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A method is provided in which stoichiometrically proportions of solid alkali metal borohydride are reacted with solid hydrated alkali metal borate. Upon heating, the borohydride hydrolyzes to generate controlled amounts of hydrogen gas and solid by-products. Water for the reaction is stored and carried in the hydrated borate, which is a hydrate of the reaction's by-product. At a suitable temperature, the hydrate melts and releases sufficient water for hydrolysis of the borohydride to molecular hydrogen.
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF THE INVENTION
The present invention provides a method of generating hydrogen gas by hydrolysis of lithium and/or sodium borohydride with water obtained from a hydrated lithium and/or sodium borate. A mixture of the borohydride and the hydrated borate is prepared in a suitable form for chemical reaction to occur between them when hydrogen is required. One suitable form is a mixture of fine particles of the two materials. However, prior to initiating the hydrolysis reaction, the reactants are preferably stored separately under temperature and pressure conditions at which water is not lost from the hydrated borate, especially if the water content of the hydrate is high. If a lower water content hydrate is used, the materials may be premixed for storage.
When hydrogen is required the mixture is heated to release water molecules from the hydrate and promote its reaction with the borohydride. Preferably, the borohydride and hydrated borate are mixed in proportions so that the hydrated borate provides enough water to fully hydrolyze the borohydride. The hydrolysis of lithium borohydride is generally shown in the following equation:
In the case of lithium or sodium borohydride, x is an integer usually having a value from 2 to 8. Thus, the amount of lithium or sodium borohydride to be mixed with a specified hydrated lithium borate, or sodium borate, depends on x, the water content of the hydrate. In order to minimize the initial borate content of the reacting system it is preferred that the value of x, i.e., the number of water molecules of crystallization be as large as can be maintained in the hydrate under the anticipated storage conditions.
Solid lithium and/or sodium borohydride is commercially available in several forms, i.e., as a powder, as pellets, or as a single coherent body like a capsule. The borohydride is fairly stable and conveniently stored in sealed vessels at ambient temperatures and atmospheric pressures.
For the present invention, several molecules of water can be stored per molecule of lithium or sodium borate in the form of a hydrate under suitable ambient conditions. Furthermore, the anhydrous form of the borate is substantially the chemical equivalent of the byproduct of the hydrolysis of the borohydride. Thus, hydrated lithium and/or sodium borate, are preferred for use in the present invention. The hydrated borate operates merely to stabilize an appreciable number of water molecules in solid form and merge with the by-product of the hydrolysis reaction. Hydrated lithium and/or sodium borate can be produced, for example, by careful evaporation and drying of aqueous solutions of their borate salts.
An optimal amount of hydrogen gas can be generated by hydrolyzing the borohydride with hydrated lithium borate having 8 water molecules. This reaction is shown in the following equation:
When hydrogen gas is desired, a stoichiometric proportion of borohydride and hydrate are placed in, or delivered to, a reaction zone comprising one or more reactor stages. In order to obtain optimal yields of hydrogen gas, the borohydride and the hydrate are preferably ground into fine particle sizes. The generation of hydrogen proceeds by mixing the components in a reaction volume and heating the mixture to release water from its borate carrier and induce hydrolysis of the borohydride.
If the generated hydrogen is to be used in a mobile operating system, such as in a motorized vehicle, a stirred, flow-through reactor may be preferred. The stirred flow through reactor permits metered amounts of reactants to be withdrawn from storage and progressively heated and ground to carry hydrogen and by-product borates from the reactor space. At suitable reaction temperatures, hydrogen is produced and increases the pressure in the reactor space. The higher pressure forces the hydrogen, as well as the reaction's by-products, out of the reactor space. Hydrogen is separated from the solid byproducts and delivered to, for example, an engine or fuel cell.
Thus the subject borohydride/hydrated borate mixture and hydrogen generation method can be used to generate hydrogen on demand as it is needed to power an engine, fuel cell or the like. Generally, a 2:1 mole ratio of water molecules to borohydride molecules is required in order to produce optimal amounts of hydrogen gas; ratios either higher or lower than this value will lower the yield of hydrogen output. Operation according to the first equation above maintains this optimum ratio regardless of the water content of the hydrate.
One of the benefits of the present invention is the ability to store water as a hydrate at sub-freezing ambient temperatures and still be able to produce demanded quantities of hydrogen gas. At temperatures below 45° C., both the borohydride and the hydrate are in solid form. Consequently, the hydrate is always manipulated as a solid.
Although a tiny fraction of the stored hydrogen will be released as the two solids come into contact, an increase in temperature will cause the hydrate will melt and release water. A spontaneous reaction then takes place at temperatures of at least 85° C. and release of almost all of the hydrogen will occur at temperatures of approximately 100° C. At this point, the hydrolysis reaction is self-sustaining and exothermic. Preferably, the reaction zone temperature should be kept slightly below 120° C. This will help minimize water loss and improve hydrogen yield.
The solid byproduct of the hydrogen producing reaction is lithium and/or sodium borate. This byproduct can be hydrogenated to regenerate the borohydride starting material. A portion of the borate can be hydrated to reconstitute the water storage medium of this invention.
As described, it is generally preferred to store the reactants separately as fine particles. However, other forms can be adapted for storage. The reactants can be stored in tubes or capsules for feeding into a reaction zone. The reactants could be embedded in an inert binding material for delivery to the reaction space.
While the invention has been described in terms of a preferred embodiment, it is not intended to be limited to that description, but rather only to the extent of the following claims.
1. A method of generating hydrogen comprising reacting alkali metal borohydride with hydrated alkali metal borate, said borate having sufficient water for the substantially complete hydrolysis of said borohydride to molecular hydrogen, said alkali metal in each instance being selected from the group consisting of lithium and sodium.
2. A hydrogen generation method as recited in claim 1 where said hydrated borate comprises at least two molecules of water per molecule of borate.
3. A hydrogen generation method as recited in claim 1 comprising reacting lithium borohydride with lithium metaborate octahydrate.
4. A hydrogen generation method as recited in claim 1 comprising reacting sodium borohydride with sodium metaborate octahydrate.
5. A hydrogen generation method as recited in claim 1 comprising reacting said borohydride with said hydrated borate at a temperature of at least 85° C.
6. A hydrogen generation method as recited in claim 3 comprising reacting said borohydride with said hydrated borate at a temperature of at least 85° C.
7. A hydrogen generation method as recited in claim 4 comprising reacting said borohydride with said hydrated borate at a temperature of at least 85° C.
8. A method of generating hydrogen comprising the steps of: mixing lithium- and/or sodium borohydride particles with hydrated lithium- and/or sodium borate particles, said borate particles having sufficient water for the substantially complete hydrolysis of said borohydride particles to molecular hydrogen; and heating said borohydride and borate mixture to release water from said hydrated borate for reaction with said borohydride to form said molecular hydrogen.
9. A hydrogen generation method as recited in claim 8 where said hydrated borate comprises at least two molecules of water per molecule of borate.
10. A hydrogen generation method as recited in claim 8 comprising reacting lithium borohydride with lithium metaborate octahydrate.
11. A hydrogen generation method as recited in claim 8 comprising reacting sodium borohydride with sodium metaborate octahydrate.
12. A hydrogen generation method as recited in claim 8 comprising heating said borohydride with said hydrated borate at a temperature of at least 85° C.
13. A hydrogen generation method as recited in claim 10 comprising heating said borohydride with said hydrated borate at a temperature of at least 85° C.
14. A hydrogen generation method as recited in claim 11 comprising heating said borohydride with said hydrated borate at a temperature of at least 85° C.
15. A composition for generating hydrogen gas, said composition consisting essentially of lithium- and/or sodium borohydride particles and hydrated lithium and/or sodium borate particles, said borate comprising sufficient water as said hydrate for the substantially complete hydrolysis of said borohydride to molecular hydrogen.
16. A composition as recited in claim 15 where said hydrated borate comprises at least two molecules of water per molecule of borate.
17. A composition as recited in claim 15 consisting essentially of lithium borohydride and lithium borate octahydrate.
18. A composition as recited in claim 15 consisting essentially of sodium borohydride and sodium borate octahydrate.
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