Summary

Wastes from industrial processes which use lignocellulosic materials as raw material are an interesting source of chemicals since they can be transformed into products of high added value. In the process described here, ozone was used to produce oxyaromatics from a lignin-rich industrial waste resulting from the production of furfural from almond shells. Ozonation, thioacidolysis and mild alkaline hydrolysis were used to determine some structural features of the raw and acid-hydrolyzed almond shell lignins. During ozone treatment of the different lignin solutions the following compounds were identified: glycolic, oxalic, malonic, glyceric (trace), malic, p-hydroxybenzoic and vanillic acids, p-hydroxybenzaldehyde, vanillin, syringaldehyde and hydroquinone. The aromatic aldehydes passed through a production maximum during the first few minutes of the reaction, while glycolic and oxalic acid (the main aliphatic acids) yields increased throughout the treatment. Raw almond shell lignin had a typical hardwood composition [made up of guaiacyl (G) and syringyl (S) phenylpropane units] and few β-O-4 structures. Acid-hydrolyzed almond shell lignins had a more condensed structure than the above. The S/G ratio obtained by ozonation was slightly lower than that obtained by thioacidolysis, with the (S/G)_Ozonation/(S/G)_{Thioacidolysis} ratio ranging from 0.52 to 0.70. The almond shell lignin-polysaccharide complex did not contain any cell wall-esterified p-hydroxycinnamic acid (p-coumaric and ferulic acids).