Porous carbons from pet waste: structure–property relationships and catalytic performance in esterification reactions

The exponential growth in plastic consumption results in unwanted accumulation on the streets of most cities due to inadequate plastic disposal, landfills overloading, and pollution of the soil, atmosphere, and oceans. This work aims to investigate the use of PET (polyethylene terephthalate) waste to produce solid acid catalysts. The catalysts were synthesized via chemical activation with ZnCl 2 or K 2 CO 3, followed by carbonization in an inert atmosphere and sulfonation. Catalytic tests were carried out on the esterification reaction of oleic acid and methanol. The best result was obtained using zinc chloride as an activating agent, functionalized at 80 °C for 2 h. Carbonaceous materials were obtained with S BET, V t, V mic, and V mes of 272 m 2 g -1, 0.36 cm 3 g -1, 0.14 cm 3 g -1, and 0.22 cm 3 g -1, respectively. An 88% conversion to methyl oleate was observed. The synthesis of a heterogeneous acid catalyst is a sustainable and promising alternative for reusing PET waste.

The global consumption of plastic has increased exponentially due to their extensive use in packaging, construction, electronics, and everyday products. As a consequence, the accumulation of plastic waste has become one of the major environmental challenges worldwide, especially in developing countries where waste management policies and recycling infrastructure remain insufficient. In addition to overloading landfills, the improper disposal of plastics contributes to soil, atmospheric, and marine pollution (Das et al., 2021; Keshta et al., 2025).

Currently, approximately 330 million tons of plastic are produced annually, and projections indicate that plastic waste generation may reach 12 billion tons by 2050 (Das et al., 2021). The persistence of these materials in the environment is particularly concerning because of their low degradation rate and their ability to release toxic compounds, such as dyes, phthalates, and bisphenol-A (BPA), which may present mutagenic and carcinogenic effects (Abou-Elyazed et al., 2022). Furthermore, the fragmentation of plastics into microplastics has intensified environmental and health concerns, since these particles have already been detected in water, soil, and atmospheric systems (Wang et al., 2021).

Among the main commercial plastics, polyethylene terephthalate (PET) is one of the most widely used polymers, accounting for approximately 30% of global plastic consumption (Sun et al., 2023). PET is extensively employed in beverage bottles, food packaging, containers, and textile materials due to its low cost, mechanical resistance, durability, and lightweight nature (Williams, 2021). Although PET recycling is encouraged through mechanical processing routes, effective recycling rates remain limited because collection, separation, and contamination issues hinder efficient reuse (Ragaert et al., 2017; Lee et al., 2022).

In this context, chemical upcycling strategies have emerged as an attractive alternative for converting PET waste into high-value functional materials (Abou-Elyazed et al., 2023). One promising approach is the production of carbon-based catalysts from plastic residues, simultaneously contributing to waste valorization and the development of sustainable catalytic materials (Rezk et al., 2026). Recently, Hazmi et al. (2022) demonstrated the potential of PET-derived heterogeneous acid catalysts for esterification reactions. However, the reported material exhibited relatively low surface area (<133 m 2 g -1) and pore volume (0.106 cm 3 g -1), while requiring severe sulfonation conditions (150 °C for 15 h). These limitations highlight the need for alternative synthesis strategies capable of producing porous sulfonated catalysts under milder functionalization conditions while maintaining high catalytic activity.

Therefore, this work investigated the conversion of PET waste into sulfonated carbon catalysts for esterification reactions related to biofuel production. The effects of activation and sulfonation conditions on the structural, textural, and catalytic properties of the materials were evaluated. The synthesized catalysts exhibited high surface area (up to 732 m 2 g -1), elevated pore volume (0.6 cm 3 g -1), and high esterification conversion (88%) under relatively mild sulfonation conditions (80 °C for 2 h). In addition, the proposed approach contributes to the valorization of PET waste through the production of functional carbon-based catalysts with potential application in sustainable biofuel processes. The materials were characterized by SEM/EDS, TG, FT-IR, N2 adsorption-desorption at 77 K, Boehm titration, and XPS.

Preparation of activated carbon

The PET bottles were collected in the streets of Manaus, Amazonas, Brazil. To ensure reproducibility, transparent 2 L bottles from a single soft drink brand were selected. These were then washed, dried, and cut into small pieces. The PET sample was subjected to activation in aqueous medium with ZnCl 2 or K 2 CO 3 in a mass ratio of 1:1 (PET/activating agent) diluted in 10 mL of distilled water at 80 °C for 3 h under magnetic stirring. This was followed by heat treatment at 100 °C for 24 h. The completion of the activation process for ZnCl 2 was performed in a tubular oven at 500, 600 and 700 °C with a heating rate of 10 °C min -1 for 2 h under an N 2 atmosphere. For the activating agent K 2 CO 3, only the temperature of 800 °C was used in the activation process. Then, the sample was washed with 0.1 M HCl solution and finally dried at 100 °C for 24 h. These temperature ranges for both activating agents were selected based on the work of De Castro et al. (2018).

Catalyst functionalization

The catalyst functionalization was performed to anchor sulfonic acid (–SO 3 H) groups to the carbon structure, conferring catalytic activity to the material. This was done by placing 1 g of the sample in 10 mL of pure sulfuric acid with a concentration of 98% under magnetic stirring at temperatures of 60, 80, 100, and 120 °C for 2, 4, 6, and 8 h. At the end of each period, the solids were separated by filtration and thoroughly washed with distilled water to ensure the removal of residual free sulfate ions, which were detected by dripping with a BaCl 2 solution. This washing procedure ensures that only stable and covalently bonded (–SO 3 H) groups remain on the surface.

The samples were then dried at 100 °C for 24 h. The samples were named CPX-T, where X represents K for K 2 CO 3 or Zn for ZnCl 2, T is the carbonization temperature, and () indicates sulfonated samples sulfonated with H 2 SO 4 at 80 °C for 2 h. For example, CPZn-6 describes PET carbon activated with ZnCl 2 at 600 °C and subsequently sulfonated under the described conditions.

Catalyst characterization

Thermogravimetric curves (TG) were determined in a thermal analyzer (DTG-60H, Shimadzu). The analyses were conducted under a nitrogen atmosphere with temperature ramp rate of 10 °C min -1 up to 900 °C, in an alumina crucible. The TG-MS curves were recorded in a thermogravimetry analyzer coupled to a mass spectrometer (QMS 403 C-Aelos, NETZSCH), under an argon atmosphere with a temperature ramp rate of 10 °C min -1. The spectra in the infrared region were determined in an FTIR spectrometer (Cary 630, Agilent), in the range of 4000–650 cm -1. SEM images were obtained using a microscope (Vega3, TESCAN) coupled to a 20 kV X-ray energy dispersive spectrometer. The adsorption-desorption isotherms of N 2 at −196 °C were measured using a volumetric analyzer (Belsorp Max, MicrotracBel). All the samples were treated at 200 °C for 2 h before measurements. The total pore volume was estimated by the amount of nitrogen at a relative pressure of p/p 0 = 0.99. The specific surface area was determined using the monolayer calculated via the Brunauer–Emmett–Teller (BET) method at a relative pressure of 0.05–0.15 p/p 0. The external surface area and the volume of micropores were estimated using the α-plot method. The DFT method was also used to calculate pore volume. X-ray photoelectron spectroscopy (XPS; VSW HA-100) measurements were performed using Al Ka radiation (hv = 1,486.6 eV). The curves were adjusted using the Gaussian modeling method at a constant pressure of 6 × 10 −8 mbar. The density of the active sites on the surface of the catalysts was determined via Boehm titration (Boehm, 2002). An aliquot of 0.5 g of catalyst was added to 17 mL of 0.05 M NaOH solution to determine the total density of active sites, and 0.5 g of catalyst was added to 20 mL of 1M Na 2 SO 4 solution to determine the density of the sulfonic groups. Both solutions were stirred for 24 h and filtered to remove the solids. A 5 mL aliquot was extracted from the solutions that were acidified with 0.05 M HCl. The solutions were titrated in triplicate with 0.05 M NaOH and using phenolphthalein as the indicator.

Catalytic tests

The performance of the catalyst was evaluated via the esterification reaction of oleic acid and methanol. The reaction was conducted in a high-pressure reactor (5,500 Compact mini bench top, Parr Instrument Company), containing 0.6667 g of catalyst, 15 mL of oleic acid and 23 mL of methanol. The experiments were performed by fixing the molar ratio at 1:12 of oleic acid and methanol and 5% catalyst. The temperature was either 25, 50, 75, 100 or 125 °C and the reaction time at 0.5, 1, 1.5 and 2 h. After the reaction, the catalyst was separated by vacuum filtration and the by-products (water and methanol) were evaporated at 120 °C. The efficiency of the catalyst was calculated using the conversion of oleic acid to methyl esters, according to Supplementary Equation S1. At the end of each reaction, the catalyst was washed with 120 mL of hexane to remove residues and dried at 100 °C for 24 h. The catalyst was used in four consecutive reaction cycles. This methodology was established based on previous studies by Araujo et al. (2021), Abou-Elyazed et al. (2024).

The activating agents are responsible for the synergistic effect involved in the formation of the carbonaceous structure and strongly influence the pore volume distribution and surface area of the catalytic support. Different activating agents promote distinct physicochemical characteristics in carbon materials. Alkaline activators, such as K 2 CO 3, are generally associated with pore development, generation of oxygenated functional groups, and gasification reactions, whereas chemical activating agents such as ZnCl 2 promote pore formation through oxidative, templating, and gasification mechanisms (Gao et al., 2020).

The choice between K 2 CO 3 and ZnCl 2 as activating agents was based on the convergence of textural properties, structural stability, density of acidic sites, and catalytic performance. The results are summarized in Supplementary Table S1. For the samples synthesized using ZnCl 2 activation, the maximum conversion to methyl oleate was 90% for CPZn-7, prepared at 700 °C. Increasing the carbonization temperature from 500 °C to 700 °C favored the incorporation of sulfonic acid groups on the catalyst surface and increased the catalytic conversion from 63% (CPZn-5) to 90% (CPZn-7). However, despite the high conversion achieved by CPZn-7, this catalyst maintained stability only up to the second reuse cycle, similarly to CPZn-5*.

The best overall catalytic performance among the ZnCl 2 -activated materials was obtained for CPZn-6* (Figure 8), which exhibited high esterification conversion (88%) and remained effective up to the third reuse cycle. In contrast, CPZn-5* and CPZn-7* showed conversion decreases of 15% and 39%, respectively, relative to the first reaction cycle. The superior catalytic performance of CPZn-6* is associated with the balance between its textural properties and the density of acidic sites (1.31 mmol g -1), which favored the incorporation and accessibility of sulfonic groups on the carbon surface (Naji and Tye, 2022). In agreement with previous studies on sulfonated carbons, the catalytic activity was more strongly related to the density and accessibility of–SO 3 H groups than to the specific surface area alone (Araujo et al., 2019; Ribeiro et al., 2024).

Table 1 summarizes the textural parameters of the activated carbon materials. As presented in Supplementary Table S1, the performance of CPZn-6* becomes more evident when compared with recently reported sulfonated carbon catalysts. The ─SO 3 H density obtained for CPZn-6* (1.31 mmol g -1) is comparable or superior to several catalysts derived from biomass residues. He et al. (2024) reported acid site densities between 0.63 and 1.18 mmol g -1 for coconut-shell-derived catalysts, whereas Duarte et al. (2025) obtained 1.12 mmol g -1 using a microwave-assisted synthesis route. Despite these similar acidity values, CPZn-6* achieved high esterification conversion under relatively mild sulfonation conditions (80 °C for 2 h), indicating that PET-derived carbons can provide efficient acidic functionalization with reduced energy demand.

Although CPZn-7* exhibited a slightly higher acidity (1.43 mmol g -1), the main advantage of CPZn-6* lies in its structural stability during reutilization. While several sulfonated catalysts reported in the literature exhibit significant decreases in conversion after the second reaction cycle, CPZn-6* remained catalytically active up to the third cycle. These results suggest that activation with ZnCl 2 at 600 °C provides an appropriate balance between acidic functionalization and structural stability of the PET-derived carbon matrix.

The CPK-8 sample activated with K 2 CO 3 showed 65% conversion of methyl oleate. Despite presenting the highest specific surface area (732 m 2 g -1), the catalytic activity was significantly lower than that of CPZn-6*. According to the literature, activation with K 2 CO 3 favors the formation of more stable carbon surfaces with lower concentrations of oxygenated functional groups (Zhao et al., 2023), which can hinder the effective incorporation of sulfonic acid groups during the functionalization process. Consequently, the reutilization of the sample decreased by 52% after the first use. These results reinforce that the catalytic behavior of sulfonated carbons is more dependent on the density and accessibility of acidic sites than exclusively on surface area.

Figure 1a presents the TG-DTG curves of the samples of activated carbon materials at different temperatures. Three mass loss events were recorded for all samples. The first mass loss was in the range of 30 °C–150 °C and is related to desorption of the water molecule (Wang et al., 2019). The second event, between 150 °C–350 °C, was related to the decomposition of the carbonaceous material. The third event started at 360 and ended at 500 °C and was attributed to thermal decomposition of the PET polymer into different volatile species such as CO 2 (Rai and Singh, 2018).

Figure 1b represents the TG-DTG curves of sulfonated carbon materials. Three mass loss events were observed for all the sulfonated samples. The first mass loss was in the range of 34 °C–150 °C and is related to the loss of the sulfonic groups that are weakly anchored to the carbon surface (Araujo et al., 2019). The second loss is more pronounced and occurs between 150 °C–360 °C and is attributed to the decomposition of the sulfonic group that is strongly attached to the catalyst surface (Correa et al., 2023). The mass losses associated with these groups were 14%, 10%, and 15% for CPZn-5, CPZn-6, and CPZn-7*, respectively. These results are consistent with the density of sulfonic groups determined by Boehm titration (Supplementary Table S1). Finally, the third event, between 360 °C and 600 °C, corresponds to decomposition of the carbonaceous matrix.

The textural properties of the carbon materials obtained from the plastic waste are summarized in Table 1. It is known that chemical activation with ZnCl 2 and K 2 CO 3 promotes pore formation in the micropore region and develops materials with a high surface area and pore volume, in addition to an adjustable chemical surface with ideal characteristics for carbonaceous materials. Therefore, the activation process, in conjunction with pyrolysis, significantly influences the textural properties (Sayğili and Güzel, 2016).

Increasing the activation temperature from 500 °C to 600 °C for the samples activated with ZnCl 2 increased the values of S BET, Vt, V mic and V mes from 23 to 272 m 2 g -1, from 0.11 to 0.36 cm 3 g -1, from 0.01 to 0.14 cm 3 g -1 and 0.09–0.22 cm 3 g -1, respectively. It appears that ZnCl 2 functions as an efficient catalyst in C-O and C-C bond scission during pyrolysis. Increasing the process temperature causes thermal dehydration of zinc oxide chloride hydrate, which results in gaseous ZnCl 2 and a solid phase of zinc oxide. The gaseous phase of ZnCl 2 reacts with the carbon atoms via reduction reactions, eliminating parts of the carbon atoms and leaving behind channels, known as a pore network (Gao et al., 2020; Dos Reis et al., 2022).

The specific surface area was higher for the sample activated with K 2 CO 3 (732 m 2 g -1). The use of the alkaline activating agent also favored the formation of a larger volume of micropores V mic (α-plot) = 0.28 cm 3 g -1 (CPK-8) when compared to the amount of V mic (α-plot) = 0.14 cm 3 g -1 for the sample activated with ZnCl 2 (CPZn-6). The same upward trend was observed for the V mes of the CPK-8 sample (0.32 cm 3 g -1). The V mes was calculated from the subtraction of V mic and Vt. It should be remembered that the value of V mes is overestimated by the method used.

The excellent results in terms of textural properties for the catalysts activated using K 2 CO 3 corroborate the results found in the works published (Sayğili and Sayğili, 2019; Wang et al., 2020; Tay et al., 2009). The development of pores and greater volume of micro- and mesopores is associated primarily with the dehydration reaction, followed by the water-gas reaction and, finally, the formation of gaseous potassium metal via the reduction of K 2 CO 3 by carbon atoms at a temperature above 760 °C. The reaction speed of potassium metal is much faster when compared to acidic or neutral activators. This is also the reason why the specific surface area of carbon materials activated by potassium sources is much larger than those activated via acidic or neutral agents (Gao et al., 2020).

The sample activated at the lowest temperature (CPZn-5) showed a lower specific surface area and negligible V mic and V mes results. The values estimated by the DFT method are similar to those discussed earlier. The N 2 adsorption-desorption isotherms (Figure 2a) of the samples activated with ZnCl 2 are classified by IUPAC as type I (a) isotherms with a predominance of micropores, which is also confirmed by the pore size distribution (PSD) values (see Table 1). The CPK-8 sample has isotherms that are classified as type I (b), with hysteresis type H4 attributed to micro-mesoporous carbons. It is observed that, in the region of low relative pressure, there was high nitrogen adsorption, indicating the filling of micropores. The PSD (Figure 2b) shows the diameter centered at 0.7 nm, and a small amount at 1 nm for the CPK-8 sample. For samples CPZn-6 and CPZn-5, the PSD curves show pores with diameters between 0.9 and 1.3 nm, respectively. Therefore, based on the texture parameters data described previously, the catalyst CPZn-6* was chosen to perform the characterization study is complemented by its surface chemistry, as evidenced by the functional groups identified in the FT-IR analysis discussed below.

Figure 3 presents the FT-IR spectra for the CPZn-6 and CPZn-6* samples. Absorption bands in the range of 1,625–2,168 cm -1 are common for both samples. The bands 1,625 and 2,168 cm -1 are assigned to the vibrational mode of the C=C bond present in the aromatic carbon. The bands positioned at 1977 and 2038 cm -1 are attributed to the vibrations of the O=C=O bond (Saka, 2012; Mennani et al., 2023). In the CPZn-6 sample, there is a band at 934 cm -1, which is related to the vibrations of the C─H bond present in the aromatic carbon structure (Yu et al., 2018). The presence of the sulfonic groups covalently bonded to the polyaromatic carbon structure of the sulfonated catalyst was confirmed by the vibration band at 1,025 cm -1 associated with the sulfonic group in the bond O=S=O. In addition, the band at 934 cm -1 (C─H) is reduced after the sulfonation process and this demonstrates the deformation of the cyclic structure (Abukhadra et al., 2022). The presence of these functional groups and their respective thermal decomposition profiles will be further validated and discussed through TG-MS analysis in the following section.

The TG-MS was used to identify the gases released from the decomposition of the sulfonated sample (CPZn-6). The peaks m/z 18, 64 and 80 correspond to the chemical species H 2 O, SO 2 and SO 3 respectively. The m/z 18 peak (Figure 4a) originates from the desorption of water molecules adsorbed on the surface of the carbon materials. Water has also been identified at temperatures above 140 °C from surface reactions (Santos et al., 2022). The peaks m/z 64 (Figure 4b) and m/z 80 (Figure 4c) are associated with the decomposition of sulfonic groups into SO 2 gases at 200 °C–350 °C and SO 3 gases at 100 °C. However, there are indications that SO 2 also decomposes at temperatures greater than 400 °C–550 °C and SO 3 in the range of 200 °C–350 °C (Zhao et al., 2010). The results confirm the data found via the FT-IR and prove the presence of sulfonic groups present on the surface of the sample CPZn-6. In addition to these chemical and thermal insights, the morphological features and surface topography of the samples were examined using Scanning Electron Microscopy (SEM).

The morphology of the catalysts CPZn-6 and CPZn-6* was studied using SEM. The image of the catalyst CPZn-6 (Figure 5a) showed different surface properties, such as roughness and irregularities, which were revealed by the presence of cavities of various sizes and shapes that developed after the activation process with ZnCl 2. These macropores facilitate the diffusion of the reagents in the aqueous phases in the meso- and micropores of the carbon to the adsorption sites. Such characteristics favors the anchoring of the acid groups on the catalyst’s surface (De Castro et al., 2018), however, the CPZn-6* sample presented a smooth and compact surface (Figure 5b) after the sulfonation process. The chemical composition of the surface of the sulfonated sample (CPZn-6*) was determined qualitatively using EDS. The EDS spectrum (Figure 5a) confirmed the presence of carbon and oxygen in the non-sulfonated sample (CPZn-6). After the sulfonation process, there was a decrease of % by weight of carbon in the structure (Figure 5b). There was also an increase in the amount of oxygen (16% by weight) and sulfur (5% by weight), which was attributed to the presence of sulfonic groups (−SO 3 H) attached to the carbon structure (Ribeiro et al., 2023). These results indicate the effectiveness in the anchoring of the acid sites, which directly cause the best performance of the catalyst. While EDS provides a preliminary qualitative overview of the elements, the precise chemical states and bonding environments of these atoms were further elucidated by XPS analysis.

As observed in the XPS spectra, the CPZn-6 sample is predominantly composed of carbon; however, its surface reactivity is revealed by the deconvolution of the C1s signal (Supplementary Figure S1a). This spectrum presents four binding energy peaks at 284.5, 287.6, 289.4, and 291.2 eV, corresponding to C-C (sp 2 and sp 3), C─O, C=O, and O─C=O bonds, respectively. The presence of these oxygenated groups in the original carbon matrix is particularly important because they act as reactive sites and structural defects that facilitate the subsequent incorporation of acidic groups during the sulfonation process. Complementarily, the O1s spectrum (Supplementary Figure S1b) exhibits three peaks at 531.3, 533, and 534.9 eV, characteristic of C─O, O─H, and C=O bonds (Araujo et al., 2021).

Significant chemical modifications were observed after sulfonation of the material. In the C1s spectrum of CPZn-6* (Figure 6a), in addition to the carbon- and oxygen-containing groups, the appearance of a peak at 286.1 eV assigned to C─S bonds confirms the incorporation of sulfur into the carbon structure (Zhao et al., 2016). This result strongly suggests that sulfur was chemically bonded to the carbonaceous matrix rather than merely physically adsorbed on the surface. Further evidence of the acidic nature of the catalyst is provided by the high-resolution S2p spectrum (Figure 6b). The deconvolution revealed the characteristic doublet S2p 3/2 and S2p 1/2 at 167.8 and 168.9 eV, respectively. These binding energy values are characteristic of sulfur in the S 6+ oxidation state and are associated with sulfonic acid groups (─SO 3 H) attached to the catalyst surface (Yang et al., 2019).

The O1s spectrum (Figure 6c) also supports the successful functionalization of the carbon surface. The deconvoluted peaks centered at approximately 530.3 and 532.4 eV were assigned to oxygen-containing functionalities such as C=O and C–O/O–H groups, respectively. The higher binding energy component at ∼534.0 eV was attributed to O=S=O species associated with sulfonic acid groups (–SO 3 H), providing additional evidence for the successful incorporation of sulfur-containing acidic functionalities into the carbon matrix. The low binding energy contribution observed at ∼528.8 eV may be associated with highly conjugated oxygen species or defects in the carbon structure and is discussed as a minor component of the spectrum (Min et al., 2007).

According to Mei et al. (2026), the presence of C-S bonds in highly defective carbons derived from polymeric precursors contributes to improved thermal and chemical stability of sulfonated catalysts. The combined interpretation of the FT-IR and XPS results indicates that activation with ZnCl 2 promoted the formation of a porous carbon structure with suitable surface defects for stable incorporation of ─SO 3 H groups. This structural characteristic is directly related to the high catalytic activity and the resistance to leaching observed during the catalyst reuse cycles.

Catalytic test

After defining CPZn-6* as the sample with the best results in terms of textural properties, the sulfonation temperature and time were evaluated because these parameters strongly influence the esterification yield. Supplementary Figure S1a presents the effect of sulfonation temperatures of 60, 80, 100 °C and 120 °C on the conversion of oleic acid. Maximum conversion (88%) was observed at the sulfonation temperature of 80 °C. Reducing the sulfonation temperature to 60 °C did not favor the conversion into methyl oleate, and the reaction yield decreased to approximately 70%. A similar effect was observed at temperatures above 100 °C.

According to the literature, increasing temperature can favor the incorporation of sulfonic groups; however, above the characteristic sulfonation limit of each material, the density of sulfonic groups decreases due to partial degradation of the carbonaceous structure and the consequent difficulty in anchoring the acidic groups (Ibrahim et al., 2020). Deng et al. (2026) also reported that maintaining the integrity of the carbon framework is essential for effective functionalization, since excessive temperatures may degrade anchoring sites, which explains the reduction in yield observed at 100 °C and 120 °C. Furthermore, Ribeiro et al. (2024) demonstrated that, in biomass-derived carbons, there is a critical temperature at which the density of active sites is optimized and increasing temperature beyond this point does not necessarily result in higher acidity.

Therefore, 80 °C was selected as the optimal sulfonation temperature for subsequent studies because it resulted in the highest conversion into methyl oleate. The influence of sulfonation time was evaluated from 2 to 8 h, and the catalytic conversion results are presented in Supplementary Figure S2b. Increasing sulfonation time did not significantly alter the reaction yield, maintaining conversion values around 85%. Maximum conversion (88%) was achieved at 2 h and 6 h. These results indicate that sulfonation time is not the predominant parameter in catalyst preparation, although its evaluation remains important for optimization of the synthesis conditions. This stabilization of catalytic activity over time suggests rapid saturation of sulfonic groups on the PET-derived carbon matrix, as also discussed by Ahmed and Huddersman (2022), who reported that the accessibility and quality of surface–SO 3 H groups are often more important for esterification than prolonged synthesis periods.

The catalyst CPZn-6* prepared under the optimized sulfonation conditions of 80 °C for 2 h, was subsequently applied in the study of temperature and reaction time effects on the esterification process. The temperatures investigated range from 25 °C to 125 °C, while the remaining reaction conditions were fixed at a reaction time of 1 h, oleic acid:methanol molar ratio of 1:12 and catalyst loading of 5wt%. As shown in Figure 7a, increasing temperature from 25 °C to 100 °C significantly improved the conversion of oleic acid. However, after reaching maximum conversion at 100 °C (88%), the reaction yield decreased at higher temperatures, suggesting that the reaction equilibrium was reached and that further temperature increase did not favor esterification.

One possible explanation for this behavior is the accelerated evaporation of methanol at higher temperatures, reducing its availability in the reaction medium and consequently decreasing methyl ester formation (Mardhiah et al., 2017; Araujo et al., 2021). Zhang et al. (2023) also discussed this limitation and demonstrated that replacing liquid methanol with methanol vapor may minimize these losses and improve catalyst stability by maintaining reactant availability at the active sites.

The influence of reaction time on the esterification of oleic acid was evaluated at 100 °C over periods of 0.5, 1, 1.5, and 2 h. According to Figure 7b, oleic acid conversion increased up to 1 h, reaching a maximum value of 88%. After this period, the reaction yield decreased. The literature reports that prolonged reaction times favor the interaction of water, generated as a byproduct, with the hydrophilic sulfonic groups of the catalyst, contributing to partial deactivation of the acidic sites (Araujo et al., 2019). Mei et al. (2026) also reported that, in highly defective sulfonated carbons, water may hinder the accessibility of fatty acids to the–SO 3 H groups due to competitive adsorption and site hydration. Despite the hydrophobic character of the carbon structure, this deactivation effect was not completely avoided, as further confirmed in the catalyst reutilization study.

Reutilization of the catalyst

Figure 8 illustrates the reutilization of the CPZn-6* catalyst for four consecutive reaction cycles. The catalyst CPZn-6* remained stable after repeated use for two reactions. In the third cycle, the conversion decreases significantly to less than 50%. This decrease in catalytic performance is associated with partial deactivation of the catalyst caused by leaching of sulfonic acid groups (─SO 3 H).

Evidence of this behavior is observed in the FT-IR spectra (Supplementary Figure S3a), where the band at 1,025 cm -1, attributed to the symmetric stretching of the O=S=O bond of sulfonic groups, disappears after the fourth reaction cycle. Similar behavior was observed in the thermogravimetric analysis. Comparison between the TG profiles of the reused catalyst after the fourth cycle (Supplementary Figure S3b) and the fresh CPZn-6* catalyst (Figure 1b) revealed a reduction in the mass loss associated with decomposition of sulfonic groups, corroborating the decrease in catalytic conversion observed in Figure 8. Therefore, the catalytic activity of CPZn-6* is strongly associated with the preservation of surface acidic groups during the reuse cycles.

The stability of CPZn-6* presents important advantages compared to recent studies involving plastic-derived sulfonated catalysts. Hazmi et al. (2022), for example synthesized a sulfonated carbon catalyst from plastic waste for esterification of palm fatty acid distillate and reported a more pronounced decrease in conversion after the second reuse cycle due to acidic site leaching and pore blockage by fatty acid chains. In contrast, CPZn-6* maintained high esterification conversion for a longer period, which may be associated with the optimized activation using ZnCl 2 at 600 °C. This activation condition likely promoted more stable incorporation of–SO 3 H groups within the porous structure of the PET-derived carbon, contributing to improved resistance against leaching. Consequently, CPZn-6* demonstrated an improved balance between catalytic activity and structural stability when compared with similar plastic-derived catalysts reported in the literature.

The use of the activating agents ZnCl 2 and K 2 CO 3 was investigated in the process of chemical activation of PET to produce carbon materials, which were applied as catalytic support in the esterification reaction. The best catalytic result was for the catalyst activated with ZnCl 2. The CPZn-6* sample had excellent conversion (88% yield in methyl ester) and was effective until the third reaction cycle. The justification for the excellent results of the CPZn-6* sample is the high surface area (272 m 2 g -1), the pore volume in the micro- and mesopore region and the greater density of acid sites (1.31 mmol g -1), which favored the anchoring of the sulfonic groups on the surface of the carbon. The CPK-8 sample activated with K 2 CO 3 showed excellent textural properties of S BET and Vt of 732 m 2 g -1 and 0.61 cm 3 g -1, respectively; however, the conversion was of 65% into methyl oleate and, therefore, the study of synthesis optimization was stopped. The presence of the sulfur of the sulfonic group on the surface of the catalyst was confirmed via FT-IR, SEM-EDS and TG-MS analyses. XPS analysis showed how sulfur is attached to the carbon structure and the analysis of the density of sulfonic groups corroborated its existence in the sulfonated catalyst. The study of the variations in sulfonation time and temperature was also performed using the CPZn-6* sample. Only a significant change was observed in relation to the sulfonation temperature, while increasing time did not the alter the conversion. The same trend was not observed for the variations in temperature and esterification reaction time, as both parameters had a significant impact on oleic acid conversion. Therefore, the proposed use of plastic waste has potential for use as a source of carbon material to produce solid acid catalysts. The use of plastic waste as a catalytic support offers a sustainable solution to mitigate plastic waste accumulation and its associated environmental problems.