Molecular Simulation Study Of Diffusion Of N-Alkanes In Cu-BTC Metal-Organic Framework

Metal-Organic Frameworks(MOFs),which have extremely high surface area,porosities,thermal stability,and structure diversity,are promising new nanoporous materials with potential applications in gas storage,separation,and catalysis,etc.Cu-BTC,having similar three-dimensional pore structure with typical MOF of MOF-5 and with unique pocket structure,is a well known representative of MOFs.Alkanes are raw materials of chemical industry.Therefore,it is important to study alkanes' diffusion properties in Cu-BTC material.In this paper,molecular simulation was used to study the diffusion properties of n-alkanes C1～C4 in Cu-BTC.The main contents and findings are summarized as follows.1.The cell of Cu-BTC was divided into three regions:A-Cell,B-Cell and Pocket(denoted as P).The A- and B-Cell were connected to form the main channel,defined as AB with a type of three-dimensional orthogonal. The second channel was formed by A-Cell and Pocket,defined as AP.2.The total and local self-diffusivities of n-alkanes C1～C4 in Cu-BTC were calculated by molecular dynamics simulations,which decrease with the increase of chain length.In general,the total self-diffusivities are mainly affected by the local self-diffusivities in the main channel AB,and the effect becomes more evident with increasing chain length.3.The center of mass(COM) probability distributions of n-alkanes were calculated to study the diffusion in main channel AB.A diffusion path of A(?)B type exists in the main channels:the guest molecules pass the A-Cell and hop from B-Cell to the next B-Cell.And the resistance of this diffusion path increases with increasing chain length.4.The COM probability distributions were also used to study the diffusion in the second channel AP.A diffusion path of A(?)P type existes in the second channels,and the resistance of this diffusion path increases with the chain length as well.Besides,in the system of butane diffusion,the pocket was blocked by butane to make the diffusion path interrupted.5.The diffusion trajectories of n-alkanes were calculated to verification and further analysis diffusion mechanisms.The resistance of A(?)P type diffusion path increases faster than that of A(?)B type, leading to percentage of A(?)P type diffusion path increases with increasing chain length.6.In summary,the simulation results above indicate that the diffusion of various n-alkanes in Cu-BTC follow their preferential path, which has distinct diffusion resistance,leading to the difference in diffusivity.The information obtained provides a better understanding of the diffusion of n-alkanes in MOFs,as well as to guide the practical applications and future design of new MOFs.