Relationship between Acid–Base Properties and the Activity of ZrO₂‐Based Catalysts for the Cannizzaro Reaction of Pyruvaldehyde to Lactic Acid

Abstract

The Cannizzaro reaction of pyruvaldehyde to lactic acid is investigated in a flow reactor with ZrO2 catalysts with different structures and acid–base properties. The results show that a difference in the crystalline structures of two ZrO2 polymorphs strongly affects the conversion of pyruvaldehyde. The monoclinic phase of zirconia is the most active for this reaction. A good correlation is observed between the reaction rate and the concentration of Lewis acid sites of sufficient strength, which shows that these sites play a major role in the reaction. A reaction mechanism is proposed involving coordinatively unsaturated Zr4+ cations as sites for activating pyruvaldehyde molecules, whereas Zr4+−O2− pairs generate terminal OH groups through water dissociation.

The Role of Brønsted and Water‐Tolerant Lewis Acid Sites in the Cascade Aqueous‐Phase Reaction of Triose to Lactic Acid

“…The significant production of formaldehyde over the catalysts with no or low concentration of water‐tolerant acid sites, SnO 2 and ZrO 2 , demands some attention as well. Formaldehyde has not been observed as a product in pyruvaldehyde conversion at this reaction temperature, and thus, the reaction pathway leading to it could start as a sequential reaction from lactic acid or as a parallel reaction path for glyceraldehyde conversion. No conversion was observed at all over these two catalysts when lactic acid was used as starting material under the same reaction conditions (130 °C, 10 bar, flow of 0.3 mL min −1 and W/F=1 g min mL −1 ) in a control experiment (Figure S3).…”

“…The concomitant drop in pyruvaldehyde selectivity evidences that the Cannizzaro reaction in the end of the cascade reaction path is accelerated. Albuquerque et al . have indeed shown that pyruvaldehyde rearrangement to lactic acid is improved at high temperature.…”

“…The first step is claimed to involve a keto‐enol tautomerization followed by dehydration promoted by both Brønsted and Lewis acid sites . Pyruvaldehyde is then rearranged to lactic acid via a hydride‐shift mechanism with addition of a water molecule on Lewis acid sites …”

“…Commercial ZrO 2 (Saint Gobain Norpro), TiO 2 (Evonik) and Nb 2 O 5 ⋅ n H 2 O (CBMM) oxides were used in this work. Nb 2 O 5 ⋅ n H 2 O was used both without any treatment, i. e. as niobic acid hereinafter labeled simply as Nb 2 O 5 , and after calcination in a muffle at 400 °C (Nb 2 O 5 ‐400) following a heating rate of 10 °C min −1 for 4 h. All oxides were ground and sieved to obtain particles size ranging between 53 and 63 μm to avoid any effect of mass transfer limitations …”

“…It can also be initially dehydrated to acetol over acid catalyst followed by oxidation/Cannizzaro reactions . Alternatively, glycerol can be dehydrogenated to trioses that are further dehydrated to pyruvaldehyde on acid catalysts, followed by Cannizzaro reaction to lactic acid …”