Dehydroxylation, mechanism

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-... 

Ruthbeig S, Barnes MW, Noyce RH (1963) Correlation of muscovite sheet mica on the basis of color, apparent optic angle, and absorption spectrum. J Res NatT Bur Std A, Phys Chem 67A 309-324 Sanz J, Gonzales-Carreno T, Cancedo R (1983) On dehydroxylation mechanisms of a biotite in vacuo and in oxygen. Phys Chem Minerals 9 14-18... 

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

While there is agreement that the rates of clay dehydroxylations are predominantly deceleratory and sensitive to PH2G, there is uncertainty as to whether these reactions are better represented by the first-order or by the diffusion-control kinetic expressions. In the absence of direct observational evidence of interface advance phenomena, it must be concluded that the presently available kinetic analyses do not provide an unambiguous identification of the reaction mechanisms. The factors which control the rates of dehydroxylation of these structurally related minerals have not been identified. 

However, the inverse correlation between activity and hydroxyl concentration [4] and the fact that excellent catalysts can be obtained with systems completely dehydroxylated by chemical means [126] (e.g., by fluorination) makes this mechanism unhkely. The only viable direction is to hypothesize that the starting structure for polymerization may evolve directly from a re-... 

G.J.C. Carpenter, Z.S. Wronski, NanocrystaUine NiO and NiO-Ni(OH) composite powders prepared by thermal and mechanical dehydroxylation of nickel hydroxide, Nanostructured Mater. 11(1) (1999) 67-80. 

Importantly, under CVD conditions the temperature can be sufficiently high to induce dehydroxylation of the alumina surface, so that the mechanism could follow reaction routes other than the one observed when GPID is carried out. This phenomenon can be even more pronounced when experiments are performed under dynamic UHV. 

Scheme 14.15 Possible grafting mechanism of[W(=Ar)(=CH Bu)(CH5Bu)j] on silica dehydroxylated at 700°C vs silica dehydroxylated at 200°C, proposed through analogy with silsesquioxane chemistry. Intermediates that have not been observed...

Figure 13.6 (a) Mechanism for the hydroxy I ation of lysine N-methyl group leading to dehydroxylation and release of formaldehyde, succinate and CO2. 

Goethite Hematite Thermal or mechanical dehydroxylation Hydrothermal dehydroxylation Gas/vacuum Solution... 

Thermal dehydroxylation of FeOOH has been studied both in vacuum and under various atmospheres. Kinetic studies of these transformations must be carried out under vacuum (Giovanoli Briitsch, 1974) and at a constant temperature. The temperature at which a phase transformation occurs, however, is determined by increasing the temperature of the sample in a controlled manner, i.e. by using a thermobalance (DTA or TGA method, see Ghap. 7). Mechanical and mechanochemical dehydroxylation of FeOOH at room temperature can also be achieved by grinding. 

Organothorium complexes such as [Th(r 3-allyI )4] supported on dehydroxylated y-alumina have been shown to exhibit activities rivaling those of the most active platinum metal catalysts.123 Thorium maintains its original +4 oxidation states at all times that is, the mechanism does not follow the usual oxidative addition-reductive elimination pathway. Partially hydrogenated products cannot be detected... 

When zeolite H-Y obtained by decationation of NH4-Y is heated further, water is irreversibly lost from the framework. The dehydroxylated zeolite Y displays Bronsted and Lewis acid properties. The mechanism for this process... 

Ward showed that the maximum hydroxyl group intensity was reached at 350°C and remained constant to 500°C. (148). As the temperature was raised above 500°C, the 3650- and 3550-cm-1 bands decreased until at 800°C, very few hydroxyl groups were observed on the surface. The loss of hydroxyl group absorption intensity upon heating above 500°C was accompanied by marked weight loss of the sample. This phenomenon was attributed to the loss of water by dehydroxylation by a mechanism resulting in formation of tricoordinated aluminum atoms (Lewis acid sites) and tricoordinated silicon atoms (146), as represented by structure I. This... 

The observed catalytic behavior in the case of l-methyl-2-ethylbenzene isomerization (158) was not so straightforwardly related to the Brdnsted site concentration. The maximum activity was observed with samples activated at temperatures where significant dehydroxylation had occurred. This reaction occurs more readily over acid catalysts than toluene disproportionation or xylene isomerization and may require fewer Brdnsted acid sites, or the reaction mechanism may involve Lewis sites. 

The formation of structural hydroxyl groups in the presence of divalent cations has been explained on the basis of a hydrolysis mechanism (148) involving water initially coordinated to the metal ions (210, 214-216). The formation of a nonacidic hydroxyl group on the metal ion and an acidic hydroxyl on the zeolite framework by dissociation of the water molecule is consistent with the observed IR spectra and pyridine adsorption experiments. Further calcination at higher temperatures results in dehydroxylation and formation of Lewis acid sites at tricoordinate aluminum atoms in the zeolite framework (149). 

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