one-pot sequential aldol condensation and hydrogenation of

One-Pot Sequential Aldol Condensation and Hydrogenation ofn‑Butyraldehyde to 2‑EthylhexanolYing 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 andTechnology, Hebei University of Technology, Tianjin 300130, China

ABSTRACT: 2-Ethylhexanol (2EHO) is an important organicchemical. The industrial production of 2EHO comprises threeunits: propylene hydroformylation to n-butyraldehyde, n-butyr-aldehyde self-condensation to 2-ethyl-2-hexenal (2E2H), and2E2H hydrogenation to 2EHO. In the present work, 2EHO wassynthesized by one-pot sequential aldol condensation andhydrogenation of n-butyraldehyde. Among a series of metal−solid acid bifunctional catalysts, Ni/La−Al2O3 showed a bettercatalytic performance. The effect of reaction conditions on theone-pot sequential synthesis of 2EHO catalyzed by Ni/La−Al2O3was investigated, and the suitable reaction conditions wereobtained as follows: weight percentage of Ni/La−Al2O3 = 15%, self-condensation reaction conducted at 180 °C for 8 h, andthen hydrogenation reaction conducted at 180 °C for 6 h under 4 MPa H2 pressure. Under the above reaction conditions, n-butyraldehyde conversion attained 100% at a 2EHO selectivity of 67.0%. The inhibition of Ni to n-butyraldehyde self-condensation reaction is responsible for the low selectivity of 2EHO. On the basis of the analysis of the reaction system, someside reactions in the one-pot sequential synthesis of 2EHO were proposed. The deactivation of Ni/La−Al2O3 was due to theagglomeration of Ni and La2O3 particles and the occurrence of γ-Al2O3 hydration. Introduction of some hydrophobic groups onthe surface of γ-Al2O3 could effectively inhibit the hydration of γ-Al2O3.

1. INTRODUCTION2-Ethylhexanol (2EHO), an important organic chemical, ismainly used in the manufacture of plasticizers such as dioctylterephthalate (DOTP), dioctyl phthalate (DOP), and dioctyladipate (DOA).1 In addition, 2EHO is extensively applied inthe production of soaps, detergents, solvents, adhesives, anddiesel additives. The industrial production of 2EHO comprisesthree reaction steps: propylene hydroformylation to n-butyraldehyde, n-butyraldehyde self-condensation to 2-ethyl-2-hexenal (2E2H), and 2E2H hydrogenation to 2EHO. Sinceseparation and purification are required between two steps, theproblems of long operation time, high equipment expense, andlarge energy consumption inevitably exist. One-pot synthesis of2EHO can solve the problems mentioned above to someextent. At present, there are few reports in the literature aboutone-pot sequential synthesis of 2EHO from n-butyraldehyde.However, the integration of n-butyraldehyde self-condensationand 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 aldolcondensation and selective hydrogenation of the CC bondof 2E2H to 2EH using Pd/Na/SiO2 catalyst in a fixed bedreactor. The maximum selectivity of 2EH was 94.9%, but theconversion of n-butyraldehyde was only 42.4%. They alsoemployed a dual bed system using a Na/SiO2 self-condensationcatalyst followed by a Cu/Zn hydrogenation catalyst to producea mixture of 2EHO and n-butanol (BO). The highestconversion of n-butyraldehyde was 31.7%, while the yield of

2EHO was merely 23.3%. Liang et al.5 studied the reaction fordirect synthesis of 2EHO from n-butyraldehyde catalyzed byNi/Ce−Al2O3. Under the suitable reaction conditions ofreaction temperature = 170 °C, reaction pressure = 4.0 MPa,and reaction time = 8 h, the conversion of n-butyraldehyde andthe selectivity of 2EHO were 100% and 66.9%, respectively.However, the reusability of Ni/Ce−Al2O3 was poor due to thehydration of γ-Al2O3.Our previous research indicated that La−Al2O3 showed

excellent catalytic performance for n-butyraldehyde self-condensation.6 In this work, Ni/La−Al2O3 was prepared byimpregnation method and then one-pot sequential synthesis of2EHO from n-butyraldehyde was realized over Ni/La−Al2O3.This reaction process comprises two steps: n-butyraldehydeself-condensation to 2E2H and then 2E2H hydrogenation to2EHO without a separation operation between the two steps.Then the effect of reaction conditions on the one-potsequential synthesis of 2EHO was investigated. Based on theanalysis of the reaction system and compared with the productsin the reaction system catalyzed by Ni/Ce−Al2O3,

5 some sidereactions in the one-pot sequential synthesis of 2EHO catalyzedby Ni/La−Al2O3 were proposed. In addition, the reusability ofNi/La−Al2O3 catalyst was studied.

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

Article

pubs.acs.org/IECR

© 2016 American Chemical Society 6293 DOI: 10.1021/acs.iecr.6b00828Ind. Eng. Chem. Res. 2016, 55, 6293−6299

pubs.acs.org/IECR

http://dx.doi.org/10.1021/acs.iecr.6b00828

2. EXPERIMENTAL SECTION

2.1. Catalyst Preparation. La−Al2O3 was prepared by acolloidal chemical method. In a typical procedure, 4 g ofpseudoboehmite and 32 mL of water were put into a beaker,and an aqueous solution of lanthanum nitrate was added intothe beaker while stirring. Then, nitric acid was added dropwise.The resultant gel mixture was aged at 95 °C for 5 h, dried at110 °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 nickelnitrate. Then the sample was aged at room temperature for 24h, dried at 110 °C for 8 h, calcinated at 500 °C for 4 h, andreduced 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 diffrac-tometer 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 theobservation, the samples were fixed on an aluminum stub withdouble-sided adhesive carbon tabs. The SEM was operated atan accelerating voltage of 5.0 kV.2.3. One-Pot Sequential Synthesis of 2EHO. One-pot

sequential synthesis of 2EHO from n-butyraldehyde wasconducted in a 100 mL stainless steel autoclave. In a typicalprocedure, the self-condensation of n-butyraldehyde wasconducted first. A 40 mL volume (about 30 g) of n-butyraldehyde and 4.5 g of catalyst were added into theautoclave, and then the air inside was replaced by nitrogen. Theself-condensation reaction was conducted at 180 °C for 8 hwith stirring. After the first step, the reaction mixture wasdirectly hydrogenated without cooling and separation. Thehydrogenation reaction was carried out at 180 °C for 4 h under4.0 MPa of H2 pressure. After the completion of reaction, themixture was cooled to room temperature. The catalyst wasseparated by vacuum filter, and the liquid was quantitativelyanalyzed 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 electronionization (EI) source was used in mass spectrometry with anion source temperature of 200 °C. The mass spectrum wasrecorded in the range 40−500 amu. The temperatures of boththe vaporizing chamber and the transmission line werecontrolled at 250 °C. A BPX5 capillary column was used forseparation of components, and the column temperature was

controlled according to the following program: started at aninitial temperature of 40 °C and then heated to 250 °C in aramp 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 AnalyticalInstrument Co., Ltd.). Nitrogen was used as a carrier gas, andits flow rate was 30 mL·min−1. The product mixture wasseparated in a KB-1 capillary column, and the components wereanalyzed quantitatively in a flame ionization detector (FID).The temperature of the KB-1 capillary column for theseparation of n-butyraldehyde and 2E2H was controlledaccording to the following program: started at an initialtemperature of 80 °C and held for 3 min and then heated to160 °C in a ramp of 10 °C·min−1 and held for 10 min. Thetemperature of the KB-1 capillary column for the separation ofBO, 2EH, and 2EHO was controlled according to the followingprogram: started at an initial temperature of 80 °C and held for3 min, heated to 160 °C in a ramp of 10 °C·min−1 and held for2 min, and then heated to 200 °C in a ramp of 10 °C·min−1 andheld for 6 min.

3. RESULTS AND DISCUSSION3.1. Screening of Catalyst. Several metal−solid acid

bifunctional catalysts were separately prepared by impregnating

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

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 90.7 0.69 82.2 0.83 − 91.5 0.67Co/La−Al2O3 95.2 1.00 87.9 − − 92.3 1.05Ni/La−Al2O3 100 12.8 − 23.4 45.0 68.4 57.8Cu/La−Al2O3 99.0 12.3 68.4 5.09 0.47 74.7 13.0Ru/La−Al2O3 100 12.8 15.6 20.3 27.9 63.8 40.7Pt/La−Al2O3 95.8 6.81 54.4 6.45 3.16 64.0 10.4Pd/La−Al2O3 89.6 0.63 46.6 28.3 4.71 88.9 5.96Rh/La−Al2O3 90.5 0.83 73.5 3.81 − 85.4 0.92

aReaction 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.

Table 2. Effect of Ni on n-Butyraldehyde Self-CondensationReactiona

catalyst XBA/% Y2E2H/% S2E2H/% YBO/% Y2EH/%

La−Al2O3 91.6 81.9 88.7 0.32 0.51Ni/La−Al2O3 83.2 62.7 75.4 3.27 5.13

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

Table 3. Effect of Catalyst Amount on n-Butyraldehyde Self-Condensation Reactiona

Ni/La−Al2O3/wt % XBA/% Y2E2H/% S2E2H/% YBO/% Y2EH/%

5 72.3 43.7 60.4 1.69 1.1810 78.8 53.7 68.1 2.21 1.9215 83.2 62.7 75.4 3.27 5.1320 85.4 62.7 73.4 4.27 5.41

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

Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.6b00828Ind. Eng. Chem. Res. 2016, 55, 6293−6299

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

was formed while no 2EHO was found. Cu, Pt, and Rh showedlow activity for hydrogenation, and the main product was2E2H. As for Pd/La−Al2O3 catalyst, the products contained asmall amount of saturated alcohols and a large amount of 2EH,indicating that Pd had high catalytic activity for the hydro-genation of CC bond, consistent with the studies of Zhang etal.7 Ru/La−Al2O3 had a good catalytic activity: the yields of2EH and 2EHO were separately 20.3% and 27.9% at a n-butyraldehyde conversion of 100%. The results indicated thatRu could catalyze the hydrogenation of both CC bond andCO bond. Since the highest 2EHO yield of 45.0% wasreached over Ni/La−Al2O3 catalyst, Ni species was determinedas the active component for hydrogenation in the bifunctionalcatalyst. Then the influence of preparation parameters on thecatalytic performance of Ni/La−Al2O3 was investigated and thesuitable preparation conditions were obtained as follows: Niloading = 25 wt %, calcination temperature = 500 °C,calcination time = 5 h, and reduction at 550 °C for 3 hunder an atmosphere of 20 vol % H2/N2.

3.2. Inhibition of Ni to Self-Condensation of n-Butyraldehyde. The effect of Ni on the self-condensation ofn-butyraldehyde was discussed, and the results are listed inTable 2. The n-butyraldehyde conversion and 2E2H yieldseparately decreased by 8.4% and 19.2% over Ni/La−Al2O3compared with La−Al2O3. On the contrary, the yields of BOand 2EH increased slightly. The results demonstrated that Ni

Table 4. Effect of Reaction Temperature on n-ButyraldehydeSelf-Condensation Reactiona

reaction temp/°C XBA/% Y2E2H/% S2E2H/% YBO/% Y2EH/%

160 78.5 54.1 68.9 2.45 3.35170 80.8 56.5 69.9 3.04 3.68180 83.2 62.7 75.4 3.27 5.13190 87.9 61.2 69.5 4.68 6.53

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

Table 5. Effect of Reaction Time on n-Butyraldehyde Self-Condensation Reactiona

reaction time/h XBA/% Y2E2H/% S2E2H/% YBO/% Y2EH/%

6 76.7 53.3 69.5 3.02 4.717 80.8 57.9 71.7 3.15 4.778 83.2 62.7 75.4 3.27 5.139 83.8 60.2 71.8 3.52 5.68

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

Table 6. Effect of Reaction Temperature on HydrogenationReactiona

reactiontemp/°C XBA/% YBO/% Y2EH/% Y2EHO/% SC8/% SBO+2EHO/%

160 100 7.53 64.1 15.7 79.8 23.2170 100 14.3 5.73 57.9 63.6 72.2180 100 15.8 4.07 61.5 65.6 77.3190 100 14.8 2.65 60.9 63.5 75.7

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

Table 7. Effect of Reaction Pressure on HydrogenationReactiona

reactionpress./MPa XBA/% YBO/% Y2EH/% Y2EHO/% SC8/% SBO+2EHO/%

3.0 100 9.03 23.1 48.8 71.9 57.83.5 100 12.4 6.36 59.8 66.2 72.24.0 100 15.8 4.07 61.5 65.6 77.34.5 100 12.5 6.02 59.4 65.4 71.9

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

Table 8. Effect of Reaction Time on HydrogenationReactiona

reactiontime/h XBA/% YBO/% Y2EH/% Y2EHO/% SC8/% SBO+2EHO/%

4 100 15.8 4.07 61.5 65.6 77.35 100 13.4 0.65 67.5 68.2 80.96 100 13.9 − 67.0 67.0 80.97 100 14.4 − 63.0 63.0 77.4

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

Table 9. Reusability of Ni/La−Al2O3a

run XBA/% YBO/% Y2EH/% Y2EHO/% SC8/% SBO+2EHO/%

1 100 13.9 − 67.0 67.0 80.92 100 13.5 3.60 64.6 68.2 78.13 100 13.2 36.9 39.2 76.1 52.44 100 10.8 45.7 27.6 73.3 38.4

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

Figure 1. XRD patterns of Ni/La−Al2O3 catalysts before and afterreaction: 1, fresh; 2, after the first use; 3, after the second use; 4, afterthe 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

Ni particle size/nm

catalyst fresh first recovered

Ni/La−Al2O3 7.1 10.4Ni/Ce−Al2O3 9.4 >100



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species inhibited the self-condensation of n-butyraldehyde andpromoted the hydrogenation of n-butyraldehyde and 2E2H.Idriss et al.8 studied the self-condensation of acetaldehyde andfound that carbon, hydrogen, and oxygen were generated by thedecomposition of acetaldehyde. Therefore, it was speculatedthat the hydrogen for hydrogenation reaction was derived fromthe decomposition of a small amount of n-butyraldehyde in ourstudy. The GC−MS analysis result showed that manybyproducts were found such as BO, 2EH, 4-heptanone, butylbutyrate, and 2-ethyl-3-hydroxyhexyl butyrate in the productsof n-butyraldehyde self-condensation over La−Al2O3. Inaddition to those substances, however, n-butyric acid and 3-heptene were found in the presence of Ni/La−Al2O3 catalyst.We thought butyl butyrate was derived from the esterificationof BO with n-butyric acid and which was found by theCannizzaro reaction of n-butyraldehyde.9 According to thestudy of Liang et al.,5 we proposed that butyl butyrate wasketonized to form 4-heptanone, and then 4-heptanone washydrogenated to 4-heptanol, and finally 4-heptanol wasdehydrated to form 3-heptene.Comparing the byproducts of n-butyraldehyde self-con-

densation catalyzed by La−Al2O3 with those formed over Ni/La−Al2O3, we found that Ni species could promote hydro-genation, esterification, and other side reactions. Therefore, the

yield and selectivity of target product 2E2H declined, affectingthe succeeding hydrogenation reaction and reducing theselectivity of 2EHO. Therefore, Ni species inhibited the self-condensation of n-butyraldehyde indeed.Since one-pot sequential synthesis of 2EHO comprises the

reactions of n-butyraldehyde self-condensation and 2E2Hhydrogenation, we separately investigated the effect of thetwo respective reactions.

3.3. Effect of Reaction Conditions on n-ButyraldehydeSelf-Condensation Stage. 3.3.1. Effect of Catalyst Dosage.The effect of Ni/La−Al2O3 dosage on n-butyraldehyde self-condensation was investigated, and the results are listed inTable 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, andthe selectivity of 2E2H rose first and then dropped. The yieldsof BO and 2EH generated from hydrogenation of n-butyraldehyde and 2E2H increased gradually where hydrogenwas 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−Al2O3for self-condensation of n-butyraldehyde. Therefore, the self-condensation of n-butyraldehyde proceeded favorably. Whenthe weight percentage was 15%, the yield and selectivity of2E2H were the highest. Along with a further increase of theamount of Ni/La−Al2O3, the increase of Ni strengthened theinhibition on self-condensation of n-butyraldehyde and theincrement of n-butyraldehyde conversion decreased slowly.Additionally, the excessive active sites of Ni/La−Al2O3promoted the Tishchenko side reaction of n-butyraldehyde,10

resulting in the decrease of 2E2H selectivity. Therefore, thesuitable weight percentage was 15%.

3.3.2. Effect of Reaction Temperature. Table 4 shows theeffect of reaction temperature on n-butyraldehyde self-

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

Table 11. Catalytic Performance of Functionalized γ-Al2O3for n-Butyraldehyde Self-Condensation Reactiona

catalyst XBA/% S2E2H/% Y2E2H/%

γ-Al2O3 87.5 87.5 76.6FAS−Al2O3 73.4 93.4 68.6CPTEOS−Al2O3 73.0 86.5 63.1

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

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



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condensation. When the temperature was lower, the reactionwas controlled by kinetics. With the increase of reactiontemperature, the reaction rates rose and the conversion of n-butyraldehyde and the yield of 2E2H increased. When thereaction temperature was higher than 180 °C, high temperaturepromoted the Tishchenko reaction of n-butyraldehyde and theketonization of the byproduct butyl butyrate,11 resulting in theincrease of n-butyraldehyde conversion and the decrease of2E2H selectivity. Additionally, the catalytic hydrogenationactivity of Ni rose with the increase of reaction temperature,so the yields of BO and 2EH increased correspondingly. Whenthe reaction temperature was 180 °C, the yield and selectivityof 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 theprolonging of reaction time, the conversion of n-butyraldehydeincreased gradually while the yield and selectivity of 2E2Hincreased first and then decreased. The yields of BO and 2EHincreased gradually. When the reaction time was 8 h, the yieldand selectivity of 2E2H were the highest, 62.7% and 75.4%,respectively. With a further prolonging of reaction time, 2E2Hcould react with water to produce 2-ethyl-3-hydroxyhexanal,and then 2-ethyl-3-hydroxyhexanal could react with n-butyraldehyde to produce 2-ethyl-3-hydroxyhexyl butyrate bythe Tishchenko reaction,10 reducing the selectivity of 2E2H.Therefore, the suitable reaction time was 8 h.Therefore, the suitable reaction conditions of n-butyralde-

hyde self-condensation were obtained as follows: the weightpercentage of Ni/La−Al2O3 = 15%, the reaction temperature =180 °C, and the reaction time = 8 h. Afterward, the effect ofreaction conditions on the hydrogenation reaction stage wasinvestigated under the suitable reaction conditions of the n-butyraldehyde self-condensation stage.3.4. Effect of Reaction Conditions on Hydrogenation

Reaction Stage. 3.4.1. Effect of Reaction Temperature. Theeffect of reaction temperature on the hydrogenation reactionfor one-pot sequential synthesis of 2EHO from n-butyralde-hyde was investigated, and the results are listed in Table 6.When the hydrogenation reaction temperature was 160 °C, thehydrogenation of CC bond was the main reaction while thecatalytic activity for the hydrogenation of CO bond was low.Therefore, the yield of 2EH generated from partial hydro-genation 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 CO bond wasimproved 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, thesuitable 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 pressurewas 3.0 MPa, the rate of the hydrogenation reaction was lowand there was a certain amount of 2EH left in the reactionsystem after the completion of reaction. With the increase ofreaction pressure, the catalytic activity for the hydrogenation ofCO bond was improved obviously and the yields of BO and2EHO increased. With a further increase of reaction pressureabove 4.0 MPa, the selectivities of BO and 2EHO decreasedslightly due to the increase of byproducts such as butyl butyrateand 2-ethylhexyl butyrate generated from some side reactions.5

Therefore, the suitable hydrogenation reaction pressure was 4.0MPa.

3.4.3. Effect of Reaction Time. The effect of reaction timeon the hydrogenation reaction stage was investigated, and theresults are listed in Table 8. With the prolonging of the reactiontime, 2EH generated from partial hydrogenation transformed to2EHO, so the yield of 2EH decreased while the yield of 2EHOincreased. When the reaction time was 6 h, 2EH wascompletely hydrogenated to 2EHO. With a further prolongingof reaction time, the yield of 2EHO decreased slightly while theyield of BO changed little. The possible reason for the declineof 2EHO yield was that 2EHO was consumed by reacting withn-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 wereobtained as follows: weight percentage of Ni/La−Al2O3 = 15%,self-condensation reaction conducted at 180 °C for 8 h, andthen hydrogenation reaction conducted at 180 °C for 6 h under4 MPa of H2 pressure. Under the above reaction conditions, theyield of 2EHO attained 67.0% at a 100% conversion of n-butyraldehyde, almost the same as the result in the reactionintegration of n-butyraldehyde self-condensation and 2E2Hhydrogenation catalyzed by Ni/Ce−Al2O3.

5

3.5. Analysis of Reaction System. The products obtainedfrom Ni/La−Al2O3 catalyzed one-pot sequential synthesis of2EHO from n-butyraldehyde were indentified by GC−MS.Besides the target product 2EHO, many byproducts were foundsuch 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 byproductswere generated in the n-butyraldehyde self-condensation stage.However, 2-ethylhexyl butyrate, 3-ethyl ketone, and 2-methylbutanol were not detected by GC−MS in the n-butyraldehyde self-condensation stage. Therefore they must beformed in the hydrogenation stage. Compared with theproducts in the reaction integration of n-butyraldehyde self-condensation and 2E2H hydrogenation catalyzed by Ni/Ce−Al2O3,

5 2-methylbutanol and ethyl acetoacetate were formedunder the conditions for one-pot sequential synthesis of 2EHOfrom n-butyraldehyde catalyzed by Ni/La−Al2O3.We conjectured that BO in the products decomposed to 1-

hydroxybutyl radical, 1-hydroxymethyl propyl radical, andmethyl radical, according to the study of the BO decompositionmechanism by Harper et al.12 1-Hydroxymethyl propyl radicalcould combine with methyl radical to produce 2-methylbutanol.Meanwhile, 1-hydroxybutyl radical could decompose to vinylalcohol and ethyl radical. Vinyl alcohol then transformed toacetaldehyde followed by a subsequent Tishchenko reaction toethyl acetate. Ethyl acetate was converted to ethyl acetoacetateby the Claisen condensation.

3.6. Reusability of Ni/La−Al2O3. After the completion ofreaction, Ni/La−Al2O3 was separated from the reaction systemby filtering and then was washed with absolute alcohol, dried at110 °C for 8 h, calcinated at 500 °C for 4 h, and finally reducedat 550 °C for 3 h in the atmosphere of a mixture of H2 and N2with a H2 volumetric percentage of 20%, just as the fresh Ni/La−Al2O3 was. The recovered and treated Ni/La−Al2O3catalysts were reused in the reaction for one-pot sequentialsynthesis of 2EHO, and the results are listed in Table 9. It wasfound that the yield of 2EHO almost remained unchanged forthe second use. The catalytic activity of Ni/La−Al2O3 usedthree times decreased, and the yield of 2EHO declined by



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27.8% and a lot of 2EH was left in the products. The yield of2EHO was merely 27.6% when the catalyst was used four times.However, compared with the Ni/Ce−Al2O3 catalyst reported inour previous paper,5 the reusability of Ni/La−Al2O3 wasimproved, 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°) diffractionpeaks 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 thefresh Ni/La−Al2O3, indicating that Ni and La−Al2O3 particleswere 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 thepeaks of metal Ni became narrow and sharp, indicating theagglomeration and growth of metal Ni and La2O3 particles inthe process of reuse. To explain the difference in the reusabilitybetween Ni/La−Al2O3 and Ni/Ce−Al2O3, Ni metal particlessize of the fresh and the recovered Ni/La−Al2O3 and Ni/Ce−Al2O3 after the first run were calculated by the Scherrer formulausing XRD measurement data and the results are listed in Table10. The sizes of the fresh Ni/La−Al2O3 and Ni/Ce−Al2O3

were similar, while the recovered Ni/La−Al2O3 showedparticles of smaller size and the recovered Ni/Ce−Al2O3

showed larger particles, indicating that La can inhibit theagglomeration of Ni efficiently. Navarro et al.13 obtained thesame conclusion in their studies. In addition, new γ-AlO(OH)(2θ = 14.4 and 38.3°) diffraction peaks appeared in therecovered Ni/La−Al2O3 sample, indicating that γ-Al2O3 washydrated with the byproduct water from n-butyraldehyde self-condensation, 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 surfacemorphology of the recovered Ni/La−Al2O3 changed obviously.The fresh Ni/La−Al2O3 presented a clear and uniform particlestructure. However, a new hydration structure was observed inthe recovered Ni/La−Al2O3 and its branch structure wasdistributed desultorily. Liang et al.5 studied the deactivation ofNi/Ce−Al2O3 and found that the new surface hydrate structurewas a mixture of γ-AlO(OH) and γ-Al2O3, which covered upparts of Ni species on the surface and decreased the catalytichydrogenation activity of Ni/La−Al2O3. Therefore, weproposed that the new structure of γ-AlO(OH) formed fromhydration of γ-Al2O3 would cover up parts of Ni species on thesurface, which was another reason for the decrease of thecatalytic hydrogenation activity of Ni/La−Al2O3, consistentwith the results of XRD analyses.Two kinds of hydrophobic substances, γ-chloropropyl

triethoxysilane (CPTEOS) and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FAS), were introduced onto the surface of γ-Al2O3 in order to inhibit its hydration. Then the catalyticperformance for self-condensation of n-butyraldehyde and theantihydration effect were evaluated. It was found from Table 11that the catalytic activity of the modified γ-Al2O3 decreased tosome degree. It is obvious that γ-AlO(OH) diffraction peakswere not found from Figure 3, indicating that CPTEOS andFAS could inhibit the generation of boehmite γ-AlO(OH)completely. In the light of the low catalytic activity ofCPTEOS−Al2O3 and FAS−Al2O3, the improvement of theircatalytic performance is the next goal in our research.

4. CONCLUSIONS

One-pot sequential synthesis of 2EHO from n-butyraldehydecan simply the present production process and has a industrialpractice value. A metal−solid acid bifunctional catalyst Ni/La−Al2O3 was prepared, and its catalytic performance for one-potsequential synthesis of 2EHO was investigated. The yield of2EHO attained 67.0% at a 100% conversion of n-butyraldehydeunder suitable reaction conditions. However, Ni could inhibitthe aldol condensation of n-butyraldehyde, resulting in thedecrease of the yield and selectivity of 2E2H and then thedecline in the selectivity of target product 2EHO. Thedeactivation of Ni/La−Al2O3 was due to the agglomerationof Ni and La2O3 and the coverage of Ni by γ-AlO(OH) formedfrom the hydration of γ-Al2O3. The introduction of somehydrophobic groups on the surface of γ-Al2O3 could effectivelyinhibit the hydration; however, the improvement of thecatalytic performance of the modified γ-Al2O3 is required next.

■ AUTHOR INFORMATION

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

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

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

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