Preparation of Methyl Ester Sulfonates Based on Sulfonation in a

Article pubs.acs.org/IECR

Preparation of Methyl Ester Sulfonates Based on Sulfonation in a Falling Film Microreactor from Hydrogenated Palm Oil Methyl Esters with Gaseous SO3 Tianming Xie,† Changfeng Zeng,‡ Chongqing Wang,† and Lixiong Zhang*,† †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, and ‡College of Mechanic and Power Engineering, Nanjing University of Technology, Nanjing 210009, P. R. China ABSTRACT: Fatty acid methyl ester sulfonate (MES) is an environmentally friendly anionic surfactant. It contains RCH(CO2Me)SO3Na (α-MES) as the active component and RCH(CO2Na)SO3Na (the disodium salt) whose content must be lower than 5%. In this paper, MES was prepared first in a falling film microreactor (FFMR) by sulfonation of hydrogenated palm oil methyl ester (ME) with SO3 diluted with N2, followed by aging in a tubular reactor and subsequent bleaching and neutralization with NaOH. The influences of the ME flow rate, the channel size, the N2 flow rate, the SO3-to-ME molar ratio, and the sulfonation temperature in the sulfonation step, as well as the influences of the SO3-to-ME molar ratio, the aging residence time, and the aging temperature in the aging step, were all examined. The results indicated that the ME flow rate had no evident influence on the ME conversion and the α-MES concentration both without overflow and with overflow. In the sulfonation step, the disodium salt (disalt) concentration increased with increase in the ME flow rate, the channel size, the SO3-to-ME molar ratio, and the sulfonation temperature or decrease in the N2 flow rate. In the aging step, the ME conversion, the α-MES concentration, and the disalt concentration increased with increase in the SO3-to-ME molar ratio, the aging residence time, and the aging temperature. The results of 1H NMR spectra indicated that RCH(CO2Me)SO3H was mainly formed in the sulfonation step in the microreactor, while it was formed in the aging step when the sulfonation was carried out in the conventional falling film reactor. Accordingly, a sulfonation reaction mechanism in the FFMR different from that in the conventional FFR was proposed. The final products with an α-MES concentration of 86.3% and a disalt concentration of 1.2% were obtained under the conditions of a ME flow rate of 2 mL/min, a channel size of 300 × 100 μm, a SO3-to-ME molar ratio of 1.2, a N2 flow rate of 900 mL/min, a sulfonation temperature of 85 °C, an aging temperature of 90 °C, and an aging residence time of 19.7 min. as demonstrated by Weil et al.10−12 The other is direct sulfonation of fatty acid methyl ester with SO3. Initially, direct sulfonation of ME with SO3 was carried out by adding liquid SO3 dropwise into a stirring batch reactor containing ME at 0 °C, followed by 60 °C for a certain time.13 Later on, a continuous sulfonation process was developed by passing ME and gaseous SO3 diluted to 5 vol % in air at different SO3-toME molar ratios through four cascade vessels with the temperatures of 50, 60, 70, and 80−85 °C successively and reacting in the fifth vessel for 30−60 min at 80−90 °C.14,15 Obviously, the above reactors are not as efficient for sulfonation reaction as it is a strong exothermic reaction and the operation is quite complex. Thus, the falling film reactors (FFRs), widely used for gas−liquid absorption and reaction processes such as sulfonation, chlorination, ethoxylation, and photocatalysis,16−20 were consequently employed in this reaction for high capacity of mass and heat transfer and low pressure drop in the reactor. Various raw materials, such as methyl stearate21 and ME derived from coconut, palm kernel, palm stearin, tallow, and soya,22 were used. The sulfonations were usually conducted with a SO3-to-ME molar ratio of 1.2−1.3 and diluted SO3 by air (about 7 vol %) at 50−100 °C. Further aging of the sulfonated

1. INTRODUCTION Today, most of the main active matters added into commodities such as detergent, soap, shampoo, and facial cleanser come from petroleum-derived products, such as linear alkylbenzene sulfonate (LAS), alpha olefin sulfonate (AOS), primary alcohol sulfate (PAS), alcohol ethoxy sulfate (AES), etc. These products may soon face the problem of shortage of the raw materials as the increasing consumption and decreasing natural reserves of petroleum. Methyl ester sulfonate (MES), an anionic surfactant prepared from natural renewable resources, such as vegetable oil, is considered to be the substitute for the petroleum-derived counterparts.1−8 Furthermore, they exhibit many advantages of being nontoxic, being a low excitant to humans, and demonstrating more excellent performance of environmental compatibility, emulsification, and foamability. Thus, MES has gained more and more attention. MES is prepared by neutralization of fatty acid methyl ester sulfonic acid (MESA) with NaOH, producing two kinds of active matters of RCH(CO2Me)SO3Na (α-MES) and RCH(CO2Na)SO3Na (disalt) in the MES product. Since the Krafft point of the disalt is much higher than that of α-MES and it is more sensitive to hardness, its concentration in MES must be lower than 5%.9 Otherwise, the solubility of MES in cold and hard water may be affected, and there would be a severe negative impact on its application in detergent preparation. There are mainly two preparation methods for MESA. One is sulfonation of fatty acid with SO3 and subsequent esterification, © 2013 American Chemical Society

Received: Revised: Accepted: Published: 3714

October 22, 2012 January 29, 2013 February 20, 2013 February 20, 2013 dx.doi.org/10.1021/ie3028763 | Ind. Eng. Chem. Res. 2013, 52, 3714−3722

Industrial & Engineering Chemistry Research

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Scheme 1. Proposed Reaction Mechanism of Sulfonation of ME with SO3.

Table 1. Composition of Hydrogenated Palm Oil Methyl Ester

ME at 85−120 °C for a certain time was necessary to improve the ME conversion. Under optimal reaction conditions, MES with an α-MES concentration of 83% and a disalt concentration of 3.5% could be obtained after bleaching and neutralization of the resultant MESA in a pilot plant.22 Although this is the best result reported so far, the α-MES concentration is expected to be further increased with decrease in the disalt concentration. Recently, the falling film microreactor (FFMR), designed by the Institut fur Mikrotechnik Mainz, has already been demonstrated to be an efficient reactor for the highly exothermic reactions such as direct fluorination of toluene and aromatics,23,24 chloration of alkylaromatics,25 sulfonation of toluene,26 and ozonolysis of acetic acid 1-vinylhexyl ester and 1decene.27,28 The conversions were enhanced and the side reactions were effectively inhibited compared to the results in the conventional tank reactors for ozonolysis reactions.27,28 It thus should be a good reactor for sulfonation of ME with SO3. In this paper, we conducted sulfonation of hydrogenated palm oil fatty acid methyl ester with SO3 in the FFMR without and with overflow and compared the results with those obtained in

the conventional FFR. Since aging is an important step after reaction in FFR, we also examined the effect of aging conditions in a tubular reactor with an inner diatmer of 3 mm. Microchannels were not used for aging because of high viscosity of the resultant sulfonated ME. We thus elucidated the reaction process in the FFMR, based on the widely accepted reaction mechanism of sulfonation of ME with SO313,21,22,29−31 shown in the Scheme 1. Briefly, the ME (I) reacts fast with SO3 adding to the carbonyl oxygen to form intermediate II, which is then quickly changed to intermediate III with a SO3-to-ME stoichiometry of 2:1. The intermediate III undergoes a rearrangement to release SO3 and the desired MESA (IV) is slowly formed, and this reaction is reversible. The intermediate III and MESA (IV) would react to disalt VI and α-MES (V) during neutralization, respectively. It is believed that intermediate III is mainly formed in the sufonation step in the conventional FFR and MESA is the main product during aging.29−31 However, results obtained in FFMR demonstrate that MESA is the main product in the sulfonation step. Furthermore, disalt is also considered to be formed primarily 3715

dx.doi.org/10.1021/ie3028763 | Ind. Eng. Chem. Res. 2013, 52, 3714−3722


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Figure 1. Flowchart of sulfonation and aging steps: 1, SO3 cylinder; 2, oven, 68 °C; 3 and 5, globe valves; 4, pressure gauge; 6, needle regulator; 7, SO3 glass rotameter; 8, N2 cylinder; 9, computer; 10, N2 flow controller; 11, mixer; 12, hydrogenated palm methyl ester; 13, thermostat, 90 °C; 14, gear pump; 15, FFMR; 16 and 23, thermostats; 17 and 18, thermocouples; 19, three-way valve; 20 and 24, erlenmeyer flask; 21, gas washing bottle; 22, tubular reactor. All the red lines were heated at a temperature of about 90 °C.

gas was fed from a N2 cylinder 8 by a mass flow controller 10 with a computer 9. The SO3 gas and N2 gas were mixed in a mixer 11 and then flowed into the FFMR 15. The temperature of the FFMR was controlled by the recycle loop of a thermostat 16 and measured by thermocouples 17 and 18. The sulfonated product could be collected by using a three-way valve 19 in the erlenmeyer flask 20 after gas−liquid separation. The residuary SO3 in the offgas was absorbed by the deionized water in the gas washing bottle 21, and its content was determined by neutral titration of a standard potassium hydroxide solution. Thus, the amount of the SO3 absorbed by ME could be calculated. The sulfonation product underwent further aging in the tubular reactor 22 with an inner diameter of 3 mm immersed in a thermostat 23. Noteworthy, the gas phase was not separated from the liquid phase before the aging, since the N2 gas in the reaction system can facilitate the flow in the tube since the reaction mixture is viscous. The aging time could be adjusted by changing the length of the tubular reactor. The aged product was brown black and collected in the erlenmeyer flask 24 after gas−liquid separation. Bleaching and neutralization of the product were conducted as follows. About 20 g of the sulfonated or aged product and 2 g of ethanol were mixed in a 100 mL three-neck flask at 60 °C for 5 min under reflux condensation and stirring. Afterward, 2 g of H2O2 was charged, and the reaction was continued for 30 min. The bleached sample was placed in a 250 mL beaker immersed in a 30 °C thermostat, and 20 wt % NaOH solution was dripped slowly under stirring until the pH of the reactant became 7−9. Special care should be taken so that the residual NaOH from the neutralizing solution was as small as possible. After drying at 100 °C for 1−2 h, white powder was obtained. 2.3. Analysis. The concentration of unreacted ME (wME) in the sulfonated or aged product was analyzed by a gas chromatograph (GC2014) equipped with a flame ionization detector (FID), an AOC-20i autoinjector, and a HPINNOWAX column (30 m × 0.32 mm × 0.25 μm). About 0.1 g (accurate to 0.0001 g) of the sample was dissolved in ethanol in a 10 mL volumetric flask, and about 1 μL of the solution was injected into the column. The injection temperature was 280 °C. The oven temperature was started at 200 °C, increased to 240 °C at a rate of 15 °C/min, and held at this temperature for 8 min. The FID temperature was set at 380 °C. The wME in the sample was calculated by external standard method, and the conversion of ME (XME) was calculated using eq 1

from hydrolysis of the MES.30 This work will help more understanding of chemistry of the sulfonation reaction in FFMR.

2. EXPERIMENTAL SECTION 2.1. Materials. Liquid sulfur trioxide with a purity of about 95% stored in a stainless steel tank was purchased from Nantong Kaitian Trading Co. Ltd., Jiangsu, China. Dehydrated methanol, hydrogen peroxide (30 wt %), and sodium hydroxide (AR-grade) were supplied by Shanghai Chemical Reagent Co. Ltd., Shanghai, China. Hydrogenated palm oil with an iodine value of 0.46 g I2/100 g and an acid value of 0.75 mg KOH/g was purchased from China Top Chemical (Jiangsu) Co. Ltd., Jiangsu, China. Hydrogenated palm oil methyl ester with a concentration above 99% was prepared by KOH-catalyzed transesterification of the hydrogenated palm oil with methanol in a microreactor,32 whose composition is listed in Table 1. 1H NMR (CDCl3, 400 MHz): δ 0.88 (t, J = 6.60 Hz, 3H, CH3− CH2), 1.26 (d, J = 11.32 Hz, 2H, CH2), 1.62 (t, J = 7.10 Hz, 2H, CH3−CH2), 2.30 (t, J = 7.48 Hz, 2H, CH2−CO), 3.66 (s, 3H, CH3−O). 2.2. Preparation Procedure of MES. The MES was prepared following four steps: sulfonation of hydrogenated palm methyl ester with SO3, aging of the sulfonation product, bleaching of the sulfonated or aged product with H2O2, and neutralization of the bleached product with NaOH. The sulfonation step was carried out in a FFMR (HT-07030, IMM, Mainz, Germany). Three microstructured stainless steel plates (64 straight, parallel microchannels, width × thickness: 300 × 100 μm; 32 microchannels, 600 × 200 μm; 16 microchannels, 1200 × 400 μm;) with a size of 89.4 × 46 mm (length × width) were used. The reaction mixture flowed out of the FFMR into a tube, which was defined as the aging step. This step was conducted in a tubular reactor with an inner diameter of 3 mm, which was connected right to the outlet of the FFMR. During the aging step, no extra reactants were supplied into the system. Figure 1 shows the flowchart of the whole sulfonation and aging steps. Before the experiment, the SO3 tank 1, which was placed in an oven 2, must be heated at 68 °C for more than 12 h. The pressure in the tank was about 0.2 MPa, as shown on the pressure gauge 4. In the preparation, the ME 12 preheated by a thermostat 13 was pumped into the FFMR 15 by a gear pump 14 (Ismatec, Sweden). The SO3 gas was fed after opening the globe valves 3 and 5, and its flow rate was adjusted with a needle valve 6 through a glass rotameter 7. Meanwhile, the N2 3716


Industrial & Engineering Chemistry Research XME = 1 −

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(281.2 + 80nSO3 /ME)wME

× 100% (1) 281.2 where 281.2 and 80 are the molecular weights of the ME and SO3 and nSO3/ME is the molar ratio of SO3 to ME. The concentrations of α-MES (xα‑MES) and disalt (xdisalt) in MES were determined by a two-phase titration method.33 1H NMR spectra were recorded on a BRUKER AV-400 MHz instrument (Bruker, American) using CDCl3 as a solvent.

3. RESULTS AND DISCUSSION 3.1. The Sulfonation Step. 3.1.1. Effect of the ME Flow Rate. Generally, reactions in FFMRs are carried out without overflow of the liquid. Thus, the liquid flow rate has to be controlled so that the thickness of the liquid film (δ, μm) is smaller than the channel depth. Table 2 lists some typical data Table 2. Film Thickness and Liquid Residence Timea ME flow rate (mL/min)

film thickness (μm)

residence time (s)

0.5 1 2 5 10

79.4 100 126 171 215.4

12.1 7.6 4.8 2.6 1.6

Channel size: 300 × 100 μm. N2 flow rate: 900 mL/min. SO3-to-ME molar ratio: 1.2. Sulfonation temperature: 85 °C.

a

of liquid film thickness and the liquid residence time (t, s) in the FFMR calculated from eqs 2 and 3:34

δ=

t=

3

Figure 2. ME conversion andα-MES and disalt concentration versus ME flow rate: (a) without overflow in the FFMR (SO3-to-ME molar ratio, 1.2; N2 flow rate, 900 mL/min; sulfonation temperature, 85 °C; channels size, 600 × 200 μm); (b) with overflow in the FFMR (SO3to-ME molar ratio, 1.2; N2 flow rate, 900 mL/min; sulfonation temperature, 85 °C; channels size, 300 × 100 μm).

3VLμL ρL nbg

(2)

nbδl VL

(3)

reaction conditions, referring that the film thickness has no influence on the diffusion rate of SO3 which should be quite fast. These results indicate that the film thickness and the residence time have no significant effect on the ME conversion and α-MES concentration without overflow but lead to obvious change in the disalt concentration. This is different from the results in ozonolysis of acetic acid 1-vinylhexyl ester and 1decene and nitrobenzene hydrogenation in the same reactor using microstructured plates with channel sizes of 600 × 200 μm and 300 × 100 μm, respectively.27,28,36 In those reactions, the increase in the flow rates of liquids results in reduced conversion, originating from mass transfer limitation of the gases under the reaction conditions. Thus, we can deduce that mass transfer of SO3 from the gas phase to the liquid phase has no significant influence. The above deduction is also verified by further experimental results from the reaction under overflow conditions. Figure 2b shows the effect of the ME flow rate on the ME conversion and the α-MES and disalt concentrations in the FFMR with the channel size of 300 × 100 μm at a SO3-to-ME molar ratio of 1.2, a N2 flow rate of 900 mL/min, and a sulfonation temperature of 85 °C. With increase in the ME flow rate from 2 to 10 mL/min, the ME conversion is almost constant and the α-MES concentration just slightly decreases from 59.6% to 58.8%. These results clearly indicate that the influence of mass transfer is also not significant under overflow conditions. Under

where VL is the ME flow rate (m /s), μL is the liquid film viscosity (Pa s), ρL is the liquid film density (kg/m3), n is the channel number, b is the channel width (m), g is the acceleration of gravity (m/s2), and l is the channel length of 0.0664 m. The film was composed mainly of the unreacted ME and α-MESA, whose viscosity and density were dependent on the conversion of ME (XME) and the temperature T (K) and calculated from eqs 4 and 5:35 3

μL = 3.7 × 10−10 e 4850/ T + 5.22XME + 0.0030.6 < XME < 1 (4)

ρL = 980 + 192XME − 0.66T

(5)

It can be seen from the Table 2 that, as the ME flow rate exceeds 1 and 10 mL/min, flooding in the channels with depths of 100 and 200 μm may occur, respectively. To examine the sulfonation reaction in broad ME flow rate, we used the plate with channels of 600 × 200 μm. Figure 2a shows the ME conversion and the α-MES and disalt concentrations in MES for different ME flow rates at a SO3-to-ME molar ratio of 1.2 and a sulfonation temperature of 85 °C. Both the ME conversion and α-MES concentration decrease slightly from about 61.9% and 59.4% to 60% and 57.4% with increasing ME flow rate from 0.5 to 6 mL/min, respectively, while the disalt concentration increases from 0.3% to 1.9%. Besides, above 99% of SO3 was measured to be absorbed by ME under these 3717



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the disalt concentration decreases from 1.2% to 0.2% as the N2 flow rate was increased from 50 to 300 mL/min and keeps almost constant with further increase in the N2 flow rate to 900 mL/min. Since the reactions of ME with SO3 are all highly exothermic, the reactions with high concentration of SO3 should release more heat, which will facilitate the formation of the disalt byproduct (vide infra). Thus, the disalt concentration increased with the increase of the SO 3 concentration. Nevertheless, the disalt concentration did not change with a high N2 flow rate (i.e., 600, 900 mL/min). This can be attributed to that (1) the reaction with low concentration of SO3 releases less heat; (2) the high N2 flow will efficiently remove the released heat. Thus, as for these results, we consider that it may be correlated to the greater closeness of the reactant temperatures to the set reaction temperature at higher N2 flow rates, as they may bring out more heat produced during the reaction, avoiding more increase in the reaction temperature. 3.1.4. Effect of the Sulfonation Temperature. Figure 4 shows the ME conversions and the α-MES and disalt

these reaction conditions, over 99% of SO3 was measured to be absorbed by ME, further demonstrating fast diffusion rate of SO3 in the ME liquid film. The disalt concentration increases from 0.2% to 0.8% in the ME flow rate range examined, possibly ascribed to higher liquid temperatures at higher ME flow rates. 3.1.2. Effect of the Channel Size. Table 3 shows the ME conversion and the α-MES and disalt concentrations in MES Table 3. ME Conversion, α-MES Concentration, and Disalt Concentration versus Channel Size in the FFMRa channel size (μm)

ME convesion (%)

α-MES concentration (%)

disalt concentration (%)

300 × 100 600 × 200 1200 × 400

62.1 61.4 60.9

59.7 58.8 57.9

0.2 0.6 1..0

ME flow rate: 1 mL/min. N2 flow rate: 900 mL/min. SO3-to-ME molar ratio: 1.2. SO3 flow rate: 7.2 mmol/min. Sulfonation temperature: 85 °C. a

obtained with the three microstructured stainless steel plates. Both the ME conversion and α-MES concentration decrease slightly with the increase in the channel size from 300 × 100 μm to 1200 × 400 μm. This may result from the approximate same film thicknesses and liquid residence times for the three plates at the same ME flow rate in the FFMR.37 However, the disalt concentration increases from 0.2% to 1% with the increase in the channel size. This may be correlated with the decreased heat transfer efficiency of the liquid film with increased liquid film width formed in the channel, although the exact reason is unknown. We later on used the plate with a channel size of 300 × 100 μm since much lower disalt concentration is obtained in this plate. 3.1.3. Effect of the N2 Flow Rate. Generally, the SO3 is diluted with N2 or air to concentrations of 5−10% to reduce reactivity and avoid strong reaction.14,15,21,22 Figure 3 shows the effect of N2 flow rates in the range from 50 to 900 mL/min. The increase in the N2 flow rate leads to almost no change in the ME conversion and α-MES concentration, suggesting no dependence of the reaction on the SO3 concentration. This may be related to the fact that this reaction is not controlled by mass transfer at those examined reaction conditions. However,

Figure 4. ME conversion and α-MES and disalt concentration versus sulfonation temperature in the FFMR (ME flow rate, 1 mL/min; channels size, 300 × 100 μm; SO3-to-ME molar ratio, 1.2; SO3 flow rate, 3.6 mmol/min; N2 flow rate, 900 mL/min).

concentrations at different sulfonation temperatures. The ME conversion almost linearly increases from 48.4% to 63.6% with elevation of the sulfonation temperature from 45 to 85 °C. The α-MES concentration increases from 43.9% at 45 °C to 59.3% at 85 °C first, and then slightly decreases at 90 °C. The disalt concentration remains at 0.1−0.2% between 45 and 85 °C and quickly increases to 0.6% with further increase in the sulfonation temperature to 90 °C. For sulfonation of ME with SO3 in the FFMR, the sulfonation rate is limited either by the rate of diffusion of SO3 into the liquid phase (reaction in the diffusion (mass transfer) controlled regime) or the rate of sulfonation reaction of the reactants in the homogeneous liquid phase (reaction in the kinetic-controlled regime).38 As 99% of the SO3 in the gas phase was detected to be absorbed in the liquid phase and the ME conversion and MES concentration increase with increasing temperature, suggesting increasing reaction rate with increasing temperature, we can derive that the operation of the falling film microreactor is in the kineticcontrolled regime. For comparison, the ME conversion and active matter concentration reach the highest at 60 °C after examining the sulfonation temperature between 50 and 70 °C in the batch reactor,39 indicating that the sulfonation

Figure 3. ME conversion and α-MES and disalt concentration versus N2 flow rate in the FFMR (ME flow rate, 1 mL/min; channels size, 300 × 100 μm; SO3-to-ME molar ratio, 1.2; SO3 flow rate, 3.6 mmol/ min; sulfonation temperature, 85 °C). 3718



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temperature in the FFMR is higher than that in batch reactors to obtain the same reaction results. This may be due to the better heat transfer performance of FFMR. 3.1.5. Effect of the SO3-to-ME Molar Ratio. The theoretical SO3-to-ME molar ratio of the reaction of ME and SO3 is 1.0, and a suitable increase of SO3-to-ME molar ratio is beneficial to improve the ME conversion.5,21 Figure 5 shows the effect of the

Figure 5. ME conversion and α-MES and disalt concentration versus SO3-to-ME molar ratio in the FFMR (ME flow rate, 1 mL/min; channels size, 300 × 100 μm; N2 flow rate, 900 mL/min; sulfonation temperature, 85 °C).

SO3-to-ME molar ratio on the ME conversion and the α-MES and disalt concentrations in the FFMR. With increasing the SO3-to-ME molar ratio from 0.4 to 1.6, the ME conversion nearly linearly increases from 7.3% to 77.9%, the corresponding SO3 conversion decreases from 62.5% to 46.9%, and the α-MES concentration increases from 24.9% to 67.5%, further indicating a kinetic-controlled reaction.27,28,36 By employing the conventional falling film reactor, Nagayama et al.21 also demonstrated that the ME conversion increased with the SO3-to-ME molar ratio, and the ME conversion was 45% at a SO3/ME molar ratio of 1.2 and 80 °C. In this work, the ME conversion was about 60% at the same SO3/ME molar ratio and sulfonation temperature. The disalt concentration slowly increases from 0.1% to 0.2% as the SO3-to-ME molar ratio was increased from 0.4 to 1.2, and then rapidly increased to 1.2% at the SO3-to-ME molar ratio of 1.6. This is related to the reaction mechanism of sulfonation of ME with SO3, which will be discussed later. 3.2. The Aging Step. Although the aging temperature is selected as 85−90 °C and a residence time of 0.5−1 h after sulfonation of ME in a conventional FFR,22 the effect of the aging conditions on the ME conversion and the α-MES and disalt concentrations are not reported. We hereafter examined the influence of the SO3-to-ME molar ratio, aging residence time, and aging temperature in the tubular reactor with an inner diameter of 3 mm after sulfonation in the FFMR with the channel size of 300 × 100 μm at a ME flow rate of 2 mL/min, a N2 flow rate of 900 mL/min, a sulfonation temperature of 85 °C. 3.2.1. Effect of the SO3-to-ME Molar Ratio and Aging Residence Time. Figure 6 shows the influence of the aging residence time on the ME conversion and the α-MES and disalt concentrations at an aging temperature of 90 °C and SO3-toME molar ratios of 1.1, 1.2, 1.4, and 1.6. At the same aging residence times, both the ME conversion and the disalt concentration increase with increase in the SO3-to-ME molar

Figure 6. ME conversion (a) and α-MES (b) and disalt concentrations (c) versus the aging residence time during aging in the tubular reactor at different SO3-to-ME molar ratios. The aging temperature was 90 °C.

ratio (Figures 6a and 6c), while the α-MES concentration changes randomly (Figure 6b). At the same SO3-to-ME molar ratio, the ME conversion increases rapidly within about 5 min and slowly with prolongation of the residence time to 15−20 min (Figure 6a). The α-MES concentration also increases rapidly within about 5 min. It still increases slowly at the SO3to-ME molar ratios of 1.1 and 1.2 but slightly decreases at the SO3-to-ME molar ratios of 1.4 and 1.6 with prolongation of the residence time to 20 min. The slight decrease in the α-MES concentrations at higher SO3-to-ME molar ratios may result from the unreacted SO3 which reacts with NaOH during 3719



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netralization to form sodium sulfate.19 The decrease of MES with higher SO3-to-ME ratio also could come from the reaction of MES with unreacted SO3 in the liquid phase, as the reaction of intermediate III with SO3 to MESA (IV) is reversible and high concentration of unreacted SO3 in the liquid phase would push the reaction toward the formation of intermediate III. The highest ME conversions at the SO3-to-ME molar ratios of 1.1, 1.2, 1.4 and 1.6, occurring at about 20 min, are 87.3%, 91.5%, 92.6%, and 93.8%, respectively. The disalt concentration increases with prolongation of the residence time but with different degrees (Figure 6c). At the SO3-to-ME molar ratios of 1.1, 1.2, and 1.4, it increases slowly with a highest value of 4% after aging for 20 min. At the SO3-to-ME molar ratio of 1.6, it increases rapidly to 5% after a residence time of 15 min. This value is higher than the required disalt concentration of first grade MES product (5%). The highest α-MES concentration of 86.3% can be obtained at an aging residence time of 20 min, which is much shorter than the aging time after sulfonation of ME in conventional FFR. Furthermore, the α-MES concentration is higher and the disalt concentration is lower. 3.2.2. Effect of the Aging Temperature. Figure 7 shows the effect of the aging temperature on the ME conversion and the α-MES and disalt concentrations at a SO3-to-ME molar ratio of 1.2 during the sulfonation reaction. At the same aging residence times, the ME conversion and the disalt concentration increase with increase in the aging temperature (Figures 7a and 7c). The α-MES concentration also increases with increase in the aging temperature in the range of 70−90 °C (Figure 7b). However, when aged at 100 °C, the α-MES concentration is just slightly higher than that aged at 90 °C within 7 min and becomes lower afterward, resulting from formation of a large amount of disalt at this temperature after longer aging time. At the same aging temperature, the ME conversion increases first with prolongation of the residence time and reaches a plateau after 20 min of aging (Figure 7a). The α-MES concentration of the products aged at 70 and 80 °C also increases first with prolongation of the residence time and reaches a plateau after 20 min of aging, while those aged at 90 and 100 °C increase first and slightly decrease. The disalt concentration slowly increases with prolongation of the residence time at 70−90 °C, but it increases sharply at 100 °C, indicating that higher aging temperature favors formation of disalt. Thus, aging of the sulfonated products produced in FFMR can enhance ME conversion, similar to the result in the conventional FFR. 3.3. Discussion. The MES products produced from the sulfonation reaction in the FFMR contain about 60% of α-MES under reasonable reaction conditions, indicating that α-MESA is the main product after sulfonation reaction in the FFMR. This result is distinct from that observed in the conventional FFR wherein the intermediate III as shown in Scheme 1 is the main product after sulfonation reaction while α-MESA mainly formed at the stage of aging.29−31 Thus, we can conclude that the reaction rate from the intermediate III to α-MESA is much faster in the FFMR than in the conventional FFR. As indicated in Scheme 1, the reaction rate from the intermediate III to αMESA is important in the whole process. A relative fast rate of this step is beneficial since the intermediate III would react with NaOH to produce a large mount of undesired disalt VI in the neutralization step.21 To further verify that α-MESA is the main product after sulfonation reaction in the FFMR, we examined the product by 1H NMR spectrometer and found that it contains α-MESA with the highest peak intensity in the 1 H NMR spectra (Figure 8a), along with intermediates II, III,

Figure 7. Aging step in the tubular reactors at different aging temperatures: (a) the ME conversion, (b) the α-MES concentration, (c) the disalt conversion (SO3-to-ME molar ratio: 1.2).

and ME. The relative fast conversion rate from the intermediate III to α-MESA in the FFMR possibly results from its high heat transfer efficiency. The liquid film thickness in the FFMR is between 79 and 215 μm (Table 2), while that in the conventional FFR is usually 500−3000 μm.40 Futhermore, the liquid film width in the FFMR is equal to or less than 1200 μm, which should be much less than that in the conventional FFR. Thus, the heat transfer efficiency in the former should be much higher than that in the latter. As all the three reactions during the conversion of ME to α-MESA are highly 3720



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films formed at such high ME flow rates. As a result, the sulfonation reaction of ME with SO3 can be carried out in the FFMR at much higher ME flow rates than those reactions limited by mass transfer,27,28,36 increasing the output of the reactor. The chemistry in the aging step of the sulfonated ME conducted in the FFMR and the conventional FFR should be different as the compositions of the sulfonated ME from the two reactors are different. In the former case, aging results in the increase in the ME conversion and the α-MES concentration, indicating that sulfonation of ME with residue SO3 to form α-MESA mainly occurs in the aging step. To further verify this, we analyzed the composition of the aged sulfonated ME at the aging time of 20 min by 1H NMR and found disappearance of the ME peak (Figure 8b). In the latter, the reaction of intermediate III to α-MESA mainly occurs in the aging of the sulfonated ME since intermediate III is the main component in the sulfonated ME.29−31 Finally, aging of the sulfonated ME produced in the FFMR at longer residence time leads to formation of more disalt. To explain this, we analyzed the composition of the aged sulfonated ME at the aging time of 24 min by 1H NMR and compared the result with that aged for 20 min. We found a decrease in the α-MESA concentration and an increase in the intermediate III concentration (Figure 8c), with the sum of the α-MESA and intermediate III concentrations equaling that in the sample aged for 20 min. The result suggests that increase in the intermediate III concentration results from the reverse reaction of α-MESA to intermediate III after aging for longer time since the ME conversion is almost invariant. On the other hand, as the reactions between SO3 and ME (I) and intermediate II are much faster than the reaction between MES and SO3, longer aging time can produce more intermediate III from the unconverted ME and SO3 flowed out of the FFMR, which is gradually conversed to MESA by a slow reaction with SO3, thus resulting in higher disalt concentrations.

Figure 8. 1H NMR spectra of the productions after sulfonation (a) and sulfonation and aging (b and c). (a) The SO3-to-ME molar ratio was 1.2, and the sulfonation temperature was 85 °C. (b) The SO3-toME molar ratio was 1.2, and the sulfonation temperature was 85 °C. The aging residence time was 19.7 min with the aging temperature of 90 °C. (c) The SO3-to-ME molar ratio was 1.2, and the sulfonation temperature was 85 °C. The aging residence time was 24.3 min with the aging temperature of 90 °C. ○ and △, intermediate II; ●, intermediate III; ◇, ME.16,20.

exothermic,29 with the first two being fast and the last being slow, efficient removal of the heat in the liquid film released by the first two timely may facilitate the occurrence of the last reaction. Therefore, α-MESA is the main product in the sulfonated ME produced in the FFMR. The mass transfer of SO3 from the gas phase to the liquid phase has no significant influence when the liquid film thickness ranged from 79.4 to 215.4 μm in FFMR, as more than 99% of SO3 was absorbed in the liquid phase. Regarding to the mass transfer of SO3 inside the liquid phase, we could not provide further evidence of the influence. However, as the liquid film thickness in the conventional FFR is about several millimeters,40 we can deduce from big difference of the liquid film thickness between the FFMR and the conventional FFR that influences of the mass transfer of SO3 inside the liquid phase in the FFMR should be much less than that in the conventional FFR, thus exerting influence on differences in performance of the two types of reactors. The disalt concentration in the MES product produced from sulfonated ME without aging increases with increase in the ME flow rate, sulfonation temperature, and channel size and decrease in the N2 flow rate. In addition, we found that higher aging temperature favors formation of disalt (Figure 7b). As mentioned above, disalt is mainly formed from intermediate III. Thus, it is reasonable to postulate that the inefficient heat transfer efficiency at higher ME flow rates and sulfonation temperatures and larger channel sizes results in much slower conversion of intermediate III to α-MESA, leading to formation of more disalt after neturalization. Sulfonation of ME in the FFMR can be operated at high ME flow rates so that the liquid is flooding from the microchannels, without significant effect on the ME conversion and the α-MES and disalt concentrations in the final MES product. This is because of the fast mass transfer rate of SO3 through the liquid

4. CONCLUSIONS We carried out sulfonation of hydrogenated palm methyl ester and SO3 in a FFMR, followed by aging of the product in a tubular reactor to produce α-MESA, which was further conversed to α-MES by bleaching and neutralization. The sulfonation reactions operated with and without liquid overflow did not have obvious difference, suggesting that mass transfer in FFMR was not overwhelming. This may be ascribed to the fast diffusion rate and absorption of SO3 in the ME liquid film. High N2 flow rates could restrain formation of disalts, while increase in the ME flow rate, the channel size, the SO3-to-ME molar ratio, and the sulfonation temperature could result in increase in the disalt concentration. In the aging step, increase in the SO3-to-ME molar ratio could improve the ME conversion, but a too high SO3-to-ME molar ratio could lead to the increase of the disalt concentration. A similar trend was also observed on the influences of aging residence time and aging temperature. α-MESA was mainly formed in the sulfonation step in FFMR, while it was formed in the aging step when the sulfonation was carried out in the conventional FFR. Under optimized reaction conditions, the final product with an α-MES concentration of 86.3% and a disalt concentration of 1.2% could be produced, in which the α-MES concentration is slightly higher and the disalt concentration is much lower than those in the commercial product. 3721



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 20733009), the Common Colleges Postgraduate Innovation Plan of Jiangsu province (No. CX10B_173Z), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

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Preparation of Methyl Ester Sulfonates Based on Sulfonation in a

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