No abstract

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.

Nonoxidative activation of methane was carried out over Ag/ZSM-5 catalysts, prepared by the wetness impregnation method, and the catalysts were characterized by various techniques. Theoretical calculation was further employed to clarify the reaction mechanism of methane coupling over the catalysts. Ag + ions were found to be the main Ag species at low loading (1-5 wt %) whereas higher loading (10 wt %) resulted in metal clusters. Combined with experimental evidence and computer modeling, it is concluded that isolated Ag + ions play a crucial role in the catalytic coupling of methane under nonoxidative conditions.