Preparation of Vanillin from Lignin
Preparation of vanillin is probably the most widely studied chemical transformation of lignin [43-46]. Currently, approximately half of the vanillin produced in the world is used as a flavoring material in the food industry and the remaining half is utilized as an intermediate in the chemical and pharmaceutical industries for the production of herbicides, antifoaming agents or drugs such as papaverine, L-dopa, L-methyldopa and the antimicrobial agent, trimethoprim [94]. This 8-carbon phenolic-aldehyde was initially isolated from the vanilla bean extract derived from orchids of the genus Vanilla, and primarily from the Mexican species Vanilla planifolia. Until the 1920s, vanillin was commercially produced from another natural product eugenol. Later it was synthesized from the lignin-containing paper industry byproduct "brown liquor." Even though this process uses industrial waste materials, the brown liquor-based process is no longer popular because of environmental concerns, and today most of the vanillin is produced from the petroleum-based raw material guaiacol [94]. However, recent advancements in the production of vanillin by catalytic air oxidation of abundant lignin has made it an easily accessible material, and has promoted the status of this flavor chemical to another promising renewable resources-based feedstock material. Oxidation of lignin to vanillin can be carried out in acidic as well as in basic media, in the presence of oxygen or air, usually under high pressure and temperatures in the 120-150°C range [43,49]. Furthermore, transition metal salts of Fe, Co, Mn, and Mo are known to catalyze the depolymerization and oxidation of complex lignin structure to vanillin [95-98].