Molecular Dynamics Studies On Water Separation Properties By Membrane Materials Based On Graphene And Polymer Of Intrinsic Microporosity

Membrane separation is a new,highly-efficient and precise separation technology.Membrane separation technology not only possesses the functions of separation,concentration,and purification,but also features energy-saving,environmental protection,molecular-level filtration.More importantly,membrane separation processes are simple and easy to control.The design of separation membrane often considers steric hindrance effects,electrostatic interaction and chemical interaction.In this dissertation,we introduce the separation characteristics of graphene-based membranes and polymer of intrinsic microporosity(PIM)membranes,with perspectives in potential applications of water desalination and biofuel dehydration.In chapter one,the general classification of separation membrane materials with their advantages,disadvantages and applications are briefly reviewed.Two membrane separation technologies:reverse osmosis(RO)and pervaporation(PV)are further introduced.Finally,two applications of membrane separation in environment and energy fields are stated:seawater desalination and ethanol dehydration.In chapter two,we introduce molecular dynamics(MD)method,which includes equations of motion,force fields,periodic boundary conditions,potential energy cut-offs,transmission and diffusion,etc.In chapter three,separation of ethanol/water mixture through single-layer graphene with designed nanoscale pores is investigated.It is observed that graphene nanosheets with suitable nanopores can selectively and preferentially pass water.The diameter and chemical functionalization of pore are found to govern the ethanol/water separation.In particular,separation performance is more sensitive to the pore size than pore's chemistry.Due to lower potential barrier for water diffusion in the pore vicinity of hydrophilic graphene membrane,water flux is higher than that of the hydrophobic membrane.This simulation unveils the governing factors that are important to design graphene separation membranes.In chapter four,we explore the water desalination performance by bilayer graphene with ripples(BGR)membranes.According to the simulation results,the size and shape of the ripples determine the water transport properties.For small and middle ripples,water molecules need to rotate with certain entrance angle and exit angle to pass through the bilayer membrane.The BGR-1.6 membrane with the best performance exhibits water permeance of 1 020 kg/(m2·h·bar)and 98.1%salt rejection,which is three orders higher than those of available commercial seawater RO membranes.In chapter five,we propose a strategy to construct polymer of intrinsic microporosity(PIM)membrane.The swelling test of the PIM-1 membrane shows that PIM-1 prefers adsorbing ethanol,and the swelling degrees in ethanol/water azeotropic mixture and pure water are found to be in good agreement with the measured values in experiments.Further,we investigate the performances of water desalination via RO and biofuel dehydration via PV using PIM-1 membrane.MD simulation results show that the water permeability through PIM-1 is on the order of 8.64×10-7 kg-m/(m2·h·bar)while the salt rejection is 100%,which is similar to that of single-atom-thickness graphene and graphyne.Moreover,the performance of PIM-1 as a PV membrane is better than other PV membranes in the literature,such as poly(vinyl alcohol).In chapter six,we study the swelling behavior in water as well as water desalination performance of PIM membranes with different functional groups.The density,swelling degree and void size of functionalized PIMs determine the water flux in RO process.The PIM-F4 membrane outperforms other PIM membranes with a water permeability of 1.2×10-6 kg·m/(m2·h·bar)and 100%salt rejection.Therefore,suitable chemical functionalization in PIMs can be used to improve water desalination.