The chemistry covered to this stage shows that representative transformations of cellulosic biomass lead mostly to the formation of low‐molecular‐weight saccharides and furan derivatives. Evidently, the processing into these products requires somewhat similar reaction conditions (Table 2.1), albeit that there is a requirement for the presence of Lewis acid activity for efficient conversions into furanoids (Scheme 2.2). The production of functionalized organic acids or their derivatives via the rehydration of furans, or via retro‐aldol reaction of monosaccharides, is often favored by more forcing processing parameters (temperature 160–280 °C) [4,7]. Such cellulose‐derived products (e.g. LevA, formic acid (ForA), or LacA) are another group of platform chemicals with a broad scope for commercial applications [3,4]. Even though ILs are shown to be efficient for certain hydrolytic transformations, the need for more forcing conditions for the production of the named acids potentially causes issues with the ionic media: some of them may decompose at elevated temperatures, while others may increase the rates of side reactions, reducing the selectivity of the targeted product(s) [86,93,94]. Instead, water and alcohols become suitable media for the preparation of cellulose‐derived acids and esters, respectively. For example, the production of LevA and ForA, rehydration products derived from HMF, is commonly conducted in aqueous media. These platform chemicals are produced commercially from cellulosic materials by the Biofine process [95–97]. The technology involves the two‐step hydrolytic processing of low‐value cellulosic materials catalyzed by sulfuric acid (Figure 2.2). The first stage is a rapid depolymerization into low‐molecular‐weight sugars and their dehydration products in a plug flow reactor at a temperature of 210–220 °C. The second stage is the subsequent rehydration in a back‐mix reactor at temperature 170–200 °C [95–97]. The process ultimately yields LevA (70–80 wt% of theoretical maximum based on hexoses), ForA (70–80 wt% of theoretical maximum based on hexoses), and tars (biochar). Sometimes, FF arises if hemicellulose is present in the substrate (up to 70 wt% of theoretical maximum based on pentoses) [95–97]. Importantly, the ratio of products may be manipulated by changing the processing parameters at the first and at the second stages of the process [95–97]. Many works have attempted to outperform the Biofine process but fail to provide similar outputs and levels of flexibility to the original technology [4]. However, there remain problems to be solved. The most apparent downside of the Biofine process is the use of sulfuric acid catalyst that is typically consumed during the reaction and cannot be reused, along with the challenging recovery of the products from acidic aqueous media [98,99]. These identify a need for further developments to efficiently produce LevA and ForA from cellulosic biomass.
Figure 2.2 Acid‐catalyzed valorization of cellulose via the Biofine process. Source: From Hayes et al. [97]. © 2006, John Wiley & Sons.
Although the production of ForA has been substantially improved with commercialization of OxFA process (catalytic oxidation of carbohydrates using a catalyst H5PV2Mo10O40·35H2O [100]), LevA and its derivatives remain in the center of the biorefinery research. Some issues that relate to the recovery of the catalyst and products may be resolved by conversions of saccharides in alcohol media instead of water. Such processes generate alkyl levulinates as principal products, which are relatively easy to recover by vacuum distillation [101]. These products may be converted into LevA or are directly used for the production of fuels and other specialty chemicals [101,102]. Lewis acidic metal trifluoromethanesulfonates (metal triflates) and their mixtures with sulfonic acids appeared to be effective and recoverable catalysts, providing high output of methyl levulinate (MLev) during the processing of MCC in methanol [101]. Sulfonic acids presumably improve the reaction rates of the Brønsted acid‐catalyzed cellulose solvolysis into low‐molecular‐weight saccharides, while the metal triflates promote further conversions of saccharides into MLev, likely via Lewis acid‐catalyzed isomerization (Scheme 2.2) [101]. In combination, these two acids (Brønsted and Lewis) favor the overall cascade of reactions and selectivity of MLev. In(OTf)3 and aromatic sulfonic acids (1 : 5 molar ratio, respectively) provide the highest MLev yield (75 mol%, 180 °C, five hours, Table 2.2).
Our recent study employs an integrated technology comprising consecutive processing of unrefined low‐value cellulose in the DES ChCl/oxalic acid and then in ethanol to form ethyl levulinate (ELev) [57,103]. The first step generates fine cellulosic powder from bulk cellulose during the processing in the DES under mild conditions (80 °C, two hours). The product possesses a structure and properties consistent with MCC [103]. The second step is a high‐temperature transformation (160–180 °C) in ethanol under the action of metal triflates. We discovered that soft Lewis acidic metal triflates form synergistic Lewis acid‐assisted Brønsted acid complexes with phosphoric acid, among which a composition