Dehydroxylation processes

Water is present in the materials of interest as free water or water of crystallization, or as combined water. The process of dehydration refers to the removal of the water of crystallization, while the removal of combined water is called dehydroxylation because hydroxyl groups in the material are broken down to form water vapor. The dehydroxylation process is very often alternately described as calcination. The drying process used in the present text pertains to both dehydration and dehydroxylation. In the processing of ores for metal extraction, drying essentially implies the removal by evaporation of water which a material holds in it in various forms. 

The kinetic data for the dehydroxylation process, i. e. the degree of transformation, a, as a function of time could be fitted to both a random nudeation model, i. e. 

It has to be pointed out that Lewis acid and base sites produced during the regular dehydroxylation process can hardly be all involved in catalytic reactions as active sites. It has already been indicated [44] that only defect sites can be considered as active sites because of their low site density. The configuration of such defect sites can hardly be predicted from idealizing model considerations. 

The same procedures and calculations, described in the previous section on pyridine desorption, can be applied to the natural desorption of water from silica as a function of temperature.32 The thermogram of silica (figure 5.19) shows two distincts areas. Water desorption in the region (298-423 K) is due to a dehydration process, desorption in the region (423-1173 K) is due to a dehydroxylation process. 

In 1993, L.T. Zhuravlev35 published a review article of work performed in the former USSR on the surface characterization of amorphous silica. This review article is a very important document in the study of the silanol distribution. Also, the energetical aspects of the dehydration and dehydroxylation processes are discussed in detail. The determination of the silanol number as a function of treatment temperature has already been discussed in chapter 4. 

The way in which Zhuravlev determined the desorption energies for the dehydration and dehydroxylation process is very similar to the model of Gillis-D Hamers. 

These results are presented in figure 5.27, showing the absolute distribution of the three silanol types on the silica surface as a function of temperature. The absolute concentration of the free silanols increases in the low temperature region, due to dehydroxylation processes of bridged silanols (reaction 5.1). 

The CO2 peak shown in Fig. 8 is due to the oxidation of acetate groups. This peak, centered at 400 °C in the CO2 mass loss is accompanied by a similar one in the same region in the water mass curve (Fig. 7). This fact indicates, as expected, that combustion of acetate gives carbon dioxide and water. Fig. 7 also shows the departure of a large quantity of physisorbed water at 150 °C. The secondary maximum at 500°C can arise from dehydration and dehydroxylation processes of the zirconium-containing pillars. Water evolution stops only at very high temperatures because of the condensation of hydroxyl groups of the clay. 

As a consequence of the 7a-dehydroxylation process, the bile acid composition of bile in healthy subjects usually comprises around 30 to 40% conjugated cholic acid, 30 to 40% conjugated chenodeoxycholic acid, 10 to 30% conjugated deoxycholic acid, and less than 5% conjugated lithocholic acid, of which the majority is sulfated (H18). 

These facts are indicative of a complex mechanism of the dehydroxylation process, which is not surprising if we keep in mind a high oxygen binding energy for this catalyst (see Table II). Also, they are suggestive of the possibility of reoxidation without intermediate formation of oxygen vacancies. 

The interpretation of the dehydroxylation process is consistent with the advancing interface concept rather than the inhomogeneous mechanism since the tunneling of protons explains the reaction of hydroxyl groups in adjacent layers. This mechanism also maintains electroneutrality, so the postulate of Mg2+ countermigration is no longer required. 

Guggenheim and co-workers considered only /ra 5-vacant dioctahedral micas in the dehydroxylation process. The possibility of dioctahedral 2 1 layers with the vacant site located in the cis position ( cA-vacanf or cv 2 1 layers) had been suggested for mont-morillonite and other dioctahedral smectites (e g., Mering and Glaeser 1954 Mering and Oberlin 1971 Besson 1980 Besson et al. 1982 Drits et al. 1984 Tsipursky and Drits... 

High-temperature studies. In this section we discuss in situ high-temperature investigations, and studies with heat-treated samples to induce oxidation and dehydration/ dehydroxylation processes. 

Advances in pore structure control of the porous active aluminas have resulted in major improvements in commercial adsorbents. Zeolites have their pore structures determined simultaneously with the precipitation process and are constrained in size by the configuration of the sodalite cage. In contrast, active alumina porosity is relatively independent of the bulk phase formation process and is usually engineered following material synthesis. Microporosity is controlled via kinetics of the dehydroxylation process, whereas macroporosity is usually developed in the agglomeration process. 

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