Introduction

Over the past few decades, the rapid increase in greenhouse gas emissions, such as CO2 emissions, has resulted in the global threat of extreme climate change. To mitigate this problem, global initiatives are being made to reduce anthropogenic CO2 emissions from various industries [1,2,3]. Aviation CO2 emission accounts for approximately 3% of the total global CO2 emissions [2]. To reduce the amount of CO2 produced by aircrafts, a method of blending conventional jet fuel with sustainable aviation fuel, which can be obtained from lignin-derived pyrolysis oil, was introduced.

Lignin is one of the most abundant carbon resources that can be obtained from nature and comprises approximately 15% to 40% of the dry weight of terrestrial plants [3]. However, most of the lignin, which is commercially generated from paper and ethanol production industries, is used for low-value-added purposes such as burning as a fuel, thereby valorization of lignin into value-added chemicals is being extensively investigated [4]. Lignin is composed of oxygenated aromatic polymers, which can be properly degraded to yield value-added chemical building blocks. Lignin-derived pyrolysis oil can be obtained by the fast pyrolysis of lignin in the absence of air [5,6,7,8]. Pyrolysis oil is a mixture of various hydrocarbon oxygenates and is an attractive potential alternative green and carbon–neutral source to replace fossil fuel-based chemicals. However, several drawbacks originating from its high oxygen content, such as high viscosity, corrosiveness, and low energy density, limit its direct use as a sustainable aviation fuel. Therefore, catalytic hydrodeoxygenation (HDO) of pyrolysis oil, an oxygen removal process conducted under high temperature and high hydrogen pressure, is required to obtain aviation biofuel, which can be utilized as an alternative to conventional petroleum-derived fuels.

The HDO of pyrolysis oil and its model compounds has been extensively investigated using various heterogeneous catalysts. Early research on HDO catalysts was predominantly conducted by incorporating sulfide-based catalysts that had already been commercialized for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) process, such as Co-Mo and Ni-Mo sulfide catalysts [9]. However, these sulfide-based catalysts not only required high reaction temperature (generally above 300 °C), but were also vulnerable to water and suffered from severe catalyst deactivation due to the leaching of sulfur. Noble metals (Pt, Pd, and Ru), non-noble metals (Ni, Co, and Fe), metal phosphides and nitrides, and various bimetal-loaded catalysts have also been investigated as active catalysts for the HDO.

HDO is a combination of a series of reactions including hydrogenation, hydrogenolysis, and dehydration. Thus, designing high-performance HDO catalysts requires proper coupling of multiple catalytic properties. As the main objective of the HDO reaction is to remove oxygen-containing components, the oxophilic nature of the active metal or the acidity of the catalyst can significantly contribute to the oxygen removal ability of the catalyst. The hydrogen activation ability of the active metal is also an essential property of HDO catalysts as it affects the hydrogenation reaction of aromatic rings and unsaturated aliphatic hydrocarbons. In addition to the properties related to high catalytic performance, the design of a catalyst with high stability, which could be modulated by varying the preparation method or the support materials, is also a requisite for HDO catalysts because HDO reactions are generally conducted under harsh reaction conditions, such as high reaction temperature and hydrogen pressure. The accessibility of the active sites to the reactant should also be considered for HDO catalysts, because lignin-derived pyrolysis oil is composed of bulky aromatic compounds.

Among the various HDO catalysts, noble metal-loaded catalysts have been widely studied owing to their superior hydrogen activation ability, enabling them to exhibit high HDO performance even under relatively mild reaction conditions. However, the high cost of noble metals hampers their practical utilization in industry. Ru has the advantage of exhibiting excellent activity in a variety of reactions, including HDO, despite being more than three times more cost-effective than other noble metals such as Pt or Pd, highlighting it as an attractive active metal in various reactions [10,11,12,13]. The physicochemical properties of Ru and its applications in the field of catalysis have been well organized in a recent review article by Axet and Philippot [10]. To briefly summarize, Ru is one of the least expensive noble metals, possessing various oxidation states ranging from metallic Ru to RuO4. Owing to these versatile properties, Ru is utilized as a highly active metal in various reactions, such as ammonia synthesis and decomposition [11, 12], biomass hydrogenation [14,15,16,17], CO and CO2 conversion [18, 19], and the Fischer–Tropsch process [20,21,22]. There are a number of detailed review papers on various reactions catalyzed by Ru catalysts [10,11,12]. However, the review of recent advances in the various Ru catalysts used in HDO is relatively scarce, even though the number of publications related to HDO and HDO catalyzed by Ru-based catalysts has steadily increased (Fig. 1). Here, we organized and critically reviewed the recently reported literatures on the HDO of lignin-derived pyrolysis oil and its model compounds over Ru-based catalysts and intended to address the various strategies applied to enhance and modify the catalytic activities of Ru-based HDO catalysts.

Fig. 1
figure 1

Number of publications per year on the publications on and related to hydrodeoxygenation obtained from Web of Science with the keywords “hydrodeoxygenation” and “hydrodeoxygenation + Ru.” (Data searched on September 7, 2023.)

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Overview of the HDO of Lignin-Derived Pyrolysis Oil

Lignin-Derived Pyrolysis Oil

Pyrolysis oil, also commonly known as bio-oil, is a dark brown liquid composed of oxygenated organic compounds that can be obtained through the fast pyrolysis of biomass [6, 23]. During fast pyrolysis, the substrates are quickly heated to the desired temperature (in general, 450–600 °C) with a short residence time (0.5–5 s) under oxygen–free conditions, followed by rapid quenching to yield a thermally decomposed, highly oxygenated organic liquid as the products [5]. Rapid quenching after the pyrolysis of biomass produces numerous intermediates from further reactions, resulting in the unstable and corrosive nature of the pyrolysis oil and necessitating an additional upgrading process for commercial utilization. The chemical composition of pyrolysis oil can vary depending on pyrolysis conditions such as residence time, heating rate, pyrolysis temperature, and species of the biomass [6, 24].

The pyrolysis of each component of lignocellulosic biomass, such as cellulose, hemicellulose, and lignin, results in pyrolysis oils with different compositions [7]. Owing to their distinctive compositions, pyrolysis products of lignin, cellulose, and hemicellulose are composed of different components. Although the product distributions also vary with the pyrolysis conditions (such as pyrolysis temperature and residence time), the pyrolysis of lignin results in various phenolic compounds with large amount of char as products, while the pyrolysis of cellulose leads to yield anhydro-saccharides (such as levoglucosan) with furan and aromatic compounds, and the pyrolysis of hemicellulose results in the high char yield [7, 25]. Among those, lignin is a complex polymer composed of three phenylpropane units (4-hydroxyphenyl, guaiacyl, and syringyl) and lignin-derived pyrolysis oil is mainly composed of alkylated phenolic compounds [7]. In particular, demethoxylated phenolic compounds are preferred in lignin-derived pyrolysis oils obtained at higher pyrolysis temperature [5]. Owing to the complex composition of lignin-derived pyrolysis oil, the performance of HDO catalysts is generally evaluated using various model compounds to simplify the reaction. For example, phenol, guaiacol, cresols, anisole, vanillin, syringol, and eugenol are widely used as model reactants. Although most of the literatures on Ru-based HDO catalysts also uses various model compounds to reduce the complexity of the lignin-derived pyrolysis oil, it should be noted that Ru-based catalysts exhibit outstanding performance not only in the HDO of model compounds but also in the HDO of crude pyrolysis oil [26,27,28].

Reaction Pathways for HDO of Pyrolysis Oil Model Compounds Using Ru Catalysts

The phenolic HDO reaction pathway can be described as a combination of various complex steps. These reaction pathways can be roughly categorized as hydrogenation (HYD), direct deoxygenation (DDO), and tautomerization (TAU) pathways [29] (Fig. 2). The DDO pathway proceeds by the direct cleavage of the Caryl-O bond, whereas the HYD pathway proceeds by the hydrogenation of the aromatic ring, followed by successive dehydration and hydrogenation to obtain oxygen-free hydrocarbons. The removal of oxygen functional groups via the DDO pathway has the advantage of using only an equivalent amount of hydrogen molecules for each oxygen atom removed, thus consuming less hydrogen than the HYD pathway. However, the DDO pathway is generally preferred over the HYD pathway at higher reaction temperatures because the bond dissociation energy between C(sp2) and heteroatom is greater than that between C(sp3) and heteroatom [30]. For catalysts with oxophilic supports such as TiO2 and ZrO2, the removal of oxygen functional groups can proceed through the TAU pathway, where keto-tautomer intermediates play a key role in the production of arene compounds rather than direct cleavage of strong Caryl-O bonds by hydrogenolysis (DDO) [31].

Fig. 2
figure 2

Simplified reaction pathways of phenol HDO. Figure reproduced from the information in Ref. [29]

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Especially for the Ru-based catalysts, several groups have investigated the detailed reaction mechanisms of phenolic HDO conducted over a metallic surface using DFT calculations. Chiu et al. conducted a computational study on the HDO of guaiacol on a Ru(0001) surface using DFT calculation [32]. According to the calculations obtained by assuming a low hydrogen pressure (1 atm), a higher energy barrier for Caryl–O bond cleavage (more than 100 kJ/mol) than its aliphatic counterparts (69 kJ/mol) was obtained, which resulted in the removal of the methyl group from the methoxy group of guaiacol to yield catecholate. Within the subsequent removal of the residual oxygen functional groups, the removal of the first oxygen center of the catecholate resulted in a notably lower barrier (106 kJ/mol) than the cleavage of Caryl–O in phenolate (189 kJ/mol). Lu and Heyden also conducted DFT calculations and microkinetic modeling to reveal the reaction mechanism of guaiacol HDO over a Ru(0001) model surface [33]. According to the results calculated under the assumption of low hydrogen pressure, the dominant pathway of guaiacol HDO was initiated by the O–H bond cleavage of guaiacol to yield C6H4(O)(OCH3), followed by sequential dehydrogenation, decarbonylation, and hydrogenation to yield C6H4(O)(OC), C6H4O, and phenol, respectively.

In addition to DFT calculations, Chin et al. performed a series of systematic kinetic measurements combined with isotope experiments to reveal the detailed reaction pathways for the Ru-catalyzed conversion of guaiacol under hydrogen pressure [30, 34, 35]. According to kinetic measurements conducted in a semi-batch reactor, the conversion of guaiacol can be divided into two independent pathways; C–OCH3 bond cleavage pathway yields phenol, cyclohexanone, cyclohexanol, and cyclohexane, whereas the H-addition pathway leads to the formation of 2-methoxycyclohexanol (Fig. 3) [34, 35]. The pathway selectivity was determined by the coverage of the H-adatom on the catalyst but was not dependent on the guaiacol concentration. Higher H-adatom coverage reduced the selectivity toward the C–OCH3 bond cleavage pathway, but increased the overall turnover rate. They also elucidated the role of protons and water solvents on guaiacol HDO over Ru nanoparticles, where water promotes HDO by reducing the activation free energies [30]. Hensley et al. explored the role of solvent in C–O bond cleavage and the ring saturation reaction of guaiacol over the H2O–Ru(0001) interface. It has been reported that protic polar solvents (such as water) selectively facilitate C–O bond cleavage, and the activation of guaiacol in either pathway is influenced by various factors, including reactive hydrogen identity, enol/keto equilibrium, and the accessibility of the transition state intermediate [34].

Fig. 3
figure 3

Copyright 2021 Elsevier

Reactive intermediates and their Gibbs energy for the two competitive pathways of C–O cleavage and ring saturation during guaiacol–H2 reactions at the H2O–Ru(0001) interface. Reprinted with permission from [34].

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Reaction Conditions for HDO

In addition to the properties of HDO catalysts, the reaction conditions significantly affect the performance of the catalysts used in the HDO of lignin-derived pyrolysis oil and its model compounds. The HDO of phenolic compounds generally requires high reaction temperature and hydrogen pressure [9, 36]. Although a higher hydrogen pressure can promote the overall HDO reaction, it induces the excessive hydrogenation of the aromatic ring to yield aliphatic hydrocarbons; therefore, arene compounds are preferred under the reaction conditions of high temperature and low hydrogen pressures [36].

The solvent composition also plays a key role in controlling the catalytic performance because it can control the stability of the reaction intermediate, the dispersion of the reactant and catalyst, and the separation of the target products [30, 34, 37]. Zhang et al. reported that Ru/Nb2O5-M catalyst used in a decalin/water biphasic solvent system, which enabled the selective HDO of phenol to yield benzene. They pointed out that the synergistic effect of the biphasic system and the Nb2O5-MC composite enabled the formation of an emulsion, and the presence of water improved benzene selectivity by inhibiting the hydrogenation of benzene [37]. The effect of the solvent on guaiacol HDO over Ru catalysts was interpreted from DFT calculations, and it was confirmed that the production of cycloalkanes from phenol is preferred in the aqueous phase over in the vapor phase or liquid phase reactions using other less protic solvents [38]. In addition, protic solvent such as 2-propanol, methanol, ethanol, and formic acid can provide hydrogen in the transfer hydrogenation (CTH) or transfer HDO catalyzed by Ru-based catalysts [17, 29, 39, 40]. Especially, 2-propanol has been reported as an effective hydrogen donor in the HDO of lignin-derived phenolic monomers catalyzed by Ru-based catalysts [39, 40].

Especially for the case of Ru catalysts, molecular nitrogen, which is generally utilized as an inert gas in most of the reactions, can participate in the reaction and enhance the HDO activity [41]. Duan et al. conducted HDO of p-cresol to yield toluene using Ru/TiO2 catalyst under the reaction condition of 160 °C and 1 bar H2, and it was shown that 4.3-fold increase in the HDO performance could be achieved, simply by adding 6 bar of N2 instead of 6 bar of He [41]. Combined with various in situ catalyst characterizations and DFT calculations, the authors suggested that molecular N2 adsorbs on the surface of metallic Ru and is converted into hydrogenated nitrogen species, which eventually promotes Caryl-O bond cleavage and –OH group hydrogenation. This enhancement via N2 molecules was also observed for other Ru catalysts such as Ru/Al2O3, Ru/ZrO2, and Ru/C, demonstrating that this approach of promoting HDO activity via N2 can be universally applied to Ru-based HDO catalysts.

Ru-Based HDO Catalysts

Ru Catalysts Used in Various Reactions

Similar to the various active metals used in HDO catalysts, Ru has hydrogen activation ability, where metallic Ru can readily dissociate molecular hydrogen into H atoms at all temperatures [42]. Similar to the Pd(111) surface, it has been reported that the dissociative adsorption of H2 occurs on the aggregate of at least three H-vacancies on Ru(001), where single H-vacancies or pairs of adjoining vacancies are inactive during H2 dissociation [43]. Owing to their superior hydrogen activation ability, Ru-based catalysts are widely utilized in various hydrogen-participating reactions [15], such as biomass hydrogenation [14, 16, 17], Fischer–Tropsch synthesis [20,21,22], and CO2 hydrogenation reaction [18, 19].

Hydrogen atoms which are dissociatively adsorbed on Ru can spillover to the neighboring support, thus the activity of the Ru-based catalysts catalyzing hydrogen-participating reactions can also be substantially affected by hydrogen spillover [44,45,46,47,48,49]. Especially for the Ru catalysts loaded on reducible oxide supports, hydrogen atoms spilt from the metallic Ru surface to the metal oxide and diffused through the surface (TiO2 and CeO2) or bulk (WO3) of the metal oxide support in the form of proton (H+) and electron (e−) pair [46]. By utilizing hydrogen spillover in Ru-based catalysts, modulation of product selectivity or enhancement of metal efficiency could be achieved in various reactions [44,45,46,47,48,49]. For instance, the selective hydrogenation of benzene to cyclohexene can be achieved by encapsulating Ru/TiO2 with porous TiO2 coating ((Ru/TiO2)@p-TiO2), where the direct contact between benzene and Ru is prevented by the TiO2 coatings [48]. The hydrogen atom activated on the Ru surface spillover to the porous TiO2 reacted with benzene adsorbed on the TiO2 coating, and high selectivity for cyclohexene was achieved because further hydrogenation of cyclohexene to cyclohexane is less favored on TiO2 than on the Ru surface [48]. The spillover of hydrogen also enhanced the catalytic performance of the 3D pompon-like RuCo catalyst used in guaiacol HDO, where hydrogen atom spilt-over from the electron-rich Ru sites significantly enhanced the HDO process [47].

Our group demonstrated that the metal efficiency of Ru/TiO2 in the guaiacol hydrogenation reaction could be greatly enhanced by the simple addition of a pristine TiO2 support to Ru/TiO2 (Fig. 4 a and b), where spilt-over hydrogen generated additional active sites on the pristine TiO2, which was inert in the hydrogenation reaction when solely used in the reaction [45]. Recently, we also observed that this strategy of enhancing the catalytic activity via hydrogen spillover could also be applied to the HDO of guaiacol, and the degree of enhancement was influenced by the degree of hydrogen spillover and the adsorption behavior of the reactants, which varied depending on the phase of TiO2 (Fig. 4c and d) [44]. Combined with the catalytic reactions and various characterizations, such as H2–TPR, H2 chemisorption, H2–DRIFT, and ATR–IR, it was confirmed that both the hydrogen spillover and the adsorption of guaiacol were facilitated more on rutile TiO2 than on its anatase counterpart, thereby promotion of the HDO performance by hydrogen spillover was more pronounced for the rutile catalysts.

Fig. 4
figure 4

a Comparison of the guaiacol conversion when Ru(x)/TiO2 catalysts were solely used and 0.1 g of pristine TiO2 was added. b Proposed model of active sites considering the hydrogen spillover in a hydrogenation reaction conducted using the Ru/TiO2 catalysts. Reprinted with permission from ref. [45]. Copyright 2022 Elsevier. c Oxygen removal obtained from Ru/TiO2(R) and Ru(5)/TiO2(A) catalysts in guaiacol HDO. d Effect of TiO2 crystal phase on the hydrogen spillover induced guaiacol HDO. Reprinted with permission from Ref. [44]. Copyright 2022 American Chemical Society

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In the case of Ru catalysts loaded on reducible oxide supports such as TiO2, modification of the Ru dispersion and morphology can be conducted by altering various metal–support interactions (MSIs), which could result in enhanced catalytic performance or the catalyst stability. The MSI in Ru-based catalysts can be controlled by modifying various factors during the catalyst preparation steps, such as the surface area of the support, temperature of the thermal treatment, phase of the support, methods for the reduction treatment, and preparation methods.

Controlling the catalytic activity by modifying the MSIs has recently been reported for CO and CO2 hydrogenation reactions using Ru/TiO2 catalysts [18, 50, 51]. Abdel-Magged et al. prepared Ru catalysts loaded on various commercially available TiO2 supports with different surface areas (ranging from 20 to 235 m2/g) for use in CO methanation [51]. The activity of the catalyst increased with enhanced CO adsorption strength for the higher surface area of TiO2, but decreased for TiO2 with excessively high surface area due to the strong metal–support interaction (SMSI) [51]. The MSIs can also be modified by varying the calcination temperature of the Ru/TiO2 catalysts, where a higher calcination temperature induces the extensive formation of Ru–O–Ti bonds in the RuxTi1−xO2 interphase, which results in a higher degree of Ru nanoparticle encapsulation by the TiOx overlayer (Fig. 5) [50]. The SMSI-induced Ru/TiO2 catalyst exhibited higher catalytic activity for CO2 methanation with 100% methane selectivity. The MSIs between Ru and TiO2 were also strongly affected by the phase of TiO2. According to Zhou et al., the identical lattice structures of rutile TiO2 and RuO2 resulted in high interfacial compatibility, which led to the formation of epitaxial overlayers of RuO2 on rutile TiO2 by annealing the Ru/TiO2 catalyst in air [18]. The MSI-controlled annealed Ru-loaded rutile TiO2 catalyst showed significantly enhanced CO2 hydrogenation activity compared to directly reduced Ru/TiO2 catalysts. In contrast, annealing of the Ru–loaded anatase TiO2 catalyst induced SMSI, which resulted in a severe decrease in catalytic activity.

Fig. 5
figure 5

a Classic support migration mechanism for SMSI formation, as in the case of the Ru/TiO2-200Air sample. b Proposed Ru–O–Ti co-reduction mechanism for the facile formation of SMSI, as in the case of the Ru/TiO2–xAir (x = 300, 400, or 500 °C) samples. Reprinted with permission from Ref. [50]. Copyright 2022 American Chemical Society

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The modification of MSIs to control catalytic activity can also be applied to Ru–based catalysts used in the synthesis and decomposition of ammonia [52, 53]. Li et al. prepared Ru/CeO2 catalysts using two different reduction methods (high-temperature hydrogen reduction and NaBH4 treatment) to control the nature of the interfacial perimeter between Ru and CeO2 [52]. Thermal reduction under a hydrogen atmosphere induced SMSI, which negatively affected the catalytic activity by reducing the number of exposed Ru sites. However, NaBH4 treatment caused electronic MSI (EMSI) between Ru and CeO2, resulting in an increase in metallic Ru, proportion of Ce3+, exposed Ru sites, and amount of surface oxygen species. The authors claimed that the reduced hydrogen adsorption strength and enhanced nitrogen dissociation ability of the EMSI-induced NaBH4-treated catalyst resulted in higher ammonia synthesis rates than that of SMSI-induced catalyst prepared by high-temperature hydrogen reduction. The MSI between the Ru nanoparticles and the support also differed according to the preparation method for Ru/Sm2O3 catalysts used in ammonia decomposition; the catalyst prepared by the precipitation method showed superior MSI and catalytic performance compared to other catalysts prepared by impregnation or solid milling methods [53].

Because Ru-based catalysts show excellent hydrogenation activity, the HDO of phenolic compounds can be conducted even under mild reaction conditions when combined with proper additional catalytic functionalities [54,55,56,57,58,59,60,61]. Table 1 summarizes the reaction conditions and catalytic activities of the Ru-based catalysts, which showed high HDO performances at low reaction temperatures of 200 °C or below. Numerous studies have reported Ru-based HDO catalysts that exhibit high HDO performance at low reaction temperatures through the incorporation of various acid catalysts such as zeolites [54, 55, 58,59,60] and heteropoly acids [56, 57] to enhance the oxygen removal ability, and each case of enhanced activity of Ru-based HDO catalysts upon the introduction of such acidic functionalities will be discussed in more detail later. In addition to the introduction of an acidic catalyst, efficient HDO of phenolic monomers and dimeric esters can be achieved at low temperatures by the addition of an ionic liquid to the catalytic system [61].

Table 1 Summary of the reaction conditions and product yields of phenolic HDO catalyzed by Ru-based catalysts at reaction temperatures of 200 °C or below
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Components Affecting the Performance of Ru-Based HDO Catalysts

The activity and stability of the catalytic active sites can be significantly affected by a variety of components, such as the change in the metal dispersion, electronic states of active metals, MSIs and acidic properties that vary with the type of the supports, and changes in the acidic properties and adsorption behaviors of reactants resulting from the introduction of a promoter. This is also true for the Ru-based HDO catalysts. Newman et al. prepared a series of Ru-loaded catalysts supported on various carbon, silica, alumina, and titania supports and evaluated the effects of different supports and metal dispersions on the HDO of phenol [62]. The authors controlled the dispersion of the Ru metal by differing the support materials, Ru contents, and calcination treatment, and compared the selectivity of the DDO and HYD pathways according to the Ru dispersion under the reaction conditions of 300 °C and 650 psi of H2 pressure. Most of the catalysts preferred the HYD pathway, which was attributed to the high hydrogen pressure and excellent hydrogenation ability of Ru. However, Ru/TiO2 with high Ru dispersion showed exceptional selectivity toward the DDO pathway. The authors suggested that the strong interaction between the hydroxyl group of phenol and the Ru neighboring Ti3+ sites, which was reduced by hydrogen spillover, was responsible for its high affinity toward DDO pathways.

In addition to the change in the Ru dispersion, the acidic properties of the HDO catalysts could be modulated by selecting the proper support material. Zeolites are representative materials used as acidic supports for Ru-based HDO catalysts, and various strategies, such as the introduction of mesoporosity or encapsulation of Ru nanoparticles to enhance catalytic performance or stability, have been applied [59, 63]. For catalysts with metal oxide supports, the interfacial sites of the Ru nanoparticles and the supports or the oxygen vacancies, created by the spilt-over hydrogen migrated from the Ru, are also responsible for the HDO performance of the catalysts [44, 47].

Promoters are also key components of numerous Ru-based HDO catalysts that customize the catalyst to achieve the desired catalytic properties. Non-metallic promoters such as P, S, and Cl can modify the morphology of Ru particles or alter the adsorption behavior of the reactants, which can result in the selective conversion of the reactants to the desired products [64,65,66]. Oxophilic metal oxides such as W, Mo, and Re oxides can enhance the oxygen removal ability of catalysts while suppressing the hydrogenation reaction ability of Ru, resulting in an increase in arene selectivity [26, 40, 67,68,69,70,71,72,73]. To enhance the acidity of the catalyst, super acids, such as heteropoly acids, can be introduced into Ru-based catalysts [56, 57, 74]. Various transition and noble metals can also be introduced into Ru-based HDO catalysts as promoters, which can be used to partially replace expensive Ru or enhance the activity of monometallic catalysts [60, 75,76,77,78,79,80,81,82,83]. Each of the above-mentioned strategies is discussed in more detail in the following sections.

Selection of Support Materials

The choice of support materials in metal-loaded heterogeneous catalysts used for the conversion of various types of biomass can affect the dispersion of the active metal and stability of the catalyst. Furthermore, the use of acidic supports or support materials with good redox properties can significantly affect the activity of catalysts by providing additional active sites with the desired catalytic functionality. Acidic supports (such as zeolites), metal oxides, and carbon supports are widely used as support materials for Ru-based HDO catalysts.

Zeolites

In 2010, Vispute et al. developed an efficient catalytic system for the production of light olefins and aromatic hydrocarbons from pyrolysis oil by combining Ru/C and Pt/C with the ZSM-5 zeolite [84]. Although zeolite was not used as a support material in this study, the incorporation of zeolites for hydroprocessing reduced the oxygen content of the product mixture by converting the hydrogenated products into oxygen-free hydrocarbons. Owing to the excellent low-temperature acetic acid hydrogenation ability of Ru, as well as the introduction of zeolites after the hydrogenation reaction, the authors were able to avoid coke formation, which severely occurs when biomass feedstock reacts directly with zeolites, and proposed an efficient catalytic system for the hydroprocessing of pyrolysis oil through the combination of noble metals and zeolites.

Table 2 shows the summary of the reaction conditions and product yields obtained from the HDO of phenolic model compounds catalyzed by various Ru-based HDO catalysts loaded on zeolite supports. ZSM-5 is one of the most extensively investigated zeolites used as Ru-based HDO catalysts. Zhang et al. reported that bifunctional Ru/HZSM-5 with Si/Al ratio of 25 showed excellent catalytic performance in the aqueous phase HDO of phenolic monomers and dimers, where 99.9% conversion of phenol and 96.3% cyclohexane selectivity could be obtained from Ru/HZSM-5(25) under the reaction conditions of 150 °C and 5 MPa H2 pressure (Fig. 6 a) [85]. The Ru/HZSM-5 catalyst resulted in a higher cyclohexane yield than its Pd- or Pt-loaded counterparts, suggesting that Ru catalysts have an advantage over Pd- or Pt-loaded catalysts given that Ru is much cheaper than other noble metals. This outstanding HDO performance is attributed to the combined hydrogenation and dehydration functionalities of Ru and HZSM-5. The authors also confirmed that ZSM-5 with a lower Si/Al ratio resulted in higher hydrocarbon selectivity, implying that the dehydration of cyclohexanol intermediates proceeded more efficiently on a more acidic catalyst. However, the catalyst after the reaction showed decrease in the specific surface area from 254 m2/g to 204 m2/g, and the crystallinity of HZSM-5 also decreased in the recovered catalysts, which was presumably attributed to the insufficient hydrothermal stability of ZSM-5 or the adsorption of reactants. An increase in the average Ru particle size was also detected after four reaction cycles, which induced a loss of catalytic activity with repeated usage of the catalyst.

Table 2 Summary of the reaction conditions and product yields of phenolic HDO catalyzed by Ru-loaded zeolite catalysts
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Fig. 6
figure 6

a Product distributions of phenol HDO using Ru/HZSM-5(25) under the reaction conditions of 140 °C and 50 bar H2. Reprinted with permission from Ref. [85]. Copyright 2014 American Chemical Society. b, c Dependences of phenol conversion and product selectivity on time over Ru/HZSM-5 and Ru/HZSM-5-OM catalysts at 150 °C and 40 bar H2. Reprinted with permission from Ref. [63]. Copyright 2015 American Chemical Society

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In contrast to the reference above, Luo et al. reported Ru/HZSM-5 as a hydrothermally stable catalyst for the aqueous phase conversion of lignin-derived substituted phenols to arene compounds [86]. Under the reaction conditions of 240 °C and 2 bar H2 + 6 bar N2, Ru loaded on HZSM-5 (Si/Al=100) showed nearly quantitative formation of benzene and methanol from guaiacol. Other Ru-loaded catalysts, including Ru/Al2O3, Ru/TiO2, Ru/activated clay, Ru/ZrO2, and Ru/SiO2 exhibited significantly lower benzene yields under identical reaction conditions. After the repeated recycling test, a slight increase in the average Ru nanoparticle size (4.1 nm to 6.3 nm after four reaction cycles) was observed, but no severe damage to the crystallinity of the zeolite or significant decrease in specific surface area was observed. The deposition of the coke species was not observed after the reaction under these reaction conditions.

Although ZSM-5 has been widely investigated as a support material for HDO catalysts owing to its excellent acidic properties, the poor accessibility of bulky biomass reactants due to its microporous structure is considered a significant challenge. To overcome these limitations, Wang et al. developed a hierarchically porous ZSM-5 zeolite with micropores and b-axis-aligned mesopore-supported Ru nanoparticles (Ru/HZSM-5-OM), which possessed a significantly increased number of accessible acid sites owing to their open mesopore structure (Fig. 6 b and c) [63]. Under the reaction conditions of 150 °C and 4 MPa H2, Ru/HZSM-5-OM showed high conversion of 97.5% and 70.0% cyclohexane selectivity from the HDO of 2,6-dimethoxyphenol, whereas 70% conversion and 55.4% cyclohexane selectivity were obtained from the conventional microporous Ru/HZSM-5. Ru/HZSM-5-OM also showed excellent reusability with 95.8% 2, 6-dimethoxyphenol conversion and 69.0% cyclohexane selectivity, even after ten reaction cycles.

Ru/HZSM-5 also showed high catalytic performance in the HDO of diphenyl ether (DPE), where a 100% cyclohexane yield was obtained under reaction conditions of 210 °C and 1 MPa H2 [87]. The conversion and product selectivity differed significantly according to the solvent, where non-polar aprotic solvents such as n-hexane and tridecane resulted in higher conversion and cyclohexane selectivity compared to the cases when alcohols were utilized as solvent. Our group recently identified that Ru/HZSM-5 exhibits high activity in the complete HDO of vanillin to cycloalkanes, and the product distribution is highly dependent on the balance between the metal and the acid [88]. The catalyst with a higher metal/acid ratio preferred Caryl–C cleavage, resulting in a higher yield of cyclohexane, whereas the catalyst with a lower metal/acid ratio afforded a higher methylcyclohexane yield. We also confirmed the synergistic effect between the acid sites of HZSM-5 and Ru, where a ~ 40-fold increase in the metal efficiency for the aromatic ring hydrogenation reaction was observed for acidic Ru(0.5)/HZSM-5(23) (Si/Al2=23) compared to the merely acidic Ru(0.5)/HZSM-5(300) (Si/Al2=300). Ru loaded on Ga-doped HZSM-5 (Ru/Ga-HZSM-5) has also been reported as a catalyst with high performance in the HDO of DPE under the reaction conditions of 180 °C and 1 MPa H2 [58]. The Ru/Ga-HZSM-5 catalyst showed a higher HDO performance than the conventional Ru/HZSM-5 catalyst, which was attributed to the enhanced interaction between the Ru metal and the support.

In addition to the Ru-loaded HZSM-5 catalysts, Ru catalysts supported on beta zeolites also exhibited high HDO activity under mild reaction conditions. The Ru/H-beta catalysts showed higher DPE HDO activity compared to other noble metal-loaded H-beta catalysts or Ru catalysts loaded on different zeolites or metal oxide supports [54]. By utilizing Ru/H-beta catalysts, a high HDO performance was achieved under the mild reaction conditions of 140 °C and 4 MPa H2, even with a low Ru loading of 0.5 wt.%.

Stockenhuber’s group compared two Ru catalysts loaded on ZSM-5 and BEA zeolites for the HDO of guaiacol and bio-crude oil [28, 89]. For guaiacol HDO conducted under the reaction conditions of 250 °C and 4 MPa H2, Ru/BEA and Ru/ZSM-5 with a low Si/Al ratio showed a high cyclohexane yield of ca. 70% with nearly complete conversion, but the catalysts with high Si/Al ratios showed lowered hydrocarbon yield [28]. The notable point was that certain amount of bicyclic compound (1,1’-bicyclohexyl) was obtained from Ru/BEA catalysts, but not from Ru/ZSM-5 catalysts. It was also noteworthy that Ru/BEA showed high performance for the HDO of bio-crude oil conducted at reaction temperature of 330 °C, while Ru/ZSM-5, Ru/SiO2, and Ru/Al2O3 showed poor catalytic activity under the identical reaction conditions. The difference in the catalytic activity between Ru/BEA and Ru/ZSM-5 was attributed to the difference in the extent of mesoporosity and the concentration of the Brønsted acid sites. Ru/BEA was deactivated by active site poisoning with phenolic adsorption (which was confirmed through the chemisorption and BET analysis), but could be successfully regenerated by calcination treatment at 500 °C in air for 200 min.

The same group also examined the guaiacol HDO activity of Ru catalysts with extremely low Ru loadings (about 0.2 wt.%) loaded on ZSM-5 and BEA zeolites, prepared by ion-exchange or impregnation method [89]. Both the guaiacol conversion and cyclohexane formation rate were higher on the ion-exchanged catalysts compared to the catalysts prepared by impregnation method, and the cyclohexane formation rate was determined by the total amount of Brønsted acid rather than the structure of the zeolite. The partially reduced Ru species (with oxidation states ranging from Ru(0) to Ru(III)) in the ion-exchanged catalysts exhibited higher H2 adsorption activity, resulting in superior hydrogenation activity compared to their counterparts prepared by the conventional impregnation method. In addition to the catalyst prepared by ion-exchange method, Ru/Hβ catalyst prepared by deposition–precipitation method was also reported to exhibit higher HDO performance than the catalyst prepared by direct wet impregnation method [55].

Kumar et al. prepared series of Ru catalysts loaded on various types of zeolites (ZSM-5, β zeolite, Y zeolite, and mordenite) and metal oxides, and compared its catalytic performance in the m-cresol HDO [90]. Among them, the highest methylcyclohexane yield was obtained from Ru/ZSM-5, and the activity was maintained up to 240 h of continuous reaction at 250 °C. Ru/ZSM-5 showed high HDO performance with a WHSV of 0.5 and 1 h−1, but the conversion and target product yield significantly decreased at a WHSV of 1.5 h−1. To enhance the catalytic activity and sintering resistance of Ru nanoparticles, catalysts with Ru nanoparticles encapsulated in the cavities of MWW zeolites were prepared through in situ hydrothermal crystallization method [59]. The encapsulation of Ru nanoparticles resulted in highly dispersed Ru metal clusters with particle sizes of < 1.5 nm, and a high HDO performance was obtained even under mild reaction conditions of 160 °C and 3 MPa H2. The authors also pointed out that close proximity between the Ru metal cluster and Brønsted acid sites induced the synergistic effect, and excellent sintering and leaching resistance could be achieved by encapsulation of the Ru nanoparticles. A recent article reported Ru oxide catalysts loaded on three different types of micro-mesoporous materials (S1: MCM-41-like mesoporous material with ZSM-5 crystallites, S2: mixed phase of MCM-41-like mesoporous material and MFI nanosheets, and S4: MFI nanosheets) used in the HDO of guaiacol and real bio-oil [91]. The authors suggested that amount of acid sites determined the production rate of the deoxygenated products, whereas the morphology of the support affected the degree of coke formation. They also pointed out that the adsorption of guaiacol was favored over the oxidized Ru species over the reduced Ru during hydrogenation, which was confirmed by in situ DRIFT study.

Metal Oxides

Along with the zeolites, various reducible or non-reducible metal oxides have been extensively used as supports for Ru-based HDO catalysts (as shown in Table 3), with the activities varying greatly depending on the type of metal oxides. The catalytic performances of Ru-based catalysts loaded on various metal oxides (including mesoporous TiO2, Al2O3, SBA-15, and P25) were compared in anisole HDO [92]. The reaction pathway was significantly influenced by both the reaction conditions and the supporting materials. Benzene, which could be obtained from the DDO pathway, was favored at lower hydrogen pressure and TiO2 loaded catalysts, whereas higher hydrogen pressures and Ru-loaded Al2O3 or SBA-15 resulted in a higher yield of HYD pathway products. The authors proposed that the abundance of Ti3+ defect sites and oxygen vacancies, formed by the spillover of hydrogen, were key factors in facilitating the DDO pathway. Within the TiO2 supports, mesoporous TiO2 showed higher DDO pathway selectivity than nonporous commercial TiO2 (P25), which resulted from the stronger interaction between Ru nanoparticles and the support.

Table 3 Summary of the reaction conditions and product yields of phenolic HDO catalyzed by Ru-loaded metal oxide catalysts
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Boonyasuwat et al. compared the activities of Ru catalysts loaded onto various metal oxides (SiO2, Al2O3, and TiO2) in guaiacol HDO [93]. Among the various metal-oxide-loaded Ru catalysts, Ru-loaded TiO2 showed superior activity in guaiacol HDO, which was attributed to the Lewis acid sites present on the support. The same group reported that the stability and guaiacol conversion were strongly dependent on the calcination temperature and TiO2 support phase, where the rutile TiO2 phase played an important role in stabilizing the Ru particles during the calcination step compared to its anatase phase counterpart [94]. The authors prepared Ru loaded on pure anatase TiO2 and P25, where P25 is a commercial TiO2 with a mixed crystal phase of anatase and rutile with an anatase/rutile ratio of about 4/1, and modified the calcination temperature after impregnation of the Ru precursor onto each TiO2 support. The average size of the Ru nanoparticles increased at higher calcination temperatures for the anatase-loaded catalyst, whereas the degree of change with elevated calcination temperature was relatively insignificant for the P25-loaded catalysts. Considering that the specific surface area of the anatase support was much higher than that of P25, it can be indicated that presence of rutile phase of P25 played a role as an anchoring site for RuO2, which resulted in higher dispersion and thermal stability than when RuO2 was loaded on pure anatase TiO2. In addition, they proposed that the high guaiacol HDO performance of Ru/TiO2 was a result of defect sites generated via hydrogen spillover from the Ru metal to the reducible TiO2 support.

The mechanistic details and active sites of Ru/TiO2 catalysts in phenol HDO were further elucidated by Nelson et al. using D2O isotope experiments and DFT calculation [95]. They pointed out that the interfacial sites between Ru and the TiO2 support played a crucial role in the HDO of phenol, and that water could act as a co-catalyst, facilitating the direct deoxygenation pathway. In order to synthesize the Ru/TiO2 with highly dispersed Ru, Shu et al. employed a photochemical method at room temperature, which resulted in a high Ru dispersion of 70% for Ru loading of 2 wt.% [96]. The resulting catalyst showed high activity of 100% conversion and 91.3% cyclohexane selectivity under the reaction conditions of 260 °C and 1 MPa H2, while the other metal-loaded TiO2 catalysts showed lower HDO performance. In addition to the crystal phase or preparation method, the morphology of the TiO2 also affects the HDO activity of Ru/TiO2 catalysts. Zhong et al. prepared Ru catalysts loaded on TiO2 supports with different TiO2 morphology [97]: TiO2 nanosphere (TNP), nanobelt (TNB), nanosheet (TNS), and nanocluster (TNC). Among the various TiO2 supported Ru catalysts, Ru/TNP exhibited the best performance in the HDO of guaiacol, which was attributed to tis high surface area and large amount of medium strength acid sites of TNP.

To modify the properties of TiO2, several mixed oxide supports have been utilized in Ru-based HDO catalysts to enhance the support acidity or inhibit the SMSI-induced coverage of Ru [98,99,100]. Lu et al. synthesized a series of Ru/TiO2–ZrO2 with different Ti/Zr ratios as catalysts for guaiacol HDO [100]. Compared to the Ru/TiO2 catalyst, which showed relatively low HDO activity due to the SMSI-induced reduction of exposed Ru sites, Ru/TiO2–ZrO2 showed superior HDO activity at all temperature ranges tested (from 200 °C to 260 °C). Higher Ru dispersion was obtained for the Ru/TiO2–ZrO2 catalysts because ZrO2 hindered the migration of Ti3+ species over the Ru surface during the reduction treatment. The acidity of the TiO2 support can be modified by adding different types of metal oxides such as ZrO2, SiO2, and Al2O3, to TiO2 [98]. Among the various Ru catalysts loaded on various TiO2–based mixed oxide supports, Ru/TiO2–Al2O3 exhibited the highest catalytic performance for the HDO of phenolic compounds, which was attributed to the synergistic effect of the high Ru dispersion and moderate acidity of the TiO2–Al2O3 support.

For Ru/TiO2–based catalysts, defective TiOx adjacent to Ru can act as an adsorption site for oxygen functional groups and as an active site for C–O bond cleavage in the HDO of phenolic compounds. In this regards, Wang et al. prepared Ti-modified SiO2 supported Ru catalysts (Ru/Ti–SiO2) with high defect concentrations and conducted the selective HDO of guaiacol to yield phenol [99]. The Ru/Ti–SiO2 catalysts showed higher C–O bond cleavage rate compared to the Ru/SiO2 and Ru/TiO2, where 83.6% guaiacol conversion and 70.4% phenol selectivity could be achieved under the mild reaction conditions of 240 °C and 0.4 MPa H2. The presence of the Ti–O–Si linkages induced the formation of highly defective TiOx species, which contributed to the high Ru dispersion and HDO activity.

As for the case of TiO2, CeO2 is also a reducible metal oxide support, where the oxygen vacancy on the surface can also act as an active site in HDO reactions [101, 102]. To inhibit the undesired C–C hydrogenolysis and aromatic ring hydrogenation, which competitively occur during the HDO reaction catalyzed by Ru-based catalysts, a solid solution of Ru and Ce oxides (Ru0.05Ce0.95O2) was prepared using a hydrothermal method and utilized in m-cresol HDO to yield toluene [102]. The Ru particles were highly dispersed as isolated atoms or sub-nanometer clusters with rich oxygen vacancies, and unwanted C–C hydrogenolysis and hydrogenation could be avoided in the solid solution catalyst. The authors proposed that high selectivity toward the direct HDO of m-cresol to toluene could be achieved because of the decreased Ru ensemble size, which originated from the strong Ru–O–Ce interaction. The solid solution showed significantly enhanced intrinsic activity compared to conventional Ru/CeO2 and Ru/SiO2 catalysts, and Ru–Ov–Ce sites were suggested as active sites for deoxygenation. In addition to the Ru-Ce mixed oxide catalysts, CeO2 supported Ru single atoms showed high activity in the selective HDO of aromatic oxygenates to yield cyclohexanols [101]. Zhang et al. prepared Ru single-atom catalyst loaded on a CeO2 sheet (denoted as CeO2–S), with Ru loading of 0.1 wt.%. Notably, the Ru single-atom catalyst facilitated hydrogenation of aromatic ring which typically does not proceed with most other single-atom catalysts. Furthermore, selective cleavage of the etheric C–O(R) bond could be conducted while preserving the C–O(H) bond, which enabled a high cyclohexanol selectivity of 99.9% under the reaction conditions of 200 °C and 1 MPa H2. Combined with control experiments and DFT calculations, the authors confirmed that Ru–O–Ce sites were formed in the single-atom catalyst and one Ru atom coordinated with about four O atoms. These Ru–O–Ce sites activated the benzene ring and molecular hydrogen, which facilitated the HDO reactions.

In addition to the aforementioned reducible metal oxide-supported catalysts, Ru supported on niobium-based catalysts has also been reported to be highly effective in HDO reactions [27, 103, 104]. Shao et al. reported that Ru-loaded on a porous niobium oxide catalyst (Ru/Nb2O5) showed superior activity in the one-pot direct HDO of lignin compounds compared to other Ru-loaded catalysts such as Ru/ZrO2, Ru/Al2O3, Ru/TiO2, Ru/HZSM-5, and Ru/C [103]. Ru/Nb2O5 resulted in complete oxygen removal from birch lignin, obtaining 35.5 wt.% of total mass yield of C7–C9 hydrocarbon monomer with 71 wt.% of arene selectivity under the reaction conditions of 250 °C and 0.7 MPa H2. The authors utilized inelastic neutron scattering (INS) combined with DFT calculations to confirm that the exclusive catalytic activity of Ru/Nb2O5 was attributed to the strong adsorption and selective activation of aromatic C–O bonds in phenolic compounds. The same group investigated the effect of Nb2O5 morphology on the HDO activity of the Ru-loaded Nb2O5 catalysts [104]. The authors prepared four types of Nb2O5 by modifying the preparation methods; Nb2O5 flowers (F–Nb2O5), hollowed Nb2O5 (H–Nb2O5), mesoporous Nb2O5 (M–Nb2O5), and layered Nb2O5 (L–Nb2O5). Among the four different types of Ru-loaded Nb2O5 catalysts, Ru-loaded L–Nb2O5 showed the highest activity for the HDO of 4-methylphenol, which was attributed to its high surface area, high Ru dispersion, and large amount of unsaturated NbOx sites (Nb=O groups). The L–Nb2O5 with lower crystallinity and more surface Nb=O groups obtained by reducing the hydrothermal time showed high activity in 4-methylphenol HDO, where hydrocarbon yield of 99.6% and 94.8% of arene selectivity under the reaction conditions of 250 °C and 0.7 MPa H2. The acidity of the Nb-based support could be further enhanced by introducing heteroatom such as P, where Ru-loaded NbOPO4 was reported to be highly active in the HDO of aromatic ethers, phenols, and real bio-oil [27]. High hydrocarbon yields of up to 88.2% were obtained from the HDO of real bio-oil under reaction conditions of 170 °C and 5 MPa H2. However, the catalyst showed slight deactivation caused by carbon deposition and Ru sintering, which could be mostly regenerated after the calcination and reduction treatment.

Owing to their suitable acidic properties, several studies have reported the use of Zr-based materials as supports for Ru-based HDO catalysts [105,106,107]. Multifunctional Ru-loaded sulfated zirconia (SZ) showed high activity in the selective cleavage and HDO of phenethoxy-benzene (PBE), when water was utilized as a solvent [105]. The authors proposed an innovative route for selective HDO of aryl ether to yield aromatic hydrocarbons, which could be achieved by combining solid acidic support with high cyclohexanol dehydration activity. The cascade reactions of C–O bond cleavages proceeded with inhibited hydrogenation reactions, which enabled nearly quantitative aromatic hydrocarbon yields. A suitable metal site (Ru) combined with a solid acid (SZ), and the reaction conditions of low hydrogen pressure (2–8 bar) with high temperature (240 °C) were required to obtain a high aromatic hydrocarbon yield. Ru-loaded tungstated zirconia (Ru/WZr) has also been reported as a highly active catalyst for guaiacol HDO [106]. The authors prepared Ru-loaded WZr catalysts with different W loadings (from 5 to 20 wt.%), where the appropriate amount of W loading (10 wt.%) resulted in the highly dispersed Ru particles with optimal HDO performance. The increase in the tungsten loading enhanced the surface acidity and dispersion of Ru, but a further increase in the W loading beyond the optimal value inhibited the formation of highly dispersed Ru nanoparticles. Shu et al. reported a Ru catalyst loaded on a SiO2-ZrO2 support used in the pyrolysis lignin oil HDO, where the support had the properties of both SiO2 with a large specific surface area and ZrO2 with moderate acidity [107]. Although the main topic emphasized in the literature was the preparation of highly dispersed Ru catalysts via polyol reduction methods, the large amount of acid sites in SiO2-ZrO2 was also responsible for its high catalytic activity. When Ru is loaded onto a support that does not have a sufficiently strong acidity, such as SiO2, demethoxylation can be accomplished readily, but the removal of the hydroxyl group does not occur efficiently, resulting in cyclohexanol being obtained as the major product [108].

Carbon Materials

Owing to their high surface area and hydrothermal stabilities, carbon-based materials are commonly utilized as supports for heterogeneous catalysts in various biomass conversions, and they have also been widely used as supports in Ru-based HDO catalysts. When carbon is used as a support for Ru-loaded catalysts, high Ru dispersion is obtained due to the large specific surface area of carbon. However, due to its lack of acidic functionalities, Ru/C alone does not exhibit sufficient HDO performance, and many studies have reported the introduction of additional catalytic functionalities to enhance oxygen removal.

The addition of Nb2O5 to Ru/C combined with a trace amount of methanol resulted in an effective catalytic system for HDO of lignin monomers under the reaction temperatures of 250 °C and low hydrogen pressure [109]. The pretreatment of the as-received Ru/C under high-pressure hydrogen resulted in a significant increase in the oxygen-free hydrocarbon selectivity from 15.9% to 72%, which could be further enhanced by the addition of Nb2O5. Acidic Nb2O5 served as an active site for the removal of oxygen functional groups, and 100% dihydroeugenol (DHE) conversion and hydrocarbon selectivity were achieved under optimized reaction conditions. The authors proposed that the preparation of Nb2O5 by low-temperature calcination at 450 °C resulted in a Nb2O5 with large amount of acid sites than commercial Nb2O5. They also suggested that methanol additives provided hydrogen to promote HDO at low hydrogen pressures. The combination of homogeneous boric acid (H3BO3) with Ru/C also resulted in a catalytic system with high HDO activity [110]. The addition of H3BO3 to Ru/C resulted in higher HDO performance compared to the case when other organic and mineral acids (such as formic acid, acetic acid, sulfuric acid, phosphoric acid, and hydrochloric acid) were added. Through this bifunctional catalyst system, the hydrocarbon content of raw lignin oil was significantly enhanced from 7.9% to 93.1% under the reaction conditions of 260 °C and 1 MPa H2. Notably, the H3BO3 turned into solid at room temperature, which allowed for easy separation and recovery. When combined with H3BO3, Pd/C and Pt/C also showed high HDO performance, whereas Ni/C resulted in slightly deterred oxygen removal.

Ru-loaded CNT exhibited distinct HDO activity when applied with the biphasic H2O/n-dodecane solvent system [111]. Among the series of supported Ru catalysts, Ru/CNT showed superior activity in eugenol HDO, with > 99% conversion and 98% alkane selectivity under the reaction conditions of 220 °C and 5 MPa H2. The authors explained that the superior catalytic activity of Ru/CNT was a combined effect of the mesoporosity of CNT, specific metal–support interactions, and specific adsorption properties, which were attributed to the unique structure of the CNT. When methanol was used as a solvent, Ru nanoparticles supported carbon nanofiber (Ru@CNF) converted guaiacol, commercial lignin, corn cob-derived lignin, and pine bark-derived lignin at exceptionally low reaction temperature of 80 °C [112]. The authors explained that the surface –COOH groups facilitated the adsorption of oxygenated molecules. A Ru single-atom catalyst supported on mesoporous graphitic carbon nitride (Ru1/mpg-C3N4) was reported for the conversion of vanillin to yield vanillyl alcohol and creosol [113]. The single-atom catalyst showed excellent selectivity for hydrogenation and HDO depending on the reaction conditions; a higher selectivity for hydrogenation was obtained at low reaction temperatures, and HDO was preferred at high reaction temperatures.

Incorporation of Promoter

The choice of a promoter to modulate catalytic performance is one of the most widely utilized strategies in the field of heterogeneous catalysts and has also been extensively employed in the investigations about Ru-based HDO catalysts. Through the addition of appropriate promoter, the adsorption behavior of the reactant can be modified by altering the morphology of the Ru, and the acidic properties of the catalyst can be enhanced by the incorporation of oxophilic metal oxides or heteropoly acids. Transition metal or noble metal precursors can also be added to Ru-based HDO catalysts to create additional active sites with the desired catalytic properties.

Non-Metallic Promoters (P, Cl, S)

The incorporation of non-metallic promoters modifies the adsorption behavior of the reactant or reaction intermediates. Furthermore, the introduction of an electronegative non-metallic promoter could induce modification of electronic properties of the active Ru metal, resulting in improved target product selectivity.

Transition metal phosphides, such as molybdenum and nickel phosphides (MoP and NiP) have been reported to be more active than their sulfide counterparts as HDO catalysts. Analogous to MoP and NiP, ruthenium phosphide (RuPx) also showed high activity in furan HDO, and the selectivity for C4 hydrocarbon dramatically increased compared to monometallic Ru- or NiP-loaded catalyst [66]. The authors suggested that the difference in product selectivity between the Ru and Ru-phosphide catalysts could be attributed to the increased hydrogen availability on Ru-phosphide and the different adsorption configurations of the ring-opened species.

The impregnation of Cl onto the Ru/TiO2 catalyst has been reported to significantly modify the product selectivity in the guaiacol HDO (Fig. 7 a) [64]. The introduction of electronegative Cl species, which were localized at the interfacial site of the metal and support, changed the electronic states of both the Ru and Ti species. The electron-enriched Ru restrained aromatic ring hydrogenation and the phenol selectivity increased, whereas the guaiacol conversion decreased as the amount of Cl increased. Via in situ DRIFT measurements of guaiacol on the catalyst, the authors also confirmed that adsorption of the aromatic ring was suppressed by the introduction of Cl species.

Fig. 7
figure 7

a Effects of the Cl/Ru molar ratio on the catalytic performance of Ru/Cl(x)/TiO2 catalysts in the HDO of guaiacol under the reaction conditions of 240 °C an10 bar H2. Reprinted with permission from Ref. [64]. Copyright 2021 American Chemical Society. b Conversion and oxygen removal obtained from the guaiacol HDO by Ru(5)/TiO2 and Ru(5)S/TiO2 catalysts under the reaction conditions of 100 °C and 10 bar H2. Reprinted with permission from Ref. [65]. Copyright 2023 Elsevier B.V. All rights reserved

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Our group recently reported that S introduced Ru/TiO2 showed enhanced oxygen removal ability in guaiacol HDO compared to the pristine Ru/TiO2, even at low reaction temperature of 100 °C (Fig. 7 b) [65]. When S species were introduced into Ru/TiO2, the oxygen removal obtained at 100% conversion in guaiacol HDO gradually increased, although the conversion obtained at identical reaction time was sacrificed as the loaded amount of S increased. By CO–DRIFT, XPS, and NH3-TPD-MS analysis, it was confirmed that the distribution of the surface Ru species was modified with the introduction of S species, which was responsible for the enhanced oxygen removal ability at low temperature of 100 °C for the S doped Ru/TiO2 catalysts.

W, Mo, Re

Bifunctional catalysts in which oxophilic metal oxides such as WOx, MoOx and ReOx combined with Ru have been investigated for the HDO of bio-oil and various bio-oil model compounds. According to the number of literatures, the introduction of oxophilic metal oxides can be used to increase the selectivity of arene compounds in the HDO of bio-oil or bio-oil model compounds by reducing the hydrogenation ability of Ru and promoting C–O bond cleavage, thereby reducing the consumption of hydrogen and obtaining value-added BTX products.

The introduction of WOx into Ru-based HDO catalysts enabled high arene selectivity. For example, in the Ru-loaded SiO2–Al2O3 catalyst used for phenolic HDO, the introduction of WOx suppressed aromatic ring hydrogenation and promoted direct cleavage of the aromatic C–O bond instead [68]. Under the reaction conditions of 270 °C and 2 MPa H2, the Ru–WOx/SiAl catalyst resulted in 100% conversion and 81% arene selectivity in the HDO of p-tert-buthylphenol, while Ru/SiAl without the WOx promoter showed a much lower arene selectivity of 10% under the same reaction conditions. The introduction of tungsten oxide enhanced the yield of arenes obtained from a variety of phenyl ethers, including dimeric lignin model compounds. The authors attributed this high arene selectivity to the combination of the hydrogenation ability of Ru and the Lewis acidity of the W species. Ru loaded onto a WOx/ZrO2 solid acid catalyst (Ru–WOx/ZrO2) was investigated for the HDO of phenolic model compounds [26]. The authors suggested that WOx provides oxophilic sites that facilitate the cleavage of the aromatic C-O bond. When the Ru–WOx/ZrO2 catalyst was used in the HDO of pretreated bio-oil, arenes were obtained as the major products under the reaction conditions of 240 °C and 1 MPa H2. Recently, Kong et al. reported bifunctional catalysts with Ru metal sites and WOx oxophilic sites loaded on nitrogen-doped carbon (Ru/WOx/N–C) for the conversion of Kraft lignin [67]. The bio-oil yield and char formation were dependent on the loaded amount of WOx, and Ru/WOx/N–C with optimized Ru and WOx loadings resulted in 96.89 wt.% oil yield and 20.85 wt.% arene monomer yield in the HDO of Kraft lignin at 310 °C with 5 h of reactions. The catalyst exhibited high reusability with no significant deactivation after five reaction cycles.

Similar to WOx, MoOx also exhibits high oxophilicity, which could result in enhanced ability for C–O bond cleavage. Therefore, bifunctional catalysts combining oxophilic MoOx and hydrogen-activating metals such as Ni, Pd, and Pt have been widely investigated for HDO reactions. Because Ru is economically more favorable than other noble metals such as Pd or Pt and more active in hydrogenation and deoxygenation reactions than Ni, bifunctional catalysts combining Ru and MoOx have also been extensively investigated [69,70,71,72]. Xiang et al. prepared HDO catalysts with uniformly dispersed Ru nanoclusters and defective MoOx clusters on nanosized tetragonal zirconia (t-ZrO2) [72]. The Ru catalysts combined with defective MoOx clusters showed significant enhancement in benzene selectivity up to 84.7% in anisole HDO, compared to the Mo-free catalyst, which showed a benzene selectivity of 25.1% under the reaction conditions of 250 °C and 1 MPa H2. However, anisole conversion was sacrificed with the increasing amount of Mo. The authors suggested that the interfacial Ruδ+–Ov–Mo5+ sites rather than Mo5+–Ov–Mo6+ sites were responsible for the enhanced benzene selectivity. The catalyst retained its catalytic activity for four successive runs, where the selectivity remained almost unchanged, although a slight decrease in anisole conversion was observed. Lv et al. reported that Ru and MoOx supported on activated carbon catalysts prepared by the sequential impregnation of a precursor, which showed high activity in the HDO of lignin oil [71]. The mild reaction temperature of 160 °C and high hydrogen pressure of 30 bar resulted in the aliphatic hydrocarbon as major products. According to the authors, stepwise impregnation of molybdenum precursor, followed by impregnation of the ruthenium precursor, resulted in highly dispersed Ru and MoOx nanoparticles, and the activity and product selectivity could be controlled by regulating the reduction temperature of the MoOx species. In addition to the above-mentioned studies, it has also been reported that phosphate RuMo catalysts loaded on carbon supports or the Ru-loaded Mo-AlPO4 loaded on amorphous silica-alumina catalysts show high activity in the HDO of bio-oil [69, 70].

Re has also been introduced as an oxophilic promoter that can enhance the rate of C-O bond cleavage during the HDO of bio-oil model compounds [40, 73, 114]. In the catalytic transfer hydrogenation of guaiacol using isopropanol, the introduction of Re to Ru/C resulted in increased cyclohexane selectivity, which was attributed to enhanced surface acidity [40]. The same group reported RuRe bimetallic catalysts loaded on various carbon supports, and RuRe loaded on multi-walled carbon nanotube (MWCNT) showed superior activity in guaiacol HDO compared to activated carbon (AC) loaded catalyst [73]. The authors suggested that close proximity between Ru and Re is essential for the efficient hydrogenolysis of phenols.

Heteropoly Acids

In order to impart the acidic function to the Ru catalysts loaded on non-acidic support, heteropoly acid loaded bifunctional catalysts were investigated as an efficient catalytic system for the HDO of lignin-derived phenolic model compounds [56, 57, 74]. To introduce acidity into Ru/C catalysts, Yang et al. loaded phosphotungstic acid (HPW) onto Ru/C by taking advantage of the temperature-controlled phase-transfer behavior of HPW [56]. The Ru/C–HPW catalyst showed comparable guaiacol HDO performance as Ru/C combined with homogeneous acid such as H3PO4. The catalyst resulted in 78.9% hydrocarbon yield from the HDO of lignin oil under the reaction conditions of 200 °C and 1 MPa H2 with reaction time of 4 h. Especially in the guaiacol HDO, the authors confirmed that the content of bicyclohexyl increased at higher HPW dosage. Same group combined silicotungstic acid (HSiW) as an acidic catalyst with Ru-based bio-char catalysts [57]. The Ru/C–bamboo combined with HSiW exhibited high cyclohexane yield of 87.7% in guaiacol HDO even at low reaction temperature of 140 °C. The authors also confirmed that the C–bamboo bio-char enabled high Ru metal dispersion among various bio-char-derived carbon supports, which was attributed to the abundant mesoporous structure and large pore volume, resulting in the notable hydrogen adsorption ability. Ru nanoparticles loaded on mixed metal oxides of Zn and Al layered double hydroxides intercalated by phosphotungstate anions has been also investigated in the guaiacol HDO, which obtained 86% cyclohexanol yield under the reaction conditions of 250 °C and 2 MPa H2 [74]. The authors suggested that strong methoxy group removal ability was attributed to the oxygen vacancies and acidic sites.

Other Bimetallic Catalysts

In order to partially replace Ru, which is the least expensive among noble metals but still quite expensive compared to the non-noble metals, various bimetallic catalysts have been investigated to reduce the cost and modulate the activity of the catalyst. The guaiacol HDO performance of HY zeolite-supported bimetallic and bifunctional catalysts was examined, in which Ru was co-loaded with inexpensive earth-abundant metals such as Fe, Ni, Cu, and Zn [75]. The authors intended to control the hydrogenolysis activity of Ru by introducing secondary metals to reduce the yield of low-molecular-weight gaseous products as well as reducing the cost of the catalyst. For bimetallic catalysts, the electronic environment of the Ru can be altered, and the geometry of the bimetallic structure can be modified owing to the heterogeneous metal–metal bonds. As a result, a decrease in the yield of gaseous products was observed for the bimetallic catalysts compared to that of the monometallic Ru/HY catalyst, which originated from the inhibition of excessive hydrogenolysis. Furthermore, the highest hydrocarbon yield and lowest gaseous product yield were obtained with the Ru–Cu/HY catalyst. The authors suggested that superior HDO performance of Ru–Cu/HY was attributed to its strong and abundant acid sites, highly dispersed metal species accompanied by the limited segregation, and high adsorption capacity for the polar fractions of reactants on the catalysts.

Bimetallic catalysts of Ru and transition metal-loaded ZSM-5 catalysts have also been evaluated and have shown enhanced HDO performance compared to the monometallic Ru/HZSM-5 catalyst [76, 77]. Lin et al. investigated bimetallic Ni-Ru/HZSM-5 catalysts for the conversion of lignin into phenolic monomers [76]. Combining the Ni and Ru on HZSM-5 significantly enhanced the Lewis acidity of the catalyst, which resulted in the synergistic effect on the catalytic performance in the C–C and C–O bond cleavage of straw lignin. The lignin conversion of 88.1% and monophenol yield of 19.5% could be achieved by Ni-Ru/HZSM-5 catalyst under the reaction conditions of 230 °C and 2.5 MPa H2. Jaya et al. reported a Cu-Ru bimetallic catalyst loaded onto HZSM-5 (Cu–Ru/Z) and utilized it in the one-pot conversion of raw woody biomass to cyclic ketones and aromatic monomers [77]. The authors suggested that the close proximity of Cu to Ru promoted the spillover of hydrogen and that the Cu1+/Cu0 sites contributed to the adsorption and activation of reaction intermediates with C=O bond. The authors also encapsulated Cu and Ru nanoparticles and coated the zeolite with a carbon layer to enhance the hydrothermal stability of the catalyst.

Among the various combinations of Ru and transition metal bimetallic HDO catalysts, RuCo catalysts have been reported to exhibit enhanced HDO performance by various researchers [79, 80]. Shu et al. prepared bimetallic RuCo/SiO2–ZrO2 catalysts and utilized the catalysts in the HDO of guaiacol and raw lignin–oil [79]. The bimetallic catalyst showed superior HDO performance compared to the monometallic catalysts, where 100% hydrocarbon yield was obtained in guaiacol HDO from the bimetallic catalyst under the reaction conditions of 260° and 1 MPa H2. The authors suggested that the different adsorption behaviors of the substrate and hydrogen on the bimetallic catalyst resulted in the enhanced HDO performance of the RuCo/SiO2–ZrO2 catalyst. In addition, it has been also reported that the trace amount of Ru (ca. 0.5 wt.%) on the RuCo bimetallic catalysts could significantly modify the surface density of oxygen vacancies, which resulted in the catalyst with high performance for guaiacol HDO, obtaining cyclohexanol yield of ~ 94% under the reaction conditions of 200 °C and 1 MPa H2 [80]. The addition of Ru on the Co nanoparticles resulted in stronger active sites for hydrogenolysis, and the density of surface oxygen vacancies could be modified by varying the reduction temperature.

Chen et al. prepared RuMn loaded on Al2O3–SiO2 multifunctional catalyst, which showed promising performance in the partial HDO of guaiacol [78]. A cyclohexanol yield of 96.8% was obtained using a bimetallic catalyst under the reaction conditions of 180 °C and 2 MPa H2. The authors suggested that the incorporation of Mn not only enhanced the dispersion of Ru but also provided moderate acid–base sites, which resulted in the high performance of the catalyst. The addition of Fe to the Ru catalyst also enhanced the catalytic activity for the selective HDO of lignin-derived phenols into cyclohexanols. Liu et al. prepared a highly dispersed RuFe bimetallic catalyst loaded on Al2O3 support by ethylene glycol reduction method and performed guaiacol HDO [81]. The increased amount of introduced Fe2O3 gradually improved the cyclohexanol selectivity, but an excessive amount of Fe2O3 resulted in the partial coverage of Ru, which resulted in the reduced conversion of guaiacol. By providing an appropriate amount of the oxophilic Fe2O3 promoter, hydrogenolysis of the C–OCH3 bond could be promoted while suppressing further dihydroxylation of cyclohexanol to cyclohexane, thereby resulting in the enhanced cyclohexanol selectivity of 81.35% under the reaction conditions of 240 °C and 3 MPa H2.

Ru-based bimetallic HDO catalysts in combination with noble metals, such as Pt and Pd, have also been reported [60, 82, 83]. These catalysts are typically used under very mild reaction conditions, and the introduction of two noble metals results in a synergistic effect. Bimetallic Pt-Ru supported by the co-impregnation on hierarchical HZMS-5 nanoparticles showed excellent HDO activity even under mild reaction conditions of 110 °C and atmospheric pressure of H2 [60]. Although the monometallic Ru-loaded HZSM-5 showed higher selectivity for oxygen-free cycloalkanes, bimetallic Pt-Ru supported HZSM-5 catalyst showed a remarkable enhancement in the conversion of the phenolic monomer to 100%, from 11.4% and 28.2% for the monometallic Ru and Pt catalyst. The authors suggested that synergistic effect was attributed to the high hydrogenation ability of Pt and the predominant direct hydrogenolysis ability of Ru. Arora et al. investigated Pd/Ru bimetallic catalyst supported on graphene oxide and utilized it in the HDO of vanillin at room temperature and 145 psi H2 [83]. According to the authors, monometallic Ru-loaded graphene oxide and bimetallic Pd/Ru-loaded graphene oxide both showed high vanillin conversion with high selectivity to p-creosol. Pd-Ru bimetallic catalyst loaded on hydroxyapatite (Pd-Ru/HAP) has been also employed with zirconium phosphate for the HDO of C15 oxygenates at low reaction temperature of 150°C [82]. The C8–C15 alkane yield of 84.24% was obtained under the optimized reaction conditions of 150 °C and 4 MPa H2. The authors proposed that the bimetallic catalyst possessed a superior adsorption capacity for substrates, which was a major reason for the enhanced HDO performance of the bimetallic catalyst.

Concluding Remarks

The development of efficient HDO catalysts is a critical factor to achieve carbon emission reductions through the utilization of sustainable aviation fuels. As mentioned in this review, Ru-based catalysts have also been extensively investigated as highly active catalysts for the HDO of lignin-derived bio-oil and its model compounds, and various strategies have been utilized to enhance the activity of the catalyst, selectively produce the target products, and achieve high catalyst stability. Especially, since HDO reactions are generally conducted under harsh reaction conditions of high reaction temperature and pressure, it is important to develop the HDO catalyst with sufficient stability under such reaction conditions. Although it depends on the catalysts, many Ru catalysts experience irreversible deactivation (such as the sintering of Ru nanoparticles) after HDO reactions, and several strategies, such as the selection of appropriate support materials or the encapsulation of Ru nanoparticles, have been introduced to address these issues. While these efforts have resulted in improved catalyst stability, it is anticipated that further research should be conducted to achieve higher catalyst stability using a more affordable and simplified approach for its practical application in industrial fields.

Improving the cost-effectiveness of the HDO reaction process using Ru-based catalysts is also an area where further research is required. In order to reduce the operating costs of the process, it is necessary to develop catalysts that allow the reaction to proceed at lower temperatures or that use less hydrogen while maintaining high activity. Ru-based catalysts have the advantage of higher activity at lower temperatures than non-noble metal-based catalysts, which originates from its superior hydrogen activation ability. Since the Ru-based catalysts are relatively expensive compared to the non-noble metal-based catalysts, it is expected that the development of Ru-based HDO catalysts with high activity at low temperatures is necessary for them to be competitive as efficient HDO catalysts. However, the oxophilicity of Ru alone cannot completely remove oxygen from lignin-derived bio-oil at low temperatures; therefore, it is necessary to develop catalysts with high oxygen removal abilities at low temperatures by introducing proper acidic functionalities or promoters. The development of catalysts that exhibit high activity even under the very mild reaction conditions (e.g., reaction temperatures below 100 °C) through proper catalyst design will be a difficult but promising challenge for researchers investigating Ru-based HDO catalysts.

Another major advantage of Ru-based HDO catalysts over other HDO catalysts is that the HDO performance of Ru catalysts can be promoted with the addition of molecular nitrogen, where it adsorbs on the surface of the metallic Ru. Since this nitrogen-induced enhancement of the HDO performance can be universally applied to Ru-based catalysts, it is anticipated that it can be applied to various previously reported and upcoming studies using only high-pressure hydrogen as the gas phase to further enhance the activity of Ru-based HDO catalysts. Further advances in Ru-based HDO catalysts include the enhancement of Ru efficiency or the partial replacement of Ru with inexpensive transition metals to reduce the cost of the catalyst. In particular, the enhancement of Ru metal efficiency or the modulation of catalytic performance through hydrogen spillover, which can be controlled through the modification of supporting materials or the introduction of promoters, is expected to be an approach that is not only academically valuable but also of great industrial importance.