Abandoned fishing net and clamshell waste (CSW) were obtained from a harbor located in the city of Buan, Republic of Korea. The obtained plastic net was washed to remove soil, debris, and salt, after which it was soaked in water for 24 h and then dried in a drying oven at 60 ºC for 48 h. Dichloromethane (DCM; 99.9% purity) supplied by Samchun chemical (Seoul, Republic of Korea) was used as the solvent. 5-methlyfurfural (5MF; 99% purity) was purchased from Sigma–Aldrich (St. Louis, MO, USA), and N₂ gas was purchased from DK gas (Hwaseong, Republic of Korea).

CSW catalyst was prepared in this study using the following procedure. First, the purchased CSW was cleaned using water to remove surface impurities, after which it was dried at 60 ºC for 24 h. Subsequently, the dried shells were broken using a mortar and pestle, and then ground into a fine powder using a pulverizer (New Korea Metal Company, Republic of Korea). Thereafter, the powders were separated based on their particle size using a sieve shaker (SS-D-S, Woo Ju Scientific, Republic of Korea) to ensure that the small particles (>600 mm) of CSW powders were obtained. Subsequently, the powders were placed in a tube furnace for carbonization at 400 ºC for 2 h under flowing N₂ gas at 100 mL min⁻¹. Thereafter, the powders were mixed with KOH solution (6 M) at a ratio of 1:3, and then dried at 60 ºC for 48 h. The dried powders were heated again using a tube furnace from 400 to 700 ºC with a holding time of 30 min for 1 h at a heating rate of 3 ºC min⁻¹ under N₂ gas. Lastly, the obtained activated materials were mixed with HCl solution (2 M) in a water bath at 90 ºC to purify and remove the metal-oxide produced during the carbonization process. The solution containing the powder was washed with deionized water several times until the pH of the solution was neutral, after which they were dried at 60 ºC.

Proximate analysis of the wasted fishing net was performed using a batch furnace. First, the moisture content was measured by placing the fishing net on an uncovered crucible in the furnace at 105 ºC for 24 h. Subsequently, the volatile matter content of the feedstock was determined by heating the covered crucible at 450 ºC for 1 h. Thereafter, the ash content was estimated when the open crucible was heated at 750 ºC for 1 h. Lastly, the fixed matter content was calculated by subtracting the moisture, volatile matter, and ash contents from the original mass.

The ultimate analysis of the elemental compositions of the fishing net was performed using a Thermo Scientific FlashSmart 2000 elemental analyzer. The C, H, N, and S contents were calculated using a copper wire and tungstic anhydride catalysts at 1000 ºC. The content of O was estimated by calculating the difference between the initial mass and the sum of ash, C, N, H, and S contents. Table 1 shows the result of proximate and ultimate analysis.

Three types of pyrolysis of fishing net (i.e., without catalyst, ex-situ, and in-situ) were conducted using a tube furnace (Tube furnace-60, Hantech, Republic of Korea). Figure S1 shows the scheme of the pyrolysis process. The feedstock was placed in a quartz tube located at the center of the heating zone with a gas mask. For the ex-situ experiment, the catalyst was loaded next to the feedstock between quartz wools. In contrast, for the in-situ process, the catalyst was mixed with the fishing net, after which the mixture was placed between quartz wools. Mass flow controller (KOFLOC, Japan) was connected to a gas mask, through which N₂ flows into the quartz tube at 100 mL min⁻¹ to achieve an oxygen-free atmosphere. The pyrolysis temperature was controlled using the temperature controller attached to the tube furnace.

The pyrolytic oil produced during the pyrolysis process was collected using four inpingers. Because the oil was composed of volatile compounds, the inpingers were located in the cold region. The first inpinger contained 40 mL of DCM and it was placed in an ice bath where the temperature was maintained at −1 ºC. Next to the first inpinger, three inpingers were consecutive soaked in cold traps where the temperature was sustained at −55 ºC using a mixture of acetone and dry ice. To collect all the pyrolytic oil, the inpingers, quartz tube, and line were washed with DCM. The collected oil was dried at 60 ºC for 24 h to remove the DCM.

The pyrolytic oil produced during the pyrolysis process was analyzed using gas chromatography/mass spectrometry (GC/MS; Agilent 5975C, USA). The components of the pyrolytic oil were qualified and quantified using an Agilent HP-5ms column (30 m × 0.25μm × 0.25 mm). The detailed conditions, including the temperature, heating rate, and flow rate, are listed in Table S1. The quantitative analysis was conducted via internal standard method using Methylfurfural (5 μg mL⁻¹).

The pyrolytic gas was analyzed using micro GC (Inficon, Switzerland). In addition, H₂ and carbon monoxide (CO) gases were quantified using Rt-Molsieve 5A (0.25 mm × 10m); carbon dioxide (CO₂), methane (CH₄,), and hydrocarbons (from C1 to C4) were identified using the Rt-Q-Bond (0.25 mm × 8 m). The specific conditions are listed in Table S2.

Fig. 1 shows the change in the weight of the fishing net samples with a change in temperature under N₂ flow. Fig. 1 shows the TGA (thermogravimetric analysis) and DTG (Derivative Thermogravimetry) curves of the fishing net. The first weight loss of the fishing net was observed at 400 ºC, after which the weight rapidly decreased until 500 ºC. With an increase in the temperature to 900 ºC, more than 97 wt.% of the fishing net was thermally degraded, which could be attributed to the thermal decomposition of volatile matter (i.e., devolatilization occurred). However, approximately 3.0% of the fishing net was not thermally decomposed owing to the presence of fixed carbon. This result is very consistent with the proximate analysis result presented in Table 1: the fishing net sample consisted of fixed carbon (0.01 wt.%), volatile matter (97 wt.%), and moisture (2 wt.%) without ash. This indicates the high consistency of the TGA and proximate analysis results. Ultimate analysis also revealed that fishing net were composed of 63.06 wt.% carbon, 12.27 wt.% nitrogen, 10.28 wt.% hydrogen, and 15.35 wt.% oxygen

Fig. 2a shows the yield of non-condensable gas obtained from the pyrolysis of fishing net with and without catalyst with a change in temperature. The total yield of non-condensable gas increased with an increase in the pyrolysis temperature. For example, there was no significant difference in the total yield of gas at 500 and 700 ºC; however, with a further increase in temperature to 800 °C, the total yield from the ex-situ process dramatically increased from 7.7 to 29.3%, and increased to 37.2% with a further increase in temperature to 900 ºC. This could be attributed to the enhancement of the pyrolysis of volatile substances through gas and gas–solids reactions at high temperatures [40]. Further, the gas with the highest concentration obtained during the pyrolysis processes was CO₂ (Fig. 2b). Although there was no significant difference in the evolution of the gas species in the absence of a catalyst, the difference was enhanced when the catalyst was loaded. Particularly, the ex-situ loading method produced the highest quantity of CO₂ compared to the non-catalytic and in-situ process. This may be attributed to the generation of CO₂ by the decomposition of the CSW catalyst. The catalysts were prepared using CSW, which consists of calcium carbonate (CaCO₃). With an increase in temperature, CaCO₃ was decomposed into CO₂, which was enhanced in the presence of a catalyst. Accordingly, as more CSW was used during the ex-situ process compared to the in-situ reaction, more CO₂ was produced during the ex-situ reaction, which was evident at temperatures of 700 ºC or higher.

As shown in Fig. 3, the major gaseous products from the pyrolysis of fishing net were H₂, CO, CH₄, and C₂H₄. In addition, C₂H₆, C₃H₆, and C₃H₈ were observed in the product, but their concentration was not comparable to those of the four major gas (Fig. 2). Further, the concentration of H₂, CO, and CH₄ increased with an increase in temperature under all conditions; however, that of C₂H₄ decreased between 500 and 600 ºC, and then increased until 900 ºC. The H₂ produced from catalytic pyrolysis was higher than that produced during the non-catalytic pyrolysis at all temperatures, and the difference between the results was not negligible (Fig. 3a). However, at temperatures above 700 ºC, the difference in the H₂ yield increased significantly. This could be attributed to the fact that an increase in the pyrolysis temperature promoted the decomposition of vaporized species released from the feedstock during the pyrolysis process [41]. In addition, compared to the non-catalytic pyrolysis, the ex-situ pyrolysis method generated more H₂, but this was lower than that generated from the in-situ pyrolysis method. This indicates that the use of CSW catalyst for the pyrolysis of fishing net can increase the production of H₂ gas, and the in-situ method generated a higher quantity than the ex-situ method. The quantity of generated CO from the pyrolysis of fishing net is shown in Fig. 3b. Further, the use of CSW enhanced the generation of CO. It was expected that CO formation via reverse water-gas-shift reaction (rWGS, H₂ + CO₂ −> CO + H₂O) could be realized using the CO₂ produced from the CSW catalysts. Thus, the CO₂ produced by the calcination reaction of CSW catalyst (CaCO₃ −> CaO + CO₂ +183kJ/mol) was used as a source of the WGS reaction [42]. Compared to the aforementioned gases (Fig. 3c and 3d), there was no significant in the amount of CO₂ generated from non-catalytic and ex-situ catalytic pyrolysis with an increase in the pyrolysis temperature. In contrast, the quantity of gas generated by the in-situ method was higher than those produced by other methods. These results offer two messages. First, the effect of catalysts is evidence at temperatures above 700 ºC. Second, the in-situ method is more effective than the ex-situ method for producing combustible gas except CO.

The weight of condensable products produced by the fishing net was 48.5 to 88.8 wt.%, and the yield of liquid was highest at 500 and 600 ºC and was lowest at 900 ºC at all conditions. The condensable gas produced from the pyrolysis of fishing net was composed of 3-pyrrolidinopropionitrile, four kinds of amine compounds (azepan-2-one, 7-butyl-3,4,5,6(2H)-tetrahydroazepine, 2,3,4,5,6,7-hexahydro-2-octylimino-1H-azepine, and oleylamine), 4-hexyl-2,5-dihydro-2,5-dioxo-3-furanacetic acid, 13-heptadecyn-1-ol, and 1,8-diazacyclotetradecane-2,9-dione.

Fig. 4 shows the total yield of condensable products (Fig. 4a) and the distribution (Fig. 4b) of the condensable gas obtained from the fishing net with and without catalysts at various pyrolysis temperatures. The in-situ and non-catalytic pyrolysis exhibited similar trend, in which the concentration of condensable products decreased with increasing the pyrolysis temperature. However, there was a slight difference in the trend of the ex-situ pyrolysis compared to the other methods. With an increase in temperature from 500 to 600 ºC, the concentration of condensable products increased, and then decreased until 900 ºC. For example, the amount of condensable products generated during the non-catalytic pyrolysis and in-situ pyrolysis decreased from 45.1 to 33.7 wt.% and from 63.6% to 53.3 wt.%, respectively. In contrast, the concentration of condensable products produced during the ex-situ pyrolysis increased from 46.2 to 50.2 wt.%, and then decreased to 39.3 wt.%. Particularly, the quantity of condensable products generated by the in-situ pyrolysis was higher than those generated by other methods at all temperatures. This result implies that the mixture of solid particles and feed would exhibit an effect on heat and mass transfer [43], as well as increase the intimate contact of the feed with the catalyst and the change in the vapor residence time in the reactor [44]. This is consistent with the result of a recent study on the comparison of the in-situ and ex-situ co-catalytic pyrolysis of high-density polyethylene and torrefied yellow poplar, which revealed that in-situ catalytic co-pyrolysis exhibited higher performance than ex-situ catalytic co-pyrolysis [45]. The distribution of the products obtained from the Non, in-situ and ex-situ catalytic pyrolysis of fishing net at 500 ºC is shown at Figure 4b. Condensable products were composed of amide, amine, acid, alcohol, and ketone compounds. Particularly, amine compounds exhibited the highest proportion (amine compounds consisted of 95.5 wt.%% of the total products) at all conditions. However, the remained products were not produced as high as amine compound. Particularly, in in situ loading method, the proportion of amide, acid, alcohol, and ketone compounds in the condensable gas product was 0.6, 1.1, 0.2, and 3.7 wt.%, respectively. This could be attributed to the composition of fishing net: fishing nets are composed of polyamide, so the high-temperature depolymerization reaction enabled the extraction of the major compounds from the complete product.