Tungsten-Vanadium mixed oxides for the oxidehydration of glycerol into acrylic acid

Abstract

In this paper we report on the one-pot transformation of glycerol into acrylic acid, catalyzed by W/V mixed oxides, with hexagonal tungsten bronze (HTB) structure. The reaction requires two different catalyst functions, i.e., an acid one, which is given by W oxide, and an oxidizing one, given by the V ions incorporated within the WO₃ lattice. W–O bronze is very active and moderately selective in acrolein formation, but yields only traces of acrylic acid. The incorporation of increasing amounts of V inside the hexagonal tungsten bronze structure, with the development of a monophasic compound, allows the consecutive oxidation of acrolein into acrylic acid. An optimal atomic ratio between W and V equal to V/(W + V) = 0.12–0.21 made it possible to obtain an acrylic acid yield of 25% (with selectivity to residual acrolein of 11%). However, during reaction under the oxygen-containing feed, the V⁴⁺ incorporated into the hexagonal bronze structure underwent a slow oxidation into V⁵⁺, which caused a progressive decline of selectivity to acrylic acid and a concomitant increase of CO_x formation; the hexagonal structure however was stable during lifetime experiments. On the other hand, in the absence of oxygen a very rapid deactivation of the catalyst occurred, with a decrease in selectivity to acrolein and increase in heavy by-products.