Molecular Simulation Study On MOF@polymer Composite Membranes For Gas Separation

Metal-organic frameworks(MOFs)have received remarkable attentation as a new class of porous solid materials.They usually have high porosity,adjustable pore size,relatively large specific surface area,and good mechanical stability;these make them ideal membrane materials for gas separation.Considering the large number of MOFs that are available for gas separation,it is practically infeasible to test the gas separation performance of every single MOF film or MOF@polymer composite membrane by purely using experimental methods.Therefore,molecular simulation methods are of great usefulness to complement experiments because they can be used to study the mechanisms of gas separation.In recent years,molecular simulation methods have been used to screen a large number of MOFs suitable for gas separation to select the optimal MOF membranes.In this way,the effort,time,and resources used for the experiment can all be directed to the MOFs with the best separation performance.In this thesis,based on the molecular simulation methods,numbers of excellent MOF@polymer composite membranes for CH4-N2 and O2-N2 separation are found out for the first time,through large-scale screening of MOFs.The main contents and foundings are as follows.(1)The adsorption capacity,adsorption selectivity,diffusion selectivity,and membrane selectivity of more than 20,000 MOFs,were predicted under practical conditions of 303 K,5 bar,and CH4:N2=1:1.Compared with traditional nitrogen-preferentially polymer membranes and zeolites,it is found that most of the existing MOFs preferentially penetrate methane gas.Relation between separation performances of methane-preferred pure MOF films and the structural properties were also investigated.Results showed that MOFs having the PLD in the range of 2.4?3.7 A,the LCD in the range of 4.6-5.4 A,the porosity in the range of 0.35?0.45,and small specific surface area are the most promising metarials for CH4-N2 separation.On the other hand,a variety of nitrogen-preferentially infiltrated pure MOF films exceeded the upper bound.(2)The results of large-scale screening of MOFs are combined with experimental data of polymer membranes for CH4-N2 separation by using the Maxwell model.The separation perfomances of composite membranes with different MOFs and polymers were investigated.Among them,methane-preferred composite membranes with the highest selectivity and permeation flux were YAVVEX@Pebax-2588 composite membrane and QUCFIF@Poly(trimethylsilylpropyne-cophenylpropyne)composite membrane respectly;the nitrogen-preferred composite membranes with best separation performances was VEYVIG@ Hyflon(?)AD60X composite membrane.In practical conditions,YAVVEX@PDMS composite membranes are optimal for industrial production.The results of this work will be useful to guide the experiments to the most promising MOFs and polymers for efficient separation of CH4-N2 mixture.(3)Towards O2-N2 separation,MOFs with a PLD in the range of 2?3 A,a LCD in the range of 3?4.8 A,and a porosity in the range of 0.42?0.47 were chosen to computionally prepare composite membrane with polymers.The oxygen selectivity and permeation flux of these MOF@polymer composite membranes were increased at the same time,and some MOF@polymer composite membranes suitable for industrial production were listed.