Stoichiometry of reactions of ozone and hypochlorous acid with lignin and hexenuronic acid and its chlorination

Abstract

The stoichiometry of ozone and hypochlorous acid reactions with lignin and hexenuronic acid (HexA) was measured in bleaching experiments of Eucalyptus sp. kraft pulp. The progress of the reactions was followed by UV Resonance Raman spectroscopy that can quantify lignin and HexA based on the Raman scattering intensities of the carbon–carbon double bond in HexA and the aromatic ring in lignin. Here, one mol of ozone converted 0.16 mol of lignin (C9 monomer units) and 0.28 mol of HexA, whereas 1 mol of hypochlorous acid converted 0.09 mol of lignin and 0.23 mol of HexA. The use of a tertiary amine catalyst with the hypochlorous acid treatments did not affect these stoichiometries. The stoichiometric ratios showed that ozone was more efficient in oxidizing lignin than hypochlorous acid, while both electrophiles reacted with HexA to a similar extent. HexA reaction by hypochlorous acid was concluded to involve initial electrophilic chlorination of the carbon–carbon double bond, contributing to significant organochlorine (OX) formation in the pulp. Evidence on this was the linear correlation between the initial HexA content and OX (0.59 mol OX per mol HexA) and the high OX content in the xylan extracted from the bleached pulp. The 2D NMR HSQC and TOCSY spectra of the isolated xylans showed the disappearance of HexA signals after the treatment with hypochlorous acid and the appearance of a new spin system, yet to be fully identified.