Discussing Lewis and Brønsted acidity on continuous pyruvaldehyde Cannizzaro reaction to lactic acid over solid catalysts

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

“…Despite the co-existence of Brønsted acidity on Nb 2 O 5 , their contribution to the activity for the Cannizzaro reaction appear to be limited, since only small amounts of lactic acid were formed over solely Brønsted acidic Amberlyst 35 (Figure 4c) in line with previous studies focused specifically on pyruvaldehyde Cannizzaro reaction into lactic acid. [25] 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, [24,25] 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.…”

“…Structural and textural properties of all commercial and synthesized catalysts were formerly investigated and the results are reported and discussed elsewhere. [25] It was shown that, apart from hydrated niobia (Nb 2 O 5 ) and calcined Nb 2 O 5 -400 (89 m 2 g À 1 ), ZrO 2 (103 m 2 g À 1 ), TiO 2 (54 m 2 g À 1 ) and SnO 2 (28 m 2 g À 1 ) are all crystalline powders. ZrO 2 presents essentially a monoclinic structure (96 %), while TiO 2 is characterized by its anatase phase (83 %).…”

“…ZrO 2 and TiO 2 had similar concentrations of acid sites (3000 and 2900 mol g À 1 ). [25] Even though such assessment was successfully correlated to catalyst activity in Cannizzaro reaction, it does not allow to discuss the nature and the strength of acid sites. Furthermore, the impact of the wellknown interaction of water with surface acid centers cannot be assessed either, limiting any further discussion on the role of each site in aqueous-phase reactions.…”

“…[18,19] Alternatively, glycerol can be dehydrogenated to trioses [20][21][22] that are further dehydrated to pyruvaldehyde [23] on acid catalysts, followed by Cannizzaro reaction to lactic acid. [24,25] [a] K. M. A. Santos Catalytic chemical conversion of trioses to lactic acid or alkyl lactates has been drawing large interest over the last years.…”

“…Recently, we reported the behavior of metal oxides in the Cannizzaro reaction of pyruvaldehyde, the very last step in the cascade conversion from trioses. [25] It was shown that Lewis acid Scheme 1. Pathways to obtain lactic acid from biomass derived molecules.…”