Hydrolysis of lignin

Hydrolysis of lignin in acidic and basic media has received attention due to the rather few and simple degradation products obtained. Acid-catalyzed hydrolysis reactions applied to isolated lignin have been studied by a number of workers. Lundquist (1), for example, subjected Bjorkman lignin to acidolysis and obtained significant yields of monomeric products. A review of the work prior to 1971 has been made by Wallis (2). 

Another product that is produced commercially from the alkaline hydrolysis of lignin is vanillin. It was found that the yield could be improved by the presence of oxygen during the alkaline hydrolysis reaction. A number of commercial ventures have been based on this procedure (8,9). 

TFA has been used for about three years in our laboratory, and it has become our exclusive reagent for the total hydrolysis of polysaccharides. Moreover, dilute TFA is applied to partial hydrolysis of lignin-polysaccharide complexes (13). 

Vanillin is obtained from sulfite waste liquor by further alkaline hydrolysis of lignin. It is the same substance that can be obtained from vanilla bean extract and is the common flavoring in foods and drinks. Natural and synthetic vanillin can be distinguished from each other by a slight difference in the amount of 13C in their structure, since one is biosynthetic in the bean and the other is isolated from a second natural product, wood, by hydrolysis of the lignin. 

Alkaline hydrolysis of lignin increases the number of reactive benzylic hydroxyl groups and may also be important in further depolymerizing the lignin once the oxidative-cleavage reaction has occurred. The formation of a p-electron-withdrawing -CHO substituent on aryl lignin units should increase the rate of hydrolysis of the ether bonds (26). Hydrolysis also forms p-phenylate ions, which then protect the benzaldehyde from further reaction via the Cannizzaro reaction, as mentioned earlier. 

Vanillin (Cgl O ) which is produced firom alkaline hydrolysis of lignin is also a trihalomethane precursor. Both Resorcinol and Vanillin have been investigated as model compoimds to further studies in understanding the role played by aquatic bacteria in the decomposition of complex naturally occurring humic acids (3). 

Sakakibara A, Nakayama N 1962 Hydrolysis of lignin with dioxane and water. II. Identification of hydrolysis products. Mokuzai Gakkaishi 8 153-162... 

A most important aroma compound worldwide, vanillin, is obtained primarily by alkaline hydrolysis of lignin (sulfite waste of the wood pulp industry), which yields coniferyl alcohol. It is converted to vanillin by oxidative cleavage ... 

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