Vanadium(III) and vanadium(IV) catalysts in a membrane reactor for benzene hydroxylation to phenol and study of membrane material resistance

Benzene hydroxylation to phenol by using vanadium based catalysts and product recovery were performedin a two-phase membrane reactor. Benzene permeates, through the hydrophobic polypropylenemembrane, in the aqueous phase containing the catalyst while phenol permeates back accumulating inthe organic phase. The following fundamental aspects have been studied: dose of hydrogen peroxide,initial oxidation states of vanadium catalysts, duration of catalytic tests and lifetime of the membranein terms of physical and chemical resistance. It was observed that feeding the oxidant by a micro pump,working in the “bulk tube” mode, phenol yield, final phenol concentration in the organic phase, phenolturnover number and system productivity increased, and no tar was formed. Initial oxidation state ofvanadium catalysts influenced system performance: indeed improved results in terms of yield (35.2%vs. 25.1%), conversion of hydrogen peroxide to phenol (36.6% vs. 25.9%), productivity (0.97 g g−1cat h−1 vs.0.78 g g−1cat h−1) were obtained by using vanadium(III) chloride compared to vanadium(IV) acetyl acetonate.Higher phenol extraction/recovery in the organic phase (61.1% vs. 46.3%) and then higher selectivity(97.5% vs. 92.8%) were obtained by increasing test duration from 270 to 510 min. A weak membraneresistance was observed after 246 h of consecutive catalytic runs on the same membrane piece, showingdegradation of the membrane material (polypropylene) caused by the OH• radical generated in thereacting mixture.