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Hydrogenation of 2-Ethylhexenal Using Thermoregulated Phase-Transfer Catalyst for Production of 2-Ethylhexanol Shiwei Liu,† Congxia Xie,*,‡ Shitao Yu,† Fusheng Liu,† and Zhanqian Song† †
College of Chemical Engineering and ‡College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, No. 53 Zhengzhou Road, Qingdao 266042, People's Republic of China ABSTRACT: The hydrogenation of 2-ethylhexenal was investigated in the presence of thermoregulated phase-transfer catalysts to produce 2-ethylhexanol. The thermoregulated catalytic system Pd/IV (IV: P-ligand, tri(methoxyl polyethylene glycol)-phosphite) was an efficient catalyst for the hydrogenation, and the conversion of 2-ethylhexenal and the selectivity of 2-ethylhexanol were 96.9% and 95.5%, respectively, when the reaction was carried out at 220 °C for 8 h. The product was easily separated from the catalytic system and the reusability of the separated catalyst was good. It was also found that the steric resistance of the P-ligand affected the performance of the catalytic system.
’ INTRODUCTION 2-Ethylhexanol is an important raw material and is predominately converted to diethylhexyl phthalate (generally known as dioctyl phthalate, DOP), which is an excellent and physiologically harmless plasticizer for poly(vinyl chloride) (PVC). 2-Ethylhexanol is also widely used in the product of adhesives, surfactants, antioxidants, and cosmetics, and it is used as a diesel and lubricating oil additive, etc. At present, most 2-ethylhexanol producers start from the hydroformylation of propene to give n-butanal. n-Butanal is converted to 2-ethylhexenal by a basecatalyzed aldol condensation reaction, and then it is hydrogenated to obtain 2-ethylhexenol.1 The hydrogenation of 2-ethylhexenal is very important in the above processes, and its result directly affects the quality and yield of 2-ethylhexanol. Usually, the hydrogenation is carried out in the gas phase over a nickel or copper catalyst. This process has some disadvantages, such as high consumption of energy, severe conditions, and low selectivity to the product. Although hydrogenation in the liquid phase can overcome some of the above shortcomings, the catalysts used, such as Ni/SiO2 and Ni-Mo/Al2O3,2-4 have their own disadvantages such as low activity, easy deactivation, and difficult separation from the product. Therefore, it is necessary to explore a new approach for the hydrogenation of 2-ethylhexanol. Based on the property of the cloud point of a nonionic P-ligand, a novel aqueous biphasic homogeneous catalysis system;called thermoregulated phase-transfer catalysis (TRPTC); was recently proposed.5-8 The introduction of TRPTC is free from the shortcomings of classical biphasic catalysis, in which the scope of application is restrained by the water solubility of the substrate. The character of this catalytic process is a monophasic reaction combined with a biphasic separation, which allows for the separation and reusability of the catalyst. TRPTC has been successfully used in the hydrogenation of high-carbon olefins with excellent conversion and selectivity.9,10 However, the reported hydrogenations mainly use linear olefins with little steric hindrance as reagents, and the reaction temperature was less than 150 °C. To the best of our knowledge, no article about r 2010 American Chemical Society
the hydrogenation of an olefinic aldehyde compound over TRPTC has been reported. Therefore, we first studied the hydrogenation of 2-ethylhexanol in the presence of TRPTC. To our surprise, the TRPTC had excellent catalytic activity and reusability in the hydrogenation of 2-ethylhexanol, even using a reaction temperature of 220 °C.
’ MATERIALS AND METHODS Materials. Tetrahydrofuran, triethylamine, and catechol were distilled over calcium hydride or a 3A molecular sieve under reduced pressure, and poly(ethylene glycol) alkyl ethers were dehydrated using a 3A molecular sieve before use. Other chemicals were obtained commercially and used without pretreatment. Preparation of P-ligands. P-ligands I-IV (see Figure 1) were synthesized according to the literature.9,10 For example, the general process for the preparation of P-ligand I was as follows. Under nitrogen, 0.15 mol of phosphorus trichloride was mixed with 0.1 mol of catechol under 20 °C with an ice bath and reacted for 30 min at room temperature. Another 0.1 mol of phosphorus trichloride then was added and reacted for 2 h at 80 °C. The obtained mixture was distilled to remove the unreacted phosphorus trichloride at 80 °C under reduced pressure (70-90 mmHg), giving 1,2-phenylene phosphorochloridite with a yield of 82.5%. Next, 0.01 mol of 1,2-phenylene phosphorochloridite was dissolved in a mixture of tetrahydrofuran and triethylamine (V/V = 3), and 0.075 mol of fatty alcohol polyoxyethylene (RO-(CH2CH2O)n-H) was dissolved in tetrahydrofuran. Both were mixed and reacted at 5 °C for 6 h, then further reacted at room temperature for 12 h. The mixture was filtered to remove ammonium salt and distilled to remove the solvents. Ethyl ether (100 mL) was added to the residue and filtered to Received: September 29, 2010 Accepted: December 19, 2010 Revised: December 15, 2010 Published: December 28, 2010 2478
dx.doi.org/10.1021/ie101991z | Ind. Eng. Chem. Res. 2011, 50, 2478–2481
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Scheme 2. Hydrogenation of 2-Ethylhexenal
Table 1. Effect of Catalysts on the Hydrogenation a entry
Figure 1. Structures of the P-ligands used.
Scheme 1. Synthesis of P-ligand IV (n = 12-18)
get the white precipitate, giving P-ligand I with a yield of 72% (see Scheme 1). The cloud point was measured according to the literature.10 I: IR (KBr disk, cm-1): ν 2923, 2860, 1600, 1498, 1467, 1108, 1032, 953, 749. 1H NMR (500 MHz, D2O, ppm): δ 0-1.50(m, C15H31-C20H41), 3.0-4.0(s, OCH2), 6.7-7.2(m, Ar-H). 31P NMR (200 MHz, D2O, ppm, external standardization: 85% H3PO4): δ 7.40(s). Cloud point: 103 °C. II: IR (KBr disk, cm-1): ν 3056, 1721, 1645, 1455, 997, 950, 699. 1H NMR (500 MHz, D2O): δ 1.1-1.2(m, CH3), 3.5-3.7(s, OCH2), 7.3-7.8(m, Ar-H). 31P NMR (200 MHz, D2O, ppm, external standardization: 85% H3PO4): δ-10.26(s). Cloud point: 112 °C. III: IR (KBr disk, cm-1): ν 2924, 2869, 1604, 1489, 953, 694, 757. 1H NMR (500 MHz, D2O): δ 1.1-1.3(m, C15H31C 20 H 41 ), 3.40-3.80(s, OCH 2 ), 6.70-7.4(m, Ar-H). 31 P NMR (200 MHz, D2O, ppm, external standardization: 85% H3PO4): δ -10.10(s). Cloud point: 106 °C. IV: IR (KBr disk, cm-1): ν 2920, 2868, 1110, 951. 1H NMR (500 MHz, D2O): δ1.0-1.2(m, CH3), 3.2-3.6(s, OCH2). 31P NMR (200 MHz, D2O, ppm, external standardization: 85% H3PO4): δ 7.18 (s). Cloud point: 107 °C. Hydrogenation of 2-Ethylhexenal. A mixture of 5 g of 2-ethylhexenal, 3 mmol metal chloride (PdCl2, RhCl2, NiCl2, CoCl2, or RuCl2), 1 g of P-ligand, 2.5 g of water, and 2.5 g of toluene was reacted in a 100-mL stainless-steel autoclave with 7 MPa of H2 at 200 °C for 4 h with stirring agitation (400 rpm), and then cooled to room temperature and depressurized. The upper organic phase was separated from the aqueous phase by decantation and the catalyst layer was reused directly in the recycle experiments. The sample of the upper layer was characterized qualitatively with a Hewlett-Packard Model HP6890/5973 GC/MS gas chromatography/mass spectroscopy system that was equipped with a Hewlett-Packard Model HP-5MS column (30 m 0.25 mm 0.25 μm), and its quantitative analysis was determined via gas chromatography using a Hewlett-Packard Model HP6890 GC gas chromatograph that was equipped with a Hewlett-Packard Model HP-5 column (30 m 0.32 mm 0.25 μm). The contents of the reactants and products were directly shown by the system of GC chemstation, according to the area of each chromatograph peak. The 2-ethylhexenal conversion was defined as C (as a percentage), which is the weight
catalyst
conversion, C (%)
selectivity, S (%)
1
PdCl2 (3 mg)
35.3
88.6
2 3
Co(CH3COO)2 (3 mg) Pd/C (5%, 60 mg)
36.8 40.5
90.6 96.7
4
Pd/I
46.9
93.5
5
Pd/II
36.5
96.3
6
Pd/III
25.5
86.3
7
Pd/IV
50.0
97.8
8
Rh/IV
48.6
94.0
9
Ni/IV
35.2
98.7
10 11
Co/IV Ru/IV
34.9 50.5
96.1 95.9
a
Composition: 2-ethylhexenal, 5 g; P-ligand, 1 g; metal chloride, 3 mmol; toluene, 2.5 g; and water, 2.5 g. T = 200 °C, t = 4 h.
percentage of 2-ethylhexenal consumed in the reaction. The 2-ethylhexanol selectivity (S, given as a percentage) was calculated by Ws 100 S ð%Þ ¼ WALL where Ws is the amount of 2-ethylhexanol, and WALL is the total amount of the products, including 2-ethylhexanol, ethylhexyl aldehyde, and octenoic alcohol, etc. The reaction formula is shown in Scheme 2.
’ RESULTS AND DISCUSSION Effects of Different Catalysts on the Hydrogenation. As can be seen from Table 1, compared to traditional catalysts, some TRPTCs exhibited better catalytic performances. In particular, Pd/IV exhibited good catalytic properties, with 50% conversion and 97.8% selectivity. The good catalytic performance was due to the thermoregulated character of TRPTC. When the catalyst was TRPTC, and the reaction temperature was higher than the cloud point of the P-ligand, the reaction occurred in the gas-liquid biphasic system. When Pd/C or PdCl2 was a catalyst, the reaction was carried out in the gas-solid-liquid triphasic system.11,12 Among the investigated TRPTCs, Pd/IV showed the best catalytic performance, and the results of other TRPTCs were not satisfied. This may be because of the steric resistance of P-ligand. With the increase of the volume/size of the substituent group in the P-ligand, the steric resistance of the catalyst was higher, which impeded the contacting of the catalyst and reagent, leading to an unfavorable hydrogenation reaction. The metal had a decisive influence on the catalytic performance of the TRPTCs. Among these catalyst systems, Pd/IV showed the best catalytic performance, while Ni/IV and Co/IV were poorer. These results can be explained by the different catalytic activity of the metal, relative to the hydrogenation, which are consistent with the results reported in the literature.13 2479
dx.doi.org/10.1021/ie101991z |Ind. Eng. Chem. Res. 2011, 50, 2478–2481
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Table 2. Effects of Reaction Conditions on the Hydrogenation a
a
entry
PdCl2 (mmol)
temperature, T (°C)
time, t (h)
pressure, P (MPa)
conversion, C (%)
selectivity, S (%)
1
2.0
200
4
5
31.6
98.4
2
3.0
200
4
5
50.0
97.8
3
4.0
200
4
5
51.5
96.1
4
3.0
240
4
5
63.0
74.9
5
3.0
220
4
5
55.8
96.4
6
3.0
180
4
5
35.2
98.1
7
3.0
220
6
5
68.0
97.2
8 9
3.0 3.0
220 220
8 10
5 5
86.9 87.3
96.0 97.5
10
3.0
220
8
6
90.4
96.2
11
3.0
220
8
7
96.9
95.5
12
3.0
220
8
8
97.3
95.7
Composition: 2-ethylhexenal, 5 g; P-ligand, 1 g; toluene, 2.5 g; and water, 2.5 g.
Table 3. Reusability of the Catalyst times used
conversion, C (%)
selectivity, S (%)
1
96.2
94.5
2 3
95.9 97.6
95.2 95.0
4
93.8
97.2
5
94.6
96.5
6
95.0
96.7
7
96.9
94.4
Effects of Reaction Conditions on the Hydrogenation.
Table 2 shows the effects of reaction conditions on the hydrogenation using Pd/IV as a catalyst. When the PdCl2 dosage was 3.0 mmol (entry 2), the conversion of the reagent and the selectivity of the product were 50.0% and 97.8%, respectively. Further increase of the PdCl2 dosage did not increase the conversion and selectivity. However, reaction temperature had an important effect on the hydrogenation. With the decrease of the reaction temperature from 240 °C to 180 °C (for entries 3-6), the conversion of 2-ethylhexenal expectedly decreased. It was suggested that too high a temperature was unfavorable to the hydrogenation. The effects of reaction time and H2 pressure on hydrogenation are also shown in Table 2 (see entries 5 and 7-9). It was found that both are very important for the hydrogenation of 2-ethylhexenal. With the increase of reaction time, the conversion of the reagent and the selectivity of the product were significantly increased. When the reaction time was 8 h (entry 8), the conversion of the reagent and the selectivity of the product were 86.9% and 96.0%, respectively. However, both did not further change when the time was increased further (see entry 10). With the increase of H2 pressure (see entries 8 and 10-12), similar reaction results were obtained. However, when the H2 pressure was increased to 7 MPa, the conversion and selectivity did not show further increases. Based on the above experiments, the following optimum conditions were obtained: 2-ethylhexenal, 5 g; P-ligand, 1 g; PdCl2, 3 mmol; toluene, 2.5 g; water, 2.5 g; reaction temperature, 220 °C; and reaction time, 8 h. Under these conditions, TRPTC Pd/IV showed good catalytic performance for the hydrogenation, and the conversion and selectivity were 96.9% and 95.5%, respectively. Reusability of Catalyst. When the reaction was finished, the upper organic phase was separated from TRPTC phase by
decantation. Then, by adding fresh solvent and reagent, the TRPTC phase was directly used to investigate its reusability under optimum reaction conditions. Results are shown in Table 3. The catalytic system was reused seven times without an obvious decrease in the conversion and selectivity, which indicated that the catalyst system was reusable for multiple times. The good reusability was attributed to the good thermal stability and effective phase-transfer performance, which could well avoid the loss of catalyst during the reaction and separation processes.
’ CONCLUSIONS The hydrogentation of 2-ethylhexenal was investigated in the presence of the thermoregulated phase-transfer catalysts (TRPTCs). The steric resistance of the P-ligand affected the performance of the catalytic system, and TRPTC Pd/IV with small steric resistance was an efficient catalyst for the hydrogenation. Using Pd/IV as the catalyst, the conversion of the reagent and selectivity of the desired product were, respectively, 96.9% and 95.5% after the reaction was carried out for 8 h at 220 °C. In addition, the product could be easily separated from the catalytic system and the catalyst system had good reusability. Hence, a clean and environmentally friendly strategy for the synthesis of 2-ethylhexanol was developed.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 30571463). The authors are grateful for the financial support. ’ REFERENCES (1) Hamilton, C. A.; Jackson, S. D.; Kelly, G. J. Solid base catalysts and combined solid base hydrogenation catalysts for the aldol condensation of branched and linear aldehydes. Appl. Catal., A 2004, 263, 63. (2) Niklasson, C.; Smedler, G. Kinetics of adsorption and reaction for the consecutive hydrogenation of 2-ethylhexenal on a nickel/silica catalyst. Ind. Eng. Chem. Res. 1987, 26, 403. 2480
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