Mixed metal oxide bronzes as catalysts for the gas-phase aerobic transformation of glycerol to acrylic acid: Single or double catalytic bed approaches

Metal-doped hexagonal tungsten oxide bronzes (h-WVO_x and h-WNbO_x) and metal-doped orthorhombic molybdenum oxide bronzes-M1 type (MoVO_x, MoVTeO_x and MoVTeNbO_x) have been synthesized hydrothermally and heat-treated at 400, 550 or 600 °C in N₂. The catalysts were characterized by several techniques and tested in the one-pot aerobic transformation of glycerol at 350 °C. From all of them, h-WNbO_x resulted as the most selective to acrolein (80 % yield), whereas both h-WVO_x and MoVTeNbO_x were more effective to acrylic acid (with a yield lower than 25 %). Subsequently, a double-bed reactor comparative study was conducted, with h-WNbO_x acting as the first catalytic bed. In this case, the yield to acrylic acid decreased as follows: MoVTeNbO_x > MoVTeO_x > MoVO_x > h-WVO_x. IR spectroscopy of acrolein adsorbed on these catalysts allows to explain the catalytic performance of these catalysts.