Characterization of mesoporous MCM-41 adsorbents by computer simulation and adsorption isotherm

In this paper, two potential models representing the interaction between the pore wall and the molecules inside the pore have been compared. One of the models is the potential function proposed by Tjatjopoulos for cylindrical surface, the other is proposed by our previous work for cylindrical pores with a wall thickness being not negligible. Grand Canonical Monte Carlo (GCMC) method has been used to obtain the adsorption isotherms of the nitrogen in MCM-41 by these two potential models, when both the models have the same well depths. It is interesting to find that the change in the potential energies nearby the pore center strongly affects the capillary condensation pressure. Then, a mixed potential model is proposed, which consists of two parts: the potential energies from the oxygen atoms inside the wall, which are represented by our previous proposed potential model and the potential energies from the silanol coverage and/or other unknown factors in the surface of the channels of MCM-41, which are represented by the potential function of Tjatjopoulos et al. To test the proposed mixed potential, we have carried out GCMC simulation of the nitrogen in the MCM-41 pore at 77 K, and compared the simulated adsorption isotherm with the experimental nitrogen isotherm. The simulated isotherm from our model is in good agreement with the experiment, which indicates that the method proposed in this work can be used for characterization of MCM-41 adsorbents.