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Laser-induced oxidation reactions in supercritical ethaneO2 mixtures
1. Photochem. Photobiol. A: Chem., 80 (1994) 43-4
Laser-induced mixtures
oxidation
reactions
439
in supercritical
ethane-0,
Kayoko Iguchi, Yoshito Oshima and Seiichiro Koda Lkpament
of Chemical Engineering, Faculty of Engineering, Universiryof Tokyo, Honga 7-3-1, Bunkyo-ky Torlyo 133 &ww)
Abstract The laser-induced oxidation of ethane by 0, at 318 K was investigated with varying the pressure between 12-91 atm. The reaction condition was regarded as the supercritical phase above 50-60 atm, depending on the 0, fraction. Ethanol, acetaldehyde, and CO, were mainly produced at any reaction conditions, together with small amounts of C, compounds and formic esters. The kinetic discussion for the time dependence indicated that the consecutive photolysis of primary products takes place during the subsequent laser irradiation period. The branching ratio to CO, formation in the primary process in the supercritical phase is much smaller than that in the gas phase, and the selectivities for ethanol and acetaldehyde show a discontinuous change near the critical point. These facts show that the supercritical phase affects this complex radical reaction system. The primary photoabsorption process is also discussed.
1. Introduction
Supercritical fluids exhibit many unique properties and are used in various chemical reactions as solvents. Fluids near or above the critical point exhibit characteristic behaviour in certain chemical properties, such as viscosity, dielectric constant and dissociation constant, and these values are strongly dependent on the fluid density. Recently, investigations of the effect of these properties on the reaction rate have been reported. In ionic reactions, the reaction rate is affected by these solvent properties [I]. Blyumberg et al. [2] have described the oxidation of n-butane in both the supercritical phase and the liquid phase. They reported that the reaction rate in the supercritical phase was higher than that in the liquid phase, and that a broader product spectrum was obtained. According to Subramanian and McHugh [3], the reason for the increased reaction rate in the supercritical phase may be associated with the more efficient production of free-radical pairs. In the critical region, it is expected that the radical pair will more readily diffuse apart. Baumgartner [4] reported that tert-butyl hydroperoxide (TBHP) formation by oxidation of isobutane was enhanced in the supercritical phase compared with the liquid phase. Our objective in this work is to determine whether or not the supercritical phase has an lOlO-6030/94/$07.00 8 1994 Elsevier SSDI 1010-6030(94)01049-L
Sequoia.
All rights reserved
overall effect on complex radical reactions. As a model reaction, we selected the reactions of ethane0, mixtures for the following reasons: (1) the critical condition of ethane is such that it is not difficult to achieve a supercritical condition experimentally; (2) a small reactant is convenient for the analysis of the reaction mechanism, but the reactivity of methane is much lower than that of ethane; (3) there have been many investigations of the kinetic parameters in the gas phase oxidation of hydrocarbons, which can be compared with the reaction mechanism in the supercritical phase; (4) the laser-induced oxidation of ethane in the gas phase has been studied previously [5]. In this paper, the effects of the reaction conditions on the product distribution are clarified experimentally, and, from the experimental results, a plausible reaction scheme, including the photoreactions of the reactant and the products, is discussed. The dependence of the product distribution on the fluid density is also examined.
2. Experimental
details
The experimental apparatus is shown in Fig. 1. The reaction cell consists of a stainless-steel cylinder (length, 5 cm; inner diameter, 1 cm) with quartz windows (thickness, 1 cm) at both ends for light introduction. Each window was sealed with an O-ring.
440
K Iguchi et al. / Laser-induced oxidation reactions in supercritical
Fig. I. Experimental apparatus. 1, Cylinder; 2, massflowcontroller; 3, thermocouple; 4, pressure sensor, 5, refrigerator; 6, cooling pump; 7, liquidfeedpump; 8, reaction cell; 9, constant temperature air bath; 10, sampling bag.
After evacuating the cell, 0, (Suzuki Shokan Co. Ltd, 99.9%) was introduced into the reactor to a fixed pressure, followed by the addition of ethane (Takachiho Commercial Co. Ltd, 99.9%) up to the reaction pressure. Both ethane and 0, were used as received. The mixture was left for about 30 min to achieve complete mixing. A IGF (248 nm) excimer laser (Lambda Physik EMGlOl) was used to irradiate the mixture at 10 Hz for 15-60 min. The reaction cell was placed in a constant temperature bath to keep the temperature constant (318 K). No increase in temperature was observed during laser irradiation. The contents were expanded into a container at 1 atm for quantitative and qualitative analysis by gas chromatography (Shimadzu GC-8A) and/or mass spectroscopy (Shimadzu QP-1000). Photoabsorption measurements were also performed using a D2 lamp (Hamamatsu Photonics L544) as a light source. The transmitted light was resolved by a monochromator (N&on P-250) and detected by a photomultiplier (Hamamatsu Photonics R928).
3. Results and discussion 3.1. Crifical bcus of ethane--02 mixture It is very important to determine the critical points of individual ethane-Q, mixtures in order to establish the reaction conditions. No information is available in the literature, and hence an estimation of the critical locus of this system was made according to the method of Prausnitz and Chueh [6]. In the calculation, the values of the interaction parameters (k, 7 and V) of the ethane-@ system were estimated by comparison
ethane-9,
O2 mole fraction [-] Fig. 2. Critical locus of ethane-0, system method of Prausnitz and Chueh [6].
caiculated
by the
100 Pressure [atm] Fig. 3. Plot of density VS. pressure of the ethane-0, mixture at 318 K. The partial pressure of O2 is constant (5 kg cm-‘).
with other hydrocarbon-Q, and hydrocarbon-N, systems. The interaction parameters k, 7 and v are characteristic for each binary system in the calculation of the critical pressure, critical temperature and critical density of the mixture respectively. The result of the calculation is shown in Fig. 2. At an O2 mole fraction below 0.15, the critical temperature and critical pressure were estimated to be 297-303 IS and 50-60 atm respectively, depending on the 0, mole fraction. To check the validity of this calculation, the dependence of the density of the mixture on the pressure at a fixed composition was measured experimentally. The plot of density against pressure shows a discontinuous increase at a pressure of 50-60 atm, which is in good agreement with the estimation, as shown in Fig. 3.
442
X &chi
solving
the differential
W&W
equations
k,
dt
=” k,+k,+k,
d[CH,CHO] =ri k +;+k
dt
1
et al. I Laser-induced oxidation reactions in supercritical ethhoneO,
2
3
-WY-WW
01
+W3WHl
PI
- (ks + k#ZH&HO]
WO,l
k3
=r’ kl +k,+k,
-T--
+k,[CH,CHO]
(3)
The kinetic parameters in the equations are obtained by fitting the calculated curve to the experimental results (Figs. 4 and 5). The most plausible values are presented in Table 1. The primary reaction rate of ethane with O2 in the supercritical phase (71 atm) is much greater than that in the gas phase (41 atm). This result may be caused primarily by the difference in the amount of light absorbed, and a detailed discussion is given in Section 3.5. The branching ratio of the primary reaction is quite different for the two reaction phases. The ratio of the formation rate constants of ethanol (k,), acetaldehyde (k,) and CO2 (k3) is equal to 0.21:0.72:0.07 in the supercritical phase and 0.15:0.40:0.45 in the gas phase. The branching ratio to CO1 in the primary reaction in the supercritical phase is about six times Iess than that in the gas phase. The other important observation is that the secondary photolysis of the primary products takes place in both reaction phases. If our assumption that secondary reactions of acetaldehyde other than photolysis are negligible is correct, the rate of photolysis of acetaldehyde would be much larger in the supercritical phase than in the gas phase. The photolysis of acetaldehyde in the gas phase has been reported [7] to proceed as follows CH,CHO
TABLE
-%
CH, + CO
-
CH,+CHO
1. Calculated
Rate constant
kinetic
The experimental observation that the yields of methane and formic esters increased with an increase in time can be explained by the secondary photolysis of acetaldehyde. The generation of radicals in the photoinduced initiation reaction makes it possible to explain the product distribution. A plausible reaction scheme can be summarized as follows. The primary photoreaction leads to the formation of the main products: ethanol, acetaldehyde and CO,. Although the detailed mechanism is now under investigation, the main reaction for the formation of these products is presumed to be the consecutive oxidation of the ethyl radical by an oxygen molecule. The rate constants kl, k2 and k3 can be combined to give the photolysis rate. The secondary photolysis of the primary products also takes place over longer reaction times, producing C, fragments, such as CO, methane and the formyl radical. Part of the CO, is expected to be produced from these fragments of the secondary photolysis of the main products, in particular acetaldehyde. The rate constant k, corresponds to such reactions. The formation of formic esters can be explained by the radical recombination of the formyl radical and the alkoxy radical_ The high radical concentration, generated by laser irradiation at high energy density, presumably favours such radical-radical recombination reactions. 3.4. Dependence of the product yields on pressure and fluid density Figure 7 shows the total pressure dependence of the product yields. The product yields are also plotted against the reduced density in Fig. 8. In
rate constants
41 atm (gas)
71 atm (supercritical)
ri (mol cme3 min-‘)
3.0 x 10-s
6.2x10-’
WWI +&+B) M& +&+&I W& +k, +b)
0.15 0.40 0.45
0.21 0.72 0.07
kr (min-‘)
2.1 x10-2
2.1 x10-2
k5 +k, (min-‘)
3.0 x 10-J
8.9~ lo-’
Pressure [atm] Fig. 7. Dependence of the product yields on the reaction pressure at 318 K. Each symbol corresponds to the same product as shown in Fig. 4.
443
Reduced density ( P’/P 3 [-] Fig. 8. Dependence of the product yields on the reduced density at 318 K. Each symbol corresponds to the same product as shown in Fig. 4.
these experiments, the partial pressure of 0, was lixed to 4 atm and the total pressure was varied between 12 and 91 atm. The total product yield, as well as those of acetaldehyde and ethanol, greatly increase in the neighbourhood of the critical pressure, while that of CO, increases linearly. The behaviour of the total product yield as a function of the reduced density suggests that the initial photoabsorption by the ethane0, mixture is strongly affected by the fluid density. The discontinuity of the yield of partially oxidized products (acetaldehyde and ethanol) near the critical pressure, which is evident for both the pressure (Fig. 7) and the reduced density {Fig. S), suggests that the high density and the characteristic structure of the supercritical fluid affect the reaction process. The high local density or “cage effect”, due to cluster formation surrounding the reactants or intermediates in the neighbourhood of the critical point, may affect the reaction mechanism, which results in the discontinuous change in the overall branching ratio. 3.5. hitiafion reaction Since no product was obtained under the same reaction conditions without laser irradiation, the initial elementary reaction is considered to be a photoinduced reaction, even though ethane and O2 in the gas phase (1 atm) have been reported to exhibit negligibly small absorption coefficients near 248 nm. As shown in Fig. 7, the total product yield increases with an increase in total pressure in the neighbourhood of the critical pressure. Figure 9 shows the dependence of (1,~-r)M, on the total pressure of the ethane-0, mixture at
.uo Pressure [atm]
Fig. 9. Dependence of (1,-1)/10 on the pressure of the ethane-O2 mixture at 248 nm. Partial pressure of O2 is constant (5 kg cm-‘).
248 run. This indicates that (I,-T)/I, also shows a discontinuous increase at around 50 atm, and no light can be detected at the end of the reaction cell when the pressure exceeds 60 atm. A similar pressure dependence was observed in the pure ethane (without 02) system, and no products were detected after irradiation (248 mu) of pure ethane at high density. These experimental observations suggest that the pressure dependence of the total product formation in the ethane-0, mixture is due to the dispersion of light. Under supercritical conditions, the light will be dispersed by enhanced fluctuation of the local density in the supercritical fluid, which makes the path length of the light larger, and consequently the light can be effectively absorbed by species exhibiting very small absorption coefficients. What type of species absorbs the 248 nm light to initiate the reaction? It is very difficult to identify the species responsible, but O2 seems to play an important role in the photoinitiation step, because no photoinitiated reaction takes place in the absence of 0,. There are several possibilities for the initiation reaction, e.g. increase in the 0, absorption coefficient, red shift of the OZ absorption band and photoabsorption by a charge transfer (CT) complex of ethane and 0,. Shurlock and Ogilby [8] have reported that singlet molecular oxygen (‘As), formed on irradiation of an oxygen-organic molecule (such as aromatic and aliphatic hydrocarbons), produces a CT absorption band in the liquid phase. Another possibility is the photoabsorption by an 02-0, dimer, or the reaction of O2 with an excited oxygen molecule. In either case, the products are presumed to be an oxygen atom and ozone. The mechanism of the initial
444
K &u&i et al. I Loser-induced
oxidution reactions in supercritical ethane-Ol
photoreaction has not yet been identified in detail, but the unique absorption behaviour combined with the dispersion by the high-density fluid should be noted.
behaviour of the total product yield as a function of the reduced density is suggested to be due to the dispersion of light. Acknowledgment
4. Conclusions As a model of radical reactions in the supercritical phase, we investigated the laser-induced oxidation of ethane with 02, and the results can be summarized as follows. (1) The dependence of the product distribution on the reaction conditions was clarified experimentally. Ethanol, acetaldehyde and CO*, together with small amounts of CH4, CO and formic ester, were produced irrespective of the reaction pressure. (2) The time dependence of the product yields was studied. To fit the experimental results to the theoretical model, the branching ratio in the primary process and the rate of secondary photolysis of the products were quantitatively estimated. (3) The dependence of the product yields on the total pressure suggests that the selectivities for ethanol and acetaldehyde show a discontinuous change near the critical point, The characteristic
This work was financially supported by a Research Grant for Promotion of Science from the Ministry of Education, Culture and Science of Japan (Grant No. 04238103). References K.P. Johnston and C. Haynes, AIChE J., 35 (12) (1987) 2017. E.R. Blyumberg, Z.K. Maizus and N.M. Emanuel, The Oxidation of Hydrocarbons in theLiquid Phase, Macmillan, New York, 1965. B. Subramanian and M.A. McHugh, Ind. Eng. Process Des. Dev., 25 (1986) 1. H.J. Baumgartner, European Patent Application, EP 76533, 1983. Y. Oshima, M. Saito, S. Koda and H. Tominaga, Nippon figakukatihi, 5 (1989) 588. J.M. Prausnitz and P.L. Chueh, Computer Calcuiotion for High-Pressure Vapor-L@idEquiiibniz, Prentice-Hall, NJ, 1968. J.G. Calvart and J.N. Pitts, Jr., Photochemistry, Wiley, New York, 1966. R.D. Shurlock and P.R. Ogilby,J. Am. Chem. Sac., 110 (1988) 640.
Laser-induced mixtures
oxidation
reactions
439
in supercritical
ethane-0,
Kayoko Iguchi, Yoshito Oshima and Seiichiro Koda Lkpament
of Chemical Engineering, Faculty of Engineering, Universiryof Tokyo, Honga 7-3-1, Bunkyo-ky Torlyo 133 &ww)
Abstract The laser-induced oxidation of ethane by 0, at 318 K was investigated with varying the pressure between 12-91 atm. The reaction condition was regarded as the supercritical phase above 50-60 atm, depending on the 0, fraction. Ethanol, acetaldehyde, and CO, were mainly produced at any reaction conditions, together with small amounts of C, compounds and formic esters. The kinetic discussion for the time dependence indicated that the consecutive photolysis of primary products takes place during the subsequent laser irradiation period. The branching ratio to CO, formation in the primary process in the supercritical phase is much smaller than that in the gas phase, and the selectivities for ethanol and acetaldehyde show a discontinuous change near the critical point. These facts show that the supercritical phase affects this complex radical reaction system. The primary photoabsorption process is also discussed.
1. Introduction
Supercritical fluids exhibit many unique properties and are used in various chemical reactions as solvents. Fluids near or above the critical point exhibit characteristic behaviour in certain chemical properties, such as viscosity, dielectric constant and dissociation constant, and these values are strongly dependent on the fluid density. Recently, investigations of the effect of these properties on the reaction rate have been reported. In ionic reactions, the reaction rate is affected by these solvent properties [I]. Blyumberg et al. [2] have described the oxidation of n-butane in both the supercritical phase and the liquid phase. They reported that the reaction rate in the supercritical phase was higher than that in the liquid phase, and that a broader product spectrum was obtained. According to Subramanian and McHugh [3], the reason for the increased reaction rate in the supercritical phase may be associated with the more efficient production of free-radical pairs. In the critical region, it is expected that the radical pair will more readily diffuse apart. Baumgartner [4] reported that tert-butyl hydroperoxide (TBHP) formation by oxidation of isobutane was enhanced in the supercritical phase compared with the liquid phase. Our objective in this work is to determine whether or not the supercritical phase has an lOlO-6030/94/$07.00 8 1994 Elsevier SSDI 1010-6030(94)01049-L
Sequoia.
All rights reserved
overall effect on complex radical reactions. As a model reaction, we selected the reactions of ethane0, mixtures for the following reasons: (1) the critical condition of ethane is such that it is not difficult to achieve a supercritical condition experimentally; (2) a small reactant is convenient for the analysis of the reaction mechanism, but the reactivity of methane is much lower than that of ethane; (3) there have been many investigations of the kinetic parameters in the gas phase oxidation of hydrocarbons, which can be compared with the reaction mechanism in the supercritical phase; (4) the laser-induced oxidation of ethane in the gas phase has been studied previously [5]. In this paper, the effects of the reaction conditions on the product distribution are clarified experimentally, and, from the experimental results, a plausible reaction scheme, including the photoreactions of the reactant and the products, is discussed. The dependence of the product distribution on the fluid density is also examined.
2. Experimental
details
The experimental apparatus is shown in Fig. 1. The reaction cell consists of a stainless-steel cylinder (length, 5 cm; inner diameter, 1 cm) with quartz windows (thickness, 1 cm) at both ends for light introduction. Each window was sealed with an O-ring.
440
K Iguchi et al. / Laser-induced oxidation reactions in supercritical
Fig. I. Experimental apparatus. 1, Cylinder; 2, massflowcontroller; 3, thermocouple; 4, pressure sensor, 5, refrigerator; 6, cooling pump; 7, liquidfeedpump; 8, reaction cell; 9, constant temperature air bath; 10, sampling bag.
After evacuating the cell, 0, (Suzuki Shokan Co. Ltd, 99.9%) was introduced into the reactor to a fixed pressure, followed by the addition of ethane (Takachiho Commercial Co. Ltd, 99.9%) up to the reaction pressure. Both ethane and 0, were used as received. The mixture was left for about 30 min to achieve complete mixing. A IGF (248 nm) excimer laser (Lambda Physik EMGlOl) was used to irradiate the mixture at 10 Hz for 15-60 min. The reaction cell was placed in a constant temperature bath to keep the temperature constant (318 K). No increase in temperature was observed during laser irradiation. The contents were expanded into a container at 1 atm for quantitative and qualitative analysis by gas chromatography (Shimadzu GC-8A) and/or mass spectroscopy (Shimadzu QP-1000). Photoabsorption measurements were also performed using a D2 lamp (Hamamatsu Photonics L544) as a light source. The transmitted light was resolved by a monochromator (N&on P-250) and detected by a photomultiplier (Hamamatsu Photonics R928).
3. Results and discussion 3.1. Crifical bcus of ethane--02 mixture It is very important to determine the critical points of individual ethane-Q, mixtures in order to establish the reaction conditions. No information is available in the literature, and hence an estimation of the critical locus of this system was made according to the method of Prausnitz and Chueh [6]. In the calculation, the values of the interaction parameters (k, 7 and V) of the ethane-@ system were estimated by comparison
ethane-9,
O2 mole fraction [-] Fig. 2. Critical locus of ethane-0, system method of Prausnitz and Chueh [6].
caiculated
by the
100 Pressure [atm] Fig. 3. Plot of density VS. pressure of the ethane-0, mixture at 318 K. The partial pressure of O2 is constant (5 kg cm-‘).
with other hydrocarbon-Q, and hydrocarbon-N, systems. The interaction parameters k, 7 and v are characteristic for each binary system in the calculation of the critical pressure, critical temperature and critical density of the mixture respectively. The result of the calculation is shown in Fig. 2. At an O2 mole fraction below 0.15, the critical temperature and critical pressure were estimated to be 297-303 IS and 50-60 atm respectively, depending on the 0, mole fraction. To check the validity of this calculation, the dependence of the density of the mixture on the pressure at a fixed composition was measured experimentally. The plot of density against pressure shows a discontinuous increase at a pressure of 50-60 atm, which is in good agreement with the estimation, as shown in Fig. 3.
442
X &chi
solving
the differential
W&W
equations
k,
dt
=” k,+k,+k,
d[CH,CHO] =ri k +;+k
dt
1
et al. I Laser-induced oxidation reactions in supercritical ethhoneO,
2
3
-WY-WW
01
+W3WHl
PI
- (ks + k#ZH&HO]
WO,l
k3
=r’ kl +k,+k,
-T--
+k,[CH,CHO]
(3)
The kinetic parameters in the equations are obtained by fitting the calculated curve to the experimental results (Figs. 4 and 5). The most plausible values are presented in Table 1. The primary reaction rate of ethane with O2 in the supercritical phase (71 atm) is much greater than that in the gas phase (41 atm). This result may be caused primarily by the difference in the amount of light absorbed, and a detailed discussion is given in Section 3.5. The branching ratio of the primary reaction is quite different for the two reaction phases. The ratio of the formation rate constants of ethanol (k,), acetaldehyde (k,) and CO2 (k3) is equal to 0.21:0.72:0.07 in the supercritical phase and 0.15:0.40:0.45 in the gas phase. The branching ratio to CO1 in the primary reaction in the supercritical phase is about six times Iess than that in the gas phase. The other important observation is that the secondary photolysis of the primary products takes place in both reaction phases. If our assumption that secondary reactions of acetaldehyde other than photolysis are negligible is correct, the rate of photolysis of acetaldehyde would be much larger in the supercritical phase than in the gas phase. The photolysis of acetaldehyde in the gas phase has been reported [7] to proceed as follows CH,CHO
TABLE
-%
CH, + CO
-
CH,+CHO
1. Calculated
Rate constant
kinetic
The experimental observation that the yields of methane and formic esters increased with an increase in time can be explained by the secondary photolysis of acetaldehyde. The generation of radicals in the photoinduced initiation reaction makes it possible to explain the product distribution. A plausible reaction scheme can be summarized as follows. The primary photoreaction leads to the formation of the main products: ethanol, acetaldehyde and CO,. Although the detailed mechanism is now under investigation, the main reaction for the formation of these products is presumed to be the consecutive oxidation of the ethyl radical by an oxygen molecule. The rate constants kl, k2 and k3 can be combined to give the photolysis rate. The secondary photolysis of the primary products also takes place over longer reaction times, producing C, fragments, such as CO, methane and the formyl radical. Part of the CO, is expected to be produced from these fragments of the secondary photolysis of the main products, in particular acetaldehyde. The rate constant k, corresponds to such reactions. The formation of formic esters can be explained by the radical recombination of the formyl radical and the alkoxy radical_ The high radical concentration, generated by laser irradiation at high energy density, presumably favours such radical-radical recombination reactions. 3.4. Dependence of the product yields on pressure and fluid density Figure 7 shows the total pressure dependence of the product yields. The product yields are also plotted against the reduced density in Fig. 8. In
rate constants
41 atm (gas)
71 atm (supercritical)
ri (mol cme3 min-‘)
3.0 x 10-s
6.2x10-’
WWI +&+B) M& +&+&I W& +k, +b)
0.15 0.40 0.45
0.21 0.72 0.07
kr (min-‘)
2.1 x10-2
2.1 x10-2
k5 +k, (min-‘)
3.0 x 10-J
8.9~ lo-’
Pressure [atm] Fig. 7. Dependence of the product yields on the reaction pressure at 318 K. Each symbol corresponds to the same product as shown in Fig. 4.
443
Reduced density ( P’/P 3 [-] Fig. 8. Dependence of the product yields on the reduced density at 318 K. Each symbol corresponds to the same product as shown in Fig. 4.
these experiments, the partial pressure of 0, was lixed to 4 atm and the total pressure was varied between 12 and 91 atm. The total product yield, as well as those of acetaldehyde and ethanol, greatly increase in the neighbourhood of the critical pressure, while that of CO, increases linearly. The behaviour of the total product yield as a function of the reduced density suggests that the initial photoabsorption by the ethane0, mixture is strongly affected by the fluid density. The discontinuity of the yield of partially oxidized products (acetaldehyde and ethanol) near the critical pressure, which is evident for both the pressure (Fig. 7) and the reduced density {Fig. S), suggests that the high density and the characteristic structure of the supercritical fluid affect the reaction process. The high local density or “cage effect”, due to cluster formation surrounding the reactants or intermediates in the neighbourhood of the critical point, may affect the reaction mechanism, which results in the discontinuous change in the overall branching ratio. 3.5. hitiafion reaction Since no product was obtained under the same reaction conditions without laser irradiation, the initial elementary reaction is considered to be a photoinduced reaction, even though ethane and O2 in the gas phase (1 atm) have been reported to exhibit negligibly small absorption coefficients near 248 nm. As shown in Fig. 7, the total product yield increases with an increase in total pressure in the neighbourhood of the critical pressure. Figure 9 shows the dependence of (1,~-r)M, on the total pressure of the ethane-0, mixture at
.uo Pressure [atm]
Fig. 9. Dependence of (1,-1)/10 on the pressure of the ethane-O2 mixture at 248 nm. Partial pressure of O2 is constant (5 kg cm-‘).
248 run. This indicates that (I,-T)/I, also shows a discontinuous increase at around 50 atm, and no light can be detected at the end of the reaction cell when the pressure exceeds 60 atm. A similar pressure dependence was observed in the pure ethane (without 02) system, and no products were detected after irradiation (248 mu) of pure ethane at high density. These experimental observations suggest that the pressure dependence of the total product formation in the ethane-0, mixture is due to the dispersion of light. Under supercritical conditions, the light will be dispersed by enhanced fluctuation of the local density in the supercritical fluid, which makes the path length of the light larger, and consequently the light can be effectively absorbed by species exhibiting very small absorption coefficients. What type of species absorbs the 248 nm light to initiate the reaction? It is very difficult to identify the species responsible, but O2 seems to play an important role in the photoinitiation step, because no photoinitiated reaction takes place in the absence of 0,. There are several possibilities for the initiation reaction, e.g. increase in the 0, absorption coefficient, red shift of the OZ absorption band and photoabsorption by a charge transfer (CT) complex of ethane and 0,. Shurlock and Ogilby [8] have reported that singlet molecular oxygen (‘As), formed on irradiation of an oxygen-organic molecule (such as aromatic and aliphatic hydrocarbons), produces a CT absorption band in the liquid phase. Another possibility is the photoabsorption by an 02-0, dimer, or the reaction of O2 with an excited oxygen molecule. In either case, the products are presumed to be an oxygen atom and ozone. The mechanism of the initial
444
K &u&i et al. I Loser-induced
oxidution reactions in supercritical ethane-Ol
photoreaction has not yet been identified in detail, but the unique absorption behaviour combined with the dispersion by the high-density fluid should be noted.
behaviour of the total product yield as a function of the reduced density is suggested to be due to the dispersion of light. Acknowledgment
4. Conclusions As a model of radical reactions in the supercritical phase, we investigated the laser-induced oxidation of ethane with 02, and the results can be summarized as follows. (1) The dependence of the product distribution on the reaction conditions was clarified experimentally. Ethanol, acetaldehyde and CO*, together with small amounts of CH4, CO and formic ester, were produced irrespective of the reaction pressure. (2) The time dependence of the product yields was studied. To fit the experimental results to the theoretical model, the branching ratio in the primary process and the rate of secondary photolysis of the products were quantitatively estimated. (3) The dependence of the product yields on the total pressure suggests that the selectivities for ethanol and acetaldehyde show a discontinuous change near the critical point, The characteristic
This work was financially supported by a Research Grant for Promotion of Science from the Ministry of Education, Culture and Science of Japan (Grant No. 04238103). References K.P. Johnston and C. Haynes, AIChE J., 35 (12) (1987) 2017. E.R. Blyumberg, Z.K. Maizus and N.M. Emanuel, The Oxidation of Hydrocarbons in theLiquid Phase, Macmillan, New York, 1965. B. Subramanian and M.A. McHugh, Ind. Eng. Process Des. Dev., 25 (1986) 1. H.J. Baumgartner, European Patent Application, EP 76533, 1983. Y. Oshima, M. Saito, S. Koda and H. Tominaga, Nippon figakukatihi, 5 (1989) 588. J.M. Prausnitz and P.L. Chueh, Computer Calcuiotion for High-Pressure Vapor-L@idEquiiibniz, Prentice-Hall, NJ, 1968. J.G. Calvart and J.N. Pitts, Jr., Photochemistry, Wiley, New York, 1966. R.D. Shurlock and P.R. Ogilby,J. Am. Chem. Sac., 110 (1988) 640.