Highly Selective Photochemical and Thermal Chlorination of Benzene by Cl2 in NaZSM-5

Loading of zeolite NaZSM-5 with benzene and chlorine from the gas phase at −100 °C resulted in spontaneous reaction to form chlorobenzene and 1,4-dichlorobenzene as the sole products. Thermal reaction at elevated temperature (up to 0 °C) accelerates the rates toward these products and yields, in addition, some 1,3-dichlorobenzene and a small (<3%) amount of 1,2-dichlorobenzene. No higher chlorinated benzenes or chlorocyclohexanes are formed even upon 90% conversion of the loaded benzene. Reactions were monitored by in situ FT-infrared spectroscopy. Irradiation of 1,4-dichlorobenzene in the presence of Cl2 in the zeolite with green or blue light produced 1,2,4-trichlorobenzene as the only product at high conversion. Such selectivities of benzene chlorination by elemental chlorine are much higher than obtained currently by traditional methods. The sharp increase of the reaction rate in NaZSM-5 compared to that in homogeneous liquid or gas phase and the effect of electronic excitation by visible light are attributed to an enhanced electrophilic character of the chlorine molecule.