169
catalyst [118]. Greater than 41% of the biocrude oil generated had extraordinarily
little N. The extreme lowering of biocrude oil N was attained at 550 °C, 10 K/min
heating rate, and 0.75 catalyst/substrate ratio.
There is little question that heterogeneous catalysts offer a superior task in forming high-yield and high-quality biocrude oil. However, biocrude oil is not the sole
basis in their large-scale industry adoption. Other parameters that are directly or
indirectly affected by the type of heterogeneous catalyst need further evaluation.
These catalyst properties include (1) stabilization, (2) effects on gaseous products,
and (3) other factors.
Stabilization. Heterogeneous catalysts can precipitate, sinter, dissolve, or diffuse
as a result of microalgal impurities and reaction condition [14]. High N and O concentrations, extreme temperatures and pressures, and prolonged reaction times contribute to the catalyst’s deactivation [127]. The studies detailed prior are bench-scale
investigations that pertain mostly to catalytic effects on biocrude yield and quality
but not on the heterogeneous catalyst’s recoverability from its solid products [123].
More studies concerning fundamental knowledge on recyclability and methods of
preventing deactivation are warranted.
Effects on Gaseous Products. Gas yield (approximately 5 wt% of product yield)
and composition are considerably affected by heterogeneous catalyst. The composition of H 2 , CH 4 , CO, CO 2 , and C 2 varies depending upon the catalyst employed
[128, 129]. Metal catalysts such as Ru/C, Pd/C, Pt/Al 2 O 3 , Pt/C, Raney-Ni, Ni-Mo/
Al 2 O 3 , and HZSM-5 generally increase CO 2 concentration in the gas phase and
hydrocarbons in the biocrude oil via decarboxylation reaction [1, 105, 120]. Ru/C
and Ni/SiO 2 -Al 2 O 3 are also responsible for increasing H 2 , CH 4 , and C 2 H 6 fractions
in the gas phase, also via decarboxylation reaction [122]. The presence of significant amount of CH 4 and C 2 H 6 in the gas phase is also linked to the use of Pt/C catalyst which is known to promote cracking during microalgal liquefaction [130].
Other Factors. The nature of material support base by heterogeneous catalyst
dictates which reaction is favored in an HTL setup. Catalysts with similar support
base influence similar chemical reactions, but when they are altered, biocrude oil
yield and quality are modified. For example, the catalysts Pt/Al 2 O 3 , Ni/Al 2 O 3 , and
Co/Mo/Al 2 O 3 all share similar aluminum oxide base which at 350 °C generated
individual specific biocrude oil yield from Nannochloropsis HTL [31]. Nonetheless,
when Al 2 O 3 was changed to Pt/C, Ni/SiO 2 -Al 2 O 3 , or CoMo/γ-Al 2 O 3 , the yield for all
three catalyst-mediated experiments were increased [41, 131]. Certain metals are
linked to certain reactions. For instance, Pt-based catalyst enhances hydrogenation,
while Co-Mo promotes deoxygenation [41]. Changes in reaction temperature using
a particular metal catalyst affect biocrude oil yield [31]. A study verified that the
biocrude oil yield for all experiments using Pd/C, Pd/Al 2 O 3 , Pt/C, Pt/Al 2 O 3 , and
Raney-Ni catalysts at 240 °C increased when temperature is raised to 280 °C [99].
Lastly, the reaction atmosphere (i.e., presence or absence of H 2 ) in conjunction with
metal catalyst or zeolite dictated the yield and quality of biocrude oil. For example,
zeolite with H 2 increased the biocrude oil yield, while zeolite without H 2 decreased
it [41] (Table 2).
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae
catalyst [118]. Greater than 41% of the biocrude oil generated had extraordinarily
little N. The extreme lowering of biocrude oil N was attained at 550 °C, 10 K/min
heating rate, and 0.75 catalyst/substrate ratio.
There is little question that heterogeneous catalysts offer a superior task in forming high-yield and high-quality biocrude oil. However, biocrude oil is not the sole
basis in their large-scale industry adoption. Other parameters that are directly or
indirectly affected by the type of heterogeneous catalyst need further evaluation.
These catalyst properties include (1) stabilization, (2) effects on gaseous products,
and (3) other factors.
Stabilization. Heterogeneous catalysts can precipitate, sinter, dissolve, or diffuse
as a result of microalgal impurities and reaction condition [14]. High N and O concentrations, extreme temperatures and pressures, and prolonged reaction times contribute to the catalyst’s deactivation [127]. The studies detailed prior are bench-scale
investigations that pertain mostly to catalytic effects on biocrude yield and quality
but not on the heterogeneous catalyst’s recoverability from its solid products [123].
More studies concerning fundamental knowledge on recyclability and methods of
preventing deactivation are warranted.
Effects on Gaseous Products. Gas yield (approximately 5 wt% of product yield)
and composition are considerably affected by heterogeneous catalyst. The composition of H 2 , CH 4 , CO, CO 2 , and C 2 varies depending upon the catalyst employed
[128, 129]. Metal catalysts such as Ru/C, Pd/C, Pt/Al 2 O 3 , Pt/C, Raney-Ni, Ni-Mo/
Al 2 O 3 , and HZSM-5 generally increase CO 2 concentration in the gas phase and
hydrocarbons in the biocrude oil via decarboxylation reaction [1, 105, 120]. Ru/C
and Ni/SiO 2 -Al 2 O 3 are also responsible for increasing H 2 , CH 4 , and C 2 H 6 fractions
in the gas phase, also via decarboxylation reaction [122]. The presence of significant amount of CH 4 and C 2 H 6 in the gas phase is also linked to the use of Pt/C catalyst which is known to promote cracking during microalgal liquefaction [130].
Other Factors. The nature of material support base by heterogeneous catalyst
dictates which reaction is favored in an HTL setup. Catalysts with similar support
base influence similar chemical reactions, but when they are altered, biocrude oil
yield and quality are modified. For example, the catalysts Pt/Al 2 O 3 , Ni/Al 2 O 3 , and
Co/Mo/Al 2 O 3 all share similar aluminum oxide base which at 350 °C generated
individual specific biocrude oil yield from Nannochloropsis HTL [31]. Nonetheless,
when Al 2 O 3 was changed to Pt/C, Ni/SiO 2 -Al 2 O 3 , or CoMo/γ-Al 2 O 3 , the yield for all
three catalyst-mediated experiments were increased [41, 131]. Certain metals are
linked to certain reactions. For instance, Pt-based catalyst enhances hydrogenation,
while Co-Mo promotes deoxygenation [41]. Changes in reaction temperature using
a particular metal catalyst affect biocrude oil yield [31]. A study verified that the
biocrude oil yield for all experiments using Pd/C, Pd/Al 2 O 3 , Pt/C, Pt/Al 2 O 3 , and
Raney-Ni catalysts at 240 °C increased when temperature is raised to 280 °C [99].
Lastly, the reaction atmosphere (i.e., presence or absence of H 2 ) in conjunction with
metal catalyst or zeolite dictated the yield and quality of biocrude oil. For example,
zeolite with H 2 increased the biocrude oil yield, while zeolite without H 2 decreased
it [41] (Table 2).
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae