In fossil refinery, succinic acid is currently produced from butane/butadiene via maleic acid and has a production volume of 30-50 kilotons/year (Bos et al., 2010). This process is relatively expensive and the existing market for succinic acid is limited. However, if a more economic production route could be established, it has a potential market of hundreds to thousands tons, thanks to its many possible derivatives (Sauer et al., 2008). Succinic acid can be efficiently produced from fermentation of sugars, on condition that low-cost fermen­tation routes are established. The basic chemistry of succinic acid is similar to that of the petrochemically derived maleic acid/anhydride. These compounds can be converted via hydrogenation/reduction to butanediol, tetrahydrofuran, and gamma-butyrolactone.

In the case of succinic acid, the technical challenge is the development of catalysts that would not be affected by impurities in the fermentation. Noteworthy is the possibility to produce pyrrolidinones, so addressing a large solvent market (Werpy and Petersen, 2004).

6.2.4 C5 Bulk Chemicals

Furfural is the starting material for industrial production of furan compounds and today it is completely produced from biomass feedstocks rich in C5 sugars. The market volume is

0. 2-0.3 million ton/year. It is obtained from hydrolysis of C5 sugars along with other degra­dation products. Removal of these impurities is expensive and industrial uses of furfural will benefit of an optimization of the furfural production process (Patel, 2006). Many valuable chemicals can be derived from furfural (e. g., maleic anhydride, furfuryl alcohol, etc.), and the chemistry for the conversions is well developed (Kamm et al., 2006b).

Itaconic acid has a chemistry similar to the fossil-derived chemicals maleic acid and maleic anhydride, which are used as monomers in the production of acrylate-based polymers and thermoset resins in oil refinery (Bos et al., 2010). Itaconic acid is currently produced via fungal fermentation and is used primarily as a specialty monomer. The major applications include the use as a copolymer with acrylic acid and in styrene-butadiene systems. The major techni­cal hurdles for the development of itaconic acid as a bulk chemical include the development of very low-cost fermentation routes. The primary elements of improved fermentation include increasing the fermentation rate, improving the final titer, and potentially increasing the yield from sugar. Besides important chemical derivatives, itaconic acid can also undergo polymerization, but the properties of polyitaconic polymers need to be ascertained in order to evaluate its use as a polymer (Werpy and Petersen, 2004).

Xylitol is commercially produced from hydrogenation of xylose, the most abundant C5 sugar in hemicellulose. At the moment, there is limited commercial production of xylitol, but once a cheaper production route is established a large potential for production of ethylene glycol and 1,2-propanediol via hydrogenation is expected.

Another promising C5 bulk chemical is LA. It is produced from dehydration by means of acid treatment of C6 sugars like glucose and fructose. LA is one of the most important build­ing blocks available from carbohydrates and has attracted interest from a number of large chemical industry firms: it has frequently been suggested as a starting material for a wide number of compounds (Bozell et al., 2000; Hayes et al., 2006; Kamm et al., 2006b; Werpy and Petersen, 2004). The technical barriers for this option include improvement of the process for LA production itself, even if the LA yield is already at 70% (Hayes et al., 2006). The family of chemical compounds available from LA is quite broad, and addresses a number of large volume chemical markets. Besides chemicals, LA shows promising efficiency in the conver­sion to methyltetrahydrofuran and ethyl levulinate, two fuel additives which can be blended up to 20% with gasoline and diesel (without requiring any modification of the engine).

Lignin is the most abundant renewable source which has aromatic units in its structure. As shown in Figure 3, the world demand for aromatics is consistent and increasing over the years. The possibility to establish a direct and efficient conversion of lignin to high — volume, low-molecular weight aromatic molecules is therefore extremely attractive. How­ever, there are important technological barriers which must be overcome, given the resistant and robust lignin structure.

The basic chemical units of lignin shows very high potential for making BTX chemicals (Figure 5). Technologies able to efficiently depolymerize the polymer by breaking the C-C and C-O bonds are necessary. An aggressive, nonselective, depolymerization would bring to a mixture of BTX, phenols, and aliphatic fractions (C1-C3). These chemicals should be suitable for being directly used by the conventional petrochemical processes which convert the bulk aromatics into nylons, resins, polymers, and others. Development of the required aggressive and nonselective chemistries is part of the long-term opportunity but is likely to be achievable sooner than highly selective depolymerizations (presented below; Holladay et al., 2007).

A related technological challenge for the production of chemicals from lignin is the elabo­ration of proper separation techniques for the mixture intermediates from which the aromatic chemicals are to be isolated (Huang et al., 2008).

Sorbitol is produced by catalytic hydrogenation of glucose on a large industrial scale (1.1 million tons/year; Patel, 2006). Besides the food industry, it can be used for production of surfactants and polyurethanes. Sorbitol has potential for the production of isosorbide at low costs (if higher yields are achieved through optimization of process conditions and dehydration catalysts). Isosorbide is a very effective monomer for raising the glass transition

temperature of polymers. The major applications are as a copolymer with PET for the use in bottle production. Hydrogenolysis of sorbitol leads to glycols, while direct polymerization forms polyesters for the resin market, whose characteristics need to be properly tested.

2,5-FDCA is formed by an oxidative dehydration of glucose, where side reactions still need to be minimized. FDCA has a large potential as a replacement for terephthalic acid, a widely used component in various polyesters, such as polyethylene terephthalate (PET) and polybutyleneterephthalate (PBT). This bulk chemical has high versatility in production of derivatives through simple chemical reactions: selective reduction leads to partially or fully hydrogenated products (with applications as new polyesters), combination with diamines produces new nylons, etc. (Werpy and Petersen, 2004). Like the other sugar-derived products, the primary technical barriers to production and use of FDCA include development of effec­tive and selective dehydration processes for sugars.

Glucaric acid is the product of catalytic oxidation (with nitric acid, which should be replaced by oxygen) of glucose. Glucaric acid can serve as starting point for the production of a wide range of products with applicability in high volume markets, like new nylons (e. g., polyhydroxypolyamides) or new surfactants.

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.

The success of the chemical industry in biomass conversion to chemical products is highly dependent on the development of new catalysts. Since the original molecular structure of bio­mass components is supposed to be preserved, the focus of catalysis research will have to shift from building functional structures out of simple building blocks to the re-functionalization of complex molecular structures (Marquardt et al., 2010). A crucial role is played by the next research achievements for basic chemical reactions like dehydration, condensation, hydrogenation, and so on, which require high selectivity to be implemented at commercial scale. Enzymatic or whole-cell biocatalysts are often high-performance alternatives resulting in high selectivity and yield (Stephanopoulos, 2007). Hybrid catalysts, combining enzymes with chemocatalysts in a complex molecular or nanoparticulate structure, constitute even more sophisticated options (Marquardt et al., 2010). In particular, the specific developments needed in the main conversion reactions are:

• Hydrogenation/reduction: this reaction is generally used to add hydrogen, e. g. to an acid functional group to form alcohols. Research developments should ensure the possibility to operate at milder conditions (pressure, temperature, etc.) giving high selectivity, by means of the improvements in catalyst performances. Catalysts should also improve their tolerance to inhibitory compounds and lifetime.

• Oxidation: this reaction oxidizes carbon and converts alcohols into acid functional groups. In future biorefineries, mineral oxidants like sulfuric acid and nitric acid should be replaced by air, molecular oxygen, dilute hydrogen peroxide, and others. Tolerance to inhibitory components of biomass processing streams should also be enhanced.

• Dehydration: this reaction removes oxygen from the substrate and it is fundamental for biomass processing. It requires improvements in the selectivity, needed to avoid side reactions. New heterogeneous catalysts (solid acid catalysts) are preferred over liquid catalysts.

• Fermentation: fermentation processes convert sugars into valuable products. In general, an improvement of microbial biocatalysts to reduce acetic acid coproducts and increase yields is needed. Lower costs to recover the products are necessary to scale-up.

• Polymerization: it is usually done through esterification to produce innovative polymers, whose applications need to be tested. Issues of selectivity and control of molecular weight and properties are still open.

The combination of new catalysts and new substrates offers innovative and largely unex­plored opportunities to establish novel production pathways and novel innovative products with particular properties which must be still explored (Vennestr0m et al., 2010). The flexi­bility in tailoring the value chain, from feedstocks to the desired products (or vice versa), com­bined with the several possible uses of side streams, may lead to different options. These options must be systematically evaluated and screened to identify those with the best performances, including carbon efficiency, energy consumption, environmental impacts, and production cost. Ideally, such an evaluation should precede laboratory experiments in catalysis and production processes, in order to specifically focus research activities on the most promising alternatives.

Over the last decade, prices of fossil fuel feedstocks have increased, whereas prices of biomass resources have slowly and steadily decreased. This situation makes the possibility to produce the existing bulk chemicals from biomass rather than fossils an attractive option. In the following paragraphs, the current state of the art in the production of the bulk chemicals previously highlighted is investigated. The possible reaction pathways are summarized in Figure 6.

1.4 Ethylene

The production of this chemical from biomass sources can be achieved through dehydra­tion of ethanol. This dehydration is favored at high temperatures (300-600 °C) and can be car­ried out over a wide variety of heterogeneous catalysts (Arenamnarta and Trakarnpruk, 2006; Takahara et al., 2005). There are no technological barriers to be faced for the production of ethene from ethanol at a commercial scale; this production is initially most likely to happen in regions with cheap and easy access to bioethanol (Haveren et al., 2008).

1.5 Propylene

Direct production of propene from sugars can be carried out via fermentation (Fukuda et al., 1987). Product yields are very low: the productivity needs to be improved by orders of magni­tude to make this process economically viable (Haveren et al., 2008). An alternative production pathway consists in the dehydration of 2-propanol, which is produced by reduction of acetone. The latter can be obtained via the acetone, butanol, ethanol (ABE) fermentation process, which is largely studied in the scientific and industrial community (Ezeji et al., 2007). In addition, propene can be produced from dehydration of 1,2-propanediol (either called propene glycol). This glycol can be effectively produced from reduced sugars as sorbitol and xylitol or lactic acid, and such conversion routes have strong commercial potential (Haveren et al., 2008).