Aluminum Siting in Silicon‐Rich Zeolite Frameworks: A Combined High‐Resolution ²⁷Al NMR Spectroscopy and Quantum Mechanics / Molecular Mechanics Study of ZSM‐5

Abstract

Dedicated to Süd-Chemie on the occasion of its 150th anniversary Zeolites are crystalline microporous aluminosilicates widely used as molecular sieves and catalysts in industrial chemical processes. Silicon-rich zeolites (Si/Al > 12) such as ZSM-5 (MFI framework) have found particular attention. The catalytically active species, that is, protons, metal cations, and metal-oxo cations, compensate the negative charge of the microporous aluminosilicate frameworks [Si nÀm Al m O 2n ] mÀ made of corner-sharing TO 4 tetrahedra (T = Si, Al À ). A typical feature of many silicon-rich zeolites is a high number of crystallographically distinguishable T sites. Since the cationic species bind to the AlO 4 À tetrahedra, the crystallographic position of aluminum in zeolite frameworks governs the location of the active sites, which in turn affects the catalytic activity and selectivity.Thus, understanding the Al siting in zeolite structures is a priority, but it has remained a challenge. Diffraction methods are of limited use because of the similar scattering properties of Si and Al but also because of the low Al content in the most active zeolite catalysts. Solid-state 29 Si magic-angle spinning (MAS) NMR spectroscopy succeeded early in distinguishing between Si in different crystallographic positions of the MFI framework, [1] but for the quadrupolar 27 Al nucleus, the development of multiple quantum (MQ) NMR spectroscopy experiments [2] opened such possibilities only in the last decade. [3][4][5][6] It is also not clear if there are preferred T sites for Al substitution or whether the T sites are occupied statistically. An X-ray diffraction study found three Cs + sites in extraframework positions of ZSM-5, thus indicating nonrandom Al siting, [7] which is also supported by the effect of the Al concentration on the 27 Al MQ MAS NMR spectra of ZSM-5 [4] and zeolite b.[5] As lattice energy minimizations with reliable force fields, for example, for MFI, [8,9] yielded only small energy differences for Al in different positions, the Al distribution might be kinetically controlled. This assumption implies that different synthesis procedures and different templates and cations could lead to different Al substitution patterns, thus increasing the total number of resolved 27 Al signals in the MQ MAS NMR spectra of a variety of samples. This strategy is followed herein.For a set of 11 differently synthesized ZSM-5 samples, ten distinct resonances have been identified by 27 Al MQ MAS NMR spectroscopy, extending over a shift range of Dd = 13.6 ppm. Quantum-chemical calculations for simulated structures with Al in 24 different T sites yield a shift range of Dd = 14.1 ppm and show that the observed resonances belong to Al in different crystallographic sites. We conclude that the Al siting in ZSM-5 is not random and can be substantially varied by the conditions of zeolite syntheses.A set of Na-ZSM-5 samples (A-K) with Si/Al framework ratios from 14 to 45 was prepared by using different silicon, aluminium, and sodium sources as well as differe...

Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts

“…A thorough examination of all possible active sites is clearly beyond the scope of this study. This is furthermore not straightforward, as it was recently shown for H-ZSM-5 that the actual aluminum distribution in a zeolite material does not only depend on thermodynamic stability, but rather on the conditions applied during the synthesis of the catalyst [34][35][36].…”

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

“…The catalytically active species in zeolites balance the negative charge of AlO 4 − tetrahedra. Silicon-rich zeolites exhibit a high number of crystallographically distinguishable framework T sites, occupied by Si or Al atoms, resulting in a variability of the Al sitting in the framework [4,5].…”

“…The catalytically active species in zeolites balance the negative charge of AlO 4 − tetrahedra. Silicon-rich zeolites exhibit a high number of crystallographically distinguishable framework T sites, occupied by Si or Al atoms, resulting in a variability of the Al sitting in the framework [4,5]. Since the proton bind to the AlO 4 − tetrahedral, forming the Brönsted acid site, the position of Al in frameworks of zeolites controls the distribution of the Brönsted acid sites, which, in turn, affects the catalytic activity and selectivity.…”

“…More specifically, temperature programmed desorption (TPD), solid-state nuclear magnetic resonance (SSNMR), fourier transform infrared spectroscopy (FTIR), and catalytic evaluation of Brönsted acidity are currently applied to obtain information about the distribution of Al atoms and the relative acidities of the zeolites [4,[12][13][14][15]. The conditions of zeolite synthesis and Al concentration in the framework were the main factors determining the distribution of Al in zeolites.…”

“…The nature of the acid sites in CHA zeolite were studied and the presence of additional Al substitutions in the zeolite framework were found to have a significant effect on the deprotonation energies and base adsorption energies of acid sites [18]. Particular attention was paid to the presence of Al-O-Si-O-Al and Al-OSiOSiO-Al sequences in ZSM-5; the presence of Al-O-(Si-O) n -Al (n = 1 or 2) sequences could result in a charge of properties of the AlO 4 − tetrahedral [19]. Sazama et al [20] have also discussed the distribution of Al in Al-rich BEA, via experimental and theoretical methods, and revealed that high Al content can lead to a high content of Al-OSiO-Al.…”