Bridging hydroxy groups zeolites

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. ...

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). 

It is unanimously recognized that the bridging hydroxy groups Al-(OH)-Si, which are located in the walls of the zeolitic cavities constitute the strong acidic sites... 

According to several authors in the 1990s [5,210], the active site for silica-alumina and protonic zeolites is the same (i.e., it is constituted by the bridging hydroxy groups bonded to a silicon and an aluminum atom) in contrast to the fact that the catalytic activity of protonic zeolites in proton-catalyzed reactions is by far higher than that of silica-aluminas. 

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