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Applied Catalysis A: General 335 (2008) 7–14 www.elsevier.com/locate/apcata
Kinetics of o-chlorotoluene hydrogenolysis in the presence of 3%, 5% and 10% Pd/C catalysts Tadeusz Janiak * Faculty of Chemistry, University of Gdan´sk, J. Sobieskiego 18, 80-952 Gdan´sk, Poland Received 24 May 2007; received in revised form 18 October 2007; accepted 25 October 2007
Abstract The kinetics of o-chlorotoluene hydrogenolysis in the presence of 3%, 5% and 10% Pd/C catalysts was experimentally investigated in an alkaline – n-heptane – gaseous hydrogen system. The main product of hydrogenolysis was toluene. Kinetic characteristics for the reaction were obtained at temperatures of 20, 40 and 60 8C at atmospheric pressure of hydrogen for all catalysts; the effectiveness of the latter in ochlorotoluene hydrogenolysis was compared. The pseudo-zero-order rate constants and times of 90% conversion for each of the catalysts at 40 and 60 8C were quite similar. The overall kinetic constants were obtained assuming the Langmuir–Hinshelwood mechanism. The activation energies of o-chlorotoluene hydrogenolysis for these catalysts were calculated. The selectivity of the reacting system was evaluated in competitive experiments with o-chlorotoluene, 1,2-dichlorobenzene and 4-chlorobiphenyl. The stability of the catalysts in a hydrogenolytic environment was tested. # 2007 Elsevier B.V. All rights reserved. Keywords: O-chlorotoluene; Catalytic hydrogenolysis; Palladium on carbon; Kinetics
1. Introduction Chloroaromatic compounds can be converted to parent hydrocarbons over Ru, Rh, Ni, Pd and Pt metal catalysts during hydrogenolysis with gaseous hydrogen [1–4]. Palladium is the most active hydrodechlorination catalyst in such conditions [5]; rhodium is the most effective in transfer hydrogenolysis with propanol-2 as the hydrogen source [6]. Hydrodechlorination of chlorobenzene, chlorotoluenes, chlorobiphenyls and 1,1-bis (4-chlorophenyl)-2,2,2-trichloroethane in the presence of Pd catalyst has been carried out in the gaseous [7–9] and liquid [6,10–33] phases. Hydrogenolysis has been done in the presence of Pd/Al2O3 [7,8,12], Pd/ZrO2 [9], Pd/AlPO4–SiO2 PM2 [17–19], Pd/zeolites [21], Pd/poly(dimethylsiloxane) [22] and Pd/C [6,10,11,13–16,20,23–33] using gaseous hydrogen [7–9,11,13–22,24–33], propanol-2 [6,10–12], potassium formate [11], formic acid [12], hydrazine [12] or hypophosphite [23] as the source of hydrogen. Liquidphase hydrodechlorination has been investigated in one liquid
* Tel.: +48 58 523 53 21; fax: +48 58 523 53 57. E-mail address: [email protected]. 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.10.027
medium with [13–17,19] and without a base [18,20–22], as well as in two liquids [23–33]. In these two liquids – multiphase systems – hydrodechlorination proceeds effectively even under mild conditions, that is, at temperatures up to 50 8C and with hydrogen at atmospheric pressure. Experiments in these systems have been performed in the presence of a transfer phase catalyst (PT) [23–29] and in its absence [32,33]. Our previous papers reported on the results of the hydrodechlorination of chlorobenzene, dichlorobenzenes and chlorotoluenes in a batch-type multiphase reactor. The process was carried out in the presence of a 10% Pd/C catalyst dispersed in an n-heptane or n-hexane solution of the chlorinated compound and an aqueous solution of NaOH. We studied the influence of in situ generated and gaseous hydrogen [32], the stirring rate, NaOH concentration, temperature, and the impact of preconditioning Pd/C on the effectiveness of the hydrogenolysis [33]. Little has been published on the kinetics of the hydrodechlorination of chloroaromatic compounds in multiphase systems [25,29–31]. In this paper, the effect of the palladium content in the catalyst on the kinetics of o-chlorotoluene hydrogenolysis is explored. The investigations on the hydrogenolysis of this model system may be of utilitarian value in that it creates a framework for developing methods of treating organochlorine wastes.
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T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
2. Experimental 2.1. Materials The catalysts – 3% Pd/C, 5% Pd/C and 10% Pd/C, as well as o-chlorotoluene (99.9% purity), p-chlorotoluene (98% purity), 1,2-dichlorobenzene (99% purity) and n-dodecane (>99.9% purity) were purchased from Sigma–Aldrich; the n-heptane (99.5% purity) solvent was from Lab-Scan. 4-chlorobiphenyl was synthesised in our laboratory. Sodium hydroxide (p.a. purity) was supplied by POCH-Gliwice, the nitrogen and hydrogen (>99.5% purity) by Linde. The characteristics of the catalysts are as follows: 3% Pd/C – particle size 10 mm, BET surface area 780 m2 g1; 5% Pd/C – particle size 45 mm, BET surface area 870 m2 g1; 10% Pd/C – particle size 90% <60 mm, 10% <5 mm BET surface area 880 m2 g1. The Pd dispersions determined from hydrogen chemisorption measurements were 0.11 for 3% Pd/C, 0.20 for 5% Pd/C and 0.09 for 10% Pd/C. 2.2. General procedure Experiments were carried out in a 50 cm3 two-walled, threenecked, round-bottomed reactor connected to a gas burette supplying hydrogen (or nitrogen). Thermostatted water was pumped between the reactor walls to control the reaction temperature. In a typical experiment, the dry reactor containing the stirring bar was initially purged with nitrogen for 5 min. The catalyst, previously stored in a desiccator over NaOH pellets, was then pre-conditioned prior to dechlorination by being soaked and stirred for 30 min at 300 rpm in 10 cm3 of a solution containing 0.25 M o-chlorotoluene and 0.025 M n-dodecane (internal standard) in n-heptane (method A) or 15 cm3 aq. NaOH (method B). During this procedure, the burette was filled with nitrogen to ensure the apparatus was leak proof. The stirrer was then stopped, and 15 cm3 of 10% aq. NaOH (method A) or 10 cm3 of a solution containing 0.25 M o-chlorotoluene and 0.025 M n-dodecane in n-heptane (method B) were added to the reactor. Next, the burette and the reactor were flushed with hydrogen. Finally, the burette was filled with hydrogen at atmospheric pressure. The instant the stirrer was turned on again was taken to be the starting point of the process. The suspension was stirred with an egg-shaped 9.5 mm 19.1 mm magnetic stirring bar (Aldrich). The stirring rate was maintained at 2000 rpm with an IKA RH digital magnetic stirrer. All experiments were conducted in triplicate. 2.3. Analyses The reaction kinetics was usually monitored by measuring the hydrogen consumption, but occasionally by determining the o-chlorotoluene and toluene contents in a sample of the nheptane layer. The moment the 50 mm3 of n-heptane solution was withdrawn via the sampling port, the stirrer was stopped for about 15 s. As both techniques provided similar results (Fig. 1), the n-heptane layer was in most cases analysed only at the end of the reaction to ensure whether hydrogenolysis had proceeded
Fig. 1. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 10% Pd/C catalyst preconditioned by method A and B at 20 8C, at atmospheric pressure of hydrogen. (&, &) toluene concentration – method A (Experiment 1), (*, *) toluene concentration – method B (Experiment 2). Filled symbols – toluene concentrations obtained from hydrogen intake; empty symbols – toluene concentrations obtained from gas chromatographic assays.
to completion and to track the final stage of the process. The substrates and the main dechlorination products were analysed by gas liquid chromatography (GLC) using columns packed with 10% 1,2,3-tris(2-cyanoethoxy)propane (TCEP) on 100/ 120 Chromosorb P-AW or 10% OV-101 on 80/100 Gas Chrom Q. Chromatographic analyses were carried out on a MeraElwro 504 gas chromatograph equipped with a flame ionisation detector (FID) and a Laboratornije Pristroje electronic integrator CI-100. The chloride ions in the aqueous layers separated following the completion of dechlorination were analysed by the Mohre method. The extent of hydrodechlorination resulting from assays of these ions agreed within 99% with that determined from measurements of hydrogen consumption and within 92% with that determined from GLC-measured toluene contents. The precision of the chromatographic assays was better than 8%, that of the chloride ion assays was better than 0.2%. 3. Results and discussion In a multiphase, heterogeneous catalytic system containing two immiscible liquids with different polarities, pre-conditioning of the catalyst influences the kinetics of the reaction. In our previous paper [33] we studied the kinetics of chlorobenzene hydrodechlorination over 10% Pd/C in an n-heptane – aqueous solution of NaOH. We found that when the catalyst was preconditioned by soaking and stirring it in an n-heptane solution of chlorobenzene and n-dodecane prior to hydrogenolysis, the time relationship of benzene production was linear for almost the entire reaction period. Moreover, hydrogenolysis proceeded more rapidly than in the presence of the catalyst pre-treated by soaking and stirring it in an aqueous solution of NaOH. In the latter conditions, the time relationship of benzene production was sigmoid.
T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
In preliminary experiments we ascertained whether the effect of catalyst preconditioning on the course of ochlorotoluene hydrodechlorination was similar to that observed for chlorobenzene. The results are shown in Fig. 1. The reaction conditions were as follows: 100 mg 10% Pd/C catalyst, temperature 20 8C, hydrogen at atmospheric pressure. The relations between toluene concentration and time in methods A (Experiment 1) and B (Experiment 2) differ. In the former case, the concentration of toluene (the main reaction product) rises quickly and linearly with time as far as 70% conversion (k0 = 2.4 104 Ms4), after which it levels out to a plateau. In the latter, no clear reaction periods are distinguishable, and the whole reaction proceeds more slowly (r0 = 5.4 105 Ms1). There is no simple explanation for this difference. It is likely that the catalyst remains coated by the organic solvent (method A), and that o-chlorotoluene diffuses better through an organic solvent than water. Pre-conditioning of the palladium catalyst by method A was chosen for the remainder of the kinetics experiments, since it was essential to ensure compatible conditions for all experiments. We also assessed the influence of a mixture of n-heptane and n-dodecane on the kinetics of the investigated multiphase system. The solvents used in our investigations are essential for analytical procedures and kinetic investigations with solid chloroaromatic compounds. We carried out separate experiments without n-heptane and ndodecane. In these experiments the catalyst was first soaked and stirred in 2.5 103 mol o-chlorotoluene (Experiment 3) or 1.25 102 mol o-chlorotoluene (Experiment 4). Next, the experiments were carried as in method A. In (Experiment 3) the time course of hydrogen consumption during hydrogenolysis was sigmoid. Initially, the rate of hydrogen consumption was 9.7 107 mol s1, thereafter increasing to 1.6 106 mol s1. In (Experiment 4) the rate of hydrogen consumption rose from 1.4 106 mol s1 to 2.0 106 mol s1, thereafter remaining constant within 32% to 97% degree of conversion. The latter value is similar to the one obtained in (Experiment 1) (hydrogen rate consumption 2.4 106 mol s1). In our view, the rate of hydrogenolysis is not substantially influenced by the presence of a solvent (nheptane) or the internal chromatographic standard (n-dodecane). Several papers deal with the mass transfer of substrates and products between gaseous, liquid and solid phases during heterogeneous hydrodechlorination [16,18,31]. It has been postulated that hydrogenolysis rates are free of transport effects when stirring or shaking rates are sufficiently high. Dechlorination of p-chlorotoluene, conducted under conditions identical to the ones described in this paper for o-chlorotoluene, showed that the pseudo-zero-order rate constant increases when the stirring rate is raised to 1400 rpm, then reaches a plateau. So in our experiments we set the stirring rate at 2000 rpm, which we think was sufficient to minimise the influence of mass transfer on hydrogenolysis. The kinetics of o-chlorotoluene dechlorination in the presence of 100 mg 3%, 5%, 10% Pd/C catalyst is shown in Figs. 2–4. In all the experiments o-chlorotoluene was converted to toluene (the main reaction product) with >99% effectiveness, but there were
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Fig. 2. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 10% Pd/C catalyst preconditioned by method A at different temperatures: (&) 20 8C, (*) 40 8C, (~) 60 8C; atmospheric pressure of hydrogen.
essential differences in the reaction kinetics. Fig. 2 shows kinetic curves of o-chlorotoluene hydrodechlorination in the presence of 10% Pd/C catalyst pre-conditioned by method A. Represented by experiments carried out at temperatures of 20, 40 and 60 8C, the time relationships of the toluene concentration display some characteristic features. Two stages of the process are distinguishable: one in which the kinetic curves can be approximated by a straight line, and another in which they are not linear. The latter stage reflects a considerable drop in the reaction rate. Raising the reaction temperature increases the rate of o-chlorotoluene hydrogenolysis. Temperature also affects the shape of the toluene concentration vs. time curves: when the temperature increases, the linear stage of hydrogenolysis is prolonged. The time relationships of the toluene concentration for hydrogenolysis carried out over 5% Pd/C (Fig. 3) and 3% Pd/C (Fig. 4) at different temperatures are similar. Table 1 presents the reaction
Fig. 3. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 5% Pd/C catalyst preconditioned by method A at different temperatures: (&) 20 8C, (*) 40 8C, (~) 60 8C; atmospheric pressure of hydrogen.
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T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
Fig. 4. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 3% Pd/C catalyst preconditioned by method A at different temperatures: (&) 20 8C, (*) 40 8C, (~) 60 8C; atmospheric pressure of hydrogen.
kinetics parameters of the above experiments. Initial reaction rates (ro) were calculated by determining the slope of the tangents to the curves at time t = 0, and pseudo-zero-order rate constants (ko) were calculated by determining the slope of the linear fits to the first stage of the kinetic curves. The time t90% denotes the time required for o-chlorotoluene to be 90% dechlorinated. Some conclusions can be drawn from the data in Table 1. With a few exceptions, the values of ro are quite similar to the corresponding values of ko for all catalysts and temperatures. However, with 10% Pd/C at 20 8C the value of ro was twice that of ko. On the other hand, in experiments with 3% Pd/C and 5% Pd/C carried out at 60 8C the values of ro are slightly lower than the corresponding ko values. The differences between ro and ko in all the experiments may be due to minor changes in the catalysts’ properties at the beginning of the hydrogenolysis. So the kinetic constants ko describe the kinetic properties of this multiphase system better than ro; this is because the abovementioned changes are compensated. The ko values for the hydrogenolysis of o-chlorotoluene in the presence of 3%, 5%, 10% Pd/C catalysts carried out at a given temperature do not differ substantially, especially at 60 8C. This suggests that the numbers of active sites in the same mass of each of these catalysts may be comparable. The assayed dispersions of the catalysts and turnover frequencies (TOF, number of reactant molecules transformed per surface Pd atom and per second) calculated on the basis of k0 at 60 8C for 5% Pd/C (dispersion
0.20, TOF = 0.95 s1) and for 10% Pd/C (dispersion 0.09, TOF = 1.01 s1) endorse this suggestion. However, the dispersion and TOF obtained for 3% Pd/C catalyst (dispersion 0.11, TOF = 3.08 s1) appear to contradict it. The dispersion assayed for the 3% Pd/C catalyst goes against the general rule that the dispersion of a lightly loaded supported metal catalyst is larger than that of a more heavily loaded one. We hypothesised that the properties of the 3% Pd/C catalyst might have changed during the measurement of dispersion. So we carried out a set of experiments to test whether the dispersion measurement procedure does indeed affect catalytic activity towards hydrogenolysis. In these experiments 100 mg each of the 3% Pd/C and 10% Pd/C catalysts, which had been analysed for dispersion, were used in the general procedure and method A at 60 8C. The result obtained for the 3% Pd/C catalyst (k0 = 3.2 104 Ms1) shows that the rate of reaction is about three times slower following the dispersion assay. The value of the TOF (1.04 s1) calculated for this rate (3.2 104 Ms1) and the dispersion (0.11) are analogous to those obtained for 5% Pd/C and 10% Pd/C catalysts. In a comparable experiment with 10% Pd/C catalyst, the rate of hydrogenolysis did not change (k0 = 9.6 104 Ms1; TOF = 1.11 s1). These facts endorse our hypothesis that the number of surface palladium atoms decreases during the dispersion measurement process for the 3% Pd/C catalyst and our idea that the numbers of active sites in the same mass of catalysts differently loaded with palladium may be comparable. Additional experiments were performed to compare the effectiveness of o-chlorotoluene hydrogenolysis with the same masses of palladium. In these experiments 100 mg 3% Pd/C, 60 mg 5% Pd/C and 30 mg 10% Pd/C catalyst were used, with the other reaction conditions in method A being retained. The results are shown in Table 2 and Fig. 5. The latter presents the results of the experiments at 60 8C. It is seen from Table 2 that the kinetic constants k0 at 40 and 60 8C for 30 mg 10% Pd/C catalyst are approximately three times lower than that for 100 mg 3% Pd/C catalyst and 2.5 times lower than that for 60 mg 5% Pd/C. This can be explained as follows: even though the same quantity of palladium was present in the reactor, the active metal surface in 100 mg 3% Pd/C may be larger than in 30 mg 10% Pd/C and in 60 mg 5% Pd/C. The rate of reaction for 100 mg 10% Pd/C is about three times faster than for 30 mg 10% Pd/C at 60 8C. This relation suggests that the process is controlled kinetically [18]. Comparison of reaction TOF for 60 mg 5% Pd/C catalyst (k0 = 7.7 104 Ms1, TOF = 1.3 s1) and 30 mg 10% Pd/C catalyst
Table 1 Kinetic characteristics of o-chlorotoluene hydrogenolysis at different temperatures in the presence of 100 mg 10% Pd/C, 5% Pd/C and 3% Pd/C catalysts at atmospheric pressure of hydrogena % Pd
t = 20 8C 1
3 5 10 a
t = 40 8C 1
1
t = 60 8C 1
ro [Ms ]
k0 [Ms ]
t90% [s]
ro [Ms ]
k0 [Ms ]
t90% [s]
ro [Ms1]
k0 [Ms1]
t90% [s]
1.6 104 3.4 104 4.8 104
1.7 104 3.4 104 2.4 104
1705 1102 1330
4.0 104 7.4 104 7.0 104
6.3 104 6.8 104 6.2 104
465 397 427
7.4 104 6.2 104 8.7 104
9.5 104 9.2 104 8.7 104
253 257 264
The values were obtained from the time relationships of the toluene concentration during hydrogenolysis.
T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
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Table 2 Kinetic characteristics of toluene hydrogenolysis at different temperatures in the presence of 100 mg 3% Pd/C, 60 mg 5% Pd/C and 30 mg 10% Pd/C catalysts at atmospheric pressure of hydrogena Catalyst quantity and loading
t = 20 8C 1
100 mg 3% Pd/C 60 mg 5% Pd/C 30 mg 10% Pd/c a
t = 4 0 8C 1
1
t = 60 8C 1
r0 [Ms ]
k0 [Ms ]
r0 [Ms ]
k0 [Ms ]
r0 [Ms1]
k0 [Ms1]
1.6 104 2.9 104 1.8 104
1.7 104 2.2 104 1.4 104
4.0 104 8.0 104 6.0 104
6.3 104 5.7 104 2.2 104
7.4 104 8.1 104 3.4 104
9.5 104 7.7 104 3.1 104
The values were obtained from the time relationships of the toluene concentration during hydrogenolysis.
(k0 = 3.1 104 Ms1, TOF = 1.2 s1) at 60 8C with respective particle sizes of 5.5 and 12.1 nm for these catalysts suggests that the hydrogenolysis in the applied conditions is structure-insensitive [34]. Two basic mechanisms of the hydrodechlorination of chloroaromatic compounds over palladium-supported catalysts have been postulated for gaseous [7,8,35] and liquid phases [14,16,20,22,31]: both adsorbed hydrogen and chloroaromatic molecules may be involved in the subsequent dechlorination reaction (the Langmuir–Hinshelwood (L–H) mechanism [34]) [7,31,35]. In the so-called Rideal–Eley mechanism [34] chloroaromatic compounds may react with adsorbed hydrogen [8]. The L–H mechanism was applied in the present work. The following assumptions were made: (a) hydrogen is adsorbed noncompetitively on the Pd surface, and its surface concentration remains constant throughout the experiments; (b) desorption of toluene and HCl is rapid and does not substantially affect the kinetics of the hydrogenolysis; (c) the reaction between hydrogen and o-chlorotoluene is the slowest process in the system investigated. It was also assumed that, because the Pd/C catalyst particles covered with n-heptane/n-dodecane film travel very quickly between the aqueous and organic phases, the transport of mass between the catalyst surface and the two phases is rapid and has little influence on the kinetics of the hydrodechlorination [16,18]. So the reactant concentrations on the catalyst surface are close to the equilibrium ones.
The hydrogenolysis reaction is considered to depend on the surface concentration of reactant according to Equation (1) [31]. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kr C0 K oClT C oClT CH0 2 K H2 C H2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r¼ (1) ð1 þ K oClT C oClT þ K Tol C Tol Þð1 þ K H2 C H2 Þ where kr is the true kinetic constant, C0 is the total concentration of adsorption sites for o-chlorotoluene, CH0 2 is the total concentration of absorption sites for hydrogen, Ko-ClT is the adsorption constant for o-chlorotoluene, KTol is the adsorption constant for toluene, K H2 is the adsorption constant for hydrogen, Co-ClT is the concentration of o-chlorotoluene in n-heptane, CTol is the concentration of toluene in n-heptane, and CH2 is the concentration of hydrogen in n-heptane, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi CH0 2 K H2 C H2 0 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kr ¼ kr (2) ð1 þ K H2 CH2 Þ At a stable hydrogen pressure of 1 atm and temperatures in the 293–333 K range, the concentration of hydrogen may be regarded as constant [36,37]. Hence, after substituting Eq. (2) in Eq. (1) and converting C0 to C0 we obtained Eq. (3) r¼
k0r C 0 K oClT C oClT 1 þ K oClT C oClT þ K tol C Tol
(3)
where k0r is the overall kinetic constant, and C0 is the virtual concentration of o-chlorotoluene in n-heptane equivalent to the maximum amount of o-chlorotoluene adsorbed on the catalyst. Eq. (3) may be further simplified [31] because of the small concentration of toluene in the first stage of the process. On the basis of the above assumptions, the following kinetic model for interpreting the experimental results is proposed: r¼
k0r C 0 K oClT CoClT 1 þ K oClT CoClT
(4)
where C0 and Ko-ClT are constants and Co-ClT is the concentration of o-chlorotoluene in n-heptane. In Eq. (4) the term describing the kinetics with respect to hydrogen is included in the overall kinetic constant. For the purpose of the above model, the o-chlorotoluene concentration vs. time data were differentiated [38] using the Origin 7.5 program, the data set being obtained in the form of Eq. (5): Fig. 5. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 3% Pd/C catalyst (~), 60 mg 5% Pd/C catalyst (&) and 30 mg 10% Pd/C catalyst (*) at 60 8C; atmospheric pressure of hydrogen.
r¼
dC oClT ¼ f ðtÞ dt
(5)
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T. Janiak / Applied Catalysis A: General 335 (2008) 7–14 Table 3 Kinetic characteristics of o-chlorotoluene hydrogenolysis at different temperatures in the presence of 100 mg 10% Pd/C, 5% Pd/C and 3% Pd/C catalystsa 3% Pd/C catalyst 20 8C k0r Ko-ClT C0
40 8C 2 1
2.3 10 s 12 1.1 102 M
60 8C 2 1
9.4 10 s 19 8.0 103 M
2.0 101 s1 70 5.4 103 M
40 8C
60 8C
5% Pd/C catalyst 20 8C k0r Ko-ClT C0
2 1
4.2 10 s 9 1.2 102 M
2 1
9.1 10 s 26 8.5 103 M
1.7 101 s1 69 6.0 103 M
40 8C
60 8C
10% Pd/C catalyst Fig. 6. The rate of o-chlorotoluene hydrogenolysis versus o-chlorotoluene concentration in the presence of 3% Pd/C catalyst preconditioned by method A at 60 8C; atmospheric pressure of hydrogen. The solid line represents the least-squares fit of Eq. (4) to differentiated experimental data, (&) values not used in the fitting procedure.
where dCo-ClT/dt is the rate of o-chlorotoluene dechlorination and f(t) is an unknown function of time. The data set in the form of Eq. (5) was then transformed to data set in the form of Eq. (6): r¼
dC oClT ¼ f ðCoClT Þ dt
(6)
in which f(Co-ClT) is the kinetic model of the process under scrutiny. The theoretical kinetic model described by Eq. (4) was fitted to the experimental data altered according to the above procedure. By way of example, the curve obtained for the hydrogenolysis conducted in the presence of 3% Pd/C at 60 8C is shown in Fig. 6. Eq. (3) was also fitted to the experimental data. The values generated in these fittings differ only slightly from those obtained with Eq. (4): for example, in the experiment with 100 mg 10% Pd/C at 60 8C we obtained k0r ¼ 2:5 101 s1 , C0 = 3.8 103 M, Ko-ClT = 54 using Eq. (4) and k0r ¼ 2:4 101 s1 , C0 = 4.0 103 M KoClT = 53, KTol = 40 using Eq. (3). The constants k0r , C0, and Ko-ClT given in Table 3 were obtained by the least squares fitting of Eq. (4) to the differentiated kinetic data [39] and combining the results with the o-chlorotoluene adsorption data for a given catalyst. The latter were obtained by equilibration of the n-heptane solution containing 0.25 M o-chlorotoluene and 0.025 M n-dodecane with each of the catalysts investigated at 60, 40 and 20 8C. The kinetic characteristics k0r and Ko-ClT for a given catalyst increase with rising temperature, whereas C0 decreases with rising temperature. The k0r values in Table 3 for 3%, 5% and 10% Pd/C catalysts at 40 and 60 8C are quite similar within the range of experimental precision. This may suggest that the number of active sites on the palladium ‘crystallites’ are similar, despite the different masses of metal.
20 8C k0r Ko-ClT C0
2 1
3.1 10 s 5.4 1.4 102 M
1 1
1.2 10 s 17 6.8 103 M
2.5 101 s1 54 3.8 103 M
a Values obtained from the least squares fitting of Eq. (4) to the differentiated experimental data.
The activation energies (DEr) were calculated on the basis of the Arrhenius equation, separately for k0r and ko. The adsorption enthalpies of o-chlorotoluene (DHa) were also calculated using the van’t Hoff equation and adsorption constants Ko-ClT from Table 3. Table 4 lists the calculated values of (DEr) and (DHa). The adsorption enthalpies (DHa) increase with increasing palladium content in the catalyst; the activation energies (DEr) calculated using both k0r and k0 display a minimum for the 5% Pd/ C catalyst. There are no simple explanations as to why the activation energy for the 3% Pd/C, 5% Pd/C and 10% Pd/C catalysts display a trend with a minimum for 5% Pd/C. It is possible that at least one of the DEa values is influenced by some physical process. Transport phenomena could well affect the kinetics of hydrogenolysis at a lower temperature. It is also likely that the kinetic constant k0 at a lower temperature (20 8C) is affected by some unknown process taking place on the catalyst during hydrogenolysis. A white deposit was observed during the dispersion measurements of the catalyst (ca. 13% mass loss for Table 4 Activation energies (DEa) of o-chlorotoluene hydrogenolysis in the presence of 10% Pd/C, 5% Pd/C and 3% Pd/C catalysts; adsorption enthalpies (DHad) of ochlorotoluene on these catalysts Catalyst
DEa kJ mol1
3% Pd/C 5% Pd/C 10% Pd/C
44.9a 28.8a 43.1a
DHad kJ mol1 35.6b 20.6b 26.8b
33d 27d 32d
35.8c 41.9c 47.3c
a Values calculated using k0r from Table 3 and the Arrhenius equation for 20, 40 and 60 8C. b Values calculated using k0 from Table 1 and the Arrhenius equation for 20, 40 and 60 8C. c Values calculated using Ko-ClT from Table 3 and the van’t Hoff equation for 20, 40 and 60 8C. d Values calculated using k0 from Table 1 and the Arrhenius equation for 40 and 60 8C.
T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
10% Pd/C catalyst). Also a white substance, a silicon-based organic polymer, precipitated in the aqueous layer separated after hydrogenolysis when this had been neutralized to pH 7. It is seen from Fig. 6 that the rate of reaction for hydrogenolysis over the 3% Pd/C catalyst at 60 8C rises from r0 = 7.1 104 Ms1 to r = 9.1 104 Ms1. This is probably due to minor changes in the catalyst’s properties. Hence, the leaching of some catalyst additive may influence the reaction kinetics, especially at the beginning of the hydrogenolysis, and this effect may be pronounced at lower temperatures. To verify this we calculated the activation energy (DEa) for two temperatures 40 and 60 8C only, obtaining 32 kJ mol1 for 10% Pd/C; 27 kJ mol1 for 5% Pd/C and 33 kJ mol1 for 3% Pd/C. These values are close to each other within experimental error. The lower activation energy, 27 kJ mol1 for 5% Pd/C, should induce quicker hydrogenolysis of o-chlorotoluene over this catalyst in comparison with the 3% and 10% Pd/C catalysts. The kinetic characteristics given in Table 1 confirm this. For o-chlorotoluene hydrogenolysis over 5% Pd/C at 20 8C, t90% = 1102 s; for 3% Pd/ C, t90% = 1705 s; and for 10% Pd/C, t90% = 1330 s. The small differences between the activation energies and adsorption enthalpies for the same catalyst, shown in Table 4, may suggest that the main energy threshold of the process is linked to the dissociative adsorption of o-chlorotoluene on the catalyst. The stability of catalytic activity in the hydrogenolysis reaction was assessed in separate experiments for all catalysts. In these experiments, after the typical procedure (method A), an extra portion of o-chlorotoluene (2.5 103 mol) was added to the reaction mixture immediately before reaction completion. The results for 10% Pd/C catalyst at 60 8C are presented in Fig. 7. It shows that the pseudo-zero-order rate constant has decreased from k0 = 8.2 104 Ms1 to k0 = 4.5 104 Ms1 (1.8 times) after the addition of a fresh portion of o-chlorotoluene. This suggests a loss of catalytic activity. It is probable that the catalyst is deactivated as a result of the palladium being etched by the strong alkaline medium used in these experiments. It has been
Fig. 7. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 10% Pd/C catalyst preconditioned by method A at 60 8C, atmospheric pressure of hydrogen. An extra portion of o-chlorotoluene (2.5 103 mol) was added immediately before reaction completion.
13
Fig. 8. Hydrogenolysis of a mixture of 0.166 M p-chlorotoluene and 0.083 M o-chlorotoluene in the presence of 100 mg 10% Pd/C catalyst preconditioned by method A at 20 8C. (&) concentration of toluene, (*) concentration of ochlorotoluene (~) concentration of p-chlorotoluene.
postulated that one of the main causes of catalyst deactivation in the liquid phase is etching of palladium, a process that depends on the palladium particle size and intensifies with increasing palladium particle size [40]. The average particle size (dav) obtained from dispersion measurements (D) (using the relation dav [nm] = 1.12/D [41]) for the catalysts used in our investigations are: 12.1 nm for 10% Pd/C and 5.5 nm for 5% Pd/C. Accordingly, catalytic activity decreased 1.8 times for 10% Pd/C and 1.5 times for 5% Pd/C, which tallies well with the above postulates. The 3% Pd/C catalyst activity decreases 1.2 times, and this tendency, a 1.2 times slower reaction rate, is retained after addition of the next 2.5 103 mol of o-chlorotoluene. The catalysts and the reaction conditions used were also tested in competitive experiments to study their selectivity in hydrogenolysis. Two tests were carried out. A mixture of nheptane, n-dodecane, o-chlorotoluene and p-chlorotoluene was hydrogenolysed by method A at 20 8C in the presence of 100 mg Pd/C catalyst. The results indicate that the catalyst – 10% Pd/C – does not exhibit any noticeable selectivity in the hydrogenolysis reaction. It was also demonstrated experimentally that the reaction rate for a given compound is proportional to its concentration in the mixture. The results are presented in Fig. 8. The values obtained are: r0 = 3.5 104 Ms1 for 0.166 M pchlorotoluene and r0 = 1.8 104 Ms1 for 0.083 M o-chlorotoluene. In another experiment, a mixture of n-heptane, n-dodecane, 0.082 M o-chlorotoluene, 0.082 M 1,2-dichlorobenzene and 0.082 M 4-chlorobiphenyl was hydrogenolysed under the conditions described above. The initial rates of hydrogenolysis of these compounds differed: r0 = 2.4 104 Ms1 for 4-chlorobiphenyl, r0 = 8.1 105 Ms1 for 1,2 dichlorobenzene and r0 = 3.2 105 Ms1 for o-chlorotoluene. 4. Conclusions The hydrogenolysis of o-chlorotoluene in the presence of Pd/C catalyst removes chlorine from it with a yield of up to
14
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100%, which is a promising result if the intention is to dechlorinate this particular compound. All three catalysts used exhibit a similar catalytic activity. The extent of dechlorination increases with time and temperature, and depends on the amount of catalyst in the system. Kinetic analysis based on the Langmuir–Hinshelwood approach enabled the kinetic constants to be determined. This may be useful for modelling the path of o-chlorotoluene dechlorination. The results of these investigations have extended our knowledge on an important group of reactions, and they form a convenient framework within which to consider the treatment of specific simple wastes, by-products and even complex organochlorine wastes. Acknowledgements The authors wish to thank Dr J. Okal from the Institute of Low Temperature and Structure Research of the Polish Academy of Sciences in Wroclaw for the dispersion measurements of the catalysts. The financing of this work from the State Funds for Scientific Research (grant No. DS/8000-4-0026-7) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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[12] F.-D. Kopinke, K. Mackenzie, R. Koehler, A. Georgi, Appl. Catal. A Gen. 271 (2004) 119. [13] V.A. Yakovlev, V.V. Terskikh, V.I. Simagina, V.A. Likholobov, J. Mol. Catal. A Chem. 153 (2000) 231. [14] H. Sajiki, A. Kume, K. Hattori, K. Hirota, Tetrahedron Lett. 43 (2002) 7247. [15] H. Sajiki, A. Kume, K. Hattori, H. Nagase, K. Hirota, Tetraedrom Lett. 43 (2002) 7251. [16] G. Del Angel, J.L. Benitez, J. Mol. Catal. A Chem. 165 (2001) 9. [17] M.A. Aramendia, R. Burch, I.M. Garcia, A. Marinas, J.M. Marinas, B.W.L. Southward, F.J. Urbano, J. Appl. Catal. B Environ. 31 (2001) 163. [18] M.A. Aramendia, V. Borau, I.M. Garcia, C. Jimenez, A. Marinas, J.M. Marinas, F.J. Urbano, Appl. Catal. B Environ. 43 (2003) 71. [19] M.A. Aramendia, V. Borau, I.M. Garcia, C. Jimenez, F. Lafont, A. Marinas, J.M. Marinas, F.J. Urbano, J. Mol. Catal. A Chem. 184 (2002) 237. [20] K. Konuma, N. Kameda, J. Mol. Catal. A Chem. 178 (2002) 239. [21] C. Schu¨th, S. Disser, F. Schu¨th, M. Reinhard, Appl. Catal. B Environ. 28 (2000) 147. [22] D. Fritsch, K. Kuhr, K. Mackenzie, F.-D. Kopinke, Catal. Today 82 (2003) 105. [23] C.A. Marques, M. Selva, P. Tundo, J. Chem. Soc. Perkin Trans I (1993) 529. [24] C.A. Marques, M. Selva, P. Tundo, J. Org. Chem. 58 (1993) 5256. [25] C.A. Marques, M. Selva, P. Tundo, J. Org. Chem. 59 (1994) 3830. [26] C.A. Marques, M. Selva, P. Tundo, J. Org. Chem. 60 (1995) 2430. [27] C.A. Marques, O. Rogozhnikova, M. Selva, P. Tundo, J. Mol. Catal. A Chem. 96 (1995) 301. [28] P. Tundo, A. Perosa, React. Funct. Polym. 54 (2003) 95. [29] G. Evdokimova, S. Zinovyev, A. Perosa, P. Tundo, Appl. Catal, A Gen. 271 (2004) 129. [30] F. Murena, F. Gioia, Catal. Today 75 (2002) 57. [31] F. Murena, F. Gioia, Appl. Catal. A Gen. 271 (2004) 145. [32] T. Janiak, J. Blazejowski, Chemosphere 48 (2002) 1097. [33] T. Janiak, J. Blazejowski, Appl. Catal. A Gen. 271 (2004) 103. [34] R.I. Masel, Chemical Kinetics and Catalysis, Wiley- Interscience, New York, 2001, 287, 869. [35] C. Menini, C. Park, E.-J. Shin, G. Tavoularis, M.A. Keane, Catal. Today 62 (2000) 355. [36] A. Anson, E. Lafuente, E. Urriolabeitia, R. Navarro, A.M. Benito, W.K. Maser, M.T. Martinez, J. Phys. Chem. B 110 (2006) 6643. [37] S.-Y. Huang, Ch-D. Huang, B.-T. Chang, Ch-T. Yeh, J. Phys. Chem. B 110 (2006) 21783. [38] R.W. Missen, C.A. Mims, B.A. Saville, Introduction to Chemical Reaction Engineering and Kinetics, John Wiley&Sons, New York, 1999, pp. 49. [39] D.W. Marquart, J. Soc. Ind. Appl. Math. 11 (1963) 431. [40] J. Panpranot, K. Pattamakomsan, P. Praserthdam, J.G. Goodwin Jr., Ind. Eng. Chem. Res. 43 (2004) 6014. [41] P. Aben, J. Catal. 10 (1968) 224.
Applied Catalysis A: General 335 (2008) 7–14 www.elsevier.com/locate/apcata
Kinetics of o-chlorotoluene hydrogenolysis in the presence of 3%, 5% and 10% Pd/C catalysts Tadeusz Janiak * Faculty of Chemistry, University of Gdan´sk, J. Sobieskiego 18, 80-952 Gdan´sk, Poland Received 24 May 2007; received in revised form 18 October 2007; accepted 25 October 2007
Abstract The kinetics of o-chlorotoluene hydrogenolysis in the presence of 3%, 5% and 10% Pd/C catalysts was experimentally investigated in an alkaline – n-heptane – gaseous hydrogen system. The main product of hydrogenolysis was toluene. Kinetic characteristics for the reaction were obtained at temperatures of 20, 40 and 60 8C at atmospheric pressure of hydrogen for all catalysts; the effectiveness of the latter in ochlorotoluene hydrogenolysis was compared. The pseudo-zero-order rate constants and times of 90% conversion for each of the catalysts at 40 and 60 8C were quite similar. The overall kinetic constants were obtained assuming the Langmuir–Hinshelwood mechanism. The activation energies of o-chlorotoluene hydrogenolysis for these catalysts were calculated. The selectivity of the reacting system was evaluated in competitive experiments with o-chlorotoluene, 1,2-dichlorobenzene and 4-chlorobiphenyl. The stability of the catalysts in a hydrogenolytic environment was tested. # 2007 Elsevier B.V. All rights reserved. Keywords: O-chlorotoluene; Catalytic hydrogenolysis; Palladium on carbon; Kinetics
1. Introduction Chloroaromatic compounds can be converted to parent hydrocarbons over Ru, Rh, Ni, Pd and Pt metal catalysts during hydrogenolysis with gaseous hydrogen [1–4]. Palladium is the most active hydrodechlorination catalyst in such conditions [5]; rhodium is the most effective in transfer hydrogenolysis with propanol-2 as the hydrogen source [6]. Hydrodechlorination of chlorobenzene, chlorotoluenes, chlorobiphenyls and 1,1-bis (4-chlorophenyl)-2,2,2-trichloroethane in the presence of Pd catalyst has been carried out in the gaseous [7–9] and liquid [6,10–33] phases. Hydrogenolysis has been done in the presence of Pd/Al2O3 [7,8,12], Pd/ZrO2 [9], Pd/AlPO4–SiO2 PM2 [17–19], Pd/zeolites [21], Pd/poly(dimethylsiloxane) [22] and Pd/C [6,10,11,13–16,20,23–33] using gaseous hydrogen [7–9,11,13–22,24–33], propanol-2 [6,10–12], potassium formate [11], formic acid [12], hydrazine [12] or hypophosphite [23] as the source of hydrogen. Liquidphase hydrodechlorination has been investigated in one liquid
* Tel.: +48 58 523 53 21; fax: +48 58 523 53 57. E-mail address: [email protected]. 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.10.027
medium with [13–17,19] and without a base [18,20–22], as well as in two liquids [23–33]. In these two liquids – multiphase systems – hydrodechlorination proceeds effectively even under mild conditions, that is, at temperatures up to 50 8C and with hydrogen at atmospheric pressure. Experiments in these systems have been performed in the presence of a transfer phase catalyst (PT) [23–29] and in its absence [32,33]. Our previous papers reported on the results of the hydrodechlorination of chlorobenzene, dichlorobenzenes and chlorotoluenes in a batch-type multiphase reactor. The process was carried out in the presence of a 10% Pd/C catalyst dispersed in an n-heptane or n-hexane solution of the chlorinated compound and an aqueous solution of NaOH. We studied the influence of in situ generated and gaseous hydrogen [32], the stirring rate, NaOH concentration, temperature, and the impact of preconditioning Pd/C on the effectiveness of the hydrogenolysis [33]. Little has been published on the kinetics of the hydrodechlorination of chloroaromatic compounds in multiphase systems [25,29–31]. In this paper, the effect of the palladium content in the catalyst on the kinetics of o-chlorotoluene hydrogenolysis is explored. The investigations on the hydrogenolysis of this model system may be of utilitarian value in that it creates a framework for developing methods of treating organochlorine wastes.
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2. Experimental 2.1. Materials The catalysts – 3% Pd/C, 5% Pd/C and 10% Pd/C, as well as o-chlorotoluene (99.9% purity), p-chlorotoluene (98% purity), 1,2-dichlorobenzene (99% purity) and n-dodecane (>99.9% purity) were purchased from Sigma–Aldrich; the n-heptane (99.5% purity) solvent was from Lab-Scan. 4-chlorobiphenyl was synthesised in our laboratory. Sodium hydroxide (p.a. purity) was supplied by POCH-Gliwice, the nitrogen and hydrogen (>99.5% purity) by Linde. The characteristics of the catalysts are as follows: 3% Pd/C – particle size 10 mm, BET surface area 780 m2 g1; 5% Pd/C – particle size 45 mm, BET surface area 870 m2 g1; 10% Pd/C – particle size 90% <60 mm, 10% <5 mm BET surface area 880 m2 g1. The Pd dispersions determined from hydrogen chemisorption measurements were 0.11 for 3% Pd/C, 0.20 for 5% Pd/C and 0.09 for 10% Pd/C. 2.2. General procedure Experiments were carried out in a 50 cm3 two-walled, threenecked, round-bottomed reactor connected to a gas burette supplying hydrogen (or nitrogen). Thermostatted water was pumped between the reactor walls to control the reaction temperature. In a typical experiment, the dry reactor containing the stirring bar was initially purged with nitrogen for 5 min. The catalyst, previously stored in a desiccator over NaOH pellets, was then pre-conditioned prior to dechlorination by being soaked and stirred for 30 min at 300 rpm in 10 cm3 of a solution containing 0.25 M o-chlorotoluene and 0.025 M n-dodecane (internal standard) in n-heptane (method A) or 15 cm3 aq. NaOH (method B). During this procedure, the burette was filled with nitrogen to ensure the apparatus was leak proof. The stirrer was then stopped, and 15 cm3 of 10% aq. NaOH (method A) or 10 cm3 of a solution containing 0.25 M o-chlorotoluene and 0.025 M n-dodecane in n-heptane (method B) were added to the reactor. Next, the burette and the reactor were flushed with hydrogen. Finally, the burette was filled with hydrogen at atmospheric pressure. The instant the stirrer was turned on again was taken to be the starting point of the process. The suspension was stirred with an egg-shaped 9.5 mm 19.1 mm magnetic stirring bar (Aldrich). The stirring rate was maintained at 2000 rpm with an IKA RH digital magnetic stirrer. All experiments were conducted in triplicate. 2.3. Analyses The reaction kinetics was usually monitored by measuring the hydrogen consumption, but occasionally by determining the o-chlorotoluene and toluene contents in a sample of the nheptane layer. The moment the 50 mm3 of n-heptane solution was withdrawn via the sampling port, the stirrer was stopped for about 15 s. As both techniques provided similar results (Fig. 1), the n-heptane layer was in most cases analysed only at the end of the reaction to ensure whether hydrogenolysis had proceeded
Fig. 1. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 10% Pd/C catalyst preconditioned by method A and B at 20 8C, at atmospheric pressure of hydrogen. (&, &) toluene concentration – method A (Experiment 1), (*, *) toluene concentration – method B (Experiment 2). Filled symbols – toluene concentrations obtained from hydrogen intake; empty symbols – toluene concentrations obtained from gas chromatographic assays.
to completion and to track the final stage of the process. The substrates and the main dechlorination products were analysed by gas liquid chromatography (GLC) using columns packed with 10% 1,2,3-tris(2-cyanoethoxy)propane (TCEP) on 100/ 120 Chromosorb P-AW or 10% OV-101 on 80/100 Gas Chrom Q. Chromatographic analyses were carried out on a MeraElwro 504 gas chromatograph equipped with a flame ionisation detector (FID) and a Laboratornije Pristroje electronic integrator CI-100. The chloride ions in the aqueous layers separated following the completion of dechlorination were analysed by the Mohre method. The extent of hydrodechlorination resulting from assays of these ions agreed within 99% with that determined from measurements of hydrogen consumption and within 92% with that determined from GLC-measured toluene contents. The precision of the chromatographic assays was better than 8%, that of the chloride ion assays was better than 0.2%. 3. Results and discussion In a multiphase, heterogeneous catalytic system containing two immiscible liquids with different polarities, pre-conditioning of the catalyst influences the kinetics of the reaction. In our previous paper [33] we studied the kinetics of chlorobenzene hydrodechlorination over 10% Pd/C in an n-heptane – aqueous solution of NaOH. We found that when the catalyst was preconditioned by soaking and stirring it in an n-heptane solution of chlorobenzene and n-dodecane prior to hydrogenolysis, the time relationship of benzene production was linear for almost the entire reaction period. Moreover, hydrogenolysis proceeded more rapidly than in the presence of the catalyst pre-treated by soaking and stirring it in an aqueous solution of NaOH. In the latter conditions, the time relationship of benzene production was sigmoid.
T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
In preliminary experiments we ascertained whether the effect of catalyst preconditioning on the course of ochlorotoluene hydrodechlorination was similar to that observed for chlorobenzene. The results are shown in Fig. 1. The reaction conditions were as follows: 100 mg 10% Pd/C catalyst, temperature 20 8C, hydrogen at atmospheric pressure. The relations between toluene concentration and time in methods A (Experiment 1) and B (Experiment 2) differ. In the former case, the concentration of toluene (the main reaction product) rises quickly and linearly with time as far as 70% conversion (k0 = 2.4 104 Ms4), after which it levels out to a plateau. In the latter, no clear reaction periods are distinguishable, and the whole reaction proceeds more slowly (r0 = 5.4 105 Ms1). There is no simple explanation for this difference. It is likely that the catalyst remains coated by the organic solvent (method A), and that o-chlorotoluene diffuses better through an organic solvent than water. Pre-conditioning of the palladium catalyst by method A was chosen for the remainder of the kinetics experiments, since it was essential to ensure compatible conditions for all experiments. We also assessed the influence of a mixture of n-heptane and n-dodecane on the kinetics of the investigated multiphase system. The solvents used in our investigations are essential for analytical procedures and kinetic investigations with solid chloroaromatic compounds. We carried out separate experiments without n-heptane and ndodecane. In these experiments the catalyst was first soaked and stirred in 2.5 103 mol o-chlorotoluene (Experiment 3) or 1.25 102 mol o-chlorotoluene (Experiment 4). Next, the experiments were carried as in method A. In (Experiment 3) the time course of hydrogen consumption during hydrogenolysis was sigmoid. Initially, the rate of hydrogen consumption was 9.7 107 mol s1, thereafter increasing to 1.6 106 mol s1. In (Experiment 4) the rate of hydrogen consumption rose from 1.4 106 mol s1 to 2.0 106 mol s1, thereafter remaining constant within 32% to 97% degree of conversion. The latter value is similar to the one obtained in (Experiment 1) (hydrogen rate consumption 2.4 106 mol s1). In our view, the rate of hydrogenolysis is not substantially influenced by the presence of a solvent (nheptane) or the internal chromatographic standard (n-dodecane). Several papers deal with the mass transfer of substrates and products between gaseous, liquid and solid phases during heterogeneous hydrodechlorination [16,18,31]. It has been postulated that hydrogenolysis rates are free of transport effects when stirring or shaking rates are sufficiently high. Dechlorination of p-chlorotoluene, conducted under conditions identical to the ones described in this paper for o-chlorotoluene, showed that the pseudo-zero-order rate constant increases when the stirring rate is raised to 1400 rpm, then reaches a plateau. So in our experiments we set the stirring rate at 2000 rpm, which we think was sufficient to minimise the influence of mass transfer on hydrogenolysis. The kinetics of o-chlorotoluene dechlorination in the presence of 100 mg 3%, 5%, 10% Pd/C catalyst is shown in Figs. 2–4. In all the experiments o-chlorotoluene was converted to toluene (the main reaction product) with >99% effectiveness, but there were
9
Fig. 2. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 10% Pd/C catalyst preconditioned by method A at different temperatures: (&) 20 8C, (*) 40 8C, (~) 60 8C; atmospheric pressure of hydrogen.
essential differences in the reaction kinetics. Fig. 2 shows kinetic curves of o-chlorotoluene hydrodechlorination in the presence of 10% Pd/C catalyst pre-conditioned by method A. Represented by experiments carried out at temperatures of 20, 40 and 60 8C, the time relationships of the toluene concentration display some characteristic features. Two stages of the process are distinguishable: one in which the kinetic curves can be approximated by a straight line, and another in which they are not linear. The latter stage reflects a considerable drop in the reaction rate. Raising the reaction temperature increases the rate of o-chlorotoluene hydrogenolysis. Temperature also affects the shape of the toluene concentration vs. time curves: when the temperature increases, the linear stage of hydrogenolysis is prolonged. The time relationships of the toluene concentration for hydrogenolysis carried out over 5% Pd/C (Fig. 3) and 3% Pd/C (Fig. 4) at different temperatures are similar. Table 1 presents the reaction
Fig. 3. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 5% Pd/C catalyst preconditioned by method A at different temperatures: (&) 20 8C, (*) 40 8C, (~) 60 8C; atmospheric pressure of hydrogen.
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T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
Fig. 4. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 3% Pd/C catalyst preconditioned by method A at different temperatures: (&) 20 8C, (*) 40 8C, (~) 60 8C; atmospheric pressure of hydrogen.
kinetics parameters of the above experiments. Initial reaction rates (ro) were calculated by determining the slope of the tangents to the curves at time t = 0, and pseudo-zero-order rate constants (ko) were calculated by determining the slope of the linear fits to the first stage of the kinetic curves. The time t90% denotes the time required for o-chlorotoluene to be 90% dechlorinated. Some conclusions can be drawn from the data in Table 1. With a few exceptions, the values of ro are quite similar to the corresponding values of ko for all catalysts and temperatures. However, with 10% Pd/C at 20 8C the value of ro was twice that of ko. On the other hand, in experiments with 3% Pd/C and 5% Pd/C carried out at 60 8C the values of ro are slightly lower than the corresponding ko values. The differences between ro and ko in all the experiments may be due to minor changes in the catalysts’ properties at the beginning of the hydrogenolysis. So the kinetic constants ko describe the kinetic properties of this multiphase system better than ro; this is because the abovementioned changes are compensated. The ko values for the hydrogenolysis of o-chlorotoluene in the presence of 3%, 5%, 10% Pd/C catalysts carried out at a given temperature do not differ substantially, especially at 60 8C. This suggests that the numbers of active sites in the same mass of each of these catalysts may be comparable. The assayed dispersions of the catalysts and turnover frequencies (TOF, number of reactant molecules transformed per surface Pd atom and per second) calculated on the basis of k0 at 60 8C for 5% Pd/C (dispersion
0.20, TOF = 0.95 s1) and for 10% Pd/C (dispersion 0.09, TOF = 1.01 s1) endorse this suggestion. However, the dispersion and TOF obtained for 3% Pd/C catalyst (dispersion 0.11, TOF = 3.08 s1) appear to contradict it. The dispersion assayed for the 3% Pd/C catalyst goes against the general rule that the dispersion of a lightly loaded supported metal catalyst is larger than that of a more heavily loaded one. We hypothesised that the properties of the 3% Pd/C catalyst might have changed during the measurement of dispersion. So we carried out a set of experiments to test whether the dispersion measurement procedure does indeed affect catalytic activity towards hydrogenolysis. In these experiments 100 mg each of the 3% Pd/C and 10% Pd/C catalysts, which had been analysed for dispersion, were used in the general procedure and method A at 60 8C. The result obtained for the 3% Pd/C catalyst (k0 = 3.2 104 Ms1) shows that the rate of reaction is about three times slower following the dispersion assay. The value of the TOF (1.04 s1) calculated for this rate (3.2 104 Ms1) and the dispersion (0.11) are analogous to those obtained for 5% Pd/C and 10% Pd/C catalysts. In a comparable experiment with 10% Pd/C catalyst, the rate of hydrogenolysis did not change (k0 = 9.6 104 Ms1; TOF = 1.11 s1). These facts endorse our hypothesis that the number of surface palladium atoms decreases during the dispersion measurement process for the 3% Pd/C catalyst and our idea that the numbers of active sites in the same mass of catalysts differently loaded with palladium may be comparable. Additional experiments were performed to compare the effectiveness of o-chlorotoluene hydrogenolysis with the same masses of palladium. In these experiments 100 mg 3% Pd/C, 60 mg 5% Pd/C and 30 mg 10% Pd/C catalyst were used, with the other reaction conditions in method A being retained. The results are shown in Table 2 and Fig. 5. The latter presents the results of the experiments at 60 8C. It is seen from Table 2 that the kinetic constants k0 at 40 and 60 8C for 30 mg 10% Pd/C catalyst are approximately three times lower than that for 100 mg 3% Pd/C catalyst and 2.5 times lower than that for 60 mg 5% Pd/C. This can be explained as follows: even though the same quantity of palladium was present in the reactor, the active metal surface in 100 mg 3% Pd/C may be larger than in 30 mg 10% Pd/C and in 60 mg 5% Pd/C. The rate of reaction for 100 mg 10% Pd/C is about three times faster than for 30 mg 10% Pd/C at 60 8C. This relation suggests that the process is controlled kinetically [18]. Comparison of reaction TOF for 60 mg 5% Pd/C catalyst (k0 = 7.7 104 Ms1, TOF = 1.3 s1) and 30 mg 10% Pd/C catalyst
Table 1 Kinetic characteristics of o-chlorotoluene hydrogenolysis at different temperatures in the presence of 100 mg 10% Pd/C, 5% Pd/C and 3% Pd/C catalysts at atmospheric pressure of hydrogena % Pd
t = 20 8C 1
3 5 10 a
t = 40 8C 1
1
t = 60 8C 1
ro [Ms ]
k0 [Ms ]
t90% [s]
ro [Ms ]
k0 [Ms ]
t90% [s]
ro [Ms1]
k0 [Ms1]
t90% [s]
1.6 104 3.4 104 4.8 104
1.7 104 3.4 104 2.4 104
1705 1102 1330
4.0 104 7.4 104 7.0 104
6.3 104 6.8 104 6.2 104
465 397 427
7.4 104 6.2 104 8.7 104
9.5 104 9.2 104 8.7 104
253 257 264
The values were obtained from the time relationships of the toluene concentration during hydrogenolysis.
T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
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Table 2 Kinetic characteristics of toluene hydrogenolysis at different temperatures in the presence of 100 mg 3% Pd/C, 60 mg 5% Pd/C and 30 mg 10% Pd/C catalysts at atmospheric pressure of hydrogena Catalyst quantity and loading
t = 20 8C 1
100 mg 3% Pd/C 60 mg 5% Pd/C 30 mg 10% Pd/c a
t = 4 0 8C 1
1
t = 60 8C 1
r0 [Ms ]
k0 [Ms ]
r0 [Ms ]
k0 [Ms ]
r0 [Ms1]
k0 [Ms1]
1.6 104 2.9 104 1.8 104
1.7 104 2.2 104 1.4 104
4.0 104 8.0 104 6.0 104
6.3 104 5.7 104 2.2 104
7.4 104 8.1 104 3.4 104
9.5 104 7.7 104 3.1 104
The values were obtained from the time relationships of the toluene concentration during hydrogenolysis.
(k0 = 3.1 104 Ms1, TOF = 1.2 s1) at 60 8C with respective particle sizes of 5.5 and 12.1 nm for these catalysts suggests that the hydrogenolysis in the applied conditions is structure-insensitive [34]. Two basic mechanisms of the hydrodechlorination of chloroaromatic compounds over palladium-supported catalysts have been postulated for gaseous [7,8,35] and liquid phases [14,16,20,22,31]: both adsorbed hydrogen and chloroaromatic molecules may be involved in the subsequent dechlorination reaction (the Langmuir–Hinshelwood (L–H) mechanism [34]) [7,31,35]. In the so-called Rideal–Eley mechanism [34] chloroaromatic compounds may react with adsorbed hydrogen [8]. The L–H mechanism was applied in the present work. The following assumptions were made: (a) hydrogen is adsorbed noncompetitively on the Pd surface, and its surface concentration remains constant throughout the experiments; (b) desorption of toluene and HCl is rapid and does not substantially affect the kinetics of the hydrogenolysis; (c) the reaction between hydrogen and o-chlorotoluene is the slowest process in the system investigated. It was also assumed that, because the Pd/C catalyst particles covered with n-heptane/n-dodecane film travel very quickly between the aqueous and organic phases, the transport of mass between the catalyst surface and the two phases is rapid and has little influence on the kinetics of the hydrodechlorination [16,18]. So the reactant concentrations on the catalyst surface are close to the equilibrium ones.
The hydrogenolysis reaction is considered to depend on the surface concentration of reactant according to Equation (1) [31]. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kr C0 K oClT C oClT CH0 2 K H2 C H2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r¼ (1) ð1 þ K oClT C oClT þ K Tol C Tol Þð1 þ K H2 C H2 Þ where kr is the true kinetic constant, C0 is the total concentration of adsorption sites for o-chlorotoluene, CH0 2 is the total concentration of absorption sites for hydrogen, Ko-ClT is the adsorption constant for o-chlorotoluene, KTol is the adsorption constant for toluene, K H2 is the adsorption constant for hydrogen, Co-ClT is the concentration of o-chlorotoluene in n-heptane, CTol is the concentration of toluene in n-heptane, and CH2 is the concentration of hydrogen in n-heptane, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi CH0 2 K H2 C H2 0 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kr ¼ kr (2) ð1 þ K H2 CH2 Þ At a stable hydrogen pressure of 1 atm and temperatures in the 293–333 K range, the concentration of hydrogen may be regarded as constant [36,37]. Hence, after substituting Eq. (2) in Eq. (1) and converting C0 to C0 we obtained Eq. (3) r¼
k0r C 0 K oClT C oClT 1 þ K oClT C oClT þ K tol C Tol
(3)
where k0r is the overall kinetic constant, and C0 is the virtual concentration of o-chlorotoluene in n-heptane equivalent to the maximum amount of o-chlorotoluene adsorbed on the catalyst. Eq. (3) may be further simplified [31] because of the small concentration of toluene in the first stage of the process. On the basis of the above assumptions, the following kinetic model for interpreting the experimental results is proposed: r¼
k0r C 0 K oClT CoClT 1 þ K oClT CoClT
(4)
where C0 and Ko-ClT are constants and Co-ClT is the concentration of o-chlorotoluene in n-heptane. In Eq. (4) the term describing the kinetics with respect to hydrogen is included in the overall kinetic constant. For the purpose of the above model, the o-chlorotoluene concentration vs. time data were differentiated [38] using the Origin 7.5 program, the data set being obtained in the form of Eq. (5): Fig. 5. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 3% Pd/C catalyst (~), 60 mg 5% Pd/C catalyst (&) and 30 mg 10% Pd/C catalyst (*) at 60 8C; atmospheric pressure of hydrogen.
r¼
dC oClT ¼ f ðtÞ dt
(5)
12
T. Janiak / Applied Catalysis A: General 335 (2008) 7–14 Table 3 Kinetic characteristics of o-chlorotoluene hydrogenolysis at different temperatures in the presence of 100 mg 10% Pd/C, 5% Pd/C and 3% Pd/C catalystsa 3% Pd/C catalyst 20 8C k0r Ko-ClT C0
40 8C 2 1
2.3 10 s 12 1.1 102 M
60 8C 2 1
9.4 10 s 19 8.0 103 M
2.0 101 s1 70 5.4 103 M
40 8C
60 8C
5% Pd/C catalyst 20 8C k0r Ko-ClT C0
2 1
4.2 10 s 9 1.2 102 M
2 1
9.1 10 s 26 8.5 103 M
1.7 101 s1 69 6.0 103 M
40 8C
60 8C
10% Pd/C catalyst Fig. 6. The rate of o-chlorotoluene hydrogenolysis versus o-chlorotoluene concentration in the presence of 3% Pd/C catalyst preconditioned by method A at 60 8C; atmospheric pressure of hydrogen. The solid line represents the least-squares fit of Eq. (4) to differentiated experimental data, (&) values not used in the fitting procedure.
where dCo-ClT/dt is the rate of o-chlorotoluene dechlorination and f(t) is an unknown function of time. The data set in the form of Eq. (5) was then transformed to data set in the form of Eq. (6): r¼
dC oClT ¼ f ðCoClT Þ dt
(6)
in which f(Co-ClT) is the kinetic model of the process under scrutiny. The theoretical kinetic model described by Eq. (4) was fitted to the experimental data altered according to the above procedure. By way of example, the curve obtained for the hydrogenolysis conducted in the presence of 3% Pd/C at 60 8C is shown in Fig. 6. Eq. (3) was also fitted to the experimental data. The values generated in these fittings differ only slightly from those obtained with Eq. (4): for example, in the experiment with 100 mg 10% Pd/C at 60 8C we obtained k0r ¼ 2:5 101 s1 , C0 = 3.8 103 M, Ko-ClT = 54 using Eq. (4) and k0r ¼ 2:4 101 s1 , C0 = 4.0 103 M KoClT = 53, KTol = 40 using Eq. (3). The constants k0r , C0, and Ko-ClT given in Table 3 were obtained by the least squares fitting of Eq. (4) to the differentiated kinetic data [39] and combining the results with the o-chlorotoluene adsorption data for a given catalyst. The latter were obtained by equilibration of the n-heptane solution containing 0.25 M o-chlorotoluene and 0.025 M n-dodecane with each of the catalysts investigated at 60, 40 and 20 8C. The kinetic characteristics k0r and Ko-ClT for a given catalyst increase with rising temperature, whereas C0 decreases with rising temperature. The k0r values in Table 3 for 3%, 5% and 10% Pd/C catalysts at 40 and 60 8C are quite similar within the range of experimental precision. This may suggest that the number of active sites on the palladium ‘crystallites’ are similar, despite the different masses of metal.
20 8C k0r Ko-ClT C0
2 1
3.1 10 s 5.4 1.4 102 M
1 1
1.2 10 s 17 6.8 103 M
2.5 101 s1 54 3.8 103 M
a Values obtained from the least squares fitting of Eq. (4) to the differentiated experimental data.
The activation energies (DEr) were calculated on the basis of the Arrhenius equation, separately for k0r and ko. The adsorption enthalpies of o-chlorotoluene (DHa) were also calculated using the van’t Hoff equation and adsorption constants Ko-ClT from Table 3. Table 4 lists the calculated values of (DEr) and (DHa). The adsorption enthalpies (DHa) increase with increasing palladium content in the catalyst; the activation energies (DEr) calculated using both k0r and k0 display a minimum for the 5% Pd/ C catalyst. There are no simple explanations as to why the activation energy for the 3% Pd/C, 5% Pd/C and 10% Pd/C catalysts display a trend with a minimum for 5% Pd/C. It is possible that at least one of the DEa values is influenced by some physical process. Transport phenomena could well affect the kinetics of hydrogenolysis at a lower temperature. It is also likely that the kinetic constant k0 at a lower temperature (20 8C) is affected by some unknown process taking place on the catalyst during hydrogenolysis. A white deposit was observed during the dispersion measurements of the catalyst (ca. 13% mass loss for Table 4 Activation energies (DEa) of o-chlorotoluene hydrogenolysis in the presence of 10% Pd/C, 5% Pd/C and 3% Pd/C catalysts; adsorption enthalpies (DHad) of ochlorotoluene on these catalysts Catalyst
DEa kJ mol1
3% Pd/C 5% Pd/C 10% Pd/C
44.9a 28.8a 43.1a
DHad kJ mol1 35.6b 20.6b 26.8b
33d 27d 32d
35.8c 41.9c 47.3c
a Values calculated using k0r from Table 3 and the Arrhenius equation for 20, 40 and 60 8C. b Values calculated using k0 from Table 1 and the Arrhenius equation for 20, 40 and 60 8C. c Values calculated using Ko-ClT from Table 3 and the van’t Hoff equation for 20, 40 and 60 8C. d Values calculated using k0 from Table 1 and the Arrhenius equation for 40 and 60 8C.
T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
10% Pd/C catalyst). Also a white substance, a silicon-based organic polymer, precipitated in the aqueous layer separated after hydrogenolysis when this had been neutralized to pH 7. It is seen from Fig. 6 that the rate of reaction for hydrogenolysis over the 3% Pd/C catalyst at 60 8C rises from r0 = 7.1 104 Ms1 to r = 9.1 104 Ms1. This is probably due to minor changes in the catalyst’s properties. Hence, the leaching of some catalyst additive may influence the reaction kinetics, especially at the beginning of the hydrogenolysis, and this effect may be pronounced at lower temperatures. To verify this we calculated the activation energy (DEa) for two temperatures 40 and 60 8C only, obtaining 32 kJ mol1 for 10% Pd/C; 27 kJ mol1 for 5% Pd/C and 33 kJ mol1 for 3% Pd/C. These values are close to each other within experimental error. The lower activation energy, 27 kJ mol1 for 5% Pd/C, should induce quicker hydrogenolysis of o-chlorotoluene over this catalyst in comparison with the 3% and 10% Pd/C catalysts. The kinetic characteristics given in Table 1 confirm this. For o-chlorotoluene hydrogenolysis over 5% Pd/C at 20 8C, t90% = 1102 s; for 3% Pd/ C, t90% = 1705 s; and for 10% Pd/C, t90% = 1330 s. The small differences between the activation energies and adsorption enthalpies for the same catalyst, shown in Table 4, may suggest that the main energy threshold of the process is linked to the dissociative adsorption of o-chlorotoluene on the catalyst. The stability of catalytic activity in the hydrogenolysis reaction was assessed in separate experiments for all catalysts. In these experiments, after the typical procedure (method A), an extra portion of o-chlorotoluene (2.5 103 mol) was added to the reaction mixture immediately before reaction completion. The results for 10% Pd/C catalyst at 60 8C are presented in Fig. 7. It shows that the pseudo-zero-order rate constant has decreased from k0 = 8.2 104 Ms1 to k0 = 4.5 104 Ms1 (1.8 times) after the addition of a fresh portion of o-chlorotoluene. This suggests a loss of catalytic activity. It is probable that the catalyst is deactivated as a result of the palladium being etched by the strong alkaline medium used in these experiments. It has been
Fig. 7. Hydrogenolysis of o-chlorotoluene in the presence of 100 mg 10% Pd/C catalyst preconditioned by method A at 60 8C, atmospheric pressure of hydrogen. An extra portion of o-chlorotoluene (2.5 103 mol) was added immediately before reaction completion.
13
Fig. 8. Hydrogenolysis of a mixture of 0.166 M p-chlorotoluene and 0.083 M o-chlorotoluene in the presence of 100 mg 10% Pd/C catalyst preconditioned by method A at 20 8C. (&) concentration of toluene, (*) concentration of ochlorotoluene (~) concentration of p-chlorotoluene.
postulated that one of the main causes of catalyst deactivation in the liquid phase is etching of palladium, a process that depends on the palladium particle size and intensifies with increasing palladium particle size [40]. The average particle size (dav) obtained from dispersion measurements (D) (using the relation dav [nm] = 1.12/D [41]) for the catalysts used in our investigations are: 12.1 nm for 10% Pd/C and 5.5 nm for 5% Pd/C. Accordingly, catalytic activity decreased 1.8 times for 10% Pd/C and 1.5 times for 5% Pd/C, which tallies well with the above postulates. The 3% Pd/C catalyst activity decreases 1.2 times, and this tendency, a 1.2 times slower reaction rate, is retained after addition of the next 2.5 103 mol of o-chlorotoluene. The catalysts and the reaction conditions used were also tested in competitive experiments to study their selectivity in hydrogenolysis. Two tests were carried out. A mixture of nheptane, n-dodecane, o-chlorotoluene and p-chlorotoluene was hydrogenolysed by method A at 20 8C in the presence of 100 mg Pd/C catalyst. The results indicate that the catalyst – 10% Pd/C – does not exhibit any noticeable selectivity in the hydrogenolysis reaction. It was also demonstrated experimentally that the reaction rate for a given compound is proportional to its concentration in the mixture. The results are presented in Fig. 8. The values obtained are: r0 = 3.5 104 Ms1 for 0.166 M pchlorotoluene and r0 = 1.8 104 Ms1 for 0.083 M o-chlorotoluene. In another experiment, a mixture of n-heptane, n-dodecane, 0.082 M o-chlorotoluene, 0.082 M 1,2-dichlorobenzene and 0.082 M 4-chlorobiphenyl was hydrogenolysed under the conditions described above. The initial rates of hydrogenolysis of these compounds differed: r0 = 2.4 104 Ms1 for 4-chlorobiphenyl, r0 = 8.1 105 Ms1 for 1,2 dichlorobenzene and r0 = 3.2 105 Ms1 for o-chlorotoluene. 4. Conclusions The hydrogenolysis of o-chlorotoluene in the presence of Pd/C catalyst removes chlorine from it with a yield of up to
14
T. Janiak / Applied Catalysis A: General 335 (2008) 7–14
100%, which is a promising result if the intention is to dechlorinate this particular compound. All three catalysts used exhibit a similar catalytic activity. The extent of dechlorination increases with time and temperature, and depends on the amount of catalyst in the system. Kinetic analysis based on the Langmuir–Hinshelwood approach enabled the kinetic constants to be determined. This may be useful for modelling the path of o-chlorotoluene dechlorination. The results of these investigations have extended our knowledge on an important group of reactions, and they form a convenient framework within which to consider the treatment of specific simple wastes, by-products and even complex organochlorine wastes. Acknowledgements The authors wish to thank Dr J. Okal from the Institute of Low Temperature and Structure Research of the Polish Academy of Sciences in Wroclaw for the dispersion measurements of the catalysts. The financing of this work from the State Funds for Scientific Research (grant No. DS/8000-4-0026-7) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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