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A Differential Scanning Calorimetric (DSC) Study on Heavy Ozonized C60 Fullerene ab

Franco Cataldo

& Susana Iglesias-Groth

c

a

Actinium Chemical Research srl, Via Casilina, Rome, Italy

b

Università della Tuscia, Dipartimento di Scienze Ecologiche e Biologiche, Viterbo, Italy

c

Departamento de Astrofısica, Universidad de La Laguna (ULL), La Laguna, Spain Accepted author version posted online: 07 Jul 2014.Published online: 04 Sep 2014.

To cite this article: Franco Cataldo & Susana Iglesias-Groth (2015) A Differential Scanning Calorimetric (DSC) Study on Heavy Ozonized C60 Fullerene, Fullerenes, Nanotubes and Carbon Nanostructures, 23:3, 253-258, DOI: 10.1080/1536383X.2014.905771 To link to this article: http://dx.doi.org/10.1080/1536383X.2014.905771

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Fullerenes, Nanotubes and Carbon Nanostructures (2014) 23, 253–258 Copyright © Taylor & Francis Group, LLC ISSN: 1536-383X print / 1536-4046 online DOI: 10.1080/1536383X.2014.905771

A Differential Scanning Calorimetric (DSC) Study on Heavy Ozonized C60 Fullerene FRANCO CATALDO1,2 and SUSANA IGLESIAS-GROTH3 1

Actinium Chemical Research srl, Via Casilina, Rome, Italy Universita della Tuscia, Dipartimento di Scienze Ecologiche e Biologiche, Viterbo, Italy 3 Departamento de Astrofısica, Universidad de La Laguna (ULL), La Laguna, Spain 2

Downloaded by [Dr. Franco Cataldo] at 13:31 15 September 2014

Received 8 March 2014; accepted 15 March 2014

The DSC (Differential Scanning Calorimetry) analysis of heavy ozonized C60 fullerene (known also as “fullerene ozopolymer”), shows the exothermal decomposition of this compound with a peak at 158 C with a decomposition enthalpy DHdec ¡322 kJ/mol and an activation energy for the decomposition E# D 71 kJ/mol. These experimental facts confirm the secondary ozonide/peroxidic and polymeric nature of fullerene ozopolymer originally suggested by the infrared spectroscopy analysis and solid state 13C-NMR spectroscopy. It is also shown that the secondary ozonide group of fullerene ozopolymer can be reduced selectively by a treatment with hydrogen iodide. Keywords: fullerene, ozonation, ozonide, DSC, HI, reduction, FTIR

Introduction The steady interest on fullerene oxides and on the reaction products between fullerenes C60 and C70 and ozone is testified by a series of review articles appeared in the last decade, showing also the complexity of the topic (1–4). The latest review (1), just published, is comprehensive of all literature appeared on this field but is not including the fate of fullerenes in simulated release in the environment (environmental degradation) (5, 6) and the fullerene radiolysis in aqueous medium which yields an oxidized fullerene resembling for certain instances heavy ozonized fullerene (7). The purpose of the present article is to focus on heavy ozonization product of C60 fullerene, referred sometimes as “fullerene ozopolymer” (also hereinafter in this paper), which forms spontaneously as a consequence of prolonged ozonization of C60 solutions for example in toluene or other solvents (8–12). The early stages of ozonation of C60 were clarified some time ago and involve the formation of a molozonide which is unstable and decomposes into C60 epoxide without yielding the secondary ozonide (13). Fullerene polyepoxides were found by prolonging the reaction with ozone (2) and only at later ozonation stages there is the precipitation of an insoluble brown product very rich in oxygen content (9, 10). Although the chemical structure of this reaction product of fullerene and

Address correspondence to Franco Cataldo, Actinium Chemical Research srl, Via Casilina 1626A, 00133 Rome, Italy. E-mail: [email protected] Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/lfnn.

ozone is not yet fully understood (1), the product is fully reproducible and well characterized in its physical properties like, for example, the infrared spectrum. Based on the excellent literature review made by Bulgakov and colleagues (1), it is possible to affirm that the fullerene ozopolymer is made by open cage fullerene units (like for example that with fullerene diketone or dicarboxyl structure) but kept together by secondary ozonide bridges and/or other peroxide species (9, 10). After all, the laser ablation-mass spectrometry of fullerene ozopolymer has demonstrated that the fullerene cages are restored back (14), thus the degradation of the fullerene structure should not be considered radical otherwise there should not be the re-formation of C60 and other carbon clusters by laser irradiation. Another aspect often neglected is the C13-NMR spectrum of the fullerene ozopolymer which clearly suggests that it is made prevalently by sp2-hybridized carbon atoms with a resonance at 127.5 ppm, while the other resonance at 170 ppm it must attributed to carboxylic carbon (8) rather than to ketones and aldehydes which instead have their NMR resonances at lower fields (15). At the time of earlier investigation of fullerene ozopolymer, the presence of secondary ozonides and other peroxides linking together the (open) fullerene cages, together was detected by FT-IR spectroscopy as an absorption band at 1090 cm¡1 which was removed by titration with iodine (9), reduction with Zn dust and acetic acid or with hydrogen sulphide (10), and decomposed by thermal treatment (9, 10). DSC is a very powerful analytical technique for the detection of ozonides and peroxides (16) and recently was used successfully in monitoring the formation and concentration of the ethyl oleate ozonides (17). Furthermore, the thermochemistry of ozonide (and related peroxides) is well defined (18) and

254 can be used for quantitative analysis. Thus, the present work is essentially dedicated to the DSC analysis of fullerene ozopolymer.

Experimental

Cataldo and Iglesias-Groth mixture was left diluted with 500 mL of distilled water, stirred and left overnight. A precipitate was formed of the reduced former fullerene ozopolymer. Part of the water in excess was decanted and the precipitate was collected by filtration, washed repeatedly with water, and left to dry in air at 45 C. The recovered sample weights 57 mg.


Materials and Equipment Fullerene C60 was high purity grade from MTR Ltd. (USA). Hydrogen iodide 67% solution and toluene analytical grade were obtained from Aldrich (USA). Chloroform (trichloromethane) was obtained from Fluka (Switzerland); it was a spectrophotometric grade stabilized with 1% ethanol. Ozone was produced in the usual manner by passing a stream of dried oxygen through a corona discharge. The differential scanning calorimetry (DSC) study was performed on a Mettler-Toledo DSC-1 Star System. Different heating rates were used as stated in the text and the samples were heated under nitrogen flow 3 L/hour using completely sealed medium pressure stainless steel crucibles (sample size was approximately 5 mg). To observe the fullerene ozopolymer decomposition at the DSC, it is necessary to use the stainless steel medium pressure crucible. In fact, the use of conventional aluminum crucible with open cap does not permit to detect very clearly and quantitatively the fullerene ozopolymer decomposition. The FT-IR spectra were recorded on Nicolet 6700 spectrometer from Thermo-Scientific. The mid infrared spectra were recorded in transmittance mode with the samples embedded in a KBr pellet or in reflectance mode using a ZnSe crystal and a horizontal attenuated total reflectance attachment. Synthesis of C60 Fullerene Ozopolymer in Toluene It was performed as described in earlier works (9, 10). In brief, 210 mg of C60 were dissolved in 250 mL of toluene. Ozone with oxygen was bubbled into the solution at a nominal rate of 0.5 g O3/hour for 105 min, until it was evident the precipitation of a product from the solution. The reaction mixture was left overnight and then it was filtered recovering 310.4 mg of dried, dark-brown product with a characteristic penetrating smell. Synthesis of C60 Fullerene Ozopolymer in CHCl3 C60 (90 mg) was sonicated for 1 hour with 250 mL of CHCl3 in order to obtain a solution/suspension of C60 in the solvent. Ozone with oxygen was bubbled into the solution at a nominal rate of 0.5 g O3/hour for 110 min. After this treatment chloroform was evaporated in a water bath at 75 C leaving 124 mg of a brown residue which was characterized by DSC and FT-IR. Reduction of C60 Fullerene Ozopolymer with HI C60 fullerene ozopolymer (91 mg) prepared in toluene was finely ground in a mortar and transferred into a flask. It was treated with 10 mL of HI 67% and heated at about 100 C for 10 hours under magnetic stirring. After cooling, the reaction

Results and Discussion DSC Analysis of C60 Fullerene Ozopolymer The C60 fullerene ozopolymer was analyzed by DSC using special sealed stainless steel medium pressure crucibles. Figure 1 shows the DSC trace of fullerene ozopolymer prepared in toluene (blue trace) taken at a heating rate of 40 C/min. An exothermal decomposition occurs with the onset at 130 C and peak at 158 C followed by a long tail and a shoulder at about 238 C. An important amount of heat is released by this exothermal decomposition DHdec D ¡305 J/g. Fortunately, the exothermal decomposition shown in Figure 1, is very broad meaning that it occurs in a nonexplosive and safe way. Figure 1 also shows a second DSC scan (red trace) of the same fullerene ozopolymer sample which has already undergone a first DSC scan. After a first DSC scan with the exothermal peak, the second DSC scan is completely featureless demonstrating that the exothermal transition at 158 C is necessarily linked to the decomposition of a labile chemical specie like a secondary ozonide or a peroxide. Also the peak temperature at 158 C is the typical decomposition peak temperature of an ozonide (16, 17,19). In order to verify the solvent effect on the C60 fullerene ozopolymer properties, another ozopolymer sample was prepared in chloroform instead of toluene. Figure 1 also reports the DSC trace of this sample scanned at 20 C/min in a sealed stainless steel crucible (black trace). Also in this case, an exothermal decomposition occurs with an onset at 109 C and peak at 157 C. Although the exothermal decomposition peak is coincident with that of the fullerene ozopolymer sample prepared in toluene, the onset of the decomposition starts at lower temperature 109 C against 130 C for the previous sample. Also the amount of heat released in the decomposition reaction is lower for the fullerene ozopolymer prepared in CHCl3 DHdec D ¡223 J/g against DHdec D ¡305 J/g of the sample prepared in toluene. These differences may be explained by the lower solubility of C60 in CHCl3 than in toluene so that the ozonation starts in an heterogeneous solution in the former case and in an homogeneous solution in the latter case. Consequently, the ozopolymer prepared in CHCl3 is not fully ozonated to the same extent as the ozopolymer prepared in toluene leading necessarily to a lower decomposition enthalpy linked to a lower secondary ozonide/peroxide groups. Because of the above considerations, the following discussion will now focus only to the results on C60 fullerene ozopolymer prepared in toluene. The decomposition enthalpy of the fullerene ozopolymer prepared in toluene, DHdec D ¡305 J/g is the average of four different measurements made on the fullerene ozopolymer sample. Furthermore, since the weight increase of the

A Differential Scanning Calorimetric (DSC) Study on Heavy Ozonized C60 Fullerene

normalized Onset Peak

42-38 C60 ozopolymer 40°/min steel, 6,6 mg

normalized Onset Peak

2

255

304,90 J/g 130,15 °C 158,27 °C

223,40 J/g 109,46 °C 156,80 °C


W/g

77-38b C60 ozopolymer CHCl3 20°C/m steel, 4,3 mg

42-38 C60 ozopolymer exhausted 40°/m steel 6,6 mg 50

100

150

200

250

300

°C

Fig. 1. DSC blue trace of C60 fullerene ozopolymer prepared in toluene and heated in a sealed medium pressure steel crucible 40 C/ min under N2 flow. Note the exothermal peak at 158 C with onset at 130 C. The exothermal decomposition is shaded in blue. The red DSC trace at the bottom of the figure refers to fullerene ozopolymer prepared in toluene heated for the second time. This time, the DSC trace is featureless without any transition. The black DSC trace in the middle of the figure is due to a C60 fullerene ozopolymer sample prepared in chloroform.

fullerene ozopolymer sample prepared in the present work is C100.4 mg it is possible to calculate a C/O molar ratio of 2.8 in the product corresponding to an empirical formula of C60O20 to C60O22. It is important to note here that such empirical compositions were also found in our earlier work with a more affordable and sophisticated analysis made by an electron probe micro-analyzer (9). Assuming that we are dealing with a ozopolymer, then each monomeric unit could

be the oxygenated unit C60O20 to C60O22 with molecular weight comprised between 1040 and 1072 Da. Since the average decomposition enthalpy is DHdec D ¡305 J/g, we can transform this value in kJ/mol by multiplying it with the molecular weight of the monomer unit getting ¡317 to ¡327 kJ/mol. Thermochemical calculations show that the decomposition enthalpy of secondary ozonides as well as peroxides is comprised between ¡280 and ¡300 kJ/mol (18). Thus, the value found for the monomeric unit of fullerene ozopolymer DHdec D ¡317 to ¡327 kJ/mol matches surprisingly well the theoretical calculations, confirming by the entity of the released heat and by the decomposition temperature peak at 158 C that we are dealing with a secondary ozonide or with a peroxide. The activation energy for the decomposition of a given compound can be determined by DSC using different heating rates b and the Ozawa (20): .2:15Logb/.1/Tpeak / ¡ 1 D ¡ E/R

(1)

or Kissinger equation (19): [2:303Log.b=T2 /].1/Tpeak / ¡ 1 D ¡ E/R Fig. 2. Calculation of the activation energy for the decomposition of the fullerene ozopolymer using the Ozawa equation. From the slope, an activation energy of 71 kJ/mol was obtained.

(2)

Figure 2 shows a graph of the dispersion of the temperature decomposition peak as measured by DSC at 10, 20, 40,

256

Cataldo and Iglesias-Groth

and 60 C/min on the fullerene ozopolymer as function of the reciprocal temperature. A straight line is obtained whose slope gives the value E#/R and hence E# D 71 kJ/mol. Using the Kissinger equation the similar data treatment gives E# D 69 kJ/mol.

The FT-IR spectroscopy of C60 fullerene ozopolymer was discussed in more occasions, also comparing the spectra of the ozopolymer synthesized in different solvents (9–11). Here we limit ourselves to some key observations. The ketonic absorption band of pristine C60 fullerene ozopolymer (see Figure 3) is a broad band centered at 1736 cm¡1. Such a band position suggests the presence of aldehyde, ketones, and carboxylic acid groups. The aldehyde groups must be present as suggested by the broad aldehyde combination bands at 2600 and 2800 cm¡1 (21) and mainly by the aldehyde C–H rocking at 1384 cm¡1 sharp absorption band (21). Figure 3 shows that after the thermal treatment of C60 fullerene ozopolymer at 350 C in a sealed crucible under N2, the ketone band originally located at 1736 cm¡1 is split into three different bands

0.6 44-38 C60 ozopolymer produced in toluene 3406

1736

3218

0.5 0.4 Abs

1384 0.3

1623

2922

1102 1189

2792

734 695 671

0.2 0.1 0.0 0.6 44-38 C60 ozopolymer fm toluene heated 350°C N2 in DSC

1724

0.5 3414 0.4 Abs


FT-IR Spectroscopy on the Fullerene Ozopolymer Before and After the DSC Heating

at 1832, 1750, and 1724 cm¡1. This suggests, respectively, the presence of anhydride, lactone, carboxylic acid, and ketone groups. Since the C–H rocking band of aldhehyde group at 1380 cm¡1 appears drastically reduced in the spectrum of thermally treated fullerene ozopolymer, it is evident that the thermal treatment has reduced the relative concentration of this moiety. The consequence of the thermal treatment of fullerene ozopolymer is the disappearance of the bands at 1188 cm¡1 and mainly at 1102 cm¡1, a phenomenon already noticed in earlier works and already assigned to the decomposition of a secondary ozonide structure (9, 10). Based on the new DSC results presented in the preceding paragraph, this interpretation was correct. Furthermore, there is abundant literature about the infrared absorption bands of secondary ozonides (22) and the assignment of the 1102 cm¡1 band of the spectrum in Figure 3 to a secondary ozonide is now certain. Another interesting aspect of the fullerene ozopolymer spectrum after the thermal treatment and the removal of the ozonide group is the development of new infrared bands at 916 and 742 cm¡1 accompanied by the strong band at 1280 cm¡1. All these bands can be associated to the formation of epoxide groups as a consequence of the thermal treatment which has decomposed the ozonide/peroxydic groups.

1619

0.3

525

1228

3211

669 632

0.2 2918

1832

2579

701 742

1380

916

0.1 0.0 3500

3000

2500

2000

1500

1000

500

Wav enumbers (cm-1)

Fig. 3. FT-IR spectra in KBr: (top) C60 fullerene ozopolymer prepared in toluene; (bottom) C60 fullerene ozopolymer after the thermal decomposition in a DSC crucible. Note the complete disappearance of the infrared bands at 1185 and 1102 cm¡1 attributed to secondary ozonide and/or peroxides, after the thermal treatment.

A Differential Scanning Calorimetric (DSC) Study on Heavy Ozonized C60 Fullerene In fact, the 1280 cm¡1 could be associated to the ring breathing (21), while the band at 916 cm¡1 to the antisymmetric epoxy ring deformation while the band at 742 is due to the symmetric epoxy ring deformation (21). Thus, the thermal treatment of fullerene ozopolymer leads to an increase of its polyoxyde character.

Hydrogen iodide is a very versatile reagent and reducing agent. It was successfully used for the synthesis of hydrogenated fullerene C60H18 (23) and in the safe reduction of graphite oxide to graphene nanoribbons (24). In the present work, HI was used to selective reduce the secondary ozonide group of C60 fullerene ozopolymer. Figure 4 shows that the reduction was successful since the ozonide band at 1102 cm¡1 originally present in the fullerene ozopolymer was removed as a consequence of the HI treatment. The reduction was specifically directed toward the ozonide band with little changes on the other spectral features. The hydrogenation action exerted by HI is also manifested in Figure 4 by the development of a strong C–H stretching band at about 2900–3000 cm¡1 suggesting that HI has hydrogenated also some of the residual and available double bonds.

Conclusions When C60 fullerene undergoes a prolonged ozonation in organic solvents (for example, toluene or CHCl3), it yields a precipitate of an insoluble brown product which is known as “fullerene ozopolymer”. The chemical structure of this product is not yet fully understood as recently masterfully reviewed (1). To throw more light about this chemical puzzle, we were able to analyze with DSC the fullerene ozopolymer discovering that it decomposes exothermally at 158 C a typical decomposition temperature of the secondary ozonides (17,22) releasing a decomposition enthalpy of DHdec ¡322 kJ/mol and with an activation energy for the decomposition E# D 71 kJ/mol. The decomposition enthalpy is again in line with the typical decomposition enthalpy of ozonides as measured experimentally (17) and as calculated theoretically (18). Thus, the originally proposed structure of oxidized and partially opened fullerene cages kept together by secondary ozonide moieties or bis-peroxide structures appears confirmed by these new results. Furthermore, it is shown that the FT-IR band at 1102 cm¡1 is certainly due to a secondary ozonide group or to a bis-peroxide since it disappears as a consequence of the thermal annealing which destroys the ozonide in the DSC. It is also shown that the FT-IR band at 1102 cm¡1 can be selectively reduced by a treatment with HI.

0.55

0.50

0.45

0.40

0.35 Absorbance


Reduction of Fullerene Ozopolymer with Hydrogen Iodide

257

0.30

0.25

0.20

0.15

0.10

0.05

0.00 4000

3500

3000

2500

2000

1500

1000

Wav enumbers (cm-1)

Fig. 4. FT-IR spectra in KBr: (red curve) pristine C60 fullerene ozopolymer; (green curve) C60 fullerene ozopolymer after reduction with hydrogen iodide. Note the disappearance of the infrared band at 1102 cm¡1 (indicated by an arrow) after the HI treatment.

258


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