A DFT study on isomorphously substituted MCM-22 zeolite

The density functional theory has been used to study the isomorphously substituted MCM-22 zeolite for the first time. The effect of the basis sets on the calculation results is discussed in details. Data of several index properties for characterizing the relative acidity of T-MCM-22 (T = B, Al, Ga, and Fe), including proton affinity, bond length and bond angle, OH stretching frequency, and charge on the acidic proton, show that the acidity of T-MCM-22 increases in the sequence of B-MCM-22 < Fe-MCM-22 < Ga-MCM-22 < Al-MCM-22. After making a correction, the calculated OH stretching frequencies for Al-MCM-22 and Fe-MCM-22 show a reasonable agreement with the experimental data. On the basis of an equilibrium structure of the B-MCM-22 zeolite, the effect of the B element in the synthesis of the Ti-MCM-22 is also discussed. The adding of the B element during the synthesis of the Ti-MCM-22 can decrease greatly the Ti substitution energy because of the forming of a structure quite similar to the terminal silanol group. The results can provide some constructively information for zeolite synthesis.