Effect of supercritical water on the stability of WOX/TiO2 and NbOX/TiO2 catalysts during glycerol dehydration

• Catalytic activity of NbOX/TiO2 less decreased than WOX/TiO2.
• Medium acid strength of NbOX/TiO2 enabled dissolution of coke precursors.
• High pressure suppressed coking due to effective transport of coke precursors.
• High ion products of water at high pressure caused leaching of W from WOX/TiO2.

The catalytic stabilities of WOX/TiO2 (WTi) and NbOX/TiO2 (NbTi) during glycerol dehydration in 400 °C supercritical water at 25 MPa and 33 MPa were investigated. The effects of coking and active component leaching on catalytic activities are quantitatively discussed. The catalytic activity of NbTi did not decrease significantly with time at both pressures. At 25 MPa, the coke precursors on the NbTi surface dissolved in the supercritical water because of the middle-strength acidity of NbTi, thus preventing loss of catalytic activities due to coking. At 33 MPa, coking was more suppressed, most likely because the coke precursors were transferred from the catalyst pores more effectively than at 25 MPa. Leaching of WOX from WTi occurred at the higher reaction pressure due to high ion products of water, but leaching of Nb from NbTi did not occur because of the high chemical stability of NbOX in acid/base solutions.

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