Combined DFT Theoretical Calculation and Solid-State NMR Studies of Al Substitution and Acid Sites in Zeolite MCM-22

Abstract

The distributions of Brönsted acidic protons and their acid strengths in zeolite H-MCM-22 have been characterized by density functional theory (DFT) calculations as well as magic angle spinning (MAS) NMR experiments. The embedded scheme (ONIOM) that combines the quantum mechanical (QM) description of active sites and semiempirical AM1 treatment of the neighboring environment was applied to predict the aluminum substitution mechanism and proton affinity (PA), as well as adsorption behaviors of acetone and trimethylphosphine oxide (TMPO) onto the zeolite. Our theoretical results indicate that the Al substitution takes place in the order of Al1-OH-Si2 > Al8-OH-Si8 > Al5-OH-Si7. The DFT theoretical and NMR experimental results suggest that the acid strength of the three Brönsted acid sites in H-MCM-22 zeolite is slightly lower than that of H-ZSM-5 zeolite and the accessible Brönsted acidic protons most likely reside in both the supercages (at the Al8-OH-Si8 and Al1-OH-Si2 sites) and external surface pocket (at the Al8-OH-Si8 site) rather than in the sinusoidal channels (Al5-OH-Si7), with the Al1-OH-Si2 site having the strongest acid strength (as probed by TMPO). This may partially explain the special selectivity of acid-catalyzed reactions occurring inside the channels of H-MCM-22 zeolite.

Solid-State NMR Studies of Zeolites

“…With the aid of DFT calculation, it was demonstrated that the 31 P chemical shifts of various trialkylphosphine probe molecules (triethylphosphine oxide, TEPO; tributylphosphine oxide, TBPO; and trioctylphosphine oxide, TOPO) could be utilized as scales for quantitatively measuring the Brønsted acid strength, and a H MAS NMR to distinguish the internal and external acidic sites in zeolites. By using DFT calculations and solidstate NMR of adsorbed TMPO, Zheng and coworkers [ 100 ] found that the accessible Brønsted acidic protons reside in both the supercages (at the Al8-OH-Si8 and Al1-OH-Si2 sites) and the external surface pockets (at the Al8-OH-Si8 site) rather than in the sinusoidal channels (at the Al5-OH-Si7 site) of zeolite H-MCM-22, with the Al1-OH-Si2 site having the strongest acid strength. This fi nding may partially explain the special selectivity of acid-catalyzed reactions occurring inside the channels of the zeolite.…”

Layer selection effect on solid state 13C and 15N chemical shifts calculation using ONIOM approach

“…Therefore, ONIOM method is very applicable in solid state chemical shifts calculations. There are several reports of using ONIOM method to calculate solid state NMR chemical shifts [29][30][31][32][33][34][35][36][37][38][39][40][41][42]. Although in most of the investigations the results of two or more ONIOM computations have been compared, there is no specific study to show the effect of selected layers and their level of theory on the computation results.…”

Effective ONIOM schemes for modeling MCM-22 zeolite

“…MCM-22 is a relatively new member in zeolite family and has attracted many theoretical attentions [4][5][6][7]. Although periodic calculations can treat crystalline of zeolites accurately [8][9][10], it is limited for zeolites with huge unit cells such as MCM-22 (large supercages with dimension of 7.1 Â 7.1 Â 18.1 Å defined by 12MR) due to the computational costs.…”

“…Using separated 8T clusters Zhou et al [4,5] found that the most favorable sites for framework Al substituted MCM-22 are T1, T3, T4, and the most preferable Brönsted acid sites are Al1-O3H-Si4, Al3-O11H-Si2 and Al4-O3H-Si1. Considering that the small 8T clusters ignore structural and long-range influences of the zeolite lattice, Zheng et al [6] used a 22T cluster with all eight non-equivalent tetrahedral sites of MCM-22 to study Al substitution. In their work, partial optimization was carried out with only 5T (Al(OSi) 4 H) cluster relaxed while the rest atoms were fixed at their crystallographic locations.…”

“…Zheng et al [6] predicted the stability order directly by comparing the total energies of different T sites embedded in the same large cluster, while Zhou et al [4] estimated the Al substitution order indirectly by calculating the substitution energy among many different 8T clusters. Considering the long-range interaction and electrostatic effects, the 22T model of Zheng et al is apparently more reasonable than the small 8T cluster of Zhou et al, but the 22T cluster is obviously not big enough to incorporate all important T sites without suffering boundary effects.…”