ring hydrogenation

Kinetic simulation of different processes of benzenering hydrogenation: the common reaction mechanism Ostrovskii Victor E. Sector of Thermodynamics and Calorimetry Karpov Institute of Physical Chemistry Moscow, Russia [email protected];[email protected] Abstract—The notion on the reaction mechanisms, according to which heterogeneous catalytic processes proceed with ratedetermining step (RDS) and equilibrium relative to the remaining portion of reactions, is applied to deduction of kinetic equation for the reactions of liquid-phase hydrogenation of benzene ring. The deduced kinetic equation is capable of description of hydrogenation of C6H6 ring with different substituents at Ni, Pt, Pd, Ru, Ir, and Rh catalysts, when the H2order is either less or higher than unity. The work favors the prevalence of the catalytic mechanisms, at which surface-gas equilibrium establishes much faster than reactions proceed.

Meanwhile, in the early 20th century, some “Founding Fathers” of the catalytic science believed that any stationary heterogeneous catalytic reaction involves the gas-catalyst equilibrium and one reaction step that is RDS (e.g., [5–7]). In other words, they believed that the gas–surface equilibrium recommences much faster than the reaction products form. The kinetic equations for a number of processes were obtained on the basis of this simple notion [5, 7]. Later, the same approach was applied for description of dehydrogenation of different organic substances and surface transformations of chemisorbed hydrocarbons were taken as the RDSs [8, 9].

Keywords-component: kinetic simulation of heterogeneous catalytic processes; benzene ring hydrogenation catalytic mechanism

Meanwhile, it is possible to read in many works that such an approach is insufficient and that multi-step models are necessary for kinetic description of different reactions. For the last decades, a number of important scientific conclusions that influence the notions on the nature of catalysis were made; in our opinion, they should also help the correct theoretical approach to the procedure of deduction of kinetic equations.

I. INTRODUCTION There is an opinion that the heterogeneous catalytic reactions between two or more source substances proceed by steps, one of which can determine the rate (rate-determining step (RDS)) and the other ones can be equilibrium or not equilibrium. The kinetics of a number of reactions are described on this basis. Namely, researchers select empirically such a set of simple stoichiometric equations (according to their opinion, it translates “the reaction mechanism” or represents “the kinetic model”) that allows deduction of the gross-reaction kinetic equation, which gives a possibility for description of a set of experimental data for the rate dependence on the gas reacting mixture composition and on the temperature. (Needless to say, the experiments should be performed over the so-called kinetic field.) On frequent occasions, authors term such a procedure theoretical, contrary to the procedure according to which kinetic equations are obtained through formal algebraic processing of the data on the reaction rates without formulation of any “kinetic model”.

These conclusions are as follows. First, thermally stabilized surfaces of crystalline bodies are homogeneous relative to the chemisorption and catalytic activity of their surface atoms [2, 3, 10]. Second, chemisorption proceeds very quickly either at mono-crystals or at poly-crystals and the sticking coefficients are, usually, as high as 1.0–0.01 [11, 12]; the smaller sticking coefficients that, sometimes, are observable can be caused by the surface contaminations, effects of surface diffusion, neglect of multi-centricity of chemisorption of gas molecules, etc. Calculations show that, at any pressure, for the time of formation of n gas molecules in any catalytic reaction at a unit surface, (103÷105)·n molecules of source products can chemisorb at the surface of this chemical composition of the same area [3, 10].

However, it is well known that a number of different equations deduced from different “kinetic models” can be applied with almost the same success for description of kinetic data on any chosen catalytic reaction [1–3]. In [4], 13 different kinetic equations for C2H4 epoxidation are reviewed, and their quantity is past 20 at present time; several principally different mechanisms are proposed for their deductions. It is clear that new modified theoretical approaches to deduction of grounded kinetic equations for engineering calculations are topical. ___________________________________

Third, chemisorbed molecules at rather high temperatures are distributed over a surface not randomly but form twodimensional “islands”, each of which is capable of changing its form and size under gas–solid equilibriums. Nothing proves that catalyzing surfaces under stationary conditions are covered with the randomly distributed particles. This position is also fixed in [13]. The available electron micrographs of

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stationary with no diffusion control depends, under the condition of correct consideration of the inverse-reaction rate, only on the chemical nature of the RDS and chemical nature of the chemadphases. This last statement is confirmed in [10].

opaque adsorbed layers [14–19] demonstrate two-dimensional “island” chemadphases against the background of solid surfaces. These three conclusions forced us to return, on the modern scientific ground, to the “Founding Fathers” concept as the general mechanism of heterogeneous catalysis.

On the basis of this approach, we described kinetics of a number of catalytic processes, such as NH3 synthesis at Fe/K2O/Al2O3 [2, 3, 20, 21], SO2 oxidation at Pt [2, 3, 21], CH3OH synthesis at ZnO/Cr2O3 [2, 3, 21], shift-reaction at Fe3O4 [2, 3, 21], oxygen isotopic exchange between CO and CO2 [2, 3, 21], and CH3OH synthesis at ZnO/CuO/Al2O3 [10]. For all these processes, the kinetics were described on the basis of the notion that the reaction proceeds with the ratedetermining step and that the remaining portion of the reaction represents the gas–surface equilibrium. The RDS can be revealed from chemisorption–desorption studies of the rates of the constituent parts of the reactions under study, as in [10], or can be presumed with subsequent comparison of the measured rates of the gross reaction with the deduced kinetic equation.

There are grounds to assume that the surfaces are covered with a great number of two-dimensional crystal-like islands of different sizes and of definite chemical compositions rather than with isolated chemadparticles. According to our notion of the mechanism of catalytic processes, the islands of each of the chemadphases occurring at the surfaces under stationary conditions change continuously their forms and sizes, appear and disappear in such a way that the total surface area of each chemadphase remains unchangeable in time. Under a stationary catalytic process, each of the chemadphases is in equilibrium with the reacting medium. The occurrence of these island chemadphases is supported by a gain in the free energy that evolves as a result of the two-dimensional crystallization of individual chemadparticles. The overwhelming majority of the two-dimensional chemadphase crystals consists of a great number of chemadmolecules. Therefore, chemisorption or desorption of a small number of molecules, which somewhat decrease or increase any of such two-dimensional crystals and changes their molar energy almost not at all (analogously to sublimation or vaporization, which doesn’t influence the molar energy of large threedimensional crystals). Besides, the valence electrons of any island cannot be energetically isolated from the electrons of the valence band of the three-dimensional crystal. In other words, when any island increases or decreases in its twodimensional sizes as a result of chemisorption or desorption, it doesn’t change its specific reaction ability. Thus, there are no grounds for mutual effect of the chemadmolecules on their reaction ability.

Below, the approach considered above is applied to different reactions, in which benzene-ring hydrogenation to the cyclohexane ring proceeds. II. KINETIC SIMULATION OF BENZENE RING HYDROGENATION The long-term history of the studies of reactions of hydrogenation of benzene ring gives an example of unavailability of kinetic data for unambiguous conclusions on the mechanisms of catalytic processes. In [22], the authors concluded that the benzene ring hydrogenation mechanism proposed several years earlier by some of them is incorrect but, for all that, they certificated the earlier-proposed kinetic equation r = k PH2 N /[ N + k’ PH2 N + k”(1 – N)],

In our opinion, the essence and specificity of the catalytic ability of a crystalline solid in any reaction consists in the capability of this solid to form a key chemadphase that stationary exists at the surface and is in equilibrium with the reaction product. At a constant temperature, the higher is the surface coverage by this chemadphase, the higher is the partial pressure of the product. Realization of the acts of the RDS leads immediately to an increase in the total size of the key chemadphase over its equilibrium value; this induces desorption of the product from the key chemadphase and retrieval of this chemadphase to its equilibrium surface coverage. Chemisorption of the molecules of source substances and desorption of the molecules of the product (or products) are separated in the two-dimensional surface space of the catalyst; the equilibrium realizes through fast interaction between the chemadphases and the gaseous medium.

(1)

where r is the rate of the gross reaction, N = MA / (MA + MB), and MA and MB are the molar fractions of A (source substance) and B (product) in liquid reaction mixtures; k and k’ are the combinations of some rate constants, and k’’ is an equilibrium constant. The authors of [22] applied this equation to description of experimental data on liquid-phase hydrogenation of C6H6 at Pt, Pd, Ru, Ir, and Rh, benzoic acid at Pd, phenol at Ni and Pd, and hydrogenation of cyclo-hexene at Pt and Pd, i.e., for description of more than ten different catalyst/reaction systems, including hydrogenation of not only benzene but also some other substances. According to [23], (1) is also applicable for description of some other reactions of C6H6 ring hydrogenation. The authors of [22] relate this equation to the following mechanism (mechanism III in their numbering).

It is important that the chemadphases characterized by disparate values of the decrease in the free energy at their formation from gas components cannot coexist at a surface. It is also important that the mechanism of the maintenance of gas-surface equilibriums in the course of stationary catalytic processes cannot influence the form of kinetic equations. The form of a kinetic equation for a catalytic reaction that proceeds

ZA + H2 → ZAH2 , ZAH2 + H2 ↔ ZA(H2)2, ZA(H2)2 → ZY, ZY + H2 → ZB, ZB + A ≡ ZA + B.

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(2) (3) (4) (4') (5)

hybridization), respectively, and the ZAH2 and ZB (nonaromatic cyclohexene and cyclohexane particles) are bound rather weakly with the surface atoms as a result of the absence of the π-electron sextet. We are not the first to consider surface transformations of hydrocarbons as the RDS in the processes of catalytic hydrogenation; as was noted above, a similar approach for other hydrocarbons was applied in works [8] and [9]. According to the mechanism IV, the equilibrium establishes very quickly as compared to the RDS and, therefore, it exists continuously under the conditions of stationary reactions. In accordance with the principle of detailed balancing (principle of microscopic reversibility) [24], under the conditions of chemical equilibrium over any entire system, all possible chemical processes in this system are self-balanced. Therefore, reactions (10)–(12) are equilibriums. Three equations for the equilibrium constants for each of these reactions are as follows.

The following notation is applied: Z is the surface center; A and B are the source and product molecules, respectively; ZA, ZB, ZAH2, and ZA(H2)2 are the chemisorbed particles; ZY is the chemisorbed isomer of the ZA(H2)2 chemadparticle; → means that the reaction proceeds from left to right only; ↔ means that the rates from left to right and from right to left are comparable in their values; and ≡ means the equilibrium. We corrected the misprints that occur in [22]: in [22], it is mistakenly written ZA(H2) instead of ZA(H2)2 and, in (2), it is written (↔), though the authors take in their subsequent consideration that k–2 = 0. Mechanism III was taken in [22] instead of a previously proposed and then rejected mechanism II, from which (1) was originally deduced. It was shown that the same equation (1) can be deduced on the basis of mechanism III supplemented with several additional limitations (taken in the process of deduction), among which the most principal are that θZA(H2)2 and θY are negligible [22]. In [23], the applicability of (1) is confirmed.

K10 = θZA·H2 / P2 H2 θZA; K11 = θZB / PH2 θZAH2; K12= PB θZA/PA θZB.

We do not present here mechanism II, which was first used to deduce (1), because it was rejected by the authors of [22] as the thermodynamically inadmissible one. In [22], it is taken that the system is ideal. Therefore, the substrate and product concentrations can be replaced in the kinetic equation by their partial pressures. We follow this assumption. Because of ideality of the system, the relation N = MA / (MA + MB), which is used in (1) for liquid phases, is also right for gaseous phases. To simplify subsequent transformations, we use the following notation, which is accurate within a constant. N = PA / (PA + PB) (1 – N) = 1 – [PA / (PA + PB)],

These equations together with the balance equation for the degrees of the surface coverages by all chemadphases, θZA + θZB + θZA·H2 + θZAH2 = 1,

(6) (7)

r = k+9 K10PH2 /[1 +K10 PH2 +PB (1 +1/K11 PH22) /K12 PA)]

(17)

(k+9 is the rate constant of the direct reaction (9), K10, K11, and K12 are the equilibrium constants for the corresponding reactions). At K11PH22 >> 1, (17) is identical to (8) deduced from mechanism III. If K11PH22 1, are known [25].

(8)

Assume now that the gross reaction proceeds under stationary conditions in conformity with the mechanism IV: ZA·H2 ↔ ZAH2 H2 + ZA ≡ ZA·H2 ZAH2 + 2H2 ≡ ZB ZB + A ≡ ZA + B.

(16)

allow presentation of θZA·H2 as a function of the partial pressures of the reaction components and formulation of the following kinetic equation for the reaction of benzene ring hydrogenation:

where PA and PB are the partial pressures of A and B, respectively, in the gas phase. Let us substitute (6) and (7) into (1). After elementary transformations, we have r = k PH2 / [1 + k' PH2 + k'' (PB / PA )].

(13) (14) (15)

III. DISCUSSION

(9) (10) (11) (12)

The mechanism IV is formulated in compliance with our notion on heterogeneous catalysis [2, 3, 10] as on the phenomenon at which the catalyst surface is, under stationary conditions, in equilibrium with the gaseous (or liquid) reacting medium and contains a two-dimensional phase capable of decomposition with desorption of desirable gaseous (or liquid) product; therewith, the RDS rate and the surface coverage by the chemadphase, over which a pressure of the product exists, determine the observable rate of the catalytic reaction. It is sufficient to study or to guess right the chemical natures of the chemadphases and RDS to deduce the appropriate kinetic equation. On this theoretical basis, we earlier described a number of the above-listed reactions with RDSs of adsorption nature; in this paper, we successfully described a number of reactions of the same type with the RDS of surface nature.

The upper catalyst layer is in equilibrium with gas H2 and contains metal and polarized H-atoms. The reacting monolayer located over it consists of the ZAH2, ZA·H2, ZA, and ZB chemadphases, which are in equilibrium with the gas phase. The reaction rate is determined by the RDS (9) and by the ratio between the areas of the chemadphases. Let us put emphasis on the fact that this is factually one-step mechanism and that, according to it, the surface is covered with four chemadphases. Apparently, the aromatic ZA and ZA·H2 particles are bound with metal atoms and with metal atoms and two polarized H-atoms (e.g., as a result of the s-p

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We mentioned above that the kinetics of the catalytic syntheses of CH3OH at ZnO/Cr2O3, NH3, SO3, shift-reaction, and oxygen isotopic exchange between CO and CO2 were described by us on the basis of the notions on the nature of catalysis, which are presented in this paper. For all these reactions, kinetic equations were earlier deduced by other authors from quite other mechanisms, and it was shown that these equations describe well experimental data. We obtained the same kinetic equations on the basis of our approach. The same approach was used by us for deduction of kinetic equation for the process of CH3OH synthesis at ZnO/CuO/Al2O3 catalyst. In this case, we studied the individual steps of the process by chemisorption–desorption and calorimetric methods and formulated the mechanism and kinetic model on this basis. It was shown that the resulted kinetic equation describes well the data obtained by different authors under atmospheric conditions and under industrial conditions [10].

[3]

The kinetics for a number of catalytic reactions were described by different authors on the basis of the notions on the occurrence of RDS with the equilibrium for the remaining portion of the reaction. These works are compatible with our notions on the nature of catalysis. Some of them are cited here.

[11]

Along with such works, there are many kinetic studies in which multi-step mechanisms of catalytic processes are postulated. It may seem that these works contradict our concept. However, it is quite possible that this is not the case. In our days, it is well known that any set of kinetic data can be described on the basis of different reaction mechanisms. A number of examples of such a kind are presented in [1], in this paper, and in other publications. I believe that there are grounds for returning to the kinetic descriptions of the reactions described earlier on the basis of multi-step kinetic models. It is useful to study the possibility of description of these kinetic data on the basis of the notions similar to those presented in this work. It may result that the notions on the nature of heterogeneous catalysis that were prevalent in the early 20th century reflect correctly the essence of this phenomenon. Apparently, the data presented in this paper, including the results of the studies of catalysts and catalytic processes by the surface-science methods, count in favor of such a possibility.

[14]

[4] [5] [6] [7] [8]

[9] [10]

[12] [13]

[15] [16] [17]

[18]

[19]

[20] [21] [22]

[23]

IV. CONCLUSION The reactions of catalytic hydrogenation of aromatic compounds are of relevance in general chemistry, for example, in caprolactam production, in improvement of diesel fuel quality, and in syntheses of different important chemical reagents for special purposes [26, 27, 28]. Therefore, the mechanism and kinetics of benzene ring hydrogenation is not only of academic but also of industrial importance.

[24] [25] [26] [27]

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[28]

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