Microenvironment Modulation of Metal-Organic Frameworks (MOFs) for Coordination Olefin Oligomerization and (co)Polymerization.

The majority of commercial polyolefins are produced by coordination polymerization using early or late transition metal catalysts. Molecular catalysts containing these transition metals (Ti, Zr, Cr, Ni, and Fe, etc.) are loaded on supports for controlled polymerization behavior and polymer morphology in slurry or gas phase processes. Within the last few years, metal-organic frameworks (MOFs), a class of unique porous crystalline materials constructed from metal ions/clusters and organic ligands, have been designed and utilized as excellent supports for heterogeneous polymerization catalysis whose high density and uniform distribution of active sites would benefit the modulations of molecular weight distributions of high-performance olefin oligomers and (co)polymers. Impressive efforts have been made to modulate the microenvironment surrounding the active centers at the atomic level for improved activities of MOFs-based catalysts and controlled selectivity of olefin insertion. This review aims to draw a comprehensive picture of MOFs for coordination olefin oligomerization and (co)polymerization in the past decades with respect to different transition metal active centers, various incorporation sites, and finally microenvironment modulation. In consideration of more efforts are needed to overcome challenges for further industrial and commercial application, a brief outlook is provided.

Toward Lower Overpotential through Improved Electron Transport Property: Hierarchically Porous CoN Nanorods Prepared by Nitridation for Lithium-Oxygen Batteries.

To lower the overpotential of a lithium-oxygen battery, electron transport at the solid-to-solid interface between the discharge product Li2O2 and the cathode catalyst is of great significance. Here we propose a strategy to enhance electron transport property of the cathode catalyst by the replace of oxygen atoms in the generally used metal oxide-based catalysts with nitrogen atoms to improve electron density at Fermi energy after nitridation. Hierarchically porous CoN nanorods were obtained by thermal treatment of Co3O4 nanorods under ammonia atmosphere at 350 °C. Compared with that of the pristine Co3O4 precursor before nitridation, the overpotential of the obtained CoN cathode was significantly decreased. Moreover, specific capacity and cycling stability of the CoN nanorods were enhanced. It is assumed that the discharged products with different morphologies for Co3O4 and CoN cathodes might be closely associated with the variation in the electronic density induced by occupancy of nitrogen atoms into interstitial sites of metal lattice after nitridation. The nitridation strategy for improved electron density proposed in this work is proved to be a simple but efficient way to improve the electrochemical performance of metal oxide based cathodes for lithium-oxygen batteries.