Indirect ammoxidation of glycerol into acrylonitrile via the intermediate acrolein

Due to the depleting reserves of coal, oil and natural gas and to their negative impact on the environment, the humanity is forced to find renewable alternatives to replace the fossil feedstocks for the production of energy and chemical products. An example for an area of application where renewables are already used to substitute fossil feedstocks is the production of fuels. Biodiesel is one of the most popular biofuels nowadays. It is produced by transesterification of vegetable oils and fats. In this process, glycerol is formed as a by-product (approximately 10 wt.%). Glycerol is a versatile starting material which has up to 2000 applications. One very promising use of glycerol as starting material would be the dehydration of glycerol into acrolein. It could then be further converted into acrylonitrile – one of the most important monomers in the polymer production worldwide – by ammoxidation in the presence of ammonia and oxygen over mixed metal oxide catalysts. Today, acrylonitrile is exclusively synthesized from fossil feedstocks like propene and propane on an industrial scale. Therefore, a process combining the dehydration of glycerol to acrolein and the ammoxidation of the latter to acrylonitrile would be an alternative to the production processes based on fossil feedstocks. Thus, both reaction steps were studied separately at first – with focus on the ammoxidation of acrolein – and connected in a tandem reactor setup finally. For the first step of dehydration of glycerol to acrolein, we used previously optimized WO3/TiO2 catalysts, while oxide catalysts containing antimony, iron, vanadium and molybdenum were developed and used for the second ammoxidation step. Especially, the Sb-Fe-O catalysts were found highly selective and the influence of Sb/Fe ratio was subsequently studied. The presence of a FeSbO4 mixed phase on the synthesized samples was correlated to a high selectivity to acrylonitrile. Further, an increase in selectivity to acrylonitrile with the reaction time was observed, which was explained by the progressive formation of additional amounts of FeSbO4 over the catalysts during the reaction. After optimizing the key reaction parameter (reaction temperature, catalyst amount, NH3/acrolein ratio, O2/acrolein ratio) within a design of experiments, both reaction steps were connected in a tandem reactor. A maximum yield in acrylonitrile of 40% (based on glycerol) was obtained