Hydrogenation of Toluene
....
Raney Nickel Catalyst React i o n Kinetics
I
H O W A R D LITTMAN' and HARDING BLISS Yale University, New Haven, Conn.
TOLLENE
hydrogenation provides a good system for studying kinetics. T h e reaction is clean-cut without undesirable side reactions and its rates are moderate and can be precisely measured. Commercial Raney nickel is easily treated to yield a nonpyrophoric. stable catalyst which gives reproducible rates of convenient magnitude. T h e liquid phase hydrogenation of toluene was studied between 131.3' and 171.4' C. for pressures u p to 100 atm. using this catalyst.
Equipment and Materials An agitated 2620-m1. Struthers-Wells autoclave made of Type 316 stainless steel and provided with a n internal cooling coil, was heated by four 750-watt Chromolox heaters, controlled through a Powerstat. The agitator, driven by a totally enclosed motor, could be operated a t 100 to 580 r.p.m. Conventional temperature and pressure instruments, a dry ice trap, a dryer, and a rupture disk rated to fail at 2725 p.s.i. were used. Hydrogen was supplied through a regulator. A small continuous purge maintained by a gas outlet prevented inerts in the hydrogen from building u p in the gas space. A sampling tube extending well into the liquid contents of the autoclave permitted Trithdraival of small portions of the reaction mass at intervals. Ordinarily, the sample contained the catalyst, but a glass wool filter could be added when desired to retain the catalyst. T h e sampling tube outside the autoclave led to a valve and a graduated cylinder for collection of the cooled samples containing the catalyst. This was used in determining hydrogenation rates. The sampling tube also led through a valve and a glass wool fiIter to a separate apparatus for determination of hydrogen solubility: a heated section of tubing in which the liquids were vaporized, throttle valves, two dry ice-acetone cooled traps in which the liquids Lvere condensed, and a gas buret for measuring the volume of hydrogen. A carbon dioxide tank was Present address, Syracuse University, Syracuse, N. Y .
attached to the system to permit flushing with this gas. A Burrell contact pipet was provided, so that the carbon dioxide could be removed from the region next to the gas buret when desired. Additional details of the equipment are given by Littman ( 3 ) . Hydrogen made by the iron-contact process was purchased from the Linde Air Products Co. T h e gas analysis, supplied by the company, was 99.64% hydrogen, 0.3070 nitrogen, 0.04570 methane. 0.01% carbon monoxide, 0.002Oj, oxygen, and O . O O l 7 0 carbon dioxide. ,4nalytical reagent grade toluene was obtained from the J. T. Baker Chemical Co., and further purified. Density and refractive index varied among the lots.
Toluene Density and Refractive Index Density, Lot Number G./MI. n9z AR grade 2361
8368 9049 NBS-API
0.8646 0.8669
1.4959 1.4951 1.4959 1.4969
Molybdenum trioxide, used in only a few runs, was prepared by the method of Liebert ( 5 ) .
Hydrogen solubility was determined in some runs by diverting the liquid sample to the solubility apparatus. T h e m t alyst was removed by filtration, the liquids were condensed and weighyd, and the hydrogen was measured in a gas buret. T h e portion of this apparatus between a valve near the buret and the autoclave was originally filled with carbon dioxide at atmospheric pressure; the buret and the portion u p to the valve mere filled with air. T h e hydrogen sample was brought to the buret, the accompanying carbon dioxide removed with potassium hydroxide in the Burrell contact pipet, and the lines were filled several times \\ ith carbon dioxide to m sure that all the hydrogen v a s transferred to the buret. Each time thc carbon dioxide was removed Thus, at the time of measurement the portion of the apparatus between the valve and the autoclave was again filled with carbon dioxide, and the other portion with hydrogen and air. T h e hydrogen volume was determined by difference. Appropriate corrections were made for the fact that the liquid traps were \veighed first containing air and finally containing carbon dioxide. T h e solubility of carbon dioxide in toluene at dry ice temperatures is considerable. but it is vaporized in warming u p to weighing temperature with negligible loss of toluene. T h e solubility was thus determined in pure toluene and in a mixture containing 27y0 methvlcvclohexane under reaction conditions. It was also determined when
Procedure and Analysis Hydrogenation rate runs were begun by charging toluene and catalyst to the autoclave, admitting hydrogen, testing for leaks, and applying heat. After 20 minutes agitation was begun, and after 60 minutes a liquid sample was withdrawn for analysis (60 minutes was sufficient to attain the desired reaction temperature in all cases). Eight milliliters of liquid were withdrawn to flush the line and 2 ml. more for the sample proper. Usually about 10 samples were withdrawn over the course of the run. Pressure, temperature, and purge gas flow rate were measured frequently. The samples, after cooling and filtration to remove the catalyst, were analyzed by determination of refractive index at 20' C. T h e amount of catalyst and the volume of the liquid removed were measured; the catalyst concentration in the samples was within 296 of the concentration charged to the autoclave. I n separate experiments, the catalyst concentration was checked on 100- and 200-ml. samples withdrawn from the autoclave. I t was concluded that the catalyst was totally and uniformly suspended during the run.
Figure 1. Solubility of hydrogen in toluene and in toluene containing 2770 methylcyclohexane is proportional to hydrogen pressure VOL. 51, NO. 5
MAY 1959
659
no reaction was occurring-Le., in the absence of catalyst. I n sampling for the hydrogen-solubility determination agitation was stopped to prevent the carryover of gas bubbles dispersed throughout the liquid.
Hydrogen Solubility Data T h e solubility of hydrogen in toluene and in toluene containing 27y0 of methylcyclohexane is proportional to the hydrogen pressure (Figure 1). The data at 32.2" C. for toluene in mole fractions of hydrogen per atmosphere of partial pressure agree with those of Cook ( 2 ) within 2%; this is within the precision of the data. The hydrogen s o h bility deter mined without catalyst was the same as under reaction conditions with catalyst.
Kinetic Data Eighty-nine runs were made ( 3 ) at 131.3" to 171.4" C., 135 to 1463 p.s.i.a., and catalyst concentrations of 0.00122 to 0.0178 gram per liter of toluene. T h e toluene conversion was about 2070 in most of the runs, although a few runs were carried to higher conversions. The primary concern of this work was with initial rates of reaction from 0.0124 to 1.137 moles of toluene hydrogenated per liter per hour. T h e initial rate, Ro, for any run is defined as the slope of the concentration time plot taken at the concentration of pure toluene or mathematically:
"",( C t ) o[SI
at constant P and T
R, = C1
I n each run, temperature and pressure were held constant and conversion of toluene to methylcyclohexane was measured several times. T h e precision of the data, as indicated by the standard deviation for the rate, averaged within about 1%. T h e reproducibility, established by repetition of runs under identical conditions, was within 27c (3). Mass Transfer Effects. The measured rate includes mass transfer: adsorption, and reaction steps. However, it is be-
Table l. Data for a Typical Run Indicate Zero-Order in Toluene
(Temp., 131.3' C. Press., 1009 p.s.i.a. Catalyst concentration, 6.55 g./L Initial rate, 0.2245 i 0.0009 mole/l.)
lieved that all mass transfer steps were rapid enough with respect to the reaction rate not to influence the results. T h e rate of agitation was set a t 300 r.p.m., above which the rate of reaction was not further influenced by it. T h e concentration of hydrogen in the liquid phase during a run was the same as in the absence of catalyst. Thus mass transfer of hydrogen from the gas to the liquid phase cannot be important. Mass transfer of hydrogen to the catalyst surface is probably not important because of the hydrogen order (1/2) and the calculated activation energy (20 kcal. per gram mole). T h e vigorous agitation, finely divided catalyst, and slow reaction rate should ensure the absence of this mass transfer effect.
Factors Affecting Initial Rates. TOLUENE ORDER.For a typical run the concentration-time plot is linear, indicating zero order in toluene (Table I). In a few runs carried to higher conversions, no deviation from linearity appeared until about 3oyO conversion. Three factors other than nonzero order can bring about curvature.
1. Volume increases as the reaction proceeds, because methylcyclohexane is less dense than toluene. This reduces catalyst concentration and decreases rate. 2. Hydrogen is more soluble in methylcyclohexane than in toluene ; this increases the rate somewhat. 3. Slow desorption ofmethylcyclohexane can also cause curvature. T h e magnitude below 30% conversion is considered negligible, as the first two factors account for any observed curvature. Below 20'3 conversion, the first two effects are small and approximately equal. .4bove 2 07'0 conversion, catalyst concentration becomes more important. At 30% conversion, the unbalance of the factors causes the rate to fall about 3 to 670 less than the initial rate, depending on the temperature and pressure of the run. This decrease is large enough to be detected in the rate data. To obtain the true curvature (if any) above 30% conversion, data on the absorption of methylcyclohexane as well as additional specific volume and solubility data would be necessary, conversions were kept at about 2076. The tentative conclusion of zero order in toluene due to the linearity of the concentration-time plot is weakened by the fact that the logarithm of concentrationtime plot is also straight over the limited conversions of concern. Accordingly,
the rates proper-X',l/A~-were next considered (last column of Table I). These rates do not decline as the conversion proceeds, and the most reasonable conclusion is that the reaction is approximately zero order in toluene in the initial stages and cannot be first order. This conclusion was confirmed in this way for all runs. T h e equation for concentration us. time was found for each run by the method of least squares. The slope of such a linear equation is the initial rate on which all the ensuing treatments and interpretations are based. Zero order in the hydrocarbon has been observed by Smith and Pennekamp ( 8 ) and Smith and Meriwether ( 7 ) , using benzene, cyclohexadiene, and cyclohexene with platinum catalysts, Lozovoi and D'yakova ( 4 ) with toluene over nickel, and de Ruiter and Jungers (6) with Raney nickel catalyst. HYDROGEN ORDER.Several series of runs were made varying only the hydrogen pressure; the initial rates varied approximately as the 0.5 power of the pressure. T h e initial rates were then related to the initial hydrogen concentration (defined as the concentration of hydrogen in pure toluene at the temperature and pressure of the run) with the aid of the solubility data. T h e steady-state value of the solubility was equal to the equilibrium value in all runs (Figure 2). There is considerably more disagreement on this point in the literature. Smith and others found the hydrogen order to be 1, as did Baker and Scheutz (7). Lozovoi and D' yakova found it to be between 0 and 1. depending on the temperature, and de Ruiter and Jungers found it to be 0 above 20 atm. EFFECT OF CATALYSTCONCENTRATION. In several series of runs the initial rate was found to depend on the initial catalyst concentration (defined as the mass of catalyst per unit volume of pure toluene charged, measured at the pressure and temperature of the run) to a considerable degree. T h e catalyst concentration changes slightly during the run as a result of the lower density of the product rnethylcyclohexane and the catalyst removed during sampling (Figure 3). T h e slopes of these lines are 1.62, 1.63, 1.67, and 1.52 and thus an order with respect to catalyst of about 1.6 independent of pressure is indicated. Previous investigators concluded from their measurements that the rate was proportional to catalyst concentration.
Av.
Time, Min.
Mole Fraction Toluene
Toluene Concn., Moles/L.
L./Hr.
60 120 180 240 300 360 420 480
0.9903 0.9679 0.9461 0.9228 0.9002 0.8764 0.8517 0.8358
8.171 7.952 7.735 7.511 7.288 7.052 6.830 6.603
0.219 0.217 0.224 0.223 0.236 0.222 0.227
660
Rate, Mole/
INDUSTRIAL AND ENGINEERING CHEMISTRY
Different'Lots of Toluene Have Different Reaction Rates Initial Catalyst Baker Temp., Pressure, Concn., Rate, G./L. hTole/L./Hr. P.S.I.A. c. Lot No.
Table II.
Run 135 125 115 95
2361 8368 9049 8368
131.3 131.3 171.4 171.4
1009 1009 1009 1009
6.61 6.63 3.59 3.59
0.120 0.224 0.203 0.896
TOLUENE H Y D R O G E N A T I O N
4
, 50,
~
-
_
_ I
I
Figure 2. Initial rate of hydrogenation varies directly with initial hydrogen concentration
b Figure 3. Effect of initial catalyst concentration on initial rate of hydrogenation Order with respect to catalyst i s about 1.6 independent of pressure ..
EFFECTO F TEMPERATURE. T h e effects of toluene, hydrogen, and catalyst concentration may be concisely summarized by the following empirical equations: 131.3 O C.
R, = 0.0193(C,),1.62(Ch),o.63
151.4" C.
R,
171.4" C.
R , = 0.1964(C,),'~j2(C~),0~~6
=
0.0560(c',),'~6'(C~),0~4~
Only the points forming the straightline portion of the 171.4" C. line in Figure 3 \vere used. T h e coefficients of the above equations depend on the temperature in such a way That the usual Arrhenius plot yielded an apparent activation energy of 20 kcal. per gram mole. EFFECTO F TOLUENE SOURCE. T h e above observations as to orders and apparent activation energy were obtained using one batch of toluene (lot 8368). The general nature of these results is probably independent of the toluene source, but absolute rates of reaction differ among different batches of toluene, although every batch of toluene was analytical reagent grade (Table 11). Such differences in rates led to a study of treatment methods. PURIFICATION O F TOLUENE. I n the first treatment toluene was brought into contact with Drierite for 3 hours at room temperature with intermittent stirring, decanted, and similarly treated with catalyst. T h e toluene was then decanted and 6.6 grams of catalyst per liter were added. T h e toluene was then distilled in a 30-plate Oldershaw column, the first 5 and the last 15%, of the toluene being discarded. T h e refractive index, unchanged before distillation, increased from 1.4959 to 1.4961. I n the second treatment toluene was distilled in the Oldershaw column without pretreatment to obtain material with a refractive index of 1.4969 (NBS-API value). Both treatments increased the initial rate of hydrogenation somewhat, but neither when applied to lots 2361 and 9049 brought the rate up to that of lot 8368 without treatment. T h e third treatment was partial hydrogenation at temperature, pressure: and catalyst concentration within the usual operating range. T h e product of this treatment,
I5 20 30 50 70 I03 I50 converted about 370, was removed from ( C ~ ) ~ - l N Al li CATALYST CGNCEYTRAT,ON.Gri.nrr',11.r the autoclave, separated from the catalyst, then returned to the autoclave, fresh catalyst added, and the rate measured denum trioxide \vas added to the charge, again. This was the most effective the reaction \vas initially inhibited and method of purification. the color change occurred. Thus it EFFECT O F TOLUENE PURIFICATION. appears that molybdenum trioxide conAfter partial hydrogenation the initial sumes the hydrogen atoms and inhibits rate at 131.3' C . increased by about a the reaction. After the color change factor of 3 in the three lots tested. M a was completed, the reaction began again terial from lot 8368, which \vas parand there was evidence that the blue tially hydrogenated twice using a catamolybdenum oxide is also a catalyst. lyst concentration of 4.20 grams per When yellow oxide was used without liter, showed an increase of 3070 in Raney nickel, no color change of the initial rate over the singly treated maoxide nor any reaction occurred. The terial. T h e analytical reagent grade action of the oxide is complex and further toluene apparently contains inhibitors work is planned to elucidate its exact which can be removed with considerable behavior in the system. effect on the hydrogenation rate. Additional work is planned. Discussion Three runs were made using toluene from lot 8368 subjected once to partial A chain mechanism involving hyclrohydrogenation. In these runs the order gen atoms and /or free radicals is proposed with respect to catalyst concentration for the hydrogenation based on two obwas 1.3. There is evidence ( 3 ) that servations from the experiments using this order was even further reduced to molybdenum trioxide. about 1.1 in the hydrogenation of toluene Presence of hydrogen atoms and/or purified by two partial hydrogenations. free radicals in solution as determined by T h e toluene is zero order for both color change of molybdenum trioxide. purified and unpurified toluene. Inhibition of the reaction during the T h e effect of toluene purity on the hyperiod in which the color change takes drogen order was not studied directly. place. However, because the hydrogen order was essentially independent of catalyst Such a chain mechanism was postuconcentration (Figure 2), and the only lated [details of the individual steps are result of impurities is to reduce the effecgiven.by Littman ( 3 ) ]in which the zero tive catalyst concentration, it is probably order in toluene results from a rapid unaffected by the impurities. (relative to the other steps) toluene chemEXPERIMENTSWITH MOLYBDENUM isorption and the half order in hydrogen TRIOXIDE. An order with respect to from a slow dissociative chemisorption catalyst concentration greater than 1 of hydrogen on the surface. T h e inindicates a chain mechanism. Because crease in toluene purity makes more the data indicate such a possibility, sevsites available, increasing the rate witheral chain mechanisms \%ere explored; out changing these orders. those found most promising involved the T h e mechanism leads to the rate equaexistence of hydrogen atoms in the solution : tion. This unusual feature prompted experiments using molybdenum trioxide. Molybdenum trioxide reacts with hydrogen atoms and free radicals with a [CY is small] collision efficiency of 1 but not with hydrogen molecules. The presence of This is consistent with the experisuch atoms is indicated by a color change mental orders for toluene and hydrogen. from yellow to blue (5). When molybT h e catalyst order is not absolutely es-
VOL. 51, NO. 5
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