Ionic Liquids as Homogeneous Catalysts for Glycerol Oligomerization

Ionic liquids (ILs) were used for the first time as catalysts for the glycerin condensation reaction. A series of imidazolium and ammonium ionic liquids differing in the length of the alkyl substituent (C₂, C₁₂, and C₁₄) and the type of anion (Br^-, CH₃COO^-, and NaHPO₄^-) were synthesized using a typical two-step method. The structure of the obtained ILs was confirmed by nuclear magnetic resonance ¹³C NMR, and their base power was determined on the basis of the Hammett function. The oligomerization of glycerin with the participation of the obtained ionic liquids and, for comparison, in the presence of a homogeneous basic catalyst Na₂CO₃, was carried out for 3 h at 180 °C, under a pressure of 0.4 bar, where the highest conversion, i.e., 92%, was obtained against 1-dodecyl-N,N,N-triethylammonium acetate. The course of the reaction was monitored using a reaction system coupled with a FTIR spectrometer, which allowed for the tracking of changes in product concentration over time and the assessment of glycerin oligomerization kinetics. The reaction products were analyzed by positive electrospray ionization mass spectrometry (ESI-MS), ¹³C NMR, and infrared absorption spectroscopy (FTIR).