DEALUMINATION OF ZEOLITES .8. ACIDITY AND CATALYTIC PROPERTIES OF HEMT ZEOLITES DEALUMINATED BY STEAMING

IR spectroscopy has been used to characterize the hydroxy groups of a series of HEMT samples dealuminated by steaming (framework Si/Al ratio s between 4.5 and 52) and their interaction with pyridine. Three OH ba nds are observed in the spectrum of the non-dealuminated sample (HEMT 4.5): at 3631 cm(-1) [assigned to (HF)OH located in the large cages]; at 3551 cm(-1) [(LF)OH located in the sodalite cages]; and at 3740 cm( -1) (terminal defect SiOH groups). A large number of new OH bands appe ar in dealuminated samples; two of them, present in mildly dealuminate d samples and located at 3599 and 3525 cm(-1) correspond to protonic s ites stronger than those corresponding to the (HF) and (LF)OH bands. T hese bands were assigned to (HF) and (LF)OH groups interacting with ca tionic extraframework aluminium species located in sodalite cages. The other bands which correspond to extraframework species or to SiOH gro ups are generally slightly acidic or non-acidic. However, a band appea ring at 3610 cm(-1) in the more dealuminated samples is due to very ac idic OH groups of silica alumina debris. With all the samples, the num ber of Br<empty set>sted sites was found to be lower than the number o f framework aluminium atoms, the difference being particularly pronoun ced for the less dealuminated samples. This can be explained by the in accessibility to pyridine of some of the (LF)OH groups, by the partial exchange of protonic sites by cationic extraframework aluminium speci es and, with HEMT 4.5, by a partial dehydroxylation during pretreatmen t. The maximum activity per protonic site for m-xylene transformation, found with the mildly dealuminated samples, was attributed to the pre sence of very strong acid sites resulting from the interaction of the OH groups with extraframework species. Transition state shape selectiv ity in the hypocages is proposed to explain the low value of the dispr oportionation/isomerization ratio and the preferential formation of 1, 2,4-trimethylbenzene found with all the HEMT samples (compared to that with HFAU zeolites).