The Attempted Depolymerization of Hc1 Lignin By Catalytic Hydrogenolysis

HC1 lignin, prepared from sweetgum wood (Liquidambar styraclflua L.) by hydrolysis of the carbohydrates with super concentrated hydrochloric acid, was reductively depolymerized in alkali using various heterogeneous catalysts. A maximum yield of 18% distillable products was obtained by this alkaline hydrolysis-catalytic hydrogenolysis treatment. Surprisingly, it was found that few covalent bonds were being cleaved by the catalysts, with the majority of the bonds having been broken by alkaline hydrolysis. GC analysis showed that a large portion of the monomeric phenolic products had been dealkylated at the para-position. A reaction scheme has been proposed to explain the alkaline hydrolysis mechanism involved. Lignin tar, which was formed by alkaline hydrolysis of HC1 lignin alone, was subjected to subsequent reduction using selected catalysts. This sequential hydrolysis-hydrogenolysis treatment caused the lignin to recondense, apparently because of the inability of the catalysts to reduce groups which are capable of polymerizing together.