The Effect of Proton Cation Exchange Level

Ion exchange is a characteristic property manifested by most molecular sieves. This property is used routinely for post synthesis modification during the preparation of molecular sieves for major industrial applications [10, 11]. 

Similar study was performed on decationated mordenites [107], which have narrower pores and a higher Si/Al ratio than Y zeolites. From microcalorimetric measurements it was found that the number of acid sites increases nearly linearly with an increase of decationation level. Growth of the exchange level increased the acid strength, and removal of the last NH ions resulted in the formation of strong sites (Odiff 130 kJ moP ). This increased concentration of strong acid sites explained that mordenites are more active in the cracking of n-octane than HY zeolites. However with bulkier reactants such as isooctane and ethylbenzene, the difference in catalytic activity decreases due to shape selectivity effects. 

The acidic properties of a series of X faujasites exchanged with Li, Na, K, Rb and Cs have been studied by adsorption microcalorimetry, using ammonia as acidic probe. The heats of NH3 adsorption were found to decrease in the sequence from Li to Cs (Fig. 9.12). 

Li and Na zeolites presented much higher heats of NH3 adsorption and greater coverage at the same pressure in comparison with the other zeolites. The acid-base properties of alkali-metal ion exchanged X and Y zeolites have also been investigated by ammonia and sulphur dioxide adsorption microcalorimetry, in parallel with the study of a catalytic reaction, viz. 4-methylpentan-2-ol conversion [111]. 

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