Fast and reliable quantification of lignin reactivity via reaction with dimethylamine and formaldehyde (Mannich reaction)

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.