Direct hydroxylation of aromatic compounds by a palladium membrane reactor

Direct hydroxylation of aromatic compounds was effectively achieved by using a newly developed Pd membrane reactor in which the Pd membrane is thin enough (ca. 1 μm) to allow permeation of hydrogen below 500 K. In this reactor, the active oxygen species is formed on the surface of Pd via the reaction between oxygen and permeated hydrogen from opposite sides of the membrane. Hydroxylation occurs on the surface of Pd via reaction of the aromatic compound and active oxygen. In the reaction of benzene at a reaction temperature of 423 K, the reactor achieved a benzene conversion of 15% and a phenol selectivity of 95%. An increase in reaction temperature, however, caused simultaneous hydrogenation. In the reaction of methyl benzoate, the reaction products were not only methyl salicylate, which is a hydroxylation product, but also numerous hydrogenation and oxidation compounds via side reactions. These side reactions were related to the gas balance between oxygen and hydrogen; oxygen-rich conditions caused complete oxidation, whereas oxygen-poor conditions (i.e., high amount of permeated hydrogen) induced high hydrogenation activity.