Oxidative degradation of carbon blacks with nitric acid

Carbon 40 (2002) 1447–1455

Oxidative degradation of carbon blacks with nitric acid II. Formation of water-soluble polynuclear aromatic compounds Katsumi Kamegawa a , *, Keiko Nishikubo a , Masaya Kodama b , Yoshio Adachi a , Hisayoshi Yoshida c a

National Institute of Advanced Industrial Science and Technology (AIST) Kyushu, Shuku, Tosu, Saga 841 -0052, Japan b AIST Tsukuba, Onogawa, Tsukuba, Ibaraki 305 -8569, Japan c Department of Industrial Chemistry, Tohwa University, Chikushigaoka, Minamiku, Fukuoka 815 -8510, Japan Received 8 August 2001; accepted 28 November 2001

Abstract Furnace black and acetylene black were oxidized with concentrated nitric acid at 100 8C for prolonged periods. The oxidized carbon black was dissolved / dispersed into alkaline solution and was size-fractionated into six fractions by ultrafiltration. The yields of the fractions revealed that oxidized furnace black contains oxygenated polynuclear aromatic compounds with a variety of molecular sizes, but oxidized acetylene black consists of only a great quantity of the largest size fraction, probably carbon black particles, and a scarce amount of the smallest size fraction. With oxidized furnace black, elemental compositions of all fractions except the largest molecular-size fraction were independent of the period of oxidation, suggesting that each fraction possesses a similar molecular structure. Noncarbon constituents such as oxygen and hydrogen increased with decreasing molecular size. The mean molecular weights of fractions were estimated to be in a range from ca. 400 to 1200 and more on the basis of elemental and functional group analyses. 13 C-NMR and IR analysis showed that the molecules of fractions comprise phenolic, carboxylic, nitro, perhaps quinonic carbonyl groups, and aromatic carbons, but no aliphatic carbons. Ultraviolet and visible spectra of fractions denoted that absorption at higher wavelengths increased with increasing the molecular weights, indicating extension in the conjugated aromatic ring system. On the basis of the experimental results molecular structure models for the fractions were proposed.  2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon black; B. Oxidation; C. Infrared spectroscopy; Nuclear magnetic resonance; D. Functional groups

1. Introduction A large number of experiments concerning the oxidation of carbon blacks have been carried out mostly to gain some insight into the surface and interior carbon structures [1–10], or rarely to yield degradation products such as mellitic acid [11]. In these studies formation of various aromatic compounds with mean molecular weights of ca. 400 and more was reported [2,5,7–11]. Akamatsu et al. described that on oxidation with potassium dichromate in phosphoric acid, carbon black and graphite were dispersed into solutions in the form of crystallites [2]. Voet et al. reported that oxidation of carbon black particles with nitric *Corresponding author. Tel.: 181-942-813-654; fax: 181942-813-696. E-mail address: [email protected] (K. Kamegawa).

acid yielded a formless material with a polyaromatic polyfunctional acidic nature [5]. These reports suggest the formation of large-size polynuclear aromatic compounds on oxidative degradation of carbon blacks. On the other hand, Fujii et al. oxidized mesophase pitch with a mixture of sulfuric and nitric acids to yield oxygenated polynuclear aromatic compounds, which was named ‘aqua-mesophase’ [12]. Ariwahjoedi et al. characterized the aqua-mesophase by titrimetry, IR spectrometry, elemental analysis, electrophoresis, TGA, and electron microscopy [13]. Authors have studied structural change of carbon blacks during oxidation with nitric acid [14] in an attempt to prepare water-soluble and large polynuclear aromatic compounds whose hydrophobic polynuclear aromatic rings are hemmed with many hydrophilic oxygenated functional groups. The aromatic compounds are expected to possess specific interfacial characters because of the peculiar

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00310-4

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structural arrangement of the hydrophobic and hydrophilic moieties. The present paper describes the formation and structures of water-soluble polynuclear aromatic compounds from carbon black on oxidation.

2. Experimental

2.1. Materials Two carbon blacks, SEAST300 (oil furnace black, Tokai Carbon Co., abbreviated as F-black) and DENKA BLACK (acetylene black, Denki Kagaku Kogyou Co., abbreviated as A-black), were used as starting materials. These carbon blacks have almost the same particle size, but their crystallographic structures are different; F-black (La : 1.7 nm, Lc : 1.5 nm) has a less developed carbon structure than A-black (La : 3.2 nm, Lc : 3.6 nm). These carbon blacks were air-dried at 120 8C for 2 h. For detailed physical properties of the carbon blacks, the preceding paper should be consulted [14]. Two commercially available humic acids, named Humic S (Sigma-Aldrich) and Humic W (Wako Pure Chemical Industries, Ltd.), were also employed as reference materials. In general, humic acid is an alkali soluble organic acid formed by degradation of natural organic materials under aerobic conditions. They consist of a skeleton of aliphatic / aromatic units substituted by oxygenated functional groups and interconnected through aliphatic, alicyclic, and ether linkages [15]. It is then interesting to compare the structure and character with those of the fractions separated from oxidized carbon blacks because it would highlight the influence of the presence of polynuclear aromatic moieties in the fractions. Humic S and W were fractionated before usage mainly to reduce inorganic impurities as follows: humic acid (5 g) was dissolved into 0.1 N NaOH solution (500 cm 3 ), and the pH of the solutions were adjusted to pH 3 for Humic S, and to pH 7 for Humic W by adding 0.1 N HCl. The precipitate formed at each pH was separated by centrifugation, and the supernatant solution was acidified to pH 1 for Humic S, and to pH 3 for Humic W. The humic acid precipitated was washed repeatedly with 1 N HCl to exchange the sodium ions held on the humic acid for hydrogen ions and dried at 60 8C under vacuum. The yields of Humic S and W were 50 and 64%, respectively.

2.2. Oxidation Carbon black (10 g for F-black and 6 g for A-black) was mixed with concentrated nitric acid (content 61%, 100 cm 3 ), and the mixture was heated at 100 8C for a prescribed period up to 1000 h while stirring. The oxidation product was centrifuged to separate acid-insoluble substances (AI) from acid-soluble ones (AS). AI was washed with water several times, and the residual was vacuumdried at 60 8C. AS was also recovered by evaporating a

mixture of the supernatant solution and washing water of AI under a reduced pressure at 60 8C.

2.3. Ultrafiltration AI was fractionated by using ultrafilters (Filtron Co.) with nominal molecular-weight cut-offs of 100,000, 30,000, 10,000, and 3000. The filters were reported to have channels of ca. 5, 2.5, 2, and 1 nm in diameter, respectively [15]. The separation procedure is as follows: AI (1 g) was dissolved / suspended into a 0.05 mol / dm 3 Na 2 CO 3 aqueous solution (100 cm 3 ) and was shaken at room temperature for 20 h. The solution / suspension was at first treated with a filter (100,000) under stirring. At this filtration, deposit on the filter was washed off repeatedly in order to avoid filtration of the constituents with the deposit. The filtrate was subsequently treated with a filter having the next lower cut-off character. By repeating a similar procedure AI was divided into six fractions as follows: fraction 1 (nominal molecular weight .100,000), fraction 2 (100,000–30,000), fraction 3 (30,000–10,000), fraction 4 (10,000–3000), fraction 5 (,3000, insoluble in 1 N HCl), and fraction 6 (,3000, soluble in 1 N HCl). Fractions were collected by acidifying suspension / solution to ca. 1 N HCl, washing with 1 N HCl, and drying at 60 8C under a reduced pressure. Fraction 6, which is soluble in 1 N HCl, was collected by evaporation at 60 8C after ionexchanging the sodium ions for hydrogen ions by using a H-type cation exchange resin. The yields of these fractions were presented on the basis of AI. These samples will be referred to as, e.g. F150F1; the first alphabetical letter, F, represents the type (furnace) of carbon black, the succeeding number, 150, is the period (hours) of oxidation, and the last one, F1, is fraction number.

2.4. Analysis 2.4.1. Functional group analysis Functional groups of fractions were quantitatively determined by both alkalimetry and elemental analysis by assuming that (1) hydrogen atoms present in the form of carboxylic and hydroxyl groups and not as aliphatic or aromatic hydrogen, (2) the acidic groups neutralized by pH 7 on alkalimetry are attributable to carboxylic groups [16–18], (3) nitrogen atoms present as nitro groups [19,20], and (4) the other functional groups are carbonyl groups, and other types of functional groups such as lactone and ester are negligible. Alkalimetry was carried out as follows: a sample (10 mg) was dispersed / dissolved 3 into 0.1 N KCl solution (20 cm ), and it was automatically titrated with 0.01 N NaOH solution with constant stirring in an atmosphere of nitrogen. The pHs of most suspensions / solutions before titration were about pH 3, and deflection points in the titration curves lay in a pH range of 7–8.

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2.4.2. Mass spectrometry A variety of mass analyses such as laser desorption ionization time of flight (LDI-TOF), matrix-assisted laser desorption ionization time of flight (MALDI-TOF), atmospheric pressure chemical ionization (APCI), electron spray ionization (ESI), and field desorption (FD) mass spectrometry were attempted to determine the molecularweight distributions of the fractions. As a result, only LDI-TOF-MS gave us suggestive mass spectra where a low molecular-weight region is reliable. A sample was dissolved into a dilute NaOH aqueous solution (100 mg / dm 3 ), and an aliquot (5 mm 3 ) was placed on a sample plate and dried at room temperature. LDI-TOF-MS was conducted in linear and positive ion modes by using a Voyager-DE RP Mass Spectrometer (PerSeptive Biosystems) equipped with nitrogen laser (337 nm). 2.4.3. NMR Solid-state 13 C-NMR spectra were obtained using a Bruker DSX-300 NMR spectrometer with a single pulse excitation pulse sequence [21,22] and high-speed magicangle spinning. A sample was packed in a 4-mm ceramic capsule, and the capsule was spun at rates up to 12.5 kHz using an air drive. A total of 1024 free induction decays with 2048 data points were accumulated with a 60-s repetition time. Chemical shifts were calibrated with respect to tetramethylsilane using glycine as a secondary standard. The curve-fitting of the NMR spectrum was performed using a Bruker line-fitting program with 70% Gaussian and 30% Lorenzian mixture functions for minimizing the error. 2.4.4. Other analyses X-ray scattering was obtained using a Rigaku RAD-B X-ray diffractometer system with Cu K a radiation. The least-squares analysis derived by Diamond [23] was applied for the 11 band to determine the size-distribution of graphitic planes of the pristine carbon black, F-black. IR spectra of fractions were recorded in air on an XNXUS 470 FT-IR spectrometer. A minimum of 32 scans were accumulated at a resolution of 2 cm 21 . The spectra were measured with KBr disks containing 0.03–0.3% samples; for a higher molecular weight fraction a reduced concentration disk was used in order to suppress high background absorption.

3. Results and discussion

3.1. Yields of fractions Oxidized carbon black was mixed with an alkaline solution and fractionated by ultrafiltration. F-blacks oxidized for long periods up to 200 h were found to comprise a wide variety of fractions, but A-black oxidized for 1000 h consisted of only fraction 1 (97%) and fraction 6 (3%).

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No formation of fractions 2–5 from A-black would be attributed to the developed crystallographic structure; that is, despite the prolonged oxidation for 1000 h the oxidation product still possesses larger graphitic planes compared with those for F-black. The characterization of fractions was, therefore, conducted only for the fractions separated from oxidized F-blacks. The yields of the fractions as a function of oxidation time are shown in Fig. 1. The yield of fraction 1, which might consist of oxidized carbon black particles and very large oxygenated aromatic compounds, decreased with the period of oxidation. The yields of fractions 2 and 3 showed maximums at about 70 and 150 h, respectively, and decreased thereafter. Fractions 4 and 5 increased steadily. These findings indicate progress of oxidative degradation of the carbon black particles into large polynuclear aromatic compounds and further into smaller ones. We carried out further experiments mainly with the fractions extracted from F150AI.

3.2. Elemental compositions of fractions The elemental compositions of fractions separated from both F30AI and F150AI are shown in Table 1 together with those for F150AS and the humic acids. Comparison of the compositions between the corresponding fractions of different oxidation periods revealed that, except for fraction 1 that might contain carbon particles, the elemental compositions of each fraction are kept almost constant irrespective of the oxidation time. This finding clearly shows that fractions with the same fraction number possess a similar molecular structure and the structure of each fraction is independent of the period of oxidation. The molecular size decreases in the following order, F1.F2. F3.F4.F5.F6.AS. Furthermore, the oxygen and hydrogen contents of the fractions increased with decreasing size of the molecules. This can be understood by presuming that the fractions consist of polynuclear aromatic compounds and the peripheries of the aromatic rings are full of functional groups. Thus, the smaller the molecular size, the larger the ratio of functional group content to the carbon content. The above speculation is reasonable because carbon blacks consist of graphitic layers, and functional groups are known to form preferentially at the edges of graphitic layers [24–26]. The carbon and oxygen contents of Humic W and S were similar to those of fractions 2 and 5, respectively, but their hydrogen contents were considerably higher than those of the corresponding fractions. The high hydrogen contents of the humic acids came from the structure that the humic acids consist of aliphatic / aromatic units and the unit size is small.

3.3. Functional groups of fractions Results of the functional group analysis of the fractions from F150AI are listed in Table 2. It indicated that as the

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Fig. 1. Yields of fractions from oxidized furnace blacks.

molecular size decreased, carboxylic groups increased markedly, and nitro groups increased gradually. Hydroxyl groups also increased from fraction 1 to fraction 5, but decreased greatly thereafter. The content of carbonyl groups did not show a regular change. These tendencies resemble the findings of other researchers [27,28]. IR spectra of F150F2, F4, F6, AS and Humic S are shown in Fig. 2. The assignment for typical bands was: 2920 and 2850 cm 21 (Humic S), stretching vibrations of aliphatic C–H species [29–32]; 1740–1720 cm 21 , stretching of C=O [29–35]; 1620–1600 cm 21 , stretching of C=C Table 1 Elemental analyses of fractions and humic acids C a (%)

O a (%)

H a (%)

N a (%)

Ash (%)

F30F1 F30F2 F30F3 F30F4 F30F5

77.2 63.8 62.4 61.4 57.3

21.5 33.8 35.0 36.1 40.4

0.5 0.9 1.0 1.1 1.2

0.8 1.5 1.6 1.4 1.1

0.4 0.3 0.1 0.1 0.1

F150F1 F150F2 F150F3 F150F4 F150F5 F150F6 F150AS

73.9 63.3 62.5 61.2 57.9 47.0 42.7

24.6 34.7 35.1 36.3 39.4 50.3 53.6

0.7 0.9 1.0 1.1 1.2 1.4 1.6

0.8 1.1 1.4 1.4 1.5 1.3 2.1

0.1 0.1 0.2 0.2 0.2 1.2 0.3

Humic W Humic S

64.7 57.5

31.4 38.0

2.5 3.7

1.4 0.8

0.5 1.3

a

Dry ash free.

[29–31,33–39]; 1560 and 1350 cm 21 , stretching of –NO 2 [34,40,41]; 1450–1430 cm 21 , in-plane deformation of O– H [32,42]; 1250–1200 cm 21 , stretching of C–O [30– 35,41–43]; and 917–860 cm 21 , out-of-plane deformation of O–H. The assumption that nitrogen atoms in fractions are present in the form of nitro groups was certified with the typical bands, 1560 and 1350 cm 21 , of nitro groups. Those bands were definitely observed especially for lowermolecular-weight fractions. The other assumption that no presence of aliphatic or aromatic hydrogen was not clearly supported by the IR spectra due to a broad absorption band, 3200–2600 cm 21 , by the stretching vibrations of O–H species. The background levels of the IR spectra increased as the molecular weight of the fractions increased. This is related to the development in the polynuclear aromatic ring system of the fractions [35]. With F150AS the IR spectrum in addition to the elemental composition [14] and the X-ray diffraction pattern [14] Table 2 Functional group analyses of fractions and AS

F150F1 F150F2 F150F3 F150F4 F150F5 F150F6 F150AS

Carboxyl (mmol / g)

Hydroxyl (mmol / g)

Carbonyl (mmol / g)

Nitro (mmol / g)

3.0 5.1 5.3 5.7 6.7 10.3 14.9

4.0 3.9 4.7 5.3 5.3 3.7 1.1

4.2 6.0 4.6 4.0 3.8 5.3 20.4

0.6 0.8 1.0 1.0 1.1 0.9 1.5

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Fig. 2. FT-IR spectra of fractions, AS, and Humic S.

was quite similar to that of mellitic acid, and then it was concluded that AS contains a greater amount of mellitic acid. Humic S showed a similar spectrum to those of the fractions except for the presence of the bands, 2920 and 2850 cm 21 , due to aliphatic C–H species, and no bands due to nitro groups. In an attempt to verify the functional group analysis, solid-state 13 C-NMR spectra of the crude oxidation product before fractionation, F150AI, and Humic S were

Fig. 3.

13

C NMR spectra of F150AI and Humic S.

acquired. The NMR bands shown in Fig. 3 were attributable as follows: 179 ppm, probably quinonic carbon [44], 166 ppm, carboxyl carbon, 156 ppm, phenolic carbon, 129 ppm, aromatic carbon, 87 ppm, ether or alcoholic carbon, 32 ppm, aliphatic carbon. The spectrum certified that the oxidation product, F150AI, is a mixture of aromatic compounds and contains no aliphatic moieties. This result contradicts the finding that these present aliphatic chains in the oxidative degradation products yielded in a carbon black / aqueous ozone system [9]. The curve-fitting of the

Fig. 4. Relationship between E /T and number of conjugated rings.

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Table 3 Average molecular weights of fractions and AS

F150F1 F150F2 F150F3 F150F4 F150F5 F150F6 F150AS

Edge C / total C

Number of rings

Molecular weight

0.202 0.332 0.334 0.354 0.407 0.701 0.828

(60) 19 19 17 12 3 1.7

(2550) 1150 1150 1100 900 500 400

NMR spectrum for F150AI quantitatively determined the percentages of various types of carbons as follows: quinonic carbon 9.5%; carboxylic acid 13.0%; phenolic carbon 11.6%, and aromatic carbon 65.9%. These contents were considerably consistent with those (quinonic carbon,

9.0%; carboxylic acid, 12.8% and phenolic carbon, 9.0%) calculated from the functional group analysis. This agreement proves the validity of the functional group analysis although this result does not exclude the possibility of the presence of other functional groups such as lactone, aldehyde, and ether.

3.4. Molecular weights and structures of fractions Since the fractions might be mixtures of polynuclear aromatic compounds whose edge carbon atoms are full of functional groups, a rough estimation of the molecular structures of the fractions can be made from the results of the elemental (Table 1) and functional group (Table 2) analyses. The ratio of the edge carbons to the total carbon atoms of polynuclear aromatic rings in a fraction, E /T, was calculated by the following equation: E /T 5 (COOH 1

Fig. 5. Average molecular structures of fractions.

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OH 1 CO 1 NO 2 ) /(C / 1.201 2 COOH), where C is the carbon content in percent, and COOH, OH, CO, and NO 2 are in mmol / g. A calibration curve of E /T to number of conjugated rings for typical aromatic hydrocarbons is shown in Fig. 4. The E /T calculated by the above equation, numbers of conjugated rings obtained from Fig. 4, and the average molecular weights for all fractions and AS are listed in Table 3. The structural models for fractions are depicted in Fig. 5. The structures were determined from both the numbers of conjugated rings (Table 3) and the result of functional group analysis (Table 2), and the average molecular weights in Table 3 were calculated from those molecular structures. The number of conjugated rings and molecular weight for fraction 1 are written in parentheses as the fraction is likely to contain oxidized carbon black particles. No difference in the structures of fractions 2 and 3 might be attributable to the result that small difference in analysis values such as E /T did not reflect the modeling of the structures. In an attempt to verify the molecular weights, laserdesorption-ionization time-of-flight mass spectrometry (LDI-TOF-MS) was applied and the results are shown in

Fig. 6. They present clear tendencies that the intensities of peaks of 300–500 m /z decreased as the molecular weights of the fractions increased and that molecular species larger than 500 m /z were hardly desorbed or ionized. Then, this mass spectrometry cannot be used to determine the whole molecular-weight distributions. However, as the spectra of fractions 2–5 having average molecular weights of 1150– 900 showed slight increases in the backgrounds in a mass range of ca. 500–1000 and as the spectra of fractions 1, 6 and AS (average molecular weights: 2550, 500, and 400, respectively) did not show such increases, we can say that the spectra of fractions 2–5 suggest the presence of molecules with the corresponding molecular weights. Some intense peaks in a range of 800–1200 were observed especially in fractions 1–5, but those peaks might be caused by pyrolysis of the samples by laser irradiation. The pyrolysis was presumed from the finding that Humic S showed almost similar peaks despite its quite different molecular structure from those of the fractions. Other experimental results described below also supported the molecular weights. The distribution of size of aromatic layers in the pristine carbon black, F-black, was estimated from X-ray measurements by using the Diammond method [23] and its result is denoted in Fig. 7. The figure revealed that the carbon black consists of aromatic layers with a variety of sizes and the aromatic layers with 1.5–2.0 nm in diameter occupied about one-half of the total carbon. On the other hand, the size of aromatic layers in fractions 2 and 3 was about 1.5 nm. This size is reasonable since the fractions were formed as a result of oxidative degradation of aromatic layers in the carbon black presumably from their edges. Ultraviolet and visible spectra of aqueous solutions (10 mg-sample / dm 3 -0.01 N NaOH) of fractions 1–6, AS, and Humic S and W are presented in Fig. 8. Fractions 1–6 and AS showed that the absorption decreased gradually as wavelength increased and that at higher wavelengths the

Fig. 6. LDI-TOF mass spectra of fractions, AS, and Humic S.

Fig. 7. Aromatic-layer-size distribution of pristine furnace black.

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Fig. 8. UV–Vis spectra of fractions and humic acids.

absorbance increased with increasing the molecular weight of the fractions. Development in a conjugated aromatic ring system seems to be well reflected in the absorption around 400–700 nm. This finding is consistent with the relation between development in an aromatic ring system and UV–Vis spectra [45]. The proximity of the absorption spectra of fractions 2–4 corresponds well with the close average molecular weights for those fractions, 1150–1110. The spectra of the humic acids presented different profiles. Consequently, the estimated molecular weights and structural models for fractions 2–6 would be appropriate.

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