DFT insight into Brønsted acidity in MCM-22 zeolite by ³¹P NMR of adsorbed trimethylphosphine oxide

The Brønsted acid (BA) strength plays an important role in the catalytic performance of protonic MCM-22 zeolite. Although many studies including both theoretical calculations and experiments have been performed, it still remains somewhat unclear and needs to understand completely. By using density functional theory (DFT) calculation including dispersion contribution, DFT-D3, the characteristics of bare protonic MCM-22 zeolite and trimethylphosphine oxide (TMPO) adsorption complexes were optimized. The BA strength of H-MCM-22 zeolite was evaluated through chemical shift of ³¹P NMR. The calculated ³¹PNMR chemical shifts of adsorbed TMPO were in good agreement with those of experiment. The strength of BA sites in protonic MCM-22 is mostly decided by the location of TMPO complexes with BA sites. The high chemical shifts (79-85 ppm) corresponding to the high BA acid strength were observed for TMPO complexes located in 12-membered ring supercage and the lower chemical shifts (73-78 ppm) corresponding tolower BA strength, were due to TMPO interaction with BA sites inside supercage or in 10-member ring crossing window or in 10-member ring sinusoidal channels. Furthermore, the relationship between adsorption energies of TMPO with BA sites and ³¹P NMR chemical shift of adsorbed TMPO was also evaluated. (author)