Adsorption/Desorption Properties And Mechanisms Of Volatile Organic Compounds On Metal-organic Framework And Its Composites

The strong irritant and pathogenicity or even carcinogenicity of volatile organic compounds?VOCs?caused a great damage to human health.In order to protect the environment and our human health,governments have been established the relevant rules and regulations.However,accurately determination of the contaminant composition,confirmation of its source and removal method are the keys of monitoring and capture VOCs.Therefore,according to the properties of VOCs,several novel porous materials of zeolitic imidazolate framework?ZIF-7?and its composites were successfully synthesized.Synthetic method of ZIF-7,ZIF-7/expanded graphite?EG?,SiO₂@ZIF-7 and SiO₂@ZIF-8?which were self-assembled ZIF layer on mesoporous silica microsphere surface?were developed.In addition,adsorption/desorption properties and mechanisms of VOCs on these porous materials were investigated.And a method without any chemical derivation based on on-line thermal desorption-gas chromatography?TD-GC?has been developed to detect trace amounts of toxic and harmful gases in the air.Chapter 1:A brief overview of VOCs had been introduced,including its definition,classification and harms to the ecological environment and our human health,and the progress of common detection methods of VOCs.Besides,the characteristics and applications of metal-organic frameworks?MOFs?and their composites are summarized.Adsorption mechanisms between MOFs and their composites and VOCs were also highlighted.Chapter 2:ZIF-7 was successfully prepared by the complexation reaction of zinc ion and benzimidazolem,and use it as adsorbent to optimize the adsorption and desorption conditions of itself.Adsorption mechanisms between ZIF-7 and VOCs molecules were investigated.It was found that?-?stacking and hydrophobic interaction played a leading role,and it was also affected by pore/molecular selective effects.As a result,ZIF-7 is a great adsorbent that superior to Tenax-TA and activated carbon.A method of ZIF-7-based TD-GC offered a good performance with relative standard deviation values?RSD,n=5?of 1.69⁷.97%,limit of detection of 1.22².38 ng,limit of quantification was 4.05⁷.94 ng and a good linearity?R²=0.9970⁰.9994?.And this method could detect VOCs level effectively in indoor air.Chapter 3:SiO₂@ZIF-7 and SiO₂@ZIF-8 core-shell microspheres were successfully prepared via seeding process on the surface of SiO₂ microspheres.Adsorption properties of these two materials had been compared with single-phase ZIF-7,ZIF-8 and commercial adsorbent of Tenax-TA and activated carbon.As a result,the strategy of combining SiO₂microspheres and ZIF materials not only solved the disadvantage of low desorption efficiency on single-phase ZIF,but also improved the adsorption efficiency.The property of organic ligands has a greater impact on adsorption.Benzimidazole help enhanced the?-?stacking effect between VOCs with?-conjugated systems and ZIF-7/SiO₂@ZIF-7,while 2-methylimidazole facilitated the interaction between ZIF-8/SiO₂@ZIF-8 and hydrophobic VOCs molecules.In addition,it was also proved that the ZIF and its composites were superior to Tenax-TA and activated carbon through normalized peak area ratio.Chapter 4:Expanded graphite?EG?was synthesized from flake graphite by high temperature expansion after intercalation.ZIF-7/EG composites were prepared by mixing EG with different mass fractions of 0,10,30,50 and 100%into ZIF-7.The adsorption effects of molecular size and polarity of VOCs on ZIF-7 and ZIF-7/EG composite materials were explored via TD-GC method.The results showed that VOCs molecules with smaller size and larger polarity were easier to be adsorbed on ZIF-7 and ZIF-7/EG composite.However,the adsorption of VOCs on the synthesized-materials were more susceptible to?-?stacking and hydrophobic interactions.In addition,among ZIF-7/EG composites with different mass fractions,the adsorption efficiency and desorption efficiency of ZIF-7/EG-10 were higher than the others.