Catalytic Oxidation of Toluene with Molecular Oxygen Over Manganese Tetraphenylporphyrin Supported on Chitosan

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Applied Catalysis A: General 338 (2008) 83–86 www.elsevier.com/locate/apcata

Catalytic oxidation of toluene with molecular oxygen over manganese tetraphenylporphyrin supported on chitosan Guan Huang a,*, Jin Luo a, Cao Cheng Deng b, Yong An Guo a, Shu Kai Zhao a, Hong Zhou a, Shan Wei a a College of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China b Guangxi Traditional Chinese Medical University, Nanning 530001, PR China Received 13 September 2007; received in revised form 18 December 2007; accepted 23 December 2007 Available online 8 January 2008

Abstract Catalysis by simple manganese tetraphenylporphyrin [Mn TPP] supported on chitosan [CTS] for liquid phase aerobic oxidation of toluene has been investigated. The toluene conversion depends on the reaction temperature, the air pressure and the amount of catalyst, but the selectivity for aldehyde and alcohol is little affected by these three parameters. Using the Mn TPP/CTS containing 2 mg of Mn TPP as a catalyst, the aerobic oxidation of toluene under the optimum conditions of 195 8C and 0.6 MPa produced benzaldehyde and benzyl alcohol at 96% selectivity with 5.9% conversion of toluene. The catalyst can be reused once. Chitosan played an important role in the catalytic oxidation of toluene. # 2008 Elsevier B.V. All rights reserved.

Keywords: Chitosan; Manganese tetraphenylporphyrin; Toluene oxidation; Air

1. Introduction catalysts have great advantages because of their easy separation and recovery from the reaction mixture, widespread application At present, stringent ecological standards require extensive and effectiveness in environmentally friendly conditions [19,20]. attention to environmentally friendly and clean production Based on these considerations, we have successfully supported processes. Therefore, the use of molecular oxygen and a simple metal tretraphenylporphyrins on polysaccharides and recoverable catalyst for the catalytic oxidation of hydrocarbons is have employed them as catalysts for the aerobic oxidation of a wise choice. Until now, the oxidation of toluene with air has cyclohexane [13,21–23]. In this paper, we report the aerobic been mainly used to synthesize benzyl acid. However, the oxidation of toluene over [Mn TPP] Cl/CTS in environmentally synthesis of benzaldehyde and benzyl alcohol by the chlorination benign reaction conditions and in the absence of reductants and of toluene followed by hydrolysis (as used industrially world- solvents to gain a further understanding into the capability of wide), is a seriously polluting process [1–3]. Although the hydrocarbon oxidation catalyzed by a supported catalyst. aerobic oxidation of toluene over vanadium-containing catalysts or the simple cobalt tetraphenylporphyrin provide the optimum 2. Experimental selectivity for benzaldehyde and benzalcohol, at present this does not exceed 60% [4–6]. Highly selective catalytic oxidation of 2.1. Materials toluene into aldehyde and alcohol using various oxidants has been an attractive ﬁeld as well as an important chemical The chemical reagents used were of analytical grade and challenge [7–9]. were employed without further puriﬁcation. No impurities were There exists much research into the aerobic oxidation of found in toluene by GC analysis before use. Manganese TPP hydrocarbons catalyzed by metalloporphyrins under mild was synthesized according to documented procedures [24,25]. conditions [10–20]. Of these, the supported metalloporphyrin Manganese TPP supported on chitosan was prepared according to a procedure similar to that previously described in reference * Corresponding author. Tel.: +86 771 3237868; fax: +86 771 2851043. [13]. The amount of chloro[tetraphenylporphinato mangane- E-mail address: [email protected] (G. Huang). se(III)] supported per 1 g of chitosan was 0.00427 mmol.

2.2. Characterization of the supported catalyst

The supported catalyst was characterized by UV–Vis spectroscopy, using a method similar to that previously described [26]. The electronic spectra of the supported Mn TPP was measured in a glycerol mull in a quartz vessel, and the UV–Vis spectra of Mn TPP/CTS was compared to that of Mn TPP in the region of 280–580 nm (see Fig. 1). The chloroform extraction from the powdered solid catalyst was measured using UV–Vis spectrophotometry, with the result shown in Fig. 1a. Fig. 2. The aspects of chitosan-supported manganese porphyrins: original (a); recovered after 2.5 h reaction time at 195 8C (b) and at 205 8C (c) under 2.3. Procedure for the catalytic oxidation of toluene 0.6 MPa air pressure, respectively.

The catalytic aerobic oxidation of toluene was carried out Fig. 1a, again indicating that the Mn TPP had been adsorbed on according to the following typical reaction procedure: toluene the support. (200 ml) and Mn TPP/CTS catalyst (containing 2 mg Mn TPP) were added into a 500-ml autoclave reactor charged with 3.2. Chitosan-supported Mn TPP catalysis of toluene nitrogen to maintain a pressure of 0.6 MPa. While the reaction oxidation mixture was heated to 195 8C under stirring, air was continuously injected at a speed of 0.020 m3/h to maintain We have investigated the aerobic oxidation of toluene over the reaction system pressure at 0.6 MPa. The reaction mixture Mn TPP and over the same complex anchored on chitosan was sampled at regular intervals and analyzed by GC–MS. without the addition of reductants and solvents. It was found Quantification of the oxygenated products was obtained by GC that both catalysts could selectively catalyze the aerobic using chlorobenzene as the internal standard [27]. The oxidation of toluene to benzaldehyde and benzyl alcohol with supported catalyst was recovered by simple separation from our reaction conditions. The aerobic oxidations of toluene the reaction mixture, followed by washing with ethanol and catalyzed by the unsupported and the supported catalyst, drying in air. It was reused in subsequent toluene oxidations. respectively, were as follows (Scheme 1). GC–MS and LC–MS analysis of the samples indicated that 3. Results and discussion the by-products of toluene oxidation were mostly benzoic acid and a little benzyl benzoate, which can be ignored. In the 3.1. Impregnation of manganese porphyrin absence of the catalysts, toluene cannot be aerobically oxidized under the same conditions. Therefore, the two catalysts play an Manganese TPP was supported on chitosan (which was important part in the aerobic oxidation of toluene. This originally white) to give a green solid (see Fig. 2a). This may indicates that the supported catalyst retains the catalytic activity indicate the presence of manganese porphyrin on the support, of manganese porphyrin for toluene oxidation. which can be established by UV–Vis spectroscopy. In the solution of the Mn TPP in chloroform, a Soret band is visible at 3.3. Influence of reaction temperature on toluene oxidation 479 nm (see Fig. 1a). After immobilization of the Mn TPP on CTS, no significant change in the Soret band position was Fig. 3 shows the influence of the reaction temperature on the observed in the UV–Vis spectrum (see Fig. 1b), indicating that toluene conversion and product selectivity (aldehyde + alco- the structure of the porphyrin ring had not changed during the hol) in the toluene oxidation. First, the toluene conversion anchoring procedure. The chloroform extraction from the generally increased with reaction time to a maximum at about powdered solid catalyst showed the same bands as those of 2–2.5 h. Secondly, a suitable increase of reaction temperature can bring about an increase of toluene conversion. However, too high a temperature results in very low toluene conversion, as shown by the toluene oxidation at 205 8C. It is probable that the supported catalyst is burned out in the oxidation process (see Fig. 2c); when the recovered catalyst was reused to catalyze further aerobic oxidations of toluene, no products were detected

Fig. 1. UV–Vis spectra at room temperature. (a) Chloroform solution of Mn Scheme 1. The aerobic oxidation of toluene catalyzed by the unsupported and TPP and (b) in glycerol mull of Mn TPP on chitosan. the supported catalyst, respectively. G. Huang et al. / Applied Catalysis A: General 338 (2008) 83–86 85

But the toluene conversion decreases to 3.2% when the pressure is further increased up to 0.8 MPa. One possible reason is that Mn TPP/CTS tends to break up and lose activity at 0.8 MPa. Under the optimum reaction conditions of 195 8C and 0.6 MPa, the heterogeneous catalytic system gives about 96% selectivity of aldehyde and alcohol, 5.9% conversion of toluene and a catalyst turnover number of 3.7 Â 105.

3.5. Inﬂuence of the amount of catalyst on toluene oxidation

For a 2 h reaction time of toluene oxidation at 195 8C and Fig. 3. Changes in toluene conversion ((~) 205 8C, (*) 195 8C, (^) 190 8C, 0.6 MPa, the product selectivity was not greatly affected by the (&) 180 8C) and in selectivity (aldehyde + alcohol) ((~) 205 8C, (*) 195 8C, amount of supported catalyst (Table 1). However, the toluene (^) 190 8C, (&) 180 8C) with reaction time for toluene oxidation catalyzed by conversion increased about 2 times with an increase in catalyst Mn TPP/CTS at different temperatures. Reaction conditions: toluene: 200 ml; from 1 to 2 mg, but decreased by the same factor with a further catalyst: 2 mg (containing Mn TPP); air pressure: 0.6 MPa; airflow: 0.02 m3/h. increase in catalyst to 4 mg. A similar phenomenon was observed in our previous work [13,28]. Black [29] and Guo by GC. Therefore, the most appropriate reaction temperature et al. [6] reported this catalytic behavior and identified it with for the aerobic oxidation of toluene catalyzed by Mn TPP/CTS the so-called ‘catalyst inhibitor conversion’ phenomenon is about 195 8C. Thirdly, the selectivity for aldehyde and already known for some auto-oxidations catalyzed by transition alcohol remains at 90 Æ 5% for each of the four different metal salts in low polar media. reaction temperatures. Therefore, Mn TPP/CTS is a highly selective catalyst for the aerobic oxidation of toluene. 3.6. Comparison between Mn TPP and Mn TPP/CTS in catalysis 3.4. Influence of reaction pressure on toluene oxidation Although the two catalysts have close selectivity for The effects of the reaction pressure on the product selectivity benzaldehyde and benzyl alcohol, Mn TPP/CTS is a more (aldehyde + alcohol) and toluene conversion in the toluene active catalyst than Mn TPP under the same reaction conditions oxidation are shown in Fig. 4. The influence of air pressure on (see Fig. 5). When the catalytic oxidation was allowed to the selectivity in catalytic toluene oxidation is unremarkable proceed for 2 h, the catalytic activity for the supported catalyst and similar to the effect of reaction temperature, i.e. the reached its maximum, 1.03 Â 106% molÀ1 hÀ1, which was selectivity remains at 90 Æ 5% throughout with each of the greater than the corresponding value (0.35 Â 106% molÀ1 hÀ1) three air pressures, and the molar ratios of aldehyde/alcohol are for the unsupported catalyst; at this time, the supported catalyst all about 1. However, when the pressure in the reaction system gave the highest selectivity and the greatest toluene conversion. is increased from 0.4 to 0.6 MPa, the toluene conversion Such results indicate that Mn TPP/CTS has a higher catalytic increases from 2.4 to 5.9%. This is because the higher the air activity for the aerobic oxidation of toluene than Mn TPP. The pressure, the higher the oxygen solubility in the liquid phase. result is in good agreement with the catalytic activity order for cyclohexane oxidation reported in our previous work [23]. Therefore, we suggest that there exists a mechanism involving chitosan-offered assistance to the Mn TPP in aerobic oxidation of toluene catalyzed by Mn TPP/CTS. The assistance action can be seen from Fig. 5; it evidently shortens the reaction time, achieving the highest selectivity and the greatest toluene conversion (compared with that of Mn TPP). Chitosan allows Mn TPP to be easily recovered (see Fig. 2b) and the recovered catalyst can catalyze the aerobic oxidation of toluene to the

Table 1 Effect of the amount of catalyst on the toluene oxidation The supported Selectivity Toluene Turnover catalyst* (mg) (%) conversion (%) numbers (Â105) Fig. 4. Changes in toluene conversion ((~) 0.4 MPa, (*) 0.6 MPa, (^) 4 92.3 2.86 0.92 0.8 MPa) and in selectivity (aldehyde + alcohol) ((~) 0.4 MPa, (*) 2 95.8 5.85 3.74 0.6 MPa, (^) 0.8 MPa) with reaction time for toluene oxidation catalyzed 1 92.3 2.71 3.48 by Mn TPP /CTS at different air pressures. Reaction conditions: toluene: 200 ml; catalyst: 2 mg (containing Mn TPP); temperature: 195 8C; airﬂow: Experimental conditions: *catalyst: amount of Mn TPP contained; toluene: 0.02 m3/h. 200 ml; temperature: 195 8C; air pressure: 0.6 MPa; reaction time: 2 h. 86 G. Huang et al. / Applied Catalysis A: General 338 (2008) 83–86

Startup Foundation (No. DD040025) and the Experimental Innovation Project Foundation of Guangxi University, P.R. China.

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