Journal
HOLZFORSCHUNG
Volume 71, Issue 1, Pages 27-34Publisher
WALTER DE GRUYTER GMBH
DOI: 10.1515/hf-2016-0054
Keywords
LC-MS; lignin; Mannich reaction; nitrogen content; pH dependence; phenol-formaldehyde (PF) resin; P-31-NMR; reactivity
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Funding
- AForsk foundation [13-299]
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The influence of pH on the Mannich reaction (amino alkylation in the presence of formaldehyde) has been analyzed by liquid chromatography-mass spectrometry (LC-MS) with vanillin (VA) as a model compound and a purified softwood kraft lignin (SKL) as a substrate. The reaction products of VA were studied at pH 5, 7, and 9 at 60 degrees C for 4 h. The Mannich adduct and side reaction products with methylene bridge were found at both pH 7 and 9, while only di-substituted by-products were observed at pH 5. Nitrogen contents determined from blank runs were substantial at pH 5 and negligible at pH 7. In VA or SKL, the resulting N-contents at pH 7 corresponded to a 76 or 62 mol% of the theory, respectively, i.e. based on the available C-5 positions in phenolic guaiacyl units (G-units). In the case of SKL, P-31-NMR analysis confirmed a 77% conversion of all phenolic G-units into their C-5 substituted derivatives. The Mannich reaction should be performed on lignin at pH 7 for 1 h to suppress unwanted side reactions, which could be observed by LC-MS under other pH conditions. The reaction is suitable for fast and reliable determination of reactive C-5-positions in lignin by multiplication of the N-content of the reaction products with a factor of 1.6.
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