**1. Introduction**

Energy shortages and environment issues are global problems and challenges that are faced by human beings today [1–6]. The development of renewable energy technologies to reduce the pollutants emission has become an important research topic to maintain the sustainable development of our planet [7–12]. Artificial photosynthesis is an ideal way to effectively solve the energy and environmental problems by decomposing water to produce hydrogen or reducing CO2 to high value-added chemicals or fuels [13–16]. Accordingly, searching for highly efficient materials that can convert solar energy and store it in chemicals is desired. Metal-organic frameworks (MOFs), which are known as coordination porous polymer, is a class of crystalline porous materials constructed by the coordination bond between metal ions or metal cluster nodes [17–21]. These materials have been widely used in gas separation/storage, catalysis, sensing, proton conductors, and drug delivery because of their structural diversity, design/modification, and ultra-high specific surface areas [22–25]. Based on the previous results, it is proven that the multifunctional organic ligands in the MOFs structure can play the role of "light capture antenna" [26,27]. It can effectively accept photons, generate band gap transition, and transfer electrons to metal center units. Thus, MOFs are usually used as efficient photocatalysts [28–30]. When comparing to other photocatalytic materials, MOFs exhibit big specific surface area, high porosity, and supervised capturing capability of CO2 molecules, which endows them with grea<sup>t</sup> application prospect in the field of photocatalysis for CO2 reduction. In recent years, MOFs and their composite materials are widely used in water decomposition, hydrogen production, CO2 reduction, and photocatalytic organic conversion [31].

Yaghi group first proposed the concept of the metal-organic frameworks in 1995, and MOFs materials were then intensively explored as new functional materials [32]. In 1997, Kitagawa group reported a three-dimensional MOF material and found its ability to adsorb gas at room temperature [33]. After that, two landmark cases of MOFs, MOF-5 and HKUST-1, were reported by Yaghi group and Williams group in 1999 [34–36]. Among them, MOF-5 is a three-dimensional skeleton that formed by coordination of Zn4O(CO2)6 clusters and terephthalic acid ligands. Through the gas adsorption experiments, the authors found that MOFs-5 showed high specific surface area, large pore size, and a certain adsorption capacity for hydrogen. HKUST-1, as reported, is a three-dimensional skeleton that is formed by the coordination of Cu2(CO2)4 clusters with benzotriformic acid ligands [37]. The authors found that HUKST-1 with unsaturated ligand sites can be obtained by heating water molecules that can be removed and coordinated on metal clusters [38]. Jinhee et al. report the the OCS-activation ability of chloromethanes to remove precoordinated solvent molecules from open coordination sites (OCSs) in MOFs [39]. A water molecule in HKUST-1 can easily access open metal site (OMS)with high coordination strength due to the specific coordination geometry around Cu2+ [40]. In particular, MOFs with OCSs have potential applications in chemical separation, molecular sorption, catalysis, ionic conduction, and sensing areas [39,41]. Since these two MOF structures were reported, the synthesis and potential applications of MOFs in gas separation, storage, catalysis, sensing, drug transportation, and so on have become hot research topics [42,43].

MOFs are extensively studied for the capture and conversion of CO2 due to their high porosity and strong interaction with CO2 molecules. At present, some MOFs have already been explored for their high catalytic performance in the field of photocatalysis for CO2 reduction [44]. As photocatalysts, MOFs exhibit the following advantages. Firstly, the high specific surface area of MOFs is helpful for the gas reactants adsorption around the active site. This is beneficial to the molecule activation and catalytic transformation in the subsequent process [45,46]. Secondly, the metal-oxygen units in MOFs exhibit semiconductor-like structure due to the existence of organic ligands. MOFs with larger energy than the band gap can be excited by photons to create electron and hole pairs [47,48]. Through selectively choosing di fferent organic ligands and metal centers, one can improve the absorption and utilization e fficiency of sunlight via MOFs as light absorbing agents [49]. Besides, the separation and transfer of electrons can be promoted by changing the crystal structure, thereby which thereby inhibits the recombination of photo-induced electrons and holes [50]. In addition, MOFs, as heterogeneous catalysts, can be easily separated and recycled from the reaction system, which is beneficial for prolonging the service life of the catalyst and avoiding any pollution to the environment [51–53].

In this paper, the advances of MOFs materials for photocatalytic CO2 reduction is systematically reviewed. This review paper starts from the research background why CO2 reduction is important, and the mechanism studies on the photocatalytic CO2 reduction process were then summarized. After that, the research progress of photocatalytic CO2 reduction using MOFs were reviewed, followed by the summary of the applications of MOFs-based composite materials for photocatalytic reduction of CO2. Finally, the current challenges and future development trend of MOFs-based materials for photocatalytic CO2 reduction are anticipated.
