The role of sodium montmorillonite on bounded zeolite-type catalysts

摘要

Zeolites with different pore structure and Si/Al ratio (ZSM--5 and Mordenite) have been pelletized with a cheap and abundant clay (sodium montmorillonite). Changes in acid properties of final catalysts have subsequently been analysed. The concentration of the binder varied between 35 and 65/100 wt. Zeolites bounded with montmorillonite present high sodium concentration (which is found to influence in weak acidity measurements) and a lower number of strong sites than unbound catalysts. This is due to partial solid state ion-exchange between Na~+ in the clay and H~+ in the zeolite matrix. Thus, additional H~+ exchange steps are necessary if agglomerated catalysts are to be used under the same conditions than unbound zeolites. Changes due to blocking of channels have been rejected since pore size and specific surface data in bound catalysts are in agreement with those calculated from the information of the parent zeolites HZSM-5, H-Mordenite and H-Montmorillonite. Reaction tests confirm that the amount of clay in the final catalyst influences the initial activity and the product selectivity. A lower isomerization and a higher disproportionation activity was found in Mordenite based catalysts when the strong acid site density is increased. For ZSM-5 based catalysts. typical cracking products were obtained, and no major changes in the product distribution were observed, since in contrast to large-pore zeolites like mordenite, medium-pore zeolites such as HZSM-5 have reduced hydrogen transfer activity.