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Heterogeneous photocatalysis: Photo-oxidation of methylbutanols
Heterogeneous A.
Laboratoire
WALKEIL,~
Photo-oxidation
Photocatalysis: M.
FOH~IENTI,
1’.
MEHIAUBEAU,
of Methylbutanols AAX
S.
J.
TEICHNER
de ‘I’hermodynamique et Cine’tiqlte ChimiqlLes, L. A. CNRS no %Tf, Universitk Claude Bernard-Lyon I, ~$3Boulevard da 11 Novembre 1918, 69621 Villeurbanne, France
Received October 20, 1976; revised May 23, 1977 Secondary and tertiary methylbutanols are photo-oxidized on nonporous TiOz (anatase) via an olefin, which is formed by dehydration, to give an aldehyde or a ketone. Primary met,hylbutanol is photo-oxidized without previous dehydration. The mechanism of the photo-oxidation of paraffins advanced previously is completed. EXPERIMENTAL
INTRODUCTION
METHODS
The partial photo-oxidation of paraffins on titanium dioxide under uv irradiation, producing ketones and aldehydes, has been widely investigated in this laborat,ory (I-6). An intermediate formation of an alcohol by the addition of atomic oxygen to the paraffin was postulated (3). The alcohol may be either directly oxidized into an aldehyde or ketone or dehydrated into an olefin which, in turn, is oxidized at the ethylenic bond into aldehydes (primary carbon) or ketones (secondary carbon). Because of the possibility of an alcohol undergoing these two types of reaction, either a direct oxidation, or, first, a dehydration, it was considered worthwhile to investigate the photo-oxidation of various methylbutanols (primary, secondary, and tertiary) in order to elucidate this part of the mechanism of the photo-oxidation of paraffins. The oxidation of alcohols on titanium dioxide under uv irradiation in a static system was mentioned a few years ago (6, ‘Q, but its mechanism was investigated only in the case of a secondary alcohol. 1 Present address: University County, Republic of Ireland.
The differential photocatalytic reactor used in this work was described previously (2, 3). The effluents were analyzed by gas chromatography. A mercury lamp (Philips Hl’K 1%) was used as a uv source. The catalyst \vas a nonporous titania powder (anatasc) prepared in an oxygen-hydrogen flame reactor (S), and it presented a surface arca of 70 m2 g-l. The temperature of the rcactor was 95°C and, as for paraffins (3, 4), the photocatalytic activity was a linc>ar function of the mass of the catalyst (up to a certain limit of the order of 20 mg), spread out in the reactor as a thin uniform layer 50 mm in diameter. For all experiments, S mg of TiOz was used, and, except where otherwise stated, the composition of the feed at atmospheric pressure was SO’% oxygen and 20% of a mixture of helium and alcohol vapor. This last variable was adjusted to a few values in order to study the influence of the parOia1 pressure of alcohol upon photocatalytic activity. The flow rate of the mixture of reagents was fixed at 40 cm3 min-I. The measurements of the activity were carried out after the achievement of a stationary state of the of Cork, Cork reaction (ca. 30 min). 237
Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in rtny form reserved.
ISSN 0021-9517
WALKER ET MT,.
2.38
Pressure of alcohol or- of &fin (tort-l
Fro. 1. Total conversion at 95°C of primary, secondary, and tertiary methylbut~anols and of 2-met)hyl-2-hutene, as a fun&ion of t,he partial pressure of the reagent. RESULTS
AND DISCUSSION
For the three methylbutanols considered (primary : 3-methyl-l-butanol ; secondary : 3-methyl-2-butanol ; tertiary : 2-methyl-2butanol), the total conversion (including the formation of COz and HzO) at 95°C as a function of their partial pressure, for a constant pressure of oxygen (600 Torr), is given in Fig. 1. The ease of oxidation of these alcohols, thus, follows the sequence: secondary > tertiary > primary. This sequence is different from that obtained in the case of the oxidation of paraffins: tertiary > secondary > primary carbon atoms (4). But for paraffins the oxidation involves one step more than for alcohols, namely, the insertion of an oxygen atom into the chain of the paraffin to form an alcohol. The ease of this insertion for various carbon atoms (primary, secondary, tertiary) may be different from the ease of oxidation of alcohols. Figure 1 also shows experimental data concerning conversion of the olefin (2-methyl-2-butene), which would be the dehydration product of the tertiary alcohol (see below). The decrease of the total conversion with increasing partial pressure of the alcohol is due to a decrease in the formation of COz and Hz0 (total oxidation) when the proportion of oxygen in the feed decreases. The mechanism of the partial oxidation reaction was traced
through the distribution of the products obtained from the three alcohols. I. Tertiary Alcohol For 2-methyl-2-butanol three partial oxidation products were detected : acetone, ethanal, and 2-butanone. The partial pressures of these products are plotted in Fig. 2 as a function of the partial pressure of the alcohol. This diagram, therefore, represents the relative catalytic activity (and the selectivity) when the feed is enriched with alcohol at a constant pressure of oxygen. It also gives information on the partial order (with respect to the alcohol) of the reaction. The initial, almost linear, relationship (partial order 1) in Fig. 2, followed by the horizontal (partial order 0) portion, would be interpreted as indicative of a Langmuir-Hinshelwood mechanism with no competition between reagents in the adsorbed state. However, for a smaller oxygen partial pressure (380 Torr), a maximum in the rate becomes visible, as the rate decreases when the partial pressure of the alcohol increases (Fig. 3), giving evidence of a competitive adsorption of reagents, alcohol now displacing oxygen. Figure 4 indicates that the influence of oxygen pressure on the products (ethanal and acetone) for a constant alcohol pressure is the same as that mentioned for the photo-oxidation
PHOTOCATALYTIC Pressure
of
239
OXIDSTION
products
(torr)
0 -acetone .
- ethanal
A - 2- butanone
Pressure (torr)
FIG. 2. The rate of formation of products of the partial as a function of the partial pressure of the alcohol.
of alkanes (5). This kinetic behavior is in favor of a Langmuir-Hinshelwood mechanism with competitive adsorption, rather than a “redox” mechanism (9) or stationary state oxygen-adsorption mechanism (SSA Model) (10). The first step in the oxidation of a tertiary alcohol should be its dehydration into an olefin which is then oxidized (4). It was indeed shown that, under uv irradiation on TiOz, even in the presence of Pressure
of alcohol
oxidation
of 2-methyl-2-butanol
(80% 02)
oxygen, alcohols may be dehydrated before their oxidation (11). The distribution into partial oxidation products (Figs. 2 and 3) is easily explained if the dehydration of the alcohol is the first step. The following scheme accounts for the results.
F
C-C-C-C AH
-H,O -
c C-C=C-C
‘5
7
o,
c=c-c-c
-
-+
C yC=O
+ C-CHO
7 o=c-c-c
+
co,
of products (brr)
\
.¶
1 h
0
10.0
20.0
30.0 Pressure
FIG. 3. The rate of formation of products of the partial as a function of the partial pressure of the alcohol.
50.0
40.0
oxidation
of
alcohol
(tow)
of 2-methyl-2-butanol
(50% OS)
240
WALKER
0
oxygen 0
150
300
FIG. 4. The rate of formation function
ET AL.
of the partial
of products pressure of oxygen.
of the partial
According to this scheme the amount of acetone should be equal to the amount of ethanal. This is not exactly the case in Figs. 2 and 3. However, the fact that ketones are stable under the experimental conditions used in this work, while aldehydes are less stable and are partially converted into lower compounds and COZ was tested. It was checked that formed (or injected) water vapour is not an inhibitor of the reaction. The ratio of acetone to 2-butanone reflects the ease of dehydration of the tertiary alcohol into two isomeric olefins, as shown Pressure
of products
pressure
450
(brr) 600
oxidation
’
of Z-methyl-2-butanol
as a
in the previous scheme. This ratio is of the order of 10 which is very close to the ratio of corresponding olefins obtained by conventional thermal dehydration of 2-methyl2-butanol. In order to check the proposed mechanism (olefin intermediate) the corresponding olefins, 2-methyl-2-butene and 2-methyl1-butene (see the previous scheme), were fed into the reactor under the same experimental conditions as used for the tertiary alcohol. The total conversion of one of the olefins (2-methyl-2-butene) is shown in Fig. 1. Its reactivity is smaller than that
&orrI
. 0Ab-
ethanal acetone 3-methyl 2 methyl
6.0
FIG. 5. The rate of formation of products of the partial as a function of the partial pressure of the alcohol.
2-butonone propond
70 Pressure
oxidation
8.0 9.0 of alcohol (torr)
of 3-methyl-Z-butauol
(80% 02)
PHOTOCATALYTIC
of the tertiary alcohol. However, the same partial oxidation products were obtained as previously for the tertiary alcohol. The decreased reactivity of the olefin may be explained by its decreased adsorption coefficient on TiOa, compared to that of the alcohol. II. Secondary Alcohol The 3-methyl-2-butanol is oxidized mainly into ethanal and acetone, but small amounts of 3-methyl-2-butanone and 2-methylpropans1 are also recorded (Fig. 5). The formation of the ketone is the result of direct oxidation of the secondary alcohol, whereas the formation of other products can be only explained by previous dehydration of the alcohol, according to the following scheme. -H,O 7 F c-c-c-c - c-c-c-c
0, +
;:
C-ECHO
explanation of these discrepancies would imply a different reactivity of ethanal toward oxygen (giving lower compounds) in the presence of a tertiary or secondary alcohol. The following experiments were performed to check this hypothesis. The tertiary alcohol (10 Torr) was photooxidized as done previously, but in the presence of 10 Torr of ethanal. The magnitude of the COZ peak was twice as large as in the case of the oxidation of the same alcohol without ethanal. The secondary alcohol oxidized in the presence of ethanal showed a CO2 peak of the same order of magnitude (X 1.1) as in the oxidation of this alcohol without ethanal. Therefore, these results confirm the hypothesis of the enhanced ability of ethanal to be oxidized in the presence of the tertiary alcohol, in comparison to its reactivity in the presence of the secondary alcohol.
+ CO,
III. c-c=c-c-
$c=o
241
OXIDATION
+
CXHO
The dehydration of the alcohol into 2-methyl-2-butene proceeds roughly 20 times faster than the dehydration into 3-methyl-1-butene involving the primary carbon atom. This figure may be calculated from the relative activities for the conversion into acetone and 2-methyl-propanal shown in Fig. 5. On the other hand, the ratio of acetone to 3-methyl-2-butanone (16: 1) shows that the main reaction path involves the dehydration of the secondary alcohol, rather than its direct oxidation into a ketone. The amount of ethanal is almost equal to that of acetone (Fig. 5), whereas, for the tertiary alcohol, a deficit of ethanal was observed (Figs. 2 and 3). Now, the mechanism for formation of acetone and ethanal would be the same for both alcohols, tertiary and secondary (see the schemes), and one would expect an equal proportion of these two products from both alcohols. One
Primary
Alcohol
The conversion of 3-methyl-1-butanol is lower than that of the two other alcohols (Fig. 1). The main product is 3-methyl-lbutanal (62%) which is formed by a direct oxidation. The possibility of dehydration of the primary alcohol followed by oxidation of the olefin was envisaged, as before, but it was discarded for the following reasons. The other reaction products are 2-methyl-1-propanal (la%), acetone (19%) and ethanal (Ga/,). Only 2-methyl-l-propanal could be formed by oxidation of the olefin resulting from the dehydration of the primary alcohol. Now, direct oxidation of 3-methyl-1-butanal under the same experimental conditions gives 2-methyl-lpropanal, acetone, and ethanal, roughly in t.he same proportions. Therefore, for the primary alcohol, apparently no dehydration step occurs under these conditions, and t,he degradation product,s (lower aldehydes and acetone) are formed by a direct oxidation of alcohol.
242
WALKER
ET AL.
Temperature 100
FIG. 6. Total conversion of 2-methyl-2-butanol the reaction.
150
OF the reactor 200 ’ (’ Cl
(20% 02) as a function of the temperature of
02- (and, by interaction with positive holes, of atomic oxygen) becomes unnecessary. It was indeed shown (5, S) that alcohols may It was shown that, in the case of paraffins be photo-oxidized in the absence of molecu(2, S), the efficient wavelength range of uv lar oxygen, by oxygen from the lattice, this irradiation has an upper limit of 360 nm, interaction still requiring uv energy equal corresponding to the width of the forbidden to the width of the forbidden band of TiOz band of titanium dioxide. The same condi(electrons and positive holes) (5). Theretions also prevail for the photo-oxidation fore, the activation of the surface lattice of of alcohols, giving evidence that electrons the catalyst by uv irradiation allows the and positive holes are simultaneously re- oxidation of alcohols by lattice oxygen quired for the process (5). However, a which is refilled by oxygen from the gas different behavior is observed concerning phase. The lattice oxygen is unable to the temperature threshold above which photo-oxidize paraffins (S), and the &(ads)the reaction stops. In the case of paraffins, species formed from molecular oxygen are this upper limit of the temperature range required. If, however, the “redox” mechawas about 15O”C, and it was correlated nism does not apply for alcohols (see with the vanishing of 02(ads)- species formed above), it is probably because the stationthrough the irradiation of TiOz in the pres- ary state of the oxidation-reduction of the ence of oxygen (2). For the tertiary alcohol, surface is not achieved due to the chemithe reaction still has a high efficiency at sorption of the alcohol (Fig. 3). 200°C (Fig. 6) and probably continues at In summary, these results tend to demoneven higher temperatures. The selectivity strate that, in the photo-oxidation of parafinto partial oxidation products (2-butanone, fins, the main step is formation of the acetone, ethanal) remains the same throughalcohol requiring atomic oxygen, probably out the whole temperature range in Fig. 6. formed by interaction of 02(a&)- species The reactivity of paraffins, correlated with with positive holes (5) ; whereas, in the the presence of 02(&)-, positive holes, and photo-oxidation of alcohols, the activation finally atomic 0 (5), accounts for the first of lattice oxygen by uv irradiation allows step of photo-oxidation, which is the addieither a direct oxidation of the alcohol or, tion of 0 to the paraffin to form an alcohol. first, its dehydration on some acid sites, Since this step is no longer required in the probably formed by the departure of lattice oxygen (unscreened Ti4f ions), followed by photo-oxidation of alcohols, the presence of
IV.
Comparison between Photo-oxidation Alcohols and Paraflins
of
PHOTOCATALYTIC
the oxidation of the olefin resulting thermal dehydration.
from
CONCLUSIONS
The photo-oxidation of methylbutanols on TiOz requires oxygen (probably from the lattice) activated by uv irradiation and proceeds mainly, for secondary and tertiary alcohols, via olefins which result from a dehydration step. ACKNOWLEDGMENT The authors are grateful to the CNRS for its financial support under Contract A.T.P. No. 2045. REFERENCES 1. Formenti, M., Juillet, F., and Teichner, S. J., C. R. Acad. Sci. 27OC, 138, (1970). 2. Formenti, M., Juillet, F., Mbriaudeau, P., and Teichner, S. J., Chem. !Z’echnoZ. 1, 680 (1971).
OXIDATION
243
S. Formenti, M., Juillet, F., Mbriaudeau, P., and Teichner, S. J., in “Proceedings, 5th International Congress on Catalysis, Miami Beach, August 20-26, 1972” (J. W. Hightower, Ed.), p. 1011. North-Holland, Amsterdam, 1973. 4. Djeghri, N., Formenti, M., Juillet, F., and Teichner, S. J., Faraday Discuss. Chem. Sot. 58, 185 (1974). 6. Formen& M., Juillet, F., and Teichner, S. J., RulE. Sot. Chim. FT., 1031 (1976). 6. Filimonov, V. N., Dokl. Akad. Nauk. SSSR 154, 922 (1964). 7. Bickley, R. I., Munuera, G., and Stone, F. S., J. Catal. 31, 398 (1973). 8. Juillet, F., Lecomte, F., Mozzanega, H., Teichner, S. J., Thevenet, A., and Vergnon P., Faraday Discuss. Chem. Sot. 7, 57 (1973). 9. Mars, P., and Van Krevelen, W., Chem. Eng. Sci. Spec. Suppl. 9, 41 (1954). 10. Shelstad, K. A., Downie, J., and Graydon, W. F., Canad. J. Chem. Eng. 35, 102 (1960) ; 39, 201 (1961). il. Cunningham, J., and MBriaudeau, P., J. Chem. Sot. Faraday I’rans. I 72, 1499, (1976).
Laboratoire
WALKEIL,~
Photo-oxidation
Photocatalysis: M.
FOH~IENTI,
1’.
MEHIAUBEAU,
of Methylbutanols AAX
S.
J.
TEICHNER
de ‘I’hermodynamique et Cine’tiqlte ChimiqlLes, L. A. CNRS no %Tf, Universitk Claude Bernard-Lyon I, ~$3Boulevard da 11 Novembre 1918, 69621 Villeurbanne, France
Received October 20, 1976; revised May 23, 1977 Secondary and tertiary methylbutanols are photo-oxidized on nonporous TiOz (anatase) via an olefin, which is formed by dehydration, to give an aldehyde or a ketone. Primary met,hylbutanol is photo-oxidized without previous dehydration. The mechanism of the photo-oxidation of paraffins advanced previously is completed. EXPERIMENTAL
INTRODUCTION
METHODS
The partial photo-oxidation of paraffins on titanium dioxide under uv irradiation, producing ketones and aldehydes, has been widely investigated in this laborat,ory (I-6). An intermediate formation of an alcohol by the addition of atomic oxygen to the paraffin was postulated (3). The alcohol may be either directly oxidized into an aldehyde or ketone or dehydrated into an olefin which, in turn, is oxidized at the ethylenic bond into aldehydes (primary carbon) or ketones (secondary carbon). Because of the possibility of an alcohol undergoing these two types of reaction, either a direct oxidation, or, first, a dehydration, it was considered worthwhile to investigate the photo-oxidation of various methylbutanols (primary, secondary, and tertiary) in order to elucidate this part of the mechanism of the photo-oxidation of paraffins. The oxidation of alcohols on titanium dioxide under uv irradiation in a static system was mentioned a few years ago (6, ‘Q, but its mechanism was investigated only in the case of a secondary alcohol. 1 Present address: University County, Republic of Ireland.
The differential photocatalytic reactor used in this work was described previously (2, 3). The effluents were analyzed by gas chromatography. A mercury lamp (Philips Hl’K 1%) was used as a uv source. The catalyst \vas a nonporous titania powder (anatasc) prepared in an oxygen-hydrogen flame reactor (S), and it presented a surface arca of 70 m2 g-l. The temperature of the rcactor was 95°C and, as for paraffins (3, 4), the photocatalytic activity was a linc>ar function of the mass of the catalyst (up to a certain limit of the order of 20 mg), spread out in the reactor as a thin uniform layer 50 mm in diameter. For all experiments, S mg of TiOz was used, and, except where otherwise stated, the composition of the feed at atmospheric pressure was SO’% oxygen and 20% of a mixture of helium and alcohol vapor. This last variable was adjusted to a few values in order to study the influence of the parOia1 pressure of alcohol upon photocatalytic activity. The flow rate of the mixture of reagents was fixed at 40 cm3 min-I. The measurements of the activity were carried out after the achievement of a stationary state of the of Cork, Cork reaction (ca. 30 min). 237
Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in rtny form reserved.
ISSN 0021-9517
WALKER ET MT,.
2.38
Pressure of alcohol or- of &fin (tort-l
Fro. 1. Total conversion at 95°C of primary, secondary, and tertiary methylbut~anols and of 2-met)hyl-2-hutene, as a fun&ion of t,he partial pressure of the reagent. RESULTS
AND DISCUSSION
For the three methylbutanols considered (primary : 3-methyl-l-butanol ; secondary : 3-methyl-2-butanol ; tertiary : 2-methyl-2butanol), the total conversion (including the formation of COz and HzO) at 95°C as a function of their partial pressure, for a constant pressure of oxygen (600 Torr), is given in Fig. 1. The ease of oxidation of these alcohols, thus, follows the sequence: secondary > tertiary > primary. This sequence is different from that obtained in the case of the oxidation of paraffins: tertiary > secondary > primary carbon atoms (4). But for paraffins the oxidation involves one step more than for alcohols, namely, the insertion of an oxygen atom into the chain of the paraffin to form an alcohol. The ease of this insertion for various carbon atoms (primary, secondary, tertiary) may be different from the ease of oxidation of alcohols. Figure 1 also shows experimental data concerning conversion of the olefin (2-methyl-2-butene), which would be the dehydration product of the tertiary alcohol (see below). The decrease of the total conversion with increasing partial pressure of the alcohol is due to a decrease in the formation of COz and Hz0 (total oxidation) when the proportion of oxygen in the feed decreases. The mechanism of the partial oxidation reaction was traced
through the distribution of the products obtained from the three alcohols. I. Tertiary Alcohol For 2-methyl-2-butanol three partial oxidation products were detected : acetone, ethanal, and 2-butanone. The partial pressures of these products are plotted in Fig. 2 as a function of the partial pressure of the alcohol. This diagram, therefore, represents the relative catalytic activity (and the selectivity) when the feed is enriched with alcohol at a constant pressure of oxygen. It also gives information on the partial order (with respect to the alcohol) of the reaction. The initial, almost linear, relationship (partial order 1) in Fig. 2, followed by the horizontal (partial order 0) portion, would be interpreted as indicative of a Langmuir-Hinshelwood mechanism with no competition between reagents in the adsorbed state. However, for a smaller oxygen partial pressure (380 Torr), a maximum in the rate becomes visible, as the rate decreases when the partial pressure of the alcohol increases (Fig. 3), giving evidence of a competitive adsorption of reagents, alcohol now displacing oxygen. Figure 4 indicates that the influence of oxygen pressure on the products (ethanal and acetone) for a constant alcohol pressure is the same as that mentioned for the photo-oxidation
PHOTOCATALYTIC Pressure
of
239
OXIDSTION
products
(torr)
0 -acetone .
- ethanal
A - 2- butanone
Pressure (torr)
FIG. 2. The rate of formation of products of the partial as a function of the partial pressure of the alcohol.
of alkanes (5). This kinetic behavior is in favor of a Langmuir-Hinshelwood mechanism with competitive adsorption, rather than a “redox” mechanism (9) or stationary state oxygen-adsorption mechanism (SSA Model) (10). The first step in the oxidation of a tertiary alcohol should be its dehydration into an olefin which is then oxidized (4). It was indeed shown that, under uv irradiation on TiOz, even in the presence of Pressure
of alcohol
oxidation
of 2-methyl-2-butanol
(80% 02)
oxygen, alcohols may be dehydrated before their oxidation (11). The distribution into partial oxidation products (Figs. 2 and 3) is easily explained if the dehydration of the alcohol is the first step. The following scheme accounts for the results.
F
C-C-C-C AH
-H,O -
c C-C=C-C
‘5
7
o,
c=c-c-c
-
-+
C yC=O
+ C-CHO
7 o=c-c-c
+
co,
of products (brr)
\
.¶
1 h
0
10.0
20.0
30.0 Pressure
FIG. 3. The rate of formation of products of the partial as a function of the partial pressure of the alcohol.
50.0
40.0
oxidation
of
alcohol
(tow)
of 2-methyl-2-butanol
(50% OS)
240
WALKER
0
oxygen 0
150
300
FIG. 4. The rate of formation function
ET AL.
of the partial
of products pressure of oxygen.
of the partial
According to this scheme the amount of acetone should be equal to the amount of ethanal. This is not exactly the case in Figs. 2 and 3. However, the fact that ketones are stable under the experimental conditions used in this work, while aldehydes are less stable and are partially converted into lower compounds and COZ was tested. It was checked that formed (or injected) water vapour is not an inhibitor of the reaction. The ratio of acetone to 2-butanone reflects the ease of dehydration of the tertiary alcohol into two isomeric olefins, as shown Pressure
of products
pressure
450
(brr) 600
oxidation
’
of Z-methyl-2-butanol
as a
in the previous scheme. This ratio is of the order of 10 which is very close to the ratio of corresponding olefins obtained by conventional thermal dehydration of 2-methyl2-butanol. In order to check the proposed mechanism (olefin intermediate) the corresponding olefins, 2-methyl-2-butene and 2-methyl1-butene (see the previous scheme), were fed into the reactor under the same experimental conditions as used for the tertiary alcohol. The total conversion of one of the olefins (2-methyl-2-butene) is shown in Fig. 1. Its reactivity is smaller than that
&orrI
. 0Ab-
ethanal acetone 3-methyl 2 methyl
6.0
FIG. 5. The rate of formation of products of the partial as a function of the partial pressure of the alcohol.
2-butonone propond
70 Pressure
oxidation
8.0 9.0 of alcohol (torr)
of 3-methyl-Z-butauol
(80% 02)
PHOTOCATALYTIC
of the tertiary alcohol. However, the same partial oxidation products were obtained as previously for the tertiary alcohol. The decreased reactivity of the olefin may be explained by its decreased adsorption coefficient on TiOa, compared to that of the alcohol. II. Secondary Alcohol The 3-methyl-2-butanol is oxidized mainly into ethanal and acetone, but small amounts of 3-methyl-2-butanone and 2-methylpropans1 are also recorded (Fig. 5). The formation of the ketone is the result of direct oxidation of the secondary alcohol, whereas the formation of other products can be only explained by previous dehydration of the alcohol, according to the following scheme. -H,O 7 F c-c-c-c - c-c-c-c
0, +
;:
C-ECHO
explanation of these discrepancies would imply a different reactivity of ethanal toward oxygen (giving lower compounds) in the presence of a tertiary or secondary alcohol. The following experiments were performed to check this hypothesis. The tertiary alcohol (10 Torr) was photooxidized as done previously, but in the presence of 10 Torr of ethanal. The magnitude of the COZ peak was twice as large as in the case of the oxidation of the same alcohol without ethanal. The secondary alcohol oxidized in the presence of ethanal showed a CO2 peak of the same order of magnitude (X 1.1) as in the oxidation of this alcohol without ethanal. Therefore, these results confirm the hypothesis of the enhanced ability of ethanal to be oxidized in the presence of the tertiary alcohol, in comparison to its reactivity in the presence of the secondary alcohol.
+ CO,
III. c-c=c-c-
$c=o
241
OXIDATION
+
CXHO
The dehydration of the alcohol into 2-methyl-2-butene proceeds roughly 20 times faster than the dehydration into 3-methyl-1-butene involving the primary carbon atom. This figure may be calculated from the relative activities for the conversion into acetone and 2-methyl-propanal shown in Fig. 5. On the other hand, the ratio of acetone to 3-methyl-2-butanone (16: 1) shows that the main reaction path involves the dehydration of the secondary alcohol, rather than its direct oxidation into a ketone. The amount of ethanal is almost equal to that of acetone (Fig. 5), whereas, for the tertiary alcohol, a deficit of ethanal was observed (Figs. 2 and 3). Now, the mechanism for formation of acetone and ethanal would be the same for both alcohols, tertiary and secondary (see the schemes), and one would expect an equal proportion of these two products from both alcohols. One
Primary
Alcohol
The conversion of 3-methyl-1-butanol is lower than that of the two other alcohols (Fig. 1). The main product is 3-methyl-lbutanal (62%) which is formed by a direct oxidation. The possibility of dehydration of the primary alcohol followed by oxidation of the olefin was envisaged, as before, but it was discarded for the following reasons. The other reaction products are 2-methyl-1-propanal (la%), acetone (19%) and ethanal (Ga/,). Only 2-methyl-l-propanal could be formed by oxidation of the olefin resulting from the dehydration of the primary alcohol. Now, direct oxidation of 3-methyl-1-butanal under the same experimental conditions gives 2-methyl-lpropanal, acetone, and ethanal, roughly in t.he same proportions. Therefore, for the primary alcohol, apparently no dehydration step occurs under these conditions, and t,he degradation product,s (lower aldehydes and acetone) are formed by a direct oxidation of alcohol.
242
WALKER
ET AL.
Temperature 100
FIG. 6. Total conversion of 2-methyl-2-butanol the reaction.
150
OF the reactor 200 ’ (’ Cl
(20% 02) as a function of the temperature of
02- (and, by interaction with positive holes, of atomic oxygen) becomes unnecessary. It was indeed shown (5, S) that alcohols may It was shown that, in the case of paraffins be photo-oxidized in the absence of molecu(2, S), the efficient wavelength range of uv lar oxygen, by oxygen from the lattice, this irradiation has an upper limit of 360 nm, interaction still requiring uv energy equal corresponding to the width of the forbidden to the width of the forbidden band of TiOz band of titanium dioxide. The same condi(electrons and positive holes) (5). Theretions also prevail for the photo-oxidation fore, the activation of the surface lattice of of alcohols, giving evidence that electrons the catalyst by uv irradiation allows the and positive holes are simultaneously re- oxidation of alcohols by lattice oxygen quired for the process (5). However, a which is refilled by oxygen from the gas different behavior is observed concerning phase. The lattice oxygen is unable to the temperature threshold above which photo-oxidize paraffins (S), and the &(ads)the reaction stops. In the case of paraffins, species formed from molecular oxygen are this upper limit of the temperature range required. If, however, the “redox” mechawas about 15O”C, and it was correlated nism does not apply for alcohols (see with the vanishing of 02(ads)- species formed above), it is probably because the stationthrough the irradiation of TiOz in the pres- ary state of the oxidation-reduction of the ence of oxygen (2). For the tertiary alcohol, surface is not achieved due to the chemithe reaction still has a high efficiency at sorption of the alcohol (Fig. 3). 200°C (Fig. 6) and probably continues at In summary, these results tend to demoneven higher temperatures. The selectivity strate that, in the photo-oxidation of parafinto partial oxidation products (2-butanone, fins, the main step is formation of the acetone, ethanal) remains the same throughalcohol requiring atomic oxygen, probably out the whole temperature range in Fig. 6. formed by interaction of 02(a&)- species The reactivity of paraffins, correlated with with positive holes (5) ; whereas, in the the presence of 02(&)-, positive holes, and photo-oxidation of alcohols, the activation finally atomic 0 (5), accounts for the first of lattice oxygen by uv irradiation allows step of photo-oxidation, which is the addieither a direct oxidation of the alcohol or, tion of 0 to the paraffin to form an alcohol. first, its dehydration on some acid sites, Since this step is no longer required in the probably formed by the departure of lattice oxygen (unscreened Ti4f ions), followed by photo-oxidation of alcohols, the presence of
IV.
Comparison between Photo-oxidation Alcohols and Paraflins
of
PHOTOCATALYTIC
the oxidation of the olefin resulting thermal dehydration.
from
CONCLUSIONS
The photo-oxidation of methylbutanols on TiOz requires oxygen (probably from the lattice) activated by uv irradiation and proceeds mainly, for secondary and tertiary alcohols, via olefins which result from a dehydration step. ACKNOWLEDGMENT The authors are grateful to the CNRS for its financial support under Contract A.T.P. No. 2045. REFERENCES 1. Formenti, M., Juillet, F., and Teichner, S. J., C. R. Acad. Sci. 27OC, 138, (1970). 2. Formenti, M., Juillet, F., Mbriaudeau, P., and Teichner, S. J., Chem. !Z’echnoZ. 1, 680 (1971).
OXIDATION
243
S. Formenti, M., Juillet, F., Mbriaudeau, P., and Teichner, S. J., in “Proceedings, 5th International Congress on Catalysis, Miami Beach, August 20-26, 1972” (J. W. Hightower, Ed.), p. 1011. North-Holland, Amsterdam, 1973. 4. Djeghri, N., Formenti, M., Juillet, F., and Teichner, S. J., Faraday Discuss. Chem. Sot. 58, 185 (1974). 6. Formen& M., Juillet, F., and Teichner, S. J., RulE. Sot. Chim. FT., 1031 (1976). 6. Filimonov, V. N., Dokl. Akad. Nauk. SSSR 154, 922 (1964). 7. Bickley, R. I., Munuera, G., and Stone, F. S., J. Catal. 31, 398 (1973). 8. Juillet, F., Lecomte, F., Mozzanega, H., Teichner, S. J., Thevenet, A., and Vergnon P., Faraday Discuss. Chem. Sot. 7, 57 (1973). 9. Mars, P., and Van Krevelen, W., Chem. Eng. Sci. Spec. Suppl. 9, 41 (1954). 10. Shelstad, K. A., Downie, J., and Graydon, W. F., Canad. J. Chem. Eng. 35, 102 (1960) ; 39, 201 (1961). il. Cunningham, J., and MBriaudeau, P., J. Chem. Sot. Faraday I’rans. I 72, 1499, (1976).