Zeolites acid-base character

Two important zeolite properties are (1) the intra-pore electrostatic field, and (2) its acid-base character. As discussed below post-synthetic modifications of many zeolites to fine-tune these properties are possible and provide a unique opportunity to influence reaction outcome. 

As described above, immersion calorimetry constitutes a powerful technique for the textural and chemical characterization of porous solids. In the absence of specific adsorbate-adsorbent interactions, heats of immersion can be related to the surface area available for the molecules of the liquid. However, the use of polar molecules or molecules with functional groups produces specific adsorbent-adsorbate interactions related to the surface chemical properties of the solid. An adequate selection of the immersion liquid can be used to study hydrophilicity, acid-base character, etc. Table 2 reports the enthalpies of immersion (J/g) into different lineal and branched hydrocarbons (n-hexane, 2-methyl-pentane and 2,2-dimethyl-butane) for Zn exchanged NaX zeolites. 

Zeolites are crystalline aluminosilicates materials that possess ordered and interconnected microporous channels with diameters ranging from 0.2-20 A. Their unique properties (microporosity, high surface area, acid-base character, shape) have made than a material of choice in a great number of applications. Zeohtes are intensively used in gas separation due to their ability to adsorb selectively a large variety of molecules and are also known as molecular sieves. Furthermore, these materials are also used as ion exchangers (water softeners) and catalysts in petrochonistry. Currently, the world s annual production of natural zeoUte is about 4 million tons. Of this quantity, 2.6 milUon tons are shipped to Chinese markets to be used in the concrete industry. The amount of synthetic zeohtes produced is about 1.5 miUion tons (Figure 5.1). 

In another study, two series of alkali-metal ion-exchanged zeolites have been investigated in order to analyze the possible correlations between the acidity and basicity of the X and Y zeolite structures and their catalytic properties [90]. The catalytic results for the conversion of 4-methylpentan-2-ol show that the activity and selectivity are both affected to some extent by the acid-base character of the catalysts. The main reaction that takes place is dehydration, giving 4-methylpent-l-ene and 4-methylpent-2-ene in variable amounts. Skeletal isomers of Ce-alkenes were also formed in some cases. Simultaneous dehydrogenation to 4-methylpenten-2-one may also occur. 

Alumina is an amphoteric catalyst, which can difficult to characterize via chemical and physic methods. The transformation of cyclopentanol/cyclohexanone mixture allows us to estimate at the same time the acid-base properties of aluminas. From this transformation, it was shown that aluminas can be classified into two families only basic aluminas, such as theta, which were more basic than MgO, and acido-basic aluminas, eta, gamma and delta, which possess an acidic character less pronounced than dealuminated HMOR zeolite... 

EF material free, alkali exchanged zeolites are used as quite mild basic catalysts. Light alkali and alkaline earth metal zeolites, such as Na-X, Na-Y [165], alkali-MOR, Na-A and Ca-A [166], have a mild Lewis acid behavior and do not appear to have strong basic character. The same occurs for Na-silica-alumina [167]. However, heavy alkali metal zeolites such as Cs-Y actually act as base catalysts, or rather as acid-base catalysts, for example for toluene side-chain alkylation. Stronger basic character arises from impregnation of alkali zeolites with alkali salts, later... 

In zeolite cages the atoms accessible to adsorbates are either the cations (protons, alkaline, alkaline-earth. .. cations) or the framework oxygen. The first species have an acidic character, protonic or Lewis, and the second ones are basic. It follows that, as in most oxides, acids and bases form conjugate acid-base pairs in zeolites (62). 

Several publications have been dedicated to the study of In-based zeolites as catalysts in different catalytic processes (e.g., SCR of NO, alkylaromatic transformation, etc.) and the role played by the acidic character in their good performances was followed mostly by IR technique [256-259] or NH3-TPD [260], It was observed... 

Whereas the number of Brpnsted acid sites can be easily determined,146 149 their acidity may vary depending on their position in the framework and also by interaction with Lewis acid sites. For these reasons and despite the various techniques tested, there are no general and reliable methods to measure the acidity of solid acids. The synthesis of active acidic zeolites used in industry still relies on a very empirical base, the most important character being their catalytic activity. 

As described above for the amorphous aluminosilicates the acid strength of a zeolite increases with increasing Si Al ratios. Species with particularly high ratios have been prepared by removing some of the aluminum by reaction with materials such as silicon tetrachloride. On the other hand, the basic character of these materials increases with increasing numbers of A104 species so aluminum-rich zeolites are better bases, particularly when the Na" " is replaced with a large cation such as Cs. 52... 

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