Study On Preparation And Catalytic Hydrogenation/Dehydrogenation Performance Of Mesoporous Molybdenum Trioxide And Its Palladium Loaded Catalysts

In the background of the global energy shortage, environmental pollution problems become increasingly grim, the use of efficient, clean and sustainable new energy technology has become a central issue. Liquid organic hydrogen carriers (LOHCs) has now become one of the most promising way due to its high hydrogen storage capacity, high safety, good cycle performance and other advantages, besides, the catalytic hydrogenation/dehydrogenation is always an important part of using hydrogen storage materials effectively. However, the traditional precious metal catalysts such as Pt, Pd, Ru, Rh, etc., followed by several problems like the limited resources, expensive costs, catalyst poisoning, disproportionation reaction and other issues, in which makes exploring the design and synthesis of new efficient catalyst materials become necessary. Wherein, the mesoporous metal oxide material with its high specific surface area, the uniform ordered pore structure, the adjustable pore size and the modifiable inner surface, has attracted more and more attention. An increasing number of ordered mesoporous materials with different morphologies, pore structures and the framework components were successfully synthesized and widely used in catalysis. Mesoporous transition metal oxides have gained an increasing interest as alternatives to precious metal catalysts for hydrogenation of unsaturated organic molecules. Among them, molybdenum oxide has been used as catalyst for hydrogenation of olefin molecules early in the 1970s. However, MoO3 is orthorhombic metal oxide and has a layered structure, these properties make it a dense bulk materials, which restrict its use in the field of catalysis, meanwhile, the synthesis of a highly crystalline mesoporous transition metal oxide with a high surface area and an ordered pore structure still presents a major technical challenge.In this paper, molybdenum trioxide was selected as the potential transition metal oxide catalyst. To investigate and discuss its catalytic activity and selectivity for the unsaturated small organic molecules and LOHCs under mild conditions, and explore new ways to enhance its catalytic properties, we start the study from the synthesis of catalysts, the control of morphology and structure, and the performance of catalytic hydrogenation/dehydrogenation. The main contents are as follows:(1) The study on the preparation and catalytic activity of Mesoporous MoO3 catalyst. A series of mesoporous molybdenum trioxide (MoO3) with tunable pore sizes were successfully synthesised by using hard template method, the cubic mesoporous silica KIT-6 was used as a template, by regulating the temperature of the hydrothermal reaction during the synthesis process of KIT-6, the pore sizes of MoO3 can be tuned as well.In the process of removing the template KIT-6, the problem that molybdenum trioxide was easily dissolved with the KIT-6 was solved by adjusting the concerntration of hydrofluoric acid solution. By using XRD, SEM, FESEM, GC-MS, XPS and other characterization and analysis, it is found that the as-synthesized mesoporous MoO3 materials exhibit crystalline frameworks, high surface areas up to 142 m2/g, and uniform and tunable pore sizes in the range of 6.0-12.7 nm. The mesoporous MoO3 catalysts display excellent catalytic activity with 100% conversion in 6 hours, much higher than that of bulk MoO3 (conversion rate 28%), and with the increase of the specific surface area, the catalytic activity enhances as well. The mesostructure and the high surface area enable the reaction to proceed with an enhanced hydrogen spillover rate and an increased concentration of hydrogen bronze.(2) The study on the catalytic hydrogenation of N- ethyl carbazole of mesoporous MoO3and Pd/MoO3catalyst. Firstly, the pure mesoporous MoO3 was used as a catalyst to hydrogenate organic liquid hydrogen storage materials N- ethyl carbazole, and to verify its catalytic hydrogenation activity; secondly, with a minor loading content of the precious metal Pd (0.5 wt%), and using commercial 5 wt% Ru/Al2O3 and 5 wt% Pd/Al2O3 catalysts for comparison, the role of HxMoO3 concentration in the hydrogenation of NEC under different reaction temperatures, as well as the possibility for the transition metal oxide mesoporous molybdenum trioxide as an alternate catalyst for the traditional precious metal catalysts were discussed. The results shows that NEC can be hydrogenated by the pure mesoporous MoO3 at 180℃, elevate the temperature may further increase the conversion rate. Under the temperature 220℃, the hydrogenation conversion achieves 16.8%; The loading of the minor Pd on MoO3 surface can make its catalytic activity close to or achieve the efficient of the commercial catalysts like 5 wt% Pd/Al2O3 and 5 wt% Ru/Al2O3, of which the conversion of hydrogenation can reach 91.2% in 6 h under the temperature of 170℃; Meanwhile, the higher of the HxMoO3 concentration in mesoporous MoO3, the faster of the reaction rate. Besides, it has a completely different mechanism of hydrogenation to the noble metal catalyst, and its catalytic efficiency is much higher than conventional noble metal catalyst, which makes it an excellent potential catalyst to replace the noble metal catalyst.(3) The study of the catalytic dehydrogenation of Perhydro-N-ethylcarbazole on mesoporous Pd/MoO3catalyst. The dehydrogenation catalysts with different Pd loadings (1-5 wt%) for the perhydro-N-ethylcarbazole (PNEC) dehydrogenation were prepared by using the MoO3 as a support, for comparison, the catalytic performance of the commercial catalyst 5 wt% Pd/Al2O3 were testedas well. Then, to observe the change of the H content and Mo valence in HxMoO3, and discuss the impact of the presence of MoO3 in dehydrogenation, we took the consideration of the different noble metal loadings, the different reaction temperatures and the reaction time conditions. Finally, we summarized the relationship between the HxMoO3 concentration and its catalytic dehydrogenation efficiency, and discussed its possibilities as a fuel cell electrode catalyst. It was found that 5 wt% Pd/MoO3 catalyst has the highest catalytic efficiency, PNEC can achieve 100% conversion to NEC in 5 h under the reaction temperature of 180℃; The initial reaction rate of PNEC on the Pd/MoO3 (180℃,5 min, conversion rate 44.6%) is much higher than the conventional commercial catalyst 5 wt% Pd/Al2O3 (180℃,5 min conversion rate 19%), and the higher of the reaction temperature, result in the larger of H content in HxMoO3 and the greater of the valence change in Mo; The concentration of HxMoO3 rapidly increased in the initial stage of the reaction, and would be able to reach an equilibrium value within 30 min, as the dehydrogenation reaction proceeds, only a little change in the value of X. These results suggest that the MoO3 is primarily existing as a H receptor during the catalytic dehydrogenation reaction, so that H atoms can present in the form of protons in the initial stage of the reaction without forming H2, result in the decrease of the dehydrogenation activation energy barrier, and somehow enhance the catalytic efficiency; With the increase of X value in HxMoO3, its conductance will increase as well.In short, there are obvious advantages for the mesoporous molybdenum trioxide in catalytic hydrogenation and dehydrogenation. The studiesin this paper provide new ideas and theoretical reference for the development of high-performance transition metal oxide catalyst, and expand its application in the field of new energy materials.