The present invention relates to a method for the preparation of materials suitable for hydrogen storage, as well as materials prepared by said method. Such materials are able to effectively store and release large amounts of hydrogen. More specifically, the invention relates to the preparation of metal hydrides by mechanical mixing. These metal hydrides have also other uses such as reduction agent, starting substance for the preparation of metal coatings and as reactant for the preparation of new metal hydrides.
Restricted amounts of fossil fuels such as oil and natural gas have stimulated considerable efforts to find alternative energy sources and alternative energy carriers. Hydrogen is of great interest as energy carrier due to high energy density and because, like electricity, it can be produced in several ways without any influence on the user of the hydrogen. Energy can more easily be stored in large quantities as hydrogen than electric energy.
As a chemical fuel, hydrogen is unique since the reaction product of a fuel cell or internal combustion engine will be pure water and will not result in any local pollution. This provides a potential for environmental benefits, since either hydrogen can be produced from renewable energy or the CO2 generated as a by-product in the hydrogen production can be deposited from centralized production facilities.
Never the less, the storage of hydrogen gas is still a challenge, which may be accomplished under high pressure or as liquid hydrogen (−250° C.). This is, however, energy demanding and impractical, and the attention is therefore focused on the storage of hydrogen in solid substances which absorb hydrogen in their crystal lattice. This hydrogen is released by increasing the temperature, and the effort is concentrated on obtaining the largest possible hydrogen density in respect of weight and volume, as well as obtaining satisfactory kinetics and costs.
Many so-called interstitial metal hydrides have been made, where hydrogen molecules are absorbed and distributed in voids in the metal structure as single atoms, but such hydrides have so far not been able to store more than about 2.5% by weight of reversible hydrogen. The knowledge thereof has the last decade lead to the study of new materials, in particular so-called complex metal hydrides, where the hydrogen atoms are bound in anionic metal-hydrogen complexes with metals as counter-ions. In particular, this concerns AlH4−, AlH63−, BH4−, NH2−, NH2− and MgH3−, but also other possibilities for complex hydrides exist. Many of is these materials have a higher gravimetric hydrogen content and some also have suitable thermodynamic properties so that the pressure/temperature conditions, in theory, are well suited. However, for the present they do posess kinetic problems because complex hydrides often involves two or more solid phases in dehydrogenated or rehydrogenated state so that diffusion of metal containing species is necessary for the reactions to take place. Considerable research has been invested to find better catalysts and to understand how said catalysts are functioning, but so far NaAlH4 having about 4 by weight reversible hydrogen capacity at 150° C. with near acceptable kinetics is the best that has been obtained. Complex metal hydrides based on nitrogen and boron do in theory have a higher capacity, but the temperature for reversibility with acceptable kinetics is substantially higher, especially for the boron compounds. Complex hydrides are therefore still not satisfactory for hydrogen storage systems for inter alia vehicles. Thus, this leaves plenty of room for improving the storage of hydrogen in solid substances.
Another compound of considerable interest as hydrogen storage material is aluminium hydride, AlH3. This compound has a hydrogen content of 10.1% by weight and this is released in one step. This may inter alia be utilized in rocket engines and considerable research has been carried out regarding AlH3 for this purpose [F. M. Brower, J. Am. Chem. Soc. 98 (1976) 2450; N. E. Matzek et al, U.S. Pat. No. 3,819,819; F. M. Brower et al, U.S. Pat. No. 3,823,226; N. E. Matzek et al, U.S. Pat. No. 3,883,644; J. A. Scruggs, U.S. Pat. No. 3,801,657; W. M. King, U.S. Pat. No. 3,810,974; R. D. Daniels, U.S. Pat. No. 3,819,335]. Other areas of utilization is the use of AlH3 as a chemical reduction agent, pyrotechnic components, polymerization catalyst and for making Al-coatings [M. A. Petrie et al, U.S. Pat. No. 6,228,338]. Moreover, it is well suited as a reactant for making new metal hydrides, e.g. by grinding/ball milling [T. N. Dymova et al., Russ. J. Coord. Chem. 26 (2000) 531]. AlH3 can crystallize in at least six different crystal structures [F. M. Brower et al., J. Am. Chem. Soc. 98 (1976) 2450], of which complete crystal structure is published only for one of the phases, α-AlH3 [J. W. Turley et al., Inorg. Chem. 8 (1969) 18]. It consists of corner-sharing AlH6 octahedrons. Therefore, α-AlH3 may to a large extent be considered as a complex hydride, but does not have the problem of diffusion of metal atoms as an obstacle to the kinetics of other complex hydrides, such as Na3AlH6 and Na2LiAlH6.
The challenges of AlH3 as hydrogen storage material are the thermodynamic properties which, in practice, makes impossible reversibility by means of gas pressure at or above room temperature, and that AlH3 must be produced by a relatively cumbersome chemical procedure under inert atmosphere.
AlH3 has typically been synthesized from LiAlH4 and AlCl3 in dietylether [F. M. Brower et al., J. Am. Chem. Soc. 98 (1976) 2450]. In a 3:1 proportion, LiCl and AlH3x0.25Et2O is formed. LiCl is filtered off. Et2O cannot be removed by heating without AlH3 being hydrogenated, but can be removed under heating with excess of LiAlH4, optionally in combination with LiBH4, and often with the use of other solvents in addition. Then, AlH3 is precipitated and dried. The crystal structure of the precipitated AlH3 strongly depends on how this is performed (mixing ratio, temperature and time) and, then, the product must be purified and dried.
It is difficult to purely produce other structure modifications than α-AlH3 with this method. In addition, it has also been tested some other combinations of hydrides and chlorides which give AlH3 by corresponding methods [Ashby et al., J. Am. Chem. Soc. 95 (1973) 6485].
There is a need for a simpler method of preparation for AlH3 and AlH3-like phases, preferably also a method of preparation where several of the AlH3-modifications can be prepared.
The object of this invention is to find a simpler and more inexpensive synthesis method for the preparation of AlH3 and related phases. This can be accomplished by mechanical mixing of hydrides together with halogenides such as chlorides preferably in solid phase and without use of any solvent which may bind to the product. Preferably, both the hydride and the chloride should contain aluminium, but it can be sufficient that one of them contains aluminium. Due to low thermal stability of AlH3 and the often exothermic character of these reactions, the mechanical methods are preferably carried out at a lower temperature than room temperature.
The present invention provides a method for the preparation of material of the type AlH3 in one of its structure modifications or structurally related aluminium containing hydrides, characterized in that one or more metal hydrides and one or more halogenides react chemically under mechanical mixing thereof.
In an embodiment of the method, the mechanical mixing is carried out by crushing, milling and/or mortaring.
In a further embodiment of the method, the mechanical mixing is carried out at a temperature which is lower than room temperature.
In a further embodiment of the method, the mechanical mixing is carried out without use of solvent.
In a further embodiment of the method, the mechanical mixing is carried out in solid state.
In a further embodiment of the method, at least one of the metal hydrides or halogenides contains aluminium, preferably both at least one of both the metal hydrides and halogenides contain aluminium.
In a further embodiment of the method, the metal hydride used as starting substance is selected among complex hydrides containing AlH4—, AlH63−, AlH52−, BH4− and NH2− with alkali metals, alkaline-earth metals and transition metals as counter-ions, particularly alkali metals and alkaline-earth metals, or binary metal hydrides of alkali metals, alkaline-earth metals and 3d transition metals, particularly alkali metals and alkaline-earth metals.
In a further embodiment of the method, the halogenide used as starting substance is halogenide of alkali metal, alkaline-earth metal, transition metal, Al, Ga or In.
In a further embodiment of the method, the structure modifications of AlH3 are selected among α-AlH3, α′-AlH3, β-AlH3 and γ-AlH3.
In a further embodiment of the method, seed crystals are added together with the starting substances to speed up the formation of product having desired crystal structure.
In a further embodiment of the method, the by-product, a halogenide, is removed by means of a solvent without the material produced being dissolved.
In a further embodiment of the method, the aluminium containing hydrides, having a composition different from AlH3, which are structurally related to the structure modifications of AlH3, are obtained by the stabilization of AlH3 by partly substituting Al therein with one or more metals selected among alkali metals, alkaline-earth metals, transition metals, B, Ga and In and/or by placing one or more metals selected among alkali metals, alkaline-earth metals, transition metals, B, Ga and In in interstitial positions.
The invention relates to the preparation of metal hydrides of the type AlH3 or metal hydrides which structurally may be related to one of the structure modifications of AlH3. Previously, in most cases this has been done by a reaction between 3LiAlH4 and AlCl3 in dietylether with the formation of AlH3 bounded to diethylether: AlH3x0.25Et2O, with a subsequent filtration and addition of LiAlH4/LiBH4 to remove Et2O during heating with a subsequent presipitation and drying [F. M. Brower et al., J. Am. Chem. Soc. 98 (1976) 2450].
The invention relates to a substantial simplification of the method of synthesis of AlH3 and structurally related compounds and does also make a less demand to laboratory equipment. The object of the method is chemical reaction by mechanically mixing the reactants in the form of powder by means of a crushing/grinding/mortaring process. This may be carried out e.g. by a planetary ball mill where a beaker filled with balls and powder rotates in an asymmetric manner so that the powders become mixed and crushed or a mill where a piston reciprocates in a cylindrical test chamber. In both cases there is obtained good mixing and formation of new clean surfaces and defects leading to good reactivity. The desired chemical reactions may therefore take place during the mill procedure itself.
The desired reactions for the synthesis of AlH3 must be thermodynamically favourable to take place, and because gas evolution normally does not occur in these reactions, they will in many cases be exothermic. AlH3 is not very thermally stable and dehydrogenation during ball milling must be avoided. This may be accomplished by cooling, e.g. by means of liquid nitrogen (−196° C.). Then a lower local temperature is obtained where the crushing process takes place and the mobility of the atoms is smaller so that the decomposition is less likely to occur. A positive additional effect of cooling is that the materials become more brittle and thereby become crushed into smaller particles so that the diffusion paths for the solid-state reactions become shorter.
The present invention also provides a material prepared according to the above method, characterized in that the aluminium containing hydrides have a composition different from AlH3, but are structurally related to the structure modifications of AlH3 in that Al is partially substituted with one or more metals selected among alkali metals, alkaline-earth metals, transition metals, B, Ga and In and/or in that one or more metals selected among alkali metals, alkaline-earth metals, transition metals, B, Ga and In are placed in interstitial positions in the actual AlH3 structure modification.
In an embodiment of the material, an AlH3 structure modification is stabilized as a consequence of the addition of one or more metals thereto.
The new materials prepared according to the invention are structurally related to one of the structure modifications of AlH3, e.g. in that parts of Al are exchanged with other metals and/or that other metals are taken up in interstitial positions in the crystal structure. The metals can be one or more alkali metal, alkaline-earth metal, transition metal, B, Ga or In and will principally be added by replacing parts of the halogenide so that this metal is absorbed in the AlH3 structure. This will lead to a change of stability. Increased stability would be strongly favourable for reversible hydrogen storage.
The α-AlH3 structure is known [J. W. Turley et al., Inorg. Chem. 8 (1969) 18]. In addition thereto, the present inventors have identified the structure of two of the other structure modifications, α′-AlH3 and β-AlH3. All these phases consist of AlH6 octahedra connected by corner-sharing of all corners with one other octahedron. In these three phases, the binding is done in different ways. The crystallization of the known AlH3 phases in AlH6 octahedra shows that AlH3 has much in common with complex hydrides such as Na3AlH6 [E. Rønnebro et al. J Alloys Compd. 299 (2000) 101], Na2LiAlH6 [H. W. Brinks et al., J. Alloys Compd. 392 (2005) 27] and Li3AlH6 [H. W. Brinks et al. J. Alloys Compd. 354 (2003) 143] which are all based on isolated AlH63− ions. It can be observed e.g. from the crystal structure that if starting with α-AlH3 and replacing half of the Al with Li, and then inserting Na in interstitial positions as a charge compensation, Na2LiAlH6 having correct crystal structure is is obtained. Thus, Na2LiAlH6 is to be regarded as stabilized α-AlH3.
The material prepared according to the invention is useful for hydrogen storage for use in fuel cell or internal combustion engine, rocket fuel, pyrotechnic compounds, reduction agent in any connection where a hydride-donor is suitable to generate a reduction, metal coating, polymerization catalyst and as starting substance for the synthesis of other metal hydrides.
The present invention also provides the use of the material prepared by the above method, or the above material, for reversible or irreversible hydrogen storage.
Further, the present invention provides the use of the material prepared by the above method, or the above material, for rocket fuel, pyrotechnic components, reduction agent, metal coating and polymerization catalyst.
Further, the present invention provides the use of the material prepared by the above method, or the above material, as starting substance for making new metal hydrides.
In addition to mixing 3LiAlH4+AlCl3, many other combinations of hydrides and halogenides which may give AlH3 exist. Both of the reactants may be replaced, either separately or together, but one of the reactants must contain aluminium. LiAlH4 may be replaced by other complex hydrides containing AlH4—, AlH63−, AlH52−, BH4− and NH2− with alkali metals, alkaline-earth metals and transition metals as counter ions, particularly alkali metals and alkaline-earth metals. LiAlH4 may also be replaced by binary hydrides of alkali metals, alkaline-earth metals and 3d transition metals, particularly alkali metals and alkaline-earth metals. AlCl3 may be replaced by AlBr3 and AlI3 or halogenides from alkali, alkaline-earth, transition metals, Ga or In. In all these reactions between halogenide and hydride, in addition to AlH3, a halogenide as by-product will also be obtained.
In some areas of utilization like e.g. as reduction agent, metal coating, catalyst or starting substance for other metal hydrides, it is in many cases likely that the product after ball milling may be used directly without further purification. For other areas of utilization, a purification would be favourable. This can be done by selectively dissolving the by-product without dissolving AlH3, e.g. dissolving LiCl may be envisioned by using crown ethers.
As mentioned above, AlH3 may be formed from many combinations of starting substances, and this will result in different structure modifications of AlH3. It is also probable that which structure modification that crystallizes may be influenced by seed crystals of the correct structural type, either by adding to the reactants some of the desired product or by adding other compounds having the same structure. I.a., FeF3 may have the same crystal structure as β-AlH3 (X-ray diffraction data show that it has pyrochloro-type structure), so that it is likely that finely divided FeF3 will lead to larger amounts of β-AlH3 in the product. For the same reason, it is likely that seed crystals of β-AlF3 will lead to larger amounts of α′-AlH3 in the product.
In Example 1, ball milling of 3LiAlD4+AlCl3 at room temperature is described. The formation of 4AlD3+3LiCl (using the 1H isotope) has an enthalpy of −213 kJ/mol and a Gibbs' free energy of −191 kJ/mol. This spontaneous reaction leeds to a local increase of temperature which may reach several hundred degrees Celcius in a confined and independent system, i.e. if the heat is not led away. Measurements of pressure during ball milling clearly shows that the temperature suddenly increases, i.e. that when the reaction occurs it proceeds quickly and it takes some time for the ball mill equipment to absorb this heat. It is observed from X-ray diffraction characterization of the product that some α-AlH3 and α′-AlH3 is present in the product afterwards, in addition to Al (and LiCl which is a by-product). This indicates that the desired reaction has taken place, but that a partial thermal decomposition has taken place thereafter as a result of the heat from the first reaction. The temperature during the crushing may also reach about 60° C. without any chemical reaction taking place due to heat of friction in case of crushing with high intensity. Therefore, cooling is desirable during the mechanical mixing/crushing.
In Example 2 a strong cooling was selected in this process, by using liquid nitrogen as cooling agent. Liquid nitrogen which has a boiling point of −196° C. Also at this temperature, the desired reaction is spontaneous. Characterization by neutron diffraction shows that this reaction has taken place without the formation of Al metal, which occurs in thermal decomposition.
There are several reasons why the invention works. The reaction is spontanous. Milling/crushing yields a smaller particle size, clean surfaces, defects and local increase of temperature which all together makes the reaction possible. At reduced temperature even smaller particle size is obtained due to the brittleness of the materials, reduced mobility and thereby reduced chance for decomposition of a relatively unstable product in addition to a lower maximum temperature during the entire process. It is therefore possible to carry out solid-state reactions at about −200° C. in 5 minutes. In industrial processes, a smaller extent of cooling than what appears from Example 2 will be of relevance.
The material prepared according to the invention is particularly useable for the storage of hydrogen in connection with vehicles and filling stations. It is also useful for rocket fuel, pyrotechnic components, reduction agent in any connection where a hydride-donor is suitable to generate a reduction, metal coating, polymerization catalyst and as starting substance for the synthesis of other metal hydrides.
As a first example, LiAlD4 (0.972 g) and AlCl3 powder (1.028 g) (3:1 molar ratio) were mixed in Ar atmosphere and mechanically milled/crushed in a planetary ball mill of the type Fritsch P6 with 100 balls of 4 g. Consequently, ball to powder mass ratio was 200:1. The ball milling was carried out under Ar atmosphere at room temperature for one hour with 500 revolutions per minute. During the ball milling, the development of the pressure was monitored by means of a built-in pressure gauge which transmits the pressure readings by means of radio waves. In the case of changes in the pressure, the pressure is measured more frequently and max speed of measurement every 22 milliseconds.
The ball milling did not result in any substantial development of pressure until after about one minute, cf.
A second example that AlH3 may be formed by mixing/crushing/milling of a hydride and a halogenide is by using lower temperature during the crushing process. LiAlD4 (0.486 g) and AlCl3 powder (0.514 g) (3:1 molar ratio) was blended in Ar atmosphere and mechanically milled/crushed in a SPEX 7650 Freezer Mill with a piston of 32 g to 1 g sample. In the freezer mill liquid nitrogen at about −196° C. was used as cooling agent and the milling time was 5 minutes.
After having transferred the sample in Ar to a vanadium sample holder, powder neutron diffraction was carried out on the sample, cf.
In a third example that AlH3 may be formed by chemical reaction during mechanical mixing, NaAlH4 (0.549 g) and AlCl3 (0.451 g) in 3:1 molar ratio was blended in Ar atmosphere and crushed in a SPEX 7650 Freezer Mill with a piston of 32 g. In the freezer mill liquid nitrogen at about −196° C. was used as cooling agent and the milling time was 60 min. Characterization by powder X-ray diffraction showed that AlH3 and LiCl was formed and AlH3 divided itself between about 50% α-AlH3 and 50% α′-AlH3.
Number | Date | Country | Kind |
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2006 2210 | May 2006 | NO | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/NO2007/000173 | 5/15/2007 | WO | 00 | 7/23/2008 |