Fullerene C60 Derivatives as High-Temperature ... - ACS Publications

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Fullerene C60 Derivatives as High-Temperature Inhibitors of Oxidative Degradation of Saturated Hydrocarbons Robert Czochara,* Jarosław Kusio, Michał Symonowicz, and Grzegorz Litwinienko* University of Warsaw, Faculty of Chemistry, Pasteura 1, 02-093 Warsaw, Poland S Supporting Information *

ABSTRACT: Fullerene C60 is a free-radical scavenger, but its antioxidant activity is limited to some polymers and hydrocarbons at high temperatures. Here we demonstrate that for hightemperature oxidation of saturated hydrocarbons, the conjugates of C60 with simple phenols are more active antioxidants than the building blocks (pristine C60 and phenols) used alone. The overall kinetic parameters calculated by the Ozawa−Flynn−Wall method for nonisothermal oxidation of model hydrocarbons: saturated stearic acid (STA) and polyunsaturated linolenic acid (LNA) indicate that C60 and its derivatives are effective antioxidants during oxidation of pure STA, but not during oxidation of LNA. These findings indicate that conjugates of C60 and phenols are candidates for potential use in base oils or lubricants as new hybrid antioxidants able to work at temperatures above 100 °C.

1. INTRODUCTION In the presence of oxygen, organic materials undergo autoxidationa process resulting in deterioration of a broad variety of goods like food, plastics, gasoline, oils, lubricants, and rubbers. All these materials contain hydrocarbons or lipids as a major constituent, and thus, their autoxidation can be described by the same mechanism with three main stages typical for the chain reaction. Primary free radicals (alkyl, hydroxyl, peroxyl, and other reactive oxygen species) are formed during the initiation as an effect of photolysis, thermolysis, or oxidation/ reduction reactions. These primary radicals can abstract hydrogen atom from C−H bond of a hydrocarbon and a new C-centered radical immediately reacts with another O 2 molecule (with the rate constant kox of about 109 M−1 s−1)1 giving peroxyl radical LOO•. The peroxyl radical abstracts a hydrogen atom from another lipid molecule (LH), and during the propagation step (described by the rate constant kp), the consecutive reactions of radical L• with oxygen (reaction 1, with reversible arrows for unsaturated hydrocarbons at higher temperatures)1 are followed by H atom abstractions (reaction 2).2 Reactions 1 and 2 are repeated several to hundreds of times, and thus, a large fraction of organic material can be converted into hydroperoxide, LOOH. L• + O2 ⇌ LOO•

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LOO• + LH → LOOH + L•

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impact, some efficient methods of protection are necessary. One of the possible ways is application of antioxidantsa broad group of compounds able to decrease the oxidation rate,3 even when they are added in a very small amount compared to the amount of a substance to be protected. Several classes of antioxidants can be applied to improve oxidative stability.1 Their effectiveness depends on the mechanism of inhibition and on the stabilized material. Hindered amines (e.g., N,N′-di2-butyl-1,4-phenylenediamine) or phenolic derivatives are substances most commonly used to enhance the stability of different materials especially polymers, plastics, resins, and lubricants. Phenol derivatives are used as antioxidants in food protection but also in machinery and the plastics industry. The typical synthetic phenolic antioxidants like 2,6-di-tert-butyl-4methylphenol (BHT), 3-tert-butyl-4-hydroxyanisole (BHA), and 2-tert-butylhydroquinone (TBHQ)4,5 are able to break the kinetic chain of the propagation because they react with peroxyl radicals: LOO• + PhOH → LOOH + PhO• k inh

This process is competitive to reaction 2 and is described by the inhibition rate constant kinh. If kinh is at least 1000 times bigger than kp, the radicals LOO• are removed before the reaction 2 occurs, and thus, the propagation cycle and the kinetic chain of peroxidation are stopped. The autoxidation is suppressed by reaction 3 until the antioxidant is consumed.

The kinetic chain is terminated when free radicals combine into nonradical products or when they form stable (nonreactive) radicals. Because autoxidation of industrially important hydrocarbons like polymers and lubricants has a large negative economic © XXXX American Chemical Society

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Received: July 5, 2016 Revised: August 22, 2016 Accepted: August 22, 2016

A

DOI: 10.1021/acs.iecr.6b02564 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Chart 1. Structures of Obtained Fullerene C60 Derivatives: Penta-(4-hydroxyphenyl)-hydro[60]fullerene (C60−I), Tetra-2,6methyl-4-hydroxyphenyl[60]fullerene (C60−II), N-Methyl-2-[4-hydroxy-3-methoxyphenyl]-3,4-[60]fulleropyrrolidine (C60− III), and N-Methyl-2-[2-(4-hydroxy-3-methoxyphenyl)vinyl]-3,4-[60]fulleropyrrolidine (C60−IV)

stability of lubricants.30 There is some experimental evidence that fullerene derivatives might also be active antioxidants. C60 with attached amine group exhibits higher efficiency than conventional antioxidants during autoxidation of cumene.31 Enes et al. found that [60]fullerene-flavonoid or [60]fullereneBHT conjugates exhibit moderate antioxidant activity during autoxidation of cumene (or styrene) in homogeneous system at 30 °C.32,33 On the basis of these previous reports, we suggest that additional antioxidant potency should be gained when some phenol derivatives are bonded to C60 because the new adducts will be able to react with wider range of radicals at higher temperatures. We checked this starting hypothesis, and herein we report the effect of presence of four C60 adducts with phenols I−IV (see Chart 1) on the kinetic parameters of thermooxidation (70−200 °C) of saturated hydrocarbon (stearic acid, STA) and unsaturated lipid (linolenic acid, LNA). We have chosen those fatty acids to test the antioxidant behavior in two systems of different sensitivity toward oxidation because allyl and bis-allyl hydrogens in LNA are easily abstractable by peroxyl radicals (see eq 2) making a dramatic decrease of oxidative stability of LNA comparing to STA.34 In this paper, we compared the kinetic parameters determined for autoxidation of pure STA and pure LNA with the ones determined for STA and LNA containing: (i) pristine C60, (ii) phenols I−IV, (iii) C60 with pyrrolidine ring only, and (iv) conjugates of C60 with phenols I−IV.

Simple phenolic derivatives are relatively volatile, and their antioxidant efficiency is limited to moderate temperatures,6,7 because at higher temperatures they can migrate to the surface (even in solid polymers) and evaporate. Some nonvolatile derivatives of phenol find applications as more efficient antioxidants (e.g., Irganox 1330: 1,3,5-trimetyl-2,4,6-tris(3,5di-tert-butyl-4-hydroksybenzyl)-benzene).8 Irganox 1010 activity was determined during oxidation of cumene initiated by azobis(isobutyronitrile), AIBN, (volumetric measurements).9 Zeinalov et al. reported a high stabilizing effect of Irganox 1076 during oxidation of cumene and polystyrene.10 Oxidation of polypropylene stabilized with Irganox 1076 (0.01 mass %) was monitored by means of differential scanning calorimetry (the induction time and oxidation induction temperature was determined). Unfortunately, Irganox is a worse stabilizer than 4-(2-hydroxyethyl)-1,2-benzenediol and α-tocopherol,11 with a relatively moderate level of toxicity (LD50/rats/oral >5000 mg/ kg)12 compared to natural antioxidants. Another disadvantage of the Irganox compounds is their low solubility in polymers. Fullerenes13 and their derivatives are promising compounds for future applications in science, industry, and medicine.14 Unfortunately, poor solubility in almost all solvents15 is a major barrier limiting the applicability of C60; there are a few reports concerning solubility of fullerenes in lipids (for example, 909 mg/L for olive oil),16−18 and in order to increase their solubility, some polar groups have to be attached.19,20 The carbon sphere can be a good starting structure for design and development of new radical-scavenging compounds with specific functionalities. Thermally stable (up to 600 °C)21 C60 molecule can trap several radicals22−24 and can potentially be used as protective substance against reactive oxygen species, ROS.25,26 There are several reports concerning potential antioxidant activity of C60.27−29 Our previous work described the antioxidant behavior of C60 during the autoxidation of stearic acid (STA) over a wide range of temperatures, as a noncorrosive agent for effectively increasing the oxidative

2. EXPERIMENTAL SECTION 2.1. Materials. Stearic acid, STA, (99%, Sigma-Aldrich) and linolenic acid, LNA, (POCH, 99%) were stored at 0 °C in darkness. Fullerene C60 was of 99+% purity (MER Corporation, Tucson). 4-Hydroxy-3-methoxybenzaldehyde, 4-hydroxy-3-methoxycinnamaldehyde, N-methylglycine (sarcosine), potassium hydroxide, 2,6-xylenol, aluminum chloride (98.5%), isobutyraldehyde, carbon disulfide, dichloromethane, copper(I) bromide dimethyl sulfide complex, acetonitrile, methanol, ethanol, B


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Industrial & Engineering Chemistry Research and hexane were purchased from Sigma-Aldrich. 4-(2Tetrahydropyranyloxy)phenyl magnesium bromide (0.5 M in 2-methyltetrahydrofuran) was purchased from ABCR. Toluene (POCH 99.5%), THF (Sigma-Aldrich, 99.5%), and 1,2dichlorobenzene (Sigma-Aldrich, 99.5%) were dried and distilled before use. Other solvents were analytical grade reagents and were used as received. 2.2. Experimental Methods. Proton nuclear magnetic resonance (1H NMR) spectra were recorded using Bruker AVANCE 300 MHz or Varian 200 MHz instruments. Fourier transform infrared (FT-IR) spectra were obtained using Shimadzu FTIR-8400S spectrometer in the 4000−400 cm−1 range. Thermogravimetry was performed using a TA Q50 instrument at a heating rate of 5 K/min in nitrogen (platinum vessels were used). Oxidation process of STA and LNA was monitored by differential scanning calorimetry (Du Pont 910 apparatus with Du Pont 9900 thermal analyzer and normal pressure, a recently refurbished cell was used). Temperature and cell constant were calibrated with an ultrapure indium standard. TA Instruments software (General V4.01) was used for collecting the data and for determination of temperatures from DSC curves. The oxidations were performed under oxygen flow 6 dm3/h. Samples (3.0−3.5 mg) were heated from 50 to 250 °C in open aluminum pan with linear heating rate β (2.5; 5.0; 7.5; 10.0; 12.5; 15.0; 17.5; 20.0 K/min). As a reference material, an empty aluminum pan was used. Temperatures of extrapolated start of oxidation, Te (see Figure 1), were determined from the plots of heat flow versus temperature dependence for each β. 2.3. Synthesis. Four phenols were used as precursors for synthesis of conjugates with C60 presented in Chart 1: unsubstituted phenol, I, 2,6-dimethylphenol, II, 4-hydroxy-3methoxybenzaldehyde, III, and 4 hydroxy-3-methoxycinnamaldehyde, IV. Derivative C60−I was obtained using organomagnesium compounds (Grignard compounds). Compound C60−II was synthesized by electrophilic addition of 2,6dimethylphenol to C60 in the presence of a strong Lewis acid, AlCl3. Two C60 adducts: N-methyl-2-[4-hydroxy-3-methoxyphenyl]-3,4-[60]fulleropyrrolidine, C60−III, and N-methyl-2[2-(4-hydroxy-3-methoxyphenyl)vinyl]-3,4-[60]fulleropyrrolidine, C60−IV, were prepared using the modified 1,3-dipolar cycloaddition of azomethine ylides to olefins (the Prato reaction, with the ylides formed by reaction of the corresponding aldehyde and N-methylglycine). The influence of unsubstituted pyrrolidine nitrogen on oxidation kinetics was studied by application of fullerene C60 containing pyrrolidine ring only, C60−Py. 2.3.1. Synthesis of N-Methyl-3,4-[60]fulleropyrrolidine, C60-Py.35 A mixture of C60 (70 mg, 0.1 mmol), sarcosine (43 mg, 0.48 mmol, 5 equiv), formaldehyde (79 mg, 1 mmol, 10 equiv), and 60 mL of toluene was stirred in reflux for 2 h in a 250 mL flask. The reaction mixture was cooled, and the solvent was removed under reduced pressure. The residue was purified by column chromatography (silicagel, toluene) to give 35 mg of product C60-Py as a solid. Analysis: 1H NMR (300 MHz, toluene-d8) δ: 2.59 (s. 3H), 3,89 (s, 4H) ppm. 2.3.2. Synthesis of Penta-(4-hydroxyphenyl)hydro[60]fullerene, C60−I. Derivative C60−I was synthesized using a method reported by Matsuo and Nakamura.36 A 50 mL two neck round-bottom flask was filled with CuBr·SMe2 (576 mg, 2.8 mmol, 20 equiv), THF (20 mL), and a 0.5 M solution of 4(THPO)C6H4MgBr (5.6 mL, 2.8 mmol, 20 equiv) in 2MeTHF. The dark red suspension was stirred for 10 min, and a

Figure 1. DSC curves (shifted vertically for clarity) of oxidative decomposition of pure STA (panel A) and STA containing: 2.0 mM 4hydroxy-3-methoxycinnamaldehyde, IV (panel B), and 2.0 mM Nmethyl-2-[2-(4-hydroxy-3-methoxyphenyl)vinyl]-3,4-[60]fulleropyrrolidine, C60−IV (panel C). An example of the extrapolated temperature of the start of oxidation (Te) is indicated on plot C. All measurements were carried out by DSC in nonisothermal mode, and the numbers above each DSC curve indicate the heating rate β (in K min−1).

solution of C60 (100 mg, 0.139 mmol) in 1,2-dichlorobenzene (7.5 mL) was added. Stirring was continued for 2 h. The reaction was stopped by addition of NH4Cl(aq) (0.5 mL), and the mixture was diluted with toluene (20 mL). The mixture was filtered through a pad of silica gel, and concentrated. After addition of EtOH, precipitate was formed, collected, and washed with EtOH. The orange solid was dissolved in CH2Cl2/ MeOH (20 mL, 50% v/v), and p-toluene sulfonic acid (10 mg, 0.053 mmol) was added. The mixture was stirred for 24 h, then neutralized with NaHCO3 and concentrated. Next, the mixture was purified by filtration with a pad of Celite. To obtain precipitates, the solvent was evaporated. The powder was dissolved in few milliliters of EtOH. After addition of hexane, precipitates formed and were collected, washed with hexane, and dried to give 83 mg of product C60−I (50%, based on converted C60). Analysis: 1H NMR (200 MHz, acetone-d6) δ: 5.49 (s, 1H), 6.64−6.68 (d, 2H), 6.72−6.76 (d, 2H), 6.87−6.91 (d, 2H), 7.24−7.28 (d, 2H), 7.55−7.59 (d, 2H), 7.74−7.79 (d, 2H), FTIR (KBr) [cm−1]: 3300 (O−H), 2959 (C−H), 1508 C


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521 (C−C, C60). Percentage weight loss at a temperature range of 150−600 °C was 21%, which corresponds to one group attached to the fullerene molecule.

(C−C), 1429 (C−C, C60), 1236 (C−O), 1174 (C−C, C60), 1015 (C−O), 538 (C−C, C60). Percentage weight at a temperature range of 150−550 °C was 35%, which corresponds to five groups attached to the fullerene molecule. 2.3.3. Synthesis of Tetra-2,6-methyl-4-hydroxyphenyl[60]fullerene, C60−II. C60 derivative (C60−II) was obtained using the synthetic procedure reported by Shi et al.37 A roundbottom flask (100 mL, two necks) was filled with C60 (43 mg, 0.9 mmol), 2,6-xylenol (100 mg, 0.9 mmol, 1 equiv), aluminum chloride (100 mg, 0.74 mmol, 0,8 equiv) and 20 mL of carbon disulfide. This mixture was stirred at ambient temperature for 24 h. Then 30 mL of dichloromethane and 15 mL of water were added. The water layer was extracted with dichloromethane (2 × 20 mL). The combined organic layers were dried (MgSO4) overnight, concentrated, and purified on a silica gel column. The product was eluted with methanol/toluene (1:10 v/v) and dried overnight in 40 °C in a vacuum oven. The product C60−II was obtained as a brown solid (20.5 mg, 30%, based on converted C60). Analysis: 1H NMR (200 MHz, CDCl3, TMS) δ: 2.17 (s, 6H), 2.30 (s, 12H), 2.44 (s, 6H), 4.58 (s, 1H), 4.70 (s, 1H),), 4.83 (s, 1H), 7.15 (s, 2H), 7.83 (s, 2H); MS TOF ESI−: m/z expected: 1205.3 found: 708, 724, 781, 797, 855, 873, 929, 1004. FTIR (KBr) [cm−1]: 3440 (O−H), 2963 (C−H), 1489 (C−C), 1261 (C−O), 1198 (C−C, C60), 1026 (C−O), 527 (C−C, C60). Percentage weight loss at a temperature range of 150−550 °C was 40%, which corresponds to four groups attached to the fullerene molecule. 2.3.4. Synthesis of N-Methyl-2-[4-hydroxy-3-methoxyphenyl]-3,4-[60]fulleropyrrolidine, C60−III.35 A mixture of C60 (100 mg, 0.14 mmol), sarcosine (61.4 mg, 0.69 mmol, 5 equiv), 4-hydroxy-3-methoxybenzaldehyde (21.1 mg, 0.14 mmol, 1 equiv), and 90 mL of toluene was stirred under reflux for 24 h in a 250 mL flask. The reaction mixture was cooled, and the solvent was removed under reduced pressure. The residue was purified by column chromatography to give 48 mg of product C60−III as a brown solid (39% yield based on converted C60). Analysis: 1H NMR (200 MHz, CDCl3, TMS) δ: 2.81 (s, 3H), 3.90 (s, 3H), 4.21−4.26 (d, 1H), 4.85 (s, 1H), 4.96−4.99 (d, 1H), 6.91−6.95 (d, 1H), 7.18−7.22 (d, 1H), 7.38 (s, 1H) ppm; MS TOF ESI − : m/z expected (C60(C10H13O2N)): 899.8577, found: 899.9668; FTIR (KBr) [cm−1]: 3300 (O−H); 2922 (C−H); 1512 (C−C); 1431 (C− C, C60); 1269 (C−O); 1180 (C−C, C60); 1032 (C−O); 768 (C−C, C60); 527 (C−C, C60). The weight loss was 27% at a temperature range of 250−650 °C, which corresponds to one group per one fullerene molecule. 2.3.5. Synthesis of N-Methyl-2-[2-(4-hydroxy-3methoxyphenyl)vinyl]-3,4-[60]fulleropyrrolidine, C60−IV.35 A mixture of C60 (100 mg, 0.14 mmol), sarcosine (61.4 mg, 0.69 mmol, 5 equiv), 4-hydroxy-3-methoxycinnamaldehyde (21.1 mg, 0.14 mmol, 1 equiv), and 100 mL of toluene was stirred in reflux for 24 h in a 250 mL flask. The reaction mixture was cooled, and the solvent was removed under reduced pressure. The residue was purified by column chromatography to give 35 mg of product C60-IV as a brown solid (28% yield based on converted C60). Analysis: 1H NMR (200 MHz, CDCl3, TMS) δ: 2.91 (s, 3H), 3.90 (s, 3H), 4.12−4.17 (d, 1H), 4.42−4.46 (d, 1H), 4.88−4.93 (d, 1H), 5.02 (s, 1H), 6.47−6.60 (dd, 1H), 6.85−6.90 (d, 1H), 6.94−7.02 (d, 1H), 6.99 (s, 1H), 7.18−7.22 (d, 1H), ppm; MS TOF ESI−: m/z expected (C60(C12H15O2N): 899.8577, found: 763.42. FTIR (KBr) [cm−1]: 3300 (O−H), 2926 (C−H), 1510 (C−C), 1422 (C−C, C60), 1273 (C−O), 1175 (C−C, C60), 1026 (C−O),

3. RESULTS AND DISCUSSION The thermal effect of oxidation of STA and LNA was monitored by differential scanning calorimetry (DSC). In general, the exothermal effect of oxidation is strong enough to be recorded as the heat flow versus temperature (see Figure 1), and the extrapolated start of the oxidation temperature (Te) can be determined. Example of DSC thermograms for pure STA and for STA containing 2.0 mM IV and C60−IV are shown in Figure 1. The plots clearly demonstrate that compound IV improves the oxidative stability of STA (Te is shifted to higher temperatures) and that compound C60−IV causes a further increase of the oxidation stability of STA. Figure 2 presents a comparison of Te obtained for oxidation of pure STA with the Te values for STA containing 2.0 mM

Figure 2. Comparison of the extrapolated temperature of start Te (°C) obtained for oxidation of STA containing: pristine C60 (A), C60−Py (B), phenols I−IV (white bars), and C60 derivatives with phenols I− IV (black bars) at a concentration of 2.0 mM. The heating rates β were 5 K min−1 (upper panel) and 10 K min−1 (lower panel). For comparison, the Te for oxidation of pure STA is marked as red dashed line in the background.

additives (phenols I−IV, C60, and their conjugates). Phenols II and III cause an increase of Te, while phenols I and IV do not. Addition of pristine fullerene C60 improves the oxidation stability of STA manifested as a substantial increase of Te. A similar increase can be observed when C60−Py is added to STA sample. At a heating rate of 5 K min−1, the value of Te for C60 is about 4 degrees higher than for C60−Py, but when the rate is 10 K min−1, the difference is only 2 °C. When the STA sample contains C60−(I−IV), the values of Te are higher by approximately 10 °C than for pure STA and higher than obtained for STA + starting phenols but without C60. The highest values of Te are observed for STA with C60− III (differences in Te are 26 and 30 °C for β = 5 and 10 K min−1, respectively). This simple comparison of temperatures is a good qualitative method to estimate the oxidative stability of lipids; however, such results cannot be extrapolated to other D


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Table 1. Values of the Overall Activation Energya (Ea) and Pre-Exponential Factor (Z), Overall Rate Constants (k, at 50, 100, 150, 200°C) for Oxidation of Pure STA and STA Containing Phenols I−IV and Fullerene C60 Derivatives C60-Py, C60−(I− IV)b k [min−1] Z [min−1]

Ea [kJ/mol]

STA + additive  I II III IV C60 C60−Py C60−I C60−II C60−III C60−IV

116 108 109 100 111 145 85 123 113 108 132

± ± ± ± ± ± ± ± ± ± ±

8 8 3 7 11 9 5 8 16 13 11

1.34 1.51 1.53 1.82 2.34 1.10 1.82 6.53 1.04 6.45 3.09

× × × × × × × × × × ×

50 °C

1013 1012 1012 1011 1012 1016 109 1013 1012 1016 1014

2.50 4.76 3.32 1.23 2.82 3.69 3.04 7.18 4.78 2.05 1.35

× × × × × × × × × × ×

100 °C

10−6 10−6 10−6 10−5 10−6 10−8 10−5 10−7 10−7 10−6 10−7

8.07 × 10−4 1.05 × 10−3 7.72 × 10−4 1.80 × 10−3 7.09× 10−4 5.15 × 10−5 2.13 × 10−3 3.39 × 10−4 1.34 × 10−4 4.54 × 10−4 9.80 × 10−5

150 °C 6.66 6.48 4.96 8.12 4.84 1.30 5.48 3.73 1.03 2.80 1.50

× × × × × × × × × × ×

200 °C

10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

2.16 1.67 1.37 1.64 1.32 1.02 0.71 1.50 0.31 0.72 0.79

a

The errors of the activation energy Ea estimated on the basis of the standard deviations of the slopes calculated for confidence level 90%. bFull data including statistic and kinetic parameters are given in the SI.

Table 2. Relative Values of Overall Rate Constants, krel, at 50 and 200°C for Oxidation of Pure STA (krel= 1.0) and for STA Containing Phenols I−IV, Fullerene C60, and Its Derivatives C60−Py, C60−(I−IV)

a

additive



I

II

III

IV

C60

C60−Py

C60−I

C60−II

C60−III

C60−IV

krel(50 °C) krel (200 °C)

1.0 1.0

1.9 a

1.3 a

4.9 a

1.1 a

0.01 0.5

12 0.3

0.3 0.7

0.2 0.1

0.8 0.3

0.1 0.4

Not calculated because of high volatility of simple phenols.

mol)30,46 and linolenic acid (70 ± 8 kJ/mol).43 The obtained values are also in reasonable agreement with Ea values for isothermal oxidation of fatty acids. The overall kinetic parameters determined for nonisothermal oxidation of pure STA (Table 1) were compared with the parameters obtained for the oxidation of STA containing compounds I−IV and their adducts with C60 at a concentration of 2.0 mM. The rate constants, k, for the overall oxidation process were calculated for temperatures 50, 100, 150, and 200 °C from the Arrhenius equation:

temperatures. In order to obtain quantitative information on the oxidation stability of STA, the values of activation energy (Ea) and pre-exponential factor Z for oxidation processes were calculated by the Ozawa−Flynn−Wall method.38−40 This method has been successfully applied by us41−43 and other researchers44,45 for studies of nonisothermal oxidation of hydrocarbons and lipids. For increasing β, an increase of Te is observed; thus, after several experiments for different β, the linear dependence can be obtained:

log β = a × Te−1 + b

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k = Z exp( −Ea /RT )

with the slope a = −0.456Ea/R and intersection b = −2.315 + log(ZEa/R), where R is gas constant (8.314 J mol−1 K−1). The Ea can be calculated from the slope a, and this overall (global) parameter is a sum of three activation components:46 propagation Ep, initiation Ei, and termination Et: Ea = Ep + (E i − Et)/2

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Addition of phenols I−IV to STA do not substantially change the parameters Ea, and k, thus, they do not possess antioxidant properties in this model system. This effect may be caused by high volatility of phenols. Pristine fullerene C60 causes an increase of activation energy and pre-exponential factor and a decrease of oxidation rate constants, in good agreement with our previous results.30 When STA contains C60 with pyrrolidine ring only (C60-Py), the parameters Ea, Z are lower than for pure STA. At 50 and 100 °C, C60−Py exhibits pro-oxidant properties (increased k) but at 200 °C inhibits oxidation of STA (decreased k). The oxidation rate constants collected in Table 1 show that STA containing any of the four C60 derivatives C60−(I−IV) is oxidized slower than pure STA. This inhibitory effect is more clear when we compare the relative rates of oxidation calculated for temperatures 50 and 200 °C collected in Table 2. For oxidation at 50 °C the phenols I−IV and C60−Py show a pro-oxidative effect: the rate constants are about 2 times higher than for pure STA (for C60−Py, the value is 12 times higher). For oxidation carried out at the same temperature the addition of C60 and conjugates C60−(I−IV) causes a decrease of the k value, with the best result for C60−II and C60−IV (5 and 10 times slower oxidation, respectively). The same comparison was done for the

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Determination of the kinetic parameters for nonisothermal autoxidation should be based on the start of oxidation (Te) as the thermal effect at this region of DSC curve is not contaminated by the heat of decomposition of hydroperoxides formed during autoxidation.42,47 Addition of phenolic antioxidant breaks the propagation step and increases the induction period. In nonisothermal DSC mode, the longer time of induction is manifested as higher Te, and therefore, the values of Te can be applied for calculation of the overall kinetic parameters of inhibited and uninhibited oxidation.46−48 For oxidation of pure STA parameters Ea = 116 ± 8 kJ/mol and Z = 1.34 × 1013 min−1 were obtained while oxidation of pure LNA is described by Ea = 78 ± 6 kJ/mol and Z = 3.10 × 1010 min−1, see Figures S15, S37 and Tables S1, S12 in the Supporting Information, being consistent with the kinetic parameters reported previously for stearic (120 ± 15 kJ/ E


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Table 3. Values of the Overall Activation Energya (Ea) and Pre-Exponential Factor (Z), Overall Rate Constants (k, at 50, 100, 150, 200°C) of Oxidation of Pure LNA and LNA Containing Fullerene C60 Derivatives C60−(I−IV) and Phenols I−IVb k [min−1] LNA + additive  I II III IV C60 C60−I C60−II C60−III C60−IV

Ea[kJ/mol] 78 75 82 87 84 74 78 87 77 86

± ± ± ± ± ± ± ± ± ±

6 6 7 4 6 7 6 12 2 10

−1

50 °C

Z [min ] 3.10 8.72 7.22 5.07 1.54 6.32 3.76 3.61 1.93 2.60

× × × × × × × × × ×

1010 109 1010 1011 1011 1010 1010 1011 1010 1016

6.81 5.91 4.80 4.32 4.39 7.54 7.49 2.82 6.74 3.31

× × × × × × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3

100 °C

150 °C

200 °C

0.338 0.252 0.280 0.331 0.286 0.298 0.379 0.219 0.314 0.241

6.68 4.43 6.24 9.12 6.97 4.95 7.51 6.08 5.91 6.36

70.2 42.5 72.2 125.0 86.4 45.4 79.5 71.5 59.8 84.1

a

The errors of the activation energy Ea estimated on the basis of the standard deviations of the slopes calculated for confidence level 90%. bFull data including statistic and kinetic parameters are given in the SI.

relative rate constants calculated for oxidation at 200 °C. At this temperature, all C60 derivatives (including C60−Py) behave as inhibitors of oxidation with a significant decrease of k value. The best effect (10-fold decrease) is observed for sample containing C60−II. In contrast to simple phenols, the conjugates of C60 are not volatile and they are thermally stable (thermogravimetric data are presented in the Supporting Information), and thus, the calculated rate constants give a representation of oxidative stability of STA at 200 °C, being in reasonable agreement with the temperatures of the extrapolated start of oxidation presented in Figure 1. Another model of hydrocarbon applied in our studies was linolenic acid (LNA). At 100 °C, this polyunsaturated fatty acid is oxidized 170 times faster than STA,49 and due to this reason, LNA is often used for studies of antioxidant activity of phenolic antioxidants by the DSC method (it has a clearly pronounced start of thermal effect of oxidation at relatively low temperature, about 100 °C lower than for oxidation of STA). The kinetic parameters for oxidation of neat LNA are presented in Table 3 and are in good agreement with our earlier observations.41 Comparison of the kinetic parameters determined for the oxidation of pure LNA with the ones determined for oxidation of LNA containing compounds C60−(I−IV) or their substrates I−IV at concentration 2 mM (see Table 3) show rather disappointing results: the addition of compounds I−IV or the adducts C60−(I−IV) had no inhibiting effect on the oxidation of LNA. The lack of stabilizing effect can be rationalized, because many physical and chemical factors are involved in the antioxidant protection. Depending on the substrate of oxidation or a method used to monitor the progress of oxidation for the same antioxidant under different conditions, the results might be very inconsistent and sometimes opposite to each other.50,51 The choice of model system that would clearly indicate antioxidant efficiency in quantitative manner is still discussed.1,52−54 Among several features of a molecule to be an effective chain-breaking antioxidant, there is one condition regarded as dependent on the nature of the protected hydrocarbon/lipid: the antioxidant effect is experimentally detected as induction time period when the rate constant kinh is at least 3 orders of magnitude higher than kp.55 Therefore, depending on the kp for lipid or hydrocarbon, the same molecule can behave as a good antioxidant (kinh/kp > 1000) or can exhibit no antioxidant activity (kinh/kp ≪ 1000). This rule is applied during determination of kinh: hydrocarbons like styrene, LNA, and

tetralin are applied for kinetic studies of highly active antioxidants, whereas cumene or oleic acid are much more suitable as model compounds for studies of moderate antioxidants.56 There is 4 orders of magnitude difference between kp for oxidation LNA (48 M−1 s−1 at 37 °C57) and for oxidation of hexadecane (3.4 × 10−4 M−1 s−1at 30 °C58) and, due to proper kinh/kp ratio, the fullerene derivatives show inhibitory effect during autoxidation of saturated hydrocarbons but not during the autoxidation of unsaturated fatty acid. Another possible but less convincing explanation of different antioxidant behavior of C60 derivatives in STA and in LNA is formation of epoxides (C60O) from C60 and peroxyl radicals.28 4C60 + 2LO2• → 4C60O + L − L k 7

(7)

This plausible mechanism should be more effective in STA than in LNA for the same reasons as mentioned above (i.e., due to kinh/kp ratio). In this particular case, the kinh/kp ratio is much higher for STA than for LNA system. Although this mechanism was proposed for pristine C60, we suppose that it might operate for derivatives of C60.

4. CONCLUSIONS Conjugates of C60 with four simple phenols: unsubstituted phenol, 2,6-dimethylphenol, 4-hydroxy-3-methoxybenzaldehyde, and 4 hydroxy-3-methoxycinnamaldehyde were prepared, and we checked their ability to inhibit the high temperature oxidation of saturated and unsaturated fatty acids. Kinetic parameters of nonisothermal oxidation of STA containing the conjugates indicate an improvement of the oxidative stability compared to the oxidative stability of STA containing the separately added compounds: pristine C60 or phenols. Thus, the conjugates are more effective antioxidants than their building blocks. Although the C60 derivatives increases the oxidative stability of STA and efficiently inhibits the oxidation of saturated hydrocarbons that occurs at high temperatures (above 150 °C), they do not inhibit thermal oxidation of linolenic acid. Therefore, fullerene and its conjugates are typical moderate antioxidants able to protect saturated hydrocarbons. Taking into account the thermal stability and low volatility of such conjugates, they can be effective antioxidants for lubricants working at severe oxidative conditions at relatively high temperatures. F


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02564. 1 H NMR; FTIR spectra of C60−Py, C60−(I−IV); TGA curves of the thermal decomposition of C60 derivatives; DSC curves of oxidative decomposition of STA, LNA; and STA, LNA containing C60 or C60 derivatives (C60− Py, C60−(I−IV)) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for G.L.: [email protected]. *E-mail for R.C.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the project (grant nos. IP 2012 009772 and NN 507 452937) by the Ministry of Science and Higher Education of Poland. J.K. thanks Warsaw Chemical Academic Consortium (WAKCh) for a KNOW scholarship.

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