Читать книгу Biomass Valorization - Группа авторов - Страница 25

Lignin is a promising source of aromatic polymers for the commercial production of bulk chemicals [8,9]. Until recently, developments in lignin refineries have somewhat lagged behind, in comparison to the processing of carbohydrates, and as any emerging field, it lacks deep fundamental understanding of the chemistry. It is considered that lignin is originated in plant cells by the polymerization of sinapyl alcohol, coniferyl alcohol, and sometimes p‐coumaryl alcohol, which are addressed to the formation of S‐units, G‐units, and H‐units, respectively [109,110]. These basic units are chemically bonded together by various types of C–O (β‐O‐4, α‐O‐4, and 4‐O‐5) and C–C (β‐5, 5‐5, β‐1, and β‐β) linkages, as portrayed in Scheme 2.7, and their ratio usually varies for different plants [109,110]. Among them, the β‐O‐4 motif is the most common in nature (up to 70% of the total) [8,32]. Various strategies have been devised for the valorization of commercially available lignin, but the most useful methods to date are based on the pyrolytic conversion of aromatic polymers into a range of phenol derivatives (e.g. phenol, catechol, and cresols), gaseous products (e.g. CO, H₂, and CH₄), and solid char [9,111]. Pyrolysis is frequently promoted by acidic catalysts, such as metal chlorides and zeolites [9], and although some of these can be considered to be sustainable technologies, here, we omit these methods and discuss only acid‐catalyzed processing at moderate temperatures (pyrolysis of biomass is discussed in Chapters 6 and 7). Transition metal‐catalyzed reduction of lignin into low‐molecular‐weight derivatives is also of great industrial interest, especially in efforts to develop continuous processing of biomass [109,112].

Scheme 2.7 Monolignols and possible linkages in lignin (specific bond types are highlighted in bold).

Scheme 2.8 Proposed acid‐catalyzed cleavage of lignin models (specific bond types are highlighted in bold). Reaction conditions: 1,4‐dioxane (solvent), ethylene glycol (4 eq., based on the substrate), Fe(OTf)₃ (10 mol%, based on the substrate), 140 °C, 15 minutes. Source: Based on Deuss et al. [113].

In distinct contrast to cellulose, lignin is a structurally branched biopolymer that is soluble in many organic solvents and can be cleaved under relatively mild conditions. The principal problem in the processing of lignin is that depolymerization products tend to rapidly repolymerize in the process forming thermodynamically stable C–C linkages [8,33,109]. For example, reactions of various model substances in 1,4‐dioxane in the presence of metal triflates demonstrated that β‐O‐4 lignin models can be fully depolymerized into derivative aromatic aldehyde with a carbonyl group at the β‐position and guaiacol (140 °C, 15 minutes, Scheme 2.8) [113]. The reaction may also occur toward a derivative ketone, analogous to Hibbert ketones (products of the acidolysis of lignin) [114]; this pathway has not been confirmed (Scheme 2.8) [113]. In contrast, a β‐5 model undergoes ring‐opening reactions into the corresponding trans‐stilbene derivatives (Scheme 2.8), while a β‐β model undergoes only epimerization in the acidic media (Scheme 2.8 does not show this transformation). Fe(OTf)₃ was found to possesses optimal activity for the cleavage of β‐O‐4 linkages. It has been hypothesized that the activity of metal triflates relates to the generation of Brønsted acidic triflic acid in the reaction media [113]. This is nonetheless unlikely, as metal triflates are known to generate Brønsted acidity via an assisted acidity mechanism (Scheme 2.1), not by the hydrolysis into triflic acid [7,21,103]. Additionally, targeted studies have demonstrated that triflic acid is not causative [115,116]. Another important observation in the study [113] is that repolymerization can be effectively suppressed by the addition of ethylene glycol to form stable acetals from reactive products. The subsequent acid‐catalyzed depolymerization of varied substrates under defined optimal conditions (solvent dioxane, catalyst Fe(OTf)₃, 140 °C, 15 minutes [113,117]) showed the relationship between the amount of produced monomers and the β‐aryl ether content of lignin. The conversion of lignins obtained by organosolv processes (extraction of the product with low molecular weight alcohols) using substrates with high β‐aryl ether content led to improved yields of phenolic acetals (up to 36 wt% of monomers), while the processing of industrial kraft lignins (lignins obtained by treatment of cellulose with aqueous Na₂S and NaOH) with their high content of C–C linkages provided lower yields of product (around 10 wt%) [117]. This apparently relates to the repolymerization of labile C—O bonds into stable C—C linkages during the kraft pulping process. Although there are isolated instances where the ratio between C–O and C–C linkages is not so clearly linked to reaction outcomes [109], it is clear that the composition of the substrate influences the overall progress of the reaction. The selection of lignin types is therefore essential for the development of specific catalytic methods.

There has been some effort to promote the valorization of lignin in ILs. The transformations of varied β‐O‐4 model compounds in common imidazolium salts in the presence of Brønsted or Lewis acidic catalysts demonstrate good activity of such systems for the hydrolytic processing of the substrates [118–121]. The presence of some form of Brønsted acidity is requisite for the cleavage of β‐O‐4 bonds, whether this is achieved via the addition of protic acids, hydrolysis of metal salts, or by Lewis acid‐assisted Brønsted acidity. Hydrolysis and reduction of reactive intermediates into stable alcohol derivatives may be accomplished by the simultaneous hydrolysis/reduction of 2‐(2‐methoxyphenoxy)‐1‐phenylethanone into guaiacol and 2‐phenylethanol (yields nearly 60% each [122]) in the mixed ionic system comprising 1‐butyl‐2,3‐dimethylimidazolium bis(trifluoromethanesulfonyl)imide, the Brønsted acidic IL 1‐(4‐sulfobutyl)‐3‐methylimidazolium triflate, and IL‐stabilized ruthenium nanoparticles (a catalyst for the hydrogenation step) [122]. These studies have improved fundamental understanding of the processes involved [118–122]; translation of such model reactions to the conversion of macromolecular lignin is yet to be done.

Lastly, zinc chloride hydrate solvents under hydrogen pressure have been successfully applied to the conversion of unrefined pinewood lignin into value‐added alkyl phenols or cycloalkanes [123]. Alkyl phenols and cycloalkanes hold potential for the production of biosurfactants and biofuel, respectively [110,111,123]. It has been identified that the processing of lignin in 63 wt% ZnCl₂ aqueous solution (corresponding to ZnCl₂·4.5H₂O; reaction conditions: H₂ pressure 4 MPa, 200 °C, and six hours) in the presence of HCl cocatalyst yields a range of alkyl phenol products (47 wt%, based on lignin input) [123]. These results suggest that the acidic reaction media is capable of catalyzing the cleavage of all types of linkages of lignins, despite the relatively low selectivity at present. Importantly, the addition of Ru/C catalyst (instead of hydrochloric acid; C = activated carbon support) promotes subsequent hydrogenation of alkyl phenol derivatives into cycloalkanes, mostly containing two ring structures (yield 54 wt%, based on lignin input) [123]. It is worth noting that the study utilized lignin obtained after sulfuric acid‐assisted fractionation of pinewood [123], i.e. unrefined lignin. This represents a step forward in lignin processing toward an industrially implementable process.