Aqueous catalysis with Oxidorhenium(V) Complexes

The nitrate anion NO3 is, together with the phosphate anion PO4, one of the two main ingredients of artificial fertilizers. Without them, feeding by now over 8 billion people on this planet would simply be impossible. Nitrate itself is ubiquitous in nature, occurring in form of minerals and salts, but for a long time, profitable mining was only possible in a few places in the world. Soon it became clear that mining could not satisfy the growing global demand of nitrates, especially as nitrates are also the starting material for manufacture of explosives. Therefore, research into alternative methods for nitrate production were initiated, which came to a breakthrough due to groundbreaking work of Fritz Haber Carl Bosch. From now on, ammonia NH3 could be produced via metal catalysis directly from N2 and H2, which could then be oxidized to nitrates. Nowadays though, the overuse of fertilizers by modern agriculture leads to increasing levels of nitrate in ground water, which can lead to health related issues for seniors and small children. Therefore, regulations for limits of nitrate in tap water are in place. Technologically, nitrate removal processes fully rely on physical separation techniques like ion exchange chromatography or osmosis. The common problem with these technologies is that the nitrate is only being filtered out, but is still there afterwards, often as a high-nitrate containing sludge that needs special disposal. In a first funded project by the FWF (P-37178-N), the fundamentals of this challenging chemical reaction were investigated. In this second granted project, the switch-over from organic solvents to running the whole catalytic reaction in water will be investigated. The goal would be to bring the chemistry out of the lab (where mainly organic solvents are used) closer to the real world. This means, the complexes need to modified to attain water solubility and stability. Especially the latter requirement surmounts to a huge chemical challenge, as hydrolysis, that is decomposition by reaction with water, now becomes a major obstacle, in contrast to organic solvents. Hence, a collaboration with the group of Prof. M. Gallei, Saarland University in Germany, who has developed several water-soluble membranes based on special polymer, that will then be modified with the Re(V) complexes from Graz, is also part of this project.