Chap. 11. Toward a Greener Anthrosphere through Industrial Ecology 297number of environmental and sustainability considerations. The source may consist of
a depleting resource, such as petroleum, in which case the lifetime of the resource and
the environmental implications of obtaining it must be considered. A preferable source
consists of recycled materials, although not many recycled materials are suitable for
chemical feedstocks. A third source that is very desirable consists of renewable resources,
particularly from materials made by photosynthesis and biological processes.
The second major aspect of converting feedstocks to final products is separation
and isolation of the desired substance. An example of this step is the isolation of specific
organic compounds from crude oil to provide a feedstock for organic chemical synthesis.
It may be necessary to process raw materials from a source to convert it to the specific
material used as a feedstock for a chemical process. Often most of the environmental
harm in providing feedstocks comes during the isolation process, in large part because
of the relatively large amount of waste material that must be discarded in obtaining the
needed feedstock.
Once a suitable chemical feedstock is obtained, it is subjected to chemical processes
that give the final product. As addressed below, this may consist of reactions with various
kinds of reagents in media such as organic solvents, often using catalysts.
Reagents
The term reagents is used here to describe the substances that act upon basic
chemical feedstocks to convert them to new chemicals in synthetic processes. The
kinds of reagents used have a very strong effect upon the acceptability of a chemical
process with respect to green chemical aspects. Much of the work that has been done
in developing and using green reagents has involved organic chemical processes, many
of which are beyond the scope of this book. However, some of the general aspects of
chemical reagents from a green chemical perspective are discussed here.
The most obvious characteristic required of a good chemical reagent is that it do what
it is supposed to do, completely, and at an acceptable rate. A reagent with a high product
selectivity produces a high percent age of the desired product with a low percentage of
undesired byproducts. Another desirable characteristic of a good reagent is high product
yield meaning that most of the feedstocks are converted to product. The use of reagents
that provide high selectivity and yield means that less unreacted feedstock and byproduct
material have to be handled or disposed.
One of the most common measures taken in implementing green chemical processes
is selection of alternative reagents. The criteria used in selecting a reagent include whether
or not it is available, how efficient it is, and its effects. Important considerations with the
chemical transformation are whether it is stoichiometric or catalytic, the degree to which
it is atom economical, and the quantities and characteristics of any wastes produced.
One of the main kinds of reactions for which reagents are used is oxidation, which
usually consists of the addition of oxygen to a chemical compound or a functional
group on a compound. (See Chapter 4, Section 4.7 for a discussion of oxidation and
its accompanying phenomenon, reduction.) An example of an oxidation reaction is the
conversion of ethanol to acetic acid,