Direct carbonylation of nitrobenzene to phenylisocyanate using gas–liquid slug flow in microchannel

A microreaction system was developed for non-phosgene direct isocyanate synthesis from nitrobenzene and CO gas at pressure (9.5 bar) much lower than those in conventional ones (>100 bar). A gas–liquid slug flow of the reactant mixture was formed in the microchannel for efficient mass transfer across the gas–liquid interfaces. The isocyanate yield of the microflow reaction was shown to be three to six times higher than that of the batch reaction, depending on the inner diameter (i.d.) of the microtube. Higher isocyanate yield was obtained in a narrow-bore tube (0.5 mm i.d.) than in a wide-bore tube (1.0 mm i.d.). The results were interpreted in terms of the length of the liquid slug monitored through transparent PFA tubes. The liquid slug length in the narrow-bore tube (0.6 ± 0.2 mm) was found to be shorter than that in the wide-bore tube (1.1 ± 0.1 mm). This is consistent with the higher isocyanate yield obtained in the narrow-bore tube, because the shorter liquid slug has the larger gas–liquid interfacial area per unit volume.

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► Microreaction system for non-phosgene direct isocyanate synthesis was developed.
► Gas–liquid slug flow of the reactant CO gas and nitrobenzene was formed in the microchannel.
► The isocyanate yield of the microflow reaction was three to six times higher than that of the batch reaction.
► Higher isocyanate yield was obtained in a narrow-bore tube (0.5 mm i.d.) than in a wide-bore tube (1.0 mm i.d.).
► The results were interpreted in terms of the length of the liquid slug monitored through transparent PFA tubes.