Comparison of the hydrophobicity of carbon supports (left) and oxidic supports (right) ; the pore size of zeolites (Beta, Y, ZSM-5, Silicalite-1 ) and mesoporous materials (MCM-41 ) is the maximum diameter of a sphere that can diffuse through the channels.
With the rising emphasis on efficient and highly selective chemical transformations, the field of ruthenium-catalysed hydrogenation and hydrogenolysis reactions has grown tremendously over recent years. The advances are triggered by the detailed understanding of the catalytic pathways that have enabled researchers to improve known transformations and realise new transformations in biomass conversion. Starting with the properties of ruthenium, this chapter introduces the concept of the catalytic function as a basis for rational design of ruthenium catalysts. Emphasis is placed on discussing the principles of dissociative adsorption of hydrogen. The principles are then applied to the conversion of typical biomolecules such as cellulose, hemicellulose and lignin. Characteristic features make ruthenium catalysis one of the most outstanding tools for implementing sustainable chemical transformations.
- reaction network
- sequential reactions
- hydrogen dissociation
- biomass conversion
Life on earth inherently depends on the element carbon creating the heart of a myriad of chemical compounds that, together with water and some inorganic compounds, build living matter. Over geologic periods, life has established a dynamic equilibrium of the flows of carbon through the different geo-habitats . With the rise of mankind, this balance has been undermined through the exploitation of vast amounts of fossil resources for generating heat and materials. The carbon dioxide (CO2) emissions from combustion of fossil resources have resulted in rising atmospheric CO2 concentrations and an increasingly evident change in the climate worldwide. Replacing fossil resources that at present make up more than 90% of the energy demand and the feedstock for the chemical industry  is one of the most pressing challenges of mankind. All our primary energy demand of annually 12.5 TW a−1 could be covered by harnessing a fraction of the 8405 TW a−1 renewable energy available annually that comprises solar, wind, geothermal, tidal and wave energy . Nevertheless, a sustainable energy supply will be needed for carbon-based compounds in order to close carbon recycle streams. Biomass is a globally available resource that is considered a suitable alternative feedstock for producing basic chemical building blocks, so-called platform molecules , that could substitute the current fossil-based platform chemicals .
Biomass largely consists of complex molecules comprising mostly oxygen and other heteroatoms. Lignocellulose, the structural component of plants and the largest fraction of plant biomass, is essentially composed of cellulose, hemicellulose and lignin. Break down of the structure by depolymerisation of the corresponding molecular entities, followed by oxygen removal, yields fuels and platform chemicals for the value-chain of the chemical industry. Sustainable conversion depends on efficient conversion steps obtained ideally via catalytic processes. In this context, the catalytically highly active element ruthenium provides unique properties. Despite ruthenium being counted among the noble metals, it resembles a non-noble metal in many aspects. In metallic form, ruthenium atoms are highly polarisable. Unlike the higher homologue platinum, e.g., that has similar atomic radius, ruthenium has a much higher average electric dipole polarizability. Consequently, distinct catalytic functions can be realised with ruthenium catalysts.
To help readers understand why ruthenium catalysts are so frequently employed in biomass conversion, this chapter will first investigate the properties of ruthenium. Here, the catalytic properties of ruthenium are linked with its propensity to adsorb certain molecular entities. After exploring the interaction of adsorbed molecules with ruthenium surfaces, we will discuss the nature of selected adsorption states, the corresponding binding energies and structures of the adsorption complexes including ordering phenomena observed for molecules co-adsorbed on the ruthenium metal surface. This sets the scene for rational design of catalysts that are specific for the conversion of chemical entities in biomass. Last but not least, we will discuss selected examples for intriguing transformations of biomolecules.
To note here is that this chapter does not aim to comprehensively review the available data on catalysis with ruthenium. Nor does it attempt to summarise all data on the conversion of biomass with ruthenium catalysts. The extensive interest in this field is reflected presently by the more than 800 articles published each year on catalysis with ruthenium, more than 110 of which focus on biomass conversion. Instead, this chapter aims to summarise the catalytic principles governing hydrogenation and hydrogenolysis reactions with heterogeneous ruthenium catalysts with particular focus on applications in biomass conversion. Cited data and papers were selected to exemplify the field and illuminate the discussion.
Ruthenium, from Latin
With regard to its chemical properties, ruthenium is stable in the absence of oxygen against non-oxidising acids. Consequently, it counts as a noble metal. Even so, ruthenium resembles a non-noble metal in many respects. Similar to the other metals of the 7th, 8th and 9th group of the periodic table, ruthenium does not form stable binary hydrides under ambient conditions; this region of the periodic table is called the “hydride gap” . For these elements a positive value is obtained for the heats of formation calculated for the binary hydrides (Figure 1) . Nonetheless, ruthenium monohydride (RuH) is formed by reaction of the elements at pressures above 14 GPa at room temperature. It transforms to Ru3H8 at pressures of more than 50 GPa and temperatures exceeding 1000 K, adopting a cubic structure, and RuH4, when the pressure is increased above 85 GPa, crystallising in a structure comprising corner-sharing H6 octahedra . Interestingly, the hydride ligand exerts a strong
Due to its ability to dissociate hydrogen on the metal surface, ruthenium, in its metallic form, finds numerous applications as a catalyst in chemical processes such as ammonia synthesis, methanation, hydrogenation or hydrogenolysis (
3. Concept of catalytic function
At first sight, most catalytic systems appear to be unnecessarily complex. A look at biologic systems, however, reveals that many biological systems are built on chains of different catalysts. There, substrate molecules are passed from one enzyme to another. Thus, in the conversion of molecular oxygen, about ten different catalysts are involved before the oxidising equivalents are reacted with carbon compounds .
Thinking in terms of sequences of consequential reaction steps is a useful strategy to rationally design heterogeneous catalysts. A good starting point is considering the catalytic functions  necessary for realising the desired transformations. The dissociative adsorption of molecular hydrogen is one of the key steps for hydrogenation and hydrogenolysis reactions, the focus of this chapter. In the case of transfer hydrogenation, the concepts equally apply to suitable hydrogen surrogates. As such, dissociative adsorption of hydrogen, as one of the important steps of catalysis, will be elucidated below. With the Langmuir-Hinshelwood mechanism most prominent in catalysis with late transition metals, co-adsorption of the substrate and transfer of hydrogen atoms to an unsaturated substrate need to be considered next. Other catalytic functions important for biomass conversion are the ability of a catalyst to either cleave or form C-C, C-O or C-N bonds. This results in a list of complementary catalytic functions that are required for realising the desired transformation. Thereby it is useful to consider orthogonal catalytic functions that do not interfere with each other. Rather molecules ought to be passed from one catalytic function to the next, like in a molecular assembly line. Noteworthy, such assembly lines may involve a single material comprising different functions. Frequently the support plays an important role even when the actual transformation occurs on supported metal nanoparticles. One aspect to be considered regarding hydrogenation and hydrogenolysis reactions is spill-over of hydrogen to surface sites on the support. Another concept for realising such assembly lines involve mechanical mixtures of two or more materials that comprise different catalytic functions. An example is given below. Whereas heterogeneous ruthenium catalysts can accommodate many of these catalytic functions, homogeneous ruthenium catalysts enable unique, highly distinct catalytic transformations. Once the necessary catalytic functions have been identified, it is useful to derive the link to the desired active state and the structure of the pre-catalysts that is to be used. This provides a straightforward path for rationally designing a particular catalyst for the desired transformation.
4. Sequential reactions
Rational and straight-forward catalyst design is the foundation of systematic conceptualisation of highly active catalysts that provide extraordinary specificity for a given transformation. Such specificity is essential upon designing catalysts for biomass transformations, because the chemist typically encounters many different molecules or molecular entities rather than single types of molecules that are to be converted. If chosen in the appropriate way, the catalyst will adsorb and convert only one type of molecule or chemical entity while leaving all other molecules and chemical entities untouched. This concept is also valuable for devising catalysts for sequentially connected, mutually exclusive catalytic reactions. To develop such catalysts, the chemist needs to fundamentally understand the nature and catalytic role of active sites to guide the design of new and improved catalysts. Two examples are described here. The general principle is exemplified for a radical reaction with a MOF catalyst; the potential is then demonstrated for the hydrogenation of a multifunctional substrate over a Ru/CNT-Pt/CNT catalyst mixture.
Metal organic framework (MOF) compounds are porous materials commonly obtained by hydrothermal reaction of metal ions and bridging organic ligands . MOFs combine the high porosity of a heterogeneous catalyst with the tunability of molecular functional groups. This combination of features has been exploited for the sequential oxidation of alcohols to carboxylic acids with molecular oxygen in the presence of TEMPO modified MOF UiO-68 . The conversion involves two sequential oxidation steps, i.e., the aerobic oxidation of alcohols to aldehydes, and the consequential autoxidation of the aldehydes to carboxylic acids. Whereas the first step is a radical reaction, the second step is inhibited by radicals. Thus, the two reactions are mutually exclusive. Complete removal of the MOF catalyst after the first radical-catalysed aerobic oxidation step by filtration provides the radical scavenger-free conditions that are necessary for the second radical-inhibited autoxidation step. This is a beautiful example of the use of a functional heterogeneous catalyst for a sequential organic transformation.
The concept of connecting consecutive one-pot reactions with a “molecular assembly line” has been explored for the hydrogenation of bifunctional substrates A-B to products AH-BH . Two catalysts were chosen in such a way that one catalyst (M1) preferentially adsorbs one of the substrate moieties, and the other catalyst (M2) preferentially adsorbs the second substrate moiety (Figure 2). In this case both catalysts function optimally, thereby yielding improved rates and selectivities compared to single or conventional bimetallic catalysts . Moreover, substrate inhibition can be avoided. By adjusting the relative quantity of the two catalysts, the relative rates of the two sequential transformations can be adjusted to be equal, because this results in the highest overall rate at the lowest catalyst concentration.
This concept has been applied successfully to the full hydrogenation of nitroaromatics to cycloaliphatic amines over a mechanical mixture of carbon nanotube (CNT)-supported Ru/CNT - Pt/CNT catalysts . Noteworthy is that the aromatic ring, considered to be “soft” due to the aromatic π-system delocalised over six carbon atoms, preferentially adsorbs on ruthenium that is readily polarizable. The nitro group, considered to be “hard” due to the negative charge which is delocalised over only two oxygen atoms, preferentially adsorbs on platinum with highly shielded d-electrons. Notably, metallic Ru and Pt have similar atomic radii of 133 and 137 pm, while differing in the static average electric dipole polarizability of 9.6 and 6.4 10−24 cm3, respectively. A 95:5 mixture of the Ru/CNT (M1) and Pt/CNT (M2) catalysts provides the required equal rates for hydrogenation of the two respective moieties and optimum selectivity to the target product cyclohexylamine (Figure 3).
5. Catalytic transformations with ruthenium catalysts
Based on the unique catalytic functions given by heterogeneous and homogeneous ruthenium catalysts, a large number of important transformations have been realised. Many of these transformations are applied on an industrial scale. For hydrogenation and hydrogenolysis reactions, in particular, heterogeneous ruthenium catalysts are among the most frequently applied catalysts, because they provide outstanding activities and excellent selectivities.
5.1 Ammonia synthesis and methanation with ruthenium catalysts
Analogous to iron and osmium, ruthenium catalyses the formation of ammonia from nitrogen and hydrogen (Eq. 1). Ruthenium has superior catalytic activity compared to iron  and results in enhanced NH3 yields at lower pressures. A ruthenium catalyst, which is supported on a carbon matrix and improved by barium and caesium as promoters, has been in industrial use in two production sites in Trinidad since 1998 . As the slow methanation of the carbon support  interferes with the process, alternative supports are preferred for ruthenium catalysts applied in ammonia synthesis. Efficiencies as close as possible to the theoretical limit are highly relevant for decentralised, islanded ammonia production plants [33, 34], where round-trip efficiencies of up to 61% can be reached . An example for a highly active and stable low-temperature ammonia catalyst are ruthenium nanoparticles on a Ba-Ca(NH2)2 support . At a weight hourly space velocity (WHSV) of 36 L g−1 h−1, a rate of 23.3 mmolNH3 g−1 h−1 is obtained at 300 °C and 9 bar. Such catalytic activity is about 6 times higher than that of industrial iron-based benchmark catalysts (at 340 °C) and 100 times higher than that of industrial ruthenium-based benchmark catalysts (Cs-doped Ru/MgO, at 260 °C) . In addition, for the reverse reaction of ammonia cleavage, high activities are likewise important [37, 38] and imply the use of ruthenium catalysts for the upcoming production of COx-free hydrogen by ammonia cleavage in energy applications.
Analogous to nickel, ruthenium catalyses methanation, the production of methane from hydrogen and carbon dioxide (Eq. 2) or carbon monoxide (Eq. 3), the so-called Sabatier reaction. Water is obtained as by-product. Carbon dioxide methanation could be seen as the combination of the reverse water gas shift reaction that converts a mixture of carbon dioxide and hydrogen to carbon monoxide and water (Eq. 4), and methanation. Over ruthenium catalysts, such as Ru/Al2O3, the coproduction of CO is negligible . This suggests a different reaction pathway not involving the intermediate formation of CO. Both reactants, H2 and CO2, are strongly adsorbed on the surface  giving rise to a Langmuir-Hinshelwood mechanism. Ruthenium catalysts are highly selective to methane and provide a very low fraction of side products, such as higher hydrocarbons, alcohols, or formic acid. Due to the exothermicity and volume reduction, the reaction is thermodynamically favoured at low temperatures and high pressures. Typical operation conditions are 200–500 °C and pressures of 10–30 bar . Since ruthenium catalysts have a higher activity than nickel catalysts, they enable higher conversions at low temperature. Methanation has long been used for removing COx from the hydrogen-nitrogen syngas mixture used in ammonia production . Carbon dioxide methanation is an option for biogas upgrading that constitutes an alternative to the removal of carbon dioxide . Carbon dioxide methanation has also been discussed in the context of storing intermittent energy generated as a result of electricity production from renewable resources. Methane can be transported and stored in the existing natural gas grid. Therefore, methanation of carbon dioxide is being discussed as one of the promising Power-to-X technologies .
5.2 Hydrogenation with ruthenium catalysts
Ruthenium is an efficient catalyst for hydrogenating aromatics, acids, ketones and unsaturated nitrogen compounds. The selective hydrogenation of aromatic amines to cycloaliphatic primary amines is an industrially relevant transformation, but is impaired by formation of secondary amines and other side products. Modification of carbon nanotube (CNT)-supported ruthenium catalysts Ru/CNT catalysts with a base (LiOH) significantly improves selectivity in toluidine hydrogenation [44, 45] without decreasing the activity of the catalysts. LiOH-modified Ru/CNT catalysts can efficiently convert also other challenging substrates, such as methylnitrobenzenes . The effect of LiOH is understood as (i) LiOH reducing acidic sites on the catalyst support, (ii) enhancing hydrogen dissociation and reducing hydrogen spillover from ruthenium to the support (
5.3 Hydrogenolysis with ruthenium catalysts
The hydrogenolysis of alkanes is an important unit operation in refineries for reducing the chain length of acyclic alkanes. It also serves as a model for the hydrogenolysis of C-O and C-N bonds in various applications relevant for oil refining and biofuel generation. Cleavage of the C-C bond is preceded by a series of quasi-equilibrated dehydrogenation steps (see Figure 4 for ethane hydrogenolysis ). Desorption of two chemisorbed hydrogen atoms generates the necessary adsorption sites on the surface. Physisorbed ethane dissociates stepwise via CH3CH2*, *CH2CH2*, *CH2CH* to form *CHCH*. Activation of the C-C bond in *CH-CH* has a lower intrinsic barrier in further dehydrogenation. Cleavage of the C-C bond in the *CH-CH* surface intermediate is thought to be the rate limiting step. During the entire process, the surface is covered to a large extent with chemisorbed hydrogen (H*). The high hydrogen coverage also enhances the re-hydrogenation of the unsaturated fragments to produce methane that is desorbed from the surface.
Similar to Ru, C-C bond cleavage in more deeply dehydrogenation intermediates is preferred for Os, Rh, Ir, and Pt relative to cleavage of the C-C bond in more saturated intermediates (Figure 4, right). Cleavage of the C-C bond in more saturated intermediates starts to compete as one moves more to the right of the periodic table. For the group 10 metals (Ni, Pd, Pt), the most favourable mechanism is C-C activation in *CHCH*, while other intermediates have activation energies of about 40 kJ mol−1 suggesting that multiple routes may coexist. For the coinage metals (Cu, Ag, Au), there is a preference for cleavage of the C-C bond in the most saturated intermediate CH3CH2*. The overall free-energy barrier for C-C bond activation is lowest for Ru providing the highest turnover rate for *CHCH* bond cleavage. Thus, the less noble metal Ru is more active than the more noble metals. This is also consistent with experimental data that show a decrease in the turnover rate in the sequence Ru > Rh > Ir > Pt .
5.4 Catalysis with molecular ruthenium catalysts
Some very active molecular ruthenium (pre)catalysts were developed for catalytic hydrogenation and transfer hydrogenation. Selected examples are shown in Figure 5. Ruthenium hydride complexes  with phosphine or diamine ligands are active for the hydrogenation of many substrates. Transfer hydrogenation with ruthenium catalysts is frequently used for the reduction of ketones to alcohols  and amides, imines and nitriles to amines [52, 53]. Isopropanol is commonly employed as hydrogen donor . The hydrogenation and transfer hydrogenation can be stereoselective if the starting material is prochiral and a chiral complex is employed [52, 55]. However, chiral BINAP catalysts can reduce only functionalised ketones in a stereoselective manner. Whereas Noyori precatalysts of the type [RuCl2(diphosphane)(diamine)] enable the asymmetric hydrogenation of ß-keto esters as well as the reduction of prochiral ketones and aldehydes, olefins are usually not converted. The stereoselectivity is enhanced, when the substituents on the ligands differ in size. The concept of bifunctional asymmetric catalysis with ruthenium complexes has later been transferred to a variety of C-C, C-O and C-N forming reactions .
Ruthenium is also the central metal in the Grubbs catalysts , which are among the most important precatalysts for olefin metathesis. There are two generations of Grubbs catalysts (Figure 5). The first generation is often employed for ring-opening polymerisation (ROMP ) and for the synthesis of large rings by metathesis. The second generation  has a much higher activity. In Grubbs-Hoveyda catalysts, one of the tricyclohexylphosphine (PCy3) ligands of the Grubbs catalysts is replaced by an aromatic ether that links to the carbon substituent. There is a wide field for ruthenium-catalysed cyclisation reactions . Ruthenium
6. Hydrogen adsorption on metallic ruthenium
As for the other platinum group elements, metallic ruthenium is characterised by excellent catalytic results for a variety of transformations. The interaction of molecular hydrogen with the surface of ruthenium is particularly interesting as far as catalytic hydrogenation or hydrogenolysis reactions are concerned; it will be analysed in further detail here. Accordingly, the fundamental concepts discussed here likewise are valid for transfer hydrogenation reactions.
Dissociative chemisorption of hydrogen on the surface is a pivotal step of the transformation and is often rate-limiting. The adsorption of hydrogen may be considered as competing molecular and dissociative adsorption of hydrogen (Figure 6) . Molecular adsorption is governed by the van der Waals interactions between molecular dihydrogen and the ruthenium surface . On a Ru(0001) surface with point group symmetry C3v , the four high-symmetry adsorption sites involve binding of the hydrogen molecule to a single ruthenium atom (
For coordination of hydrogen in the molecular state , the
Dissociative adsorption occurs when the bonds formed between the two hydrogen atoms and the ruthenium surface are stronger than the strength of the hydrogen–hydrogen bond (460 kJ.mol−1). This is the case when the hydrogen atoms adsorb at either the
At low temperature, the catalytically active ruthenium surface is normally covered to a large extent with hydrogen. The surface coverage remains incomplete under reaction conditions even at elevated pressures. Thus, at 100 bar, a coverage
Under catalytic conditions, surfaces are saturated by hydrogen or one or more adsorbed intermediates. This leads to strong co-adsorbate interactions. These interactions are not accounted for in kinetic models built on Langmuir isotherms. In real catalysts, however, mostly supported metal nanoparticles are employed, where these co-adsorbate interactions are lessened. The curvature of the nanoparticles allows for adlayer relaxation . Thus, CO hydrogenation rates on Ru clusters are much higher at high CO coverage than predicted based on a Langmuir approach . Activation of adsorbed CO by reaction with surface hydrogen results in transition states that occupy less space than  the pair of surface moieties that they replace. This causes the overall activation energy to decrease with increasing CO* coverage.
Interestingly, species co-adsorbed on a ruthenium surface may show a strong tendency to segregate. Thus, with carbon monoxide and hydrogen co-adsorbed on a Ru(0001) surface, the carbon monoxide molecules form islands that are surrounded by hydrogen atoms . At cryogenic temperatures, the carbon monoxide molecules form triangular islands of up to 21 molecules located on the
7. Supports for heterogeneous ruthenium catalysts
For applicable heterogeneous catalysts, metallic ruthenium is supported in form of ruthenium nanoparticles on a suitable support. This ensures a high dispersion and a large surface area of ruthenium. Carbon supports, in particular active carbons and carbon nanotubes, and oxidic supports are frequently employed. To ensure that the ruthenium nanoparticles are immobile on the support surface under the catalytic conditions, there has to be a sufficiently strong interaction between metal nanoparticles and the support. Otherwise, there would be pronounced sintering of the ruthenium nanoparticles that would lead to gradual loss of the catalytic activity. The support also influences the electron density in the ruthenium nanoparticles, thereby lowering or increasing the Fermi level. For oxidic supports, the interaction between nanoparticles and the support cannot be too strongly pronounced, because ruthenium cations tend to diffuse into the bulk of the support material.
For carbonaceous materials anchoring sites have to be generated on the surface to anchor the ruthenium nanoparticles. Providing high surface area, active carbons and carbon nanotubes thus usually undergo an oxidative pre-treatment. As a result, oxygenated moieties are generated to which the ruthenium nanoparticles strongly bind. In this aspect, the property of ruthenium being at the borderline between noble and non-noble metals is exploited. Under more driving reductive conditions of a hydrogen atmosphere, however, the susceptibility of carbon carriers to methanation is challenging for carbon-supported ruthenium catalysts, because it leads to degradation of the carrier and sintering of the ruthenium clusters. Compared to active carbons, carbon nanotubes lend a more defined support and higher stability.
Carbon nanotubes combine physicochemical properties that make them interesting as support for ruthenium, such as high surface area, good mechanical strength, chemical and thermal stability, high heat and electric conductivity. So far, the high costs incurred by elaborate synthesis procedures [85, 86, 87, 88, 89] hinder their more widespread use as well-defined catalyst supports . For immobilisation of metal nanoparticles, anchoring sites need to be generated on the surface of the carbon support. A method of preparing a Ru/CNT catalyst with supported ruthenium nanoparticles involves treatment of the CNT in refluxing nitric acid . Deposition-precipitation of the ruthenium precursor Ru(NO)(NO3)x(OH)y followed by reduction of the precursor to the metal with molecular hydrogen provides well-dispersed surface-anchored Ru nanoparticles (Figure 7) . Such catalysts are excellent hydrogenation and hydrogenolysis catalysts (see below).
Oxidic supports that are frequently employed comprise silica, alumina (mostly γ-Al2O3), zirconia, ceria and the corresponding mixed oxides. Even though amorphous materials provide the necessary high surface area, they often are associated with certain distribution of surface functions. Yet as surface groups, they may be harmful in catalysis. The presence of different surface sites often leads to alternative catalytic pathways that result in reduced selectivity of the transformation. Instead, more defined support materials are nanoporous zeolites, such as zeolite Y, Beta, and ZSM-5, or mesoporous materials, such as MCM-41. The internal pore system (Table 1) provides a uniform environment for the catalytic transformation. Nevertheless, many biomolecules are too large to enter the pore system and need to be cut to molecular entities first. Catalysis at the pore mouth or using molecular catalysts is an option for the depolymerisation step.
|Adsorbent||Hydrophobicity index (HI)||Adsorbent||Pore size [Å]||Hydrophobicity index (HI)|
|F300||160||ZSM-5, Silicalite-1||4.46||8, 15.2|
The hydrophobicity index (HI) is a good measure for assessing the internal hydrophobicity of porous materials. HI can be determined by the competitive adsorption of a toluene-water mixture. The hydrophobicity index is defined by the ratio of the adsorption capacity for toluene (
Unsupported metal nanoparticles can be employed as quasi-homogeneous catalysts but need to be stabilised by ligation or generation of an electric double layer to prevent agglomeration of the nanoparticles . Upon decreasing the size of the metal nanoparticles, the boundary of the metallic state is obtained for two-shell clusters of about 1.5 nm in diameter . Ruthenium nanoparticles stabilised with a thin layer of ionic liquid tartaric acid tetraoctylammonium [TA2−][N+ 8888]2 or glycine tetraoctylammonium [Gly−][N+ 8888] have shown excellent catalytic properties for the hydrogenation of challenging substrates. One example is the conversion of nitrobenzene to cyclohexylamine. Catalytic activity and selectivity of the quasi-homogeneous nanoparticle catalyst resemble that of a corresponding supported Ru/C catalyst. Upon switching to the less polar ionic liquid dimethylglycine tetraoctylammonium [Me2Gly−][N+ 8888], the selectivity changes to the reaction intermediate aniline. This is attributed to the relative binding strength of ionic liquid and intermediates to the ruthenium surface. Thus, the use of ionic liquids as stabiliser lends a ready method to tailor the properties of the catalyst. Interestingly, ionic liquid-stabilised nanoparticles are readily supported on a mesoporous support [96, 97] thus turning the quasi-homogeneous catalyst into a true heterogeneous catalyst. Noteworthy, the catalytically active site remains in the flexible environment of the ionic liquid  which imparts beneficial properties to the catalyst . During the chemical transformation, the active species can easily adapt to the geometry changes that occur during the path from reactant to transition and product state. Moreover, the equivalence of all catalytically active sites is readily maintained which can render enhanced selectivity. The ionic liquid then again provides a polar medium for tailoring the adsorption of the substrate molecules and desorption of the product molecules  that precede and succeed the catalytic reaction, respectively. Interestingly, in supported films of ionic liquid. Rates as well as chiral induction can be enhanced, as was demonstrated for the hydrogenation of the prochiral substrate acetophenone over [Ru((R)-BINAP)(PPh3)nCl3-n], n = 0, 1 . A useful feature of such supported catalysts is that fixed-bed reactor technology common in continuous chemical processes can be employed .
8. Biomass conversion with ruthenium catalysts
About 1% of the incoming solar radiation on earth is captured for generating biomass . This energy is utilised in photosynthesis  to build a myriad of complex molecules  such as carbohydrates, lignin, proteins, fats and oils, and terpenes. In this way about 170 x 109 t/a of complex substances are produced annually . In plants, the radiation use efficiency is controlled by the net-photosynthetic capacity and the canopy structure [106, 107]. Cultivars with a heavy canopy and long growth period are able to harness more solar radiation . A large fraction of the produced biomass is characterised by a high oxygen content (Table 2). Cellulose, a polymeric carbohydrate, and lignin a randomly linked phenolic polymer constitute a major fraction of plant biomass (around 95% ). Their oxygen content is much higher than that of fossil resources such as crude oil, natural gas and coal (Table 2). About 56% of the oil extracted from the resources is utilised to make liquid fuels (70.6%) for transport purposes . About 14% of the oil and 8% of the gas extracted from these resources is utilised to make petrochemicals. Both fuels and many petrochemical products are characterised by a low oxygen-to-carbon ratio. Some examples are given in Table 2. Consequently, in order to exploit biomass, a controlled de-functionalisation is necessary. In particular, efficient strategies are needed to decrease the oxygen-to-carbon ratio.
At present, biorefinery routes [118, 119] have been improved to more efficiently exploit biomass feedstock. In the production of bioethanol from lignocellulosic biomass, e.g., by hydrolysis of wood with dilute acids, hexoses are obtained that are good feedstock for fermentation . The target product then needs to be separated from the aqueous fermentation broth. By producing ethanol in this way, about 8.7% of the mass and 11% of the energy contained originally in the wood are found in the product . The remainder are 37% by-products and 40% waste products, mostly carbon dioxide (36%) that need to be utilised or disposed. Green chemistry metrics , notably the E-factor and atom economy, clearly need to be improved further. One option is the direct chemical conversion of lignocellulosic biomass in a single reaction step over a multifunctional catalyst as outlined below. Such transformation follows the principles of a molecular assembly line. Thus, efficient and frequently multistep catalysis is one of the keys for realising fast and highly selective conversion of biomass . Before the particular aspects of ruthenium catalysts in biomass conversion are considered, the general architecture and the availability of biomass is analysed briefly. Lignocellulose makes up the structural components of plants and a large fraction of the plant biomass available for producing platform chemicals. Wood, e.g., is essentially composed of cellulose (39–45%), hemicelluloses (27–32%) and lignin (22–31%) .
9. Sustainable feedstock from biomass
Cellulose is an important structural component of the cell wall of green plants, many forms of algae and the oomycetes. Many bacteria secrete it to form biofilms . Plants build about 1011–1012 t/a of cellulose annually mostly in combination with hemicelluloses and lignin . This makes cellulose the most abundant organic polymer on Earth . Cellulose is a polysaccharide, a linear chain with the formula (C6H10O5)n consisting of 7,000–15,000 of β(1 → 4) linked D-glucose units .
Even though hemicellulose is a polysaccharide often associated with cellulose, cellulose and hemicellulose have distinct compositions and structures. Hemicellulose is a branched polymer but cellulose is unbranched. Whereas hemicellulose is built from diverse sugars, cellulose is derived exclusively from glucose. For instance , besides glucose, sugar monomers in hemicelluloses can include hexose sugars, such as mannose and galactose, and pentose sugars, such as xylose and arabinose. Unlike cellulose, the side chains in hemicelluloses are often modified with acetyl and glycosyl groups.
Lignin is a randomly linked polymer (Figure 8) comprising phenolic
The components of plant biomass are normally fractionated using biochemical [129, 130], thermochemical  and/or catalytic methods . Lignin is particular is that respect that it is highly resistant to depolymerisation. Consequently, at present the lignin fraction is often used to a large extent as fuel for heat generation. Methods have been developed to utilise the phenolic structure for producing polymers, resins, additives, fuels and chemicals. Common methods for depolymerisation of lignin into monomeric phenolic compounds involve pyrolysis [133, 134, 135, 136], enzyme [137, 138], acid or base [139, 140] catalysed hydrolysis, and hydrogenolysis [141, 142, 143]. Catalysts based on metallic ruthenium are frequently employed in hydrolysis and hydrogenolysis of the ether linkages or hydrodeoxygenation [110, 144] of the phenol products (
In subsequent downstream processing, the biomass fractions are converted to platform chemicals. Based on the generally accessible biomass, platform molecules (Figure 9 ) include organic acids, such as propionic acid, 3-hydroxypropionic acid, succinic acid, fumaric acid, itaconic acid, and levulinic acid , fat and oil-derived polyols, in particular glycerol, as well as sugar-derived polyols such as sorbitol and xylitol. Additional platform chemicals are alcohols such as methanol, ethanol, and propanol, cyclic ethers, such as furfural and 5-hydroxymethylfurfural, and terpenes, such as isoprene. Such platform molecules can be exploited as fuels and industrially relevant chemicals or are readily transformed into such fuels and chemicals. Ruthenium-based catalysts are frequently employed in key transformations such as hydrogenation, hydrogenolysis, and oxydehydrogenation . Compared to nickel-based catalysts, ruthenium-based catalysts provide higher activity and better stability that result in lower catalyst loadings, longer lifetimes and less pronounced deactivation. Although ruthenium-based catalysts are more expensive, these costs are offset by their higher activity and their lower tendency to leach.
10. Ruthenium catalysts in cellulose conversion
While first-generation bioethanol is produced on the million t/a scale, production of second-generation bioethanol from cellulosic biomass is still in its infancy . The challenge is the enzyme- or acid-catalysed hydrolysis of lignocellulosic materials to simple sugars that can be fed into fermentation, from which ethanol is separated by distillation . A one-step catalytic conversion of cellulosic biomass (bagasse and corn stalk) to bioethanol has been realised with a ruthenium-based catalyst . The catalyst comprises well-dispersed Ru and WOx nanoparticles on a H-ZSM-5 solid acid support. Under catalytic conditions, also highly dispersed Ru3W17 alloy nanoparticles are formed. In a cascade reaction cellulose undergoes hydrolysis on moderately acidic sites of the H-ZSM-5 support, followed by glucose retro-aldol condensation to glycolaldehyde over WOx and hydrogenation over Ru to yield ethylene glycol that is dehydrated and finally hydrogenated to ethanol on the Ru3W17 alloy sites.
Interestingly, subcritical water is an efficient reaction medium for cellulose conversion [149, 150]. Thus, cellulose is converted to polyols over ruthenium supported on crosslinked polystyrene [149, 151]. Swelling of the polymer  thereby facilitates access of the substrate to the catalytic sites.
A carbon-supported ruthenium hybrid catalyst with a specific surface area of 1200 m2 g−1 was employed for the direct hydrogenolytic cleavage of cellulose to sorbitol . High microporosity and low acidity of the carbon support favour high dispersion of the metallic ruthenium. Interestingly, ball-milling of cellulose with carbon supported ruthenium provides enhanced conversions and selectivities to sorbitol [154, 155].
Selective conversion of cellulose to sorbitol is achieved
Hydrogenolysis of sorbitol to ethylene glycol and 1,2-propanediol is obtained over bifunctional Ru-WOx/CNT catalysts . Furthermore, addition of Ca(OH)2 proved beneficial for the hydrogenolysis activity.
11. Ruthenium catalysts in lignin conversion
Hydrogenolysis of lignin involves reductive bond cleavage of C-O bonds linking the phenolic moieties, thereby generating hydrogenated and therefore less reactive monomeric species. For the reduction step, ruthenium catalysts are frequently employed. A variety of reducing agents have been suggested [141, 160, 161], such as hydrogen , carbon monoxide, formic acid (HCOOH/NEt3 ), methanol, ethanol, isopropyl alcohol , acetonitrile, acetone. The energy needed for producing the reductant and the associated CO2-footprint ought to be taken into account when the lignin-derived products are utilised as biofuels . Supercritical fluids as solvent have been claimed to produce fewer solid residues and provide higher biomass conversions [163, 164]. Catalytic transfer hydrogenolysis of corn stover lignin in supercritical ethanol with a Ru/C catalyst yields bio-oil with a high fraction of monomeric moieties . The key transformation is the reductive cleavage of ether linkages. Sequential extraction with a series of solvents differing in polarity results in monomer fractions that are enriched in alkylated phenols, guaiacols, syringols and hydrogenated hydroxycinnamic acid derivatives (Figure 10).
For using bio-oils as fuel, hydrotreating is necessary for lowering the oxygen content. Hydrotreating increases stability and energy density while decreasing the viscosity of the bio-oils. Ruthenium catalysts are often used in this hydrogenolytic upgrading of bio-oils. Even though zeolites are a good support material, substituted phenols cannot enter the micropores of typical zeolites. One concept for overcoming this challenge are catalysts comprising hierarchical pore systems. Thus, Ru supported on mesoporous ZSM-5 with a characteristic pore size of 4.5–4.7 Å of the MFI lattice channels (Figure 11)  and the mesopore system aligned to the
12. Conclusions on biomass transformation with ruthenium catalysts
Ruthenium, a late transition element, provides catalytic pathways that make it highly promising for catalysts applied for biomass conversion. Biomass, a globally available resource, is a sustainable feedstock for producing platform chemicals, that could substitute the current fossil-based platform chemicals in the chemical industry. However, in order to implement further processes in small and large-scale biorefineries, more efficient transformations will be required. Here, the distinct catalytic functions provided by ruthenium and ruthenium complexes could open new pathways. Biomass largely consists of complex molecules that comprise oxygen and other heteroatoms. Catalytic transformations need to accommodate for these heteroatoms, because molecules with heteroatoms tend to adsorb strongly to catalytic sites possibly causing substrate- or product inhibition. However, the preferential adsorption of chemical moieties associated with heteroatoms on the catalytically active site can be exploited for directing catalytic transformations. The principle has been explored for the consecutive hydrogenation of unsaturated moieties on a molecular assemble line. In this context, it is useful considering the concept of orthogonal catalytic functions, where a catalyst preferentially binds and transforms a selected chemical entity without hindering other catalysts that may be added for realising preceding or subsequent catalytic transformations.
In this context, the catalytically highly active element ruthenium embodies unique features. Ruthenium does not form binary hydrides that are stable under usual catalytic conditions. Nevertheless, metallic ruthenium can dissociate molecular hydrogen. Under an atmosphere of hydrogen, the surface of metallic ruthenium is covered with hydrogen atoms. Adsorption states and chemical reactivity of this hydrogen is well understood. Desorption of a fraction of the hydrogen provides the empty coordination sites necessary for co-adsorption of reactant molecules. Typically following a Langmuir-Hinshelwood-type mechanism, hydrogen atoms can be transferred to unsaturated moieties. Remarkably, ruthenium can also form and cleave C-C, C-O and C-N bonds. Combined with its strong propensity for hydrogenation, this ability gives rise to hydrogenation, hydrogenolysis and hydrodeoxygenation transformations that make ruthenium catalysts so interesting for biomass conversion. Noteworthy are the distinct catalytic transformations that can be realised with ruthenium catalysts. Selected examples for intriguing transformations of biomolecules and bio-derived molecules have been discussed above.
Understanding the interaction of adsorbed molecules with ruthenium surfaces, the nature of adsorption states, binding energies and structures of the adsorption complexes lies at the heart of rational design of catalysts that are specific for the conversion of the chosen chemical entity in biomass. It is anticipated that new transformations will be realised based on the unique catalytic functions provided by heterogeneous and homogeneous ruthenium catalysts. Serving as important tools for the synthetic chemist, these transformations will bolster the use of biomass as sustainable feedstock for the chemical industry.
With financial support from the state government of North Rhine-Westphalia and the Indo-German Science & Technology Centre (IGSTC). TEM acknowledges the support of Fraunhofer-Institut für Umwelt-, Sicherheits- und Energietechnik UMSICHT. P. Tomkins and E. Gebauer-Henke are gratefully acknowledged for the characterisation of the Ru/CNT catalyst, C. Herwartz (GFE) for TEM measurements, M. Hermesmann for the literature searches on environmental impact, as well as D. Panke for drawing the chemical structure of biomass fractions.
Conflict of interest
There are no conflicts of interest to declare.
For compiling Figure 2, image 1961095 was obtained with the standard licence from Shutterstock.
Abbreviations and nomenclature
Surface adsorbed species Coverage Chemisorption stoichiometry Adsorption Aniline Cyclohexylamine Carbon nanotube Dissociation Gemeinschaftszentrum für Elektronenmikroskopie Hydrophobicity index Metal Nitrobenzene Nuclear magnetic resonance Ring opening metathesis polymerisation Zeolite Beta with BEA structure Zeolite ZSM-5 with MFI structure in the proton form Mesoporous material Zeolite Y with FAU structure bridging adsorption site three-fold coordination three-fold coordination terminal adsorption site annum kilojoule metric ton Terawatts
Surface adsorbed species
Gemeinschaftszentrum für Elektronenmikroskopie
Nuclear magnetic resonance
Ring opening metathesis polymerisation
Zeolite Beta with BEA structure
Zeolite ZSM-5 with MFI structure in the proton form
Zeolite Y with FAU structure
bridging adsorption site
terminal adsorption site