- Email: [email protected]
A novel catalyst of copper hydroxyphosphate (Cu2(OH)PO4) with high activity in hydroxylation of phenol by hydrogen peroxide
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
791
A Novel Catalyst of Copper Hydroxyphosphate (Cu2(OH)PO4) with High Activity in Hydroxylation of Phenol by Hydrogen Peroxide Feng-Shou Xiao*, Jianmin Sun, Ranbo Yu, Xiangju Meng, Hongming Yuan, Dazhen Jiang, Ruren Xu Department of Chemistry, Jilin University, Changchun 130023, P. R. China Fax: (0431) 5671974, Email: [email protected]
A novel catalyst of copper hydroxyphosphate (Cu2(OH)PO4) that has not microporous and mesoporous pores (surface area <0.01 m2/g) has been successfully synthesized from hydrothermal method by using ethylenediamine, phosphoric acid, and copper acetate. Catalytic data in hydroxylation of phenol by hydrogen peroxide as a model reaction for oxidation catalysis showed that the copper hydroxyphosphate is very active catalyst, and its activity is even higher than that of microporous TS-1 catalyst that is known as one of the most effective catalysts. Furthermore, we observed that the Cu2(OH)PO4 catalyst is readily regenerable to its active state by recalcining the expired form in air. Comparison of various catalysts suggests that the unusual catalytic activity on the Cu2(OH)PO 4 catalyst may be related to unique structure of as-synthesized Cu2(OH)PO4. Characterization of catalytic process by ESR method gives very strong signals assigned to radical .OH species, showing their possible catalytic mechanism. 1. INTRODUCTION Because catalysis by metal or by metal oxide is a surface phenomenon, many technological catalysts have very high metal dispersion. Catalytic oxidation, as an important industrial reaction for production of fine chemicals, has been investigated extensively recently [1-3]. In catalytic oxidation reactions a large amount of catalysts are focused on microporous and mesoporous materials such as TS-1, TS-2, and Ti-MCM-41, because of their large surface area with high metal dispersion. But some disadvantages of these catalysts will limit their applications as a popular tool for oxidation reactions in organic chemistry. For examples, the synthesis of microporous and mesoporous titanosilicates is somewhat complicated in order to avoid precipitation of TiO2 as a separate phase, which often acts as a catalyst poison in the subsequent oxidation reactions by hydrogen peroxide. Small crystal size (0.1-10 ~tm) of microporous and mesoporous titanosilicates has a trouble in separation of catalysts mixed with
792 the reaction products. Small pore size of microporous titanosilicates restricts their use in oxidation reactions for larger organic molecules. Thus new materials with much higher reaction rate have always been sought [3]. Notably, although a series of microporous and mesoporous catalysts in oxidation reactions have been investigated intensively, catalysts with small surface area have not been studied yet, which were usually considered as low catalytic conversion or inactive catalysts in oxidation reactions [1-3]. Here we reported a novel catalyst of copper hydroxyphosphate (Cu2(OH)PO4) that has not microporous and mesoporous pores with very small surface area (< 0.01 m2/g) in hydroxylation of phenol by hydrogen peroxide as a model reaction for catalytic oxidation. Catalytic data show that the Cu2(OH)PO4 catalyst is very active with advantages of simple preparation method, high reaction rate, large crystal size, suitable solvent, and good catalyst regeneration. Furthermore, catalytically active sites are discussed. 2. EXPERIMENTAL The copper hydroxyphosphate of nominal composition Cu2(OH)PO 4 was hydrothermally synthesized using ethylenediamine (H2NCH2CHRNH2), phosphoric acid (H3PO4, 85 mass%), copper acetate (CuAc2) in a Teflon-lined stainless-steel autoclave for 3 days at 140-170~ The crystalline product was filtered, washed with distilled water and dried at ambient temperature, the final product being deep green with crystal size of very uniformed 900 ~tm. The X-ray powder diffraction patterns of the copper hydroxyphosphate, recorded with CuKa radiation, are similar to those of Libethenite mineral [4]. Single crystal diffraction data were collected with a Siemens P4 with Mo(Kct) radiation. All computations were carried out using the SHELXTL program (Version 5.0) [5]. Differential thermal analysis (DTA) and thermogravimetry (TGA) were performed on a Perkin-Elmer DTA 1700 analyzer and TGA7 analyzer with heating rate of 20K/min, respectively. Nitrogen isotherm was performed on ASAP 2010M at 77 K after degassing at 673 K. Catalytic phenol hydroxylation experiments were run in a 50 ml glass reactor and stirred with a magnetic stirrer. In a standard run, 1.6 g of phenol, 0.08 g of catalyst, and 15 ml of solvent were mixed, followed by addition of 0.58 ml of H202 (30% aqueous, molar ratio of phenol/H202=3.0, weight ratio of catalyst/phenol =0.05). After the reaction for 4 h at 80~ the products were hydroquinone (HQ), catechol (CAT), 1,4-benzoquinone (BQ), and tar. The products of hydroquinone, catechol, 1,4-benzoquinone were analyzed by GC, and the amount of tar was determined on products from which the catalyst was removed by filtration and the volatile products by evaporation in vacuo [the amount of tar in various runs was at 8-18 wt.-% (weight percentage of tar with hydroquinone, catechol, and 1,4-benzoquinone)]. Electron spin resonance (ESR) spectra were recorded on Bruker RE 200D spectrometer with microwave frequecy of 9.77 GHz and microwave power of 6.5 mW, equipped with 100 KHz modulation. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was chosen as a spin-trap reagent. To determine relative intensity of radicals quantitatively, all reagents in this study were calculated, and a quartz tube with diameter of lmm was used. While the reactants were added
793 into the reaction quartz tube, the ESR spectra were measured. 3. RESULTS AND DISCUSSION 3.l.Structure of Copper Hydroxyphosphate The structure of copper hydroxyphosphate was resolved by direct method and refined by full-matrix least squares to R=0.043 and Rw=0.060. The deep green copper hydroxyphosphate crystallizes in the orthorhombic space group Pnnm with a=8.058(2), b=8.393(2), c=5.889(2) A. The structure consists of PO4 tetrahedra, Cu(1)O5 pentahedra, Cu(2)O6 octahedra, and OH group between two Cu species, in which oxygen atoms are shared each other but no P-O-P chains (Fig. 1). In Cu(1)O5 pentahedra, two oxygen atoms are close to the Cu atom, giving Cu-O distances at averaged 1.932 A, and three oxygen atoms are far from the Cu atom, giving Cu-O distances at averaged 2.05 A. In Cu(2)O6 octahedra, four oxygen atoms are closed to the Cu atom, giving Cu-O distances at averaged 1.970 A, and two oxygen atoms are far from the Cu atoms, giving Cu-O distances at averaged 2.399 A. As a result, the Cu(2)O 6 octahedra becomes longer due to John-Teller distortion [6]. In PO 4 tetrahedra, P-O distances vary from 1.517-1.568 A (averaged 1.538 A), and O-P-O angles are within range 106.6-110.5 ~ suggesting that the tetrahedra is highly symmetrical.
4..a ~ r~
01B
04A
OIA 4-a
04
F/////~ Cu2
O3D
03A
02A 03B
03C
I 5
10
I
I
20
30
40
20 (degrees) Fig. 1. Asymmetric unit of Cu2(OH)PO 4.
Fig.2. XRD patterns of (a) as-synthesized, (b) calcination at 550~ for 2h, (c) 850~ for 6h of the as-synthesized Cu2(OH)PO4.
794
3.2.Characterization of Copper Hydroxyphosphate Isotherms of nitrogen on the sample show no adsorption, suggesting that the framework structure is not microporous. DTA curve of the copper hydroxyphosphate does not exhibit any peaks in the ranges of 25-650~ and at the same range of temperatures TGA curve has not any weight loss. These results suggest that the copper hydroxyphosphate is thermal stable during 25-650~ When the temperature is over 650~ the curve exhibits a peak at 690~ which is assigned to a formation of new copper compound. Furthermore, XRD patterns (Figure 2) of the samples have confirmed the same assignment. Figure 2a shows a characteristic of assynthesized copper hydroxyphosphate. However, when the copper hydroxyphosphate is calcined at 850~ for 6 h, the calcined sample XRD pattern gives quite different peaks with that of the as-synthesized sample, as shown in Figure 2c. These results have demonstrated that calcination at 850~ for the copper hydroxyphosphate leads to a formation of new copper compound (Cu40(PO4)2) [7].
3.3.Catalytic Phenol Hydroxylation Catalytic activities of various samples in phenol hydroxylation are presented in Table 1. Na3PO4 is catalytically inactive. However, a series of copper compounds such as CuC12, CuO, Cu2(OH)PO4, Cu40(PO4)2, Cu(OH)2 are catalytically active, giving the conversion in the range of 7-28%. These results suggested that copper species are catalytic active sites in the catalysis. In particular, the phenol conversion on the CUE(OH)PO4 catalyst prepared from hydrothermal crystallization is highest (28%), which is even slightly higher than that of TS-1 catalyst that was well known as an effective catalyst in catalytic hydroxylation of phenol [ 1-3]. It is very interesting to note that the best solvent on the Cu2(OH)PO4 catalyst is water. In contrast, the suitable solvents on a series of microporous titanosilicate catalysts are methanol, acetone, and acetonitrile [1-3]. Furthermore, we observed that the crystal size of the Cu2(OH)PO4 catalyst is very large as compared with that of TS-1 catalyst, indicating that the separation from the mixture of catalyst and products is relatively easy. Additionally, the preparation of the CUE(OH)PO4 catalyst with a pure phase is relatively simple and easy. In contrast, the synthesis of titanosilicate catalysts such as TS-1 is somewhat complicated in order to avoid the formation of TiO2 as a separate phase. Moreover, the Cu2(OH)PO4 is prepared by using ethylenediamine as an organic alkali. In contrast, TS-1 catalyst is usually synthesized in presence of organic templates such as TPAOH which are very expensive. Consequently, the price for preparation of the CUE(OH)PO4 is much lower than that of TS-1 catalyst. The Cu2(OH)PO4 catalyst is readily regenerable to its active state by recalcining the expired form in air at 550~ for 2h. The X-ray diffraction and infrared spectroscopy, as well as the catalytic characterization of the recycled catalyst, show neither detectable structural degradation nor loss in activity of any significance. The unusual catalytic activity in hydroxylation of phenol on the Cu2(OH)PO4 catalyst may be related to unique structure of as-synthesized CUE(OH)PO4. Notably, the catalytic activities of CuCI 2, CuO, and Cu40(PO4)2 are very similar, giving conversion at 7.5, 7.9, and 11.8%, respectively (Table 1). The structure of Cu40(PO4) 2 prepared from calcination of Cu2(OH)PO4
795 Table 1 Catalytic activities in hydroxylation of phenol by H202 over various catalysts Catalyst Crystal Surface area, Solvent Phenol H202 efficiency, % size4tm m2/g conv., % Na3PO 4 . . . . . . water . . . . . . CuCI 2 . . . . . . water 7.5 28.1 Cu2(OH)PO4 900 2.4x 10.3 water 28.3 82.0 Cu2(OH)PO4 900 2.4x 10.3 acetonitrile 5.9 20.5 Cu2(OH)PO4 900 2.4x 10.3 acetone . . . . . . Cu2(OH)PO4 s 900 2.4x 10.3 water 16.1 51.2 Cu2(OH)PO4s~ 900 2.4x 10.3 water 28.2 81.6 Cu40(PO4)2" 50 2.4x 10.3 water 11.8 39.6 CuO 5 0.44 water 7.9 30.5 Cu(OH)2 5 18.6 water 9.2 35.3 TS-1 0.4 380 acetone 25.5 72.3 s Used catalyst. s, Regeneration of used catalyst by calcination at 550~ for 2h. Prepared by calcination of Cu2(OH)PO4 at 850~ for 6 h. (Figure 2) also consisting of CuO5 and C u O 6 units, is similar to that of Cu2(OH)PO 4. Only difference in structure between Cu2(OH)PO4 and Cu40(PO4)2 is the OH species attached on Cu sites [7]. Relating to a large difference in catalytic conversion for Cu2(OH)PO4 and Cu40(PO4)2 catalysts (Table 1), we suggest that the OH species attached on Cu sites play an important role for the catalysis. However, Cu(OH)2 catalyst with both OH and Cu sites shows low catalytic conversion (9.2%) as compared with Cu2(OH)PO4 catalyst (Table 1), suggesting that the conventional Cu(OH)2 sample is not good catalyst. Therefore, we proposed that the high activity for Cu2(OH)PO4 in the catalysis could be mainly attributed to synergetic effect of OH and Cu species of Cu2(OH)PO4 catalyst with unique structure.
3.4.ESR Spectra Figure 3 shows ESR spectra of H202 on CuO and Cu2(OH)PO 4 with DMPO at room temperature, respectively. Notably, very strong signals (aN= a.=1.49 mT) appeared on Cu2(OH)PO4 catalyst, which are assigned to the adduct of hydroxyl radical with DMPO (DMPO-OH) [8]. On the CuO catalyst, the signals were much weaker, suggesting much lower concentration of hydroxyl radicals. In the contrast, the ESR spectrum of H202 on TS-1 with DMPO shows typical signals assigned to DMPO-O2, which are well consistent with those of reported mechanism [2]. Comparison of the intensity of hydroxyl radicals with catalytic activity on various catalysts, it was reasonably suggested that hydroxyl radicals may be important intermediates in the catalysis. The easier the production of .OH, the faster the production of diphenols. Possibly, OH species in Cu2(OH)PO4 catalyst promote production of hydroxyl radicals (.OH) in the catalysis.
796
15 Gauss
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-
-
.
.
.
.
-
f
_
Figure 3. ESR spectra of H202 on the samples with DMPO: (a) CuO & (b) Cu2(OH)PO4 ACKNOWLEDGEMENTS: The financial support of this research by the National Natural
Science Foundation of China, the Education Ministry, and National Advanced Materials Committee of China (NAMMC) is acknowledged. REFERENCES
1. M. Taramasso, G. Perego, and B. Notari, US Patem No. 4410501 (1983). 2. D.R.C. Huybrechts, L. De Bruycker, and P. A. Jabobs, Nature, 345 (1990) 240. 3. R. Murugavel, and H. W. Roesky, Angew. Chem. Ira. Ed. Engl., 36 (1997) 477. 4. Natl. Bur. Stand. (US) Monogr., 25 (1980) 30. 5. G.M. Sheldrick, SHELXTL PLUS, Program packed for structure solution and refinement, version 4.2, Simens Analytical Instruments Inc., Madison, WI, 1990. 6. F.A. Cotton and G. Wilkinson (eds.), Advanced Inorganic Chemistry, John Wiley and Sons, 1988, 54 ed., p766-767. 7. M. Brunel-Laugt, A. Durif, and J. C.Guitel, J. Solid State Chem., 25 (1978) 39-47. 8. H, Xiao, W. Fu, B. Zhao, F. Yang, and W. Xin, Acta Biophysica Sinica, 8 (1992) 334.
791
A Novel Catalyst of Copper Hydroxyphosphate (Cu2(OH)PO4) with High Activity in Hydroxylation of Phenol by Hydrogen Peroxide Feng-Shou Xiao*, Jianmin Sun, Ranbo Yu, Xiangju Meng, Hongming Yuan, Dazhen Jiang, Ruren Xu Department of Chemistry, Jilin University, Changchun 130023, P. R. China Fax: (0431) 5671974, Email: [email protected]
A novel catalyst of copper hydroxyphosphate (Cu2(OH)PO4) that has not microporous and mesoporous pores (surface area <0.01 m2/g) has been successfully synthesized from hydrothermal method by using ethylenediamine, phosphoric acid, and copper acetate. Catalytic data in hydroxylation of phenol by hydrogen peroxide as a model reaction for oxidation catalysis showed that the copper hydroxyphosphate is very active catalyst, and its activity is even higher than that of microporous TS-1 catalyst that is known as one of the most effective catalysts. Furthermore, we observed that the Cu2(OH)PO4 catalyst is readily regenerable to its active state by recalcining the expired form in air. Comparison of various catalysts suggests that the unusual catalytic activity on the Cu2(OH)PO 4 catalyst may be related to unique structure of as-synthesized Cu2(OH)PO4. Characterization of catalytic process by ESR method gives very strong signals assigned to radical .OH species, showing their possible catalytic mechanism. 1. INTRODUCTION Because catalysis by metal or by metal oxide is a surface phenomenon, many technological catalysts have very high metal dispersion. Catalytic oxidation, as an important industrial reaction for production of fine chemicals, has been investigated extensively recently [1-3]. In catalytic oxidation reactions a large amount of catalysts are focused on microporous and mesoporous materials such as TS-1, TS-2, and Ti-MCM-41, because of their large surface area with high metal dispersion. But some disadvantages of these catalysts will limit their applications as a popular tool for oxidation reactions in organic chemistry. For examples, the synthesis of microporous and mesoporous titanosilicates is somewhat complicated in order to avoid precipitation of TiO2 as a separate phase, which often acts as a catalyst poison in the subsequent oxidation reactions by hydrogen peroxide. Small crystal size (0.1-10 ~tm) of microporous and mesoporous titanosilicates has a trouble in separation of catalysts mixed with
792 the reaction products. Small pore size of microporous titanosilicates restricts their use in oxidation reactions for larger organic molecules. Thus new materials with much higher reaction rate have always been sought [3]. Notably, although a series of microporous and mesoporous catalysts in oxidation reactions have been investigated intensively, catalysts with small surface area have not been studied yet, which were usually considered as low catalytic conversion or inactive catalysts in oxidation reactions [1-3]. Here we reported a novel catalyst of copper hydroxyphosphate (Cu2(OH)PO4) that has not microporous and mesoporous pores with very small surface area (< 0.01 m2/g) in hydroxylation of phenol by hydrogen peroxide as a model reaction for catalytic oxidation. Catalytic data show that the Cu2(OH)PO4 catalyst is very active with advantages of simple preparation method, high reaction rate, large crystal size, suitable solvent, and good catalyst regeneration. Furthermore, catalytically active sites are discussed. 2. EXPERIMENTAL The copper hydroxyphosphate of nominal composition Cu2(OH)PO 4 was hydrothermally synthesized using ethylenediamine (H2NCH2CHRNH2), phosphoric acid (H3PO4, 85 mass%), copper acetate (CuAc2) in a Teflon-lined stainless-steel autoclave for 3 days at 140-170~ The crystalline product was filtered, washed with distilled water and dried at ambient temperature, the final product being deep green with crystal size of very uniformed 900 ~tm. The X-ray powder diffraction patterns of the copper hydroxyphosphate, recorded with CuKa radiation, are similar to those of Libethenite mineral [4]. Single crystal diffraction data were collected with a Siemens P4 with Mo(Kct) radiation. All computations were carried out using the SHELXTL program (Version 5.0) [5]. Differential thermal analysis (DTA) and thermogravimetry (TGA) were performed on a Perkin-Elmer DTA 1700 analyzer and TGA7 analyzer with heating rate of 20K/min, respectively. Nitrogen isotherm was performed on ASAP 2010M at 77 K after degassing at 673 K. Catalytic phenol hydroxylation experiments were run in a 50 ml glass reactor and stirred with a magnetic stirrer. In a standard run, 1.6 g of phenol, 0.08 g of catalyst, and 15 ml of solvent were mixed, followed by addition of 0.58 ml of H202 (30% aqueous, molar ratio of phenol/H202=3.0, weight ratio of catalyst/phenol =0.05). After the reaction for 4 h at 80~ the products were hydroquinone (HQ), catechol (CAT), 1,4-benzoquinone (BQ), and tar. The products of hydroquinone, catechol, 1,4-benzoquinone were analyzed by GC, and the amount of tar was determined on products from which the catalyst was removed by filtration and the volatile products by evaporation in vacuo [the amount of tar in various runs was at 8-18 wt.-% (weight percentage of tar with hydroquinone, catechol, and 1,4-benzoquinone)]. Electron spin resonance (ESR) spectra were recorded on Bruker RE 200D spectrometer with microwave frequecy of 9.77 GHz and microwave power of 6.5 mW, equipped with 100 KHz modulation. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was chosen as a spin-trap reagent. To determine relative intensity of radicals quantitatively, all reagents in this study were calculated, and a quartz tube with diameter of lmm was used. While the reactants were added
793 into the reaction quartz tube, the ESR spectra were measured. 3. RESULTS AND DISCUSSION 3.l.Structure of Copper Hydroxyphosphate The structure of copper hydroxyphosphate was resolved by direct method and refined by full-matrix least squares to R=0.043 and Rw=0.060. The deep green copper hydroxyphosphate crystallizes in the orthorhombic space group Pnnm with a=8.058(2), b=8.393(2), c=5.889(2) A. The structure consists of PO4 tetrahedra, Cu(1)O5 pentahedra, Cu(2)O6 octahedra, and OH group between two Cu species, in which oxygen atoms are shared each other but no P-O-P chains (Fig. 1). In Cu(1)O5 pentahedra, two oxygen atoms are close to the Cu atom, giving Cu-O distances at averaged 1.932 A, and three oxygen atoms are far from the Cu atom, giving Cu-O distances at averaged 2.05 A. In Cu(2)O6 octahedra, four oxygen atoms are closed to the Cu atom, giving Cu-O distances at averaged 1.970 A, and two oxygen atoms are far from the Cu atoms, giving Cu-O distances at averaged 2.399 A. As a result, the Cu(2)O 6 octahedra becomes longer due to John-Teller distortion [6]. In PO 4 tetrahedra, P-O distances vary from 1.517-1.568 A (averaged 1.538 A), and O-P-O angles are within range 106.6-110.5 ~ suggesting that the tetrahedra is highly symmetrical.
4..a ~ r~
01B
04A
OIA 4-a
04
F/////~ Cu2
O3D
03A
02A 03B
03C
I 5
10
I
I
20
30
40
20 (degrees) Fig. 1. Asymmetric unit of Cu2(OH)PO 4.
Fig.2. XRD patterns of (a) as-synthesized, (b) calcination at 550~ for 2h, (c) 850~ for 6h of the as-synthesized Cu2(OH)PO4.
794
3.2.Characterization of Copper Hydroxyphosphate Isotherms of nitrogen on the sample show no adsorption, suggesting that the framework structure is not microporous. DTA curve of the copper hydroxyphosphate does not exhibit any peaks in the ranges of 25-650~ and at the same range of temperatures TGA curve has not any weight loss. These results suggest that the copper hydroxyphosphate is thermal stable during 25-650~ When the temperature is over 650~ the curve exhibits a peak at 690~ which is assigned to a formation of new copper compound. Furthermore, XRD patterns (Figure 2) of the samples have confirmed the same assignment. Figure 2a shows a characteristic of assynthesized copper hydroxyphosphate. However, when the copper hydroxyphosphate is calcined at 850~ for 6 h, the calcined sample XRD pattern gives quite different peaks with that of the as-synthesized sample, as shown in Figure 2c. These results have demonstrated that calcination at 850~ for the copper hydroxyphosphate leads to a formation of new copper compound (Cu40(PO4)2) [7].
3.3.Catalytic Phenol Hydroxylation Catalytic activities of various samples in phenol hydroxylation are presented in Table 1. Na3PO4 is catalytically inactive. However, a series of copper compounds such as CuC12, CuO, Cu2(OH)PO4, Cu40(PO4)2, Cu(OH)2 are catalytically active, giving the conversion in the range of 7-28%. These results suggested that copper species are catalytic active sites in the catalysis. In particular, the phenol conversion on the CUE(OH)PO4 catalyst prepared from hydrothermal crystallization is highest (28%), which is even slightly higher than that of TS-1 catalyst that was well known as an effective catalyst in catalytic hydroxylation of phenol [ 1-3]. It is very interesting to note that the best solvent on the Cu2(OH)PO4 catalyst is water. In contrast, the suitable solvents on a series of microporous titanosilicate catalysts are methanol, acetone, and acetonitrile [1-3]. Furthermore, we observed that the crystal size of the Cu2(OH)PO4 catalyst is very large as compared with that of TS-1 catalyst, indicating that the separation from the mixture of catalyst and products is relatively easy. Additionally, the preparation of the CUE(OH)PO4 catalyst with a pure phase is relatively simple and easy. In contrast, the synthesis of titanosilicate catalysts such as TS-1 is somewhat complicated in order to avoid the formation of TiO2 as a separate phase. Moreover, the Cu2(OH)PO4 is prepared by using ethylenediamine as an organic alkali. In contrast, TS-1 catalyst is usually synthesized in presence of organic templates such as TPAOH which are very expensive. Consequently, the price for preparation of the CUE(OH)PO4 is much lower than that of TS-1 catalyst. The Cu2(OH)PO4 catalyst is readily regenerable to its active state by recalcining the expired form in air at 550~ for 2h. The X-ray diffraction and infrared spectroscopy, as well as the catalytic characterization of the recycled catalyst, show neither detectable structural degradation nor loss in activity of any significance. The unusual catalytic activity in hydroxylation of phenol on the Cu2(OH)PO4 catalyst may be related to unique structure of as-synthesized CUE(OH)PO4. Notably, the catalytic activities of CuCI 2, CuO, and Cu40(PO4)2 are very similar, giving conversion at 7.5, 7.9, and 11.8%, respectively (Table 1). The structure of Cu40(PO4) 2 prepared from calcination of Cu2(OH)PO4
795 Table 1 Catalytic activities in hydroxylation of phenol by H202 over various catalysts Catalyst Crystal Surface area, Solvent Phenol H202 efficiency, % size4tm m2/g conv., % Na3PO 4 . . . . . . water . . . . . . CuCI 2 . . . . . . water 7.5 28.1 Cu2(OH)PO4 900 2.4x 10.3 water 28.3 82.0 Cu2(OH)PO4 900 2.4x 10.3 acetonitrile 5.9 20.5 Cu2(OH)PO4 900 2.4x 10.3 acetone . . . . . . Cu2(OH)PO4 s 900 2.4x 10.3 water 16.1 51.2 Cu2(OH)PO4s~ 900 2.4x 10.3 water 28.2 81.6 Cu40(PO4)2" 50 2.4x 10.3 water 11.8 39.6 CuO 5 0.44 water 7.9 30.5 Cu(OH)2 5 18.6 water 9.2 35.3 TS-1 0.4 380 acetone 25.5 72.3 s Used catalyst. s, Regeneration of used catalyst by calcination at 550~ for 2h. Prepared by calcination of Cu2(OH)PO4 at 850~ for 6 h. (Figure 2) also consisting of CuO5 and C u O 6 units, is similar to that of Cu2(OH)PO 4. Only difference in structure between Cu2(OH)PO4 and Cu40(PO4)2 is the OH species attached on Cu sites [7]. Relating to a large difference in catalytic conversion for Cu2(OH)PO4 and Cu40(PO4)2 catalysts (Table 1), we suggest that the OH species attached on Cu sites play an important role for the catalysis. However, Cu(OH)2 catalyst with both OH and Cu sites shows low catalytic conversion (9.2%) as compared with Cu2(OH)PO4 catalyst (Table 1), suggesting that the conventional Cu(OH)2 sample is not good catalyst. Therefore, we proposed that the high activity for Cu2(OH)PO4 in the catalysis could be mainly attributed to synergetic effect of OH and Cu species of Cu2(OH)PO4 catalyst with unique structure.
3.4.ESR Spectra Figure 3 shows ESR spectra of H202 on CuO and Cu2(OH)PO 4 with DMPO at room temperature, respectively. Notably, very strong signals (aN= a.=1.49 mT) appeared on Cu2(OH)PO4 catalyst, which are assigned to the adduct of hydroxyl radical with DMPO (DMPO-OH) [8]. On the CuO catalyst, the signals were much weaker, suggesting much lower concentration of hydroxyl radicals. In the contrast, the ESR spectrum of H202 on TS-1 with DMPO shows typical signals assigned to DMPO-O2, which are well consistent with those of reported mechanism [2]. Comparison of the intensity of hydroxyl radicals with catalytic activity on various catalysts, it was reasonably suggested that hydroxyl radicals may be important intermediates in the catalysis. The easier the production of .OH, the faster the production of diphenols. Possibly, OH species in Cu2(OH)PO4 catalyst promote production of hydroxyl radicals (.OH) in the catalysis.
796
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.
.
.
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f
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Figure 3. ESR spectra of H202 on the samples with DMPO: (a) CuO & (b) Cu2(OH)PO4 ACKNOWLEDGEMENTS: The financial support of this research by the National Natural
Science Foundation of China, the Education Ministry, and National Advanced Materials Committee of China (NAMMC) is acknowledged. REFERENCES
1. M. Taramasso, G. Perego, and B. Notari, US Patem No. 4410501 (1983). 2. D.R.C. Huybrechts, L. De Bruycker, and P. A. Jabobs, Nature, 345 (1990) 240. 3. R. Murugavel, and H. W. Roesky, Angew. Chem. Ira. Ed. Engl., 36 (1997) 477. 4. Natl. Bur. Stand. (US) Monogr., 25 (1980) 30. 5. G.M. Sheldrick, SHELXTL PLUS, Program packed for structure solution and refinement, version 4.2, Simens Analytical Instruments Inc., Madison, WI, 1990. 6. F.A. Cotton and G. Wilkinson (eds.), Advanced Inorganic Chemistry, John Wiley and Sons, 1988, 54 ed., p766-767. 7. M. Brunel-Laugt, A. Durif, and J. C.Guitel, J. Solid State Chem., 25 (1978) 39-47. 8. H, Xiao, W. Fu, B. Zhao, F. Yang, and W. Xin, Acta Biophysica Sinica, 8 (1992) 334.