Modifying the acidic properties of PtZSM-5 and PtY zeolites by appropriately varying reduction methods

Abstract

PtZSM-5 and PtY catalysts were prepared by the wet ion-exchange method from their Na-forms. The modified zeolites were reduced by either NaBH, or gaseous H, Reduction with NaBH, did not alter the original Lewis acidity of the samples (Lewis acidity due to the sodium ions), while reduction with H, generated new Bronsted and Lewis acid sites. Reduction with NaBH, did not modify the ZSM-5 crystal lattice, however, treatment with H-2 led to some structural decomposition. Thus, Lewis sites due to extraframework alumina and Bronsted sites due to hydroxyl groups of the heeled defect sites were formed. For the Y zeolite, after reduction the platinum atoms moved out from the ion-exchange positions and started to migrate toward the supercage. On their way they aggregated and the large crystallites pushed apart portions of the zeolite crystal. The detrimental effect was of a higher extent when reduction occurred with gaseous H-2. The resulting SiO2-Al2O3 supported catalyst contained many "true" Lewis sites due to extraframework alumina species as well as acidic OH groups connected to defect sites, however, it has lost shape-selective properties connected to the zeolitic structure