Dehydroxylation value

One of the factors responsible for the rather wide variation in a values for benzene is the presence of ji-clectrons in the molecule, which can cause its adsorption to acquire a specific character if the adsorbent is polar (Chapter 1, p. 11). On hydroxylated silica, for example, the heat of adsorption is much higher than on the dehydroxylated material - on the latter solid indeed the interaction is so weak that a Type HI isotherm results (Fig. 2.19). Unfortunately c-values are rarely quoted in the literature, but... 

Novel aromatic carboxylation reactions have been observed in the anaerobic transformation of phenols to ben2oates (82). A mixed anaerobic microbial consortium apparentiy transforms phenol (33) through an intermediate to ben2oic acid (34) via dehydroxylation. This reaction has not yet been widely exploited for its obvious synthetic value. 

Kodama and Brydon [631] identify the dehydroxylation of microcrystalline mica as a diffusion-controlled reaction. It is suggested that the large difference between the value of E (222 kJ mole-1) and the enthalpy of reaction (43 kJ mole-1) could arise from the production of an amorphous transition layer during reaction (though none was detected) or an energy barrier to the interaction of hydroxyl groups. Water vapour reduced the rate of water release from montmorillonite and from illite and... 

Daw et al. [632] made a microscopic and crystallographic study of talc dehydroxylation. Nucleation, to yield enstatite, occurs inhomogeneously within the particles, perhaps at dislocations. Later, this product is topo-tactically orientated with respect to the reactant lattice, though with extensive faulting on the (010) plane owing to misfit, in addition to the attempt to preserve the oxygen lattice. In an isothermal study (1100— 1160 K) of the same reaction, Ward [633] found first-order obedience and the value of E determined (422 kJ mole-1) is close to that estimated for... 

Clearly, strong 0-H interaction occurs leading to dissociation of ammonia, formation of OH groups, dehydroxylation, and surface imide formation. The NH(a) species has a characteristic N(ls) value of 398 eV, i.e., 1 eV greater than N(a) and 1 eV less than NH2(a). 

Synthetic 5-FeOOH has a surface area which ranges from 20-300 m g depending on the thickness of the crystals. In a series of seven synthetic feroxyhytes the surface area increased from 140 to 240 m g (EGME method) as the crystallinity decreased (Garlson and Schwertmann, 1980). 5-EeOOH displays interpartide porosity, i.e. slitshaped micro- or mesopores between the plate like crystals (Jimenez-Mateos et al., 1988 Ishikawa et al., 1992). Both TEM observations and t-plot analysis showed that 0.8 nm micropores formed upon dehydroxylation at 150 °G in vacuo. The surface area rose steeply as the temperature exceeded 100 °G and reached a value close to 150 m g at 200 °C at which temperature, the sample was completely converted to hematite. 

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

The rate coefficients of reactions (15)-(27) were taken from the results of ab initio calculations. Reactions (28) and (29) describe the process of surface dehydroxylation/hydroxylation. We used a value of 1013 sec-1 as an estimation of the preexponential factor (this value corresponds to the characteristic frequency of internal vibrations of the reaction center) for the desorption reaction. To describe the experimental dependence of the growth rate vs. temperature adequately, we considered that the water adsorption energy is a linear function of the hydroxylation degree / ... 

The deuterium exchange method was also used to determine the average value of the silanol number as a function of dehydroxylation temperature (473 - 1373 K in vacuo). The experimental results are shown in figure 4.1. 

H30+ per gram and calculated pH values, for the filtrate samples, are shown in Table V. The molar H+/A1 ratio is also tabulated, demonstrating that a third of the proton sites are dehydroxylated by 400°C and another third by 700°C. This is further supported by the observation of decreased absorbance of the OH band at 3610 cm- in the infrared spectrum as the temperature is increased from 400°C to 800°C. Assuming that all strong acidity is derived from the exchange of H+ in the zeolite by Na+ in the concentrated NaCl solution, the filtrate pH is a direct measure of that exchange, using the simple relation pH = -log [H+]. For H30+-ZSM-5 calcined at 400°C, the filtrate sample titer is 0.431 meq. H30+/g of... 

NO and O rearrange into structural nitrite and nitrate (limiting step) which further react with NH3 to give N2 and H20. Lewis sites formed from dehydroxylation of a Bronsted site are proposed as the active centres. According to Brandin et al. (81), in the SCR over H-MOR the NOx conversion goes through a maximum value at ca. N02/NO = 1. From l5NMR studies of the N0+NH3+02 reaction over H-MFI,... 

The low-coverage energy data for the adsorption of n-hexane and benzene on various non-porous solids in Table 1.4 illustrate the importance of the surface structure of the adsorbent and the nature of the adsorptive. Since n-hexane is a non-polar molecule, Em > Esp, and therefore the value of E0 is dependent on the overall dispersion forces and hence on the density of the force centres in the outer part of the adsorbent (i.e. its surface structure). Dehydroxylation of a silica surface involves very little change in surface structure and therefore no significant difference in the value of E0 for n-hexane. However, replacement of the surface hydroxyls by alkylsilyl groups... 

The early work of Kiselev (1957) revealed that the adsorption isotherms of n-pentane and n-hexane on non-porous quartz were intermediate in character between Types II and m. Values of C(BET) <10 were obtained and the differential enthalpies of adsorption decreased steeply at low surface coverage. More recently, the isotherms of isobutane (at 261 K) and neopentane (at 273 K) on TK800 have been found to be of a similar shape (Carrott et al., 1988 Carrott and Sing, 1989). Unlike those of benzene, these alkane isotherms do not undergo a pronounced change of shape as a result of surface dehydroxylation. This is consistent with the non-specific nature of their molecular interactions (see Chapter 1). 

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