Dissociative adsorption of water in CaNaA zeolites studied by TG, DRIFTS and 1H and 27Al MAS NMR spectroscopy

Three CaNaA zeolites, in which, respectively, 49%, 65% and 83% of the Na+ cations were exchanged with Ca2+ ions, along with a NaA sample were studied by thermogravimetry, temperature-programmed DRIFT spectroscopy and MAS NMR. Thermogravimetric desorption profiles of the Ca2+ exchanged zeolites show a low-temperature peak between 423 and 508 K as well as two high-temperatures peaks at 853 and 973 K, which are missing in case of the NaA sample. Three main bands at 3603-3611, 3495-3510 and 3552-3565 cm−1, of which the first two are due to bridging Si-OH-Al groups located in 8- and 6-rings, respectively, whereas the third corresponds to Ca(OH)+ groups dominate the DRIFT spectra of zeolite samples exchanged with Ca2+ ions. The intensity of the band at 3603-3611 cm−1 decreases significantly until it vanishes almost completely at 823 K. A simultaneous decreases of the intensity of the band of Ca(OH)+ groups is indicative of a low-temperature dehydroxylation process between these two species. The intensity of the other Si-OH-Al group bands at 3495-3510 cm−1 shows almost no change up to 823 K, thus suggesting that these groups participate in a high-temperature dehydroxylation process. A weak band in the region 3670-3680 cm−1 is attributed to Al-OH groups of extra-framework aluminum species formed during dehydration of the CaNaA zeolites. Peaks at 4.3 and 5.2 ppm in the 1H MAS NMR spectra are assigned to Si-OH-Al groups in 8- and 6-rings, respectively. 27Al MAS NMR spectra show two main peaks at 57.3 and 78.7 ppm, which are due to tetrahedrally coordinated aluminum and calcium alumosilicate groups. A weak signal between 12 and 13 ppm ascribed to octahedrally coordinated aluminum disappears at higher temperatures.