Photosynthesis, which uses sunlight to convert CO₂ and H₂O into carbohydrates and O₂, is the prime example of a reaction that can produce chemical fuels from waste CO₂ and mitigate climate change. Artificial photosynthesis aims to achieve similar CO₂ photoreduction reactions using chemical systems. Metal–organic frameworks (MOFs), for example, can promote CO₂ photoreduction in the liquid phase, but usually require organic reagents as the electron donor.

Babu Joseph, University of South Florida, Tampa, USA, Shengqian Ma, University of North Texas, Denton, USA, and colleagues have synthesized a mixed-metal (Ti/Zr) porphyrinic MOF (pictured) that can promote CO₂ photoreduction in the gas phase without organic sacrificial reagents. The team started from the zirconium-based MOF-525, which is composed of tetrakis(4‐carboxyphenyl)porphyrin (H₄TCPP) linkers and Zr₆O₄(OH)₄ units. They used a metal metathesis procedure to partially replace Zr with Ti, resulting in a Ti/Zr atomic ratio of 9:1. The structure of the MOF was retained after the exchange.

The researchers then performed CO₂ photoreduction experiments using the mixed-metal MOF as the catalyst in the presence of water vapor under irradiation with visible light. The MOF reduced CO₂ into CH₄, with high selectivity toward the formation of CH₄ over CO. The MOF uses the porphyrin units to absorb visible light, the mixed-metal clusters act as reactive sites, and water vapor is used as an electron donor. This first example could be a basis for the development of other MOF‐based photocatalysts for gas-phase CO₂ reduction that do not require organic sacrificial reagents.

• A Mixed-Metal Porphyrinic Framework Promoting Gas-Phase CO₂ Photoreduction without Organic Sacrificial Agents,
  Wen-Yang Gao, Huong T. Ngo, Zheng Niu, Weijie Zhang, Yanxiong Pan, Zhongyu Yang, Venkat R. Bhethanabotla, Babu Joseph, Briana Aguila, Shengqian Ma,
  ChemSusChem 2020.
  https://doi.org/10.1002/cssc.202001610