Investigations into the influence of nickel loading on MoO3-modified catalysts for the gas-phase hydrodeoxygenation of anisole

Abstract

Molybdenum oxide-based catalysts are promising catalysts for the gas-phase hydrodeoxygenation (HDO) of lignocellulosic pyrolysis oils with high selectivities to arenes. The gas-phase HDO is conducted at temperatures between 300 °C and 400 °C at ambient pressure, resulting in a low hydrogen consumption and high energy efficiency. The loading of nickel forming a binary structure consisting of MoO₃ and nickel molybdate (NiMoO₄) during calcination results in a significant increase of the catalytic activity connected with a drastic shortening of the induction period observed for the unpromoted catalyst system. The promotional effect of nickel seems to be related to its enhanced reduction behaviour and hydrogen dissociation leading eventually to an improved formation of a molybdenum oxycarbohydride phase (MoO_xC_yH_z). The MoO_xC_yH_z phase plays an important role in stabilizing active Mo⁵⁺ sites and prevents over-reduction to inactive MoO₂. The activity and benzene selectivity are maximal when pure NiMoO₄ is used as a catalyst. However, the selectivity towards undesired methane increases significantly indicating the decomposition of arenes. This effect is considerably reduced for catalysts with lower nickel contents (3–5%) which still exhibit a highly improved activity compared to MoO₃. In situ XRD studies revealed that the population of the MoO_xC_yH_z phase is strongly affected by the nickel content, the structure of the hydrocarbon substrate and the hydrogen content during pre-reduction and catalysis.