Environmental Engineering Reference
In-Depth Information
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Exploration and Development of New Polyesters
. A recent comprehensive study by
the DOE has identified 12 promising low molecular-weight materials that can be
produced by fermentation in commercial quantities from plant sugars (succinic,
fumaric, malic, 2,5-furandicarboxylic, 3-hydroxypropionic, aspartic, glutaric,
glutamic, itonic, and levulinic acids, and the alcohols 3-hydroxybutyrolactone,
glycerol, sorbitol, and xylitol). Combination of these acids and alcohols can produce
polyesters by direct condensation. In particular, reactive intermediates that can be
produced by anaerobic fermentations are desirable, because anaerobic processes
typically lose much less of the feedstock carbon to CO2 than do aerobic processes.
The success of the DuPont SoronaTM material, a polyester of such low molecular-
weight precursors (1 ,4-benzenedicarboxylic acid-dimethyl ester with 1 ,3-
propanediol) shows that development of sustainable processes to take advantage of
readily available, renewable substances to produce additional biodegradable plastics
deserves high priority for its great potential to yield both homopolymeric and
copolymeric materials with new ranges and combinations of desirable properties.
F.
P
OLYHYDROXYAKANOATES
PHA development is proceeding in a number of promising directions on both metabolic
engineering and chemical engineering fronts. Fortunately, most of these have the potential for
success both individually and in combination with others, such that no particular obstacle is
currently forming a bottleneck to further progress. The range of physical and thermal
properties achievable with PHAs is still expanding rapidly as new configurations of
copolymers and blends are explored. An on-going challenge will be the ability of the
metabolic engineers to keep pace with the discoveries of the materials scientists, enabling
microbes to synthesize the desired polymers both conveniently and inexpensively. These
efforts can be categorized as follows.
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Investigation of Novel Polymers and Properties
. Clearly, a number of modifications
of PHA composition have the potential to improve the plasticity, moldability, heat
tolerance, and durability of the resulting plastics to approach those of conventional
thermoplastics. Because of the promising availability and flexibility of routes to PHA
synthesis, and because of increasing oil prices that will enable PHA polymers to
become increasingly cost-competitive, it is a valuable effort to explore the properties
of new PHA-based homopolymers, copolymers, and blends even before microbial
pathways to their syntheses are in place.
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Metabolic and Genetic Engineering
. The increasing availability of mathematical
modeling tools, genomic and proteomic data and techniques, and microarray and
antisense RNA technology will allow increasingly accurate prediction of useful
targets for metabolic engineering. At the same time, genetic manipulation within
both microorganisms and plants is becoming increasingly possible and rapid. Several
enzymes central to PHA synthesis are just beginning to be explored through
combinatorial and rational design mutagenesis approaches, and efforts to understand
their catalytic mechanisms, substrate specificities, modes of competition with other
