环境化学与生态毒理学国家重点实验室知识库

    共价有机框架（covalent organic frameworks，COFs）是一类结晶多孔聚合物，其允许有机单元从原子层面精确整合成具有周期性骨架和有序纳米孔道的延伸结构。COFs的一个重要特征是它们是可设计的，也就是说，可以控制构筑基元的几何形状和尺寸以指导结构周期性的拓扑演变。构筑基元的多样性和共价连接的拓扑方案使 COFs成为结构控制和功能设计的新兴材料平台，非常有望用来解
决实际环境问题。本文围绕 COFs的设计制备及其在富集和光催化降解有机污染物中的应用展开研究，论文共分以下三个部分。
    1）介绍了 COFs的特点、磁性固相萃取和光催化领域的研究进展及其COFs在磁性固相萃取和可见光光催化领域中的应用前景。
    2）将 COF多孔、大比表面积的优势与磁性材料易于磁分离的特点结合起来，合成了一种含有高比例 COF的磁性固相萃取剂，并将其用于富集环境水样中的有机污染物。通过简单的策略将 COF（TpPa-1）接枝到表面修饰的 Fe3O4纳米颗粒（Fe3O4 NPs）上，制备了新型花束状磁性多孔纳米复合材料。磁性 TpPa-1（由1,3,5-triformylphloroglucinol（Tp）和 p-phenylenediamine（Pa-1）合成的 COF）材料包含核-壳型磁性纳米颗粒簇和相互连接的多孔 TpPa-1纳米纤维。由于这种特殊的形貌，它具有更大的比表面积、更高的孔隙率和超磁性，使其成为富集痕量分析物的理想吸附剂。在高效液相色谱分析之前，通过磁性固相萃取（MSPE）环境样品中痕量多环芳烃（PAHs）评估其性能。结果表明，磁性 TpPa-1材料对于这类有机化合物具有很强的富集能力。
    3）考虑到 COFs结构的多样性和可定制性，COFs材料被认为是有机半导体材料分子设计的诱人平台。它们的能带结构和物理化学特性可以通过定制构筑基元来轻松调节。我们通过分子设计和精确构筑定向制备了三种高效的可见光光催化剂，并深入探索了影响光催化剂性能的结构因素及其 COFs光催化降解有机污染物的机理。
利用 1,3,5-triformylphloroglucinol（Tp）和 melamine（MA）合成内嵌有 C3N4构建模块的 COF，将其作为一种高效可见光响应型无金属光催化剂。将促发光催化性能的三个因素巧妙地整合到 COF骨架中，其中三嗪单元作为光活性中心，环酮单元作为吸电子基团，共轭结构作为光电子转移平台。这种缺电子-给电子单元交错排列的 COF对于降解有机污染物表现出卓越的可见光光催化能力。研
究结果表明，通过聚合物分子结构的设计，可以精确调节其能带结构和光电性能，从而增强光催化活性。
    利用相同的醛和三种不同的含氮官能团单体合成了一系列亚胺键连接的COFs，研究了前体中化学组成和结构的变化对所得 COFs光电性能的转变，并随之对光催化降解能力的影响。研究结果表明，可见光光催化性能随着活性中心密度和骨架中的共轭程度的增加而增强，这将为其结构和物理化学性质的系统微调提供有力依据，从而拓宽了 COFs在光催化领域中的应用。
    利用高稳定的 TTB-TTA（4,4',4''-(1,3,5-triazine-2,4,6-triyl) tribenzaldehyde TTB）和 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline（TTA）合成的 COF）包覆金属有机框架（metal-organic framework，MOF）构建了一系列新型的金属有机框架/共价有机框架（MOF/COF）杂化材料。这类杂化材料继承了 MOF和COF结晶度高、比表面积大、可见光响应强和带隙可调的优势。将 NH2-MIL-125(Ti)引入到杂化材料中，可以调控它们的光电和氧化还原特性，同时形成的异质结可以有效地促进电荷分离，从而显著地提高了材料的光催化性能。因此，NH2-MIL-125(Ti)/TTB-TTA复合物具有极高的甲基橙（MO）光降解动力学常数，其速率分别高于母体 NH2-MIL-125(Ti)和 TTB-TTA的  9倍和  2倍。这种通用的分子-材料平台可以实现 COF分子工程和  MOF类型筛选双重调控模式，在许多领域都能得到应用。

    Covalent organic  frameworks (COFs)  are a  class of crystalline  porous polymer that allows the atomically precise integration  of organic units into extended structures with periodic skeletons and ordered nanopores.  One important feature of COFs is that they are  designable; that is,  the geometry and  dimensions of the  building blocks can be controlled to direct the topological evolution of structural periodicity. The  diversity of building  blocks and  covalent linkage  topology schemes make  COFs an  emerging materials platform  for structural  control and  functional design,  which are  promising for  handling   environmental  issues.  This   dissertation  focuses  on   the  design   and preparation   of   COFs  and   their   applications   in   enrichment   and   photocatalytic degradation of organic pollutants. It consists of the following three sections.
     The first  part  describes the  properties of  COFs, the  progresses  in the  fields of magnetic  solid-phase   extraction  (MSPE)  and  photocatalysis,   and  the  application prospects of COFs in MSPE and photocatalysis.
      In the second part, combining the advantages of porous and large specific surface area  of   COFs  with  the   magnetic  separation  of   magnetic  materials,   a  magnetic solid-phase  extraction  material  with  a  high  proportion of  COF  is  synthesized  for enrichment of organic pollutants in environmental water samples. A facile strategy for the fabrication of  novel bouquet-shaped magnetic  porous nanocomposite via grafting a  COF  (TpPa-1)  onto  the surface-modified  Fe3O4  nanoparticles  (Fe3O4  NPs)  was reported.  The magnetic TpPa-1(a COF synthesized from 1,3,5-triformylphloroglucinol  (Tp)  and p-phenylenediamine  (Pa-1)) contains  clusters of  core-shell magnetic  nanoparticles  and interconnected  porous  TpPa-1  nanofibers.
 Due  to this  particular  morphology, it  possesses  larger specific  surface  area,  higher porosity,  and  supermagnetism, making  it  an  ideal  sorbent  for  enrichment of  trace analytes.  Its   performance  was  evaluated   by  the  magnetic   solid-phase  extraction (MSPE)  of   trace  polycyclic  aromatic   hydrocarbons  (PAHs)   from  environmental samples  prior   to  high-performance  liquid   chromatographic  analysis.   The  results indicated that  the  magnetic TpPa-1  possessed superior  enrichment capacity  of  such organic compounds.
        In  the  third  part,  given  the  structural  diversity  and  tailorability,  COFs  were regarded as an appealing platform for the molecular design  of organic semiconductors. The band  structure and  physicochemical  properties can  be easily  tuned by  tailoring the  building   blocks.   We  synthesized   three  visible-light-driven   covalent  organic framework photocatalysts  via molecular design  and precise  construction, and further explored  the   structural  factors  affecting   the  photocatalytic  performance   and  the photocatalytic mechanism for the organic pollutants degradation.
      A COF inlaid with C3N4 building block was synthesized from 1,3,5-triformylphloroglucinol  (Tp) and  melamine  (MA),  which was  employed  as  a high-performance   metal-free   photocatalyst    with   visible   light   response.    Three functional factors  inducing photocatalytic ability  were artfully integrated  in the COF skeleton,  where triazine  units  served as  photoactive  centers, cyclic  ketone  units  as electron-withdrawing  moieties,  and   conjugated  structure  as  a   photoelectron  shift platform, respectively. This  COF with segregated  donor-acceptor alignments exhibits an excellent visible-light  photocatalytic capacity for organic pollutant  decomposition.
      Our  findings demonstrate  that  by  the judicial  design  of the  molecular  structure  of polymers,   it   is   possible   to   precisely  regulate   their   band   gap   structures   and optoelectronic properties, thus creating an enhanced photocatalytic performance.
       We synthesized a series of  imine-linked COFs from the same aldehyde and  three different  monomers  of  nitrogen-containing functional  groups,  and  investigated  the translation  of  chemical  and  structural  variation  in  the  precursors  to  photoelectric properties  of  the resulting  COFs,  with  the  consequent  influence on  photocatalytic degradation.   We  found   that   the  visible-light   photocatalytic   performances   were enhanced  with  increasing  density of  active  centers  and  conjugation  degree  in  the networks,  which  provide  a   forceful  basis  for  the  systematic  fine-tuning  of   their structural and physicochemical properties thus broadening the applications of COFs in photocatalysis.
       Series    of    novel    metal-organic    framework/covalent    organic     framework (MOF/COF)   hybrid  materials   were   constructed  by   encapsulating   metal-organic framework（  MOF）  with   highly  stable   TTB-TTA   (a   COF   synthesized   from 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde(TTB)and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA)) layer, inheriting their merits of  high crystallinity, large  surface  area, outstanding  visible-light  response and  tunable band gaps. The introduction  of NH2-MIL-125(Ti) into the hybrid  materials could modulate their  optical,  electronic  and  redox  properties,  and   promote  the  charge  separation owing to the formation of heterojunction, thus  resulting in an enhanced photocatalytic performance for organic pollutant decomposition. As such,NH2-MIL-125(Ti)/TTB-TTA composite  has a  much higher  photodegradation kinetic of   methyl   orange   (MO)  which   is   over   9   and   2   times   the   rates   of  parent NH2-MIL-125(Ti)  and   TTB-TTA,   respectively.  This   versatile  molecular-material platform can achieve dual adjusting modes consisting of a COF molecular engineering and a MOF type screening strategy, which will be applicable to a wide range of fields.