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Selective oxidation of ethylbenzene by a biomimetic combination: Hemin and N-hydroxyphthalimide (NHPI)
Catalysis Communications 8 (2007) 27–30 www.elsevier.com/locate/catcom
Selective oxidation of ethylbenzene by a biomimetic combination: Hemin and N-hydroxyphthalimide (NHPI) Hong Ma, Jie Xu *, Qiaohong Zhang, Hong Miao, Wenhai Wu State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China Received 9 January 2006; received in revised form 16 May 2006; accepted 16 May 2006 Available online 25 May 2006
Abstract A biomimetic catalytic system consisting of hemin and N-hydroxyphthalimide (NHPI) for the selective oxidation of ethylbenzene with dioxygen is reported. 90.32% conversion of ethylbenzene with 94.30% selectivity for acetophenone (AcPO) can be obtained at 100 °C under 0.3 MPa O2 for 9 h. Hemin can efficiently decompose in situ formed 1-phenylethyl hydroperoxide (PEHP) with AcPO as the main product. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Ethylbenzene; Hemin; N-hydroxyphthalimide; Oxidation
1. Introduction For the controllable oxyfunctionalization of hydrocarbons at lower temperature, molecular oxygen is more attractive than NaClO, PhIO, H2O2 [1] and considered to be an ultimate oxidant. However, the triplet nature of molecular oxygen hampers the reaction with hydrocarbons in singlet states and the reported results are far from satisfaction at present. The biomimetic catalytic system seems to be a fundamentally new approach. Metalloporphyrins, found to be the analogues of the prosthetic group of heme-containing monooxygenases, such as cytochrome P-450, are generally chosen for biomimetic chemical simulation [2–5]. It has been reported that metalloporphyrins successfully catalyzed the oxidation of alkanes, alkenes and other organic compounds by dioxygen with surprisingly high yield and selectivity under mild conditions [6,7]. In metalloporphyrin/O2 system, the active species is recognized to be high-valent metal-oxo species. Its generation is through two single-electron reductions. Sacrificial coreductants, such as H2/metal, NaBH4, CO, and zinc amalgam *
Corresponding author. Tel./fax: +86 411 84379245. E-mail address: [email protected] (J. Xu).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.05.023
etc., are needed [8–11]. Obviously, the major drawback is the consumption of stoichiometric amounts of co-reductants. Further, researchers found that high-halogenated metalloporphyrins can catalyze the reaction of alkanes with oxygen to give alcohols and/or carbonyl compound without co-reductants [12,13], but the difficulties in synthesis and the high price limited their large-scale application. Recently, Guo and his coworkers [14,15] reported the simple metalloporphyrins, such as iron, manganese, and cobalt tetraphenylporphyrins, could catalyze the hydrocarbons oxidation with air at high temperature. A potential alternative for alkane oxidations by metalloporphyrin/O2 system is to utilize the in situ generated alkyl hydroperoxide. Traylor and other researchers revealed that the reaction of alkyl hydroperoxide with iron porphyrins would produce high-valent iron-oxo species [16,17]. The 1 H NMR spectroscopic study gave evidence for the formation of iron-oxo as intermediate in the reaction of t-BuOOH with iron porphyrins [18]. Recently, NHPI serving as a radical catalyst promoter, usually combined with Co(II) or Mn(II) salts, was found to effectively catalyze autoxidation of hydrocarbons through alkyl hydroperoxide as transient intermediate [19–22]. The in situ formed alkyl hydroperoxide in NHPI-catalyzed hydrocarbon oxidations
28
H. Ma et al. / Catalysis Communications 8 (2007) 27–30
can be used as oxidant. Ishii has succeeded in epoxidation of alkenes by alkyl hydroperoxide generated in situ in NHPIcatalyzed hydrocarbons oxidation [23]. Based on the above, we reasoned that iron porphyrins and NHPI could couple to be an effective catalytic system for hydrocarbons oxidation. In this system, hemin was expected to cleave in situ generated alkyl hydroperoxide to form alkoxyl radical and high-valent iron-oxo. This iron-oxo intermediate can abstract hydrogen from NHPI, affording the chain propagating phthalimide N-oxyl radical (PINO). In this paper, we combined hemin and NHPI to constitute a biomimetic oxidation system and investigated the catalytic performance in the selective oxidation of ethylbenzene by dioxygen under mild conditions. To the best of our knowledge, it is the first time to use this catalytic system for hydrocarbons oxidation. 2. Experimental Hemin (ferriprotoporphyrin IX chloride) was obtained from Alfa Products, Thiokol/ventron Division. NHPI (99%) was synthesized as previous described [24,25]. Ethylbenzene (99%) and acetonitrile (99.9%) were purified before use. 1-Phenylethanol (PEA, 97%) was purchased from Acros Organics. The typical catalytic reaction was performed in a 50 mL stainless steel autoclave equipped with a magnetic stirrer. 2 mL ethylbenzene (16 mmol), 10 mL CH3CN, 0.25 mol% hemin and 10 mol% NHPI were added into the autoclave. After the given temperature reached, O2 was pressurized (ca. 0.3 MPa) into the reactor and the pressure was kept constant by supplying dioxygen during the reaction. The reaction under atmospheric pressure (0.1 MPa) was carried out in a two-neck flask equipped with a condenser and an oxygen bubbler. The oxidation products were identified by Agilent 6890 N GC/5973 MS detector and quantitated by Agilent 4890 D GC equipped with FID detector. AcPO, 1-phenylethanol, and benzoic acid were determined by the internal
standard method using 1,2,4,5-tetramethylbenzene as the internal standard. The yield of PEHP could not be directly measured by GC because of its thermolability. According to literature, PEHP can be converted quantitatively to PEA with excessive Ph3P at room temperature [26,27]. Thus the amount of PEHP can be accurately quantified by treatment of the mixture with excess Ph3P for 1 h and a second GC measurement. 3. Results and discussion Ethylbenzene was oxidized by molecular oxygen to AcPO, PEA, and PEHP (Scheme 1). The reaction results are listed in Table 1. In the absence of NHPI or hemin/ NHPI, less than 1% of ethylbenzene was directly oxygenated as a result of autoxidation at 100 °C for 6 h. When NHPI was used alone, the conversion of ethylbenzene was 38.30% with PEHP as main product at 100 °C for 6 h, which was consisted with the previous literature [24]. However, after the addition of hemin, the reaction was promoted significantly. At the same reaction conditions, it gave 71.47% conversion of ethylbenzene with 87.36% selectivity for AcPO. When the reaction time was prolonged to 9 h, 90.32% conversion of ethylbenzene with 94.30% selectivity for AcPO was obtained. The catalytic performances of NHPI and hemin/NHPI at decreased temperature were also investigated. With NHPI alone, oxidation cannot proceed at 40 °C, and gave comparably low conversion at 60 and 80 °C. With hemin/NHPI, the conversion was 33.70% at 40 °C and increased with elevating temperature. The selectivity for PEHP was 19.91% at 40 °C, and drastically decreased with the further increase of temperature. The phenomena that PEHP was produced as main product HO
O
CH2CH3 hemin/NHPI
C
CH3
HOO CH CH3
CH CH3
O2,CH3CN
Scheme 1. Oxidation of ethylbenzene to AcPO, PEA, and PEHP.
Table 1 Selective oxidation of ethylbenzene with molecular oxygen Entry
1 2 3 4 5a 6 7 8 9 10 11
Feed catalysts
– Hemin NHPI Hemin/NHPI Hemin/NHPI NHPI Hemin/NHPI NHPI Hemin/NHPI NHPI Hemin/NHPI
Temperature (°C)
100 100 100 100 100 80 80 60 60 40 40
Conversion (%)
Traces Traces 38.30 71.47 90.32 33.42 54.94 25.17 45.89 Traces 33.70
Selectivity (%) AcPO
PEA
PEHP
– – 27.08 87.36 94.30 23.00 87.00 11.41 87.21 – 70.53
– – 13.68 4.05 0.30 15.38 6.94 14.49 6.43 – 9.56
– – 59.24 5.94 0 61.62 5.68 74.10 6.37 – 19.91
Reaction conditions: 16 mmol ethylbenzene, 0.25 mol% hemin, 10 mol% NHPI, 10 mL CH3CN, 0.3 MPa O2, 6 h. a 9 h.
H. Ma et al. / Catalysis Communications 8 (2007) 27–30 Table 2 Comparison of various iron salts/NHPI systems on ethylbenzene oxidation
1 2 3 4a 5b
Feed catalysts
Iron oxalate/NHPI Iron citrate/NHPI Hemin/NHPI Hemin/NHPI Hemin/NHPI
Conversion (%)
37.34 46.92 54.94 37.82 47.95
100
80
Selectivity (%) AcPO
PEA
PEHP
50.84 48.43 87.00 78.27 81.68
25.92 13.05 6.94 9.43 7.79
23.24 38.51 5.68 12.30 10.53
Reaction conditions: 16 mmol ethylbenzene, 0.25 mol% iron ions, 10 mol% NHPI, 10 mL CH3CN, 80 °C, 0.3 MPa O2, 6 h. a 1 h. b 2 h.
Conv./Select. (mol%)
Entry
29
60
40
20
0 0
120
240
360
480
Time (min.) Fig. 1. Time course of ethylbenzene oxidation by hemin/NHPI system:(n) Ethylbenzene; (c) AcPO; (m) PEA; (.) PEHP. Reaction conditions: 16 mmol ethylbenzene, 0.25 mol% hemin, 10 mol% NHPI, 10 mL CH3CN, 100 °C, 0.3 MPa O2.
under 0.1–0.5 MPa oxygen pressures (Fig. 2). It can be seen that the conversion of ethylbenzene and distribution of products did not change with the increase of oxygen pressures. It indicated the pressure of oxygen hardly affected ethylbenzene oxidation. In NHPI-catalyzed ethylbenzene oxidation, as suggested in previous literature, NHPI was easily abstracted hydrogen by molecular oxygen to form PINO, which initiated the radical propagation of autoxidation with PEHP as the main product [25,28]. When hemin was added, in situ produced PEHP was decomposed efficiently, and AcPO was the main product. The similar phenomenon was observed in the investigation about catalytic cleavage of hydroperoxide by Fe(III)TPPCl [29]. It revealed that Fe(III)TPPCl cleaved 10-OOH-18:2 homolytically to ketones with the generation of [PFeIV=O]. Here we have
100
80
Conv./Select. (mol%)
by NHPI alone, and then decomposed by hemin strongly implied that hemin played an important role in decomposing PEHP and contributed to the production of AcPO. On the basis of these results, the oxidation of ethylbenzene with dioxygen was examined with various iron salts/ NHPI systems (Table 2). The selectivities for PEHP in iron oxalate/NHPI system and iron citrate/NHPI system were 23.24% and 38.51%, respectively. Under the same conditions, lower selectivity for PEHP (5.68%) was obtained using hemin/NHPI system. Additionally, hemin/NHPI system showed to be preferred to produce AcPO than the other two iron salts/NHPI systems. Considering more PEA would converted to AcPO at higher conversion due to subsequent reaction, the results for hemin, iron oxalate, and iron citrate were compared at comparable conversions. For hemin/NHPI system, the selectivities for AcPO in the reactions run for 1 and 2 h were 78.27% and 81.68%, respectively. These results indicated that AcPO was mainly from the decomposition of PEHP, not from the subsequent oxidation of PEA. In order to confirm this point, PEA was used as substrate. It was found that no PEA was converted to AcPO under the same conditions as entry 3, Table 2. In another experiment, after the reaction had been performed for 2 h (entry 5, Table 2), PEA was added. And then the reaction continued to run for additional 4 h. The molar content of PEA in the reaction mixture was 19.96% after PEA was added, and only decreased to 18.38% after the subsequent run. It indicated that PEA was difficult to convert to AcPO under such reaction conditions. In order to further study the process, the influence of reaction time on the catalytic performance of hemin/NHPI was investigated. As illustrated in Fig. 1, the conversion of ethylbenzene increased very rapidly in the initial 10 min, then increased steadily. When the reaction time was up to 9 h, the conversion of ethylbenzene was increased to 90.32%. In the whole reaction process, the selectivity for AcPO was higher than 80% and increased with time. Correspondingly, the selectivity for PEHP was decreased with time. At high conversion (90.32%), PEHP was decomposed completely. To evaluate the effect of oxygen pressure on ethylbenzene oxidation, the catalytic performance was investigated
60
40
20
0 0.1
0.2
0.3
0.4
0.5
Pressure (MPa) Fig. 2. Effect of pressure on ethylbenzene oxidation. (n) Ethylbenzene; (c) AcPO; (m) PEA; (.) PEHP. Reaction conditions: 16 mmol ethylbenzene, 0.25 mol% hemin, 10 mol% NHPI, 10 mL CH3CN, 60 °C, 6 h.
30
H. Ma et al. / Catalysis Communications 8 (2007) 27–30
ROOH
R-H
PINO
R=O R-OH
ROO
ROOH PFe(III)-OH
PFe(III) ROOH
R=O R-OH
ROO
R
NHPI
O2
PFe(IV)=O
RO
PFe(III)-OOR
R=O
Scheme 2. Catalytic cycle of hemin/NHPI system for ethylbenzene oxidation.
not identified the short-lived intermediate derived from hemin, but the candidates are the iron-oxo complexes ([PFeIV=O]). On the basis of the above, the mechanism of catalytic cycle of hemin/NHPI system in the aerobic oxidation of ethylbenzene is proposed (Scheme 2). Hemin cleaves PEHP to [PFeIV=O] and alkoxyl radial. The alkoxyl radical further converts to ketone by 1 e-oxidation process. The intermediate [PFeIV=O] readily abstracts hydrogen from NHPI to form PINO. Then PINO propagates radical oxidation and returned to NHPI by the abstraction of hydrogen from hydrocarbon. And hemin regenerates via the reaction of PFe(III)-OH and PEHP. 4. Conclusion In conclusion, hemin and NHPI coupled to be an active and selective biomimetic catalytic system for hydrocarbons oxidation. It realized the coreductant-free biomimetic oxidation of ethylbenzene by dioxygen under mild conditions (40–100 °C). It was suggested that intermediate PEHP was decomposed by hemin via a homolytic cleavage with AcPO as the main product. Furthermore, potential catalytic applications in selective oxidation of various hydrocarbons are currently under investigation. Acknowledgements We gratefully thank National Natural Science Foundation of China (20433078), and the National Hi-Tech Research and Development Program of China (2004AA32G020) for the financial support of this work. References [1] U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R.S. da Cruz, M.C. Guerreiro, D. Mandelli, E.V. Spinace´, E.L. Pires, Appl. Catal. A: Gen. 211 (2001) 1. [2] B. Meunier, Chem. Rev. 92 (1992) 1411. [3] B. Meunier, S.P. de Visser, S. Shaik, Chem. Rev. 104 (2004) 3947.
[4] M. Sono, M.P. Roach, E.D. Coulter, J.H. Dawson, Chem. Rev. 96 (1996) 2841. [5] I.D. Cunningham, T.N. Danks, J.N. Hay, I. Hamerton, S. Gunathilagan, Tetrahedron 57 (2001) 6847. [6] R.A. Sheldon, Metalloporphyrins in Catalytic Oxidation, Marcel Dekker, Basel, 1994. [7] J.T. Groves, J. Porphyrins Phthalocyanines 4 (2000) 350. [8] H. Tang, C.Y. Shen, M.R. Lin, A. Sen, Inorg. Chim. Acta 300–302 (2000) 1109. [9] P.A. Gosling, R.J.M. Nolte, J. Mol. Catal. A: Chem. 113 (1996) 257. [10] H. Ohtake, T. Higuchi, M. Hirobe, J. Am. Chem. Soc. 114 (1992) 10660. [11] L. Gaillion, F. Bedioui, P. Battioni, J. Devynck, J. Mol. Catal. 78 (1993) L23. [12] M.W. Grinstaff, M.G. Hill, J.A. Labinger, H.B. Gray, Science 264 (1994) 1311. [13] J. Haber, L. Matachowski, K. Pamin, J. Poltowicz, J. Mol. Catal. A: Chem. 198 (2003) 215. [14] C.C. Guo, Q. Liu, X.T. Wang, H.Y. Hu, Appl. Catal. A: Gen. 282 (2005) 55. [15] C.C. Guo, Q.J. Peng, Q. Liu, G.F. Jiang, J. Mol. Catal. A: Chem. 192 (2003) 295. [16] D. Mansuy, J.F. Bartoli, J.C. Chottard, M. Lange, Angew. Chem. Int. Ed. 19 (1980) 909. [17] T.G. Traylor, C. Kim, W.P. Fann, C.L. Perrin, Tetrahedron 54 (1998) 7977. [18] R.D. Arasasingham, C.R. Cornman, A.L. Balch, J. Am. Chem. Soc. 111 (1989) 7800. [19] Y. Ishii, S. Sakaguchi, T. Iwahama, Adv. Synth. Catal. 343 (2001) 393. [20] B.B. Wentzel, M.P.J. Donners, P.L. Alsters, M.C. Feiters, R.J.M. Nolte, Tetrahedron 56 (2000) 7797. [21] X. Baucherel, L. Gonsalvi, I.W.C.E. Arends, S. Ellwood, R.A. Sheldon, Adv. Synth. Catal. 346 (2004) 286. [22] Y. Ishii, T. Iwahama, S. Sakaguchi, K. Nakayama, Y. Nishiyama, J. Org. Chem. 61 (1996) 4520. [23] T. Iwahama, G. Hatta, S. Sakaguchi, Y. Ishii, J. Chem. Soc. Chem. Commun. (2000) 163. [24] G.Y. Yang, Y.F. Ma, J. Xu, J. Am. Chem. Soc. 126 (2004) 10542. [25] G.Y. Yang, Q.H. Zhang, H. Miao, X.L. Tong, J. Xu, Org. Lett. 7 (2005) 263. [26] S. Evans, J.R.L. Smith, J. Chem. Soc. Perkin Trans. 2 (2000) 1541. [27] S. Evans, J.R.L. Smith, J. Chem. Soc. Perkin Trans. 2 (2001) 174. [28] Y. Ishii, K. Nakayama, M. Takeno, S. Sakaguchi, T. Iwahama, Y. Nishiyama, J. Org. Chem. 60 (1995) 3934. [29] R. Labeque, L.J. Marnett, J. Am. Chem. Soc. 111 (1989) 6621.
Selective oxidation of ethylbenzene by a biomimetic combination: Hemin and N-hydroxyphthalimide (NHPI) Hong Ma, Jie Xu *, Qiaohong Zhang, Hong Miao, Wenhai Wu State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China Received 9 January 2006; received in revised form 16 May 2006; accepted 16 May 2006 Available online 25 May 2006
Abstract A biomimetic catalytic system consisting of hemin and N-hydroxyphthalimide (NHPI) for the selective oxidation of ethylbenzene with dioxygen is reported. 90.32% conversion of ethylbenzene with 94.30% selectivity for acetophenone (AcPO) can be obtained at 100 °C under 0.3 MPa O2 for 9 h. Hemin can efficiently decompose in situ formed 1-phenylethyl hydroperoxide (PEHP) with AcPO as the main product. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Ethylbenzene; Hemin; N-hydroxyphthalimide; Oxidation
1. Introduction For the controllable oxyfunctionalization of hydrocarbons at lower temperature, molecular oxygen is more attractive than NaClO, PhIO, H2O2 [1] and considered to be an ultimate oxidant. However, the triplet nature of molecular oxygen hampers the reaction with hydrocarbons in singlet states and the reported results are far from satisfaction at present. The biomimetic catalytic system seems to be a fundamentally new approach. Metalloporphyrins, found to be the analogues of the prosthetic group of heme-containing monooxygenases, such as cytochrome P-450, are generally chosen for biomimetic chemical simulation [2–5]. It has been reported that metalloporphyrins successfully catalyzed the oxidation of alkanes, alkenes and other organic compounds by dioxygen with surprisingly high yield and selectivity under mild conditions [6,7]. In metalloporphyrin/O2 system, the active species is recognized to be high-valent metal-oxo species. Its generation is through two single-electron reductions. Sacrificial coreductants, such as H2/metal, NaBH4, CO, and zinc amalgam *
Corresponding author. Tel./fax: +86 411 84379245. E-mail address: [email protected] (J. Xu).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.05.023
etc., are needed [8–11]. Obviously, the major drawback is the consumption of stoichiometric amounts of co-reductants. Further, researchers found that high-halogenated metalloporphyrins can catalyze the reaction of alkanes with oxygen to give alcohols and/or carbonyl compound without co-reductants [12,13], but the difficulties in synthesis and the high price limited their large-scale application. Recently, Guo and his coworkers [14,15] reported the simple metalloporphyrins, such as iron, manganese, and cobalt tetraphenylporphyrins, could catalyze the hydrocarbons oxidation with air at high temperature. A potential alternative for alkane oxidations by metalloporphyrin/O2 system is to utilize the in situ generated alkyl hydroperoxide. Traylor and other researchers revealed that the reaction of alkyl hydroperoxide with iron porphyrins would produce high-valent iron-oxo species [16,17]. The 1 H NMR spectroscopic study gave evidence for the formation of iron-oxo as intermediate in the reaction of t-BuOOH with iron porphyrins [18]. Recently, NHPI serving as a radical catalyst promoter, usually combined with Co(II) or Mn(II) salts, was found to effectively catalyze autoxidation of hydrocarbons through alkyl hydroperoxide as transient intermediate [19–22]. The in situ formed alkyl hydroperoxide in NHPI-catalyzed hydrocarbon oxidations
28
H. Ma et al. / Catalysis Communications 8 (2007) 27–30
can be used as oxidant. Ishii has succeeded in epoxidation of alkenes by alkyl hydroperoxide generated in situ in NHPIcatalyzed hydrocarbons oxidation [23]. Based on the above, we reasoned that iron porphyrins and NHPI could couple to be an effective catalytic system for hydrocarbons oxidation. In this system, hemin was expected to cleave in situ generated alkyl hydroperoxide to form alkoxyl radical and high-valent iron-oxo. This iron-oxo intermediate can abstract hydrogen from NHPI, affording the chain propagating phthalimide N-oxyl radical (PINO). In this paper, we combined hemin and NHPI to constitute a biomimetic oxidation system and investigated the catalytic performance in the selective oxidation of ethylbenzene by dioxygen under mild conditions. To the best of our knowledge, it is the first time to use this catalytic system for hydrocarbons oxidation. 2. Experimental Hemin (ferriprotoporphyrin IX chloride) was obtained from Alfa Products, Thiokol/ventron Division. NHPI (99%) was synthesized as previous described [24,25]. Ethylbenzene (99%) and acetonitrile (99.9%) were purified before use. 1-Phenylethanol (PEA, 97%) was purchased from Acros Organics. The typical catalytic reaction was performed in a 50 mL stainless steel autoclave equipped with a magnetic stirrer. 2 mL ethylbenzene (16 mmol), 10 mL CH3CN, 0.25 mol% hemin and 10 mol% NHPI were added into the autoclave. After the given temperature reached, O2 was pressurized (ca. 0.3 MPa) into the reactor and the pressure was kept constant by supplying dioxygen during the reaction. The reaction under atmospheric pressure (0.1 MPa) was carried out in a two-neck flask equipped with a condenser and an oxygen bubbler. The oxidation products were identified by Agilent 6890 N GC/5973 MS detector and quantitated by Agilent 4890 D GC equipped with FID detector. AcPO, 1-phenylethanol, and benzoic acid were determined by the internal
standard method using 1,2,4,5-tetramethylbenzene as the internal standard. The yield of PEHP could not be directly measured by GC because of its thermolability. According to literature, PEHP can be converted quantitatively to PEA with excessive Ph3P at room temperature [26,27]. Thus the amount of PEHP can be accurately quantified by treatment of the mixture with excess Ph3P for 1 h and a second GC measurement. 3. Results and discussion Ethylbenzene was oxidized by molecular oxygen to AcPO, PEA, and PEHP (Scheme 1). The reaction results are listed in Table 1. In the absence of NHPI or hemin/ NHPI, less than 1% of ethylbenzene was directly oxygenated as a result of autoxidation at 100 °C for 6 h. When NHPI was used alone, the conversion of ethylbenzene was 38.30% with PEHP as main product at 100 °C for 6 h, which was consisted with the previous literature [24]. However, after the addition of hemin, the reaction was promoted significantly. At the same reaction conditions, it gave 71.47% conversion of ethylbenzene with 87.36% selectivity for AcPO. When the reaction time was prolonged to 9 h, 90.32% conversion of ethylbenzene with 94.30% selectivity for AcPO was obtained. The catalytic performances of NHPI and hemin/NHPI at decreased temperature were also investigated. With NHPI alone, oxidation cannot proceed at 40 °C, and gave comparably low conversion at 60 and 80 °C. With hemin/NHPI, the conversion was 33.70% at 40 °C and increased with elevating temperature. The selectivity for PEHP was 19.91% at 40 °C, and drastically decreased with the further increase of temperature. The phenomena that PEHP was produced as main product HO
O
CH2CH3 hemin/NHPI
C
CH3
HOO CH CH3
CH CH3
O2,CH3CN
Scheme 1. Oxidation of ethylbenzene to AcPO, PEA, and PEHP.
Table 1 Selective oxidation of ethylbenzene with molecular oxygen Entry
1 2 3 4 5a 6 7 8 9 10 11
Feed catalysts
– Hemin NHPI Hemin/NHPI Hemin/NHPI NHPI Hemin/NHPI NHPI Hemin/NHPI NHPI Hemin/NHPI
Temperature (°C)
100 100 100 100 100 80 80 60 60 40 40
Conversion (%)
Traces Traces 38.30 71.47 90.32 33.42 54.94 25.17 45.89 Traces 33.70
Selectivity (%) AcPO
PEA
PEHP
– – 27.08 87.36 94.30 23.00 87.00 11.41 87.21 – 70.53
– – 13.68 4.05 0.30 15.38 6.94 14.49 6.43 – 9.56
– – 59.24 5.94 0 61.62 5.68 74.10 6.37 – 19.91
Reaction conditions: 16 mmol ethylbenzene, 0.25 mol% hemin, 10 mol% NHPI, 10 mL CH3CN, 0.3 MPa O2, 6 h. a 9 h.
H. Ma et al. / Catalysis Communications 8 (2007) 27–30 Table 2 Comparison of various iron salts/NHPI systems on ethylbenzene oxidation
1 2 3 4a 5b
Feed catalysts
Iron oxalate/NHPI Iron citrate/NHPI Hemin/NHPI Hemin/NHPI Hemin/NHPI
Conversion (%)
37.34 46.92 54.94 37.82 47.95
100
80
Selectivity (%) AcPO
PEA
PEHP
50.84 48.43 87.00 78.27 81.68
25.92 13.05 6.94 9.43 7.79
23.24 38.51 5.68 12.30 10.53
Reaction conditions: 16 mmol ethylbenzene, 0.25 mol% iron ions, 10 mol% NHPI, 10 mL CH3CN, 80 °C, 0.3 MPa O2, 6 h. a 1 h. b 2 h.
Conv./Select. (mol%)
Entry
29
60
40
20
0 0
120
240
360
480
Time (min.) Fig. 1. Time course of ethylbenzene oxidation by hemin/NHPI system:(n) Ethylbenzene; (c) AcPO; (m) PEA; (.) PEHP. Reaction conditions: 16 mmol ethylbenzene, 0.25 mol% hemin, 10 mol% NHPI, 10 mL CH3CN, 100 °C, 0.3 MPa O2.
under 0.1–0.5 MPa oxygen pressures (Fig. 2). It can be seen that the conversion of ethylbenzene and distribution of products did not change with the increase of oxygen pressures. It indicated the pressure of oxygen hardly affected ethylbenzene oxidation. In NHPI-catalyzed ethylbenzene oxidation, as suggested in previous literature, NHPI was easily abstracted hydrogen by molecular oxygen to form PINO, which initiated the radical propagation of autoxidation with PEHP as the main product [25,28]. When hemin was added, in situ produced PEHP was decomposed efficiently, and AcPO was the main product. The similar phenomenon was observed in the investigation about catalytic cleavage of hydroperoxide by Fe(III)TPPCl [29]. It revealed that Fe(III)TPPCl cleaved 10-OOH-18:2 homolytically to ketones with the generation of [PFeIV=O]. Here we have
100
80
Conv./Select. (mol%)
by NHPI alone, and then decomposed by hemin strongly implied that hemin played an important role in decomposing PEHP and contributed to the production of AcPO. On the basis of these results, the oxidation of ethylbenzene with dioxygen was examined with various iron salts/ NHPI systems (Table 2). The selectivities for PEHP in iron oxalate/NHPI system and iron citrate/NHPI system were 23.24% and 38.51%, respectively. Under the same conditions, lower selectivity for PEHP (5.68%) was obtained using hemin/NHPI system. Additionally, hemin/NHPI system showed to be preferred to produce AcPO than the other two iron salts/NHPI systems. Considering more PEA would converted to AcPO at higher conversion due to subsequent reaction, the results for hemin, iron oxalate, and iron citrate were compared at comparable conversions. For hemin/NHPI system, the selectivities for AcPO in the reactions run for 1 and 2 h were 78.27% and 81.68%, respectively. These results indicated that AcPO was mainly from the decomposition of PEHP, not from the subsequent oxidation of PEA. In order to confirm this point, PEA was used as substrate. It was found that no PEA was converted to AcPO under the same conditions as entry 3, Table 2. In another experiment, after the reaction had been performed for 2 h (entry 5, Table 2), PEA was added. And then the reaction continued to run for additional 4 h. The molar content of PEA in the reaction mixture was 19.96% after PEA was added, and only decreased to 18.38% after the subsequent run. It indicated that PEA was difficult to convert to AcPO under such reaction conditions. In order to further study the process, the influence of reaction time on the catalytic performance of hemin/NHPI was investigated. As illustrated in Fig. 1, the conversion of ethylbenzene increased very rapidly in the initial 10 min, then increased steadily. When the reaction time was up to 9 h, the conversion of ethylbenzene was increased to 90.32%. In the whole reaction process, the selectivity for AcPO was higher than 80% and increased with time. Correspondingly, the selectivity for PEHP was decreased with time. At high conversion (90.32%), PEHP was decomposed completely. To evaluate the effect of oxygen pressure on ethylbenzene oxidation, the catalytic performance was investigated
60
40
20
0 0.1
0.2
0.3
0.4
0.5
Pressure (MPa) Fig. 2. Effect of pressure on ethylbenzene oxidation. (n) Ethylbenzene; (c) AcPO; (m) PEA; (.) PEHP. Reaction conditions: 16 mmol ethylbenzene, 0.25 mol% hemin, 10 mol% NHPI, 10 mL CH3CN, 60 °C, 6 h.
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ROOH
R-H
PINO
R=O R-OH
ROO
ROOH PFe(III)-OH
PFe(III) ROOH
R=O R-OH
ROO
R
NHPI
O2
PFe(IV)=O
RO
PFe(III)-OOR
R=O
Scheme 2. Catalytic cycle of hemin/NHPI system for ethylbenzene oxidation.
not identified the short-lived intermediate derived from hemin, but the candidates are the iron-oxo complexes ([PFeIV=O]). On the basis of the above, the mechanism of catalytic cycle of hemin/NHPI system in the aerobic oxidation of ethylbenzene is proposed (Scheme 2). Hemin cleaves PEHP to [PFeIV=O] and alkoxyl radial. The alkoxyl radical further converts to ketone by 1 e-oxidation process. The intermediate [PFeIV=O] readily abstracts hydrogen from NHPI to form PINO. Then PINO propagates radical oxidation and returned to NHPI by the abstraction of hydrogen from hydrocarbon. And hemin regenerates via the reaction of PFe(III)-OH and PEHP. 4. Conclusion In conclusion, hemin and NHPI coupled to be an active and selective biomimetic catalytic system for hydrocarbons oxidation. It realized the coreductant-free biomimetic oxidation of ethylbenzene by dioxygen under mild conditions (40–100 °C). It was suggested that intermediate PEHP was decomposed by hemin via a homolytic cleavage with AcPO as the main product. Furthermore, potential catalytic applications in selective oxidation of various hydrocarbons are currently under investigation. Acknowledgements We gratefully thank National Natural Science Foundation of China (20433078), and the National Hi-Tech Research and Development Program of China (2004AA32G020) for the financial support of this work. References [1] U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R.S. da Cruz, M.C. Guerreiro, D. Mandelli, E.V. Spinace´, E.L. Pires, Appl. Catal. A: Gen. 211 (2001) 1. [2] B. Meunier, Chem. Rev. 92 (1992) 1411. [3] B. Meunier, S.P. de Visser, S. Shaik, Chem. Rev. 104 (2004) 3947.
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