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Hydrogenation of aliphatic carboxylic acids to corresponding aldehydes over Cr2O3-based catalysts
Applied Catalysis A: General 276 (2004) 179–185 www.elsevier.com/locate/apcata
Hydrogenation of aliphatic carboxylic acids to corresponding aldehydes over Cr2O3-based catalysts Toshiharu Yokoyama*, Naoko Fujita Mitsubishi Chemical Group Science and Technology Research Center Inc. 1000 Kamoshida-cho, Aoba-ku, Yokohama, Kanagawa 227-8502, Japan Received 16 April 2004; received in revised form 28 July 2004; accepted 3 August 2004 Available online 12 September 2004
Abstract Direct hydrogenation of aliphatic carboxylic acids to the corresponding aldehydes catalyzed by Cr–ZrO2 and supported Cr2O3 catalysts were investigated. A variety of Cr–ZrO2 catalysts were prepared by doping of Cr2O3 into ZrO2, which itself catalyzes the intermolecular ketonization of aliphatic carboxylic acids. This Cr–ZrO2 catalyst exhibited higher catalytic performance toward the hydrogenation of caprylic acid than pure Cr2O3, having maxima both of the catalyst activity and of specific surface area of the catalyst at Cr:Zr = 15:100 in atomic ratio. It is estimated that high dispersion of chromia on the Cr–ZrO2 catalyst surface will improve the catalyst performance. Furthermore, the aAl2O3-supported Cr2O3 catalyst showed over 90% aldehyde selectivity in the hydrogenation of stearic acid. However, employment of other supports such as g-Al2O3, SiO2 and TiO2 to Cr2O3 resulted in poor aldehyde selectivity and also encouraged undesirable ketonization to give predominantly C35-ketone species. Pre-treatment of the g-Al2O3 support at or above 1080 8C, enabled the Cr2O3 (10 wt.%)/g-Al2O3 catalyst to catalyze the hydrogenation over 90% selectivity. The novel catalyst of 10 wt.% Cr2O3 supported by commercial a-Al2O3 was found to hydrogenate various aliphatic carboxylic acids as well as benzoic acid and methyl nicotinate in moderate to high conversion and in high aldehyde selectivity. The effects of catalyst supports and their thermal pre-treatment in promoting the hydrogenation are also discussed. # 2004 Elsevier B.V. All rights reserved. Keywords: Carboxylic acid (aliphatic); Hydrogenation; Aldehyde; Chromia; Zirconia; Alumina
1. Introduction Aldehydes are widely recognized as important intermediates or final products in medicines, in pesticides and in some flavor industries. Some aliphatic aldehydes are also used as fragrances. Common methods for the production of aldehydes (except the oxo-aldehydes) are Rosenmund reduction of acid chlorides [1] and dehydrogenation or partial oxidation of alcohols, for example [2–4]. Although these methods can be utilized easily in a laboratory, these also have some problems: prior synthesis of reactive organohalides or acyl halides, requirement of a large amount of a base or halides, undesirable byproducts to be discarded and low yield and poor quality of aldehyde produced. * Corresponding author. Tel.: +81 45 9633197; fax: +81 45 9633974. E-mail address: [email protected] (T. Yokoyama). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.08.004
We have already succeeded in the development of direct hydrogenation of carboxylic acids to the corresponding aldehydes in a vapor-phase over efficient catalysts, Cr–ZrO2 for aromatic acids and Cr2O3 for aliphatic acids, as shown in Eq. (1). This hydrogenation process generates only water as a by-product and is widely applicable to various types of carboxylic acids, aromatic, alicyclic, aliphatic and even unsaturated aliphatic carboxylic acids such as 10-undecenoic acid, which is converted selectively into 10-undecenal without hydrogenation or migration of the terminal carbon– carbon double bond. We have also accomplished the commercial production of aldehydes by the hydrogenation process [5–9]. RCOOH þ H2 ! RCHO þ H2 O
(1)
Recently, Yamamoto and co-workers have been developed a similar reaction of liquid-phase hydrogenation of
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carboxylic acids catalyzed by a palladium–phosphane complex in the presence of excess pivalic anhydride [10]. This hydrogenation method can be carried out in a laboratory and can also convert various carboxylic acids including thermally unstable ones to vapor. Some types of di- and tribasic carboxylic acids, which did not give aldehydes by our hydrogenation, could be converted selectively and almost quantitatively into the corresponding aldehydes in one pot under mild conditions (typically at 80 8C for several hours is enough to complete the hydrogenation) except for a high pressure of hydrogen (>3 MPa). However, this method possesses some disadvantages: use of an excess amount of pivalic anhydride needs an additional separation process of aldehyde from remaining pivalic anhydride and pivalic acid, onerous handling of an air-sensitive palladium– phosphane catalyst is required, and the reaction must be performed in an autoclave. Thus, this method is considered to be complementary to our present hydrogenation process. For the vapor-phase direct hydrogenation of aliphatic carboxylic acids into aldehydes, Fe2O3 [11–12], V2O5/TiO2 [13], lanthanide-modified ZrO2 [14], Cr2O3/TiO2 [15] and Ru–Sn/Al2O3 [16] catalysts have been developed in addition to the pure Cr2O3 catalyst found by us. Furthermore, Grootendorst et al. [11] have reported that the metal oxide catalysts having moderately strong metal–oxygen bonds such as Fe, Ga, Sn and V show high aldehyde selectivity in the hydrogenation of acetic acid, whereas the catalysts with a strong metal-oxygen bond such as ZrO2 and TiO2 poorly catalyze the hydrogenation [12]. Among the metal oxide catalysts, Fe2O3 exhibits the highest catalytic activity toward the hydrogenation to give acetaldehyde in >80% selectivity. We consider, however, that the catalytic performances of these catalysts are insufficient to apply to commercial processes in view of the aldehyde selectivity and the catalyst life. It is already revealed that the simultaneous occurrence of intermolecular decarboxylative condensation to give ketones causes a lowering of aldehyde selectivity in the hydrogenation of the aliphatic carboxylic acids having two a-hydrogen atoms catalyzed over ZrO2 or ThO2, as shown in Eq. (2) [17–19]. This undesirable ketonization is thought to be a major problem in these direct hydrogenation reactions, even in our commercialized process. 2RCOOH ! R2 CO þ H2 O þ CO2
(2)
To surmount this problem, we attempted further improvement of our present pure Cr2O3 catalyst by introduction of some other metal oxide and by utilization of a catalyst support. Here, we report the investigation of the effect of chromia-doping to zirconia catalyst and of utilization of a support such as alumina, silica and titania, for the hydrogenation of aliphatic carboxylic acids to aldehydes. This study led us to a novel and highly efficient catalyst system, 10 wt.% Cr2O3/a-Al2O3. The novel catalyst is also found to be applicable to the hydrogenation of
benzoic acid, pivalic acid cyclohexanecarboxylic acid and methyl nicotinate. We also describe the effects of the surface of a catalyst and that of a support on the hydrogenation.
2. Experimental 2.1. Catalyst preparation 2.1.1. Cr–ZrO2 catalyst A variety of Cr–ZrO2 catalysts were prepared as follows. After isolation of ZrO(OH)2 precursor prepared by hydrolysis of ZrOCl2 with NH4OH, an aqueous solution of Cr(NO3)39H2O (Cr:Zr = 5:100, 10:100, 15:100 and 20:100 in atomic ratio) was added to the solid ZrO(OH)2, this material was evaporated to dryness and then calcined. Pure Cr2O3 was also obtained by hydrolysis of Cr(NO3)39H2O with NH4OH in water and following washing and calcination. These catalysts were further calcined at 700 8C for 3 h and sized between 10–20 meshes before use. 2.1.2. Supported Cr2O3 catalyst Supported Cr2O3 catalysts were obtained by addition of the support (sized in 10–20 mesh) to an aqueous solution of Cr(NO3)39H2O, followed by evaporation to dryness and calcination at 700 8C. As catalyst supports, g-Al2O3 (Rhone Poulenc, SPHERALITE 518B), a-Al2O3 (Rhone Poulenc, SPHERALITE 512A), SiO2 (Nikki Chemical Co. N608) and TiO2 (Mitsubishi Chemical Co.) were used as received. 2.1.3. Catalyst characterization These prepared catalysts and alumina supports were analyzed by the specific surface area measurements and Xray diffraction analysis. The specific surface areas of the catalysts were obtained with an Ohkura Riken AMS1000A by means of usual BET methods. The XRD analysis was performed with a PANalytical PW1880. 2.2. Hydrogenation The vapor-phase hydrogenation was performed with a fixed-bed reactor that consisted of a Pyrex glass tube (19 mm i.d. 450 mm) filled with 7 ml of the catalyst. The reactant was preheated to vaporize and the vapor was mixed with atmospheric hydrogen before introduction to the reactor. The gaseous mixture of hydrogen and the reactant was flowed through the reactor at H2 GHSV = 1250/h and the reactant LHSV = 0.13 kg/l-catalyst h, respectively, as standard conditions. The hydrogenated products were analyzed by gas chromatography and the carboxylic acids that remained unreacted were determined by titration with an aqueous solution of NaOH. Carboxylic acids and methyl nicotinate were used without further purification.
T. Yokoyama, N. Fujita / Applied Catalysis A: General 276 (2004) 179–185 Table 1 Hydrogenation of lauric acid over Cr–ZrO2 catalyst Catalysta
Cr:Zr atomic ratio
Conversion (%)
Selectivity (%) Laurylaldehyde
C23-ketone
ZrO2 Cr–ZrO2
0:100 5:100 10:100 15:100 20:100 100:0
91.9 67.2 74.9 65.6 73.4 30.3
23.2 83.8 90.4 92.6 86.1 88.7
76.1 15.4 5.0 7.0 13.5 8.3
Cr2O3
Reaction conditions: H2 GHSV=1250/h; LHSV=0.12 kg/l-catalyst h, 310 8C. a 10–20 mesh.
3. Results and discussion 3.1. Cr-doping effect of ZrO2 catalyst Results of hydrogenations of lauric acid catalyzed over pure Cr2O3, pure ZrO2 and Cr–ZrO2 are summarized in Table 1 and Fig. 1. Lauric acid was hydrogenated to laurylaldehyde accompanied by 12-tricosanone (C23-ketone) via intermolecular ketonization in all runs. Pure ZrO2 catalyzed preferentially undesirable ketonization at a high conversion of 91.9% to give C23-ketone (76.1% selectivity) as a major product together with a small amount of laurylaldehyde (23.2% selectivity). In contrast, pure Cr2O3 catalyst exhibited high aldehyde selectivity (88.7%) but lower reactivity (30.3% conversion). Surprisingly, Cr–ZrO2 prepared by doping Cr into the ZrO2 was found to catalyze the hydrogenation effectively, showing higher aldehyde selectivity with somewhat lower conversion of lauric acid as compared with that of pure ZrO2. Over the Cr–ZrO2 catalyst
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at Cr:Zr = 15:100 in atomic ratio, the aldehyde selectivity reached up to 92.6%. To characterize these catalysts, we also measured the specific surface areas (SA) of the Cr–ZrO2 catalysts. The SA is plotted against the Cr content (Cr/[Cr+Zr]) in Fig. 2. The SAs of pure ZrO2 and pure Cr2O3 were found to be 31.0 and 21.7 m2/g, respectively. Despite lower SA, pure ZrO2 showed high ketonization selectivity with high conversion of lauric acid. Doping Cr to ZrO2 catalyst caused considerable increase of the SA, having a maximum (99.0 m2/g) at Cr:Zr = 15:100 where the aldehyde selectivity was also highest. Expansion of SA of Cr–ZrO2 over three times larger than those of pure ZrO2 and pure Cr2O3 will also cause considerable suppression of ketonization and then high aldehyde selectivity. These results suggest that the crystalline microparticles of doped Cr2O3 will deposit so as to cover almost the entire surface of Cr–ZrO2 catalyst at around Cr:Zr = 15:100, causing remarkable suppression of ketonization. It is also estimated that the hydrogenation activity of Cr–ZrO2 catalysts will become closer to that of pure Cr2O3 with an increase of Cr content, in accord with a decreasing tendency of SA of Cr–ZrO2 over the range Cr:Zr = 15:100. Fig. 3 represents the relationship between the calcination temperature of the Cr–ZrO2 (Cr:Zr = 15:100) and the hydrogenation activity. As the calcination temperature rose from 600 to 800 8C the hydrogenation activity decreased, while the aldehyde selectivity increased, showing over 90% selectivity at 700 8C. The optimum calcination temperature is considered to be 700 8C in terms of the balance of hydrogenation activity and aldehyde selectivity. Some mechanisms for the ketonization of aliphatic carboxylic acid have been suggested: (1) by thermolysis of the corresponding alkali salt of carboxylic acid [20] (2) via
Fig. 1. Effect of Cr:Zr atomic ratio on the hydrogenation of lauric acid over Cr–ZrO2 catalyst.
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3.2. Supported Cr2O3 catalysts
Fig. 2. Surface area of Cr–ZrO2 catalyst.
carboxylic anhydride intermediate [21], and (3) via bketocarboxylic acid [22]. Pestman et al. have proposed a mechanism for the ketonization from acetic acid which involves the coupling reaction between one adsorbed molecule of acetic acid that strongly interacts with the catalyst through a-hydrogen and another adjacent adsorbed acetic acid [23]. According to their mechanism, the number of active sites on a catalyst surface which can interact the a-hydrogen of carboxylic acid will decrease with increase in the calcination temperature to cause suppression of the ketonization. Meanwhile, it is accepted that the carboxylate species adsorbed on the catalyst surface is the key intermediate to form aldehyde in the direct hydrogenation of carboxylic acids [6,24–26]. Thus, appropriate design of the catalyst surface suited for carboxylate absorption and not for interaction with the a-hydrogen of carboxylic acid, if possible, would enable the catalyst to be more efficient.
The idea above described that high dispersion of chromia on the Cr–ZrO2 catalyst surface will improve the catalyst activity prompted us to further investigate the effect of supports on the activity of the Cr2O3 catalyst. Results of hydrogenation of stearic acid over Cr2O3 at 330 8C on various supports are summarized in Table 2. Two percent Cr2O3/a-Al2O3 showed the stearylaldehyde selectivity of 82.9% and C35-ketone selectivity of 8.8% at a conversion of 47.4%. The catalyst performance increased gradually with an increase in the amount of Cr2O3 on the a-Al2O3 supports and the aldehyde selectivity showed a maximum around a range 5–10 wt.%. However, the Cr2O3 catalysts supported by g-Al2O3, TiO2 and SiO2 were found to be more prone to ketonization of stearic acid to C35ketone and exhibited poor aldehyde selectivity of below 30%, although Fisher et al. have reported that the hydrogenation over Cr2O3/TiO2 catalyst converted caprylic acid into caprylaldehyde in selectivity over 90% [15]. These results indicate that the type of support largely affects the performance of Cr2O3 catalyst. Table 3 shows the thermal effect of pre-treatment of the g-Al2O3 support of Cr2O3 on the hydrogenation of ncaprylic acid. The 10 wt.% Cr2O3/g-Al2O3 catalyst without pre-treatment of alumina support showed poor aldehyde selectivity. The activity of 10 wt.% Cr2O3/g-Al2O3 catalyst decreased as the temperature of pre-treatment of g-Al2O3 support rose up to 900 8C, above which it increased, showing a maximum at 1080 8C as shown in the upper part of Fig. 4. The selectivity of caprylaldehyde became over 95% by use of g-Al2O3 support pretreated above 1080 8C. On the contrary, the SA of the g-Al2O3 support itself decreased with increasing temperature of pre-treatment
Fig. 3. Effect of calcination temperature of Cr–ZrO2 on hydrogenation of lauric acid.
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Table 2 Hydrogenation of stearic acid over supported Cr2O3 catalyst Catalysta
Temperature
Conversion (%)
Selectivity (%)
(8C) Cr2O3 2%Cr2O3/a-Al2O3 5%Cr2O3/a-Al2O3 10%Cr2O3/a-Al2O3 20%Cr2O3/a-Al2O3 20%Cr2O3/g-Al2O3 20%Cr2O3/TiO2 20%Cr2O3/SiO2
330 330 330 330 330 330 330 330
84.7 47.4 55.1 59.2 65.8 31.3 94.7 47.2
Stearylaldehyde
C35-ketone
95.1 82.9 89.0 89.2 83.3 20.3 23 29.6
1.2 8.8 4.7 6.8 11.9 72.4 64.2 56.7
Reaction conditions: H2 GHSV=1250/h; LHSV=0.13 kg/l-catalyst h. a 10–20 mesh. Table 3 Hydrodenation of caprylic acid over 10%Cr2O3/g-Al2O3 catalyst Catalysta
Pre-treatment temperature of Al2O3 (8C)
Conversion (%)
10% Cr2O3/g-Al2O3
None 900 980 1080 1200 1400
45.1 34.2 55.3 93.3 52.2 41.2
SA (m2/g)
Selectivity (%) Caprylaldehyde
C17-ketone
Al2O3
Catalyst
36.4 43.9 78.6 96.2 95.5 97.6
57.4 55 20.5 2 3.9 2
273 131 96 27.3 9.2 2.2
190 117 89 25.7 9 3.2
Crystal-phase of Al2O3 g d d, u, a d, u, a – –
Reaction conditions: H2 GHSV=1250/h; LHSV=0.12 kg/l-catalyst h, 370 8C. a 3 mmf, calcined at 700 8C.
of g-Al2O3 support over the whole temperature range measured, as shown in Table 3 and the lower part of Fig. 4. The SA of 10 wt.% Cr2O3 catalyst supported by g-Al2O3 pretreated at 1080 8C was found to be 25.7 m2/g, somewhat smaller than that of g-Al2O3 support itself (27.3 m2/g). In
addition, the powder diffraction X-ray analysis also revealed the existence of the a-, d- and u-Al2O3-phases in the gAl2O3 pretreated at 1080 8C. Moreover, the 10 wt.% Cr2O3/ commercial a-Al2O3 catalyst (SA 9.7 m2/g) showed a conversion of 92.8% and aldehyde selectivity of 97.8%,
Fig. 4. Hydrogenation of caprylic acid over 10%Cr2O3/g-Al2O3 catalyst. (Effects of pre-treatment temperature of support on the catalytic activity and SA).
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Table 4 Hydrogenation of caprylic acid over Cr2O3/Al2O3 catalyst and Al2O3 supports Catalyst
Temperature (8C)
Conversion (%)
Selectivity (%) Caprylaldehyde
C17-ketone
10%Cr2O3/g-Al2O3a g-Al2O3a
370 370 380 380 380
45.1 59.3 97.4 96.4 13.9
36.2 2.8 2.2 95.7 24.7
57.4 88.3 94.9 1.9 75.3
10%Cr2O3/a-Al2O3b a-Al2O3b
Reaction conditions: H2 GHSV=1250/h, LHSV=0.11 kg/l-catalyst h. a 3 mm f. b 4–6 mm f.
Table 5 Hydrogenation of various carboxylic acids over 10%Cr2O3/a-Al2O3 catalysta Carboxylic acids
Temperature (8C)
Conversion (%)
Selectivity of aldehyde (%)
Caprylic acid Cyclohexane carboxylic acid Pivalic acid Benzoic acid Methyl nicotinate
380 400 410 410 410
96.4 94.5 97.9 77.8 59.4
95.7 97.9 97.9 98.4 85.0
Reaction conditions: H2 GHSV = 1250/h, LHSV=0.11 kg/l-catalyst h. a 4–6 mmf.
almost equal to those of 10 wt.% Cr2O3/g-Al2O3 pretreated at 1080 8C. These results suggest that the nature of the catalyst surface rather than the crystal-phase of alumina support will play a quite important role in the catalyst activity. Table 4 illustrates the comparison of catalytic performance of 10 wt.% Cr2O3/Al2O3 with that of Al2O3 itself toward the hydrogenation of caprylic acid. Ketonization of caprylic acid dominated the hydrogenation by use of pure gAl2O3 catalyst [9], showing C17-ketone selectivity of 88.3% at 370 8C. The 10 wt.% Cr2O3/g-Al2O3 catalyst also exhibited poor aldehyde selectivity (36.2%) with lowering of C17-ketone selectivity. a-Al2O3 itself showed somewhat lower selectivity of C17-ketone than g-Al2O3 did, but the activity became poor. However, the 10 wt.% Cr2O3/a-Al2O3 catalyzed the hydrogenation in high conversion (96.4%) and aldehyde selectivity (95.7%). These results will be explained by an idea similar to that mentioned before that high dispersion of Cr2O3 particle on the a-Al2O3 surface causes suppression of ketonization originating in the character of pure a-Al2O3 and enhancement of aldehyde selectivity derived from pure Cr2O3 catalyst. 3.3. Hydrogenation of various carboxylic acids Results of hydrogenation of various carboxylic acids over the 10 wt.% Cr2O3/a-Al2O3 are summarized in Table 5. Hindered aliphatic carboxylic acids, cyclohexanecarboxylic acid and pivalic acid were hydrogenated to cyclohexanecarbaldehyde and pivalaldehyde, respectively, in high conversion and high aldehyde selectivity under
somewhat higher temperatures than that of caprylic acid. In the case of aromatic carboxylic acids, benzoic acid was also converted into benzaldehyde in high selectivity at moderate conversion. Since nicotinic acid forms an intramolecular zwitterionic salt and can not vaporize, prior esterification of nicotinic acid to the hydrogenation is required. The Cr2O3/a-Al2O3 catalyst afforded to hydrogenate methyl nicotinate to give nicotinaldehyde at a low conversion of 59.4% but in moderate selectivity (85.0%). These results indicate that the 10 wt.% Cr2O3/a-Al2O3 catalyst is widely applicable to hydrogenation of various types of carboxylic acids into aldehydes.
4. Conclusions Direct hydrogenation of aliphatic carboxylic acids to the corresponding aldehydes catalyzed by Cr–ZrO2 and supported Cr2O3 catalysts was investigated. Cr–ZrO2 (Cr:Zr = 15:100) exhibited highest catalyst performance. When we used a-Al2O3 as a support of Cr2O3, the aldehyde selectivity became over 90%. The Cr2O3 (10 wt.%)/a-Al2O3 catalyst also hydrogenates various types of aliphatic carboxylic acids as well as aromatic carboxylic acids and their esters in moderate to high conversion and selectivity. In conclusion, the use of a support that itself is less prone to undesirable ketonization such as a-Al2O3 is found to be very effective in improvement of the hydrogenation performance of Cr2O3 catalyst. We believe that the Cr2O3 (10 wt.%)/a-Al2O3 catalyst will have a potential to produce both aromatic and
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aliphatic aldehydes in the same reactor without requirement of change of the catalysts.
References [1] K.W. Rosenmund, Berichte 51 (1918) 585. [2] R. Brettle, in: J.F. Stoddard (Ed.), Comprehensive Organic Chemistry,, 1, Pergamon Press, Oxford, 1979, pp. 943–1015. [3] D.J. Miller, 4th Edition, Kirk-Othmer Encyclopedia of Chemical Technology,, 1, Wiley, New York, 1991, pp. 926–937. [4] T. Maki, T. Yokoyama, Org. Synth. Chem. 49 (1991) 195. [5] T. Maki, T. Yokoyama, US Patent 4,613,700 (1986). [6] T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima, T. Maki, Appl. Catal. 88 (1992) 149. [7] T. Yokoyama, K. Fujii, Petrotech (Tokyo) 14 (1991) 633. [8] N. Yamagata, T. Yokoyama, T. Maki, Stud. Surf. Sci. Catal. 121 (1998) 441. [9] T. Yokoyama, N. Yamagata, Appl. Catal. 221 (2001) 227. [10] K. Nagayama, I. Shimizu, A. Yamamoto, Bull. Chem. Soc. Jpn. 74 (2001) 1803. [11] E.J. Grootendorst, R. Pestman, R.M. Koster, V. Ponec, J. Catal. 148 (1994) 261.
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[12] R. Pestman, R.M. Koster, J.A.Z. Pieterse, V. Ponec, J. Catal. 168 (1997) 255. [13] D.C. Hargis, US Patent 4,950,799 (1990). [14] W.Sheurr, R. Fisher, J. Wulff-Do¨ ring, M. Hesse, European Patent 716,070 (1995). [15] R. Fisher, S. Liang, E. Schwab, H. Sterzel, J. Wulff-Doering, Ger Offen. Patent 1,979,381 (1997). [16] R.M. Ferrero, European Patent 539,274 (1992). [17] T. Yokoyama, N. Fujita, T. Maki, Stud. Surf. Sci. Catal. 92 (1994) 331. [18] T. Okamoto, T. Ohashi, I. Koga, Japan Kokai Patent 49-48614 (1974). [19] A.L. Miller, N.C. Cook, F.C. Whitmore, J. Am. Chem. Soc. 72 (1950) 2732. [20] E.R. Squbb, J. Am. Chem. Soc. 17 (1895) 187. [21] E. Bamberger, Berichte 43 (1910) 3517. [22] O. Neunhoeffer, P. Paschke, Berichte 72 (1939) 919. [23] R. Pestman, R.M. Koster, A.V. Duijine, J.A.Z. Pieters, V. Ponec, J. Catal. 168 (1997) 265. [24] S.T. King, E.J. Strojny, J. Catal. 76 (1982) 274. [25] J. Kondo, N. Ding, K. Maruya, K. Domen, T. Yokoyama, N. Fujita, T. Maki, Bull. Chem. Soc. Jpn. 66 (1993) 3085. [26] N. Ding, J. Kondo, K. Maruya, L. Domen, T. Yokoyama, N. Fujita, T. Maki, Catal. Lett. 17 (1993) 309.
Hydrogenation of aliphatic carboxylic acids to corresponding aldehydes over Cr2O3-based catalysts Toshiharu Yokoyama*, Naoko Fujita Mitsubishi Chemical Group Science and Technology Research Center Inc. 1000 Kamoshida-cho, Aoba-ku, Yokohama, Kanagawa 227-8502, Japan Received 16 April 2004; received in revised form 28 July 2004; accepted 3 August 2004 Available online 12 September 2004
Abstract Direct hydrogenation of aliphatic carboxylic acids to the corresponding aldehydes catalyzed by Cr–ZrO2 and supported Cr2O3 catalysts were investigated. A variety of Cr–ZrO2 catalysts were prepared by doping of Cr2O3 into ZrO2, which itself catalyzes the intermolecular ketonization of aliphatic carboxylic acids. This Cr–ZrO2 catalyst exhibited higher catalytic performance toward the hydrogenation of caprylic acid than pure Cr2O3, having maxima both of the catalyst activity and of specific surface area of the catalyst at Cr:Zr = 15:100 in atomic ratio. It is estimated that high dispersion of chromia on the Cr–ZrO2 catalyst surface will improve the catalyst performance. Furthermore, the aAl2O3-supported Cr2O3 catalyst showed over 90% aldehyde selectivity in the hydrogenation of stearic acid. However, employment of other supports such as g-Al2O3, SiO2 and TiO2 to Cr2O3 resulted in poor aldehyde selectivity and also encouraged undesirable ketonization to give predominantly C35-ketone species. Pre-treatment of the g-Al2O3 support at or above 1080 8C, enabled the Cr2O3 (10 wt.%)/g-Al2O3 catalyst to catalyze the hydrogenation over 90% selectivity. The novel catalyst of 10 wt.% Cr2O3 supported by commercial a-Al2O3 was found to hydrogenate various aliphatic carboxylic acids as well as benzoic acid and methyl nicotinate in moderate to high conversion and in high aldehyde selectivity. The effects of catalyst supports and their thermal pre-treatment in promoting the hydrogenation are also discussed. # 2004 Elsevier B.V. All rights reserved. Keywords: Carboxylic acid (aliphatic); Hydrogenation; Aldehyde; Chromia; Zirconia; Alumina
1. Introduction Aldehydes are widely recognized as important intermediates or final products in medicines, in pesticides and in some flavor industries. Some aliphatic aldehydes are also used as fragrances. Common methods for the production of aldehydes (except the oxo-aldehydes) are Rosenmund reduction of acid chlorides [1] and dehydrogenation or partial oxidation of alcohols, for example [2–4]. Although these methods can be utilized easily in a laboratory, these also have some problems: prior synthesis of reactive organohalides or acyl halides, requirement of a large amount of a base or halides, undesirable byproducts to be discarded and low yield and poor quality of aldehyde produced. * Corresponding author. Tel.: +81 45 9633197; fax: +81 45 9633974. E-mail address: [email protected] (T. Yokoyama). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.08.004
We have already succeeded in the development of direct hydrogenation of carboxylic acids to the corresponding aldehydes in a vapor-phase over efficient catalysts, Cr–ZrO2 for aromatic acids and Cr2O3 for aliphatic acids, as shown in Eq. (1). This hydrogenation process generates only water as a by-product and is widely applicable to various types of carboxylic acids, aromatic, alicyclic, aliphatic and even unsaturated aliphatic carboxylic acids such as 10-undecenoic acid, which is converted selectively into 10-undecenal without hydrogenation or migration of the terminal carbon– carbon double bond. We have also accomplished the commercial production of aldehydes by the hydrogenation process [5–9]. RCOOH þ H2 ! RCHO þ H2 O
(1)
Recently, Yamamoto and co-workers have been developed a similar reaction of liquid-phase hydrogenation of
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carboxylic acids catalyzed by a palladium–phosphane complex in the presence of excess pivalic anhydride [10]. This hydrogenation method can be carried out in a laboratory and can also convert various carboxylic acids including thermally unstable ones to vapor. Some types of di- and tribasic carboxylic acids, which did not give aldehydes by our hydrogenation, could be converted selectively and almost quantitatively into the corresponding aldehydes in one pot under mild conditions (typically at 80 8C for several hours is enough to complete the hydrogenation) except for a high pressure of hydrogen (>3 MPa). However, this method possesses some disadvantages: use of an excess amount of pivalic anhydride needs an additional separation process of aldehyde from remaining pivalic anhydride and pivalic acid, onerous handling of an air-sensitive palladium– phosphane catalyst is required, and the reaction must be performed in an autoclave. Thus, this method is considered to be complementary to our present hydrogenation process. For the vapor-phase direct hydrogenation of aliphatic carboxylic acids into aldehydes, Fe2O3 [11–12], V2O5/TiO2 [13], lanthanide-modified ZrO2 [14], Cr2O3/TiO2 [15] and Ru–Sn/Al2O3 [16] catalysts have been developed in addition to the pure Cr2O3 catalyst found by us. Furthermore, Grootendorst et al. [11] have reported that the metal oxide catalysts having moderately strong metal–oxygen bonds such as Fe, Ga, Sn and V show high aldehyde selectivity in the hydrogenation of acetic acid, whereas the catalysts with a strong metal-oxygen bond such as ZrO2 and TiO2 poorly catalyze the hydrogenation [12]. Among the metal oxide catalysts, Fe2O3 exhibits the highest catalytic activity toward the hydrogenation to give acetaldehyde in >80% selectivity. We consider, however, that the catalytic performances of these catalysts are insufficient to apply to commercial processes in view of the aldehyde selectivity and the catalyst life. It is already revealed that the simultaneous occurrence of intermolecular decarboxylative condensation to give ketones causes a lowering of aldehyde selectivity in the hydrogenation of the aliphatic carboxylic acids having two a-hydrogen atoms catalyzed over ZrO2 or ThO2, as shown in Eq. (2) [17–19]. This undesirable ketonization is thought to be a major problem in these direct hydrogenation reactions, even in our commercialized process. 2RCOOH ! R2 CO þ H2 O þ CO2
(2)
To surmount this problem, we attempted further improvement of our present pure Cr2O3 catalyst by introduction of some other metal oxide and by utilization of a catalyst support. Here, we report the investigation of the effect of chromia-doping to zirconia catalyst and of utilization of a support such as alumina, silica and titania, for the hydrogenation of aliphatic carboxylic acids to aldehydes. This study led us to a novel and highly efficient catalyst system, 10 wt.% Cr2O3/a-Al2O3. The novel catalyst is also found to be applicable to the hydrogenation of
benzoic acid, pivalic acid cyclohexanecarboxylic acid and methyl nicotinate. We also describe the effects of the surface of a catalyst and that of a support on the hydrogenation.
2. Experimental 2.1. Catalyst preparation 2.1.1. Cr–ZrO2 catalyst A variety of Cr–ZrO2 catalysts were prepared as follows. After isolation of ZrO(OH)2 precursor prepared by hydrolysis of ZrOCl2 with NH4OH, an aqueous solution of Cr(NO3)39H2O (Cr:Zr = 5:100, 10:100, 15:100 and 20:100 in atomic ratio) was added to the solid ZrO(OH)2, this material was evaporated to dryness and then calcined. Pure Cr2O3 was also obtained by hydrolysis of Cr(NO3)39H2O with NH4OH in water and following washing and calcination. These catalysts were further calcined at 700 8C for 3 h and sized between 10–20 meshes before use. 2.1.2. Supported Cr2O3 catalyst Supported Cr2O3 catalysts were obtained by addition of the support (sized in 10–20 mesh) to an aqueous solution of Cr(NO3)39H2O, followed by evaporation to dryness and calcination at 700 8C. As catalyst supports, g-Al2O3 (Rhone Poulenc, SPHERALITE 518B), a-Al2O3 (Rhone Poulenc, SPHERALITE 512A), SiO2 (Nikki Chemical Co. N608) and TiO2 (Mitsubishi Chemical Co.) were used as received. 2.1.3. Catalyst characterization These prepared catalysts and alumina supports were analyzed by the specific surface area measurements and Xray diffraction analysis. The specific surface areas of the catalysts were obtained with an Ohkura Riken AMS1000A by means of usual BET methods. The XRD analysis was performed with a PANalytical PW1880. 2.2. Hydrogenation The vapor-phase hydrogenation was performed with a fixed-bed reactor that consisted of a Pyrex glass tube (19 mm i.d. 450 mm) filled with 7 ml of the catalyst. The reactant was preheated to vaporize and the vapor was mixed with atmospheric hydrogen before introduction to the reactor. The gaseous mixture of hydrogen and the reactant was flowed through the reactor at H2 GHSV = 1250/h and the reactant LHSV = 0.13 kg/l-catalyst h, respectively, as standard conditions. The hydrogenated products were analyzed by gas chromatography and the carboxylic acids that remained unreacted were determined by titration with an aqueous solution of NaOH. Carboxylic acids and methyl nicotinate were used without further purification.
T. Yokoyama, N. Fujita / Applied Catalysis A: General 276 (2004) 179–185 Table 1 Hydrogenation of lauric acid over Cr–ZrO2 catalyst Catalysta
Cr:Zr atomic ratio
Conversion (%)
Selectivity (%) Laurylaldehyde
C23-ketone
ZrO2 Cr–ZrO2
0:100 5:100 10:100 15:100 20:100 100:0
91.9 67.2 74.9 65.6 73.4 30.3
23.2 83.8 90.4 92.6 86.1 88.7
76.1 15.4 5.0 7.0 13.5 8.3
Cr2O3
Reaction conditions: H2 GHSV=1250/h; LHSV=0.12 kg/l-catalyst h, 310 8C. a 10–20 mesh.
3. Results and discussion 3.1. Cr-doping effect of ZrO2 catalyst Results of hydrogenations of lauric acid catalyzed over pure Cr2O3, pure ZrO2 and Cr–ZrO2 are summarized in Table 1 and Fig. 1. Lauric acid was hydrogenated to laurylaldehyde accompanied by 12-tricosanone (C23-ketone) via intermolecular ketonization in all runs. Pure ZrO2 catalyzed preferentially undesirable ketonization at a high conversion of 91.9% to give C23-ketone (76.1% selectivity) as a major product together with a small amount of laurylaldehyde (23.2% selectivity). In contrast, pure Cr2O3 catalyst exhibited high aldehyde selectivity (88.7%) but lower reactivity (30.3% conversion). Surprisingly, Cr–ZrO2 prepared by doping Cr into the ZrO2 was found to catalyze the hydrogenation effectively, showing higher aldehyde selectivity with somewhat lower conversion of lauric acid as compared with that of pure ZrO2. Over the Cr–ZrO2 catalyst
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at Cr:Zr = 15:100 in atomic ratio, the aldehyde selectivity reached up to 92.6%. To characterize these catalysts, we also measured the specific surface areas (SA) of the Cr–ZrO2 catalysts. The SA is plotted against the Cr content (Cr/[Cr+Zr]) in Fig. 2. The SAs of pure ZrO2 and pure Cr2O3 were found to be 31.0 and 21.7 m2/g, respectively. Despite lower SA, pure ZrO2 showed high ketonization selectivity with high conversion of lauric acid. Doping Cr to ZrO2 catalyst caused considerable increase of the SA, having a maximum (99.0 m2/g) at Cr:Zr = 15:100 where the aldehyde selectivity was also highest. Expansion of SA of Cr–ZrO2 over three times larger than those of pure ZrO2 and pure Cr2O3 will also cause considerable suppression of ketonization and then high aldehyde selectivity. These results suggest that the crystalline microparticles of doped Cr2O3 will deposit so as to cover almost the entire surface of Cr–ZrO2 catalyst at around Cr:Zr = 15:100, causing remarkable suppression of ketonization. It is also estimated that the hydrogenation activity of Cr–ZrO2 catalysts will become closer to that of pure Cr2O3 with an increase of Cr content, in accord with a decreasing tendency of SA of Cr–ZrO2 over the range Cr:Zr = 15:100. Fig. 3 represents the relationship between the calcination temperature of the Cr–ZrO2 (Cr:Zr = 15:100) and the hydrogenation activity. As the calcination temperature rose from 600 to 800 8C the hydrogenation activity decreased, while the aldehyde selectivity increased, showing over 90% selectivity at 700 8C. The optimum calcination temperature is considered to be 700 8C in terms of the balance of hydrogenation activity and aldehyde selectivity. Some mechanisms for the ketonization of aliphatic carboxylic acid have been suggested: (1) by thermolysis of the corresponding alkali salt of carboxylic acid [20] (2) via
Fig. 1. Effect of Cr:Zr atomic ratio on the hydrogenation of lauric acid over Cr–ZrO2 catalyst.
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3.2. Supported Cr2O3 catalysts
Fig. 2. Surface area of Cr–ZrO2 catalyst.
carboxylic anhydride intermediate [21], and (3) via bketocarboxylic acid [22]. Pestman et al. have proposed a mechanism for the ketonization from acetic acid which involves the coupling reaction between one adsorbed molecule of acetic acid that strongly interacts with the catalyst through a-hydrogen and another adjacent adsorbed acetic acid [23]. According to their mechanism, the number of active sites on a catalyst surface which can interact the a-hydrogen of carboxylic acid will decrease with increase in the calcination temperature to cause suppression of the ketonization. Meanwhile, it is accepted that the carboxylate species adsorbed on the catalyst surface is the key intermediate to form aldehyde in the direct hydrogenation of carboxylic acids [6,24–26]. Thus, appropriate design of the catalyst surface suited for carboxylate absorption and not for interaction with the a-hydrogen of carboxylic acid, if possible, would enable the catalyst to be more efficient.
The idea above described that high dispersion of chromia on the Cr–ZrO2 catalyst surface will improve the catalyst activity prompted us to further investigate the effect of supports on the activity of the Cr2O3 catalyst. Results of hydrogenation of stearic acid over Cr2O3 at 330 8C on various supports are summarized in Table 2. Two percent Cr2O3/a-Al2O3 showed the stearylaldehyde selectivity of 82.9% and C35-ketone selectivity of 8.8% at a conversion of 47.4%. The catalyst performance increased gradually with an increase in the amount of Cr2O3 on the a-Al2O3 supports and the aldehyde selectivity showed a maximum around a range 5–10 wt.%. However, the Cr2O3 catalysts supported by g-Al2O3, TiO2 and SiO2 were found to be more prone to ketonization of stearic acid to C35ketone and exhibited poor aldehyde selectivity of below 30%, although Fisher et al. have reported that the hydrogenation over Cr2O3/TiO2 catalyst converted caprylic acid into caprylaldehyde in selectivity over 90% [15]. These results indicate that the type of support largely affects the performance of Cr2O3 catalyst. Table 3 shows the thermal effect of pre-treatment of the g-Al2O3 support of Cr2O3 on the hydrogenation of ncaprylic acid. The 10 wt.% Cr2O3/g-Al2O3 catalyst without pre-treatment of alumina support showed poor aldehyde selectivity. The activity of 10 wt.% Cr2O3/g-Al2O3 catalyst decreased as the temperature of pre-treatment of g-Al2O3 support rose up to 900 8C, above which it increased, showing a maximum at 1080 8C as shown in the upper part of Fig. 4. The selectivity of caprylaldehyde became over 95% by use of g-Al2O3 support pretreated above 1080 8C. On the contrary, the SA of the g-Al2O3 support itself decreased with increasing temperature of pre-treatment
Fig. 3. Effect of calcination temperature of Cr–ZrO2 on hydrogenation of lauric acid.
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Table 2 Hydrogenation of stearic acid over supported Cr2O3 catalyst Catalysta
Temperature
Conversion (%)
Selectivity (%)
(8C) Cr2O3 2%Cr2O3/a-Al2O3 5%Cr2O3/a-Al2O3 10%Cr2O3/a-Al2O3 20%Cr2O3/a-Al2O3 20%Cr2O3/g-Al2O3 20%Cr2O3/TiO2 20%Cr2O3/SiO2
330 330 330 330 330 330 330 330
84.7 47.4 55.1 59.2 65.8 31.3 94.7 47.2
Stearylaldehyde
C35-ketone
95.1 82.9 89.0 89.2 83.3 20.3 23 29.6
1.2 8.8 4.7 6.8 11.9 72.4 64.2 56.7
Reaction conditions: H2 GHSV=1250/h; LHSV=0.13 kg/l-catalyst h. a 10–20 mesh. Table 3 Hydrodenation of caprylic acid over 10%Cr2O3/g-Al2O3 catalyst Catalysta
Pre-treatment temperature of Al2O3 (8C)
Conversion (%)
10% Cr2O3/g-Al2O3
None 900 980 1080 1200 1400
45.1 34.2 55.3 93.3 52.2 41.2
SA (m2/g)
Selectivity (%) Caprylaldehyde
C17-ketone
Al2O3
Catalyst
36.4 43.9 78.6 96.2 95.5 97.6
57.4 55 20.5 2 3.9 2
273 131 96 27.3 9.2 2.2
190 117 89 25.7 9 3.2
Crystal-phase of Al2O3 g d d, u, a d, u, a – –
Reaction conditions: H2 GHSV=1250/h; LHSV=0.12 kg/l-catalyst h, 370 8C. a 3 mmf, calcined at 700 8C.
of g-Al2O3 support over the whole temperature range measured, as shown in Table 3 and the lower part of Fig. 4. The SA of 10 wt.% Cr2O3 catalyst supported by g-Al2O3 pretreated at 1080 8C was found to be 25.7 m2/g, somewhat smaller than that of g-Al2O3 support itself (27.3 m2/g). In
addition, the powder diffraction X-ray analysis also revealed the existence of the a-, d- and u-Al2O3-phases in the gAl2O3 pretreated at 1080 8C. Moreover, the 10 wt.% Cr2O3/ commercial a-Al2O3 catalyst (SA 9.7 m2/g) showed a conversion of 92.8% and aldehyde selectivity of 97.8%,
Fig. 4. Hydrogenation of caprylic acid over 10%Cr2O3/g-Al2O3 catalyst. (Effects of pre-treatment temperature of support on the catalytic activity and SA).
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Table 4 Hydrogenation of caprylic acid over Cr2O3/Al2O3 catalyst and Al2O3 supports Catalyst
Temperature (8C)
Conversion (%)
Selectivity (%) Caprylaldehyde
C17-ketone
10%Cr2O3/g-Al2O3a g-Al2O3a
370 370 380 380 380
45.1 59.3 97.4 96.4 13.9
36.2 2.8 2.2 95.7 24.7
57.4 88.3 94.9 1.9 75.3
10%Cr2O3/a-Al2O3b a-Al2O3b
Reaction conditions: H2 GHSV=1250/h, LHSV=0.11 kg/l-catalyst h. a 3 mm f. b 4–6 mm f.
Table 5 Hydrogenation of various carboxylic acids over 10%Cr2O3/a-Al2O3 catalysta Carboxylic acids
Temperature (8C)
Conversion (%)
Selectivity of aldehyde (%)
Caprylic acid Cyclohexane carboxylic acid Pivalic acid Benzoic acid Methyl nicotinate
380 400 410 410 410
96.4 94.5 97.9 77.8 59.4
95.7 97.9 97.9 98.4 85.0
Reaction conditions: H2 GHSV = 1250/h, LHSV=0.11 kg/l-catalyst h. a 4–6 mmf.
almost equal to those of 10 wt.% Cr2O3/g-Al2O3 pretreated at 1080 8C. These results suggest that the nature of the catalyst surface rather than the crystal-phase of alumina support will play a quite important role in the catalyst activity. Table 4 illustrates the comparison of catalytic performance of 10 wt.% Cr2O3/Al2O3 with that of Al2O3 itself toward the hydrogenation of caprylic acid. Ketonization of caprylic acid dominated the hydrogenation by use of pure gAl2O3 catalyst [9], showing C17-ketone selectivity of 88.3% at 370 8C. The 10 wt.% Cr2O3/g-Al2O3 catalyst also exhibited poor aldehyde selectivity (36.2%) with lowering of C17-ketone selectivity. a-Al2O3 itself showed somewhat lower selectivity of C17-ketone than g-Al2O3 did, but the activity became poor. However, the 10 wt.% Cr2O3/a-Al2O3 catalyzed the hydrogenation in high conversion (96.4%) and aldehyde selectivity (95.7%). These results will be explained by an idea similar to that mentioned before that high dispersion of Cr2O3 particle on the a-Al2O3 surface causes suppression of ketonization originating in the character of pure a-Al2O3 and enhancement of aldehyde selectivity derived from pure Cr2O3 catalyst. 3.3. Hydrogenation of various carboxylic acids Results of hydrogenation of various carboxylic acids over the 10 wt.% Cr2O3/a-Al2O3 are summarized in Table 5. Hindered aliphatic carboxylic acids, cyclohexanecarboxylic acid and pivalic acid were hydrogenated to cyclohexanecarbaldehyde and pivalaldehyde, respectively, in high conversion and high aldehyde selectivity under
somewhat higher temperatures than that of caprylic acid. In the case of aromatic carboxylic acids, benzoic acid was also converted into benzaldehyde in high selectivity at moderate conversion. Since nicotinic acid forms an intramolecular zwitterionic salt and can not vaporize, prior esterification of nicotinic acid to the hydrogenation is required. The Cr2O3/a-Al2O3 catalyst afforded to hydrogenate methyl nicotinate to give nicotinaldehyde at a low conversion of 59.4% but in moderate selectivity (85.0%). These results indicate that the 10 wt.% Cr2O3/a-Al2O3 catalyst is widely applicable to hydrogenation of various types of carboxylic acids into aldehydes.
4. Conclusions Direct hydrogenation of aliphatic carboxylic acids to the corresponding aldehydes catalyzed by Cr–ZrO2 and supported Cr2O3 catalysts was investigated. Cr–ZrO2 (Cr:Zr = 15:100) exhibited highest catalyst performance. When we used a-Al2O3 as a support of Cr2O3, the aldehyde selectivity became over 90%. The Cr2O3 (10 wt.%)/a-Al2O3 catalyst also hydrogenates various types of aliphatic carboxylic acids as well as aromatic carboxylic acids and their esters in moderate to high conversion and selectivity. In conclusion, the use of a support that itself is less prone to undesirable ketonization such as a-Al2O3 is found to be very effective in improvement of the hydrogenation performance of Cr2O3 catalyst. We believe that the Cr2O3 (10 wt.%)/a-Al2O3 catalyst will have a potential to produce both aromatic and
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aliphatic aldehydes in the same reactor without requirement of change of the catalysts.
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