Ismail Badrana,
Nashaat N. Nassar*b,
Nedal N. Mareibc and
Azfar Hassanb
aDepartment of Chemistry, An-Najah National University, PO Box 7, Nablus, Palestine
bDepartment of Chemical and Petroleum Engineering, University of Calgary, 2500 University Street NW, Calgary, Alberta, Canada T2N 1N4. E-mail: nassar@ucalgary.ca; Fax: +1 403 210 3973; Tel: +1 403 210 9772
cDepartment of Chemical Engineering, An-Najah National University, PO Box: 7, Nablus, Palestine
First published on 2nd June 2016
In this study, the thermal oxidation of an asphaltene model molecule, Quinolin-65, was investigated using the density functional theory (DFT) and the second-order Møller–Plesset (MP2) perturbation theory. The reactions studied involved thermal decompositions as well as the interactions between the model molecule and singlet atomic (O1D) and molecular (O21Δ) oxygen. The theoretical study was performed under conditions similar to those of the uncatalyzed thermal oxidation of asphaltenes. A new reaction pathway for the loss of the olefin chain in Quinolin-65 via a 1,3-hydrogen shift mechanism was revealed. Thermogravimetric analysis of Quinolin-65 was also performed and the reaction products were probed by a mass spectrometer. Both the theoretical study and the thermogravimetric analysis concluded that the thermo-oxidative decomposition of Quinolin-65 is a complex multi-step reaction process, which involves different reaction pathways. The thermodynamic parameters obtained in this study showed that the reaction process should start with the loss of the olefin chain in the Quinolin-65 molecule, followed by the oxidation of the aromatic chain, to produce mainly, H2O, CO2, and SO2.
For instance, Song and Parish have employed theoretical methods to study the pyrolysis mechanisms of thiophene and methylthiophene, as asphaltene model molecules.18 The reaction mechanism involved C–C, C–S, and C–H bond ruptures, in addition to H-migration. The major products were found to be ethane, thioketene, propyne, CS, and CO. Similarly, Song considered the recombination reaction between 2-thienylmethyl and HO2 radical to play an important role in asphaltenes decomposition.19 Using high-level theoretical calculations, Song's study concluded that the most favorable pathway in the reaction mechanism was the insertion of carbonyl group leading to the formation of H2O. Simple nitrogen-containing structures such as quinoline and isoquinoline have also been used as asphaltene model molecules in many experimental and theoretical studies.20–22 Full reaction mechanisms were drawn for quinoline that showed the important role of indene as an intermediate. Similar to the thiophene studies, H-migration has also played a significant role in the reaction mechanism.
Nevertheless, simple molecules cannot completely simulate or mimic the complex nature of asphaltenes. Hence, in this study, a heavy model molecule, like Quinolin-65 (Q65), has been proposed to better mimic asphaltenes and understand their chemical properties.23 Q65 is not only more representative heavy molecule, but also its structure is composed of polyaromatic cores containing heteroatoms such as S, N, and O with attached aliphatic chains. These features are commonly reported in a typical asphaltenes structure.2 Actually, Q65 was proven to resemble asphaltenes in their chemical and physical properties and adsorption behaviour on solid surfaces10,23,24 as well as thermal cracking.24
Herein, we are practically interested in the thermo-oxidative decomposition of asphaltenes using Q65 as a model molecule. Oxidation of hydrocarbons had gained an intense interest for long time.25–28 Although the ground state of molecular oxygen has a spin multiplicity of three, designated as triplet oxygen (O23Σ+g), singlet oxygen (O21Δg) has gained special interest due to its high reactivity.29,30 Oxygen addition to organic molecules such as ethane,28 ethanol,31 1,3-butadiene,32 benzene,25,32 and toluene,26 have been extensively studied using high-level theoretical methods. Widely speaking, addition of singlet molecular oxygen was shown to proceed by both stepwise and concerted mechanisms. While the former involves the formation of a diradical intermediate, the latter proceeds to form a peroxide moiety (benzene oxide). Generally, the addition of triplet oxygen to hydrocarbons and aromatic rings is highly endergonic, such as in the cases of ethane28 and benzene.32 However, addition of the more reactive singlet oxygen is usually exergonic.25,29,30 The study of Chen et al. on the reaction dynamics of singlet oxygen (O1D) with benzene shapes a milestone in this field.25 Using laser photolysis, TOF-MS, and theoretical methods, the authors proposed a full mechanism for the O(1D) reactions with benzene to produce, mainly, CO, C5H5, and H species.
Accordingly, we explored in this study the interaction of Q65 with singlet atomic (O1D) and molecular (O21Δg) oxygen. The study aimed to construct a thermo-oxidative reaction mechanism of Q65. For which, we employed the density functional theory (DFT) and second-order Møller–Plesset perturbation theory (MP2) to examine different possible reaction pathways for Q65 thermal oxidation and decomposition. In addition to the theoretical study, thermo-oxidative behavior of virgin Q65 was investigated by using a thermogravimetric analyzer coupled with an online mass spectrometer. The main products formed upon the thermo-oxidative decomposition of Q65 were CO2, H2O and SO2 gases. Therefore, these experimental and theoretical investigations provide a comprehensive reaction mechanism, which provides both full understanding of the reactions involved, as well as vital kinetic parameters such activation energies, rate constants, and pre-exponential factors. In addition, theoretical calculations could predict the transition state structures, which cannot be observed directly during experiments. Hence, a general reaction mechanism for the non catalyzed Q65 thermal oxidation can be drawn.
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Scheme 1 Q65 molecular structure drawn with ChemDraw V14.33 The resemblance with an asphaltene molecule can be seen by the archipielago aromatic region, the alkyl chain and the presence of heteroatoms like sulfur, nitrogen and oxygen. |
Enthalpies at 0 K (H0) were obtained by adding the single-point energy to the scaled ZPE. Enthalpies and Gibbs free energies at 298 K (H298 and G298) were obtained by adding a value of x to the enthalpies or Gibbs free energies at 0 K (H0, or G0), which is equivalent to the energies after the scaled ZPE corrections. x was calculated by subtracting the unscaled ZPE from the thermal correction to enthalpy (Hcorr.) or thermal correction to Gibbs free energy (Gcorr.). For calculating the thermodynamic values at different temperatures, the same calculation method mentioned above was used by utilizing different corrections (Hcorr. and Gcorr.) obtained at a given temperature. All calculations in this work were performed using the Gaussian 09 program.38
The enthalpies of activation (ΔH‡) and Gibbs free energies of activation (ΔG‡) were determined by computing the difference between the H‡ or G‡ for the reactant and those of the transition state. The values were scaled as described above prior to obtaining the difference. The entropies of activation (ΔS‡) was calculated using eqn (1)
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The reaction enthalpies (ΔH), Gibbs free energies (ΔG), and entropies (ΔS) were determined similarly using H, G, and S of the reactants and products.
Using our calculation method at 0 K, the aliphatic (route a) and the aromatic (route b) bond ruptures in Q65 were endothermic by 192.4 and 234.6 kcal mol−1, respectively. In order to simplify the highly demanding calculations of Q65, we shortened the aliphatic chain down into two carbon atoms. This new structure is referred hereinafter by Q65b. Calculations on Q65b provided close values of 190.8 and 233.4 kcal mol−1 for routes (a) and (b), respectively. Using these values, and the thermal and the Gibbs free energies corrections from the Gaussian program, we calculated the Gibbs free energy at room temperature (ΔG298) for the two reactions, (a) and (b), to be 178.2 and 222.1 kcal mol−1, respectively. The energy level diagram for these reactions is shown in Fig. 1.
In addition to the loss of the olefinic chain by homolytic cleavages, we also considered the 1,3-hydrogen migration from the ethyl group to the O atom in Q65b, as illustrated in Scheme 3. The H-shift results in the formation of ethylene and the aromatic alcohol (I). A similar migration in the parent Q65 molecule will lead to the formation of 1-octene and (I). Using our calculation method, a transition state (TS1) was located for this mechanism. The transition state was confirmed by using the intrinsic reaction coordinate (IRC) as explained earlier. An optimized structure for TS1 and product (I) are shown in Fig. 2. The strong imaginary frequency of 672.4i cm−1 in TS1 corresponds to H migration from C to O as expected. Also, Fig. 1 shows the Gibbs free energy of activation (ΔG‡298) for the 1,3-H shift mechanism to be 70.8 kcal mol−1. This barrier is the lowest among all reaction routes calculated in this study. This suggests that the thermal decomposition of Q65b to form an olefin and (I) is the most favorable among all other routes. This is also supported by the low value of free Gibbs energy of reaction (ΔG298) of 6.3 kcal mol−1, as seen in Fig. 1. Later, the thermochemical parameters of this mechanism are correlated to those obtained from the experimental thermogravimetric analysis of Q65, in order to show the importance of this reaction pathway.
The next reaction pathway we considered in the Q65b thermal decomposition is the abstraction of aromatic hydrogen by an oxygen atom. We have explored this type of abstraction on both singlet and triplet energy surfaces. Under the level of theory used in this work, we have only located the transition state of abstraction at the triplet surface. The abstraction has resulted in the formation of a hydroxyl radical and a Q65b free radical stabilized by the large aromatic system, as illustrated in Scheme 4. The barrier height between Q65b + triplet O and the triplet transition state, TS2(t), was found to be 151.4 kcal mol−1, as seen in Fig. 1. The figure also shows a ΔG298 of 134.1 kcal mol−1. Clearly, these values are much higher than those of the 1,3-H shift mechanism, due to the triplet nature of the transition state and the products of the reaction. In addition, one need to keep in mind that the bond dissociation energy of the triplet ground state of the O2(3Σ+g) molecule needed to form two triplet O(3P) atoms is estimated to be 119 kcal mol−1.29 This suggests that H abstraction by a triplet O is highly unfavorable in the course of Q65b thermal decomposition. This agrees with previous studies on the abstraction reaction of hydrogen by O(3P) in benzene, where the reaction does not play an important role at room temperature.27,42
Scheme 5 summarizes the results of our explorations for the O(1D) additions on different sites of Q65b, at MP2/6-311g(d,p)//B3LYP/6-31+g(d) level of theory. The negative values shown in the scheme represent ΔG298 for each addition reaction. Interestingly, we found the most favorable site of addition to take place on C2, which represent the LUMO of Q65b, in perfect agreement with our previous speculation. Also, we were unable to locate any transition state for the additions shown in Scheme 5, in concert with previous studies on benzene.25 Hence, we propose that any addition of O(1D) to Q65b to be barrierless, and highly exothermic.
Fig. 4 shows optimized structures for the different addition products of O(1D) on Q65b at different sites as discussed above. The addition of singlet oxygen on the LUMO of Q65b, i.e., on the C2 atom, has led to the formation of an aromatic oxide product (II), similar to benzene oxide. Analogue oxides were produced from the attacks on C3 and C5, named IV and V, respectively. In their study on O(1D) reactions with benzene, Chen et al. computed ΔH for the formation of benzene oxide to be −106.8 kcal mol−1. This value is less exothermic than the ΔH values for the addition of O(1D) to Q65b obtained in this work. This suggests that the addition of singlet oxygen to the large aromatic system of Q65b produces relatively stabilized products, and therefore, larger negative values of ΔH.
The O(1D) attack on the C1 atom of Q65b has caused a ring opening to form (III), as depicted in Fig. 4a. This attack, which happens to be on the LUMO+1 of the Q65b molecule, has led to the formation of a stable “ketonic” terminal, and also freed the sulfur atom forming a phenyl sulfur terminal. Under the level of theory used in this work, we located a transition state, named TS3, that connects II with III. An optimized structure for TS3 is shown in Fig. 5. The imaginary frequency of this transition state corresponds to swinging of the newly attached O atom between the ketonic and the oxide forms. In addition, the small magnitude of the imaginary frequency (69.3i cm−1) corresponds to a small energy barrier of 28.7 kcal mol−1. From the intermediate (III), a homolytic C–C bond cleavage can lead to the formation of two other intermediates, VII, and VIII, as shown in Scheme 6. Under the level of theory used in this work, we were unable to locate any transition state that connects III with the intermediates VII and VIII. Nevertheless, the phenyl sulfur intermediate (VIII) can either undergo further thermal decompositions or react with the readily available oxygen atoms to form SO2 gas. An energy level diagram for this reaction mechanism is shown in Fig. 6. Experimental evidence for the production of SO2 will be provided later.
Another interesting route for Q65b oxidation is the one evolving from the attack of O(1D) on C5, which led to the formation of the oxide V, as explained earlier in Scheme 5. In a similar fashion to the ring opening caused by TS3, we have located a transition state, TS4, shown in Fig. 7, that transformed the oxide intermediate (V) into the intermediate (IX), which can easily decompose into carbon monoxide and intermediate X through a ring closure.43 This mechanism is illustrated in Scheme 7. The importance of this reaction path lies in the possible formation of CO2 gas out of IX, via further oxidation with molecular oxygen.43
In addition to the reactions of Q65 with atomic oxygen, we also explored its reactions with molecular oxygen in its singlet state (O21Δ). In contrast to the barrier-less reactions with O(1D), we found that reactions with O2(1Δ) proceed through a transition state, which implies the need for an activation barrier. Fig. 8a shows an optimized geometry for the transition state (TS5) involved in the addition of O2(1Δ) to the carbon atoms C1 and C3 in Q65b. The imaginary frequency of TS5 (216.5i cm−1) shows a synchronized bond stretching between the O2 molecule and the two carbon atoms in order to form an aromatic peroxide (XI). The peroxide, shown in Fig. 8b, resembles the one obtained from the reaction of O2(1Δ) with benzene. Under the level of theory used in this work, the activation barrier (ΔG‡298) was determined to be only 0.9 kcal mol−1, and the reaction energy was determined to be slightly exergonic, i.e., ΔG298 = −9.9 kcal mol−1. In their study on singlet oxygen reactions with benzene, Bobrowski et al.32 determined the addition of O2(1Δ) to benzene to form benzene peroxide moiety to be remarkably endergonic. This was attributed to the high strain in the bicyclic compound formed and the loss of aromaticity. With a large aromatic system like Q65b, these two effects can be largely released, which accounts for the high stability of the peroxide (XI) and the exothermicity of the reaction.
So far, we discussed the reactions of atomic and molecular oxygen with the asphaltene model molecule, Q65b, at 0 K and room temperature. In order to gain more insights into the reactions at higher temperatures, and to better compare the outcomes of this theoretical study with those from the experimental investigation of Q65, we computed the thermochemical parameters for the main reactions of Q65b with O(1D) and O2(1Δ) at temperatures ranging from 298 to 873 K at 100 K intervals. Fig. 9 shows the reaction Gibbs free energies (ΔG) for some of the main reaction routes of Q65b. Generally, changing the reaction temperature from room temperature to 800 K has little effect on ΔG. Nevertheless, one noticeable reaction route is the 1,3-H shift mechanism, where ΔG switches to exergonic at around 473 K (200 °C). In addition, the figure shows a slight increase in the endergonicity of the reactions involving addition of either O(1D) or O2(1Δ) to Q65b as temperature rises. This can be explained by the changes in the translational entropy, which affects the TΔS term in eqn (1).
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Fig. 9 Reaction Gibbs free energies (ΔG) for some reactions of Q65b with singlet oxygen as a function of temperature. |
Similar to the calculations on the reaction Gibbs free energies (ΔG), we computed the Gibbs free energies of activation (ΔG‡) for the main reaction routes of Q65b. Fig. 10 shows changes of ΔG‡ for five main reactions described in this work. In the course of this analysis, we considered ΔG of the barrier-less aliphatic rupture reaction to be equal to the activation barrier, in accordance with the principles of the variational transition state theory.39,40 As seen in Fig. 10, the values of ΔG‡ for the aliphatic rupture and the 1,3-H shift (TS1) reaction routes are decreasing with temperature, and increasing for other reaction routes. This can be explained by the tight nature of the transition state, such as in the case of the 1,3-H shift mechanism, or the loose nature of it, such as in the case of TS3 and TS5.
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Fig. 10 Activation Gibbs free energies (ΔG‡) for some reactions of Q65b with singlet oxygen as a function of temperature. |
Based on the above findings, Q65 thermal oxidation in the gas phase, in the absence of any catalyst, should start with highly endothermic reactions; such as attacks of triplet oxygen on the aromatic part of Q65, or those responsible for producing singlet oxygen from its ground state triplet, or hemolytic bond ruptures that lead to the loss of the aliphatic “olefin” chain. Once these primary endothermic initiating steps are completed, exothermic attacks of singlet O and O2 should follow, accompanied by a series of propagation reactions with low activation barriers (e.g., TS3 and TS4).
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Fig. 11 TG-DTA curve for Q65 heat flow and rate of mass loss as a function of temperature, air flow is 30 cm3 min−1 and heating rate 10 °C min−1. |
Fig. 12 shows the evolution profiles of CO2, H2O and SO2 as a function of the temperature during the thermo-oxidative process of Q65. Evolved gas analysis by MS showed very small peak for CO2 below 400 °C suggesting very little oxidation. Interestingly, the MS data in the high temperature oxidation region showed not only H2O and CO2 peaks, but also a small peak for SO2. The formation of SO2 supports our theoretical mechanism as previously explained in Section 4.1.2. The fact that insignificant CO2 and H2O form below 400 °C suggests that formation is preceded by primary decomposition reactions, such as those corresponding to the loss of the olefin chain, and O and O2 addition, as proposed in our theoretical assessment earlier. It should be noted here that the mass spectrometric analysis of gases produced during the reaction did not show any formation of octene nor methanol. This could be due to the low mass content present in the sample, which was due to the crucible size (50 to 60 μL). Thus, the concentration of octene or methanol remained very dilute and too low to be detected.
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Hence, the effective activation energy is obtained from the slope of the best-fit line of the plot of log(β) against 1/T. The error in the estimation of Eα due to the Doyle's approximation is corrected using a correction factor as follows
Ecorrected = Eα/F(xm) | (5) |
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Fig. 13 shows plots of α as a function of temperature and at three different heating rates. As anticipated, the rate of mass loss changes with the heating rate. At a fixed temperature, α is different for different heating rates with an exception (between 400 and 450 °C). These results are consistent with our description of the TGA/DSC thermo-oxidative and evolution gas profiles in Section 4.2.1. This clearly indicates that Q65 thermal oxidation passes through multi-steps.16
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Fig. 13 Percent conversion of Q65 during oxidation in air at flow of 100 cm3 min−1 and heating rates of 5, 10 and 20 °C min−1, respectively. |
Recall that during our theoretical assessment the loss of the olefin chain by 1,3-H shift mechanism is both thermodynamically and kinetically favorable overall other reaction pathways, including the formation of singlet oxygen itself. Therefore, we propose that the loss of the olefin chain is responsible of producing the first peak on the thermoanalytical curve. This step resembles the first stage in Fig. 10, where α is between 0 and 0.28. This is supported by the fact that the olefin chain is almost 28% by weight of the whole Q65 molecule. The next stage, where α is between 0.35 and 1, depicts the oxidation of the aromatic rings, as already mentioned in the theoretical and the gas analysis sections. In the region where α is between 0.3 and 0.35, due to the overlapping of the conversion profiles, the OFW isoconversional method cannot be applied here because the method is used only for estimation of the data sets with well-separated steps. Therefore, activation energy was calculated separately from α = 0.1 to 0.3 and from 0.35 to 0.9, to avoid the overlapping limitations.48 This overlapping, in the region of α between 0.3 and 0.35, is attributed to the fact that the total reaction effect is the same and independent of the heating rate within this region.48 This suggests the consecutive reaction sequence and the oxidation of aromatic ring (Step 2, resembled by the second peak in the TGA profile) must be always after the oxidation of olefin chain (Step 1, resembled by the first peak in the TGA profile). Hence, Step 1 will be the rate-limiting step for the reaction.
Fig. 14 shows the estimated effective activation energy as a function of conversion. It is obvious that a constant value of effective activation energy, at various values of α, is an indication of a single-step reaction, while a change of activation energy with an increase in α is an indication of a complex reaction mechanism.16 Clearly, the Eα profile in the figure shows a multi-step reaction mechanism (high temperature oxidation region), as different effective activation energies are obtained at various stages of the oxidation process.44,48 Here, Eα exhibited two distinct trends of variation. For low α region, Eα appears almost constant showing a single step reaction mechanism confirming cracking of olefin chain. During the region α = 0.35 to 0.9, oxidative cracking of aromatic as well as formed char/coke is occurring with gradual decrease in Eα, an indication that the presence of oxygen at high temperature alters the reaction mechanism and facilitates its rate.49 Again, this is consistent with our description of the TGA/DSC thermo-oxidative decomposition and evolution gas profiles in Section 4.2.1, where two major peaks appear. It is also interesting to notice the correlation between our theoretical determination for the different values of activation energies, as seen in Fig. 10, and the effective Eα values obtained from the experimental study, as seen in Fig. 14. Clearly, the differences in Eα values obtained by both approaches support the multi-step reaction mechanism. Worth noting here that the Eα values obtained from the thermogravimetric analysis and our theoretical treatment are quite different. The Eα values obtained from the experimental part were in the range of 24–33 kcal mol−1, while those obtained from the theoretical assessment ranged between 0 and 180 kcal mol−1. This difference can be explained by the fact that not all reactions from our theoretical study can be part of the Q65 thermo-oxidative decomposition, i.e., the olefin loss by homolytic cleavage. Further treatment of the Eα values will be discussed later in this article.
T (°C) | α at 5 °C | Function | g(α) | f(α) | R2 | Slope | |
---|---|---|---|---|---|---|---|
Tp1 | 333 | 0.2 | One and a half order | (1 − α)−1/2 − 1 | 2(1 − α)3/2 | 0.98 | −1.53 |
Tp2 | 517 | 0.75 | Three-quarters order | 1 − (1 − α)1/4 | 4(1 − α)3/4 | 0.99 | −0.72 |
After calculating the effective activation energies and the most probable mechanism function, the pre-exponential factor (Aα) is estimated from the intercept of the best-fit-line of the plots of ln(g(α)) against ln(β). The obtained pre-exponential factors are shown in Fig. 15. As seen, to some extent, the Aα trend had followed that of that activation energy, especially in the high temperature oxidation zone. This can be explained by the fact that the pre-exponential factor reflects the nature of the transition state. In other words, a loose transition state will convey a higher Aα value. Because of the different types of transition states involved in the Q65 thermal oxidation, the changes in Aα values can be rationalized.
ΔH‡ = E‡p − RTp | (7) |
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Exp. (Tp1)a | Exp. (Tp2)b | Theo. aliphatic rupture | Theo. 1,3-H shiftc | Theo. TS3c | Theo. O(1D) additionc,d | Theo. O2(1Δ) additionc,e | |
---|---|---|---|---|---|---|---|
a Tp1 = 333 °C.b Tp2 = 517 °C.c Theoretical values were obtained at 300 °C.d Attack on C2.e Attack of O2 at C1–C3 of Q65b. | |||||||
ΔS‡ (cal mol−1 K−1) | −27.9 | −32.5 | 26.4 | 4.6 | −0.4 | −27.4 | −43.1 |
ΔH‡ (kcal mol−1) | 23.7 | 24.6 | 192.0 | 73.3 | 28.6 | — | — |
ΔG‡ (kcal mol−1) | 40.6 | 50.3 | 165.6 | 68.7 | 28.8 | — | — |
The negative values of ΔS‡ obtained experimentally can be explained by the fact that the entropy of the activated complex, i.e. transition state, is lower than that of the reactants. From the theoretical values for ΔS‡ presented in Table 2, we can see that the addition reactions of atomic and molecular oxygen to Q65 can explain the sign and the magnitude of ΔS‡. In addition, it is clear that the ΔH‡ and ΔG‡ values obtained experimentally are more close to those of the 1,3-H shift mechanism, suggested in our theoretical assessment. This again supports our finding on this mechanism, and its important role during the Q65 thermo-oxidative decomposition. Finally, the increase in ΔH‡ and ΔG‡ values between the two stages, Tp1 and Tp2, is unexpected. However, we showed during our thermal analysis for the different reactions of Q65 that some of these reactions might involve an increase in the thermodynamic values, as shown earlier in Fig. 9 and 10.
The significance of this work lies in the fact that the thermodynamic parameters obtained, ΔG in particular, are highly endogenic, including those involved in the formation of singlet oxygen. In addition, and because of the complexity and size of the Q65 molecule, its oxidation reaction mechanism suggests slow and tough reaction pathways, which strongly suggests the need for a catalyst. Therefore, the outcomes of this study will be used to shape the framework for future studies that aim to understand Q65 reactions on nanoparticle surfaces as catalysts, in order to allow better optimization and control of the catalytic recovery and upgrading of asphaltenes.
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