Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
pubs.acs.org/IECR
Calcium Stearate as an Acid Scavenger for Synthesizing High Concentrations of Bromobutyl Rubber in a Microreactor System Pei Xie, Kai Wang, and Guangsheng Luo* The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: The preparation of bromobutyl rubber (BIIR) faces two challenges, the high viscosity of the reaction system and the accumulation of generated HBr during the reaction, which result in lower productivity, higher energy consumption, and lower selectivity for BIIR-1. In this work, we developed an efficiently intensified technology using calcium stearate as the acid scavenger and a microreactor as the reaction platform. We successfully realized both high productivity and high selectivity for synthesizing high concentrations of bromobutyl rubber. The optimized results indicate that using predispersed calcium stearate in a butyl rubber (IIR) solution (xCa of about 0.8 wt %) as the acid scavenger could guarantee sufficiently high selectivity for BIIR-1 (w of almost 95%). A T-junction microreactor with slitlike microchannels is suitable for mixing a high-viscosity IIR solution and a Br2 solution. As the concentration of IIR increases from 10 wt % to 15 wt %, the conversion of IIR could increase by 13%, and the residence time could be shortened to less than 1 min. The utility of the reactants and the reaction efficiency have both been largely improved.
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INTRODUCTION Bromobutyl rubber is a butyl rubber (IIR) derivative containing active bromine.1 Compared with IIR, it has many other beneficial properties, such as, for instance, enhanced vulcanizing properties, the ability to more easily blend with other rubbers, increased adhesion performance, and a more excellent thermal stability.2−4 With these advantages, BIIR becomes an indispensable and irreplaceable material for manufacturing tubeless tires, medicalgrade pharmaceuticals, and so forth.5−7 BIIR is usually prepared by brominating IIR in n-hexane rather than by extruding IIR to react with bromine in a solid state.8−10 The bromination reaction only occurs in the 1,4-isoprene units, and the reaction route can be seen in Scheme 1. According to many studies,8,11−15 in this bromination process, IIR produces predominantly substitution products (BIIR-1 and BIIR-2) with a small amount of addition product (BIIR-3). This phenomenon could be explained by considering its intermediate, a bromonium ion of the type B+. Most of the positive charge is still located on the tertiary atom. The existence of the sterically bulky methyl groups in the β position prevents the nucleophilic attack of the bromide ion. As a result, the elimination of hydrogen bromide is preferred over the addition of Br2.15 Thus, the main reaction of this bromination process should be the substitution reaction of IIR and Br2, labeled as r1 in Scheme 1. Actually, there exist two substitution products, BIIR-1 and BIIR-2. The desired target product is only the brominated secondary allylic structure, BIIR-1. Although BIIR-1 is not very stable, it can easily isomerize to BIIR-2 under the effect of HBr. This, labeled as r2 in Scheme 1, is the main side reaction © XXXX American Chemical Society
and can reduce the selectivity for the product. Of course, the addition reaction between IIR and Br2 cannot be avoided completely and is shown as r3 in Scheme 1. Indeed, there are some side reactions not displayed in Scheme 1. For example, polymer chains can easily break down under the catalysis of HBr,8,12,13 which can affect the mechanical strength of the rubber. As for the synthesis of BIIR, mainly two problems exist in the traditional process, the high viscosity of the reactant (about 260 mPa·s for a 15 wt % IIR solution) and the accumulation of HBr generated during the reaction. In a traditional tank-reactor system, IIR reacts with Br2 via strong mechanical agitation. The solution is usually not mixed very well because of the high viscosity difference and the large volume ratio between IIR and Br2. As a result, the reaction rate is limited, and a long residence time is required. Additionally, HBr seriously accumulates in this system, which can decrease the selectivity for the product. Because the addition reaction is only a small proportion of the processes, the most common parameter to evaluate the selectivity for a product in industry is the molar ratio of BIIR-1 to the sum of BIIR-1 and BIIR-2 (w). One study on the bromination of IIR in a tank reactor indicates that a 2.5 min residence time is required to get the highest selectivity for BIIR-1 Received: Revised: Accepted: Published: A
January 19, 2018 March 2, 2018 March 2, 2018 March 2, 2018 DOI: 10.1021/acs.iecr.8b00285 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Scheme 1. Mechanism of the IIR-Bromination Process
but also have no influence on the subsequent application of BIIR. Calcium stearate is a weak base and is often used as a stabilizer after bromination because it has little influence on BIIR’s eventual use.26 Thus, calcium stearate is chosen as the in situ acid scavenger. A modified T-junction microreactor is used to construct the reaction platform. The influences of varied parameters on the new technology have been investigated, and the optimized conditions for the new technology have been determined.
(w of approximately 80%), which does not meet the industrial standard very well, in which w should be larger than 90% (more detailed data from different companies can be seen in the Supporting Information).16 To control the quality of product well, the residence time must be shortened, and the micromixing efficiency must be enhanced. Until now, a lot of work has been done on stirred tanks,17 static mixers,18 rotating packed beds,19 and microreactors20 from a micromixing point of view. Recently, because of their excellent mixing and mass-transfer performances, microreactors have been widely used in many reaction processes to intensify conversion rates, yields, and selectivities.21−23 With the advantages of short diffusion distances as well as high contact interfacial areas, microreactors can also be applied to the intensification of highviscosity polymer systems.24 In our previous work, the microreactor system was successfully used for synthesizing polyvinyl butyral (PVB) from the condensation of poly(vinyl alcohol) (PVA) and n-butanal. The microreactor system provided highly effective mixing of the hydrochloric acid and PVA-n-butanal aqueous solutions with a shorter reaction time, milder reaction conditions, and better products.24,25 We also developed a microreactor system for the bromination process in our previous work. Because of the better mixing situation of IIR and Br2 in the system, the reaction rate is enhanced, and the residence time was shortened to less than 1 min. Additionally, to completely avoid the accumulation of HBr, we considered predispersing 1 wt % H2O in 10 wt % IIR to extract HBr. The generated substitution products were almost 100% the secondary allylic structure (w ∼ 100%).8 However, this in situ extraction method is connected with the mass-transfer resistance and extraction efficiency. It is not suitable for all cases, especially for higher-viscosity systems. The common concentration of IIR in industry is about as much as 15 wt %. The viscosity of such an IIR solution is approximately 260 mPa·s, which is almost 3 times the viscosity of a 10 wt % IIR solution (80 mPa·s). Therefore, a new technology that could be operated under high concentrations is highly required to intensify the process. This study focuses on developing a new intensified microreaction technology for high-concentration systems. We consider predispersing an acid scavenger into the IIR solution to neutralize HBr in situ. In this case, HBr is not available to participate in further isomerization and degradation side reactions. The chosen scavenger should not only consume HBr
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EXPERIMENTAL SECTION Materials. The IIR1 (molar percentage of the 2-methylbutene unit in the polymer chain, U0,1 = 1.75%; number-average molecular weight, Mn0,1 = 305 kDa; polydispersity index, PDI0,1 = 1.53) and IIR2 (U0,2 = 1.67%, Mn0,2 = 283 kDa, PDI0,2 = 1.84) used in this study were both provided by Cenway New Materials Company Ltd. Another reactant, 99.5% Br2, was purchased from Sinopharm Chemical Reagent Company Ltd. The solvent for this system, 98% n-hexane, was bought from Eastern Chemical Company Ltd. The neutralization reagents, 98.5% sodium hydroxide and 96% sodium metabisulfite, were acquired from J&K Scientific Company Ltd. The mobile phase used in the GPC analysis was 99.9% tetrahydrofuran from Fisher Scientific Company Ltd. The stabilizers, 95% epoxidized soya-bean oil and calcium stearate (Ca: 6.6−7.4%), were supplied by Aladdin Industrial Company Ltd. Deionized water was provided by the laboratory water-purification system from Beijing Boxinyuanyang Scientific Company Ltd. Preparation of Solutions. Almost all of the solutions used in this bromination process were prepared before the experiment. IIR, a solid polymer, was predissolved in n-hexane to form a solution, and the solution was stored for more than 2 weeks before the experiment to precipitate the macromolecular and undissolved substances. The upper and transparent solution was used for the experiments. Br2 in an n-hexane solution was prepared by mixing bromine with n-hexane quantitatively in a brown bottle. Because Br2 could react with n-hexane under light, the Br2 solution was always prepared just before the start of the experiment and had to be transferred to a polytetrafluoroethylene (PTFE)-lined pressure vessel to avoid corrosion and light completely. Sodium hydroxide and sodium metabisulfite were mixed together to form the quenching solution, which was B
DOI: 10.1021/acs.iecr.8b00285 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Experimental setup of the IIR-bromination microreactor system.
Figure 2. Detailed structure of T-junction microreactor. (a) 3D image of a T-junction microreactor of the kind used to mix the reactants. (b) Detailed structure of the T-junction microreactor.
providing a cross section with two slitlike microchannels for the IIR solution. The width of each channel, w2, is 0.4 mm, and the length, l2, is 4 mm. Our group previously studied this kind of channel, showing that with the same mass-transfer efficiency, the throughput of a slitlike device was comparable to that of a microchannel array with 300 channels.27 Therefore, this kind of structure could provide more excellent mixing performance, which might be very suitable for the IIR-bromination process. Part 4 serves as a buffer tank for the IIR solution. The width of this channel, w3, is 2 mm, and its length, l3, is 10 mm. The mixed solution then came to a coiled 1.5 m PTFE tube (2.5 mm inner diameter), which was used to further mix the IIR and Br2 solutions with a secondary flow.28 The outlet fluid was conducted into the following PTFE tubular system to continue the reaction. The inner and outer diameters of the following tubes were 4 and 6 mm, respectively. The tube lengths were changed via a three-way switching valve and a five-way switching valve, and the different tube lengths led to different residence times. We calculated the volumes of the tubes between the Tjunction microreactor and the outlet to be 13.9, 38.6, 53.5, 71.3, and 84.2 mL, respectively. Indeed, the reaction was finished in two steps: (1) the T-junction microreactor gave the first mixing, and (2) the delayed loops provided sufficient reaction time. When the device ran smoothly and the fed-in fluid was more than 3 times the reaction volume, the product solution was then injected into a stirred base solution to quench the bromination reaction. After 30 min of stirring, the BIIR solution was washed by DI water three times, and 30 min of stirring was required each time. During the washing step, a droplet of stabilizer was added into the BIIR solution to keep the product stable in air. Then, the solution sat for 12 h and was centrifuged, and a transparent BIIR solution was separated from the washed liquid. Finally, the samples were dried in a vacuum-drying oven at 35 °C for 8 h to get the solid BIIR. Analysis. To evaluate the quality of the obtained BIIR, two analysis methods, nuclear magnetic resonance (1H NMR, 600 M,
used for the reduction and neutralization of the unreacted Br2 and generated HBr, respectively. The stabilizer was a mixture of epoxidized soybean oil and calcium stearate, which was added to the oil phase in the product-washing step to stabilize the BIIR in air. Experimental Setups and Bromination Process. The IIR-bromination microreactor system is shown in Figure 1. It consists of four parts used for predispersing calcium stearate, transporting reactants, and implementing and quenching the bromination reaction. In the dispersion system, the proper amount of calcium stearate was predispersed in an IIR solution via 30 min of mechanical stirring. The predispersed IIR solution was transferred to a pressure vessel, which was actually a piston column. The IIR solution was stored in the top part, and water was fed from the bottom. The IIR-solution-volume-flow rate equaled the water-volume-flow rate, which could be controlled by a metering pump. Hence, the viscous liquid could be transported quantitatively.8 Then, it came to a 1 m coiled stainless-steel tube (3 mm inner diameter), which was mainly used for preheating the IIR solution. The preheated tube and the following reaction units were all set in a water bath to control the reaction temperature. The preheated IIR solution and the Br2 solution were first mixed via a T-junction microreactor, which initiated the whole reaction. Compared with the Br2 solution, the IIR solution had a relatively high viscosity, which meant that the IIR solution had a higher inertial force and greater turbulence energy. Therefore, the Br2 solution flowed along the straight channel, and the IIR solution came from the side channel, as shown in Figure 2a. The detailed structure of the T-junction microreactor is shown in Figure 2b, which includes five parts. Parts 1 and 5 are stainless-steel modules that supply the inlets and outlet. Parts 2−4 are all 1 mm thick PTFE plates, which avoid corrosion from HBr and Br2. These three plates have different structures and different uses. Part 2 mainly supplies the mixing channel for the two reactants. The width of this mixing channel, w1, is 2 mm, and its length, l1, is 10 mm. Part 3 is mainly used for C
DOI: 10.1021/acs.iecr.8b00285 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 3. Chemical shifts of the 1H peaks and 1H NMR spectra of the product. (a) 1H NMR spectra of the product. (b) Chemical shifts of the 1H peaks in BIIR-1, BIIR-2, and IIR.
spectra and the chemical shifts of the 1H peaks in the product, respectively. According to the 1H NMR spectra of the products, some important parameters, such as the conversion rate of IIR (Xconv), the yield of BIIR-1 (YBIIR‑1), the yield of BIIR-2 (YBIIR‑2), the yield of BIIR-3 (YBIIR‑3), the selectivity for BIIR-1 (SBIIR‑1), the selectivity for BIIR-2 (SBIIR‑2), and the molar ratio of BIIR-1 to the sum of BIIR-1 and BIIR-2 (w), could be calculated using eqs 1−7:
Bruker) and gel-permeation chromatograph (GPC, Waters 1515), were used in this study. An NMR spectrometer can characterize the chemical structures of halobutyl rubbers, which was already reported in a 1987 study.29 Recently, 1H NMR spectra have become the main means to evaluate the quality of BIIR quantitatively, because its standard difference can be controlled within 1.5% (detailed data can be seen in the Supporting Information). Figure 3a,b shows the 1H NMR D
DOI: 10.1021/acs.iecr.8b00285 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 4. Comparison of two IIR-solution concentrations. (a) Conversion rate of IIR versus the residence time. (b) Molar ratio of BIIR to the sum of BIIR-1 and BIIR-2 versus the residence time. Low-concentration experimental conditions: xIIR = 10.25 wt %, QIIR = 90 mL/min, xBr2 = 4.89 wt %, QBr2 = 9 mL/min, T = 30 °C. High-concentration experimental conditions: xIIR = 14.73 wt %, QIIR = 90 mL/min, xBr2 = 7.39 wt %, QBr2 = 9 mL/min, T = 30 °C. The measuring error of NMR is
