One-Pot Sequential Aldol Condensation and Hydrogenation of n

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One-Pot Sequential Aldol Condensation and Hydrogenation of n‑Butyraldehyde to 2‑Ethylhexanol Ying Li, Xiaohong Liu, Hualiang An, Xinqiang Zhao,* and Yanji Wang Hebei Provincial Key Lab of Green Chemical Technology and Efficient Energy Saving, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China ABSTRACT: 2-Ethylhexanol (2EHO) is an important organic chemical. The industrial production of 2EHO comprises three units: propylene hydroformylation to n-butyraldehyde, n-butyraldehyde self-condensation to 2-ethyl-2-hexenal (2E2H), and 2E2H hydrogenation to 2EHO. In the present work, 2EHO was synthesized by one-pot sequential aldol condensation and hydrogenation of n-butyraldehyde. Among a series of metal− solid acid bifunctional catalysts, Ni/La−Al2O3 showed a better catalytic performance. The effect of reaction conditions on the one-pot sequential synthesis of 2EHO catalyzed by Ni/La−Al2O3 was investigated, and the suitable reaction conditions were obtained as follows: weight percentage of Ni/La−Al2O3 = 15%, self-condensation reaction conducted at 180 °C for 8 h, and then hydrogenation reaction conducted at 180 °C for 6 h under 4 MPa H2 pressure. Under the above reaction conditions, nbutyraldehyde conversion attained 100% at a 2EHO selectivity of 67.0%. The inhibition of Ni to n-butyraldehyde selfcondensation reaction is responsible for the low selectivity of 2EHO. On the basis of the analysis of the reaction system, some side reactions in the one-pot sequential synthesis of 2EHO were proposed. The deactivation of Ni/La−Al2O3 was due to the agglomeration of Ni and La2O3 particles and the occurrence of γ-Al2O3 hydration. Introduction of some hydrophobic groups on the surface of γ-Al2O3 could effectively inhibit the hydration of γ-Al2O3. 2EHO was merely 23.3%. Liang et al.5 studied the reaction for direct synthesis of 2EHO from n-butyraldehyde catalyzed by Ni/Ce−Al2O3. Under the suitable reaction conditions of reaction temperature = 170 °C, reaction pressure = 4.0 MPa, and reaction time = 8 h, the conversion of n-butyraldehyde and the selectivity of 2EHO were 100% and 66.9%, respectively. However, the reusability of Ni/Ce−Al2O3 was poor due to the hydration of γ-Al2O3. Our previous research indicated that La−Al2O3 showed excellent catalytic performance for n-butyraldehyde selfcondensation.6 In this work, Ni/La−Al2O3 was prepared by impregnation method and then one-pot sequential synthesis of 2EHO from n-butyraldehyde was realized over Ni/La−Al2O3. This reaction process comprises two steps: n-butyraldehyde self-condensation to 2E2H and then 2E2H hydrogenation to 2EHO without a separation operation between the two steps. Then the effect of reaction conditions on the one-pot sequential synthesis of 2EHO was investigated. Based on the analysis of the reaction system and compared with the products in the reaction system catalyzed by Ni/Ce−Al2O3,5 some side reactions in the one-pot sequential synthesis of 2EHO catalyzed by Ni/La−Al2O3 were proposed. In addition, the reusability of Ni/La−Al2O3 catalyst was studied.

1. INTRODUCTION 2-Ethylhexanol (2EHO), an important organic chemical, is mainly used in the manufacture of plasticizers such as dioctyl terephthalate (DOTP), dioctyl phthalate (DOP), and dioctyl adipate (DOA).1 In addition, 2EHO is extensively applied in the production of soaps, detergents, solvents, adhesives, and diesel additives. The industrial production of 2EHO comprises three reaction steps: propylene hydroformylation to nbutyraldehyde, n-butyraldehyde self-condensation to 2-ethyl-2hexenal (2E2H), and 2E2H hydrogenation to 2EHO. Since separation and purification are required between two steps, the problems of long operation time, high equipment expense, and large energy consumption inevitably exist. One-pot synthesis of 2EHO can solve the problems mentioned above to some extent. At present, there are few reports in the literature about one-pot sequential synthesis of 2EHO from n-butyraldehyde. However, the integration of n-butyraldehyde self-condensation and 2E2H selective hydrogenation to 2-ethylhexanal (2EH) have been studied by some researchers.2,3 Hamilton et al.4 studied the reaction integration of n-butyraldehyde aldol condensation and selective hydrogenation of the CC bond of 2E2H to 2EH using Pd/Na/SiO2 catalyst in a fixed bed reactor. The maximum selectivity of 2EH was 94.9%, but the conversion of n-butyraldehyde was only 42.4%. They also employed a dual bed system using a Na/SiO2 self-condensation catalyst followed by a Cu/Zn hydrogenation catalyst to produce a mixture of 2EHO and n-butanol (BO). The highest conversion of n-butyraldehyde was 31.7%, while the yield of © XXXX American Chemical Society

Received: February 29, 2016 Revised: May 16, 2016 Accepted: May 17, 2016

A

DOI: 10.1021/acs.iecr.6b00828 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Table 1. Screening of Metal in Bifunctional Catalyst for One-Pot Sequential Synthesis of 2EHOa,b catalyst

XBA/%

YBO/%

Y2E2H/%

Y2EH/%

Y2EHO/%

SC8/%

SBO + S2EHO/%

La−Al2O3 Co/La−Al2O3 Ni/La−Al2O3 Cu/La−Al2O3 Ru/La−Al2O3 Pt/La−Al2O3 Pd/La−Al2O3 Rh/La−Al2O3

90.7 95.2 100 99.0 100 95.8 89.6 90.5

0.69 1.00 12.8 12.3 12.8 6.81 0.63 0.83

82.2 87.9 − 68.4 15.6 54.4 46.6 73.5

0.83 − 23.4 5.09 20.3 6.45 28.3 3.81

− − 45.0 0.47 27.9 3.16 4.71 −

91.5 92.3 68.4 74.7 63.8 64.0 88.9 85.4

0.67 1.05 57.8 13.0 40.7 10.4 5.96 0.92

Reaction conditions: weight percentage of catalyst = 15%. n-Butyraldehyde self-condensation: T = 180 °C, t = 8 h. Hydrogenation: T = 180 °C, t = 4 h, P = 4.0 MPa. bX, conversion; Y, yield; S, selectivity; BA, n-butyraldehyde; 2E2H, 2-ethyl-2-hexenal; BO, n-butanol; 2EHO, 2-ethylhexanol, 2EH, 2-ethylhexanal. a

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. La−Al2O3 was prepared by a colloidal chemical method. In a typical procedure, 4 g of pseudoboehmite and 32 mL of water were put into a beaker, and an aqueous solution of lanthanum nitrate was added into the beaker while stirring. Then, nitric acid was added dropwise. The resultant gel mixture was aged at 95 °C for 5 h, dried at 110 °C for 10 h, and calcinated at 700 °C for 4 h to obtain La− Al2O3 finally. The bifunctional catalyst Ni/La−Al2O3 was prepared by impregnating La−Al2O3 with an aqueous solution of nickel nitrate. Then the sample was aged at room temperature for 24 h, dried at 110 °C for 8 h, calcinated at 500 °C for 4 h, and reduced at 550 °C for 4 h under an atmosphere of 20 vol % H2/ N2. 2.2. Catalyst Characterization. X-ray diffraction (XRD) patterns were recorded with a Rigaku D/MAX-2500 diffractometer using a Cu Kα radiation source at 100 mA and 40 kV. The scan range covered from 10 to 90° at a rate of 4° min−1. A scanning electron microscopy (SEM) image was taken using an FEI Nova Nano SEM 450 instrument. Before the observation, the samples were fixed on an aluminum stub with double-sided adhesive carbon tabs. The SEM was operated at an accelerating voltage of 5.0 kV. 2.3. One-Pot Sequential Synthesis of 2EHO. One-pot sequential synthesis of 2EHO from n-butyraldehyde was conducted in a 100 mL stainless steel autoclave. In a typical procedure, the self-condensation of n-butyraldehyde was conducted first. A 40 mL volume (about 30 g) of nbutyraldehyde and 4.5 g of catalyst were added into the autoclave, and then the air inside was replaced by nitrogen. The self-condensation reaction was conducted at 180 °C for 8 h with stirring. After the first step, the reaction mixture was directly hydrogenated without cooling and separation. The hydrogenation reaction was carried out at 180 °C for 4 h under 4.0 MPa of H2 pressure. After the completion of reaction, the mixture was cooled to room temperature. The catalyst was separated by vacuum filter, and the liquid was quantitatively analyzed by a gas chromatograph. 2.4. Product Analysis. A qualitative analysis of the product was conducted with gas chromatography−mass spectrometry (GC−MS) (Thermo Finnigan TRACE DSQ). An electron ionization (EI) source was used in mass spectrometry with an ion source temperature of 200 °C. The mass spectrum was recorded in the range 40−500 amu. The temperatures of both the vaporizing chamber and the transmission line were controlled at 250 °C. A BPX5 capillary column was used for separation of components, and the column temperature was

controlled according to the following program: started at an initial temperature of 40 °C and then heated to 250 °C in a ramp of 10 °C·min−1 and held for 2 min. A quantitative analysis of the product was carried out using a SP-2100 gas chromatograph (Beijing Beifen-Ruili Analytical Instrument Co., Ltd.). Nitrogen was used as a carrier gas, and its flow rate was 30 mL·min−1. The product mixture was separated in a KB-1 capillary column, and the components were analyzed quantitatively in a flame ionization detector (FID). The temperature of the KB-1 capillary column for the separation of n-butyraldehyde and 2E2H was controlled according to the following program: started at an initial temperature of 80 °C and held for 3 min and then heated to 160 °C in a ramp of 10 °C·min−1 and held for 10 min. The temperature of the KB-1 capillary column for the separation of BO, 2EH, and 2EHO was controlled according to the following program: started at an initial temperature of 80 °C and held for 3 min, heated to 160 °C in a ramp of 10 °C·min−1 and held for 2 min, and then heated to 200 °C in a ramp of 10 °C·min−1 and held for 6 min.

3. RESULTS AND DISCUSSION 3.1. Screening of Catalyst. Several metal−solid acid bifunctional catalysts were separately prepared by impregnating Table 2. Effect of Ni on n-Butyraldehyde Self-Condensation Reactiona catalyst

XBA/%

Y2E2H/%

S2E2H/%

YBO/%

Y2EH/%

La−Al2O3 Ni/La−Al2O3

91.6 83.2

81.9 62.7

88.7 75.4

0.32 3.27

0.51 5.13

a

Reaction conditions: weight percentage of catalyst = 15%, T = 180 °C, t = 8 h.

Table 3. Effect of Catalyst Amount on n-Butyraldehyde SelfCondensation Reactiona Ni/La−Al2O3/wt %

XBA/%

Y2E2H/%

S2E2H/%

YBO/%

Y2EH/%

5 10 15 20

72.3 78.8 83.2 85.4

43.7 53.7 62.7 62.7

60.4 68.1 75.4 73.4

1.69 2.21 3.27 4.27

1.18 1.92 5.13 5.41

a Reaction conditions: reaction temperature = 180 °C, reaction time = 8 h.

La−Al2O3 with an aqueous solution of metal salt. Their catalytic performance was evaluated, and the results are listed in Table 1. All the bifunctional catalysts had a lower selectivity of B

DOI: 10.1021/acs.iecr.6b00828 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 9. Reusability of Ni/La−Al2O3a

Table 4. Effect of Reaction Temperature on n-Butyraldehyde Self-Condensation Reactiona reaction temp/°C

XBA/%

Y2E2H/%

S2E2H/%

YBO/%

Y2EH/%

160 170 180 190

78.5 80.8 83.2 87.9

54.1 56.5 62.7 61.2

68.9 69.9 75.4 69.5

2.45 3.04 3.27 4.68

3.35 3.68 5.13 6.53

run

XBA/%

YBO/%

Y2EH/%

Y2EHO/%

SC8/%

SBO+2EHO/%

1 2 3 4

100 100 100 100

13.9 13.5 13.2 10.8

− 3.60 36.9 45.7

67.0 64.6 39.2 27.6

67.0 68.2 76.1 73.3

80.9 78.1 52.4 38.4

a

Reaction conditions: weight percentage of Ni/La−Al2O3 = 15%. nButyraldehyde self-condensation: T = 180 °C, t = 8 h. Hydrogenation: T = 180 °C, t = 6 h, P = 4.0 MPa.

a

Reaction conditions: weight percentage of Ni/La−Al2O3 = 15%, reaction time = 8 h.

Table 5. Effect of Reaction Time on n-Butyraldehyde SelfCondensation Reactiona reaction time/h

XBA/%

Y2E2H/%

S2E2H/%

YBO/%

Y2EH/%

6 7 8 9

76.7 80.8 83.2 83.8

53.3 57.9 62.7 60.2

69.5 71.7 75.4 71.8

3.02 3.15 3.27 3.52

4.71 4.77 5.13 5.68

a

Reaction conditions: weight percentage of Ni/La−Al2O3 = 15%, reaction temperature = 180 °C.

Table 6. Effect of Reaction Temperature on Hydrogenation Reactiona reaction temp/°C

XBA/%

YBO/%

160 170 180 190

100 100 100 100

7.53 14.3 15.8 14.8

Y2EH/% Y2EHO/% 64.1 5.73 4.07 2.65

15.7 57.9 61.5 60.9

SC8/%

SBO+2EHO/%

79.8 63.6 65.6 63.5

23.2 72.2 77.3 75.7

Figure 1. XRD patterns of Ni/La−Al2O3 catalysts before and after reaction: 1, fresh; 2, after the first use; 3, after the second use; 4, after the third use; 5, after the fourth use. •, Ni; ◇, γ-Al2O3; ▲, La2O3; ★, AlO(OH).

Table 10. Ni Metal Particle Sizes of Ni/La−Al2O3 and Ni/ Ce−Al2O3 Catalysts before and after Reaction

a

Reaction conditions: weight percentage of Ni/La−Al2O3 = 15%. nButyraldehyde self-condensation: T = 180 °C, t = 8 h. Hydrogenation: t = 4 h, P = 4.0 MPa.

Ni particle size/nm

Table 7. Effect of Reaction Pressure on Hydrogenation Reactiona reaction press./MPa

XBA/%

YBO/%

3.0 3.5 4.0 4.5

100 100 100 100

9.03 12.4 15.8 12.5

Y2EH/% Y2EHO/% 23.1 6.36 4.07 6.02

48.8 59.8 61.5 59.4

SC8/%

SBO+2EHO/%

71.9 66.2 65.6 65.4

57.8 72.2 77.3 71.9

Table 8. Effect of Reaction Time on Hydrogenation Reactiona

4 5 6 7

XBA/% YBO/% 100 100 100 100

15.8 13.4 13.9 14.4

Y2EH/% Y2EHO/% 4.07 0.65 − −

61.5 67.5 67.0 63.0

SC8/%

SBO+2EHO/%

65.6 68.2 67.0 63.0

77.3 80.9 80.9 77.4

fresh

first recovered

7.1 9.4

10.4 >100

was formed while no 2EHO was found. Cu, Pt, and Rh showed low activity for hydrogenation, and the main product was 2E2H. As for Pd/La−Al2O3 catalyst, the products contained a small amount of saturated alcohols and a large amount of 2EH, indicating that Pd had high catalytic activity for the hydrogenation of CC bond, consistent with the studies of Zhang et al.7 Ru/La−Al2O3 had a good catalytic activity: the yields of 2EH and 2EHO were separately 20.3% and 27.9% at a nbutyraldehyde conversion of 100%. The results indicated that Ru could catalyze the hydrogenation of both CC bond and CO bond. Since the highest 2EHO yield of 45.0% was reached over Ni/La−Al2O3 catalyst, Ni species was determined as the active component for hydrogenation in the bifunctional catalyst. Then the influence of preparation parameters on the catalytic performance of Ni/La−Al2O3 was investigated and the suitable preparation conditions were obtained as follows: Ni loading = 25 wt %, calcination temperature = 500 °C, calcination time = 5 h, and reduction at 550 °C for 3 h under an atmosphere of 20 vol % H2/N2. 3.2. Inhibition of Ni to Self-Condensation of nButyraldehyde. The effect of Ni on the self-condensation of n-butyraldehyde was discussed, and the results are listed in Table 2. The n-butyraldehyde conversion and 2E2H yield separately decreased by 8.4% and 19.2% over Ni/La−Al2O3 compared with La−Al2O3. On the contrary, the yields of BO and 2EH increased slightly. The results demonstrated that Ni

a Reaction conditions: weight percentage of Ni/La−Al2O3 = 15%. nButyraldehyde self-condensation: T = 180 °C, t = 8 h. Hydrogenation: T = 180 °C, t = 4 h.

reaction time/h

catalyst Ni/La−Al2O3 Ni/Ce−Al2O3

a Reaction conditions: weight percentage of Ni/La−Al2O3 = 15%. nButyraldehyde self-condensation: T = 180 °C, t = 8 h. Hydrogenation: T = 180 °C, P = 4.0 MPa.

C8 products than La−Al2O3 except for Co/La−Al2O3, indicating that the metals inhibited the self-condensation of n-butyraldehyde except Co. However, Co/La−Al2O3 showed hardly any activity for hydrogenation; a small amount of BO C

DOI: 10.1021/acs.iecr.6b00828 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. SEM images of fresh and recovered Ni/La−Al2O3 catalysts: (a) fresh; (b) recovered.

Table 11. Catalytic Performance of Functionalized γ-Al2O3 for n-Butyraldehyde Self-Condensation Reactiona catalyst

XBA/%

S2E2H/%

Y2E2H/%

γ-Al2O3 FAS−Al2O3 CPTEOS−Al2O3

87.5 73.4 73.0

87.5 93.4 86.5

76.6 68.6 63.1

yield and selectivity of target product 2E2H declined, affecting the succeeding hydrogenation reaction and reducing the selectivity of 2EHO. Therefore, Ni species inhibited the selfcondensation of n-butyraldehyde indeed. Since one-pot sequential synthesis of 2EHO comprises the reactions of n-butyraldehyde self-condensation and 2E2H hydrogenation, we separately investigated the effect of the two respective reactions. 3.3. Effect of Reaction Conditions on n-Butyraldehyde Self-Condensation Stage. 3.3.1. Effect of Catalyst Dosage. The effect of Ni/La−Al2O3 dosage on n-butyraldehyde selfcondensation was investigated, and the results are listed in Table 3. With the increase of weight percentage of Ni/La− Al2O3, the conversion of n-butyraldehyde increased gradually, the yield of 2E2H increased first and then remained stable, and the selectivity of 2E2H rose first and then dropped. The yields of BO and 2EH generated from hydrogenation of nbutyraldehyde and 2E2H increased gradually where hydrogen was derived from the decomposition of n-butyraldehyde.8 When the weight percentage of the catalyst was less than 15%, the inhibition of Ni was weaker than the catalysis of La−Al2O3 for self-condensation of n-butyraldehyde. Therefore, the selfcondensation of n-butyraldehyde proceeded favorably. When the weight percentage was 15%, the yield and selectivity of 2E2H were the highest. Along with a further increase of the amount of Ni/La−Al2O3, the increase of Ni strengthened the inhibition on self-condensation of n-butyraldehyde and the increment of n-butyraldehyde conversion decreased slowly. Additionally, the excessive active sites of Ni/La−Al2 O3 promoted the Tishchenko side reaction of n-butyraldehyde,10 resulting in the decrease of 2E2H selectivity. Therefore, the suitable weight percentage was 15%. 3.3.2. Effect of Reaction Temperature. Table 4 shows the effect of reaction temperature on n-butyraldehyde self-

a

Reaction conditions: weight percentage of catalyst = 15%, reaction temperature = 180 °C, reaction time = 8 h.

species inhibited the self-condensation of n-butyraldehyde and promoted the hydrogenation of n-butyraldehyde and 2E2H. Idriss et al.8 studied the self-condensation of acetaldehyde and found that carbon, hydrogen, and oxygen were generated by the decomposition of acetaldehyde. Therefore, it was speculated that the hydrogen for hydrogenation reaction was derived from the decomposition of a small amount of n-butyraldehyde in our study. The GC−MS analysis result showed that many byproducts were found such as BO, 2EH, 4-heptanone, butyl butyrate, and 2-ethyl-3-hydroxyhexyl butyrate in the products of n-butyraldehyde self-condensation over La−Al2O3. In addition to those substances, however, n-butyric acid and 3heptene were found in the presence of Ni/La−Al2O3 catalyst. We thought butyl butyrate was derived from the esterification of BO with n-butyric acid and which was found by the Cannizzaro reaction of n-butyraldehyde.9 According to the study of Liang et al.,5 we proposed that butyl butyrate was ketonized to form 4-heptanone, and then 4-heptanone was hydrogenated to 4-heptanol, and finally 4-heptanol was dehydrated to form 3-heptene. Comparing the byproducts of n-butyraldehyde self-condensation catalyzed by La−Al2O3 with those formed over Ni/ La−Al2O3, we found that Ni species could promote hydrogenation, esterification, and other side reactions. Therefore, the

Figure 3. XRD patterns of CPTEOS−Al2O3 and FAS−Al2O3 catalysts before and after reaction: (a) CPTEOS−Al2O3; (b) FAS−Al2O3. D

DOI: 10.1021/acs.iecr.6b00828 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research condensation. When the temperature was lower, the reaction was controlled by kinetics. With the increase of reaction temperature, the reaction rates rose and the conversion of nbutyraldehyde and the yield of 2E2H increased. When the reaction temperature was higher than 180 °C, high temperature promoted the Tishchenko reaction of n-butyraldehyde and the ketonization of the byproduct butyl butyrate,11 resulting in the increase of n-butyraldehyde conversion and the decrease of 2E2H selectivity. Additionally, the catalytic hydrogenation activity of Ni rose with the increase of reaction temperature, so the yields of BO and 2EH increased correspondingly. When the reaction temperature was 180 °C, the yield and selectivity of 2E2H were the highest, 62.7% and 75.4%, respectively. Therefore, the suitable reaction temperature was 180 °C. 3.3.3. Effect of Reaction Time. Table 5 indicates the effect of reaction time on n-butyraldehyde self-condensation. With the prolonging of reaction time, the conversion of n-butyraldehyde increased gradually while the yield and selectivity of 2E2H increased first and then decreased. The yields of BO and 2EH increased gradually. When the reaction time was 8 h, the yield and selectivity of 2E2H were the highest, 62.7% and 75.4%, respectively. With a further prolonging of reaction time, 2E2H could react with water to produce 2-ethyl-3-hydroxyhexanal, and then 2-ethyl-3-hydroxyhexanal could react with nbutyraldehyde to produce 2-ethyl-3-hydroxyhexyl butyrate by the Tishchenko reaction,10 reducing the selectivity of 2E2H. Therefore, the suitable reaction time was 8 h. Therefore, the suitable reaction conditions of n-butyraldehyde self-condensation were obtained as follows: the weight percentage of Ni/La−Al2O3 = 15%, the reaction temperature = 180 °C, and the reaction time = 8 h. Afterward, the effect of reaction conditions on the hydrogenation reaction stage was investigated under the suitable reaction conditions of the nbutyraldehyde self-condensation stage. 3.4. Effect of Reaction Conditions on Hydrogenation Reaction Stage. 3.4.1. Effect of Reaction Temperature. The effect of reaction temperature on the hydrogenation reaction for one-pot sequential synthesis of 2EHO from n-butyraldehyde was investigated, and the results are listed in Table 6. When the hydrogenation reaction temperature was 160 °C, the hydrogenation of CC bond was the main reaction while the catalytic activity for the hydrogenation of CO bond was low. Therefore, the yield of 2EH generated from partial hydrogenation was high while those of BO and 2EHO were low. With the increase of the hydrogenation reaction temperature, the catalytic activity for the hydrogenation of CO bond was improved and the yields of BO and 2EHO increased obviously. When the hydrogenation reaction temperature was over 180 °C, the yields of BO and 2EHO changed a little. Therefore, the suitable hydrogenation reaction temperature was 180 °C. 3.4.2. Effect of Reaction Pressure. The effect of hydrogen pressure on the hydrogenation reaction stage was investigated, and the results are listed in Table 7. When the reaction pressure was 3.0 MPa, the rate of the hydrogenation reaction was low and there was a certain amount of 2EH left in the reaction system after the completion of reaction. With the increase of reaction pressure, the catalytic activity for the hydrogenation of CO bond was improved obviously and the yields of BO and 2EHO increased. With a further increase of reaction pressure above 4.0 MPa, the selectivities of BO and 2EHO decreased slightly due to the increase of byproducts such as butyl butyrate and 2-ethylhexyl butyrate generated from some side reactions.5

Therefore, the suitable hydrogenation reaction pressure was 4.0 MPa. 3.4.3. Effect of Reaction Time. The effect of reaction time on the hydrogenation reaction stage was investigated, and the results are listed in Table 8. With the prolonging of the reaction time, 2EH generated from partial hydrogenation transformed to 2EHO, so the yield of 2EH decreased while the yield of 2EHO increased. When the reaction time was 6 h, 2EH was completely hydrogenated to 2EHO. With a further prolonging of reaction time, the yield of 2EHO decreased slightly while the yield of BO changed little. The possible reason for the decline of 2EHO yield was that 2EHO was consumed by reacting with n-butyraldehyde to produce 2-ethylhexyl butyrate.4 Therefore, the suitable hydrogenation reaction time was 6 h. On the basis of the above investigation, the suitable reaction conditions for one-pot sequential synthesis of 2EHO were obtained as follows: weight percentage of Ni/La−Al2O3 = 15%, self-condensation reaction conducted at 180 °C for 8 h, and then hydrogenation reaction conducted at 180 °C for 6 h under 4 MPa of H2 pressure. Under the above reaction conditions, the yield of 2EHO attained 67.0% at a 100% conversion of nbutyraldehyde, almost the same as the result in the reaction integration of n-butyraldehyde self-condensation and 2E2H hydrogenation catalyzed by Ni/Ce−Al2O3.5 3.5. Analysis of Reaction System. The products obtained from Ni/La−Al2O3 catalyzed one-pot sequential synthesis of 2EHO from n-butyraldehyde were indentified by GC−MS. Besides the target product 2EHO, many byproducts were found such as BO, n-butyric acid, butyl butyrate, 2-ethylhexyl butyrate, 4-heptanone, n-heptane, 2EH, 2-ethyl-3-hydroxyhexyl butyrate, 3-ethyl ketone, and 2-methylbutanol. Most of the byproducts were generated in the n-butyraldehyde self-condensation stage. However, 2-ethylhexyl butyrate, 3-ethyl ketone, and 2methylbutanol were not detected by GC−MS in the nbutyraldehyde self-condensation stage. Therefore they must be formed in the hydrogenation stage. Compared with the products in the reaction integration of n-butyraldehyde selfcondensation and 2E2H hydrogenation catalyzed by Ni/Ce− Al2O3,5 2-methylbutanol and ethyl acetoacetate were formed under the conditions for one-pot sequential synthesis of 2EHO from n-butyraldehyde catalyzed by Ni/La−Al2O3. We conjectured that BO in the products decomposed to 1hydroxybutyl radical, 1-hydroxymethyl propyl radical, and methyl radical, according to the study of the BO decomposition mechanism by Harper et al.12 1-Hydroxymethyl propyl radical could combine with methyl radical to produce 2-methylbutanol. Meanwhile, 1-hydroxybutyl radical could decompose to vinyl alcohol and ethyl radical. Vinyl alcohol then transformed to acetaldehyde followed by a subsequent Tishchenko reaction to ethyl acetate. Ethyl acetate was converted to ethyl acetoacetate by the Claisen condensation. 3.6. Reusability of Ni/La−Al2O3. After the completion of reaction, Ni/La−Al2O3 was separated from the reaction system by filtering and then was washed with absolute alcohol, dried at 110 °C for 8 h, calcinated at 500 °C for 4 h, and finally reduced at 550 °C for 3 h in the atmosphere of a mixture of H2 and N2 with a H2 volumetric percentage of 20%, just as the fresh Ni/ La−Al2O3 was. The recovered and treated Ni/La−Al2O3 catalysts were reused in the reaction for one-pot sequential synthesis of 2EHO, and the results are listed in Table 9. It was found that the yield of 2EHO almost remained unchanged for the second use. The catalytic activity of Ni/La−Al2O3 used three times decreased, and the yield of 2EHO declined by E

DOI: 10.1021/acs.iecr.6b00828 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

4. CONCLUSIONS One-pot sequential synthesis of 2EHO from n-butyraldehyde can simply the present production process and has a industrial practice value. A metal−solid acid bifunctional catalyst Ni/La− Al2O3 was prepared, and its catalytic performance for one-pot sequential synthesis of 2EHO was investigated. The yield of 2EHO attained 67.0% at a 100% conversion of n-butyraldehyde under suitable reaction conditions. However, Ni could inhibit the aldol condensation of n-butyraldehyde, resulting in the decrease of the yield and selectivity of 2E2H and then the decline in the selectivity of target product 2EHO. The deactivation of Ni/La−Al2O3 was due to the agglomeration of Ni and La2O3 and the coverage of Ni by γ-AlO(OH) formed from the hydration of γ-Al2O3. The introduction of some hydrophobic groups on the surface of γ-Al2O3 could effectively inhibit the hydration; however, the improvement of the catalytic performance of the modified γ-Al2O3 is required next.

27.8% and a lot of 2EH was left in the products. The yield of 2EHO was merely 27.6% when the catalyst was used four times. However, compared with the Ni/Ce−Al2O3 catalyst reported in our previous paper,5 the reusability of Ni/La−Al2O3 was improved, especially in the second run. The XRD patterns of the fresh and recovered Ni/La−Al2O3 are shown in Figure 1. It can be seen that γ-Al2O3 (2θ = 37, 46, and 66.5°) and metal Ni (2θ = 44.5, 51.7, and 76.4°) diffraction peaks were observed in both the fresh and the recovered Ni/ La−Al2O3. The metal Ni diffraction peaks in the fresh Ni/La− Al2O3 were broad and the peaks of La2O3 were not found in the fresh Ni/La−Al2O3, indicating that Ni and La−Al2O3 particles were small and distributed uniformly on the surface of γ-Al2O3. With the increase of reuse cycles, the recovered Ni/La−Al2O3 showed La2O3 (2θ = 28 and 49°) diffraction peaks and the peaks of metal Ni became narrow and sharp, indicating the agglomeration and growth of metal Ni and La2O3 particles in the process of reuse. To explain the difference in the reusability between Ni/La−Al2O3 and Ni/Ce−Al2O3, Ni metal particles size of the fresh and the recovered Ni/La−Al2O3 and Ni/Ce− Al2O3 after the first run were calculated by the Scherrer formula using XRD measurement data and the results are listed in Table 10. The sizes of the fresh Ni/La−Al2O3 and Ni/Ce−Al2O3 were similar, while the recovered Ni/La−Al2O3 showed particles of smaller size and the recovered Ni/Ce−Al2O3 showed larger particles, indicating that La can inhibit the agglomeration of Ni efficiently. Navarro et al.13 obtained the same conclusion in their studies. In addition, new γ-AlO(OH) (2θ = 14.4 and 38.3°) diffraction peaks appeared in the recovered Ni/La−Al2O3 sample, indicating that γ-Al2O3 was hydrated with the byproduct water from n-butyraldehyde selfcondensation, just as Ni/Ce−Al2O3 was. The SEM images of the fresh and the third recovered Ni/ La−Al2O3 are shown in Figure 2. It can be seen that the surface morphology of the recovered Ni/La−Al2O3 changed obviously. The fresh Ni/La−Al2O3 presented a clear and uniform particle structure. However, a new hydration structure was observed in the recovered Ni/La−Al2O3 and its branch structure was distributed desultorily. Liang et al.5 studied the deactivation of Ni/Ce−Al2O3 and found that the new surface hydrate structure was a mixture of γ-AlO(OH) and γ-Al2O3, which covered up parts of Ni species on the surface and decreased the catalytic hydrogenation activity of Ni/La−Al2O3. Therefore, we proposed that the new structure of γ-AlO(OH) formed from hydration of γ-Al2O3 would cover up parts of Ni species on the surface, which was another reason for the decrease of the catalytic hydrogenation activity of Ni/La−Al2O3, consistent with the results of XRD analyses. Two kinds of hydrophobic substances, γ-chloropropyl triethoxysilane (CPTEOS) and 1H,1H,2H,2H-perfluorooctyl triethoxysilane (FAS), were introduced onto the surface of γAl2O3 in order to inhibit its hydration. Then the catalytic performance for self-condensation of n-butyraldehyde and the antihydration effect were evaluated. It was found from Table 11 that the catalytic activity of the modified γ-Al2O3 decreased to some degree. It is obvious that γ-AlO(OH) diffraction peaks were not found from Figure 3, indicating that CPTEOS and FAS could inhibit the generation of boehmite γ-AlO(OH) completely. In the light of the low catalytic activity of CPTEOS−Al2O3 and FAS−Al2O3, the improvement of their catalytic performance is the next goal in our research.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-22-60202427. Fax: +86-22-60204294. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21476058, 21506046). The authors are gratefully appreciative of their contributions.

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REFERENCES

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DOI: 10.1021/acs.iecr.6b00828 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX