163
by Biller et al. validated that the decarboxylation of fatty acids to form alkanes is
achieved in the presence of Na 2 CO 3 [39]. The upgraded biocrude oil is found to
contain higher straight-chain alkanes, higher HHV, lower fraction of high boilers,
and lower oxygen relative to crude bio-oil. The catalysts Pd/C, Pt/C, Pt/Al 2 O 3 ,
Raney-Ni, and HZSM-5 are also beneficial in upgrading biocrude from direct catalytic HTL because they enhance alkane fractions and lower fatty acid content resulting in a better quality biocrude [41, 83, 97–99]. Alkali catalysts (NaOH, KOH) are
appropriate in the monomolecular decarboxylation of stearic acid, while metal
oxide catalysts (CeO 2 , Y 2 O 3 , ZrO 2 ) are suitable for the bimolecular decarboxylation
of stearic acid [100].
Denitrogenation. After HTL, the N from the microalgae biomass is partitioned in
the biocrude oil as N-containing compounds, in the aqueous phase as HCN and
NH 3 , and in the solid phase [72, 82]. N-containing compounds can be heterocyclic
(ring compounds with at least two different elements in its ring) such as pyrazine,
cyclic oxygenates, indole, quinoline, and pyrrole or non-heterocyclic such as aliphatic amines (straight-chain amines), aniline (amino group with phenyl group
attachment), and nitrile (C ≡ N
−
). Heterocyclic compounds and the phenyl group
for aniline need to be saturated (i.e., removal of double bonds) before the C-N bond
is destroyed. Typical denitrogenation reaction requires the addition of H 2 via catalytic hydrodenitrogenation. As an example, the mechanism of pyridine hydrodenitrogenation was proposed by Duan and Savage under hydrothermal conditions
using Pt/γ-Al 2 O 3 as the most effective catalyst [101]. The study presented four
sequential steps to (1) removal of double bonds in the cyclic ring (saturation) by
addition of H to form piperidine, (2) cleavage of C-N bond by addition of H to form
pentylamine, (3) hydrolysis by addition of H 2 O to form n-pentanol and NH 3 , and (4)
dehydration by removal of H 2 O and hydrogenation by addition of H 2 to form
n-pentane.
Desulfurization. The amino acid composition of microalgae determines the
S-containing heteroatoms in the biocrude oil, at about <1 wt% [8, 14, 102]. In reference, ULSD has a maximum diesel content of 15 ppm (0.0015 wt%) since 2006
[103]. Sulfur is significantly reduced during microalgae HTL and biocrude oil
upgrading to prevent the formation of So x during fuel use. It has been determined
that Ni/SiO 2 -Al 2 O 3 catalyst promotes more efficient removal of S in the biocrude oil
below the detection limit than Ru/C or CoMo/Al 2 O 3 catalysts [95]. Although there
are limited studies in literature regarding the desulfurization of microalgae HTL and
biocrude oil upgrading, the mechanism of S removal has been investigated in petroleum biocrude. The catalytic dehydrogenation of petroleum biocrude significantly
removes S-containing compounds such as mercaptans and disulfides and to a lesser
degree thiols (thiophenes) with benzene ring(s) (benzothiophenes or dibenzothiophenes) [104, 105]. The body of research on the desulfurization of microalgae HTL
and biocrude oil upgrading focuses on the effects heterogeneous catalysts such as
Pt/C and Pd/C on catalytic activity and regeneration. Nevertheless, further research
is warranted in developing catalysts which effectively removes S while maintaining
high-quality and high-yield biocrude oil.
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae
by Biller et al. validated that the decarboxylation of fatty acids to form alkanes is
achieved in the presence of Na 2 CO 3 [39]. The upgraded biocrude oil is found to
contain higher straight-chain alkanes, higher HHV, lower fraction of high boilers,
and lower oxygen relative to crude bio-oil. The catalysts Pd/C, Pt/C, Pt/Al 2 O 3 ,
Raney-Ni, and HZSM-5 are also beneficial in upgrading biocrude from direct catalytic HTL because they enhance alkane fractions and lower fatty acid content resulting in a better quality biocrude [41, 83, 97–99]. Alkali catalysts (NaOH, KOH) are
appropriate in the monomolecular decarboxylation of stearic acid, while metal
oxide catalysts (CeO 2 , Y 2 O 3 , ZrO 2 ) are suitable for the bimolecular decarboxylation
of stearic acid [100].
Denitrogenation. After HTL, the N from the microalgae biomass is partitioned in
the biocrude oil as N-containing compounds, in the aqueous phase as HCN and
NH 3 , and in the solid phase [72, 82]. N-containing compounds can be heterocyclic
(ring compounds with at least two different elements in its ring) such as pyrazine,
cyclic oxygenates, indole, quinoline, and pyrrole or non-heterocyclic such as aliphatic amines (straight-chain amines), aniline (amino group with phenyl group
attachment), and nitrile (C ≡ N
−
). Heterocyclic compounds and the phenyl group
for aniline need to be saturated (i.e., removal of double bonds) before the C-N bond
is destroyed. Typical denitrogenation reaction requires the addition of H 2 via catalytic hydrodenitrogenation. As an example, the mechanism of pyridine hydrodenitrogenation was proposed by Duan and Savage under hydrothermal conditions
using Pt/γ-Al 2 O 3 as the most effective catalyst [101]. The study presented four
sequential steps to (1) removal of double bonds in the cyclic ring (saturation) by
addition of H to form piperidine, (2) cleavage of C-N bond by addition of H to form
pentylamine, (3) hydrolysis by addition of H 2 O to form n-pentanol and NH 3 , and (4)
dehydration by removal of H 2 O and hydrogenation by addition of H 2 to form
n-pentane.
Desulfurization. The amino acid composition of microalgae determines the
S-containing heteroatoms in the biocrude oil, at about <1 wt% [8, 14, 102]. In reference, ULSD has a maximum diesel content of 15 ppm (0.0015 wt%) since 2006
[103]. Sulfur is significantly reduced during microalgae HTL and biocrude oil
upgrading to prevent the formation of So x during fuel use. It has been determined
that Ni/SiO 2 -Al 2 O 3 catalyst promotes more efficient removal of S in the biocrude oil
below the detection limit than Ru/C or CoMo/Al 2 O 3 catalysts [95]. Although there
are limited studies in literature regarding the desulfurization of microalgae HTL and
biocrude oil upgrading, the mechanism of S removal has been investigated in petroleum biocrude. The catalytic dehydrogenation of petroleum biocrude significantly
removes S-containing compounds such as mercaptans and disulfides and to a lesser
degree thiols (thiophenes) with benzene ring(s) (benzothiophenes or dibenzothiophenes) [104, 105]. The body of research on the desulfurization of microalgae HTL
and biocrude oil upgrading focuses on the effects heterogeneous catalysts such as
Pt/C and Pd/C on catalytic activity and regeneration. Nevertheless, further research
is warranted in developing catalysts which effectively removes S while maintaining
high-quality and high-yield biocrude oil.
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae