Computational Study Of Mechanism And Functionalization Of Metal-Organic Frameworks In Adsorption And Separation

Industrial separation and energy storage are the two of the most important research fields for industry development,in which adsorption technology,with its advantage of low energetic consumption,less financial investment and ease operation,has attracted widespread interest.With respect to organic gas separation and storage,metal-organic frameworks(MOFs)exhibit significant potential to be outstanding adsorbents.Based on the Cambridge Structural Database(CSD),the MOFs families has grown dramatically during the last decades with total number of about 70000 materials,such diversity of MOFs may hinder its development by the time-consuming nature of experiments,while computational modeling provides the possibility to access the mechanism behind the experimental behaviors from molecular level,as well as the predictions for hypothetical materials,further compensate for the limitations of experiments.In this respect,multiple computational approaches including Grand Canonical Monte Carlo(GCMC),Density Functional Theory(DFT),and Ideal Adsorbed Solution Theory(IAST),were employed in this work to study of several adsorption/separation systems that are of interest in industrial separation and energy storage.Adsorptive denitrogenation(ADN)of fuel was investigated in MIL-101(Cr),MIL-100(Fe)and Cu-BTC.The obtained results indicate MIL-101(Cr)exhibits the strongest adsorption loadings with the organonitrogen(QUI: 14.2 mg/g,IND: 8.5 mg/g)compared to other two MOFs,and the adsorptive selectivity of the components in MIL-101(Cr)follows the order of organonitrogen(QUI: 234.0,IND: 79.1)>> organosulfur(DBT: 4.0,DMDBT: 3.2)> naphthalene(1.0),indicating that MIL-101(Cr)is a promising ADN adsorbent.The DFT results clarify the varied mechanisms of ADN in MIL-101(Cr).The adsorption of basic QUI is governed by the electrostatic interaction between lone pair electrons of N and the open metal sites,while non-basic organonitrogen like IND and CAR is adsorbed through the hydrogen bonding-O sites interactions,and MCAR adsorption is dominated by the ?-open metal interaction.Apart from denitrogenation,ethane/ethane separation is another challenge in separation community.In this work,the structure-performance relationship of ZIFs adsorptive selection of ethane/ethylene mixtures were investigated.For single-component adsorption,ZIF-3 with smallest pore limiting diameter is able to provide strongest interaction affinity at low pressures(ethane: ~-6.7 kcal/mol,ethylene: ~-5.8 kcal/mol),while ZIF-6 and ZIF-10 with larger surface area and pore volume have larger adsorption capacity at high pressures(with ethane and ethylene loadings of 10.0 mmol/g).In terms of binary mixture separation,the “cooperative effect” between ethane and ethylene helps to enhance the ethane adsorption at low pressures because of the stronger interaction of ethane with ZIFs,and ZIF-3 with small pore diameter possesses highest ethane selectivity of 4.8.At high pressures,the selectivity of ethane drops accordingly as both adsorbates fill the cages,but the stronger “competitive effect” of ethane allows ethane further clusters in the pores,ZIF-10 with larger pore volume is able to accommodate more ethane and thus exhibits high ethane selectivity of 2.0.Similar as ethane/ethane mixtures,separation of benzene/toluene is more effective by means of adsorption technology.We theoretically fabricated two linker-functionalized UiO-Phe and UiO-Me2,on the basis of UiO-67,so as to improve the adsorptive selectivity of benzene/toluene mixture.Both functionalization approaches enhance the ? delocalization around the linkers,which help to increase the interaction affinity with the adsorbates at low pressures(e.g.binding energy of benzene,UiO-Phe and UiO-Me2 increase by 13.7% and 15.7%,respectively),but the addition of functional groups also leads to steric hindrance at high pressures,leading to lower saturation loadings(e.g.for toluene loadings,UiO-Phe and UiO-Me2 decrease by 25.0% and 48.5%,respectively).For binary mixture adsorption,UiO-Phe creates stronger ? delocalization without significantly compressing the cages,and thus provides highest toluene selectivity(~20.0)at 0.01 kPa.Previous study emphasizes the role of linkers on separation performance,while the open metal sites are another significant properties of MOFs during adsorption.Hence,the open metal site effects of M-BTCs(M=Ti,Fe,Cu,Co,Ru,Mo)are investigated for the adsorptive separation of methanol/acetone mixtures.The low pressure adsorption is affected by adsorbate-adsorbent interactions,as well as the electrostatics,size and mass of the open metal sites,among which methanol-open metal sites(~-36.0 kJ/mol),and methanol-methanol(~-24.0 kJ/mol)interaction are both governed by electrostatic interactions,while acetone-framework(~-24.0 kJ/mol),acetone-acetone(~-12.0 kJ/mol)interactions are determined by the dispersive interactions,although certain extent of electrostatic interactions exist between acetone-framework(~-21.0 kJ/mol).At higher pressures,the open metal sites are gradually blocked by the adsorbates and exhibit less impacts on the adsorption,while Ti-BTC with largest surface area has highest adsorption loadings.In terms of binary mixture separation,the “cooperative effect” strengthens the adsorption of both adsorbates at low pressures;as the pressure rises,the acetone tends to distribute around the structure due to its lower liquid enthalpy,while methane adsorbs around the polar sites,but has lower adsorption around the apolar space.Based on the study above,Cu-BTC exhibited relatively low loadings of methanol in the apolar pores.Thus,Li-doping was performed to strengthen the methanol adsorption in Cu-BTC.Compared to Cu-BTC,Cu-BTC-Li shows higher loadings(298 K,~500.0 cm3(STP)/cm3)and more thermally stable methanol capacity(adsorption loading only decreases 27.5% with temperature increases 70 K).DFT results indicate that Li-doping creates new adsorption sites for methanol(electrostatic interactions with binding energy of-90.5 kJ/mol)in the apolar cages.Without considering the electrostatic interactions,Cu-BTC-Li is capable of providing comparative methanol capacity(298 K,~400.0 cm3(STP)/cm3)at high pressures as the parent Cu-BTC,since the small size of Li atoms is unlikely to compress the adsorption space in the cages.Therefore,Li-doping is a promising strategy to improve methanol storage in Cu-BTC.Apart from facilitating the emrgence of methanol as clean energy resource,development of natural gas storage system is of significance,as natural gas is one of the most widely used renewable energy resources nowadays.A mathematical model is developed to simulate the performance of an adsorbed natural gas(ANG)system for storage of pipeline natural gas with up to eight components using MOFs as adsorbents.The impurities with stronger interaction affinity tend to gradually accumulate in the ANG system,diminishing the deliverable methane and energy,e.g.40% of butane was accumulated in Cu-BTC after 100 cycles,leading to 25.7% decrease of deliverable energy.Such accumulation effects become more serious with more species of impurities as well as their stronger interaction with MOFs,e.g.the deliverable energy and working time decrease 11.2% and 30 hours,respectively,when five components mixture extends to eight components.Comparing with different MOFs,MOF-5 is found to provide the most stable methane capacity and deliverable energy(maintaining 70% of deliverable methane and 75% deliverable energy after 200 cycles).These results highlight the importance of considering multicomponent effects during ANG applications,and suggesting that concerns of structural properties of MOFs should be taken into account when developing MOFs as ANG adsorbents.