We report here about an investigation of the key catalyst features and reaction parameters in glycerol oxidehydration into acrylic acid over W-V-Nb mixed oxides with the hexagonal tungsten bronze structure. The incorporation of Nb allowed to considerably improve the catalytic behaviour not only because of the higher yield to acrylic acid and acrolein compared to W-V bronzes, but also to increase of more than order of magnitude the oxygenated productivity rate, because of the greater concentration of glycerol used in the inlet feed and the lower contact time needed. Experiments were carried out by reacting acrolein, and by oxidation of methanol, as a model molecule for the determination of surface acid and redox properties. These experiments demonstrated that with the W-V-Nb catalyst the optimal ratio between the two consecutive steps of acid-catalysed glycerol dehydration and acrolein oxidation, and between the two parallel reactions of acrolein oxidation and transformation into by-products (ketals and oligomers), was achieved in the presence of defined glycerol-to-oxygen inlet ratio. Indeed, oxygen played the fundamental role of accelerating the oxidation of the intermediately formed acrolein into acrylic acid, by allowing a greater concentration of the oxidizing V 5+ sites.
Glycerol surplus from biodiesel synthesis still represents a major problem in the biofuel production chain. Meanwhile, those in the acrylic acid market are looking for new processes that are able to offer viable alternatives to propylene-based production. Therefore, acrylic acid synthesis from glycerol could be an effective solution to both issues. Among the viable routes, one-pot synthesis theoretically represents the most efficient process, but it is also highly challenging from the catalyst design standpoint. A new class of complex W--Mo--V mixed-oxide catalysts, which are strongly related to the hexagonal tungsten bronze structure, able to directly convert glycerol into acrylic acid with yields of up to 51 % are reported.
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