Rubidium- and caesium-doped silicotungstic acid catalysts supported on alumina for the catalytic dehydration of glycerol to acrolein
Dehydration of glycerol was carried out using rubidium- and caesium-doped silicotungstic acid catalysts. These catalysts were prepared by varying concentration of the dopant metal cations while keeping the concentration of heteropoly acid unchanged. High acrolein selectivity (94-96%) was observed with unsupported caesium-doped silicotungstic acid and rubidium-doped silicotungstic acid with a dilute glycerol feed (0.5 wt.% in water). These catalysts were then supported on alpha-alumina and an alumina comprising a theta-delta mixture. Caesium-doped silicotungstic acid supported on theta-delta alumina gave a maximum selectivity of ca. 90% at 100% glycerol conversion for 90-h time online, with a 10 wt.% glycerol solution. With a more concentrated glycerol feed (20 wt.%), this catalyst achieved a space time yield of 210 g (acrolein)kg(cat)-1h -1. The catalyst was investigated further to determine the origin of the long-term stability. The binding strength of the partially doped silicotungstic acid on the alumina was found to be crucial to sustain the supported Keggin structure and hence the acidity of the active sites resulting in a high acrolein yield.