Abstract
Single liquid-phase and liquid-liquid phase reactions in microdroplets have shown much faster kinetics than that in the bulk phase. This work extends the scope of microdroplet reactions to gas-liquid reactions and achieves preparative synthesis. We report highly efficient aerobic oxidation of aldehydes to carboxylic acids in microdroplets. Molecular oxygen plays two roles: (1) as the sheath gas to shear the aldehyde solution into microdroplets, and (2) as the sole oxidant. The dramatic increase of the surface-area-to-volume ratio of microdroplets compared to bulk solution, and the efficient mixing of gas and liquid phases using spray nozzles allow effective mass transfer between aldehydes and molecular oxygen. The addition of catalytic nickel(ii) acetate is shown to accelerate further microdroplet reactions of this kind. We show that aliphatic, aromatic, and heterocyclic aldehydes can be oxidized to the corresponding carboxylic acids in a mixture of water and ethanol using the nickel(ii) acetate catalyst, in moderate to excellent yields (62-91%). The microdroplet synthesis is scaled up to make it preparative. For example, aerobic oxidation of 4-tert-butylbenzaldehyde to 4-tert-butylbenzoic acid was achieved at a rate of 10.5 mg min-1 with an isolated product yield of 66%.
Highlights
Recent ndings indicate that reactions in microdroplets created by spray-based ionization/aerosols are extremely attractive, as microdroplet reactions can be many orders of magnitude faster than their conventional bulk-phase counterparts.[1,2] This phenomenon stimulates strong interest in using microdroplets as a chemical synthetic tool
The yield of product 4-tert-butylbenzoic acid was largely enhanced with an increase of the SA/V ratio of droplets from 0.033 to 1 (Fig. 3a), and reached the maximum yield when the droplet size decreased to about 3 mm
We have demonstrated that aerobic oxidation can be carried out in microdroplets much more rapidly and with higher yield compared with their bulk-phase counterpart
Summary
Recent ndings indicate that reactions in microdroplets created by spray-based ionization/aerosols are extremely attractive, as microdroplet reactions can be many orders of magnitude faster than their conventional bulk-phase counterparts.[1,2] This phenomenon stimulates strong interest in using microdroplets as a chemical synthetic tool. Non-catalytic oxidation can be achieved in microdroplets formed by dual sonic sprays within milliseconds in moderate to good yields.[4] Other reactions involving C–C,5–7 C–N,6,8–10 and C–O11 bond formation have been reported to be accelerated by factors of 10 to 106 in single liquid-phase solutions.
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