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Hydroxylation of benzene and phenol in presence of vanadium grafted Beta and ZSM-5 zeolites
Catalysis Communications 8 (2007) 693–696 www.elsevier.com/locate/catcom
Hydroxylation of benzene and phenol in presence of vanadium grafted Beta and ZSM-5 zeolites R. Dimitrova *, M. Spassova Institute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Received 3 October 2005; received in revised form 23 March 2006; accepted 28 March 2006 Available online 5 April 2006
Abstract Vanadium grafted zeolites type Beta and ZSM-5 were investigated for direct oxidation of benzene to phenol and hydroxylation of phenol with hydrogen peroxide in acetonitrile media at 353 K. The introduction of vanadium was done by reductive solid state ion exchange. The morphological characterisation of samples and the stability of the vanadium sites were examined before and after the reaction by applying FTIR, UV–Vis techniques and reduction/oxidation cycles (TPR/TPO). The results proved that the vanadium sites had retained their multiple oxidation state when used in liquid phase oxidation at high vanadium atom efficiency for the reaction product. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Benzene; Phenol; Hydroxylation; VBeta; VZSM-5
1. Introduction The possibility of stable vanadium anchoring in zeolites was investigated intensively, as the vanadium-based catalysts were known to be effective for oxidation chemistry [1–4]. Formation of SiAOAV bonds, with isolated tetrahedral vanadium environments containing V@O, was proved for different types of zeolites, zeolite-like materials and amorphous aluminosilicates [5–12]. The incorporation of vanadium in zeolite framework led to formation of weak Brønsted sites with specific properties [13–16]. It is stated, that vanadium containing zeolites, particularly the ones prepared by ion exchange, show significant vanadium leaching when used in liquid phase reactions [16]. The leaching depends on various factors such as the amount of vanadium, its distribution within zeolite framework, the properties of the porous structure and its textural characteristics.
*
Corresponding author. Fax: +359 2 8700225. E-mail addresses: [email protected], [email protected] (R. Dimitrova). 1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.03.019
In the previous paper [15], it was established that when applying the method of reductive solid state ion exchange, at temperatures up to 673 K, stable vanadium ions were introduced in boron analogues of BEA and ZSM-5 zeolites which had low acidity. A competitive replacement of boron by vanadium occurred. In a further approach to the problem, we investigated the stability and efficiency of the vanadium sites in a reaction of liquid phase benzene and phenol hydroxylation. 2. Experimental 2.1. Catalysts The vanadium containing zeolites were produced by reductive solid state ion exchange (RSSIE) with V2O5 of the parent zeolites (BBeta and BZSM-5) and of their primary deboronated forms [15]. The amount of V2O5 was equivalent to the sample acidity, determined by a temperature programmed desorption of ammonia (TPDA). Thus, the VZSM-5 sample contained 0.90 mmol V per 1 g ZSM-5 and VBeta contained 1.08 mmol V per 1 g Beta zeolite. After a ball-milling procedure, the thermal
694
R. Dimitrova, M. Spassova / Catalysis Communications 8 (2007) 693–696
pre-treatment was accomplished by heating 40 mg of the ball-milled mixture in SETARAM microbalance in a flow of dry argon (10 grad/min). It was perceived that dehydration was occurring at the temperature of about 453 K, in the region from the ambient temperature up to 573 K, for all samples. Further, temperature programmed reduction (TPR) was carried out in a H2 + Ar (1:1) flow from the ambient temperature up to 773 K. The studied samples were denoted as VBZSM-5, VZSM-5_B, VBBeta and VBeta_B. Samples morphological characterisation was fulfilled before and after a reaction run. The crystallinity was determined by powder X-ray diffraction on a Philips PW1840 diffractometer, equipped with a PW 1830 generator. FTIR spectroscopy measurements were done on a Brucker Vector 22 FTIR spectrometer. FTIR spectra of adsorbed pyridine were registered with a Nicolet IR spectrometer (Impact 400) using the wafer transmission technique [17]. The diffuse reflectance UV–Vis spectra were recorded on a Beckman 5270 spectrophotometer, equipped with a DR spectra accessory in the wavelength range 220–800 nm. BaSO4 was used as the reference. Samples characteristics were summarised in Table 1. 2.2. Catalysis The hydroxylation was carried out in a 100 ml threeneck flask, equipped with a magnetic stirrer, water condenser and a thermometer. The temperature of the reaction vessel was maintained constant using a round heated jacket. A solvent (10 ml acetonitrile), a reactant (1 g benzene or 1 g phenol), and a catalyst (0.1 g powder) were stirred at 353 K. The amount of H2O2 (26 wt%) introduced was about 0.26–0.3 g in order to attain a substrate/oxidant ratio of 3:1. After 8 h, the products were analysed. The H2O2 content was analysed using standard iodometric titration. The aromatics were analysed using HPLC with a Beckman 168 diode array detector, operating at 280 nm. The HPLC was fitted with a C18 reverse-phase column (Luna) and 25% acetonitrile in water was used as a mobile phase. The catalyst activity was determined as number of molecules (benzene or phenol) converted per V ion in 0.1 g catalyst. Blank experiments, where the catalyst was allowed to react for 8 h with benzene/phenol
and solvent in one flask and with H2O2 and solvent in a second flask, were carried out. There after the solutions have been filtered through a 0.02 lm pores ceramic filter at the reaction temperature. H2O2 and benzene/phenol have been added to the supernatants, respectively, and the reaction was continued for 8 h more. No further reaction was detected in those experiments that demonstrate the heterogeneous character of the catalyst. Almost the same amounts of vanadium, as determined by the atomic absorption spectrometry, remained in the recovered catalyst. 3. Results and discussion The XRD patterns of all samples indicated that its crystallinity was adequate and in accordance with the literature data [18,19]. At room temperature, during the ball-milling process, a partial dehydroxylation of vanadium pentoxide/ zeolite mixture occurred in accordance with the reaction: V2 O5 +4Hþ (SiO)Zeol!2[(VO)3þ (OH)]2þ (SiO)Zeol+H2 O. The calculated weight loss for the V2O5 dehydroxylation was 0.3 mg. The effect was better perceived in the case of VBeta samples (bonds in a region 940–970 cm1 in the FTIR spectra, not shown), most probably due to the greater extent of coordinatively unsaturated surface hydroxyl groups in comparison with the VZSM-5 zeolite type. During the TPR experiments in flow of H2 + Ar up to 773 K, a reduction of [(VO)3+(OH)]2+(SiO)Zeol was fully accomplished as measured by the weight loss – about 0.66 mg (Table 1). Two temperature regions were observed, related with the formation of vanadium ions as [VO]2+ and/or V4+ ions had been observed. The subsequent temperature programmed oxidation (TPO) experiments, done with the initial and the reused samples up to temperature of 800 K, showed an oxygen consumption in stoichiometric quantity, according to [O]:[V] = 1:1. So, it could be stated that the oxidation of the different vanadium species did not proceed back to V2O5 oxide. Hence, it is reasonable to conclude that the incorporated [VO]2+ and/or V4+ ions are fairly strong bonded in the framework of the investigated samples.
Table 1 Samples characteristics based on TPDA, UV–Vis and TPR analyses Samples
VBZSM-5 VZSM-5_B VBBeta VBeta_B a b
Acidity by TPDA mmol/g
UV–Vis (nm)
Weight loss during TPR (mg)a
Before RSSIE
After RSSIE
After RSSIE
423–673 Kb
673–823 K
Total
1.08 0.69 1.18 0.85
1.35 0.54 1.13 0.69
230, 230, 230, 230,
0.67 0.63 0.68 0.70
0.50 0.66 0.82 0.98
1.17 1.29 1.50 1.68
280, 280, 280, 280,
340 340, 415 340 340, 410
The amount of V2O5 is 3.78 mg for all samples. Weight loss of 0.66 mg could be obtained for each of the reactions: 2[(VO)3þ (OH)]2þ (SiO)Zeol + 1/2H2 + Hþ Zeol ! 2[VO]2þ Zeol + 2H2 O,
or 2[VO]2þ Zeol + H2 ! 2V4þ Zeol + 2H2 O
R. Dimitrova, M. Spassova / Catalysis Communications 8 (2007) 693–696
The aforementioned effects are well perceived in the spectra of sorbed pyridine (Fig. 1). In the spectra of the ball-milled samples, significant intensity of bands at 1460 and 1626 cm1 (pyridine bound to extra framework cationic species) was seen (Fig. 1A and B, curves a and b). A low intensity pyridinium band at 1546 cm1, attributed to pyridinium cations, was observed only for VBZSM-5 and revealed the presence of Brønstedacid sites (Fig. 1A, curve a). In the spectra of the reduced samples, the intensity of the bands at 1446 and 1596 cm1 (characteristic of pyridine interacting with Lewis acid sites) increased, revealing the formation of [V@O]2+ and V4+ ions. The FTIR measurements confirmed the maintenance of the vanadium ions under different conditions. The high intensity of the band at 1640 cm1 (vanadium as a compensating ion) and the bands in a region 960–970 cm1 (vanadium in silanol nest) was seen in all spectra [20,21]. The vanadium ions were preserved in the regenerated and in the recovered samples (Fig. 2, line B and C). Sample regeneration was completed in SETARAM microbalance by carrying out the procedure of TPR in a H2 + Ar (1:1) flow from ambient temperature up to 773 K. After reusing the sample, a second regeneration was done since the characteristic bands of the used sample were with almost the same intensity compared to the initial one. The existence of different types of vanadium was also displayed in the UV–Vis spectra of the reduced sample (Table 1). Besides the band at 230 nm, due to silica in a molecular sieve, a high intensive band at 280 nm and a broad absorption near 340 and 415 nm were registered. Hence, it was reasonable to conclude that the procedure of reductive solid state vanadium exchange into the low acidic BZSM-5 and BBeta zeolites was effective and vanadium was fairly strong grafted to samples’ framework. It is known from the literature, that the presence of V4+ ion is required for the formation of vanadium hydroperox-
C
Transmitance (a.u.)
B
A
1600
1400
1200
400
1460 1546
1596
1447
a 1626
-1460 -1546
600
ide (V ± OOH) which can hydroxylated benzene when using hydrogen peroxide as oxidant [22]. At the beginning of the experiment, on addition of H2O2, colour change was observed from green-blue (due to the existence of VO2+) to light yellow (presence of V5+) [23]. Further, proceeding with the reaction, the colour changed respectively from yellow to green-yellowish and back to dark blue, pointing to the complete use up of the peroxide. Benzene was hydroxylated to give mostly phenol and hydroquinone (Table 2). The higher activity, of the initially deboronated sample of both zeolite types, were most probably due to the better dispersion and accessibility of the vanadium sites. (Table 1). VBeta_B was the most active catalysts at sufficient peroxide selectivity. The results from the blank experiments revealed practically no further conversion of benzene, so
B
b
b 1447
-1609
800
Fig. 2. FTIR spectra of samples: (A) parent Vbeta_B; (B) regenerated Vbeta_B; (C) after second regeneration.
a
-1626
1000
Wavenumber (cm-1)
A
Absorbance, a.u.
695
c
c d
d
1600
1500
1400
1600
1500
1400
-1
Wavenumber, cm
Fig. 1. Spectra of pyridine absorbed at 470 K on: (A) VBZSM-5 (curves a and c) and VZSM-5_B (curves b and d); (B) VBBeta (curves a and c) and VBbeta_B (curves b and d); all curves (a, b) – ball-milled samples, thermally treated up to 770 K; all curves (c, d) – reduced samples at 770 K in H2:Ar = 1:1.
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Table 2 Catalytic performances of vanadium containing zeolites Sample
VBZSM-5 VZSM-5_B VBBeta VBeta_B a b c d e
Benzene, turn overa
10 25 15 35
Benzene product distributionb mass% PH
HQ
98 87 75 70
– 13 25 30
H2O2 selectivityc
57.5 57.4 55.4 65.0
Phenol turn overa
4.8 1.6 10.2 7.4
Phenol product distributiond mass % pBQ
CAT
HQ
30 50 25 26
55 25 57 57
14 25 17 16
H2O2 selectivitye
50.0 13.9 65.3 55.8
Moles of benzene/phenol converted per mole of vanadium. PH, phenol; HQ, hydroquinone. H2O2 utilized for phenol formation. pBQ, p-benzoquinone; CAT, catechol; and HQ, hydroquinone. H2O2 utilized for pBQ, CAT HQ formation.
the hydroxylation of benzene was mainly heterogeneous. The catalysts were regenerated in a flow of dry argon up to 573 K and reduced in a H2 + Ar (1:1) atmosphere. Nearly the same amount of vanadium was determined in the regenerated catalyst, but due to the low content of vanadium in the investigated samples, the analytical analysis was within the limits of the experimental error. There were three main reaction products of phenol conversion: catechol, hydroquinone and benzoquinone. The observed activity per V atom turned out to be rather low for phenol hydroxylation in comparison with the one of benzene. It was observed that the yield to catechol was higher that the hydroquinone in all experiments. It seemed that phenol hydroxylation occurred on the surface rather than inside the channel where low numbers of grafted vanadium sites existed. 4. Conclusions The procedure of reductive solid state vanadium exchange into BZSM-5 and BBeta zeolites led to the formation of vanadium ions fairly strong bonded to the zeolite framework, so that the latter could be used in liquid phase reaction. The grafted isolated vanadium sites, as coordinated V-oxides having a (V@O) bond and vanadium cations as compensating ions, possess activity in hydroxylation of benzene and phenol. References [1] G. Kasperek, P. Bruce, T. Bruce, H. Yagi, D. Jerina, J. Chem. Soc. Commun. (1972) 784.
[2] K. Yoshizava, Y. Shiota, T. Yumura, T. Yamabe, J. Phys. Chem. B 104 (2000) 734. [3] Wee Lee, Woo Jin Lee, Yong Ki Park, Sang-Eon Park, Catal. Today 61 (2000) 137. [4] M. Stockmann, F. Konietzni, J. Notheis, J. Voss, W. Keune, W. Maier, Appl. Catal. A 208 (2001) 343. [5] J. Kornatowski, B. Wichterlova, J. Jirkovsky, E. Loffler, W. Pilz, J. Chem. Soc., Faraday Trans. 92 (1996) 1067. [6] B. Whittington, J. Anderson, J. Phys. Chem. 95 (1991) 3306. [7] I. Moudrakovski, A. Sayari, C. Ratcliffe, J. Ripmeester, K. Preston, J. Phys. Chem. 98 (1994) 10895. [8] K. Reddy, I. Moudrakovski, A. Sayari, J. Chem. Soc., Chem. Commun. 1491 (1994). [9] M. Chatterjee, D. Bhattacharya, N. Venkatathri, S. Sivasanker, Catal. Lett. 35 (1995) 313. ˜ ckmann, F. Konietzni, J. Notheis, J. Voss, W. Keune, W.F. [10] M. A Maier, Appl. Catal. A 208 (2001) 343. [11] I. Arends, R. Sheldon, Appl. Catal. A 212 (2001) 175. [12] Whul Wee Lee, Woo Jin Lee, Yong Ki Park, Sang-Eon Park, Catal. Today 61 (2000) 137. [13] K. Yamagishi, S. Namba, T. Yashima, in: P.A. Jacobs, R.A. Van Santen (Eds.), Zeolites: Facts, Figures, Future, Elsevier Science Publishers, Amsterdam, 1989, p. 459. [14] S. Dzwigaj, P. Massiani, A. Davidson, M. Che, J. Mol. Catalysis A 155 (2000) 169. [15] R. Dimitrova, Y. Neinska, M. Miha´lyi, G. Pal-Borbe´ly, M. Spassova, Appl. Catal. A 226 (2004) 123. [16] N. Das, H. Eckert, H. Hu, I. Wachs, J. Waizer, F. Feher, J. Phys. Chem. 97 (1993) 8240. [17] H .G. Karge, W. Niessen, Catal. Today 8 (1991) 451. [18] M. Petras, B. Wichterlova, J. Phys. Chem. 96 (1992) 1905. [19] S. Narayanan, A. Sultana, Appl. Catal. A 167 (1998) 103. [20] P.R. Hari Prasad, A. Ramaswamy, V.P. Ratnasamy, J. Catal. 137 (1992) 225. [21] M.S. Rigutto, H. Van Bekkum, Appl. Catal. 68 (1991) 297. [22] H. Mimoun, L. Saussine, E. Daire, M. Postel, J. Fischer, R. Weiss, J. Am. Chem. Soc. 105 (1983) 3101. [23] J.S. Reddy, A. Sayari, J. Chem. Soc., Chem. Commun. (1995) 2231.
Hydroxylation of benzene and phenol in presence of vanadium grafted Beta and ZSM-5 zeolites R. Dimitrova *, M. Spassova Institute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Received 3 October 2005; received in revised form 23 March 2006; accepted 28 March 2006 Available online 5 April 2006
Abstract Vanadium grafted zeolites type Beta and ZSM-5 were investigated for direct oxidation of benzene to phenol and hydroxylation of phenol with hydrogen peroxide in acetonitrile media at 353 K. The introduction of vanadium was done by reductive solid state ion exchange. The morphological characterisation of samples and the stability of the vanadium sites were examined before and after the reaction by applying FTIR, UV–Vis techniques and reduction/oxidation cycles (TPR/TPO). The results proved that the vanadium sites had retained their multiple oxidation state when used in liquid phase oxidation at high vanadium atom efficiency for the reaction product. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Benzene; Phenol; Hydroxylation; VBeta; VZSM-5
1. Introduction The possibility of stable vanadium anchoring in zeolites was investigated intensively, as the vanadium-based catalysts were known to be effective for oxidation chemistry [1–4]. Formation of SiAOAV bonds, with isolated tetrahedral vanadium environments containing V@O, was proved for different types of zeolites, zeolite-like materials and amorphous aluminosilicates [5–12]. The incorporation of vanadium in zeolite framework led to formation of weak Brønsted sites with specific properties [13–16]. It is stated, that vanadium containing zeolites, particularly the ones prepared by ion exchange, show significant vanadium leaching when used in liquid phase reactions [16]. The leaching depends on various factors such as the amount of vanadium, its distribution within zeolite framework, the properties of the porous structure and its textural characteristics.
*
Corresponding author. Fax: +359 2 8700225. E-mail addresses: [email protected], [email protected] (R. Dimitrova). 1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.03.019
In the previous paper [15], it was established that when applying the method of reductive solid state ion exchange, at temperatures up to 673 K, stable vanadium ions were introduced in boron analogues of BEA and ZSM-5 zeolites which had low acidity. A competitive replacement of boron by vanadium occurred. In a further approach to the problem, we investigated the stability and efficiency of the vanadium sites in a reaction of liquid phase benzene and phenol hydroxylation. 2. Experimental 2.1. Catalysts The vanadium containing zeolites were produced by reductive solid state ion exchange (RSSIE) with V2O5 of the parent zeolites (BBeta and BZSM-5) and of their primary deboronated forms [15]. The amount of V2O5 was equivalent to the sample acidity, determined by a temperature programmed desorption of ammonia (TPDA). Thus, the VZSM-5 sample contained 0.90 mmol V per 1 g ZSM-5 and VBeta contained 1.08 mmol V per 1 g Beta zeolite. After a ball-milling procedure, the thermal
694
R. Dimitrova, M. Spassova / Catalysis Communications 8 (2007) 693–696
pre-treatment was accomplished by heating 40 mg of the ball-milled mixture in SETARAM microbalance in a flow of dry argon (10 grad/min). It was perceived that dehydration was occurring at the temperature of about 453 K, in the region from the ambient temperature up to 573 K, for all samples. Further, temperature programmed reduction (TPR) was carried out in a H2 + Ar (1:1) flow from the ambient temperature up to 773 K. The studied samples were denoted as VBZSM-5, VZSM-5_B, VBBeta and VBeta_B. Samples morphological characterisation was fulfilled before and after a reaction run. The crystallinity was determined by powder X-ray diffraction on a Philips PW1840 diffractometer, equipped with a PW 1830 generator. FTIR spectroscopy measurements were done on a Brucker Vector 22 FTIR spectrometer. FTIR spectra of adsorbed pyridine were registered with a Nicolet IR spectrometer (Impact 400) using the wafer transmission technique [17]. The diffuse reflectance UV–Vis spectra were recorded on a Beckman 5270 spectrophotometer, equipped with a DR spectra accessory in the wavelength range 220–800 nm. BaSO4 was used as the reference. Samples characteristics were summarised in Table 1. 2.2. Catalysis The hydroxylation was carried out in a 100 ml threeneck flask, equipped with a magnetic stirrer, water condenser and a thermometer. The temperature of the reaction vessel was maintained constant using a round heated jacket. A solvent (10 ml acetonitrile), a reactant (1 g benzene or 1 g phenol), and a catalyst (0.1 g powder) were stirred at 353 K. The amount of H2O2 (26 wt%) introduced was about 0.26–0.3 g in order to attain a substrate/oxidant ratio of 3:1. After 8 h, the products were analysed. The H2O2 content was analysed using standard iodometric titration. The aromatics were analysed using HPLC with a Beckman 168 diode array detector, operating at 280 nm. The HPLC was fitted with a C18 reverse-phase column (Luna) and 25% acetonitrile in water was used as a mobile phase. The catalyst activity was determined as number of molecules (benzene or phenol) converted per V ion in 0.1 g catalyst. Blank experiments, where the catalyst was allowed to react for 8 h with benzene/phenol
and solvent in one flask and with H2O2 and solvent in a second flask, were carried out. There after the solutions have been filtered through a 0.02 lm pores ceramic filter at the reaction temperature. H2O2 and benzene/phenol have been added to the supernatants, respectively, and the reaction was continued for 8 h more. No further reaction was detected in those experiments that demonstrate the heterogeneous character of the catalyst. Almost the same amounts of vanadium, as determined by the atomic absorption spectrometry, remained in the recovered catalyst. 3. Results and discussion The XRD patterns of all samples indicated that its crystallinity was adequate and in accordance with the literature data [18,19]. At room temperature, during the ball-milling process, a partial dehydroxylation of vanadium pentoxide/ zeolite mixture occurred in accordance with the reaction: V2 O5 +4Hþ (SiO)Zeol!2[(VO)3þ (OH)]2þ (SiO)Zeol+H2 O. The calculated weight loss for the V2O5 dehydroxylation was 0.3 mg. The effect was better perceived in the case of VBeta samples (bonds in a region 940–970 cm1 in the FTIR spectra, not shown), most probably due to the greater extent of coordinatively unsaturated surface hydroxyl groups in comparison with the VZSM-5 zeolite type. During the TPR experiments in flow of H2 + Ar up to 773 K, a reduction of [(VO)3+(OH)]2+(SiO)Zeol was fully accomplished as measured by the weight loss – about 0.66 mg (Table 1). Two temperature regions were observed, related with the formation of vanadium ions as [VO]2+ and/or V4+ ions had been observed. The subsequent temperature programmed oxidation (TPO) experiments, done with the initial and the reused samples up to temperature of 800 K, showed an oxygen consumption in stoichiometric quantity, according to [O]:[V] = 1:1. So, it could be stated that the oxidation of the different vanadium species did not proceed back to V2O5 oxide. Hence, it is reasonable to conclude that the incorporated [VO]2+ and/or V4+ ions are fairly strong bonded in the framework of the investigated samples.
Table 1 Samples characteristics based on TPDA, UV–Vis and TPR analyses Samples
VBZSM-5 VZSM-5_B VBBeta VBeta_B a b
Acidity by TPDA mmol/g
UV–Vis (nm)
Weight loss during TPR (mg)a
Before RSSIE
After RSSIE
After RSSIE
423–673 Kb
673–823 K
Total
1.08 0.69 1.18 0.85
1.35 0.54 1.13 0.69
230, 230, 230, 230,
0.67 0.63 0.68 0.70
0.50 0.66 0.82 0.98
1.17 1.29 1.50 1.68
280, 280, 280, 280,
340 340, 415 340 340, 410
The amount of V2O5 is 3.78 mg for all samples. Weight loss of 0.66 mg could be obtained for each of the reactions: 2[(VO)3þ (OH)]2þ (SiO)Zeol + 1/2H2 + Hþ Zeol ! 2[VO]2þ Zeol + 2H2 O,
or 2[VO]2þ Zeol + H2 ! 2V4þ Zeol + 2H2 O
R. Dimitrova, M. Spassova / Catalysis Communications 8 (2007) 693–696
The aforementioned effects are well perceived in the spectra of sorbed pyridine (Fig. 1). In the spectra of the ball-milled samples, significant intensity of bands at 1460 and 1626 cm1 (pyridine bound to extra framework cationic species) was seen (Fig. 1A and B, curves a and b). A low intensity pyridinium band at 1546 cm1, attributed to pyridinium cations, was observed only for VBZSM-5 and revealed the presence of Brønstedacid sites (Fig. 1A, curve a). In the spectra of the reduced samples, the intensity of the bands at 1446 and 1596 cm1 (characteristic of pyridine interacting with Lewis acid sites) increased, revealing the formation of [V@O]2+ and V4+ ions. The FTIR measurements confirmed the maintenance of the vanadium ions under different conditions. The high intensity of the band at 1640 cm1 (vanadium as a compensating ion) and the bands in a region 960–970 cm1 (vanadium in silanol nest) was seen in all spectra [20,21]. The vanadium ions were preserved in the regenerated and in the recovered samples (Fig. 2, line B and C). Sample regeneration was completed in SETARAM microbalance by carrying out the procedure of TPR in a H2 + Ar (1:1) flow from ambient temperature up to 773 K. After reusing the sample, a second regeneration was done since the characteristic bands of the used sample were with almost the same intensity compared to the initial one. The existence of different types of vanadium was also displayed in the UV–Vis spectra of the reduced sample (Table 1). Besides the band at 230 nm, due to silica in a molecular sieve, a high intensive band at 280 nm and a broad absorption near 340 and 415 nm were registered. Hence, it was reasonable to conclude that the procedure of reductive solid state vanadium exchange into the low acidic BZSM-5 and BBeta zeolites was effective and vanadium was fairly strong grafted to samples’ framework. It is known from the literature, that the presence of V4+ ion is required for the formation of vanadium hydroperox-
C
Transmitance (a.u.)
B
A
1600
1400
1200
400
1460 1546
1596
1447
a 1626
-1460 -1546
600
ide (V ± OOH) which can hydroxylated benzene when using hydrogen peroxide as oxidant [22]. At the beginning of the experiment, on addition of H2O2, colour change was observed from green-blue (due to the existence of VO2+) to light yellow (presence of V5+) [23]. Further, proceeding with the reaction, the colour changed respectively from yellow to green-yellowish and back to dark blue, pointing to the complete use up of the peroxide. Benzene was hydroxylated to give mostly phenol and hydroquinone (Table 2). The higher activity, of the initially deboronated sample of both zeolite types, were most probably due to the better dispersion and accessibility of the vanadium sites. (Table 1). VBeta_B was the most active catalysts at sufficient peroxide selectivity. The results from the blank experiments revealed practically no further conversion of benzene, so
B
b
b 1447
-1609
800
Fig. 2. FTIR spectra of samples: (A) parent Vbeta_B; (B) regenerated Vbeta_B; (C) after second regeneration.
a
-1626
1000
Wavenumber (cm-1)
A
Absorbance, a.u.
695
c
c d
d
1600
1500
1400
1600
1500
1400
-1
Wavenumber, cm
Fig. 1. Spectra of pyridine absorbed at 470 K on: (A) VBZSM-5 (curves a and c) and VZSM-5_B (curves b and d); (B) VBBeta (curves a and c) and VBbeta_B (curves b and d); all curves (a, b) – ball-milled samples, thermally treated up to 770 K; all curves (c, d) – reduced samples at 770 K in H2:Ar = 1:1.
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R. Dimitrova, M. Spassova / Catalysis Communications 8 (2007) 693–696
Table 2 Catalytic performances of vanadium containing zeolites Sample
VBZSM-5 VZSM-5_B VBBeta VBeta_B a b c d e
Benzene, turn overa
10 25 15 35
Benzene product distributionb mass% PH
HQ
98 87 75 70
– 13 25 30
H2O2 selectivityc
57.5 57.4 55.4 65.0
Phenol turn overa
4.8 1.6 10.2 7.4
Phenol product distributiond mass % pBQ
CAT
HQ
30 50 25 26
55 25 57 57
14 25 17 16
H2O2 selectivitye
50.0 13.9 65.3 55.8
Moles of benzene/phenol converted per mole of vanadium. PH, phenol; HQ, hydroquinone. H2O2 utilized for phenol formation. pBQ, p-benzoquinone; CAT, catechol; and HQ, hydroquinone. H2O2 utilized for pBQ, CAT HQ formation.
the hydroxylation of benzene was mainly heterogeneous. The catalysts were regenerated in a flow of dry argon up to 573 K and reduced in a H2 + Ar (1:1) atmosphere. Nearly the same amount of vanadium was determined in the regenerated catalyst, but due to the low content of vanadium in the investigated samples, the analytical analysis was within the limits of the experimental error. There were three main reaction products of phenol conversion: catechol, hydroquinone and benzoquinone. The observed activity per V atom turned out to be rather low for phenol hydroxylation in comparison with the one of benzene. It was observed that the yield to catechol was higher that the hydroquinone in all experiments. It seemed that phenol hydroxylation occurred on the surface rather than inside the channel where low numbers of grafted vanadium sites existed. 4. Conclusions The procedure of reductive solid state vanadium exchange into BZSM-5 and BBeta zeolites led to the formation of vanadium ions fairly strong bonded to the zeolite framework, so that the latter could be used in liquid phase reaction. The grafted isolated vanadium sites, as coordinated V-oxides having a (V@O) bond and vanadium cations as compensating ions, possess activity in hydroxylation of benzene and phenol. References [1] G. Kasperek, P. Bruce, T. Bruce, H. Yagi, D. Jerina, J. Chem. Soc. Commun. (1972) 784.
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