Gas phase dehydration of glycerol to acrolein: Coke on WO3/TiO2 reduces by-products

• Gas-phase dehydration of glycerol to acrolein occurred in a fluidized-bed of WO3/TiO2 catalyst.
• Acrolein selectivity increased from 36% in the first hour to 73% after 14 h.
• Coke selectivity dropped from 50% in the first hour to only 9% after 14 h.
• Strong acid sites were responsible for the formation of acrolein.

Glycerol is a renewable feedstock for specialty chemicals that is co-produced when oil and fats are transesterified to biodiesel or hydrolysed to fatty acids. Acrolein is a target specialty chemical but catalysts deactivate with time due to coke and thus require frequent regeneration cycles to maintain activity. WO3/TiO2 catalyst dehydrated glycerol for 14 h while the acrolein selectivity increased as coke formed on the catalyst. After the first hour of reaction the acrolein selectivity was 36% and it reached steady state after 6 h at a selectivity of 73%. The major by-products were acetone, propanaldehyde, acetaldehyde, acetol and formic acid. Coke selectivity dropped from 50% in the first hour to 9% after 14 h. Elemental analysis (CHNS/O) of the catalyst withdrawn from the reactor during the experiments confirmed that polyaromatics formed on the surface. Based on NH3 and SO2 adsorption calorimetry the number of strong acid sites remained constant during 14 h, while the number of the weak and medium acid sites dropped. The basic sites disappeared after 2 h; coincidentally, less by-products (acetone) formed. Samples treated with pyridine, were analysed by FT-IR and they had fewer Brønsted sites but no Lewis acid sites. Carbon deposited on walls of the cylindrical pores (based on Barrett–Joyner–Halenda method) and reduced the pore diameter.

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