168
albeit cheap to produce. Notably, molecular sieve catalysts such as HZSM-5 have
high catalytic cracking effects (i.e., produce more alkanes, alkenes, and ketones in
biocrude oil with minimal organic acids) but quite inexpensive [26, 50, 105].
Interestingly, ideal microalgae HTL and biocrude oil upgrading conditions utilizing
these heterogeneous catalysts are close to 350 °C and 60 min. Some of these recent
studies are outlined herein. They discuss catalytic effects from a purely biocrude oil
yield and quality standpoint.
In a study involving the upgrade of Chlorella vulgaris and Nannochloropsis
occulta sp., Pt/Al 2 O 3 , Ni/Al 2 O 3 , and Co/Al 2 O 3 were used as catalysts by impregnating the alumina support with the metal at 20 wt%, 6 wt%, and 20 wt%, respectively
[31]. Experimental conditions included 350 °C, 1 h, and 1:9 catalyst/water ratio.
Results for the Chlorella vulgaris showed that the use of Pt/Al 2 O 3 and Co/Al 2 O 3
increased biocrude oil yield by 3%, while the use of Ni/Al 2 O 3 decreased biocrude
oil yield by 6% from the yield without catalysts (36 wt%). In the case of
Nannochloropsis occulta sp., all three catalysts lowered biocrude oil yield. Pt/Al 2 O 3
and Co/Al 2 O 3 were effective in cleaving C-C and C-O bonds, and they also reinforced the occurrence of deoxygenation of oxygenates, which were mostly generated from carbohydrates in the biomass. In another Nannochloropsis occulta sp.
investigation, Duan and colleagues demonstrated that Pd/C and CoMo/Al 2 O 3 raised
biocrude oil yield by 22 wt% and 20 wt%, respectively, from 35 wt% without catalyst [97]. Another metal-supported catalyst study utilized Pd/C in the liquefaction of
Nannochloropsis sp. in a system that isolated the catalyst from the biocrude oil and
solid residue [98]. Similar to the Duan study, biocrude oil was increased to 40 wt%
and 38 wt% at 10-bar and 30-bar H 2 pressure, respectively, from the 35 wt% yield
in the uncatalyzed control.
Nickel supported by rare-earth and ammonia exchange (Ni/REHY) catalyst was
researched by Yang et al. in the HTL and upgrading of Dunaliella sp. at 200 °C,
2 MPa, 1 h, and 2 g Ni/REHY at 90 mL solvent [98]. A biocrude oil yield of 35 wt%
was achieved for the reaction without catalyst, 52 wt% yield for REHY-mediated
reaction, and 72 wt% yield for Ni/REHY-mediated reaction. Raney nickel
(Raney-Ni) and zeolite socony mobil-5 (H-ZSM-5) catalyst systems were utilized
by Zhang et al. in the HTL of Chlorella sp. using ethanol as solvent [125]. Reaction
conditions were as follows: 200–300 °C temperature range, 2.8–2.9 MPa pressure
range, and 30 min reaction time. Although there was no significant increase in biocrude oil yield for both catalytic experimental setups across the range of temperatures and pressures, product analysis revealed the formation of gasoline-range
hydrocarbons. Such chemicals indicate bond cleavage, depolymerization, and
hydrogenation processes took place. A rise in biocrude oil yield from 32 wt%
(uncatalyzed) to 38 wt% (H-ZSM-5) and 52 wt% (Ce/H-ZSM-5) was realized in Xu
et al.’s study that employed C. pyrenoidosa sp. liquefaction [126]. The study concluded that the incorporation of Ce in the zeolite enhanced yield in terms of both
biocrude oil and hydrocarbons.
The quality of high-yield biocrude oil was investigated with regard to its N content through the valorization of Cyanobacteria microalgae using magnesium
aluminum- layered double oxide/zeolite socony mobil-5 (MgAl-LDO/HZSM-5)
E. P. Resurreccion and S. Kumar
albeit cheap to produce. Notably, molecular sieve catalysts such as HZSM-5 have
high catalytic cracking effects (i.e., produce more alkanes, alkenes, and ketones in
biocrude oil with minimal organic acids) but quite inexpensive [26, 50, 105].
Interestingly, ideal microalgae HTL and biocrude oil upgrading conditions utilizing
these heterogeneous catalysts are close to 350 °C and 60 min. Some of these recent
studies are outlined herein. They discuss catalytic effects from a purely biocrude oil
yield and quality standpoint.
In a study involving the upgrade of Chlorella vulgaris and Nannochloropsis
occulta sp., Pt/Al 2 O 3 , Ni/Al 2 O 3 , and Co/Al 2 O 3 were used as catalysts by impregnating the alumina support with the metal at 20 wt%, 6 wt%, and 20 wt%, respectively
[31]. Experimental conditions included 350 °C, 1 h, and 1:9 catalyst/water ratio.
Results for the Chlorella vulgaris showed that the use of Pt/Al 2 O 3 and Co/Al 2 O 3
increased biocrude oil yield by 3%, while the use of Ni/Al 2 O 3 decreased biocrude
oil yield by 6% from the yield without catalysts (36 wt%). In the case of
Nannochloropsis occulta sp., all three catalysts lowered biocrude oil yield. Pt/Al 2 O 3
and Co/Al 2 O 3 were effective in cleaving C-C and C-O bonds, and they also reinforced the occurrence of deoxygenation of oxygenates, which were mostly generated from carbohydrates in the biomass. In another Nannochloropsis occulta sp.
investigation, Duan and colleagues demonstrated that Pd/C and CoMo/Al 2 O 3 raised
biocrude oil yield by 22 wt% and 20 wt%, respectively, from 35 wt% without catalyst [97]. Another metal-supported catalyst study utilized Pd/C in the liquefaction of
Nannochloropsis sp. in a system that isolated the catalyst from the biocrude oil and
solid residue [98]. Similar to the Duan study, biocrude oil was increased to 40 wt%
and 38 wt% at 10-bar and 30-bar H 2 pressure, respectively, from the 35 wt% yield
in the uncatalyzed control.
Nickel supported by rare-earth and ammonia exchange (Ni/REHY) catalyst was
researched by Yang et al. in the HTL and upgrading of Dunaliella sp. at 200 °C,
2 MPa, 1 h, and 2 g Ni/REHY at 90 mL solvent [98]. A biocrude oil yield of 35 wt%
was achieved for the reaction without catalyst, 52 wt% yield for REHY-mediated
reaction, and 72 wt% yield for Ni/REHY-mediated reaction. Raney nickel
(Raney-Ni) and zeolite socony mobil-5 (H-ZSM-5) catalyst systems were utilized
by Zhang et al. in the HTL of Chlorella sp. using ethanol as solvent [125]. Reaction
conditions were as follows: 200–300 °C temperature range, 2.8–2.9 MPa pressure
range, and 30 min reaction time. Although there was no significant increase in biocrude oil yield for both catalytic experimental setups across the range of temperatures and pressures, product analysis revealed the formation of gasoline-range
hydrocarbons. Such chemicals indicate bond cleavage, depolymerization, and
hydrogenation processes took place. A rise in biocrude oil yield from 32 wt%
(uncatalyzed) to 38 wt% (H-ZSM-5) and 52 wt% (Ce/H-ZSM-5) was realized in Xu
et al.’s study that employed C. pyrenoidosa sp. liquefaction [126]. The study concluded that the incorporation of Ce in the zeolite enhanced yield in terms of both
biocrude oil and hydrocarbons.
The quality of high-yield biocrude oil was investigated with regard to its N content through the valorization of Cyanobacteria microalgae using magnesium
aluminum- layered double oxide/zeolite socony mobil-5 (MgAl-LDO/HZSM-5)
E. P. Resurreccion and S. Kumar