Separation and characterisation of sulphur-free lignin from different agricultural residues

• Effect of different pulping procedures on lignin separation.
• Effect of process parameter on lignin composition.
• Comparison of lignin of various agricultural residues separated by alkaline pulping.
• Classification of separated lignins.

Wheat straw, as one of the most abundant agricultural residues in Europe, was subjected to alkaline pulping, microwave-assisted alkaline pulping and organosolv pulping using formic acid and hydrogen peroxide. The obtained lignins were characterised by means of Klason–lignin, FT-IR spectroscopy, elementary analysis, thioacidolysis, SEC and different wet chemical methods. The emphasis here was on their structural differences, especially the content of functional groups like total hydroxyl, phenolic hydroxyl, carboxyl, carbonyl and methoxyl groups.In addition, numerous agricultural residues, for example barley straw, maize straw, sunflower stalks and horse manure, were characterised and subjected to alkaline pulping, which was carried out based on the optimum parameters found for wheat straw: θ = 160 °C, t = 30 min and ω(NaOH) = 3 wt%. The examined raw materials showed different suitability for lignin separation using this pretreatment. The resulting lignins were characterised with the methods described above and were classified into three groups within which lignins show similar characteristics. By this, lignin with interesting structural features could be proposed for further studies.