Structural dehydroxylations

Hydrated clay surfaces are acidic. When isomorphic substitution occurs in the tetrahedral layer, acid leaching or NH thermal decomposition may generate acidic surface OH. For clays whose negative charges are produced by isomorphic substitutions in the octahedral layer, mild dehydration removes the source of acidity, because of the reversibility of reaction (3). Deamination of the ammonium exchanged clay with octahedral substitution drives protons into the octahedral layer, as evidenced by the lowered temperature at structural dehydroxylation. 

The low-coverage energy data for the adsorption of n-hexane and benzene on various non-porous solids in Table 1.4 illustrate the importance of the surface structure of the adsorbent and the nature of the adsorptive. Since n-hexane is a non-polar molecule, Em > Esp, and therefore the value of E0 is dependent on the overall dispersion forces and hence on the density of the force centres in the outer part of the adsorbent (i.e. its surface structure). Dehydroxylation of a silica surface involves very little change in surface structure and therefore no significant difference in the value of E0 for n-hexane. However, replacement of the surface hydroxyls by alkylsilyl groups... 

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-... 

Giovanoli and Briitsch [264] studied the kinetics of vacuum dehydroxylation of 7-FeO 0H(- -7 Fe203). It was not possible to demonstrate satisfactory obedience to a single kinetic expression. Microscopic examinations detected the occurrence of random nucleation over reactant surfaces and crystallographic indications of the specific structural reorganization steps, which occur at the reaction interface, are discussed. 

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. 

Scheme 2 Different siloxane bridge structures formed upon dehydroxylation of silica surface. The increasing dimension of silicon rings and, consequently, of the Si - O - Si angle reflects a decreasing of the strain of these structures...

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-... 

Scheme 2, vide infra for characterization of these structures) [15]. At an intermediate temperature of 500 °C, a 65/35 mixture of these two complexes is obtained [16]. The proposed structure is further confirmed by the mass balance analysis since hydrolysis or ethanolysis of the resulting solid yields the complementary amounts of neopentane, these are 2 and 3 equiv. of neopentane/Ta for [(=SiO)2Ta(= CHlBu)(CH2fBu)] and [(=SiO)Ta(= CH(Bu)(CH2fBu)2], respectively. Moreover, elemental analysis provides further information indeed, 4.2 wt % of Ta grafted onto sihca partially dehydroxylated at 700 °C corresponds to 0.22 mmol of Ta/g of sofid [ 17,18]. This is comparable to the amount of silanol present on this support (0.26 mmol OH/g), which shows that most of them have reacted during grafting (as observed by IR spectroscopy). 

Alumina is known to have more ionic character and its surface has a more complex structure than that of silica. Reaction of Bu3SnH with the surface of partially dehydroxylated aluminas was followed and it was found that the extreme sensitivity of tin chemical shifts to the molecular environment constitutes a method whereby surface organometallic complexes of tin can be used as molecular probes for determining surface structures of oxides.248... 

The carbonyl cluster Rh,5(CO)i,5 was initially stable as such on the completely dehydroxylated alumina surface. But as soon as hydroxyl groups were generated (e.g., by adding traces of water) it decomposed to give various surface transformations. First, the cluster structure was dismpted, with breakage of the core cluster frame, into (Al-0-)(Al-0H)Rh (C0)2, Rh > monoatomic species sigma and n-bonded to the oxygens atoms of the alumina surface, with formation of molecular... 

A structural investigation of [Ta(=CH Bu)(CH2 Bu)3] grafted on siUca partially dehydroxylated at 700 °C using EXAFS [18] has revealed a short-range interaction of 2.64 between the O-atom from a siloxane bridge and tantalum. This O-atom acts as a two-electron donor ligand to stabiUze the formally ten-electron surface complex [(=SiO)Ta(=CH Bu)(CH2 Bu)2j, yielding the more stabiUzed twelve-electron species [(=Si0)Ta(=CH Bu)(CH2 Bu)2(=Si-0-Si j (Scheme 11.9). 

As structural Al increases, the dehydroxylation temperature, the OH stretching frequency and both OH bending vibrations as well as their separation (50H - yOH)... 

The oxide surface has structural and functional groups (sites) which interact with gaseous and soluble species and also with the surfaces of other oxides and bacterial cells. The number of available sites per unit mass of oxide depends upon the nature of the oxide and its specific surface area. The specific surface area influences the reactivity of the oxide particularly its dissolution and dehydroxylation behaviour, interaction with sorbents, phase transformations and also, thermodynamic stability. In addition, specific surface area and also porosity are crucial factors for determining the activity of iron oxide catalysts. 

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). 

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. 

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