Preparation Of M-N-C Single-atom Catalysts And Catalytic Performance For Chemoselective Hydrogenation Of Nitrostyrene

Single-atom catalysis is a frontier in heterogeneous catalysis.Nevertheless,the active center of most single-atom catalysts(SACs)are based on noble metals,and great efforts should be devoted to developing non-precious metal based SACs.M-N-C(M=Fe,Co,Ni,etc)SACs represent one type of non-noble metal SACs which exhibit high catalytic performance comparable to that of Pt/C in electro-chemical reactions(e.g.,ORR),yet studies on their utilization in thermal catalytic transformations are limited.This thesis aims to develop highly efficient and selective M-N-C single-atom catalysts for chemoselective hydrogenation of functionalized nitroarenes,new approaches of fabrication of M-N-C catalysts were developed,and compressed CO2 was employed as reaction media to replace traditionally organic solvents,in order to develop green and highly efficient synthetic route for functionalized anilines.The main contents are as follows:(1)The textural properties,dispersion of metal,and electronic properties of the M-N-C catalysts were modulated by C/N source,pyrolysis temperature and atmosphere.Ni(OAc)2,Co(OAc)2,Fe(OAc)2 were used as metal sources,and C/N sources were glutamic acid.4methylimidazole.melamine and dicyandiamide.etc.Magnesium oxide,montmorillonite,sodium chloride and nano-silica are used as templates,and M-N-C single-atom catalysts were prepared by calcination them at high temperatures under N2.NH3 and H2 atmosphere,and then removing the templates.The catalysts were characterized by XRD(X-ray diffraction),XAFS(X-ray absorption fine structure spectroscopy),HAADF-STEM(spherical aberration transmission electron microscopy),etc.It is found that the metal species are dispersed as single atoms,and the loading could reach 3.5 wt%.meanwhile,the catalysts have large specific surface area(436?1042 m2 g-1).(2)The catalytic performances can be optimized through modulation of the coordination environment as what have been done in homogeneous catalysts.The coordination environment of Ni-N-C SACs and others was modulated by changing the pyrolysis temperature and atmosphere.To realize the dynamic monitoring of the pyrolysis process,the real-time changes of tail gas during the pyrolysis were quantitatively analyzed.It is found that under NH3 atmosphere,with temperature at T<700?,coordination number(CN)increases due to the doping of N species,whereas decreases abruptly with further increasing the temperature as a result of etching of carbon matrix by NH3.Further treated by H2,CN and the oxidation state of metal atom could be lowered continuously.In the hydrogenation reaction of 3-nitrostyrene,the TOFs of Ni-N-C and Co-N-C SACs reached as high as 60.2 h-1 and 152.3 h-1,which were among the highest values ever reported.even better than Au/TiO2,and the selectivity to the desired 3vinylaniline was higher than 99%.Moreover,the structure of catalysts was characterized by a series of characterization to establish the structure-performance relationship using CN as a descriptor,and it is found that the lower CN of Ni-N,the lower oxidation state,the higher catalytic activity.(3)Dual metal CoFe-N-C SAC was fabricated based on the idea of "concerted catalysis".Glutamic acid was used as C/N source,nano-silica was used as template,and both Co(OAc)2 and Fe(OAc)2 were metal resource,and then CoFe-N-C SAC is obtained by pyrolyzing them under N2 atmosphere.In the hydrogenation of 3-nitrostyrene(80?,3 MPa H2,8 h),the conversion over CoFe-N-C SAC reaches 100%,much higher than those of Co-N-C(58.7%)and Fe-N-C(negligible)catalysts,and a series of comparative experiments suggesting synergistic effect is occurred between Co and Fe sites.The possible mechanism of the synergy effect was studied by characterization such as XAFS,57Fe Mossbauer spectroscopy.It is found that compared with Co-N-C,the electron density and the bond length of Co-N(O)increases in CoFe-N-C,indicating that doping of Fe atoms modulates the coordination and electronic structure of the Co active center,thus improving its catalytic performance.(4)Compressed CO2 was employed as the reaction media of hydrogenation of 3nitrostyrene over Co-N-C SAC with the aim to develop a green and efficient synthetic route.Using 1,10-phenanthroline as the C/N source,Mg(OH)2 as the template,and Co(OAc)2 as the metal source,the Co-N-C single-atom catalyst was prepared.By adjusting the reaction temperature,H2 partial pressure and CO2 pressure,the performance of the CO2 solvent was regulated to optimize the chemoselective hydrogenation reaction of 3-nitrostyrene.It is found that as the CO2 pressure increases,the conversion of 3-nitrostyrene presents an "inverted Vshaped" trend.100%conversion is achieved at PCO2=5.0 MPa(total pressure of 8.1 MPa),below or above which the catalytic activity decreases significantly.Phase behavior and kinetic studies reveal that bi-phase composed of CO2/H2 gas-rich phase and CO2-expanded substrate liquid phase forms at PCO2=5.0 MPa(total pressure of 8.1 MPa).As a consequence,the dissolution of H2 in substrate is greatly promoted and the reaction order of H2 decreases from 0.5 to zero,which is responsible for the enhanced catalytic activity.