Epoxidation of Soybean Oil by Continuous Micro-Flow System with

Aug 19, 2013 - ABSTRACT: In the study, the epoxidation process of soybean oil by microflow system (MFS) was investigated. With formic acid as oxygen ...
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Epoxidation of Soybean Oil by Continuous Micro-Flow System with Continuous Separation Wei He,† Zheng Fang,† Dong Ji,‡ Ketao Chen,‡ Zhidong Wan,† Xin Li,‡ Haifeng Gan,‡ Shigui Tang,‡ Kai Zhang,‡ and Kai Guo*,‡ †

School of Pharmaceutical and ‡College of Biotechnology and Pharmaceutical Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, P.R. China S Supporting Information *

ABSTRACT: In the study, the epoxidation process of soybean oil by microflow system (MFS) was investigated. With formic acid as oxygen carrier and EDTA-2Na as stabilizer, the optimal result (epoxidized soybean oil (ESO) with an epoxy number (EN) of 7.3) was obtained in such conditions: temperature 75 °C, H2SO4 concentration 3%, EDTA-2Na dosage 3%, residence time 6.7 min, ratio of formic acid to hydrogen peroxide 1:1, and H2O2 to double bond molar ratio 8:1.

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INTRODUCTION Process intensification (PI) based on microdevices is a new concept, which has become one of the most interesting research topics in process engineering.1 By reducing the size of the chemical plant, investment and energy costs can be cut, accompanying safety and environmental friendliness issues. Microprocess technology, following the PI concept, is a preferred concept of MFS that integrates microstructured reactors and separation units into custom continuous process design.2 One of the most significant features of MFS is the extremely fast mixing by virtue of short diffusion path. Meanwhile, heat transfer is normally much more efficient in MFS than in conventional instruments due to the high surfaceto-volume ratio. These features are quite advantageous for conducting extremely fast and highly exothermic reactions. Easy number-up of microsystems for increasing the scale of production is also beneficial in terms of industrial production.3 As a plasticizer, ESO is a good substitute for phthalates which are banned by the EU due to toxicity.4,5 Meanwhile, ESO is also of high commercial importance as a stabilizer for plastics, ingredient of lubricants, starting material for polyol in polyurethanes industry, and intermediate for synthesis of surfactants.6 Typically in industry, epoxidation of soybean oil is performed by reacting double bonds in the oil with a peracid (peracetic or performic acid). The process can be divided into two stages (Figure 1). First, the peracid is generated in situ by treating formic or acetic acid with hydrogen peroxide. Subsequently, peracid migrates into the oil phase and reacts with the double bonds to yield epoxy rings.

However, there are several limitations in this typical procedure. First of all, a series of side reactions, especially ring-opening reactions (Figure 2), cannot be avoided in the

Figure 2. Possible oxirane ring-opening reactions.

process, since the reaction mixture is very acidic. Therefore, EN, which is the most significant parameter for quality of ESO, will be reduced. The other drawback of this reaction is that the reaction itself is extremely exothermic (ΔH = −55 kcal/mol for each double bond),7 which may cause safe risk, especially with peracid and hydrogen peroxide in the system. Thus, strict control of reaction temperature and operation is necessary.8 In industry, addition of oxidant agent is normally performed in fed-batch mode. Vigorous stirring is also demanded to diffuse the oxidant agent well, since the reaction mixture is a two-phase immiscible liquids. During the past decade, studies of the epoxidation of soybean oil in continuous mode have been reported by several groups.9−13 Volker Hessel et al.14 described a kinetics model concerning the effect of temperature, heat removal efficiency, Received: February 26, 2013

Figure 1. Process of ESO Production. © XXXX American Chemical Society

A

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might be able to improve the reaction with formic acid as oxygen carrier. Mechanistically, more oxygen carrier, which leads to higher concentration of peracid, can accelerate the reaction, although it also brings disadvantages in terms of safety issues. Generally in batch experiment, the ratio of hydrogen peroxide to acid was about 1:0.2.17 In this study, it was found that conversion of ESO could be enhanced with more oxygen carriers in the reaction mixture (Figure 5). The highest EN was obtained in

hotspot formation, and decomposition of hydrogen peroxide. However, it has still demanded more effort to improve the experimental results to make the application of MFS in preparation of ESO more practical. According to reported results,7 the conversion of ESO in MFS (with EN around 0.5− 3.0) was much less effective than in batch facility (with EN around 4.5−6.0). Another phenomenon in preparation of ESO by MFS was that the separation of the biphasic mixture was still performed on a rhythmic model, which makes the whole process complicated. This is also a common problem for most application cases of MFS. Although MFS can simplify the reaction process dramatically, purification is still a big challenge for flow chemistry. Recently, investigation about integration of separation or purification devices in MFS has been launched. A continuous extraction was employed by Bryan Li et al.15 in the syntheses of N-aryl pyrazoles with MFS, avoiding the flash column in the purification process. In this study, we reported our recent progress producing high quality ESO (with an EN of 7.3, an iodine number of 1.2) by a faster process in MFS with a continuous gravity difference miniature separator to make the process more convenient.

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RESULTS AND DISCUSSION As described above, preparation of ESO includes two stages: generation of peracid from carboxylic acid and H2O2 with catalyst; epoxidation of double bonds in oil. The carboxylic acid employed in formation of peracid can be viewed as an oxygen carrier, which plays a significant role in the whole process. Therefore, discovery of an efficient acid can improve the reaction dramatically. Catalysts, which can accelerate the formation of peracid, stabilizers, reaction temperature, residence time, and ratio of starting materials were also investigated to optimize the process in this study. 1. Studies of Oxygen Carrier. Typically, acetic acid and formic acid were chosen as oxygen carriers in epoxidation. Generally, formic acid is more reactive, since performic acid forms faster than peracetic acid.16 However, acetic acid is the common oxygen carrier in preparation of ESO in batch experiments, mainly due to safety issues. In this study, both acids were employed in MFS. The results are displayed in Table 1.

Figure 3. Process flow diagram for ESO production.

Figure 4. Experimental apparatus for ESO production.

Table 1. Comparison of Oxygen Carriersa residence time (min) EN

oxygen carrier

3

4

5

Acetic acid Formic acid no acid

0.01 6.29 0.12

0.27 5.78 0.15

1.11 5.12 0.12

a

Reaction conditions: H2O2 to HCOOH molar ratio = 1; H2O2 to double bond molar ratio = 1; temperature 110 °C; H2SO4 = 5%.

In all cases, formic acid showed a much better result than acetic acid (Table 1), which strongly suggested that formic acid was a more efficient oxygen carrier for this reaction in MFS. Interestingly, the two acids gave opposing tendencies of EN shift with increasing residence time. Acetic acid yielded better EN with longer residence time, while less ESO was detected with formic acid with extending residence time. This is probably because performic acid is less stable than peracetic acid in the reaction mixture, which indicates that a stabilizer

Figure 5. Effect of H2O2 to HCOOH molar ratio on the yield of ESO. Conditions: temperature 110 °C; H2SO4 = 5%; EDTA-2Na = 2%; residence time = 3 min. B

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the reaction with equal amount of formic acid and hydrogen peroxide. Interestingly, the reaction was inhibited with more formic acid than hydrogen peroxide used, which was probably because of acidity brought by formic acid leading to more ringopening side reactions as displayed in Figure 2. Therefore, equal amount of formic acid to hydrogen peroxide was proven to be the optimal oxygen carrier in this reaction. 2. Studies of Stabilizer. Stabilizer, which can make the peracid as well as hydrogen peroxide and ESO stable, showed good capability to improve the reaction in batch according to our previous studies. In fact, the decomposition rate might be increased in MFS, partly due to the very large internal reactor wall surface, and partly because of the presence of leached metal from the reactor walls in the solution.14 Meanwhile, heavy metal (Mn, Pb) and mineral salt (MgCl2, CaCl2) in the soybean oil can also promote oxygenolysis during the process. Generally, ethylene diamine tetraacetic acid disodium salt (EDTA-2Na) and urea were employed as stabilizers in epoxidation. As can be seen in Figure 6, the reaction was improved dramatically in the present of stabilizer. Generally, EDTA-2Na

Figure 7. Effect of H2SO4 content on the on the yield of ESO. Conditions: temperature 75 °C; HCOOH (H2O2) to double bond = 8; EDTA-2Na = 3%; residence time = 6.7 min.

because of a series of ring-opening reactions (Figure 2) can be amplified with higher concentration of sulfuric acid.9 3.2. Reaction Temperature. As discussed above, formation of ESO is an extremely exothermic reaction, which can bring risk of explosion in traditional batch facility. Meanwhile, oxygenolysis of peracid and hydrogen peroxide can be enhanced with increasing temperature, which eventually inhibits the reaction. Therefore, it is important to find suitable temperature for this reaction. Based on results displayed in Figure 8, the optimal temperature was obtained as 75 °C in MFS. Although more

Figure 6. Effect of EDTA-2Na dosage on the on the yield of ESO. Conditions: temperature 75 °C; HCOOH(H2O2) to double bond = 8; H2SO4 = 3%; residence time = 6.7 min.

showed much better result than urea. An EN of 7.2 of the product was obtained with 3% of stabilizer in the reaction mixture, while the EN was only 2.4 without addition of EDTA2Na. However, overdosage of stabilizer (more than 3%) was unable to increase formation of ESO. Constant EN was observed with more than 3% of EDTA-2Na in the system, which suggested that 3% of EDTA-2Na was the optimal condition for stabilizer in this system. 3. Optimization of Reaction Conditions. To optimize the reaction conditions, catalyst, ratios of starting materials, reaction temperature, residence time, and flow rate were studied. 3.1. Catalyst. Catalysts, which can accelerate the formation of peracid, also lead to various side reactions or decomposition of the product. According to previous research, catalyst is the most widely investigated factor in preparation of ESO. A variety of acids, both homogeneous10 and heterogeneous18−20 ones, were reported. Considering the limitation of MFS (solids need be avoided in most cases) and commercial reason, sulfuric acid was used in this work. The best EN obtained in this set of reactions was 7.2 with 3% of sulfuric acid as catalyst, although ESO was formed even without catalyst (Figure 7). As expected, excess catalyst (>3%) was found to play a negative role in this reaction. This is mainly

Figure 8. Effect of temperature on the on the yield of ESO. Conditions: HCOOH (H2O2) to double bond = 1; EDTA-2Na = 3%; residence time = 6.7 min; H2SO4 = 3%.

ESO was generated with temperature increasing from 60 to 75 °C, EN of the product was found to be reduced from 6.1 to 3.8 with the temperature raised from 75 to 105 °C (Figure 8). This, again, demonstrates that heating not only improves the epoxidation, but also increases the rate of oxirane ring-opening reactions, which is similar to the results obtained in batch experiments according to our previous research. 3.3. Residence Time and Flow Rate. The volume of microreactor used in this study is 37 mL. By adjusting the flow rate, residence time in the microreactor was altered from 2 to 11 min. In the first, EN increased as residence time increased. The maximum EN was obtained when residence time was adjusted to 6.7 min. With longer residence time, EN was decreased (Figure 9). This phenomenon is very common in MFS reactions, because a longer residence time corresponds to a smaller average velocity for fixed-length microreactors, which in turn weakens the mass transfer,21 and thus lower ENs are obtained. In consequence, with the view of maximum EN, the most advantageous residence time appeared to be 6.7 min.22 C

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Figure 11. Oil−water separator in the process.

Figure 9. Effect of residence time on the on the yield of ESO. Conditions: temperature 75 °C; HCOOH (H2O2) to double bond = 8; EDTA-2Na = 3%; H2SO4 = 3%.

high level by adding a small quantity of H2O2 in the acid solution.

3.4. Effect of H2O2-to-Double Bond Molar Ratio. The effect of H2O2-to-double bond molar ratio on the yield of ESO was studied in the ratio range 0.8−16. All the results are displayed in Figure 10. The best result obtained (with an EN of 7.3) at

Table 2. EN of Every Recovery Process recovery time

1

2

3

4

5

EN

7.07

6.93

7.03

7.1

6.89

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Figure 10. Effect of H2O2-to-double bond molar ratio on the yield of ESO. Conditions: temperature 75 °C; H2SO4 = 3%; EDTA-2Na = 3%; residence time = 6.7 min.

CONCLUSIONS In conclusion, the replacement of batch process by MFS in preparation of ESO resulted in notable progress in terms of both process intensification and product quality. Compared with batch reaction, MFS could cut the reaction time from several hours to 6.7 min, which brought significant benefit for energy consumption. The best EN obtained in MFS was 7.3 in this study, which was more than 20% higher compared to the batch experiments (with an EN of 6.0). Online separation of the reaction mixture was also realized and the obtained aqueous phase was recycled successfully, which indicated that this integrated MFS had good potential for further industry application.

the ratio of 8:1 was close to the theoretical value (7.58). Generally, better conversion was detected with increasing H2O2. However, the EN was found to be decreased to 6.4 with 16× H2O2 used, which is probably because the stability of the epoxy ring was very poor at such concentrated H2O2.10 Generally, ESO with an EN of 6.0 has satisfied the demands of the market. However, the optimal H2O2-to-double bond molar ratio was set to 8:1, aimed at obtaining higher EN in this study.23 4. Continuous Separation. As a typical two-phase reaction, the purification is a challenge for the preparation of ESO. Generally, purification of ESO was performed by a batch mode, which made for higher cost and complicates the postprocessing. In this study, an oil−water separator (Figure 11) was employed in the purification process which had been reported by Bryan Li et al.15 With this device, the purified ESO was obtained continuously and analyzed immediately. According to the results shown in Figure 10, the consumption of H2O2 was highly excessive. With regard to industrial application, we have tried the recovery process for the acid solution. By using the recyclable acid solution obtained from the oil−water separator, EN was maintained at a relatively

EXPERIMENTAL SECTION General Procedure. The instrument (Figure 3) used in this study was a Bayer sandwich reactor (Figure 4) assembled with a micromixer, a sandwich microreactor and a tailor-made oil−water separator. EN was analyzed by acetone-hydrochloride method or standards.24 Two flows were pumped into the micromixer at a set flow rate. One of the flows was pure soybean oil, while the other was a solution including hydrogen peroxide (30 wt %), formic acid (98 wt %), catalyst, and stabilizer. Temperature inside the microreactor was adjusted to 75 °C by external heating cycle.22 After reaction, the effluent went into the continuous oil−water separator. The obtained oil layer was washed with sodium carbonate solution (5%, aqueous) and distilled water. The washed organic layer was dried with anhydrous magnesium sulfate. Afterwards the obtained organic layer was concentrated under vacuum. Typical Reaction. A run with an EN of 7.2 in Figure 6, stock solution A: 30 wt % H2O2 (4.72g), 98 wt % formic acid (1.95g), 98 wt % H2SO4 (0.031g), EDTA-2Na (0.036g); stock solution B: soybean oil. Stock solution A was prepared in advance. In the instrument used in this study, the volume of microreactor is 37 mL. The volumetric flow rate of soybean oil was 0.8 mL/min, while, the volumetric flow rate of was 4.7 mL/ min. In other words, the residence time in the microreactor was

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(6) Michert, E.; Smagowica, A.; Lewandowski, G. Org. Process Res. Dev. 2010, 14, 1094. (7) Santacesaria, E.; Renken, A.; Russo, V.; Turco, R.; Tesser, R.; Serio, D. M. Ind. Eng. Chem. Res. 2012, 51 (26), 8760. (8) Czub, P.; Boncza-Tomaszewski, Z.; Penczek, P.; Pielichowski, J. Chemia technologia zywic epoksydowych; WNT: Warszawa, 2002. (9) Goud, V. V.; Patwhardan, A. V.; Dinda, S.; Pradhan, N. C. Chem. Eng. Sci. 2007, 62, 4065. (10) Dinda, S.; Patwardhan, A. V.; Goud, V. V.; Pradhan, N. C. Bioresource Technol. 2008, 99, 3737. (11) Gan, L. H.; Goh, S. H.; Ooi, K. S. J. Am. Oil Chem. Soc. 1992, 69, 347. (12) Campanella, A.; Fontanini, C.; Baltanas, M. A. Chem. Eng. J. 2008, 144, 466. (13) Sinadinovic-Fiser, S.; Jankovic, M.; Petrovic, Z. S. J. Am. Oil Chem. Soc. 2001, 78, 725. (14) Cortese, B.; Croon, M. H. J. M.; Hessel, V. Ind. Eng. Chem. Res. 2012, 51, 1680. (15) Li, B.; Widlicka, D.; Boucher, S.; Hayward, C.; Lucas, J.; Murray, J. C.; Samp, L.; VanAlsten, J.; Xiang, Y. Q.; Young, J. Org. Process Res. Dev. 2012, 16 (12), 2031. (16) Ebrahimi, F.; Kolehmainen, E.; Oinas, P. Chem. Eng. J. 2011, 167, 713. (17) Santacesaria, E.; Tesser, R.; Serio, M. D.; Turco, R.; Russo, V.; Verde, D. Chem. Eng. J. 2011, 173, 198. (18) Cozzolino, M.; Serio, M. D.; Tesser, R.; Santacesaria, E. Appl. Catal. A: Gen. 2007, 325, 256. (19) Rios, L. A.; Weckes, P.; Schuster, H.; Hoelderich, W. F. J. Catal. 2005, 232, 19. (20) Tatsumi, T.; Koyano, K. A. Chem.Commun. 1996, 145. (21) Dummann, G.; Quittmenn, U.; Groschel, L.; Agar, D. W.; Worz, O.; Morgenschweis, K. Catal. Today 2003, 433, 79. (22) To complete such an exothermic reaction in this short time, it is a big challenge to keep the system being isothermal. The microreactor has a very high surface-to-volume ratio, which offers the reactor very efficient heat transfer. This is a great benefit of MFS. In this study, heating oil was pumped into the Sandwich reactor to heat the reaction mixture. Oil can play a role as both heating and cooling medium. Set to a certain temperature (e.g., 75 °C), it can either heat the cool reaction mixture or cool the over heat reaction mixture. (23) Academically, we used H2O2/double bond = 8 as optimal ratio, just because the best EN was obtained at this ratio. But as can be seen in Figure 10, H2O2/double bond = 1 could yield an EN of 6.2, which was equal to high quality product in industry. Actually, we are using 1:2 ratio in our 500 ton/a pilot-flow system (which is not reported in this manuscript), considering cost and safty issues. (24) Standard EN-ISO 3001 ; 1999,.

6.7 min. Temperature inside the microreactor was adjusted to 75 °C by external heating cycle.22 After reaction, the effluent went into the continuous oil−water separator. The obtained oil layer was washed with sodium carbonate solution (5%, aqueous) and distilled water. The washed organic layer was dried with anhydrous magnesium sulfate. Afterwards the obtained organic layer was concentrated under vacuum. Analytical Method for EN. EN was calculated in accordance with the required standards, according to the following equation: EN =

⎡ ⎣V − V1 −

(

V2 G

⎤ × W ⎦N × 0.016

)

W

× 100%

where

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V is the volume of sodium hydroxide used to titrate blank sample. [mL] V1 is the total volume of sodium hydroxide used to titrate right sample. [mL] V2 is the volume of sodium hydroxide used to titrate right sample for acid value. [mL] N is the concentration of sodium hydroxide solution. [mol/L] W is the mass of right sample. [g] G is the mass of right sample for determining acid value. [g]

ASSOCIATED CONTENT

S Supporting Information *

DSC spectrum of the ESO and reaction mixture. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

(K.G.) Puzhu South Road No. 30 Nanjing, Nanjing, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support by the National Key Basic Research Program of China (973 Program) 2012CB725204 and 2011CB710803; Program for Changjiang Scholars and Innovative Research Team in University IRT1066.

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REFERENCES

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