Efficiency of high-loaded nickel catalysts modified by Mg in hydrogen storage/extraction using quinoline/decahydroquinoline pair as LOHC substrates

An effect of Mg introduction on efficiency of high-loaded nickel catalysts in dehydrogenation of decahydroquinoline (10HQ) was investigated. 10HQ dehydrogenation is key process for the liquid organic hydrogen carrier (LOHC) storage technology using the quinoline/10HQ pair as H₂-lean/H₂-rich substrates. An influence of synthesis technique of Ni/Mg/Al catalysts on their properties has been demonstrated. The catalysts were synthesized through coprecipitation of Ni, Mg, Al precursors to obtain layered double hydroxides (LDH) or via synthesis of (∼72 wt%) Ni-Al₂O₃ system – also through coprecipitation, followed by modifying with a magnesium-containing precursor. For the catalysts of the first series, the inclusion of magnesium into LDH lattice led to a significant increase in catalytic activity in hydrogen extraction (10HQ dehydrogenation reaction). Despite the decrease in the content of catalytically active nickel, a significant increase in the yield of the dehydrogenation product was observed. This regularity is presumably associated with appearance of basic sites, that accelerates the dehydrogenation reaction. In the case of the second series, activity of pre-reduced (600 °C, H₂) catalysts in dehydrogenation of 10HQ also significantly depends on a MgO content and is maximal at Mg:Ni weight ratio 0.056. Using an in-depth study of structure of the original and reduced catalyst samples (Ni-Al₂O₃ and Ni-MgNiO_x-Al₂O₃), it was shown that this regularity is associated with the increased resistance of catalytically active Ni particles to agglomeration during the reductive activation. Also, using the Ni-MgNiO_x-Al₂O₃ catalyst for hydrogen storage process (hydrogenation reaction), the possibility of deep quinoline hydrogenation (up to 10HQ) in a flow-type reactor was demonstrated for the first time.