(426f) Conversion of Dihydroxyacetone into Lactic Acid over Metal Phosphates Catalysts in a Packed Bed Reactor

In the last decades, biofuel research and production increased due to oil depletion and policies aiming at sustainability. Biodiesel obtained by transesterification of fatty acid (FAME) is one of the most used and commercially viable biofuels. However, large amounts of glycerol are coproduced during the transesterification process. Therefore, the economic viability of this biofuel depends also on the possibility of upgrading the coproduced glycerol into added-value products. Ideally, the upgrading is performed in a continuous-flow process.

Lactic acid (LA) is one of the most appealing target products among glycerol derivatives, since it can be successively upgraded into polylactic acid (PLA), a compostable polymer whose market is growing rapidly.

Glycerol can be first converted into dihydroxyacetone (DHA) or glyceraldehyde (GLA) over supported metal catalysts. Subsequently, DHA or GLA can be converted into LA in a cascade reaction over catalysts possessing both Bronsted and Lewis acid sites. In fact, Bronsted acid sites are more active to catalyze DHA dehydration to pyruvaldehyde (PVA), while the Lewis acid sites assist pyruvaldehyde disproportionation into LA (Cannizzaro reaction). DHA conversion to LA has been widely studied but mainly in batch reactors using alcoholic solvents. Continuous liquid flow studies are rare, even though crucially important to develop sustainable processes. In fact, the major part of the petrochemical processes is performed in continuous flow reactors. As consequence, having viable continuous flow processes to convert biomasses is essential to make biorefineries economically competitive. Furthermore, biomass derived products are usually viable in aqueous phase, hence gaining a better understanding on the catalyst stability in aqueous media is extremely important.

For all these reasons, in this contribution the behavior of three metal phosphate catalysts, which are known to be water-tolerant, is studied in the continuous aqueous phase conversion of DHA into LA.

La, Zr and Nb phosphates were fully characterized and structure-properties relationships were found. External mass transfer limitation affected the rate of reaction over the most active catalysts. To allow for better understanding of macrokinetics the intrinsic kinetic constant and the mass transfer coefficient were retrieved. The catalytic performances were studied with respect to temperature, contact time and time on stream. The catalysts showed a slow deactivation over 30 h, which was associated to Lewis acid site deactivation. Furthermore, it was observed a correlation between lactic acid concentration and carbon balance closure. Therefore, to completely understand the deactivation mechanism involved, the deactivation kinetics were modeled. Important insight into mass transport phenomena and deactivation kinetics during conversion of aqueous biomass solutions are highlighted in this work.