Research On HCDs@MOFs Adsorbent Preparation And Benzene Adsorption Performance Improvement Mechanism

Benzene is a typical volatile organic compound,which is harmful to both the environment and human body.MOFs have shown good application potential in the adsorption and capture of VOCs because of their well-ordered pore structure,large specific surface area,unique flexibility and strong designability.However,in the practical application of adsorption technology,the ubiquitous water molecules will compete with VOCs to reduce the adsorption performance,attack the ligand nodes to destroy the framework of MOFs,and make regeneration more difficult and energy intensive,Therefore,the development of hydrophobic high adsorption capacity MOFs adsorbent is still a challenging topic.In this work,Cu-BTC,Cu-BDC,MIL-100（Fe）and MIL-53（Fe）were selected for the study,and their in situ composite modification by hydrophobic carbon dots（HCDs）was used to investigate the effects of HCDs on the physicochemical properties and adsorption performance of the four MOFs systematically by some characterization means;The composite adsorbents with good performance were selected,and the effects of the amount of HCDs on the physicochemical properties and adsorption properties of each composite adsorbent were systematically investigated;On this basis,the competitive adsorption behavior of benzene and water on each adsorbent,adsorption mechanism,thermal effect of adsorption and diffusion behavior were investigated in depth,and the mechanism of compound modification of MOFs by HCDs was elucidated,and the main results are summarized as follows:（1）The results of composite modification of four MOFs by HCDs showed that the pore structure as well as the crystalline condition of HCDs@Cu-BTC-C1 and HCDs@MIL-100（Fe）-C1 were significantly enhanced,while HCDs were inhibitory to Cu-BDC and MIL-53（Fe）;The static adsorption results of benzene and water showed that when P/P₀=0.1,compared with Cu-BTC,HCDs@Cu-BTC-C1 showed a decrease of 18.98 mg/g for water and an increase of 36.66mg/g for benzene,which increased the adsorption capacity of benzene along with the hydrophobicity,The adsorption of HCDs@MIL-100（Fe）-C1 increased by 175.65 mg/g for benzene and 16.87 mg/g for water compared with MIL-100（Fe）for both benzene and water,but the increase in benzene adsorption was much greater than that of water adsorption.（2）Different concentrations of HCDs were used to compound with Cu-BTC and MIL-100（Fe）,and the test results showed that the benzene adsorption capacity of each compound adsorbent showed a trend of increasing and then decreasing with the increase of HCDs addition.Among them,HCDs@Cu-BTC-C1 and HCDs@MIL-100（Fe）-C1 had the highest specific surface area,static adsorption of benzene and dynamic breakthrough time,indicating that one times the concentration of HCDs was the optimal addition amount,and the addition of moderate amount of HCDs could promote the crystal growth of Cu-BTC and MIL-100（Fe）to produce more unsaturated metal sites,while excessive amount will cause crystal defects and blockage of pore channels,which will affect the adsorption performance.（3）The benzene/water competitive adsorption selectivity of Cu-BTC and MIL-100（Fe）could be significantly enhanced by the addition of appropriate amount of HCDs,HCDs@Cu-BTC-C1 and HCDs@MIL-100（Fe）-C1 could reach up to 2.7 and 3.4,respectively,improved their competitive benzene/water interactions;MIL-100（Fe）formed multilevel pores in the addition of HCDs led to a decrease in the heat of adsorption of benzene on each adsorbent,while Cu-BTC was a change in the distribution of micropores and increased the activation energy of benzene desorption on each adsorbent;the presence of HCDs did not change the adsorption sites of benzene on the adsorbents as verified by in situ FTIR and benzene-TPD analysis,This is mainly attributed to the low content of HCDs and the overlap ofπ-πinteractions in HCDs with those in Cu-BTC and MIL-100（Fe）;XPS and FTIR characterization results indicate that HCDs have different mechanisms of action on Cu-BTC and MIL-100（Fe）.Water contact angle experiments confirm that HCDs are hydrophobic and their hydrophobicity can be enhanced by incorporation into MIL-100（Fe）;the generation of mesopores leads to improved mass diffusion and transfer of benzene in HCDs@MIL-100（Fe）-Cx,and HCDs@MIL-100（Fe）-C1 have good dynamic cycling stability.