Dehydroxylation, hydroxides

Release of water from the crystalline hydroxides (dehydroxylation) differs from the dehydration of a crystalline hydrate (Sect. 1) in that product release must be preceded by chemical interaction between anions. 

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. 

Less detailed information is available concerning the rates of reactions of most hydroxy salts of inorganic acids indeed, the qualitative changes occurring during stepwise or overall removal of water have not been established for many systems. The behaviour characteristics of a number of hydroxy halides are mentioned below, as are the dehydroxylations of representative minerals. Some aspects of the relationships between the reactions of minerals and structurally similar metal hydroxides are critically discussed by Brett et al. [92]. 

Several other hydroxides of divalent metals crystallize in the same Cdl2 type structure as brucite, notably [610] those of Ca2+,Mn2+, Fe2+, Co2+, Ni2+ and Cd2+. The rates of dehydroxylation of these solids have, how-... 

Decompositions of crystalline mixed hydroxides to mixed oxides often occur at temperatures lower than those required to produce the same phases through the direct interaction of metal oxides. This route thus offers an attractive approach for the preparation of catalysts of high area and activity [1147]. Detailed kinetic investigations comparable with those for the dehydroxylations of a number of pure hydroxides (Sect. 2.1) are not, however, available. 

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. 

Pores may be present as structural features (e. g. between domains) or as a result of aggregation of particles. They may also be the result of partial dehydroxylation (oxide hydroxides) or dissolution. Although the shapes of pores can be quite variable, there are some definite, basic forms. The commonest of these are 1) slit shaped, the walls of which may or may not be parallel 2) ink bottle which are closed upon all sides but one from which a narrow neck opens and 3) cylindrical. Upon partial dissolution, pores bounded by well-defined crystal planes (e. g. 102 in goethite) develop (Chap. 12). 

Hematite formed by dehydroxylation of oxide hydroxides at temperatures below 500-600 °C is porous. That formed by heating goethite in vacuo at 300 °C contains slit shaped meso pores which coalesce to circular macropores at temperatures >400°C (Naono and Fujiwara, 1980). At even higher temperatures, these pores are... 

A common feature of the dehydroxylation of all iron oxide hydroxides is the initial development of microporosity due to the expulsion of water. This is followed, at higher temperatures, by the coalescence of these micropores to mesopores (see Chap. 5). Pore formation is accompanied by a rise in sample surface area. At temperatures higher than ca. 600 °C, the product sinters and the surface area drops considerably. During dehydroxylation, hydroxo-bonds are replaced by oxo-bonds and face sharing between octahedra (absent in the FeOOH structures see Chap. 2) develops and leads to a denser structure. As only one half of the interstices are filled with cations, some movement of Fe atoms during the transformation is required to achieve the two thirds occupancy found in hematite. 

Valence deficient oxygen species (O, 022, 033") produced by a) ionizing radiation and associated with lattice defects b) N20 or 6 adsorption, possibly followed by incorporation of O into the lattice c) thermal dehydroxylation of hydroxides in part via ... 

G. J. C. Carpenter, Z. S. Wronski, Nanocrystalline NiO and NiO-Ni(OH)2 Composite Powders prepared by Thermal and Mechanical Dehydroxylation of Nickel Hydroxide, Nanostructured Mater., 1999,... 

The nature of the donor site D depends on the type of oxide and its pretreatment temperature for pure oxides and, additionally, on the composition in the case of mixed oxides. The radical anion formation from TCNE (electron affinity 2.89 eV) on aluminas occurs on extraordinarily coordinated hydroxide ions on hydroxyl-rich surfaces, whereas exceptionally coordinated O2" ions play the role of the donor sites on more strongly dehydroxylated surfaces (328, 330). Accordingly, as the chemical nature of the donor site changes with the degree of surface hydroxylation, the spin concentration of the anion radical passes through two maxima the first is located between 400° and 500°C (OH- donor sites), and the second (brought about by the O2 ions) is between 600° and 700°C (328, 331). Trinitrobenzene (TNB) (electron affinity 1.0 eV) is a weaker electron acceptor than TCNE and interacts only with the 02 sites (332), thus acting more selectively than TCNE. 

Extending the carbon chain of vinylidenes by one or three carbons provides linear cumulenated ligands, LwM=(C=C)w=CR2 (n = 1,2). These compounds have been studied in considerable detail, especially allenyl-idenes (n = 1) and arise in most cases from the rearrangement and dehydration (often spontaneous) of hydroxyalkyl alkynes or diynes (Figure 5.51). Various intermediates (Figure 4.18) may be envisaged (and occasionally isolated) which are related by elimination/addition of H+ or hydroxide, e.g. y-hydroxyalkynyl complexes have been isolated and subsequently dehydroxylated with Lewis or Bronsted acids. 

Several authors [1, 18-20] have surveyed the chemical and crystallographic changes which occur during the dehydroxylation of the three forms of aluminium hydroxide, (Al(OH)3 gibbsite (y), bayerite (a) and norstrandite) and the two forms of aluminium oxyhydroxide, (AlOOH diaspore (a) and boehmite (y)). These phases... 

The texture and porosity of copper oxides are controlled by the water vapour pressure prevailing during the dehydroxylation of the hydroxide [47]. 

Although some hydroxides are precipitated from solution in a finely-divided, disordered and perhaps even amorphous state, compounds containing the OH" ion may also be prepared in a well-crystallized state, and studies of dehydroxylation reactions have often been concerned with such materials. Dehydration of compounds possessing the brucite lattice (M(OH)2 where M = Mg, Ca, Fe, Co or Ni) are... 

Peroxide formation may intervene [50] during dehydroxylation of hydroxides of the following metals Ba, Al, Cd, Zn and Pb. This conclusion was reached from the observation that during the reactions of these salts with alkali halides elemental halogen was liberated. These reactions need further investigation. 

In contrast to the decompositions of many solids (including the hydrates discussed in the previous chapter), the dehydrations of hydroxides show some common patterns of behaviour in two broad groups the dehydroxylations of (i) simple hydroxides (Mg(OH)2, Ca(OH)2, etc.) and (ii) extended silicates (clays, minerals. 

The dissociations of transition-metal oxides have often been studied as later processes following the dehydration/dehydroxylation of hydroxides. The existence of polymorphic varieties of the oxide systems has inhibited rapid development of this field. Descriptions of behaviour tend to be predominantly qualitative, devoted to the recognition of the phases involved, the sequences of changes which occur and the crystallographic relationships (if any) between reactants and products in each transformation. 

The thermal stability of these materials has been previously studied by Miyata (2) and Reichle ( 3). Upon heating, below 200°C, the Interlayer water is lost. Between 250°C and 500°C the materials dehydroxylate accompanied by the decomposition of the anion. Reichle (3) has shown that this process Is reversible up to 600°C, and takes place without exfoliation of the layers while maintaining the morphological structure of the hydroxide. In their layered form, these hydroxides have found wide use as anion exchangers (4 ), and sorbents for various hydrocarbon molecules (5) and water. Intercalation of heteropoly anions ( 6) and polymerized bidimensional silicate anions into the interlayer has also been reported ( 7). 

Reaction Studies. The reaction system consisted of a flow micro-reactor using helium as a carrier gas bubbled through a saturator containing 2-propanol. The partial pressure of 2-propanol was adjusted by controlling the temperature of the saturator. Approximately 200-400 mg of the layered hydroxide were heated In the reactor under a flow of helium In order dehydrate and dehydroxylate the material. The partial pressure of 2-propanol was maintained at 100 torr and the helium flow was varied between 10-20 ml/min. The in-situ calcination temperature was varied between 400-500°C and the reaction temperature between 150-350°C. Analyses of the reactants and the products were performed by an on-line GC fitted with a capillary column. 

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