Dehy droxylation

Tetrabutyltin reacts a similar way with the OH groups. On partially dehy-droxylated silica (treated at 200°C) the reaction shown in Scheme 7.10 takes... 

Investigations of T effects on amphibole volume are complicated by dehy-droxylation. Experimental data show that loss of OH groups is favored by the presence of Fe, based on equilibrium ... 

The disappearance of the magnetization above the Curie (or Neel) temperature (see Chap. 6) can be used to identify iron oxides. Thermal reactions - dehydration/dehy-droxylation, oxidation/reduction - with accompanying phase transformations may give additional information about phases present. Therefore, not only the heating cycle, but also the cooling cycle is usually recorded to give the so-called Js(T) curve. 

So far the discussion of the goethite/hematite equilibrium refers to aqueous systems in which the water activitY (i. e. relative humidity), anp, is unity. In many cases, however, the water activity may be <1. This applies to soils and sediments where can be lowered by the binding of water in pores. When considering the dehy-droxylation reaction. 

Figure 6. Adsorption of diethyl ether on silica at 24°C (A) volumetric isotherm on silica containing free OH groups (- -SiOSi) (B) volumetric isotherm on dehy-droxylated silica (D) spectroscopic isotherm on free OH groups (19)...

Vieira AJSC, Steenken S (1990) Pattern of OH radical reaction with adenine and its nucleosides and nucleotides. Characterization of two types of isomeric OH adduct and their unimolecular transformation reactions. J Am Chem Soc 112 6986-6994 Vieira AJSC, Steenken S (1991) Pattern of OH radical reaction with N6,N6,9-trimethyladenine. Dehy-droxylation and ring-opening of isomeric OH-adducts. J PhysChem 95 9340-9346 Vieira AJSC, Candeias LP, Steenken S (1993) Hydroxyl radical induced damage to the purine bases of DNA in vitro studies. J Chim Phys 90 881-897... 

Mixed oxides have a widespread application as magnets, catalysts, and ceramics. Often, nonstoichiometric mixtures with unusual properties can be prepared for example, Fe203 and ZnO have been milled for the production of zinc ferrite [40], while mixed oxides of Ca(OH)2 and Si02 were described by Kosova et al. [77]. Piezoceramic material such as BaTi03 from BaO and anatase Ti02 has been prepared [78], while ZnO and Cr203 have been treated by Marinkovic et al. [79] and calcium silicate hydrates from calcium hydroxide and silica gel by Saito et al. [80]. The thermal dehy-droxylation of Ni(OH)2 to NiO or NiO-Ni(OH)2 nanocomposites has also been investigated [81]. 

At the same time, specific properties (primarily the Si/Al ratio) of a zeolite should be taken into account when discussing the mechanism of its dehy-droxylation. It is quite possible that the mechanism typical of H forms of faujasites would be completely improper for high-silica-containing zeolites. Thus, in their studies of dehydroxylated forms of ZSM-5 zeolite by means of IR spectroscopy of molecular hydrogen adsorbed at low temperatures, Kazansky et al. (72, 76) have demonstrated that the Uytterhoeven-Cristner-Hall scheme seems valid in this case. 

Studies of organoactinide surface chemistry and catalysis have been carried out by the Marks group under rigorously anhydrous/anaerobic conditions on either partially dehy-droxylated alumina (PDA) or DA at surface coverages of 0.25 0.50 molecules nm. The PDA and DA surfaces are represented schematically in equation (68). Product yields, isotopic labeling, and surface reactivity indicate that the irreversible y-alumina adsorption chemistry of Cp 2AnMe2 can be described by three methane-evolving pathways (equations 69-71). Equation (69) dominates on PDA. On DA,... 

For the samples studied, no pyridine adsorbed on Lewis acid sites was detected, indicating that the zeolite had not been partially dehy-droxylated to form such sites. The band reflecting pyridine-cation interaction was detected only after about 16 alkaline earth ions had been introduced into the unit cell. It grew steadily in intensity as more divalent ions were introduced into the structure. In Figure 1, the intensity of the pyridinium ion band, expressed as absorbance/sample mass, is plotted as a function of the per cent exchange by divalent ion. The concentration of acid sites is a function of the degree of exchange. 

FIGURE 12.3 Sketch of the nucleation-densification process (b) involved with the dehy-droxylation of a gel surface (a). (After Reference 27.)... 

K on HY zeolites with a Si/Al ratio of approximately 2.4 is found to vary from 130 to 205 kJ mol 91,147,149,150,153-155). The same samples also display different acid site strength distributions, some with strong peaks at heats that vary from 125 to 176 kJ mo whereas others show completely monotonic distributions. These strong differences are caused by other variables such as the sodium content (as discussed above), the activation or dehy-droxylation temperature, and the framework Si/Al ratio. 

A major problem with Al NMR spectroscopy of steamed zeolites (and with dehy-droxylated minerals such as metakaolinite (MacKenzie et al. 1985)) is the inability to detect all the Al present at lower applied fields and slower MAS speeds. The presence... 

From the present data it is impossible to determine whether tt- (I) or CT-complex (II) formation, or some other process, is rate determining, or to define the stereochemistry when going from (II) to (III). It is certain, however, that a strongly chemisorbed aromatic is involved at some stage. It is improbable that the defect sites produced by dehy-droxylation (69,70) are directly involved in the electrophilic hydrogen-deuterium exchange process. 

Figure 5. (a) Water adsorption onto a typical hydroxylated silica surface, (b) Surface of dehy-droxylated silica. 

Hydrotalcite-like compounds (HTlcs) have attracted much attention in recent years as catalyst precursors and catalyst support. This is due to (i) their ability to accommodate a large variety of bivalent and trivalent cations (ii) the homogeneous mixture of the cations on an atomic scale and (iii) the formation of thermostable mixed oxides, often denoted as ex-HTlcs, with high surface area upon decomposition. The first two properties are a result of the precursor while the last property appears to be related to the decomposition mechanism. The transitions in the structural properties of Co-based hydrotalcites upon high temperature treatments have been extensively studied in our group.15,16 In the first decomposition step, water is removed from the structure. This transition is followed by dehy-droxylation and decarbonation, as well as carbonate reorganization in the interlayer. Thermal treatment in air finally leads to a solid solution of cobalt spinels (Co(Co, A1)2C>4). Mixtures of CoO and C0AI2O4 are formed upon treatment in inert. 

The following experiment suggests that the degree of surface dehy-droxylation is important. As shown by the results of Figure 12, the mixing... 

The origin of the 3a,7a,12a,23 -tetrahydroxy-5 -cholanic has been studied [163] with subcellular fractions of adder Vipera berua) liver with the following conclusions the active microsomal enzyme requires NADPH and O2 deoxycholate is not converted to cholate, but to 3a,7, 12a-trihydroxy-5)8-cholanate. Bitocholic acid could result from two pathways (a) cholate is converted to 3a,7a,12a,23 -tetrahy-droxy-5j8-cholanate by hepatic 23-hydroxylase the tetrahydroxy acid is 7a-dehy-droxylated by intestinal microorganisms to produce bitocholate (the 3a,12a,23 -tri-... 

Additional metabolic reactions that have been attributed to the intestinal flora are dehy-droxylations (alcoholic), decarboxylations, O-demethylations, nitro reductions, double bond reductions, azo reductions, scission of heterocyclic ring systems, and even aromati-zation of cyclohexyl carboxylic acids. 

Figure 12. Infrared spectra for Ca-exchanged Y zeolite (Si/Al of 2.5) after dehydration, dehy-droxylation, and rehydration...

The TG and DTG curves of a kaolinite standard, as determined by Earnest (111). s shown in Figure 4.3. Sorbed water is evolved at temperatures up to 200°C in this case, the amount is 0.2% of the total sample mass. The dehy-droxylation reaction occurs in the temperature range of 400-700 C. resulting in a mass-loss of 13.8%. The DTG curve is similar in symmetry to the DTA curve peak. Experimental factors, such as heating rate, larger sample-particle size, and so on, can shift the peak minima temperatures. 

Fig. 4.8. Adsoiption complexes of ilienium subcarbcmyl on cationic defects of MgO. Dehy-droxylated surface - Re(CO)3 OMg 3 (left), hydroxylated surface - Re(CO)3 HOMg)3 (right).

Research at Hoechst has led to the fixing of neutral triarylborane activators to silica surfaces. This was achieved by reacting HB(CeF5)2 with silica modified by vinyltriethoxysilane (Scheme 32) or substituted tris(polyfluorophenyl)boranes with partially dehy-droxylated silica (Scheme 33). The supported neutral activators can also be quaternized by reaction with Li(C6Fs).2"... 

The silica A surface is not fully hydroxylated. About two-thirds of the sites that are precursors of type D2 siloxane bridges are already dehy-droxylated in the initial sample. That observation is consistent with both the high manufacturing temperature of the fumed silica and its low... 

Lygin and co-workers (Moscow State University) [378-381] used the methods of quantum chemistry in calculating cluster models of the surface structure of silica. The selection of cluster models was based on experimental infrared spectral data. Quantum chemical calculations were made of models describing the defects of the dehy-droxylated silica surface that had been thermally treated. Calculations were carried out on models describing the surface structures of water molecules interacting with silanol groups on the silica surface. 

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