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Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 238-241
Photoassisted Water Cleavage and Nitrogen Fixation over Titanium-Exchanged Zeolites Faslhulla Khan and Polock Yue' School of Chemical Engineering, University of h t h , h t b BA2 7AY, England
Luclo Rlzzutl, Vlncenzo Augugllaro, and Albert0 Brucato Istituto di Ingegneria Chimica, Universita di Palermo, Palermo, Italy
The cleavage of water and fixation of nitrogen have been successfully performed using titanium-exchanged zeolites irradiated by vlsible light. The cations in Laporte type 3A, 4A, and 5A zeolites were exchanged with titanium ions. ESR analysis showed the presence of Ti3+ in the exchanged zeolites. When the exchanged zeolites were immersed in water and exposed to light irradiation, hydrogen was produced. The Ti3+ ions were oxMized to Ti4+ and replenished by a reexchange process. Introducing nitrogen into the reactor resulted in the formation of ammonia. Exchanged 4A zeolite yielded less hydrogen and ammonia than 3A and 5A zeolites because fewer titanium ions were present in 4A zeolite. Cutting off UV irradiation did not affect the results; thus the potential of utilizing solar irradiation is raised. The efficiency of the processes can be increased by improving the irradiation and reactor design.
Introduction The possibility of converting solar energy into chemical energy has generated a lot of interest and opened up a whole new area of research in the past ten years. Many of the reported studies were focused on the production of hydrogen by the photolysis of water over semiconductor materials. Exchanged zeolites also have been shown (Jacobs et al., 1977 and Kuznicki et al., 1978) to be effective in assisting the cleavage of water under the irradiation of visible light. However, the exchanged zeolite lost its activity as the cation in the zeolite lost its oxidation state. It is possible partially to regenerate the zeolite by heating, but this method does not make it possible to achieve a closed photochemical cycle that harnesses visible solar energy to produce hydrogen. Some attempts have been made to utilize the hydrogen from the water splitting reaction to carry out photoreduction reactions in situ over the same catalyst. Schrauzer and Guth (1977) reported the formation of small amounts of ammonia in a batch reactor using iron-doped titanium dioxide. Augugliaro et al. (1982a,b) were also successful in producing ammonia in a fluidized bed reactor using similar photocatalytic materials. In both cases the catalysts were subjected to near-UV irradiation. The conversion of solar energy into chemical energy will be greatly enhanced if the spectrum of the irradiation can be extended into the visible region. Recently Khan et al. (1981) have succeeded in utilizing visible light to assist the synthesis on ammonia over titanium exchanged zeolite 5A. The purpose of the present work is to study the cleavage of water and the fixation of nitrogen over different types of titanium exchanged zeolites under the irradiation of visible light. Experimental Methods and Apparatus Laporte type 3A, 4A, and 5A zeolites were used. These zeolites have a simple cubic crystalline structure. The 4A zeolite is of the basic sodium form and is the base material from which the cation-exchanged types are derived. The crystal structure has a three-dimensional pore system ca-
pable of absorbing molecules with critical diameters up to 4 A units. The 3A zeolite is the cation-exchanged potassium form of the type A crystal structure. The potassium ions, being larger than the sodium ions in the basic 4A type structure, effectively reduce the zeolite pore size to 3 8, units. The 5A zeolite is the cation-exchanged calcium form of the type A structure. This cation exchange increases the pore size to 5 8, units. The zeolites were in the shape of spherical beads of diameter varying from 1 to 2 mm. The cations in the zeolites were exchanged with titanium ions by using 30% (w/v) aqueous titanium trichloride solution in HC1. For every gram of zeolite treated, 2 mL of solution was used. The zeolite was soaked in the solution unstirred for 1 to 3 h at room temperature. After the exchange process the zeolite turned purple, which is the characteristic color of hexaaquotitanium(II1) ions. The zeolites were analyzed by ESR spectroscopy before and after the ion-exchange process. The zeolites used in the study of both the cleavage of water and the fixation of nitrogen reactions were prepared by the same method as that described above. Deactivated zeolite was reexchanged with titanium ions using the same procedure. The exchanged zeolites were washed and then immersed in water in the reactor. The reactors were made of Pyrex tubing with an internal diameter of 31.75 mm. The reactor was irradiated by a 150-W photoflood lamp. Experiments were also carried out with the wavelengths of light in the UV range filtered out so as to determine if the photoassisted reactions were due to UV or not. To achieve more uniform irradiation of the whole reactor surface, an aluminum sheet of mirror-finish quality was used as a reflector placed behind the reactor. The water used was double distilled. All experiments were conducted with 1 g of zeolite. (a) Photoassisted Cleavage of Water. The reactor tube was completely filled with degassed water after the washed titanium exchanged zeolites were placed in the reactor. A slight vacuum was applied to assist the collection of gases. After 7 h, the gases collected were ana-
0196-432118311222-0238$01.50/0 0 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 239 Ti3*
Ti3*
8 -
100 G
Figure 1. ESR spectrum of &newnTi-A zeolite.
lyzed by a gas chromatograph using a katharometer detector which was kept at 150 “C. The gases were separated by a column packed with 45 to 60 mesh 5A molecular sieves. The column oven temperature was 70 “C. Argon was used as the carrier gas and the gas flow rate was 0.75 mL s-l. ESR spectroscopic analysis was again performed on the zeolite. (b) Photoassisted Fixation of Nitrogen. The reactor was fitted with a porous sintered disk to support the zeolite and to distribute the inlet gas flow. High-purity nitrogen was bubbled through the porous disk continuously at a flow rate which partially fluidized the zeolite particles. Reactor temperature and pH were measured. The effluent gases passed through an absorber containing dilute sulfuric acid for 3 h. The amount of ammonia produced was determined by titrating the acid with dilute sodium hydroxide solution. The ammonia production was also determined by an ammonia electrode. The electrode is fitted with a thin hydrophobic membrane of microporous PTFE. Ammonia diffuses through the membrane until the partial pressure of ammonia on both sides of the membrane are equal. The concentration of ammonia is determined according to the Nernst equation. The electrode is sensitive to ammonia concentrations down to as low as 17 ppm. A further check on the ammonia level was performed in some of the experiments by a colorimetric method using a UV spectrophotometer. ESR spectroscopic analysis of the zeolite was again conducted after the reaction experiments. In all the experiments and analyses great care had been taken to eliminate all possible sources of ammonia other than that which arose from the photoassisted reactions. Control experiments were also performed under identical reactor conditions using zeolites which had not been exchanged with titanium ions. Results and Discussion The reactor temperature was between 30 and 35 “C and the pH was 7. The behavior of the zeolites was found to be dependent on the number of times of exchange with titanium ions. There was in all cases a significant difference between the results of zeolites which were exchanged with titanium for the first time, hereafter called “newn exchanged zeolite, and those which had been exchanged more than once, hereafter called “reexchanged” zeolite.
100 G
Figure 2. ESR spectrum of “reexchanged” Ti-A zeolite.
100G
’
Figure 3. ESR spectrum of spent catalyst.
(a) The Cleavage of Water. With a new exchanged zeolite, evolution of gases was immediately visible as soon as the zeolite was placed in water. Indeed, if washing was not done in the dark, gas bubbles were observed when the zeolite was being washed. Gas bubbles were also observed to evolve from the surface of the “reexchangednzeolites after they had been put in the reactor for 10-15 min. ESR spectroscopy results, as shown in Figures 1and 2, reveal Ti3+in the “new” and “reexchanged” zeolites. The ESR spectrum of Ti3+ is almost identical with that observed by Ono et al. (1974) for Ti3+ in type Y zeolites, showing a g value of 1.947 and a line width of 35 Oe at room temperature. The reaction took a considerable time, usually about 7 h, to complete. This slow rate prevailed because the zeolite was static and was not uniformly irradiated. The rate of hydrogen production could have been accelerated by bubbling an inert gas into the reactor to fluidize the particles, but, at the same time, it would have reduced the concentration of hydrogen to levels which were beyond the capability of the gas chromatograph. Figure 3 shows the ESR spectrum of the deactivated zeolite which has no Ti3+ signal. The rate of production of hydrogen was not constant. Figure 4 shows a sharp decline in the hydrogen formation using “new” type 5A zeolite after 4 h. Table I summarizes the amount of hydrogen produced in 7 h. No hydrogen
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
I
I
I
1
1
I
1
I
I
J
5.
0.iC
0.30
Figure 4. Cumulative yield of hydrogen.
t /
4
c
Table I. Hydrogen Production (mLa t STP/g of Zeolite) 3A new exchanged zeolite first reexchange second reexchange third reexchange fourth reexchange average efficiency percentage
0.32 1.12 1.80 0.17 0.42 30.0
type 4A 0.25 0.22 0.10 0.08
14.9
L
20
5A 0.40 0.78 0.50 1.20 1.15 27.6
was detected using zeolites which had not been exchanged with Ti3+. Of the three types of zeolites used, type 4A gave the poorest performance. It yielded 0.25 mL of hydrogen when it was newly exchanged. Further reexchanging with Ti3+ showed a decline in the amount of hydrogen collected. After four times of reexchange no significant quantity of hydrogen was detected in the products; 0.32 mL of hydrogen evolved from exchanged 3A zeolites when they were %ewn, but this figure was raised nearly sixfold after twice reexchanging the titanium. After four times of reexchange, the zeolite performance returned to its original level when it was “new”. “New” 5A zeolites yielded 0.40 mL of hydrogen. In this case too, an increase in the production of hydrogen was obtained after the zeolite had been exchanged with Ti3+ more than once, but this increase in hydrogen formation was not as sharp as in the case of type 3A zeolites. After three reexchanges, hydrogen production remained a t the level of approximately 1.1 to 1.2 mL. Yields of hydrogen were reproducible to within &lo%, which were within the range of experimental errors. It is important to note that irradiation through UV filter did not diminish the amount of hydrogen produced. The poor performance of type 4A zeolites is partly due to the difficulty of the sodium ions to exchange with titanium ions. Analyses of the amount of titanium present in the exchanged zeolites were made. These analyses required a set of duplicate experiments. A colorimetric method described by Willard et al. (1974) was used to determine the titanium content. For zeolites exchanged for the first time with titanium, the exchanged particles were dried and digested in concentrated sulfuric acid and then analyzed by the colorimetric method. The amount of titanium intake in subsequent exchanges was determined by analyzing the TiC13solution before and after the exchange. The Ti3+intake varied between 5.5 and 7.7 mg for 3A and 5A zeolites, but decreased rapidly from 5 to 1 mg for 4A zeolites over five times of exchange. Small amounts of a milky precipitate were obtained in the reactor, consisting of titanium (Ti4+)and zeolites that had broken up. The deactivated zeolite was discolored, and as shown in Figure 3, no ESR signal for Ti3+was detected. Probably in addition to the photodecomposition of water, further reactions occurred which oxidized the Ti3+
30LRS
Figure 5. Cumulative yield of ammonia.
ions. For example, the following reactions may have taken place hu Ti3+ + Ti4+ + e-
e-
+ H+
H.
-
+
Ti4++ 4H20
-
H.
(1) (2)
‘/zHz
(3)
Ti(OH)4 + 4H+
(4)
The titanium found in the precipitate may be due to the hydroxide or the formation of titanium oxides such as TiOz and Ti4+-O-Ti4+. The latter was suggested by van Damme and Hall (1979) in their comments on the results of Kuznicki et al. (1978). Unfortunately, the present method of analysis of titanium contents does not allow a more definite conclusion on the reaction mechanism to be drawn. The determination of quantunh yields is clearly vital for studying the efficiency of the process. The measurement of light absorption by a heterogeneous mixture necessitates a much more elaborate investigation. These results will be presented in the future. However, one can obtain some idea of this efficiency by examining the ratio of hydrogen production achieved to the maximum obtainable if all the Ti3+ were irradiated and oxidized. According to the stoichiometry of reactions 1to 4, two moles of the Ti3+are required for the formation of one mole of hydrogen. The efficienciesof 3A, 4A, and 5A exchanged zeolites averaged over five consecutive exchanges were then found to be 30, 14.9, and 27.6% respectively. These figures could be increased if the irradiation of the exchanged zeolite could be improved. The design and optimization of heterogeneous photoreactors must be fully studied if an efficient photochemical conversion process is to be developed. Some preliminary analysis and experimental results of a gas-solid photoreactor have been obtained by Rizzuti and Yue (1983). Similar work on gas-liquid-solid photoreactors is currently in progress and will provide useful information for a more thorough understanding of the process reported here. (b) The Fixation of Nitrogen. The process was made continuous by the bubbling of nitrogen into the reactor. The irradiation of the zeolite was more efficient than in the case of water cleavage because the zeolite was partially fluidized. The production of ammonia took much less time to complete. The activity of type 3A and 5A zeolites decreased rapidly after l h when the exchanged zeolites were “new”. After reexchange, the active period extended to
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 241
Table 11. Ammonia Production (mg/g of Zeolite) type 4A
3A
new exchanged zeolite first reexchange second reexchange third reexchange average efficiency percentage
0.40 0.95 1.10 1.48 43.7
0.22 0.13 0.05 18.3
5A 0.55 1.10 1.58 1.68 47.5
two hours. Figure 5 shows the progress of the cumulative amount of ammonia collected using "new" type 5A zeolite. Again the deactivated zeolite was discolored and no Ti3+ signal was found in the ESR spectroscopic analysis. Replenishment of Ti3+in the zeolites by reexchanging with titanium trichloride solution regenerated the fixation of nitrogen. Table I1 shows the amount of ammonia produced in 3 h by the three different types of exchanged zeolites. In the case of type 3A and 5A zeolites, ammonia production increased with the number of times the zeolites had been reexchanged with Ti3+. Initially "new" 5A zeolites yielded 0.55 mg of ammonia, but this figure was raised to 1.30 mg after three reexchanges. Overall the performance of 3A zeolites was about 25% below that of 5A zeolites. Type 4A zeolites again gave the poorest results, making 0.22 mg of ammonia when the exchanged zeolite was "new". The yield of ammonia decreased upon further reexchange. These results are reproducible to within the range of experimental error, which is approximately &lo%. It should again be noted that experiments carried out with the UV irradiation cutoff showed no reduction in the yield of ammonia. No ammonia was detected when the cations in the zeolites were not exchanged with Ti3+. Control experiments using no zeolite at all did not yield any ammonia. Clearly the formation of ammonia was the result of the fixation of nitrogen. Once again, small amounts of a milky precipitate were obtained in the reactor. A reaction mechanism similar to that given in (1)to (4) could be written for the fixation of nitrogen.
+
(5)
3H-
(6)
NH3
(7)
3Ti3+-k3Ti4+ 3e3e- + 3H+ 3H. + '/2N2
-
+ 12H20
3Ti4+
-+
3Ti(OH)4
+ 12H+
(8)
The efficiency of the fixation process can be expressed in terms of the ratio of ammonia production achieved to the maximum obtainable if all the titanium were irradiated and oxidized. According to reactions 5 and 8 the efficiencies of 3A, 4A, and 5A exchanged zeolites, averaged over five exchanges, were found to be 43.7,18.3, and 47.5%,
respectively. These figures are higher than the corresponding ones for hydrogen production, which confirms that fluidization improved the irradiation of the zeolites. It should also be pointed out that the zeolite beads lost weight as they were used repeatedly by reexchanging with titanium trichloride solutions. This was because the zeolites were not stable in the acidic titanium trichloride solution. The loss of weight of the zeolites was not constant, but usually was about 50% after three reexchanges. Although the zeolites lost weight, they usually maintained the external spherical shape. Their densities were therefore reduced upon repeated use, making them more uniformly fluidized than when they were "new". This phenomenon may have contributed to the increased effectiveness of the 3A and 5A zeolites with more reexchange. The processes will be greatly improved if the loss of zeolites can be minimized. Conclusions Titanium-exchanged zeolites are effective in the water cleavage and nitrogen fixation reactions when the zeolites are irradiated by visible light. It is encouraging that visible irradiation can be utilized. Ti3+ions are responsible for assisting the reactions. There is, however, a loss of Ti3+ ions because they are oxidized to Ti4+. These Ti3+ ions can be replenished by a reexchanging process, thus making it possible photochemically to synthesize ammonia from water and nitrogen continuously. Acknowledgment The authors are grateful to Mr. C. W. Roberts of Laporte Industries Ltd., for the provision of zeolites and helpful literature; Mr. E. Minshall of the University of Bath for his assistance in ESR analysis; Miss C. Bazilio of the University of Bath for her help in gas chromatographic analysis; and The Government of Pakistan and the University of Karachi for financial support and study leave for one of the authors, F. Khan. Registry No. Hydrogen, 1333-74-0; ammonia, 7664-41-7.
Literature Cited Augugllaro. V.; Lauricella, A.; Rizzuti, L.; Schiavello. M.; Sclafani, A. Int. J. Hydrogen Energy 1982a, 7(1 I),845. Augugliaro, V.; D'Aiba, F.; Rizzuti, L.; Schiavello, M.; Sclafani, A. I n t . J . Hydrogen Energy 1982b, 7(11),851. Jacobs, P. A.; Uytterhoeven, J. 6.; Beyer, H. K. J . Chem. SOC., Chem. Commun. 1977, 128. Khan, F.; Yue, P.-L.; Rlzzutl, L.; Augugliaro, V.; Schiavello, M. J. Chem. SOC.,Chem. Commun. 1981, 1049. Kuznicki, S. M.; Eyrlng, E. M. J. Am. Chem. SOC. 1978, 100, 6790. Ono, Y.; Suzuki. K.; Keii, T. J. fhys. Chem. 1974, 78, 218. Schrauzer, G. N.; Guth, T. D. J. Am. Chem. SOC. 1977, 9 9 , 7189. Wlllard, H. H.; Merrlt: L. L., Jr.; Dean, J. A. "Instrumental Methods of Analysis", 5th ed.;Van Nostrand: London, 1974;p 115. Van Damme, H.; Hall, W. K. J. Am. Chem. SOC.1979, 101, 4373. Rizzuti, L.; Yue, P.-L. Chem. Eng. Sci., accepted for publication.
Received for review June 10, 1982 Accepted December 8, 1982
