Molecular Properties of 2-Pyrone-4,6-dicarboxylic Acid (PDC) as a Stable Metabolic Intermediate of Lignin Isolated by Fractional Precipitation with Na+ Ion

Abstract

Abstract A chemically stable metabolic intermediate of lignin, 2-pyrone-4,6-dicarboxylic acid (PDC), was isolated, and the molecular properties were comprehensively investigated by using thermal analysis, optical spectroscopy, potentiometric titration, and X-ray crystallography. UV absorption spectra of PDC did not show any solvent effects, whereas the fluorescence spectra displayed distinct solvatochromism. Despite the fairly good planarity of the PDC geometry, dual fluorescence was observed in some small donor number solvents. The emission behaviors are thought to be due to the significant twist between the pyrone ring and carboxylate substituents, found in the X-ray crystal structures of the Na+ complex. The crystal structure also showed the intermolecular, centrosymmetric short hydrogen bonding of the carboxylate moieties with an O···H···O distance of 0.2452 nm, which made it possible to assign the pKa values to the molecular structure. PDC was thermally stable and could be sublimed even at atmospheric pressure, offering the preparation possibility of ultra-thin films by vapor-deposition techniques. The fact that the chemically and thermally stable compound possessing many attractive functions, such as unique emission, facile sublimation, and strong acidity, resulting in exceptionally short intermolecular contacts, can be obtained from biomass on a large quantity will lead to establishment of carbon neutral system in future industry.