A Surface Group-oriented,Condensation Cyclization-drived Nitrogen Doping Strategy For Nitrogen-doped Carbon Supported Pd Catalyst For Selective Hydrogenation Of P-chloronitrobenzene

Chloroaniline plays an important role in the fine chemical industry as a kind of important organic synthesis intermediates and mainly gained from the reduction of the corresponding chlorinated aromatic nitro compounds.Compared with other traditional methods,catalytic hydrogenation reduction method is still a more valuable method due to its green and efficient properties.However,dechlorination side reactions are very easy to take place accompanied by the production of hydrogen chloride,which is a corrosive by-product for the equipment and catalyst.When nitrogen atoms are doped into activated carbon structure,the electronic effect between carrier and metals can be effectively adjusted which benefit from the electrons in the outer orbit of nitrogen atom and the negative electronegativity difference between the N atom and C atom,which achieved the suppression of side reactions of hydrogen dichlorination.In this paper,nitric acid,hydrogen peroxide and oxalic acid were used to treat activated carbon to get different surface oxygen containing groups,and then obtained different nitrogen species via the condensation with ethylenediamine.Formation mechanism of nitrogen species that guided by the surface oxygen groups have been explored.The modulation of electron distribution between activated carbon carrier and metal was investigated,and the effect of liquid phase hydrogenation on selectivity of p-CNB was also discussed.The main results are as follows:1)Different nitrogen precursors under roasting nitrogen doping method show different catalytic properties at a small amount of nitrogen?0.5 wt%?,and the lower molecular nitrogen content?dicyandiamide>urea?ethylenediamine>alanine?,the better the catalytic performance.Among them,the catalyst with alanine as the nitrogen precursor showed the best catalytic activity,and the catalyst with ethylenediamine as the nitrogen precursor had the best dechlorination inhibition ability.2)The acid-base difference between palladium precursor and carrier affects the stability and catalytic performance of the catalyst.The active components were easily lose from the basic N-doped carbon when chlorine palladium acid solution was used as palladium precursor.When alkaline palladium and ammonia complex ions are used as metal precursors,the catalyst showed good catalytic performance and stability after reduction,and the optimum reduction temperature is 250?.3)The results indicate that CO-releasing groups are apt to form a high proportion of amino groups,whereas the CO₂-releasing groups especially the carboxyl and lactones are mainly transformed into pyrrolic-type nitrogen.Amino groups induce nitrogen-containing structure to act as electron donor for the in-situ reduction of Pd²⁺to Pd⁰ at the time of Pd²⁺adsorption on the framework.Notably,in the CN_x framework with dominant pyrrolic-type nitrogen,an electron-rich Pd activated site composed of Pd atoms,pyrrolic-type N and adjacent C atoms is built,in which electron transfer occurs from N to neighboring C and Pd atoms.This activated site contributes to the formation of electron-rich activated hydrogen?H^-?and the desorption of p-chloroaniline,which work together to inhibit the hydrodehalogenation side reaction.The superior selectivity about 99.90%of p-chloroaniline and the excellent reusable performance were eventually realized.