(597d) Continuous Aqueous Flow Conversion of Dihydroxyacetone into Lactic Acid over Metal Phosphates.

The conversion of dihydroxyacetone (DHA) into lactic acid (LA) has been widely studied, mainly in batch reactors and using alcoholic solvents. Continuous liquid flow studies are rare, although crucial to understanding complex kinetics and potential catalyst deactivation in liquid phase processes.^1,2

In this work, the behavior of three metal (La, Zr, and Nb) phosphates is studied in the continuous aqueous phase conversion of DHA into LA. NbPO and LaPO were externally mass transfer limited. To allow for a better understanding of macro kinetics, the intrinsic kinetic rate constant and the mass transfer coefficient were deconvoluted from the experimental rate constant. The reaction network over ZrPO was studied by varying the contact time and feeding DHA, pyruvaldehyde (PVA), and LA. The experimental concentrations obtained were fitted with a set of ODEs. Interestingly, the dehydration of DHA to PVA was found to be well described with a second-order reaction, and a dimeric intermediate was hypothesized. NbPO showed deactivation over 30 h, which was associated with deactivation of the Lewis acid sites. Humin-like carbon deposits were detected on NbPO by 13C-MAS-NMR and ATR-FTIR spectroscopy. It is hypothesized that these macromolecules were formed by condensation reactions catalyzed by the Lewis acid sites. Lastly, a correlation between carbon deposition and LA concentration was found. This result suggested that LA polymerization was the reason of the deficit in the carbon balance. However, the role of PVA and DHA in the formation of carbonaceous deposits cannot be ruled out based on the presence of humin-like carbon deposit on the catalyst surface.³

References

(1) Santos, K. M. A. et al. ChemCatChem 2019, 11 (13), 3054–3063.

(2) Clippel, F. De et al J. Am. Chem. Soc. 2012, 134 (24), 10089–10101.

(3) Innocenti, G. et al. ACS Catal. 2020, 10 (20), 11936–11950.