. GHARDA1 and C. M. SLIEPCEVICH2 Department of Chemical and Metallurgical Engineering, University of Michigan, Ann Arbor, Mich. Copper Catalysts in Hydrogenating Nitrobenzene to Aniline Ind. Eng. Chem. 1960.52:417-420. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/23/19. For personal use only. Up to 400° F., stable and produce nearly Industrial application is enhanced by the catalysts are theoretical yields of aniline. • availability of raw materials from petrochemical operations • possibilities for controlling catalyst poisons in the feed • ease IVÍanufacturing of maintaining the required temperature range in fluidized bed reactors aniline by cataly- tically hydrogenating nitrobenzene is attractive as raw materials are available, high, and the process incurs no by-product disposal problem. Nitrobenzene can be hydrogenated over a variety of catalysts in both liquid and vapor phases. In this study, the vapor phase hydrogenation ovfer copper catalysts was investigated. It was found that under optimum conditions copper catalysts are stable and produce nearly theoretical yields of aniline from nitrobenzene. However, sulfur compounds and dinitrobenzenes in the feed rapidly deactivated the catalyst at all operating temperatures. Furthermore, as the highly active catalysts are stable only in a very narrow temperature range, the high heat of reaction must be removed. A fluidized-bed reactor would be ideal for this purpose. The mole ratio of nitrobenzene to hydrogen and the particle size of the catalyst are important inasmuch as they affect the actual temperature of the catalyst particle. Catalyst deactivation is essentially irreversible—i.e., catalyst activity could not be restored by oxidation followed by reduction. The rate of hydrogenation of nitrobenzene in gram moles/hour/gram of barium-promoted copper chromite catalyst at 400° F. was correlated by the following equation: yields are r = adsorbed hydrogen and nitrobenzene dissociated on two active centers. Experimental Equipment and Materials. The experimental equipment is shown. The reactor and all connecting lines were of catalytic glass to eliminate extraneous effects. Further details have been reported by Gharda (2). Hydrogen and nitrogen, 99.9% pure with traces of oxygen and water, were obtained from commercial cylinders. The various catalysts used are designated below: Catalysts No. Composition A CuCrOz" B CuCrOz Preparation Vs-in. unsupported pellets l/s-in. pellets impregnated on kieselguhr (CuO, 5%; Cr203, 2%)° C Ba-promoted Vs-in. pellets D CuCrOz6 CuO Impregnated silica gelc on No. Composition E CuO F CuO -f- CrzOz G CuO “ “ Preparation Supported on silica gel prepared by coprecipitation Supported on silica gel prepared by coprecipitation on Impregnated activated AlzOa1* Harshaw Chemical Co. 6 Girdler Co. Davison Chemical Co. d Alcoa F-10. 0.475 V-PN„2 pn The rate controlling step appears to be the surface reaction between molecularly 1 Present address, 145 Bandra, Bombay 20, India. 2 E. Hill Rd., Present address, University of Okla- homa, Norman, Okla. Reactor and connecting lines were of glass, as a steel reactor introduced traneous catalytic effects and the studies were made at atmospheric pressure VOL. 52, NO. 5 · MAY 1960 ex- 417 O 1.0 2.0 3.0 TIME Figure 1. Nitrobenzene prepared in the laboratory from sulfur-free benzene gave maximum conversions and least catalyst deactivation Four grades of nitrobenzene as designated below: were used Nitrobenzenes No. Preparation A As-received analytical reagent grade® B Prepared from reagent grade benzene, HNOs, and HiSOj; free of dinitrobenzene C petroleum-derived Prepared from benzene and technical HNOs and H2SO4; about 0.1% dinitrobenzene D Nitrobenzene B + 1% (wt.) pure indinitrobenzene “ Fisher Scientific Co. was first Procedure. The reactor with catalyst. To maintain isothermal conditions to within 10° F., the catalyst was diluted with either Ve-inch glass balls or Ottawa sand. After the catalyst was charged to the reactor, the system was purged with hydrogen for 15 minutes. The reactor packed was then adjusted to the desired pretreatment temperature. Generally, the reaction temperature was less than the pretreatment temperature; therefore, following pretreatment the reactor heater had to be adjusted for the reaction heater t.0 ( 5.0 6.0 HOURS) Figure 2. The higher rate of deactivation at low hydrogen to nitrobenzene feed ratios was caused by localized temperatures and diffusion effects temperature, and the nitrobenzene feed was then introduced. The products were sampled periodically and analyzed for aniline by titration in glacial acetic acid against a standard solution of perchloric acid in glacial acetic acid, using methyl violet as an indicator. In all experiments where the catalyst stability was good the condensed reaction products contained only aniline, water, and unreacted nitrobenzene. No intermediate reduction products such as azoxybenzene or azobenzene could be detected. Results The experimental work involved finding first factors affecting catalyst stability and activity before determining the effect of space velocity on degree of Blank runs conversion. at 600° F. using only the catalyst diluents, glass balls, and Ottawa sand proved that no homogeneous reaction was taking place. Factors Affecting Catalyst Stability and Activity. Optimum conditions for catalyst stability were affected by particle size of the catalyst, time and tem- perature of pretreatment with hydrogen, reaction temperature, quality of nitrobenzene, and ratio of nitrobenzene to hydrogen in the feed. Under optimum conditions, activity of the catalyst was also reproducible with great precision. Grade A nitrobenzene, even when purified by washing with dilute sodium hydroxide and redistilling, showed less stability and activity than the grades prepared in the laboratory (Figure 1). Grade B showed some improvement over petroleum-derived nitrobenzene, Grade C, from which the olefins had been removed by sulfuric acid, but which contained about 0.1% dinitrobenzene. An experiment was made in which hydrogen gas was first passed over catalyst C in separate reactor before using to hydrogenate nitrobenzene. No improvement in catalyst stability was noted. Figure 3. The larger gradients in the large temperature catalyst particles accounted for decreased catalyst stability Figure 4. Above 400° to 450° catalyst decreased rapidly 418 INDUSTRIAL AND ENGINEERING CHEMISTRY F. HYDROGENATING 0 2.0 1.0 was The mole ratio of nitrobenzene to hydrogen was varied between 1 to 6 and 1 to 18—i.e., from twice to six times the theoretical amount (Figure 2). The deactivation at the lower ratios of hydrogen to nitrobenzene was presumably a temperature effect associated with diffusion into the pores and heat conductivity of the catalyst. Four sizes of catalyst were studied (Figure 3). The effect of particle size was concluded to be primarily the effect of localized temperature and diffusional gradients inside the pellet. Experimental runs were made at 50° F. reaction temperature intervals between 350° and 550° F. Activity increased with temperature; stability decreased above 400° to 450° F. (Figure 4). The pretreatment consisted of passing hydrogen over the catalyst for 2 hours. As shown in Figure 5, catalysts pretreated at 400° and 450° F. attained the same steady state activity; those activated at higher temperatures had a slightly initial activity which they lost rapidly. Apparently, the stable catalyst component is an unreduced complex between copper oxide and chromium oxide. At a higher temperature, copper is apparently formed which loses its activity rapidly. Catalysts A through G (see designations above) were investigated for their activity and stability under optimum reaction conditions. The results are summarized in Figure 6. Catalysts A, C, and F showed nearly the same stability; catalyst F was nearly twice as active as A or C. Catalysts D and E had slightly lower initial activity and were less stable than catalysts A or C. Catalysts B and G had rather low initial activity and lost it rather rapidly. Besides, catalyst G gave rise to a tarry product containing inter- attained by pre- S.O 4.0 S.O ad- Figure 6. Copper catalysts supported on silica were versely affected by impregnation with sodium silicate mediate reduction products. In the case of catalysts E and F, neither initial activity nor stability was affected by the following treatments: Removal of sodium ions by ion exchange with ammonium ions, followed by ignition at 900° F. Removal of sulfate ions either by starting with nitrates or by ion exchange of catalyst with ammonium and nitrate ions, followed by ignition at 900° F. However, initial activity and stability adversely affected by impregnation with additional sodium silicate. Catalyst Reactivation. Upon the completion of a run, the nitrobenzene flow was stopped and the hydrogen was kept on for another 15 minutes. The reactor was then purged with nitrogen. The temperature of the reactor was raised to 600° F., and air was passed gradually over the catalyst so that the temperature did not exceed 650° F. After initial reactivation had subsided, air was passed for another hour at 600° F. The catalyst was allowed to cool in the reactor in the current of air. It was removed from the reactor and ignited with free access to air in an electric muffle furnace at 900° F. for 2 hours. After cooling, the catalyst was returned to the reactor, pretreated with hydrogen in the standard way at 450° F., and the reaction was carried out as usual at 400° F. The activity of the regenerated catalyst was the were in tivity was restored by treatment hydrogen for 2 hours at 450° F. The regeneration procedure was carried out on fresh catalyst pretreated in hydrogen at 450° F. but which had not been deactivated by the reaction of nitroThe regenerated catalyst benzene. showed no change in activity. This experiment proved that there was nothing fundamentally wrong in the method of reactivation. Effect of Space Velocity on Degree of Conversion. This variable was investigated for catalyst C at only one ° temperature, 400 F., where the catalyst was known to be stable. The other conditions and results are summarized in Figure 7. Space velocity was varied by using different amounts of catalyst at a constant feed rate to avoid a high temperature rise and at the same time minimize mass transfer effects. For each experiment, a fresh charge of catalyst was used. Correlation of the Reaction Rate Data. The raw experimental data on conversion were graphically differentiated to obtain the rate of reaction, r. The rate was then correlated against the partial pressures of nitrobenzene, + /· />no2 + r is a 7Ch2 />h2 as when the reaction was interrupted to start the regeneration. This behavior is in contradiction with German practice (3).: When deactivation was caused by dinitrobenzene, the ac- />NOi, and hydrogen, ^. These results summarized in Figure 8. The rate equation = 0.475 V/’noj Ph, + are (1) simplified form of the equation ¿V/’NOj ¿Ha (1 same 6.0 ( HOURS I TIME Figure 5. Maximum catalyst stability treatment at 400° to 450° F. NITROBENZENE TCa Pa + · 2 / 2 )2 (2) where the K’s represent the adsorption equilibrium constants for nitrobenzene, The hydrogen, aniline, and water. denominator in the above equation reBecause presents the resistance term. VOL. 52, NO. 5 · MAY 1960 4 9 1 the data could be fitted by the simplified form, the resistance was essentially con- the range. 2 holds when the rate controlling step is the surface reaction between molecularly adsorbed hydrogen and nitrobenzene dissociated on two active centers. From purely theoretical considerations, such dissociation is difficult to visualize. stant over Equation Discussion As was expected, sulfur compounds in the feed deactivated the catalyst at all The sulfur operating temperatures. compounds, probably thiophene derivatives, could not be removed from the nitrobenzene; it was necessary to make nitrobenzene from a sulfur-free benzene. The deactivation of the catalyst by dinitrobenzene was unexpected; it is not of great technical significance because the nitrobenzene as usually prepared does not contain enough dinitrobenzene to affect catalyst stability appreciably. The deactivation was apparently due to strong adsorption of the product phenylenediamines, as the activity was restored by treatment with hydrogen at 450° F. Next to impurities in the feed, the most important variable affecting catalyst stability appeared to be the actual catalyst particle temperature. The mole ratio of nitrobenzene to hydrogen affected both the rate of reaction and the heat transfer characteristics of the bed. At lower mole ratios of nitrobenzene to hydrogen, not only was the amount ot heat release per unit weight of catalyst less but the heat transfer between the flowing gases and the particles was also Flence, stable operation improved. should be realized at a higher bed temperature. Similarly, at a higher mole ratio of nitrobenzene to hydrogen, catalyst stability should be improved by operating at a lower bed temperature. Both of these predictions were verified experimentally. The easier deactivation of larger catalyst particles could be explained by the larger temperature gradients present between the reaction site and the bulk of the catalyst in the larger particles. The stability of large catalyst particles was improved by operating at lower bed temperatures. Even when sulfur-containing impurities were absent catalyst deactivation was irreversible—i.e., catalyst activity could not be restored by oxidation followed by reduction. This behavior led to the conclusion that catalyst deactivation was not due to deposition of tars but was caused by slow, irreversible, thermal transformation of the catalyst structure. In this connection, it might be noted that copper chromite loses its activity in liquid phase hydrogenations if used at too high a temperature be- 420 0 Figure 7. Adverse effects of high temperature rise and mass transfer resistance were overcome by operating at sufficiently high space velocities W, weight of catalyst, grams feed rate, gram moles/hour F, 0.02 0.08 0.10 0.i2 · gm. mole/ hr. gm. Figure 8. Agreement between data and simplified rate equation indicates that resistance to reaction remained constant over the conditions investi- gated p, partial pressure, r, reaction of reduction to metallic copper. Hence, it is believed that the stable active catalyst is essentially unreduced copper chromite and that metallic copper rapidly loses its activity. This hypothesis was verified when it was found that catalysts pretreated at temperatures over 450° F. had both a lower initial activity and a lower stability. The only extensive published work on the catalytic hydrogenation of nitrobenzene using copper catalysts is that of Brown and Henke (7). They used copper oxide prepared either by ignition of the nitrate or by precipitation from the nitrate by hot dilute sodium hydroxide. The catalyst prepared by the latter method was superior to that prepared by the former. Their conclusions are not directly comparable with the present study. Their catalysts were not promoted or supported and hence were almost certainly reduced to metallic copper. Their catalysts were also very finely divided and yet of low activity—i.e., the amount of nitrobenzene reacted per unit weight of catalyst was low. Moreconcerned only with over, they were complete reaction, and hence their catalyst deactivation studies were not sensitive enough to detect small changes in catalyst activity. Furthermore, as they used a simple heated tube and measured only the exit temperature of the gases, no great reliance can be placed on their temperature values. The Germans have operated a process for the reduction of nitrobenzene to a aniline over copper catalyst (3). Their catalyst was made by impregnating lumps of pumice stone with a slurry of basic copper carbonate in sodium silicate solution. Their operating temperatures ranged from 200° C. at the entrance of the reactor to 350° C. at the exit; they also used 60 moles of hydrogen per mole of nitrobenzene feed—i.e., 20 times the theoretical requirement. They revivified their catalyst by controlled oxidation with air followed by reduction with 0.06 0.04 atm. rate, gram moles/hour/gram cause INDUSTRIAL AND ENGINEERING CHEMISTRY hydrogen. However, the present authors have found that initial activity and stability of copper catalysts supported on silica were adversely affected by impregnation with sodium silicate. The only published work on the kinetics of the reaction in the vapor phase is a brief mention by Wilson in connection with the design of a reactor (4). The rate of the reaction is given by the equation: 5.79 X 104 C”·57^-2·958-'7, r where r nitrobenzene reacting, gram rules/cc. hour expressed in terms of void volume in the = = reactor C T = = concentration of nitrobenzene, gram moles/cc. ° K. No mention was made about the type of catalyst used; very high ratios of hydrogen to nitrobenzene were used. At these high ratios the partial pressure of hydrogen is essentially constant, and so the equation is similar to the one proposed by the present authors. Acknowledgment A portion of this work was supported by grants from the Michigan Gas Association and Monsanto Chemical Co. Literature Cited (1) Brown, O. W., Henke, C. O., J. Phys. Chem. 26, 161, 715 (1922). (2) Gharda, K. H., Ph.D. thesis, University of Michigan, Ann Arbor, Mich., 1958. (3) Groggins, P. H., “Unit Processes in Organic Synthesis.” 4th ed., McGrawHill, New York, 1952. (4) Wilson, K. B., Trans. Inst. Chem. Engrs. (London) 24, 77 (1946). Received for review May 4, 1959 Accepted January 11, 1960 Part of the Ipatieff Award Presentation, Division of Industrial and Engineering Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959.