Hydrogenation of Chlorobenzene to Cyclohexane over Colloidal Pt

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Environ. Sci. Technol. 2009, 43, 2519–2524

Hydrogenation of Chlorobenzene to Cyclohexane over Colloidal Pt Nanocatalysts under Ambient Conditions M A N H O N G L I U , * ,† M E I F E N G H A N , † A N D W I L L I A M W . Y U * ,‡ College of Material Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China, and Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609

Received December 6, 2008. Revised manuscript received January 26, 2009. Accepted February 9, 2009.

A series of finely dispersed poly(vinyl-2-pyrrolidone)stabilized platinum colloidal nanocatalysts (PVP-Pt) were prepared and characterized by TEM. Hydrogenation of monochlorobenzene (MCB) was carried out in a batch mode using hydrogen over PVP-Pt at 298 K and atmospheric pressure. The product consisted of benzene and cyclohexane during the reaction, and nearly 100% selectivity to cyclohexane could be obtained at ∼100% conversion of MCB. The catalytic performance of the PVP-Pt colloids is dependent on the preparation conditions. The small amount of the stabilizing polymer (PVP) in the preparation of colloidal platinum could not protect the platinum colloid commendably, but the large amount of PVP hindered the contact of reactant with catalyst surface and the desorption of product. Extra PVP added in the reaction system has some inhibiting effect on the reaction activity, and also resulting in some decrease in selectivity to cyclohexane. The reaction was verified to be first order to the concentration of MCB. The polymer-stabilized noble metal nanoparticles could be suitable to be hydrodehalogenation and reduction catalysts for the remediation of various chloroaromatic compounds in the environment.

Introduction Chlorinated organic compounds are widely used as starting materials, intermediates and solvents in chemical industry, agricultural chemistry, and medical chemistry (1, 2). However, these compounds especially chloroarenes are harmful to many aspect of the environment and human life because of their acute toxicity, thermal stablility, and strong bioaccumulation potential (3). These compounds also contribute to global warming and ozone depletion (4). In order to clear these chlorinated organic compounds, or convert them to alternative materials which are environmentally acceptable, many fundamental investigations have been done in incineration (5, 6), microbial culture (7-9), radiolysis (10), and photocatalysis (11). However, incineration leads to the * Address correspondence to either author. Phone: +86-532-84022814 (M. L.); 1-508-831-4115 (W. W. Y.). Fax: +86-532-8402-2814 (M. L.); 1-508-831-4116 (W. W. Y.). E-mail: [email protected] (M. L.); [email protected] (W. W. Y.). † People’s Republic of China. ‡ Worcester Polytechnic Institute. 10.1021/es803471z CCC: $40.75

Published on Web 03/06/2009

 2009 American Chemical Society

formation of more toxic products like dioxins and phosgene due to the incomplete oxidation. Other methods are also inefficient in conversion with difficulty in scale-up. Consequently, better handling methods and decontamination techniques of the chloroaromatic chemicals have attracted increasing attention. Indeed, catalytic hydrodechlorination (HDC) (12) has emerged as a promising nondestructive technology with low energy demands proposed for remediation of chlorinated organic compounds. It can not only convert toxic substances into safer compounds, but also produce useful products from the chlorinated wastes (13-15). Catalytic HDC has been reported in both liquid and vapor phases over supported catalysts of noble metals (Pd (16), Pt (17, 18), Rh (18), Ru 19, 20) and nonnoble metals (Ni (21), Ni-Mo (13), Fe (22)). In addition, in the point of view of the “green” organic synthetic methodologies, much attention has been focused on the development of highly efficient catalytic protocols using molecular hydrogen (23-27) in order to achieve the “clean” dehalogenation of organic halides. Although HDC is widely used to treat chlorinated aliphatic hydrocarbons, there are few reports on the HDC of chloroarenes, especially with nanoscale metal catalysts. Zhu et al. (28) studied HDC of chlorobenzene with 1-3 chlorine atoms on nanoscale Pd/Fe catalysts at room temperature. Chlorinated benzenes could be completely reduced to benzene. However, the aged Pd/Fe particles exhibited significant decrease in dechlorination reactivity. Zhang et al. (29-31) explored the dechlorination of chlorinated organic compounds using Fe0 and Pd/Fe powders as catalysts. Freshly synthesized nanoscale Fe particles were more reactive than the commercial Fe powders (32), mainly due to the much higher specific surface area. Nanoscale Pd/Fe bimetallic particles (nano Pd/Fe) were even more reactive than the pure Fe because the addition of Pd onto Fe surface significantly reduced the oxidation of iron, thus preserving the reactivity of the zerovalent iron. However, transformation of Fe to Fe2+ would cause Fe depletion over time on the treatment to chlorinated organic compounds. Nakao et al. (33) studied hydrogenation and dehalogenation of chloroarenes with 1-5 chlorine atoms under aqueous conditions over an amphiphilic-polymer-supported nanopalladium catalyst. Chlorined aromatics were readily dechlorinated to give the corresponding reduced products with high yields (89-99%), where wide functional group tolerance for benzylic hydroxyl, phenolic hydroxyl, amine, ketone, amide, carboxylic acid, and carboxylic ester was noted. Liu et al. (34) studied the HDC of chlorobenzene over Pd nanoparticles supported on carbon nanofibers and found a promising HDC performance. It showed high activity and impressive time-on-stream stability in the gas- and liquidphase catalytic hydrodechlorination of chlorobenzene. However, most of the reactions are often incomplete, and produce aryl compounds, such as benzene, aniline, naphthalene, which are also the environmental pollutants. In this paper, an efficient and convenient dehalogenation and hydrodearomatisation using molecular hydrogen catalyzed by poly(N-vinyl-2-pyrrolidone)-stabilized platinum colloids (PVP-Pt) was reported. Monochlorobenzene (MCB), as a model reactant, was hydrodechlorinated to benzene and finally to cyclohexane. The present catalytic methodology possesses several attractive features: (1) high catalytic efficiency, (2) use of atmospheric pressure molecular hydrogen, and (3) aliphatics obtained by complete dehalogenation and VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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hydrodearomatisation. The above characteristics make it an ideal environmentally benign process in treating halogenated wastes.

Experimental Section Materials and Instrument. Poly(N-vinyl-2-pyrrolidone) (PVP, average molecular weight 10 000) was purchased from Shanghai Chemicals Co. Hexachloroplatinic acid of analytical grade was supplied by Beijing Chemicals Co. Hydrogen (H2) with a purity of 99.999% was supplied by an extra-pure hydrogen generator HA-300. Monochlorobenzene was distilled before use. Other reagents were of analytical grade supplied by Qingdao Chemicals Co. and were used directly. Transmission electron microscopy (TEM) photographs were taken by using a JEOL-2100 electron microscope. Specimens were prepared by placing a drop of the colloidal dispersion on a copper grid covered with a perforated carbon film and then evaporating the solvent. The particle diameters were measured from the enlarged TEM photographs. The particle size distribution histogram was obtained on the basis of the measurements of about 300-400 particles. Preparation of Polymer-Stabilized Platinum Colloids. The preparation of the PVP-Pt colloids was similar to the literature (35-42). Typically, in a 100 mL flask equipped with a condenser, an oil bath, and a magnetic stirrer,we dissolved 0.222 g PVP (2.00 × 10-3 mol as monomeric unit) and 0.052 g H2PtCl6 · 6H2O (1.00 × 10-4 mol) in a mixed solution of 30 mL methanol and 30 mL water, vigorous stirring was maintained all the time. The reaction mixture was refluxed for 3 h to give a homogeneous dark-brown solution of colloidal platinum. They were evaporated to dryness with rotated evaporator under reduced pressure below 60 °C. The resulting solid residues were redispersed in methanol with a definite concentration, giving a thoroughly homogeneous dispersion prior to the reaction. Hydrogenation Reaction. Hydrogenation of MCB was carried out at 298 K under 0.1 MPa of hydrogen. Typically, 1.00 mmol MCB, 10.0 mL PVP-Pt methanol dispersion (containing 5.00 × 10-5 mol Pt), and methanol (totaling 30 mL) were placed in a three-neck flask equipped with a magnetic stirrer and a thermostatic water bath. After H2 was charged several times to replace air, the mixture was stirred vigorously and the reaction started. The initial reaction rate was calculated by the uptake rate of hydrogen. Chemical analysis of the products was analyzed by gas chromatography (GC) equipped with a FID detector and an AC-10 column. Reactants and products were identified by comparison with authentic samples. Octane was used as an internal standard.

Results and Discussion Characterization of Polymer-Stabilized Platinum Colloidal Nanoparticles. A series of finely dispersed PVP-Pt colloidal nanocatalysts were prepared by varying the molar ratio of PVP (in monomeric unit) to Pt in the range of 10:1-40:1 and characterized by TEM. The preparation conditions of PVP-Pt colloidal nanoparticles, together with the corresponding average particle diameters and standard deviations (σ) obtained through TEM (Figure 1), are summarized in Table 1. It was found that all PVP-Pt nanoparticles were in the size range of 2.4-5.6 nm. When the molar ratio of PVP to Pt changed from 10/1 to 40/1, no significant effect on particle size (dav ) 3.60-3.63 nm) and size distribution (σ ) 0.55-0.65 nm) was observed, which was consistent with the literature (37). TEM measurements show that PVP-Pt nanoparticles with the molar ratio of PVP (in monomeric unit) to Pt in the range of 20:1-40:1 are well dispersed and no aggregation of the metallic particles can be detected. It can be seen from Figure 1 (a) that there was a slight agglomeration of colloidal particles with PVP-Pt1 in which the molar ratio of PVP to Pt was 10:1. So, more PVP placed in the Pt colloid synthesis is good for the particle’s dispersion. 2520

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Selective Hydrogenation of MCB over PVP-Pt Colloids. The prepared metallic colloids were used as catalysts in the hydrogenation of MCB in methanol at 298 K under 0.1 MPa of hydrogen. The hydrogenation products consist of benzene and cyclohexane in the beginning, and finally only cyclohexane (analyzed by GC-MS). This indicates that bezene can be further hydrogenated into cyclohexane at the ambient reaction conditions with these PVP-Pt nanocatalysts. The conversion of MCB over PVP-Pt2 colloid was investigated as a function of time. Results presented in Figure 2 (a) demonstrate that the hydrogenation of MCB is very effective over the PVP-Pt colloidal catalysts under the ambient conditions. The concentration of MCB in the reaction decreases gradually with time, and in the same time the concentration of cyclohexane in the reaction increases gradually. Benzene was immediately found in the beginning of the reaction, and started to convert to cyclohexane in the same time. That is to say, benzene is the intermediate product of the conversion of MCB to cyclohexane. It seems that only little amount of benzene diffused and leaved from Pt surface after the dechlorination of MCB, most of benzene was hydrogenated to cyclohexane immediately on the catalyst surface before it diffused to the solution, so the concentration of benzene was low in the whole reaction. To test whether benzene could be converted to cyclohexane over these PVP-Pt nanocatalysts, we incubated benzene with PVP-Pt2 under the same hydrogenation conditions. Indeed, it was indicated from Figure 2 (b) that the conversion of benzene was increased gradually with time, and only cyclohexane was detected by GC analysis. Benzene was hydrogenated to cyclohexane from 1 mmol (30 mL volume) in the beginning of the reaction, the reaction rate of benzene to cyclohexane was higher than that in the hydrogenation of chlorobenzene in the first hour for its concentration was much larger than that generated through the hydrogenation of chlorobenzene (see data points of solid squares in both figures of Figure 2). However, the reaction rate of benzene to cyclohexane was lower than that in the hydrogenation of chlorobenzene after 3 h. The average rate was 0.422 molbenzene · molPt-1 s-1, lower than the activity of benzene generated from chlorobenzene (0.646 molbenzene · molPt-1 s-1) although the latter concentration was much lower (the composition of benzene was 0.390 in Figure 2 (a) and 0.051 mmol (30 mL volume) in Figure 2 (b), respectively), indicating that in situ generated nascent benzene is much more active. Influence of the Amount of PVP. The effect of the amount of PVP in the preparation on hydrodechlorination of MCB was investigated by comparing the catalytic properties of PVP-Pt1, PVP-Pt2, and PVP-Pt3 (Table 2). First of all, the amount of PVP used in the preparation of PVP-Pt colloids is different, and extra PVP was added to reaction system till the molar ratio of monomeric unit of PVP to platinum (PVP: Pt) was 40:1. The data given in Table 2 indicated that the three catalysts were all active for the reaction, and their catalytic activities decreased as follows: PVP-Pt2 > PVP-Pt3 > PVP-Pt1. To assess the effect of metal dispersion on reaction rates, we calculated overall turn-over-frequencies (TOFs) in a 5 h reaction process. The catalytic activities, based on TOFs, still followed the same trend of PVP-Pt2 > PVP-Pt3 > PVP-Pt1. The catalytc activity of PVP-Pt1 was the lowest. Several papers reported the agglomeration of polymerstabilized metallic particles after catalytic reactions (43, 44). On hydrogenation of o-chloronitrobenzene over PVPstabilized platinum colloidal clusters, Yang et al. (43) reported that the average diameter of Pt particles had little change, but showed a tendency of coagulation of the particles after the catalytic reaction. Furthermore, a large part of metal precipitate could be found after keeping the reaction mixture with PVP-Pt for a month. In Liu’s paper (44), the hydro-

FIGURE 1. TEM photographs (left) and the corresponding particle size distribution histograms (right) of the PVP-stabilized platinum dispersions: (a) PVP-Pt1; (b) PVP-Pt2; (c) PVP-Pt3.

TABLE 1. Synthesis of PVP-Pt Colloids PVP-Pt colloids

PVPa

average diameter (nm)

standard deviation σ (nm)

PVP-Pt1 PVP-Pt2 PVP-Pt3

10 20 40

3.60 3.63 3.62

0.56 0.62 0.65

a The numbers are the molar ratio of PVP to Pt in the synthesis; Pt: 1.00 × 10-4 mol.

genation of o-chloronitrobenzene over polymer-stabilized ruthenium colloidal catalysts was studied. The reaction mixture was no longer homogeneous and partial precipitation of metallic ruthenium was observed after the reaction when small amount of the stabilizing polymer, PVP, was used in the preparation of colloidal ruthenium. TEM showed that the PVP-Ru nanoparticles had a tendency of aggregation even before the reaction. Our result in TEM (Figure 1a) showed that the PVP-Pt1 nanoparticles already had slight aggregation before reaction due to the small amount of the stabilizing polymer (PVP) in the particle synthesis which could

not protect the platinum colloid commendably. It was obvious that the tendency of coagulation of the platinum particles during the catalytic reaction would lead to a decrease of the catalytic activity of the Pt nanoparticles (45, 46). The catalytic activity of PVP-Pt2 was the highest indicating that the amount of PVP was moderate which can not only protect Pt particles, but also have little inhibiting effect on the reaction. The catalytic activity of PVP-Pt3 was moderate, perhaps the large amount of PVP hindered the contact of reactant with catalyst surface and desorption of product (44, 47, 48). As to the selectivity, it was found from Table 2 that the selectivity to cyclohexane decreased in the same trend as that of the activity: PVP-Pt2 > PVP-Pt3 > PVP-Pt1. Our results demonstrated that the catalytic properties of PVP-Pt colloids were determined by the preparation conditions which is in accordance with the literature (44). Further investigation to the effect of extra addition of PVP to the reaction system was conducted for PVP-Pt1 and PVP-Pt2. It was observed in Table 3 that the extra addition of PVP in hydrogenation results in decrease in both of activity and selectivity to cyclohexane. Comparing the hydrogenation of MCB over PVP-Pt2 (PVP: Pt ) 20:1) with that of VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (a) Hydrogenation of MCB over PVP-Pt2 catalyst. 2, b, 9: composition of MCB, benzene and cyclohexane, respectively. (b) Hydrogenation of benzene over PVP-Pt2, b, 9: composition of benzene and cyclohexane, respectively.

TABLE 2. Hydrogenation of MCB with Different PVP-Pt Colloidsa selectivity (%) TOFc PVP: conversion (molMCB · catalyst Pt (%) cyclohexane benzene molPt,surface-1 h-1) Pt1b Pt2b Pt3

10 20 40

84.94 97.75 91.13

69.36 93.60 75.86

30.64 6.40 24.14

7.08 9.07 8.13

a Reaction conditions: PH2 ) 0.1 MPa, T ) 298 K, V ) 30 mL, MCB ) 1 × 10-3 mol, Pt ) 5 × 10-5 mol, reaction time: 10 h. b Extra PVP was added to keep the molar ratio of PVP to Pt ) 40:1 in the reactions. c TOF was calculated when the reaction time was 5 h.

TABLE 3. Effect of Extra Addition of PVP in Hydrogenation of MCB with PVP-Pt Colloidsa catalysts PVP: Ptb PVP: Ptc Pt1 Pt11 Pt12 Pt2 Pt21 Pt22

10 10 10 20 20 20

10 20 40 20 40 80

selectivity (%) conversion (%) cyclohexane Benzene 96.13 84.00 84.94 99.34 97.75 97.78

84.71 71.65 69.36 98.09 93.60 89.69

15.21 28.35 30.64 1.91 6.40 10.31

a Reaction conditions: PH2 ) 0.1 MPa, T ) 298 K, V ) 30 mL, MCB ) 1 × 10-3 mol, Pt ) 5 × 10-5 mol, reaction time ) 10 h. b The molar ratio of PVP: Pt in the particle preparation. c The actual molar ratio of PVP: Pt in the reaction.

FIGURE 3. First-order reaction to MCB. PVP displayed inferior activity and selectivity in these hydrogenation reactions. Kinetics of PVP-Pt-Catalyzed Hydrogenation of MCB. The initial concentration (c0 ) 3.3 × 10-5 mol/mL) was employed in this study. Figure 3 shows the temporal evolution of the dechlorination in the reaction process. It is obviously a first-order reaction to the reactant. So the rate of the consumption of MCB in the reaction can be described by the following equation: dc/dt ) -kc

where c is the concentration of MCB present (mol/mL), k is the observed reaction rate constant (h-1) and t is reaction time (h). After integration, eq 1 changes to eq 2: ln(c/c0) ) -kt

PVP-Pt21 (PVP: Pt ) 40:1), it can be observed that the extra addition of PVP in hydrogenation results in some decrease in both conversion and selectivity to cyclohexane. This indicates that PVP has slight inhibiting effect on the reaction which in accordance with the result observed by Liu et al. (44). Furthermore, the more extra addition of PVP in hydrogenation (PVP-Pt22, PVP:Pt ) 80:1) results in decrease in selectivity to cyclohexane, but no change on conversion (the activity). These data show that PVP has more negative effect on selectivity than activity. Yu et al. (40, 42) found that PVP was harmful not only to the activity but also to the selectivity in the selective hydrogenation of o-chloronitrobenzene to o-chloroaniline over supported platinum colloids. Similar results were also given in liquidphase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol over different supported platinum colloids (40, 42). Taken together, it could be concluded that 2522

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(1)

(2)

From Figure 3, we can get the first-order reaction constant of k as 0.5026 h-1. Stability and Recovery of PVP-Pt. When 10 mmol MCB was used in the reaction, the conversion could reach 73.8% at 30 h reaction time; it means that the TON can be at least 374 mol MCB per mol surface Pt at 298 K and atmospheric pressure. The present PVP-Pt catalysts have predominant catalytic performance on the conversion of chlorobenzene. Isolation and recycle are important for the practical application of the catalysts. Indeed, the application of soluble metal nanocolloids in catalysis is limited for their obvious drawbacks in the separation and recovery of the catalyst from the reaction system (49). One method of the recovery of the catalysts is to use ultrafiltration to separate PVP-Pt colloid from the reaction system which also avoid the contamination of PVP. The catalytic properties would not decrease as demonstrated by several papers (50, 51)

if there is enough PVP in the beginning of the particle synthesis. That is to say, polymer-stabilized metal clusters are comparatively stable and can endure severe reaction conditions, such as in the hydroformylation of propylene (4.0 MPa and 363 K) (50) and the carbonylation of methanol (3.0 MPa and 413 K in acetic acid) (51). Another way is to immobilize metal clusters on supports, which could solve this problem. This investigation is underway in our group.

Acknowledgments Financial support for this work is by the Outstanding Youth Promotive Foundation of Shandong (Contract No. 2008BS09009), and the Worcester Polytechnic Institute is also gratefully acknowledged.

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