Sustainable Methods for Conversion of Cellulosic Biomass to Bio-Based Plastics: A Green Chemistry Approach

Abstract

Plastic manufacturing depends heavily on petroleum-derived monomers like terephthalic acid, the main component of polyethylene terephthalate (PET). However, the depletion of fossil resources and increasing environmental concerns have heightened the need for sustainable alternatives. Lignocellulosic biomass has emerged as a promising resource due to its renewable, abundant, and eco-friendly nature. Understanding its chemical composition enables conversion of this biomass into platform chemicals, such as 2,5-furandicarboxylic acid (FDCA) and lactic acid, derived from cellulose and hemicellulose. These can be polymerized into bio-based plastics such as polyethylene furanoate (PEF), polylactic acid (PLA), and polyhydroxyalkanoates (PHAs), offering greener alternatives to fossil-based plastics. PEF features rigid furan rings that enhance thermal stability, mechanical strength, and barrier properties, and reduce gas permeability compared to PET. PLA is a renewable, biodegradable plastic widely used in packaging and medical applications. This review covers the chemical composition of lignocellulosic biomass cellulose, hemicellulose, and lignin, and various pretreatment strategies, chemical, physicochemical, and physical, to overcome biomass recalcitrance and improve conversion efficiency. It also highlights recent catalytic advances in transforming cellulosic carbohydrates into bio-based plastic precursors such as FDCA and lactic acid. Lastly, this review discusses polymerization pathways for producing PEF and PLA, emphasizing their role in reducing the environmental impact of polymer manufacturing and promoting green chemistry principles.

1. Introduction

Plastics have revolutionized the modern world since the beginning of their large-scale production [1,2]. This remarkable material is used in thousands of applications due to its unique characteristics, i.e., light weight, flexibility, durability, and cost-effectiveness [3]. In 1833, the French chemist Henri Braconnot synthesized the first semi-synthetic plastic called xyloidine by treating plant-based materials, such as sawdust, starch, and cotton, with nitric acid. A few years later, Christian Friedrich Schönbein synthesized another cellulose derivative, pyroxylin, by treating cellulose (cotton) with a mixture of sulfuric and nitric acid, and it is commonly known as guncotton. The history of flexible plastics formally began in 1862, when Alexander Parkes introduced Parkesine, followed by the synthesis of celluloid by John Wesley Hyatt in 1870 [1]. In 1872, Baumann first developed polyvinyl chloride (PVC) by exposing tubes filled with vinyl chloride to sunlight [4]. Later, PVC showed some drawbacks, such as cracking and brittleness, when exposed to heat and light. In 1926, further research by Walter Semon led to plasticized PVC [5]. Growing research efforts and rising demand for plastic materials ultimately led to the production of the first fully synthetic plastic (Bakelite) by Leo Baekeland in 1907. Dr. Baekeland’s invention marked the beginning of the “Age of Plastics,” and he was the first to coin the term plastic [6].

In the following years, various forms of synthetic plastics, including high-density polyethylene, polyethylene terephthalate, acrylic, low-density polyethylene, polypropylene, and Styrofoam, were synthesized by different researchers, as shown in Figure 1. Plastic production expanded rapidly after World War II, fueled by industrial growth, and has now surpassed 400 million tons annually. Remarkably, half of all plastics ever produced have been manufactured in the past 15 years [7]. Global plastic production is projected to reach between 902 and 1124 million tons by 2050, two to three times higher than current levels [3,8,9]. The complete history timeline of plastic evolution over the years is summarized in Figure 1.

Conventional plastics are derived from petroleum-based products and are highly resistant to natural degradation. Due to their hydrocarbon nature, these materials retain their physicochemical properties even after remolding and thermal processing [10]. When subjected to incomplete degradation, plastics fragment into micro- and nanoscale particles that can easily infiltrate soil and aquatic environments [11]. Since the first documented scientific study on beach debris by Herms in 1907 [12], plastic waste has increasingly accumulated in marine and coastal regions worldwide [13]. The earliest recorded evidence of plastic pollution dates back to the Irish coast between 1960 and 1970 [14]. Since then, extensive research has been conducted to understand the fate, transport, and mitigation of plastics in both aquatic and terrestrial ecosystems. As shown in Figure 2, globally, about 34 million tons of plastic are produced each year, of which only about 7% is recycled [15], while the remaining 93% ends up in landfills or marine environments [16,17], leading to heavy economic losses of about 80 to 100 billion USD each year [18]. In the US, the government spends about 8.3 billion US dollars yearly to dispose of plastic waste in landfills [19]. Although plastics are widely used, they pose a significant risk to human health and water ecosystems [20]. In addition, the production of plastics accounts for almost 13 percent of the world’s carbon budget, and it also contributes significantly to fossil fuel use and the emission of greenhouse gases [21]. Given the exhaustion of fossil resources, climate change, and health-related issues, these environmental and economic problems have triggered an international shift to more sustainable solutions [22]. This has led to a focus on research on renewable natural polymers, which are environmentally benign as feedstocks, to produce bio-based plastics and other sustainable materials.

Bio-based plastics are also a promising substitute for petroleum-based plastics since they are made using natural materials that are sustainable, biodegradable, and may emit fewer greenhouse gases throughout their life cycle [24]. In recent years, researchers have shifted their attention to the use of natural, sustainable feedstocks in the form of agricultural residues, lignocellulosic biomass, and organic wastes as raw materials to produce sustainable plastics [25]. The global research trend in sustainable bio-based plastics between 2000 and 2025 was extracted from the CAS SciFinder and Scopus databases, as shown in Figure 3. One can observe a sharp increase in publications after 2020, indicating growing scientific and industrial interest in environmentally friendly materials. Major areas of research are bio-based plastics, biodegradable materials, biomass refinery, packaging materials, and biodegradability, and to a large extent, the disciplines of environmental science, engineering, and materials science have contributed to these. To align with the Sustainable Development Goals on responsible consumption and climate action, sustainable resources are being explored for the sustainable production of plastics to comply with environmental regulations.

Bio-based plastics are generally produced from biomass such as polysaccharides (cellulose, chitosan, alginate, and gelatin), proteins, and fats [26,27]. Lignocellulosic biomass, a renewable material, is abundantly available, with an estimated 130 billion tons of biomass produced each year globally, providing a low-cost and sustainable raw material [28]. However, fractionation of lignocellulosic biomass into cellulose and hemicellulose, which are tightly bound together by lignin, requires pretreatment, which often involves excessive use of harsh chemicals (organosolv, acid, alkaline, and supercritical CO₂) and harsh reaction conditions (high temperature and pressure) [29,30,31]. Abolore et al. and Sharma et al. critically reviewed green pretreatment methods that operate under mild reaction conditions, require minimal solvent use, and achieve high delignification efficiency [29,30]. Green solvents such as ionic liquids (ILs), deep eutectic solvents (DESs), and low-transition-temperature mixtures (LTTMs) are regarded as reliable alternatives to conventional solvents [31]. Their applications in cellulose modification, carbohydrate conversion, and lignin extraction have been comprehensively reviewed by Tang et al., Stalewal et al., Chen and Mu, and Ullah et al. [32,33,34,35]. Storz and Vorlop critically review the production of common bio-based plastics and discuss strengths, obstacles, and key trade-offs [36].

2,5-furandicarboxylic acid (FDCA) is one of the substances produced from lignocellulosic biomass, which can be used as a monomer for polyester, polyurethane, and polyamide production [37,38,39,40,41,42,43]. Ethylene glycol can be copolymerized with FDCA to produce polyethylene furanoate (PEF), which is a promising bio-based alternative to polyethylene terephthalate (PET), a petroleum-derived polymer [43,44,45,46]. Two-step conversion is required for converting biomass-derived cellulose into FDCA, cellulose dehydration to 5-hydroxymethylfurfural (HMF), followed by oxidation to FDCA [43,47,48,49].

With the ongoing research efforts, bio-based plastics are replacing conventional plastics in developed countries. The most common types of bio-based biodegradable plastics, i.e., polylactic acid (PLA), polybutylene succinate (PBS), polyhydroxyalkanoates (PHA), polybutylene adipate terephthalate (PBAT), and thermoplastic starch (TPS) [50], accounted for approximately 40% of the global bio-based plastics market in 2020, and this share is projected to increase to about 70% by 2026. The total bio-based plastic production capacity is expected to reach 7.6 million tons by 2026. While the production of starch-based blends is projected to decline by 72.3%, the production of other biodegradable bio-based plastics such as PBAT, PBS, and PHA is expected to increase by 500%, 226.5%, and 166.7%, respectively, as shown in Figure 4 [26,51,52,53].

This review critically examines the sustainable conversion of lignocellulosic biomass components, cellulose, and hemicellulose into bio-based plastics and the associated processing challenges. In the Section 1, different solvents used for biomass pretreatment are critically analyzed, followed by a detailed discussion of pretreatment strategies. In the Section 2, conversion pathways from sugar monomers to bio-based plastic precursors are discussed. In the Section 3, routes for the conversion of glucose into 5-hydroxymethylfurfural (HMF) (route 1), the fermentation of these sugars to lactic acid (route 2), and the production of polyhydroxyalkanoates (PHA) (route 3) using bacteria are explained. Later in this section, as mentioned in Figure 5, routes for conversion of each of the above-mentioned precursors into bio-based monomers and bio-based plastics, such as (2,5-furandicarboxylic acid) FDCA polymerization to produce polyethylene furanoate (PEF), polymerization of lactic acid to poly(lactic acid) (PLA), are explained. At the end, we discuss sustainability considerations, focusing on waste reduction through the utilization of green solvents and renewable resources, as well as the development of eco-friendly production techniques by incorporating life-cycle analysis. Finally, recommendations are made to address the current research gaps for the synthesis of bio-based plastics.

2. Lignocellulosic Biomass: Composition and Pretreatment Strategies

Lignocellulosic biomass (LCB) is the most abundant renewable resource on Earth, primarily composed of cellulose (40–50%), hemicellulose (20–30%), and lignin (15–35%), along with minor components such as pectin, resins, waxes, ash, and minerals [54]. These polymers form a complex 3D matrix where cellulose is embedded in a lignin–hemicellulose network. LCB varies by plant species, growth stage, and environment. Major sources include crop and forest residues, energy crops, waste, manure, pulp sludge, and forestry byproducts [55]. Globally, LCB production is estimated at 10-50 billion tons annually, offering a sustainable, carbon-neutral alternative to fossil fuels for bioenergy and products [56].

Cellulose is the most abundant biopolymer on Earth, with an annual production of approximately 7.5 × 10¹⁰ tons [57]. It is a linear polysaccharide consisting of β-1,4-linked D-glucose units (Figure 6) with the general formula (C₆H₁₀O₅)_n and a degree of polymerization ranging from 8000 to 10,000, depending on the biomass source [58]. In contrast, hemicellulose is an amorphous, branched heteropolymer composed of various sugar monomers, including pentoses (such as xylose and arabinose), hexoses (such as mannose, glucose, and galactose) and sugar acids. Its degree of polymerization is significantly lower than cellulose, typically ranging from 100 to 200, resulting in lower crystallinity and greater susceptibility to hydrolysis [59]. Lignin is the third major component, a complex, amorphous, and hydrophobic aromatic polymer that provides structural rigidity, impermeability, and resistance to microbial degradation [60]. It is synthesized through radical coupling of three primary monolignols: p-coumaryl, coniferyl, and sinapyl alcohol, which correspond to p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, respectively, as shown in Figure 6.

LCB is a promising renewable feedstock for sustainable bio-based plastics due to its abundance, renewability, and non-food competition. Composed mainly of cellulose, hemicellulose, and lignin, it makes up 50–75% of plant dry mass and is versatile for producing bio-based monomers and polymers [61]. These polysaccharides can be converted into fermentable sugars and bio-based plastic precursors like lactic acid, succinic acid, HMF, and other chemicals used to synthesize PLA, PHAs, and PEF [62]. Valorizing lignocellulosic residues benefits the environment by reducing petrochemical plastics and waste. Demand for bio-based polymers is rising, with lignocellulosic-derived monomers expected to replace over 30% of fossil plastics in decades. Efficient fractionation and conversion technologies are needed to isolate key components: cellulose for glucose polymers, hemicellulose for pentose derivatives, and lignin for aromatic biopolymers. Thus, lignocellulosic biomass is vital for the circular bioeconomy, advancing renewable, low-carbon, biodegradable plastics [25].

Lignocellulosic biomass pretreatment methods are classified into physical, chemical, physicochemical, and biological approaches, each targeting the disruption of cellulose-hemicellulose–lignin linkages to improve hydrolysis [63]. Physical methods like chipping, grinding, and milling reduce particle size and increase surface area, but are energy-intensive and often costly at scale [64]. Chemical methods use acids, alkalis, oxidizers, or solvents to solubilize hemicellulose and disrupt lignin, exposing cellulose; they are fast but can produce inhibitors and require costly recovery [65]. Physicochemical techniques such as steam explosion, liquid hot water, AFEX, and CO₂ explosion combine mechanical and chemical effects for delignification and hemicellulose solubilization, offering cost-effective, scalable solutions. Biological pretreatments employ fungi and bacteria to degrade lignin and hemicellulose under eco-friendly conditions with low energy, though they are slow. Each method has benefits and limitations depending on the biomass and desired product. Physical and chemical methods enable rapid biomass disruption, while biological and hybrid methods are greener, lower-energy alternatives. Optimizing pretreatment selection and operational conditions remains a critical step toward efficient biorefinery operations and sustainable bio-based plastic production from lignocellulosic feedstocks [66].

Table 1. Advantages and disadvantages of different pretreatment methods of lignocellulosic biomass.

Pretreatment Methods	Advantages	Disadvantages	Ref.
Acidic	Hydrolyzed hemicellulose sugars alter lignin structure	Corrosion of equipment, formation of degradation products, High cost, neutralization required	[67]
Alkaline	Cause cellulose swelling, disrupt lignin structure, increase surface area, remove lignin and hemicellulose	A long time and a high concentration of base are required for salt formation, which is not easily removed	[68]
Oxidative	Oxidize lignin, low inhibitors formation, decrease cellulose crystallinity	Soluble aromatic compounds formation due to the oxidation of lignin	[69]
Ozonolysis	Eco-friendly, no waste generation, mild operating conditions	Expensive, a large amount of ozone is required	[69]
Organosolv	Sulfur-free lignin yield, easily recovered by distillation	An extremely controlled environment is required. Solvent acts as an inhibitor	[70]
Ionic Liquid	High efficiency, disrupts cellulose structure, environmentally friendly	High cost, lignin condensation, recovery challenge	[71,72]
Deep Eutectic Solvents	Low melting point, high ability to break down the C-C bond in LCB	High viscosity, difficult to recover and recycle	[73,74]
Steam Explosion	Low chemical usage, effectively hydrolyzed hemicellulose, and cost-effective	Inhibitor formation at high temperature, no lignin removal	[75]
Liquid Hot Water	Environment-friendly, high pentose recovery, minimal corrosion	Inhibitor formation at high temperature	[76]
AFEX	Increase accessible area, avoid inhibitor production	Not efficient with high lignin biomass, high cost of ammonia	[77]
Ultrasound	Disrupt lignin-cellulose-hemicellulose matrix, reduced reaction time, and no inhibitor production	High Energy consumption, scalability challenge	[78]
Mechanical	Decrease cellulose crystallinity	High power consumption, difficult to scaling up.	[79]
Biological	Low energy required, degrade hemicellulose and lignin	Hydrolysis rate is slow	[67]

summarizes the pros and cons of different pretreatment techniques for lignocellulosic biomass.

3. Green Catalytic Conversion Pathways for Bio-Based Plastic Precursors

Catalytic valorization of lignocellulosic biomass into bio-based plastic precursors is a key sustainable pathway. Renewable biomass intermediates are transformed into platform chemicals used as monomers for bio-based plastics like polylactic acid (PLA) and polyethylene furanoate (PEF). Lignocellulosic feedstocks, mainly cellulose, hemicellulose, and lignin, are broken down into fermentable sugars such as glucose, xylose, and fructose. These carbohydrates are crucial for catalytic reactions that produce monomers with high selectivity, efficiency, and a lower ecological footprint.

3.1. Synthetic Pathway for FDCA Production

The goal of eco-friendly FDCA synthesis is to oxidize HMF under mild, sustainable conditions that reduce energy use, avoid harmful reagents, and generate less waste.

HMF, a key platform chemical from lignocellulosic biomass, is usually made via acid dehydration of monosaccharides like fructose or directly from glucose [80,81] as shown in Figure 7. Due to its multifunctional nature, HMF acts as a valuable precursor for producing high-value chemicals and polymer building blocks. Among these, FDCA stands out as one of the top 12 biomass-derived platform chemicals and is a key monomer used in making polyethylene furanoate (PEF) [82,83].

The catalytic oxidation of HMF to FDCA proceeds through successive transformations of its alcohol and aldehyde functionalities, with the dominant reaction pathway largely dictated by the catalyst composition and reaction environment. As illustrated in Figure 8, two principal and competing mechanisms are typically observed. In the first route, the aldehyde group is oxidized first to produce 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). This intermediate undergoes further oxidation of the hydroxyl group to form FFCA. In the alternative route, the hydroxyl group yields 2,5-furandicarbaldehyde (DFF); subsequent oxidation of the aldehyde function forms 5-formyl-2-furancarboxylic acid (FFCA), which is then transformed into FDCA [84,85]. Overall, HMF can be oxidized to FDCA using metal-based catalytic systems via aerobic, electrochemical, or photocatalytic approaches.

Using a metal catalyst for the oxidation of HMF to FDCA, the Mars–van Krevelen mechanism underscores the critical role of a catalyst’s redox properties in governing HMF oxidation pathways [86]. Transition metals like Co, Mn, Cu, Ni, and Fe are frequently utilized because of their accessible redox couples (Co³⁺/Co²⁺, Mn⁴⁺/Mn³⁺/Mn²⁺, Cu¹⁺/Cu²⁺, Ni⁴⁺/Ni²⁺, and Fe²⁺/Fe³⁺). Although noble metals, including Au, Pt, and Ru, exhibit high catalytic performance for FDCA synthesis, their high cost, environmental concerns, and challenging recovery procedures have shifted research interest toward more sustainable and economical non-noble metal catalysts, as summarized in Table 2 and Figure 9.

Numerous studies show that manganese-based catalysts exhibit excellent performance in HMF oxidation. The proposed aerobic metal-catalyzed oxidation mechanism of HMF to FDCA is illustrated in Figure 10. Beibei Liu et al. [87] described the preparation of MnO₂ nanorods via hydrothermal methods, which acted as efficient catalysts to the NaHCO₃-mediated oxidation of HMF, delivering 97.8% conversion to FDCA and maintaining performance at gram-scale reactions. Similarly, Hayashi et al. [88] conducted a comparative study of MnO₂ polymorphs and revealed that catalytic activity is strongly dependent on crystal structure. Among six examined phases (β-, α-, γ-, δ-, ε-, and λ-MnO₂), the β-phase, characterized by the lowest vacancy-formation energy, exhibited the highest activity at 100 °C under an oxygen pressure of 10 bar for 24 h, achieving a 91% FDCA yield when its surface area was enhanced. Bao et al. [89] developed porous two-dimensional Mn₂O₃ nanoflakes via high-temperature calcination of a Mn-based metal–organic framework (Mn-TPA). This catalyst enabled nearly quantitative conversion of HMF to FDCA over 24 h and retained catalytic stability over five successive cycles. However, despite their excellent FDCA yields, the relatively low productivity of manganese oxides remains a barrier to large-scale implementation. Consistent with these findings, Yao et al. [90] reported that β-MnO₂, owing to its minimal vacancy-formation energy and optimal FFCA adsorption characteristics, possesses the most reactive lattice oxygen, thereby promoting efficient FDCA formation.

Liao et al. [91] introduced an environmentally benign and straightforward method to enhance the performance of MnO₂ catalysts by treating them with dilute nitric acid, producing the modified catalyst α-MnO₂-H⁺. Their comparative evaluation showed that the acid treatment removed amorphous or poorly crystalline surface nanoparticles, increased the surface oxidation (OA) ratio, and markedly improved catalytic activity. Among the tested materials, α-MnO₂-H⁺ delivered the highest efficiency, achieving 98.5% FDCA yield and over 99.9% HMF conversion under relatively moderate reaction conditions. The catalyst further demonstrated strong durability, retaining its activity across five consecutive reaction cycles.

Table 2. HMF oxidation over different metals.

Catalyst	Reaction Condition	Base	FDCA Yield %	HMF Con. %	Ref.
α-MnO2-H+	20 bar O2, 120 °C, 8 h	NaHCO3	98.5 d	99.9	[91]
nanorods (MnO2-R)	5 bar O2, 110 °C, 12 h	NaHCO3	96.7 d	100	[87]
2 D Mn2O3 nanoflakes	14 bar O2, 110 °C, 24 h	NaHCO3	99.5 d	100	[89]
La-MnO2	5 bar O2, 140 °C, 4 h	NH4OH	95.4 d	100	[92]
Mn6Ce1Ox	10 bar O2, 120 °C, 8 h	KHCO3	97.2 d	99.4	[93]
Co@NC-a	2 bar O2, 65 °C, 16 h	Na2CO3	73.1 f	100	[94]
Co-NC	6 bar O2, 130 °C, 20 h	K2CO3	98 ad	99.9	[95]
Co(II)–meso-tetra(4-pyridyl)-porphyrin	t-BuOOH, 100 °C, 24 h	Free	60.3 e	95.6	[96]
Co@KIT-6	1 bar Air, 130 °C, 20 h	Free	99 d	100	[97]
hexagon MnCo2O4	10 bar O2, 130 °C, 20 h	KHCO3	70.9 d	99.5	[98]
CoMn-NC	t-BuOOH, 80 °C, 12 h	Free MeCN b	95 e	99	[99]
Mn-Co	20 bar Air, 130 °C, 8 h	NaHCO3	98.7 d	100	[100]
Co-Mn-0.25	10 bar O2, 120 °C, 5 h	NaHCO3	95 d	99	[101]
Co/Mn/Br	30 bar (molar CO2/O2 = 1), 100 °C, 24 h	Free CH3COOH c	90 e	99	[102]
Li2CoMn3O8	55 bar O2, 130 °C, 8 h	Free CH3COOH c	80 d	100	[103]
C-Fe3O4-Pd	30 mL/min O2, 80 °C, 4 h	K2CO3	91.8 d	98.2	[104]
Pt@Fe1.7Cr0.3O3	1 bar O2, 120 °C, 12 h	Free	78.7 d	100	[105]
FeP-Co_0.2/NC	10 bar O2, 150 °C, 24 h	Na2CO3	91.6 d	100	[106]
CoxFey@NC	5 bar O2, 100 °C, 7 h	NaHCO3	91.1 d	100	[107]
MIL-100(Fe)	BuOOH, 100 °C, 24 h	Free TEMPO d	74 e	100	[108]
Fe0.6Zr0.4O2	20 bar O2, 160 °C, 24 h	Free [Bmim]Cl b	60.6 d	99.7	[109]
SBA-NH2-VO2+	O2 rate of 20 mL/min, 110 °C, 12 h	Free 4-chlorotoluene b	62.7 d	98.8	[110]
K-10 clay-Mo	O2 rate of 20 mL/min, 110 °C, 12 h	Free Toluene b	100 d	86.9	[111]
γ-Fe2O3@HAP-Mo	O2 rate of 20 mL/min, 110 °C, 12 h	Free Trifluorotoluene b	30.8 d	67.5	[112]
Mo8O26	H2O2, 100 °C, 2 h	NaOH	100 ad	99.5	[113]
Ni foam modified (NixB)	30 min	KOH	98.5 f	100	[114]
Ni3S2-MoS2/NF	2 h	KOH	97 f	100	[115]
Au/TiO2	VIS-LED, 40 °C, 2 h	NaOH	97 g	100	[116]
CoPz–g-C3N4	UV, RT. air flow at 20 mL, 14 h	pH = 9.18	96.1 g	99.6	[117]

^a Selectivity, ^b solvent, ^c acid, ^d aerobic, ^e chemical oxidant, ^f electrooxidation, ^g photocatalytic oxidation.

Table 2. HMF oxidation over different metals.

Catalyst	Reaction Condition	Base	FDCA Yield %	HMF Con. %	Ref.
α-MnO2-H+	20 bar O2, 120 °C, 8 h	NaHCO3	98.5 d	99.9	[91]
nanorods (MnO2-R)	5 bar O2, 110 °C, 12 h	NaHCO3	96.7 d	100	[87]
2 D Mn2O3 nanoflakes	14 bar O2, 110 °C, 24 h	NaHCO3	99.5 d	100	[89]
La-MnO2	5 bar O2, 140 °C, 4 h	NH4OH	95.4 d	100	[92]
Mn6Ce1Ox	10 bar O2, 120 °C, 8 h	KHCO3	97.2 d	99.4	[93]
Co@NC-a	2 bar O2, 65 °C, 16 h	Na2CO3	73.1 f	100	[94]
Co-NC	6 bar O2, 130 °C, 20 h	K2CO3	98 ad	99.9	[95]
Co(II)–meso-tetra(4-pyridyl)-porphyrin	t-BuOOH, 100 °C, 24 h	Free	60.3 e	95.6	[96]
Co@KIT-6	1 bar Air, 130 °C, 20 h	Free	99 d	100	[97]
hexagon MnCo2O4	10 bar O2, 130 °C, 20 h	KHCO3	70.9 d	99.5	[98]
CoMn-NC	t-BuOOH, 80 °C, 12 h	Free MeCN b	95 e	99	[99]
Mn-Co	20 bar Air, 130 °C, 8 h	NaHCO3	98.7 d	100	[100]
Co-Mn-0.25	10 bar O2, 120 °C, 5 h	NaHCO3	95 d	99	[101]
Co/Mn/Br	30 bar (molar CO2/O2 = 1), 100 °C, 24 h	Free CH3COOH c	90 e	99	[102]
Li2CoMn3O8	55 bar O2, 130 °C, 8 h	Free CH3COOH c	80 d	100	[103]
C-Fe3O4-Pd	30 mL/min O2, 80 °C, 4 h	K2CO3	91.8 d	98.2	[104]
Pt@Fe1.7Cr0.3O3	1 bar O2, 120 °C, 12 h	Free	78.7 d	100	[105]
FeP-Co_0.2/NC	10 bar O2, 150 °C, 24 h	Na2CO3	91.6 d	100	[106]
CoxFey@NC	5 bar O2, 100 °C, 7 h	NaHCO3	91.1 d	100	[107]
MIL-100(Fe)	BuOOH, 100 °C, 24 h	Free TEMPO d	74 e	100	[108]
Fe0.6Zr0.4O2	20 bar O2, 160 °C, 24 h	Free [Bmim]Cl b	60.6 d	99.7	[109]
SBA-NH2-VO2+	O2 rate of 20 mL/min, 110 °C, 12 h	Free 4-chlorotoluene b	62.7 d	98.8	[110]
K-10 clay-Mo	O2 rate of 20 mL/min, 110 °C, 12 h	Free Toluene b	100 d	86.9	[111]
γ-Fe2O3@HAP-Mo	O2 rate of 20 mL/min, 110 °C, 12 h	Free Trifluorotoluene b	30.8 d	67.5	[112]
Mo8O26	H2O2, 100 °C, 2 h	NaOH	100 ad	99.5	[113]
Ni foam modified (NixB)	30 min	KOH	98.5 f	100	[114]
Ni3S2-MoS2/NF	2 h	KOH	97 f	100	[115]
Au/TiO2	VIS-LED, 40 °C, 2 h	NaOH	97 g	100	[116]
CoPz–g-C3N4	UV, RT. air flow at 20 mL, 14 h	pH = 9.18	96.1 g	99.6	[117]

^a Selectivity, ^b solvent, ^c acid, ^d aerobic, ^e chemical oxidant, ^f electrooxidation, ^g photocatalytic oxidation.

Mn-based bimetallic catalysts have demonstrated remarkable efficiency in HMF oxidation. For example, a MnO_x-CeO₂ composite exhibited outstanding catalytic activity [118]. Compared with monometallic MnO₂, the synergistic interaction between Ce³⁺ and Mn⁴⁺ enhanced performance Mn⁴⁺ ions acted as the primary active sites, while lattice oxygen transfer from CeO₂ to MnO_x promoted oxidation under mild conditions. Under optimized parameters, the MnOx-CeO₂ catalyst (Mn/Ce = 6) achieved a 91% FDCA yield and maintained stability over five successive reaction cycles without noticeable deactivation. Liu et al. [93] investigated the impact of various doping metals on the activation of lattice oxygen (OL) in Mn-based oxides, which demonstrates that Ce-doped MnO_x (Mn₆Ce₁O_x) is highly effective for aerobic HMF oxidation to FDCA, achieving 97.2% yield and high formation rates under mild conditions with stable reusability. Ce doping creates asymmetric Mn-O-Ce sites, enhancing lattice oxygen mobility and the formation of oxygen vacancies, which act as active species in the Mars–van Krevelen mechanism. Other dopants (Zr, La, Sm) showed weaker or negative effects. A range of magnetic ferrite catalysts, including CuFe₂O₄, CoFe₂O₄, MnFe₂O₄, MgFe₂O₄, and Fe₃O₄, have also been investigated for HMF oxidation [119]. Among the evaluated ferrite catalysts, MnFe₂O₄ showed the highest catalytic efficiency, reaching an 85% yield of FDCA under a base-free environment when TBHP was employed as the oxidant. The enhanced performance was ascribed to the flexible redox behavior of Mn and the cooperative interaction between Mn species and the Fe₂O₄ framework. Additionally, the magnetic characteristics of MnFe₂O₄ enabled straightforward separation and allowed reuse for up to 4 reaction cycles without noticeable loss of catalytic performance. In a related investigation, Neatu et al. [120] developed a Mn_0.75/Fe_0.25 composite catalyst for the one-pot oxidation conversion of HMF to FDCA. During the initial reaction stage, only 9% FDCA was obtained, with FFCA (69%) forming as the predominant intermediate. Interestingly, in the absence of a catalyst, using eight equivalents of NaOH under 8 bar oxygen pressure, an 87% FFCA yield was achieved in just 30 min. Subsequent steps consisting of acidification, filtration, and catalytic oxidation of the isolated FFCA using Mn_0.75Fe_0.25 under optimized conditions consisting of 0.05 g catalyst, 2 mmol NaOH, 8 bar O₂, 90 °C, and a reaction time of 24 h led to approximately 90% FDCA yield. In addition to manganese oxides, several other non-noble metal catalysts, including those based on cobalt, copper, iron, and molybdenum, are also extensively employed for the catalytic oxidation reaction of HMF. Gao et al. [94] prepared a Co-loaded activated carbon catalyst, Co@NC, which was subsequently graphitized, yielding an impressive 81.8% FDCA yield via electrooxidation. To identify the active sites, they performed acid etching to remove surface Co, revealing Co species originally bonded to the carbon matrix. The resulting catalyst, designated NC-900-a, showed a significant reduction in Co content from 28.2% to 0.71%, as confirmed by ICP analysis. Using the etched catalyst led to a 16% drop in FDCA yield after 12 h compared to the unetched sample. Interestingly, when the reaction conditions were modified using the same amount of etched catalyst with five times more HMF (50 mM), a lower O₂ pressure, and an extended reaction duration of 16 h, the FDCA yield increased to 94%. Li et al. [95] developed a Co-N-C catalyst by heating a Co-based ZIF. The process created well-dispersed cobalt sites strongly bonded with nitrogen. The catalyst shows a high conversion of HMF, with a 98% yield of FDCA within 3 h. under mild alkaline conditions, and keeps its performance steady through repeated use. Co(II) complexes also showed activity; Gao et al. [96] evaluated the catalytic performance of Co(II)-meso-tetra(4-pyridyl)-porphyrin (M-resin-Co-Py) for converting HMF to FDCA with tert-butyl hydroperoxide (t-BuOOH) as the oxidant. The catalyst yielded a 96% HMF conversion and a 90% yield of FDCA. Li. et al. [97] embedded ultrafine Co₃O₄ nanoparticles inside mesoporous KIT-6 (Co@KIT-6), reaching 99% FDCA yield within 2 h using air as oxidant and water and solvent. The nanoconfinement effect prevented Co₃O₄ aggregation and leaching, enabling stable activity over six cycles. Cobalt single-atom catalysts also performed well. Co-Mn heterogeneous catalysts have emerged as highly effective systems for HMF oxidation. Zhang et al. [98] examined a hollow-centered MnCo₃O₄ catalyst at the nanoscale, synthesized via a thermal method, which achieved full conversion of HMF and a 71% FDCA yield under mild oxygen as oxidant. The improved catalytic efficiency was attributed to increased oxygen mobility, as evidenced by the lowest reduction temperatures observed in H₂-TPR analysis. In a related investigation, Zhang et al. [99] developed a bimetallic CoMn-NC catalyst to oxidize HMF into FDCA. Under optimized conditions, the catalyst reached an outstanding FDCA yield of over 95%, highlighting its high catalytic performance. The porous hierarchical structure of CoMn-NC, along with the stable coordination of Co/Mn to N–C, effectively reduces metal leaching and maintains nanoparticle dispersion during extended cycles. The catalyst’s structural stability was further demonstrated by maintaining FDCA selectivity above 90% across five successive cycles, indicating its potential for industrial applications. Li et al. [100] investigated a Mn-doped Co-based bimetallic catalyst with abundant oxygen vacancies (MnCO₃-Mn_xCo₃-_xO₄-Ov) for converting HMF to FDCA via aerobic oxidation. The oxygen-vacancy-rich variant (MnCO₃-Mn_xCo₃-_xO₄-Ov-1.0) showed outstanding activity, yielding a peak of 98.7% of FDCA. Mechanistic analysis suggested that increased oxygen vacancies enhance HMF adsorption and activate O₂, thereby promoting efficient oxidation. Additionally, Rao et al. [101]. Prepared Co-Mn mixed oxides with various Co/Mn proportions using a solid-state grinding approach. Among the samples, Co-Mn 0.25 achieved 99% conversion of HMF and an FDCA yield of 95%. Compared with traditional liquid-state co-precipitation, this greener, more straightforward method yielded approximately 40% higher FDCA, attributed to its higher BET surface area, enhanced mobility of lattice oxygen, and multiple Mn oxidation states. The catalyst also showed excellent stability, maintaining performance over five cycles with minimal Co or Mn leaching, as verified by ICP analysis.

Zuo et al. [102] conducted a semi-continuous investigation on the oxidation of HMF, comparing its performance with the established p-xylene oxidation process Figure 11. Under the same conditions (160 °C, 60 bar, Co/Mn/Br catalytic system), the oxidation of HMF yielded around 65% FDCA, significantly lower than the 95% yield of TPA obtained from p-xylene. This difference was due to the increased reactivity of the furan core and its -CH₂OH and -CHO functionalities compared to the more stable methyl substituents on benzene. By fine-tuning the reaction parameters to minimize secondary reactions such as the esterification of HMF to AcHMF and excessive oxidation to CO_x, the yield of FDCA was increased to ~90% at 180 °C and 30 bars (CO₂/O₂ = 1) with 7 vol% added water. Future studies will focus on determining intrinsic kinetic parameters under these optimized conditions to facilitate process development and industrial scaling.

Jain et al. [103] reported that synergistic interactions between Co and Mn in the Co/Mn/Br system promote rapid generation of bromo radicals through a three-step redox cycle, enabling efficient completion of the catalytic oxidation pathway. Additional examples include Li₂CoMn₃O₈, prepared via pyrolysis with citric acid and urea, which produced an 80% isolated yield of FDCA, which was obtained in acetic acid with NaBr as a co-catalyst. As an inexpensive and earth-abundant metal, iron (Fe) has been widely employed either as a support or as an alloying component with noble metals in catalytic HMF oxidation. Representative systems include Fe-Pd alloys. Mei et al. [104] investigated a magnetically recoverable palladium supported on graphene oxide catalyst (C-Fe₃O₄-Pd), synthesized through a one-step solvothermal method procedure, for aerobic HMF oxidation. Under optimized conditions (80 °C, 4 h, K₂CO₃/HMF = 0.5), this catalyst achieved 98.2% conversion of HMF and 91.8% yield of FDCA. Its magnetic separability enabled easy recovery and reuse without a substantial decrease in activity.

Perumal et al. [105] demonstrated that Pt@Fe_2−xCr_xO₃ acts as an efficient catalyst applicable under base-free aerobic oxidation of HMF under mild conditions. Incorporating chromium into Fe₂O₃ significantly enhanced catalytic activity by tuning surface acidity. The optimized Pt@Fe_1.7Cr_0.3O₃ catalyst delivered almost complete conversion of HMF and a 78.7% FDCA yield, surpassing many noble metal and transition metal oxide catalysts. It also exhibited excellent durability, retaining over 90% activity after five cycles. Mechanistic studies indicated that improved Pt-support interactions and Cr-induced oxygen vacancies facilitated faster redox cycling and reduced overoxidation, enhancing activity and selectivity. Kalimuthu et al. [106] developed recyclable metal/metal phosphide catalysts supported by nitrogen-doped carbon (NC) via an eco-friendly synthesis approach. Among these, the FeP-Co_0.2/NC catalyst showed exceptional performance, achieving complete conversion of HMF and 91.6% FDCA yield under optimized conditions. The synergistic contributions of pyridinic nitrogen, Co-Fe alloy phases, and FeP species collectively enhanced catalytic activity and selectivity. Although the system required elevated temperatures and longer reaction times, the study emphasizes the strong potential of non-precious metal catalysts as efficient and sustainable candidates for industrial oxidation of HMF to FDCA. Liu et al. [107] prepared Co₂Fe₁@NC alloy catalysts via a simple solid-state approach, and tuning the Co/Fe composition ratio resulted in outstanding HMF oxidation performance. The catalyst achieved 96.1% FDCA yield at 100 °C and an oxygen pressure of 0.5 MPa, outperforming monometallic Fe and Co catalysts. It also maintained stability over five recycling cycles. The enhanced activity is attributed to strong Co-Fe interactions, which accelerated HMF oxidation, altered the rate-limiting step, and lowered activation energy barriers. This straightforward, cost-effective synthesis offers a promising strategy for scalable HMF-to-FDCA conversion. Fe-Co, nano Fe₃O₄-CoO_x, and a catalyst without the use of a base achieved a 68% FDCA yield using TBHP as the oxidant. Notably, when nano Fe₃O₄-SiO₂ was functionalized with –SO₃H groups for the two-stage catalytic conversion of fructose to FDCA, a 60% FDCA yield was obtained without any detectable Fe or Co leaching. The cost-effective synthesis, environmental compatibility, and strong catalytic performance make this system a promising candidate for large-scale applications reported by Wang et al. [121].

Under another base-free condition, Sha et al. [122] achieved a 79% FDCA yield using Fe³⁺-POP-1 as a catalyst under mild conditions without any base (100 °C, 10 h, 10 bar O₂). The large surface area along with the abundant Fe active centers of Fe³⁺-POP-1 were key factors in promoting HMF oxidation. Furthermore, analysis of the recovered catalyst confirmed that the Fe³⁺ oxidation state remained stable after the reaction.

Chamberlain et al. [108] report the use of MIL-100(Fe), utilized as a reusable catalyst for the oxidative transformation of HMF to FDCA under environmentally friendly conditions. In the presence of TEMPO as a co-catalyst, MIL-100(Fe) achieved complete HMF conversion in 24 h, with a maximum 57% yield of FDCA and 17% FFCA, giving a total selectivity of 74%. A mutually enhancing interaction between TEMPO and MIL-100(Fe) was proposed to drive both alcohol and aldehyde oxidation. Unlike many other systems, this reaction uses water as a solvent and mild conditions rather than pure oxygen at high pressures. MIL-100(Fe) offers advantages over enzymes and electrocatalysts due to its ease of preparation, non-toxicity, and robust performance, marking a step toward scalable, green production of FDCA. The cost-effective and environmentally friendly synthesis of this catalyst, combined with its excellent performance, makes it a strong candidate for large-scale applications. Yan et al. [109] significantly advanced the utilization of biomass-derived materials in ionic liquids (ILs). They synthesized a series of Fe-based catalysts (FexZr_0.41-_xO₂) for HMF oxidation, identifying Fe_0.6Zr_0.4O₂ as the most effective in [Bmim]Cl. Its small particle size, high surface area, and oxygen vacancies enhanced redox properties, boosting catalytic activity, while the catalyst demonstrated excellent reusability over five cycles.

Cu-based homogeneous catalysts have shown activity to produce DFF [123,124]. In recent studies, Ren et al. [124] employed DFT calculations to investigate adsorption of HMF on CuO (111) and Co₃O₄ (110) surfaces, identifying both as promising catalysts. In the adsorption process, Cu or Co ions interact with the oxygen atoms of HMF in an end-on manner, with the molecule adopting a bridging orientation. The adsorption energy is influenced by the coordination of surface oxygen to hydrogen and possible hydrogen bonding involving HMF’s hydroxy and formyl groups. Experimental validation aligned with the DFT results, as CuO achieved a 99% FDCA yield and Co₃O₄ gave 96% under mild conditions, demonstrating their effectiveness for the oxidation HMF to FDCA.

Mannam et al. [125] described a CuO-based catalytic system for HMF oxidation, using CuCl as the catalyst and tert-butyl hydroperoxide (t-BuOOH) as the oxidant to transform primary alcohols into carboxylic acids in acetonitrile (MeCN) at ambient temperature. In a related study, Hansen et al. [126] employed CuCl with TBHP under mild, base-free conditions, obtaining a 50% FDCA yield in MeCN. The limited conversion was associated with the creation of an FDCA–Cu complex within the system, which impeded further oxidation.

Ventura et al. [127] showed that using O₂ as the oxidant without any added base led to conversion of only 33% of HMF, producing 9% FDCA and 14% FFCA, illustrating the difficulties of base-free oxidation even at elevated temperatures (100 °C) or extended reaction times (15 h). In contrast, CuO-CeO₂ produced via high-energy milling (HEM) achieved complete HMF conversion with a 90% FFCA yield under the same conditions. A simple 1:1 physical mixture of CuO and CeO₂, however, gave low conversion 5% of HMF and FFCA yield of 3%, indicating that the superior activity of CuO-CeO₂ results from synergistic interactions rather than simple mixing. Characterization revealed that this improvement arises from an optimal distribution of the acid and base sites present on the surface of CuO-CeO₂.

Liu et al. [110] investigated SBA-NH₂-supported VO²⁺ (vanadyl) and Cu²⁺ (cupric) ions used for the oxidation of HMF, achieving a 29% yield of FDCA. The inclusion of Cu²⁺ facilitated the formation of active V⁵⁺ species, as Cu⁺ is easily reoxidized to Cu²⁺ by oxygen, maintaining the catalytic cycle. Recycling tests indicated minimal leaching of vanadium or copper, confirming the stable immobilization of VO²⁺ and Cu²⁺ on the SBA-NH₂ support.

Molybdenum-based catalysts are commonly used for HMF oxidation. For example, a Mo(VI) complex supported on montmorillonite K-10 (K-clay-Mo) was optimized regarding the supply of oxygen, solvent, base and temperature, achieving complete conversion of HMF and a 87% yield of HMFCA under optimal conditions (toluene as solvent, oxygen purging, 110 °C, 3 h) [111].

The magnetic γ-Fe₂O₃@HAP-Mo catalyst was fine-tuned for recyclability by adjusting the temperature and solvent, resulting in 96% conversion of HMF with product distribution of 20% FDCA, 68% DFF, and 9% HMFCA in 4-chlorotoluene under an oxygen atmosphere [112]. The utilization of aromatic solvents, however, reduced selectivity toward intermediate products, restricting wider application. In a comparative study, ammonium octamolybdate and ammonium decatungstate were tested for HMF oxidation in aqueous H₂O₂. Both catalysts feature single and double epoxy groups, with the two epoxy groups acting as the primary active sites for FDCA production. The tungsten-based catalyst experienced reduced activity due to steric hindrance from the quaternary ammonium, while the molybdenum-based system showed no such limitation, allowing [EMIM]₄Mo₈O₂₆ to achieve a 99% FDCA yield [113].

Electrochemical oxidation has recently gained attention as an environmentally friendly method for HMF converting to FDCA, providing significant benefits compared to traditional thermal catalytic approaches [128]. In these systems, electrodes serve as catalysts. Various materials have been investigated for the development of electrocatalysts that facilitate the oxidation of biomass-derived compounds [129].

Zhu et al. [130] synthesized an Au/ZnO photocatalyst via a straightforward deposition–precipitation method for the visible-light-induced conversion of HMF to FDCA. The optimized 2.5% Au/ZnO-DP-H catalyst demonstrated exceptional performance, achieving 96.9% FDCA selectivity (Figure 12a). The improved catalytic performance was ascribed to several factors: The localized surface plasmon resonance (LSPR) of Au improved absorption of visible light and generated hot electron generation; hydrogenation treatment lowered the size of Au particles particle size, strengthened metal support interactions, and introduced oxygen vacancies that promoted efficient charge separation; and the Schottky barrier at the Au-ZnO interface suppressed electron hole recombination. Despite the photocatalyst’s renewable and environmentally friendly nature, its instability and high cost limit its industrial application for HMF oxidation to FDCA [131]. In parallel, Avantium has developed a pilot-scale process for FDCA and plans to scale up to commercial production [129]. A critical challenge for large-scale implementation is the synthesis of HMF, the unstable intermediate required for FDCA production. HMF is typically obtained via acid-catalyzed dehydration of the C6 sugars such as sucrose, fructose, and glucose. Because glucose exhibits lower yield and selectivity compared to fructose, it is commonly isomerized to fructose before dehydration to enhance HMF production efficiency [132].

Among the non-noble metals, nickel-based materials have attracted attention in recent studies as they are highly effective for HMF oxidation because of their strong activity toward aldehyde and hydroxyl groups. Barwe et al. [114] developed an efficient and cost-effective electrocatalyst for HMF oxidation by modifying high surface area nickel boride (Ni_xB) deposited on Ni foam. Electrolysis conducted under constant potential coupled with HPLC analysis indicated nearly 100% faradaic efficiency and a 98.5% FDCA yield. The oxidation of HMF mainly proceeded through the intermediate 5-hydroxymethyl-2-furancarboxylic acid rather than 2,5-diformylfuran, consistent with product analysis. Additionally, vertically aligned NiSe@NiOx core–shell nanowires grown electrochemically on nickel foam acted as an effective non-precious metal electrocatalyst for HMF electrooxidation to FDCA (Figure 12b). The NiSe@NiOx core–shell architecture provided abundant exposed active sites and improved electron transfer, achieving outstanding performance with 100% FDCA yield and 99% faradaic efficiency.

A flower-like Ni₃S₂-MoS₂ nano-heterojunction catalyst was designed via interface engineering. The Ni₃S₂-MoS₂/NF catalyst exhibited excellent electrocatalytic performance toward the hydrogen evolution reaction (HER) and HMF oxidation to produce FDCA, reaching complete conversion of HMF and a 97% FDCA yield. This remarkable performance is attributed to the high density of exposed active sites, strong interfacial electronic interactions, and the synergistic effect between Ni₃S₂ and MoS₂ [115]. Despite the promise of electrochemical HMF oxidation as a sustainable route for producing value-added chemicals from biomass, substantial carbon loss often occurs because the spontaneous degradation of HMF in basic electrolytes, particularly at higher concentrations, which presents a significant challenge for achieving high efficiency [133].

Photocatalytic methods have attracted attention because of their environmentally friendly nature, mild operating conditions, and ability to harness renewable light energy. Upon light irradiation, electrons are excited from the valence band to the conduction band, where the valence band drives oxidation reactions and the conduction band enables reductions. Despite this potential, only a few studies have successfully achieved HMF oxidation to FDCA through photocatalysis. Recent findings suggest this singlet oxygen species (¹O₂) can catalyze the selective oxidation of HMF to FDCA. While noble metal-based catalysts like Ru and Au have the ability to efficiently produce ¹O₂ when exposed to visible light, their high cost restricts broader application [134]. Non-noble metal photocatalysts, such as decatungstate (DT), ZnO/polypyrrole (ZnO/PPy), and cobalt thioporphyrazine supported on graphitic carbon nitride (CoPz/g-C₃N₄), have attracted growing research interest; Yang et al. [135] demonstrated that DT under visible light in CH₃CN at room temperature oxidized HMF, yielding 67.1% DFF and 5.8% FDCA in the presence of HBr. DT was found to interact with HBr and HMF, enhancing visible-light absorption and inhibiting HMF polymerization, though FDCA selectivity remained limited. Gonzalez-Casamachin et al. [136] employed ZnO/PPy composites under visible-light irradiation, achieving 30% HMF conversion and 30% FDCA yield, with HMFCA as the primary intermediate and FFCA formation identified as the rate-limiting step. Wang et al. [137] prepared α-Fe₂O₃/Zn_0.5Cd_0.5S via hydrothermal methods, obtaining excellent activity for converting HMF selectively to DFF or FDCA in an aqueous solution. The optimized 15% Fe₂O₃/Zn_0.5Cd_0.5S catalyst reached 85% FDCA selectivity at 99% HMF conversion. Enhanced activity was attributed to efficient charge separation and reactive oxygen species (ROS) generation via a Z-scheme mechanism, with superoxide radicals (•O₂⁻) identified as key oxidizing species, demonstrating strong potential for solar-driven selective HMF oxidation.

Xu et al. [117] reported that deposition of CoPz on g-C₃N₄ markedly improved photocatalytic performance. Under alkaline conditions (pH 9.18), the system produced 96% FDCA yield, whereas under mildly acidic conditions, DFF was the predominant product. The study revealed that g-C₃N₄ generates hydroxyl radicals, which lead to CO₂ and H₂ formation, while ¹O₂ from CoPz selectively oxidizes HMF to FDCA. Strong CoPz-g-C₃N₄ interactions enhanced active site availability, promoted ¹O₂ generation, and minimized CoPz aggregation, providing an effective non-noble metal photocatalytic approach for transforming biomass-derived compounds into value-added chemicals. Additionally, a noble metal-based Au/TiO₂ catalyst with adjusted calcination allowed the preparation of materials with tunable oxygen vacancy (O_v) concentrations, in an atmosphere (H₂ or air), to investigate the effect of defects in photocatalytic selective oxidation of HMF. Hydrogen reduction generated abundant O_v sites by removing lattice oxygen, significantly enhancing photocatalytic activity. The O_v-rich Au/TiO₂-H₂ catalyst achieved a remarkable 97% FDCA yield from a concentrated 500 mM HMF solution following 20 h of visible-light exposure. Mechanistic studies revealed that singlet oxygen (¹O₂) is the key oxidizing species for hydroxyl group oxidation, with O_v sites serving dual functions, enhancing charge separation as electron traps and promoting O₂ activation to generate reactive intermediates. This work not only clarifies the defect-mediated photocatalytic mechanism but also offers a practical strategy for designing high-performance catalysts for selective photochemical oxidation [116].

3.2. Synthetic Pathway for Lactic Acid Production

Lactic acid (LA) is a widely used and chemically versatile compound, finding applications in industries such as food, cosmetics, pharmaceuticals, chemicals, and packaging [138]. The global need for lactic acid (LA) is expected to reach 130,000–150,000 tons per year. LA is present in two optical forms: D-(-)-lactic acid (D-LA) and L-(+)-lactic acid (L-LA), as illustrated in Figure 13. Chemical synthesis typically produces a racemic mixture containing both isomers, but the pure forms are more valuable due to their enhanced properties. For example, lactic acid (LA) represents a vital building block for polylactic acid (PLA); nevertheless, employing a racemic mixture of L- and D-forms produces polymers that are largely amorphous and exhibit reduced stability. Conversely, microbial fermentation techniques can yield highly enantiomerically pure LA, as the biosynthetic enzymes display pronounced stereospecificity [139].

Crop residues are among the most significant renewable resources, accounting for about 3.5 billion tons each year. Feedstock selection for lactic acid production depends mainly on cost, supply, and carbohydrate quality [140]. Lignocellulosic biomass found in materials like agricultural residues such as distillers’ grains, straw, and garden waste are rich sources of hemicellulose and cellulose. Its structure consists of cellulose molecules organized into microfibrils interconnected through hemicellulose and lignin, forming a dense, compact matrix [141]. Pretreatment is an essential stage in transforming lignocellulose into monosaccharides, as it removes lignin, increases the material’s porosity, and exposes cellulose and hemicellulose to enzymatic action, thereby boosting hydrolysis efficiency [142]. Significant research has been devoted to pretreating cellulose-rich biomass wastes before lactic acid (LA) fermentation. For example, novel pretreatment strategies combining microwave-alkali and steam-alkali methods have been employed to enhance LA production from vinasse [143]. These pretreatments significantly improved LA production.

Various microorganisms have been employed for lactic acid (LA) fermentation; among them, Bacillus coagulans consistently ranks among the top species used, reflecting its widespread application across different studies and time periods. As fermentation technologies have advanced, genetically engineered microbial strains have also gained significant attention for enhancing LA yield, productivity, and stereochemical purity. The specific traits and advantages of these microorganisms used in LA fermentation are summarized in Figure 14.

Lactic acid (LA) bacteria refer to a broad group of microbes able to produce substantial quantities of lactic acid produced from fermentable sugars. These bacteria utilize various carbohydrates, including monosaccharides such as glucose, galactose and fructose, as well as disaccharides (e.g., sucrose, maltose, and lactose) utilized as carbon sources. In homolactic fermentation, more than 80% of the products consist of lactic acid, and common species that perform this process include Lactococcus lactis, Lactobacillus delbrueckii, L. casei, and L. helveticus. In contrast, heterolactic fermentation performed by species including Leuconostoc, Lactobacillus, and Bifidobacterium produces not only lactic acid but also byproducts formed, e.g., acetic acid, carbon dioxide, and ethanol [144]. Generally, lactic acid bacteria are anaerobic or microaerophilic in nature. Because of their fast growth rate, efficient reproduction, achievement of significant lactic acid yields, and excellent productivity, these bacteria have attracted significant attention for their use in industrial lactic acid production [145].

Oonkhanod et al. [146] pretreated Sugarcane using bagasse in a two-step acid ethanolysis and alkaline peroxide process, achieving an 87.1% glucose yield. Fermentation with Lactobacillus casei produced 21.3 g/L lactic acid after 120 h (0.63 g/L·h). Lactic acid was efficiently separated using a reduced flux nanofiltration membrane, showing 93.28% glucose exclusion and 82.48 selectivity at 6 bars.

He et al. [147] use an ethanol/H₂O–oxalic acid system to demonstrate the separation of corn stover into the three primary components, cellulose, lignin, and hemicellulose (Figure 15). MgO was subsequently used to effectively convert the hemicellulose-derived fraction into lactic acid, resulting in a high yield (79.6 weight percent) and selectivity (90%). With a reusable catalyst and useful solid cellulose and lignin byproducts, the technique enables efficient biomass utilization.

Bacillus coagulans strain 36 is capable of utilizing C5 sugars through the pentose phosphate (PP) pathway, achieving yields of lactic acid as high as 1.0 g·g⁻¹ [148]. To take advantage of its thermophilic nature, open fermentation under non-sterile conditions has been developed as a simple and energy-efficient method for lactic acid production [149,150]. In addition, Bacillus coagulans can efficiently convert pentose sugars derived from cellulose hydrolysates into optically pure L-lactic acid. Lactic acid bacteria are also able to metabolize disaccharides such as maltose, sucrose, and lactose as carbon sources. Interestingly, Lacticaseibacillus rhamnosus has been reported to directly ferment cellobiose into lactic acid [151], indicating that lignocellulosic hydrolysates rich in cellobiose represent a promising and cost-effective substrate for lactic acid fermentation.

Lactic acid can also be produced by filamentous fungi like Rhizopus oryzae and Rhizopus arrhizus; however, their main fermentation product is mostly L-lactic acid. These organisms share similar metabolic pathways. Studies on glucose metabolism in R. oryzae have revealed the presence of two independently regulated pyruvate pools: a cytosolic pool, which contributes to the synthesis of ethanol, lactate, oxaloacetate, malate, and fumarate, and a mitochondrial pool, which channels pyruvate into the tricarboxylic acid (TCA) cycle [152]. Rhizopus strains have been widely investigated for their ability to produce lactic acid (LA) from diverse renewable feedstocks. Miura et al. [153] utilized Rhizopus sp. MK-96-1196, an L-lactic acid producer, and Acremonium thermophilus ATCC 24622, a cellulase-producing microbe, which were used in a mixed-culture system aimed at boosting L-lactic acid synthesis using untreated raw corncob; Bai et al. [154] achieved about 77.2 g·L⁻¹ of L-lactic acid from xylose in corncob employing Rhizopus oryzae strain HZS6. Additionally, R. oryzae has been used to produce lactic acid from a variety of renewable resources, such as waste office paper, wheat straw, xylose, chicken feather protein hydrolysate, sugar beets, and molasses. Huang et al. [155] obtained 0.85–0.92 g·g⁻¹ yields of lactic acid from potato starch wastewater using R. oryzae and Rhizopus arrhizus. Similarly, Zhang et al. [156] achieved a lactic acid yield of 88 g·L⁻¹ from the starch of potato waste using acid-resistant precultures of R. arrhizus in a reactor with a bubble column. However, some R. oryzae strains also produce minor amounts of ethanol and fumaric acid as byproducts during fermentation.

Yeasts can grow in highly acidic environments, tolerating pH levels as low as 1.5, which removes the need for alkali-based pH neutralization during fermentation, yet native yeast strains typically produce only limited amounts of lactic acid. To address this, genetic engineering has been applied to enhance lactic acid preparation in yeasts [157,158]. For example, Osawa et al. [159] engineered Candida boidinii by disrupting its ethanol fermentation pathway and introducing a bovine LDH gene expressed under control of the PDC1 promoter. Under optimized conditions, a 48 h batch fermentation produced 85.90 g·L⁻¹ of lactic acid, resulting in a productivity of 1.79 g·L⁻¹·h⁻¹. Similarly, Wakai et al. [160] modified Aspergillus oryzae to produce lactic acid from starch-based substrates. Since A. oryzae naturally secretes amylases, the engineered strain LDHΔ871 efficiently converted starch, dextrin, or maltose (each at 100 g·L⁻¹) into approximately 30 g·L⁻¹ of lactic acid, simplifying the process compared to conventional mixed-culture or enzyme-assisted methods. Zhao et al. engineered an E. coli strain, E. coli JH12, by introducing the L-lactate dehydrogenase (LDH) gene derived from Pediococcus acidilactici. When fermenting 6% xylose as the sole carbon source, this strain generated 34.73 g·L⁻¹ of lactic acid with 98% purity [161]. Although genetically modified E. coli strains offer shorter fermentation cycles than traditional lactic acid bacteria, conventional strains generally outperform them with respect to yield, productivity, and tolerance to lactic acid.

3.3. Production of Polyethylene Furanoate (PEF)

Polyethylene furanoate (PEF) is formed by copolymerizing 2,5-furandicarboxylic acid (FDCA) or its derivative dimethyl furan dicarboxylate with diol, typically monoethylene glycol (MEG). Its production can cut the use of fossil-based energy by about 40–50% and result in a 45–55% reduction in greenhouse gas emissions compared with polyethylene terephthalate (PET). Figure 16 shows the production of PEF and PET. PEF also shows better performance, with an increased glass transition temperature and a reduced melting point, and stronger resistance to oxygen, carbon dioxide, and water permeation than PET [162,163]. Research on FDCA-based polyesters has grown rapidly as advances in FDCA production have strengthened the push for renewable alternatives to petroleum-derived plastics. These bio-based polymers are typically produced via direct esterification or transesterification, with FDCA or its derivatives reacting with diols, for example, ethylene glycol, 1,4-butanediol, or 1,3-propanediol, or with other monomers like 1,4-cyclohexanedicarboxylate, 2,2,4,4-tetramethyl-1,3-cyclobutanediol, or isosorbide. The resulting polymers PEF, PPF, PTF, PBF, PCHDMF, and PEIF, shown in Figure 17, offer thermal stability, mechanical strength, and gas permeability performance that match or even exceed those of traditional terephthalate-based plastics [164]. Their processability and durability make them suitable for applications in packaging engineering and plastics.

Among them, PEF stands positioned as a sustainable substitute for PET for food and beverage packaging because of its strong gas-barrier effectiveness. At the same time, PPF and PBF offer enhanced toughness for industrial applications [38,165], as described in

Table 3. Summary of the properties and usage of different FDCA-based polymers.

Polymers	Diol	Properties	Usage	Ref
PEF	Ethylene glycol	- Superior gas (O2, CO2, H2O) barrier properties vs. PET; - Higher glass transition temperature (Tg), good mechanical strength, fully recyclable, 100% bio-based when using bio-MEG.	Food and beverage packaging (soft drinks, water bottles, beer, juices), films for fiber-based textiles, and flexible packaging.	[166]
PPF	Propylene glycol (1,3-propanediol)	Higher polarity and lower Tg than PEF, good gas barrier properties, can be amorphous, can be blended or cross-linked to form degradable networks.	This material shows strong potential for use in bio-based packaging. When cross-linked with suitable agents, it can also be adapted for biological applications, including as tissue-engineering scaffolds and drug-delivery devices.	[167]
PTF	2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO)	Amorphous, high transparency, high Tg (up to ~120 °C in some blends), good heat resistance, and high impact resistance.	High-performance engineering plastics, potentially as a BPA-free alternative to polycarbonate in durable household items and baby bottles.	[168]
PBF	1,4-butanediol (BDO)	It exhibits good crystallization behavior, a relatively high melting temperature, excellent gas barrier performance, and can be readily electrospun into fibrous mats.	Packaging materials, as a blend component to improve foam morphology in bead foams, and in biomedical applications like drug delivery mats, due to biocompatibility.	[169]
PCHDMF	1,4-cyclohexanedimethanol (CHDM)	Higher Tg and enhanced barrier properties when used as a comonomer in FDCA-based polyesters (e.g., in PBCF-68, which has a Tg of 69 °C), improved stiffness.	Potential for use in high-performance, bio-based engineering plastics and improved packaging materials.	[170]
PEIF	Ethylene glycol and Isosorbide	Inclusion of rigid diols like isosorbide leads to increased stiffness, higher Tg, and better barrier properties compared to linear diols, while maintaining bio-based content,	Enhanced performance of bio-based packaging materials requiring higher thermal resistance and stiffness.	[171]

.

Current studies focus on refining polymerization strategies of PEF to optimize the quality and scalability of FDCA-based polyesters. Traditional synthesis routes, such as direct esterification [172], transesterification [173], and solution polymerization [174], remain the foundation for PEF production. However, recent advancements have introduced ring-opening polymerization (ROP) [175] as a promising alternative. This method enables rapid polymerization, better molecular weight control, and fewer side reactions, producing high-purity polymers suitable for large-scale manufacturing. The adoption of ROP is expected to streamline industrial processing, reduce energy requirements, and accelerate the transition toward fully bio-based polyester materials [176].

3.3.1. Direct Esterification

Researchers are improving the synthesis of polyethylene furanoate (PEF) to enable industrial-scale production and expand its use in packaging and fibers. The direct esterification method is the most efficient and environmentally friendly route for PEF preparation, but it still faces challenges, such as oxidative degradation of the feedstock that can cause color changes and lower polymer quality [165,177], as shown in Figure 18a.

To overcome these issues, studies have focused on optimizing temperature, pressure, and catalyst systems to reduce side reactions. The process involves two stages: Esterification of ethylene glycol (EG) with 2,5-furandicarboxylic acid (FDCA), followed by polycondensation to extend polymer chains. Catalysts, including zinc acetate, manganese acetate, and tetrabutyl titanate, are commonly employed as shown in Table 4.

The esterification step proceeds at 165–200 °C under nitrogen until water formation ceases, after which polycondensation occurs at 230–240 °C under vacuum [179] as shown in Figure 18. The addition of catalysts such as antimony trioxide accelerates polymerization, and increased viscosity indicates polymer growth and chain elongation [180]. High reaction temperatures are required to achieve the thermal stability of FDCA, but this often results in dark-colored PEF, which affects optical and mechanical performance. Despite this drawback, direct esterification ensures high productivity and broad adaptability with different diols [166]. Stabilizers and anaerobic conditions are necessary to prevent degradation and maintain polymer integrity. Several furan-based polyesters, including PEF, polybutylene furanoate (PBF), and polytrimethylene furanoate (PTF), have been produced through this approach. The process can proceed without excess solvents or intermediates, using tetrabutyl titanate as the catalyst in a single-step, efficient, and low-energy reaction [181].

Table 4. Summary of the methods used for synthesis of PEF.

Aspect	1-Direct Esterification (DE) [166,172]	2-Transesterification (TE) [173,182]	3-Solution Polymerization (SP) [174]	4-Ring Opening Polymerization (ROP) [175]
Monomers	2,5-Furandicarboxylic acid (FDCA) + Mono ethylene glycol (MEG)	Dimethyl 2,5-furandicarboxylate (DMFD) + Mono ethylene glycol (MEG)	FDCC + MEG (in solvent medium)	Cyclic oligomers of PEF (e.g., cyclic ester or cyclic oligomer of FDCA & MEG)
Reaction Type	Direct condensation between carboxylic acid and alcohol	Ester exchange (methanol replaced by ethylene glycol)	Polycondensation is carried out in a solvent	Polymerization of cyclic monomers via ring opening
Catalysts	Metal oxides (e.g., Sb2O3, Ti(OBu)4, GeO2, Zn(OAc)2)	Metal acetates (Mn, Co, Zn) + Ti(OBu)4	Similar to DE/TE catalysts, may use organic catalysts	Tin octoate (Sn(Oct)2), organometallic catalysts, or enzymes
Reaction Conditions	165–240 °C, under vacuum or inert gas	180–220 °C, stepwise removal of methanol	100–200 °C depending on solvent; moderate pressure	150–200 °C; often in bulk or solvent-free
Byproducts	Water (H2O)	Methanol (CH3OH)	Water or methanol (depends on monomer)	None (ideal step-growth ROP)
Advantages	- Direct use of bio-based FDCA - Environmentally friendly (no methanol) - High purity product possible	- Easier control of reaction - High reactivity of DMFD - Lower risk of side reactions	- Good molecular weight control - Easy to incorporate additives - Moderate temperature	- Solvent-free and energy efficient - High molecular weight polymer - Narrow molecular weight distribution
Drawbacks	- Poor solubility of FDCA in MEG - High reaction temperature required - Difficult water removal	- DMFD preparation adds cost - Methanol byproduct handling - Possible color formation (discoloration)	- Solvent recovery needed - Lower productivity - Possible chain degradation in the solvent	- Requires a cyclic monomer synthesis step - Expensive catalyst - Limited scalability
Molecular Weight	Moderate to high	Moderate to high	Moderate	High
Polymer Quality	High clarity, good color if well-controlled	Often slight yellowing, good mechanical properties	Good control, but may contain solvent residues	Excellent control, high molecular weight, narrow distribution

Table 4. Summary of the methods used for synthesis of PEF.

Aspect	1-Direct Esterification (DE) [166,172]	2-Transesterification (TE) [173,182]	3-Solution Polymerization (SP) [174]	4-Ring Opening Polymerization (ROP) [175]
Monomers	2,5-Furandicarboxylic acid (FDCA) + Mono ethylene glycol (MEG)	Dimethyl 2,5-furandicarboxylate (DMFD) + Mono ethylene glycol (MEG)	FDCC + MEG (in solvent medium)	Cyclic oligomers of PEF (e.g., cyclic ester or cyclic oligomer of FDCA & MEG)
Reaction Type	Direct condensation between carboxylic acid and alcohol	Ester exchange (methanol replaced by ethylene glycol)	Polycondensation is carried out in a solvent	Polymerization of cyclic monomers via ring opening
Catalysts	Metal oxides (e.g., Sb2O3, Ti(OBu)4, GeO2, Zn(OAc)2)	Metal acetates (Mn, Co, Zn) + Ti(OBu)4	Similar to DE/TE catalysts, may use organic catalysts	Tin octoate (Sn(Oct)2), organometallic catalysts, or enzymes
Reaction Conditions	165–240 °C, under vacuum or inert gas	180–220 °C, stepwise removal of methanol	100–200 °C depending on solvent; moderate pressure	150–200 °C; often in bulk or solvent-free
Byproducts	Water (H2O)	Methanol (CH3OH)	Water or methanol (depends on monomer)	None (ideal step-growth ROP)
Advantages	- Direct use of bio-based FDCA - Environmentally friendly (no methanol) - High purity product possible	- Easier control of reaction - High reactivity of DMFD - Lower risk of side reactions	- Good molecular weight control - Easy to incorporate additives - Moderate temperature	- Solvent-free and energy efficient - High molecular weight polymer - Narrow molecular weight distribution
Drawbacks	- Poor solubility of FDCA in MEG - High reaction temperature required - Difficult water removal	- DMFD preparation adds cost - Methanol byproduct handling - Possible color formation (discoloration)	- Solvent recovery needed - Lower productivity - Possible chain degradation in the solvent	- Requires a cyclic monomer synthesis step - Expensive catalyst - Limited scalability
Molecular Weight	Moderate to high	Moderate to high	Moderate	High
Polymer Quality	High clarity, good color if well-controlled	Often slight yellowing, good mechanical properties	Good control, but may contain solvent residues	Excellent control, high molecular weight, narrow distribution

Studies such as those by Thiyagarajan et al. [183] demonstrate that polyesters derived from 2,4-FDCA and 3,4-FDCA exhibit thermal stability similar to or exceeding that of traditional 2,5-FDCA-based polymers, with similar glass transition temperatures. Structural analysis revealed that 2,4-PEF is amorphous while 2,5-PEF and 3,4-PEF are semicrystalline, indicating that the FDCA isomer type significantly influences morphology and crystallinity.

Wang et al. [178] demonstrated poly(ethylene furanoate) (PEF) as a highly promising bio-based polyester for advanced barrier packaging applications. BOPEF achieved a beneficial combination of high mechanical strength, good ductility, and exceptional resistance to oxygen, carbon dioxide, and water vapor permeability through optimal biaxial orientation, as shown in Figure 18b. By including 20% PEF in the PET/PEF/PET multilayer, greater performance improvements were achieved. Pilot-scale experiments demonstrated the industrial feasibility of PEF-based films by confirming the scalability and reproducibility of these improved qualities, as seen in Figure 18c. When taken as a whole, the results present PEF as a high-performance, sustainable substitute for conventional petrochemical polymers, highlighting the crucial part process optimization plays in facilitating widespread use.

Papageorgiou et al. [184] examined the effect of molecular weight on melting and crystallization kinetics. Higher-molecular-weight PEFs crystallized more slowly, while lower-molecular-weight polymers recrystallized faster during cooling. The activation energy for melting and cold crystallization shifted with temperature, showing complex thermal behavior characteristic of semicrystalline polymers. Microscopic analysis indicated denser nucleation at lower crystallization temperatures. These findings help refine polymerization parameters, improve processing stability, and enhance the mechanical and thermal performance of bio-based PEF, supporting its transition toward commercial application. Further research by Codou et al. [185] confirmed similar crystallinity and unit cell structures between glassy and melt-crystallized samples. Thermal analysis revealed consistent early crystallization dynamics and distinct polymerization (SP), transesterification (TE), ring-opening polymerization (ROP), and transitions in crystal growth below 171 °C. A study by Banella et al. [186] explored a more practical and cost-effective method involving direct esterification of 2,5-furandicarboxylic acid. Two environmentally friendly and food-safe catalysts, zinc acetate and aluminum acetylacetonate, were evaluated under controlled conditions using minimal diol excess and short reaction times as illustrated in Figure 19.

Illustrated in Figure 20 is the ¹H NMR spectrum of the polymer synthesized using the Al(acac)₃ catalyst; the resulting PEF exhibited favorable properties, including desirable viscosity, color, and low diethylene glycol content, which are crucial for its thermal and barrier performance. The non-crystalline polymers developed are excellent for creating packaging materials such as oriented sheets and bottles. Industrial-scale synthesis of PEF by Wang et al. [187] reported that when PEF and PTF were synthesized in a 150 L stainless-steel reactor, their tensile strengths increased by 33.3% and 39.2%, respectively, compared with the same polymers produced in a small 100 mL flask. This enhancement was largely due to the improved control of temperature, vacuum, and mixing that is possible in larger reactors. FDCA-based polyesters with higher intrinsic viscosities ([η] > 0.9 dL/g) showed particularly strong mechanical performance, reaching tensile strengths of 93.2 MPa for PEF and 80.2 MPa for PTF, along with elastic moduli of 2.1 GPa and 1.8 GPa. In addition to their mechanical strength, both polymers demonstrated excellent barrier properties against oxygen, CO₂, and water vapor. The study also highlighted successful ton-scale production of PEF resin using a 3000 L pilot reactor, confirming that industrial-scale manufacturing of FDCA-based polyesters is not only feasible but already well within reach.

3.3.2. Polycondensation Through Solution Polymerization

Melt polymerization can produce high-molecular-weight polymers quickly, but it demands high temperatures and strong vacuum conditions. These requirements become harder to manage as the diol chain length increases. Longer-chain diols slow down the polycondensation reaction, reduce efficiency, and increase the likelihood of thermal degradation and discoloration. In contrast, solution polymerization can be performed under gentler conditions, with better temperature control and less oxidative damage, making it easier to tune the final polymer’s average molecular weight and its distribution profile [174]. In this method, 2,5-FDCA is converted to the reactive (FDCC) 2,5-dicarbonylfuran chloride by reaction with SOCl₂ or COCl₂ (Figure 19), which combines with diols to yield lighter-colored polyesters as mentioned in Table 4. Gomes et al. [188] applied solution polymerization to take advantage of the monomers’ reactivity and volatility. Instead of using standard aliphatic diols, they employed bis(hydroxyalkyl)-2,5-furan dicarboxylates, which enabled the formation of high-molecular-weight, semicrystalline PEF with excellent thermal stability. The polymerization was carried out at 240–250 °C under vacuum in the presence of an Sb₂O₃ catalyst. The resulting semicrystalline polymers were both infusible and insoluble, behaving similarly to fully aromatic polyesters. In contrast, when nonvolatile diols such as isosorbide or isostearyl alcohol were used, the resulting polyesters were completely amorphous. Overall, these findings demonstrate that by tailoring the monomer structure, reaction conditions, and catalyst choice, it is possible to precisely tune polymer molecular weight, crystallinity, and thermal properties. Gandini et al. [189] prepared low-molecular-weight PEF oligomers by reacting FDCA dichloride with MEG in tetrachloroethane at room temperature in the presence of pyridine. Subsequent addition of 1% Sb₂O₃ followed by high-vacuum treatment at 220 °C yielded solid PEF with well-controlled molecular weight. When higher-boiling diols such as isosorbide were used, their corresponding dichlorides were formed, leading to polymers with distinctly different thermal behaviors, including glass transition temperatures spanning from 90 to 180 °C.

3.3.3. Polymerization Through RING-Opening

Most PEF described in the literature is made using condensation polymerization, the standard approach for polyesters. However, this approach has significant drawbacks: as polymer chains lengthen, the melt becomes more viscous, which makes mixing challenging and slows the elimination of byproducts like water or alcohol. Lower response efficiency, longer manufacturing times, and higher energy consumption are the results of these diffusion-related problems. Ring-opening polymerization (ROP), on the other hand, has become a viable method for creating high-molecular-weight furan-based polyesters such as PEF and PBF. In ROP, cyclic oligomers are opened by an initiator and then join to create polymers. In contrast to polycondensation, ROP is entropy-driven, low-viscosity, and creates no byproducts, allowing for quicker reactions and more consistent polymer development. The molecular weight can be precisely tuned by adjusting the initiator-to-monomer ratio, and the polydispersity can be controlled through optimized reaction parameters [190,191,192]. A critical factor for successful ROP is the high purity of the cyclic oligomers, since any residual end groups can trigger premature chain initiation, thereby lowering molecular weight. Preparing such oligomers remains a major challenge. Rosenboom et al. [186] demonstrated that cyclic depolymerization of FDCA and EG can yield cyclic PEF oligomers, but the resulting size distribution was nonuniform due to solidification and diffusion limits during the reaction. To overcome this, selective precipitation and cyclic dimer purification methods have been developed to create oligomers with excellent purity suitable for scalable ROP. Ultimately, PEF synthesized by optimized ROP has achieved molecular weights up to 50,000 g/mol and thermal properties comparable to those of melt-polymerized PEF, confirming ROP as an efficient and industrially viable alternative for producing bottle-grade PEF. ROP has been successfully applied to produce high molecular weight PEF with properties comparable to melt-polymerized material. Liu et al. [192] demonstrated a green, economically competitive ROP process using FDCA and EG, forming cyclic oligomers from prepolymerized linear PEF under dilution and depolymerization, (SEC) profile indicates that neat ROP of 99% pure cyclic oligomeric ethylene furanoate (cyOEF) at 280 °C, without any additional catalyst, rapidly converts trimers (C3) and larger rings into PEF. However, the dimer (C2) reacts much more slowly and remains only partially polymerized (black arrow). As the reaction continues, the mixture gradually becomes heterogeneous, accompanied by noticeable discoloration and a drop in molecular weight. This is reflected in Figure 21a, where the PEF peak shifts toward higher elution volumes. To accelerate polymerization at 280 °C, several ROP conditions were evaluated, as shown in Figure 21b: a neat reaction (black squares); dry grinding of cyOEF with 0.1 mol% cyclic stannoxane initiator for mechanical mixing (blue diamonds); feeding fresh cyOEF into a reactive PEF melt already converted by more than 90% and containing the same initiator level (purple triangles); and adding 33 wt% tetraglyme as an inert liquid plasticizer together with 0.1 mol% initiator (red circles). Notably, tetraglyme alone without any initiator (gray circles) provided a moderate increase in polymerization rate, as illustrated in Figure 21c,d. When ROP was performed at 260 °C using 0.1%, 0.2%, or 0.3% cyclic stannoxane in the presence of 33 wt% tetraglyme, the reaction reached nearly complete conversion (>96%) within 20 min, producing high-molecular-weight PEF (>30 kg mol⁻¹) suitable for bottle-grade applications. As shown in Figure 21e, the optimized ROP condition (0.1% cyclic stannoxane + 33% tetraglyme at 260 °C for 25 min) generated a colorless, high-molecular-weight PEF (right). In comparison, a non-optimized ROP utilizing 97% pure cyOEF at 280 °C with the same initiator for 60 min resulted in a noticeably discolored product (middle). For reference, industrial PEF produced by polycondensation (Mn ≈ 15 kg mol⁻¹) displayed similar discoloration (left).

Li et al. prepared multiblock copolymers of poly(ethylene2,5-furandicarboxylate)-block poly (tetramethylene oxide) using cascade condensation coupling ROP (PROP). Reaction of cyclic oligo(ethylene 2,5-furandicarboxylate) with PTMO diols under vacuum, producing bicrystalline structures with controlled molecular weight, block length, and phase distribution. Increasing the PEF content elevated the melting temperature and crystallinity of the PEF segments, while PTMO properties remained stable. These amphiphilic, biodegradable thermoplastic elastomers exhibit tailored mechanical properties, demonstrating the effectiveness of ROP-based strategies for producing sustainable, high-performance PEF-based polymers and providing a foundation for further polymer design and applications [193].

Huerta et al. [194] have effectively created cyclic low-molecular-weight polyesters using thermal ring depolymerization and high-dilution polycondensation techniques, producing mixtures of tiny cyclic oligomers. Using Sn(Oct)₂ as a catalyst, these cyclic oligomers, principally dimers, trimers, and tetramers, were subsequently polymerized by ring-opening polymerization (ROP) to create PEF and PBF. The oligomers, purified using semipreparative chromatography, were crystalline solids with melting temperatures between 140 and 200 °C. In both mixed and pure oligomer systems, polymerization tests showed that smaller ring diameters resulted in faster reaction rates. The ROP technique yielded high-molecular-weight polymers (50,000–60,000 g/mol), equivalent to or exceeding those made with conventional polycondensation. Additionally, PBF polymerized more quickly than PEF, and as ring size decreased, polymerization rates marginally rose. The thermal characteristics of PEF and PBF formed from ROP were similar to those of polymers produced by melt polycondensation, demonstrating that ROP is an extremely effective and commercially viable method of producing furan-based polyesters.

3.3.4. Polycondensation Through Transesterification

Discoloration and sluggish crystallization are two ongoing issues in the manufacturing of polyethylene furanoate (PEF). Impurities in FDCA, decarboxylation during polymerization, and the use of metal catalysts like titanium or manganese can all cause discoloration. Researchers investigated ring-opening polymerization and chloride-solution techniques utilizing manganese catalysts to reduce these effects. [166]. Still, these approaches involve complex steps and solvent-intensive processes, unsuitable for large-scale production. An alternative route involves converting FDCA into dimethyl 2,5-furandicarboxylate (DMFD), followed by transesterification with ethylene glycol (EG) as shown in Figure 22, to form PEF [195,196]. This approach yields lighter-colored PEF with better quality, but careful control of reaction time and temperature is essential since high thermal exposure (230–250 °C) increases costs and intensifies discoloration. Developing stable, colorless catalysts for DMFD and EG transesterification, as mentioned in Table 4, can enable industrial-scale synthesis of high-quality, colorless PEF with strong market potential.

Transesterification remains a preferred route for bio-based PEF synthesis because it offers lower activation energy, greater product purity, and improved reactant stability compared to direct esterification. In this process, DMFD reacts with EG, exchanging ester and hydroxyl groups to form polymer chains. The ideal esterification temperature is 180–190 °C, with polycondensation occurring around 235–245 °C. Catalysts such as tetrabutyl titanate, manganese acetate, zinc acetate, and stannous oxalate boost polymerization efficiency and product quality. Studies showed that PEF synthesized via transesterification exhibits a glass transition temperature (Tg) of 80 °C, higher than that of PET (75 °C), and a melting temperature (Tm) of 215 °C, lower than PET’s 260 °C. These results confirm that PEF offers suitable mechanical and thermal properties for replacing PET in packaging and industrial applications [197,198].

Figure 22. (a) Optical images and (b) UV-visible spectra of PBF, PEF, PPF, and their CBDO-modified copolyester films were captured before and after annealing at 150 °C for 30 min [199].

Figure 22. (a) Optical images and (b) UV-visible spectra of PBF, PEF, PPF, and their CBDO-modified copolyester films were captured before and after annealing at 150 °C for 30 min [199].

Knoop et al. [200] effectively synthesized high-molecular-weight, semi-aromatic FDCA-based polyesters using high-purity DMFDCA through a two-step commercial melt polymerization technique. They produced polyesters with low coloring, narrow molecular weight distribution (PDI < 2), and moderate molecular weights by reacting DMFDCA with a number of linear aliphatic diols. Poly(ethylene furanoate) (PEF), poly(propylene furanoate) (PPF), and poly(butylene furanoate) (PBF) displayed comparable molecular weights and color to their terephthalate counterparts. The molecular weight of PEF was further increased by almost an order of magnitude through subsequent solid-state polymerization (SSP), demonstrating that PEF may be made in industrial settings similar to PET. After annealing, the high-molecular-weight PEF showed enhanced mechanical strength above its glass transition temperature and a Young’s modulus of about 2450 MPa on par with PET, demonstrating its great promise for advanced material applications. PEF was prepared by Papageorgiou et al. [201] via a two-step melt polycondensation method employing dimethyl 2,5-furandicarboxylate (DMFDCA) and ethylene glycol (EG), and its thermal characteristics were compared with those of PET and PEN. The results showed that PEF had an equilibrium melting point of 265 °C and a heat of fusion of approximately 137 J g⁻¹. Crystallization kinetics were examined using several models, and the impact of synthesis parameters such as monomer ratio, catalyst type, and reaction temperature on the structure and characteristics of furanoate polyesters was carefully explored. It was revealed that the molecular chain configuration substantially impacts the final polymer quality. Wang et al. [199] Dimethyl 2,5-furandicarboxylate (DMFD) was transesterified with various diols to create PEF, PTF, and PBF, with intrinsic viscosities of 0.92, 0.88, and 0.96 dL g⁻¹, respectively. As shown in Figure 21a,b, copolymers containing 2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO) showed exceptional mechanical strength, barrier performance, and glass transition temperatures. These characteristics imply that these furan-based copolyesters could be attractive bio-based, high-performance substitutes for traditional PET, PPT, and PBT, particularly in packaging applications. Bimestre et al. [202] successfully created PEF using a transesterification melt polycondensation method, demonstrating a direct connection between the monomer structure and the polymerization process. The glass transition temperature (Tg) of the generated PEF was 80 °C higher than that of PET (75 °C), and its melting temperature (Tm) was 215 °C lower than that of PET (260 °C). These thermal properties demonstrate PEF’s great potential as a PET substitute in a variety of applications.

3.4. Production of Polylactic Acid (PLA)

Made from renewable resources, polylactic acid (PLA) is a completely biodegradable polymer. It can be generated from lactic acid using chemical or enzymatic processes. PLA is classified into three main types based on the optical isomer of lactic acid employed: Different physical and chemical properties are displayed by poly(D-lactic acid) (PDLA), poly(L-lactic acid) (PLLA), and poly(DL-lactic acid) (PDLLA). PDLA is mostly amorphous, while PLLA is semi-crystalline. PLLA stands out in particular for having a glass transition temperature of 67 °C, a melting temperature of about 180 °C, and a crystallinity of about 37% [203,204,205].

Both PLLA and PDLA are highly thermally stable polymers, making them suitable for a range of applications. PDLA, which is amorphous and optically inactive, is one of the two enantiomers derived from L(+)- and D(−)-lactic acid. When combined with PLLA through co-precipitation, it forms a stereo-complex polylactide (sc-PLA) that exhibits improved physicochemical characteristics, including a higher melting point of approximately 230 °C [206]. Polysaccharides (such as cellulose and starch), lipids (such as vegetable oils and fats), and proteins (such as gelatin) are examples of sustainable biomass sources from which PLA can be made. The first step in its manufacture is the microbial fermentation of carbohydrates into lactic acid, which is subsequently polymerized into PLA. This polymerization can proceed via three main methods (Figure 23): (i) direct polycondensation, yielding low-molecular-weight PLA that requires subsequent chain extension Table (5 entry 1); (ii) azeotropic dehydration, which similarly produces short-chain PLA Table (5 entry 2); and (iii) ring-opening polymerization (ROP) of lactide, a three-step process involving polycondensation, depolymerization, and final polymerization to produce high-molecular-weight PLA. Catalysts for ROP include metal complexes of aluminum, zinc, magnesium, calcium, iron, tin, yttrium, samarium, lutetium, titanium, and zirconium, with stannous octoate (Sn(Oct)₂) being the most commonly used for generating high-molecular-weight products [207]. Although condensation polymerization is cost-effective, it does not yield high-molecular-weight PLA directly. Additional expenses arise from the use of coupling agents and esterification accelerators. The process typically involves two stages: first, dehydration condensation between hydroxyl and carboxyl groups forms low-molecular-weight PLA; second, chain-extending additives are applied to lengthen polymer chains and modify structural properties. Byproducts and residual additives generated during this process are non-biodegradable, which can be problematic for biomedical applications. Triphosgene is sometimes employed to remove impurities and residual catalysts, but its use raises production costs and safety concerns due to flammable solvents. While new chain extenders have been developed to replace conventional agents, issues related to toxicity and non-biodegradability remain unresolved. Although condensation polymerization is cost-effective, it does not yield high-molecular-weight PLA directly. Additional expenses arise from the use of coupling agents and esterification accelerators. The process typically involves two stages: first, dehydration condensation between hydroxyl and carboxyl groups forms low-molecular-weight PLA; second, chain-extending additives are applied to lengthen polymer chains and modify structural properties. Byproducts and residual additives generated during this process are non-biodegradable, which can be problematic for biomedical applications. Triphosgene is sometimes employed to remove impurities and residual catalysts, but its use raises production costs and safety concerns due to flammable solvents. While new chain extenders have been developed to replace conventional agents, issues related to toxicity and non-biodegradability remain unresolved [208,209,210].

Senila et al. [211] developed a two-step polymerization strategy to synthesize PLA from lignocellulosic residues. In the first phase, lactic acid was produced from sugars liberated via pressurized hot water pretreatment and subsequent fermentation utilizing Lacticaseibacillus rhamnosus. The purified lactic acid then underwent two polymerization stages: initially, azeotropic dehydration at 140 °C for 24 h in xylene with tin(II) chloride (SnCl₂) to generate lactide, followed by microwave-assisted polymerization at 140 °C for 30 min using 0.4% SnCl₂. This methodology offers an efficient route to convert lignocellulosic biomass into PLA. Although the azeotropic dehydration-condensation approach avoids extensive additives, solvents such as dibasic acids and glycols are still used, and catalysts remain present in the final polymer.

In practice, lactic acid is initially distilled under reduced pressure at 130 °C for 2–3 h to eliminate most water. The mixture is then refluxed over a molecular sieve for 30 to 40 h at 130 °C after the catalyst and diphenyl ether are added. Efficient water removal increases the solvent’s boiling point, accelerating polymerization. Among the catalysts tested, tin-based compounds demonstrated the highest efficiency, though residual impurities can partially inhibit the reaction. Industrial studies later confirmed that most catalyst residues can be removed without degrading PLA [212]. To create high-molecular-weight PLA, ring-opening polymerization (ROP) of lactide is frequently utilized in industry (

Table 5. Comparison of different PLA production methods.

Method	Process Description	Catalyst(s)	Molecular Weight	Advantages	Disadvantages	Ref
Direct Polycondensation	Lactic acid monomers are condensed directly, releasing water as a byproduct.	SnO, ZnO, Ti(OBu)4	Low	Simple and low-cost process	Reaction equilibrium limits MW: water removal is difficult	[216]
Azeotropic Dehydration Polycondensation	Lactic acid polymerized under azeotropic conditions with continuous removal of water using solvents.	SnCl2, Zn(Ac)2, Ti-based catalysts	Low to Medium	Efficient water removal; improved polymer quality over direct condensation	Solvent use increases cost and purification steps	[217]
Ring-Opening Polymerization (ROP)	High-MW PLA is produced by first converting lactic acid to lactide (cyclic dimer), which then goes through ring-opening polymerization.	Sn(Oct)2, Al(O-iPr)3, Zinc lactate	High	Produces high-MW, high-purity PLA; most common industrial method	Requires multi-step process and catalyst removal	[218]
Enzymatic Polymerization	Enzymes catalyze the polymerization of lactic acid or lactide under mild, eco-friendly conditions.	Lipases (e.g., Candida antarctica lipase B), Proteinase K	Medium	Green process; no toxic catalysts	Slow reaction rate; limited scalability	[219]

, entry 3). Lactide, a cyclic dimer of lactic acid, exists as L-lactide, D-lactide, and meso-lactide, and can be produced under mild, solvent-free dehydration conditions. High-purity L- or D-lactide appropriate for ROP is produced commercially by condensing lactic acid at 115–179 °C, eliminating water, and recrystallizing to separate meso-lactide and low-molecular-weight polymers [213,214]. For instance, reduced-pressure reflux was employed by Cargill Inc [215] to eliminate partial lactide, oligomers, lactic acid, and leftover water. While prior techniques used inert-gas or gas-phase recrystallization to increase yield, Nemphos added multistage melt recrystallization to remove unreacted monomers and low-molecular-weight species. For extraction, weakly alkaline aqueous solvent systems have also been studied.

Following purification, high-purity lactide undergoes ROP through cationic, anionic, or coordination/insertion mechanisms depending on the catalyst. Cationic initiators, such as triflic acid or methyl triflate, efficiently promote polymerization, whereas anionic initiators can cause racemization and uneven molecular weight distributions [220]. To reduce toxicity, industrial processes prefer less active metal catalysts such as tin(II) or zinc compounds. Tin(II) 2-ethylhexanoate (Sn(Oct)₂) is the most widely used FDA-approved catalyst due to its high efficiency and low toxicity, though aluminum alkoxides and rare-earth compounds are also employed, the latter offering faster polymerization rates. Enzymatic polymerization (Table 5, entry 4) provides a sustainable alternative for PLA synthesis, operating under mild conditions with high specificity. Zahra et al. [221] reported that ultrasound-assisted dehydration during ROP effectively reduced water content and enhanced polymer properties. Using ultrasonic treatment at 109.6 W for 98.85 min, they achieved a moisture content of 1.9%, yielding PLA with 60.01% crystallinity, a melting temperature of 165 °C, and a molecular weight of 40,567 g/mol. These findings suggest that ultrasonic-assisted dehydration is a promising strategy for improving PLA synthesis via ROP. Enzymatic approaches are increasingly explored as environmentally friendly alternatives [222]. Unlike chemical methods, which require highly pure monomers, anhydrous conditions, and elevated temperatures to prevent side reactions, enzymatic polymerization enables the production of well-defined polymers from inexpensive raw materials. A fully biosynthetic route for PLA could offer significant advantages if remaining technical challenges are addressed [223], although reports on enzymatic PLA synthesis remain limited. Using lactic acid-polymerizing enzymes (LPEs) as alternatives to chemical catalysts represents a promising strategy. Discovering natural PLA-producing microorganisms would be ideal, but this remains an ongoing challenge requiring further research. Using lipase B from Candida antarctica (Novozyme 435), Chanfreau et al. [224] demonstrated the enzymatic synthesis of poly-L-lactide (PLLA) in the ionic liquid 1-hexyl-3-ethylimidazolium hexafluorophosphate ([HMIM][PF₄]). At 90 °C, the maximum PLLA yield was 63%, with a molecular weight of 37.8 × 10³ g/mol.

3.5. Classification and Functional Properties of Polyhydroxyalkanoates (PHAs)

PHAs are a class of aliphatic polyesters synthesized by bacteria, widely recognized for their ability to degrade safely across diverse environments. These compounds serve as intracellular energy reserves, accumulating within the bacterial cytoplasm. Their ability to decompose in soil, compost, and aquatic systems without releasing harmful byproducts has positioned them as a promising sustainable alternative to conventional synthetic plastics, drawing significant interest from industries such as packaging and single-use product manufacturing [225].

The structural diversity of PHAs arises from the varying carbon chain lengths of their 3-hydroxyalkanoate monomers, with over 100 different monomer types identified so far, opening possibilities for potentially thousands of copolymer configurations [226,227]. Based on this carbon chain length, PHAs are broadly categorized into three groups as shown in Figure 24: short chain length (4–5 carbon atoms), medium chain length (6-14 carbon atoms), and long chain length (14 or more carbon atoms) [228]. PHAs are not limited to homopolymer forms; they can also exist as copolymers that incorporate both short-chain and medium-chain units. These combined structures generally exhibit enhanced material properties compared to their homopolymer counterparts [229]. Among the most extensively studied and industrially relevant members of this family are the short-chain type polymers, particularly poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxyvalerate) (PHV) [230,231], given that short-chain PHAs dominate both industrial production and commercial availability.

One of the most readily available and reasonably priced feedstock types is lignocellulosic biomass; it is possible to turn these unused wastes into products with added value [232]. A wide variety of biomass sources have been investigated as carbon substrates for PHA biosynthesis, as mentioned in Table 6. As shown in Figure 25, the life cycle of the PHA product involves collecting waste lignocellulosic biomass from various sources, processing it, fermenting it, and transforming it into PHA materials throughout its lifespan. Ideally, these PHAs should be transferred to industrial or municipal composting facilities after use, where they will biodegrade and become part of the compost, enhancing soil quality and promoting the growth of new lignocellulosic biomass. The natural biodegradability of PHAs reduces their overall environmental impact, even if they are disposed of improperly.

Cell density levels significantly influence the accumulation of PHA as inclusion bodies within cells [233]. Various fermentation techniques, including batch, fed-batch, and continuous fermentation systems, have been investigated in numerous studies to improve PHA production [234,235]. Nevertheless, the selection of an appropriate strategy depends on multiple variables, such as the carbon source utilized, the bioreactor configuration, and the culturing method employed. The manufacture of biopolymers frequently uses batch fermentation because it is inexpensive and straightforward. This closed-system method introduces all substrates and necessary components from the outset, allows them to react within the vessel, and then recovers the finished product [236]. Batch fermentation is simple, although it is not as productive as other techniques. This restriction results from the tendency of stored PHA to break down after the carbon source is depleted, reducing the total amount of material produced. To mitigate carbon depletion, increasing substrate concentrations often inhibits microbial growth and lowers yield [237]. Additionally, because batch systems are closed, interim sampling is not possible, which makes intermediate analysis impractical [238]. Using wheat straw hydrolysate as the carbon substrate in a fed-batch system, Ces Rodríguez et al. achieved exceptionally high PHB production (105.0 g/L) and biomass concentration (135.8 g/L) [239]. Waste fiber rejects were used by Jia et al. to make PHB, which reached 14.7 g/L with 46% accumulation; growth was enhanced by greater substrate and inoculum, and a bioreactor for scale-up was recommended. In another study by mahara et al., PHB was produced from alkaline-pretreated hemp with high sugar recovery (97.9% glucose, 99.8% xylose). C. necator achieved a PHB yield of 0.433 g/g through batch fermentation under optimum nitrogen, phosphate, and pH conditions, whereas Paraburkholderia sacchari co-cultures increased xylose consumption and PHB output to 0.341 g/g. Overall, PHB production from lignocellulosic hydrolysates was improved by process improvement [240]. Saratale et al. enhanced biomass hydrolysis and allowed R. eutropha to convert sugar to PHB more effectively by using a NaOH pretreatment to remove lignin as a low-cost method for producing bioplastics, as demonstrated by the high yield (75.45%, 11.42 g/L) that was attained in 48 h with corn steep liquor boosting production [241]. A study by Wang et al. demonstrates the effect of radiofrequency (RF) heating compared with conventional heating. RF heating of switchgrass in an alkaline environment enhances biomass breakdown, resulting in hydrolysates that produce noticeably more PHB in E. coli fermentation. RF pretreatment is an effective technique for producing bioplastics, as yeast extract increases output and reduces treatment variability.

Table 6. Biosynthesis of PHAs using various biomass and strains of bacteria.

Biomass	Bacteria	PHA Type	Yield	References
Rice bran	P. aeruginosa	PHB	93.7%	[242]
Sugarcane molasses	Alcaligenes sp.	PHB	90.9%	[242]
Forestry LCB	H. pseudoflava	PHA	84%	[243]
Potato starch	Ralstonia eutropha	PHA	55%	[244]
Rice straw	Bacillus cereus	PHA	59.3%	[245]
Corn stover	Paracoccus sp. LL1	PHB	9.71 g/L	[246]
Grass biomass	Pseudomonas strains	MCL PHA	0.3 g/L	[247]
Lignin	C. necator DSM 545	PHB	4.5 g/L	[248]
Corn cob	Bacillus sp.	PHB	4.80 g/L	[249]
Wheat bran	Ralstonia eutropha NCIMB 11599	PHB	14.82 g/L	[250]

Table 6. Biosynthesis of PHAs using various biomass and strains of bacteria.

Biomass	Bacteria	PHA Type	Yield	References
Rice bran	P. aeruginosa	PHB	93.7%	[242]
Sugarcane molasses	Alcaligenes sp.	PHB	90.9%	[242]
Forestry LCB	H. pseudoflava	PHA	84%	[243]
Potato starch	Ralstonia eutropha	PHA	55%	[244]
Rice straw	Bacillus cereus	PHA	59.3%	[245]
Corn stover	Paracoccus sp. LL1	PHB	9.71 g/L	[246]
Grass biomass	Pseudomonas strains	MCL PHA	0.3 g/L	[247]
Lignin	C. necator DSM 545	PHB	4.5 g/L	[248]
Corn cob	Bacillus sp.	PHB	4.80 g/L	[249]
Wheat bran	Ralstonia eutropha NCIMB 11599	PHB	14.82 g/L	[250]

4. Sustainability Considerations

Single-use plastics are not sustainable and produce a higher level of carbon emissions. Bio-based plastics are a viable substitute for petroleum-based plastics because they have a lower carbon footprint. However, the biggest problem facing bio-based plastics adoption is ambiguity in production cost, end-of-life handling, and a lack of adequate biodegradability [251]. To promote sustainable bio-based plastic production and consumption, several key factors should be considered, as shown in Figure 26. These include the use of renewable and sustainable raw materials, along with the use of green solvents, as addressed in previous sections. Additionally, an integrated life-cycle assessment (LCA) is necessary to evaluate the energy balance and the environmental fate of bio-based plastics, specifically their biodegradation behavior after disposal. In addition, waste minimization during production can be achieved through closed-loop recycling systems, thereby maintaining resource efficiency and reducing overall carbon footprint.

4.1. Life-Cycle Assessment (LCA)

The LCA tool compares the environmental and economic footprints of bio-based plastics, whose sustainability depends on stages from raw material procurement to end of life [26]. Raw material issues include land-use change, non-renewable energy use, and eutrophication potential from fertilizers for starch-based feedstocks [252,253]. Process optimization and renewable energy use can reduce impacts. Manufacturing faces high costs, slow microbial growth, low efficiency, and energy needs; optimizing processes and minimizing waste can cut environmental impacts [254,255]. While the use phase has less environmental impact than production, material performance and durability affect sustainability. During plastic’s end of life (EOL), recycling, composting, and biodegradation are key to limiting waste and environmental effects. Circular approaches like waste recycling and environmental analysis enhance sustainability. Considering environmental and economic factors throughout the life cycle is vital for responsible bio-based plastic production and disposal. Several studies use LCA to compare the environmental footprints of different polymers. Senila et al. analyzed two bio-based plastics, PLA and PHB. They found that biomass processing and pretreatment significantly impact PLA and PHB production, with energy-intensive processes increasing the carbon footprint [50,256]. Yadav and Nikalje reviewed the LCA of bio-based plastics, highlighting benefits like biodegradability and lower carbon footprint due to renewable resources. Their environmental impact depends on full life-cycle factors such as feedstock, land use, energy, production, and waste. To minimize ecological trade-offs, sustainable sourcing, energy-efficient manufacturing, proper infrastructure, and responsible waste management are essential. Effective integration requires coordinated regulation, innovation, and stakeholder collaboration within a circular economy [257]. Menglei et al. analyzed PLA’s energy use and environmental effects in China using life-cycle assessment, from cradle to grave, across seven impact categories. They found the production stage has the greatest impact, with global warming potential, acidification, and human toxicity making up 32.63%, 24.83%, and 14.01%. Recommendations included reducing electricity, increasing renewable energy, improving production efficiency, and replacing fertilizers with greener options [258]. Benavides and Mehrjerdi conducted a life-cycle assessment (LCA) of biodegradable polylactic acid (PLA) and bio-based polyethylene (bio-PE), evaluating their environmental impacts from raw material extraction to end of life (EOL). Both bio-PE and PLA showed lower greenhouse gas emissions and fossil energy use compared to fossil-based plastics like HDPE and LDPE, with emissions of 1.0 and 1.7 kg CO₂ eq/kg and energy consumption of 29 and 46 MJ/kg. Research indicates that PLA’s environmental profile worsens in landfills or composting, with a 16–163% increase in life-cycle GHG emissions, highlighting the importance of end-of-life management for sustainable biodegradable plastics [259]. Roijen and Miller analyzed three bio-based plastics (PLA, PHA, thermoplastic starch) for EOL using composting, landfilling, and anaerobic digestion (AD). They argued that LCA studies on biodegradation do not align well with projected uses of bio-based plastics. Landfilling was used in 60% of studies, biodegradation in 12%. Assigning utilitarian outputs to AD greatly improved environmental performance, cutting carbon emissions by about 55%. Regarding composting, 38% of studies were done continuously, which is important for complete composting [260].

4.2. Environmental and Socio-Economic Impacts

Sustained economic growth over the last few years has raised grave concerns about its environmental impacts, especially in rapidly growing economies. To address these issues, Moshood et al. [261] carried out research on sustainable bio-based plastics, exploring their economic nature, environmental impact, and social qualities; among these, the environment has been declared the most critical factor. Bio-based plastics can minimize the carbon emissions by 241–316 million metric tons per year [262]. On the economic front, bio-based plastics still have an uphill climb against biofuels in terms of biomass resources. Furthermore, there are substantial financial obstacles related to biomass utilization, as research and development in the field require significant investment. Investors and governments, however, tend to be more hesitant to fund such initiatives compared to the well-established infrastructure supporting fossil-based plastic production [263]. Bio-based plastics have potential in all three pillars of sustainability (environment, economy, and social) [262].

4.3. Waste Minimization

Biorefineries consist of multiple interconnected processes that work together to convert feedstocks into a variety of valuable products. Over the past few decades, numerous studies have focused on individual bioprocesses for producing either biofuels, such as bioethanol, biomethane, biohydrogen, and biodiesel, or bioproducts, including pharmaceuticals, enzymes, and bio-based plastic monomers, from organic materials. By examining the process flow diagrams of these bioprocesses, different standalone processes can be strategically integrated to design a conceptual bio-based plastic biorefinery. Moreover, understanding the feedstock characteristics and the fate of byproducts at each stage is essential to achieving complete valorization [264]. Figure 27 shows that designing products for reuse and recycling helps keep them in use longer, with less material and energy use. When products can no longer be repaired, processes like mechanical recycling and depolymerization can recover their material value. Even if some bio-based plastics end up in the environment, they can still support a circular economy. This is because they can break down into CO₂ and water, allowing the carbon to return to nature and be used again in making new materials, which helps us move toward a zero-waste and sustainable production cycle [265].

5. Conclusions

Lignocellulosic biomass is a sustainable source to produce bio-based plastics. It not only reduces dependence on petroleum-based polymers but also mitigates plastic pollution at the source. This review comprehensively discusses green chemistry approaches for biomass pretreatment and the synthesis of precursors for the efficient utilization of biomass-derived carbohydrates and aromatics as renewable feedstocks for high-performance bio-based plastics. Moreover, chemical, physical, physicochemical, and biological pretreatment methods are critically compared based on reaction rates, working conditions, monomeric sugar yields, and their advantages and drawbacks. New green solvents, such as ILs, DESs, and LHWs, achieve higher delignification rates under milder conditions, in a more environmentally friendly setting, and produce sugar monomers that can be further processed for bio-based plastic production. In the same vein, the catalytic valorization of sugars into major platform chemicals, 5-hydroxymethylfurfural (HMF) and its oxidation product, 2,5-furandicarboxylic acid (FDCA), has been significantly advanced by the development of non-noble metal catalysts, which are highly yielding, selective, and reusable. Despite significant research, several key factors negatively affect the synthetic process, including biomass heterogeneity, enzyme inhibition, and difficulty in solvent recovery. The overall process requires significant energy inputs, which ultimately increase the cost of production. Therefore, to achieve cost competitiveness with traditional petroleum-based plastics, catalyst performance must be upgraded, solvent recycling enhanced, and reaction rates increased. However, the shift toward a circular bioeconomy, combined with increased environmental awareness and supportive policy instruments, is accelerating the exploration and industrial interest in bio-based polymers, including PLA, PHA, PBS, and PEF.

6. Future Prospects and Recommendations

The following recommendations and key points need further exploration:

• No single pretreatment is completely eco-friendly; more research is necessary to develop greener protocols that maintain yields.
• The lack of standardized analytical methods leads to yield variations caused by biomass differences, which must be addressed to ensure process consistency.
• Few studies have examined scaling laboratory processes to pilot or commercial levels, highlighting the need for further research.
• Genetically engineered plants could improve yields by tackling biomass heterogeneity and recalcitrance.
• Bio-based plastics require comprehensive cost assessments and end-of-life analyses to confirm their economic and environmental viability.

Generally, lignocellulosic biomass is a strong, renewable, and untapped source of next-generation bio-based plastics. New environmentally friendly pretreatment technologies, catalytic mechanisms, and green polymerization processes will remain important in the full exploitation of its potential. Further innovations will be based on interdisciplinary cooperation, process optimization informed by life-cycle assessment, and the development of scalable, cost-effective biorefinery models. With sustained research and technological refinement, biomass-derived bio-based plastics can play a pivotal role in shaping the future of low-carbon, circular, and environmentally responsible materials.