Reactions of neutral sodium sulphite solutions with chemical components of wood and wood pulps

Global targets for climate change mitigation require drastic reduction of fossil fuels and materials produced from petrochemicals. At the same time forests are expected to serve as carbon sinks, which should help reaching the national climate neutrality targets such as the one in Finland for 2035, as well as regional targets such as EU 2030, 2040 and 2050 climate targets. In addition to climate targets, global and regional biodiversity commitments impose increasing pressure on forest protection and restoration. Altogether the global outlook, combined with climate and nature targets for forests, indicate that we should make better use of the wood we already produce in our forests and plantations. Too large part of the valuable wood material is still burned to energy, either directly or as pulping process liquors. In Publication 1, the binding of sulphite to the reactive carbonyl groups of lignin was investigated with the help of lignin model compounds as well as lignin-rich chemithermomechamical pulp (CTMP). The model compound studies with vanillin, verataldehyde and acetovanillone established that the rate and equilibrium of the addition reaction of bisulphite to carbonyl groups depends on the carbonyl structure type: the bisulphite addition appears to be fast and extensive with aldehyde structures, while only minor part of ketones react in same conditions. In Publication 2, a spectroscopic method was developed for assessing the phenol content of lignin with the help of multiple lignin model compounds as well as pulp samples. UV resonance Raman (UVRR) spectroscopy was used for structural studies of the samples. The traditional method for phenolic hydroxyl group determination and the UVRR band shift from neutral to alkaline produced similar trends but could not be straight connected with each other. The spectroscopic method developed in Publication 2 was used to evaluate the effect of neutral sulphite treatment on the demethylation of lignin in Publication 3. In addition to lignin demethylation and dissolution, sulphite treatment at high temperature was also found to cleave cellulose chains. Sulphonated cellulose can be applied e.g. in concrete industry as viscosity increasing agent or as so-called superplasticizer, which interacts with the surface of cement particles by provoking dispersion and decreasing coagulation tendency. Cellulose sulphonic acid can be also used as solid acid catalyst by the chemical industry for example in biodiesel production. Conventional homogenous catalysts have serious limitative features due to the toxic, corrosive reagents, tedious preparation, neutralization of effluents, long reaction time, and high temperature. Publication 4 examined the applicability of neutral sulphite cooking to produce high-yield pulp from softwood. Sodium sulphite pulping experiments of pine chips under neutral and slightly alkaline pH resulted in pulps with high yields between 52 and 73% and kappa numbers ranging from 35 to 106. The number of acidic groups of the pulps correlated linearly with the residual lignin content showing highly charged fibres. The treatments resulted in pulps with a high content of hemicelluloses combined with relatively high lignin content. The high content of hemicelluloses can be beneficial in hindering hornification (i.e. stiffening of fibres) during drying, as well as in promoting fibrillation during refining. These results demonstrate the possibility to reach much higher yields than in kraft pulping, and potential for applications in bio-based materials, which require modifiable hydrophilicity and water interaction.