EFFECT OF ACID AND STEAM TREATMENT OF NA-MORDENITE AND H-MORDENITE ONTHEIR STRUCTURAL, ACIDIC AND CATALYTIC PROPERTIES

A systematic study of the effect of acid washing of sodium mordenite ( Na-MOR) and hydrogen-exchanged mordenite (H-MOR), both before and afte r steaming, has been carried out. The study investigated the formation and removal of extra-framework aluminium and the effect of dealuminat ion on the acidity and isobutane cracking activity of the mordenite. W ashing ammonia-exchanged mordenite (NH4-MOR), which has been calcined at 550 degrees C, with deionised water resulted in an increase in extr a-framework octahedral aluminium (EFAl-o). The corresponding decrease in tetrahedral aluminium and the sharp drop in catalytic activity is a scribed to the hydration of strong acid sites. Both acid washing and t hermal treatment of Na- and NH4-MOR produced EFAl-o. A systematic stud y of the washing of Na- and H-MOR with nitric acid of increasing conce ntrations showed that a maximum occurrence of EFAl-o was reached by wa shing with 0.001 N acid in the case of H-MOR. Thereafter EFA1 was remo ved with little accompanying framework dealumination up to an acid was h concentration of 1 N. Further washing up to 10 N removed large amoun ts of framework aluminium. A maximum in catalytic activity for isobuta ne conversion was observed for a sample washed with 1 N acid. Generall y, however, catalytic activity increased with the number of framework acid sites. Extra-framework tetrahedral aluminium (EFAl-t) and penta-c oordinated aluminium (Al-p), both produced by steaming, reduced both t he catalytic activity and the number of framework acid sites. These al uminium species were progressively removed by increasing strength of a cid washes. The number of Bronsted and strong Lewis acid sites as dete rmined from ammonia temperature-programmed desorption (TPD) were found to be essentially equal to the amount of framework aluminium not char ge-balanced by non-framework cations such as sodium and tetrahedral ex tra-framework aluminium.