Recent Advances in Novel Catalytic Hydrodeoxygenation Strategies for Biomass Valorization without Exogenous Hydrogen Donors—A Review

Abstract

Driven by the growing energy crisis and environmental concerns regarding the utilization of fossil fuels, biomass liquefaction has emerged as a highly promising technology for the production of renewable energy and value-added chemicals. However, due to the high oxygen content of biomass materials, biocrude oil produced from liquefaction processes often contains substantial oxygenated compounds, posing challenges for direct downstream applications. Catalytic hydrodeoxygenation (HDO) upgrading with hydrogen donors is crucial for improving the quality and applicability of biomass-derived fuels and chemicals. The costs, safety, and sustainability concerns associated with high-pressure gaseous hydrogen and organic molecule hydrogen donors are driving researchers to explore alternative and innovative biomass hydrodeoxygenation approaches without exogenous hydrogen donors. This review offers an overview of the recent developments in catalytic hydro-liquefaction and hydrodeoxygenation methods for biomass valorization without external hydrogen donation, including catalytic self-transfer hydrogenolysis using endogenous hydrogen in biomass structure, in situ catalytic hydrodeoxygenation employing water as the hydrogen donor, and in situ hydrodeoxygenation via water splitting assisted by zero-valent metals. The in situ hydrogen supply mechanisms and the impact of various hydrodeoxygenation catalysts on hydrogen donation efficiency using endogenous hydrogen are summarized in detail in this work. Furthermore, the current obstacles and future research demands are also discussed in order to provide valuable recommendations for the advancement of biomass utilization technologies.

1. Introduction

The global increase in population, rapid expansion of industrialization, and accelerated urbanization have led to a significant annual rise in human energy demand. For a prolonged period, fossil fuels represented by coal, petroleum, and natural gas have fulfilled the substantial worldwide demand for energy and chemicals [1]. However, the excessive reliance on fossil fuels has led to a range of sustainability issues, including negative environmental impacts, greenhouse gas emissions, and energy crises resulting from the uneven distribution and depletion of fossil fuel resources [2]. The imperative for pioneering technological innovations and ensuring a steadfast provision of renewable and alternative energy has emerged as paramount for facilitating sustainable development on a global scale. Due to the advantages of widespread distribution, renewability, and carbon neutrality, biomass has been recognized as one of the most promising renewable energy sources, attracting extensive attention over the past few decades and is mainly composed of woody residues, agricultural crop wastes, algae, livestock manure, and municipal organic wastes [3,4]. Biomass-derived energy and chemicals are believed to hold significant promise for establishing a sustainable circular carbon economy, yielding economic, social, and environmental benefits [5].

Efficient and cost-effective conversion methods are crucial for maximizing the economical and environmentally sustainable utilization of biomass resources. Thermochemical conversion routes, including combustion, gasification, pyrolysis, and liquefaction, have garnered significant interest due to their high energy efficiency and the capability to utilize a wide range of biomass feedstocks [6,7]. Among various thermochemical conversion methodologies, the liquefaction process employs organic solvents or environmentally friendly subcritical water as the reaction medium, facilitating the conversion of biomass feedstocks into bio-oil and high-value chemicals under comparatively mild reaction conditions. Moreover, the liquefaction process obviates the need for energy-intensive drying pretreatment of high-moisture biomass feedstocks, thereby presenting significant energy efficiency and cost advantages [8,9]. However, despite the higher calorific value and lower oxygen content of bio-oil produced through biomass liquefaction compared to fast pyrolysis oil, the abundant oxygen-containing functional groups inherent in the macromolecular structure of biomass feedstocks result in the presence of significant oxygenated derivatives in the bio-oil products. Consequently, the resulting bio-oil exhibits undesirable properties, such as high viscosity, strong corrosiveness, and poor stability, making it challenging for its further application as fuel [10,11].

Hydrodeoxygenation (HDO) presents an attractive method for upgrading bio-oil into high-quality fuels and high-value chemical products, including decarboxylation, hydrogenation, hydrogenolysis, and dehydration reactions, which collectively lead to a significant reduction in oxygen content. The overall chemistry of the HDO process involves the interaction of hydrogen with oxygenated compounds derived from biomass, resulting in the production of more deoxygenated products and water. In addition, the introduction of a hydrogen donor in the biomass liquefaction process can suppress the repolymerization of reaction intermediates by providing a reductive reaction medium, thereby reducing the formation of char by-products [5,12]. In conventional HDO processes, external gaseous hydrogen (H₂) is commonly utilized as the hydrogen donor due to its widespread availability and facile activation on numerous metal surfaces [13]. Nevertheless, owing to the limited solubility of molecular hydrogen in most liquefaction solvents, achieving favorable HDO efficiency often necessitates high hydrogen pressure. The dissociation of hydrogen molecules on metal sites on the catalyst surface also demands harsh reaction conditions, inevitably accelerating the leaching and sintering of active sites. At an industrial scale, handling high-pressure hydrogen gas and maintaining high reaction temperatures during biomass liquefaction pose significant infrastructure costs, and the considerable safety risks associated with the transportation and storage of pressured hydrogen present additional challenges for the development of biomass HDO technology [11,14,15]. Therefore, exploring new strategies as alternatives to external gaseous hydrogen would be highly advantageous.

Catalytic transfer hydrogenation (CTH) employing liquid organic molecules as hydrogen donors has attracted growing research attention due to its better sustainability [16]. Several biomass-derived organic hydrogen donors with a relatively high hydrogen storage density, such as formic acid (FA) and alcohols (e.g., methanol, ethanol, and isopropanol), have been successfully applied in the catalytic transfer hydrogenation of biomass feedstocks [17]. Unlike the HDO reaction using gaseous hydrogen, the CTH process involves the dehydrogenation of organic hydrogen donors and the hydrogenation of the oxygenated reaction intermediates, and efficient heterogeneous catalysts with multiple active sites (e.g., metals, acids, bases) are generally used together with the organic hydrogen donors [18,19]. The utilization of liquid organic hydrogen donors mitigates the safety concerns associated with handling high-pressure flammable gaseous hydrogen and enhances the solubility of the hydrogen donor within the liquid-phase reaction system, thereby reducing the complexity and cost of the HDO procedure [20,21]. Nevertheless, the primary obstacle to the current catalytic transfer hydrogenation approaches employing exogenous hydrogen donors lies in the separation of the undesired dehydrogenation products produced from organic hydrogen donors, resulting in increased energy consumption [13,22]. Moreover, the application of liquid organic molecules typically derived from fossil fuels or biomass as hydrogen donors requires further consideration in terms of cost and sustainability.

Recently, some investigations have confirmed that catalytic self-transfer hydrogenolysis can be achieved only using endogenous hydrogen originating from the inherent chemical groups in the macromolecular structure of lignocellulosic biomass [23,24,25]. This approach ensures a self-sustaining hydrogen supply, and enhances the atomic utilization efficiency. The complex biomass liquefaction process, where the energy required for the separation and purification of final products is reduced. For example, the abundant aliphatic hydroxyl and methoxy groups in the lignin structure can serve as in situ hydrogen donors, promoting biomass depolymerization and enhancing the intramolecular catalytic transfer hydrogenation reaction. Employing the inherent hydrogen within biomass macromolecules for its hydrogenolysis and hydrogenation circumvents the issues related to external gaseous hydrogen, representing another form of green hydrogen and mitigating the carbon footprint associated with the whole biomass utilization process [26,27,28]. More recently, a novel water-splitting catalytic HDO system using water as both the reaction medium and the hydrogen donor was developed, where multifunctional catalysts are also necessary for this innovative route to conduct multiple reaction steps, such as water activation and HDO of bio-compounds [29]. This water-splitting hydrogenation approach holds immense promise for establishing a biomass valorization method that fully meets the principles of green chemistry and sustainable development [30,31]. In addition, active metals, such as zinc (Zn), aluminum (Al), magnesium (Mg), and iron (Fe), can react with sub-critical water under the reaction conditions of biomass liquefaction, providing another strategy for in situ hydrogen production through water splitting [32]. The HDO reactions can be facilitated by the energy released from the exothermal metal hydrolysis process, and the resulting metallic oxides, like ZnO, AlOOH, and Fe₃O₄, may have catalytic effects on biomass depolymerization, further improving the bio-oil yield and quality [33,34,35]. Based on the above advantages, the in situ hydrogen generated by zero-valent metal hydrolysis has also been considered as another promising alternative to high-pressure hydrogen [36].

The integration of innovative hydrogen supply strategies into the biomass liquefaction process represents a notable advancement in the achievement of cost-effective, eco-friendly, and sustainable approaches to the valorization of various biomass feedstocks. Deepening the understanding of in situ hydrogen generation for biomass liquefaction is essential for the development of biomass-conversion technologies and the practical application of bio-based fuels and chemicals. In this review, the recent development of typical catalytic hydro-liquefaction and hydrodeoxygenation methods for biomass valorization without exogenous hydrogen donors is overviewed, including catalytic self-transfer hydrogenolysis using endogenous hydrogen, the in situ hydrogen induced by catalytic water splitting, and the hydrogenation of biomass assisted by zero-valent metal. Based on previous research conducted on alternative hydrogen supply approaches to gaseous hydrogen during biomass conversion, this work primarily focuses on in situ hydrogen supply mechanisms, the selection and effects of hydrogenation catalysts, and the implementation of these novel hydrogen supply strategies in biomass components and actual biomass feedstocks. In addition, this review summarizes the distinctive features of various novel hydrogen supply strategies and provides an outlook on their development prospects. The objective of this review is to fill in the knowledge gaps in this field and provide inspiration for the choice of an appropriate catalytic hydrogenation method, finally promoting the advancement of biomass utilization technologies.

2. Catalytic Self-Transfer Hydrogenolysis with Endogenous Hydrogen

2.1. Effects of Catalysts on Self-Transfer Hydrogenolysis Process

Previous studies have indicated that the inherent structure of lignocellulosic biomass can serve as a hydrogen donor in liquefaction reactions, providing an active source of hydrogen for hydrogenation and improving product quality. For example, hemicellulose can serve as a hydrogen donor through aqueous-phase reforming (APR) reactions, while the hydroxyl and methoxy groups in lignin structures hold potential as internal hydrogen sources, facilitating the hydrogenolysis of biomass macromolecules [37,38,39]. However, the effective utilization of biomass’ intrinsic hydrogen is still challenging and closely related to the employment of homogeneous and heterogeneous catalytic systems.

Bergman and Ellman [40] initially developed a novel catalytic depolymerization method for a lignin-related polymer with a homogeneous RuH₂(CO)(PPh₃)₃ catalyst. The ether linkages in the lignin model compound underwent a tandem dehydrogenation and reductive cleavage reaction, resulting in a 62–98% conversion yield without adding other reagents. Cai and co-workers reported a MIL-100(Fe)-supported Pd-Ni BMNP catalyst for the intramolecular transfer hydrogenolysis of lignin model compounds and organosolv lignin. C_αH-OH groups were explored as the hydrogen source to selectively cleave β-O-4 linkages in the lignin structure with water as the reaction medium, providing a more cost-effective option for the industrial-scale depolymerization of lignin [39]. Wang and co-workers developed a novel redox-neutral system for the depolymerization of lignin using a water-soluble binuclear Rh complex as the catalyst [41]. A homogeneous catalytic system for a “lignin-first” biorefinery was successfully constructed in water solvent and unutilized in the depolymerization of various lignin-related substrates, including alkaline lignin and raw lignocellulose samples. Mechanistic investigations showed that the reaction proceeded via a metal-catalyzed dehydrogenation step with the formation of a carbonyl intermediate, followed by the cleavage of the C-O bond, to generate ketone and phenol products. As shown in Figure 1, the hydrogen utilized for cleaving the C-O bond originated from the benzyl hydroxy groups in the side chain of aromatic units in the lignin structure.

Although homogeneous catalysts have been extensively studied in the biomass liquefaction process, challenges persist due to the complexity of catalyst recovery and the associated high cost. To address these issues, significant research efforts are now focused on employing heterogeneous catalysts for biomass liquefaction, especially for the hydrodeoxygenation reaction. The heterogeneous catalysts exhibit high activity and facilitate easy recovery, thereby mitigating the economic burden of bio-oil production. Han and co-workers [42] introduced the concept of self-supported hydrogenolysis (SSH) of aromatic ethers for arenes production using hydrogen within the reactants. With the assistance of heterogeneous RuW alloy nanoparticles, the selectivity to arenes could reach >99.9% at the complete conversion of the ethers. Moreover, lignin could also be converted into arenes through the integration of hydrogen-transfer depolymerization and hydrodeoxygenation using an inherent hydrogen source generated from methoxy groups. The RuW alloy catalyst demonstrated efficient catalysis in the SSH reaction by abstracting hydrogen from the activated C_Al-H bond within the ethers and effectively cleaving the C_Ar-O bond via hydrogenolysis. Both the experimental observations and the density functional theory (DFT) calculations corroborated the synergistic action between neighboring Ru and W species within the RuW catalyst, facilitating the acceleration of the SSH reaction. In another work by Han’s group, an in situ refining strategy for lignin was developed for the sustainable production of benzene using high-silica HY zeolite-supported RuW alloy (RuW/HY30) as a catalyst and water as the reaction solvent [43]. It was reported that the RuW/HY30 catalyst could facilitate the selective deconstruction of the C_sp2-C_sp3 and C_sp2-O bonds in the lignin structure, allowing for the production of benzene exclusively from lignin, with a maximum yield of 18.8%. Notably, the RuW component not only enabled the hydrogenolysis of the C_sp2-O bond using active hydrogen derived in situ from the lignin molecule but also promoted the prompt deconstruction of C_sp-C_sp3 bonds on the local lignin structure by leveraging the Bronsted acid sites of HY30 zeolite, without the need for reductive catalytic fractionation or competition from hydroxyl group hydrogenolysis. This work opened a novel approach to producing benzene using lignin feedstock without exogenous hydrogen.

In order to replace the precious metals with abundant and inexpensive transition metals during catalyst construction for the self-transfer hydrogenolysis of lignin, the catalytic performance of a Ni/C catalyst derived from a Ni-containing metal–organic framework (Ni-MOF) in self-transfer hydrogenolysis was evaluated by Mei et al. in detail [24]. The results showed that the Ni-NDC-500 catalyst gave the best performance, leading to the efficient hydrogenolysis of 2-phenoxy-1-phenylethanol into monomers, with a high conversion rate of up to 99% at 220 °C and 2 h. The tested catalyst was also effective in the conversion of native lignin and could be reused three times without an obvious reduction in catalytic performance. A well-preserved spatial structure and robust catalytic stability demonstrated the superior performance of the Ni-NDC-500 catalyst. The proposed reaction mechanism comprises two stages, namely the dehydrogenation of C_α-OH on Ni-NDC-500 to produce a C_α=O intermediate and a hydrogen poor, followed by the hydrogenolysis of the ether bond in the Cα=O intermediate facilitated by the hydrogen poor to form acetophenone and phenol (Figure 2).

The approach of self-transfer hydrogenolysis utilizing internal groups as hydrogen donors has been effectively applied to model compounds of lignin by most researchers, resulting in high yields and selectivity. But, the application of this method in actual lignin presents significant challenges, primarily due to the insolubility of the lignin macromolecular structure and the difficulty of catalyst accessibility. Dou et al. [23] reported a self-hydrogen transfer hydrogenolysis (STH) process of native lignin to produce monomers over a Pd-PdO/TiO₂ catalyst. When compared with Pd/TiO₂ and PdO/TiO₂ catalysts, Pd-PdO/TiO₂ showed better activity for the STH of the C_β-O bond in β-O-4 models. Using Pd-PdO/TiO₂ as the catalyst, lignin monomers were obtained from poplar lignin with a yield of 40 wt% at 180 °C, without the need for external hydrogen donors. According to the results of control experiments and density functional theory (DFT) calculations, the reaction mechanism indicates a two-step fragmentation process (Figure 3). The dehydrogenation of C_αH-OH on metallic Pd first generates a β-O-4 ketone intermediate and a “hydrogen pool”, followed by the hydrogenolysis of the C_β-O bond in the β-O-4 ketone intermediate by the “hydrogen pool”. PdO was more effective for activating the β-O-4 ketone intermediate and showed lower activation energy for the C_β-O bond cleavage than metallic Pd. Thus, the synergistic effect of Pd and PdO could promote the cleavage of the C_β-O bond in the β-O-4 linkage.

2.2. Self-Reforming-Driven Hydrogenolysis of Lignin

Under moderate conditions and in the presence of suitable catalysts, oxygen-containing compounds can be converted into H₂ and other high-value chemicals via aqueous-phase reforming (APR) reactions. The APR process involves dehydrogenation reactions, the cleavage of C-C and C-O bonds, steam reforming reactions, and dehydration reactions. Currently, numerous researchers have applied aqueous-phase reforming technology in the hydrogen production process of biomass-derived derivatives [44,45]. The structure of lignin is characterized by an abundance of aliphatic hydroxy groups and methoxy groups bonded to aromatic rings. The aliphatic hydroxy groups can undergo dehydrogenation and decarbonylation to yield in situ hydrogen, participating in the reductive depolymerization of lignin and the hydrogenolysis of methoxy groups within the aromatic structure. Subsequently, the generated methanol from methoxy groups can act as an ideal hydrogen donor through direct dehydrogenation and aqueous-phase reforming for the self-transfer hydrogenolysis of lignin, while Pt is generally considered the active metal for dehydrogenation and reforming reactions [26,27,46,47]. Li et al. [26] developed a hydrogen-free production approach for alkylphenols directly from actual lignin for the first time through self-reforming-driven hydrogenolysis and depolymerization, using the inherent aliphatic hydroxyl and methoxy groups as in situ hydrogen donors over a Pt/NiAl₂O₄ catalyst (Figure 4). Under hydrogen-free conditions, the yield of 4-alkylphenols from birch lignin was up to 17.3 wt% in a single-step reaction. A detailed investigation of the reaction pathways showed that the in situ hydrogen generated from aliphatic hydroxy groups on the lignin side chain was the initial and pivotal step, as it provided the requisite hydrogen for subsequent depolymerization and demethoxylation reactions. Subsequently, the produced methanol further supplied hydrogen through aqueous-phase reforming (APR), thereby expediting the hydrogenolysis of the ether linkages, methoxy groups, and carbonyl groups, ultimately leading to the formation of 4-alkylphenols.

Based on the above investigation, Li et al. [48] further studied the rate-determined step and catalytic issues in the self-reforming-driven hydrogenolysis of lignin or lignin oils. Compared to the Pt/NiAl₂O₄ catalyst, Ru/NiAl₂O₄ showed superior ability for the self-reforming-driven depolymerization and hydrogenolysis of lignin due to the rapid recovery of active sites for the demethoxylation reaction, which is the rate-determined step in the whole reaction process. With Ru/NiAl₂O₄ as an efficient catalyst, the yields of 4-alkylphenols could reach 40.7 mol% and 92.8 mol% from birch lignin and lignin oil, respectively. In the stability test, Ru/NiAl₂O₄ maintained its activity in converting birch lignin oil to 4-alkylphenols, even after three catalytic cycles. Furthermore, the authors investigated the size effects of Ru particles on the self-reforming driven hydrogenolysis with 4-(3-hydroxypropyl)-2-methoxyphenol as the lignin model compound and NiAl₂O₄ spinel as the catalyst support [49]. Three catalysts with varying Ru particle sizes (denoted as Ru^1.5, Ru^2.6, and Ru^3.3) were prepared, and the results showed that the difference in catalytic performance originated from the interaction between Ru colloids and the NiAl₂O₄ spinel support, leading to different Ru^δ+/Ru⁰ ratios and, ultimately, influencing the reaction pathways. Among all of the three catalysts, Ru^2.6/NiAl₂O₄ exhibited optimal catalytic performance, with the best self-reforming driven demethoxylation capability due to its appropriate ratio of Ru^δ+/Ru⁰, facilitating the dissociation of in situ generated hydrogen.

2.3. Self-Hydrogen Supplied Catalytic Fractionation

Lignocellulosic biomass, comprising cellulose (30–50%), hemicellulose (20–35%), and lignin (15–30%), stands as the most abundant and sustainable carbon resource on Earth. The fractionation of lignocellulosic biomass into cellulose, hemicellulose, and lignin typically serves as the initial step for obtaining biomaterials and high-value-added chemicals [50,51]. In recent years, the reductive catalytic fractionation (RCF) technique, which is also called the lignin-first strategy, has emerged as a highly promising approach for processing lignocellulosic biomass. This method involves the depolymerization of lignin into lignin oils while preserving the cellulose component within a hydrogen atmosphere [52]. Given the high reaction temperatures and economic concerns associated with external gaseous hydrogen, the application of a hydrogen-free process for the fractionation of lignocellulose shows significant promise. Galkin et al. [53,54] developed a novel Pd-catalyzed strategy for the fractionation of lignocellulosic biomass in an ethanol–water solvent, where the hemicellulose, cellulose, and lignin could be separated and, in parallel, the lignin was converted to aromatic monomers with only endogenous hydrogen. During the fractionation process, partial hemicellulose was consumed to supply in situ hydrogen, while the ethanol in the reaction medium could also serve as a hydrogen donor via catalytic transfer hydrogenation. Ferrini et al. [55] proposed a catalytic biorefining approach that converted lignocellulose into non-pyrolytic lignin oil and pulp (holocellulose) susceptible to enzymatic hydrolysis under mild conditions without adding pressured hydrogen. Moreover, the lignin bio-oil was able to be further upgraded through hydrogen transfer reactions over the Raney Ni catalyst using 2-PrOH as the hydrogen donor.

In very recent research, Wang and colleagues introduced an innovative strategy for lignocellulose fractionation, termed self-hydrogen supplied catalytic fractionation (SCF) [56]. Through this method, various lignocellulosic biomasses could be converted into lignin oils and cellulose-rich pulps, with structural hydrogens in hemicellulose serving as the hydrogen donor over the Pt/NiAl₂O₄ catalyst. Figure 5a illustrates a comparison between the conventional reductive catalytic fractionation (RCF) method and the self-hydrogen supplied catalytic fractionation (SCF) process, and the reported reductive catalytic fractionation conditions and results were summarized in Figure 5b. With only water as the reaction medium, this hydrogen-free SCF process achieved a theoretical yield of phenolic monomers (46.6 wt%) under a mild condition of 140 °C while preserving 90% cellulose intact in birch sawdust. The current limitation of SCF is its long reaction time, requiring up to 24 h to achieve a high yield. Moreover, as shown in Figure 5c, the Pt/NiAl₂O₄ catalyst exhibited good stability in recycling, as well as regeneration treatment. Therefore, this method made significant progress in the development of high-performance, eco-friendly, and stable catalysts for the sustainable biorefining of lignocellulosic biomass without the consumption of exogenous hydrogen [56].

3. In Situ Catalytic Hydrodeoxygenation Employing Water as the Hydrogen Donor

3.1. Application of Multifunctional Catalysts for Water Splitting and Biomass HDO

Hydrodeoxygenation (HDO) stands as a critical step in the production of renewable fuels and valuable chemicals from biomass feedstocks. Water has been considered a commonly used reaction medium, reactant, or byproduct in biomass HDO reactions. The unique property of water in subcritical or supercritical states contributes to the regulation of reaction rates, selectivity, and mass transfer during the biomass-conversion process [57,58]. Compared with the frequently employed hydrogen donors in biomass valorization, such as gaseous hydrogen, alcohols, or formic acid, water presents itself as an economical, non-toxic, safe, and eco-friendly hydrogen source, which has been largely overlooked. The implementation of a water-assisted in situ HDO pathway for biomass presents significant challenges, primarily attributed to the limitations associated with water splitting [36]. Several recent studies have aimed to develop multifunctional catalytic systems using water as both the reaction medium and as a hydrogen donor for the production of bio-derived fuels (Figure 6) [29,59,60,61]. The proposed concept involves water splitting on the catalytic surface to generate hydrogen, and the in situ produced hydrogen can subsequently engage in the biomass HDO process, facilitating the cleavage of C-O bonds and the removal of oxygen from organic molecules. Therefore, the multifunctional catalysts designed for the in situ catalytic HDO employing water as the hydrogen donor should be capable of (i) mediating water splitting under the specified reaction conditions and (ii) simultaneously catalyzing the hydrogen transfer and the deoxygenation processes using the hydrogen induced by water [30].

Jin et al. [29] showcased a novel hydrogen-free approach for biomass HDO and valorization by the in situ hydrogen generated from water splitting. In order to assess the feasibility of this innovative route, guaiacol was used as the lignin model compound for the HDO process, and a series of noble metal catalysts (i.e., Au/C, Pd/C, Rh/C, and Ru/C) were tested in a high-pressure batch reactor at 250 °C and 4 h. The results showed that the conversion of guaiacol was improved by adding noble metal-based catalysts, and Ru/C gave the best catalytic performance, with a conversion rate of 25.07 wt%, owing to the greater dispersion and smaller particle size of Ru particles on the carbon support. In another work by Jin et al., a series of Ni-based or NiMo-based catalysts supported on CeO₂ with or without activated carbon (C) were prepared, characterized, and examined for the HDO of guaiacol in water at 250 °C without the supply of external hydrogen. The Ni clusters doped with ceria and supported on activated carbon with a high surface area were demonstrated as promising materials for the overall reaction, achieving conversion yields of over 20% under the investigated conditions. The superior activity of CeO₂-C supported catalysts could be attributed to the excellent dispersion of Ni nanoparticles on the support surface, while the presence of Mo enhanced the selectivity toward partially deoxygenated products. Furthermore, the spent catalysts exhibited minimal deactivation from sintering and coking, demonstrating their promising prospects for practical utilization in the biomass upgrading process [30].

As nitrogen-dopants incorporated into the carbon structure can be viewed as advantageous anchoring sites or imperfections that promote particle nucleation and reduce particle size, nitrogen-doped carbon (NC) materials have garnered recognition as promising substrates for heterogeneous catalysts [62,63]. Therefore, Jin et al. further investigated the water-assisted in situ HDO of guaiacol over a series of Ni-based nitrogen-doped activated carbon-supported catalysts at 250 °C and 4 h. The results demonstrated that the nitrogen-doped samples were more active than the undoped catalysts during the HDO reaction. Among various nitrogen sources, such as polypyrrole (PPy), polyaniline (PANI), and melamine (Mel), Ni/PANI-AC exhibited a superior efficiency due to better dispersion and the smaller Ni particle size, increasing the conversion rate of guaiacol by 8% compared to the rate obtained by the Ni/AC catalyst [64]. The addition of N in the carbon structure could stabilize metallic Ni in the catalysts, resulting in no obvious metal sintering for the spent samples under the studied conditions. In another work, a series of Pt catalysts supported on N-doped activated carbons were developed by Pastor-Pérez et al. for the HDO of guaiacol without exogenous hydrogen [65]. Generally, the N-enhanced catalysts exhibited greater selectivity towards oxygen-depleted products, which was attributed to the modification of electronic and acid–base properties by nitrogen doping. Pt/PANI-AC demonstrated the most effective catalytic material, achieving approximately 75% guaiacol conversion at 300 °C and exhibiting commendable stability over multiple recycling cycles. The excellent catalytic performance of Pt/PANI-AC was mainly due to the following reasons: (i) enhanced dispersion of Pt, (ii) smaller Pt particle sizes, and (iii) an optimal ratio of pyrrole to pyridine groups, showcasing the advantageous influence of nitrogen in the catalyst formulation for the H₂-free HDO process.

More recently, Parrilla-Lahoz et al. synthesized a series of Ni-based catalysts to study the effects of synthesis methods, nitrogen doping, and the addition of ZrO₂ on the catalytic performance of multi-step reactions, including water splitting and guaiacol HDO. The best activity/selectivity balance was presented by the NiZr₂O/Gr-n catalyst, achieving a noteworthy conversion rate and high selectivity towards mono-oxygenated compounds under challenging reaction conditions [66]. Due to N doping and the promotion effect of Zr, the oxidation phenomenon of the Ni metallic phase was not observed in the NiZrO₂/Gr-n samples, indicating the structural and morphological stability of this catalyst during the hydrothermal upgrading of lignin model compounds. Moreover, multifunctional catalysts based on Ru supported on a commercial active carbon were prepared by Carrasco-Ruiz et al., and the effect of cerium oxide addition was also tested. The cerium oxide could significantly enhance the dispersion of ruthenium metal and improve the overall redox properties of the multicomponent system, achieving a conversion rate exceeding 40% and demonstrating selectivity towards phenol as an advanced reaction product. Notably, the multicomponent catalysts were proven to be stable under harsh reaction conditions, showing no modification in crystalline structure or nucleation of carbon deposits during the HDO reaction [59]. The above research findings all provide innovative and highly efficient strategies for the construction of HDO catalysts using water as the hydrogen donor.

3.2. Synergistic Utilization of Endogenous Hydrogen Generated from Water Splitting and Biomass Structure

Recently, an innovative depolymerization method for lignocellulosic biomass without exogenous hydrogen donors, termed self-hydrogen supplied catalytic fractionation (SCF), was proposed and successfully achieved by Zhou et al. [56] For the better utilization of the lignin oil obtained from the SCF process, Zhou and co-workers further developed a new strategy for the demethoxylation of lignin oil to produce 4-propylphenol using internal hydrogen over a RuNi/NiAl₂O₄ catalyst [28]. In this approach, a trace amount of hydrogen is generated via water splitting on the Ni surface, facilitating the removal of methoxy groups from lignin oil and resulting in the formation of 4-alkylphenol and methanol. The aqueous-phase reforming of the in situ generated methanol can subsequently produce more hydrogen, thereby accelerating the entire reaction process. The excellent catalytic performance of the RuNi/NiAl₂O₄ catalyst has been demonstrated to originate from the following reasons. (i) Metal Ni initiates hydrogen production through water splitting. (ii) The in situ formed Ru-NiOx interface can enhance the activation of C-O bonds and facilitate the demethoxylation reaction. (iii) The increased Ru content in Ru-NiOx promotes methanol dehydrogenation and accelerates the aqueous-phase reforming, ultimately expediting the entire reaction [28]. By integrating the SCF process with catalytic water splitting, the fractionation of lignocellulosic biomass and the depolymerization and upgrading of lignin were realized with endogenous hydrogen (Figure 7). This tandem process represents a promising approach for establishing a mild, eco-friendly, and sustainable biorefinery.

Owing to their excellent catalytic properties, nanoporous metals have been considered to be one kind of classical catalytic material that has been extensively utilized in the biomass HDO process [67]. According to the research by Lu et al., a nano-porous Ni catalyst demonstrated outstanding performance in selectively converting guaiacol to cyclohexanol, achieving complete guaiacol conversion with over 90% selectivity under the conditions of 180 °C and 2 MPa H₂ for 4 h [68]. To further validate the concept of employing water and endogenous hydrogen in the biomass structure as the hydrogen donors and establish a more efficient catalytic system, Ren et al. also integrated the water activation and aqueous-phase reforming of in situ generated methanol for selectively converting guaiacol to phenol over the nano-porous Ni catalyst [69]. Remarkably, a conversion rate of 41.5% was achieved along with 100% selectivity to phenol at 160 °C. Increasing the temperature to 190 °C significantly enhanced the conversion rate to 90.5% while maintaining a high selectivity to phenol of 90.3%. The study on reaction mechanisms using density functional theory (DFT) simulation showed that hydrogen initially originates from irreversible nickel oxidation by water on the catalyst surface, facilitating the conversion of guaiacol to methanol and phenol. Following this, more H₂ was generated through the aqueous-phase reforming of methanol over the Ni catalyst. Finally, the HDO process is accelerated by the in situ generated hydrogen, leading to the production of phenol with high selectivity (Figure 8). However, owing to the oxidation of Ni⁰, a significant deactivation of the spent nano-porous Ni catalyst was observed in the stability test. After reduction at 500 °C under a hydrogen atmosphere, the regenerated catalyst showed stable activity and good selectivity for phenol production, indicating that the nano-porous Ni catalyst could act as a reactant during the HDO reaction process.

Furthermore, a more stable and easily prepared Ni/MgO catalyst was employed in the upgrading process of lignin-derived monomers in near water by Ren et al., aiming to establish an efficient strategy for phenol production with high selectivity [31]. The results proved that Ni/MgO could act as a multifunctional catalyst, facilitating the water splitting, selective cleavage of aromatic ether bonds, and aqueous-phase reforming of the in situ generated methanol in a one-spot process. An impressive guaiacol conversion rate of 87.8% phenol was obtained under the optimal temperature of 190 °C, coupled with a high selectivity of 88.9% for phenol. Compared to the previously reported nanoporous Ni catalyst, Ni/MgO exhibited significantly enhanced stability due to the high dispersion of Ni nanoparticles on MgO and the strong interaction between Ni and MgO, which inhibited the oxidation of Ni⁰ to Ni²⁺. More importantly, when using industrial technical lignin as the substrate, a combined yield of 16.3 wt% phenol and 4-methylphenol was achieved under optimized conditions over a Ni/MgO catalyst. Considering the excellent catalytic performance, low cost, and stability of Ni/MgO, this hydrogen-free approach presents a promising alternative to traditional biorefinery processes, addressing the challenges associated with hydrogen supply and economic viability [31].

4. In Situ Hydrodeoxygenation Assisted by Zero-Valent Metals

4.1. Promotion Effects and Mechanism of In Situ Hydrogen Induced by Zero-Valent Metals on Biomass Liquefaction/Upgrading20

Water splitting on the surface of zero-valent metals represents a promising strategy for cost-effective and sustainable hydrogen production. Compared with other hydrogen-generation routes, this approach shows benefits, including (i) using water as an eco-friendly reaction medium and hydrogen source and circumventing the challenges associated with gaseous hydrogen; (ii) demonstrating higher activity than gaseous hydrogen by employing the in situ generated hydrogen; (iii) facilitating biomass liquefaction by the exothermal property of the reactions between zero-valent metals and water; and (iv) giving catalytic effects on biomass conversion by the metallic oxides obtained through metal hydrolysis [7,33,70]. Zero-valent metals, such as aluminum (Al), zinc (Zn), magnesium (Mg), and iron (Fe), which are relatively stable at ambient temperature conditions but reactive under hydrothermal conditions, have been widely investigated by researchers to produce in situ hydrogen through metal hydrolysis reactions for biomass liquefaction and hydrogenation [10,32,36].

Cheng et al. [71] first conducted the HDO treatment of pine sawdust bio-oil with the addition of zero-valent Zn at different temperatures in the autoclave reactor. High temperatures were proven to be beneficial for decreasing the content of oxygenated compounds and improving the bio-oil quality with the assistance of Zn, while the highest hydrocarbon content of 68.95% in bio-oil was obtained at 400 °C. Yang et al. [72] proposed a novel approach for deoxy-liquefaction of corn stalks in subcritical water with metallic Al as the hydrogen donor at 370 °C. The results demonstrated the positive effects of Al addition on the physicochemical properties of bio-oils. The highest bio-oil yield of 26.54 wt% was achieved when the aluminum content was 30 wt%, while the deoxygenation ratio of bio-oil reached 47.92%, resulting in a significantly improved quality of bio-oil product. Moreover, the HDO of crude bio-oil was conducted by Yang et al. using the in situ hydrogen generated via Al hydrolysis in the aqueous phase by-product from the hydrothermal liquefaction of rice stalks [73]. The results indicated that the Al to the aqueous-phase ratio positively influenced the HDO reactions of the bio-oil. With the Al to aqueous-phase ratio of 2/25 g/mL, the deoxygenation degree and higher heating value reached 76.38% and 38.53 MJ/kg, respectively. Li et al. [74] further investigated the effects of in situ produced via the oxidation of aluminum by supercritical ethanol on the hydro-liquefaction of rice stalks. Similarly, the Al-assisted liquefaction process in supercritical ethanol demonstrated a significant increase in deoxygenation degree, while the maximum energy recovery achieved was around 75%, with a bio-oil yield of approximately 33 wt% at the Al to RS ratio of 2:3. The possible reaction mechanism was proposed by the authors, as depicted in Figure 9. Al first reacted with ethanol to form Al(C₂H₅O)₃ and produced active hydrogen, and the unstable Al(C₂H₅O)₃ was subsequently hydrolyzed to form Al(OH)₃. Through a multistage thermal dehydration process, boehmite (AlOOH) and alumina (Al₂O₃) are ultimately generated. Moreover, the ethanol reforming occurred and was catalyzed by γ-Al₂O₃ during rice-stalk liquefaction, improving the in situ hydrogen generation in the whole reaction process.

Owing to the advantages of widespread availability and low cost, zero-valent Fe has been widely employed in biomass liquefaction and upgrading processes, with the in situ hydrogen produced through the Fe hydrolysis reaction [7,15]. De Caprariis et al. [75] studied the effects of Fe on the hydrothermal liquefaction of oak wood at 260–320 °C for 15 min. The highest bio-oil yield of about 40% was obtained using Fe powder, while the H/C ratio of biocrude oil and the presence of aliphatic compounds were both improved by Fe addition. It is noteworthy that Fe₃O₄ could be generated via the reaction of Fe and sub-critical water during biomass liquefaction, serving as a catalyst for biocrude oil production. However, the bio-oil yield was obviously decreased when using recovered Fe, indicating that the oxidized particle surface of the Fe powder inhibited the hydrogen production reaction. Zhao et al. [76] explored the role of metallic Fe in the cornstalk liquefaction process using water, ethanol, or ethanol–water mixed solvents as the reaction medium. This work demonstrated that the promotional effects of Fe on bio-oil production were primarily due to the in situ hydrogen generated from water splitting facilitated by Fe, while Fe₃O₄ exhibited only a minor catalytic role in pure water. Although the maximum biocrude oil yield of about 50 wt% was achieved in water–ethanol mixed solvents with Fe, it had been confirmed that Fe could not interact with the ethanol solvent during the liquefaction process. Furthermore, the hydrothermal liquefaction of cellulose in a batch reactor with the addition of different transition metals (Ni, Fe, or Zn) was conducted by de Caprariis et al. at 300 °C for 10 min [77]. Among the tested transition metals, the largest improvement in bio-oil yield, from 17.4% to 26.5%, was obtained with Fe. Due to higher redox properties, Zn could produce a greater amount of active hydrogen during cellulose liquefaction and, thus, result in more water phase by-products rich in organic compounds. Taking into account both bio-oil yield and quality, Fe exhibited the best performance. Given its low cost and ease of secondary reduction, Fe shows significant potential in the biomass liquefaction industry.

An innovative hydrothermal conversion strategy for oil-palm empty fruit bunches using fresh Fe powder as the in situ hydrogen-generating agent was proposed by Miyata and co-workers [15]. The hydrogen produced via the reaction between Fe and H₂O efficiently promoted the degradation of biomass to bio-oil containing water-soluble and water-insoluble fractions with high yields, and char formation was successfully inhibited. In addition, the water-soluble fractions could be further treated via catalytic cracking to produce light olefins (C₂–C₄) in good yields, while the oxidized Fe could be regenerated by heating with the solid residue from the hydrothermal liquefaction process at 1000 °C, improving the economic viability and sustainability of this biomass-conversion approach. In another work, the quantitative analysis of the water-soluble fraction produced from the Fe-assisted liquefaction process was conducted by Miyata et al. using the relative response factors estimated by the effective carbon number method [78]. The incorporation of Fe significantly modified the composition and structure of heavy water-soluble components, markedly increasing the proportion of volatile compounds. The reactivity of the water-soluble fraction in the catalytic cracking reaction was evaluated again, revealing that the volatile components of the water-soluble fraction could be efficiently converted into valuable olefins. Moreover, Miyata et al. [34] employed commercially available carbohydrates and enzymatically isolated lignin as the model substrates to explore the effect of metallic Fe on the liquefaction product’s composition, and the possible reaction mechanism was proposed by the authors (as shown in Figure 10). The results demonstrated that the oxidized Fe could facilitate the retro-aldol condensation of sugars, and the in situ generated by Fe suppressed the recondensation of reactive intermediates, synergistically enhancing the production of light compounds within the water-soluble fraction. In addition, the stabilization of these reactive intermediates could be further supported by an electron-transfer-type reduction mechanism. Conversely, Fe had a minimal effect on the degradation of enzymatic lignin, which was predominantly converted into water-insoluble products.

4.2. Synergistic Effects of Zero-Valent Metals and Hydrodeoxygenation Catalysts on Hydrothermal Liquefaction of Biomass Feedstocks

With the aim of enhancing the utilization efficiency of the hydrogen generated from zero-valent metals, researchers attempted to introduce various HDO catalysts in the metal-assisted biomass-liquefaction process, while the in situ hydrogen is expected to improve the catalyst resistance against deactivation [79]. Cheng et al. [80] investigated the in situ HDO upgrading process of pine sawdust bio-oil with the assistance of Zn, using Pd/C as the catalyst under different reaction temperatures (200 °C, 250 °C, and 300 °C). The results indicated that the combination of Zn and Pd/C efficiently promoted the HDO performance, achieving the highest heating value (30.17 MJ/kg) and hydrocarbon content (24.09%) at 250 °C. In addition, a series of bifunctional catalysts (Co/HZSM-5, Zn/HZSM-5, and Co-Zn/HZSM-5) were prepared to improve the bio-oil production from pine sawdust with Zn as the in situ hydrogen donor [81]. The utilization of Co/HZSM-5 or Zn/HZSM-5 catalyst resulted in improved bio-oil and gas yields and reduced the contents of oxygenated compounds, exhibiting favorable synergistic effects in combination with metallic Zn. Compared to the monometallic catalysts, bimetallic Co-Zn/HZSM-5 catalysts demonstrated superior promotional effects on both the yield and quality of the bio-oil, which was attributed to the synergistic interaction between Co and Zn on the HZSM-5 support.

More recently, Hamidi et al. [82] developed a novel synthesis method for Y zeolite from rice husks using TMAOH as a template to enhance mesopores for diffusion accessibility. The catalytic performance of the Ni/synthesized Y zeolites on bio-oil upgrading was investigated with the in situ hydrogen produced through Zn hydrolysis. Compared with the blank test and commercial Ni/HY catalyst, the synthesized Ni/HY led to the highest upgraded oil yield of 80.0% with the best quality. The one-pot catalytic hydro-liquefaction of oak wood was also conducted by the authors using the same reaction system, achieving a bio-oil yield of 39.0% and an HHV of 32.61 MJ/kg, which showed the potential to integrate the bio-oil production and upgrading steps with a Ni/HY catalyst and metallic Zn. However, a good catalytic activity of the synthesized Ni/HY catalyst was only maintained in the first reuse test, and then, the catalytic activity decreased rapidly due to the collapse of the zeolite structure, oxidation of Ni active site, coke deposition, and ZnO contamination. Furthermore, Zhao et al. [35] studied the hydrothermal liquefaction process of pinewood sawdust using various homogeneous/heterogeneous catalysts with Fe addition as the hydrogen producer at 300 °C for 30 min, and substantial synergistic effects between Fe and the tested catalysts were observed, leading to enhanced bio-oil yields and quality. The highest bio-oil yield of 45.58 wt% was obtained with Fe + Na₂CO₃, while Fe + Ru/C produced bio-oil with superior HDO performance and the highest HHV of 30.93 MJ/kg. Moreover, the influence of zero-valent metals (Fe and Zn) with the assistance of HDO catalysts (Ni and Co) on the hydrothermal liquefaction of oak wood was investigated by Tai et al. [83]. The active hydrogen was proved to be generated via the oxidation reaction of Fe and Zn in subcritical water, which played a crucial role in stabilizing biomass fragments and facilitated the HDO process catalyzed by Ni and Co. It is worth noting that the synergistic effects between metals and HDO catalysts were more pronounced with longer reaction durations, and the maximum oil yield of 48% was obtained by employing Fe and Ni at 330 °C for 30 min.

The distinctive textural properties of Al₂O₃, including its high surface area, moderate pore size distribution, favorable acid/base properties, and excellent hydrothermal stability, have made it an outstanding catalyst support material for biomass HDO processes [84,85]. To further understand the promotional effect of Al-water reactions on biomass conversion in subcritical conditions, Li et al. [86] investigated the HDO process of enzymolysis lignin aided by metallic Al over a NiMoS/Al₂O₃ catalyst. It was found that the reaction temperatures significantly influenced the reactivity of Al powder in subcritical water, where most of the Al underwent oxidation to form boehmite at temperatures exceeding 340 °C with a high hydrogen yield. During the in situ HDO process over the NiMoS/Al₂O₃ catalyst, bio-oil showed reduced yields of phenolics and ketones, while the formation of aromatic hydrocarbons was obviously improved. Moreover, Hirano et al. [87] studied the influence of heterogeneous metal catalysts on the Fe-assisted hydrothermal liquefaction process of cellulose, including noble metal-based hydrogenation catalysts (Pd/Al₂O₃, Pt/Al₂O₃, and Ru/Al₂O₃) and transition-metal-based hydrogenation catalysts (Cu/SiO₂ and Ni/kieselguhr). Both a high yield and the quality of the water-soluble fraction were achieved by using Fe in conjunction with a hydrogenation catalyst, and the highest water-soluble yield of 73% was obtained with Fe + Pd/Al₂O₃ at 250 °C for 1 h. A possible reaction mechanism for the synergistic effect of Pd catalysts and Fe on cellulose was proposed by the authors, as depicted in Figure 11. The Pd-based catalyst could activate gaseous hydrogen produced from the Fe-water reaction, effectively enhancing the hydrogenation of intermediate compounds. Meanwhile, metallic Fe could stabilize the unstable intermediates through electron-transfer-type reduction. Additionally, a Pd-based catalyst played a vital role in hydrogenating ketones into alcohols in collaboration with Fe, leading to the activation of carbonyl groups to improve the quality of the water-soluble fraction.

For the purpose of developing a viable and efficient approach for lignin valorization, Tai et al. [88] comprehensively studied the effects of several zero-valent metals (Zn, Fe, Ni, and Co) and metal-supported Al₂O₃ catalysts (Ni/Al₂O₃, Fe/Al₂O₃, Co/Al₂O₃, and Cu/Al₂O₃) on the hydrothermal liquefaction of lignin waste. The results showed that the active hydrogen produced through Fe or Zn oxidation could stabilize the intermediates during the liquefaction process and thereby enhance the bio-oil yields. The utilization of metal-supported Al₂O₃ catalysts in conjunction with metallic Fe or Zn demonstrated a synergistic effect on the HDO reaction, enhancing the efficiency of in situ hydrogen transfer and utilization. Wang et al. [33] compared the difference between autocatalytic and non-autocatalytic hydrogenation processes of lignin using water-splitting metals (Mg, Al, Zn, and Fe), and the interactions between the tested metals and an Al-Ni catalyst were also investigated in detail. Apart from Al, the other metals (Fe, Zn, and Mg) all exhibited favorable synergistic effects with an Al-Ni catalyst on bio-oil production, resulting in higher bio-oil yields in the non-autocatalytic process. Among the tested zero-valent metals, the optimal bio-oil yields and deoxygenation degrees were all obtained with Fe in both autocatalytic and non-autocatalytic processes, demonstrating its significant potential in the lignin-application process.

5. Conclusions and Perspectives

Liquefaction has emerged as a highly promising technology for producing renewable energy and value-added chemicals from various biomass wastes. It offers significant advantages, such as high energy efficiency, mild operating conditions, suitability for diverse biomass feedstocks, and the elimination of energy-intensive pre-drying processes. Owing to the high oxygen content inherent in biomass materials, the bio-crude obtained from the liquefaction process typically contains significant amounts of oxygenated compounds, making it challenging for direct utilization in downstream applications. Therefore, hydrodeoxygenation (HDO) upgrading is essential to further enhance the quality and utility of biomass-derived fuels and chemicals. Nevertheless, the traditional HDO approach using pressured gaseous hydrogen presents serious economic and safety concerns, while catalytic transfer hydrogenation (CTH) methods using liquid organic molecules, such as formic acid and alcohols, as hydrogen donors also face challenges, including the difficulty in the separation of by-products, cost restrictions, and sustainability issues. The integration of novel hydrogen-donation strategies into the biomass-liquefaction process signifies a significant advancement toward achieving cost-effective, environmentally friendly, and sustainable methods for valorizing diverse biomass feedstocks. This review provides an overview of recent developments in typical catalytic hydro-liquefaction and HDO methods for biomass valorization without external hydrogen donation, including catalytic self-transfer hydrogenolysis using endogenous hydrogen in the biomass structure, in situ catalytic HDO employing water as the hydrogen donor, and in situ HDO via water splitting assisted by zero-valent metals. Given the extensive research on alternative hydrogen supply strategies in biomass conversion, this work primarily focuses on the in situ hydrogen supply mechanisms and the effects of various hydrogenation catalysts during biomass HDO via these innovative approaches. A summary of the catalytic hydrodeoxygenation strategies for biomass valorization without exogenous hydrogen donors is presented in

Table 1. Summary of the catalytic hydrodeoxygenation strategies for biomass valorization without exogenous hydrogen donors.

Hydrogen Donation Strategy	Hydrogen Supply Mechanism	Advantages and Limitations
Catalytic self-transfer hydrogenolysis with endogenous hydrogen	In situ hydrogen production from potential hydrogen-donor groups in biomass structure via catalytic dehydrogenation reactions	Advantages: Reducing the additional costs associated with the utilization of high-pressured hydrogen or exogenous hydrogen donors; reducing the complexity of the reaction system; further enhancing the efficiency of self-transfer hydrogenolysis by integrating with aqueous-phase reforming (APR) reactions; avoiding side reactions caused by liquid hydrogen donors; and reducing the energy consumption in the purification and hydrogen-donor recovery process.
		Limitations: Low selectivity of hydrogenation products due to the structural heterogeneity of biomass feedstocks and difficult for the potential hydrogen-donor groups to effectively serve as inherent hydrogen donors due to dense cross-linked macromolecular structures of actual biomass and the existence of competing reactions.
In situ catalytic hydrodeoxygenation employing water as the hydrogen donor	In situ hydrogen production from catalytic water splitting under biomass upgrading conditions	Advantages: A cost-effective, non-toxic, safe, and environmentally friendly hydrodeoxygenation method with only water serving as the hydrogen donor; reducing the complexity of the reaction system; avoiding side reactions caused by liquid hydrogen donors; and reducing the energy consumption in the purification and hydrogen-donor recovery process.
		Limitations: Unsatisfactory yield and selectivity of hydrogenation products due to limited hydrogen supply capacity of water and relatively harsh reaction conditions.
In situ hydrodeoxygenation assisted by zero-valent metals	In situ hydrogen production from the reactions between zero-valent metals and sub-critical water	Advantages: High reaction efficiency due to the exothermal property of metal hydrolysis; catalytic effects of the generated metallic oxides for biomass conversion; and avoiding side reactions caused by liquid hydrogen donors.
		Limitations: Relatively high cost associated with the application of zero-valent metals and high energy input and complexity of the recycling and regenerating procedures of the metallic oxides.

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Catalytic self-transfer hydrogenolysis technology, employing endogenous hydrogen originating from the inherent chemical groups, holds considerable promise for the HDO of lignocellulosic biomass. However, several key challenges currently limit its practical efficiency and effectiveness. For example, the dense, cross-linked macromolecular structure and complex functional groups present in biomass complicate the thorough degradation of raw biomass under aqueous reaction conditions. These structural complexities not only hinder the effective contact between hydrogen-donating groups and HDO catalysts but also diminish the efficiency of the endogenous hydrogen supply and the yield of the target products. Additionally, the current research predominantly focuses on lignin model compounds, which fail to fully capture the complexities of reaction systems and actual mechanisms for real biomass. To address the above issues, future research should focus on the following directions:

• Exploring cost-effective transition-metal catalysts: identifying and developing cost-effective and abundant transition-metal catalysts as alternatives to precious metal catalysts is crucial for the practical application of catalytic self-transfer hydrogenolysis technology. Especially, innovative designs for transition-metal catalysts that can enhance both the cleavage of ether bonds and the dehydrogenation reactions of hydrogen-donating groups are essential for improving overall HDO efficiency;
• Design of efficient solvent systems: developing solvent systems that can effectively dissolve actual biomass feedstocks and facilitate the interaction of catalytic sites with hydrogen-donating groups is important for improving hydrogen transfer efficiency and HDO reactions. Moreover, employing biomass pretreatment approaches or assisted methods during the liquefaction process to promote the degradation of biomass macromolecular structures can further enhance the effectiveness of catalytic self-hydrogenation technology and improve the HDO catalyst performance;
• Advancing research on complex compounds and real biomass: research should progressively focus on more complex model compounds and real biomass materials to gain a comprehensive understanding of the reaction mechanisms and catalytic HDO pathways involving hydrogen-donating functional groups. This approach will provide a scientific basis for enhancing catalytic performance and advancing the practical application of this technology.

The unique properties of water under subcritical conditions enable it to play multiple roles in the biomass HDO process, including serving as a reaction medium, facilitating mass-transfer rates, stabilizing transition states, and actively participating in the reactions. Recent advancements in the in situ catalytic HDO approach based on water splitting highlight the innovative role of water in biomass upgrading. Compared to traditional hydrogen donors, such as pressurized gaseous hydrogen and organic hydrogen donors, water stands out as a cost-effective, non-toxic, safe, and environmentally friendly hydrogen source. Current research highlights the significant potential of water in the HDO process, while also identifying its limitations, particularly regarding its limited hydrogen supply capacity and the harsh reaction conditions required. To advance this field, the following research directions should be prioritized:

• Investigating the in situ hydrogen supply mechanism of water splitting: a deeper exploration of the water-splitting-based hydrogen supply mechanism and the reaction pathways for active hydrogen transfer is essential for enhancing the efficiency of a water-assisted HDO system. A comprehensive understanding of this mechanism will facilitate the development of more effective HDO catalysts and the design of aqueous liquefaction systems, ultimately improving hydrogen transfer efficiency;
• Combining in situ hydrogenation methods: given the limited hydrogen supply capability of water, integrating water-mediated in situ hydrogenation techniques with other alternative hydrogen supply strategies, such as catalytic self-transfer hydrogenolysis and catalytic reforming processes, can strengthen the stable supply and efficient utilization of internal active hydrogen through multiple pathways. Additionally, developing multifunctional catalysts that can simultaneously improve various in situ hydrogen supply methods will significantly reduce reaction costs and improve the overall HDO efficiency.

Zero-valent metal-assisted in situ HDO technology represents another emerging pathway for biomass conversion. Current studies indicate that utilizing active hydrogen derived from the reaction between zero-valent metals and subcritical water significantly enhances both the yield and quality of bio-oil products while optimizing the product distribution. However, the widespread application of zero-valent metal-based in situ hydrogen supply approaches still faces significant challenges. Compared to other hydrogen donors, zero-valent metals, such as Zn, Al, and Fe, are relatively expensive, posing a considerable barrier to commercialization. Furthermore, the recovery and regeneration of metal oxides post-reaction require substantial energy, further increasing the operational costs. To address these challenges, future research should focus on the following areas:

• The specific mechanisms between zero-valent metals and HDO catalysts remain inadequately understood. Further investigation is essential to elucidate their reaction pathways and catalytic efficiencies, which is crucial for developing suitable catalysts that work synergistically with zero-valent metals and their oxides during HDO reactions to enhance overall efficiency;
• Future research should also prioritize the development of cost-effective and environmentally friendly recovery technologies for unreacted zero-valent metals and generated metal oxides. This will help mitigate the economic burden and energy consumption associated with the zero-valent metal-assisted HDO process. Additionally, optimizing metal oxide regeneration technologies can further lower production costs for in situ hydrogen and facilitate the cyclic utilization of costly zero-valent metals, thus promoting the practical application of this technology.