Deactivation of titania-supported ruthenium catalysts for levulinic acid hydrogenation to gamma-valerolactone

Abstract

Hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) is an important conversion in biomass valorization. Supported-ruthenium catalysts, typically used in aqueous phase conditions, exhibit excellent activity, albeit suffering from deactivation. This work investigates the (in)stability of Ru/TiO₂ during LA hydrogenation at 100–200 °C, the deactivation mechanism and kinetics. TGA and ICP analyses ruled out coking and leaching as deactivation causes. TEM and CO chemisorption show sintering of Ru particles causing low dispersion and loss of activity. However, the deactivation rate decreases with temperature, thus indicating another deactivation mechanism. Regardless, the inherent catalytic activity, expressed as TOF, remains the same. Moreover, greater Ru dispersion loss in presence of LA as compared to pure aqueous media shows that LA accelerates deactivation. Hence, increasing LA concentration leads to quicker deactivation. XPS analysis shows a reduction of Ru and Ti species (increasing [Figure presented] and [Figure presented] content), alongside the decline in catalytic activity over time. After initial deactivation, both catalytic activity and oxidation state of surface species stabilize. The deactivation rate and final stable activity depend on total Ru loading, affecting spatial LA conversion and carboxylic acids concentration in the bed. Therefore, inadvertent operation with excess of Ru, either in total catalyst mass or Ru loadings on the support, and at higher temperatures (i.e., greater conversions and GVL selectivity) lead to masking of deactivation phenomena. Additional co-feeding experiments with the intermediate, 4-hydroxypentanoic acid, confirm LA as the carboxylic acid species predominantly causing deactivation. Furthermore, we study the kinetics of LA hydrogenation on the stabilized catalyst, and demonstrate that reaction kinetics transition from a zero to first-order dependence on LA from initial to stabilized activity. Additional experiments reveal a half-order dependence on hydrogen, valid from 100 to 200 °C.