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 Y(OTf)3/H3PO4 (1 : 1 molar ratio, respectively), affording for the highest ELev yield (up to 75 mol%, Table 2.2) [103]. Neither Y(OTf)3 nor H3PO4 can separately catalyze the conversion of MCC into ELev, and only their combination generates the active catalyst (Table 2.2). Hard Lewis acids, including In(OTf)3, show moderate activity in this process, which can be marginally improved in combination with p‐toluenesulfonic acid (TsOH), as was noted during the conversion of MCC in methanol (Table 2.2) [101]. Usefully, the conversion of wood‐derived cellulose, obtained after the processing of softwood chips in the biphasic system ChCl/oxalic acid/MIBK (Table 2.1), as disclosed earlier in the text, enabled similarly excellent conversion thereof into ELev in the presence of a combined acid catalyst Y(OTf)3/H3PO4 (Table 2.2) [57]. Such integrated methods that involve different catalytic processes, leading to a range of value‐added chemicals, have significant potential to become commercially viable. Meanwhile, engineering of (preferably) continuous processes requires further laboratory‐ and pilot‐scale research.
Table 2.2 Conditions and results of the acid‐catalyzed processing of cellulosic biomass into organic acids or estersa. Source: Bodachivskyi et al. [57,103].
| Substrate | Catalyst | T (°C) | t (h) | Yield LevA or alkyl levulinates (%) | Yield ForA (%) | Yield LacA or α‐hydroxy acid derivatives (%) | References |
|---|---|---|---|---|---|---|---|
| Paper pulp | H2SO4 | 205 185 | 15 (s) 0.42 | 61 (LevA) | 82 | — | [96] |
| MCC | TsOH | 180 | 5 | 20 (MLev) | — |
