Fundamental Studies On Novel Pd-based Catalysts For 2-Alkyl-anthraquinone Hydrogenation

The hydrogenation of 2-alkyl-anthraquinone?AQ?is the key step in the anthraquinone autoxidation process.The reaction rate and selectivity of AQ hydrogenation are responsible for the efficiency of the production equipment and level of material and energy consumption.The catalysts have been the focus of research and development in this field because of their significant effects on the rate and selectivity of AQ hydrogenation.In the present work,the design and preparation of the supported Pd-based catalysts were conducted.The catalytic performance of Pd-based catalysts for AQ hydrogenatoion was studied to have a deep understanding of the mechanism of AQ hydrogenation and structure-activity relationship of the catalysts.A density functional theory analysis combined with the experiment has been conducted to study the effects of the aromatic ring and alkyl side chain of AQ on its hydrogenation behavors.It was found that compared to 2-ethyl-anthraquinone?EAQ?,the high electron delocalization over the boundary aromatic ring and high electron localizationoverthecentersix-memberedringof2-ethyl-5,6,7,8-tetrahydro-anthraquinone?H₄EAQ?enhance the electron transfer from Pd₆H₂ to C=O,which favors the adsorption and hydrogenation of H₄EAQ.2-Tertamyl-anthraquinone exhibits a significantly slower hydrogenation rate but has a higher maximum yield of H₂O₂ and a smaller amount of degradation products than EAQ,which could be ascribed to the larger and branched tertamyl side chain.Bimetallic Pd-M/Al₂O₃ catalysts were prepared by doping of the second metal?M=CO,Ag,Cu?in Pd.It was found that the highest catalytic activity,along with the high selectivity is achieved over bimetallic Pd-Co catalysts having M/Pd=0.25reduced at 80°C,with an increase of an order of magnitude in the reaction rate of EAQ hydrogenation compared to the monometallic Pd/Al₂O₃ catalyst.It is because that the Pd-M^?+structure formed at low reduction temperature increases the surface reaction rate significantly.Pd/SBA-15 catalysts were prepared by the controllable synthesis of SBA-15mesoporous materials via the hydrothermal method,and then deposition of Pd on SBA-15 supports.It was found that the Pd catalysts supported on SBA-15 possessing22 nm pore diameter and long-range order exhibits the best catalytic performance towards EAQ hydrogenation.The incorporation of Mg into SBA-15 enhances the catalytic performance of Pd catalysts,which could be ascribed to the increase in Pd dispersion and alkaline sites amount.But,the incorporation of Al brings about the decrease in the catalytic performance due to the decreased dispersion of Pd.The graft of SBA-15 surface with the organosilane has a promoting effect on Pd dispersion,which leads to the increase of 98%in the catalytic activity.The skeleton doping Pd/Al₂O₃ catalyst possessing the large specific surface area,ordered mesopores with the narrow pore size distribution and high dispersion of Pd particles was prepared via the one-pot evaporation-induced self-assembly method.It was found that it exhibits superior performance for EAQ hydrogenation,which could be ascribed to the higher proportion of low-coordinated Pd atoms and lower amount of acid sites.Furthermore,the exposure extent of Pd particles and number of low-coordinated Pd atoms were tailored by manipulating the reduction temperature.It was unravelled that low-coordinated Pd atoms originate from the three-phase boundary of the Pd-Al₂O₃ interface,which exhibits the higher catalytic activity for EAQ hydrogenation than the surface Pd atoms.The Al₂O₃ layer was prepared on the surface of the cordierite substrate by the in-situ hydrothermal and dip-coating method respectively,and then monolithic catalysts were obtained by the deposition of Pd.As comparison,the conversion of EAQ hydrogenation over the former is 50%higher than that of the latter.