| Nitrogen oxides NOx,irritating colorless gas NH3,and C5H5N,C4H5N and C4H4O in liquid oil are all main sources of environmental pollution.Therefore,removing nitrogen-containing compounds from air and liquid oil to reduce pollution has become a hot topic.Among various denitrogenation techniques,adsorptive denitrogenation?ADN?is preferred because of its environmental friendliness and mild operating conditions.The ADN performance strongly depends on the adsorbent.As a new type of adsorbent material,metal-organic frameworks has not only the characteristics of traditional adsorbent,but also the excellent characteristics of highly adjustable pore structure,high specific surface area,high porosity,low density,and functional pore space.In this paper,the researches focus on the modification of HKUST-1,which has exhibited excellent performance in adsorptive denitrogenation through metal center substituion and ligand functionalization of HKUST-1 aiming at a better ADN adsorbent than the pristine HKUST-1.Firstly,the adsorption of nitrogen-containing compounds?NO,NO2and NH3?on 19 M-HKUST-1?M=Be,Fe,Ni,Cr,Co,Cu,V,Zn,Mo,Mn,W,Sn,Ti,Cd,Mg,Sc,Ca,Sr and Ba?was screened by the first principle calculation systematically.It is found that the adsorption of nitrogen-containing compounds?NO,NO2and NH3?on four variants of M-HKUST-1?M=Ni,Co,V and Sc?is stronger than the pristine HKUST-1.Ti-HKUST-1,Sc-HKUST-1,and Be-HKUST-1 present the strongest NO,NO2,and NH3adsorption within the screened M-HKUST-1 series,respectively.With the one exception of NO2dissociation on V-HKUST-1,dissociative adsorption of NO,NO2,and NH3molecules on the other considered M-HKUST-1 is energetically less favorable than molecular adsorption thermodynamically.The barrier calculations show that the dissociation is difficult to occur on Cu-HKUST-1 kinetically due to the very large dissociation barrier(about 458 k J mol-1).Next,the analysis of electronic properties explains the bonding nature between the adsorbed molecule and M-HKUST-1.Subsequently,to study the influence of functional groups on the adsorption behavior,the adsorption of NO,NO2,NH3,C5H5N,C4H5N and C4H4O on functionalized HKUST-1-X?X=-H,-CH3,-OCH3,-NH2,-NO2,Br?has been investigated.The geometry optimization of bulk phase shows that the introduction of some functional groups results in the changes of the most favorable bulk geometry.Different from the pristine HKUST-1with tbo topological structure,when-CH3and-OCH3functional groups are introduced by substituting H of the benzene ring of the ligand,the most stable bulk structure of HKUST-1-X?X=-CH3,-OCH3?presents fmj topological structure,while the most favorable bulk structure of-NH2,-NO2and-Br functionalized HKUST-1-X is the same with the pristine HKUST-1 being tbo.In addition,the present study found that the introduction of functional groups would affect the adsorption of NCCs on HKUST-1.For all the obove considered NCCs adsorption on HKUST-1-X,the adsorption energy keeps the same order:HKUST-1-CH3?HKUST-1-OCH3>HKUST-1>HKUST-1-NO2>HKUST-1-Br?HKUST-1-NH2,namely the introduction of-CH3and-OCH3benifits the adsorption of NCCs on HKUST-1,while the introduction of-NH2,-NO2and-Br inhibits.It is noticed that the adsorption behavior of NCCs in HKUST-1-X is related to its topological structure.In addition,although the introduction of functional groups provides extra adsorption sites,the unsaturated metal center is still one of the most stable adsorption sites.The last part of work takes the smallest non-polar molecule H2as the research object to find out the adsorption capacity descriptor formula of H2in MOFs to verify the feasibility of machine learning method in this field in order to quickly screen out the potential MOFs adsorbents from the huge metal organic framework crystal database.SISSO method is used to calculate the H2adsorption capacity of 252 MOFs,and the best descriptor formula of H2adsorption capacity is selected from about 8×108descriptor formulas.The linear relationship factor R2=0.93 between the H2adsorption capacity QSISSIOcalculated by the descriptor formula and QMCMCcalculated by GCMC indicates that the method is feasible.From the obtained descriptor equation,it can be seen that pore volume is a very important parameter in the best descriptor formula of adsorption capacity.The macro characteristics of porous materials,such as density,pore diameter,and void fraction,also appear in the formula.The best descriptor formula also includes the ion radius and ion potential of the metal center.This method can effectively reduce calculation cost.The present results provide theoretical guidance for adjusting the local structure of MOFs,improving their denitrogenation performance and developing new ADN materials. |
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