Applied Catalysis A: General 205 (2001) 79–84
Microkinetic analysis of isopropanol dehydrogenation over Cu/SiO2 catalyst Yuwang Han, Jianyi Shen∗ , Yi Chen Department of Chemistry, Nanjing University, Nanjing 210093, China Received 6 December 1999; received in revised form 14 March 2000; accepted 14 March 2000
Abstract The reaction of isopropanol dehydrogenation to acetone over the Cu/SiO2 catalyst was studied using the method of microkinetic analysis. This analysis provided a picture about the coverage of different surface species and their influences on the reaction rate. Specifically, the analysis reinforced the previously proposed mechanism and added some new findings for the reaction. It was found that under the normal reaction conditions, the catalyst surface is mostly covered by the isopropoxyl groups and the overall reaction rate is controlled by the elimination of ␣-H of the isopropoxyl groups. In addition, the elimination of the ␣-H is significantly affected by the availability of surface Cu sites. At the low hydrogen partial pressure, the number of bare surface Cu sites increases with the increasing of hydrogen partial pressure, leading to the increased overall reaction rate and thus the positive reaction order with respect to hydrogen. Thus, hydrogen can enhance the rate of this reaction via some kinetic factors under the specific coverage regime, although it is thermodynamically unfavorable for the reaction. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Microkinetic analysis; Dehydrogenation of isopropanol; Supported copper catalyst; Surface coverage
1. Introduction Dehydrogenation of alcohol on metallic copper catalysts has been studied for decades due to its practical importance in the chemical industry. Reactions of alcohols on metal surfaces are of considerable interest since they provide models for the catalytic oxidation of alcohols to aldehydes and ketones [1]. Along with much research work done to evaluate the catalytic properties, there were also many fundamental investigations related to the reaction kinetics and mechanism [2–5]. Although plausible reaction models have been proposed, there is still an interesting phenomenon that has not been properly elucidated: ∗ Corresponding author. Tel.: +86-25-3592909. E-mail address:
[email protected] (J. Shen).
the dehydrogenation rate increases with the increase of hydrogen partial pressure. Thermodynamically, the forward rate of the dehydrogenation reaction of alcohols should be limited by the presence of hydrogen. In order to enhance the equilibrium conversion of alcohol, oxygen is sometimes introduced into the reaction system to consume the hydrogen produced. However, it was surprisingly found that the rate of the formation of aldehydes and ketones could be enhanced by the presence of more hydrogen during the dehydrogenation of alcohols over metallic copper catalysts [6]. Furthermore, Venugopal et al. [7] and Uma and Kuriacose [8] found the positive reaction orders with respect to hydrogen in the dehydrogenation of ethanol on this kind of catalysts. To our knowledge, there is no plausible interpretation for these results up to now. Therefore, the main attempt of this work
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is to provide an explanation for this phenomenon by using the method of microkinetic analysis developed by Dumesic and co-workers [9–11]. The traditional catalytic reaction kinetics is not capable of doing so since it is difficult to interpret clearly the kinetic data without knowing the detailed information such as the rate constant of each elementary step, the rate limiting steps and the coverage of surface species. Now, such information could be obtained by using the method of microkinetic analysis. The quantitative analysis of the kinetic data collected for the dehydrogenation of isopropanol over a Cu/SiO2 catalyst allowed us to reach a plausible interpretation for this seeming conflict between the thermodynamics and the reaction kinetics.
below 3% for obtaining the kinetic data. The reaction reached steady state after 5 h and the kinetic data were then collected.
3. Microkinetic analysis 3.1. The reaction model The following reaction mechanism summarized according to the literature [6,10–12] was adopted for the purpose of our modeling: (CH3 )2 CHOH + Cu (CH3 )2 CHOH–Cu
(1)
(CH3 )2 CHOH–Cu + Cu 2. Experimental
(CH3 )2 CHO–Cu + Cu–H
(2)
(CH3 )2 CHO–Cu+Cu (CH3 )2 CO–Cu+Cu–H (3)
2.1. Catalyst preparation
(CH3 )2 CO–Cu (CH3 )2 CO + Cu
(4)
The catalyst containing about 8 wt.% of Cu was prepared by the incipient-wetness impregnation method. The support (Cab-o-sil EH-5) was impregnated with an aqueous solution of Cu(NO3 )2 ·3H2 O. The slurry formed was dried at 393 K for 12 h. The sample was then calcined at 573 K for 2 h, followed by reduction at 673 K for 8 h in flowing H2 ; it was stored in a desiccator for further use.
2Cu–H H2 + 2Cu
(5)
2.2. Kinetic measurements The kinetic data were collected on a conventional flowing fixed-bed micro-reactor system. The catalyst (45–60 mesh, 10 mg) was placed in a glass tube (5 mm in diameter) inserted in an electrically heated furnace and was reduced with highly pure hydrogen in situ in the reactor at 673 K for 2 h before the reaction. Then the reactor was cooled down to the reaction temperatures under flowing nitrogen. Isopropanol was fed by highly pure nitrogen bubbling with the isopropanol feed rate of 2.0 g h−1 and the total gas flow rate of 112.5 ml min−1 . The temperature of the bubbler was controlled precisely within the margin of ±0.5 K. The reaction products were monitored by on-line gas chromatography (15% PEG6000/GDX-103, FID, for organic species and TDX-01, TCD, for gaseous products). Conversion of isopropanol was controlled
Since the reaction data were collected at steady state in the flowing reactor, the coverages of different surface species can reach steady state and can be expressed as functions of the rate constants (ki ), the partial pressures of reactants and products (Pn ), and the reaction temperature (T). Then the overall reaction rate, Rovs , for the formation of acetone can be written as a function of the above parameters and the rates can be compared with the experimental observations, Rove . The simulation was accomplished by adjusting the parameters Ai and Ei for all the elementary steps to P j j minimize the sum, (Rovs − Rove )2 , where j refers to an experimental datum. Additional assumptions have to be made in order to simplify the simultaneous equations that correlate the rate constants, surface coverage, partial pressures and the reaction temperature. Since isopropanol adsorbs readily on copper to form the isopropoxyl groups, Eqs. (1) and (2) can be combined into one, i.e. Eq. (6). Since acetone is weakly adsorbed on copper (the heat of acetone adsorption on Cu (100) was reported to be about 42 kJ mol−1 [13]), it desorbs readily from the copper surface once it is formed. Thus, we also combine Eqs. (3) and (4) into one, i.e. Eq. (7). In addition, since the adsorption of acetone is kinetically insignificant, Eq. (7) can be
Y. Han et al. / Applied Catalysis A: General 205 (2001) 79–84
considered to be irreversible. Finally, Eqs. (6) and (8) are assumed to be equilibrated at the reaction conditions usually employed: (CH3 )2 CHOH+2Cu (CH3 )2 CHO–Cu + Cu–H (6) (CH3 )2 CHO–Cu + Cu → (CH3 )2 CO + Cu + Cu–H
(7)
2Cu–H H2 + 2Cu
(8)
Therefore, the following equations can be obtained according to Eqs. (6)–(8), in which θ Cu , θ Ipo , and θ CuH represent the coverage of bare Cu sites, adsorbed isopropoxyl groups, and adsorbed hydrogen atoms, respectively: 2 − k−1 θIpo θCuH = 0 k1 PIpa θCu 2 2 − k−3 PH2 θCu =0 k3 θCuH
θCu + θIpo + θCuH = 1 PIpa and PH2 are the partial pressures of isopropanol and H2 , respectively. These equations can be solved analytically for θ Cu , θ CuH and θ Ipo . θCu =
θIpo =
k−1 (k3 k−3 PH2 )1/2 k−1 (k3 k−3 PH2 )1/2 + k−1 k −3 PH2 + k1 k3 PIpa
k−1 (k3 k−3 PH2
θCuH =
k1 k3 PIpa + k−1 k −3 PH2 + k1 k3 PIpa
)1/2
k−1 k−3 PH2 1/2 k−1 (k3 k−3 PH2 ) + k−1 k −3 PH2
+ k1 k3 PIpa
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The rate of acetone formation Rovs =k2 θ Cu θ Ipo . With the analytical expressions of θ Cu and θ Ipo , we may write the equation for Rovs as below: Rovs =
k1 k−1 k2 k3 3/2 (k−3 PH2 )1/2 PIpa [k−1 (k3 k−3 PH2 )1/2 + k−1 k−3 PH2 + k1 k3 PIpa ]2
3.2. Estimation of pre-exponential factors The pre-exponential factors for the mechanism were estimated according to the transition state theory, as described in detail by Dumesic et al. [9]. The estimated and corresponding simulated values of these pre-exponential factors are given in Table 1. It should be mentioned that the units used for these pre-exponential factors should match the unit for experimental rate, Rove (mol s−1 g−1 ). The number of surface sites of the catalyst was found to be about 4.3×10−5 mol g−1 , as determined by O2 adsorption at 313 K. This value is comparable with that reported in the literature [9]. Thus, the estimated values of pre-exponential factors listed in Table 1 are obtained by multiplying the values given in [9] by 4.3×10−5 mol g−1 . 3.3. Estimation of activation energies Wachs and Madix studied the adsorption and desorption of ethanol on Cu (110) surface and found the formation of ethoxyl groups on the surface [14]. In addition, they found that the activation energy for the desorption of ethanol was between 54 and 69 kJ mol−1 . Since the dissociative adsorption of ethanol and isopropanol is similar, we used the average
Table 1 The kinetic parameters for the dehydrogenation of isopropanol (Ipa) to acetone over the Cu/SiO2 catalyst Parameters
Initial estimation
A1 E1 A−1 E−1 A2 E2 A3 E3 A−3 E−3
4.3×101
mol kPa−1 s−1 g−1
0 kJ mol−1
4.3×106 mol s−1 g−1 61 kJ mol−1 1.7×104 mol s−1 g−1 115 kJ mol−1 4.3×106 mol s−1 g−1 63 kJ mol−1 4.3×101 mol kPa−1 s−1 g−1 21 kJ mol−1
Simulated value 1.43×101 mol kPa−1 s−1 g−1 9 kJ mol−1 1.27×106 mol s−1 g−1 54 kJ mol−1 3.98×105 mol s−1 g−1 83 kJ mol−1 1.42×106 mol s−1 g−1 44 kJ mol−1 3.93×101 mol kPa−1 s−1 g−1 17 kJ mol−1
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of the above values as the initial guess for E−1 . The initial guess of E1 for the adsorption of isopropanol was zero. Since the strength of the ␣-C–H bond of isopropanol is 381±4 kJ mol−1 [15], we estimated the activation energy for E2 to be about 115 kJ mol−1 by using the 30% rule. Finally, Shustorovich and Bell estimated the activation energy for the combination desorption of hydrogen atoms from Cu (111) to be about 63 kJ mol−1 [16–19] according to the bond order conservation theory. Activation energy for the adsorption of hydrogen on Cu (111) was experimentally determined to be 21 kJ mol−1 [16]. These two values were used as the initial guesses for E3 and E−3 in our simulation.
4. Results and discussion Fig. 1 shows the experimental and simulated plots for the rates of acetone formation versus reaction temperature. The kinetic parameters used for Rovs are the simulated values given in Table 1. The agreement between the model and experiment is satisfactory. Accordingly, the findings from the simulation may be summarized as follows: 1. Dehydrogenation of isopropoxyl groups may be the rate-limiting step. 2. Reaction orders with respect to isopropanol and hydrogen calculated from simulated results are shown
Fig. 1. Results of experiment versus simulation for the overall rate of the reaction for the conversion of isopropanol to acetone over Cu/SiO2 as a function of temperature.
Fig. 2. Plot of ln Rovs (mol s−1 g−1 ) vs. ln PIpa (kPa) calculated from simulation for the reaction at 500 K, PH2 is fixed at 0.1 kPa.
in Figs. 2 and 3. The simulated reaction order with respect to isopropanol changes from 1 at low partial pressures to −1 at high partial pressures (higher than 0.14 kPa). The simulated reaction order with respect to hydrogen is 0.5 at low partial pressures, in agreement with the value reported in the literature [6–8]. The simulation also indicates that when the partial pressure of hydrogen is higher than 1.5×102 kPa, the reaction order may become negative. 3. The most abundant surface species is the isopropoxyl groups, the coverage may be as high as 0.99. The coverage of other species, i.e. θ CuH and θ Cu may be as low as 3.5×10−5 and 0.009, respectively, according to the results of simulation. The
Fig. 3. Plot of ln Rovs (mol s−1 g−1 ) vs. ln PH2 (kPa) calculated from simulation for the reaction at 500 K, PIpa is fixed at 10.0 kPa.
Y. Han et al. / Applied Catalysis A: General 205 (2001) 79–84
Fig. 4. Plot of calculated surface coverage (θ) vs. ln PIpa (kPa) for the reaction at 500 K, PH2 is fixed at 0.1 kPa.
results for the surface coverages versus the partial pressures of isopropanol and H2 are presented below. 4. The simulation results revealed the effects of partial pressures of isopropanol and hydrogen on the coverage of surface species, as shown in Figs. 4 and 5. The fraction of bare sites of copper (θ Cu ) decreases while the coverage of isopropoxyl group (θ Ipo ) increases with the increase of partial pressure of isopropanol (PIpa ); the two curves intercept at PIpa =0.14 kPa. The coverage of hydrogen (θ CuH ) does not seem to be influenced by PIpa . However, the θ CuH increases significantly while θ Ipo drops with the increase of the partial pressure of hydrogen; the two curves intercept at PH2 =403 kPa. It
Fig. 5. Plot of calculated surface coverage (θ) vs. ln PH2 (kPa) for the reaction at 500 K, PIpa is fixed at 10.0 kPa.
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is interesting to note that the simulation predicts a maximum of θ Cu at PH2 =403 kPa with the increase of the partial pressure of hydrogen. The explanation of this behavior is discussed further below. As we know, the mechanism of alcohol dehydrogenation on metal catalysts under relatively mild conditions is always assumed to follow the ‘carbonyl’ model in which adsorbed alkoxyl is an important surface species. Abstraction of the ␣-H of alkoxides is the rate-limiting step and therefore the surface alkoxyl groups are the most abundant intermediates. Our simulation results reinforced this experimental conclusion. Since Rovs =k2 θ Ipo θ Cu , according to Eqs. (6)–(8) one would expect a maximum rate when the product θ Ipo θ Cu reaches the maximum with respect to the variation of θ Ipo /θ Cu . This is the typical feature of a Langmuir–Hiselwood reaction. Specifically, when the partial pressure of isopropanol is low, the surface coverage of alkoxyl group (θ Ipo ) is low. Then, θ Ipo increases with the partial pressure of isopropanol, leading to the increase of rate Rovs . In this case, the reaction exhibits the first order with respect to isopropanol. However, the increase of θ Ipo results in the decrease of θ Cu , which is essential for the abstraction of ␣-H from the alkoxyl groups. This situation is shown in Fig. 4. When θ Cu equals θ Ipo , the reaction rate reaches the maximum. Afterwards, the further increase of θ Ipo results in the lowering of θ Cu , leading to the decrease of the reaction rate and hence the negative reaction order with respect to isopropanol. Thus, the key factor influencing the overall reaction rate at high θ Ipo is the fraction of bare copper sites, θ Cu . Fig. 5 shows that θ Ipo decreases while θ CuH increases with the hydrogen pressure. Meanwhile, θ Cu passes through a maximum corresponding to the point of intersection of θ Ipo and θ CuH . Thus, Eq. (8) will shift to the left side with the increase of hydrogen pressure. This shift will decrease the coverage of bare copper sites, θ Cu , and increase the coverage of surface hydrogen, θ CuH . The increase of θ CuH will in turn shift the Eq. (6) to the left side to increase the coverage of bare copper sites, θ Cu . The net effect of this process is the increase of θ Cu at low hydrogen pressures, and vice versa. In fact, the dissociative adsorption of one hydrogen molecule as expressed by the reversed reaction of Eq. (3) consumes two Cu sites and produces two Cu–H sites. However,
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the two Cu–H sites may produce four bare Cu sites through the reversed reaction of Eq. (1). This may be the reason why θ Cu increases with the hydrogen pressure and why the overall reaction exhibits the reaction orders of 0.5 at the low hydrogen pressures. At high hydrogen pressures, the substantial increase of θ CuH results in the decrease of both θ Ipo and θ Cu , leading to the negative reaction orders with respect to hydrogen. 5. Summary In this paper, we demonstrated that a simple microkinetic analysis could provide insight into the chemistry of the catalytic reaction for the conversion of isopropanol to acetone over the Cu/SiO2 catalyst. In addition, this analysis reinforced the previously proposed mechanism and provided some new findings for the reaction. Under the normal reaction conditions, the catalyst surface is mainly covered by the isopropoxyl groups. The overall reaction rate is controlled by the elimination of ␣-H of the adsorbed isopropoxyl groups, which is in turn limited by the availability of the bare surface Cu sites. At low partial pressures of hydrogen, an increase of partial pressure of hydrogen may increase the number of bare surface Cu sites, leading to the increased overall reaction rate. Thus, although hydrogen is thermodynamically unfavorable for the reaction, it may promote the reaction via kinetic factors under some specific coverage regimes.
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