A Comparative Evaluation of Bimetallic Alumina-Supported Catalysts: Synthesis, Characterization and Catalytic Performance in Pyrolysis of Expanded Polystyrene Waste

Abstract

Plastics are essential to technological and industrial development, yet their prevalent single-use life and poor recycling rates are contributing to escalating environmental concerns. Expanded polystyrene (EPS), although valued for being lightweight, durable, and insulating, poses a significant challenge as it is typically disposed of after a single use. Furthermore, traditional recycling is limited because it requires clean, well-separated waste. Therefore, it remains necessary to develop recycling strategies that maximize the value of plastics. To address this issue, the present work aims to provide a comparative evaluation of the synthesis and characterization of FeMg/Al₂O₃ and Fe/Al₂O₃-MgO as catalysts, along with an analysis of their catalytic performance in the pyrolysis of EPS waste at varying temperatures and catalyst loadings. The results showed an advantage in using catalysts in the pyrolysis of EPS waste; however, the FeMg/Al₂O₃ (15 wt.%) catalyst demonstrated the best efficiency in the pyrolysis of EPS waste at 400 °C, achieving 96% liquid yield and reducing reaction times by up to 45% due to its high metal dispersion and strong metal-support interaction, which promotes faster and more efficient conversion. In contrast, Fe/Al₂O₃-MgO showed lower catalytic performance, although it can offer lower synthesis costs and good thermal stability, making it more viable on a large scale. These findings represent a significant advance in catalytic EPS recycling, offering promising strategies to promote the circular economy of EPS and extend its useful life.

1. Introduction

Plastics currently provide multiple benefits to various industrial applications due to their properties, such as lightness, strength, corrosion resistance, durability, insulating properties, and moldability [1]; these properties have been essential in the evolution of the technological industry and human society. Global production of plastic is increasing due to the inherent properties of this material, representing 430.9 Mt in 2024, where 89% still comes from fossil sources [2]. However, there is no equitable relationship between production and recycling. If this problem is not solved, approximately 12,000 metric tons of plastic waste will contaminate the environment by 2050 [3].

Plastic recycling technologies could be grouped into primary, secondary, tertiary, and quaternary. Primary and secondary recycling involves the mechanical reprocessing of used materials (sorting, extrusion, segregation, grinding, and melt-processing) and they differ in terms of the feedstock used [4]. These methods are the most used in the recycling industry [5]. Nonetheless, mechanical recycling requires pure single-polymer streams to ensure good quality products, which are difficult to obtain due to the contaminated or multilayered plastic in waste, leading to high cleaning and sorting costs [6]. On the other hand, tertiary involves valuable chemical intermediate recovery (depolymerization, solvolysis, pyrolysis, gasification, hydrocracking, and others) [7,8,9,10]. Finally, quaternary recycling implicates energy recovery (incineration); however, this strategy has many drawbacks since its released emissions that are highly toxic to human health and the environment, such as dioxins, furans, heavy metal oxides, polycyclic aromatic hydrocarbons, among others [11,12,13]. In statistical terms, only 9.5% of plastic waste is recycled [2], consequently, the development of efficient plastic waste treatment technologies remains of great significance [1].

Thermo-catalytic pyrolysis has proven to be an alternative and promising technology to reduce the impact of polymeric waste on the environment. Pyrolysis offers the possibility of converting long chain polymers into carbonaceous residues, condensable hydrocarbons, and hydrogen-rich gases [14,15] and conventionally, does not require extensive pre-treatments.

Moreover, utilizing a diverse range of catalysts in various chemical processes is a well-established practice in the field, such as alumina [16,17,18], bentonite [19,20,21], and zeolite [22,23,24]. Catalysts provide an alternative, more energetically favorable pathway for the reaction, forming intermediate species involving the catalyst surface and the reactants, increasing reaction speed and efficiency, reducing activation energy, and improving selectivity of chemical products [25,26,27]. These advantages highlight the role of catalysts in degradation processes such as plastic waste pyrolysis processes [28].

Notably, activated alumina is widely used as a catalyst in the catalytic pyrolysis of EPS waste [29,30,31,32]. This is due to its excellent wear and chemical resistance, high specific surface area and thermal stability, and above all, to the abundance and affordability of aluminum, making it a cost-effective choice. Research on catalytic pyrolysis of PS waste, shows that alumina has shown favorable activity in the process [33]. Previous works have reported the impregnation of metals on alumina supports, such as Ni, Co, La, and Ce, among others, achieving liquid yields up to 90% [34]. Furthermore, non-catalyst pyrolysis still requires high temperatures (~450–600 °C) to achieve efficient degradation, increasing energy consumption [35,36,37,38].

In such a scenario, catalytic pyrolysis can operate at low-temperature pyrolysis and help to mitigate this issue. Research regarding bimetallic catalysts, have demonstrated synergy between metals in different applications, combining complementary properties of two metals improving thermal and chemical stability enhancing yield and selectivity of desired products [39]. From the literature, Yao et al. explored the influence of Ni–Fe bimetallic catalyst to produce high-value carbon nanotubes (CNTs) from plastic waste pyrolysis and clean hydrogen [40]. They reported that the amount and quality of CNTs were greatly influenced by the catalyst composition. Li et al. reported that the presence of Mo–Ni bimetal on the composite catalyst improved the catalyst reactivity and the pyrolysis oil yield from municipal waste [41]. Notably, bimetallic catalysts have been tested at low-temperatures, achieving competitive yields. A highlighted work is by Aljabri et al. [42], their objective was to evaluate the conversion of PS and its wastes into valuable products with high conversion at 250 °C with FeCo/Al₂O₃ bimetallic catalysts in a Parr stirred batch reactor, reaching up to 91% of liquid yield with a styrene monomer selectivity of up to 45 wt.% and ethylbenzene up to 55 wt.%.

Despite a long history in EPS waste catalytic recycling, the discovery of new metals as catalysts and making catalytic processes more efficient remains an active area of research. On the other hand, the search for more economically and environmentally practical sustainable processes is of significant interest today. Especially for plastics, it is necessary to generate strategies to encourage the circular economy of this material and extend its useful life.

In this study, two catalysts were synthesized and formulated (FeMg/Al₂O₃ and Fe/Al₂O₃-MgO) for comparison and evaluation in the catalytic pyrolysis of EPS waste. Catalysts are typically expensive, so alternatives were explored to lower costs without sacrificing catalyst performance. Two magnesium precursors were used: technical-grade magnesium sulfate, a lower-cost option, and reactive-grade magnesium nitrate, which is purer and more expensive. This comparison examines the influence of synthesis and formulation on the physicochemical properties and catalytic performance in the degradation of EPS waste. The analysis includes the preparation and characterization of the catalysts, as well as their catalytic performance, including their influence on the yield of liquid, solid, and gaseous fractions. The conversion rate to liquid fraction is also discussed, and finally, the production of aromatics and a comparative with literature are also analyzed.

2. Results and Discussion

2.1. Catalyst Characterization

2.1.1. N₂-Physisorption

The physicochemical and textural properties of the catalysts depend on both the raw material used and the synthesis methods employed. Nitrogen adsorption-desorption measurements were used to examine the textural properties of the synthesized supports and the formulated catalysts. Figure 1 compares the isotherms and the particle size distribution (PSD) of the samples studied.

From the results, some differences can be observed: Figure 1a illustrates type IV adsorption isotherms and H1–H2 hysteresis loops according to the classification proposed by the IUPAC [36], revealing a mixture of uniform cylindrical pores and narrow-necked pores, a common characteristic of alumina. The Al₂O₃-MgO support exhibits a slightly narrower loop, which can be attributed to the addition of MgO to the support partially blocking or modifying the alumina pores, thereby reducing the total volume and possibly generating smaller or less uniform pores. On the other hand, Figure 1b shows the isotherms of the formulated catalysts, which retain the type IV isotherm, preserving a mesoporous structure but with lower adsorption volumes and surface areas compared to the supports. In contrast, Figure 1c,d show no significant difference regarding particle size distribution.

The physicochemical and textural properties of the synthesized supports and formulated catalysts are shown in

Table 1. Surface area properties of support and bimetallic catalysts.

Sample	SBET [m2·g−1]	SBJH [m2·g−1]	Pore Volume [cm3·g−1]	Pore Size [Å]
ϒ-Al2O3	286	384	0.57	59.1
Al2O3-MgO	208	334	0.47	56.5
FeMg/Al2O3	171	242	0.34	57.0
Fe/Al2O3-MgO	162	247	0.36	58.1

. According to the literature, the activated alumina’s surface area and pore volume have been reported in several works [43,44], indicating that their average values range between 50 and 500 m²·g⁻¹ and 0.2 to 0.76 cm³·g⁻¹, respectively. In comparison, the values of the synthesized supports in this study are within these ranges, with the modified alumina (Al₂O₃-MgO) possessing the lowest surface area among them. Furthermore, it is worth noting that the synthesized alumina (ϒ-Al₂O₃) exhibits higher values than those reported in the literature for commercial Al₂O₃ [29,42,45].

Moreover, it can be observed that the surface area drops significantly when two metals are impregnated into the alumina support, FeMg/Al₂O₃ shows a greater decrease, with a difference of 115 m²·g⁻¹ (40%). By contrast, impregnating only one metal in Fe/Al₂O₃-MgO results in a much smaller decrease, just 46 m²·g⁻¹ (22%) compared to the Al₂O₃-MgO support. Nevertheless, additional characterization techniques are still required to verify the dispersion of metal species and avoid agglomeration.

2.1.2. X-Ray Diffraction (XRD)

XRD patterns of alumina supports and formulated catalysts are shown in Figure 2, along with phase references corresponding to γ-Al₂O₃ [PDF #10–0425], MgO [PDF #45–0946], MgAl₂O₄ [PDF #21–1152], and Fe₂O₃ [PDF #33–0664] [46].

Regarding the supports, both diffractograms show wide, low-intensity peaks (around 2θ ≈ 37°, 46°, 67°), characteristic of the γ-Al₂O₃ phase, confirming a mesoporous and partially amorphous nature. For Al₂O₃-MgO, the diffraction is slightly more diffuse and less intense, which may represent lower crystallinity or the formation of a spinel-type mixed phase (MgAl₂O₄). Furthermore, no defined MgO signals (2θ ≈ 43° and 62°) appear, indicating that Mg is highly dispersed or incorporated into the Al₂O₃ support. These findings are consistent with the observed decrease in BET area, which may reflect a possible structural reorganization.

On the other hand, after impregnation of Fe in FeMg/Al₂O₃, a series of moderate peaks (around 2θ ≈ 35°, 44°, 62°) are observed, typical of Fe₂O₃ or Fe₃O₄. The moderate intensity of these peaks could suggest that the Fe is highly dispersed, with small crystals or mixed in the support. In contrast, for Fe/Al₂O₃-MgO, the Fe peaks are more intense and more defined, especially between 2θ ≈ 30–50°, indicating the presence of more crystalline and larger Fe oxide particles. This difference implies less dispersion of Fe, possibly located on the outer surface of the support. Despite that, the lower BET area drop in this sample suggests that the mesoporous structure is better preserved. Moreover, the interaction with MgO could limit the dispersion of Fe within the pores.

2.1.3. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS)

The samples were examined by SEM to visualize textures formed during alumina synthesis and metal addition to the catalysts. Figure 3 shows the micrographs and EDS elemental composition by weight for each sample.

All samples presented a similar morphology. Figure 3a illustrates irregular agglomerates of micrometric size (scale: 10 µm), where only Al and O were detected in ~1:1 proportions, corresponding to Al₂O₃. No other metals were detected, indicating high purity of the support. On the other hand, Figure 3c shows a morphology similar to that of pure alumina, while EDS analysis indicates the incorporation of Mg (5.45 wt.%), confirming the modification of the support with magnesium oxide. The slight reduction in the percentage of Al could be attributed to the formation of MgAl₂O₄ spinel, as supported by the XRD results.

Furthermore, Figure 3b shows the results for FeMg/Al₂O₃, where it can be observed that impregnation does not significantly influence the agglomerates, which remain the same scale as the Al₂O₃ support. In addition, EDS analysis reveals the presence of Fe (17.9 wt.%) and Mg (7.9 wt.%), along with Al and O, indicating the effective incorporation of metal species after impregnation. Finally, for Fe/Al₂O₃-MgO, a rough but smoother surface is observed compared to the other samples, which is consistent with the lowest surface area value among the formulated catalysts. EDS analysis detected a higher Fe content (24.3 wt.%) and a lower Mg concentration (4.4 wt.%). The moderate proportion of Mg implies that MgO could act mainly as a structural promoter rather than a dominant phase.

The presence of chlorine is attributed to iron chloride, which is used as a reagent in the impregnation method.

2.2. Catalyst Evaluation

Figure 4 visualizes the coloration of the pyrolytic liquids experimented at different temperatures (300 °C, 350 °C and 400 °C), catalyst types (FeMg/Al₂O₃ and Fe/Al₂O₃-MgO), and catalyst loading percentages (10 wt.% and 15 wt.%). Although not all runs are visualized, the trend remained similar for all samples; as the temperature was elevated, the coloration became darker.

2.2.1. Comparative of Product Yields

Figure 5 illustrates the influence of temperature and catalyst type on the production of liquid, solid, and gas fractions. Product yields are compared with those obtained by thermal pyrolysis, as reported in previous work [36].

The catalyst that maximizes pyrolytic liquid production is FeMg/Al₂O₃ with a 15 wt.% loading at 400 °C, yielding a 96% liquid fraction, 3.5% solids, and 0.5% gas and losses. Furthermore, compared to other operating conditions, this catalyst favored liquid production, minimizing the gas fraction. These results are attributed to the fact that FeMg/Al₂O₃ has well-dispersed Fe within the pores. This high dispersion favors the presence of many active sites and enhances metal-support interaction, resulting in improved catalytic performance [47]. In contrast, Fe/Al₂O₃-MgO has more crystalline and superficial Fe and low dispersion, resulting in lower catalytic performance. Additionally, the results showed that the liquid yield increases with increasing temperature up to 400 °C for both catalysts. Nevertheless, there were some differences between the two catalysts, which will be discussed below.

In the absence of a catalyst, when the temperature is lowered to 300 °C, pyrolysis becomes less efficient, resulting in decreased liquid production and increased solid and gas fractions. However, high accessibility to active FeMg/Al₂O₃ sites facilitated cracking, favoring liquid production, and achieved 78% with a 15 wt.% loading, which is up to 11.5% more than that of the thermal process. Under the same conditions, the Fe/Al₂O₃-MgO catalyst increased the liquid yield by 3% over the non-catalyst process.

At 350 °C, where thermal conversion to liquid is still limited, FeMg/Al₂O₃ enhanced energy pathways and increased liquid yield. In contrast, low dispersion and limited access to Fe/Al₂O₃-MgO sites had a moderate effect compared to thermal pyrolysis.

Furthermore, it is worth noting that the maximum catalytic effect, that is, the greatest difference compared to the thermal results, was observed at a temperature of 350 °C. Under these conditions, the catalysts significantly increased liquid production, achieving a maximum difference of 15% compared to the non-catalytic process.

At 400 °C, all tests (catalytic and non-catalytic) yielded high liquid production because the thermal decomposition of EPS waste at this temperature is almost complete [36], resulting in a moderate catalytic effect compared to the tests performed at 350 °C. Notably, the FeMg/Al₂O₃ catalyst slightly outperformed with 96% at 15 wt.% loading, only 2% higher than that of Fe/Al₂O₃-MgO. Moreover, under thermal pyrolysis, high operating temperatures favor secondary cracking and gasification reactions, increasing the gas fraction. In such a scenario, the catalysts moderated these gaseous reactions; FeMg/Al₂O₃ reduced gas production, while Fe/Al₂O₃-MgO favored the formation of carbonized solids.

Although the Fe/Al₂O₃-MgO catalyst produced slightly lower liquid yields, it still offered competitive performance. Specifically, it produced 94.5% liquid at 400 °C with 15 wt.% loading. In addition, using 5 wt.% increased the liquid yield by 2.5% compared to the thermal process. Based on characterization results, the incorporation of Mg into the alumina support generates an MgAl₂O₄ spinel phase, which exhibits improved thermal stability [48]. This improved stability is particularly important under prolonged or repeated reaction conditions, as the presence of Mg increases resistance to sintering and loss of support area. Therefore, although Fe/Al₂O₃-MgO showed a slightly lower yield, the catalyst could exhibit better operational durability.

In summary, the optimal catalyst will depend on the determined goal. If maximum conversion to liquid fraction is sought, the use of FeMg/Al₂O₃ is recommended due to its higher efficiency. On the other hand, if competitive yields, low cost and operational stability are prioritized, the use of Fe/Al₂O₃-MgO is suggested.

2.2.2. Conversion Rate of Catalytic Pyrolysis vs. Thermal Pyrolysis

Analyzing the conversion rate of EPS waste to pyrolytic liquid is another method for evaluating catalyst performance in the degradation process. This approach provides insight into how efficiently the catalyst promotes reactions, helping to optimize both the pyrolysis process and catalyst design.

Figure 6 illustrates the conversion rates of the two catalysts formulated compared to thermal pyrolysis. The results are organized by temperature and catalyst loadings, where the resulting curves visualize the effect of catalysts on pyrolytic liquid production versus the complete reaction time of the pyrolysis, that is, since the heating process began and when the liquid drops end.

The liquid yield curves as a function of reaction time show significant differences between thermal and catalytic pyrolysis. The use of both catalysts demonstrated the acceleration of liquid fraction conversion and the reduction in the time required to reach maximum yield.

Figure 6a–c show the results obtained at low temperatures (300 °C), where it can be observed that both processes (thermal and catalytic) were slow and required long reaction times (200–250 min) to achieve maximum liquid fraction yield. However, catalysts moderately improved the speed (resulting in a 10–20% reduction), and their maximum yield was slightly higher than that of the thermal process; however, this improvement depends significantly on the catalyst load.

In comparison, FeMg/Al₂O₃ exhibits nearly linear kinetic improvement with increasing loading up to 15 wt.%. In contrast, Fe/Al₂O₃-MgO does not exhibit a linear relationship, as loadings above 10 wt.% reduce its catalytic performance, which is attributed to possible saturation effects.

On the other hand, Figure 6d–f show the results of the liquid fraction conversion rate when the temperature was increased to 350 °C, where the catalytic tests achieved a yield within a range of 80–90% of liquid fraction in approximately 90 to 110 min. In contrast, thermal pyrolysis required a longer time (150–180 min) to achieve similar yields. At this temperature, better catalytic effectiveness is observed, yielding highest liquid yields with a significant reduction in time, reaching a maximum reduction of 40–45%. Nevertheless, FeMg/Al₂O₃ accelerates the maximum yield in less time than Fe/Al₂O₃-MgO.

Finally, Figure 6g–i show the results at 400 °C, where it can be observed that the pyrolysis process is fast, even without a catalyst; thermal pyrolysis reached 85–90% liquid in 110–130 min. In contrast, with a catalyst, the process was accelerated, achieving 90–96% in 70–90 min, resulting in a time reduction of 25–45%, with no significant difference between the two catalysts.

From an economic perspective, the results demonstrate the advantage of catalytic pyrolysis of EPS waste at 350 °C. The use of FeMg/Al₂O₃ as a catalyst maximizes productivity, reducing reaction times by up to 45% compared to thermal pyrolysis, and allows operation at intermediate temperatures, resulting in lower thermal requirements and potential cost reductions. However, the Fe/Al₂O₃-MgO catalyst, although it had lower catalytic performance, has lower synthesis costs because the catalyst formulation is more economical due to the use of a single analytical reagent (FeCl₃·6H₂O) and the addition of Mg with technical-grade MgSO₄·7H₂O in the coprecipitation process. In addition, the textural properties of Fe/Al₂O₃-MgO demonstrated potential thermal stability, which may make it more economically attractive on a large scale. Nevertheless, an economic assessment of energy consumption, considering longer operation times or increased production costs due to the addition of catalysts, can help determinate whether suitable catalysts or a thermal process.

2.2.3. Aromatic Composition of Liquid Products

The composition of the liquid fraction obtained from the catalytic pyrolysis of EPS waste was characterized by gas chromatography (GC). The GC results showed that the degradation of EPS waste through this process yielded an average of 42 compounds; however, only compounds with the highest concentrations are reported in terms of weight percent in

Table 2. Average aromatic composition of thermal and catalytic pyrolysis of EPS waste (wt.%).

Aromatic	FeMg/Al2O3		Fe/Al2O3-MgO		Thermal
	Average	±	Average	±	Average	±
Toluene	7.41	1.33	8.04	1.89	6.09	1.80
Ethylbenzene	7.08	3.25	8.87	5.27	7.27	0.87
Cumene	0.15	0.01	0.18	0.02	n.d.	n.d.
α-Methyl styrene	11.68	2.20	12.52	4.36	6.00	0.49
Styrene	49.65	2.74	48.93	4.07	61.1	7.51
Benzene	0.25	0.05	0.36	0.11	n.d.	n.d.
Xylene	0.99	2.21	1.40	3.13	n.d.	n.d.

. To provide a comprehensive understanding, these results were compared with those obtained from thermal pyrolysis. The comparison was based on the average concentrations found at temperatures within a range of 300 °C to 400 °C for thermal and catalytic pyrolysis, and in the case of catalytic pyrolysis, over catalyst loading of 5 wt.% to 15 wt.%.

In summary, catalytic pyrolysis did not show significant differences in aromatic compound concentrations with any catalyst. Some results deviated slightly, with a maximum difference of 5.27 wt.%. These differences were caused by changes in experimental conditions. The reasons will be detailed below.

Unsaturated products, such as styrene and α-methylstyrene, were detected in high concentrations. Their corresponding saturated compounds, such as ethylbenzene and cumene, were present in low amounts or were undetectable. This suggests that the reaction conditions favored the formation of unsaturated products, consistent with Kim et al. [49]. The highest concentration for all samples was that of styrene, reaching up to 49.70 wt.% in catalytic pyrolysis, while an average of 61.1 wt.% was found in thermal pyrolysis. The difference between these two results is attributed to the fact that in catalytic pyrolysis, the production of other aromatics was promoted at higher concentrations.

On the other hand, ethylbenzene is a hydrogenated derivative of styrene, which behaves similarly to styrene. Therefore, there is a relationship between these two compounds: as the concentration of styrene increases, the concentration of ethylbenzene decreases, and vice versa. Additionally, the maximum deviation in ethylbenzene concentration was observed specifically in experiments conducted at 300 °C.

To further clarify their relationship, ethylbenzene behaved as an intermediate and was particularly sensitive to catalytic conditions. For example, in the FeMg/Al₂O₃ catalyst with a 5 wt.% loading, and at 300 °C, ethylbenzene contents of up to 8 wt.% were detected, indicating that partial hydrogenation of styrene is possible, but subsequent dehydrogenation and dealkylation reactions are still limited. However, when the loading is increased to 15 wt.%, ethylbenzene disappears from the liquid, suggesting that under these conditions it becomes a highly reactive intermediate that is converted to styrene or lighter aromatics (toluene, benzene). In contrast, the 15 wt.% of Fe/Al₂O₃-MgO catalyst promotes the accumulation of ethylbenzene to values of 18.14%, consistent with the lower dispersion of Fe and the possible basicity of the support, which favors the hydrogenation and stabilization of the styrene chain but limits its subsequent cracking [50].

As highlighted in previous work [36], toluene, ethylbenzene, cumene, and xylene play a crucial role as intermediates that donate or accept hydrogen during styrene degradation. This interaction has a significant impact on the selectivity of styrene in the samples. For instance, the formation of benzene and its subsequent hydrogenation to form toluene and ethylbenzene, or the formation of other aromatic products, can decrease the yield of the styrene monomer.

Finally, a common output response among the two catalysts was the high concentration of α-methyl styrene at low temperatures (300 °C), and at minimum catalyst loading percentages (5 wt.%), the formulation of this compound was favored up to 19.91 wt.% and 15.38 wt.% using Fe/Al₂O₃-MgO and FeMg/Al₂O₃, respectively. This statement can be attributed to the fact that the residence time in the experiments at 300 °C was longer (150 min), which could have led to secondary reactions during degradation.

2.3. Performance Comparison with Literature

Table 3. Summary of operating conditions and liquid fraction yields.

Feedstock	Reactor	Catalyst	Loading (wt.%)	Temperature (°C)	Liquid Yield (%)	Reference
Virgin PS	Batch	FeCu/Al2O3	10	250	66.0	[45]
Virgin PS	Batch	FeCo/Al2O3	10	250	91.0	[42]
EPS waste	Pyrex	Mg/Al2O3	30	450	95.4	[29]
EPS waste	Pyrex	Mg/Al2O3	20	450	87.0	[51]
EPS waste	Pyrex	Fe/Al2O3	20	450	89.2	[51]
Virgin PS	Bench-scale	MgO	10	500	93.7	[52]
EPS waste	Semi-batch	FeMg/Al2O3	15	300	78.0	This work
EPS waste	Semi-batch	FeMg/Al2O3	15	350	91.5	This work
EPS waste	Semi-batch	FeMg/Al2O3	15	400	96.0	This work
EPS waste	Semi-batch	Fe/Al2O3-MgO	15	300	69.5	This work
EPS waste	Semi-batch	Fe/Al2O3-MgO	15	350	91.5	This work
EPS waste	Semi-batch	Fe/Al2O3-MgO	15	400	94.5	This work

summarizes the best results obtained in this study compared with those reported in the literature for the catalytic pyrolysis of polystyrene. Previous works have demonstrated that monometallic catalysts based on Fe, Cu, Co, and Mg, when supported on Al₂O₃, achieved liquid yields ranging from 66% to 95%, depending on the active metal and reaction conditions. In contrast, the bimetallic catalysts developed and formulated in this work exhibited superior performance, achieving liquid yields up to 94.5%. These findings indicate a synergistic effect between Fe and Mg, promoting EPS waste conversion into liquid products.

Notably, the high yields obtained in the present work were achieved at moderate temperatures (300 °C to 400 °C), which is lower than those commonly required for comparable monometallic catalysts (≥450 °C). Operating at lower temperatures also offers practical advantages, including reduced energy consumption and avoiding possible catalyst deactivation, making FeMg/Al₂O₃ and Fe/Al₂O₃-MgO catalysts particularly attractive for plastic waste valorization.

3. Materials and Methods

3.1. EPS Feedstock

The raw material used in this work was EPS waste generated during packaging manufacturing processes and supplied by “Poliespuma del Bajío” manufacturer in Irapuato, Guanajuato, Mexico. This residue, a powder with a nominal density ranging from 8 to 12 kg m⁻³, resulted from the company’s recycling process. The samples were compacted into 100 g samples to eliminate air content. As the material came directly from the company, no cleaning or thermal pre-treatments were carried out.

3.2. Reagents

Gamma-alumina (ϒ-Al₂O₃) was synthesized using technical grade Octadecahydrate Aluminum Sulfate (Al₂(SO₄)₃·18H₂O) with a molecular weight of 666 g·mol⁻¹, distilled water, and NH₃ ammonia gas. Moreover, a modification on alumina support adding Mg was synthesized using technical grade Heptahydrate Magnesium Sulfate (MgSO₄·7H₂O). The metals used for the formulation of the catalysts were Ferric Chloride Hexahydrate (FeCl₃·6H₂O) reagent grade and Magnesium Nitrate Hexahydrate (Mg (NO₃)₂·6H₂O) reagent grade with molecular weights of 270 g·mol⁻¹ and 148 g·mol⁻¹, respectively. All reagents were supplied by Kem de Leon (León, Mexico).

3.3. Synthesis of Catalyst Supports

The flow charts illustrating the preparation of catalyst support are shown in Figure 7. As a first stage, the precursor phase (AlOOH), known as pseudo-boehmite (PSB), was synthesized by hydrolysis-precipitation [53,54] followed by heat treatment to obtain the ϒ-Al₂O₃ phase. Figure 7a shows the scheme of the synthesis process of alumina support used in this work. A solution of aluminum sulfate was prepared with 400 g of Al₂(SO₄)₃·18H₂O dissolved in 3900 mL of distilled water and then heated at approximately 40 °C with agitation for 15 min; afterwards, a filtration with a medium pore filter was conducted to eliminate possible insoluble impurities. The aluminum sulfate solution free of impurities was reacted in an ammoniacal medium (ammonia gas), dosing the addition using a peristaltic pump, maintaining a pH of 9–10, and constant stirring. The reaction was carried out between 65 °C and 70 °C. At the end of the reaction, the material was recovered by filtration. The PSB was dried to 110 °C for 24 h; subsequently, it was calcined, raising the temperature to 550 °C for 24 h, resulting in the ϒ-Al₂O₃ phase.

The present work also evaluates the modification of the ϒ-alumina support with the addition of Mg by the hydrolysis co-precipitation process, shown in Figure 7b. An aluminum sulfate solution was prepared with 350 g of Al₂(SO₄)₃·18H₂O dissolved in 3400 mL of distilled water. On the other hand, a magnesium sulfate solution was prepared with 36 g of MgSO₄·7H₂O dissolved in 345 mL of distilled water. Both solutions were heated to approximately 40 °C and stirred. The solutions were filtered separately to remove insoluble matter. Once cleaned of impurities, the solutions were dosed together to an ammoniacal medium (ammonia gas) using two peristaltic pumps. The additions of the solutions were regulated at rates of 2 mL·min⁻¹ and 10 mL·min⁻¹ for magnesium and aluminum solutions, respectively. The reaction was carried out in a temperature range of 65 °C and 70 °C. At the end of the reaction, the material was recovered by filtration with a medium pore filter. Subsequently, the precursor was allowed to dry overnight and then dried at 110 °C for 24 h. Finally, the precursor was calcined at 550 °C for 24 h to obtain the active phase of ϒ-Al₂O₃-gO.

3.4. Catalysts Formulation

For FeMg/Al₂O₃ catalyst, 12.1 g of FeCl₃·6H₂O and 11.71 g of Mg (NO₃)₂·6H₂O were dissolved into 10.8 mL of deionized water and loaded into alumina powder by wetness-impregnation technique. Subsequently, the mixture was stirred at 70 °C for two hours to eliminate excess of water and then dried in an oven at 110 °C for 24 h. Finally, the catalysts were calcinated to 500 °C for 24 h and crushed into a fine powder. On the other hand, for the formulation of Fe/Al₂O₃-MgO catalyst, a single metal was used as precursor (FeCl₃·6H₂O); moreover, unlike the other catalyst, the stirred time at 70 °C was reduced to 1 h, and subsequent procedures were the same.

3.5. Catalyst Characterization

The geometrical description of the pore size and structure, including diameter, volume, and wall roughness of a catalyst is essential to justify the catalytic activity of a catalyst in a process. An understanding of the catalyst’s structure can provide insight into how the catalyst operates and how to enhance the prepared material [55]. In brief, catalysis involves reaction at a surface; therefore, surface composition is important in determining the behavior of a catalyst.

3.5.1. N₂-Physisorption

Surface area, pore volume, and pore size distribution are important characteristics to evaluate in a catalyst since all these parameters contribute to the overall kinetic of a catalyst. The catalyst’s surface area and pore volume were determined from N₂-physisorption (adsorption-desorption isotherms) at −196 °C using a Micromeritics TriStar II Plus (Newtown Square, PA, USA) device (JEOL 6010 plus/LA, JEOL, Tokyo, Japan). Before measurements, the samples were degassed at 200 °C for two hours. The surface area was calculated using the Brunauer-Emmett-Teller (BET) method applied to the adsorption curve in the relative pressure range of 0.05 to 0.35 P/P_o. For the size distribution and pore volume, the Barret-Joyner-Halenda (BJH) method was followed based on the desorption isotherm [56].

3.5.2. X-Ray Diffraction (XRD)

The diffractogram obtained from this technique shows the radiation emitted from the sample when X-ray radiation has been applied to it. If the analyzed object is amorphous or vitreous, i.e., it does not present internal arrangement, the sample will not produce any response, but when it has a particular order, the diffractogram will show characteristic peaks of such material. Therefore, there is a corroboration of the presence of the desired material. The analysis was performed in a RIGAKU ULTIMA IV (The Woodlands, TX, USA) spectrometer with a copper X-ray tube with 2ϴ measurements from 5° to 80°.

3.5.3. Scanning Electron Microscopy (SEM)

SEM is used to obtain micrographs that show the morphological and topographical characteristics of the surface of a solid. This characterization technique reveals the shape and structure of a cross-section of the material [55]. A JEOL model JSM-6010 PLUS/LA (JEOL, Tokyo, Japan) scanning electron microscopy equipment was used for this technique.

3.5.4. Energy Dispersive X-Ray Spectroscopy (EDS)

EDS is an analytical technique that allows chemical characterization/elemental analysis of materials. The technique allows compositional analysis of a specific sample volume excited by the energy source. The position of the peaks in the spectrum identifies the element, while the signal’s intensity corresponds to the element’s concentration [55]. A JEOL model JSM-6010 PLUS/LA (JEOL, Tokyo, Japan) scanning electron microscope including an integrated energy dispersive X-ray spectrometer was used for this technique.

3.6. Liquid Fraction Characterization

The qualitative analysis and identification of the chemical aromatic compounds present in the liquid hydrocarbon obtained were performed using a Varian 450 gas chromatograph (GC) (Waltham, MA, USA). The GC was equipped with an Omegawax 250 ®® fused silica capillary column, 30 m × 25 m × 0.25 µm (Thermo Fisher Scientific, Milan, Italy). Benzene, Cumene, Styrene, Ethylbenzene, Xylene, α-Methyl styrene, and Toluene were used as reference standards. The injection temperature was 260 °C; the oven was held at a temperature of 40 °C for one minute, then increased to 200 °C at a rate of 10 °C·min⁻¹, subsequently increased to 240 °C at a rate of 5 °C·min⁻¹ and held for 35 min. For the quantitative analysis of the products, the standard internal method using C₁₉ was used.

3.7. Catalyst Performance Studies

Thermal and catalytic pyrolysis of EPS waste were performed in a semi-batch reactor. The reactor, a stainless-steel tube with a height of 17 cm and a nominal diameter of 4.5 inches, was hermetically sealed to prevent leaks without adding any solvent or any inert gas. An external band-type electric heater heated the reactor at a rate of 7 °C·min⁻¹ approximately. The experimental temperatures were obtained through a type-k thermocouple located at the gas zone. For catalytic pyrolysis, the feed consisted of a mix of EPS waste and catalyst. The gases produced by pyrolysis were dislodged through a pipe that flows into a room-temperature condenser manufactured according to the Advancing Standards Transforming Markets (ASTM D86) [57]; subsequently, the pyrolytic liquid was recollected, and the solid fraction, including the catalyst, was collected from the bottom of the reactor.

The influence of reaction temperature (300 °C, 350 °C and 400 °C), type of catalyst (FeMg/Al₂O₃ and Fe/Al₂O₃-MgO), and catalyst loading (5 wt.%, 10 wt.%, and 15 wt.%) on the catalytic pyrolysis of EPS waste is discussed. The reaction time will be indicated as the total time of the process, i.e., from the start of heating. In contrast, residence time started when the temperature at the zone gas achieved the setpoint temperature. Residence times were adjusted to the experimental runs; all times warranted the drip ended.

In addition, the yields of liquid, solid and gas + losses fractions were calculated using Equations (1)–(3), respectively:

Liquid yield (wt.%) = (Liquid mass/EPS mass) × 100

(1)

Solid yield (wt.%) = [(Solid mass − catalyst mass)/EPS mass] × 100

(2)

Gas + Losses yield (wt.%) = 100 − Liquid yield − Solid yield

(3)

4. Conclusions

It is not a novelty that expanded polystyrene is used in various industrial applications and daily life, and this is due to its beneficial properties such as lightness, resistance, and above all, low price, giving this material and advantage over others. However, the high demand leads to large production; nevertheless, the low recycling rate of this material causes waste that end up in landfills, polluting the environment. Catalytic pyrolysis has been cataloged as a good recycling strategy to reduce the amount of EPS waste in the environment. From the results, it can be concluded that:

• Global evaluation of catalytic pyrolysis showed that the incorporation of Fe significantly modifies the pyrolysis performance, improving the yield and selectivity toward the liquid phase. However, the final behavior depends significantly on the nature of the support and the method in which the metallic components are integrated into the structure.
• The highest liquid yield achieved was 96% using FeMg/Al₂O₃ with 15 wt.% of catalyst loading at 400 °C, and the lowest yield obtained was 69.5% using Fe/Al₂O₃-MgO at 300 °C at the same catalyst loading.
• In terms of process intensification, the use of 15 wt.% of FeMg/Al₂O₃ reduced the time required to achieve 91.5% of liquid yield by around 45% compared to thermal pyrolysis at 350 °C.
• Fe/Al₂O₃-MgO exhibited less dispersion and lower accessibility to active sites, requiring longer operating times to achieve competitive yields. However, the possible formation of MgAl₂O₄ spinel may lead to improved operational durability by enhancing thermal and structural stability.

In summary, it has been demonstrated that catalytic pyrolysis of EPS waste is an effective and highly successful recycling method. Therefore, catalytic pyrolysis showed the potential to solve waste management problems. It has the potential to transform large amounts of EPS waste into high-value products, with the added benefit of achieving high conversion yields with lower operation conditions. Finally, as the proposed study offers an alternative recycling strategy, it is essential to elaborate an economic assessment to evaluate whether the methods are feasible.