Direct Evidence for the Nonrandom Nature of Al Substitution in Zeolite ZSM-5: An Investigation by27Al MAS and MQ MAS NMR

Abstract

Many important properties of aluminosilicate zeolites, most notably their Br˘nsted acidity and hence catalytic performance, depend primarily on the framework Al content, that is, the extent of isomorphous substitution of Si by Al. However, even in materials with the same framework structure and composition, the location of catalytically active sites can differ according to the manner in which Al atoms are spatially distributed over the available tetrahedral sites (T-sites) in a given zeolite lattice. Therefore, detailed knowledge of the local ordering of Al atoms in these microporous materials is the starting point for systematically manipulating the distribution of acidic sites within the pore architecture.Zeolite ZSM-5 (MFI topology) is one of the industrially most important zeolites. It typically crystallizes in the orthorhombic space group Pnma, and 12 crystallographically distinct T-sites with equal populations exist in the ZSM-5 framework. [1] A fundamental, recurring question regarding this type of zeolite structure is whether the acidic sites (that is, Al atoms on T sites) are spatially ordered. Recently, Olson et al. [2] carried out Rietveld refinement of synchrotron powder X-ray diffraction (XRD) data on a Cs-ZSM-5 zeolite with 5.8 Al atoms per unit cell (Si/Al 15.6) and showed that Cs ions are located in three different extraframework sites. They speculated that Al atoms may be unequally distributed over the 12 distinct T-sites of the ZSM-5 framework. However, because of the very similar scattering powers of Si and Al for X-rays and the difficulties in obtaining large single crystals, especially at high Al concentrations, it is very difficult, or even impossible, to directly locate Al atoms by conventional diffraction methods. Although many quantum chemical studies have also addressed this issue, [3] the nature of

Selective Introduction of Acid Sites in Different Confined Positions in ZSM-5 and Its Catalytic Implications

“…In agreement with previous studies, 11,43,44 the energy values obtained for the Al-containing models are quite similar, within 4 kcal/mol at high Si/Al ratio and within 8 kcal/mol at most at higher Al content, with the only exception of T7 position that is clearly unstable at Si/Al = 23 (see Table 1 and Figures 2 and S1). This result is in accordance with the proposal that Al siting in ZSM-5 depends on the synthesis procedure, [33][34][35] because there is not any clear thermodynamic preference for occupying any given position in the framework. Indeed, at the highest Al content considered, there are six different positions whose relative stability differs by less than 3 kcal/mol: T1, T2, T3 and T12 at the channels intersections and T4 and T11 within the 10R channels.…”

“…[26][27][28][29][30][31][32] Deconvolution of the 27 Al-MAS-NMR spectra of ZSM-5 into different peaks has allowed to demonstrate that Al siting in this zeolite varies with the synthesis conditions, and that several different T sites are occupied by Al even in samples with high Si/Al ratio. [33][34][35] However, while different deconvolution patterns are indicative of a different Al distribution within the MFI framework, a clear assignation of the peaks to particular positions is not established yet. Another aspect considered in relation to Al distibution is the proximity between Al atoms, and their local arrangement as isolated, paired -defined as framework Al atoms separated by either one or two Si atoms [Al-…”

Determining the location and nearest neighbours of aluminium in zeolites with atom probe tomography

“…However, X-ray diffraction studies that include ion-exchanged Cs-ZSM-5 and Tl-ZSM-5 zeolites have shown a non-random Al distribution over crystallographic T-sites 12 13 . Correspondingly, Al nuclear magnetic resonance (NMR) studies suggest that the Al distribution over the T-sites is kinetically controlled and not random, while the distribution can vary substantially according to the conditions of zeolite synthesis 14 15 . A recent study by Vjunov et al 16 confirms that identical zeolite types may have drastically different Al distribution over T-sites.…”

New Molecular Simulation Method To Determine Both Aluminum and Cation Location in Cationic Zeolites

“…Even if direct determination is limited to very few cases, other technics such as 29 Si or 27 Al NMR technics as well as the analysis of the TO distances from the average structure measured by RX give some indications that in many cases the distribution may be nonrandom. 9,10,11,12 Silicon or aluminum NMR techniques have been applied to various zeolites. 9,10,12,13,14,15,16,17,18 They allow identifying different environments for aluminum atoms.…”

“…9,10,11,12 Silicon or aluminum NMR techniques have been applied to various zeolites. 9,10,12,13,14,15,16,17,18 They allow identifying different environments for aluminum atoms. In materials with a small number of different Tsites, the attribution of the different picks can be done but the determination of the aluminum repartition of the sites remains subject to interpretation.…”