Bio-aromatics: Revolutionizing the integrated biomass and plastic waste valorization for high-value aromatic hydrocarbons via bifunctional catalytic pathways of bio-syngas conversion

Bio-aromatics: Revolutionizing the integrated biomass and plastic waste valorization for high-value aromatic hydrocarbons via bifunctional catalytic pathways of bio-syngas conversion Maria Saif, Rub´ en Blay-Roger, Muhammad Asif Nawaz * , Luis F. Bobadilla, Tomas Ramirez-Reina , J.A. Odriozola ** Department of Inorganic Chemistry and Materials Sciences Institute, University of Seville-CSIC, 41092, Seville, Spain ARTICLE INFO Keywords: Bio-aromatics Biomass-derived syngas Co-gasification Tandem catalysis Bifunctional catalytic pathways ABSTRACT Aromatic hydrocarbons play a pivotal role in various industrial applications, serving as essential building blocks to produce polymers, resins, and specialty chemicals. Traditionally, their synthesis has been reliant on fossil fuels, raising concerns about environmental sustainability and resource depletion. However, recent advancements in the field have paved the way for a paradigm shift, with a focus on biomass-derived synthesis gas as a renewable and environmentally friendly feedstock. This review explores innovative shortcuts in the synthesis of aromatic hydrocarbons, a key area of research that holds promise for a more sustainable and efficient future. As we delve into the intricacies of biomass-derived synthesis gas conversion, we will examine breakthroughs in catalyst development, process optimization, and integrated approaches. By scrutinizing these advancements, we aim to provide a comprehensive overview of the current state of the art, highlighting both challenges and opportunities for further exploration. The urgency of addressing environmental concerns and the growing demand for renewable alternatives underscore the importance of reevaluating the methodologies. The unique characteristics of biomass-derived synthesis gas coupled with co-gasification processes present an intriguing avenue for redefining the landscape of aromatic hydrocarbon synthesis. Through this exploration, we seek to unravel the complexities of these innovative shortcuts, offering insights that may contribute to a more sustainable and greener future for the chemical industry. 1. Introduction Aromatics, characterized by their benzene ring structures, are key components in numerous industrial applications, extensively utilized in the production of synthetic materials, fine chemicals, pesticides, energy, and other industries [1]. The modern world’s resource challenges and the rapid advancement of the global techno-economic landscape have significantly amplified the demand for aromatics [2–4]. Where the key products within the aromatic sector include benzene, toluene, and xylene (BTX), followed by ethylbenzene, styrene, and cumene. Various recent reports highlight the significant growth in global consumption and demand for BTX, projecting the market size to reach 165.65 million tons by 2029, with a current Compound Annual Growth Rate (CAGR) of over 4 %. As Fig. 1 presents the global market size forecast for BTX from 2022 to 2032, highlighting a steady Compound Annual Growth Rate (CAGR) of 3.80 %. The market demand is projected to increase from 128,619.10 kilotons (kt) in 2022 to approximately 179,920.91 kt by 2031, indicating rising consumption driven by the expanding petrochemical and industrial sectors. The upward trajectory underscores BTX’s critical role in producing essential chemicals, fuels, and synthetic materials, making it a key segment in the global energy and manufacturing industries. However, its supply heavily depends on imports, underscoring the need to explore new production routes for aromatic hydrocarbons, especially BTX [4–6]. Historically, the synthesis of these vital components has been heavily dependent on fossil fuels, which typically involve catalytic reforming of naphtha, pyrolysis of gasoline in ethylene plants, and toluene disproportionation. However, these conventional methods have raised significant concerns about the This article is part of a special issue entitled: Rafael Bilbao published in Biomass and Bioenergy. * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M.A. Nawaz), [email protected] (J.A. Odriozola). Contents lists available at ScienceDirect Biomass and Bioenergy journal homepage: www.elsevier.com/locate/biombioe https://doi.org/10.1016/j.biombioe.2025.107736 Received 31 December 2024; Received in revised form 18 February 2025; Accepted 19 February 2025 Biomass and Bioenergy 196 (2025) 107736 Available online 23 February 2025 0961-9534/© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). depletion of finite resources and the reliance on volatile petroleum imports, driven by fluctuating petrochemical prices. While the urgency of addressing environmental concerns and the accelerating recent campaigns of waste valorization present a model for renewable energy resources in producing aromatic hydrocarbons, aligning with the global narrative of a carbon-free environment. In this concern, the recent advancements in green chemistry have catalyzed a transformative shift towards utilizing biomass-derived synthesis gas (syngas) as a renewable and eco-friendly feedstock for aromatic hydrocarbon production. The unique characteristics of biomass-derived syngas offer an intriguing avenue for redefining the landscape of aromatic hydrocarbon synthesis. This exploration aims to unravel the complexities of these innovative pathways, offering insights that contribute to a more sustainable and greener future for the chemical industry. In this context, integrating biomass and plastic waste as feedstocks offers a promising solution for producing valuable aromatic compounds, addressing environmental issues, and reducing reliance on fossil fuels. Biomass, being carbon-neutral, and plastic waste, rich in carbon and hydrogen, complement each other, enhancing the production of high-value aromatics. Using catalytic processes in this context enhances selectivity toward aromatic compounds, resulting in higher yields. The high hydrogen-to-carbon ratio of plastics complements the high oxygen-to-carbon ratio of biomass, improving product uniformity and minimizing coke deposition. This integration makes biomass a cost-effective feedstock for syngas production. For instance, syngas produced in co-gasification of biomass and plastic waste can be routed to syngas-based production of aromatics via Fischer Tropsch Synthesis (FTS), Reverse-Water-Gas-Shift (RWGS) or Methanol Synthesis (MTS) via indirect (two-step) or direct (one-step) processes [7–19]. Where the two-step method involves converting syngas (CO +H 2 ) into methanol/olefins/paraffins using a metal oxide catalyst, followed by aromatization on a molecular sieve (typically zeolite) [20–24]. While the one-step (often regarded as tandem or direct) route employs an integrated catalyst to directly convert syngas into aromatics using a modified metal-oxide/zeolite composite catalyst [25,26]. Recent developments in the catalytic processes, synthesis routes, and design approaches for aromatic hydrocarbons from syngas have shown significant promise, particularly utilizing FTS and MTS tandem routes in conjunction with HZSM-5 zeolites [27]. Innovations in catalyst design, especially the integration of HZSM-5, have enhanced the selectivity and yield of aromatic compounds. The FTS process, traditionally known for converting syngas into a range of lower olefinic hydrocarbons, has been optimized by incorporating HZSM-5, to increase the production of desired lower aromatic hydrocarbons (such as BTX). Concurrently, MTS from syngas followed by methanol-to-aromatics (MTA) conversion using HZSM-5 has emerged as a robust alternative to facilitate the aromatization process majorly towards tetramethyl benzene. However, recent studies have focused on improving the stability and activity of these catalysts under operating conditions, addressing challenges such as catalyst deactivation and process integration. Furthermore, advances in process optimization, including reactor design and operational parameters, have contributed to higher efficiency and lower energy consumption [28]. These developments collectively underscore the potential of utilizing syngas derived from biomass and plastic waste to produce high-value aromatic hydrocarbons, paving the way for more sustainable and efficient chemical manufacturing processes. In summary, integrating biomass and plastic waste-derived syngas for aromatics production represents a crucial step toward a sustainable chemical industry. This review explores the Bio-Tandem Aromatic (BTA) process, focusing on co-gasification/pyrolysis, syngas cleaning, and FTS/MTS-mediated routes, while highlighting recent advancements and ongoing challenges. By bridging key gaps in catalysis, process engineering, and waste management, this work emphasizes the synergy between biomass and plastic waste co-processing, advances in bifunctional catalytic pathways, and innovations in reactor design to enhance selectivity, stability, and efficiency. Additionally, it evaluates cogasification strategies, catalyst performance, and process integration, linking fundamental research with industrial feasibility. By addressing both technical and sustainability challenges, this review provides valuable insights into optimizing bio-aromatic production, promoting a greener and more resource-efficient future for chemical manufacturing. Fig. 1. Global BTX market overview and projected CAGR value during the forecast period of 2023–2031. Source: Straits research BTX market report 2023. Note: Kt =kilotons. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 2 2. Environmental Imperative for sustainable and Renewable Society Fossil fuels like petroleum, coal, and natural gas currently dominate global primary energy consumption, accounting for about 75 %, with nuclear power, hydropower, and biomass comprising the remaining 25 % [29]. However, dwindling crude oil reserves, increasing fuel demands, climate concerns, and political commitments have intensified the focus on renewable energy sources. Fig. 2 highlights the urgent need for a sustainable and renewable energy transition by illustrating the environmental impact of fossil fuel extraction, refining, and CO 2 emissions. The pressing issue of air pollution and greenhouse gas emissions, reinforce the necessity for carbon capture and conversion technologies to mitigate climate change. While the dominance of fossil fuels for global energy consumption, the depleting crude oil reserves and the growing demand for alternative fuels, necessitates the critical role of alternative sustainable processes in transforming those waste streams into sustainable fuels, paving the way for a cleaner, more resilient energy future. Among these, biomass and biowastes, derived through photosynthesis, sequester CO 2 and H 2 O into carbon-rich solids, making them an attractive renewable alternative. Their rapid production rate and carbon-neutral nature allow them to be converted into valuable products such as syngas, gaseous methane, liquid bioethanol, biodiesel, biodegradable plastics, solid briquettes, and biochemical products, offering similar functionality to those derived from fossil fuels [30]. Bioenergy stands out among renewable energy sources due to its unique ability to be collected, stored, and transported, making it suitable for on-demand use [31,32]. Biomass resources are diverse, encompassing agricultural residues (e.g., straw, stalks, and corn cobs), forestry wastes (e.g. shrubs, branches, and leaves), energy crops (e.g., algae, switchgrass, and jatropha seeds,), food industry byproducts (e.g., coffee shells), human waste from municipal plants and sewage, and animal farming byproducts (e.g., manure and fat) [33]. Solid biomass is predominantly utilized for heat and electricity production, representing the largest share of its usage. Its abundant availability and carbon-neutral profile also position biomass as a promising feedstock for biofuels and valuable chemicals, with significantly lower greenhouse gas emissions than fossil fuels. Among various types of biomass, lignocellulosic biomass, primarily sourced from agricultural and forestry wastes, is particularly notable. Over 100 billion metric tons of lignocellulosic biomass are generated annually, including 3.7–5.1 billion metric tons of agricultural residues discarded each year. Where, 0.8–10.6 EJ (EJ) of energy can be produced annually through forestry waste by 2050. In the European Union, forestry waste already contributes to half of its renewable energy portfolio. On the other hand, global municipal solid waste generation is escalating, with 1.3 billion tons produced annually and projections reaching 2.2 billion tons by 2025 [34]. Plastics, a major component of this waste, have surged in popularity due to their low cost, lightweight nature, durability, and resistance to environmental degradation [35]. However, their poor degradability poses significant environmental challenges [36]. Plastic waste has increased from 1.7 million tons in 1950 to 322 million tons in 2015, with an estimated 12 billion metric tons expected in landfills by 2050 [37]. Conventional disposal methods like landfilling and incineration exacerbate environmental issues, breaking down plastics into microplastics that pollute ecosystems and oceans, threatening marine life and ecosystems [38–42]. 3. Thermochemical processes for waste valorization Biomass or plastic valorization via thermochemical processes for encompassing the conversion of organic/plastic waste materials into higher-value products (such as fuels, chemicals, or energy applications) can play a crucial role in tackling the growing challenges of waste management to align with principles of sustainability and circular economy. Among the major thermochemical processes (combustion, incineration, pyrolysis, or gasification), combustion or incineration Fig. 2. Environmental imperative for renewable society to reduce the reliance on traditional fossil fuel resources and mitigate CO 2 footprints. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 3 involves burning plastics at high temperatures in the presence of excess oxygen, resulting in the conversion of waste into carbon dioxide, water vapor, and energy. While this method significantly reduces waste volume and generates energy, it also raises environmental concerns due to the emission of toxic pollutants and greenhouse gases. Even though, advanced incineration technologies equipped with stringent emission control can minimize these issues, offering a more sustainable approach [43]. Since, despite their benefits of high waste reduction and energy recovery, incineration can release hazardous substances, including phosgene, dioxins, and heavy metals, which pose serious environmental and health risks to nearby communities [44,45]. Furthermore, the need for extensive flue-gas cleaning increases operational costs, which can negatively impact the efficiency and sustainability of the process [46]. In this context, thermochemical processes like gasification and pyrolysis have emerged as promising solutions for converting waste into valuable energy and chemical products [47,48]. Pyrolysis, a thermal decomposition process conducted at temperatures between 300 ◦C and 900 ◦C in an oxygen-free environment, transforms organic materials into valuable products: liquid bio-oil, solid biochar, and non-condensable syngas. The yield and composition of these products are shaped by key factors, including temperature, residence time, and the characteristics of the feedstock. Fast pyrolysis, conducted at moderate temperatures with short reaction times, has gained attention for its ability to selectively produce bio-oil [49]. Catalytic pyrolysis, using catalysts like zeolite (ZSM-5), activated carbon, and various metal oxides and chlorides, enhances reaction efficiency by providing active sites for cracking, dehydrogenation, and aromatization, improving the yield and selectivity of desirable liquid products. Despite the challenges of conventional pyrolysis, such as long processing times and low bio-oil yields with high oxygen content, advancements like microwave pyrolysis have significantly improved the process, majorly yield pyrolytic oil and some syngas fractions with enhanced H 2 content and calorific value, resulting in improved oil properties. However, scalability issues remain, including feedline clogging due to the high viscosity of melted plastics and coking in reactors, which can cause operational difficulties [49]. On the other hand, gasification is an advanced thermochemical process that converts waste organic materials into high-quality syngas at high temperatures, typically ranging from 800 ◦C to 1200 ◦C. This process occurs in a controlled environment with a limited supply of oxygen or steam in the presence of a gasifying agent such as CO 2 , steam, O 2 , or air. The restricted oxygen supply prevents complete combustion, leading to the breakdown of complex carbonaceous materials into simpler molecules like carbon monoxide (CO), hydrogen (H 2 ), carbon dioxide (CO 2 ), and methane (CH 4 ). The resulting syngas, which are rich in hydrogen and carbon monoxide, serve as a valuable feedstock for electricity generation, synthetic fuel production, and various chemical processes. Gasification occurs in four key stages: drying, pyrolysis, oxidation, and reduction. The drying stage (100–200 ◦C) involves the removal of moisture without altering the chemical composition of the feedstock. Pyrolysis, or devolatilization, breaks down biomass into noncondensable gases, condensable liquids (tars), and solid char. During oxidation, the intermediate products react with air, influencing the final composition of the gas. In the reduction and gasification stage, various reforming, cracking, and heterogeneous char reactions take place, ultimately leading to the production of syngas. 3.1. Integration of thermochemical processes by employing dual functionality Co-processing of plastic waste and biomass by integrating (pyrolysis & gasification) has emerged as an effective solution for waste management, offering synergistic economic benefits and addressing waste mixing issues while optimizing resource recovery and minimizing environmental impacts [50]. Such integrated approaches not only diversify the feedstock base but also contribute to a more resilient and sustainable waste management and energy production system. Extensive research in this concern has demonstrated the synergistic effects of combining feedstocks, enhancing gas yields and reducing tar formation [51]. When biomass and plastic waste undergo co-processing, significant increase in gas volume, quality of liquid fuels and decrease in tar formation are observed, leveraging the complementary characteristics of both materials. Plastics have a high hydrogen-to-carbon (H/C) ratio and low oxygen-to-carbon (O/C) ratio, which complement biomass’s high O/C and low H/C ratio, thus efficient way of improving product uniformity and minimizing coke deposition [52–54]. The elevated hydrogen content in plastic waste enhances the energy density of biomass, positioning it as an economical and efficient feedstock for syngas production. The co-gasification process involves complex reaction dynamics influenced by factors such as temperature, gasifying agents, and feedstock composition. Even though, sufficient work has been attributed to thermochemical co-processing for fossil fuel replacement and the production of essential chemicals, thereby improving energy security, however, the complex interactions between plastics and biomass require further research to optimize the process [55]. As briefly discussed in the previous section, several important chemical reactions govern the gasification or co-gasification process, starting with thermal decomposition (pyrolysis) reactions, which break down biomass and plastics into volatile gases, tar, and char. Biomass pyrolysis typically produces CO, CO 2 , H 2 , CH 4 , tar, and solid char, while plastic pyrolysis primarily generates hydrocarbons such as ethylene, methane, and propane. These volatile compounds then undergo primary gas-phase reactions, where carbonaceous materials react with steam, CO 2 , or oxygen to form syngas. The major chemical reactions involved in gasification can be categorized into volatilization, char gasification, oxidation, and secondary gas-phase reactions. Volatilization occurs through immediate chemical reaction: Biomass → H 2 +CO +CO 2 +Tar +C n H m +Char (Eq. 1) Char gasification primarily involves two key reactions: the water-gas reaction and the Boudouard reaction. These are represented by the following equations: C +H 2 O ⇔ CO +H 2 (ΔH = +131 kJ/mol) Primary water-gas reaction (Eq. 2) C +CO 2 ⇔ 2CO (ΔH = +173 kJ/mol) - Boudouard reaction (Eq. 3) Secondary gas-phase reactions further influence the composition of syngas. One of the most important of these is the water–gas shift reaction: CO +H 2 O ⇔ CO 2 +H 2 (ΔH = − 42 kJ/mol) – Water–gas shift reaction (Eq. 4) Oxidation reactions, which are exothermic, contribute to the overall energy balance of the gasification process. The key oxidation reactions include: C +½ O 2 ⇔ CO (ΔH = − 111 kJ/mol) – Complete oxidation (Eq. 5) CO +½ O 2 ⇔ CO 2 (ΔH = − 283 kJ/mol) – Partial oxidation (Eq. 6) CH 4 +2O 2 ⇔ CO 2 +2H 2 O (ΔH = − 803 kJ/mol) (Eq. 7) H 2 +½ O 2 ⇔ H 2 O (ΔH = − 242 kJ/mol) (Eq. 8) Several reforming and cracking reactions also play a crucial role in gasification. These include steam reforming, dry reforming, and hydrocarbon cracking, which influence the final gas composition: CH 4 +H 2 O ⇔ CO +3H 2 (ΔH =206 kJ/mol) – Steam reforming(Eq. 9) CH 4 +CO 2 ⇔ 2CO +2H 2 (ΔH =247 kJ/mol) – Dry reforming(Eq. 10) M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 4 C +2H 2 ⇔ CH 4 (ΔH = − 75 kJ/mol) – Hydrogasification or methanation (Eq. 11) C n H m +2nH 2 O ⇔ (2n +m/2)H 2 +nCO 2 (ΔH <0) – Steam reforming of hydrocarbons (Eq. 12) Tar → H 2 +CH 4 +CO +CO 2 +C n H m (ΔH <0) – Tar cracking(Eq. 13) The efficiency of co-gasification and the quality of syngas depend on multiple factors, including feedstock composition, gasification technology, and operating conditions [56]. Where, understanding the distinct properties of biomass and waste plastics is essential for optimizing gasification and other conversion technologies. Table 1 and Fig. 3 shows the structure and elemental composition of biomass, waste plastics, and municipal solid waste (MSW) significantly influences syngas production, particularly their carbon, hydrogen, and oxygen content. A higher carbon content increases the calorific value of syngas since carbon serves as the primary energy source in combustion and gasification. Since the Lower Heating Value (LHV) of syngas is directly influenced by the elemental composition of carbon and hydrogen. Carbon and hydrogen are primary contributors to the energy content of the syngas, while oxygen reduces the LHV as it dilutes the combustible elements. Where, carbon contributes to the energy density by oxidizing to CO and CO 2 during combustion or gasification. The higher the carbon content in the feedstock, the greater the calorific value of the produced syngas. While Hydrogen significantly enhances LHV because of its high calorific value. Hydrogen-rich plastics, for example, are frequently used as hydrogen donors in co-gasification processes to improve the energy yield of the syngas. The LHV of syngas can be approximated as: LHV (MJ/kg) =10.8 ×X H2 +12.6 ×X CO +3.5 ×X CH4 Where “X” represents the mole fractions of hydrogen, carbon monoxide, and methane in the syngas. Since increasing the hydrogen fraction enhances LHV due to its high energy density. While carbon contributes by forming CO and CH 4 , both of which increase the syngas’ heating value. Biomass generally contains 35–50 % carbon, while plastics like polyethylene and polypropylene have a significantly higher volatile matter content (over 85 %) and very low ash content (less than 1 %), making them highly effective contributors to syngas heating value. MSW components with a higher carbon concentration, such as plastics, contribute substantially to syngas energy density,whree hydrogen plays a crucial role in determining the heating value and composition of syngas. Waste plastics, with 7–15 % hydrogen content, act as hydrogen donors in cogasification, increasing hydrogen yield and boosting syngas calorific value. In co-gasification scenarios, hydrogen-rich plastics help balance the H 2 /CO ratio, making the syngas more suitable for downstream applications like FTS. Oxygen , on the other hand, affects the efficiency of gasification reactions by influencing oxidation processes. Biomass typically contains 40–50 % oxygen, which can lower syngas heating value compared to plastics, which have minimal oxygen content. The high oxygen content in biomass leads to a lower effective hydrogen-tocarbon ratio compared to plastics, impacting syngas composition and suitability for specific applications. Biomass, primarily composed of cellulose (40–50 %), hemicellulose (15–30 %), and lignin (15–30 %), exhibits varying volatile matter and ash content, both of which influence its calorific value and conversion efficiency [57,58]. Woody biomass and nutshell-derived feedstocks generally have higher carbon and hydrogen content, leading to improved syngas quality and enhanced hydrogen-to-carbon ratios. This favors the production of aromatic hydrocarbon-rich liquid fuels through pyrolysis. Cellulosic biomass has 70–90 % volatile matter, 10–20 % fixed carbon, and varying ash content. Herbaceous biomass, in particular, has a higher ash content, with alkali and alkaline earth metals in the ash acting as catalysts during pyrolysis. These minerals promote secondary cracking of volatiles, reducing bio-oil yield while increasing gas and char production [59]. Waste plastics encompass various types with distinct chemical structures, including polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET). Among them, PE and PP are the most abundant, accounting for about 50 % of global plastic waste. These plastics primarily consist of 80–93 % carbon and 7–15 % hydrogen, with an effective hydrogen-to-carbon ratio of 1–2, which is significantly higher than biomass’s 0–0.3 ratio. Table 1 Composition and characteristics of lignocellulosic biomass, waste plastics and typical MSW. Sample Lignocellulosic biomass Ultimate Analysis wt % Proximate Analysis wt % C H O N S Cl V FC A H/C eff Q HHV /MJ⋅kg −1 Woody biomass, pine wood 49.33 6.06 44.57 0.04 0.00 0.0 73.40 16.70 0.50 0.12 19.80 Woody biomass, fir wood 49.07 6.70 44.18 0.02 0.03 0.0 81.93 17.75 0.32 0.29 20.29 Woody biomass, poplar wood 47.21 6.04 46.74 0.01 0.00 0.0 87.95 10.93 1.12 0.05 18.58 Herbaceous biomass, wheat straw 38.34 5.47 55.22 0.60 0.37 0.0 83.08 10.29 6.63 −0.45 13.96 Herbaceous biomass, rice straw 36.07 5.20 57.83 0.64 0.26 0.0 78.07 6.93 15.00 −0.67 12.53 Fruit shell biomass, walnut shell 47.30 6.10 42.00 0.50 0.10 0.0 76.60 19.40 4.00 0.22 19.11 Fruit shell biomass, palm kernel shell 48.44 6.23 44.99 0.31 0.03 0.0 71.72 25.21 3.07 0.15 20.30 Plastics Waste Ultimate Analysis wt % Proximate Analysis wt %  Sample C H O N S Cl V FC A H/C eff Q HHV /MJ⋅kg −1 Polyethylene (PE) 85.50 14.5 0.00 0.00 0.00 0.0 99.96 0.04 0.00 2.04 46.01 Low density polyethylene (LDPE) 84.21 14.2 1.47 0.03 0.00 0.0 99.79 0.21 0.00 2.01 48.83 High density polyethylene (HDPE) 85.16 14.4 0.31 0.02 0.03 0.0 99.85 0.06 0.09 2.03 49.63 Linear low-density polyethylene (LLDPE) 85.61 14.2 0.02 0.05 0.03 0.0 99.85 0.07 0.05 2.00 46.17 Polystyrene (PS) 92.20 7.80 0.00 0.00 0.00 0.0 99.50 0.50 0.00 1.02 40.49 Polypropylene (PP) 84.70 15.3 0.00 0.00 2.10 0.0 96.90 0.00 1.00 2.17 45.23 Polyethylene terephthalate (PET) 64.10 3.70 34.20 0.00 0.00 0.0 84.10 13.90 0.00 −0.01 24.15 Polyvinyl chloride (PVC) 38.70 4.80 0.00 0.00 0.00 56 95.80 4.20 0.00 1.49 19.30 Municipal Waste Component Moisture Ultimate Analysis LHV Wt % C H O N S Ash MJ/Kg Textiles 10.0 48.0 6.4 40.0 2.2 0.2 3.2 17.08 Wood 1.3 49.6 6.0 42.6 0.2 0.1 1.5 18.28 Plastics 1.2 60.0 7.2 22.8 0.0 0.0 10.0 29.0 Paper 10.2 43.4 5.8 44.3 0.3 0.2 6.0 13.43 Food Waste 70.0 48.0 6.4 37.6 2.6 0.4 5.0 3.90 Note: C represents carbon, H hydrogen, O oxygen, N nitrogen, S sulfur, Cl chlorine, V volatile, FC fixed carbon, A for ash, and H/C eff is hydrogen-to-carbon effective ratio. Where the information on ultimate analysis, moisture contents and LHV are acquired from references [61–64]. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 5 However, some plastics contain specific elements that impact their processing. For example, PVC contains 56.5 % chlorine, which poses environmental concerns, while PET contains 34.2 % oxygen, influencing its gasification behavior [60]. Therefore, the long persistence of plastics in the environment is a major global concern, particularly in marine ecosystems, where plastic pollution threatens biodiversity. Consequently, valorizing waste plastics into renewable energy sources is essential for achieving sustainability. Since co-processing plastics with biomass presents an effective strategy to mitigate operational challenges related to plastic waste while enhancing syngas production and overall process efficiency. Where, the integration of advanced cleaning and conditioning systems further enhances the purity of syngas, making biomass gasification a promising and sustainable approach for meeting energy demands and reducing reliance on fossil fuels. Zhao et al. explored the synergistic effects of co-pyrolysis and cogasification, revealing distinct gas compositions. For co-pyrolysis, the gases comprised H 2 (32–38 %), CH 4 (14–15 %), and CO (29–30 %), while co-gasification yielded H 2 (47 %), CH 4 (6–8%), and CO (26–31 %) [66]. D´ eparrois et al. documented impressive syngas production rates, achieving approximately 3.6–4.8 g/min during co-pyrolysis and an enhanced output of 5.9–8.1 g/min for co-gasification [67]. Hydrogen production rates reached approximately 0.17–0.19 g/min for co-pyrolysis and 0.09–0.13 g/min for co-gasification, with energy yields of around 1.7–2.3 g/min and 1.6–2.4 g/min, respectively, showcasing the efficiency of both processes. Such as Block et al. [68] provided a comprehensive review of co-pyro gasification of biomass and plastic waste, focusing on product distribution, operational conditions, feedstock characteristics, process pathways, and synergistic effects. Uzoejinwa et al. [69] emphasized the advantages of co-pyrolysis, particularly its enhanced operational control mechanisms and improved product yield. Wong et al. [37] delved into the technical intricacies of converting plastics into fuel via thermal and catalytic pyrolysis, while also exploring the co-pyrolysis of biomass and plastic waste, highlighting its potential for sustainable fuel production. Inayat et al. [70] investigated the influence of catalytic co-gasification on enhancing syngas performance and quality derived from various blended fuel sources, highlighting its role in optimizing energy outputs. Mishra et al. [51] and Ramos et al. [71] examined the beneficial impact of solid waste co-gasification on final product yields, with a focus on the latest advancements in gasification and co-gasification technologies for efficient waste-to-energy conversion. Brachi et al. emphasize that combining plastics with biomass effectively mitigates the challenges and operational issues associated with using plastic feedstock alone [72]. They further highlight that co-gasifying biomass and PET results in a product gas composition that eliminates the need for additional conditioning in a water-gas shift reactor for methanol production, streamlining the process. Robinson et al. advocate for the co-gasification of plastic waste, such as food containers and plastic bottles, with biomass as an effective solution to divert waste from landfills, particularly in small and remote communities [73]. These studies suggest that each technique has its strengths and weaknesses, making it challenging to definitively choose the more efficient method. However, it is crucial to evaluate the advantages of both techniques (co-pyrolysis and co-gasification) to determine the most effective method that would simultaneously deal with all the associated problems while evaluating the efficiency of these techniques. One of the main challenges in co-processing is obtaining and separating waste plastics into pure components due to inadequate infrastructure and reliance on manual sorting, consequently, plastic waste is often mixed with other materials. Developing co-pyrolysis can moderate the need for extensive waste separation and alleviate operational issues associated with pure plastic waste conversion, majorly producing bio-oil and biochar along with some fraction of syngas. Co-gasification, however, has garnered interest due to its potential to replace fossil-based fuels with particularly hydrogen-rich syngas, offering environmental benefits and enhanced syngas quality. Since it is usually regarded that the pyrolysis of biomass and plastics enhances the hydrogen content and calorific value of bio-oil, the syngas typically contains more impurities and has lower hydrogen content compared to gasification. For packaging waste that is challenging to separate from materials like cardboard, paper, or wood, co-gasification offers a practical solution. It becomes especially appealing for mixed waste processing by minimizing the need for extensive waste separation [74]. The reactions involved in single feedstock gasification are similar to those in co-gasification; however, co-gasifying biomass and plastic waste boosts both the quantity and composition of syngas, particularly increasing H 2 levels [68]. Fig. 3. Chemical composition of solid waste containing polyolefin (e.g., PE and PP), polyester (e.g., PLA and PET), polyether, polyamide, protein, lipid, and lignocellulose (i.e., cellulose, hemicellulose, and lignin), reprinted with the permission of ref. [65]. Copyright 2023 Royal Society of Chemistry. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 6 The water-gas shift (WGS) reaction and steam methane reforming (SMR) are highly effective in generating hydrogen-rich streams, while cracking reactions further contribute by breaking down heavy hydrocarbons into lighter molecules, unlocking hydrogen from tar. Similarly, when used as a gasification agent, water generates free radicals during the reforming stage. These radicals are absorbed by the catalyst, promoting the breakdown of tar into hydrogen (H 2 ) and carbon monoxide . Chai et al. [75] found that gas production, including H 2 , initially increases with the plastic percentage in the feedstock but declines after a certain concentration, suggesting a limit to the synergistic effect between plastics and biomass. Plastics provide H radicals that promote H 2 production and interact with biomass radicals to break down complex hydrocarbons into CO and lighter hydrocarbons. Incorporating H 2 O into the process boosts gas production by reacting with CO and lighter hydrocarbons, increasing hydrogen yield. While, a biomass-to-plastics ratio of 7:3, with plastic content kept below 30 % in the feed, is recommended for optimal results [75]. The resulting syngas holds immense versatility, serving applications such as electricity generation, chemicals, H 2 production, FTS/MTS liquid fuels, and dimethyl ether (DME) synthesis [76]. However, further research is essential for optimizing and unlocking these processes for sustainable energy solutions through advanced waste management platforms. A schematic diagram of biomass and plastic waste co-gasification to produce syngas is shown in Fig. 4, illustrating the integrated co-pyrolysis and co-gasification process for converting plastic and biomass waste into valuable energy products. Where, the co-pyrolysis pathway, conducted at 700 ◦C with a Ni-La/Al 2 O 3 -CaO-C catalyst under a nitrogen flow of 100 ml/min, produces intermediate products such as gas, bio-oil, and char. These intermediates undergo further catalytic treatments, including reforming, hydrodeoxygenation, and catalytic reforming, to generate hydrogen, light olefins (BTX), gasoline, and diesel. Simultaneously, co-gasification at 800 ◦C, facilitated by a CaO catalyst and a steam-to-feed ratio of 0.75, yields syngas enriched in hydrogen (up to 70 vol%) which undergoes catalytic synthesis and water gas shift reactions to produce methanol, fuels, and additional hydrogen. This integrated approach optimizes hydrogen production (up to 55.45 vol% from pyrolysis) while enhancing overall process efficiency and reducing plastic waste impact. 3.2. Co-gasification operating parameters As discussed before, the gasification process unfolds through four essential stages: drying, pyrolysis, oxidation, and reduction, where these stages encompass key mechanisms such as reforming and cracking reactions in the gaseous phase, along with the gasification of heterogeneous char. In the initial drying stage, feedstock is subjected to temperatures between 100 ◦C and 200 ◦C, resulting in moisture removal while maintaining the chemical composition of the material largely unchanged. Following the drying process, the subsequent initial pyrolysis or devolatilization involves the thermal decomposition of biomass, yielding three primary components: solid residue, condensable liquids (tars), and non-condensable gases. These products can further be classified into volatile matter (gases and liquids) and char, which are exposed to air during oxidation. The degree of oxidation depends on the air-to-solid feed ratio which dictates whether partial or complete Fig. 4. Flow Diagram of the Co Gasification Biomass and plastic waste. Reproduced with the permission of ref. [77]. Copyright 2023 Elsevier. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 7 oxidation reactions occur. This stage is critical to the gasification process, as it determines the composition of the resulting products and influences the subsequent reactions within the system [77]. While, the final stages of the gasification process, reduction/gasification reactions, are the slowest yet critical steps in the system. Key factors such as gasification agents, temperature, pressure, catalyst or bed materials, and residence time play a pivotal role in determining producer gas yield, gas quality, tar formation, and overall carbon conversion efficiency [68,78]. Since, understanding these parameters is essential for optimizing gasification efficiency and examining the interactions between biomass and thermoplastics. Reaction Temperature: The gasification temperature is a critical and highly influential factor in the co-gasification process of biomass and plastic waste. It significantly impacts the yield and composition of syngas by influencing chemical reactions and their equilibria. In here, temperature plays a crucial role in co-gasification, as higher temperatures promote the breakdown of complex polymers found in both biomass and plastics. Increased temperatures favor endothermic reactions, leading to the formation of hydrogen and carbon monoxide while reducing tar formation. However, excessively high temperatures may cause equipment degradation and excessive carbon deposition. Generally, steam gasification operates efficiently between 800 and 1000 ◦C, oxygen gasification at 900–1200 ◦C, and CO 2 gasification at 750–950 ◦C. Choosing the right temperature range is essential for maximizing syngas production and avoiding operational challenges. Higher temperatures favor endothermic reactions, increasing gas yield due to enhanced pyrolysis, cracking, gasification, and steam reforming processes [79]. Lahijani et al. [80] demonstrated that in an air-blown fluidized bed for two wood waste types, increasing the temperature from 650 to 1050 ◦C increased syngas yield from 1.36 to 2.10 Nm 3 /kg and from 1.28 to 1.95 Nm 3 /kg, respectively [81]. These findings underscore the importance of temperature optimization for maximizing syngas production. Moreover, higher gasification temperatures reduce tar content and improve carbon conversion efficiency. This is particularly critical for biomass and plastic waste, where tar formation can hinder process efficiency and downstream applications. For example, higher temperatures enhance steam reforming and WGS reactions, increasing H 2 content while decreasing CH 4 content. Gonz´ alez et al. [82] observed increased H 2 and CO concentrations with decreased CH 4 and CO 2 concentrations when the temperature was raised from 700 to 900 ◦C during air gasification. The CO/CO 2 ratio increased from 0.85 to 2.7, indicating a notable improvement in syngas quality. At temperatures above 750–800 ◦C, WGS and the steam reforming reactions become more pronounced, contributing to higher H 2 production. While at further higher temperatures (above 850–900 ◦C), the Boudouard reaction increases CO content [83]. Hern´ andez et al. [84] compared different gasifying agents and found that air gasification primarily boosts CO and H 2 concentrations through the Boudouard and steam reforming reactions. The Boudouard reaction (2CO → C +CO 2 ) is a key equilibrium in syngas chemistry, influencing catalyst stability, coke formation, and overall process efficiency in biomass-derived aromatic hydrocarbon synthesis. This reaction is particularly relevant in co-gasification and tandem catalytic pathways where syngas is used as feedstock. Recent studies have highlighted the impact of the Boudouard reaction on the efficiency and longevity of catalysts used in biomass and plastic waste co-gasification. The interplay between gasification temperature, syngas composition, and catalyst properties determines the extent of carbon deposition and its subsequent removal. During biomass and plastic waste co-gasification, the reaction conditions (temperature, pressure, and gas composition) determine whether the Boudouard reaction favors carbon deposition or carbon removal. At temperatures below 700 ◦C, the reaction tends to form solid carbon (coke), leading to catalyst deactivation. In contrast, at higher temperatures (>900 ◦C), CO formation is favored, reducing carbon buildup. Carbon formation from the Boudouard reaction can block active sites of metal/zeolite catalysts, decreasing reaction efficiency. The reaction equilibrium can shift depending on CO concentration, impacting downstream reactions such as FTS or methanol-to-aromatics conversion. At moderate temperatures (700–900 ◦C), carbon deposition via the Boudouard reaction is significant, leading to catalyst fouling and reactor clogging. Studies have shown that increasing the temperature beyond 900 ◦C shifts the equilibrium towards CO formation, thereby reducing carbon buildup. Higher pressures tend to favor solid carbon formation, exacerbating catalyst deactivation. However, introducing CO as a coreactant can counteract this by promoting carbon gasification. CO-rich syngas enhances carbon deposition, necessitating the use of catalysts with higher coke resistance. Gasifier type also plays a role: fluidized-bed reactors tend to have lower carbon buildup due to better mixing, while fixed-bed reactors may experience localized carbon deposits that hinder performance. In contrast, mixed steam/air gasification, with a significant proportion of steam (56.4 vol%), primarily enhances H 2 production via charsteam reforming and the WGS reaction, although it also increases CH 4 content. While increasing gasification temperature generally improves syngas quality by raising CO and H 2 concentrations, which are essential for the LHV of syngas and its suitability for various applications there are limitations. Excessively high temperatures can lead to reduced energy efficiency and operational challenges, such as ash sintering and agglomeration [81,85]. These issues highlight the need for a balanced approach in selecting the optimal gasification temperature. Since the gasification temperature plays a pivotal role in determining the efficiency and quality of syngas production in the co-gasification of biomass and plastic waste. A thorough understanding of the temperature’s effects on chemical reactions, syngas composition, and operational constraints is essential for optimizing the process and ensuring its sustainability. The Boudouard reaction facilitates the conversion of CO 2 generated during gasification back into CO, which is a valuable component of syngas, thus improving overall efficiency (Kaydouh & Hassan, 2022). The Boudouard reaction is integral to process modeling and optimization efforts, as it affects the thermodynamic equilibrium and the overall performance of the gasifier (Aentung et al., 2024) (Kaydouh & Hassan, 2022). Multi-objective optimization techniques, such as genetic algorithms, consider the Boudouard reaction’s impact to achieve optimal gasification conditions and maximize syngas production (Aentung et al., 2024). While the Boudouard reaction is beneficial in enhancing syngas quality and gasification efficiency, it is important to consider the potential challenges it may pose, such as the need for precise control of reaction conditions to prevent undesirable by-products. Additionally, the choice of gasifying agents and feedstock composition can significantly influence the reaction’s effectiveness, necessitating careful optimization to achieve the desired outcomes in co-gasification processes. Gasifying Agents and Feedstock Composition: The choice of gasifying agent plays a pivotal role in determining the efficiency, reaction pathways, syngas composition, environmental impact, and economic viability of the co-gasification process, particularly when addressing biomass and plastic wastes. Recent studies have delved into the influence of gasifying agents on syngas yield, heating values, and gas compositions, providing valuable insights for optimizing these processes [86]. Where, different gasifying agents (include air, steam, oxygen, and their mixtures with air) are adopted for improving the producer gas yield and quality for the most economical choices [87]. Steam (H 2 O) is commonly used to enhance hydrogen production through steam reforming and the water-gas shift reaction, leading to hydrogen-rich syngas. Oxygen (O 2 ), on the other hand, supports partial oxidation reactions, providing additional energy to drive endothermic gasification reactions. Carbon dioxide plays a unique role in promoting the Boudouard reaction (C +CO 2 → 2CO), which enhances CO formation. The choice and combination of gasifying agents determine the balance between hydrogen, carbon monoxide, and carbon dioxide in the final syngas product. Utilizing CO 2 as a gasifying agent can enhance carbon conversion rates, especially when combined with steam, which is shown to be the most effective gasifying agent (Kaydouh & Hassan, 2022). The M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 8 integration of the Boudouard reaction in co-gasification processes can lead to a more sustainable waste management approach by reducing greenhouse gas emissions and improving the quality of the produced syngas ("Co-gasification of Waste Biomass and Plastic for Syngas Production with CO 2 Capture and Utilization: Thermodynamic Investigation," 2023) (Dai et al., 2021). While the Boudouard reaction offers significant advantages in co-gasification, challenges remain, such as the need for efficient catalysts and optimized process conditions to maximize CO 2 conversion rates and syngas quality. While various types of gasifiers are also utilized depending on the gasifying agent, such as oxygen-blown, air-blown, and steam gasifiers [88–90]. Air-blown gasifiers, for instance, are a common choice due to their simplicity and cost-effectiveness. However, the resulting syngas typically has a low heating value (3.5–7 MJ/m 3 ) because of nitrogen dilution, which also reduces hydrogen content. Despite its limitations, air-blown gasification remains suitable for certain applications, particularly where the cost constraints outweigh the need for high-quality syngas [81,91]. On the other hand, steam gasification offers significant advantages by producing syngas with a higher heating value (11–20 MJ/m 3 ) and a greater hydrogen content, largely attributed to the steam reforming reactions that enhance H 2 production [87,92]. The addition of steam to air-blown systems further increases H 2 and CH 4 concentrations in the syngas, but steam also necessitates higher energy input due to the endothermic nature of the reactions, which can reduce the overall temperature. While, the steam-to-biomass ratio (SBR) is a critical parameter in steam gasification, with optimal SBR values ranging from 0.3 to 1.0 shown to improve hydrogen production, reduce tar formation, and enhance carbon conversion efficiency and cold gas efficiency [71]. While, higher SBR values (1.35–4.04), as noted by Pindoria et al. [93], boost H 2 and CO 2 production while reducing the concentrations of hydrocarbons such as C 2 H 2 , CH 4 , and CO, suggesting that steam-rich environments favor hydrogen production and tar reduction but may lower the calorific value due to the increased production of CO 2 . Excessive unreacted steam, however, can lead to thermal inefficiencies and reduced syngas quality. While, oxygen-blown gasifiers eliminate nitrogen dilution, resulting in syngas with higher heating values and a cleaner composition, though the higher cost of pure oxygen and the elevated activation energies associated with its use can limit its applicability [84]. Studies have demonstrated that increasing oxygen concentration enhances reaction rates and activation energies, altering the distribution and composition of the outputs. For example, Guo et al. [94] observed that higher oxygen levels reduce H 2 activation energy, although oxygen-based systems generally exhibit higher activation energies compared to inert environments. Combining steam and air in optimal proportions offers a balanced approach to improving gas quality and fuel conversion while minimizing energy requirements [95]. However, the selection and optimization of gasifying agents are critical for achieving efficient co-gasification of biomass and plastic wastes. Balancing energy inputs is also essential, as the endothermic nature of steam gasification demands careful energy management to maintain system efficiency while maximizing syngas quality. Tailoring gasifier designs to align with specific feedstock compositions and desired syngas characteristics, and adjusting operating parameters such as temperature, pressure, residence time, and catalyst or bed material, are equally important for enhancing carbon conversion efficiency, reducing tar formation, and improving syngas yield. While significant progress has been made in understanding the role of gasifying agents, further research is needed to optimize the steam-to-air ratios for various feedstock combinations, develop cost-effective methods for integrating pure oxygen without prohibitive expenses, enhance gasifier designs to accommodate diverse feedstocks including challenging plastic wastes, and advance catalyst technologies to improve reaction rates and syngas quality. By addressing these challenges, the co-gasification process can become a more viable and sustainable solution for managing biomass and plastic wastes while producing valuable syngas for energy and chemical applications. Feedstock composition is another critical factor in co-gasification. The biomass-to-plastic ratio directly affects syngas composition and overall process efficiency. As discussed before, plastics, being hydrocarbon-rich, contribute to higher concentrations of hydrogen and carbon monoxide, leading to a syngas with a higher heating value. In contrast, biomass contains oxygenated compounds, which can result in increased CO 2 and H 2 O production. An optimal blend of biomass and plastic can leverage synergistic effects, improving overall gasification efficiency while mitigating issues like excessive tar formation and carbon deposition. The presence of plastics, such as low-density polyethylene (LDPE) and polypropylene, in the feedstock, enhances the Boudouard reaction, leading to improved syngas quality and higher heating values (Ishak et al., 2024) (Kaydouh & Hassan, 2022). The reaction aids in increasing the carbon conversion efficiency (CCE) and cold gas efficiency (CGE) of the co-gasification process, as it facilitates the conversion of carbon-rich materials into gaseous products (Aentung et al., 2024) (Kaydouh & Hassan, 2022). Optimizing the plastic-to-biomass ratio and gasifying agents, such as steam, can further enhance the efficiency of the Boudouard reaction, leading to better gas yields and energy recovery (Regueiro, 2022) (Kaydouh & Hassan, 2022). Reaction Pressure: The reaction pressure plays a significant role in the co-gasification of plastic waste and biomass, directly influencing the syngas composition and the higher heating value (HHV) of the product gas [96]. Recent studies have provided insights into how variations in gasification pressure affect the concentrations of key syngas components, including hydrogen (H 2 ), carbon monoxide , carbon dioxide , and methane (CH 4 ) [97,98]. A comprehensive evaluation of biomass gasification and its downstream operations suggests that higher gasification pressures can enhance the overall efficiency of the process. However, the specific effects on syngas composition depend on various factors, including the type of biomass used and the gasification technology employed. Different studies highlight the strong dependence of syngas composition on biomass conditions, type of gasifier, and operating parameters such as pressure and temperature, indicating that even with the same type of biomass and gasifier, variations in pressure can lead to significant discrepancies in the final syngas composition [99]. Elevated pressures can alter the equilibrium of gasification reactions, potentially affecting the concentrations of H 2 and CO in the syngas. For instance, in certain gasification scenarios, increasing the pressure from atmospheric levels to higher values has been observed to influence the H 2 /CO ratio, which is critical for downstream applications like FTS. Higher pressures tend to favor the formation of methane and other higher hydrocarbons due to enhanced methanation reactions. This can lead to an increase in the calorific value of the syngas but may require additional processing steps to adjust the syngas composition for specific applications. While specific numerical data can vary depending on the feedstock and gasification technology used, studies have shown that: Increasing the pressure from 1 atm (atm) to approximately 1.75 atm can lead to an optimal balance between syngas yield and cold gas efficiency (CGE). At this pressure, a syngas yield of around 58 % has been reported, with a CGE near 80 %. However, further increases in pressure may result in diminishing returns or even negative effects on syngas quality. In summary, while increasing gasification pressure can improve syngas yield and modify its composition, the specific outcomes are influenced by a combination of factors, including biomass type, gasifier design, and other operating conditions. Therefore, optimizing gasification pressure requires a comprehensive understanding of these interrelated parameters to achieve the desired syngas quality and yield. Maninderjit et al. [100] demonstrated that increasing reaction pressure leads to a reduction in the amounts of hydrogen and carbon monoxide in syngas. It was further interpreted that the effect of pressure on syngas composition, the H 2 /CO ratio, and the higher heating value (HHV) of the product gas was significant in biomass-polymer co-gasification. This phenomenon occurs because higher pressure conditions favor reactions that produce fewer gas moles. Specifically, elevated pressures promote methanation reactions, which convert CO and H 2 into methane, while inhibiting steam methane reforming, which produces H 2 M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 9 and activity is key to improving CO 2 hydrogenation to methanol. A recent study explores the effect of different synthesis conditions on the structure and performance of ZnZrO x catalysts (Fig. 7) [157]. The figure illustrates a systematic approach to synthesizing ZnZrO x catalysts with tunable properties by varying key synthesis parameters such as drying methods, calcination temperatures, and additive incorporation. Starting with coprecipitation, catalysts are subjected to different drying techniques, including freeze-drying (cryogel), thermal drying (xerogel), and solvent exchange (ambigel), each influencing porosity and structural characteristics. Additionally, calcination temperatures (450–600 ◦C) are adjusted to modify the catalytic performance. Further tuning is achieved through post-synthesis ball-milling, controlling catalyst morphology for applications requiring fine powders. To enhance porosity, PVA-assisted coprecipitation and a hard-templating method using carbon black are employed, leading to catalysts with unique textural properties. This diverse synthesis strategy allows precise control over catalyst structure and performance, optimizing ZnZrO x for applications such as methanol synthesis and CO 2 conversion. It has been demonstrated that by adjusting factors like drying methods, calcination temperature, ball-milling time, and additives, they developed catalysts with varied surface areas (4.5–106 m 2 /g) and oxygen content [O/(Zn +Zr) = 1.60–2.04]. The findings reveal that MTS is structure-sensitive, with oxygen-rich surfaces crucial for H 2 activation, the rate-limiting step. Catalysts with high surface oxygen content showed enhanced activity, making oxygen-rich surfaces the primary active sites for CO 2 hydrogenation, highlighting the overlooked importance of surface oxygen in catalytic performance and suggesting that designing oxygen-rich ZnZrO x catalysts with high surface areas can significantly boost methanol production, offering a new pathway for catalyst development. On the other hand, when a mixture of MTS catalyst (usually Cu or Zn based) and aromatization catalyst (Zeolite usually HZSM-5) is employed to synthesize aromatic hydrocarbons, with methanol as an intermediate product. Where the design and development of catalysts with high selectivity, exceptional activity, and outstanding stability are critical to advancing synthesis gas to methanol technology. Usually, the copper-based catalysts mixed with MoCoK-ZSM-5 and MoNiK-ZSM-5 illustrated that the composite catalyst has higher reaction activity, the CO conversion rate has increased from 25 % to 90 %, where the higher reaction temperature was conducive to the formation of aromatics and the catalytic effects of composite catalysts [160]. Other catalysts include Zn, Zr, Co, and Mo-based catalysts, etc. [161–165], can be combined with HZSM-5 for a higher aromatics selectivity in the liquid phase product, and a good catalyst stability. Most famous bifunctional catalyst by Wang Ye’s group combining ZnO-ZrO 2 aerogel and zeolite H-ZSM-5 offers a breakthrough in CO 2 hydrogenation, converting CO 2 to aromatics with 76 % selectivity and minimal CH 4 formation (<1 %) [166]. Despite CO 2 ’s inert nature and the high energy barrier of C-C coupling, this catalyst achieves 16 % CO 2 conversion and a high space-time yield of 0.24 g goxide −1 h −1 at 340 ◦C and 40 bar, outperforming conventional Fischer-Tropsch systems. The key to its success lies in the ZnO-ZrO 2 aerogel’s large surface area and oxygen vacancies, making it highly efficient in methanol formation. Similarly, in another study they reported to achieve a great promise for commercial CO 2 -to-chemical conversion applications with a CO conversion rate of 20 %, while maintaining a stable performance of 1000h without significant drop in activity under industrial conditions [25]. In another study of a highly dispersed ZnZr metal oxide catalyst synthesized by Xu et al. for CO 2 hydrogenation to methanol [167], revealed that the ordered structure of ZnZrO x solid solution could keep the high methanol selectivity. Where it was further demonstrated that the strategy can also be applied to Pd or Pt doping to ZnZrO x by hydrogen spillover technique. A small amount of catalysts can carry out the catalytic reaction, which is economical and environmentally friendly, and can be efficiently industrialized. Arsalan et al. prepared nano ZnCr 2 O 4 for the synthesis gas to aromatics, where the aromatics selectivity reached 70 % at 250 o C by optimizing the structure and particle size of the catalyst and preventing the oversaturation of carbon pool species [168]. Mohanty et al. [169] investigated the catalytic activity of ZnCoCr-ZSM-5 for the synthesis of aromatics from syngas, and the influence of process conditions on the reaction results was studied, including reaction temperature, reaction pressure, space velocity and feed gas ratio. The results showed that under the conditions of 275 o C and 3.8 MPa, the CO conversion rate reached the highest (72 %), with the highest aromatic selectivity. Increasing temperature and pressure is conducive to the conversion of CO, while lower H 2 /CO mole ratio is conducive to the formation of aromatics. A wide variety of MTS catalysts combined with HZSM-5 have been extensively studied for synthesizing aromatics from syngas, demonstrating high selectivity for aromatics. However, these catalysts often exhibit lower conversion rates and produce significant amounts of byproducts [26,165,170–186]. 4.3. FTS route FT synthesis is a heterogeneous catalytic hydrogenation process with carbon monoxide (adopted from coal, natural gas, biomass, and other carbon-rich sources) to produce a diverse range of valuable liquid fuels and petrochemical feedstocks through a non-selective polymerization reaction [187]. These products form a mixture encompassing linear alkanes, alkenes, alcohols, aldehydes, ketones, carboxylic acids, and water. Where, the primary target products for FT synthesis are olefins and paraffins, which are crucial components to produce transportation fuels such as diesel and gasoline, as well as various petrochemical derivatives [153,188–194]. While, recent efforts focus on scaling biomass-to-liquid (BTL) technologies using FT synthesis to produce sustainable liquid fuels, including biofuels and sustainable aviation fuel (SAF) [195]. Since its inception in 1923, FTS has been governed by three primary reaction pathways: paraffin formation, olefin formation, and the WGS reaction [196,197]. Two main mechanisms underpin FTS: the carbide mechanism and the CO-insertion mechanism (Fig. 8). In the carbide mechanism, metal carbides react with H 2 to form CH 2 * species, which act as chain growth monomers [198]. Chain termination produces olefins, paraffins, and oxygenates. Variations of this mechanism propose alternative intermediates, such as C* and CH*, or suggest growing chains like alkylidynes (RC*) instead of alkyls (RCH 2 *) [199–201]. Conversely, the CO-insertion mechanism involves CO acting as a monomer, inserting into the growing chain before hydrogenation and C–O bond cleavage. This mechanism readily explains oxygenate formation. The primary difference between the two lies in the sequence of bond formation: in CO insertion, the C–C bond forms first, whereas in the carbide pathway, the C-O bond cleaves first. The CO-insertion mechanism, inspired by homogeneous catalysis, was initially proposed by Pichler and Schulz and later supported by Schweicher’s et al. [202], experiments and Zhuo’s et al. [203] theoretical studies. On cobalt catalysts, such as Co(111) and Co(0001), the energy barrier for CO insertion is lower than for direct CO dissociation [202,204,205]. Another proposed pathway, the hydroxycarbene mechanism, involves the polymerization of HCOH* with a growing RCOH* chain, though experimental support for this mechanism remains limited [206]. FTS follows a surface polymerization model governed by the Anderson–Schulz–Flory (ASF) distribution, which predicts hydrocarbon product distribution based on the chain-growth probability ( α ). While high α -values ( α >0.9) favor heavy hydrocarbons (C 21 + ), experimental deviations often occur, including excessive methane formation and variations in olefin-to-paraffin ratios due to secondary reactions like olefin re-adsorption and hydrocracking [207,208]. Since, the ASF model inherently limits direct selectivity for middle-distillate fuels (C 5 –C 20 ), requiring additional hydrocracking steps for refining Overcoming these constraints involves precise catalyst design, such as support modifications, noble metal incorporation, and tailored synthesis methods to control reaction kinetics and minimize methane formation. While the distribution between paraffins and olefins depends on the H 2 /CO ratio in the syngas and the catalyst employed. High H 2 /CO ratios and strong M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 16 hydrogenating catalysts favor paraffin production, while lower ratios and weaker hydrogenating catalysts promote olefin formation. Water, an inevitable byproduct of FTS, significantly impacts syngas conversion, hydrocarbon selectivity, and catalyst performance. Excess water can deactivate iron catalysts through oxidation, thereby reducing process efficiency. Despite extensive studies, the lack of a comprehensive molecular depiction of FTS has sparked ongoing debate and exploration using surface science, computational methods, and microkinetic models [206,209,210]. Since, addressing environmental concerns and the need to reduce fossil fuel dependency, there is a growing shift toward biomass-based and CO 2 -driven FTS processes [128]. These approaches replace CO with CO 2 , often coupled with sustainable hydrogen sources such as blue or green hydrogen. These challenges underscore the importance of bifunctional catalysis, where metal sites facilitate CO activation and chain growth, while acidic or redox sites regulate secondary transformations like isomerization and hydrocracking. The integration of bifunctional catalysts such as Co or Fe with zeolites, metal oxides, or carbides enables greater selectivity control, overcoming ASF constraints and enhancing middle-distillate production without additional refining steps. Moreover, commercial High-Temperature Fischer Tropch (HTFT) and Low-Temperature Fischer Tropch (LTFT) reactors, including fixed-bed, fluidized-bed, and slurry-phase configurations, each introduce unique mass and heat transfer challenges that influence catalyst performance and selectivity. The optimization of reactor design, alongside bifunctional catalyst advancements, remains key to achieving more efficient, cost-effective, and selective FT processes, particularly for sustainable fuel applications (Table 5) [211,212]. This perspective aligns with the ongoing shift toward bio-based FTS and carbon-neutral fuels, reinforcing bifunctional catalytic routes as a crucial strategy for next-generation synthetic fuel production [213,214]. Researchers are also exploring integrated technologies like tandem catalysis and two-stage processes that combine electrochemical syngas production with conventional FTS. These innovations aim to enhance sustainability and efficiency, aligning FTS with global efforts to transition to cleaner energy systems [215]. 4.4. Catalyst choice, promoters role and their effect on product distribution in FTS route In FTS, CO dissociation on Group VIII metal surfaces is crucial [217], influenced by the electronic structures of transition metals. Metals with fewer occupied d-orbitals [218] bind dissociated C and O atoms more strongly, enhancing CO dissociation and favoring longer-chain hydrocarbons. Ru, Co, and Fe are ideal for FT synthesis, with Ru being the most active but costly and scarce. Co catalysts offer high activity and selectivity for long-chain hydrocarbons, especially wax and diesel, with resistance to water deactivation. Fe catalysts, while cheaper, are versatile over wider conditions and beneficial for the WGS reaction, though Fig. 8. Major three mechanistic pathways of metal oxide catalysts for CO hydrogenation reaction via FTS. Reproduced with the permission from the ref [196]. Copyright 2023 American Chemical Society. Table 5 Comparison of Coand Fe-based FT catalysts. Feature Co-based Catalysts Fe-based Catalysts CO Dissociation Lower ability compared to Fe Higher ability Hydrocarbon Selectivity Higher selectivity for long-chain hydrocarbons Broad product distribution including light olefins and heavy hydrocarbons Catalyst Lifetime Generally, longer Shorter, with more deactivation issues Resistance to Deactivation Better at moderate CO conversion levels Deactivates faster at high CO conversion levels Effect of Promoters Enhanced with noble metals like Pt, Ru, Pd Enhanced with alkali metals like K, Na, transition metals like Mn, Cu Support Materials Typically used with SiO 2 , Al 2 O 3 , TiO 2 SiO 2 , Al 2 O 3 , MgO, molecular sieves, activated carbon WGS Activity Low High Operating Temperature Typically lower Higher Carburization Less important Crucial for forming active phases like FexC Active phase Metallic Co Fe Carbides Carbon Source Natural Gas Biomass & Coal H 2 /CO ratio 2 0.5–2.5 Methane Selectivity High Low Sulfur Tolerance Very Sensitive Sensitive Comparison of selected FT Catalysts [216,300]. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 17 they require significant modifications and face rapid deactivation. Table 5 provides a concise comparison of the main characteristics and differences between Coand Fe-based FTS catalysts [217]. For Co-based catalysts, metallic cobalt (Co 0 ) is the active phase for CO hydrogenation, primarily producing paraffins with some olefins and alcohols. Deactivation occurs due to oxidation and carburization, forming cobalt oxides (CoO x ) and carbides (Co 2 C), which increase methane selectivity and lower activity. Co 2 C is effective for the Fischer-Tropsch to Olefins (FTO) process, especially when sodium (Na) is added, forming Co 2 C nanoprisms that enhance C 2– C 4 = selectivity and reduce methane selectivity. On TiO 2 support, oxidized cubic CoO species show higher activity and selectivity compared to metallic Co on SiO 2 . Carbon-based supports offer stability and facilitate olefin formation. Advanced treatments like hydrogen and carbon monoxide treatments improve the activity and selectivity of Co 2 C catalysts. However, in HTFT processes, methane formation poses challenges, that demand some special confinement effect and innovative composite catalyst designs for overcoming these hurdles, improving selectivity, and reducing environmental impacts [306]. While Fe-based catalysts have garnered significant attention for FTS due to their high activity, cost-effectiveness, and low methane selectivity [260]. Unlike Co catalysts, Fe catalysts can produce a broad range of hydrocarbons (C 1 -C 60 ). However, identifying and controlling the active iron phases such as Fe 3 O 4 , Fe 2 O 3 , α -Fe, and various iron carbides during FTS is challenging. Iron carbides like ε ′ -Fe 2 C, χ -Fe 5 C 2 , and Θ-Fe 3 C are key active phases, with χ -Fe 5 C 2 being particularly stable and active at moderate temperatures [17]. Advanced characterization techniques have revealed phase transformations in iron catalysts under realistic conditions, such as the formation of a Fe 3 O 4 @ χ -Fe 5 C 2 core-shell structure, which exhibits high activity. Achieving a balance between iron carbides and oxides, especially Fe 3 O 4 , is crucial for optimizing performance. Despite extensive research, the most active phase for FTS remains debated, with factors like particle size, phase morphology, and support effects influencing the outcomes. Robust understanding requires synthesizing pure Fe-carbide and preventing phase changes during reactions. Efforts to synthesize oxide-free Fe-carbides have shown promising results. Xu et al. [219] used a rapid quenched skeletal iron precursor to prepare a catalyst primarily consisting of ε ′ -carbide, demonstrating excellent LTFT activity. Peng et al. [220] synthesized phase-pure ε ′ -carbide using Raney-Fe, achieving low CO 2 selectivity in low-temperature FT reactions. Stabilizing ε ′ -carbide into graphene layers enabled high-temperature FT reactions without transforming into χ -Fe 5 C 2 . Yang et al. [221] synthesized single-phase χ -Fe 5 C 2 with a bromine agent, and subsequent studies used mesoporous silica shells to protect χ -Fe 5 C 2 , maintaining its phase and low CO 2 selectivity. These approaches enhance the understanding of FT catalysis using well-defined, phase-pure catalysts. Promoters are crucial for enhancing the activity and selectivity of FTS catalysts [207]. They are categorized into electronic promoters (e. g., alkali metals, transition metals, rare earth metals) and structural promoters (e.g., inorganic oxides like SiO 2 and Al 2 O 3 ). Key promoters for Fe-based FT catalysts include K, Na, Mn, Cu, and S, where alkali metals like K and Na enhance C-O bond dissociation and improve iron oxide reduction and carburization, optimizing product distribution [7–19,222–225]. Usually, Mn increases olefin selectivity and catalyst surface area, while Cu aids Fe-oxide reduction and light olefin formation [7,12,18,222,226–228]. It can be seen in Fig. 9 that different catalyst systems used in FTS and CO 2 hydrogenation, highlighting the impact of supports and promoters on performance and product selectivity [18, 229–231]. Fig. 9A illustrates a Na-Fe-Mn-Si catalyst with a core-shell structure, where iron-based active sites are encapsulated within a silica framework. The inclusion of HZSM-5 zeolite enhances selectivity toward light olefins and aromatics, minimizing the production of heavier hydrocarbons. This dual-function catalyst combines Fischer-Tropsch activity with secondary reactions facilitated by the zeolite, optimizing hydrocarbon distribution. Fig. 9B compares iron-based (Fe 3 O 4 ) and cobalt-based catalysts supported on SiC for CO 2 hydrogenation, showing that the Fe 3 O 4 catalyst is effective in the Fischer-Tropsch reaction, promoting the formation of long-chain hydrocarbons, while the cobalt catalyst on SiC primarily facilitates methanation, leading to CH 4 production. The reaction mechanism involves CO 2 hydrogenation via the RWGS reaction, followed by CO conversion to hydrocarbons in FTS. The cobalt catalyst, however, favors direct CO 2 hydrogenation to methane, distinguishing it from iron-based systems. Fig. 9C focuses on iron-aluminum oxide (FeAlO 5 ) catalysts, particularly their role in enhancing light olefin production. This system involves carbonate, bicarbonate, and formate species that influence CO Fig. 9. A-D) Performance of the different catalysts with different supports and promoters on FTS catalysts, A) Reproduced with the permission from ref. [18]. Copyright 2022 Elsevier. B) Reproduced with the permission from ref. [229]. Copyright 2022 Elsevier. C) Reproduced with the permission from ref. [230]. Copyright 2020 American Chemical Society. D) Reproduced with the permission from ref. [231]. Copyright 2021 American Chemical Society, respectively. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 18 activation while exhibiting negligible WGS activity, thereby limiting CO 2 formation. The catalyst efficiently promotes C–C coupling, leading to selective production of C 5 + linear α -olefins. The suppression of secondary reactions ensures high selectivity towards valuable intermediates rather than paraffins. Fig. 9D, compares Fe 3 Al and Fe 6 Zn 1 Al 1 catalysts, demonstrating how ZnO and Al-based promoters modify catalytic performance. The Fe 3 Al spinel structure favors CO activation and alkane formation, whereas the Fe 6 Zn 1 Al 1 system, with ZnO as a promoter, enhances CO 2 hydrogenation and increases α -olefin selectivity. The ZnO-modified catalyst exhibits superior control over product distribution, making it more efficient for light hydrocarbon production. Similarly, Gong et al. [207] investigates the effect of Cu promotion on Fe-Mn-based catalysts for the Fischer-Tropsch to olefins (FTO) process, aiming for high light olefin selectivity with low CO 2 emissions (Fig. 10). A 3.0 wt% Cu-loaded Fe-Mn catalyst exhibited exceptional performance, achieving 96.9 % CO conversion and 40.1 % selectivity to light olefins, while maintaining a low CO 2 selectivity of 23.0 %. As Cu loading rises (0–3.0 wt%), light olefin selectivity improves (31.3 %→ 40.1 %), while C 5 + decreases (32.7 %→9.5 %) due to weaker surface basicity, reducing chain growth probability. Cu can also improve the catalyst reducibility and enhance H-assisted CO dissociation by modifying metal-support interactions, leading to increased activity and a shift toward short-chain hydrocarbons. Cu addition to Fe-Mn catalysts shift to faster CH x desorption and limit long-chain formation, while stable hydrogenation and β-H elimination maintain the olefin/paraffin ratio. Cu also boosts CO conversion by lowering the activation energy for CO dissociation. While, Zn and Mg promote Fe reduction, enhance catalyst dispersion, and increase selectivity for specific hydrocarbons [232]. In Co-based catalysts, noble metals like Pt, Ru, and Pd enhance cobalt oxide reduction and overall activity. Bimetallic Co-Fe catalysts have also gained attention for their enhanced FTS performance, where adding Co to Fe-based catalysts can improve CO dissociation and boosts FTS activity, promoting long-chain hydrocarbon production [233–260]. These catalysts show greater stability against deactivation than pure Co catalysts, although they can still deactivate at high CO conversion levels due to increased H 2 O partial pressure. The active metal and phase significantly influence the catalytic behavior in FTS reactions. Usually, in both cases, structural supports like SiO 2 and Al 2 O 3 improve catalyst stability, strength, and porosity. SiO 2 is particularly effective in maintaining catalyst stability without sacrificing activity, while Al 2 O 3 enhances promoter dispersion and light hydrocarbon selectivity [261–264]. 4.5. Bifunctionality of (OX-ZEO) cascade reactions to aromatics Regarding the upgrading of products to aromatics, zeolites have Fig. 10. Different reaction intermediates, chain growth scenarios and pathways on FeCuMn catalyst for light synthesis olefins via FTS route. Reproduced with the permission from Ref. [207]. Copyright 2020 Elsevier. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 19 established themselves as essential catalysts in the chemical industry for upgrading products to aromatics due to their cost-effectiveness, high performance, and versatile design [265]. These porous crystalline aluminosilicates are widely used in adsorption, ion exchange, and catalysis because of their shape-selective properties, uniform pores, adjustable acidities, and high thermal and hydrothermal stability. Zeolites can be either naturally occurring or synthesized for enhanced purity and functionality, with 252 approved frameworks as of May 2020. Their diverse channel systems, pore openings, and cavities allow for precise control over reaction intermediates and prevent metallic species aggregation, making them efficient and cost-effective catalysts for hydrocarbon modification. The acidic properties and high specific surface area (>100 m 2 /g) of zeolites significantly enhance their catalytic capabilities, especially in syngas conversion, which is becoming increasingly important. The confinement effect within zeolite cavities directs reaction intermediates, influencing pathways and enhancing selectivity toward desired products. Tailoring zeolite topology optimizes catalytic processes, promoting target product selectivity and improving stability. Popular zeolites like CHA, MFI, *BEA, MOR, and AEI are particularly favored for selective syngas conversion due to these advantageous properties. Among these, ZSM-5 stands out for its medium pore size (~5.5 Å), which is effective at controlling the formation of gasoline-range product molecules (≤C10) and resisting coke formation. It excels at cracking, isomerizing, oligomerizing, and aromatizing formed intermediates (oxygenates and olefins) into specific aromatic monomers. Thanks to its mordenite framework inverted (MFI) topology, ZSM-5 allows gasoline-range aromatic products to diffuse through linear channels while intermediate molecules enter through sinusoidal channels without counter-diffusion. The shape-selectivity of HZSM-5 favors the diffusion of linear-shaped hydrocarbons over bulky-shaped molecules, promoting the formation of p-xylene isomer among gasoline-range aromatics. In contrast, large-pore H-MOR zeolite, with a micropore size of ~7 Å, significantly alters aromatic distribution and increases methane fraction (45 wt%). Researchers have found HZSM-5 to be a highly stable catalyst for aromatic synthesis. Fujimoto et al. [266] screened various zeolites, including HZSM-5, H-MOR, Y-zeolite (HY), and dealuminated versions of H-MOR and HY for methanol and syngas conversion, concluding that HZSM-5 prevents coke precursor formation, and its dealumination leads to excellent catalytic stability and higher yields of aromatic monomers compared to other zeolites studied. Since, the remarkable ability of zeolites to enhance product upgrading to aromatics, coupled with their structural versatility and stability, underscores their vital role in the chemical industry [267]. The bifunctional catalytic route to aromatics in FTS addresses the inherent selectivity limitations of traditional FTS by integrating metal centers with acidic molecular sieves. In this approach, catalysts activate and dissociate CO into carbon intermediates, which then undergo C–C coupling. However, the challenge lies in balancing C–O bond activation and C–C coupling, often leading to a trade-off between product selectivity and syngas conversion rate [187]. The bifunctional system overcomes this by combining metal oxides or FTS metals with molecular sieves. This dual functionality allows for initial CO activation and intermediate hydrocarbon formation on metal sites, followed by selective C–C bond reconstruction and aromatization on acidic sites. This method efficiently directs the reaction pathways toward the production of high-selectivity C 2 −C 4 hydrocarbons and aromatics, breaking the traditional ASF distribution (Fig. 11). The overall figure illustrates different hybrid catalytic strategies for the selective conversion of syngas into lower olefins and aromatics while breaking the Anderson-Schulz-Flory (ASF) distribution rule in FTS. The ASF rule predicts a broad distribution of hydrocarbons with a high fraction of long-chain products; however, coupling distinct catalytic functionalities enables selectivity control. The central concept presented in the figure is hybrid catalysis, where Fe-based catalysts or metal oxides work in synergy with zeolites (ZSM-5) to achieve tailored product selectivity. Fig. 11A&B demonstrates that syngas can be converted into aromatics through two different pathways: (i) an olefin-based route, where Fe-based catalysts generate lower olefins, which are then transformed into aromatics over ZSM-5, and (ii) a methanol-mediated pathway, where Zn-ZrO 2 facilitates syngas conversion to methanol/dimethyl ether (DME), followed by C 2 -C 4 hydrocarbon formation and subsequent aromatization over ZSM-5. The pathway depicted in the middle of the image further elaborates on the bifunctional catalytic mechanism, where Zn-ZrO 2 catalyzes the conversion of H 2 and CO into methanol/DME, which is then transformed into light olefins (C 2 -C 4 ) over ZSM-5. The zeolite’s acidity plays a critical role in promoting oligomerization, cyclization, and dehydrogenation, leading to the selective formation of aromatics. This hybrid approach bypasses the ASF distribution limitation by providing an alternative reaction pathway rather than direct Fischer-Tropsch polymerization. It also presents a selectivity comparison under different CO pressures, demonstrating that increasing CO pressure shifts the product distribution towards aromatics, confirming the effectiveness of this reaction coupling. Fig. 11C demonstrate a modified FeZnMg catalyst synthesized via sol-precipitation and combined with HZSM-5 for direct aromatics synthesis from syngas. Zn induced electronic modulations, forming ZnFe 2 O 4 ferrite enhancing Fe oxide reduction, while Mg improved active site dispersion and Fe-lattice tuning, boosting Fe species carburization and C–O chemisorption. This synergy reduced CO 2 and C 1 –C 4 paraffins, achieving 52 % aromatic selectivity at 97 % CO conversion. Fig. 11D highlights the catalyst design, showcasing a Zn-ZrO 2 and H-ZSM-5 bifunctional system, where the metal oxide component governs methanol formation while the zeolite facilitates subsequent hydrocarbon transformation. The combination of these functions leads to high aromatic selectivity (>80 %) with remarkable stability (>1000 h). The synergistic interaction between metal centers and acidic sites not only enhances selectivity and yield but also reduces the need for costly secondary treatments, making this bifunctional approach both efficient and economically viable to produce aromatic hydrocarbons. The challenge lies in achieving high selectivity to aromatics, particularly BTX, through FTS integrated with HZSM-5 zeolite catalysts. Optimal conditions for FTS and aromatization/isomerization reactions often do not align well, hindering efficient BTX production. Fe-based catalysts are favored in single-bed reactors due to their ability to generate alkenes and their high-temperature tolerance, which supports aromatic formation and facilitates heat recovery for steam generation. Factors such as zeolite Si/Al ratio(acidity), pore structure, and Fe-based catalyst type significantly influence aromatic selectivity and distribution. Light olefins and paraffins from Fe-based FTS catalysts are crucial for aromatic Fig. 11. A-D) Different reaction coupling for selective conversion of syngas into lower olefins and aromatics breaking the ASF distribution. A) Reproduced with permission from ref. [273]. Copyright 2021 Elsevier. B& D) Reproduced with permission from ref. [25]. Copyright, 2017 Elsevier. C) Reproduced with the permission from the ref. [233]. Copyright, 2022 American Chemical Society. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 20 formation, with ZSM-5 zeolite proving effective due to its suitable pore structure and acidity, crucial for shape-selectivity and aromatization. Indeed, recent studies have explored various bifunctional catalyst systems for synthesizing aromatics directly from syngas. For instance, Xu et al. [268] developed a Fe/HZSM-5 catalyst, achieving 90 % CO conversion and 50 % aromatic selectivity under conditions of 320 ◦ C, 2.0 MPa, and 4000 h −1 . Yi Fu et al. [269] reported a composite Fe/ZnCr 2 O 4 &H-ZSM-5 catalyst with 57.5 % CO conversion and 74 % aromatics selectivity at 653 K and 4.0 MPa, highlighting the role of Fe in enhancing CO conversion and aromatics production through oxygen vacancy and iron carbide formation. Tang et al. [270] integrated MnFe 2 O 4 spinel and ZSM-5 zeolite, achieving over 65 % aromatics selectivity in C 5+ products [116]. Additionally, Ma et al. [271] synthesized MgZrO x catalysts combined with HZSM-5, demonstrating 66.4 % aromatic selectivity at 400 ◦C and 3 MPa by efficiently converting methanol and oxygenates into aromatics while minimizing long-chain hydrocarbon formation. Yang et al. [272] investigated Na-Fe-ZrO 2 /ZSM-5 tandem catalysts, elucidating zeolite’s role in enhancing CO hydrogenation and olefin transformation to improve aromatics selectivity. Nawaz et al. [260] studied Na-FeMnCo/HZSM-5, featuring Fe-Co alloy, achieving 98 % CO conversion and enhancing hydrocarbon and aromatic fractions (55 %), with reduced CO 2 (21 %). Uniform CoFe 2 O 4 nanoparticles crucially improved catalytic performance through unique Fe substitution in oxygen vacancy density, iron oxide reducibility, and surface adsorption, highlighting zeolite’s catalytic enhancement effects. These studies underscore the diverse strategies and catalyst combinations aimed at optimizing aromatics production from syngas, emphasizing the critical role of catalyst design and synergistic effects in enhancing performance. While the hydrogenation of waste CO 2 through heterogeneous catalysis has also gained significant attention due to advancements in CO 2 capture technologies and the production of renewable hydrogen as shown in Fig. 12. In overall the figure illustrates two primary catalytic strategies for the selective synthesis of aromatics from CO 2 hydrogenation: modified FTS and methanol-mediated pathways. These approaches integrate multifunctional catalysts to enhance selectivity, surpassing the limitations of conventional FTS. In the modified Fischer-Tropsch route (Fig. 12A), a Na-Fe 3 O 4 /zeolite bifunctional catalyst is employed to convert CO 2 into aromatics. The reaction sequence begins with the RWGS reaction, where CO 2 is reduced to CO over Fe 3 O 4 . The CO then undergoes the Fischer-Tropsch reaction on Fe 5 C 2 sites, forming shortchain hydrocarbons (C n H m ). These intermediates diffuse into the zeolite component, where Brønsted acid sites catalyze oligomerization, isomerization, and aromatization, producing C 5 -C 11 aromatic hydrocarbons with enhanced selectivity. The methanol-mediated pathway (Fig. 12B) involves CO 2 hydrogenation to methanol or its intermediates (HCOO*, H 2 CO*, CH 3 OH*) on a Zn-ZrO 2 catalyst, followed by conversion to C 2 -C 4 olefins and aromatics over zeolites. The Zn-ZrO 2 sites promote efficient CO 2 activation and hydrogenation, while the zeolite ensures selective hydrocarbon transformation. This bifunctional system achieves 73 % aromatic selectivity, demonstrating its potential for CO 2 utilization in petrochemical applications. Fig. 12C provides further insights into the reaction mechanism for ZnFeO x -Na/HZSM-5 catalysts. The Fe 3 O 4 phase enables RWGS, while Fe 5 C 2 facilitates FTS, producing short-chain olefins. These intermediates diffuse to HZSM-5, where Brønsted acid sites catalyze aromatization, yielding high xylenes (PX) selectivity (75 %). This pathway effectively enhances CO 2 conversion (~40 %) while directing selectivity toward valuable aromatics. While Fig. 12D presents a structural optimization strategy for HZSM-5 to further improve aromatic yield. A chain-like HZSM-5 morphology with enhanced b-axis diffusion channels prevents surface passivation and facilitates efficient molecular transport. This results in 74.7 % aromatic selectivity, with controlled distribution of para-xylene (PX) and tetramethylbenzene (TeMB). Since, among the promising approaches to Fig. 12. A-D) The modified FTS and methanol-mediated pathways for Aromatics synthesis from CO 2 hydrogenation. A) Reproduced with the permission from ref. [275]. Copyright 2017 Nature. B) Reproduced with the permission from ref. [180]. Copyright 2019 Elsevier, C) Reproduced with the permission from ref. [276]. Copyright 2019 American Chemical Society. D) Reproduced with the permission from ref. [277]. Copyright 2021 Elsevier, respectively. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 21 reduce CO 2 emissions and generate valuable aromatic compounds is CO 2 aromatization, which involves the same tandem catalyst system combining CO 2 hydrogenation and intermediate aromatization catalysts. Where the olefinic intermediates from CO 2 hydrogenation on the Fe-based catalysts (via the RWGS reaction followed by FTS route), or on the MTS based metal oxides (via methanol or DME), can be subsequently aromatized on H-ZSM-5. The composition of the catalysts, the acidity of H-ZSM-5, and the interaction between components play crucial roles in determining aromatics selectivity. Where the economic viability demands a higher aromatic yield at least 46 % for toluene production alongside a carbon price below $340 per ton of CO 2 to compete with fossil-based alternatives. This underscores the need for continued research and optimization to enhance the process’s economic competitiveness. Direct CO 2 aromatization faces several challenges compared to indirect methods, which use separate reactors optimized for each step. A major limitation is the narrow temperature range required for both catalytic functions to operate effectively, restricting the use of certain catalysts for methanol or olefin synthesis in direct processes. Additionally, reaction parameters exhibit complex trade-offs; for instance, while water can improve ethylene aromatization by competing for adsorption on H-ZSM-5 acid sites, excess water inhibits CO 2 hydrogenation. Despite these hurdles, direct aromatization offers advantages such as cost and energy savings through process intensification, reduced coke formation due to hydrogen’s presence, and potential thermodynamic benefits from the in-situ consumption of intermediates. Continued innovation is essential to overcome these challenges, improve yields, and establish direct CO 2 aromatization as a viable and competitive method for sustainable aromatic production. A recent study on Fe-based catalyst paired with a novel twinned HZSM-5 (LZ5) zeolite containing a high sinusoidal pore opening ratio, claimed the enhanced aromatics formation rate (40 mmol CO2gcat −1 h −1 ) with highly selective paraxylene (PX) production from CO 2 [274]. Operating under industrial conditions, the tandem catalyst reached a 31.5 % CO 2 conversion with PX selectivity of 34 % among aromatics and 90 % within xylene isomers. This enhanced selectivity was attributed to LZ5’s unique pore structure, which efficiently screens xylene isomers, while the proximity of catalyst components enables continuous CO 2 -to-PX conversion. Similarly a high-performance triple composite catalyst system using ZnZrO x , Al 2 O 3 , and H-ZSM-5 significantly improved the alkylation of benzene with CO 2 /H 2 to produce toluene and xylene, offering a promising route to reduce carbon emissions and upgrade light aromatics [158]. The system achieves 97 % selectivity for toluene and xylene at 9.7 % benzene conversion by converting CO 2 to methanol, dehydrating it to DME, and alkylating benzene with DME. This approach enhanced the alkylation activity while minimizing unwanted reactions, paving the way for more efficient CO 2 hydrogenation processes. Enhancing the synergy between catalyst components, such as using size-controlled Pt nanoparticles on H-ZSM-5, has proven effective, increasing CO 2 conversion activity significantly. MTS based metal oxides offer different advantages over Fe-based catalysts for CO 2 aromatization, notably exhibiting significantly lower CH 4 selectivity (less than 0.5 %) compared to Fe-based/H-ZSM-5 catalysts (over 4 %). Contrarily, the Fe-based/H-ZSM-5 catalysts maintain CO selectivity below 10 %, whereas MTS-based/H-ZSM-5 catalysts struggle to keep CO selectivity below ~35 %, as these metal oxides are less effective for CO-FTS reactions at high temperatures. Co-feeding CO can suppress its formation by thermodynamically intervening in the RWGS reaction, but optimizing the RWGS reaction rate through catalyst design is a more practical approach for improving the process. Where, optimizing aromatics selectivity involves adjusting the design of H-ZSM-5 catalysts, particularly the type, intensity, and placement of acid sites. Fe-based catalysts paired with H-ZSM-5 zeolites excel in CO 2 -to-olefin conversion, especially when the zeolite features a lower Si/Al ratio, corresponding to higher acid density. This enhanced acid functionality aligns with the catalysts’ need for effective CO 2 conversion. Modifying H-ZSM-5 through NaOH treatments allows fine-tuning of acid site intensity and type, but the interplay between Brønsted and Lewis acid sites remains insufficiently understood. Future research should prioritize reducing methane selectivity in Fe-based systems, optimizing Si/Al ratios and acid site distributions in H-ZSM-5, and leveraging dual-bed configurations to improve aromatization performance. Up to now, a more recent development by Wang et al. group have been reported for synthesizing a highly efficient catalytic system for the direct conversion of syngas to para-xylene (PX) and other aromatics using a bifunctional catalyst composed of CoMnAl and versatile HZSM5@silicalite-1 zeolites (Fig. 13) [4]. Under mild conditions (280 ◦C, 2 MPa), this system achieved over 70 % CO conversion and produced 63.5 % aromatics, with PX making up 34.7 % of the total. Methane formation was minimal at just 2.9 %, while CO 2 selectivity was kept under 25.6 %. The innovative design of nanosized HZSM-5 with a hollow structure enhances olefin diffusion, boosting both CO conversion and aromatic selectivity. The addition of a scaly silicalite-1 layer on the HZSM-5 prevents unwanted side reactions, improving catalyst stability, with no deactivation observed over 726 h. Crucially, oxygen-containing aromatic compounds like p-tolualdehyde play a key role in the conversion process, helping to transform syngas into valuable PX. This breakthrough offers a promising industrial solution for efficient and selective PX synthesis while advancing our understanding of syngas-to-aromatics reaction mechanisms. Though, Co-based metal oxides, when paired with zeolites, are revolutionizing syngas-to-hydrocarbon technology by enhancing performance and selectivity for C 6 + hydrocarbons. Key challenges in designing direct CO 2 aromatization processes include managing diffusion path lengths for intermediates, which impact the conversion to desired aromatics and limit unwanted side reactions like isomerization and hydrocracking. Hierarchical zeolite structures, created through alkali treatments, increase surface area and reduce diffusion limitations but can introduce heterogeneity and acid/base poisoning. Selectivity for BTX aromatics remains a critical area of focus. Techniques such as coating zeolites’ external surfaces with SiO 2 have shown promise in improving BTX yields but also risk narrowing pores and reducing internal acidity, which hampers intermediate conversion. Exploring alternative zeolite structures and advanced passivation strategies could yield significant improvements. Theoretical and computational studies play a pivotal role in unraveling the complexities of tandem catalytic systems. For Feand Co-based catalysts, density functional theory (DFT) must account for spin-polarization, magnetic properties, and dynamic thermodynamic phases during reactions. Mixed-metal oxides and multi-component catalysts add another layer of complexity due to ambiguous active sites, making atomistic modeling particularly challenging. For zeolites, accurate modeling requires incorporating long-range dispersive interactions and diverse adsorption environments. Advanced techniques such as kinetic Monte Carlo simulations and coverage-dependent mean-field models are essential to simulate high-pressure, non-ideal reaction conditions but demand significant computational resources. Comprehensive kinetic studies are vital to mapping the multistep reaction networks in CO 2 aromatization, including alternative pathways like carbon suboxide-mesitylene synthesis or methane aromatization. Coupling CO 2 -to-methanol catalysts with aromatics-alkylation catalysts presents an intriguing avenue for producing methylated aromatics, but achieving high selectivity remains elusive. Integrating kinetic, transport, and process models is essential to bridge the gap between theoretical predictions and experimental results, enabling meaningful evaluations of selectivity and yield. Despite these challenges, the direct conversion of CO 2 to BTX aromatics offers a transformative opportunity for carbon capture and utilization. Success hinges on simplifying complex reaction networks into manageable models to gain a foundational understanding. Leveraging data science and machine learning to analyze experimental and computational data can accelerate breakthroughs, offering insights into intricate reaction mechanisms and guiding catalyst design. By merging experimental insights, advanced simulations, and data-driven approaches, researchers can overcome the interdisciplinary hurdles and unlock the full potential of CO 2 -to-aromatics technologies. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 22 4.6. Mechanistic pathways of bifunctional route for aromatics The direct synthesis of aromatic hydrocarbons from syngas involves numerous complex steps and faces significant challenges, primarily due to limited selectivity for valuable aromatic monomers like BTX. The fundamental reaction mechanisms on bifunctional heterogeneous catalysts are not fully understood, impeding the optimization of existing catalysts and the development of new ones. Understanding these mechanisms is crucial for improving catalyst performance and efficiency. The process of STA typically follows two main pathways: the FTS route and the MTS route, both involving solid-acid HZSM-5 (Fig. 14). While the conversion of CO to olefins through CO hydrogenation is welldocumented, the subsequent transformation of olefins or paraffins into aromatics over HZSM-5 is particularly critical. These transformations require higher temperatures than those optimal for olefin synthesis from syngas. The overall conversion process includes a variety of reactions such as hydrogenation, oligomerization, cracking, isomerization, methylation, dehydrogenation, and hydrogen transfer. Each of these reactions contributes to the complexity of the process and poses challenges in achieving high selectivity and efficiency. A thorough understanding of each reaction mechanism is essential for optimizing the production of valuable aromatic compounds and enhancing the performance of the catalytic system. The conversion of syngas to aromatic monomers over bifunctional catalysts begins with CO being converted into olefins of varying carbon lengths (C 2 –C 5 , C 6 –C 8 , and C 9 + ), followed by the diffusion into the channels of HZSM-5 zeolite and undergoing for further transformations. Specifically, C 2 –C 5 olefins are first oligomerized into C 6 –C 8 olefins over the Brønsted acid sites (BAS) of HZSM-5. These C 6 –C 8 olefins then undergo cyclization, hydrogen transfer, and/or dehydrogenation to form C 6 –C 8 light aromatics. Some of these light aromatics may be alkylated into C 9 + heavy aromatics over the external acid sites of HZSM-5. Meanwhile, C 6 –C 8 and C 9 + olefins can be directly converted into their corresponding light and heavy aromatics through similar cyclization and dehydrogenation processes. The formation of aromatics is facilitated by a synergistic effect between BAS and Lewis acid sites (LAS); for instance, cyclic olefins generated over BAS can be converted into cyclic diolefins via hydrogen transfer and dehydrogenation over LAS. However, some of the C 9 + heavy aromatics may undergo over-dehydrogenation, leading to coke formation and the eventual deactivation of the HZSM-5 catalyst [4]. The MTA Fig. 13. Co-based/HZSM-5 bifunctional catalysts for unprecedented para-xylene production from CO hydrogenation. Reproduced with the permission from ref. [4]. Copyright 2022 Elsevier. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 23 reaction involves numerous intermediates and comprises complex steps, such as dehydration, alkylation, dealkylation, isomerization, oligomerization, cyclization, and hydride transfer [278]. Initially, methanol undergoes dehydration to produce dimethyl ether, which can be further dehydrated to form ethylene. Subsequent continuous methylation of ethylene, along with oligomerization and cyclization, leads to the formation of C 6 –C 9 cyclic olefins. Hydrogen transfer from these C 6 –C 9 cyclic olefins to C 2 –C 5 olefins generates C 6 –C 9 aromatics and C 2 –C 5 alkanes, while hydrogen transfer to methanol produces C 6 –C 9 aromatics and CH 4 . Zn-based oxides play a crucial role in this process by suppressing hydrogen transfer but facilitating the dehydrogenation of C 6 –C 9 cyclic olefins, thereby promoting the production of aromatics. Consequently, oligomerization and cyclization of low-carbon olefins are favored due to chemical equilibrium. Low-carbon alkanes also form through the cracking of C 6 –C 9 + cyclic olefins, though their aromatization is challenging at 450 ◦C. Throughout the reaction, methylation, dealkylation, and isomerization of intermediates are prevalent, with dealkylation of C 9+ aromatics contributing to the increased yields of both alkanes and BTX. While an impressive mechanistic illustration on Co-based/HZSM-5 composite catalyst has been provided by Wang et al. [4], demonstrating that using CO instead of Ar significantly enhances olefin conversion and increases selectivity toward aromatics while reducing the production of lower olefins, where CO plays a key role in olefin coupling, especially for long-chain olefins, to form aromatics over HZSM-5 zeolite (Fig. 15). It has been demonstrated that CO can be adsorbed on both weak and strong acid sites, forming carbonyl compounds that react with olefins to produce cyclic oxygenates, which are then converted to aromatics. Where, CO dissociates on the Co 2 C surface, leading to the formation of intermediate species that undergo hydrogenation and C-C coupling, promoting the conversion of olefins into aromatics. Key intermediates, such as 2,4-dimethyl-benzaldehyde and p-tolualdehyde, were identified, indicating that these oxygenated compounds play a vital role in aromatic formation. This pathway involves CO hydrogenation to olefins, their coupling with CO, and further transformation into aromatics through reactions facilitated by HZSM-5. Similarly, the study by Tian et al. [279] briefly reveals highlights how LaFeO 3 perovskite enables the selective conversion of CO 2 into valuable aromatics. The process begins with CO 2 hydrogenation on Fig. 14. Figure: General reaction pathways for the production of aromatic monomers from syngas over the FTS, and MTS with HZSM-5. Reproduced with the permission from ref. [182]. Copyright 2023 Elsevier. M. Saif et al. Biomass and Bioenergy 196 (2025) 107736 24 LaFeO 3 , producing C1 oxygenates like formate and formaldehyde. These intermediates then migrate to the H-ZSM-5 zeolite, where they undergo C-C coupling and aromatization. A key advantage of LaFeO 3 is its resistance to carburization, which prevents unwanted side reactions commonly seen in iron-based catalysts. This decoupling of CO 2 hydrogenation from aromatics formation ensures high selectivity and stability, resulting in a highly efficient CO 2 -to-aromatics conversion process. Unlike conventional iron-based catalysts that form carbides and produce a wide range of products, LaFeO 3 ’s unique surface chemistry favors the formation of precise C 1 intermediates, which are further transformed into aromatics in the acidic pores of H-ZSM-5. Through advanced in-situ spectroscopy and computational analysis, the researchers identified the key reaction pathways, demonstrating how LaFeO 3 maintains catalytic performance for over 1000 h without deactivation. This innovative perovskite-based system offers exceptional stability and efficiency, making it highly promising for scalable industrial applications. The tandem catalysis approach not only converts CO 2 into valuable chemicals but also contributes to greenhouse gas reduction, paving the way for a sustainable future in chemical manufacturing. Usually, methanol mediated pathways are attractive towards heavy aromatics such as durene, a valuable aromatic used in polyimide production and aviation kerosene additives, is traditionally derived from petroleum, which is costly and unsustainable. A recent study presents efficient method for synthesizing durene through the methylation of trimethylbenzene coupled with CO 2 hydrogenation, achieving an impressive 83.2 % durene selectivity in tetramethylbenzene (TetraMB) and 69.1 % TetraMB selectivity among aromatics, with a 1,2,4-trimethylbenzene conversion of 33.2 % [280]. Even with mixed TetraMB feed, durene selectivity remained high at 81.4 %. The 5 % CuZnZrO x -HZSM-5 catalyst demonstrated long-term stability, maintaining performance for over 1100 h. Copper’s role in the ZnZrO x composite was crucial in generating active intermediates from CO 2 hydrogenation, enhancing the methylation process. This approach provides a sustainable alternative for durene synthesis with high efficiency and durability. The distribution of products in the STA process is primarily controlled by the characteristics of acid sites and their strength on the HZSM-5 catalyst. However, it is crucial to emphasize that having a comprehensive understanding of the reaction mechanisms involved in the direct synthesis of aromatics from syngas is valuable. This understanding not only aids in the development of new, advanced heterogeneous catalysts but also contributes to enhancements in existing catalytic systems. By unraveling the intricacies of the reaction pathways, researchers can optimize catalyst design and operation, leading to improved selectivity and efficiency in the production of aromatic compounds from syngas. 4.7. Tunning the catalytic bifunctionality for product upgradation and elevation while dictating the deactivation scenarios Molecular sieves, particularly HZSM-5, are highly valued for their precise acidity and distinctive channel structure, making them critical in STA reactions and as catalysts for aromatization. The distribution of strong and weak acid centers in HZSM-5 molecular sieves is essential for alkene cyclization, which drives aromatic compound formation. Optimizing catalyst performance hinges on understanding the intricate relationship between acidity, pore structure, and aromatic selectivity. However, first understanding the deactivation scenarios in bifunctional catalyst is crucial for maintaining a higher catalytic activity. Usually an integrated catalyst (OX-ZEO) composed of two separate comparts, metal-based oxides termed as “OX” and the molecular sieve catalyst usually zeolite are termed as ZEO, which encounter the catalyst deactivation pathways in different manners. Mainly, the metal oxides (OX) such as Fe-based catalysts used in FTS reactions are susceptible to deactivation due to catalyst sintering, particle agglomeration, and attrition. 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