Controllable Synthesis Of Fe-MOFs And The Research On Photocatalytic Degradation Of VOCs

Volatile organic compounds（VOCs）have adverse effects on the ecological environment and human health due to their toxicity and environmental persistence.Aromatic VOCs（benzene,toluene,xylenes,and styrene）and aldehydes（formaldehyde and acetaldehyde）exist widely in the environment.Therefore,reducing or eliminating the above VOCs is of great significance to purify the atmospheric environment and protect human health.Photocatalytic technology is considered to be one of the most promising methods because of its low energy consumption,mild reaction conditions and easy realization.However,Ti O2 and Zn O,as the most widely used inorganic semiconductor photocatalysts,still suffer from poor affinity for aromatic hydrocarbons,the low photocatalytic activity in visible light and easy deactivation,which severely limit the application of photocatalytic technology in removing VOCs.Therefore,it is necessary to explore new photocatalytic materials for the degradation of VOCs.Metal-organic frameworks（MOFs）are new porous materials composed of metal clusters and organic ligands.Some MOFs such as iron,zirconium and titanium-based MOFs exhibit semiconductor-like photocatalytic activity and have been used in photocatalytic degradation of dyes,CO2 reduction and the splitting of water.Benefiting from the high specific surface area（～1000 m²/g）and porosity,MOFs show improved ability in the capture and adsorption of VOCs.The orderly arranged metal active centers provide abundant metal catalytic sites for photocatalytic reactions.Meanwhile,adjustable structure and easy functionalization of MOFs enable the catalysts to be designed for specific target pollutants.Therefore,the development of highly-performance MOFs for the photocatalytic degradation of VOCs is of great importance for photocatalytic air purification.However,there are still problems in the degradation of VOCs by MOFs as photocatalysts:（1）The easy recombination of photogenerated electron-hole pairs owing to their short diffusion path leads to low catalytic efficiency,which limits their application in the photocatalytic degradation of VOCs;（2）The relatively small diameter of micropore is not conducive to the mass transfer of macromolecular VOCs in the channels of MOFs,resulting in low reaction rate;（3）A large number of Lewis acid sites in MOFs are not beneficial to the adsorption and degradation of oxygenated VOCs.Therefore,how to solve the above problems through the design and optimization of materials is the key to realize the application of MOFs-based photocatalytic materials.To solve these problems of MOFs during photocatalytic remove of VOCs,works have been done in the following aspects,（1）Synthesis of Z-Scheme MIL-100（Fe）/α-Fe2O3 Heterojunction for Enhanced Adsorption and Visible-light Photocatalytic Oxidation of O-xyleneMIL-100（Fe）/α-Fe2O3 photocatalysts were fabricated through a facile one-step hydrothermal method by adjusting the coordination of Fe（III）ions for the photocatalytic oxidation of typical VOCs.The MIL-100（Fe）/α-Fe2O3 hybrid presented a high o-xylene removal efficiency of 100%under a 250 W Xenon lamp irradiation and 90%under visible light（λ≥420 nm）,which is far beyond the performance of commercial Ti O2 photocatalyst under the same conditions（23%under a 250 W Xenon lamp irradiation and 0%under visible light）.The good removal performance of the MIL-100（Fe）/α-Fe₂O₃ for o-xylene is due to the large specific surface area(763?m²?g^-1),uniformly distributed active sites,suitable pore structure and the formation of Z-Scheme heterojunction.The heterojunction betweenα-Fe2O3 and MIL-100（Fe）made the electrons in conduction band ofα-Fe2O3 transfer to the highest occupied molecular orbital（HOMO）of MIL-100（Fe）and recombined with the holes in the HOMO of MIL-100（Fe）.The holes were left on the valence band ofα-Fe2O3 to oxidize the adsorbed water molecules to produce highly oxidizing·OH,while electrons were left on the lowest unoccupied molecular orbital（LUMO）of MIL-100（Fe）to produce highly active·O2^-.In addition,the photocatalytic reaction mechanism of o-xylene were deeply explored,and the reversible conversion of Fe（III）and Fe（II）under light irradiation was found to participate in and promote the photocatalytic oxidation of o-xylene.This work not only provides a means for the synthesis and optimization of high performance photocatalysts based on MOFs for air purification,but also sheds light on the photocatalytic oxidation mechanism of o-xylene by MOFs-based photocatalysts.（2）MIL-100（Fe）MOF/MOX（metal-organic xerogel）Homojunctions with Tunable Hierarchical Pores for the Photocatalytic Degradation of BTXS（benzene,toluene,xylenes,and styrene）The construction of MOFs with hierarchical pores is of great importance for the photocatalytic oxidation of gaseous BTXS pollutants with large molecular size,but it is still challenging.Herein,by adjusting the ratio of anions（NO3^-and Cl^-）in the metal precursors,hierarchically porous MIL-100（Fe）MOF/MOX homojunctions were obtained through a one-pot solvothermal method.The existence of both micro-and mesopores（2～10 nm）enabled mass transfer of VOCs molecules and made the active sites more accessible.The formation of MOF/MOX homojunctions promoted the separation of electron-hole pairs.Additionally,the coordinatively unsaturated acidic Fe3-O sites facilitated the capture of BTXS molecules and participated in the photocatalytic oxidation process through the conversion between Fe（III）and Fe（II）.Under the irradiation of 250 W Xenon lamp,the MIL-100（Fe）MOF/MOX homojunction showed improved performance towards the photocatalytic degradation of o-xylene（83.1%）compared with the crystalline（63.4%）or xerogel（69.7%）counterparts,and the photocatalytic reaction rate was 1.73 times and 1.30 times higher than that of crystalline or xerogel counterparts,respectively.The MIL-100（Fe）MOF/MOX homojunctions exhibited great potential in the photocatalytic oxidation of aromatic air pollutants.（3）The Regulation of Lewis Acid/Basic sites in MIL-100（Fe）MOX and the Research of its Adsorption and Degradation for Electron-Rich/Deficient VOCsThe performance of metal-organic xerogels（MOXs）in the photocatalytic degradation of VOCs relies on their Lewis acid-base interaction and the generation of oxidative radicals.However,the single Lewis acid sites makes MOX not beneficial to the adsorption and photocatalytic degradation of electron-deficient oxygenated VOCs（such as aldehydes）.Therefore,the generation of Lewis basic sites and effective regulation of Lewis acid/basic sites in MOX is of great value for the controllable photocatalytic degradation of VOCs.In this chapter,the Lewis basic sites were created in MIL-100（Fe）MOX by introducing metal Na and designing the local charge imbalance of the ligand.The Lewis basic sites of Na Fe-MOXs were enhanced by synergistic effect between the carboxylic acid groups,Fe³⁺,and Na⁺,which promoted the adsorption of acetaldehyde,while the pristine Fe-MOX with abundant Lewis acid sites could capture more o-xylene.Compared with Fe-MOX（20.0%）,1Na Fe-MOX showed improved photocatalytic performance for acetaldehyde（38.0%）,and the mineralization ability was increased from 58.5%to 90.4%.Compared with Fe-MOX（69.0%）,although the removal efficiency of 0.5Na Fe-MOX in photocatalytic degradation of o-xylene（49.2%）was decreased,the high concentration of CO2 was produced and improved the mineralization ability of o-xylene.These were attributed to the generation of Lewis basic sites in 1Na Fe-MOX and 0.5Na Fe-MOX,which changed the interaction between o-xylene/acetaldehyde and photocatalysts and promoted the formation of oxidative free radicals.The successful preparation of the Na Fe-MOXs provides a new mean for adjusting the Lewis acid/basic sites of MOFs materials to realize the controllable photocatalytic degradation of electron-rich/deficient VOCs.