Structure of Lignosulphonates from Acidic Magnesium-Based Sulphite Pulping of Eucalyptus globulus

Abstract

The structure of lignosulphonates (LS) from acidic magnesium-based sulphite pulping of Eucalyptus globulus wood has been studied. LS from thin and thick spent liquors were purified by dialysis and characterised by 1D/2D Nuclear Magnetic Resonance (NMR) and Electrospray Ionization Mass Spectrometry (ESI-MS). It was suggested that major part of LS is comprised of partially sulphonated low molecular weight oligomers formed via extensive cleavage of β-O-4 bonds in eucalypt lignin upon sulphite pulping. More than ten types of LS structures derived from different lignin structural units have been identified. A significant proportion of LS (> 20%) is comprised by monomeric compounds, mainly 4-allyl-2,6-dimethoxyphenyl-α -sulphonic and 4-propyl-2,6-dimethoxyphenyl-α,γ -disulphonic acids. LS revealed partial degradation during the thin liquor evaporation leading to cleavage of ether and carbon-carbon linkages in the side chain and to the increasing of the condensation degree.

Keywords:

• Eucalyptus globulus
• ESI-MS
• NMR
• lignosulphonates
• structure
• sulphite pulping
• sulphite spent liquor

This work was financially supported by the Foundation of the Science and the Technology (FCT), grant reference SFRH/BD/31463/2006, and by the innovation PRIME Program (grant SIME I&DT no. 00/22689).

Notes

∗Analysis of LSF0 was carried out by the same methodology as previously reported.^{[ 8 ]}

∗∗Corrected for the ash content.

∗∗∗Empirical formula for LSF and LSG per phenylpropane unit: C₉H_9.77O_3.92S_0.10(SO₃H)_0.36(OCH₃)_1.51 and C₉H_13.20O_5.61S_0.02(SO₃H)_0.79(OCH₃)_1.59, respectively.^{[ 8 ]}

∗Structure assignments are presented in Figure 3.

∗The relative intensity of signals may be inaccurate due to the superposition of peaks from different structures at the same m/z.

∗Sulphonated and non-sulphonated structures.