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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