87. Ober,
C.A., Gupta, R.B., pH Control of Ionic Liquids with Carbon Dioxide and Water:
1-Ethyl-3-methylimidazolium Acetate. Ind. Eng. Chem. Res. 2012 51 (6), 2505-2820.
Abstract
In
order to use ionic liquids in pH-dependent reaction and separation processes, pH
control is necessary. The use of traditional mineral acids poses a challenge
due to the non-volatile nature of both the ionic liquid and the mineral
acid/salts, requiring additional separation steps. The use of a volatile
acidification agent, such as CO2, can be beneficial and is the
subject of this work. The pH of 1-ethyl-3-methylimidazolium acetate ([emim][Ac]), a basic ionic liquid,
and water mixtures is measured under low CO2 pressure. Water content
is varied from 0 to 25 (w/w)% and CO2 pressure
is varied from 0 to 1.5 bar. The effect of temperature is also examined for the
25% water and [emim][Ac]
mixture. Results are compared to those obtained using an alternative volatile
acid, acetic acid. The pH of [emim][Ac]
and water mixtures is found to decrease with increasing water content and
increasing CO2 pressure, while temperature has a minimal effect. Two
mechanisms are proposed to explain the pH behavior of both pure [emim][Ac] and [emim][Ac]/water
mixtures under CO2 pressure. Low pressure CO2 is capable
of providing mild and reversible acidification and offers a valuable tool for
pH control of ionic liquids.
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