Znd. Eng. Chem. Res. 1994,33,1881-1888
1881
KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of Crotonaldehyde Hydrogenation over a Titania-Supported Platinum Catalyst Romuald M. Makouangou: Dimitri Y. Murzin,t*tAnne E. Dauscher,0 and Raymonde A. Touroude*~+ Laboratoire d'Etudes de la Rkactivitb Catalytique, des Surfaces et Interfaces, U.A.1498 CNRS- ULP-EHICS, 4 rue Blake Pascal, F-67070 Strasbourg Cedex, France, and Laboratoire de M h l l u r g i e Physique et Science des Matkriaux, Ecole des Mines, Parc de Saurupt, F-54042 Nancy Cedex, France
Crotonaldehyde hydrogenation is a complex reaction that has been modeled using results from a standard gas flow system. The obtained results give rise to the formation of a reaction network and an elementary step mechanism derived from steady-state kinetics. The selectivity of the system to give crotyl alcohol is explained in terms of the surface electronic gas model.
Introduction Selective hydrogenation of a,O-unsaturated aldehydes into allylic alcohols remains a difficult task to achieve, especially in the gas phase, when working with heterogeneous catalysts. It is becoming, however, more and more studied. Monometallic catalysts supported on A1203 or Si02 lead mostly, as in the liquid phase, to the formation of the saturated aldehyde (Padley et al., 1992; Touroude, 1980; Vannice and Sen, 1989; .Waghray et al., 1992). Touroude (1980) found that hydrogenation of the C=O bond in acrolein could be achieved only on monometallic Pt and OS/A1203 catalysts. The use of additives (Marinelli et al., 1992; Raab et al., 1993; Waghray et al., 1992), bimetallic catalysts (Beccat et al., 1990; Hubaut et al., 1989; Lawrence and Schreifels,1989; Noller and Lin, 1984), or easily reducible supports (Marinelli et al., 1992; Raab and Lercher, 1993; Vannice and Sen, 1989; Yoshitake and Iwasawa, 1990) has been proposed to improve the selectivity toward unsaturated alcohols. Differing results can be obtained depending on the nature of the metal itself. Some metals polarize the C=O bond thus favoring its reactivity whereas others act so as to decreasethe reactivity of the C=C bond. Recently, an interesting study on the vapor-phase hydrogenation of crotonaldehyde over several supported Pt catalysts and on Pt powder was reported (Vannice and Sen, 1989) using different supports, precursors, and pretreatments. They observed that Pt/TiOZ catalysts, even reduced at low temperature (473 K), lead to the formation not only of the saturated aldehyde but also to the unsaturated alcohol, contraryto Pt powder and Al2O3or SiO2-supported catalysts. A higher reduction temperature (773 K) was favorable to the formation of crotyl alcohol only for one Pt loading (1.9% 1. It was nevertheless concluded that the hydrogenation of the C=O bond takes place at the Pt-titaniainterfacial sites created during high
reduction temperature. This concept was also used for the explanation of experimental data for crotonaldehyde hydrogenation over a titania-supported Pt-Ni catalyst (Jentys et al., 1992), where metal-support interactions were thought to be involved with TiO, species decorating the platinum particles. Yoshitake and Iwasawa (1990) noted changes in the reaction pathway for Pt/Nb205 catalystsreduced at 623 K, compared to lower temperature reduction. In this work, we present results obtained for the hydrogenation of crotonaldehyde over 2 and 4.7% Pt/ Ti02 catalysts reduced at low temperature (473 K) for which we have already found a good selectivity (Makouangou et al., 1992,1993). Attention was focused on the influence of the ratio of partial pressures of H2 to crotonaldehyde. The steady-state kinetics of this reaction have been studied for iron-doped platinum on silica catalysts (Simonik and Beranek, 1972). Langmuir-Hinshelwood type equations were used to describe the experimental data. In the present study, our interest was focused on the kinetic modeling of the hydrogenation of crotonaldehyde over a Pt/TiOz catalyst operating under steady-state and transient conditions. A kinetic study of the relaxation process on the rate of a heterogeneous catalytic reaction provides information additional to that obtained under steady-state conditions (Kobayashi and Kobayashi, 1974; Bennet, 1976). Hence, transient response methods are very useful for describing reaction kinetics in the case of complex reactions, where multireaction pathways lead to large uncertainties in the estimation of kinetic constants. It is of great interest, therefore, to investigate the transient kinetics of acomplex catalytic reaction. These methods can be also used to increase both the reaction yield and the selectivity to the desired product (Kobayashi et al., 1992).
+To whom correspondence should be addressed. E-mail: MAIRE@ CH1MIE.U-STRASBG.FR. U.A.1498 CNRS-ULP-EHICS. Permanent address: Department of Chemical Kinetics and Catalysis, Karpov Institute of Physical Chemistry, Obukha 10, Moscow, Russia. 8 Ecole des Mines.
Experimental Section
'*
08SS-5885/94/2633-1SS1~04.50l0
1. Catalyst Preparation and Reaction Procedure. The 2 and 4.7 w t % Pt/TiO2 catalysts used in this study were prepared by adding dropwise, followed by thorough mixing, an aqueous solution of hexachloroplatinicacid to a T i 0 2 support (2 cm3 of H~PtCls.6H20solution/g of 1994 American Chemical Society
1882 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994
support) from Tioxide (specificarea 46 m2g-l, pore volume 0.5 cm3 g-1, particle size 100-200 pm). Any small excesses of water were evaporated slowly on a hot plate. The catalysts were then dried overnight a t 393 K and stored under air until use. Each sample was dried again a t 393 K in air prior to use. “In situ” reduction of the catalyst (50 mg) was carried out at atmospheric pressure in the plug flow reactor of the catalytic apparatus previously described (Garin et al., 1981) under a flow of 50 cm3min-l H2. The catalyst was heated for 16 h at 473 K reached at the rate of 3 K min-l and was then slowly cooled to the reaction temperature (333 K)under hydrogen. For each run, 25 or 50 p L of crotonaldehyde (CROALD) (Fluka, puriss) was passed over the catalyst at constant = 95,60, partial pressure of 8,13, or 25 Torr (PHJPCROALD and 30, respectively). The hydrogen flow was varied from one run to another between 20 and 80 cm3 min-’. Therefore, the time period of each run ranged from 10 to 40 min, depending on the partial pressure and on the flow rate used. During each run, after ensuring that the pressures of the reactant and the reactant products were the same, samples were taken in the gas phase every 2 min and were analyzed by gas chromatography using a 1540 Carbowax packed stainless steel column of 4.5-m length and 118-in. diameter. Data processing was carried out using a DelsiNiermag device. Changes in activity and selectivity were monitored as a function of time during each run. The absence of any diffusional limitations was checked by the application of the Weisz criterion. No noticeable activity was detectedeither in the catalytic apparatus alone or on the Ti02 support itself. 2. Catalyst Characterization by X-ray Photoelectron Spectroscopy (XPS). The 4.7 Pt/TiOz catalyst was characterized by XPS using the Kar radiation of aluminum as the photon source. The apparatus used was a Vacuum Generators ESCA 111 spectrometer fitted with a hemispherical analyzer using a channeltron as the electron multiplier. Before XPS analyses were conducted, the samples were treated in the pretreatment chamber, using different procedures. This chamber was then evacuated to 1 P T o r r and the sample was transferred to the analyzing chamber which achieved a vacuum better than Torr. Before spectra were recorded, different pretreatments were used. Samples were reduced in the XPS preparation chamber at 473, 673, and 773 K either with or without previous reduction in catalytic apparatus. XPS spectra were also obtained for some catalysts after catalytic experiments. Spectra were recorded for the 0 Is, C Is, C12p, Pt 4f, Ti 2p, and Pt, Ti Auger regions. Several scans were accumulated to increase the signal to noise ratio if the elements were present at low concentrations. The binding energies were determined with reference to the Ti4+ 2 ~ 3 1 2transitions at 458.7 eV (Fung, 1982), correcting the XPS positions in all samples in order to avoid the influence of charging effects. Data analysis was performed by a standard personal computer linked to the spectrometer and run using a “homemade” program. Estimations of the surface concentrations and atomic ratios were obtained by comparing peak areas after background subtraction (Shirley, 1972) and correcting for differences in escape depths (using a root square approximation) and cross sections (Scofield, 1976). The XPS spectrum of the as-prepared catalyst without any reduction showed that Pt was present in an oxidized
+
state. However, after reduction in the XPS apparatus at 473 K for 1 h, Pt was completely reduced. The surface chloride concentration was seen to decrease upon reduction in hydrogen, depending on the duration of reduction. For example, the difference in Cl/Ti atomic ratio is 1 order of magnitude higher for 1-h reduction in the XPS pretreatment chamber compared to the 12-h reduction in the catalytic plug flow reactor. Similiar observations have been reported for chloride-containing Pt-alumina catalysts (Sethuraman and Davis, 1993). Chloride was removed from the surface of the catalyst in catalytically tested samples, the surface chloride concentration being below the detection limit of the XPS apparatus used. In all of the XPS spectra, no shift in the binding energy of Pt was observed for the different pretreatments (ashift toward lower binding energies could be due to interactions between the metal and the support). Moreover, only the main oxide line (Ti4+ 2 ~ 3 1 2at 458.7 eV) was detected without any transitions at lower values of binding energy. This indicates the absence of Ti3+ species even after hightemperature reduction. The presence of such Ti3+species could also have been an indication of a strong metalsupport interaction. Nevertheless, as we used a nonmonochromatized X-ray source, we cannot exclude the presence of very small amounts of Ti3+.
Results and Discussion 1. Kinetic Experiments. Experimental data which were used to model the system studied in this paper have been in part reported elsewhere (Makouangou et al., 1993). In Figure 1are presented other data obtained on a 4.7 96 Pt/TiO2 catalyst, corresponding to the variations of both overall activity and selectivities for crotyl alcohol and butyraldehyde during one run of crotonaldehyde, for a PH2/PcRofiD of 95 and various hydrogen gas flow rates. These changes occur on a stabilized catalyst presenting reproducible catalytic behaviors from one run to another. This means that the first run performed just after the reduction pretreatment, giving different results, is not considered here. The most important conclusions are resumed here: during time on stream, the overall conversion decreases to a quasi-steady-state value. The selectivity for crotyl alcohol increases while selectivities for other products decrease. The steady-state values of activity, expressed in micromoles of crotonaldehyde having reacted per second and per gram of Pt are independent of pressure and flow rate. Thus, the reaction order is zero with respect to crotonaldehyde pressure. The duration after which steady state is observed is found to be inversely proportional to the gas flow rate, but independent of crotonaldehyde partial pressure. For similar overall conversions,the selectivities for crotyl alcohol are different in the transient state (beginning of the run) and the steady state (Figure 2). The kinetics of crotyl alcohol formation are therefore different in the transient and steady states. 2. Kinetic modeling. 2.a. Reaction Network. The kinetic modeling was based upon a reaction network (Scheme 1) proposed firstly by Simonik and Beranek (1972) and supported by our observations (Makouangou et al., 1992, 1993). In modeling adsorption/desorption, equilibrium was assumed for all organic molecules. It was further assumed that the adsorption of hydrogen occurs on hydrogen-specific active sites and therefore is not influenced by the adsorption of organic species. One hydrogen site can adsorb two atoms of hydrogen. Hence
Ind. Eng. Chem. Res., Vol. 33, No. 8,1994 1883
-pa 2$
70
140
60
120
50
100
40
80
30
60
20
40
10
20
* 8
Schenie 2
Kzit
e3
g
0
0
time on stream (min) , 140
70 ,
(C) K3!
C
Table 1. Elementary Steps with Parallel Overall Reactions basic routasa 5
0
10
1s
time on stream (min)
20
10 20 30 40 1 2 3 4 5
A+Z=ZA B+Z=ZB
c+z=zc
P+Z=ZP ZA+Hz-ZB ZB+Hz-ZP ZA+Hz-ZC ZC+Hz-ZP ZB-ZC
1 -1 0 0 1 0 0 0
1 0 0 -1 1 1 0 0 0
1 0
-1 0 0 0
1 0 0
0 ON(1):A + Hz = B. N(2), N(4), N(5):A + 2Hz Hz C. 0
2
6
4
30 H transient state
10
20
30
40
60
50
conversion (%) Figure 2. Variations of selectivity for crotyl alcohol as a function of overall conversion in the transient and stable states on a 4.7% Pt/TiOz catalyst reduced at 473 K (reactiontemperature = 333 K).
Scheme 1 CH3-CH: CH-CH2- O H
+Y \ 1
crotylalcohol B
CH3-CHZCH-CH.0
-
crotonaldehyde A
+
HZ
~
\* CHS-CHZ-CHZ-CHZ-OH
CH3-CH2-CHz-CH;O
0
1 0 0 -1 1 0 0 1 1
P. N(3): A +
10
8
time on stream (min) Figure 1. Variationsof overall activity ( X ) and selectivity for crotyl alcohol (0)and butyraldehyde ( 0 )during one run on 4.7 % Pt/TiOz = 8 Torr, reduced at 473 K. Reaction temperature = 333 K, PCROALLI PH,= 762 Torr. (a) Hydrogen flow = 18 cmamin-l; (b) hydrogenflow = 31 cms min-'; (c) hydrogen flow = 67 cms min-l.
0
1 0 0 -1 0 0 1 1
/A
-
butanol P
-
butyraldehyde C
the dependence of hydrogen pressure is the same whether it is dissociated or not. It would also be the same if an Eley-Rideal mechanism were considered. The reaction order with respect to hydrogen was found to be equal to unity (Ftaab and Lercher, 1992),and in the derivation of
the kinetic equations we have used this value. The kinetic scheme used for our modeling is given in Scheme 2. 2.b. Rate Law Computation. The design of the kinetic model is based upon the theory of complex reactions (Horiuti, 1957;Temkin, 1979). This theory permits the formation of an algorithm for estimating the rates of complex multistep reactions (Ostrovskiiet al., 1987).Such an approach has been used extensively (Kiperman, 1979; Ostrovskii et al., 1987;Snagovskii and Ostrovskii, 1976; Temkin, 19791,and we wiJl recall here some basic principles of this theory. According to this theory, elementary reactions are grouped into steps (stages). Chemical equations of steps contain reaction participants and intermediates (surface species). Overall reaction equations can be obtained by the summation of chemical equations of steps multiplied by stoichiometric numbers (positive, negative, or zero). These numbers must be chosen in such a way that the overall equations contain no surface species. A set of stoichiometric numbers of steps defines a reaction route (Horiuti, 1957). Basic routes must be essentially different, and it is impossible to obtain one route through multiplication of another by a number, although their respective overall equations can be identical. If, in a set of reaction routes, none can be represented as a linear combination of others, then the routes of this set are linearly independent. Although routes can be chosen in different ways, the number of basic routes for a given mechanism is unique (Horiuti, 1957;Kiperman, 1979;Ostrovskii et al., 1987; Snagovskii and Ostrovskii, 1976; Temkin, 1979). The proposed basic reaction routes for a network of parallel reactions of crotonaldehyde hydrogenation are given in Table 1. Althoughsteps 1-5 are not elementaryreactions, they can however consist of several elementary reactions with only one of them being rate determining. In Table 1, Z is a surface site and stages lo-4O are
1884 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994 Table 2. Elementary Steps with Consecutive Overall Reactions basic routesa elementarysteps
A+Z=ZA B+Z=ZB
10 20
c+z=zc
30
P+Z=ZP ZA+Bz-ZB ZB+Hz-ZP ZA+H*-ZC ZC+Hi-ZP ZB - ZC
40
1 2
3 4
5
N(1) 1 -1 0 0 1 0 0 0 0
N(2) 0 1 0 -1 0 1 0 0 0
N(1):A + Hz = B. N(2):B + Hz C + Hz = P. N(5):B = C. a
N(3) 1 0 -1 0 0 0
N(4) 0 0 1 -1 0
1
0 0
0 0
1 0
N(5) 0 1 -1 0 0 0 0 0
1
P. N(3):A + Hz = C. N(4):
Scheme 3
forward one when writing the mechanism. In the mechanism, all the stages are considered to be linear meaning that the rate of the stages depends linearly on the concentrations of the intermediates, including free sites of the surface (Temkin, 1979). A graph of such a reaction contains only primary edges. A set of edges which continue one another is called a chain, a chain whose beginning and end coincide is called a cycle. The cyclomatic number of the graph (i.e., the largest number of independent cycles) is equivalent to the number of basic routes (Temkin, 1965). It can be seen from Scheme 3 that the representative graph contains five independent cycles and thus five basic routes. 2.c. Rate Equations under Steady-State Conditions. In the region of high surface coverages, assuming zero order with respect to crotonaldehyde, and first order with respect to hydrogen, the rate of crotonaldehyde hydrogenation, r~ can be expressed:
where PA and P represent the partial pressures of crotonaldehyde and hydrogen, respectively, Wis the weight of catalyst used, and F is the flow rate. From Scheme 2, we can express the partial pressures of products as a function of PAunder steady state. Integrating under initial conditions, t = 0, PA= PA! and PB = Pc = 0, one obtains equilibria. On the right hand side of the equations, stoichiometric numbers for the different routes are reported. We can verify that the number of basic routes is written correctly, since it was shown that the number of basic routes, P, is determined by the following equation (Horiuti, 1957; Temkin, 1979):
where S is the number of stages, W is the number of balanced equations, and J i s the number of intermediates. Balanced equations determine relationships between adsorbed intermediates; for example, such equations can correspond to a total coverage equal to unity. For the mechanism presented in Table 1,there are nine equations for steps (five for irreversible steps and four equilibria), five intermediates including unoccupied sites (ZA, ZB, ZC, ZP, and Z), one balanced equation (ZA + ZB + ZC + ZP Z = l),and therefore five basic routes. The same number of basic routes are obtained if steps 1-5 consist of several elementary reactions. The reaction mechanism can be presented in another form with another set of basic routes, e.g., as in Table 2, considering consecutivereactions. In this case basic routes will differ from that given in Table 2, but the number of basic routes is exactly the same. However,the kinetic equations are the same in both cases, parallel or consecutive reactions, in distinction from formal kinetics. This fact has been demonstrated previously (Murzin et al., 1990) and is valid only if the Bodenstein (steady-state) conditions are established. The concepts in discussion are more evident if the reaction mechanism is shown graphically (Temkin, 1965; Kamenski et al., 1992),as reported in Scheme 3 and which corresponds to the reaction mechanism of Table 2. The vertices of the graph are presented as circles and correspond to intermediates. The edges of a primary graph can be shown by ,complete lines and represent stages (steps). The edges are supplied with ordinal numbers of the steps, and the arrow at the number indicates the direction of the stage which is arbitrarily taken as the
+
(3)
and
L [fs _ f L + f d ] (4) (P2y-1
f2
-f5
1- f 5
A derivation of eqs 2-4 is explained in detail in Appendix 1 as well as the expressions for f 1 - f ~ . 2.d. ParameterEstimation. The estimation of partial pressures was performed by minimization of the sum of residual squares. The minimization was carried out numerically using the kinetic nonlinear regression program, described elsewhere (Ostrovskii et al., 1987). The mathematical structure of the kinetic model is relatively complex; therefore a strong correlation between the constants was expected. Thus, a search of the parameters fo-f5 (taking into account that f 1 = 1- f 3 ) was performed instead of searching for values of rate constants, in order to avoid correlation between constants. The results of the regression valuesfor eqs 2-4 are presented for different flow rates and Hz/crotonaldehyde ratios in Table 3, and the values of estimated parameters are given in Table 4. For calculations, several points at each initial partial pressure of A and flow rate were used. Comparison between the mean experimental values and calculated ones is given in Table 3. It can be seen that the kinetic description is acceptable. The mean relative deviation of the calculated values from those obtained experimentally is 18%,thus allowing us to use the reaction mechanism presented in Table 1 to further discuss the mechanistic aspects for crotyl alcohol selectivity. 3. Selectivity for CrotonaldehydeHydrogenation. 3.a. Selectivity Dependences in Flow System. One of the most interesting kinetic observations during the
Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994 1885 Table 3. Crotonaldehyde Hydrogenation at 333 K over 4.7% Pt/TiOz (Reduced at 473 K)at Different Initial Pressures PAO (in Torr) and Flow Rates 8 (in cma min-1): Comparison between Experimental (exp) and Calculated (cal) Data (Partial Pressures in Torr) 8 8 8 13 13 13 25 25 25
4.78 6.49 7.11 11.2 11.7 12.3 21.3 23.4 23.7
18 31 67 20 34 78 20 30 48
4.93 6.18 7.17 10.2 11.4 12.3 22.2 23.2 23.8
0.48 0.36 0.19 0.33 0.26 0.16 0.52 0.29 0.17
0.39 0.27 0.14 0.41 0.26 0.12 0.52 0.31 0.20
1.62 0.66 0.49 1.12 0.81 0.60 2.25 1.10 0.99
Table 4. Values of the Kinetic Parameters fo fl f2 fs f4 0.18
1.10
1.40
0.82
1.12 0.81 0.48 1.31 0.88 0.42 1.48 1.01 0.67
fs
0.29
2.41
course of the present study concerns the obtained selectivities for crotyl alcohol and butyraldehyde, which are different in the transient period and in the steady state, at a same overall conversion (Figure 2). Generally, a flow system is used to investigate relaxation processes although it requires a more complicated analysis than in a gradientless system, especially if a plug flow regime is not achieved and if isothermic conditions throughout the catalyst bed are not maintained. Recently, a theoretical analysiswas developed for the relaxation rate in ammonia synthesis in a flow reactor (Kuchaevet al., 1991). Following these authors, we propose, for crotonaldehyde hydrogenation, that in a perfect displacement regime (plug flow), the amount of consumed crotonaldehyde in an unit of catalyst bed is equal to the sum of crotonaldehyde both converted into products and adsorbed on the catalyst surface. The same approach is valid for all reaction participants. A set of equations in partial derivatives (eqs A21-A23) is given in Appendix 2. For discussion of the selectivityin the transient period, we will use some simple assumptions: (i) the surface coverage of organic species B and C is linearly dependent on their corresponding partial pressures; (ii) the partial pressure of each component varies linearly along the catalytic bed even at relatively high conversions. This last assumption is supported by the experimental results presented earlier using different flow rates. Therefore the following equations can be deduced (5) and QPcIRTm = rc -
-
K3 @c kif'- [(k$ k$ + [k&$d'KIPAl
+ ~&K$B/K~PA] - [P&/RTm] - [k.$$&P~l-
ZA
[PcQ/RTml (7)
+ H2
-w
products
(8)
according to the model is given by
where f A and f are activity coefficientsof A in the adsorbed state and in the transition state, respectively. eA is the surface coverage of A. The rate of reaction can thus be expressed (Snagovskii et al., 1978):
r
K3G @c dt
Here G represents the number of organic molecules at the surface, Q the volume of gas mixture passing through the reactor per unit of time, and T the thermodynamic temperature. From eqs 5 and 6 we have K2@B
It follows from eq 7 that differential dpBldPc can have a negative value, in accordance with experimental data. c be obtained by dividing In the steady state, d P ~ l d P can eq A14 by eq A17. It can be seen by comparing this ratio with eq 7 that selectivities in the transient period and in the steady state at the same conversions are different. 3.b. Application of the Surface Electronic Gas Model. Previously, the high selectivity for crotyl alcohol formation over PtITiO2 catalysts has been attributed to the existence of active sites capable of hydrogenating the C = O bond (Raab and Lercher, 1993). Vannice (1992) suggested that such sites could be oxygen vacancies present within the support material, leading to accessible TiX+ cations at the Pt-Ti02 interface. However, in the present work, we were unable to observe any spectral evidence of such species by XPS,even when the catalyst was reduced at 773 K. Another possible explanation will be proposed here to describe the influence of the support on the ease of hydrogenation of the carbonyl group. In agreement with Raab and Lercher (1993), we assume that surface polarity plays a key role in the selective hydrogenation. The surface electronic gas model, defined elsewhere (Shakhovskaya et al., 1970; Temkin, 1972a,b) can also explain the selectivities obtained with the catalyst used in the present study. The model was proposed in order to describe the inhomogeneity of the catalyst surface. It explains both mutual interactions between adsorbed particles and their interactionwith the catalyst determined by changes in the position of the Fermi level. The model is based on the assumption that a complete or partial ionization of the adsorbed particles takes place during chemisorption, with electrons being transferred to the surface layer. A peculiar characteristic of the model is that the energy of the adsorbed layer is determined only by the total number of adsorbed particles and does not depend on their arrangement (Temkin, 1972a). The concept of the surface electronic gas has been used for a discussion of chemisorption (Shakhovskaya et al., 1970) and also reaction kinetics (Murzin, 1992; Snagovskiiet al., 1978). The rate of elementary reaction such as
kPH6A exp[(l - a)uq2eAY1]
(10)
where q is the effective charge of the adsorbed species A and a the constant of proportionalitybetween the effective charges of the transition state and adsorbed state species. Constant U is given by
U = h2/4rm*k
(11)
Here h is the Planck constant, k the Boltzmann constant, and m* the effective electron mass. Let us now consider the selectivity to crotyl alcohol (SB). At high surface coverages of A , and low conversions, the equation for SBis SB
= k1/(k1 + k3)
(12)
1886 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994 Table 5. Crotonaldehyde Hydrogenation over 2% Pt/TiOz for Various Reduction Temperatures (Td)and Reaction Temperatures (T-):Overall Activity and Selectivity for Frotyl Alcohol activity (pmols-1 gR-1) at T d (K) selectivity ( % ) at T d (K) T,(K) 473 673 773 473 673 773 293 313 333 353 373
16
17 30 48
4 6 9 17 22
3 3 4 7
4
4
19 19 17
10 17 24 20
4 8
13 14
The selectivity to butyraldehyde Sc can thus be expressed Sc = k3/(k1+ k3)
(13)
Therefore, the selectivity ratio is given by SB/Sc = k1/k3
(14)
Using eq 10, the selectivity ratio S at high crotonaldehyde coverages is given by
where k1° and E1 correspond to the preexponential factor and the activation energy, respectively. In the surface electronic gas model, it is reasonable to assume that the charge transfer toward metal is lower in the adsorbed state of olefinic species than of carbonyl species. Thus, 773 can be considered as negligible in comparison with VI. The value of 11 can depend greatly on the support; thus a high selectivity can be observed in the case of the titania supported Pt catalyst where the metal-support interaction can increase the surface polarity (i.e., increase the effective charge). Equation 15 can be used to explain the temperature dependence of the selectivity. Actually, the selectivity to crotyl alcohol goes through a maximum at 353 K. Experimental results from this study are given in Table 5. Equation 15was developed assuming that the effective charge is independent of temperature. Such an assumption is not valid when reducible oxide supports are considered. For simplicity, we consider the linear dependence 11 = *T
(16)
where fi is a coefficient. Consequently, we have
we can write, assuming high surface coverages: r = k: exp[(l- C Y ~ ) U -~El/RTJ ]~~T +~ k: exp[(l - a3)U732T1- E,/RTl (19)
As it can be seen from eq 19, the reaction rate has no extremum as a function of temperature in accordance with experimental data (Table 5). Our suggestion of the application of the surface electronic gas model requires further kinetic studies on crotonaldehyde hydrogenation on Pt catalysts using different supports and a wide range of temperatures. This investigation is now in progress. 4. Active Sites for CrotonaldehydeHydrogenation. In our derivation of the kinetic equations, the nature of the active sites was not specified. As previously proposed for metals supported on reducible oxides (Vannice and Sen, 1989; Vannice, 1992; Yoshitake and Iwasawa, 1990), hydrogenation of the C-C bond could occur on metallic sites which are not influenced by the support and hydrogenation of the C=O bond could occur at the interfacial sites between the metal and support. Therefore, whatever the nature of the metallic sites, interfacial and others, the site balance equation (eq A6) is verified, allowing the reaction mechanisms in Tables 1and 2 to be presented as if the reaction occurred only on one type of site. It is worth noting that the existence of these interfacial sites does not depend on the reduction temperature and hence they are present in the catalyst even if it is not operated in strong metal-support interaction conditions.
Conclusions A kinetic model was developed, and successfully used, to describe the complex behavior of crotonaldehyde hydrogenation over a titania-supported platinum catalyst, giving a reaction network and elementary step model. Selectivity for crotyl alcohol in the transient period and steady state was discussed and explained using the surface electronic gas model.
Acknowledgment
D.Y .M. thanks the Ministry of Research and Technology (France) and NATO for financial support during his time in France. Appendix 1. Rate Equations under Steady-State Conditions For adsorption/desorption processes, a quasi-equilibrium approximation was used, thus allowing the equilibrium constants of these steps to be easily expressed. For example, step locan be written as K, = eAievpA
The value of temperature at which selectivity is maximum is given thus by
T = [(E3- El)/(l - CY,)U$~IO.~
(18)
We note, however, that this derivation is only valid if the coefficient CY is not equal to unity. If the proposed explanation is correct, the maximum selectivity as a function of the temperature can be observed with reducible oxides as supports, but not with other types of materials. As opposed to the selectivity, the activity obeys the usual Arrhenius dependence. For crotonaldehyde consumption,
(AI)
where OA is the surface coverage of A, Ov is the fraction of vacant sites and PAdenotes the partial pressure of A. The surface coverage of OB can be expressed as K2 = eB/eVPB
(A21
OB = KpBOA/KiPA
(A31
leading to
The surface coverages of C and P can be obtained using similar derivations, thus
Ind. Eng. Chem. Res., Vol. 33, No. 8,1994 1887 f4
klkd(!P = K,(k,P + kJ')[k,P+ k$- K2(k$
+ k,)/K,]
(A191
After integration of eq A17 under initial conditions,PA = PAO and Pc = 0, eq 4 in the main text can be obtained.
Appendix 2. Kinetic Modeling of the Transient Period in the Flow System For a flow-type tubular reactor we will symbolize the volume of the bed preceding a given cross section of the reactor as x . Under a perfect displacement regime (plug flow), the amount of consumed crotonaldehyde in an element of the catalyst bed from x to ( x dx) is equal to the sum of the amount of the substrate reacted and the change in the amount of crotonaldehyde on the surface. From this, we have following Kuchaev et al. (1991)
+
Hence, from eq A9 we deduce eq 2 given in the main text, which was used for the kinetic modeling. The rate equation for B is given by rB = rl - r2- rs
where Q is the volume flow rate, i.e., the volume of the mixture passing through the reactor per unit of time, T is the thermodynamic temperature, and r A is defiied by eq A9. The volume flow rate and temperature were assumed to remain constant over the bed. Corresponding equations for partial pressures of crotyl alcohol and butyraldehyde are as follows:
(All)
Therefore the final rate equation for B is obtained rB
= dA[kiP- (k$
~~)K$'$(K~PA)I
(A121
A similar rate equation can be developed for C: rc = e A [ k $
(kd($dKiPA) - (k$'K$c/Kif'~)I
(AW
From eqs 2, A12, and A13, we can obtain differential equations, which correspond to the dependence of partial pressures of the components as a function of PA: @$ @A = -f 1 + (f$dpA)
(A141
where
However, the set of differential equations (A21)-(A23) cannot be solved analytically; therefore, some simplified assumptions must be applied. If one considers that the surface coverage of adsorbed species of B and C is proportional to the corresponding partial pressure, we can use partial derivativesof partial pressures instead of partial derivatives of the coverage as a function of time. As shown in Makouangou et al. (1992) conversion is a linear function of contact time (W/F;W, weight of catalyst; F, flow rate). Hence for the kinetic model we assume that the partial pressures vary linearly along the catalyst bed. Therefore, using eqs A22, A23, 3, and 4, we arrive at eqs 5 and 6 in the main text.
Literature Cited
Under initial conditions, PA = PAO,PB= 0, and fz is not equal to unity. Thus, integration of eq A14 gives eq 3 in the main text. The differential equation for the dependence of C as a function of A is given by d P C / d P A = -f3
+ f4[1 - (~~~')/(~Ao)'T'l
+ f 6 p J p A (A171
where
Beccat, P.; Bertolini, J. C.; Gauthier, Y.; Maesardier, J.; Ruiz, P. Crotonaldehyde and MethylcrotonaldehydeHydrogenation over Pt(ll1) and PQem (111) Single Crystals. J. Catal. 1990, 126, 451. Bennet, C. 0. The Transient Method and Elementary Steps inheterogeneoue Catalysis. Catal. Rev.-Sci. Eng. 1976,13,121. Fung, S. F. XPS Studies of Strong-Metal Support Interactions (SMSI) on PtITiOz catalysts. J. Catal. 1982, 76,225. Garin, F.;Maire, G.; Gault, F. G. Hydrogholyee et Isom6risation des Hydrocarbures sur Mbtaux. Hydrogholyse des Pentanes sur Catalyseur Pt-AlzOs 0,2% . (Hydrogenolysieand Isomerization of Hydrocarbons on Metal Catalysts. Pentane Hydrogenolysis on 0.2% PtlAl~OsCatalyst.) Nouv. J. Chim. 1981,5,553. Horiuti, J. Stochiometrieche W e n und die Kinetic der Chemiechen Reactionen. (Stoichiometric Number and Kinetics of Chemical Reactions.) Res. Znst. Catal., Hokkaido Univ. 1967,5, 1.
1888 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994 Hubaut, R.; Daage, M.; Bonnelle, J. P. Selective Hydrogenation on Copper Chromite Catalysts. IV. Hydrogenation Selectivity for a,& Unsaturated Aldehydes and Ketones. Appl. Catal. 1986,22,231. Jentys, A. J.; Raab, C. G.; Lercher, J. A. Design of Platinum Based Metallic Catalysts for SelectiveHydrogenation of Crotonaldehyde. Proc. Znt. Congr. Catal. 1992, loth, Vol. C, 2301. Kamenski, D. I.; Temkin, 0. N.; Bonchev, D. G. Reaction network for the epoxidation of alkenes with hydroperoxides. Appl. Catal. A 1992,88, 1. Kiperman, S. L. In Introduction into the Kinetics of Heterogeneous Catalytic Reactions (in Russian); Chimia: Moscow, 1979. Kobayashi, H.; Kobayashi, M. Transient Response Method in Heterogeneous Catalysis. Catal. Rev.-Sci. Eng. 1974, 10, 139. Kobayashi, M.; Kanno, T.; Hakozaki, M. Discrimination andRegulation of Multi-Reaction Pathways in Heterogeneous Catalysis. Proc. Znt. Congr. Catal. 1992, loth, Vol. B, 1555. Kuchaev, V. L.; Shapatina, E. N.; Zyskin, A. G.; Temkin, M. I. Establishing of the Steady-State Reaction Rate in Ammonia Synthesis. Flow Reactor. Kinet. Katal. 1991,32, 1098. Lawrence, S. S.; Schreifels, J. A. Time Dependence for the Selective Hydrogenation of Crotonaldehyde. J. Catal. 1989,119, 272. Makouangou, R.; Dauscher, A,; Touroude, R. Crotonaldehyde Hydrogenation over Pt/TiOz Catalysts. Influence of the Catalysts Pretreatments. Proc. Znt. Congr. Catal. 1992, loth, Vol. C, 2475. Makouangou, R.; Touroude, R.; Dauscher, A. Crotonaldehyde Hydrogenation over Pt/TiOz Catalysts: Variations of Activity and Selectivity with the Partial Pressure of Crotonaldehyde. Stud. Surf. Sci. Catal. 1993, 78, 219. Marinelli, T. B.; Vleeming, J. H.; Ponec, V. Reactions of Multifunctional Organic Compounds. Hydrogenation of Acroleinon Modified Pt-Catalysts. Proc. Znt. Congr. Catal. 1992, loth, Vol. B, 1211. Murzin, D. Yu. Application of the Surface Electronic Gas Model to the Kinetics of Heterogeneous Catalytic Reactions. Dokl. Acad. Nauk Russ. 1992, 326,-475. Murzin. D. Yu.: Kul'kova. N. V.: Temkin. M. I. Kinetics of Hvdiogenation of Benzene into Cyclohexene and Cyclohexane. KEnet.Kata1. 1990, 31, 983. Noller. H.: Lin. W. M. Activitv and Selectivitv of Ni-Cu/AlzO.? Ca&lys& for Hydrogenation of Crotonaldehyde and Mechanism of Hydrogenation. J. Catal. 1984,85,25. Ostrovskii, G. M.; Zyskin, A. G.; Snagovskii, Yu. S. A System for Automation of Kinetic Computation. Comput. Chem. 1987, 11, ~
85.
Padley, M. D.; Rochester, C. H.; Hutchings, G. J.; Okoye, I. P.; King, V. FTIR Catalytic Studies of the Effects of Sulphur Poisons on Cu/A1203Catalyst Selectivity. Proc. Znt. Congr. Catal. 1992, loth, Vol. C, 2503. Raab, C. G.; Lercher, J. A. Activity and Selectivity of NiPt/SiOz Catalysts for Hydrogenation of Crotonaldehyde. J. Mol. Catal. 1992, 75, 71.
Raab, C. G.; Lercher, J. A. Activity and Selectivity of PtNi/TiOz Catalysts for Hydrogenationof Crotonaldehyde. Catal. Lett. 1993, 18, 99.
Raab, C. G.; English, M.; Marinelli, T. B.; Lercher, J. A. Selective Hydrogenation of Crotonaldehyde over Pt derived catalysts. Stud. Surf.Sci. Catal. 1993, 78, 211. Scofield, V. Hartree-Slater Subshell Photoionization Cross-Sections at 1254 and 1487 eV. J.Electron Spectrosc. Relat. Phenom. 1976, 8, 129.
Sethuraman, A. R.; Davis, B. H. The State of Platinum in Pt-Al203 Catalysts Containing high Chloride Loading. Catal. Lett. 1993, 18,401.
Shakhovskaya, L. I.; Rudnitskii, L. A.; Kul'kova, N. V.; Temkin, M. I. Reactivity of Oxygen Chemisorbed on Silver and the Work Function. Kinet. Katal. 1970,11, 467. Shirley, D. A. High Resolution X-ray Photoemission Spectrum of the Valence Band of Gold. Phys. Rev. B 1972,5,4709. Simonik,J.; Beranek, L. Kinetics of Complex HeterogeneousCatalytic Reactions. V. Parallel-Consecutive Hydrogenation of Crotonaldehyde. Collect. Czech. Chem. Commun. 1972,37,353. Snagovskii, Yu. S.; Ostrovskii, G. M. In Kinetic Modelling of Heterogeneous CatalyticReactions (inRussian);Chimia: Moscow, 1976.
Snagovskii, Yu. S.; Khomenko, A. A.; Ivanova, R. A.; Shub, F. S.; Temkin, M. I. Comparison of Models of Uniform and Nonuniform Surfaces of Catalyst in the Case of Kinetics of Methane Conversion by Vapour of Water on Nickel. Kinet. Katal. 1978,19,942. Temkin, M. I. Graphical Method of Derivation of Kinetic Equations of Complex reactions. Dokl. Acad. Nauk 1965,152, 615. Temkin, M. I. Theoretical Models of Kinetics of Heterogeneous Catalytic Reactions. Kinet. Katal. 19728, 13, 555. Temkin, M. I. Description of the Interaction Effects of Chemisorbed Particles Based on the Surface Electronic Gas Model. In Some Theoretical Problems of Catalysis; University of Tokyo Press: Tokyo, 1972b; p 83. Temkin, M. I. The Kinetics of Some Industrial Heterogeneous Catalytic Reactions. Adu. Catal. 1979, 28, 173 Touroude, R. Catalytic Behaviour of Group VI11 transition Metals in the Deuterium-Acrolein Reaction. J. Catal. 1980,65, 110. Vannice, M. A. Hydrogenation of CO and Carbonyl Functional Groups. Catal. Today 1992,12,255. Vannice, M. A.; Sen, B. H. Metal-Support Effects on the Intramolecular Selectivity of Crotonaldehyde Hydrogenation over Platinum. J. Catal. 1989,115,65. Waghray, A.; Wang, J.; Okaci, R.; Blackmond, D. G. Influence of Alkali Promoters in the Selective Hydrogenation of 3-Methyl-2butenal over Ru/SiOz Catalysts. J. Phys. Chem. 1992,96, 5954. Yoshitake, H.; Iwasawa, Y. Cooperative Behaviour of Two Kinds of Reaction Sites and Reaction Mechanisms for Deuteration of Acrolein on SMSI-Pt/NbzOb Catalyst. J. Catal. 1990, 125, 227.
Received for review November 15, 1993 Revised manuscript received April 20, 1994 Accepted May 10, 1994' Abstract published in Advance ACS Abstracts, July 1,1994.
