The key to achieving the goal of reducing the generation of environmentally unfriendly waste and the use of toxic solvents and reagents is the widespread substitution of "stoichiometric" technologies by greener, catalytic alternatives. Examples include catalytic hydrogenation, carbonylation, and oxidation. The first two involve 100% atom efficiency, while the latter is slightly less than perfect owing to the coproduction of a molecule of water. The longer-term trend is toward the use of the simplest raw materials—H2, O2, H2O, H2O2, NH3, CO, and CO2—in catalytic, low-salt processes. Similarly, the widespread substitution of classic mineral and Lewis acids by recyclable solid acids, such as zeolites and acidic clays, and the introduction of recyclable solid bases, such as hydrotalcites (anionic clays) will result in a dramatic reduction of inorganic waste.
CN CN By-product of Nylon 6,6 manuf.
Oxide cat. v\i
Whole cells Rh. rhodocrous
Whole cells Rh. rhodocrous
Figure 9.11 Lonza nicotinamide process.
A possible alternative for the use of organic solvents (many of which are on the black list), is the extensive utilization of water as a solvent. This provides a golden opportunity for biocatalysis, since the replacement of classic chemical methods in organic solvents by enzymatic procedures in water, at ambient temperature, can provide both environmental and economic benefits. Similarly, there is a marked trend toward organometallic catalysis in aqueous biphasic systems and other nonconventional media, such as fluorous biphasic, supercritical carbon dioxide, and ionic liquids.
In conclusion, the widespread application of chemo- and biocatalytic methodologies to the manufacture of fine chemicals has enormous potential for creating greener, environmentally benign processes.
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