Selective Activation Of C-H Bonds By Single-Atom Catalysts And Perovskite-Based Photocatalysts

C-H bond widely exists in natural products and other organic compounds.Selective oxidation of C-H bond is the key to the production of chemical raw materials,and is also one of the most important and challenging processes in organic synthesis.Compounds prepared by such reactions are widely used as precursors for the synthesis of various special chemicals.However,due to the high bond dissociation energy of C-H(355-439 kJ mol-1),it usually needs to be broken at high temperature or using a large amount of hydrogen peroxide as an oxidant,which poses a severe challenge to the safety and sustainability of the production process.The key factor restricting the efficiency of C-H bond activation reaction is the activity of catalyst,and the development of catalysts with high catalytic activity,a wide range of suitable substrates and good selectivity is still the key to activate C-H bond.Based on this,in order to improve the activation ability of C-H bond and product selectivity,three kinds of catalysts were designed and synthesized:(1)Co-N-C single atom catalyst was prepared by calcination of bimetallic Zn/Co metal-organic skeleton material(ZnCo-ZIF)at high temperature;(2)Using g-C3N4 and 2,2’-bipyridine as template and ligand,Zn-N-C single atom catalyst was prepared by high temperature calcination.(3)Cs2AgBiBr6/ZSM-5 photocatalyst was prepared by in-situ synthesis method.The catalytic activities of the prepared catalyst for oxidative dehydrogenation of 1,2,3,4-tetrahydroquinoline and selective oxidation of ethylbenzene were investigated,respectively.1.Co-N-C single atom catalyst was prepared by high temperature pyrolysis and carbonization using ZnCo-ZIF as precursor.The imidazole ligand of ZIF provided sufficient N coordination atoms for metal Co,and the high content of metal Zn could be gasification of evaporation at 1000℃,which could not only isolate the metal Co site to prevent its agglomeration,but also play a role of pore-forming.The results of high angle ring dark field correction scanning transmission electron microscopy(HAADF-STEM)confirm that the metal Co is uniformly distributed in the form of single atom on the nitrogen-doped porous carbon.The prepared Co-N-C-1000 catalyst can efficiently catalyze the oxidative dehydrogenation of C-H bond of tetrahydroquinoline and its derivatives,and has been recycled for 10 times with no obvious decrease in activity and selectivity.This work provides a new strategy for loading stable Co single atoms on nitrogen-doped porous carbon.2.Nitrogen-doped carbon supported single atom Zn catalyst(Zn-N-C)was prepared by ligand stable pyrolysis using 2,2’-bipyridine as the ligand and graphite carbon nitride(g-C3nN4)as a two-dimensional template and nitrogen source.HAADF-STEM and X-ray absorption fine structure spectroscopy(EXAFS)results show that Zn exists as a single atom in the Zn-N4 structure.The optimized Zn-C-900 catalyst shows excellent catalytic performance for the selective oxidation of benzyl C-H bond of ethylbenzene and its derivatives.The Zn-N-C-900 catalyst could be reused for 15 times,and the activity and selectivity were not significantly decreased.The reaction mechanism was revealed by density functional theory calculation.The results show that the high-valence Zn=O species formed on Zn-N-C-900 are the key active centers of C-H bond activation.3.In order to improve the stability and photocatalytic activity of Cs2AgBiBr6,the Cs2AgBiBr6/ZSM-5 photocatalyst was prepared by in-situ synthesis using ZSM-5 molecular sieve as carrier and Cs2AgBiBr6 was loaded into the channels of ZSM-5 molecular sieve.Under visible light irradiation,Cs2AgBiBr6/ZSM-5 can efficiently catalyze the selective oxidation of ethylbenzene and its derivatives,and the conversion rate is up to 120,000 μmol gcat-1 h-1.