Multi-scale Simulations Of Gas Adsorption And Separation In Functional Porous Materials

Gas adsorption and separation widely exist in the fields of chemical engineering,petroleum,food,light industry and environmental protection,and the efficiency of adsorption and separation are mainly determined by the choice of adsorbent,MOF and COF materials,as the newly-developed materials with high surface areas,large void,big pore volume and easily-functionalized pore suface,are widely used for the application of adsorption and separation.However,due to the enormous number and diversity of MOF and COF,it is difficult to directly find out the desirable one for specific gas adsorption and separation.Focusing on this challendge,this thesis prepares to obtain COF and MOF materialsby functional modification,i.e.,metal dopant and topological control.Further based on the multiscale methods of grand canonical Monte Carlo(GCMC)simulation combined with density functional theory,some key scientific problems on gas adsorption and separation have been systematically investigated,such as the adsorption,storage and separation of CO2 and C4 hydrocarbons,as well as H2/D2 separation.Overall,some applicable and instructive results have been achieved,which are introduced as following:(1)Based on the reported structures of 2D squarine-involved covalent organic frameworks(SQ-COP),lithium atoms are quantificationly and orderly doped on the naked oxygen of squarine unit to get the new materials,SQ-COP-Li.From DFT calculations,it is found the atomic charge of framework atoms are remarkably changed because of lithium dopant,which obviously enhances the adsorption ability to CO2,further GCMC simulation confirm that CO2 adsorption uptake can reach 202.0 mmol/g at 298 K and 100 bar,which is comparable with the best reported 3D COF material;moreover,even at 298 K and 30 bar,the CO2 uptake can also reach 61.5 mmol/g,which is twice those of MOF-177 and IRMOF-10.Such high CO2 storage capacity not only owes to the lithium dopant,but also due to the super-high surface area.Beyond the CO2 storage,SQ-COP-Li also reveals high CO2 selectivity,the CO2/H2 and CO2/CH4 selectivities can reach 10.3 and 3.6 at 298 K,both higher than traditional COF materials.What s more important,we designed two possible pathways to experimentally synthesize SQ-COP-Li,and lithium di-isopropylamide as the lithium resource is more energetically feasible.(2)Based on the conclusion that metal dopant can enhance the gas adsorption and separation properties,and by employing the Chemical Affinity Quantum Sieving(CAQS)mechanism,D2/H2 separtion has been investigated.The lithium-doped cluster,alkaline earth metal-doped clusters and MOF clusters with open metal sites have been calculated for H2 adsorption energy,and the alkaline earth metal dopant especially beryllium can signicficantly enhance the H2 adsorption,and the energy decomposition analysis(EDA)further reveals that orbital interaction mainly contributed to the strong H2 adsorption energy,namely higher contribution of orbital interaction,the larger zero point adsorption energy difference between H2 and D2,leading to higher efficiency of D2/H2 separation.Such clusters were further introduced into peridoc MOF material UiO-67,producing UiO-67-COOLi and UiO-67-O4Be2,and the GCMC simulations based on FEYNMAN HIBBS potential function reveal that the D2/H2 selectivity of UiO-67-O4Be2 can reach 40.7,which is higher than 25.3 of UiO-67-COOLi and 12.1 of UiO-67.These results indicate the great improvement of D2/H2 selectivity by beryllium dopant in MOF,from both microscopic and macroscopic views.(3)Aside frommetal dopant,topologycontrolled pore property is also an important strategy to improve adsorption prroperties.The effects of pedal motion,interpenetration and substituting link groups to control the pore and channel of COF have been employed,and the C4 hydrocarbons adsorption and selectivity has been investigated.Based on periodic DFT calculations,it is indicated that the interlayer space and specific angle of COF-300 determine the interpenetration number,and the pedal motion on COF-320 would change the pore shape from the square pore to dumbbell pore,while-N=N-linkage of azo-COF-2 can reduce the channel dimension from 2D to 1D.Further GCMC simulations revealed that such changes of pore and channel in COFs would significantly influence the adsorption and separation of C4 hydrocarbons,and COF-300(dia-c5)shows obviously higher isobutene adsorption property than those of other COFs.The density distribution,EDA and radial distribution function(RDF),further suggested that isobutene molecules would be adsorbed in the interlayer of COF-300(dia-c5)by lamellar stacking configuration at low pressure,and there are π-π stacking interaction between isobutene and-CH=N-of COF-300(dia-c5);with pressure increasing,isobutene molecules then to gather in pore and channel by end-to-end configuration,which is due to the contribution of guest-guest interaction.Besides,by GCMC simulations,the selectivity of isobutene/other C4 hydrocarbons in COF-300(dia-c5)follows the order of isobutene/2-trans-butene(38.4)>isobutene/2-cis-butene(35.8)>isobutene/n-butane(13.4)>isobutene/1-butene(6.3)>isobutene/isobutane(2.6)>isobutene/1,3-butadiene(2.5).The high isobutene selectivities in COF-300(dia-c5)is of underlying value for industrial processes.