Hydrogenation of Dibenzyltoluene Catalyzed by Magnesium-Based Hydrides AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY21040043

Chinese Library Classification Number:TK91

Citation Information:

Hydrogen，as one of the most promising energy carriers in the 21st century，it has many properties such as being clean，abundant and sustainable，there has three segments of hydrogen energy industry，include hydrogen production，storage and transportation of hydrogen，usage of hydrogen. The intermediate part is the most urgent to be developed，owing to hydrogen is difficult to be stored by a low-cost and reliable storage method. Nowadays，except for storage in liquid organic hydrogen carriers （LOHCs） or hydrides，hydrogen can be stored in hydrogen slurry （HS） which composed of LOHCs with hydrides，HS has attracted interests of avoids the drawback of hydrides which were pulverize to plug the gas pipe，improved the sluggish reaction rate of LOHCs，can be transported with exist fuels infrastructure. Currently，dibenzyltoluene （DBT） is considered as one of the most promising LOHCs，has the advantages of high boiling point，high hydrogen storage capacity （6.2%），non-toxic，non-flammable and cheap，but it also has slowly hydrogenation rate. In order to accelerate the rate of hydrogenation，magnesium-based hydride （16%LaNi₅-Mg hydride，replaced by LM） was chosen as the hydrogenation catalyst of DBT，the kinetics of hydrogenation reaction of HS （LM-DBT） were studied systematically. The properties of MH_x were characterized by X-ray diffraction （XRD），meanwhile，the fluctuation of weight and heat change of MH_x during programly heating were captured by thermalgravimetric analysis-differential scanning calorimetry （TG-DSC），the hydrogenation productions of DBT （H_n-DBT） were analyzed by ¹H nuclear magnetic resonance （¹H NMR）. The effect of reaction temperature was studied，and the results showed that the kinetics of hydrogenation reaction of DBT was continue to accelerate with rising reaction temperature，the kinetics of hydrogenation reaction of HS （LM-DBT） increased first and then slowed down with the rising temperature，the optimal hydrogenation temperature of HS （LM-DBT） was 300 ℃，hydrogenation absorption could reach 2.9% in 10 h. The effect of solid-liquid ration in HS （LM-DBT） was studied，the kinetics of hydrogenation reaction of HS （LM-DBT） increased first and then slowed down with the more and more addition of LM，the optimal solid-liquid ratio in HS （LM-DBT） was 1∶15，the hydrogenation absorption of HS （LM-DBT） was up to 4.8% in 10 h. LM was composed of MgH₂，Mg₂NiH₄ and LaH₃，the main catalytic hydrogenation phase in LM was Mg₂NiH₄，when reaction temperature was higher than critical dehydrogenation temperature of LM and Mg₂NiH₄，they could promote the hydrogenation of DBT at hydrogen atmosphere. DBT had three unsaturated rings （side ring-middle ring-side ring），there existed five pathways during hydrogenation progress： in order of middle ring，side ring，side ring；in order of side ring，middle ring，side ring；in order of side ring，side ring，middle ring；hydrogenation occurred at all three rings at the same time or hydrogenation in random. Hydrogenation productions of DBT were analyzed by ¹H NMR，and the results showed that hydrogenation pathway of DBT was constantly in the order of the side ring，side ring and middle ring which was not influenced by reaction temperature，solid-liquid ratio and component of HS. The consequences provided that when the reaction temperature was higher than critical dehydrogenation temperature of MH_x，MH_x （LM and Mg₂NiH₄ that contained transition metal element-Ni） could catalyze the hydrogenation of DBT at hydrogen atmosphere. A hypothesis could be put forward was that MH_x （contained transition metal elements such as Mn，Co，Fe，Pt，Pd，et al.） could catalyze the hydrogenation of LOHCs at suitable reaction temperature and hydrogen atmosphere. DBT had three benzene rings，and the results showed that the hydrogenation order was always at the side ring，side ring and middle ring，at the same time，DBT had not been completely hydrogenated during these experiment，compared with the saturated hydrogenation capacity of 6.2%，there was still a definite gap. It was speculated that the spatial structure of benzene rings at both ends changed after hydrogenation，owing to the hydrogenation progress contained two steps： DBT was activated by MH_x and MH_x provided active H atom to promote the hydrogenation of DBT，two side hydrogenation rings provided additional steric hindrance effect for the hydrogenation process of intermediate benzene ring. It was assumed that when the diameter of MH_x particles were small to several nanometer，MH_x could fully contact with DBT in slurry，at this time，the kinetics of hydrogenation of HS would be improved and DBT could be hydrogenated completely. The dehydrogenation of HS was endothermic reaction，dehydrogenation progress was accompanied with rising system pressure which meant when system pressure was too high that HS would hardly to dehydrogenate completely. When system pressure was too low，the metal or alloy was difficult to combine with H atom which released from H_n-DBT to keep form hydrides. Therefore，the dehydrogenation catalyst for slurry was MH_x with high temperature and a low platform pressure.