Fluolab【Angew.Chem.】东南大学汪勇教授|阳光+水+空气：光催化制备双氧水新突破，量子产率高达15.7%
通讯作者： Yong Wang
文章概要
过氧化氢（）作为一种绿色环保的氧化剂，在工业合成和医疗消毒领域需求巨大。然而，传统的蒽醌法生产过程复杂且伴随环境风险。本研究开发了一类新型的离子型乙烯基连接共价有机框架（ivCOFs），利用阳光、水和空气，实现了高效且可持续的光催化合成。通过创新的水相EDA络合合成策略，研究者成功解决了离子型框架材料在合成中易出现的结晶度低、孔隙塌陷等难题，为实现“零碳”化学品生产提供了新的设计范式。
Design and advantages of ionic covalent organic frameworks for photosynthesis of H2O2. (a) Schematic diagram of ionic covalent organic frameworks. (b) Schematic diagram of Pauling-type O2 adsorption at ionic sites. (c) Effects of electronic band structures on the thermodynamic driving force of 2e− ORR. (d) Chemical structures of ivCOFs and vCOF with different mass transfer and charge separation abilities.
引言
目前利用光催化技术直接从水和氧气中合成面临着多重技术瓶颈。传统的共价有机框架（COFs）大多呈现电中性且具有疏水性，这导致催化剂与水溶液的接触较差，且反应物在通道内的扩散动力学受限。更为关键的是，大多数COF材料缺乏对氧气的特异性吸附位点，往往导致非选择性的4电子还原过程，生成的是水而非目标产物。为了克服这些障碍，离子型共价有机框架（iCOFs） 因其带电骨架和亲水性纳米通道而备受关注。然而，由于带电基团之间的强静电斥力，制备具有高结晶度和有序离子通道的iCOFs一直是一项极具挑战性的任务。
Formation of EDA complexes and synthesis of ivCOFs by EDA-mediated aqueous synthesis strategy. (a) Comparisons of EDA-mediated synthesis and direct synthesis. (b) Reaction paths and the structural transformation through intermolecular electron transfer and nucleophilic substitution. (c) PXRD patterns of ivCOFs. (d) Nitrogen adsorption-desorption isotherms of ivCOFs. (e) The corresponding pore-size distributions of ivCOFs. (f) Comparisons of the BET surface area with most of the reported iCOFs since 2017. (g) PXRD patterns of ivCOF-I subjected to various treatments.
主要实验及结论
研究团队提出了一种基于电子供体-受体（EDA）络合的水相合成策略。通过引入催化性的电子供体DMAP，与带正电的离子单体形成稳定的EDA中间体，有效屏蔽了离子间的静电斥力。实验结果显示，这种方法制备出的ivCOF-I具有极高的结晶度和高达1035 的比表面积，远超以往报道的离子型COF材料。这些有序的离子纳米通道不仅实现了超快的水分子传输能力（比非离子型COF高出11倍以上），还诱导了高效的Pauling型氧气吸附，显著降低了生成中间产物的反应能垒。
Ultrafast mass transfer and accelerated charge separation. (a) Water vapor sorption isotherms of ivCOF-I, ivCOF-Br, ivCOF-Cl, and vCOF measured at 298 K. Solid dots, adsorption; open dots, desorption. Insets: water contact angles of ivCOF-I, ivCOF-Br, ivCOF-Cl, and ivCOF-I. (b) The corresponding transfer rate-time variation curves. (c) Optimized adsorption configuration and corresponding energy of ivCOFs (Pauling-type) and vCOF (Yeager-type). (d) O2 physical adsorption isotherms at 298 K. (e) Dynamic O2 adsorption profiles at 298 K and 1 atm. (f) Schematic illustration of ionic nanochannels for water transport and O2 adsorption. (g) Electrostatic potential maps of the conjugative fragments in the ivCOF-I and vCOF. (h) Transient absorption spectra of ivCOF-I recorded at the indicated delay times following excitation at 400 nm. (i) Transient absorption traces for ivCOF-I normalized to the 600 nm exciton band.
Photophysical properties and photosynthesis of H2O2 from H2O and air. (a) Solid-state UV-vis diffuse reflectance spectra of ivCOF-I, ivCOF-Br, ivCOF-Cl, and vCOF. (b) The corresponding Tauc plots. (c) Experimentally derived electronic band structures of COFs compared to potentials for oxygen reduction and water oxidation. (d) Batch reactor performance for photosynthesis of H2O2 from ambient air-equilibrated water over 120 min under visible light. (e) Schematic diagram of the apparent activation energy of photocatalytic H2O2 production. (f) Wavelength-dependent AQY and SCC efficiency of photocatalytic H2O2 evolution for ivCOF-I. (f) Schematic of manufacturing H2O2 solution via a continuous flow reactor. Inset: image for converting water and O2 into H2O2 through the ion nanochannels. (g) Flow reactor performance for continuous manufacture of H2O2 aqueous solution. The blue dashed line indicates an average concentration of about 0.155 mM, while the gray dashed line denotes the flow rate. Inset: image for collecting ∼10 L H2O2 aqueous solution.
在光物理性质方面，ivCOFs展示了优异的可见光吸收和显著增强的光生电荷分离能力。瞬态吸收光谱证实，乙烯基连接增强了电荷的离域化，延长了激子寿命。在性能测试中，ivCOF-I在仅以环境空气为氧源、不添加牺牲剂的条件下，展现出6.9 的产率，其在420 nm处的表观量子产率（AQY）高达15.7%，太阳能到化学能的转化效率（SCC）达到1.08%。研究还通过原位红外光谱和电化学分析证实，该反应同时通过2电子水氧化和2电子氧还原两条路径并行产氢。此外，通过搭建蛇形流路反应器，研究者实现了超过120小时的连续稳定生产，产出的溶液总量达到10升，充分证明了其工业应用潜力。
Reaction pathways of photosynthesis of H2O2. (a) Photocatalytic H2O2 production rates of ivCOF-I with different scavengers and operation conditions. (b) The DMPO-•OOH signal in methanol with ivCOFs and vCOF under light irradiation. (c) The DMPO-•OH signal in water with ivCOFs and vCOF under light irradiation. (d) Transferred electron number of ivCOFs and vCOF based on RDE analysis. (e) H2O2 selectivity of ivCOFs and vCOF under different applied potentials. (f) Time-dependent in situ DRIFTS spectra of ivCOF-I for photosynthetic H2O2 processes in the O2 atmosphere.
总结及展望
这项研究成功地构建了具有高度有序离子通道的乙烯基COF材料，并通过卤素离子调节能带结构，实现了对副反应析氢的有效抑制。ivCOF-I凭借其卓越的亲水性、极强的氧气吸附能力以及优异的电子迁移动力学，打破了光催化合成过氧化氢在环境条件下的性能瓶颈。这一成果不仅丰富了离子型框架材料的合成方法学，也为未来开发低成本、高效率的分布式太阳能化学品合成系统奠定了坚实的科学基础。