Direct production of H 2O 2 from H 2 and O 2 in a biphasic H 2O/scCO 2 system over a Pd/C catalyst: Optimization of reaction conditions

Abstract

The direct synthesis of hydrogen peroxide has been achieved using only water as solvent in a semi-continuous stirred reactor system; this is a greener alternative over the traditional process using anthraquinone or the direct synthesis process using organic solvents (e.g. alcohols). The use of an inert co-solvent such as scCO 2 is advisable due to safety issues. Moreover, the use of scCO 2 presents additional advantages due to its acidification effect and the important enhancements it causes in the mass transfer coefficients for both the gas and the liquid phase when compared to N 2. The catalyst used, a commercial 5 wt.% Pd/C, was especially active and amounts as low as 15 mg per 200 mL of reaction volume produced remarkable results with turnover frequencies after 2 h (TOF 120) over 2 mol H 2 O 2 h − 1 g cat − 1 . A maximum productivity of 0.84 mmol H 2 O 2 min − 1 in our system was obtained for the optimized conditions, obtaining selectivities around 60% and yields around 20%. A maximum concentration of 4.3% (w/v) H 2O 2 was obtained after 8 h. An on-line monitoring Raman technique was used for H 2O 2 quantification.