Modeling Of Intracrystalline Adsorption And Diffusion In Zeolite

The objectives of this paper are related to the modeling of intra-crystalline adsorption and diffusion in zeolite with its potential applications to the shape selective catalysis. To this end, a new thermodynamic model is developed to predict diffusivities of hydrocarbons in zeolites, and molecular simulations are also carried out to predict adsorption isotherms of hydrocarbons with long chains in zeolites.In the first part of this paper, first, the well-known Xiao and Wei's GT model for prediction of diffusivity of hydrocarbons in zeolites was critically evaluated based on a database set of the diffusional activation energy and the diffusional coefficients reported in the literature. It was demonstrated that while the GT model predicts quantitatively the diffusional coefficients of C3-C6 hydrocarbons, it yields large errors of several orders of magnitude in calculating the diffusional coefficients of hydrocarbons with complicated molecular structures. Secondly, a new model for prediction of activation energy and diffusivity of hydrocarbons in zeolites was developed based on the concept of group contribution method. Compared with the calculated diffusivity of GT model and experimental data, the present group-contribution-based model gives the same order of magnitude diffusivities of alkenes, while the predictions for most aromatic hydrocarbons, cyclohexane and it's derivatives are in the same order of magnitude with the largest difference being 1～2 orders of magnitude. Finally, based upon an analysis of the physical meaning of the diffusional activation energy, the activation energy was found, for the first time, to be correlated with boiling points of the guest molecules. In most cases investigated in this paper, the correlation coefficient of diffusional activation energy of linear alkenes is over 0.90.In the second part of the paper, absorption in zeolites has been researched by using molecular simulation. The third chapter introduces basic theory and technique of molecular stimulation, and makes an example about phase-equilibrium of simple molecule to test the dependability of molecular stimulation. In addition, random sample is related to in this method, and stand or fall of its property influence the result of simulation directly. So addressing a method for generating quasi-random numbers may be timely and useful. To this end, in the fourth chapter a novel method has been presented to overcome the limitation existed in the conventional method. Based on a special property of prime numbers, two property-choiceness methods have been developed for generating quasi-random numbers, including optimum multiplicator method and long period method. Statistical results show that the new methods have advantages of satisfactory statistic properties over the multiplicative or mixed congruential methods. In the last part of the thesis, the grand canonical Monte Carlo (GCMC) method has been used to investigate the phenomenon of absorption for nitrogen in 5A zeolite. Satisfactory results has been obtained by comparing values of the simulation with those of the literature. At the same time, distributing of the adsorbent capability is discussed by investigating the absorption of nitrogen on some sections of 5A zeolite. Generally speaking, it is the linear and branched alkanes or benzene and its homologues that are interested in petrochemical reactions. And as one kind of catalyst for shape-selective reactions, ZSM-5 plays an important role in the chemical and the petroleum industries for its special apertures having the size with the adsorbent molecular. While, for long chain molecules with strong intra-molecular potentials GCMC algorithm will not be efficient because a displacement of a single atom will not change the conformation of molecule very much, then this will lead to poor sampling statistics. It is worse that the probability for adding a molecule in system successfully is almost equals zero because the molecule stand a good chance to overlap with other molecules having existed in the system and causes p...