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Study on Preparation of Mg-Based Hydrogen Storage Materials and Its Hydrogenation Properties to Carbon Disulfide and Thiophene

Author: YangMinJian
Tutor: ZhouShiXue
School: Shandong University of Science and Technology
Course: Mineral Processing Engineering
Keywords: Mg-based hydrogen storage materials hydrogenation carbon disulfide thiophene
CLC: TQ223.121
Type: PhD thesis
Year: 2010
Downloads: 181
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Abstract

The key step for synthesis of methanol from coke oven gas lies in the deep purification of the gas, especially hydrogenation of organic sulfur. This has remarkable influence on long-term and stable operation of the catalysts for steam mathane reforming and methanol synthesis. It is a basic research to solve the problem that Mg-based hydrogen storage materials were used in hydrodesulfurization to the sulfocompounds in coke oven gas.Mg-based hydrogen storage materials were prepared by hydrogen reaction ball-milling and the key factors that affect the preparation were investigated including the kinds of carbon raw materials, the mass ratio of Mg/C, the variety of metal catalysts and the ball-milling time. Carbon disulfide and thiophene were taken as the model compounds for the hydrogenation by the hydrogen storage materials and the hydrogenation activities of the two compounds were compared. At last, the influence factors to the hydrogenation reaction of the sulfocompounds were studied and the reaction mechanisms between hydrogen donating of the Mg-based hydrogen storage materials and hydrogenation of sulfocompounds were discussed.Carbon materials were fine dispersants, grinding aids and modifiers in the preparation of Mg-based hydrogen storage materials. When the anthracite was treated by deashing and carbonizing, the hydrogen atoms and oxygen atoms were removed from the carbon skeleton and a kind of crystallitic carbon with graphite-like structure was made. When the additive content of crystallitic carbon was 40 wt.%, the material called 60Mg40C was well-distributed and its particle size was at a range of 30～50 nm. Moreover, the crystallitic carbon was also even-distributed in the hydrogen storage material without any agglomeration. When the crystallitic carbon content was 30 wt.% or 40 wt.%, the hydrogen storage materials showed a relatively higher hydrogen density of 4.93 wt.% and 4.78 wt.%, respectively. The hydrogen density of the materials would decline with too much or too little crystallitic carbon. Needle coke performed a superior dispersibility to crystallitic carbon and 10 wt.% of needle coke could prevent the powders from forming cold welding and adhering, but the needle coke was esay to occur amorphization. The influence of needle coke content on the hydrogen density was prominent. When the content of needle coke was 5 wt.%, the hydrogen density could reach to 4.35 wt.%, while the content was increased to 10 wt.%, the hydrogen density declined sharply. Active carbon was also a good dispersant, which could avoid cold welding and adhering when the additive content is only 20 wt.%. However, the hydrogen density was decreased with the active carbon content increasing under the premise of good dispersity. XRD analysis showed that Mg was the main hydrogen storage body in the prepared Mg-based hydrogen storage materials with the carbon raw materials mentioned above. In addition, whichever carbon raw material was added, the dehydrogenation temperature of the materials could be lowered.The addition of metal catalysts Al, Co, Fe, Mo could reduce the dehydrogenation temperature of hydrogen storage materials, and the dehydrogenation properties of the materials were improved. The initial dehydrogenation temperature of hydrogen storage material 50Mg40C10Co was 207.1℃, which was the lowest among the prepared materials with various metal catalysts. The content of metal catalyst within 4 wt.%-7 wt.% was thought to be appropriate, with which the hydrogen storage materials prepared had a clear dehydrogenation endothermic peak and a lower dehydrigenation temperature.Milling time significantly affected the properties of hydrogen storage materials, and the suitable milling time was 4 h. The hydrogen adsorption and desorption cycles of hydrogen storage materials didn’t improve the hydrogen storage properties distinctly, and hydrogen density was falling with the increasing number of the cycles. The synergy of Co, Mo and Ni could effectively reduce the dehydrogenation temperature. The formability of hydrogen storage materials were related to the content of metals, and the inlay and joint of metals with different hardness could improve the formability to be molded.When the Mg-based hydrogen storage material 60Mg35C5Mo reacted with carbon disulfide, Mg and H2S were formed in the product. And when the reaction temperature was 350℃, the proportion of hydrogen transferred from the hydrogen storage material to H2S reached to the highest value of 20.55%. When the material reacted with thiophene, MgS and H2S were formed, and the proportion of hydrogen transferred from the hydrogen storage material to H2S got to the highest value at 300℃. Neither the higher temperature nor the lower temperature was good for the hydrogenation between the hydrogen supplied by hydrogen storage material and sulfocompound. Only the specified range of temperatures could the dehydrogenation of hydrogen storage material and hydrogenation of sulfocompound facilitate, and form more H2S.The reaction temperature had an important impact on the hydrogenation, influencing the dehydrogenation of hydrogen storage materials as well as the degree of the hydrogenation between hydrogen supplied by hydrogen storage materials and the sulfocompounds. The hydrogenation activity was different for the carbon disulfide and thiophene. The C-S bond energy in carbon disulfide is lower, and it is easy to break down, while hydrogenation to thiophene is difficult to happen due to aromaticity of sulfur heterocyclic.The hydrogenation to carbon disulfide was hanppened in consecutive tube reactor. The carrier gas flowed in continuum, which caused a short contact time of hydrogen storage materials and sulfocompounds. The temperature of the carrier gas in the reaction tube was influenced by the velocity of the carrier gas flow, the sulfocompounds in which could not rise to reaction temperature before heated. Although the high pressure reactor, used in the hydrogenation to thiophene, could extend the contact time of thiophene and hydrogen storage materials, the concentration of reactants reduced as the reaction went on as a result of the reaction process was airtight, which caused adverse impact on the reaction.The hydrogen storage materials with crystallitic carbon added had the best hydrogenation performance on sulfocompounds. The metal catalyst Mo had the best catalysis to the hydrogenation. The Mg-based hydrogen storage materials supplied the hydrogen to react with the sulfocompounds, and crystallitic carbon supplied the diffusion channels for the hydrogen in Mg-based hydrogen storage materials, and the metal catalysts catalyzed the hydrogen donating by Mg-based hydrogen storage materials.

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