In Situ Removal of HBr via Microdroplets for High Selectivity

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Kinetics, Catalysis, and Reaction Engineering

In-situ removal of HBr via micro droplets for high selectivity bromobutyl rubber synthesis in a microreaction system Pei Xie, Kai Wang, Jisong Zhang, Yunpeng Hu, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01884 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Industrial & Engineering Chemistry Research

In-situ removal of HBr via micro droplets for high selectivity bromobutyl rubber synthesis in a microreaction system Pei Xie, Kai Wang*, Jisong Zhang, Yunpeng Hu and Guangsheng Luo* The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China. Corresponding authors [email protected] FAX (8610)62788568, TEL (8610)62788568; [email protected]

FAX (8610)62783870, TEL (8610)62783870

Abstract To obtain high quality bromobutyl rubber product, a higher selectivity of brominated secondary allylic structure is necessary, but it is difficult to realize in traditional reaction devices. We develop a microreaction system and used micro water droplets as an extractant to in-situ remove HBr, the catalyst of isomerization side reaction, from the organic reacting solution to prevent converting brominated secondary allyl to brominated primary allyl and degradation of the polymer chain. The microreaction system contains two micromixers, including a membrane dispersion micromixer for dispersing micro water droplets in the butyl rubber solution, and a cross-junction micromixer for blending butyl rubber and Br2 solutions. A volume adjustable delayed loop after the micromixers is used to carry out the reaction. The results show that 1 wt% water in the organic phase is satisfied to remove HBr, and 97% selectivity of brominated secondary allyl is successfully obtained under optimized operating conditions.

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords: bromobutyl rubber, microreactor, micro droplets, process intensification. 1. Introduction Bromobutyl rubber is a halogenated product of butyl rubber (isobutylene isoprene rubber, IIR). It retains the major structure of butyl rubber, and therefore exhibits similar physical properties,1,2 such as thermal stability, oxidation resistance, and low permeability to air and moisture.3,4 Owing to the brominated allylic group, bromobutyl rubber has better properties during vulcanization and good compatibility with other elastomers, overcoming the limitation of common butyl rubber,5 and contributing to various applications in tires, tubes, and electric cables. 6 Bromobutyl rubber is usually prepared via the bromination reaction of butyl rubber. Because the reaction products in the alkane solution exhibit more stable and homogeneous properties than those from the extrusion reaction of solid butyl rubber, solvent method is more commonly used in the industrial process. 7,8 The molecular structures of bromobutyl rubber and the mechanism of the bromination process have been reported in literatures.1, 7, 9-11 As shown in Figure 1, when Br2 meets the 2-methyl-butene group in the polymer chain, labelled with IIR in the figure, an intermediate bromonium ion named with B+ is generated. In this B+, most of the positive charge locates on the tertiary atom. The sterically bulky methyl groups in the β position makes against the nucleophilic attack of the bromide ion. Therefore, the following main reaction r1 is a Br2 substitution reaction of the 2-methyl-butene group,12-14 and the product is a brominated secondary allyl, named to BIIR1 in this study. However, the brominated secondary allyl is unstable in acidic environment, and it will quickly turn to the brominated primary allyl, which is named to BIIR-2 in Figure 1, under the


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catalytic effect of the side product HBr.15 This reaction labelled with r2 is the most important side reaction. In addition, addition reaction of Br2 to double bonds occurs, shown by r3 in Figure 1. The addition reaction product BIIR-3 reduces the content of double bonds for vulcanization and brings less Br2 utilization.9, 12, 16 Besides the Br isomerization reaction, another negative effect from HBr is the degradation of the polymer, which significantly reduces the molecular weight of bromobutyl rubber.17

Figure 1. Mechanism of butyl rubber bromination reaction. IIR is the abbreviation of butyl rubber and it is also used to specifically represent the 2-methyl-butene group in this paper. BIIR-1 represents the brominated secondary allylic structure as the main product; BIIR-2 represents the side product brominated primary allylic structure; and BIIR-3 shows addition reaction product. Traditionally, bromobutyl rubber is synthesized in consecutive reactors. The butyl rubber and bromine solutions are first mixed in a small vessel mixer with short residence time, and then the reaction keeps on implementing in pipelines. The halogenated product is neutralized by an aqueous 3 ACS Paragon Plus Environment


alkaline solution after the reaction. The bromobutyl rubber is finally aggregated by low pressure steams and washed by water, and the waste solution is subsequently separated. One of the most important parameters of bromobutyl rubber is the molar percentage of BIIR-1 in total substituted bromides, representing the structure selectivity of the main product. Typically, for high industry application of bromobutyl rubber, this structure selectivity should be larger than 90%. However, it is very difficult to obtain such a high value in traditional reactor due to the limitation on mixing ability and the relatively long reaction time. In the traditional reaction process, the high viscosity of the butyl rubber solution and the large flow rate ratio of the rubber solution and Br2 solution reduce the mixing performance of mixers and slow down the working efficiency. As a result, the overall reaction is mixing limited, and a long residence time has to be required, which usually calls for large volume reactor, and the side reactions keep proceeding under the catalytic effect of HBr.16 One study compared the bromination reaction in a stirred tank reactor (STR) and a rotating packed bed (RPB) indicated that the STR needed 5 min reaction time, but the RPB only needed 2 min for the similar yield of the target product.16 Furthermore, because of the low diffusion rate of HBr in the polymer solution, HBr is easy to accumulate in the reaction and aggravate the isomerization reaction. For these reasons, a more highly efficient reaction system with strong mixing ability and short reaction time is necessary for developing bromobutyl rubber production technology. Microreactors have a much better mixing ability than traditional vessel mixers or reactors. They have confined mixing channels, which shorten the mixing distance and reduce the concentration gradient.18-20 In addition to rapid mixing, microreactors also provide efficient heat transfer,19, 21


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accurate residence time control,22, 23 and excellent anticorrosion properties via coatings with costeffective resistant materials.24 With these features, microreactors have been widely used in many chemical processes for improving reaction efficiency or the selectivity of target products.25-27 In our previous work, a microreaction system for synthesizing bromobutyl rubber was developed.16 The microreaction system allows the reaction to be finished in 1 min at 30ºC, much shorter than the 5 min reaction time in STR.16, 28 However, less than 80% selectivity of BIIR-1 in that microreaction system is still not good enough. This is because HBr is in-situ generated in the reaction and the nutrition at the end of the reaction is rather late, although a highly efficient micromixer has been used. To further eliminate the negative effect of HBr, we must remove it from the reaction immediately. Considering the partition coefficient of HBr between water and alkane is rather high, (KHBr is approximately equal to 100 at 25ºC24) we believed water extraction will be an advanced method to in-situ remove HBr from the organic reacting system. Therefore, we propose an idea to use micro water droplets as tinny extractors in the reaction system to rapidly remove HBr from the organic phase, as shown in Figure 2. An improved microreaction system is then developed based on this consideration, and tested in the following experiment.

Figure 2. Coupling of bromide reaction and water extraction in bromobutyl rubber reaction. 5 ACS Paragon Plus Environment


Small dots are water droplets and the arrows show the mass transfer of HBr. 2. Experimental Section 2.1 Materials and equipment To improve the mixing efficiency and avoid corrosion of HBr and Br2, a polytetrafluoroethylene (PTFE) microreaction system is developed for the bromination reaction, whose structure is shown in Figure 3. Before the experiment, butyl rubber (molar percentage of 2-methyl-butene unit U0 = 1.74%, number average molecular weight Mn0 = 273 kD, and polydispersity index PDI0 = 2.19, provided by Cenway New Materials Co. Ltd., China) was dissolved in n-hexane (98.0%, Eastern Chemical Co. Ltd., China), and stored for more than 2 weeks to precipitate the insoluble impurities. Only the transparent liquid in the upper layer of the solution was applied. Br2 solution was prepared by Br2 (99.5%, Sinopharm Chemical Reagent Co. Ltd., China) and n-hexane before experiment. Because Br2 can slowly reacts with n-hexane under the light condition, after preparation, the Br2 solution was immediately transferred to a PTFE-lined piston vessel with complete darkness. The high viscous butyl rubber solution was also transferred by a piston vessel, which was made from 316L stainless steel. The IIR and Br2 solutions were stored on the top of the vessels and the water was fed in from the bottom. PTFE and 316L stainless steel pistons separated the IIR/Br2 solutions and water, and the IIR/Br2 solutions were pressed out by the feeding water. The flow rates of those solutions were controlled by water metering pumps. Hence, the IIR and Br2 solutions transferred quantitatively.24 The base solution was also prepared before the experiment, for the extraction and consumption of HBr and Br2 in the sampling process. In this study, 5% sodium hydroxide (98.5%,


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J&K Scientific Co. Ltd., China) and 2.5% sodium metabisulfite (Na2S2O5, 96%, J&K Scientific Co. Ltd., China) were mixed with equal volume to form a quenching solution. Epoxidized soybean oil (ESO, 95%, Aladdin Industrial Co. Ltd., China) and calcium stearate (Ca: 6.6-7.4%, Aladdin Industrial Co. Ltd., China) were pre-mixed (weight ratio 3:1) as the stabilizer and added into the product during the washing step.

Figure 3. Microreaction platform of the butyl rubber bromination experiment. The delivery water was feed by metering position pumps (Xingda, 2PB100, China). The switching valves only connect the inlet to one outlet and close the others. During the experiment, water was pre-dispersed into the butyl rubber solution to form micro droplets via a membrane dispersion micromixer, whose structure are shown in Figure 4(a) and 4(b). The micromixer consists of five parts. Parts 1 and 5 are stainless steel (SST) modules, which supply the inlets and outlet. Between these modules, two PTFE plates and one SST filtration membrane (316L, 5 μm average pore size and 50% porosity) are assembled to form the flow path. The width, length and the height of the upper chamber are 2 mm, 8 mm and 1 mm, respectively, and this 7 ACS Paragon Plus Environment


chamber is used as a buffer tank for the water phase, which was pressed through the membrane and dispersed to micro droplets. The lower channel is mainly used as the mixing chamber, whose length, width and height are 12 mm, 1 mm and 1 mm, respectively. 1 m long SST coiled tube (inner diameter 2 mm) is connected to the outlet of the membrane dispersion micromixer, used for preheating the butyl rubber solution as shown in Figure 3. The pre-heating tube and the following reaction devices are all placed in a water bath with controlled temperature. The butyl rubber solution containing water droplets and Br2 solution are then mixed in a crossjunction micromixer. This micromixer included 6 parts, as shown in Figures 4(c) and 4(d). Parts 1 and 6 are SST modules, which serve to support the inner structure. Parts 2, 3, 4 and 5 are all PTFE plates, which supply a mixing channel for the reactant solutions. Both the height and width of the crossed channels are 1 mm, and the lengths of the channels are 8 mm and 12 mm, respectively. After the cross-junction micromixer, 1 m PTFE-lined SST coiled tube (inner diameter 1.1 mm) is applied to further mix the rubber and Br2 solutions. The out coming fluid then enters to a complex delayed loop system with switching valves (Beijing XiongChuan Technology Co. Ltd., China) and PTFE pipes, providing different reaction volumes. The inner diameter of the pipes in the delayed loops are 4 mm. The volumes of the coiled tubes and pipes between the cross-junction micromixer and the outlet can be adjusted among 12.0, 33.1, 66.3, 99.3 and 132.4 mL. The residence time in these volumes can be further calculated by dividing the flow rate.


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Figure 4. Details of the micromixers. (a) 3D image of the membrane dispersion micromixer, used for water pre-dispersion. (b) Detailed structures. (c) 3D image of the cross-junction micromixer, used for the bromination reaction. (d) Detailed component structures. During the experiment, as the system pressure became stable and the fed volume of fluid had been more than three times of the reactor, the product solution would be injected into a stirred base solution, terminating the reaction. In this sampling process, HBr and Br2 were quickly extracted by water, reduced and neutralized by Na2S2O5 and NaOH. After 30 min stirring, a peristaltic pump was used to suck the water out. The organics were then washed three times by deionized water (DI water, laboratory water purification from Beijing Boxinyuanyang Scientific Co. Ltd., China), with 30 min oil-water mixing for each step. During the second washing step, a drop of the mixture of epoxidized soybean oil and calcium stearate was added into the sample solution to stabilize the



bromobutyl rubber from oxidation. After 12 h static standing in the last washing step, the organic phase was separated. Finally, the products were dried 8 h in a vacuum drying oven at 35ºC. 2.2 Measurement and analysis In the membrane dispersion micromixer water were dispersed to micro droplets in the rubber solution. The two-phase mixture were recorded by a microscope (XSP-BM21AY, Shanghai, China) with a high-speed CMOS camera (742U, PixeLINK, Canada). After drying, the bromobutyl rubber products are characterized by hydrogen nuclear magnetic resonance (1H-NMR, 600M, Bruker) and gel permeation chromatography (GPC, Waters 1515). Deuterated chloroform (J&K Scientific Co. Ltd., China) was used as the solvent in the 1H-NMR analysis, and tetrahydrofuran was used as the solvent in GPC. A literature2 in 1987 first reported the characterization of the chemical structures of halo-butyl rubbers using NMR. Recently, other research groups16, 24, 28 have also quantitatively evaluated the molar ratios of different groups in the bromobutyl rubber using the 1H-NMR method. This method has shown good repeatability in the present study with less than 1.5% standard deviations. (Detailed data are in the Supplementary Information). Figure 5(a) shows the chemical shifts of 1H peaks in the product, and Figure 5(b) shows a sample of the 1H-NMR spectrum.


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Figure 5. Chemical shifts of 1H peaks and 1H-NMR spectra of the products. (a) The chemical shifts of 1H peaks of BIIR-1, BIIR-2 and IIR. (b) 1H-NMR spectrum of the bromobutyl rubber product. Peaks are A: δ = 5.4 ppm (BIIR-1); B: δ = 5.07 ppm (IIR); C: δ = 5.02 ppm (BIIR-1); D: δ = 4.34 ppm (BIIR-1); E: δ = 4.09 ppm (BIIR-2); F: δ = 4.05 ppm (BIIR-2); G: δ = 1.42 ppm (– CH2–); H: δ = 1.11 ppm (–CH3); I: δ = 4.28 and 4.30 ppm (ESO); J: δ = 4.13, 4.14, 4.15, and 4.16 ppm (ESO). In addition to the important selectivity parameter, i.e., the molar percentage of BIIR-1 in total BIIR-1 and BIIR-2 (S), we also characterized the conversion of 2-methyl-butene group (X), yields of BIIR-1 (YBIIR-1), BIIR-2 (YBIIR-2), and BIIR-3 (YBIIR-3), and total selectivity of BIIR-1 in BIIR-1, BIIR-2, and BIIR-3 (ST) using the 1H-NMR analysis. Based the areas of all peaks in the spectra of the original butyl rubber (labelled with superscript IIR,0) and the bromobutyl rubber (labelled with superscript BIIR), the following parameters are calculated: X 

nIIR，0  nIIR [B]/[G]IIR，0  [B]/[G]BIIR = nIIR，0 [B]/[G]IIR，0

(1) 11

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YBIIR-1 

nBIIR-1 [D]/[G]BIIR  nIIR,0 [B]/[G]IIR,0

(2)

YBIIR-2 

nBIIR-2  [E]  [F] /[G]BIIR  nIIR,0  2[B]/[G]IIR,0

(3)

YBIIR-3  X  YBIIR-1  YBIIR-2 S

(4)

YBIIR-1 YBIIR-1  YBIIR-2

ST 

(5)

YBIIR-1 YBIIR-1  YBIIR-2  YBIIR-3

(6)

where [B] to [G] are the peak areas, nIIR,0 is the molar flow rate of original 2-methyl-butene groups, nIIR is the molar flow rate of residual 2-methyl-butene groups. nBIIR-1, nBIIR-2 and nBIIR-3 are the molar flow rates of different bromide groups. The molar flow rate of the methylene groups in butyl rubber is the reference in calculation, which does not change during the reaction. In the traditional process, polymer chains break down under the effect of HBr; therefore, some errors may exist as using Equations (1) to (6). However, the polymer delegation could be ignored in this study. The delegation degree can be observed from the variation of number average molecular weight, Mn, and polydispersity index, PDI, which were determined by GPC. 3. Results and Discussion 3.1 Selectivity problem of bromobutyl rubber synthesis Because high percentage of BIIR-1 in the bromobutyl rubber is necessary in the vulcanization process, we give a short discussion of the selectivity problem of BIIR-1 without the assistant of water droplets at the beginning of this section. Results in Table 1 show the selectivity of BIIR-1 in


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both substitution products (BIIR-1 and BIIR-2) decreases with the increase of residence time (τ) and butyl rubber concentration (w0) in the reactant solution. In other words, the accumulation of HBr intensifies the isomerization side reaction from BIIR-1 to BIIR-2, reducing the selectivity of the main substitution product. To improve the selectivity, the amount of HBr in the system needs to be controlled. As shown in Table 1, low original butyl rubber concentration and short residence time can improve the selectivity of BIIR-1, because of less HBr generation in these cases. However, except for the high selectivity, high yield of BIIR-1 is also required in practice, which calls for higher concentration of reactants and longer residence time, but the results in Table 1 show less than 90% structure selectivity without any assistant. Table 1. Working performances of the microreactor system without water droplets w0 (wt%)

τ (min)

X (%)

YBIIR-1 (%)

S (%)

0.13

29.6

16.5

100.0

0.35

28.5

16.9

100.0

0.69

56.7

29.0

94.9

1.39

61.6

32.1

84.3

0.12

48.5

33.6

88.1

0.34

57.6

40.4

88.6

0.67

62.7

42.9

87.9

1.01

64.2

41.8

82.2

0.14

65.8

39.1

77.8

0.39

75.7

38.3

65.4

0.72

75.6

36.6

68.4

0.85

77.0

36.8

66.0

2.86

10.34

14.73



Experimental conditions are (a) butyl rubber concentration in the reactant solution w0 = 2.86 wt%, bromide concentration in the other reactant solution wBr2 = 4.33 wt%, feeding flow rate of rubber solution Qrubber = 90 mL/min, flow rate of Br2 solution QBr2 = 6 mL/min, temperature T = 30 ºC; (b) w0 = 10.34 wt%, wBr2 = 5.23 wt%, Qrubber = 90 mL/min,QBr2 = 9 mL/min, T = 30 ºC; (c) w0 = 14.73 wt%, wBr2 = 7.39 wt%, Qrubber = 90 mL/min, QBr2 = 9 mL/min, T = 30 ºC. 3.2 Droplet dispersion state observation Using membrane dispersion micromixers to generate micro droplets in other polymer solutions has been tested in our previous work.29, 30, 31 In this study, a 5 μm average pore size membrane was used to disperse DI water into the butyl rubber solution. The liquid-liquid mixture and droplet size distribution are shown in Figure 6. We collected the liquid-liquid dispersion systems at the outlet of the membrane dispersion micromixer and observed them with the microscope introduced above. To obtain the statistical law, at least 40 pictures with more and 300 droplets were counted for each test. According to our observations, the micro water droplets in 10 wt% butyl rubber solution are very stable without obvious coalescence phenomenon in more than 10 min. Therefore, the water droplets can maintain their initial dispersion state during the following less than 1.5 min reaction time. The average droplet diameter was 36 μm, with a standard deviation of 15 μm.


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Figure 6. Statistics of droplet size distribution and microscope picture. Experiment was at w0 =10 wt%; Qrubber = 90 mL/min; QH2O = 0.66 mL/min. 3.3 Influence of water percentage on reaction performance There are two aspects to the pre-dispersion of water in the suspension. On one hand, water can rapidly extract HBr from the organic phase to improve the selectivity of product. On the other hand, water droplets contained Br- also extract Br2 from the organic phase forming Br3- in water, which reduces the amount of Br2 joins in the bromination reaction. Considering both the positive and negative effects of water in the reaction process, we conducted two groups of experiments to obtain the relatively ideal addition amount of water. As shown in Table 2, with the addition of water (1 wt% and 3 wt% addition), increased selectivity of brominated secondary allyl in the substitution product (S) and the total brominated reaction product (ST) are clearly exhibited, compared with the control experiment without water. However, the conversion of the IIR decreases with adding water, particularly in the 3 wt% H2O experiment, with a maximum value of X = 54.3%. Thus, 1 wt% water in the butyl rubber solution (Qrubber/QH2O = 99/0.66) is sufficient enough to obtain the desired high selectivity with a minimal reduction in IIR conversion, and obtains the highest yield of BIIR15 ACS Paragon Plus Environment


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1 at 48.0%. Table 2. Effect of water amount on the reaction performances. Amount of Additives

None H2O

1 wt% H2O

3 wt% H2O

τ (min)

X (%)

0.34

57.6

0.67

YBIIR-1 (%)

ST (%)

S (%)

40.4

70.1

88.6

62.7

42.9

68.3

87.9

1.01

64.2

41.8

65.2

82.2

0.33

49.9

38.6

77.4

97.3

0.67

58.3

45.3

77.7

98.5

1.00

62.0

48.0

77.4

97.0

0.33

47.5

39.4

82.8

95.9

0.66

51.1

41.1

80.3

93.8

1.00

54.3

43.4

80.0

94.2

Experimental conditions are (a) w0 = 10.34 wt%, wBr2 = 5.23 wt%, Qrubber = 90 mL/min, QBr2 = 9 mL/min, T = 30ºC; (b) w0 = 9.65 wt%, wBr2 = 4.75 wt%, Qrubber = 90 mL/min, QBr2 = 9 mL/min, QH2O = 0.66 mL/min, T = 30ºC; (c) w0 = 10.45 wt%, wBr2 = 5.32 wt%, Qrubber = 90 mL/min, QBr2 = 9 mL/min, QH2O = 2.01 mL/min, T = 30ºC. In addition to the isomerization and addition side reactions, there is a significant breakdown of polymer chains in the traditional bromination process, which has been described in our previous study.24 HBr is the catalyst that accelerates the breakdown of polymer. Here, the 1 wt% pre-added water prevents the rubber delegation. As shown in Figure 7, the number average molecular weight Mn is stabilized around 270 kD (Mn0), and PDI also has little variation compared to the original value 2.19. The possible extraction ratio of water for HBr can be evaluated by Equation 7. 16 ACS Paragon Plus Environment

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E % 

K HBr 100% K HBr  Vo / Vw

(7)

where E is the extraction ratio, which shows the percentage of residual HBr in the organic phase; KHBr is the partition coefficient of HBr between water and n-hexane; and Vo and Vw are the volumes of the oil and water, respectively. According our previous study,24 KHBr is approximately equal to 100, and in this study, Vo/Vw was mainly at 148. Therefore, E is approximately equal to 40%, which indicates that approximately 40% of HBr has been extracted to the water phase in time. This is important in the early stage of the reaction, where large amount of HBr was quickly generated. In general, this result shows that using a small amount of green and cheap extractant, the selectivity and yield of BIIR-1 can be improved.

Figure 7. Effect of water on the molecular weight of polymer product. Experiment was at w0 = 9.65 wt%, wBr2 = 4.75 wt%, Qrubber = 90 mL/min, QBr2 = 9 mL/min, QH2O = 0.66 mL/min, and T = 30ºC. The results have some fluctuations from the errors of GPC. 3.4 Influence of operating conditions on reaction results To in-depth understand the performance of the microreaction system on the bromination results, 17 ACS Paragon Plus Environment


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the influences of other operation parameters, including residence time (τ), flow rate (Q), reactant molar ratio (nBr2/nIIR,0) and reaction temperature (T), on the conversion, yield, and selectivity of different products were experimentally investigated. First of all, the mixing efficiency is vital for the fast bromination process in this highly viscous reaction system. As a passive mixing device, the mixing intensity in a microreactor is mainly dependent on flow rate and higher flow rate causes better mixing performance. We therefore tested the butyl rubber solution from 22.5 to 90.0 mL/min in the experiment and control the residence time constant at 1.4 min by changing the reaction tube volume. Correspondingly, the flow rates of the Br2 solution and water were varied from 2.2 to 9.0 mL/min and 0.17 to 0.66 mL/min, respectively, maintaining the flow rate ratios. The results with different total flow rates are shown in Table 3. The conversion of IIR and the yield of BIIR-1 both increase greatly with rising flow rate. The enhanced mixing in both micromixers also contributes to highly efficient extraction of HBr, which results in the decrease of BIIR-2 yield. However, the BIIR-3 yield initially increases and then decreases to a stable value at about 11%, which does not show clearly variation rules. This might be because the yield of BIIR-3 is both mixing and kinetic dependent at low flow rate condition, but the kinetic difference from the parallel substitution and addition reactions gives low yield of BIIR-3 as the mixing performance becomes strong enough at high flow rate. Totally from the results in Table 3, the highest flow rate is the best to obtain high BIIR-1 yield and selectivity. Table 3. Effect of total flow rate on the reaction results. QT (mL/min)

X (%)

YBIIR-1 (%)

YBIIR-2 (%)

YBIIR-3 (%)


ST (%)

S (%)

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24.9

43.6

33.1

3.6

7.0

75.8

90.2

49.8

54.4

35.1

3.6

15.8

64.4

90.7

74.8

54.4

39.9

2.9

11.6

73.3

93.2

99.7

60.9

49.2

0.7

11.0

80.8

98.5

Experiment conditions are w0 = 9.46 wt%, wBr2 = 4.81 wt%, Qrubber = 22.5, 45.0, 67.5, and 90.0 mL/min, QBr2 = 2.2, 4.5, 6.8, and 9.0 mL/min, QH2O = 0.17, 0.34, 0.51, and 0.66 mL/min, τ = 1.4 min and T = 30ºC, QT = Qrubber + QBr2 + QH2O. Residence time is another important parameter. As shown in Table 4, increasing the residence time with longer delayed tubes improves the IIR conversion and the BIIR-1 yield. The results show that 1.0 min residence time can provide more than 60% IIR conversion, 48% BIIR-1 yield, higher than 98% selectivity of BIIR-1 in the substitution products, and larger than 77% BIIR-1 selectivity in all brominated products. In addition to those substitution products, the reaction also produces addition product BIIR-3 with 10-14% yield, which is agree with the results in Table 3. The results in Table 4 also show that BIIR-3 is mainly generated at the beginning of the reaction process. We did not find any exact reason for this phenomenon from the literatures, but we guess the kinetic order of Br2 in the addition reaction might be >1, and the low concentration of Br2 at the following reaction time strongly reduce the rate of addition reaction. Table 4. Effect of residence time on the reaction results. τ (min)

X (%)

YBIIR-1 (%)

YBIIR-2 (%)

YBIIR-3 (%)

ST (%)

S (%)

0.12

39.7

28.3

0.4

10.9

71.4

98.5



Page 20 of 33

0.33

49.9

38.6

0.5

10.8

77.4

98.7

0.67

58.3

45.3

0.5

12.5

77.7

98.9

1.00

62.0

48.0

0.8

13.2

77.4

98.4

Experimental conditions are w0 = 9.65 wt%, wBr2 = 4.75 wt%, Qrubber = 90 mL/min, QBr2 = 9 mL/min, QH2O = 0.66 mL/min, and T = 30ºC. High BIIR-1 yield is sometimes also necessary for applying bromobutyl rubber, because it can accelerate the vulcanization process. One feasible method to increase the yield of BIIR-1 is to use excess Br2 during the reaction. We thus implemented an experiment with constant w0, Qrubber, QH2O and QBr2 and varied wBr2 from 4.92 to 7.64 wt%, which meant the molar ratio of nBr2/nIIR,0 increased from 0.98 to 1.55. Table 5 shows that excess Br2 does not only improve the BIIR-1 yield, but also intensify the addition and isomerization side reactions, which results in an obviously decrease of the selectivity. In particular, if nBr2/nIIR,0 is larger than 1.2, S will be less than 90% and ST will be less than 70%. Therefore, an appropriate amount of Br2 is important for the reaction. Table 5. Effect of reactant molar ratio on the reaction results. nBr2/nIIR,0

X (%)

YBIIR-1 (%)

YBIIR-2 (%)

YBIIR-3 (%)

ST (%)

S (%)

0.98

52.0

40.9

0.9

10.2

78.7

97.9

1.10

62.5

45.4

2.1

15.0

72.6

95.5

1.19

65.0

45.3

4.5

15.1

69.8

90.9

1.41

67.8

43.0

7.3

17.5

63.4

85.4

1.55

91.7

57.3

10.1

24.2

62.5

85.0


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Experimental conditions are w0 = 9.93 wt%, wBr2 = 4.92, 5.52, 5.94, 6.96, and 7.64 wt%, Qrubber = 90 mL/min, QBr2 = 9 mL/min, QH2O = 0.67 mL/min, with τ = 1.4 min and T = 30ºC. Reaction temperature is the last parameter we optimized in the experiment. Higher temperature reduces the viscosity of the polymer solution and intensifies mixing. However, the results in Table 6 indicate that as temperature rises from 30 to 50ºC, yield and selectivity of BIIR-1 as well as the conversion of IIR decrease sharply. Br2 should be excessively extracted to the water phase at higher temperature, which directly reduces the IIR conversion. The BIIR-1 is also easier to turn to BIIR2 at high temperature, as the major reason for the decrease of selectivity. However, proper increase of reaction temperature reduces the addition product of BIIR-3. As shown in Table 6, there is nearly no BIIR-3 as the temperature rising to 40ºC. Table 6. Effect of temperature on the reaction results. T (ºC)

X (%)

YBIIR-1 (%)

YBIIR-2 (%)

YBIIR-3 (%)

ST (%)

S (%)

30

55.9

51.2

0.7

3.9

91.6

98.6

35

53.4

48.6

2.6

2.2

90.9

94.9

40

44.5

41.7

2.4

0.4

93.7

94.5

45

38.4

32.7

5.0

0.8

85.1

86.8

50

13.0

7.4

5.6

0

57.1

56.9

Experiment was at w0 = 9.93 wt%, wBr2 = 5.17 wt%, Qrubber = 90.0 mL/min, QBr2 = 9.0 mL/min, QH2O = 0.67 mL/min, and τ = 1.4 min. 3.5 Comparisons with commercial products In the above experiment, we have obtained high selectivity of BIIR-1 by controlling the amount 21 ACS Paragon Plus Environment


Page 22 of 33

of water droplets and optimizing the operation condition of the microreaction system. To show the potential value in the industrial application of the microreaction technology, selectivity data of different bromobutyl rubber samples from ExxonMobil, Lanxess and Cenway Co. Ltd. as well as literature results are compared in Table 7. The highest selectivity of BIIR-1 is from the present microreaction technology with the assistant of micro water droplets. Furthermore, to evaluate the working efficiency of the microreaction system, we calculated the space-time yield of the lab scale microreaction process, which was as high as 2320 kg/(m3·h) bromobutyl rubber, a relatively higher value for reaction of bromobutyl rubber synthesis. Table 7. Comparison of the selectivity of BIIR-1 in the substitution reaction.

S (%)

This study

Tank reactor 28

Exxon 2222

Lanxess 2031

Cenway 2302

97.0

84.2

92.1

91.3

93.9

The samples in the table has similar bromine content.

Conclusions We herein described a microreaction system and a pre-dispersed micro water droplets method for synthesizing bromobutyl rubber with high selectivity of the brominated secondary allylic group. The micro water droplets stored in the original butyl rubber solution can in-situ extract HBr from the reacting organic phase, preventing side isomerization reaction and the degradation of polymer. The microdispersed water droplets was generated from a membrane dispersion micromixer with a 5 μm pore size filtration membrane as the dispersion media, and then the water contained butyl 22 ACS Paragon Plus Environment

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rubber solution was mixed with the Br2 solution in the second micromixer with cross channels. The whole reaction was then taken place in the delayed loops following the micromixers within less than 1.5 min residence time. 1 wt% amount of pre-dispersed water was balanced by the effects of HBr extraction and Br2 extraction. After optimizing the other operation parameters, including the residence time, flow rate, reactant ratio and temperature, the selectivity of the brominated secondary allyl was controlled to 97%, higher than the industrial samples. Since we focused on the selectivity of BIIR-1 from the substitution reaction in this study, the addition reaction generated BIIR-3 was less discussed. However, experimental results also showed very interesting kinetics of the addition reaction, whose mechanism behind phenomena is still unclear. Further studies on an advanced microreaction system with higher mixing performance and much shorter reaction time will be considered to in-depth exhibit the laws of the addition reaction. Acknowledgements We gratefully acknowledge the financial support from National Key R&D Program of China (2017YFB0307102), National Natural Science Foundation of China (91334201) and Tsinghua University Initiative Scientific Research Program (20151080361). Supporting Information The Supporting Information illustrates the repeatability of the 1H-NMR spectra and the viscosity of IIR solution. Notations E

HBr extraction ratio, %



KHBr

partition coefficient of HBr between water and n-hexane

Vo

volume of the organic phase

Vw

volume of the water phase

Mn

Number average molecular weight, kD

Mn0

Number average molecular weight of reactant rubber, kD

nBIIR-1

molar flow rate of brominated secondary allylic group, mol/mim

nBIIR-2

molar flow rate of brominated primary allylic group, mol/mim

nBIIR-3

molar flow rate of di-bromo product group, mol/mim

nBr2

molar flow rate of bromine, mol/mim

nIIR

molar flow rate of 2-methyl-butene group in product, mol/mim

nIIR,0

molar flow rate of 2-methyl-butene group in reactant, mol/mim

PDI

Polydispersity index of molecular weight

PDI0

Polydispersity index of reactant rubber molecular weight

QBr2

Volume flow rate of Br2 solution, mL/min

Qrubber

Volume flow rate of butyl rubber solution, mL/min

QH2O

Volume flow rate of H2O, mL/min

QT

Total flow rate, mL/min

S

Selectivity of BIIR-1 in total BIIR-1 and BIIR-2, %

ST

Selectivity of BIIR-1 in all products of BIIR-1 2 and 3, %

T

Reaction temperature, ºC


Page 24 of 33

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U0

molar percentage of 2-methyl-butene unit in polymer chain, %

w0

Mass concentration of butyl rubber in reactant solution, wt%

wBr2

Mass concentration of Br2 in reactant solution, wt%

X

Conversion of 2-methyl-butene group, %

YBIIR-1

Yield of BIIR-1 group, %

YBIIR-2


YBIIR-3


τ

Residence time in reaction system, min

References (1) Vukov R. Halogenation of butyl rubber-a model-compound approach. Rubber Chem. Technol. 1984, 57(2):275-283. (2) Chu, C. Y., Watson, K. N., Vukov R. Determination of the structure of chlorobutyl and bromobutyl rubber by NMR-spectroscopy. Rubber Chem. Technol. 1987, 60(4):636-646. (3) Malmberg, S. M., Parent, J. S., Pratt,

D. A., Whitney, R. A. Isomerization and elimination

reactions of brominated poly(isobutylene-co-isoprene). Macromolecules. 2010, 43(20):8456-8461. (4) Pazur, R. J., Petrov, I. The thermo-oxidation of chlorinated and brominated isobutylene-coisoprene polymers: Activation energies and reactions from room temperature to 100 ºC. Polym. Degrad. Stab. 2015, 121:311-320. (5) Sathi, S. G., Jang, J. Y., Jeong, K. U., Nah, C. Thermally stable bromobutyl rubber with a high crosslinking density based on a 4,4′-bismaleimidodiphenylmethane curing agent. Journal of



Applied Polymer Science. 2016, 133(42):1-14. (6) Jipa, S.; Giurginca, M.; Setnescu, T.; Setnescu, R.; Ivan, G.; Mihalcea, I. Thermo-oxidative behaviour of halobutyl and butyl elastomers. Polym. Degrad. Stab. 1996, 54:1-6. (7) Parker, P. T.; Rouge, B.; L. Bryan, J.; B. Small, A.; Pugh, D. W.; J. Buchmann, F.; Rouge, B. Continuous Chlorination and Bromination of Butyl Rubber. U. S. Patent 3,099,644. July 30, 1963. (8) Wang, W.; Zou, H. K.; Chu, G. W.; Weng, Z.; Chen, J. F. Bromination of butyl rubber in rotating packed bed reactor. Chem. Eng. J. 2014, 240:503-508. (9) Baade, W.; Konigshofen, H.; Kaszas, G. Process for the bromination of alkyl rubbers. U. S. Patent 5,569,723. October 29, 1996. (10) Chu, Y.C.; Vukov, R. Determination of the structure of butyl rubber by NMR spectroscopy. Macromolecules. 1985, 18:1423-1430. (11) Gardner, I. J.; Fusco, J. V.; Newman, N. F.; Kowalski, R. C.; Davis, W. M.; Baldwin, F. P. Halogenated butyl rubber. U. S. Patent 4,703,091. October 27, 1987. (12) Kaszas, G. Bromination of butyl rubber in the presence of electrophilic solvents and oxidizing agents. Rubber Chem. Technol. 2000, 73(2), 356-365. (13) Kuntz, I.; Park, R.; P. Baldwin, F.; Summit; M. Thomas, R.; Mountainside; E. Serniuk, G.; Roselle. Nitrogen cured halogenated butyl rubber compositions. U. S. Patent 3,104,325. September 17, 1963. (14) W. Powers, K.; C. Wang, H.; N. Webb, R.; V. Fusco, J.; F. Vanbrackle, H.; F. McDonald, M. Halogenation of star-branched butyl rubber with improved neutralization. U. S. Patent 5,286,804.


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February 15, 1994. (15) Shan B. Preparation of bromobutyl rubber and characterization of its structure and property (in Chinese). Beijing: Chemical Engineering and Technology, Beijing University of Chemical Technology, 2010. (16) Wang, W.; Zou, H. K.; Chu, G. W.; Xiang, Y.; Peng, H.; Chen, J. F. Effects of Assistant Solvents and Mixing Intensity on the Bromination Process of Butyl Rubber. Chin. J. Chem. Eng. 2014, 22(4):398-404. (17) Zhang, Y.; Li, S. X.; Guo, W. L.; Deng, Z. W. The effects of bromination condition on the microstructure of brominated butyl rubber (in Chinese). Journal of Petrochemical Universities. 2010, 23(1):27-29. (18) Fanelli, F.; Parisi, G.; Degennaro, L.; Luisi, R. Contribution of microreactor technology and flow chemistry to the development of green and sustainable synthesis. Beilstein J. Org. Chem. 2017, 13:520-542. (19) Wiles, C.; Watts, P. Recent advances in micro reaction technology. Chem. Commun. 2011, 47(23):6512-6535. (20) Wang, K.; Zhang, J. S.; Zheng, C.; Dong, C.; Lu, Y. C.; Luo, G. S. A consecutive microreactor system for the synthesis of caprolactam with high selectivity. AIChE J. 2015, 61(6):1959-1967. (21) Abdollahi, A.; Sharma, R. N.; Vatani, A. Fluid flow and heat transfer of liquid-liquid two phase flow in microchannels: A review. International Communications in Heat and Mass Transfer. 2017, 84:66-74.



(22) Qiu, L.; Wang, K.; Zhu, S.; Lu, Y. C.; Luo, G. S. Kinetics study of acrylic acid polymerization with a microreactor platform. Chem. Eng. J. 2016, 284:233-239. (23) Wang, P. J.; Wang, K.; Zhang, J. S.; Luo, G. S. Kinetic study of reactions of aniline and benzoyl chloride in a microstructured chemical system. AIChE J. 2015, 61(11):3804-3811. (24) Xie, P.; Wang, K.; Wang, P.; Xia, Y.; Luo, G. S. Synthesizing bromobutyl rubber by a microreactor system. AIChE J. 2017, 63(3):1002-1009. (25) Wiles, C.; Watts, P. Continuous flow reactors: a perspective. Green Chem. 2012, 14(1):38-54. (26) Yoshida, J.; Kim, H.; Nagaki, A. Green and Sustainable Chemical Synthesis Using Flow Microreactors. Chemsuschem. 2011, 4(3):331-340. (27) Zuidhof, K. T.; Croon, M.; Schouten, J. C. Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam in microreactors. AIChE J. 2009, 56(5): 1297-1304. (28) Wang, X. G.; Jiao, H. Y.; Liu, X. L.; Peng, R. P.; Hu, K. F.; Liu, Z. Q.; Wang, C. L.; Xu, H. Y.; Pan, G. Q. Controling structure and mechanical properties of brominated butyl rubber. China Elastomerics. 2015, 25(3):24-28. (29) Lin, X. Y.; Yan, S.; Zhou, B. Y.; Wang, K.; Zhang, J. S.; Luo, G. S. Highly efficient synthesis of polyvinyl butyral (PVB) using a membrane dispersion microreactor system and recycling reaction technology. Green Chem. 2017, 19(9):2155-2163. (30) Lin, X. Y.; Wang, K.; Zhang, J. S.; Luo, G. S. Process Intensification of the Synthesis of Poly(vinyl butyral) Using a Microstructured Chemical System. Ind. Eng. Chem. Res. 2015, 54(14), 3582-3588.


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(31) Xu, J. H.; Tan, J.; Li, S. W.; Luo, G. S. Enhancement of mass transfer performance of liquidliquid system by droplet flow in microchannels. Chem. Eng. J. 2008, 141(1-3):242-249.



For Table of Contents Only


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Prof. Guangsheng Luo is Cheung Kong distinguished professor and Head of State Key Laboratory of Chemical Engineering. He received his Ph.D. and B.Sc. degrees in 1993 and 1988, respectively, both from Tsinghua University. His research interests include microstructured chemical systems, separation science and technology, and functional materials. He has published more than 300 papers in peer-reviewed journals and holds more than 100 Chinese patents. He was awarded the National Science Fund for Distinguished Young Scholars and he is the recipient of several awards, including the second prize of China State Technological Invention Award. His is also a Fellow of the Royal Society of Chemistry.



Dr. Kai Wang obtained his Ph. D degree and worked as a 2-year post doctor in Department of Chemical Engineering Tsinghua University, Beijing, China. During 2015 to 2016, he worked as a visiting scholar in Department of Chemical Engineering, Massachusetts Institute of Technology. He is now working as an associate professor in the State Key Laboratory of Chemical Engineering, Department of Chemical Engineering Tsinghua University. His research interest is flow chemistry principle and technology, especially for syntheses of organic chemicals, polymers and nano-particles by using microreactor. He has published more than 40 research papers as first and corresponding authors and revised more than 30 Chinese patents.


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Miss Pei Xie is a Ph. D. candidate in Department of Chemical Engineering Tsinghua University. She received her Bachelor degree from Xiamen University in 2014. Her research topic mainly focuses on microreaction technology and bromination process. She has published 4 research papers and applied for 3 Chinese patents.


In Situ Removal of HBr via Microdroplets for High Selectivity

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