Catalytic Hot-Gas Filtration with a Supported Heteropolyacid Catalyst for Preconditioning Biomass Pyrolysis Vapors

During ex situ catalytic fast pyrolysis (CFP) of biomass, the separation of reactive char and alkali/alkaline particulates from biomass pyrolysis vapors by hot-gas filtration (HGF) leads to improved vapor stability and quality. HGF in tandem with chemical tailoring (e.g., partial deoxygenation) of the clean pyrolysis vapors, denoted as catalytic hot-gas filtration (CHGF), has the potential to further improve vapor composition by removing reactive oxygen moieties and protect downstream upgrading catalysts from fouling. Downstream upgrading refers to both vapor phase upgrading (e.g., ex situ CFP) and condensed phase upgrading (e.g., hydrotreating). Consequently, CHGF (as a single unit operation) was evaluated for preconditioning pyrolysis vapors for downstream upgrading processes. In order to understand the effective operating conditions that successfully filter and partially deoxygenate pyrolysis vapors, a titania-supported molybdenum heteropolyacid (Mo-HPA/TiO₂) catalyst was studied for use in CHGF. Here, pine pyrolysis vapors were generated in a small pilot-scale pyrolyzer and transferred to a CHGF unit via a continuous-flow slipstream. In the CHGF unit, the pyrolysis vapors were filtered and upgraded over a packed Mo-HPA/TiO₂ catalyst bed. Real-time monitoring and identification of the products formed were achieved by molecular beam mass spectrometry. The results showed that under a hydrogen-rich environment, the pine vapors were partially deoxygenated and alkylated over the Mo-HPA/TiO₂ catalyst. Reactivity studies revealed that an increase in hydrogen concentration and a reduction in weight-hourly space velocity enhanced deoxygenation and alkylation. Time-on-stream (TOS) studies showed stable product formation up to 1 h with little change in catalyst activity. Additionally, the liquid product was collected using a custom fractional condensation unit (built in-house) and analyzed by gas chromatography mass spectrometry to confirm that the product was partially deoxygenated and alkylated. The combination of CHGF and fractional condensation allowed for chemical and physical removal of both foulant and value-added compounds (e.g., phenols, alkylphenols, methoxyphenols, cyclopentenones) for additional enhancement of downstream upgrading processes. The pre- and postreaction catalysts were characterized using temperature-programmed desorption, N₂ physisorption, and elemental analysis with results indicating some catalyst coking.