Zeolites bridging hydroxyls

Taking into account the above discussion, the substantial heterogeneity of zeolite bridging hydroxyls that is inferred in many articles could be, at least in part, an artifact related to the probe, which produces multiple bands for steric reasons and/or Fermi resonance. However, benzene unambiguously confirms the heterogeneity of the silanols in sUica—alumina materials. 

Operando DRIFTS measurements suggest that bridged hydroxyl groups are in extensive interaction with hexane molecules during the reaction even at 553 K. However adsorbed alkene or surface alkoxide could not be detected. These findings questions, whether the Haag-Dessau mechanism [4] gives true description of the alkane activation process over zeolite catalysts. 

By measuring the shifts of the various hydroxyl bands of the zeolite, a direct measure of the relative acid site strengths can be made without the need for thermal desorption. Table 4.6 lists the measured hydroxyl band shifts for a variety of hydroxyl groups on different zeolites using low temperature CO adsorption. This data indicates that there is indeed a difference in the intrinsic acid strength of the bridging hydroxyl groups in different zeolites as well as in the same zeolite structure with different framework aluminum content. 

In this review, we focus on cluster models of Br0nsted acid sites, bridging hydroxyl groups that result from the incorporation of trivalent aluminum atoms into the siliceous framework during synthesis. These sites are by no means the only active sites within zeolites, but they are among the best characterized. 

For the first time, a systematic uniform ab initio study of endohedral complexes of C60, Si60 and Ge60 with monoatomic cations (Li+, Na+, K+ and Rb+) and anions (F-, Cl% Br and P) was performed. The 3-21 G level (3-21+G for the anions) was used. This could be achieved by application of the multiplicative integral approximation [86], as implemented in the program BRABO [87]. The sequence studied for the cages bears a close resemblance with isomorphic substitution in zeolites modifying its catalytic activity via bridging hydroxyls. 

Beran S (1984) Quantum chemical study of the effect of the structural characteristics of zeolites on the properties of their bridging hydroxyl groups, J Mol Catal 26 31—36... 

Bronsted acidity, characteristic of H forms of zeolites, is associated with bridged hydroxyl groups. Another type of center is represented by the cations in the cationic forms of zeolites. They are mainly located within 4- and 6-membered rings. Just these two types of sites have comprised the principal subject of quantum-chemical studies. They will be discussed below. 

A simple cluster model of a bridged hydroxyl group in a zeolite is cluster 3. Such a cluster with A = H was used by Chuvylkin et al. (70) as early as 1975 to discuss the properties of possible intermediate structures in the catalytic isomerization of butenes on aluminosilicate surfaces in terms of CNDO/2 approximation. Mikheikin et al. (34) have used a similar cluster with terminal pseudo-atoms A to study the Bronsted acidity of zeolites and its dependence on the Si/AI ratio. 

Extraframework aluminum species created by mild steaming were shown to increase the catalytic activity of zeolites. This increase in activity was ascribed to the creation of sites exhibiting enhanced acidity through interaction of bridging hydroxyl groups (BrOnsted sites) and neighboring small extraframework aluminum species (Lewis acid sites) (16). Their exact nature is still a matter of debate, but is schematically represented as ... 

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