**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 [122]. Plants build about 1011–1012 t/a of cellulose annually mostly in combination with hemicelluloses and lignin [123]. This makes cellulose the most abundant organic polymer on Earth [124]. 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 [125].

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 [126], 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 *p*-hydroxyphenyl (**H**), guaiacyl (**G**) and syringyl (**S**) moieties (see also **Figure 9**) that are linked *via* ether linkages (*ß-O-4*', *α-O-4*', *4-O-5*'), biphenyl (*5–5*'), resinol *(ß-ß')*, and other condensed linkages (*ß-5*′, *ß-1*′) as well as dibenzodioxocin, and phenylcoumaran linkages [109, 127]. The complex structure of lignin is the result of the biosynthetic pathway that involves oxidation of phenolic precursors to radicals followed by radical coupling that leads to stepwise build-up of the lignin structure [128].

The components of plant biomass are normally fractionated using biochemical [129, 130], thermochemical [131] and/or catalytic methods [132]. 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 [133–136], enzyme [137, 138], acid or base [139, 140] catalysed hydrolysis, and hydrogenolysis [141–143]. Catalysts based on metallic ruthenium are frequently employed in hydrolysis and hydrogenolysis of the ether linkages or hydrodeoxygenation [110, 144] of the phenol products (*vide supra*).

In subsequent downstream processing, the biomass fractions are converted to platform chemicals. Based on the generally accessible biomass, platform molecules (**Figure 9** [4]) include organic acids, such as propionic acid, 3-hydroxypropionic acid, succinic acid, fumaric acid, itaconic acid, and levulinic acid [145], 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

#### **Figure 8.**

*Chemical structure of important biomass fractions, lignin (left), cellulose with 1,4-glycosidic linkages and selected hydrogen bonds (right, top) and the common molecular motif of hemicellulose (right, bottom). For the structure of lignin, the characteristic aromatic p-hydroxyphenyl (***H***), guaiacyl (***G***) and syringyl (***S***) moieties as well as aromatic ether linkages (***-O-***), dibenzodioxocin, biphenyl, resinol, and phenylcoumaran linkages [109, 127] were marked.*

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

**Figure 9.**

*Biomass derived platform chemicals.*

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 [146]. 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 [4]. 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 [147]. A one-step catalytic conversion of cellulosic biomass (bagasse and corn stalk) to bioethanol has been realised with a ruthenium-based catalyst [148]. 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 [152] thereby facilitates access of the substrate to the catalytic sites.

A carbon-supported ruthenium hybrid catalyst with a specific surface area of 1200 m<sup>2</sup> g−1 was employed for the direct hydrogenolytic cleavage of cellulose to sorbitol [153]. 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 *i.a*. by use of bi-functional ruthenium catalysts supported on sulphated zirconia and sulphated silicazirconia [156]. Tetragonal zirconia, associated with generation of superacidity, is the active phase for cellulose depolymerisation that accompanies the hydrogenation function of ruthenium. Also, zeolite- [146, 157] and silica- [158] supported ruthenium nanoparticles are suitable for the hydrogenation of glucose to the sugar alcohol sorbitol.

Hydrogenolysis of sorbitol to ethylene glycol and 1,2-propanediol is obtained over bifunctional Ru-WOx/CNT catalysts [159]. 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 [142], carbon monoxide, formic acid (HCOOH/NEt3 [53]), methanol, ethanol, isopropyl alcohol [54], 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 [162]. 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 [163]. 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**) [92] and the mesopore system aligned to the *b*-axis was found to be effective for the hydrodeoxygenation of phenolic biomolecules [144]. For comparison, the Van-der-Waals radius of the syringol molecule is estimated to

#### **Figure 10.**

*Phenol-, guaiacol-, syringol- and hydroxycinnamic acid (top row)-derived monomers typically found in lignin hydrogenolysates (bottom row).*

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

**Figure 11.**

*Comparison of the characteristic dimensions of syringol (top right) and the three-dimensional MFI pore system of zeolite ZSM-5, here viewed down the b-direction (left; not to scale; red, oxygen atoms; orange, silicon atoms), and maximum diameter of a sphere that can diffuse through the channel system (bottom right).*

be 9.88 × 7.61 Å (**Figure 11**) based on the distance of the outermost hydrogen atoms [165] and a Van-der-Waals radius for hydrogen of 1.04 Å [166]. Only at the channel entries do the open mesopores expose acid sites to the approach of bulky molecules necessary for catalysing the cleavage of the phenolic C-O bonds. This type of catalyst was found to effectively catalyse the hydrodeoxygenation of phenol and 2,6-dimethoxyphenol at 4.0 MPa H2-pressure and a temperature of 150 °C [144]. Conversions were > 99.5 and 97.5% after a 4 h reaction time, respectively; product selectivities to cyclohexane were accordingly 95.0 and 70.0%.
