The Origins of Phenol Produced in the Rapid Hydrothermolysis and Alkaline Hydrolysis of Hybrid Poplar Lignins

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Abstract
Hybrid poplar lignins were isolated from selected hybrid poplar clones by an enzymatic/solvent extraction method. The lignins were subjected to both rapid hydrothermolysis (RHT) and alkaline hydrolysis. Phenol (C6H6OH) was the predominant component of hydrothermolysis products, whilst p-hydroxybenzoic acid (PHBA) was the chief product of the alkaline hydrolysis. Phenol yields were 2–9 wt.% of the lignin, whilst PHBA yields ranged from 3–6 wt.% of the lignin. The yields of both products were sensitive to the poplar clone. When PHBA was similarly subjected to the RHT process, it decarboxylated to phenol quantitatively. Thus the phenol observed in the RHT products derives mostly from the PHBA esters in the lignin. The PHBA is linked to the lignin by ester and/or ether bonds. PHBA linkages are cleaved hydrolytically and the resultant acid then decarboxylates to phenol.