Dealumination of zeolites Part VIIIAcidity and catalytic properties of HEMT zeolites dealuminated by steaming

Abstract

IR spectroscopy has been used to characterize the hydroxy groups of a series of HEMT samples dealuminated by steaming (framework Si/Al ratios between 4.5 and 52) and their interaction with pyridine. Three OH bands 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 appear in dealuminated samples; two of them, present in mildly dealuminated samples and located at 3599 and 3525 cm^-1, correspond to protonic sites stronger than those corresponding to the (HF) and (LF)OH bands. These bands were assigned to (HF) and (LF)OH groups interacting with cationic extraframework aluminium species located in sodalite cages. The other bands which correspond to extraframework species or to SiOH groups are generally slightly acidic or non-acidic. However, a band appearing at 3610 cm^-1 in the more dealuminated samples is due to very acidic OH groups of silica alumina debris. With all the samples, the number of Brønsted sites was found to be lower than the number of framework aluminium atoms, the difference being particularly pronounced for the less dealuminated samples. This can be explained by the inaccessibility to pyridine of some of the (LF)OH groups, by the partial exchange of protonic sites by cationic extraframework aluminium species and, with HEMT 4.5, by a partial dehydroxylation during pretreatment. The maximum activity per protonic site for m-xylene transformation, found with the mildly dealuminated samples, was attributed to the presence of very strong acid sites resulting from the interaction of the OH groups with extraframework species. Transition state shape selectivity in the hypocages is proposed to explain the low value of the disproportionation/isomerization ratio and the preferential formation of 1,2,4-trimethylbenzene found with all the HEMT samples (compared to that with HFAU zeolites).

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