Due to the shortage of fossil fuels the use of lignin as a renewable resource is of great global importance.
In this work two representative spent liquors from the two dominant processes in the paper and pulp industry were examined.
First sum parameters (e.g. content of lignin and carbohydrates) and inorganic components of the liquors were analyzed. Because of large amounts of inorganic compounds in sulfate spent liquor and high sugar content in sulfite spent liquor lignin degradation products were isolated by well-known methods.
The development of analytical methods was performed by lignin model compounds. In addition to commercially available monomeric substances several dimeric, trimeric and tetrameric compounds with lignin typical linkages were synthesized. Due to the large number of lignin degradation products in spent liquors a GCxGC-(TOF)MS method was developed. With this method monomeric and dimeric lignin model compounds can be analyzed. Even after derivatization trimeric and tetrameric substance cannot be recognized.
The optimized method was applied to the examination of the isolated lignin degradation products. It was confirmed that spent liquors are complex mixtures of many compounds which even cannot be separated by comprehensive gas chromatography.
So a fractionation of the degradation products via a successive soxhlet-extraction method using different solvents became necessary. After the fractionation some monomers were identified by GCxGC-(TOF)-MS.
Afterwards a liquid chromatography method for the identification of high molecular compounds was developed.
The developed and optimized HPLC-method shows signals in the DAD-chromatogram. With a mass spectrometric detection no signals were available.
Therefore a mass spectrometric analysis method with direct injection using model substances was developed and transferred to the real samples. The mass spectrometric method enables a qualification of many substances. Using a high-resolution mass spectrometer it is possible to determine the molecular formula by detecting masses with a precision of ± 0.02 Da. According to the molecular formula double-bound-equivalents were calculated. In particular MS/MS experiments could obtain information about the number of methoxygroups and the presence of ß-O-4´-linkages and sulfonic acid groups.
According to this method several proposals for the structures of 35 monomers in isolated lignosulphonates were evaluated. Eleven monomeric structures include sulfonic acid groups. Additionally eight C-C-linked dimers were identified.
Furthermore masses of trimers, tetramers and pentamers were detected. In kraft lignin, isolated from the sulphate liquor, 27 monomers and 21 dimers were identified. In addition, high molecular weight substances with masses up to 680 Da were determined in this liquor. MS/MS experiments have shown that there are no ß-O-4'- linkages in the identified compounds.
Up to now the masses of detected trimers, tetramers and pentamers were only postulated in literature. So these exact masses were discovered for the first time.
Various determined substances, for example long-chain aromatic structures or condensation products, do not originate from wood and can only be formed during the pulping process.
All identified substances have a uniform basic structure. Therefore the use of the uniform lignin degradation products seems reasonable. For example unification could be performed by hydrogenation. The received substances could serve as renewable raw materials for other chemical products.