Aluminium siting in the ZSM-5 framework by combination of high resolution 27Al NMR and DFT/MM calculations

Abstract

The Al siting in the ZSM-5 zeolite was investigated by (27)Al 3Q MAS NMR spectroscopy and QM/MM calculations. It was found that the occupation of the framework T-sites by Al and the concentration of Al in these T-sites are neither random nor controlled by a simple rule. They both depend on the conditions of the zeolite synthesis. At least 12 out of the 24 distinguishable framework T-sites of ZSM-5 are occupied by Al in the set of the investigated zeolite samples. A partial identification of the Al sites is possible. The calculated (27)Al NMR shielding values were converted to (27)Al isotropic chemical shifts using the experimental isotropic chemical shift of 60.0 ppm referenced to the aqueous solution of Al(NO(3))(3) and the corresponding calculated NMR shielding of 490.0 ppm of a silicon rich (Si/Al 38) chabazite structure zeolite as a secondary internal standard. The observed (27)Al isotropic chemical shifts of 50.0 and 54.7 ppm correspond to Al atoms in the T20 and T6 sites, respectively. The pair of measured isotropic chemical shifts of 52.9 and 53.7 ppm can be assigned to the T4, T8 pair. At the low-shielding end, two assignments are plausible. The smallest deviations between the calculated and observed isotropic chemical shifts are reached for the assignment as follows: T24 (64.8 ppm) is not occupied in the samples and that the observed isotropic chemical shifts 63.6, 62.8, and 60.0 ppm belong to T1, T17, and T7, respectively. It follows then that T-sites T12 (60.8 ppm), T3 (61.7 ppm), and T18 (62.0 ppm) are most likely not occupied by Al in our ZSM-5 samples. If we assume that the calculated isotropic chemical shifts are systematically larger than the observed ones then we can assign the largest observed isotropic chemical shifts of 63.6 and 62.8 ppm to the least shielded T24 and T1 sites, respectively, and 60.0 ppm to T12. Then the sites T3 (61.7 ppm), T18 (62.0 ppm), and T17 (62.5 ppm) would be unoccupied by Al in our ZSM-5 samples. It was further shown that there is no simple linear relationship between the observed (27)Al isotropic chemical shifts and the average Al-O-Si angles.

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

“…These results agree with the previous experimental study by Zhao et al [34]. We note that in the present study we only considered an ideal model for Al substitution and may not reproduce the realistic condition where the distribution of Brönsted acid sites could be affected by many other factors [4,14]. …”

“…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.…”

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].…”

Theoretical Study of the Adsorption of the Butanol Isomers in H-ZSM-5

“…Because of the dominant stabilizing role of vdW interactions, the pbc[DFT-D] approach 33,34 in which the Grimme's dispersion correction 33 is included in the periodic DFT calculations 34 is used in the present work for obtaining optimized structures, energies, and for determining harmonic vibrational frequencies of the adsorbed systems. 35,36 show a variability of the Al siting for ZSM-5 samples with the Si/Al ratio of 95 (one Al per unit cell). However, performing calculations for adsorption of the four butanols on all the different possible T sites is computationally very expensive and practically unfeasible.…”