The role of metal–organic frameworks in a carbon-neutral energy cycle

1 Around 86% of all energy used globally comes from burning fossil fuels1, generating 35 billion tons of carbon dioxide annually2. The continuing reliance on fossil fuels by developed countries3 and the increasing demand for energy by emerging countries4 make the emission of carbon dioxide into the atmosphere a serious global problem. This has provided impetus for finding alternative energy from solar, wind, geothermal, hydropower, biomass and nuclear fission sources. Although these are active areas of research and development, and are being used in some countries, they still constitute a minority of the global energy supply because of various issues related to cost, storage, scalability and safety5. It is widely believed that we will continue to be reliant on fossil fuels for the foreseeable future6 and so it is urgent in the short term to address carbon dioxide emissions, while continuing to develop alternative fuels as a long-term solution (Fig. 1). In this Review, we examine the use of metal–organic frameworks (MOFs; Box 1) in the development of a carbon-neutral energy cycle involving the use of hydrogen as a long-term objective, methane as a transitional fuel with lower carbon dioxide emission than petroleum and the capture of carbon dioxide as an immediate solution. Specifically, we outline the progress in using MOFs to: store hydrogen, which is the ultimate fuel because it burns cleanly and produces only water as a by-product; store and deliver methane for use in automobile fueling; and capture carbon dioxide from flue gas in power plants and potentially other combustion sources (Fig. 1). We also include recent results on the emerging field of using MOFs and related frameworks not only to capture, but also to convert carbon dioxide to highvalue chemicals. For each of the three gases (hydrogen, methane and carbon dioxide), we present the various means of their production, and the potential and limitations of the technologies and materials being pursued for their capture, storage and utilization. A running theme of this Review pertains to how the flexibility with which MOFs can be designed, assembled and precisely modified on the atomic and molecular levels provides unparalleled opportunities for solving these problems and ultimately achieving a carbon-neutral energy cycle.