Direct fluorination of toluene using elemental fluorine in gas/liquid microreactors

Direct fluorination of toluene, pure or dissolved in either acetonitrile or methanol, using elemental fluorine was investigated in gas/liquid microrea ctors, namely a falling film microreactor and a micro bubble column. The ex periments included measurements at high substrate concentrations and at hig h fluorine contents diluted in a nitrogen carrier gas, e.g. up to 50 vol.% fluorine. Results obtained were compared to the performance of a laboratory bubble column which served as a technological benchmark. Due to the formation of liquid layers of only a few tens of micrometers thi ckness, the microreactors provide very large interfacial areas, e.g. up to 40,000 m(2)/m(3). These values exceed by far those of the laboratory bubble column as well as all other devices applied in practice. The potential for enhancing mass and heat transfer was verified by several experiments resulting in an increase in conversion and selectivity for the microreactors compared to the laboratory benchmark. For the falling film mi croreactor, yields of up to 28% of monofluorinated ortho and para products for a degree of toluene conversion of 76% were obtained. These values are o f the same order as described for the industrially applied Schiemann proces s. Space-time yields of the microreactors, when referred to the reaction ch annel volume, were orders of magnitude higher than those of the laboratory bubble column. Taking into account the construction material needed, the co rresponding figures of merit, for an idealized geometry as well as the exis ting total reactor geometry, still indicate technological and economic bene fits. A variation of operating conditions for the direct fluorination revealed th at conversion can be increased in the microreactors by using higher fluorin e-to-toluene ratios and reaction temperatures. The choice of solvent is als o essential, with acetonitrile yielding much better results than methanol. (C) 2000 Elsevier Science S.A. All rights reserved.