Aluminum Siting in the ZSM-22 and Theta-1 Zeolites Revisited: A QM/MM Study

Abstract

The Al siting in the silicon rich ZSM-22 and Theta-1 zeolites of the TON structure was investigated analyzing already published 27Al 3Q MAS NMR experimental data using QM/MM calculations. The results of our computations show that Al atoms can be located in 6 framework T positions because the two eightfold sites (T1 and T2) split into four fourfold T sites after an Al/Si substitution. The observed resonance at 55.5 ppm corresponds to the T4 site which is predominantly occupied by Al. This site is not located on the surface of the TON ten-membered ring channel and thus the protonic sites related with the majority of Al atoms in the TON structure exhibit a significantly limited reaction space. The 27Al NMR signals centered at 57.6 and 58.7 ppm correspond to either the T2 and T3 sites, respectively, or only to T2. The T2 and T3 sites accommodate some 40% and up to 10%, respectively, of Al while the T1 site is unoccupied by Al. Isotropic shifts of 61.1 and 61.6 ppm were calculated for Al atoms located in the T1-1 and T1-2 sites, respectively. The effect of a silanol "nest" as a next-next-nearest neighbor on the 27Al isotropic chemical shift of Al located in the T4 site is calculated to be less than 1 ppm.

Exploring Scaling Relations for Chemisorption Energies on Transition‐Metal‐Exchanged Zeolites ZSM‐22 and ZSM‐5

“…This gives rise to a large number of different sites where extraframework copper can be positioned within the zeolite. Copper exchanging the proton on all possible T sites exchanged by aluminum in ZSM‐22 and ZSM‐5 has been considered in the present study and the most stable configurations were chosen. In Figure , the optimized structures are shown along with the relative stabilities referenced to the most stable Cu I –ZSM‐22 compound.…”

Unraveling the Relationship between Zeolite Structure and MTO Product Distribution by Theoretical Study of the Reaction Mechanism

“…The H-ZSM-5 structure contains one 10-MR straight (5.3 × 5.6 Å) and one 10-MR zigzag channel (5.1 × 5.5 Å) that are intersected to form a void space allowing to place a maximum sphere of 6.4 Å in diameter and with one Si atom replaced by one Al atom at the T12 site . The H-ZSM-22 structure has a one-dimensional 10-MR straight channel (5.7 × 4.6 Å) with an Al atom occupying T4 . All peripheral Si (Al and P for H-SAPO-34) atoms are saturated by H atoms along the pre-existing Si–O bonds with the terminal Si–H distance of 1.47 Å (Al–H and P–H distances of 1.55 and 1.35 Å respectively for H-SAPO-34).…”

Local Structure of Cationic Sites in Dehydrated Zeolites Inferred from 27Al Magic-Angle Spinning NMR and Density Functional Theory Calculations. A Study on Li-, Na-, and K-Chabazite

“…Diffraction techniques cannot distinguish between Al and Si atoms and thus do not allow direct identification of the Al siting in zeolites. Conversely, high-resolution 27 Al solid state NMR spectroscopy and applications of multiple-quantum methods and ultrahigh field magnets led to the recent success in determining the Al siting in the zeolites of the FER, 16 *BEA, 17 TON, 18 and MFI 19,20 (partial determination) structures. Tetrahedral framework Al atoms are in this case characterized by the nuclear quadrupolar coupling product, P Q , around 2.5 MHz resulting in narrow 27 Al NMR signals.…”