Preparation Of G-C₃N₄@Zn-MOF And Study Of Its Photocatalysis

Efficient removal of pollutants from wastewater is very significant for remediation and protection of environment.Traditional methods,like adsorption,chemical oxidation,and biological treatment,generally play an important role in cleaning wastewater,but they still need to be improved to enhance efficiency and to avoid the secondary pollution.Photocatalysis,one of water treatment methods,can degrade supermolecule organic pollutants into biodegradable compounds as well as inorganic CO₂,H2 O under solar irradiation.Besides,photocatalysis can be used to deoxidaed toxic Cr???into Cr?III?,which is less toxic and requisite for organism.It can be seen that photocatalysis is low cost,environmental protection and ecology.Nowadays,TiO₂ has been reported as and less toxic and low-cost photocatalyst,although it suffers from post-separation,which limites its application.Therefore,it is necessary to develop noval photocatalysts.Metal-Organic Frameworks?MOFs?is porous materils built by metal ions and organic ligands,which have been used extensively in many fields,like photocatalysis,gas separation,CO₂ capture,sensor,fluorescence,drug delivery and adsorptive removal of pollutants in wastewater.Taking photocatalysis as example,MOFs have attracted much attention due to their tunable and designable structures,and available composites.1.A noval MOF?Zn?bpy?L,namely BUC-21?was synthesized based on Zn and linked by cis-1,3-dibenzyl-2-imidazolidone-4,5-dicarboxylic acid and 4,4'-bipyridine.BUC-21 was characterized by single X-ray diffraction,powder X-ray diffraction?PXRD?,Fourier transform infrared spectra?FTIR?,UV-Vis diffuse reflectance spectra?UV-vis DRS?and thermogravimetric analyses?TGA?.Cr???and four organic dyes?methylene blue,Rhodamine B,methyl orange and reactive X-3B?were selected as polluted modes to test the photocatalytic performance of BUC-21.The effect of pH on photocatalysis was studied,and photocatalytic reaction under mixed solution was also carried out.The resuts showed that the structure of BUC-21 can not only be stable at the pH range of 2 – 12,but also showed highly efficient photocatalytic performance,which was much better than commercial TiO₂?P25?under the same conditions.It is interesting to note that BUC-21 was unable to achive simultaneous reduction of Cr???and degradation of dye in the Cr???/dye system.The phenomenon was resulted from the competition between Cr???and the organic dye for access to the photo-excited electrons,which was proven by adding scavenger into the photocatalytic system.The possible mechanism was proposed based on the transient photocurrent response of BUC-21.2.It has been proven that the photocatalytic performance of BUC-21 is excellent,and its structure is highly stable.However,BUC-21 can only be used under UV light irradiation,which limits its application area.In order to solve the question,g-C₃N₄,a semiconductor responded to vis light,was selected to composite with BUC-21 through mechanochemistry.The composite,g-C₃N₄@BUC-21,was characterized by PXRD,TGA,FTIR,UV-vis DRS and TEM?Transmission Electron Microscope?.It can be seen from TEM images that g-C₃N₄ was covered on the surface of BUC-21,which was beneficial for g-C₃N₄ to access light.Photocatalytic performance of g-C₃N₄@BUC-21 towards Cr???and MB was carried out under visible light.The results showed that the reduction rate of Cr???on g-C₃N₄@BUC-21 was more 82% than that of pure g-C₃N₄.The removal rate of MB was enhanced with increasing pH,which can reach 100% as pH = 12,while degradation rate downed to 50% at pH = 3.It can be attributed to concentration of hydroxyl free radicals?·OH?,which increases as increasing hydroxide ions at higher pH.3.g-C₃N₄@BUC-21 can be used as photocatalyst under visible light,which broadens the application of BUC-21.The ligand constructing BUC-21,however,is expensive.In this chapter,a cheap and stable MOF,ZIF-8,was selected to prepare composite with g-C₃N₄.g-C₃N₄@ZIF-8 was prepared by in-situ method,and characterized by PXRD,TGA,FTIR,UV-vis DRS and TEM.The photocatalytic activity of g-C₃N₄@ZIF-8 was tested by degradation of MB under vis light irradiation.The results indicated that the photocatalytic activity of g-C₃N₄@ZIF-8 was affected by the ratio of g-C₃N₄ and ZIF-8.When ZIF-8 : g-C₃N₄ = 4 : 1,the composite showed highly photocatalytic activity,and MB can be degradaded more 43% than g-C₃N₄.The effects of pH and concomitant ions on photocatalysis were also studied.The results showed that faster degradation happened under higher pH value.In this system,different from g-C3N-4@BUC-21,·OH was the major active radicals to degrade MB even under neutral conditions.The removal of MB was decreased significantly when Cl-?NO3-or CH3COO-were added into the photocatalytic system.