【作者】 赵鑫；

【导师】 李映伟；

【作者基本信息】 华南理工大学 ， 化学， 2023， 博士

【摘要】 原子级分散金属-碳材料因其独特的尺寸效应和接近100%的原子利用率,已成为催化领域的研究前沿。金属有机骨架（MOFs）是由金属节点与有机配体通过自组装形成的一类多孔配位聚合物,具有周期性结构、原子级分散和多样化的金属节点、可调变的孔径尺寸和孔隙率。近年来,MOFs热解已逐渐成为制备原子级分散金属-碳材料的常用策略,但仍然存在一系列问题有待解决。例如,MOFs的金属原子具有很高的表面自由能,在热解过程中极易烧结形成不规则的金属纳米颗粒,直接导致催化活性、选择性的严重下降;此外,MOFs的有机配体在热解过程中的分解碳化具有高度随机性,也会造成原有的孔结构破坏和比表面积锐减,降低活性位可及性,进而影响催化性能。致力于解决以上问题,本论文提出并发展了多种有效的新型热解制备策略,通过模板辅助、配位诱导、空间限域等方式,在热解过程中对各组分的迁移演化实现精准调控并归纳出一般规律,成功制得一系列具有多级孔结构和可调配位环境的原子级分散金属-碳材料。进一步地,以生物质平台化合物（如糠醛等）的高值转化反应为研究对象,考察所得材料的催化性能;通过控制实验、原位实验、理论计算等方式,全面揭示底物转化路径,并阐明可能的反应机理,最终建立材料理化性质与催化性能之间的联系和相应调节机制。论文的主要研究内容和结果如下:1.发展了一种新型阳离子交换-扩散策略,首先将NENU-5转化为微孔和介孔共存的三层中空结构同时引入原子级Fe,再经热解制得锚定在原子级Fe掺杂的Mo O₂载体上的Cu亚纳米簇(Cu₄/Fe_0.3Mo_0.7O₂@C)。系统的表征结果表明,Fe原子掺杂显著改变了Mo O₂载体的电子结构,进而通过金属-载体相互作用使Cu亚纳米簇内部的电荷实现重新分布。所得Cu₄/Fe_0.3Mo_0.7O₂@C材料在糠醛与C₃～C₁₀伯/仲醇的氧化偶联合成C₈～C₁₅醛/酮（生物航空燃料中间体）的反应中表现出优异的催化性能,能够实现糠醛的完全转化和>99%的产物收率。进一步地,通过密度泛函理论计算和控制实验揭示了最适条件下的反应过程和相应中间体,阐明了Cu₄/Fe_0.3Mo_0.7O₂@C材料的组成和结构对催化性能的影响机制。2.发展了一种熔融盐辅助热解新策略,首先将Cu-BDC前体转化为封装在多级孔碳中的Cu亚纳米簇（<0.8 nm）;再通过超声辅助置换得到封装在多级孔碳中的双金属亚纳米簇（Cu-M@HPC,M=Pd,Pt,Ru）,金属负载量可达11.2 wt%。通过调变熔融盐用量、热解温度、热解时间等条件,实现了对所得材料结构和组成的可控调节。所得Cu-Pd@HPC材料在糠醛选择性氧化合成马来酸的反应中表现出优异的催化活性和稳定性:在最适条件下,转化频率（TOF值）可达20.1 h^-1,是文献报道最优水平,且循环利用6次后未出现明显活性降低。密度泛函理论计算表明,Pd的引入使Cu的局域态密度向费米能级移动,能够显著增强糠醛的化学吸附并提升催化活性。此外,Cu-Pd@HPC材料的多级孔结构也能够强化底物传质,进一步提升催化性能。3.提出了一种错位沉积策略,首次制得具有不同配位环境的异核双金属单原子材料（Cu C₄/Co N₄@HC）。表征结果显示,所得Cu和Co单原子分别以Cu C₄和Co N₄的形式锚定于氮掺杂的多级孔碳载体中。密度泛函理论计算表明,两种位点之间的强相互协同显著促进了电荷的极化分布,即分别实现Cu C₄和Co N₄位点周围的电子积聚和缺失。与常规Cu和Co单原子催化剂相比,Cu C₄/Co N₄@HC材料具有更强的底物吸附和氧气活化能力,因而在糠醛的氧化酯化反应中表现出明显优于已报道催化剂的活性:在常温、常压、空气条件下可实现糠醛、苯甲醛等底物的几乎定量转化,合成包括2-糠酸甲酯、苯甲酸甲酯等26种芳香酯类化合物。4.发展了一种介质诱导渗透-沉积策略,使用M_a金属有机骨架（M_a-MOF）和M_b金属酞菁复合物为模板,以KCl-KBr混合熔融盐为介质,制得一系列具有均一配位环境的双金属单原子复合材料（M_aN₄/M_bN₄@NC,M_a=Cu、Co、Ni、Mn;M_b=Co、Cu、Fe;NC=氮掺杂碳）。通过原位实验、控制实验以及密度泛函理论计算等方式,从多角度揭示了熔融盐在程序升温条件下的相变过程对模板的形貌、孔结构、金属和碳组分演化过程的影响规律;阐明了高温条件下熔融盐对M_a和M_b两种金属迁移运动、金属-氮配位键断裂和形成的作用机制;验证了熔融盐对金属-金属键形成的抑制作用。所得Cu N₄/Co N₄@NC材料成功实现了以2’-羟基苯乙酮和苯甲醛等大宗化学品为原料,经一锅多米诺反应定量合成33种黄酮化合物。并且在连续流动相反应器中运转6 h后催化活性无明显衰减。控制实验和密度泛函理论计算结果表明,Cu N₄与Co N₄位点间的强协同作用能够增强氧气活化能力并降低反应能垒,因此提升Cu N₄/Co N₄@NC的整体性能。此外,催化剂内部相互贯通的多级孔结构也能够促进底物传质扩散,提升双金属单原子位点的可及性,进而提升催化活性。更多还原

【Abstract】 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.更多还原