Complex Analysis of the Aluminum Siting in the Framework of Silicon-Rich Zeolites. A Case Study on Ferrierites

Abstract

We present a multistep method combining multispectroscopic experiments with DFT calculations to determine the complete Al distribution in silicon-rich zeolites, independent of the presence of Al–O–(Si–O)n–Al (n = 1, 2) sequences in their frameworks. 29Si MAS NMR spectroscopy is employed to confirm the absence of Al–O–Si–O–Al in the framework of silicon-rich zeolites while 27Al 3Q MAS NMR spectroscopy and DFT computations of 27Al isotropic chemical shifts serve to determine the locations of isolated Al atoms. The maximum ion-exchange capacity of zeolites for [Co2+(H2O)6]2+ reveals the presence of close Al atoms (i.e., those Al atoms which are able to balance [Co2+(H2O)6]2+ ions). Then visible spectroscopy of the bare Co(II) ion in the dehydrated zeolite of the samples with close Al is utilized to identify the locations of the corresponding Al–O–(Si–O)2–Al pairs in a ring. Subsequently, their 27Al isotropic chemical shifts are evaluated at DFT and the complete Al distribution is determined. The complete Al siting in three ferrierite samples with only isolated framework Al atoms and two ferrierites with Al–O–(Si–O)2–Al sequences was determined. Our results reveal that the Al siting in the former samples varies with the conditions of the zeolite synthesis; Al is present in three or four sites (T1b, T2, T3, and T4) depending on the sample while T1a is never occupied by Al and the concentrations of Al atoms in various T sites are very diverse. For ferrierites with both isolated and close Al atoms, isolated Al atoms occupy the T2, T3, and T4 sites and the close Al atoms are arranged in Al(T1a)–O–(Si–O)2–Al(T1a) and Al(T2)–O–(Si–O)2–Al(T2) sequences forming the α and β – 2 cationic sites, respectively. Isolated Al atoms do not occupy the T1b site and close Al atoms do not form Al(T4)–O–(Si–O)2–Al(T4) sequences of the β – 1 site. The differences between the concentrations of Al in T sites are not as pronounced as those for the ferrierite samples with only isolated framework Al atoms. In addition, our results reveal that the Al siting in ferrierite is not random and depends on the conditions of the zeolite synthesis.