A Review on Catalytic Co-Pyrolysis of Biomass and Plastics Waste as a Thermochemical Conversion to Produce Valuable Products

Abstract

In order to satisfy the increasing need for renewable chemicals and fuels, it is important to replace petroleum-based products with alternative feedstocks. Lignocellulosic biomass is considered to be the most capable alternative source for producing sustainable biofuels. Catalytic co-pyrolysis (CCP) is a process that involves simultaneously pyrolyzing biomass and plastics to produce a combination of liquid and gaseous products, such as bio-oil and syngas. Catalysts are used to raise the reaction degree and the selectivity of the co-pyrolysis process, with the choice of catalyst dependent on the physico-chemical features of the feedstock. Catalytic pyrolysis is a useful method for producing high-quality biofuels directly from biomass, although it typically yields a modest amount of aromatic hydrocarbons (HCs) and a large amount of coke, even with highly effective catalysts. Adding a co-reactant high in hydrogen to the CCP process can significantly increase the yield of aromatics while reducing coke formation. The use of CCP can help to address the environmental issues related to waste plastic disposal and improve energy security. This review article thoroughly discusses the process and mechanism of catalytic co-pyrolysis, the influence of plastics on the process, and how the addition of plastics can improve the quality and output of bio-oil while reducing the production of oxygenated compounds and coke. The importance of various catalysts (such as biochar, activated carbon, and acid and base catalysts) in improving the production and quality of obtained products is also compared and discussed.

1. Introduction

The transformation of solid biomass into valuable products such as chemical and liquids through thermochemical processes shows promise as a solution to the global energy crunch and environmental contamination [1]. Various technologies such as chemical, biochemical, and thermochemical platforms can transform biomass into energy precursors such as bio-oil, biocrude, biosyngas, biodiesel, bioethanol, charcoal, and biomethane [2]). Biomass is a common form of solid waste that can be sustainably utilized to create a circular bioeconomy focused on energy use [3]. The thermochemical treatment of lignocellulose using pyrolysis can produce bio-oils or syngas, along with solid char (biochar), tar (bio-oil), and pyrolysis vapor, which can be condensed to generate tar and product gas [4]. However, the quality of byproducts (bio-oil) at present is not comparable to that of fossil fuels, likely owing to the lower heating value of around 50% and the high oxygen concentration, corrosiveness, and volatility resulting from cellulose/hemicellulose processing methods [5].

After undergoing pyrolysis, biomass experiences an increase in its oxygen level, resulting in a bio-oil of low quality that is unstable and has a poor calorific value due to the increased water content. However, the quality of this bio-oil can be enhanced by co-feeding it with polymers that are rich in hydrogen. The worldwide production of plastic garbage is 150 million tons [6], though less than 10% of it is recycled, and recycling is prohibitively expensive due to the contaminants present in the feedstocks from various sources such as domestic, industrial, packaging, and agricultural operations [7]. To tackle this waste management problem, polymers can be utilized during the fast pyrolysis of biomass to generate hydrogen, and, by using suitable catalysts, the co-processing of biomass and waste plastic can produce high-value aromatic chemicals, including benzene, toluene, xylenes, and ethylene (BTXE), which are crucial precursors in several sectors such as pharmaceuticals, textiles, polymers, automobiles, and food, as well as essential additives in fuel [7,8]. This approach eliminates the need for conventional recycling methods to filter out plastic contaminants. The global market for aromatics, including BTXE, was valued at USD 185.9 billion in 2017 [9].

Plastics are a readily available co-reactant during co-pyrolysis, providing a viable option for long-term sustainable fuel production [10]. Plastic usage is widespread globally, with Western Europe seeing a 4% annual growth rate in plastic consumption from 1996 to 2007, increasing from 33.4 million tons to 48.3 million tons, respectively [11]. However, less than 10% of plastics can be recycled after their initial use, and approximately 60% of solid plastic waste (PW) worldwide is either dumped in open areas or landfills. According to the World Bank, waste plastics constitute between 8% and 12% of the municipal solid waste (MSW) generated in different regions [11]. It is projected that by 2025, the percentage of global waste plastics in MSW will increase to between 9% and 13%. Despite the environmental damage instigated via plastic waste disposal, the significant availability of PW presents a potential opportunity for its utilization in co-pyrolysis in order to decrease the amount of valuable plastics being disposed of in landfills [12].

Co-pyrolysis, which is a simple and effective method, involves utilizing two or more materials to produce high-quality pyrolysis products [13]. This process is significant in waste management as it allows for the simultaneous management of multiple types of waste, leading to an increase in waste usage while reducing response time and energy consumption [14]. In order to cut operating costs, it is essential to comprehend co-pyrolysis and address several unwanted managing concerns. In order to enhance the conversion of biomass into HCs, plastics that contain higher ratios of H/C are typically incorporated into co-pyrolysis systems involving biomass that possess lower H/C ratios. For example, research on lignin co-pyrolysis (which has an H/C ratio of 0–0.3) and plastics revealed that their interaction can lower the deceptive activation energy and hinder the end products [15]. Several studies indicated that co-pyrolysis can enhance the properties of pyrolysis oil by improving its caloric value, minimizing the water level, and increasing the oil supply. In addition to bio-oil, the hydrogen-rich syngas generated during pyrolysis is considered another potential and dependable energy source. Due to its high energy and lack of carbon emissions, hydrogen is a versatile substance that may be utilized in various industries, including energy production and heating [15].

The co-pyrolysis of plastics and biomass leads to a synergistic reaction, as the H/C_eff ratio increases, and chemical interactions occur between the co-reactants, resulting in the formation of highly interesting compounds. There is extensive research on this topic, as evidenced by numerous works. Jin et al. [16] conducted an investigation into the co-pyrolysis of lignin with plastic polymers and found that the incorporation of polycarbonate (PC) or low-density polyethylene (LDPE) reduces the production of certain aromatic compounds, while enhancing the production of monomeric aromatic HCs during lignin—polystyrene (PS) co-pyrolysis without using a catalyst. These results demonstrate that combining biomass and plastics in a pyrolysis process has the potential to produce liquid products of a higher quality and yield than using either material alone.

The increasing attention toward co-pyrolysis has encouraged scholars to explore the intricate mechanisms and kinetics of the reaction, along with the synergistic interplay between plastics and biomass during pyrolysis, in order to enhance the process, system, and product yields. Additionally, premium bio-oil can be produced using a catalytic co-pyrolysis process that combines two or more feedstock materials with an acidic catalyst. This review article thoroughly discusses the process and mechanism of catalytic co-pyrolysis, the influence of plastics on the process, and how the addition of plastics can improve the quality and output of bio-oil while reducing the production of oxygenated compounds and coke. The importance of various catalysts (such as biochar, activated carbon, and acid and base catalysts) in improving the production and quality of obtained products is also compared and discussed.

2. Methodology

The review paper employed a two-step method: firstly, literature was collected and categorized using predetermined guidelines through database searches. A total of 114 papers including both research and review papers were collected for this review. Secondly, expertise was evaluated and debated by extracting knowledge on the technological effectiveness of CCP of biomass–plastic mixtures. Relevant terms and keywords were used to conduct a detailed search on Elsevier, Web of Science, Science Direct, Springerlink, Taylor and Francis, American Chemical Society, Google Scholar books, and Scopus catalogs. The research papers were evaluated to determine their relevance to the current research agenda, and available reviews on CCP of biomass–plastic mixtures were also gathered. The authors excluded reports with controversial findings or that lacked scientific reasoning. This could be because the studies have findings that are inconsistent with the current scientific consensus, or the studies have significant methodological flaws that make their results unreliable. Excluding such reports is a common practice in scientific research, as it is important to ensure that the studies being cited are reliable, valid, and replicable. By excluding reports with controversial findings or lacking scientific reasoning, the authors are attempting to maintain the scientific rigor of their research and to provide a more accurate and trustworthy representation of the current state of knowledge in their field.

3. Influence of Feedstock Characteristics on CCP of Biomass and Plastic

The characteristics of biomass and plastic feedstocks play a crucial role in the co-pyrolysis process and can significantly influence the outcomes. By considering these feedstock characteristics and utilizing catalysts, certain issues encountered during the co-pyrolysis of biomass and plastics can be potentially improved. Here is the essential information on the influence of feedstock characteristics and the potential benefits of catalysts.

Biomass feedstock typically comprises 40–50 wt% of cellulose, followed by hemicellulose (15–30 wt%) and lignin (15–30 wt%) [17]. The minor components of biomass, such as ash and extractives/volatiles, may have a considerable influence on the conversion process that is used. The three major categories of renewable feedstocks—lignocellulosic, herbaceous biomass, and municipal solid wastes—show notable differences in composition, as revealed in Table 1. Although differences in composition are visible among terrestrial feed stocks, this variation becomes more evident when municipal solid wastes are included. Due to a lack of data and the challenges of obtaining similar analysis techniques across different institutions, algal biomass was not integrated into this study. Due to the three natural polymers present in biomass, it is a heterogeneous polymer, as illustrated in Figure 1. The diverse composition of feedstocks affects the reaction pathways, product yields, and quality during co-pyrolysis. Catalysts can aid in optimizing the interaction among different components, enhancing conversion efficiency, and promoting desirable product formation [18]. Biomass and plastic feedstocks possess different reactivities due to their chemical structures. For instance, the lignin in biomass is less reactive compared to that in cellulose and hemicellulose. Plastics also exhibit varying degrees of reactivity depending on their polymer type. Catalysts can modify the reaction kinetics and improve the reactivity of feedstocks, enabling better conversion and increasing the yield of desired products [19].

Numerous studies investigated the catalytic co-pyrolysis (CCP) of diverse biomass and plastic feedstocks, resulting in a significant improvement in the properties of the resulting bio-oil, including its heating value (HHV) and oxygen content, compared to the pyrolysis of each feedstock individually. Biomass for CCP can be classified into four categories: biomass components, whole biomass, pretreated biomass, and composites of biomass and plastic.

Yuan et al. (2018) [20] conducted pyrolysis of cellulose with plastics such as PE, HDPE, and LDPE in the absence of a catalyst. They found that HDPE reduced carbohydrates, aldehydes, ketones, and furan groups in the resulting bio-oil by breaking down cellulose-derived compounds. Moreover, the presence of cellulose promoted the formation of alkane and alkene groups from plastics. In a separate study, Gunasee et al. [21] reported an increase in bio-oil yield during cellulose and LDPE co-pyrolysis, leading to 15% total energy recovery through increased devolatilization and condensation processes. In the case of lignin, there are fewer studies on co-pyrolysis due to a lack of synergy, which results in a decreased aromatic yield compared to cellulose co-pyrolysis. Jin et al. [16] investigated the co-pyrolysis of lignin with LDPE, polycarbonate (PC), and polystyrene (PS). They found that co-pyrolysis with PC led to the production of phenolic-type compounds while inhibiting aromatic compounds.

Table 1. The makeup of substances used as raw material for woody, herbaceous, and waste materials; average (standard deviation) ^{number of samples}.

Feedstock Composition	Wastes	Herbaceous	Woody
Proximate
Volatiles (%)	76.7 (5.5) 21	79.1 (5.8) 284	84.0 (2.1) 193
Ash (%)	6.6 (6.7) 21	5.5 (3.2) 284	1.3 (0.9) 193
Fixed carbon (%)	14.8 (5.0) 21	15.4 (4.0) 284	14.7 (1.6) 193
Ultimate
Hydrogen (%)	5.9 (0.4) 21	5.8 (0.3) 276	6.0 (0.1) 192
Carbon (%)	46.0 (4.0) 21	47.4 (1.9) 276	50.7 (4.71) 192
Nitrogen (%)	1.3 (1.6) 21	0.75 (0.49) 276	0.32 (0.01) 192
Oxygen (%)	38.3 (4.2) 7	41.0 (2.4) 107	41.9 (1.4) 134
Sulfur (%)	0.15 (0.16) 7	0.10 (0.32) 107	0.03 (0.01) 135
Structural
Cellulose (%)	28.4 (13.2) 27	32.1 (4.5) 2425	51.2 (8.7) 241
Hemicellulose (%)	16.4 (5.5) 27	18.6 (3.4) 2425	21.0 (8.7) 241
Lignin (%)	12.5 (2.7) 15	16.3 (3.3) 2425	26.1 (5.3) 241

Note: Reproduced and edited from Williams et al. (2017) [22].

Table 1. The makeup of substances used as raw material for woody, herbaceous, and waste materials; average (standard deviation) ^{number of samples}.

Feedstock Composition	Wastes	Herbaceous	Woody
Proximate
Volatiles (%)	76.7 (5.5) 21	79.1 (5.8) 284	84.0 (2.1) 193
Ash (%)	6.6 (6.7) 21	5.5 (3.2) 284	1.3 (0.9) 193
Fixed carbon (%)	14.8 (5.0) 21	15.4 (4.0) 284	14.7 (1.6) 193
Ultimate
Hydrogen (%)	5.9 (0.4) 21	5.8 (0.3) 276	6.0 (0.1) 192
Carbon (%)	46.0 (4.0) 21	47.4 (1.9) 276	50.7 (4.71) 192
Nitrogen (%)	1.3 (1.6) 21	0.75 (0.49) 276	0.32 (0.01) 192
Oxygen (%)	38.3 (4.2) 7	41.0 (2.4) 107	41.9 (1.4) 134
Sulfur (%)	0.15 (0.16) 7	0.10 (0.32) 107	0.03 (0.01) 135
Structural
Cellulose (%)	28.4 (13.2) 27	32.1 (4.5) 2425	51.2 (8.7) 241
Hemicellulose (%)	16.4 (5.5) 27	18.6 (3.4) 2425	21.0 (8.7) 241
Lignin (%)	12.5 (2.7) 15	16.3 (3.3) 2425	26.1 (5.3) 241

Note: Reproduced and edited from Williams et al. (2017) [22].

Figure 1. The lignocellulosic biomass structure includes lignin, hemicellulose, and celluloses. Adapted from Suriapparao and Tejasvi (2022) [23].

Figure 1. The lignocellulosic biomass structure includes lignin, hemicellulose, and celluloses. Adapted from Suriapparao and Tejasvi (2022) [23].

4. Influence of Plastic Composition on Catalytic Co-Pyrolysis

Understanding the influence of plastic composition on CCP is crucial for optimizing product yields and quality. Plastic, a synthetic or semi-synthetic material, is made up of a diverse range of organic polymers sourced from petroleum. It has become a common material for manufacturing everyday objects such as bottles, packaging, toys, and numerous other items. Plastics are composed of various organic polymers with high molecular mass, typically derived from petrochemicals. There are two main types of plastics: thermoplastics, which can be melted and reformed, and thermosetting plastics, which form irreversible chemical bonds when heated. Common examples of plastics include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyester (PET) [24]. Plastics can have different properties such as transparency, opacity, reflectivity, or fluorescence, and they are available in various colors, textures, and sizes. These properties are dictated by both the polymer type and the additives incorporated during the manufacturing process. Different forms of plastics are manufacture based on their intended application, such as polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), and more [25].

The composition of plastic waste significantly impacts the efficiency and selectivity of CCP. Different types of plastics have different chemical structures and properties, that affect their reaction during pyrolysis. On the one hand, plastics with a high content of aromatic compounds, such as PET or PS, can produce a high yield of aromatic (HCs) and are used in the production of plastics, fuels, and chemicals. On the other hand, plastics with a high content of aliphatic compounds, such as PE or PP, can produce a high yield of liquid fuels [26]. The presence of impurities such as metals, chlorine, and sulfur in plastic waste can also impact the efficiency of the CCP process. These impurities can deactivate or poison the catalyst, leading to a decrease in product yield and quality [27]. Understanding the composition of plastic waste and its effects on the CCP process is essential for optimizing product yields and quality.

Numerous studies explored the utilization of plastic waste through CP and gasification to produce value-added products, including but not limited to liquid oil, porous C, solid CNTs, and synthesis gas. Researchers proposed and explored a two-stage method aimed at enhancing the conversion efficiency of the thermochemical reaction of plastic. In the first phase, PW was subjected to pyrolysis, which transformed it into HC vapors [28]. In the second phase, these vapors underwent catalytic reforming at high temperatures. Polyolefins, including polyethylene, polypropylene, and polystyrene, account for more than 50% of PW and are generated, consumed, and discarded at a greater pace than other types of polymers. To address the issue of plastic waste, researchers explored various approaches to transform these waste polymers into value-added products, such as HC fuels, carbon nanotubes (CNTs), and other materials [29].

5. Process and Mode of Catalytic Pyrolysis for Bio-Oil Production

Catalytic pyrolysis (CP) is a promising thermochemical process that converts solid biomass into valuable liquid fuels and chemicals in a single step. It involves subjecting the biomass to high heating rates (ranging from 10–100 °C/s) and medium temperatures ranging from 350–700 °C in anaerobic or low-oxygen environments with a varying residence time depending on several factors such as the type of catalyst used, the reaction temperature, and the properties of the feedstock. Typically, the residence time ranges from a few seconds to several minutes [30]. The reactor operates under mild temperature, under atmospheric pressure, and without hydrogen. CP of biomass can produce several essential petrochemicals such as toluene, benzene, xylene, ethylene, and propylene. However, enhancing the yield of useful petrochemicals remains a significant challenge in CP, as stated in [31].

The pyrolysis of biomass feedstock generates solid, liquid, and gaseous byproducts, as depicted in Figure 2 (adapted from Wang et al. (2022) [32], with permission). Through catalytic pyrolysis, a thermochemical process, lignin, which is among the primary components of biomass, along with cellulose and hemicellulose, can be transformed into valuable aromatic HCs or other phenolic compounds [9].

When catalysts are used, the quantity of the aromatic HCs produced significantly increases in bio-oil. Figure 3 depicts the process of creating aromatics from lignocellulosic biomass. Previous studies showed that multiple chemical processes, such as dehydration, depolymerization, isomerization, decarboxylation, aromatization, and char formation, simultaneously occur during biomass pyrolysis. Anhydrosugars and furan compounds are the main initial pyrolysis products formed during the degradation of cellulose and hemicellulose. The acid-catalyzed dehydration of cellulose generates anhydrosugar at the catalyst’s acid sites, along with the production of furanic species such as furan, methylfuran, furfural, small aldehydes, and water in the gaseous state, via dehydration, bond cleavage, and bond rearrangement, as reported in Wang et al. (2022) [32].

Using catalysts can significantly increase the production of bio-oil and the quantity of aromatic HCs. Figure 3 (adapted from (Wang et al. (2022) [32], with permission) illustrates the process of generating aromatics from lignocellulosic biomass. Anhydrosugars and furan compounds are the primary initial pyrolysis products that form when cellulose and hemicellulose degrade. The acid-catalyzed dehydration of cellulose results in the formation of anhydrosugar at the acid sites of the catalyst. This process is accompanied by the creation of furanic species, including furan, methylfuran, furfural, small aldehydes, and water, through dehydration, bond cleavage, and bond rearrangement in the gaseous state, as mentioned by Wang et al. (2022) [32].

According to a source (Wang et al. (2023) [33]), in situ and ex situ catalytic pyrolysis are currently the two most commonly used types of catalytic pyrolysis. In the in situ method, the catalyst and biomass raw material are mixed in a specific ratio before pyrolysis. In contrast, the ex situ method involves the addition of a separate catalytic layer to upgrade the initial pyrolysis vapors of the biomass. The reactor configurations for both in situ and ex situ upgradings are shown in Figure 4, which has been reproduced with the permission of Wang et al. (2023) [33].

Xu et al. (2017) [34] conducted a comparison of in situ and ex situ catalytic pyrolysis methods. They found that in situ catalytic pyrolysis could reduce the production of solid biochar products and facilitate the breakdown of lignin macromolecular fragments. However, the high amount of catalyst required for the process resulted in a shorter interval between the pyrolysis vapors and the catalyst, leading to a significant increase in production costs as the catalyst had to be repeatedly recycled for continual industrial output. In contrast, ex situ catalytic pyrolysis involved layering the catalyst and lignin and the catalyst and biochar could be easily separated after the reaction. The separated biochar could be used to create a high-value supplementary product [35]. The combination of ex situ catalytic pyrolysis with other upgrading technologies, such as catalytic hydrodeoxygenation, could enable accurate control of lignin pyrolysis parameters and catalytic upgrading, resulting in the production of high-quality target products from lignin.

6. Co-Pyrolysis and Catalytic Co-Pyrolysis (CCP) of Biomass–Plastic Mixtures

Growing environmental concerns and increasing global demand for fuel have spurred the exploration of alternative, clean, and renewable energy sources [36]. By more efficiently using renewable feedstocks and reducing our reliance on finite, non-renewable energy sources [37], this advanced technique offers a viable alternative to traditional fossil-fuel-based production methods. CCP has become a popular method for converting biomass and plastics into high-quality liquid fuels and chemicals. Different catalysts are used to enhance the reaction rate and selectivity and reduce undesirable byproducts. Compared to non-catalytic pyrolysis, CCP produces higher yields of liquid products with better quality, making it an energy-efficient and cost-effective process. Lignocellulose biomass has attracted attention as a cheap, non-edible, renewable, and environmentally beneficial source for producing liquid biofuels and other useful compounds [38], making it a promising solution to the financial and environmental challenges associated with fossil fuels [36].

The selection of an appropriate catalyst is crucial for successful pyrolysis, as it influences product distribution, chemical homogenization, and fractional product yield, ultimately improving the quality of the end products. Catalysts are cost-effective, accessible, and compatible with a variety of raw materials. They can lead to a higher conversion rate, shorter reaction time, and lower temperature. The liquid HCs generated through the catalytic pyrolysis of cellulose have boiling points similar to gasoline and kerosene. The use of various types of catalysts can alter the selectivity and yields of the final product, thus affecting the content and physicochemical properties of the bio-oil. For instance, when cellulose was subjected to heating in the presence of both acidic and basic catalysts, several reaction paths were observed [39].

Research studies explored the potential of catalytic pyrolysis in generating oxygen-containing organic compounds, diesel, and gasoline. In in situ pyrolysis, the feedstock and catalyst are added together into the reactor, resulting in the formation of both solid and gas phases. Conversely, in ex situ pyrolysis, the feedstock and catalyst are separately introduced, with the solid and gas phases occurring in the reactor and catalyst bed, respectively [40]. According to Sharifzadeh et al. (2019) [41] and Duan et al. (2020) [42], the yield and reliability of bio-oil obtained from catalytic pyrolysis depend on various operational factors such as temperature, reactor type, catalyst-to-biomass ratio, heating rate, residence time, and the size of the biomass particles. The biomass substock and catalyst type also play a crucial role in determining the outcome of the process. Researchers explored numerous catalysts, including metals and their oxides (ZnO and FeOx), acidic zeolites (ZSM-5, HZSM-5, ZSM-11, HB, HY, and H-mordenite), alumina, basic catalysts (CaO, KOH, and MgO), bentonite clay, and red mud, which have unique potentials for catalytic pyrolysis [43]. Operating at high temperatures results in excessive vapor cracking, which increases gas production. Conversely, lower temperatures promote the yield of char. The temperature range of 400–600 °C produces the highest yield of bio-oil in various reactors [44]. In catalytic pyrolysis, the size of biomass particles affects the yield of bio-oil; smaller biomass particles (<2 mm) result in higher yields, while larger particles (>2 mm) increase the temperature gradient, which affects the local heating rate [45]. Deoxygenation processes such as dehydration, decarbonylation, and decarboxylation release oxygen from water and carbon oxides (COx) and regulate both yield and quality in the process [46]. The yield of bio-oil produced by catalytic pyrolysis is somewhat lower than that of non-catalytic pyrolysis, so there is a need to improve the process parameters of catalytic pyrolysis. Although various studies focused on increasing pyrolysis rates, only a few concentrated on optimizing the variables to improve catalytic pyrolysis efficiency [32].

By utilizing a suitable catalyst during co-pyrolysis, the thermochemical degradation of both biomass and plastic can be improved, leading to tailored product compositions and lowered activation energy for the reaction. The inclusion of a catalyst during the degradation process offers several benefits, such as shorter reaction times, reduced degradation temperatures, increased degradation extent, minimized solid residue content in the final products, and a more focused distribution of end products [40,46]. The catalyst also helps to guide the reaction in the desired direction through interactions among its composition, the reaction pyrolyzates, and the intended products [47]. The effectiveness of the catalyst depends on its porosity, redox, and acidity qualities. Therefore, it is crucial to adjust the acidity of the catalyst based on its density, strength, and type, as each of these factors has a unique impact on the activity, product selectivity, and reaction route.

7. Synergistic Effects and Mechanism of CCP

The interaction between biomass and plastic feedstocks plays a crucial role in enhancing the yields of co-pyrolysis products. This interaction occurs through the exchange of radicals and elemental particles during the co-pyrolysis process. When biomass and plastic synergistically interact, there is a significant improvement in both the quality and quantity of the produced products. However, the understanding of these positive interactions is limited due to various factors such as heating rate, temperature, catalyst type, and feedstock blending ratios [48]. In this section, we explore the synergistic mechanisms involved in biomass and plastic co-pyrolysis and examine their impacts on the quality and yield of biofuels.

The specific composition of the products obtained from catalytic co-pyrolysis can vary depending on the type and proportion of feedstocks used, the catalyst used, and the reaction conditions. For example, the use of a catalyst can increase the yield of certain products, such as aromatic HCs, while reducing the yield of others, such as oxygenated compounds. The use of biomass as a feedstock can also increase the yield of oxygenated compounds in bio-oil [49]. In general, the products obtained through catalytic co-pyrolysis exhibit promising prospects as sustainable energy and chemical sources. By optimizing the process, it becomes possible to enhance both the efficiency and selectivity of the product yield, further maximizing its potential benefits. .

The process of CCP reaction can become intricate due to the use of various materials. There are two main categories of CCP: the thermal decomposition of biomass and plastics caused by heat and the pyrolytic volatiles that are catalyzed by catalysts [50]. The co-pyrolysis of biomass–plastic model complexes during catalytic fast pyrolysis exhibited a synergistic impact upon the introduction of cellulose and LDPE together. This interaction resulted from the oxygenates originating from cellulose and olefins stemming from LDPE [51]. It was discovered that by using a ZSM-5 catalyst, the oxygenates generated from cellulose could interact with light olefins produced from LDPE [51].

The radical interaction mechanism is frequently put forward as a potential explanation for the observed synergistic effects in co-pyrolysis studies involving biomass, plastics, and other materials. Despite some researchers who found these synergies to be ambiguous or challenging to identify in certain cases, many research groups viewed the radical interaction mechanism as a key factor in co-pyrolysis [52,53]. Typically, the phrase “synergistic effect” denotes the phenomenon where two or more factors interact, resulting in an overall effect that surpasses the sum of the individual effects or contributions of the co-feeding factors. This interaction can either enhance the quality and quantity of the end products or reduce the properties of the resulting blend or its yield products [54]. Many researchers, including Lopez et al. (2017) [52], acknowledged this synergistic interaction between biomass and plastics in co-pyrolysis as the primary cause of the improvement in oil yield and quality. It has been observed that to attain a synergistic effect, it is crucial to have effective interaction between the co-feeding components, including biomass and plastics, as well as the compositions and proportions of the individual components, in addition to pyrolysis temperature and reaction time. Hence, fixed-bed and auger reactors are more appropriate for carrying out co-pyrolysis, since achieving good interaction among the co-feeding elements is the primary determinant in achieving favorable synergistic results, rather than fluidized-bed reactors.

To generate pyrolysis oil of superior quality, it is important to have a proper grasp of the co-pyrolysis mechanism. Therefore, the mockup preparation/pretreatment, co-pyrolysis, and condensation stages are the primarily three rudimentary processes or mechanism routes involved in co-pyrolysis for the aim of producing oil. In order to ensure efficient and stable conversion in the co-pyrolysis process, feedstock resources must be appropriately arranged and pretreated after harvesting or acquiring from suppliers. This includes drying for 24 h in an oven at 105 °C and grinding the material into minor units, typically < 2–3 mm, in order to increase the heating rate of biomass [54].

Due to variations in their compositions, the thermal degradation of biomass alone differs from the co-pyrolysis of biomass mixed with other raw materials. As a result, the degradation mechanisms for plastic and biomass during the thermal pyrolysis process are distinct. Biomass undergoes thermal decomposition through a number of exothermic and endothermic reaction mechanisms, while plastic pyrolysis occurs through radical mechanisms. These radical mechanisms involve initiation, propagation/secondary radicals’ formation, and revocation through the use of radicals’ disproportionation or recombination, as indicated by previous studies [55,56].

When biomass and plastics are burned together, typical free radical events occur, including depolymerization, hydrogen transfer reactions, the synthesis of monomers, intermolecular hydrogen transfer (which results in the production of paraffin and dienes), and isomerization via vinyl groups. Moreover, Demirbas, (2009) [57] found that biomass does have an inferior thermal stability compared to plastics, which may alter the radical degradation process of these materials by encouraging the breakdown of synthetic macromolecules. It is essential to bear in mind that the co-pyrolysis mechanism of biomass and plastics at low heating rates, particularly in co-pyrolysis studies conducted in TGA instruments, significantly differs from that under high heating rates, such as in industrial reactors and Py-GC/MS devices, where the thermal degradation of each module of biomass and plastics simultaneously occurs.

The whole reaction network shown in Figure 5 was proven by Zhang et al. (2016) [19], using the co-feed CP of real lignocellulosic biomass with HC-based polymers over zeolite-based catalysts. For the predominant pathway in lignocellulosic biomass, the thermal degradation of cellulose could involve a series of dehydration, decarbonylation, and decarboxylation processes to produce furan compounds [58].

The potential yield of biofuels can be estimated by analyzing the moisture, ash, carbon, and volatile matter content in the feedstocks, known as proximate analysis. Generally, a higher proportion of volatile matter in the mix enhances reactivity during pyrolysis, leading to increased biofuel production. On the other hand, a higher ash content decreases the biofuel yield, resulting in more biochar production. Woody biomass, rich in lignin, tends to yield a higher percentage of char compared to other biomass types. Therefore, biomass with a higher proportion of cellulose and hemicellulose is preferred as it generates more volatiles during pyrolysis, thereby increasing the biofuel yield [59]. In contrast, plastics mainly produce volatiles and liquid products during pyrolysis. To optimize the yields of pyrolysis liquid and volatiles from plastics, it is crucial to employ high temperatures and rapid heating rates, which are the key influential parameters to consider [60].

A high higher heating value (HHV) of pyrolysis oil indicates the production of high-quality biofuels and signifies good synergy between the chosen biomass and plastic feedstocks [61]. Achieving a high HHV involves reducing the oxygen content in oil, while a high calorific value (CV) is attained by increasing the ratio of HCs in oil. Conversely, a high water or moisture content negatively affects the quality of biofuels and contributes to a lower energy density. Excessive moisture from biomass can decrease the efficiency of pyrolysis in the reactor, resulting in a lower biofuel yield and compromised quality. Hence, biomass feedstocks often require drying before undergoing pyrolysis. As plastic wastes generally have lower water content compared to biomass, increasing the blending ratio of plastic feedstock reduces the overall water content while simultaneously boosting the biofuel yield [62].

Figure 5. Possible reaction pathways of catalytic pyrolysis over zeolite catalyst, adapted based on Zhang et al. [19,58].

Figure 5. Possible reaction pathways of catalytic pyrolysis over zeolite catalyst, adapted based on Zhang et al. [19,58].

8. Role of Catalyst in CCP

In order to limit coke production and boost the quantity of aromatics, catalysts are particularly important in the transformation of plastic and biomass mixing in CP [54]. Catalysts can dramatically lower energy input and tar production while lowering the oxygen level and improving the quality of biofuel [12]. Acid catalysts are renowned for their ability to enhance the production of valuable compounds in CP and promote dehydration in cellulose pyrolysis deoxygenation. However, when they interact with alkali and alkaline earth metals (AAEMs), which are present in biomass ash, certain drawbacks arise. The catalyst’s acidity sites can accumulate metals, rendering the catalyst inactive [63]. During catalytic CP, the feedstock is converted into bio-oils through catalytic cracking and thermal cracking (pyrolysis). The surface and internal pores of the catalyst experience thermal cracking. In comparison to regular pyrolysis, catalytic cracking is offered as an efficient and selective technique that enables the production of precious materials. A catalyst could be added to pyrolysis to lower the desirable temperature, change the distribution of products with more selectivity, or raise the yield of the desired HCs’ economic worth. Catalysts are crucial in the conversion of the volatile mixtures of oxygenated species produced by thermal degradation into olefins and aromatics by a number of processes, including cracking, aromatization, oligomerization, and deoxygenation [64]. Different catalysts have been used during the CCP of plastics and biomass.

Importantly, zeolite serves as the process’s main catalyst. Various scholars found that CCP raises the C output of petrochemicals (especially monocyclic aromatic HCs) and reduces coke formation because the proper amount of suitable zeolite catalyst is added during the process [65]. Similarly, it was discovered that adding the right catalysts reduces the temperature and activation energy during the CCP of biomass and polymers, increasing conversion efficiency. Yet it was observed that the presence of heteroatoms during the catalytic co-pyrolysis process might have a considerable impact on a zeolite-based catalyst’s life. As a result, to reduce the issues related to catalytic co-pyrolysis, it is necessary to produce a more acceptable catalyst with acid sites and an acidity that are more suited to the target products via specific reaction pathways. However, it is determined that CCP is a very auspicious approach, particularly for reducing the over-dependence on and depletion of fossil fuels, mitigating the consequences of global warming, improving the setup for waste management, and enhancing energy security. The process also produces gas and char, two extremely useful byproducts. The CCP of biomass with PW for the synthesis of valued aromatics is unquestionably noteworthy, being a considerably more encouraging technology than the CP of biomass alone [66,67]. According to numerous research findings, the catalyst HZSM-5 is the most efficient method for pyrolyzing biomass and polymers in order to produce aromatics [30]. The synthetic polymer–biomass mixture undergoes catalytic thermal degradation; however, the catalyst HZSM-5 exhibits variable properties depending on how quickly or slowly it is heated. When the catalyst HZSM-5 is used under moderate heating, the thermal degradation of the biomass–PW blend into useful products is greatly reduced because acid catalyzed reactions took place on the surface of the zeolite [30].

8.1. Activated Carbon (AC)

Due to their significant benefits in the CFP of plastics waste and lignocellulosic materials, activated carbon (AC) catalysts have gained more and more attention in the past few years. Comparatively speaking, ZSM-5 zeolite is more expensive than AC made from agrarian biomass residues or coal [68]. Additionally, AC is less vulnerable to deactivation because of its hierarchical pore structure, which contains both mesoporous and microporous particles, as well as its non-acidic or slightly acidic features [69].

As phosphoric acid activates AC, several P-containing functional groups (such as -C-P-O and -C-O-P) may be introduced. AC is also rich in O-containing functional groups. Due to the strong catalytic reactivity of these O- and P-containing functional groups, which serve as reactive sites in the AC catalyst, the deoxygenation and cracking reactions are promoted [70]. During the simultaneous pyrolysis of biomass and polymers, the micro-mesoporous AC catalyst with active functional groups demonstrated good catalytic performance. In the co-pyrolysis of biomass and plastic blends, Duan et al. (2020) [71] discovered that AC catalysts with various pore shapes displayed various catalytic effects on the yield and selectivity of aromatics and phenols. They performed the initial, commercial, activated carbon catalytic co-pyrolysis of Douglas fir and low-density polyethylene. They evaluated and compared six different kinds of activated carbon catalysts. Depending on the different co-pyrolysis settings, the resulting liquid product had physically distinct components that were in the oil and water phases, with the percentage of the oil phase varying from 10.10 to 64.4 wt%. Up to 98.6% of the oil phase of bio-oil contained C₈-C₁₆ aromatics and aliphatics, which are compatible with jet fuel for transportation. In addition, phenols and guaiacols made up the majority of the bio-oil in the water phase, with significant phenol selectivity (up to 92.9 area%) and phenol concentration (up to 26.4 mg/mL) being attained. The predominant components of gaseous products were hydrogen, methane, carbon dioxide, and carbon monoxide, where methane (23.6 vol%) and carbon monoxide (39.1 vol%) could be found in large concentrations. The process was optimized by taking into account the production of bio-oil, the selectivity of C₈-C₁₆ HCs, and the concentration of phenol.

In a recent study by Lin et al. (2023) [72], the authors investigated the co-pyrolysis of Douglas fir and low-density polyethylene using different activated carbons (AC) as catalysts. The study focused on understanding the influence of AC pore structures and active functional groups on the product yields and compositions. The highest yield was achieved using an H₃PO₄-activated AC catalyst, even though the liquid output varied between 36.7% and 50.0%. Steam-activated AC catalysts exhibited remarkable selectivity for mono-aromatic compounds and had well-developed micro-mesoporous structures dominated by micropores. In contrast, H₃PO₄-activated AC catalysts had additional mesoporous characteristics and P-containing functional groups, which provided enough room and reactive sites for converting bulky co-pyrolytic intermediates into double-ring aromatics, which were mostly followed by the discharge of hydrogen (up to 80.6 vol%). During co-pyrolysis, a synergistic catalytic action boosted gas formation at the expense of liquid. The yield of oxygenates and >C16 aliphatic HCs was decreased, while the production of aromatics was improved by all AC catalysts. Steam-activated AC catalysts, in contrast to H₃PO₄-activated AC catalysts, had a detrimental impact on hydrogen production, indicating various synergistic routes during co-pyrolysis. The possible reaction mechanism for Douglas fir (DF)/low-density polyethylene (LDPE) co-pyrolysis over an AC catalyst is depicted in Figure 6 (adapted with the permission of Lin et al. (2023) [72]). Their research showed that AC catalysts have a significant potential for transforming waste biomass and plastics into aromatics with value-added and hydrogen-rich syngas by controlling pore shape and functional groups.

According to Lin et al. (2023) [72], the addition of sulfonic active sites to an AC-based sulfonated catalyst increased the synthesis of bio-jet fuels (aromatics and C_9-16 alkanes) from the catalytic co-pyrolysis of biomass and plastics. These experiments collectively demonstrated the effective catalytic performance of AC’s pore structure and active sites on the development of co-pyrolysis products. However, prior research largely focused on optimizing co-pyrolysis parameters such as temperature and the ratio of AC to feedstock for the increased synthesis of jet-fuel-range HCs.

Potential critical aspects of the study include the limited range of the activated carbons used as catalysts, as well as the specific conditions and parameters employed in the fixed-bed reactor. Further research could explore a broader selection of catalysts and optimize the reaction conditions to enhance the desired product yields and selectivity, including hydrogen production.

8.2. Biochar-Based Catalyst

An alternative, lower-energy method for targeted production yield is provided by catalysts. Without a catalyst, the thermal degradation of solid biomass and plastics inevitably necessitates escalating the temperatures and yields of non-selective byproducts. Zeolites and metal catalysts such as Ni, Zn, Al, and K were used in co-pyrolysis to improve H₂ production. Unfortunately, those catalysts encountered a number of issues due to chemical poisoning, metal sintering, or inadequate metal dispersion [73]. Biochar is a charcoal-like material created through the pyrolysis of biomass with a large number of sources, enriched porosity, and a customizable structure, which are all signs of a good catalyst candidate. In tar reforming, esterification/transesterification, hydrogenation, and oxygen reduction reactions, research demonstrated that biochar catalysts have remarkable catalytic activity [74,75]. Biochar may be used as a catalyst in the co-pyrolysis of biomass and plastics, resulting in the simultaneous production of syngas and bio-oil. In this process, biochar acts as a source of heat, causing the feedstock materials to decompose into combustible gases and liquids. The syngas that is generated can be used to generate electricity, while the bio-oil can be used to fuel vehicles and other applications. According to studies, using biochar enhanced the output of the acetic acid and phenols in bio-oil, and high-quality syngas was also produced [76]. Scientists concur that the use of biochar instead of traditional activated carbon (AC) is much more economically sound because it can provide similar physicochemical characteristics to ACs but requires very little energy and chemical products during the preparation method. Although some studies preferred to use activated carbon (AC) as catalysts, scientists agree that biochar can offer these properties [77]. Pyrolysis is an important step in the synthesis of biochar, since it has an impact on both the effectiveness and the cost of production. Furthermore, this biochar has unique surface properties, such as high porosity, surface area, and increased aromatic and mineral content [78].

In a work by Zou et al. (2022) [15], biochar was used as a catalyst in the co-pyrolysis of biomass and plastics (Douglas fir (DF) and low-density polyethylene (LDPE)) with microwave assistance. In an effort to create a straightforward, effective, affordable, and environmentally friendly approach for raising the values of biomass pyrolysis products, the authors also made an effort to investigate the effect of applying biochar to pyrolysis products... The impact of different ratios of biochar/feedstock on the composition of liquid products is presented in Figure 7. The experimental groups conducted at temperatures of 600 °C and 650 °C demonstrated that the yield of medium-weight aromatic HCs (MAHs) increased as the biochar/feedstock ratio increased. Specifically, at 600 °C, the yield of MAHs rose from approximately 7 area% to 56 area% with increasing ratio. However, when the ratio exceeded 4, the beneficial effect on MAHs yield diminished, and a negative trend was observed. In contrast, the selectivity of aliphatic HCs showed a decreasing trend with increasing biochar/feedstock ratio, but it started to increase again when the ratio surpassed 4. Oxygenated compounds present in the liquid products include phenolic compounds and other oxygenated compounds. The addition of biochar in higher proportions effectively reduced the proportion of oxygenates in the liquid phase. This reduction in oxygen content is beneficial as it enhances the calorific value of the liquid product. Reduced oxygen concentration in the liquid form would increase its calorific value, and high HC contents are a key sign of premium liquid products [79].

Figure 8 displays the outcomes of the micro-GC analysis used to determine the syngas composition. Syngas mostly consists of H₂ and CH₄, and, when compared to a process without biochar, biochar improves the hydrogen enrichment of syngas. The reduction in C₂, CH₄, and CO adds to the boost in H₂ generation. When the ratio of BC to feedstock was 4, the greatest output of H₂ reached 72 vol% at 650 °C. The yield of H₂ was increased from 47 vol% to 70 vol% when the temperature rose from 529 °C to 671 °C.

8.3. Zeolite Catalysts

A zeolite catalyst is an effective tool used to accelerate the thermal disintegration of biomass and plastics in a process known as catalytic pyrolysis. This type of catalyst is made from natural zeolite minerals and can be used to break down long-chain molecules into shorter-chain molecules or into solid char or gases. By using a zeolite catalyst, the pyrolysis process can be made much more efficient and cost-effective. When using catalytic fast pyrolysis (CFP) to convert plastic–biomass mixtures into desirable aromatic HCs, zeolite-based catalysts are effective [80].

The high acidity of zeolites can catalyze the degradation of biomass and plastic components, decreasing the temperature required for co-pyrolysis and improving the yields of the desired end products. Zeolites with high acidity can also reduce the formation of tar and other undesired byproducts, resulting in a better overall quality of the end products. In addition, zeolites with high acidity can be used to selectively remove polymers from the pyrolysis vapor stream and, thus, improve the quality of pyrolysis oil. CP comprising combinations of biomass (xylan, cellulose, lignin, and switchgrass) and plastic (PP, LDPE, HDPE, PET, and PS) was frequently treated with HZSM-5 [7]. It was shown that combinations of biomass and PE, PP, and PET were co-pyrolyzed in the presence of H-ZSM5, which boosted the generation of total aromatic compounds. According to Xue et al. [50], lignin more strongly interacts with PE than xylan and cellulose because hydrogen is transferred from the aliphatics formed by PE to the phenolic compounds derived from lignin. Additionally, during ex situ CCP, deactivation behavior for switchgrass and HDPE was seen. It was observed that when the cumulative feed to catalyst loading reached a 2:1 ratio, the catalyst deactivation rates were lower; however, the beneficial effects were no longer present at a 4:1 ratio [81].

A study conducted by Park et al. (2019) [82] demonstrated that the production of aromatics can be increased by pyrolyzing microporous zeolite. They co-pyrolyzed Quercus variabilis, and plastic films (PFs) were co-pyrolyzed using two distinct zeolites, HZSM-5 and HY. The results demonstrated that HZSM-5, with its higher and stronger acidity, promoted the synthesis of aromatics more effectively at 600 °C compared to HY. This can be attributed to HZSM-5’s properties, which include its medium pore size, suitable pore window size, internal pore volume, and steric hindrance [83]. Although both HZSM-5 and HY produced the same aromatics from the CP of Q. variabilis and waste PFs, the pathways of aromatic development appeared to differ based on the yields of the major products derived from each feedstock. The HC pool [84] and phenolic pool [85] processes are believed to be involved in the production of aromatics from the CP of biomass using acid zeolites. When biomass undergoes decomposition with acid catalysts, it undergoes catalytic decarbonylation, decarboxylation, and dehydration processes. This transforms the typical pyrolysis products of hemicellulose and cellulose, such as acids and levoglucosan, into light olefins [82]. The addition of hydrogen-rich polymers to the catalytic processing of biomass can extend the catalyst lifetime by preventing the accumulation of coke caused by the oligomerization of oxygen-containing pyrolysis intermediates from biomass. This is achieved through the efficient conversion of oxygen-containing intermediates to aromatics, with the help of plastic-derived intermediates [86]. However, the potential critical aspects of the study include the need for further investigation into the specific mechanisms and reaction conditions involved in the production of aromatics from biomass using acid zeolites. Additionally, the impact of different feedstock compositions and the scalability of the process should be explored to assess the feasibility of this approach on a larger scale.

Mesoporous zeolite catalysts are used for the co-pyrolysis of biomass and plastics because they provide the optimal balance between reactivity and selectivity. These catalysts are known to have a high surface area and high pore volume, which makes them ideal for the co-pyrolysis of biomass and plastics. They also have a high thermal stability which allows them to withstand the high temperatures of the pyrolysis process. Additionally, they are highly selective, meaning that they only react with specific molecules in the reaction mixtures, thus improving the yield of desired products. These catalysts provide greater access to active sites and enhance the catalytic interaction between the co-pyrolyzed reactants, and are also relatively inexpensive, making them a cost-effective choice for the co-pyrolysis of biomass and plastics. Hong et al. (2017) [87] demonstrated that by employing ZSM-5, desilicated ZSM-5, and Al-SBA-15 as catalysts, the co-pyrolysis of cellulose and polypropylene resulted in higher yields of aromatic HCs compared to microporous zeolites. This outcome can be attributed to two primary factors: the augmented hydrogen content of the feedstock and the synergistic interaction between furans derived from cellulose and olefins originating from polypropylene.

Sirous et al. (2017) [88] used three mesoporous solid acid materials—hierarchical mesoporous MFI (meso MFI), hierarchical mesoporous Y (meso Y), and Al-SBA-15—to co-pyrolyze yellow poplar (YP) and high-density polyethylene. Out of the three catalysts, meso MFI was the most effective for the generation of aromatic HCs, thanks to its excellent pore structure, broad channels, and high acidity. Furthermore, due to the hydrogen-donating properties of HDPE and the catalytic interactions between the pyrolyzates produced from YP and HDPE, meso MFI showed the greatest synergistic production of aromatic HCs during the catalytic co-pyrolysis of YP and HDPE. The results from the catalytic co-pyrolysis of YP and HDPE showed that microporous HZSM-5 and HY produced lower amounts of aromatic HCs than mesoporous MFI and mesoporous Y. This indicates that mesoporous catalysts with a higher degree of acidity can increase the yield of aromatics. The presence of strong acids and mesopores both contributed to an enhanced Diels–Alder reaction, as they facilitate molecule diffusion and plastic cracking, respectively [84]. A prior study conducted by Hong et al. (2017) [87] also found that mesoporous HZSM-5 had a higher aromatic generation activity than microporous HZSM-5.

Zeolites, which are acid catalysts, can improve the efficiency of dehydration, decarbonylation, cracking, and aromatization. The strong acid sites of these catalysts are particularly conducive to the aromatization reaction, which creates high-value monocyclic aromatic HCs such as benzene, toluene, ethylbenzene, and xylene—all of which are crucial petrochemicals [51]. ZSM-5 is a type of zeolite molecular sieve catalyst used for various petrochemical processes. It is known for its high activity and selectivity, making it a popular choice for petrochemical applications such as the production of gasoline and diesel fuels. Zeolite ZSM-5 is regarded as the best material for the generation of monocyclic aromatic HCs among the different types of acid catalysts because of its distinctive microporous structure and relatively high Brønsted acidity [83,89].

The study’s findings, which were obtained through the catalytic fast pyrolysis (CFP) of a variety of biomass, plastics (low-density polyethylene (LDPE), polyethylene (PP), and polystyrene (PS)), and their mixtures with ZSM-5 zeolite, show that CFP is a reliable conversion method that can transform various biomasses. Additionally, cellulose or pine wood have a strong synergy with LDPE in CFP that increases petrochemical synthesis (particularly aromatic) and lowers coke formation. The Diels–Alder reactions of furans coming from cellulose with olefins from LDPE can form aromatics when catalyzed by ZSM-5 zeolites and were primarily blamed for this synergy [7]. The primary aromatic formation route during the CCP of plastics and biomass corresponded to the Diels–Alder reaction between olefins from plastics and furans from biomass. Cheng and Huber (2012) [90] demonstrated that the use of ZSM-5 in the conversion of biomass-derived furan, furfural, and 2-methylfuran with propylene led to a higher selectivity of toluene and xylenes than the sole conversion of furans via Diels–Alder reaction-assisted aromatic synthesis. Additionally, oxygenate pyrolyzates, such as levoglucosan, double anhydrosugar, xylose, acetic acids, and furans generated from cellulose and xylan, extracted hydrogen from PE to aid in the depolymerization of PE into tiny olefins and alkanes [91].

In biomass residual ash, alkali and alkaline earth metals including Ca, K, Mg, and Na are typically present. These metals are simple to deposit on the acid sites of catalysts, resulting in the quick deactivation of acid catalysts [63]. In contrast, base active sites are more resilient to the deactivation caused by AAEMs compared to those with acid sites. Carbon coupling reactions such as the transformation of organic acids, ketones, and aldehydes into gasoline and diesel-range products can occur on acid sites. Through CO₂ production during acids’ ketonization, base catalysts such as MgO [92] and CaO (Chen et al. (2019) [93]), encourage the deoxygenation of pyrolysis products [37]. Since acid catalysts encourage CO production, a deoxygenation process is more effective than using an acid catalyst, in terms of conserving carbon. To preserve the H₂ level in bio-oil and boost its energy level and quality, base catalysts also reduce H₂O generation [63].

Activated carbon was identified as a potential catalyst for the co-pyrolysis (CP) of biomass, in addition to the acid–base catalyst. Mateo et al. (2020) [94] found that sulfonated activated carbon produced bio-oil that was suitable for bio-jet fuels. The presence of SO₃H groups induced acidity that promoted catalytic cracking, hydrogenation, and aromatization. Furthermore, Chai et al. (2020) [95] used activated carbon as a support for the pyrolysis/gasification of pine sawdust and LDPE. Specifically, Ni-CaO-C exhibited a higher H₂ yield and lower coke formation due to its increased pore structure and surface area.

Table 2. Summary of recent progress on catalytic co-pyrolysis of biomass with plastics.

Biomass	Plastic	Catalyst	Reactor and Operating Conditions	Optimized Conditions	Ref.
Pine sawdust	LDPE	HZSM-5	Fixed-bed reactor 450 °C	The addition of LDPE inhibited the coking reaction of biomass effectively, resulting in an increase in the volatile content.	[96]
Sugarcane bagasse	HDPE	Mesoporous FAU	Fixed-bed reactor, 400–700 °C	Catalyst-to-feedstock ratio of 1:6, HDPE-to-SCB ratio of 40:60, and temperature of 500 °C; maximum bio-oil yield was achieved.	[25]
Sugarcane bagasse	PET	HZSM-5/Na2CO3/γ-Al2O3	Tandem μ-reactor coupled with GC 400–800 °C	700 °C, biomass-to-PET ratio of 4, and HZSM-5-to-Na2CO3/γ-Al2O3 ratio of 5; maximum BTX yield of 18.3% was obtained.	[80]
Bamboo sawdust	LLDPE	HZSM-5, CeO2/γ-Al2O3	Pyroprobe pyrolyzer coupled with GC 600 °C	Catalyst/biomass ratio of 4, CeO2/γ-Al2O3-to-HZSM-5 mass ratio of 1:3, 75% LLDPE percentage; maximum contents of aromatic HCs were obtained.	[97]
Pine sawdust	HDPE	MgCl2, HZSM-5	Fixed-bed reactor 400–700 °C	Pyrolysis temperature: 600 °C, biomass to-HDPE ratio: 1:2, and feedstock-to-catalyst ratio: 1:1; maximum oil-phase product yield of 20.6% and aromatics’ selectivity (area %) of 95.9% were obtained.	[98]
Grape seeds	Waste tire	CaO	Auger reactor (pilot scale)	Catalyst calcination temperature: 900 °C, 20 wt% waste tires, feedstock: CaO mass ratio of 2:1; deoxygenated bio-oil (0.5 wt% of oxygen content) was obtained with a heating value of 41.7 MJ/kg.	[99]
Sugarcane bagasse	PS	HZSM-5, MgO, CaO	Fixed-bed reactor 500 °C	Mass ratio of 1:3 HZSM-5:MgO; maximum (56.8 wt%) MAHs’ yield and lowest (20.8 wt%) PAHs’ content was observed.	[100]
Camellia shell	Take-out solid waste	HZSM-5, CaO, MgO	Pyroprobe pyrolyzer coupled with GC 700 °C	The mixing ratio of biomass and plastic was 3:7, and the mixture of HZSM-5 and CaO (mixing ratio of 1:1); aliphatic HCs and MAHs were generated, and acids’ formation was inhibited.	[101]

(adapted with the permission of [12]) provides a summary of the studies conducted on the CP of biomass with plastics under different catalysts.

9. Role of Metal Oxides in Catalytic Co-Pyrolysis

Metal oxides, such as iron oxide, nickel oxide, and copper oxide, have been widely used as catalysts in co-pyrolysis processes due to their high activity, selectivity, and stability. The role of metal oxides in catalytic co-pyrolysis can be described in terms of the physical and chemical interactions between the catalyst and the feedstock during the pyrolysis process. The physical interactions involve the adsorption of the feedstock molecules on the catalyst surface, which increases the contact between the reactants and the catalyst and facilitates the chemical reactions. The chemical interactions involve the activation of the biomass molecules by the catalyst, which lowers the activation energy required for the pyrolysis reaction to occur [17,102].

One of the primary functions of metal oxides in co-pyrolysis is to promote the deoxygenation and decarboxylation reactions of the feedstock molecules. During pyrolysis, biomass molecules undergo a series of complex chemical reactions, which result in the formation of a wide range of products, including bio-oil, gases, and solid char. However, the bio-oil produced by traditional pyrolysis methods contains high levels of oxygen and water, which reduces its energy content and stability. Metal oxide catalysts can help to reduce the oxygen content of bio-oil by promoting the removal of oxygen atoms from the biomass molecules. This process, known as deoxygenation, results in the production of bio-oil with a lower oxygen content and higher energy density [103].

Metal oxide catalysts can also promote the decarboxylation of biomass molecules during co-pyrolysis. Decarboxylation is the process of removing a carboxyl group (-COOH) from a molecule, which can improve the quality of bio-oil produced by co-pyrolysis. Metal oxide catalysts can facilitate decarboxylation by providing a suitable reaction environment, such as high temperature and pressure, which can break the chemical bonds between the carboxyl group and the biomass molecule [104]. Alkaline metal oxides such as MgO and CaO were successfully used to deoxygenate aldehydes and carboxylic acids to produce light HC compounds via ketonization and aldol condensation reactions [92,105].

Another important role of metal oxides in catalytic co-pyrolysis is to prevent coke formation during the pyrolysis process [106]. Coke is a byproduct of pyrolysis that can accumulate on the catalyst surface and reduce its activity and selectivity. Metal oxide catalysts can prevent coke formation by promoting the gasification of the solid char produced during pyrolysis. Gasification is the process of converting solid char into gas in the presence of a gasifying agent such as water vapor or carbon dioxide. Metal oxide catalysts can facilitate gasification by providing a suitable reaction environment and promoting the reaction between the gasifying agent and the solid char [107].

Metal oxides, including CaO, CuO, ZnO, Fe₂O₃, Al₂O₃, MgO, and SiO₂, are readily available and cost-effective catalysts. They have been shown to effectively remove oxygen from co-pyrolysis products. In one study, the co-pyrolysis of poplar wood and polypropylene with metal oxides in a reactor at 600 °C resulted in a lower yield of carboxylic acid and an increased yield of alkene compared to non-catalytic pyrolysis. Furthermore, the presence of ZnO, CaO, and MgO was found to improve ketone formation compared to non-catalytic pyrolysis [108]. When black-liquor lignin was co-pyrolyzed with PE/PP/PS using Al₂O₃ as a catalyst, the selectivity for naphthalene and its derivatives was higher compared to non-catalytic pyrolysis [109]. Unlike zeolites, which can suppress oil product yields, mesoporous SiO₂-Al₂O₃ is an attractive option to increase the yield of liquid products.

CaO and HZSM-5 were developed by Zheng et al. [110] to convert xylan and LDPE into valuable hydrocarbons using thermogravimetric analysis (TGA) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). In Figure 9 (adapted with the permission of Zheng et al. (2020) [110]), the noncatalytic pyrolysis of xylan and LDPE resulted in the highest bio-oil yield (56.37%) and the lowest solid yield (8.22%). The presence of a catalyst significantly decreased the bio-oil yield (42.67% and 37.55%), leading to increased solid and gas yields. This suggests that the catalyst promoted biomass transformation and secondary reactions [111]. Gases such as CO, CO₂, and CH₄ and C₂ alkanes/alkenes were produced more by catalytic pyrolysis compared to noncatalytic pyrolysis. The ex situ process increased the solid content (from 39.64% to 46.61%) and decreased the liquid content (from 42.67% to 37.55%) compared to the in situ process. Three explanations for the higher bio-oil yields in in situ catalytic pyrolysis are (1) the slower heating rate in the bulk catalyst/biomass mixture; (2) the rapid catalyst deactivation due to char buildup, leading to higher yields of oxygenated liquid products; and (3) the longer residence/catalyst-contact times for biomass pyrolysis vapors.

Figure 10 illustrates the product yield and aromatic selectivity during the catalytic co-pyrolysis of xylan and LDPE using varying CaO/HZSM-5 mass ratios. Under the conditions of a xylan-to-LDPE ratio of 1:2 and pyrolysis/catalytic temperatures of 450 °C and 550 °C, respectively, Figure 10a shows that without a catalyst, oxygen-containing compounds were the main products, followed by hydrocarbons, which were primarily derived from LDPE degradation. Aromatics were not formed without catalysts. However, with the introduction of HZSM-5, the aromatic content significantly increased while the alkane and olefin contents decreased. Dual-catalytic-stage CaO/HZSM-5 further enhanced the aromatic content. Decreasing the CaO catalyst quantity from 1:0 to 1:2 increased the hydrocarbon and aromatic hydrocarbon contents from 63.27% to 96.01% and 88.35%, respectively, while the oxygenates and polycyclic aromatic hydrocarbon contents decreased. Lowering the CaO amount further decreased the hydrocarbons but increased the polycyclic aromatic hydrocarbons. A CaO-to-HZSM-5 mass ratio of 1:2 maximized aromatic hydrocarbon formation, with a 10.98% enhancement compared to pure HZSM-5. This combined use of CaO and HZSM-5 facilitated hydrocarbon production and oxygen removal during catalytic co-pyrolysis. Similar results were found in studies conducted by Liu et al. (2016) [112] and Zhang et al. (2017) [113].

In summary, metal oxide catalysts play a critical role in catalytic co-pyrolysis by promoting the deoxygenation and decarboxylation of biomass molecules, preventing coke formation, and enhancing the yield and quality of bio-oil, gas, and solid char.

10. Future Directions and Outlook

The production of high-quality liquid fuels and chemicals from biomass and plastic waste through catalytic co-pyrolysis is a promising technology that can provide an alternative to fossil fuels. This process has the potential to convert biomass and plastic waste into valuable HCs that can be used as fuels or feedstocks for other valuable products. The process is still in its early stages of development, but there is significant potential for further research and development.

Observations reveal that the heteroatoms present during the catalytic co-pyrolysis process can significantly impact the lifetime of zeolite-based catalysts. Consequently, there is a need to develop more appropriate catalysts that possess suitable acid sites and acidity for the desired products through specific reaction pathways and that exhibit a longer lifespan. This is necessary to address the challenges associated with catalytic co-pyrolysis.

The main challenges to this technology are related to the optimal selection of the catalyst materials and reaction conditions. Different catalysts and reaction conditions can significantly increase the yield of resultant products and also reduce the formation of undesirable compounds. Future research should focus on the development of efficient catalysts that can effectively convert biomass and plastic waste into useful HCs. Additionally, research should focus on improving the understanding of the catalytic co-pyrolysis process and the effects of different reaction conditions.

Exploring an alternative feedstock for CCP systems, such as waste plastic and biomass mixtures, would increase the economic viability of the process. The co-pyrolysis of biomass and plastics is an energy-intensive process due to the high temperatures and pressures required. Therefore, research should focus on developing new strategies to improve the energy efficiency of this process. Developing predictive models to effectively monitor and control the CCP process in order to maximize the desired product yields is also desirable. Developing a machine learning model for co-pyrolysis problems can establish the groundwork for subsequent optimization and integration with sophisticated simulation tools. This, in turn, can facilitate the identification of the strengths and limitations of machine learning in addressing co-pyrolysis problems [114]. Moreover, there are also challenges related to the scale and cost of production. Furthermore, investigating the economic viability of these processes, particularly regarding the ratio of plastic and biomass, could offer promising prospects for commercializing such technologies. Another crucial aspect is to explore the link between the quality of the produced liquid oil and the composition of combustible gases to gain deeper insights into the process. Moreover, to facilitate integration, there is a pressing need to delve deeper into the current challenges and issues concerning biomass and plastic waste management [102]. Further research is needed to develop efficient processes that can be scaled up to industrial levels at a reasonable cost.

Understanding the relationship between the biomass–plastic mixture composition and product distribution in catalytic co-pyrolysis is crucial for future research and process optimization. The composition directly affects product yields, enabling control over bio-oil, biochar, and syngas distribution through feedstock ratio variation. Synergistic effects arise from plastics’ higher carbon content and distinct properties, enhancing the yield of HC-rich products and valuable gases/liquids. The complementarity between biomass and plastics optimizes distribution, with biomass contributing oxygen-containing compounds for bio-oil production and plastics favoring high-energy-density HC-rich products. Catalyst selection and interaction further influence selectivity, promoting the desired components’ decomposition. Adjusting the biomass–plastic ratio provides process flexibility, allowing tailored production to meet commercial value, market demand, and application suitability. Moreover, optimizing the mixture composition aligns with sustainability goals, mitigating plastic waste disposal’s environmental impact while producing valuable energy products, thereby supporting circular economy principles and resource conservation.

A future perspective about the relation between the biomass–plastic mixture composition and the product distribution, which can be useful, is as follows:

Feedstock Ratios: The influence of biomass–plastic mixture ratios on product distribution (e.g., bio-oil, gases, and char) and their yields, quality, and properties still needs to be investigated.

Synergistic Effects: It is also important to highlight the potential synergies between biomass and plastics during co-pyrolysis, leading to enhanced product yields, altered chemical compositions, and improved product characteristics.

Feedstock Properties: The impact of biomass and plastic properties (e.g., moisture content, particle size, plastic type, and polymer composition) on product distribution is worth exploring, by considering the influence on thermal degradation behavior and product formation pathways.

Catalyst Effects: Assessing the role of catalysts in co-pyrolysis and their influence on product distribution, including catalyst selection, loading, and the properties affecting the yields and selectivity of specific products may also be crucial for future research.

These points highlight the key areas of investigation and considerations related to the relation between the biomass–plastic mixture composition and product distribution in catalytic co-pyrolysis.

11. Conclusions

Overall, the catalytic co-pyrolysis (CCP) of biomass and plastics offers a promising alternative to conventional thermochemical conversion methods for producing high-quality liquid fuels and chemicals. By using different catalysts, CCP can achieve higher yields of products while reducing the quantity of byproducts. This makes it an attractive option for industries looking to reduce emissions and maximize the efficiency of their processes. CCP is a promising tool for obtaining valuable products; however, in order to commercialize this process, several steps must be taken. First, the feedstock/catalyst ratio must be increased in order to maximize the efficiency of the process. This requires the development of a catalyst that can suppress coke formation while still promoting the desirable reactions. Additionally, the process must be tested on a bench-scale reactor to ensure that the process is operating at the desired rate and with the desired efficiency. Finally, the economic feasibility of the process must be assessed. This involves analyzing the costs of the process and the potential revenue from the products it produces. Once these steps are taken, the co-pyrolysis process can be commercialized and used for waste management and energy production.