Activation and Catalysis of Methane over Metal-Organic Framework Materials

Methane (CH4), which is the main component of natural gas, is an abundant and widely available carbon resource. However, CH4 has a low energy density of only 36 kJ L-1 under ambient conditions, which is significantly lower than that of gasoline (ca. 34 MJ L-1). The activation and catalytic conversion of CH4 into value-added chemicals [e.g., methanol (CH3OH), which has an energy density of ca. 17 MJ L-1], can effectively lift its energy density. However, this conversion is highly challenging due to the inert nature of CH4, characterized by its strong C-H bonds and high stability. Consequently, the development of efficient materials that can optimize the binding and activation pathway of CH4 with control of product selectivity has attracted considerable recent interest. Metal-organic framework (MOF) materials have emerged as particularly attractive candidates for the development of efficient sorbents and heterogeneous catalysts due to their high porosity, low density, high surface area and structural versatility. These properties enable MOFs to act as effective platforms for the adsorption, binding and catalytic conversion of CH4 into valuable chemicals. Recent reports have highlighted MOFs as promising materials for these applications, leading to new insights into the structure-activity relationships that govern their performance in various systems.In this Account, we present analysis of state-of-the-art MOF-based sorbents and catalysts, particularly focusing on materials that incorporate well-defined active sites within confined space. The precise control of these active sites and their surrounding microenvironment is crucial as it directly influences the efficiency of CH4 activation and the selectivity of the resulting chemical products. Our discussion covers key reactions involving CH4, including its activation, selective oxidation of CH4 to CH3OH, dry reforming of CH4, nonoxidative coupling of CH4, and borylation of CH4. We analyze the role of active sites and their microenvironment in the binding and activation of CH4 using a wide range of experimental and computational studies, including neutron diffraction, inelastic neutron scattering, and electron paramagnetic resonance, solid-state nuclear magnetic resonance, infrared and X-ray absorption spectroscopies coupled to density functional theory calculations. In particular, neutron scattering has notable advantages in elucidating host-guest interactions and the mechanisms of the conversion and catalysis of CH4 and CD4. In addition to exploring current advances, the limitations and future direction of research in this area are also discussed. Key challenges include improvements in the stability, scalability, and performance of MOFs under practical conditions, as well as achieving higher selectivity and yields of targeted products. The ongoing development of MOFs and related materials holds great promise for the efficient and sustainable utilization of CH4, transforming it from a low-density energy source into a versatile precursor for a wide range of value-added chemicals. This Account summarizes the design and development of functional MOF and related materials for the adsorption and conversion of CH4.