Research On The Construction Of Nano-sheet Supported Metal Catalyst And Its Application In The Hydrodeoxygenation Of Lignocellulose

Lignocellulose is the most abundant biomass resource in nature.It is an effective way for high-value utilization of lignocellulose to achieve high-efficiency hydrodeoxygenation for preparing fuels by constructing new catalysts with high activity,which can significantly reduce the dependency on the consumption of fossil energy.However,the hydrodeoxygenation of lignocelluloses faces great challenges due to its complicated structure and chemical inertness.In this work,three kinds of catalysts,including Pd/Co₃O₄-Ns,Ni/Ni-PS and Ni/NiPS-ZSM-5 were fabricated to be used in hydrodeoxygenation of phenolics,lignin and lignocelluloses,respectively.The technical process and mechanism of hydrodeoxygenation were investigated to provide theory and application guidance for efficient conversion of lignocellulose into fuels.Aiming at solving the problems of low selectivity of target products and harsh reaction conditions during selectively producing cyclohexanol fuels from lignin-based phenolic compounds by HDO,a 2D-carrier supported Pd/Co₃O₄-Ns catalyst was prepared.Under mild reaction conditions of 0.3 MPa H₂ and 140℃,the catalyst had an excellent catalytic performance for the selective conversion of phenolic monomers to cyclohexanols and its derivatives,reaching as higher as 97.4%of cyclohexanol yield from guaiacol.Through investigating variations of intermediate products yield during the process,it was found that the excellent selectivity of the catalyst is due to the existence of a reaction path that preferentially generates phenol in the conversion process of guaiacol.Even under the low reaction pressure of 0.1 MPa H₂,the catalyst exhibited high activity and selectivity to convert guaiacol into cyclohexanol.The phase structure change,the H₂-TPR characterization and the discoloration experiment of WO₃ powder indicated that Pd/Co₃O₄-Ns catalyst can realize the activation and spillover of hydrogen at low temperature.Through the control variable experiment of active sites and H₂-TPD characterization,it was found that the highly selective conversion of guaiacol over Pd/Co₃O₄-Ns resulted from the dual-site hydrogenation mechanism.Wherein,hydrogen molecules are adsorbed and activated on the Pd surface to produce active hydrogen species which overflow to the Co O_x surface.Consequently,guaianol molecule can be converted into phenol by demethoxylation on the Co O_x interface with the presence of active hydrogen species,resulting in a phenol-preferred reaction path in the HDO process.Because of hydrogen spillover,Pd/Co₃O₄-Ns catalyst exposes a large number of active sites under the dual-site hydrogenation mechanism,thus achieving highly selective conversion of phenolic compounds under mild conditions.The one-pot conversion of lignin biomass into high-grade hydrocarbon biofuels via catalytic HDO uses precious metal catalysts mostly,while transition metal catalyst still faces problems like harsh reaction conditions and yield of hydrocarbons.A reduction-oxidation strategy was adopted to prepare new Ni phyllosilicate like nanosheets（Ni-PS）at room temperature and constructed Ni/Ni-PS catalyst through partial in-situ reduction.The Ni-PS precursors are partly converted into Ni⁰ nanoparticles by in situ reduction and the rest remained as supports.The Si-containing supports are found to have strong interactions with the nickel species,hindering the aggregation of Ni⁰ particles and minimizing the Ni⁰ particle size.The catalyst contains abundant surface defects,weak Lewis acid sites and highly dispersed Ni⁰particles.The catalyst exhibits excellent catalytic activity towards the depolymerization and HDO of the lignin model compound,2-phenylethyl phenyl ether（PPE）,and the enzymatic hydrolysis of lignin under mild conditions,with 98.3%cycloalkane yield for the HDO of PPE under 3 MPa H₂ pressure at 160°C and 40.4%hydrocarbon yield for that of lignin under 3 MPa H₂ pressure at 240°C,and its catalytic activity can compete with reported noble metal catalysts.Based on the excellent performance of Ni phyllosilicate catalyst in the direct HDO reaction of lignin,ZSM-5 molecular sieve was introduced as a functional active support to improve the versatility of the catalyst in the direct HDO of raw lignocellulose into biofuels.The introduced ZSM-5 molecular sieves provide new active sites in the catalyst as both a silicon source and a support.Due to the existence of molecular sieves,the aggregation of nickel based two-dimensional nanosheets is hindered.The monodisperse two-dimensional nanosheets,the interaction between the nanosheets and the support,and the formation of nickel phyllosilicate-like phase jointly promote the decreasing nickel particle size.Ni/NiPS-ZSM-5 catalyst has a large specific surface area abundant nickel vacancies and oxygen vacancies.This defect rich structure may be beneficial to the formation of Lewis acid sites and the adsorption and activation of reactant molecules.ZSM-5 also provides the catalyst with additional microporous structure and Br?nsted acid sites.The catalyst has excellent HDO performance for birch sawdust.It can achieve 97.7%conversion of birch sawdust at 270°C,5h and 5MPa H₂.The total carbon yield of low-carbon fuel is 75.1%,and that of volatile fuel,CH₄ and bio-oil are33.6%,29.5%and 12.0%respectively.Even if the birch sawdust concentration is as high as250 g/L,it can still achieve direct and efficient HDO to the whole components of biomass,achieve 88.2%conversion of birch sawdust at 270°C,10 h,7 MPa H₂,and the total carbon yield of low-carbon fuel is still 60.2%.