Molecular simulations of the dynamics and adsorption thermodynamics of hydrocarbons in zeolites.

摘要

The dynamics and adsorption thermodynamics of hydrocarbons in the zeolite silicalite were studied using molecular simulations. New techniques were developed and applied to examine a range of industrially-relevant problems.; Methods for constructing a molecular model were reviewed, as were several of the simulation techniques useful for investigating adsorption and diffusion in zeolites. A survey of the sorbate/zeolite simulation studies performed over the last twenty years was also given.; A nonequilibrium molecular dynamics technique for directly computing the Fickian transport diffusivity of sorbates in zeolites was developed. The technique was applied to study methane diffusion in silicalite. The transport diffusivity was predicted to increase with increased loading, in agreement with the phenomenological Darken model.; A configurational-bias Monte Carlo technique was developed that enables the adsorption thermodynamics of long alkanes to be calculated. This technique is much more efficient than standard methods. Computed Henry's law constants and isosteric heats agreed well with experimental data. The results indicated that, at high temperature, n-alkanes probe all accessible regions of the zeolite. At low temperature, short chains populate all regions of the zeolite, while chains longer than n-octane align along the straight channels in localized, low-energy conformations.; A hierarchical simulation methodology was developed which enables the diffusivities of long alkanes to be computed using modest computational resources. The approach starts with an atomistic representation of both the zeolite and alkane and utilizes a systematic coarse-graining procedure along with concepts from Brownian motion theory and transition-state theory to arrive at a simpler dynamical model. The long-time behavior of this model can be tracked using a fraction of the computer time required to perform molecular dynamics. Results showed that the self-diffusivity of n-alkanes decreases as a function of chain length up to n-decane, after which the diffusivity remains essentially constant. Activation energies for diffusion were found to be about 5 kJ/mol for chains shorter than n-decane, and 12 kJ/mol for n-decane and longer chains. These trends have been observed experimentally, although the magnitudes of the experimental and simulation values are different.