Selective oxidation of 1,2-propanediol over gold, palladium and platinum heterogeneous catalysts

Biodiesel is one of the most commonly used non petroleum-derived fuels in modem society. Glycerol, a by-product from bio-diesel manufacture, is an excellent starting material for the production of 1,2-propanediol by catalytic hydrogenolysis, which can be further oxidized to lactic acid - a major platform molecule which is used in the manufacturing of biodegradable polymers, food additives and moisturizing agents. Lactic acid can be produced by several routes, which are non-green and require the use of toxic and corrosive reagents, such as acetaldehyde and lactonitrile, as well as high-pressure apparatus, whereas the fermentation process is affected by low productivity and severe purification problems. In this work it is demonstrated that the oxidation of 1,2-propanediol can be carried out selectively under mild conditions using mono- and bimetallic catalysts comprising of Au and Pt group metals. These catalysts were obtained via the sol immobilization technique, while previous studies have used monometallic Au and Au-Pd alloyed catalysts at elevated temperatures and pressures. In fact, although it is known that 1,2-propanediol can be successfully oxidized to lactic acid, no literature is available for the oxidation of this compound under mild conditions and/or in the absence of a base. This prompted us to extend previous findings of 1,2-propanediol conversion and to investigate the possibility of 'green' routes for lactic acid production. A range of different Au, Pd and Pt activated carbon supported nanoparticles have been tested for 1,2-propanediol oxidation reaction. In the case of basic conditions, it was possible to reach 100% conversion with selectivity to lactate of ca. 96% using AuPt/C catalyst under 3 bar oxygen pressure and 40 C, which are significantly lower parameter values than those reported in literature. Interestingly, a synergistic effect was observed when comparing the efficiency of monometallic catalysts and their bimetallic combinations. A series of tests with physical mixtures of Au/C and Pt/C catalysts proved that introducing the second metal led to an enhanced catalytic activity. It has also been shown that decreasing the amount of metal does not lead to drastic decreases in conversion. A range of activated carbons has been tested with the Darco KB-B (Sigma-Aldrich) giving the highest conversion values. This charcoal was used as a support for all further studies. Finally, it has been shown that when improving the reactor design by enhancing stirring capability, even air can be used as an oxidant giving similar results to the ones when using molecular oxygen and pressurizing the system. The possibility of base-free 1,2-propanediol oxidation has also been evaluated, and Au, Pd and Pt supported nanoparticles appeared also to be active giving ca. 75% conversion. Surprisingly, when the addition of the base is omitted, the reaction profile changes to the formation of another product, namely hydroxyacetone, which is the result of a secondary hydroxyl group oxidation. This trend was further investigated and a series of mechanistic studies have been carried out, including attempts to synthesize lactaldehyde. These results provided an insight into the origin of formic, acetic and pyruvic acids which are the by-products of the investigated reaction. Finally, a full spectrum of C4 isomeric diols have been screened to evaluate the activity of the catalysts having proved to be efficient for 1,2-propanediol oxidation. In the absence of a base, a similar trend was observed: both alcohol groups in diols were oxidized, though at different rates. Promising results were obtained for the oxidation of vicinal 2,3-butanediol to acetoin (100% selectivity). In fact, at present this compound can be produced by fermentation and a reusable heterogeneous catalyst can be advantageous.