Oct
, 1960
KINETICS O F METHYLCYCLOHEX.INE DEHYDROGENA4TIOSOVER
indicate that hyperconjugation bond” structures such as
involving “no
/-I
H+&C,
P
does not play an important role in stabilization of this ion. Experimental Reagents.-Triphenylcarbinol, m.p. 162-163’, waa prepared by recrystallization of Eastman White Label material.
I’T--~L?OI
1539
Diphenylethylene ( n 1.6093) ~ was prepared by distillation of Aldrich Chemical Co. DPE. Deuteriosulfuric acid was prepared by deuteriolysis of anhydrous sulfur trioxide using D20 (99.5%) from Stewart Oxygen Co. Measurements.-N.m.r. measurements were made with a Varian Associates model V-4300A spectrometer and 12 inch electromagnet. Chemical shifts were measured by the audio sideband technique with the use of a frequency counter. The chemical shifts of Tables I, I1 and VI were measured by use of trimethylsilane in a capillary placed in the tube which contained the material under study.
Acknowledgment.-The portion of this work carried out a t the Mellon Institute was sponsored by the Gulf Research & Development Company as a part of the research program of the Multiple Fellowship on Petroleum.
KINETICS O F METHYLCYCLOHEXAXE DEHYDROGEYATION OVER PTBY J. H. SINFELT, H. HURWITZ AND R.il. SHULMAN ESSOResearch and Engineering Company, Linden, -1-J . Receiuod .!fag 1 8 , 1360
The kinetics of methylcyclohexane dehydrogenation over a platinum-on-alumina catalyst were investigated over the temperature range 315 to 372O, a t methylcyclohexane partial pressures ranging from 0.07 to 2.2 atmospheres and hydrogen pressures ranging from 1.1to 4.1 atmospheres. The reaction was found to be nearly zero order with respect to methylcyclohexane and hydrogen over the range of conditions studied. The activation energy for the reaction was found to be 33 kcal./ mole. The near zero order behavior of the reaction suggests that the active catalyst sites are heavily covered with adsprbed molecules or radicals at reaction conditions. It is suggested that adsorption equilibria are not established at the condJtions used in this study. .4 simple kinetic scheme, according to which the reaction rate corresponds to the rate of desorption of toluene, is proposed t o account for the observed kinetics.
Introduction The formation of aromatics by the catalytic dehydrogenation of saturated six-membered ring hydrocarbons is a well-known reaction. The transition metals and their oxides are active catalysts for such dehydrogenation reactions. Supported metal catalysts, such as platinum or palladium-on-alumina, are particularly active. It has been shown by hydrogen chemisorption measurements that freshly prepared platinum on alumina catalysts are characterized by extremely high dispersion of the platinum on the support.’ The high dispersion of the platinum is an important factor contributing to the very high activity of these catalysts. Although a number of investigations on the dehydrogenation of cyclohexanes over supported platinum catalysts hare been r e p ~ r t e d ~the - ~ kinetics of the reaction have not been extensively investigated. Therefore, it was decided to study the kinetics of methylcyclohexane dehydrogenation over a platinum-on-alumina catalyst to gain some insight into the nature of the surface phenomena involved in the reaction. (1) L. Spenadel and 1RI Boudart, THISJOURYAL,64, 204 (1960). (2) V. Haensel and G R Donaldqon, I n d Eng Chem 43, 2102 (1951).
(3) W. P. Hettingei, C D Keith, J L Gring and J. W Teter, z b z d , 47, 719 (19j6). (4) 9. I. M. Keulernans and H. H. Vope, THISJOURNAL,63, 476 (1959).
Experimental Procedure.-Reaction rates were measured in a flow system in the presence of added hydrogen using a l / z inch i.d. stainless steel reactor with a volume of about 20 cc. The reactor was surrounded by an electrically heated aluminum block to maintain isothermal operation. The runs were made with a catalyst charge of 6 g. diluted with inert ceramic beads to fill the reactor volume. Prior to introducing the methylcyclohexane feed, the catalyst was pretreated with flowing hydrogen for three hours at 527”. Reaction products were analyzed by a inch i.d. chromatographic column coupled directly to the outlet of the reactor. The column was 4 meters long, packed with firebrick impregnated with polyethylene glycol, and operated at a temperature of 90’. This gave excellent resolution of the methylcyclohexane, toluene and other aromatics used in this study. Reaction periods of 30 minutes were employed in all of the runs to ensure the attainment of steady-state conditions prior to sampling the reaction products. Several reaction temperatures were used in this work, ranging from 315 to 372 Total pressure was varied from 1.4to 6.3 atmospheres and hydrogen to methylcyclohexane mole ratio from 2 to 20. Molal space velocities ranged from 0.2 to 1.0 g. mole of methylcyclohexane per hour per g. of catalyst, depending on the reaction temperature. Materials.-Phillips pure grade methylcyclohexane (> 99 mole % purity) was used in all the experiments. The methylcyclohexane was dried with Drierite ( CaS04) to less than 5 p.p.m. by weight of water before using. The benzene and meta-xylene which were used in binary mixtures with methylcyclohexane in some of the experiments were also Phillips pure grade and were dried in the same way. The hydrogen was passed through a Deoxo cylinder containing palladium catalyst to convert trace amounts of oxygen t o water, and then dried over Linde 5A molecular sieves. The catalyst used in this study contained 0.3 w t . yo platinum
.
J. H. SINFELT,H. HURWITZ AND R.* I SHULMAN .
1560
T'ol. 64
and was prepared by impregnation of alumina (surface area = 155 ~ i i . ~ / g with . ) aqueous chloroplatinic acid.
nearly independent of the methylcyclohexane partial pressure; for example, a tenfold increase in the Results methylcyclohexane partial pressure increased the The met hylcyclohexane dehydrogenation ex- rate less than twofold. In addition, the rate was periments were carried out at low conversion levels found to be essentially independelit of hydrogen t o obtain initial reaction rates, For sufficiently partial preswre. Thus, over the range of temperalow conversions in a flow system the reaction rate tures and pressures studied, the reaction is nearly zero order with respect to both methylcyclohexane is given by and hydrogen. r = F Az Some measurements were made to determine !1! W how the presence of aromatics affects the rate of where F is the feed rate in moles per unit time, I+' dehydrogenation of methylcyclohesane. This was is the amount of catalyst, and Ax is the extent of done to determine if inhibition by the reaction conversion. The quantity F/W is the molal space product toluene was appreciable when determining velocity, and hence the rate a t low conversions is rates a t low conversions. In these experiments simply the product of F/W and Ax. 1:1 molal mixtures of methylcyclohexane with The dehydrogenation of methylcyclohexane was either benzene or mefa-xylene were passed over the found to be an extremely clean reaction, with catalyst in the preseiiee of hydrogen, and the detoluene being the only observed product. Data hydrogenation rates compared with that of pure showing the initial rate of reaction as a function methylcycloh~xane in the presence of hydrogen of the partial pressures of methylcyclohexane and alone. The methylcyclohexane and hydrogen parhydrogen are summarized in Table I. Rate data tial pressures were maintained constant in this are presented for three different temperatures: comp:trison, so that we have a direct measure of 315, 344 and 372". The data in Table I mere all the effects of the added components on the rate obtained over a single charge of catalyst. Cat,a- of dehydrogenation. The partial pressures of lyst act,ivity was checked periodically throughout methylcyclohesane, hydrogen and the third tomthe run at a, standa,rd set of condit>ions. No loss ponent (benzene or metn-xylene) were 0.36, 1.4 in a c t i d v W I E S observed. and 0.36 atmospheres, respectively. The effects of the added aromatics on the rates at 315" are shown TABLE I by the ratio 4 r 2 , in which rl and r2 are the rates in SUMMABY OF IlATE DATAFOR METHYLCYCLOHEXANE DE- the presence and absence of the aromatics, reHYDROGENATION~ spectively. The rate of dchydrog~nation of Temp., '(2.
p ~ atm. ,
p ~ atm. ,
F/W
Az
r
0.36 .C@
Feed
TdTZ
1.1 0.21 5.6 0.012 Methylcyclohexane 1.0 3.0 .20 5.8 .012 Methylcyclohexane henzme, 1:1 os 1.4 .21 .(I7 4.1 .0086 Methylcpclohexane xylene, 1 :1 0.8 .24 1.4 .21 5.2 .011 methylcyclohexane in a 1 :1 mixture with benzene .72 1.4 .21 6.2 .013 or metu-xylene is thus 80% of the rate in the .36 1.1 .53 5.7 ,030 presence of hydrogen alone. In a 1:l mixture .36 3.1 .52 6.2 .032 with toluene the rate would undoubtedly be affected .08 1.4 .52 3.9 .020 to the same extent. In any case, at, the low toluene .24 1.4 .55 6.2 .034 concentrations (4-12%) encountered in the ex.68 1.4 .52 6.5 .034 periments on methylcyclohexane, the inhibiting .36 1.1 1.02 7.4 .076 effect of the toluene would be small. The effect .36 4.1 1.02 7.8 .080 of toluene itself on the rate cannot be measured 1.1 4.1 1.05 11.8 .124 very accurately by our technique, since in the 2.2 4.1 1.05 12.5 .131 presence of a large amount of toluenc the small cona pbf := methylcyclohexane partial pressure; pa = hyversion of methylcyclohexane to toluene would drogen partial pressure; F/W = molar space velocity, g . have moles of methylcyclohexane/hr./g. catalyst; Ax = mole ? l , to be obtained as the difference between two conversion to toluene; r = rate of dehydrogenation, g. large numbers (tnluene out minus toluene in). moles t'oluene formed/hr./g. catalyst. Some other technique, such as the use of radio315 315 315 315 3 15 344 344 344 344 344 372 372 372 372
Thermodynamic calculations using free energy data from rlPI Project 445 show better than 90% conversion of methylcyclohexane to toluene at equilibrium for all conditions studied except the one point a t 315" and 3.0 atmospheres hydrogen partial pressure, at which equilibrium corresponds to about 50% conversion to toluene. Since the conversion levels were lorn (4-12%) in all these experiments, th.e effects of the reverse reaction can be neglected. The rate of dehydrogenation was found to be (5) "Selected V,elues of PhyEiCd and Thermodynamio Properties of Hydrocarbons andl Related Compounds." API Research Project 44, Carnegie I'ress, Inc.. New York, N. Y.,1953.
+ +
active tracers, would be better fur this type of measurement. Discussion As already pointed out, the rate of dehydrogenation of methylcyclohexane is nearly aero order with respect to methylcyclohexane and is zero order with respect to hydrogen. The small effect of methylcyclohexane pressure which is observed can be accounted for by a rate expression of the form where k' is a rate constant, p~ is the methylcyclo-
Oct., 1960
KINETICS OF METHYLCYCLOHEXANE DEHYDROGENATION OVER PT-AL~O~
1561
is 2.6 X 1014. Substituting this value into equation 3, the activation energy E is calculated t o be 37 kcal./mole. The a eement with the experimental value of 33 kcal.gole is fair. According to the Lsngmuir-Hinshelwood mechanism of surface reactions, the reactant molecules are considered to be in adsorptive equilibrium with the surface, so that the reaction rate is deT e m p , OC. k’ b termined by the rate of transformation of the ad315 0.013 27 sorbed molecules on the surface. Applying this to 344 043 11 methylcyclohexane dehydrogenation, the near zero372 .I54 3 order behavior with respect t o methylcyclohexane where k’ is expressed in g. mole/hr./g. catalyst would suggest that the active catalyst sites are and b is expressed in reciprocal atmospheres. quite heavily covered with adsorbed niethylcycloFrom an Arrhenius plot of In k’ z’s. 1/T, the ac- hexane. The parameter b in equation 2 would tivation energy for the reaction is found t o be then be an adsorption equilibrium constant for 33 kcal./mole. Since the effect of methylcyclo- the adsorption of methylcyclohexane, and k’ hexane pressure on the rate is small, the intercept a specific rate constant for the conversion of adof a plot of 1,lr us. l / p can ~ be determined more sorbed methylcyclohexane to toluene on the surface. accurately than the slope, so that the values of It.’ From the temperature dependence of b, a value of are defined more precisely than the corresponding the order of 30 kcal./mole for the heat of adsorption of methylcyclohexane would be indicated. HowvaIues of b. ever, it is highly unlikely that adsorption of At sufficiently high methylcyclohexane partial methylcyclohexane as such would involve a heat pressures, the rate given by equation 2 approaches of adsorption of this particularly a t the value k’, which represents the rate at condi- high surface coverage.magnitude, Interpreting the kinetic tions where the reaction is truly zero order. At data on the basis of adsorption equilibria also the pressures used in the present work, the meas- leads to other difficulties. Since the adsorption ured rates are only slightly lower than the values of methylcyclohexane probably involves dissociaof k’, since the kinetics are close to zero order. tion of hydrogen from the molecule, increasing The near zero-order behavior of the reaction hydrogen should reduce the concentrasuggests that the active platinum sites are almost tion of the pressure adsorbed species in question, and hence completely covered with adsorbed hydrocarbon decrease the rate of toluene formation. However, molecules or radicals formed from the methyl- this was not found. Furthermore, the small cyclohexane. According to the transition state inhibiting effects of benzene and mcta-xylene on theory of reaction rates, the rate of a zero order the rate are not readily explained in terms of adsurface reaction is given by sorption equilibria. One would expect that arokT matics, by virtue of their unsaturation, would be r = C, - exp( -E/RT) (3) h more strongly adsorbed than methylcyclohexane. is evidence of this a t a much lower temperawhere Ca represents the number of adsorbed mole- There (-22.5’) based on kinetic studies of the reture cules or radicals per cm.2 of active surface, kT/h action of benzene with deuterium over platinum is a frequency factor of the order of 1013, and E films a t benzene pressures the order of 1 mm.’ is the activation energy.6 Assuming 1.1 X 1015 The rate of deuteration wasofzero order in benzene sites per cm.2 of platinum surface,l we can take Ca = 1.1 X 1015 for the number of hydrocarbon and uninhibited by the deuterocyclohexanes formed, on a fully covered indicating strong adsorption of benzene relative molecules or radicals per surface, assuming that one molecule or radical to cyclohexane. Extending this conclusion to the present study, benzene and meta-xylene would be is adsorbed on each site. Using the experimental expected to inhibit methylcyclohexane dehydrorate data we can calculate the activation energy genation substantially. Since this was not obfrom equation 3 and compare it with the value determined from the observed temperature de- served, it is suggested that adsorption equilibria pendence of the rate constant. To make this are not established at the conditions used in this calculation the experimental rate must be expressed study. The small inhibiting effects of the benin molecules/sec./cm.2 of platinum. Taking the zene and metu-xylene are then interpreted t o mean platinum surface area as 276 m.?/g.,l and recall- that their rates of adsorption are small compared ing that the weight fraction of platinum on the to the rate of adsorption of methylcyclohexane, catalyst is 0.003, the rate in g. mole/hr./cm.2 of so that their coverage of the active surface is small. In view of the above considerations, an alternaplatinum a t 315” becomes tive mechanism is proposed. It is suggested that 0.013 the active catalyst sites are heavily covered with r = - 1.57 x 10-5 0.003 x 276 x 104 adsorbed toluene and that the reaction rate is the Muitmiplyingthis by 6.02 X loz3and dividing by rate of desorption of toluene from the surface. 3600, the rate in molecules/sec./cm.* of platinum To account for the observed kinetics, a simple kinetic scheme is proposed hexane partial pressure, and b is a parameter which is a function of temperature. The physical significance of k’ and b will be discussed later. The value of k‘ can be determined from the intercept , b can then be evaluof a plot of l / r us. l / p ~ and ated from the slope. The values of IC’ and b at the temperatures investigated were found to be
(6) 8. Glssstone, K. J . Laidler and H. Eyring, “The Theory of R a b Procems,” McGraw-HU1 Book Co., New York. N. Y..1941, Chapter
VIX.
(7) J. R. Anderson and C. Kernball, “Advanoee in Catalysis,” Academic Preen, Ino., New York. N. Y.,1957. Vol. IX. p. 51.
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J. H. SIXFELT,H. HURWITZ .4ND R.. A. SHULMAN
Vol. 64
I n the simple kinetic scheme just described, the exact nature of the surface reaction leading in which M and T represent methylcyclohexane t o the formation of adsorbed toluene (step 1) was and toluene in the gas phase, and T, represents ad- not considered. The detailed mechanism of such sorbed toluene. It is postulated that adsorption a step has been the subject of considerable discusequilibria are not established, and that the in- sion. As a special case of his more general multidividual steps are effectir-ely irreversible. Step 1 plet theory of catalysis, Balandin8 originally represents the adsorption of methylcyclohexane suggested (in 1929) that cyclohexane is converted with subsequent reaction to form toluene on the to benzene over metals like platinum, palladium, surface whih step 2 is the desorption of toluene or nickel in a single step by the simultaneous from the surface. The first step is likely a combi- removal of six hydrogen atoms. In order for nation of steps in.\-olving partially dehydrogenated this t o occur it is necessary that the cyclohexane hydrocarbon molecules or radicals. The present ring be adsorbed in a very definite manner such treatment is therefore a simplification, but is that a sextet of metal atoms is involved in the adequal;e for our purposes. The parameters kl simultaneous rupture of six carbon-hydrogen and kz are r,ite constants for steps 1 and 2, re- bonds. According to this theory the active cataspectively. Alssumingthat coverage of the active lyst unit is thus an aggregate of metal atoms which sites by components other than toluene is very must be spaced within certain definite limits consmall, we can write the following steady-state sistent with the geometry of the cyclohexane ring. expression for the net rate of formation of ad- While there is a great deal of experimental evidence attesting t o the importance of geometrical sorbed toluene factors in the catalytic dehydrogenation of sixdl, membered rings,'j more recent findings indicate -- = k$\l(l - 6") - k& 0 (-1) at that chemisorption of the cyclohexane ring need where BT represents the fraction of the active sites not involve the simultaneous rupture of six carboncovered by toluene. The rate of formation of the hydrogen bonds. For example, in the exchange of deuterium with cyclohexane over platinum product toluene is giren by metal films,g the initial product is predominantly r = k& (5) CGH~~ rather D than C6H6De. The latter would Solving equation 4 for BT and substituting in have been the only primary product if chemisorpequatioq 5 , we obtaiq the rate expression tion of cyclohexane occurred strictly by the sextet mechanism. Furthermore, for platinum-onalumina catalysts similar to that used in the present study, the work of Spenadel and Boudart' showed that if any platinum crystallites are present, they must on the arerage consist of blocks containing which is of the same form as equation 2 with less than two unit cells on a side. It is questionk' = k : and b = k l / k 2 . The experimental acti- able whether crystallites such as these could acvation energy of 33 kcal./mole now corresponds to commodate the cyclohexane ring in the manner the activation energy E2 for desorption of toluene envisioned in the sextet mechanism. from the surface. From the observed temperaAlternatively, it seems probable in methylture dependence of b we can estimate the difference cyclohexane dehydrogenation that t,he formation in activation energies (E2 - El) of steps 1 and 2 of toluene on the surface proceeds by a stepwise t o be about 30 kcal./mole. The value of El is mechanism, involving intermediate surface species then about 3 kcal./mole, which may be interpreted of varying degrees of dehydrogenation. as an activation energy for chemisorption of Acknowledgment.-The authors wish to express methylc yclohexane. their appreciation to the Esso Research and EngiThe :hove kinetic scheme leads to a rate ex- neering Company for permision to publish the repression which satisfactorily accounts for the sults of this work. They also wish to acknowledge observed rate data. At the conditions of the helpful discussion with Professor 11. Boudart of present study, it appears that methylcyclohexane dehydrogenation over a platinum on alumina Princeton Vn iverPity . (8) B. M. 15'. Trapnell, 'Advances in Catalysis," Academic Pres*, catalyst is an example of a surface reaction in New York, N. Y., 1961, Vol. 111, p. 1. which the assumption of adsorption equilibria Inc., (9) J. R . Anderson and C . Kemhail. Proc. Roy. SOC.(London) A226, may n o apply. 472 (1954).
