Bridged hydroxyl groups

Hill, J.-R., Freeman, C. M., Delley, B., 1999, Bridging Hydroxyl Groups in Faujasite Periodic vs Cluster Density Functional Calculations , J. Phys. Chem. A, 103, 3772. 

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. 

Breit-Winger-Fano representation of edge structure, 34 261 Brewer-Engel theory, 38 62-63 Bridged hydroxyl groups, 34 159-174 acidity versus... 

DRIFT-IR Spectroscopy—Study of Surface Hydroxyl Groups DRIFT-IR spectroscopy at 298 K of self-supported wafers in the region 3500 to 3900 cm indicated the presence of four different types of terminal and bridged hydroxyl groups... 

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. 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

Fig. 20. Schematic diagram of indirect carboxylate-zinc interactions through bridging hydroxyl groups, as observed in the (a) zinc proteases and (b) carbonic anhydrases.

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. 

Both displacement and condensation reactions occur when M(MeCOCHCOMe)4 (M = Zr, Hf) is treated with (3)-l,2- or (3)-l,7-B9C2Hff. The cation in the product [M4(OH)n(MeCOCHC-OMe)4]+ B9C2Hf2 is proposed to contain terminal acetylacetonate and bridging hydroxyl groups.269... 

Meradotos and Barthomeuf (31) found enhanced activity in hydrothermally treated mordenites, which they accredited to bridging hydroxyl groups interacting with extra-framework Al. Lago et al. (32) found a large increase in the activity of HZSM-5 after mild steaming at 540°C, which they attributed to paired Al atoms. 

Figure 5.1 shows the estimated distribution of free and bridged hydroxyl groups as a function of the pretreatment temperature for Kieselgel 60, as obtained by the numerical band integration of the respective FTIR spectra. 

The relative coverage of the free hydroxyls as a function of temperature is equal to one minus the relative coverage of the bridged hydroxyl groups. 

As calculations show, the activation energy of dehydrogenation from bridge hydroxyl group of nanocrystal zirconia particle change at anion doping. 

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... 

On the other hand, the bands at 490 and 570 nm can be attributed to the radiative decay process of two types of photoexcited Cu(I) ion, a and // respectively, formed upon reduction of Cu(II) ions introduced by exchange of the protons of bridging hydroxyl groups (Si-OH-Al). Cu(I) ions of type j> are assumed to be coordinated by -Si0(A10P0A10) Si-groups with relatively large values of n as compared to Cu(I) ions of type a. 

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