Application of organic porous polymer in visible light catalysis

Organic porous polymers (POPs) are widely concerned and studied in the fields of substance storage / separation, biomedicine, pollution treatment, energy, catalysis, etc. due to their high porosity, low density, diverse components, and ease of functionalization. . Photocatalysis is driven by light energy and can produce a variety of chemical products and energy molecules, which is an important choice for future industrial production. The molecular level design can give POPs a specific catalytic activity. The high thermal / chemical / photostability provides a guarantee for high cycle stability. The structural characteristics such as porosity make POPs more efficient than molecular catalysts. The above advantages make photocatalysis a POPs application Research hotspots.

     Recently, the National Center for Nanoscience Hanbo Air Task Force summarized recent years POPs research in the field of visible light catalysis, was invited to the Royal Society (RSC) Journal of Materials Chemistry A Journal on entitled Emerging applications of porous organic polymers in visible -light photocatalysis Review article. The article begins with a photocatalytic development and a brief introduction to the characteristics of POPs . It mainly introduces the sources and advantages of POPs photocatalytic properties, and their applications in organic synthesis, hydrogen production, carbon dioxide reduction, and organic degradation ( mainly introduces CMPs , CTFs , COFs , HCPs ) And summarize the challenges that the material development may face.

1. Principles and advantages of POPs photocatalysis

     In the photocatalytic reaction, after the photogenerated electrons and holes migrate to the surface of the material, they participate in the reduction and oxidation processes respectively ( Figure 1) . POPs constructed by molecular units can obtain photocatalytic activity, and can be easily modified and constructed with organic dye molecules or metal organic catalysts to obtain materials with catalytic sites; and organic frameworks with semiconductor properties, such as The body - receptor (D-A) structure can be precisely controlled by adjusting the electronic characteristics ( heteroatoms, substituents ) , composition ratio, and spatial configuration of the monomer . The high porosity facilitates the contact of reactants with catalytic sites and mass transport. For covalent organic frameworks (COFs) and partially covalent triazine frameworks (CTFs) , high crystallinity helps the separation and transfer of photogenerated charges. The composite material composed of POPs and other photocatalysts generally has higher catalytic activity than a single type of material.

Figure 1.  The basic principle and process of photocatalysis.

     In long-term or cycling experiments, high chemical / photostability is essential to maintain the integrity and catalytic activity of the porous framework of POPs . Construction of a covalent bond POPs Photocatalytic reaction environment has a good tolerance; partially reversible linkages COFs less stable, modified groups, chemically converted, chemically stable construct COFs stability can improve material ( Figure 2) . At the same time, the excellent thermal stability makes POPs have great application potential in high-temperature photocatalytic reactions.

Figure 2.  The chemical reaction formula for constructing POPs and the conversion of imine bonds in COFs .

2. Photocatalytic organic synthesis

    In the photocatalytic oxidation reaction of conjugated microporous polymers   , the oxidative active substances are generated by electron transfer or energy transfer between POPs and oxidants ( for example, oxygen ) . A variety of monomers have been used to construct conjugated microporous polymers (CMPs) as catalysts for photocatalytic organic synthesis ( Figure 3) . Sonogashira-Hagihara coupling reaction containing benzene and benzothiadiazole units prepared The CMPs , blue light at the α- terpinene (terpinene) oxidation of ascaridole hormone (ascaridole , FIG . 4A) ^{[. 1]} . 3- mercaptopropionic acid modified CMPs (WCMP_X , Figure 5) catalyze the oxidation of furoic acid to 5- hydroxy- 2 ( 5H ) -furanone in the aqueous phase , showing a stronger photocatalytic ability than unmodified CMPs ^[2] .

Figure 3.  Construction monomers for CMPs used in photocatalytic organic synthesis .

Figure 4. Schematic diagram of POPs photocatalyzed organic reaction.

Figure 5.  Thiol - alkyne reaction modified CMPs ^[2] .

    The energy band structure of CMPs with D–A structure can be adjusted by changing the type and composition of the electron-withdrawing unit and the electron-donating unit. The poly ( benzodiazole ) network containing different chalcogen elements has an adjustable band gap structure. B-BT containing benzothiadiazole has the best photocatalytic ability in the oxidation of benzylamine. Its catalytic mechanism is as follows As shown in Figure 6 ^[3] .

Figure 6.  Reaction mechanism of photocatalytic oxidation coupling of benzylamine ^[3] .

     1,3,5 -triethynylbenzene and 2,5 -diiodo -1,4- benzenediol were prepared by a series of Sonogashira coupling reaction and intramolecular cyclization reaction to prepare a microporous organic network with photocatalytic activity (BDF-MON , Figure 7) ^[4] . Under blue LED irradiation ( λ  <460 nm) , BDF-MON catalyzes the oxidation of primary amines to imines at room temperature and oxygen atmosphere ( Figure 4c) , and has excellent catalytic activity for various substrates.

FIG. 7.  Polymer Synthesis benzene series and two policy furan ^[4] .

    Carbazole and its derivatives have strong electron donating ability. Carbazole-based porous polymers (CPOPs) prepared with different carbazole units ( Figure 8 ) can catalyze a variety of chemical reactions. CPOP-1 constructed with 1,3,5- tris (9- carbazolyl ) -benzene not only has a high specific surface area and adsorption capacity ^[5] , but also can catalyze a variety of chemical reactions, including the oxidation of primary amines to imines, sulfur Oxidation of the substance to sulfoxide and so on ^[6] .

Figure 8.  Carbazole monomers used to construct CPOPs .

    Metal or dye-sensitized polymers   as photocatalysts, POPs containing ruthenium (Ru) , iridium (Ir) complex structures and metal phthalocyanine units ( Figure 9) have been extensively studied. Ir- fixed porous polycarbazole can be used as a catalyst for sulfide oxidation, arylboronic acid hydroxylation and aerobic cross-dehydrocoupling (CDC) reaction ^[7] . POPs containing iron (II) phthalocyanine show high catalytic activity in the photooxidation of aromatic hydrocarbons ^[8] .

Figure 9.  Conjugated microporous polymer containing metal phthalocyanine.

    Organic dye molecules ( Figure 10) , including Rose Bengal ^[9] , BODIPY ^[10] and Eosin Y ^[11] , have been used as building blocks to prepare POPs with photocatalytic activity .

Figure 10.  Organic dye molecules used to construct functionalized POPs .

    The structure of BF 2 in BODIPY dye molecules has the function of protecting dipyrrolene. CMPs constructed with BODIPY monomers , in which dipyrrolene is deprotected, combined with [Ru (bpy) 2 ] ²⁺ to obtain a catalyst that catalyzes the aza-Henry reaction ( Figure 11) ^[12] .

Figure 11.  Post-modification preparation of conjugated microporous polymers.

      During the ionothermal preparation of covalent triazine framework CTFs , the partial carbonization at high temperature leads to the uncertainty of the position of the valence band and conduction band of the material. Lower temperature (100 ° C) , the steam to trifluoromethanesulfonic acid as catalyst, prepared benzo thiadiazolyl two benzonitrile CTF-BT ( FIG. 12) can catalyze 4- nitrophenol is reduced to 4- Aminophenol ^[13] .

Figure 12.  Solid-phase steam synthesis of hollow CTFs and nanopore structure ^[13] .

     In addition to the trimerization of nitrile compounds, CMPs containing triazine units can be obtained without high-temperature and strong acid arylation polymerization ( Figure 13) , the higher the proportion of triazine structures in the polymer (P4-1 and P4-1.5 ) , The catalytic conversion of benzylamine to imine is higher (> 99%) ^[14] .

Figure 13.  Monomer molecular structure and stoichiometric ratio used in arylation polymerization.

    Multi-component photocatalysts have strong catalytic capabilities due to the diverse forms of charge and energy transfer. The structurally asymmetric asy-CTF constructed by 5- (4- cyanophenyl ) thiophene -2- carbonitrile ( Figure 14) contains four types of electron donor - acceptor structures, which can promote the continuity of charge / energy in various forms Transfer, thereby enhancing the catalytic performance of the polymer ^[15] . In the actual reaction, photooxidation, ASY the CTF- catalyzed 1,2,3 triphenylphosphine indol- 1- yield oxide of 93% , far exceeding having a symmetric structure CTFs reaction involved, CTF- Th (22%) , CTF-Th-Ph (63%) .

Figure 14. (a)  Representative structure of asymmetric asy-CTF ; (b) Symmetrical counterpart of asy-CTF , CTF-Th and CTF-Th-Ph ; (c) Four types of D–A in asy-CTF System ^[15] .

    The ordered structure of covalent organic framework   COFs is conducive to the absorption of visible light and the separation of photogenerated electron - hole pairs. The controlled molecular structure and chemical composition make it possible to adjust the photocatalytic performance at the molecular level. COF-JLU5 synthesized from 1,3,5- tris (4 -aminophenyl ) triazine and 2,5 -dimethoxyterephthalaldehyde exhibits excellent thermal / chemical / light stability, D–A structure The formation of COF-JLU5 shows good photocatalytic performance in aerobic cross-dehydrocoupling (DCC) reaction; in N - phenyl -1,2,3,4 -tetrahydroisoquinoline and nitro In the reaction of methane, the yield of the reaction catalyzed by COF-JLU5 reaches 99% , while the yield of TPB-DMTP COF without triazine structure catalyzes the reaction under the same conditions is only 44% ^[16] . Knoevenagel reaction and Aldol reactionSP ² - carbon-linked COFs having a high stability of the whole structure and a conjugated, 5,10,15,20 four - (4- benzaldehyde ) - porphyrins and 1,4- benzene acetonitrile two Por-sp ² c-COF is a highly efficient photocatalyst with amine oxide as imine ^[17] .

    The porous composite   to POPs and other combinations of materials are effective strategy to improve the catalytic properties of the polymer. The composite (Pd / TiATA @ LZU1) of MOFs @ COFs material with core - shell structure and metal palladium (Pd ) can catalyze the dehydrogenation of NH 3 BH 3 and the hydrogenation of styrene in series to obtain ethylbenzene ^{[ 18]} . Growing polybenzothiadiazole on glass fiber, the resulting B-BT @ glass heterogeneous photocatalyst can efficiently catalyze the dehalogenation of α- bromoacetophenone and the α- alkylation of aldehyde compounds ^{[19 ]} .

3. Photocatalytic hydrogen production

    In the process of photocatalytic water decomposition to produce hydrogen, photogenerated electrons and holes undergo hydrogen reduction and water oxidation reactions, respectively. As a photolysis water catalyst, the minimum value of the conduction band (CB) of the catalyst is lower than the redox potential of H ⁺ / H 2 (0 V , vs.  NHE) , and the maximum value of the valence band (VB) is higher than O 2 / H 2 O redox potential (1.23 V , vs.  NHE) . Therefore, the band gap width of the catalyst should theoretically be greater than 1.23 eV . The band gap structure of POPs can be adjusted by adjusting the electronic characteristics of the building units, composition ratio, etc. to meet the requirements of the experiment.

    Conjugated microporous polymer   sulfur-containing units ( thiophene, benzothiadiazole, dibenzothiophene sulfone ) , nitrogen-containing units ( pyridine, triazine, carbazole ), etc. are common in the construction of CMPs with photocatalytic hydrogen production performance Compound. Heteroatoms in polymers such as nitrogen and sulfur act as active sites for catalytic reactions. Adjusting the proportion of dibenzothiophene dioxide (FSO) in the polymer results in a series of polymers ( Figure 15) . Among them, P- FSO with the highest FSO content ( Figure 15e) has the highest photocatalytic hydrogen production rate ^[20] . In addition to the types and proportions of monomers, the substitution mode of the building unit also has a great influence on the performance of the polymer . PyDOBT-1 ( Figure 15e) constructed with 3,7- substituted FSO units and those with 2,8- substituted structure PyDOBT-2 ( Figure 15f) compared to (2113 μmol h ^–1  g ^–1) , Its conjugated framework is more planar, has a higher charge transfer rate, and exhibits higher photocatalytic performance (5697 μmol h ^–1  g ^–1 ) without the participation of other catalysts ^[21] .

Figure 15.  Structural fragments of conjugated microporous polymers.

    By adjusting the types and proportions of the electron donor ( carbazole, diphenylamine ) and electron acceptor ( cyano ) substituents in the building block , the D–A system of CMPs can be adjusted at the molecular level to obtain the energy band structure (1.64 –2.29 eV) different nitrogen-containing polymers ( Figure 16) ^[22] . Among them, 4-CzPN constructed by the ortho-dicyano substitution unit has an appropriate energy band structure (HOMO , +1.30 V ; LUMO , –0.81 V , vs.  NHE) , and shows the highest hydrogen generation rate.

Figure 16.  Monomer structure for the construction of nitrogen-containing polymers :  the position ( black ) , number ( blue ) and type ( red ) of the substituents are different.

    Covalent organic frameworks   COFs are closely related to the composition of the photocatalytic properties of the building blocks, containing different number of nitrogen atoms (N X , X = 0,. 1, 2,. 3) azine prepared monomers linked N X -COFs ( Figure 17) , the rate of hydrogen evolution increases with the increase in the number of nitrogen atoms, respectively 23 , 90 , 438 and 1703 μmol h ^{– 1 [23]} . The gC 40 N 3 -COF prepared by 3,5 -dicyano -2,4,6 -trimethylpyridine and 4,4 ″ -diformyl terphenyl has semiconductor characteristics. Under visible light irradiation, gC 40 N 3 -COF can catalyze the two half reactions of water decomposition at the same time, and the hydrogen evolution rate reaches 2.9 μmol h ^–1 , and can be further optimized to 206 μmol h ^{–1 [24]}.

Figure 17. (a)  The structure of triphenyl aromatic hydrocarbons, the planarity of which can be changed by replacing C–H with nitrogen atoms ; (b)  N x -COF prepared by hydrazine and aldehyde ^[23] .

    Covalent triazine framework   polymerization of different monomers can obtain CTFs with appropriate optical properties for photocatalytic hydrogen production. Through the strategy of stepwise polymerization, CTF-BT / Th containing thiophene (Th) and benzothiadiazole (BT) units with molecular heterostructure can be obtained ( Figure 18) , which can effectively prevent photogenerated electrons - holes The right compound ^[25] . Therefore, compared with single component CTF-BT and CTF-Th , CTF-BT / Th-1 has more superior hydrogen evolution performance (6.6 mmol h ^–1 g ^–1 ) , and exhibits good photocatalytic stability.

Figure 18.  Stepwise polymerization to construct CTF-BT / Th with molecular heterogeneity ^[25] .

Structural defects in      CTF , such as terminal groups, can affect the band gap structure of the polymer and hinder the photocatalytic performance. After heat treatment of CTP-1 at 300 ° C ( Figure 19) , the obtained CTP 300 not only retains the structural characteristics of the original polymer; the rearrangement of the structure also makes the defects disappear, resulting in a new energy band structure, giving CTP 300 Excellent visible light absorption capacity and effective charge separation and transfer performance ^[26] . Under visible light irradiation, the hydrogen release rate catalyzed by CTP 300 is three times higher than that of CTP-1 .

Figure 19. Possible structural defects in CTP-1 and possible structural reorganization in CTP T after heat treatment ^[26] .

    The combination of other types of catalysts in the porous composite   and POPs with matching band gaps can theoretically transfer photogenerated carriers between different materials to improve the photocatalytic capability. In the [alpha]-of Fe 2 O . 3 in the presence, between the three acyl phloroglucinol and 2,5- dimethyl- 1,4 -phenylenediamine is made by reacting [alpha]-of Fe 2 O . 3 and TpPa-2-COF composed of Z -type hybrid material to achieve electrons from [alpha]-of Fe 2 O . 3 of CB to COFs of VB transfer, promote photogenerated electrons - efficient separation hole pairs, so that material having excellent photocatalytic hydrogen production performance (3.77 mmol H ^{- 1}  g ^–1 ) ^[27] . Connect NH 2 -UiO-66 to TpPa-1-COF through a covalent bondAt the interface of the heterojunction, charge separation becomes more efficient, making the rate of hydrogen evolution (23.41 mmol g ^–1  h ^–1 ) of NH 2 -UiO-66 / TpPa-1-COF (4/6 ) high a TpPa-1-COF 20 -fold ^[28] .

4. Photocatalytic carbon dioxide reduction

    Carbon dioxide photoreduction faces many challenges. First, CO 2 is a stable linear molecule, and the cleavage energy of the carbon-oxygen double bond is about 750 kJ mol ^–1 ; second, at –0.42 V ( vs. NHE) , the water reduction reaction that generates hydrogen is CO 2 reduction Competitive response. Table  1 lists the redox potential and corresponding products of CO 2 reduction in the aqueous phase (pH = 7) ^[29] . Therefore, POPs used to catalyze the reduction of CO 2 need to have an appropriate energy band structure, visible light absorption performance and CO 2 capture capability.

Table  1. Redox potential of CO 2 reduction and corresponding products

    The metal-containing polymer   metal rhenium (Re) is coordinated with the nitrogen atoms of POPs to construct POPs for photocatalytic CO 2 reduction . Porous po