165
(non-water- soluble). The role of catalysts is to facilitate the degradation of the biomass into biocrude oil at hydrothermal conditions or to upgrade biocrude oil into
fuel grade with appropriate hydrocarbon composition. Homogeneous catalysts such
as salts of Na and K have been extensively investigated in the HTL of microalgae
[41, 56, 57]. Na and K salts promote the formation of CO 2 and H 2 by hastening the
water gas shift reaction. These salts also increase the HHV of biocrude oil [18].
These catalysts maintain water in the biomass, thereby promoting decarboxylation
(deoxygenation via the removal of CO 2 ). However, evidence suggests that homogeneous catalysts increase the oxygen content of biocrude oil [72]. In contrast, heterogeneous catalysts are used in the valorization or upgrade of biocrude oil. They
inadvertently reduce biocrude oil yield due to increased gas phase formation, and
there are also challenges associated with intra-particle diffusion limitations, fouling,
and sintering [12]. Despite this, a research confirmed that a slight increase in biocrude oil yield and a pronounced gain in HHV were realized when a heterogeneous
catalyst was employed [31].
Catalytic microalgae HTL uses water as solvent. Water below the critical point
(i.e., subcritical water or hot compressed water) makes the hydrophobic organic
components of the microalgae more soluble and the presence of ions conducive for
intermediate reactions necessary in the production of biocrude [56]. Water above
the critical point (i.e., supercritical water (SCW)), on the contrary, promotes the
production of radical species and gaseous hydrocarbons (CH 4 , C2–C4 gases).
Regardless of the type of catalyst or solvent, HTL involves the degradation of the
macrocomponents of microalgae, the rate of which depends on the species/strain
[110]. Specifically, lipids degrade into fatty acids and glycerol, while carbohydrates
depolymerize into aldehydes and ketones [32, 111]. The fatty acids produced from
lipids are further decarboxylated to form corresponding straight- chain hydrocarbons (alkanes, alkenes), depending on the R-structure. Proteins are hydrolyzed at
hydrothermal conditions to produce amino acid monomers which are further converted into N-containing compounds such as amines, ammonia, etc. The algaenan
component of microalgae can be converted into alkanes, a potential “drop- in” fuel
in replacement for gasoline, diesel, and jet fuel [17, 61]. The mechanisms of these
reactions are yet to be fully understood, although catalysts’ role has been regarded
by studies to promote or deter the intermediate reactions of depolymerization [61,
112]. Thus, the actual composition of biocrude oil is dependent not only on microalgal species/strain but also on catalyst’s activity.
7.1 Homogeneous Catalysis
Homogeneous catalysts are catalysts that are soluble at room temperature in the
solvent medium used in the reaction system. They were first developed for use in the
HTL of algae biomass and are almost exclusively used for that purpose. They exist
in the form of organic acids (e.g., CH 3 COOH, HCOOH), their organic salts (e.g.,
CH 3 COONa, HCOONa), alkali salts (e.g., Na 2 CO 3 , KOH), and cations of transition
elements (e.g., Co
3+
, Zn
2+
). Homogeneous catalysts aid in breaking the C-C bond in
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae
(non-water- soluble). The role of catalysts is to facilitate the degradation of the biomass into biocrude oil at hydrothermal conditions or to upgrade biocrude oil into
fuel grade with appropriate hydrocarbon composition. Homogeneous catalysts such
as salts of Na and K have been extensively investigated in the HTL of microalgae
[41, 56, 57]. Na and K salts promote the formation of CO 2 and H 2 by hastening the
water gas shift reaction. These salts also increase the HHV of biocrude oil [18].
These catalysts maintain water in the biomass, thereby promoting decarboxylation
(deoxygenation via the removal of CO 2 ). However, evidence suggests that homogeneous catalysts increase the oxygen content of biocrude oil [72]. In contrast, heterogeneous catalysts are used in the valorization or upgrade of biocrude oil. They
inadvertently reduce biocrude oil yield due to increased gas phase formation, and
there are also challenges associated with intra-particle diffusion limitations, fouling,
and sintering [12]. Despite this, a research confirmed that a slight increase in biocrude oil yield and a pronounced gain in HHV were realized when a heterogeneous
catalyst was employed [31].
Catalytic microalgae HTL uses water as solvent. Water below the critical point
(i.e., subcritical water or hot compressed water) makes the hydrophobic organic
components of the microalgae more soluble and the presence of ions conducive for
intermediate reactions necessary in the production of biocrude [56]. Water above
the critical point (i.e., supercritical water (SCW)), on the contrary, promotes the
production of radical species and gaseous hydrocarbons (CH 4 , C2–C4 gases).
Regardless of the type of catalyst or solvent, HTL involves the degradation of the
macrocomponents of microalgae, the rate of which depends on the species/strain
[110]. Specifically, lipids degrade into fatty acids and glycerol, while carbohydrates
depolymerize into aldehydes and ketones [32, 111]. The fatty acids produced from
lipids are further decarboxylated to form corresponding straight- chain hydrocarbons (alkanes, alkenes), depending on the R-structure. Proteins are hydrolyzed at
hydrothermal conditions to produce amino acid monomers which are further converted into N-containing compounds such as amines, ammonia, etc. The algaenan
component of microalgae can be converted into alkanes, a potential “drop- in” fuel
in replacement for gasoline, diesel, and jet fuel [17, 61]. The mechanisms of these
reactions are yet to be fully understood, although catalysts’ role has been regarded
by studies to promote or deter the intermediate reactions of depolymerization [61,
112]. Thus, the actual composition of biocrude oil is dependent not only on microalgal species/strain but also on catalyst’s activity.
7.1 Homogeneous Catalysis
Homogeneous catalysts are catalysts that are soluble at room temperature in the
solvent medium used in the reaction system. They were first developed for use in the
HTL of algae biomass and are almost exclusively used for that purpose. They exist
in the form of organic acids (e.g., CH 3 COOH, HCOOH), their organic salts (e.g.,
CH 3 COONa, HCOONa), alkali salts (e.g., Na 2 CO 3 , KOH), and cations of transition
elements (e.g., Co
3+
, Zn
2+
). Homogeneous catalysts aid in breaking the C-C bond in
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae