English

• 1.   Introduction

  The oxidation of anilines to azoxybenzenes is an important transformation that has been widely applied in the synthesis of their oxygenated derivatives such as hydroxylamine, oxime, azoxy, nitro, nitroso and azo compounds (Scheme 1).^[1] Additionally, azoxybenzenes are used as reducing agents, polymerization inhibitors, chemical stabilizers, and dyes. Therefore, many methods have been reported for the synthesis of azoxybenzene. Traditional methods heavily rely on stoichiometric reduction and oxidation such as reduction of nitro and nitroso aromatic compounds by using sodium bis(trimethylsilyl)amide^[2] and potassium borohydride, ^[3] and oxidation of anilines with stoichiometric oxidants of sulfonic peracids, ^[4] Bu₄N- HSO₅, ^[5] and m-chloroperoxybenzoic acid.^[6] However, the use of stoichiometric reagents makes these processes environmentally unfriendly.

  Scheme 1

  

  Scheme 1.  Pathways for the synthesis of azoxybenzene

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  Various attempts were made for the oxidative coupling of aniline to azoxybenzene by catalytic protocols. For instance, homogeneous ruthenium-catalyzed oxidation of aniline to provide azoxybenzene with the aid of environmentally clean and cheaper oxidant H₂O₂, has been reported.^[7] However, homogeneous catalysis has a common problem of recycling of the catalyst. In order to address this drawback, heterogeneous catalysts like zeolites, vanadium silicates, and titanium containing molecular sieves, and cobalt oxide supported mesoporous silica for the synthesis of azoxybenzene from aniline are descibed in the literature.^[8] Additionally, silver supported tungsten oxide nanostructured catalyst^[9] and CuCr₂O₄ spinel nano catalyst^[10] were found to be suitable for the oxidation of aniline to azoxybenzene with 91% and 92% selectivities, respectively. Recently, Zhang et al. reported the synthesis of azoxybenzenes using mesoporous titania microspheres.^[11] Although various catalysts have been efficiently used for the preparation of azoxybenzene, a successful catalytic system with very high selectivity^[12] and yield of azoxybenzene using lower hydrogen peroxide concentration under mild conditions has not been achieved. Herein, we develop a general and high yielding method for the synthesis of various azoxybenzenes via titanium silicate molecular sieves-catalyzed oxidation of anilines using H₂O₂ as an oxidant. Notably, we observed that the molar ratios of silicon to titanium can adjust the selectivity of the reaction.^[13]

  2.   Results and discussion

  2.1   Scanning electron microscope images of TS-1 (80)

  The scanning electron microscope (SEM) images of TS-1 (80) are shown in Figure 1. From the images, it is observed that all the particles are spherical in shape with size of 150~200 nm.

  Figure 1

  

  Figure 1.  Scanning-electron micrographs of TS-1 (80)

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  2.2   Influence of catalysts

  The influence of several catalysts on the oxidation of aniline to azoxybenzene was studied. As shown in Table 1 are the results for the reactions performed in acetone and at 60 ℃ for 5 h with aniline to H₂O₂ molar ratio of 1:1. The catalyst plays a decisive role in this reaction according to the blank experimental result (Table 1, Entry 1). As can be seen from Entries 2~5 in Table 1, TS-1 gave higher reactivity than any of the other catalysts. Under the same conditions, the influence of different Si/Ti ratios was also explored. When the ratio of Si/Ti was decreased from 100 to 80, the yield would increase from 49.1% to 52.6%. However, the Si/Ti ratio further decreased to 33 and even to 10, leading to an apparent decrease in the yield. This is likely due to higher titanium content producing more white precipitate from the hydrolysis of the titanium source during the catalyst synthesis that adheres to the surface of the catalyst and prevents the incorporation of the titanium into the TS-1 framework.

  Table 1

  

  Table 1.  Optimization of reaction conditions ^a

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  Entry	Catalyst	H2O2/Substrate	Solvent	Yield/%
  1	No catalyst	1.0	Acetone	No product
  2	TS-1 (33)	1.0	Acetone	36.6
  3	Mg-TS-1	1.0	Acetone	28.1
  4	Al-TS-1	1.0	Acetone	31.1
  5	Fe-TS-1	1.0	Acetone	1.4
  6	TS-1 (80)	1.0	Acetone	52.6
  7	TS-1 (10)	1.0	Acetone	35.0
  8	TS-1 (100)	1.0	Acetone	49.1
  9	TS-1 (80)	1.5	Acetone	84.9
  10	TS-1 (80)	2.0	Acetone	96.6
  11	TS-1 (80)	2.5	Acetone	96.6
  12	TS-1 (80)	2.0	Methanol	85.0
  13	TS-1 (80)	2.0	Acetonitrile	95.3
  a Reaction conditions: substrate (1 mmol), catalyst (0.1 g), solvent (2 mL), water bath 60 ℃, 5 h.

  2.3   Dose of H₂O₂

  Next, we have chosen TS-1 (80) as the catalyst for investigating the influence of the molar ratios of H₂O₂ to aniline on the oxidation reaction (Table 1, Entries 6 and 9~11). With increasing the ratio of H₂O₂ to aniline in the reaction mixture, the yield was found to increase. When the ratio of H₂O₂ to aniline reaches 2.0, the optimal yield of azoxybenzene was obtained (96.6%). High concentration of H₂O₂ didn't lead to significant change in the yield.

  2.4   Influence of solvent

  In a series of reactions with titanium silicate as the catalyst, the solvent was explored for the activity and selectivity.^[14] Table 1 (Entries 10 and 12~13) shows the effect of three different solvents on the oxidation of aniline. The reactions were proceeded at 60 ℃ for 5 h with keeping molar ratio of H₂O₂ to aniline. Acetone gave a higher yield (96%) for the desired product 2a than acetonitrile (95%) or methanol (85%).

  2.5   Reaction applicability

  The optimized reaction conditions were then tested with a wide range of anilines (Table 2). Varying the substituents on the aromatic unit showed that electron-withdrawing (2e~2h and 2k~2l) and electron-rich (2b~2d, 2i~2j, 2m~2t) aniline were competent substrates. The results indicate that the electronic effects of the substituents have little influence on the transformation. In contrast, the steric effect of the substituents was observed. For instance, 2-methylaniline (1d) gave the desired product 2d in moderate yield, and extremely bulky 2, 6-dimethyl aniline (1p) only led to a poor yield of 2p.

  Table 2

  

  Table 2.  Oxidation of different anilines to azoxybenzenes^a

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  Entry	Aniline	Product	Yield/%
  1			96
  2			98
  3			60
  4			70
  5			80
  6			97
  7			70
  8			96
  9			98
  10			96
  11			90
  12			98
  13			98
  14			90
  15			92
  16			50
  17			85
  18			96
  19			85
  20			88
  a Reaction conditions: 1 (1 mmol), H2O2 (2 mmol), catalyst (0.1 g), acetone (2 mL), water bath 60 ℃, 5 h. b Conversion was determined by separation.

  2.6   Reusability test

  The reusability of TS-1 (80) was tested in the oxidation system for subsequent cycles. At the end of reaction, the catalyst was centrifuged from reaction mixture. The catalyst was calcined in a muffle furnace at 550 ℃ for 6 h and then reused in the next reaction. Three runs were performed under the same conditions. There is no significant loss of catalyst activity and the yield for each run was 94%, 93%, and 93%, respectively.

  3.   Conclusions

  In summary, we achieved the selective oxidation of anilines to azoxybenzenes by adjusting the titanium content. It has been demonstrated that TS-1 with the Si/Ti molar ratio of 80 is superior to other ratios of titanium silicon molecular sieves in the oxidation of aniline to azoxybenzene. The catalyst also has excellent performance for the selective oxidation of a wide scope of substituted anilines to the corresponding azoxybenzenes. The reactions worked well (yields: 50%~98%) with different anilines at 60 ℃ with environmentally benign H₂O₂ as the oxidation and acetone as the solvent. In addition, the catalyst maintained its catalytic activity even after three recycles. These features make the protocol attractive for broad applications.

  4.   Experimental section

  The TS-1 was prepared by hydrothermal method reported in Ref. [13, 15]. The TS-1 samples with different Si/Ti molar ratios were synthesized by adjusting the molar ratio of silicon source (tetraethyl orthosilicate, TEOS) to titanium source (titanium isopropoxide, TTIP). In a synthesis of Si/Ti＝80, as an example, TPAOH (32 g, 25% aq.) was dissolved in deionized water (8 g) completely. TEOS (21 g, 98.5%) was added to the solution under stirring, and then the stirring was continued for 1 h. To a solution of TTIP (0.4 g) in dry isopropanol (2 g) was slowly dropped into the reaction mixture with stirring for 1 h. Note that in the above process the formation of white solid titanium dioxide by hydrolysis of isopropyl titanate has to be avoided. Finally, phosphoric acid (0.3 g, 85%, aq.) in distilled water (9.7 g) was added to the above mixed system and the reaction mixture was stirred for one more hour. The molar composition of the mixture was n(TEOS):n(TTIP):n(TPAOH):n(IPA):n(H₃PO₄): n(H₂O)＝80:1:27.9:23.6:1.8:1646.

  The above mixture was transferred to the autoclave and crystallized for 6 h at 433 K. The solid product was separated from the mother liquor by centrifugation, followed by washing thoroughly with deionized water, 1 wt% aqueous solution of ammonium acetate, and deionized water. The precipitate was collected, dried in the oven at 393 K, and calcined in a muffle furnace at 823 K for 16 h.

  The supported metal (Al, Fe, and Mg) catalyst was synthesized by an impregnation method.^[16] First, the above TS-1 sample was kept dry in dryer. A 5 mL of solution of corresponding metal salts [Al₂(NO₃)₃, FeCl₂•4H₂O, and Mg(OAc)₂)] with 0.2 mol•L^－1 was used, and the TS-1 (1 g) was impregnated into the support under room temperature for 2 h. Then, the catalyst was dried in an oven for 6 h at 120 ℃ to remove water. Finally, the catalyst was calcined at 540 ℃ for 6 h in a muffle furnace.

  All the catalytic reactions were carried out in an oven dried reaction tube with a magnetic stir bar. The reaction tube was charged with TS-1 (Si/Ti＝80, 0.1 g), and a solution of substrate (1 mmol) in solvent (acetone, 2 mL). H₂O₂ (30%, 2 mmol) was added successively. Then, the reaction mixture was heated in a water bath at 60 ℃ for 5 h. Conversion was calculated on the basis of the substrate. The progress of the reaction was monitored by TLC. After that, the catalyst was centrifuged from the reaction mixture for recycling. The mixture was washed with saturated brine and extracted with ethyl acetate (15 mL×3). The organic solution was dried over anhydrous Na₂SO₄ and concentrated to 1~2 mL by rotary evaporator. Then, by means of column chromatography, the target products 2a~2t were obtained.

  1, 2-Diphenyldiazene oxide (2a):^[17] 99.1 mg, 96% yield. Yellow liquid, m.p. 34~35 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.32 (d, J＝8.0 Hz, 2H), 8.17 (d, J＝7.5 Hz, 2H), 7.57~7.40 (m, 6H); ¹³C NMR (100 MHz, CDCl₃) δ: 148.3, 143.9, 131.6, 129.6, 128.8, 128.7, 125.5, 122.3.

  1, 2-Di-p-tolyldiazene oxide (2b):^[17] 113.1 mg, 99% yield. Orange solid, m.p. 67~68 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.18 (d, J＝8.6 Hz, 2H), 8.12 (d, J＝8.4 Hz, 2H), 7.28 (d, J＝8.1 Hz, 4H), 2.44 (s, 3H), 2.41 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 146.1, 141.9, 141.7, 140.0, 129.3, 129.3, 125.6, 122.1, 21.6, 21.3.

  1, 2-Di-m-tolyldiazene oxide (2c):^[6] 113.1 mg, 60% yield. Orange solid, m.p. 35~36 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.10 (d, J＝12.8 Hz, 2H), 7.98 (d, J＝7.2 Hz, 2H), 7.41~7.35 (m, 3H), 7.22 (d, J＝7.8 Hz, 1H), 2.47 (s, 3H), 2.44 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 148.4, 144.0, 139.0, 138.4, 132.3, 130.4, 128.6, 126.0, 122.7, 122.5, 119.5, 21.5, 21.4.

  1, 2-Di-o-tolyldiazene oxide (2d):^[6] 113.1 mg, 75% yield. Orange solid, m.p. 57~58 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.03 (d, J＝7.9 Hz, 2H), 7.67 (d, J＝7.7 Hz, 2H), 7.40~7.28 (m, 6H), 2.52 (s, 3H), 2.38 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 149.4, 142.7, 134.1, 131.8, 131.2, 130.8, 128.6, 126.6, 126.0, 123.5, 121.5, 18.4, 18.4.

  1, 2-Bis(3-bromophenyl)diazene oxide (2e):^[18] 178.0 mg, 80% yield. Yellow solid, m.p. 110~111 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.48 (t, J＝2.0 Hz, 1H), 8.42 (t, J＝1.9 Hz, 1H), 8.26~8.23 (m, 1H), 8.07~8.04 (m, 1H), 7.73~7.70 (m, 1H); 7.56~7.53 (m, 1H), 7.43~7.35 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 148.8, 144.6, 134.9, 132.8, 130.2, 130.0, 128.3, 125.7, 124.5, 122.4, 122.3, 121.1.

  1, 2-Bis(4-bromophenyl)diazene oxide (2f):^[17] 178.0 mg, 99% yield. Yellow solid, m.p. 169~170 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.18 (d, J＝8.9 Hz, 2H), 8.08 (d, J＝8.8 Hz, 2H), 7.65 (d, J＝9.0 Hz, 2H), 7.61 (d, J＝8.8 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 147.0, 142.6, 132.0, 132.0, 127.2, 126.5, 123.9, 123.6.

  1, 2-Bis(3-chlorophenyl)diazene oxide (2g):^[18] 133.6 mg, 70% yield. Yellow solid, m.p. 95~96 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.32 (s, 1H), 8.27 (s, 1H), 8.21 (d, J＝8.2 Hz, 1H), 8.01 (d, J＝7.6 Hz, 1H), 7.56 (d, J＝8.0 Hz, 1H); 7.49~7.38 (m, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 153.0, 148.8, 144.5, 134.8, 134.4, 132.0, 129.9, 129.7, 125.4, 124.1, 122.8, 120.6.

  1, 2-Bis(4-chlorophenyl)diazene oxide (2h):^[18] 133.6 mg, 96% yield. Yellow solid, m.p. 151~152 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.26 (d, J＝9.0 Hz, 2H), 8.16 (d, J＝9.0 Hz, 2H), 7.50~7.44 (m, 4H); ¹³C NMR (100 MHz, CDCl₃) δ: 146.5, 142.2, 138.1, 135.2, 129.0, 129.0, 127.1, 123.7.

  1, 2-Bis(4-methoxyphenyl)diazene oxide (2i):^[17] 129.1 mg, 98% yield. Yellow solid, m.p. 115~116 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.29~8.24 (m, 4H), 6.99~6.95 (m, 4H), 3.89 (s, 3H), 3.88 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 161.8, 160.2, 141.7, 138.0, 127.8, 123.7, 113.7, 113.6, 55.7, 55.5.

  1, 2-Bis(3-methoxyphenyl)diazene oxide (2j):^[19] 129.1 mg, 96% yield. Yellow solid, m.p. 50~51 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.91~7.89 (s, 1H), 7.84 (s, 1H), 7.80 (m, 1H), 7.75~7.73 (m, 1H), 7.41~7.39 (m, 2H), 7.12~7.09 (m, 1H), 6.99~6.96 (m, 1H), 3.90 (s, 3H), 3.87 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 159.8, 159.5, 149.4, 144.9, 129.4, 129.3, 118.4, 116.3, 114.6, 110.0, 107.4, 55.7, 55.4.

  1, 2-Bis(3-fluorophenyl)diazene oxide (2k):^[6] 117.1 mg, 95% yield. Yellow solid, m.p. 50~51 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.13 (dd, J＝8.2 Hz, 1H), 8.09~8.02 (m, 2H), 7.86~7.84 (m, 1H), 7.53~7.43 (m, 2H), 7.32~7.28 (m, 1H), 7.16~7.11 (m, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.6 (d, J＝7.7 Hz), 161.1 (d, J＝5.2 Hz), 149.3 (d, J＝9.7 Hz), 144.8 (d, J＝9.2 Hz), 130.1 (d, J＝8.3 Hz), 129.8 (d, J＝8.7 Hz), 122.1 (d, J＝3.0 Hz), 119.0 (d, J＝21.3 Hz), 118.1 (d, J＝3.2 Hz), 116.9 (d, J＝21.5 Hz), 112.2 (d, J＝24.5 Hz), 110.3 (d, J＝26.6 Hz).

  1, 2-Bis(4-fluorophenyl)diazene oxide (2l):^[17] 117.1 mg, 98% yield. Yellow solid, m.p. 89~90 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.34~8.30 (m, 2H), 8.28~8.24 (m, 2H), 7.19~7.15 (m, 4H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.8 (d, J＝251.3 Hz), 162.5 (d, J＝250.9 Hz), 144.2 (d, J＝2.1 Hz), 140.2 (d, J＝3.3 Hz), 128.0 (d, J＝8.4 Hz), 124.5 (d, J＝9.2 Hz), 115.8 (d, J＝5.8 Hz), 115.6 (d, J＝4.9 Hz).

  1, 2-Bis(4-ethylphenyl)diazene oxide (2m):^[18] 127.2 mg, 98% yield. Orange liquid, ¹H NMR (400 MHz, CDCl₃) δ: 8.22 (d, J＝8.5 Hz, 2H), 8.16 (d, J＝8.4 Hz, 2H), 7.32 (d, J＝8.4 Hz, 4H), 2.77~2.69 (m, 4H), 1.29 (t, J＝15.6 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃) δ: 148.1, 146.3, 146.2, 142.0, 128.1, 128.0, 125.7, 122.2, 28.8, 28.6, 15.3, 15.3.

  1, 2-Bis(4-isopropylphenyl)diazene oxide (2n): 141.2 mg, 90% yield. Orange liquid, ¹H NMR (400 MHz, CDCl₃) δ: 8.18~8.01 (m, 4H), 7.44~7.40 (m, 3H), 7.28 (d, J＝7.7 Hz, 1H), 3.08~2.97 (m, 2H), 1.32 (dd, J＝6.9 Hz, 6.9 Hz, 12H); ¹³C NMR (100 MHz, CDCl₃) δ: 150.0, 149.4, 148.5, 144.1, 129.7, 128.7, 128.5, 127.9, 123.8, 122.7, 120.3, 119.9, 34.1, 23.9, 23.8.

  1, 2-Bis(3-isopropylphenyl)diazene oxide (2o): 141.2 mg, 92% yield. Orange liquid, ¹H NMR (400 MHz, CDCl₃) δ: 8.17~8.00 (m, 4H), 7.43~7.39 (m, 3H), 7.28~7.23 (m, 1H), 3.06~2.95 (m, 2H), 1.32~1.29 (m, 12H); ¹³C NMR (100 MHz, CDCl₃) δ: 150.0, 149.4, 148.5, 144.1, 129.7, 128.6, 128.5, 127.9, 123.8, 122.7, 120.3, 119.8, 34.1, 23.9, 23.8.

  1, 2-Bis(2, 6-dimethylphenyl)diazene oxide (2p):^[20] 127.2 mg, 50% yield. Orange solid, m.p. 79~81 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.18~7.13 (m, 6H), 2.47 (s, 6H), 2.32 (s, 6H); ¹³C NMR (100 MHz, CDCl₃) δ: 149.5, 141.7, 130.8, 130.3, 129.0, 129.0, 128.5, 127.5, 19.4, 18.0.

  1, 2-Bis(4-ethoxyphenyl)diazene oxide (2q):^[17] 143.2 mg, 85% yield. orange solid known. m.p. 134~135 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.28~8.22 (m, 4H), 6.97~6.93 (m, 4H), 4.11 (q, J＝6.9 Hz, 4H), 1.47~1.43 (m, 6H); ¹³C NMR (100 MHz, CDCl₃) δ: 161.2, 159.6, 141.5, 137.8, 127.8, 123.7, 114.1, 114.0, 63.9, 63.7, 14.8, 14.7.

  1, 2-Bis(4-(tert-butyl)phenyl)diazene oxide (2r):^[6] 155.2 mg, 96% yield. Yellow solid, m.p. 141~142 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.21 (d, J＝8.9 Hz, 2H), 8.15 (d, J＝8.8 Hz, 2H), 7.50 (dd, J＝8.9 Hz, 8.8 Hz, 4H), 1.37 (s, 9H), 1.36 (s, 9H); ¹³C NMR (100 MHz, CDCl₃) δ: 155.0, 153.0, 146.0, 141.7, 125.7, 125.6, 121.9, 35.0, 31.2, 31.2; m.p. 141~142 ℃.

  1, 2-Bis(3-chloro-4-methylphenyl)diazene oxide (2s):^[19] 147.6 mg, 85% yield. Yellow solid, m.p. 116~117 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.31 (q, J＝2.5 Hz, 2H), 8.08 (dd, J＝8.4 Hz, 8.4 Hz, 1H), 7.97 (dd, J＝8.3 Hz, 8.3 Hz, 1H), 7.37~7.32 (m, 2H), 2.46 (s, 3H), 2.43 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 146.7, 142.5, 140.3, 138.2, 134.6, 134.3, 130.9, 130.8, 126.0, 124.2, 123.0, 120.4, 20.3, 20.1.

  1, 2-Bis(3, 5-dimethylphenyl)diazene oxide (2t): 127.2 mg, 88% yield. Yellow solid, m.p. 106~107 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.9 (s, 2H), 7.78 (s, 2H), 7.17 (s, 1H), 7.04 (s, 1H), 2.42 (6H, s), 2.39 (6H, s); ¹³C NMR (100 MHz, CDCl₃) δ: 148.5, 144.1, 138.7, 138.2, 133.1, 131.2, 123.0, 120.0, 21.4, 21.3.

  Supporting Information   ¹H NMR and ¹³C NMR spectra for compounds 2a~2t. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

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  Scheme 1  Pathways for the synthesis of azoxybenzene

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  Figure 1  Scanning-electron micrographs of TS-1 (80)

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  Table 1.  Optimization of reaction conditions ^a

  Entry	Catalyst	H2O2/Substrate	Solvent	Yield/%
  1	No catalyst	1.0	Acetone	No product
  2	TS-1 (33)	1.0	Acetone	36.6
  3	Mg-TS-1	1.0	Acetone	28.1
  4	Al-TS-1	1.0	Acetone	31.1
  5	Fe-TS-1	1.0	Acetone	1.4
  6	TS-1 (80)	1.0	Acetone	52.6
  7	TS-1 (10)	1.0	Acetone	35.0
  8	TS-1 (100)	1.0	Acetone	49.1
  9	TS-1 (80)	1.5	Acetone	84.9
  10	TS-1 (80)	2.0	Acetone	96.6
  11	TS-1 (80)	2.5	Acetone	96.6
  12	TS-1 (80)	2.0	Methanol	85.0
  13	TS-1 (80)	2.0	Acetonitrile	95.3
  a Reaction conditions: substrate (1 mmol), catalyst (0.1 g), solvent (2 mL), water bath 60 ℃, 5 h.

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  Table 2.  Oxidation of different anilines to azoxybenzenes^a

  Entry	Aniline	Product	Yield/%
  1			96
  2			98
  3			60
  4			70
  5			80
  6			97
  7			70
  8			96
  9			98
  10			96
  11			90
  12			98
  13			98
  14			90
  15			92
  16			50
  17			85
  18			96
  19			85
  20			88
  a Reaction conditions: 1 (1 mmol), H2O2 (2 mmol), catalyst (0.1 g), acetone (2 mL), water bath 60 ℃, 5 h. b Conversion was determined by separation.

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