Diesel and gasoline are blends of aromatic and aliphatic compounds that contain significant levels of heterocyclic aromatic contaminants. Nitrogen compounds form NO_x exhaust gases which contribute to smog while the sulfur equivalent, SO_x, contributes to acid rain. Hydrodesulfurization catalysts can produce fuels with sulfur levels down to 5 ppmw (parts per million by weight), but only in the absence of nitrogen compounds.

Dirk E. De Vos and co-workers, Katholieke Universiteit Leuven, Belgium, have found that metal-organic frameworks (MOFs) can be used to remove nitrogen compounds from fuel feeds. Good uptake of a range of aromatic nitrogen compounds was seen with the iron-based MOF, MIL-11, which contains trimers of metal octahedra with Lewis acid sites.

This provides a cheap, non-toxic, and biodegradable material that can be regenerated with ease and could be used to remove nitrogen compounds before they poison hydrodesulfurization catalysts and pollute the environment.

• Selective Removal of N-Heterocyclic Aromatic Contaminants from Fuels by Lewis Acidic Metal–Organic Frameworks
  M. Maes, M. Trekels, M. Boulhout, S. Schouteden, F. Vermoortele et al.,
  Angew. Chem. Int. Ed. 2011.
  DOI: 10.1002/anie.201100050