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 (SiOH^supAl) 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 (Al^IV/H⁺) and aluminum cations (Al ^IV/Al^x+), 3.6-4.4 MHz for tetrahedrally coordinated framework aluminum atoms compensated by sodium cations (AI^IV/Na ⁺), and 5.6-7.6 MHz for pentacoordinated extraframework aluminum cations (Al^x+ cat.) 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)₂⁺ and AlO⁺ species 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.