Zeolites basicity

Thus, methylacetylene and H2S lead to a classification of the zeolite basicity NaLSX < KLSX < CsLSX that again, well accounts for the change of the negative charge of the basic framework oxygen atoms with change of the nature of the cations. 

Blaszkowski et al. (221) demonstrated that the methanol molecule is capable of adsorbing in a physisorbed state in two different modes, the end-on mode, shown in the first part of Fig. 12, and a side-on mode, shown in Fig. 13a. In this side-on mode, a C-H bond of the methanol CH3 group is directed toward the zeolitic basic oxygen site, while the acidic zeolite proton retains its strong hydrogen bond with the methanol oxygen. The authors used TST (4) to determine the equilibrium constants for the two modes of adsorption from the computed adsorption energies. The equilibrium constant for the side-on mode is a factor of 106 smaller than that for the end-on mode at 300 K. Thus, nearly all methanol molecules adsorb in an end-on manner, but the dehydration reaction necessitates conversion to the side-on form. 

Zeolite basicity can be further increased by the deposition of metallic clusters with excess electrons in the cages [39]. While there are many procedures to form clusters such as Na43+ or Na65+, one of the best accessible procedures seems the thermal decomposition of zeolite-adsorbed NaN3. In an alternative procedure, ammoniacal metal solutions can be used for impregnation of the zeolite (40). 

Different charge-compensating cations in zeolite L have been tested for their promotional effect in n-hexane aromatization. Apparently, high basicity of the alkaline and alkaline earth promoter favors n-hexane aromatization. Basicity and selectivity both increase from Li and Cs 331) and from Mg to Ba (22,25). Bezouhanova et al. studied the FTIR bands of linearly adsorbed CO in the range of 2060-2075 cm . One band at 2075 cm", which is also found on unsupported Pt, is attributed to extrazeolite Pt particles, a second band shifts from 2060 cm" for Li to lower wavenumbers with K and Rb 331). Another criterion, used by Larsen and Haller, is the measured rate of competitive hydrogenation of benzene and toluene, which has been found to correlate with the zeolite basicity (25). As described in a previous section, this method had previously been used by Tri el al. to probe for the electron deficiency of Pt particles in acidic zeolites 332). The rate data are analyzed in terms of a Langmuir-Hinshelwood model and the ratio of the adsorption coefficients of toluene and benzene, A, /b, is determined. It was found to decrease from 8.6 for Pt/Si02, and 5.4 for Pt/MgL, to 4.4 for Pt/BaL. As direct electron transfer from the cations to neutral Pt particles is unlikely, an interaction of Pt with the zeolite framework or with... 

Photoelectron spectroscopy (XPS) and other surface analysis XPS of zeolites Acidity in zeolites Basicity in zeolites... 

In this paper, we will focus on the zeolite basic catalysts, other solid superbases will be considered in detail elsewhere [6]. 

Hunter and Hutchings [33] approached this issue by examining the reaction of TM0+SbClg" with lithium tetraisopropoxide, which they regarded as a reasonable model of the zeolite basic site, with isopropoxyl replacing siloxyl groups. No MeOEt was detected, but only products of isopropoxide decomposition, or nucleophilic chloride and isopropoxide attack. 

A variety of porous 3-dimensional (3-D) 4-connected AIPO4 nets have been synthesised, some of which have the same topologies as their zeolite counterparts while others have novel topologies [13, 14]. As with porosils and silica-rich zeolites, basic organic species are involved in their synthesis, to occupy channels and cavities as space fillers and stabilisers. AlPO s have... 

XPS and IR indicate that Pd is electron deficient in acid zeolites, and has an electron excess in alkaline zeolites EXAFS gives some evidence of this in PtHL.104 Pt particles outside the channels are not significantly modified by the zeolite basicity, and hence give unselective aromatisation, like Y zeolite.174... 

It should be remarked that for very strong zeolite basic catalysts the presence of CO2 and H2O should be avoided in any stage of preparation and reaction, because these lead to rapid catalyst deactivation. 

Metal Species and Oxide Clusters Encapsulated in Zeolites Basicity and General Aspects... 

FIGURE 9.16 Two-dimensional representation of zeolite basic framework. 

Comparison of CH3OH adsorption on the first two models allows one to evaluate the effect of the A1 content and of the zeolite basicity on the adsorption mode. The third model was used to analyze the influence of the type of the alkali cation that also affects the basicity of the zeolite O atoms [145]. 

Figure 10 Dehydrocyclization of n-hexane to benzene at 733K on Pt-MLZeolites. Influence of zeolite basicity (45,46).

This work reviews the use of pyrrole, chloroform, methanol, as well as methoxy and nitrosonium groups generated in situ , as infrared and NMR probe molecules to characterize zeolites basicity. ... 

Because of the negatively charged framework of zeolites, basic framework hydroxyl groups do not exist [55]. Moreover, positively charged frameworks have not been reported for molecular sieve structures so far. Basic hydroxyl... 

External templates were used to structure silicates and generate porous materials for the first time in the early 1990s by independent groups in Waseda University in Japan [7] and at Mobil in the United States [8]. These syntheses took place under hydrothermal conditions similar to those for the synthesis of zeolites basic media and in the presence of cationic surfactants such as cetyltrimethylam-monium bromide. After this pioneer work, numerous silicates have been synthesized using a wide variety of surfectants. They have allowed the preparation of many mesoporous silicates such as MCM-41 with a hexagonal pore structure, MCM-48 with a cubic structure, and MCM-50 with a lamellar structure. 

The influence of the basicity of the zeohte and nature of the alkaU-metal cations on the decomposition of Pd(NH3) + cations and on the redudbility of Pd + cations was recently investigated by Sauvage et al. [187] using TPO, TPR and UV-VIS-near IR spectroscopy. These studies showed that the decomposition of the complexes and Pd + cation reduction occurred at lower temperatures when the zeolite basicity increased. 

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