Hydroxylation and dimerization of para-dihydroxylated aromatic compounds mediated by cold atmospheric-pressure plasma in comparison with chemically catalyzed reactions

The synthesis of valuable organic compounds is a novel possibility for the application of physical plasma. As model reactants 2,5-dihydroxybenzoic acid derivatives were used and the resulting reactions investigated in detail. The formation of methoxylated and hydroxylated products as well as dimers (biaryls) was mediated by physical plasma in aqueous solution at atmospheric pressure and room temperature. Additionally, nitrated or nitrosated products were assumed. The direct (reactant is part of the liquid which is plasma-treated) and indirect (liquid is treated by plasma and afterwards reactant is added) plasma treatment resulted in a partly different product pattern, but both reaction concepts are suitable for the synthesis of organic compounds. The influence of operating parameters such as duration of plasma treatment, volume of the liquid, or concentration of reactants was varied to study the reaction course in order to increase product yields. Nitrite, nitrate, hydrogen peroxide, a decreased pH-value and an increased temperature was determined for the plasma-treated liquid. Consequently, the reactions with the 2,5-dihydroxybenzoic acid derivatives were pursued in the presence of these compounds at acidic pH-value and 50 degrees C. As result, nitrite, a low pH-value (pH = 2.2) and an elevated temperature were at least in part responsible for the formation of the products. The advantages of plasma-mediated synthesis are discussed in detail. The environmentally friendly reaction conditions without the need for catalysts or buffer components recommend physical plasma as novel mean in green chemistry.