Carbohydrates are obtained from lignocellulosic resources after depolymerization of cellulose and hemicelluloses. Glucose (a sugar containing six carbons) is produced via hydrolysis of cellulose, whereas xylose and mannose are the main products obtained by hydrolysis of hemicellulose.

Lignin

FIGURE 5 The production of BTX from lignin requires the development of a new technology.

Carbohydrates have the possibility to be converted to a wide spectrum of products by means of biochemical (e. g., fermentation) or chemical transformations. Fermentation of sugars to ethanol is already established in the market: nowadays more than 90% of the world ethanol production is derived from biomass feedstocks, while the remaining 10% is produced from oil or gas refinery (Patel, 2006). Further promising sugar derivatives through fermenta­tion are organic acids like succinic, fumaric, malic, glutamic, aspartic, and others (Werpy and Petersen, 2004). Because of their functional groups, organic acids are extremely useful as starting materials for the chemical industry and may act as intermediate to production of fine chemicals. For many organic acids, the actual market is small, but an economical production process will create new markets by providing new opportunities for the chemical indus­try (Sauer et al., 2008). For example, succinic, fumaric, and malic acid could replace the petroleum-derived commodity chemical maleic anhydride in its applications. The market for maleic anhydride is huge, whereas the current market for the organic acids mentioned is small owing to price limitations. Once a competitive microbial production process for one of these acids is established, the market for that acid is expected to consistently increase. The technological barriers which keep these conversion routes at a precommercial stage con­cern microbial biocatalysts, which need to be improved to simultaneously reduce formation of byproducts and increase yields and selectivity. Issues of scale-up and system integration are also to be addressed (Werpy and Petersen, 2004).

In addition to microbial conversions, there are several catalytic transformations for carbohydrates, like oxidations, dehydration, hydrogenations, alkylations, among others, which are industrially feasible. Oxidation leads to valuable intermediates like gluconic acid, which is used for synthesis of pharmaceuticals, food additives, cleaning agents, and others (van Bekkum, 1998). Dehydration of sugars is a promising option for producing important platform chemicals like levulinic acid (LA; from glucose) and furfural (from xylose) which can be converted into a large portfolio of chemicals having many applications in the chemical industry and transportation sector (i. e., fuel additives; Bozell et al., 2000; Hayes et al., 2006). The technical barriers for this pathway concern the necessity to increase yields through more selective dehydration processes, perhaps supported by the development of new catalysts. Catalytic hydrogenation of sugars gives sugar alcohols, such as xylitol and sorbitol. Sorbitol is used as a sweetener as well as an intermediate for synthesis of vitamin C, food additives,
and C4-C6 polyols for synthesis of alkyds (Blanc et al., 2000). Alkyds are polyesters formed via esterification between polyhydric alcohols and di — or poly-basic carboxylic acids or their anhydrides (Maki-Arvela et al., 2007). These reaction pathways have a larger degree of devel­opment than fermentation routes, and some of them are already at a commercial stage. For instance, the production of sorbitol is practiced by several companies and has a production volume on the order of 0.1 million tons/years (Werpy and Petersen, 2004). These productions are usually based on batch technology, and the only technical development needed would be the use of a continuous process.