A series of Brönsted acidic ionic liquids based on 2-pyrrolidonium and 1-methylimidazolium cations ([Hnhp]＋and [Hmim]＋) with anions , and , were synthesized. Their thermal stabilities and acidities were characterized, and the catalytic activity of the acidic ionic liquids for the esterification of acetic acid with iso-pentanol was studied. When n(i-PentOH)∶n(MeCO2H)∶n([Hnhp]HSO4)＝1.2∶1∶0.2, T＝100 ℃ (oil bath) and t＝2 h, the yield of iso-amyl acetate could be 93.6% without removal of water during the esterificaion. After the reaction, the [Hnhp]HSO4 system achieved phase separation successfully, but [Hnhp]BF4 system did not. When using water separator, n(i-PentOH)∶n(MeCO2H)∶n([Hnhp]BF4)＝1.2∶1∶0.01, T＝120 ℃ (oil bath) and t＝1.5 h, the yield of iso-amyl acetate could reach 96.8%, and the [Hnhp]BF4 system could exhibit good biphasic behavior. There is no appreciable decrease of catalyst activity for both the systems after 6 cycle catalyst repetition. The acidity/biphasic behavior-catalytic activity relationships were also investigated. The results showed that the effects produced by acidity and immiscibility of the acidic ionic liquids with the ester on the both esterification systems, whether the resultant water separated or not, were different. In addition, Austenitic stainless steel 316 was immersed in four kinds of ionic liquids (containing and ) and sulfuric acid respectively for corrosion tests under
the above conditions of esterificaion without removal of water, i.e., n(i-PentOH)∶n(MeCO2H)∶
n(catalyst)＝1.2∶1∶0.2, oil bath 100 ℃. After 6 h of reflux immersion, it was observed that there was lower corrosion of the steel sample by each ionic liquid solution system than by sulfuric acid. For [Hnhp]HSO4, [Hmim]HSO4 and [Hmim]BF4, there was a decrease of corrosion rate with the decreasing acidity of ionic liquids. [Hnhp]BF4 was more corrosive than other ionic liquids, and the corrosion rates of [Hnhp]BF4 and sulfuric acid were 20.1 and 41.8 g/(m2•h), respectively.

Key words: pyrrolidonium cation, ionic liquid, Brö, nsted acidity, acetic acid, iso-pentanol, esterification, iso-amyl acetate, corrosion