Strong acidity of silanol groups of zeolite beta: Evidence from the studies by IR spectroscopy of adsorbed CO and 1H MAS NMR

Acidic OH groups of acid-form and Zn and Ga-modified zeolite beta (BEA) have been characterized with H-1 MAS NMR and by IR spectroscopy of adsorbed carbon monoxide. It is demonstrated that OH groups, which exhibit a vibration band at 3740 cm(-1), reveal acidity, which is similar to that of the OH groups with the band at 3610 cm(-1) according to the value of the low frequency shift of OH vibrations with adsorbed CO (Delta nu(OH/CO) = 300 cm(-1)). The IR band 3740 cm(-1) corresponds to the signal at ca. 2.1 ppm in H-1 MAS NMR spectrum. The OH groups with signal 2.1 ppm are involved in H/D exchange with methane-d(4) similar to acidic OH groups with the signals 4.0-5.1 ppm. The signals at 2.1 ppm in H-1 MAS NMR and at 3740 cm(-1) in IR are attributed to the strongly acidic silanol groups of the faulted structure of the zeolite. The silanols with the signals at 1.8 ppm and 3745 cm(-1) are weakly acidic (Delta nu(OH/CO) = 85 cm(-1)) and are not involved in the H/D exchange. Loading of the zeolite with Zn affords a notable decrease of the concentration of strongly acidic SiOHAl groups, whereas the quantity of these groups does not decrease upon loading the zeolite with Ga. (C) 2009 Elsevier Inc. All rights reserved.