The current study examined the carbon recycling application of waste materials. Thermal catalytic cracking reactions were carried out in a fixed bed to synthesize gasoline-range hydrocarbon fuels from used plastics. Titanium (IV) oxide (TiO2) and zeolite were tested as catalysts for pyrolysis using low-density polyethylene (LDPE), polyvinylchloride (PVC), and polystyrene (PS) reactants. In addition to the catalyzed pyrolysis reactions, we also investigated non-catalyzed thermal degradation of the plastic substrates for negative control. The liquid yield, reaction temperature profile, and physical appearance of the synthesized liquid products were determined. The pyrolysis reactions demonstrated that the optimum catalyst–polymer ratio is 40%. The distillate collection temperatures ranged between 82 and 198 °C (LDPE), 68–172 °C (PVC), and 40–168 °C (PS). Our experiments showed that LDPE, PVC, and PS can readily be pyrolyzed to produce 44% (LDPE), 13% (PVC), and 89% (PS) hydrocarbon liquid products using zeolite catalyst. Gas chromatography–mass spectrometry (GC–MS) was used to analyze the structure and chemical composition of the products. The main products were C5 (1,2-dimethylcyclopropane), C6 (2-methylpentane), C7 (1,3-dimethylcyclopentene, 1-heptene), and C8 (2-octene, 4-octene, octane, 3-ethylhexane), indicating gasoline-range hydrocarbon molecules. The highest liquid yield of 89.3% was obtained from zeolite catalyst over polystyrene in comparison to all plastics cracked while the lowest liquid yield of 3.9% was obtained from the cracking of PVC under no catalyst condition.
Plastics have become one of the largest products and the most profitable molecules of the chemical industry, fetching the top 50 global chemical companies more than $950 billion in annual revenues . Plastics are integrated into our daily lives as they are used in packaging, plumbing, furniture, and vehicle parts . Many plastics are resistant to heat and water, therefore, degrade slowly. The versatile and inexpensive nature of plastics, coupled with their non-biodegradability makes them exist for a very long period in the oxidative environment. Chemical companies produce an estimated amount of 8.3 billion tons of plastics and 6.3 billion tons become waste with a recycling efficiency of 9% .
The majority of the produced and used plastics are dumped into water bodies, polluting the environment, and posing a health threat to wildlife and human. It is predicted that by 2050, there will be more plastics than live fish in the ocean [3, 4]. Disposal of plastics through landfills and incineration are inefficient, expensive, and pollutant, therefore more environmentally friendly methods are required. Among the recent methods for treating contaminated wastes, pyrolysis showed a promising alternative approach. Pyrolysis is a cracking reaction of the plastic waste under high temperatures, in the range of 400–500 °C, and pressure into useful energy which can be in the forms of solid, liquid, or gas [5, 6]. Plastics are thermally degraded to produce useful liquid hydrocarbons which can be added into existing fuel, solvent product, or returned to a refinery where they can be added to the feedstocks . To improve the energy efficiency of pyrolysis processes, catalysts are introduced in the cracking reactions.
Catalytic pyrolysis involves the degradation of the polymeric materials by heating them in the absence of oxygen and the presence of a Lewis acid catalyst. The catalyst in pyrolysis affects specific parameters including reaction temperature, reaction time, gas formation, the solid residue, byproduct impurities, the aromaticity, paraffin formation, reactivity, and distribution of molecular weight . Catalytic pyrolysis is characterized by shorter reaction time, increased concentration of short to medium chain distillates, less solid product yield, removal of impurities via adsorption, and high formation of aromatics with better octane rating. Zeolites are hydrated crystalline aluminosilicate mineral widely utilized as a catalyst material . Zeolites are microporous, allowing ions to fit for the cracking reaction and are characterized by the Si/Al ratio which determines the acidity, making it versatile and suitable for a wide range of reactions [10, 11]. The high silica content of the zeolite gives the suitable property for the pyrolysis reactions. The high Si/Al ratio in zeolite means a high Lewis acidity, which is optimized for the polyolefins cracking . Even though zeolite catalysts can effectively convert plastic wastes to fuel products at a lower temperature, however, they produce more gas than liquid products .
Titanium dioxide (TiO2) is a naturally occurring metal oxide applicable in the catalyzed pyrolysis of waste plastics [14, 15]. Titanium dioxide is from ilmenite, rutile, and anatase which are natural minerals with titanium in its core . Due to its high refractive index, titanium dioxide can be used for pigment making to provide whiteness and high opacity to paint coatings, plastics, and paper. It can be used for thin-film production and UV blocking. Titanium dioxide is often used as heterogeneous catalysts to stabilize them. The metal oxide is highly efficient in redox reactions with low pressure and low temperatures. Besides, TiO2 has strong chemical stability, acid–base characteristics, as well as a high metal–support interaction, and all of these properties making it an efficient metal oxide catalyst .
Plastics are characterized by volatility and combustibility, making them adequate as feedstock for the production of fuel oils . Low-density polyethylene is one of the most mass-produced plastics and contributes significantly to the global plastic waste pollution. It has a density of 0.910–0.940 g/cm3 and is made up of many branching short-chain and long-chain structures. Its increased ductility and low tensile strength make it a good substrate for catalytic cracking. LDPE has been cracked over ZSM-5 zeolite in a fixed-bed reactor system to yield C4–C8 aliphatic and C7–C12 aromatic compounds . Polyvinylchloride (PVC) is produced via the radical addition polymerization of chloroethene. PVC is heat, fire, and water-resistant, so mostly used for electrical wiring covers and water pipes in construction. The various chemical methods for recycling PVC are pyrolysis, catalytic de-chlorination, and hydrothermal treatment . These treatments have been carried out with transition metal oxides . PVC has proven to be an excellent substrate for pyrolysis having exhibited a high average laminar burning velocity of 178.6 cm/s [22, 23]. Polystyrene (PS) is an inexpensive plastic popularly known as Styrofoam. PS is a vinyl polymer and it is synthesized by free radical vinyl polymerization of the styrene monomer. The efficacy of polystyrene as a pyrolysis feedstock was studied under a temperature range of 400–600 °C in the presence of nitrogen, yielding liquid products in the range of C2–C15 .
Previous studies have established that zeolite is the most promising catalyst for catalytic cracking of waste polymers . The objective of the current study was to test the hypothesis of carbon recycling on the catalytic efficiency of zeolite and metal oxides such as titanium dioxide on the thermal cracking of plastic wastes including LDPE, PVC, and PS to generate useful gasoline-range products. The original aspects of our study are: (a) Comparison of the well-established zeolite catalyst with other metal oxides and our previously reported novel potash catalyst for catalytic cracking of polymers; and (b) Generation of gasoline-range liquid hydrocarbon products alongside natural gas production. Furthermore, we assessed the effects of the different catalysts on the yield and composition of liquid hydrocarbon products, product collection temperatures, and the quality of the collected products compared to positive controls.
Materials and methods
All reactions were conducted in the ventilated fume hood. Reactions were repeated duplicate or triplicate, and the mean average with the standard deviation is presented. The plastic samples were considered to represent the majority of waste plastics pollutants in the environment. Our feedstocks for the experiments were recycled LDPE, PVC, and PS which were washed, dried, and cut into the form of cubes (2 mm × 2 mm) and were obtained from the environment in Yola, Nigeria (Fig. 1). An analytical grade zeolite (powder; particle size: < 45 µm) and titanium dioxide (matrix material; density: 4.26 g/mL at 25 °C; melting point: > 350 °C) were obtained from Sigma Aldrich from New Jersey, USA.
Pyrolysis–catalysis reactor and yield analysis
The one-stage fixed-bed reactor consisted of a heating mantle, batch Pyrex round bottom flask and stoppers, thermometer, waste plastics, catalyst materials, condenser, and oil collection bottle (Fig. 2). Each of the shredded waste plastics (6.0 g) was placed into a Pyrex round bottom flask which was mounted on a heating mantle. Into each flask, different ratios of metal oxide catalysts were added (1, 5, 10, 20, 30, 40, 50% catalyst/reactant). The apparatus was set up with the condenser held with a retort stand, and the collection flask placed at the bottom of the condenser to obtain the liquid products. An extra condenser was attached to the top of the Pyrex round bottom flask to extract gas with the aid of a syringe. The heating mantle was set to 100 °C, and the reaction was allowed to run for 75 min. The temperature ranges for the expulsion of visible gas were recorded, and that of the liquid extraction was recorded for each feedstock. After the gas and liquid extraction, the batch Pyrex round bottom flask was measured for solid products. The experiment was repeated with the titanium dioxide (TiO2, 40 wt%) as well as under no catalyst conditions. All reactions were conducted under atmospheric pressure and the yield percentage (% Yield) was determined in terms of liquid conversion of the different polymer feedstocks (LDPE, PVC, and PS) used in this study as seen in the formula below in Eq. 1.
GC–MS analysis of products
The obtained products were captured in a 50 mL glass syringe and a micro-needle was used for the injection into a GC–MS instrument (Agilent 7890A GC, 5975C MS). The sample (1 µL) was injected into the inlet of the gas chromatography where it was vaporized into the column (DB-35MS 30 m × 0.25 mm × 0.25 µm) using split mode into the inlet at 250 °C. Helium gas of high purity was used as the carrier gas at a flow rate of 1.1 mL/min. The oven temperature was programmed to start from 60 °C, hold for 1 min, increase 10 °C/min to 280 °C, and hold for 21 min at an electron ionization mode (EI) of 70 eV. The same conditions were used to analyze liquid products. The identity, structure, and molecular weight of the samples were obtained by the interpretation of the mass spectrum using the National Institute Standard and Technology database (NIST 2014 and NIST 2011). The unknown component of the mass spectrum was compared to the known compounds stored in the database.
Viscosity and density analysis
The viscosity of the LDPE liquid products was analyzed to determine its rheology and internal friction capacity using a viscometer. Testing the viscosity is significant as a fuel property because it reveals the fluid’s internal resistance. The liquid product (5 mL) was poured into the viscometer tube and inserted into the viscometer, followed by setting the temperature of the viscometer and stabilization. With the aid of a pipette pump, the liquid was dragged to the start mark on the viscometer tube, then the pump was released and the liquid was allowed to drop to the stop mark of the viscometer tube. The time for the liquid to drip from the start mark to stop mark was recorded. The viscosity (V) was calculated with the formula in Eq. 2 below where c is a constant from the viscometer instrument and T is the time for the liquid to pass the stop point mark.
We also considered the other essential properties including density. Density (D) is the ratio of mass (m) to volume (v) of the liquid according to the formula in Eq. 3. It is significant as it gives information on the properties of the liquid and how to transport the fluids, as well as setting prices for the liquid.
Results and discussion
Liquid product yield
The catalytic cracking of waste LPDE, PVC, and PS over zeolite and titanium (IV) oxide catalysts were investigated in the one-stage pyrolysis-catalysis reactor (Figs. 1, 2). The liquid hydrocarbon yield in the catalytic degradation of LDPE, PS, and PVC over zeolite and TiO2 is shown in Fig. 3. PVC, with an average liquid yield of 12.8%, produced more gas hydrocarbon products, while PS, with an average liquid yield of 87.7%, generated mostly liquid products. Among the reactants, the yield of gas products is in the order of PVC > LDPE > PS. The variation in liquid yield is attributed to the structures of the polymers and the stability of the intermediates. PVC has chlorine attached to an ethyl group while PS has a benzene ring attached to an ethyl group. The resonance structure of a benzene ring is more stable than chlorine, contributing to the high stability of PS which results in an overall reduction in cracking reactions. Our data indicate that cracking rates of gas to liquids are reduced with more stable structures. Similar results were reported on the catalytic degradation of HDPE, LDPE, polypropylene, and PS over an H-Y zeolite in a two-stage batch reactor at 500 °C over a reaction time of 43.75 min. The PS feedstock yielded the highest liquid products (71%) while LDPE showed 42% liquid yield .
In our previous studies, we examined catalytic cracking of waste plastics feedstocks over the following selected polymer feedstocks and metal oxide catalysts including; (1) low-density polyethylene over Ca(OH)2, Al2O3, and ZnO, (2) high-density polyethylene (HDPE) over zeolite and a novel potash catalyst, and (3) polypropylene (PP) over MgCO3, CaCO3, Al2O3, and MgO at a catalyst loading of 40 wt%. The liquid yields were 73.5% (MgCO3), 90.5% (CaCO3), 90.1% (Al2O3), and 86.3% (MgO) on PP; and 34.7% (potash) and 32.4% (zeolite) on HDPE, respectively. For the catalysts including ZnO, Ca(OH)2, Al2O3, we also tested the reusability of these various metal oxides catalysts. The re-used catalysts generated lower liquid products and require a higher temperature for the reaction.
It is noteworthy that the catalytic pyrolysis using zeolite consistently gave higher yields for all reactants than the titanium dioxide catalyst. For PS, LDPE, and PVC pyrolysis using zeolite, the liquid product yield was 3.3, 17, and 0.3 percentage points higher than that of titanium dioxide. Zeolite and titanium dioxide are among the most commonly used metal oxide catalysts with several advantages including Lewis acidity, large surface area, and non-toxicity. The surface of titanium dioxide is suitable for pyrolysis and can give rise to different structures for diverse applications . For zeolites, the numerous acid sites as determined by the Si/Al ratio enhance C–C bond scission catalytic activities. Other studies have reported an improvement in catalytic activity over TiO2/zeolite catalyst composites [27, 28].
The GC–MS analysis of hydrocarbon products from the pyrolysis-catalysis of LDPE over zeolite and titanium dioxide is presented in Figs. 4 and 5, respectively. For the retention times (RT) of the LDPE products over zeolite (Fig. 4), Fig. 4a is the m/z analysis of an unknown molecule at RT 1.182 min showing m/z = 55.1 (base peak) and m/z = 84.1 (M+); Fig. 4b is the NIST database search showing 2-methyl-1-pentene with m/z = 56.0 (base peak) and m/z = 84.0 (M+); Fig. 4c is the m/z analysis of an unknown molecule at RT 1.478 min showing m/z = 56.1 (base peak) and m/z = 100 (M+); while Fig. 4d is the NIST database search showing 1,3-dimethylcyclopentene with m/z = 56.0 (base peak) and m/z = 100 (M+). In addition, Fig. 5 shows the LDPE products cracked over TiO2. Figure 5a is the m/z analysis of an unknown molecule at RT 1.027 min showing m/z = 55 (base peak) and m/z = 71 (M+); while Fig. 5b is the NIST database search showing 1,2-dimethylcyclopropane with m/z = 55 (base peak) and m/z = 71 (M+), The liquid products were C5 (1,2-dimethylcyclopropane), C6 (2-methylpentane), C7 (1,3-dimethylcyclopentane), and C8 (2-octene, 4-octene, octane, 3-ethylhexane), in addition to C9 and C10 with less amount. The liquid products from catalytic pyrolysis using zeolite and titanium catalysts were mainly aliphatic. The production of a high amount of gasoline-range hydrocarbon fractions can be attributed to the use of the metal oxide catalysts compared to non-catalytic pyrolysis.
The current experiments demonstrate that the liquid products mainly consisted of hydrocarbons in the gasoline-range hydrocarbon fractions of C5–C8. Our data indicate that the one-stage pyrolysis system is effective for the synthesis of gasoline-range hydrocarbons from LDPE using metal oxide catalysts including zeolite and titanium dioxide. GC–MS data indicate that a specific catalyst may determine the chemical composition of the liquid products. Our previous experiments using other metal oxide catalysts have typically yielded kerosene and diesel-range distillates within the range on C8–C22.
The impact of catalysts on the hydrocarbon composition was reported by Almustapha et al.  on the cracking reaction of waste HDPE plastics over sulfate modified zirconium wherein the products were mainly hydrocarbon gas and light liquids within the range of C1–C7, with methane and ethane appearing in negligible amounts while the major components were C3 (propane and propene), C4 (n-butane, 1-butene, iso-butane), and C5 (n-pentane, pentene, and iso-pentane). Microwaves-induced pyrolysis of LDPE over ZSM-5 has also been reported to produce gasoline-range hydrocarbons of mono-ring aromatic structures with a liquid yield of 32.58% at the reaction temperature of 450 °C and catalyst to polymer ratio of 50% .
The physical appearance of the liquid products from LDPE, PS, and PVC is shown in Fig. 6. A white or colorless gas expulsion was noticed inside the round bottom flask for all feedstocks, suggesting C1–C4 products. For LDPE and PS, a pale-yellow transparent liquid was extracted from the pyrolysis with zeolite and TiO2 catalysts. However, the liquid extraction without catalyst turned into wax after 2 min of extraction. The production of waxes has been previously reported to correlate with low residence times and high heating rates that reduce secondary reactions and increase the yield of waxes [30,31,32]. For PVC, a dark green opaque liquid was extracted using zeolite and titanium dioxide catalysts, while a dark brownish liquid was extracted from the pyrolysis of PVC without the use of catalysts.
Effects of catalyst on the pyrolysis temperature
The temperature profile and corresponding yields for different reactants in the experiments are presented in Table 1. Initially, we performed the pyrolysis experiment on the solid waste plastics before introducing catalysts to the reactor system with a catalyst/polymer ratio of 40%. According to Table 1, it was observed that the temperatures did not follow a regular trend and the presence or identity of the catalyst had a relatively smaller effect on the reaction temperatures in comparison to the reactant type. For example, the liquid collection temperature for LDPE and PS were lower over zeolite (LDPE: 90–178 °C; and PS: 62–164 °C) and no catalyst (LDPE: 82–140 °C; and PS: 40–140 °C) conditions than over titanium dioxide (LDPE: 99–198 °C; and PS: 68–168 °C). Whereas the reverse was the case for PVC which was lower over titanium dioxide (68–168 °C) compared to zeolite (70–172 °C) and no catalyst conditions (80–154 °C).
However, the liquid yield (wt%) varied according to the starting materials. Regardless of the use of a catalyst, polystyrene produced the highest average liquid yield of 82.4% and at lower temperatures compared to LDPE and PVC. Using the zeolite catalyst, the gas expulsion temperature of PS (62 °C) was 8 °C and 28 °C lower than that of PVC and LDPE, respectively. This observation can be attributed to the high level of stability of the structure of polystyrene as the benzene ring plays a key role in stabilizing the thermal cracking intermediates formed during the pyrolysis reaction .
LDPE generally favored higher cracking temperature in comparison to PVC and PS. Under the zeolite catalyst condition, the gas expulsion temperature was 90 °C compared to 70 °C and 62 °C for PVC and PS, respectively. A similar trend is also observed using titanium dioxide where the gas expulsion temperature for LDPE (99 °C) was 31 °C higher than that of PVC and PS. Onwudili et al.  reported that similar higher pyrolysis product collection temperature for LDPE at 425 °C against PS products which were expelled at a lower temperature of 350 °C.
Products viscosity and density analysis
The quality of liquid products from the LDPE pyrolysis was characterized by the physicochemical properties of petroleum fuel including density and kinematic viscosity. The density of liquid products from LDPE, PVC, and PS at 25 °C is presented in Table 2. Our measurements demonstrated that the density of LDPE, PVC, and PS liquid fuels ranged from 0.783 to 0.803 g/mL, 0.63 to 1.142 g/mL, and 0.876 to 0.906 g/mL, respectively. Comparing with the properties of commercial gasoline fuels in Nigeria with a range of 0.728–0.746 g/mL , it was observed that the synthesized liquid fuels are generally within the standard gasoline density range. The liquid products obtained from PVC over zeolite and titanium dioxide showed higher density values of 1.142 g/mL and 1.052 g/mL compared to those of LDPE and PS. This result is consistent with the physical appearance (Fig. 6) of the liquid products wherein the PVC liquids were darker and most viscous, attributable to the presence of chlorine atoms in the structure of the polymer.
The kinematic viscosity of the liquid fractions from LDPE over different catalysts at 75 °C is shown in Table 3. The viscosity in centistokes (cSt) ranged from 3.1749 to 3.5143 cSt. Even though there is no direct relationship between density and viscosity, a lowering of the °API is associated with higher viscosity .
The current study demonstrates that the catalytic pyrolysis of LDPE, PVC, and PS to liquid hydrocarbons can be performed using zeolite and titanium dioxide in a one-stage reaction system for carbon recycling application. The PVC and LDPE generated an average yield of liquid products as 12.8% and 35.7%, respectively, while PS yielded a higher average yield of liquid products as 87.7% using metal oxide catalysts. The catalytic cracking led to the formation of liquid, gas, and solid residue as reaction products, in addition to the production of waxes from LDPE. In the case of PS, the stability provided by the delocalized benzene ring which is part of its structure helped increase the yield of liquid products.
Furthermore, our results show that the reactant types strongly affect the product yields and the quality of the liquid products. PVC waste produced the highest gas and solid fractions compared to liquid products. The use of metal oxide catalysts, especially zeolite, improved the liquid fraction yield across all feedstocks deployed. Pyrolysis with zeolite catalyst produced higher liquid products, including C5–C8 gasoline-range hydrocarbon products, compared to titanium dioxide catalyst. Overall, we have shown that there is a huge potential to extract useful mid-range hydrocarbon fuels from waste plastics via the catalytic cracking of recycled LDPE, PVC, and PS in a fixed-bed reactor.
All data generated or analyzed during this study are included in this published article.
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The current research was supported in part by Research Assistantship and Teaching Assistantship from the American University of Nigeria and Julia Foundation. The Research Fund was generously awarded from the Dean’s office of Arts and Sciences at the American University of Nigeria.
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Nwankwor, P.E., Onuigbo, I.O., Chukwuneke, C.E. et al. Synthesis of gasoline range fuels by the catalytic cracking of waste plastics using titanium dioxide and zeolite. Int J Energy Environ Eng (2020). https://doi.org/10.1007/s40095-020-00359-9
- Plastic pyrolysis
- Thermal catalytic cracking
- Metal oxide catalysts
- Fixed bed reactor
- Carbon recycle