1 Introduction
Driven by the dual forces of accelerating global energy transition and the urgency of the "dual carbon" goals, seeking renewable energy to replace fossil resources has become an inevitable requirement for contemporary development
[1]. As an important renewable organic carbon resource, biomass is increasingly highlighting its strategic position with the growing depletion of fossil resources and continuous innovation in conversion technologies
[2-3]. Among these, lignin is the second largest component in plant cell walls after cellulose and the most abundant renewable aromatic biopolymer in nature
[4]. Although its mass fraction in biomass is typically only 15%~30%, thanks to its unique aromatic structure, the energy it contains can account for nearly 40% of the total energy of biomass
[5]. However, in stark contrast to its huge resource potential, lignin consists of phenylpropane units randomly cross-linked through highly complex and robust C—O and C—C bonds, forming a three-dimensional network structure that is extremely difficult to selectively depolymerize (
Figure 1). This structural recalcitrance has long made it difficult to efficiently depolymerize and directionally convert, often resulting in its disposal as a low-value byproduct. Therefore, achieving precise cleavage of its chemical bonds and efficient reconstruction of its molecular skeleton to produce high-value fuels and chemicals is the core challenge in breaking through the bottleneck of full-value utilization of biomass
[6].
Hydrodeoxygenation (HDO) technology is widely recognized as the key solution to the challenge of high-value lignin utilization. Its essence is not a single chemical process, but a complex network transformation composed of multiple steps including hydrogenation, hydrogenolysis, dehydration, and alkylation.
[8-9]. To effectively drive this complex process, constructing multifunctional catalysts with metal active components supported on acidic porous carriers has become a core strategy. Among these, metal sites (such as Ru, Pt, Pd, Ni, etc.) are primarily responsible for hydrogen activation, aromatic ring hydrogenation, and C—O bond hydrogenolysis.
[10-12], while the Brønsted/Lewis acid sites provided by the carrier (such as zeolites) undertake functions of dehydration, rearrangement, and intermediate stabilization.
[13-15]. The diffusion behavior of reactants within the catalyst pores, along with the adsorption configuration and stability of intermediates, jointly determine the final product distribution of the reaction.
Based on the aforementioned bifunctional catalytic mechanism, the evolution of the reaction pathway first depends on the regulation of the hydrodeoxygenation (HYD) process (
Figure 2). Under thermodynamically dominant conditions of low temperature and high hydrogen pressure, the reaction tends to follow the HYD pathway: first saturating the aromatic ring via hydrogenation to generate cycloalkanol/ketone intermediates, followed by synergistic steps of acid-site-catalyzed dehydration and metal-site-catalyzed hydrogenation, ultimately achieving deep deoxygenation to produce cycloalkane fuels
[16]. In this process, the strength, quantity, and spatial distribution of acidic sites are crucial, as they regulate the competitive balance between dehydration rates and cracking side reactions
[17]. Meanwhile, hierarchical pore design and the spatial proximity arrangement of active sites have also been proven to significantly improve mass transfer efficiency and inhibit coke formation, thereby enhancing the overall stability and service life of the catalyst
[18-19]. However, it is worth noting that with the continuous upgrading of fuel performance requirements in the aerospace sector, the energy density of simple monocyclic alkanes prepared solely via traditional HYD pathways can no longer meet the stringent specifications for aviation kerosene.
To break through this energy density bottleneck, introducing C—C coupling reactions into the HDO system has become a cutting-edge innovative direction
[22-23]. This strategy utilizes the precise matching of metal and acidic sites to induce in situ alkylation or condensation of phenolic derivatives or their hydrogenated intermediates (such as cyclohexanol/cyclohexanone) on acidic sites, achieving a one-pot cascade transformation of "hydrogenation-deoxygenation-coupling"
[24-25]. This strategy not only accomplishes deep deoxygenation but also, through carbon chain growth and polycyclic skeleton construction, directly synthesizes bicyclic or polycyclic alkanes with densities significantly higher than conventional jet fuel in a single step, opening a new pathway for the leapfrog conversion of lignin into high-performance renewable aviation fuels.
Significantly different from the aforementioned pathways targeting saturated fuels, obtaining high-value-added aromatic chemicals such as benzene and toluene requires shifting the reaction to a direct deoxygenation (DDO) pathway dominated by high-temperature and low-hydrogen-pressure kinetics. The core challenge of this pathway lies in selectively cleaving C—O bonds while preserving the integrity of the aromatic ring skeleton.
[26]To this end, a dual strategy of electronic and geometric regulation is often adopted: on one hand, introducing oxophilic promoters (such as WO
x, MoO
x) to anchor oxygen-containing groups and regulate the metal d-band center, significantly reducing the C—O bond dissociation energy barrier.
[27-28]; on the other hand, utilizing the geometric cutting effect of alloying to break continuous metal arrays, eliminating the spatial sites required for flat-lying hydrogenation of aromatic rings, thereby physically blocking the hydrogenation pathway and achieving highly selective deoxygenation.
[29].
Based on the aforementioned background, this paper systematically reviews the research progress of metal-zeolite bifunctional catalytic systems in the directional conversion of lignin. The article first explores the HYD pathway, focusing on analyzing strategies employed by microporous and hierarchical zeolites to address mass transfer limitations, regulate product distribution, and enhance catalytic stability. Subsequently, it delves into the synergistic mechanisms of HDO and C—C coupling, elucidating how optimizing the spatial arrangement of active sites enables the directional construction of high-density polycyclic fuels. Finally, targeting the synthesis of high-value aromatics, it summarizes catalyst design principles for strengthening the DDO pathway by modulating the electronic properties of metals and the acidic environment of supports. By systematically organizing reaction networks and catalyst structure-activity relationships, this study aims to provide theoretical support for developing next-generation efficient and stable technologies for synthesizing lignin-based fuels and chemicals.
2 Hydrodeoxygenation of lignin phenolic derivatives to produce alkane compounds
The HYD pathway of lignin phenolic derivatives is essentially a tandem reaction channel of "hydrogenation saturation first, followed by stepwise deoxygenation," which can be regarded as a series of cascade transformation processes involving the synergy between metal sites and acid sites.
[30](
Figure 3). Under typical conditions, the substrate first undergoes aromatic ring/carbonyl hydrogenation on metal sites to generate saturated oxygen-containing intermediates; subsequently, dehydration, rearrangement, and C—O bond cleavage occur on the Brønsted/Lewis acid sites of the support, alternating with re-hydrogenation processes on the metal sites, ultimately converting primarily into oxygen-free cycloalkanes and short-chain alkanes.
[31]. The core to achieving efficient conversion via this pathway lies in the spatiotemporal matching of metal-acid functions and the precise control of reaction conditions: on one hand, higher hydrogen partial pressure and appropriate low temperatures are kinetically favorable for prioritizing aromatic ring saturation; on the other hand, the strength and distribution of acid sites on the support must be strictly controlled to drive dehydration/hydrogenolysis while avoiding excessive cracking and coking deactivation.
[32]. In short, only by establishing a dynamic balance between metal hydrogenation capacity and acid-site deoxygenation rates under specific process conditions can deep deoxygenation be achieved while suppressing carbon loss.
2.1 Regulation of Hydrodeoxygenation Performance of Metal Catalysts Supported on Microporous Molecular Sieves
Microporous molecular sieves (such as Beta, ZSM-5, and Y-type) have become classic supports for constructing metal-acid bifunctional hydrodeoxygenation (HDO) catalysts, owing to their regular crystalline pore channels and highly tunable acidity.
[33-34]The efficient performance of their catalytic activity hinges on the synergistic matching between acidic characteristics and topological structure: acidity determines the activation of functional groups and reaction pathways, while the pore topological structure influences the accessibility of reactant molecules, mass transfer efficiency, and selectivity changes induced by confinement effects.
In constructing an acidic microenvironment, fine-tuning the Si/Al molar ratio is an effective means to reshape the distribution and strength of Brønsted (B) and Lewis (L) acids. Existing studies indicate that suitable acid properties can significantly promote the activation and cleavage of oxygen-containing bonds. For example, Yao et al.
[35]pointed out that increasing L acid site density facilitates the coordination activation of oxygen-containing functional groups such as ether bonds, enabling Ru/HBeta to achieve efficient deoxygenation of phenols and their dimers under mild aqueous conditions. This trend has also been verified in Ni/zeolite systems; Feng et al.
[36]and Yan et al.
[37]have all confirmed that moderately increasing acid content accelerates intermediate dehydration, thereby reducing residual oxygenated species and improving hydrocarbon yields. Furthermore, the roles of B and L acids in the HDO process are not equivalent. Kinetic studies confirm that B acids, as key proton donors, exhibit a turnover frequency (TOF) far higher than L acids when catalyzing the dehydration of saturated alcohol intermediates, making them the absolute driving force for dehydration/dealcoholization steps
[38]. In contrast, the more important role of L acids lies in acting as electron pair acceptors to coordinate with functional groups such as hydroxyl and methoxy groups. This interaction not only polarizes C—O bonds but also strongly anchors oxygenated intermediates during the reaction process, significantly lowering their deoxygenation energy barriers to promote complete C—O bond cleavage
[39-40]. Thus, the B/L acid ratio not only determines the surface acid distribution but also directly constitutes a regulatory switch for the evolution of reaction pathways: abundant L acid sites provide strong anchoring effects, driving intermediates to completely convert into cycloalkanes; whereas a decrease in L acid density makes partially hydrogenated oxygenated species more prone to desorb and escape from the catalyst surface, thereby offering possibilities for obtaining high-value-added chemicals
[41]. However, enhancing acidity is not always better. A subtle competitive balance exists between deep deoxygenation and side reactions: on one hand, excessively strong B acids easily induce deep cracking and condensation leading to coke formation; on the other hand, overly strong or excessive L acids cause strong adsorption of substrates on the surface, even shielding neighboring B acid sites, thereby inhibiting further intermediate dehydration and inducing pore blocking and deactivation. Therefore, acid quantity and strength must be strictly controlled within an effective deoxygenation window to balance conversion efficiency and catalyst stability
[37,42].
In addition to acidity regulation, the spatial matching between topological structure and reactant size also determines the quality of catalytic performance. For bulky phenolic substrates, although MFI-type zeolites (such as ZSM-5) exhibit significant shape-selective effects, their narrow pores are highly prone to diffusion limitations and pore mouth blocking. In contrast, supports with larger pore sizes and high connectivity are more favorable for allowing large molecules to enter the pores and undergo continuous conversion. Wang et al.
[43]'s comparative study shows that when using HBeta zeolite with three-dimensional interconnected channels as a support, its excellent pore connectivity can significantly reduce mass transfer resistance, thereby making the Ni/HBeta catalyst superior to Ni/SiO
2and other microporous zeolite systems in terms of activity, selectivity, and stability.
Overall, metal-acid bifunctional systems based on microporous molecular sieves have demonstrated excellent performance in the conversion of model phenols and small molecular dimers. However, this system has a clear size adaptability boundary: when substrates are extended to macromolecules or complex oligomers in real bio-oil, mass transfer limitations within the micropores and insufficient site accessibility will gradually become efficiency bottlenecks. Therefore, subsequent chapters will focus on introducing mesopores/macropores into the microporous framework to construct hierarchical pore systems, alleviating diffusion limitations through structural design, thereby expanding the applicability of catalysts in real lignin oil HDO reactions.
2.2 Mass transfer regulation of metal catalysts supported on hierarchical zeolites in hydrodeoxygenation
The key to resolving the inherent contradiction between mass transfer limitations in microporous frameworks and insufficient site utilization lies in constructing a hierarchical pore system integrating micropores, mesopores, and macropores. This strategy marks a fundamental upgrade in catalyst design philosophy: shifting from one-dimensional "site intensity regulation" to multi-scale structural engineering emphasizing "mass transfer-reaction synergy"
[44-45].
First, in the dimension of physical mass transfer optimization, the introduction of mesopores and macropores plays a dual role: on one hand, they provide interconnected rapid diffusion channels, significantly shortening the diffusion path for large-molecule substrates; on the other hand, they construct buffer spaces to accommodate bulky intermediates and coke precursors, thereby effectively reducing the risk of pore blockage while retaining the shape-selective advantages of micropores
[46-47]. Existing research provides ample evidence for this: Wang et al.
[48]developed a nano-sponge-like Ni/NPZ catalyst, and Gan et al.
[49]constructed a connected macroporous ZSM-5 zeolite; thanks to their open pore networks, these materials not only significantly enhanced the accessibility of substrates to deep active sites but also served as effective buffers to reduce the coking rate; Tian et al.
[50]'s research also confirmed that the mass transfer enhancement brought about by the introduction of mesopores can significantly improve the cyclic stability of catalysts.
A more critical breakthrough lies in the fact that the hierarchical pore configuration overcomes the spatial limitations of micropores, providing a more accessible chemical space proximity platform for coupling bifunctional sites in macromolecular systems. Through precise encapsulation or confinement strategies, metal species can be anchored within specific cavities and constructed into tight interfaces with neighboring Brønsted acid centers, thereby achieving an efficient relay of hydrogenation and dehydration steps at the atomic scale.
[51-52]. This in-situ conversion mechanism based on nanoconfinement is a significant source for enhancing target selectivity: He et al.
[53]and Jin et al.
[54]respectively achieved synchronous encapsulation of Ru clusters or Pd with acid sites inside MWW and Y zeolite crystals (
Figure 4), utilizing extremely close spatial proximity to ensure that intermediates are rapidly converted immediately after formation, avoiding energy and mass losses as well as side reactions caused by long-range migration. Furthermore, the confined environment can effectively suppress metal sintering, thereby further amplifying the bifunctional synergistic effect between metal and acid.
[55-56].
In summary, the design philosophy of HDO catalytic systems is undergoing a profound evolution from "single microporous bifunctionality" to "hierarchical pore structures and precise spatial synergy." Facing the complex macromolecular system of real lignin oil, constructing hierarchical pore networks has been proven to be the inevitable path to breaking through diffusion bottlenecks and delaying deactivation due to coke deposition. However, process optimization cannot mask the intrinsic limitations of product performance. We must clearly recognize that mono-cyclic or linear alkanes obtained solely through traditional HDO pathways are often unable to meet the stringent requirements for volumetric energy density in high-performance aviation kerosene due to their molecular structure constraints. Therefore, efficient deoxygenation is by no means the endpoint, but rather a new starting point. The core challenge for the next stage of catalyst and reaction system design lies in how to introduce C–C coupling or alkylation mechanisms while achieving deep deoxygenation, simultaneously driving carbon skeleton reconstruction and carbon chain growth, thereby accomplishing the leap from simple fuels to high-energy fuels.
3 Preparation of polycyclic alkanes via HDO and C—C coupling
To overcome the structural limitations of monocyclic alkanes in volumetric energy density, constructing polycyclic skeletons has become an inevitable direction for developing high-performance aviation fuels
[57-58]. Polycyclic alkanes, with their higher packing density and volumetric heating value, effectively compensate for the performance deficiencies of traditional hydrodeoxygenation (HDO) products and are regarded as key alternatives surpassing conventional fuel components
[59]. Taking bicyclohexyl as an example, its room-temperature density reaches as high as 0.89 g/mL, significantly superior to conventional aviation kerosene RP-3 (approximately 0.78 g/mL), thus making it a benchmark for the molecular design of high-density jet fuels
[33].
3.1 Kinetics and microenvironment regulation via metal-acid site synergy
Unlike traditional petrochemical routes, modern biomass conversion technologies provide new pathways for the synthesis of polycyclic alkanes: using lignin-derived phenolic compounds and platform molecules such as cyclohexanone/cyclohexanol as feedstocks, synergistic conversion via hydrodeoxygenation and C—C coupling enables the production of polycyclic alkanes with both high density and good low-temperature fluidity on a renewable route
[60-62]. The key to achieving this process lies in establishing a kinetic balance between metal hydrogenation and acid-catalyzed coupling, rather than relying solely on strong acid or strong metal sites. Specifically, metal sites must moderately hydrogenate oxygen-containing aromatic fragments to generate intermediates that can be further activated by acid sites; the acid sites then convert them into electrophilic species such as olefins or carbocations, thereby triggering alkylation or condensation reactions to extend the carbon chain
[63]. This balance is extremely delicate: if metal hydrogenation proceeds too rapidly, the intermediates will be quickly deep-hydrogenated into inert monocyclic alkanes, thus cutting off the coupling pathway
[64].
Existing studies have confirmed that precise regulation of the metal-acid synergistic effect and the reaction microenvironment can effectively direct the target pathway. Zhao et al.
[65]In an aqueous Pd/HBeta system, by coordinating the hydrogenation rate of Pd with the acid-catalyzed alkylation rate within the pores, a tandem reaction of carbon chain growth and deep deoxygenation was achieved. To elucidate the microscopic essence of the alkylation step, the team combined spectroscopy and theoretical calculations to confirm that the cyclohexyl carbocation is the key electrophilic intermediate for aromatic ring alkylation. Its formation energy barrier highly depends on the solvent microenvironment: the coupling pathway dominates in non-polar media, whereas solvation effects in the aqueous phase significantly inhibit carbocation formation.
[24]. Based on this mechanistic understanding, the research team further proposed a segmented regulation strategy: utilizing the spacious channels of large-pore zeolites (such as HBeta and HY), combined with a process of "front-end dehydration to create electrophiles and back-end low-alcohol concentration alkylation," effectively avoiding the accumulation of inert species and catalyst deactivation.
[25]. Furthermore, Shen et al.
[66]utilized hierarchical pore HBeta supported with trace amounts of Pt to enhance metal-acid synergy through the mass transfer advantages of hierarchical channels, completing the construction of polycyclic skeletons in a one-pot system.
3.2 Spatial Synergy of Metal-Acid Sites and Reaction Pathway Orientation
To achieve the aforementioned kinetic balance in a multi-step tandem system, relying solely on the matching of intrinsic activities between metal and acid sites remains insufficient. The spatial proximity of metal-acid sites, as a key geometric parameter converting physical distance into reaction time, also profoundly influences intermediate migration, pathway competition, and product distribution. For the pathway involving the complete conversion of lignin derivatives into monomeric cycloalkanes via HDO, higher spatial proximity generally favors shortening the migration distance of oxygen-containing intermediates between the two types of sites, thereby strengthening the "hydrogenation-dehydration-rehydrogenation" cascade relay. Cho et al.
[67]Through a microporous confinement encapsulation strategy, it was discovered that embedding metal nanoparticles into zeolite channels can induce the enrichment of framework acid sites around them, forming a halo effect. This significantly accelerates the tandem deoxygenation process and suppresses condensation deactivation caused by long-range diffusion of intermediates.
However, when the target shifts from monomeric HDO to HDO coupled with C—C coupling to construct high-density polycyclic fuels, the optimal spatial configuration no longer simply follows the rule that closer is better. If metal and acid sites are excessively close, intermediates with coupling potential may be prematurely hydrogenated by adjacent metal sites before completing condensation, thereby terminating carbon chain growth. Cho et al.
[68]demonstrated in the tandem conversion of furfural and acetone that when Pt is directly loaded onto the outer surface of a molecular sieve, substrates tend to undergo monomeric hydrogenation preferentially, hindering C—C coupling; whereas when Pt is confined deep within micropores, the outer acidic channels act as a physical barrier, forcing substrates to complete aldol condensation first before diffusing inward to metal sites for subsequent hydrogenation, thereby achieving an 87% C8 coupling yield. Based on similar understanding, He et al.
[69]further pointed out that moderately increasing the spatial distance between metal and acid centers, or even constructing physical isolation at the micrometer or macroscopic scale, helps avoid premature intervention by the strong hydrogenation capability of metals on the carbon-extension pathway. Meanwhile, in situ spectroscopy provides direct evidence for revealing coupling mechanisms within pores. Zhao et al.
[70]utilized In situ
13C MAS NMR to dynamically track the alkylation mechanism of phenol and cyclohexanol on H-BEA molecular sieves under real liquid-phase conditions, directly capturing transient cyclohexyl carbocation intermediates formed by olefin protonation, and revealing that protonated alcohol dimers/trimers competitively occupy Brønsted acid sites, inhibiting further adsorption and protonation of olefins. This result indicates that efficient C—C coupling does not simply rely on acid site promotion, but depends on whether key electrophilic intermediates can be generated, captured, and converted in time, and whether surface-accumulated species are effectively suppressed.
In summary, the HDO process based on C—C coupling is evolving from mere phenomenological observation to rational directional design. An ideal catalyst must simultaneously possess moderate hydrogenation capability to provide couplable precursors and acid catalytic ability to drive chain growth. Building on this, macroporous/hierarchical pore structures provide the necessary reaction space and accessibility to active sites, while the optimization of the spatial configuration between metal and acid sites supports the synthesis system of lignin-based high-density aviation fuels by regulating the migration sequence and residence time of intermediates between these two types of sites. It is worth emphasizing that the value of lignin as a natural aromatic polymer extends far beyond fuels: if the pathway shifts from deep hydrogenation to selectively retaining aromaticity, it holds promise for further expansion into the synthesis of high-value-added aromatic chemicals such as benzene and toluene, which is the focus of the next section.
4 Hydrodeoxygenation of lignin phenolic derivatives to produce aromatic compounds
Unlike the fuel synthesis pathways pursuing deep hydrogenation mentioned above, if high-value aromatics such as benzene, toluene, and xylene (BTX) are targeted as products, the reaction pathway must be strictly confined to the direct deoxygenation (DDO) channel, which removes oxygen atoms while preserving the aromatic ring structure
[71]. This presents a challenge requiring extremely high chemical selectivity, as the catalytic system must not only precisely cleave high-bond-energy C—O bonds but also effectively suppress the inherent tendency of aromatic rings to undergo excessive hydrogenation
[72]. This process faces dual constraints of thermodynamics and kinetics: on typical metal catalyst surfaces, the activation energy for aromatic ring hydrogenation is usually far lower than the energy barrier for aryl C—O bond cleavage; meanwhile, under suitable hydrogen-rich conditions, cycloalkanes possess high thermodynamic stability, causing the reaction to easily fall into the "potential well" of cycloalkane formation. Therefore, to achieve DDO pathway dominance in such a competitive landscape, it is necessary to finely tune reaction conditions as well as the electronic structure and geometric configuration of the catalyst, lowering the C—O bond cleavage energy barrier and narrowing or even reversing the energy barrier difference between this and the aromatic ring hydrogenation pathway, thereby establishing the kinetic advantage of preferential C—O bond activation
[73].
Mechanistic studies at the microscopic scale provide theoretical guidance for solving this challenge. DFT calculations and surface science experiments consistently indicate that the adsorption configuration of the substrate is key to determining the reaction pathway. When the aromatic ring adsorbs in a flat-lying mode, strong interactions between the π system and the metal d-band readily induce hydrogenation; whereas when the substrate adopts a tilted or vertical configuration, the C—O bond is more likely to approach active sites and undergo cleavage. For example, Duong et al.
[74]calculations revealed that on Rh(533) step sites, the activation barrier for C—O bond cleavage in m-cresol (approximately 122 kJ/mol) is significantly lower than that on flat Rh(111) surfaces (approximately 164 kJ/mol), demonstrating the significant regulatory effect of specific geometric sites on reaction barriers. Furthermore, tunable Lewis acid sites have also been confirmed to interact with lone pair electrons on oxygen atoms, inducing the substrate to adopt a non-coplanar adsorption configuration favorable for C—O bond cleavage, thereby suppressing aromatic ring saturation at the source. As a supplementary example of acid site-regulated deoxygenation branching, Pan et al.
[75]combined Operando PEPICO with in situ solid-state NMR techniques to dynamically track the deoxygenation process of guaiacol on FAU zeolites (
Figure 5). Operando PEPICO directly captured short-lived intermediates such as methyl radicals, catechol, and fulvenone, revealing a reaction network where guaiacol, via catechol, competes between the phenol formation pathway and the fulvenone formation pathway. In situ solid-state NMR further confirmed that optimizing the distribution of adjacent Brønsted acid sites by adjusting the silicon-to-aluminum ratio can alter the surface coordination configuration of catechol and suppress fulvenone formation. This greatly alleviates catalyst deactivation due to coking and achieves a five-fold increase in the selectivity of target aromatic products, providing strong evidence for understanding pathway switching during the direct deoxygenation of lignin-derived phenols into aromatics.
Based on the above mechanistic understanding, introducing oxophilic promoters has become the preferred strategy for regulating the electronic environment of metals and enhancing DDO selectivity. Such species (e.g., WO
x, MoO
x, NbO
x, etc.) play a dual role in catalytic systems: on one hand, they preferentially anchor oxygen-containing groups in the substrate, significantly lowering the dissociation energy barrier of C—O bonds; on the other hand, they modify the d-band center of active metals through electronic modulation, weakening their excessive feedback bonding with the aromatic π system. Multiple studies have confirmed the effectiveness of introducing WO
x, MoO
xand other oxophilic species. For example, Huang et al.
[28]and Yang et al.
[76-77]have all confirmed the effectiveness of this strategy: introducing W/Mo oxides into Ru- and Ni-based catalysts utilizes their preferential anchoring of oxygen-containing groups and electronic modulation of the metal d-band center to successfully suppress aromatic ring hydrogenation while significantly reducing the C—O bond dissociation energy barrier, achieving highly selective aromatic hydrocarbon synthesis; changes in XPS binding energies and a significant decrease in CO chemisorption amounts corroborate from electronic/adsorption perspectives that the metal surface properties are regulated, favoring reduced excessive adsorption and over-hydrogenation of aromatic rings (
Figure 6). Furthermore, Wang et al.
[27]demonstrated that WO
x-Pt/SBA-15 can stably couple alkane dehydrogenation with m-cresol HDO under high-pressure conditions with in situ hydrogen supply from light alkanes, highlighting the process adaptability and potential for coupled application of the WO
x-metal interface.
In addition to the regulation of electronic effects, reconstructing the geometric arrangement of the surface via alloying or intermetallic compounds is also an effective strategy for suppressing aromatic ring hydrogenation. Its essence lies in using geometric isolation to weaken the multi-atom continuous sites required by aromatic rings. Flat adsorption and stepwise hydrogenation of aromatic rings usually rely on large continuous metal arrays, whereas C—O bond cleavage has lower requirements for site scale and can often occur on small atomic clusters. Therefore, when a second metal (or an inert component) is inserted into the active metal lattice, the originally coherent metal surface is segmented into isolated sites or small clusters, making it difficult for aromatic rings to maintain flat adsorption and forcing them to adopt adsorption configurations more favorable for C—O activation. Taking Zheng et al.
[29]'s Ni
3Ga/SiO
2as an example, after Ga insertion, the continuous Ni array is cut into smaller active domains, accompanied by certain charge transfer, thereby significantly suppressing flat adsorption of aromatic rings and markedly improving benzene selectivity in the HDO of anisole. Similarly, Xu et al.
[78]utilized the synergy between FeNi alloy sites and support acidity in Fe/Ni/HBeta to preferentially promote C—O bond cleavage and methyl transfer processes, thereby achieving the directional conversion of guaiacol to BTX while effectively suppressing aromatic ring hydrogenation.
In summary, the conversion of lignin-derived phenolics into high-value aromatics is essentially a precise game between the competing reactions of "C—O bond activation" and "aromatic ring hydrogenation." Whether by introducing oxygen-affinity promoters to modulate electronic properties or utilizing alloying strategies to reconstruct the distribution of surface geometric sites, the core objective is to construct a catalytic microenvironment that disfavors aromatic ring adsorption while highly activating C—O bonds. Combined with molecular sieve supports possessing appropriate pore sizes and Lewis acid strengths, this rational design strategy based on microscopic reaction mechanisms provides solid theoretical support and a practical paradigm for overcoming technical bottlenecks in producing aromatic chemicals from biomass.
5 Hydrodeoxygenation for Upgrading Real Lignin-Derived Oils
Although significant progress has been made in research on the metal-zeolite bifunctional catalytic mechanism based on model systems, the direct conversion of real lignin-derived oils with extremely complex compositions into high-quality liquid fuels is the key to advancing toward industrial applications. In recent years, catalytic upgrading of real systems has shown initial success. Existing studies indicate that multifunctional catalytic systems can not only act on native lignin depolymerization oils but also further regulate the products to the C8–C16 naphthene range required for aviation fuels.
[79]. Regarding the inert C—C bonds widely present in real lignin, Luo et al.
[80]constructed a catalytic system synergizing Pt dehydrogenation/hydrogenation functions with zeolite Brønsted acid sites. After initially cleaving easily breakable C—O bonds, this system induced the formation of active alkenyl groups adjacent to inert C—C bonds. Subsequently, via an acid-catalyzed β-scission pathway, it successfully cleaved typical stubborn C—C bonds such as 5-5′, β-1′, β-5′, and β-β′, thereby significantly enhancing the conversion efficiency of real lignin to monocyclic hydrocarbons and demonstrating potential for producing naphthenes and aromatics within the gasoline/aviation kerosene range.
However, compared to model systems, the coexistence of monomers, oligomers, strongly polar oxygenated compounds, and in-situ generated water in real oil products poses more severe challenges for catalytic upgrading. On one hand, disordered cross-condensation of reactive intermediates on acid sites occurs more readily, rapidly generating coke precursors; on the other hand, the large kinetic diameter of oligomers induces diffusion limitations and pore mouth blocking on the surface of microporous molecular sieves, while the high-temperature hydrothermal environment further accelerates dealumination of the molecular sieve framework and loss of active metals, ultimately leading to rapid catalyst deactivation.
[81]. Thus, the key contradiction in real systems is no longer merely insufficient activity, but rather how to simultaneously achieve deep deoxygenation, smooth mass transfer, and long-term stable operation under complex feedstock conditions.
Addressing the aforementioned bottlenecks, recent studies have begun to propose solutions from two aspects: catalyst spatial configuration and reaction timing. In the spatial dimension, Zhang et al.
[82]constructed a "spatially orthogonal" MoCu/HZSM-5@SiO
2core-shell catalyst. Among them, the outer layer Mo
0efficiently removes methoxy groups via planar adsorption, avoiding excessive hydrogenation of the aromatic ring; the inner ultra-thin HZSM-5 nanosheets provide strong acid sites for dealkylation, significantly shortening the diffusion path of intermediates; the intermediate SiO
2shell acts as a physical barrier, forcibly blocking oligomers from poisoning internal acid centers, fundamentally mitigating condensation coking. Real birch lignin oil achieved a phenol mass yield of 20% in one-step conversion and could operate stably for 50 h. In the temporal dimension, addressing the limitation that one-pot methods easily trigger side reactions, Zhu et al.
[83]proposed a stepwise control strategy. First, a Ni/CaO-HZSM-5 bifunctional catalyst was used to preferentially cleave large-molecule C—O ether bonds to achieve initial depolymerization; subsequently, the liquid-phase intermediates were transferred to Ni/HZSM-5 for deep hydrogenation and upgrading. By separating the processes of macromolecular depolymerization and monomer deoxygenation in the temporal dimension, disordered competition of reactive intermediates within the same stage was effectively avoided, increasing the overall deoxygenation rate to 71% and achieving the directional synthesis of n-hexanol.
Overall, the hydroconversion of real lignin-derived oils has progressed from feasibility verification at the model molecule level to targeted upgrading and stable operation under complex feedstock conditions; however, its core still lies in constructing a metal-zeolite bifunctional system that balances site accessibility, spatial isolation, and resistance to deactivation. In the future, only by integrating feedstock fractionation, catalyst configuration optimization, and continuous stable operation can the catalytic upgrading of real lignin oils truly advance from laboratory research to practical application.
6 Conclusion and Outlook
In metal-zeolite bifunctional catalytic systems, the hydrodeoxygenation (HDO) process of lignin-derived phenolics is essentially a complex reaction network synergistically coupled by hydrogenation/hydrogenolysis at metal sites and dehydration, rearrangement, and other steps mediated by zeolite acid sites; under specific conditions, this network can be further coupled with C—C coupling processes to achieve carbon skeleton growth. By regulating the competitive relationship between HYD and DDO pathways, directional switching of products among cycloalkane fuels, polycyclic high-density components, and aromatic chemicals can be realized. Specifically, in alkane preparation, the key to achieving deep deoxygenation and suppressing coke formation lies in the dynamic matching of metal hydrogenation and acid-site deoxygenation rates, as well as the regulation of intermediate diffusion by channel topology. Hierarchical pore networks and confined encapsulation can effectively break through the size limitations of microporous zeolites, enhancing site accessibility and amplifying cascade relay effects through high-speed mass transfer channels and nanoscale neighboring sites. In constructing polycyclic fuels, the controllability of C—C coupling depends on the subtle balance between moderate metal hydrogenation (generating coupling precursors) and acid-catalyzed condensation rates, significantly influenced by the solvent microenvironment and intra-pore confinement effects. For the directional synthesis of aromatics, the core of DDO is to establish a kinetic advantage prioritizing C—O activation while suppressing aromatic ring hydrogenation; this can be achieved through strategies such as alloying geometric cutting to weaken continuous hydrogenation sites, oxygen-affinity promoters to regulate electronic structures, and Lewis acid-induced non-planar adsorption of reactants. To advance HDO technology for lignin-derived phenolics from model studies to real-world applications, future research must focus on the following four key directions to achieve the leap of catalytic systems from mechanistic exploration to industrial feasibility.
(1) Spatial confinement and synergistic enhancement of dual-functional sites: To improve catalytic efficiency and suppress side reactions, catalyst design should shift from macroscopic co-loading to nanoscale spatial confinement. By employing core-shell structures or intra-pore confinement techniques, the spatial proximity and electronic interactions between metal and acid sites can be precisely regulated. This leverages the nano-proximity effect to achieve efficient synergy in hydrogenation, deoxygenation, and coupling. This strategy not only avoids efficiency losses caused by long-range migration of intermediates but also effectively inhibits active phase sintering and coke formation, thereby simultaneously enhancing the selectivity for deep deoxygenation and C—C coupling.
(2) Deep integration of reaction kinetics and in-situ characterization: Given the intense competition between DDO and HYD pathways during HDO and the involvement of multi-step tandem reactions, future research must closely combine macroscopic reaction kinetics (such as apparent energy barriers and hydrogen partial pressure sensitivity analysis) with microscopic in-situ spectroscopic characterization. The focus lies in quantitatively elucidating the specific impacts of intrinsic properties—such as metal hydrogenation capability, acid site dehydration/alkylation activity, and substrate adsorption configurations—on the rate-determining steps of the reaction, thereby dynamically revealing the evolution mechanisms of key intermediates. This will facilitate the construction of predictive "structure-performance" mapping relationships, providing universal theoretical guidelines for the rational design of catalysts.
(3) Construction of Hierarchical Pore Systems and Synergy between Mass Transfer and Reaction: Addressing the challenges of mass transfer limitations and coking caused by the complex composition and large molecular size of real lignin oil, it is crucial to create a hierarchical pore system integrating micropores, mesopores, and macropores. This strategy retains the shape selectivity of micropores while utilizing the meso/macropore network to provide rapid diffusion channels and buffer space for coke deposition. The design focus should lie in the synergy between pore structure and active site distribution: anchoring metal sites within open pores to enhance accessibility for large molecules, and precisely regulating acid sites in confined regions to suppress excessive condensation, thereby achieving high activity, high selectivity, and long-term stability under complex operating conditions.
(4) Low-cost, high-stability non-noble metals and process optimization: To overcome the industrial bottlenecks of high costs and insufficient hydrothermal stability in noble metal systems, future efforts should focus on developing highly active non-noble metal alloy catalysts based on nickel, cobalt, and iron, while enhancing their resistance to coking and poisoning through surface and interface modification. Simultaneously, medium engineering and hydrogen supply strategies must be integrated into the holistic design of catalysts. This includes exploring biphasic solvent systems, mild in-situ hydrogen supply, and continuous flow processes, thereby advancing HDO technology from laboratory performance validation toward the goal of optimal cost-effectiveness in industrial applications.
Natural Science Foundation of Shandong Province， China(ZR2023MB101)
Special Program for Basic Research of CNPC(2023ZZ36)
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