Hydroxy groups of zeolites

Hydroxy Groups of Zeolites Characterized by IR Fundamental Stretching Bands... 

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...

Marie, O., Thibault-Starzyk, F. and Lavalley, J.C. (2000) Confirmation of the strongest nitriles-hydroxy groups interaction in the side pockets of mordenite zeolites, Phys. Chem. Chem. Phys., 2, 5341. 

Other covalently bonded catalysts are those on inorganic supports such as silica, molecular sieves (zeolites), and alumina, the complexes being held either directly via the oxygen of surface hydroxy groups or via an intermediate functional group, again especially phosphino. 

This would indicate that (3 has stronger acid sites than dealuminated HYD samples, something which agrees with IR data of both solids, where the wavenumber of the accesible acidic hydroxy groups is 3620-3630 cm-1 on dealuminated HY, and 3612 cm- in HJ3 zeolites (ref. 8), A similar conclusion was reached by measuring the desorption of pyridine on these two types of large pore zeolites. 

Typically the supports most widely used are alumina, silica and aluminosilicates (clays and zeolites). These inorganic solids all contain surface hydroxy groups and it can be valuable to examine activation of these surfaces by dehydration. 

The reaction was carried out in acetonitrile at 353 K using TBHP as oxidant. Conversions as high as 80 % were obtained. As shown in Scheme 6, it was postulated that the reaction takes place via epoxidation over Ti sites foUowed by acid catalyzed intramolecular opening of the epoxide ring by the 3-hydroxy group. Ti-6 zeolite gave somewhat lower conversions in addition to the preferential formation of furans over pyrans (ratio of ca. 1.5) due to shape selectivity. Ti-MCM-41 and gave furan to pyran ratios of ca. 0.9, comparable to those obtained by the epoxidase conversion of linalool. 

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). 

---