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 Bronsted acidic bridging OH groups in supercages ((SiOHAl)-Al-sup) was reached upon dehydration of zeolites AI,Na-X and AI,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 Al-27 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 (Al-IV/H+) and aluminum cations (Al-IV/Alx+), 3.6-4.4 MHz for tetrahedrally coordinated framework aluminum atoms compensated by sodium cations (Al-IV/Na+), and 5.6-7.6 MHz for pentacoordinated extraframework aluminum cations (Alx+ cat.) were obtained. Comparison of the number of AlOH groups with the number of pentacoordinated extraframework aluminum cations determined by one-dimensional high-field Al-27 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)(2)(+) and AlO+ species could exist. The enhanced acid strength of bridging OH groups in zeolites AI,Na-X and AI,Na-Y in comparison with zeolites Al,Na-X and Al,Na-Y, as found by adsorption of acetonitrile, may be due to a polarizing effect of cationic extraframework aluminum species in the vicinity of Bronsted acid sites.