【Small】华北电力大学王祥科|扩展的π共轭共价有机框架：实现可见光下铀污染高效清除的新突破
文章标题：Rational Synthesis of Extended π-Conjugated Covalent Organic Frameworks for Enhanced Photocatalytic Uranium(VI) Removal
通讯作者：Hui Yang, Shengqian Ma, Xiangke Wang
文章概要
本文开发了一种创新的一锅法串联Scholl环化脱氢反应策略，成功合成了具有扩展π共轭结构的新型共价有机框架（COF） photocatalysts。通过在分子水平上精细调控化学键的连接方式，将传统的亚胺键转化为环化共轭结构，并在框架中引入萘环等大π共轭单元，显著提升了光生电子的离域能力与传输效率。该系列COF材料在光催化去除水体中的六价铀（U(VI)）实验中展现出极佳的清除性能，为理性设计高活性COF光催化剂提供了全新的普适性思路。
(a) Schematic diagram of different types of conjugation in COFs. (b) One-pot tandem Scholl cyclization-dehydrogenation reaction used to prepare extended π-conjugated COFs. (c) Design and synthesis of extended π-conjugated COFs. (d) Schematic illustration of the relationship between π-conjugation and photocatalytic performance in COFs.
引言
随着核能的快速发展，含铀放射性废水的环境污染问题引起了广泛关注，高效清除水体中的可溶性六价铀是当前环境治理领域的重大科学挑战。共价有机框架（COF） 作为一种新兴的结晶性多孔聚合物，凭借其可调的结构、高比表面积以及明确的通道，成为了光催化领域的理想平台。然而，传统COF在光催化应用中面临着光生载流子极易复合的瓶颈，这主要归结于其结构中常见的亚胺键具有一定的柔性，限制了电子的长程离域。如何通过化学键的本质转变，构建具有连续共轭和高电子传输速率的刚性骨架，是提升COF光催化效率的核心科学问题。
Characterization of synthesized COFs. (a) FT-IR spectra for PyTTA, PDA, NDA COF-1, and COF-2. (b) Solid-state 13C CP/MAS NMR spectra of COF-1 and COF-2. (c), (e) Experimental PXRD patterns of COF-1, COF-2 with corresponding Pawley refinement (orange), simulated results (light blue), and Bragg positions (blue) showing a good fit for the experimental data (gray) with minimal differences (purple). (d), (f) Top and side views of the eclipsed AA stacking crystal structure of COF-1 and COF-2. The C, N, and H atoms are represented by gray, blue, and white spheres, respectively. (g) SEM image of COF-1. (h), (i) HRTEM images of COF-1 and COF-2 (insets highlight the honeycomb-like pores and channels).
主要实验及结论
为了解决载流子复合快的难题，研究团队提出了一种直接在 reticular 骨架中构建全共轭刚性结构的设想。如图1所示，他们设计并制备了扩展π共轭的COF-1与COF-2，作为对比，还合成了具有传统p-π共轭的亚胺环化结构及常规亚胺连接的对照组。这种策略的核心在于利用一锅法串联Scholl环化脱氢反应，将不连续的电子结构转化为多环芳烃高能通路。这种精细的分子编辑不仅克服了骨架刚性与自组装柔性之间的冲突，还完美保留了材料的结晶完整性。
Characterization of synthesized COFs. (a) N2 sorption isotherms measured at 77 K for COF-1 and COF-2. (b) Pore size distributions of COF-1 and COF-2 from N2 isotherms measured at 77 K. (c) FT-IR spectra for COF-3 and COF-4. (d) Solid-state 13C CP/MAS NMR spectra of COF-3 and COF-4. (e), (f) Experimental PXRD patterns of COF-3, COF-4 with corresponding Pawley refinement (orange), simulated results (light blue), and Bragg positions (blue) showing a good fit for the experimental data (gray) with minimal differences (purple). The inset shows the structural fragments of COF-3 and COF-4, assuming the eclipsed AA stacking mode. (g) N2 sorption isotherms measured at 77 K for COF-3 and COF-4. (h) Pore size distributions of COF-3 and COF-4 from N2 isotherms measured at 77 K. (i) HRTEM image of COF-3 (insets highlight the honeycomb-like pores and channels).
新材料的物理化学特性得到了系统表征。如图2所示，红外光谱与固体核磁共振谱图明确证实了喹啉环结构的成功构建，表明环化反应进行得非常彻底。X射线粉末衍射（PXRD）与Pawley精修结果进一步表明，COF-1和COF-2均具有高度结晶的二维层状结构，并呈现出经典的AA堆叠模式，形成了整齐的蜂窝状纳米通道。高分辨透射电镜下清晰可见的孔道结构，为反应物的扩散与吸附提供了坚实的结构保障。
Physical and photoelectrochemical characterization of synthesized COFs. (a) Experimental PXRD patterns of COF-1, COF-3, and COF-5 after immersion in 6 M NaOH and 6 M HCl solution. (b) Experimental PXRD patterns of COF-2, COF-4, and COF-6 after immersion in 6 M NaOH and 6 M HCl solution. (c) EIS spectra for COF-1 to COF-6. (d) Tauc plots for COF-1 to COF-6. (e) Mott-Schottky plots for COF-1 to COF-6. (f) Band alignments for COF-1 to COF-6. (g) Photocurrent responses for COF-1 to COF-6. (h) EPR conduction band e-signals for COF-1, COF-2, COF-3, and COF-5. (i) Photoluminescence decay curves for COF-1, COF-2, COF-3, and COF-5.
Uranium extraction capacity and product analysis. (a), (b) Uranium extraction from uranyl-spiked deionized water and uranyl-spiked groundwater with initial uranium concentrations of ∼20 ppm, using COF-1 to COF-6 as photocatalysts. (c) Uranium extraction from uranyl-spiked organic wastewater with different initial uranium concentrations, using COF-1 and COF-2 as photocatalysts. (d) Uranium cycle extraction from uranyl-spiked groundwater with initial uranium concentrations of ∼20 ppm, using COF-2. (e) Removal efficiency of uranium by COF-2 under pH conditions ranging from 1 to 7. (f) Zeta potentials of COF-1, COF-2, COF-3, and COF-5. (g) U 4f XPS spectrum of COF-2 after photocatalysis. (h) HRTEM image of COF-2 after photocatalysis (inset highlights the lattice fringes). (i) PXRD pattern of COF-2 after photocatalysis.
光电化学性能测试表明，这种扩展的π共轭体系显著增强了可见光吸收范围，并极大地降低了电荷转移阻抗。在模拟可见光照射下的光催化去除六价铀实验中，COF-2表现出了最为优异的催化活性，其去除速率远远超过了缺乏离域电子的传统亚胺连接对照组，证明了连续的平面π共轭网络在促进电荷分离、延长激发态寿命方面的绝对优势。
Uranium extraction mechanism. (a) Photocatalytic H2O2 production performance of COF-1 to COF-6 in deionized water. (b) Quenching experiments for photocatalysis. (c) EPR spectra for ·O2−-DMPO adducts formed under visible light irradiation of COF-1 and COF-2. (d) The ELF diagrams of COF-2, COF-4, and COF-6. (e) LUMO and HOMO frontier molecular orbitals for the models of COF-2, COF-4, and COF-6.
总结及展望
本研究成功开辟了一条构建全共轭刚性COF光催化材料的全新路径。通过精准的原子级调控，将局部受限的电子解放出来，在整个RETICULAR网络中实现了高效的长程输运。这不仅使材料在放射性核素铀的清除上展现出巨大的应用潜力，也为未来设计用于光催化制氢、二氧化碳还原等大范围电子转移反应的高性能催化剂奠定了坚实的理论与实验基础。未来，团队将进一步优化该一锅法反应的普适性，以期将该策略推广至更多低维功能材料的构筑中。