Dehydration of glycerol in gas phase using heteropolyacid catalysts as active compounds

Different silica-, alumina-, and aluminosilicate-supported heteropolyacid catalysts were prepared using phosphomolybdic acid H3PMo12O40⋅xH2O, phosphotungstic acid H3PW12O40⋅xH2O, silicotungstic acid H4SiW12O40⋅xH2O, and ammonium phosphomolybdate (NH4)3PMo12O40⋅xH2O as precursor compounds. The as-synthesised solids were characterised by nitrogen adsorption, XRD, TG/DTA, Raman spectroscopy, and TPD of ammonia. Silica-supported heteropolyacids are rather well crystallised, whereas alumina-supported samples are X-ray amorphous. Investigations using Raman spectroscopy of calcined samples and TG/DTA revealed that molybdenum-containing heteropolyacids tend to decompose partly close to 400 °C into molybdates and MoO3 whereas tungsten-containing samples are stable. This makes in particular tungsten-based materials interesting acid catalysts for the dehydration of glycerol in the gas phase. In particular, the influence of selected support materials, catalyst loading, and temperature on acrolein formation was studied at standardised reaction conditions (10% by weight of glycerol in water, 225–300 °C, modified contact time 0.15 kg h mol−1). Surprisingly, alumina is found to be superior to silica as support material with regard to catalyst activity and selectivity. Nevertheless, tungsten based heteropolyacids showed outstanding performance and stability. Acrolein was always the predominant product with maximum selectivity of 75% at complete conversion over silicotungstic acid supported over alumina and aluminosilicate.