Correlation Between Adsorption Heat and Catalytic Activity

3 Correlation Between Adsorption Heat and Catalytic Activity 

Finding correlations in between catalysts surface properties and catalytic behaviour is the main reason to study adsorption of probe molecules on catalysts surfaces. If the probe molecules are chosen to resemble possible reaction intermediates of the catalytic cycle, measurements of heat of adsorption of such probe molecules can provide essential information about reaction mechanisms. 

In a study performed by Auroux et. al. [Ill] microcalorimetry experiments of ammonia and sulfur dioxide were performed in order to analyze the possible correlations between the acidity and basicity of the alkali-metal ion-exchanged X and Y zeolite structures and their catalytic properties. The catalytic results for the 4-methylpentan-2-ol conversion show that activity and selectivity are both affected to some extent by the acid-base character of the catalysts. The activity was found to increase in order Cs Rb K Na Li for both X and Y zeolites. The dehydrogenation reaction occurs only on CsX + CS2O, which presents very strong basicity. The product selectivity of the reaction depends on both Lewis acidity and basicity Lewis basic or acidic sites of zeolites can be considered as acid-base pairs, in which both framework basic oxygens and neighboring cations are important. The selectivity ratio between the 1-alkene and (2-alkene+isomers) increases linearly with the ratio between basic and acid sites number, so2/ nh3, for both X and Y zeolites, as shown in Fig. 9.14. 

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 

In a study on ZSM-5 based materials [139], Nicolaides et. al. have shown that the number of Brdnsted acid sites, determined by adsorption mlcrocalorimetry of ammonia, increases with increasing XRD cristallinity. A strong correlation was observed between the catalytic activity of these materials, in n-hexane cracking reaction, and the number of strong acid sites, as presented in Fig. 9.16. 

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