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

The eventual role played by Bronsted and Lewis acid sites on pyruvaldehyde Cannizzaro reaction to lactic acid was investigated in a flow reactor using different catalysts bearing different acidity. It was shown that Lewis acid sites on heterogeneous catalysts are crucial for the reaction while Bronsted sites play no role at all. The results revealed that catalytic activity is indeed directly related to catalyst acidity. TiO2, ZrO2 and Nb2O5 were the most promising catalysts and their activity was related to their water-tolerant Lewis acid sites. Nb2O5 was shown to be the most active and stable catalyst while ZnO underwent a fast deactivation process ascribed to the formation of amorphous carbonaceous materials as revealed by TG and laser Raman spectroscopy. (C) 2017 Elsevier B.V. All rights reserved.