tion and polymerization over catalysts due to the presence of
excessive acidity or too strong acid sites, leading to an increase in
the viscosity and
ﬂash point of bio-oil, making it unsuitable for
application as transportation fuel.
Therefore, we need to
consider the properties of a catalyst before utilizing it in the bio-
oil upgrading process. Also, the price and the long-term stability
of the catalyst must be controlled for speci
ﬁc applications in a
practical process. Commercially, alumina and silica have been
widely used for catalytic cracking of bio-oil molecules, but the
problem is rapid deactivation of the spent catalyst after the
run. To date, commercial zeolites show excellent activity for
upgrading bio-oil via various reactions such as cracking,
decarboxylation, decarbonylation, dehydration, aromatization,
and others, leading to the facile formation of aromatic
hydrocarbons as the main components in bio-oil.
kinds of zeolites have also been tested for bio-oil deoxygenation.
Du et al.
found that Zeolite Socony Mobil-5 (ZSM-5), an
aluminosilicate zeolite in the protonic form with the pore size in
the range of about 5.4
−5.6 Å, showed better deoxygenation/
aromatization activities than others for promoting deoxygena-
tion, resulting in the rich formation of aromatic hydrocarbons in
bio-oil. However, due to the limitation of pore size on the zeolite
structure, the large components of bio-oil cannot access or
disperse to the active site of zeolite.
Mesoporous materials are
a good choice to solve such a problem. Kaewpengkrow et al.
found that mesoporous acid catalysts had high ability for
selective conversion of oxygenated compounds into hydro-
carbons. To increase the catalytic deoxygenation performance,
doping of transition metals such as Ni, Cu, Ga, Fe, Co, and Mo
into mesoporous supports has been systematically tried and
found that new acid sites are well generated, which can highly
promote the bio-oil upgrading process.
Among them, Cu,
Fe, and Ni, being abundantly available and cheap metals, exhibit
the same activity, as compared to noble metals such as Pt and Ru
for the bio-oil upgrading process. To date, the main problems
during the deoxygenation process are facile formation of coke on
commercial catalysts and metal-doped catalysts having too
much acidity, which lead to rapid deactivation.
To solve the above problems, we developed a green, cheap,
and stable mesoporous acid catalyst by eco-friendly production
from waste biomass derived from industry. Activated carbon is
found to be an e
ﬀective catalyst/support material because it has
good ability to improve the bio-oil quality.
Herein, waste palm
kernel cake (WPKC) derived from the palm oil industry was
applied as feedstock for the sustainable production of catalysts
and bio-oil. In this study, metal (Cu, Ni, or Fe)-doped activated
carbon catalysts were prepared from WPKC using impregnation
and pyrolysis methods. The activity of the as-prepared catalysts
was systematically tested for deoxygenation of bio-oil derived
from WPKC pyrolysis. The physical and chemical characteristics
of catalysts were characterized to support the catalytic results. A
statistical experiment was systematically performed to obtain the
highest yield of bio-oil. The product distributions in the
upgraded bio-oils from biomass feedstock were investigated by
using metal-doped activated carbon with di
amounts. The catalytic performance and reusability without
further regeneration of Ni-carbon were also compared with
those of commercial materials such as zeolite and alumina. It is
expected to provide higher-performance, longer-stability, and
lower-cost green catalysts for the bio-oil deoxygenation process.
2. RESULTS AND DISCUSSION
2.1. Characterization of Catalysts. As shown in
the pure activated carbon without metal doping exhibited a
higher surface area (356 m
/g) than metal (Cu, Ni, or Fe)-
doped activated carbon with various loading amounts of 5
wt % (207
/g). The surface area and the pore volume
ﬁcantly decreased to some extent with the increase in the
metal-loading amount, suggesting that the surface and pore
structures of the supports are partly covered and occupied by the
dispersed metal species.
Meanwhile, the mesopore volume in
Fe-doped activated carbon severely decreased when compared
with those of the others, indicating that the iron oxide formed
during thermal treatment was easily distributed into the porous
structure, resulting in the facile occurrence of mesopore
blockage. Here, an average pore size of around 2.2 nm was
clearly observed for all carbon catalysts.
shows the XRD patterns of metal-loaded activated
carbon catalysts. One can see weak di
ﬀraction peaks appearing at
5 wt % loading amount, corresponding to the metal (Cu, Ni, or
Fe) crystalline phase. This indicates that metals with a small