Mesoporous Carbon-supported Pd Catalysts For Selective Hydrogenation Of Phenol

Cyclohexanone is the key raw material for the synthesis ofε-caprolactam and adipic acid,andε-caprolactam and adipic acid are the starting materials for the production of nylon 6 and nylon 66,respectively.Selective hydrogenation of phenol is one of the main technologies for the production of cyclohexanone,which has the merits of low energy consumption,high atomic economy and environmental friendliness.Palladium catalysts have excellent H₂ dissociation ability and are commonly used hydrogenation catalysts.However,in the selective hydrogenation of phenol,the C=O functional group in the target product cyclohexanone is a very active functional group,which is easily adsorbed on the surface of palladium,and over-hydrogenation generates by-products such as cyclohexanol.Therefore,it is still very challenging to develop palladium catalysts with high selectivity for the selective hydrogenation of phenol.In this thesis,a series of N-doped ordered mesoporous carbon-supported palladium catalysts（Pd/N/MCN）were prepared by low-concentration hydrothermal synthesis method combined with reduction technology using low-molecular-weight phenolic resin precursors as carbon source,urea as nitrogen source,and palladium chloride ethanol solution as palladium precursor.The catalyst has a good spherical morphology with a diameter of about 100 nm and a pore size of about 2 nm.Using the interaction between N species and Pd,and adjusting the loading of Pd,the continuous regulation of Pd particle size from single-atom,two-dimensional rafts to sub-nanoclusters was achieved.The results of aberration-corrected electron microscopy showed that the Pd catalysts had uniform size and no obvious aggregation phenomenon.Combined with X-ray photoelectron spectroscopy（XPS）,CO-diffuse reflection Fourier transform infrared spectroscopy（CO-DRIFTS）and X-ray absorption fine structure spectroscopy（XAFS）,the electronic structure and coordination environment of palladium catalysts were studied.It is found that as the size decreases to a single atom,Pd loses more electrons and the Pd-C/N coordination number increases significantly.The d charge of Pd were further measured,and the results showed that the d charge of the catalyst was significantly reduced.The phenol selectivity of the Pd/N/MCN catalyst was investigated under mild conditions of 80°C and 0.1 MPa.The experimental results show that the catalyst has high catalytic activity,and the selectivity of cyclohexanone increases with the decrease of Pd particle size.Among them,the single-atom catalyst 0.2-Pd/N/MCN has the highest selectivity,maintaining 100%cyclohexanone selectivity without controlling the reaction time and conversion.The Pd site d charge of the catalyst was correlated with the turnover frequency（TOF）and apparent activation entropyΔS^0*of phenol hydrogenation performance,and a good linear relationship was found.At the same time,the d charge was correlated with the selectivity of cyclohexanone,and the two showed a positive correlation.The catalyst with the most d charge loss had the highest selectivity to cyclohexanone.A similar linear relationship was found in the further hydrogenation of cyclohexanone.Accordingly,the use of d charge to describe the selective hydrogenation performance of phenol of Pd/N/MCN catalyst was proposed,and the influence of the electronic structure of the catalyst on the catalytic activity and target product selectivity was clarified.In addition,0.2-Pd/N/MCN showed excellent stability in the hydrogenation of phenol,and after 10 cycles,it still maintained a turnover frequency（TOF）comparable to that of the fresh catalyst and nearly 100%cyclohexanone selectivity without loss of any Pd species was detected.The experimental results show that the pore walls of ordered mesoporous carbon provide a curved coordination structure Pd N₅C₄ for Pd single atoms.Compared with the ordinary Pd N₄ structure,due to the curved coordination structure,the second-layer coordination distance of the Pd-C bond is shortened,the Pd-C coordination number is increased,and the axial pyridine nitrogen ligand in Pd N₅C₄ further reduces the Pd d charge,which weakens the adsorption of cyclohexanone,thereby realizing the desorption of cyclohexanone and the high selectivity of cyclohexanone.This work provides a new idea for studying the structure-activity relationship between single-atom catalysts and phenol hydrogenation.