DEALUMINATION OF ZEOLITES .7. INFLUENCE OF THE ACID TREATMENT OF A HBEA ZEOLITE ON THE FRAMEWORK COMPOSITION AND ON THE POROSITY

The dealumination of a HBEA sample (total and framework Si/Al ratios o f 11.15 and 15.5 respectively) by treatment with a chlorhydric acid so lution was investigated under various operating conditions : HCl conce ntrations of 0.1, 0.5, 1.0 and 1.5 N, temperatures of 30, 60 and 100 d egrees C, times between 5 minutes and 6 hours. Examination by IR spect roscopy of hydroxyl groups in the parent zeolite and in a series of de aluminated samples shows that tile acid treatment causes a preferentia l elimination of the extraframework aluminium species and of transient -state framework aluminium species. A linear correlation was obtained between the wavenumber of the asymmetric stretching vibration band aid the number of framework aluminium atoms of the most dealuminated samp les which did not contain extraframework aluminium species. This corre lation can be used to determine the composition of the zeolite framewo rk of HBEA samples dealuminated by acid treatment. The parent zeolite presents a large mesopore volume due to intercrystalline voids created by agglomeration of the very small zeolite crystallites. The acid tre atment causes a slight decrease in the microporous volume but also a s ignificant decrease In the mesopore volume, which indicates a reorgani zation of the zeolite crystallites. Two periods can be distinguished i n tile dealumination of the HBEA framework. The initial period during which the dealumination is very rapid corresponds to the easy eliminat ion of the Al species partially disconnected from the framework. The s econd period which corresponds to the elimination of the normal framew ork aluminium species is much slower. For both periods, the reaction o rder with respect to the number of framework aluminium species is clos e to 1 and the effect of temperature shows the existence of diffusion limitations. From the results of the kinetic study, operating conditio ns can be chosen so as to prepare HBEA samples with a desired framewor k Si/Al ratio, a good crystallinity and no extraframework aluminium sp ecies.