Direct synthesis of hydrogen peroxide in a trickle bed reactor: comparison of Pd-based catalysts

Palladium catalysts based on different supports, silica (SiO 2), zirconia (Z), sulfated ceria (CeS), and sulfated zirconia (ZS), previously tested in a semibatch reactor, were chosen to demonstrate how the direct synthesis process can be improved by continuous operation in a three-phase fixed bed. The gas and liquid flow rates were systematically varied to find suitable combinations for a maximum hydrogen peroxide production rate and selectivity. Different catalysts gave the same results in terms of selectivity and production rate with different operating conditions. A selectivity of 60% and 70% was found with two different catalysts (namely, Pd-ZS and Pd-CeS). Very interestingly those selectivities were found with different gas and liquid flow rates (i.e., 70% of selectivity for Pd-ZS with liquid 1 mL/min and gas 2.7 mL/min, liquid 0.5 mL/min MeOH and gas 4 mL/min, and finally liquid 2 mL/min MeOH and gas 1 mL/min 65% of selectivity for Pd-CeS with liquid 1 mL/min and gas 4 mL/min). Moreover, the same maximum production rate of H 2O 2 around 3 μmol/min for Pd-CeS, Pd-ZS, and Pd-Z was achieved with different liquid and gas flow rates. Continuous operation and reactor technology play important roles in this green synthesis: optimization of gas and liquid flow rates and contact time between the liquid and the solid (catalysts) phase lead to a dramatic selectivity improvement in a continuous reactor, raising the value obtained in the semibatch reactor from 30% to 70%. © 2012 American Chemical Society.