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 of 11.15 and 15.5 respectively) by treatment with a chlorhydric acid solution was investigated under various operating conditions : HCl concentrations of 0.1, 0.5, 1.0 and 1.5 N, temperatures of 30, 60 and 100 degrees C, times between 5 minutes and 6 hours. Examination by IR spectroscopy of hydroxyl groups in the parent zeolite and in a series of dealuminated samples shows that the acid treatment causes a preferential 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 samples which did not contain extraframework aluminium species. This correlation can be used to determine the composition of the zeolite framework 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 treatment causes a slight decrease in the microporous volume but also a significant decrease In the mesopore volume, which indicates a reorganization of the zeolite crystallites. Two periods can be distinguished in the dealumination of the HBEA framework. The initial period during which the dealumination is very rapid corresponds to the easy elimination of the Al species partially disconnected from the framework. The second period which corresponds to the elimination of the normal framework aluminium species is much slower. For both periods, the reaction order with respect to the number of framework aluminium species is close to 1 and the effect of temperature shows the existence of diffusion limitations. From the results of the kinetic study, operating conditions can be chosen so as to prepare HBEA samples with a desired framework Si/Al ratio, a good crystallinity and no extraframework aluminium species.