Stability assessment of sulfided NiMo-based catalysts for continuous flow supercritical water hydrodeoxygenation applications

Sulfided NiMo-based catalysts are widely used in hydroprocessing, including hydrotreating and hydrocracking in petroleum refining and renewable fuel production via biomass conversion. The development of hydrothermal liquefaction (HTL) for biocrude production has increased interest in direct hydrothermal upgrading processes, such as (hydro)deoxygenation and desulfurisation. This study evaluated the stability of NiMo- and NiW-based catalysts under continuous-flow conditions to assess their suitability for hydrothermal upgrading. After just 2 h in supercritical water (SCW), significant metal leaching was observed: Mo losses ranged from 47 % to 97 %, and Ni losses from 35 % to 80 %. Sulfidation provided only modest improvement, with losses still at 56–80 % for Mo and 52–75 % for Ni. Tungsten loss reached 90 % in the sulfided NiW catalyst. Mo leaching was particularly severe during the heating phase, with over 60 % of Mo and around 20 % of Ni lost during heating and cooling, peaking at 200 °C. At 400 °C, leaching rates from unreduced NiMo/AC were 0.4 mg h⁻¹ for Ni and 10 mg h⁻¹ for Mo. Sulfided NiMo showed better Mo stability, with leaching rates of 0.24 mg h⁻¹ for Ni and 3.6 mg h⁻¹ for Mo. These findings highlight the importance of assessing catalyst stability under realistic flow conditions. NiMo-, NiW-, and by extension, CoMo- and CoW-based catalysts are unsuitable for hydrothermal upgrading due to poor stability in SCW, particularly during heat-up and cool-down phases.