The advantageous application of a falling-film microreactor for a photochemical gas/liquid reaction was demonstrated by the selective photochlorination of toluene-2,4-diisocyanate (TDI). In the microstructured reactor the selectivity to the side-chain chlorinated product 1-chloromethyl-2,4-diisocyanatobenzene (1Cl-TDI) achieved a value of 80% at 55% TDI conversion, whereas the side product toluene-5-chloro2,4-diisocyanate (5Cl-TDI) was formed with only 5% selectivity. The yield of 1Cl-TDI was enhanced by increasing the residence time from 24% after 5 s to 54% after 14 s. At the same time the formation of consecutive products increased and the selectivity to 1Cl-TDI decreased to 67% after 14 s residence time. The influence of the reactor material was shown. In presence of a Lewis acid such as FeCl3, formed by chlorination using a reaction plate made of iron, consecutive products were formed and the selectivity to 1Cl-TDI was lowered. The microstructured reactor led to remarkably higher selectivities than the conventional batch reactor, where the selectivity to 1Cl-TDI was only 45% at 65% TDI conversion and the side product 5Cl-TDI was formed with 50% selectivity. The space-time yield of 1Cl-TDI achieved in the microstructured reactor (400 mol l?1 h?1) clearly exceeded the performance of the batch reactor (space-time yield 1.3 mol l?1 h?1). Based on the microreactor data, a kinetic model for the TDI chlorination including by-product formation was suggested and used to predict product selectivity at full TDI conversion.

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