Characterisation of lignins from different sources by appropriate analytical methods: Introducing thermogravimetric analysis-thermal desorption-gas chromatography–mass spectroscopy

Abstract

Lignin is the most abundant natural source of renewable aromatic units and therefore, detail characterisation to unveil its chemical properties is a critical step for its utilisation. Nine black liquor samples from different plant origins namely sugarcane bagasse, Eucalyptus grandis, and Pinus gregii extracted from the Kraft, soda, soda-anthraquinone and sulphite pulping processes were considered. After lignin purification, when applicable, the samples were characterised by several common methods (wet chemical methods, Fourier Transformed Infra-Red spectroscopy, Gel Permeation Chromatography). Lignin monomer composition (H:G:S) was determined by thioacidolysis as well as a new pyrolysis method based on the use of an analytical set-up which couples Thermo Gravimetric Analysis (TGA) for lignin devolatilisation, the capture of released volatile compounds in thermal desorption (TD) tubes, and the quantification of the captured phenols by TD-GC–MS (gas chromatography–mass spectroscopy). The TGA-TD-GC–MS, with the use of internal calibration, allowed the quantification of 5.5–12.9wt.% of monomeric products based on dry weight of purified lignin. Pyrolysis of sugarcane lignin resulted in significant yield of furfural, which was explained by the conversion of residual sugar. Pyrolysis of pine lignin gave the lowest yield of syringyl-type phenols, which was consistent with characterisation results (low methoxy content and absence of FT-IR band characteristic of syringyl unit). Pyrolysis method had the advantage to break different types of chemical bonds, which is likely to give a product distribution more representative of the lignin. With TGA-TD-GC–MS the monomer proportion for purified lignin from the same plant species were found to be very comparable (deviation lower than 10% for each unit). Compared to thioacidolysis (known to be selective towards bond cleavage and suspected to overestimate S content), TGA-TD-GC–MS gave lower S/G ratio. The TGA-TD-GC–MS method has demonstrated to be a good alternative technique to study the H:G:S proportions of lignins with low ash content (<5%).