Mordenite acid-base properties

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

As indicated above (Table I), 2- and 3-valent cation forms of mordenite are considerably less active in cyclohexane isomerization than that of hydrogen. It is this property of mordenite-supported catalysts that distinguishes them sharply from zeolites of the faujasite type. Thus, in the case of faujasites, the activity of H-form (decationized form) and 2-valent modifications (such as Ca and Mg ) in acid-base reactions (cracking, isomerization, alkylation) is the same, while H-mordenite is many times as active as any cation form under study. 

Cg Aromatic Reactions with Hydrogen. The mild acid nature of the family of aluminophosphate based sieves renders them selective for a number of rearrangements as observed in the reactions of olefins and paraffins described above. This property as well as their apparent low disproportionation activity observed in the alkylation of toluene suggests that they be evaluated as the acid function in bifunctional Cg aromatic isomerization. As described above, cyclo-olefins are most likely involved in the conversion of ethylbenzene to xylenes. Strong acid functions, such as in mordenite, actively isomerize cyclo-olefinic intermediates but also catalyze ring-opening reactions which lead to loss of aromatics. A more selective acid function must still effectively interconvert ethyl cyclohexene to dimethylcyclohexenes but must leave the cyclohexene rings intact. 

In another study [138], authors presented evidence that heats of adsorption of ammonia on three different kinds of zeolite structure (Y, mordenite, ZSM-5) can be used to obtain the correlation plots that describe relationships between acidic and catalytic properties of these catalysts. Catalytic properties were tested in simple carbonium ion reactions. An approach to search acidity-catalytic activity correlations was based on the straightforward concept which implies that the heat of adsorption of the base is directly related to the energy needed for the protonation of hydrocarbon molecules leading to the carbocation formation. For example it was... 

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