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Production of hydrogen peroxide by the reaction of hydroxylamine and molecular oxygen over activated carbons
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Catalysis Communications 9 (2008) 831–836 www.elsevier.com/locate/catcom
Production of hydrogen peroxide by the reaction of hydroxylamine and molecular oxygen over activated carbons Wei Song, Juan Li, Junlong Liu, Wenjie Shen
*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Received 30 April 2007; received in revised form 7 September 2007; accepted 8 September 2007 Available online 14 September 2007
Abstract In situ production of hydrogen peroxide was achieved by the reaction of hydroxylamine and molecular oxygen over reusable activated carbon catalysts at ambient temperature with environmentally benign by-products (nitrogen and water). The yield of hydrogen peroxide was about 46% with the concentration of 0.39 wt% (114 mmol/L). Temperature-programmed desorption analysis and Fourier transformation infrared spectroscopy revealed that the oxygen-containing species, especially quinone/semiquinone-like structures, on the surface of activated carbon played a crucial role in the generation of H2O2, probably through the effective activation of oxygen molecule. 2007 Elsevier B.V. All rights reserved. Keywords: In situ production; Hydrogen peroxide; Hydroxylamine; Oxygen; Activated carbon
1. Introduction Hydrogen peroxide (H2O2) has long been used as environmentally benign and selective oxidizing agent in the synthesis of organic compounds and fine chemicals as well as the paper bleaching process. The only reaction product of its use is water, which make it one of the most versatile chemical oxidants in green chemistry. H2O2 is commercially produced by the anthraquinone oxidation (AO) process, as recently reviewed by Fierro et al. [1]. However, the AO process is a multistep route that requires significant energy input and generates liquid waste mainly from overhydrogenated anthraquinone and toxic organic solvents. Moreover, the transport, storage, and handling of bulk H2O2 involve hazards and escalating expenses. On the other hand, the use of in situ production of H2O2 is attracting growing interests owing to its high efficiency and environmentally benign nature. Particularly, the selective oxidation of organic compounds coupling with the in situ *
Corresponding author. Tel.: +86 411 84379085; fax: +86 411 84694447. E-mail address: [email protected] (W. Shen). 1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.09.006
generation of H2O2 as oxidant is of great practical importance, as demonstrated by epoxidation olefins [2,3] and benzene oxidation to phenol [4,5]. The essential work by Sheriff et al. [6–8] revealed a most successful method for the local and in situ production of H2O2 through an enzyme-like process, in which the reaction of hydroxylamine and oxygen molecule was catalyzed by homogeneous Mn complexes. Although the turnover frequency of hydroxylamine was as high as 104 h 1 due to tiny quantities manganese used in the reaction system, this homogeneous route was still limited for practical application because of the difficulties in separating and reusing the Mn complexes. Furthermore, the manganese species also showed high performance on the decomposition of H2O2, which may decrease the concentration of H2O2 significantly. Very recently, Choudhary and coworkers [9– 11] used supported Au and Pt nanoparticles to catalyze the above reaction and obtained satisfying yield of H2O2, but the concentration of H2O2 (0.01–0.14 wt%) was still relatively low and the cost of the catalysts was also high due to the limited availability of precious metals. In this work, we used activated carbon as catalyst for the in situ production of H2O2 by the reaction of hydroxylamine
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W. Song et al. / Catalysis Communications 9 (2008) 831–836
aqueous solution with oxygen molecule at ambient temperature. The effects of temperature, pH value and reaction time on the production of H2O2 were extensively investigated. The surface properties of the activated carbons were characterized by Fourier transformation infrared (FT-IR) spectroscopy and Temperature-programmed desorption (TPD) analysis, which were further correlated with the catalytic performance.
concentration of hydrogen peroxide was analyzed using the colorimetric method based on titanium (IV) sulphate [12]. The yield of H2O2 was calculated according to the reaction stoichiometry (2NH2OH + O2 = N2 + 2H2O + H2O2). Namely, H2O2 yield (%) = (moles of H2O2/moles of NH2OH Æ HCl) · 2 · 100.
2. Experimental
3.1. Effect of reaction temperature
2.1. Pretreatment of activated carbon
Fig. 1 shows the temperature dependence of H2O2 yield over the ACH catalyst. The yield of H2O2 increased rapidly with reaction temperature and reached the maximum (40%) at 35 C, then declined to 27% at 45 C. This can be understood by considering the fact that the ratio between the rates of H2O2 formation and decomposition decreased significantly when increasing the temperature from 35 C to 45 C, mainly due to an appreciable increase in the rate of H2O2 decomposition at higher temperature.
2.2. Characterization Nitrogen adsorption–desorption isotherms were recorded on a Micrometrics ASAP 2000 instrument at 196 C. Prior to measurement, the sample was outgassed at 110 C overnight. The specific surface area (SBET) was calculated from a multipoint Braunauer–Emmett–Teller (BET) analysis. FT-IR spectra of the samples were recorded with a Fourier transformation spectrometer (Bruker Vector-22) using KBr pallet containing 0.5 wt% of AC. Temperature-programmed desorption (TPD) was conducted with a conventional apparatus equipped with a quadrupole mass spectrometer (Omnistar, Balzers). About 40 mg of AC was placed in a U-shaped quartz vessel. Once the samples were stabilized under a helium stream (30 ml/ min) at room temperature, they were heated up to 900 C at a rate of 10 C /min. The outlet gases were monitored by the mass spectrometer. 2.3. Production of hydrogen peroxide The reaction of O2 and hydroxylamine over the AC catalysts was conducted in a magnetically stirred jacketed glass reactor (250 ml). Typically, O2 (25 ml/min) was introduced through a mass flow controller into 100 ml aqueous solution containing 3.48 g (50 mmol) hydroxylammonium chloride (NH2OH Æ HCl) and 0.3 g AC at 25 C. Prior to AC addition, the pH value of the NH2OH Æ HCl solution was adjusted to 8.6 with sodium hydroxide. Aliquots of the reaction mixture were withdrawn periodically and the
3.2. Effect of pH value The pH value of the reaction medium also greatly affected the yield of H2O2, as shown in Fig. 2. The yield of H2O2 monotonously increased with increasing the pH value and stabilized at pH of 8.6. Afterwards, a slight decrease in H2O2 yield was observed by further increasing the pH value. There was a little or almost no H2O2 formation at low pH values (below 6.0). This was because that the reactive species in the reaction mixture was NH2OH (pKb = 8.18) which was released from the hydroxylammonium salt in the neutral or basic medium. However, the cationic species (NH3OH)+ existing in the acidic medium were not or much less reactive for reducing oxygen to H2O2 [10]. Hence, the H2O2 yield decreased drastically with decreasing the pH of the reaction medium. At higher pH values, such as 9.0, a slight decrease of H2O2 yield was observed. Since H2O2 decomposition was much faster in basic medium than in acid, which was confirmed by the recent work of 40
H2O2 yield (%)
Activated carbon (AC, 200–300 mesh, pHIEP 5, Aldrich) was initially treated with concentrated HCl solution to eliminate the inorganic impurities and ashes. AC (1 g) and 10 ml of HCl aqueous solution (37%) was mixed and stirred at room temperature for 3 h. After filteration, the AC was thoroughly washed with hot deionized water until the filtrate was free of Cl detected by AgNO3. The AC was then dried at 110 C overnight in a vacuum oven, which was nominated as ACH. The ACH was also treated at 400–800 C for 1 h in a quartz tube reactor under N2 flow. These samples were denoted as ACH 400, ACH 600 and ACH 800, respectively.
3. Results and discussion
35
30
25
20 0
10
20
30
40
50
Temperature (oC) Fig. 1. Effect of reaction temperature on the yield of H2O2 over the ACH catalyst (Reaction time = 180 min, pH 8.6).
W. Song et al. / Catalysis Communications 9 (2008) 831–836
H2O2 yield (%)
40
30
20
10
0 5
6
7
8
9
10
pH value Fig. 2. Effect of pH value of the reaction mixture on the yield of H2O2 over the ACH catalyst (Reaction time = 180 min, T = 25 C).
Choudhary et al. [10], the decrease in H2O2 yield may be caused by the faster decomposition of H2O2 in more basic solution. Additionally, the pH value of the reaction mixture was found to decrease during the course of reaction owing to the continuous consumption of NH2OH. 3.3. Effect of reaction time
50 16 40 12 30 8 20 4
H2O2 Yield (%)
Formation rate of H2O2 (mmol/g/h)
Fig. 3 shows the yield and the formation rate of H2O2 as a function of reaction time. The formation rate of H2O2 rapidly increased and approached to the maximum value of 16.7 mmol/g/h at about 60 min. Then, it decreased with prolonging the reaction time, due to the consumption of NH2OH in the reaction. Simultaneously, the yield of H2O2 remarkably increased during the initial stage and gradually stabilized at around 46% when reacted for 300 min. After that, the decrease of H2O2 yield was found at longer reaction periods. This phenomenon indicated that there was an equilibrium between H2O2 formation and its sequential decomposition (H2O2 = H2O + 0.5 O2). In fact, AC was also active for H2O2 decomposition, as previously
10
0
0 0
100 200 300 400 0
100
200
300
400
Reaction time (min) Fig. 3. Effect of reaction time on the formation rate and the yield of H2O2 over the ACH catalyst (solid symbols) and the recycling ACH catalyst (hollow symbols) (T = 25 C, pH 8.6).
833
reported [13]. After two cycles, there was almost no decrease in the activity of the reused ACH with H2O2 yield of about 47% (118 mmol/L).Therefore, it was essential to adjust the temperature, the pH value and the reaction time in a proper range for getting higher formation rate and/or yield of H2O2. Compared with the homogeneous reaction system using Mn complexes as catalysts [6–8], this heterogeneous approach using ACH as catalysts also offered satisfying yield of H2O2. The optimal concentration of H2O2 was about 0.39 wt% (114 mmol/L), which was not only comparable with the value (0.75 wt% or 220 mmol/L) of homogeneous system, but also much higher than the most promising result (0.14 wt% or 40 mmol/L) obtained over Au and Pd catalysts [9,10]. Moreover, the ACH catalysts can be easily separated by filtration from the reaction mixture and reused. 3.4. Effect of thermal treatment Since the catalytic activity of activated carbon was strongly influenced by the surface chemistry, especially the surface oxygen-containing groups, the parent ACHs were further heated at 400–800 C under nitrogen stream. Table 1 shows the results of textural characteristics and the yield of H2O2 over the ACH catalysts. It is obvious that both the BET specific surface area (SBET) and the total pore volume (TVP) of the ACH catalysts decreased with increasing the temperature of thermal treatment. Meanwhile, the H2O2 yield also decreased with increasing the temperature of thermal treatments. The parent ACH gave the highest yield of H2O2 (46%), while the corresponding values over the ACH 400 and the ACH 600 catalysts were 33% and 17%, respectively. No H2O2 formation could be detected over the ACH 800 catalyst. 3.5. FT-IR Fig. 4 shows the FT-IR spectra of the ACH catalysts. There were three bands at about 1710 cm 1, 1580 cm 1 and 1240 cm 1. The band at 1710 cm 1 may be assigned to the C@O stretching vibration from lactones and carboxyl groups, mainly involved in aromatic rings. The band at 1580 cm 1 may be attributed to the C@C double bonds in quinone-like structure [14,15]. The band around 1240 cm 1 may be associated with CAO stretching and OAH bending
Table 1 Textural characteristics and catalytic performance of the ACH catalysts Catalyst
SBET (m2/g)
TPV (cm3/g)
H2O2 Yield (%)
ACH ACH 400 ACH 600 ACH 800
1767 1442 1293 1131
1.57 1.35 1.17 1.04
46 33 17 –
SBET: BET specific surface area; TVP: total pore volume. Results at 25 C for five-hour reaction.
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W. Song et al. / Catalysis Communications 9 (2008) 831–836
0.6
1580 ACH 1710
CO 2 (μmol/g/s)
1240
CO2
ACH
0.4
ACH 400 ACH 800
0.2 ACH 600 0 3
CO (μmol/g/s)
Absorbance
ACH 400
200
400
600
800 ACH 400
CO
2 ACH
1
ACH 600 ACH 800
ACH 600 0 0
200
400
600
800
Temperature (oC) ACH 800
1800
1500
1200
900
-1
Wave number (cm ) Fig. 4. FT-IR spectra of the ACH catalysts.
molds in ethers, lactones, phenols and carboxylic anhydrides [16]. The three bands in the parent ACH exhibited the strongest intensities, and they were slightly decreased in the ACH 400 sample. The band at 1710 cm 1, associated with carboxyl group, became very weak or almost disappeared in the ACH samples thermally treated at 600 C and 800 C, the intensities of the other two peaks at 1240 cm 1 and 1580 cm 1 also simultaneously decreased. 3.6. TPD Fig. 5 shows the TPD profiles of the ACH catalysts. Upon heating, the surface oxygen-containing species of activated carbon would decompose into CO and CO2 [16–18]. For the parent ACH, the significant evolution of CO2 at 360 C and 540 C was due to the decomposition of carboxylic acids/anhydrides and lactones, respectively. The extensive generation of CO at 650 C might be caused by the decomposition of carboxylic anhydrides, phenols, ethers, carbonyls and quinones. For the ACH 400 sample, desorption of CO2 was only observed at around 550 C, representing the decomposition of lactones and carboxylic anhydrides. The evolution of CO at 640 C also decreased due to the decomposition of the remaining carboxylic anhydrides, phenols, ethers, carbonyls and quinones. CO2 evolution was almost disappeared in the ACH 600 sample, and the small desorption of CO at about 750 C might be assigned to the decomposition of carbonyls and quinones remaining on the surface. Concerning the ACH 800, there
Fig. 5. CO2 and CO evolutions in TPD profiles of the ACH catalysts.
were almost no evolutions of CO2 or CO. These results agreed with the FT-IR measurements. It may be concluded that heating up to 400 C mainly removed the carboxylic acids together with part of carboxylic anhydrides, lactones, phenols and ethers. The thermal treatment at 600 C would completely remove carboxylic acids/anhydrides and lactones and most of phenols and carbonyls. Consequently, the thermal treatment at 800 C completely removed almost all of the oxygen-containing groups on the surface of AC. In order to determine the amount of each surface group, the TPD profiles of the ACH catalysts were deconvoluted by using a multiple Gaussian function procedure [16], as shown in Fig. 6. The desorption spectra of CO2 were decomposed into three peaks, the first corresponding to carboxylic acid groups, the second to the anhydrides and the third to lactones. Similarly, the CO profiles of the samples were also decomposed into three peaks corresponding to anhydrides, phenols and carbonyl-quinones, respectively. The amounts of each surface group calculated from the areas of the deconvoluted peaks are given in Table 2. For each oxygen-containing surface group, the maximum peak temperature (TM), width at half height (W) and the amount (A) are given. The data clearly showed that the TM values corresponded closely to the peak maxima observed in the TPD profiles, and were within the range of values reported by Figueiredo et al. [16]. Thus, the decomposition temperatures were 250 C for carboxylic acids, 380 C for anhydrides, 560 C for lactones, 655 C for phenols and 770 C for carbonyls/quinones. The observations of FT-IR spectroscopies and TPD measurements were consistent with the reaction results, which indicated that the catalytic performance of the ACH samples decreased with the removal of surface oxygen-containing groups. Accordingly, it is reasonable to assume that the surface oxygen-containing groups in the ACH catalysts played a crucial role in the generation of
W. Song et al. / Catalysis Communications 9 (2008) 831–836
2.5
ACH
ACH
2.0
0.4
CO (μmol/g/s)
CO2 (μmol/g/s)
0.5
835
0.3 0.2
1.5 1.0 0.5
0.1
0.0
0.0 0
200
400
600
800
0
200
Temperature (oC)
400
600
800
Temperature (oC) 3.0
0.4
ACH 400
ACH 400
2.5
CO (μmol/g/s)
CO2 (μmol/g/s)
0.3
0.2
0.1
2.0 1.5 1.0 0.5 0.0
0.0 0
200
400
600
800
0
200
Temperature (oC)
400
600
800
Temperature (oC) 2.0
ACH 600 CO (μmol/g/s)
1.5
1.0
0.5
0.0 0
200
400
600
800
Temperature (oC) Fig. 6. Deconvolution of TPD profiles of the ACH catalysts.
Table 2 Deconvolution of TPD profiles for the ACH catalysts Sample
CO2 desorption Carboxylic
ACH ACH 400 ACH 600
Anhydride
Lactone
TM (C)
W (C)
A (lmol/g)
TM (C)
W (C)
A (lmol/g)
TM (C)
W (C)
A (lmol/g)
247.6 – –
142.0 – –
192.2 – –
373.6 383.4 –
177.4 157.0 –
503.1 120.6 –
558.4 558.5 –
177.4 157.0 –
361.5 355.2 –
CO desorption Anhydride ACH ACH 400 ACH 600
458.4 481.5 –
Phenol 177.4 157.0 –
503.1 120.6 –
635.1 633.5 691.8
Carbonyl-quinone 196.4 188.4 90.6
2613.5 2538.5 492.4
788.3 767.3 761.7
196.4 188.4 90.6
854.0 821.3 811.3
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W. Song et al. / Catalysis Communications 9 (2008) 831–836
4. Conclusions
70
H2O2 yield (%)
60
ACH and Tiron
50 40 ACH
30 20 Tiron
10 0 100
200 300 Reaction time (min)
400
Fig. 7. Production of H2O2 over (m) ACH and Tiron, (d) ACH alone and (j) Tiron alone (T = 25 C, pH 8.6).
H2O2 was successfully produced by reaction of NH2OH aqueous solution and molecular oxygen at ambient temperature only using AC as catalyst. Under optimal conditions, the yield of H2O2 was about 46% with concentration of 0.39 wt% (114 mmol/L). The oxygen-containing groups, especially quinone/semiquinone-like structures, on the surface of AC played a crucial role in the generation of H2O2 through the efficient activation of oxygen molecule. In addition to this environmentally friendly process (nitrogen and water as the only by-products) for the local and in situ production of H2O2, the AC catalyst also offered significant advantages of low cost and easy separation and recycling. References
H2O2. By recalling the earlier observations of Sheriff et al. [6] that there was almost no H2O2 formation in the absence of Tiron (1,2-dihydroxybenzene-3,5-disulphonate, disodium salt) in the homogenous reaction system catalyzed by Mn-complexes, it seems true that the oxygen-containing groups on the surface of ACH may play a similar role as Tiron in the Mn-complexes, which can be easily oxidized to semiquinone and quinone [8]. This can be further confirmed by adding a small amount of Tiron (0.06 mmol) to the reaction medium containing 0.3 g ACH. As shown in Fig. 7, the presence of ACH alone resulted in H2O2 concentration of 0.39 wt% (114 mmol/L), and the addition of Tiron caused the concentration of H2O2 nearly doubled (0.56 wt%, 184 mmol/L). Comparatively, the use of Tiron alone could only give H2O2 concentration of 0.05 wt% (15 mmol/L). Moreover, there was almost no H2O2 formation under similar reaction conditions by using N2 instead of molecular oxygen, suggesting that molecular oxygen was the only source for the formation of H2O2, instead of the surface oxygen of ACH. As a matter of fact, activated carbons were also previously used as catalysts for ferrous ions oxidation [19] and propanethiol oxidation to dipropyl disulfide [20] in liquid phase at ambient temperature due to their high ability in activating molecular oxygen, mainly through the surface oxygen-containing species. Therefore, it is highly probable that the ACH also enhanced the activation of oxygen molecule in a similar way and thus offered satisfying performance for in situ production of H2O2.
[1] J.M. Campos-Martin, G. Blanco-Brieva, J.L.G. Fierro, Angew. Chem. Int. Ed. 45 (2006) 6962. [2] C. Perego, A. Carati, P. Ingallina, M.A. Mantegazza, G. Bellussi, Appl. Catal. A 221 (2001) 63. [3] M.G. Clerici, P. Ingallina, Catal. Today 41 (1998) 351. [4] S. Niwa, M. Eswaramoorthy, N. Jalajakumari, R. Anju, I. Naotsugu, S. Hiroshi, N. Takemi, M. Fujio, Science 295 (2002) 105. [5] G.D. Vulpescu, M. Ruitenbek, L.L. van Lieshout, L.A. Correia, D. Meyer, P.P.A.C. Pex, Catal. Commun. 5 (2004) 347. [6] T.S. Sheriff, J. Chem. Soc., Dalton Trans. (1992) 1051. [7] T.S. Sheriff, P. Carr, B. Piggott, Inorg. Chim. Acta 348 (2003) 115. [8] T.S. Sheriff, P. Carr, S.J. Coles, M.B. Hursthouse, J. Lesin, M.E. Light, Inorg. Chim. Acta 357 (2004) 2494. [9] V.R. Choudhary, P. Jana, S.K. Bhargava, Catal. Commun. 8 (2007) 811. [10] V.R. Choudhary, P. Jana, Catal. Commun. 8 (2007) 1578. [11] V.R. Choudhary, P. Jana, C. Samanta, Appl. Catal. A 323 (2007) 202. [12] P.A. Clapp, D.F. Evans, T.S. Sheriff, Anal. Chim. Acta 218 (1989) 332. [13] L.C.A. Oliveira, C.N. Silva, M.I. Yoshida, R.M. Lago, Carbon 42 (2004) 2279. [14] A. Macı´as-Garcı´a, M.A. Dı´az-Dı´ez, E.M. Cuerda-Correa, M. Olivares-Marı´n, J. Gan˜an-Go´mez, Appl. Surf. Sci. 252 (2006) 5972. [15] G. de la Puente, J.J. Pis, J.A. Mene´ndez, P. Grange, J. Anal. Appl. Pyrolysis 43 (1997) 125. ´ rfa˜o, [16] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. O Carbon 37 (1999) 1379. [17] H.P. Boehm, Carbon 40 (2002) 145. [18] U. Zielke, K.J. Hu¨ttinger, W.P. Hoffman, Carbon 34 (1996) 983. [19] E. Ahumada, H. Lizama, F. Orellana, C. Sua´rez, A. Huidobro, A. Sepu´lveda-Escribano, F. Rodrı´guez-Reinoso, Carbon 40 (2002) 2827. [20] R.V.R.A. Rios, J. Silvestre-Albero, A. Sepu´lveda-Escribano, F. Rodrı´guez-Reinoso, Colloids Surf. A 300 (2007) 180.
Catalysis Communications 9 (2008) 831–836 www.elsevier.com/locate/catcom
Production of hydrogen peroxide by the reaction of hydroxylamine and molecular oxygen over activated carbons Wei Song, Juan Li, Junlong Liu, Wenjie Shen
*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Received 30 April 2007; received in revised form 7 September 2007; accepted 8 September 2007 Available online 14 September 2007
Abstract In situ production of hydrogen peroxide was achieved by the reaction of hydroxylamine and molecular oxygen over reusable activated carbon catalysts at ambient temperature with environmentally benign by-products (nitrogen and water). The yield of hydrogen peroxide was about 46% with the concentration of 0.39 wt% (114 mmol/L). Temperature-programmed desorption analysis and Fourier transformation infrared spectroscopy revealed that the oxygen-containing species, especially quinone/semiquinone-like structures, on the surface of activated carbon played a crucial role in the generation of H2O2, probably through the effective activation of oxygen molecule. 2007 Elsevier B.V. All rights reserved. Keywords: In situ production; Hydrogen peroxide; Hydroxylamine; Oxygen; Activated carbon
1. Introduction Hydrogen peroxide (H2O2) has long been used as environmentally benign and selective oxidizing agent in the synthesis of organic compounds and fine chemicals as well as the paper bleaching process. The only reaction product of its use is water, which make it one of the most versatile chemical oxidants in green chemistry. H2O2 is commercially produced by the anthraquinone oxidation (AO) process, as recently reviewed by Fierro et al. [1]. However, the AO process is a multistep route that requires significant energy input and generates liquid waste mainly from overhydrogenated anthraquinone and toxic organic solvents. Moreover, the transport, storage, and handling of bulk H2O2 involve hazards and escalating expenses. On the other hand, the use of in situ production of H2O2 is attracting growing interests owing to its high efficiency and environmentally benign nature. Particularly, the selective oxidation of organic compounds coupling with the in situ *
Corresponding author. Tel.: +86 411 84379085; fax: +86 411 84694447. E-mail address: [email protected] (W. Shen). 1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.09.006
generation of H2O2 as oxidant is of great practical importance, as demonstrated by epoxidation olefins [2,3] and benzene oxidation to phenol [4,5]. The essential work by Sheriff et al. [6–8] revealed a most successful method for the local and in situ production of H2O2 through an enzyme-like process, in which the reaction of hydroxylamine and oxygen molecule was catalyzed by homogeneous Mn complexes. Although the turnover frequency of hydroxylamine was as high as 104 h 1 due to tiny quantities manganese used in the reaction system, this homogeneous route was still limited for practical application because of the difficulties in separating and reusing the Mn complexes. Furthermore, the manganese species also showed high performance on the decomposition of H2O2, which may decrease the concentration of H2O2 significantly. Very recently, Choudhary and coworkers [9– 11] used supported Au and Pt nanoparticles to catalyze the above reaction and obtained satisfying yield of H2O2, but the concentration of H2O2 (0.01–0.14 wt%) was still relatively low and the cost of the catalysts was also high due to the limited availability of precious metals. In this work, we used activated carbon as catalyst for the in situ production of H2O2 by the reaction of hydroxylamine
832
W. Song et al. / Catalysis Communications 9 (2008) 831–836
aqueous solution with oxygen molecule at ambient temperature. The effects of temperature, pH value and reaction time on the production of H2O2 were extensively investigated. The surface properties of the activated carbons were characterized by Fourier transformation infrared (FT-IR) spectroscopy and Temperature-programmed desorption (TPD) analysis, which were further correlated with the catalytic performance.
concentration of hydrogen peroxide was analyzed using the colorimetric method based on titanium (IV) sulphate [12]. The yield of H2O2 was calculated according to the reaction stoichiometry (2NH2OH + O2 = N2 + 2H2O + H2O2). Namely, H2O2 yield (%) = (moles of H2O2/moles of NH2OH Æ HCl) · 2 · 100.
2. Experimental
3.1. Effect of reaction temperature
2.1. Pretreatment of activated carbon
Fig. 1 shows the temperature dependence of H2O2 yield over the ACH catalyst. The yield of H2O2 increased rapidly with reaction temperature and reached the maximum (40%) at 35 C, then declined to 27% at 45 C. This can be understood by considering the fact that the ratio between the rates of H2O2 formation and decomposition decreased significantly when increasing the temperature from 35 C to 45 C, mainly due to an appreciable increase in the rate of H2O2 decomposition at higher temperature.
2.2. Characterization Nitrogen adsorption–desorption isotherms were recorded on a Micrometrics ASAP 2000 instrument at 196 C. Prior to measurement, the sample was outgassed at 110 C overnight. The specific surface area (SBET) was calculated from a multipoint Braunauer–Emmett–Teller (BET) analysis. FT-IR spectra of the samples were recorded with a Fourier transformation spectrometer (Bruker Vector-22) using KBr pallet containing 0.5 wt% of AC. Temperature-programmed desorption (TPD) was conducted with a conventional apparatus equipped with a quadrupole mass spectrometer (Omnistar, Balzers). About 40 mg of AC was placed in a U-shaped quartz vessel. Once the samples were stabilized under a helium stream (30 ml/ min) at room temperature, they were heated up to 900 C at a rate of 10 C /min. The outlet gases were monitored by the mass spectrometer. 2.3. Production of hydrogen peroxide The reaction of O2 and hydroxylamine over the AC catalysts was conducted in a magnetically stirred jacketed glass reactor (250 ml). Typically, O2 (25 ml/min) was introduced through a mass flow controller into 100 ml aqueous solution containing 3.48 g (50 mmol) hydroxylammonium chloride (NH2OH Æ HCl) and 0.3 g AC at 25 C. Prior to AC addition, the pH value of the NH2OH Æ HCl solution was adjusted to 8.6 with sodium hydroxide. Aliquots of the reaction mixture were withdrawn periodically and the
3.2. Effect of pH value The pH value of the reaction medium also greatly affected the yield of H2O2, as shown in Fig. 2. The yield of H2O2 monotonously increased with increasing the pH value and stabilized at pH of 8.6. Afterwards, a slight decrease in H2O2 yield was observed by further increasing the pH value. There was a little or almost no H2O2 formation at low pH values (below 6.0). This was because that the reactive species in the reaction mixture was NH2OH (pKb = 8.18) which was released from the hydroxylammonium salt in the neutral or basic medium. However, the cationic species (NH3OH)+ existing in the acidic medium were not or much less reactive for reducing oxygen to H2O2 [10]. Hence, the H2O2 yield decreased drastically with decreasing the pH of the reaction medium. At higher pH values, such as 9.0, a slight decrease of H2O2 yield was observed. Since H2O2 decomposition was much faster in basic medium than in acid, which was confirmed by the recent work of 40
H2O2 yield (%)
Activated carbon (AC, 200–300 mesh, pHIEP 5, Aldrich) was initially treated with concentrated HCl solution to eliminate the inorganic impurities and ashes. AC (1 g) and 10 ml of HCl aqueous solution (37%) was mixed and stirred at room temperature for 3 h. After filteration, the AC was thoroughly washed with hot deionized water until the filtrate was free of Cl detected by AgNO3. The AC was then dried at 110 C overnight in a vacuum oven, which was nominated as ACH. The ACH was also treated at 400–800 C for 1 h in a quartz tube reactor under N2 flow. These samples were denoted as ACH 400, ACH 600 and ACH 800, respectively.
3. Results and discussion
35
30
25
20 0
10
20
30
40
50
Temperature (oC) Fig. 1. Effect of reaction temperature on the yield of H2O2 over the ACH catalyst (Reaction time = 180 min, pH 8.6).
W. Song et al. / Catalysis Communications 9 (2008) 831–836
H2O2 yield (%)
40
30
20
10
0 5
6
7
8
9
10
pH value Fig. 2. Effect of pH value of the reaction mixture on the yield of H2O2 over the ACH catalyst (Reaction time = 180 min, T = 25 C).
Choudhary et al. [10], the decrease in H2O2 yield may be caused by the faster decomposition of H2O2 in more basic solution. Additionally, the pH value of the reaction mixture was found to decrease during the course of reaction owing to the continuous consumption of NH2OH. 3.3. Effect of reaction time
50 16 40 12 30 8 20 4
H2O2 Yield (%)
Formation rate of H2O2 (mmol/g/h)
Fig. 3 shows the yield and the formation rate of H2O2 as a function of reaction time. The formation rate of H2O2 rapidly increased and approached to the maximum value of 16.7 mmol/g/h at about 60 min. Then, it decreased with prolonging the reaction time, due to the consumption of NH2OH in the reaction. Simultaneously, the yield of H2O2 remarkably increased during the initial stage and gradually stabilized at around 46% when reacted for 300 min. After that, the decrease of H2O2 yield was found at longer reaction periods. This phenomenon indicated that there was an equilibrium between H2O2 formation and its sequential decomposition (H2O2 = H2O + 0.5 O2). In fact, AC was also active for H2O2 decomposition, as previously
10
0
0 0
100 200 300 400 0
100
200
300
400
Reaction time (min) Fig. 3. Effect of reaction time on the formation rate and the yield of H2O2 over the ACH catalyst (solid symbols) and the recycling ACH catalyst (hollow symbols) (T = 25 C, pH 8.6).
833
reported [13]. After two cycles, there was almost no decrease in the activity of the reused ACH with H2O2 yield of about 47% (118 mmol/L).Therefore, it was essential to adjust the temperature, the pH value and the reaction time in a proper range for getting higher formation rate and/or yield of H2O2. Compared with the homogeneous reaction system using Mn complexes as catalysts [6–8], this heterogeneous approach using ACH as catalysts also offered satisfying yield of H2O2. The optimal concentration of H2O2 was about 0.39 wt% (114 mmol/L), which was not only comparable with the value (0.75 wt% or 220 mmol/L) of homogeneous system, but also much higher than the most promising result (0.14 wt% or 40 mmol/L) obtained over Au and Pd catalysts [9,10]. Moreover, the ACH catalysts can be easily separated by filtration from the reaction mixture and reused. 3.4. Effect of thermal treatment Since the catalytic activity of activated carbon was strongly influenced by the surface chemistry, especially the surface oxygen-containing groups, the parent ACHs were further heated at 400–800 C under nitrogen stream. Table 1 shows the results of textural characteristics and the yield of H2O2 over the ACH catalysts. It is obvious that both the BET specific surface area (SBET) and the total pore volume (TVP) of the ACH catalysts decreased with increasing the temperature of thermal treatment. Meanwhile, the H2O2 yield also decreased with increasing the temperature of thermal treatments. The parent ACH gave the highest yield of H2O2 (46%), while the corresponding values over the ACH 400 and the ACH 600 catalysts were 33% and 17%, respectively. No H2O2 formation could be detected over the ACH 800 catalyst. 3.5. FT-IR Fig. 4 shows the FT-IR spectra of the ACH catalysts. There were three bands at about 1710 cm 1, 1580 cm 1 and 1240 cm 1. The band at 1710 cm 1 may be assigned to the C@O stretching vibration from lactones and carboxyl groups, mainly involved in aromatic rings. The band at 1580 cm 1 may be attributed to the C@C double bonds in quinone-like structure [14,15]. The band around 1240 cm 1 may be associated with CAO stretching and OAH bending
Table 1 Textural characteristics and catalytic performance of the ACH catalysts Catalyst
SBET (m2/g)
TPV (cm3/g)
H2O2 Yield (%)
ACH ACH 400 ACH 600 ACH 800
1767 1442 1293 1131
1.57 1.35 1.17 1.04
46 33 17 –
SBET: BET specific surface area; TVP: total pore volume. Results at 25 C for five-hour reaction.
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W. Song et al. / Catalysis Communications 9 (2008) 831–836
0.6
1580 ACH 1710
CO 2 (μmol/g/s)
1240
CO2
ACH
0.4
ACH 400 ACH 800
0.2 ACH 600 0 3
CO (μmol/g/s)
Absorbance
ACH 400
200
400
600
800 ACH 400
CO
2 ACH
1
ACH 600 ACH 800
ACH 600 0 0
200
400
600
800
Temperature (oC) ACH 800
1800
1500
1200
900
-1
Wave number (cm ) Fig. 4. FT-IR spectra of the ACH catalysts.
molds in ethers, lactones, phenols and carboxylic anhydrides [16]. The three bands in the parent ACH exhibited the strongest intensities, and they were slightly decreased in the ACH 400 sample. The band at 1710 cm 1, associated with carboxyl group, became very weak or almost disappeared in the ACH samples thermally treated at 600 C and 800 C, the intensities of the other two peaks at 1240 cm 1 and 1580 cm 1 also simultaneously decreased. 3.6. TPD Fig. 5 shows the TPD profiles of the ACH catalysts. Upon heating, the surface oxygen-containing species of activated carbon would decompose into CO and CO2 [16–18]. For the parent ACH, the significant evolution of CO2 at 360 C and 540 C was due to the decomposition of carboxylic acids/anhydrides and lactones, respectively. The extensive generation of CO at 650 C might be caused by the decomposition of carboxylic anhydrides, phenols, ethers, carbonyls and quinones. For the ACH 400 sample, desorption of CO2 was only observed at around 550 C, representing the decomposition of lactones and carboxylic anhydrides. The evolution of CO at 640 C also decreased due to the decomposition of the remaining carboxylic anhydrides, phenols, ethers, carbonyls and quinones. CO2 evolution was almost disappeared in the ACH 600 sample, and the small desorption of CO at about 750 C might be assigned to the decomposition of carbonyls and quinones remaining on the surface. Concerning the ACH 800, there
Fig. 5. CO2 and CO evolutions in TPD profiles of the ACH catalysts.
were almost no evolutions of CO2 or CO. These results agreed with the FT-IR measurements. It may be concluded that heating up to 400 C mainly removed the carboxylic acids together with part of carboxylic anhydrides, lactones, phenols and ethers. The thermal treatment at 600 C would completely remove carboxylic acids/anhydrides and lactones and most of phenols and carbonyls. Consequently, the thermal treatment at 800 C completely removed almost all of the oxygen-containing groups on the surface of AC. In order to determine the amount of each surface group, the TPD profiles of the ACH catalysts were deconvoluted by using a multiple Gaussian function procedure [16], as shown in Fig. 6. The desorption spectra of CO2 were decomposed into three peaks, the first corresponding to carboxylic acid groups, the second to the anhydrides and the third to lactones. Similarly, the CO profiles of the samples were also decomposed into three peaks corresponding to anhydrides, phenols and carbonyl-quinones, respectively. The amounts of each surface group calculated from the areas of the deconvoluted peaks are given in Table 2. For each oxygen-containing surface group, the maximum peak temperature (TM), width at half height (W) and the amount (A) are given. The data clearly showed that the TM values corresponded closely to the peak maxima observed in the TPD profiles, and were within the range of values reported by Figueiredo et al. [16]. Thus, the decomposition temperatures were 250 C for carboxylic acids, 380 C for anhydrides, 560 C for lactones, 655 C for phenols and 770 C for carbonyls/quinones. The observations of FT-IR spectroscopies and TPD measurements were consistent with the reaction results, which indicated that the catalytic performance of the ACH samples decreased with the removal of surface oxygen-containing groups. Accordingly, it is reasonable to assume that the surface oxygen-containing groups in the ACH catalysts played a crucial role in the generation of
W. Song et al. / Catalysis Communications 9 (2008) 831–836
2.5
ACH
ACH
2.0
0.4
CO (μmol/g/s)
CO2 (μmol/g/s)
0.5
835
0.3 0.2
1.5 1.0 0.5
0.1
0.0
0.0 0
200
400
600
800
0
200
Temperature (oC)
400
600
800
Temperature (oC) 3.0
0.4
ACH 400
ACH 400
2.5
CO (μmol/g/s)
CO2 (μmol/g/s)
0.3
0.2
0.1
2.0 1.5 1.0 0.5 0.0
0.0 0
200
400
600
800
0
200
Temperature (oC)
400
600
800
Temperature (oC) 2.0
ACH 600 CO (μmol/g/s)
1.5
1.0
0.5
0.0 0
200
400
600
800
Temperature (oC) Fig. 6. Deconvolution of TPD profiles of the ACH catalysts.
Table 2 Deconvolution of TPD profiles for the ACH catalysts Sample
CO2 desorption Carboxylic
ACH ACH 400 ACH 600
Anhydride
Lactone
TM (C)
W (C)
A (lmol/g)
TM (C)
W (C)
A (lmol/g)
TM (C)
W (C)
A (lmol/g)
247.6 – –
142.0 – –
192.2 – –
373.6 383.4 –
177.4 157.0 –
503.1 120.6 –
558.4 558.5 –
177.4 157.0 –
361.5 355.2 –
CO desorption Anhydride ACH ACH 400 ACH 600
458.4 481.5 –
Phenol 177.4 157.0 –
503.1 120.6 –
635.1 633.5 691.8
Carbonyl-quinone 196.4 188.4 90.6
2613.5 2538.5 492.4
788.3 767.3 761.7
196.4 188.4 90.6
854.0 821.3 811.3
836
W. Song et al. / Catalysis Communications 9 (2008) 831–836
4. Conclusions
70
H2O2 yield (%)
60
ACH and Tiron
50 40 ACH
30 20 Tiron
10 0 100
200 300 Reaction time (min)
400
Fig. 7. Production of H2O2 over (m) ACH and Tiron, (d) ACH alone and (j) Tiron alone (T = 25 C, pH 8.6).
H2O2 was successfully produced by reaction of NH2OH aqueous solution and molecular oxygen at ambient temperature only using AC as catalyst. Under optimal conditions, the yield of H2O2 was about 46% with concentration of 0.39 wt% (114 mmol/L). The oxygen-containing groups, especially quinone/semiquinone-like structures, on the surface of AC played a crucial role in the generation of H2O2 through the efficient activation of oxygen molecule. In addition to this environmentally friendly process (nitrogen and water as the only by-products) for the local and in situ production of H2O2, the AC catalyst also offered significant advantages of low cost and easy separation and recycling. References
H2O2. By recalling the earlier observations of Sheriff et al. [6] that there was almost no H2O2 formation in the absence of Tiron (1,2-dihydroxybenzene-3,5-disulphonate, disodium salt) in the homogenous reaction system catalyzed by Mn-complexes, it seems true that the oxygen-containing groups on the surface of ACH may play a similar role as Tiron in the Mn-complexes, which can be easily oxidized to semiquinone and quinone [8]. This can be further confirmed by adding a small amount of Tiron (0.06 mmol) to the reaction medium containing 0.3 g ACH. As shown in Fig. 7, the presence of ACH alone resulted in H2O2 concentration of 0.39 wt% (114 mmol/L), and the addition of Tiron caused the concentration of H2O2 nearly doubled (0.56 wt%, 184 mmol/L). Comparatively, the use of Tiron alone could only give H2O2 concentration of 0.05 wt% (15 mmol/L). Moreover, there was almost no H2O2 formation under similar reaction conditions by using N2 instead of molecular oxygen, suggesting that molecular oxygen was the only source for the formation of H2O2, instead of the surface oxygen of ACH. As a matter of fact, activated carbons were also previously used as catalysts for ferrous ions oxidation [19] and propanethiol oxidation to dipropyl disulfide [20] in liquid phase at ambient temperature due to their high ability in activating molecular oxygen, mainly through the surface oxygen-containing species. Therefore, it is highly probable that the ACH also enhanced the activation of oxygen molecule in a similar way and thus offered satisfying performance for in situ production of H2O2.
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