PerspectiveChemistry

Chemicals from Biomass

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Science  23 Nov 2007:
Vol. 318, Issue 5854, pp. 1250-1251
DOI: 10.1126/science.1146356

Recently, there has been a strong political and technical focus on using biomass to produce transportation fuels (1, 2). Much less attention has been given to biomass as a feedstock for organic chemicals. Replacement of petroleum-derived chemicals with those from biomass will play a key role in sustaining the growth of the chemical industry.

Currently, ∼13% of the crude oil consumed by the United States is used for nonfuel chemical production (3). Biomass-based processes that could replace crude oil harness enzymatic methods, microbiology, and metabolic engineering to direct the transformation of sugars, lipids, and other biomass-derived molecules to the desired small molecules and polymers.

The biological production of commodity chemicals has considerable history. Between 1945 and 1950, 66% of the n-butanol and 10% of acetone in the United States were produced by fermentation of molasses and starch. Other commodity products produced by fermentation in the first half of the 20th century include acetic acid, citric acid, lactic acid, and itaconic acid (4). Increased prices of sugar feedstock and decreased prices of petrochemical feedstock ended the fermentative production of these commodities.

Now, the situation has reversed. A 2004 report by the U.S. Department of Energy (DOE) identified high-volume commodity chemicals that could be produced from biomass and can serve as starting materials for many chemical products via biological processes (5). A joint report by the DOE and the U.S. Department of Agriculture (6) concluded that U.S. agricultural and forest sources can renewably supply a billion tons per year of lignocellulosic biomass. This amount of biomass would not satisfy U.S. fuel demands, but could theoretically replace petrochemical feedstocks for chemical production. Several commodity chemicals are already produced by fermentation, including glutamic acid (∼1.7 billion kg/year worldwide), citric acid (∼1.6 billion kg/year), and lysine (∼850 million kg/year).

New feedstocks for the chemical industry.

Ferulic acid, a precursor for numerous aromatic chemicals used in the chemical industry, can be extracted from corn fiber. Many other important precursors can also be obtained from biomass.

PHOTO CREDIT: JUPITER IMAGES

In addition to these commodity chemicals, commodity polymers may also be produced using biologically produced monomers combined with classical chemical methods, as well as through biological polymerization methods, either with isolated enzymes or in actively growing cells.

For example, lactic acid produced by fermentation (7) can be converted chemically to methyl lactate, lactide, and polylactic acid (PLA). The latter polymer—commercially available under different trade names—is a fully biodegradable replacement for polyethylene terephthalates (PETs) (8). Efforts are also under way to develop efficient processes for converting biologically produced lactic and hydroxypropionic acids to methacrylic and acrylic acids, respectively, both major commodity monomers consumed at ∼1.6 billion kg/year worldwide.

Another well-known example is work by scientists at Genencor and DuPont, who have developed a cost-effective fermentative route to 1,3-propanediol (1,3PDO), the key building block in poly-(propyleneterephthalate) (PPT, Sorona 3GT), which is not readily available from petrochemical feedstocks (9).

Immobilized enzyme catalysts have also been used to polymerize bio-derived monomers. For example, by using a commercially available lipase catalyst, direct polycondensations can be performed between sorbitol and/or glycerol with chemically or biologically produced diacids (10). This method dramatically reduces reaction temperatures and energy consumption relative to chemical polymerization processes, while also controlling branching during polymerization.

Fermentation by various microorganisms has been used to produce succinic acid (11, 12), which may potentially replace maleic anhydride, now produced from butane at ∼1.8 billion kg/year worldwide. (Maleic anhydride is the starting material for various polymers and industrial solvents.) In another example of microorganism-based production, the shikimic acid pathway in Escherichia coli has been reengineered to transform glucose to catechol and other aromatic alcohols (13); these industrial chemicals are currently produced from benzene. Further manipulation of this metabolic pathway leads to cis, cis-muconic acid, which can be converted chemically to adipic acid. The latter is one of two components required to produce Nylon 6,6 (14).

Polymers from biomass have also been produced completely within microbial cells. A notable example is the microbial synthesis of poly-3-hydroxyalkanoates (PHAs) (15).

Nor is fermentation a required action for the use of biomass-derived feedstock. Fischer-Tropsch chemistry has been run on pyrolysed biomass, and some commodity chemicals could be derived directly from biomass via conventional extraction methods. For example, corn fiber contains a high percentage of ferulic acid (see the figure), a flexible feedstock for various fine chemicals such as vanillin and guaicol. An estimated 1 billion kg/year of ferulic acid could be recovered from corn fiber separated in current corn-milling operations (16).

The rapid growth in the biodiesel industry, which uses chemical methods to synthesize its product, has decreased the market price of glycerol; many biodiesel production facilities now view crude glycerol as waste. Chemical companies can use this glycerol as a low-cost chemical building block. Dow Chemical Company, Huntsman Corporation, Cargill, and Archer Daniels Midland Corporation have begun, or announced plans, to chemically convert glycerol to propylene glycol (17). Dow Chemical Company and Solvay plan to build plants that use glycerol to produce epichlorohydrin (18).

Other innovative chemical methods convert fatty acids to polymer building blocks. For example, Cargill, working with the Kansas Polymer Research Center, has developed a bio-derived polyol (BiOH) by the chemical conversion of triglyceride carbon-carbon double bonds to alcohol and methoxy groups. Polyols are important precursors to numerous polyurethane materials (19).

Despite these achievements, the transition of industrial chemical production from petrochemical to biomass feedstock faces real hurdles. Biological processes do not require the high pressures and temperatures associated with most nonbiological chemical processes and thus have the potential to reduce costs. However, current processes for production of commodity chemicals have evolved through considerable investment to become highly efficient, often continuous, and well integrated. To be successful, new biological processes must rapidly approach similar levels of efficiency and productivity. Nevertheless, economic opportunities, available technologies, and environmental imperatives make the use of biomass and biological methods for industrial chemical production not only feasible but highly attractive from multiple perspectives.

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