**2. Ruthenium**

Ruthenium, from Latin *ruthenia* ("Russia"), is one of the late transition metals and is located in the periodic table in the 5th period and group 8 (**Figure 1**). With an abundance of 7.0 ± 0.9 ng. g−1 in the silicate shell [6], ruthenium is one of the rarest non-radioactive elements on earth. Its low abundance is due to segregation of the platinum group elements in the core of Earth that was partially compensated by addition of 0.3–0.8% of chondritic material after core formation had been complete [6]. Ruthenium is found mostly in deposits associated with the other platinumgroup elements [7] and as the rare RuS2 mineral called laurite [8]. Ruthenium is a silvery white, extraordinarily hard and brittle metal. With a density of 12.45 g. cm−3 [9], ruthenium is the second lightest platinum group metal after palladium. In the electronics industry, it is used in devices for perpendicular recording [10], a technology applied in hard disks that enables high-density data storage on magnetic media.

*Hydrogenation and Hydrogenolysis with Ruthenium Catalysts and Application to Biomass… DOI: http://dx.doi.org/10.5772/intechopen.97034*

#### **Figure 1.**

*Calculated heats of formation of the (unstable) binary hydrides MnHy (n > y) for group 8–10 transition metals [12] (left) and position of ruthenium (circle) in the periodic table (right) indicating the group of noble metals (bold, grey) and the platinum group metals (dark grey).*

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" [11]. For these elements a positive value is obtained for the heats of formation calculated for the binary hydrides (**Figure 1**) [12]. 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 [13]. Interestingly, the hydride ligand exerts a strong *trans* influence in ruthenium complexes (*vide infra*), thereby weakening the binding of ligands located in *trans* position [14].

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 (*vide infra*). Moreover, it can catalyse the oxidation of alcohols to aldehydes and carbonic acids. Ruthenium compounds are distinguished for their rich coordination chemistry, and compounds with ruthenium in oxidation states between −2 and + 8 are known. The most stable and most common oxidation states are +3 and + 4. With ruthenium at an intermediate oxidation state, +2, +3 or + 4, complexes have also been obtained that, similar to other late transition metal clusters (e.g., Ni, Pd, Pt [15]; Pt [16]; Co, Rh, Ir [17]; Au [18]), comprise ruthenium-ruthenium bonds. Ruthenium complexes, like Grubbs catalyst and Noyori catalyst (*vide infra*), play a significant role in chemical syntheses. Likewise, ruthenium compounds are employed in olefin metathesis polymerisation of cyclic alkenes [19–21]. The perovskite mixed oxide Ba2LaRuO6 is used in automotive exhaust gas catalysts [22]. Titanium electrodes covered with a layer of RuO2 are applied in the chloralkaline electrolysis [23]. Moreover, ruthenium nanoparticles are interesting Deacon catalysts for the gasphase oxidation of hydrogen chloride to chlorine [24].

### **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 [25].

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 [26] 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 *Hydrogenation and Hydrogenolysis with Ruthenium Catalysts and Application to Biomass… DOI: http://dx.doi.org/10.5772/intechopen.97034*

ligands [27]. 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 [28]. 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 [29]. Two catalysts were chosen in such a way that one catalyst (M1 ) preferentially adsorbs one of the substrate moieties, and the other catalyst (M<sup>2</sup> ) 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 [29]. 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 [29]. 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**).


#### **Figure 2.**

*Concept of a molecular assembly line for catalysing the consecutive one-pot reaction of a bifunctional subtract A-B to product aH-BH with a mixture of orthogonal catalysts M1 and M2 (right) and requirements concerning the affinity for binding of the respective moieties to the metal centres M<sup>1</sup> and M2 (table, left).*

#### **Figure 3.**

*Concept of a molecular assembly line applied to the hydrogenation of nitrobenzene (NB, A-B, blue) to cyclohexylamine (CA, aH-BH, brown) over a mixture of orthogonal catalysts Pt/CNT (M1 ) and Ru/CNT (M<sup>2</sup> ) and time-concentration profile showing also the intermediate aniline (AN, AH-B, green) and the side product dicyclohexylamine (DA, purple) (right).*

## **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 [30] 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 [31]. As the slow methanation of the carbon support [32] 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 [35]. An example for a highly active and stable low-temperature ammonia catalyst are ruthenium nanoparticles on a Ba-Ca(NH2)2 support [36]. 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 rutheniumbased benchmark catalysts (Cs-doped Ru/MgO, at 260 °C) [36]. 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.

$$\text{NH}\_2 + \text{3H}\_2 \rightleftharpoons 2\text{NH}\_3 \quad \rightarrow \qquad \Delta \text{H}\_r\text{°=} - 91.8 \text{kJ} \,\text{mol}^{-1} \tag{1}$$

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

*Hydrogenation and Hydrogenolysis with Ruthenium Catalysts and Application to Biomass… DOI: http://dx.doi.org/10.5772/intechopen.97034*

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 [39]. This suggests a different reaction pathway not involving the intermediate formation of CO. Both reactants, H2 and CO2, are strongly adsorbed on the surface [39] 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 [40]. 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 [41]. Carbon dioxide methanation is an option for biogas upgrading that constitutes an alternative to the removal of carbon dioxide [42]. 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 [43].

$$\text{CH}\_2 + 4\text{ H}\_2 \rightleftharpoons \text{CH}\_4 + 2\text{ H}\_2\text{O} \quad \rightarrow \quad \Delta\text{H}\_r\text{ }^\circ = -165.12 \text{ kJ mol}^{-1} \tag{2}$$

$$\text{CO} + 3\,\text{H}\_2 \rightleftharpoons \text{CH}\_4 + \text{H}\_2\text{O} \quad \rightarrow \qquad \Delta\text{H}\_r\text{ }^\circ = -20\text{€}.28 \text{ kJ mol}^{-1} \tag{3}$$

$$\text{CO}\_2 + \text{H}\_2 \rightleftharpoons \text{CO} + \text{H}\_2\text{O} \quad \rightarrow \qquad \Delta \text{H}\_r\text{}^\circ = \text{ } + \text{41.16 kJ mol}^{-1} \tag{4}$$

#### **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 [46]. 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 (*vide infra*) and (iii) shifting the adsorption mode of the substrate on the ruthenium metal nanoparticles from binding of the amine group to the aromatic ring. In a similar manner, nitro compounds are able to change the binding mode of aromatic amines to the ruthenium surface [47, 48].

#### **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

**Figure 4.**

*Calculated reaction enthalpies for the elementary steps in the hydrogenolysis of ethane on a Ru(001) surface (593 K, left) and intermediates with the lowest activation free-energy barrier relative to \*CH-CH\* bond activation (right) [49]. Energies are relative to a surface covered with chemisorbed hydrogen (H\*); \* denotes coordination to the ruthenium surface.*

and biofuel generation. Cleavage of the C-C bond is preceded by a series of quasiequilibrated dehydrogenation steps (see **Figure 4** for ethane hydrogenolysis [49]). 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 [49].

#### **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 [50] 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 [51] and amides, imines and nitriles to amines [52, 53]. Isopropanol is commonly employed as hydrogen donor [54]. 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

*Hydrogenation and Hydrogenolysis with Ruthenium Catalysts and Application to Biomass… DOI: http://dx.doi.org/10.5772/intechopen.97034*

**Figure 5.**

*Examples of molecular ruthenium complexes that are used in homogeneously catalysed hydrogenation and metathesis reactions.*

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 [56].

Ruthenium is also the central metal in the Grubbs catalysts [57], 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 [21]) and for the synthesis of large rings by metathesis. The second generation [58] 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 [59]. Ruthenium *N*-heterocyclic carbene (NHC) complexes based on the second-generation Grubbs catalysts have also been applied in a variety of related transformations, such as hydrogenation [60], hydrosilylation, and isomerization [61]. Metathesis can also be combined with a second chemical transformation to tandem reaction sequences [61]. Likewise, living free radical polymerizations are feasible with ruthenium complexes [62]. An example is the polymerisation of methyl methacrylate with [RuCl2(PPh3)3] as a catalyst [63, 64].

## **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**) [65]. Molecular adsorption is governed by the van der Waals interactions between molecular dihydrogen and the ruthenium surface [66]. On a Ru(0001) surface with point group symmetry C3v [67], the four high-symmetry adsorption sites involve binding of the hydrogen molecule to a single ruthenium atom (*on top*), a position bridging two ruthenium atoms (*brg*) or three-fold coordination at *fcc* or *hcp* sites (**Figure 6**) [67, 68].

#### **Figure 6.**

*Physisorption and chemisorption of molecular hydrogen on extended ruthenium surfaces and the C3v highsymmetry adsorption sites on the Ru(0001) surface on top (uCM = 0; vCM = 0), brg (1/2;1/2), fcc (2/3;1/3) and hcp (1/3,2/3) (top) [67] and changes in adsorption energy for physisorption of hydrogen Eads(H2), dissociation barrier* Δ*Ediss and chemisorption of hydrogen Eads(H) (physisorption vdW-DF2 + PBE level, [66], chemisorption GGA with periodic plane-wave basis set, 1 monolayer coverage, no correction for zero point energy [78].*

Physisorption attracts a charge of −0.04 electrons to the hydrogen molecule [66]. This is consistent with promotion of dissociative adsorption of hydrogen in the presence of alkaline metal cations [69]. Electron transfer to the anti-binding orbitals of hydrogen and upward shift of the transition metal d-band centre towards the Fermi level are likely explanations.

For coordination of hydrogen in the molecular state [70], the *on top* site provides the highest adsorption energy of −20.3 kJ. mol−1 and the lowest dissociation barrier of 16.4 kJ. mol−1 [66]. Consequently, an entrance channel barrier is missing, and this dissociation channel appears to be active even for dissociation of H2 molecules with negligible incident energy. Nevertheless, a suitable approach of the H2 molecules to the ruthenium surface is essential for such a low dissociation barrier. Dissociation of molecular hydrogen on ruthenium is a rather slow process [69], and equilibrium is obtained only after several hours [71]. Point-like defect structures, like Ru vacancies or Ru adatoms on the surface do not seem to provide comparably low dissociation barriers. Other defects that are present at finite temperatures on the surface include steps, kinks and adatom islands [72, 73]. Low coordinated defect sites may be the preferential sites for a direct dissociative adsorption pathway on ruthenium nanoparticles [74]. Due to the low barrier, the *on top* site is likely the most reactive site for hydrogen dissociation on extended ruthenium surfaces [67]. For supported ruthenium catalysts, a rapid H2/D2 isotopic equilibration reaction has been reported [69]. Even so, the isotope exchange is slowed down considerably in the presence of alkaline metal cations that prevent spillover [70, 75] of hydrogen atoms to the support.

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

#### *Hydrogenation and Hydrogenolysis with Ruthenium Catalysts and Application to Biomass… DOI: http://dx.doi.org/10.5772/intechopen.97034*

the *fcc* or the *hcp* hollow site (−258.6 and − 258.2 kJ. mol−1, respectively). Noteworthy is the relatively small difference in energy between the *fcc* and *hcp* hollow sites. As for extended surfaces of other late transition metals, hydrogen, thus, has a pronounced preference for binding to multi-fold coordination sites [76, 77]. As far as metal clusters and nanoparticles are concerned, the number of adsorption sites can differ, whereby specific 2-, 3-, and 4-fold coordination to surface atoms has been reported [76]. The barrier for surface diffusion [70] of hydrogen is rather small and was estimated to 13–21 kJ. mol−1. There is a small decrease of −12.1 kJ. mol−1 in adsorption energy with coverage θ increasing from partial (1/3) to monolayer coverage.

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 *θ* of ca. 85% was calculated at room temperature, whereby it decreased to ca. 70% at increasing temperature (500 °C) [78]. Temperature-programmed desorption of hydrogen from ruthenium catalysts shows two distinct desorption peaks as a characteristic feature [71, 74]. The peaks represent strongly and weakly chemisorbed hydrogen, consistent with distinct NMR signals at −60 and − 30 ppm [79]. The corresponding heats of adsorption were determined to be 40–70 kJ. mol−1 (αH) and 10 kJ. mol−1 (βH), respectively, by microcalorimetry [69]. This suggests that part of the hydrogen is not dissociated over real samples. Consequently, a chemisorption stoichiometry xM exceeding unity is frequently considered (xM = 1.4 [74]; 2 [71, 80]; 5 [79]). Although surface processes dominate, subsurface hydrogen cannot be ruled out [70, 81]. Furthermore, the support can act as a reservoir for hydrogen [69].

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 [82]. Thus, CO hydrogenation rates on Ru clusters are much higher at high CO coverage than predicted based on a Langmuir approach [83]. Activation of adsorbed CO by reaction with surface hydrogen results in transition states that occupy less space than [82] 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 [84]. At cryogenic temperatures, the carbon monoxide molecules form triangular islands of up to 21 molecules located on the *on top* sites. Through this type of island formation, long-range lateral CO-H repulsive interactions are minimised. With an increase in temperature, the carbon monoxide molecules shift to the *hcp* sites and the island size decreases to 3–6 molecules [84]. Through this decrease in domain size, repulsive CO–CO interactions that become more prominent upon increasing the temperature are reduced. The proximity of the carbonyl and hydride adsorbate species to one another (3.0–3.7 Å distance) [84] explains the propensity of ruthenium surfaces for Fischer-Tropsch reactions. The ensuing CO bond cleavage is facilitated by the formation of partially hydrogenated CHO and COH intermediates.

#### **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 [85–89] hinder their more widespread use as well-defined catalyst supports [90]. 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 [91]. 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**) [29]. 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

**Figure 7.**

*Particle size distribution of the ruthenium nanoparticles for a typical Ru/CNT catalyst and representative transmission electron microscopy images [29]. The carbon nanotubes are Baytubes C 150 P.*

*Hydrogenation and Hydrogenolysis with Ruthenium Catalysts and Application to Biomass… DOI: http://dx.doi.org/10.5772/intechopen.97034*


#### **Table 1.**

*Comparison of the hydrophobicity of carbon supports (left) and oxidic supports (right) [27]; the pore size of zeolites (Beta, Y, ZSM-5, Silicalite-1 [92]) and mesoporous materials (MCM-41 [93]) is the maximum diameter of a sphere that can diffuse through the channels.*

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.

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 (*Q*Toluene) to that of water (*Q*Water). For comparison, the reported HIs of some activated carbons, microporous zeolites, and mesoporous materials are listed in **Table 1** [27]. The hydrophobicity index (HI) of typical zeolites, such as beta, Y and ZSM-5 is low (HI = 1.4, 0, 8, respectively), which is consistent with the hydrophilic nature of the pore walls. This is attributed to a certain polarity of the zeolite walls that results from the aluminium atoms substituting a certain part of the silicon atoms. All-silica zeolites, such as Silicalite-1, are clearly more hydrophobic (HI = 15.2) and more resemble activated carbons which are commonly regarded as hydrophobic adsorbents.

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 [94]. 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 [95]. 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 [98] which imparts beneficial properties to the catalyst [99]. 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 [100] 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 [101]. A useful feature of such supported catalysts is that fixed-bed reactor technology common in continuous chemical processes can be employed [97].

### **8. Biomass conversion with ruthenium catalysts**

About 1% of the incoming solar radiation on earth is captured for generating biomass [102]. This energy is utilised in photosynthesis [103] to build a myriad of complex molecules [104] such as carbohydrates, lignin, proteins, fats and oils, and terpenes. In this way about 170 x 10<sup>9</sup> t/a of complex substances are produced annually [105]. In plants, the radiation use efficiency is controlled by the netphotosynthetic capacity and the canopy structure [106, 107]. Cultivars with a heavy canopy and long growth period are able to harness more solar radiation [108]. 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% [109]). 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 [2]. 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-tocarbon 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 [4]. 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 [109]. The remainder are 37% by-products and 40% waste products, mostly carbon dioxide (36%) that need to be utilised or disposed. Green chemistry metrics [120], 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 [109]. 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%) [121].


*Hydrogenation and Hydrogenolysis with Ruthenium Catalysts and Application to Biomass… DOI: http://dx.doi.org/10.5772/intechopen.97034*

*\*1 [110]; \*2 increasing degree of coalification relates with decreasing O/C ratio; \*3 originating from Pennsylvania [4]; \*4 [111];\*5 [112]; \*6 [113]; \*7 [114]; \*8 [115]; \*9 [116]; \*10 [117]; \*11 for global mass flows refer to [5];*

#### **Table 2.**

*Oxygen content of typical components of biomass in comparison to fossil resources and selected derived products.*
