Catalytic fast pyrolysis (CFP) enables the production of stabilized bio-oil feedstocks for direct insertion into refineries, hydrotreating to finished fuels, and extraction of monomers for materials production. Bifunctional metal-acid catalysts are favored for upgrading bio-oil intermediates because they activate hydrogen at ambient pressure and promote hydrodeoxygenation (HDO) reactions to eliminate oxygen as water with limited C-C bond cleavage. Recent reviews have highlighted the effectiveness of bifunctional metal-acid catalysts (e.g. Mo₂C, MoO₃, Ru/TiO₂ and Pt/TiO₂) for HDO of bio-oil model compounds. We separately evaluated performances of Mo₂C and Pt/TiO₂ catalysts during CFP of biomass using a large bench-scale reactor. This reactor enables exploration of practical aspects such as product speciation, activity, deactivation and regeneration of the catalysts. The results suggest that the H₂-activating sites on catalysts are responsible for significant deoxygenation of pyrolysis vapors at low biomass-to-catalyst ratios (B:C), and these reactions lead to rapid consumption of the H*-sites. As a result, catalysts lose their bifunctionality and starts to behave like a typical acid catalyst, catalyzing some deoxygenation reactions and aromatic and eventually polycyclic aromatic hydrocarbons (PAHs). Catalyst deactivation occurs because of buildup of the nonvolatile PAHs likely on the catalyst surface, blocking access to active sites. We will show that the Pt/TiO₂ catalyst was easily regenerated by thermal oxidation followed by H₂ pre-reduction. However, Mo₂C regeneration was not straightforward. We will show that flowing H₂ at reaction temperature (400 °C) can mitigate the buildup of the PAHs at low B:C and prevent Mo₂C deactivation. If the Mo₂C catalyst was exposed to excess biomass pyrolysis vapors, extensive deactivation occurred, and the catalyst required an offline recarburization to regenerate it. Additional experiments conducted with Pt/TiO₂ showed that the TiO₂ support disintegrated during regeneration due to thermal excursions, indicating that using this catalyst in a fixed-bed reactor will be challenging.

Kristiina Iisa

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