WILLIAM H. CLINGMAN, Jr.l Research and Development Department, American Oil Co., Texas City, Tex.
Photosensitized Oxidation of Propane with X-Rays A zinc oxide photosensitizer increases absorption of gamma radiation by the reaction system and gives a more selective distribution of the oxidation products T H E PHOTOSENSITIZED OXIDATION of propane with high-energy radiation has been investigated as a selective process for producing propyl alcohols and acetone. A solid sensitizer was used in the reaction system, which absorbed almost all the radiation and then induced a selective oxidation reaction in the radiation field. Oxidation of paraffins normally produces a great variety of products and degradation of the original hydrocarbon skeleton. Photosensitized oxidation, however, offers the possibility of a selective reaction on the solid surface. This phenomenon is to be distinguished from permanently modifying a catalyst with radiation. In the present study, the radiation was of insufficient energy to cause atomic displacements in the solid. The radiation, however, produces an electronically excited state in the solid, and it is this state which sensitizes the reaction.
Experimental Materials a n d Sensitizer Preparation. Phillips pure grade propane and isobutane were used, and the radioactive propane labeled with carbon-14 was supplied by the New England Nuclear Corp. Air was pretreated over Ascarite and dried over calcium sulfate before use. Most of the experimental results were obtained using Mallinckrodt or Baker analytical reagent grade zinc oxide. Before use, the powdered solid was stirred with water, filtered, dried a t 95' to 105' C. for 20 hours, and then granulated to 10-16 mesh. This granulated material was then heated in a nitrogen stream for 1 hour a t 600" C. The surface area of this sensitizer was 3.5 sq. meter per gram as determined by the BET method using nitrogen adsorption. The apparatus was especially designed with a small free volume so that surface areas less than Present address: Central Research Laboratories, Texas Instruments, Inc., Dallas 21. Tex.
Oxidation Literature Subject Vapor-phase oxidation of propane gives methanol, carbon monoxide, and formaldehyde a s major oxygenated carbon compounds Radiation-induced oxidation (a particles) of methane and ethane give carbon dioxide and water as only products Radiation-induced, liquid-phase oxidation of n-heptane, isooctane, and cyclohexane gives mixture of peroxides, carbonyls, and acids
Ref. (12, 1 9 )
(13)
(1, 2 )
1 sq. meter per gram could be measured to + O . l sq. meter per gram. A lower surface-area (0.3 sq. meter per gram) zinc oxide was prepared by thermal decomposition of analytical reagent grade zinc nitrate. After evolution of nitrogen oxides was complete, the solid was treated in a nitrogen stream for 1 hour a t 600' C. A zinc oxide sensitizer doped with alumina was prepared in a similar manner from a melt of zinc and aluminum nitrates.
Photosensitization Literature Subject Ref. Photooxidation (ultraviolet) of (4, 6,33) water to hydrogen peroxide is sensitized by zinc oxide, cadmium sul5de, and other semiconductors Photooxidation (ultraviolet) of ($0) methanol, ethyl alcohol, and xylene may also be sensitized with semiconductors Benzene and ethylene may be (9) hydrogenated over zinc oxide in presence of ultraviolet light Zinc oxide is a photosensitizer for (9,14) the polymerization of vinyl monomers Water and oxygen give hydrogen (23) peroxide in the presence of oxide semiconductors and y radiation Radiolysis of pentane adsorbed on (3) solids has been described
T h e latter catalyst contained 0.3 weight yo of aluminum. and its area was 0.1 sq. meter per gram. The highest surface-area zinc oxide (10.7 sq. meter per gram) was prepared by thermal decomposition of zinc carbonate in a nitrogen stream a t 600' C. The carbonate was precipitated by slowly adding a solution of ammonium carbonate to a zinc nitrate solution. Irradiation Procedure. Propane conversion was determined by two different methods, one using radioactive propane and the other using gas chromatography. In the latler technique, known amounts of propane and air were blended together in a borosilicate glass tube, which served as an irradiation cell. The latter usually contained about 3 grams of solid and had a volume of 6 to 7 ml. O n e end of the cell was attached to a stopcock to enable removal of propane after irradiation. Before each run the catalyst was degassed in a vacuum at 450' C. €or half an hour. The sample was irradiated with200-kv. x-rays in a water bath which was kept a t the desired reaction temperature. The cell was then placed in a liquid nitrogen bath and evacuated in order to remove most of the air. The residual gas was then transferred to a solid-free sample tube by means of a Toepler pump. Residual air was removed from the propane by freezing the latter with liquid nitrogen and evacuating. A known amount of isobutane was blended with the unreacted propane in the sample tube, and the resulting mixture was analyzed by gas chromatography. T h e amount of unreacted propane could be calculated from the ratio of propane to isobutane. T h e propane conversion was also measured using a radiochemical method (7). Mixtures of air, propane-2-carbon14, and zinc oxide were prepared in sealed ampoules and irradiated as described previously. The total product including zinc oxide was dissolved in dilute hydrochloric acid, and the solution equilibrated with inactive propane to remove dissolved, unreacted proVOL. 52, NO. 1 1
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NOVEMBER 1960
9 15
pane-2-carbon-14. The remaining carbon-14 in the solution resulted from oxygenated products and \vas equivalent to the converted propane. Some activity was present when the sample had not been irradiated, and the results were corrected for this apparent “dark reaction.” The individual products in the solution were determined by reverse isotope dilution analysis (7). Dosimetry. The x-rays used were generated by a 200-kv. electron beam impinging upon a tungsten target and were filtered through 1/2 mm. of copper and 1 mm. of aluminum. In conjunction with each irradiation experiment, intensity of the x-ray beam was measured with a Victoreen ionization chamber at a fixed position behind the samples. The relationship between the intensity a t this position and a t the samples \vas determined by using a ferrous dosimeter. In most cases, the samples were irradiated with an intensity of about 450 roentgens per minute. The effective wave length of the primary beam was 0.135 A., which was obtained by deLermining the thickness of a copper filter which would decrease the radiation intensity by a factor of two. After entering the water bath, this primary beam is scattered. From independent studies of x-ray scattering in water, which have been reviewed by Shalek (27), it was estimated that about 39y0 of the radiation impinging on the sample was scattered from the surrounding water bath. This radiation had an effective wave length of 0.216 A. From this data, the energy absorbed by the zinc oxide was calculated according to a method previously described (6). In this calculation, it was assumed that all of the radiation scattered by the zinc oxide was reabsorbed. Electrical Measurements. The 60cycle, alternating current impedance of the zinc oxide was measured under reaction conditions. The solid was placed in a normal irradiation tube between two concentric, cylindrical electrodes. The outer electrode was sufficiently thin that it did not significantly alter the intensity of the radiation impinging on the zinc oxide. This point was verified by irradiating a ferrous solution in place of the solid. The tube was open at both ends so that gas could be passed over the zinc oxide during the electrical measurements. The composition of this gas and the intensity of the radiation could be varied without even disturbing the arrangement of the solid in the cell. Thus, although absolute values of the impedance could not be determined, relative changes due to variations in the zinc oxide environment could be measured. The cell was connected in series with a low resistance and a 60-cycle, a.c. power source. Almost the entire applied voltage occurred across the sample
9 16
Table I. Radiation-Induced O x i d a tion of Propane over Zinc Oxide Gives Propyl Alcohols and Acetone as initial Oxidation Products hlole % Yield Based on Reacted Propane 1.4 x 104 5.2 x 104 roentgens roentgens
Acetone n-Propanol 2-Propanol Ethyl alcohol Total
25 8.8 36 5.3 75
6.4 5.2 14.2 9.8 36
cell and was constant during variations in the sample impedance. The voltage drop across the low resistance was proportional to the current through the sample. This latter voltage was rectified and the direct current output continuously recorded on a Varian 0- to 10mv. recording potentiometer. T h e entire circuit was calibrated using known resistances in place of the sample cell. Although the above technique did not rcsolve the sample impedance into its resistive and capacitive parts, it did enable rapid adsorption and desorption to be followed after sudden changes in the zinc oxide environment.
Results Oxidation Products. Photosensitized oxidation of propane over the standard zinc oxide sensitizer gave acetone, ethyl alcohol, n-propanol, and 2-propanol as the initial products. As irradiation continued, these initial products were oxidized further. T h e yields from the reaction a t 25’ C. are summarized in Table I. These values were determined by reverse isotope dilution analysis. All of the product could not be identified even though analyses for several oxygenated compounds were made. The isotopic method confirmed the absence of propylene glycol, pinacol, acetaldehyde, propionaldehyde, acetic acid, and propionic acid after an integrated dose of 5.2 X l o 4 roentgens. Furthermore, it was shown that the unidentified product was insoluble in 2-propanol or aqueous pinacol. Thus, this product is probably either a strongly adsorbed polymer or is chemically combined with the zinc oxide. The three-carbon atom products account for a lower percentage of the converted propane a t the higher radiation dose. This result implies that 2-propanol, n-propanol, and acetone are initial oxidation products and that these continue to react as irradiation continues. T o form ethyl alcohol, a compound containing one carbon atom must also be produced. This latter product would not contain carbon-14 and thus would not be detected when using propane-2carbon-14 as the reactant.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Effect of Reaction Variables on Propane Conversion. Data showing the propane conversion as a function of radiation dose are given in Figure 1. The conversion is given as the volume of propane measured under standard conditions that reacted per gram of zinc oxide. After the reactants have received an initial dose of about 1 X lo4 roentgens, the conversion becomes a linear function of the radiation dose. Also the conversion per unit weight of zinc oxide is independent of the total weight, as expected. The slope of the conversion us. dose curvc mcasurcs efficiency of the zinc oxide in producing product from absorbed radiation. During the initial stages the efficiency decreases but then reaches a constant value. The latter was 0.84 mole of propane converted per kilowatt-hour of radiant energy absorbed by the srandard zinc oxide. At a given radiation dose, propane conversion was essentially independent of reactant pressures and temperature for the standard zinc oxide. This point is illustrated by the data in Table 11, which were measured by the chromatographic method; in Figure 2, the same pressure independence is illustrated with data obtained by the more accurate radiochemical method. Conversion is plotted as a function of dose, and the experimental points were obtained at different partial pressures of propane and air. If it is assumed that the conversion is proportional to the partial pressures and each point is corrected to 300 mm. of air and 200 mm. of propane, the dark circles are obtained. The open circles represent the uncorrected data, and give a better fit to a linear relationship between conversion and dose. In the case of zinc oxide produced from zinc carbonate, the conversion increased with either increasing air or propane partial pressures. Since there
Table II. Pressure and Temperature Have No Significant Effect on Radiation-Induced Oxidation of Propane over ZnO a t 5.2 X l o 4 Roentgens
Temp.,
c.
70-73 70-73 70-73 70-73 21 21 21 21
Hydrocarbon Press., Mm.
85 199 324 416 233 259 269 220
Conver., Moles of Hydrocarbon/ Air Kw.-Hr. of Press., Radiation Mm. Absorbed
241 246 290 249 143 336 3 76 400a
a jO:50 oxygen-nitrogen instead of air.
0.85 0.76 0.98 0.98 1.07 1.16 1.07 0.94
mixture
used
PROPANE OXIDATION 10.0
0
I10.0 2 '
2'o ,
t
00.0
O
r
1.0
2 .O
I
3.0
4.0
5.0
6.0
DOSE (ROENTGENS X
I .o
2.0
3.O
4.0
DOSE (ROENTGENS
5 .O
Figure 2. With the standard zinc oxide, propane conversion i s essentially independent of reactant pressure and temperature
6.0
x
Figure 1. After an initial dose of 1 X l o 4 roenygens, conversion becomes a linear function of radiation dose
was no agitation during the irradiation experiments, the reaction rate may have been partially limited by diffusion of the reactants to the surface. The sensitizer prepared by zinc carbonate decomposition had an average pore diameter of 580 A. but, for the standard zinc oxide, the diameter was 1370 A. Also the adsorption rate on the carbonate-derived zinc oxide may be slow enough to be partially rate-determining. Other variables affecting propase conversion include the zinc oxide surface area and method of preparation (Table 111). Propane conversion with four different zinc oxide preparations was measured after an initial line-out period of about 30 minutes. Differences in surface area will account for a large part of the difference in sensitizer activity, particularly with the three undoped forms of zinc oxide. Since the reaction rate should be proportional to the surface area, a specific activity has been calculated for each sensitizer. The data were obtained a t a propane pressure of about 200 mm. and an air pressure of about 300 mm. At higher pressures, the specific activity of the carbonate-derived zinc oxide would approach that of the standard zinc oxide. Thus, the three undoped sensitizers are all comparable in activity. Adding a small amount of aluminum appears to increase significantly the specific activity of zinc oxide. The increase in reaction rate may result from enhanced chemisorption of oxygen on the doped solid.
Photodesorption of Oxygen from Zinc Oxide. Oxygen desorption from zinc oxide was studied under different conditions by measuring changes in the electrical impedance of the solid. This
Table 111. Propane Conversion Varies with Zinc Oxide Preparation and Surface Area ZNO Spec. Propane Activity, Conver- (Moles/ Surface
sion, Kw.-Hr.)/
Area, Moles/ (Sa. Mi./ Sensitizer Sq. M./G. Kw.-Hr. G.) ZnO f AlnOa (0.3% AI) (from nitrate) 0 . 1 ZnO (from ZnCOa) 10.7 ZnO (standard) 3 . 5 ZnO [from Zn-
(Nod21
0.26
0.099
1.0
1.3 0.84
0.12 0.24
0.041
0.16
technique for studying the kinetics of adsorption has been discussed by Gray ( 9 ) . I n the present case: the continuous response of the a x . impedance to changes in the ambient gas composition was measured a t different temperatures and in the presence and absence of the radiation. Chemisorbed oxygen is present on the surface as anions, and these repel conduction electrons from the surface of the zinc oxide and lower its electrical conductivity (8, 77, 77). The electrical measurements indicate changes in adsorbed oxygen concentration with variations in environment. I t was concluded that oxygen could not be desorbed a t room temperature by passing nitrogen over the solid. This was true even though the last traces of oxygen were removed from the nitrogen with Oxosorb. At 100' C., however, slow desorption did occur. When the surface was again brought into contact with oxygen, readsorption took
place almost instantaneously. These rate differences and temperature effect are characteristic of an exothermic chemisorption of oxygen on the surface, and are consistent with the results of other investigators. I n the presence of x-ray irradiation, however, a different behavior was observed. Oxygen desorption now took place a t room temperature when nitrogen was passed over the solid. I n Figure 3, typical data are shown for three forms of zinc oxide. Air was passed over the solid under irradiation conditions until a constant electrical impedance was obtained. Pure nitrogen was then passed over the zinc oxide and the decrease in impedance continuously recorded as oxygen was desorbed. Two independent experiments on the same sample showed the desorption to be both reversible and reproducible. Weller and Voltz (24) have reported that the concentration of chemisorbed oxygen on chromia is linearly related to the logarithm of the conductivity. The exact relation between measured impedance and adsorbed oxygen concentration in the present experiments, however, is not known. From the general shape of the curves, however, it appears probable that the half life of adsorbed oxygen is about 1 minute under desorption conditions. I t can also be concluded that the desorption time constants for the three forms of zinc oxide are nearly the same.
Discussion The absence of aldehydes, esters, and acids, which are normally produced in paraffin oxidation, shows that the mechanism is different from the usual oxidation reaction. It is likely that the active oxidizing agent is a species which is not normally present in either thermally or radiation-induced oxidaVOL. 52, NO. 1 1
NOVEMBER 1960
9 17
2.01
I
I
8.0
9.0
A
0 0.0
I
I 2.0
1.0
I 3.0
I
I
4.0
5.0
I 6.0
I 7.0
Time, Minutes Figure 3. During irradiation, air was passed over the solid until a constant electrical impedance was obtained. The air was then replaced with pure nitrogen and, as oxygen was desorbed, the decrease in impedance was continuously recorded
tions. This unique oxidant would be produced from chemisorbed oxygen by the radiation. In the presence of oxygen, zinc oxide contains adsorbed oxygen anions. These are denoted by 0-; although other forms probably exist. Since zinc oxide in the dark is not a good oxidation catalyst, the oxygen anions must not be the active oxidizing agent in the radiation-induced reaction. Upon irradiation, however, the reaction can occur (76, 76, 77); 0 - (ads)
+
-+
0“(ads)
(1)
where IT(is an electrical hole produced by the radiation and O* is a new species of adsorbed oxygen, possibly in an excited state. If the species, 0*,is less strongly bound to the solid surface than 0-, Reaction 1 can explain the greatly enhanced oxygen desorption rate in the presence of radiation. In the present study, it is postulated that O h reacts with propane to form water and the observed oxidation products. With standard zinc oxide, it is likely that Reaction 1 is rate-determining. If chemisorption of oxygen or reaction of O* with propane were rate-determining, increasing partial pressures would be expected to increase the reaction rate, but the latter is independent of pressure. The observed rates of propane oxidation and oxygen desorption may be used to test the consistency of this mechanism. The rate of oxygen desorption in a nitrogen stream and in the absence of propane may be used to measure the rate of Reaction 1. Assuming that the desorption rate is proportional to the adsorbed oxygen concentration gives : d [O*]/dt
918
=
k [O-]
(2)
If the half life for oxygen desorption is 1 minute, then k is 0.7 min.-’ A value for IO-] may be estimated from data given by Morrison for the chemisorption of oxygen on zinc oxide ( 7 8 ) . The surface area of the solid was not given, but the particle size was 0.25 micron. Assuming the particles to be nonporous spheres gives an area of 4.4 sq. meters per gram. This value agrees with the area of 3.5 sq. meters per gram found for analytical reagent grade zinc oxide in the present work. Using Morrison’s adsorption data and a surface area of 4.4 sq. meters per gram gives an adsorbed oxygen concentration of 3.6 X 1OI2 atoms per sq. cm. a t room temperature. The corresponding rate of formation of O* given by Equation 2 is then 2.5 X 1 O I 2 atoms per sq. cm. per minute. This value is a maximum rate, because probably all of the oxygen is not adsorbed as 0-.The experimental oxidation rate over the standard catalyst is 3.4 X 10” molecules per sq. cm. per min. This value is consistent with the above estimate for the rate of O* formation. The specific activity differences in Table 111probably result fromdifferences in the value of [ O - ] rather than k . Doping the solid with alumina increases the chemisorbed oxygen (70, 77). In addition, the conductivity experiments show that the rate constants for Reaction 1 are nearly the same for the three undoped zinc oxide sensitizers. The data obtained in this study illustrate the effectiveness of using a sensitizer in radiation-induced reactions. Under the experimental conditions, no propane oxidation was observed except over zinc oxide. This is because in-
INDUSTRIAL AND ENGINEERING CHEMISTRY
sufficient radiation was absorbed by the system in the absence of the solid, since radiation-induced oxidation of hydrocarbons is known to take place. The second advantage of the photosensitized process is product composition. There were only four major products, and three of these contained the same number of carbon atoms as the starting hydrocarbon. I n the field of oxidation reactions, it has been shown possible to produce a new intermediate, 0*,not formed by other means. Additional new intermediates might be formed by using solid sensitizers in other radiation-induced reactions, leading to desirable changes in the product distribution. literature Cited
(1) Bakh, N. A., “Symposium on Radiation Chemistry,” p. 145, Division of Chemical Science, Academy of Sciences, U.S.S.R., Moscow, 1955. (2) Bakh, N. A., Popov, N. I.; Zbid., p. 156. (3) Caffrey, J. M., Allen, A. 0.; J . Phys. Chem. 62, 33 (1958). (4) Calvert, J. G., Rept. 2, No. 3, p. 86, Air Pollution Foundation (Los Angeles), 1956. (5) Calvert, J. G., others, J . A m . Chem. SOC.76, 2575 (1954). (6) Clingman, W. H., Jr., J . Chem. Phys. 27, 322 (1957). (7) Clingman, W. H., Jr., Hammen, H. H., Anal. Chem. 32, 323 (1960). (8) Dyrue, J., Helv. Chim. Acta. 39, 812 (1936). (9) Gray, T. J., “Chemical-Physics of Catalyst Surfaces,” Div. of Petroleum Chem., 135th Meeting, ACS, Boston, Mass., April 1959. (IO) Hauffe, K.: “Advances in Catalysis,” Vol. 7, p. 213, Academic Press, New York, 1955. (11) Heiland, G.. Z. Phys. 138, 459 (1954). (12) Lewis, B.: von Elbe, G., “Combustion, Flames, and Explosions of Gases,” p. 133, Academic Press, New York, 1951. (13) Lind, S. C., Bardwell, D. C.; J. Am. Chem. SOC.48, 2335 (1926). (14) Markham, M. C., Laidler, K. J . , J . Phvs. Chem. 57. 363 (1953’1. M‘edved, D. ’B., j . Chin. Phps. 28, 870 (1958). 16) Melnick, D. A.: I t i d . , 26, 1136 (1957). (17) Morrison, S. R., “Advances in Caysis,” Vol. 7, p. 259. Academic Press,
‘ I h . ENG.CHEW 46, YO01 (1954). (20) Schwab, G. M., “Advances in Catalysis,” Vol. 9, p. 229, Academic Press, New York, 1957. (21) Shalek, R. J., Sinclair, W. K., (‘Radiation Biology and Cancer,” p. 149, University of Texas Press, Austin, 1959. (22) Stephans. R. E., Ke, B., Trivich, D., J . Phys. Chem. 59, 966 (1955)‘; (23) Veselovsky, V. I., others, Symposium on Radiation Chemistry,” p. 119, Div. of Chem. Science, Academy of Sciences of the U.S.S.R., Moscow, 195.5. (24) Weller, S. W., Voltz, S. E., “Advances in Catalysis,” Vol. 9, p. 215, Academic Press, New York, 1957.
RECEIVED for review December 21, 1959 ACCEPTEDAugust 1, 1960
