Ethanol is readily dehydrated by chemical reactions to ethylene (ethene):

C2H5OH ^ C2H4 + H2O

in a reversal of the chemistry used to manufacture “industrial” alcohol from petrochemical sources. Ethylene per se is a key intermediate in organic chemistry for plastics (polyethylenes); in 2005, its worldwide production was estimated to be 113 million tonnes.9 From ethylene (a compound with no direct end uses), the vast “hinterland” of petrochemical production rapidly opens up by way of

• Ethylene oxide (C2H4O), a starting material for the manufacture of acrylo­nitrile, nonionic surfactants, and others, and a ripening agent for fruits

• Ethylene glycol (C2H5O2), a solvent as well as an intermediate in the syn­thesis of synthetic fibers

• Ethylene chlorohydrin (C2H5ClO), another solvent and an intermediate in the production of agrochemicals

• Ethyl bromide (C2H5Br), an ethylating agent in organic syntheses

Approximately 0.9 million tonnes of glycerol is synthesized via chemical routes annu­ally; two-thirds of this could be isolated from bioethanol fermentation broths as glyc­erol-rich stillage.3 Biodiesel wastes and direct fermentations could, however, supply far more glycerol (chapter 6, section 6.3.2) — in fact, so much glycerol is presently being produced as a coproduct of biodiesel production that, although not a biological process, biodiesel manufacture is an imminent development of the biorefinery concept (see section 8.3.3). Important derivatives of glycerol prepared chemically include10

• Glycerol trinitrate, that is, nitroglycerin for explosives

• Epichlorohydrin, the most important material for the production of epoxy resins (and historically the immediate precursor of glycerol in chemical manufacture)11

• Oxidation products as intermediates for pharmaceutical synthesis, including glyceric, tartronic, hydroxypyruvic, and mesoxalic acids and dihydroxyacetone

• Tertbutyl ethers formed by the reaction between glycerol and alkenes such as 2-butene

Approximately 36% of global glycerol production is directed toward such chemi­cal intermediates and products, most of the remainder finding uses in the food and cosmetic industries.

Butanol (chapter 6, section 6.3.3) is a major solvent and finds numerous applica­tions during the manufacture of plastics and textiles.11 Butanol could be catalytically dehydrated to yield l-butene, one of four butene isomers (1, cis-2, trans-2, and iso forms); none of these highly volatile hydrocarbons exist naturally but are produced from petroleum refineries; the complexity of the composition of the butene-contain­ing C4 streams from crude oil “cracking” has limited the chemical exploitation of butanes as intermediates and feedstocks.12 The availability of pure 1-butene in feed­stock quantities may herald new horizons for industrial synthetic chemistry.

These examples demonstrate one fundamental feature of biorefineries, that is, the ability to replace petroleum refineries as sources of both liquid fuels (ethanol, biobutanol, etc.) and (by catalytic transformation of these compounds) feedstock chemicals. This model implies a short linear chain of sequential biochemical and chemical processes:

biomass substrate ^ ethanol, methanol, glycerol, butanol ^ feedstock chemicals

This could operate on an either/or basis, either producing biofuels for transportation uses or proceeding straight to the production of bulk chemicals. The development of a full infrastructure for biofuels distribution would bring in parallel a means of transporting the liquid biofuels to existing chemical industrial sites, thus reducing the total investment cost associated with the transition from oil dependency to a biobased commodity economy, and, although the economic analysis of biorefiner­ies is poorly developed, the price tag will be high: one estimate puts the cost of a biorefinery capable of processing 2000 tonnes/day as $500 million.13 Approximately 500 such facilities will be required to process the “billion tons” annually required for the mass production of lignocellulosic ethanol in the United States (chapter 2, section 2.7), thus representing a $200 billion total investment.

A more elaborate model for biorefineries entails the dual bioproduction of biofuels and other fermentation products.14 The logistic basis for this design is the multiple nature of lignocellulosic and whole-plant carbohydrate streams (chapter 2, section 2.3), that is, hemicellulose-derived pentoses, cellulose-derived glucose and oligoglucans, and starch-derived glucose if grains are processed. Given the wide spectrum of naturally occurring and genetically engineered ethanologens (chapter 3), ethanol could be produced entirely from one of these carbohydrate streams, leaving the others as substrates for different types of fermentations — potentially as wide a choice as that of fermentations known or already used for industrial production. To substantially narrow the field, twelve building block chemicals that can be produced from sugars via biological or chemical conversions and that can be subsequently converted to a number of high-value bio-based chemicals or materials have been identified in a report pre­pared for the U. S. DOE.15 These building block chemicals are molecules with multiple functional groups that can transformed into new families of useful molecules:

• 1,4-Diacids (succinic, fumaric, and malic), all intermediates of the tricar­boxylic acid cycle and easily bioproduced by microbes

• 2,5-Furan dicarboxylic acid, chemically produced by the oxidative dehy­dration of C6 sugars

• 3-Hydroxypropionic acid, a microbial product

• Aspartic acid, an amino acid biosynthesized by all living organisms

• Glutamic acid, another amino acid and one of the major products of indus­trial fermentations for fine chemicals (as monosodium glutamate, MSG)

• Glucaric acid, chemically produced by the nitric acid oxidation of starch

• Itaconic acid, a tricarboxylic acid manufactured on an industrial scale by the fungus Aspergillus oryzae

• Levulinic acid, chemically produced by the acid-catalyzed dehydration of sugars

• 3-Hydroxybutyrolactone, chemically produced by the oxidative degrada­tion of starch by hydrogen peroxide

• Glycerol (chapter 6, section 6.3.2)

• Sorbitol, a sugar alcohol derived from glucose (chemically by hydrogenation) but also known as an enzyme-catalyzed product of glucose metabolism

• Xylitol/arabinitol (chapter 3, section 3.2)

These compounds were chosen by consideration of the potential markets for “build­ing blocks” and their derivatives and the technical complexity of their (bio)synthetic pathways. A second-tier group of building blocks was also identified as viable candi­dates, most of which are microbial products: gluconic acid, lactic acid, malonic acid, propionic acid, the two triacids, citric and aconitic, xylonic acid, acetoin, furfural, levoglucosan, and the three amino acids, lysine, serine, and threonine (table 8.1).

The building block compounds were clearly differentiated from two other potential biorefinery products: direct product replacements and novel products. The former might include acrylic acid manufactured from lactic acid rather than from fossil-derived propylene; markets already existed for the compound and the cost structures and growth potential of these markets were well understood, thus reduc­ing the risks involved in devising novel production routes. On the other hand, novel products such as polylactic acid (section 8.1) had no competing routes from fossil reserves, had unique properties (thus rendering cost issues less crucial), and were intended to meet new markets.

Microbial Routes Known for Future Biobased Chemical Building Blocks

TABLE 8.1 Building block Aerobic fermentation Anaerobic fermentation Glycerol Three-carbon compounds Yeast/fungal and bacterial Yeast/fungal and bacterial Lactic acid Yeast/fungal Commercial bacterial process Propionic acid None Bacterial Malonic acid Yeast/fungal None 3-Hydroxypropionic acid Yeast/fungal and bacterial None Serine Commercial bacterial process None 3-Hydroxybutyrolactone Four-carbon compounds None None Acetoin Yeast/fungal and bacterial Bacterial Aspartic acid Yeast/fungal and bacterial None Fumaric acid Yeast/fungal and bacterial None Malic acid Yeast/fungal and bacterial None Succinic acid Yeast/fungal and bacterial Bacterial Threonine Commercial bacterial process None Arabitol Five-carbon compounds Yeast/fungal Yeast/fungal Xylitol Yeast/fungal Yeast/fungal and bacterial Furfural None None Glutamic acid Commercial bacterial process None Itaconic acid Commercial fungal process None Levulinic acid None None 2,5-Furan dicarboxylic acid Six-carbon compounds None None Aconitic acid Yeast/fungal None Citric acid Commercial fungal process None Glucaric acid Yeast/fungal and bacterial None Gluconic acid Commercial fungal process None Levoglucosan None None Lysine Commercial fungal process None Sorbitol Yeast/fungal and bacterial None Source: Data from Werpy and Petersen.15

In contrast, the building block compounds were envisaged as being the starting points for diverse portfolios of products, both replacing existing fossil-based compounds and offering novel intermediates for chemical syntheses. This combina­tion has three advantages:

• The market potential is expanded.

• Multiple possible markets can reduce risks.

• Capital investment can be spread across different industrial sectors.

To illustrate these points, the 12 “finalists” could give rise to many derivatives that would find immediate or short-term uses in fields as diverse as transportation (polymers for automobile components and fittings, anticorrosion agents, and oxygenates), recreation (footgear, golf equipment, and boats), and health and hygiene (plastic eyeglasses, suntan lotions, and disinfectants).15 Three of them will now be considered in greater detail to explore possible key features and likely variables in the development of biorefineries.