Dehydroxylation thermal

The most intensively investigated dehydroxylation is probably the reaction of Mg(OH)2, though detailed results are also available for the hydroxides of certain other divalent cations. Several summaries of the mechanistic deductions obtained from such work, including literature sources, were presented at a conference at Dijon in 1974 [87]. The extensive literature concerned with the thermal analysis of hydroxides has been reviewed by Dollimore [79] who has also included the behaviour of oxides. Water elimination can be regarded as the first in a sequence of structurally related steps through which the hydroxide is converted into the thermally most stable oxide. 

Figure 1.9 TG, DTG, and DTA profiles for an amorphous catalyst precursor obtained by coprecipitation of Fe(N03)3 and Mg(N03)2 in solution [65], This precursor is heated at high temperatures to produce a MgFe204 spinel, used for the selective oxidation of styrene. The thermal analysis reported here points to four stages in this transformation, namely, the losses of adsorbed and crystal water at 110 and 220°C, respectively, the decomposition and dehydroxylation of the precursor into a mixed oxide at 390°C, and the formation of the MgFe204 spinel at 640°C. Information such as this is central in the design of preparation procedures for catalysts. (Reproduced with permission from Elsevier.)...

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. 

Shono and Kise have investigated the electroreductive coupling reaction of y- and -cyanoketones, yielding bicyclic a-hydroxy ketones 218 and their dehyd-roxylated equivalents 221 [198], Optimized yields are obtained when the electroreduction is carried out in i-propanol at a controlled potential of — 2.8 V using a divided cell equipped with a ceramic diaphragm and an Sn or Ag cathode. The product ratio is controlled by the reaction temperature. When the reaction is carried out at 25°C, almost exclusively the a-hydroxy ketone 218 is obtained, whereas at 65°C the obviously thermally dehydroxylated ketone 221 is the predominant product (Scheme 42). Furthermore, this methodology has been... 

One portion of the oxygen in clay minerals occurs as the hydroxyl ion. Hamza and Epstein (1980), Bechtel and Hoemes (1990) and Girard and Savin (1996) have attempted to separate the hydroxyl and nonhydoxyl bonded oxygen for separate isotope analysis. Techniques include thermal dehydroxylation and incomplete flu-orination, both of which indicate that hydroxyl oxygen is considerably depleted in 0 relative to nonhydroxyl oxygen. 

Girard JP, Savin S (1996) IntercrystaUine fractionation of oxygen isotopes between hydroxyl and non-hydroxyl sites in kaoUnite measured by thermal dehydroxylation and partial fluorination. 

Re2Pt(CO)n Dehydroxylated AI2O3 Adsorption from solution and thermal treatment under H2 Re4Pt2 entities, model catalyst for methylcyclohexane dehydrogenation [57, 60]... 

Fe(CO)s Dehydroxylated y-AfOj Adsorption from solution and thermal treatment under Ft2 at moderate T Higher dispersion, reduction degree and activity in Fischer-Tropsch than conventional catalysts [68]... 

Mainly Fe aggregates form when highly dehydrated magnesia is impregnated with Fe3(CO)i2 or Fe(CO)5 and the resulting surface species are thermally decomposed conversely, when MgO is not dehydroxylated the thermal treatment generates Fe and H2 because of the electrophihc attack by surface protons on the carbonyhc surface species [73]. 

Furthermore, the removal of these groups by thermal treatment (dehydroxyl-ation) yields coordinatively unsaturated oxygens and anions in which coordina-tively unsaturated aluminum atoms are exposed (Lewis acid sites). In general, the total concentration of OH groups on a alumina support ranges between 10 and... 

A detailed study on the immobilization of Ln[N(SiMe3)2]3 on fumed silica showed that the nature of the grafted species is considerably affected by the thermal pretreatment of the support [117]. Whereas at low (AS-380.2so) and intermediate dehydroxylation temperatures (AS-380.5oo) mixtures of bipodal [(=SiO)2Ln N(SiMe3)2 ] and monopodal [(=SiO)Ln N(SiMe3)2 2] species prevailed... 

In the case of oxide catalysts or alkali metal-doped oxide catalysts, basic surface sites can be generated by decarboxylation of a surface metal carbonate exchange of hydroxyl hydrogen ions by electropositive cations thermal dehydroxylation of the catalyst surface condensation of alkali metal particles on the surface and reaction of an alkali metal with an anion vacancy (AV) to give centers (e.g., Na + AV — Na + e ). 

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

Lepidocrocite Maghemite, Hematite Thermal dehydroxylation Gas/vacuum... 

The end product of the dehydroxylation of pure phases is, in all cases, hematite, but with lepidocrocite, maghemite occurs as an intermediate phase. The amount of water in stoichiometric FeOOH is 10.4 g kg , but adsorbed water may increase the overall amount released. Thermal dehydroxylation of the different forms of FeOOH (followed by DTA or TG) takes place at widely varying temperatures (140-500 °C) depending on the nature of the compound, its crystallinity, the extent of isomorphous substitution and any chemical impurities (see Fig. 7.18). Sometimes the conversion temperature is taken from thermal analysis data (e. g. DTA), but because of the dynamic nature of the thermoanalysis methods, the temperature of the endothermic peak is usually higher than the equilibrium temperature of conversion. 

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. 

Paterson, E. Swaffield, R. (1980) Influence of adsorbed anions on the dehydroxylation of synthetic goethite. J. Thermal Analysis 18 161-167... 

Wells, M.A. Gilkes, R.J. Anand, R.R. (1989) The formation of corundum and aluminous hematite by the thermal dehydroxylation of aluminous goethite. Clay Min. 24 513-530 Wells, M.A. Gilkes, R.J. Fitzpatrick, R.W. (2001) Properties and acid dissolution of metal-substituted hematites. Clays Clay Min. 49 60-72... 

An additional complexity arises from the dissociation of water molecules which occurs when alkaline-earth-exchanged zeolites are thermally activated since several modes of dehydroxylation are possible. This problem has been extensively investigated by IR spectroscopy, in particular by Ward (268,269) and Uytterhoeven et ai (270), and by X ray (271). They concluded that the electrostatic field associated with the cation causes dissociation of adsorbed water to produce acidic hydroxyl groups. The dissociation reaction may occur according to the following reactions ... 

The chemistry and structure of the hydrogen form of zeolite Y have been thoroughly investigated 82) and are not considered further. The structure of the dehydroxylated zeolite proposed by Uytterhoeven, Christ-ner, and Hall 15) remains unchanged. Recently Ward, on the basis of infrared studies, suggested that this form may be amorphous 27). The extreme instability of dehydroxylated zeolite Y to moisture complicates detailed study 19). The elucidation of the detailed nature of this material lies in the future. At present, completely dehydroxylated Y is little understood and presents a challenging void in our knowledge of the nature of ammonium zeolite Y thermal decomposition products. 

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