GB2108402A

GB2108402A - Improvements relating to the conversion of solar energy into chemical energy

Info

Publication number: GB2108402A
Application number: GB08229250A
Authority: GB
Inventors: Fasihullah Khan; Po-Lock Yue
Original assignee: University of Bath
Current assignee: University of Bath
Priority date: 1981-10-13
Filing date: 1982-10-13
Publication date: 1983-05-18
Also published as: GB2108402B

Abstract

Hydrogen and oxygen produced from splitting the water with exchanged zeolites (preferably titanium exchanged zeolites) acting as a catalyst immersed in water in the presence of solar energy or other visible radiation is used for the synthesis of chemical products and the process therefore serves as a method of converting solar energy into chemical energy. Possible processes use powdered coal, nitrogen and carbon oxides as starting materials for reaction with the products of water splitting.

Description

SPECIFICATION Improvements relating to conversion of solar energy into chemical energy The discovery of photolysis of water in the presence of semiconductor materials has inspired a great deal of interest in the possibility of the conversion of solar into chemical energy. Some attempts have been made to utilise the photolysis products to carry out photoreduction reactions in situ over catalysts containing semiconductor materials. Thus small amounts of ammonia have been produced in a batch reactor using iron doped titanium dioxide, and in a fluidised bed reactor using similar photocatalytic materials supported on 8 alumina. In both cases the catalysts were subjected to near ultraviolet irradiation.

Zeolites underthe irradiation of visible light have been shown to be effective for water splitting. Partial thermal regeneration of these catalysts is possible.

However this method does not make it possible to achieve a closed photochemical cycle that harnesses visible solar energy to produce hydrogen.

According to the invention, there is provided a method of splitting water to provide hydrogen and oxygen for the synthesis of chemical products wherein gas containing as a major constituent nitrogen, carbon dioxide or carbon monoxide or other gas which will react with the water splitting products, is passed continuously over exchanged zeolites as a catalyst immersed in water in the presence of solar energy or other visible radiation, resulting in photolysis of the water with concomintant reaction with the major constituent of the gas.

From a further aspect, there is provided a method of creating chemical products with the use of solar energy or other visible radiation wherein powdered carbon is agitated in water with exchanged zeolites acting as a catalyst in the presence of solar energy or other visible radiation resulting in photolysis of water and reaction of the water splitting products with the carbon.

Preferably in this particular example the carbon will be provided in the form of powdered coal. With advantage the carbon and zeolites are fluidised, such as by bubbling a gas through the water.

The term "exchanged zeolites" as used herein is meantto define a class of zeolites which can have and have had their cations exchanged with other ions, preferably titanium ions, although silver ions are also suitable, the zeolites containing micropores which favour the production of substances (such as ammonia) using the hydrogen and/or oxygen produced by the water splitting reaction. The preferred titanium exchanged zeolite is 5A zeolite although 3A zeolite is also useful but has smaller pore sizes. 4A zeolite is poor because sodium ions in 4A zeolite do not exchange readily with Ti3+ ions. Other suitable zeolites are Y type and possibly SM zeolites.

The exchanged zeolites may readily be formed by exchanging calcium or other metal cations with titanium ions in a titanium trichloride solution.

Whilst 30% titanium trichloride solution may be used, ideally the solution of titanium trichloride will be in the range of 5% to 20%, with a preferred value of around 7%. The lower values reduce the losses of the zeolite which occur.

When the catalyst becomes spent it may be re-exchanged with the exchange ions. It is possible to perform this re-exchange of the catalyst continuously or batchwise thus resulting in continuous synthesis of the chemical products. The soaking of the exchanged zeolite in a base solution before re-exchange improves the exchange process and releases some of the gaseous or other products of the synthesis reaction which have been absorbed on the zeolites.

In the preferred method of operation, the exchanged zeolite catalyst is supported in a fluidised bed reactor. Ideally the exchanged zeolites are in the form of beads suitable for use in the fluidised bed.

Ideally the particle size lies between 0.1 mm and 10 mm, with the preferred range being between 1 to 2 mm. Whilst hydrogen production is favoured by reducing the particle size further to 0.2 to 0.5 mm this would be at the expense of ammonia formation, in an ammonia synthesis plant. The exchanged zeolite catalyst may readily be supported on a porous sintered disc.

Where the process is to be used to synthesise ammonia, although air can be used, best results are achieved if high purity nitrogen is bubbled through the exchanged zeolite catalyst immersed in the water.

Whilst the process is ideally suited for the production of ammonia by passing nitrogen into the water, other reactions may also be carried out utilising the hydrogen and oxygen produced by the water splitting reaction, such as where carbon dioxide or carbon monoxide is passed into the water and is used in a water-gas shift reaction, and a methanation reaction to form methane or methanol. It is also envisaged that coal could be used together with the exchanged zeolites to promote the photoreactions.

The invention may be performed in various ways and preferred examples illustrating methods of carrying out this invention will now be described.

EXAMPLE 1 The calcium ions in 5A zeolite are exchanged with titanium ions in a 30% by weight of titanium trichloride solution. The zeolite to solution ratio is 1 g to 2 ml. 5A zeolite beads of 1 to 2 mm diameter are allowed to soak in an unstirred titanium trichloride solution for one to three hours at room temperature.

Atomic absorption studies show that about 10 mg of of titanium are exchanged per gram of zeolite. The exchanged zeolite is now purple in colour which is characteristic of hexaquotitanium (III) ions. ESR spectroscopy reveals Ti3+ in the zeolite 5A.

The washed catalyst is immersed in water in the reactor and supported by a porous sintered disc.

High purity nitrogen is bubbled through the porous disc continuously and the catalyst is partially fluidised. The fluidised catalyst is irradiated by a 150 watt light bulb. 0.35 mg of ammonia is produced per gram of catalyst per hour. The activity of the catalyst decreases after about two hours. The deactivated catalyst is discoloured and the ESR spectrum gives no signal of Ti3+. The spent catalyst is re-exchanged by the same method as in the preparation of fresh exchanged zeolite. The ESR spectrum again shows Ti3+ in the re-exchanged catalyst. The re-exchanged catalyst has an activity which is higher than that of a fresh catalyst. The rate of production of ammonia is increased to 0.5 mg of ammonia per gram of catalyst per hour. After three hours of operation the catalyst begins to deactivate again, and requires further replenishing of Ti3+.The catalyst may be re-exchanged continuously or batchwise. Thus the present method makes it possible to synthesise ammonia continuously. Soaking the deactivated zeolite in 1N or less potassium hydroxide solution (not sodium hydroxide) for six to ten hours in between exchanges with titanium releases some of the hydrogen and/or ammonia which have been absorbed on the zeolite and improves the subsequent titanium exchange process.

The rate of ammonia production per gram of catalyst is much higher than in any presently known method.

Experiments without the introduction of nitrogen show vigorous bubbling when the catalyst is active.

Analysis of the resulting gas shows that hydrogen is obtained, together with oxygen and nitrogen which arise from air absorbed in the higher porous catalyst.

Small amounts of precipitate are obtained in the reactor. This milky precipitate is found to be titanium oxides and zeolite that has broken up. Thus there is a net loss of catalyst over a long period of time.

EXAMPLE 2 The process of Example 1 was carried out but with variation of the parameters, namely type of zeolite, particle size, exchange solution concentration and nitrogen source and the following Table illustrates variation in the rate of production of ammonia and hydrogen.

Table 1 Average production of ammonia and hydrogen

Particle Exchange Zeolite Size Solution Source of NH3 H2 Type (mm) (% TiC 13) Nitrogen 3A 1 - 2 30 Pure N2 0.42 0.84 3A 1 - 2 30 Air 0.458 N.D.

3A 1-2 7.5 Pure N2 0.42 1.06 3A 1-2 7.5 Air 0.31 N.D.

3A 0.25-0.50 7.5 Pure N2 0.21 7.33 3A 0.25- 0.50 7.5 Air 0.30 N.D.

5A 1-2 30 Pure2 0.53 0.12 5A 1-2 30 Air 0.37 N.D.

5A 1 - 2 7.5 Pure N2 0.67 I 0.96 5A 1-2 7.5 Air 0.38 N.D.

5A 0.25 - 0.50 7.5 Pure N2 0.60 1.42 5A 0.25-0.50 7.5 Air 0.43 N.D.

Note: N.D. means not detected The ammonia production is in mglg zeolite The hydrogen production is in ml/g zeolite The results show that with titanium exchanged 3A zeolites slightly higher production of ammonia can be achieved by using air instead of pure nitrogen as the source of nitrogen. However, in the former case no hydrogen was found in the product gases, whereas a significant amount of hydrogen was formed if pure nitrogen was used. With titanium exchanged 5A zeolites more ammonia was produced using pure nitrogen as the feed.

The exchanged zeolites can be used as a catalyst in connection also with the following reactions wherein carbon monoxide or carbon dioxide are bubbled through water containing zeolite beads to react with the water or with hydrogen formed by the water splitting reaction, namely: Methanation Reactions

Water-Gas shift Reaction

The exchanged zeolites may also be used with fluidised carbon (such as powdered coal), the carbon reacting with the water in the following reactions promoted by the zeolites, namely

and the reaction products may further be subjected to methanation reactions as described above. In these reactions the carbon and zeolites will be agitated to promote the reactions either by a mechanical agitation or by bubbling a gas through the solution, either in the form of a fluidised bed or by sparging into a container provided with a closed bottom.

Claims

1. A method of splitting water to provide hydrogen and oxygen for the synthesis of chemical products wherein gas containing as a major constituent nitrogen, carbon dioxide or carbon monoxide or other gas which will react with the water splitting products, is passed dver exchanged zeolites as a catalyst immersed in water in the presence of solar energy or other visible radiation, resulting in photo lysis of the water with concomitant reaction with the major constituent of the gas.

2. A method according to claim 1, wherein the gas is bubbled through the exchanged zeolite catalyst immersed in the water.

3. A method according to claim 1 or claim 2, wherein high purity nitrogen comprises the major gas constituent or air is used as the gas and ammonia is formed by photoreduction of the nitrogen.

4. A method according to claim 1 or claim 2, wherein carbon dioxide or carbon monoxide comprises the major gas constituent and is used in a water-gas shift reaction or a methanation reaction to form methane or methanol.

5. A method of creating chemical products with the use of solar energy or other visible radiation, wherein powdered carbon is agitated in water with exchanged zeolites acting as a catalyst in the presence of solar energy on other visible radiation resulting in photolysis of water and reaction of the water splitting products with the carbon.

6. A method according to claim 5, wherein the carbon is provided as powdered coal.

7. A method according to claim 5 or claim 6, wherein the carbon and the zeolites are fluidised, such as by bubbling a gas through the water.

8. A method according to any one of claims 1 to 7, wherein the zeolite selected from 3A, 5A, Y type and ;S SM zeolite.

9. A method according to any one of claims 1 to 8, wherein the zeolite is exchanged with titanium or silver ions.

10. A method according to any one of claims 1 to 9, wherein spent catalyst is re-exchanged with the exchange ions.

11. A method according to claim 10, wherein the catalyst is re-exchanged batchwise or continuously.

12. A method according to claim 10 or claim 11, wherein the exchanged zeolite is soaked in a base solution before re-exchange.

13. A method according to any one of claims 1 to 12, wherein the exchanged zeolite catalyst is supported in a fluidised bed reactor.

14. A method according to claim 13, wherein the exchanged zeolites are in the form of beads suitable for use in the fluidised bed.

15. A method according to claim 14, wherein the particle sizes of the beads lie between 0.1 mm and 10 mm.

16. A method according to claim 15, wherein the particle sizes of the beads lie between 1 to 2 mm.

17. A method according to claim 15, wherein the particle sizes of the beads lie between 0.2 to 0.5 mm.

18. A method according to any one of claims 1 to 17, wherein the exchanged zeolite catalyst is supported on a porous sintered disc.

19. A method according to any one of claims 1 to 18, wherein the exchanged zeolites are formed by exchanging calcium or other metal cations with titanium ions in a titanium solution.

20. A method according to claim 19, wherein the titanium trichloride solution will not be in excess of 30% and preferably within the range of 5% to 20%.

21. A method according to claim 20, wherein the titanium trichloride solution is about 71/2%.

22. A method according to claims 1 or 5 and substantially as herein described with reference to the accompanying examples.