Enzymes for Fuels and Chemical Feedstocks - ACS Symposium

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Enzymes for Fuels and Chemical Feedstocks K. Grohmann and Michael E. Himmel

Downloaded by 80.82.77.83 on December 26, 2017 | http://pubs.acs.org Publication Date: April 30, 1991 | doi: 10.1021/bk-1991-0460.ch001

Applied Biological Sciences Section, Biotechnology Research Branch, Solar Fuels Research Division, Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401 The increase in the price of petroleum feedstocks has created opportunities for the development of combined biological and chemical processes that will produce liquid fuels and chemicals from alternate feedstocks such as biomass, coal, and gas. Near-term opportunities exist in the bioconversion of cellulosic materials contained in urban, agricultural, and forestry wastes. This large reservoir of raw materials can be augmented in the future by short-rotation woody and herbaceous crops as well as by specialty crops. Enzymatic conversions will play an increasing role in future biochemical processes because enzymes achieve relatively high catalytic activities and high selectivity, and they have a low impact on the environment. The wide-scale industrial application of enzymes will require development of enzymes with long-term stability and high activity under different use-conditions. Such a broad spectrum of requirements will be difficult to satisfy by a single enzyme for each transformation, but can be supplied by families of related enzymes isolated from microorganisms adapted to diverse environments and improved by site-directed mutagenesis and chemical modification.

Biopolymers of a carbohydrate and polyphenolic nature represent the largest reservoir of organic carbon fixed annually by plants (1). This pool of raw material can be obtained at very reasonable costs, comparable to the cost of major fossil feedstocks (Figure 1). The biopolymers (in aggregate termed biomass) have many uses, mainly in food, feed, fiber, and as materials of construction. Starch has a unique position among polymeric carbohydrates because it is digestible by man and livestock. The largest part of the worlds agricultural production is thus devoted to the production of starchy crops (i.e., grains and tubers) from which only starch-containing organs are utilized and the rest of the plant is usually wasted. Figure 2 illustrates the ranges of agricultural and other residues generated annually in the United States over the past decade (2-4). The annual production of the basic chemical feedstocks from which practically all other organic chemicals are derived, by one route or another, is included in the same figure for comparison (5). These data show that many segments of the annual biomass pool easily exceed the total annual requirements for basic petrochemicals. However, the data in Figures 1 and 2 can be misleading. Biomass feedstocks are highly oxygenated, whereas fossil raw materials are mainly pure hydrocarbons. All processes for the conversion of carbohydrates to hydrocarbon-type materials thus suffer a penalty of high

0097-6156/91/0460-0002$06.00/0 © 1991 American Chemical Society

Leatham and Himmel; Enzymes in Biomass Conversion ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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GROHMANN AND HIMMEL

Fuels and Chemical Feedstocks

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Figure 1. Approximate ranges of the major chemical and fuel feedstock costs in the United States between 1980 and 1989 (projected price only). 500-1

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Figure 2. Approximate ranges of U.S. annual production of major carbohydrate feedstocks and organic chemical feedstocks in the 1980s. The data for organic chemicals are plotted only for 1988.



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weight losses, with the exception of short chain organic acids, where high weight yields are possible (6). The oxygen content effects are easily shown when various feedstocks are compared on an energy content basis (Figure 2) rather than a weight basis. The differences in fuel costs become much smaller than was evident in Figure 1. Figure 3 illustrates the discount of solid fuels, versus gaseous and especially liquid fuels, which receive a premium for their exclusive dominance in the transportation sector. Figure 3 further shows that lignocellulosic biomass was competitively priced as a solid fuel over the past decade and as a result, its fuel use via combustion has increased (2). Such direct use for heat generation does not allow •Valorization" of other useful properties which are contained in the spectrum of biomass resources. The bioconversion of biomass and generation of profits necessary for any viable commercial enterprise should concentrate on conversion into higher value products in chemical, food, feed, and liquid fuel industries where higher priced products are marketed at the present time. Some of these higher valued products can then be combusted, recycled, or converted to other uses such as production of transportation fuels or chemicals. Multiple recycle of original raw materials will decrease pressure on finite natural resources, such as land, fresh water, and mineral fertilizers. The lowest cost fuel currently available at a negative cost is municipal solid waste (Figure 1), which is a collection of discarded products from the public and various industries. Biopolymers The major components of terrestrial plants are two families of carbohydrates - cellulose and hemicelluloses. Cellulose fibers account for approximately 40% to 50% of the total dry weight of stems, roots, and leaves (7). Cellulose fibers are embedded in the matrix of hemicelluloses and phenolic polymers, commonly identified as lignin-carbohydrate complexes (LCCs)(5). There is evidence for covalent ether, and possibly ester, bonds between hemicellulose and lignin chains. Covalent crosslinking renders both hemicelluloses and lignin insoluble in nonreactive chemical solvents. The extraction of LCC components requires relatively mild alkaline cleavage in case of grasses and hardwoods; however, harsh oxidative pretreatments are required for extraction of hemicelluloses from softwoods (9-11). These treatments destroy the structure of lignin. Alternate pulping processes for extraction of lignin from softwoods hydrolyze (i.e., bisulfite pulping) or destroy (i.e., Kraft pulping) carbohydrates and modify lignin in the same process (12). Starch, a widespread storage polymer in the plant kingdom, is widely available because of intensive cultivation of starch-producing grain and tuberous crops for human food and animal feeds. This starch is present in specialized tissues in the form of granules that can be separated by a steepmg-milling-sedimentation sequence from grains and by a similar process from tubers (13). Grains are a favorable source of industrial starch, because they have a low moisture content and are therefore storable for long periods of time. An underutilized storage polymer in plant tubers is the polyfructosan, inulin, which is not digestible by humans but which can provide the sweet monomer, fructose (14). Other industrially useful polymeric carbohydrates are pectins (75), which are present in the tissues of fruits, tubers, and immature plants and are usually obtained as a byproduct of food processing. There are many other specialized carbohydrates of plant origin, but their availability is rather low, so they will not be addressed in this chapter. Chitin, a polymer of N-acetylglucosamine, is present in large amounts in the exoskeletons of crustaceans and other arthropods and is a major marine biopolymer (16). Its availability is, however, limited to byproducts from seafood industries (16).


GROHMANN AND HIMMEL

Fuels and Chemical Feedstocks


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Figure 3. Ranges of relative fuels costs in dollars/million BTU in the United States in the 1980s.


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Summary of Major Uses Because mankind, like other nonphotosynthetic organisms, developed survival strategies based on the exploitation of photosynthetic plants, there are many traditional uses for biopolymers that are unlikely to be replaced in a foreseeable future. The largest use of cultivated plants is as human food, as animal feed, and in fiber production, with a relatively tiny acreage devoted to specialty crops for spices, herbs, drugs, and textile fibers (17). With the exception of starch, many biopolymers cannot provide nutrition for humans and other omnivorous animals. In human foods biopolymers are used as additives that can improve texture, viscosity, fiber content, and other properties of prepared foods, without providing direct nutritional values. Examples of such utilization are the addition of pectins, agar, and other gums to foods to achieve thickening and gelling effects. Another example of potential large-scale utilization of cell wall biopolymers is the dramatic improvement in the texture and rising of breads prepared from corn and other starches by the addition of xylans (18). Many biopolymers have very large utilization in traditionally chemical areas, such as fiber, film, and adhesive production, where they often successfully compete with more modern petrochemical products. Paper pulp utilization, for example, exceeds the usage of any other polymer (4,5). Cotton fiber has also maintained a significant position in the textile industry over the past ten years. Two biopolymers, cellulose and chitin, were endowed by nature with high tensile strength (7,16,19). They are linear molecules of high molecular weight that have a strong interchain bonding through multiple repetitive hydrogen bonds. Both polymers are also highly crystalline. The linear component of starch, amylose, also shows a high tensile strength in the dry state (13). Amylose is unfortunately a minor (

Enzymes for Fuels and Chemical Feedstocks - ACS Symposium

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