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Formation of dihydrogen and carboxylic acid from water and aldehyde over metal oxide catalysts
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APPLIED
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CATALYSIS
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A: GENERAL
ELSEVIER
Applied Catalysis A: General 125 (1995) 159-167
Formation of dihydrogen and carboxylic acid from water and aldehyde over metal oxide catalysts Toshiharu Yokoyama *, Naoko Fujita, Takao Maki Yokohama Research Center, Mitsubishi Chemieal Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227, Japan Received 12 September 1994; revised 30 November 1994; accepted 22 December 1994
Abstract The formation of dihydrogen and benzoic acid was observed from water vapor and benzaldehyde over Cr 3 ~ -promoted ZrO2, ZrO2 and Cr203 catalysts. Cr ~+-promotedZrO2 was the most active catalyst tbr this reaction and an equimolecular amount o f dihydrogen and benzoic acid was obtained. Dihydrogen and the corresponding carboxylic acids were also obtained from the reaction of 3-phenoxybenzaldehyde or 1-heptanal with water. It is suggested that a surface carboxylate species formed by aldehyde adsorption is an intermediate of these reactions. Keywords." Benzoic acid; Chromium promotion; Hydrogen; Zirconia
1. Introduction We have previously demonstrated that aromatic carboxylic acids can be hydrogenated to the corresponding aldehydes in good yield using a promoted ZrO2 catalyst [ 1 ]. It is important to study the mechanism of this reaction and several papers have been published on this subject [ 1-3]. While studying this reaction, we found that the reaction rate was significantly affected by water vapor [4]. This suggested that a strong interaction of the catalyst surface with water vapor exists during the reaction. We examined the influence of water on the hydrogenation reaction of carboxylic acid in the presence of reaction products. We found that carboxylic acid and dihydrogen were formed by the reaction between aldehyde and water vapor on the catalyst at elevated temperature, as shown by Eq. ( 1 ). * Corresponding author. E-mail [email protected], tel. ( + 81-45) 9633197, fax. ( + 81-45) 9633974. 0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO926-860X( 95 ) 0 0 0 0 3 - 8
T. Yokoyama et al. /Applied Catalysis A." General 125 (1995) 15~167
160
Table 1 Dihydrogen evolution from water on Cr ~ +-promoted ZrO_, catalyst Reaction conditions: N2 GHSV = 200 h ~, benzaldehyde feed rate catalyst volume = 5 cm ~, 400°C, under atmospheric pressure Reactant
H20 + benzaldehyde H~O benzaldehyde
ArCHO +
Conversion of benzaldehyde
43.2 9.0
H20
Selectivity to acid / %
95.7 88. I
-->
ArCOOH + H2
7.5 mmol/h, HeO feed rate = 110 mmol/h,
Formation rate (mmol h) Acid
H:
3.4 0.18
3.1 0 0.43
H2/acid tool ratio
0.9 I 2.40
( 1)
The present paper describes the study of this dihydrogen evolution reaction over metal oxide catalysts.
2. Experimental The catalysts used in this study were Cr 3 +-promoted ZrOz, ZrO2 and Cr203. The Cr 3 +-promoted ZrOz catalyst (Cr/Zr = 5/100 atomic ratio) was prepared from zirconyl hydroxide and chromium nitrate as previously described in detail [1 ]. ZrO2 and CrzO3 were prepared from the hydroxide. These catalysts were calcined at 700°C for 3 h in an air stream. Benzaldehyde (Kishida, 98%), 3-phenoxybenzaldehyde (Aldrich, > 95%) and l-heptanal (Tokyo Kasei, > 95%) were used without further purification. Reactions were carried out under atmospheric pressure in an ordinary flow system. The volume of the catalyst employed ( 10-20 mesh) was 5 or 10 cm 3. Nitrogen was used as a carrier gas. The reaction products were analyzed by gas-liquid chromatography (GLC) and titration.
3. Results 3.1. Reaction of benzaldehyde with water The reaction between benzaldehyde and water vapor over Cr 3 +-promoted ZrOz was studied first. The results are shown in Table 1. Benzaldehyde reacted with water, forming selectively benzoic acid and dihydrogen. Benzene was detected as the main by-product which was formed by decarboxylation. The molar ratio of dihydrogen to acid was nearly unity. The formation of the acid was fairly fast with a rate of 0.68 mmol/gc~,t h. The reaction rate was drastically decreased when water was absent, and only a small amount of dihydrogen was formed, probably because
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of coke formation from benzaldehyde. Dihydrogen was not detected in the absence of benzaldehyde. Simple water decomposition did also not occur. Fig. 1 shows the catalytic activity changes with time-on-stream. Changes in activity and selectivity during the reaction were not observed. These results show that benzaldehyde is oxidized to benzoic acid by water in the presence of Cr 3+-promoted ZrO2 catalyst. The catalytic activities of ZrO2 and Cr203 are shown in Table 2. Both catalysts showed low activity compared to the promoted Z r O 2. The correlation between surface area of the catalyst and the activity, which is expressed in terms of the formation rate of benzoic acid (mmol/h), is shown in Fig. 2. The catalytic activity increased linearly with increasing surface area. These results suggest that the enhancement of the catalytic activity of Cr 3+-promoted ZrO2 is mainly due to the increase in its surface area by addition of the chromium component, which is the same phenomenon as for the hydrogenation of benzoic acid. The temperature dependency of this reaction is shown in Fig. 3. The conversion of benzaldehyde increased linearly with increasing reaction temperature from 350 Table 2 Reaction of benzaldehyde with water on metal oxide catalysts. Reaction conditions: N~ GHSV = 200 h ~, benzaldehyde feed rate = 7.5 mmol/h, H20 feed rate= 110 retool/h, catalysts volume = 5 cm 3, 400°C, under atmospheric pressure Catalyst
Cr 3 +-modified ZrO2 ZrO2 Cr203
Surface area (m2/g)
Conversion of benzaldehyde (%)
Selectivity to acid (%)
Formation rate (mmol/h) Acid
Hz
H2/acid mol ratio
72.0
43.2
95.7
3.4
3.1
0.91
31.0 17.5
14.8 13.8
91.2 93.0
1.1 1.0
0.9 1.2
0.93 1.17
162
T. Yokoyama et all/Applied Catalysis AI" General 125 (1995) 159-167
0
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Fig. 2. Correlation between surface area of the catalyst and activity: ( 0 , 0 ) ([],m) Cr20>
Cr 3" -promoted ZrO> ( A, • ) ZrO 2,
to 450°C. However, over 400°C, the selectivity for benzoic acid decreased and considerably amounts of benzene were formed. Fig. 4 shows the correlation between the water-to-benzaldehyde feed ratio and activity. Conversion and selectivity increased with increasing water-to-benzaldehyde ratio at the expense of benzene formation. The molar ratio of dihydrogen to acid was approximately one, and was unaffected over the wide water-to-benzaldehyde ratio range of zero to 16.5.
3.2. Reaction of 3-phenoxvbenzaldehyde and l-heptanal with water We also examined the influence of the substrates on this reaction. 3-Phenoxybenzaldehyde and 1-heptanal were chosen. The results are shown in Tables 3 and 4. 3-Phenoxybenzaldehyde was also converted to the corresponding acid accompanied by formation of an equivalent amount of dihydrogen. However, the selec100
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T. Yokoyama et al. / Applied Catalysis A: General 125 (1995) 159-167 100
163
2 ./
/&--" ./
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H 2 0 / b e n z a l d e h y d e ratio ~m0t/mo0 Fig. 4. The effect of H20/benzaldehyde ratio on the reaction of H20 and benzaldehyde over Cr ~+-promoted ZrO2: (©) conversion of benzaldehyde, selectivity to benzoic acid ( A ) and benzene ( V ), ( • ) H2/acid ratio• Table 3 Reaction of 3-phenoxybenzaldehyde with water on Cc~+-promoted ZrO2 catalyst. Reaction conditions: N, GHSV = 200 h ~, 3-phenoxybenzaldehyde feed rate = 8.7 mmol/h, H20 feed rate = 226 mmol/h, catalyst volume = 10 cm 3. 400°C, under atmospheric pressure Reactant
Conversion of aldehyde (%)
3-phenoxybenzaldehyde 3-phenoxybenzaldehyde+H20
3.9 36.5
Selectivity to acid (%)
52.8 85.4
Formation rate (mmol/h) Acid
H2
0.18 2.73
l.ll 3.97
Table 4 Reaction of 1-heptanal with water on Cr-~+-promoted Z r O 2 catalyst. Reaction conditions: N2 GHSV = 200 h 1-heptanal feed rate ~ 12.2 mmol/h, H20 feed rate = 231 mmol / h, catalyst volume = I0 cm 3, temperature = 350°C, under atmospheric pressure Reactant
1-heptanal 1-heptanal + H20
Conversion of aldehyde (%)
68.3 56.7
Selectivity (%)
Formation rate (mmol/h)
acid
ketone
acid
ketone
H2
0 61.3
62.3 32.4
0 4.2
2.6 1.1
7.1 6.7
tivity was low compared to benzaldehyde. Major by-products were phenol and diphenyl ether. The formation rate of acid and dihydrogen was also drastically reduced when water was absent, and diphenyl ether was obtained as the main product. 1-Heptanal also reacted with water to form the corresponding acid and dihydrogen. Selectivity for the acid was low, as compared to the case of the aromatic aldehyde. The main by-product was tridecanone. In the absence of water, the conversion of 1-heptanal and the formation rate of dihydrogen slightly increased,
T. Yokoyama et al. / Applied Catalysis A: General 125 (1995) 159-167
164
unlike the aromatic aldehyde. Tridecanone was formed as the main product and the acid was not obtained.
4. Discussion The present reaction of dihydrogen evolution has a special character, i.e., the reaction rate is dependent on the specific surface area of the catalyst, and rather independent of chemical species of the catalyst, ZrO2 or Cr203. The free energy changes of the reaction between benzaldehyde and water were calculated using the CHETAH program (The ASTM Thermodynamic and Energy Release Evaluation Program, version 4.4). The literature value was used for the entropy change of benzaldehyde [5 ]. Values of free energy changes of this reaction are - 9.4 (kcal/ mol) at 300°C and - 5.1 (kcal/mol) at 400°C. The equilibrium constants (K) are calculated to be 4.0.103 for 300°C and 5.1 • 103 for 400°C, respectively. These data show that the reaction equilibrium is towards products' side, though this reaction cannot occur without catalyst under the present conditions. Possible reaction mechanisms considered are as follows. Benzaldehyde is known to form benzylbenzoate over CaO [6], as expressed by Eq. (2). 2PhCHO ---> PhCOOCH2Ph
(2)
The active sites for ester formation are considered to be calcium benzyloxide whose formation is facilitated by both basic (02 ) and acidic (Ca 2+ ) sites on the surface. ZrO2 is also known as an acid-base multifunctional catalyst [7]. Benzoic acid and benzyl alcohol can be obtained by hydrolysis of benzylbenzoate [ Eq. (3)1. PhCOOCH2Ph + H20 ---> PhCOOH + PhCHzOH
(3)
It is well known that benzyl alcohol is easily dehydrogenated to form an aldehyde under the present reaction conditions [ 1 ], as shown in Eq. (4). PhCH2OH ---> PhCHO + H2
(4)
Equal molar amounts of benzoic acid and dihydrogen can be obtained from Eqs. (2) to (4) when considering benzylbenzoate as the intermediate of this reaction. However, the formation of benzylbenzoate was hardly observed over ZrO2 and Cr203 even if water was absent. These results suggest that the benzylbenzoate intermediate is doubtful in the present case. It has been known that low-valent metal species such as Cr 2 + decompose water to form dihydrogen as expressed in Eq. (5). 2CrO + HzO --> Cr203 + H2
(5)
T. Yokoyama et al. / Applied Catalysis A: General 125 (1995) 159-167
[• Ar C HO
-1.12, . H20
Q,:,.C.,-, 0 . ,,-'
165
H20 , - H 2 ArCOOH
M Scheme 1.
This reaction has been used to determine the amount of Cr~+ species formed during the reduction of a Cr/AI203 catalyst [ 8 ]. According to Eq. (5), dihydrogen evolution is supposed stop when all Cr 2+ species are consumed. However, the regeneration of Cr 2+ by benzaldehyde does not seem to be plausible in the present case and the Zr 3 + redox cycle will be more difficult than in the case of Cr 2+. Anyway, we did not detect any sign of low-valent metal species on the catalyst's surface. It has been reported that when benzaldehyde is adsorbed over ZrO2 [ 3 ], y-A1203 or a-Mn304 [9], surface benzoate species were formed [Eq. (6)] by abstracting a lattice oxygen.
o:(-;:o Zr
(6)
The same species is also formed by adsorption of benzoic acid o v e r Z r O 2 [ 1-3 ] and is considered to be an intermediate for the aldehyde formation reaction. This surface benzoate seems to play an important role in the present dihydrogen evolution reaction. The reverse reaction path of the hydrogenation of aromatic carboxylic acid seems to take place (Scheme 1 ). The rate of dihydrogen evolution was proportional to the partial pressure of water. It has been reported that surface hydroxyl groups of ZrO2 are converted to deuteroxyl groups by contacting with D20 [ 10]. It has also been reported that surface hydroxyl groups originated from dissociative water chemisorption on Cr203 [11 ]. So, we assume that benzoic acid and dihydrogen are formed by the reaction between surface benzoate species and dissociated water species on the catalyst surface, i.e., the Langmuir-Hinshelwood mechanism. Benzaldehyde is adsorbed on the metal center of the catalyst to form surface benzoate species by abstracting lattice oxygen and by leaving a hydrogen atom on another oxygen site. Water dissociates on the catalyst surface to form H and OH. Surface OH attacks the carboxyl carbon of the benzoate species to form benzoic acid, releasing lattice oxygen. Adsorbed hydrogen atoms desorb as dihydrogen from the surface. The rate comparison between the dihydrogen evolution reaction and hydrogenation is as follows. The formation rate of benzaldehyde due to the hydrogenation of benzoic acid was 0.54 (mmol/gca, h) at 330°C, whereas the formation rate of
166
T. Yokoyama et al. / Applied Catalysis A: General 125 (1995) 159-167
benzoic acid due to the reaction of benzaldehyde and water was 0.68 (mmol/gcat h) at 400°C and 0.28 (mmol/gc,~ h) at 350°C. The reason why the rate of the dihydrogen evolution is slow compared to the hydrogenation reaction is considered to be as follows. We previously reported that the dissociative adsorption of hydrogen molecules is related to the rate determining step and a strong interaction of carboxylic acid with the catalyst surface inhibits the consecutive reaction of aldehyde in the hydrogenation reaction [ 1 ]. In the present dihydrogen evolution reaction, the dissociative adsorption of water molecules seems to be the rate determining step since surface benzoate species are stable under the reaction conditions and the formation rate of benzoic acid is strongly affected the by water/benzaldehyde ratio. It is suggested that the activation of water molecules on the catalyst surface requires a higher reaction temperature, as compared to that of hydrogen molecules when the catalyst surface is covered with benzoate species, although water vapor inhibits the dissociative adsorption of hydrogen molecules on the catalyst's surface. It has also been proposed that oxidative dehydrogenation of furfural in the presence of a Pt/Pb catalyst and water proceeds via a gem-diol type intermediate, followed by dehydrogenation to form 3-furoic acid with the oxidation of hydrogen to water [ 12]. This gem-diol type intermediate might be formed in the present case; however, further mechanistic studies are needed. The reaction mechanism seems to be different in the case of l-heptanal. In this case, it is well known that a primary aliphatic aldehyde can undergo a Tishchenkotype reaction in the presence of a basic catalyst [Eq. (7) ]. 2RCH2CHO ~ RCH2CH2OCOCH2R
(7)
The ester undergoes hydrolysis to form the carboxylic acid and the alcohol which can be immediately regenerated to the aldehyde. It is also known that a primary aliphatic carboxylic acid undergoes a ketonization reaction over a thoria catalyst [ 13]. We think that tridecanone is formed in this manner. When water is absent, the ester is decarboxylated in a complex manner to form the condensed ketone simultaneously [ 14], co-producing a certain amount of dihydrogen.
5. Conclusions
An equimolecular formation of aromatic carboxylic acid and dihydrogen was observed during the reaction of aromatic aldehydes and water over ZrO2 and Cr203 catalysts. The Cr 3 +-promoted ZrO2 was the most active for this reaction. Aliphatic carboxylic acid and dihydrogen were also produced by the reaction of the corresponding aldehyde and water. It is suggested that carboxylic acid and dihydrogen are formed from the reaction between surface carboxylate species and adsorbed water on the catalyst's surface.
T. Yokoyama et al./Applied Catalysis A: General 125 (1995) 159-167
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References [ 1] T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima, T. Maki and K. Fujii, Appl. Catal. A, 88 (1992) 149. [2] A. Aboulay, A. Chambellan, M. Marzin, J. Saussey, F. Mauge, J.C. Lavally, C. Mercier and R. Jacquot, Proc. 3rd International Symposium on Heterogeneous Catalyst and Fine Chemicals (Poitiers), C 115, (1993). [3] J. Kondo, N. Ding, K. Maruya, K. Domen, T. Yokoyama, N. Fujita and T. Maki, Bull. Chem. Soc. Jpn., 66 (1993) 3085. [4] T. Yokoyama and K. Fujii, Petrotech (Tokyo), 14 ( 1991 ) 633. [5 ] D. Ambrose, J.E. Connet, J.H.S. Green, J.L. Hales, A.J. Head and J.F. Martin, J. Chem. Thermodynamics, 7 (1975) 1143. [6] T. Tanabe and K. Saito, J. Catal., 35 (1974) 247. [ 7] T. Yamaguchi, Y. Nakano, T. Iizuka and K. Tanabe, Chem. Lett., (1976) 677. [8] K.I. Slovitskaya, K.M. Gitis, R.V. Dmitrieve and A.M. Rubinshtein, Izv. Akad. Nauk, SSSR, Ser. Khim., 18 (1969) 196. [ 9] C.A. Koutstall, P.A.J.M. Angevare and V. Ponec, J. Catal., 143 (1993) 573. [ 10] N.F. Tretyakov, D.V. Fozdnyakov, O.M. Oranskaya and V.N. Filimonov, Russ. J. Phys. Chem., 44 (1970) 596. [ 11 ] A. Zecchina, S. Coluccia, F. Guglielminotti and G. Ghiotti, J. Phys. Chem., 75 ( 1971 ) 2774. [ 12] H. Misuh, H. Khim and H. Paik, Appl. Catal. A, 96 (1993) L7. [ 13] W. Winkler, Chem. Ber,, 81 (1948) 258. [ 14] C.W. Hargis, H.S. Young, J.W. Reynolds, Kingsport and Term, US Patent 3 453 331 (1969).
APPLIED
.,
CATALYSIS
-~,,~ ~,~:~
A: GENERAL
ELSEVIER
Applied Catalysis A: General 125 (1995) 159-167
Formation of dihydrogen and carboxylic acid from water and aldehyde over metal oxide catalysts Toshiharu Yokoyama *, Naoko Fujita, Takao Maki Yokohama Research Center, Mitsubishi Chemieal Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227, Japan Received 12 September 1994; revised 30 November 1994; accepted 22 December 1994
Abstract The formation of dihydrogen and benzoic acid was observed from water vapor and benzaldehyde over Cr 3 ~ -promoted ZrO2, ZrO2 and Cr203 catalysts. Cr ~+-promotedZrO2 was the most active catalyst tbr this reaction and an equimolecular amount o f dihydrogen and benzoic acid was obtained. Dihydrogen and the corresponding carboxylic acids were also obtained from the reaction of 3-phenoxybenzaldehyde or 1-heptanal with water. It is suggested that a surface carboxylate species formed by aldehyde adsorption is an intermediate of these reactions. Keywords." Benzoic acid; Chromium promotion; Hydrogen; Zirconia
1. Introduction We have previously demonstrated that aromatic carboxylic acids can be hydrogenated to the corresponding aldehydes in good yield using a promoted ZrO2 catalyst [ 1 ]. It is important to study the mechanism of this reaction and several papers have been published on this subject [ 1-3]. While studying this reaction, we found that the reaction rate was significantly affected by water vapor [4]. This suggested that a strong interaction of the catalyst surface with water vapor exists during the reaction. We examined the influence of water on the hydrogenation reaction of carboxylic acid in the presence of reaction products. We found that carboxylic acid and dihydrogen were formed by the reaction between aldehyde and water vapor on the catalyst at elevated temperature, as shown by Eq. ( 1 ). * Corresponding author. E-mail [email protected], tel. ( + 81-45) 9633197, fax. ( + 81-45) 9633974. 0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO926-860X( 95 ) 0 0 0 0 3 - 8
T. Yokoyama et al. /Applied Catalysis A." General 125 (1995) 15~167
160
Table 1 Dihydrogen evolution from water on Cr ~ +-promoted ZrO_, catalyst Reaction conditions: N2 GHSV = 200 h ~, benzaldehyde feed rate catalyst volume = 5 cm ~, 400°C, under atmospheric pressure Reactant
H20 + benzaldehyde H~O benzaldehyde
ArCHO +
Conversion of benzaldehyde
43.2 9.0
H20
Selectivity to acid / %
95.7 88. I
-->
ArCOOH + H2
7.5 mmol/h, HeO feed rate = 110 mmol/h,
Formation rate (mmol h) Acid
H:
3.4 0.18
3.1 0 0.43
H2/acid tool ratio
0.9 I 2.40
( 1)
The present paper describes the study of this dihydrogen evolution reaction over metal oxide catalysts.
2. Experimental The catalysts used in this study were Cr 3 +-promoted ZrOz, ZrO2 and Cr203. The Cr 3 +-promoted ZrOz catalyst (Cr/Zr = 5/100 atomic ratio) was prepared from zirconyl hydroxide and chromium nitrate as previously described in detail [1 ]. ZrO2 and CrzO3 were prepared from the hydroxide. These catalysts were calcined at 700°C for 3 h in an air stream. Benzaldehyde (Kishida, 98%), 3-phenoxybenzaldehyde (Aldrich, > 95%) and l-heptanal (Tokyo Kasei, > 95%) were used without further purification. Reactions were carried out under atmospheric pressure in an ordinary flow system. The volume of the catalyst employed ( 10-20 mesh) was 5 or 10 cm 3. Nitrogen was used as a carrier gas. The reaction products were analyzed by gas-liquid chromatography (GLC) and titration.
3. Results 3.1. Reaction of benzaldehyde with water The reaction between benzaldehyde and water vapor over Cr 3 +-promoted ZrOz was studied first. The results are shown in Table 1. Benzaldehyde reacted with water, forming selectively benzoic acid and dihydrogen. Benzene was detected as the main by-product which was formed by decarboxylation. The molar ratio of dihydrogen to acid was nearly unity. The formation of the acid was fairly fast with a rate of 0.68 mmol/gc~,t h. The reaction rate was drastically decreased when water was absent, and only a small amount of dihydrogen was formed, probably because
T. Yokoyama et al. /Applied Catalysis A: General 125 (1995) 159-167 100
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of coke formation from benzaldehyde. Dihydrogen was not detected in the absence of benzaldehyde. Simple water decomposition did also not occur. Fig. 1 shows the catalytic activity changes with time-on-stream. Changes in activity and selectivity during the reaction were not observed. These results show that benzaldehyde is oxidized to benzoic acid by water in the presence of Cr 3+-promoted ZrO2 catalyst. The catalytic activities of ZrO2 and Cr203 are shown in Table 2. Both catalysts showed low activity compared to the promoted Z r O 2. The correlation between surface area of the catalyst and the activity, which is expressed in terms of the formation rate of benzoic acid (mmol/h), is shown in Fig. 2. The catalytic activity increased linearly with increasing surface area. These results suggest that the enhancement of the catalytic activity of Cr 3+-promoted ZrO2 is mainly due to the increase in its surface area by addition of the chromium component, which is the same phenomenon as for the hydrogenation of benzoic acid. The temperature dependency of this reaction is shown in Fig. 3. The conversion of benzaldehyde increased linearly with increasing reaction temperature from 350 Table 2 Reaction of benzaldehyde with water on metal oxide catalysts. Reaction conditions: N~ GHSV = 200 h ~, benzaldehyde feed rate = 7.5 mmol/h, H20 feed rate= 110 retool/h, catalysts volume = 5 cm 3, 400°C, under atmospheric pressure Catalyst
Cr 3 +-modified ZrO2 ZrO2 Cr203
Surface area (m2/g)
Conversion of benzaldehyde (%)
Selectivity to acid (%)
Formation rate (mmol/h) Acid
Hz
H2/acid mol ratio
72.0
43.2
95.7
3.4
3.1
0.91
31.0 17.5
14.8 13.8
91.2 93.0
1.1 1.0
0.9 1.2
0.93 1.17
162
T. Yokoyama et all/Applied Catalysis AI" General 125 (1995) 159-167
0
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Cr 3" -promoted ZrO> ( A, • ) ZrO 2,
to 450°C. However, over 400°C, the selectivity for benzoic acid decreased and considerably amounts of benzene were formed. Fig. 4 shows the correlation between the water-to-benzaldehyde feed ratio and activity. Conversion and selectivity increased with increasing water-to-benzaldehyde ratio at the expense of benzene formation. The molar ratio of dihydrogen to acid was approximately one, and was unaffected over the wide water-to-benzaldehyde ratio range of zero to 16.5.
3.2. Reaction of 3-phenoxvbenzaldehyde and l-heptanal with water We also examined the influence of the substrates on this reaction. 3-Phenoxybenzaldehyde and 1-heptanal were chosen. The results are shown in Tables 3 and 4. 3-Phenoxybenzaldehyde was also converted to the corresponding acid accompanied by formation of an equivalent amount of dihydrogen. However, the selec100
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T. Yokoyama et al. / Applied Catalysis A: General 125 (1995) 159-167 100
163
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/&--" ./
to"
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6
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H 2 0 / b e n z a l d e h y d e ratio ~m0t/mo0 Fig. 4. The effect of H20/benzaldehyde ratio on the reaction of H20 and benzaldehyde over Cr ~+-promoted ZrO2: (©) conversion of benzaldehyde, selectivity to benzoic acid ( A ) and benzene ( V ), ( • ) H2/acid ratio• Table 3 Reaction of 3-phenoxybenzaldehyde with water on Cc~+-promoted ZrO2 catalyst. Reaction conditions: N, GHSV = 200 h ~, 3-phenoxybenzaldehyde feed rate = 8.7 mmol/h, H20 feed rate = 226 mmol/h, catalyst volume = 10 cm 3. 400°C, under atmospheric pressure Reactant
Conversion of aldehyde (%)
3-phenoxybenzaldehyde 3-phenoxybenzaldehyde+H20
3.9 36.5
Selectivity to acid (%)
52.8 85.4
Formation rate (mmol/h) Acid
H2
0.18 2.73
l.ll 3.97
Table 4 Reaction of 1-heptanal with water on Cr-~+-promoted Z r O 2 catalyst. Reaction conditions: N2 GHSV = 200 h 1-heptanal feed rate ~ 12.2 mmol/h, H20 feed rate = 231 mmol / h, catalyst volume = I0 cm 3, temperature = 350°C, under atmospheric pressure Reactant
1-heptanal 1-heptanal + H20
Conversion of aldehyde (%)
68.3 56.7
Selectivity (%)
Formation rate (mmol/h)
acid
ketone
acid
ketone
H2
0 61.3
62.3 32.4
0 4.2
2.6 1.1
7.1 6.7
tivity was low compared to benzaldehyde. Major by-products were phenol and diphenyl ether. The formation rate of acid and dihydrogen was also drastically reduced when water was absent, and diphenyl ether was obtained as the main product. 1-Heptanal also reacted with water to form the corresponding acid and dihydrogen. Selectivity for the acid was low, as compared to the case of the aromatic aldehyde. The main by-product was tridecanone. In the absence of water, the conversion of 1-heptanal and the formation rate of dihydrogen slightly increased,
T. Yokoyama et al. / Applied Catalysis A: General 125 (1995) 159-167
164
unlike the aromatic aldehyde. Tridecanone was formed as the main product and the acid was not obtained.
4. Discussion The present reaction of dihydrogen evolution has a special character, i.e., the reaction rate is dependent on the specific surface area of the catalyst, and rather independent of chemical species of the catalyst, ZrO2 or Cr203. The free energy changes of the reaction between benzaldehyde and water were calculated using the CHETAH program (The ASTM Thermodynamic and Energy Release Evaluation Program, version 4.4). The literature value was used for the entropy change of benzaldehyde [5 ]. Values of free energy changes of this reaction are - 9.4 (kcal/ mol) at 300°C and - 5.1 (kcal/mol) at 400°C. The equilibrium constants (K) are calculated to be 4.0.103 for 300°C and 5.1 • 103 for 400°C, respectively. These data show that the reaction equilibrium is towards products' side, though this reaction cannot occur without catalyst under the present conditions. Possible reaction mechanisms considered are as follows. Benzaldehyde is known to form benzylbenzoate over CaO [6], as expressed by Eq. (2). 2PhCHO ---> PhCOOCH2Ph
(2)
The active sites for ester formation are considered to be calcium benzyloxide whose formation is facilitated by both basic (02 ) and acidic (Ca 2+ ) sites on the surface. ZrO2 is also known as an acid-base multifunctional catalyst [7]. Benzoic acid and benzyl alcohol can be obtained by hydrolysis of benzylbenzoate [ Eq. (3)1. PhCOOCH2Ph + H20 ---> PhCOOH + PhCHzOH
(3)
It is well known that benzyl alcohol is easily dehydrogenated to form an aldehyde under the present reaction conditions [ 1 ], as shown in Eq. (4). PhCH2OH ---> PhCHO + H2
(4)
Equal molar amounts of benzoic acid and dihydrogen can be obtained from Eqs. (2) to (4) when considering benzylbenzoate as the intermediate of this reaction. However, the formation of benzylbenzoate was hardly observed over ZrO2 and Cr203 even if water was absent. These results suggest that the benzylbenzoate intermediate is doubtful in the present case. It has been known that low-valent metal species such as Cr 2 + decompose water to form dihydrogen as expressed in Eq. (5). 2CrO + HzO --> Cr203 + H2
(5)
T. Yokoyama et al. / Applied Catalysis A: General 125 (1995) 159-167
[• Ar C HO
-1.12, . H20
Q,:,.C.,-, 0 . ,,-'
165
H20 , - H 2 ArCOOH
M Scheme 1.
This reaction has been used to determine the amount of Cr~+ species formed during the reduction of a Cr/AI203 catalyst [ 8 ]. According to Eq. (5), dihydrogen evolution is supposed stop when all Cr 2+ species are consumed. However, the regeneration of Cr 2+ by benzaldehyde does not seem to be plausible in the present case and the Zr 3 + redox cycle will be more difficult than in the case of Cr 2+. Anyway, we did not detect any sign of low-valent metal species on the catalyst's surface. It has been reported that when benzaldehyde is adsorbed over ZrO2 [ 3 ], y-A1203 or a-Mn304 [9], surface benzoate species were formed [Eq. (6)] by abstracting a lattice oxygen.
o:(-;:o Zr
(6)
The same species is also formed by adsorption of benzoic acid o v e r Z r O 2 [ 1-3 ] and is considered to be an intermediate for the aldehyde formation reaction. This surface benzoate seems to play an important role in the present dihydrogen evolution reaction. The reverse reaction path of the hydrogenation of aromatic carboxylic acid seems to take place (Scheme 1 ). The rate of dihydrogen evolution was proportional to the partial pressure of water. It has been reported that surface hydroxyl groups of ZrO2 are converted to deuteroxyl groups by contacting with D20 [ 10]. It has also been reported that surface hydroxyl groups originated from dissociative water chemisorption on Cr203 [11 ]. So, we assume that benzoic acid and dihydrogen are formed by the reaction between surface benzoate species and dissociated water species on the catalyst surface, i.e., the Langmuir-Hinshelwood mechanism. Benzaldehyde is adsorbed on the metal center of the catalyst to form surface benzoate species by abstracting lattice oxygen and by leaving a hydrogen atom on another oxygen site. Water dissociates on the catalyst surface to form H and OH. Surface OH attacks the carboxyl carbon of the benzoate species to form benzoic acid, releasing lattice oxygen. Adsorbed hydrogen atoms desorb as dihydrogen from the surface. The rate comparison between the dihydrogen evolution reaction and hydrogenation is as follows. The formation rate of benzaldehyde due to the hydrogenation of benzoic acid was 0.54 (mmol/gca, h) at 330°C, whereas the formation rate of
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benzoic acid due to the reaction of benzaldehyde and water was 0.68 (mmol/gcat h) at 400°C and 0.28 (mmol/gc,~ h) at 350°C. The reason why the rate of the dihydrogen evolution is slow compared to the hydrogenation reaction is considered to be as follows. We previously reported that the dissociative adsorption of hydrogen molecules is related to the rate determining step and a strong interaction of carboxylic acid with the catalyst surface inhibits the consecutive reaction of aldehyde in the hydrogenation reaction [ 1 ]. In the present dihydrogen evolution reaction, the dissociative adsorption of water molecules seems to be the rate determining step since surface benzoate species are stable under the reaction conditions and the formation rate of benzoic acid is strongly affected the by water/benzaldehyde ratio. It is suggested that the activation of water molecules on the catalyst surface requires a higher reaction temperature, as compared to that of hydrogen molecules when the catalyst surface is covered with benzoate species, although water vapor inhibits the dissociative adsorption of hydrogen molecules on the catalyst's surface. It has also been proposed that oxidative dehydrogenation of furfural in the presence of a Pt/Pb catalyst and water proceeds via a gem-diol type intermediate, followed by dehydrogenation to form 3-furoic acid with the oxidation of hydrogen to water [ 12]. This gem-diol type intermediate might be formed in the present case; however, further mechanistic studies are needed. The reaction mechanism seems to be different in the case of l-heptanal. In this case, it is well known that a primary aliphatic aldehyde can undergo a Tishchenkotype reaction in the presence of a basic catalyst [Eq. (7) ]. 2RCH2CHO ~ RCH2CH2OCOCH2R
(7)
The ester undergoes hydrolysis to form the carboxylic acid and the alcohol which can be immediately regenerated to the aldehyde. It is also known that a primary aliphatic carboxylic acid undergoes a ketonization reaction over a thoria catalyst [ 13]. We think that tridecanone is formed in this manner. When water is absent, the ester is decarboxylated in a complex manner to form the condensed ketone simultaneously [ 14], co-producing a certain amount of dihydrogen.
5. Conclusions
An equimolecular formation of aromatic carboxylic acid and dihydrogen was observed during the reaction of aromatic aldehydes and water over ZrO2 and Cr203 catalysts. The Cr 3 +-promoted ZrO2 was the most active for this reaction. Aliphatic carboxylic acid and dihydrogen were also produced by the reaction of the corresponding aldehyde and water. It is suggested that carboxylic acid and dihydrogen are formed from the reaction between surface carboxylate species and adsorbed water on the catalyst's surface.
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