The Excellent Hydrogenation Selectivity Of Quinoline Under D-charge Controlled Pd Catalyst

1,2,3,4-tetrahydroquinoline and its derivatives have important research value in medicinal chemistry and dye synthesis,so the direct hydrogenation of quinoline to1,2,3,4-tetrahydroquinoline has caused widespread concern.However,due to the strong adsorption between quinoline or its hydrogenation products and the metal active center of the catalyst,the loss of activity or poor selectivity in the reaction process generally results in the formation of 5,6,7,8-tetrahydroquinoline and dehydroquinoline.In this paper,we use the coordination of nitrogen species with Pd to synthesize a nitrogen-doped carbon supported Pd nanocatalyst in one step and use it as a model catalyst for selective hydrogenation of quinoline.C and N co-modified Pd nanocatalysts have good dispersibility,and the particle size is about 3-4 nm.Through the XPS,EXAFS and other characterization methods,the coordination environment and electronic structure of Pd nanoparticles were explored.There is an interaction between nitrogen species and Pd.Electron-rich nitrogen transfers electrons to Pd,increasing the charge density on the surface of Pd.Pd nanocatalysts with different nitrogen contents have adjustable Pd-C,Pd-N and Pd-Pd coordination,and the electronic performance can be adjusted.Under the reaction conditions of 60?and 1 MPa H₂,the C,N?4.3?-Pd/OMC catalyst has the highest catalytic activity for the selective hydrogenation of quinoline.The TOF value is 188 h^-1 and the selectivity of 1,2,3,4-tetrahydroquinoline is 100%.The catalyst also has high stability.After performing cyclic reactions and trapping capture experiments,the TOF value in the initial stage of the reaction didn't change and still maintains a high catalytic activity.After the reaction,no obvious aggregation and loss of Pd nanoparticles were found,and the ordered mesostructure was still maintained.Based on the measurable d-charge density on the surface of the Pd site,the descriptor is quantitatively related to the catalytic activity and selectivity of Pd nanocatalysts for the selective hydrogenation of quinoline.In the C,N?4.3?-Pd/OMC catalyst,the d charge transfer amount at the Pd site is the largest,reaching 0.29 eV,with the highest hydrogenation activity and selectivity.At the same time,there is a positive linear dependence between the d-charge density on the surface of the Pd site and the catalytic activity,activation entropy of the quinoline selective hydrogenation reaction.The C,N?4.3?-Pd/OMC catalyst is also suitable for the dehydrogenation of1,2,3,4-tetrahydroquinoline.It also has a high catalytic activity and selectivity in the reversible selective hydrogenation/dehydrogenation of quinoline,realizing that a single catalyst can catalyze two different types of reactions simultaneously.