Zeolites Br (?) Nsted Acidic Theoretical Calculations Characterized

Theoretical calculations are powerful tools to analyze the target from the atomic level and have been applied to the zeolite studies. In this work density functional theory calculations have been used to study the Br?nsted acidity of MCM-22 zeolite. Based on the calculated substitution energies, proton affinity and the interaction energies of probe molecules adsorbed on the acidic centers, the localization and acid strength of different Br?nsted acid sites are investigated. All calculations were performed with 8T cluster model (H₃SiO)₃Si-O(H)-T(OSiH₃)₃, where T = Si or Al, and at BLYP/DNP or B₃LYP/6-31G** levels. According to the calculations it was proposed that the most favorable sites for Al substitution are T1, T4 sites in 12MR supercages and T3 , T8 sites in 10MR sinusoidal channels. The preferable Bronsted acid sites locate at Al1-O3H-Si4, Al4-O3H-Si1, Al3-O11H-Si2 and Al8-O10H-Si2 bridging groups. The acidity at T1 and T4 sites is similar, and stronger than T3 and T8 sites. The Br?nsted acidity in ZSM-5 is also evaluated at the same calculating conditions. It draws the conclusion that the acidity at T6, T9 and T12 sites shows no singnificent difference. Comparing to MCM-22 zeolite, the acidity of ZSM-5 lies between the 12MR supercage and 10MR sinusoidal channel, but more closer to T1 and T4 sites. Our theoretical calculations confirmed that the two zeolites have similar acidity, which is in good agreenment with the experinements. The protonation reaction of C2H4 at supercages and sinusoidal channels in MCM-22 are studied. The calculated acitive energies and reaction free energies reveal that C2H4 molecule is preferably protonated in 12MR supercages.