Dehydration, temperatures

Catalyst acid properties depend on several parameters, including method of preparation, dehydration temperature, silica-to-alumina ratio, and the ratio of Bronsted to Lewis acid sites. 

Keywords zeolite, SR-XRPD, dehydration, temperature resolved. 

High Dehydration Temperature - In contrast to other forms of zinc borates, the water of hydration of the zinc borate is retained up to 290°C, thus allowing it to be used in polymers requiring high processing temperatures. The proposed molecular structure for the zinc borate is depicted in Fig. 

The high dehydration temperature can be explained by the absence of interstitial water in the crystal lattice. 

Table 11.2 Characteristics of silica Aerosil 200 as a function of the dehydration temperature [56]. ...

Borates, through their ability to act as glass network formers, can act as excellent char formers and drip suppressants in fire retardant applications. In many cases this involves processing into polymeric materials, leading to specific requirements for thermal stability and particle size. Most common borate materials, however, exhibit relatively low dehydration temperatures and may be unsuitable for use in many polymer systems. Zinc borates are often used because they have unusually high dehydration onset temperatures and can be produced as small particle size powders. 

XPS has been used by Inoue and Yasumori (425a) to study the surface of MgO, CaO, and BaO after exposure to water vapor followed by in situ dehydration at various temperatures. Signals attributed to O2- lattice ions were observed after all treatments with peaks at 531.6 (MgO), 529.6 (CaO), and 528.5 (BaO) eV. Immediately after hydration, a second series of peaks at 533.7 (MgO), 532.2 (CaO), and 532.1 (BaO) eV were observed, which progressively disappeared as the dehydration temperature increased these were assigned to OH- ions. In addition, peaks at 532.1 (CaO) and 530.6 (BaO) eV which appeared during dehydration were assigned to O- ions formed from the following reaction ... 

Both forms of anhydrous sodium triphosphate are unstable with respect to combination with water, and cannot therefore be prepared from aqueous solution. Likewise they cannot be obtained by dehydration of the hexahydrate (19, 141, 142, 326) as long as the dehydration temperature is less than 150°C. When the hexahydrate is heated in an open atmosphere to about 120° it first loses only about five molecules of water (238, 317). The residual molecule of water simultaneously causes hydrolysis and splits the triphosphate anion to crystalline diphosphate and amorphous monophosphate, perhaps according to the equation ... 

Figure 5. ESR spectrum (low field components) of CuNaY zeolite (a = 29%) in the Q-band taken al room temperature as a function of dehydration temperature (numbers on the right are pretreatment temperatures)...

In the spectrum of zeolite KL, at the temperature of evacuation (100°-500°C), stronger water-bound bands (3665, 3685, 3700 cm 1 and 1602, 1630, and 1650-1660 cm 1) are found. Absorbance of these bands decreases above 400°C. A parallelism in the absorbance decrease is shown for the 1602 and 3700 cm 1 bands (preserved to 600°C) at increased dehydration temperature. These bands are not recovered on rehydration, at either low or at high temperatures. Water molecules are, evidently, localized in the secondary system of channels (8). Perhaps water is localized in cancrinite cells. 

Fig. 23. Effect of dehydration temperature on the MBssbauer spectrum of Fe2+-M. (a) 15 Torr of H20, 273 K. (b) evacuated 24 hr, 273 K (c) evacuated 3 hr, 517 K (d) evacuated 8 hr, 800 K (e) 15 Torr of H20, 273 K. All spectra at room temperature and on the same sample. Zero velocity is with respect to a 57Co in chromium source. Figure according to Garten et cd.

Cu Y. The absorption spectra of hydrated and dehydrated CuIJY zeolites are shown in Figs. 5 and 6, respectively. The dehydrated Cu Y zeolite also displayed a weak photoluminescence at 540 nm, in qualitative accord with the reports of partial autoreduction of Cu to Cu upon dehydration, which amounts to approximately 20% of Cu converted to Cu at the dehydration temperature of 400°C (3). The sharp peaks at 5200 and 7000 cm- in Fig. 5 are the (v+6) and (2v) vibrational bands of water (14). Their absence in Fig. 6 demonstrates that the dehydration of Cu Y is complete. Also, absence of the silanol (2v) band at 7300 cm- (I5) shows that hydroxyl groups are absent in the dehydrated Cu Y as well as in all subsequently treated copper zeolites. The broader bands between 9000 and 16000 cm and above 30000 cm- are electronic absorption spectra of the copper species in the hydrated and dehydrated Cu Y, as follows from their comparison with the spectra of NaY and CuxY. 

Nutrient composition. The nutritional quality of dried citrus pulp may be affected by processing conditions, particularly dehydration temperatures. Pulp dried with dryer exit-stack gas temperatures greater than 143°C shows caramelization or browning. Ammerman et al. (6J and Chapman et al. ( 7) have shown that a... 

The results of Maciel and Sindorf show that, for dehydrated samples, the relative apparent population of the surface geminal sites decreases from the initial value of about 15% at room temperature to a value of about 12% at 673 K. Above this temperature, there is a sharp increase in the relative population of geminal sites, which reaches a maximum of approximately 24% at 923 K and then decreases again at higher dehydration temperatures. 

A mixture of ammonium chloride and borax was one of the treatments of cellulosic fabrics reported by Gay-Lussac in 1821. Due to its low dehydration temperature and water solubility, sodium borates are only used as flame retardants in cellulose insulation (ground-up newspaper— see Sections 9.2.1.2 and 9.2.2.1), wood timber, textiles, urethane foam, and coatings. For example, a mixture of urethane (100 parts), borax (100 phr), and perlite (30phr) was claimed to provide flame-retardant urethane foam.8 Borax in conjunction with boric oxide, silica, ammonium chloride, and APB as ceramizing additives and volume builders, are claimed in a fire-protection coating based on polybutadiene and silicone microemulsion.9 Using a modified DIN 4102 test, the chipboard with the coating showed a loss of mass less than 1% and there was no pyrolysis of the wood sample. 

Due to their low dehydration temperatures and water solubilities, boric acid and sodium borates (borax pentahydrate and borax decahydrate) are mostly used as fire retardants in wood/cellulosic products such as timbers, plywood, particle board, wood fiber, paper products, and cotton products. In recent years, boric acid has also been used as fire retardant in epoxy intumescent coating, pheno-lics, urethane foam, and so on. When necessary, boric acid can be coated with silicone oil such as silicone to alleviate its water solubility in water-based coating. 

Its usage in polymers, however, is limited by its high water solubility (10.9%) and low dehydration temperature. PPG reported the use of APB in conjunction with APP, zinc borate, silica, talc, and so on, in a flame-retardant intumescent coating for protection against hydrocarbon fires.78... 

Melamine diborate (MB), known in the fire-retardant trade as melamine borate, is a white powder, which can be prepared readily from melamine and boric acid. It is partly soluble in water and acts as an afterglow suppressant and a char promoter in cellulosic materials. Budenheim Iberica79 claims that, in a 1 1 combination with APP, MB (10%-15%) can be used for phenolic bound nonwoven cotton fibers. In general, melamine borate can be used as a char promoter in intumescent systems for various polymers including polyolefins or elastomers. However, its low dehydration temperature (about 130°C) limits its application in thermoplastics that are processed at above 130°C. Melamine borate is also reported to suppress afterglow combustion in flame-proofing textiles with APP or monoammonium phosphate to meet the German DIN 53,459 and Nordtest NT-Fire 002.80... 

Salt Number of molecules of water of crystallization Dehydration temperature °C Initiation temperature °C Inflammability from flame... 

Tucholski determined the dehydration temperatures of picric acid salts hydrates, their melting points, initiation temperatures and temperatures preceding explosion. T. Urbanski and Sion determined their sensitiveness to impact and flame. 

Thermal analysis shows that dehydration temperature of Ca(OH) decreases from 520 to 440°C in activated mixtures. Nevertheless, the fine product, Ca2Fe20s, is formed after heat treatment at 600°C. 

Under general conditions, rare earth hydroxides RE(OH)3 H20 precipitate from a high pH solution as a gel. However, they are unstable during heating and usually lose water to become REO(OH) or RE2O3 when the temperature approaches or exceeds 200 °C. From lanthanum to lutetium, the dehydration temperature decreases with an increase in atomic number because of a decrease in the ionic ratio. 

Comparison of the calculated P-T curves of the thermal stability of grunerite, anthophyllite, and tremolite shows a systematic increase in dehydration temperature on passing from iron to magnesian amphiboles and beyond to magnesian-calcic (Fig. 90). Obviously, substitutions of Mg and Ca... 

Figure 89 shows schematic P-T curves of the upper temperature limit of stability of Mg-Fe amphiboles. The region of overlap here is arbitrarily retained. From the experimental data it follows that the iron content of cummingtonite does not fall below 30-40%, while ferroanthophyllite does not contain more than 60-70% iron molecule. Therefore the interpolation was not strictly linear, and the P-T curves obtained may contain an error of the order of 5-15°. From these approximate formulations it follows that a 10% increase in the magnesium content of cummingtonite would cause a 10-15° rise in dehydration temperature. As in the carbonate reactions, the product of decomposition will be olivine in the case of a high iron content and orthopyroxene in the case of an iron content below 70-90%. 

Niobium oxide surfaces are also very dependent on the dehydration temperature. An illustration is given in Figure 9.5, which represents the differential heats of NH3 adsorption versus ammonia coverage for a niobium oxide from CBMM pre-treated at 423, 523, 623, 723 and 823 K [41]. 

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