Zeolites water chemisorption

Strained Bonds on Oxides and Zeolites. Many dry oxides and silicates, including zeolites, contain oxide linkages which are highly "strained . Such strain can be relieved by chemisorption of water, ammonia, methanol, etc., which opens strained linkages by... 

The catalytic activity of aluminosilicate zeolites and aluminas appears to be directly related to the concentration of 30 ppm sites present, leading to the development of super-five materials displaying large NMR signals at this position (Wood et al. 1990). Since catalysis depends on the chemical nature of the Al at the surface, which may not be the same as in the bulk, cross-polarisation experiments between H and Al have been used to distinguish between the surface and bulk species (Coster et al. 1994). Since chemisorption of water provokes extensive surface reconstruction, a more suitable proton source for the CP experiments was found to be ammonia adsorbed on the surface. The results showed the presence of two kinds of surface Lewis sites associated with the non-framework Al (a tetrahedral site at ca.58 ppm with a xq of about 6 MHz, and an Af site at ca. 40 ppm with a slightly smaller xq)- Lewis sites either... 

Carbenic mechanisms. Venuto and Landis [10] were the first to address the question of mechanism of hydrocarbon formation from methanol over zeolites, in this case zeolite X [11]. These workers proposed a scheme involving a-elimina-tion of water and polymerization of the resultant methylcarbenes to olefins. Swabb and Gates [12], elaborating on Venuto-Landis, proposed that concerted action of acid and basic sites in the zeolite (mordenite) facilitates a-elimina-tion of water from methanol. According to Salvador and Kladnig [13], carbenes are generated through decomposition of surface methoxyls (a-el imination of silanol) formed initially upon chemisorption of methanol on the zeolite (zeolite Y). Hydrocarbons are assumed to form, in the latter two schemes, also by carbene polymerization. 

O—C02) are formed together with carbonate ions in some cases. The interaction is believed to result from the chemisorption of C02 in a bent configuration on surface oxygen atoms. Since the stable species are not formed on the monovalent cation zeolites, the divalent cations must promote the chemisorption. Furthermore, since the frequencies observed are a function of the exchanged cation, the adsorption must occur in the vicinity of the cations. Adsorption of water and ammonia removed the 1575 and 1380 cm-1 bands and formed bands at 1620 and 1450 cm"1, respectively. The carbonate bands were restored by evacuation at 300°C. Benzene had no effect. The observations are believed to reflect hydrogen bonding. 

Chemisorption of COj leads to carbonate structures. Carbonate formation is accelerated by preadsorbed water (cf. also [543]). According to Boese et al. [608], the assignment of the corresponding bands is difficult because of band overlap. However, the authors tentatively attributed a band pair found with CO2 on Nai jLiio.y-A at 1660 and 1365 cm to a bidentate structure and a second one at 1588 and 1421 cm to a monodentate structure (cf. [543,641]). From alkali ion-exchanged zeolites, carbonates could be removed by pumping at elevated temperatures, which was, however, not possible in the case of alkaline earth-exchanged zeolites. 

Though a clear decision between physisorption and chemisorption states is not always possible - examples for this are ammonia or water sorption on hydrophilic zeolites - it seems to be worthwhile to illustrate their basic differences qualitatively in Table 1.1 as follows [1.2, 1.3, 1.11] ... 

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