Ind. Eng. Chem. Res. 2007, 46, 3443-3445
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RESEARCH NOTES Direct Amination of Benzene to Aniline by Aqueous Ammonia and Hydrogen Peroxide over V-Ni/Al2O3 Catalyst with Catalytic Distillation Changwei Hu,*,† Liangfang Zhu,† and Yunsheng Xia†,‡ Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan UniVersity, Chengdu, 610064, People’s Republic of China, and College of Chemistry and Chemical Engineering, Bohai UniVersity, Jinzhou, 121003, People’s Republic of China
Catalytic distillation (CD) technology was used in the direct amination of benzene to aniline with aqueous ammonia and hydrogen peroxide over a V-Ni/Al2O3 catalyst. The effects of several parameters (including the packing manners of the catalytic column, the feed ratio, the distillation temperature, and the reaction time) on the amination were studied. It was found that the yield of aniline was significantly improved in the CD process, in comparison to the kettle-type reaction. 1. Introduction Aniline is an important organic chemical and is widely used in industry. The existing methods for aniline production involve multiple reactions, which has many disadvantages1 and disobeys the “greening” trends of global chemical manufacturing.2 Onestep production of aniline via the direct amination of benzene, turning the multistep reaction into one step, which significantly improves the atomic efficiency, is an attractive and challenging method from the viewpoint of both green chemistry and synthetic chemistry. Therefore, this type of investigation has attracted much attention,3-14 most of which has focused on the development of effective catalysts and optimal aminating conditions using gaseous ammonia as the aminating agent and gaseous oxygen as the oxidizing agent. However, these methods generally suffer from critical operation conditions of high temperature (from ∼373 K to 1273 K), high pressure (from 10 atm to 1000 atm), relatively low aniline yield, and/or relatively low selectivity to aniline. In our previous work, hydrogen peroxide was found to be suitable for the direct amination of benzene on V-Ni/Al2O3,15 Ni-Zr-Ce/Al2O3,16 Mo-Ni/Al2O3, and MnNi/Al2O317 under mild conditions (low temperature, atmospheric pressure); however, only modest yields were obtained. Thus, enhancement of both the aniline yield and selectivity is needed. The technology of process intensification, as Stankiewicz and Moulijn have noted,18 is a sound way to solve the aforementioned problems. One example of such an application is catalytic distillation (CD), which combines a heterogeneous catalytic reaction and distillation into a single-unit operation.19 In the CD process, the reaction products can be separated from reactants and catalysts simultaneously as the reaction proceeds, thus shifting the chemical equilibrium in the desired direction and resulting in a higher conversion of reactants and a greater yield of products.20-22 Recently, the CD technology has been gaining increasing attention in both industrial practice and scientific research, because of its advantages, such as high * To whom correspondence should be addressed. Tel: +86-2888835525. Fax: +86-28-85411105. E-mail address:
[email protected],
[email protected]. † Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University. ‡ College of Chemistry and Chemical Engineering, Bohai University.
selectivity and yield, energy savings, lower capital investment, and simpler operation.23 In this paper, CD was designed for the direct amination of benzene to aniline with aqueous ammonia and hydrogen peroxide over a V-Ni/Al2O3 catalyst to ameliorate the yield of aniline. Different catalytic column intervals and different operating conditions have been studied. 2. Experimental Section 2.1. Reagent and Catalyst. Analytical-grade benzene, aqueous ammonia (NH3‚H2O) (25%-28%), and hydrogen peroxide (H2O2) (30%) were commercially obtained. Benzene was used without further purification. A V-Ni/Al2O3 catalyst was prepared via a two-step wet impregnation method, and a detailed description of the preparation was given in our previous paper.15 The actual vanadium and nickel loadings on the catalyst used were 2.2% and 13.9%, respectively, determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). 2.2. Catalytic Reaction. A glass column with an inner diameter of 10 mm, an outer diameter of 30 mm, and a height of 500 mm was used to perform CD. The experimental setup is shown in Figure 1. The top of the column was equipped with a condenser and a mixer for mixing NH3‚H2O and H2O2. A 100-mL reboiler, which was equipped with a temperature controller, was installed at the bottom of the column, together with an atmospheric valve. Benzene was stored in the reboiler. One gram of solid V-Ni/Al2O3 catalyst was loaded in a designed manner, and over and below it, an appropriate amount of glass beads was packed. A heating tape (300 W) was wrapped around the glass column to control the reaction temperature. The CD was performed at atmospheric pressure. Because no distillate overflowed out the top of the column, the process was conducted in total-reflux operation. Some parameters were controlled: d/D ) 1:10-1:20, d/d′ ) 0.18:1-0.36:1, l′ /L ) 3:1, l/L ) 0.14:1-0.84:1. Before the reaction, small quantities of NH3‚H2O and H2O2 were added dropwise into the column for moistening and the reboiler was strongly heated. The CD column was maintained at flooding for 3 min under reflux to get the catalyst and the packing thoroughly wetted. The power of the reboiler then was reduced to the required value and the temperature of the CD column was kept at 323 K. A heating program of 353 K-
10.1021/ie070103r CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 10, 2007 Table 2. Influence of the Packing Mode of the Catalyst on the Catalytic Aminationa column
Yaniline (mg)
Saniline (%)
one bisected trisected
4.7 4.9 5.2
84 81 80
a Conditions: l/L ) 0.68:1 and l′′/D ) 2:1; the other parameters were the same as those stated for Table 1.
Table 3. Influence of the Distillation Temperature and Reaction Time on the Catalytic Aminationa No.
temperature program
reaction time at each stage (min)
Yaniline (mg)
1 2 3 4
353 K-358 K-363 K 353 K-358 K-363 K 358 K-363 K-368 K 358 K-363 K-368 K
60 100 60 100
5.2 5.6 5.3 5.8
a Conditions: l/L ) 0.68:1; trisectedly packed; l′′/D ) 2:1; 5 mL benzene; RNH3/C6H6/H2O2 ) 3.5:1:2.6; column temperature, 353 K.
Figure 1. Scheme of the catalytic distillation setup. Table 1. Influence of the Location of Catalyst Bed on the Catalytic Aminationa l/L
Yaniline (mg)
Saniline (%)
0.14 0.32 0.50 0.68 0.84
1.5 3.1 4.2 4.7 4.3
83 85 84 84 83
a Experimental conditions: 5 mL benzene; R NH3/C6H6/H2O2 ) 3.5:1:2.6; column temperature, 353 K; heating program of the reboiler, 353 K358 K-363 K; reaction time, 180 min.
358 K-363 K or 358 K-363 K-368 K was performed for the reboiler during the reaction process. The reactants NH3‚H2O and H2O2 were fed to the column in a dropwise manner at room temperature. The atmospheric valve (labeled as “10” in Figure 1) was used to make the reaction system nonpressurized and, thus, the amination could occur at the catalyst zone. Some typical experimental conditions are listed in Table 1. 2.3. Product Analysis. After the CD process, the reaction system was cooled to room temperature and the resulting reaction mixture in the bottom kettle was stratified. The organic phase and the aqueous phase were analyzed separately using gas chromatography (GC) (Varian, CP-3800, FID detector). The target product was identified using coupled gas chromatography and mass spectroscopy (GC-MS) (Agilent, 5973 Network 6890N). The yield of aniline was expressed as the amount of aniline produced (in milligrams), as determined by a calibration curve. The conversion of benzene was estimated as follows:
conversion of benzene ) amount of aniline + amount of byproducts initial amount of benzene The selectivity to aniline was calculated as follows:
selectivity to aniline ) amount of aniline amount of aniline + amount of byproducts 3. Results and Discussion The present CD process was a gas-liquid-solid three-phase reaction. The vaporing benzene could react with the dropwise additions of NH3‚H2O and H2O2 in the catalyst zone. Because the temperature of the CD column was kept at 353 K, the water (with a boiling point (bp) of 373 K) and aniline (bp ) 455 K) that formed from the reaction would drop into the reboiler, because of their weight. The unreacted benzene was refluxed back to the column to participate further in the amination. The boiling point of the target product aniline was much higher than
that of the starting material benzene (353 K); therefore, the catalytic amination and separation can occur instantaneously in the same catalytic column through the CD process, shifting the chemical equilibrium to the formation of aniline. 3.1. Influence of the Catalyst Packing. The packing manner of the catalyst zone in the column was investigated in detail. First, the beginning of the catalyst layer from the bottom of the column was varied to test the catalytic performance of the system (see Table 1). It was shown that a sufficient distance of the catalyst layer from the bottom of the column was needed for the CD process. The highest yield of aniline was obtained when the l/L value ranged from 0.6:1 to 0.8:1. This may be due to the fact that the ascending benzene vapor was too intense to come into contact with the downflowing NH3‚H2O and H2O2 in the catalyst layer when the catalyst was placed too close to the bottom of the column. However, a liquid film of NH3‚H2O and H2O2 would form at the surface of the catalyst when the catalyst was placed near the top of the column, blocking the mass transfer among the reactants and further resulting in the decrease of the aniline yield. Table 1 also shows that the location of the catalyst zone had no obvious influence on the selectivity to aniline in the CD process. Then, keeping the l/L ratio at 0.68:1, three different packing manners were tested (see Table 2). The catalyst was divided into two or three parts, separated by a definite interval (l′′/D ) 2:1) packed with glass beads. It was shown that the aniline yield was increased when the catalyst was packed segmentally. This result indicated that the well-dispersed catalyst in the column was advantageous for the amination, probably because it increased the contacting time and favored the mass transfer. 3.2. Influence of the Distillation Temperature. While the catalyst zone was packed in the best way (l/L ) 0.68:1, trisectedly packed, l′′/D ) 2:1), the heating program of the reboilder was varied. The result is presented in Table 3. It is shown that the heating program of the reboilder had no obvious effect on the yield of aniline. That is, even when the temperature of the uprising benzene vapor was increased, the temperature of the catalytic column was not impacted by properly tuning the drop rate of NH3‚H2O and H2O2, as well as the pressure of the system. The yield of aniline should be affected by the actual temperature of the catalytic column. The influence of the column temperature on the amination deserves further investigation. 3.3. Influence of the Reaction Time. The influence of the reaction time on the amination was investigated (see Table 3). By prolonging the reaction time at each heating step to 100 min, the total reaction time was increased from 180 min to 300
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Acknowledgment The authors are grateful for the financial support of the National Natural Science Foundation of China (No. 200720024) and the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, PRC (2002). Literature Cited Figure 2. Influence of the feed ratio on the amination.
min. It was found that the increase in the reaction time resulted in an increase in the aniline yield. This may be attributed to the fact that prolonging the reaction time increased the amount of the refluxed benzene and thus enhanced the contact ratio of the reactants. Because of the fact that some benzene still remained in the reboilder after 300 min of reaction, the yield of aniline could be further enhanced by prolonging the reaction time until no benzene was detected. 3.4. Influence of the Feed Ratio. The dependence of the feed ratio on the yield of aniline is illustrated in Figure 2. The molar ratio of NH3/C6H6 was varied from 1:1 to 6:1. Obviously, the yield of aniline increased as a function of the feed ratio, reached a maximum at RNH3/C6H6 ) 3.5:1-4.5:1, and then remained constant. The highest aniline yield was ∼5.8 mg aniline/(5 mL of benzene) (0.11 mol %). In comparison to the kettle-type reaction,15 the conversion of benzene was improved by a factor of ∼2 (increasing from 0.08% to 0.15%). The increasing tendency of the aniline yield was in agreement with our previous work.16 Because the temperature of the column was maintained at 353 K, the decomposition of NH3‚H2O and H2O2 could not be avoided. An RNH3/C6H6 value of 3.5-4.5 was deemed capable of acting as a sufficient source of ammonia for the present amination. Further increases in the ammonia feed were unnecessary. 4. Conclusion In summary, aniline was synthesized directly from benzene and aqueous ammonia with hydrogen peroxide through a catalytic distillation (CD) process. The aniline yield and benzene conversion were increased, in comparison to the kettle-type reaction. In addition, the CD process prevented the catalyst from being crushed as it was in the previous kettle-type reaction.16 Thus, the efficiency and the recycle of the catalyst could be enhanced by the CD process. The packing manner of the catalyst zone in the column was proven to be important to the catalytic amination. The favorable location of the catalyst zone was at l/L ) 0.6:1-0.8:1, where the catalyst was trisectedly arranged with the l′′/D ) 2:1. The heating program of the reboiler had no obvious influence on the amination, but a prolonging reaction time at each temperature favored the production of aniline. The optimal RNH3/C6H6 was 3.5:1 to 4.5:1. Because the present catalytic amination was conducted at a relatively low temperature and under atmospheric pressure, this observation brings new possibilities for the “green” synthesis of aniline from an industrial viewpoint. Further work is currently underway in our laboratory. Notation d ) size of catalyst (mm) d′ ) size of glass bead (mm) D ) inner diameter of column (mm) l ) distance of the catalyst layer from the bottom of column (mm) l′ ) length of heating tape (mm) l′′ ) interval of catalyst packing (mm) L ) total length of column (mm)
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ReceiVed for reView January 16, 2007 ReVised manuscript receiVed March 13, 2007 Accepted March 27, 2007 IE070103R
