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SiO2)
Applied Catalysis A: General 177 (1999) 205±217
Kinetic studies of methylcyclopentane ring opening on EuroPt-1 (Pt/SiO2) Yiping Zhuang*, Alfred Frennet1 Catalyse HeÂteÂrogeÁne, Universite Libre de Bruxelles, CP243, 1050 Brussels, Belgium Received 23 April 1998; received in revised form 17 July 1998; accepted 17 July 1998
Abstract Kinetic studies of methylcyclopentane (MCP) ring opening on EuroPt-1 (a 6.3 wt% Pt/SiO2) were carried out under both steady state and concentration step change conditions. At steady state, the activity changes monotonically on increasing MCP pressure and exhibits a maximum with hydrogen pressure. The product ratio of 2-methylpentane (2MP) to n-hexane (nH) increases with increasing MCP pressure and decreases with hydrogen pressure. No change was observed for the product ratio of 2MP to 3-methylpentane (3MP). Following a step change up of MCP pressure from 0 to ca. 9 Torr, an overshoot response at 60 Torr H2 and a monotonic increase at 750 Torr H2 were observed for all the products. The height of the overshoot response of nH is more signi®cant than that of 2MP and 3MP and decreases as the temperature increases. Both 2MP/nH and 2MP/3MP change with time during a step change up of MCP pressure. A maximum for 2MP/3MP and a value less than unity for 2MP/nH were observed. The results were discussed on the basis of a parallel reaction model: one forming nH and the other forming MPs (2MP3MP). A reactive adsorption mechanism for the reaction forming nH and a dissociative adsorption mechanism for the reaction forming MPs were proposed for the MCP adsorption±dehydrogenation process. A molecular hydrogen associated with an active site is most likely involved in the C±C bond rupture, which is concluded to be the rate-controlling step in this work. # 1999 Elsevier Science B.V. All rights reserved. Keywords: EuroPt-1 (Pt/SiO2); Methylcyclopentane ring opening; Steady state and transient kinetics; Reaction mechanism
1. Introduction Alkane hydrogenolysis on metal catalysts has been a long-time interesting research subject due to its importance in petroleum industry and fundamental *Corresponding author. Present address: Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1. Tel.: +519-8851211-2879; fax: +519-7464979; e-mail: [email protected] 1 Senior Research Associate, National Fund for Scientific Research, Belgium.
science. It is generally accepted that the reaction passes through three consecutive steps: alkane adsorption±dehydrogenation, C±C bond rupture and product hydrogenation±desorption. However, many arguments are provided in the literature for the mechanism involved in each step [1±4]. Methylcyclopentane (MCP) is an important probe molecule in the exploration of the properties of a catalyst surface. The possible products for MCP reaction with hydrogen are n-hexane (nH), 2-methylpentane (2MP) and 3-methylpentane (3MP) from ring opening (MCP hydrogenolysis) as well as cyclohex-
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00261-0
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ane and benzene from ring enlargement. Metal facilitates MCP ring opening and acid sites on supports promote MCP ring enlargement [5]. Much attention was paid to MCP ring opening on platinum catalysts because a methylcyclopentane-like intermediate was proposed to be involved in the isomerisation reaction of nH and its isomers [2,3]. It was found that the product distribution of MCP ring opening is quite sensitive to the properties of the catalyst surface. Numerous reaction mechanisms were proposed to correlate the observed experimental results [2,3,6±13]. Gault and coworkers [7,8] postulated two reaction mechanisms for MCP ring opening on platinum catalysts. On a catalyst with high Pt dispersion, the nonselective mechanism dominates, producing nH, 2MP and 3MP in a statistical ratio of 2:2:1. A diadsorbed species on a metal site was proposed to be the intermediate. When the particle size of platinum is larger than 1.8 nm, the selective mechanism prevails producing 2MP and 3MP only. The intermediate was considered to be a tetra-adsorbed species on two metal sites. The product distribution obtained in some cases cannot be accounted for by a simple combination of the above two mechanisms. A partially selective mechanism producing more nH was proposed by Maire et al. [3,6]. This mechanism is more effective at high temperature. Kramer and Zuegg [9,10] and also PaaÁl [11] found that the selectivity towards nH increases with the interface length between the Pt and support. They postulated that two parallel reactions take place: one mainly producing 2MP and 3MP on the Pt surface and the other forming additional nH on the interface. The pairs of the metallic Pt0 and ionic Pt sites were suggested to be the active sites for the latter reaction [14]. Although extensive work has been carried out for MCP ring opening on platinum catalysts, no work was performed under transient conditions. It was demonstrated that more information about the reaction mechanism could be provided by transient kinetic studies, because the elemental steps of a reaction may be kept from equilibrium during a transient procedure [15±18]. The effect of these steps on the overall reaction can be derived from the product response [18]. In this work, MCP ring opening was studied in a ¯ow reaction system under both steady state and transient conditions. The catalyst used is
EuroPt-1. It is a reference catalyst and has been extensively characterised by several European laboratories [19±24]. The catalyst contains 6.3 wt% Pt on a silica support and has a dispersion of ca. 60%. The transient condition was realised by suddenly introducing MCP or stopping its supply into a reaction stream with a ®xed hydrogen concentration. The effects of temperature, H2 and MCP pressures on the reaction were investigated and some interesting results are reported here. 2. Experimental 2.1. Catalyst and reactants The catalyst used is EuroPt-1 (6.3 wt% Pt/SiO2). Before the reaction, the catalyst was treated in situ in a 50 ml/min ¯ow of 10% O2/Ar at 623 K for 1 h and then of H2 at 573 K for 1 h, to eliminate possible contaminants such as carbon and oxygen on the catalyst surface. MCP (purity>99%) was purchased from Fluka. The major impurity is nH (0.64%) as measured by GC. Hydrogen (H2 5.5) was purchased from UCAB and helium (grade A) from BOC. 2.2. Apparatus and activity measurement The reaction was performed in a ¯ow reaction apparatus at atmospheric pressure. The reaction stream could be composed of three gas streams: hydrogen, helium, and hydrogen or helium. By switching a 4-way valve, the ®nal gas could pass through or pass by a saturator ®lled with liquid MCP. The transient process is therefore initialised. The rotameters calibrated by a soap ®lm ¯ow meter were used to measure the ¯ow rate of each gas stream. The saturator was inserted in a liquid thermostat at 273 K. Equilibrium between liquid MCP and its vapour was always kept within a range of the gas ¯ow between 5 and 50 ml/min through the saturator. The partial pressure of MCP at the inlet of the reactor was then calculated by the following formula PMCP Torr
P0MCP ; P0MCP =760 Ft =Fs 1 ÿ P0MCP =760 (1)
Y. Zhuang, A. Frennet / Applied Catalysis A: General 177 (1999) 205±217
where Ft is the total gas ¯ow rate, Fs the gas ¯ow rate that passes through the saturator, and P0MCP is the vapour pressure of MCP, which is 43.53 Torr at 273 K [25]. The catalyst was placed in a U-type Pyrex glass reactor with an inside diameter of 4 mm. The reactor was heated by an electrical furnace. The temperature was measured by a chromel±alumel thermocouple just bound to the outside of the reactor. The desired temperature was maintained by Eurotherm 808, a temperature controller. A multiposition valve (ST16P, Valco) with 16 loops of 0.5 ml was placed just after the reactor, in which 15 of the loops can be used to store the ef¯uent gases. The products were analysed by a gas chromatograph (HP5890), which was equipped with a 50 m fused silica PLOT Al2O3/KCl capillary column and a TCD detector. Both the gas chromatograph and multiposition valve were controlled by an IBM compatible personal computer through the HP Chemstation Program. In order to avoid hydrocarbon condensation, the tubes and valves through which MCP was ¯owing were heated to ca. 368 K and the multiposition valve to 423 K. For a transient process, the time zero was de®ned at the time when the hydrocarbon was detected in the ef¯uent gases after a step change up of MCP or when the concentration of the hydrocarbon begins to decrease during a step change down of MCP. The real time zero is unknown, because the time interval was found to suffer an error of 3 s from the time to switch the 4-way valve for a step change of MCP to the time at which the change of product concentration was detected in the ef¯uent gases with an empty reactor. The residence time of the reaction system was measured at a total gas ¯ow rate of 50 ml/min by a step change up or down of MCP pressure from 0 to 9 Torr. It is about 9 s no matter whether the reactor was empty or ®lled with glass beads. A longer residence time of about 12 s was observed if the multiposition valve was only heated to 373 K. The formation rate of products was calculated by the following equation rj F Cj;out ÿ Cj;in =A;
207
¯ow rate, Cj,in and Cj,out are the jth product concentration at the inlet and the outlet of the reactor, respectively. The conversion rate of MCP was obtained by summation of the formation rate of all the products. The product selectivity was expressed as the product ratio of 2MP/3MP and 2MP/nH as documented in the literature [6±13]. The time on stream in one reaction run was less than 8 min. The hydrogen was always kept ¯owing through the catalyst bed between two successive runs for at least 5 min. Under these conditions, no signi®cant deactivation of the catalyst was observed. 3. Results 3.1. Steady state kinetics 3.1.1. Activity Fig. 1 shows the effect of hydrogen pressure on activity, which was measured at ca. 9 Torr MCP. A maximum was reached at ca. 90 Torr hydrogen at 473 K and its location shifted to higher hydrogen pressure as the temperature increased. Similar results can be found in the literature for both MCP ring opening [27] and other alkane hydrogenolysis on Pt catalysts [28±32]. The apparent activation energy was derived based on the data presented in Fig. 1. It is a strong function
(2)
where A is the number of surface platinum atoms per gram catalyst. It is 2.341020 for EuroPt-1 as measured by hydrogen adsorption [21,26]. F is the total
Fig. 1. Effect of hydrogen pressure on the rate of MCP conversion at ca. 9 Torr MCP: () 473; (^) 496; and (*) 513 K.
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surface must be signi®cantly covered by hydrocarbon species and the MCP adsorption is not the rate controlling step. By changing the amount of the catalyst at a given total gas ¯ow rate, the effect of MCP conversion on activity and selectivity was measured. No signi®cant effect was observed with a variation of MCP conversion from 7.1% to 31.6% at 9.2 Torr MCP, 150 Torr hydrogen and 496 K. This result is consistent with the above observation that the reaction exhibits a low reaction order with respect to MCP pressure.
Fig. 2. Effect of MCP pressure on the rate of MCP conversion at ca. 740 Torr H2: () 473; (^) 496; and (*) 513 K.
of hydrogen pressure, and increases monotonically from ca. 90 kJ/mol at 60 Torr H2 to ca. 220 kJ/mol at 750 Torr H2, a value much higher than that required for dehydrogenation and hydrogenation of hydrocarbon on Pt catalyst [4]. This may indicate that the MCP dehydrogenation and product hydrogenation are not the rate controlling step in this work. As depicted in Fig. 2, the reaction rate increases with increasing MCP pressure at about 740 Torr hydrogen. The reaction order with respect to MCP pressure, as presented in Table 1, increases as hydrogen pressure increases, indicating that hydrogen inhibits the adsorption of MCP [32]. As the reaction order with respect to MCP pressure is much smaller than unity even at about 740 Torr hydrogen, the catalyst
3.1.2. Selectivity By using a pulse-microcatalytic reactor, PaaÁl [11] observed that both the product ratio 2MP/nH and 2MP/3MP changed with reaction temperature, and a maximum appears for the ratio 2MP/nH. In our work, the ratio 2MP/nH changes with the reaction conditions. While the ratio 2MP/3MP keeps constant. The value is ca. 3, which is consistent with that reported by Vaarkamp et al. [27]. Fig. 3 shows the temperature effect on the ratio 2MP/nH, which was obtained at ca. 9 Torr MCP. The deviation, which was derived from at least four independent experiments, is also presented in this ®gure. From this ®gure one may note that the ratio 2MP/nH is not only a function of temperature, but also a function of hydrogen pressure. At 750 Torr hydrogen, the ratio 2MP/nH is about 1.2, and does not change signi®cantly with temperature. At 150 Torr hydrogen a
Table 1 The reaction order with respect to MCP pressure (r kP MCP ; P MCP varies from ca. 5 to 45 Torr) Temperature (K)
H2 pressure (Torr) 60
150
740
473 496 513
0.050.02 n* n
0.260.02 n n
0.370.03 0.590.04 0.590.03
n*: No measurement.
Fig. 3. Effect of temperature on product ratio 2MP/nH at ca. 9 Torr MCP: (*) 60 () 150; and (^) 750 Torr H2.
Y. Zhuang, A. Frennet / Applied Catalysis A: General 177 (1999) 205±217
Fig. 4. Effect of MCP pressure on product ratio 2MP/nH: 473 K ((&) 150 and (*) ca. 740 Torr H2), 496 K (() ca. 740 Torr H2), 513 K ((^) ca. 740 Torr H2).
maximum is observed at ca. 480 K and at 60 Torr hydrogen it decreases as the temperature increases. The effect of MCP pressure on the product distribution is seldom reported in the literature. As shown in Fig. 4, the ratio 2MP/nH increases signi®cantly with MCP pressure. A value equal to 1, corresponding to the non-selective mechanism, could only be observed with an extrapolation of MCP pressure to zero. 3.2. Transient kinetics The transient kinetic studies were realised by suddenly introducing MCP or stopping its supply into the reaction stream as previously mentioned in Section 2. The total gas ¯ow rate was kept around 50 ml/min and the MCP pressure around 9 Torr at steady state. As only a 1.3% change of the total volumic ¯ow rate was caused by introducing MCP or stopping its supply into the system, its effect on the reaction was neglected in this work. 3.2.1. MCP pressure step change up Before introducing MCP into the reactor, the catalyst was equilibrated with a given hydrogen pressure. During the step change up of MCP pressure from 0 to ca. 9 Torr, the concentration of hydrocarbons in the ef¯uent was monitored as a function of time. Fig. 5
209
depicts the results obtained at 473 K, where p(t) is the partial pressure of hydrocarbon at time t and p(st) is the steady state value. A monotonic response at 750 Torr H2 (Fig. 5(a)) and an overshoot response at 60 Torr H2 (Fig. 5(b)) were observed for all of the products. A signi®cant difference exists between MP (2MP or 3MP) and nH responses. This difference is accentuated at 60 Torr hydrogen: the height of p(t)/ p(st) is ca. 1.5 for 2MP and 3MP, while it is ca. 6 for nH. The maximum height of the overshoot responses for nH and MP do not occur at the same time, and it is about 6 s later for nH. The responses of 2MP and 3MP seem to follow the same curve as shown in Fig. 5(a) and (b). While by plotting mole ratio of the products, a signi®cant difference between 2MP and 3MP can also be found (Fig. 5(c) and (d)), a maximum of 2MP/3MP appears at 60 Torr hydrogen. The product ratio 2MP/ nH changes with the reaction conditions under steady state conditions, however, the value is never smaller than 1. In contrast, during the step change up of MCP pressure from 0 to ca. 9 Torr, a value of 2MP/nH less than 1 is observed at 750 Torr hydrogen (Fig. 5(c)). Even more interesting at 60 Torr hydrogen, the ratio 2MP/nH ®rst increases from 1 to 2 then decreases to 0.62, and ®nally increases again to a steady state value of 3 (Fig. 5(d)). One may note that MCP appears later than the products in the ef¯uent. This becomes more apparent at 60 Torr H2, where no MCP was detected in the ®rst three samples. This result again con®rms that MCP adsorption is not the rate controlling step, and that hydrogen inhibits MCP adsorption. For comparison purposes, the results obtained at 458 and 496 K at 60 Torr H2 are shown in Fig. 6. These results are quite similar to Fig. 5(b) and (d). However, as the temperature increases, the overshoot response becomes less signi®cant. The maximum value of 2MP/3MP decreases from 3.8 to 3.2 and the ratio 2MP/nH is no longer lower than 1 at 496 K as observed at 458 and 473 K. 3.2.2. MCP pressure step change down After the steady state was reached, the supply of MCP was suddenly stopped and the hydrocarbon concentration at the outlet of the reactor was monitored as a function of time. Fig. 7 shows the typical results that were obtained at 496 K. The responses of 2MP and 3MP follow the same curve but differ from nH. Such a difference becomes more signi®cant as the
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Fig. 5. Product response after a MCP step change up from 0 to ca. 9 Torr at 473 K: (a) and (c) 750 Torr H2, (b) and (d) 60 Torr H2. (*) 2MP, () 3MP, (*) nH, () MCP, (^) 2MP/nH, and (}) 2MP/3MP.
hydrogen pressure decreases (Fig. 7(a) and (b)). The ratio 2MP/3MP does not change signi®cantly with time, while the ratio 2MP/nH decreases from the steady state value to a value less than 1 (Fig. 7(d) and (e)). Comparing Fig. 7(b) and (c), one may note that the MCP conversion affects the desorption of the products. Although the steady state rate does not change with a high MCP conversion, the product desorption becomes fast. The apparent fast desorption of the products may be attributed to the readsorption of the products. At steady state the adsorption of the product can be neglected because the adsorption of MCP is stronger than its products and further the concentration of MCP is high in the reactor. While during a step change down of MCP, the product
concentration in the gas phase becomes relatively high after a certain time with a high MCP conversion at steady state. The active sites originally occupied by MCP are now available for product readsorption. The readsorption of nH may be easier than its isomers, because the ratio 2MP/nH is no longer less than 1 as shown in Fig. 7(f). Similar results were also observed at 60 Torr hydrogen. It is worth mentioning that no quantitative difference was observed for MCP decay under different reaction conditions, with or without the catalyst in the reactor. The amount of MCP desorbed from the catalyst surface should be small compared with the amount of MCP in the gas phase. Therefore, its effect on MCP decay is invisible.
Y. Zhuang, A. Frennet / Applied Catalysis A: General 177 (1999) 205±217
211
Fig. 6. Product response after a MCP step change up from 0 to ca. 9 Torr at 60 Torr H2. (a) and (c) 458 K, (b) and (d) 496 K. (*) 2MP, () 3MP, (*) nH, ()MCP, (^) 2 MP/nH, (}) 2MP/3MP and Wcat0.86 g.
4. Discussion 4.1. Steady state kinetics It is generally accepted that alkane hydrogenolysis on metal catalysts takes place through three consecutive steps: alkane adsorption±dehydrogenation; the C± C bond rupture and product hydrogenation±desorption. All the steps have been proposed to be rate controlling, depending on the reaction conditions and the nature of the catalyst used [28±35]. In this work, the reaction rate is controlled by the C±C bond rupture because the reaction order with respect to MCP pres-
sure is less than unity and the apparent activation energy observed is higher than that required for hydrogenation and dehydrogenation of alkanes. This conclusion is further supported by the fact that no MCP was detected in the initial ef¯uent gases during the MCP step change up. The volcano dependence in activity on hydrogen pressure was often observed for alkane hydrogenolysis on metal catalysts. This can be accounted for by two possible reaction mechanisms: the C±C bond rupture involving a molecular hydrogen (Eley±Rideal mechanism) [28±30], or the C±C bond rupture involving a vacant site or an adsorbed hydrogen atom
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Fig. 7. Product response at different hydrogen pressure and MCP conversion after a MCP step change down from ca. 9 to 0 Torr at 496 K: (a) and (d) 750 Torr H2, with 16.3% MCP conversion; (b) and (e) 60 Torr H2 with 7.1% MCP conversion; (c) and (f) 60 Torr H2 with 31.6% MCP conversion. (*) 2MP, () 3MP, (*) nH, ()MCP, (^) 2MP/nH, and (}) 2MP/3MP.
(Langmuir±Hinshelwood mechanism) [4,27,34]. The ®rst one cannot predict a maximum activity with a variation of hydrocarbon pressure while the second one does. As in most cases only a monotonic increase in activity was observed with increasing hydrocarbon pressure, the Eley±Rideal mechanism was widely accepted [28±30]. However, due to the fact that the experiments were often carried out at a high reactant ratio of hydrogen to alkane, a maximum activity may not be observed with a limited range of alkane concentration. In other words, even such a maximum is observed, it is still dif®cult to conclude that the Langmuir±Hinshelwood mechanism takes place because a decrease in activity may result from catalyst deactivation caused by carbon deposition during reaction.
In order to draw more information from the steady state kinetic data, kinetic modelling was performed. A parallel reaction model, one producing MPs and the other forming nH, was considered, and different active sites are assumed to be involved for these two reactions [9,10,14]. Both the steady state and transient kinetic results obtained in this work support such a model. The reaction scheme may be written as: H2 2 2H ; C6 H12
C6 H12ÿ2a
(a) aH2 ;
(b)
C6 H12ÿ2a H2 ! C6 H14ÿ2a ;
(c)
C6 H14ÿ2a aH2 ! C6 H14 ;
(d)
where is a vacant active site. In this scheme, step
Y. Zhuang, A. Frennet / Applied Catalysis A: General 177 (1999) 205±217
(b) may not be an elementary one but a sum of a series of successive dehydrogenation equilibrium steps, such as
C6 H12 2 C6 H11
C6 H11
C6 H10
H ;
H ;
.. .
r
ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ (b0 )
0
By combining step (b ) with step (a) and letting n/2a, one can have step (b). As only step (b) is considered in the reaction scheme, the amount of the surface species C6 H11 , C6 H10 C6 H12ÿ n1 should be small compared to C6 H12ÿn , and does not affect the reaction rate. The C±C bond rupture may take place at any stage of the dehydrogenation process [34]. The parameter 2a may be regarded as an average amount of the lost hydrogen atoms of MCP before the C±C bond rupture. The C±C bond rupture, step (c), is rate-controlling, involving possibly a molecular hydrogen H2 or a vacant site , or an adsorbed hydrogen atom H* or a combination form of H2 (equivalent to an adsorbed H2). This reaction scheme has been widely used to simulate the reaction rate of alkane hydrogenolysis including MCP ring opening on metal catalysts [27,29,30]. In our work, it was found that the rate equations derived on the basis of this reaction scheme can account for the effect of hydrogen pressure but not the effect of MCP pressure on the reaction rate. A similar conclusion was also reached for light alkane hydrogenolysis on metal catalysts by Bond et al. [29] and Shang and Kenney [30]. The above reaction scheme was then modi®ed with step (b) being rewritten as follows: C6 H12 C6 H12 ; C6 H12
C6 H12ÿ2a
aH2 :
(3)
k KKA PaH2 PMCP
PaH2 1 KA PMCP KKA PMCP 2
;
(4)
where k k 1 ÿ 0H or k k0H, respectively, 3. H2 involved in the C±C bond rupture
Scheme 1
C6 H12 n 1 C6 H12ÿn nH
kKKA PH2 PMCP ; PaH2 1 KA PMCP KKA PMCP
2. or H* involved in the C±C bond rupture r
C6 H12ÿ n1 C6 H12ÿn H
r
213
(b1) (b2)
This means that the amount of adsorbed MCP is comparable to C6 H12ÿn , and its effect on the reaction cannot be neglected. According to this reaction scheme, the following rate equations were derived: 1. H2 involved in the C±C bond rupture
k KKA Pa1 H2 PMCP
PaH2 1 KA PMCP KKA PMCP 2
;
(5)
where k k 1 ÿ 0H . In all the equations above, KA KA 1 ÿ 0H . KA and K are the equilibrium constants of steps (b1) and (b2), respectively. k is the rate constant of step (c). In the derivation of the above equations, the following equation was used [31,32]: H 0H 1 ÿ C6 H12 ÿ C6 H12ÿ2a ; where 0H and H are the hydrogen coverage in the absence and in the presence of MCP, respectively. C6 H12 and C6 H12ÿ2a are the coverage of C6 H12 and C6 H12ÿ2a , respectively. 0H does not change with hydrogen pressure signi®cantly on EuroPt-1. @ln 0H =@ln PH2 presents a value between 0.05 and 0.09 as measured previously [21,26]. Thus, both 0H and 1 ÿ 0H were treated as constant with a variety of hydrogen pressure from 60 to 760 Torr at a ®xed reaction temperature. The simulation was performed by a modi®ed Marquardt method [36]. Since the reaction rate exhibits a maximum as a function of hydrogen pressure, which may contain more information about the reaction mechanism, the sum of square of residues, (estimated rateÿexperimental rate)2, was selected as the objective function. The simulation shows that Eq. (4) does not ®t with the data as well as Eq. (3) and Eq. (5). The sum of square of residues for Eq. (4) is several times larger than that of Eqs. (3) and (5). Furthermore at 496 and 513 K, the value of KA and K is negative, which has no physical meaning. Thus, Eq. (4) should be ruled out from the rival rate equations. Fig. 8 shows how Eqs. (3) and (4) ®t with the formation rate of MPs obtained at 473 K. A deviation appears at high MCP pressure between these two equations. The parameter value estimated according to Eqs. (3) and (5) is given
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Fig. 8. Model fitness with the formation rate of MPs obtained at 473 K: (*) experimental data; (- - -) Eq. (3); and (ÐÐÐ) Eq. (5).
Table 2 Parameter values estimated by model fitting for the reaction forming MPs (and nH) Temperature (K)
473
496
k KA K a
7.22 (2.67) 35.4 (166) 0.0233 (0.0148) 2.80 (2.52)
17.9 (5.56) 31.2 (46.4) 0.115 (0.170) 2.40 (2.04)
k* KA K a
32.8 (12.7) 11.0 (20.4) 0.0187 (0.0150) 2.44 (2.26)
83.9 (26.5) 14.9 (17.2) 0.057 (0.094) 2.05 (1.71)
in Table 2. The value of KA decreases with increasing temperature, indicating that MCP adsorption is an exothermic process. The value of K increases with increasing temperature, showing that the dehydrogenation is an endothermic process. Increasing temperature leads to a decrease in the value of a, which may indicate that fewer hydrogen atoms are required to be released before the C±C bond rupture at a relatively high temperature. The value of a for the
513 47.3 (17.4) 22.6 (18.4) 0.368 (0.858) 2.42 (2.06) 211 (74.2) 10.4 (8.60) 0.18 (0.408) 1.98 (1.66)
Eq. (3)
Eq. (5)
reaction forming nH is about 0.3 lower than that of the reaction forming MP, probably indicating that fewer hydrogen atoms are lost before the C±C bond rupture in the former reaction [2,3]. As both Eqs. (3) and (5) ®t the data well, it may be concluded that the C±C bond rupture involves a molecular hydrogen. The estimated activation energy and enthalpy are presented in Table 3 for Eqs. (3) and (5). No signi®cant difference in activation energy was found for the
Table 3 Activation energy and enthalpy (kJ/mol) estimated based on Table 2 for the reaction forming MPs (and nH)
Eq. (3) Eq. (5)
E for k or k*
H for KA
H for K
H for KA K
93.4 (92.0) 92.8 (87.0)
ÿ21.7 (ÿ111) 0 (ÿ43.5)
138 (205) 113 (166)
116 (94.0) 112 (124)
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C±C bond rupture between the reaction forming nH and that forming MPs with a given rate equation. A signi®cant difference in enthalpy exists for the MCP adsorption and dehydrogenation between the two parallel reactions no matter what rate equation was employed. For example with Eq. (3), the apparent adsorption heat is 21.7 kJ/mol in the reaction forming MPs and 111 kJ/mol for the reaction forming nH; the enthalpy for dehydrogenation is 138 kJ/mol for the former reaction and 205 kJ/mol for the latter reaction. These results indicate that different adsorption±dehydrogenation mechanism is probably involved for these two parallel reactions. A dissociative adsorption mechanism as shown previously (Scheme 1) for a series of successive adsorption±dehydrogenation of MCP and a reactive adsorption mechanism have been proposed for alkane adsorption±dehydrogenation process [1±4,37]. The reactive adsorption mechanism may be expressed as C6 H12 H C6 H11 ÿH2 ; C6 H11 ÿH2 C6 H11 H2 ; followed by a series of dehydrogenation steps until the C±C bond rupture. A series of arguments in favour of the reactive adsorption mechanism for alkane adsorption±dehydrogenation has been addressed recently by Frennet [37] on the basis of experimental data. Concerning this work, we suggest that the reaction forming nH may involve the reactive adsorption mechanism. The form of the rate equation does not change with different adsorption mechanism. But once the reactive adsorption mechanism is applied, KA KA 0H . It should be stressed that with dissociative adsorption mechanism the real adsorption heat of MCP is higher than the value presented in Table 3, and with the reactive adsorption mechanism, it is lower than the value shown in the same table. 4.2. Transient kinetics More information about the reaction mechanism is contained in the response curve from the transient kinetic studies. The characterisation of the response models during a step change of reactant concentration has been conducted by Kobayashi [18]. With the technique used in this work, an overshoot response at 60 Torr H2 and a monotonic increase at 750 Torr H2
215
may be explained by a reaction model in which the adsorption between hydrogen and MCP is competitive, the C±C bond rupture is rate control involving other surface species such as an adsorbed hydrogen atom, a vacant site, or a molecular hydrogen associated with a vacant site. During a step change up of MCP, the coverage of adsorb-dehydrogenated MCP increases while the other surface species decreases with time. If the coverage of the adsorb-dehydrogenated MCP exceeds a certain value, a maximum rate can be expected because the rate is proportional to the product of the coverage of the adsorb-dehydrogenated MCP and the other surface species. A high coverage of the adsorb-dehydrogenated MCP can be obtained at a low hydrogen pressure and a high temperature because the adsorbed MCP easily dehydrogenates. As the coverage of the adsorb-dehydrogenated MCP increases with an increase of temperature at a given hydrogen pressure, the overshoot response should become more signi®cant. However, it is surprising that the height of the overshoot response decreases with increasing temperature (see Fig. 6). This means that the mechanism for the C±C bond rupture involving an adsorbed hydrogen atom or a vacant site is unlikely, which is consistent with the conclusion drawn from the steady state kinetic modelling. A possible explanation is that the C±C bond rupture may involve an adsorbed molecular hydrogen (H2 ) at low temperature, while at a high temperature a molecular hydrogen directly from the gas phase (H2) may, at least partly, participate in the C±C bond rupture. According to Kobayashi [18], a more signi®cant overshoot response could be expected if the formation rate of H2 is low compared to that of MCP adsorption and dehydrogenation. A less signi®cant overshoot response for MPs may be attributed to the difference in MCP adsorption and dehydrogenation mechanism, or/and H2 participating in the C±C bond rupture to a lesser extent compared to the reaction forming nH. It is also possible that it is the nH conversion that causes the overshoot response of MPs because of a high concentration of nH in the reactor at initial time of a step change up of MCP. The ratio 2MP/3MP changing with time initially during a step change up of MCP at 60 Torr hydrogen may support such an assumption, but one should note that the maximum of MPs appears earlier than that of nH.
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An important ®nding appeared in this work. The ratio 2MP/nH changes with time during a step change up or down of MCP, the value less than 1 was often observed (Figs. 5±7). Both the selective and non-selective mechanisms cannot explain this observation. It may be accounted for by the partial selective mechanism in which more nH is produced. However according to Maire et al. [2,3,6], this mechanism is more effective at higher temperatures. This contradicts the results we observed here that the ratio 2MP/nH was smaller at lower temperature. Thus a parallel reaction model, one producing nH and the other producing MPs must be considered. The results from the desorption of the products during a step change down of MCP strongly support this model because the responses of MPs and nH follow different curves, indicating that different intermediates are involved for these two reactions. Unfortunately we are unable to rule out the possibility that part of nH may be produced from the reaction forming MPs and/or part of MPs from the reaction forming nH. It should be pointed out that during a step change of MCP, the surface environment of active sites changes, the activity of the active sites changes either. This can be evidenced by Fig. 7, where due to the readsorption of the product, the product desorption becomes fast with an increase of MCP steady state conversion. The change of surface environment, in fact, restricted our effort to draw more information about the reaction mechanism from the transient response curves obtained in this work. 5. Conclusion Steady state and transient kinetic studies were carried out on a well-characterised Pt/SiO2 catalyst. The results can be explained by two parallel reactions: one forming nH and the other forming MP. A reactive adsorption mechanism for the reaction forming nH and a dissociative adsorption mechanism for the reaction forming MPs were proposed to be involved in MCP adsorption±dehydrogenation process. Under the experimental conditions used in this work, the C±C bond rupture is the rate controlling step, in which a molecular hydrogen associated with an active site is most likely involved.
Acknowledgements The research was ®nancially supported by stimulation contract ST2J-0467-C(TT) with EU. YZ thanks the foundation ``van Buuren'' for a research grant. Bene®cial discussions with Dr. A. Crucq and Dr. L. Guczi are gratefully acknowledged.
References [1] A. Frennet, Catal. Rev.-Sci. Eng. 10 (1974) 34. [2] F.G. Gault, Adv. Catal. 30 (1981) 1. [3] G. Maire, F. Garin, in: J.R. Anderson, M. Boudart (Eds.), Catalysis ± Science and Technology, vol. 6, Springer, Berlin, 1984, p. 161. [4] L. Guczi, A. Frennet, V. Ponec, Acta Chim. Hung. 112 (1983) 127. [5] M. Chow, S.H. Park, W.M.H. Sachtler, Appl. Catal. 19 (1985) 349. [6] G. Maire, G. Plouidy, J.C. Prudhomme, F.G. Gault, J. Catal. 4 (1965) 556. [7] J.-M. Dartiques, A. Chambellan, F.G. Gault, J. Am. Chem. Soc. 98 (1976) 856. [8] F.G. Gault, V. Amir-Ebrahimi, F. Garin, P. Parayre, F. Weisang, Bull. Soc. Chim. Belg. 88 (1979) 475. [9] R. Kramer, H. Zuegg, J. Catal. 80 (1983) 446. [10] R. Kramer, H. Zuegg, J. Catal. 85 (1984) 330. [11] Z. PaaÁl, Catal. Today 2 (1988) 595. [12] E. Fulop, V. Gnutzmann, Z. PaaÁl, V. Vogel, Appl. Catal. 66 (1990) 319. [13] O.V. Bragin, Z. Karpinski, K. Matusek, Z. PaaÁl, P. TeÂteÂnyi, J. Catal. 56 (1979) 216. [14] R. Kramer, M. Fischbacher, J. Mol. Catal. 51 (1989) 247. [15] K. Tamaru, in: J.R. Anderso, M. Boudart (Eds.), Catalysis ± Science and Technology, vol. 9, Springer, Berlin, 1991, p. 88. [16] A. Frennet, in: R.W. Joyner, R.A. van Santen (Eds.), Elementary Reaction Steps in Heterogenous Catalysis, Kluwer Academic Publishers, 1991, p. 423. [17] J.L. Margitfalvi, P. Szedlacsek, M. Hegedus, F. Nagy, Appl. Catal. 15 (1985) 69. [18] M. Kobayashi, Chem. Eng. Sci. 37 (1982) 403. [19] G.C. Bond, P.B. Well, Appl. Catal. 18 (1985) 221, 225. [20] J.W. Geus, P.B. Wells, Appl. Catal. 18 (1985) 231. [21] A. Frennet, P.B. Wells, Appl. Catal. 18 (1985) 243. [22] P.B. Well, Appl. Catal. 18 (1985) 259. [23] G.C. Bond, F. Garin, G. Maire, Appl. Catal. 41 (1988) 313. [24] G.C. Bond, Z. PaaÁl, Appl. Catal. A 86 (1992) 1. [25] T. Earl Jordan, Vapor Pressure of Organic Compounds, Interscience Publishers, New York, 1954. [26] A. Crucq, L. Degols, G. Lienard, A. Frennet, Acta Chim. Acad. Sci. Hung. 111 (1982) 547. [27] M. Vaarkamp, P. Dijkstra, J. van Grondelle, J.T. Miller, F.S. Modica, D.C. Koningsberger, R.A. van Santen, J. Catal. 151 (1995) 330.
Y. Zhuang, A. Frennet / Applied Catalysis A: General 177 (1999) 205±217 [28] G. Leclercq, L. Leclercq, R. Maurel, Bull. Soc. Chim. Belg. 88 (1979) 599. [29] G.C. Bond, R.H. Cunningham, E.L. Short, in: L. Guczi, F. Solymosi, P. TeÂteÂnyi (Eds.), New Frontiers in Catalysis ± Proceedings of the Tenth International Congress on Catalysis, vol. 2, Budapest, 1992, p. 849. [30] S.B. Shang, C.N. Kenney, J. Catal. 134 (1992) 134. [31] A. Frennet, G. Lienard, A. Cruck, L. Degols, J. Catal. 53 (1978) 150.
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[32] A. Frennet, in: Z. PaaÁl, P.G. Menon (Eds.), Hydrogen Effect in Catalysis, Marcel Dekker, New York, 1988, p. 399. [33] R. Larsson, Catal. Lett. 11 (1991) 137. [34] B.S. Gudkov, L. Guczi, P. Tetenyi, J. Catal. 74 (1982) 207. [35] J.H. Sinfelt, Catal. Lett. 9 (1991) 159. [36] D.W. Marquart, J. Soc. Ind. Appl. Math. 11 (1963). [37] A. Frennet, Catal. Today 12 (1992) 131.
Kinetic studies of methylcyclopentane ring opening on EuroPt-1 (Pt/SiO2) Yiping Zhuang*, Alfred Frennet1 Catalyse HeÂteÂrogeÁne, Universite Libre de Bruxelles, CP243, 1050 Brussels, Belgium Received 23 April 1998; received in revised form 17 July 1998; accepted 17 July 1998
Abstract Kinetic studies of methylcyclopentane (MCP) ring opening on EuroPt-1 (a 6.3 wt% Pt/SiO2) were carried out under both steady state and concentration step change conditions. At steady state, the activity changes monotonically on increasing MCP pressure and exhibits a maximum with hydrogen pressure. The product ratio of 2-methylpentane (2MP) to n-hexane (nH) increases with increasing MCP pressure and decreases with hydrogen pressure. No change was observed for the product ratio of 2MP to 3-methylpentane (3MP). Following a step change up of MCP pressure from 0 to ca. 9 Torr, an overshoot response at 60 Torr H2 and a monotonic increase at 750 Torr H2 were observed for all the products. The height of the overshoot response of nH is more signi®cant than that of 2MP and 3MP and decreases as the temperature increases. Both 2MP/nH and 2MP/3MP change with time during a step change up of MCP pressure. A maximum for 2MP/3MP and a value less than unity for 2MP/nH were observed. The results were discussed on the basis of a parallel reaction model: one forming nH and the other forming MPs (2MP3MP). A reactive adsorption mechanism for the reaction forming nH and a dissociative adsorption mechanism for the reaction forming MPs were proposed for the MCP adsorption±dehydrogenation process. A molecular hydrogen associated with an active site is most likely involved in the C±C bond rupture, which is concluded to be the rate-controlling step in this work. # 1999 Elsevier Science B.V. All rights reserved. Keywords: EuroPt-1 (Pt/SiO2); Methylcyclopentane ring opening; Steady state and transient kinetics; Reaction mechanism
1. Introduction Alkane hydrogenolysis on metal catalysts has been a long-time interesting research subject due to its importance in petroleum industry and fundamental *Corresponding author. Present address: Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1. Tel.: +519-8851211-2879; fax: +519-7464979; e-mail: [email protected] 1 Senior Research Associate, National Fund for Scientific Research, Belgium.
science. It is generally accepted that the reaction passes through three consecutive steps: alkane adsorption±dehydrogenation, C±C bond rupture and product hydrogenation±desorption. However, many arguments are provided in the literature for the mechanism involved in each step [1±4]. Methylcyclopentane (MCP) is an important probe molecule in the exploration of the properties of a catalyst surface. The possible products for MCP reaction with hydrogen are n-hexane (nH), 2-methylpentane (2MP) and 3-methylpentane (3MP) from ring opening (MCP hydrogenolysis) as well as cyclohex-
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00261-0
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ane and benzene from ring enlargement. Metal facilitates MCP ring opening and acid sites on supports promote MCP ring enlargement [5]. Much attention was paid to MCP ring opening on platinum catalysts because a methylcyclopentane-like intermediate was proposed to be involved in the isomerisation reaction of nH and its isomers [2,3]. It was found that the product distribution of MCP ring opening is quite sensitive to the properties of the catalyst surface. Numerous reaction mechanisms were proposed to correlate the observed experimental results [2,3,6±13]. Gault and coworkers [7,8] postulated two reaction mechanisms for MCP ring opening on platinum catalysts. On a catalyst with high Pt dispersion, the nonselective mechanism dominates, producing nH, 2MP and 3MP in a statistical ratio of 2:2:1. A diadsorbed species on a metal site was proposed to be the intermediate. When the particle size of platinum is larger than 1.8 nm, the selective mechanism prevails producing 2MP and 3MP only. The intermediate was considered to be a tetra-adsorbed species on two metal sites. The product distribution obtained in some cases cannot be accounted for by a simple combination of the above two mechanisms. A partially selective mechanism producing more nH was proposed by Maire et al. [3,6]. This mechanism is more effective at high temperature. Kramer and Zuegg [9,10] and also PaaÁl [11] found that the selectivity towards nH increases with the interface length between the Pt and support. They postulated that two parallel reactions take place: one mainly producing 2MP and 3MP on the Pt surface and the other forming additional nH on the interface. The pairs of the metallic Pt0 and ionic Pt sites were suggested to be the active sites for the latter reaction [14]. Although extensive work has been carried out for MCP ring opening on platinum catalysts, no work was performed under transient conditions. It was demonstrated that more information about the reaction mechanism could be provided by transient kinetic studies, because the elemental steps of a reaction may be kept from equilibrium during a transient procedure [15±18]. The effect of these steps on the overall reaction can be derived from the product response [18]. In this work, MCP ring opening was studied in a ¯ow reaction system under both steady state and transient conditions. The catalyst used is
EuroPt-1. It is a reference catalyst and has been extensively characterised by several European laboratories [19±24]. The catalyst contains 6.3 wt% Pt on a silica support and has a dispersion of ca. 60%. The transient condition was realised by suddenly introducing MCP or stopping its supply into a reaction stream with a ®xed hydrogen concentration. The effects of temperature, H2 and MCP pressures on the reaction were investigated and some interesting results are reported here. 2. Experimental 2.1. Catalyst and reactants The catalyst used is EuroPt-1 (6.3 wt% Pt/SiO2). Before the reaction, the catalyst was treated in situ in a 50 ml/min ¯ow of 10% O2/Ar at 623 K for 1 h and then of H2 at 573 K for 1 h, to eliminate possible contaminants such as carbon and oxygen on the catalyst surface. MCP (purity>99%) was purchased from Fluka. The major impurity is nH (0.64%) as measured by GC. Hydrogen (H2 5.5) was purchased from UCAB and helium (grade A) from BOC. 2.2. Apparatus and activity measurement The reaction was performed in a ¯ow reaction apparatus at atmospheric pressure. The reaction stream could be composed of three gas streams: hydrogen, helium, and hydrogen or helium. By switching a 4-way valve, the ®nal gas could pass through or pass by a saturator ®lled with liquid MCP. The transient process is therefore initialised. The rotameters calibrated by a soap ®lm ¯ow meter were used to measure the ¯ow rate of each gas stream. The saturator was inserted in a liquid thermostat at 273 K. Equilibrium between liquid MCP and its vapour was always kept within a range of the gas ¯ow between 5 and 50 ml/min through the saturator. The partial pressure of MCP at the inlet of the reactor was then calculated by the following formula PMCP Torr
P0MCP ; P0MCP =760 Ft =Fs 1 ÿ P0MCP =760 (1)
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where Ft is the total gas ¯ow rate, Fs the gas ¯ow rate that passes through the saturator, and P0MCP is the vapour pressure of MCP, which is 43.53 Torr at 273 K [25]. The catalyst was placed in a U-type Pyrex glass reactor with an inside diameter of 4 mm. The reactor was heated by an electrical furnace. The temperature was measured by a chromel±alumel thermocouple just bound to the outside of the reactor. The desired temperature was maintained by Eurotherm 808, a temperature controller. A multiposition valve (ST16P, Valco) with 16 loops of 0.5 ml was placed just after the reactor, in which 15 of the loops can be used to store the ef¯uent gases. The products were analysed by a gas chromatograph (HP5890), which was equipped with a 50 m fused silica PLOT Al2O3/KCl capillary column and a TCD detector. Both the gas chromatograph and multiposition valve were controlled by an IBM compatible personal computer through the HP Chemstation Program. In order to avoid hydrocarbon condensation, the tubes and valves through which MCP was ¯owing were heated to ca. 368 K and the multiposition valve to 423 K. For a transient process, the time zero was de®ned at the time when the hydrocarbon was detected in the ef¯uent gases after a step change up of MCP or when the concentration of the hydrocarbon begins to decrease during a step change down of MCP. The real time zero is unknown, because the time interval was found to suffer an error of 3 s from the time to switch the 4-way valve for a step change of MCP to the time at which the change of product concentration was detected in the ef¯uent gases with an empty reactor. The residence time of the reaction system was measured at a total gas ¯ow rate of 50 ml/min by a step change up or down of MCP pressure from 0 to 9 Torr. It is about 9 s no matter whether the reactor was empty or ®lled with glass beads. A longer residence time of about 12 s was observed if the multiposition valve was only heated to 373 K. The formation rate of products was calculated by the following equation rj F Cj;out ÿ Cj;in =A;
207
¯ow rate, Cj,in and Cj,out are the jth product concentration at the inlet and the outlet of the reactor, respectively. The conversion rate of MCP was obtained by summation of the formation rate of all the products. The product selectivity was expressed as the product ratio of 2MP/3MP and 2MP/nH as documented in the literature [6±13]. The time on stream in one reaction run was less than 8 min. The hydrogen was always kept ¯owing through the catalyst bed between two successive runs for at least 5 min. Under these conditions, no signi®cant deactivation of the catalyst was observed. 3. Results 3.1. Steady state kinetics 3.1.1. Activity Fig. 1 shows the effect of hydrogen pressure on activity, which was measured at ca. 9 Torr MCP. A maximum was reached at ca. 90 Torr hydrogen at 473 K and its location shifted to higher hydrogen pressure as the temperature increased. Similar results can be found in the literature for both MCP ring opening [27] and other alkane hydrogenolysis on Pt catalysts [28±32]. The apparent activation energy was derived based on the data presented in Fig. 1. It is a strong function
(2)
where A is the number of surface platinum atoms per gram catalyst. It is 2.341020 for EuroPt-1 as measured by hydrogen adsorption [21,26]. F is the total
Fig. 1. Effect of hydrogen pressure on the rate of MCP conversion at ca. 9 Torr MCP: () 473; (^) 496; and (*) 513 K.
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surface must be signi®cantly covered by hydrocarbon species and the MCP adsorption is not the rate controlling step. By changing the amount of the catalyst at a given total gas ¯ow rate, the effect of MCP conversion on activity and selectivity was measured. No signi®cant effect was observed with a variation of MCP conversion from 7.1% to 31.6% at 9.2 Torr MCP, 150 Torr hydrogen and 496 K. This result is consistent with the above observation that the reaction exhibits a low reaction order with respect to MCP pressure.
Fig. 2. Effect of MCP pressure on the rate of MCP conversion at ca. 740 Torr H2: () 473; (^) 496; and (*) 513 K.
of hydrogen pressure, and increases monotonically from ca. 90 kJ/mol at 60 Torr H2 to ca. 220 kJ/mol at 750 Torr H2, a value much higher than that required for dehydrogenation and hydrogenation of hydrocarbon on Pt catalyst [4]. This may indicate that the MCP dehydrogenation and product hydrogenation are not the rate controlling step in this work. As depicted in Fig. 2, the reaction rate increases with increasing MCP pressure at about 740 Torr hydrogen. The reaction order with respect to MCP pressure, as presented in Table 1, increases as hydrogen pressure increases, indicating that hydrogen inhibits the adsorption of MCP [32]. As the reaction order with respect to MCP pressure is much smaller than unity even at about 740 Torr hydrogen, the catalyst
3.1.2. Selectivity By using a pulse-microcatalytic reactor, PaaÁl [11] observed that both the product ratio 2MP/nH and 2MP/3MP changed with reaction temperature, and a maximum appears for the ratio 2MP/nH. In our work, the ratio 2MP/nH changes with the reaction conditions. While the ratio 2MP/3MP keeps constant. The value is ca. 3, which is consistent with that reported by Vaarkamp et al. [27]. Fig. 3 shows the temperature effect on the ratio 2MP/nH, which was obtained at ca. 9 Torr MCP. The deviation, which was derived from at least four independent experiments, is also presented in this ®gure. From this ®gure one may note that the ratio 2MP/nH is not only a function of temperature, but also a function of hydrogen pressure. At 750 Torr hydrogen, the ratio 2MP/nH is about 1.2, and does not change signi®cantly with temperature. At 150 Torr hydrogen a
Table 1 The reaction order with respect to MCP pressure (r kP MCP ; P MCP varies from ca. 5 to 45 Torr) Temperature (K)
H2 pressure (Torr) 60
150
740
473 496 513
0.050.02 n* n
0.260.02 n n
0.370.03 0.590.04 0.590.03
n*: No measurement.
Fig. 3. Effect of temperature on product ratio 2MP/nH at ca. 9 Torr MCP: (*) 60 () 150; and (^) 750 Torr H2.
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Fig. 4. Effect of MCP pressure on product ratio 2MP/nH: 473 K ((&) 150 and (*) ca. 740 Torr H2), 496 K (() ca. 740 Torr H2), 513 K ((^) ca. 740 Torr H2).
maximum is observed at ca. 480 K and at 60 Torr hydrogen it decreases as the temperature increases. The effect of MCP pressure on the product distribution is seldom reported in the literature. As shown in Fig. 4, the ratio 2MP/nH increases signi®cantly with MCP pressure. A value equal to 1, corresponding to the non-selective mechanism, could only be observed with an extrapolation of MCP pressure to zero. 3.2. Transient kinetics The transient kinetic studies were realised by suddenly introducing MCP or stopping its supply into the reaction stream as previously mentioned in Section 2. The total gas ¯ow rate was kept around 50 ml/min and the MCP pressure around 9 Torr at steady state. As only a 1.3% change of the total volumic ¯ow rate was caused by introducing MCP or stopping its supply into the system, its effect on the reaction was neglected in this work. 3.2.1. MCP pressure step change up Before introducing MCP into the reactor, the catalyst was equilibrated with a given hydrogen pressure. During the step change up of MCP pressure from 0 to ca. 9 Torr, the concentration of hydrocarbons in the ef¯uent was monitored as a function of time. Fig. 5
209
depicts the results obtained at 473 K, where p(t) is the partial pressure of hydrocarbon at time t and p(st) is the steady state value. A monotonic response at 750 Torr H2 (Fig. 5(a)) and an overshoot response at 60 Torr H2 (Fig. 5(b)) were observed for all of the products. A signi®cant difference exists between MP (2MP or 3MP) and nH responses. This difference is accentuated at 60 Torr hydrogen: the height of p(t)/ p(st) is ca. 1.5 for 2MP and 3MP, while it is ca. 6 for nH. The maximum height of the overshoot responses for nH and MP do not occur at the same time, and it is about 6 s later for nH. The responses of 2MP and 3MP seem to follow the same curve as shown in Fig. 5(a) and (b). While by plotting mole ratio of the products, a signi®cant difference between 2MP and 3MP can also be found (Fig. 5(c) and (d)), a maximum of 2MP/3MP appears at 60 Torr hydrogen. The product ratio 2MP/ nH changes with the reaction conditions under steady state conditions, however, the value is never smaller than 1. In contrast, during the step change up of MCP pressure from 0 to ca. 9 Torr, a value of 2MP/nH less than 1 is observed at 750 Torr hydrogen (Fig. 5(c)). Even more interesting at 60 Torr hydrogen, the ratio 2MP/nH ®rst increases from 1 to 2 then decreases to 0.62, and ®nally increases again to a steady state value of 3 (Fig. 5(d)). One may note that MCP appears later than the products in the ef¯uent. This becomes more apparent at 60 Torr H2, where no MCP was detected in the ®rst three samples. This result again con®rms that MCP adsorption is not the rate controlling step, and that hydrogen inhibits MCP adsorption. For comparison purposes, the results obtained at 458 and 496 K at 60 Torr H2 are shown in Fig. 6. These results are quite similar to Fig. 5(b) and (d). However, as the temperature increases, the overshoot response becomes less signi®cant. The maximum value of 2MP/3MP decreases from 3.8 to 3.2 and the ratio 2MP/nH is no longer lower than 1 at 496 K as observed at 458 and 473 K. 3.2.2. MCP pressure step change down After the steady state was reached, the supply of MCP was suddenly stopped and the hydrocarbon concentration at the outlet of the reactor was monitored as a function of time. Fig. 7 shows the typical results that were obtained at 496 K. The responses of 2MP and 3MP follow the same curve but differ from nH. Such a difference becomes more signi®cant as the
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Fig. 5. Product response after a MCP step change up from 0 to ca. 9 Torr at 473 K: (a) and (c) 750 Torr H2, (b) and (d) 60 Torr H2. (*) 2MP, () 3MP, (*) nH, () MCP, (^) 2MP/nH, and (}) 2MP/3MP.
hydrogen pressure decreases (Fig. 7(a) and (b)). The ratio 2MP/3MP does not change signi®cantly with time, while the ratio 2MP/nH decreases from the steady state value to a value less than 1 (Fig. 7(d) and (e)). Comparing Fig. 7(b) and (c), one may note that the MCP conversion affects the desorption of the products. Although the steady state rate does not change with a high MCP conversion, the product desorption becomes fast. The apparent fast desorption of the products may be attributed to the readsorption of the products. At steady state the adsorption of the product can be neglected because the adsorption of MCP is stronger than its products and further the concentration of MCP is high in the reactor. While during a step change down of MCP, the product
concentration in the gas phase becomes relatively high after a certain time with a high MCP conversion at steady state. The active sites originally occupied by MCP are now available for product readsorption. The readsorption of nH may be easier than its isomers, because the ratio 2MP/nH is no longer less than 1 as shown in Fig. 7(f). Similar results were also observed at 60 Torr hydrogen. It is worth mentioning that no quantitative difference was observed for MCP decay under different reaction conditions, with or without the catalyst in the reactor. The amount of MCP desorbed from the catalyst surface should be small compared with the amount of MCP in the gas phase. Therefore, its effect on MCP decay is invisible.
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211
Fig. 6. Product response after a MCP step change up from 0 to ca. 9 Torr at 60 Torr H2. (a) and (c) 458 K, (b) and (d) 496 K. (*) 2MP, () 3MP, (*) nH, ()MCP, (^) 2 MP/nH, (}) 2MP/3MP and Wcat0.86 g.
4. Discussion 4.1. Steady state kinetics It is generally accepted that alkane hydrogenolysis on metal catalysts takes place through three consecutive steps: alkane adsorption±dehydrogenation; the C± C bond rupture and product hydrogenation±desorption. All the steps have been proposed to be rate controlling, depending on the reaction conditions and the nature of the catalyst used [28±35]. In this work, the reaction rate is controlled by the C±C bond rupture because the reaction order with respect to MCP pres-
sure is less than unity and the apparent activation energy observed is higher than that required for hydrogenation and dehydrogenation of alkanes. This conclusion is further supported by the fact that no MCP was detected in the initial ef¯uent gases during the MCP step change up. The volcano dependence in activity on hydrogen pressure was often observed for alkane hydrogenolysis on metal catalysts. This can be accounted for by two possible reaction mechanisms: the C±C bond rupture involving a molecular hydrogen (Eley±Rideal mechanism) [28±30], or the C±C bond rupture involving a vacant site or an adsorbed hydrogen atom
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Fig. 7. Product response at different hydrogen pressure and MCP conversion after a MCP step change down from ca. 9 to 0 Torr at 496 K: (a) and (d) 750 Torr H2, with 16.3% MCP conversion; (b) and (e) 60 Torr H2 with 7.1% MCP conversion; (c) and (f) 60 Torr H2 with 31.6% MCP conversion. (*) 2MP, () 3MP, (*) nH, ()MCP, (^) 2MP/nH, and (}) 2MP/3MP.
(Langmuir±Hinshelwood mechanism) [4,27,34]. The ®rst one cannot predict a maximum activity with a variation of hydrocarbon pressure while the second one does. As in most cases only a monotonic increase in activity was observed with increasing hydrocarbon pressure, the Eley±Rideal mechanism was widely accepted [28±30]. However, due to the fact that the experiments were often carried out at a high reactant ratio of hydrogen to alkane, a maximum activity may not be observed with a limited range of alkane concentration. In other words, even such a maximum is observed, it is still dif®cult to conclude that the Langmuir±Hinshelwood mechanism takes place because a decrease in activity may result from catalyst deactivation caused by carbon deposition during reaction.
In order to draw more information from the steady state kinetic data, kinetic modelling was performed. A parallel reaction model, one producing MPs and the other forming nH, was considered, and different active sites are assumed to be involved for these two reactions [9,10,14]. Both the steady state and transient kinetic results obtained in this work support such a model. The reaction scheme may be written as: H2 2 2H ; C6 H12
C6 H12ÿ2a
(a) aH2 ;
(b)
C6 H12ÿ2a H2 ! C6 H14ÿ2a ;
(c)
C6 H14ÿ2a aH2 ! C6 H14 ;
(d)
where is a vacant active site. In this scheme, step
Y. Zhuang, A. Frennet / Applied Catalysis A: General 177 (1999) 205±217
(b) may not be an elementary one but a sum of a series of successive dehydrogenation equilibrium steps, such as
C6 H12 2 C6 H11
C6 H11
C6 H10
H ;
H ;
.. .
r
ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ (b0 )
0
By combining step (b ) with step (a) and letting n/2a, one can have step (b). As only step (b) is considered in the reaction scheme, the amount of the surface species C6 H11 , C6 H10 C6 H12ÿ n1 should be small compared to C6 H12ÿn , and does not affect the reaction rate. The C±C bond rupture may take place at any stage of the dehydrogenation process [34]. The parameter 2a may be regarded as an average amount of the lost hydrogen atoms of MCP before the C±C bond rupture. The C±C bond rupture, step (c), is rate-controlling, involving possibly a molecular hydrogen H2 or a vacant site , or an adsorbed hydrogen atom H* or a combination form of H2 (equivalent to an adsorbed H2). This reaction scheme has been widely used to simulate the reaction rate of alkane hydrogenolysis including MCP ring opening on metal catalysts [27,29,30]. In our work, it was found that the rate equations derived on the basis of this reaction scheme can account for the effect of hydrogen pressure but not the effect of MCP pressure on the reaction rate. A similar conclusion was also reached for light alkane hydrogenolysis on metal catalysts by Bond et al. [29] and Shang and Kenney [30]. The above reaction scheme was then modi®ed with step (b) being rewritten as follows: C6 H12 C6 H12 ; C6 H12
C6 H12ÿ2a
aH2 :
(3)
k KKA PaH2 PMCP
PaH2 1 KA PMCP KKA PMCP 2
;
(4)
where k k 1 ÿ 0H or k k0H, respectively, 3. H2 involved in the C±C bond rupture
Scheme 1
C6 H12 n 1 C6 H12ÿn nH
kKKA PH2 PMCP ; PaH2 1 KA PMCP KKA PMCP
2. or H* involved in the C±C bond rupture r
C6 H12ÿ n1 C6 H12ÿn H
r
213
(b1) (b2)
This means that the amount of adsorbed MCP is comparable to C6 H12ÿn , and its effect on the reaction cannot be neglected. According to this reaction scheme, the following rate equations were derived: 1. H2 involved in the C±C bond rupture
k KKA Pa1 H2 PMCP
PaH2 1 KA PMCP KKA PMCP 2
;
(5)
where k k 1 ÿ 0H . In all the equations above, KA KA 1 ÿ 0H . KA and K are the equilibrium constants of steps (b1) and (b2), respectively. k is the rate constant of step (c). In the derivation of the above equations, the following equation was used [31,32]: H 0H 1 ÿ C6 H12 ÿ C6 H12ÿ2a ; where 0H and H are the hydrogen coverage in the absence and in the presence of MCP, respectively. C6 H12 and C6 H12ÿ2a are the coverage of C6 H12 and C6 H12ÿ2a , respectively. 0H does not change with hydrogen pressure signi®cantly on EuroPt-1. @ln 0H =@ln PH2 presents a value between 0.05 and 0.09 as measured previously [21,26]. Thus, both 0H and 1 ÿ 0H were treated as constant with a variety of hydrogen pressure from 60 to 760 Torr at a ®xed reaction temperature. The simulation was performed by a modi®ed Marquardt method [36]. Since the reaction rate exhibits a maximum as a function of hydrogen pressure, which may contain more information about the reaction mechanism, the sum of square of residues, (estimated rateÿexperimental rate)2, was selected as the objective function. The simulation shows that Eq. (4) does not ®t with the data as well as Eq. (3) and Eq. (5). The sum of square of residues for Eq. (4) is several times larger than that of Eqs. (3) and (5). Furthermore at 496 and 513 K, the value of KA and K is negative, which has no physical meaning. Thus, Eq. (4) should be ruled out from the rival rate equations. Fig. 8 shows how Eqs. (3) and (4) ®t with the formation rate of MPs obtained at 473 K. A deviation appears at high MCP pressure between these two equations. The parameter value estimated according to Eqs. (3) and (5) is given
214
Y. Zhuang, A. Frennet / Applied Catalysis A: General 177 (1999) 205±217
Fig. 8. Model fitness with the formation rate of MPs obtained at 473 K: (*) experimental data; (- - -) Eq. (3); and (ÐÐÐ) Eq. (5).
Table 2 Parameter values estimated by model fitting for the reaction forming MPs (and nH) Temperature (K)
473
496
k KA K a
7.22 (2.67) 35.4 (166) 0.0233 (0.0148) 2.80 (2.52)
17.9 (5.56) 31.2 (46.4) 0.115 (0.170) 2.40 (2.04)
k* KA K a
32.8 (12.7) 11.0 (20.4) 0.0187 (0.0150) 2.44 (2.26)
83.9 (26.5) 14.9 (17.2) 0.057 (0.094) 2.05 (1.71)
in Table 2. The value of KA decreases with increasing temperature, indicating that MCP adsorption is an exothermic process. The value of K increases with increasing temperature, showing that the dehydrogenation is an endothermic process. Increasing temperature leads to a decrease in the value of a, which may indicate that fewer hydrogen atoms are required to be released before the C±C bond rupture at a relatively high temperature. The value of a for the
513 47.3 (17.4) 22.6 (18.4) 0.368 (0.858) 2.42 (2.06) 211 (74.2) 10.4 (8.60) 0.18 (0.408) 1.98 (1.66)
Eq. (3)
Eq. (5)
reaction forming nH is about 0.3 lower than that of the reaction forming MP, probably indicating that fewer hydrogen atoms are lost before the C±C bond rupture in the former reaction [2,3]. As both Eqs. (3) and (5) ®t the data well, it may be concluded that the C±C bond rupture involves a molecular hydrogen. The estimated activation energy and enthalpy are presented in Table 3 for Eqs. (3) and (5). No signi®cant difference in activation energy was found for the
Table 3 Activation energy and enthalpy (kJ/mol) estimated based on Table 2 for the reaction forming MPs (and nH)
Eq. (3) Eq. (5)
E for k or k*
H for KA
H for K
H for KA K
93.4 (92.0) 92.8 (87.0)
ÿ21.7 (ÿ111) 0 (ÿ43.5)
138 (205) 113 (166)
116 (94.0) 112 (124)
Y. Zhuang, A. Frennet / Applied Catalysis A: General 177 (1999) 205±217
C±C bond rupture between the reaction forming nH and that forming MPs with a given rate equation. A signi®cant difference in enthalpy exists for the MCP adsorption and dehydrogenation between the two parallel reactions no matter what rate equation was employed. For example with Eq. (3), the apparent adsorption heat is 21.7 kJ/mol in the reaction forming MPs and 111 kJ/mol for the reaction forming nH; the enthalpy for dehydrogenation is 138 kJ/mol for the former reaction and 205 kJ/mol for the latter reaction. These results indicate that different adsorption±dehydrogenation mechanism is probably involved for these two parallel reactions. A dissociative adsorption mechanism as shown previously (Scheme 1) for a series of successive adsorption±dehydrogenation of MCP and a reactive adsorption mechanism have been proposed for alkane adsorption±dehydrogenation process [1±4,37]. The reactive adsorption mechanism may be expressed as C6 H12 H C6 H11 ÿH2 ; C6 H11 ÿH2 C6 H11 H2 ; followed by a series of dehydrogenation steps until the C±C bond rupture. A series of arguments in favour of the reactive adsorption mechanism for alkane adsorption±dehydrogenation has been addressed recently by Frennet [37] on the basis of experimental data. Concerning this work, we suggest that the reaction forming nH may involve the reactive adsorption mechanism. The form of the rate equation does not change with different adsorption mechanism. But once the reactive adsorption mechanism is applied, KA KA 0H . It should be stressed that with dissociative adsorption mechanism the real adsorption heat of MCP is higher than the value presented in Table 3, and with the reactive adsorption mechanism, it is lower than the value shown in the same table. 4.2. Transient kinetics More information about the reaction mechanism is contained in the response curve from the transient kinetic studies. The characterisation of the response models during a step change of reactant concentration has been conducted by Kobayashi [18]. With the technique used in this work, an overshoot response at 60 Torr H2 and a monotonic increase at 750 Torr H2
215
may be explained by a reaction model in which the adsorption between hydrogen and MCP is competitive, the C±C bond rupture is rate control involving other surface species such as an adsorbed hydrogen atom, a vacant site, or a molecular hydrogen associated with a vacant site. During a step change up of MCP, the coverage of adsorb-dehydrogenated MCP increases while the other surface species decreases with time. If the coverage of the adsorb-dehydrogenated MCP exceeds a certain value, a maximum rate can be expected because the rate is proportional to the product of the coverage of the adsorb-dehydrogenated MCP and the other surface species. A high coverage of the adsorb-dehydrogenated MCP can be obtained at a low hydrogen pressure and a high temperature because the adsorbed MCP easily dehydrogenates. As the coverage of the adsorb-dehydrogenated MCP increases with an increase of temperature at a given hydrogen pressure, the overshoot response should become more signi®cant. However, it is surprising that the height of the overshoot response decreases with increasing temperature (see Fig. 6). This means that the mechanism for the C±C bond rupture involving an adsorbed hydrogen atom or a vacant site is unlikely, which is consistent with the conclusion drawn from the steady state kinetic modelling. A possible explanation is that the C±C bond rupture may involve an adsorbed molecular hydrogen (H2 ) at low temperature, while at a high temperature a molecular hydrogen directly from the gas phase (H2) may, at least partly, participate in the C±C bond rupture. According to Kobayashi [18], a more signi®cant overshoot response could be expected if the formation rate of H2 is low compared to that of MCP adsorption and dehydrogenation. A less signi®cant overshoot response for MPs may be attributed to the difference in MCP adsorption and dehydrogenation mechanism, or/and H2 participating in the C±C bond rupture to a lesser extent compared to the reaction forming nH. It is also possible that it is the nH conversion that causes the overshoot response of MPs because of a high concentration of nH in the reactor at initial time of a step change up of MCP. The ratio 2MP/3MP changing with time initially during a step change up of MCP at 60 Torr hydrogen may support such an assumption, but one should note that the maximum of MPs appears earlier than that of nH.
216
Y. Zhuang, A. Frennet / Applied Catalysis A: General 177 (1999) 205±217
An important ®nding appeared in this work. The ratio 2MP/nH changes with time during a step change up or down of MCP, the value less than 1 was often observed (Figs. 5±7). Both the selective and non-selective mechanisms cannot explain this observation. It may be accounted for by the partial selective mechanism in which more nH is produced. However according to Maire et al. [2,3,6], this mechanism is more effective at higher temperatures. This contradicts the results we observed here that the ratio 2MP/nH was smaller at lower temperature. Thus a parallel reaction model, one producing nH and the other producing MPs must be considered. The results from the desorption of the products during a step change down of MCP strongly support this model because the responses of MPs and nH follow different curves, indicating that different intermediates are involved for these two reactions. Unfortunately we are unable to rule out the possibility that part of nH may be produced from the reaction forming MPs and/or part of MPs from the reaction forming nH. It should be pointed out that during a step change of MCP, the surface environment of active sites changes, the activity of the active sites changes either. This can be evidenced by Fig. 7, where due to the readsorption of the product, the product desorption becomes fast with an increase of MCP steady state conversion. The change of surface environment, in fact, restricted our effort to draw more information about the reaction mechanism from the transient response curves obtained in this work. 5. Conclusion Steady state and transient kinetic studies were carried out on a well-characterised Pt/SiO2 catalyst. The results can be explained by two parallel reactions: one forming nH and the other forming MP. A reactive adsorption mechanism for the reaction forming nH and a dissociative adsorption mechanism for the reaction forming MPs were proposed to be involved in MCP adsorption±dehydrogenation process. Under the experimental conditions used in this work, the C±C bond rupture is the rate controlling step, in which a molecular hydrogen associated with an active site is most likely involved.
Acknowledgements The research was ®nancially supported by stimulation contract ST2J-0467-C(TT) with EU. YZ thanks the foundation ``van Buuren'' for a research grant. Bene®cial discussions with Dr. A. Crucq and Dr. L. Guczi are gratefully acknowledged.
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