Journal of Energy Chemistry 22(2013)659–664

Sulfonated carbon catalyzed oxidation of aldehydes tocarboxylic acids by hydrogen peroxide

Lipeng Zhoua, Beibei Donga, Si Tanga, Hong Maa, Chen Chenb, Xiaomei Yanga∗, Jie Xuba. College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China;

b. State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,Chinese Academy of Sciences, Dalian 116023, Liaoning, China[Manuscript received June 6, 2012; revised September 9, 2012 ]

AbstractSulfonated carbon as a strong and stable solid acid catalyst exhibited excellent catalytic performance in various acid-catalyzed reactions. Here,sulfonated carbon, as catalyst for oxidation reaction, was prepared via the carbonization of starch followed by sulfonation with concentratedsulfuric acid. N2 physisorption, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray fluorescence and acid-base titration wereused to characterize the obtained materials. The catalytic activity of sulfonated carbon was studied in the oxidation of aldehydes to carboxylicacids using 30 wt% H2O2 as oxidant. This oxidation protocol works well for various aldehydes including aromatic and aliphatic aldehydes.The sulfonated carbon can be recycled for three times without obvious loss of activity.

Key wordsaldehyde; carboxylic acid; oxidation; hydrogen peroxide; sulfonated carbon

1. Introduction

The transformation of aldehydes to carboxylic acids iswidely used in organic synthesis and fine chemical produc-tion. The conventional oxidants used for this reaction aremainly Mn and Cr based reagents, which produce copiousamounts of undesirable wastes [1−3]. Other oxidants suchas hydrogen peroxide [4−7], H5IO6 [8], oxone [9], calciumhypochlorite [10], 2-hydroperoxyhexafluoro-2-propanol [11]and O2 [12,13] have been reported as oxidants in this reac-tion. Among these oxidants, aqueous hydrogen peroxide hasgained much interest because of its safety, low cost, high oxy-gen content, and environmentally friendly nature as water isthe sole by-product [14]. However, the reactivity of aque-ous H2O2 is insufficient for the direct oxidation of aldehy-des to carboxylic acids. Therefore, hydrogen peroxide wasoften used as oxidant for the oxidation of aldehydes to car-boxylic acids in combination with other reagents as cata-lysts including base [15], acid [5], transition metals [4,6,16]and N-methylpyrrolidin-2-one hydrotribromide [7]. However,these systems are associated with the drawbacks such as therequirement of strong acidic/basic conditions, use of toxic andheavy metals, and difficult regeneration and reuse. Therefore,

mild, catalytic, economic and efficient alternativemethods arerequired.

Recently, sulfonated carbon materials have attractedmuch attention due to their strong and stable solid acid prop-erties, which have been employed as adsorbents [17], com-ponents for proton-conducting electrolyte membranes [18,19]and polymer electrolyte fuel cells [20]. As acid cata-lysts, sulfonated carbon materials exhibited excellent catalyticperformance in various acid-catalyzed reactions, includingetherification of alkene with alcohol [21], esterification re-action [22−24], hydrolysis reaction [25], Aldol condensation[26], dehydration [27], silylation of alcohols and phenols [28],Friedel-Crafts reaction [29,30] and Biginelli reaction [31]. Asit is known, peroxysulfuric acid is a strong oxidant which canbe formed through the reaction of sulfuric acid with hydro-gen peroxide [32]. Sulfonated materials have been reportedas catalysts in the oxidation of sulfides to sulfones, tertiaryamines to N-oxide, secondary alcohols to esters/lactones andaldehydes to methyl esters [33,34].

Here, we studied the catalytic performance of sulfonatedcarbon in the oxidation of aldehydes to the corresponding car-boxylic acids with H2O2. The sulfonated carbon is found tobe an efficient, easy recyclable heterogeneous catalyst for theoxidation of aldehydes to carboxylic acids by H2O2.

∗ Corresponding author. Tel: +86-371-67781780; E-mail: yangxiaomei@zzu.edu.cnThis work was supported by the National Nature Science Foundation of China (J1210060, 21143002).

Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.

660 Lipeng Zhou et al./ Journal of Energy Chemistry Vol. 22 No. 4 2013

2. Experimental

2.1. Materials

All the aldehydes were purchased from Aladdin-reagentCompany (China) and used without further purification.Starch (AR grade) was used as the starting material in thepreparation of carbon material.

2.2. Preparation and characterization of sulfonated carboncatalysts

In a typical procedure, starch was heated in a quartz tubefrom room temperature to 723 K at a rate of 10 K·min−1 inthe flowing N2. And then the temperature was maintained at723 K for 5 h. The obtained carbon material (5 g) was heatedfor 15 h in concentrated sulfuric acid (98%, 100 mL) at de-sired temperature under N2 to introduce sulfonic acid groups(–SO3H). After cooling to room temperature, the suspensionwas filtered to yield black precipitate which was washed re-peatedly with boiling distilled water until the sulfate ions inthe washed water were no longer detected by the solution ofbarium chloride (0.02 mol·L−1). The sample was finally driedovernight at 353 K to afford the sulfonated carbon catalyst.The obtained sulfonated carbon was denoted as CS-x, wherex was denoted as the temperature for sulfonation.

The density of –SO3H was calculated according to thesulfur content of the sample determined by X-ray fluorescence(XRF) using a Philips Margix X-ray fluorescence spectrome-ter. The total strong acid density (–SO3H and –COOH) wasestimated by the exchange of protons with Na+ in aqueoussolution of NaCl (2 mol·L−1). After the sulfonated carbonwas filtrated, the acid amount in the solution was measuredby the titration with NaOH (0.1 mol·L−1). The total con-tent of –SO3H, –COOH and –OH was obtained by titratingthe suspension of sulfonated carbon in excess 0.1 mol·L−1NaOH with 0.1 mol·L−1 HCl [35]. Powder X-ray diffraction(XRD) was performed on a Panalytical X’pert PRO instru-ment with Cu Kα (λ = 0.15418 nm) radiation. Tube voltageand tube current were 40 kV and 40 mA, respectively. Fouriertransform infrared (FT-IR) spectroscopy was performed on aBruker Tensor 27 FT-IR spectrometer with 32 scans for a reso-lution of 4 cm−1 in KBr media at room temperature. The BETsurface area and pore volume of the samples were determinedby N2 physisorption at 77 K on an Autosorb-1 Quantachromeinstrument.

2.3. Oxidation of aldehydes

The oxidation of aldehydes was performed in a 25 mLround-bottomed flask equipped with a magnetic stirring barand a reflux condenser. Initial attempts to optimize the reac-tion conditions for the oxidation of aldehydes were performedwith benzaldehyde as a model reactant. 20 mmol benzalde-hyde, desired amount of oxidants and catalysts were added

into the flask. The flask was heated to the reaction tempera-ture with stirring, and the temperature was held until the re-action was stopped. After the reaction, the flask was cooleddown to room temperature and the yield of benzoic acid wasdetermined by the internal standard method using methylben-zene as the internal standard on a gas chromatograph witha flame ionization detector. For the oxidation of other alde-hydes, aldehyde (10 mmol), 30 wt% H2O2 (15 mmol), sul-fonated carbon (5 wt%) and acetic acid (5 mL) as solventwere charged into the flask. The mixture was heated at 363 Kwith stirring for 7 h and then cooled down to room temper-ature. The solvent was removed through rotary evaporation.The pH value of the residual was adjusted to 8.0 with NaOH(2 mol·L−1), and then it was extracted with ethyl acetate. Theaqueous layer was acidified to pH=2.0 using HCl (6 mol·L−1)and extracted with ethyl acetate. The organic layer was re-moved ethyl acetate through rotary evaporation to get the car-boxylic acid.

3. Results and discussion

3.1. Samples characterization

The XRD patterns of carbon before and after sulfonationare shown in Figure 1. For each sample, two broad peakswere observed. One broad peak at 10o−30o is correspondingto the amorphous carbon composed of aromatic carbon sheetoriented in a considerably random fashion [35]. The distanceof (0 0 2) plane was 4.13 A calculated from Bragg equation.After sulfonation, the (0 0 2) peak shifted to higher 2θ an-gle, and the distance of (0 0 2) plane was 3.75 A. This impliesthat the sample tends to form graphene in the sulfonation pro-cess; however, the framework is still amorphous and far fromgraphitization (d002 = 3.354 A). The weak and broad peak at2θ of 35o−50o is ascribed to the (1 0 1) plane (d101 = 2.14 A),representing the a axis of the graphite structure [36]. Thistype of XRD patterns is known to be characteristic to the car-bon states described as clusters composed of small fragmentsof graphene planes plus some amount of disorganized carbon.

Figure 1. XRD patterns of CS-423 (1) and carbon (2)

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Figure 2 presents the FT-IR spectra of carbon sulfonatedat different temperature. The –SO3H group can be identifiedby the peaks at 1226, 1167 and 1127 cm−1. Additionally,the peak at 1031 cm−1 is attributed to the S = O stretchingmode of –SO3H group [36]. These absorption bands related to–SO3H group imply that –SO3H groups are functionalized oncarbon. Additionally, these bands gradually became apparentwith the increase of sulfonation temperature, indicating theincrease of the density of –SO3H group.

Figure 2. FT-IR spectra of carbon (1), CS-353 (2), CS-403 (3) and CS-423 (4)

The absorption bands of other functional groupswere alsoobserved in the spectra. The band at 1403 cm−1 is relatedto the stretching of C = C. The absorption bands at 3436 and1573 cm−1 can be assigned to O-H stretching and bend vibra-tions, respectively. The bands at 1632 and 1702 cm−1 are as-cribed to the stretching and bend vibrations of C = O of thecarboxylic groups [23,34]. The absorption bands of C = Oalso became stronger after sulfonation; the reason is probablythat more carboxylic groups were formed through the oxida-tion of carbon surface by the concentrated sulfuric acid duringsulfonation.

The BET surface area and total pore volume of the sam-ples determined by N2 physisorption are summarized in Ta-ble 1. The BET surface area and pore volume of the par-ent carbon were 100 m2·g−1 and 0.06 mL·g−1, respectively,which would decrease significantly after sulfonation. Thedrop of these parameters is attributed to partial oxidation, con-densation, carbonization and partial destruction of the porousstructure during sulfonation. CS-403 had the lowest surfacearea (∼0). With further increasing sulfonation temperature,the surface area increased slightly which can be attributed tothe generation of some pores by deep oxidation of the concen-trated sulfuric acid at high temperature. The small surface areaof sulfonated carbon (2 m2·g−1) prepared by sulfonation ofthe partially carbonized cellulose was also observed by Sug-anuma et al. [35]. The acid densities of the sulfonated materi-als are also listed in Table 1. The amount of –SO3H increasedwith elevating the sulfonation temperature. Meanwhile, the

contents of –COOH and –OH also showed the similar changetrend due to the oxidation of the surface groups by concen-trated sulfuric acid, which was consistent with the drop of thesurface area after sulfonation.

Table 1. Physical properties of sulfonated carbon materials

Density of acid sites Pore BETCatalysts (mmol·g−1) volume surface area

–SO3H –COOH –OH (mL·g−1) (m2·g−1)Carbon – 0.08 1.10 0.060 100CS-353 0.44 0.02 1.51 0.014 2.1CS-403 0.66 0.14 1.91 0.035 ∼0CS-423 0.75 0.25 2.12 0.010 1.2

3.2. Catalytic activity

Sulfonated carbon is usually used as a strong protonicacid catalyst in various acid-catalyzed reactions. Here, sul-fonated carbon was applied as catalyst for oxidation reaction.The catalytic performance of sulfonated carbon was firstly in-vestigated in the oxidation of benzaldehyde to benzoic acidwith H2O2. It can be seen from Figure 3 that sulfonated car-bon can catalyze the oxidation of benzaldehyde under mildconditions. And the conversion of benzaldehyde increasedwith the elevation of sulfonation temperature. Using 5.0 wt%of CS-423 as catalyst, the conversion of benzaldehyde reached68% at 333 K with the reaction time of 7 h. As discussedabove, the sulfonation temperature influenced the density of–SO3H, which was increased with the sulfonation tempera-ture. The dependence of the activity on the sulfonation tem-perature proved that –SO3H groups were the main active sitesof the catalyst. Additionally, sulfonated carbon had plenty of–COOH and –OH. To verify the function of these groups inthe reaction, the catalytic activities of NaHSO4, carbon andcarbon-NO (synthesized by oxidation of carbon with HNO3;densities of –COOH and –OH were 1.22 and 2.44 mmol·g−1,respectively) were studied. It can be seen that carbon showed

Figure 3. The performance of benzaldehyde oxidation catalyzed by differentcatalyst. Reaction conditions: benzaldehyde (20 mmol), catalyst (5 wt%),30 wt% H2O2 (30 mmol), 333 K, 7 h. Blank denoted without catalyst;carbon-NO denoted carbon oxidized by concentrated nitric acid at 343 K for2 h

662 Lipeng Zhou et al./ Journal of Energy Chemistry Vol. 22 No. 4 2013

slightly catalytic activity as compared with the blank experi-ment. The catalytic activity of carbon-NO was much higherthan that of carbon. NaHSO4 exhibited similar activity withCS-423. These results suggest that –COOH and –OH alsoshow activity in the oxidation of aldehyde to some extent, butthe catalytic activity of –COOH and –OH is lower than that of–SO3H.

Since CS-423 presented the best catalytic activity in theoxidation of benzaldehyde, it was selected for further stud-ies. The reaction parameters, such as the amounts of catalystand oxidant, temperature and time, were optimized for the ox-idation of benzaldehyde, and the results are summarized inTable 2. The conversion of benzaldehyde was 45% in thepresence of 0.5 wt% CS-423. With the increase of catalystamount to 2.5 wt%, the conversion of benzaldehyde was im-proved to 64%. When the amount of the catalyst was furtherincreased, the conversion of benzaldehyde improved slowly,especially at the amount of catalyst exceeding 5 wt%. In viewof the cost and efficiency of the catalyst, 5 wt% catalyst wasmore suitable for the reaction. It can also be observed fromTable 2 that the conversion of benzaldehyde increased withthe increase of H2O2 amount. When its amount exceeded 1.5equiv., the increase of the conversion was negligible. So 1.5equiv. of H2O2 was the most proper amount for the reaction.As the temperature increased, the conversion of benzaldehydealso increased. In the initial stage, the increase of the conver-sion was quick; when the temperature was above 353 K, theactivity dropped slightly. It is known that high temperaturemakes hydrogen peroxide decompose fast, which renders theconversion of benzaldehyde decreasing at high temperature.When the reaction time was concerned, during the initial 7 hthe conversion of benzaldehyde increased steadily and then re-mained unchanged with further increasing the reaction time.So, the best reaction time for the oxidation of benzaldehyde tobenzoic acid was 7 h.

Table 2. Oxidation of benzaldehyde to benzoic acid with 30 wt% H2O2catalyzed by CS-423 under different conditionsa

CS-423 H2O2 Temperature Time ConversionEntry(wt%) (mmol) (K) (h) (%)

1 0.5 30 333 7 452 2.5 30 333 7 643 5 30 333 7 664 10 30 333 7 695 5 20 333 7 546 5 25 333 7 607 5 35 333 7 678 5 30 343 7 719 5 30 353 7 8510 5 30 373 7 8111 5 30 353 5 7512 5 30 353 6 8113 5 30 353 9 85a 20 mmol of benzaldehyde was used.

To explore the range of suitable substrates for this cat-alytic system, the oxidation of various aldehydes catalyzedby sulfonated carbon was investigated, and the results are

given in Table 3. It can be seen that benzaldehyde deriva-tives with an electron-withdrawing groups, such as –Cl, –Brand –NO2 (Table 3, Entries 2,3,7−10), were more reac-tive than those with electron-donating groups like –CH3, –OCH3 and –OH (Table 3, Entries 11−13). In addition,the position of the substituent also affected the reactivityof benzaldehyde derivatives. For example, the reactivity of2-chlorobenzaldehyde (Table 3, Entry 3) was lower than that

Table 3. Oxidation of aldehydes to the corresponding acids with H2O2catalyzed by CS-423a

Entry Organic substrates Products Yield (%)b

1 92, 90c

2 94

3 87

4 93

5 79

6 34

7 65

8 97

9 95

10 92

11 54

12 13

13 40

14 CH3(CH2)7CH2CHO CH3(CH2)7CH2COOH 7115 CH3(CH2)2CH2CHO CH3(CH2)2CH2COOH 54a Reaction conditions: substrate (10 mmol), CS-423 (5 wt%) and30 wt% H2O2 (15 mmol) in acetic acid (5 mL) at 363 K for 7 h;b Isolated yield; c Addition of 2 wt% hydroquinone as free-radicalscavenger

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of 4-chlorobenzaldehyde (Table 3, Entry 2); the conversionof monosubstituted nitrobenzene increased in the order of 2-nitrobenzene<3-nitrobenzene<4-nitrobenzene (Table 3, En-tries 8−10). For benzaldehyde derivatives with two sub-stituents, similar phenomenon was observed. The reactiv-ity decreased in the order of 3,4-dichlorobenzaldehyde>2,4-dichlorobenzaldehyde>2,6-dichlorobenzaldehyde (Table 3,Entries 4−6). These results indicate that the substituent at theortho-position of aldehyde group hinders its reactivity con-siderably. Same phenomenon was also observed in the ox-idation of benzaldehyde derivatives with hydrogen peroxidecatalyzed by N-methylpyrrolidin-2-one hydrotribromide [7].

Our catalytic system was also effective for the oxidationof aliphatic aldehydes. For example, the yields of 71% and54%were obtained for the oxidation of decanal and valeralde-hyde to their corresponding acids (Table 3, Entries 14,15).

The reusability of a solid catalyst is very important, so therecycling of the catalyst was performed. After the first reac-tion run using CS-423 catalyst at 333 K for 7 h, the solid cat-alyst was filtered. The separated catalyst was washed twicewith acetone, dried at 373 K for overnight. The regeneratedcatalyst was used for the recycling study. Figure 4 shows thereusability of sulfonated carbon in the oxidation of benzalde-hyde with H2O2. The catalytic activity slightly decreased inthe first recycling study, and almost kept constant in the fol-lowing two runs. The amount of –SO3H decreased about 10%after the first run, which should account for the decrease ofactivity of recycled catalyst. These results demonstrate thatthe sulfonated carbon is a recyclable catalyst for the oxidationof aldehyde to acid.

Figure 4. Recycling studies of sulfonated carbon in the oxidation of ben-zaldehyde to benzoic acid with H2O2 catalyzed by CS-423. Reaction condi-tions are similar to those given in Figure 3

To identify the reaction mechanism, hydroquinone, a free-radical scavenger, was added in the reactant (see Table 3, En-try 1). The oxidation of benzaldehyde was not affected byradical scavenger, implying that the reaction is a non-radicalmechanism, which is different from Fenton oxidation processusing H2O2 as oxidant [37]. Considering the electronic andsteric effects of substituents, a possible mechanism was sug-gested (Figure 5). At first step, H2O2 and sulfonated car-bon form the intermediate peroxysulfuric acid, which reacts

with aldehyde to give the intermediate peroxyacetal. Finally,peroxyacetal decomposes to acid and supported sulfonic acidvia a six member ring. The substituent at the ortho-positionof aldehyde group would restrict the formation of peroxyac-etal, thus the reactivity of aldehyde decreased. The electron-withdrawing group on benzene ring resulted in the electrondeficiency on the carbon of aldehyde group, which promotedthe homolysis of O–O bond to form acid.

Figure 5. Possible reaction mechanism for the oxidation of aldehyde to acidwith H2O2 catalyzed by sulfonated carbon

4. Conclusions

In conclusion, sulfonated carbon is an effective cata-lyst for the oxidation of aldehydes, including aromatic andaliphatic aldehydes, to the corresponding acids by 30 wt%H2O2. The primary active center of sulfonated carbon forthe oxidation of aldehyde using H2O2 as oxidant is –SO3H.Sulfonic group reacts with H2O2 to form peroxysulfuric acidfirstly, which in turn oxidizes aldehyde. To further improvethe catalytic activity, the increase of the density of sulfonicgroup on carbon material is under investigation.

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