The chemistry of several novel bioconversion lignins

Fourteen lignin-rich residues from six different bioconversion technology schemes currently under investigation for converting lignocellulosic resources into chemicals were analyzed with regard to their chemical structures. Process schemes included (a) acid hydrolysis with sulfuric acid, (b) acid hydrolysis with hydrofluoric acid, (c) steam explosion followed by solvent or alkali extraction, (d) organosolv pulping, and (e) digestion with cellulase or (f) digestion with cellulase-producing organisms. The quantitative analysis regime tested for elemental composition, functionally, interunit linkages, molecular mass, and glass transition temperature. The results were computed on the basis of the composition of average lignin-building, phenylpropane (C/sub 9/) structures. They suggest (a) that bioconversion lignins are subject to, in part, severe hydrolytic depolymerization of their alkyl aryl ether bonds, (b) that the degree of depolymerization differs significantly with the process, (c) that significant secondary condensation occurs only during acid hydrolysis, and, to a minor extent, during steam explosion and organosolv pulping, (d) that, except for phenolic hydroxyl groups, only minor variations in functionality exist, (e) that molecular mass ranges between 500 and 20,000 (g/M) and was lowest with the organosolv and steam explosion lignins, (f) that glass transition temperatures vary between 95 and 160 degrees C, and (g) that the isolation of homogeneous, solvent-or alkali-soluble, lignin-rich fractions from the residues of the HF acid hydrolysis and the cellulolytic enzyme process options is extremely difficult. 68 references.