Characterization and Acidic Properties of Aluminum-Exchanged Zeolites X and Y

Zeolites Al,Na−X and Al,Na−Y with defined numbers of extraframework aluminum cations were prepared by exchange in an aqueous solution of aluminum nitrate. A maximum concentration of Brønsted acidic bridging OH groups in supercages (SiOHsupAl) was reached upon dehydration of zeolites Al,Na−X and Al,Na−Y at 423 K. Further raising of the dehydration temperature led to a dehydroxylation of zeolites due to the recombination of aluminum hydroxyl groups with hydroxyl protons of bridging OH groups. High-field 27Al multiple-quantum magic-angle spinning (MQMAS) NMR spectroscopy was utilized to study zeolites Al,Na−X/61 and Al,Na−Y/63 dehydrated at 423 K. Second-order quadrupolar effect parameters of 10.1−11.0 MHz for tetrahedrally coordinated framework aluminum atoms, compensated in their negative charge by hydroxyl protons (AlIV/H) and aluminum cations (AlIV/Alx), 3.6−4.4 MHz for tetrahedrally coordinated framework aluminum atoms compensated by sodium cations (AlIV/Na), and 5.6−7.6 MHz for pentacoordinated extraframework aluminum cations (Alxcat.) were obtained. Comparison of the number of AlOH groups with the number of pentacoordinated extraframework aluminum cations determined by one-dimensional high-field 27Al MAS NMR spectroscopy gave a ratio near 1:1. This finding and the five-fold coordination of the cationic extraframework aluminum species hint to the presence of HO−Al−O−Al−OH compounds, but also a minor number of Al(OH)2and AlOspecies could exist. The enhanced acid strength of bridging OH groups in zeolites Al,Na−X and Al,Na−Y in comparison with zeolites H,Na−X and H,Na−Y, as found by adsorption of acetonitrile, may be due to a polarizing effect of cationic extraframework aluminum species in the vicinity of Brønsted acid sites.