Study On The Hydrogenation Of Carbonates To Methane By The Liquid Organic Hydrogen Carriers

With the issue of global warming brought by the emission of greenhouse gases,various countries and regions have put forward the goal of carbon neutrality.Therefore,CO₂ capture,effective utilization and storage（CCUS）has received a lot of attention.Direct air capture of CO₂（DAC）has become an integral part of helping to solve climate problems.The carbonation reaction of metal oxides（such as Mg O,Ca O）and hydroxides（such as KOH,Ca（OH）₂）to absorb CO₂ is a capture method that does not require enrichment and purification of CO₂,and can be used in absorption of low concentrations of CO₂in air（0.04%）or flue gas（4–15%）to generate corresponding metal carbonates.The utilization of chemically stable CO₂ is converted into the utilization of carbonate.Most of the industrialized carbonate chemical conversion methods are high-temperature calcination,such as ironmaking,refractory production,calcium carbide production,cement production,etc.Calcination produces metal oxides and releases a large amount of CO₂.Compared with high-temperature calcination,carbonate hydrogenation can reduce the reaction temperature by100–200 ^oC while avoiding the generation of CO₂.There are few studies on carbonate hydrogenation,and the hydrogen sources that have been studied are limited to gas-phase H₂ and metal hydrides.The development of high-energy-density liquid organic hydrogen carriers（LOHCs）in recent years has provided a flexible medium for Green Hydrogen to facilitate storage and transportation.But the LOHCs include two steps of H₂storage and H₂ releasing as a transport medium.In this work,we try to couple the release of H₂ with the hydrogenation reaction of carbonate to avoid the cumbersome and energy-intensive steps of releasing H₂ and then use it in the hydrogenation reaction,and to replace H-H bonds that activate high bond-dissociation energy（BDE）with C-H bonds that activate lower BDE.This work focuses on LOHCs as an effective and recyclable hydrogen source in hydrogenation reaction,coupling dehydrogenation process with hydrogenation process,aiming at establishing low temperature and efficient conversion process of carbonate in CO₂ absorption and utilization cycle,and realizing the preparation of highly selective CH₄ by catalytic hydrogenation of carbonate.In order to find out the reaction network of carbonate and LOHCs,using CaCO₃ and NiCO₃ mechanically mixed as raw material,using tetrahydronaphthalene（THN）,dihydrophenanthrene（DHP）,dihydroanthracene（DHA）as LOHCs,to systematically study the liquid phase hydrogenation reaction of carbonate and explore the catalytic nature of Ni in this process.The reactivity of three LOHCs in the system was evaluated.The following main conclusions are drawn:（1）NiCO₃ alone has good reactivity with THN,and can generate a large amount of CH₄ and a small amount of CO₂.At the same time,NiCO₃ is reduced to Ni,which has the potential to catalyze the dehydrogenation of LOHCs and the hydrogenation of CaCO₃.The preferred temperature is 400 ^oC.The reaction of mixed carbonates with LOHCs at 400 ^oC can generate CH₄ with a selectivity greater than 90%and a small amount of CO₂ from the decomposition of NiCO₃.CH₄ is generated from the direct hydrogenation of carbonates,rather than the hydrogenation of CO₂ released from carbonate decomposition.（2）LOHCs are dehydrogenated under the catalysis of Ni first,and then H atoms combine to form H₂,H atoms reduce carbonates to form CH₄ and H₂O,and H atoms hydrogenate LOHCs raw materials.Among them,the reduction of carbonate by H atoms to generate CH₄ and H₂O also requires Ni catalysis,and the initial dehydrogenation rate of Ni-catalyzed LOHCs is greater than the hydrogenation rate of CaCO₃.The conversion of CaCO₃ can reach 15%after 20min reaction.（3）The size of raw NiCO₃ is about 14–25μm,and it has the behavior of agglomeration and dispersion at the same time during the reaction.The higher the proportion of NiCO₃ in the mixed carbonate,the more serious the agglomeration and the increase in particle size.At the same time,200–900 nm particles of Ni can be formed in-situ from NiCO₃,and the Ni particles attach to the surface of CaCO₃,which can catalyze the hydrogenation reaction.