Structural properties of flame-made Rh/Al2O3 and catalytic behavior in chemoselective hydrogenation

Rhodium/alumina catalysts with Rh loadings of 0.5–5 wt% were prepared by flame spray pyrolysis, and their structural and catalytic properties were compared with that of commercial 5 wt% Rh/Al2O3 reference catalysts. The structure of the materials was investigated using STEM, XRD, H2 chemisorption, TPR, and CO adsorption combined with DRIFT spectroscopy. The as-synthesized flame-made material was composed of well-dispersed oxidized 1–2 nm rhodium particles deposited on virtually nonporous agglomerated 10–20 nm alumina particles. The oxidic rhodium phase in this material showed considerably higher resistance against reduction compared with the reference catalysts up to 600 °C. On reduction, metallic rhodium and cationic Rh species were uncovered by DRIFTS. The flame-made catalysts showed an increase in the fraction of cationic species with increasing reduction temperature, whereas similar behavior was not observed with the commercial reference catalysts. The high stability of the oxidic Rh species against reduction seems to be an intrinsic structural property of the flame-made Rh/Al2O3 and is indicative of a strong interaction between the rhodium and alumina constituents. Although metallic Rh was a prerequisite for activity in the chemoselective and enantioselective hydrogenation of 3,5-di-(trifluoromethyl)-acetophenone, the presence of cationic Rh species seemed to positively affect catalytic performance, as shown by the superior catalytic behavior of the flame-made material compared with the commercial reference catalysts.