Structure Regulation And Catalytic Performances Of MOFs-Derived Atomically-Dispersed Metal-Carbon Materials

Atomically dispersed metal-carbon materials have become a frontier in catalysis field due to their unique size effects and approximately 100%atomic utilization.Metal-organic frameworks（MOFs）are a class of porous coordination polymers fabricated through the self-assembling of metal nodes and organic ligands.MOFs feature perodic structure,atomically dispersed and diverse metal nodes,adjustable pore size and porosity.In recent years,the pyrolysis of MOFs has emerged as a promising synthetic strategy for fabricating atomically dispersed metal-carbon materials,however there are still some key issues remained to be solved.For instance,the metal atoms in MOFs feature extremely high surface free energy and tend to aggregate into irregular nanoparticles during pyrolysis,resulting in dramatical decrement in catalytic activity and selectivity.Besides,the carbonization process of organic ligands of MOFs is highly random,resulting in destruction of porosity and severe loss in specific surface areas,which lowers their catalytic performances through reducing accessibility of active sites.Aiming at addressing these issues,this thesis focused on the transferring tendency and conversion process of metal and carbon components during MOFs pyrolysis,and developed various preparation strategies including template assistant,coordination induction and space confinement,to realize the precise regulation upon the transformation of different components.Subsequently,a series of metal-carbon materials composing of metal atoms with adjustable coordination environments embedded in hierarchical pores were successfully fabricated.Further,the catalytic performances of the as-prepared materials were evaluated in the catalytic conversions of biomass-based small molecules（e.g.,furfural）to value-added chemicals.The reaction pathways,plausible mechanisms were thoroughly disclosed via control experiments,in situ experiments and theoretical calculations.Finally,the relationships between physicochemical properties and catalytic performances were established.The main contents and results of this thesis are as follows:1.A novel cation exchange-diffusion strategy was developed.First,Fe atoms were introduced into NENU-5,during which the latter was transformed into a three-layer hollow structure with abundant micro-and mesopores.Afterwards,Cu sub-nanoclusters were obtained which were embedded on Fe-doped Mo O₂ support(Cu₄/Fe_0.3Mo_0.7O₂@C)after pyrolysis.Systematical characterizations suggest the atomic doping of Fe significantly modified the electronic structure of Mo O₂ support,resulting in charge redistribution within Cu sub-nanoclusters through metal-support interactions.The obtained Cu₄/Fe_0.3Mo_0.7O₂@C showed outstanding catalytic performance in oxidative coupling of furfural and C₃～C₁₀primary/secondary alcohols into C₈～C₁₅ aldehydes/ketones（aviation biofuel intermediates）,achieving complete furfural converision and>99%product yield.Further,the reaction process and intermediates under optimized conditions were clearly revealed through density functional theory（DFT）calculations and control experiments.The influence mechanisms of composition and structure of Cu₄/Fe_0.3Mo_0.7O₂@C upon catalytic performances were also investigated.2.A molten salt assisted pyrolysis strategy was developed to prepare sub-nanoclusters.Cu-BDC was first transformed into Cu sub-nanoclusters（<0.8 nm）encapsulated in hierarchical porous carbons（HPC）.Then,a secondary metal specie was introduced via galvanic replacement to fabricate bimetallic Cu-M sub-nanoclusters（Cu-M@HPC,M=Pd,Pt,Ru）at high loadings（up to 11.2 wt%）.The composition and structure were controllable regulated through tuning molten salt content,pyrolysis temperature and time.The obtained Cu-Pd@HPC exhibited superior catalytic activity and stability in the selective oxidation of furfural into maleic acid,achieving a turnover frequency（TOF）as high as 20.1 h^-1 under the optimized conditions,outperforming the previously reported heterogeneous catalysts.No significant decrease in activity was detected within six runs.DFT calculations revealed the introduction of Pd shifted the partial density of states of Cu toward Fermi level,significantly strengthened the chemisorption of furfural and therefore catalytic reactivity.Besides,the hierarchical pores of Cu-Pd@HPC also contributed to the catalytic performances by promoting mass transfer during the reaction.3.A misplaced deposition strategy was proposed for the fabrication of dual-metal single-atoms with different coordinations.Characterization results revealed the Cu and Co single atoms were affixing to N-doped hierarchical carbons as Cu C₄ and Co N₄ sites（Cu C₄/Co N₄@HC）.Density functional theory studies revealed the strong synergistic interactions between Cu C₄ and Co N₄ sites led to remarkable charge polarization with electron accumulations and depletions over Cu C₄ and Co N₄ sites,respectively.In comparison with conventional Cu and Co single atom catalysts,Cu C₄/Co N₄@HC featured stronger substrate adsorption and activation capability,hence showed promoted performances in the oxidative esterification of furfural,almost quantitatively converted furfural and benzaldehyde into 26kinds of aromatic aldehydes including methyl 2-furoate and methyl benzoate.4.A series of dual-metal single-atom composites（M_aN₄/M_bN₄@NC;M_a=Cu,Co,Ni,Mn,M_b=Co,Cu,Fe,NC=N-doped carbon）were synthesized from M_a MOF and M_bphthalocyanin templates in KCl-KBr medium via a medium-induced infiltration deposition strategy.In situ and control experiments,and DFT calculations were employed to thoroughly reveal the impact of the phase transformation of molten salts upon the morphology,porosity,and evolution of metal and carbon components in templates under programmed heating conditions.The affecting mechanisms of molten salts upon the migration of M_a and M_b,the cleavage and formation of metal-nitrogen coordination bonds at high temperature were clarified.And the inhibitory effects of molten salts toward metal-metal bond generation were also verified.The obtained Cu N₄/Co N₄@NC successfully achieved the quantitative one-pot domino synthesis of up to 33 natural flavonoids from commodity chemicals such as2’-hydroxyacetophenone and benzaldehyde.No significant degradation in catalytic activity was detected after running 6 hours in a continuous flow reactor.Control experiments and DFT calculations suggested the synergistic interactions between Co N₄ and Cu N₄ could facilitate O₂activating-splitting process and significantly reduce energy barriers.Besides,the interconnecting structure with hierachical pores also benefitted mass transfer and diffusion,hence contributed to the accessibility of dual-metal single-atoms for performance promotion.