Cu2+-Exchanged Zeolites as ... - ACS Publications

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Energy & Fuels 2004, 18, 470-476

Cu2+-Exchanged Zeolites as Catalysts for Phenol Hydroxylation with Hydrogen Peroxide Jun Wang,†,| Jung-Nam Park,† Han-Cheol Jeong,‡ Kwang-Sik Choi,‡ Xian-Yong Wei,†,⊥ Suk-In Hong,§ and Chul Wee Lee*,† Advanced Chemical Technology Division, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea, Aekyung Chemical Technology Research Institute, Daejeon 305-345, Korea, and Department of Chemical & Biological Engineering, Korea University, Seoul 136-701, Korea Received April 16, 2003. Revised Manuscript Received December 10, 2003

The Cu2+-exchanged NaY, HY, USHY, Hβ, and HZSM-5 zeolites were prepared and evaluated in phenol hydroxylation with hydrogen peroxide using an atmospheric batch reactor. CuNaY, CuHY, and CuHβ catalysts were found to be more active than TS-1 or a simple homogeneous copper nitrate catalyst under similar reaction conditions. Both zeolite type and copper content in the zeolite catalyst were revealed to exert critical impact upon the catalytic activity in phenol hydroxylation. Reaction time, reaction temperature, and the molar ratio of phenol to hydrogen peroxide also remarkably influenced the reaction results. The addition of a small amount of hydrochloric acid to the reaction systems significantly enhanced the phenol conversion, hydroxylation selectivity, and reaction rate. The used catalysts can be regenerated completely by calcination at 450 °C for 4 h in air. On the basis of ESR spectroscopy, the relationship between catalytic activity and copper loading is explained and the hydroxyl radical is suggested as the reaction intermediate.

Introduction Phenol is known to be present in coal liquids1,2 and petroleum asphaltenes3 in a relatively high concentration. The main industrial products from phenol include phenol resins, caprolactam, bisphenol A, adipic acid, alkylphenols, and miscellaneous other products.2 Catechol and hydroquinone are two of the many phenolic derivatives of high value. They are widely used as photographic chemicals, antioxidants, and polymerization inhibitors and are also used in pesticides, flavoring agents, and medicine. Catechol was also used as an organic sensitizer in a photoelectrochemical cell.4 It is most desirable that the dihydroxybenzenes could be produced by the direct hydroxylation of phenol with the environmentally benign oxidant, hydrogen peroxide (H2O2). Mineral acids,5-7 simple metal ions,8,9 and metal * Corresponding author. Tel: (82)-42-860-7381. Fax: (82)-42-8607388. E-mail: [email protected]. † Korea Research Institute of Chemical Technology. ‡ Aekyung Chemical Technology Research Institute. § Korea University. | Permanent address: College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, China. ⊥ Permanent address: Department of Applied Chemistry, School of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China. (1) Butala, S. J. M.; Medina, J. C.; Hulse, R. J.; Bartholomew, C. H.; Lee, M. L. Fuel 2000, 79 (13), 1657-1664. (2) Schobert, H. H.; Song, C. Fuel 2002, 81 (1), 15-32. (3) Demirbas, A. Energy Convers. Manage. 2002, 43 (8), 1091-1097. (4) Tennakone, K.; Kumara, G. R. R. A.; Kumarasinghe, A. R.; Sirimanne, P. M.; Wijayantha, K. G. U. J. Photochem. Photobiol. A: Chem. 1996, 94 (2-3), 217-220. (5) Bourdin, F.; Costantini, M.; Jouffret, M.; Latignan, G. Ger. Patent 2,064,497, 1971.

complexe,10 are the traditional catalysts for this reaction, but these homogeneous catalysts are difficult to be separated and recovered from the reaction mixture, which prevents their practical utilization in phenol hydroxylation. Therefore, heterogeneous catalysis over various metal oxides and complexes, such as pure metal oxides or supported ones,11-13 metal complex oxides,14,15 zeolite-encapsulated metal complexes,16 and hydrotalcite-like compounds,17,18 has been of great interest to many researchers for a long time. However, most of these catalysts show either low catalytic activity or unsatisfactory product selectivity, and some of them require very complicated synthesis. With the first commercial application of the TS-1 for phenol hydroxylation realized by Enichem in 1986 in (6) Skepalik, C. Ger. Patent 2,138,735, 1973. (7) Varagnat, J. Ind. Eng. Chem. Prod. Res. Dev. 1976, (15), 212215. (8) Hamilton, G. A.; Hamfin, J. W.; Friedman, J. P. J. Am. Chem. Soc. 1966, 88 (22), 5269-5272. (9) Brook, M. A.; Gatle, L.; Lindsay, I. R. J. Chem. Soc., Perkin Trans. 1982, (2), 687-689. (10) Hytbrechts, D. R. C.; Vaesen, I.; Li, H. X.; Jacobs, P. A. Catal. Lett. 1991, 8 (1), 237-244. (11) Ai, M. J. Catal. 1978, 54 (2), 223-229. (12) Al-Hayck, N. Water Res. 1985, (19), 657-666. (13) Goldstein, S.; Czapski, G.; Robani, J. J. Phys. Chem. 1994, 98 (26), 6586-6591. (14) Yu, R.; Xiao, F.; Wang, D.; Sun, J.; Liu, Y.; Pang, G.; Feng, S.; Qiu, S.; Xu, R.; Fang, C. Catal. Today 1999, 51 (1), 39-46. (15) Sun, J.; Meng, X.; Shi, Y.; Wang, R.; Feng, S.; Jiang, D.; Xu, R.; Xiao, F. J. Catal. 2000, 193 (2), 199-206. (16) Maurya, M. R.; Titinchi, S. J. J.; Chand, S.; Mishra, I. M. J. Mol. Catal. A: Chem. 2002, 180 (1-2), 201-209. (17) Zhu, K.; Liu, C.; Ye, X.; Wu, Y. Appl. Catal. A: Gen. 1998, 168 (2) 365-372. (18) Dubey, A.; Rives, V.; Kannan, S. J. Mol. Catal. A: Chem. 2002, 181 (1-2), 151-160.

10.1021/ef0300904 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/06/2004

Cu2+-Exchanged Zeolites in Phenol Hydroxylation

Italy,19-21 an increasing impetus of studies on various metallosilicates,22 such as TS-2,23 Ti-β,24 TAPO-5, TAPO11,25 Ti-ZSM-48,26 VS-2,27 and Cu-AlPO4-5,28 has been shown by many publications. Although some of these transition metal framework-substituted zeolites have exhibited attractive catalytic conversion, selectivity and stability in phenol hydroxylation, their complicated synthesis, high cost, and low reaction rate will limit their commercial applications. On the other hand, metal-modified mesoporous materials, MCM-41 and MCM-48, have also become target catalysts aiming at accelerating the reaction rate of phenol hydroxylation.29-31 In addition, some novel catalysts, such as copper hydroxyl phosphate 32 and copper Keggin-type heteropoly compounds,33 have been investigated for this reaction recently. Despite numerous studies on the transition metals in various catalysts mentioned above, the simple ionexchanged zeolites have not been paid much attention so far. Usually, the metal ion exchange is used to adjust the zeolite pore size and/or acidity. However, considering that the well-known oxidative Fenton’s reagent with hydrogen peroxide as oxidant is the low-valence transition metal ion, such as Fe2+, Cu2+, and Co2+,34,35 it is reasonable to suppose that the transition metal ions exchanged into the cages and/or channels of zeolite could be the oxidant for phenol hydroxylation if the zeolite pore size is large enough to allow the reaction to occur inside the zeolite pores. Indeed, we have recently found that an Fe2+, Co2+ ion-exchanged Naβ zeolite catalyst is very active in phenol hydroxylation with H2O2 at room temperature.36 In this paper, we report the conversion, selectivity, stability, and reproducibility of Cu2+-exchanged zeolites in phenol hydroxylation with H2O2. Electron spin resonance (ESR) spectra have been attempted to characterize the Cu-zeolite and also to propose a reaction mechanism involving hydroxyl radical as the reaction intermediate. (19) Esposito, A.; Taramasso, M.; Neri, C. U.S. Patent 4,396,783, 1983. (20) Thangaraj, A.; Kumar, R.; Ratnasamy, P. J. Catal. 1991, 131 (1), 294-297. (21) Martens, J. A.; Buskens, P.; Jacobs, P. A. Appl. Catal. A: Gen. 1993, 99 (1), 71-84. (22) Arends, I. W. C. E.; Sheldon, R. A.; Wallau, M.; Schuchardt, U. Angew. Chem., Int. Ed. Engl. 1997, 36 (11), 1144-1163. (23) Reddy, J. S.; Sivasanker, S.; Ratnasamy, P. J. Mol. Catal. 1992, 71 (3), 373-381. (24) Camblor, M. A.; Corma, A.; Martinez, A.; Perez-Pariente, J. J. Chem. Soc. Chem. Commun. 1992, (8), 589-590. (25) Ulagappan, N.; Krishnasamy, V. J. Chem. Soc. Chem. Commun. 1995, 373-374. (26) Serrano, D. P.; Li, H. X.; Davis, M. E. J. Chem. Soc. Chem. Commun. 1992, (10), 745-747. (27) Hari, P. R.; Rao, P.; Ramaswamy, A. V. Appl. Catal. A: Gen. 1993, 93 (2), 123-130. (28) Chou, B.; Tsai, J. L.; Cheng, S. Microporous Mesoporous Mater. 2001, 48 (1-3), 309-317. (29) Lee, C. W.; Ahn, D. H.; Wang, B.; Hwang, J. S.; Park, S. E. Microporous Mesoporous Mater. 2001, 44-45, 587-594. (30) Zhao, W.; Luo, Y.; Deng, P.; Li, Q. Catal. Lett. 2001, 73 (2-4), 199-202. (31) Norena-Franco, L.; Hernandez-Perez, I.; Aguilar-Pliego, J.; Maubert-Franco, A. Catal. Today 2002, 75 (1-4), 189-195. (32) Xiao, F. S.; Sun, J.; Meng, X.; Yu, R.; Yuan, H.; Jiang, D.; Qiu, S.; Xu, R. Appl. Catal. A: Gen. 2001, 207 (1-2), 267-271. (33) Zhang, H.; Zhang, X.; Ding, Y.; Yan, L.; Ren, T.; Suo, J. New J. Chem. 2002, 26, 376-377. (34) Fenton, H. J. H. J. Chem. Soc. London 1894, 65, 899-910. (35) Masarwa, M.; Cohen, H.; Meyerstein, D.; Hickman, D. L.; Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1988, 110 (13), 42934297. (36) Wang, J.; Park, J.-N.; Wei, X.-Y.; Lee, C. W. Chem. Commun. 2003, (5), 628-629.

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Experimental Section Preparation of Catalysts. HY was prepared from NaY (ZEOCAT, Z6-01-01, SiO2/Al2O3 ) 5.5) by the ammonium exchange. The NaY powder was suspended in the 0.1 M aqueous solution of NH4NO3 (Aldrich, G. R. purity) with the liquid/solid weight ratio of 100, at 80 °C for 8 h, followed by filtration, washing with water, drying at 120 °C for 8 h, and calcination at 500 °C for 8 h in air. This procedure was repeated 3 times. The Cu2+-exchange reaction was conducted by treating the corresponding zeolite, i.e., NaY, HY, USHY (Zeolyst, CBV780, SiO2/Al2O3 ) 80), NH4-β (Zeolyst, CP814E, SiO2/Al2O3 ) 25), and HZSM-5 (Zeolyst, CBV5020, SiO2/Al2O3 ) 50), respectively, with the aqueous solution of Cu(NO3)2‚3H2O (Junsei, Extra Pure) at the liquid/solid weigh ratio of 100, and the reaction temperature of 25 °C for 8 h. This was followed by filtration, washing with water until the filtrate became free from copper ions which was analyzed by ICP, drying at 100 °C for 8 h, and calcination at 450 °C for 5 h in air. After the thermal treatment, the ammonium form of β zeolite was converted into hydrogen form (Hβ). The resulting Cu2+exchanged sample is designated as nCuZ, where n stands for the concentration (mM) of the Cu(NO3)2‚3H2O aqueous solution used in Cu2+-exchange, and Z is the name of corresponding zeolite. The copper content in solid powders was analyzed by an inductively coupled plasma (ICP) spectrometer (J. Y. Ultima C, Jobin Yvon) according to the literature.37 Catalytic Activity Tests. Catalytic tests of prepared zeolites for phenol hydroxylation with H2O2 were carried out in a 100 mL three-necked glass flask batch reactor equipped with a magnetic stirrer, a reflux condenser, and a temperature controllable oil-bath. Two grams of phenol was dissolved in 60 mL of water, followed by the addition of 0.2 g of catalyst. After the desired temperature was reached, a calculated amount of aqueous H2O2 solution (30 wt %) was added dropwise into the reaction mixture through the septum using a syringe pump within 10 min at the beginning of the reaction. Water was usually employed as the reaction solvent, because it is considerably safer, cheaper, and more environmentally benign than organic solvents. A dilute hydrochloric acid aqueous solution (0.01 M) was also employed as the reaction solvent, as comparison with the water solvent. The product was sampled periodically through the septum by a syringe and filtered in order to remove any catalyst particles. Both reactant mixture and product were analyzed by high-performance liquid chromatograph (HPLC, Shimadzu, LC-10ADVP, equipped with a RP, C18 column) using 4-fluorophenol as external standard, and UV/Vis as the detector (ICI, LC1200). The conversion and selectivity were calculated as follows. Xphenol (%) ) 100 × ([phenol]i - [phenol]f)/[phenol]i, where Xphenol is the conversion of phenol, [phenol]i is the molar concentration of phenol before reaction, and [phenol]f is the molar concentration of phenol after sampling. Shydrox (%) ) 100 × ([CAT]f + [HQ]f + [BQ]f)/([phenol]i - [phenol]f), where Shydrox is the selectivity for hydroxylation, and [CAT]f, [HQ]f, and [BQ]f are the molar concentration of catechol, hydroquinone, and 1,4benzoquinone, respectively, after reaction. Product distribution (%) ) 100 × [product]f/([phenol]i - [phenol]f), where [product]f is the molar concentration of product (catechol, hydroquinone, or 1,4-benzoquinone) after reaction. CAT/HQ ) [CAT]f /[HQ]f. H2O2eff (%) ) 100 × ([CAT]f + [HQ]f + 2[BQ]f)/[H2O2]add, where H2O2eff is the effective conversion of H2O2, and [H2O2]add is the molar concentration of the total added H2O2 in the reaction mixture. In this case, the balance H2O2 remained either unreacted or decomposed to water and oxygen.20 Characterization of Catalysts. For spin trapping, ESR spectra were measured at room temperature and DMPO (5,5′(37) Evmerides, N. P.; Dwyer, J. Chim. Chron. New Ser. 1982, 11, 331-336.

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Table 1. Activities of Various Cu2+-Exchanged Zeolites in Phenol Hydroxylation with H2O2 under the Same Reaction Conditionsa Cuc catalyst

W%

Xphenol %

Shydrox %

5CuNaY 5CuHY 5CuUSHY 5CuHβ 10CuHZSM-5 TS-1b

2.39 1.08 0.26 1.05 1.04 none

45.3 49.3 15.2 49.3 47.0 36.0

62.9 74.5 78.8 73.0 14.6 -d

product distributione % CAT HQ BQ 43.6 50.4 45.1 51.1 8.8 51

18.3 21.8 15.7 18.8 4.2 5.0

1.0 2.3 18.0 3.0 1.5 44.0

CAT/HQ mol/mol

H2O2eff %

2.38 2.31 2.87 2.71 2.1 10.2

29.8 37.9 14.7 37.5 7.6 28.0

a Reaction temperature 60 °C; reaction time 3 h; phenol/H O (molar ratio) ) 1; phenol/catalyst (weight ratio) ) 10; water as a solvent. 2 2 From ref 20, reaction conditions: reaction temperature 57 °C; reaction time 6 h; phenol/H2O2 (molar ratio) ) 1; phenol/catalyst (weight ratio) ) 10; acetone as a solvent. c Weight percentage of copper in catalysts measured by ICP. d Not mentioned in ref 20. e Selectivity (%) of CAT, HQ, and BQ for the TS-1 catalyst, without considering byproducts.

b

dimethyl-1-pyrroline N-oxide) was used as a spin trapping agent. The detailed experimental procedure was described elsewhere.15 For copper-containing samples, ESR spectra were recorded at liquid nitrogen temperature. DPPH (diphenylpicryl hydrazyl) was used as a g-marker.

Results and Discussion According to a previous report,38 in wet oxidation of phenol, the deep oxidation products such as maleic acid, acrylic acid, acetic acid, oxalic acid, and oligomerization products could be possibly formed in addition to desired catechol, hydroquinone, and 1,4-benzoquinone. Some of these deep oxidation products were also detected in this work as byproducts. Catalytic Activities of Various Cu2+-Exchanged Zeolites. The conversion and selectivity of various Cu2+exchanged zeolite catalysts for phenol hydroxylation with H2O2 are presented in Table 1. It can be seen in Table 1 that all the Cu2+-exchanged zeolites are catalytically active in phenol hydroxylation. Among them, CuNaY, CuHY, and CuHβ exhibit the highest conversion of phenol, at 45.3%, 49.3%, and 49.3%, respectively, with considerable selectivity for hydroxylation of 62.9%, 74.5%, and 73.0%, respectively, and very low selectivity for 1,4-benzoquinone of less than 3%. Moreover, these three catalysts are very selective for catechol with the molar ratio of catechol to hydroquinone being larger than 2. The Cu2+ ions in Cu2+-exchanged Y Zeolite have been revealed to be located in the supercage, main channel, sodalite, and hexagonal prism of the zeolite;39-41 thus, for CuY, the phenol hydroxylation reaction must take place in the intracrystalline zeolite cages and channels. When compared with TS-1 in Table 1 at very similar reaction conditions, except that TS-1 was evaluated in acetone, which has been verified to be the best solvent for TS-1,20,21 the employed Cu2+-exchanged large-pore zeolites (Y and β) obviously possess both high conversion of phenol and high selectivity for dihydroxybenzenes. It should be pointed out that the molar ratio of phenol to H2O2 of 1 was not the optimum reaction condition for TS-1, which led to very high CAT/HQ ratio of 10.2, as shown in Table 1. Actually, a CAT/HQ ratio of nearly 1 without detection of 1,4-benzoquinone could (38) Santos, A.; Yustos, P.; Quintanilla, A.; Rodriguez, S.; GarciaOchoa, F. Appl. Catal. B: Environ. 2002, 39 (2), 97-113. (39) Gallezot, P.; Taarit, Y. B.; Imelik, B. J. Catal. 1972, 26 (1), 295302. (40) Maxwell, I. E.; de Boer, J. J. J. Phys. Chem. 1975, 79 (17), 1874-1879. (41) Haniffa, R. M.; Seff, K. Microporous Mesoporous Mater. 1988, 25 (1-3), 137-149.

be achieved when phenol/H2O2 g 3, according to which, the selective production of hydroquinone on TS-1 was asserted by the authors because of the shape selectivity of the narrow 10-membered ring channels of TS-1 for hydroquinone.20 Nevertheless, the selective formation of hydroquinone is not observed in this study owing to the 12-membered ring open-pore structure of Y and β zeolites, the channels of which are large enough not only to allow phenol and H2O2 to approach catalytic centers freely, but also for all products to diffuse out easily. On the other hand, a slightly higher conversion of phenol and selectivity for hydroxylation is observed on CuHY than that on CuNaY. It has been demonstrated38,39 that the hydrogen form Y zeolite itself was active in this oxidation reaction due to the zeolite acidity. Thus, the high activity in CuHY here might be explained by the coexistence of acidic protons and oxidative Cu2+ ions in intracrystalline Y zeolite pores. The observation of the low conversion of phenol at 15.2% on 5CuUSHY catalyst might be ascribed to its exclusive low copper content, as shown in Table 1, that only 0.26 wt % of copper can be detected in 5CuUSHY. By contrast, more than 1% of copper exists in other zeolites. This might be the result of the poor ability to ion exchange caused by the very high SiO2/Al2O3 ratio of 80 in USHY. Table 1 also shows the very low selectivity for hydroxylation over CuHZSM-5 of only 14.6% and the very low efficiency in the utilization of H2O2 of only 7.6%, despite its high conversion of phenol at 47.0%. It has been proposed in phenol hydroxylation 21 that the external surface of zeolite is favorable to the formation of catechol, as well as deep oxidation products. It is thus probable for CuHZSM-5 that the phenol oxidation reaction mainly occurs on its external surface without exhibiting the selectivity of the narrow 10-membered ring channels of ZSM-5 for hydroquinone and with the CAT/HQ ratio being 2.1. Because the objective of the present work is to investigate the oxidation behavior of Cu2+ ions in ionexchanged zeolites rather than the function of zeolite acidities, further study is focused on CuNaY. Influence of Copper Content. To test the effect of the copper content in Y zeolite on the catalytic reactivity of phenol hydroxylation, various CuNaY catalysts were measured and compared in Table 2. These CuNaY catalysts were obtained by using different Cu(NO3)2‚ 3H2O concentrations in aqueous solution when running the Cu2+-exchange reaction. As shown in Table 2, the copper content in zeolites increases gradually with the

Cu2+-Exchanged Zeolites in Phenol Hydroxylation

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Table 2. Activities of CuNaY Catalysts with Different Copper Content in Phenol Hydroxylation with H2O2 under the Same Reaction Conditionsa

catalyst none NaY 2.5CuNaY 5CuNaY 10CuNaY 100CuNaY 500CuNaY Cu(NO3)2 Cu(NO3)2b

Cuc W%

Xphenol %

Shydrox %

1.56 2.39 3.38 3.61 3.94

0 1.7 44.9 45.3 50.3 56.2 66.5 47.6 18.9

0 60.7 62.9 74.7 62.3 45.5 71.7 76.1

product distribution % CAT HQ

BQ

0 42.5 43.6 50.3 41.9 30.0 50.3 46.6

0 1.6 1.0 2.1 1.0 1.0 2.3 12.3

0 16.7 18.3 22.3 19.4 14.5 18.5 17.1

CAT/HQ mol/mol

H2O2eff %

2.54 2.38 2.26 2.16 2.07 2.71 2.72

0 0 28.5 29.8 38.6 35.6 30.9 35.2 18.4

a Reaction temperature: 60 °C; reaction time: 3 h; phenol/H O (molar ratio) ) 1; phenol/catalyst (weight ratio) ) 10; water as a 2 2 solvent. b Phenol/catalyst (weight ratio) ) 417; others are the same as above. c Weight percentage of copper in catalysts measured by ICP.

increment of Cu(NO3)2‚3H2O concentration in the ion exchange reaction mixture. Table 2 indicates that no product is detected without catalyst, and NaY alone only generates slight minor products. However, all the Cu2+-exchanged NaY zeolites lead to high activities with phenol conversions above 44%. This implies that copper cations are active centers for hydroxylation of phenol in this work. It is also found in Table 2 that the conversion increases with the enhancement of copper content in CuNaY, and the selectivity for hydroxylation gets its maximum at 74.7% over 10CuNaY. Furthermore, all the CuNaY zeolites are very selective for catechol with the molar ratio of catechol to hydroquinone larger than 2, although this ratio decreases gradually with the increase of copper content in zeolite. The effective conversion of H2O2 is between 28% and 39% at this reaction condition. Pure copper nitrate is also observed to be active in this reaction, as shown in Table 2, which is consistent with previous results.33 When carried out under similar reaction conditions with the weight ratio of phenol to copper nitrate being 10, the activity of copper nitrate is comparable with CuNaY. However, with the weight ratio of phenol to copper nitrate being 417, i.e., with the same molar ratio of phenol to copper in the hydroxylation reaction mixture as that for the 5CuNaY catalyst, a much lower phenol conversion at 18.9% is observed over copper nitrate than that over 5CuNaY at 45.3%. This observation indicates evidently that the oxidation activity of Cu2+ is significantly improved by the introduction of Cu2+ into zeolite Y pores by the ion exchange (see also ESR measurement). Influence of Reaction Time. The dependence of activity of 5CuNaY in phenol hydroxylation on the reaction time is displayed in Table 3. It is pointed out in Table 3 that the phenol conversion, the effective conversion of H2O2, and the selectivity for hydroxylation are enhanced with the increase of the contact time, and the reaction acquires steady state after 3 h. It is also found that 1,4-benzoquinone dominates the reaction products at the early stage of reaction, and with the prolongation of reaction time, the selectivity for 1,4benzoquinone decreases, whereas those for catechol and hydroquinone increase. When phenol conversion is at a high level of 47.9%, 1,4-benzoquinone selectivity is only 2.9%. Similar results have also been observed on other kinds of catalysts such as TS-1,20 acidic zeolite,42,43 and metal complex oxide.15 It is proposed that at the early

Table 3. Activity of 5CuNaY Catalyst in Phenol Hydroxylation with H2O2 as a Function of Reaction Timea time Xphenol Shydrox h % % 0.5 1 2 3 5

2.1 6.8 26.8 46.8 47.9

3.5 4.7 77.2 70.8 71.3

product distribution % CAT HQ BQ 0 0 42.7 46.7 47.7

0 1.3 16.7 20.1 20.7

CAT/HQ H2O2eff mol/mol %

3.5 3.4 17.8 4.0 2.9

0 2.56 2.32 2.30

0.1 0.6 25.5 35.0 35.5

a Reaction temperature: 50 °C; phenol/H O (molar ratio) ) 1; 2 2 phenol/catalyst (weight ratio) ) 10; water as a solvent.

Table 4. Activity of 5CuNaY Catalyst in Phenol Hydroxylation with H2O2 as a Function of Reaction Temperaturea temperature Xphenol Shydrox °C % % 40 50 60 80

25.8 47.9 51.1 54.7

69.1 71.3 67.1 63.1

product distribution % CAT

HQ

BQ

42.7 47.7 45.0 43.7

16.2 20.7 19.3 18.4

10.4 2.9 2.8 1.0

CAT/HQ H2O2eff mol/mol % 2.64 2.30 2.33 2.38

20.6 35.5 35.7 32.9

a Reaction time: 5 h; phenol/H O (molar ratio) ) 1; phenol/ 2 2 catalyst (weight ratio) ) 10; water as a solvent.

reaction stage, a fast over oxidation of hydroquinone in the reaction medium by the large concentration of H2O2 could cause the formation of 1,4-benzoquinone in relatively large amount. The subsequent disappearance of 1,4-benzoquinone can be explained by the decomposition of the 1,4-benzoquinone into deeper oxidation or degradation byproducts,23,44 and/or the oxidation of H2O2 by the 1,4-benzoquinone with the formation of hydroquinone and oxygen.42 Influence of Reaction Temperature. Table 4 shows the influence of reaction temperature on the catalytic activity of 5CuNaY in phenol hydroxylation with H2O2. The increase of phenol conversion is observed in Table 4 when the reaction temperature increases from 40 °C to 80 °C. At low reaction temperature of 40 °C, the phenol conversion is only 25.8%, while the 1,4-benzoquinone selectivity is comparatively high (10.4%). However, the hydroxylation selectivity (42) Allian, M.; Germain, A.; Figueras, F. Catal. Lett. 1994, 28 (24), 409-415. (43) Germain, A.; Allian, M.; Figueras, F. Catal. Today 1996, 32 (1-4), 145-148. (44) Tuel, A.; Taarit, Y. B. Appl. Catal. A: Gen. 1993, 102 (1), 6977.

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Table 5. Activity of 5CuNaY Catalyst in Phenol Hydroxylation with H2O2 as a Function of the Molar Ratio of Phenol to Hydrogen Peroxidea phenol/H2O2 Xphenol Shydrox mol/mol % % 1 2 3

47.9 25.7 17.4

71.3 78.7 90.1

product distribution % CAT

HQ

BQ

47.7 48.0 47.7

20.7 21.5 17.2

2.9 9.2 25.2

CAT/HQ H2O2eff mol/mol % 2.30 2.23 2.77

35.5 45.2 60.2

a Reaction temperature: 50 °C; reaction time ) 5 h; phenol/ catalyst (weight ratio) ) 10; water as a solvent.

finds its lowest value of 63.1% at the reaction temperature of 80 °C, despite the high conversion at 54.7%. Considering both conversion and selectivity, the reaction temperature between 50 °C and 60 °C is suitable over this catalyst. Influence of the Molar Ratio of Phenol to H2O2. The activity of 5CuNaY is changed significantly with the change in molar ratio of phenol to H2O2, as presented in Table 5. When the molar ratio of phenol to H2O2 was increased from 1 to 3, the phenol conversion decreased while the efficiency of H2O2 increased remarkably, which is in good agreement with the previous results on other catalysts.15,20 Moreover, hydroxylation selectivity also increased very much with the increase of the reactant ratio. At the phenol-to-H2O2 molar ratio of 3, although the hydroxylation selectivity and H2O2 effective conversion are high, the 1,4-benzoquinone selectivity is also at a high level of 25.2%. On the other hand, the molar ratio of catechol to hydroquinone is somewhat insensitive to the change of the molar ratio of phenol to H2O2, and the 1,4-benzoquinone increases clearly with the enhancement of the molar ratio of phenol to H2O2. These results are opposite with those on TS-1,20 where both molar ratio of catechol to hydroquinone and concentration of 1,4-benzoquinone decreased sharply with the increase of the molar ratio of phenol to H2O2 from 1 to 3. Solvent Effect between Water and Aqueous 0.01 M HCl. Since the catalytic activity can be significantly improved by decreasing the pH value of the reaction mixture using iron(II)-quinolinol/MCM-41 as the catalyst for phenol hydroxylation, the mechanism of which is via the formation of hydroxyl radicals, the intermediates in the reaction, by the interaction of the metal complex with H2O2,45 we compare the effect between water and dilute HCl aqueous solution as the reaction solvent on the activity of 5CuNaY. It is indicated in Table 6 that by the addition of a small amount of HCl in the water solvent (0.01 M), the reaction rate is enhanced drastically, with the reaction steady state being gained after only 1 h of reaction time. Moreover, the phenol conversion increases and 1,4-benzoquinone decreases in the HCl solution, with the hydroxylation selectivity increasing from 71.3% to 91.2%, which is altered by reaction time. On the other hand, the molar ratio of catechol to hydroqinone is around 2.0, and the H2O2 effective conversion possesses a high value of around 60%. This observation suggests that the dilute HCl aqueous solution is also an effective solvent for the CuNaY catalyst in phenol hydroxylation with H2O2. It seems that the reaction intermediate, such as a hydroxyl

radical, can be produced easily in a weak acidic medium.45,46 The hydroxyl radical was trapped by DMPO and observed by ESR (see below). Catalyst Regeneration. Catalyst regeneration is evaluated with the 2.5CuNaY catalyst, as shown in Table 7. By simple washing using water, the used catalyst in run 2 exhibits a very similar activity to that of the fresh catalyst, with only slight decrease of conversion and increase of 1,4-benzoquinone selectivity. This might be assigned to the coke formed inside the Y zeolite channels during reaction, which cannot be removed by the simple washing and drying, or only caused by the error range of the product analyses. By calcining the catalyst at 450 °C for 4 h in air, the activity of the used catalyst in run 3 is recovered completely, with the selectivity for 1,4-benzoquinone even lower than that over the fresh one (9.5% vs 14.6%). This result indicates that the Cu2+-exchanged catalyst is stable and renewable in phenol hydroxylation, and is potentially important for the industrial production of dihydroxybenzenes. ESR Measurement. The ESR spectrum of DMPO with H2O2 over copper ion-exchanged zeolites is demonstrated in Figure 1a. With 10CuNaY, for example, the spectrum with parameters of aN ) aH ) 15.0G is observed and assigned to the adduct of DMPO with hydroxyl radical.46 The calculated ESR spectrum with parameters of aN ) aH ) 15.0G is also plotted and compared in Figure 1b. Spin trapping experiment implies that hydroxyl radical is a possible intermediate

(45) Liu, C.; Shan, Y.; Yang, X.; Ye, X.; Wu, Y. J. Catal. 1997, 168 (1), 35-41.

(46) Tamagaki, S.; Sakai, M.; Tagaki, W. Bull. Chem. Soc. Jpn. 1989, 62 (1), 153-158.

Figure 1. (a) ESR spectrum of DMPO with H2O2 over 10CuNaY. (b) Calculated ESR stick diagram of DMPO-OH adduct with aN ) aH ) 15.0G.

Cu2+-Exchanged Zeolites in Phenol Hydroxylation

Energy & Fuels, Vol. 18, No. 2, 2004 475

Table 6. Catalytic Comparison of 5CuNaY Catalyst in Phenol Hydroxylation with H2O2 between Water and 0.01 M Hydrochloric Acid Aqueous Solution as the Reaction Solvent at Different Reaction Timea

a

time h

solvent

Xphenol %

0.5 0.5 1 1 3 3 5 5

H2O 0.01 M HCl H2O 0.01 M HCl H2O 0.01 M HCl H2O 0.01 M HCl

0 8.5 0 23.7 10.6 25.8 17.4 23.1

product distribution % CAT HQ

BQ

CAT/HQ mol/mol

88.7

29.1

3.7

55.9

7.9

91.2 77.9 71.3 90.1 77.8

46.3 30.6 43.0 47.7 46.8

16.4 7.3 21.4 17.2 24.2

28.5 40.0 6.8 25.2 6.8

2.82 4.2 2.0 2.77 1.93

Shydrox %

H2O2eff % 0 34.6 0 85.1 37.5 60.4 60.2 58.6

Reaction temperature: 50 °C; phenol/H2O2 (molar ratio) ) 3; phenol/catalyst (weight ratio) ) 10.

Table 7. Regeneration of 2.5CuNaY Catalyst in Phenol Hydroxylation with H2O2a

catalyst fresh (run 1) usedb (run 2) usedc (run 3)

Xphenol Shydrox % % 22.1 20.5 21.7

88.8 89.7 87.6

product distribution % CAT

HQ

BQ

51.2 49.9 52.1

23.0 20.7 25.8

14.6 19.2 9.5

CAT/HQ H2O2eff mol/mol % 2.23 2.41 2.02

68.6 67.0 63.2

a Reaction temperature: 50 °C; reaction time: 3 h; phenol/H O 2 2 (molar ratio) ) 3; phenol/catalyst (weight ratio) ) 10; 0.01 N HCl as a solvent. b Catalyst regenerated by washing with water, followed by drying at 110 °C. c Catalyst regenerated by washing with methanol, followed by drying at 110 °C, and calcinations at 450 °C for 4 h in air.

Figure 2. ESR spectra at 77 K of (a) 2.5CuNaY, (b) 5CuNaY, (c) 10CuNaY, (d) 100CuNaY, and (e) 500CuNaY.

in the catalytic hydroxylation of phenol with H2O2 over copper-containing samples prepared in this work, which is in a good agreement with previous work elsewhere 15 with Cu-Bi-V-O complex as the catalyst. To elucidate the presence of isolated Cu2+ and clustered copper in the Cu-NaY samples, their ESR spectra were recorded and compared in Figure 2, indicating that ESR line shape becomes broader with increasing copper loading.

Assuming that there is no magnetic interaction between the copper(II) species in CuNaY catalyst, the relative concentration of each species could be calculated by simulating ESR spectra with two different components where isolated copper species are associated with the component of g| ) 2.396, g⊥ ) 2.109, and A| ) 136.4G and the clustered copper is responsible for g| ) 2.276, g⊥ ) 2.081, and unresolved hyperfine splitting. After simulation, it was found that 4%, 10%, and 24% of the clustered copper(II) species are included in the 10CuNaY, 100CuNaY, and 500CuNaY, respectively, while a negligible amount (less than 3%) of clustered copper(II) species exist in the sample of 2.5CuNaY and 5CuNaY. For instance, the ESR spectrum of 10CuNaY could be simulated by mixing 90% of isolated copper(II) species and 10% of clustered copper(II) species. It is not clear to understand the line broadening. However, Bahranowski et al. has reported the preparation of Zn,Cu,Al-layered double hydroxides and catalytic activity dependence as a function of copper loading in the oxidation of aromatic hydrocarbons, such as xylene and toluene.47 They explained that the line broadening in the ESR spectra is due to the presence of clustered copper which catalyzes the decomposition of H2O2 and is inactive in the oxidation reaction. In this study, it seems that not only isolated copper(II) species but also a small amount of clustered copper(II) species (less than 10% of total content) is necessary to improve the selectivity and the effective conversion of H2O2. However, the Cu2+ ion-exchanged catalyst without clustered copper and with an excess amount of clustered copper shows lower selectivity and the effective conversion of H2O2. As shown in Table 2, selectivity increases from 60.7% over 2.5CuNaY to 74.7% over 10CuNaY, and decreases to 45.5% over 500CuNaY. The same trend is observed for the effective conversion of H2O2. Since it is known49-51 that during the ion-exchange process the relative composition of copper species is closely related to the concentration and type of copper source and ionexchange temperature, a better catalyst can be obtained by precise control of experimental conditions of the ionexchange process. (47) Finkelstein, E.; Rosen, G. M.; Ranckman, E. J. J. Am. Chem. Soc. 1980, 102 (15), 4994-4999. (48) Bahranowski, K.; Dula, R.; Gaisor, M.; Labanowska, M.; Michalik, A.; Vartikian, L. A.; Serwicka, E. M. Appl. Clay Sci. 2001, 18 (1-2), 93-101. (49) Iwamoto, M.; Yahiro, H.; Tanda, K.; Mizuno, N.; Mine, Y.; Kagawa, S. J. Phys. Chem. 1991, 95 (9), 3727-3730. (50) Dedecek, J.; Sobalik, Z.; Tvaruzkova, Z.; Kaucky, D.; Wichterlova, B. J. Phys. Chem. 1995, 99 (44), 16327-16337. (51) Hass, K. C.; Schneider, W. F. J. Phys. Chem. 1966, 100 (22), 9292-9301.

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Energy & Fuels, Vol. 18, No. 2, 2004

Summary The Cu2+-exchanged NaY, HY, and Hβ zeolites are very active for phenol hydroxylation with H2O2 with the phenol conversion at ca. 47%, hydroxylation selectivity at ca. 70%, 1,4-benzoquinone selectivity being less than 3% and the molar ratio of catechol to hydroquinone being larger than 2.0. To obtain these results, the reaction temperature should be 60 °C, with the molar ratio of phenol to hydrogen peroxide of 1 and water as the solvent. Moreover, under similar reaction conditions the Cu2+-exchanged zeolites are more active than TS-1 or homogeneous copper nitrate catalyst. It is also observed that the zeolite type, copper content in zeolite catalyst, and reaction conditions, such as reaction time, reaction temperature, and the molar ratio of phenol to hydrogen peroxide in the reaction mixture, influence the catalytic activity remarkably. On the other hand, significant enhancement of phenol conversion, hydroxylation selectivity, and reaction rate are observed when using dilute HCl aqueous solution as the reaction solvent compared with water as the solvent, implying that hydroxyl radical is more easily formed in a weak acidic medium.45,46 By calcining the used catalyst at 450 °C for 4 h in air, its activity can be completely recovered. These results indicate that the Cu2+-exchanged zeolite catalysts are potentially important for the industrial

Wang et al.

production of dihydroxybenzenes through the environmentally benign process of phenol hydroxylation with hydrogen peroxide. ESR of the Cu2+-exchanged sample suggests that the hydroxyl radical is the reaction intermediate in phenol hydroxylation, and a proper amount of isolated Cu2+ and clustered copper are necessary to improve the selectivity and the effective conversion of H2O2. Acknowledgment. This work was supported by Korea National Cleaner Production Center and the Grant R01-2003-000-10069-0 from the Basic Research Program of the Korea Science & Engineering Foundation. Dr. Jun Wang acknowledges the financial support of the Foreign Scientist Invitation Program sponsored by Korea Institute of S&T Evaluation and Planning (KISTEP). Note Added after ASAP Posting. This article was released ASAP on 02/06/2004. Additional minor changes were made in Tables 1 and 2, in Figure 1b, and in the footnote containing permanent address information for Professor Xian-Yong Wei. The revised version was posted on 02/24/2004. EF0300904