Formation Mechanism and Structural Evolution of Mesophase Pitch via Phase Separation

3.1. Impact of Reaction Time on Yield The variation in yields with reaction time across different heat treatment stages shows distinct trends, as shown in Table 3 and Figure 1. Shorter non-stirring holding times at a certain stage (S3–M3.1) notably increase the solid phase yield, in contrast to the yield gains observed with extended reaction durations (S3–S4). This phenomenon can be attributed to the varying concentrations of free radicals at different stages; maintaining reactions at points where free radicals are more concentrated enhances macromolecular radical binding, thus maximizing solid phase production [40,41]. During the initial stage, extending the thermal polycondensation reaction time led to a progressive decrease in liquid phase yield and an increase in solid phase yield, the total yield experienced a gradual reduction. This is because the formation of a new mesophase pitch requires the interplay of pyrolysis reactions, nucleation, and liquid crystallinity coupled with the diffusion of mesogen from the surrounding isotropic phase into the anisotropic phase [9]. Notably, the rate of increase in the solid phase was not uniform; it increased by 9.20 wt.% from S1 to S3 but showed a marginal decrease between S3 and S4. This irregularity is due to the slower conversion rate of monocyclic and bicyclic aromatic hydrocarbons to polycyclic aromatic hydrocarbons compared to their transformation into anisotropic phases [42]. As the reaction progresses, extensive consumption of active free radicals prevents the formation of new solid phase components. Instead, when no new components are formed, hydrogen transfer is weakened, and polycyclic aromatic hydrocarbons undergo condensation to form smaller molecules [43,44]. This will reduce the weight of the polycyclic aromatic hydrocarbons, resulting in a slight decrease in solid phase yield. Pressure polymerization can effectively incorporate some lighter components in the formation of mesogen [19,25], thereby increasing the yield of mesophase pitch. At stage S5, as the pressure increases, the original gas-liquid equilibrium will be disrupted, and the molecules that have turned into the gas phase will return to the liquid phase, thus boosting the total yield. In the second stage, the most substantial increase in solid phase yield occurred when non-stirring heating was preserved at the stage of S3, where the yield rose by 14.20 wt.%. At this juncture, a significant amount of solid phase product was formed (5.80 wt.%), displaying enhanced condensation activity and a robust capacity for free radical absorption. Under non-stirring conditions, the density gradient promoted increased interactions between free radicals and macromolecules, expediting the formation of anisotropic phases centered around polycyclic aromatic. In contrast, lesser increases were observed at S2, attributed to the lack of generation of large amounts of polycyclic aromatic hydrocarbons and the lower polycondensation activity of the components. The molecules with higher reactivity can break their side chains and release free radicals earlier, while molecules with lower condensation reactivity require a longer time to transform into flat, disc-shaped polycyclic aromatic hydrocarbons [45]. Therefore, after one hour of holding, yield increases were observed; however, during the second hour, yield gains decelerated markedly due to substantial free radical consumption in the initial hour. Further examination of the reaction processes in M2.1–M2.3 and M3.1–M3.2 illustrates that a similar reaction pattern to the first stage (S2–S4) occurs when free radical content is low, characterized by condensation reactions alongside cleavage to form smaller molecules, thereby slightly increasing or reducing the solid phase yield due to intensified cyclization and deepening carbonation [46]. 3.2. Impact of Extended Holding on QI Formation The content of four subfractions in solid phase under different reaction conditions shows distinct trends, as shown in Table 4 and Figure 2. In the initial stage of the heat treatment process, aromatic molecules within the reaction system undergo dehydrogenation and condensation, forming polycyclic aromatic. This results in a gradual increase in the QI yield, which stabilizes around 69.00 wt.%. The QI (anisotropy), behaving chemically as a solute and the other three components as solvents, correlates with an increase in solid phase yield. According to reports [18,27], the fusible components within mesophase pitch possess a specific quantity of alkyl side chains, and the proportion and properties of these components determine the extent to which the non-fusible components can be fully dissolved. This dynamic equilibrium is crucial for indicating excessive carbonization, as depicted by the trends shown in Figure 2. In the reaction dynamics, HS (isotropic) and HI-TS (isotropic) serve as initial feedstock components. These undergo cleavage, producing small molecule radicals while simultaneously condensing to form TI-QS (partial anisotropic). TI-QS further transforms, breaking down to regenerate lighter components such as HS and HI-TS and condensing into more complex QI [6]. At a point where no new QI is produced (S4), the existing QI undergoes further dehydrogenation and condensation, leading to a deeper degree of carbonization and a slight decrease in mass. Extending the holding time beyond optimal parameters can induce structural disparities between newly formed QI and the original components, thereby potentially compromising the quality of the final product [47]. During the second stage of the heat treatment, non-stirring heat preservation improves the solid phase yield by promoting free radicals to bind more to generate lightweight components, which disrupts the homogeneity of the reaction mixture, leading to fluctuations in the proportion of QI. Although the solid-phase yield exhibits a marginal increase during the second hour of holding, the formation of new QI ceases, reflecting a similar reaction pattern observed between stages S3 and S4. This cessation is attributable to the substantial depletion of macromolecular radicals, resulting from the extensive conversion of TI-QS to QI and the production of lightweight components at the preceding stage, which proves insufficient for new component synthesis [42,45]. Prolonged holding times exacerbate these structural differences, further degrading product quality. Conversely, heat preservation at points where the generation of new QI is minimal, such as stage S2, exerts negligible effects on QI yield. This phenomenon is due to the diminished polycondensation activity of the original components, where the promoting effect on the condensation of other components is relatively small [22,48]. Notably, the most pronounced augmentation in QI yield is achieved by maintaining heat at a stage characterized by high production of new QI (S3). At this juncture, gravitational forces and enhanced condensation of macromolecular radicals contribute significantly to an increase in molecular weight, thereby increasing the yield of the final product [49,50]. 3.3. Impact of QI Formation on Anisotropic Phase Textures Isotropic absorbs light and shows a completely black color, while anisotropic reflects light and shows a yellow color (as shown in Figure 3). During the heat treatment stage, the anisotropic phase texture morphology undergoes significant transformations. It can be observed in combination with the polarization image in Figure 3 and the ratio of the components of the subfractions in Table 4. At the outset (S1), with a minimal amount of newly formed QI, the texture is notably refined characterized by a streamlined optical structure. As the reaction advances to S2, the increase of QI is slow (Table 4), and the isotropy is further transformed into anisotropy (Figure 3, HS + HI-TS → QS-TI + QI), the increase in new components enhancing the texture’s structure (S1 → S2). However, at stage S3, a surge in QI production disrupts this structure due to structural discrepancies between new and original components, reintroducing isotropic characteristics into the solid phase [51,52]. Lou [42] has reported that during the initial stage of carbonization, the formation of TI promotes the aggregation of macromolecules with poor planarity, thereby disrupting the layer orientation and texture of the mesophase pitch. In the absence of new QI formation (S4), the ongoing carbonization process continues to convert isotropic components into the anisotropic phase. Here, the dehydrogenation and condensation processes enhance the structural alignment of the newly formed QI with the original, reducing steric hindrances and leading to the formation of a wide-area streamlined mesophase structure. Nevertheless, by stage S5, the cumulative effects of QI regeneration disrupt the textural balance once more, deteriorating the structure as the reaction cycle progresses. In the second stage of heat treatment, displayed in Figure 4, the optimal holding time is identified as two hours. Similar to the first stage, structural disruptions occur after one hour due to significant new QI generation. However, the texture morphology improves significantly in the second hour. The substantial consumption of macromolecular radicals during this period halts further QI formation, allowing the polycyclic aromatic more time to align and fuse. This alignment enhances the overall structure of the mesophase [46]. Nonetheless, due to gravitational forces and stacking effects, newly formed QI tends to accumulate on the surface of the original component. These results in excessive carbonization at the bottom layer, leading to compositional and structural differences across the lengthways of the anisotropic phase [53]. 3.4. Thermodynamic Analysis of Mesophase Pitch TGA analysis (as shown in Figure 5) presents the thermal behavior of solid phase products from various stages of heat treatment. Guo [54] demonstrated by thermogravimetric analysis that the thermal cracking activity of the four fractions isolated from petroleum residual oils is in the order of saturates > aromatics > resins > asphaltenes. In Figure 5, the TGA curve features two primary peaks. The first peak, observed around 240 °C, is attributed to the evaporation of light components such as HS, HI-TS, and TI-QS, with the peak intensity reflecting the proportion of light components. The second peak, appearing around 470 °C, relates to the amount of newly formed QI and the extent of carbonization; a higher newly formed QI content enhances this peak due to increased condensability [53]. Conversely, a lower newly formed QI content and reduced polycondensation activity result in a diminished peak due to decreased reactivity. Throughout the first heat treatment stage, the first peak exhibits periodic trends with the extension of reaction time, suggesting that the isotropic phase undergoes periodic changes driven by the heat shrink polymerization activities of the molecules. The intensity of this second peak progressively increases with QI accumulation from S1 to S3 but decreases at S4 when the rate of QI formation slows, reflecting reduced reactivity of the newly formed QI. Conversely, as new QI continues to form at S5, the peak ascends once again. During the second stage of heat treatment, after one hour of non-stirring heat preservation, a notable enhancement in both peaks suggests that non-stirring preservation under low pressure effectively promotes solid phase formation. However, the molecular mass remains relatively low with higher molecular activity, resulting in less uniform molecular structures. In the subsequent one hour, the extensive consumption of free radicals limits new component formation, leading to condensation among existing molecules [18], which increases the overall molecular weight and decreases structural disparities, as reflected by a reduction in both peak intensities. Prolonged holding times, such as seen in M2.3, intensify structural differences due to gravitational effects under non-stirring conditions [53]. Nonetheless, during the first hour of holding at S3, a significant amount of QI is newly formed, absorbing macromolecular free radicals, which further increases the molecular weight and reduces light component formation. Prolonged holding times lead to a phenomenon similar to other reaction processes, where dehydrogenation and condensation improve the ordering of polycyclic aromatic hydrocarbons, resulting in a more refined texture structure of the mesophase pitch. 3.5. Functional Group Analysis Evolution during Heat Treatment The FTIR spectroscopy analysis reveals detailed vibrational characteristics (As shown in Figure 6) indicative of the structural evolution within the solid phase products of mesophase pitch obtained from the first and second stages of heat treatment. Key to the findings, bending vibration peaks of aromatic hydrogen C-H bonds are observed at 750 cm⁻¹, 810 cm⁻¹, and 877 cm⁻¹. The peak at 750 cm⁻¹, signifying the presence of a kata-condensed aromatic nucleus, suggests a robustly condensed aromatic structure, while the isolated C-H bond peak at 877 cm⁻¹ indicates a peri-condensed aromatic nucleus. Throughout the reaction stages S1 to S5, the increasing intensity of these peaks highlights a dominant polycondensation reaction, progressively enriching the aromatic content of the product [55,56]. Moreover, the FT-IR spectra include skeletal stretching vibrations of aromatic rings at 1610 cm⁻¹, indicating the increase of aromatic compounds as the reaction progresses. Deformation vibration peaks of CH₃ at 1380 cm⁻¹ and CH₂ at 1460 cm⁻¹, along with their respective stretching vibrations at 2850 cm⁻¹ and 2920 cm⁻¹, indicating that the solid phase product still contains methyl, methylene, or cycloalkyl-substituted branched chains. Additionally, the appearance of aromatic ring C-H stretching vibration peaks at 3050 cm⁻¹ (as shown in Figure 6) suggests that unsubstituted aromatic hydrogen structures are still present in the solid phase product, indicating a degree of structural integrity in the aromatic framework. However, a significant decrease in the intensity of these peaks suggests thermal cracking and condensation of aromatic side chains, reducing methyl and methylene structures and deepening the condensation process, which ultimately leads to the formation of extensive polycyclic aromatic hydrocarbons [57]. A critical observation relates to the C-H out-of-plane bending vibration peak at 670 cm⁻¹, which closely correlates with the quantity of newly formed QI and its polycondensation activity. As new QI is produced, this peak’s intensity becomes pronounced, leading to a deterioration in texture structure. Conversely, when QI undergoes self-polycondensation, the peak intensity diminishes, and the texture structure improves. During the first stage of thermal treatment, the peak intensity increases, reaching a maximum at stage S3, where extensive QI formation occurs. When no new QI forms at stage S4, the original QI undergoes condensation reactions, enhancing the aromaticity and causing the peak to vanish. This disappearance coincides with improvements in the texture structure. However, when a substantial amount of QI forms again at stage S5, the peak reappears, indicating a cyclic pattern of chemical transformations. In the second stage of heat treatment, QI exhibits distinct behavior patterns. After one hour at the S2 holding temperature, the amount of QI generated is comparable to that observed during the first stage, and the peak values are lower, suggesting differences in QI formation under non-stirring conditions. Non-stirring heat preservation enhances the likelihood of condensation between molecules, which modifies the condensation dynamics. The newly formed QI at S2 absorbs less macromolecular free radicals from the lower layers, displaying relatively low polycondensation activity compared to S3. This finding underscores the complex interplay of heat treatment conditions, molecular interactions, and chemical reactions that influence the structural and textural properties of the mesophase pitch. 3.6. Crystal Phase and H/C Analysis The XRD analysis delineates the evolution of crystal structures within solid phase products throughout various heat treatment stages. The XRD patterns (as illustrated in Figure 7) show a pronounced peak at 2θ = 25° and the calculation results are listed in Table 5. It confirms a high degree of ordering among polycyclic aromatic, which correlates with the spectral data of molecular interlayer spacing (d₀₀₂), stacking heights (Lc), layer numbers (n) and orientation degrees (O_g). Lee [58] points out that the smaller the d₀₀₂ and the greater the Lc, the more ordered the mesophase pitch. Analysis reveals that the molecular layer spacing and the number of carbon layer stacks vary as the polycondensation reactions intensify. This trend is mirrored by the intensity changes in the C-H out-of-plane bending vibration at 670 cm⁻¹, providing a marker for QI activity. When QI formation is extensive, the structural disparity disrupts the orderly arrangement of aromatic, reducing the Lc and n. Conversely, no new QI formation facilitates closer molecular stacking and improved texture by enhancing the planarity [25]. Throughout the heat treatment phase, the mesophase structure develops rapidly, with polycyclic aromatic initially hindered by long side chains from asphaltenes and heavy gums. As these side chains cleave from S1 to S2, molecular orientation and stacking improve significantly. However, the newly formed QI at stages of S3 obstructs complete aromatic interpenetration, affecting molecular stacking and orientation negatively [59]. From S3 to S4, the absence of new QI formation allows other molecular aromatization and cyclization, which further consolidates the molecular structure, enhancing stacking. A resurgence in new QI formation at S5 disrupts this order, highlighting the cyclic nature of structural transformations. In the second stage, optimal structural properties are achieved with a two-hour holding period. Initially, increased QI content enhances structural difference, but without stirring, the uniformity of the component distribution suffers, reducing the Lc and n. As reaction dynamics slow down, the molecular structure stabilizes, which improves texture and molecular orientation. The reason is that polycyclic aromatic gain sufficient energy to align and interpenetrate more effectively under no-stirring conditions [22]. The H/C ratio analysis across these stages illustrates a decrease with prolonged reaction time, reflecting increased carbonization and condensation. The first stage shows superior homogeneity and molecular structure, lessening the risk of over-carbonization [53]. In contrast, the second stage demonstrates that prolonged holding intensifies condensation, intensifying the vertical differences in molecular structure and complicating control in industrial settings. 3.7. Formation Mechanism and Structural Evolution of Mesophase Pitch As shown in Figure 8, it can be seen that mesophase pitch forms through a two-stage heat treatment of catalytic cracking oil slurry, wherein controlled polycondensation and condensation reactions of aromatic hydrocarbons create an organized structure with anisotropic properties essential for advanced carbon materials. In the first stage, polycyclic aromatics self-assemble into liquid crystal domains. The aromatic hydrocarbons are mostly semi-rigid, have a single alkyl side chain, and the molecular spacing is large, which contains mesogens that are not transformed into anisotropy [60]. Reaction time is pivotal: extended durations promote the transformation of semi-rigid molecules into rigid ones by enhancing radical interactions and dehyroaromatization. According to Lee [57], polycyclic aromatic hydrocarbons with semi-rigid structures do not have the same degree of molecular stacking and ordering in the liquid crystal phase as compared to rigid structures. When the increase in QI stopped, the transition from semi-rigid to rigid structures in polycyclic aromatic hydrocarbons led to an enhancement in molecular order. In the second stage, non-stirring heat preservation enables macromolecular condensation to generate larger aromatic structures and promote mesophase molecule integration [18], refining the anisotropic texture and yield essential for pitch quality. Thermal and structural analyses reveal that heat treatment lowers the hydrogen-carbon (H/C) ratio. FTIR spectra show increasing aromatic bonds and fewer aliphatic side chains, indicating deep carbonization. XRD analysis further confirms reduced interlayer spacing and increased molecular stacking. Overall, optimized heat treatment yields mesophase pitch with high structural integrity and anisotropy, which is ideal for carbon-based applications.