Design Of Cobalt-based Hybrids And Their Applications In Chemoselective Hydrogenation Of Nitroarenes

Functionalized amines are industrially important intermediates for the manufacture of pharmaceuticals,agrochemicals,dyes,and fine chemicals.Recently,many non-precious metal based catalysts have been applied in the transformation,whereas the process suffer from more or less drawbacks such as troublesome synthesis of catalysts,poor reusability,elevated reaction temperature and pressure,not satisfying selectivity.To address these issues,we design a series of efficient catalysts with low cost.First,we report a novel and straightforward method to prepare CoO,Co₃O₄ and N-doped carbon nanotubes hybrids?CoO_x@NCNTs?through thermal condensation of inexpensive starting materials,that is,D-glucosamine hydrochloride?GAH?,melamine,and Co?NO₃?₂·6H₂O.GAH,a nitrogen-containing biomass is selected as both the carbon and nitrogen sources.Melamine,a common industrial chemical,acts as the soft template.The active component cobalt is derived from the abundant and readily available Co?NO₃?₂·6H₂O.The specific surface areas were calculated to be 254 m² g^-1 and the mean pore size was 9.6 nm.Most cobalt-based nanoparticles?NPs?dispersed effectively with a mean size of 12.9 nm.The strategy is easy,straightforward,scalable and environmentally benign.CoO_x@NCNTs furnish excellent catalytic activity and perfect chemoselectivity?>99%?for a wide range of substituted nitroarenes?21 examples?.The high catalytic performance and durability is attributed to the synergistic effects between each component,the unique structure of graphene Iayers-coated CoO,and the electronic activation of doped nitrogen.Density functional calculations indicate that the inner CoO core and N species on the carbon shell can significantly decrease the dissociation energies of H₂,giving evidence of the ability of carbon shell in the hybrids to enable H₂ activation.What is more,carbon-caoated CoO can be recycled up to five times with unconspicuous loss of activity,overcoming the poor durability of cobalt-based catalyst for the hydrogenation of nitrobenzene in liquid phase.Considering that the hydrogenation of nitroarenes using CoO_x@NCNTs should proceed in high temperature and pressure,development of more efficient catalysts is necessary.To solve this problem,we combine non-precious metal,N-doped porous carbon and tiny Pt.Pyrolysis GAH,melamine and Co?OAc?2·4H₂O,CoO,CO₃O₄ and N-doped carbon hybrids?CoO,@CN?was obtained.CoO_x@CN possessed a bread-like architecture assembled from graphene layers and CoO_x NPs are well encapsulated inside the carbon layers with obvious defects.The surface area was as high as 458 m² g^-1.We choosed CoO_x@CN as a support and loaded tiny Pt?0.2 wt%?on the support through ultrasonic-assisted reduced with NaBH4 or galvanic replacement reaction.The prepared Pt/CoO_x@CN can enable the efficient transformation of 1-chloro-3-nitrobenzene under mild conditions?50??0.5 MPa H₂?.An impressive turnover frequency of 114660 h^-1 was obtained and the selectivity was also perfect,which is one of the best among the reported catalysts.The catalyst could be recycled for 4 consecutive runs exhibiting no obvious activity decay.The synergistic effect among the each component in the hybrids greatly promoted the raction.In detail,Pt is responsible for dissocation of H₂;the electron transfer from CoO to Pt made Pt electronic-enriched,which caused electrostatic repulsion between C-Cl and Pt and inhibited dechlorination;Co₃O₄ could provide more sites to anchor Pt NPs and improved the dispersibility.Moreover,the catalyst prepared by galvanic replacement reaction also performed well in the reaction and the prosess did not use any reductants,which is efficient,green and economic.This work significantly improved the activity of non-precious metal-based catalyst and made full use of the tiny Pt.Although the selectivity of the reported non-noble metal catalysts was comparable to that obtained by noble metal catalysts,in terms of 3-nitrostyrene,the selectivity towards the desired 3-aminostyrene was 82%-93%and the conversion should be precisely controlled.Once prolong the reaction time to further improve the conversion,byproduct 3-vinylaniline will produce.Therefore,there is still room for improvement.We reported the straightforward,low-cost synthesis of CoS₂ on porous carbon?CoS₂/PC?to study the synergic effects of single-atom active site for heterogeneous catalysis.The "synergic single-atom active site pair" in CoS₂/PC endows the catalyst with high activity and superior selectivity in the hydrogenation of nitroarenes.For hydrogenation of 3-nitrostyrene,CoS₂/PC furnishes a superior yield of 99%,higher than those reported noble-metal-free catalysts.The selectivity still remains as high as 97%,even if prolonging the reaction time.The face to face located Co3 and Co4 sites in CoS₂ formed "synergic single-atom active site pair" and they are occupied by 3-nitrostyrene and H₂,respectively.Then the reaction proceeds in a very efficient local catalytic unit,which is beneficial to the superior activity and selectivity.The molecular adsorbed H₂ serves as more active hydrogen source in the reaction than atomic H.The priority for nitro group adsorption guarantees the exclusive hydrogenation of nitro group and the single atomic adsorption greatly inhibit the formation of coupling products.The strong intermediates binding,especially for nitrosobenzene,is another key factor that makes the reaction proceeding along the direct route.This work not only improves the selectivity for the hydrogenation of 3-nitrostyrene,but also paves the way to explore synergic interaction between thesingle-atom active sites for heterogeneous catalysis.In summary,we focused on the design of efficient cobalt-based catalysts and succesufully applied them into the chemoselective hydrogenation of nitrarones.The synergic effects among the each component were investigated in detail,which greatly improved the performance of catalysts in trms of activity,selectivity and staability.We also did in-depth research on the reaction mechanism.Our work settled the problems existed in the hydrogenation of nitroarenes using the reported catalysts and opened up an avenue to design more powerful catalysts.