Despite their industrial relevance, the mechanistic rationalization of selective oxidations often trails the industrial state-of-the-art.[1] Nitric acid based oxidations are a classical example of a poorly understood process of tremendous industrial importance. The production of adipic acid (3 Mt/year), a building block for nylon-6,6, is just one example from bulk industry. The reason why HNO3-based oxidations are so valuable is the fact that nitric acid is a rather inexpensive oxidant with no toxic potential. One disadvantage of this technology is the stoichiometric reduction of the HNO3 to NOx and N2O, a severe greenhouse gas, the formation of which is still poorly understood.
In this contribution, we unravel the reaction mechanism of HNO3-induced alcohol oxidations, and illustrate how such stoichiometric reactions can be turned into catalytic systems in which HNO3 initiates a selective oxidation with O2 as terminal oxidant.[2]
Mass spectrometry and transmission IR spectroscopy were used to monitor the formation of N2O in the gas phase, stemming from HNO dimerization.[2,3] With the help of these measurements, the mechanism for the forma-tion of N2O and the detrimental role of H2O that reduces the turnover of (H)NOx species in the system could be elucidated (Scheme 1).
These micro-kinetic insights, combined with tailored reaction engineering, allow the design of a promising sustainable oxidation system for alcohols by reducing the amount of N2O formed and by boosting the turnover in (H)NOx species.
[1] F. Cavani, J. H. Teles, ChemSusChem 2009, 2, 508-534
[2] C. Aellig, C. Girard, I. Hermans, Angew. Chem. Int. Ed. 2011, 50, 12355-12360
[3] C. Aellig, I. Hermans, ChemCatChem 2012, 4, 525-529