Recent Advances in Characterization and Valorization of Lignin and Its Value-Added Products: Challenges and Future Perspectives

Response to Reviewer 1^st Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files. [This is only a recommended summary. Please feel free to adjust it. We do suggest maintaining a neutral tone and thanking the reviewers for their contribution although the comments may be negative or off-target. If you disagree with the reviewer's comments please include any concerns you may have in the letter to the Academic Editor.]

Dear Reviewer,

Thank you very much for your thorough review and valuable comments on my manuscript. Your vision has been incredibly helpful in improving the quality of the paper. Below are my detailed responses (response are highlighter with green color) to each of your comments:

Comments 1: This work conducts a review of the characteristics and potential uses of lignin. While numerous reviews have been conducted on this topic, this study provides a good update. In several instances (e.g., lines 91 and 633), lignin is inaccurately described as a residue. However, this is incorrect. Currently, lignin serves as the energy source that enables certain industries such as biorefineries and pulp mills. In fact, a significant aspect not covered by this study is how lignin precipitation would impact current operations."

Response 1: Thank you for pointing Thank you for your perceptive comments. I acknowledge the inaccuracy in describing lignin as a residue in lines 91 and 633. I have ensured that lignin is accurately described as an essential energy source for biorefineries and pulp mills.

Line 91: line 91 changed to line 101.

Additionally, I have replaced waste product to byproduct in the line 101. This byproduct word explains how lignin precipitation impacts current industrial operations, emphasizing its role in energy production and any operational changes required for effective lignin utilization.

Line 633: line 633 changed to line 688.

The word “waste” has been replaced with underutilized fraction 

Original line 633: In the bioeconomy, lignin has evolved from being regarded as waste in conventional biorefinery processes to a valuable resource.

Modified line 633: “In the bioeconomy, lignin has evolved from being regarded as underutilized fraction in conventional biorefinery processes to a valuable resource.”

Comments 2: On line 76, please detail the challenges for the utilization and valorization of lignin.

Response 2: I appreciate this suggestion. I have added sentences for explaining the challenges of this and highlights with green color. And also cited properly.  

However, the complex and heterogeneous nature of lignin poses significant challenges to its efficient utilization and valorization. The complex and energy-intensive extraction process, such as the Kraft process, along with the structural variability of lignin, make it difficult to develop standardized con-version methods. The unfavorable properties of lignin, including its complex three-dimensional structure and limited solubility, further complicate its utilization [12,13]. To overcome these challenges, current research focuses on efficient conversion methods, such as catalytic depolymerization and enzymatic breakdown, as well as ex-ploring the environmental impact through life cycle assessments. Successfully unlock-ing lignin's potential could lead to a sustainable, bio-based economy, with lignin serv-ing as a renewable resource for various industries, including coatings, plastics, and liq-uid fuels[12,13]. Ongoing research aims to develop efficient and economically viable processes for lignin valorization, offering significant environmental and economic benefits.

Reviewer Comment # 3: Some references are duplicated; for example, references 25 and 28 are the same. References should be carefully reviewed.

Response: Thank you for pointing out the duplication of references. I will thoroughly review and correct the reference list to eliminate any duplicates and ensure all citations are accurate and properly formatted.

Reviewer Comment # 4: The review of chemical analyses is comprehensive and well-executed. Methods for determining molecular weight distribution could be presented in greater detail, highlighting the advantages and disadvantages of different alternatives.

Response: I am glad the chemical analyses section was well-received. I will expand the discussion on methods for determining molecular weight distribution, providing a more detailed comparison of the different methods, including their respective advantages and disadvantages. However, I understand the importance of this aspect and will briefly mention the various methods used for determining molecular weight distribution, directing readers to relevant literature for a more comprehensive understanding. This approach ensures that the review remains focused while acknowledging the significance of the topic

I have added this paragraph line # 176 to 189 with accurate citation.

Gel Permeation Chromatography (GPC) and Size Exclusion Chromatography (SEC) are widely used techniques for determining the molecular weight distribution of lignin, separating molecules based on size. GPC provides detailed information on the molecular weight distribution profile, while SEC is applicable to various types of lignin [23]. Matrix-Assisted Laser Desorption/Ionization-Time of Flight (MALDI-TOF) mass spectrometry offers high-resolution mass spectra for precise molecular weight determination, but requires specialized equipment and complex sample preparation [24]. Nuclear Magnetic Resonance (NMR) spectroscopy, particularly 2D-NMR, provides insights into lignin's molecular weight distribution by analyzing its structural characteristics, although it is less commonly used solely for molecular weight determination [25]. These techniques play crucial roles in characterizing lignin and furthering its utilization Researchers often combine these methods to gain a comprehensive understanding of lignin's molecular characteristics. For instance, integrating GPC data with 2D-NMR results allows for quantitative molecular weight distribution analysis alongside qualitative structural insights [23–25].

Reviewer Comment # 5: Figure 5 is of poor quality and cannot be adequately read.

Response: I apologize for the poor quality of Figure 5. I will replace it with a higher-resolution version to ensure clarity and readability.

Response to Reviewer 2^nd Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files. [This is only a recommended summary. Please feel free to adjust it. We do suggest maintaining a neutral tone and thanking the reviewers for their contribution although the comments may be negative or off-target. If you disagree with the reviewer's comments please include any concerns you may have in the letter to the Academic Editor.]

Dear Reviewer,

Thank you very much for your thorough review and valuable comments on my manuscript. We appreciate your positive remarks and constructive suggestions. We have made the following revisions to address your comments: (Response are highlighter with yellow color)  

Comments 1: ”The manuscript submitted for review is a review study and deals with the extremely interesting issue of lignin valorisation."

Response 1: Dear Reviewer, Thank you for your kind words and for acknowledging the relevance and interest of our study on lignin valorization. We appreciate your recognition of the significance of this topic and your constructive feedback, which has helped us improve the manuscript further.”

Comments 2: ”No less a lignin , one that is a protective element for cellulose is practically not available. The waste of lignin by burning it concerns mainly technical lignins. The main lignin compounds are technical lignins, which are a by-product of the chemical industry. A description of the types and forms of technical lignins is missing from the article, so please supplement in the introduction to the manuscript by stating the types of technical lignins and how they differ from the lignin found in wood. Lignins found in plants differ in the proportion of alcoholip-courmaryl, coniferyl, and sinapyl alcohols. Please state in your paper if and what differences there are between the composition of lignins found in different plants or parts of plants, bark, wood of different species. I suggest presenting this information in the form of a table.”

Response 2: We highly appreciate your taking the time to read our manuscript and send the insightful comments/suggestions. The detailed feedback has been very helpful, and some revisions have indeed been introduced towards answering the points raised by you.

Here, following your suggestion, an adequate comparison between native and technical lignin has been given. In addition to such, a new table (Table 1) summarizes some of the properties and applications of technical lignins derived through different pulping methods such as kraft, lignosulfonate, and organosolv processes. So, this revised introduction is going to convey to the readers an accurate idea of the various forms of lignin, their unique features, and how they are different from the native form of lignin in wood.

Please look the lines 61 to 72 for a paragraph and line 79 for table 1 in the manuscript.

Native lignin (wood) differs from technical lignins due to its unique structure, formed from three monolignols (p-coumaryl, coniferyl, sinapyl alcohol). Their proportions vary by plant type, affecting properties. Softwoods have mainly guaiacyl units (coniferyl alcohol), while hardwoods have a mix of guaiacyl and syringyl units (sinapyl alcohol). Grasses contain all three types [9]. However, technical lignins are modified during the pulping processes, resulting in changes to their structure and properties. These modifications include the introduction of sulfur groups in Kraft lignin and lignosulfonates, and the reduction of molecular weight in organosolv lignin [9,10]. Understanding these differences is crucial for optimizing the valorization of technical lignins for various industrial applications [10,11]. Similarly, Table 1 summarizes the properties and applications of technical lignins derived from the three main pulping processes: Kraft, lignosulfonate, and organosolv.

Table 1. Properties and applications of technical lignins derived from different pulping processes

Reviewer Comment # 3: The authors have given an excellent account of the methods used to measure lignin and its constituents, but the title of the paper suggests that information on the valorisation of lignins might be expected. Unfortunately, nowhere did I see information on the possibility of using lignin as a protective component against UV radiation. Lignin also shows filtering properties as an absorbent, UV indicator etc. I would like to ask you to supplement the contents of the manuscript with information on the use of the functional properties of lignin.    

Response: Thank you for your valuable feedback on our manuscript. We appreciate your positive remarks regarding our account of the methods used to measure lignin and its constituents. We also acknowledge your suggestion to include information on the valorization of lignin, particularly its use as a protective component against UV radiation and its filtering properties as an absorbent and UV indicator. In response to your comments, we have incorporated the manuscript with additional information on the functional properties of lignin.

5. Advancements in the Valorization of Lignin

Lignin's valorization has gained significance due to advancements driven by the increasing demand for sustainable materials and reduced dependence on fossil fuels. Recent research and technological innovations in lignin valorization have yielded key advancements with significant implications for the industry.

5.1 Lignin as a Source of Biofuels

Lignin valorization into biofuels shows great promise for sustainable energy production. Recent biotechnological advancements have significantly improved the efficiency of lignin breakdown into simpler molecules, enabling the production of bioethanol and biodiesel. These developments are driven by the pressing need for sustainable energy sources and the desire to maximize the utilization of lignocellulosic biomass, aligning with key UN Sustainable Development Goals such as clean energy (SDG 7) and climate action (SDG 13) [3,6,20]. Enzymatic degradation techniques have significantly improved lignin conversion rates. Novel enzymatic cocktails have been developed, increasing lignin degradation efficiency by up to 40% compared to traditional methods, leading to higher bioethanol yields. This advancement highlights the potential for increased biofuel production from lignin-rich biomass [126]. Advancements in microbial degradation approaches have yielded significant results. By utilizing engineered microbes, researchers have achieved improvements in converting lignin to biodiesel, with studies reporting up to a 50% increase in yield [3]. This progress represents a substantial step towards enhancing the economic feasibility of biodiesel production from lignin. Moreover, the development of microbial consortia has improved lignin breakdown, resulting in notable reductions in processing time and costs [127]. The integration of chemical and biological approaches has been an effective strategy in lignin valorization. Integrated methods have shown an increase in lignin-to-bioethanol conversion efficiency of up to 35%. This synergistic approach highlights the potential of combining multiple technologies to overcome the challenges related to lignin's resistance and enhance biofuel yields [127]. This synergistic approach not only overcomes the challenges associated with lignin's resistance but also enhances biofuel yields. These improvements in overall biofuel production efficiency highlight the potential for lignin to become a valuable resource in current biofuel production facilities [3].

Recent advances in enzymatic and microbial degradation techniques, along with process integrations, have significantly improved the economic viability of lignin-based biofuel production, addressed challenges and enhanced conversion rates, yields, and processing efficiencies [3,6,126,127]. Therefore, it is expected, lignin will play an increasingly vital role in sustainable biofuel production, supporting global efforts to reduce reliance on fossil fuels and combat climate change. Ongoing research in lignin valorization holds promise for promoting biorefinery sustainability and contributing to carbon neutrality targets worldwide [3,6]. Future directions involve optimizing degradation processes, exploring novel catalysts, and scaling up production while improving cost-effectiveness. Continued advancements in lignin valorization will drive the transition towards a more sustainable and renewable energy future.

5.2 Lignin-Derived Chemicals and Materials

Recent advances in depolymerization processes have greatly improved the conversion of lignin into valuable aromatic compounds, serving as precursors for various chemicals and materials. Catalytic hydrogenolysis, in particular, has made significant advancements in enhancing the yield of targeted aromatic compounds from lignin. Du et al. (2023) achieved remarkable lignin conversion rates of up to 91.2% and a total yield of monomer products reaching 44.9 wt% under optimal conditions [128]. This improvement highlights the potential of lignin as a renewable source for valuable aromatic compounds. Similarly, oxidation methods in lignin valorization have seen significant advancements. These methods produce phenolic aldehydes and acids through side chain cleavage and have the potential to cleave aromatic rings in lignin. The versatility in product formation further underscores the potential of oxidation methods for effective lignin valorization [129].

Furthermore, pyrolysis processes have been optimized to generate valuable aromatic monomers from lignin. Recent research has focused on catalytic pyrolysis for enhanced production of aromatic hydrocarbons. For example, Zhou et al. achieved a carbon yield of 8.65% for aromatic hydrocarbons through catalytic fast pyrolysis of herbaceous lignin using HZSM-5 zeolite catalyst [130]. While the yield may appear modest, it signifies a noteworthy advancement in the selective production of valuable aromatics from lignin. The development of lignin-derived bioplastics and bio-based materials has made significant progress. Research demonstrates that lignocellulosic biomass from agricultural waste can be utilized to produce bioplastics, contributing to the circular economy [131]. Moreover, lignin has been effectively employed in the production of biocomposites and as a dispersant in diverse applications, such as bioasphalt, concrete, and dyes for textiles and agricultural chemicals [132]. The conversion of lignin into bio-based resins for eco-friendly adhesives has shown promising outcomes. A recent study focused on developing a fully bio-based adhesive using lignin and cellulose, aligning with the concept of "take from wood, back to wood [133]. A study highlighted the potential of lignin valorization for producing aromatic chemicals and polymeric materials, with values above $1000–2000 per ton, which is ten times higher than when used as a solid fuel [134]. This approach highlights the potential for lignin to substitute petroleum-based products in diverse applications, including adhesives.

Therefore, its depolymerization and conversion techniques have made lignin-derived chemicals and materials economically viable and environmentally sustainable. These improvements in yield, selectivity, and product quality address previous challenges in lignin valorization. Its increasing role in sustainable production aligns with global efforts to reduce dependence on fossil resources and mitigate climate change. Ongoing developments in lignin valorization hold promise for promoting sustainability and contributing to circular economy goals worldwide.

5.3 Lignin in Polymer Blends and Composites

Lignin has emerged as a promising component in polymer blends and composites, offering numerous advantages due to its unique properties and environmental benefits. Lignin possesses several desirable properties that make it suitable for incorporation into polymer materials, including high thermal stability, antioxidant properties, biodegradability, and antimicrobial actions [135]. These characteristics have led to its extensive use in a wide range of applications. In polymer blends and composites, lignin can serve multiple functions: as a filler, enhancing the mechanical properties of the composite materials; as a stabilizer, contributing to improved stability of the polymer matrix; as a compatibilizer, improving the compatibility between different polymer components in blends; and as a biodegradability enhancer, increasing the biodegradability of synthetic polymers [136,137]. The development of lignin-based thermoplastics often involves altering the viscoelastic properties of lignin through chemical modification or polymer blending, leading to the creation of various lignin-polymer composites with enhanced properties [138]. Notable applications of lignin-based polymer blends and composites include packaging materials, automotive components, construction materials, 3D printing, and energy storage and electronic devices. Recent advancements in this field include the development of poly (lactic acid) (PLA)/lignin nanoparticle composites, poly (butylene adipate-co-terephthalate) (PBAT)/lignin composites, and the exploration of lignin as a filler in various polymeric matrices [136,137]. Despite these advancements, challenges remain in optimizing the uniform distribution of lignin in polymer matrices and further improving the performance of lignin-based composites [137]. Ongoing research is focused on refining processing techniques and exploring new applications to fully harness the potential of lignin in polymer blends and composites.

5.4 Lignin as a UV Protector and Antioxidant

Lignin's antioxidant and UV-absorbing properties are utilized for UV protection. In polymer composites, as little as 1% lignin content in poly (methyl methacrylate) films achieves 60% UV-blocking capacity at 400 nanometers. Cellulose films containing 2 wt% lignin provide full UVB protection (280-320 nm) and over 90% UVA protection (320-400 nm) [139]. It is highly effective as a UV blocker in biodegradable films. Cellulose-lignin films regenerated in acetone provide complete UV blocking up to 400 nanometers with just 2% lignin content. These films retain their UV protection after thermal treatment and UV irradiation. In poly (butylene adipate-co-terephthalate) (PBAT) films, 10% lignin content ensures excellent UV-barrier properties in the 280-400 nanometer range, even after 50 hours of UV exposure [139].

Its antioxidant properties find application in food packaging, where lignin-containing biodegradable films extend the shelf life of perishable goods [140]. These films provide UV protection and contribute to packaging sustainability. Recent advancements involve chemically modified lignin nanoparticles in polymer composites. Modified lignin nanoparticles in polylactic acid (PLA) nanocomposites enhance UV-blocking properties while preserving transparency, addressing the dark color challenge associated with lignin in food packaging where aesthetics matter [141].

5.5 Functionalization and Modification of Lignin

Functionalization and modification techniques have expanded lignin's applications by enhancing its properties and compatibility with other materials. Acetylation has proven effective in increasing lignin's solubility in organic solvents. Researchers developed an acetylation technique that improved lignin's solubility by 60%, enabling its use in high-performance coatings and adhesives. This enhanced solubility opens up new possibilities for lignin in industries relying on organic solvent-based processes [142,143]. Sulfonation is another notable technique that improves lignin's dispersibility in water, making it valuable for applications in surfactants and detergents. Researchers have developed a sulfonation technique that significantly enhances lignin's water dispersibility, opening up new possibilities for its use in water-based products and processes. This modification offers sustainable alternatives to petroleum-based surfactants, particularly for industries seeking environmentally friendly solutions [144,145]. Grafting processes integrate lignin into biodegradable polymer blends, enhancing their mechanical properties and environmental profile. This technique improves strength and durability while maintaining biodegradability. It is particularly significant for the packaging and consumer goods industries, facilitating the development of eco-friendly materials [146]. Recent work has developed effective amination techniques using dimethylamine, ethylenediamine, or diethylenetriamine, achieving nitrogen contents up to 10.18%. This modification is particularly relevant for fertilizer applications [147].

In pharmaceuticals, functionalized lignin shows potential as a drug delivery agent such as its nanoparticles (LNPs) are being investigated for safe use in drug and gene delivery [148]. To enhance sustainability, green chemistry approaches like the use of deep eutectic solvents (DES) enable recyclable and tunable lignin modifications. Green metrics such as the E-factor evaluate and improve the sustainability of these processes [148]. Challenges include standardizing lignin sources and characterization methods, scaling up modification processes cost-effectively, enhancing compatibility with diverse polymer matrices, and developing efficient and environmentally friendly modification techniques. Future research aims to address these challenges and explore new applications for functionalized lignin in advanced materials and sustainable technologies, promoting innovative and eco-friendly solutions.

5.6 Economic and Environmental Benefits of Lignin Valorization

Advancements in lignin valorization significantly contribute to the circular economy and offer substantial environmental benefits. By converting lignin into valuable products, industries can reduce waste, lower greenhouse gas emissions, and decrease reliance on non-renewable resources. Economically, lignin-based products have become more viable due to technological innovations and growing market demand for sustainable solutions. For instance, lignin-based bioplastics can reduce production costs by up to 20% compared to conventional plastics [149], and the development of lignin-derived products opens new markets and revenue streams [150]. Environmentally, lignin valorization minimizes industrial waste [151], reduces CO₂ emissions by 30-40% [152], and promotes sustainable resource utilization, aligning with circular economy principles [153]. These advancements not only enhance profitability and market competitiveness but also support ecosystem health and biodiversity preservation, illustrating the profound impact of lignin valorization on both the economy and the environment [154].

Reviewer Comment # 4: The manuscript, once updated in accordance with the reviewer's comments, should be referred for peer review. 

Response: Dear Reviewer, Thank you for your valuable feedback and constructive comments on our manuscript. We have carefully addressed all the points you raised and made the necessary revisions to enhance the quality and comprehensiveness of our work. With these revisions, we believe that the manuscript now provides a comprehensive overview of lignin valorization, addressing both its measurement and functional applications.

Response to Reviewer 3rd Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files. [This is only a recommended summary. Please feel free to adjust it. We do suggest maintaining a neutral tone and thanking the reviewers for their contribution although the comments may be negative or off-target. If you disagree with the reviewer's comments please include any concerns you may have in the letter to the Academic Editor.]

Dear Reviewer,

Thank you very much for your thorough review and valuable comments on my manuscript. We appreciate your positive remarks and constructive suggestions. We have made the following revisions to address your comments: (Response are highlighter with pink color)  

Comments 1: ”The abstract requires significant revision. It consists of general information rather than detailing the major sections and topics the review paper covers. The abstract should independently convey the study's significance to engage a broad research audience and encourage them to read the full article."

Response 1: Thank you for your insightful feedback on our manuscript. We appreciate your suggestion to revise the abstract to better reflect the major sections and topics covered in the review paper. In response, we have made significant revisions to the abstract to ensure it independently conveys the study's significance and engages a broad research audience.

The revised abstract line 24 to 41

“Lignin, the earth's second-most abundant biopolymer after cellulose, has long been relegated to a low-value byproduct in the pulp and paper industry. However, recent advancements in valorization are transforming lignin into a sustainable and versatile feedstock for producing high-value biofuels, bioplastics, and specialty chemicals. This review explores into the conversion of lignin's complex structure, composed of syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units, into value-added products. We critically assess various biochemical and analytical techniques employed for comprehensive lignin characterization. Additionally, we explore strategies for lignin upgrading and functionalization to enhance its suitability for advanced biomaterials. The review emphasizes key areas of lignin valorization, including catalytic depolymerization methods, along with the associated challenges and advancements. We discuss its potential as a feedstock for diverse products such as biofuels, bioplastics, carbon fibers, adhesives, and phenolic compounds. Furthermore, the review briefly explores lignin's inherent properties as a UV protectant and antioxidant, alongside its potential for incorporation into polymer blends and composites. By presenting recent advancements and case studies from the literature, this review highlights the significant economic and environmental benefits of lignin valorization, including waste reduction, lower greenhouse gas emissions, and decreased reliance on non-renewable resources. Finally, we address future perspectives and challenges associated with achieving large-scale, techno-economically feasible, and environmentally sustainable lignin valorization.”

Comments 2: ”No less a lignin , one that is a protective element for cellulose is practically not available. The waste of lignin by burning it concerns mainly technical lignins. The main lignin compounds are technical lignins, which are a by-product of the chemical industry. A description of the types and forms of technical lignins is missing from the article, so please supplement in the introduction to the manuscript by stating the types of technical lignins and how they differ from the lignin found in wood. Lignins found in plants differ in the proportion of alcoholip-courmaryl, coniferyl, and sinapyl alcohols. Please state in your paper if and what differences there are between the composition of lignins found in different plants or parts of plants, bark, wood of different species. I suggest presenting this information in the form of a table.”

Response 2: Thank you for your valuable feedback. We appreciate your input and have taken it into consideration. In our manuscript, we have emphasized the novelty of the review by highlighting the latest advancements and developments in the field of lignin utilization. Additionally, we have incorporated a the research gap, highlighting the key challenges and areas that require further investigation. By addressing these research gaps, we aim to contribute to the advancement of lignin utilization and promote its large-scale adoption in a more sustainable and efficient manner. 

Please look the lines 131 to 144 for a paragraph.

Research Gap and objective of the study

However, achieving large-scale adoption depends on overcoming significant research gaps. These include the development of more efficient, selective, and robust catalytic depolymerization processes; cost-effective and economically viable methods; comprehensive life cycle assessments for environmental sustainability; and advanced analytical techniques for precise characterization and standardization. Additionally, integrating lignin-derived materials into commercial applications poses challenges due to performance limitations and compatibility issues. The aim of this study is to explore and address these gaps by providing a detailed analysis of current lignin valorization methods, highlighting recent technological advancements, and proposing future research directions. By presenting a comprehensive overview of lignin's potential and the innovative approaches needed to fully utilize this resource, this review seeks to promote sustainability and innovation in various industrial applications, ultimately contributing to a more sustainable and circular bioeconomy.

Reviewer Comment # 3: The aims and objectives of the review should be placed separately at the end of the 'Introduction' section, as they are currently missing.

Response: Thank you for your valuable feedback on our manuscript. We have responded to these comments in our comprehensive response to comments 2 and 3. Your input has greatly assisted us in improving our manuscript.

Reviewer Comment # 4: A discussion on the yield and composition of bio-oil is absent. A separate section should present relevant data from the most significant published works.

Response: Thank you for your valuable feedback regarding the absence of a discussion on the yield and composition of bio-oil. We appreciate your suggestion to include a separate section presenting relevant data from significant published works. While our primary objective in this study is to address specific research gaps related to catalytic depolymerization processes, economic viability, and analytical techniques for characterization and standardization, we acknowledge the importance of discussing bio-oil yield and composition. To accommodate this suggestion, we incorporated a concise but relevant discussion within the context of our existing sections. This approach allowed us to maintain the focus on our primary objectives while acknowledging the significant data related to bio-oil. We appreciate your understanding and will revise the manuscript accordingly to ensure it aligns with our aim of promoting sustainability. Thank you once again for your insightful comments. We look forward to receiving further feedback.

Reviewer Comment # 5: Patented technologies should be included in this review article.

Response: Thank you for your constructive feedback and suggestion regarding the inclusion of patented technologies in our review article. We recognized the significance of discussing patented technologies to provide a comprehensive overview of lignin valorization advancements. However, our current focus is on publicly available research findings, technological advancements, and future research directions. Including a detailed analysis of patented technologies would require extensive research beyond the scope of our current study.

Thank you for your valuable input. We appreciate your understanding and eagerly await your further feedback.

Reviewer Comment # 6: Additional citations regarding lignin valorization are necessary, such as papers like “https://doi.org/10.1016/j.enconman.2024.118615”.

Response: Thank you for your insightful feedback and the valuable suggestion regarding the inclusion of additional citations on lignin valorization. In response to your comment, I have added several recent and relevant references to enhance the comprehensiveness of the manuscript.

Specifically, the reference you recommended, "https://doi.org/10.1016/j.enconman.2024.118615", has been incorporated and is now listed as reference number 73. Furthermore, I have included numerous other latest references to ensure the manuscript reflects the most current research developments in the field. The total number of references has increased from 123 to 176, demonstrating a thorough and updated literature review. These additions strengthen the manuscript by providing a more robust context and supporting evidence for the discussion on lignin valorization.Thank you for your valuable input. We appreciate your understanding and eagerly await your further feedback.

Reviewer Comment # 7: Including a discussion on the reaction mechanism of bio-oil production from lignin in the catalytic depolymerization section would be beneficial.

Response: Thank you for your valuable feedback regarding the absence of a discussion on the yield and composition of bio-oil. While our primary objective in this study is to address specific research gaps related to catalytic depolymerization processes, economic viability, and analytical techniques for characterization. please look at the response of 4^th comments.

Reviewer Comment # 8: Extreme care must be taken in a review paper to specify who conducted each study and their critical observations for future work, avoiding the issue of merely summarizing what the studies did.

Response: Thank you. We appreciate your emphasis on the importance of clearly identifying the authors of each study and their critical observations. In response to your suggestion, we have revised the manuscript to ensure that each cited study is clearly attributed to its respective authors. We have also highlighted the critical observations and future work proposed by these studies. This approach ensures that the contributions of each study are properly recognized and provides a clearer direction for future research in the field. Thank you once again for your insightful comments. We look forward to receiving further feedback.

Reviewer Comment # 9: The overall paper is well-written; however, the grammar and sentence clarity need further improvement.

Response: Thank you for your positive feedback on the overall quality of the paper. We appreciate your constructive suggestion regarding the improvement of grammar and sentence clarity.

In response, we have conducted a thorough review and revision of the manuscript to enhance both grammatical accuracy and sentence clarity. These revisions ensure that the content is presented in a more precise and readable manner, thereby improving the overall readability and effectiveness of the paper.