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R, H or organyl.

      Although acid catalysis prevails in most processes with, for example, lignocellulose, it is not the only means that adds value to biomass, especially with noncellulosic substances. Sometimes, other strategies are more advantageous, brought about in large part because of the limitations of the chemistry under acid‐catalyzed conditions. For example, lipids, such as triacylglycerides and free fatty acids, can form manifold oleochemicals (functional products based on oleaginous materials) in the presence of an appropriate acid catalyst (Brønsted or Lewis) [1,2,10–12]. In particular, acid‐catalyzed esterification of free fatty acids with low‐molecular‐weight alcohols is beneficial for the production of biodiesel fuel from waste cooking oils [26,27]. However, base catalysts or transition metal catalysts are more efficient for the transformation of triacylglycerides into fuel products (fatty acid alkyl esters or varied hydrocarbons) and other functional oleochemicals [24,26–28]. Acid catalysts fare less well for such transformations [26,27]. Additionally, the proteinaceous portion of biomass may be hydrolyzed into amino acids in solutions of concentrated Brønsted acids [13]. Nevertheless, the depolymerization of keratinous waste is often more efficient by other means than acid catalysis [25].

      Polysaccharides, such as glucans and xylans, can be depolymerized into monomer sugars in the presence of Brønsted acid catalysts [4,6]; such monosaccharides have many applications from organic synthesis [29] to energy sources for fermentation [30]. In the context of valorization chemistry toward platform molecules, for example, the depolymerization step is followed by the Lewis acid‐catalyzed aldose–ketose isomerization of monosaccharides, or their retro–aldol reactions, into other derivative carbohydrates [4,7]. These intermediate products are typically convertible into furan‐type molecules or carboxylic acid derivatives under suitable processing parameters (catalyst, solvent, and reaction temperature). Indeed, the reaction conditions can be determinative of the selectivity toward specific types of products [4,7].

      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

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