LITMIR.BIZ

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rel="nofollow" href="#ulink_815ad749-f171-59de-a8da-59c4ac1807e2">[57] MCC Er(OTf)₃ 240 0.5 4 (LevA) 3 90 (LacA) [104] MCC Ga‐doped Zn/H‐nanozeolite Y 280 1 5 (MLev) — 58 (MLac) 13 (MMP) [105] MCC Zr‐SBA‐15 260 6 2 (ELev) — 30 (ELac) 14 (EHB) [106] Scenedesmus Sn‐beta 210 2 0 — 83 (LacA) [107]

      ^a) We note that this table shows yields of products either in mol% or in wt%; it is recommended to refer to the given references if the accurate evaluation of yields is sought. “—”, not specified; “0”, not detected, or detected in trace amounts; T, reaction temperature; t, reaction time; MCC, microcrystalline cellulose; OTf, trifluoromethanesulfonate; TsOH, p‐toluenesulfonic acid; 2‐NSA, 2‐naphthalenesulfonic acid; LevA, levulinic acid; ForA, formic acid; MLev, methyl levulinate; ELev, ethyl levulinate; LacA, lactic acid; MLac, methyl lactate; MMP, methyl 2‐methoxypropanoate; ELac, ethyl lactate; and EHB, ethyl‐2‐hydroxybutanoate.

      ^b) Cellulose treated in the DES ChCl/oxalic acid [103].

      ^c) Cellulose treated in the DES ChCl/oxalic acid/MIBK [57].

      Finally, we turn now to the valorization of cellulosic biomass into α‐hydroxy acids. The formation of these products occurs via Lewis acid‐catalyzed retro–aldol transformation of cellulose‐derived 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

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