Process for the production of lithium aluminum hydride

Abstract

The invention relates to a process for the preparation of lithium aluminum hydride, LiAlH4, by decomposition of lithium aluminum hexahydride, Li3AlH6, in an aprotic solvent or an aprotic solvent mixture, wherein the solvent or solvent mixture contains at least one cyclic ether and / or at least one polyfunctional ether.

Description

[0001] Process for the production of lithium aluminum hydride

[0002] The invention relates to a process for the production of lithium aluminum hydride (UAIH4) from lithium aluminum hexahydride (LisAIHe).

[0003] Background

[0004] Lithium aluminum hydride, LiAIH4, is a complex hydride that is highly soluble in etheric solvents and rich in hydrogen. It is mainly used in organic synthesis chemistry for the reduction of functional groups such as esters, carboxylic acids, carboxylic acid amides or nitriles to the corresponding alcohols or amines (see also J.S. Pizey "Lithium Aluminum Hydride", John Wiley, 1977, New York; and J. Magnano, Org. Process Res. Dev. 2012, 16, 1156-1184). Furthermore, due to its high hydride content of 10.5% by weight, it is an excellent storage medium for hydrogen. The hydridic hydrogen it contains can be released either hydrolytically (E. Ashton et al., ACS Appl. Energy Mater. 2022, 5, 8336-8345) or thermolytically (P. Corbo et al., Journal of Power Sources 193 (2009) 285-291 ). In contrast to UAIH4, the hexahydride LisAIHe is typically insoluble in aprotic organic solvents. For this reason, it is disadvantageous as a reducing agent and, unlike LiAIH4 , it has not found industrial application.

[0005] The first stage of the thermal decomposition of LiAIH4 produces the hexahydride Li3AIH6:

[0006] R. Santhanam and G. Sean McGrady describe that LiAIH4 can also be converted to LisAIHe in the presence of lithium hydride either mechanochemically (i.e. by grinding a mixture of the two solids) or in various solvents, for example dimethyl ether (Me20), according to Reaction (2) (Inorganica Chimica Acta 361 (2008) 473-478):

[0007] Me2O

[0008] MAIH4+ 2 MH - ► M3AIH6(M = Li, Na, K) (2)

[0009] T = 80 / 160°C

[0010] LisAIHe is also formed from AIBr3ZLiH in a diethyl ether / toluene mixture in the presence of AIEt3catalyst (FR 2043692) and according to Reaction (2) in a diethyl ether / toluene mixture also in the presence of catalytically active triethylaluminum (FR 2093124) and in cyclohexane / diethyl ether or toluene / diethyl ether under a high H2pressure of 300 bar (FR 1604706).

[0011] Rehydration of Li3AIHe and formation of UAIH4 in the reverse reaction (1 ) is not observed even at high H2pressures of 4 MPa (J. Chen et al., J. Phys. Chem. B 2001 , 105, 11214-11220). It is therefore not possible to produce LiAIH4 from LisAIHe using processes known from the state of the art.

[0012] Problem

[0013] Accordingly, the present invention is based on the problem of utilizing the availability of LisAIHe and producing LiAIH4 from LisAIHe in an industrially suitable process.

[0014] Description of the invention

[0015] The problem is solved by the process according to claims 1 to 13 and the use according to claim 14.

[0016] The process according to the invention is characterized in that lithium aluminum hexahydride (LisAIHe ) is decomposed to lithium aluminum hydride (LiAIH4 ) and lithium hydride (LiH) by suspending LisAIHe in an aprotic solvent or aprotic solvent mixture, wherein the solvent or solvent mixture contains at least one cyclic ether and / or at least one polyfunctional ether, and this suspension is maintained at a reaction temperature of O to 180°C.

[0017] It was surprisingly found that in the solvents used in the process according to the invention, LisAIHe splits into LiAIH4 and LiH under the reaction conditions defined according to the invention in a reversal of reaction (2):

[0018] Solvent according to the present invention

[0019] Li3AIH6 - ► LiAIH4+ 2 LiH I (3)

[0020] T = 0 to 180°C '

[0021] It was therefore surprisingly found that the process according to the invention can be used to convert LisAIHe to LiAIH4 by selecting a suitable solvent and suitable reaction conditions, in particular a suitable reaction temperature.

[0022] For the purposes of the present invention, the term "splitting" covers in particular a reaction of the disproportionation type.

[0023] LisAIHe can be used in the process according to the invention as a pure substance or in the form of a LisAIHe-containing mixture for the production of LiAIH4.

[0024] In a preferred embodiment of the process according to the invention, LiAIH4 present in dissolved form after splitting is separated from LiH and optionally unreacted LisAIHe by a solid / liquid separation operation. In the solvents according to the invention, which contain at least one cyclic and / or one polyfunctional ether, the LiAIH4 formed has a significant solubility of typically >5% by weight, while the lithium hydride formed as a coupling product is insoluble. It is therefore possible to separate the valuable product UAIH4 from any remaining insoluble reactant (LisAIHe) and the co-product LiH by a solid / liquid separation operation and isolate it in the form of a solution.

[0025] Preferably, the LiAIH4 is separated by filtration, sedimentation or centrifugation, particularly preferably by filtration.

[0026] In a particularly preferred embodiment of the process according to the invention, solid LiAIH4 is produced from LiAIH4 , which has been separated by a solid / liquid separation operation, preferably by total evaporation or displacement crystallization. Suitable process steps for the production of solid LiAIH4 from dissolved LiAIH4 are known from the prior art, e.g. from EP 1 394 106 B1.

[0027] The process according to the invention comprises first suspending LisAIHe in an aprotic solvent or an aprotic solvent mixture, wherein the solvent or solvent mixture contains at least one cyclic ether and / or at least one polyfunctional ether.

[0028] In a preferred embodiment, the solvent or solvent mixture comprises at least one cyclic ether selected in particular from the group consisting of tetrahydrofuran, 2-methyl- tetrahydrofuran, tetrahydropyran, 2-methyltetrahydropyran and 4-methyl- tetrahydropyran.

[0029] It is also preferred that the solvent or solvent mixture contains at least one polyfunctional ether, which is selected in particular from the group consisting of ethylene glycol dimethyl ether, propylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and 1 ,4-dioxane.

[0030] Accordingly, a solvent for suspending LisAIHe which is a cyclic or a polyfunctional ether can be used in the process according to the invention. Preferably, the solvent is a cyclic ether, in particular selected from the group consisting of tetrahydrofuran, 2- methyltetrahydrofuran, tetrahydropyran, 2-methyltetrahydropyran and 4- methyltetrahydropyran, or a polyfunctional ether, in particular selected from the group consisting of ethylene glycol dimethyl ether, propylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and 1 ,4-dioxane.

[0031] In the process according to the invention, however, solvent mixtures can also be used for suspending LisAIHe and these solvent mixtures can contain several ethers and particularly preferably consist of these, the ethers being selected from cyclic and polyfunctional ethers, in particular from the group consisting of tetrahydrofuran, 2- methyltetrahydrofuran, tetrahydropyran, 2-methyltetrahydropyran, 4-methyl- tetrahydropyran, ethylene glycol dimethyl ether, propylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and 1 ,4-dioxane. The process according to the invention comprises maintaining the suspension of LisAIHe and the solvent or solvent mixture at a reaction temperature of 0 to 180°C, preferably 20 to 180°C.

[0032] In a particularly preferred embodiment, the suspension is maintained at a temperature for reflux conditions.

[0033] The duration for the contacting of LisAIHe with the solvent or solvent mixture is typically selected depending on the selected solvent, the temperature, the grain size of the LisAIHe and possibly other reaction conditions. Preferably, the suspension is kept at the reaction temperature for at least 1 min, in particular at least 10 min and particularly preferably for at least 1 h. Typically, the reaction time is 1 min to 24 h, in particular 10 min to 20 h and especially preferably 1 to 15 h. When tetrahydrofuran is used as the solvent, for example, the reaction time under reflux conditions is preferably at least 10 min, more preferably 1 to 15 hours.

[0034] Preferably, the splitting of LisAIHe in the process according to the invention takes place at a pressure of 4 bar or less, in particular 2 bar or less, and particularly preferably essentially without pressure, i.e. at atmospheric pressure or ambient pressure, i.e. a pressure of about 1 bar. Surprisingly, it has been shown that LiAIH4 can be produced from LisAIHe by the process according to the invention without the need to apply high pressures.

[0035] In particular, it is not necessary for a hydrogen atmosphere to be present for the desired splitting. Rather, it is preferred that less than 10 mol%, in particular less than 5 mol%, especially preferably less than 1 mol%, such as less than 0.5 mol%, of H2, based on the molar amount of LisAIHe, is added to the suspension. Consequently, it is preferred not to add hydrogen before and during the decomposition reaction, which simplifies the process control and is conducive to the safety of the process.

[0036] Furthermore, it is not necessary for the desired splitting that elemental aluminum is present. Rather, it is preferred that the suspension comprises less than 10 mol%, in particular less than 5 mol%, especially preferably less than 1 mol%, such as less than 0.5 mol%, of Al, based on the molar amount of LisAIHe. Thus, it is preferred not to add aluminum before and during the decomposition reaction.

[0037] It is further preferred that the suspension is homogenized during all or part of the splitting reaction, in particular by stirring and / or ultrasonic treatment.

[0038] In a preferred embodiment of the process according to the invention, the LisAIHe is ground before or during the entire splitting reaction or a part thereof, preferably in a mill or by means of an agitating or dispersing tool, particularly preferably in a ball mill, a rod mill or by means of a dispersing tool (e.g. of the ULTRA-TURRAX® type from IKA). It was surprisingly found that grinding can accelerate the splitting of LisAIHe to LiAIH4and LiH. According to a preferred embodiment of the invention, the LisAIHe can be ground before the splitting reaction, so that the average particle diameter D50, measured by laser diffraction method according to ISO 13320:202, is preferably <100 pm and particularly preferably <50 pm. The LisAIHe can be ground before suspension, i.e. in dry form. However, it is also possible to first grind the LisAIHe after suspending and then expose the suspension with the ground LisAIHe to the desired reaction conditions, i.e. in particular to the desired temperature.

[0039] In a further preferred embodiment, the LisAIHe is milled during all or part of the splitting reaction. This means that the suspension is milled, preferably in a mill or with a dispersing tool, particularly preferably in a ball mill, a rod mill or by means of a dispersing tool, while the suspension is kept at the reaction temperature.

[0040] Grinding can also be carried out using magnetic stirrer cores. Reaction vessels partially filled with glass, ceramic or metal beads can also be used, which stir the mixture of beads and LisAIHe suspension with the aid of an agitator (e.g. horizontal or vertical agitator).

[0041] Grinding during the splitting can be carried out up to the desired degree of conversion, preferably up to at least 80%, in particular at least 90%, particularly preferably at least 95% conversion according to reaction (3). However, it is also possible to first prepare the suspension, then grind the LisAIHe during the splitting reaction and then transfer the suspension for the purpose of further splitting into a reaction vessel, preferably a stirred vessel, in which the splitting reaction continues under the reaction conditions according to the invention.

[0042] Grinding typically takes place under inert conditions, i.e. in the absence of air and moisture. Preferably, grinding takes place in a vacuum or under a protective gas atmosphere, whereby the protective gas atmosphere consists in particular of nitrogen or a noble gas, particularly preferably argon.

[0043] The exact reaction conditions, i.e. in particular the minimum time required for the splitting according to reaction (3) with the selected solvent according to the invention and the selected temperature, and as a function of the particle size and the specific surface area of the LisAIHe used, can be determined empirically by the skilled person on the basis of the invention disclosed herein in routine tests. In general, almost complete or even complete splitting can be achieved under economically relevant conditions. However, it may be advantageous from an economic point of view, e.g. taking into account the synthesis costs and the material value of the products, to terminate the splitting reaction before reaching complete or almost complete decomposition. In a further aspect, the invention is directed to the use of LisAIHe for the production of UAIH4 in the process according to the invention described above.

[0044] The invention also relates to the separation of the coupling product LiH formed in the process according to the invention described above and its use as a strong base in organic synthesis, for the production of lithium-containing amides or alcoholates or for the production of other lithium-containing hydrides, preferably UAIH4 or lithium trialkyl borohydrides. It has been shown that this use according to the invention also allows the solid lithium hydride produced as a by-product to be reused in an economically viable manner. In particular, lithium-containing alcoholates can be produced by reaction with alcohols. Lithium trialkyl borohydrides are prepared in particular by reaction with boron alkyls to give trialkyl borohydrides of the type Li[HBRs], where each R is particularly preferably selected independently from aliphatic radicals containing 1 to 6 atoms. The use of LiH for the preparation of LiAIH4 can be carried out in particular according to the Schlesinger process, for example as described by Finholt et al. (J. Amer. Chem. Soc. 69 (1947), 1199).

[0045] The invention is explained in more detail below with reference to examples.

[0046] Examples

[0047] General information:

[0048] All process steps were carried out under inert conditions, i.e. in an Ar-filled glove box or in dried and nitrogen-filled glass or metal containers.

[0049] Example 1 : Preparation of LiAIH4from Li3AIH6in tetrahydrofuran (THF)

[0050] In an inertized, i.e. in a dry and nitrogen-filled, 250 ml two-necked flask with reflux condenser and gas inlet valve, 6.6 g of LisAIHe (content according to gas volumetry = 96 %) was suspended in 93.4 g of THF and stirred for 7 hours under reflux conditions at ambient pressure using a magnetic stirrer. The product solution was then cooled to room temperature and filtered. The filter residue was washed with 15 ml THF and the filtrates were combined.

[0051] Yield: 105 g essentially clear colorless solution

[0052] The hydride content was determined by titration according to Felkin and resulted in 3.85 mmol / g (96 mmol LiAIH4 , corresponding to 89% of the theory)

[0053] The identity of the product LiAIH4 was confirmed by27AI-NMR spectroscopy:

[0054] 27AI-NMR (1H-coupled): Quintet, 5 = 99.6 ppm,1J(AI-H) = 173 Hz Example 2: Preparation of LiAIH4from Li3AIH6in ethylene glycol dimethyl ether (= 1,2-di methoxyethane)

[0055] In an Ar-filled glove box, 0.2 g of LisAIHe (content = 96 % according to gas volumetry) and 1.5 ml of 1 ,2-dimethoxyethane (1.32 g) were filled into a steel autoclave from SYSTAG Systemtechnik with a capacity of approx. 3 ml and hermetically sealed. The steel autoclave was heated to 180°C within 3 h and cooled back to room temperature after a holding time of 10 min.

[0056] The steel autoclave was opened in a glove box and the resulting product suspension was filtered using a syringe filter (pore diameter 0.45 pm).

[0057] The determination of the hydride content in the filtrate using Felkin titration resulted in a hydride activity of 0.85 mmol / g. This corresponds to a conversion of approx. 33% of the theory.

[0058] The identity of the product LiAIH4was confirmed by27AI-NMR spectroscopy:

[0059] Quintet, 6 = 99.7 ppm

[0060] Comparative example 1: No splitting / decomposition of Li3AIH6to LiAIH4and LiH in diethyl ether

[0061] 11 g of LisAIHe (content = 96 % according to gas volumetry) were suspended in 100 g of diethyl ether and stirred at ambient pressure, first for 2 hours at room temperature, then for 34 hours under reflux conditions. After cooling to room temperature, the suspension was filtered.

[0062] Yield: 95 g of a slightly cloudy solution

[0063] A hydride activity of 0.07 mmol / g was measured in the filtrate using Felkin titration. This corresponds to a conversion of approx. 0.5 % of the theory.

[0064] No signal for a dissolved aluminum species could be detected by27AI-NMR spectroscopy, especially no signal in the range of 6 = 99.7 ppm.

[0065] This comparative example shows that no significant disproportionation of LisAIHe to LiAIH4and LiH takes place in diethyl ether. It is assumed that the very low hydride activity measured is due to finely dispersed LisAIHe that could not be retained by the filter and thus entered the filtrate.

Claims

Claims1. Process for the preparation of lithium aluminum hydride (LiAIH4), characterized in that lithium aluminum hexahydride (LisAIHe) is decomposed to LiAIH4 and lithium hydride (LiH) by suspending LisAIHe in an aprotic solvent or aprotic solvent mixture, said solvent or solvent mixture containing at least one cyclic and / or at least one polyfunctional ether, and maintaining said suspension at a reaction temperature of O to 180°C.

2. Process according to claim 1 , wherein LiAIH4 present in dissolved form after decomposition is separated from LiH and optionally from LisAIHe by a solid / liquid separation operation, preferably by filtration, sedimentation or centrifugation, particularly preferably by filtration.

3. Process according to claim 2, wherein solid LiAIH4 is produced from separated LiAIH4 , preferably by total evaporation or displacement crystallization.

4. Process according to any one of claims 1 to 3, wherein the solvent or solvent mixture comprises at least one cyclic ether preferably selected from the group consisting of tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 2- methyltetrahydropyran and 4-methyltetrahydropyran.

5. Process according to any one of claims 1 to 4, wherein the solvent or solvent mixture comprises at least one polyfunctional ether preferably selected from the group consisting of ethylene glycol dimethyl ether, propylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and 1 ,4-dioxane.

6. Process according to any one of claims 1 to 5, wherein the suspension is maintained at a reaction temperature of 20 to 180°C.

7. Process according to any one of claims 1 to 6, wherein the suspension is maintained at a temperature for reflux conditions.

8. Process according to any one of claims 1 to 7, wherein the suspension is kept at the reaction temperature for a reaction time of at least 1 min and preferably at least 10 min.

9. Process according to any one of claims 1 to 8, wherein the decomposition is carried out at a pressure of 4 bar or less, in particular 2 bar or less and particularly preferably at a pressure of about 1 bar.

10. Process according to any one of claims 1 to 9, wherein the suspension is homogenized during all or part of the decomposition reaction, preferably by stirring and / or an ultrasonic treatment.11 . Process according to any one of claims 1 to 10, wherein the LisAIHe is milled before and / or during all or part of the decomposition reaction, preferably in a mill or by means of an agitating or dispersing tool, in particular in a ball mill, a rod mill or by means of a high energy dispersing tool.

12. Process according to claim 11 , wherein the LisAIHe is ground before the decomposition reaction, so that the average particle diameter D50 , measured by means of the laser diffraction method, is preferably <100 mm and in particular <50 mm.

13. Process according to claim 11 or 12, wherein the LisAIHe is milled during the decomposition reaction.

14. Use of LisAIHe for the preparation of LiAIH4 in a process according to any one of claims 1 to 13.

Images