Phosphoric acid dehydration effect

The performance of a HT-PEMFC depends mainly on the amount of phosphoric acid in the polymer membrane and in the porous catalyst layer as well as on the temperature. Furthermore, it is well known that phosphoric acid dehydrates at low water vapour partial pressure and rehydrates with increasing partial pressure, effects which can be observed in HT-PEMFCs under operating conditions [1, 2]. The composition change of phosphoric acid results in a variation of the ionic conductivity as well as of the viscosity. 

Dehydration of ethanol has been effected over a variety of catalysts, among them synthetic and naturally occurring aluminas, siUca-aluminas, and activated alumina (315—322), hafnium and 2irconium oxides (321), and phosphoric acid on coke (323). Operating space velocity is chosen to ensure that the two consecutive reactions. 

The use of dehydrating agents such as sulfuric or phosphoric acid on (555 X = OH) was also successful, and these closures may proceed via mixed anhydrides 67AHC(8)277, 75MIP41600). Carbonyldiimidazole effected the conversion of hydroxamic acid (557) into a 3-hydroxy-1,2-benzisoxazole derivative (79JHC1277). The mixed anhydride (558) where... 

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. 

Cellulose pyrolysis kinetics, as measured by isothermal TGA mass loss, were statistically best fit using 1st- or 2nd-order for the untreated (control) samples and 2nd-order for the cellulose samples treated with three additives. Activation parameters obtained from the TGA data of the untreated samples suggest that the reaction mechanism proceeded through an ordered transition state. Sample crystallinity affected the rate constants, activation parameters, and char yields of the untreated cellulose samples. Various additives had different effects on the mass loss. For example, phosphoric acid and aluminum chloride probably increased the rate of dehydration, while boric acid may have inhibited levoglucosan... 

Sodium tripolyphosphate is made by the reaction of phosphoric acid and sodium carbonate in the proper amounts to give a 1 2 ratio of monosodium and disodium phosphates and then heating to effect dehydration at 300-500°C. 

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. 

The mixed anhydride of phosphoric acid and glyceric acid then is used to convert ADP to ATP and form 3-phosphoglycerate. Thereafter the sequence differs from that in photosynthesis. The next few steps accomplish the formation of pyruvate by transfer of the phosphoryi group from C3 to C2 followed by dehydration to phosphoenolpyruvate. Phosphoenolpyruvate is an effective phosphorylating agent that converts ADP to ATP and forms pyruvate ... 

This dehydration proceeds under acidic conditions and is a widely used olefin-forming reaction. In the laboratory phosphoric acid is the reagent of choice sulphuric acid, which is often used, can lead to extensive charring and oxidation and hence to lower yields of alkene. Other reagents include potassium hydrogen sulphate and anhydrous copper(n) sulphate.17 Passage over heated alumina is also effective. 

The dehydration of hydroxyl groups in cellulose is enhanced by boric acid, which increases the char yield of borate-treated cellulose. As it is a weak acid, this effect is not as strong as that for phosphoric acid. 

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... 

S-Hydfoxy ketones obtained by the condensation of ketones or by the crossed condensation of aldehydes with ketones are important sources of olefinic ketones. Dehydration is effected by warming the ketols with oxalic acid, dilute sulfuric acid, hydrobromic acid, phosphoric acid, or a trace of iodine. A typical example is the dehydration of diacetone alcohol obtained from the self-condensation of acetone. The product is an equilibrium mixture of the conjugated and unconjugated isomers, (CHj)jC=CHCOCHj and CHj=C(CHj)CHjCOCHj, in a ratio of 91 to 9, respectively. ... 

The preferred feedstock for chemical activation (see Fig. 5.7-1) is wood, in the form of sawdust. These processes are based on the dehydration of the feedstock by reaction with dehydration agents and are particularly effective with cellulose-containing materials. Chemicals such as phosphoric acid or zinc chloride are utilized. Sawdust is mixed with phosphoric acid or zinc chloride, optionally formed, and then heated to 400 to 600°C when phosphoric acid is utilized or to 600 to 700°C when zinc chloride is utilized. 

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