Chemistry Reference
In-Depth Information
becomes more evident that as waste, energy, etc. are
reduced the cost of the process also normally will be
reduced. This economic advantage undoubtedly will
be the biggest driver for change. There will, however,
be other advantages for industry, not least of which
will be an improvement in the public image, which
is at an all time low in many countries [6] mainly
due to the perception that the industry is environ-
mentally unfriendly. We can see now how green
chemistry becomes connected to the increasingly
important business concept of the 'triple bottom line'
in which business performance is measured not only
in terms of profitability but also in terms of the envi-
ronmental and social performance of the company.
Although it is easy to buy into the concepts of sus-
tainable development, it is often more difficult to
achieve the objectives in practice. Many of these dif-
ficulties are to do with culture and the way chem-
istry and related disciplines are taught and practised.
What is really required is a culture change both in
education and industry. In education the principles
of green chemistry need to be the underlying theme,
not taught in isolation. In industry the principles
of sustainability should form part of the company
ethos and be reflected in management systems and
procedures.
A + B Æ C + D + E
(2.1)
in which A and B react to give product C in high
yield and high purity, also leads to the formation of
by-products (or waste) D and E in stoichiometric
quantities. For many years phenol was manufac-
tured via the reaction of sodium benzene sulfonate
(from benzene sulfonation) with sodium hydroxide;
the products of this reaction are sodium phenolate
(which is hydrolysed subsequently to phenol),
sodium sulfite and water. Even if the reaction pro-
ceeds in quantitative yield, it is evident from looking
at the molecular weights of the product (sodium
phenolate) and unwanted by-products (sodium
sulfite and water) that, in terms of weight, the reac-
tion produces more waste than product. Historically,
however, the chemist would not consider the pro-
duction of this aqueous salt waste to be of any
importance when designing the process.
The atom economy concept proposed by Trost [7]
is one of the most useful tools available for design of
reactions with minimum waste. The concept is that
for economic and environmental reasons reactions
should be designed to be atom efficient, i.e. as many
of the reacting atoms as possible should end up in
useful products. In the example shown in Fig. 2.3 all
the carbon atoms present in the starting material are
incorporated into the product, giving a carbon atom
efficiency of 100%, but none of the sulfur ends up
as useful product and hence the atom efficiency for
sulfur is 0%. Overall, the atom efficiency of the reac-
tion is defined as the ratio of the molecular weights
of desired product to the sum of the molecular
weights of all materials produced in the process. In
the above example the atom efficiency would be
116/260 or 44.6%.
The concept of atom economy has been expanded
usefully by Sheldon [8,9], by the introduction of the
term 'E factor', which is the ratio of the kilograms of
3 Waste Minimisation and Atom Economy
3.1 Atom economy
Generations of chemists, especially organic chemists,
have been educated to devise synthetic reactions
to maximise yield and purity. Although these are
worthy goals, reactions may proceed in 100% yield
to give a product of 100% purity and still produce
more waste than product. In simplistic terms, Equa-
tion 2.1:
SO
3
Na
ONa
+ 2NaOH
+ Na
2
SO
3
+ H
2
O
2
¥
40
MWts 180
116
126
18
Fig. 2.3
Benzene sulfonate route to
phenol.
