Molecular Dynamics Studies of Diffusion in Confined Regions

Abstract

Diffusion of guest molecules under confinement of zeolites shows many interesting properties due to guest-host interactions. This thesis explores some of these interesting behaviours relating to the diffusion of guest molecules in zeolites using molecular dynamics simulations. These include diffusion of hexane isomers, role of diffusion in pentene isomerization reaction, effect of inter-crystalline regions on experimentally measured diffusivities and diffusion and permeability of charged species inside zeolites. Adsorption and diffusion properties of the five hexane isomers – n-hexane, 2-methyl pentane, 3-methyl pentane, 2,3-dimethyl butane and 2,2-dimethyl butane – in zeolite Y have been obtained from molecular dynamic simulations. The self-diffusivities of the isomers exhibit an anomalous dependence on their molecular diameter where large doubly branched isomer, 2,2-dimethyl butane, shows the maximum diffusivity among all isomers. This anomalous dependence of diffusivity as well as the computed activation energies are in excellent agreement with the predictions of Levitation Effect (LE). The order of exit of different isomers from a zeolite Y column depends on the order of their diffusivities. This order can be controlled by varying the temperature. Also, n-hexane shows a tendency to bend by increasing its gauche conformer population to reduce the energetic barrier it experiences at the 12-ring window of the zeolite Y cage. Transport properties of the reactant 1-pentene along with those of the products: cis- 2-pentene and trans-2-pentene formed during isomerization of 1-pentene in zeolite Y is investigated using molecular dynamics simulations. Of these three, cis-2-pentene diffuses the fastest followed by trans-2-pentene and finally the reactant 1-pentene. The rate determining process in this reaction is the diffusion of 1-pentene inside the zeolite. Calculations with different charge distributions but identical short range interactions on 1-pentene suggest that the Coulombic interactions play a key role in the diffusivities. Simulations with fixed dihedral angles of the isomers show that diffusion is significantly aided by the changes in the torsional motion around the bonds. The activation energy for diffusion is highest for 1-pentene, followed by trans-2-pentene and finally cis-2-pentene. This suggests that the order of diffusivities are determined by the activation energy barrier for the isomers which in turn is determined by the Levitation effect. The reorientational motion of the isomers appears to be determined by their radius of gyration where cis-2-pentene reorients the fastest. Extensive molecular dynamics simulations of xenon in two classes of zeolite crystal systems, one consisting of purely intra-crystalline space and the other with both intra- and inter-crystalline space are reported. The latter mimics a typical powder sample of zeolite. Comparison of results from these two systems provides insights into the structure and dynamics in the presence of inter-crystalline space. The temperature, as well as the distance between the crystallites, have been varied. The density distribution and diffusivities calculated inside the polycrystalline system show that the interfacial region between the crystal and the inter-crystalline region act as a bottleneck for diffusion through the system. At lower temperatures, the particles are trapped at the interface due to the pronounced energy minima present in that region. With the increase in temperature, the particles are able to overcome this barrier frequently and the transport across the inter-crystalline region is increased. A ballistic or super diffusive motion is seen in the inter-crystalline region along all the axes except along the axis which has inter-crystalline space. The transition time for ballistic to diffusive motion increases with the increase in the length of the inter-crystalline space. Velocity auto- and cross-correlation functions exhibit strong oscillations and exchange of kinetic energy along directions perpendicular to the direction of inter-crystalline space. These results explain why uptake and PFG-NMR measurements exhibit lower values for diffusivity for the same system when compared to QENS. Thus, using molecular dynamics simulations, the difference of diffusivity values measured using various experimental methods can be correlated where these inter-crystalline regions are common. Diffusion properties of linear hexane isomer – n-hexane – and doubly branched hexane isomer – 2,2-dimethyl butane in zeolite Beta (BEA) are investigated using molecular dynamics simulations. Zeolite BEA has 2 straight channel systems along x- and y- axes and a zig zag channel system along z-axis. The density distribution function of the isomers within the zeolite shows that both isomers prefer to stay in channels rather than pore intersections. Diffusion is seen to be highly anisotropic in these systems. n-hexane has a higher diffusivity than 2,2-dimethyl butane in zeolite Beta. However, ZLC experimental measurements by B𝑎́rcia et al. suggest that n-hexane has a lower diffusivity than 2,2-dimethyl butane in zeolite BEA pellets. This difference is attributed to the presence of inter-crystalline regions in the experimental sample. The reorientational motion is also calculated for the isomers and it is found that at lower temperatures 2,2-dimethyl butane reorients faster due to globular shape. At higher temperatures, n-hexane bends to assume globular shape which will help the molecule to reorient with similar times as 2,2-dimethyl butane. There have been considerable efforts at designing efficient membranes for reverse osmosis for water desalination. A molecular dynamics investigation into real and model ions in SPC/E water in zeolite Y is reported to understand the factors that are related to the ionic diffusivities. Non-equilibrium simulations of salt solutions in dealuminated zeolite DON and of NaCl solution in zeolite Y have also been carried out. The results suggest that for real ions, there is a good correlation between the lifetime of water molecules in the first solvation shell (FSS) and diffusivity with and without the zeolite. However, for model ions, there is no such correlation in several instances leading to a breakdown of solventberg model. Non-equilibrium simulations suggest that zeolite DON is impermeable to the ions whereas zeolite Y exhibits non zero permeability to Na+ and Cl- ions. However, equilibrium simulations suggest non zero diffusivity of all ions (real and model), although the diffusivity values are of the order of 10-10 m2/s which is one order less that the bulk ion diffusivities in salt solutions