摘要/Abstract

近几十年来，气候变暖、海平面上升等全球性气候问题日益严重，对人们赖以生存的自然环境造成了巨大的威胁.为了缓解并最终解决温室效应，多年来人们一直着手研究以二氧化碳（CO₂）为主的温室气体的处理方法.CO₂捕获和转化是一项新的技术，将捕获得到的CO₂直接转化成甲酸、甲醇、甲烷等小分子有机物或药物中间体等高附加值的化合物.卟啉金属-有机框架（Porphyrin Metal-Organic Frameworks，PMOFs）是一种基于卟啉配体和金属节点的多孔配位框架材料.卟啉配体具有良好热稳定性、化学稳定性以及优异独特的光学性能，结合MOFs框架的多孔性带来的对CO₂等气体分子的良好吸附性，使得PMOFs在CO₂捕获与转化上具有巨大的潜力.首先，介绍了PMOFs合成中常用的构筑策略，包括拓扑导向、柱层策略以及金属-有机笼策略.然后，根据次级结构基元对常见的PMOFs结构进行系统分类，包括基于低价态金属离子、桨轮状M₂（COO）₄、金属-氧无限长链和硬酸金属-氧簇四类，叙述了各类PMOFs的结构特性和稳定性.随后，通过一些代表性的实例分类总结了PMOFs在CO₂捕获与转化上的应用，包括CO₂的捕获、环加成反应、光催化反应和电催化反应.最后，总结了PMOFs在四大类应用中具有的优势与挑战，并展望了PMOFs在CO₂捕获与转化中的机遇和发展前景.
关键词: 卟啉金属-有机框架, CO₂捕获与转化, CO₂环加成反应, CO₂光还原, CO₂电还原
The worldwide climate issues such as the global warming and the sea level rising are becoming serious. In order to relieve the stress of environment, a lot of attempts have been made to reduce the emission of CO₂, which is the main component of greenhouse gases. CO₂ capture and conversion (C3) is an emerging technology, which directly converts the captured CO₂ into high value-added compounds or fuels such as formic acid, methanol and methane. Porphyrin metal-organic frameworks (PMOFs) are based on porphyrin or metalloporphyrin ligands and metal nodes. The combination of excellent thermal/chemical stability, strong absorption of visible light and long lifetime of excited state, and high CO₂ capture capacity paves the way for the applications of PMOFs in C3. In this review, we have firstly introduced the synthesis strategies of PMOFs, which are guided by framework topology, pillar-layer and metal-organic cage (MOC). With the good control of the pore sizes and thermal/chemical stability, the catalytic performances of PMOFs can be easily tuned:PMOFs that are prepared via the pillar-layer and MOC strategies are of relatively lower stability, and the ones that are guided by framework topology are of higher stability. Next, we have classified the types of PMOFs according to the secondary building units (SBUs). There are four types of PMOFs, and the SBUs include (1) the low-valence metal ions such as Cu²⁺ and Cd²⁺; (2) the paddle-wheel M₂(COO)₄ (M=Cu²⁺, Zn²⁺) units; (3) the infinite metal (such as Al³⁺, Ga³⁺ and In³⁺) oxide chains; (4) the hard metal (such as Cr³⁺, Fe³⁺, Ti⁴⁺, Zr⁴⁺, Hf⁴⁺, and rare earth metals) oxide clusters. The structure characters and stability have been described afterwards. The coordination bonds in the first and second types of SBUs are relatively weak. For comparison, most of the PMOFs based on the infinite metal oxide chains and hard metal oxide cluster exhibit high thermal/chemical stabilities, which could be used for practical applications towards C3. Then, we have summarized the recent works about applications of PMOFs in C3, which are divided into four parts, including the selective capture of CO₂, organic transformations with CO₂, CO₂ photoreduction and CO₂ electroreduction. Selective capture of CO₂ from a mixture of gases is one of the most important applications, considering that less energy and lower temperatures/pressures are required. Through the catalytic cycloaddition reaction of CO₂ and epoxides, the important products of cyclic carbonates can be produced. Some of the catalytic reactions can be carried out at 0.1 MPa and room temperature with high yields. With the assistance of environmentally friendly visible light, CO₂ can be photoreduced into fuels such as formate ion, methanol and methane. In addition, two typical examples of CO₂ electroreduction have been discussed in this review. Through the process of photoreduction and electroreduction, clean energies such as solar light and electricity can be employed to help transfer the green gas CO₂ into fuels. At the end, we have discussed the merits and challenges of PMOFs in the applications of C3. Selective adsorption of CO₂ from other gases, especially NO_x, SO_x and other flue gases, is highly required. The efficiency of the catalytic cycloaddition reaction should be further improved, especially cutting down the reaction time. Reaction efficiency and product selectivity of photoreduction and electroreduction should be improved. Photoelectrocatalytic reduction of CO₂, which combines both advantages of photoreduction and electroreduction, should be a hot topic in the future. The ideal system should include both a photoanode for water oxidation and a photocathode for CO₂ reduction that are linked by a wire without external applied bias, achieving the dream of artificial photosynthesis.
Key words: porphyrin metal-organic framework, CO₂ capture and conversion, CO₂ cycloaddition reaction, CO₂ photoreduction, CO₂ electroreduction


PDF全文下载地址:

点我下载PDF