Green Chemistry Principles - Process Intensification for Green Chemistry - page 49

2.6 An Overview of Green Chemical Processing Technologies
The application of green chemistry to reaction systems tends to fall into one of two
categories from a hardware perspective. First, there is equipment associated with utilizing
alternative reaction solvents, such as supercritical or high-pressure solvents, which allow
high mass-transfer and high energy-transfer techniques such as microwave reactors,
ultrasonication reactors or microreactors and separation equipment. Second, there is a
large family of equipment associated with high energy transfer to the reactants. New
technology such as microreactors, microwave reactors, radio frequency heating, electric
pulses, ultrasonication and SDRs are becoming part of the technologist's arsenal.
2.6.1 Alternative Reaction Solvents for Green Processing
Ionic liquids, fluorous biphasic systems and supercritical fluids have all been studied as
alternatives to conventional organic and aqueous solvents. However, due to their nature,
some of these novel systems require additional hardware for utilization. For example,
some suppliers have designed advanced mixing paddles and blades to enable polyphasic
systems to be intimately mixed at the laboratory scale [53]. There has also been
considerable rethinking of the green credentials of some of these alternative solvents,
and many ionic liquids are no longer considered suitable due to their complex syntheses,
toxicities or other unacceptable properties, or due to difficulties in separation and
purification. Fluorous solvents are not considered to be environmentally compatible (as
they persist in the environment).
Supercritical solvents are more difficult to manipulate, due to the high pressures and
temperatures sometimes employed. In the case of supercritical water, equipment had to be
designed which could contain the highly corrosive liquid; this was achieved through the use
of alloyed metals. Supercritical water has the unique property of being able to oxidize most
organic compounds, and for this reason it is being used in wastewater treatment [54]. Vessels
for creating supercritical solvents such as scCO 2 are now available; these are capable of
making fine adjustments to temperature and pressure in order to affect the solvents' properties
[55]. Excessive pressure and temperature are thus not required to produce scCO 2 , as shown in
Figure 2.9. scCO 2 is an increasingly popular reaction medium, as its properties can be
controlled by varying the temperature and pressure or by using a cosolvent. The main
environmental benefit of scCO 2 lies in the work up, as the product mixture is obtained free
from solvent simply by returning to atmospheric conditions. Additionally, carbon dioxide is
nontoxic, nonflammable, recyclable and a byproduct of other processes. However, there are
energy and safety concerns associated with the elevated temperatures and pressures
employed, which must be balanced against the benefits of its use.
Supercritical solvents are finding uses in a number of fields [57]. For example, they are
being used as replacements for organic solvents in several conventional reactions [58]. This
was considered at commercial scale by Thomas Swan for the multiton synthesis of 3,3,5-
trimethylcyclohexanone [59], which is used as a solvent in the manufacture of resins and
varnishes, and it has been proven for other compounds [60].
Solvent intensive processes such as decaffeination [61] and extraction of natural
products [57] are being carried out with the more gentle technique of supercritical
fluid extraction. This enables fragile compounds to be removed selectively without