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Catalytic steam reforming of ethanol to produce hydrogen and acetone
Applied Catalysis A: General 279 (2005) 273–277 www.elsevier.com/locate/apcata
Catalytic steam reforming of ethanol to produce hydrogen and acetone Toshiya Nishiguchia, Tomoaki Matsumotoa, Hiroyoshi Kanaia, Kazunori Utania, Yasuyuki Matsumurab, Wen-Jie Shenc, Seiichiro Imamuraa,* a
Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan b National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda, Osaka 563-8577, Japan c State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
Received 29 June 2004; received in revised form 21 October 2004; accepted 24 October 2004 Available online 8 December 2004
Abstract Steam reforming of ethanol over CuO/CeO2 was studied. Acetaldehyde and hydrogen were mainly produced at 260 8C. At 380 8C, acetone was the main product, and 2 mol of hydrogen was produced from 1 mol of ethanol. The formation of hydrogen accompanied by the production of acetone was considered to proceed through the following consecutive reactions: dehydrogenation of ethanol to acetaldehyde, aldol condensation of the acetaldehyde, and the reaction of the aldol with the lattice oxygen [O(S)] on the catalyst to form a surface intermediate, followed by its dehydrogenation and decarboxylation. The overall reaction was expressed by 2C2H5OH + H2O ! CH3COCH3 + CO2 + 4H2. Ceria played an important role as an oxygen supplier. The addition of MgO to CuO/CeO2 resulted in the production of hydrogen at lower temperatures by accelerating aldol condensation. # 2004 Elsevier B.V. All rights reserved. Keywords: Ethanol; Steam reforming; Acetone; Hydrogen; CuO/CeO2
1. Introduction Shortage of fossil fuel in the near future will cause serious energy problems. Therefore, the development of the technology utilizing biomass energy resources attracts much attention. Hydrogen is a clean energy source, which emits neither CO2 nor NOx, and its demand as a fuel for fuel cells is increasing. Steam reforming of lower organic compounds such as methanol and propane is an effective method of manufacturing hydrogen. However, if biomass ethanol can be used, it is desirable from the environmental standpoint. Ethanol is obtained as an aqueous solution by fermentation of biomasses. As this aqueous ethanol can be used for steam
* Corresponding author. Tel.: +81 75 724 7534; fax: +81 75 724 7534. E-mail address: [email protected] (S. Imamura). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.10.035
reforming without extensive elimination of water, the use of biomass ethanol is very profitable for energy saving. Steam reforming of ethanol has been studied using various catalysts [1–8]. It is an endothermic reaction and produces only hydrogen and CO2 if ethanol reacts in a most desirable way. However, usually by-products such as acetaldehyde, and ethyl acetate are formed. Steam reforming of ethanol to produce only hydrogen and CO2 favors high temperatures, while by-product formation is rather dominant at low temperatures. The amount of hydrogen produced in the steam reforming at high temperatures is larger than that accompanied by by-product formation at lower temperatures. However, from the standpoint of energy saving, low temperature reaction accompanied with the formation of useful by-products is preferable. From this consideration, we tried to develop steam reforming catalysts which are active at low temperatures and can also produce useful by-products. Copper was used as a
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main catalyst component. A promoter component was ceria, which has oxygen storage action [9–14]. Owing to this function, it promotes the action of various precious metals in the reactions in which hydrogen is involved as a reactant or product [15–19].
2. Experimental 2.1. Catalyst preparation The 20 mol% CuO/CeO2 composite oxide was prepared as follows. An aqueous solution containing known amounts of Cu(NO3)23H2O and Ce(NO3)36H2O was stirred at 80 8C; 3N NaOH was added until the pH of the solution was 11. The resultant precipitate was filtered, washed with deionized water three times, and dried overnight at 80 8C, followed by calcination at 250 8C in air for 3 h. The 20 mol% CuO/Al2O3 and CuO/SiO2 were prepared as follows. A known amount of Al2O3 or SiO2 was added to an aqueous solution of Cu(NO3)2 under stirring. The mixture was evaporated to dryness. The obtained solid was dried overnight at 80 8C followed by calcination at 250 8C in air for 3 h. A mechanical mixture of 20 mol% CuO/CeO2 and MgO (CuO/CeO2:MgO = 1:1 weight ratio) was also used as a catalyst. Acid sites of 20 mol% CuO/Al2O3 were eliminated by treating with KOH. The 20 mol% CuO/Al2O3 was stirred in 0.1N KOH, filtered, and dried at 80 8C overnight, followed by calcination at 250 8C in air for 3 h.
Fig. 1. Steam reforming of ethanol over 20 mol% CuO/Al2O3 (0.32 g): (*) ethanol, (*) hydrogen, (~) acetaldehyde, (^) ethylene and (^) others.
their retention time on the gas chromatogram, their amounts were determined on the basis of their relative sensitivity for FID detector estimated from their carbon content. Therefore, this procedure inevitably brought about some inaccuracy in the material balance of the reaction. The amount of the catalyst used was determined so as to use 0.05 g of CuO in all experiments.
3. Results and discussion 3.1. Steam reforming of ethanol over CuO, CuO/SiO2, CuO/Al2O3, and CuO/CeO2 The effect of the supports on the action of CuO was investigated. It was found that almost selective dehydrogenation of ethanol to acetaldehyde and hydrogen occurred in the temperature range of 200–400 8C over CuO and CuO/ SiO2:
2.2. Reaction procedure
C2 H5 OH ! CH3 CHO þ H2
Reactions were carried out using a flow-type reactor under atmospheric pressure. Catalysts were molded into disks under 20 MPa for 5 min and each disk was crushed and sieved into 14–28 mesh size. The samples were reduced with 10% H2 in Ar (100 ml/min at 25 8C) at 300 8C for 1 h before reaction. Ethanol (1.2 104 mol/min) and water (6.0 104 mol/min) [H2O/ethanol = 5 molar ratio] were supplied to the catalyst bed with Ar as a carrier gas (20 ml/ min at 25 8C). Reacted gases were analyzed with two on-line gas chromatographs. Organic compounds were analyzed with a Shimadzu GC-14B FID gas chromatograph equipped with a capillary column (CBP-10, 25 m). Gaseous products were analyzed with a Shimadzu GC-3BT TCD gas chromatograph equipped with a packed column (Porapak Q, 2 m) after water was eliminated with an ice water trap. In addition to hydrogen, acetone, and acetaldehyde, other byproducts were formed. They are butanal, methyl ethyl ketone, ethyl acetate, acetal (1,1-diethoxyethane), and some unknown compounds. The expression ‘‘others’’ appearing in the text includes these unknown compounds. Determination of the unknown compounds was carried out as follows. By assuming appropriate compounds for them judging from
Steam reforming of ethanol over CuO/Al2O3 produced about 1.2 mol of hydrogen per 1 mol of ethanol below 300 8C (Fig. 1). Above 350 8C, ethylene was the main product without hydrogen production: C2 H5 OH ! C2 H4 þ H2 O
(1)
(2)
Moreover, a considerable amount of various by-products is formed. Since ethylene and these by-products seemed to
Fig. 2. Steam reforming of ethanol over KOH-treated 20 mol% CuO/Al2O3 (0.32 g): (*) ethanol, (*) hydrogen, (~) acetaldehyde, (!) acetone, (&) CO2 and (^) others.
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As the amount of hydrogen production was the largest over CuO/CeO2, the action of this catalyst was investigated in detail hereafter. 3.2. Reaction over CuO/CeO2
Fig. 3. Steam reforming of ethanol over 20 mol% CuO/CeO2 (0.5 g): (*) ethanol, (*) hydrogen, (~) acetaldehyde, (!) acetone, (&) CO2 and (^) others.
be formed on the acid sites of alumina, its acid sites were neutralized with KOH. Then, a maximum of 1.5 mol of hydrogen was formed from 1 mol of ethanol and other byproducts decreased above 350 8C (Fig. 2). The amount of hydrogen produced was not large over CuO, CuO/SiO2 and CuO/Al2O3, indicating that water is not effectively utilized in the reaction. Fig. 3 shows that 2 mol of hydrogen was formed from 1 mol of ethanol over CuO/CeO2 above 380 8C; the amount of hydrogen was found to be twice that over CuO/SiO2 and CuO/Al2O3 without KOH treatment. Acetone and CO2 were also produced. By-products were ethylene, butanal, ethyl acetate, acetal (1,1-diethoxyethane), and a minute amount of unknown compounds. Molar ratios of acetone, CO2, and H2 produced per reacted ethanol were 1/2, 1/2, and 2, respectively. Therefore, the following reaction should have occurred above 380 8C:
The reaction with water accelerated the reaction and increased the production of hydrogen and acetone (Table 1). The formation of acetone even in the absence of water seems to be caused by water liberated in the formation of other products (ethylene, butanal, acetal, etc.). In order to confirm that acetone was formed via acetaldehyde, we carried out steam reforming of ethanol over a CuO bed and a CeO2 bed arranged in a series (catalyst B in Table 1). Ethanol and water were passed through a CuO catalyst bed to produce acetaldehyde, which was then allowed to react over CeO2 catalyst. Selective formation of acetaldehyde on CuO was confirmed as described before. Acetone and hydrogen were formed with high selectivity on the two-bed system as in the case of one-bed CuO/CeO2 catalyst system. Therefore, acetone is formed via acetaldehyde: acetaldehyde is first produced mainly over CuO and its subsequent reaction over CeO2 leads to acetone formation. The conversion of ethanol was low on the single component CeO2 (catalyst C in Table 1) although it produced hydrogen and acetone. The combination of both is necessary for steam reforming of ethanol to proceed effectively. On the other hand, acetal was not formed over the CuO and CeO2 two-bed system. Acetal is produced by the cooperative function of CuO/CeO2 [23]. 3.3. Mechanism of the formation of the products
2C2 H5 OH þ H2 O ! CH3 COCH3 þ CO2 þ 4H2
(3)
Formation of acetone, which leads to high selectivity of hydrogen, needs water. Since the amount of acetaldehyde decreased and that of acetone increased as the temperature increased, acetone should be formed via acetaldehyde. The formation of acetone from ethanol was also observed on Fe2O3/ZnO [20] and ZnO/CaO [21,22] at around 400 8C, although the amount of hydrogen production was not addressed in the literature.
Here, we briefly address the mechanisms of the formation of the main products. Acetone was formed via acetaldehyde and its formation required water. Therefore, the following mechanism proposed by Elliott and Pennella [24] should be applied, where (S) means surface of the catalyst. Dehydrogenation of ethanol occurs first on CuO to produce acetaldehyde: C2 H5 OH ! CH3 CHO þ H2
(4)
Table 1 Steam reforming of ethanol over CuO-based catalysts Catalyst
H2O/EtOH
Conversion (%)
Selectivity (%) H2
A A A B C
0 5 10 5 5
55.9 95.7 75.5 89.7 16.1
66.4 84.4 87.6 77.8 71.5
CO2 11.0 16.2 18.7 13.4 17.2
CH3CHO 11.4 27.7 12.7 47 0
Acetone
35.6 37.5 54.7 31.5 54.9
Others C2H4
BA
AcOEt
MEK
Acetal
Unknown
2.5 0 0 0 5.1
2.4 0.5 0.1 0 0
1.1 0.2 0.1 0.1 0
0.7 0 0 0 0
16.8 14.7 6.0 0 0
18.4 3.2 7.7 8.0 22.9
Catalyst: (A) 20 mol% CuO/CeO20.5 g; (B) CuO 0.05 g + CeO20.45 g [CuO was charged in the front part of the reactor and CeO2 in the back, with glass wool separator between the two]; (C) CeO2 0.45 g, temperature: 320 8C, BA: butanal, AcOEt: ethyl acetate, MEK: methyl ethyl ketone, acetal: 1,1-diethoxyethane. For convenience’s sake, when 2 mol of hydrogen were formed from 1 mol of ethanol reacted, the selectivity of hydrogen was defined as 100%. The selectivity of other carbon-containing compounds was expressed on the basis of carbon balance of the reacted ethanol.
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Acetaldehyde suffers aldol condensation: 2CH3 CHO ! CH3 CHðOHÞCH2 CHO
(5)
The reaction of aldol with lattice oxygen O(S) on the catalyst follows to give the surface intermediate: CH3 CHðOHÞCH2 CHO þ OðSÞ ! CH3 CHðOHÞCH2 COOðSÞ þ HðSÞ
(6)
Finally, dehydrogenation and decarboxylation of the intermediate give acetone:
Fig. 4. Steam reforming of ethanol on 20 mol% CuO/CeO2 + MgO (1:1 weight ratio) (1.0 g): (*) ethanol, (*) hydrogen, (~) acetaldehyde, (!) acetone, (&) CO2 and (^) others.
CH3 CHðOHÞCH2 COOðSÞ þ HðSÞ ! CH3 COCH3 þ CO2 þ H2
(7)
Inui et al. also proposed a similar mechanism [25]. Aldol condensation (5) is catalyzed by acids or bases, so it occurs on the basic sites of CeO2. In reaction (7), dehydrogenation should occur prior to decarboxylation, because decarboxylation tends to occur for b-ketocarboxylate [26]. Lattice oxygen O(S) in reaction (6) must be constantly supplied for reactions (4)–(7) to proceed continuously. The lattice oxygen O(S) is formed by the decomposition of water (H2O ! O(S) + H2) over CeO2 [25]. Although some ambiguity is left about the decomposition of water, oxygen was surely supplied from water, considering the stoichiometry of the reaction. Therefore, the overall reaction is expressed by Eq. (3) and it can explain the observed outlet gas composition. The routes to the formation of ethyl acetate and acetal are deduced as follows. Hemiacetal is formed from ethanol and acetaldehyde: C2 H5 OH þ CH3 CHO ! CH3 CHðOHÞOC2 H5
(8)
By the dehydrogenation of hemiacetal, ethyl acetate is formed [25,27]: CH3 CHðOHÞOC2 H5 ! CH3 COOC2 H5 þ H2
(9)
However, when hemiacetal reacts with another molecule of ethanol, acetal is produced: CH3 CHðOHÞOC2 H5 þ C2 H5 OH ! CH3 CHðOC2 H5 Þ2 þ H2 O
(10)
As described previously, the formation of acetal occurred by the cooperative action of CuO and CeO2 [23]. 3.4. Effect of the addition of MgO Since, aldol condensation seems to be promoted by bases, steam reforming of ethanol was carried out on Cu/CeO2 mixed with MgO as a base component (Fig. 4). Larger amounts of hydrogen and acetone were produced at lower temperatures, compared with the results in the absence of MgO. This surely supports the mechanism of acetone formation via aldol condensation.
4. Conclusion Two moles of hydrogen were produced per 1 mol of ethanol in the steam reforming of ethanol over CuO/CeO2. Formation of acetone accompanied by high selectivity of hydrogen proceeded in the following three sequential steps: dehydrogenation of ethanol to acetaldehyde, aldol condensation from two molecules of acetaldehyde on a base catalyst, and the reaction of the aldol with lattice oxygen on CeO2 to form the surface intermediate, followed by its dehydrogenation and decarboxylation. The addition of base catalyst (MgO) produced acetone at lower temperatures.
Acknowledgement This research has been conducted as part of the project ‘‘Research and Development of Polymer Electrolyte Fuel Cell’’ under the entrustment contract with the New Energy and Industrial Technology Development Organization (NEDO).
References [1] V.V. Galvita, G.L. Semin, V.D. Belyaev, V.A. Semikolenov, P. Tsiakaras, V.A. Sobyanin, Appl. Catal. A 220 (2001) 123. [2] J. Llorca, P.R. Piscina, J. Sales, N. Homs, Chem. Commun. (2001) 641. [3] J. Llorca, N. Homs, J. Sales, P.R. Piscina, J. Catal. 209 (2002) 306. [4] F. Haga, T. Nakajima, K. Yamashita, S. Mishima, S. Suzuki, Nippon Kagaku Kaishi (1997) 33. [5] F. Haga, T. Nakajima, K. Yamashita, S. Mishima, Nippon Kagaku Kaishi (1997) 758. [6] M. Morita, A. Sato, Y. Matsuda, Nippon Kagaku Kaishi (1993) 164. [7] F.J. Marin˜ o, E.G. Cerrella, S. Duhalde, M. Jobbagy, M.A. Laborde, Int. J. Hydrogen Energy 23 (1998) 1095. [8] F. Marin˜ o, M. Boveri, G. Baronetti, M. Laborde, Int. J. Hydrogen Energy 26 (2001) 665. [9] T. Miki, T. Ogawa, M. Haneda, N. Kakuta, A. Ueno, S. Tateishi, S. Matsuura, M. Sato, J. Phys. Chem. 94 (1990) 339. [10] C. Padeste, N.W. Cant, D.L. Trimm, Catal. Lett. 18 (1993) 305. [11] S. Kacimi, J. Barbier Jr., R. Taha, D. Duperz, Catal. Lett. 22 (1993) 343. [12] G.S. Zafiris, R.J. Gorte, J. Catal. 143 (1993) 86.
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Catalytic steam reforming of ethanol to produce hydrogen and acetone Toshiya Nishiguchia, Tomoaki Matsumotoa, Hiroyoshi Kanaia, Kazunori Utania, Yasuyuki Matsumurab, Wen-Jie Shenc, Seiichiro Imamuraa,* a
Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan b National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda, Osaka 563-8577, Japan c State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
Received 29 June 2004; received in revised form 21 October 2004; accepted 24 October 2004 Available online 8 December 2004
Abstract Steam reforming of ethanol over CuO/CeO2 was studied. Acetaldehyde and hydrogen were mainly produced at 260 8C. At 380 8C, acetone was the main product, and 2 mol of hydrogen was produced from 1 mol of ethanol. The formation of hydrogen accompanied by the production of acetone was considered to proceed through the following consecutive reactions: dehydrogenation of ethanol to acetaldehyde, aldol condensation of the acetaldehyde, and the reaction of the aldol with the lattice oxygen [O(S)] on the catalyst to form a surface intermediate, followed by its dehydrogenation and decarboxylation. The overall reaction was expressed by 2C2H5OH + H2O ! CH3COCH3 + CO2 + 4H2. Ceria played an important role as an oxygen supplier. The addition of MgO to CuO/CeO2 resulted in the production of hydrogen at lower temperatures by accelerating aldol condensation. # 2004 Elsevier B.V. All rights reserved. Keywords: Ethanol; Steam reforming; Acetone; Hydrogen; CuO/CeO2
1. Introduction Shortage of fossil fuel in the near future will cause serious energy problems. Therefore, the development of the technology utilizing biomass energy resources attracts much attention. Hydrogen is a clean energy source, which emits neither CO2 nor NOx, and its demand as a fuel for fuel cells is increasing. Steam reforming of lower organic compounds such as methanol and propane is an effective method of manufacturing hydrogen. However, if biomass ethanol can be used, it is desirable from the environmental standpoint. Ethanol is obtained as an aqueous solution by fermentation of biomasses. As this aqueous ethanol can be used for steam
* Corresponding author. Tel.: +81 75 724 7534; fax: +81 75 724 7534. E-mail address: [email protected] (S. Imamura). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.10.035
reforming without extensive elimination of water, the use of biomass ethanol is very profitable for energy saving. Steam reforming of ethanol has been studied using various catalysts [1–8]. It is an endothermic reaction and produces only hydrogen and CO2 if ethanol reacts in a most desirable way. However, usually by-products such as acetaldehyde, and ethyl acetate are formed. Steam reforming of ethanol to produce only hydrogen and CO2 favors high temperatures, while by-product formation is rather dominant at low temperatures. The amount of hydrogen produced in the steam reforming at high temperatures is larger than that accompanied by by-product formation at lower temperatures. However, from the standpoint of energy saving, low temperature reaction accompanied with the formation of useful by-products is preferable. From this consideration, we tried to develop steam reforming catalysts which are active at low temperatures and can also produce useful by-products. Copper was used as a
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main catalyst component. A promoter component was ceria, which has oxygen storage action [9–14]. Owing to this function, it promotes the action of various precious metals in the reactions in which hydrogen is involved as a reactant or product [15–19].
2. Experimental 2.1. Catalyst preparation The 20 mol% CuO/CeO2 composite oxide was prepared as follows. An aqueous solution containing known amounts of Cu(NO3)23H2O and Ce(NO3)36H2O was stirred at 80 8C; 3N NaOH was added until the pH of the solution was 11. The resultant precipitate was filtered, washed with deionized water three times, and dried overnight at 80 8C, followed by calcination at 250 8C in air for 3 h. The 20 mol% CuO/Al2O3 and CuO/SiO2 were prepared as follows. A known amount of Al2O3 or SiO2 was added to an aqueous solution of Cu(NO3)2 under stirring. The mixture was evaporated to dryness. The obtained solid was dried overnight at 80 8C followed by calcination at 250 8C in air for 3 h. A mechanical mixture of 20 mol% CuO/CeO2 and MgO (CuO/CeO2:MgO = 1:1 weight ratio) was also used as a catalyst. Acid sites of 20 mol% CuO/Al2O3 were eliminated by treating with KOH. The 20 mol% CuO/Al2O3 was stirred in 0.1N KOH, filtered, and dried at 80 8C overnight, followed by calcination at 250 8C in air for 3 h.
Fig. 1. Steam reforming of ethanol over 20 mol% CuO/Al2O3 (0.32 g): (*) ethanol, (*) hydrogen, (~) acetaldehyde, (^) ethylene and (^) others.
their retention time on the gas chromatogram, their amounts were determined on the basis of their relative sensitivity for FID detector estimated from their carbon content. Therefore, this procedure inevitably brought about some inaccuracy in the material balance of the reaction. The amount of the catalyst used was determined so as to use 0.05 g of CuO in all experiments.
3. Results and discussion 3.1. Steam reforming of ethanol over CuO, CuO/SiO2, CuO/Al2O3, and CuO/CeO2 The effect of the supports on the action of CuO was investigated. It was found that almost selective dehydrogenation of ethanol to acetaldehyde and hydrogen occurred in the temperature range of 200–400 8C over CuO and CuO/ SiO2:
2.2. Reaction procedure
C2 H5 OH ! CH3 CHO þ H2
Reactions were carried out using a flow-type reactor under atmospheric pressure. Catalysts were molded into disks under 20 MPa for 5 min and each disk was crushed and sieved into 14–28 mesh size. The samples were reduced with 10% H2 in Ar (100 ml/min at 25 8C) at 300 8C for 1 h before reaction. Ethanol (1.2 104 mol/min) and water (6.0 104 mol/min) [H2O/ethanol = 5 molar ratio] were supplied to the catalyst bed with Ar as a carrier gas (20 ml/ min at 25 8C). Reacted gases were analyzed with two on-line gas chromatographs. Organic compounds were analyzed with a Shimadzu GC-14B FID gas chromatograph equipped with a capillary column (CBP-10, 25 m). Gaseous products were analyzed with a Shimadzu GC-3BT TCD gas chromatograph equipped with a packed column (Porapak Q, 2 m) after water was eliminated with an ice water trap. In addition to hydrogen, acetone, and acetaldehyde, other byproducts were formed. They are butanal, methyl ethyl ketone, ethyl acetate, acetal (1,1-diethoxyethane), and some unknown compounds. The expression ‘‘others’’ appearing in the text includes these unknown compounds. Determination of the unknown compounds was carried out as follows. By assuming appropriate compounds for them judging from
Steam reforming of ethanol over CuO/Al2O3 produced about 1.2 mol of hydrogen per 1 mol of ethanol below 300 8C (Fig. 1). Above 350 8C, ethylene was the main product without hydrogen production: C2 H5 OH ! C2 H4 þ H2 O
(1)
(2)
Moreover, a considerable amount of various by-products is formed. Since ethylene and these by-products seemed to
Fig. 2. Steam reforming of ethanol over KOH-treated 20 mol% CuO/Al2O3 (0.32 g): (*) ethanol, (*) hydrogen, (~) acetaldehyde, (!) acetone, (&) CO2 and (^) others.
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As the amount of hydrogen production was the largest over CuO/CeO2, the action of this catalyst was investigated in detail hereafter. 3.2. Reaction over CuO/CeO2
Fig. 3. Steam reforming of ethanol over 20 mol% CuO/CeO2 (0.5 g): (*) ethanol, (*) hydrogen, (~) acetaldehyde, (!) acetone, (&) CO2 and (^) others.
be formed on the acid sites of alumina, its acid sites were neutralized with KOH. Then, a maximum of 1.5 mol of hydrogen was formed from 1 mol of ethanol and other byproducts decreased above 350 8C (Fig. 2). The amount of hydrogen produced was not large over CuO, CuO/SiO2 and CuO/Al2O3, indicating that water is not effectively utilized in the reaction. Fig. 3 shows that 2 mol of hydrogen was formed from 1 mol of ethanol over CuO/CeO2 above 380 8C; the amount of hydrogen was found to be twice that over CuO/SiO2 and CuO/Al2O3 without KOH treatment. Acetone and CO2 were also produced. By-products were ethylene, butanal, ethyl acetate, acetal (1,1-diethoxyethane), and a minute amount of unknown compounds. Molar ratios of acetone, CO2, and H2 produced per reacted ethanol were 1/2, 1/2, and 2, respectively. Therefore, the following reaction should have occurred above 380 8C:
The reaction with water accelerated the reaction and increased the production of hydrogen and acetone (Table 1). The formation of acetone even in the absence of water seems to be caused by water liberated in the formation of other products (ethylene, butanal, acetal, etc.). In order to confirm that acetone was formed via acetaldehyde, we carried out steam reforming of ethanol over a CuO bed and a CeO2 bed arranged in a series (catalyst B in Table 1). Ethanol and water were passed through a CuO catalyst bed to produce acetaldehyde, which was then allowed to react over CeO2 catalyst. Selective formation of acetaldehyde on CuO was confirmed as described before. Acetone and hydrogen were formed with high selectivity on the two-bed system as in the case of one-bed CuO/CeO2 catalyst system. Therefore, acetone is formed via acetaldehyde: acetaldehyde is first produced mainly over CuO and its subsequent reaction over CeO2 leads to acetone formation. The conversion of ethanol was low on the single component CeO2 (catalyst C in Table 1) although it produced hydrogen and acetone. The combination of both is necessary for steam reforming of ethanol to proceed effectively. On the other hand, acetal was not formed over the CuO and CeO2 two-bed system. Acetal is produced by the cooperative function of CuO/CeO2 [23]. 3.3. Mechanism of the formation of the products
2C2 H5 OH þ H2 O ! CH3 COCH3 þ CO2 þ 4H2
(3)
Formation of acetone, which leads to high selectivity of hydrogen, needs water. Since the amount of acetaldehyde decreased and that of acetone increased as the temperature increased, acetone should be formed via acetaldehyde. The formation of acetone from ethanol was also observed on Fe2O3/ZnO [20] and ZnO/CaO [21,22] at around 400 8C, although the amount of hydrogen production was not addressed in the literature.
Here, we briefly address the mechanisms of the formation of the main products. Acetone was formed via acetaldehyde and its formation required water. Therefore, the following mechanism proposed by Elliott and Pennella [24] should be applied, where (S) means surface of the catalyst. Dehydrogenation of ethanol occurs first on CuO to produce acetaldehyde: C2 H5 OH ! CH3 CHO þ H2
(4)
Table 1 Steam reforming of ethanol over CuO-based catalysts Catalyst
H2O/EtOH
Conversion (%)
Selectivity (%) H2
A A A B C
0 5 10 5 5
55.9 95.7 75.5 89.7 16.1
66.4 84.4 87.6 77.8 71.5
CO2 11.0 16.2 18.7 13.4 17.2
CH3CHO 11.4 27.7 12.7 47 0
Acetone
35.6 37.5 54.7 31.5 54.9
Others C2H4
BA
AcOEt
MEK
Acetal
Unknown
2.5 0 0 0 5.1
2.4 0.5 0.1 0 0
1.1 0.2 0.1 0.1 0
0.7 0 0 0 0
16.8 14.7 6.0 0 0
18.4 3.2 7.7 8.0 22.9
Catalyst: (A) 20 mol% CuO/CeO20.5 g; (B) CuO 0.05 g + CeO20.45 g [CuO was charged in the front part of the reactor and CeO2 in the back, with glass wool separator between the two]; (C) CeO2 0.45 g, temperature: 320 8C, BA: butanal, AcOEt: ethyl acetate, MEK: methyl ethyl ketone, acetal: 1,1-diethoxyethane. For convenience’s sake, when 2 mol of hydrogen were formed from 1 mol of ethanol reacted, the selectivity of hydrogen was defined as 100%. The selectivity of other carbon-containing compounds was expressed on the basis of carbon balance of the reacted ethanol.
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Acetaldehyde suffers aldol condensation: 2CH3 CHO ! CH3 CHðOHÞCH2 CHO
(5)
The reaction of aldol with lattice oxygen O(S) on the catalyst follows to give the surface intermediate: CH3 CHðOHÞCH2 CHO þ OðSÞ ! CH3 CHðOHÞCH2 COOðSÞ þ HðSÞ
(6)
Finally, dehydrogenation and decarboxylation of the intermediate give acetone:
Fig. 4. Steam reforming of ethanol on 20 mol% CuO/CeO2 + MgO (1:1 weight ratio) (1.0 g): (*) ethanol, (*) hydrogen, (~) acetaldehyde, (!) acetone, (&) CO2 and (^) others.
CH3 CHðOHÞCH2 COOðSÞ þ HðSÞ ! CH3 COCH3 þ CO2 þ H2
(7)
Inui et al. also proposed a similar mechanism [25]. Aldol condensation (5) is catalyzed by acids or bases, so it occurs on the basic sites of CeO2. In reaction (7), dehydrogenation should occur prior to decarboxylation, because decarboxylation tends to occur for b-ketocarboxylate [26]. Lattice oxygen O(S) in reaction (6) must be constantly supplied for reactions (4)–(7) to proceed continuously. The lattice oxygen O(S) is formed by the decomposition of water (H2O ! O(S) + H2) over CeO2 [25]. Although some ambiguity is left about the decomposition of water, oxygen was surely supplied from water, considering the stoichiometry of the reaction. Therefore, the overall reaction is expressed by Eq. (3) and it can explain the observed outlet gas composition. The routes to the formation of ethyl acetate and acetal are deduced as follows. Hemiacetal is formed from ethanol and acetaldehyde: C2 H5 OH þ CH3 CHO ! CH3 CHðOHÞOC2 H5
(8)
By the dehydrogenation of hemiacetal, ethyl acetate is formed [25,27]: CH3 CHðOHÞOC2 H5 ! CH3 COOC2 H5 þ H2
(9)
However, when hemiacetal reacts with another molecule of ethanol, acetal is produced: CH3 CHðOHÞOC2 H5 þ C2 H5 OH ! CH3 CHðOC2 H5 Þ2 þ H2 O
(10)
As described previously, the formation of acetal occurred by the cooperative action of CuO and CeO2 [23]. 3.4. Effect of the addition of MgO Since, aldol condensation seems to be promoted by bases, steam reforming of ethanol was carried out on Cu/CeO2 mixed with MgO as a base component (Fig. 4). Larger amounts of hydrogen and acetone were produced at lower temperatures, compared with the results in the absence of MgO. This surely supports the mechanism of acetone formation via aldol condensation.
4. Conclusion Two moles of hydrogen were produced per 1 mol of ethanol in the steam reforming of ethanol over CuO/CeO2. Formation of acetone accompanied by high selectivity of hydrogen proceeded in the following three sequential steps: dehydrogenation of ethanol to acetaldehyde, aldol condensation from two molecules of acetaldehyde on a base catalyst, and the reaction of the aldol with lattice oxygen on CeO2 to form the surface intermediate, followed by its dehydrogenation and decarboxylation. The addition of base catalyst (MgO) produced acetone at lower temperatures.
Acknowledgement This research has been conducted as part of the project ‘‘Research and Development of Polymer Electrolyte Fuel Cell’’ under the entrustment contract with the New Energy and Industrial Technology Development Organization (NEDO).
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