TNU‐9 Zeolite: Aluminum Distribution and Extra‐Framework Sites of Divalent Cations

Abstract

The TNU-9 zeolite (TUN framework) is one of the most complex zeolites known. It represents a highly promising matrix for both acid and redox catalytic reactions. We present here a newly developed approach involving the use of Si and Al (3Q) MAS NMR spectroscopy, Co as probes monitored by UV/Vis and FTIR spectroscopy, and extensive periodic DFT calculations, including molecular dynamics, to investigating the aluminum distribution in the TUN framework and the location of aluminum pairs and divalent cations in extra-framework cationic positions. Our study reveals that 40 and 60 % of aluminum atoms in the TNU-9 zeolite are isolated single aluminum atoms and aluminum pairs, respectively. The aluminum pairs are present in two types of six-membered rings forming the corresponding α and β (15 and 85 %, respectively, of aluminum pairs) sites of bare divalent cations. The α site is located on the TUN straight channel wall and it connects two channel intersections. The suggested near-planar β site is present at the channel intersection.

Tuning the Aluminum Distribution in Zeolites to Increase their Performance in Acid‐Catalyzed Reactions

“…Single Al atoms represent 40 % and Al in Al pairs represent 60 % of all the Al atoms for Si/Al=14. AlSiAl sequences and close unpaired Al atoms are not observed . The Al pairs form three cationic sites, α, β, and γ, and they are highly similar to those in ZSM‐5.…”

“…The methods described in this chapter result from the need to analyze the organization of Al in some zeolites (ZSM‐5, ferrierite, mordenite, beta zeolite, TNU‐9, and SSZ‐13) that have appeared during the last decades . The developed methods are, therefore, without doubt affected by the nature of the investigated matrices.…”

“…The advantage of this method is simple analysis of the IR spectrum of [D 3 ]acetonitrile adsorbed in a dehydrated Co‐zeolite. The concentration of Al in Al pairs is then given by Equation , and the concentration of close unpaired Al atoms is given by Equation : …”

“…However, the frequencies of perturbed T–O–T vibrations related to bare M 2+ cations have been reported predominantly for Co‐zeolites, and the ion exchange of Co cations to the zeolite is robust and easily monitored. The main obstacle associated with this approach is the quality of the window in the region of the antisymmetric T–O–T vibrations, which depends on the topology of the zeolite (excellent for ferrierite; good for ZSM‐5 the beta zeolite, TNU‐9, and SSZ‐13) and on the crystallinity of the sample . Figure shows the effect of Co 2+ coordination on the geometry of the zeolite 6‐ring, the FTIR spectrum of dehydrated Co‐TNU‐9 in the region of T–O–T vibrations with simulation of the spectrum to the individual bands, and the effect of Co loading in TNU‐9 on the total intensity of the T–O–T vibrations.…”

“…Thus, analysis of Al pairs in the zeolite first requires detailed analysis of the spectrum of the dehydrated Co‐zeolite, including identification of spectroscopic species corresponding to the base Co 2+ cations (one to four bands have been reported to reflect one coordination of Co 2+ cations) and estimation of the corresponding extinction coefficients. Nevertheless, for several zeolites this analysis has been done and reported (mordenite, ferrierite, ZSM‐5, the zeolite beta, and SSZ‐13) . The concentration of Al in Al pairs is then given by Equation , and the concentration of close unpaired Al atoms is given by Equation : …”