Enhancement of light hydrocarbon production from polypropylene waste by HZSM-11-catalyzed pyrolysis

Abstract

Herein, a mixture of real polypropylene (PP) waste was pyrolyzed with a HZSM-11 catalyst as a potential method to recover light hydrocarbons (C _≤ 12), the potential feedstock for value-added chemicals and fuels, from polyolefin plastic waste. Using the HZSM-11 in the PP waste mixture pyrolysis noticeably improved the yield of gas pyrolysate and oil in compensation for the yield of wax (i.e. hydrocarbons of C _> 20) and solid residue particularly at a higher temperature. In addition, the selectivity of C₃–C₁₂ in the PP-waste mixture-derived pyrolysate was markedly increased by the HZSM-11. The highest yield of light hydrocarbons was ≈40 wt% (per mass of the feedstock) achieved at 700 °C with the HZSM-11 catalyst. Despite 7.9 wt% coke deposition on the HZSM-11 after its use in the pyrolysis of the PP waste mixture, the catalyst could be reusable for at least three times after regeneration. The experimental results demonstrate that HZSM-11 has the potential for being a promising catalyst to valorize polyolefin waste into value-added chemicals.

Keywords

Plastic waste waste treatment wax reduction gasoline jet fuel zeolite

Introduction

Polypropylene (PP) has strong popularity due to its lower density than other commodity plastics.¹ The 2022 global PP market size was $123.5 billion, which growth from 2023 to 2030 is expected.² Note that 1.2 gallons of crude oil are used to make petrochemical feedstocks (e.g. for plastic production) per one barrel of crude oil.³ The expectation of the PP market growth is associated with gradually increasing PP consumption in various end-use industries (e.g. building and construction, automotive, and packaging). The ever-growing consumption of plastics (e.g. PP) inevitably generates tremendous amounts of plastic waste, causing world plastic pollution crisis.

More than half of plastic waste have gone straight to landfills, and only ≈9% of plastic waste could be recycled.⁴ Plastic waste can cause slow-but-certain havoc throughout the environment in multiple ways. For example, leaching toxic chemicals from plastic waste into the soil and groundwater directly chokes or poisons animals who unwittingly ingest it. Moreover, approximately 1% of plastic waste ultimately ends up in the ocean, where it creates even larger problems.⁵ Plastic waste in the ocean can injure sea animals outright and is frequently—and fatally—mistaken for food by others. Microplastics pose hazards to ecological systems, and their presence, in particular in aquatic environments, has adverse impacts on the ecosystem and ultimately human health.⁶

Proper recycling of plastic waste (e.g. PP waste) has thus become more and more essential. Plastics like PP originate from high calorific petroleum-derived hydrocarbons; hence, they can be valuable feedstocks to produce alternative fuels and chemicals.⁷ Therefore, the recovery of value-added chemicals and fuels from plastic waste has gained great interest as a new class of plastic waste upcycling strategy. Among various petroleum-derived products, light hydrocarbons (typically C _≤ 12) are derived from various refinery reactors such as atmospheric distillation and gas plants. The light hydrocarbon streams are used to produce various commodity chemicals or utilized for fuel applications. For example, C₅–C₁₂ hydrocarbons are categorized as gasoline, while C₈–C₁₂ alkanes and aromatics are classified as jet fuel.⁸ Developing alternatives to petroleum-derived hydrocarbon products has been prompted by constantly tightening environmental regulations. From the above two points of view, the recovery of light hydrocarbons from plastic waste is regarded as a promising option. As PP is composed of multiples of propylene (a simple light hydrocarbon), PP waste should be a suitable source of light hydrocarbon streams.⁹

A range of chemicals alternative to fossil-based chemicals have been made from plastic waste by catalytic pyrolysis processes. For instance, pyrolysis of polyamide waste (e.g. abandoned fishing nets and nylon textile waste) on solid base catalysts^10,11 and supported metal catalysts¹² produced caprolactam with high yields of >70 wt%. When bioplastic waste (e.g. biodegradable straw waste^13,14 and used mulch film¹⁵) was pyrolyzed in the presence of solid base catalysts, lactic acid was obtained with the yield of up to 15.2 wt%. Pyrolysis has also shown its effectiveness of the treatment of polyester waste in terms of energy recovery¹⁶ and monomeric chemical production.¹⁷ Moreover, pyrolysis process was found to be effective at recovering hydrocarbons (e.g. olefins) from plastic waste.¹⁸

Various catalysts have been used in plastic waste pyrolysis to lower required process temperature and optimize the pyrolytic product distribution.¹⁹ Among them, zeolite catalysts have the ability to separate chemicals by their molecular sizes, thereby increasing target product selectivity.²⁰ Zeolite catalysts also have large surface area, high thermal stability, and controllable acidity.²¹ Among various zeolites, ZSM-5 and ZSM-11 are typical zeolites acting as solid acid catalysts that can be applied to fuel upgrading and petrochemical production.^22–24 Although both zeolites have the similarity of pore size and framework density, their topologies are different: ZSM-5 having both intersectional straight and sinusoidal channels and ZSM-11 having intersectional straight channels only.²⁵ Also, the diffusion resistance of ZSM-5 is higher than that of ZSM-11.²⁶ ZSM-11 has shown better catalytic performance than ZSM-5 for different reactions including aromatization,²⁷ catalytic cracking,²⁸ isomerization,²⁷ and methanol-to-hydrocarbons.²⁹ It has been demonstrated how ZSM-11 works in pyrolysis of various substances. For example, a hierarchical ZSM-11 catalyst led to high light olefin selectivity and low coke yield in pyrolysis of heavy oil.³⁰ The use of hierarchical ZSM-11 led to the enhancement of producing hydrocarbon-rich bio-oil from maize straw biomass.³¹ For pyrolysis of waste teabags, an H-form of ZSM-11 (HZSM-11) produced the pyrolytic products having higher calorific values.³²

Despite many advantages, however, HZSM-11 zeolite is still undervalued as a catalyst for pyrolysis processes. Particularly, HZSM-11 catalysts have hardly ever been used in pyrolysis of plastic waste, so its importance in the recovery of value-added hydrocarbon compounds from plastic waste is still unknown. This study has aimed to recover light hydrocarbons from real PP waste by catalytic pyrolysis on a HZSM-11 catalyst. The effects of the HZSM-11 catalyst on the yields and product distributions of the PP waste-derived pyrolysates were explored. The results of this study have demonstrated that HZSM-11 is a catalyst applicable to the valorization of plastic waste as a sustainable source of value-added light hydrocarbons.

Experimental

Polypropylene waste feedstock

Toothpaste caps, wet wipes pack caps, and cosmetic packaging products were collected, used as PP waste feedstocks. After cleaning and drying, they were shredded to the size of 1 mm × 1 mm and then thoroughly mixed. The PP waste mixture was used as the feedstock for the pyrolysis experiment.

Composition of the PP waste feedstock was determined as follows. A sample was dried in an open batch at 105 °C for 1 d. For measuring the volatile matter content, the sample was heated in a closed batch at 450 °C for 1 h. For measuring the ash content, the sample was heated in an open batch at 750 °C for 1 h. The fixed carbon content was calculated by taking away the contents of above three substances from the initial sample weight.

The degree of thermal degradation of the PP waste feedstock was determined using a thermogravimetric analyzer (Discovery TGA 55, TA Instruments) at 25–900 °C (10 °C min⁻¹) in N₂. The sample of ≈5 mg was used for the TGA.

The infrared spectrum of absorption of the PP waste feedstock was obtained using a Fourier transform infrared spectrometer (IRTracer-100, Shimadzu) to ensure the feedstock consisting of PP and detect any impurities.

Catalyst characterization

In order to obtain H-form zeolite (i.e. HZSM-11), calcination of a ZSM-11 zeolite (Vision Chemicals) was carried out at 550 °C for 3 h. A series of techniques were used to characterize the catalyst as follows.

A physisorption instrument (ASAP 2020, Micromeritics) was employed to obtain N₂ physisorption isotherm of the HZSM-11 at −196 °C. Right before the physisorption, a sample was degassed under vacuum at two temperatures: 90 °C (0.5 h) and 148 °C (4 h). Note that the t-plot method was applied to determining external surface area.

A temperature-programmed desorption (TPD) instrument (Autochem II 2920, Micromeritics) was used to measure the HSZM-11's surface acidity using ammonia as an adsorbate (i.e. NH₃-TPD). Right before the NH₃-TPD, a sample was degassed under He flow (50 mL min⁻¹) at 500 °C for 1 h. The degassed sample was then saturated at 150 °C with 15 vol% NH₃ (balance He) for 0.5 h, followed by purging at 150 °C under He (50 mL min⁻¹) for 1.2 h. The amount of desorbed NH₃ was measured from 150 °C to 500 °C (5 °C min⁻¹). The NH₃-TPD results were used to quantify acid amount of the catalyst.

The bulk crystalline structure of the HZSM-11 was characterized using an X-ray diffraction (XRD) instrument (Ultima IV, Rigaku) that uses copper K-α.

The amount of coke formed on the used HZSM-11 catalyst (≈5 mg) was measured by TGA from 30 °C to 900 °C (10 °C min⁻¹) under air flow (100 mL min⁻¹) using the thermogravimetric analyzer (same model used for the PP waste feedstock characterization (Section “Polypropylene waste feedstock”).

Pyrolysis experiment

A quartz tube in which the PP waste mixture was loaded was heated by a tube furnace. The feedstock sample (≈1 g) was loaded on the center of heat source. For catalytic pyrolysis, the HZSM-11 catalyst (10 wt% of the feedstock) was used. Prior to pyrolysis, any air and other impurity gases were removed by flowing N₂ (100 mL min⁻¹) for 15 min. The N₂ gas flow (100, 200, or 300 mL min⁻¹) was then kept flowing until the pyrolysis was completed to ensure inert atmosphere during the pyrolysis. Pyrolysis experiments were performed at 500 °C, 600 °C, and 700 °C.

Condensable species were collected in a series of condensers (an ice bath at −1 °C and four acetone/dry ice baths at −55 °C each). The condensable compounds were analyzed using a GC/MS (GC: 7890A; MS: 5975C, Agilent Technologies) equipped with a DB-5MS column. For in-situ quantification of non-condensable species, a micro GC (Fusion Gas Analyzer, INFICON) was used.

Results and discussion

Polypropylene waste feedstock characteristics

Thermochemical composition analysis on dry basis results showed that the PP waste mixture is composed of 98.3 wt% volatile matter and 1.7 wt% fixed carbon and ash. The infrared spectrum and peak assignments for the PP waste mixture are shown in Figure 1(a). The CH₃ stretching peaks at 2956 cm⁻¹ and 2875 cm⁻¹, CH₂ stretching peaks at 2921 cm⁻¹ and 2840 cm⁻¹, CH₃ symmetrical bending peaks at 1457 cm⁻¹ and 1377 cm⁻¹, C–C stretching peaks at 1166 cm⁻¹, 973 cm⁻¹, and 808 cm⁻¹, and C–H rocking peak at 840 cm⁻¹.^33,34 The infrared spectrum in Figure 1(a) is very similar to the characteristic peaks of PP,^35,36 confirming that the waste mixture is made of PP. As shown in Figure 1(b), thermal degradation of the PP waste mixture under N₂ atmosphere exhibited a single weight loss of ≈98 wt% between 320 °C and 480 °C most likely associated with the evolution of volatile matter. After the thermal degradation until 900 °C, approximately 2 wt% of the feedstock sample remained, consistent with the thermochemical composition analysis result. It is worth mentioning that the thermal degradation of the PP waste mixture obtained by TGA only reflects its physical decomposition. Pyrolysis temperature greatly affects the composition of the PP-waste mixture-derived pyrolysates in a lot more susceptible manner.^37,38

Figure 1.

(a) Infrared spectrum and peak assignments and (b) thermogram (TG) and differential thermogram (DTG) for the PP waste mixture. PP: polypropylene.

Characteristics of HZSM-11 catalyst

The physicochemical properties of the fresh HZSM-11 and the used HZSM-11 (i.e. the HZSM-11 catalyst collected after the PP waste mixture pyrolysis) are summarized in Table 1. The fresh catalyst showed type I isotherm having H4 hysteresis loop (Figure 2(a)), which indicates co-existence of mesopores with micropores. However, the HZSM-11's BET surface area decreased by 90% after being used in the PP waste mixture pyrolysis (Table 1). Given no structural degradation of the HZSM-11 after the pyrolysis (Figure 2(c)), the loss of micropores was mainly due to coke formation. The HZSM-11's surface acidity decreased by the surface coke formation (Table 1 and Figure 2(b)). In the TPD profile, the fresh HZSM-11 exhibited two peaks at ≈220 °C (ascribed to weakly adsorbed NH₃) and ≈350 °C (ascribed to strongly adsorbed NH₃).³⁹ The HZSM-11 catalyst lost its surface acidity by 60% after the pyrolysis, most likely due to the blockage of micropores by coke deposition.

Figure 2.

(a) N₂ adsorption–desorption isotherms, (b) NH₃-TPD profiles, and (c) XRD patterns for the fresh and used HZSM-11 catalysts. (d) Thermogram (TG) and differential thermogram (DTG) profiles for the used HZSM-11. TPD: temperature-programmed desorption; XRD: X-ray diffraction.

Table 1.

The difference in physicochemical properties of the HZSM-11 before and after use.

Catalyst	BET surface area (m² g⁻¹)	External surface area^a (m² g⁻¹)	Micropore volume^a (cm³ g⁻¹)	Acid amount (µmol_NH3 g⁻¹)	Coke deposition (wt%)
Catalyst	BET surface area (m² g⁻¹)	External surface area^a (m² g⁻¹)	Micropore volume^a (cm³ g⁻¹)	Acid amount (µmol_NH3 g⁻¹)	Soft coke^a	Hard coke^a	Total
HZSM-11 Fresh	420	72	0.15	270	-	-	-
HZSM-11 Used	34	24	0.004	120	2.4	5.5	7.9

Soft and hard cokes were determined by the DTG profile shown in Figure 2(d).

The results for XRD analysis of the fresh and used HZSM-11 catalysts are shown in Figure 2(c). Both samples exhibited sharp diffraction peaks at 2θ = 23°, 24°, 30°, and 45° that agree with aluminum hydrogen silicate. The XRD spectra well support that hydrogen atom exchange is well achieved by the calcination step (Section “Catalyst characterization”). Although the color of the used HZSM-11 catalyst turned into dark grey (due to carbon deposition), a carbon diffraction peak was not observed. However, the used catalyst showed slight baseline lifting around 2θ = 20–30° which is a distinct characteristic of amorphous structure. In addition, minor XRD peaks of aluminum hydrogen silicate turned into smaller and wider peaks. This indicates that the coke deposited on the catalyst surface during the pyrolysis has an amorphous structure.

The carbon deposition on the used HZSM-11 catalyst is further characterized by TGA. Figure 2(d) shows the TG and DTG profiles for the used HZSM-11 catalyst, obtained by the TGA to characterize thermal stability of coke formed on the catalyst surface and its quantitative amount. As found in Figure 2(d), coke removal occurred between 200 °C and 380 °C and rest of carbon species were entirely removed at temperature around 500 °C. The coke removed at temperatures lower than 350 °C imply the predominant formation of hydrogen-rich deposits (i.e. soft coke). Hard coke may originate from stable aromatic hydrocarbons, decomposed between 400 °C and 600 °C. Hard coke is relatively difficult to be thermally decomposed compared to soft coke.⁴⁰ The carbon deposition on the HZSM-11 catalyst was 7.9 wt% with the hard/soft cokes ratio of 2.3 (Table 1).

HZSM-11-catalyzed pyrolysis of polypropylene waste mixture

To explore the effect of HZSM-11 on the PP-waste mixture-derived pyrolysates, non-catalytic pyrolysis and catalytic pyrolysis were carried out, which compared the yields and product distributions of the pyrolysates. UHP N₂ was kept flowing during the pyrolysis to sustain an inert atmosphere. The N₂ flow rate was varied between 100 mL min⁻¹ and 300 mL min⁻¹ to investigate its impact on the compositions of the PP-waste mixture-derived pyrolysates. No change in the pyrolysate compositions was observed; thus, 100 mL min⁻¹ was applied to further tests.

Mass balances of the pyrolysates produced from the PP waste mixture with and without the HZSM-11 catalyst at 500–700 °C are shown in Figure 3. The yield of pyrolysis oil was always highest in both non-catalytic and catalytic pyrolysis at all tested temperatures. The highest oil yield was 80.9 wt% achieved in the catalytic pyrolysis at 700 °C. Approximately 2 wt% of the feedstock remained after its TGA (Figure 1(b)); thus, the residue of ≥2 wt% after the pyrolysis was observed in all experiments (Figure 3). In non-catalytic pyrolysis, wax (i.e. heavy hydrocarbons (C _> 20)) was formed at all tested temperatures with the yields of >20 wt%. The HZSM-11 catalyst greatly increased the yields of gas pyrolysate and pyrolysis oil in compensation for the yield of wax. This means that the HZSM-11 promotes thermal cracking of heavy compounds evolved from the PP waste mixture. In addition, increasing the pyrolysis temperature enhanced the production of the gas pyrolysate and pyrolysis oil and reduced the formation of wax and solid residue in the catalytic pyrolysis of the PP waste mixture. This is because thermal cracking of pyrolysis vapors that occurs during the pyrolysis may favor at higher temperatures due to promoted gas–solid heterogeneous reactions and gaseous homogeneous reactions.^41,42

Figure 3.

Overall mass balance of pyrolysates made from the PP waste mixture with and without the HZSM-11 at 500–700 °C. PP: polypropylene.

The gas pyrolysate is a mixture of hydrogen gas and non-condensable hydrocarbons (i.e. C _< 4). Specific selectivities of the PP-waste mixture-derived gas pyrolysates obtained at different temperatures are shown in Figure 4. In non-catalytic pyrolysis of the PP waste mixture, the selectivity of non-condensable gases was not markedly affected by varying the pyrolysis temperature. The C₃ content, in particular propylene (C₃H₆), was noticeably increased by the HZSM-11 compared with the non-catalytic pyrolysis (e.g. 400% more propylene production at 700 °C). The enhancement of propylene production by the HZSM-11 is most likely because zeolites promote dehydrogenation of alkanes (e.g. propane) to alkenes (e.g. propylene) through monomolecular and protolytic pathways.⁴³

Figure 4.

Specific selectivity of gas pyrolysates made from the PP waste mixture with and without the HZSM-11 at 500–700 °C. PP: polypropylene.

Table 2 shows that a range of hydrocarbons (e.g. C₄–C₂₀) were found in the pyrolysis oil derived from the PP waste mixture. The use of the HZSM-11 catalyst increased the selectivity of C₄–C₁₂ hydrocarbons at 500–700 °C compared with the non-catalytic pyrolysis. For example, the pyrolysis of the PP waste mixture over the HZSM-11 led to >97% selectivities of C₄–C₁₂ hydrocarbons at 500–700 °C, while the non-catalytic pyrolysis led to <86% selectivities of C₄–C₁₂ hydrocarbons. In particular, the selectivity of C _≤ 10 hydrocarbons obtained with the HZSM-11 catalyst was higher than 95%, but that obtained without any catalyst was less than 67%. This is an indication of the H-ZSM-11 catalyst affecting thermal cracking of higher-molecular weight hydrocarbons into lower-molecular weight hydrocarbons.

Table 2.

Specific hydrocarbon selectivity (%) in the oil made from the PP waste mixture with and without the HZSM-11 at 500–700 °C.

Carbon number	Non-catalytic			Catalytic
Carbon number	500 °C	600 °C	700 °C	500 °C	600 °C	700 °C
C₄	3.7	3.6	3.7	29.5	29.0	29.5
C₅	8.4	8.4	8.4	33.3	33.5	33.3
C₆	5.7	5.7	5.4	19.2	19.3	19.2
C₇	4.7	4.7	4.7	4.7	4.7	4.6
C₈	3.4	3.4	3.4	2.0	2.0	2.0
C₉	33.7	33.6	33.7	5.6	5.6	5.6
C₁₀	6.6	6.7	6.7	1.1	1.1	1.0
C₁₁	5.4	5.4	5.4	0.9	0.9	0.9
C₁₂	13.9	13.9	14.0	1.1	1.1	1.1
C₁₄	0.8	0.8	0.8	0.5	0.5	0.5
C₁₅	1.5	1.5	1.5	0.3	0.3	0.3
C₁₇	2.0	2.0	2.0	0.9	1.0	1.0
C₁₈	2.7	2.8	2.8	0.2	0.2	0.2
C₁₉	1.8	1.8	1.8	0.4	0.4	0.4
C₂₀	5.7	5.7	5.8	0.3	0.3	0.3

PP: polypropylene.

The light hydrocarbons (C _≤ 12) identified in the PP-waste mixture-derived pyrolysates (Table 2) could be produced mainly via mid-chain β-scission and intramolecular hydrogen transfer.⁴⁴ As shown in Figure 5(a) and (b), intramolecular hydrogen transfer is key to form hydrocarbons which molecular weights are lower than PP (e.g. C₆–C₁₂). The backbiting reactions associated with 1,3-, 1,4-, 1,5-, and 1,6-end-hydrogen transfer (Figure 5(a)) originate from an end-chain radical, energetically more probable than the backbiting reactions occurring at the 7^th (or greater) C positions.⁴⁵ The 1,3-, 1,4-, and 1,5-end-hydrogen transfers lead to the formation of mid-chain radicals that further experience additional hydrogen transfer. This results in forming specific radicals on the 7^th–9^th C positions.⁴⁶ Moreover, the end-hydrogen transfer occurs followed by 1,3-, 1,4-, and 1,5-mid-hydrogen transfer (Figure 5(b)). The 1,3-, 1,4-, and 1,5-end-hydrogen transfers tend to occur faster than the 1,3-, 1,4-, and 1,5-mid-hydrogen transfers owing to entropic factors. Figure 5(c) describes mid-chain β-scission reactions. Saturated low-molecular weight radicals and unsaturated oligomers are produced by the β-scission of mid-chain radicals at 2^nd–9^th carbons from a chain end. The specific mid-chain radicals near unsaturated chain ends undergo β-scission, leading to the formation of dimeric/trimeric radicals and allylic radicals/dienes.⁴⁷ As a result, light hydrocarbons (C _≤ 12) can result mainly from the three mechanisms.

Figure 5.

Mechanisms associated with light hydrocarbon (C _≤ 12) production in pyrolysis of polypropylene.

Figure 6 shows the yields of light hydrocarbons (C _≤ 12) produced from the PP waste mixture through non-catalytic and catalytic pyrolysis over the HZSM-11 at 500–700 °C. Among the light hydrocarbons, the yield of C₉ and C₁₂ was higher (e.g. the C₉ yield of 10.2 wt% at 700 °C) in non-catalytic pyrolysis of the PP waste mixture, while the yield of C₃, C₄, and C₅ was higher (e.g. the C₃ yield of 14.6 wt% at 700 °C) in the catalytic PP waste mixture pyrolysis. In the catalytic pyrolysis, an increase in the pyrolysis temperature increased the overall yield of light hydrocarbons (in particular C₃ hydrocarbons). In association with the light hydrocarbon production mechanisms (Figure 5), the HZSM-11 may expedite the reactions associated with mid-chain β-scission and intramolecular hydrogen transfer on the PP polymeric bond during the pyrolysis of the PP waste mixture between 500 °C and 700 °C.

Figure 6.

Yield of light hydrocarbons made from the PP waste mixture with and without the HZSM-11 at 500–700 °C. PP: polypropylene.

Reusability of the HZSM-11 catalyst was assessed as follows. For the regeneration of the used HZSM-11 catalyst, it was calcined at 500 °C (10 °C min⁻¹) for 3 h under air flow (100 mL min⁻¹) to remove the carbon deposited on the catalyst surface (Section “Characteristics of HZSM-11 catalyst”). As shown in Figure 7, the overall mass balance of the PP-waste mixture-derived pyrolysates was not noticeably changed with the reuse cycle. This is most likely because the deposited carbon species are removed by oxidation in the catalyst regeneration. This means that the HZSM-11 is reusable with proper regeneration at least three consecutive cycles in pyrolysis of plastic waste such as PP.

Figure 7.

Overall mass balance of pyrolysates produced by catalytic pyrolysis of the PP waste at 700 °C with the HZSM-11 catalyst used for three consecutive cycles. PP: polypropylene.

Conclusions

In this work, catalytic pyrolysis of a mixture of real PP waste (e.g. toothpaste caps, wet wipes pack caps, and cosmetic packaging products) from 500 °C to 700 °C over HZSM-11 was conducted. The flow rate of N₂ gas (to sustain an inert atmosphere during pyrolysis) did not noticeably affect the yield and selectivity of pyrolysates. The HZSM-11 catalyst markedly decreased the formation of wax (heavy hydrocarbons of C _> 20) and enhanced the production of light hydrocarbons (C _≤ 12) especially at higher temperatures. In particular, the yield of C₃–C₅ dramatically increased by using the HZSM-11 catalyst. The enhanced production of light hydrocarbons is most likely due to the HZSM-11 catalyst not only promoting thermal decomposition of heavy vaporized species evolved during the pyrolysis but also expediting intramolecular hydrogen transfer and mid-chain β-scission reactions of polymeric bonds present on the PP waste mixture. After use in the pyrolysis, 7.9% coke deposition was observed on the HZSM-11 catalyst. The catalyst reusability test at 700 °C indicated no marked change in the yield of pyrolysates for three consecutive cycles. It is hoped that this study contributes to expanding the application of relatively less used catalytic materials (e.g. HZSM-11) to waste upcycling process.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Research Foundation of Korea (grant no. RS-2023-00209044). This work was also supported by the Carbon Neutral Industrial Strategic Technology Development Program (No. RS-2023-00261088) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

Heesue Lee has been received her master's degree at the Department of Global Smart City, Sungkyunkwan University, Suwon, South Korea. Her research interest includes value-added product production from renewable resources.

Sam Yeol Lim is a master's degree student at present in the Department of Global Smart City, Sungkyunkwan University, Suwon, South Korea. His research interest includes monomer recovery from waste materials in an eco-friendly way.

Shuting Fu is a master's degree student at present in the Department of Global Smart City, Sungkyunkwan University, Suwon, South Korea. Her research interest includes environmentally-friendly chemical recycling of waste.

.Yong Tae Kim received his PhD in Chemical Engineering in 2011 from Ajou University with Professor Eun Duck Park. After completing postdoctoral research in 2014 at University of Wisconsin-Madison, he is currently a principal researcher at Korea Research Institute of Chemical Technology and an associate professor at University of Science & Technology. His research interests are heterogeneous catalysis and reaction engineering of C₁ (CO, CO₂, and CH₄) and biomass conversion for hydrogen, fuel, and chemical production.

Jechan Lee is received his PhD in chemical engineering from University of Wisconsin-Madison in 2015 and MS in environmental engineering from Columbia University in 2010. After his PhD, he worked as a postdoctoral researcher at the Catalysis Center for Energy Innovation at the University of Delaware (2015-2016) and Sejong University (2016-2018). He was a faculty member of Ajou University from 2018 to 2022. He is now an associate professor in Sungkyunkwan University (SKKU), South Korea. His research interests are in the areas of waste upcycling, waste-to-energy, biorefinery, and green chemistry. He has currently authored more than 270 peer-reviewed SCI(E) papers. Furthermore, he is a recipient of Highly Cited Researcher (HCR) by Clarivate in 2022-2023.

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