| In the catalytic reaction,catalyst active center is the main factor that determines the performance of catalyst.By adjusting the electronic and geometric structure of the metal active center,as well as the synergistic effect between the metal active center and the support,it is of great significance to significantly change the catalytic activity and product selectivity of the enhanced catalyst.Selective hydrogenation of phenol reaction to cyclohexanone reaction has some problems,such as harsh reaction conditions,poor catalytic process efficiency and easily occuring side reaction.Although metal catalysts(Pd,Pt,Ni,etc.)have made some progress in the hydrogenation of phenol and show certain advantages,it is still necessary to further optimize the structure of the catalyst on the basis of deconstruction activity relationship and reaction mechanism,so as to realize the enhancement of performance and efficiency.Pd-based catalyst is easily hydrogenated to ethane under low acetylene coverage,reducing the selectivity of ethylene.Therefore,it is very important to develop a more efficient catalyst by adjusting the structure of Pd active site for acetylene hydrogenation.Aiming at the problems in selective hydrogenation of phenol,this paper carried out the structure regulation of Pd-based catalyst and explored the synergistic effect between carrier elements and metal active components and influence of electronic structure of active center on hydrogenation of phenol.The enhancement mechanism of selective hydrogenation of phenol to cyclohexanone was revealed.In the selective hydrogenation of phenol,the layered structure of hydrotalcite was grown on alumina by hydrothermal synthesis method.Compared with the traditional alumina catalyst,the catalyst prepared by this method has larger specific surface area,higher average pore diameter,higher adsorption capacity,and abundant acid and base sites on the surface of the catalyst.The results of catalyst performance evaluation showed that the Pd/MgAl-LDO@Al2O3 and Pd/NiAl-LDO@Al2O3 still have good catalytic performance under the high substrate ratio.Compared with Pd/Al2O3 catalyst,the phenol conversion of Pd/MgAl-LDO@Al2O3 and Pd/NiAl-LDO@Al2O3 increased significantly.Under the conditions of phenol/Pd=1000:1 mol/mol,80℃,0.4 MPa H2 and 800 rpm,Pd/MgAl-LDO@Al2O3 catalyzed phenol with a conversion rate of 96.6%and a selectivity of 87.7%after 5 h.The improvement of performance can be attributed to the fact that the introduction of metal Mg increased the number and strength of base sites on the carrier surface,and the presence of base sites affected the adsorption form of phenol,thus greatly increasing the selectivity of cyclohexanone.The electronic structure and geometrical structure of catalysts were analyzed,the study found that PdNi alloy formed in the Pd/NiAl-LDO@Al2O3 catalyst.The binding energy of Pd/NiAl-LDO@Al2O3 catalyst moves to high direction compared with Pd/Al2O3 catalyst,which lead to low Pd electron cloud density and resulted in a direct hydrogenation path to cyclohexanol from the original phenol hydrogenation path.In the selective hydrogenation of acetylene,the Pd/α-Al2O3 catalyst was prepared by simple impregnation method using PdCl42-,PdSO4 and Pd(acac)2 as metal precursors.The performance of the catalysts were evaluated in a fixed-bed reactor,with emphasis on the influence of different active precursors on the performance of the catalyst under the relative pressure of 0.4 MPa at 70℃.It was found that the Pd/Al2O3 catalyst prepared with Pd(acac)2 as the precursor had better ethylene selectivity and stability than the catalysts prepared with PdC142-and PdSO4 at a certain conversion rate.The study demonstrated that the remaining species form during the preparation of catalyst were obviously different for different Pd precursor.For PdCl42-metal salts,the residual Cl still mainly exists in the form of PdCl42-complexes,while S residue embedded in Pd lattice for PdSO4 precursor.As for Pd(acac)2 precursor,C elements mainly in the form of carbon residue on the surface of the catalyst.According to the analysis of the electronic environment of the above catalysts,the Pd site in Pd-ac catalysts has higher electron cloud density compared with Pd-Cl and Pd-S,which is conducive to the desorption of ethylene molecules.In addition,to further explore on the deactivation mechanism of three kinds of catalyst,Pd catalyst atomic environment and the nanoparticles morphology after time-steam for 24 h were analysed,which indicates that the Pd-Cl,Pd-S the main reason for catalyst deactivation are sintering and carbon deposition.Pd-ac catalyst during the reaction process formed the Pd-C phase,which is beneficial to keep a good activity and selectivity.In this paper,the synergistic effect between the active center and the support as well as the electronic influence of the active center on the catalytic performance for typical selective hydrogenation were investigated by regulating the structure of Pd based catalyst support.The performance enhancement mechanism of target products in serial reactions was also revealed.The results of this study provide a new theoretical basis and research idea for the design and construction of high efficient active centers in serial hydrogenation. |
PDF Full Text Request