Characterization of high molecular mass material in ECF and TCF bleaching liquors by Py-GC/MS with and without TMAH methylation

High molecular weight (HMW) fractions (MW > 1000) of effluents were obtained by ultrafiltration of elemental chlorine-free (ECF) bleaching (O-D-EOP-D-ED) and totally chlorine-free (TCF) bleaching (O-Z-Q-P-Z-Q-P-P) liquors of softwood (Pinus sylvestris) pulp. The fractions were studied by analytical pyrolysis coupled to gas chromatography/mass spectrometry (Py-GC/MS) with and without TMAH (tetramethyl ammonium hydroxide) derivatization. The pyrograms showed a number of pyrolysis products mainly derived from lignin and carbohydrates. The comparison of 4-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) lignin units in both bleaching effluents showed that the G unit content was clearly higher than the S unit content. Also the amounts of pyrolysis products derived from lignin were higher than those derived from carbohydrates in the beginning of both bleaching sequences. The chemical composition and the nature of functional groups of HMW material after ECF and TCF bleaching sequences differ significantly. Typical major pyrolysis products formed from lignin in the ECF bleaching liquor were phenol, 2-methylphenol, guaiacol and 4-vinylguaiacol, whereas crotonolactone and 2-furfural were characteristic degradation products of carbohydrates. Guaiacol, 2-furfural, 2-methylphenol, 2- or 3-ethylphenol, 2-methyl-2-cyclopenten-1-one, 5-methyl-2-furaldehyde, 2,4-dimethylphenol and 2-methoxy-3-methylphenol were major degradation products formed from HMW TCF bleaching liquors. Only small amounts of chlorinated degradation products were formed from HMW material in the ECF bleaching liquors. The amounts of HMW carbohydrates (such as hemicelluloses) were slightly higher in the liquor obtained after the last step of the TCF than that of the ECF bleaching sequence. Both these liquors consist mainly of lignin matter.