Effect of the Functional Group on the Hydrodechlorination of

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Effect of the Functional Group on the Hydrodechlorination of Chlorinated Aromatic Compounds over Pd, Ru, and Rh Supported on Carbon Jose Luis Benítez*,† and Gloria Del Angel‡ † ‡

Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, Mexico D.F. Departamento de Química, Universidad Autonoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco No. 186, C. P. 09340, Mexico D.F. ABSTRACT: The effect of a functional group on the hydrodechlorination chlorinated aromatic compounds on Pd/C, Ru/C, and Rh/C catalysts was studied in liquid phase at 30 °C and 2 kg/cm2 of hydrogen pressure, using 200 ppm of the chlorinated organic compound. An effect of particle size on Pd catalysts was observed, small particles showed to be more resistant to the deposit of chloride from the reaction. The activity in the hydrodechlorination of organic compounds as a function of the functional group increases in the following order p-chlorophenol > p-chlorotoluene > p-dichlorobenzene. The activity in the hydrodechlorination of these molecules increases with the presence of donating groups as -CH3 and -OH. The p-chlorotoluene hydrodechlorination on Ru and Rh leads also to the hydrogenation of the aromatic ring (toluene, methylcyclohexane). On the other hand, the p-chlorophenol hydrodechlorination over Pd, Ru, and Rh produces deoxygenated products (chlorobenzene, phenol, and benzene). Pd showed the highest activity followed by Ru and Rh.

’ INTRODUCTION It has been estimated that around 2000 chlorinated hydrocarbons (CHC’s) are discharged into our biosphere, either by man or natural causes.1 In addition, at the industrial level more than 250 organic chemicals have been detected in effluents, some of which are considered to be dangerous to the environment.2 Examples are the chlorophenols,3 dibenzofurans,4 polychlorinated biphenyls,5 etc.; all of these compounds require special handling, because they can cause groundwater contamination. The CHC’s have been detected both in natural and drinking water; therefore, it is considered an environmental problem that the scientific society has tried to solve.6 Due to its toxicity and accumulation potential,7 these compounds are environmentally persistent, accumulating in the fatty tissues showing carcinogenic and mutagenic activity.8 There are a number of compounds have in their structure not only chlorine atoms but also nitrogen, sulfur, oxygen, and other atoms; some of these compounds are also toxic for humans. They have been used for decades in the industry, specifically in oil refining processes.9,10 For the destruction of these pollutants, different processes have been studied finding incineration as the most used; nevertheless, this process has certain limitations in its effectiveness.11,12 Nowadays moderate conditions are being studied, such as catalytic hydrodechlorination using noble metals especially palladium and rhodium.13 This technology eliminates the formation of dangerous products; also, this technology is simple, secure, and effective. Amorim et al.14 reported the effect of the functional group in the hydrodechlorination on chlorobenzenes and chlorophenols compounds with Pd/C catalysts. Kawabata15 et al. studied the effect of functional groups in several chlorinated organic molecules on Pd catalysts using MCM-41 as support, observing that the chlorobenzaldehydes are less active. r 2010 American Chemical Society

Table 1. Characterization of Different Catalysts

a

metal content

dispersion

mean particle

ASNa

catalyst

(wt %)

chemisorptions

size (Å)

( 1019)

Pd/C-A Pd/C-B

0.72 0.88

30 10

35 85

1.24 50.0

Ru/C

2.65

40

25

8.02

Rh/C

2.85

9

117

1.41

Active sites number.

In the present paper is reported a study on the effect of functional group on the activity and selectivity of catalytic hydrodechlorination of p-chlorobenzene (p-diClBz), p-chlorotoluene (p-Cltol), and p-chlorophenol (p-ClPh) on Pd, Ru, and Rh supported on carbon.

’ EXPERIMENTAL SECTION The Pd/C, Ru/C, and Rh/C catalysts were prepared by impregnation of the GAC 1240 carbon support (ELF-Atochem) with a surface area of 1200 m2/g. The support was ground in a china mortar and passed through a sieve with the following characteristics: mesh number, 150; wire number, 46; diameter, 0.06 mm; opening, 0.105 mm; opening area, 38.4%. Aqueous ammonium hydroxide (pH 11) solutions of Pd(NH3)4(NO2)2, [Rh(NH3)5Cl]Cl2, and [Ru(NH3)5Cl]Cl2 were used as precursors. Special Issue: IMCCRE 2010 Received: March 22, 2010 Accepted: July 30, 2010 Revised: July 26, 2010 Published: September 15, 2010 2678

dx.doi.org/10.1021/ie100702s | Ind. Eng. Chem. Res. 2011, 50, 2678–2682

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Figure 1. Hydrodechlorination of p-dichlorobenzene in (a) Pd/C-A and (b) Pd/C-B catalysts.

Carbon Support Treatment. To 50 g of ground and sieved carbon, in an Erlenmeyer flask with a capacity of 250 mL, 125 mL of HNO3 (1.5 M) was added; this mixture was heated at 50 °C for 2 h with slow stirring. Afterward, it was cooled down at room temperature, filtered, and washed with deionized water until reaching a pH of 6. The carbon support was dried overnight at 120 °C in an oven. The dried carbon was placed in a fixed-bed glass reactor; and a N2 flow of 3.6 L/h was passed for 8 h at 100 °C. The carbon then was calcined under air at 300 °C with a rate of 6 °C/min for 2 h. Catalyst Preparation. In an Erlenmeyer flask (250 mL), 5 g of the treated carbon and 5 mL of deionized water were placed; afterward, the corresponding metallic solution was added little by little with magnetic stirring during 4 h at room temperature. The mixture then was heated to evaporate the water in a heating plate until the solid was almost dried. Then, it was placed in an oven at 120 °C under nitrogen flow. The catalyst was calcined under air at 200 °C for 4 h using a temperature ramp of 3 °C/min with a flow rate of 3.6 L/h. Subsequently, the catalyst was cooled down at room temperature and the solid was purged for 30 min with nitrogen flow at a rate of 1 L/h. After this, the nitrogen was substituted by hydrogen, and the temperature was increased to 500 °C and kept for 2 h. Then the catalyst was cooled down at room temperature and placed in a desiccator until being used. The catalysts prepared were labeled as Rh/C, Ru/C, and two Pd/C catalysts with different dispersion were prepared named: Pd/C-A and Pd/C-B (see Table 1). The load metals in the catalysts were determined by atomic absorption (Table 1). The metallic dispersion percentage was determined by CO chemisorption at 70 °C in a conventional gravimetric apparatus.16 To calculate the number of active sites, stoichiometric ratios of CO/Pd, CO/Rh, and CO/Ru equal to 1 were assumed. The particle size was estimated by the independent transmission electron microscopy (TEM) technique using a JEOL 100CX apparatus. The mean particle size was calculated using the expression Σdnidi3/Σdnidi2, where di is the diameter measured directly from the micrograph and ni is the number of particles having the diameter di. The measurements concerning the catalytic activity were performed in a glass reactor (150 mL) with magnetic stirring. First, 0.08 g of previously reduced catalyst was placed in the reactor; then, 70 mL of a solution methanol/water 50:50 to 200 ppm of the organic compound in the study were added. The reactor was locked, and magnetic stirring was started at 872 rpm for 20 min (necessary time to reach the adsorption equilibrium between the chloroaromatic and the catalyst) before adding hydrogen.

Table 2. Activity Per Site (TOF) for Pd/C (A,B) in the Hydrodechlorination of p-Dichlorobenzene

a

catalyst

TOF1min (1/s)

TOF50%a (1/s)

mean particle size (Å)

Pd/C-A

3.99

2.02

35

Pd/C-B

5.19

0.09

85

50% of conversion.

The reaction was carried out at room temperature (30 °C); then, the reactor was purged five times with hydrogen (oxygen free) at a pressure of 2 kg/cm2; finally, it was set at the same purge pressure. The reaction was started when stirring at 872 rpm was reached. The reaction sequence was followed by analyzing samples at time intervals. The samples were analyzed by coupled gas chromatography-mass spectrometry (CG/MS) with a model 5971A Hewlett-Packard apparatus, equipped with a HewlettPackard Ultra-2 capillary column of (cross-linked 5% Ph Me silicone) 30 m  0.25 mm, 0.25 μm film thickness. The results were plotted like concentration in percent (We used 200 ppm of the organic molecule, which represents 100%.) as a function of time. In this way, we can follow, in the same figure, the evolution of the reactant and the formation of the subproducts. The activities expressed as turnover frequency (TOF, 1/s; number of molecules of reactant transformed per unit time per surface metal atom) were determined from metal dispersion values and the rate of reaction (1 min and 50% of transformation). The absence of external and internal transfer limitations was corroborated and reported elsewhere.16-18

’ RESULTS AND DISCUSSION p-Dichlorobenzene Hydrodechlorination. The hydrodechlorination of p-dichlorobenzene on Pd/C-A and Pd/C-B catalysts at moderated conditions of pressure and temperature follows a consecutive mechanism, where the chlorobenzene is observed as an intermediate product and benzene as final product (Figure 1). It was found that the initial activity (TOF) of p-dichlorobenzene depends of the palladium particle size; big particles are more active than the smaller ones (Table 2). However, comparing the activities to constant conversion (50%), it is observed that for Pd catalyst with smaller particle size, the activity per site is higher. This means that small Pd particles are more resistant to the deposit of chlorine arising from the hydrodechlorination reaction. Small particles are considered as electron deficient species, therefore these particles are less reactive toward electrophilic 2679

dx.doi.org/10.1021/ie100702s |Ind. Eng. Chem. Res. 2011, 50, 2678–2682

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Figure 2. Hydrodechlorination of p-dichlorobenzene in (a) Ru/C and (b) Rh/C catalysts.

Table 3. Activity Per Site (TOF) for Pd/C (A,B) in the Hydrodechlorination of p-Chlorotoluene TOF1min (1/s)

TOF50%a (1/s)

mean particle size (Å)

Pd/C-A

4.5

0.65

35

Pd/C-B

11.42

0.68

85

catalyst

a

50% of conversion.

species (Cl1-), this explains why the small particles are more resistant to the deposit of chlorine on the metal surface.16,19 Ru/C and Rh/C presented the same reaction products, chlorobenzene and benzene (Figure 2). However, Ru/C presented a higher reaction conversion reaching 92% of transformation at 120 min whereas Rh/C presented 58% at the same time of reaction; this can be related to the differences in particle size. Ru shows a lower particle size (25 Å) than Rh (97 Å). Nevertheless, the activity of these two catalysts is lower than the Pd/C-A catalyst. This difference in activity of Ru and Rh could be explained by the presence of the chlorine residual coming from the precursors of Ru and Rh, which can influence the activity of these catalysts. p-Chlorotoluene Hydrodechlorination. The activity per site for the hydrodechlorination of p-chlorotoluene on Pd/C-A and Pd/C-B catalysts is shown in Table 3. The main product of the reaction is toluene; although traces of methylcyclohexane, a product of the hydrogenation of the aromatic ring, were also detected. In this reaction is observed the same behavior in terms of activity as in the case of the hydrodechlorination of p-dichlorobenzene, higher initial activity of the large Pd particles (Figure 3) for the hydrodechlorination of p-chlorotoluene than smaller ones. The activity per site of p-chlorotoluene at 50% of transformation is similar in both cases. However, as it can be seen in Figure 3, the Pd/C-A catalyst present a total transformation of the p-dichlorobenzene at 45 min while for Pd/C-B at 120 min only an 87% transformation is obtained. Using the same argument, Pd particles sizes are more active due to the resistance to deactivation by the chlorine release in the reaction media. Comparing the results of hydrodechlorination of p-dichlorbenzene and p-chlorotoluene on small Pd particles (Pd/C-A catalyst), an effect of functional group is observed; the hydrodechlorination of p-chlorotoluene is faster than that of p-dichlorbenzene. On the other hand, the amount of chlorine released in this last case is 2 times higher than for p-dichlorbenzene.20 An effect on the nature of the metal as catalyst in the p-chlorotoluene hydrodechlorination is observed. Contrary to Pd/C catalysts, where only the toluene was detected as product, the

Figure 3. Effect of particle size in hydrodechlorination of p-chlorotoluene on Pd/C (A,B) catalysts.

hydrodechlorination of p-chlorotoluene on Ru and Rh catalysts leads to a consecutive mechanism, where toluene is the intermediate and methycyclohexane the final product. The hydrogenation of the aromatic ring is more easily on Ru/C catalysts (Figure 4).

p-Chlorophenol Hydrodechlorination. The initial activity per site on Pd/C-B catalysts for the hydrodechlorination of p-chlorophenol is higher than that for the Pd/C-A catalyst (Table 4). The same behavior at 50% of conversion was observed. In the reaction of p-chlorophenol on Pd/C catalysts, three reaction products were identified, chlorobenzene, phenol, and benzene. This reaction is a parallel mechanism taking place: on the one hand, the hydrodechlorination reaction produces phenol, meanwhile deoxygenation is obtaining chlorobenzene; the formation of benzene as final product comes from a competitive reaction between chlorobenzene and phenol, Figure 5 (Pd/C-B catalyst). The same behavior is observed in both Pd/C catalysts: the hydrodechlorination reaction (phenol) is bigger than the deoxygenation of p-chlorophenol reaction; this contrasts with the report by Hoke21 et al. in the same reaction, who found only 2680

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Figure 4. Hydrodechlorination and hydrogenation of p-chlorotoluene for the catalysts: (a) Ru/C and (b) Rh/C.

Table 4. Activity Per Site (TOF) for Pd/C (A,B) in the Hydrodechlorination of p-Chlorophenol

a

catalyst

TOF1min (1/s)

TOF50%a (1/s)

mean particle size (Å)

Pd/C-A Pd/C-B

6.59 19.46

1.03 4.32

35 85

50% of conversion.

Figure 6. Hydrodechlorination and hydrodeoxygenation of p-chlorophenol on Rh/C catalyst.

Figure 5. Hydrodechlorination and hydrodeoxygenation of p-chlorophenol on Pd/C catalyst.

the hydrodechlorination reaction with the formation of phenol, at moderate conditions of pressure and temperature in basic medium using a Pd/C to 5% as catalyst. On the contrary, Hagh et al.22 using NiMo/Al2O3 as catalyst at temperatures between 225-275 °C for the same reaction reported the simultaneous hydrodechlorination and desoxygenation, the rate of the hydrodechlorination of chlorophenol being 2 orders of magnitude bigger than that of the deoxygenation. The Rh/C and Ru/C catalysts showed less activity than the Pd/C catalyst (Figures 6 and 7) and presented different selectivity in the products. There is a competition between the hydrodechlorination, deoxygenation, and hydrogenation of the aromatic ring, the Rh/C being more active than Ru/C. The hydrodechlorination reaction and the hydrogenation of the aromatic ring are more important on an Rh/C catalyst, while on the Ru catalyst the hydrogenation of the aromatic ring predominates. The effect of the functional group on the catalytic activity can be explained by the type of substituents, whether it is donating to or subtracting from electronic density. The donating groups, in

Figure 7. Hydrodechlorination and hydrodeoxygenation of p-chlorophenol on Ru/C catalyst.

this case the -CH3 and -OH,23 increase the electronic density of the aromatic ring; the substituent that subtracts electronic density weakens the aromatic ring. In the molecule of p-dichlorobenzene the presence of the two -Cl substituents decreases the electronic density of the aromatic ring so this would be more reactive on sites with higher electron density such as large metal particles; this agrees with the major activity that presents the catalyst of palladium with lower dispersion. 2681

dx.doi.org/10.1021/ie100702s |Ind. Eng. Chem. Res. 2011, 50, 2678–2682

Industrial & Engineering Chemistry Research In the case of p-chlorotoluene, both effects are present: on the one hand, -Cl that subtracts electronic density and, on the other hand, the radical methyl -CH3 donating to electronic density. The methyl group causes a weakening of the bond C-Cl, making this one more reactive, as it is observed comparing the activity of p-chlorotoluene with the p-dichlorobenzene. The hydrodechlorination is favored with respect to the demethylation of p-chlorotoluene as we see with Pd catalysts. This might be also related to the differences in dissociation energies that have the aromatic carbon-chlorine and aromatic carbon-methyl whose values are 406 and 427 kJ/mol, respectively. That shows that the molecule of toluene is more stable than chlorobenzene. In p-chlorophenol hydrodechlorination, the -OH substituent is more electronegative, therefore, it causes a further weakening of the C-Cl. Chlorine increases its reactivity, thus favoring the hydrodechlorination with respect to the deoxygenation as it can be seen from the data in Figure 5. The dissociation energy of the aromatic carbon-oxygen bond (469 kJ/mol) is higher than the corresponding carbon-chlorine (406 kJ/mol). In this case, the first bond is more stable than the latter. However, the specificity of the metal can be taken into account as we saw that, on Ru and Rh catalysts, the hydrogenation of the aromatic ring is done easily.

’ CONCLUSION The initial activity per site for the hydrodechlorination of p-dichlorobenzene, p-chlorotoluene, and p-chlorophenol is higher in larger particles of Pd/C. Pd small particles are more resistant to deposits of chlorine from the hydrodechlorination of p-dichlorobenzene. The activity of the hydrodechlorination depends on the functional group, the order in activity is as follows: p-chlorophenol > p-chlorotoluene > p-dichlorobenzene. The activity of hydrodechlorination of these molecules is increased by the presence of donor groups such as -CH3 and -OH. The order of activity in the hydrodechlorination of p-chlorophenol, p-chlorotoluene, and p-dichlorobenzene with respect to the nature of the metal decreases in the following order: Pd . Ru > Rh. The p-chlorotoluene hydrodechlorination on Ru and Rh catalysts is produced besides hydrogenation of the aromatic ring. In the p-chlorophenol hydrodechlorination reaction on Pd, Ru, and Rh catalysts, reactions of hydrodechlorination and deoxygenation are detected giving chlorobenzene, phenol, and benzene as products following a parallel mechanism.

’ AUTHOR INFORMATION

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*Tel.: 91756871. E-mail: [email protected].

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dx.doi.org/10.1021/ie100702s |Ind. Eng. Chem. Res. 2011, 50, 2678–2682