Metal-free catalysis on the reactions of nitric acid with aliphatic aldehydes: A new potential source of organic nitrates

The photochemical reaction of peroxy radical (RO2·) and NO has been identified by field and forest studies as important source of organic nitrates (RONO2) in the atmosphere. However, this traditional pathway is not sufficient to explain the high concentration of RONO2. Hence, a new source of the tropospheric RONO2 from the dark reactions of nitric acid (HNO3) with aliphatic aldehydes (C1–C5) under catalysis is provided and examined for the first time by high-level quantum chemistry. The findings show that the reaction between HCHO and HNO3, which produces HOCH2ONO2, can be catalyzed by a series of metal-free catalysts (NH3, CH3NH2, CH3NHCH3, H2O, HNO3, H2SO4, HCOOH, HOOCCOOH). At 296 K, the effective rate constant for the bimolecular HNO3–HCHO reaction under the catalysis of CH3NH2 or CH3NHCH3 can be sufficiently accelerated by 5–8 order of magnitudes through this new loss pathway for HNO3 or HCHO to become competitive with the conventional loss pathway for their photochemical reactions with ·OH radical. Significantly, this new HOCH2ONO2 formation pathway from the dark reaction of HCHO with CH3NH3+NO3−/(CH3)2NH + NO3− was more favorable than the recognized source of RO2· with NO. Efficient catalysis performance of CH3NH2 and CH3NHCH3 is mainly attributed to their excellent proton receptivity capacity by activating the O–H bond of HNO3 to form stable organic nitrates (CH3NH3+NO3− and (CH3)2NH + NO3−) in the rate-determining step transition states. In the case of only considering the barrier, H2SO4 is the best catalyst among the investigated inorganic and organic acids, and dicarboxylic acid (HOOCCOOH) is stronger than monocarboxylic acid HCOOH in facilitating the RONO2 formation reaction. These new findings deepen our understanding on the unexpected source of organic nitrate and loss pathway of HNO3 or HCHO under catalysis in highly polluted regions.