Lubricant Base Oils From Polyethylene via Pyrolysis and Catalytic Upgrading

Abstract

The escalating proliferation of plastic waste necessitates transformative and sustainable valorization pathways. Herein, we report an efficient two‐step process for converting virgin and postconsumer high‐density polyethylene (HDPE) into high‐value lubricant base oils. The approach integrates thermal pyrolysis in a conical spouted bed reactor with subsequent catalytic oligomerization of olefin‐rich pyrolyzates. This methodology optimizes wax and liquid hydrocarbon yields while suppressing undesirable by‐products. Following wax separation, the liquid fractions underwent oligomerization over solid acid catalysts to generate branched hydrocarbons predominantly in the base oil range (≥C₂₀). Among the tested catalysts, MCM‐22, owing to its unique pore structure, exhibited superior performance, achieving ≥C₂₀ yields of 65 and 49 wt% in the oligomerization of the virgin and postconsumer waste HDPE pyrolysis liquids, respectively. Moreover, the zeolite proved reusable and largely regenerable, with a simple calcination restoring the majority of its initial activity. The oligomeric products displayed compositional and physicochemical characteristics analogous to Group III base oils, with minimal aromatic content. Post hydrofinishing and distillation, the final products exhibited favorable viscosity and cold‐flow properties, underscoring their potential as sustainable lubricant base stocks.

Virgin and postconsumer high‐density polyethylene (HDPE) plastics are converted into high‐quality base oils via thermal pyrolysis, followed by catalytic upgrading. The combined oligomerization and hydrogenation strategy enables molecular growth and branching, yielding lubricant‐range hydrocarbons.

1. Introduction

Plastics have become a pervasive part of modern life, with global production reaching an astonishing 413.8 million tons in 2023. Only ca. 9% of postconsumer waste (PCW) plastic was recycled that year. The remainder typically ends up in landfills, is incinerated, or leaks into the environment [1, 2]. Polyolefins, the majority of plastics, make up more than 40% of total plastic production [3]. To effectively manage PCW plastics and meet circular economy objectives, two main recycling methods can be used: mechanical and chemical recycling. Chemical recycling complements mechanical processing by enabling the handling of heterogeneous plastic mixtures and contaminated polymers, often requiring minimal pretreatment [4, 5, 6]. Thermal pyrolysis is a highly effective chemical recycling technique that breaks down polyolefins into valuable products by applying heat in an inert atmosphere, typically nitrogen or steam.

Polyolefins can be thermally decomposed into gases, liquids, and waxes, depending on factors such as contact time, temperature, and reactor type [7]. While chemical recycling has primarily focused on converting plastics into monomers or naphtha fractions ultimately for repolymerization, producing high molecular weight products, such as waxes and base oils, offer additional opportunities to diversify the product range. In addition, manufacturing these products is economically feasible and it shows promising life cycle and technoeconomic benefits [8, 9, 10, 11].

Different reactor types are used in the chemical recycling of plastics [11]. The conical spouted bed reactor (CSBR) is a type of fluidized bed reactor featuring a sand bed located in a conically shaped section (Figure 1). This design offers several advantages: The cyclic movement of the sand bed ensures efficient mass and heat transfer, creating a uniform coating of the sand particles with the fused plastic, while the high carrier gas flow rate minimizes defluidization by promoting vigorous sand movement. Crucially, the resulting short residence time of the volatiles limits secondary reactions such as hydrogen transfer, aromatization, cyclization, and deep cracking. Polyolefin pyrolysis in CSBRs has accordingly been reported to yield products dominated by primary chain scission species, namely linear α‐olefins and α,ω‐diolefins, with markedly lower aromatic content than obtained in reactors with longer vapor residence times such as fixed beds or conventional fluidized beds [11, 12]. This olefin‐rich, aromatic‐lean product slate is not only attractive for producing crude waxes that can be refined into value‐added products, but its liquid fraction is also particularly well suited as a feedstock for acid‐catalyzed oligomerization; this is because terminal olefins function as the reactive coupling species, whereas aromatics act as coke precursors that degrade base oil quality.

FIGURE 1.

(a) Schematic representation of the CSBR with a zoom‐in on the sand bed where the thermal pyrolysis on plastics is conducted. (b) General setup scheme of CSBR including: (I) a feeding system, (II) the CSBR with gas preheater, and (III) the product recovery system consisting of a cyclone, a condensation box, and a filter system.

In addition to heavier fractions, thermal pyrolysis produces lighter products such as gases and liquids [11, 12, 13, 14]. These liquids are often used as fuels or can be further transformed by cracking processes into monomers or oligomers. Importantly, the high olefin content of pyrolysis liquids can be exploited for a broad range of chemical transformations, enabling the production of valuable compounds such as aldehydes, alcohols, carboxylic acids, amines, and sulfate detergents [15, 16]. This work goes beyond the current product scope by transforming the olefin‐rich pyrolysis liquids into base oils, the main constituent of lubricants.

In this work, the base oils were produced through the ex situ catalytic oligomerization of pyrolysis oils derived from CSBR. The olefins in this mixture, primarily α‐olefins and α,ω‐diolefins, are coupled to form dimers and higher oligomers using acidic heterogeneous catalysts. The use of heterogeneous acid catalysts for oligomerization has been scarcely studied, and most prior work has focused on C₆–C₁₈ α‐olefin model compounds. Catalysts investigated include ZSM‐5, MCM‐41, Y, beta, ZSM‐12, mordenite, acid clays, and ion exchange resins. Among these, acid clays and wide‐pore zeolites, such as MCM‐41, Y, beta, and ZSM‐12, have shown notable activity in oligomerization reactions. However, these studies were conducted primarily in the context of Fischer–Tropsch applications, which differ significantly from the catalytic oligomerization of pyrolysis liquids in terms of feedstock composition [17, 18, 19, 20, 21, 22, 23, 24]. Our acid‐catalyzed approach also differs from traditional polyalphaolefin (PAO) production, which typically uses Ziegler–Natta, metallocene, or BF₃ catalysts. While Ziegler–Natta catalysts are primarily used for polymerizing light gaseous olefins into polymers, other homogeneous catalysts, such as BF₃, pose significant corrosion and health risks, and metallocene catalysts are mostly limited to the oligomerization of lower olefins [25, 26, 27]. Other methods for converting plastics into liquids or base oils include hydrogenolysis, metathesis, and alkylation catalysis. However, these processes are typically performed on a small scale and they often require costly noble metals and metal complexes, together with long reaction times [28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38]. Although base oils have been produced from pyrolysis waxes via hydroisomerization [39, 40, 41], producing them through the (acid catalyzed) oligomerization of liquid pyrolysis products remains an unexplored yet promising pathway with ideal physical properties due to the long branches.

2. Results and Discussion

The following sections detail the thermal pyrolysis of virgin (i.e., pristine or clean) HDPE and PCW HDPE, followed by the ex situ oligomerization of the resulting pyrolysis oils to generate lubricant‐range base oils. The study of virgin HDPE serves to avoid potential deactivation effects caused by impurities, thereby enabling a more accurate assessment of the intrinsic reaction chemistry. This structured investigation highlights how each transformation influences molecular architecture and ultimately governs base oil quality.

2.1. Thermal Pyrolysis

The selection of the pyrolysis temperature is a crucial factor that significantly influences the yield of waxes and liquids [42]. A previous design‐of‐experiments optimization of this reactor system (500–650 °C) in our earlier work showed that the liquid yield increases strongly with temperature, reaching its highest value within the studied range at 650 °C and 8 NL·min⁻¹ N₂, as secondary scission shifts the product distribution from waxes toward the liquid range [7]. Since this liquid fraction is the targeted oligomerization feedstock, these conditions were adopted here, with a feeding rate of 3 g·min⁻¹ to ensure sufficient throughput for the downstream steps. Two pyrolysis streams were produced using the CSBR (Figure 1) for further liquid upgrading: one from virgin HDPE and the other from PCW HDPE plastic, both processed at a reactor temperature of 650 °C. Complete characterization of both feedstocks, comprising the polymer composition, attenuated total reflectance infrared spectroscopy, thermogravimetric analysis/differential scanning calorimetry, and elemental analyses, is provided in the Supporting Information (Table S1, Figures S1 and S2).

The pyrolysis products include waxes (Figure S3), liquids (Figure 2), and gases (Figure S4). Pyrolysis at 650 °C yielded liquid fractions of 35 wt% from virgin HDPE and 38 wt% from PCW HDPE. The PCW HDPE produced a slightly higher proportion of lighter products compared to the virgin sample, a trend also observed in the wax fraction, where the virgin HDPE yielded fewer heavy components. The increased cracking observed in the PCW HDPE sample may be attributed to the presence of heteroatom‐containing contaminants, including polyethylene terephthalate (PET, 6.96 wt%), polyamide (PA, 0.06 wt%), polycarbonate (PC, 0.33 wt%), and metal‐containing compounds (0.17 wt%; Table S1), as well as to intrinsic differences in the HDPE material itself. These oxygenated polymers contributed to CO₂ formation, which is observed in the gas fraction (Figure S4).

FIGURE 2.

Thermal pyrolysis of virgin and PCW HDPE in the CSBR at 650 °C, using an N₂ flow rate of 8 NL·min⁻¹ and an HDPE feeding rate of 3 g·min⁻¹. (a) Yields of waxes (≥C₂₀), liquids (C₅–C₁₉), gases (C₁–C₄), and impurities, referenced to the mass of plastic fed (see Mass balance, Experimental Section). (b) Carbon number distribution and molecular composition (n‐alkanes, isomers, α‐olefins, and α,ω‐diolefins) of the distilled pyrolysis liquids, with a photograph of the virgin pyrolysis liquid.

The liquid fraction was subsequently isolated from the wax fraction by vacuum distillation up to an atmospheric‐equivalent temperature of 342.7 °C, the boiling point of a C₂₀ hydrocarbon. This cut point marks the lower boundary of the lubricant base oil range: The sub‐C₂₀ olefin fraction is considered as the reactive feed that shifts to the C₂₀₊ base oil range upon successful oligomerization (Figure 2b), and the same boundary consistently defines the wax fraction and the ≥C₂₀ oligomerization yield throughout this work. The liquid and wax yields were quantified via SimDist analysis of the recovered pyrolysate after thermal pyrolysis (Figure 2a), as accurate gravimetric quantification after distillation was hampered by wax holdup in the distillation column and transfer lines. The liquid fraction consists primarily of α‐olefins (ca. 53 and 56 wt% in the virgin and PCW stream, respectively), α,ω‐diolefins (ca. 27 and 24 wt%, respectively), n‐alkanes (12 and 13 wt%, respectively), and a small amount of branched hydrocarbons (ca. 7 wt% in both cases) (Figure 2b). The carbon number distributions of the liquids range from C₅ to C₂₅, with the weighted carbon number being C₁₆ and C₁₇ for the virgin and PCW pyrolysis liquid, respectively. The remaining waxes (Figure S3) exhibit a broad carbon distribution, with detected peaks up to C₆₇ and C₅₆ for the virgin and PCW wax samples, respectively. This stream could be utilized as a feedstock for cracking processes or for various industrial wax applications [11, 43]; however, these possibilities are beyond the scope of the present study.

2.2. Oligomerization of Model Compounds

Before oligomerizing the real pyrolysis liquids to produce base oils, model compounds were studied to enable more controlled product analysis and quantification compared to the complex composition of real pyrolysis mixtures. Since liquid olefin‐rich pyrolysis streams can range from C₅ to C₂₀ and beyond (Figure 2b), three olefinic model compounds were selected to represent this range: 1‐octene, 1‐tetradecene, and 1‐octadecene. Additionally, 1,7‐octadiene was chosen to simulate reactions with diolefins. A 60 wt% model solution in dodecane simulated the olefin‐rich pyrolysis liquids. Various acid catalysts were first tested with 1‐octene, and then, top catalyst performers from each group were chosen for reactions with the heavier model compounds (Figures S5–S10). The catalyst groups tested included: 2D‐12‐membered ring (MR) zeolites, 2D‐10MR zeolites, 2D‐12MR zeolites, a 1D‐12MR zeolite, a 1D‐10MR zeolite, amorphous silica–aluminas, and an acid clay. The resulting products were classified into four categories: (I) cracked products (smaller than the original model compounds), (II) dimers, (III) trimers and higher oligomers, and (IV) skeletal isomers of the model compound (i.e., carbon backbone rearrangement). Due to difficulties in distinguishing trimers from higher oligomers, these were grouped as a “trimer+” fraction. Reversible double‐bond isomerization (i.e., positional olefin isomerization without skeletal rearrangement) was not included in the yield calculations.

The oligomerization reactions with the tested catalysts (Figure 3a) resulted in clear differences in product yields. The results show that longer olefins yield fewer oligomers, likely due to steric hindrance during oligomerization or limitations in product desorption causing pore blockage. Increasing the reaction temperature could potentially improve desorption and alleviate pore blockage, although this also promotes undesired cracking. A notable exception is the MCM‐22 catalyst, which produced a relatively constant oligomer yield, regardless of the olefin chain length. Although characterization shows that MCM‐22 possesses a high S _BET (553 m²·g⁻¹) and V _meso (0.81 cm³·g⁻¹), these values are not exceptional compared to the other catalysts examined (Table S3). Its superior performance instead arises from its distinctive pore architecture. MCM‐22 features two distinct nonintersecting pore systems: a 2D network of 10‐membered‐ring (10 MR) channels and a second pore system containing large 12 MR supercages [44]. These supercages are accessible directly from the external surface, creating 12 MR pockets (named as emisupercage pockets or half supercages) that allow larger olefins to react at or near the outer surface without requiring diffusion into the internal pore system. This structural feature explains the consistently high oligomer yields observed for MCM‐22 [45]. In contrast, the USY zeolite has most of its supercages located internally. Reactant molecules access the active catalytic sites by diffusing through the mesopores and 12‐membered ring (12MR) pores, where large products form in the spacious supercages. These products face diffusion limitations when exiting. This is consistent with the high initial reaction rate observed for USY, followed by a slower build‐up of oligomers due to mass transfer limitations and progressive pore blockage from coke formation (Figure 3b). One‐dimensional zeolites such as MOR or zeolites with 10MR openings such as ZSM‐5 display a low oligomer yield which is in line with slow diffusion and restricted accessibility of the reactants and produced oligomers. The acid and textural properties of the catalysts (Table S3) were also examined, but no correlation was found between the Brønsted acid site density (titrated by pyridine), its accessibility (titrated by the bulkier lutidine probe), or the surface area and the observed rate constants. This indicates that the pore topology and the associated diffusion of reactants and products are the dominant factors governing the oligomerization performance.

FIGURE 3.

Oligomerization of various model compounds using different acid catalysts: (a) The yield of dimers, trimers and higher oligomers (trimer+ yield), all oligomers, the isomer yield, the unreacted species, and the apparent rate constant for each catalyst for the 1‐octene (C8), 1‐tetradecene (C14), and 1‐octadecene (C18) model compounds. (b) The yield of oligomers in function of time for different catalysts for 1‐octene oligomerization reactions. (c) The different yields for mixtures in 40 wt% dodecane containing (I) 60 wt% 1‐octene, (II) 60 wt% 1,7‐octadiene, and (III) 30 wt% 1‐octene + 30 wt% 1,7‐octadiene. Reactions were conducted in a 10‐mL batch reactor with 2 g model compound mixture and 10 wt% catalyst at 200 °C with 600 rpm stirring speed.

The initial reaction kinetics could be empirically described by an apparent second‐order rate law in olefin concentration (see Figure S11; in the Supporting Information for fitting details), yielding apparent rate constants that allowed a quantitative comparison of the catalysts under identical conditions. Given the heterogeneous nature of the reaction, involving olefin adsorption and protonation, surface carbenium‐ion chain growth, hydride transfer, β‐scission, product desorption, and possible diffusional and deactivation contributions, these constants are lumped empirical parameters and are not interpreted as intrinsic kinetics. Notably, the apparent rate constant increased with the use of larger olefins, which is attributed to stronger surface adsorption interactions of the heavier molecules. These interactions result in a longer surface residence time, leading to faster initial chemical reactions. Among the tested catalysts, USY zeolites exhibited exceptionally high apparent reaction rates (Figure 3a), but their oligomer yield leveled off at low values despite abundant remaining reactant (Figures S6–S10), indicating a progressive loss of activity. Together with the high coke loadings measured on these catalysts (Figure 4a), this points to deactivation by coke deposition, although in a batch system a contribution of strong product inhibition cannot be fully excluded. In contrast, MCM‐22, montmorillonite K10, and SIRAL 70 HPV displayed lower apparent rate constants but still achieved oligomer yields above 50 wt% for all model compounds after longer contact time. The combination of moderate initial activity and high overall yields indicates that these catalysts retain their activity over time, due to enhanced resistance to coking and deactivation.

FIGURE 4.

Catalytic upgrading of the pyrolysis liquids to base oil. (a) Yields of ≥C₂₀ products (bars) and the associated coke deposition on the catalyst (dots) after oligomerization of the virgin (green) and PCW (orange) pyrolysis liquids over MCM‐22, montmorillonite K10, SIRAL 70 HPV, and USY (10 wt% catalyst, 225 °C, 600 rpm, 10 mL batch reactor). A photograph of the oligomerized virgin pyrolysis liquid obtained with MCM‐22 is included. (b) Photograph of the final base oil after hydrofinishing of the oligomerized product over Ni/silica–alumina and subsequent distillation, with representative branched alkane structures indicated. (c) SimDist profiles of the virgin pyrolysis liquid (1), the corresponding oligomerized product obtained with MCM‐22 (2), and the hydrofinished and distilled base oil (3), illustrating the progressive shift toward the lubricant boiling range.

Since the polyolefin pyrolysis liquids in this study contained a high fraction of α,ω‐diolefins (24–27 wt%), 1,7‐octadiene was also selected as a model compound to evaluate its potential for oligomer formation and catalyst fouling on USY (Si/Al = 30), MCM‐22, montmorillonite K10, and SIRAL 70 HPV. Mixtures of n‐dodecane with 1‐octene (40:60 wt%), n‐dodecane with 1,7‐octadiene (40:60 wt%), and n‐dodecane with 1‐octene and 1,7‐octadiene (40:30:30 wt%) were evaluated. The results (Figure 3c) clearly show altered yields. Given that diolefins are more reactive, a higher oligomer yield might be expected, as seen with SIRAL 70 HPV. However, when using the USY (Si/Al = 30) zeolite and montmorillonite K10 clay, a high yield of isomers and a lower yield of oligomers were obtained. Gas chromatography–mass spectrometry (GC–MS) analysis revealed that a significant fraction of these isomers consisted of ring structures when diolefins were used. The high cyclization tendency of these catalysts is attributed to their structural features such as the large and accessible mesopores of USY or the layered structure of montmorillonite K10 (see Table S3), which provide sufficient space or surface accessibility for 1,7‐octadiene to adopt conformations that bring its two double bonds into close proximity for intramolecular reaction. These ring structures are likewise seen in the oligomerization of the pyrolysis liquids, as explained below. In contrast, the MCM‐22 catalyst showed similar yields across all three olefin oligomerization cases without significant formation of the ring structures. This is explained by its unique pore structure, which combines 10MR pores with highly accessible 12MR pockets, along with catalytic activity at the external surface.

2.3. Oligomerization of Liquid Pyrolysis Products

The successful oligomerization of the model compounds led us to apply a similar approach to the crude pyrolysis liquids to obtain base oils. The four best‐performing catalysts, such as USY (Si/Al = 30), MCM‐22, montmorillonite K10, and SIRAL 70 HPV, were used to oligomerize the genuine pyrolysate liquids. These reactions were performed under conditions analogous to the model compound study, albeit at a slightly higher temperature (225 °C). Since the real feeds contain substantially heavier olefins (up to C₂₅, Figure 2b) than the model compounds, the higher temperature was applied to facilitate the desorption of the bulkier oligomerization products and counteract the diffusion limitations observed for longer olefins, while remaining low enough to limit undesired cracking. The resulting oligomerization products of the virgin and PCW pyrolysis oils (Figure 4a) clearly show an increase in molecular weight, providing evidence of oligomerization. Since base oils generally consist of hydrocarbons starting at approximately C₂₀, the yields of ≥C₂₀ fractions were calculated as a reference point. The MCM‐22 catalyst produced the highest increase in ≥C₂₀ fraction, achieving 65 and 49 wt% yields for the virgin and PCW pyrolysis liquids, respectively. These were the highest yields of all tested catalysts for both feeds (Figure 4a). This superior performance aligns with the model compound study, where MCM‐22 demonstrated high effectiveness in promoting the oligomerization of both synthetically pure α‐olefins and α,ω‐diolefins.

A reduced oligomerization activity efficiency was observed when the PCW pyrolysis oil was used, which is at least partially attributed to impurities in the PCW feedstock, viz. metallic and inorganic elements (Al, Ba, Cr, Cu, Ni, Si, Zn, Fe, Ca, K, Mg, and Na; Table S2) and heteroatom‐containing polymer contaminants (Table S1). Alkali and alkaline earth metal ions are known to ion exchange with Brønsted acid sites, while transition metals and their oxides, such as Ni, Fe, and Cu, catalyze dehydrogenation reactions that generate coke precursors on acid catalysts [46]. Although these largely nonvolatile contaminants are expected to be at least partially retained in the sand bed during pyrolysis, some carryover into the pyrolysis oil, e.g., as entrained fines or volatile metal species, is expected. This, however, could not be quantified in the present work owing to the limited amount of oil available for trace metal analysis. In addition, volatile heteroatom‐containing compounds formed upon decomposition of the polymer contaminants, such as PET, PA, and PC, likely contribute: The PCW feedstock contains more oxygen (0.7 vs. 0.3 wt%) and sulfur (0.13 wt% vs. not detected) than the virgin material (Table S1), and while part of this oxygen leaves as CO₂ in the gas fraction (Figure S4), polar O‐ and S‐containing molecules may partition into the liquid, adsorb strongly on the acid sites, and act as coke precursors. Both contributions are consistent with the systematically higher coke deposition observed for the PCW‐derived oil over all catalysts (Figure 4a), although their relative importance remains to be established. Notably, calcination of the spent MCM‐22 restored the majority of its activity (Figure 5), indicating that the deactivation is dominated by carbonaceous and strongly adsorbed species rather than by irreversible metal poisoning or structural collapse.

FIGURE 5.

Reusability and regenerability of MCM‐22 and montmorillonite K10 in the oligomerization of the PCW pyrolysis liquid (5 h, 10 wt% catalyst, 225 °C, 600 rpm, 10 mL batch reactors). Yields of ≥C₂₀ products are given over fresh, reused (washed and dried), and regenerated (washed and calcined) catalysts. The fresh MCM‐22 value is the mean of three parallel reactions (49.2 ± 2.6 wt%). Reuse and regeneration conditions are given in the Experimental Section.

The differences in catalyst deactivation behavior can provi‐sionally be explained by considering the pore architecture and accessibility of the materials. SIRAL 70HPV (amorphous silica–alumina) exhibited the greatest sensitivity to coke formation and deactivation, which can be attributed to its highly open, nonstructured mesoporous architecture and broad acid site distribution (Table S3). This open structure provides unrestricted access to acid sites, facilitating high initial activity but also promotes unselective secondary reactions such as oligomerization, aromatization, and condensation. These reactions lead to the rapid formation and deposition of coke on the exposed surface, accelerating deactivation. In contrast, montmorillonite K10 possesses a more constrained layered structure with fewer mesopores, which limits the accessibility of larger molecules and may suppress the formation of bulky coke precursors. MCM‐22, which possesses a well‐defined 10‐membered ring (10MR) microporous framework with well accessible 12 MR pockets, exhibited relatively stable coke content across both feedstocks. Although its micropores can, in principle, be susceptible to blockage, the formation of large coke precursors is limited by, e.g., transition state shape‐selective constraints. This reduces the material’s sensitivity to deactivation. This behavior is consistent with the high initial activity observed for MCM‐22, followed by minimal changes in performance, suggesting a balance between catalytic efficiency and resistance to fouling with this zeolite topology. It should be noted that the yield–time profiles (Figures S6–S10) were recorded with the model compounds, while for the real pyrolysis oils, catalyst stability was assessed through coke quantification of the spent catalysts (Figure 4a).

Given the appreciable coke formation, the reusability and regenerability of two catalysts were evaluated with the PCW pyrolysis liquid as feedstock (Figure 5). For MCM‐22, the fresh catalyst yielded 49.2 ± 2.6 wt% of ≥C₂₀ products (mean of three parallel reactions with standard deviation). Upon direct reuse after washing and drying, the yield decreased to 28.9 wt%, i.e., 59% of the initial activity, in good agreement with the fraction of accessible Brønsted acid sites retained in the spent catalyst (95 vs. 154 µmol·g⁻¹, Table S3), indicating that washing alone does not remove the strongly adsorbed species and coke blocking the acid sites. Calcination at 550 °C in air, in contrast, restored the yield to 39.9 wt% (81% of the fresh activity), demonstrating that MCM‐22 is largely regenerable by a simple coke burn‐off. Another tested catalyst, montmorillonite K10, showed a smaller relative activity loss (22.7 and 25.6 wt% upon reuse and regeneration, vs. 28.4 wt% fresh), but at a substantially lower activity level throughout.

To assess the overall process performance, the yields of the individual steps can be combined into a mass balance referenced to the plastic feed (Table 1). Per 100 g of virgin HDPE, pyrolysis delivers 49 g of waxes, 35 g of liquids, and 15 g of gases; oligomerization of the liquid fraction over MCM‐22 subsequently converts 22.8 g into ≥C₂₀ base oil range product, with only 0.5 g of coke, while the residual 11.7 g of <C₂₀ material is removed in the final distillation and could be recycled or valorized otherwise such as fuel. For the PCW feed, the corresponding overall yield amounts to 18.7 g of ≥C₂₀ product from 38 g of liquids. In total, ca. 72 g (virgin) and 64 g (PCW) per 100 g of HDPE thus end up as ≥C₂₀ products (wax + lubricants) when the directly valorizable wax fraction is included. Note that these overall yields are calculated by combining the experimentally determined yields of each step and assume ideal separations; physical losses during distillation and hydrofinishing, which are equipment and scale dependent, could not be quantified at batch scale.

TABLE 1.

Overall mass balance per 100 g of HDPE feed, combining the measured yields of the individual process steps (MCM‐22 route).

Per 100 g HDPE feed	Virgin	PCW
Pyrolysis (650 °C)
Gases (C1–C4)	15	16
Pyrolysis liquids (C5–C19)	35	38
Waxes (≥C20)	49	45
Inorganic residue	<1	2
Oligomerization of the liquid fraction
≥C20 oligomerizate (≈base oil)	22.8	18.7
Remaining <C20	11.7	18.7
Coke	0.5	0.6
Total ≥C20 products (incl. waxes)	71.8	63.7

Note: Calculated values assuming ideal separations; physical losses during vacuum distillation and hydrofinishing (e.g., wax holdup in the column and transfer lines) could not be quantified at batch scale.

Analysis by GC×GC after oligomerization with MCM‐22 and montmorillonite K10 catalysts on both virgin and PCW oils revealed the presence of mono‐naphthenes (6–11 wt%), polynaphthenes (1–6 wt%), mono‐aromatics (2–4 wt%), and traces of diaromatics (0.0–0.1 wt%) (Figure S12 and Table S4). It should be noted that the naphthene contents over MCM‐22 somewhat exceed those over montmorillonite K10 for both feeds (e.g., 12.3 vs. 8.2 wt% for the virgin oil, Table S4). The suppressed diolefin cyclization observed for MCM‐22 in the model study thus does not translate directly into lower naphthene contents with the real feeds. This comparison is, however, made at different conversion levels: The higher oligomer yield over MCM‐22 implies a larger converted product fraction, whereas the composition over the less active clay is more strongly diluted by unreacted feed olefins. In addition, the diverse C5–C25 olefin/diolefin mixture in the PCW pyrolysis oil offers cyclization pathways beyond those probed by the synthetically pure 1,7‐octadiene. These cyclic components are expected to decrease further upon distillation to obtain the desired base oil cut ranges. Interestingly, the resulting lubricant oils exhibit a composition similar to that of highly refined API (American Petroleum Institute) Group III base oils, and their beneficial low aromatic and naphthenic content may approach the levels typically found in Group IV polyalphaolefin (PAO) base oils [47].

The oligomerization reaction was scaled up to a 180 g batch using crude virgin pyrolysis oil and MCM‐22 as selected catalyst based on the earlier beneficial results in this work. The resulting product was subsequently hydrogenated and distilled, with the fraction below approximately C₂₀ removed to allow for physical property assessment. The starting pyrolysis oils had a cloud point of approximately 16 °C, a pour point of 13 °C, and a kinematic viscosity of 3 mm²/s at 40 °C. After oligomerization, hydrogenation, and distillation, these values changed to −3, −11 °C, and 64 mm²/s, respectively, demonstrating a significant transformation in physical properties (see SI). These changes in physical properties are attributed to the formation of high molecular weight, isomeric hydrocarbons with long alkyl chains. This is further supported by SimDist analysis, which shows a final boiling point of approximately 720 °C, indicative of the presence of extremely high molecular weight hydrocarbon products, likely in the C₆₀–C₇₀₊ range. The elevated viscosity reflects the incorporation of these larger molecules, while the improved low‐temperature flow behavior is due to the presence of branched structures, which reduces the tendency to crystallize.

These properties allow a first benchmarking against commercial base oils. The kinematic viscosity of 64 mm²/s at 40 °C places the product in the range of heavy neutral base stocks, while the pour point of −11 °C provides adequate low‐temperature performance for such heavy grades. Compositionally, the hydrofinished product contains ≥96% saturates (2–4% aromatics, Table S4), thereby meeting the API saturates criterion for Group II/III base oils (>90%), and its low naphthene content approaches that of Group IV polyalphaolefins. A complete base oil qualification, including the viscosity index, Noack volatility, and oxidative stability, requires larger product quantities and is the subject of ongoing work [47]. Nevertheless, the combination of base oil range viscosity, favorable cold‐flow properties, and a highly saturated, aromatics‐lean composition demonstrates that the targeted transformation of plastic pyrolysis liquids into lubricant base oil quality has been achieved at the compositional level. In addition, the final properties are strongly influenced by the distillation cutoff (approximately C₂₀), which determines the boiling range and molecular weight distribution of the product. Selecting a different cutoff (e.g., C₁₅) or adjusting the catalytic conversion would lead to different physical properties, highlighting the tunability of the process depending on the desired base oil characteristics.

3. Conclusion

In summary, this study demonstrates the viability of transfor‐ming plastic‐derived pyrolysis oils into premium quality base oils through catalytic oligomerization. Utilizing both virgin and PCW high‐density polyethylene (HDPE) oils, the research employed a suite of solid acid catalysts to achieve substantial molecular upgrading. Among the catalysts tested (USY (Si/Al = 30), MCM‐22, montmorillonite K10, and SIRAL 70 HPV), MCM‐22 proved particularly effective, delivering the highest yields of C₂₀ and higher oligomers in both the virgin and PCW‐derived oils. Moreover, MCM‐22 proved largely regenerable: After deactivation with the PCW‐derived oil, a classic air calcination restored the majority of its initial activity, underscoring the practical feasibility of the process. In the model compound study, the distinctive pore architecture of MCM‐22 suppressed the intramolecular cyclization of α,ω‐diolefins through transition‐state shape selectivity. This effect was more pronounced than over USY and montmorillonite K10, which aligns with the spatial constraints of the catalysts (i.e., 10‐MR channels and 12‐MR pockets in MCM‐22 versus the larger supercages of USY and the unconstrained space of montmorillonite K10). Upon oligomerization of the real pyrolysis liquids, moderate naphthene (8–16 wt%) and low aromatic (2–4 wt%) contents were obtained yielding products compositionally comparable to Group III base oils. This behavior is advantageous for producing high‐performance lubricant base stocks where minimizing cyclic content is desirable. Scaling up the process and applying hydrofinishing yielded a base oil fraction with favorable properties: a base oil range viscosity (64 mm²/s at 40 °C), markedly improved cold‐flow behavior (pour point −11 °C vs. 13 °C for the feed), and a ≥96% saturated, aromatics‐lean composition meeting the API Group II/III saturates criterion. Altogether, the findings highlight the transformative potential of catalytic upcycling for plastic waste. Through careful feedstock conditioning and strategic catalyst selection, this approach paves the way for producing customized base oil fractions from waste plastics, contributing meaningfully to circular economy goals and the development of sustainable lubrication technologies.

4. Experimental Section

4.1. Chemicals

Following zeolite catalysts were used with its corresponding supplier and product name: USY zeolites (Zeolyst): CBV712, CBV720, CBV760, and CBV780; ZSM‐5 zeolites (Zeolyst): CBV2314, CBV3024E, CBV5524G, CBV8014, and CBV28014; ferrierite (Zeolyst): CP914C; ZSM‐35 (XFNANO); Beta (Zeolyst): CP814E*; Mordenite (Zeolyst): CBV21A; MCM‐22 (Bonding Chemicals); ZSM‐22 (ACS Material); ZSM‐23 (ACS Material); SAPO‐11 (ACS Material); and SAPO‐34 (ACS Material).

Following non‐zeolitic catalysts were used with its corresponding supplier and product name: amorphous silica–alumina (SASOL): SIRAL 10, SIRAL 40, and SIRAL 70 HPV; clay (Acros Organics): montmorillonite K10; and Ni 64% on silica–alumina (Strem Chemicals).

Following chemicals were used with its corresponding supplier: 1‐octene (Acros Organics); 1‐tetradecene (Alfa Aesar); 1‐octadecene (Fujifilm), 1,7‐octadiene (Alfa Aesar); n‐dodecane (Thermo Scientific); carbon disulfide (VWR); HDPE (Resinex): DOW HDPE 25055 E; and postconsumer HDPE (Indaver).

4.2. Apparatus and Experimental Procedure

4.2.1. Conical Spouted Bed Reactor

A CSBR system was built and employed for HDPE pyrolysis; a detailed reactor scheme is shown in Figure 6. The reactor system consists of three main units: (I) a plastic feeding system where plastics are stored and transported to the pyrolysis reactor using a screw feeder, set at approximately 3 g·min⁻¹. Plastics were continuously flushed using 470 mL·min⁻¹ N₂; (II) the conical spouted bed thermal pyrolysis reactor with a 30‐g sand bed in a conical section, heated to 650 °C with active heating of the conical shaped reaction part and the lower heating zone. Gases were preheated using a gas preheater before entering the reactor at 500 °C. A gas flow rate of 8 NL·min⁻¹ N₂ was used during the reactions; and (III) a product recovery system existing out of a cyclone (set at 120 °C), a condensation box (water‐ice cooled) with internal oil filters, and a separate coalescence filter. Gases leaving the condensation system were analyzed using an online gas TraceGC.

FIGURE 6.

Schematic representation of the CSBR system with the used dimensions (in cm).

4.2.2. Distillation

The collected pyrolysis samples were distilled using a vacuum distillation setup, obtained from Pilodist. Pyrolysate samples were distilled in 1‐L round‐bottom flask, using a packed Sulzer column. Continuous heating was used at a pressure of ca. 30 m bar. Products were distilled until an atmospheric equivalent head temperature of 342.7 °C was reached, representing a boiling point of a C₂₀ hydrocarbon. This atmospheric equivalent temperature was calculated using the Maxwell–Bonnell correlation. Owing to wax holdup in the column and transfer lines, the fractions were not quantified gravimetrically; the liquid and wax yields (Figure 2a) were determined by SimDist analysis of the combined pyrolysate (see Mass balance).

4.2.3. Catalytic Oligomerization Reactions in Batch Reactors

The model compound oligomerization tests were performed in 10‐mL batch reactors. The reactors were filled with a cross‐shaped magnetic stirrer, the hydrocarbon products, and catalysts. A 60:40 wt% mixture of n‐dodecane and model compound was used. This mixture was used for having a representative degree of olefins, representing a typical thermal pyrolysis sample. The reactor was flushed with nitrogen for oxygen removal and put in a stirred heating plate with a preheated copper block. Most reactions were performed at the standard conditions of 10 wt% catalyst, 200 °C and 600 rpm stirring.

Liquid thermal pyrolysis products that were isolated using vacuum distillation were oligomerized analogously in these 10‐mL reactors, at 10 wt% catalyst, 225 °C and 600 rpm stirring for 5 h. Additionally, a 600‐mL batch Parr reactor was used for bigger quantity production. Equally, reactions were performed at 10 wt% catalyst, 225 °C and 600 rpm stirring for 5 h.

Secondary hydrogenation steps were performed in the 600‐mL Parr reactor using 10 wt% Ni/silica–alumina catalyst. These reactions were performed at 80 °C, using 21 bar H₂ for a time of 6 h.

4.2.4. Catalyst Reuse and Regeneration

The fresh catalyst reactions were performed in three parallel reactors, after which the recovered material was pooled to provide sufficient catalyst for the consecutive cycles. The catalyst was recovered by centrifugation and washed with toluene through repeated cycles of agitation, centrifugation, and removal of the supernatant (three to four cycles, until the supernatant remained clear), followed by an analogous washing with acetone, and dried overnight at 60 °C. The washed and dried catalyst was employed in a subsequent oligomerization run under identical conditions (reuse). For regeneration, the catalyst recovered after this second run was calcined at 550 °C for 6 h and tested again (regeneration).

4.3. Chemical and Physical Analysis

4.3.1. Analysis of Liquid Pyrolysis Products

Liquid pyrolysis products were analyzed by GC–MS/FID. GC analysis was performed on a GCMS‐QP2010 SE (single quadrupole) from Shimadzu with an AOC‐20i Plus auto‐injector, injecting 1 μL sample with a split ratio of 1:20 into a 350 °C injection chamber. The GC was equipped with an SH‐I‐5HT (30 m × 0.25 mm × 0.25 μm) column. Following temperature program was used in the oven: 5 min at 40 °C, heat up to 300 °C (heat rate: 3 °C/min), followed by a 30‐min isothermal phase at 300 °C, and ended with a heating up to 350 °C (heat rate: 5 °C‍/‍min). He (150 kPa, total flow = 61.2 ml·min⁻¹) served as a carrier gas. After elution, the eluate was split to both detectors using a passive Silflow 4‐port splitter with an FID/MS split ratio of 1/0.15. The temperature of the flame ionization detector (FID) was kept constant at 400 °C. The retention time of linear alkanes, with a chain length ranging from 4 to 40 carbon atoms, was used to appoint a chain length to each detected peak. By summing all the areas between two consecutive calibrated retention times of linear alkanes, the absolute area of hydrocarbons with the same carbon number could be determined. The MS detector uses a 200 °C ion source and was used to identify peaks in the chromatogram. This detector was used as a qualitative assessment, whereas the FID detector was used as a quantitative assessment. GCxCG‐FID of oligomerized samples was measured externally at the TotalEnergies facilities.

4.3.2. Analysis of Solid Hydrocarbon Pyrolysis Products

The pyrolysis products recovered from the CSBR contained heavy molecular weight waxes. Therefore, analysis was needed to quantify the complete hydrocarbon mixture as classical GC methods undercalculate or fail to detect the higher molecular weight waxes. To overcome this, products were analyzed on a simulated distillation (SimDist) analysis tool. The products were analyzed according to the ASTM D7169 procedure on a TRACE 1600 (Thermo Scientific) GC.

4.3.3. Analysis of Gas Pyrolysis Products

Analysis of the gas fractions was performed online on a TraceGC 1610 (ThermoScientific) with three different detectors: (I) an FID for hydrocarbon detection, (II) a thermal conductivity detector (TCD) with He as reference gas for N₂, CO, and CO₂ detection, and (III) a TCD with Ar as reference gas for H₂ detection.

(I) FID channel for detection of hydrocarbons:

A sample loop (20 µL volume) was filled with the gas sample and injected on a Al₂O₃ MAPD column (30 m × 0.32 mm, 0.5 µm), followed by an RTX‐1 column (30 m × 0.32 mm). Argon was used as carrier gas with a flow of 3 mL·min⁻¹. Sample injection was stopped after 0.1 min. Then, Ar was flowed over the RTX‐1 column and Al₂O₃ MAPD column (same direction) for 1.10 min, after which a backflush was set over the Rtx‐1 in opposite direction, leading to the FID. A temperature program was applied to the oven where a 2‐min isothermal period was installed at 60 °C. Next, temperature was increased from 60 to 220 °C (30 °C·min⁻¹), followed by an isothermal section at 220 °C for 11 min. The FID detector was set at 250 °C, supplied by 350 mL·min⁻¹ air flow, 30 mL·min⁻¹ make up gas, and 35 mL/min H₂.

(II) TCD channel for N₂, CO, and CO₂ detection:

A sample loop (250 µL volume) was filled with the gas sample and injected on a HayeSep N packed column (60–80 mesh, 0.25 m × 1/16″ SS), followed by a XL sulfur packed column (60–80, 1 m × 1/16″ SS) and Molsieve (5A, 60–80, 2 m × 1/16″ SS). Helium is used as carrier gas at a constant pressure of 280 kPa. 1.35 min after the start of sample injection, the Molsieve is bypassed. 1.50 min after the start of sample injection, the injection is stopped and the compounds are backflushed from the HayeSep column and sent to vent. For the next 4.90 min, compounds further proceed through the XL sulfur column and are sent to the TCD, which is set at 110 °C with a filament temperature of 210 °C. The reference gas flow equals 1 mL·min⁻¹. Finally, components are eluted from the Molsieve. Valves and columns were located in an oven kept at 200 °C.

(III) TCD channel for H₂ detection:

A sample loop (250 µL volume) was filled with the gas sample and injected on a HayeSep N packed column (60–80 mesh, 1 m × 1/16″ SS), followed by a SC‐ST (60–80 mesh, 3 m × 1/16″ SS). Argon was used as carrier gas at a constant pressure of 350 kPa. 1.00 min after the start of sample injection, the HayeSep column was backflushed and sent to vent. The compounds on the SC–ST column were further sent to a TCD at 110 °C and a filament temperature of 230 °C. The reference gas flow was set at 1 mL·min⁻¹. Valves and columns were located in an oven kept at 200 °C.

4.3.4. Mass Balance

Product yields are reported relative to the weighed mass of plastic fed to the reactor. The condensed products collected in the cyclone and condensation box were combined and quantified gravimetrically, and the yields of liquids (C₅–C₁₉) and waxes (≥C₂₀) were obtained by resolving this condensate by simulated distillation (ASTM D7169). The impurities fraction corresponds to the inorganic content of the feedstock as determined by TGA (2.1 wt% for the PCW sample, Table S1). The gas yield was obtained by difference, with its composition verified by online GC analysis.

4.3.5. N₂‐Physisorption

N₂‐physisorption was measured on a TriStar (Micromeritics) gas adsorption analysis instrument. Samples were pretreated under N₂ atmosphere at 300 °C for at least 5 h using a SmartPrep degassing system (Micromeritics). Hereafter, the catalysts were analyzed at −196 °C. The surface areas were calculated using the Brunauer–Emmett–Teller (BET) method according to Rouquerol et al. [48]. The t‐plot method was used to determine the micropore and mesopore volume.

4.3.6. Pyridine and Lutidine‐Fourier‐Transform Infrared (FT‐IR) Spectroscopy

The nature and density of the acid sites were determined using pyridine (∅ = 0.57 nm) and lutidine (∅ = 0.67 nm) (2,6‐dimethylpyridine) adsorption, followed by Fourier‐transform infrared (FT‐IR) spectroscopy. A Nicolet 6700 spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector was used for this purpose [49, 50]. Catalyst wafers were made by pressing them (at 107 Pa) and cutting them into ca. 5–10 mg·cm⁻² materials. They were degassed in situ for 1 h at 400 °C (5 °C· min⁻¹) under vacuum (<1 m bar). After cooling to 150 °C, spectra of the materials were recorded as a reference with an accumulation of 128 scans at a resolution of 2 cm⁻¹. The samples for the pyridine adsorption were subjected to pyridine until saturation at 50 °C. After saturation, vacuum thermal desorption was performed for 1 h at 150 °C (and optionally at 250 and 350 °C). Spectra were recorded at 150 °C after each desorption step. The amount of Brønsted (BAS) and Lewis (LAS) acid sites were determined by integrating the area of the characteristic bands: ν8a of the pyridinium ions (PyH+) at 1545 cm⁻¹ and ν19b of the coordinated pyridine species (PyL) at ~1445 cm⁻¹ and using their corresponding molar absorption coefficients: ε (PyH+) = 1.67 cm·µmol⁻¹ and ε (PyL) = 2.22 cm·µmol⁻¹, respectively [51]. The accessibility of BAS was measured by gas‐phase lutidine adsorption at 50 °C. Small doses of almost 3 m bar lutidine were added onto the samples after activation until saturation. A spectrum was recorded after each dose with 2 min delay to give time for the diffusion of lutidine molecules inside the pores. Thermal desorption was first performed at 50 °C for 10 min under vacuum, in order to remove the excess lutidine species from the surface of the catalyst, followed by a second desorption step of 1 h at 150 °C. The amount of lutidine titrated BAS was estimated from the sum of the bands at 1643 and 1627 cm⁻¹ (respectively, ν8a and ν8b vibrations) using the integrated molar absorption coefficient ε (Lut‐H+) = 6.8 cm µmol⁻¹ [52].

4.3.7. Near‐Infrared (NIR) Spectroscopy

A near‐infrared (NIR) spectroscopy analysis was performed on the PCW polymer granules using a sIRoPAD analysis tool (GUT Environmental Technologies). In this way, a polymer composition could be calculated, differentiated into PE (polyethylene), PET (polyethylene terephthalate), PA (polyamide), PVC (polyvinyl chloride), PMMA (poly(methyl methacrylate)), and PC (polycarbonate). Measurements were conducted by Indaver.

4.3.8. Metal Analysis

Metals in the postconsumer samples were determined via inductively coupled plasma–optical emission spectrometry. Following elements were measured: Al, As, B, Ba, Be, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, Se, Si, Sn, Tl, V, Zn, Fe, Ca, K, Mg, Na, P, and S. Measurements were conducted by Indaver.

4.3.9. Elemental (CHNS/O) Analysis

CHNS/O analysis of solid plastic feedstocks was performed using a Flash EA2000 elemental analyzer (Interscience) equipped with a TCD. Samples were measured following the ASTM D5291 method in combustion mode for determining. The ASTM D5622 method was used for oxygen determination and was performed in a separate reactor in pyrolysis mode.

4.3.10. Physical Property Assessment

The cloud point, pour point, and kinematic viscosity (at 40 °C) of produced bigger quantity base oil were measured at TotalEnergies facilities. Due to limited sample availability, the initial properties were measured on postconsumer pyrolysis oil, while the final values correspond to a transformed virgin pyrolysis oil. These data are included to illustrate the typical shift in physical properties following the applied upgrading process.

Funding

This work was supported by the Agentschap Innoveren en Ondernemen (HBC.2019.0001, HBC.2021.0683, HBC.2021.0752, HBC.2020.2609, HBC.2023.0550, HBC.2024.0628); Fonds Wetenschappelijk Onderzoek (G0B7218N); Vlaamse Overheid (VV021); KU Leuven (KA/16/064, KA/20/038) and Bio‐Based Industries Joint Undertaking (744310).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supplementary Material

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.