Density functional theory was applied to study the structure of Beta zeolite. A model cluster containing 41Si atoms, 1 Al atom, 70 O atoms and 29 H atoms was constructed. The model structures were optimized using the Becke’s three-parameter hybrid method with the Lee-Yang-Parr correlation functional (B3LYP) and the 6-31G basis set applying the Gaussian03 program package. The NMR parameters were calculated to validate the rationality of the model. It was found that in the optimization models, all O–H bond lengths were in range of 0.984～0.985 , among which the model with O–H bond length of 0.98478 was more stable than the others. The 1H and 27Al chemical shifts of the most stable model were 4.03434 and 55.74 ppm, which were pretty consistent with Larry’s experimental data of 4.1 and 54 ppm. The relationship between other structure parameters and total relative electric energy has also been found. All the results exhibit that the 42 T (the total number of Si and Al atoms is 42) model has common properties of the standard of zeolite Beta.
The adsorption behavior of benzene and propylene in zeolite ZSM-5 was studied by Grand Canonical Monte Carlo(GCMC) simulations. It could be found that benzene and propylene molecules showed different adsorption behavior in the zeolite cavities. The loadings of propylene were significantly larger than those of benzene at 100 kPa. From the figures of potential energy distribution,the potential energy of benzene/zeolite was more negative than that of propylene/zeolite,so benzene could be adsorbed more stably than propylene. When the temperature was increased from 298 to 443 K at 100 kPa,the loading of propylene was reduced from 99 to 82 molecules,whereas that of benzene changed little. When benzene and propylene were adsorbed in zeolite simultaneously,the competitive adsorption of them occurred; therefore,the potential energy distribution could be changed significantly. Besides,the adsorption isotherms of benzene and propylene in ZSM-5 at 298 and 443 K were simulated. The results exhibit that the different factors influenced the molecular adsorption at various temperatures and pressures,leading to the different rules for the adsorption of benzene and propylene molecules in the zeolite. At a low pressure,the unfavorable energy of propylene/zeolite and the "commensurate freezing" phenomenon of benzene would make the loadings of itself higher than those of propylene. When pressure was higher than 5 kPa,the adsorption of benzene in ZSM-5 would nearly reach saturation.
SUN Xiao-yanLI Jian-weiLI Ying-xiaYAN Shi-chengCHEN Biao-hua
