Liquid Phase Direct Synthesis of H2O2: Activity and Selectivity of Pd-Dispersed Phase on Acidic Niobia-Silica Supports

In this work, acidic niobia-silica (NbS, 4-14 wt % Nb) materials used as supports of dispersed Pd particles (1.0-2.0 wt % Pd) have been prepared from different Nb-precursors (niobium ethoxide, NBE, and ammonium niobium oxalate, ANBO) and techniques (coprecipitation and deposition), characterized, and tested in the direct synthesis of H2O2 in water and methanol solvents. In particular, on a typical NbS sample, the evolution of morphology (by N-2-adsorption desorption), crystalline-phase (by XRD), electronic structure (by UV-vis-DRS), and surface acidity with time/temperature of treatment (350-800 C for 4-100 h) has been investigated. Surface acidity was measured by titrations with 2-phenylethylamine adsorption in various liquids: cyclohexane, for the intrinsic acidity, and water, methanol, and water methanol mixtures for the effective acidities. Direct H2O2 synthesis reaction was performed in semibatch slurry reactor with continuous feeding of the gaseous mixture (H-2, O-2, and N-2), under pressure (5 X 10(3) kPa or 10(4) kPa) at 5 degrees C in methanol or in water. In both solvents, reaction rates only little decreased with time on stream (ca. 5% of rate decrease after 4 h of reaction from initial rate of ca. 0.5 g(H2O2)center dot(kg(solution).min)(-1), according with the slight Pd sintering observed by TEM images. Catalysts prepared by deposition of NBE on silica gave better performances than those prepared from ANBO. In general, selectivity to H2O2 in water and in methanol was observed to be similar; the unexpected good selectivity in water was due to the higher effective acid strengths of the catalytic surfaces in water than in methanol, as experimentally proven.