| This thesis consists of two parts, i.e. a hybrid potential model for characterization of MCM-41 and the adsorption of simple fluids in it by the Grand Canonical Monte Carlo simulation (GCMC) and density functional theory (DFT).Porous materials have been applied in many fields, such as gas separation, purification, and reaction processes, etc. In the meantime, when a fluid is confined to a region of a molecular scale, its phase behavior can be strongly affected, and a rich variety of new types of phase transitions can occur. Therefore, investigations on characterization and the adsorption behavior of porous materials are of great importance from both scientific and practical points of view. In this thesis, MCM-41 zeolites are chosen to study for their special characteristics and widespread use. The phase behavior and the adsorption isotherms of simple fluids in it at different conditions are investigated. In both the DFT and MC methods, a potential model for the wall-fluid interactions is required, which plays a key role in the accuracy of the calculated properties. Previous works show that the experimental isotherms can not be reproduced very well by using the existing potential models in the whole pressure range if the surface heterogeneity of MCM-41 is not taken into account. Therefore, a hybrid potential model was proposed in this work to consider the surface heterogeneity of MCM-41, which consists of two parts: (1) the potential energies from the oxygen atoms inside the wall, represented by the complete analytical model developed by our group recently;(2) the potential energies from the silanol coverage and/or other unknown factors in the surface of the channels of MCM-41, represented by the cylindrical surface potential function suggested by Tjatjopoulos et al. To test the new model, the GCMC method was used to calculate the adsorption isotherm of nitrogen in MCM-41 at 77 K first .The isotherm calculated is compared with the experimental data as well as the calculated results of Maddox et al., which divided the surface of MCM-41 into eight sectors and adopted different parameters for each sectors to consider the heterogeneity of the surface. Our model shows good agreement with the experimental data and comparable results with the work of Maddox et al. with much less parameters and computation efforts. In addition, adsorption and phase behavior of methane and ethane are studied by this potential, and the calculation results also coincide well with the experimental data. As a result, this work demonstrates that the hybrid potential model can consider the surface heterogeneity of MCM-41, and the partition of the interactions between the fluid molecules and the wall into two parts is reasonable and useful.Furthermore, this idea is extended to the DFT method to study the possibility of incorporating the hybrid model into the DFT method for characterization of MCM-41. First we compare the adsorption isotherms of nitrogen at the same condition using the hybrid potential model, the complete analytical model and the model of Tjatjopolous. It is found that the hybrid model can incorporate the advantages of the two constituent models, leading to a better description of the experimental adsorption isotherm in the pressure ranges. Then, the relationship between the reduced pressure at which capillary condensation takes place and the pore diameter is determined with our model. As is shown the results are in good agreement with that obtained by the method of Maddox et al. using Monte Carlo method. To further test the new model, we use DFT method to predict the isotherms of methane and ethane in MCM-41 pores. This again proves to be that the hybrid potential model incorporating into the DFT method provides a useful tool for characterization of MCM-41. |
PDF Full Text Request