LITMIR.BIZ

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ChCl/oxalic/MIBK — 100 2 0 68 13 9 44 4 0 32 0 5 72 Softwood ChCl/oxalic/MIBK — 100 5 0 0 0 1 55

      ^a We note that this table show 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; HMF, 5‐(hydroxymethyl)furfural; FF, furfural; MCC, microcrystalline cellulose; [C₂mim]Cl, 1‐ethyl‐3‐methylimidazolium chloride; [C₂mim]OAc, 1‐ethyl‐3‐methylimidazolium acetate; [C₄mim]Cl, 1‐butyl‐3‐methylimidazolium chloride; [C₄SO₃Hmim]CH₃SO₃, 1‐(4‐sulfobutyl)‐3‐methylimidazolium methanesulfonate; ChCl, choline chloride; DMA, dimethylacetamide; EtOAc, ethyl acetate; and MIBK, methyl isobutyl ketone.

      ^b The substrate was subjected to sulfuric acid‐assisted ball milling.

      ^c ILs were diluted with water during processing to a total water up to 10–43 wt% based on the reaction system.

      ^d t does not include the time for the dissolution of the substrate.

      ^e Substrate was treated with aqueous sodium hydroxide.

The rates and (often) selectivities of the chemical process improve if the process can be performed under homogeneous conditions. As mentioned above, cellulosic materials are essentially insoluble in aqueous systems and most common organic solvents. However, ionic liquids (ILs) in their many manifestations are potentially key to improved chemical transformations of cellulosic materials. ILs are a class of green solvents that consist solely of ions and, under certain conditions, are able to fully dissolve cellulosic polysaccharides [48,60,61]. This ability enables significant progress toward milder reaction conditions and better yields of some targeted products, making ILs outstanding reaction systems for the acid‐catalyzed valorization of native carbohydrates [4,61]. For example, quaternary ammonium salts, especially imidazolium derivatives such as 1‐alkyl‐3‐methylimidazolium chloride ([C_nmim]Cl, n = integer), have been effectively employed in the catalytic hydrolysis of cellulose and cellulosic biomass of diverse origin [34,49,62,63]. A seminal study [34] explored transformations of polysaccharides into monomer sugars (glucose and xylose) in a range of ILs in the presence of hydrochloric acid, a Brønsted acid catalyst. The study identifies that the molecular formula of ILs substantially influences the reaction outcomes, mostly related to the solubility of the substrate. There is an apparent need for the chloride anion to coordinate hydroxyl groups and to disrupt the extensive hydrogen bonding between polysaccharides, thus promoting their dissolution. In contrast, non‐coordinating, or weakly coordinating, anions, such as tetrafluoroborate (BF₄⁻), nitrate (NO₃⁻), bromide (Br⁻), and trifluoromethanesulfonate (triflate, OTf⁻), dissolve cellulose only poorly, slowing or preventing reactions from taking place [34,64]. The cation component of the ILs also influences the chemical reactivity of the cellulose in ILs: imidazolium salts or alkylpyridinium‐based solvents with longer alkyl chains reduce the solubility of cellulose in such ILs, hampering the reactivity [58,64]. Interestingly, imidazolium ILs with acetate (OAc⁻) and dimethylphosphate ((MeO)₂P(O)O⁻) counterions, which are excellent solvents for cellulose [64], provide zero yield of glucose after exposure to intended hydrochloric acid‐catalyzed processing of cellulose [34]. This is likely because of the reaction between the strong Brønsted acidic hydrochloric acid and the conjugate base of weaker acids, leading to formation of imidazolium chloride and a weaker Brønsted acid (acetic acid or dimethylphosphoric acid) that is unable to promote the hydrolysis under the selected processing conditions [62].

Another important finding is that acid‐catalyzed conversion of cellulosic biomass in IL media can be improved and effectively promoted by the gradual addition of water [34]. The optimized method provides impressive yields of glucose (up to 87 mol% based on the glucose content in the substrate) and xylose (up to 79 mol% based on the xylose component of the substrate) in [C₂mim]Cl solvent in the presence of hydrochloric acid catalyst at 105 °C, whose mixture was gradually diluted with 43 wt% of water (Table 2.1). Previous methods, which were based on the processing of cellulose in [C₄mim]Cl without addition of water, provided significantly lower yields of monosaccharides [48]. Although glucose is the terminal product of hydrolysis chemistry of cellulose, the hydrolysis of cellulose proceeds predominantly into glucose oligomers (cellotetraose, cellotriose, and cellobiose) from which glucose emerges (Scheme 2.4) [49]. There is also evidence that water suppresses the subsequent acid‐catalyzed conversion of saccharides into furanoids and by‐products in ionic solvents [34,49,62], thereby enhancing the yields of the desirable monosaccharides. The addition of water enhances the high yielding and selective transformation of polysaccharides into monomer sugars in ILs: it helps to promote the formation of monosaccharides and suppresses unwanted processes. However, care must be taken as to the timing of the addition of water. If this is done at the start of the process, then the substrate can remain undissolved because of the negative influence of the added water on the ability of ILs to dissolve biomass [34, Скачать книгу