Studies On Catalytic Hydrogenation Of Nitrobenzene With Heterojunction Between N-doped Carbon And Transition Metal

The industrial preparation of p-aminophenol is mainly realized within an one-pot method by Bamberger rearrangement from hydroxylbenzeneamine(PHA)which is an intermediate product form catalytic hydrogenation of nitrobenzene under strong acid condition(usually sulphuric acid),and aniline is the major by-product obtained from further hydrogenation of PHA.Although catalysts generally used in the process are supported noble metal hydrogenation catalysts such as Pt,Pd and Rh exhibit an excellent catalytic activity,precious metals would possibly suffer from poisoning of sulphur and loss of active components under the strong acidic reaction condition.Considering the resource scarcity of precious metal and its high costs,we focus on developing a new structure of nonprecious metal-based catalyst which could function well in strong acidic environmentN-doped carbon as an inexpensive semiconductor generally shows an unique electronic characteristics as well as a high chemical stability and its electron and band structure could be adjusted by additional heteroatom doping(B,P,S and so on)or organic groups modification,which making it widespread applications in various catalytic field.It has been demonstrated that N-doped carbon could act as an effective metal-free catalyst for oxygen reduction reaction.Moreover,N-doped carbon is also an outstanding catalytic support substrate benefiting from its high surface area and tunable pore structure and it has been used to support metal or metal oxide for many catalytic reactions which would possibly exhibit a higher catalytic activity due to a strong interaction or synergy effect between the two components.An unusual and reverse loaded N-doped carbon on either nickel or molybdenum carbides in a construction of heterojunction based on the semiconductor characteristic of N-doped carbon and the strong electron-donating ability of metallic nickel as well as molybdenum carbides,with which N-doped carbon is modified to be able to adsorb and active hydrogen by accepting electron from nickel or molybdenum carbides which are isolated from the ambient environment by the inert N-doped carbon.Different characterization methods such as X-ray diffraction(XRD),transmission electronic microscopy(TEM),X-ray photoelectron spectroscopy(XPS)and hydrogen-deuterium exchange(H/D exchange)etc.are carried out to give a sight into the structure of the catalyst and its catalytic performance for hydrogenation of nitrobenzene to p-aminophenol under strong acidic condition is investigated.The contents are summoned as following:(1).A N-containing porous organic polymer is synthesized from glycoluril and terephthalaldehyde as a support for impregnation of nickel precursor and the novel structure of N-doped carbon encapsulated nickel is obtained after a thermal process.Then various techniques are applied for a better understand of its structure and composition and its catalytic performance are tested to prepare P-aminophenol from nitrobenzene under strong acid condition.The results show that supported nickel species is reduced to metallic nickel during the thermal treatment of the complex and N-doped carbon encapsulation is realized at the same time,leading to a N-doped carbon-nickel heterojunction,with which electron flows from metallic nickel to N-doped carbon,making it electron-rich to be able to adsorb and activate hydrogen and the catalyst exhibits a well stability in 1.5 M sulphuric acid according to the protection of N-doped carbon.Furthermore,catalytic performance of the catalyst for hydrogenation nitrobenzene to p-aminophenol is studied under different conditions and the results show that catalyst possesses a high catalytic activity and an excellent selectivity under the optimum reaction eonditions.(2).Silica oxide supported nickel oxide is obtained by calcination in air after impregnation of nickel on silica oxide,then,precursor of N-doped carbon is deposited form carbon tetrachloride and ethanediamine followed by a high temperature treatment to give the catalyst with N-doped carbon loaded on nickel and various characterization methods are applied for a better understanding of structure and composition about the catalyst.Then,sulfonic acid groups are introduced to the surface of encapsulated N-doped carbon for the purpose of making it able to supply an acidie environment necessary for Bamberger rearrangement as well as the new active center for hydrogen activation,which would avoid the use of large amount of sulphuric acid.The modified catalyst is employed to catalyze hydrogenation of nitrobenzene to p-aminophenol in water and results show that the catalyst does exhibit a certain catalytic activity and selectivity,indicating the construction of such an integrated structure is available.(3).Molybdenum carbide which is a superior electron donator possesses similar catalytic properties with precious metals and often exhibits excellent performance in many catalytic hydrogenation reactions.Therefore,N-doped carbon-molybdenum carbides heterojunction would also exhibit catalytic activity for hydrogenation of nitrobenzene using molybdenum carbides for the replacement of metallic nickel.Several layers N-doped carbon encapsulated molybdenum carbide is obtained from molybdenum pentachloride,urea and glucose through an one-pot solid reaction and its structure characteristics and composition are studied by various techniques.The catalyst is used to catalyze nitrobenzene to p-aminophenol under sulphuric acid solution in the presence of hydrogen and the results show that such a catalyst also exhibits a high catalytic ability and selectivity with a good stability.