Solvents

One of the key components of any green industrial process has to be the selection of the solvent, since working with conventional solvents results in the emission of volatile organic compounds (VOCs), a major source of environmental pollution (see Figure 5.4).8,9 There are fourt principal strategies to avoid conventional organic solvents:10,11

1. No solvent (heterogeneous catalysis)

2. Water

3. Supercritical fluids

4. Ionic liquids

The first of these, the solventless option, is the best established,12 and is central to the petrochemical industry, the greenest chemical sector.13 The second, to use water as the solvent, can be an ideal approach,14 but suffers from the difficulty of dissolving many organic compounds in water, undemonstrated scale-up, and the cost of the cleanup of organic contaminated water. Supercritical fluids represent a

^It is conventional to include perfluorocarbons (fluorous phases) in this list. However, these have extremely high global warming potentials: they are from 140-23,900 times more potent than CO2 in their abilities to trap heat in the atmosphere over a century. Moreover, they remain in the atmosphere almost indefinitely, and so will accumulate as long as they are used on a significant scale.9 We do not consider their use as a green option.

D Commercial, Institutional

■ Production processes D Fossil fuel extraction D Solvent use

■ Road transport D Agriculture

D Commercial, Institutional

■ Production processes D Fossil fuel extraction D Solvent use

■ Road transport D Agriculture

Figure 5.4 UK VOC emissions (1970-2002). Note the predominance of emissions from solvent usage in the latest data.

new phase of matter for chemical synthesis, and wonderful developments have been described elsewhere,15 including the recent commercialization of this technology by Thomas Swan, in a newly commissioned plant in Consett (County Durham, United Kingdom).16 The final choice, ionic liquids (which, like supercritical fluids, are neoteric solvents, but unlike supercritical fluids, have immeasurably low vapor pressures at room temperature) are the subject of this overview. If must be stressed, however, even to the ad nauseum limit, that these methodologies are not in competition with each other, they are complementary. They form a toolbox of alternative solvent strategies (sometimes referred to as alternative reaction media or green solvents), giving a significant choice to the industrialist looking for the best option for his or her green process.

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Responses

  • bellisima
    What is the harm in using conventional solvents in green chemistry?
    2 years ago

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