Molecular Simulation Study Of Diffusion Of N-Alkanes In MOF-5 Metal-Organic Framework

Metal-Organic Frameworks (MOFs) is a new type of nanoporous material. It has many advantages:structure diversity, high surface area and porosities. Therefore, in the field of separation, catalysis and gas storage it has very broad prospect. In this kind of materials, MOF-5 material is very typical, which has very regular three-dimensional pore structure. In industry, as the basic raw materials, it is important to study alkanes' diffusion properties in porous materials. In this work, MOF-5 and the C1-C4 alkanes are adopted to study the factors that affect molecular diffusion.In this paper, molecular dynamic simulation was used to study the diffusion properties, and the framework of MOF-5 was treated both flexible and rigid for comparison. The main contents and findings are summarized as follows:(1) Testing of the reliability of the force field parameter. The force fields and the simulation program were tested by comparing with literature, and good agreement was obtained.(2) Analysis of the influencing factors. The effects of the four factors: temperature, the number of molecules, flexibility of the framework and the chain length of alkanes, were analysized. The self-diffusion coefficients of the alkanes are larger in flexible framework than that in the rigid one, illustrating that flexible framework is more convenient to diffuse for molecular.(3) When the other factors are the same, the longer the chain, the slower the diffusion.(4) Temperature and the number of the molecules are two factors that interplay. At lower temperature of 200 K, the kinetic energy is smaller, but the adsorption force between the material and molecules is stronger, so it is a major role, which causes the self-diffusion of molecules increase with the number of molecules increase. And after reached a peak the self-diffusion begin to reduce because of inhibitions between molecules. At higher temperatures like 300 K and 500 K, the kinetic energy of the molecules rises with the increase of temperature, and the diffusion is the main role.(5) To understand better the diffusion mechanism, we further calculated the activated energy. The activation energy in the flexible framework is smaller than that in the rigid one, and thus the Ds in the former is bigger than that in the later. Furthermore, the activation energy increases with increasing the length of chain, resulting a decrease of diffusivity with increasing chain length.