Introduction

Plastics have broadly benefited our modern society with indispensable uses in packaging [1], automotive [2], medical supplies [3], and many other applications. To date, the cumulative global production of plastics has exceeded 9 billion metric tons (MTs), however, only less than 6% of them were recycled; this rate was among the least of all recyclable materials. In fact, plastic waste largely ends up with incineration and landfilling, leading to significant negative impacts on environment and society. For example, after experiencing degradation events through a combination of physical, chemical, and biological processes, landfilled plastics become primary pollution in natural environments, with potential links to various human diseases [4, 5]. Alternatively, incineration of plastic waste can release harmful emissions into the atmosphere, potentially causing damage to nervous, reproductive, and immune systems [6]. Therefore, reducing and remediating plastic waste are of paramount importance for establishing materials circularity toward development of a sustainable future.

Among many hurdles in plastic recycling, a major challenge is associated with crosslinked polymers, as these materials are inherently non-processable and non-recyclable; their permanent network structures completely hinder materials flowability even at elevated temperatures, and therefore conventional mechanical recycling method is not applicable [7]. To combat this issue, many chemical upcycling approaches have been developed, including hydrogenolysis [8], solvolysis [9, 10], and pyrolysis-based methods [11], allowing the conversion of plastic wastes to new products with comparable or enhanced value. For example, Tour et al., developed a flash joule heating approach to produce turbostratic graphene through using waste rubbers as the feedstock [12], which can then be used as nanofillers and/or reinforcing agents. Additionally, Furgal et al., reported room temperature depolymerization of silicone-based crosslinked polymers through introducing low amounts of fluorides in organic solvents [13]. The recovered monomer can then be re-polymerized through using acid, base, and fluoride catalysts. These methods demonstrate efficient upcycling of crosslinked polymers, as their derived products can be further utilized with enhanced value and properties.

Interestingly, most research works for upcycling of crosslinked polymers have been focused on rubber tires [14], silicon elastomers [15], benzoxazine [16], and other thermoset materials as feedstock precursors/resources [17]; much less can be found concerning crosslinked polyethylene (x-PE) foam materials. x-PE foams have with an estimated global market value of approximately 1.3 billion USD in 2021, with a compound annual growth rate of 6.4%. These foams typically contain closed-cell structures and high porosity, which have strong impact damping and chemical resistance properties [18], with broad use in construction [19], automotive parts [20], and sport products [21]. Several developments were made to valorize x-PE wastes, primarily through depolymerization approaches [22, 23]. For example, supercritical methanol, propanol, and water were employed to de-crosslink x-PE using a twin-screw extruder at temperatures ranging from 300 to 340 °C [24,25,26]. Moreover, this process can be enhanced through the assist of ultrasonication, and it was found that increasing the ultrasonic amplitude and flow rate can result in further reduced gel fraction and crosslink density of x-PE materials [27, 28], suggesting ultrasonic-assisted extrusion is a viable approach to obtain melt-processable and de-crosslinked PE resins.

Notably, PE materials can be employed as carbon precursors through sulfonation-induced crosslinking, a concept originally established for economic carbon fiber manufacturing [29, 30]. Briefly, by reacting PE in sulfuric acid at elevated temperatures, polymer backbones can be aromatized and crosslinked through radical-based couplings. As the reaction is a diffusion-controlled process, it is often employed for PE fibers with diameters less than 50 μm [31, 32]. Building on these efforts, this strategy was extended to convert PE waste to carbonaceous materials. Specifically, after plastic grinding, sulfonation, and pyrolysis, PE-derived carbon materials show demonstrated use in energy storage, such as lithium-ion battery and supercapacitors [33,34,35]. In general, these studies primarily focus on using linear and/or branched PE as precursors, and the applicability of sulfonation-crosslinking chemistry in x-PE remains not clear. Additionally, to best use the waste materials, it would be ideal to retain high porosity of the x-PE foams, arguably the most important feature, throughout the upcycling process. If successful, x-PE foam can be converted to functional porous carbons with enhanced value for many applications, such as catalysis [36], gas capture and separation [37, 38], energy storage [39], and environmental remediation [40, 41].

Herein, we demonstrate a simple and catalyst-free approach for the upcycling of x-PE foams to functional carbon sorbents containing both micropores and macropores. Due to the highly porous nature of starting materials, sulfonation-crosslinking reaction can be accomplished within only 30 min, resulting in carbons with a high surface area of 372 m2/g as well as doped carbon framework containing up to 3.7 at.% sulfur. It is found that x-PE foam-derived carbons can exhibit a high CO2 sorption capacity of 3.1 mmol/g at 25 °C and 1 bar, which can further increase to 4.5 mmol/g at 0 °C. Furthermore, incorporation of sulfur heteroatoms enables high selectivity of these porous carbon sorbents for CO2 uptake against N2 gas molecules, an important feature for their practical use to address carbon capture from flue gas emissions. This work represents a viable pathway for upcycling of crosslinked polyolefin materials, collectively addressing plastic waste and carbon emission challenges toward sustainable development of environment and society.

Results and discussion

The broad use of x-PE foams in various applications, including product packaging, protective padding, health care, and thermal insulation [42, 43], leads to an enormous amount of plastic waste generated after their end-of-life, which imposes significant challenges due to the lack of appropriate recycling methods. Particularly, mechanical recycling, a common method for reprocessing of single-stream PE waste, is not applicable for waste x-PE materials; the permanently crosslinked network completely hinders the polymer processability for being re-engineered into new products. As shown in Fig. 1(A), the DSC thermogram of x-PE indicates a melting temperature of approximately 107 °C and a degree of crystallinity of 31%. Upon exposure to an elevated temperature of 150 °C for 24 h, x-PE foams only exhibited approximately 60% volumetric shrinkage with no characteristic flow feature, indicating their inability to be mechanically recycled.

Figure 1
figure 1

(A) DSC thermogram of a neat x-PE foam sample. (B) Photos indicate that the shrinkage of x-PE foam after heating at 150 °C for 24 h. Polymer flow above their melting point did not occur due to the crosslinked nature of x-PE foams.

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This work presents a simple, scalable, and catalyst-free upcycling method for converting x-PE foams to porous carbon-based environmental sorbents with over 50 wt% product yields. The chemical reaction scheme is demonstrated in Fig. 2(A), which is similar to previous reports of synthesizing carbon fiber and other carbonaceous products from polyolefin materials [33, 44,45,46]. Briefly, immersing x-PE in sulfuric acid at elevated temperatures would first result in the attachment of sulfonic acid groups onto the polymer backbones, followed by the homolytic dissociations of sulfonyl groups. The elimination of sulfonic acid groups leads to the formation of double bonds within x-PE samples, which can further react through multiple mechanisms, including secondary addition and dissociation. Subsequently, radical species from unsaturated groups in x-PE backbones enable coupling reactions, which eventually yield further crosslinked networks. During this reaction, we found that macroscopic shape and structure of x-PE foams can be retained, while a high degree of volumetric shrinkage was observed.

Figure 2
figure 2

(A) Simplified reaction schemes of sulfonation-induced crosslinking of x-PE foams using fuming sulfuric acid. (B) FTIR spectra and (C) mass gain and crystallinity of x-PE foam as a function of sulfonation time at 140 °C.

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Figure 2(B) shows the FTIR spectra of x-PE foams after heating at 140 °C for different periods of time with the presence of fuming sulfuric acid. Noticeably, bands in the region of 1023 cm−1 rapidly appear after only 5 min of reaction, corresponding to the attachment of sulfonic groups onto the x-PE backbones, while bands at 1633 cm−1, associated with the formation of C=C bonds, were also developed rapidly as the reaction progresses. Further evidence confirming the sulfonation of x-PE foams includes the diminishing of C–H stretching bands at 2913 cm−1 and the appearance of –OH band at 3350 cm−1, which is associated with the presence of sulfonic acid groups in sulfonated x-PE samples. The reaction kinetics of x-PE sulfonation was further studied through understanding changes in mass and degree of crystallinity of foams as a function of reaction time. Specifically, sulfonation-induced crosslinking of polyolefin materials can result in mass increase due to the incorporation of sulfonic acid groups, which is shown in Fig. 2(C). The mass of x-PE foams quickly increases within the first 30 min of reaction, then reaching a plateau value of approximately 60%. Additionally, the degree of crystallinity of x-PE foams significantly decreases after 5 min, followed by a complete loss of crystallinity after 15 min due to the combined effects of further hindered chain mobility from crosslinking and the presence of bulky acid groups on the x-PE backbones (Figure S1). Figure 2(B) and (C) indicates very rapid sulfonation-crosslinking kinetics of x-PE foams using fuming acid, with at least an order of magnitude faster than previous work associated with PE crosslinking using concentrated acid [32, 47]. While fuming acid is known to have higher reactivity than concentrated sulfuric acid due to the presence of free SO3 groups, porous nature of x-PE foams also plays a critical role in facilitating the acid diffusion, allowing more efficient and rapid crosslinking to occur. The significantly improved reaction kinetics is advantageous for potential scale-up with lower energy consumption.

While the foam shape was retained after sulfonation reaction, it is important to understand how the microscopic structures of x-PE foams evolve as a function of reaction time. As shown in Fig. 3(A), the pristine PE foam exhibits a typical closed-cell structure, with an averaged wall thickness of 15.2 μm. Figure 3(B) and (C) shows the morphology of x-PE foams after reacting with fuming sulfuric acid for 15 min and 2 h, respectively. Similar macroporous structures were observed in comparison to pristine x-PE foams with slight disruptions in closed-cell walls. In a previous study, it was found that sulfonation-enabled crosslinking of polyolefins can generate gaseous byproducts that effectively alter the surface area and porosity of polymer precursors [48]. To examine the impact of sulfonation time on the materials microporosity, physisorption measurements were performed on sulfonated x-PE samples. As shown in Fig. 3(D) and Figure S2, x-PE foams exhibit a general trend of slightly increased surface areas with extended reaction time, specifically from 18 m2/g for untreated sample (pristine x-PE foams) to 29 m2/g after 2 h of reaction. Despite the very low surface area of all sulfonated samples, these results indicate that crosslinking reaction of x-PE using fuming acid could introduce a low amount of additional micropores, which may facilitate the diffusion of acids for crosslinking.

Figure 3
figure 3

SEM images of (A) pristine x-PE foam, and after sulfonation at 140 °C for (B) 15 min, and (C) 2 h. (D) Pore volume and surface area of sulfonated x-PE foams with increasing sulfonation time.

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The conversion of sulfonated x-PE foams to carbons was performed using a direct pyrolysis approach, heating the samples up to 800 °C under N2 atmosphere using an electric furnace [Fig. 4(A)]. For control samples, x-PE without any sulfonation reaction leads to no carbon yield, which is consistent with the previous studies [49]. After sulfonation for 15 and 30 min, the carbon yield of x-PE foam was increased to approximately 24 and 41 wt%, respectively. Further extending the sulfonation time to 1 and 2 h results in a plateau value of carbon yield, slightly varied between 49 and 51 wt% (Figure S3). It is important to note that the x-PE samples employed here are thick foams [as shown in Fig. 1(B)], without grinding or any pre-treatment steps before their crosslinking in fuming acid. Thus, high carbon yields obtained on a relatively short reaction time, attributed to the rapid acid diffusion and crosslinking kinetics, are highly advantageous toward their practical implementation in scaled systems; note that similar polyolefin-based systems in previous reports typically require a grinding step prior to sulfonation, which could be time and energy consuming [50].

Figure 4
figure 4

(A) Carbon yield of x-PE foams as a function of sulfonation time at 140 °C. (B), (C), (E), and (F) are associated with gas-phase outlet during temperature-programmed-carbonization (TPC); (B) and (E) show SO2 and CO2 outlets, respectively; (C) and (F) show the hydrocarbon formation over 5 min (crosslink) and the neat PE foam, respectively; (D) is associated with the liquid outlet from TPC.

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To further understand the temperature-dependent thermal carbonization process, TPC (temperature-programmed-carbonization) experiments were performed for selected samples. The gas-phase outlet information during the TPC is provided in Fig. 4(B–C) and (E–F), while the liquid outlet information is included in Fig. 4(D). As shown in Fig. 4(B), while SO2 is absent from the neat foam, the sulfonated x-PE samples show SO2 release at temperatures between 150 and 300 °C, consistent with the presence of sulfonic acid groups in polymer backbones after sulfonation-crosslinking. Additionally, CO2 release from samples can also be observed at the same temperature, which might originate from the oxidation of carbon atoms by the sulfonic acid groups. More interesting observations can be found related to hydrocarbon species formation during the TPC. As shown in Fig. 4(F), TPC of neat x-PE shows the formation of C2–C5 hydrocarbon (olefins) as major thermally decomposed products at temperatures between 450 and 600 °C, indicating that x-PE (without sulfonation) was pyrolyzed to mixed olefins rather than carbons. Besides the gas-phase outlet, the TPC of neat x-PE foam produces significant amounts of mixed wax according to the liquid/solid analysis determined by gas chromatography–mass spectrometry (GC–MS). The formation of wax and light olefins during the TPC of neat PE foam is in line with zero carbon yield shown in Fig. 4(A). While x-PE with 5 min sulfonation-crosslinking time also shows the formation of light olefins (at the same temperature ranges of neat x-PE foam), their peak intensities decreased significantly [Fig. 4(C)]. Notably, wax was absent from the GC–MS analysis for the x-PE samples sulfonated for 5 min and longer. For the x-PE with 30 min and 2 h of crosslinking, the light olefins as decomposed products were totally absent. The significant differences between the neat x-PE foam and their sulfonation-crosslinked counterparts in the gas-phase outlet and liquid outlet during the TPC are consistent with the different carbon yields shown in Fig. 4(A).

To characterize the pore textures of x-PE foam-derived carbons, liquid nitrogen sorption isotherms were obtained and analyzed for samples sulfonated for at least 15 min. As demonstrated in Fig. 5(A), upon carbonization at 800 °C it is found that these porous carbons materials (with varied sulfonation time at 140 °C) exhibit very similar surface areas and micropore volume, which are in the range from 367 to 388 m2/g, and from 0.90 to 1.22 cm3/g, respectively. Their respective sorption isotherms and pore size distributions were included in Figure S4. As sulfonated x-PE foams exhibit very limited surface areas of only up to 29 m2/g, it is clear that the carbonization process was responsible for developing micropores, and thus high surface areas, in their derived carbons, which involve polymer degradations as well as the potential activation reactions between decomposed products and carbon frameworks. For x-PE samples crosslinked for 20 and 40 min, their increase in pore volume is consistent with the trend shown in Fig. 3(D), while further extending reaction leads to reduced pore volume, particularly a low pore volume of ~ 0.2 cm3/g for 2 h of sulfonated sample. We attributed this change to their high crosslinking degree from sulfonation reaction, and thus the denser structures can lead to a more significant framework contraction upon polymer to carbon conversion [46]. The limited influence of crosslinking time on the pore surface areas (only varied from 360 to 390 m2/g and much higher than 29 m2/g from sulfonated samples) was similar to previous reports using PP masks and SIS as carbon precursors [37, 48]. Additionally, we attribute the low pore volume of sample, which was crosslinked for 2 h, to the reduced framework integrity from over-sulfonation, causing a high degree of shrinkage. To further confirm this point, we investigated the pore volume of x-PE-derived carbons, which have been crosslinked for 4 and 8 h under the same reaction temperature. It was observed that from 8 h of sulfonation, resulting carbon shows a very low pore volume less than 0.1 cm3/g (Figure S5).

Figure 5
figure 5

(A) Pore volume and surface area of x-PE foam-derived carbons, which were sulfonated for different periods of time at 140 °C and carbonized at 800 °C. (B) SEM image of a carbonized foam, which was crosslinked for 30 min at 140 °C and carbonized at 800 °C.

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Additional experiments were performed to understand how carbonization temperature impacts the surface area of x-PE foam-derived carbons. Figure S6 shows the surface area and pore volume of sulfonation-crosslinked x-PE foam (30 min at 140 °C) as a function of carbonization temperature, including 600, 700, and 900 °C. A clear trend was observed, which 600 °C carbonized sample has a surface area of 144 m2/g, increasing to 266 and 367 m2/g, when 700 and 800 °C were, respectively, employed. Further increasing pyrolysis temperature to 900 °C led to a reduced surface area of 187 m2/g. These results further confirm that micropore development is through carbonization and the resulting micropore surface area is dependent on pyrolysis conditions. Similarly, it was found that the pore volume of x-PE foam-derived carbons was 0.68, 1.34, 1.22, and 1.19 cm3/g, for samples obtained from different carbonization temperatures, ranging from 600 to 900 °C. Both high surface area and micropore volume are important features for determining the performance of porous carbon as sorbent materials in many applications. For example, Vinu et al. prepared activated porous carbons using alligator weed as the precursor, which was first pyrolyzed at 600 °C and followed by a chemical activation step at 800 °C, leading to high surface area microporous carbons (up to 3106 m2/g). The resulting material has a high CO2 uptake capacity of 6.4 mmol/g at 1 bar [51]. Alternatively, microporous and mesoporous carbon sorbents, prepared by a scalable roll-to-roll process [52], can exhibit a high surface area up to 2455 m2/g, These materials show efficient adsorption performance for the removal of bulky organic dyes from aqueous media, with a high capacity of 0.436 and 0.378 g/g against methylene green and methyl blue, respectively [53].

Figure 5(B) shows the successful retention of macroporous structures after converting sulfonated x-PE foams to carbon, while a high degree of shrinkage in macropore sizes was observed from crosslinked to carbonized states (approximately 39%); carbonization-induced pore shrinkage has been often observed in many systems. For example, soft-templated ordered mesoporous materials, including carbon, silica, and metal oxides, often exhibit more than 30% reduction in their pore sizes upon pyrolysis [54]. Our method takes the advantage of porous structure in starting materials, while additional processing handles (such as crosslinking time and carbonization temperature) exist to enable control over their final pore textures. Collectively, results in Fig. 5(A) and (B) suggest that the x-PE foam-derived porous carbons contain both micropores and macropores, which is important for their application for CO2 capture.

An important feature of our process is the use of fuming acid to crosslink x-PE foams, which introduces sulfonic acid groups into polymer precursors and thus can result in a sulfur-doped carbon framework upon pyrolysis. The acid–base interactions between sulfur-doped carbons (sorbent) and CO2 molecules (sorbate) can enhance their affinity, thus improving the overall CO2 uptake efficiency [55, 56]. Figure 6(A) shows an XPS survey spectra of all x-PE foam-derived carbon materials after being exposed to different crosslinking times. These samples were carbonized at 800 °C. Specifically, it was found that the doping content of sulfur heteroatoms in the carbons increased from 1.1 at.% with 15 min of crosslinking time, to approximately 3.7 at.% (sulfonated for 30 min). The doping content was then decreased to 2.6 and 2.2 at.% for samples reacting with fuming sulfuric acid for 1 and 2 h, respectively. In our process, increasing sulfonation time can lead to potential elimination of sulfonic acid groups from the polymer backbones as the crosslinking step further occurs. Additionally, oxygen heteroatoms were also present in the resulting carbon materials, showing an increase from 7.1 to 12.9 at.% when increasing sulfonation time from 15 min to 1 h, followed by a decrease to 6.4 at.% after 2 h. For the use of sulfur-doped carbon for CO2 capture, it was found that bond type of sulfur heteroatoms plays a key role in determining their binding affinity with CO2 molecules, through combined investigations of experiments and simulations [57]. Figure 6(B) shows two different sulfur types were found in the resulting carbon frameworks (crosslinked for 30 min), including C–S–C and C–S–O forms, which are 59.6 and 40.4 at.% of the total sulfur content, respectively. Changing the sulfonation time can alter the bond formation of sulfur heteroatoms during the carbonization process. Specifically, the relative content of C–S–O first increases from 20.6 to 40.4 at.% by extending sulfonation reaction time from 15 to 30 min, which was then decreased to 33.4 and 8.9 at.% after 1 and 2 h of sulfonation, respectively (Figure S7). We note that through density function theory (DFT) simulation, it was found that C–S–O type bond can more favorably interact with CO2 molecules compared to its counterpart of C–S–C. The presence of sulfur in the carbon framework, and the micropores formed upon carbonization represent two desired features for synthesizing carbon-based CO2 capture sorbents using x-PE foams as the precursors.

Figure 6
figure 6

XPS survey scans of x-PE foam-derived porous carbon samples with different sulfonation times at 140 °C. (B) High-resolution S2p scan of XPS spectra of x-PE foam-derived carbon, which was crosslinked for 30 min. All samples were carbonized at 800 °C.

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The performance of x-PE foam-derived carbons for CO2 capture was investigated at 0 °C and room temperature (25 °C). Figure 7(A) and (B) demonstrates the CO2 uptake capacity of these porous carbon sorbents as a function of crosslinking time at 0.1 and 1 bar, respectively. The complete CO2 adsorption isotherms of respective carbon samples are included in Figure S8. Among all, 30 min of sulfonation resulted in the porous carbon with the highest CO2 uptake capacity, which was 1.9 mmol/g at 0.1 bar and 4.5 mmol/g at 1 bar (at 0 °C), and 1.1 mmol/g at 0.1 bar and 3.1 mmol/g at 1 bar (at 25 °C), respectively. This performance is comparable to our previous work using PP mask as the starting materials [37], while it is noteworthy mentioning that a much shorter crosslinking time was required for sulfonation-enabled crosslinking of x-PE foams using fuming sulfuric acid. As the crosslinking time is extended, the CO2 sorption capacity of these carbon materials decreases, which can be attributed to the reduced content of sulfur heteroatoms, since these samples have comparable surface areas. Particularly, the content of C–S–O binding type in derived carbons became significantly lower after extending the sulfonation time beyond 30 min, ultimately dropping to only 6.6 at.% for samples crosslinked for 2 h. This sample (sulfonation for 2 h) exhibits a room temperature CO2 uptake capacity of 0.9 mmol/g at 0.1 bar and 2.5 mmol/g at 1 bar. The optimized performance of the sample sulfonated for 30 min, using our reported approach, is comparable to other recent reports in literature [58,59,60,61], while this work employs non-recyclable x-PE foams as the feedstock. Furthermore, we investigated how carbonization temperature impacts their CO2 sorption performance, in which the sulfonated x-PE samples carbonized at 600 and 700 °C exhibit a CO2 sorption capacity of 1.6 to 2.4 mmol/g at room temperature and 1 bar, respectively, while a sharp drop to 0.9 mmol/g is observed when increasing carbonization temperature to 900 °C. Furthermore, practical application of carbon capture sorbents requires them to selectively and efficiently uptake CO2 from mixed gas streams/environments, which are very common in industries. Therefore, sorbent sorption selectivity of CO2 over N2 is an important performance feature. As shown in Fig. 7(C), it was found that a high selectivity of CO2 in comparison to N2 can be obtained, reaching up to 58 at 0 °C and 46 at 25 °C (determined through the use of Henry’s law constants). Notably, the carbon derived from x-PE foams crosslinked by fuming sulfuric acid for 30 min not only exhibits the highest sorption capacity for CO2 capture but also shows the highest selectivity among all samples studied in this work.

Figure 7
figure 7

CO2 adsorption capacity studies of carbonized PE foams with different crosslinking times at 140 °C at 0.1 and 1 bar, including at (A) 0 °C and (B) 25 °C. (C) CO2/N2 selectivity of these carbon materials at 0 and 25 °C.

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Figure 8 provides a brief schematic summary about how this reported work could make a transformative impact on the fields of plastic recycling and carbon capture. In general, plastic manufacturing is a carbon-intensive process, including key activities associated with petroleum extractions, refining, cracking, and product manufacturing. In 2019, plastic industry contributes to approximately 3.4% of global greenhouse gas emissions [62]. Toward development of a carbon–neutral society, it is necessary to design plastic recycling technologies that can also effectively address the large-scale emission challenges. By directly converting non-recyclable x-PE foams to porous carbon sorbents, our method could enable their use for CO2 capture, attributed to the combined advantages of relatively high surface areas and sulfur-doped carbon frameworks. Therefore, the use of non-recyclable plastic as the feedstock, paired with simple and scalable process to synthesize CO2 sorbents, renders our method to potentially provide a system-level solution for enabling a circular carbon economy.

Figure 8
figure 8

A schematic illustration of how catalyst-free upcycling of x-PE foams addressing CO2 emission challenges in plastic industry.

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Conclusion

In this work, we demonstrate a simple, two-step method for upcycling of x-PE foams into porous carbon sorbents by exposing the precursors to fuming acid at elevated temperatures for crosslinking, enabling their conversion to carbons with over 50 wt% yield upon pyrolysis. Due to the high porosity of x-PE foams, the reaction can be rapidly accomplished within 30 min, without requiring a pre-treatment step such as grinding. The resulting porous carbons exhibit several features that are favorable for sorption-related applications, including relatively high surface area and sulfur-doped carbon framework. Using these waste-derived carbons for CO2 capture, it is found that x-PE foam-derived carbon prepared under the optimized condition can uptake CO2 molecules as high as 4.5 mmol/g at 0 °C and 3.1 mmol/g at 25 °C, both at 1 bar, while exhibiting a high CO2/N2 gas selectivity up to 58. Together, this work demonstrates an important concept and a simple and practically viable method for employing non-recyclable plastic waste as feedstock materials to fabricate environmental sorbents for addressing grand-scale carbon emission challenge.

Methods

PE foam crosslinking and carbonization

Crosslinked polyethylene (x-PE) foams (purchased from the Foam Factory) were first cut into pieces (~ 0.2 g per piece) and placed in a 50 mL glass beaker, where they were exposed to 20 mL of fuming sulfuric acid, containing 18–24% of free SO3 groups (obtained from Thermo Fisher). These materials were then heated at 140 °C on a thermal plate with varied reaction time. Subsequently, samples were carefully collected from the beaker and washed with deionized water (obtained using a Milli-Q IQ 7003 ultrapure lab water purification system) for at least three times. The sulfonation-crosslinked x-PE foams were then dried under vacuum at 140 °C for overnight. For carbonization, samples were placed in an electric tube furnace (MTI Corporation OTF-1200x) using a heating ramp of 5 °C /min to desired carbonization temperatures (600–900 °C) under N2 atmosphere.

Sample characterization

Changes in the chemical composition of x-PE foams after different sulfonation times were determined using a PerkinElmer Frontier attenuated total reflection Fourier transform infrared (FTIR) spectrometer, with a scan range of 4000–600 cm−1 running over 32 scans in average at a resolution of 4 cm−1. Micrographs of x-PE foam samples after sulfonation and carbonization processes were obtained using a Zeiss Ultra 60 field-emission scanning electron microscope (SEM). A Micromeritics Tristar II instrument was used to determine the pore textures of x-PE foam-derived carbons, including their adsorption and desorption isotherms under N2 at 77 K. Surface area was determined using Brunauer–Emmett–Teller (BET) analysis, while the pore size distribution was calculated using the non-local density function theory (NLDFT) model. Additionally, this instrument was also utilized to determine the CO2 and N2 uptake performance of x-PE foam-derived carbons at 0 and 25 °C by measuring their respective sorption isotherms from 0.01 to 1 bar. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Fisher ESCALAB Xi+ spectrometer, equipped with a monochromatic Al X-ray source (1486.6 eV) and a MAGCIS Ar+/Arn+ gas cluster ion sputter gun. Binding energies were calibrated in respect to the peak of C1s, which is at 284.8 eV. Thermo Avantage analysis software was used to analyze high-resolution XPS spectra for understanding the heteroatom type and content of x-PE foam-derived carbon samples. Thermogravimetric analysis (TGA) was conducted using a Discovery Series TGA 550 (TA Instruments) to determine the carbon yield of polymer precursors. Differential scanning calorimetry (DSC) was performed utilizing a Discover Series DSC 250 (TA Instruments) in order to determine the melting temperature of polymer precursors as well as the change in crystallinity as a function of sulfonation reaction time. The reference enthalpy of PE is 293 J/g. Mass gain (from sulfonation reaction) was determined by comparing the mass of polymer precursors to the sulfonation-crosslinked samples, and product yield was calculated by comparing the initial mass of the polymer precursors to the final mass of carbons.

The conversion process from sulfonated x-PE foams to carbons was investigated through temperature-programmed carbonization (TPC). Specifically, 10 mg of sample was mixed with 1 g silica sand and loaded into a quartz reactor with a volume of 2 ml (i.d. Φ = 1/2″). The sample was first heated in Argon (Ar, 30 ml/min) to 120 °C for 1 h to remove the moisture and impurities. Subsequently, temperature of the reactor was increased at a constant ramp (10 °C/min) to 650 °C under the same Ar flow. The reactor outlet was passed through an ice-cooled gas/liquid separator before the gas effluent enters the online mass spectrometry (Agilent 5973 MS equipped with MS Sensor 2.0 software, Diablo Analytical, Inc.). Critical mass-to-charge ratios, such as m/z = 44 (for CO2), m/z = 64 (for SO2), and various other m/z ratios for olefins, paraffins, and aromatics were recorded during the TPC. Additionally, the liquid sample collected from the gas/liquid separator was diluted with 1 mL of dichloromethane (Sigma-Aldrich, 99.9%), which was then analyzed on a GCMS-QP2010S (SHIMADZU) equipped with a Rxi-5 ms (30 m, 0.25 mmlD, 0.25 μm df) column. The injection temperature is 250 °C, and the oven rises from 40 °C to 250 °C at a ramp of 10 °C/min.