A Review of Recent Research on Catalytic Biomass Pyrolysis and Low-Pressure Hydropyrolysis

Direct
catalytic upgrading of biomass-derived fast pyrolysis vapors
can occur in different process configurations, under either inert
or hydrogen-containing atmospheres. This review summarizes the myriad
of different catalysts studied and benchmarks their deoxygenation
performance by also taking into account the resulting decrease in
bio-oil yield compared to a thermal pyrolysis oil. Generally, catalyst
modifications aim at improving the initial selectivity of the catalyst
to more desirable oxygen-free hydrocarbons and/or improving the catalysts’
stability against deactivation by coking. Optimizing the pore structure
and acid site density/distribution of solid acid catalysts can slow
down deactivation and prolong activity. Basic catalysts such as MgO
and Na2O/γ-Al2O3 are excellent
ketonization catalysts favoring oxygen removal via decarboxylation,
whereas solid acid catalysts such as zeolites primarily favor decarbonylation
and dehydration. Basic catalysts can therefore produce bio-oils with
higher H/C ratios. However, since their coke formation per surface
area is higher, compared to a microporous HZSM-5 zeolite, precoking
(or imperfect regeneration) of these basic catalysts and operating
for longer time-on-stream can be approaches to improve the oil yield.
In-line vapor-phase upgrading with a dual bed comprising a solid acid
catalyst followed by a basic catalyst active in ketonization and aldol
condensation further improves deoxygenation while maintaining high
bio-oil carbon recovery. Also low-cost catalysts such as iron-rich
red mud have deoxygenation activity. An improved bio-oil carbon recoverycompared
at a similar level of oxygen removalcan be obtained when changing
from an inert atmosphere to a hydrogen-containing atmosphere and using
an effective hydrodeoxygenation (HDO) catalyst. To keep costs low,
this can be conducted at near-atmospheric pressure conditions. Pt/TiO2 and MoO3/TiO2 showed high activity
and reduced coke formation. Stable performance has been demonstrated
using Pt/TiO2 for 100+ reaction/regeneration cycles with
woody biomass feedstock. If future works can demonstrate the same
durability for lower-cost biomass containing higher contents of ash,
N, and S, this would considerably boost the commercial viability of
near-atmospheric pressure HDO. Further research should be directed
to testing the durability of lower-cost HDO catalysts such as MoO3/TiO2 and further improving the activity and stability
of lower-cost catalysts.