Apart from carbohydrate polymers, lignin is known to transform into a range of useful phenol derivatives under acid‐catalyzed conditions, but the overall role of the catalysis is less well understood for these processes [5,8,9]. These bio‐based molecules may potentially generate a renewable platform that produces large volumes of green replacements to crude oil‐based products, such as fuels, monomers for plastics, detergents, and other commodity products [1–13]. In the rest of this chapter, we will concentrate only on those transformations where acid catalysts display notable advantages over other types of catalysts. Effectively, this approach focuses on lignocellulosic materials.
2.2 Acid‐Catalyzed Processing of Cellulosic Polysaccharides
The valorization of lignocellulose is an intricate chemical problem. The challenge relates largely to the rigid supramolecular structure of native plant cell walls, formed by inter‐ and intramolecular bonding of polysaccharides, lignins, and sometimes other macromolecules [31]. A common strategy to improve reactivity is to separate carbohydrate polymers and lignin [32]. Fractionation of biomass is performed on large scale during commercialized pulping processes and may also be conducted by the selective catalytic transformation of one particular class of biomacromolecules [16,32–35]. For example, a carbohydrate fraction can be recovered by catalytic hydrogenolysis of lignin into low‐molecular‐weight phenolic substances [33]. It is also possible to retain the lignin fraction by the direct acid‐catalyzed processing of polysaccharides present in native biomass [34,35]. To complicate matters, the acid‐catalyzed transformation of individual macromolecules is a complex cascade of reactions that require both Brønsted acid and Lewis acid catalysts (Scheme 2.2) [4,7]. In addition, acidic catalysts may simultaneously promote side reactions of the products into undesirable by‐products, compromising selectivity of the targeted processes [4,7]. Consequently, judicious selection of the catalysts and processing conditions are essential to ensure the efficient processing of biomass into targeted value‐added products. This section will deal with major developments in the processing of cellulosic substances.
Cellulose is the most naturally abundant macromolecule on the Earth [36]. Even if this view is contested, lignocellulose is the only large volume biomass to which we have ready access on large industrial scale and where there is a globally distributed large volume industry and supply chain [4]. Cellulose consists of β(1 → 4) linearly linked glucose units and is a principal portion of plant cell walls. Hemicellulose is another polysaccharide present in lignocellulose and is often made of structurally branched xylose units and sometimes other moieties [6,16,31]. The past few decades have witnessed a significant interest in the acid‐catalyzed processing of cellulosic substances into organic building block chemicals (platform molecules), such as 5‐(hydroxymethyl)furfural (HMF), furfural (FF), levulinic acid (LevA), LacA, and their many derivatives [4].
A key first step in the valorization of polysaccharides is the depolymerization thereof into low‐molecular‐weight sugars, from which other value‐added chemicals are generated (Scheme 2.2) [6,37]. In their own right, low‐molecular‐weight saccharides are valuable ingredients in the food manufactory and also used as substrates for the fermentative production of ethanol or LacA [38–40]. Polysaccharides are commonly hydrolyzed in aqueous mineral acids in industrial processes [41]. Most technologies are based on thermal hydrolysis of cellulosic biomass in dilute aqueous sulfuric acid solution in batch or percolation reactors, with a typical glucose yield of 55–70% [41]. The glucose yield may be improved to 85%, when conducting the hydrolytic processing of biomass in a countercurrent shrinking bed reactor system at elevated temperatures (up to 225 °C) [42]. This technology has been engineered at National Renewable Energy Laboratory (NREL) by designing a cascade of percolation reactors simulating countercurrent. The available information suggests that this specific process has been demonstrated to bench scale at present [42]. The sustainability of such methods at industrial scale may be compromised by the need for forcing processing parameters [40,43]. On the other hand, complexities arise because of the formation of large amounts of acidic wastewater and solid waste (typically, calcium sulfate after neutralization of sulfuric acid) and also because of the requirement for corrosion‐resistant manufacturing equipment [4,40]. These ongoing challenges have led to the exploration of new efficient methods for the hydrolytic processing of polysaccharides into low‐molecular‐weight derivatives, where high yields and selectivities, accompanied by low levels of waste production, are key parameters.
Solid acid‐catalyzed reactions of polysaccharides in aqueous media have drawn particular interest [40]. The use of solid catalysts is attractive because of the ease of their recovery and sometimes recyclability. An interesting strategy has been devised using a cellulase mimetic catalyst [44], containing both cellulose binding and cellulose hydrolyzing sites, similar to cellulase enzymes. The catalyst is a sulfonated chloromethyl polystyrene resin (CP‐SO3H) bearing chloride groups (–Cl) with which saccharides and sulfonic groups (–SO3H) are coordinated to generate the Brønsted acidity requisite for the hydrolysis of the glycosidic linkages (Scheme 2.3) [44]. The catalyst CP‐SO3H demonstrated impeccable activity during the processing of microcrystalline
