monosaccharides into C2–C4 sugars and sequential retro‐Michael dehydration into pyruvic aldehyde, rehydration, and isomerization into 2‐hydroxy carboxylic acids catalyzed by both Brønsted and Lewis acids (Scheme 2.2) [3,4,7]. LacA is usually the desired product of such transformations; however, other derivatives may also appear. The conversion of sugars into LacA requires hydrothermal conditions and Lewis acidic catalysts that remain stable in water at elevated temperature. Metal triflates are water‐tolerant Lewis acids and so are suitable catalysts for the task of hydrothermal conversion of cellulose into products [7,21]. In a study of a series of lanthanide triflates, it has been established that yields of LacA increase with a reduction of the ionic radius of the metal center La3+ < Ce3+ < Pr3+ < Nd3+ < Dy3+ < Ho3+ < Er3+ and is approximately equal for Er3+ ≈ Yb3+ ≈ Lu3+ [104]. Among these, Er(OTf)3 was found to be the most active catalyst, providing an outstanding yield of 90 mol% yield of LacA during the reaction of MCC (Table 2.2) [104]. Importantly, the catalyst may be recovered by distillation of the product and be reused in the next cycle.
There is also a great deal of interest in zeolite‐catalyzed conversions of cellulosic saccharides into α‐hydroxy acids and esters [105–108]. For example, Ga‐doped Zn/H‐nanozeolite Y catalysts are active in the transformation of MCC in supercritical methanol, yielding methyl lactate (MLac) as the major product (yield 58%, Table 2.2) and small amounts of methyl‐2‐methoxypropionate (MMP, yield 13%, Table 2.2) and MLev (5%, Table 2.2) [105]. Mesoporous Zr‐SBA‐15 silicate was reported to catalyze the transformation of cellulose in supercritical aqueous ethanol (95%), providing moderate yields of ethyl lactate (ELac, yield 30%), ethyl‐2‐hydroxybutanoate (EHB, yield 14%), and ELev (yield 2%, Table 2.2) [106]. A Sn‐beta zeolite has been employed in the valorization of the carbohydrate‐rich microalga Scenedesmus sp. into LacA in an aqueous ForA solution (in this instance, formic acid is a catalyst, not a product) [107]. The catalytic system enabled selective and high‐yielding conversion of algal sugars into LacA in 83% yield under optimal conditions (210 °C, two hours, Table 2.2). It is proposed that ForA helps to degrade the algal cell walls, whereas the Sn‐beta catalyst promotes the retro–aldol reaction of glucose and other reactions leading to LacA [107]. Most likely, the reported excellent yields relate to the ease of the depolymerization of structurally branched algal sugars under acid‐catalyzed conditions.
2.3 Acid‐Catalyzed Processing of Lignin
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].
