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.
A hydrogen-based catalyst regeneration procedure restored the reacted
catalyst activity to that of fresh Mo-HPA/TiO2.