Catalytic Transformation of LDPE into Aromatic-Rich Fuel Oil

Abstract

The present study investigates the catalytic conversion of low-density polyethylene (LDPE) into high-grade fuel oil using a semi-batch reactor at 350 °C under ambient pressure, with a catalyst-to-LDPE ratio of 1:20. Zeolite-based catalysts were synthesized by impregnating different metals (Fe, Zn, Cr, Mn, and Ga) onto ZSM-5 with a silica-to-alumina ratio of 30 (Z30). These catalysts were characterized using BET, XRD, and NH3-TPD techniques to evaluate their physicochemical properties. The results showed that catalytic pyrolysis of LDPE yielded less pyrolytic oil compared to non-catalytic pyrolysis. The obtained pyrolytic oil was analysed through elemental composition, gross calorific value (GCV), Simulated Distillation, and GC-DHA. The elemental analysis revealed a high carbon (85–86%) and hydrogen (13–14%) content, resulting in a high GCV of approximately 42 MJ/kg. GC-DHA analysis indicated that the pyrolytic oil was rich in aromatic and olefinic compounds. Among the catalysts, 5Fe/Z30 exhibited the highest aromatic selectivity (35%), a research octane number of 91, and 100% LDPE conversion. These findings underscore the potential of low-cost iron-based catalysts for efficiently converting LDPE waste into valuable chemicals and fuels.

1. Introduction

Global plastic production has experienced exponential growth over recent decades, driven by diverse applications, leading to substantial fossil fuel consumption and unprecedented plastic waste generation. The global accumulation of plastic waste is anticipated to reach approximately 26 billion tons by 2050 [1]. Most of the plastic materials are non-biodegradable and their improper disposal has led to catastrophic problems in the aquatic and land environments. Effective end-of-life plastic waste management has emerged as a pressing public priority. Developing innovative recycling strategies and controlling pollution at its source are crucial for mitigating plastic waste issues. Conventional mechanical recycling techniques, involving melting and re-extrusion, are limited by high pre-sorting costs, energy consumption, and compromised product quality. Energy recovery through incineration, while beneficial for processing mixed plastics, poses environmental risks due to uncontrolled pollutant emissions, including polychlorobiphenyls, dioxins and elevated emission of carbon dioxide [2,3]. Therefore, development of a technology for upcycling plastic waste materials causing reduction in carbon emissions and environmental problems is inevitable. The thermochemical recycling of plastic waste is gaining more importance as these technologies allow for the treatment of plastic mixtures, thereby bypassing the need for upstream separation of different plastic materials. Consequently, the development of novel catalytic approaches for plastic waste upcycling, including hydrogenolysis, hydrocracking, pyrolysis, and oxidative upcycling, has gained significant momentum in recent years, focusing on producing valuable monomers and petrochemicals like aromatics. Among these, Hydrogenolysis has shown potential for depolymerization of PE into paraffins. Notable examples include Pt/SrTiO₃ for producing lubricants and waxes [4] and ruthenium-based catalysts [5,6,7] exhibiting higher activity than platinum-based catalysts. A study demonstrated hydrocracking of mixed plastic over Fe/Mo-Al₂O₃ catalyst, yielding a fuel product with 97% similarity to diesel fuel [8]. Recent advancements have highlighted catalytic pyrolysis approaches for producing aromatics from waste plastics without using hydrogen, although it generally requires high reaction temperatures (500–700 °C) [9,10]. Zhang et. al. [11] have reported upcycling of polyethylene to high boiling liquid and wax fraction predominated with alkyl aromatics using Pt/Al₂O₃ catalyst in absence of H₂ or solvents. Du et al. [12] used Ru/ZSM-5 to convert HDPE into cyclic hydrocarbons including high percentage of aromatics without requiring H₂ and any solvent. Islam et. al. [13] have reported microwave-based pyrolysis of LDPE using metal impregnated ZSM5 with high aromatic contents in liquid pyrolytic oil. Riaz et al. [14] conducted a kinetic study on catalytic pyrolysis of HDPE using ZSM-5 catalyst, employing a machine learning-based Boosted Regression Tree (BRT) model to predict activation energy. A significant drop in activation energy from 323 kJ/mol (non-catalytic) to 164 kJ/mol (catalytic) was observed.

This study aims to develop a sustainable, hydrogen-free process for converting low-density polyethylene (LDPE) into aromatic-rich fuel oil using cost-effective, metal-modified ZSM-5 catalysts under mild pyrolysis conditions (350 °C). We systematically evaluate Fe, Cr, Ga, Zn, and Mn on ZSM-5—focusing on their bifunctional (acid-redox) properties—to correlate catalyst design with product selectivity. Unlike prior studies, we highlight the unprecedented efficiency of Fe/ZSM-5 and Cr/ZSM-5 in achieving near-complete LDPE conversion to aromatics without solvents, precious metals, or external hydrogen. A suite of characterization techniques (NH₃-TPD, XRD, ICP-OES, GC-DHA) links metal-specific cracking-aromatization mechanisms to performance. This work provides actionable insights for scaling plastic waste upcycling to drop-in fuels

2. Results and Discussion

2.1. Characterization of LDPE

LDPE was characterized using various analytical instruments, and the corresponding data are presented in

Table 1. Analysis of LDPE.

Ultimate Analysis (%)		Proximate Analysis (%)
Carbon (C)	86.05	Volatile matter (VM)	99.60
Hydrogen (H)	13.95	Ash	0.33
Oxygen (O)	0.00	Fixed carbon (FC)	0.00
Nitrogen (N)	0.00	Moisture content (MC)	0.07
Sulphur (S)	0.00
GCV (MJ/kg)	42.27
Size of pellets (mm)	2–4
Mn (Daltons)	5000	Mw (Daltons)	98,000

. The proximate and ultimate analysis play a critical role in understanding the properties of any potential feedstock to produce fuel. The proximate analysis indicated the presence of 99.60% volatile matter (VM) in LDPE along with 0.33% ash and 0.07% moisture content (MC). The pyrolysis of plastics exhibits a positive correlation between volatile matter content and liquid yield, whereas high ash content favors char formation. The high volatile matter content in LDPE demonstrates significant potential for producing high liquid oil yields under optimal process conditions. The negligible amount of fixed carbon (FC) in polymeric materials suppresses tar formation during thermal conversion via pyrolysis. Similarly, the presence of MC in the material increases energy requirements subsequently reducing the GCV of the material. Several studies have documented similar results while performing the proximate analysis of LDPE [15,16].

The ultimate analysis indicated a significant content of carbon (~86.05%) and hydrogen (~13.95%) in LDPE, contributing to both the GCV of LDPE itself and the fuel derived from it. The GCV of the LDPE sample was estimated to be 42.27 MJ/kg. Furthermore, the ultimate analysis results demonstrated that LDPE is rich in hydrocarbon content, suggesting its potential for generating value-added chemicals through depolymerization processes. The results of ultimate analysis of LDPE are aligned with values available in the literature [16,17]. The results of both proximate and ultimate analyses support the suitability of LDPE for fuel production and the generation of high-value organic chemicals.

The Thermogravimetric Analysis (TGA) of LDPE was conducted to investigate its thermal characteristics concerning time and temperature. During the TGA analysis, the temperature was raised from 25 °C to 800 °C. The TGA and DTG curves obtained from LDPE are depicted in Figure 1, which illustrates that nearly all of the LDPE underwent degradation, reaching approximately 99.60% degradation at around 550 °C. The results of the TGA analysis were utilized to determine three critical temperatures in LDPE degradation: the initial temperature (T_i) at which degradation initiates, the final temperature (T_F) at which LDPE was completely degraded, and the temperature (T₅₀) at which 50% of the LDPE sample was degraded. The T_i, T₅₀ and T_F of LDPE were 320, 432 and 550 °C respectively. The TGA analysis revealed minimal weight loss below 200 °C, indicating the minimal amount of moisture in the LDPE. Likewise, quite small weight was observed after the complete degradation of LDPE, confirming the very small amount of ash in LDPE. The results from the proximate analysis of LDPE are consistent with these findings The TGA data of LDPE aligns with previously reported [18,19,20] results from kinetic studies conducted under diverse conditions. In conclusion, the TGA analysis results suggest that the temperature range of 340–550 °C is optimal for achieving the highest conversion of LDPE into liquid oil products.

2.2. Characterization of Catalysts

The synthesized catalysts underwent comprehensive characterization via BET, NH3-TPD, and XRD analyses to determine their physicochemical properties. The textural properties of all catalysts are given in

Table 2. Characteristics of Parent and metal Impregnated Z30 catalysts.

Entry	Catalyst	Textural Properties				NH3-TPD Results (mmol NH3/g)
		SBET	PVtotal	PDavg	LTP	HTP	Total	HTP/LTP
		(m2/g)	(cm3/g)	(nm)	(100–300 °C)	(300–625 °C)	(100–625 °C)
1	Z30	342.00	0.17	5.05	0.78	0.62	1.39	0.79
2	2Fe/Z30	333.00	0.14	6.41	0.62	0.55	1.17	0.88
3	5Fe/Z30	319.00	0.17	7.87	0.57	0.54	1.11	0.95
4	2Zn/Z30	332.00	0.15	7.30	0.68	0.15	0.83	0.22
5	5Zn/Z30	293.00	0.11	7.14	0.53	0.05	0.58	0.08
6	2Cr/Z30	320.00	0.18	8.00	0.63	0.52	1.15	0.84
7	5Cr/Z30	315.00	0.20	7.10	0.48	0.42	0.90	0.89
8	2Mn/Z30	317.00	0.14	7.00	0.74	0.60	1.34	0.81
9	5Mn/Z30	291.00	0.12	7.00	0.70	0.59	1.29	0.84
10	2Ga/Z30	325.00	0.16	7.57	0.76	0.52	1.28	0.68
11	5Ga/Z30	316.00	0.13	7.00	0.75	0.44	1.19	0.59

S_BET = BET surface area; PV_total = Total Pore Volume; PD_avg = Average Pore Diameter.

. The N₂ adsorption and desorption isotherms displayed a type-I isotherm, suggesting that all catalyst samples possess a microporous structure. Isotherms of N₂ desorption and adsorption are presented in Figure S2 of the Supplementary Material. The BET surface area and total pore volume of catalyst samples dropped on impregnation of metals on Z30. The increase in mass percent of metals caused successive drop in both surface area and pore volume due to the distribution of metal ions on the surface of zeolites and causing partial blockage of pores [21]. The average pore diameter of all metal impregnated catalysts was slightly higher than that of parent Z30 catalysts. The probable reason for the increase in pore diameter could be due to collapse of micropores and formation of large pores during impregnation of metals ions and successive calcinations. Typically, moderate metal loading on Z30 does not show any remarkable effect on the BET surface area, and the structure of Z30 is preserved. However, excessive loading of any metal on Z30 could lead to pore blockage or inflict damage on the crystal structure, resulting in a decrease in BET surface area [21]. The pore size distribution of the different catalysts is presented in Figure S3 of the Supplementary Material.

2.2.1. NH₃-TPD Analysis

NH₃-TPD was performed on all the synthesized catalysts along with parent Z30 to examine their acidity and the outcomes are presented in Figure 2 and Table 2. Typically, when examining a profile of NH₃-TPD on HZSM-5 zeolite, two desorption peaks are observed. These include a low-temperature peak (LTP) below 300 °C and a high-temperature peak (HTP) above 300 °C. These peaks are attributed to the adsorption of ammonia on the weak and strong acid sites, respectively [22]. The loading of metal precursors into the parent Z30 resulted in a change in acidic characteristics by decreasing both weak and strong acid sites. All catalysts exhibited weak acidity within the range of 0.48–0.78 NH₃ mmol/g, and the strong acidity in the range of 0.05–0.62 NH₃ mmol/g. This variation in acidity is mainly attributed to the electronic interaction between the metal precursors and the Z30 structural framework. During metal impregnation on Z30, some protons may have been replaced by some metal cations within the Z30 zeolite structural framework. These ions exhibit lower positive charge density than protons, resulting in the formation of weak electrostatic contacts with the oxygen atoms of the structural framework [23]. Consequently, Z30 catalysts impregnated with metal ions exhibit lower acidity (both weak and strong) due to the interaction of metal ions with framework Bronsted acid sites and partial blockage of micropores [23]. The 5Fe/Z30 exhibited the highest strong/weak acid ratio (0.95) as compared to all other catalysts. A similar result has been reported by Rahimi et al. [24]. Whereas 5Zn/Z30 exhibited lowest strong/weak acid ratio 0.08 probably due to interaction of Zn species with conventional strong acid sites, and changed to medium acidity [25].

2.2.2. XRD Analysis

The XRD analysis of all the catalysts was compared with the parent Z30 to evaluate the influence of metal impregnation on the crystallographic structure. The XRD results are illustrated in Figure S4. All catalyst samples exhibited diffraction peaks within the range of 5°–50°. These peaks, observed at 2θ values of 7.94°, 8.78°, 14.77°, 23.09°, 23.91°, and 24.39°, correspond to the [011], [020], [031], [051], [303], and [313] planes, respectively, which are characteristic of the MFI structure and align with the reference ZSM-5 (PDF#44-0003) [26]. A subtle decrease in X-ray diffraction (XRD) peak intensities, accompanied by a minor shift in 2θ values, is attributed to the metal impregnation on ZSM-5. However, no significant differences were noted between the XRD patterns of the parent Z30 and the metal-impregnated Z30. This confirms that the metal impregnation process did not alter the fundamental crystalline structure of Z30, ensuring that the Z30 framework remained intact even after the addition of metal precursors [21,27,28,29,30,31].

2.3. Catalytic Pyrolysis of LDPE

A semi-batch reactor was used for the catalytic pyrolysis of LDPE under an argon flow to maintain an inert atmosphere. Prior to the reactor experiments, thermogravimetric analysis (TGA) was conducted on LDPE both with and without the catalyst (Z30). The resulting TGA and DTG curves are presented in Figure 1 and Figure S1. As discussed in Section 3.1 and Figure 1, pristine LDPE (without catalyst) showed minimal thermal degradation, with less than 5% weight loss at 350 °C. In contrast, when mixed with Z30, LDPE exhibited significant catalytic degradation, with over 80% weight loss at the same temperature. To validate these findings, LDPE alone was heated in the batch reactor at 350 °C for 2 h, but no substantial pyrolysis occurred. Based on these results, all catalytic pyrolysis experiments were performed at 350 °C under atmospheric pressure. For comparison, non-catalytic (thermal) pyrolysis was conducted at a higher temperature (450 °C) due to the lower reactivity of LDPE in the absence of a catalyst. A similar reduction in pyrolysis temperature with catalytic assistance has been reported by Marcilla et al. [32] using ZSM-5 and HUSY zeolites.

2.3.1. Effect of the Catalyst-to-Feed (Z30/LDPE) Ratio

The effect of the catalyst-to-feed (Z30/LDPE) ratio on product distribution and conversion was systematically examined. The ratio varied from 1:10 to 1:100 during the catalytic pyrolysis of LDPE at 350 °C. The results are presented in Figure 3. At a Z30/LDPE ratio of 1:10, a liquid yield of 46% and a gas yield of 54% were achieved, with no solid residue, indicating 100% conversion. Increasing the ratio to 1:15 resulted in a liquid yield of 56% and a gas yield of 44%. At a ratio of 1:20, similar liquid and gas yields were observed, maintaining 100% conversion. However, further increases in the ratio caused a decline in both liquid and gas yields and an increase in solid residue, indicating reduced conversion. The highest ratio of 1:100 produced 58% solid residue, reflecting only 42% conversion of LDPE. Based on these findings, a catalyst/feed ratio of 1:20 was identified as the optimum ratio for subsequent catalytic evaluations.

2.3.2. Catalytic Performance of Metal/Z30 Catalysts

A series of catalysts was synthesized by impregnating Z30 with five different metals (Fe, Cr, Mn, Zn, and Ga) at 2 and 5 wt.% loadings, selected through a systematic approach combining literature benchmarks and experimental optimization. Studies suggest 1–5 wt.% balances metal dispersion and activity, while ≥5 wt.% risks pore blockage [33]. Our preliminary characterization of Fe/Z30 catalysts confirmed this trend: increasing the Fe loading from 2 to 5 wt.% resulted in only a modest (<5%) reduction in BET surface area, whereas 10 wt.% loading led to a significant 15% decrease due to pore blockage. Correspondingly, catalytic performance tests revealed that 2Fe/Z30 achieved 99% LDPE conversion with 32.7% aromatic selectivity, while 5Fe/Z30 showed slightly improved performance (100% conversion, 34.57% aromatics). In contrast, the 10Fe/Z30 catalyst exhibited substantially reduced activity (90% conversion) and selectivity (30.27% aromatics).

All synthesized catalysts were evaluated for their ability to catalyse the pyrolysis of LDPE at 350 °C, using a catalyst/feed ratio of 1:20. The results are summarized in

Table 3. Effect of different catalysts on octane number, elemental analysis, and GCV.

Entry	Parent Catalyst	Metal Precursor	Metal Loading (wt%)	Catalyst Name	Octane Number	Elemental Analysis			GCV (MJ/kg)
						Carbon (%)	Hydrogen (%)	Oxygen (%)
1	Non-Catalytic	ꟷ	ꟷ	Non-Cat	75.20	85.26	14.74	0.00	42.81
2	ZSM-5	ꟷ	ꟷ	Z30	83.40	85.56	14.43	0.00	42.60
3	ZSM-5	Fe(NO3)3·9H2O	2.00	2Fe/Z30	89.10	85.94	14.05	0.00	42.34
4	ZSM-5		5.00	5Fe/Z30	91.00	86.02	13.92	0.05	42.22
4	ZSM-5	Zn(NO3)2·6H2O	2.00	2Zn/Z30	87.30	85.75	14.24	0.00	42.46
5	ZSM-5		5.00	5Zn/Z30	88.40	86.15	13.79	0.05	42.13
6	ZSM-5	Cr(NO3)3·9H2O	2.00	2Cr/Z30	89.20	86.17	13.82	0.00	42.18
7	ZSM-5		5.00	5Cr/Z30	90.20	86.45	13.51	0.03	41.96
8	ZSM-5	MnCl2·4H2O	2.00	2Mn/Z30	85.50	85.43	14.49	0.07	42.60
9	ZSM-5		5.00	5Mn/Z30	84.30	85.58	14.41	0.00	42.58
10	ZSM-5	Ga(NO3)3 xH2O	2.00	2Ga/Z30	89.80	86.15	13.80	0.04	42.14
11	ZSM-5		5.00	5Ga/Z30	90.10	86.21	13.78	0.00	42.15

and illustrated in Figure 4.

Thermal pyrolysis of LDPE at 450 °C yielded 28% gas, 65% liquid, and 7% polymer residue. In contrast, catalytic pyrolysis (at 350 °C) using Z30 produced 44.5% gas and 55.5% liquid, with no polymer residue, indicating complete conversion. Although the liquid yield from thermal pyrolysis was higher, the quality of the liquid oil obtained with the Z30 catalyst was significantly superior. The lower oil yield and higher gas yield in the catalytic process are attributed to the acidic properties of Z30, which enable efficient cracking and selective hydrocarbon production at the optimal process temperature [24].

Among the metal-impregnated catalysts, 2Fe/Z30 and 5Fe/Z30 exhibited the highest gas yields (61% and 58%, respectively) and complete conversion (99% and 100%, respectively). All other metal-based catalysts left some polymer residue, indicating incomplete conversion. Furthermore, increasing the metal loading from 2% to 5% generally resulted in lower conversion and higher residue formation. The superior performance of 5Fe/Z30, which achieved the highest gas yield and 100% conversion, is attributed to its optimal ratio (0.95) of strong to weak acid sites, where the synergistic interplay between strong Brønsted acid sites (NH₃ desorption at 300–625 °C) and weak Lewis acid sites (150–300 °C, Fe³⁺) facilitated a cascade of reactions: Brønsted sites initiated C-C bond cleavage in LDPE to generate C₃-C₅ olefinic intermediates while stabilizing carbocations for cyclization (alkene → cycloalkene → aromatic), while Lewis sites subsequently promoted the dehydrogenation of these cycloalkenes (e.g., cyclohexene → benzene) to yield the final aromatic products.

While the use of metal-based catalysts reduced the liquid oil yield, they significantly improved the oil’s quality, as evidenced by enhanced gross calorific value (GCV), flow properties, and chemical composition [34].

Figure 5 illustrates the effect of various catalysts on LDPE conversion during the pyrolysis process. The non-catalytic pyrolysis (conducted at 450 °C) achieved an LDPE conversion of 93.4%. In comparison, the catalytic pyrolysis process using Z30 and 5Fe/Z30 achieved complete conversion (100%) of LDPE. Iron serves dual roles in cracking and aromatization during plastic pyrolysis. Mechanistically, Fe generates both Brønsted and Lewis acid sites that facilitate C-C bond scission while promoting dehydrogenation reactions that convert olefins to aromatic compounds through Diels-Alder pathways. The strong Fe-O-ZSM-5 bonding enhances catalyst stability and increases overall acidity. These properties make Fe/ZSM-5 particularly effective at converting long-chain hydrocarbons into valuable light aromatic compounds like benzene, toluene, and xylenes (BTX).

In general, LDPE conversion was lower for metal-impregnated catalysts compared to pristine Z30, and the conversion decreased further with increasing metal loading. This trend can be attributed to a reduction in surface area and a decline in Brønsted acid sites, which are essential for effective cracking reactions. Interestingly, the Cr-based catalysts demonstrated an opposite trend, with LDPE conversion increasing from 91% to 93% as the chromium loading increased from 2% to 5%. This improvement is likely due to the ability of chromium to exist in multiple oxidation states, enabling redox-based cracking reactions [35]. Additionally, Cr-based catalysts exhibit superior thermal stability compared to other metals [35,36]. Chromium primarily acted as a cracking catalyst through its unique redox cycling between Cr³⁺ and Cr⁶⁺ states, which facilitated radical formation. Compared to Fe, Cr shows weaker hydrogen transfer capability, resulting in less aromatization activity. The moderate interaction between Cr and ZSM-5 typically forms isolated Cr³⁺ sites rather than extensive clusters. This leads to product distributions richer in light gases (C₂-C₄ hydrocarbons) with only moderate aromatic yields.

Gallium- and zinc-based catalysts showed a more significant drop in conversion as the metal loading increased from 2% to 5%. These metals specialize in aromatization but demonstrate poor cracking performance for large polymer molecules. Ga and Zn promote cyclization reactions, with Zn²⁺ particularly effective at stabilizing reaction intermediates for aromatic formation. When incorporated into ZSM-5, Ga and Zn preferentially occupy ion-exchange sites, which reduces the catalyst’s strong acidity. While selective for aromatic production, these metals often suffer from rapid pore blockage and consequently show lower overall conversion rates compared to Fe or Cr catalysts.

In contrast, Manganese based catalysts (2Mn/Z30 and 5Mn/Z30) demonstrates minimal catalytic activity in this system due to incomplete decomposition of its nitrate precursor during preparation. This anomalous behaviour may be explained by the preparation method, as manganese catalysts were synthesized using MnCl₂ as the precursor. It has been reported that MnCl₂ does not decompose effectively at calcination temperatures below 600 °C [37]. Consequently, the incomplete decomposition of MnCl₂ likely prevented the formation of fully active manganese oxide, limiting the catalyst’s performance.

The resulting MnO_x clusters physically block zeolite pores while nitrate residues poison active sites. Mn exhibits weak interaction with the ZSM-5 framework, ultimately producing catalytic performance similar to unmodified ZSM-5 with no significant enhancement of pyrolysis products.

The spent catalyst samples were analyzed to find coke in terms of carbon %. Quantitative measurements revealed significant variations in coke formation among catalysts, with 5Cr/Z30 exhibiting the highest carbon content (5.3%) and 2Ga/Z30 showing the lowest (2.9%). The observed coke deposition trend followed the order: Cr/Z30 > Fe/Z30 > Mn/Z30 > Zn/Z30 > Z30 > Ga/Z30, as illustrated in Figure S5 (Supplementary Materials). This analysis provides valuable insights into the deactivation behaviour of different metal-modified catalysts.

2.4. Characterization of Pyrolytic Oil

2.4.1. Elemental Analysis, GCV, and Octane Number of Pyrolytic Oil Samples

The pyrolytic oil obtained from different catalysts underwent extensive analysis to evaluate its fuel properties and chemical composition. Carbon and hydrogen content play a crucial role in determining the fuel’s energy density and combustion efficiency, while high oxygen content negatively impacts combustion efficiency, leading to a lower gross calorific value (GCV). The GCV represents the energy content of the fuel and is a key indicator of its efficiency during combustion. Fuels with higher GCV values are more energy-dense and efficient.

The elemental composition and GCV of the pyrolytic oil samples obtained using different catalysts are summarized in Table 3. The carbon and hydrogen contents in all pyrolytic oil samples were significantly high, ranging from 85–87% and 13–14%, respectively. This composition contributed to the high GCV of all oil samples, which fell within the range of 41–42 MJ/kg, comparable to commercial fuels such as gasoline and diesel (42–47 MJ/kg) [38]. The octane number of a fuel is a critical parameter that ensures smooth engine performance by preventing knocking. A higher-octane number indicates greater fuel stability. According to the Energy Information Administration (EIA), gasoline is categorized into three main grades based on octane number: Regular (87), Mid-grade (89–90), and Premium (91–94) [15]. The octane numbers of pyrolytic oil samples obtained from different catalysts are presented in Table 3. The results showed that most metal-impregnated catalysts produced pyrolytic oil with octane numbers in the range of 87–91, aligning with EIA recommendations. However, Mn-based catalysts (2Mn/Z30 and 5Mn/Z30) exhibited relatively lower octane numbers of 85.5 and 84.3, respectively. The incomplete decomposition of MnCl₂ precursor material at calcination temperatures below 600 °C [37] is believed to be responsible for the observed behaviour, as it restricts the complete transformation into an active metal oxide catalyst. In contrast, non-catalytic pyrolysis resulted in pyrolytic oil with an octane number of 42.8, significantly lower than the recommended EIA range, highlighting the effectiveness of catalytic pyrolysis in producing high-quality fuel.

2.4.2. Simulated Distillation Analysis of Pyrolytic Oil Samples

The Simulated Distillation (SimDist) analysis of pyrolytic oil samples was conducted to evaluate the effect of different catalysts on the fraction composition of the produced oil. This was achieved by comparing the boiling point distribution of various petroleum fractions using GC-SimDist, which categorizes the fractions into light distillate (20–200 °C), medium distillate (200–350 °C), and heavy distillate (>350 °C) [39].

The light distillate fraction, which includes naphtha, gasoline, kerosene, and jet fuel, is particularly significant due to its high volatility, small molecular size, and ease of flow. Figure 6 presents the SimDist curves of pyrolytic oil obtained from different catalysts. The non-catalytic pyrolytic oil exhibited the highest proportion of heavy (40%) and medium (35%) fractions compared to all other pyrolytic oil samples.

In contrast, pyrolytic oil samples obtained using different catalysts demonstrated a significantly higher percentage of light fractions, ranging from 70–85%. The use of acidic catalysts facilitated the thermal cracking of reaction intermediates formed during the pyrolysis process, leading to an increased yield of light fractions while reducing the formation of medium and heavy fractions. This highlights the catalytic role in enhancing product quality and maximizing the production of valuable light distillates.

2.4.3. GC-DHA Analysis of Pyrolytic Oil Samples

The chemical composition of pyrolytic oil samples was analysed using GC-DHA to determine the selectivity of catalysts toward specific hydrocarbon groups, including paraffins, olefins, naphthenes, and aromatics. The results are presented in

Table 4. Detailed hydrocarbon analysis (DHA) of pyrolytic oil using different catalysts.

Entry	Catalyst	Paraffins	Olefins	Naphthenes	Aromatics	C14+
1	Non-catalyst	19.47	54.60	0.00	12.00	13.90
2	Z30	27.75	46.97	6.96	18.28	0.00
3	2Fe/Z30	23.65	35.60	7.48	32.70	0.60
4	5Fe/Z30	23.16	34.48	7.00	34.57	0.80
5	2Cr/Z30	24.50	36.00	7.20	31.90	0.40
6	5Cr/Z30	24.60	34.40	7.10	33.40	0.48
7	2Mn/Z30	23.00	45.40	6.60	24.80	0.16
8	5Mn/Z30	24.45	52.40	6.35	16.76	0.00
9	2Zn/Z30	23.85	43.06	7.29	25.60	0.20
10	5Zn/Z30	22.39	42.10	6.20	29.07	0.20
11	2Ga/Z30	22.59	37.39	8.00	31.68	0.30
12	5Ga/Z30	21.90	35.35	7.14	34.77	0.79

and Figure 7.

Non-catalytic pyrolysis produced the highest proportion (13.9%) of C₁₄₊ hydrocarbons, indicating a heavier composition. In contrast, all catalytic pyrolysis runs resulted in less than 1% C₁₄₊, a trend consistent with SimDist analysis, which showed the highest heavy distillate fraction in non-catalytic pyrolysis.

Pristine Z30 exhibited the highest selectivity for paraffins and olefins and the lowest selectivity for aromatics. In contrast, all metal-impregnated Z30 catalysts (except Mn/Z30) demonstrated higher aromatic selectivity, with a more pronounced effect observed in catalysts with 5% metal loading compared to those with 2%. This trend can be attributed to the bifunctional nature of metal-impregnated Z30 catalysts. Metal oxides exhibit Lewis acidity, while zeolite protons provide Brønsted acidity. Brønsted acid sites facilitate cracking, isomerization, oligomerization, and cyclization reactions, whereas Lewis acid sites promote dehydrogenation reactions [40]. This bifunctional property enhances aromatic formation, explaining the higher aromatic selectivity in metal-impregnated catalysts. The highest aromatic selectivity (35%) was observed in 5Fe/Z30 and 5Ga/Z30 samples. However, 5Fe/Z30 also demonstrated 100% conversion and a higher liquid yield, which can be attributed to its high ratio of strong to weak acid sites and superior cracking activity [30,41]. In contrast, 2Mn/Z30 and 5Mn/Z30 catalysts exhibited the lowest aromatic selectivity among all metal based catalysts. The observed behavior of Mn/Z30 catalysts is believed to result from the use of MnCl₂ as the precursor material, which exhibits incomplete decomposition at calcination temperatures below 600 °C [37]. This incomplete decomposition prevents the formation of an active metal oxide catalyst with bifunctional properties, thereby impeding aromatic formation.

Although the high aromatic selectivity of 5Fe/Z30 meets petrochemical feedstock requirements, the environmental and health risks associated with benzene derivatives in the pyrolytic oil (Table S1) must be addressed. These compounds are toxic and strictly regulated in transportation fuels under EPA/EU standards. To ensure compliance, downstream refining strategies—such as partial hydrogenation or blending with aliphatic fractions—can effectively reduce aromatic content. Alternatively, directing this aromatic-rich pyrolytic oil exclusively toward chemical feedstock applications (avoiding fuel combustion entirely) presents a safer and more sustainable utilization pathway.

The reaction mechanism for the catalytic pyrolysis of LDPE over bifunctional catalyst is proposed and illustrated in Figure 8, based on the observed product distribution. In non-catalytic pyrolysis, thermal cracking leads to the random scission of C–C bonds, generating long-chain hydrocarbon fragments, which typically result in a broad distribution of heavy hydrocarbons, often in the form of waxy products [42]. In contrast, during catalytic pyrolysis, hydrocarbon chains undergo chain scission reactions over the Bronsted acid sites of catalyst via a carbocation mechanism, leading to the formation of olefins [43]. The presence of Lewis acid sites facilitates hydrogen transfer reactions, producing short-chain alkanes. These generated olefins and alkanes diffuse into the pores of Z30, where they undergo secondary reactions such as isomerization, cyclization, dehydrogenation, and aromatization to produce aromatics (BTEX). Light aromatics (BTEX) further undergo oligomerization, leading to coke formation.

3. Experimental

3.1. Materials

LDPE pellets with particle sizes ranging from 2–4 mm (Mn: 5000 & Mw: 98,000) were obtained from SABIC in Saudi Arabia with a commercial code of HP0823. ZSM-5 (SiO₂/Al₂O₃: 30) zeolite was obtained from Zeolyst International (CBV3024E). High purity metal precursors, including Fe(NO₃)₃·9H₂O, Zn(NO₃)₂·6H₂O, Cr(NO₃)₃·9H₂O, Ga(NO₃)₃·xH₂O, and MnCl₂·4H₂O, were procured from Sigma–Aldrich and used without any further purification.

3.2. Catalyst Synthesis

ZSM-5 zeolite with ammonium cation was converted to acid form by calcination at 550 °C for 5 h prior to its subsequent use in the process. Various metals, namely Fe, Zn, Cr, Ga, and Mn, were impregnated on ZSM-5 using different concentration via wet impregnation method. In typical procedure, a 5 g of ZSM-5 zeolite was suspended in 50 mL of water with continuous stirring at 500 RPMs in a beaker placed on a hot plate. Subsequently, 50 mL of aqueous solution containing required amount of metal precursor was gently added to ZSM-5 slurry drop by drop and stirred continuously for three hours. The mixture was left overnight in a fume hood to evaporate the water. The prepared material was dried at 80 °C for 12 h and then calcined at 550 °C for 5 h. The different concentration of metal on the ZSM-5 was designated as %M/Z30. The resulting catalysts were designated as listed in

Table 5. Details about different catalysts used in the pyrolysis of LDPE.

Entry	Parent Catalyst	Metal Precursor	Metal Loading (wt. %)	Catalyst Name
1	ZSM-5	ꟷ	ꟷ	Z30
2	ZSM-5	Fe(NO3)3·9H2O	2.00	2Fe/Z30
3	ZSM-5		5.00	5Fe/Z30
4	ZSM-5	Zn(NO3)2·6H2O	2.00	2Zn/Z30
5	ZSM-5		5.00	5Zn/Z30
6	ZSM-5	Cr(NO3)3·9H2O	2.00	2Cr/Z30
7	ZSM-5		5.00	5Cr/Z30
8	ZSM-5	MnCl2·4H2O	2.00	2Mn/Z30
9	ZSM-5		5.00	5Mn/Z30
10	ZSM-5	Ga(NO3)3 xH2O	2.00	2Ga/Z30
11	ZSM-5		5.00	5Ga/Z30

.

3.3. Pyrolysis Process

A semi-batch reactor (Novoclave), fabricated and supplied by büchiglasuster in Switzerland, was employed for the pyrolysis of LDPE. A 50 g sample of LDPE either alone or mixed with catalyst in a certain ratio was added to the reactor. The reactor was adequately sealed and flushed continuously with pure Argon gas (99.999%) to establish a non-oxidative environment. The temperature control and monitoring within the reactor were facilitated by an RTD-type temperature sensor integrated with the reactor assembly. The reactor was heated to the desired temperature. An integrated stirrer was used to facilitate the agitation and mixing inside the reactor. The vapors generated during the pyrolysis process were passed through a condenser connected to a chiller set at the temperature of −5 °C. The condensed liquid oil was collected in a flask attached to the condenser, while non-condensable gases were collected in a gas bag for subsequent analysis. All experiments were performed in duplicate, with each run lasting approximately 40–45 min. Following each reaction, the spent catalyst—which contained residual coke deposits and unreacted LDPE—was treated with toluene at 100 °C for 30 min to dissolve the polymeric residues. The catalyst was then filtered, thoroughly washed, and dried for subsequent characterization. This separation protocol ensured effective removal of organic deposits while preserving the catalyst’s structural integrity for further analysis.

The sketch of reactor setup is shown in Figure 9.

The weight of the collected oil and residue was determined using a precise weighing balance, enabling the calculation of their yields through Equations (1) and (2). The yield of gas products was obtained by the difference using Equation (3). The conversion of LDPE was calculated using Equation (4). To ensure the reliability of the results, all experimental runs were conducted a minimum of three times, and the average values are reported.

$$\mathrm{O}\mathrm{i}\mathrm{l} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{\%}={\displaystyle \frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{e}\mathrm{d} \mathrm{l}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}}{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{L}\mathrm{D}\mathrm{P}\mathrm{E}}}\times 100$$

(1)

$$\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{u}\mathrm{e} \mathrm{\%}={\displaystyle \frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{r}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{u}\mathrm{e} \mathrm{l}\mathrm{e}\mathrm{f}\mathrm{t}}{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{L}\mathrm{D}\mathrm{P}\mathrm{E}}}\times 100$$

(2)

$$\mathrm{G}\mathrm{a}\mathrm{s} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{\%}=100-(\mathrm{O}\mathrm{i}\mathrm{l} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\mathrm{\%}+\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{u}\mathrm{e}\mathrm{\%})$$

(3)

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{\%}\right)={\displaystyle \frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{L}\mathrm{D}\mathrm{P}\mathrm{E} \mathrm{f}\mathrm{e}\mathrm{e}\mathrm{d}-\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{r}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{u}\mathrm{e} \mathrm{l}\mathrm{e}\mathrm{f}\mathrm{t}}{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{L}\mathrm{D}\mathrm{P}\mathrm{E} \mathrm{f}\mathrm{e}\mathrm{e}\mathrm{d}}}\times 100$$

(4)

3.4. Characterization of Raw Materials, Catalysts and Products

The ultimate compositional analysis of LDPE was ascertained utilizing a CHNS analyzer (Elementar Vario Micro Cube, Langenselbold, Germany). In contrast, the elemental composition of the resulting pyrolytic oil was determined employing a GC equipped with Detailed Hydrocarbon Analysis (DHA) Dragon software (Version 1.1.0.1779). The proximate analysis of LDPE was conducted in accordance with ASTM methods 3173 and 3175.

A Gel permeation chromatograph (GPC) model PL-GPL-220 was used to determine the average molecular weights of the LDPE, using trichlorobenzene as solvent at 160 °C. Additionally, a Thermogravimetric Analysis (TGA) of LDPE was carried out to evaluate its thermal properties concerning both time and temperature. The TGA instrument, manufactured by Diamond Spectrum, was employed for the investigation of LDPE’s thermal degradation. A 30 mg aliquot of the LDPE sample was placed into the crucible of the TGA. Nitrogen (N₂) gas was employed as a purging agent to eliminate any atmospheric oxygen from the sample environment. The temperature was ramped from 25 °C to 800 °C, and the corresponding percentage of weight loss was recorded as a function of temperature.

A gas chromatograph (Shimadzu GC Model 2020) equipped with a Flame Ionization Detector (FID) was used to perform the simulated distillation (SimDist) analysis of the pyrolytic oil. A capillary column (MXT 2887) with a diameter of 0.53 mm and a length of 10 m was used in this GC. SimDist analysis is a chromatographic technique that relates retention times to the boiling points of various fractions. The detector was maintained at a temperature of 400 °C. The temperature program for the GC column included an initial hold at 40 °C for 2 min, followed by a linear ramp with a heating rate of 5 °C/min until reaching a final temperature of 380 °C. High-purity N₂ gas (10 mL/min) was utilized as the carrier gas.

A gas chromatograph (Shimadzu Nexis GC-2030) equipped with a FID was used to perform detailed hydrocarbon analysis (DHA) of the pyrolytic oil. The GCV of pyrolytic oil samples was calculated using Equation (5) based on the ultimate analysis [44].

$$\mathrm{G}\mathrm{C}\mathrm{V}\left({\displaystyle \frac{\mathrm{M}\mathrm{J}}{\mathrm{k}\mathrm{g}}}\right)=4.18\times (78\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n}+241.3\left(\mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}-{\displaystyle \frac{\mathrm{O}\mathrm{x}\mathrm{y}\mathrm{g}\mathrm{e}\mathrm{n}}{8}}\right)+22.1\mathrm{S}\mathrm{u}\mathrm{l}\mathrm{f}\mathrm{u}\mathrm{r})$$

(5)

Powder XRD analysis of catalysts was performed utilizing a Rigaku Minifix II benchtop diffractometer equipped with a Cu Kα X-ray source (λ = 1.5405 Å). The analysis was performed with a scanning rate of 2°/min over a 2θ range of 5° to 60°.

Micromeritics ASAP-2020 apparatus was used to get nitrogen adsorption-desorption isotherms at a liquid nitrogen temperature of 195 °C. Prior to analysis, each catalyst sample was subjected to a high vacuum at 220 °C for 80 min. The T-Plot techniques were adopted to determine the micropore surface area and pore volume.

Ammonia temperature programmed desorption (NH₃-TPD) experiments were executed using a BELCAT system to determine the acidity of the catalyst samples. The samples were first heated to 500 °C for 1 h under flow of He (50 mL/min) and subsequently cooled to room temperature. Following this, a mixture of 5% ammonia in He was added at 100 °C for 30 min and then heated to 120 °C under He (50 mL/min) alone to remove loosely bonded ammonia. Subsequently, the samples were cooled to room temperature, and then heated to 600 °C under flow of He (25 mL/min). The quantity of released ammonia was determined using a thermal conductivity detector (TCD).

To ensure data accuracy, all analyzers were calibrated using standard values.

4. Conclusions

This study investigated the catalytic conversion of LDPE into pyrolytic oil using a semi-batch pyrolysis reactor at 350 °C. Non-catalytic pyrolysis was performed at 450 °C for comparison, as pyrolysis did not occur at 350 °C in the absence of a catalyst. Various catalysts were synthesized by impregnating different metals (Zn, Cr, Ga, Fe, and Mn) onto ZSM-5 zeolite using the wet impregnation method.

The non-catalytic process yielded the highest liquid pyrolytic oil (65%), whereas catalytic processes resulted in liquid yields ranging from 24% to 55.5%. The non-catalytic pyrolytic oil contained the highest proportion of middle and heavy distillate fractions, while catalytic pyrolysis primarily produced light distillate fractions. GC-DHA analysis revealed that non-catalytic pyrolytic oil exhibited the highest selectivity for C₁₄₊ hydrocarbons, whereas all catalytic pyrolysis runs produced less than 1% of C₁₄₊.

Among the catalysts, 5Fe/Z30 demonstrated the highest cracking efficiency, achieving 100% LDPE conversion and the highest aromatic selectivity (35%). In contrast, 5Mn/Z30 exhibited the lowest aromatic selectivity (17%), likely due to the incomplete decomposition of MnCl₂. This study demonstrated that metal impregnation on Z30 enhances aromatic production at relatively low temperatures (350 °C) during LDPE catalytic cracking.

Simulated distillation (SimDist) analysis confirmed that the pyrolytic oil primarily consisted of gasoline- and diesel-range hydrocarbons, with a gross calorific value (GCV) of approximately 42 MJ/kg. These findings suggest that pyrolytic oil derived from LDPE holds significant potential for producing valuable chemicals and fuels.