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Keywords:

  • aldol reaction;
  • catalytic upgrading;
  • ketones;
  • lactic acid;
  • niobium

Abstract

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Aqueous solutions of lactic acid derived from biomass can be catalytically upgraded over Pt(0.1 %)/Nb2O5 into an organic phase rich in C4–C7 ketones that spontaneously separates from water. The single-reactor process retains 50 % of the feed carbon in the organic layer. The niobia support catalyzes C[BOND]C coupling reactions and plays a key role in directing the synthesis towards these valuable products.

Dilute aqueous solutions of lactic acid (30 %wt.) can be catalytically processed at 573 K and 57 bar over a low-metal-content Pt(0.1 %)/Nb2O5 catalyst in a spontaneously separating organic phase rich in valuable products such as C4–C7 ketones. An increase in the lactic acid concentration to 60 wt % allows conversion of approximately 50 % of the carbon feed in this organic layer, while maintaining good stability of the catalyst. Experiments at low conversion showed that lactic acid reacts first over Pt(0.1 %)/Nb2O5 to produce acetaldehyde and propanoic acid (along with CO and CO2 in the gas phase). These compounds (less oxygenated than lactic acid but still reactive) are the key intermediates in the overall process, and they react differently depending on the nature of the catalyst support. In particular, reaction kinetics studies with propanoic acid as feed showed that Pt(0.1 %)/Nb2O5 favored the formation of pentanones by ketonization reactions, whereas a monofunctional Pt(0.1 %)/carbon catalyst produced ethane and COx by decomposition reactions. In the same manner, acetaldehyde was preferentially hydrogenated to ethanol over Pt(0.1 %)/carbon, whereas the presence of niobia allowed this intermediate to react (by successive aldol condensations) to form C4–C7 condensation products stored in the organic phase. Finally, reaction pathways are proposed to explain the catalytic processing of lactic acid over bifunctional Pt(0.1 %)/Nb2O5. In this scheme, metal sites catalyze hydrogenation reactions and niobia promotes C[BOND]C coupling processes (ketonization and aldol condensation), in contrast to C[BOND]C cleavage reactions which take place preferentially over Pt(0.1 %)/carbon and lead to loss of carbon in the gas effluent as CO, CO2, and methane.