Brønsted ionic liquids: study of physico-chemical properties and catalytic activity in aldol condensations

Abstract

A serie of substituted amines based ionic liquids was prepared using an aliphatic linear carboxylic acid (pentanoic acid). The ionic liquids synthesised are 2-hydroxy ethylammonium pentanoate (2-HEAPE), 2-hydroxy diethylammonium pentanoate (2-HDEAPE) and 2-hydroxy triethylammonium pentanoate (2-HTEAPE). FT-IR spectra establishes their ionic salt structure. The influence of structural variations on their thermodynamic properties (density, ultrasonic velocity, ionic conductivity, refractive index, viscosity and surface tension) was investigated, as a function of temperature in the range 278.15–338.15 K. In the compounds studied here, the anion has strong effect on the properties if compared with previous studies of the authors, when shorter chains were used. It was found that the elongation of the alkyl chain causes the pronounced depression of fluidity and ionic conductivity. The observed temperature trend of the studied properties points out the special packing of these ionic liquids, as well as, the strong influence of the steric hindrance among linear aliphatic residues. We also describe the catalytic activity of these type of ionic liquids for a representative serie of aldol condensation processes. Conversions higher than 98% and selectivities higher than 85% were obtained. Moreover the catalysts are easily separated from reaction mixture and re-used, obtaining good conversions for at least three consecutive cycles. As a conclusion, it was found that simple structural modifications into anion length and degree of substitution provide a mechanism to manipulate the catalytic potency of these ionic liquids, as well as, tailor physico-chemical properties for potential end-use applications.