The unveiling of a levulinic acid isomer in high performance liquid chromatography analysis, a key point for the reliable determination of the catalytic conversion of levulinic acid to γ-valerolactone

Abstract

This work was motivated by the difficulties found in the analysis and quantification of levulinic acid (LA) and the reaction products resulting from the catalyzed transformation of LA into γ-valerolactone (GVL) using High Performance Liquid Chromatography (HPLC). The starting study carried out with the Zorbax Eclipse XDB-C8 column and UV/Vis detector (or Diode Array Detector, DAD) was puzzling, mainly because two different signals appeared in the analysis of aqueous LA standards. The use of C8-UV/Vis-MS and the measurements with DAD in range wavelength mode (RWL) allowed to suggest the presence of pseudo-LA (sometimes regarded as unidentified impurities in the reactant) together with LA. The finding of an additional LA species was fundamental for the accurate quantification of LA, necessary to provide reliable results in the analysis of the catalytic LA transformation products. The study carried out also allowed the identification of 4-hydroxypentanoic acid (HPA), a labile intermediate compound. The analysis of the target solution (resulting from the explained catalytic system) was also performed with a REZEX ROA-Organic Acid H⁺ column and Refractive Index Detector (RID) in order to determine its applicability and to check if the results obtained with the two configurations were coherent. The REZEX-RID configuration led to a simpler analysis, as it did not involve the separation of the LA species, and allowed the detection of 1,4-pentanediol (not possible by UV/Vis analysis). The results obtained with REZEX-RID helped to support the need of considering the presence and quantification of an LA isomer in the standard solutions for the right quantification of LA using the C8-UV/vis (or DAD) configuration. The present study clarifies relevant practical aspects of the HPLC-UV/Vis (or DAD) analysis of LA present in aqueous solutions.