Stationary and Nonstationary Kinetic Studies Related to the

Ind. Eng. Chem. Res. 1997, 36, 3211-3222

3211

Stationary and Nonstationary Kinetic Studies Related to the Mechanism of Heterogeneous Catalytic Reactions† S. L. Kiperman* and K. Kumbilieva N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Science, Leninsky Prosp. 47, 117913 Moscow, Russia

Some results on the kinetics and mechanism of catalytic processes obtained by use of a complex strategy, including combined stationary and nonstationary investigations, are discussed. The approach developed is grounded on exact kinetic studies by gradientless methods, isotopic exchange, IR spectroscopy of surface compounds, isotopic effects, adsorption measurements, investigations of transition processes, and catalyst deactivation. The reaction kinetics is related to the specificity in the mechanism of processes under investigation. Introduction Studies devoted to the mechanism of catalytic reactions draw the interest of many researchers. Yet, knowledge about the mechanism of most of the heterogeneous catalytic reactions is far from sufficient. One of the reasons for such an inadequacy seems to be that little attention has been paid to the importance of wedding the resources which stationary and nonstationary kinetic studies put at our disposal to cast light on the reaction mechanism. We commend the contribution of Professor Froment to developing the theory of complex catalytic reactions and their mechanism (1). In the present review, the approach developed in our laboratory for studying the reaction mechanism will be discussed. The approach is based on the understanding that conclusions drawn from steady-state data will acquire reliability when they are supported by nonstationary studies. A complex strategy has been applied involving exact gradientless kinetic experiments, isotopic exchange methods, IR spectroscopy of surface compounds, adsorption measurements, as well as studies in nonstationary regimes (2-4). Experimental techniques were developed (4) to examine in a gradientless system the kinetics of catalyst deactivation in conjunction with the main reaction kinetics. Such studies give grounds for conclusions elucidating the mechanism of the main and deactivating reactions. The use of the transient response method, which was developed by Kobayashi et al. (5, 7) and Bennett (8, 9) and successfully applied by many other researchers (see e.g. refs 10-12) made available important information concerning the character of the interactions, surface coverages, characteristics of intermediates, strength of bonds, values of the rate coefficients of some elementary steps, and prehistory of the catalyst. Pieces of this approach are scattered in different studies carried out for various reactions. In this paper, we take the effort to summarize into a whole the experience gained and make some common conclusions. * FAX: (7.095)135-5328. E-mail: [email protected]. † Dedicated to Prof. G. F. Froment on the occasion of his 65th birthday. S0888-5885(96)00693-8 CCC: $14.00

The details of the experiments can be found in the original works cited. Some details concerning the conditions of the transient experiments realized in our laboratory are in the Appendix. The approach can be illustrated by Scheme 1. One of us (S.L.K.) had the pleasure to discuss some of the results presented in a lecture delivered in the laboratory of Professor Froment.

Dehydrogenation of Paraffins and Hydrogenation of C3-C5 Olefins over Platinum Catalysts Dehydrogenation of the low paraffins C3-C5 as well as hydrogenation of olefins C3-C5 over platinumalumina catalysts (0.35-0.60% Pt) with various additives was studied at 500-600 °C and 1 atm in refs 1327. The approach developed in ref 28 was supplied to consider the influence of the back reaction. The processes were carried out in a flow of either H2 or a mixture of H2O and H2. The common reaction scheme may be presented as follows: i–CnH2n+2

n–CnH2n+2

CnH2n (1) CnH2n–2 coke

cracking

The efficiency of catalysts was determined on considering their activity, selectivity, and stability. With this regard, the catalysts containing different additives can be ranged in the orders Pt < Pt-Cu ≈ Pt-Se , Pt-Ge < Pt-Pb < Pt-In < Pt-Sn and Pt-K < Pt-K-Sn < Pt-K-In. The kinetic equations derived on the basis of gradientless experiments appear as follows: © 1997 American Chemical Society

3212 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Scheme 1 primary description

kinetic equations

step scheme

kinetic experiment (steady state and unsteady state)

physicochemical experiments

(I)

hypotheses about the mechanism

dehydrogenation in H2 flow rI ) kIP1γ/(PH20.5 + k′P2 + k′1P3)

(II)

(2)

cracking rII ) (kIIP1 + k′IIP2)/(PH20.5 + k′P2 + k′1P3)

(III)

mechanism of the process

(3)

isomerization

rIII ) (kIIIP1PH20.25 + k′IIIP2)/(PH20.5 + k′P2 + k′1P3) (4) (IV)

coke formation

rC ) (kCP2 + k′CP3)/(1 + k4PH20.5)

while c < c* (5)

rCC ) (kCCP2 + k′CCP3 - k*CP2PH20.5)/ (P2 + k5PH2 + k*5P3 + k*5(c* - c))

at c > c* (6)

The coefficient γ considers the influence of the back reaction (28); Pi (i ) 1, 2, 3, ..., H2) stand for the partial pressures of the reactants and c for the surface concentration of coke. When c attains the threshold value of c*, the contribution of deactivation becomes noticeable and the rate of dehydrogenation is described by the following equation:

r ) k′IPIγ(PH2 + k6P2 + k′6P3 + k′′6c2/3)-1

(7)

The rate of dehydrogenation in H2 + H2O flow is described by

r′′ ) k′′P1γ(PH20.5 + k′P2 + k′1P3 + k′′1PH2O2)-1 (8) It is essential that the kinetic equations characterizing the reactions on Pt catalysts with different promoters are similar by form and vary by the values of the rate coefficients. To examine the reaction mechanism, the transient response method was applied in combination with isotopic experiments and kinetic studies. The typical relaxation curves for hydrogenation-dehydrogenation transformations of hydrocarbons C3 and C4 are presented in Figures 1 and 2. The reversibility of these reactions makes the information obtained for the back reactions applicable for the forward reactions and vice versa. Henceforth, any curve describes the response of the reaction rate to the final jump change in the composition of the mixture introduced. The plots on Figure 1 characterize the changes in the reaction rate of propene hydrogenation to propane. The

Figure 1. Relaxation curves of responses in the reaction of propene hydrogenation on Pt-K-Sn catalyst. (A) At 450 °C: 1, H2/(C3H6 + H2); 2, (C3H6 + He)/(C3H6 + H2). (B) At 400 °C: 1, (C3H6 + He)/He; 2, (C3H6 + He)/H2.

relaxation curves in Figure 1A refer to the jump (H2 + C3H6) substitution for either pure H2 (curve 1) or propene (curve 2) initially introduced into the reaction system. The curves on Figure 1B refer to the changes of the reaction rate when preliminary introduced pure propene is replaced by either H2 (curve 1) or He (curve 2). It follows from the monotonous form of the curves that (i) both H2 and propene interact from the adsorbed state and (ii) they occupy the same sites on the catalyst surface. At that, propene may be partially replaced by H2. The relaxation curves in Figure 2A show the changes in the rate of butene hydrogenation over Pt with different promoters after substituting H2 with a H2 + C4H8 mixture. The curve corresponding to the unpromoted catalyst exerts an extremum, while the other curves are monotonous. This peculiarity can be linked to the action of the promoters present. The point is that

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3213

Figure 2. Relaxation curves of responses in the reaction of isobutane hydrogenation. (A) H2/(i-C4H8 + H2): 1, on Pt at 450 °C; 2, on Pt-Sn; 3, on Pt-Ge; 4, on Pt-In at 500 °C. (B) He/ (C4H8 + H2): 1, on Pt; 2, on Pt-Sn; 3, on Pt-Ge, 4, on Pt-In at 500 °C.

when butene is introduced into the system, initially the reaction rate increases with the increase of its surface concentration. However, the adsorption affinity of the unpromoted catalyst is stronger for butene than for H2. For this reason, the system comes to a point when the surface fraction of butene exceeds the optimal concentration. This results in a resistance effect, and the reaction rate decreases. The absence of the extremum in the reaction rate over promoted catalyst samples is indicative that the promoters are conductive to maintaining an optimum in the ratio of the surface concentrations of the reactants. Figure 2B refers to the case when the H2 + C4H8 mixture is introduced over a catalyst preliminary treated by He. The unpromoted catalyst is characterized by a monotonous relaxation curve. The increase of the reaction rate is due to the increase of the surface concentrations of the reactants; at the same time, the stronger adsorption affinity of the olefin prevents attaining the optimal ratio of the surface fractions. This explains the monotonous form of the relaxation curve. The manifestation of the maximum in the reaction rate when the catalyst is promoted by Sn or Ge is evidence that these additives reduce the adsorption affinity for the olefin and, thus, contribute to attaining the optimal surface concentrations. The relaxation curves on Figure 3 indicate that propene and isobutene occupy the same active sites. The indications are that the adsorption of propene is higher than the adsorption of isobutene. The comparison of

Figure 3. Relaxation curves of responses in the reaction of propene and isobutane dehydrogenation on Pt-K-Sn catalyst. (a) 1 and 2′, (i-C4H8 + H2)/(C3H6 + H2); 1′ and 2, (C3H6 + H2)(i-C4H8 + H2) with formation of C3H8 (curves 1 and 1′ relate to the left ordinate) and i-C4H10 (curves 2 and 2′ relate to the right ordinate at 475 °C. (b) He/(C3H6 + i-C4H8 + H2) at 350 °C; 1, i-C4H10; 2, C3H8 formation.

(C4H8 + H2)/He/H2 and (C3H6 + He)/H2 responses as well as isotopic data discussed in our original papers (14, 18-22) gives grounds to conclude that the hydrogenation reaction proceeds via a half-hydrogenation form. The coverages by such a form are remarkably higher on Pt-Sn than on unpromoted Pt. In the same works, the response (C3H6 + H2)/(C3H6 + D2) was investigated. It followed from the form of the relaxation curves that an appreciable isotopic exchange was realized in the molecules of the olefins, while no isotopic exchange occurred in the paraffin species. Such a result confirms the conclusion drawn from steady-state data that the adsorption of the olefin species is not the rate-determining step. The relaxation curves describing the responses (iC4H8 + He)/He/D2 for different longitudes of the He treatment show that both the isotopic exchange and the hydrogenation reaction take place in the system. Anyhow, the picture observed is just the opposite for the curves illustrating the response D2/He/(C4H8 + He) at short-time treatment by He. Only isotopic exchange takes place in this case, and no hydrogenation products are formed. It may be concluded that under such conditions, only hydrogen weakly bound to the platinum surface participates in the hydrogenation reaction. This conclusion was proved in more detail for the hydrogenation of aromatic hydrocarbons; see below.

3214 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997

The sum of the results fits best the following scheme (Z is a site on the catalyst surface):

(1)

CnH2n+2 + 2Z ) [CnH2nZ]‚Z(H2)

(2)

[CnH2nZ]‚Z(H2) ) CnH2nZ + Z(H2)

(3)

CnH2nZ ) CnH2n + Z

(4)

Z(H2) ) H2 + Z

(9)

According to this unusual scheme, the rate-determining step is supposed to form a complex of the olefin with weakly bound adsorbed hydrogen. Such a complex can be treated as a “half-hydrogenated form”. The back reaction follows the inverse scheme. We understand that such a scheme can provoke a lot of objections, but we hope to obtain further additional data to support it. Summarizing the data presented, we would like to stress the main principles of the kinetic version for the action of promoters. It can be considered in supplement to the generally recognized physicochemical aspect of their influence. The most important kinetic features in the presence of efficient promoters can be defined as follows. Promoters accelerate the rate-determining steps, reduce the retardation by reaction products, increase the selectivity and stability of the catalyst, conduce the optimal ratio of surface intermediates, reduce the initial rate of coke formation and raise the threshold coke coverage, increase the hydrogen content of coke, and thus facilitate removal of coke. We do hope that the kinetic version for the action of promoters is at service for understanding the behavior of other catalytic processes as well. Hydrogenation of Aromatic Hydrocarbons and Dehydrogenation of Cycloparaffins These processes were studied on platinum-alumina catalysts within three temperature intervals, namely, 60-100, 130-180, and 210-270 °C (29-32). The kinetic description of benzene and toluene hydrogenation changes on raising the temperature and follows the equations

(10)

at 60-110 °C

r ) kPH2

at 130-180 °C

r ) kP1PH22/(P1 + k′PH20.5 + k′′PH2)n (11)

at 210-270 °C

r ) kP1PH23γ/(P1 + k′PH20.5 + k′′PH2)n (12)

for n equal to 1 in the reaction of benzene hydrogenation and 2 in the hydrogenation of toluene. Data obtained by use of the transient response method were of help in understanding the specificity of the reaction kinetics and mechanism. The relaxation curves in Figure 4 show that the reaction proceeds via surface interactions in the adsorbed layer. Figure 5 show the relaxation curves characterizing the interaction of benzene (preliminary adsorbed on the catalyst) with hydrogen introduced after He treatment

Figure 4. Relaxation curves of responses in the reactions of benzene and toluene hydrogenation. (a) H2/(C6H6 + H2) at T (°C) (1) 82, (2) 140, (3) 180, and (4) 260. (b) (C7H8 + He)/(C7H8 + H2) at T (°C) 140 and (2) 173.

of the catalyst. As is seen, even long-term treatment of the system with He does not prevent the hydrogenation reaction leading to the formation of cyclohexane. The picture observed is quite different when the catalyst surface, with H2 (or D2) preliminary adsorbed, is treated for a short time by He, and then benzene is introduced. As seen from Figure 5b, after the system is treated by He up to 15 s, only intensive isotopic exchange takes place, but not the hydrogenation of benzene. These results indicate that only hydrogen weakly bound to the surface of platinum catalyst takes part in hydrogenation reactions, while the more tightly adsorbed hydrogen participates only in isotopic exchange. Therefore, at least two different reactive forms of adsorbed hydrogen participate in the hydrogenation and isotopic-exchange reactions on platinum catalysts. Earlier it was found (33) that only isotopic exchange occurs between benzene and deuterium when both reactants were adsorbed on Ni catalyst. Hydrogenated products were formed only after the appearance of deuterium in the gas phase. It can be supposed that weakly bound hydrogen molecularly adsorbed participates in the hydrogenation reaction. By contrast, hydrogen adsorbed more tightly in atomic form is reactive for the isotopic exchange. Some data confirming the existence of the weakly bound forms of molecularly adsorbed hydrogen can be found in the literature (see, e.g., refs 34-36). Anyhow, knowledge concerned with the reactivity of such species is extremely limited. Theoretical calculations (37)


plained by the relaxation curves on Figure 6 relevant to mixtures containing both hydrocarbons. It follows from the forms of the relaxation curves in Figure 6a,b that the adsorption affinity of toluene exceeds that of benzene, and by this reason, benzene gets partly displayed by toluene. The difference of orders n ) 1 and n ) 2 for the denominators of eqs 11 and 12 is indicative of the fact that the hydrogenation of benzene occurs in the region of medium coverages, while the hydrogenation of toluene is realized at large coverages. A new reaction mechanism was proposed to explain the whole complex of data obtained by different methods and considering the preceding detailed investigations of benzene hydrogenation and cyclohexane dehydrogenation of Ni catalyst (32, 38). The step scheme refers to the region of low temperatures and considers the phenomena of reversibility. The reaction is assumed to proceed via reactive complexes with molecular hydrogen

Figure 5. Relaxation curves of responses in the reaction of benzene hydrogenation by deuterium (curves 1-4) and isotopic exchange in benzene d0-d6). (a) (C6H6 + He)/He/H2, (1, 2, 3) at 90 and (4) 260 °C. (b) D2/He/(C6H6 + He) at 90 °C. (c) D2/He/ (C6H6 + He) at 260 °C. Longtitude of the treatment with He: (a) (curves 1, 3, 4) 15 s, (curve 2) 60 s; (b and c) 15 s.

showed that the bond energy of H-H at the adsorption of hydrogen on group VIII metals can be significantly reduced owing to the formation of the ion (H2)ads-. These calculations can be considered to be an argument in support of our interpretation. The existence of two forms of adsorbed hydrogen in the system under investigation can explain the unusual forms of eqs 13 and 14. It can be supposed that the kinetic equations reflect the availability of two adsorbed forms of hydrogen on the catalyst surface. Another specificity of the kinetic description of the hydrogenation of aromatic hydrocarbons can be ex-

(1)

C6H6 + Z ) C6H6Z

(2)

H2 + Z ) Z(H2)

(3)

C6H6Z + Z(H2) ) C6H6‚Z(H2) + Z

(4)

C6H6‚Z(H2) + Z(H2) ) C6H6‚2Z(H2)

(5)

C6H6‚2Z(H2) + Z(H2) ) C6H6‚3Z(H2)

(6)

C6H6‚3Z(H2) ) C6H8‚2Z(H2) + Z(H2)

(7)

C6H8‚2Z(H2) ) C6H10‚Z(H2) + Z

(8)

C6H10‚Z(H2) ) C6H12 + Z

(13)

The possibility for such reactive complex formations to occur is accounted by analogy with arene complexes formed in homogeneous catalysis (39-41) as well as the alkyl complexes supposed for paraffin dehydrogenation (see above). Step 3 is supposed to be the rate-determining step in case the reaction occurs in the temperature interval 60100 °C, at large coverages of the catalyst surface by benzene. Step 4 is supposed to be the rate-determining step in the interval 130-180 °C at medium coverage of benzene (the adsorption of toluene at these temperatures corresponds to large coverages). Step 5 is assumed to be the rate-determining step in the temperature interval 210-270 °C. The rate-determining step is followed by fast isomerization of these complexes up to the final product (at 130-270 °C or by fast interaction of C6H6‚Z(H2) with H2 at 60-110 °C). The assumption that the nature of the rate-determining step is changed with temperature is essential. The whole complex of experimental data is in accord with this unusual scheme. Other possible interpretations run counter to some of the experimental data. Transition Processes in Dehydrocyclization Reactions Dehydrocyclization of isooctane on Pt catalysts in the presence of hydrogen was investigated by nonstationary methods at 260-310 °C in combination with kinetic and isotopic studies under steady-state conditions (42, 43). The reaction is accelerated by hydrogen. The rate is described by this kinetic equation


r ) (kP1PH20.5 - k′P2PH21.5)/(P1 + k2P2 + k3PH2 + k4P1/PH2) (14) (P1 and P2 are the partial pressures of the initial and final hydrocarbons, respectively.) A considerable kinetic isotopic effect was observed after hydrogen was replaced by deuterium; rH/rD ) 2. The relaxation curves in Figure 7 show that hydrogen takes part in a number of reactions steps: it interacts with surface intermediates and also participates in the cycle formation. These results give proof of the “concert” mechanism scheme. Mechanism of the Low-Temperature Water Gas Shift Reaction Kinetic studies of this reaction over copper-containing catalysts were carried out under steady- and unsteadystate regimes at 140-220 °C (44, 45). The results obtained under stationary conditions are in accord with the kinetic equation

r ) kPCOPH2O/(PH2O0.5 + k′PCO + k′′PCO2)2

(15)

The reaction mechanism can be described by the scheme

(1)

CO + Z ) COZ

(2)

H2O + Z + ZO ) 2HOZ

(3)

COZ + HOZ ) HCOOZ + Z

(4)

HCOOZ + HOZ ) COOZ + H2 + ZO

(5)

COOZ ) CO2 + Z

(16)

The correctness of both the reaction scheme and the kinetic description has been proved by measurements of the kinetic isotopic effects on substituting D2O for H2O. Studies in the nonstationary regimes were also carried out, and the relaxation curves are presented on Figure 8. In particular, the relaxation curves gives evidence that both the reactants participate in the process by their adsorbed forms, contrary to the “hightemperature mechanism” (46, 47). Another mechanism was recently proposed for this reaction in refs 48 and 49. However, these authors have not carried out kinetic studies. For this reason, we adhere to the mechanism stated above as being supported by kinetic investigations. Reactions of Total Oxidation The total oxidation of different organic compounds on Pt catalysts was investigated in a series of studies carried out in our laboratory in connection with problems of environmental protection (50-60). We shall discuss here some of the systems studied. The oxidation of aromatic hydrocarbons is of special interest in connection with the heterogeneous-homogeneous character of the process on Pt (53, 60) and some oxide catalysts (61) at low temperatures. In a serious of studies (49, 53-55), the mechanism of these reactions was proved by use of the transient response method. The relaxation curves in Figure 9a show that the reaction mechanism changes depending on the quantity of glass packing introduced into the system. In the case,

Figure 6. Relaxation curves of responses in the reactions of benzene and toluene hydrogenation. (a) (C7H8 + He)/(C6H6 + He) at 120 °C. (b) He/(C6H6 + C7H8 + H2) at 175 °C. Curves 1 and 1′, responses of C6H6; 2 and 2′, responses of C7H8.

where the glass packing is of a small quantity or absent, oxygen from the gas phase interacts with the benzene adsorbed on the catalyst. When the quantity of glass packing is high enough, the heterogeneous-homogeneous mechanism is suppressed, and oxygen interactions only from the adsorbed state. The relaxation curve in Figure 9b indicate that the extent of the heterogeneous-homogeneous reaction increases with the decrease of the concentration of benzene. This means that the radicals C6H6O desorbed from the catalyst into the gas phase lose their activity because of collisions with benzene molecules (60). Figure 10a shows that the oxygen taking part in the reaction is adsorbed on Pt very weakly and can be removed by short-time treatment with He. Therefore, only weakly bound oxygen is reactive in the reactions of total oxidation on Pt (50-60). Benzene is bound to Pt more tightly but weaker than the surface intermediates formed from benzene with oxygen in the course of the reaction. This can be seen from the relaxation curves in Figure 10b. Some other hydrocarbons, e.g., isooctane and cyclohexane, also exhibit weak bonds with Pt catalysts (50, 55, 58). The relaxation curves presented in Figure 11 refer to the reaction of total oxidation of isooctane. These hydrocarbons form two kinds of bonds with the catalyst: weak bonds with the oxygen preliminary adsorbed and tight bonds with the free sites. A short-time treatment of the system with He is sufficient to remove from the catalyst surface the fraction of the weakly bound hydrocarbon. The hydro-


Figure 7. Relaxation curves of responses in isooctane dehydrocyclization on Pt catalyst at 260 °C. (a) 1, H2/(C8H18 + H2); 2, (C8H18 + He)/(C8H18 + H2). (b) 1, (C8H18 + H2/H2); 2, (C8H18 + H2)/He; 3, (C8H18 + H2)/(C8H18 + H2 + He).

Figure 8. Relaxation curves of responses in the water gas shift reaction on low-temperature copper catalyst at 140 °C. (a) H2O + He)He/(CO + He), change of H2O vapors by CO. (b) (CO + He)/ He/(H2O + He), change of CO by H2O vapors.

carbon molecules which are tightly bound to the platinum sites undergo dissociative adsorption and take no part in the reaction. The adsorbed layer formed can be removed only by hydrogen treatment. The data on the total oxidation of acetic acid under nonstationary conditions (50, 58, 59) are presented on Figure 12. The acid reacts only from the adsorbed state at 160 and 260 °C. However, oxygen reacts at 160 °C both from the adsorbed state and from the gas phase. Oxygen at 260 °C interacts only from the surface layer. Similar changes in the mechanism of the total oxidation reactions on Pt catalysts obviously take place in some other systems (58). The heterogeneous-homogeneous character of benzene oxidation on Pt was confirmed in ref 61. As is seen from Figure 12, some surface intermediates formed in the total oxidation of acetic acid get spontaneously converted into the reaction products. Such a specific reaction mechanism we observed only for this system.

mechanism in particular. For this reason, we consider it desirable to develop common kinetic models including the kinetic description of both the main process and the accompanying deactivation reaction. By way of example, we shall quote some of our kinetic models describing the deactivation as an integral part of the kinetics of the main process. Dehydrogenation of Low Paraffins on PlatinumAlumina Catalysts. The catalyst deactivation is caused by (i) coke deposits and (ii) poisoning by impurities (22, 71, 72). Initially, the rate of coke formation is constant and follows eq 5. Upon attaining a threshold coke coverage, c*, the rate of coke formation abruptly drops and follows eq 6. To examine the influence of poisoning by impurities, small quantities of H2S were introduced into the system. The reaction rate decreased with the increase of H2S, in agreement with the equation

Studies of Catalyst Deactivation

The value of k′ in eq 17 turned out to be equal to the value of the coefficient k′ appearing in eqs 2-4, relevant to realizing the process in H2 flow. This is evidence that the principle of simple mutual influence (62) is observed. By contrast, when the reaction is lead in an atmosphere of H2 + H2O, the determined value of k′ appearing in eq 8 is quite different. The latter is indicative of the fact that some of the Pt sites on the catalyst surface undergo partial oxidation to form Pt4+ ions. Such a

Our approach sticks to the concept that the reaction mechanism plays a guiding role in the evolution of processes accompanied by catalyst deactivation. Examining the deactivating kinetics in conjunction with the reaction mechanism can be a tool for reducing the effect of deactivation. On the other side, research into the laws of catalyst deactivation can be conductive to elucidating the specificity of the basic process, its

rIV ) kIVP1γ/(PH20.5 + k′P2 + k′′PH2S)

(17)


Figure 9. Relaxation curves of responses in the reaction of benzene total oxidation on platinum catalysts at 250 °C with different quantities of glass packing: (a) 1 and 2, O2/(C6H6 + O2) at 0 and 75% packing; 3-5, (C6H6 + He)/(C6H6 + O2) at 0, 50, and 75% packing. (b) (C6H6 + O2)/O2; 1 and 1′, 23 Torr with and without packing, respectively; 2 and 2′, the same at 65 Torr.

suggestion was confirmed by both chemical analysis and kinetic data. Dehydrogenation of Isoamylenes on CalciumNickel-Phosphate Catalysts. A common kinetic model was developed in refs 3 and 65 correlating the rate of the reaction on the fresh catalyst with the evolution of the process under conditions of catalyst deactivation. The model describes all the routes of the process realized in agreement with the assumption that they are constituent parts of a common mechanism. Dehydrogenation of High Linear Paraffins C10C12 over Platinum-Alumina Catalysts. All the variety of reactions occurring, such as the formation of olefins, cracking processes, and coke generation, can be described by a common kinetic model (22, 66, 67). It has been supported by additional physicochemical data. The laws of catalyst deactivation are an integral part of the common kinetic description, in line with the reaction mechanism supposed. When analyzing the mechanism of coke formation, the key point is the origin of coke precursors. Froment and Bischoff (68) proposed to classify the deactivation reactions as parallel, when the coke deposits are formed from the initial species, or consecutive, in case the product species are responsible for the coke formation. Wolf and Petersen (69, 70) related the model to the cases when surface intermediates are the contributors of coke precursors, to classify the deactivation as parallel or

Figure 10. Relaxation curves of responses in the reaction of benzene total oxidation on platinum catalyst at 350 °C and different longtitudes of helium treatment: 1, 0s; 2, 5s; 1′, 60s; 2′, 180s; 3, 15s. Responses: (a) O2/He/(C6H6 + He); (b) (C6H6 + O2)/ He/O2.

consecutive, depending on whether the coke precursors originate from intermediates generated before or after the rate-determining step. Taking into account the possibility that precursors often arise as a result of the interaction of surface intermediates with gaseous reactants, we considered it reasonable to extend the latter classification, specifying four complementary mechanism types (71, 72). The latter may be described by the following schemes for the simplest reaction A ) B with simultaneous formation of coke precursors P:

nA + Z f (PZ)

“parallel-parallel”

nB + [AZ] f (PZ)

“parallel-consecutive”

nA + [BZ] f (PZ)

“consecutive-parallel”

nB + [BZ] f (PZ)

“consecutive-consecutive” (18)

Each of these steps may bring specific kinetic peculiarities, depending on the nature of the gas contributors, on the one hand, and the power of the bonds between the responsible intermediates and the catalyst surface, on the other. Lability of bonds reduces the probability that coke precursors come into existence. Tight bonds favor the formation of carbon deposits. In this connection, it is appropriate to note that the introduction of additives which weaken the bonds of the catalyst surface with the intermediates, thus increasing


Figure 11. Relaxation curves of responses in the reaction of isooctane total oxidation on platinum catalysts. (a) Pt-Al2O3, 120 °C: 1, O2/(C8H18 + He)/O2; 2, H2/(C8H18 + He)/O2. Interaction of O2 with C8H18 adsorbed on the catalyst preliminarally treated by O2 (curve 1) or by H2 (curve 2); 3, O2/(C8H18 + He)/He/O2; 4, H2/ (C8H18 + He)/He/O2, interaction of O2 after intermediate treatment by He with C8H18 adsorbed on the catalyst preliminarally treated by O2 (curve 3) or H2 (curve 4); 5, O2/(C8H18 + He); 6, (C8H18 + He)/H2/O2, interaction of C8H18 with O2 adsorbed on the catalyst (curve 5) and O2 with C8H18 after intermediate treatment of catalyst by H2 (curve 6). (b) Pt wire, 350 °C; 1′, O2/(C8H18 + O2); 2, (C8H18 + He)/(C8H18 + O2), interaction of the reaction mixture with preliminarally adsorbed O2 or C8H18.

their reactivity, results in decreased coke formation. This statement is supported by experimental findings. The introduction of the scheme (18) provides a good service in understanding the influence of diffusion on deactivation kinetics. The gradient of partial pressures along the pellet radius superimposes with the gradient of surface concentrations of the intermediates. According to the mechanism of generation of coke precursors, the interdependence of these gradients may be accentuating or opposing. For this reason, when modeling the deactivation kinetics of processes realized on large catalyst grains, it is of significance to taken into account both the diffusion factors and the inherent distribution of surface intermediates along the radius of the catalyst pellet. In view of this, we have developed a model (73) which includes a set of coupled nonlinear differential equations relating the mass-transfer and the deactivation kinetics with the particular position inside the catalyst grain. The numerical solution of the model gives a picture of the discrete distribution of the surface concentrations, blockage, and rate gradients inside the pore and their evolution in the course of the process. The results of the theoretical analysis indicate that, in case the deactivation is caused by consecutive coke

Figure 12. Relaxation curves of responses in the reaction of acetic acid total oxidation on platinum catalysts. (a) Pt-Al2O3 at 160 °C; (b) Pt wire at 260 °C. 1 (CH3COOH + He)/(CH3COOH + O2); 2 and 1′, O2/(CH3COOH + O2; 2′, (CH3COOH + O2)/(CH3COOH + He); 3, (CH3COOH + O2)/He/O2; 3′, (CH3COOH + O2)/He; 4, CH3COOH + O2)/He interaction of the adsorbed reaction mixture with O2, He, or CH3COOH.

formation, the decrease of the reaction rate with time should be smoother under diffusion control, due to the diffusion-induced retardation of deactivation. As a result, the rate of the process affected by intraparticle resistance may come to a point to exceed the rate which would be attained within the same period in the absence of diffusion restrictions. These theoretical conclusions (74) found experimental support for the two processes of isoprene production investigated (75). The problem becomes still more complicated when the effects on selectivity are to be considered in addition. In ref 76, we specified three types of selectivity, associated with the intimate peculiarites of the mechanism of complex reactions. Detailed analysis of selectivityconversion curves for the different selectivity types gave grounds for conclusions about the expected changes of selectivity in the course of deactivation, depending on the mechanism of coke formation. Particular process mechanisms call forth a much more abrupt decrease of the selectivity with the drop of conversion in the absence of diffusion limitations, so that the selectivity exerted under diffusion control is appreciably higher. In such a case, it may be of benefit to lead the process on catalyst grains, ensuring moderate intraparticle resistance. On the other side, some process mechanisms make for an increase of the selectivity with the fall of conversion. Then, in the course of deactivation, the target yield will be influenced by two opposing factors: the delay in the reaction rate and the increase of selectivity. The effects


are more noticeable under kinetic control, the rise in selectivity being expected to overcompensate the effect of rate retardation. Hence, it seems more advantageous to realize processes of such a type on smaller catalyst grains, preventing the diffusion effects. In such a way, the kinetic predictions may be of practical use with respect to elucidating the resources for reducing the harmful effect on the yield of the desired product. Optimal Bond Strengths of Reactive Surface Compounds It follows from some fundamental theories of catalysis (e.g., ref 77) that the medium values of the bond strengths between the reactive surface compounds and the catalyst surface should be optimal. A similar conclusion arises from the theory of processes on inhomogeneous catalyst surfaces (78) where medium surface coverages are supposed to be optimal. Nonetheless, the data presented in this review indicate that some surface compounds which are characterized by weak bonds with the catalyst exhibit high reactivity. In particular, this is the case with the hydrogenation of aromatic and olefin hydrocarbons on Pt and Ni. Also, the data obtained for oxygen adsorbed on Pt give evidence that only weakly bound oxygen participates in the total oxidation of aromatics. The activation energy for desorption for such adsorbed forms estimated is about 40 kJ/mol. The aromatic hydrocarbons are bound more tightly. The value of adsorption heat for benzene on platinum is about 140-150 kJ/mol. It is evident that weakly bound surface compounds are formed in heterogeneous-homogeneous reactions. By way of example, we mentioned above the data obtained for the total oxidation of benzene. For benzene, the total oxidation on platinum is supposed to be the slow step C6H6OZ + O2 + Z ) C6H6O + 2OZ, where the radicals C6H6O passed into the gas phase from the surface are formed (55, 56, 60). Weakly bound surface intermediates were detected in the hydrogenation and isomerization of C6-C9 olefins on sulfided Pd catalyst in an excess of aromatic hydrocarbons (79). Since the olefins have been displaced by aromatic hydrocarbons, these olefins were not indicated in the IR spectra of surface compounds. Nevertheless, the olefins undergo very quick transformations on the free sites. The heats of adsorption of such olefins are estimated to be about 16-18 kJ/mol. On the other side, the high reactivity may be also exerted by surface compounds tightly bound to the catalyst. Such a case was observed on studying the isomerization and hydrogenolysis of C5 paraffins on platinum-alumina catalyst (80). The data discussed point out the necessity to revise the assumption that middle-tight bond strength would always be optimal for the reactivity of surface intermediates. It is necessary to consider that the optimal values can vary within certain limits, depending on the reaction kinetics and mechanism, on the one hand, and on the operation conditions, on the other. It seems desirable to extend such a revision to the concepts developed by different theories of catalysis as they concern the problems of optimal catalyst action. Conclusions It was our aim to present data and results in support of the concept that the combination of stationary and nonstationary methods of investigation may turn out

to be a mighty tool for the elucidation of a reaction mechanism. New, important information was obtained concerning the peculiarities of the reaction mechanism in different systems. Some weakly bound surface intermediates can be reactive in the hydro-dehydrogenation and total oxidation processes on group VIII metals. However, this problem needs further thorough studies. Another conclusion drawn by use of this approach is the kinetic version of the mechanism of promoter action for some reactions. It is desirable to consider it side by side with the physicochemical basis of promoter effects in different processes. The changes in the reaction mechanism as well as the influence of the prehistory of the catalyst are particularly noticeable in the course of unsteady-state experiments, for oxidation processes in particular. The combination of steady- and unsteady-state experiments with other physicochemical studies is especially efficient. In conclusion, we stress once more the necessity for a common approach linking the problems of catalytic process mechanism, kinetic models, and catalyst deactivation. Thorough studies of Prof. Froment brought an appreciable contribution to such an approach. Acknowledgment We are obliged to the Russian Foundation of Fundamental Investigations (Grant 94-03-09526a) and to the Bulgarian National Foundation for Scientific Research for providing funding to these studies. Appendix The transient experiments were carried out in a special kinetic unit of small volume (about 10-15 mL) connected with a time-of-flight mass spectrometer and gas chromatograph. The quantity of the catalyst loaded varied in the different experiments from 0.6 to 2 cm3. The reactants (diluted by He when necessary) used were admitted by jump into the reactor with a flow rate about 8 L/h, ensuring a residence time of 2-5 s. The act of such a jump we note in the legends of the A/B or a/b were A/a and B/b stand for the compositions of the reaction mixture before and after the jump, respectively. When more than one jump had been realized in a given experiment, e.g., a/b/c, the relaxation curves correspond to the final jump, i.e., b/c (nonetheless, the influence of a has been considered in interpreting the form of the relaxation curves). The concentrations of the different reactants specific to the particular experiments can be found in the original works cited. The initial points of the relaxation curves coincide with the end of the residence time. The experimental points relevant to the mass-spectrometric measurements have been obtained from the corresponding peaks registrated by 1-s intervals. The samples of the reaction mixture used were selected in the course of the experiments for posterior chromatographic analysis. The classification proposed by Kobayashi et al. (6, 7) was applied to relate the forms of the relaxation curves with the reaction mechanism. Such an interpretation was supported by the results obtained by other physicochemical methods. To avoid the disguising influence of possible side processes on the form of the relaxation curves, special precautions were taken, providing operating conditions under which the value of the relax-


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Received for review November 1, 1996 Revised manuscript received April 1, 1997 Accepted April 12, 1997X IE960693L

Abstract published in Advance ACS Abstracts, June 15, 1997. X

Stationary and Nonstationary Kinetic Studies Related to the

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