I n d Eng ('hem. lirs 1990. 2s. 754 766
754
Chemical Society: Washington, DC, 1973; p 140. Derouane, E. G.; Detremmerie, S.; Gabelica, Z.; Blom, N. Synthesis and Characterization of ZSM-5 Type Zeolites, I. Physico-Chemical Properties of Precursors and Intermediates. ,4pp!. Cntni. 1981, I , 201. Dutta, P. K.; Shieh, D. C. Crystallization of Zeolite A: A Spectroscopic Study. J . Phys. Chem. 1986, 90. 2331. Engelhardt, G.; Fahlke, B.; Magi, M.; Lippmaa, E. High-Resolution Solid-state 29Siand 27AlNMR of Aluminosilicates in Zeolite A Synthesis. Zeolites 1983, 3, 292. Gahelica, Z.; Blom, N.; Derouane. E. G. Synthesis and Characterization of ZSM-5 Type Zeolites. I i I . A Critical Evaluation of the Role of Alkali and Ammonium Cations. Appl. Cntal. 1983,5, 227. Huang, C. L.; Yu, W. C.; Lee, T. Y. Kinetics of Nucleation and Crystallization of Silicalite. Chem. Eng. Sci. 1986. 41 (41, 625. Jarman, R. H.; Melchior, M. T.; Vaughan, D. E. W.Synthesis and Characterization of A-Type Zeolites. In Intrazeolite Chemistry; Stucky, G. D., Dwyer, F. G., Eds.; ACS Symposium Series 218; American Chemical Society: Washington, DC. 1983; p 267. Kacirek, H.; Lechert, H. Investigations on the Growth of the Zeolite Type N a y . J . Phys. Chem. 1975, 79 (15). 1589. Kacirek, H.; Lechert, H. Kinetic Studies of the Growth of Zeolites of the Faujasite and N-A Type. In Moieculur Sieve I J ; Katzer, J. R., Ed.; ACS Symposium Series 40; American Chemical Society: Washington, DC, 1977; p 244. Kerr, G. T. Chemistry of Crystalline Aluminosilicates. 1. Factors Affecting the Formation of Zeolite A. J . Ph.xs. Chent. 1966, 70 (41. 1017. Klinowski, J.; Thomas, J. M.; Fyle, C. A,; Hartman, J. S. Applications of Magic-Angle-Spinning Silicon-29 Nuclear Magnetic Resonance. Evidence for Two Different Kinds of Silicon-Aluminum Ordering in Zeolitic Structures. .I. Phys. Chrm. 1981>85. 2590. Kostinko, ,I. A. Factors Influencing the Synthesis of Zeolites A, S
and E'. In Intrazeolite Chemistry; Stucky, G. D., Dwyer, F. G.; Eds.; ACS Symposium Series 218; American Chemical Society: Washington, DC, 1983; p 3 . Iippmaa, E.; Magi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A.-R. Structural Studies of Silicates by Solid-state High-Resolution %i NMR. J . Am. Chem. Soc. 1980, 102, 4889. McNicol, B. D.; Pott, G. T.: Loos, K. R. Spectroscopic Studies of Zeolite Synthesis. J . Phys. Chem. 1972, 76, 3388. hlcn'icol, R. D.: I'ott, G. T.; Loos, R.; Mulder, N. Spectroscopic Studies of Zeolite Synthesis: Evidence for a Solid-state Mechanism. In Molrcular Sieue; Meier, W. M., Uytterhoven, J . B., Eds.; Advances in Chemistry 121: American Chemical Society: Washington, DC, 1973; p 152. Meise, W.;Schwochow, F. E. Kinetic Studies on the Formation of Zeolite A. In Molecular Sieue; Meier, W. M., Uytterhoven, J . B., Eds.; Advances in Chemistry 121; American Chemical Society: Washington, DC, 1973; p 169. Melchior, M. T.; Vaughan, D. E. W.; Jarman, R. H.; Jacobson, A. J. The Characterization of Si-AI Ordering in A-Type Zeolite (ZK4) hy %i NMR. 'Vature 1982, 298, 455. Polak, F.; Cichocki, A. Mechanism of Formation of X and Y Zeolites. In Molecular Sieue: Meier, W. M., Uytterhoven, J. B., Eds.; Advances in Chemistry 121; American Chemical Society: Washington DC, 1973; p 209. Roozehoom, F.; Robson, H. E.; Chan, S. S. Laser Raman study on the crystallization of zeolites A, X and Y. Zeolites 1983, 3, 321. Zhdanov. S.1'. Some Problems of Zeolite Crystallization. In Molecular S L ~ Zeolite-l; L,~ Flanigen, E. M., Sand, L. B., Eds.; Advances in Chemistry 101; Americn Chemical Society: Washington. DC. 1971; p 20.
Received for revieu May 30, 1989 Accepted December 26, 1989
Kinetics of the Catalytic Hydrogenation of 2,4-Dinitrotoluene. 1. Experiments, Reaction Scheme, and Catalyst Activity Henk J. Janssen,f Arjen J. Kruithof,l Gerard J. Steghuis,§and K. Roe1 Westerterp* Chemical Reaction hngineering Laboratories, F a t u l t j of Chemical Engineering, I'niuersitJ of T r c ~ n t ~ ,
P I ) Box 217, 7850@ A E Enrchede, The Nethrrlandb
There is a great need for quantitative data of a complex reaction network that can be used to evaluate reactor performance and selectivity in heterogeneous reactors. T o this end, the chemistry and the kinetics of the catalytic hydrogenation of 2,4-dinitrotoluene over a 5% P d / C catalyst in methanol have been studied in a batch slurry reactor a t isothermal and isobaric conditions, in the temperature range 308-357 K and a t hydrogen pressures ranging up to 4 MPa. T h e relevant reacting species have been characterized and analyzed quantitatively. A reaction scheme is proposed with two parallel pathways and three stable intermediates. During a short induction period a t the initial stage of the reaction, a rapid deactivation of the catalyst has been observed. It has been shown experimentally t h a t exposure of the catalyst to oxygen and hydrogen plays an important role in this effect. For the five identified main reactions, the quantitative kinetic data will be given in another article. In our laboratories we investigate, among other reactor types, the possible use for the fine chemicals industry of a continuous stirred three-phase slurry reactor for catalytic hydrogenations with high heat effects and selectivity problems. As in industrial practice where optimal selectivity gains more and more interest, there is a growing desire for model reactions to test reactor performance at temperatures and pressures in conventional process equipment. Some studies have been published on complex three-phase reaction systems (Kohler and Richarz, 1986; Chaudhari et ai., 1987), but regretfully kinetic data on
complex three-phase hydrogenation systems with high heat effects are not readily available. After considering several hydrogenation reactions, we have decided upon the catalytic hydrogenation of 2.4,6-trinitrotoluene and of 2,4-dinitrotoluene (2,4-DNT)in methanol using 5% palladium on active carbon (Pd/C) as a catalyst to be used as model reactions to test the performance of our reactors. In the present article we focus on the hydrogenation of 2.4-DNT. The overall reaction equation is CH3
_______
* Author to whom correspondence should be addressed. ' P r e s e n t address: D S M Research B.V , Process Technolog\ Department, P T - C P , Geleen, T h e Netherland-. Present address. Andeno B.V., Venlo, T h e Netherland3 Present address: Akzo Engineering, Arnhem. T h e Netherland5
NO, 24
- DNT
NH2 2 , 4 - DAT
A&
= -1200 MJ kmol-'
(1)
Ind. Eng. Chem. Res., Vol. 29, No. 5, 1990 755 HSC6-N02
HZ +H,C,-NO
mrrobenzene
\
u
\
f-
HZ
"'
bHSC6-NHOH
I I
murrosobenzene
b
HSC6-w
hvdroxvlarmnobenzene
amline
,magnetic
valve
H~C~-NO
XI"
H s c 6 - N ~N - C ~ H T ,
azobrnzmr
Figure 1. Haber reaction scheme for the electrochemical reduction of nitrobenzene to aniline (Haber, 1898).
where 2,4-DAT is the end product, 2,4-diaminotoluene. A large amount of literature has been published in recent years on the catalytic hydrogenation of several mononitro-substituted aromatic compounds. Although some studies have been published with respect to the catalytic hydrogenation of 2,4-DNT, in none of these previous studies have the relevant reaction intermediates been characterized and quantitatively analyzed; also a mathematical model for the rates in this reaction system has not yet been developed. Commonly, for the catalytic hydrogenation of nitrosubstituted aromatic compounds, Pd, Pt, and Ni are used as catalysts. Often the reaction rates found are of the order zero in the nitro compound and between zero and one in hydrogen. In general, the nitro compounds are converted to the amines quantitatively. Burge et al. (1980) reported that the hydrogenation of nitrobenzene using Raney nickel as a catalyst did involve the formation of azoxy- and of azobenzene as stable intermediates. The formation of azoxybenzene as an intermediate also has been found by Yao and Emmet (1962) for the hydrogenation of nitrobenzene over degassed Raney nickel. Burge concluded that the reaction mechanism followed a reaction pathway as proposed by Haber (1898) for the electrochemical reduction of nitrobenzene; see Figure 1. Collins (1982) reported that the reaction pathway of the reduction of nitrobenzene also obeyed Haber's scheme when nickel boride was used as a catalyst. The maximum amounts of azoxy- and of azobenzene that built up during the reaction, however, were negligible. Bird and Thompson (1980) stated that the reduction of any nitro group in an aromatic ring, using supported palladium as a catalyst, will follow the same pathway as chemical reduction but that it is very difficult to produce the intermediates with a high selectivity. Yuculen (1984) studied the kinetics of the hydrogenation of 2,6-dinitrotoluene using Pt/C and Pt/Al,O, as catalysts. The reaction proceeds according to a simple scheme of two consecutive reactions: N 0 2CH3 ~ N 0 22 H J+20 N
0
2,4 - DNT
4
CH3
0
2 - A - 6 - NT
NH2&NH2 (2)
--c
2,6 - DAT
in which 2-amino-6-nitrotoluene (2-A-6-NT) is the only intermediate found and where 2,6-DAT is the end product, 2,6-diaminotoluene. Acres and Cooper (1972) and Bird and Thompson (1980) reported about the occurrence of two intermediates in half- hydrogenated samples of 2,4-DNT: 2-amino-4-nitrotoluene and 4-amino-Z-nitrotoluene, respectively. Unfortunately they did not give any further information, neither on the change of the concentrations of the substrate and the products as a function of the course of the reaction nor on the analytical technique used. Pawlowski and Kricsfalussy (1981) presented a rate equation for the direct conversion of 2,4-DNT to 2,4-DAT, which is first order in the DNT concentration. Although
electrical hcaring element
Figure 2. Experimental setup. M = motor; PI = bourdon-type pressure manometer; PT = electronic pressure transducer; TC = thermocouple; W = value open in normal position; b4 = value closed in normal position.
they indicated the use of a GC analysis, no details have been given on the possible occurrence of stable intermediate compounds. In this article we report on the experimental investigation of the catalytic hydrogenation of 2,4-dinitrotoluene in a batch slurry reactor. The course of the reaction has been monitored by measuring the consumption of hydrogen during the reaction. Our main objective was to determine the influences on the hydrogen consumption rate of the reactor temperature, the hydrogen pressure, and the catalyst concentration. Furthermore, samples of the reaction mixture have been analyzed in order to investigate the reaction path and to determine as a function of time the change of the concentrations of the substrate, of any relevant intermediates, and of the end product. In a later second article we will present a mathematical model of the reaction kinetics.
Experimental Section Experimental Set up. The experimental setup is outlined in Figure 2. Nitrogen gas and hydrogen gas, both of a purity of above 99.9%, are drawn from gas cylinders without further pretreatment. The nitrogen gas is used a t the beginning of an experiment to purge the reactor to remove all oxygen from the system. The hydrogen gas is filled into a calibrated high-pressure storage cylinder of 532 mL up to a pressure of about 12 MPa, and both of the valves in the line from the gas cylinder to the storage cylinder are closed tightly. The reactor is operated under isobaric conditions by means of a Tescom pressure regulator, Type 44-2264-241, with a range of 0-3.5 MPa. The hydrogen consumed in the reactor is supplied from the hydrogen storage cylinder, where the pressure decreases. A detailed drawing of the reactor is presented in Figure 2. The reactor has been constructed of 316 stainless steel. The thickness of the reactor wall is 3 mm, and it has an inner diameter of 75 mm and a volume of 500 mL. The reactor is equipped with four baffles, which have a space of about 1 mm between the wall of the reactor and the back of the baffles. This will prevent accumulation or settling of catalyst particles in the dead zones near the baffles. A Rushton turbine impeller with six blades is driven by a magnetic coupling MRK 3 of Medimex, which has been fitted in the top lid of the reactor and has been connected to a rotary current motor. The speed of the motor can be adjusted between 10 and 50 rps by means of a Microsyn 3011 electronic frequency controller. The speed employed normally is 20-25 rps.
756
Ind. Erg. Chem. Res., Vd. 29, No. 5 . 1990
1$ I
‘K
mdgnetii Id\?
Figure 4 Data-acquisition and process control system.
t Figure 3. Batch slurry reactor. 1, gas inlet; 2, magnetic coupling: 3, PTFE gaskets, 4,cooling air out; 5 , baffles; 6, Thermocoax electrical heating element; 5, ceramic insulation shell; 8, copper cylinder with cooling fins; 9, turbine impeller; 10,cooling air in.
Liquid samples can be taken by means of a thin dip pipe, which extends through the top lid to about half-way down the round bottom of the reactor. The samples are taken manually through a valve. The reactor is heated electrically with a Thermocoax element that has been imbedded in the outside wall of the reactor vessel. The reactor temperature is controlled by a Eurotherm electronic temperature controller, Type 810. Around the reactor vessel and the heating element, a tightly fitting copper cylinder with 24 cooling fins has been brazed, see Figure 3, and the whole has been mounted into an air cooling jacket. Electronic pressure transducers of Bell & Howell are used to measure the pressure in the reactor and in the hydrogen storage cylinder. The pressure transducers are calibrated regularly against bourdon-type high-precision manometers. Calibrated chromel-alumel thermocouples are used for the temperature measurements. The temperature in the hydrogen storage cylinder is measured by a thermocouple that extends directly into the hydrogen gas. The temperature of the reaction mixture is measured with a thermocouple that has been inserted into a dip pipe filled with silicon oil and extending through the top lid about half-way into the liquid phase in the reactor. The reactor system has been automated by using a pMac-5000 programmable, stand-alone process control computer of Analog Devices. The pMac has no screen, keyboard, or mass-storage facilities. These are provided by coupling a personal computer system (PC) to the pMac via an RS-232 interface. During an experiment all data are stored in the &Mac‘smemory. Operator attention is required only to start up the reactor control program and to retrieve the data with the PC and store them on a disk, pending further processing and analysis. During the rest of the experimental procedure, the PC can be disconnected and used for other purposes. A schematic representation of the data acquisition and control system is given in Figure 4. For a detailed description of the automation of the reactor, we refer to Janssen (1989). Experimental Procedure. The experiments have been carried out with 2,4-dinitrotoluene, of a purity of above 98%, obtained from Janssen Chimica, Belgium, and a commercial catalyst of 5% Pd on active carbon, article
20568, also from Janssen Chimica. As a solvent, technical grade methanol has been used. To check for the influence of the purity of the methanol on the kinetics of the reaction, some preliminary experiments have been reproduced with methanol pro analyse as a solvent: comparison of the experimental results revealed no influence on the overall reaction rate of the two grades of the solvent. The clean and dry reactor is filled with some methanol, carefully weighed amounts of 2,4-dinitrotoluene and catalyst, and the remainder of the measured amount of methanol, in that sequence; as a rule, about 10 g of DNT, 0.25 g of catalyst, and 250 mL of methanol have been used. After the reactor has been closed tightly, for safety reasons a checklist in the control program has to be fulfilled before the control of the reactor is completely turned over to the process computer. The control program is supplied with the desired operating conditions for the experiment. Then the program prepares the reactor for the experiment. The gas phase of the reactor is flushed four times with nitrogen gas. It can be checked that the amount of methanol lost by evaporation in the purging steps is negligible. Next the agitator and the electrical heating are turned on. When the desired reactor temperature has been reached, the system is allowed to stabilize for 10 min. Then the agitator is turned off, and after 1 min, the magnetic valve in the hydrogen supply is opened to pressurize the reactor with hydrogen. After 30 s, in which the temperature in the hydrogen storage cylinder can stabilize again, the reaction is started by turning on the agitator. Immediately the first set of measurements is made. For each measurement set, the reaction time ( t ) ,the temperature (7K) of the liquid phase, the pressure (pR)in the reactor, and the temperature (Tsc)and the pressure ipse) in the hydrogen storage cylinder are recorded. During the first 2 min of an experiment, the measurements are made at fixed intervals of 10 or 15 s. Later on, the time intervals are doubled each time the decrease of the pressure in the hydrogen storage cylinder between two measurements becomes less than 4 kPa. The maximum time interval is 640 s. The end of the reaction is detected when four measurements have been made a t the maximum time interval without further decrease of the pressure in the hydrogen storage cylinder. Then the hydrogen supply is closed and the electrical heating is turned off. The reactor is depressurized, filled with nitrogen to 0.6 MPa again, and iooied down quickly to 30 O C , after which the agitator is turned off too. Processing of the Experimental Data. The experimental temperature and pressure data are used to calculate the molar amounts of hydrogen present in the storage cylinder and in the reactor. The molar amount of hydrogen (rzsc) in the high-pressure storage cylinder is calculated with
where ZH2 is the compressibility factor for hydrogen. Because the reactor is operated under isobaric conditions, the amount of hydrogen gas in the reactor itself is virtually
Ind. Eng. Chem. Res., Vol. 29, No. 5, 1990 757 constant throughout an experiment. Moreover, pressures are much lower in the reactor than in the storage cylinder so that the ideal gas law is used to calculate the molar amount of hydrogen gas in the reactor: (4) Here Vg,Ris the volume of the gas phase and p H z is the true hydrogen pressure in the reactor. To obtain the hydrogen pressure the total reactor, p R is corrected according to p H 2 = pR- po, where p o accounts for the vapor pressure of the methanol and the pressure of the nitrogen gas remaining in the reactor after the rinsing. The pressure (PO) in the reactor is measured a t the end of the temperature equilibration period, just before the reactor is pressurized with hydrogen gas. The volume of the gas phase in the reactor ( Vg,R)is obtained by subtracting the volume of the methanol used in the experiment from the total volume in the reactor. At the instant the agitator is turned on, the hydrogen gas immediately will dissolve into the liquid. Preliminary experiments with hydrogen and with 250 mL of pure methanol charged into the reactor showed that equilibrium is reached within 1 s. To determine the amount of hydrogen gas consumed by the reaction, the molar amount of hydrogen that is available in the reactor system a t the starting point of the reaction has to be corrected for the amount of hydrogen gas that dissolved in the methanol. T o this end, we derived a relation for the solubility constant of hydrogen in pure methanol as a function of the temperature from data given by Radhakrishnan et al. (1983). Furthermore, since the mole fractions of the aromatic compounds in the liquid phase are very low, we assumed that the solubility of hydrogen is not influenced by these compounds. Analysis of Reaction M i x t u r e Samples. To take a sample of the reaction mixture, the process control computer can be instructed to break off a hydrogenation experiment at a specified hydrogen consumption or a certain reaction time. Then agitation is stopped, the hydrogen supply to the reactor is shut off, and the hydrogen gas is purged from the reactor to nearly atmospheric pressure. Next a sample is drawn off manually through the sampling valve. After around 10 mL of the liquid has been discarded to rinse the sampling line, about 5 mL of the reaction mixture is collected in a brown flask. After the sample has been taken, the experiment is terminated following the normal experimental procedure of cooling down the reactor and rinsing with nitrogen gas. Consequently, to take a sample of the liquid reaction mixture, the reactor each time is charged with fresh DNT and catalyst and a new hydrogenation experiment is executed. A Varian 3400 flame-ionization gas chromatograph equipped with a 6-ft X 1/8-in. column packed with Tenax-GC, 60-80 mesh, has been used for routine analysis of the samples. Nitrogen a t 30 mL/min is used as the carrier gas. The temperatures of both the injector and the detector have been set to 270 "C. The temperature of the column is kept at 230 "C for 5 min, programmed from 230 to 235 "C at 1.3 "C/min, and finally is kept at 235 "C until elution is complete, usually within 10 min. A sample has been analyzed three times in a series, caring for a min'i m W delay between the analyses because the samples showed to be sensitive to disproportionation reactions. During the time between the injections into the gas chromatograph, the sample flask has been stored in a dark place. By comparing retention times of the eluents in the liquid samples and of the pure compounds, four major peaks in
Table I. Retention Times of t h e E l u e n t s i n t h e S t a n d a r d GC Analysis retention time, min compd in reaction mixture of pure compd 2,4-DNT 3.9-4.4 4.0-4.5 5.1-5.7 4-A-2-NT 5.0-5.4 7.1-7.9 2-A-4-NT 6.8-7.3 3.2-3.4 2,4-DAT 3.O-3.4 1.8-2.0 x2 2.3-2.4 x3 3.0-3.1
x,
t mule
n
1000
21~x1
"5
30
reactlun ume -~ 5
hoixi
7(m)
Figure 5. Total molar amount of hydrogen gas (nH ) present in the storage cylinder as a function of the reaction time Por a typical experiment at 308 K and 2 MPa.
the gas chromatograms have been identified as DNT, 2amino-4-nitrotoluene, 4-amino-2-nitrotoluene, and DAT. Furthermore, three minor peaks have been found that have not been identified yet. A summary of the retention times of the eluents is presented in Table I. In addition to the routine analysis, some samples have been analyzed by GC-MS. T o this end, the gas chromatograph has been equipped with a CP-wax-51 capillary column with a length of 25 m and a diameter of 0.22 mm. Nitrogen gas has been used as the carrier gas. The column was kept at 50 "C for 4 min and programmed from 50 to 240 "C a t 8 deg/min. The gas chromatograph has been connected to a Finnigan MAT311A double-focus mass spectrometer. Experimental Results Typical Experiment. In Figure 5 the total molar amount of hydrogen, nH = nsc + nR, as a function of the reaction time has been plotted for a typical experiment a t 308 K and 2 MPa. In the insert in the upper right corner of Figure 5, the first 150 s has been enlarged. The results in Figure 5 show a smooth curve with very little experimental noise. In the insert, the effect of dissolving the hydrogen in the first seconds of the experiment can be seen clearly. The arrow indicates the corrected initial amount of hydrogen a t the starting point of the reaction. The pressure and the temperature in the reactor of the experiment are plotted in parts a and b of Figure 6, respectively. Throughout the course of the reaction, the reactor pressure slowly increased by about 0.5% to 2.06 MPa, whereas at the very end of the reaction the pressure increased to about 2.07 MPa. This effect is caused by the decrease of the flow of the hydrogen gas through the pressure control valve during the reaction. Nevertheless, we can conclude that the pressure has been virtually constant throughout the experiment. The mean temperature of the experiment has been 308.4 K. In Figure 6b, we can see that the deviations from the mean temperature remained less then 0.5 K, even at the start of the reaction where suddenly a large amount of reaction heat was liberated.
758 Ind. Eng. Chem. Res., Vol. 29, No. 5 , 1990
1
‘0’
L I
Figure 6. Total pressure and temperature of the liquid phase in the reactor as a function of the reaction time during the experiment shown in Figure 5. I iJ
(1 2
ill
11 3
--A
xmoie
m‘
Figure 8. Peak areas in arbitrary units versus the concentrations of DNT, 4-A-2-NT, 2-A-4-NT, and DAT as used for calibration of the routine GC analysis
0k
t
Oh
’1%
I
O4 ,I 2
0
__ reaction ome
A
\
Figure 7 . Hydrogen converstion (,fH2)of the experiment of Figure 5 as a function of the reaction time.
The hydrogen conversion (i;I ) is defined as the amount of hydrogen actually consumeh by the reaction over the amount required for a complete conversion of the DNT to DAT:
In Figure 7 the hydrogen conversion ( { H J of the typical experiment has been plotted as a function of the reaction time. The overall hydrogen consumption rate is represented by the slope of the hydrogen conversion curve. By close inspection of Figure 7, we can assign four regions to the course of the reaction. In the first region up to a conversion of around 0.15, the hydrogen consumption rate decreases gradually. In the second region up to a conversion of around 0.4, the rate exhibits a zeroth-order behavior. The rate drops sharply around a conversion of 0.4 and exhibits a zeroth-order behavior again up to a conversion of 0.75. In the fourth region, the hydrogen consumption rate slowly drops to zero as the reaction nears completion. When we compare our results to the experiments of Yuculen (1984) for the hydrogenation of the isomer 2.6DNT, we observe two pronounced differences: (1)As shown in eq 2, in the first reaction step the 2,tiDNT is converted to the only intermediate, 2-amino-6nitrotoluene, until nearly all 2,6-DNT has reacted away and zeroth-order behavior has been found directly from the beginning to the end of the first reaction step. (2) The transition between the first and the consecutive reaction steps of the hydrogenation of 2,6-DNT occurs at a hydrogen conversion of around 0.50. In contrast to the 2,6 compound, we must realize that in the case of 2,4-DNT two chemically distinct nitro groups can react simultaneously, which may give rise to a different course of the hydrogen consumption sate. When the sharp
XI
Figure 9. Relative concentrations of DNT, 4-A-2-NT, 2-A-4-NT, and DAT as a function of the reaction time at 308 K and 2 MPa.
drop in the reaction rate of the typical experiment a t a conversion of around 0.4 represents the point at which nearly all the 2,4-DNT has been converted and where consecutive reactions will start, whether or not via parallel pathways, we must conclude that the reactions do not proceed through amino-nitro compounds exclusively. Furthermore, the gradual decrease of the hydrogen consumption rate at the beginning of the reaction may be explained by a complex reaction pathway as well as by a rapid deactivation of the catalyst. However, from hydrogen consumption data alone, we cannot answer these questions; to this end, an analysis of the liquid reaction mixture is required. Elaboration of the Reaction Pathway. To study the reaction pathway, hydrogenation experiments have been performed at a constant reactor pressure of 2 MPa and a t reactor temperatures of 308,353, and 357 K, respectively. Samples of the reaction mixture have been analyzed by using the routine GC analysis already described. The experimental results of the experiments at 308 K have been summarized in Table 11. By means of calibration, from the peak areas the concentrations of DNT, 2-A-4-NT, 4-A-2-NT, and I>AT have been calculated. In Figure 8 the I alibration curves of these compounds have been plotted, where the peak area is given in arbitrary units. In Figure 9 the relative concentrations (c,/co) a t 308 K, obtained by dividing the concentrations (c,) of the compounds by the initial concentration (c,) of DNT, have been plotted as a function of the reaction time. By comparing Figures 7 and 9, we can see immediately that after about 900 s and around a conversion of 0.4 nearly all DNT has reacted away. Then also the concentration of 2-A-4-NT reaches a maximum, and the production of DAT starts. However, at this point, the maximum concentration of 4-A-2-NT has not been reached yet. This indicates that
Ind. Eng. Chem. Res., Vol. 29, No. 5 , 1990 759 Table 11. Basic Analytical Data for the Experiments at 308 K and 2 MPa peak area, au"
tred,
no. 1 2 3 4 5 6 m
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
s kg/mol
Hzconv
0.59 1.39 1.93 2.79 3.86 4.34 4.90 5.03 6.49 6.64 7.97 8.37 8.93 9.04 9.33 11.68 13.28 13.91 14.32 16.93 17.27 22.41 33.54
0.117 0.198 0.243 0.308 0.382 0.406 0.432 0.438 0.505 0.512 0.573 0.591 0.616 0.621 0.634 0.729 0.787 0.808 0.821 0.894 0.903 0.987 1.000
DNT 4.481 3.317 2.643 1.419 0.173 0.067 0.028 0.030 0.018 0.021 0.021 0.032 0.026 0.012 0.018 0.031 0.000 0.000 0.000 0.008 0.039 0.000 0.000
4,2-ANT 0.671 0.833 1.067 1.251 1.543 1.581 1.644 1.804 1.802 1.934 2.224 2.499 2.250 2.310 2.304 2.059 1.959 1.854 1.852 1.295 1.032 0.226 0.016
2,4-ANT 0.255 0.350 0.482 0.551 0.656 0.632 0.624 0.678 0.532 0.554 0.554 0.583 0.500 0.457 0.470 0.342 0.273 0.242 0.204 0.119 0.100 0.003 0.000
DAT 0.034 0.095 0.170 0.178 0.287 0.264 0.405 0.511 0.758 0.858 1.579 1.785 1.830 2.284 1.961 2.736 3.507 3.533 3.127 4.930 4.276 6.109 6.723
XI
X,
X,
0.149 0.248 0.268 0.324 0.319 0.253 0.292 0.246 0.197 0.197 0.159 0.129 0.120 0.073 0.107 0.044 0.017 0.008 0.003 0.002 0.000 0.000 0.000
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.003 0.011 0.013 0.007 0.010 0.007 0.008 0.010 0.011 0.005 0.013 0.014 0.025 0.036
0.000 0.000 0.000 0.001 0.002 0.000 0.007 0.006 0.016 0.009 0.030 0.030 0.030 0.023 0.026 0.030 0.031 0.021 0.013 0.000 0.024 0.000 0.000
au = arbitrary units.
co
I
O4
n?
a.u.
a.u
I
I
0
n
02
0.4
0.6
b
deficit
I
1.n
0.8
-1;
02
04
-r
06
na
0
02
04
-5%
06
08
i n
Figure 11. Peak areas of the unidentified eluents XI, X2, and X3 as a function of the hydrogen conversion for the experiments a t 308 K and 2 MPa.
I 10
4
H2 Figure 10. Relative concentrations of DNT, 4-A-2-NT, 2-A-4-NT, and DAT and the material balance deficit as a function of the hydrogen conversion for the experiments a t 308 K and 2 MPa.
4-A-2-NT is not formed directly from DNT and that another intermediate must be involved. Furthermore, a careful examination of Figure 9 reveals that there is a significant deficit in the material balance of the aromatic compounds. For convenience in Figure loa, the relative concentrations have been replotted as a function of the hydrogen conversion. We assume that a t a constant reactor temperature the composition of the reaction mixture is a unique function of the hydrogen conversion and that by plotting the concentrations versus the hydrogen conversion the scattering of the experimental results, caused by differences between experiments with regard to the hydrogen pressure in the reactor, the catalyst concentration and the initial DNT concentration, is eliminated. In Figure 10b the deficit in the material balance for aromatic compounds has been plotted. From Figure 10 we can read that the maximal deficit is reached when nearly all DNT has reacted away. Therefore, we assume that the unknown intermediate is formed from DNT in parallel to the reaction from DNT to 2-A-4-NT. In Figure 11the response signals
of the three unidentified eluents have been plotted as a function of the hydrogen conversion. Since the data for eluent X2 show the same path as the relative concentration of DAT in Figure loa, we presume that X2 corresponds to another fully hydrogenated species of dinitrotoluene, probably 2,5-DNI', which is present as an impurity in the 2,4-DNT. A t this point, eluent X, appears to be related to the unknown intermediate in the reaction path of DNT to 4-A-2-NT. Analogously we presume that eluent X, plays a role in the reaction of 2-A-4-NT to DAT. In order to check this hypothesis concerning X, and X,, separate hydrogenation experiments have been performed with the pure substances 2-A-4-NT and 4-A-2-NT. In liquid samples taken during the hydrogenation of 2-A-4-NT to DAT, eluent X, has been found, whereas in liquid samples taken during the hydrogenation of 4-A-2-NT no other peaks than those corresponding to the reagent and the end product DAT have been detected. These results indicate that the hypothesis concerning X, and X, is correct. Note that apparently the intermediate compounds related to X, and X3 are formed during the hydrogenation of the p-nitro group of DNT exclusively. According to the Haber reaction scheme, the most direct route to an amine is via the corresponding nitroso compound and hydroxyl compound. Since nitroso compounds are highly reactive, we assume that the unknown intermediates related to X, and X3 are the N-hydroxy-4-
760 Ind, Eng. Chem. Res.. Vol. 29, No. 5 , 1990
HcpHbd< 03
>&-\.
j,
--
Figure 12. Two possible pathways for the formation of DNAT from 4-HA-2-NT.
methyl-$nitrobenzeneamine and the N-hydroxy-3amino-4-methylbenzeneamine, which are referred as 4(hydroxyamino)-2-nitrotoluene(4-HA-2-NT) and 4-(hydroxyamino)-2-aminotoluene(4-HA-2-AT),respectively. Brand and Steiner (1923) have described as follows a method for the synthesis of 4-HA-2-NT by a catalytic hydrogenation of D N T over P d j C as a catalyst: 70 mL of ethanol, 10 mL of water, 0.02 mol of DNT, and 0.1 g of a P d / C catalyst are shaken in a flask with 0.04 mol of hydrogen at room temperature and atmospheric pressure. Via extraction with benzene. 4-HA-2-NT is obtained with a melting point of 99-100 "C and a yield of 70%. Making minor adjustments to this procedure, we have tried to synthesize 4-HA-2-NT in the batch slurry reactor by using 10 g of DNT and 0.3 g of catalyst in 250 mL of methanol at a temperature of 25 "C and at a total reactor pressure of 200 kPa. The hydrogenation has been interrupted at a hydrogen conversion of 33%. The methanol was removed by using a rotating film evaporator, and the remaining residue has been dissolved in benzene. After cooling, crystals formed that have been isolated by filtration and have been washed with cold benzene. Thin-layer chromatography on silica gel with CHC1, as eluent revealed that the crystals consisted of a mixture of two components. This has been confirmed by NMR analyses in CDC1,. By use of a silica gel column and CHCI, as eluent, two fractions could be separated, from which crystals were isolated after removal of the solvent. The melting trajects of the crystals of the first and the second fraction were 156-158 and 99-102 "C, respectively. The crystals have been analyzed with NMR and mass spectroscopy. The first fraction could be characterized as the 3,3'-dinitro-4,4'-dimethylazoxybenzene ( M + = 316.0832, CI4Hl2N405:M + = 316.08081, which are referred to as dinitroazoxytoluene (DNAT). The second fraction could be identified as 4-HA-2-NT ( M + = 168.0533, C7H8N203:M + = 168.0535). After the isolation. the yield of 4-HA-2-NT was 18.6%. Taking into account that DNAT contains two aromatic rings, the yield for this compound was 15.270. The formation of DNAT as a byproduct during the synthesis of 4-HA-2-NT has been reported by Brand and co-workers (Brand and Steiner, 1923; Brand and Modersohn, 1928; Brand and Mahr, 1931). They have presented two possible reaction pathways for the formation of DNAT from 4-HA-2-NT as given in Figure 12. Furthermore. DNAT is known to be formed from 4-HA-2-NT under influence of oxygen or air (Houben and Weyl, 1971). Because the nitroso compound tends to react immediately to the hydroxylamine, we think that the DNAT is formed during the isolation procedure according to the second reaction in Figure 12. In that case. since for each DNAT
F i g u r e 13. (a) X I peak area as a function of the concentration of solutions of pure 4-HA-2-NT in methanol. (b) XI peak area as a function of the 4-HA-2-NT concentration in mixtures of 4-A-2-NT and 4-HA-2-NT in methanol. 12, plre4-HA-2Xl
mixrureoi4-HA-2.NTand4.A-2-NT mixrureoi4-HA-2.NTand 4.A-2-NT
I
o n?
--flM
OM
o nR
O IO
kmole m
Figure 14. (a) 4-A-2-NT peak area from 4-HA-2-NT, as a function of the concentration of solutions of pure 4-HA-2-NT in methanol. (b) 4-A-2-NT peak area from 4-HA-2-NT, as a function of the 4HA-2-NT concentration in mixtures of 4-A-2-NT and 4-HA-2-NT in methanol.
molecule three molecules of 4-HA-2-NT are involved, the overall yield for the synthesis of the hydroxylamine from DNT has been 41.4%. Brand and Steiner arrived a t a yield for 4-HA-2-NT of 70%. We presume that we arrived at a much lower yield mainly because we did not optimize the isolation procedure of the raw reaction mixture. Analysis of a solution of pure 4-HA-2-NT in methanol by the standard GC procedure reveals both an XI peak and a 4-A-2-NT peak. We presume that the hydroxylamine is not stable in the injector of the gas chromatograph and decomposes instantaneously according to the second reaction in Figure 12. In Figures 13 and 14, the measured peak areas of XI and 4-A-2-NT are plotted as a function of the 4-HA-2-NT concentration. Note the strongly curved line of the X1 peak area. The slope observed for line a in Figure 14 is around one-third of the slope observed a t the calibration of pure 4-A-2-NT. This agrees very well with the stoichiometry of the second reaction in Figure 12. Unfortunately, a t this moment we still have not been able to identify the structure of XI. Because the true 4-A-2-NT as well as the 4-HA-2-NT in the reaction mixtures contributes to the 4-A-2-NT peak area, we must perform a correction on the analytical results to find the real 4-A-2-NT concentration. To check whether the 4-HA-2-NT and the 4-A-2-NT show interference in the analysis, we have analyzed several mixtures with different concentrations of both compounds in methanol. The results are presented in Table 111. In this table, the 4-A2-NT peak area resulting from 4-HA-2-NT in the solution is calculated by subtracting the expected 4-A-2-NT peak area of the true 4-A-2-NT from the total 4-A-2-NT peak area measured. The results have been plotted in Figures
Ind. Eng. Chem. Res., Vol. 29, No. 5, 1990 761 Table 111. 4-A-2-NT and X, Peak Areas of Mixtures at Different Concentrations of 4-HA-2-NT and 4-A-2-NTa 4-A-2-NT concn, calcd 4-A-2-NT measd 4-A-2-NT 4-A-2-NT peak area 4-HA-2-NT measd XI kmol/m3 peak area, au peak area, au from 4-HA-2-NT, au concn, kmol/m3 peak area, au 0.0150 0.094 0.207 6.83 7.13 0.30 0.0189 0.116 0.064 2.11 2.35 0.24 0.0189 0.107 0.120 3.96 4.32 0.36 0.0374 0.142 0.078 2.56 3.14 0.58 0.0919 0.278 0.091 3.02 4.22 1.20 au = arbitrary units.
Table IV. Processed Analytical Results for the Experiments at 308 K and 2 MPa relative concentration, au tred, 4,2-ANT 2,4-ANT DAT H, conv DNT no. s kg/mol 1 0.59 0.117 0.633 0.030 0.043 0.005 2 1.39 0.198 0.468 0.000 0.060 0.014 3 1.93 0.082 0.026 0.243 0.373 0.000 2.79 0.027 4 0.308 0.200 0.094 0.000 3.86 0.112 0.044 5 0.382 0.024 0.027 4.34 0.108 0.040 6 0.079 0.406 0.009 4.90 0.106 0.062 7 0.060 0.432 0.004 5.03 0.115 0.078 8 0.115 0.438 0.004 6.49 0.091 0.115 9 0.150 0.505 0.003 6.64 0.512 0.003 0.094 0.130 0.168 10 7.97 0.094 0.240 0.235 11 0.573 0.003 12 8.37 0.099 0.271 0.294 0.591 0.005 8.93 13 0.616 0.004 0.085 0.278 0.266 14 9.04 0.078 0.347 0.296 0.621 0.002 9.33 0.283 15 0.634 0.003 0.080 0.298 0.058 0.416 0.269 16 11.68 0.729 0.004 0.046 0.263 17 0.787 0.000 13.28 0.533 0.251 13.91 0.808 0.000 0.041 0.537 18 14.32 0.035 0.475 0.252 19 0.821 0.000 0.176 0.894 0.001 16.93 0.020 0.749 20 0.017 0.650 0.141 17.27 21 0.903 0.006 22.41 22 0.929 0.031 0.987 0.000 0.001 1.022 0.002 33.54 23 1.000 0.000 0.000
13 and 14 too. Apparently the X1peak area is strongly influenced by the presence of 4-A-2-NT; the points for the 4-A-2-NT peak have moved to systematically higher values, whereas the slope of the line through the points has remained equal. The experimental results s h o w in Figures 10a and 11 indicate that only very low concentrations of the 4-HA-2-AT can be expected in the reaction mixture. A t this moment, the analytical results do not allow determination of the concentration of the 4-HA-2-NT within the experimental error. Therefore, we further neglect this compound and assume that for practical purposes a satisfactory reaction pathway for the hydrogenation of DNT can be described as s h o w in Figure 15. As a consequence, the following procedure has been followed to calculate the concentrations of the components in the reaction mixture: (1)The concentrations of DNT, 2-A-4-NT, and DAT are calculated from their respective peak areas by use of the calibration constants according to Figure 8. (2) At X1peak areas higher than 0.09, the concentration of 4-HA-2-NT is calculated according to line b in Figure 13, which is given by Sx, = 0.063 + 2.32Cq-HA.Z.NT; a t Xl peak areas between 0 and 0.09, the concentration of 4HA-2-NT is calculated according to Sx, = ~ . ~ ~ C & H A . ~ . N T . (3) Next the contribution of 4-HA-2-NT to the total 4-A-2-NT peak area is calculated using the calibration constant for line a in Figure 14; this peak area is then subtracted from the total 4-A-2-NT peak area, and from this result, the true 4-A-2-NT concentration is calculated by use of the calibration constant for 4-A-2-NT. (4) We assume that any remaining deficit in the material balance of the aromatic compounds is caused by uncontrolled disproportionation of the hydroxylamine, and therefore, the remaining deficit is added to the concentration of the 4-HA-2-NT.
4-HA-2NT 0.289 0.458 0.519 0.679 0.793 0.763 0.768 0.688 0.642 0.605 0.428 0.331 0.367 0.277 0.337 0.252 0.158 0.171 0.238 0.053 0.187 0.000 0.000
calcd conv 0.138 0.197 0.240 0.300 0.377 0.388 0.401 0.422 0.449 0.463 0.547 0.578 0.576 0.626 0.592 0.664 0.740 0.740 0.698 0.865 0.791 0.944 1.023
parity 1.180 0.993 0.986 0.974 0.987 0.955 0.927 0.963 0.889 0.904 0.955 0.978 0.935 1.009 0.933 0.910 0.941 0.916 0.850 0.968 0.876 0.957 1.023
KHOH
2 4-diormnoioiuene ioiuene
NO2 2-mno4.nmoioluene
Figure 15. Reaction pathway for the hydrogenation of 2,4-DNT. IO*
'b
0
0'2
'\ A -
