Handbook of Solvents - George Wypych - ChemTech - Ventech!

1460 D.W. Rooney, K.R. Seddon
application in commercial sectors as diverse as the nuclear industry, 1,2 pharmaceuticals and
fine chemicals, 3,4 as well as in mainstream petrochemical processes. 5-7 This contrasts with
the unique perspective of Takahashi et al. who state that the two major applications are batteries
and electrolytes, clearly unaware of the modern literature. 8 In a number of indicated
cases, these processes have been taken through the development process to a point of industrial
commercialization and represent first generation ionic liquid processes, principally
based on chloroaluminate(III) ionic liquids which are currently ready for industrial uptake.
Following this line, second generation ionic liquid processes based on other, more benign,
ionic liquids are currently under investigation and development in a variety of laboratories
around the world. Many of these processes utilize the ability of a range of ionic liquids to selectively
immobilize transition metal catalysts for liquid-liquid two-phase catalysis while
permitting easy, often trivial, extraction of products. 9-14
So why have these solvents not been used to rapidly replace the current volatile
organics currently found in industry? There are a number of answers to this, but the most
significant reason is economics. By looking through any chemical catalogue, it is obvious
that 1-methylimidizole (a precursor for the manufacture of ionic liquids) is considerably
more expensive than standard solvents. By the time this compound is processed to the final
ionic liquid product, its cost will have increased to many times that of normal solvents. This
is not helped by the fact that at present there is a very limited market for these compounds,
keeping retail prices high. It is envisaged that if ionic liquid technology does become widely
accepted, then the cost of production will decrease rapidly. Other quaternary ammonium
salts and those that are based around pyridinium are cheaper alternatives (principally due to
the scale of manufacture), but economics is not the only problem. As yet, there is a significant
deficit of raw physical property data for engineers to use when designing new processes
or retrofitting old plants.
For the efficient design of any new industrial process incorporating ionic liquid technology,
a complete understanding of the behavior of the solvent during operation is necessary.
Physical properties such as viscosity, density, heat capacity and surface tension are all
important during these early design stages. Others like electrochemical windows and electrical
conductivities, will be important for more specific applications. With the advent of
computers, chemical engineers have been able to use powerful process simulation software
packages to estimate how a particular process will behave under certain operating conditions.
These software packages, like the manual
calculations which preceded them,
predict the physical properties of organic solvents
by using a number of empirical and
semi-empirical equations which are available.
Unfortunately these equations where
developed for molecular compounds and
tend to require the critical temperature and
pressure data, information which does not
apply to ionic liquids. In addition, other techniques
for predicting physical properties like
Figure 21.2.1. Showing the relationship between lattice
energy 26 and melting point 27 for the Group 1 halide
salts.
surface tension involve using group contribution
methods, but again these fail to account
for organic salts and therefore cannot