(34a) Direct Carbonylation of Nitrobenzene to Phenylisocyanate with Microreaction System

Organic isocyanates have achieved great commercial importance as chemical intermediates in the manufacture of thermoplastic foams, elastomers, adhesives, and agrochemicals. Aromatic isocyanates are currently synthesized via the two-step process: a catalytic reduction of the corresponding nitro compound to aromatic amine followed by a phosgenation of the amine to isocyanate. The phosgenation route has a number of serious problems; e.g., use of extremely toxic phosgene gas, and formation of a large amount of corrosive hydrochloric acid as a side product. It has been thus desired to develop alternative phosgene-free route for the isocyanate synthesis. Reductive carbonylation with carbon monoxide (CO) is an attractive alternative, because it enables a direct conversion of nitro aromatic to isocyanate. The direct carbonylation, however, needs very high CO pressure (> 10 MPa) to solubilize CO gas into liquid solvents. In the present study, microreaction technology is applied to the direct isocyanate synthesis in order to improve the mass-transfer across the gas-liquid interface. Here we present a direct carbonylation of nitrobenzene to phenylisocyanate in microflow reactor at CO pressure lower than 1 MPa. The isocyanate yield in microflow reactor is compared with that in batch reactor, and the effect of the microtube diameter on the isocyanate yield is discussed.