Metal-organic framework (MOF) structure, molecular model. MOFs are highly crystalline porous materials made up of 3D networks of metal ions connected by organic molecules. This is the mesoporous MOF PCN-222 which has porphyrin connecting zirconium-oxygen clusters (red and purple). Up to 91 per cent of their volume can be made up by nanoscale pores. Uses of MOFs include gas storage, gas purification, gas separation, or as catalysts.