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 1H 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 (ΔVOH/CO = 300 cm-1). The IR band 3740 cm-1 corresponds to the signal at ca. 2.1 ppm in 1H MAS NMR spectrum. The OH groups with signal 2.1 ppm are involved in H/D exchange with methane-d4 similar to acidic OH groups with the signals 4.0-5.1 ppm. The signals at 2.1 ppm in 1H 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 (Δ VOH/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 SiOHAI groups, whereas the quantity of these groups does not decrease upon loading the zeolite with Ga.