A MOF for nitrogen dioxide

03 December 2019 - Clean air

Schröder and Yang of the University of Manchester report on a new way to remove nitrogen dioxide from an exhaust using MOF's (Li et al. DOI). NOx compounds are hazardous to human health and damage eco systems. The diesel emissions scandal made clear new ways to tackle NOx are in need (previous report here). The authors point out that selective catalytic reduction with ammonia requires high temperatures, toxic chemicals and metals, is inefficient and easily poisoned by water or sulfur dioxide.
The use of MOF's in NOx capture was first reported in 2010 (DOI), the MFM-520 metal-organic framework (MOF) used in the study has been around since 2006 (DOI) from the same research group and can be synthesized from zinc chloride and 4,4-bipyridyl-3,3,5,5-tetracarboxylate as ligands. In the framework zinc is pentacoordinated with links to the carboxylate units and the pyridyl units. Key to the innovation is the presence of bowtie-shaped pores with dimensions of 6 by 4 Angstrom. The nitrogen dioxide uptake is 4.2 mmole/g at 0,01 bar and very selective: up to 600 times more nitrogen dioxide is adsorbed than carbon dioxide. X-ray diffraction revealed that the bowtie pores are occupied by dinitrogen tetroxide which is the the dimerization product. Removing the nitrogen dioxide and resetting the MOF is surprisingly simple, all it takes is immersing the MOF in water and the compound is converted to nitric acid. This acid has practical and commercial uses and perhaps somewhere in the future diesel drivers can regularly deposit a bucket of it at recycling stations.