Fabrication Of P-doped Pd Nanoparticles-and P-C-Pd Framework Structure Over The Activated Carbon For The Chemoselective Hydrogenation Of Para-chloronitrobenzene

The selective hydrogenation of aromatic halonitro compounds to the corresponding aromatic haloamines is one of the most essential transformations in the synthesis of fine chemicals and intermediates.But,the dehalogenation may be together with the hydrogenation of chloronitrobenzene.The selective hydrogenation of aromatic halonitro compounds to the corresponding aromatic haloamines is considered as the most environment-friendly and efficient method instead of conventional chemical reduction involving the use of Fe,Na2 S,hydrazine hydrate etc.However,up to now,it remains a major challenge to avoid the undesired hydrodehalogenation because the catalytic hydrodehalogenation rates of the C-X(Cl,Br)bond is enhanced as a result of the electron donating effect of amino groups in the aromatic ring.It is the only way to solve this scientific problem to explore new catalytic materials and to study the mechanism of inhibiting dehalogenation,which is also a hotspot in the process of catalytic hydrogenation synthesis of halogenated aromatic amines.The doping of heteroatoms has significant influence on the structure and electron distribution of the particles adjacent to them due to the difference in bond length and atomic size and rich electrons in their outer electronic orbit.In this paper,a series of phosphorus-modified palladium-carbon catalysts have been synthesized via an ammonium pyrophosphate high temperature roasting method.The interaction between heteroatom phosphorus and palladium metal cluster particles has been extensively studied.The relationship between the structure and electron distribution of activated carbon has been explored.The effects of these catalysts on the catalytic hydrogenation of chloronitrobenzene have also been discussed.The main contents include the following parts:1)The crystal morphology of carbon-supported palladium clusters has a significant influence on the phosphating process.It is found that when palladium cluster particles are in the form of zero valent palladium,positively divalent palladium ion and palladium oxide,only palladium oxide is easily phospholized to form new phase crystal such as phosphonium triphosphate.The optimum conditions were as follows: phosphating temperature was 400 ?,the molar ratio of phosphorus and palladium molar ratio was 32.2)In view of the carbon-supported palladium oxide precursors,the mechanism of palladium oxide phosphating process was investigated.It is found that the process is closely related to the calcination temperature and the amount of phosphorus.With the increase of temperature,the palladium oxide clusters gradually agglomerate.When the temperature is increased to 330 ?,the reductive substances such as PH3,which are produced by the decomposition of hypophosphite,react with palladium oxide.The higher the temperature,the deeper the degree of phosphating becomes.It was found that it can form a more complete phase of Pd3 P when the phosphating temperature reached above 400 ?.Surprisingly,due to the infiltration of phosphorus,the agglomerated palladium oxide cluster particles split again to form particles of palladium phosphorus with small particle size.3)The phosphorus-modified palladium-carbon catalyst has a role in the inhibition of dehalogenation.When the phosphorus doping amount n(P)/n(Pd)is equal to 8 and the calcination temperature is 400 ?,the selectivity of para-chloroaniline is highest,which can be attributed to the synergistic effect between palladium phosphate and palladium as well as the size effect of the large particle size palladium.4)A novel Pd-P-C framework structure was fabricated by supporting Pd on the P-doped carbon layer coated on the activated carbon,which was generated via the calcination of the sodium hypophosphite and ethanediol under inert gas.In this structure electrons were transferred to P atoms from carbon atoms adjacent to the P atoms,which is in favor of forming the stable Pd-P species such as Pd15P2 phase.Pd possesses electron-rich property because of electrons from C to Pd atoms via P atoms,in which the phosphorus seems to play a role in electron storage medium.As a result,the adsorbed hydrogen on the Pd particles similarly possesses electrons and becomes a very nice nucleophile.The produced electron-rich H-might prefer the nucleophilic attack on the nitro group rather than the electrophilic attack on the C-Cl bond.We suggest that it is responsible for the superior selectivity of up to 99.9% to p-CAN for the hydrogenation of p-CNB with the excellent stability.