Dehydroxylation activation energies

A linear dependence with pore diameter is observed. The intermolecular distance, directly dependent on the curvature of the pores, i.e. the average pore diameter, is linearly related to the activation energy for dehydroxylation. Extrapolation to smaller pores suggests activation energies of approximately 100 kJ mol 1 for dehydroxylation of hydroxyl groups in e.g. zeolite channels, if the hydroxyls are of a comparable type. Rees published corresponding activation energies for water desorption in dealuminated Y-zeolites.34... 

Figure 5.20 Average activation energies for water desorption originating from dehydration (+) and dehydroxylation ( ) respectively.

It is conceivable, however, that deactivation of Tung s more active catalysts at lower temperatures is caused by coke formation which has accumulated on the more active catalyst at lower temperatures. The lack of return of activity upon regeneration by air remains to be explained. The jump of framework oxygen as the rate-controlling step in catalysis is not likely in view of the large apparent activation energy of dehydroxylation reported by Venuto (25), > 70-100 kcal/mole, a substantial... 

The diffusion of an H atom over the very well dried, but not dehydroxylated, support surface can be described as hopping from one site to another, or alternatively as a synchronous hopping of a proton and an electron (25., 28). Each hopping consists of a simultaneous reduction of the receiving titanium cation and an oxidation of the leaving site, which may explain the similarity in activation energy for H2 chemisorption on and desorption from the support. 

The Ahrrenius plot., figure 6, which is preliminary, shows time constants at three anneal temperatures. It suggests that, as with loss of crystallinity (ref. 5), there are two regimes of dehydroxylation with activation energies of 0.7 eV above and J3.4 eV below about 750°C. The latter value is not inconsistent with the activation energy of 4.3 eV for diffusion of framework aluminiums deduced from thermal decomposition studies (ref. 5). Further measurements are in progress. 

MacKenzie (95,96) has developed a method for theTG study of solids in the presence of applied electrical fields. Electric fields of the order of 05 V/m lower the initial decomposition temperature for the dehydroxylation of kaolinite by 60° in some cases. The activation energy for the process is reduced by 3-12 kcal mole-1. Rate constants for the material are increased by electrolysis but this effect falls off at higher temperatures as the normal processes begin to predominate. 

There are three traditional structures usually adopted as probable BASs in amorphous aluminosilicates a water molecule coordinated by an electron-acceptor center (I), a bridged OH group (II), and a surface H30 + ion (III) (125,126). The catalytic activity of these sites is obviously determined by their properties and surface concentrations. Pelmenshchikov et al. (127) have attempted to compare these characteristics for the above types of BAS in aluminosilicates in terms of the cluster approach. For this purpose they considered a sequence of states of the model fragment of a dehydroxylated surface plus two water molecules (Fig. 15). State S0 corresponds to a dehydroxylated surface, states S, Sn, and Sm represent the sites of the I, II, and III types and states Sla and SIla correspond to centers I and II at a higher coverage. The relative energies of these structures obtained using the CTP scheme and the CNDO/BW technique are presented in Fig. 15. The relative surface density of the sites, og(nJnf), was estimated as the relative probability of their occurrence ... 

The rehydroxylation of a wide-pore silica gel sample calcined in air at 850 °C and held in water at 100 °C for periods covering 1 to 100 h was found to take 5-10 h for complete rehydroxylation. These and other results indicate that rehydroxylation of dehydroxylated silica (calcined at >400 °C) in the presence of water requires considerable energy to activate the process of dissociative adsorption, Ed. Chemisorption of water appears to take place, resulting in the formation of hydroxyl groups bound through... 

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