zinc chloride hydrate solvents under hydrogen pressure have been successfully applied to the conversion of unrefined pinewood lignin into value‐added alkyl phenols or cycloalkanes [123]. Alkyl phenols and cycloalkanes hold potential for the production of biosurfactants and biofuel, respectively [110,111,123]. It has been identified that the processing of lignin in 63 wt% ZnCl2 aqueous solution (corresponding to ZnCl2·4.5H2O; reaction conditions: H2 pressure 4 MPa, 200 °C, and six hours) in the presence of HCl cocatalyst yields a range of alkyl phenol products (47 wt%, based on lignin input) [123]. These results suggest that the acidic reaction media is capable of catalyzing the cleavage of all types of linkages of lignins, despite the relatively low selectivity at present. Importantly, the addition of Ru/C catalyst (instead of hydrochloric acid; C = activated carbon support) promotes subsequent hydrogenation of alkyl phenol derivatives into cycloalkanes, mostly containing two ring structures (yield 54 wt%, based on lignin input) [123]. It is worth noting that the study utilized lignin obtained after sulfuric acid‐assisted fractionation of pinewood [123], i.e. unrefined lignin. This represents a step forward in lignin processing toward an industrially implementable process.
2.4 Conclusions and Perspectives
Ongoing research toward the catalytic valorization of biomass is creating a foundation for sustainable chemical development. In numerous bench‐scale processes, acid catalysis has been shown to be a universal tool to source value‐added products from reactions of low value or waste cellulosic materials. The abundance of residual biomass derived from existing manufactories, such as in agriculture, algal aquaculture, forestry, and the food processing industry, can readily provide a sizeable amount of renewable substrates for the production of chemicals. Importantly, many products derived from such sources under acid‐catalyzed conditions, products such as low‐molecular‐weight carbohydrates, furaldehydes, functional acids and esters, phenols, and alkanes can be immediately employed in extant commercial settings. For example, cellulose‐derived alkyl (poly)glucosides can form a part of detergent formulas, alkyl levulinates and lignin‐based cycloalkanes can be used in the production of automotive fuels, and hydroxy acids can be polymerized into commodity polyesters. Nevertheless, most laboratory methods need substantial improvements before they may fully translate into commercial production. On the one hand, this translation requires further scientific research to render targeted catalytic reactions as efficient as possible. These studies will involve improved reaction conditions, separation processes, and, arguably most importantly, the development of catalysts capable of achieving highly selective processes involving transformations of biomacromolecules into known and new products. The designed systems must be robust and be easily recoverable for the next catalytic cycle. On the other hand, there remain technological difficulties. New technologies based on native substrates should optimally employ the whole biomass, not individual or refined components. Although this task may be partially solved by catalytic fractionation in lignocellulose refineries, as discussed in the chapter, the use of algal biomass remains less investigated. The separation and recovery of all valuable products in a given biomass, such as lipids, pigments, and proteins, necessarily enhances the value and sustainability of the approach and represents a significant opportunity to maximize value, but this approach embodies significant engineering, technological, and commercial barriers. For the foreseeable future, biorefineries are likely to focus on predictable and (essentially) uniform feedstocks with scale, such as lignocellulose or, possibly, algal biomass.
References
1 1. Corma, A., Iborra, S., and Velty, A. (2007). Chemical routes for the transformation of biomass into chemicals. Chemical Reviews 107 (6): 2411–2502.
2 2. Gallezot, P. (2012). Conversion of biomass to selected chemical products. Chemical Society Reviews 41 (4): 1538–1558.
3 3. Dusselier, M., Mascal, M., and Sels, B.F. (2014). Top chemical opportunities from carbohydrate biomass: a chemist's view of the biorefinery. In: Selective Catalysis for Renewable Feedstocks and Chemicals (ed. K.M. Nicholas), 1–40. Cham: Springer.
4 4. Bodachivskyi, I., Kuzhiumparambil, U., and Williams, D.B.G. (2018). Acid‐catalyzed conversion of carbohydrates into value‐added small molecules in aqueous media and ionic liquids. ChemSusChem 11 (4): 642–660.
5 5. Han, X., Guo, Y., Liu, X. et al. (2019). Catalytic conversion of lignocellulosic biomass into hydrocarbons: a mini review. Catalysis Today 319: 2–13.
6 6. Negahdar, L., Delidovich, I., and Palkovits, R. (2016). Aqueous‐phase hydrolysis of cellulose and hemicelluloses over molecular acidic catalysts: insights into the kinetics and reaction mechanism. Applied Catalysis B: Environmental 184: 285–298.
7 7. Bodachivskyi, I., Kuzhiumparambil, U., and Williams, D.B.G. (2019). A systematic study of metal triflates in catalytic transformations of glucose in water and methanol: identifying the interplay of Brønsted and Lewis acidity. ChemSusChem 12 (14): 3263–3270.
8 8. Galkin, M.V. and Samec, J.S. (2016). Lignin valorization through catalytic lignocellulose fractionation: a fundamental platform for the future biorefinery. ChemSusChem 9 (13): 1544–1558.
9 9. Agarwal, A., Rana, M., and Park, J.H. (2018). Advancement in technologies for the depolymerization of lignin. Fuel Processing Technology 181: 115–132.
10 10. Corma, A., Hamid, S.B., Iborra, S., and Velty, A. (2008). Surfactants from biomass: a two‐step cascade reaction for the synthesis of sorbitol fatty acid esters using solid acid catalysts. ChemSusChem 1 (1–2): 85–90.
11 11. Cermak, S.C., Isbell, T.A., Bredsguard, J.W., and Thompson, T.D. (2017). Estolides: synthesis and applications. In: Fatty Acids (ed. M.U. Ahmad), 431–475. London: AOCS Press.
12 12. Jamil, M.A., Siddiki, S.H., Touchy, A.S. et al. (2019). Selective transformations of triglycerides into fatty amines, amides, and nitriles by using heterogeneous catalysis. ChemSusChem 12 (13): 3115–3125.
13 13. De Schouwer, F., Claes, L., Vandekerkhove, A. et al. (2019). Protein‐rich biomass waste as a resource for future biorefineries: state of the art, challenges, and opportunities. ChemSusChem 12 (7): 1272–1303.
14 14. Hagen, J. (2015). Industrial Catalysis: A Practical Approach, 3e. Weinheim: Wiley‐VCH.
15 15. Loque, D., Scheller, H.V., and Pauly, M. (2015). Engineering of plant cell walls for enhanced biofuel production. Current Opinion in Plant Biology 25: 151–161.
16 16. Heinze, T., El Seoud, O.A., and Koschella, A. (2018). Production and characteristics of cellulose from different sources. In: Cellulose Derivatives (eds. T. Heinze, O.A. El Seoud and A. Koschella), 1–38. Cham: Springer.
17 17. Adhikari, S. and Ozarska, B. (2018). Minimizing environmental impacts of timber products through the production process “From Sawmill to Final Products”. Environmental Systems Research 7 (1): 6.
18 18. Cornils, B., Herrmann, W.A., and Zanthoff, H.W. (eds.) (2013). Catalysis from A to Z. New York: Wiley‐VCH.
19 19. Lowry, T.H. and Richardson, K.S. (1987). Mechanism and Theory in Organic Chemistry. New York: Harper & Row.
20 20. Yamamoto, H. and Futatsugi, K. (2005). “Designer acids”: combined acid catalysis for asymmetric synthesis. Angewandte Chemie International Edition 44 (13): 1924–1942.
21 21. Williams, D.B.G. and Lawton, M. (2010). Metal triflates: on the question of Lewis versus Brønsted acidity in retinyl carbocation formation. Journal of Molecular Catalysis A: Chemical 317 (1–2): 68–71.
22 22.