Harnessing High-Density-Polyethylene-Derived Liquid as a Model Solvent for the Co-Liquefaction of Low-Rank Coals: Toward Sustainable Mesophase Pitch for Making High-Quality Carbon Fibers from Waste Plastics

Abstract

The accumulation of polyolefin waste, particularly high-density polyethylene (HDPE), presents a growing environmental challenge due to limited recycling options and poor end-of-life recovery. This study explores a strategy to convert HDPE into mesophase pitch (MP), a valuable carbon precursor, by integrating polyolefin recycling with the mild solvolysis liquefaction (MSL) of low-rank coals. HDPE was first hydrogenolyzed into a hydrogen-rich aromatic liquid (HDPE-liquid), which was then used as the liquefaction solvent. Under identical conditions (400 °C, 60 min), Utah Sufco coal co-liquefied with HDPE-liquid produced tar that formed mesophase pitch with a higher mesophase content (84.5% vs. 78.6%) and a lower softening point (~302 °C vs. >350 °C) compared to pitch from conventional tetralin (THN). The approach was extended to Illinois #6 and Powder River Basin coals, increasing the mesophase content from 12.4% to 32.6% and 17.8% to 62.1%, respectively. These improvements are attributed to differences in tar composition: HDPE-derived tars had lower terminal methyl (Hγ) contents, reducing cross-linking during thermal upgrading. This work demonstrates that HDPE-derived liquids can act as functional solvents for coal liquefaction, enabling an effective route to recycle polyolefin waste into durable carbon products, while also reducing reliance on fossil-based solvents for mesophase pitch production.

1. Introduction

Plastic waste accumulation poses serious environmental and resource efficiency challenges. With the annual production of plastics exceeding 400 million tons, the need for scalable and carbon-negative recycling solutions is increasingly urgent [1]. Polyolefins, such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP), are the most widely used plastics in the world, especially in packaging. These materials are produced in large quantities due to their low cost and durability, but their durability also makes them difficult to manage at the end of their life [2,3]. Most polyolefin waste is not recycled. It is either landfilled or discarded in the environment, with only a small portion processed through mechanical or chemical recycling [2]. Each year, over USD 80 billion worth of material is lost due to single-use packaging waste alone [3].

The recycling processes that are currently used, such as incineration or fuel production, release CO₂ and, therefore, fail to meet emission reduction goals [4]. These methods also release secondary pollutants, which raise additional environmental and public health concerns, prompting regulatory restrictions by local governments and the U.S. Environmental Protection Agency (EPA) [5]. As a result, there is growing interest in carbon-sequestering plastic valorization strategies that convert plastic waste into durable carbon products instead of releasing carbon to the atmosphere.

However, although many thermal processes have been developed to repurpose or convert polyolefins, most of them yield only gaseous or liquid products, rather than solid products [6,7,8]. This is due to the chemical structures of polyolefins, which tend to break down into light hydrocarbons when heated, rather than directly converting into solid carbon materials [9]. Extensive research has explored co-processing polyolefins with various hydrocarbons to produce carbon-based materials. For instance, Melendi et al. demonstrated the co-processing of HDPE with lube oil and coal tar to produce furnace coke [10,11], while LDPE and PP have been co-processed with polystyrene (PS) and polyethylene terephthalate (PET) for metallurgical coke production [12]. Similarly, Xu et al. employed metal-doped mesoporous biochar catalysts to co-process biomass with HDPE, yielding methane and carbon nanotubes [13]. These studies primarily treated polyolefins as reactive additives or hydrogen-rich diluents in high-temperature thermal or catalytic systems, rather than isolating functional intermediates for targeted carbon material synthesis. However, considering the broad distribution of the molecular weight and the high aromatic content in polyolefin-derived liquid, an alternative approach is to repurpose these liquids as functional solvents in thermochemical liquefaction. This opens the possibility of producing high-value feedstocks such as mesophase pitch, which can be used to produce high-performance carbon fibers [14].

Mesophase pitch is a solid carbonaceous material with optically anisotropic, aligned polyaromatic structures that make it suitable for producing high-performance carbon fibers [15]. Mesophase pitch can be produced by the thermal treatment of different aromatic-rich precursors, such as coal tar or coal tar pitch, which can be produced via metallurgical coking, pyrolysis, gasification, and liquefaction [16]. Liquefaction involves thermally treating coal in organic liquefaction solvents under heat and pressure. This provides a controlled method to generate liquefaction tar, a heavy aromatic-rich liquid, by breaking down solid feedstocks such as coal into polyaromatic intermediates [17]. When hydrogen-donating solvents are used in coal liquefaction, the resulting tars are more reactive and promote the formation of mesophase domains during subsequent thermal treatment [16], as hydrogenation can partially saturate aromatic rings into naphthenic structures that facilitate molecular alignment and domain growth during thermal treatment of liquefaction tar [17].

Our group has previously developed a mild solvolysis liquefaction (MSL) process that uses tetralin (THN) to convert low-rank coal into coal tar suitable for making mesophase pitch [18]. However, while this method produces high-quality pitch, it depends on THN, a costly petroleum-derived hydrogen-donor solvent, typically produced via the high-pressure hydrogenation of naphthalene, an energy-intensive process that contributes to fossil carbon emissions [19]. Replacing THN with a solvent made from waste plastic would reduce costs and environmental impacts.

Waste HDPE is known to produce aromatic-rich liquids through hydrogenolysis [20]. These hydrocracking processes typically produce heavy fractions rich in aromatic and naphthenic fractions as byproducts [21,22]. Similar fractions in industrial direct coal liquefaction (DCL) are commonly recycled and repurposed as liquefaction solvents [23]. Therefore, such structural similarities suggest that these HDPE-derived liquids could have the right chemical features to act as process solvents for coal liquefaction [24,25]. However, the use of hydrogenolysis-derived plastic liquids as functional solvents in coal liquefaction, particularly for mesophase pitch production, remains largely unexplored in the open literature.

While various studies have explored the use of coal- and petroleum-derived solvents in coal conversion [26,27] and pitch synthesis [28,29], the application of plastic-derived liquids as process solvents in these contexts remains underexplored. This presents a novel integration point between plastic recycling and carbon material production. By valorizing plastic-derived aromatic liquids as liquefaction solvents, it may be possible to simultaneously advance plastic waste mitigation and carbon sequestration through durable material formation.

In this proof-of-concept work, we used hydrogenolysis to convert virgin HDPE into a liquid product, then applied this HDPE-derived liquid as a model solvent in the MSL of Utah Sufco coal. We studied how temperature and reaction time affected the yield and composition of the resulting coal tar, and then thermally treated the tar to produce mesophase pitch. To test whether this method works more broadly, we also applied the same HDPE-liquid to two other types of coal: Illinois #6 and Powder River Basin (PRB). We compared the performance of HDPE-liquid with THN in terms of the mesophase formation, softening point, and tar composition. This study provides foundational engineering insights into how hydrogenolysis-derived plastic liquids can be repurposed as functional solvents for mesophase pitch production, offering a step toward integrating plastic recycling with advanced carbon material manufacturing, helping reduce plastic pollution while creating useful products that store carbon in solid form.

2. Materials and Methods

2.1. Materials and Reagents

HDPE powder was purchased from Sigma-Aldrich. Three types of low-rank coal, including Utah (Sufco) Coal, Illinois #6 coal, and Wyoming PRB (Black Thunder) coal, were used in this study. The proximate and ultimate analyses of the HDPE and coal samples are presented in

Table 1. Proximate and ultimate analysis of polyolefin plastic HDPE and three types of coals.

Sample	HDPE	UT Sufco	IL #6	WY PRB
Coal Rank	N.A.	bituminous	bituminous	Sub-bituminous
Moisture (wt%)	N.A.	6.1	9.6	23.7
Ash (wt%, db)	1.0	8.9	10.4	6.5
Volatile Matter (wt%, db)	N.A.	41.0	39.9	43.7
Element Content (wt%, daf)
C	85.2	79.4	78.4	75.3
H	14.4	5.6	5.4	5.0
N	0.1	1.3	1.5	1.1
S	0.0	0.4	3.9	0.3
O a	0.3	13.3	10.8	18.3

db: dry basis; daf: dry-ash-free basis; N.A.: not available; a: calculated by difference.

. Prior to testing, all the coal samples were ground, sieved with a 100-mesh sieve, and dried. Although virgin HDPE was used for consistency and reproducibility, it is recognized that real-world waste HDPE may contain impurities, degradation products, and polymer blends that could influence the solvent composition and performance. This limitation is discussed further in Section 3.5.

2.2. Experimental Procedures

To aid clarity, a schematic diagram of the experimental procedure is presented in Figure 1. The process begins with the hydrogenolysis of HDPE under high-pressure hydrogen to produce a complex liquid–solid mixture. The THF-soluble portion of this mixture, referred to as HDPE-liquid, was isolated and used as the co-liquefaction solvent. This solvent was mixed with various coals in a high-pressure reactor to generate a liquefaction product rich in heavy tar. The hexane-insoluble fraction of this product, referred to as CLT, was subsequently thermally upgraded under nitrogen to form mesophase pitch (CLTP). Light fractions, gases, and solid residues generated during the process were separated and excluded from further analysis.

2.2.1. Preparation of the HDPE-Derived Liquid as the Liquefaction Solvent

The HDPE-derived liquid was prepared by hydrogenolysis in a high-pressure batch reactor. The reaction was carried out in a 1.0 L 316 stainless-steel reactor (Parr 4571, Parr Instruments Co., Moline, PA, USA) at 450 °C for 60 min under an initial hydrogen pressure of 6 MPa, with a stirring rate of 100 rpm. The conditions were selected based on our previous studies [30,31] to maximize the yield of THF-soluble heavy liquid fractions relevant to further liquefaction. After the reaction, the liquid products were collected, and the tetrahydrofuran (THF)-soluble fraction was isolated using Soxhlet extraction. This THF-soluble fraction was labelled as the HDPE-derived liquid and used as the solvent in subsequent mild coal liquefaction experiments.

2.2.2. Co-Liquefaction Procedure and Product Separation Procedure

The solvolysis co-liquefaction of the HDPE-derived liquid and Sufco coal was carried out in a 1.0 L 316 stainless-steel high-pressure batch reactor (Parr 4571, Parr Instruments Co., Moline, PA, USA). Dry coal powder was mixed with the HDPE-derived liquid at a 1:1 mass ratio (coal-to-liquid). The reaction was conducted at temperatures ranging from 350 °C to 450 °C for durations of 0 to 480 min under an initial hydrogen pressure of 6 MPa, with a stirring rate of 100 rpm. After the reaction, the system was rapidly cooled to room temperature. Given the low hydrogen consumption (<2 wt% of coal) [32,33,34] and the current focus on liquid products, the gas phase was vented without further analysis, and only the solid and liquid products were collected.

The combined product mixture underwent solvent extraction to isolate the hexane-insoluble, THF-soluble (HI-THFS) liquid fraction. This fraction was defined as co-liquefaction heavy tar (CLT). Each sample was labeled as A-CLT-X-Y, where A represents the coal source, X the liquefaction temperature, and Y the reaction time. For example, UT-CLT-400-60 refers to the sample produced from Utah Sufco coal at 400 °C for 60 min. The reactions were performed in triplicate to ensure reproducibility, and only the average values are reported. A summary of the sample designations used throughout the manuscript is provided in

Table 2. Sample designation and corresponding experimental conditions.

Sample Code	Coal	Solvent	Liquefaction Temperature (°C)	Time (min)	Notes
UT-CLT-400-60	Utah Sufco	HDPE-liquid	400	60	Co-liquefaction tar
UT-MSLT-400-60	Utah Sufco	Tetralin	400	60	THN-based tar
IL-CLT-400-60	Illinois #6	HDPE-liquid	400	60	Co-liquefaction tar
IL-MSLT-400-60	Illinois #6	Tetralin	400	60	THN-based tar
WY-CLT-425-120	Wyoming PRB	HDPE-liquid	425	120	Co-liquefaction tar
WY-MSLT-425-120	Wyoming PRB	Tetralin	425	120	THN-based tar

for clarity.

2.2.3. Thermal Upgrading Procedure

The thermal upgrading of the coal liquefaction tar samples was carried out in a custom-designed 316 stainless-steel stirred reactor with a total volume of 739.3 mL (25 US fl oz), as detailed in our previous work [35]. The samples were thermally treated under a continuous nitrogen flow at temperatures ranging from 380 to 440 °C for 1 to 6 h. The evaporation of volatile fractions occurred during heating, and only the condensed pitch residue was recovered post-reaction. A mechanical stirring bar maintained uniform mixing at 100 rpm throughout the process. The resulting solid product was defined as the co-liquefaction tar pitch (CLTP) and used for further analysis. The pitch yield and the mesophase pitch yield were calculated using Equations (1) and (2):

$$pitch yield\left(Y_{CLTP},\mathrm{w}\mathrm{t}\mathrm{\%}\right)={\displaystyle \frac{mass of pitch}{mass of coal tar}}\times 100 \mathrm{w}\mathrm{t}\mathrm{\%}$$

(1)

$$mesophase pitch yield\left(Y_{MP},\mathrm{w}\mathrm{t}\mathrm{\%}\right)=Y_{CLTP}\times mesophase content$$

(2)

All the thermal upgrading experiments were performed in triplicate to ensure data reproducibility. Standard deviations for yield and softening point measurements are included where relevant.

2.3. Characterization Approaches and Calculation Methods

2.3.1. Solvent Fractionation Approach

The co-liquefaction tar samples were sequentially fractionated into an n-hexane-soluble fraction (HS), a hexane-insoluble/toluene-soluble fraction (HI-TS), a toluene-insoluble/THF-soluble fraction (TI-THFS), and a THF-insoluble fraction (THFI) using Soxhlet extraction following a modified ASTM D-6560-2022 method [36]. In the following sections, the HS, HI-TS, and TI-THF fractions are referred to as oil (O), asphaltene (A), and pre-asphaltene (P) fractions, respectively. The quinoline-insoluble (QI) fraction content in the co-liquefaction samples and obtained products was determined according to ASTM D2318-20 [37]. All the analyses were conducted in triplicate and reported with average results and standard deviation.

2.3.2. Thermogravimetric Analysis (TGA)

Boiling point distributions of co-liquefaction tar samples were determined using a thermogravimetric analyzer (Q-600, TA Instruments, New Castle, DE, USA). Approximately 20–30 mg of each sample was heated in nitrogen (flow rate: 50 mL/min) at a constant rate of 10 °C/min. Since the thermal upgrading process was conducted at 380–425 °C, particular attention was given to the fractions of materials with boiling points of above 400 °C. The mass fraction of components with boiling points of above 400 °C was used as a proxy for the pitch-forming potential.

2.3.3. Proton Nuclear Magnetic Resonance (¹H NMR)

A ¹H NMR analysis was performed on a 500 MHz 1H NMR spectrometer (AVANCE III HD, Bruker, Billerica, MA, USA). Each tar sample was dissolved in deuterochloroform (CDCl₃) with 1 vol% tetramethylsilane (TMS) as an internal reference. Proton chemical shifts were assigned to the following categories: H_ar represents the percentage of protons in aromatic structures (chemical shift at 8.5 to 6.6 ppm). H_α represents the percentage of protons near the carbon atom in the α-position of aliphatic substituents of aromatic structures (chemical shift at 4.0 to 2.2 ppm). H_β represents the percentage of protons in methylene groups (chemical shift at 2.1 to 1.1 ppm). H_γ represents the percentage of protons in the terminal methyl groups of the alkyl moieties of molecules (chemical shift at 1.1 to 0.3 ppm). These structural proton fractions were used to assess solvent reactivity and potential hydrogen-donating behavior. All the spectra were acquired at 25 °C with a 64-scan average.

2.3.4. Polarizing Microscopy (POM)

Mesophase textures were examined using a polarized light microscope (BH200-MR, Ningbo Sunny Instruments Co., Ltd., Ningbo, China), equipped with a polarizer, wave retarder, and oil-immersion objectives. Samples were embedded in epoxy resin and polished sequentially with silicon carbide papers (360, 600, 800, 1200 grit), followed by a 2 min polish with a 1 μm diamond suspension (Buehler MetaDi). Images were taken at 10× magnification under crossed polarizers. For each sample, 12 images from different fields were analyzed. The mesophase content was quantified based on a modified ASTM D4616-95 (2018) method [38] using an image analysis tool developed in our previous work [39]. The results are reported as the percentage mesophase coverage, averaged over all the fields.

2.3.5. Dynamic Mechanical Analysis (DMA)

The softening points of the mesophase pitch (MP) samples were measured using a hybrid rheometer/DMA (HR 20, TA Instruments, New Castle, DE, USA) under a nitrogen atmosphere. The samples were formed into 1/8-inch diameter discs and loaded under parallel plate geometry with 0.2 N compression. The temperature ramp was 5 °C/min. This test is critical to determine whether the pitch falls within the target spinnability window (210–350 °C) for melt-spun carbon fiber production [40]. Each sample was tested in duplicate; the values reported are the averages.

3. Results and Discussion

3.1. Characterization of HDPE-Liquid

In coal liquefaction under high-pressure, high-temperature, and hydrogen-rich conditions, the choice of solvent plays a critical role. Effective liquefaction solvents disrupt the inter- and intra-molecular forces within the coal’s macromolecular structure, facilitating swelling and the depolymerization of coal into smaller molecular fragments [41,42,43]. These reactions enhance coal hydrogenolysis and aid in generating liquid products with lower-molecular-weight liquid products [44,45,46]. In addition to physical dispersion, solvents participate chemically by donating hydrogen atoms to coal fragments, thereby stabilizing reactive intermediates through hydrogenation and saturation reactions [47,48,49]. As a result, the chemical structure of the liquefaction solvent significantly affects both reaction behavior and product distribution. In this study, a liquid product derived from the hydrogenolysis of HDPE was evaluated as a potential coal liquefaction solvent. The HDPE-derived liquid was characterized using proton nuclear magnetic resonance (¹H NMR) and solvent fractionation. Its structural and compositional features were then compared with those of tetralin (THN) and several DCL-related solvents to assess suitability.

The ¹H NMR and fractionation results are summarized in

Table 3. The ¹H NMR proton content and solvent fractionation of HDPE-derived liquid compared to representative liquefaction solvents.

Sample	Relative Content of Proton (%)				Solvent Fractionation (wt%)
	Har	Hα	Hβ	Hγ	HS	HI-TS	TI-THFS
HDPE-liquid	21.1	35.8	27.7	15.4	42.5	33.2	24.3
THN	33.3	33.3	33.3	0	100	0	0
DCL products [40]	46.3	31.5	19.4	2.8	0	100	0
DCL products [40]	44.3	31.4	21.5	2.8	0	0	100
DCL solvents [42]	11	20	50	19	NA	NA	NA

HS: n-hexane-soluble fraction; HI-TS: n-hexane-insoluble but toluene-soluble fraction; TI-THFS: toluene-soluble but THF-insoluble fraction; NA: not available.

. The HDPE-derived liquid contains both light and heavy fractions, with 42.5 wt% n-hexane-soluble (HS) and 57.5 wt% heavy fractions (33.2 wt% HI-TS and 24.3 wt% TI-THFS). The NMR analysis reveals that the HDPE-liquid contains 21.1% aromatic protons (H_ar), along with 35.8% H_α, 27.7% H_β, and 15.4% H_γ. The relatively high content of H_β suggests abundant naphthenic or cycloalkyl structures, which are known to contribute positively to proton transfer and hydrogen donation in liquefaction reactions [50,51].

Compared with other liquefaction solvents reported in the literature [51,52,53,54,55], the HDPE-derived liquid shows a chemical composition that is within the range of effective hydrogen-donating solvents. For example, the Har content is moderate, while the combination of H_β and H_γ indicates a balance of cycloalkane and alkyl chain components, a feature associated with favorable hydrogen-transfer characteristics. Therefore, based on this characterization, the HDPE-derived liquid is considered a suitable candidate for use in coal liquefaction, particularly in mild solvolysis liquefaction (MSL) systems.

3.2. The Effect of Co-Liquefaction Conditions on the Production of Tar Intermediates

We investigated how temperature and reaction time influence the formation of tar intermediates (UT-CLT) during the co-liquefaction of Utah Sufco coal and HDPE-derived liquid. As shown in Figure 2a, the tar yield at 350 °C was low. Increasing the temperature to 375 °C and 400 °C resulted in significant increases in the tar yield, reaching the highest yield at 400 °C. However, further increasing the temperature above 400 °C led to reduced tar yields. This is attributed to enhanced thermal cracking, which converts heavy tar fractions into lighter liquids and gaseous products [56].

Similarly, the reaction time also affected tar yields, as shown in Figure 2b. Extending the reaction time initially improved the tar yields, but prolonged times beyond the optimum caused a reduction in the yield. This is likely due to the secondary cracking of heavier fractions into lighter compounds, accompanied by increased free radical reactions over time. Thus, carefully controlling temperature and reaction time is necessary to maximize heavy tar yields for producing mesophase pitch.

Based on these results, three co-liquefaction tar samples were selected for further evaluation. These samples include UT-CLT-400-60 (400 °C, 60 min), UT-CLT-425-60 (425 °C, 60 min), and UT-CLT-400-120 (400 °C, 120 min). Additionally, a control sample produced from Utah Sufco coal and tetralin (THN) at 400 °C for 60 min (UT-MSLT-400-60) was prepared for comparison.

Table 4. Characterization of co-liquefaction tar samples produced at selected reaction conditions.

Sample	Relative Content of Proton (%)				Boiling Point Distribution (wt%)
	Har	Hα	Hβ	Hγ	≥400 °C	<400 °C
UT-CLT-400-60	70.50	17.99	10.07	1.44	26.5	73.5
UT-CLT-425-60	70.45	17.93	10.09	1.53	21.3	78.7
UT-CLT-400-120	67.88	18.66	12.41	1.05	22.9	77.1
UT-MSLT-400-60	60.93	19.87	15.23	3.97	41.9	58.1

H_ar: proton with a chemical shift at 8.5 to 6.6 ppm; H_α: proton with a chemical shift at 4.0 to 2.2 ppm; H_β: proton with a chemical shift at 2.1 to 1.1 ppm; H_γ: proton with a chemical shift at 1.1 to 0.3 ppm.

presents the characterization results from the ¹H NMR and boiling point analyses. The three UT-CLT samples show similar proton distributions, featuring relatively high aromatic proton (H_ar) contents and low terminal methyl proton (H_γ) contents. Among these, UT-CLT-400-60 had the highest heavy fraction content, with 26.5 wt% of its components having boiling points above 400 °C. This heavy fraction is important because lighter fractions tend to evaporate during thermal upgrading, reducing mesophase pitch yield [57].

The proton distribution, especially the content of terminal methyl groups (H_γ), significantly affects pitch formation. High H_γ levels can cause excessive cross-linking reactions, resulting in isotropic pitch, which is less desirable for making carbon fibers [58,59]. Compared to UT-CLT-400-60, the reference tar sample UT-MSLT-400-60 contained a much higher H_γ content (3.97%), approximately 176% greater. This difference suggests that UT-CLT-400-60 would be more suitable for mesophase pitch production. However, previous work has shown that even UT-MSLT-400-60 can yield pitch with a moderate mesophase content (see Figure 3i), indicating that favorable pitch formation can still occur under THN-based conditions. Nevertheless, due to its more favorable composition of a lower H_γ content and a higher heavy fraction, the UT-CLT-400-60 sample was selected for detailed thermal upgrading in subsequent experiments.

3.3. Evaluation of Mesophase Pitch Formation from Co-Liquefaction Tar from Utah Sufco Coal and HDPE-Liquid

The mesophase pitch formation behavior of UT-CLT-400-60 was evaluated by thermally treating the tar at temperatures ranging from 380 to 425 °C and holding times between 1 and 6 h. A reference pitch (UT-MSLT-400-60), produced from Utah Sufco coal using tetralin at 400 °C for 60 min and thermally treated at 425 °C for 3 h, was also included for comparison. Mesophase development was assessed via polarized microscopy (Figure 3), and key pitch properties (

Table 5. Anisotropic contents, QI contents, softening points, and tar-to-pitch yields of the obtained pitch derived from the UT-CLT-400-60 tar.

Temperature (°C)	Holding Time (hours)	Mesophase Content (%)	QI Content (%)	Softening Point (SP, °C)	Tar-to-Pitch Yield (wt%)
410	3	42.1	48.6	210	25.1
410	6	70.4	54.8	273	23.9
425	1	66.7	56.8	284	24.3
425	3	84.5	63.4	302 ± 3	22.7
UT-MSLT-400-60 @425 °C for 3 h		78.6	79.4	>350	31.9

) were analyzed to identify optimal thermal treatment conditions.

At 380 °C and the shortest holding time of 1 h, no observable mesophase domains were formed (Figure 3a). Under these mild conditions, the aromatic molecules in UT-CLT-400-60 lacked sufficient mobility and thermal energy to reorganize into mesophase structures [60]. Increasing the treatment duration at 380 °C to 3 h initiated the formation of small spherical mesophase domains, as referring with the yellow arrow in Figure 3b. Extending the duration further to 6 h resulted in slightly larger domains, but mesophase formation remained limited, as the yellow arrows pointed out in Figure 3c. However, overall mesophase formation remained limited, indicating that 380 °C was insufficient for substantial molecular reorganization.

At higher thermal treatment temperatures, like 395 °C and especially 410 °C (both for 3 h), larger and more densely packed spherical mesophase domains formed, as the yellow arrows pointed out in Figure 3d,e. Higher temperatures promoted molecular mobility, facilitating the growth and coalescence of mesophase spheres into larger anisotropic domains [61].

The most well-developed mesophase texture was achieved at 425 °C with a holding time of 3 h, resulting in large, well-developed lamellar mesophase structures greater than 100 μm in size, covering the entire pitch surface (Figure 3h). This indicates that 425 °C provides sufficient thermal energy and reaction time to achieve complete mesophase domain coalescence. At a slightly lower temperature (410 °C) with an extended treatment time (6 h), a uniform flow-domain mesophase structure formed, covering most of the pitch surface (Figure 3f). In this case, the longer reaction time partially compensated for the lower temperature, allowing partial coalescence into larger flow-domain structures. However, these domains did not fully merge into the uniformly large lamellar structures observed at higher temperatures and adequate durations [62]. Thermal treatment at a higher temperature (425 °C) for a shorter duration (1 h) resulted in a coarse mosaic mesophase structure dispersed throughout the pitch sample (Figure 3g). While 425 °C initiated rapid mesophase nucleation, the short treatment time limited the extent of domain growth and coalescence compared to the sample treated at 425 °C for 3 h (Figure 3h).

Table 5 summarizes the mesophase contents, quinoline-insoluble (QI) fractions, softening points, and pitch yields from tar under different thermal conditions. The pitch obtained at 425 °C for 3 h exhibited the highest mesophase content (84.5%), a relatively high QI content (63.4%), and a favorable softening point around 302 °C. In comparison, the reference UT-MSLT-400-60 pitch sample, despite similar mesophase content, exhibited a higher QI content and a significantly higher softening point (above 350 °C), making it less suitable for fiber spinning. The lower softening point (~302 °C) of the mesophase pitch indicates improved flowability during thermal processing, which is generally favorable for fiber spinning applications [63]. This enhancement is likely associated with the compositional features of the HDPE-derived tar, particularly its relatively high H_β content (Table 4) [64], which quenches the over-polymerization and dehydrogenation during thermal treatment, thereby limiting the formation of semi-coke and QI species that can increase the SP of pitch.

Meanwhile, the superior mesophase pitch formation observed with HDPE-derived tars is primarily attributed to their favorable molecular composition. As shown in Table 4, tar derived from HDPE-liquid exhibits a reduced terminal methyl proton content (H_γ), which helps suppress excessive cross-linking reactions [65,66], and an elevated aromatic proton content (H_ar), which promotes aromatic stacking and molecular alignment [67]. These features support the early nucleation and coalescence of mesophase domains, leading to the formation of larger, more ordered lamellar structures. The resulting pitch shows a higher mesophase content, a lower QI content, and a significantly lower softening point compared to pitch derived from THN-based tars (Table 5). These outcomes demonstrate that the compositional tuning of the tar, particularly increased aromaticity and minimized alkyl branching, is critical to enabling high-quality mesophase pitch formation [68].

These differences in tar composition can be traced back to the chemical and solubility characteristics of the HDPE-derived solvent itself. While THN is fully aromatic and consists entirely of light, hexane-soluble components (100% HS), the HDPE-derived liquid contains a more chemically diverse distribution, including substantial fractions of hexane-insoluble but toluene- and THF-soluble compounds (HI-TS and TI-THFS, totaling 57.5 wt%) (Table 3). This broader solubility profile enhances coal–solvent interactions during liquefaction, facilitating the depolymerization of coal macromolecules and stabilizing intermediate fragments [69]. Although the HDPE-derived solvent contains slightly lower intrinsic aromaticity than THN, it enables the formation of tars with higher H_ar and lower H_γ values after liquefaction. This suggests that the solvent’s role lies not only in solvating and dispersing coal structures but also in controlling the reactivity and composition of the resulting tar [70]. These structural advantages, originating at the solvent–coal interface, ultimately drive the superior mesophase behavior of the resulting pitch.

Although direct rheological data were not collected in this study, the measured softening point and optical texture development are consistent with favorable spinning behavior, as reported in prior research [15,64]. Future work will incorporate rheology testing and fiber-spinning trials to confirm this performance. Given these favorable mesophase and thermal properties, the UT-CLT-400-60 pitch treated at 425 °C for 3 h was selected as the benchmark condition for subsequent evaluations.

3.4. Expanding the Application of HDPE-Liquid for Co-Liquefaction of Varying Types of Coals for Making MP

To assess the broader applicability of HDPE-derived liquid, co-liquefaction experiments were conducted using two additional coal types: Illinois #6 bituminous coal and Wyoming Powder River Basin (PRB) sub-bituminous coal. The influence of liquefaction temperature and reaction time on the heavy tar fraction (CLT) yield was systematically studied.

As shown in Figure 4, at low temperatures, insufficient thermal energy limited coal depolymerization, resulting in low CLT yields. Conversely, excessively high temperatures or prolonged reaction times also decreased the CLT yield, likely due to increased hydrocracking that converts heavier components into lighter fractions, gases, or water. Similar observations have been reported previously for other coal and hydrocarbon liquefaction processes [71,72].

To maximize mesophase pitch (MP) production, the tar samples with the highest CLT yields—IL-CLT-400-60 from Illinois #6 coal (400 °C, 60 min) and WY-CLT-425-120 from Wyoming PRB coal (425 °C, 120 min)—were chosen for thermal upgrading. Corresponding reference samples using tetralin solvent, labeled IL-MSLT-400-60 and WY-MSLT-425-120, were also prepared under identical liquefaction conditions. All four tar samples were thermally treated at 425 °C for 3 h to produce pitch.

Figure 5 and

Table 6. Pitch yields, QI contents, and mesophase contents of pitch samples generated at 425 °C from co-liquefaction of tar of Illinois #6 and Wyoming PRB coal with different liquefaction solvents.

Coal	Illinois #6		Wyoming PRB
Solvent	Tetralin	HDPE-Liquid	Tetralin	HDPE-Liquid
Mesophase Content (%)	12.4	32.6	17.8	62.1
QI Content (wt%)	6.1	19.7	13.9	66.8
Softening Point (°C)	134	215	168	265
Pitch Yield (wt%)	26.7	24.3	22.8	17.1

show that mesophase formation was observed in all the pitch samples, confirming HDPE-liquid’s effectiveness as a solvent for producing MP from various US coals. For Illinois #6 coal, pitch produced with THN exhibited limited mesophase formation, with small (<20 µm) mosaic textures (Figure 5a). In contrast, using HDPE-liquid significantly enhanced mesophase formation, resulting in larger lamellar and coarse-flow domains (>20 µm) that covered considerable portions of the pitch surface (Figure 5b). The enhancement was even more pronounced for Wyoming PRB coal. THN yielded mostly coarse mosaic and granular-flow textures (Figure 5c), while HDPE-liquid produced extensive large lamellar domains (Figure 5d), indicating superior mesophase growth.

Because all the pitch samples were thermally upgraded under identical conditions, observed differences in mesophase formation are attributed to the chemical composition of the tar precursors. The ¹H NMR analysis (

Table 7. Characterization of co-liquefaction tar generated from different liquefaction solvents with Illinois #6 coal and Wyoming PRB coal.

Coal	Illnois #6		Wyoming PRB
Liquefaction Conditions	400 °C for 60 min		425 °C for 120 min
Solvent	Tetralin	HDPE-Liquid	Tetralin	HDPE-Liquid
Relative H Content (%)
Har	68.53	64.54	68.62	72.44
Hα	16.42	21.89	17.53	14.69
Hβ	9.08	9.29	10.41	11.15
Hγ	5.97	4.28	3.44	1.72

H_ar: proton with a chemical shift at 8.5 to 6.6 ppm; H_α: proton with a chemical shift at 4.0 to 2.2 ppm; H_β: proton with a chemical shift at 2.1 to 1.1 ppm; H_γ: proton with a chemical shift at 1.1 to 0.3 ppm.

) confirms compositional differences between HDPE-liquid and THN-derived tar. In both coal systems, HDPE-liquid reduced the terminal methyl group content (H_γ) while slightly increasing H_β. A lower H_γ content suppresses excessive cross-linking during thermal treatment [65], thereby promoting the formation of larger, ordered mesophase domains, which would be beneficial for producing spinnable pitch.

Interestingly, the Wyoming PRB coal showed far better mesophase formation than the Illinois #6 coal under the same HDPE-liquid conditions. which may relate to compositional differences between these coals. Illinois #6 coal has a notably higher sulfur content (as shown in Table 1), which could hinder effective mesophase formation [73,74,75]. Future research should further examine how sulfur-containing species and other heteroatoms influence tar reactivity and pitch evolution. Additionally, understanding how these structural and compositional differences affect pitch spinnability and downstream carbon fiber properties is important. These considerations, along with broader techno-economic and environmental implications, are outlined in Section 3.5.

3.5. Prospective and Future Work

The findings from this study provide a promising foundation for integrating plastic waste recycling with carbon material manufacturing. The demonstrated ability of HDPE-derived liquids to serve as functional solvents for coal liquefaction introduces a sustainable route for producing mesophase pitch, a key precursor in high-performance carbon materials. This approach not only valorizes polyolefin waste but also has the potential to reduce reliance on petroleum-derived solvents like tetralin, thereby lowering the environmental and economic footprint of pitch production.

Despite these promising results, several limitations must be addressed before large-scale implementation. The current work used virgin HDPE as a model feedstock to ensure reproducibility and compositional control. However, post-consumer HDPE waste often contains additives, degradation products, and polymer contaminants that may influence hydrogenolysis behavior and solvent performance. Future studies should investigate the yield, composition, and functionality of HDPE-derived liquids obtained from real-world waste streams, including mixed or contaminated plastics.

Furthermore, while improved softening points and mesophase development suggest enhanced spinnability, no direct rheological measurements or fiber-spinning trials were conducted. These tests are necessary to fully validate the applicability of the obtained mesophase pitches for industrial carbon fiber production. To advance this concept toward practical application, future work should focus on four main areas. First, evaluate the hydrogenolysis of mixed plastic waste to determine solvent robustness and scalability under real-world conditions. Second, the rheological properties of the resulting mesophase pitches should be systematically characterized using shear and extensional flow tests to confirm fiber processability. Third, the environmental and economic feasibility of the integrated process should be assessed through a life cycle assessment (LCA) and a techno-economic analysis (TEA). Fourth, detailed mechanistic studies on pitch formation from HDPE-derived tars, including the influence of sulfur, oxygenates, and molecular weight distribution, should be pursued.

Additionally, the reuse and regeneration of HDPE-derived solvents, or their catalytic modification to improve selectivity, could further enhance process efficiency and reduce operating costs. Such strategies would help close the loop on solvent use and align the process with circular economy principles. Collectively, these efforts will advance the development of an integrated and scalable pathway for upcycling plastic waste into high-value carbon products.

4. Conclusions

This study demonstrates that hydrogenolysis-derived HDPE-liquids can serve as effective solvents for coal liquefaction, offering a sustainable alternative to tetralin. Co-liquefaction using HDPE-derived liquids enhanced mesophase pitch formation across multiple coal types. Compared to tetralin, HDPE-liquid-derived tars yielded pitches with improved mesophase textures and lower softening points, suggesting enhanced thermal processability. These performance gains were linked to favorable tar composition, particularly reduced terminal methyl groups, known to suppress excessive cross-linking during thermal upgrading.

This strategy offers a scalable route to valorize plastic waste into high-performance carbon materials. By repurposing waste plastics as liquefaction solvents, the process reduces the environmental burdens associated with both plastic disposal and fossil solvent use. Ultimately, the approach contributes to carbon sequestration by embedding plastic-derived carbon into durable mesophase pitch and supports broader sustainability goals in carbon-intensive applications, such as composites, electrodes, and thermal materials. Future work should explore solvent performance using real-world waste plastics, evaluate pitch spinnability through rheological testing, and quantify the environmental and economic impacts through LCA and TEA.