Combined DFT theoretical calculation and solid-state NMR studies of Al substitution and acid sites in zeolite MCM-22

The distributions of Bronsted acidic protons and their acid strengths in zeolite H-MCM-22 have been characterized by density functional theory (D]FT) 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 C 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 A1 substitution takes place in the order of A11-OH-Si2 > A18-OH-Si8 > A15-OH-SiT The DFr theoretical and NMR experimental results suggest that the acid strength of the three Bronsted acid sites in H-MCM-22 zeolite is slightly lower than that of H-ZSM-5 zeolite and the accessible Bronsted acidic protons most likely reside in both the supercages (at the A18-OH-Si8 and A11-OH-Si2 sites) and external surface pocket (at the A18OH-Si8 site) rather than in the sinusoidal channels (A15-OH-Si7), with the A11-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.