Oligomeric products and formation mechanisms from acid-catalyzed reactions of methyl vinyl ketone on acidic sulfate particles |
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Authors: | Ka Man Chan Dan Dan Huang Yong Jie Li Man Nin Chan John H Seinfeld Chak K Chan |
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Institution: | 1. Division of Environment, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong 2. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA 5. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 3. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA 4. Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong
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Abstract: | Methyl vinyl ketone (MVK) is a key first-generation product from atmospheric isoprene photo-oxidation, especially under high-NOx conditions. In this work, acid-catalyzed reactions of gas-phase MVK with ammonium sulfate (AS), ammonium bisulfate (ABS), and sulfuric acid (SA) particles were investigated in a flow reaction system at relative humidity (RH) of 40 % and 80 %. Ultra-performance liquid chromatography with electrospray ionization time-of-flight mass spectrometry (UPLC/ESI-TOFMS) and gas chromatography-mass spectrometry (GC-MS) are utilized to identify particle-phase products for developing the reaction mechanisms. High-order oligomers such as dimers and tetramers were detected when ABS and SA particles were used, while no oligomeric products were found when AS particles were used. Particle-phase oligomeric products were formed via i) acid-catalyzed aldol reaction with or without dehydration and/or ii) acid-catalyzed hydration followed by oligomerization. Reactions on SA particles yield more abundant and higher-order oligomers up to hexamers than on ABS particles. Moreover, aldol reaction occurred only on SA particles, but hydration followed by oligomerization occurred in both ABS and SA particles. The high RH condition with the same type of acidic particles was found to favor hydration and facilitate the subsequent oligomerization, while the low RH condition with the same type of acidic particles was found to favor aldol reaction with dehydration (aldol condensation). Overall, the findings suggest acidic particles can facilitate the formation of high-order oligomers in the particle phase, with particle acidity and RH as key factors. |
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