phinating of the polymer and from attempts to obtain alternate rhodium complexes which will attach to the polymer in a manner to provide the least restriction to free movement of the bonded complex. Literature Cited Birch, A. J., Walker, K. A. M., Tetrahedron. 20, 1935 (1967). Burwell, R. L., Jr., Chem. Techno/., 370 (June 1974). Delmon. E., James. G., "Catalysis, Heterogeneousand Homogeneous," Elsevier, New York, N.Y., 1975. Grubbs, R. H., Kroil, L. E., J. Am. Chem. SOC.,93, 3062 (1971). Grubbs, R. H.,Kroli, L. E., Sweet, E. M., J. Macromol. Sci. Chem., 7 (5), 1047 (1973). Hamer, A. D., Walton, R . A., Inorg. Chem., 13 (6), 1446 (1974). Heinemann, H., Chem. Techno/., 286 (1971). Horner, L., Buthe, H., Siegel, H., Tetrahedron Lett., 4023 (1968). Houghton. R. R., Chem. Ind., (4), 155 (1973). Lee, H., Stoffey, D., Neville, K., "New Linear Polymer", p 134, McGraw-Hill, New York, N.Y., 1967. Lemke, H., Hydrocarbon Process., 45 (2), 27 (1966).
Mague, J. T., Wilkinson, G., J. Chem. SOC.,A, 1736 (1966). Manassen, J., Chim. Ind. (Milan),51, 1058 (1969). Michalska, 2. M., Webster, D. E., Chem. Techno/., 117 (Feb 1975). Mitchell, R. W., Ruddick, J. D.,Wilkinson, G., J. Chem. SOC.,A, 3224 (1971). Montelatici, S.,Osborn, J. A., Wilkinson, G., J. Chem. SOC.,A, 1054 (1968). Morrison, J. D., Burned, R. E., J. Am. Chem. Soc., 93, 1301 (1971). Morrison, J. D., "Asymmetric Organic Reactions", Prentice-Hall, Englewood Cliffs, N.J., 1971. O'Connor, C., Wilkinson, G., Tetrahedron Lett., 1375 (1969). Osborn. J. A., Jardine, F. H., Young, J. F., Wilkinson, G., J. Chem. SOC.,A, 1711 (1966). Pittman, C. U., Jr., Evans, G.O., Chem. Techno/., 560 (Sept 1973). Sasse, K., "Methoden der Organischen Chemie (Houben-Weyl)", p 213, G. Thieme Verlag, Stuttgart, 1963. Scroggins, L. H., Microchem. J., 13, 385 (1968). Stern, R., Chevallier, Y., Sajus, L.. Compt. Rend., 264, 1740(1967). Van Beekkum, H., Van Rantwijk, F., Van de Putte, T., Tetrahedron Lett., 1 (1969).
Received for review October 20, 1915 Accepted June 21,1976
Platinum/Alumina Catalysts in Reforming Methylcyclopentane Bjirn B. Donnis' lnstituttet for Kemiindustri, The Technical University of Denmark, DK-2800 Lyngby, Denmark
Reactions of methylcyclopentane on commercial R-AI2O3-CI reforming catalysts have been studied. Kinetic data (activation energies, reaction orders for methylcyclopentane and hydrogen)are presented for the formation of: (1) cyclohexane and benzene, (2) 2-methylpentane and 3-methylpentane, and (3) n-hexane. The reactions were studied at temperatures from 470 to 515 OC, partial pressures of methylcyclopentane from 0.02 to 0.14 atm, and partial pressures of hydrogen from 6 to 40 atm. The conversion of methylcyclopentane was kept below 10%. The kinetic data combined with results from varying Pt content of the catalyst, water vapor pressure, and catalyst age imply that for the formation of (1) and (3) the rate-determining step is catalyzed by acidic centers, while the formation of (2) is catalyzed by platinum; i.e., there are two different ring opening mechanisms.
Introduction This paper presents a study of the catalytic reforming of methylcyclopentane (MCP) with a conventional platinumalumina-chloride catalyst in a differential fixed-bed reactor. The study appears warranted, since the reforming of MCP should provide information about the dual-function character of this type of catalyst. Moreover, an extensive study of the kinetics of the MCP reactions has not been reported previously. Experimental Section The experimental reactor used in this study was made from a 13-mm i.d. stainless steel pipe, surrounded by a 100-mm 0.d. copper block, surrounded in turn by three independent 2000-W heaters. The reactor was equipped with two axial thermocouple wells, ending 25 mm over and 13 mm below the catalyst bed, respectively. By proper adjustment of three PID regulators, controlling the power of the heaters, the temperatures measured at four different points in the thermocouple wells showed that the reactor during use was isothermal (f0.3 "C) over 300 mm of length. The hydrogen used was electrolytic grade and was deoxygenated over a Pd catalyst and dried by molecular sieves (4A). After having passed a reduction valve, a known amount of
Address correspondence to the author a t Haldor Topsie A D , Nymillevej 55, P.O. Box 213, Dk-2800 Lyngby, Denmark. 254
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
water vapor was added by letting the hydrogen equilibrate with ice in a vessel cooled in a cryostat (normally operated a t -36 OC). The Phillips pure grade MCP used was stored over molecular sieves and refluxed under nitrogen prior to use. The analysis of the MCP after purification is: MCP 99.79%, nHx 0.14%, and CH 0.07%. The product analyses are corrected for the nHx and CH content of the feed (the CH is subtracted from the Bz of the product, because the reaction CH Bz is fast). The MCP was added to the hydrogen by letting the hydrogen bubble through the liquid in a vessel, immersed in a constant-temperature bath, this temperature thus determining the partial pressure of MCP. (By varying the hydrogen flow (to low values) and analyzing samples of constant volume by gas chromatography it was shown that the samples always contained the same amount of hydrocarbon; therefore it seems justified to consider the hydrogen saturated by MCP. After discharge from the reactor, the pressure was relieved by letting the gases pass a reduction valve and a calibrated needle valve, and the gas volume was finally measured in a wet gas meter. Catalysts from Ketjen, Netherlands, were used with 0.3% and 0.6% Pt on y-alumina, originally containing 0.67% chlorine. Before use the catalysts were crushed and screened to 140/170 mesh and mixed with (catalytically) inactive sand. The catalyst bed contained about 2.5 cm3 of this mixture. The start-up procedure including reduction was standardized in such a way that removal of the water vapor evolved occurred a t a rate which kept the water vapor pressure constant. The start-up procedure is listed in Table I.
-
Table 111. Formation of Bz
Table I Time, h
Temp, “C
HZpressure atm
HZflow, g-mol/h
0- K %-1Y2 lfi,-2% 2 I/>-3Y*
Room temp.-250 250 250-490 490 490-515 515 515-540 540
5 5 25 25 25 25 40 40
4 4 4 4 4 1.6 4 4
31/*-3% 3:Y4-20 20 20-21
+ CH
Eubs,
% Pt
kcal/mol
a
P
0.3 0.6 0.6 initial 0.3 wet
51 f 4 55 f 8
0.8 f 0.1 0.7 f 0.2* 0.6 f 0.3 0.9 f 0.1*
-0.9 f 0.1 -0.8 f 0.2 -1.0 f 0.6 -0.9 Zk 0.1*
-
61 f 5*
Table IV.Selectivity for the Formation of Bz and CH. SBdCH Eobs,
Table 11. Typical Product Analysis Compound “Small” molecules (mainly CHJ 2,2-Dimethylbutane 2-Methylpentane (with 2,3dimethylbutane) 3-Methylpentane n-Hexane 3-Methylcyclopentene 4-Methylcyclopentene Methylcyclopentane 1-Methylcyclopentene Cyclohexane Benzene
% Pt
Abbrevia- % C atoms in tion the compound Cg-
-
2MP 3MP nHx -
MCP -
CH Bz
0.17 f 0.01 0.004 f 0.003 0.273 f 0.006 0.140 f 0.004 1.22 f 0.006 0.31 f 0.04 0.15 f 0.02 94.8 f 0.01 1.36 f 0.02 0.203 f 0.007 1.34 f 0.03 Total 99.97
The products were analyzed by a Perkin-Elmer F-11 gas chromatograph with flame-ionization detector. The column used was a 50-ft squalane SCOT-column operated a t 50 “C.
Discussion of Results Catalyst Aging. During preliminary studies it was established that the catalyst activity for all reactions declined during a period after startup. Therefore, after reduction, the catalysts were kept a t 520 “C and 20 atm a t a low hydrogen flow (approximately 0.8 g-mol/h, no hydrocarbon present) giving a gas velocity in the empty tube of 2 mm/s for a period during which the activity was checked regularly until it became constant, after approximately 3 weeks. Kinetic Studies, General. Measurements of the effects of temperature and partial pressures of MCP and hydrogen on reaction rates constitute the major part of this work. The applied ranges of conditions were as follows: temperature, 470-515 “C; MCP partial pressure, 0.02-0.14 atm; H2 partial pressure, 6-40 atm. Space velocity was kept high enough to assure that conversion of MCP stayed below 10% (hydrogen flow: 8-50 g-mole&). (Calculations show that mass and heat transfer effects are negligible a t these conditions.) The major products for all conditions are given in Table 11. The number of experiments (each experiment consists of 3-5 analyses) performed for the kinetic measurements amount to 24 for the 0.3% Pt catalyst, 28 for the 0.6% catalyst, 7 a t “initial” conditions, and 6 a t “wet” conditions. 1-,3-, and 4-Methylcyclopentene were always found to be in equilibrium with MCP, even a t space velocities three times higher than those normally used in the present kinetic experiments. In a few experiments very small amounts of cyclohexene (CH’), cyclopentane, and cyclopentene were detected. The following figures (Table 11, mean values of four analyses) represent a typical analysis (run MCP 217 B, 46.4 mg catalyst with 0.3% Pt; temperature, 490.5 “C; MCP and
0.3 0.6 0.6 initial 0.3 wet
kcal/mol 17 f 11
6 f 14 Bz/CH near equilibrium 34 f 33*
a
P
0.0 f 0.3 -0.1 f 0.5*
-1.1 f 0.3 -0.9 f 0.4
0.3 f 0.8*
-1.0 f 0.6*
H ? partial pressures, 0.118 and 21.15 atm; H2 flow, 11.17 gmol/h). All experimental data are correlated by linear regression of the logarithmic form of the expression
r = ho exp(-E,b,/RT)
(PMCP)~(PHJ@
(1)
In this expression r is the reaction rate in g-moles formed per hour and gram of catalyst, ho is the preexponential factor, Eobs is the apparent energy of activation (kcal/g,mol), R is the gas constant (1.987 X 10-:3 kcal/g-mol K), T is the temperature (K), p x is the partial pressure of x (atrn),and a and /3 are apparent reaction orders. The logarithmic form is used for the regression analysis because there is reason to believe that a constant error variance can be found in log r rather than in r itself. The reason for this is that the major part of the error in the rates originates in the error a t the integration of the peaks of the chromatograms, the peaks being of approximately the same area (because the attenuation of the gas chromatograph is changed according to the amount present). The use of a simple power law rate function is to be considered as a short way of presenting the experimental results, more than as a rate function based on some mechanistic understanding. Kinetics of the Ring Isomerization Reaction. This reaction consists of the formation of Bz and CH and possibly also CH= (generally not detected).,The results in Table I11 were obtained for the formation of Bz CH. All confidence limits are a t the 0.05 level (95%), except when noted by an asterisk, where the results correspond to the 0.2 level (80%), because too few degrees of freedom are present to justify the higher level of confidence. The recorded selectivity S B ~ / C is H defined as the ratio between the rate of formation of Bz and the rate of formation of CH ( r R z / r C H ) . The results of Table IV are obtained by performing linear regressions of the logarithmic form of expression (1)(with S B ~ / C replacing H r). For the results marked “initial” the catalyst was not aged to constant activity. The results marked “wet” were obtained a t a vapor pressure of water of 0.43 mmHg (being 0.15 mmHg in all other experiments). The CH concentration always corresponded to an amount of CH much in excess of the amount valid for equilibrium with the Bz formed except for the “initial” catalyst. Data from experiments, where the Bz concentration reached 30% or more of the equilibrium values, have been omitted from the selectivity calculations. Contrary to Selman and Voorhies (1973) the amount of CH
+
Ind. Eng. Chern., Prod. Res. Dev., Vol.
15,No. 4, 1976 255
Table V. Comparison of the Rate of Formation and the Selectivity at Different Conditions for the Ring Isomerization Condition I/condition 11 All -?:st!= other variables than the T ( ' I i 4 B7'II) SHI/CH(I~) quoted are kept constant P t contents: 0.6% Pt/0.3% Pt 0.8 20 Vapor pressure of water: 0.43 0 50 1.1 mmHg/0.15 mmHg Aged catalyst/"initial" (0.6O6 07 0.1 Pt) The activity is fully restoied by lowering the vapor pressure of water to 0.15 mmHg. formed (at the low conversions, 0 on a catalyst, where nHx is the major ring opening product. Selectivity has been investigated much more thoroughly, and several different ratios between the products have been reported. Of greatest interest for the present study is the work by Iijima et al. (1963), who observed that the ring opening products are formed in the ratios 2MP:3MP:nHx = 2:1:2 on a fresh catalyst, while an aged (coked) catalyst and a sulfur poisoned catalyst gave higher percentages of nHx in the products. These observations are nearly identical with those obtained in the present study. The high percentage of nHx was also observed on a thiophene poisoned Pd catalyst (Iijima et al., 1970). Both in their work and the work by Smith et al. (1971) (referred to below) one or both of the reaction paths fpr nHx formation on a acidic site shown in Figure 4 are presented without further comments. Some similar results were obtained by Smith et al. (1971) on a fresh catalyst with npropylamine in the feed (ratios being 2.4:1:1.6) and a fresh catalyst with thiophene in the feed, nHx being the only ring opening product. It is of particular interest that Selman and Voorhies (1973) in their study of a Pt-Re catalyst have found the same shift in selectivity during aging, going from a nearly “statistical” ring opening (2MP:3MP:nHx being -2:1:2) to a selective nHx-producing ring opening. Unfortunately, they have not given kinetic data for the individual reactions, but it is clearly seen from their figures that the nHx formation has a higher fl value than the isoHx formation.
Conclusions The data given in the present study of the reforming of MCP on a Pt-Al203-Cl catalyst support the view that the reactions occur within the network shown below with the rate-determining step of vertical reaction paths occurring on (CH/CH=),? n
+
Bz
lt isoHx
(MCP/MCP=),,
It
nHx S ”nHx-structure”(n-hexadiene?)
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Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
acidic catalytic sites and the rate-determining step of horizontal paths on Pt sites. For the ring opening to nHx on acidic sites, the most probable mechanism has as its rate-determining step the pbreakage of a carbonium ion resembling MCP structure (with the positive charge at the 3 position). Kinetic data are given for the reactions shown above as exponents of a power-law equation (1).The most interesting results are the strongly deviating reaction orders of hydrogen and activation energies in the different reactions, while the reaction order for the reacting hydrocarbon does not vary much (between 0.5 and 1.0).
Acknowledgment The support of this study by the Danish Statens tekniskvidenskabelige Forskningsrttd is gratefully acknowledged. All calculations were performed at the NEUCC (Northern Europe University Computing Center) on an IBM 370/165 computer. Literature Cited Anderson, J. R., Shimoyama, Y., Prepr. Proc. Fifth lnt. Congr. Catal., Amsterdam, 1972,NO. 48 (1972). Compagnon, P. A., HoanpVan. C., Teichner, S. J., Bull. Soc.Chim. Fr., 11,2311,
2317 (1974). Flynn, P. C..Wanke. S. E., J. Catal., 37,432 (1975). Ganguli. N. C. et al., Technology(Sindri, India), 11(1),25 (1974). Granhoj, M., Ph.D. thesis, instituttet for Kemiindustri, The technical University of Denmark, Lyngby, Denmark, 1972. Haensel, V., et at., Proc. 3rd lnt. Congr. Catal., Amsterdam, 1, 294 (1964). Hettinger, W. P., Jr., et al., lnd. Eng. Chem., 47(4),719 (1955). Hindin, S.G., Weiler, S. W.. Mills. G. A., J. Phys. Chem., 62,244 (1958). lijima, K., et al., Jpn. Petrol. lnst. Bull., 5, 1 (1963). lijima, K., et al., Jpn. Petrol. lnst. Bull., 12,21 (1970). Keulemans, A. I. M., Voge, H. H., J. Phys. Chem., 83,476 (1959). Maat, H. J., Moscou, L., Proc. lnt. Congr. Catal., 3rd, 1964, 1277 (1965). Pajonk, G., Teichner, S. J., Bull. SOC.Chim. Fr., 9-10,2650 (1973). Pausescu, P.,et at., J. Appl. Cryst., 7,281 (1974). Selman. D. M., Voorhies. A., Jr.. Am. Chem. SOC.,Div. Petrol. Chem., Prepr., IS, 171 (1973);lnd. Eng. Chem., Prod. Res. Dev., 14, 118 (1975). Sinfelt, J. H., Rohrer, J. C., J. Phys. Chem., 85,978 (1961). Sinfelt, J. H., et al., J. Catal., 1, 481 (1962). Smith, R. L., et al., J. Catal., 20, 359 (1971).
Received yor reuiew M a r c h 23, 1976 Accepted August 17,1976
