BROAD-LINE AND HIGH-RESOLUTION NMR-STUDIES CONCERNING THE HYDROXONIUM ION IN HZSM-5 ZEOLITES

Bronsted sites must be able to react with adsorbed water molecules to form hydroxonium ions. This study shows by line-shape simulation of the H-1 NMR signal measured at 4 K that for water concentrations about equal to the concentration of bridging hydroxyl groups in defect-free HZSM-5 zeolites two configurations exist: water molecules hydrogen-bonded to the bridging OH group and the hydroxonium ion. The results is confirmed by H-1 MAS NMR investigations at room temperature. In addition, Al-27 and Si-29 MAS NMR were used to characterize HZSM-5 zeolites under study synthesized without template, with TPA+, with butylamine as template. All samples contain 4-bridging hydroxyl groups per unit cell. For the zeolite synthesized with butylamine and loaded with 2, 4, and 8.4 water molecules per unit cell the concentration of hydroxonium ions per unit cell is 0.28, 0.64, and 1.56, respectively.