Catalytic performance of brønsted and lewis acid sites in phenylglyoxal conversion on flame-derived silica-zirconia

Flame-derived silica-zirconia has been used for the promising one-step catalytic conversion of phenylglyoxal (PG) to mandelates, important intermediates in pharmacy and fine chemistry. In the literature it was proposed that Lewis acid sites (LAS) on solid acids promote mandelate production, whereas Brønsted acid sites (BAS) only generate acetal byproducts during PG conversion. Herein it is shown that ZrO₂, which contains only LAS and no BAS, exhibits a very low activity for PG conversion to afford a yield of mandelate of only 8% after 6h. The activity of this ZrO₂ catalyst was confirmed by the condensation of acetone, in which its LAS could activate the carbonyl groups immediately for the fast condensation of all acetone molecules. The introduction of BAS by the admixing of silica precursor to the feed in the flame-spray pyrolysis enhanced the yield of mandelate up to 52-67%. This indicates that BAS are mainly responsible for the PG conversion to mandelate on silica-zirconia catalysts and that their LAS are nearly inactive for this reaction.