Coke promoters improve acrolein selectivity in the gas-phase dehydration of glycerol to acrolein

Commercial processes partially oxidize propylene to acrolein but glycerol can replace the petroleum derived feeds as a bio-feedstock for this large volume specialty chemical. Metal oxides, particularly acid catalysts, dehydrate glycerol at elevated temperature. Dehydration of glycerol to acrolein is usually performed in traditional fixed-bed reactors. We tested phosphotungstic acid (HPW) loaded on titania catalysts (a weight fraction of 10, 20 and 30%) at 280 degrees C in a fluidized-bed reactor. With time-on-stream, the acrolein selectivity increased and the coke selectivity decreased. As much as 85% of the glycerol formed coke in the first hour and less than 20% acrolein. We developed a new method to spray the liquid feed directly on the catalyst bed and the reaction occurred in the gas phase. We also compared the performance of pure titania supports with respect to the acrolein selectivity. Acrolein selectivity was 13% higher on TiO2 with an average pore diameter of 17.3 nm versus the TiO2 support with a pore of 5.6 nm. Three times mote coke formed on the small pore diameter TiO2. We treated the catalyst with hydrogen rich coke promoters - tetralin and decalin - to passivate non-selective catalytic sites and to evidence hydrogen transfer reactions. This treatment increased acrolein selectivity from 10% to 30% in the first 30 min of reaction. As expected, they hydrogenated the intermediates to undesired compounds including acetone and propanaldehyde (propanal). (C) 2016 Elsevier B.V. All rights reserved.