Study on V(III) and V(IV) Catalysts and Membrane Resistance in the Conversion of Benzene to Phenol in a Membrane Contactor

The search for new routes for phenol synthesis, such as the direct hydroxylation of benzene,becomes more intensive in the last decades [1, 2]. Such a process presents various potentialadvantages compared to the industrial process, but its selectivity is usually poor since phenol can beoxidized easier than benzene. The prompt removal of the produced phenol from the reactionenvironment represents a key point for developing such a method and, on this aspect, membranescan play an important role.In our previous works [3, 4] we proposed the use of a membrane contactor, for the one-step benzeneoxidation to phenol, consisting of a flat-sheet hydrophobic polypropylene membrane that separatestwo compartments containing an aqueous and an organic phase (only benzene). Benzene permeates,through the membrane, in the aqueous phase containing the iron catalyst and hydrogen peroxide asthe oxidant while phenol permeates back in the organic phase where it is protected by overoxidations.Despite the high phenol selectivity (98%), obtained thanks to phenol extraction in theorganic phase, the low permeation rate of phenol across the membrane causes its further oxidationto over-oxidized products as benzoquinone, biphenyl and tar (black solid).In the present work, to improve system performance: i) a micro pump was used to feed the oxidantto reduce its degradation increasing phenol yield and overall phenol production; ii) vanadium basedcatalysts vanadium(III) chloride (VC) and vanadium(IV) acetyl acetonate (VAAC), were testedwith the aim to achieve a phenol production comparable with that one obtained by using iron-basedcatalysts, preserving system selectivity and avoiding tar formation; iii) different test duration (270and 510 minutes) and membrane lifetime were evaluated in view of long time operation for largescale applications.