Hydroisomerization and hydrocracking activity enhancement of a hierarchical ZSM-5 zeolite catalyst via atomic layer deposition of aluminium

Abstract

Desilication of a ZSM-5 zeolite in basic medium creates intracrystalline mesoporosity advantageous for catalysis, but a decrease in acidity often limits the gain by improved accessibility of sites. Atomic layer deposition of aluminium (Al-ALD) using cycles of reaction and hydrolysis of trimethylaluminum was applied to desilicated ZSM-5 samples to enhance acidity. Framework as well as extra-framework aluminium species were introduced. Removal of extra-framework aluminium via mild acid leaching was beneficial to the acidity. The Al-ALD modified ZSM-5 specimen showed improved dispersion of platinum introduced via impregnation of a Pt(NH₃)₄Cl₂ precursor followed by oxidation and reduction. Hydroisomerization and hydrocracking performance was very sensitive to the ZSM-5 zeolite modification with Al-ALD. The hierarchical sample from which the excess of deposited aluminium was removed showed the best penetration of platinum to the interior and was most active. It outperformed the original ZSM-5 zeolite in activity and hydroisomerization selectivity. Alumina deposited by Al-ALD on the external surface of the ZSM-5 zeolite particles is a preferential location for platinum deposition. Such a catalyst with externally positioned Pt was most selective for hydroisomerization.