Cellulose degradative pathway

Figure 5.2 Probable thermal degradation pathways for cellulose, according to Fengel and Wegener...

The general pyrolysis mechanisms of polysaccharides have been determined from model studies on cellulose and involve the splitting of the polysaccharide structure by three basic chemical reaction mechanisms dehydration, retroaldolization, and decarboxylation. Using these basic pyrolysis mechanisms, it is possible to explain the pyrolysis of polysaccharides and evolved pyrolysis products. The hexose degradation pathway for cellulose results in formation of furan- and pyran-type fragments and smaller acyclic aldehyde and ketone fragments. ... 

Conventional organie systems are based on the acid catalysed dehydration of carbonifics sueh as dipentaerythritol. Metal oxides also have a use as catalytic flame retardants. For example, both antimony and tin have been used to impart flame retardancy to cellulosics without any assistanee from halogen eompounds. They appear to alter the condensed phase thermal degradation pathways in such a way that more non-volatile ehar and less flammable gases are generated. It has been found that low levels of potassium biearbonate significantly modify the pyrolysis kinetics of a cellulose to yield more ehar. 

Chemiluminescence has also been proposed as a novel tool in paper conservation studies [654,655]. CL phenomena can be used for assessing the thermal and oxidative degradation pathways of paper-based historical documents. In contrast with the usual accelerated degradation experiments in climatic chambers, measurement of isothermal CL is quick. The influence of all paper components (alkalinity, metal content, cellulose peroxides and carbonyl groups, moisture) and exposure to light will be investigated in the framework of the PAPYLUM project (ending October 2004). 

The series of degradation steps comprising mineralization is similar, whether the carbon source is a simple sugar (e.g., glucose), a plant polymer (e.g., cellulose), or a pollutant molecule [49,50,62 - 64,72,73]. Each degradation step in the pathway is facilitated by a specific catalyst (i. e., an enzyme) made by the degrading cell. Enzymes are found mostly within a cell (i. e., internal enzymes),... 

Several products were also detected in base-degraded D-fructose solution acetoin (3-hydroxy-2-butanone 62), l-hydroxy-2-butanone, and 4-hydroxy-2-butanone. Three benzoquinones were found in the product mixture after sucrose had been heated at 110° in 5% NaOH these were 2-methylbenzoquinone, 2,3,5-trimethylbenzoquinone, and 2,5-dimethyl-benzoquinone (2,5-dimethyl-2,5-cyclohexadiene-l,4-dione 61). Compound 62 is of considerable interest, as 62 and butanedione (biacetyl 60) are involved in the formation of 61 and 2,5-dimethyl-l,4-benzenediol (63) by a reduction-oxidation pathway. This mechanism, shown in Scheme 10, will be discussed in a following section, as it has been proposed from results obtained from cellulose. 

Pyrolysis of biomass is defined as the chemical degradation of the biopolymers (cellulose, lignin and hemicellulose) constituting the wood fuel which initially requires heat. As can be seen in Figure 51, all reaction pathways making up the pyrolysis are not endothermic, which implies that some of the pyrolysis reactions generate heat. However, overall the pyrolysis process is endothermic. 

Carbohydrates ranging from cellulose to simple sugars are subject to thermal alteration. Factors such as temperature, pH, compound concentration, and other reactants present can alter both the rate and complexity of decomposition reactions. Carbohydrate types are reviewed relative to degradation/carameli-zation pathways and endproducts. Some of the resulting typical food flavors produced are also discussed. 

---