Ind. Eng. Chem. Res. 1988,27, 21-24 Horiuchi, J. J. Res. Inst. Catal., Hokkaido Uniu. 1948, 1, 8. Horiuchi, J.; Ikushima, M. Proc. Imp. Acad. (Tokyo) 1939, 15, 39. Ivanov, A. A.; Boreskov, G . K.; Beskov, V. S. Presented a t the IVth International Congress on Catalysis, Moscow, June 1968. Kaneko, Y.; Odanaka, H. J. Res. Inst. Catal., Hokkaido Uniu. 1965, 13, 29. Kaneko, Y.; Oki, S. J . Res. Inst. Catal., Hokkaido Univ. 1965a, 13, 55. Kaneko, Y.; Oki, S. J . Res. Inst. Catal., Hokkaido Univ. 1965b,13, 169. Kaneko, Y.; Oki, S. J . Res. Inst. Catal., Hokkaido Uniu. 1967, 15, 185. Kaneko, Y.; Oki, S. Utsunomiya Daigaku Kyoyobu Kenkyu Hokoku, Dai-2-bu 1969a, 2, 19. Kaneko, Y.; Oki, S. Utsunomiya Daigaku Kyoyobu Kenkyu Hokoku, Dai-2-bu 1969b, 2, 33. Kulkova, N . V.; Temkin, M. I. Zh. Fiz. Khim. 1949, 23, 695. Livbjerg, H.; Villadsen, J. Chem. Eng. Sci. 1972, 27, 21. Mezaki, R.; Oki, S. J . Catal. 1973, 30, 488.
21
Nakanishi, J.; Tamaru, K. Trans. Faraday SOC.1963, 59, 1470. Oki, S.; Happel, J.; Hnatow, M. A,; Kaneko, Y. Presented at the Vth International Congress on Catalysis, Palm Beach, FL, Aug 1972. Oki, S.; Kaneko, Y.; Arai, Y.; Shimada, M. Shokubai 1969,11,184. Oki, S.; Mezaki, R. J . Phys. Chem. 1973a, 77,447. Oki, S.; Mezaki, R. J. Phys. Chem. 1973b, 77, 1601. Shchibrya, G. G.; Morozov, N. M.; Temkin, M. I. Kinet. Katal. 1965, 6(6), 1057. Tanaka, K. J . Res. Inst. Catal., Hokkaido Univ. 1965, 13, 119. Tanaka, K. J . Res. Int. Catal., Hokkaido Uniu. 1966, 14, 153. Tanaka, K. J . Res. Int. Catal., Hokkaido Uniu. 1971, 19, 63. Tinkle, M.; Dumesic, J. A. J. Catal. 1987, 103, 65. Wagner, C. Advances in Catalysis; Academic: New York, 1970: Vol. 21, p 323. Weisz, P. B.; Hicks, J. S. Chem. Eng. Sci. 1962, 17, 265. Yang, K. H.; Hougen, 0. A. Chem. Eng. Prog. 1950,46, 146 Received for review November 3, 1986 Accepted August 14, 1987
Selective Catalytic Hydrogenation of Nitrobenzene to Hydrazobenzene Shrikant L. Karwa and Rajeev A. Rajadhyaksha* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
Selective hydrogenation of nitroaromatics to corresponding hydrazobenzenes is an industrially useful class of reactions. The dependence of the selectivity on a variety of reaction parameters is investigated in the present study. T h e results lend considerable insight into the reaction behavior. Platinumon-carbon was observed to be a significantly more selective catalyst than palladium-on-carbon. Addition of dimethyl sulfoxide is shown t o improve the selectivity, although the rates of various reactions were observed t o be considerably reduced. T h e hydrogenation of a few substituted nitroaromatics is also investigated. T h e presence of electron-releasing substituents was observed to reduce the selectivity to hydrazobenzene. Complete hydrogenation to amines was the only side reaction. Hydrazobenzene and substituted hydrazobenzenes are important intermediates in the manufacture of a variety of drugs and dyes. They are usually synthesized by reduction of appropriate nitro compounds in the presence of alkali. The formation of hydrazobenzene is believed to occur by the following sequence of reactions: PhN02
2H
nitrobenzene PhNO
+
PhNHOH
alkali
-
PhNHOH
PhNH2
phe ny Ihyd rox y Iam i ne
an I I Ine
2H
PhNO
nitrosobenzme
PhNa'Ph
I
2H
PhNZNPh azobenzene
0azoxybenzene /2H
PhNHNHPh hydrazobenzene
12.
product, and lesser handling of materials. However, most of the reports on hydrogenation are in the form of patents, and the understanding of the reaction behavior is clearly limited. In the present work, catalytic hydrogenation of nitrobenzene and substituted nitrobenzenes is investigated. The important aspect of the reaction for practical application is the selectivity to hydrazobenzene. The dependence of the selectivity on various operating parameters such as temperature, hydrogen pressure, alkali concentration, and type of alkali is studied. The addition of dimethyl sulfoxide (DMSO) is known to improve the selectivity for the formation of phenylhydroxylamine (Rylander et al., 1970; Karwa and Rajadhyaksha, 1987) in preference to aniline. Hence, the effect of addition of DMSO on the selectivity for hydrazobenzene was also investigated. The reaction was also carried out employing o-nitrotoluene and o-nitroanisole as reactants in order to study the effect of substituent on the reaction selectivity.
2PhNH2
The above reduction is reported to be carried out using a wide variety of reducing agents including zinc/alkali, iron/alkali, formaldehyde, dextrose, sulfides, and sodium amalgam, etc. (Stratz, 1984). A few reports on catalytic hydrogenation using metallic catalysts have also appeared (Groogins, 1958; Ackermann et al., 1978). The catalytic hydrogenation seems to be favored (Lubs, 1955) in industrial practice due to lower cost of hydrogen as compared to many other reducing agents, ease of separation of
* Present address: Institut fur Physikalische Chemie der Universitat Munchen, 8 Munchen 2, West Germany.
Experimental Section Experiments were conducted in a high-pressure, stainless steel 316, magnetically driven autoclave of 1-L capacity fitted with a gas-inducing impeller. In a typical experiment, the appropriate quantities of methanol, nitroaromatic, alkali, and catalyst were added to the clean, dry autoclave. A typical charge consisted of 400 g of methanolic solution of the nitroaromatic compound. The autoclave was repeatedly purged with hydrogen at room temperature. After the contents were heated to a desired temperature, the autoclave was pressurized with hydrogen to a required pressure. As the reaction commenced, the
0888-5885/88/2627-0021$01.50/0 0 1988 American Chemical Society
22 Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988
-
-ANILINE AZOXYBENZENE
--
I
CO- N I T R O B E N Z E N E
8-
2
60
+ANILINE
0
AZOXYBENZENE
K 8-
k!
--C AZOBENZENE
10
&A Z O B E N Z E N E
8
HYORAZOBENZENE
W
z
Y
;20
mw
? = s
o o
1200
2100 TIME
,
3600 SIC
4800
6000
7200 TIME
__+
Figure 1. Typical product concentration profile. Reaction conditions: temperature, 363 K; pressure, 18 atm; initial concentration of nitrobenzene, 13% (w/w);concentration of NaOH, 2.23% (w/w); catalyst loading, 0.23% (w/w); impeller speed, 33 rps.
roo
-ANILINE
u
AZOXYBENZENE
80
U
consumed hydrogen was replenished so as to maintain a constant pressure. Temperature was controlled within f1 K by regulating cooling water flow and the rate of heating. Samples of 2-3 mL were withdrawn through the sample outlet a t regular time intervals and were analyzed on a Perkin-Elmer Series 10 HPLC using a 3 - ~ mC-18 , reversed phase column. Methanol-water mixture was used as the eluent at a flow rate of 0.014 mL/s. The initial 50% (v/v) methanol in eluent was linearly increased to 90% (v/v) in 300 s. A mixture of aniline, nitrobenzene, hydrazobenzene, azoxybenzene, and azobenzene could be clearly resolved under these conditions. All the reagents used were of analytical grade
Results and Discussion Partial hydrogenation of nitrobenzene to phenylhydroxylamine has been investigated on a variety of metal catalysts (Rylander et al., 1970). Platinum-on-carbon has been found to be most selective for the reaction. In view of this, the present reaction was first investigated on platinum-on-carboncatalyst. Since alkaline conditions are required for the reaction, it was necessary to choose a solvent in which sodium hydroxide as well as reactant and products is soluble. Methanol was, therefore, selected as solvent. Figure 1 shows typical product composition vs time profile for the reaction carried out at 363 K and 18 atm of hydrogen pressure. The concentrations of nitrobenzene and sodium hydroxide in methanol were 13% (w/w) and 2.23% (w/w), respectively. The catalyst loading employed was 0.23% (w/w) of 5% platinum-on-carbon based on nitrobenzene. The figure clearly indicates simultaneous formation of azoxybenzene, azobenzene, and aniline from nitrobenzene. No nitrosobenzene and phenylhydroxylamine were observed in the product, indicating that conversion of these compounds to azoxybenzene is very rapid. This is in agreement with the known chemistry (Russel et al., 1967). Further hydrogenation of azoxybenzene to azobenzene occurred simultaneously. However, hydrogenation of azobenzene to hydrazobenzene seems to be poisoned in the presence of nitrobenzene. Thus, no formation of hydrazobenzene could be observed until all the nitrobenzene was consumed. The product profile appears to indicate that after complete conversion of nitrobenzene, azoxybenzene is directly hydrogenated to hydrazobenzene. This is supported by the following observations. 1. As azoxybenzene is converted, no increase in azobenzene concentration is observed. The concentrations
-
-
r
2
, sec
-CAZOBENZENE
W Ix
o 60 0 W
c W U
g
10
0 Y
W z
; 20 W
m
0
? z $
0
1200
2100 TIME,
3600 SIC
-*
LBO0
6000
7200
Figure 2. Effect of alkali concentration. Reaction conditions: temperature, 363 K; pressure, 18 atm; initial concentration of niimpeller trobenzene, 13% (w/w);catalyst loading, 0.23% (w/w); speed, 33 rps.
of both azobenzene and azoxybenzene decrease continuously, resulting in an increase in concentration of hydrazobenzene. 2. The rate of increase of hydrazobenzene concentration shows a distinct decrease at the point where azoxybenzene is fully converted. The concentration of aniline shows no further increase after nitrobenzene is fully converted, which indicates that conversion of azo- and azoxybenzene to hydrazobenzene occurs with total selectivity. No increase in concentration of aniline was observed even after complete conversion of azobenzene under the conditions employed in the experiment. To investigate the role of gas-liquid and solid-liquid diffusional processes, the impeller speed was varied in the range of 18-33 rps. The rates of all the reactions remained unchanged, which implies absence of the diffusion resistances. These experiments were carried out at 373 K and 11.2 atm of pressure, which represent the highest temperature and lowest pressure employed in the study (where the effect of transport limitations is likely to be maximum). In the subsequent set of experiments, catalyst loading was varied between 0.15% and 0.4% (w/w) based on nitrobenzene. The rate of reaction was found to increase with catalyst loading. These experiments indicate that under the conditions employed in the study, the reaction rates were governed by intrinsic kinetics. Figure 2 shows the progress of reaction when 1% and 3.23% (w/w) alkali concentrations were employed. These are to be compared with the results shown in Figure 1 where the alkali concentration was 2.23% (w/w). The final selectivity to hydrazobenzene increased from 59% to 86% with the increase in alkali Concentration. The selectivity seems to depend on the initial product distribution from
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 23 Table I. Effect of Type of Alkali on Hydrazobenzene SelectiviW selectivity to hydrazobenzene, alkali mol % sodium hydroxide 83.3 potassium hydroxide 85.0 90.0 sodium methoxide
-0- N I T R O B E N Z E N E
+A Z O X Y B E N Z E N E W
.-CAZOBENZENE
c
60
HYORAZOBENZENE
Y
a Reaction conditions: temperature, 363 K; pressure, 18 atm; impeller speed, 33 rps; nitrobenzene concentration, 13% (w/w); alkali concentration, 0.14 mol; catalyst loading, 0.23% (w/w).
W
%
8
10
Y W
z 20
OMS0 CONCENTRATION
m
a
-z
s o
2LOO
1200 ...
TIME
,
SeC
=
--
1% lw/wl
NITROBENZENE
-
00
AZOXYBENZENE
a
Figure 3. Hydrogenation of nitrobenzene at 343 K. Reaction conditions: pressure, 18 atm; initial concentration of nitrobenzene, 13% (w/w);concentration of NaOH, 2.23% (w/w);catalyst loading, impeller speed, 33 rps. 0.23% (w/w);
3-AZOBENZENE
Y
I 0
HYDRAZOBENZENE
60
I
A100r
PRESSURE = I I a t m -ANILINE
a
AZOXYBENZENE
--C A Z O B E N Z E N E
+H Y D R A Z O B E N Z E N E
60
0
3600 TIME ,src
I
I
1
1800
6000
7200
Figure 5. Hydrogenation of nitrobenzene in presence of DMSO. Reaction conditions: temperature, 363 K; pressure, 18 atm; initial concentration of NaOH, concentration of nitrobenzene, 13% (w/w); catalyst loading, 0.23% (w/w). 2.23% (w/w);
- "1 5000
TIME
1 lool
PRESSURE
= 25
, src
atm
7200
-
-ANILINE AZOXYBENZENE
-CAZOBENZENE
& HYORAZOBENZENE
TIME,
1 3600
,
1800
I
6000
T I M E , SIC
Figure 4. Effect of hydrogen pressure. Reaction conditions: temperature, 363 K; initial concentration of nitrobenzene, 13% (w/w); concentration of NaOH, 2.23% (w/w);catalyst loading, 0.23% (w/w); impeller speed, 33 rps.
nitrobenzene. After complete conversion of nitrobenzene, there is no increases in aniline and all other products are quantitatively converted to hydrazobenzene. A further increase in alkali concentration to 5% (w/w) did not result in any further improvement in the selectivity. Thus, an alkali concentration around 3% (w/w) appears to be optimum. The effect of temperature on reaction behavior was investigated in the temperature range 343-373 K. Figure 3 shows the product distribution observed a t 343 K. Comparison with Figure 1 (363 K) clearly indicates improvement in selectivity with an increase in temperature. Further improvement of selectivity from 363 to 373 K was found to be marginal.
SIC
Figure 6. Hydrogenation of nitrobenzene by palladium catalyst. Reaction conditions: temperature, 363 K; pressure, 18 atm; initial concentration of NaOH, concentration of nitrobenzene, 13% (w/w); catalyst loading, 0.58% (w/w). 2.23% (w/w);
Figure 4 shows the reaction behavior at 11 and 25 atm of hydrogen. It is apparent that the rates of all the reactions are significantly dependent on hydrogen pressure: however, the selectivity essentially remains unchanged with the change in pressure. The type of alkali can also have an influence on the reaction behavior. The reaction was, therefore, studied employing equimolar concentrations of potassium hydroxide and sodium methoxide. The reaction behavior was similar with different alkali compounds. The selectivity, however, was found to be significantly higher when sodium methoxide was employed (Table I). Addition of DMSO is known to poison selectively the complete hydrogenation of nitrobenzene to aniline which results in increased selectivity for the partially hydrogenated products. An experiment was, therefore, conducted by adding 1%(w/w) DMSO based on nitrobenzene. The results are shown in Figure 5 . The combined selectivity
I n d . Eng. Chem. Res. 1988, 27, 24-30
24
insight into the reaction behavior. The results show that the progress of reaction can be clearly divided into two parts. In the first part, simultaneous conversion of nitrobenzene to azobenzene, azoxybenzene, and aniline occurs. This is accompanied by further hydrogenation of azoxybenzene to azobenzene. In the second part, hydrogenation of azoxy- and azobenzene to hydrazobenzene occurs. The ultimate selectivity to hydrazobenzene,which is the principal consideration in these types of reactions, is governed by the selectivity to azoxy- and azobenzene in the first part of the reaction. The results lead to the following specific conclusions regarding the reaction behavior. (i) Platinum-on-carbon gives significantly higher selectivity for hydrazobenzene as compared to palladiumon-carbon. (ii) An increase in hydrogen pressure increases the rates of all the reactions without affecting the selectivity. (iii) Addition of DMSO improves the selectivity to hydrazobenzene; however, the rates reduce very significantly. (iv) The presence of electron-releasing substituents in the ring results in a decrease in rate as well as selectivity to the partially hydrogenated products.
Table 11. Effect of Substituent on the Selectivity to Partially Hydrogenated Productsa combined selectivity to azoxy and azo compds, nitro comDd mol % 58 o-nitrotoluene 25 o-nitroanisole a Reaction conditions: temperature, 363 K; pressure, 18 atm; impeller speed, 33 rps; nitroaromatic concentration in methanol, 8% (w/w); sodium hydroxide concentration, 2.23% (w/w); catalyst loading, 0.23 (w/w).
to azoxy- and azobenzene increased from 83.3% to 90.2% by addition of DMSO (compare with Figure 1). The rates of the reactions were, however, considerablydecreased due to the poisoning effect of DMSO. The reaction was also studied using 2% palladium-oncarbon catalyst. The results are shown in Figure 6. As was expected, the selectivity to hydrazobenzene was observed to be considerably lower. Several substituted hydrazobenzenes are also of practical importance. To investigate the effect of substituents on rate and selectivity, the reaction was carried out employing o-nitrotoluene and o-nitroanisole as reactants. The reaction rates were lower at least by a factor of 2 as compared to nitrobenzene. The selectivity to the partially hydrogenated products was also lower. The selectivities to azoxy and azo compounds a t complete conversion of nitro compounds are compared in Table 11. In the case of onitrotoluene, the azo and azoxy compounds were quantitatively converted to hydrazo compounds, while for onitroanisole no further hydrogenation to hydrazo compound could be observed. The decrease in rate and selectivity with electron-releasing substituents is also observed in partial hydrogenation of nitroaromatics to the correspondinghydroxylamines (Karwa and Rajadhyaksha, 1987). This may be attributed to stronger bonding of the nitrogen atom with the catalyst with an increase in the electron density on the aromatic ring.
Registry No. DMSO, 67-68-5; Pt, 7440-06-4; Pd, 7440-05-3; NaOH, 1310-73-2; KOH, 1310-58-3; CH30Na, 124-41-4; P h N 0 2 , 98-95-3; 2-02NCsHkCH3, 88-72-2; 2-02NCGHdOCH3, 91-23-6; PhNHNHPh,122-66-7; PhNH2,62-53-3; PhN=N(O)Ph,495-48-7; P h N G N P h , 103-33-3.
Literature Cited Ackermann, 0.;Bonsel, P.; Neff, U. Chem. Abstr. 215052. Groggins, P. H. Unit Processes in Organic Synthesis, 5th ed.; McGraw-Hill: Kogakusha, Tokyo, 1958. Karwa, S. L.; Rajadhyaksha, R. A. Ind. Eng. Chem. Res. 1987, 26, 1746-1750. Lubs, H. A. The Chemistry of Synthetic Dyes and Pigments; Robert E. Krieger Publishing: New York, 1955. Russel, G. A.; Geels, E. J.; Smentowski, F. J.; Chang, K. Y.; Reynolds, J.; Kaupp, G. J . Am. Chem. SOC.1967,89(15),3821-3828. Rylander, P. N.; Karpenko, I. M.; Pond, G. R. Ann. N . Y . Acad. Sei. 1970, 172(9),266-275. Stratz, A. M. In Catalysis of Organic Reactions; Kosak, J. R., Ed.; Marcel Dekker: New York. 1984.
Concluding Remarks The paper, which is perhaps the most elaborate study to be reported on this class of reactions, gives considerable
Received f o r review December 23, 1986 Accepted August 11, 1987
A Method for Improving Load Turndown in Fluidized Bed Combustors Robert C. B r o w n * and James E. Foley Department of Mechanical Engineering, Iowa State University, Ames, Iowa 5001 1
We are investigating a new concept in the design of fluidized bed combustors that improves load turndown capability. This design is based on control of heat-transfer rate independent of combustion rate. T h e configuration under investigation consists of a central combustion bed surrounded by an annular fluidized bed that establishes overall heat-transfer rate from the inner combustion bed. The two beds are fluidized independently by separate air plenums. Experiments were performed to evaluate the usefulness of this device in improving load turndown. For a given combustor firing rate and air-to-fuel ratio, a wide variation in combustion temperature could be achieved by controlling the flow of fluidization air to the annular fluidized bed. In other tests, a load turndown ratio of 8.7 was achieved while combustion temperature was held constant. 1. Introduction
Use of fluidized bed combustion (FBC) has increased with the realization that it can burn coal and low-grade fuels in an environmentally acceptable manner. Long fuel-residence times in fluidized beds provide fuel flexibility, while use of inexpensive sorbents for bed material and staged firing reduce emissions of sulfur dioxide and 0888-5885/88/2627-0024$01.50/0
nitrogen oxides, respectively. Unfortunately, technical problems remain that must be overcome before wider markets are developed. Prominent among these difficulties is the poor load turndown capability of fluidized bed combustors; this capability is defined here as the ratio of maximum to minimum firing rates. Even modest load turndowns are frequently accompanied by degradations 0 1988 American Chemical Society
