Monday, May 4, 2009
12-21
Structural changes in lignin and cellulose resulting from the two-step dilute acid pretreatment of Loblolly pine
Two-step dilute sulfuric acid pretreatment was performed on Loblolly pine to enhance the overall efficiency of the enzymatic conversion of lignocellulosic biomass to monomeric sugars prior to their fermentation to bioethanol. Lignin, cellulose and hemicellulose the major components of lignocellulosic biomass, are closely associated with each other at the plant cell level. This close association, together with the partly crystalline nature of cellulose protects it from enzymatic hydrolysis of native biomass. In the overall conversion of biomass to bioethanol, the structure of lignin is also of importance as it may physically hinder cellulase access to cellulose microfibrils and participate in non-productive binding to enzymes. Detailed structural characterization of cellulose and milled wood lignin isolated from Loblolly pine before and after the two-step dilute sulfuric acid pretreatment elucidates the modifications taking place as a result of this pretreatment. Solid-state 13C NMR spectroscopy coupled with line shape analysis has been used to determine cellulose crystallinity and ultrastucture. The results indicate an increase in the degree of crystallinity and reduced relative proportion of less ordered cellulose allomorphs. These changes may be attributed to a preferential degradation of amorphous cellulose and less ordered crystalline forms during the treatment. Milled wood lignin structural elucidation by quantitative 13C and 31P NMR reveals an increase in the degree of condensation. This is accompanied by a decrease in the number of β-O-4 linkages which are fragmented and subsequently recondensed during high temperature acid-catalyzed reactions. The impact of these changes on pine recalcitrance and enzymatic deconstruction will be reviewed.
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