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Ind. Eng. Chem. Fundam. 1986. 25.
Solomon. P. R.: Colket, M. 6.Symp. ( I n ? . )Combust. [Proc.]. 17th 1978b, 131. Solomon, P. R.; Hambien, D. G.; Carangelo, R. M.;Krause, J. L. Symp. ( I n t . ) Combust., [Proc.], 79th 1982. Solomon, P. R.; Hambien, D. G.; Goetz. G. J.; Nsakala, N. Y, P r e p Pap .A m . Chem. S O ~ .Div. , Fuel Chem. 1981, 2 6 , 6 . Solomon, P. R.: Serio, M. A . ; Carangelo, R. M.: Markham. J. R. Fuel 1986, 65, 182. Solomon, P. R.; Squire, K. R.; Carangelo, R. M. Prepr Pap.-Am. Chem. Soc.. Div. FuelChem. 1984, 2 9 , 10. Speight, J. G. The Chemisfry and Technology of Coal: Marcel Dekker: New York. 1983
544-553
Suuberg. E. M. PhD. Dissertation. Massachusetts Institute of Technology, 1977.
Suuberg. E. M.:Unger, P. E.;Lilly, W. D. Fuel 1985, 6 4 , 956. Ubhayakar, S. K.:Stickier, D. 6.; Von Rosenburg, C. W., Jr.; Gannon. R. E. Symp. ( I n f . )Combust., [Proc.], 16th 1976. Unger, P. E. Ph.D. Thesis, Carnegie-Mellon University, 1983. Weast, R. C., Ed. CRC Handbook of Chemistry and Physics: Chemical Rubber Company: Boca Raton. FL, 1979.
Recezued for reuiea June 12, 1986 Accepted July 24 1986
Kinetics of Methylcyclohexane Dehydrogenation on Sulfided Commercial Platinum/Alumina and Platinum-Rhenium/Alumina Catalysts P. A. Van Trimpont, G. B. Marin, and G. F. Froment' Laboratorium voor Petrochemische Techniek, Rijksuniversiteit Gent, Krijgslaan 28 1, 89000 Gent, Belgium
The kinetics of the dehydrogenation of methylcyclohexane were studied in a tubular reactor on commercial Pt/AI,O, and PtRe/Al,O, reforming catalysts. The partial substitution of Pt by Re causes drastic changes. Pt/AI,O, was continuously exposed to a H,S/H, molar ratio of whereas PtRe/Al,O, was presulfided only. The inlet partial pressures of methylcyclohexane and hydrogen varied from 0.15 to 1.5 bar and from 4 to 20 bar, respectively. The temperature ranged from 582 to 683 K on R/AI,O, and from 627 to 719 K on PtRe/Al,O,. Increasing hydrogen partial pressures caused a decrease of the rate of dehydrogenation on Pt/AI,O, but had no effect on the rate of dehydrogenation on PtRe/Al,O,. This different behavior is attributed to a shift of the ratedetermining step in the reaction sequence. Competitive adsorption by toluene is important only with PtRe/Al,O,. The rate of dehydrogenation is higher on Pt/AI,O, than on PtRe/Al,O,. The apparent activation energy is 60 kJ/mol higher on PtRe/Al,O, than on Pt/AI,O,.
Introduction The combination of platinum with an acid alumina support has resulted in a catalytic process for the reforming of naphtha (Haensel, 1949; Ciapetta, 1951) in which the catalyst operates in a bifunctional way. Platinum takes care of the dehydrogenation-hydrogenation reactions (Weisz and Prater, 1957). The dehydrogenation of hydrocarbons by platinum is generally considered to be structure insensitive (Cusumano et al., 1966; Boudart, 1969). A second generation of reforming catalysts has been introducted by adding a second metal, in the first instance rhenium (Kluksdahl, 1968). These second generation catalysts are less subject to deactivation by coke formation. Although immediately recognized by industry (Haensel and Sterba, 1976), the beneficial effects of sulfur on the activity and selectivity of reforming catalysts were first demonstrated by Menon and Prasad (1977). Since then, the effects of sulfur adsorption on the catalytic properties of a metal have been discussed mainly in terms of a geometric effect. Ligand effects caused by sulfur are generally considered to be of minor importance (Oudar, 1980; Barbier, 1982). Alloying effects can be closely related to effects of sulfur or coke deposition (Ponec, 1983; Menon and Froment, 1984). A ligand effect was invoked by Betizeau et al. (1976) to describe the synergism observed for hydrogenation and hydrogenolysis by adding rhenium to platinum. Most authors, however, invoke geometric effects to describe changes in activity and selectivity by alloying (Ponec, 1983). In particular, Biloen et al. (1980) suggested that adding both sulfur and rhenium to platinum results 0196-4313/86/1025-0544$01.50/0
in the division of the platinum into small sulfur-free platinum ensembles, separated by rhenium atoms which are covered by sulfur. Another group of authors questions the existence of bimetallic clusters (Johnson and LeRoy, 1974; Kelley et al., 1982). Bertolacini and Pellet (1980) demonstrated that physical mixtures of Pt/A1,0,, and Re/Al,O, also resulted in a better stability and suggested that Re eliminates volatile coke precursors. Recently, Somorjai and co-workers (Davis et al., 1982) have shown that, even in the presence of hydrogen at atmospheric pressure, hydrocarbon reactions lead to the formation of a carbonaceous layer on single-crystal platinum surfaces. At temperatures between 350 and 750 K this layer is disordered and acts as a nonselective poison, blocking sites from incident reactant molecules. A fraction of the surface sites always remain uncovered. The disordered layer can also store and exchange hydrogen with reacting surface species and provide desorption sites for product molecules. This is in contrast to the graphitic coke which is formed a t higher temperatures or a t lower hydrogen pressures. Coughlin et al. (1984b) explained the conversion, selectivity. and coking behavior of commercial reforming catalysts during the dehydrogenation of methylcyclohexane with the concepts of Somorjai and co-workers (1982), emphasizing the role of rhenium and sulfur in the formation of a carbonaceous layer. The dehydrogenation of cyclohexane or methylcyclohexane on supported platinum was investigated by several authors (Sinfelt et al., 1960; Barnett et al., 1961; Cusumano et al., 1966; Mencier et al., 1969; Maatman et al., 1971; 0 1986 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986
545
Nagata, 1973; Jossens and Petersen, 1982b; Coughlin et al., 1984a; Coughlin et al., 1984b). Most of the published work was performed at atmospheric pressure, some even without addition of hydrogen. The present paper reports on a kinetic study of the dehydrogenation of methylcyclohexane into toluene on a sulfided commercial Pt/A1203catalyst and on a presulfided commercial PtRe/A1203catalyst. Care was taken to work under partial pressures of hydrogen sufficiently high to avoid coke deposition.
Table I. Range of Reaction Conditions for the Kinetic Study of the Dehydrogenation of Methylcyclohexane Pt /A1,0, P tRe/Al2O3 temp, K 582-683 627-719 4.5-20 4.0-12 PH, bar 0.15-1.5 0.15-1.5 PMCH,bar 0-0.5 0-0.15 P T ~ I ,bar P ~ P , bar , 0-0.5 0-0.5 0.5-12 0.35-3.6 W/FoHc, kg of cat. h/kmol H2S to H2molar ratio 10-5 0
Experimental Section Equipment and Catalysts. The experimental setup and the construction of the tubular reactor have been described before (Marin and Froment, 1982). Conversion vs. W / F o H ccurves were obtained by on-line gas chromatographic analysis of samples withdrawn a t three to five heights in the catalyst bed. The separation of the hydrocarbons was performed by means of a squalane (length 50 m, i.d. 0.25 mm) or a OV-101 capillary column (length 54 m, i.d. 0.24 mm) at 303 K, and the detection by means of flame ionization. Both reforming catalysts were commercial. The monometallic catalyst contained 0.59 w t % Pt and 0.67 wt '70 C1. The bimetallic catalyst contained 0.3 wt % Pt, 0.31 wt % Re, and 0.95 wt % C1. The metal dispersion measurements were performed in a pulse microreactor made of quartz and containing between 0.5 and 1 g of catalyst. Procedure and Conditions. The mass of catalyst loaded in the tubular reactor amounted to 3.54 g for Pt/A1203and 1.40 g for PtRe/A1203. In the first case the amount of catalyst was divided into five sections separated by inert material, in the second case into three sections. In addition, each catalyst layer was diluted with inert material up to a volume ratio of 10. To avoid mass- and heat-transfer limitations, the catalyst particle diameter did not exceed 0.4 mm. The catalyst pretreatment consisted of a calcination in air at 763 K, followed by a reduction in H2 at the same temperature and by presulfidation. More details about the catalyst pretreatment are given elsewhere (Van Trimpont et al., 1985). To suppress the deleterious effects of hydrogenolysis, sulfur was continuously added to the hydrocarbon feed in the Pt/A1203case. A constant H2S to H2molar ratio of was maintained for each experiment. The experiments performed on PtRe/A1203did not require continuous sulfur addition to the hydrocarbon feed. The weakly held sulfur was removed after presulfidation by stripping with H2 The bimetallic catalyst contained only the strongly held sulfur, and this was sufficient to suppress hydrogenolysis and achieve high selectivities for dehydrogenation (Van Trimpont et al., 1985). The catalyst stabilization lasted for 20 h on both catalysts. The Pt/A1203catalyst activity was stabilized at 623 K, a total pressure of 15.5 bar, and a H2 to hydrocarbon molar ratio of 30. The hydrocarbon flow rate was fixed at 1 mol/h. The bimetallic catalyst stabilization was achieved at 673 K, 9 bar, and a H2to hydrocarbon molar ratio of 8. The actual kinetic experiments were performed after catalyst stabilization. The catalyst activity level was regularly checked at reference conditions. At the end of a run the hydrocarbon flow was switched off and the total pressure was lowered to 6 bar. The hydrogen flow was raised to 12 mol/h for desorption of the adsorbed hydrocarbons and, in the case of Pt/A1203,of the weakly held sulfur. The stripping with H2 was performed at 773 K on Pt/A1203. For PtRe/A1203the temperature was maintained at the level of the last experiment. In between runs the hydrogen flow was reduced to 1.3 mol/h
after H2Sstripping, while the temperature was maintained at 623 K. The hydrogen (purity of 99.999%) was further deoxygenated over a Pt catalyst a t 573 K and dried over a Ca-A zeolite at room temperature. The purity of the hydrocarbons fed to the reactor exceeded 99 mol %. The reaction conditions are listed in Table I. For most of the reaction conditions falling in the ranges given in Table I, the toluene selectivity exceeded 99%. The catalyst pretreatment prior to the measurement of the metal dispersion was identical with the pretreatment procedure for the kinetic study in the tubular reactor but did not involve a presulfidation. After reduction, the catalyst was cooled down to 298 K under a flow of H2. At 298 K, hydrogen was replaced by a helium stream of 0.04 NL/min. Oxygen pulses of 0.433 NmL were introduced, and the nonadsorbed O2 was detected at the outlet of the reactor by a thermal conductivity detector. The injection series was continued as long as O2 adsorption took place. The first H2-02 titration cycle was ended by switching back to H2 and performing a reduction at 298 K. Next, the chemisorbed hydrogen was titrated by a new injection series of O2as long as O2 adsorption lasted. Several H2-02 titration cycles were performed at 298 K on the same batch. The gases used were dry and more than 99.99 mol 9'0 pure. Exposed Fraction of the Metal Function The determination of the exposed fraction of Pt atoms of the monometallic catalyst has been reported earlier and amounts to 0.48 (Menon et al., 1982). The corresponding specific metal surface area amounts to 0.73 m2/g of catalyst. The O2 uptake per gram of PtRe/A1203 amounts to 0.37 NmL/g after reduction at 763 K and is markedly higher than the O2 uptake after reduction at room temperature, which amounts to 0.27 NmL/g. This is attributed to the nonreducibility of oxidized Re a t room temperature (Menon et al., 1973; Charcosset, 1979; Eskinazi, 1982; Isaacs and Petersen, 1984). Menon et al. (1973) and Eskinazi (1982) assumed that oxidized Re cannot be reduced at room temperature, even when Re is closely interacting with Pt. According to these authors, the O2 uptake subsequent to a reduction at room temperature occurs with the following stoichiometry PtsH
-
+ 3/402 PtsO + l/*H20
(1)
and allows the separate determination of the exposed fraction of the Pt atoms. According to (l),the O2 uptake during the first titration after a reduction at 295 K corresponds to a ratio of the number of exposed to the total number of Pt atoms of 0.9 and to a specific Pt surface area of 0.7 m2/g of catalyst. Bolivar et al. (1976) and Isaacs and Petersen (1984), however, reported results suggesting that oxidized Re can be reduced at room temperature if it is in close interaction with Pt. Hence, the values of 0.9 and 0.7 m2 of P t / g of catalyst should be considered as upper limits.
546
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986
4
c 1.0
0.5
15
5
10
15
20
pi, ( b a r )
pHLR ( b a r )
Figure 1. Dehydfogenation rate aa a function of the partial pressure of methylcyclohexane at 623 K and a hydrogen pressure of 15 bar. Catalyst Pt/AI2O9.
Bolivar et al. (1976) and Charcosset (1979) assume that a t 773 K H2 chemisorption occuf8 on both the exposed Pt and the exposed Re atoms and that the stoichiometry of the subsequent O2titration a t room temperature is identical for Pt and Re:
3 4
mPhH + n h H 4- -(m
-
+ n)02 m+n m P k O + nResO + -HzO (2) 2
According to (2), the O2 uptake observed during the titration subsequent to a reduction at 763 K results in a ratio of the number of exposed metaI atoms to the total number of metal atoms of 0.69. The corresponding specific metal surface area amounts to 1.2 m2/g of catalyst. These values should be considered as a lower limit. Indeed, Isaacs and Petersen (1984) concluded from a comparison of Hz chemisorption data, obtained a t room temperature on mono- and bimetallic ca€alysts,that a significant part of the exposed Pt atoms on a bimetallic catalyst do not chemisorb H2. This was explained by the dilution of Pt by Re. Pt atoms isolated by Re on the bimetallic surface were considered to be active for the dissociative chemisorption of O2but not for the dissociative chemisorption of Hp Hence, the O2uptake by isolated Pt surface atoms subsequent to a reduction is one-third less than the O2 uptake expected from (1) or (2). The diluent effect may also decrease the amount of chemisorbed H2 a t 763 K. Bolivar et al. (1976) and Charcosset (1979) attribute the difference between the O2uptake after reduction a t 773 K and after reduction at 298 K to the presence of exposed Re atoms which are not interacting with Pt but which are present in the form of monometallic clusters. The observed differences then indicate that one-third of the exposed metal atoms are Re a t o m which do not interact with Pt. This value should be considered as a lower limit, since a t room temperature titration of up to 10% of the oxygen adsorbed on pure Re has been observed (Bolivar et al., 1976). A continuous decrease of the O2 uptake was observed as the number of the titration cycle increased. Bolivar et al. (1976) and Isaacs and Petemln (1984) reported analogous results and suggested that O2 titrations at room temperature cause some segregation of the bimetallic clusters. Dehydrogenation Kinetics of Pt/Al2OS Inflwnce of Partial Pressures on t h e Initial Rate of Dehydrogenation. Figures 1 and 2 show the influence
Figure 2. Dehydrogenation rate as a function of the hydrogen partial pressure at 623 K and a methylcyclohexane pressure of 0.5 bar. Catalyst: Pt/A1203.
of the partial pressure of methylcyclohexane and hydrogen on the initial dehydrogenation rate at 623 K on Pt/A1203. The dehydrogenation rate'increases with increasing methylcyclohexane partial pressures. At a hydrogen partial pressure of 15 bar, the rate increase per partial pressure unit levels off for methylcyclohexane partial pressures higher than 0.5 bar. A t a methylcyclohexane partial pressure of 0.5 bar, the initial dehydrogenation rate sharply decreases with increasing hydrogen partial pressures. Another series of experiments were performed to assess the importance of competitive adsorption between methylcyclohexane and a second hydrocarbon. Binary mixtures of methylcyclohexane and toluene and methylcyclohexane and It-heptane were fed. Adsorption of toluene can be neglected. This is in agreement with the results of Sinfelt et al. (1960). There is competitive adsorption between n-heptane and methylcyclohexane, however. Power-Law Rate Equation. The experimental data for which the rate of the reverse reaction could be neglected were first regressed with a power-law rate equation by using the integral method of kinetic analysis: (3) The maximum-likelihood estimates of the parameters in the reaction rate equation were obtained by minimization of the sum of squares of the deviations between the experimental and calculated values for the fractional conversion into toluene. This minimization was achieved with a single-response Marquardt algorithm (Froment and Hosten, 1981). The parameter estimates were tested for significance by means of their t values. A parameter estimate is different from zero with a probability of 95% when its t value exceeds the corresponding tabulated value. The tabulated t value corresponding to a probability level of 95% and 60 degrees of freedom amounts to 2.0. The significance of the global regression was expressed by means of the ratio of the mean regression sum of squares to the mean residual sum of squares, which is distributed according to F (Draper and Smith, 1966). A high value of the F ratio corresponds to a high significance of the global regression. The preexponential factor was reparametrized according to
A. = AO'eElRTm
(4)
The preexponential factor, the apparent activation energy, and the partial orders with respect to methylcyclohexane, toluene, and hydrogen are given in the first row of Table 11. The zero reaction order with respect to toluene reflects
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986 547 Table 11. Parameter Estimates and Approximate Individual t Values (in Parentheses) for a Rate Equation of the Power-Law Type" catalvst F An E nMrU nTnl nu Pt/Al,O, 807 1.22 X 10" (8.3) 133.3 (39) 0.705 (12) -1.28 (-30) 195.8 (20) 0.83 (27) -0.36 (-13) PtRe/A1,03 1986 2.96 x 1013 (7.3) "Units: A. in kmol/(kg of cat. h bar"), where n = nMCH+ nTai + nH; E in kJ/mol. Table 111. Reaction Sequences for the Dehydrogenation of Methylcyclohexane Sequence I. Dual-Site Surface Reactions, Generation of Atomic Hydrogen MCH + L z MCH-L (1) MCH-L + L F? AI-L H-L (2) AI-L + L z MCH1-L + H-L (3) H-L MCH1-L + L F? A,-L (4) A2-L + L z MCH2-L H-L (5) MCH2-L + L s As-L H-L (6) AB-L + L z Tol-L H-L (7) 2H-L P H2 + 2L (8)
+
+
+ + +
Sequence 11. Dual-Site Surface Reactions, Generation of Molecular Hydrogen MCH L P MCH-L (1) MCH-L + L 9 MCH1-L + H2-L (2) MCH1-L + L s MCH2-L + H2-L (3) MCH2-L + L e Tol-L H2-L (4) Tol-L P To1 + L (5) H2-L P H2 + L (6)
+
+
Sequence 111. Single-Site Surface Reactions, No Adsorption-Desorption of Hydrogen MCH + L s MCH-L (1) MCH-L P MCH1-L HZ (2) MCHl-L 2 MCH2-L + H2 (3) MCHB-L F? Tol-L + H2 (4) Tol-L s To1 L (5)
+
+
the absence of competitive adsorption of toluenes. The reaction order with respect to methylcyclohexane is positive but smaller than one, which can be interpreted as an approximation to the Langmuir isotherm (Boudart, 1956). The negative reaction order with respect to hydrogen can be explained either by the involvement of a dehydrogenated reaction intermediate as reactant in the rate-determining step or by competitive adsorption of hydrogen. Discrimination between these possibilities is possible by deriving the corresponding Hougen-Watson rate equations. Hougen-Watson Rate Equations. Hougen-Watson rate equations contain an adsorption term accounting for the competitive coverage of active sites by reactants, reaction intermediates, reaction products, and, eventually, by species not involved in the reaction sequence. Sequence I, shown in Table 111, basically consists of elementary steps originally proposed by Horiuti and Polanyi (1934). The adsorption of methylcyclohexane, step I(1), is followed by six abstractions of a hydrogen atom, steps I(2)-I(7). The sequence is closed by the associative desorption of the hydrogen atoms, step I(8), considered to be in dynamic equilibrium. Sequence I considers eight surface intermediates: the adsorbed reactants and reaction products and five reaction intermediates with increasing degree of unsaturation. Hence, the adsorption term of the HougenWatson rate equations derived from sequence I contains eight apparent adsorption coefficients. Considering that the stability of the adsorbed olefinic and diolefinic intermediates, MCH1-L and MCH2-L, is much greater than the stability of the intermediates A1-L, A,-L, and AB-L leads to the simplified sequences I1 and I11 with three consecutive surface reactions. In sequence 11, weakly adsorbed molecular hydrogen is considered and postulated to be in dynamic equilibrium with hydrogen in the gas
phase. The adsorption term of the Hougen-Watson rate equations corresponding to sequence I1 contains five apparent adsorption coefficients. In sequence 111, H2 adsorption is not considered at all and, more importantly, a single-site mechanism is postulated for the consecutive surface reactions. Rate equation II(4), corresponding to step (4)of sequence I1 as rate determining, is given for illustrative purposes by rMCH-T~l
= kMCH-T~l(PMCH - PTo#H3/
KMCH-Tol)
/PH202 (5)
In the above rate equation kMCH4Tol represents the kinetic factor
-
~ M C H - T ~-~
k4KMCHK2K3
KH2
c,2
(6)
and 6' is the adsorption term
KMCHK2K3
PMCH
+ K T ~ ~ +P KHPH T ~ ~ (7) KH2 PH The coefficients KMCH, KTol, and KH are the adsorption equilibrium coefficients corresponding to the adsorption of methylcyclohexane, toluene, and hydrogen; K, is the equilibrium constant of the surface reaction II(i). Taking the adsorption-desorption steps of the reaction intermediates into account (Siegel, 1966; Gland et al., 1975) and assuming dynamic equilibrium for these steps lead to the following equivalent expressions for the kinetic factor and the adsorption term: kMCH-Tol =
k4KMCH-MCH2KMCH2Ct
2
(8)
PMCH 6' = 1 + KMCHPMCH + KMCH-MCHIKMCHIPH PMCH K M C H - M C H ~ K M+CKT~IPT~I H ~ ~ + KHPH (9) PH
+
The coefficients KMCH-MCHI and KMCH-MCH~ are the equilibrium coefficients corresponding to the dehydrogenation of methylcyclohexane into methylcyclohexene and of methylcyclohexane into methylcyclohexadiene. The coefficients KMCH1 and K M ~are~ the 2 adsorption equilibrium coefficients corresponding to the adsorption of methylcyclohexene and methylcyclohexadiene. Discrimination among the rival rate equations was based on the complete set of 82 experimental conversions into toluene obtained over the range of conditions listed in Table I. A first discrimination was performed without accounting for the temperature dependence of the apparent adsorption coefficients, Le., the products of adsorption and dehydrogenation equilibrium coefficients. The maximum number of parameters amounted to 10: a preexponential factor and an apparent activation energy, together with eight apparent adsorption coefficients. Apparent adsorption coefficients which were not significantly different from zero at the 95% probability level were set equal to zero. Most of the rate equations were rejected, either because of significantly negative values of the pa-
548
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986
test on the global regression or the t test on the parameter estimates. For further discrimination the temperature dependence of the adsorption coefficient of methylcyclohexane and of the apparent adsorption coefficient of the most abundant reaction intermediates was accounted for:
Table IV. Four Best Rate Equations and the Corresponding Rate-Determining Step on Pt/AI2O3
K, =
(12)
Ae-JH'IRT
The standard entropy changes can be obtained from the corresponding preexponential factors by ASo = R In A
(13)
The parameter estimates are listed in Table V. The preexponential factor and the standard enthalpy change for the adsorption of methylcyclohexane are significantly different from zero at the 95% probability level for rate equation 1(7) only. The standard adsorption enthalpy for methylcyclohexane in eq I(7) is positive, however. This would correspond to an endothermic adsorption. Hence, introduction of the temperature dependence of the adsorption coefficient for methylcyclohexane does not lead to statistically or physically meaningful results. The preexponential factor and the apparent standard enthalpy change corresponding to the adsorption of the most abundant reaction intermediate are significantly different from zero a t the 95% probability level, with the exception of the apparent standard enthalpy change in rate equation II(3). The standard adsorption enthalpy of the most abundant surface intermediate is a sum of the apparent standard enthalpy change given in Table V and a standard dehydrogenation enthalpy. The reported positive standard enthalpy changes indicate that the endothermic effect of the dehydrogenation reactions is larger than the exothermic effect of the adsorption. For rate equation II(4), by way of example, the standard adsorption enthalpy is given by
rameters or because of the lack of significance of the global regression. The four remaining rate equations are given in Table IV, together with the corresponding rate-determining step and the F value for the global regression. The F values listed in Table IV are a t least twice that of the F values corresponding to the rejected rate equations. The form of the selected rate equations shows striking similarities. Adsorption on the active sites for dehydrogenation of hydrogen or of surface intermediates not involved as reactants in the rate-determining step can be neglected. Clearly, the lowering of the reaction rate with increasing partial pressure of hydrogen is caused by the involvement of a dehydrogenated surface intermediate in the rate-determining step and not by the competitive adsorption of hydrogen on the active sites for dehydrogenation. There are two most abundant surface intermediates: the adsorbed methylcyclohexane and the reaction intermediate involved as reactant in the rate-determining step. The apparent adsorption coefficients listed in Table IV are products of equilibrium coefficients for dehydrogenation and of the adsorption coefficient of the most abundant reaction intermediate. This can be illustrated by comparing the coefficient of pMC+pH-' in eq 9 and in the adsorption term of rate eq II(3) of Table IV. All four rate equations correspond to a dual-site rate-determining step. The corresponding reaction mechanisms differ only by the degree of unsaturation of the most abundant reaction intermediate. Adsorbed methylcyclohexene is the most abundant reaction intermediate in mechanisms I(5) and II(3), and adsorbed methylcyclohexadiene in mechanisms I(;) and II(4). The corresponding adsorption steps are dissociative for mechanisms I(5) and I(7): MCHl + 2L . A,-L + H-L (10) MCH2 + 2L .+A,-L + H-I, (11) A further discrimination between the four rate equations listed in Table IV was not possible on the basis of the F
~ ' M C H 2= u'MCH2a
-
u'MCH-MCH2
(14)
and the standard adsorption entropy by LS'MMCH~ = -~LS'WX-W' - LS'MCH-MCH~ (15) The apparent standard adsorption entropies are obtained from Table V by applying eq 13. The standard reaction enthalpies and entropies are obtained from tabulated data or computed according to Benson (1976). Table VI lists the calculated standard adsorption enthalpies and entropies for the most abundant reaction intermediate corresponding to each of the four retained rate equations. The limits for AS', in rate equations I(5) and I(7) correspond to the minimum and maximum values of the standard adsorption entropy of H2 given by Vannice et al. (1979).
+
Table V. Parameter Estimates for the Four Best Rate Equations on Pt/A1203" rate eauation kinetic uarameters 3.86 X lo5 3.5
173.04 3.6 5.11x 101 1.1 Ai, bar" ih
Itjoin. kJ/mol I
F
62.83 0.4 4.18x 109 5.0 100.46 2.9 581
I('i) 9.09x 10'8 4.7 179.88 6.3 1.15 x 10' 2.1 90.16 2.1 2.45 x 10'2
5.1
84.03 2.5 6 10
II(3) 4.24 x 1013
4.6 180.43 6.0 5.10 x 105 1.6 74.61 1.1
6.8 121.70 2.1 0.34
5.18 X 10'
1.47 X 10'O
2.0 99.55 1.1 545
4.6 99.77 2.6 607
69 8.9
3.6 160.62 3.9 6.80x lo-" 1.5 49.33 0.5
2.0
833
Rate equations I(5), I(7), II(3): temperature-dependent adsorption coefficients. Rate equation II(4): with and without temperaturedependent adsorption coefficients. On the reparametrized preexponential factors
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986 549
Table VI. Standard Adsorption Enthalpy and Entropy for the Most Abundant Reaction Intermediate Corresponding to Each of the Four Best Rate Eauations on Pt/AI,O, rate equation I(7) II(3) II(4) I(5) - M o I , kJ/mol 20 21 20 128 -21 81 -ASoI, J/(mol K) 18-37 108-150 ~~
~
According to Everett (1950a) the standard entropy change for Langmuir adsorption is given by
AS',
= So, - Sog
(16)
where Sogis the entropy in the gas phase taken at unit pressure and So, is the entropy in the adsorbed state at a degree of coverage of half of a monolayer. Boudart et al. (1967, 1972) formulated two strict rules which must be satisfied by ASo, for nondissociative Langmuir adsorption -AS", -ASo,
>0 < So,
(17) (18)
as well as two relations which can be used as guidelines to assess the meaningfulness of the values obtained for AS", and AH",
-AS",
> 42
Table VII. Standard Activation Enthalpies and Entropies for the Rate-Determining Steps Corresponding to Each of the Four Best Rate Equations on Pt/Al,O,"
(19)
-AS", I 5 1 - 1.4AH0, (20) The equality sign in (20) corresponds to physical adsorption on charcoal at 273 K (Everett, 1950b). Vannice et al. (1979) reported that relations (17)-(20) are also valid for dissociative adsorption. All four of the standard adsorption enthalpies reported in Table VI are negative, as expected. The standard adsorption entropy obtained in rate equation II(3) is positive, however, in contradiction with the strict rule (17). The standard adsorption entropy obtained in rate equation 1(5) fulfills the strict rule (17), but not the guideline (19). The standard adsorption entropy corresponding to rate equation 1(7) meets the strict rule (18), since the standard entropy of methylcyclohexadiene amounts to 460 J f (mol K) a t temperatures around 650 K. The guideline (20) is not met, however. The standard adsorption entropy of the most abundant reaction intermediate in mechanism II(4), i.e., of adsorbed methylcyclohexadiene, meets all four of the requirements (17)-(20). Values for the standard activation enthalpy and entropy of each of the selected rate-determining steps can be obtained from the corresponding estimates of the preexponential factor and of the apparent activation energy in Table V. An expression for the standard activation enthalpy is obtained by differentiating the logarithm of both sides of, by way of example, eq 8 with respect to temperature (Gerasimov, 1978): = E a - AfIoIa - RT, (21) Introduction of the temperature dependence according to Arrhenius and van't Hoff in, by way of example, both sides of eq 8 and equating the preexponential factors lead to
where I is the reaction intermediate involved as reactant in the rate-determining step. The preexponential factor A o S , expressed in m2/h, is obtained from AO,RDS by
S o t , kJ/mol ASot, J / ( m o l K )
68 -311
91 -146
57 -139
76 -114
" z = 2, [L] = 1OI8 m-* of Pt, and T,,, = 643 K.
Transition-state theory leads to following expression for the preexponential factor of a dual-site reaction (Boudart and Dj6ga-Mariadassou, 1982; Gerasimov, 1978)
where z is the number of nearest neighbors of an active site and [L] is the active site density per unit P t surface area. The concentration of active sites follows from the site density by ct
=
ASILl/NA
(25)
Equations 22-25 allow the calculation of the standard activation entropy of the rate-determining step in the reaction mechanism, starting from the estimated value of the preexponential factor, A O , M ~ + T o l , of the rate coefficient in the corresponding Hougen-Watson rate equation. The atom density on a clean platinum surface amounts to m-2 of Pt. Because of the continuous sulfidation, an important fraction of the platinum surface is covered by sulfur, however (Menon et al., 1982; Van Trimpont et al., 1985). Table VI1 shows the obtained standard activation enthalpies and entropies assuming an average number of nearest neighbors of two and an active site density of 1OI8 m-2 of Pt. The standard activation enthalpies are positive for each of the four retained rate equations. The negative standard activation entropies indicate a strong steric requirement for the rate-determining step. Maatman et al. (1971) reported a standard activation enthalpy around 70 kJ/mol and a standard activation entropy around -100 J/ (mol K) for the dehydrogenation of cyclohexane in the absence of hydrogen at 420 K and atmospheric pressure. When the thermodynamic properties of the adsorption and rate coefficients are accounted for, the rate equation II(4) appears to be the one preferred. It is the only rate equation for which all the thermodynamic quantities which can be significantly determined are meaningful. Introduction of the temperature dependence of the adsorption coefficients leads to a lower F value, however. The last column of Table V shows the estimates for the kinetic parametors of rate equation II(4) when the temperature dependence of the adsorption coefficients over the investigated temperature range is neglected. Figure 3 shows the calculated and experimental conversions as a function of W / F o H cfor several inlet partial pressures of methylcyclohexane. The complete set of experimental conversions into toluene is compared with the corresponding set of calculated conversions in Figure 4. The absence of any systematic deviation illustrates the adequacy of the selected rate equation II(4). Dehydrogenation Kinetics on PtRe/A1,03 Influence of Partial Pressures on the Initial Rate of Dehydrogenation. Figures 5 and 6 show the influence of the partial pressure of methylcyclohexane and hydrogen on the initial dehydrogenation rate at 673 K on PtRe/ A120,. The dehydrogenation rate increases with increasing methylcyclohexane partial pressure. At a hydrogen partial pressure of 8 bar, the rate increase per partial pressure unit
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986
550
A
4
10-
5-
".
10.
i
cc
_I
0
1
2
*
3
, i
0
0 -- ----------_----_-------0 Q
1 E
W/FiC ( k g c a t . h / k m o l ) Figure 3. Conversion into toluene a t 623 K, PMCH = 0.5 bar, and PH = 4.5 ( O ) , 15 (X), and 20 bar (+}: points, experimental values; full lines, rate equation II(4). Parameter estimates are from the last column of Table V. Catalyst: Pt/Al,O,.
30
L
f
5
10
pH ( b a r )
Figure 6. Dehydrogenation rate as a function of the hydrogen partial pressure at 673 K and a methylcyclohexane pressure of 0.5 bar. Catalyst PtRe/A120B.
E X P E R I M E N T 9 L CONVLKSION
(%)
Figure 4. Calculated vs. experimental conversions into toluene. The calculated values are from rate equation II(4),and the parameter estimates are from the last column of Table V. Catalyst: Pt/ Al,O,.
levels off for methylcyclohexane partial pressures higher than 0.5 bar. At a methylcyclohexane partial pressure of 0.5 bar, a variation of the hydrogen partial pressure from 4 to 1 2 bar has no effect on the dehydrogenation rate, in contrast with the decrease observed on Pt/A1203. A series of experiments investigating the effect of competitive adsorption on the dehydrogenation rate were performed. In contrast with Pt/A1203, there is a pronounced competitive adsorption between toluene and methylcyclohexane. The extent of the competitive adsorption between n-heptane and methylcyclohexane is
similar to that on Pt/A1203 and much less pronounced than the competitive adsorption with toluene. Power-Law Rate Equation. The experimental data on the bimetallic reforming catalyst consist of 119 conversions into toluene which were obtained over the range of conditions listed in Table I. The result of the regression with a power-law rate equation, eq 3, of the data set for which the rate of the reverse reaction can be neglected is shown in Table 11. The apparent activation energy is 60 kJ/mol higher on PtRe/A1,03 than on Pt/A1203,which is compensated to some extent by a higher preexpontential factor. The reaction order with respect to methylcyclohexane is similar to the corresponding reaction order on Pt/A1,03. The reaction order with respect to toluene is negative and corresponds to strong toluene adsorption. The most striking difference with the corresponding rate equation on Pt/A1,0, is made up by the reaction order with respect to hydrogen, which is not significantly different from zero at the 95% probability level. The zero reaction order with respect to hydrogen and the negative order with respect to toluene can be understood from the Hougen-Watson rate equations. Hougen-Watson Rate Equations. Only the HougenWatson rate equations corresponding either to the adsorption of methylcyclohexane or to the first surface reaction in sequences 1-1' . of Table I11 as rate-determining step can conform to a ro reaction order with respect to hydrogen. This requi the concentrations of adsorbed H2 and dehydrogenatt action intermediates to be small when compared with L;.B concentrations of adsorbed meI -
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986 551 Table VIII. Two Best Rate Equations and the Corresponding Rate-Determining Step on PtRe/Al2O2 reaction mechanism Houpen-Watson rate eauation
III(2)
TMCH-T~I
-- kMCH-T~l(PMCH - P T o ~ P H ' / ~ M C H - T O ~ ) 1 + KMCHPMCH + K T ~ ~ P+TK n~PI7 P n P 7
Table IX. Parameter Estimates for the Best Rate Equations on PtRe/A1203 rate eauation kinetic parameter 1(2)-11(2) III(2) A O , M C H - T ~ ~ , kmol/(kg of cat. h bar) 2.18 X 1013 2.89 X l O I 3 ta 28 21 181.4 180.5 EMCH-T~I, kJ/mol t 25 26 K M C H , bar-' 1.73 0.53 t 11 15 KTol, bar-' 17.9 5.01 t 11 7.9 1.84 Knp7,bar-' 0.58 t 3.5 4.1 F 2092 2155 a
On the reparametrized preexponential factor.
thylcyclohexane and toluene. Of the investigated rate equations, those corresponding to the first hydrogen abstraction as the rate-determining step lead to the highest F values for the global regression. Table VI11 shows the retained rate equations. The mechanisms 1(2) and II(2) lead to identical rate equations at small concentrations of reaction intermediates and adsorbed hydrogen. The parameter estimates are shown in Table IX. In the present case a significant estimation of the adsorption coefficient of the n-heptane added to the feed was possible. The statistical tests reported in Table IX do not allow a discrimination between the two retained rate equations. They differ only by the exponent of the adsorption term. Rate equation 1(2)-11(2) corresponds to a dual-site rate-determining step, whereas rate equation III(2) corresponds to a single-site rate-determining step. Taking into account the temperature dependence of the adsorption coefficients resulted in estimates for the standard adsorption enthalpies of methylcyclohexane and n-heptane which were not significantly different from zero at the 95% probability level. The standard adsorption enthalpy of methylcyclohexane was significantly negative at the 50% probability level. The estimated values of the standard adsorption enthalpy and entropy of methylcyclohexane satisfy the relations (17)-(20). They are listed in the first part of Table X. The estimated value of the standard adsorption enthalpy of toluene was significantly negative a t the 95% probability level and amounted to -462 kJ/mol for rate equation 1(2)-11(2) and to -510 kJ/mol for rate equation III(2). The corresponding values for the adsorption entropy loss are higher than the standard entropy of toluene in the gas phase, viz. rule (18), however. Since there is a strong correlation between the standard adsorption enthalpy and entropy, the estimated values for the standard adsorption enthalpy of toluene are more negative than the difference between the standard enthalpies of toluene in the adsorbed state and in the gas phase. The standard activation enthalpies of the rate-determining step corresponding to the retained rate equations follow from eq 21, with adsorbed methylcyclohexane as reaction intermediate. The obtained values are also shown in Table X. They are substantially higher than the
Table X. Standard Adsorption Enthalpies and Entropies for Methylcyclohexane and Standard Activation Enthalpies and Entropies for the Rate-Determining Step Corresponding to the Retained Rate Equations on PtRe/Al,O," rate equation 1(2)-11(2) III(2) - A H O M C H I kJ/mol 86 79 -ASOMCH, J/(mol K) 124 123 AHo*, kJ/ mol 138 117 ASo*, J/(mol K) -303 -321 a z = 3, [L] = 1019 m-2 of Pt, and T,,, = 663 K.
standard activation enthalpy for the rate-determining step on Pt/A1203, viz. Table VII. The standard activation entropy for the dual-site ratedetermining step of mechanism 1(2),II(2), follows from eq 22-25. For a single-site the step following equation holds:
AO,RDS=
AO,MCH-T~~
AIG On the other hand, transition-state theory leads to the following expression for A O , ~ D(Boudart S and DjBga-Mariadassou, 1982; Gerasimov, 1978):
Equations 25-27 allow the calculation of the standard activation entropy of the single-site rate-determining step in reaction mechanism III(2). There are both experimental (Biloen et al., 1980) and thermodynamic (Bartholomew et al., 1982) arguments to picture a presulfided PtRe bimetallic surface as consisting of clean ensembles of platinum atoms surrounded by rhenium atoms covered by sulfur. Hence, the active site density has been taken equal to the atom density of a platinum surface. A value of three was assigned to the average number of nearest neighbors. The value of 0.7 m2 of Pt/g of cat. obtained from the 02-H2 titration data was used as an estimate for the specific active surface area. The calculated standard activation entropies are also listed in Table X. The difference between the standard entropy of the rate-determining transition complex and of the adsorbed reactants is even more pronounced than with Pt/Al,O,. Such high negative values are usually attributed to a solvent effect, in particular when the transition complex is polarized and the reactants are not (Barrow, 1979). Clearly, the thermodynamic analysis of the rate coefficients does not allow to reject either one of the two retained rate equations. A plot of residual deviations against the calculated values of the conversion into toluene shows a more uniform band for rate equation III(2), however. Hence, the single-site mechanism is considered as the most probable. Figure 7 shows the experimental and calculated conversions vs. W/FoHcfor several inlet partial pressures of methylcyclohexane. A parity plot for the complete set of experimental conversions is shown in Figure 8. Comparison of Pt/A1203 and PtRe/A120a In the investigated range of temperatures and partial pressures, the dehydrogenation rate of methylcyclohexane is much higher on the sulfided Pt/A1203 catalyst than on the presulfided PtRe/A1203catalyst. The apparent activation energy is higher on the bimetallic than on the monometallic catalyst, however (viz. Tables 11,V, and IX), so that the difference in activity is less pronounced under typical industrial conditions, i.e., 730-800 K and hydrogen
Ind. Eng. Chern. Fundarn., Vol. 25, No. 4, 1986
10
1
Figure 7. Conversion into toluene at 673 K, phlcH = 0.15 ( O ) ,0.5 ( X ) , and 1.0 bar (+), and p H = 8 bar: points, experimental values; full lines. rate eauation III(2). Parameter estimates are from Table Catalyst: &Re/Al,Oi.
5 1
r " L . l i
10
(kLIk-
*
\
Figure 8. Calculated vs. experimental toluene conversions. The calculated values are from rate equation III(2), and the parameter estimates are from Table IX. Catalyst: PtRe/A1,03.
partial pressures higher than 10 bar. The most important difference between the reaction mechanisms on the two catalysts consists in the location of the rate-determining step, which causes a different dependence of the rate of dehydrogenation on the partial pressure of hydrogen. On Pt/A1203the concentration of adsorbed methylcyclohexadiene, which is the reaction intermediate involved in the rate-determining step, decreases when the hydrogen partial pressure is increased. On PtRe/A1203, the concentration of the reaction intermediate, adsorbed methylcyclohexane, is independent of the hydrogen partial pressure. Two sites are involved in the
rate-determining step on Pt/A1203. With PtRe/A1,03, a single-site mechanism seems to be favored. The adsorption of toluene is pronounced on PtRe/Al,O, only. The differences between the behavior of Pt/A1203and PtRe/A1203show striking analogies with the differences observed between the behavior of unsulfided platinum surfaces without or with steps and kinks (Somorjai, 1981). Steps and kinks contain atoms with a low degree of coordination. Gland et al. (19751, Blakely and Somorjai (1976), and Herz et al. (1981) have investigated the dehydrogenation of cyclohexane on platinum single crystals at atmospheric pressure and at pressures around bar. On surfaces essentially free of steps and kinks the dehydrogenation of cyclohexane into benzene mainly takes place on the high-coordination atoms of the terraces and the dehydrogenation of cyclohexene is rate-determining. On stepped surfaces the dehydrogenation of cyclohexane into benzene mainly occurs on the steps and the dehydrogenation of cyclohexane to cyclohexene is rate-determining. Benzene is mainly adsorbed on the terraces. Hydrogenolysis of cyclohexane is associated with stepped surfaces. Sulfur deposition selectively poisons the hydrogenolysis of hydrocarbons by platinum (Menon et al., 1982) and, hence, preferentially occurs on steps and kinks. On platinum surfaces essentially free of steps and kinks ordering of the sulfur adsorbed on the terraces occurs when the coverage exceeds one-fourth of a monolayer (Fischer and Kelemen, 1978; Abon and Billy, 1985). On PtRe surfaces, sulfur adsorption preferentially occurs on the rhenium atoms (Biloen et al., 1980; Bartholomew et al., 1982). The observations made in the present work can be related to the above. The rate-determining step on a sulfided commercial Pt/A1203catalyst is one of the reaction steps following the formation of cyclohexene, similar to what Gland et al. (1975) observed on unsulfided platinum surfaces essentially free of steps and kinks. The adsorption of sulfur on terraces hampers the adsorption of toluene by enhancing steric constraints. On a presulfided commercial PtRe/Al,O, catalyst the rate-determining step is one of the steps preceding the formation of methylcyclohexene, similar to what Blakely and Somorjai (1976) observed on unsulfided stepped platinum surfaces. On a presulfided commercial PtRe/A1203catalyst the adsorption of toluene on platinum becomes pronounced, since it is not hindered by sulfur coverage of the platinum terraces. The combined effect of sulfur and rhenium divides the platinum into small ensembles and favors a single-site mechanism.
Conclusions A kinetic analysis according to the Hougen and Watson approach has provided a better understanding of the dehydrogenation of methylcyclohexane on sulfided commercial Pt/A1,03 and PtRe/A1203catalysts under quasiindustrial conditions. The partial substitution of platinum by rhenium primarily causes a shift of the rate-determining step from the dehydrogenation of methylcyclohexadiene into toluene to the dehydrogenation of methylcyclohexane into methylcyclohexene. This leads to a significant change in the hydrogen partial pressure dependence of the rate of dehydrogenation. The latter strongly decreases with increasing hydrogen partial pressures on Pt/A120, but is independent of the hydrogen partial pressure on PtRe/A1,03. The partial substitution of platinum by rhenium enhances the adsorption of toluene to such an extent that it becomes competitive. This is a result of the preferential adsorption of sulfur on rhenium, keeping the platinum surface free
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986 553 of sulfur. Hence, toluene strongly adsorbs on the platinum surface of a bimetallic catalyst.
Registry No. PhMe, 108-88-3;H,, 1333-74-0;S, 7704-34-9; Pt, 7440-06-4;Re, 7440-15-5;methylcyclohexane, 108-87-2.
Acknowledgment
P. A. Van Trimpont is grateful to IWONL-IRSIA for a Fellowship (1981-1984). The Belgian Ministry of Scientific Affairs is acknowledged for a “Center of Excellence” grant awarded within the framework of the “Concerted Actions on Catalysis“. Glossary A,, A AOS
AS
ct
e E FHC
h H HC k K
L [LI MCH MCHl MCH2 n, N A
PI
r R
4 SO
T Tln To1 W XI
preexponential factor, kmol bar”/(kg of cat. h) or barn preexponential factor of reaction rate coefficient, m2/h specific platinum surface area, m2 of Pt/g of cat. molar concentration of active sites on the metal function per unit catalyst mass, kmol/kg of cat. Euler number activation energy, kJ/mol hydrocarbon molar flow rate, kmol/h Planck constant, J h hydrogen or molar enthalpy, kJ/mol hydrocarbon reaction rate coefficient adsorption coefficient of a component or equilibrium constant of a reaction active site on the metal function active site density, m-2 of Pt methylcyclohexane methylcyclohexene methylcyclohexadiene partial reaction order with respect t o component i Avogadro constant, mol-’ partial pressure of component j , bar reaction rate, kmol/(kg of cat. h) ideal gas constant, kJ/(kmol K) or kJ/(mol K) production rate of component i, kmol/(kg of cat. h) standard molar entropy, J/(mol K) temperature, K average temperature of the experimental data, K toluene catalyst mass, kg of cat. fractional conversion of component j
Greek Symbols reaction product minus reactant adsorption term for the metal function in the Hougen-Watson rate equation
A 0
Subscripts a adsorption A component A A B reaction from A to B I reaction intermediate involved as reactant in the rate-determining step g gas phase L active metal site RDS rate-determining step S surface
-
Superscripts a apparent 3 transition state reparametrized 0 inlet condition, standard
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Received for review September 23, 1985 Accepted February 24, 1986
