To break, or not to break: is selective depolymerization of lignin a Riemann hypothesis rather than a solution?

Abstract

Lignin—nature's most complex, frustratingly stubborn macromolecule—has long been the poster child for biomass valorization's unrealized potential. Despite decades of hand-wringing over selective depolymerization to produce aromatic monomers, progress remains embarrassingly slow. This perspective article tackles the elephant in the room: is chasing the Holy Grail of high selectivity really the best use of our time and resources? Or should we finally admit that lignin's complexity demands a more pragmatic approach? We argue for a radical shift in perspective, advocating for a “liquefy-first” strategy that ditches the impossible dream of perfect depolymerization in favor of producing a heterogeneous liquid feedstock. As such, this feedstock could be fed into existing industrial processes, bypassing the tedious obsession with monomer purity. Maybe it is time to re-evaluate what success looks like in lignin research and embrace solutions that could move us faster toward a carbon-neutral future—without chasing the unicorn of selective breaking down.

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Adam Slabon

Adam Slabon grew up in Germany and Poland and graduated in Chemistry at the Jagiellonian University. He earned his doctoral degree from ETH Zurich (supervisor: Reinhard Nesper) in 2013. After a postdoctoral stay at UC Berkeley (supervisor: Peidong Yang), he moved to RWTH Aachen, Germany, where he obtained his habilitation (mentor: Richard Dronskowski). He joined Stockholm University as Assistant Professor (tenure-track) in 2019 and was appointed Wallenberg Academy Fellow in 2021. Since 2022, he is Professor at the University of Wuppertal (BUW), Germany, leading the Chair of Inorganic Chemistry.

Bruno V. Manzolli Rodrigues

Bruno V. M. Rodrigues is a Brazilian chemist with extensive expertise in the valorization of lignocellulosic biomass and the development of high-value-added materials. He earned his undergraduate degree in Chemistry from the University of São Paulo in 2010, where he also completed his Ph.D. in 2014. In 2016, he joined Universidade Brasil (São Paulo) as a Professor, a role he held until 2022. Concurrently, from 2020 to 2022, he served as a Guest Researcher at Stockholm University in the Department of Materials and Environmental Chemistry. Since 2022, he has been a Senior Research Associate at Bergische Universität Wuppertal (BUW). His current research focuses on the study of organic–inorganic hybrid materials and biomass conversion to produce high-value-added materials.

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Green foundation

1. This perspective challenges the paradigm of selective lignin depolymerization toward monoaromatic compounds by proposing a ‘liquefy-first’ strategy as a complementary pathway to the ‘lignin-first’ concept. It proposes alternatives such as electrocatalytic liquefaction, which align with industrial practices, reduce energy inputs, and leverage lignin's complexity to produce chemically heterogeneous liquid feedstocks.

2. Lignin valorization is pivotal for a sustainable bioeconomy. The proposed strategies address global energy challenges by offering scalable solutions, integrating lignin-derived feedstocks into high-volume industrial applications, like biofuels and bulk chemicals, while reducing reliance on fossil-based resources.

3. The future lies in scalable, pragmatic solutions that consider lignin's heterogeneity. Shifting focus from high selectivity depolymerization to liquefaction strategies, this perspective lays a foundation for greener, more efficient biomass utilization. It advocates developing catalytic systems and refining processes aligned with industrial needs, shaping the green chemistry's future through practical innovation.

The lignin paradox: high hopes, low yields

Chasing aromatic monomers: an endless quest?

The Riemann hypothesis, one of the greatest unsolved problems in mathematics, posits that the non-trivial zeros of the Riemann zeta function are complex numbers with a real part of 1/2. This hypothesis is widely regarded as one of the most important unsolved problems in pure mathematics. Despite many attempts, a solution remains elusive. We wonder how many of us—whether chemists or related enthusiasts—opened this article out of a genuine interest in biomass breakdown or simply due to unfamiliarity with the Riemann Hypothesis. To clarify, this discussion is not about the latter and we apologize to the 0.1% of readers seeking insights into it. Instead, this article will tackle an equally complex challenge: selective lignin (Fig. 1) depolymerization. Would it be an insurmountable problem or merely one that needs a fresh perspective?

Fig. 1 A representation of the chemical structure of the insoluble fraction of Kraft lignin, proposed by Crestini et al.¹

Some chemists have devoted their entire careers to tailoring complex structures from simple units. Leo Baekeland, for instance, would likely criticize those who linger over unsolved puzzles. When asked about his motivation for entering the field of synthetic resins, he candidly stated that his goal was ‘to make money’. This pragmatic approach proved successful, as also previously demonstrated by his profitable sale of Nepera to Kodak. Conversely, Richard Feynman took a different path, believing that the world had seen enough of bulk materials. To him, taking a dive into the unexplored space of the nanoscale was far more pressing. He was also right, which is reflected in today's semiconductor industry producing commercially transistors on the nanoscale. As chemists devoted to biomass upgrading, we find ourselves at a crossroads: striving to convert the macro world towards the micro. While we shy away from fully understanding the principles of quantum confinement, we propose a shift towards more sustainable carbon sources, with lignin offering a promising path to an oil-free world.

You can make anything out of lignin, except money. This assertion, arguably one of the most classic refrains heard at scientific conferences, serves as a common rhetorical device. PhD candidates up to Senior Researchers in the field alike employ it in their opening remarks, attempting to convince their audiences that lignin has been unjustly relegated to the status of a waste material – with no significant advances to find a noble use. In fact, almost all lignin produced today is burned to generate energy. According to recent statistics, the annual production of lignocellulosic biomass is approximately 181.5 billion tons.² In 2021, the global domestic supply of biomass reached nearly 54 EJ, with 85% derived from solid biomass sources, including wood chips, pellets, and traditional biomass, accordingly to 2024 Global Bioenergy Statistics reports.³ In 2022, 1.28 EJ of heat was produced from biomass-based sources – with 51% accounting from solid biomass sources.³ In 2023, 697 TWh of bioenergy electricity was generated worldwide, accounting 8% of total renewable electricity generation.³ These figures underscore the prevalent use of lignin—a significant component of solid biomass—in combustion processes. Despite its recognized potential as a versatile and high-value raw material, lignin's predominant role as a combustion source reflects the ongoing challenge of transitioning from traditional energy paradigms. Combustion remains the most economically viable pathway for biomass utilization, highlighting a critical barrier in achieving its more sustainable and value-added applications. This trend not only underscores the global prioritization of energy recovery but also emphasizes the urgency to reimagine lignin's role within bioenergy strategies, moving beyond this economically and environmentally suboptimal pathway.

Returning to the crux of our discussion, how does systematically and selectively breaking lignin into smaller and functional units seem? What if we valorise lignin to create an endless feedstock of selected aromatics – with way higher aggregated value – rather than merely burning it for energy generation? More than a provocation, we would like to invite you, for just a moment, to consider how much effort has been dedicated to a problem that should be like solving a Rubik's cube—not easy, but doable – yet has ended up as complex and unresolved as the Riemann hypothesis. This raises critical questions: Have we misjudged the nature of lignin itself? Should we shift focus from high selectivity depolymerization toward a more pragmatic solution?

Misunderstanding lignin: a non-polymer puzzle

We shall at this point address the elephant in the room: selective lignin depolymerization toward monoaromatic compounds. The very concept seems flawed, as it involves attributing a polymeric character to lignin (Fig. 1), which does not justify the unexpected difficulties encountered in breaking it down into – the same – repeating units. It is important to recognize that lignin is not a true polymer in the conventional sense, even though we often speak of ‘depolymerization’, which sounds way more appealing than ‘demacromolecularization’ – an awkward neologism anyway. This mischaracterization might be at the heart of the challenges we face. Lignin is a complex polyphenolic macromolecule, and acknowledging this complexity is crucial. Rather than simplifying it into repetitive units, we need to understand its heterogeneous nature to make meaningful progress in breaking it down.

According to the IUPAC guidelines, lignins are defined as macromolecular constituents composed of phenolic propylbenzene skeletal units linked randomly at various sites.⁵ Interpreting lignin as a macromolecule made up of multiple lignans—low molecular weight units formed by coupling moieties at their β-carbon atoms—may lead to the mistaken conclusion that lignin behaves like a traditional polymer. This misinterpretation masks lignin's inherent complexity and heterogeneity, emphasizing the need for rigorous scrutiny when defining its macromolecular nature. As Souto and Calado eloquently argued in a recent critical review,⁶ lignin's classification as a polymer is rooted in outdated conventions that fail to reflect its true structural nature. Lignin lacks the uniform repeating units that are characteristic of polymers, a point underscored by its high polydispersity and irregular bonding patterns. The distinction between native and isolated lignin emphasizes lignin's structural heterogeneity. Isolation processes, such as Kraft pulping or Organosolv methods, often alter the native lignin's structure, leading to condensed or crosslinked products that deviate significantly from its in situ form. This variability further challenges the misclassification of lignin as a polymer, which relies on a presumption of structural uniformity. Therefore, it is time to accurately recognize lignin as the second most abundant macromolecule in the world, an acknowledgment that could pave the way for more realistic and impactful valorization strategies.

Going to the core of the problem, lignin complexity arises not just from its macromolecular structure but from the diversity of inter-unit linkages formed during its macromolecularization.⁴ As illustrated in Fig. 2, lignin's structure involves various ether bonds (such as β-O-4′, α-O-4′, 4-O-5′) and C–C bonds (such as β-5′, β-1′, and 5-5′), resulting in an intricate network.⁷ These linkages, particularly the β-O-4′ bonds, dominate the lignin structure and are a primary target for depolymerization processes due to their relative lability.⁸

Fig. 2 Chemical structures of lignin phenylpropanoid units (hydroxycinnamyl alcohols or monolignols) and sub-units. Colour representation of the main lignin inter-unit linkage motifs with relative occurrence in different types of feedstocks based on Parthasarathi et al.,⁴ Rinaldi et al.,⁹ Wang and Wang,¹⁴ and adapted from Brienza et al.⁷

Native lignin, or protolignin, predominantly features β-O-4′ inter-unit linkages, accounting for 45%–84% of its structure.⁹ However, during lignin isolation, processes targeting ether bonds (such as α-O-4′, β-O-4′, and 4-O-5′) often cause disruption due to their significant bond dissociation energies (ranging from 209 to 346.9 kJ mol⁻¹).^6,9,10 The cleavage of these bonds initiates competitive condensation reactions, leading to the formation of carbon–carbon linkages and even complex structures such as lactones, which are particularly challenging to address.^6,11–13 This rearrangement highlights the difficulty in retaining the native structure of lignin post-cleavage. Moreover, lignin macromolecules with a higher content of C–C bonds, typical of feedstocks enriched in G-units, pose additional challenges, as the methoxy substitution in S-units limits the formation of these bonds.^8,9 The structural transformations and condensation reactions following β-O-4′ cleavage underline the heterogeneity of lignin and the necessity to recognize that cleaved and rearranged macromolecules are fundamentally altered.

The relative abundance of C–O and C–C linkages must be considered when selecting a depolymerization method, especially for processes aimed at maximizing the “yield of aromatic monomers”.¹⁴ Feedstocks with higher β-O-4′ content are ideal for such depolymerization processes, while others may require more robust catalytic methods to break the stronger C–C bonds. Therefore, lignin's structural complexity and the various factors affecting its macromolecularization, including the transport of monolignols to the cell wall, play crucial roles in determining the composition of inter-unit linkages.¹⁵ In sum, this variability in lignin composition across different biomass feedstocks implies that no universal depolymerization approach can be ever applied.^9,14

A pragmatic shift: from depolymerization to liquefaction?

Economic and Technological Realities

The depolymerization of lignin has been explored using a variety of methods, including thermal, chemical, biological, and electrochemical approaches.^8,16,17 Each method presents unique advantages and challenges, ranging from high energy demands in thermal processes to selectivity issues in chemical routes and scalability concerns in biological systems. Among these, electrocatalytic methods stand out as a promising pathway due to their ability to operate under mild conditions, their compatibility with renewable electricity, and the potential for tuneable product distributions. Given these advantages, this section focuses on electrocatalysis as a representative method to discuss the potential and challenges of lignin valorization, particularly in the context of a “liquefaction approach”. While other methods remain relevant and complementary, we believe electrocatalysis offers a compelling framework for addressing the complexity and heterogeneity of lignin in a scalable and sustainable manner.

It is unquestionable that electrocatalytic methods have significantly improved the selectivity of lignin depolymerization. However, recent investigations into bulk and thin film electrocatalysts for lignin breakdown have lagged the remarkable progress seen in the field of electrochemical CO₂ reduction.^18,19 For instance, in CO₂ electroreduction, faradaic efficiencies exceeding 80% have been achieved in some systems, with well-defined reaction pathways for producing high value-added chemicals. In contrast, lignin depolymerization struggles with much lower efficiencies and unpredictable product distributions. The complexity of lignin's structure—lacking the clear reaction pathways present in CO₂ reduction—suggests that we may need to reconsider whether lignin depolymerization can realistically achieve the same level of selectivity. This unavoidably brings us to the question of whether this disparity underscores an urgent need for a paradigm shift towards developing more complex electrocatalysts for lignin conversion.¹⁸ The answer is maybe. There is certainly room to enhance selectivity and control over specific bond cleavages within lignin macromolecules. Expanding the complexity and functionality of electrocatalysts could pave the way for significant breakthroughs in biomass conversion technologies. Nevertheless, a re-evaluation regarding the practical applications might be more pragmatic and realistic than pursuing a combination of high yields and selectivity towards aromatic monomers.

In this scenario, the lignin-first strategy^20–22 has garnered significant attention as it emphasizes the selective extraction and stabilization of lignin during biomass processing, producing high-purity aromatic monomers. However, lignin-first processes often require specific reactor designs, advanced catalysts, and significant hydrogen input, which can limit scalability and economic feasibility.²⁰ Despite these challenges, lignin-first strategies have been supported by techno-economic analyses (TEAs) and life-cycle assessments (LCAs), which show their potential to enhance overall biorefinery economics when integrated with polysaccharide valorization.^23,24 Excellent critical and tutorial reviews of TEAs can be found on recent publications.^23,24

At this point, we would argue that it is not necessarily the lignin-first^20,21 approach that will (not as a lonely fighter, at least) trigger future lignin-based technologies, but rather a “liquefy-first”-combined pathway, as a complementary and pragmatic alternative that prioritizes scalability and industrial compatibility. This strategy would focus on transforming low-purity, industrial lignin waste streams into chemically heterogeneous liquid feedstocks through processes, such as electro- or photocatalytic liquefaction. It is important to clarify that this approach is not meant diminish the value of existing technologies that utilize lignin in its solid form for high value-added products, such as carbon fibers, adhesives, or adsorbents. These applications effectively capitalize on lignin's inherent macromolecular properties. We emphasize the importance of addressing the inefficiencies of approaches that attempt selective depolymerization to produce specific compounds in high yields. By proposing liquefaction, we aim to expand the utility of lignin to a broader array of industrial applications, rather than confining its use to high-selectivity pathways that struggle with scalability and economic viability.

A liquefy-first strategy would align with existing industrial practices for processing heterogeneous feedstocks. For example, in the field of bio-oil refining, liquefaction processes such as hydrothermal liquefaction have successfully converted complex biomass into versatile liquid intermediates, which are then upgraded to fuels or chemicals.²⁵ Similarly, many studies have demonstrated that liquefaction of lignin can yield stable feedstocks that integrate with hydrodeoxygenation and refining units.^26,27 By adapting such approaches, a liquefy-first strategy would provide a scalable pathway to valorise industrial lignin waste streams. Unlike lignin-first approaches, liquefaction could leverage lignin's inherent heterogeneity to produce a broader spectrum of products, including aliphatic acids,²⁸ phenolic oligomers,²⁹ and other intermediates. These liquid feedstocks could then be refined using conventional chemical processes, bridging the gap between high-volume, low-cost applications and niche, high-value markets.

Electrocatalytic liquefaction is particularly suited to lignin due to its ability to operate under mild conditions, reduce energy inputs, while still targeting specific bond types within the macromolecule. Depending on the catalysts and reaction conditions, product distributions can be tuned toward not only phenolics^29,30 but also aliphatic acids.²⁸ Until recently, it was commonly believed that lignin depolymerization should always yield (specific) aromatic compounds, allowing for their separation and conversion into well-defined products.¹⁷ However, we have recently observed an unexpected and previously unreported phenomenon: full dearomatization during the reductive electrocatalytic hydrogenation of lignin in an aqueous medium using carbon electrodes.²⁸ It was found that this process produces four major aliphatic compounds—formic acid, acetic acid, 4-hydroxyvaleric acid, and levulinic acid, which can be easily separated from the depolymerization products. These acids are likely to be formed through hydrogenation and cleavage of key ether and C–C bonds within the lignin structure, reflecting the tuneable nature of electrocatalytic processes. Nonetheless, this approach still falls short of the ideal, which is to achieve selective depolymerization that yields (phenolic) compounds in high amounts.

In sum, the dichotomy in lignin markets—combustion for energy versus refined specialty products—underscores the need for intermediate solutions. Deconstructing lignin into smaller, functional units, while promising, faces economic hurdles due to the high costs of depolymerization and product refinement – on top of all cost related to delignification processes. For instance, delignification processes such as kraft or soda extractions incur annual costs of $47.09–$60.84 million, whereas Organosolv extraction is slightly more economical at $36.21 million per year.³¹ Membrane filtration techniques for lignin separation, while promising, have historically been associated with high costs due to significant challenges such as low flux and the large membrane area required, often exceeding 100 000 m². This has resulted in annualized membrane and equipment capital costs accounting for approximately $7.64 per kg, bringing the total separation cost to $8.20 per kg of lignin dry product—well above the product's market value of $1.11 per kg.²² However, advancements in process optimization have led to the development of a three-stage membrane system, which dramatically reduces the cost to just $0.38 per kg.²² This significant cost reduction addresses one of the key economic bottlenecks in lignin separation, making the process far more commercially viable and paving the way for broader industrial adoption.

While lignin-derived chemicals like vanillin, for example, with costs ranging from a minimum of USD 1250 per kg to 4400 per kg and a global market of USD 627 million in 2022,³² demonstrate market competitiveness, refined lignin chemicals often remain costlier than their fossil-derived counterparts. With high-purity lignin serving to niche markets, such as laboratory applications, commanding a premium price of up to $153 per 100 grams,³³ a liquefaction approach would obviously target low-purity lignin streams, typically priced between USD 100–300 per metric ton.³³ By converting these low-value streams into versatile liquid feedstocks, it would be feasible to integrate it into high-volume industrial markets, such as biofuels, bioplastics, and bulk chemicals.

Conclusions

Rethinking selective lignin depolymerization is not merely a scientific pursuit but an economic and technological necessity. Rather than adhering to rigid, high-selectivity pathways, a “liquefy-first” strategy embraces lignin's inherent complexity to produce heterogeneous liquid feedstocks suitable for diverse industrial applications. This approach sidesteps the limitations of achieving monomeric purity, instead leveraging existing industrial processes to refine these feedstocks at later stages. The production of monoaromatic substrates from lignin may lack competitiveness against fossil feedstocks in established industrial processes. However, liquefied lignin-derived feedstocks would present a more viable alternative, requiring fewer fundamental changes to industrial workflows. This might require rethinking the 170 years old well-established chemical processes; a scenario that may be still easier to realize than the depolymerization of lignin to monoaromatic compounds without the usage of fossil-based hydrogen.

Data availability

As this is a perspective article, a data availability statement is not applicable.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to thank the University of Wuppertal for the research support. We also extend our gratitude to the reviewers for their valuable insights, which have undoubtedly enriched the discussion in the final version of this perspective article.

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