Construction Of Iron And Nitrogen Co-doped Carbon And Its Catalytic Performance In Selective Hydrogenation

The catalytic application of traditional carbon materials is limited by their surface chemical inertness,lack of anchoring metal active sites and mostly microporous pore structure.After nitrogen(N)doping,the surface polarity and electronic structure of carbon materials will be changed,resulting in structural defects,and the coordination or anchoring effect between N and metal is beneficial to the uniform distribution and stability of metal particles.The catalytic performance can be further improved by adjusting the pore structure,surface morphology,metal composition and the interaction between metal and N species.In this paper,different metal(M=Fe,Co or Ni)and N co-doped carbon materials(M–N–C)were prepared first,and the selective transfer hydrogenation of halogenated nitrobenzenes was used as the probe reaction.It was found that Fe–N–C with Fe as the active species showed the best catalytic activity,and the hydrogenation pathways catalyzed by different M–N–C were also different.On this basis,a series of Fe–N–C catalysts were designed and constructed by using different preparation methods,their catalytic performance in the transfer hydrogenation of halogenated nitrobenzenes were studied,and the reasons for the different catalytic performance of the prepared catalysts were also discussed.(1)By the simple pyrolysis of Fe²⁺,Co²⁺or Ni²⁺and 1,10-phenanthroline(Phen)complexes under the presence of activated carbon and dicyandiamide in nitrogen atmosphere,different nonprecious transition metal-based species that well confined in nitrogen doped carbon(M–N–C,M=Fe,Co or Ni)were prepared.The addition of N-containing precursor Phen and dicyandiamide can not only incorporate N into activated carbon during the catalyst preparation process,but also obtain small sized and uniformly dispersed Fe species due to the confined effect of N species.In addition,the generation of corrosive gases in the pyrolysis process can transform the microporous structure into mesoporous structure,which was conducive to the mass transfer.When the prepared M–N–C was applied for the selective transfer hydrogenation of halogenated nitrobenzenes,Fe–N–C showed the best catalytic performance.Nearly 100%conversion and selectivity could be obtained under 80 ^oC in 30 min.The reaction samples with different reaction time were analyzed by gas chromatography-mass spectrometry(GC-MS).Different M–N–C may show different transfer hydrogenation pathway.(2)Using SiO₂ spheres as the templates,polydopamine(PDA)nanolayers were coated on the surface of SiO₂(PDA@SiO₂).Then calcined PDA@SiO₂ at high temperature in an inert atmosphere,a core-shell structure of SiO₂ covered with N-doped mesoporous carbon layer(NC@SiO₂)was firstly prepared.Then the NC@SiO₂ was used as catalyst carrier to adsorb Phen-Fe²⁺complex,then mixed with dicyandiamide.Finally,the target catalyst,denoted as Fe–N–C,was prepared by the calcination of the above solid mixture at high temperature in an inert atmosphere.Fe–N–C showed good activity and selectivity for catalytic transfer hydrogenation of halogenated nitrobenzenes under mild conditions(Reaction temperature:80 ^oC,reaction time:60min,conversion and selectivity:>99%).The NC layer with mesoporous structure and N-doping was beneficial to the adsorption of Phen-Fe²⁺complex.During the pyrolysis process,the doped N heteroatoms could provide anchors and confinement for the growth of Fe active sites,which can form small Fe species with good dispersity and stability.The interaction/synergistic effect between NC and Fe species might also have beneficial effects on the catalytic activity of the catalyst.(3)In the assistance of dicyandiamide,nitrogen doped carbon confined uniformly dispersed Fe species(Fe–N–C)with interconnected mesoporous pores was prepared by a facile one-step pyrolysis process of Fe-doped zeolitic imidazolate framework(Fe-doped ZIF).Through simply adjusting the amount of dicyandiamide added,Fe–N–C with various morphologies,pore structures and chemical compositions could be obtained.The specific surface area of the materials increased first and then decreased with the increase content of dicyandiamide,and the pore structure also changed accordingly.Fe–N–C possessed the largest specific surface area with interconnected pore structure when the mass ratio of dicyandiamide to Fe-doped ZIF reached 1:1(Fe–N–C–1).And Fe–N–C–1 showed the optimal catalytic activity compared with other catalysts for the transfer hydrogenation of halogenated nitrobenzenes.Most of the halogenated nitrobenzenes could be completely converted with>99%selectivity in 30min under 80 ^oC reaction temperature.Moreover,the Fe–N–C–1 exhibited good stability and could be reused without obvious decrease of catalytic activity.(4)By using different Fe precursors to replace part of Zn²⁺in the growth process of ZIF-8 or by the incorporation of Fe precursors into the pore structure of ZIF,Fe precursor@ZIF-8 doped with different Fe precursors were prepared.Then through the high temperature calcination and acid-washing process,a series of catalysts with good dispersion of Fe single atoms in nitrogen-doped mesoporous carbon(FeSAs–N–C)were prepared.When applied for the transfer hydrogenation of halogenated nitrobenzenes,the FeSAs–N–C prepared with iron phthalocyanine(FePc)as the Fe precursor reflected certain superiority.This may be due to the efficient formation of larger pore structures(mesopores or even macropores)and the introduction of more atomically dispersed Fe-N_x active sites when using FePc as the Fe precursor.The FeSAs–N–C–24 prepared with the addition of 24 mg FePc showed the best catalytic performance in the transfer hydrogenation of various halogenated nitrobenzenes,showing good catalytic activity,selectivity and stability compared with other catalysts.