171
and CO) and the aqueous phase (e.g., organics). Second, it determines the amount
of alkanes necessary to satisfy the requirements for gasoline, jet fuel, and diesel.
Such amount is a function of the nature of the metal species in the zeolite. Third, it
determines the degree of the dehydration process which determines the amount of
water vapor in the gaseous phase. The goal of biocrude oil valorization is to satisfy
the ASTM D 6751 biodiesel standard [138] in terms of the following properties: S
content, total acid number, viscosity, density, HHV, and distillation temperature at
which 90 wt% of O-containing compounds are converted to fuel-range hydrocarbons (alkanes and aromatics). The O from oxygenates, mainly produced from carbohydrates, lowers the HHV and energy density of the fuel and therefore must be
diminished (deoxygenation). The N and S from these compounds must also be
reduced since (1) they cause the release of NO x and SO x that pollutes air and (2) they
degrade combustion engines through corrosion (denitrogenation and desulfurization) [97, 139, 140]. One of the most efficient and effective upgrading methods is
hydrothermal valorization, a process implemented at high temperatures with water
as exclusive H donor (or in combination with added H 2 ) and heterogeneous catalyst
to eliminate O, N, and S heteroatoms [141, 142].
Recent studies on biocrude valorization primarily centered on zeolites and metal
catalysts, as these catalysts have been shown to consistently produce high-quality
(yield, hydrocarbons) fuels. Zeolites and metal-modified zeolites, in particular, are
effective in generating alkanes and aromatics [143, 144]. Zeolites enhance octane
properties of fuels through mediating hydrogenation and isomerization reactions
critical to increasing isomer concentration in the fuel [145, 146]. Zeolites were
shown (1) to effectively mediate dehydration, isomerization, and alkylation of HTLderived biocrude oil from algae, (2) to not get deactivated by coking, and (3) to
possess high tolerance to steam [78]. One of these studies compared the catalytic
activities of HZSM-5 and Pt/Al 2 O 3 catalysts in upgrading Scenedesmus almeriensis- and Nannochloropsis gaditana-derived biocrude oil [14]. Temperature was set
at 400 °C, reaction period at 4 h, and catalyst concentration at 0.3 g/0.55 mL DI
water. Results indicated that the yields for both catalysts employed to both algae
were remarkably similar under the experimental condition applied. Specifically, the
fuel products contained significant concentrations of pentadecane and hexadecane,
while the gaseous products composed of C1–C4 alkanes, CO 2 , and CO. The authors
hypothesized that pentadecane, hexadecane, and C1–C4 alkanes were produced
from the hydrogenation of alkenes, while CO 2 and CO were produced from decarboxylation of fatty acids.
The zeolite HZSM-5 was compared to metal catalysts Pt/C and Mo 2 C by Duan
and Savage [143]. Results showed that at 430 °C, S-, O-, and N-containing compounds were greatly reduced using the three catalysts, with Mo 2 C mediating the
highest saturated hydrocarbons at 76 wt%. However, when the temperature was
raised to 530 °C, Pt/C and HZSM-5 mediated 90 wt% aromatics, while Mo 2 C mediated only 88 wt% aromatics. Duan and colleagues again tested zeolite in terms of
the effect of the catalyst’s structure on its activity and selectivity [147]. The results
revealed that activity is independent of structure, while selectivity is structuredependent. Duan and colleagues followed this study with an evaluation of different
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae
and CO) and the aqueous phase (e.g., organics). Second, it determines the amount
of alkanes necessary to satisfy the requirements for gasoline, jet fuel, and diesel.
Such amount is a function of the nature of the metal species in the zeolite. Third, it
determines the degree of the dehydration process which determines the amount of
water vapor in the gaseous phase. The goal of biocrude oil valorization is to satisfy
the ASTM D 6751 biodiesel standard [138] in terms of the following properties: S
content, total acid number, viscosity, density, HHV, and distillation temperature at
which 90 wt% of O-containing compounds are converted to fuel-range hydrocarbons (alkanes and aromatics). The O from oxygenates, mainly produced from carbohydrates, lowers the HHV and energy density of the fuel and therefore must be
diminished (deoxygenation). The N and S from these compounds must also be
reduced since (1) they cause the release of NO x and SO x that pollutes air and (2) they
degrade combustion engines through corrosion (denitrogenation and desulfurization) [97, 139, 140]. One of the most efficient and effective upgrading methods is
hydrothermal valorization, a process implemented at high temperatures with water
as exclusive H donor (or in combination with added H 2 ) and heterogeneous catalyst
to eliminate O, N, and S heteroatoms [141, 142].
Recent studies on biocrude valorization primarily centered on zeolites and metal
catalysts, as these catalysts have been shown to consistently produce high-quality
(yield, hydrocarbons) fuels. Zeolites and metal-modified zeolites, in particular, are
effective in generating alkanes and aromatics [143, 144]. Zeolites enhance octane
properties of fuels through mediating hydrogenation and isomerization reactions
critical to increasing isomer concentration in the fuel [145, 146]. Zeolites were
shown (1) to effectively mediate dehydration, isomerization, and alkylation of HTLderived biocrude oil from algae, (2) to not get deactivated by coking, and (3) to
possess high tolerance to steam [78]. One of these studies compared the catalytic
activities of HZSM-5 and Pt/Al 2 O 3 catalysts in upgrading Scenedesmus almeriensis- and Nannochloropsis gaditana-derived biocrude oil [14]. Temperature was set
at 400 °C, reaction period at 4 h, and catalyst concentration at 0.3 g/0.55 mL DI
water. Results indicated that the yields for both catalysts employed to both algae
were remarkably similar under the experimental condition applied. Specifically, the
fuel products contained significant concentrations of pentadecane and hexadecane,
while the gaseous products composed of C1–C4 alkanes, CO 2 , and CO. The authors
hypothesized that pentadecane, hexadecane, and C1–C4 alkanes were produced
from the hydrogenation of alkenes, while CO 2 and CO were produced from decarboxylation of fatty acids.
The zeolite HZSM-5 was compared to metal catalysts Pt/C and Mo 2 C by Duan
and Savage [143]. Results showed that at 430 °C, S-, O-, and N-containing compounds were greatly reduced using the three catalysts, with Mo 2 C mediating the
highest saturated hydrocarbons at 76 wt%. However, when the temperature was
raised to 530 °C, Pt/C and HZSM-5 mediated 90 wt% aromatics, while Mo 2 C mediated only 88 wt% aromatics. Duan and colleagues again tested zeolite in terms of
the effect of the catalyst’s structure on its activity and selectivity [147]. The results
revealed that activity is independent of structure, while selectivity is structuredependent. Duan and colleagues followed this study with an evaluation of different
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae