Performance Of Carbon-coated Nickel-based Catalysts For In-situ Aqueous Phase Hydrodeoxygenation Of Methyl Palmitate

The utilization of biomass can mitigate energy and environmental problems.Therein,the hydrodeoxygenation of fatty acid esters has been developed to produce diesel-like hydrocarbons.Usually,the hydrodeoxygenation is performed under an external supply of hydrogen,while there is a potential safety issue during the H₂ storage and transportation.To circumvent this,in-situ aqueous phase hydrodeoxygenation,i.e.,the coupling of in-situ H₂ generation via aqueous phase reforming and hydrodeoxygenation,has attracted attention.In present work,the in-situ aqueous phase hydrodeoxygenation of methyl palmitate on nickel-based catalysts was investigated using methanol as hydrogen donor.Because the harsh hydrothermal condition can result in the sintering and leaching of the active centers,the carbon-coated nickel-based bimetallic catalysts were prepared to improve the catalyst stability by taking advantage of the carbon confinement effect.First,the carbon-coated Ni,Co and Ni-Co alloy catalysts（i.e.,Ni@C,Co@C and Ni-Co@C）were prepared by the carbonization of the metals doped resorcinol-formaldehyde（RF）resins synthesized by the one-pot extended St?ber method.It was found that the RF resins microsphere size was remarkably reduced due to the Co²⁺doping,while it did not obviously change when the Ni²⁺species were introduced.During the carbonization at 800 ^oC in a N₂ flow,the Ni²⁺and Co²⁺species were reduced and the RF resin microspheres were slightly shrunk.The metallic Ni,Co and Ni-Co alloy particles were uniformly distributed in the carbon microspheres and their size distribution was very narrow（mainly between 10 and 12 nm）.Also,a charge transfer from Ni to Co appeared in the Ni-Co alloy.Compared with the d-band center of metallic Ni and Co,those of Ni-Co alloy shifted away from and toward Fermi level,respectively.In the in-situ aqueous phase hydrodeoxygenation（HDO）of methyl palmitate with methanol as hydrogen donor at 330℃,the decarbonylation/decarboxylation pathway was dominating on all the catalysts.The Ni-Co@C catalysts gave higher deoxygenation activity than the Ni@C and Co@C ones,and the yields of n-pentadecane and n-C₆～n-C₁₆ reached 71.6%and 92.6%,respectively.We suggest that the high performance of Ni-Co@C is attributed to the electronic interaction between Ni and Co and the small carbon microspheres.Interestingly,the metal particles were scarcely sintered and there was very little metal loss during the reaction,which is ascribed to the confinement effect of carbon.The catalyst deactivation is due to the carbonaceous deposition,and the regeneration by CO₂ treatment can completely recover the catalyst reactivity.In addition,a variety of carbon-coated nickel-based bimetallic catalysts（Ni-Cu,Ni-Zn,Ni-Ga,Ni-Sn and Ni-In）are prepared,and their hydrothermal stability and reaction performance in the in-situ aqueous phase hydrodeoxygenation of methyl palmitate are preliminarily investigated.It was found that Ni Cu,Ni₃Sn₂ and Ni In intermetallic compounds showed good hydrothermal stability.However,the Ni Ga intermetallic compound was destroyed to form Ni and Ni Ga₂O₄,and the Ni₂In₃intermetallic compounds was converted to Ni In intermetallic compounds.In the in-situ aqueous phase hydrodeoxygenation,the Ni-In and Ni-Sn bimetallic catalysts gave higher performance than other catalysts.On the Ni-Cu,Ni-Ga,and Ni-Zn catalysts,the main deoxygenation pathway was decarbonylation/decarboxylation.The formation of Ni₃Sn₂ and Ni In intermetallic compounds inhibited decarbonylation/decarboxylation while promotes the selective hydrogenation to hexadecanol.Due to the confinement effect of carbon,the metal nanoparticles in the catalysts did not undergo significant sintering during the reaction.