Theoretical Simulations On Gas Adsorption And Separation Of Three Composite Carbon Nanoporous Materials

The rapid development of both society and economy leads to the exhaustion of natural resources as well as severe environmental pollution problems.Therefore,it is imperative to develop a kind of clean energy with abundant storage to adapt to the development of the new era.Hydrogen(H₂)is regarded as one of the best energy carriers thanks to its advantages of abundant reserves,clean combustion product,high energy density and renewable nature.Besides,the main component of natural gas,methane(CH₄),is considered as another potential transportation fuel for future clean energy applications because of its substantial deposits and lower carbon emission than petroleum.However,finding suitable materials for the safe and efficient storage of H₂ and CH₄ is a key issue in the utilization of such energy at present.With high specific surface area,metal–organic frameworks(MOFs)have been considered as a kind of materials very suitable for gas storage.However,pure MOFs have poor storage capacity of H₂ and CH₄ at room temperature and normal pressure,which cannot meet the requirement for the effective utilization.On the other hand,CO₂ capture and separation is very important for reducing environmental problems such as greenhouse effect.In order to find materials with good gas storage and separation performance,we designed and modified three porous carbon materials,then studied their gas storage and separation performance,including the following parts:In chapter one,the advantages of hydrogen and natural gas as well as their limitations in the application process are introduced,while the importance of CO₂ capture and separation are also discussed.Then,the research progress in the gas storage of carbon nanoporous materials was reviewed.In chapter two,the theoretical basis,software applied in this dissertation,and the research methods were introduced,including the development of quantum chemical methods,the density functional theory,molecular dynamics method,grand canonical Monte Carlo method,as well as software packages used in the simulation.In chapter three,we modified graphdiyne by B doping as well as Li or Na decoration,and then studied its H₂ storage performance.The first principle molecular dynamics simulations show that,each metal atom,for double side Li and Na decorations of 1B-GDY,can adsorb up to 5 H₂ molecules.The GCMC simulations show that the gravimetric H₂uptakes of Li and Na modified 1B-GDY,at 233 K and 100 bar,are correspondingly 7.10 wt%and 5.80 wt%,both of which reach the 2025 DOE target.The results show that Li or Na decoration after B doping is an effective way to improve the adsorption capacity of two-dimensional materials such as graphdiynes.In chapter four,we designed a novel MOF structure,using heterofullerene as linker(C₄₈B₁₂-MOF),and then study their gas storage and separation performance.The density functional theory calculation and first-principles molecular dynamics simulation show that the C₄₈B₁₂-MOF has good struactrual and thermal stabilities.The grand canonical Monte Carlo(GCMC)simulations shows that at 298 K and 100 bar,the gravimetric and volumetric H2uptakes of C₄₈B₁₂-MOF after Li decoration are 7.09 wt%and 31.36 g/L respectively.The gravimetric uptakes reach the ultimate target,and the volumetric uptakes reach the 2020 DOE target.Thus,it is an excellent material for H₂ storage.At 298 K and 40 bar,the gravimetric CH₄ uptake also achieves the DOE target.Meanwhile,the designed material also shows excellent adsorption and separation ability of CO₂.The results show that the new material is stable at room temperature,and its gas adsorption and separation performance can be greatly improved after Li decoration.In chapter five,we used multiple modifications to MOF-205 and studied their gas storage and separation performance.One C atom in every benzene ring of MOF-205 is substituted by B atom to enhance the binding energy of Li atom(B-MOF-205),and then B-MOF-205 was modified by Li(B-MOF-205-Li)decoration and heterofullerene(C₄₈B₁₂)impregnating(C₄₈B₁₂@B-MOF-205).We also consider the case that C₄₈B₁₂-Li was impregnated into B-MOF-205-Li(C₄₈B₁₂-Li@B-MOF-205-Li).GCMC simulations shows that at 233 K and 100 bar,the H₂ uptakes of B-MOF-205-Li are 5.18 wt%and 31.09 g/L,which reach the 2020 DOE target.While the H₂ uptakes are 5.99 wt%and 41.07 g/L for C₄₈B₁₂-Li@B-MOF-205-Li,which reach the 2025 DOE target.The gravimetric CH₄ uptakes are 0.59 g/g and 0.53 g/g,while the volume uptakes are 364.84 L/L and 402.47 L/L,respectively,which also exceed the DOE target.Meanwhile,the multiple modified MOF-205shows satisfactory performance of CO₂ separation in different ratios of CO₂/CH₄ and CO₂/H₂mixtures.The results show that B doping,C₄₈B₁₂ insertion,and Li modification are effective methods to improve gas adsorption and separation ability of MOFs.