Cellulose dehydration reactions

Condensed-Phase Mechanisms. The mode of action of phosphorus-based flame retardants in cellulnsic sy stems is probably best understood. Cellulose decomposes by a noncalalyzed route lo tarry depolymerization products, notably levoglucosan, which then decomposes to volatile combustible fragments such as alcohols, aldehydes, ketones, and hydrocarbons. However, when catalyzed by acids, the decomposition of cellulose proceeds primarily as an endothermic dehydration of the carbohydrate to water vapor and char. Phosphoric acid is particularly efficaceous in this catalytic role because of its low volatility (see Phosphoric Acids and Phosphales). Also, when strongly heated, phosphoric acid yields polyphosphoric acid which is even more effective in catalyzing the cellulose dehydration reaction. The flame-retardanl action is believed to proceed by way of initial phosphory lation of the cellulose. 

Additives, such as fire retardants, can have a major effect on pyrolysis, and even trace amounts of ash have been shown to influence pyrolysis (6 ). Generally, fire retardants work by increasing the dehydration reaction rate to form more char and as a direct result give fewer flammable volatile compounds (1,3,7). Several papers have noted that phosphoric acid and its salts decrease the Efl (13,18,22,29), aluminum chloride has little effect (22) on Efl and boric acid increases the Efl (12,18). The reaction order for treated samples has been generally reported as lst-order (12,13,18,29) which is also the most commonly used rate expression for analysis of TGA data of untreated cellulose. 

The addition of a Lewis acid, i.e., ZnC significantly decreases the production of tar and enhances the production of char due to the enhanced dehydration reactions. At higher temperatures the glycosyl units and the random condensation products are further degraded to a variety of volatile products, as shown in Table V (9). Comparison of this table with the high temperature pyrolysis products listed for cellulose in Table III shows that the products of both fractions are basically similar. The significant increase in the yields of 2-furaldehyde, water and char and decrease in the yield of tar by the addition of ZnCl verifies the enhanced dehydration and is similar to observed effects in cellulose pyrolysis. 

Cotton Ammonium phosphates are the most effective FRs for cotton as first identified by Gay-Lussac in 1821 and still widely used. All phosphates on heating release phosphoric acid, which catalyses dehydration reactions of cellulose to yield char at the expense of volatiles formation reactions.50 However, ammonium phosphates like mono- or diammonium phosphates are water soluble, hence applicable as nondurable treatments only. Ammonium bromide can be used in combination with ammonium phosphates to provide some vapor-phase FR action. Other examples include borax and boric acid, ammonium sulfamate, and sulfates. These nondurable finishes are useful for disposable fabrics, insulation, wall boards, theatrical scenery, packaging material, paper, etc. Ammonium polyphosphates (APPs) are used in combination with urea to provide semidurable finishes and by curing at 160°C, when some phosphorylation can occur. Semidurable finishes are very useful for materials that may not need frequent washings, e.g., mattresses, drapes, upholstery, carpets, etc. Some commercial examples of semidurable finishes include Flammentin FMB (Thor Specialities), Pyrovatim PBS (Ciba, now marketed by Huntsman), etc.26... 

Dobele, G., Rossinskaja, G., Telysheva, G., Meier, D., and Faix, O., Cellulose dehydration and depolymerization reactions during pyrolysis in the presence of phosphoric acid. J Analytical Appl Pyrolysis 1999, 49 (1-2), 307-317. 

The body image on the cloth is the result of an oxidation-dehydration reaction of the cellulose material of the cloth itself rather than the result of an applied pigment, stain, or dye. 

In the presence of acid additives thermal degradation of cellulose is intensified at lower temperatures, due to the occunence of the dehydration reactions [3], Condensation reactions result in the decrease of volatile products at 450"C combined... 

Harada H and Cote WA (1985) Structure of wood. In Higuchi T (ed). Biosynthesis and biodegradation of wood components. Academic Press, London, 1-42 Harrington 11 (2002) Hierarchical modelling of softwood hygro-elastic properties. Ph.D thesis. University of Canterbury, Christchurch, New Zealand Harris IF (1976) Acid hydrolysis and dehydration reactions for utilizing plant carbohydrates. In Timell TE (ed). Proceedings of 8th cellulose conference. Wiley, New York, Vol. 1 131-44... 

An early approach toward chiral heterogeneous catalysts was the deposition of the catalyticaUy active metal or metal oxide particles onto intrinsically chiral supports such as quartz [24], cellulose [25], or synthetic chiral polymers [26-28]. Hydrogenation and dehydration reactions were tested, but enantioselective performance was found to be poor. In a recent review, Mallat et al. [29] attributed this poor enantioselectivity to the fact that only a small fraction of the metal atoms would... 

In Fig. 12, a modified van Krevelen plot is shown, which contains cellulose and common sugars as well as reaction arrows. The latter are drawn to emphasize the impact of dehydration (in blue). As can be seen, a typical dehydration reaction preserves the level of functionality in our assessment while decreasing the H index of the compound. In general, dehydration involves a horizontal move to the left-hand-side of the plot. 

Many phosphorous additives act as flame retardants in this way in hydroxyl containing polymers such as cellulose. During the polymer degradation process, phosphorus acids are produced which lead to char via phosphorylation and dehydration reactions. Relatively low quantities of phosphorus compounds are needed to impart a reasonable degree of flame retardancy. Several boron additives behave in a similar way by the promotion of carbonaceous char through esterification and dehydration reactions. 

It is interesting to note that cotton-based products can be treated against combustion by boron- or phosphorus-containing products that function as catalysts for the endothermic dehydration reaction of cellulose [4] ... 

Depolymerisation is initiated by the scission of acetal bonds of the glycosidic xmits, followed by splitting of volatile fuel-forming levoglucosan, the cyclic monomer of cellulose, from ensuing chain ends. Competing dehydration reactions lead to thermally stable aliphatic structures (char I), which subsequently are converted via Stage II into... 

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