Hardwood Cell Wall Modifications by Acid Hydrolysis and Their Effects on Alkaline Delignification

Although the hydrolysis of wood has been investigated in the past there is renewed interest in the topic due to the global energy crisis that appears imminent. Acid hydrolysis of wood can be used for extracting wood components that can be utilized to produce valuable commercial chemicals currently obtained from crude oil fractionation and distillation. Key amongst these chemicals are alternative transportation fuels such as ethanol and butanol. The acid hydrolysis of wood has benefits to the conventional alkaline pulping industry that could be employed solely or in parallel with a biorefinery approach. The processes would include improved reaction or delignification rates, bleaching chemicals use, chemical recovery and power/steam generation. Much research is needed to explore the potential mechanisms for the observed improvement in alkaline pulping, but based on this initial research; the mechanisms appear to be more chemical in nature than physical. Soda-anthraquinone (AQ) pulping so far is observed to be the most significantly affected by using a pre-hydrolysis stage prior to pulping. This is due to AQ's dependency on polysaccharide reducing end groups in its redox catalytic cycle to form anthrahydroquinone (AHQ), a nucleophile capable of cleaving beta-O-4 bonds. Acid hydrolysis of wood increases these reducing groups without a significant loss of fiber strength by selectively cleaving the hemicelluloses hydrolytically at mild conditions that do not degrade cellulose to any significant extent. Although not conclusive the results also point to the cleavage of acid-labile lignin carbohydrate bonds. This is based on the observation that lignin leaches out of acid pre-hydrolyzed chips into dilute NaOH at a rate that is approximately twice as high as untreated chips.