Lactic acid (LA) is one of the most attractive products from glycerol or glucose. It can be upgraded into polylactic acid, a compostable polymer whose market is rapidly growing. The cascade reaction of dihydroxyacetone (DHA), which a key intermediate regardless of the feedstock, into LA has been widely studied, mainly in batch reactors and using alcoholic solvents. Continuous liquid flow studies are rare but will be necessary to understand complex kinetics and potential catalyst deactivation in this reaction. Aqueous phase processes are of particular interest because water is abundantly available and readily dissolves many biomass species.<br/><br/>The conversion of DHA to LA in aqueous phase requires catalysts with a combination of Brønsted acid sites for the dehydration of DHA to pyruvaldehyde (PA) and water-tolerant Lewis acid sites for the Cannizzaro reaction of PA to LA. Metal phospoate (La, Zr and Nb) catalysts generally meet this requirement. A kinetic study using a continuously operated packed-bed reactor showed that these catalysts operated free from internal mass transfer limitations, while NbPO and LaPO were partially externally mass transfer limited. Intrinsic kinetic rate constants and the mass transfer coefficients were deconvoluted from the observed rate constants. ZrPO was used to perform an evaluation of the reaction network. The reaction was studied with respect to the contact time feeding DHA, pyruvaldehyde, and LA. The experimental concentrations obtained were fitted with a set of ODEs to retrieve the reaction order and optimized values of the kinetic rate constants. Interestingly, the dehydration of DHA to PA was found to be well described with a second-order reaction. Consequently, the formation of a dimeric intermediate is suggested. NbPO and ZrPO showed a slow deactivation over 30 h, which was mostly associated with deactivation of Lewis acid sites. Humin-like carbon deposits were detected on NbPO by <sup>13</sup>C-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 LA concentration and carbon loss was found. This result suggested that the deficit of the carbon balance was due to LA polymerization, as also suggested by the 13C MAS-NMR and ATR-FTIR spectra. A 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.

Lactic acid production over solid acid catalysts in aqueous phase

ACS Spring Meeting 2021

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