Lactic acid (LA) is one of the most appealing value-added products from glycerol, since it can be upgraded into polylactic acid, a compostable polymer whose market is rapidly growing. The cascade conversion of dihydroxyacetone (DHA), which can be easily obtained from glycerol or gluocse, into LA has been widely studied, mainly in batch reactors and using alcoholic solvents. Continuous liquid flow studies are rare, even though crucially important to understand complex kinetics and potential catalyst deactivation in these liquid phase processes. Aqueous phase processes are of particular interest because water is abundantly available and readily dissolve many biomass species. In this contribution, the behavior of three water-tolerant metal (La, Zr and Nb) phosphate catalysts is studied in the continuous aqueous phase conversion of DHA into LA. All the catalysts were free from internal mass transfer limitations, while 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 catalytic performance were studied with respect to temperature, contact time and time on stream. 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 pyruvaldehyde 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 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 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 metal phosphates catalysts in a liquid-flow packed-bed reactor

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.