167
chemical properties. First, carbonate-, hydroxide-, and carboxylic acid-based catalysts have low effectiveness in catalyzing fatty acid decarboxylation, isomerization,
and aromatization [78]. These reactions improve biocrude oil yield. Second, they
inhibit denitrogenation, deoxygenation, and desulfurization [78]. These reactions
improve biocrude oil quality. Third, they are consumed in the reaction and are difficult to recycle [60]. Fourth, they advance corrosion of tanks and equipment by
increasing the pH of the HTL reaction [122]. Insomuch as all these disadvantages
are concerned, scientists direct their research efforts to heterogeneous catalysts.
7.2 Heterogeneous Catalysis
To overcome the negative consequences of homogenous catalyst-mediated HTL of
microalgae biomass, heterogeneous catalysts are widely adopted. They are primarily employed in the valorization or upgrading of biocrude oil. They are solid and
insoluble in the reaction medium [81, 123, 124], have high recyclability after several uses [24, 68], have low corrosion rate [10, 113, 115], and have high catalytic
activity. Heterogeneous catalysts elicit intermediate reactions [13, 109] through
subsequent adsorption-desorption of reactants into their surface. Extensively
researched heterogeneous catalysts include metals (e.g., Ru, Pd, Pt, Ni) supported
on Al 2 O 3 , activated carbon, or zeolite, metal oxides, and molecular sieves [35].
Heterogeneous catalysts come in many forms such as oxides (SiO 2 and Al 2 O 3 ), zeolites, and metals supported by C (e.g., Pd/C and Pt/C). Of primary interest to
researchers are their catalytic activity and their effects on the biocrude oil products
including gaseous phase, aqueous phase, and solid residue.
In a review by Xu and colleagues [35], these following findings are highlighted
about heterogeneous catalysts. The catalytic deoxygenation performance of metal
catalysts such as Ru/C, Pt/Al 2 O 3 , Pt/C, NiO, Ni/SiO 2 -Al 2 O 3 , Ni-Mo/Al 2 O 3 , CoMo/
γ-Al 2 O 3 , Co/Mo/Al 2 O 3 , and cesium on zeolite socony mobil-5 (Ce/HZSM-5) were
excellent at 2–51 wt% (i.e., percent of microalgae components that are deoxygenized). Co/Mo/Al 2 O 3 was found to have the highest deoxygenating efficiency. In
terms of hydrogenation, Pd/C, Pt/Al 2 O 3 , Pt/C, NiO, Ni-Mo/Al 2 O 3 , and Ce/HZSM-5
demonstrated 1–52 wt%. The ability of Pt/Al 2 O 3 , Ni/Al 2 O 3 , Co/Mo/Al 2 O 3 , and Ce/
HZSM-5 in increasing carbon content of the biocrude oil was demonstrated to be
9–15 wt%. Moreover, these heterogeneous catalysts were shown to increase C and
H and to lower O (i.e., raise HHV): Ru/C, Pt/Al 2 O 3 , Pt/C, NiO, Ni-Mo/Al 2 O 3 , Co/
Mo/Al 2 O 3 , Ce/HZSM-5, and Ca 3 (PO 4 ) 2 . With respect to desulfurization, Ru/C,
Pd/C, Pt/C, Ni/SiO 2 -Al 2 O 3 , CoMo/γ-Al 2 O 3 , and Ce/HZSM-5 showed 6–100 wt%
efficiency, with Ni/SiO 2 -Al 2 O 3 demonstrating the highest activity with or without
H 2 . Meanwhile, NiO and Ca 3 (PO 4 ) 2 exhibited unfavorable desulfurization activity.
The removal of N (denitrogenation), on the other hand, was positively mediated by
Ru/C, Pt/Al 2 O 3 , Pt/C, Ni/SiO 2 -Al 2 O 3 , Ni/Al 2 O 3 , Ni-Mo/Al 2 O 3 , Ce/HZSM-5, and
Ca3(PO 4 ) 2 at approximately 10 wt% efficiency. In comparison to metal catalysts,
zeolite has lower catalytic efficiency with respect to biocrude oil yield and quality,
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae
chemical properties. First, carbonate-, hydroxide-, and carboxylic acid-based catalysts have low effectiveness in catalyzing fatty acid decarboxylation, isomerization,
and aromatization [78]. These reactions improve biocrude oil yield. Second, they
inhibit denitrogenation, deoxygenation, and desulfurization [78]. These reactions
improve biocrude oil quality. Third, they are consumed in the reaction and are difficult to recycle [60]. Fourth, they advance corrosion of tanks and equipment by
increasing the pH of the HTL reaction [122]. Insomuch as all these disadvantages
are concerned, scientists direct their research efforts to heterogeneous catalysts.
7.2 Heterogeneous Catalysis
To overcome the negative consequences of homogenous catalyst-mediated HTL of
microalgae biomass, heterogeneous catalysts are widely adopted. They are primarily employed in the valorization or upgrading of biocrude oil. They are solid and
insoluble in the reaction medium [81, 123, 124], have high recyclability after several uses [24, 68], have low corrosion rate [10, 113, 115], and have high catalytic
activity. Heterogeneous catalysts elicit intermediate reactions [13, 109] through
subsequent adsorption-desorption of reactants into their surface. Extensively
researched heterogeneous catalysts include metals (e.g., Ru, Pd, Pt, Ni) supported
on Al 2 O 3 , activated carbon, or zeolite, metal oxides, and molecular sieves [35].
Heterogeneous catalysts come in many forms such as oxides (SiO 2 and Al 2 O 3 ), zeolites, and metals supported by C (e.g., Pd/C and Pt/C). Of primary interest to
researchers are their catalytic activity and their effects on the biocrude oil products
including gaseous phase, aqueous phase, and solid residue.
In a review by Xu and colleagues [35], these following findings are highlighted
about heterogeneous catalysts. The catalytic deoxygenation performance of metal
catalysts such as Ru/C, Pt/Al 2 O 3 , Pt/C, NiO, Ni/SiO 2 -Al 2 O 3 , Ni-Mo/Al 2 O 3 , CoMo/
γ-Al 2 O 3 , Co/Mo/Al 2 O 3 , and cesium on zeolite socony mobil-5 (Ce/HZSM-5) were
excellent at 2–51 wt% (i.e., percent of microalgae components that are deoxygenized). Co/Mo/Al 2 O 3 was found to have the highest deoxygenating efficiency. In
terms of hydrogenation, Pd/C, Pt/Al 2 O 3 , Pt/C, NiO, Ni-Mo/Al 2 O 3 , and Ce/HZSM-5
demonstrated 1–52 wt%. The ability of Pt/Al 2 O 3 , Ni/Al 2 O 3 , Co/Mo/Al 2 O 3 , and Ce/
HZSM-5 in increasing carbon content of the biocrude oil was demonstrated to be
9–15 wt%. Moreover, these heterogeneous catalysts were shown to increase C and
H and to lower O (i.e., raise HHV): Ru/C, Pt/Al 2 O 3 , Pt/C, NiO, Ni-Mo/Al 2 O 3 , Co/
Mo/Al 2 O 3 , Ce/HZSM-5, and Ca 3 (PO 4 ) 2 . With respect to desulfurization, Ru/C,
Pd/C, Pt/C, Ni/SiO 2 -Al 2 O 3 , CoMo/γ-Al 2 O 3 , and Ce/HZSM-5 showed 6–100 wt%
efficiency, with Ni/SiO 2 -Al 2 O 3 demonstrating the highest activity with or without
H 2 . Meanwhile, NiO and Ca 3 (PO 4 ) 2 exhibited unfavorable desulfurization activity.
The removal of N (denitrogenation), on the other hand, was positively mediated by
Ru/C, Pt/Al 2 O 3 , Pt/C, Ni/SiO 2 -Al 2 O 3 , Ni/Al 2 O 3 , Ni-Mo/Al 2 O 3 , Ce/HZSM-5, and
Ca3(PO 4 ) 2 at approximately 10 wt% efficiency. In comparison to metal catalysts,
zeolite has lower catalytic efficiency with respect to biocrude oil yield and quality,
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae