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 AlÀOÀ(SiÀO) n ÀAl (n = 1, 2) sequences in their frameworks. 29 Si MAS NMR spectroscopy is employed to confirm the absence of AlÀOÀSiÀOÀAl in the framework of silicon-rich zeolites while 27 Al 3Q MAS NMR spectroscopy and DFT computations of 27 Al isotropic chemical shifts serve to determine the locations of isolated Al atoms. The maximum ion-exchange capacity of zeolites for [Co 2þ (H 2 O) 6 ] 2þ reveals the presence of close Al atoms (i.e., those Al atoms which are able to balance [Co 2þ (H 2 O) 6 ] 2þ 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 AlÀOÀ(SiÀO) 2 ÀAl pairs in a ring. Subsequently, their 27 Al 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 AlÀOÀ(SiÀO) 2 ÀAl 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)ÀOÀ(SiÀO) 2 ÀAl(T1a) and Al(T2)ÀOÀ(SiÀO) 2 ÀAl(T2) sequences forming the R and β À 2 cationic sites, respectively. Isolated Al atoms do not occupy the T1b site and close Al atoms do not form Al(T4)ÀOÀ(SiÀO) 2 À Al(T4) sequences of the β À 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.

The Distribution and Strength of Brönsted Acid Sites on the Multi-Aluminum Model of FER Zeolite: A Theoretical Study

“…The Al4-O-(SiO) 2 -Al4 site has the lowest S E (Si/Al), suggesting the most stable configuration of Brönsted acid formation in the 2-Al substitution models. Herein the Al4-O-(SiO) 2 -Al4 sequence is located in one 6-membered ring; the experiments carried out by Dedecek and his co-workers [23] identified the locations of the Al-O-(SiO) 2 -Al in a ring, which is excellently consistent with our result. The Al1-O-(SiO) n -Al4 (n = 2, 3) configurations have a similar S E (Al/Si) of around 15 kcal/mol, indicating that the stability exhibits little dependency on the number of SiO groups between the two Al sites.…”

“…Therefore it is necessary to study the distribution of different amount of Al atoms and the acid strength of Brönsted acid sites in FER zeolite. Although the experiment research on these issues was mentioned a little, several DFT calculations were also carried out to investigate the distribution of Al in FER zeolite [23,24]. However their cluster model was so extremely small that it could not represent the various types of Al/Si distributions, which are believed to be significant in determining the catalytic reactivity in butene isomerization.…”

Mechanism of Framework Oxygen Exchange in Fe‐Zeolites: A Combined DFT and Mass Spectrometry Study

“…The first two models (a and bÀ2) represent isolated cationic sites while the other two models (bÀ1 + bÀ1 and bÀ2 + bÀ2) feature two cooperating adjacent Fe II cationic sites. [9]) revealed that bÀ2 and a cationic sites are present in the sample. Moreover, our recent study [9] of the parent ferrierite zeolite (FER/D in ref.…”

“…[7] The four models, possessing P1 symmetry, were utilized to investigate four distinct possible arrangements of the active sites in ferrierite. Moreover, our recent study [9] of the parent ferrierite zeolite (FER/D in ref. The bÀ1 model was not used in this study since N 2 O does not adsorb via the O atom on Fe II accommodated in isolated bÀ1 sites, [7] and therefore, no isotope exchange can occur.…”

Location of Framework Al Atoms in the Channels of ZSM‐5: Effect of the (Hydrothermal) Synthesis

“…The Al siting in the zeolite frameworks [i.e., the crystallographically distinguishable tetrahedral (T) sites that are occupied by Al] was clearly shown to be neither random nor controlled by simple rules, but to depend on the conditions of their hydrothermal synthesis . This means that Al atoms can be located in zeolites of the same topology and Al concentration at various crystallographically distinguishable T sites ,,,. Therefore, obtaining insights into the Al locations in the channel system of ZSM‐5 is of crucial importance for development of highly selective catalysts.…”