302 Green Chemistry, 2nd ed
a reaction mixture can be captured and recycled for the same application. Carbon dioxide
can be obtained at low cost from biological fermentation processes.
Ionic liquids present another alternative to organic solvents for use as media for
chemical synthesis. Inorganic salts consisting of ions, such as NaCl composed of Na+
and Cl
-
ions, are normally hard, high-boiling solids. However, when one or both of the
ions are composed of large charged organic molecules, as shown by the cation in the
example below,
H
H
H
C C
H
H
C
H
H
N N
H 3 C C
H
H
1-Butyl-3-methylimidazolium hexafluorophosphate-
P
F
F
F
F
F
+ F
the salts can be liquids at room temperature and are called ionic liquids. These materials
have the potential to act as suitable media in which substances can be dissolved and
undergo reactions, and active research is underway to explore this possibility. There is an
enormous variety of such ionic liquids with widely varying solvent properties because
of the large number of kinds of ions that can be combined leading to almost limitless
possibilities for various ionic liquids.
The ultimate approach to eliminating problems with solvents in chemical synthesis
is to do reactions without solvents of any kind. Some reactions can be performed in
which the reactants are simply mixed together or are held on solid supports, such as
clays. Microwave heating of such reaction mixtures has proven effective in providing
energy to enable reactions to occur rapidly.
Catalysts
Recall from Chapter 4, Section 4.5, that catalysts are materials that enable reactions
to occur without themselves being changed. Large numbers of different catalysts are
used in chemical processes and their potential toxicities, production of byproducts and
contaminants, recycling, and disposal are matters of considerable importance in the
chemical industry. Catalysts are divided into the two major categories of heterogeneous
catalysts that are held upon some sort of support where they interact with reactants and
homogeneous catalysts that are actually mixed with the reactants, often in solution
in the media in which the reactions are carried out. Heterogeneous catalysts offer the
advantage of being readily separated from reaction products, whereas homogeneous
catalysts require measures such as distillation to remove them from the product. In many
cases, however, homogeneous catalysts are much more effective in carrying out reactions.
One of the objectives of green chemistry, therefore, has been to develop heterogeneous
catalysts that equal homogeneous catalysts in their performance.
An important area of endeavor in the development of improved catalysts with
respect to green chemistry is selectivity enhancement. Basically, this means developing
a catalyst that is very selective in what it does, ideally making the right product and
nothing else. A highly selective catalyst increases the percentage utilization of raw