Gas / Liquid Microreactors for Direct Fluorination of Aromatic Compounds Using Elemental Fluorine

Two different concepts of contacting gas and liquid media were realized in miniaturized reaction devices. The first one, a micro bubble column, utilizes dispersed systems consisting of small gas bubbles in a liquid phase. A large specific interfacial area of about 9,000 m2/m3 was determined by applying the sulfite oxidation method. Flow equipartition of a high number of parallel operating channels was achieved by using a pressure barrier. First visual observations of the liquid and gaseous flows indicate that this principle leads to a uniform flow equipartition. The second concept was based on the generation of thin stable films in a falling film microreactor. Very narrow residence time distributions of the flows in the channels were achieved also by using a pressure barrier.The direct fluorination of toluene was feasible in both types of microreactors as proven by the synthesis of quantitative amounts of monofluorinated products with yields up to 20%. The yields, and in particular the conversion, of the microreactors were much larger compared to a laboratory bubble column which was operated at similar process conditions, e.g. equal ratios of fluorine to toluene, temperature, toluene and fluorine concentration. The falling film microreactor showed a very high conversion up to 50%, whereas the bubble column gave high selectivities reaching 49%. An analysis of the substitution pattern obtained in both microreactors is consistent with an electrophilic substitution mechanism.