Supported molybdenum oxides as effective catalysts for the catalytic fast pyrolysis of lignocellulosic biomass

Abstract

The catalytic fast pyrolysis (CFP) of pine was investigated over 10 wt% MoO₃/TiO₂ and MoO₃/ZrO₂ at 500 °C and H₂ pressures ≤0.75 bar. The product distributions were monitored in real time using a molecular beam mass spectrometer (MBMS). Both supported MoO₃ catalysts show different levels of deoxygenation based on the cumulative biomass to MoO₃ mass ratio exposed to the catalytic bed. For biomass to MoO₃ mass ratios <1.5, predominantly olefinic and aromatic hydrocarbons are produced with no detectable oxygen-containing species. For ratios ≥1.5, partially deoxygenated species comprised of furans and phenols are observed, with a concomitant decrease of olefinic and aromatic hydrocarbons. For ratios ≥5, primary pyrolysis vapours break through the bed, indicating the onset of catalyst deactivation. Product quantification with a tandem micropyrolyzer–GCMS setup shows that fresh supported MoO₃ catalysts convert ca. 27 mol% of the original carbon into hydrocarbons comprised predominantly of aromatics (7 C%), olefins (18 C%) and paraffins (2 C%), comparable to the total hydrocarbon yield obtained with HZSM-5 operated under similar reaction conditions. Post-reaction XPS analysis on supported MoO₃/ZrO₂ and MoO₃/TiO₂ catalysts reveal that ca. 50% of Mo surface species exist in their partially reduced forms (i.e., Mo⁵⁺ and Mo³⁺), and that catalyst deactivation is likely associated to coking.