Preparation Of Molybdenum-tungsten Composite Oxide Catalysts With Highly Efficient Hydrogen Spillover Effect And Its Application In Catalytic Hydrogenation

As an important secondary energy source,hydrogen energy has attracted more and more attention for its high energy density,clean and environment-friendly properties.However,the safety and efficient storage and transportation of hydrogen at normal temperature and pressure is becoming one of the major bottlenecks restricting the economic development of hydrogen energy.Organic liquid hydrogen storage technology has broad commercial application prospects due to advantages such as good safety and cycle performance,convenient storage and transportation,etc.,which is used in the catalytic hydrogenation of hydrogen storage materials.The catalytic hydrogenation of organic liquid hydrogen storage materials requires a large amount of catalyst.The traditional noble metal catalysts such as Pd,Pt,Ru,Rh,etc.cannot be used on a large scale because of their limited resources and high price.Therefore,the development of cheap and efficient alternative catalysts has become a research hotspot.Among them,a class of transition metal oxides represented by MoO₃ and WO₃ have been applied to the catalytic hydrogenation of olefins due to the hydrogen spillover effect.However,the study of these two kinds of oxides and their composite oxides has not been reported in the literature in the field of catalytic hydrogenation of organic liquid hydrogen storage.In view of the above situation,molybdenum and tungsten oxides were studied for the catalyst synthesis,morphology control and catalytic hydrogenation properties.And the mechanism of the catalytic hydrogenation reactions was discussed.The main research work and results are as follows:?1?Hydrothermal synthesis,structure characterization and catalytic activity evaluation of nanolayered MoO₃ and nanorod-like WO₃.MoO₃ synthesized under the experimental conditions has a lamellar structure with an orthorhombic phase,while WO₃ has a monoclinic rod-like structure.A batch of catalysts loaded with 0.1 wt%of Ru and Pd was also prepared.The hydrogenation of cyclohexene was selected to evaluate the catalytic activity of catalysts,and the experimental conditions were all reacted at 150°C and 3 MPa hydrogen pressure for 6 hours.The result showed that the first and second cycle catalytic conversion rates of MoO₃ and WO₃ were 18.1%/26.4%and 18.3%/38.6%,respectively.By analyzing the XRD,XPS,and Raman of the catalysts before and after the two cycles,it was found that the increase in catalytic activity was due to hydrogen spillover in the catalytic hydrogenation process,which was conformed the formation of the H_x MoO₃ and H_x WO₃.In contrast,the catalytic conversion rates of 0.1 wt%Ru/MoO₃ and 0.1 wt%Pd/MoO₃ were 61.5%and 70%,respectively;0.1 wt%Ru/WO₃ and 0.1 wt%Pd/WO₃ were 94.1%and 97%,respectively.The results show that WO₃ has better catalytic activity than MoO₃.?2?Hydrothermal synthesis,structure characterization of nanorod-like molybdenum-tungsten composite oxide and the evaluation of catalytic hydrogenation activity for cyclohexene.The nanorod-shaped molybdenum-tungsten composite oxides with ordered structure were prepared by hydrothermal method.The morphology of the rods was variable by changing the hydrothermal experimental conditions such as the pH value of solution and calcining temperature.The highest specific surface area of the material was 188 m²/g.The molybdenum-tungsten composite oxide(Mo_0.6W_0.4O₃)was used as a modle to evaluate the catalytic activity of cyclohexene.The results show that the conversion of cyclohexene reaches 100%after 3 hours reaction at 150°C and 3 MPa.The comparison shows that the catalytic hydrogenation performance is comparable to that of commercial Ru/Al₂O₃ catalyst and is much better than MoO₃ and WO₃ monomer catalysts.The XPS,TPD,and XRD tests of Mo_0.6W_0.4O₃ before and after hydrogenation show that the formation of H_xMo_0.6W_0.4O₃ has excellent catalytic hydrogenation activity during the catalytic hydrogenation reaction.In addition,the catalytic hydrogenation rate increases with the increase in the number of cyclic hydrogenation,indicating that Mo_0.6W_0.4O₃ has excellent cyclic hydrogenation performance.?3?The use of liquid phase reduction method for the preparation of 0.1 wt%Ru/Mo_0.6W_0.4O₃ and 0.1 wt%Pd/Mo_0.6W_0.4O₃ to verify its effect on styrene selective hydrogenation performance.High-resolution transmission electron microscopy showed that the Ru particle size is between 8 and 9 nm.The Fourier-transform calculation shows that the lattice spacing of Ru nanoparticles is 0.235 nm and grows well along the?100?crystal plane;The size of Pd nanoparticles is in the range of 10 nm,and the crystal plane distance of Pd nanoparticles is 0.226 nm calculated by Fourier transform,and grows well along the?111?crystal plane.The catalyst was prepared for selective catalytic hydrogenation of styrene.The results showed that Mo_0.6W_0.4O₃ performed 100%selective catalytic hydrogenation of styrene at 150°C and 3 MPa for 7 hours,while the time required for 100%conversion of styrene after Ru and Pd loading was shortened to120 min and 90 min,respectively.0.1 wt%Pd/Mo_0.6W_0.4O₃ was used as a modle to test and characterize the samples before and after hydrogenation.The analysis showed that during the catalytic hydrogenation reaction,due to the formation of H_xMo_0.6W_0.4O₃ with higher H concentration,Mo and W is reduced from the+6 partially to+5 and+4,and the catalytic hydrogenation activity of Mo_0.6W_0.4O₃ is enhanced to exhibit more excellent catalytic hydrogenation activity and selectivity.?4?The Mo_0.6W_0.4O₃ catalyst was used in the catalytic hydrogenation of organic liquid hydrogen storage materials.The typical hydrogen storage molecules N-ethylcarbazole and N-propyl carbazole were studied to evaluate the catalytic hydrogenation.The results showed that the reaction of N-ethyl carbazole has been converted to 40.5%after reaction at 170°C and 8 MPa for 11 h,confirming that Mo_0.6W_0.4O₃ catalyst exhibits a certain catalytic hydrogenation activity.To improve the hydrogenation reaction rate,the 0.1 wt%Pd/Mo_0.6W_0.4O₃ was used as a modle for further study.The results showed that the raw materials were completely converted under the same conditions for 7 h.Then the temperature gradient experiment was performed at 150¹80°C.The results show that the hydrogenation rate of the catalyst increases and the yield of the product increases,with the increase of hydrogenation temperature.And after five hydrogenation cycles,the efficiency of catalytic hydrogenation is increasing.The calculated hydrogenation activation energy of 0.1 wt%Pd/Mo_0.6W_0.4O₃ for N-ethylcarbazole is 30.4 kJ/mol.Under the same hydrogenation conditions,the N-propyl carbazole reaction was completely converted in 5 h and the apparent activation energy of hydrogenation was 53.5 kJ/mol.The above calculated activation energies are lower than those reported in the literature for precious metal Ru catalysts,therefore,the Mo_0.6W_0.4O₃ catalyst is expected to replace precious metal catalysts for further research and development in industrial production.