Fe Bimetallic Systems

of 5.0 mM aqueous phenol solutions to Pd (2.6 ppt m/m)/Mg (1.00 g 20 mesh) ... Pd (0.5%)/alumina in contact with 1.00 g 20 mesh Mg results in 74% and ...
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Ind. Eng. Chem. Res. 2002, 41, 3071-3074

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Hydrogenation of Phenol by the Pd/Mg and Pd/Fe Bimetallic Systems under Mild Reaction Conditions Jose´ Morales, Ryan Hutcheson, Christina Noradoun, and I. Francis Cheng* Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343

Three palladium-catalyzed zerovalent metal systems were found to be able to hydrogenate phenol to cyclohexanol and cyclohexanone under room temperature and pressure conditions. Exposure of 5.0 mM aqueous phenol solutions to Pd (2.6 ppt m/m)/Mg (1.00 g 20 mesh) and to 0.53 g of 1 /8 in. Pd (0.5%)/alumina in contact with 1.00 g 20 mesh Mg results in 74% and 24% destruction of the reactant after 6 h of reaction time. The latter system was found to be greatly enhanced in the presence of 2% (v/v) glacial acetic acid, resulting in an 84% reduction of phenol with a carbon balance of 93%. Palladized iron and unmodified metals systems were much less effective at hydrogenating phenol. The advantage of the 1/8 in. Pd/alumina/20 mesh Mg system is that it gives a readily recoverable form of the catalysts when compared to the bimetallic systems. Introduction The hydrogenation of phenol is significant both from a commercial standpoint as a synthesis route to cyclohexanol and for the destruction of a pollutant.1-3 Because of their toxicity, persistence, and bioaccumulation in aquatic organics, phenolic compounds in industrial waste streams are of great concern.4 Phenols may occur in concentrations in the parts per hundred in the wastewater of oil refineries, petrochemical units, polymeric resin manufacturing, and plastics. It is also noteworthy to add that oxidations by either electrochemical methods, supercritical water, dioxygen, or hydrogen peroxide have also been examined as a method for phenol/phenolic compound destruction.5-16 In all cases, treatment of phenolic waste streams requires harsh reaction conditions or special reactors (i.e., high temperatures or high pressures). An ideal method would be one in which phenol would be destroyed under conditions close to that of the waste streams, which are taken to be room temperature and pressure (RTP). The demonstrated catalytic hydrogenation of phenolic compound at RTP may be a viable approach where hazardous substances are transformed into benign useful products. In previous investigations, we have examined the hydrodechlorination characteristics of Pd/Mg bimetallic systems for the destruction of chlorinated phenols, DDT, and PCB.17-19,23 In this system and with Pd/Fe systems, complete dechlorination of the halocarbon to the hydrocarbon backbone is rapid and conducted under RTP conditions.20,21 During the course of our studies, it was found that 0.2% Pd/Mg (m/m) and, to a lesser extent, 0.05% Pd/Fe systems were able to hydrogenate phenol and biphenyl. This property may be explained by the following sequence of reactions:

Mg(0) f Mg2+ + 2e-

(1)

2H+ + 2e- f H2

(2)

Pd

H2 + R ) R 98 RH-RH

(3)

* Corresponding author. Phone: 208-885-6387. E-mail: [email protected].

Such hydrogenation of phenol under aqueous RTP conditions has not yet been reported in the literature. In this investigation, we report on the efficiency and conditions for the hydrogenation of aqueous phenol by the 0.2% (m/m) Pd/Mg system under RTP conditions. We also investigated the use of the 0.5% (m/m) Pd/ alumina system in contact with 20 mesh Mg. The latter gives the advantage of a readily recoverable form of the Pd hydrogenation catalyst. Experimental Section Chemicals. Ethyl acetate (certified ACS), iron powder (99%+, electrolytic and finer than 100 mesh), and hydrochloric acid (certified ACS Plus) were obtained from Fisher (Fair Lawn, NJ). Magnesium granules (20 mesh, 98%), cyclohexanol (99+%), biphenyl (sublimed), and phenol (99+%) were from Aldrich Chemical Co. (Milwaukee, WI). Potassium hexachloropalladate (Pd 26.59%) was obtained from Alfa Aesar (Ward Hill, MA). Cyclohexanone was obtained from EM Science (Gibbstown, NJ). Palladium on 1/8 in. alumina pellets (0.5% Pd) was from Acros (Pittsburgh, PA). Experimental Setup. A phenol stock solution (5.00 mM) was prepared in doubly distilled water. For these reactions, 1.000 ((0.0001) g of magnesium granules or 0.655 g of iron powder was transferred to a 15 mL glass vial. These quantities of zerovalent metals represented the same surface area of 0.079 m2 as measured by Brunauer-Emmett-Teller analysis (Porous Materials, Ithaca, NY). In studies where palladium modification of the metals was conducted, dry K2PdCl6(s) ((0.001 g) was added. Palladium deposits were formed spontaneously on the zerovalent metal surface upon the addition of water. Caution is urged because the reaction of the phenol solution with the bimetal system is quite violent. Therefore, care should be taken, and the phenol solution should be added slowly to the Mg(0)/K2PdCl6 powders. In runs using 1/8 in. Pd/alumina pellets, 0.533 (( 0.001) g was added to the magnesium particles. This quantity of Pd/alumina pellets contains the same mass of Pd as the Pd/Mg and Pd/Fe preparations. Reaction time started once the aqueous phenol stock solution aliquot of 5.00 mL was transferred to the vial containing the metal systems. Extraction. At the end of each reaction, the supernatant in the reaction vial was transferred to another

10.1021/ie0200510 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/01/2002

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Figure 1. Time-dependent phenol concentration after reaction with Pd-modified systems. In this graph, (() 1/8 in. Pd/alumina with 20 mesh Mg; (9) Mg/Pd(2.6 ppt); and (2) 1/8 in. Pd/alumina with 20 mesh Mg (2% (v/v) glacial acetic acid) at different reaction times. Pd mass was 2.6 ppt relative to Mg in all three cases. Presented concentrations were normalized using an internal standard. Each point represents the average of three measurements performed on the same sample.

vial free of metals. Then samples were vigorously shaken with 2.00 mL of 6.21 mM biphenyl in ethyl acetate (Fisher, certified ACS) along with 3 drops of concentrated hydrochloric acid to protonate any phenolates left in the aqueous phase. Analysis then took place once both phases achieved complete separation. GC-FID Analysis. Sublimed biphenyl (6.21 mM) in ethyl acetate was selected as the internal standard for these experiments and kept as a stock solution. An HP5890 GC-FID was used for the separation and analysis of the components. The separation column was an Alltech EC-5 (0.32 mm i.d., 0.25 µm film). The temperature program used was 2 min at 50 °C, with a temperature ramp of 10 °C/min up to 160 °C for 1 min. Injection was performed in splitless mode using helium as a carrier gas at a rate of 3.35 mL/min (37.5 mL/min including makeup gas). Calibration curves for phenol and cyclohexanone/cyclohexanol were produced, both showing a linear response in the concentration range of interest. For phenol, the detection limit was approximately 0.34 mM, while for cyclohexanol and cyclohexanone, which coeluted as one peak in the temperature program used, it was 0.70 mM. Results and Discussion The reactions with the Pd/Mg systems were noticeably exothermic. Upon the addition of the phenol solutions to the Mg(0) and K2PdCl6 solids, the temperature rose to 66 °C within 15 s followed by a decay back to room temperature within 2.5 min. Attempts at controlling the temperature of this reaction failed at 20 °C. The temperature of this reaction rose to 51 °C within 5 s in a water bath followed by a decay back to 20 °C within 30 s. However, the rate of phenol volatilization at 66 °C was found to be negligible during the timescales of the described experiments as followed by GC-FID on thermostated control experiments. The hydrogenation

reactions were much less violent with unmodified Mg(0) and Fe(0) in the presence and absence of 1/8 in. Pd/alumina pellets and with the Pd/Fe bimetallic systems. The degradation of phenol by the various 2.6 ppt (m/m) Pd/1.00 g Mg systems is demonstrated in Figure 1. The disappearance of phenol (5.00 mM, 5.00 mL) reaches approximately 80% after 6 h of treatment for both Pd/Mg and 1/8 in. Pd/alumina/Mg, 2% (v/v) glacial acetic acid systems. GC-MS indicated that both cyclohexanol and cyclohexanone were both produced from the hydrogenation of phenol. These species proved to be exceedingly difficult to separate by GC and thus are treated as a single product, cyclohexanol/cyclohexanone. The 1/8 in. Pd/alumina/Mg system was found to be less effective in the absence of added acetic acid (Figure 1). Table 1 highlights the results of the exposure of each metallic system to 5.00 mM aqueous phenol solution. None of the Fe series were able to conduct hydrogenation of phenol as effectively as the Pd/Mg and 1/8 in. Pd/ alumina/Mg series. In both bimetal systems, the extent of palladization did not greatly affect phenol hydrogenation efficiencies. The approximate order of hydrogenation efficiencies follows as Pd/Mg > 1/8 in. Pd/alumina/ Mg > Pd/Fe . Mg ≈ Fe ≈ 1/8 in. Pd/alumina/Fe ≈ 1/ in. Pd/alumina. The latter four systems were unable 8 to hydrogenate phenol after reaction times of 6 h. The extraction efficiencies for the Fe(0) were always greater than 100%, such observations have been noted with other investigators examining chlorinated phenol systems.19,22 The carbon balance for the Pd/Mg systems were found to be far less than 100%. The Pd/Fe and 1/8 in. Pd/ alumina/Mg systems gave carbon balances that generally ranged from 90% to 100% (Table 1). In all of the systems summarized in Table 1, no other products were detected by GC-FID analysis of the ethyl acetate extract of the aqueous reaction solutions. The possibility of the production of benzene, which coelutes with ethyl ac-

Ind. Eng. Chem. Res., Vol. 41, No. 13, 2002 3073 Table 1. Phenol and Cyclohexanone/Cyclohexanol (mM) Yields after Reaction with the Metallic Systems with 6 h of Reaction Timea systems control 1.00 Mg 0.3 ppt Pd/1.00 g Mg 1.3 ppt Pd/1.00 g Mg 2.6 ppt Pd/1.00 g Mg 5.2 ppt Pd/1.00 g Mg 0.6552 g Fe 4.1 ppt Pd/0.655 g Fe 8.2 ppt Pd/0.655 g Fe 1/ in. Pd/alumina 8 2.6 ppt Pd/alumina 0.655 g Fe 2.6 ppt Pd/alumina 1.00 g Mg 2.6 ppt Pd/alumina 1.00 g Mg + 2% (v/v) glacial acetic acid a

[phenol] mM ( std dev 5.00 ( 0.12 5.20 ( 0.13 3.34 ( 0.01 1.47 ( 0.04 1.29 ( 0.01 1.32 ( 0.04 5.58 ( 0.04 4.38 ( 0.08 4.34 ( 0.03 5.51 ( 0.02 5.378 ( 0.05 3.79 ( 0.03 0.82 ( 0.02

cyclohexanone/ol mM ( std dev nd nd 0.68 ( 0.03 0.87 ( 0.01 0.95 ( 0.01 0.74 ( 0.01 nd nd 0.27 ( 0.00 nd nd 1.36 ( 0.04 3.81 ( 0.01

carbon balance 100% 104% 82% 47% 45% 41% 112% 88% 92% 110% 108% 103% 93%

nd indicates no detection. Concentrations were calculated from calibration curves and normalized with an internal standard.

Figure 2. Disappearance of phenol (9), appearance of cyclohexanone/cyclohexanol ((), and carbon balance (2) after reaction with 1/8 in. Pd/alumina with 20 mesh Mg as a function of reaction time. Presented concentrations (mM) were normalized using an internal standard. Each point represents the average of three measurements performed to the same sample.

etate, was examined by the extraction of the aqueous reaction mixtures with cyclohexanol, which has a longer retention time than benzene. No benzene was observed to the detection limit of GC-FID with each of the runs summarized in the table. Figure 2 demonstrates the concentration-time relationships for both phenol and the product for the acidified 1/8 in. Pd/alumina/Mg system. The carbon balance decreased from 100% to 93% during the course of 6 h. The carbon balances of the Pd/Mg series (Table 1) may indicate three possibilities: (i) irreversible adsorption of phenol or cyclohexanol/cyclohexanone, (ii) volatilization of those species, or (iii) further reaction of the cyclohexanol/cyclohexanone. The latter two cases were ruled out with the results of control experiments in this investigation, and the first case was ruled out from a previous study, which examined this possibility with thermal desorption-direct probe MS analysis of Pd/Mg particles used for the dechlorination of chlorinated phenols.23 We hypothesize as we did in that previous study that the Pd/Mg system forms a product more

volatile than cyclohexanol/cyclohexanone. The carbon balance for the Pd/Fe and 1/8 in. Pd/alumina/Mg systems were much closer to unity, indicating that the primary product formed was cyclohexanol/cyclohexanone. The enhancement of the reaction rate of the 1/8 in. Pd/ alumina/Mg system is probably attributable to the consumption of acid in the hydrogenation reaction (eqs 1-3). Such attempts at enhancing the phenol hydrogenation rates of the Pd/Fe and Pd/Mg systems failed. This observation may be due to the release of Pd from the substrate metal under conditions of enhanced metal corrosion (see eqs 1-2). To date, this is the only known phenol hydrogenation system capable of operating at room temperature and pressure. The 1/8 in. Pd/alumina/Mg system offers the ability to recover easily Pd once the reductant, Mg, is spent. Furthermore, 1/8 in. Pd/alumina is a common commercially available catalytic preparation. This feature may offer rapid scale-up in order to accommodate industrial waste streams and transformation processes.

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Acknowledgment The authors wish to acknowledge financial support from the University of Idaho Research Office seed grant program. Literature Cited (1) Shin, E.-J.; Keane, M. A. Ind. Eng. Chem. Res. 2000, 39, 883-892. (2) Galvagno, S.; Donato, A.; Neri, G.; Pietropaolo, R. J. Chem. Technol. Biotechnol. 1991, 51, 145-153. (3) Mukumoto, M.; Mashimo, T.; Tsuzuki, H.; Tsukinoki, T.; Uezo, N.; Mataka, S.; Tashiro, M.; Kakinami, T. J. Chem. Res., Synop. 1995, 412-413. (4) Dojlido, J. R.; Best, G. A. Chemistry of Water and Water Pollution; Ellis Horwood: New York, 1993; pp 295-301. (5) Yu, J.; Savage, P. E. Environ. Sci. Technol. 2000, 34, 31913198. (6) Santos, A.; Yustos, P.; Durban, B.; Garcia-Ochoa, F. Ind. Eng. Chem. Res. 2001, 40, 277-2781. (7) Santos, A.; Yustos, P.; Durban, B.; Garcia-Ochoa, F. Environ. Sci. Technol. 2001, 35, 2828-2835. (8) Martino, C. J.; Savage, P. E. Ind. Eng. Chem. Res. 1997, 36, 1385-1390. (9) Krajnc, M.; Levec, J. Ind. Eng. Chem. Res. 1997, 36, 34393445. (10) De, A. K.; Bhattacharjee, S.; Dutta, B. K. Ind. Eng. Chem. Res. 1997, 36, 3607-3612. (11) Sun B.; Sato, M.; Clements, J. S. Environ. Sci. Technol. 2000, 34, 509-513.

(12) Martino, C. J.; Savage, P. E. Environ. Sci. Technol. 1999, 33, 1911-1915. (13) Oshima, Y.; Tomita, K.; Koda, S. Ind. Eng. Chem. Res. 1999, 38, 4183-4188. (14) Rodgers, J. D.; Jedral, W.; Bunce, N. Environ. Sci. Technol. 1999, 33, 1453-1457. (15) Can˜izares, P.; Domı´nguez, J. A.; Rodrigo, M. A.; Villasen˜or, J.; Rodrı´guez, J. Ind. Eng. Chem. Res. 1999, 38, 3779-3785. (16) Yu, J.; Savage, P. E. Ind. Eng. Chem. Res. 1999, 38, 37933801. (17) Doyle, J. G.; Miles, T.; Parker, E.; Cheng, I. F. Microchem. J. 1998, 60, 290-295. (18) Engelmann, M. D.; Doyle, J. G.; Cheng, I. F. Chemosphere 2001, 43, 195-198. (19) Morales J.; Hutcheson, R.; Cheng, I. F. J. Hazard. Mater. 2001, submitted for publicaiton. (20) Muftikian, R.; Fernando, Q.; Korte, N. Water Res. 1995, 29, 2434. (21) Grittini, C.; Malcomson, M.; Fernando, Q.; Korte, N. Environ. Sci. Technol. 1995, 29, 2898-2900. (22) Kim, Y.-H.; Carraway, E. R. Eniviron. Sci. Technol. 2000, 34, 2014-2017. (23) Morales, J.; Hutcheson, R.; Cheng, I. F. Journal of Hazardous Waste 2001, in press.

Received for review January 17, 2002 Revised manuscript received April 25, 2002 Accepted April 30, 2002 IE0200510