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kinds of zeolites and SAPO-x catalysts with various structures and acidities [147].
They utilized these catalysts to upgrade biocrude oil from HTL of algae. They found
out that at 400 °C, 6 MPa H 2 , 10 wt% catalyst, and supercritical H 2 O, yield was
highly dependent on the structure. The HSAPO-11 catalyst had the lowest upgrading efficiency at 42.4 wt% and the lowest hydrocarbon yield at 80 wt%, while the
HMCM-41 catalyst had the highest upgrading efficiency at 54.5 wt% and the highest hydrocarbon yield at 95.6 wt%. In another study, the zeolite HZSM-5 was used
in upgrading biocrude oil with H 2 at high pressure [148]. Temperatures (400–500 °C)
and catalyst loadings (5–50 wt%) were varied. All S-, O-, and N-containing compounds were significantly reduced, and the highest alkane yield was achieved at
400 °C. The authors hypothesized that the dominant upgrading reactions were
hydrotreating (hydrodesulfurization, hydrodeoxygenation, hydrodenitrogenation),
hydrogenation, hydrocracking, and aromatization [97].
In terms of metal catalysts, Raney-Ni catalyst was found to be more effective
than Ru/C or a combination of Raney-Ni and Ru/C because it mediated a fuel product with 87 wt% alkanes, in contrast to the other two catalysts which mediated fuel
products with <60 wt% alkanes [141]. Zhang et al. has shown in their experiment
that the yields mediated by these three catalysts were as follows: Ru/C > Pt/C > Mo 2 C
at 350 °C and 6 MPa H 2 pressure [144]. It has been shown that the deoxygenation
activities of Pt/γ-Al 2 O 3 , Ru/C, Ru/C + alumina, MCM-41 (100% Si), and
Ru/C + Mo 2 C in algae valorization were high [35]. Ru/C + alumina, MCM-41
(100% Si), Pt/γ-Al 2 O 3 , Pt/C, and Pd/C were excellent for hydrogenation, and
Ru/C + Pt/C was ideal for desulfurization [35].
A method of improving a heterogeneous catalyst’s activity is to combine two or
more of them, known as the two-component or multicomponent catalyst system,
respectively. This is done to improve the catalyst’s ability to remove heteroatoms
from biocrude oil (i.e., better-quality fuel with high concentration of fuel-range
hydrocarbons) or increase fuel yield [95, 147]. An example is the combination of
Ru/C with Raney-Ni (Ru/C + Raney-Ni). Introducing H 2 to the biocrude oil at
400 °C in the presence of the two-component catalyst system boosted the fuel’s
yield and C and H contents. In fact, fuel yield reached 77.2 wt%, HHV was determined to be 45.2 MJ/kg, and energy yield (i.e., ratio of the energy contained in fuel
versus the energy contained in biocrude oil) reached 86 wt% [141]. In a similar
manner, a multicomponent system of catalysts containing Ru/C + Pt/C, Ru/C + Mo 2 C,
and Ru/C + Pt/γ-Al 2 O 3 achieved enhanced deoxygenation, denitrogenation, and
desulfurization activities [95].
Lastly, solvent plays an important role in microalgae valorization. Water is the
commonly used solvent, and it influences the catalytic activities of heterogeneous
catalysts. Some of the documented effects of water on catalyst and fuel are as follows: (1) Using subcritical water, increasing Pt/C-sulfide catalyst from 0 to 20 wt%,
lowers S and O contents, raises C and H contents, and increases the energy density
and HHV of the fuel [142]; (2) using water, the catalytic activity of HZSM-5 is
improved [107] which stimulates decarboxylation to generate more CO 2 [143].
Some of the challenges associated with heterogeneous catalysts in microalgae
valorization include (1) deactivation due to solid residue (coke) formation [115,
E. P. Resurreccion and S. Kumar
kinds of zeolites and SAPO-x catalysts with various structures and acidities [147].
They utilized these catalysts to upgrade biocrude oil from HTL of algae. They found
out that at 400 °C, 6 MPa H 2 , 10 wt% catalyst, and supercritical H 2 O, yield was
highly dependent on the structure. The HSAPO-11 catalyst had the lowest upgrading efficiency at 42.4 wt% and the lowest hydrocarbon yield at 80 wt%, while the
HMCM-41 catalyst had the highest upgrading efficiency at 54.5 wt% and the highest hydrocarbon yield at 95.6 wt%. In another study, the zeolite HZSM-5 was used
in upgrading biocrude oil with H 2 at high pressure [148]. Temperatures (400–500 °C)
and catalyst loadings (5–50 wt%) were varied. All S-, O-, and N-containing compounds were significantly reduced, and the highest alkane yield was achieved at
400 °C. The authors hypothesized that the dominant upgrading reactions were
hydrotreating (hydrodesulfurization, hydrodeoxygenation, hydrodenitrogenation),
hydrogenation, hydrocracking, and aromatization [97].
In terms of metal catalysts, Raney-Ni catalyst was found to be more effective
than Ru/C or a combination of Raney-Ni and Ru/C because it mediated a fuel product with 87 wt% alkanes, in contrast to the other two catalysts which mediated fuel
products with <60 wt% alkanes [141]. Zhang et al. has shown in their experiment
that the yields mediated by these three catalysts were as follows: Ru/C > Pt/C > Mo 2 C
at 350 °C and 6 MPa H 2 pressure [144]. It has been shown that the deoxygenation
activities of Pt/γ-Al 2 O 3 , Ru/C, Ru/C + alumina, MCM-41 (100% Si), and
Ru/C + Mo 2 C in algae valorization were high [35]. Ru/C + alumina, MCM-41
(100% Si), Pt/γ-Al 2 O 3 , Pt/C, and Pd/C were excellent for hydrogenation, and
Ru/C + Pt/C was ideal for desulfurization [35].
A method of improving a heterogeneous catalyst’s activity is to combine two or
more of them, known as the two-component or multicomponent catalyst system,
respectively. This is done to improve the catalyst’s ability to remove heteroatoms
from biocrude oil (i.e., better-quality fuel with high concentration of fuel-range
hydrocarbons) or increase fuel yield [95, 147]. An example is the combination of
Ru/C with Raney-Ni (Ru/C + Raney-Ni). Introducing H 2 to the biocrude oil at
400 °C in the presence of the two-component catalyst system boosted the fuel’s
yield and C and H contents. In fact, fuel yield reached 77.2 wt%, HHV was determined to be 45.2 MJ/kg, and energy yield (i.e., ratio of the energy contained in fuel
versus the energy contained in biocrude oil) reached 86 wt% [141]. In a similar
manner, a multicomponent system of catalysts containing Ru/C + Pt/C, Ru/C + Mo 2 C,
and Ru/C + Pt/γ-Al 2 O 3 achieved enhanced deoxygenation, denitrogenation, and
desulfurization activities [95].
Lastly, solvent plays an important role in microalgae valorization. Water is the
commonly used solvent, and it influences the catalytic activities of heterogeneous
catalysts. Some of the documented effects of water on catalyst and fuel are as follows: (1) Using subcritical water, increasing Pt/C-sulfide catalyst from 0 to 20 wt%,
lowers S and O contents, raises C and H contents, and increases the energy density
and HHV of the fuel [142]; (2) using water, the catalytic activity of HZSM-5 is
improved [107] which stimulates decarboxylation to generate more CO 2 [143].
Some of the challenges associated with heterogeneous catalysts in microalgae
valorization include (1) deactivation due to solid residue (coke) formation [115,
E. P. Resurreccion and S. Kumar