Improved Catalytic Lifetime of H2SO4 for Isobutane Alkylation with

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Improved Catalytic Lifetime of H2SO4 for Isobutane Alkylation with Trace Amount of Ionic Liquids Buffer Qian Huang,†,‡ Guoying Zhao,*,† Suojiang Zhang,*,† and Feifei Yang† †

Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China ‡ College of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, P.R. China S Supporting Information *

ABSTRACT: Trace amounts of ionic liquids have been mixed in sulfuric acid to enhance the catalytic performance for the alkylation of isobutane with butene. The experimental results from batch reactors indicated that the reaction efficiency was significantly improved. The effective catalytic lifetime of concentrated H2SO4 mixed with [Bmim][SbF6] was twice compared with pure H2SO4. Under the optimal conditions, the alkylate research octane (RON) reached 98, and the selectivity of C8 was 90%. The ionic liquids with SbF6 anion worked similar to buffer agents, which were in favor of keeping the acid strength of catalytic system, slowing the growth of acid soluble oil, and reducing acid consumption. In conclusion, the new catalytic system of acid and trace amounts of ionic liquids is very promising to substitute the old catalytic system of concentrated H2SO4 alone for the alkylation. tris(n-alkyl) phosphoric triamide,14,15 sulfoamide,16 and trifluoromethanesulfonic acid17 have been proposed as effective additives. Wenshing Chen and co-workers have proved N,N′dimethyl-1,4-phenylenediamine and naphthalenesulfonic acid can work as better additives.18 Most of the reported additives functioned as surfactants to enhance the interfacial area of sulfuric acid/hydrocarbon dispersion and thus alkylate qualities. Other functional additives have rarely been reported. It is well-known that the alkylation of isobutane with C3−C5 olefins with strong acid as catalyst is generally accompanied by many side reactions that all deteriorate the product. The acid strength and composition play a key role in maximizing the desired trimethylpentane (TMP) yield.19 Butylene and amylene feeds produce their best alkylate in the range 94−91 wt % H2SO4.20 Lower acidity of the catalyst leads to oligomerization and generation of red oil, which inactivates the catalyst. Higher acidity of the catalyst promotes product cracking and increases the light-end production. Therefore, the optimum acid strength of the catalyst should be maintained to produce the best alkylate product and reduce the acid consumption. However, for isobutane alkylation, small amounts of acid soluble oil (ASO) in the feed-stream decreased the acid strength of the catalyst dramatically, and the presence of the toxicants finally resulted in the catalyst deactivation. For instance, the acidity of triflic acid decreased one unit with only 0.03 ASO mole fraction increased.21 The Hammett acidity function of sulfuric acid changed from −10.0 to −9.34 as the mass fraction of H2SO4 changed from 96.7% to 92.1%.22 A buffering agent is widely used to resistant the PH change of the solution in many other chemical applications, including food

1. INTRODUCTION Alkylate oil, the products of isobutane/butene alkylation, are the ideal high-octane blending components of gasoline due to their low vapor pressure as well as low contents of aromatics, alkenes, and sulfur.1,2 More and more strict environmental protection requirement and growing market space for high quality and environmental friendly gasoline lead to increasing demand of alkylate oil. Concentrated sulfuric acid (H2SO4) is the most dominant commercial catalyst for the current industrial isobutane alkylation process. However, the drawbacks like the considerable consumption of H2SO4 and huge severe environmental pollution have limited the wide application of sulfuric acid alkylation technology.3,4 Several advanced liquid catalysts, such as chloroaluminate ionic liquids,5 binary mixture of TFOH, ionic liquids, and so on,6,7 have been tested as potential alternatives for isobutane alkylation because of their better performances in lab-bench experiments. However, the higher cost may weaken their industrial application in near future. Therefore, there are much drives to improve the current sulfuric acid alkylation technology. Several approaches have been investigated to improve the quality of alkylate and reduce sulfuric acid consumption during the last 60 years. On the one hand, new technologies for the reaction zone, including the improvement and redesign of alkylation reactor to enhance liquid−liquid dispersion and stabilize reaction temperature, have been applied by the related engineers to further optimize the alkylation system.8 Moreover, feed-stream purifying, optimization of the varied operating conditions for different olefins, regulation of isoparaffin/olefin ratio, temperature, the feeding speed ratio of acid to hydrocarbon, and so on also have been studied intensively.9−11 On the other hand, some additives have been found and industrialized to enhance the performance of sulfuric acid catalyst. Cities Service Oil Co. proposed dodecylbenzenesulfonic acid and p-phenylenediamine as additives.12,13 N,N′,N″© 2015 American Chemical Society

Received: Revised: Accepted: Published: 1464

October 21, 2014 January 16, 2015 January 18, 2015 January 19, 2015 DOI: 10.1021/ie504163h Ind. Eng. Chem. Res. 2015, 54, 1464−1469

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Industrial & Engineering Chemistry Research

temperature. UV spectra were recorded on a SHIMADZU UV2550 UV spectrophotometer. 2.2. Ionic Liquid Synthesis. 1-Butyl-3-methyl-imidazolium hexafluoroantimonate ([Bmim][SbF6]) was prepared according to the following process: NaSbF6 was added to the solution of [Bmim]Cl in dichloromethane and stirred for 24 h. The suspension liquid was filtered to remove the precipitated chloride salt, and the filtrate was washed repeatedly with water until no precipitation of AgCl appeared in the aqueous phase titrated with standard AgNO3 solution. Then, the organic phase was washed with additional two times with water in addition to ensure removal of all the chloride salt completely. The solvent was removed by rotary evaporation and then dried at 60 °C in vacuum for 24 h until no visible signs of water were present in the IR spectrum. The remaining solution was ionic liquid. [Bmim][SbF6] was detected by 1H NMR (JEOL ECA-600 NMR), ESI/MS (Bruker microTOF Q). 1H NMR (600 MHz, DMSO, 25 °C), δ = 0.86 (t, 3H), 1.23 (m, 2H), 1.73 (m, 2H), 3.80 (s, 3H), 4.11 (t, 2H), 7.64 (s, 1H), 7.70 (s, 1H), 9.05 (s, 1H). ESI/MS: m/z (+) 139.1, m/z (−) 234.9. 2.3. Alkylation Reactions. Alkylation reactions took place in a 200 mL autoclave equipped with a cooling groove, as described in literature.6 A two-stage propellant stirrer with three paddles on the shaft provided the effective agitation. In a typical procedure, the appropriate amount of catalysts was put into the reactor which was purged with nitrogen for three times and pressured with nitrogen up to 0.4 MPa. When the reactor was equilibrated to the desired temperature, a previously prepared isobutane/butene mixture (50 mL, mole ratio was 8.5:1) was then introduced into the stirred reactor (1000 r/min) using precision metering pump at given flow rate. Then, the reaction mixture was stirred for another 10 min at the desired temperature under nitrogen. After the reaction completed, the hydrocarbon phase was separated from the acid and washed several times with NaHCO3 saturated solution. For recycling studies, the catalyst was put back into the reactor and mixed with fresh C4 feed, and then, the procedures were repeated. The alkylate product was qualitatively analyzed by the HP 6890/5975 gas chromatography/mass spectrometry (GC/MS) system. Furthermore, alkylate product was quantitatively analyzed by a gas chromatograph (SHIMADZU GC 2014) equipped with a capillary column (DB-Petro,100 m × 0.25 mm) and a flame ionization detector. The analysis conditions were as follows: split ratio = 100:1, injector temperature = 250 °C, detector temperature = 280 °C, carrier gas flow rate = 1 mL/min. The temperature program for GC analysis was as follows: initial column temperature 40 °C, 2 °C/min to 100 °C, then 25 °C/min to 250 °C hold for 20 min. Nitrogen was used as carrier gas. The research octane number (RON) of alkylate was calculated according to the method detailed previously.5 2.4. Acidity Determination. The Hammett acidity of catalyst system was determined by 13C-NMR according to the literature.22,31,32 In a typical procedure, various amount of acidity indicator (mesityloxide,α, β-unsaturated ketone) was added to the recycled catalyst to make three samples of different concentration varied between 0.05 and 0.8 mol/L. All the liquids were carefully shaken to ensure a full dissolution of the indicators. Subsequently, the sample was loaded into a NMR tube assembled with another coaxial tube, which was filled with tetramethylsilane as an internal standard. The 13C NMR spectra were acquired at 300 K on a Bruker Avance III 600 MHz spectrometer. The chemical shift differences of Cß and Cα (Δδ) in the mesityloxide molecular at three varied

processing, biochemistry, medicine, and so on. Thus, it would be better to resist the acidity change and improve the efficiency of sulfuric acid during the reaction process by using some additive as a buffer agent. Nowadays, ionic liquids have been widely investigated in many fields such as catalytic reactions, materials, electrochemistry, and polymerization due to their special property.23−26 They have the advantages of low-vapor pressure, nonflammability, a wide liquid-phase temperature range, and, more importantly, good designability and functional diversity, which are believed as a potential platform for many industrial process transformations.26 In addition, the ionic liquids as potential alternative catalyst or cocatalyst for isobutane alkylation have been explored a lot more.27−29 The ionic liquids exhibited excellent catalytic performance for isobutane alkylation in that they can improve the acid strength, acid composition, solubility, interfacial properties between acid and hydrocarbon, and so on. Especially, we found the composite Brønsted acidic ionic liquids composed of 1-alkyl-3-methylimidazolium hexafluoroantimonate ([Cnmim][SbF6], n = 4, 6, or 8) and trifluoromethanesulfonic acid (TFSA) catalyzed the isobutane alkylation efficiently in a wide concentration range of TFSA.29 In follow-up work,30 the acidity of a series of composite Brønsted acidic ionic liquids was determined by using 13C NMR with an unsaturated acetone as molecular probe. A far slower acidity change with the TFSA concentration was observed with the ionic liquids containing SbF6 anion than others such as BF4−, HSO4−, and so on. These results indicated that the ionic liquids with SbF6 anion may be mixed with sulfuric acid and function as a buffer agent to alleviate the acidity change of sulfuric acid during the isobutane alkylation. Herein, several kinds of ionic liquids including [Bmim][SbF6] were investigated as buffer agent for sulfuric acid in isobutane alkylation. The catalytic performances of different catalyst system for isobutane alkylation were tested and compared at the same reaction conditions. Then, lifetime tests of pure sulfuric acid catalyst system and H2SO4/ [Bmim][SbF6] catalyst system were studied respectively at the optimized operating condition. The mechanism of ionic liquids buffer was also discussed in this work. All the results indicated that the ionic liquids with SbF6 anion worked similar to buffer agents, which resist the acidity changes of catalytic system, slower the growth of acid soluble oil.

2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. All chemicals (AR grade) were purchased commercially and were used as received unless otherwise noted. Sulfuric acid (A.R 95%−98%) was purchased from Beijing Chemical Works. Isobutane/butene (8.5:1) was purchased from LvLingGas Co., Ltd. The butene used in the experiment is a mixture of 1-butene (36%), 2-butene (62%), and isobutene (2%). Table S1, a supplementary table of components, has been added in Supporting Information. NaSbF6 was purchased from Aladdin Chemistry Co. Ltd. [Bmim][SbF6] was synthesized in lab. The other ionic liquids were purchased from the Linzhou Keneng Material Technology Co., Ltd. 1 H NMR spectra were measured with a JEOL ECA-600 (600 MHz) spectrometer. 13C NMR spectra were measured with a Bruker Avance III (600 MHz) spectrometer. Mass spectra (MS) in ESI mode were recorded on a Bruker microTOF-QII spectrometer. Infrared (IR) spectra were recorded on a Thermo Nicolet 380 spectrometer using KBr pellets at room 1465

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Industrial & Engineering Chemistry Research concentrations were used to determine the infinite dilution (Δδ0) by exploration. The Hammett acidity (H0) could be calculated through the equations as reported in literature.32 2.5. Corrosion Tests. According to national standards of the corrosion resistance reported,33,34 we have designed this experiment. Three different materials (carbon steel, 304 stainless steel, 316 stainless steel) were polished using 400 mesh Roving paper first and then scrubbed by ethanol and acetone. After being weighed and dried, they were immersed in 180 mL catalyst, respectively, for 30 days at room temperature under atmospheric condition. After this process, the materials were scrubbed, dried and weighed. The formula of corrosion rate is R = 87600 × (M − M1)/(S × T × D). In which R = corrosion rate, mm/a M = quality of the sample before test, g M1 = quality of the sample after test, g S = surface area of the sample cm2 T = test time, h D = density of the sample, g/cm3

Figure 1. Effect of the mass ratio of ionic liquids to sulfuric acid on the alkylate composition.

3. RESULTS AND DISCUSSION 3.1. Various Ionic Liquids as Catalyst Additives for Alkylation. In this experiment, 40:50 is the optimum value of acid (H2SO4)/hydrocarbon volume ratio. The stirring speed was fixed on 1000 r/min considering the energy consumption and catalytic effect. More details such as determining the optimize acid/hydrocarbon volume ratio and stirring speed were shown in Supporting Information, Figures S1 and S2. Several kinds of ionic liquids (0.5 wt % in the H2SO4) were investigated as promoters for alkylation at appropriate conditions. The results are listed in Table 1. The catalytic

3.3. Regulation of Feed Rate. The RON of alkylate obtained in our previous work was relatively low. Various parameters (acid/hydrocarbon volume ratio, stirring speed) were optimized to increase RON in laboratory. After considerable experimentation, we reached the conclusion that feed rate had considerable effect on alkylate quality. The results from Table 2 exhibited the RON was raised from 91.8 to 97.6, Table 2. Effect of the Feed Rate on the Alkylate Compositiona

Table 1. Various Ionic Liquids as Additives for Alkylationa ILs (0.5 wt %)

alkylate composition (%)

alkylate composition (%)

(total vol. 40 mL)

C5−C7

C8

C9+

TMPs

RON

none BmimSbF6 BmimPF6 BmimBF4 BmimCF3SO3 N2222PF6 N2222BF4 N2224SbF6 N2224PF6

13.78 10.28 12.55 13.96 12.82 12.72 13.37 13.09 10.08

58.38 62.97 60.94 59.08 58.78 60.61 59.34 59.00 61.44

27.84 26.76 26.52 26.96 28.40 26.67 27.30 27.92 28.48

49.01 54.13 51.65 50.06 49.95 51.67 50.20 49.56 51.56

91.7 92.4 92.1 92.0 91.8 92.2 91.9 91.8 91.8

feed rate (mL/h)

C5−C7

C8

C9+

TMPs

RON

max temp. (°C)

500 400 300 200 150 100 50

13.09 12.92 11.46 9.49 9.95 6.04 4.83

59.00 63.81 74.15 76.33 78.57 87.92 88.60

27.92 23.28 14.40 14.18 11.48 6.05 6.56

49.56 54.46 58.55 66.88 69.52 79.78 80.85

91.8 92.8 93.4 94.9 95.6 97.6 97.8

10.5 9.8 8.5 7.5 6.9 6.3 6.0

a

Conditions: reaction time 10 min, stirring rate 1000 r/min, 40 mL catalyst, 50 mL C4 feed, mass ratio of ILs/H2SO4 0.5 wt %.

a

Conditions: reaction time 10 min, stirring rate 1000 r/min, 40 mL catalyst, 50 mL C4 feed, feed rate 500 mL/h, mass ratio of ILs/H2SO4 0.5 wt %.

when the feed rate decreased from 500 mL/h to 100 mL/h. During the reaction, higher feed speed resulted in higher heat release within a short time and higher local temperature because isobutane alkylation is a highly exothermic rapid reaction. In this case, olefin polymerization increased, byproducts increased, and the quality of products declined. In contrast, lower feed speed resulted in the complete removal of reaction heat and thus nearly no rise in local temperature. Therefore, the lower feed speed lead to the alkylate products with better quality. There was no significant improvement when feed rate changed from 100 to 50 mL/h and extra time was wasted on feeding, which indicated that the feed rate lower than 100 mL/h was unnecessary, so the feed rate was fixed on 100 mL/h in later experiments. 3.4. Lifetime Test for Catalytic Systems under the Optimal Feed Rate. Based on the optimal feed rate (100 mL/ h) lifetime tests were carried out to study the reusability of

performance of catalyst was enhanced with the addition of liquids, which led to a higher selectivity of C8 and TMP (trimethylpentane), higher alkylate RON, and lower selectivity of C9+. Among these ionic liquids, [Bmim][SbF6] showed best performance. 3.2. Mass Ratio of Ionic Liquids to Sulfuric Acid. Different amounts of ionic liquid [Bmim][SbF6] were mixed with sulfuric acid to catalyze the isobutane alkylation. Figure 1 shows the result. The performance of H2SO4/[Bmim][SbF6] was better than that of sulfuric acid alone obviously. The selectivity of C8 and the value of RON reached a maximum when added 0.5 wt % additive. The optimal mass ratio of ILs/ H2SO4 (0.5 wt %) was applied in follow-up experiments. 1466

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Industrial & Engineering Chemistry Research H2SO4 and H2SO4/[Bmim][SbF6]. The results shown in Figure 2 indicated that ionic liquid [Bmim][SbF6], which

Figure 4. ESI-MS spectra of washed alkylation product catalyzed by H2SO4/BmimSbF6.

liquid in alkylation product was so tiny that anion SbF6−, which should have two peaks at 234.9 and 236.9 m/z, could not be detected. 3.5. Function Mechanism of Ionic Liquid Additive. The unexpected results of lifetime tests aroused our interest. A series of experiments were designed to reveal the function mechanism of ionic liquid additive. The trend of Hammett acidity of two kinds of catalyst systems, concentrated sulfuric acid and concentrated sulfuric acid added [Bmim][SbF6] (0.5 wt %), was determined by 13C- NMR after every cycle according to the literatures.23,32,33 The results were shown in Supporting Information Table S2 and Figure 5. The Hammett acidity of catalysts after first cycle

Figure 2. Lifetime test for catalyst systems.

mixed in the traditional catalyst H2SO4 as additive, showed excellent performance. The RON, the selectivity of C8 and C9+ were all improved, as well as the catalyst lifetime. The catalyst contained [Bmim][SbF6] could be reused for at least 52 times, and the RON, the selectivity of the C8 were still high. In contrast, the catalyst of sulfuric acid only could be reused for 28 times before the activity of the catalyst decreased and the alkylate quality worsen irreversibly. After 28 times, the catalyst tended to deactivate, which leads to RON declining and C9+ increasing. Before catalysts became obvious inactive, these two kinds of catalyst systems produced 120 g alkylate (cumulative amount for 28 times) and 228 g alkylate (cumulative amount for 52 times), respectively. In addition, the result of lifetime test under 500 mL/h feed rate, shown in Supporting Information Figure S3, also demonstrated the reusability of H2SO4/ [Bmim][SbF6] was better than that of H2SO4. In other words, the service lifetime of H2SO4 based catalytic system was prolonged when mixed with trace amount of [Bmim][SbF6]. The total produced alkylate were also enhanced. These improvements indicated the added environmental benefits and economic benefits of the current catalytic system. The residual quantity of ionic liquid in alkylation product was measured using ESI-MS spectra. The ESI-MS spectra of products unwashed and washed showed in Figures 3 and 4, respectively. The SO42− was detected at 96.96 m/z under the anionic pattern, and the intensity was declined after washed by NaHCO3 saturated solution. The residual quantity of ionic

Figure 5. Trend of Hammett acidity for catalyst systems.

was higher than fresh catalysts, because tiny H2O would help activating the catalysts. During the reaction the acidity of H2SO4/[Bmim][SbF6] had a milder downward trend than H2SO4 alone. After the second cycle, the acidity of H2SO4/ [Bmim][SbF6] even surpassed the acidity of H2SO4. Sulfuric acid strength can be maintained after adding [Bmim][SbF6], and therefore, the catalyst lifetime was prolonged. Through these comparisons, it was clear that ionic liquids such as [Bmim][SbF6] could work as buffer in C4 alkylation to help keeping the acidity. The color of catalyst changed from light to dark during the reaction progress. It was found that the color of H2SO4/ [Bmim][SbF6] catalyst system changed more slowly than H2SO4 catalyst system. It can be seen from Figure S4, in

Figure 3. ESI-MS spectra of unwashed alkylation product catalyzed by H2SO4/BmimSbF6. 1467

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stainless steel. The 316 stainless steel had the minimum corrosion rate. The results of corrosion test supports the feasibility of H2SO4/[Bmim][SbF6] catalyst system, because the corrosion rate was not increase after adding ionic liquids.

Supporting Information. UV spectra were used to explain this phenomenon further. After every cycle, the same amount of catalyst was taken out for UV detection at same dilution ratio with water. The results are shown in Figures 6 and 7. It is

4. CONCLUSIONS In this work, trace amounts of ionic liquids have been mixed in sulfuric acid to enhance the catalytic performance of isotbuane alkylation. The optimal conditions of isobutane/butene alkylation in 200 mL autoclave were studied. The optimal reaction time, stirring rate, acid/hydrocarbon volume ratio, feed rate, and mass ratio of ILs/H2SO4 were 10 min, 1000 r/min, 40 mL/50 mL, 100 mL/h, 0.5 wt %, respectively. Under these optimal reaction conditions, the yields of C8, TMP, and RON were 90.07%, 82.03%, and 98.00, respectively. In addition, all the results indicated that the [Bmim][SbF6] significantly improved the catalytic performance of sulfuric acid. First, [Bmim][SbF6] was helpful to improve the selectivity of trimethylpentane (TMP), the selectivity of C8 and C9+, and the research octane number (RON). Second, functioning as buffer, it resisted the acidity change of the catalytic system, slowed the production of acid-soluble oil, and prolonged the lifetime of H2SO4 up to double its original one. In summary, adding [Bmim][SbF6] can increase catalyst utilization ratio and thus reduce the consumption of sulfuric acid. In flow-up investigation, the changes in acidity and UV-spectra of catalyst could prove the buffer function of [Bmim][SbF6]. The results of corrosion test and ionic liquid residual quantity test also support the feasibility of H2SO4/[Bmim][SbF6] catalyst system. Based on our research, we believe that H2SO4/[Bmim][SbF6] with better environmental and economic benefits is a particularly promising catalyst system for isobutane alkylation reactions.

Figure 6. UV spectra of recycled catalyst system (H2SO4).

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ASSOCIATED CONTENT

S Supporting Information *

The composition of isobutane/butene mixture, the detailed results of optimizing acid/hydrocarbon volume ratio and stirring speed, the result of lifetime test under 500 mL/h feed rate, the detailed results of Hammett acidity, color change of catalyst systems and corrosion rate. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 7. UV spectra of recycled catalyst system (H2SO4/BmimSbF6).

AUTHOR INFORMATION

Corresponding Authors

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

obvious that the absorbance ranging from 220 to 260 nm of H2SO4 catalyst system increased more rapidly than that of H2SO4/[Bmim][SbF6] during the reactions. The reason is the structure of acid soluble oil was more complicated and the production rate of acid soluble oil was faster in the H2SO4 catalyst system. Acid-soluble oil, whose characteristic peaks range from 220 to 260 nm, would ruin catalytic activity considerably.35 It is proposed that ionic liquid added in H2SO4 could suppress the formation of acid soluble oil in catalyst and therefore help keeping the acid strength as buffer. 3.6. Corrosion Test of Different Materials in Catalysts. The corrosion rate in H2SO4/[Bmim][SbF6] and H2SO4 were similar, as shown in Supporting Information Table S3. The corrosion rate of different materials in H2SO4 had the same changing tendency as they did in H2SO4/[Bmim][SbF6]. The corrosion of carbon steel was more serious than that of 304

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (No. 2015CB251401), National Natural Science of Foundation of China (No. 51374193), Beijing Municipal Natural Science Foundation (No. 2122052), and International S&T Cooperation Projects of China (No. 2014DFA61670).

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REFERENCES

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DOI: 10.1021/ie504163h Ind. Eng. Chem. Res. 2015, 54, 1464−1469