, the obtained product is Mg(BH₄)₂ ½Me₂S. In this product Mg(BH₄)₂ coordinates weakly to sulphur in Me₂S providing a solvate structure with a stoichiometry of one Me₂S for each two Mg(BH₄)₂ molecules. This structure can easily be desolvated to give a structure in which the connectivity of Mg(BH₄)₂ is the same, thus affording a cage-like structure (y-Mg(BH₄)₂), in which the cages contained the Me₂S molecules before the desolvation. Depending on the specific desolvation procedure it is possible to obtain the well known a-Mg(BH₄)₂ phase from the Me₂S solvate, and from there to obtain the p-Mg(BH₄)₂ upon heating.

The products obtained by the process of the present invention may be used in any known industrial exploitation known within the technical area. Preferably, the products obtained by the process of the present invention are used as materials for hydrogen storage or as reducing agents in organic chemical processes. It is well-known that NaBH₄ is the only borohydride produced in ton scale, due to its use in the paper and pulp industry. It is used also as a precursor in a number of chemicals productions; e.g. NaCNBH₃ and LiBH₄.

Examples

The invention is further illustrated with reference to the following examples, which are not intended to be in any way limiting the scope of the present invention. Example 1. Preparation of Mg(BH₄)₂ in small scale

0.658 g (25.0 mmol) of MgH₂ was ball milled in a Fritsch Pulverisette 4 planetary ball mill for approximately 2 hours (10 min milling, 2 min pause x 12). The ball milled MgH₂ and 30.0 ml of 2M Me₂S BH₃ (2.4 equiv.) was then mixed at room temperature, and the reaction mixture was left to react for 4 days. Samples were collected after 1 , 3 and 4 days, respectively. The samples were dried in vacuo using a Schlenk line and then analysed using X-ray diffraction techniques. The resulting x-ray diffraction spectrum is shown in Figure 1. The figure clearly shows a decrease in intensity of Bragg peaks from unreacted MgH₂, which can be seen by following the dotted line throughout the graph. The products are y-Mg(BH₄)₂ and Mg(BH₄)₂ ½Me₂S. The plot is depicted w. 1/d spacing as the X-axis since the data sets are recorded at different wavelengths.

Depending on the desolvation conditions applied after synthesis, both γ or a - polymorphs of Mg(BH₄)₂ can be obtained.

Example 2. Preparation of Ca(BH₄)₂ in small scale

1.052 g (25.0 mmol) of CaH₂ was ball milled in a Fritsch Pulverisette 4 planetary ball mill for approximately 2 hours (10 min milling, 2 min pause x 12). The ball milled CaH₂ and 27.5 ml of 2M Me₂S BH₃ (2.2 equiv.) was then mixed at room temperature, and the reaction mixture was left to react for 6 days. Samples were collected after 1 , 3 and 6 days, respectively. The samples were dried in vacuo using a Schlenk line and then analysed using X-ray diffraction techniques. The resulting x-ray diffraction spectrum is shown in Figure 2. The decrease in intensity of Bragg peaks from unreacted CaH₂ is seen by following the dotted line throughout the graph. The products are solely a- Ca(BH₄)_2. Example 3. Preparation of LiBH₄ in small scale

0.197 g (25.0 mmol) of LiH was ball milled in a Fritsch Pulverisette 4 planetary ball mill for approximately 2 hours (10 min milling, 2 min pause x 12). The ball milled LiH and 15.0 ml of 2M Me₂S BH₃ (1.2 equiv.) was then mixed at room temperature, and the reaction mixture was left to react for 6 days. Samples were collected after 1 , 3 and 6 days, respectively. The samples were dried in vacuo using a Schlenk line and then analysed using X-ray diffraction techniques. The resulting x-ray diffraction spectrum is shown in Figure 3. The decrease in intensity of Bragg peaks from unreacted LiH is seen by following the dotted line throughout the graph. Example 4. Preparation of LiBH₄ in an small upscaling procedure (= x 10)

LiH was activated as disclosed in example 3. 5.15 g (0.649 mol) of ball milled and filtered LiH was suspended in 75.0 mL toluene (anhydrous) after which 75.0 mL 10M Me₂S BH₃ (0.750 mol, 1.16 equiv.) was added dropwise while cooling and stirring. The reaction mixture was left to react at room temperature for 6 days. A total of 8 ml of sample was collected after 1 , 3, 4 and 5 days, respectively (2 mL / day). The samples were analysed using X-ray diffraction (1 day, 3 day, product). ICP (elemental analysis) data is shown in table 1 below.

Table 1. ICP (elemental analysis) data for the relative contents of B and Li in the reaction after 1 , 3 and 4 days, respectively. The ratio of Li and B is measured and the relative amounts compared to the ideal composition of 1 :1 (LiBH₄ contains one B/Li) and the purity estimated. The total collected quantities of material (13.3 g) indicate that all LiH has reacted (Molar ratios of 99% LiBH₄ and 1 % LiH corresponds to 13.3 g). The SR-PXRD spectrum (figure 4) agrees well with this since no residual LiH is observed. An impurity of WC is observed - probably arising from a transient pollution in our laboratories (ball milling). The quantity of WC as indicated by the intensities of the Bragg peaks should not be considered significant since WC is known to diffract extremely strongly and the occurrence was accidental and is avoidable. This point is proven in the previous examples (examples 1-3) using the same ball mill and conditions, still avoiding WC contamination. FTIR shows no sign of any hydrocarbon residue from the synthesis, i.e. the mass of the collected material is not falsely quantified by the existence of solvent residue impurities. The validity and usefulness of the method has been demonstrated by the analysis of the end products, showing that no amount of unreacted LiH is present, and thus the yield should be considered to be essentially quantitative.

Perspectivations

Depending on the identity of M in the general reaction concerning Mⁿ⁺H_n, the reaction speed, reagent concentration, and degree of hydride activation can be altered in any way practically feasible to provide the most complete conversion. In certain cases, re- activation procedures may be applied, i.e. for synthesis of Sr(BH₄)₂ it has proven successful to ball mill the collected material again and repeating the reaction. As stated earlier a large number of different metal borohydrides can be produced using this method. For example, the production of materials with various compositions of isotopes (¹¹ B, ¹⁰B ²D, ⁷Li), for use in e.g. neutron diffraction experiments, is performed equally well. The supply of suitably enriched starting materials is the only prerequisite. The current extreme costs of such materials justify the use of the method enclosed herein, due to the potential for cutting costs.

Claims

1. A process for the preparation of metal borohydrides characterized by comprising the steps of:

a) providing a metal hydride as the starting material,

b) activating said metal hydride,

2. The process according to claim 1 , where the starting material is selected from the group consisting of LiH, NaH, KH, RbH, CsH, BeH₂, MgH₂, CaH₂, SrH₂, BaH₂, YH₃, LaH₃, EuH₃, YbH₃, ErH₃, TbH₃, ScH₃ or an isotopically enriched analogue thereof.

3. The process according to any of the preceding claims, where activating the starting material is performed by ball milling, nanoconfinement or hand grinding, optionally followed by removing aggregates.

4. The process according to any of the preceding claims, where the borane reagent is Me₂S BH₃.

5. The process according to any of the preceding claims, where the mixing in step c is performed by adding all activated metal hydride and borane reagent in one step.

6. The process according to any of the preceding claims, where the stirring at inert conditions is performed by use of substantially dry equipment in an atmosphere selected among N₂ and argon.

7. The process according to any of the preceding claims, were the stirring in step d is continued for a period ranging from 24 to 144 hours.

8. The process according to any of the preceding claims, where the collection of metal borohydride is obtained by filtration of the mixture obtained in step d.

9. The process according to any of the preceding claims, where the obtained product is dried in vacuo at room temperature or at elevated temperature.

10. The process according to any of the preceding claims, where the activated metal hydrides and the borane reagent is mixed in equivalent amounts.

1 1. The process according to any of the preceding claims, where the activated metal hydrides and the borane reagent is mixed in a ratio where the borane reagent is slightly in excess of the activated metal hydride, such as in a ratio of borane reagent : activated metal hydride of 1.1 : 1.

12. The process according to any of the preceding claims, where the product produced is selected from the group consisting of LiBH₄, NaBH₄, KBH₄, RbBH₄, CsBH_4, Be(BH₄)₂, Mg(BH₄)₂, Ca(BH₄)₂, Sr(BH₄)₂, Ba(BH₄)₂, Y(BH₄)₃, La(BH₄)₃, Eu(BH₄)₃, Yb(BH₄)₃, Er(BH₄)₃, Tb(BH₄)₃, Sc(BH₄)₃ or an isotopically enriched analogue thereof.

13. Use of the product obtained by the process according to any of claims 1 -12 as a material for hydrogen storage or as a reducing agent in organic chemical processes.