n-Butyraldehyde Self-Condensation Catalyzed by Sulfonic Acid

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n‑Butyraldehyde Self-Condensation Catalyzed by Sulfonic Acid Functionalized Ionic Liquids Xiaolong Zhang, Hualiang An, Hongqi Zhang, Xinqiang Zhao,* and Yanji Wang Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, Hebei University of Technology, Tianjin, 300130, China ABSTRACT: Self-condensation of n-butyraldehyde to 2-ethyl-2-hexenal is one of the important processes for the industrial production of 2-ethylhexanol. In the present work, several sulfonic acid functionalized ionic liquids (SFILs) were synthesized. Their acid strengths were determined by the Hammett method combined with UV−vis spectroscopy, and their catalytic performances in n-butyraldehyde self-condensation were investigated. The results show that the conversion of n-butyraldehyde correlated well with the acid strength of the SFILs with the same cation. The SFILs with triethylammonium cations showed a better catalytic performance than those with imidazolium cations or pyridinium cations, and [HSO3-b-N(Et)3]p-TSA (“b”, butyl) exhibited the highest selectivity. Under the optimal reaction conditions of the mass ratio of [HSO3-b-N(Et)3]p-TSA to n-butyraldehyde = 0.1, reaction temperature = 393 K, and reaction time = 6 h, the conversion of n-butyraldehyde was 89.7% and the selectivity to 2-ethyl-2-hexenal was 87.8%. [HSO3-b-N(Et)3]p-TSA could be reused four times without a significant loss in its catalytic performance. A kinetic analysis result showed that this is a reversible second-order reaction. Compared with the kinetic parameters from the reaction catalyzed by an aqueous base or acid catalyst, the pre-exponential factor is lower due to the restriction of the high viscosity of [HSO3-b-N(Et)3]p-TSA. Finally, a possible reaction mechanism for n-butyraldehyde self-condensation catalyzed by [HSO3-b-N(Et)3]p-TSA was proposed.

1. INTRODUCTION 2-Ethylhexanol (2EHO) is an important organic chemical, and its worldwide production is greater than that of all other alcohols containing from one to four carbon atoms, mainly due to the widespread use of its carboxylic acid esters as a plasticizer, especially in polyvinyl chloride manufacture. Other uses of 2EHO include the production of intermediates for acrylic surface coatings, diesel fuel, and lube oil additives and surfactants.1 The industrial production of 2EHO comprises three reaction steps: propylene hydroformylation to n-butyraldehyde, n-butyraldehyde self-condensation to 2-ethyl-2-hexenal (2E2HA), and 2E2HA hydrogenation to 2EHO. The self-condensation of n-butyraldehyde is a typical aldol condensation reaction, which can be catalyzed by an acid, a base, or an acid−base bifunctional catalyst. At present, the industrial technology for the manufacture of 2E2HA using an aqueous caustic alkali as catalyst possesses many disadvantages.2 First of all, if the liquid alkali concentration is lower, n-butyraldehyde self-condensation reaction proceeds incompletely. However, the aqueous alkali with rather high concentration is prone to the formation of trimer or polymer products, leading to a lower selectivity of 2E2HA. Second, the emission of a lot of alkali wastewater is harmful to the environment and a costly treatment operation is needed. Finally, the production cost is raised from a high consumption of the aqueous caustic alkali catalyst, from the treatment of the alkali wastewater, and from the recovery of 2E2HA dissolved in the water phase. Several kinds of solid base catalysts including alkaline earth metal oxides and supported alkali catalysts have been studied for n-butyraldehyde selfcondensation.3−6 The solid base catalysts were focused on due to their characteristics such as high activity, high selectivity, and easy separation. However, they are sensitive to air and water and show poor stability, hindering their industrial application. © XXXX American Chemical Society

As we know, aldol condensation can be catalyzed by either a basic catalyst or an acidic catalyst. Unfortunately, very few studies on n-butyraldehyde self-condensation catalyzed by an acidic catalyst have been published in the literature.7,8 During the past decades, ionic liquids (ILs) have received considerable attention as environmentally friendly and effective reaction media for a wide variety of organic reactions and other applications. The first Brønsted acid functionalized ionic liquids were prepared by Cole and co-workers.9 The introduction of acidic groups (such as carboxyl group, sulfonic group, etc.) to the cation of ILs can obtain an adjustable Brønsted acid system, which can be used in the conventional acid-catalyzed reactions, such as esterification,10,11 etherification,12 acetalization,13 hydrolysis,14 Prins,15 Henry,16 and other reactions.17−22 Acidic ILs possess the advantages of both liquid acid and solid acid catalysts such as good mobility, high acid density, uniformly distributed acid strength, easy separation, and reusability. To the best of our knowledge, there are no reports published about the n-butyraldehyde selfcondensation reaction catalyzed by sulfonic acid functionalized ionic liquids (SFILs). In this paper we synthesized a series of SFILs, characterized their structures, determined the acid strength and acid amount, and evaluated the catalytic performance and reusability. In addition, a qualitative analysis and a kinetic study were made and a possible catalytic reaction mechanism was proposed. Received: July 22, 2014 Revised: October 7, 2014 Accepted: October 8, 2014

A

dx.doi.org/10.1021/ie5029254 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. n-Butyraldehyde (AR, Tianjin Damao Chemical Reagent Factory, China), N-methylimidazole (99%, Zhejiang Linhai Kaile Chemical Factory, China), pyridine (AR, Tianjin Fengchuan Chemical Reagent Science and Technology Co. Ltd., China), triethylamine (AR, TCRY, China), 1,4-butane sultone (AR, Wuhan Fengfan Chemical Co. Ltd., China), sulfuric acid (CP, Tianjin Jiangtian Chemical Technology Co. Ltd., China), trifluoromethanesulfonic acid (AR, Shanghai Ever-thriving Trading Co. Ltd., China), and p-toluenesulfonic acid and 4-nitroaniline (AR, Sinopharm Chemical Reagent Co. Ltd., China) were used. 2.2. Preparation of SFILs. The SFILs used in this work (as shown in Figure 1) were synthesized according to the

the scanning rate was 0.2 cm/s in the wavenumber range 400− 4000 cm−1. The acid strengths of the SFILs were determined by the Hammett method combined with UV−vis spectroscopy. The spectra were recorded on an Agilent Cary 100 UV−vis spectroscope in the scanning range of 200−800 nm at room temperature. 4-Nitroaniline was selected as an indicator (pKa = +0.99), and methylene dichloride was used as a solvent. The acid amounts of the SFILs were determined by an acid− base titration method. Phenolphthalein was used as an indicator, and the concentration of NaOH aqueous solution was 0.1 mol/L. 2.4. n-Butyraldehyde Self-Condensation Reaction. Samples of 40 g of n-butyraldehyde and 4 g of [HSO3-bN(Et)3]p-TSA were placed in a 100 mL stainless steel autoclave. After purging with dry nitrogen, the mixture was heated and the reaction was carried out at 393 K for 6 h. After the completion of the reaction, the mixture was cooled to room temperature and transferred into a separatory funnel containing 10 mL of water. The upper layer containing condensation products and unreacted n-butyraldehyde was separated from an aqueous solution of [HSO3-b-N(Et)3]p-TSA located at the bottom. The products were quantitatively analyzed by gas chromatography and the [HSO3-b-N(Et)3]p-TSA was recovered after removal of water under vacuum. 2.5. Kinetic Experiment. Samples of 120 g of nbutyraldehyde and 12 g of [HSO3-b-N(Et)3]p-TSA were placed in a 300 mL Parr autoclave. After purging by dry nitrogen, the mixture was heated. When the reaction mixture reached the target temperature, the reaction time was set as t = 0. The first sample was withdrawn when the reaction time reached 1200 s, and the succeeding samples were taken out at a time interval of 1200 s until the sixth sample was withdrawn at 7200 s. Each sample was extracted by distilled water to remove [HSO3-bN(Et)3]p-TSA, and the organic layer was quantitatively analyzed by gas chromatography. 2.6. Product Analysis. A qualitative analysis of the product was conducted with gas chromatography−mass spectrometry (GC−MS; Thermo Finnigan TRACE DSQ). An electron ionization source was used in mass spectrometry with the ion source temperature of 473 K. The mass spectrum was recorded in the range 40−500 amu. The temperature of both the vaporizing chamber and the transmission line was controlled at 523 K. A BPX5 capillary column was used for separation of components and the column temperature was controlled according to the following program: initial temperature of 313 K, then heated to 523 K in a ramp of 10 K·min−1 and held for 5 min. A quantitative analysis of the product was carried out using an SP-2100 gas chromatograph (Beijing Beifen-Ruili Analytical

Figure 1. Structures of sulfonic acid functionalized ionic liquids.

method described in the literature.10,23 The synthesis of N,N,Ntriethyl-N-(4-butanesulfonic acid) 4-methylbenzenesulfonate ([HSO3-b-N(Et)3]p-TSA) is given here as an example: 0.5 mol of triethylamine and 0.45 mol of 1,4-butane sultone were charged into a 250 mL three-necked flask equipped with a stirrer and the mixture was stirred at 333 K for 14 h to obtain a white solid zwitterionic salt [SO3-b-N(Et)3]. The zwitterion salt was washed five times with ether to remove nonionic residues and then dried in a vacuum. After that, 0.5 mol of [SO3-b-N(Et)3] and 0.5 mol of p-toluenesulfonic acid were placed in another 250 mL threenecked flask equipped with a stirrer and the mixture was stirred at 353 K for 7 h to form [HSO3-b-N(Et)3]p-TSA. The resultant product was also washed five times with ether and then dried in a vacuum for 12 h. The structure of [HSO3-b-N(Et)3]p-TSA was confirmed by the Fourier transform infrared (FT-IR) spectral data, and the recorded spectrum was coincident with that in the literature.24 Other SFILs were prepared by the same method, and their FT-IR spectral data were also in accord with those in the literature.11,24 2.3. Characterization of SFILs. The FT-IR spectra of the SFILs were recorded on a Bruker VECTOR22 infrared spectrometer with the potassium bromide emulsion liquid membrane method. The instrument resolution was 4 cm−1 and

Table 1. Catalytic Performance of the SFILs in n-Butyraldehyde Self-Condensation Reactiona

a

entry

catalystb

n-butyraldehyde conv (%)

2E2HA yield (%)

2E2HA selectivity (%)

1 2 3 4 5 6 7 8 9

[HSO3-b-mim]HSO4 [HSO3-b-mim]CF3SO3 [HSO3-b-mim]p-TSA [HSO3-b-Py]HSO4 [HSO3-b-Py]CF3SO3 [HSO3-b-Py]p-TSA [HSO3-b-N(Et)3]HSO4 [HSO3-b-N(Et)3]CF3SO3 [HSO3-b-N(Et)3]p-TSA

34.5 76.0 51.5 46.7 94.0 54.2 78.2 95.5 89.4

18.6 55.5 33.1 23.9 66.2 32.4 55.7 72.8 78.4

54.0 73.1 64.3 51.3 70.4 59.8 71.3 78.1 87.8

Mass ratio of catalyst to n-butyraldehyde = 0.1, reaction temperature of 393 K, and reaction time of 6 h. bb, butyl. B



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order CF3SO3− < p-TSA− < HSO4−, in accordance with their catalytic performance. The selectivity of 2E2HA is almost in the same order except for [HSO3-b-N(Et)3]p-TSA, possibly due to the cooperative effect of its cation and anion. The acid strength of the SFILs with p-TSA anion is relatively strong, favoring the n-butyraldehyde self-condensation reaction. Furthermore, the [HSO3-b-N(Et)3]+ cation makes proton transfer more easily between n-butyraldehyde and the sulfonic group. Therefore, the conversion of n-butyraldehyde correlated well with the acid strength of the SFILs with the same cation. Since [HSO3b-N(Et)3]p-TSA showed the highest selectivity, it was chosen as the catalyst for the further investigation. 3.3. Identification of Byproducts. After the completion of reaction, the reaction mixture was collected and analyzed by GC−MS, and the main byproducts were identified as 5-ethyl2,4-dipropyl-1,3-dioxane and its isomer. On the basis of the product analysis results, we proposed a possible reaction network for n-butyraldehyde self-condensation catalyzed by [HSO3-b-N(Et)3]p-TSA (shown in Scheme 1). According to prior studies,28,29 a small amount of hydrogen can be generated in the n-butyraldehyde self-condensation reaction system. The intermediate product 2-ethyl-3-hydroxyhexanal (2E3OHA) might be hydrogenated to 2-ethyl-1,3-hexanediol followed by an acetalization reaction with another n-butyraldehyde molecule to form the byproduct 5-ethyl-2,4-dipropyl-1,3-dioxane. In the initial stage of the n-butyraldehyde self-condensation reaction, the resistance to the forward reaction was small and the self-condensation reaction could occur easily by the catalysis of [HSO3-b-N(Et)3]p-TSA because of the high concentration of n-butyraldehyde and low concentrations of 2E2HA and water. During this period, the intermediate product 2E3OHA tended to undertake a dehydration reaction. However, a small amount of 2E3OHA would be hydrogenated and then form 5-ethyl-2,4-dipropyl-1,3-dioxane. At this point 2E2HA was formed with high selectivity and each reaction proceeded rapidly and steadily. When a lot of n-butyraldehyde was consumed, the resistance to the forward reaction would be enhanced with the increase of the concentration of 2E2HA and water.30 By so doing, the dehydration of 2E3OHA would be impeded and, what is more, the 2E2HA concentration would decrease due to the enhancement of the backward reaction, promoting the side reaction to 5-ethyl-2,4-dipropyl-1,3-dioxane. In order to understand the influence of reaction conditions on the catalytic performance of [HSO3-b-N(Et)3]p-TSA, the experiments in section 3.4 were conducted. 3.4. Effect of Reaction Conditions. 3.4.1. Effect of Catalyst Dosage. The effect of [HSO3-b-N(Et)3]p-TSA dosage on n-butyraldehyde self-condensation reaction was studied, and the results are shown in Figure 2. With the increase of [HSO3b-N(Et)3]p-TSA dosage, the conversion of n-butyraldehyde increased monotonically and the yield of 2E2HA rose first and then remained even while the selectivity of 2E2HA increased first, then declined, and finally went steadily. When the mass ratio of [HSO3-b-N(Et)3]p-TSA to n-butyraldehyde was less than 0.1, the increase of [HSO3-b-N(Et)3]p-TSA dosage means the increase of the active sites, favoring the n-butyraldehyde self-condensation to 2E2HA. When the mass ratio of [HSO3-bN(Et)3]p-TSA to n-butyraldehyde was 0.1, [HSO3-b-N(Et)3]pTSA provided enough active sites and the yield and selectivity of 2E2HA reached the maximum. With a continual increase of the dosage of [HSO3-b-N(Et)3]p-TSA, the conversion of n-butyraldehyde rose slowly while the yield and selectivity of 2E2HA decreased. The high concentration of 2E2HA would

Instrument (Group) Co., Ltd.). The product mixture was separated in a KB-1 capillary column whose temperature was controlled according to the following program: initial temperature of 353 K and held for 3 min, and then heated to 433 K in a ramp of 10 K·min−1 and held for 10 min. Nitrogen was used as a carrier gas and its flow rate was 30 mL·min−1. The components were analyzed with a flame ionization detector.

3. RESULTS AND DISCUSSION 3.1. Screening of SFIL Catalysts. The catalytic performance of the prepared SFILs was evaluated, and the results are listed in Table 1. There was a marked influence of anion on the catalytic performance of SFILs if the cation was the same. Similarly, the influence of different cations on the catalytic performance of SFILs was significant if the anion was the same. The conversion of n-butyraldehyde decreased in the order [HSO3-b-N(Et)3]+ > [HSO3-b-Py]+ > [HSO3-b-mim]+, while the selectivity of 2E2HA declined in the order [HSO3-bN(Et)3]+ > [HSO3-b-Py]+ ≈ [HSO3-b-mim]+. The SFILs with triethylammonium cation showed a better catalytic performance than the other SFILs. Wu et al.25 studied the selfcondensation of propanal catalyzed by some kinds of SFILs and found that the structure of the cation could affect the masstransfer resistance. In addition, the distances of O−H bonds in the sulfonic acid group of the different cations with the same anions were studied by both acidity measurements and theoretical calculations.12,21,26,27 The results showed that the distances of O−H bonds in the sulfonic acid functionlized triethylammonium ionic liquids were longer, making the proton in the sulfonic acid group much more active and exhibiting a better catalytic activity. This may be why the triethylammoniumtype SFILs showed a better catalytic performance than the imidazolium-type or pyridinium-type SFILs in this work. 3.2. Relationship between Catalytic Performance and Acid Strength or Acid Amount of SFILs. In order to explore the relationship between the catalytic performance and the properties of SFILs, the acid amount and acid strength of these SFILs were measured. The acid amount was determined by an acid−base titration method, and the moles of NaOH consumed per mole of SFILs are listed in Table 2. Combined Table 2. Acid Amount and H0 Values of Sulfonic Acid Functionalized ILsa

a b

entry

ILsb

n(NaOH):n(SFILs)

H0

1 2 3 4 5 6 7 8 9

[HSO3-b-mim]HSO4 [HSO3-b-mim]CF3SO3 [HSO3-b-mim]p-TSA [HSO3-b-Py]HSO4 [HSO3-b-Py]CF3SO3 [HSO3-b-Py]p-TSA [HSO3-b-N(Et)3]HSO4 [HSO3-b-N(Et)3]CF3SO3 [HSO3-b-N(Et)3]p-TSA

1.74 0.88 0.88 1.92 0.90 0.94 1.86 0.94 0.99

1.50 0.95 1.22 1.47 0.93 1.21 1.92 1.01 1.23

Indicator 4-nitroaniline (5 mg/L, pKa = +0.99) and SFILs (8 mmol/L). b, butyl.

with the data in Table 1, it is inferred that the acid amount of SFILs is not related to the catalytic performance. The acid strength of these SFILs was measured according to the method reported previously23 and their H0 values are listed in Table 2. Each type of SFILs with different anions exhibited the same change tendency on H0; i.e., H0 decreased according to the C



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Scheme 1. Possible Reaction Network for n-Butyraldehyde Self-Condensation Catalyzed by [HSO3-b-N(Et)3]p-TSA

Figure 2. Effect of catalyst amount on n-butyraldehyde selfcondensation reaction. Reaction conditions: T = 393 K; t = 6 h. B, n-butyraldehyde; 2E2HA, 2-ethyl-2-hexenal. X, conversion; Y, yield; S, selectivity.

Figure 3. Effect of reaction temperature on n-butyraldehyde selfcondensation reaction. Reaction conditions: m(ILs)/m(n-butyraldehyde) = 0.1; t = 6 h. B, n-butyraldehyde; 2E2HA, 2-ethyl-2-hexenal. X, conversion; Y, yield; S, selectivity.

restrict the dehydration of the intermediate 2E3OHA and promote the side reaction to 5-ethyl-2,4-dipropyl-1,3-dioxane. Therefore, the suitable mass ratio of [HSO3-b-N(Et)3]p-TSA to n-butyraldehyde was 0.1. 3.4.2. Effect of Reaction Temperature. The effect of reaction temperature on the n-butyraldehyde self-condensation reaction was investigated. As can be seen from Figure 3, with the increase of reaction temperature ranging from 373 to 413 K, the conversion of n-butyraldehyde increased monotonically and the selectivity of 2E2HA grew steadily first and then declined while the yield of 2E2HA first increased and then decreased. When the temperature was below 393 K, the reaction rate increased at a higher temperature with the unchanged selectivity of 2E2HA, so the yield of 2E2HA increased. When the temperature was above 393 K, the conversion of n-butyraldehyde rose slowly but the selectivity of 2E2HA declined because of serious side reactions. Therefore, the suitable reaction temperature was 393 K. 3.4.3. Effect of Reaction Time. The effect of reaction time on n-butyraldehyde self-condensation was studied, and the results are shown in Figure 4. Along with the prolonging of the

Figure 4. Effect of reaction time on n-butyraldehyde self-condensation reaction. Reaction conditions: m(ILs)/m(n-butyraldehyde) = 0.1; T = 393 K. B, n-butyraldehyde; 2E2HA, 2-ethyl-2-hexenal. X, conversion; Y, yield; S, selectivity. D



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n-butyraldehyde self-condensation to a certain degree and cannot be ignored. Assuming that both the forward and backward reactions are second order, the reaction rate equation can be expressed as31 n B dx B dx 1 dnB −rB = − = 0 = c B0 B V dt V dt dt

reaction time, the conversion of n-butyraldehyde increased slowly, and the selectivity of 2E2HA grew steadily first and then declined while the yield of 2E2HA increased first and then decreased. When the reaction time was shorter than 6 h, n-butyraldehyde underwent self-condensation reaction to produce 2E2HA and the selectivity of 2E2HA maintained steadily. When the reaction time was longer than 6 h, the reaction equilibrium moved backward and the production of the byproduct 5-ethyl-2,4-dipropyl-1,3-dioxane was promoted. In this period, the conversion of n-butyraldehyde increased slowly while the yield and selectivity of 2E2HA decreased accordingly. Therefore, the suitable reaction time is 6 h. On the basis of the above experiments, the optimal reaction conditions were obtained as follows: mass ratio of [HSO3-bN(Et)3]p-TSA to n-butyraldehyde = 0.1, reaction temperature of 393 K, and reaction time of 6 h. The conversion of n-butyraldehyde, the yield of 2E2HA, and the selectivity of 2E2HA were 89.4%, 78.4% and 87.8%, respectively. 3.5. Reusability of [HSO3-b-N(Et)3]p-TSA. In order to investigate the reusability of [HSO3-b-N(Et)3]p-TSA, a recycle experiment was conducted under suitable reaction conditions. After the completion of the reaction, the reaction mixture was separated into two phases by the addition of water. The aqueous phase containing [HSO3-b-N(Et)3]p-TSA was extracted from the lower layer, and [HSO3-b-N(Et)3]p-TSA could be recovered after removal of most of the water by vacuum distillation and then drying in a vacuum. Each time about 91% of [HSO3-b-N(Et)3]p-TSA could be recovered. The result of the reusability of [HSO3-b-N(Et)3]p-TSA is shown in Table 3. [HSO3-b-N(Et)3]p-TSA could be reused four times

= k+c B 2 − k −c 2E2HAc w

Since c2E2HA= cw = cB0xB/2, eq 2 can be expressed as follows: dx B = k+c B0(1 − x B)2 − k −c B0(x B/2)2 dt

XB (%)

Y2E2HA (%)

1 2 3 4 5

89.4 89.4 88.6 88.0 87.3

78.4 78.4 78.2 77.5 77.2

(3)

where k+ and k− are separately the rate constants for the forward and backward reactions. xB is the conversion of n-butyraldehyde. Substituting the Arrhenius equation into eq 3, the reaction rate equation can be expressed as eq 4. ⎛ Ea, + ⎞ ⎛ Ea, − ⎞ dx B = k 0 + exp⎜ − ⎟c B0(1 − x B)2 − k 0 − exp⎜ − ⎟c (x /2)2 ⎝ ⎠ ⎝ RT ⎠ B0 B RT dt (4)

where k0+ and k0− are separately the pre-exponential factor for the forward and backward reaction. Ea,+ and Ea,− are separately the activation energies for the forward reaction and the backward reaction. 3.6.2. Kinetic Experiment. The kinetic experiment was conducted in a 300 mL Parr autoclave. Since high temperature would lead to a side reaction easily, the reaction temperatures in the kinetic experiment were selected as 363, 373, 383, and 393 K. The conversion of n-butyraldehyde versus reaction time at different temperatures and with the mass ratio of [HSO3-bN(Et)3]p-TSA to n-butyraldehyde of 0.1 is separately shown in Figures 5−8.

Table 3. Reusability of [HSO3-b-N(Et)3]p-TSAa run

(2)

a

Reaction conditions: m(ILs)/m(n-butyraldehyde) = 0.1; T = 393 K; t = 6 h. B, n-butyraldehyde; 2E2HA, 2-ethyl-2-hexenal; b, butyl. X, conversion; Y, yield.

and the yield of 2E2HA remained almost unchanged. FT-IR analyses result showed that the characteristic peaks of the recovered [HSO3-b-N(Et)3]p-TSA are almost the same as the fresh, indicating that the structure of [HSO3-b-N(Et)3]p-TSA remained unchanged during the reaction. Therefore, [HSO3-bN(Et)3]p-TSA not only showed a high catalytic performance but also exhibited good reusability. 3.6. Kinetic Study. 3.6.1. Establishment of Kinetic Model. The reaction equation for n-butyraldehyde self-condensation can be described as follows:

Figure 5. Conversion of n-butyraldehyde versus reaction time at reaction temperature of 393 K.

3.6.3. Determination of Kinetic Parameters. Since the time consumed for elevating the reaction temperature from room temperature to the target temperature was rather short, the reaction taking place in this period could be ignored. Therefore, the initial condition for the kinetic equation was set as t = 0, xB = 0. The solution of the rate equation is an initial-value problem of a first-order ordinary differential equation. If the initial values of the parameters k0+, Ea,+, k0−, and Ea,− are given, the conversion of n-butyraldehyde at t = 1200 s can be obtained by integrating the set of first-order ordinary differential equations

The thermodynamic calculation results showed that the reaction equilibrium constant varied from 10.75 to 2.438 in the reaction temperature range 363−393 K under ordinary pressure. Therefore, the backward reaction should affect E



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analyses. Then the objective function for parameter estimation is expressed as follows: T

f=

t

∑ ∑ (xexp − xcal)2

(5)

where T represents the reaction temperature and t represents different sampling times. The parameters k0+, Ea,+, k0−, and Ea,− can be estimated by minimizing this function using MATLAB software. The estimated kinetic parameters are listed in Table 4. Table 4. Values of Activation Energy and Pre-Exponential Factor of Forward and Reverse Reactions k0+ (L·(mol·s)−1)

Ea,+ (kJ·mol−1)

k0− (L·(mol·s)−1)

Ea,− (kJ·mol−1)

1.999 × 10

60.29 ± 2.856

1.001 × 10

62.94 ± 0.323

4

4

Therefore, the rate equation was obtained as follows:


⎛ 6.029 × 104 ⎞ dx B 2 = 1.999 × 104 exp⎜ − ⎟c B0(1 − x B) dt RT ⎝ ⎠ ⎛ 6.294 × 104 ⎞ 2 − 1.001 × 104 exp⎜ − ⎟c B0(x B/2) RT ⎝ ⎠ (6)

3.6.4. Test of Kinetic Model. Figure 9 shows a comparison between the predicted values of the kinetic model and the


Figure 9. Comparison between experimental data and predictions of conversion of n-butyraldehyde. x, conversion of n-butyraldehyde; pre, predicted values; exp, experimental values.

experimental data. What can be seen is that the points scatter on both sides of the diagonal line and the predicted values accord well with the experimental data. The results of variation analysis and the F-test on the kinetic model are listed in Table 5. As we know from the variation analysis theory, if F < Fα (F0.05(23,23) = 2.05) and the correlation coefficient is larger than 0.9, the model is considered to be suitable to the α level. In the present work, the kinetic model is


using a fourth-order Runge−Kutta method in the time interval of [0, 1200]. Similarly, the conversion of n-butyraldehyde at t = i can be attained in the time interval of [0, i]. These obtained conversions of n-butyraldehyde were used as the predicted values of the reaction kinetics model while the experimental data at t = i, conversions of n-butyraldehyde, were obtained by GC

Table 5. Model Statistics free variation no. 4 F

regression square sum 2.367

residual error square sum −4

3.94 × 10

correln coeff

F0.05(23,23)

0.973

1.0049



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the α carbon atom can form the σ−π hyperconjugated effect with the carbonyl group. The influences of both the induction effect and the hyperconjugation effect make the hydrogen in the α carbon atom more active. First, the proton in the sulfonic acidic group of [HSO3-b-N(Et)3]p-TSA combines with the carbonyl oxygen of n-butyraldehyde. The protonated carbonyl group has a stronger electron-withdrawing effect; thus the acidity of the α hydrogen is enhanced and the dissociation of the α hydrogen to enol is promoted. Second, [HSO3-b-N(Et)3] p-TSA could protonate and activate the carbonyl group of another n-butyraldehyde molecule, making the carbonyl group show a strong eletrophilicity. The α carbon atom in the enol molecule has nucleophilicity and can attack the protonated carbonyl carbon. Then 2E3OHA is formed after deprotonation. Since the α hydrogen of 2E3OHA is active, 2E3OHA is prone to losing one molecule of water to form 2E2HA, which has a stable structure with a conjugated double bond.

significant to the level α = 0.05 and thus is able to describe the n-butyraldehyde self-condensation reaction process. 3.6.5. Verification of Reaction Order. Integrating eq 3, we can obtain its integral form as eq 7. 1 2 a2 − a

ln

(x B − a) −

a2 − a

(x B − a) +

a2 − a

= c B0t + C (7)

where a = k+/[k+ − (k−/4)]. Substituting the experimental data xB and t into eq 7 and plotting ln |[(xB − a) − (a2 − a)1/2]/ [(xB − a) + (a2 − a)1/2)]| vs t, a straight line was obtained, indicating that the hypothesis for second-order reaction is correct. Up until now, the reaction kinetics for n-butyraldehyde self-condensation has been studied using conventional liquid acid or base catalyst. Lee et al.32 studied a biphasic aldol condensation of n-butyraldehyde catalyzed by an aqueous solution of sodium hydroxide and estimated that the activation energy was 56.457 ± 1.673 kJ/mol and the pre-exponential factor was 1.712 × 108 L·(mol·s)−1 in the temperature range 383−423 K. Casale and co-workers33 studied the kinetics of the acid-catalyzed aldol condensation reaction of a range of aliphatic aldehydes (C2−C8). They estimated that the activation energy was 53.792 ± 7.815 kJ/mol and the pre-exponential factor was 3.594 × 108 L·(mol·s)−1 for n-butyraldehyde self-condensation catalyzed by an aqueous solution of H2SO4 (85 wt %) in the temperature range 249−315 K. In comparison with the kinetic data from the conventional inorganic acid and base catalysts, the difference in activation energy is small but the pre-exponential factor is much smaller for the case of n-butyraldehyde condensation under catalysis of [HSO3-b-N(Et)3]p-TSA due to the restriction of high viscosity and low intermolecular collision frequency. 3.7. Reaction Mechanism of n-Butyraldehyde SelfCondensation Catalyzed by [HSO3-b-N(Et)3]p-TSA. The mechanism of n-butyraldehyde self-condensation catalyzed by [HSO3-b-N(Et)3]p-TSA is proposed in Scheme 2. Due to the strong electronegativity of the carbonyl oxygen atom in an n-butyraldehyde molecule, the carbonyl group has an induction effect for electron withdrawal. In addition, the C−H bond in

4. CONCLUSIONS As compared with the dilute liquid alkali catalyst used in industry, the SFIL used in this work is a highly efficient green catalyst with the following characteristics: noncorrosiveness, smaller dosage, easy separation from the reaction products, reusability, no emission of wastewater, and no requirement of neutralization operation. Among the three kinds of SFILs prepared, the ones with triethylammonium cation showed a better catalytic performance and [HSO3-b-N(Et)3]p-TSA exhibited the highest selectivity. The optimal reaction conditions for n-butyraldehyde self-condensation were as follows: mass ratio of [HSO3-b-N(Et)3]p-TSA to n-butyraldehyde = 0.1, reaction temperature of 393 K, and reaction time of 6 h. Under the optimal conditions the conversion of n-butyraldehyde, the yield of 2E2HA, and the selectivity of 2E2HA were 89.4%, 78.4%, and 87.8%, respectively. [HSO3-b-N(Et)3]p-TSA can be reused four times without a significant loss in its catalytic performance. The reaction kinetic model was established for n-butyraldehyde self-condensation catalyzed by [HSO3-b-N(Et)3]p-TSA. It is a second-order reversible reaction. The activation energy is 60.29 ± 2.856 kJ/mol and the pre-exponential factor is

Scheme 2. Possible Mechanism of n-Butyraldehyde Self-Condensation Catalyzed by [HSO3-b-N(Et)3]p-TSA

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Article

1.999 × 104 L·(mol·s)−1 for the forward reaction while the activation energy is 62.94 ± 0.323 kJ/mol and the preexponential factor is 1.001 × 104 L·(mol·s)−1 for the backward reaction in the temperature range 363−393 K. As compared with the kinetic parameters from the reaction catalyzed by an aqueous base or acid catalyst, the pre-exponential factor in the present study is lower due to the restriction of the high viscosity of [HSO3-b-N(Et)3]p-TSA. Finally, a possible reaction mechanism of n-butyraldehyde self-condensation catalyzed by [HSO3-bN(Et)3]p-TSA was proposed.

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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, 21236001) and the Key Basic Research Project of Applied Basic Research Plan of Hebei Province (Grant 12965642D). The authors are gratefully appreciative of their contributions.

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n-Butyraldehyde Self-Condensation Catalyzed by Sulfonic Acid

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