Synthesis of 4, 4′-Methylenedianiline Catalyzed by SO3H

Jul 14, 2015 - of aniline with formaldehyde for the synthesis of 4,4′-MDA. The result showed that SFILs had excellent catalytic activity, and their ...
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Synthesis of 4,4′-Methylenedianiline Catalyzed by SO3H‑Functionalized Ionic Liquids Jinping Tian, Hualiang An, Xiaomeng Cheng, 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: 4,4′-Methylenedianiline (4,4′-MDA) is a key intermediate for the production of polyurethanes. A series of SO3Hfunctionalized ionic liquids (SFILs) were synthesized, and their catalytic performance was evaluated in the condensation reaction of aniline with formaldehyde for the synthesis of 4,4′-MDA. The result showed that SFILs had excellent catalytic activity, and their catalytic activity was consistent with their acid strength. [HSO3-bmim]CF3SO3 (SImTf) with the highest acid strength showed the best catalytic activity. Then, the influence of reaction conditions on the condensation reaction of aniline with formaldehyde to 4,4′-MDA was investigated using SImTf as the catalyst, and suitable reaction conditions were obtained as follows: molar ratio of aniline to formaldehyde = 5, mass ratio of SImTf to formaldehyde = 3.5, reaction temperature of 80 °C, and reaction time of 8 h. Under the above reaction conditions, the conversion of aniline was 36.3%, and the yield and selectivity of 4,4′-MDA were 79.4% and 87.9%, respectively. However, the catalytic activity of the recovered SImTf declined dramatically. The results of UV−vis and FT-IR spectroscopy analyses showed that there was a chemical interaction between SImTf and aniline. The deactivated SImTf could be regenerated by acidization with trifluoromethanesulfonic acid, and the acidized SImTf could be reused four times with a tolerable loss of its catalytic activity. Meanwhile, a plausible catalytic reaction mechanism for the synthesis of 4,4′-MDA was proposed.

1. INTRODUCTION As an important organic chemical, 4,4′-methylenedianiline (4,4′MDA) is mainly used for the manufacture of 4,4′-methylene diphenyl diisocyanate (4,4′-MDI). It is also used for the production of epoxy resin tough curing agent, chain extension agents, insulation materials, and organic dyes.1,2 The synthesis of 4,4′-MDA via the condensation reaction of aniline with formaldehyde is a typical acid-catalyzed reaction. At present, 4,4′-MDA is industrially manufactured using hydrochloric acid as the catalyst, in a simple technological process with several advantages such as mild reaction conditions and high product yield, etc., and the maximum yield of 4,4′-MDA was just about 70%.3−6 However, there existed some drawbacks in this process, e.g., corrosiveness of hydrochloric acid and emission of a lot of acid wastewater. In order to overcome these disadvantages, researches focused on solid acid catalysts, and some good results have been achieved.3−12 However, a solid acid-catalyzed reaction process requires a high reaction temperature. Therefore, a novel green catalyst with the advantages of both liquid acid catalysts and solid acid catalysts is required. During the past decades, ionic liquids have received considerable attention due to their good solubility, negligible vapor pressure, thermal stability, and chemical stability. As environmentally friendly catalysts, acidic ionic liquids possess the advantages of both liquid acid catalysts and solid acid catalysts, such as good mobility, high acid density, uniformly distributed acid strength, easy separation, and reusability. Functionalized acid ionic liquids are novel green catalysts and have been applied in a wide variety of organic synthesis reactions.13−18 To the best of our knowledge, there are no reports published about the application of SO3H-functionalized ionic liquids (SFILs) in the condensation reaction of aniline with formaldehyde to 4,4′-MDA. In this paper, we synthesized a series of SFILs and © 2015 American Chemical Society

evaluated their catalytic performance in the condensation reaction of aniline with formaldehyde. The reason for the deactivation of the SFILs was analyzed, and a regeneration procedure was found. On the basis of identification of the reaction components, a plausible catalytic reaction mechanism for the synthesis of 4,4′MDA was proposed.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. N-methylimidazole (99%, Zhejiang Linhai Kaile Chemical Factory, China), 1,4-butane sultone (AR, Wuhan Fengfan Chemical Co., Ltd., China), trifluoromethanesulfonic acid (AR, Shanghai Ever-thriving Trading Co., Ltd., China), aniline, phenylsulfonic acid and p-toluene sulfonic acid (AR, Sinopharm Chemical Reagent Co., Ltd., China), dimethyl yellow, tetrafluoroboric acid and 4-nitroaniline (AR, Tianjin Kemiou Chemical Reagent Co., Ltd., China), methanesulfonic acid and trifluoroacetic acid (AR, Tianjin Jiangtian Chemical Technology Co., Ltd., China), trichloroacetic acid (CP, Shanghai SSS Reagent Co., Ltd., China), formaldehyde (AR, Tianjin Fengchuan Chemical Reagent Science and Technology Co., Ltd., China), 4-(phenylazo)diphenylamine and 4,4′-methylenedianiline (AR, J & K Chemical Technology Co., Ltd., China), ethanol and diethyl ether (AR, Tianjin Fuchen Chemical Reagent Factory, China) were used. 2.2. Preparation of SFILs. The SFILs used in this work (shown in Scheme. 1) were synthesized according to the method described in the literature.13−15 The synthesis of Received: Revised: Accepted: Published: 7571

April 22, 2015 July 9, 2015 July 14, 2015 July 14, 2015 DOI: 10.1021/acs.iecr.5b01519 Ind. Eng. Chem. Res. 2015, 54, 7571−7579

Article

Industrial & Engineering Chemistry Research

The mixture was heated to 50 °C while stirring. Then, 14.244 g of formaldehyde aqueous solution (37 wt %) was added dropwise. After about 15 min, the reaction temperature was heated to 80 °C and maintained for 8 h. After completion of the reaction, the mixture was cooled to room temperature for HPLC analysis. The reaction equation of aniline with formaldehyde to 4,4′-MDA is shown in Scheme 2.

Scheme 1. Structure of SFILs

Scheme 2. Reaction of Aniline with Formaldehyde to 4,4′-MDA

[HSO3-bmim]CF3SO3 is given here as an example: 0.15 mol of N-methylimidazole and 0.15 mol of 1,4-butane sultone were charged into a 100 mL three-necked flask equipped with a stirrer, and the mixture was stirred at 60 °C for 5 h to obtain a white solid zwitterion salt. The zwitterion salt was washed five times with ethanol to remove nonionic residues and then dried in vacuum at 80 °C for 6 h. After that, an equal mol of trifluoromethanesulfonic acid was added dropwise to the zwitterion salt. The mixture was stirred for 30 min at room temperature and then stirred at 80 °C for 5 h to form [HSO3bmim]CF3SO3. The resultant product was washed three times with ether and then dried in vacuum at 80 °C for 6 h. Other SFILs were prepared by the same method. The structures of all the prepared SFILs were determined by Fourier transform infrared (FT-IR) spectrum analysis, and their recorded spectra were consistent with those given in the literature.19 2.3. Characterization and Acid Strength Measurement of SFILs. The FT-IR spectra of the SFILs were recorded on a Bruker VECTOR 22 infrared spectroscopy using the potassium bromide emulsion liquid membrane method. The instrument resolution was 4 cm−1 and scanning rate was 0.2 cm/s in the wavenumber range of 400 to 4000 cm−1. The acid strength of the SFILs was determined by the Hammett method combined with UV−vis spectroscopy.15 First, the ionic liquids and the indicators were dissolved separately in a dried dichloromethane to attain an ionic liquid concentration of 5 mmol/L and an indicator concentration of 5 mg/L. Then, the solutions were shaken vigorously and left to stand for 6 h. Finally, the spectra were recorded on an Agilent Cary 100 UV−vis spectroscope in the scanning range of 200 to 800 nm at room temperature. Since the colors of SFILs are light, some Hammett indicators were simply screened to determine the possible H0 extent of the SFILs before measuring their acid strength. The selected indicators were as follows: 4-nitroaniline with a pKa value of +0.99 as the indicator for [HSO3-bmim]CF3SO3; 4-(phenylazo)-diphenylamine with a pKa value of +1.4 as the indicator for [HSO3-bmim]p-TSA, [HSO3-bmim]C6H6SO3, [HSO3-bmim]BF4 ,and [HSO3-bmim]CH3SO3; dimethyl yellow with a pKa value of +3.3 as the indicator for [HSO3-bmim]CF3COO and [HSO3-bmim]CCl3COO. 2.4. Synthesis of 4,4′-MDA from Aniline and Formaldehyde. The condensation reaction of aniline with formaldehyde was carried out in a 250 mL three-necked flask equipped with a thermometer and a condenser. SFILs and aniline were charged at first, and then, formaldehyde aqueous solution was added dropwise. The condensation reaction was stopped after the required reaction time was reached at the desired reaction temperature. A typical operation procedure is described as follows: 18.446 g of [HSO3-bmim]CF3SO3 was charged into the flask, and then 81.720 g of distilled aniline was added.

2.5. Product Analysis. 4,4′-MDA was quantitatively analyzed by Waters HPLC (Waters Co., U.S.A.) with a Waters 1515 pump, a Kromasil C18 column (4.6 mm × 150 mm), and a Waters 2489 UV−vis detector operated at 254 and 232 nm. Methanol−water with a volumetric ratio of 5/5 was used as mobile phase whose flow was 0.4 mL/min. The yield of 4,4′-MDA was calculated on the basis of formaldehyde. The conversion of aniline and the selectivity of 4,4′-MDA were calculated on the basis of aniline. The calculation equations are as follows: 4,4′‐MDA yield (%) moles of 4,4′‐MDA formed = × 100% moles of HCHO charged 4,4′‐MDA selectivity (%) moles of 4,4′‐MDA formed × 2 = × 100% moles of aniline converted The yield and selectivity of 4,4′-MDA and conversion of aniline given in this paper are the average calculated from the repeated tests of three times. The error bars for aniline conversion, 4,4′-MDA yield, and 4,4′-MDA selectivity are given in all figures and tables. The reaction components in the reaction of aniline with formaldehyde to 4,4′-MDA was identified by LCQ Deca XP MAX HPLC-MS (ThermoFisher Co., U.S.A.) with a Thermo BOS Hypersil C18 column (2.1 mm × 150 mm). Methanol− water with a volumetric ratio of 5/5 was used as mobile phase whose flow was 0.4 mL/min. An electrospray ion source (ESI) was used. The scan was operated in both a positive and a negative ion mode with a mass scanning range of 50 to 600 m/z. The spray voltage was 4.5 kV and capillary temperature was 250 °C.

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance of SFILs. The catalytic performance of the prepared SFILs was evaluated in the condensation reaction of aniline with formaldehyde to 4,4′-MDA, and the results are listed in Table 1. No reaction occurred in the absence of SFILs. To determine whether the immiscibility of aniline with formaldehyde limited the occurrence of the reaction, methanol was introduced as a solvent, but 4,4′-MDA could not be formed either. The results show that the reaction of aniline with formaldehyde cannot proceed without SFILs. The condensation reaction of aniline with formaldehyde proceeds in two steps: the reaction of aniline with formaldehyde to form an intermediate, aminal, at room temperature first and then the rearrangement of aminal to 4,4′-MDA at high 7572

DOI: 10.1021/acs.iecr.5b01519 Ind. Eng. Chem. Res. 2015, 54, 7571−7579

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Industrial & Engineering Chemistry Research Table 1. Catalytic Performance and Hammett Acidity Functions H0 of Different SFILsa SFILs blank [HSO3-bmim]CF3SO3 [HSO3-bmim]p-TSA [HSO3-bmim]C6H6SO3 [HSO3-bmim]BF4 [HSO3-bmim]CH3SO3 [HSO3-bmim]CF3COO [HSO3-bmim]CCl3COO

X (%) 36.3 34.1 33.4 35.1 35.9 33.7 34.6

0 (±0.61) (±0.30) (±0.40) (±0.91) (±0.61) (±0.21) (±1.08)

Y (%)

S (%)

0 79.4 (±0.40) 64.4 (±0.70) 63.8 (±0.40) 59.2 (±1.20) 57.8 (±0.38) 51.9 (±1.25) 8.3 (±0.75)

0 87.9 (±1.03) 75.7 (±0.70) 75.5 (±0.44) 68.6 (±0.52) 63.9 (±0.77) 62.0 (±1.48) 9.7 (±0.59)

H0 0.28 1.21 1.54 1.98 2.40 3.95 4.18

(±0.03) (±0.04) (±0.02) (±0.03) (±0.05) (±0.06) (±0.04)

Reaction conditions: molar ratio of aniline/formaldehyde = 5:1, mass ratio of SFILs/formaldehyde = 3.5, 80 °C, and 8 h. X, conversion of aniline; Y, yield of 4,4′-MDA; S, selectivity of 4,4′-MDA. a

temperature.2 The conversion of aniline could reach 36.2% in the presence of SImTf at 50 °C for only 15 min. However, the yield and selectivity of 4,4′-MDA were only 26.7% and 29.5%, respectively. This indicates that the synthesis of aminal from aniline and formaldehyde is a fast reaction, while the rearrangement of aminal to 4,4′-MDA is the rate-controlling step. So the conversion of aniline was determined mainly by the first step. The experimental results show that this step needs an acidic catalyst, but the acidity of catalyst is not important because there was little difference in the conversion of aniline over different SFILs. The yield and selectivity of 4,4′-MDA, however, decreased in the following order: [HSO3-bmim]CF3SO3 > [HSO3-bmim]p-TSA > [HSO3-bmim]C6H6SO3 > [HSO3-bmim]BF4 > [HSO3-bmim]CH3SO3 > [HSO3-bmim]CF3COO > [HSO3-bmim]CCl3COO. The difference in the catalytic performance of the SFILs might result from their different acidity. In order to explore the relationship between the catalytic performance of the SFILs and their acidity, the acid strength of the SFILs was measured. 3.2. Correlation of Catalytic Performance of SFILs with Their Acid Strength. The acid strength of the SFILs was measured by the Hammett method combined with UV−vis spectroscopy. The UV−vis absorption spectra of the indicators in the dichloromethane solution of the SFILs were shown in Figures 1 to 3. As shown in the figures, the stronger the acid

Figure 2. UV−vis absorption spectra of 4-(phenylazo)-diphenylamine in the dichloromethane solution of SFILs.

Figure 3. UV−vis absorption spectra of dimethyl yellow in the dichloromethane solution of SFILs.

peak could not be observed because it is too weak. In Figure 2, the alkali and acid absorption peak of 4-(phenylazo)diphenylamine is separately observed at 400 and 550 nm. In Figure 3, the alkali and acid absorption peak of dimethyl yellow is observed at 410 and 530 nm, respectively. The Hammett acidity functions (H0) of the SFILs were calculated according to the equation:

Figure 1. UV−vis absorption spectra of 4-nitroaniline in the dichloromethane solution of [HSO3-bmim]CF3SO3.

strength is of the SFILs, the weaker the alkali absorption peak is of the indicator and the stronger the acid absorption peak is of the indicator. The alkali absorption peak of 4-nitroaniline was observed at 350 nm in Figure 1, whereas its acid absorption

H 0 = pK a + log([I]/[HI])) 7573

DOI: 10.1021/acs.iecr.5b01519 Ind. Eng. Chem. Res. 2015, 54, 7571−7579

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Industrial & Engineering Chemistry Research where pKa is the dissociation constant of an indicator; [I] is the absorbance of alkali absorption peak; [HI] is the absorbance of acid absorption peak. The H0 of different SFILs was also listed in Table 1. Since the cation of the SFILs was identical, the acid strength of the SFILs varied with their anions. This means that the acid strength of the SFILs is not only related to the H proton in the sulfonic acid group of the cation but also is affected by the anion. According to the experimental results, the catalytic activity of the SFILs was consistent with their acid strength: the higher the acid strength, the better the catalytic activity. Among the SFILs, [HSO3-bmim]CF3SO3 (SImTf) with the highest acid strength showed the best catalytic activity. Therefore, SImTf was chosen as the catalyst for the further investigation. 3.3. Influence of Reaction Condition on the Condensation of Aniline with Formaldehyde. 3.3.1. Influence of Reaction Temperature. The influence of reaction temperature on the condensation reaction of aniline with formaldehyde was investigated, and the results are shown in Figure 4. As

Figure 5. Influence of molar ratio of aniline/formaldehyde on the condensation reaction of aniline with formaldehyde. Reaction conditions: mass ratio of SFILs/formaldehyde = 3.5, 80 °C, and 8 h. X, conversion of aniline; Y, yield of 4,4′-MDA; S, selectivity of 4,4′-MDA.

3.3.2. Influence of Molar Ratio of Aniline to Formaldehyde. Figure 5 showed the influence of molar ratio of aniline to formaldehyde on the condensation reaction of aniline with formaldehyde. The conversion of aniline decreased monotonously, while the yield and selectivity of 4,4′-MDA increased first and then declined with the increase in molar ratio of aniline to formaldehyde. When the molar ratio was 5, the yield and selectivity of 4,4′-MDA separately reached their maxima, 79.4% and 87.9%, respectively. When the molar ratio was less than 5, the concentration of formaldehyde was much higher, and some polynuclear compounds (such as tri-MDA and tetraMDA) were formed easily,2 leading to higher conversion of aniline but lower yield and selectivity of 4,4′-MDA. When the molar ratio was larger than 5, the concentration of formaldehyde was so low that the reaction proceeded very slowly, and the yield and selectivity of 4,4′-MDA decreased. Therefore, the suitable molar ratio of aniline to formaldehyde was 5. 3.3.3. Influence of SImTf Dosage. The influence of SImTf dosage on the condensation reaction of aniline with formaldehyde is shown in Figure 6. As shown in the figure, the conversion of aniline was almost unchanged, while the yield and selectivity of 4,4′-MDA increased first and then kept even with the increase in SImTf dosage. When a mass ratio of SImTf to formaldehyde = 3.5, the yield and selectivity of 4,4′-MDA separately reached their maxima, 79.4% and 87.9%, respectively. When the mass ratio was less than 3.5, the reaction may be restrained by an insufficient number of catalytic active centers. When the mass ratio was larger than 3.5, the number of catalytic centers was enough for the reaction, and the yield and selectivity of 4,4′-MDA kept even. Therefore, the suitable SImTf dosage was a mass ratio of SImTf to formaldehyde = 3.5. 3.3.4. Influence of Reaction Time. The influence of reaction time on the condensation reaction of aniline with formaldehyde is shown in Figure 7. The conversion of aniline was almost unchanged with an increase in reaction time due to the two-step reaction mechanism. The first step to aminal could be completed at room temperature in about 15 min. Since the conversion of aniline was determined by the first step, the prolonged reaction time could not affect the conversion of aniline. However, the yield and selectivity of 4,4′-MDA increased first and then

Figure 4. Influence of reaction temperature on the condensation reaction of aniline with formaldehyde. Reaction conditions: molar ratio of aniline/formaldehyde = 5, mass ratio of SFILs/formaldehyde = 3.5, and 8 h. X, conversion of aniline; Y, yield of 4,4′-MDA; S, selectivity of 4,4′-MDA.

shown in the figure, there is little effect of reaction temperature on the conversion of aniline. This may be explained from the reaction mechanism.2,20 The condensation reaction of aniline with formaldehyde proceeds in two steps: the reaction of aniline with formaldehyde to form aminal at room temperature and then the rearrangement of aminal to MDA at high temperature. The conversion of aniline was determined by the first step, so the change of reaction temperature cannot affect it. When the temperature was below 80 °C, the yield and selectivity of 4,4′-MDA increased gradually with the rise in temperature. At 80 °C, they separately reached their maxima, 79.4% and 87.9%, respectively. However, the yield and selectivity of 4,4′-MDA decreased above 80 °C. When the reaction temperature was below 80 °C, aminal could not be converted to MDA quickly, and hence, the yield and selectivity of 4,4′-MDA were lower. At temperature above 80 °C, 2,2′-MDA, 2,4′-MDA, and some polynuclear compounds were formed easily, decreasing the yield and selectivity of 4,4′-MDA. 7574

DOI: 10.1021/acs.iecr.5b01519 Ind. Eng. Chem. Res. 2015, 54, 7571−7579

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Industrial & Engineering Chemistry Research Table 2. Reuse of SImTfa no. of experiment 1 2 3 4

X (%) 36.3 33.2 33.8 35.5

(±0.61) (±0.64) (±0.23) (±0.30)

Y (%) 79.4 68.7 57.1 47.1

(±0.40) (±0.76) (±0.25) (±1.10)

S (%) 87.9 82.5 67.7 53.0

(±1.03) (±1.07) (±0.52) (±0.81)

a

Reaction conditions: molar ratio of aniline/formaldehyde = 5, mass ratio of SImTf/formaldehyde = 3.5, 80 °C, and 8 h. X, conversion of aniline; Y, yield of 4,4′-MDA; S, selectivity of 4,4′-MDA.

recovered SImTf declined. However, the selectivity of 4,4′-MDA declined with the number of reuse experiment. In order to find out the reason, the recovered SImTf was analyzed using HPLC, FT-IR, and UV−vis technique. The analysis of HPLC showed that there were identical concentrations of aniline and 4,4′-MDA remainiing in the recovered SImTf every time, which were 8.2% and 3.3%, respectively. Because the catalytic performance always declined with the number of reuse experiment, the aniline and 4,4′-MDA remaining in the recovered SImTf were not the factors responsible for the decline in catalytic performance. The result of the FT-IR analysis showed that there were slight differences in the spectra of the fresh and recovered SImTf in the range from 1200 to 1300 cm−1 (not given). What is more, the bands involved the stretching vibration absorption peak of SO bond. The acid strength of the recovered SImTf was measured by the Hammett method combined with UV−vis spectroscopy. It was found that the alkali absorption peak of dimethyl yellow indicator increased, suggesting that the acid strength declined greatly in the recovered SImTf. On the basis of the above analyses, we supposed that there might be a chemical interaction between alkali aniline and the acidic SImTf. Kang21 used sulfonic acid-functionalized ionic liquids supported on silica coated with [bmim]PF6 as a scavenger for the removal of alkali agents such as aniline by an acid−base interaction. It was observed that the efficiency of the silica-supported scavenger was significantly improved when coated with [bmim]PF6, probably due to a better compatibility of the sulfonic acid group with aniline. Inspired by the work of Kang, we hypothesized that there was a chemical interaction between alkali aniline and acidic SImTf in our work. In order to verify this hypothesis, the fresh SImTf was treated by aniline under the reaction conditions and then used to catalyze the reaction for the synthesis of 4,4′-MDA. The yield of 4,4′-MDA was 67.5%, while it was 68.7% using the recovered SImTf as catalyst, illustrating that the catalytic performance of as-treated SImTf was almost the same as the recovered SImTf. So it was certain that there must be a chemical interaction between alkali aniline and the acidic SImTf. In order to further explain the interaction between aniline and SImTf, the FT-IR spectrum of the SImTf treated by aniline under reaction conditions was compared with that of the recovered SImTf and with that of the SImTf physically treated by aniline, as shown in Figure 8. As compared with the spectrum of the fresh SImTf and the SImTf treated physically by aniline, the peak centered at 1226 cm−1 became smaller, and furthermore, a new peak centered at 1042 cm−1 appeared in the spectrum of the recovered SImTf and the SImTf treated by aniline under reaction conditions. Those peaks were the characteristic peaks of sulfonic acid group. There were no obvious changes in the characteristic peaks of sulfonic acid group of the fresh SImTf and the SImTf treated physically by aniline, demonstrating that the deactivation of the SImTf was

Figure 6. Influence of SImTf dosage on the condensation reaction of aniline with formaldehyde. Reaction conditions: molar ratio of aniline/ formaldehyde = 5, 80 °C, and 8 h. X, conversion of aniline; Y, yield of 4,4′-MDA; S, selectivity of 4,4′-MDA.

Figure 7. Influence of reaction time on the condensation reaction of aniline with formaldehyde. Reaction conditions: molar ratio of aniline/ formaldehyde = 5, mass ratio of SImTf/formaldehyde = 3.5, and 80 °C. X, conversion of aniline; Y, yield of 4,4′-MDA; S, selectivity of 4,4′-MDA.

declined with the prolonging of reaction time. The yield and selectivity of 4,4′-MDA reached their maxima, 79.4% and 87.9%, respectively, at a reaction time of 8 h. When the reaction time was less than 8 h, the conversion of aminal was affected by the shortage of reaction time, and the yield and selectivity of 4,4′-MDA were lower. When the reaction time surpassed 8 h, 4,4′-MDA may undergo a further condensation to some polynuclear compounds, decreasing the yield and selectivity of 4,4′-MDA. Therefore, the suitable reaction time was 8 h. 3.4. Reusability of SImTf. Since SImTf showed rather high catalytic activity, its reusability appeared much more important. After the completion of reaction, SImTf was recovered and reused in the reaction. The results are listed in Table 2. There was little influence of the reuse of SImTf on the conversion of aniline due to the two-step reaction mechanism. The conversion of aniline was determined by the first-step reaction and was influenced little by the acidity variation, so the conversion was kept unchanged even if the acidity of the 7575

DOI: 10.1021/acs.iecr.5b01519 Ind. Eng. Chem. Res. 2015, 54, 7571−7579

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Industrial & Engineering Chemistry Research

Figure 8. FT-IR spectra of the fresh and the recovered SImTf: (1) fresh, (2) treated physically by aniline, (3) treated by aniline under reaction conditions, and (4) recovered.

trifluoromethanesulfonic acid at 80 °C for 6 h, and then, the mixture was extracted by diethyl ether until the layer of diethyl ether was not acidic. After that, the mixture was dissolved in absolute ethanol, filtered, and dried in vacuum. Finally the acidized (i.e., regenerated) SImTf was attained. The catalytic activity of the acidized SImTf was evaluated, and the results are listed in Table 3. As shown in the table, the acidized SImTf could be reused four times with a tolerable loss of its catalytic activity, suggesting that the catalytic activity of the recovered SImTf was restored after acidized. The fresh, recovered, and acidized SImTf were analyzed using FT-IR, and the results are shown in Figure 9. The spectrum was basically the same, meaning that all of them were SImTf. However, there were some small differences in the three spectra. Compared with the fresh SImTf, the peak centered at 1226 cm−1 attributed to the antisymmetric stretching vibration of the SO bond became weaker and even disappeared, while the peak centered at 1031 cm−1 attributed to the symmetric stretching vibration of SO bond became wider in the recovered SImTf. Those changes demonstrated that the sulfonic acid group must have undergone some variations, leading to the decline in catalytic activity. The FT-IR spectrum of the acidized SImTf almost was consistent with that of the fresh, suggesting that its sulfonic acid group had been restored. However,

not due to a physical interaction between SImTf and aniline. Since the FT-IR spectrum of the recovered SImTf and the SImTf treated by aniline under reaction conditions was basically the same, it was clear that there must be a chemical interaction between aniline and the sulfonic acid group in the recovered SImTf. Since the deactivation of SImTf was caused by the chemical interaction of the sulfonic acid group with aniline, we tried to regenerate the deactivated SImTf according to the theory that stronger acids can prepare weaker acids. Trifluoromethanesulfonic acid was used to treat the recovered SImTf to avoid introducing other kind of anions. First the recovered SImTf was treated with Table 3. Reuse of Recovered SImTf after Acidizeda no. of experiment 1 2 3 4 5

X (%) 36.3 32.4 33.6 32.8 33.3

(±0.61) (±0.38) (±0.23) (±0.50) (±0.40)

Y (%) 79.4 72.7 71.3 71.2 72.2

(±0.40) (±0.45) (±1.35) (±0.30) (±0.85)

S (%) 87.9 89.8 84.8 85.4 87.2

(±1.03) (±0.60) (±1.12) (±0.97) (±1.07)

a

Reaction conditions: molar ratio of aniline/formaldehyde = 5, mass ratio of SImTf/formaldehyde = 3.5, 80 °C, and 8 h. X, conversion of aniline; Y, yield of 4,4′-MDA; S, selectivity of 4,4′-MDA.

Figure 9. FT-IR spectra of the fresh, recovered, and acidized SImTf: (1) fresh, (2) recovered, and (3) acidized. 7576

DOI: 10.1021/acs.iecr.5b01519 Ind. Eng. Chem. Res. 2015, 54, 7571−7579

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Industrial & Engineering Chemistry Research Scheme 3. Plausible Reaction Mechanism for Synthesis of MDA from Aniline and Formaldehyde

compared with the fresh, there were two additional peaks centered at 1499 and 695 cm−1 in the acidized SImTf. The two small peaks were attributed to the vibration of benzene ring and outer plane bending vibration of C−H in a monosubstituted benzene ring. This showed that there was some aniline remaining, resulting in the difference in the catalytic performance of the acidized and the fresh SImTf. 3.5. Identification of Condensation Reaction Components. The reaction components in the condensation reaction catalyzed by SImTf were identified using HPLC-MS. The reaction mixture was extracted and distillated in vacuum prior to analyses. The ESI-MS1 spectrum could tell us the relative molecular weight of the components, while the ESI-MS2 spectrum could give the fragment ion peaks of the components. (1) In the negative ion mode, there was only one peak at RT = 3.19 min (marked substance B) in the ion total flow diagram. In the ESI-MS1 spectra of substance B, there was a main peak with a molecular weight of 149.3. The peaks of the ESI-MS1 spectra always were attributed to the substance after abscission of H or after a combination of a formic acid molecule and then abscission of H. If the peak with a molecular weight of 149.3 was the substance after abscission of H, the relative

molecular weight of substance B should be 150.3. If it was the substance after a combination of a formic acid molecule and then abscission of H, the relative molecular weight of substance B should be 104.3. Thus, it was inferred that substance B was phenylmethanimine, which was formed by the Schiff base reaction of aniline with formaldehyde. Liang22 described the preparation of aromatic methylene amine from aniline and formaldehyde and Sprung23 isolated phenylmethanimine and measured its melting point, which gave supports for our speculation of substance B. (2) In the positive ion mode, the structure of the substance (marked as C) with a relative molecular weight of 303.4 at RT = 1.65 min was determined as follows:

Three peaks appeared at RT = 1.24, 1.56, and 2.45 min, which were attributed to the substances with a molecular weight ranging from 198.9 to 199.9. Their ESI-MS2 spectra were the same, and only one fragment ion peak with a molecular weight of 106.2 (methylene aniline) appeared. Consequently, the three substances were speculated as MDA isomers. 7577

DOI: 10.1021/acs.iecr.5b01519 Ind. Eng. Chem. Res. 2015, 54, 7571−7579

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Industrial & Engineering Chemistry Research 3.6. Plausible Catalytic Reaction Mechanism. On the basis of the identification of the reaction components and some relative literature,3,5,24,25 a plausible catalytic reaction mechanism was proposed and is shown in Scheme 3. A formaldehyde molecule was protonated in the presence of acid. Thus, the carbonyl carbon atom of the protonated formaldehyde showed more positive charges and was easily attacked by aniline, which served as a nucleophilic reagent. Because the nitrogen atom of aniline molecule had one lone-pair of electrons, aniline donated an electron pair to the carbonyl carbon atom of the protonated formaldehyde to form substance A and released a hydrogen proton simultaneously. Substance A can undergo a succeeding reaction in two ways: an intramolecular dehydration to substance B or an intermolecular dehydration with a molecule of aniline to aminal. In the presence of acid sites, aminal was protonated to form a carbenium ion (D) through the breakage of C−N and released an aniline molecule simultaneously. Species D with strong electrophillicity was prone to an attack on the para or ortho carbon atom of an aniline molecule to form para or ortho aminobenzylaniline (PABA or OABA). PABA or OABA was protonated in the presence of acid to form para or ortho amino benzyl carbonium ion (E or F) and released an aniline molecule simultaneously. Then, species E or F attacked the para or ortho carbon atom of an aniline molecule to form 4,4′-MDA, 2,4′-MDA, or 2,2′-MDA. The attack of species E on the ortho carbon atom of 4,4′-MDA leads to the formation of the substance C, a trimer. As shown from the entire catalytic process, the catalytic active sites of SImTf are mainly from the hydrogen proton in the sulfonic acid group of the cation. On the basis of the result of the acid strength measurement, the acid strength of SFILs varies with anions when the cation was identical. Xing et al.26 measured the minimum-energy geometries of some SFILs and manifested that anions have strong interactions with the sulfonic acid proton through hydrogen band. Due to these strong interactions, the H−O bond of the sulfonic acid group was lengthened to a different degree, and hence, it varied with the anions. Jiang et al.27 analyzed the formation principle of the hydrogen bonds. It was considered that the hydrogen bond was formed by sharing a lone-electron pair of O or F atoms with an H atom. If the lone-electron pair was shared in a greater degree, the hydrogen bond became stronger and H atom was more stable and the acidity was weaker. Hence the catalytic activities of SFILs varied with the kinds of anions. Therefore, the influence of anion on the acidity and then the catalytic performance of the SFILs was achieved through the action to the H proton of the sulfonic acid group. This is the reason why the action of the anions did not appear in the mechanism.

reaction time of 8 h. Under the above conditions, the conversion of aniline was 36.3%, and the yield and selectivity of 4,4′-MDA were 79.4% and 87.9%, respectively. (3) FT-IR analyses showed that there was a chemical interaction between aniline and the sulfonic acid group of SImTf, which caused the deactivation of SImTf. The acidization could regenerate the deactivated SImTf; it could be reused four times with a tolerable loss of its catalytic activity. (4) The reaction components were identified by using HPLCMS, and a plausible catalytic reaction mechanism for the synthesis of 4,4′-MDA 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 National Natural Science Foundation of China (Grant No. 21236001, 21476058) and Key Basic Research Project of Applied Basic Research Plan of Hebei Province (Grant No. 12965642D). The authors are gratefully appreciative of their contributions.

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REFERENCES

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4. CONCLUSIONS (1) A series of SO3H-functionalized ionic liquids (SFILs) were synthesized, and their catalytic performance was evaluated in the condensation reaction of aniline and formaldehyde to 4,4′-methylenedianiline (4,4′-MDA). The result showed that the acid strength of SFILs was consistent with their catalytic activity. Among the SFILs, SImTf with the highest acid strength showed the best catalytic activity. (2) The influence of reaction conditions on the synthesis of 4,4′-MDA was investigated using SImTf as the catalyst. Suitable reaction conditions were as follows: molar ratio of aniline/formaldehyde = 5, mass ratio of SImTf/ formaldehyde = 3.5, reaction temperature of 80 °C, and 7578

DOI: 10.1021/acs.iecr.5b01519 Ind. Eng. Chem. Res. 2015, 54, 7571−7579

Article

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DOI: 10.1021/acs.iecr.5b01519 Ind. Eng. Chem. Res. 2015, 54, 7571−7579