Co-pyrolysis of biomass and waste plastics for production of chemicals and liquid fuel: A review on the role of plastics and catalyst types

2.1 High-density polyethylene

High density polyethylene (HDPE) is a common waste in municipal solid waste (MSW), with a H/C_eff ratio of 2 (He et al., 2021). HDPE is widely used to produce sturdy bottles, flexible pipes, toys, geomembranes, ropes, cutting boards and others. Due to its less fixed carbon with no oxygen, the addition of HDPE in co-pyrolysis of biomass can lower the formation of coke on catalyst and oxygenated compound in pyrolysis oil. Furthermore, HDPE is favourable for pyrolysis process since it possesses greater than 99 % volatile content with nil moisture (Rahman et al., 2021).

Few studies have reported that the incorporation of HDPE can considerably improve the quality of pyrolysis oil in terms of olefins and monoaromatic hydrocarbon while reducing the undesired product of polycyclic aromatic hydrocarbons (PAHs), which are toxic, carcinogenic, and mutagenic (Chen et al., 2016). He et al. (2021) investigated the co-pyrolysis of HDPE and corn stalk over HZSM-5 using Py-GC/MS in the temperature range and biomass-to-HDPE ratio of 550–800 °C and 1.0–0:1, respectively. The result revealed that the addition of HDPE to corn stalk sharply reduced the oxygenated compounds from 97.02 % to 42.03 %. In addition, significant synergistic effect on condensable volatile organic products (CVOPs) and hydrocarbon was observed as the experimental value of CVOPs and hydrocarbon was higher than the calculated value. During co-pyrolysis, the corn stalk-derived oxygenates could interact with HDPE-derived olefins to form hydrocarbon via Diels-Alder reactions, enhancing the hydrocarbon production while reducing the oxygenated compounds. In addition, the hydrogen atoms transferred from HDPE could promote the hydrocarbon production by enhancing the cracking and deoxidation (decarbonylation, decarboxylation and dehydration) reaction of corn stalk-derived oxygenates to hydrocarbon. Furthermore, the addition of HDPE to corn stalk as co-reactant could inhibit the coke formation by stabilizing the corn stalk-derived oxygenates (lignin-derived phenolic compound) and preventing it from undergoing polymerization on the surface of HZSM-5. A comparable trend was observed by Rahman et al. (2021) who attributed the improvement in gasoline range hydrocarbon to proton supplement provided by HDPE. The highest selectivity of gasoline range hydrocarbon (77 %) was found at pine-to-HDPE ratio and temperature of 50:50 and 550 °C, respectively. The authors also highlighted that the amount of oxygenated compounds reduced from 31.11 % to 0 as the pine-to-HDPE ratio surged from 100/0 to 0/100 because H/C_eff ratio increased as the HDPE ratio in the feedstock increased, promoting the cracking of oxygenated compounds such as phenolic to gasoline-equivalent hydrocarbons (C₆-C₁₂).

Yuan et al. (2018) carried out the co-pyrolysis of cellulose and HDPE at different ratios and reported that the synergistic effects in the co-pyrolysis accelerated the generation of small molecule volatiles, including H₂O, CO/C₂H₄, and CO₂. The decomposition of HDPE via chain-end and random scission can transfer hydrogen for the decomposition of cellulose-derived anhydrosugars to aldehyde and ketone while cellulose-derived oxygenated compounds, which act as acceptor, promote the scission of HDPE to alkane and alkene groups. During co-pyrolysis, aldehyde and ketone can be further decomposed to hydrocarbon. Fig. 2 shows the reaction mechanism between cellulose and HDPE at different biomass-to-plastic ratio. In co-pyrolysis of biomass with HDPE, HDPE generally provides positive synergistic effect on bio-oil yield (Hassan et al., 2020; Önal et al., 2014; Rahman et al., 2021). Co-pyrolysis of discarded newspaper and HDPE produced more bio-oil and less gas than the theoretical value due to cross reaction between newspaper and HDPE which interfered with the degradation of functional groups attached to the cellulose structure of WP (Chen et al., 2016). This condition inhibited the production of gases of low molecular weight and favoured the production of oil. The highest oil yield of 68.43 wt% was achieved at newspaper-to-HDPE blend ratio of 1:2, and it was 31.59 % higher than the theoretical data based on weighted averages. Positive synergistic effects on fuel properties were also observed in terms of significant reduction of total acid number and viscosity by 216 % and 76 %, respectively. In addition, the quality of bio-oil was also enhanced with maximum hydrocarbon and alcohol yield of 85.88 %, which was obtained at WP:HDPE ratio of 50:50.

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Fig. 2

Chemical structure of lignocellulosic biomass (a) Cellulose; (b) Hemicellulose; (c) Lignin..

Reproduced with permission from (Hansen and Plackett, 2008; Shahzadi et al., 2014)

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Fig. 3

Reaction mechanism between cellulose and HDPE at different biomass-to-plastic ratio.

Reproduced with permission from (Yuan et al., 2018)

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2.2 Low density polyethylene (LDPE)

LDPE represents the second biggest portion of plastic waste with the approximate consumption of 415 million in 2015. This value is expected to increase by 4 % in the following years. LDPE is widely used as plastic bags and packaging due to its excellent characteristics of flexibility, ease of processing and low cost (Duan et al., 2021). Compared to HDPE, LDPE has more branching (2 % of carbon atom) and weaker intermolecular forces. Suriapparao and Vinu, (2021) examined the co-pyrolysis of LDPE with five different biomass and found that the experimental bio-oil yield (13.2 – 32.3 wt%) was less than the theoretical value (42–47.5 wt%). Excessive cracking of heavier molecules into lighter gases contributed to low bio-oil yield. Although the yield of bio-oil was low, the heating value of bio-oil (37.6–41 MJ/kg) was better than the theoretical value (32.6–37.8 MJ/kg) due to the interaction between oxygen transfer from condensable phase to gas phase and hydrogen release from LDPE vapours. Substantial progress has been observed in the selectivity of naphthalene derivatives, including methylnaphthalene and 2-methylnaphthalene produced from catalytic co-pyrolysis of LDPE, and cellulose and pine wood. LDPE was a high molecular weight polyolefin, and its pyrolysis and chain scission were incomplete, resulting in the production of larger molecules. Furthermore, the addition of LDPE could inhibit the coke formation during pyrolysis of biomass due to the breakdown of LDPE and biomass via free radical. Compared to LDPE, biomass could decompose earlier due to its poor thermal stability to produce primary radicals. As the temperature increased, the LDPE started to decompose to hydrocarbon that was rich in hydrogen (H) radical. The LDPE-derived H radical could promote secondary decomposition of biomass to generate volatile substances. These volatile substances prevented the LDPE from covering the biomass by melting down at high temperature. Furthermore, the coke growth was also hindered due to the inhibition of free radical polymerization by the precipitation of volatile substances (Zheng et al., 2018).

Al-Maari et al. (2021) studied the co-pyrolysis of empty fruit bunch (EFB) and oil palm frond (PF) with LDPE for bio-oil production. The results showed positive synergistic interaction on the production of aliphatic hydrocarbons and inhibition of oxygenated compounds. The hydrogen released from LDPE enhanced the decarbonylation of carbonyls and sugar, and decarboxylation of acid to hydrocarbon due to oxygen removal via CO and CO₂, respectively. In addition, significant synergistic interaction between EFB and PF with LDPE on the production of bio-oil has also been observed. The positive synergistic effect could be attributed to the secondary radical reaction, leading to the condensation of non-condensable fragments. Furthermore, LDPE that acted as the hydrogenation medium for biomass could inhibit the cross-linking reactions and polymerization of biomass, leading to greater biomass weight loss (Aboulkas et al., 2012; Yuan et al., 2018). Co-feeding LDPE and sugarcane bagasse yielded pyrolysis oil which mainly consisted of aliphatic compounds with fewer aromatic compounds as compared to individual biomass with high calorific value of 40 MJ/kg. The addition of LDPE to sugarcane bagasse enhanced the H/C ratio from 1.25 to 1.47 and boosted the formation of saturated hydrocarbon in the range of C₆ – C₂₅ (Dewangan et al., 2016).

2.3 Polyethylene terephthalate (PET)

PET is the third largest thermoplastic consumed in Europe after polypropylene and LDPE. PET is usually used in a variety of consumer goods, including synthetic polyester fibres, bottles and films due to its characteristics of clear and strong thermoplastic (Choi et al., 2021; Dhahak et al., 2020). Özsin and Pütün, (2018) investigated the co-pyrolysis of PET blended with peach stones and walnut shells using a fixed bed reactor, and observed increased ester and acid compounds and decreased phenolic compound. Maximum acid and ester yield of 65.87 % and 63.11 % were achieved in co-pyrolysis of PET with walnut shells and peach stones, respectively. The liquid was dominated by benzenecarboxylic acid with more than 40 % yield for both co-pyrolysis blend. Benzenecarboxylic acid and vinyl benzoate were formed when the ester link of carboxylic group was broken via beta scission, initiating the decomposition of PET. One of the biggest challenges regarding pyrolysis oil from PET is the high acid content such as benzoic acid. The acidic characteristic of pyrolysis oil can lead to corrosiveness, depreciating the fuel quality. In addition, benzoic acid can clog the pipelines and heat exchanger, triggering issues during operation at industrial scale (Lee et al., 2017). Despite the disadvantages, it is noteworthy that benzoic acid is a valuable precursor/feedstock for various industries (Çepelioǧullar and Pütün., 2014).

Chen et al. (2017) investigated the synergistic interaction effects on char morphology and thermal behaviour during co-pyrolysis of PET with paulownia wood (PAW) using TGA. Their result showed a remarkable deviation between the experimental and calculated value on volatile release. Higher char yield was obtained from the PAW/PET blends at final decomposition temperature of 530 °C with ΔW above zero. In addition, the char yield increased as the PET blending ratio increased. With the increment of PET ratio in the feedstock, more cross-link reaction between PET-derived products and PAW-derived char occurred, leading to greater char production. The PET decomposition played a role as a limiting factor for the cross-linking reaction. Meanwhile, the addition of PET to PAW resulted in the agglomeration morphology of char. Ablative surface and granule cohesion were observed on the char topography as the PET blending ratio increased due to the reaction between PET decomposition products and PAW-derived initial char. PET typically decomposed at temperature between 370 °C and 460 °C. Non-catalytic pyrolysis of PET produced a liquid containing terephthalic acid and benzoic acid along with CO and CO₂ gas whereas co-pyrolysis of PET and biomass formed mainly acid and esters. The upgraded bio-oil from catalytic co-pyrolysis of biomass and PET demonstrated high content of aromatic compounds in the range of C₅-C₁₂ of carbon number fuel range (Dyer et al., 2021).

2.4 Polycarbonate (PC)

Polycarbonates (PC) are a group of thermoplastic polymers that contain carbonate groups in their chemical structures. PC is a prominent engineering plastic due to its characteristics, such as high impact strength, superb thermal resistance, and exceptional electrical insulation properties; and it is widely used in automobile industry, building and construction, and data storage devices such as compact disc and DVDs (Antonakou et al., 2014; Bai et al., 2020). In 2017, the global PC production has reached 6 million metric tons (Do et al., 2018). PC is unrecyclable due to its superior opposition to chemical attacks and difficulty to be extracted from the waste stream. Landfilling PC could pose environment threats due to the leaching of bisphenol. Bisphenol A (BPA) and diphenyl carbonate (DPC) substance contained in PC are regarded as endocrine disruptors that cause serious illnesses, including cancer, threaten adult health and interfere with infant hormones (Bai et al., 2020).

Liu et al. (2021) researched the co-pyrolysis of pinewood blended with PC to determine the synergistic effect. The extent of synergistic effect was determined via comparison between the experimental result of co-pyrolysis of pinewood-PC mixture with the weighted average values from individual feedstock pyrolysis. Positive synergy between pinewood and PC was obtained due to the enhancement of H₂, CO and total syngas yield of 33 %, 36 % and 19 %, respectively, compared to the theoretical value from individual pyrolysis. However, negative synergistic effect was noticed in the formation of C_nH_m. The variation in synergistic behaviour of different gas components could be attributed to the interactions between PW and PC intermediates during co-pyrolysis, producing more oxygenated compounds (alcohols, carboxylic acids, and aldehydes) with less hydrocarbons. In addition, co-pyrolysis of PW and PC remarkably enhanced the gas production yield (from 67.6 wt% to 77.2 wt%) but reduced the tar and char yield compared to the theoretical values from individual feedstock pyrolysis. This phenomenon suggested that the synergistic effects of co-pyrolysis of PW and PC involved both mutual interaction of volatile in gas phase and volatile-solid interaction which enhanced the total conversion of solid feedstock to gases. The pyrolysis of PC tends to generate more phenol via oxygen removal as CO and CO₂ (Burra and Gupta, 2018). Decomposition of PC mainly occurs via chain scission mechanism which can be divided into two main reactions: primary step in cyclic oligomers production by an intramolecular exchange reaction and hydrolytic cleavage of the carbonate group, generating hydroxyl-terminated oligomers and CO₂ at 400 to 500 °C temperature (Jin et al., 2016). Blending pinewood (PW) with PC could enhance this pathway, and stable phenolic intermediates could be formed with the lignin portion, enhancing the breakdown and conversion of PW to low molecular weight aromatics that exist as volatiles, and decreasing the char formation at about 10 % (Burra and Gupta, 2018). On the other hand, addition of lignin to PC pyrolysis can escalate the decomposition of PC to phenolic type compounds by enhancing the release of C⚌O during co-pyrolysis while inhibiting the aromatic compound (Jin et al., 2016).

2.5 Polyvinyl chloride (PVC)

Polyvinyl chloride (PVC) is widely used in the production of cable and wire insulation, fashion and footwear, packaging, window frames, and water pipes. PVC has a longer lifespan than other packaging plastics. About 44.3 million metric tons of PVC was produced globally in 2018, and by 2025 the world’s market size of PVC is expected to grow to nearly 60 million metric tons (Statistica.com, 2021). PVC is the main source of chlorine in municipal solid waste (MSW) and one of the problematic plastics in the feed. Its presence in the feedstock is limited to less than 5 % and generally around 1 to 2 %. The release of chlorinated hydrocarbons and HCl in PVC results in corrosion in the reactor and renders the oil halogenated (Qureshi et al., 2020). As there is no public recycling system for PVC, the proportion of the recovered PVC is relatively low. Moreover, PVC needs to be treated using hydrochloride scrubber for PVC cracking as chloride is not desired in the fuels (Xue et al., 2017).

Özsin and Pütün, (2018) analysed the synergistic effects during co-pyrolysis of PVC with two solid biomasses (walnut shell and peach stones). Negative synergistic interaction on the liquid yields were observed as the liquid yield during the co-pyrolysis (14.70 – 17.60 wt%) was lower than the aggregate values (17.21 – 18.64 wt%). On the other hand, positive synergistic effect was observed with higher aromaticity of tars in co-pyrolysis yields than biomass alone. ¹H NMR result showed that both aromatic protons comprised of guaiacyl units (ArH and HC = C-(conjugated)); and ɑ-hydrogen atoms of the branched chain of aromatic ring carbons, methoxy and aliphatic hydroxyl were increased when PVC was added into the biomasses. Polyenes condensation and aromatization during PVC decomposition contributed to the enhanced formation of tars aromatic. It has been well established that chlorine radicals generated during PVC decomposition could initiate condensation reaction, cyclization and aromatization. In addition, considerable value of PAHs was observed during co-pyrolysis of PVC with walnut shell (64.40 %) and peach stones (59.06 %). The decomposition of PVC favoured aromatization reaction and creation of heavier tar compounds via dichlorination, followed by inter-molecular chain transfer; the aromatic chain scission generated two or three aromatic-ring side chain before the coke deposition. HCl release during co-pyrolysis of PVC blends escalated the progression of light tar portions to heavy portions, resulting in the generation of higher molecular weight substances, such as PAHs (Tang et al., 2018).

The addition of PVC could instigate the decomposition of pinewood (PW) at lower temperature range due to the acceleration of PW decomposition by HCl from the dehydrochlorination of PVC. In addition, the co-pyrolysis of PW and PVC yielded more char and less liquid compared to the theoretical data. HCl generation from PVC at lower temperature range (230–300 °C) promoted the dehydration of cellulose to aldehyde compound which was confirmed from the cleavage of glycosidic units. The hydrogen and oxygen atoms in cellulose were lessened due to the dehydration at low temperature, leading to higher char yield. Furthermore, the dehydration also reduced the tendency of depolymerization, consequently reducing the liquid yield. Furthermore, the PW-derived solid char could also act as a catalyst owing to the presence of some inorganics, such as Cao, K₂O and NaO that promoted the secondary cracking of PVC oil to generate more char and gas (Lu et al., 2018a). The presence of PVC could influence the reactivity and activation energy of lignocellulosic biomass. The magnitude of reactivity of co-pyrolysis of cherry seed (CS) and PVC was nearly-two orders higher than the pyrolysis of CS at all heating rates. This observation was credited to the chemical structure of PVC which contained high electronegative chloride ions. The activation energy of co-pyrolysis of CS/PVC fell between CS and PVC value. The deviation between theoretical and experimental value of activation energy signifies the occurrence of synergistic effect between CS and PCV during co-pyrolysis (Özsin and Pütün, 2019).

2.6 Polystyrene

Generally, the addition of PS to biomass can enhance the liquid yield while decreasing the gas and char yield. (Stančin et al., 2021) reported that an addition of 25 % of PS to sawdust (SD) could double the yield of pyrolysis oil from 31 % to 62 %, specifically on the expense of gas formation, indicating the occurrence of synergistic effect in the process. Moreover, blending 25 % of PS with SD could enhance the quality of bio-oil in terms of reduction of oxygenates and PAHs while promoting the aromatic hydrocarbon. However, when the ratio of PS exceeded 25 %, a higher generation of undesired benzene derivatives and toxic PAHs became noticeable due to the secondary cracking of PS-derived styrene monomer accelerated by the interaction with biomass feedstock. Benzene derivatives in bio-oil limit its further utilization since such compounds are categorized as carcinogenic.

Samal et al. (2021) examined the co-pyrolysis of eucalyptus biomass and polystyrene waste on the physiochemical and thermal characteristic of the solid char. Two distinct physiochemical and thermal characteristics of char have been observed basically at temperature below and above 450 °C. The char generated below 450 °C has high heating value and volatile content with low fixed carbon because of the polystyrene coating on the char surface. The melting polystyrene waste could deposit over biomass at temperature below 450 °C, go through volatilization with additional increase in temperature, and be transformed to liquid oil and syngas. Solid fuels with high volatile content and low fixed carbon generally possess low ignition and burnout temperatures and a higher mass-loss rate, making them unstable. However, the increased high heating value due to the existence of waste plastic coating could ease in enhancing the combustion efficiency of the fuel. In contrast, the produced chars at temperature 450 °C and above possessed more high heating value and fixed carbon with low volatile content. This kind of solid fuel demonstrates superior combustible behaviour with broader temperature range and longer time for complete combustion, all of which signify an excellent solid fuel.

The addition of PS enhances the yield and property of pyrolysis oil. In contrast to pyrolysis oil from biomass (Mahua seeds) alone, the addition of 20 wt% of PS in co-pyrolysis enhanced the liquid yield from 39.26 wt% to 45.89 wt%(Mishra and Mohanty, 2020). At 20 % blending ratio, the plastic could have a maximum synergistic interaction between particles which subsequently maximize the generation of hot volatiles that could be further transformed to liquid form. At this state, greater heat and mass interaction happened between biomass and plastic particles. However, at 10 wt% and 30 wt% blending ratios, the interaction between biomass and plastic particles created negative synergistic effect, reducing the formation of hot volatiles and the production of liquid oil. Furthermore, the higher plastic ratio in the feedstock could cause the plastic melting, which would coat the biomass surface, eventually creating resistance for the discharge of hot volatiles and reducing the liquid yield. The NMR study showed the increment of aromatic and olefinic percentage in the co-pyrolysis oil (as confirmed in the FTIR diagnostics showing the peak of 1650 cm⁻¹ –1580 cm⁻¹ attributed to C⚌C stretching vibration). Meanwhile, the GC–MS results revealed that an addition of 20 wt% of PS as co-reactant substantially enhanced the hydrocarbon compounds and reduced the oxygenate derivatives such as acid, making it attractive compared to thermal pyrolysis oil. However, further upgrading technique is needed due to higher viscosity value than diesel fraction. Van Nguyen et al. (2021) examined the co-pyrolysis of waste PS and coffee-grounds at various blending ratio of 75:25, 50:50, and 25:75. The results revealed that co-pyrolysis could accelerate the deoxygenation reaction, causing a reduction of oxygenated compounds and enhancement in carbon content. The effect was strongest at the PS ratio of 75 % with reduction of oxygen content to 5.68 wt%. This condition contributed to an improvement in the calorific value (39.66 MJ/kg) of pyrolysis oil which was comparable to the heating value of conventional fuel. Table 1 shows the yields and quality of bio-oil obtained from co-pyrolysis of various biomasses and plastics.

3.1 Microporous zeolite

Zeolite is recognized as the most efficient catalyst to produce high-value chemicals because of its high acidity, high specific surface area, high adsorption ability and shape selectivity (Han et al., 2020; Ryu et al., 2020). Its unique pore structure with strong acidity favours aromatic selectivity with excellent cracking and deoxygenation ability (Hassan et al., 2016). The acidity of zeolite which is expressed by the Si/Al ratio determines their reactivity and affects the end products of pyrolysis process with low ratio, indicating high acidity (Chi et al., 2018). Generally, the introduction of microporous zeolite in the pyrolysis is usually favourable to enhance the aromatic production.

It is well established that the introduction of microporous zeolite in the pyrolysis is favourable to enhance the aromatic production. Park et al. (2019b) investigated the co-pyrolysis of Quercus variabilis (Q. variabilis) and waste plastic films (PFs) over two microporous zeolites (HZSM-5 and HY) of different acidity and surface area. The acidity (SiO2/Al2O3) of HZSM-5 and HY zeolite was adjusted to 30 and 23, respectively. The result showed that HZSM-5 with higher and stronger acidity could enhance the aromatics production than HY catalyst during the co-pyrolysis at 600 °C due to higher cracking efficiency of pyrolyzates. In addition, more appropriate shape selectivity of HZSM-5 which has medium pore size, appropriate pore window size and internal pore volume together with steric hindrance characteristic could favour the production of aromatics (Jae et al., 2011). On the other hand, higher formation of coke was observed for HY catalyst due to the more space provided as it had higher surface area (780 m²/g) than HZSM-5 (425 m²/g). In contrast, Kim et al. (2016) observed greater aromatic production over HZSM-5 catalyst at high temperature and catalyst-to-reactant ratio compared to HY catalyst during catalytic co-pyrolysis of cellulose-PP/LDPE mixture. HZSM-5 which had strong acidity was advantageous for aromatic production while high catalyst-to-reactant ratio of 1:10 could provide a large number of active sites for aromatization reaction. The authors emphasized that the properties of catalyst, specifically acidity and pore size, are crucial in determining the aromatic production efficacy during catalytic co-pyrolysis reaction. On the other hand, low temperature and less catalyst-to-reactant ratio were applied for HY catalyst since the reaction intermediates could diffuse easily into its pore and make intimate contact with active sites to undergo further reaction to form aromatic.

Coke deposition and limitation of mass transfer and reactant flow diffusion are among the major challenges of pyrolysis over microporous zeolite (Kim et al., 2017b). The small pore size (less than2 nm) of microporous HZSM-5 zeolite inhibits the diffusion of large biomass and plastic reaction intermediates produced during the initial stage of pyrolysis into its internal acidic sites. The large molecules of biomass and plastic pyrolysis intermediates formed during the initial stage of pyrolysis cannot pass through the inner pores and contact the active sites of HZSM-5 since their kinetic diameter is greater than the pore size of ZSM-5 (Hassan et al., 2019). Furthermore, pore blockage from polymerization and polycondensation reactions due to acidic properties of zeolite causes deactivation of the catalyst and reduces the catalyst lifetime. Shao et al. (2017) reported that the parallel side reactions of anhydrosugars, furans, and other organic molecules in the hydrocarbon pool could lead to the coke formation on the interior surface of zeolite while Custodis et al. (2014) stated that the competing side reaction of phenol repolymerization and lignin polycondensation could cause the coke deposition.

3.2 Mesoporous zeolite

Mesoporous zeolite catalysis has been recognized as an efficient approach to attenuate the diffusion restriction of bulky biomass and plastic molecules and expand the production of aromatic hydrocarbon through the larger pore size. High surface area of mesoporous zeolite provides greater access to active sites and enhance the catalytic interaction between the co-pyrolyzed reactants, causing higher conversion rate of oxygenates to aromatic hydrocarbon. Hong et al. (2017) reported the influence of microporous and mesoporous HZSM-5 during co-pyrolysis of cellulose and polypropylene on the aromatic formation efficiency. The result showed that mesoporous HZSM-5 by ZSM-5 desilication could offer better catalytic activity than microporous HZSM-5 in term of aromatic yield. Larger pore opening obtained by desilication can enhance the diffusion of bulky intermediates to active sites of catalyst to undergo further reactions to aromatics. In addition, mesoporosity can be allocated into the zeolite core-structure via post-synthesis treatments, such as steaming and leaching with acidic or basic media (Zhu et al., 2013).

MCM-41 is a type of mesoporous zeolite that has bigger pore size, making it suitable for adsorption, separation and macromolecular catalysis. Its larger pore size could ease the diffusion limitation in pores. MCM-41 could provide enough active sites for adsorption and catalytic reaction due to its high specific surface area greater than 1000 m²/g. Chi et al. (2018) conducted co-pyrolysis of cellulose and PP in the presence of MCM-41 and Al-MCM-41. The cracking of oxygenated compounds was heightened by the strong acidity originated from the inclusion of Al onto the mesoporous MCM-41. The results indicated that the production of olefins and aromatics were enhanced by using Al-MCM-41, inferring that Al-MCM-41 had superior cracking and deoxygenation effect. The aromatic formation during the co-pyrolysis was governed by internal acid sites, hydrocarbon pool, and Diels-Alder reaction (Fig. 3). Cellulose was decomposed earlier compared to polypropylene as it had lower decomposition temperature. Numerous oxygenated compounds and penta heterocyclic furans were produced via ring cleavage and catalytic cracking to break its hexa heterocyclic, followed by dehydration and cyclization. During the catalytic co-pyrolysis, olefin was produced from direct cracking of polypropylene via carbonium ion and β-scission and deoxygenation of oxygenated compounds at acid sites via dehydration, decarbonylation, and decarboxylation reactions. These intermediates (olefins and oxygenates) participated in deoxygenation and oligomerization to form carbocation hydrocarbon pool where the aromatic and olefins were formed. Along with hydrocarbon pool mechanism, the monocyclic aromatic hydrocarbon can be formed via Diels-Alder reaction between cellulose-derived furans and polypropylene-derived olefin.

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Fig. 4

Chemical mechanism of catalytic co-pyrolysis of cellulose and polypropylene over Al-MCM-41.

Reproduced with permission from (Park et al., 2019)

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Kim et al. (2017c) investigated the impact of acidity and molecular diameter on the formation of aromatic hydrocarbon in co-pyrolysis of carbohydrates with linear LDPE. They assessed the catalytic activity of microporous and mesoporous ZSM-5 with high mesoporosity and poor acidity Al-SBA-15. Higher yield of monoaromatic hydrocarbons was obtained under catalysis of ZSM-5 due to the combination of micropores and mesopores structure. This framework is suitable for the shape selectivity of aromatic production and to improve diffusivity of bulky molecular pyrolysis intermediates into the catalyst pore. The finding of this study indicated that catalyst with higher acidity together with an appropriate structure and pore diameter was an ideal catalyst for aromatic formation in co-pyrolysis reaction. Similar trend was found in the catalytic co-pyrolysis of yellow poplar and HDPE over three types of mesoporous catalysts, including hierarchical mesoporous MFI, hierarchical mesoporous Y, and Al-SBA-15 (Rezaei et al., 2017). Hierarchical mesoporous MFI which had large mesopores and strong acidity delivered the highest yield of olefins and aromatic hydrocarbons attributable to the efficient hydrocarbon pool mechanism. The yield of solid residue (char/coke) decreased for all three types of mesoporous catalyst. The lifespan of catalyst could be enhanced by reducing the coke deposition.

3.3 Metal modified zeolite

Metal addition could modify the textural characteristics and acid sites, and enhance thermal stability of the catalyst. This process aids in decreasing the rate of coke growth over the catalyst and enhancing the liquid production (Botas et al., 2014; Iliopoulou et al., 2012). Razzaq et al. (2019) observed that anchoring of Ni, Co, Zn and Fe oxides onto the HZSM-5 framework by wet impregnation technique could reduce the coke yield by 50 % compared to intrinsic HZSM-5 during the co-pyrolysis of wheat straw and polystyrene. This was due to the moderate acidic strength of metal-modified zeolite which was helpful in decreasing the coke formation over the zeolite. In addition, pyrolytic oil catalysed over metal-modified zeolite contained relatively higher organic phase yield instead of aqueous phase as compared to unmodified HZSM-5. The presence of metal-modified zeolite could enhance the decarboxylation and decarbonylation while inhibiting the dehydration reaction. French and Czernik, (2010) reported that incorporation of metal sites onto the zeolite framework could alter the deoxygenation pathway so that it favourably released more oxygen in the form of carbon monoxide instead of carbon dioxide and water, thereby offering more hydrogen available for aromatic production. The presence of metals boosted the aromatic selectivity towards high value mono-aromatic hydrocarbon (MAHs) and supressed the formation of oxygenated compounds.

Kim et al. (2017b) investigated HZSM-5, mesoporous MFI, Pt/mesoporous MFI and Al-SBA-16 catalyst effect for the Laminaria Japonica and PP co-pyrolysis. Pt/mesoporous MFI showed higher aromatic yield and oxygenate removal efficiency than the other catalysts due to the strong Brönsted acid sites and large pore size as well as catalytic effect resulted from the incorporation of Pt. Pt promoted the cracking and deoxygenation of oxygenated compounds to aromatic. The authors also highlighted that the strength of acidity played more essential role than the pore size in the production of aromatic hydrocarbon. Coupling of weak acid sites and large mesopores lowered the catalytic performance of Al-SBA-16. Conversely, mesoporous Al-SBA-15 with weak acidity showed a better oxygen removal efficiency than HZSM-5, concluding that the pore size played an important role during the cracking of large oxygenate molecules.

Impregnation of phosphorous onto the zeolite framework could enhance the hydrothermal stability and anti-coking properties of zeolite and ease the transformation of alkane to olefin, which was subsequently converted to aromatic. Yao et al. (2015) found that the modification of ZSM-5 with phosphorous (P) and nickel (Ni) increased the production of valuable aromatic hydrocarbons and olefins in the catalytic fast co-pyrolysis of pine wood and LDPE due to the enhanced zeolite’s Lewis acid sites which acted as electron pair acceptor and which had a high tendency to accept the hydride ions generated during the conversion of alkanes to olefins. Higher content of aromatic hydrocarbon boosts the commercial value of bio-oil as the aromatic compound is vastly used as additives in transportation fuel and feedstock materials in the petrochemical industry (Kim et al., 2017a). In addition, the rate of coke-induced catalyst deactivation, which is the main concern in catalytic fast pyrolysis, has also been reduced due to the impregnation of ZSM-5 with P and P/Ni cation. The incorporation of P and Ni onto the ZSM-5 significantly decreased the strong Bronsted acid sites of zeolite, in turn reducing the coke deposition. Gallium (Ga) altered the texture characteristic and acidity of zeolite by reducing the pore volume and surface area of zeolite (Li et al., 2015). Ga was introduced into the zeolite framework via incipient wetness impregnation. The Ga decreased the density of Brönsted acid sites due to the replacement of some Brönsted acid sites by Ga. Ga-containing zeolite substantially increased the production of olefin and/or monoaromatic hydrocarbons at the expense of less valuable alkane during the catalytic co-pyrolysis of pine wood and LDPE. Non-framework Ga provided a new route for dehydrogenation of alkane to olefin, which is subsequently converted to aromatic.

3.4 Hierarchical zeolite

In an effort to enhance the catalytic activity of the zeolite catalysts, the incorporation of hierarchical porosity or alteration through metals and oxide supplement has been frequently reported (Han et al., 2020; Jin et al., 2016; W. Wang et al., 2019). Although the mesoporous materials are synthesized to solve the problem of diffusion limitations, it has poor surface acidity and unstable structure property, bringing about unsatisfactory activity in acid-catalysed reactions. To solve this shortcoming, researchers have combined the advantages of microporous molecular sieve and mesoporous material, producing zeolites with hierarchical micro-mesoporous composite (Talebian-Kiakalaieh and Tarighi, 2020). It works in the way that the external mesopores capture molecules in several directions and concentrate them towards the zeolite micropores. The mesoporous structure could enhance mass transfer and cracking of large molecular pyrolysis vapours, which are hard to diffuse into the microporous zeolite (Kim et al., 2019). Furthermore, every mesopore behaves as a funnel and enables the effective penetration of molecules within the narrow one-dimensional micropore system. Such a combination of the properties of both porous systems would make the hierarchical aluminosilicates a versatile material for many applications. Five different approaches to synthesize hierarchical micro-mesoporous include recrystallization of ordered mesoporous silicas, zeolite-seeding, mesoporous carbon templating during crystallization, alkaline extraction of zeolites, and combining mesostructure and microstructure-directing agents (Enterría et al., 2014).

Several studies reported that the hierarchical zeolites could substantially resolve the limitations of the conventional zeolite, such as low mass transfer problem, deactivation of catalytic activity and low activity to bulky substrates in different chemical reactions due to significant deoxygenation and excellent aromatic selectivity (Ahmed et al., 2020; Chi et al., 2018). Moreover, Song et al. (2018) mentioned that more advantages from hierarchical zeolites could be observed, such as shortened diffusion path length, abundant external acid sites and surface area, and excellent hydrothermal stability. Combination of mesoporosity and traditional zeolites of hierarchical zeolite could broaden its application in catalysis.

The catalytic activity of hierarchical zeolite is mainly dependent on the synthesis method. Desilication (removing silica) and dealumination (removing aluminum) are an efficient approach to generate mesoporosity though it may result in a considerable shift in acidic properties (Ahmed et al., 2020). The alteration of zeolite structure during desilication and dealumination of zeolite is shown in Fig. 4. Proper acid sites distribution and mesoporosity resulted from the alteration in acidity could benefit the reaction pathway and intermediates stabilization as reported by (Hong et al., 2017). The desilicated ZSM-5 showed superior catalytic activity in term of aromatic selectivity (33.50 wt%) compared to parent ZSM-5 during co-pyrolysis of cellulose and polypropylene. The treatment enlarged the pore for better diffusion while retaining its strong acidity. The desilication enhanced the weak acid sites, thus improving the liquid products yield. In addition, the weak acid sites also fostered the deoxygenation of furan via reaction with olefins to produce more aromatics. Hierarchical zeolite has great potential in catalytic reactions related to bulky molecules due to the presence of microporous and mesoporous structure (Lv et al., 2020). A hierarchical pore structure could be created in ZSM-5 by including larger pore structures namely mesopore linked to the core microporous framework as an endeavour to inhibit the coke deposition and attain higher transformation of bulky oxygenates (Feliczak-Guzik, 2018). The mesoporous structure could enhance mass transfer and cracking of large molecular pyrolysis vapours, which were hard to be diffused into the microporous zeolite (Kim et al., 2019). Alkaline treatment is well-known and established as a post-synthetic technique comprised of fractional desilication of the zeolite structure to create secondary mesopores in ZSM-5 with bigger pore opening and outer surface area (Li et al., 2014). Apart from alkaline treatment, re-assembly aided with organic templating agent permits restructuring and redeposition of silicate and aluminosilicate fragment into the mesoporous material while maintaining the weight and/or acidity in basic medium (Chen et al., 2018).

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Fig. 5

Desilication and dealumination method.

Reproduced with permission from (Feliczak-Guzik, 2018)

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Lin et al. (2021) developed a series of hierarchical HZSM-5 with various alkaline solutions ranging from 0.2 to 0.4 mol/L and found that low alkaline solution (≤0.3 mol/L) accelerated the formation of monoaromatics from 63.79 % catalysed by HZSM-5 to 71.75 % for 0.3-HZSM-5 while higher alkaline solution diminished the framework of HZSM-5, leading to reduction of aromatic production. The alkaline treatment enhanced the mesoporosity of the zeolite so that the larger intermediates including oxygenated compounds and aliphatic hydrocarbons could effortlessly access the acid sites of hierarchical zeolite to form aromatics. Furthermore, the alkaline treatment reduced the polyaromatic hydrocarbons (PAHs) formation due to shorter diffusion path distance of molecules in the hierarchical HZSM-5 zeolites, retarding the secondary polymerization reactions of mono-aromatics inside the catalyst channel. Conversely, the selectivity of aliphatic hydrocarbons and oxygenated compounds were reduced as the alkaline concentration reached 0.3 mol/L, probably due to the conversion to aromatics at the catalyst pores via a series of reactions.

Li et al. (2020) investigated the catalytic fast co-pyrolysis of waste greenhouse plastic films and rice husk over hierarchical micro-mesoporous zeolite with HZSM-5 as core and MCM-41 as shell (HZSM-5/MCM-41). The result showed that the relative content of hydrocarbons and CO₂ were higher than for the non-catalytic pyrolysis, suggesting that HZSM-5/MCM-41 promoted the conversion of pyrolyzates to aromatic and decarboxylation becoming one of the routes that governed the conversion. The addition of MCM-41 mesopore around the HZSM-5 crystal particles assisted in cracking the large-molecular weight volatile to small molecular compound (Lin et al., 2021). Zhang et al. (2018) reported that hierarchical HZSM-5/MCM-41 which contained a moderate amount of mesopore was effective for pyrolysis intermediate upgrading while reducing the coke formation simultaneously.

Qian et al. (2021) synthesized a novel hierarchical zeolite with the aid of alkaline lignin in the re-organization of alkaline treatment core material. The result revealed that the yield of bio-oil and gas was enhanced at the expense of solid residue. Higher transformation of main pyrolyzates derived from co-reactant and inhibition of char was observed due to higher acidity and hierarchical pore system of the catalyst. In addition, coke yield also decreased due to the enhanced diffusion capability of the feedstock and coke precursor, and shorter diffusion path length in the ZSM-5 structure. Deactivation rate could be reduced as no secondary reaction was produced resulting from the short residence time (Serrano et al., 2013). More particularly, the abundant reactive species of pyrolyzates from biomass-plastic mixture rapidly traverse the catalyst layer by hierarchical pore structure prior to absorption, producing solid residue. Diffusion through hierarchical zeolite crystals is faster in a manner that is closely related to Knudsen regime since the diffusion through mesoporous materials proceeds by molecule-to-molecule interaction as well as molecule-to-pore interaction.

3.5 Low-cost, mineral-based catalyst

Extensive efforts are being made to develop new catalyst with low-cost, good catalytic performance and environmental friendliness. The utilization of natural ore and industrial waste as low-cost and high-activity catalyst in the production of value-added bio-oil can pave ways for recycling and reusing those mineral and waste. Red mud (RM) is a waste residue generated from aluminium industries by the Bayer process of alumina production from bauxite (Wang et al., 2019). It comprises a complex mixture of metal oxides, notably iron oxides and small amounts of alkali earth metals (Das and Mohanty, 2019). Recently, there are significant interest in making use of red mud as a catalyst in pyrolysis of biomass due to its compositional properties containing metal oxides, including CaO, TiO₂, Fe₂O₃, Al₂O₃, MgO and SiO₂. (Chang et al., 2020) investigated the catalytic pyrolysis of palm kernel shell over red mud using a bench scale fixed bed reactor. The result indicated that the presence of RM could enhance the cleavage of oxygen-containing double bonds and functional groups in-side chains on benzene ring to phenol and aromatic. Duman et al. (2013) studied the catalytic pyrolysis of safflower oil cake over RM in a dual reactor system and found that the RM was an effective catalyst in deoxygenation reaction, enhancing the aromatic selectivity. Although the base property of RM could provide the additional cracking efficiency, high production of aromatic could not be achieved since RM did not possess strong acid sites and shape selectivity that were able to limit the diffusivity of longer chain intermediates into the active sites and to foster the secondary reactions including isomerization and aromatization to produce aromatic hydrocarbon (Kelkar et al., 2015). Therefore, the combination of low-cost alkaline catalyst and acidic catalyst is regarded as an ideal approach to achieve higher formation of aromatic hydrocarbon and enhance the zeolite lifetime. Yathavan and Agblevor, (2013) pyrolyzed pinyon − juniper (PJ) woody biomass over HZSM-5 and RM catalyst. The addition of RM as fractional catalyst could enhance the deoxygenation process in which the oxygen was rejected via decarboxylation (CO₂) process instead of decarbonylation (CO) and dehydration (H₂O) process. This process could enhance the overall carbon and hydrogen efficiency and thus, more hydrogens are available for aromatic production. Furthermore, the pyrolysis oil catalysed by RM has relatively lower viscosities than HZSM-5 catalyst. In another study, in-situ RM was used in the catalytic fast co-pyrolysis (CFCP) of organosolv lignin (OL) and polypropylene (PP) over ex-situ HZSM-5. The authors reported an increase in the cracking efficiency of OL/OP intermediates as well as an enhancement of aromatic selectivity due to the effective interaction between pyrolyzates. The presence of RM in the in-situ catalytic reactor could improve the formation of selected hydrocarbon that acted as precursor to produce aromatics over ex-situ HZSM-5 in second reactor (Ryu et al., 2020a).

Coal fly ash (CFA), a by-product of coal-fired thermal power plants (TPP) is often disposed in the landfill, causing environmental and economic issues. One of the key features of CFA is that it consists of aluminosilicates, such as SiO₂ and Al₂O₃, making it appealing as a precursor to produce zeolite-based catalysts (Supelano et al., 2020). Vichaphund et al. (2019) successfully synthesized ZSM-5 from CFA (HZSM5-FA) via consecutive alkaline fusion and hydrothermal treatment (Fig. 5). The zeolite crystallization time was varied at 24 hr (HZSM5-FA −24) and 72 hr (HZSM5-FA-72). The catalytic activity of the HZSM5-FA was determined in catalytic fast pyrolysis of Jatropha waste at the temperature of 500 °C and Jatropha-to-catalyst ratio of 1:1–1:10. The addition of HZSM5-FA considerably enhanced the aromatic selectivity up to 97.2 % and reduced the undesired oxygenated and N-containing compounds via deoxygenation and denitrogenation reactions. HZSM5-FA promoted the cracking of large oxygenates and nitrogenated species and further converted them to olefins and aromatic via a series of reactions, including decarbonylation, decarboxylation, dehydration, cyclization, aromatization, dehydronitration, deamination, and hydrogenation. On the other hand, HZSM5-FA-72 produced low amounts of aromatics compared to HZSM5-FA-24 due to both low acidity and high mesopore volume. Low acidity zeolite had low number of active sites (Bronsted acid sites) which were responsible to convert oxygenated compounds to aromatic compounds within the framework of zeolite catalyst while high mesopore volume could limit the molecular diffusion of pyrolyzates to inner pore of zeolite to further undergo the series of reactions for aromatic formation. Based on this result, it can be concluded that the pore structure and type of acidity play an important role for aromatic formation.

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Fig. 6

The schematic HZSM- 5 derived from fly ash by alkali fusion followed by hydrothermal treatment.

Reproduced with permission from (Vichaphund et al., 2019).

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Steel-slag is a waste by-product derived from steel-making process which accounts about 15 % of the total crude steel output. Most of the steel slags are accumulated heavily in landfill, becoming environmental hazards due to the leaching of heavy metals, particularly mercury (Hg), lead (Pb), chromium (Cr), cadmium (Cd), and arsenic (As) (Song et al., 2021). The higher activity of Faujasite zeolite derived from steel slag in hydrocarbon production was described by Hassan et al. (2019) in the co-pyrolysis of sugarcane bagasse and HDPE. The origin of Faujasite zeolite influenced the formation of mesopore with an average size of 45 nm and a modest surface area of 39.6 m²/g. Even though the surface area of the zeolite is quite low, the NH₃-TPD measurement showed a relatively strong acidity. Promotion to the hydrocarbon pool and deoxygenation reaction takes place due to the fact that the microporous structure enables intimate contact to the strong acid sites. Due to the lack of weak acidity in the catalyst, thermal condition is thought to be responsible for the decomposition and cracking of biomass and HDPE molecules. In increasing the pyrolysis temperature, they have been able to compensate the inadequacy of weak acidity while the strong acidity contributes to the upgrading of bio-oil pyrolyzates through a succession of dehydration, decarbonylation, decarboxylation, and oligomerization reactions. Nonetheless, at the reaction temperature of 500 °C and above, reverse Diels-Alder reaction would begin to occur, hindering further upgrading of the product by favouring the generation of olefins in place of aromatics.

3.6 Summary on catalyst types

With its distinctive pore structure and high acidity, microporous zeolite has exceptional cracking and deoxygenation abilities which favour the aromatic selectivity. However, a very small pore size (0.54–0.56 nm) of microporous zeolite resulted to coke formation, mass transfer limitation, and slow diffusion of large molecules into its inner active sites, preventing further reactions of macromolecules to valuable aromatic. Acidity and porosity are two paramount factors that influence the catalytic activity of zeolite catalyst. Mesoporous zeolite has been recognised as an effective method for reducing the diffusion restriction of bulky biomass and plastic molecules, and increasing aromatic hydrocarbon production due to the larger pore size. Although mesoporous materials are synthesised to address the issue of diffusion limitations, they have poor surface acidity and an unstable structure, resulting in inadequate activity in acid-catalysed reactions. To solve this challenge, introduction of new mesoporosity in the micropore of zeolite produces zeolites with hierarchical micro-mesoporous composite. Hierarchical zeolites can substantially resolve the limitations of the conventional zeolite, such as low mass transfer problem, deactivation of catalytic activity and low activity to bulky substrates in different chemical reactions due to the combination of two levels of porosity. Desilication and dealumination are the effective methods to create mesoporous structure with large pore size for better diffusion of bulky intermediates to active sites of catalyst to undergo further reactions to aromatic. In addition, the acidity could also be altered to foster the deoxygenation reaction. Incorporation of metal into the zeolite could alter the pore size and total number of acidic sites, and enhance the thermal stability of the catalyst, all of which are helpful to decrease the coke formation of zeolite catalyst. With the addition of metal, new enhanced Lewis acid sites of zeolite were generated, which boosted the aromatic production. Ubiquitous, low cost and a complex mixture of metal oxides, natural ores such as red mud could be utilized as a catalyst in pyrolysis of biomass. The base properties of red mud could provide additional cracking and deoxygenation for aromatic production. However, the result was still unsatisfactory due to the lack of strong acid sites and shape selectivity as compared to the conventional zeolite catalyst. Therefore, it is advisable to combine low-cost base catalyst with acidic catalyst to achieve higher production of aromatic. Natural mineral wastes including coal fly ash and steel slag consisting of aluminosilicates, such as SiO₂ and Al₂O₃ could be exploited as precursors for the synthesis of zeolite. However, two paramount factors that need to be considered to ensure high production of aromatics are pore structure and type of acidity. A zeolite catalyst with high acidity with an appropriate pore structure needs to be tailored to obtain high cracking and deoxygenation efficiency for the production of aromatics. The performance of different types of catalysts in the catalytic co-pyrolysis of biomass and plastic is summarized in Table 2.