Hydroxy groups, bridging

For the adsorption of two methanol molecules per bridging hydroxy groups (2 1 loading) studies on all zeolites, SOD [29], CHA [22, 30], FER [22] agree that a pro-tonated methanol dimer is formed (Fig. 22.3). The obvious reason is the high PA of the methanol dimer that exceeds even the PA of ammonia (Table 22.1). 

Figure 22.4 IR spectrum of and H2 0 adsorbed on H-ZSM-5 (adapted from Fig. 2 in Ref [37]). Shown is the assignment based on frequency calculations for models of the neutral adsorption complex [34]. The overtone of the in-plane SiOH bending ( zqh) hilh ooto the red-shifted OH stretching frequency of the bridging hydroxy group of the zeolite, vOH. ...

Proton loss is, in some cases, accompanied by the formation of dinuclear or polynuclear species in aqueous solution. For example, after the dissociation of from [Cr(H20)g] +, the product undergoes an intermolecular condensation (equation 6.38). The resulting dichromium species (Figure 6.7) contains bridging hydroxy groups. 

Cl2],H20 was also obtained at low pH and high chloride ion concentration however, the nature of the co-ordination in this complex could not be ascertained. [Cu3L3(0H)2]C104,2H20 was obtained at pH 8.5 and this is also probably polymeric. Both polymers have bridging hydroxy-groups. ... 

Mo(0) exhibits very little chemistry with oxygen donor ligands however, Beyerholm et al. reported NMR data for a series of Mo clusters which are held together by bridging hydroxy groups (see Table 14). Other compounds with oxygen donors are discussed above (see Section 2.11). 

Brunner [389] showed by H MAS NMR that, in complete agreement with the findings of Czjzek et al. [385], the ratio of the concentration of the bridging hydroxy groups pointing into the large cavities (Une b, HF band) to those vibrating into the six-membered rin (Une c, LF band) is 0.9 0.1. 

Fig. 28. The normal modes (v stretching, 6 in-plane, and y out-of-plane bending modes) of a bridging hydroxy group of a zeolite structure...

IR spectroscopy of adsorbed CO probe molecules was employed by Manoilova et al. [776] to characterize silanol groups, acid bridging hydroxy groups (Brpn-sted acid sites) and coordinatively unsaturated aluminum and titanium atoms (Lewis acid sites) in Ti-containing zeolites (TS-l,Ti-Beta). 

Acid properties were further detected by Romannikov et al. [778] via IR spectroscopy with and without CO adsorption in the case of zirconium-containing mesoporous materials, e.g.,ZBS mesophases with texture characteristics close to those of MCM-41. However, the acidity was ascribed to terminal Zr-OH groups rather than to bridging hydroxy groups. But Lewis acid sites, which were moreover convertible by interaction with H2O into bridging Bronsted acid hydroxy groups, were detected by, e.g., Viale et al. [779] in [Si,Al]MCM-41. These authors observed, besides silanol peaks, bands at aroimd 3688 [=Si-Al(OH)-Sis], 3610 [sSi-(OH)-Al=] and, after contact with NHj,at 1465 cm (ammonium band). 

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