Recovery of a solvolytic lignin: Effects of spent liquor/acid volume ratio, acid concentration and temperature

Abstract

A lignin has been isolated from a prototype hardwood, Populus deltoides, by organosolv delignification using ethylene glycol, followed by dilute acidification of the spent black liquor. Various approaches for product separation are described. It was found that separation of the product slurry was best achieved if suction filtered whilst hot (> 100°C). The effects of temperature, volume ratio of black liquor/acidic aqueous solution, and acid concentration for precipitation of the lignin are discussed. Optimum conditions, in terms of yields, filterability, texture and color of the lignin were found to be: temperature range 50–60°C, liquor/acid volume ratio 1:3 and an acid concentration of 0·05%wt aqueous hydrochloric acid. The glycol lignin so isolated was soluble or partly soluble in a number of organic solvents. Gel permeation chromatography showed the weight average molecular weight to be 4762 g/mole with a polydispersity of 4·8 and the elution profile indicated a significant proportion of this lignin consists of a low molecular weight material. ¹³C NMR spectroscopy suggested that some cleavage of β-aryl ether linkages during the solvolysis process had occurred. This lignin seems to be rich in aliphatic hydroxyl groups and syringyl units. The presence of a strong signal at low field in the aliphatic region of the NMR spectrum suggests its origin from the solvent due to a possible ‘lignin-solvent interaction’ that occurred during the delignification step. Electron microscopy showed the lignin to exist as aggregates of mostly spherical particles of size range 0·5–2·5 μm.