Unsupported nanoporous palladium‐catalyzed chemoselective hydrogenation of quinolines:Heterolytic cleavage of H_2 molecule

An efficient and highly chemoselective heterogeneous catalyst system for quinoline hydrogenation was developed using unsupported nanoporous palladium(PdNPore).The PdNPore‐catalyzed chemoselective hydrogenation of quinoline proceeded smoothly under mild reaction conditions(low H2 pressure and temperature)to yield 1,2,3,4‐tetrahydroquinolines(py‐THQs)in satisfactory to excellent yields.Various synthetically useful functional groups,such as halogen,hydroxyl,formyl,ethoxycarbonyl,and aminocarbonyl groups,remained intact during the quinoline hydrogenation.No palladium was leached from PdNPore during the hydrogenation reaction.Moreover,the catalyst was easily recovered and reused without any loss of catalytic activity.The results of kinetic,deuterium‐hydrogen exchange,and deuterium‐labeling experiments indicated that the present hydrogenation involves heterolytic H2 splitting on the surface of the catalyst.

Reaction kinetics and phase behavior in the chemoselective hydrogenation of 3‐nitrostyrene over Co‐N‐C single‐atom catalyst in compressed CO_(2)

Single‐atom catalysts(SACs)have demonstrated excellent performances in chemoselective hydrogenation reactions.However,the employment of precious metals and/or organic solvents compromises their sustainability.Herein,we for the first time report the chemoselective hydrogenation of 3‐nitrostyrene over noble‐metal‐free Co‐N‐C SAC in green solvent—compressed CO2.An interesting inverted V‐curve relation is observed between the catalytic activity and CO2 pressure,where the conversion of 3‐nitrostyrene reaches the maximum of 100%at 5.0 MPa CO2(total pressure of 8.1 MPa).Meanwhile,the selectivities to 3‐vinylaniline at all pressures remain high(>99%).Phase behavior studies reveal that,in sharp contrast with the single phase which is formed at total pressure above 10.8 MPa,bi‐phase composed of CO2/H_(2)gas‐rich phase and CO2‐expanded substrate liquid phase forms at total pressure of 8.1 MPa,which dramatically changes the reaction kinetics of the catalytic system.The reaction order with respect to H_(2)pressure decreases from~0.5 to zero at total pressure of 8.1 MPa,suggesting the dissolved CO2 in 3‐nitrostyrene greatly promotes the dissolution of H_(2)in the substrate,which is responsible for the high catalytic activity at the peak of the inverted V‐curve.

Zero-oxidation state precursor assisted fabrication of highly dispersed and stable Pt catalyst for chemoselective hydrogenation ofα,β-unsaturated aldehydes

The chemoselective hydrogenation ofα,β-unsaturated aldehydes is a key strategy for the synthesis of fine chemicals.Herein,we developed an efficient method of depositing Pt particles on FeO_(x)/SBA-15.This strategy is dependent on using a platinumdivinyltetramethyldisiloxane complex(Pt^(0)-DVTMS)as the precursor,which we demonstrate can be removed through a H_(2)-treatment under mild conditions.This,in turn,allowed for the synthesis of catalysts with well dispersed Pt particles.The presence of FeO_(x) species also aided Pt dispersion;when coated onto SBA-15,FeO_(x) strongly interacted with dissociated Pt species,inhibiting both Pt aggregation and metal leaching.Using cinnamaldehyde as a modelα,β-unsaturated aldehyde,it was demonstrated that this catalyst was highly selective towards the unsaturated alcohol and no obvious loss in activity was observed over five recycles.This catalyst was determined to be significantly more effective than an analogous catalyst prepared using chloroplatinic acid as a precursor,evidencing the importance of using the Pt0-DVTMS precursor.We corroborate the excellent catalytic performance to highly dispersed Pt-species,whereby Pt0 and Pt^(2+) play a critical role in activating H_(2) and the C=O bond.This research demonstrates that the Pt precursor can have a significant impact on the physicochemical properties and thus,the performance of the final catalyst.It also evidences how metal support interactions can dramatically influence selectivity in such hydrogenation reactions.This novel catalyst preparation protocol,using a DVTMS ligand for Pt impregnation,offers a facile approach to the design of multi-component heterogeneous catalysts.

Single-atom cobalt catalysts for chemoselective hydrogenation of nitroarenes to anilines

Single-atomic catalysts(SACs)caught considerable attention due to their unique structural properties,complete exposed active site,and 100%atom utilization efficiency with remarkable catalytic activity.Mesoporous single-atomic cobalt catalyst with Co-N_(4) active sites was synthesized by using nitrogen-doped graphene derived from acrylonitrile.Single-atomic cobalt was observed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)in Co@Nx-C-800.Notably,the density functional theory(DFT)calculation and the extended X-ray absorption fine structures(EXAFS)fitting results indicate that the coordination structure of Co-N is four-coordinated.In this work,the practical hydrogenation of nitroarenes to anilines enabled by Co@Nx-C-800 was established with excellent yields and selectivity,which proved its advantages and potential applications.

Chemoselective Catalytic Hydrogenation of Nitroarenes Using MOF-Derived Graphitic Carbon Layers Encapsulated Ni Catalysts

Replacement of precious noble metal catalysts with cost-effective,non-noble heterogeneous catalysts for chemoselective hydrogenation of nitroarenes holds tremendous promise for the clean synthesis of nitrogen-containing chemicals.Graphitic carbon layers encapsulated Ni catalysts(Ni@CN)are generated by a facile,scalable and straightforward strategy via the pyrolysis of 2,5-pyridinedicarboxylic acid coordinated Ni-MOF acting as the precursor.Physicochemical properties of the Ni@CN catalysts have been investigated by X-ray diffraction,scanning electron microscopy,transmission electron microscopy,elemental analysis and N2 adsorption-desorption analysis.The Ni@CN catalysts were found to be highly efficient in the chemoselective hydrogenation of various nitroarenes with other functional groups towards corresponding anilines under mild reaction conditions(85℃,1.0 MPa of H2 pressure).Based on the results of controlled tests,the catalytic activity can be attributed to the Ni NPs,while the presence of graphitic carbon layers favors the preferential adsorption of the nitro groups.The recyclability and anti-sulfur poisoning capability of Ni@CN were also investigated.

Tuning the coordination environment of single-atom catalyst M-N-C towards selective hydrogenation of functionalized nitroarenes

Fine-tuning of the coordination environment of single-atom catalysts(SACs)is effective to optimize their catalytic performances,yet it remains challenging due to the vulnerability of SACs.Herein,we report a new approach to engineering the coordination environment of M-N-C(M=Fe,Co,and Ni)SACs by using glutamic acid as the N/C source and pyrolysis atmosphere as a regulator.Compared with that in N2,NH3 was able to promote the doping of N at 7<700℃yet etch the N-species at higher temperatures,by which the M-N coordination number(CN)and the electronic structure were delicately tuned.It was found that the electron density of Ni single atoms increased with the decrease of Ni-N CN.As a consequence,the capability of Ni-N-C to dissociate H2 was greatly enhanced and a higher catalytic activity in chemoselective hydrogenation of functionalized nitroarenes was achieved.Moreover,this modulation method could be applied to other transition metals including Fe and Co.In particular,the as-synthesized Co-N-C SAC afforded a turnover frequency of 152.3 h~1 with 99%selectivity to 3-vinylaniline in the hydrogenation of 3-nitrostyrene,which was the highest ever reported thus far and was at least one order of magnitude more active than state-of-the-art noble-metal-free M-N-C catalysts,demonstrating the great potential of engineering the coordination environment of SACs.

Carbon dots-incorporated CuSeO_(3)rationally regulates activity and selectivity of the hydrogen species via light-converted electrons

The chemoselective hydrogenation of structurally diverse nitroaromatics is a challenging process.Generally,catalyst activity tends to decrease when excellent selectivity is guaranteed.We here present a novel photocatalyst combining amino-functionalized carbon dots(N-CDs)with copper selenite nanoparticles(N-CDs@CuSeO_(3))for simultaneously improving selectivity and activity.Under visible light irradiation,the prepared N-CDs@CuSeO_(3)exhibits 100%catalytic selectivity for the formation of 4-aminostyrene at full conversion of 4-nitrostyrene in aqueous solvent within a few minutes.Such excellent photocatalytic performance is mainly attributed to the precise control of the hydrogen species released from the ammonia borane by means of light-converted electrons upon N-CDs@CuSeO_(3).Besides,the defect states at the interface of N-CDs and CuSeO_(3)enable holes to be trapped for promoting separation and transfer of photogenerated charges,allowing more hydrogen species to participate in catalytic reaction.