Cellulose-hemicellulose precursor

The model of a wood cell includes cellulose, hemicelluloses, metal ions, pectin analogues, lignin precursors and water however inclusion of all of these components is computationally not feasible these days. Thus in order to delimit our study we have chosen the secondary wood cell wall, in which lignin and cellulose are dominant. 

The pentosan polysaccharides, xylan and arabinan, commonly known as hemicellulose, are the principal precursors of furfural and are always found together with lignin and cellulose in plant materials. 

The destruction and dehydration of hemicelluloses and cellulose starts already at 120 to 130°C. At 200T, 70 % of acids and more than 80% of furfural are evolved. The formation of wood tar is decreased by an acidic catalyst promoting the process of cellulose dehydration. The dehydration of cellulose in the temperature range iq> to 280°C causes the formation of precursors of charcoal structures and provides a high charcoal yield. 

Cellulosic and hemicellulosic feedstocks may be converted into 100+ chemicals, among them drop-in products but also several novel chemicals. Aside from biofuels and their precursors, they may be classified as end products, platform chemicals, monomers, and chemical intermediates (leading to a specific end-product). Some of these are already produced commercially from carbohydrates whereas others are currently petroleum or natural gas derived. Most of the chemicals described here are not produced commercially to date. 

Cellulose microfibrils are usually embedded in a continuous phase of lignin, pectin, and hemicellulose hemicellulose usually predominates (Whistler and Richards, 1970). Most of the hemicellulose is found mixed with cellulose in both the primary and secondary cell walls. Hemicellulose is now known not be a precursor to cellulose. In general, hemicellu-loses are the cell wall polysaccharides other than cellulose and pectin. These polymers are classified on the basis of the type of sugar residues present. For example, o-xylan is made up of D-xylose, D-mannan of D-mannose, and D-galactan of D-galactose units (Whistler and Richards, 1970). In practice, few hemicelluloses contain only one sugar, most contain two to four. Furthermore most hemicelluloses have branched structures. The major types of hemicellulose and their distribution have been reviewed (Whistler and Richards, 1970). Methods for the analysis of mannans have been reviewed (Matheson, 1990). 

Levulinic acid (4-oxopentanoic acid) obtained from cellulose and hemicellulose materials by acid-catalysed hydrolysis can be hydrogenated to y-valerolactone (5-methyldihydrofuran-2(3H)-one) which is a useful platform chemical precursor to liquid fuels, polymers and fine chemicals. Carbon-supported tin-ruthenium catalysts were active and selective in the hydrogenation of levulinic acid (Scheme 21.4). ... 

Impregnation may lead to fragmentation of cellulose and other components of the botanic precursor such as hemicellulose and lignin. Although the three chemicals react with the precursor, some clear differences can be observed at the end of the impregnation step (see Section 6.2). 

In this communication we extend our prior observations and demonstrate the use of xylan-rich, hemicellulosic residual fractions of wood for the production of P(3HB-co-3HV) by B. cepacia. Levulinic acid, the secondary carbon source utilized in this bioconversion process, can be produced cost-effectively from a vast array of renewable carbohydrate-rich resources including cellulose-containing forest and agricultural waste residues 24,25). This five-carbon cosubstrate (4-ketovaleric acid) serves as a precursor to the 3-hydroxyvalerate (3HV) component of the B. cepacia-dtnved P(3HB-co-3HV) copolymer (Figure 1). Further, the mol % 3HV composition and associated physical/mechanical properties of the copolymer can be manipulated as a function of the substrate concentrations provided in the fermentation. Physical-chemical characterizations of such PHA copolymers are reported herein, as evidence supporting the potential of these biodegradable thermoplastics to serve as viable replacements for conventional, environmentally recalcitrant commodity plastics. 

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